Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<function> |
a0d0e21e LW |
3 | |
4 | perlfunc - Perl builtin functions | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
8 | The functions in this section can serve as terms in an expression. | |
9 | They fall into two major categories: list operators and named unary | |
10 | operators. These differ in their precedence relationship with a | |
11 | following comma. (See the precedence table in L<perlop>.) List | |
12 | operators take more than one argument, while unary operators can never | |
13 | take more than one argument. Thus, a comma terminates the argument of | |
14 | a unary operator, but merely separates the arguments of a list | |
8f1da26d | 15 | operator. A unary operator generally provides scalar context to its |
2b5ab1e7 | 16 | argument, while a list operator may provide either scalar or list |
3b10bc60 | 17 | contexts for its arguments. If it does both, scalar arguments |
18 | come first and list argument follow, and there can only ever | |
19 | be one such list argument. For instance, splice() has three scalar | |
2b5ab1e7 TC |
20 | arguments followed by a list, whereas gethostbyname() has four scalar |
21 | arguments. | |
a0d0e21e LW |
22 | |
23 | In the syntax descriptions that follow, list operators that expect a | |
3b10bc60 | 24 | list (and provide list context for elements of the list) are shown |
a0d0e21e LW |
25 | with LIST as an argument. Such a list may consist of any combination |
26 | of scalar arguments or list values; the list values will be included | |
27 | in the list as if each individual element were interpolated at that | |
28 | point in the list, forming a longer single-dimensional list value. | |
8bdbc703 | 29 | Commas should separate literal elements of the LIST. |
a0d0e21e LW |
30 | |
31 | Any function in the list below may be used either with or without | |
32 | parentheses around its arguments. (The syntax descriptions omit the | |
3b10bc60 | 33 | parentheses.) If you use parentheses, the simple but occasionally |
34 | surprising rule is this: It I<looks> like a function, therefore it I<is> a | |
a0d0e21e | 35 | function, and precedence doesn't matter. Otherwise it's a list |
3b10bc60 | 36 | operator or unary operator, and precedence does matter. Whitespace |
37 | between the function and left parenthesis doesn't count, so sometimes | |
38 | you need to be careful: | |
a0d0e21e | 39 | |
5ed4f2ec | 40 | print 1+2+4; # Prints 7. |
41 | print(1+2) + 4; # Prints 3. | |
42 | print (1+2)+4; # Also prints 3! | |
43 | print +(1+2)+4; # Prints 7. | |
44 | print ((1+2)+4); # Prints 7. | |
a0d0e21e LW |
45 | |
46 | If you run Perl with the B<-w> switch it can warn you about this. For | |
47 | example, the third line above produces: | |
48 | ||
49 | print (...) interpreted as function at - line 1. | |
50 | Useless use of integer addition in void context at - line 1. | |
51 | ||
2b5ab1e7 TC |
52 | A few functions take no arguments at all, and therefore work as neither |
53 | unary nor list operators. These include such functions as C<time> | |
54 | and C<endpwent>. For example, C<time+86_400> always means | |
55 | C<time() + 86_400>. | |
56 | ||
a0d0e21e | 57 | For functions that can be used in either a scalar or list context, |
8f1da26d TC |
58 | nonabortive failure is generally indicated in scalar context by |
59 | returning the undefined value, and in list context by returning the | |
3b10bc60 | 60 | empty list. |
a0d0e21e | 61 | |
5a964f20 TC |
62 | Remember the following important rule: There is B<no rule> that relates |
63 | the behavior of an expression in list context to its behavior in scalar | |
64 | context, or vice versa. It might do two totally different things. | |
80d38338 | 65 | Each operator and function decides which sort of value would be most |
2b5ab1e7 | 66 | appropriate to return in scalar context. Some operators return the |
5a964f20 | 67 | length of the list that would have been returned in list context. Some |
a0d0e21e LW |
68 | operators return the first value in the list. Some operators return the |
69 | last value in the list. Some operators return a count of successful | |
70 | operations. In general, they do what you want, unless you want | |
71 | consistency. | |
d74e8afc | 72 | X<context> |
a0d0e21e | 73 | |
d1be9408 | 74 | A named array in scalar context is quite different from what would at |
5a964f20 TC |
75 | first glance appear to be a list in scalar context. You can't get a list |
76 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
77 | the context at compile time. It would generate the scalar comma operator | |
78 | there, not the list construction version of the comma. That means it | |
79 | was never a list to start with. | |
80 | ||
3b10bc60 | 81 | In general, functions in Perl that serve as wrappers for system calls ("syscalls") |
5dac7880 | 82 | of the same name (like chown(2), fork(2), closedir(2), etc.) return |
5a964f20 TC |
83 | true when they succeed and C<undef> otherwise, as is usually mentioned |
84 | in the descriptions below. This is different from the C interfaces, | |
5dac7880 | 85 | which return C<-1> on failure. Exceptions to this rule include C<wait>, |
19799a22 | 86 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> |
5a964f20 TC |
87 | variable on failure. Other functions do not, except accidentally. |
88 | ||
88e1f1a2 JV |
89 | Extension modules can also hook into the Perl parser to define new |
90 | kinds of keyword-headed expression. These may look like functions, but | |
91 | may also look completely different. The syntax following the keyword | |
92 | is defined entirely by the extension. If you are an implementor, see | |
93 | L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such | |
94 | a module, see the module's documentation for details of the syntax that | |
95 | it defines. | |
96 | ||
cb1a09d0 | 97 | =head2 Perl Functions by Category |
d74e8afc | 98 | X<function> |
cb1a09d0 AD |
99 | |
100 | Here are Perl's functions (including things that look like | |
5a964f20 | 101 | functions, like some keywords and named operators) |
cb1a09d0 AD |
102 | arranged by category. Some functions appear in more |
103 | than one place. | |
104 | ||
13a2d996 | 105 | =over 4 |
cb1a09d0 AD |
106 | |
107 | =item Functions for SCALARs or strings | |
d74e8afc | 108 | X<scalar> X<string> X<character> |
cb1a09d0 | 109 | |
22fae026 | 110 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
1dc8ecb8 | 111 | C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>, |
945c54fd | 112 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> |
cb1a09d0 AD |
113 | |
114 | =item Regular expressions and pattern matching | |
d74e8afc | 115 | X<regular expression> X<regex> X<regexp> |
cb1a09d0 | 116 | |
ab4f32c2 | 117 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
cb1a09d0 AD |
118 | |
119 | =item Numeric functions | |
d74e8afc | 120 | X<numeric> X<number> X<trigonometric> X<trigonometry> |
cb1a09d0 | 121 | |
22fae026 TM |
122 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, |
123 | C<sin>, C<sqrt>, C<srand> | |
cb1a09d0 AD |
124 | |
125 | =item Functions for real @ARRAYs | |
d74e8afc | 126 | X<array> |
cb1a09d0 | 127 | |
a5ce339c | 128 | C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values> |
cb1a09d0 AD |
129 | |
130 | =item Functions for list data | |
d74e8afc | 131 | X<list> |
cb1a09d0 | 132 | |
1dc8ecb8 | 133 | C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack> |
cb1a09d0 AD |
134 | |
135 | =item Functions for real %HASHes | |
d74e8afc | 136 | X<hash> |
cb1a09d0 | 137 | |
22fae026 | 138 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
cb1a09d0 AD |
139 | |
140 | =item Input and output functions | |
d74e8afc | 141 | X<I/O> X<input> X<output> X<dbm> |
cb1a09d0 | 142 | |
22fae026 TM |
143 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, |
144 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
0d863452 | 145 | C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>, |
22fae026 TM |
146 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, |
147 | C<warn>, C<write> | |
cb1a09d0 | 148 | |
5dac7880 | 149 | =item Functions for fixed-length data or records |
cb1a09d0 | 150 | |
22fae026 | 151 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
cb1a09d0 AD |
152 | |
153 | =item Functions for filehandles, files, or directories | |
d74e8afc | 154 | X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink> |
cb1a09d0 | 155 | |
22fae026 | 156 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, |
5ff3f7a4 | 157 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, |
1e278fd9 JH |
158 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>, |
159 | C<umask>, C<unlink>, C<utime> | |
cb1a09d0 | 160 | |
cf264981 | 161 | =item Keywords related to the control flow of your Perl program |
d74e8afc | 162 | X<control flow> |
cb1a09d0 | 163 | |
98293880 JH |
164 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, |
165 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> | |
cb1a09d0 | 166 | |
8f1da26d | 167 | =item Keywords related to the switch feature |
0d863452 | 168 | |
4a904372 | 169 | C<break>, C<continue>, C<default>, C<given>, C<when> |
0d863452 | 170 | |
4a904372 FC |
171 | Except for C<continue>, these are available only if you enable the |
172 | C<"switch"> feature or use the C<CORE::> prefix. | |
8f1da26d | 173 | See L<feature> and L<perlsyn/"Switch statements">. |
4a904372 FC |
174 | Alternately, include a C<use v5.10> or later to the current scope. In Perl |
175 | 5.14 and earlier, C<continue> required the C<"switch"> feature, like the | |
176 | other keywords. | |
0d863452 | 177 | |
54310121 | 178 | =item Keywords related to scoping |
cb1a09d0 | 179 | |
8f1da26d | 180 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use> |
36fb85f3 | 181 | |
4a904372 FC |
182 | C<state> is available only if the C<"state"> feature |
183 | is enabled or if it is prefixed with C<CORE::>. See | |
8f1da26d | 184 | L<feature>. Alternately, include a C<use v5.10> or later to the current scope. |
cb1a09d0 AD |
185 | |
186 | =item Miscellaneous functions | |
187 | ||
36fb85f3 | 188 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, |
834df1c5 | 189 | C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray> |
cb1a09d0 AD |
190 | |
191 | =item Functions for processes and process groups | |
d74e8afc | 192 | X<process> X<pid> X<process id> |
cb1a09d0 | 193 | |
22fae026 | 194 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
1dc8ecb8 | 195 | C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
22fae026 | 196 | C<times>, C<wait>, C<waitpid> |
cb1a09d0 | 197 | |
3b10bc60 | 198 | =item Keywords related to Perl modules |
d74e8afc | 199 | X<module> |
cb1a09d0 | 200 | |
22fae026 | 201 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
cb1a09d0 | 202 | |
353c6505 | 203 | =item Keywords related to classes and object-orientation |
d74e8afc | 204 | X<object> X<class> X<package> |
cb1a09d0 | 205 | |
22fae026 TM |
206 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
207 | C<untie>, C<use> | |
cb1a09d0 AD |
208 | |
209 | =item Low-level socket functions | |
d74e8afc | 210 | X<socket> X<sock> |
cb1a09d0 | 211 | |
22fae026 TM |
212 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
213 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
737dd4b4 | 214 | C<socket>, C<socketpair> |
cb1a09d0 AD |
215 | |
216 | =item System V interprocess communication functions | |
d74e8afc | 217 | X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> |
cb1a09d0 | 218 | |
22fae026 TM |
219 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
220 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
cb1a09d0 AD |
221 | |
222 | =item Fetching user and group info | |
d74e8afc | 223 | X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> |
cb1a09d0 | 224 | |
22fae026 TM |
225 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
226 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
227 | C<getpwuid>, C<setgrent>, C<setpwent> | |
cb1a09d0 AD |
228 | |
229 | =item Fetching network info | |
d74e8afc | 230 | X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> |
cb1a09d0 | 231 | |
22fae026 TM |
232 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
233 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
234 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
235 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
236 | C<setnetent>, C<setprotoent>, C<setservent> | |
cb1a09d0 AD |
237 | |
238 | =item Time-related functions | |
d74e8afc | 239 | X<time> X<date> |
cb1a09d0 | 240 | |
22fae026 | 241 | C<gmtime>, C<localtime>, C<time>, C<times> |
cb1a09d0 | 242 | |
37798a01 | 243 | =item Functions new in perl5 |
d74e8afc | 244 | X<perl5> |
37798a01 | 245 | |
834df1c5 SP |
246 | C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>, |
247 | C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>, | |
1dc8ecb8 | 248 | C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>, |
834df1c5 SP |
249 | C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, |
250 | C<ucfirst>, C<untie>, C<use>, C<when> | |
37798a01 | 251 | |
3b10bc60 | 252 | * C<sub> was a keyword in Perl 4, but in Perl 5 it is an |
5a964f20 | 253 | operator, which can be used in expressions. |
37798a01 | 254 | |
255 | =item Functions obsoleted in perl5 | |
256 | ||
22fae026 | 257 | C<dbmclose>, C<dbmopen> |
37798a01 | 258 | |
cb1a09d0 AD |
259 | =back |
260 | ||
60f9f73c | 261 | =head2 Portability |
d74e8afc | 262 | X<portability> X<Unix> X<portable> |
60f9f73c | 263 | |
2b5ab1e7 TC |
264 | Perl was born in Unix and can therefore access all common Unix |
265 | system calls. In non-Unix environments, the functionality of some | |
8f1da26d | 266 | Unix system calls may not be available or details of the available |
2b5ab1e7 | 267 | functionality may differ slightly. The Perl functions affected |
60f9f73c JH |
268 | by this are: |
269 | ||
270 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
271 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
272 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
ef5a6dd7 JH |
273 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>, |
274 | C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
54d7b083 | 275 | C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>, |
60f9f73c JH |
276 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, |
277 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
278 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
2b5ab1e7 | 279 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, |
60f9f73c JH |
280 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
281 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
282 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
737dd4b4 | 283 | C<shmwrite>, C<socket>, C<socketpair>, |
80cbd5ad JH |
284 | C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, |
285 | C<times>, C<truncate>, C<umask>, C<unlink>, | |
2b5ab1e7 | 286 | C<utime>, C<wait>, C<waitpid> |
60f9f73c JH |
287 | |
288 | For more information about the portability of these functions, see | |
289 | L<perlport> and other available platform-specific documentation. | |
290 | ||
cb1a09d0 AD |
291 | =head2 Alphabetical Listing of Perl Functions |
292 | ||
3b10bc60 | 293 | =over |
a0d0e21e | 294 | |
5b3c99c0 | 295 | =item -X FILEHANDLE |
d74e8afc ITB |
296 | X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> |
297 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> | |
a0d0e21e | 298 | |
5b3c99c0 | 299 | =item -X EXPR |
a0d0e21e | 300 | |
5228a96c SP |
301 | =item -X DIRHANDLE |
302 | ||
5b3c99c0 | 303 | =item -X |
a0d0e21e LW |
304 | |
305 | A file test, where X is one of the letters listed below. This unary | |
5228a96c SP |
306 | operator takes one argument, either a filename, a filehandle, or a dirhandle, |
307 | and tests the associated file to see if something is true about it. If the | |
7660c0ab | 308 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
19799a22 | 309 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or |
a0d0e21e | 310 | the undefined value if the file doesn't exist. Despite the funny |
d0821a6a | 311 | names, precedence is the same as any other named unary operator. The |
a0d0e21e LW |
312 | operator may be any of: |
313 | ||
5ed4f2ec | 314 | -r File is readable by effective uid/gid. |
315 | -w File is writable by effective uid/gid. | |
316 | -x File is executable by effective uid/gid. | |
317 | -o File is owned by effective uid. | |
a0d0e21e | 318 | |
5ed4f2ec | 319 | -R File is readable by real uid/gid. |
320 | -W File is writable by real uid/gid. | |
321 | -X File is executable by real uid/gid. | |
322 | -O File is owned by real uid. | |
a0d0e21e | 323 | |
5ed4f2ec | 324 | -e File exists. |
325 | -z File has zero size (is empty). | |
326 | -s File has nonzero size (returns size in bytes). | |
a0d0e21e | 327 | |
5ed4f2ec | 328 | -f File is a plain file. |
329 | -d File is a directory. | |
330 | -l File is a symbolic link. | |
331 | -p File is a named pipe (FIFO), or Filehandle is a pipe. | |
332 | -S File is a socket. | |
333 | -b File is a block special file. | |
334 | -c File is a character special file. | |
335 | -t Filehandle is opened to a tty. | |
a0d0e21e | 336 | |
5ed4f2ec | 337 | -u File has setuid bit set. |
338 | -g File has setgid bit set. | |
339 | -k File has sticky bit set. | |
a0d0e21e | 340 | |
5ed4f2ec | 341 | -T File is an ASCII text file (heuristic guess). |
342 | -B File is a "binary" file (opposite of -T). | |
a0d0e21e | 343 | |
5ed4f2ec | 344 | -M Script start time minus file modification time, in days. |
345 | -A Same for access time. | |
346 | -C Same for inode change time (Unix, may differ for other platforms) | |
a0d0e21e | 347 | |
a0d0e21e LW |
348 | Example: |
349 | ||
350 | while (<>) { | |
a9a5a0dc VP |
351 | chomp; |
352 | next unless -f $_; # ignore specials | |
353 | #... | |
a0d0e21e LW |
354 | } |
355 | ||
4fb67938 FC |
356 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
357 | C<-exp($foo)> still works as expected, however: only single letters | |
358 | following a minus are interpreted as file tests. | |
359 | ||
360 | These operators are exempt from the "looks like a function rule" described | |
4d0444a3 FC |
361 | above. That is, an opening parenthesis after the operator does not affect |
362 | how much of the following code constitutes the argument. Put the opening | |
4fb67938 FC |
363 | parentheses before the operator to separate it from code that follows (this |
364 | applies only to operators with higher precedence than unary operators, of | |
365 | course): | |
366 | ||
367 | -s($file) + 1024 # probably wrong; same as -s($file + 1024) | |
368 | (-s $file) + 1024 # correct | |
369 | ||
5ff3f7a4 GS |
370 | The interpretation of the file permission operators C<-r>, C<-R>, |
371 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
372 | of the file and the uids and gids of the user. There may be other | |
ecae030f MO |
373 | reasons you can't actually read, write, or execute the file: for |
374 | example network filesystem access controls, ACLs (access control lists), | |
375 | read-only filesystems, and unrecognized executable formats. Note | |
376 | that the use of these six specific operators to verify if some operation | |
377 | is possible is usually a mistake, because it may be open to race | |
378 | conditions. | |
5ff3f7a4 | 379 | |
2b5ab1e7 TC |
380 | Also note that, for the superuser on the local filesystems, the C<-r>, |
381 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
5ff3f7a4 GS |
382 | if any execute bit is set in the mode. Scripts run by the superuser |
383 | may thus need to do a stat() to determine the actual mode of the file, | |
2b5ab1e7 | 384 | or temporarily set their effective uid to something else. |
5ff3f7a4 GS |
385 | |
386 | If you are using ACLs, there is a pragma called C<filetest> that may | |
387 | produce more accurate results than the bare stat() mode bits. | |
5dac7880 FC |
388 | When under C<use filetest 'access'> the above-mentioned filetests |
389 | test whether the permission can(not) be granted using the | |
3b10bc60 | 390 | access(2) family of system calls. Also note that the C<-x> and C<-X> may |
5ff3f7a4 GS |
391 | under this pragma return true even if there are no execute permission |
392 | bits set (nor any extra execute permission ACLs). This strangeness is | |
ecae030f MO |
393 | due to the underlying system calls' definitions. Note also that, due to |
394 | the implementation of C<use filetest 'access'>, the C<_> special | |
395 | filehandle won't cache the results of the file tests when this pragma is | |
396 | in effect. Read the documentation for the C<filetest> pragma for more | |
397 | information. | |
5ff3f7a4 | 398 | |
a0d0e21e LW |
399 | The C<-T> and C<-B> switches work as follows. The first block or so of the |
400 | file is examined for odd characters such as strange control codes or | |
61eff3bc | 401 | characters with the high bit set. If too many strange characters (>30%) |
cf264981 | 402 | are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file |
3b10bc60 | 403 | containing a zero byte in the first block is considered a binary file. If C<-T> |
9124316e | 404 | or C<-B> is used on a filehandle, the current IO buffer is examined |
3b10bc60 | 405 | rather than the first block. Both C<-T> and C<-B> return true on an empty |
54310121 | 406 | file, or a file at EOF when testing a filehandle. Because you have to |
4633a7c4 LW |
407 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
408 | against the file first, as in C<next unless -f $file && -T $file>. | |
a0d0e21e | 409 | |
5dac7880 | 410 | If any of the file tests (or either the C<stat> or C<lstat> operator) is given |
28757baa | 411 | the special filehandle consisting of a solitary underline, then the stat |
a0d0e21e LW |
412 | structure of the previous file test (or stat operator) is used, saving |
413 | a system call. (This doesn't work with C<-t>, and you need to remember | |
3b10bc60 | 414 | that lstat() and C<-l> leave values in the stat structure for the |
5c9aa243 | 415 | symbolic link, not the real file.) (Also, if the stat buffer was filled by |
cf264981 | 416 | an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). |
5c9aa243 | 417 | Example: |
a0d0e21e LW |
418 | |
419 | print "Can do.\n" if -r $a || -w _ || -x _; | |
420 | ||
421 | stat($filename); | |
422 | print "Readable\n" if -r _; | |
423 | print "Writable\n" if -w _; | |
424 | print "Executable\n" if -x _; | |
425 | print "Setuid\n" if -u _; | |
426 | print "Setgid\n" if -g _; | |
427 | print "Sticky\n" if -k _; | |
428 | print "Text\n" if -T _; | |
429 | print "Binary\n" if -B _; | |
430 | ||
fbb0b3b3 RGS |
431 | As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file |
432 | test operators, in a way that C<-f -w -x $file> is equivalent to | |
3b10bc60 | 433 | C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use |
fbb0b3b3 RGS |
434 | the return value of C<-f $file> as an argument to another filetest |
435 | operator, no special magic will happen.) | |
436 | ||
bee96257 | 437 | Portability issues: L<perlport/-X>. |
ea9eb35a | 438 | |
a0d0e21e | 439 | =item abs VALUE |
d74e8afc | 440 | X<abs> X<absolute> |
a0d0e21e | 441 | |
54310121 | 442 | =item abs |
bbce6d69 | 443 | |
a0d0e21e | 444 | Returns the absolute value of its argument. |
7660c0ab | 445 | If VALUE is omitted, uses C<$_>. |
a0d0e21e LW |
446 | |
447 | =item accept NEWSOCKET,GENERICSOCKET | |
d74e8afc | 448 | X<accept> |
a0d0e21e | 449 | |
3b10bc60 | 450 | Accepts an incoming socket connect, just as accept(2) |
19799a22 | 451 | does. Returns the packed address if it succeeded, false otherwise. |
2b5ab1e7 | 452 | See the example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e | 453 | |
8d2a6795 GS |
454 | On systems that support a close-on-exec flag on files, the flag will |
455 | be set for the newly opened file descriptor, as determined by the | |
456 | value of $^F. See L<perlvar/$^F>. | |
457 | ||
a0d0e21e | 458 | =item alarm SECONDS |
d74e8afc ITB |
459 | X<alarm> |
460 | X<SIGALRM> | |
461 | X<timer> | |
a0d0e21e | 462 | |
54310121 | 463 | =item alarm |
bbce6d69 | 464 | |
a0d0e21e | 465 | Arranges to have a SIGALRM delivered to this process after the |
cf264981 | 466 | specified number of wallclock seconds has elapsed. If SECONDS is not |
d400eac8 JH |
467 | specified, the value stored in C<$_> is used. (On some machines, |
468 | unfortunately, the elapsed time may be up to one second less or more | |
469 | than you specified because of how seconds are counted, and process | |
470 | scheduling may delay the delivery of the signal even further.) | |
471 | ||
472 | Only one timer may be counting at once. Each call disables the | |
473 | previous timer, and an argument of C<0> may be supplied to cancel the | |
474 | previous timer without starting a new one. The returned value is the | |
475 | amount of time remaining on the previous timer. | |
a0d0e21e | 476 | |
2bc69794 BS |
477 | For delays of finer granularity than one second, the Time::HiRes module |
478 | (from CPAN, and starting from Perl 5.8 part of the standard | |
479 | distribution) provides ualarm(). You may also use Perl's four-argument | |
480 | version of select() leaving the first three arguments undefined, or you | |
481 | might be able to use the C<syscall> interface to access setitimer(2) if | |
482 | your system supports it. See L<perlfaq8> for details. | |
2b5ab1e7 | 483 | |
80d38338 TC |
484 | It is usually a mistake to intermix C<alarm> and C<sleep> calls, because |
485 | C<sleep> may be internally implemented on your system with C<alarm>. | |
a0d0e21e | 486 | |
19799a22 GS |
487 | If you want to use C<alarm> to time out a system call you need to use an |
488 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
f86cebdf | 489 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
19799a22 | 490 | restart system calls on some systems. Using C<eval>/C<die> always works, |
5a964f20 | 491 | modulo the caveats given in L<perlipc/"Signals">. |
ff68c719 | 492 | |
493 | eval { | |
a9a5a0dc VP |
494 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
495 | alarm $timeout; | |
496 | $nread = sysread SOCKET, $buffer, $size; | |
497 | alarm 0; | |
ff68c719 | 498 | }; |
ff68c719 | 499 | if ($@) { |
a9a5a0dc | 500 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
5ed4f2ec | 501 | # timed out |
ff68c719 | 502 | } |
503 | else { | |
5ed4f2ec | 504 | # didn't |
ff68c719 | 505 | } |
506 | ||
91d81acc JH |
507 | For more information see L<perlipc>. |
508 | ||
ea9eb35a BJ |
509 | Portability issues: L<perlport/alarm>. |
510 | ||
a0d0e21e | 511 | =item atan2 Y,X |
d74e8afc | 512 | X<atan2> X<arctangent> X<tan> X<tangent> |
a0d0e21e LW |
513 | |
514 | Returns the arctangent of Y/X in the range -PI to PI. | |
515 | ||
ca6e1c26 | 516 | For the tangent operation, you may use the C<Math::Trig::tan> |
28757baa | 517 | function, or use the familiar relation: |
518 | ||
519 | sub tan { sin($_[0]) / cos($_[0]) } | |
520 | ||
a1021d57 RGS |
521 | The return value for C<atan2(0,0)> is implementation-defined; consult |
522 | your atan2(3) manpage for more information. | |
bf5f1b4c | 523 | |
ea9eb35a BJ |
524 | Portability issues: L<perlport/atan2>. |
525 | ||
a0d0e21e | 526 | =item bind SOCKET,NAME |
d74e8afc | 527 | X<bind> |
a0d0e21e | 528 | |
3b10bc60 | 529 | Binds a network address to a socket, just as bind(2) |
19799a22 | 530 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
531 | packed address of the appropriate type for the socket. See the examples in |
532 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 533 | |
fae2c0fb | 534 | =item binmode FILEHANDLE, LAYER |
d74e8afc | 535 | X<binmode> X<binary> X<text> X<DOS> X<Windows> |
1c1fc3ea | 536 | |
a0d0e21e LW |
537 | =item binmode FILEHANDLE |
538 | ||
1cbfc93d NIS |
539 | Arranges for FILEHANDLE to be read or written in "binary" or "text" |
540 | mode on systems where the run-time libraries distinguish between | |
541 | binary and text files. If FILEHANDLE is an expression, the value is | |
542 | taken as the name of the filehandle. Returns true on success, | |
b5fe5ca2 | 543 | otherwise it returns C<undef> and sets C<$!> (errno). |
1cbfc93d | 544 | |
8f1da26d | 545 | On some systems (in general, DOS- and Windows-based systems) binmode() |
d807c6f4 | 546 | is necessary when you're not working with a text file. For the sake |
d7a0d798 FC |
547 | of portability it is a good idea always to use it when appropriate, |
548 | and never to use it when it isn't appropriate. Also, people can | |
8f1da26d | 549 | set their I/O to be by default UTF8-encoded Unicode, not bytes. |
d807c6f4 JH |
550 | |
551 | In other words: regardless of platform, use binmode() on binary data, | |
d7a0d798 | 552 | like images, for example. |
d807c6f4 JH |
553 | |
554 | If LAYER is present it is a single string, but may contain multiple | |
3b10bc60 | 555 | directives. The directives alter the behaviour of the filehandle. |
d7a0d798 | 556 | When LAYER is present, using binmode on a text file makes sense. |
d807c6f4 | 557 | |
fae2c0fb | 558 | If LAYER is omitted or specified as C<:raw> the filehandle is made |
0226bbdb NIS |
559 | suitable for passing binary data. This includes turning off possible CRLF |
560 | translation and marking it as bytes (as opposed to Unicode characters). | |
749683d2 | 561 | Note that, despite what may be implied in I<"Programming Perl"> (the |
3b10bc60 | 562 | Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>. |
563 | Other layers that would affect the binary nature of the stream are | |
564 | I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the | |
0226bbdb | 565 | PERLIO environment variable. |
01e6739c | 566 | |
3b10bc60 | 567 | The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the |
d807c6f4 JH |
568 | form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to |
569 | establish default I/O layers. See L<open>. | |
570 | ||
fae2c0fb RGS |
571 | I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" |
572 | in "Programming Perl, 3rd Edition". However, since the publishing of this | |
573 | book, by many known as "Camel III", the consensus of the naming of this | |
574 | functionality has moved from "discipline" to "layer". All documentation | |
575 | of this version of Perl therefore refers to "layers" rather than to | |
576 | "disciplines". Now back to the regularly scheduled documentation...> | |
577 | ||
8f1da26d | 578 | To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>. |
6902c96a | 579 | C<:utf8> just marks the data as UTF-8 without further checking, |
8f1da26d | 580 | while C<:encoding(UTF-8)> checks the data for actually being valid |
6902c96a | 581 | UTF-8. More details can be found in L<PerlIO::encoding>. |
1cbfc93d | 582 | |
ed53a2bb | 583 | In general, binmode() should be called after open() but before any I/O |
3b10bc60 | 584 | is done on the filehandle. Calling binmode() normally flushes any |
01e6739c | 585 | pending buffered output data (and perhaps pending input data) on the |
fae2c0fb | 586 | handle. An exception to this is the C<:encoding> layer that |
d7a0d798 | 587 | changes the default character encoding of the handle; see L</open>. |
fae2c0fb | 588 | The C<:encoding> layer sometimes needs to be called in |
3874323d JH |
589 | mid-stream, and it doesn't flush the stream. The C<:encoding> |
590 | also implicitly pushes on top of itself the C<:utf8> layer because | |
3b10bc60 | 591 | internally Perl operates on UTF8-encoded Unicode characters. |
16fe6d59 | 592 | |
19799a22 | 593 | The operating system, device drivers, C libraries, and Perl run-time |
8f1da26d TC |
594 | system all conspire to let the programmer treat a single |
595 | character (C<\n>) as the line terminator, irrespective of external | |
30168b04 GS |
596 | representation. On many operating systems, the native text file |
597 | representation matches the internal representation, but on some | |
598 | platforms the external representation of C<\n> is made up of more than | |
599 | one character. | |
600 | ||
8f1da26d TC |
601 | All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use |
602 | a single character to end each line in the external representation of text | |
603 | (even though that single character is CARRIAGE RETURN on old, pre-Darwin | |
604 | flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other | |
605 | systems like OS/2, DOS, and the various flavors of MS-Windows, your program | |
606 | sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the | |
607 | two characters C<\cM\cJ>. That means that if you don't use binmode() on | |
608 | these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on | |
609 | input, and any C<\n> in your program will be converted back to C<\cM\cJ> on | |
610 | output. This is what you want for text files, but it can be disastrous for | |
611 | binary files. | |
30168b04 GS |
612 | |
613 | Another consequence of using binmode() (on some systems) is that | |
614 | special end-of-file markers will be seen as part of the data stream. | |
d7a0d798 FC |
615 | For systems from the Microsoft family this means that, if your binary |
616 | data contain C<\cZ>, the I/O subsystem will regard it as the end of | |
30168b04 GS |
617 | the file, unless you use binmode(). |
618 | ||
3b10bc60 | 619 | binmode() is important not only for readline() and print() operations, |
30168b04 GS |
620 | but also when using read(), seek(), sysread(), syswrite() and tell() |
621 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
622 | in L<perlvar> for how to manually set your input and output | |
623 | line-termination sequences. | |
a0d0e21e | 624 | |
ea9eb35a BJ |
625 | Portability issues: L<perlport/binmode>. |
626 | ||
4633a7c4 | 627 | =item bless REF,CLASSNAME |
d74e8afc | 628 | X<bless> |
a0d0e21e LW |
629 | |
630 | =item bless REF | |
631 | ||
2b5ab1e7 TC |
632 | This function tells the thingy referenced by REF that it is now an object |
633 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
19799a22 | 634 | is used. Because a C<bless> is often the last thing in a constructor, |
2b5ab1e7 | 635 | it returns the reference for convenience. Always use the two-argument |
cf264981 SP |
636 | version if a derived class might inherit the function doing the blessing. |
637 | See L<perltoot> and L<perlobj> for more about the blessing (and blessings) | |
638 | of objects. | |
a0d0e21e | 639 | |
57668c4d | 640 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
2b5ab1e7 | 641 | Namespaces with all lowercase names are considered reserved for |
cf264981 | 642 | Perl pragmata. Builtin types have all uppercase names. To prevent |
2b5ab1e7 TC |
643 | confusion, you may wish to avoid such package names as well. Make sure |
644 | that CLASSNAME is a true value. | |
60ad88b8 GS |
645 | |
646 | See L<perlmod/"Perl Modules">. | |
647 | ||
0d863452 RH |
648 | =item break |
649 | ||
650 | Break out of a C<given()> block. | |
651 | ||
8f1da26d | 652 | This keyword is enabled by the C<"switch"> feature: see |
4a904372 FC |
653 | L<feature> for more information. You can also access it by |
654 | prefixing it with C<CORE::>. Alternately, include a C<use | |
8f1da26d | 655 | v5.10> or later to the current scope. |
0d863452 | 656 | |
a0d0e21e | 657 | =item caller EXPR |
d74e8afc | 658 | X<caller> X<call stack> X<stack> X<stack trace> |
a0d0e21e LW |
659 | |
660 | =item caller | |
661 | ||
5a964f20 | 662 | Returns the context of the current subroutine call. In scalar context, |
80d38338 TC |
663 | returns the caller's package name if there I<is> a caller (that is, if |
664 | we're in a subroutine or C<eval> or C<require>) and the undefined value | |
5a964f20 | 665 | otherwise. In list context, returns |
a0d0e21e | 666 | |
ee6b43cc | 667 | # 0 1 2 |
748a9306 | 668 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
669 | |
670 | With EXPR, it returns some extra information that the debugger uses to | |
671 | print a stack trace. The value of EXPR indicates how many call frames | |
672 | to go back before the current one. | |
673 | ||
ee6b43cc | 674 | # 0 1 2 3 4 |
f3aa04c2 | 675 | ($package, $filename, $line, $subroutine, $hasargs, |
ee6b43cc | 676 | |
677 | # 5 6 7 8 9 10 | |
b3ca2e83 | 678 | $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) |
ee6b43cc | 679 | = caller($i); |
e7ea3e70 | 680 | |
951ba7fe | 681 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
19799a22 | 682 | call, but an C<eval>. In such a case additional elements $evaltext and |
7660c0ab | 683 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
19799a22 | 684 | C<require> or C<use> statement, $evaltext contains the text of the |
277ddfaf | 685 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, |
cc1c2e42 | 686 | $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that |
0fc9dec4 RGS |
687 | each C<use> statement creates a C<require> frame inside an C<eval EXPR> |
688 | frame.) $subroutine may also be C<(unknown)> if this particular | |
689 | subroutine happens to have been deleted from the symbol table. | |
690 | C<$hasargs> is true if a new instance of C<@_> was set up for the frame. | |
691 | C<$hints> and C<$bitmask> contain pragmatic hints that the caller was | |
692 | compiled with. The C<$hints> and C<$bitmask> values are subject to change | |
693 | between versions of Perl, and are not meant for external use. | |
748a9306 | 694 | |
b3ca2e83 NC |
695 | C<$hinthash> is a reference to a hash containing the value of C<%^H> when the |
696 | caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values | |
697 | of this hash, as they are the actual values stored in the optree. | |
698 | ||
748a9306 | 699 | Furthermore, when called from within the DB package, caller returns more |
7660c0ab | 700 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 701 | arguments with which the subroutine was invoked. |
748a9306 | 702 | |
7660c0ab | 703 | Be aware that the optimizer might have optimized call frames away before |
19799a22 | 704 | C<caller> had a chance to get the information. That means that C<caller(N)> |
80d38338 | 705 | might not return information about the call frame you expect it to, for |
b76cc8ba | 706 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the |
19799a22 | 707 | previous time C<caller> was called. |
7660c0ab | 708 | |
8f1da26d | 709 | Be aware that setting C<@DB::args> is I<best effort>, intended for |
ca9f0cb5 NC |
710 | debugging or generating backtraces, and should not be relied upon. In |
711 | particular, as C<@_> contains aliases to the caller's arguments, Perl does | |
712 | not take a copy of C<@_>, so C<@DB::args> will contain modifications the | |
713 | subroutine makes to C<@_> or its contents, not the original values at call | |
714 | time. C<@DB::args>, like C<@_>, does not hold explicit references to its | |
715 | elements, so under certain cases its elements may have become freed and | |
716 | reallocated for other variables or temporary values. Finally, a side effect | |
d7a0d798 | 717 | of the current implementation is that the effects of C<shift @_> can |
8f1da26d TC |
718 | I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a |
719 | reference to C<@_> has been taken, I<and> subject to the caveat about reallocated | |
ca9f0cb5 NC |
720 | elements), so C<@DB::args> is actually a hybrid of the current state and |
721 | initial state of C<@_>. Buyer beware. | |
722 | ||
a0d0e21e | 723 | =item chdir EXPR |
d74e8afc ITB |
724 | X<chdir> |
725 | X<cd> | |
f723aae1 | 726 | X<directory, change> |
a0d0e21e | 727 | |
c4aca7d0 GA |
728 | =item chdir FILEHANDLE |
729 | ||
730 | =item chdir DIRHANDLE | |
731 | ||
ce2984c3 PF |
732 | =item chdir |
733 | ||
ffce7b87 | 734 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
0bfc1ec4 | 735 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
ffce7b87 | 736 | changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the |
b4ad75f0 | 737 | variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If |
80d38338 | 738 | neither is set, C<chdir> does nothing. It returns true on success, |
b4ad75f0 | 739 | false otherwise. See the example under C<die>. |
a0d0e21e | 740 | |
3b10bc60 | 741 | On systems that support fchdir(2), you may pass a filehandle or |
34169887 | 742 | directory handle as the argument. On systems that don't support fchdir(2), |
3b10bc60 | 743 | passing handles raises an exception. |
c4aca7d0 | 744 | |
a0d0e21e | 745 | =item chmod LIST |
d74e8afc | 746 | X<chmod> X<permission> X<mode> |
a0d0e21e LW |
747 | |
748 | Changes the permissions of a list of files. The first element of the | |
8f1da26d | 749 | list must be the numeric mode, which should probably be an octal |
4ad40acf | 750 | number, and which definitely should I<not> be a string of octal digits: |
3b10bc60 | 751 | C<0644> is okay, but C<"0644"> is not. Returns the number of files |
8f1da26d | 752 | successfully changed. See also L</oct> if all you have is a string. |
a0d0e21e | 753 | |
3b10bc60 | 754 | $cnt = chmod 0755, "foo", "bar"; |
a0d0e21e | 755 | chmod 0755, @executables; |
3b10bc60 | 756 | $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to |
f86cebdf | 757 | # --w----r-T |
3b10bc60 | 758 | $mode = "0644"; chmod oct($mode), "foo"; # this is better |
759 | $mode = 0644; chmod $mode, "foo"; # this is best | |
a0d0e21e | 760 | |
3b10bc60 | 761 | On systems that support fchmod(2), you may pass filehandles among the |
762 | files. On systems that don't support fchmod(2), passing filehandles raises | |
763 | an exception. Filehandles must be passed as globs or glob references to be | |
764 | recognized; barewords are considered filenames. | |
c4aca7d0 GA |
765 | |
766 | open(my $fh, "<", "foo"); | |
767 | my $perm = (stat $fh)[2] & 07777; | |
768 | chmod($perm | 0600, $fh); | |
769 | ||
3b10bc60 | 770 | You can also import the symbolic C<S_I*> constants from the C<Fcntl> |
ca6e1c26 JH |
771 | module: |
772 | ||
3b10bc60 | 773 | use Fcntl qw( :mode ); |
ca6e1c26 | 774 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; |
3b10bc60 | 775 | # Identical to the chmod 0755 of the example above. |
ca6e1c26 | 776 | |
ea9eb35a BJ |
777 | Portability issues: L<perlport/chmod>. |
778 | ||
a0d0e21e | 779 | =item chomp VARIABLE |
d74e8afc | 780 | X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> |
a0d0e21e | 781 | |
313c9f5c | 782 | =item chomp( LIST ) |
a0d0e21e LW |
783 | |
784 | =item chomp | |
785 | ||
2b5ab1e7 TC |
786 | This safer version of L</chop> removes any trailing string |
787 | that corresponds to the current value of C<$/> (also known as | |
28757baa | 788 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
789 | number of characters removed from all its arguments. It's often used to | |
790 | remove the newline from the end of an input record when you're worried | |
2b5ab1e7 TC |
791 | that the final record may be missing its newline. When in paragraph |
792 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
4c5a6083 | 793 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
34169887 | 794 | a reference to an integer or the like; see L<perlvar>) chomp() won't |
b76cc8ba | 795 | remove anything. |
19799a22 | 796 | If VARIABLE is omitted, it chomps C<$_>. Example: |
a0d0e21e LW |
797 | |
798 | while (<>) { | |
a9a5a0dc VP |
799 | chomp; # avoid \n on last field |
800 | @array = split(/:/); | |
801 | # ... | |
a0d0e21e LW |
802 | } |
803 | ||
4bf21a6d RD |
804 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. |
805 | ||
a0d0e21e LW |
806 | You can actually chomp anything that's an lvalue, including an assignment: |
807 | ||
808 | chomp($cwd = `pwd`); | |
809 | chomp($answer = <STDIN>); | |
810 | ||
811 | If you chomp a list, each element is chomped, and the total number of | |
812 | characters removed is returned. | |
813 | ||
15e44fd8 RGS |
814 | Note that parentheses are necessary when you're chomping anything |
815 | that is not a simple variable. This is because C<chomp $cwd = `pwd`;> | |
816 | is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as | |
817 | C<chomp( $cwd = `pwd` )> which you might expect. Similarly, | |
818 | C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than | |
819 | as C<chomp($a, $b)>. | |
820 | ||
a0d0e21e | 821 | =item chop VARIABLE |
d74e8afc | 822 | X<chop> |
a0d0e21e | 823 | |
313c9f5c | 824 | =item chop( LIST ) |
a0d0e21e LW |
825 | |
826 | =item chop | |
827 | ||
828 | Chops off the last character of a string and returns the character | |
5b3eff12 | 829 | chopped. It is much more efficient than C<s/.$//s> because it neither |
7660c0ab | 830 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
4bf21a6d RD |
831 | If VARIABLE is a hash, it chops the hash's values, but not its keys. |
832 | ||
5b3eff12 | 833 | You can actually chop anything that's an lvalue, including an assignment. |
a0d0e21e LW |
834 | |
835 | If you chop a list, each element is chopped. Only the value of the | |
19799a22 | 836 | last C<chop> is returned. |
a0d0e21e | 837 | |
19799a22 | 838 | Note that C<chop> returns the last character. To return all but the last |
748a9306 LW |
839 | character, use C<substr($string, 0, -1)>. |
840 | ||
15e44fd8 RGS |
841 | See also L</chomp>. |
842 | ||
a0d0e21e | 843 | =item chown LIST |
d74e8afc | 844 | X<chown> X<owner> X<user> X<group> |
a0d0e21e LW |
845 | |
846 | Changes the owner (and group) of a list of files. The first two | |
19799a22 GS |
847 | elements of the list must be the I<numeric> uid and gid, in that |
848 | order. A value of -1 in either position is interpreted by most | |
849 | systems to leave that value unchanged. Returns the number of files | |
850 | successfully changed. | |
a0d0e21e LW |
851 | |
852 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
853 | chown $uid, $gid, @filenames; | |
854 | ||
3b10bc60 | 855 | On systems that support fchown(2), you may pass filehandles among the |
856 | files. On systems that don't support fchown(2), passing filehandles raises | |
857 | an exception. Filehandles must be passed as globs or glob references to be | |
858 | recognized; barewords are considered filenames. | |
c4aca7d0 | 859 | |
54310121 | 860 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
861 | |
862 | print "User: "; | |
19799a22 | 863 | chomp($user = <STDIN>); |
5a964f20 | 864 | print "Files: "; |
19799a22 | 865 | chomp($pattern = <STDIN>); |
a0d0e21e LW |
866 | |
867 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
a9a5a0dc | 868 | or die "$user not in passwd file"; |
a0d0e21e | 869 | |
5ed4f2ec | 870 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
871 | chown $uid, $gid, @ary; |
872 | ||
54310121 | 873 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
874 | file unless you're the superuser, although you should be able to change |
875 | the group to any of your secondary groups. On insecure systems, these | |
876 | restrictions may be relaxed, but this is not a portable assumption. | |
19799a22 GS |
877 | On POSIX systems, you can detect this condition this way: |
878 | ||
879 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
880 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
4633a7c4 | 881 | |
ea9eb35a BJ |
882 | Portability issues: L<perlport/chmod>. |
883 | ||
a0d0e21e | 884 | =item chr NUMBER |
d74e8afc | 885 | X<chr> X<character> X<ASCII> X<Unicode> |
a0d0e21e | 886 | |
54310121 | 887 | =item chr |
bbce6d69 | 888 | |
a0d0e21e | 889 | Returns the character represented by that NUMBER in the character set. |
a0ed51b3 | 890 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
2575c402 | 891 | chr(0x263a) is a Unicode smiley face. |
aaa68c4a | 892 | |
8a064bd6 | 893 | Negative values give the Unicode replacement character (chr(0xfffd)), |
80d38338 | 894 | except under the L<bytes> pragma, where the low eight bits of the value |
8a064bd6 JH |
895 | (truncated to an integer) are used. |
896 | ||
974da8e5 JH |
897 | If NUMBER is omitted, uses C<$_>. |
898 | ||
b76cc8ba | 899 | For the reverse, use L</ord>. |
a0d0e21e | 900 | |
2575c402 JW |
901 | Note that characters from 128 to 255 (inclusive) are by default |
902 | internally not encoded as UTF-8 for backward compatibility reasons. | |
974da8e5 | 903 | |
2575c402 | 904 | See L<perlunicode> for more about Unicode. |
bbce6d69 | 905 | |
a0d0e21e | 906 | =item chroot FILENAME |
d74e8afc | 907 | X<chroot> X<root> |
a0d0e21e | 908 | |
54310121 | 909 | =item chroot |
bbce6d69 | 910 | |
5a964f20 | 911 | This function works like the system call by the same name: it makes the |
4633a7c4 | 912 | named directory the new root directory for all further pathnames that |
951ba7fe | 913 | begin with a C</> by your process and all its children. (It doesn't |
28757baa | 914 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 915 | reasons, this call is restricted to the superuser. If FILENAME is |
19799a22 | 916 | omitted, does a C<chroot> to C<$_>. |
a0d0e21e | 917 | |
ea9eb35a BJ |
918 | Portability issues: L<perlport/chroot>. |
919 | ||
a0d0e21e | 920 | =item close FILEHANDLE |
d74e8afc | 921 | X<close> |
a0d0e21e | 922 | |
6a518fbc TP |
923 | =item close |
924 | ||
3b10bc60 | 925 | Closes the file or pipe associated with the filehandle, flushes the IO |
e0f13c26 | 926 | buffers, and closes the system file descriptor. Returns true if those |
8f1da26d | 927 | operations succeed and if no error was reported by any PerlIO |
e0f13c26 RGS |
928 | layer. Closes the currently selected filehandle if the argument is |
929 | omitted. | |
fb73857a | 930 | |
931 | You don't have to close FILEHANDLE if you are immediately going to do | |
3b10bc60 | 932 | another C<open> on it, because C<open> closes it for you. (See |
01aa884e | 933 | L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line |
19799a22 | 934 | counter (C<$.>), while the implicit close done by C<open> does not. |
fb73857a | 935 | |
3b10bc60 | 936 | If the filehandle came from a piped open, C<close> returns false if one of |
937 | the other syscalls involved fails or if its program exits with non-zero | |
938 | status. If the only problem was that the program exited non-zero, C<$!> | |
939 | will be set to C<0>. Closing a pipe also waits for the process executing | |
940 | on the pipe to exit--in case you wish to look at the output of the pipe | |
941 | afterwards--and implicitly puts the exit status value of that command into | |
942 | C<$?> and C<${^CHILD_ERROR_NATIVE}>. | |
5a964f20 | 943 | |
2e0cfa16 FC |
944 | If there are multiple threads running, C<close> on a filehandle from a |
945 | piped open returns true without waiting for the child process to terminate, | |
946 | if the filehandle is still open in another thread. | |
947 | ||
80d38338 TC |
948 | Closing the read end of a pipe before the process writing to it at the |
949 | other end is done writing results in the writer receiving a SIGPIPE. If | |
950 | the other end can't handle that, be sure to read all the data before | |
951 | closing the pipe. | |
73689b13 | 952 | |
fb73857a | 953 | Example: |
a0d0e21e | 954 | |
fb73857a | 955 | open(OUTPUT, '|sort >foo') # pipe to sort |
956 | or die "Can't start sort: $!"; | |
5ed4f2ec | 957 | #... # print stuff to output |
958 | close OUTPUT # wait for sort to finish | |
fb73857a | 959 | or warn $! ? "Error closing sort pipe: $!" |
960 | : "Exit status $? from sort"; | |
5ed4f2ec | 961 | open(INPUT, 'foo') # get sort's results |
fb73857a | 962 | or die "Can't open 'foo' for input: $!"; |
a0d0e21e | 963 | |
5a964f20 | 964 | FILEHANDLE may be an expression whose value can be used as an indirect |
8f1da26d | 965 | filehandle, usually the real filehandle name or an autovivified handle. |
a0d0e21e LW |
966 | |
967 | =item closedir DIRHANDLE | |
d74e8afc | 968 | X<closedir> |
a0d0e21e | 969 | |
19799a22 | 970 | Closes a directory opened by C<opendir> and returns the success of that |
5a964f20 TC |
971 | system call. |
972 | ||
a0d0e21e | 973 | =item connect SOCKET,NAME |
d74e8afc | 974 | X<connect> |
a0d0e21e | 975 | |
80d38338 TC |
976 | Attempts to connect to a remote socket, just like connect(2). |
977 | Returns true if it succeeded, false otherwise. NAME should be a | |
4633a7c4 LW |
978 | packed address of the appropriate type for the socket. See the examples in |
979 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 980 | |
cb1a09d0 | 981 | =item continue BLOCK |
d74e8afc | 982 | X<continue> |
cb1a09d0 | 983 | |
0d863452 RH |
984 | =item continue |
985 | ||
4a904372 FC |
986 | When followed by a BLOCK, C<continue> is actually a |
987 | flow control statement rather than a function. If | |
cf264981 | 988 | there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or |
98293880 JH |
989 | C<foreach>), it is always executed just before the conditional is about to |
990 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
cb1a09d0 AD |
991 | it can be used to increment a loop variable, even when the loop has been |
992 | continued via the C<next> statement (which is similar to the C C<continue> | |
993 | statement). | |
994 | ||
98293880 | 995 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
3b10bc60 | 996 | block; C<last> and C<redo> behave as if they had been executed within |
19799a22 | 997 | the main block. So will C<next>, but since it will execute a C<continue> |
1d2dff63 GS |
998 | block, it may be more entertaining. |
999 | ||
1000 | while (EXPR) { | |
a9a5a0dc VP |
1001 | ### redo always comes here |
1002 | do_something; | |
1d2dff63 | 1003 | } continue { |
a9a5a0dc VP |
1004 | ### next always comes here |
1005 | do_something_else; | |
1006 | # then back the top to re-check EXPR | |
1d2dff63 GS |
1007 | } |
1008 | ### last always comes here | |
1009 | ||
3b10bc60 | 1010 | Omitting the C<continue> section is equivalent to using an |
1011 | empty one, logically enough, so C<next> goes directly back | |
1d2dff63 GS |
1012 | to check the condition at the top of the loop. |
1013 | ||
4a904372 | 1014 | When there is no BLOCK, C<continue> is a function that |
8f1da26d TC |
1015 | falls through the current C<when> or C<default> block instead of iterating |
1016 | a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>. | |
4a904372 FC |
1017 | In Perl 5.14 and earlier, this form of C<continue> was |
1018 | only available when the C<"switch"> feature was enabled. | |
8f1da26d TC |
1019 | See L<feature> and L<perlsyn/"Switch statements"> for more |
1020 | information. | |
0d863452 | 1021 | |
a0d0e21e | 1022 | =item cos EXPR |
d74e8afc | 1023 | X<cos> X<cosine> X<acos> X<arccosine> |
a0d0e21e | 1024 | |
d6217f1e GS |
1025 | =item cos |
1026 | ||
5a964f20 | 1027 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
34169887 | 1028 | takes the cosine of C<$_>. |
a0d0e21e | 1029 | |
ca6e1c26 | 1030 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
28757baa | 1031 | function, or use this relation: |
1032 | ||
1033 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
1034 | ||
a0d0e21e | 1035 | =item crypt PLAINTEXT,SALT |
d74e8afc | 1036 | X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> |
f723aae1 | 1037 | X<decrypt> X<cryptography> X<passwd> X<encrypt> |
a0d0e21e | 1038 | |
ef2e6798 MS |
1039 | Creates a digest string exactly like the crypt(3) function in the C |
1040 | library (assuming that you actually have a version there that has not | |
bb23f8d1 | 1041 | been extirpated as a potential munition). |
ef2e6798 | 1042 | |
34169887 | 1043 | crypt() is a one-way hash function. The PLAINTEXT and SALT are turned |
ef2e6798 MS |
1044 | into a short string, called a digest, which is returned. The same |
1045 | PLAINTEXT and SALT will always return the same string, but there is no | |
1046 | (known) way to get the original PLAINTEXT from the hash. Small | |
1047 | changes in the PLAINTEXT or SALT will result in large changes in the | |
1048 | digest. | |
1049 | ||
1050 | There is no decrypt function. This function isn't all that useful for | |
1051 | cryptography (for that, look for F<Crypt> modules on your nearby CPAN | |
1052 | mirror) and the name "crypt" is a bit of a misnomer. Instead it is | |
1053 | primarily used to check if two pieces of text are the same without | |
1054 | having to transmit or store the text itself. An example is checking | |
1055 | if a correct password is given. The digest of the password is stored, | |
cf264981 | 1056 | not the password itself. The user types in a password that is |
ef2e6798 | 1057 | crypt()'d with the same salt as the stored digest. If the two digests |
34169887 | 1058 | match, the password is correct. |
ef2e6798 MS |
1059 | |
1060 | When verifying an existing digest string you should use the digest as | |
1061 | the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used | |
cf264981 | 1062 | to create the digest is visible as part of the digest. This ensures |
ef2e6798 MS |
1063 | crypt() will hash the new string with the same salt as the digest. |
1064 | This allows your code to work with the standard L<crypt|/crypt> and | |
8f1da26d TC |
1065 | with more exotic implementations. In other words, assume |
1066 | nothing about the returned string itself nor about how many bytes | |
1067 | of SALT may matter. | |
85c16d83 JH |
1068 | |
1069 | Traditionally the result is a string of 13 bytes: two first bytes of | |
1070 | the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only | |
bb23f8d1 | 1071 | the first eight bytes of PLAINTEXT mattered. But alternative |
ef2e6798 | 1072 | hashing schemes (like MD5), higher level security schemes (like C2), |
e1020413 | 1073 | and implementations on non-Unix platforms may produce different |
ef2e6798 | 1074 | strings. |
85c16d83 JH |
1075 | |
1076 | When choosing a new salt create a random two character string whose | |
1077 | characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', | |
d3989d75 CW |
1078 | '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of |
1079 | characters is just a recommendation; the characters allowed in | |
1080 | the salt depend solely on your system's crypt library, and Perl can't | |
1081 | restrict what salts C<crypt()> accepts. | |
e71965be | 1082 | |
a0d0e21e | 1083 | Here's an example that makes sure that whoever runs this program knows |
cf264981 | 1084 | their password: |
a0d0e21e LW |
1085 | |
1086 | $pwd = (getpwuid($<))[1]; | |
a0d0e21e LW |
1087 | |
1088 | system "stty -echo"; | |
1089 | print "Password: "; | |
e71965be | 1090 | chomp($word = <STDIN>); |
a0d0e21e LW |
1091 | print "\n"; |
1092 | system "stty echo"; | |
1093 | ||
e71965be | 1094 | if (crypt($word, $pwd) ne $pwd) { |
a9a5a0dc | 1095 | die "Sorry...\n"; |
a0d0e21e | 1096 | } else { |
a9a5a0dc | 1097 | print "ok\n"; |
54310121 | 1098 | } |
a0d0e21e | 1099 | |
9f8f0c9d | 1100 | Of course, typing in your own password to whoever asks you |
748a9306 | 1101 | for it is unwise. |
a0d0e21e | 1102 | |
ef2e6798 | 1103 | The L<crypt|/crypt> function is unsuitable for hashing large quantities |
19799a22 | 1104 | of data, not least of all because you can't get the information |
ef2e6798 | 1105 | back. Look at the L<Digest> module for more robust algorithms. |
19799a22 | 1106 | |
f2791508 JH |
1107 | If using crypt() on a Unicode string (which I<potentially> has |
1108 | characters with codepoints above 255), Perl tries to make sense | |
34169887 | 1109 | of the situation by trying to downgrade (a copy of) |
f2791508 JH |
1110 | the string back to an eight-bit byte string before calling crypt() |
1111 | (on that copy). If that works, good. If not, crypt() dies with | |
1112 | C<Wide character in crypt>. | |
85c16d83 | 1113 | |
ea9eb35a BJ |
1114 | Portability issues: L<perlport/crypt>. |
1115 | ||
aa689395 | 1116 | =item dbmclose HASH |
d74e8afc | 1117 | X<dbmclose> |
a0d0e21e | 1118 | |
19799a22 | 1119 | [This function has been largely superseded by the C<untie> function.] |
a0d0e21e | 1120 | |
aa689395 | 1121 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 1122 | |
ea9eb35a BJ |
1123 | Portability issues: L<perlport/dbmclose>. |
1124 | ||
19799a22 | 1125 | =item dbmopen HASH,DBNAME,MASK |
d74e8afc | 1126 | X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> |
a0d0e21e | 1127 | |
01aa884e KW |
1128 | [This function has been largely superseded by the |
1129 | L<tie|/tie VARIABLE,CLASSNAME,LIST> function.] | |
a0d0e21e | 1130 | |
7b8d334a | 1131 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
19799a22 GS |
1132 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
1133 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
aa689395 | 1134 | is the name of the database (without the F<.dir> or F<.pag> extension if |
1135 | any). If the database does not exist, it is created with protection | |
19799a22 | 1136 | specified by MASK (as modified by the C<umask>). If your system supports |
80d38338 | 1137 | only the older DBM functions, you may make only one C<dbmopen> call in your |
aa689395 | 1138 | program. In older versions of Perl, if your system had neither DBM nor |
19799a22 | 1139 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
aa689395 | 1140 | sdbm(3). |
1141 | ||
1142 | If you don't have write access to the DBM file, you can only read hash | |
1143 | variables, not set them. If you want to test whether you can write, | |
3b10bc60 | 1144 | either use file tests or try setting a dummy hash entry inside an C<eval> |
1145 | to trap the error. | |
a0d0e21e | 1146 | |
19799a22 GS |
1147 | Note that functions such as C<keys> and C<values> may return huge lists |
1148 | when used on large DBM files. You may prefer to use the C<each> | |
a0d0e21e LW |
1149 | function to iterate over large DBM files. Example: |
1150 | ||
1151 | # print out history file offsets | |
1152 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
1153 | while (($key,$val) = each %HIST) { | |
a9a5a0dc | 1154 | print $key, ' = ', unpack('L',$val), "\n"; |
a0d0e21e LW |
1155 | } |
1156 | dbmclose(%HIST); | |
1157 | ||
cb1a09d0 | 1158 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 1159 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 1160 | rich implementation. |
4633a7c4 | 1161 | |
2b5ab1e7 TC |
1162 | You can control which DBM library you use by loading that library |
1163 | before you call dbmopen(): | |
1164 | ||
1165 | use DB_File; | |
1166 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
a9a5a0dc | 1167 | or die "Can't open netscape history file: $!"; |
2b5ab1e7 | 1168 | |
ea9eb35a BJ |
1169 | Portability issues: L<perlport/dbmopen>. |
1170 | ||
8f1da26d TC |
1171 | =item default BLOCK |
1172 | ||
1173 | Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when> | |
1174 | that's always true. Only available after Perl 5.10, and only if the | |
4a904372 FC |
1175 | C<switch> feature has been requested or if the keyword is prefixed with |
1176 | C<CORE::>. See L</when>. | |
8f1da26d | 1177 | |
a0d0e21e | 1178 | =item defined EXPR |
d74e8afc | 1179 | X<defined> X<undef> X<undefined> |
a0d0e21e | 1180 | |
54310121 | 1181 | =item defined |
bbce6d69 | 1182 | |
2f9daede | 1183 | Returns a Boolean value telling whether EXPR has a value other than |
3b10bc60 | 1184 | the undefined value C<undef>. If EXPR is not present, C<$_> is |
2f9daede TP |
1185 | checked. |
1186 | ||
1187 | Many operations return C<undef> to indicate failure, end of file, | |
1188 | system error, uninitialized variable, and other exceptional | |
1189 | conditions. This function allows you to distinguish C<undef> from | |
1190 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 1191 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 1192 | false.) Note that since C<undef> is a valid scalar, its presence |
19799a22 | 1193 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
2f9daede TP |
1194 | returns C<undef> when its argument is an empty array, I<or> when the |
1195 | element to return happens to be C<undef>. | |
1196 | ||
f10b0346 GS |
1197 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
1198 | has ever been defined. The return value is unaffected by any forward | |
80d38338 | 1199 | declarations of C<&func>. A subroutine that is not defined |
847c7ebe | 1200 | may still be callable: its package may have an C<AUTOLOAD> method that |
3b10bc60 | 1201 | makes it spring into existence the first time that it is called; see |
847c7ebe | 1202 | L<perlsub>. |
f10b0346 GS |
1203 | |
1204 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
34169887 | 1205 | used to report whether memory for that aggregate had ever been |
f10b0346 GS |
1206 | allocated. This behavior may disappear in future versions of Perl. |
1207 | You should instead use a simple test for size: | |
1208 | ||
1209 | if (@an_array) { print "has array elements\n" } | |
1210 | if (%a_hash) { print "has hash members\n" } | |
2f9daede TP |
1211 | |
1212 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 1213 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 1214 | purpose. |
a0d0e21e LW |
1215 | |
1216 | Examples: | |
1217 | ||
8f1da26d | 1218 | print if defined $switch{D}; |
a0d0e21e LW |
1219 | print "$val\n" while defined($val = pop(@ary)); |
1220 | die "Can't readlink $sym: $!" | |
a9a5a0dc | 1221 | unless defined($value = readlink $sym); |
a0d0e21e | 1222 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 1223 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 1224 | |
8f1da26d | 1225 | Note: Many folks tend to overuse C<defined> and are then surprised to |
7660c0ab | 1226 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
2f9daede | 1227 | defined values. For example, if you say |
a5f75d66 AD |
1228 | |
1229 | "ab" =~ /a(.*)b/; | |
1230 | ||
80d38338 | 1231 | The pattern match succeeds and C<$1> is defined, although it |
cf264981 | 1232 | matched "nothing". It didn't really fail to match anything. Rather, it |
2b5ab1e7 | 1233 | matched something that happened to be zero characters long. This is all |
a5f75d66 | 1234 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 1235 | it's an admission that it couldn't give you an honest answer. So you |
3b10bc60 | 1236 | should use C<defined> only when questioning the integrity of what |
7660c0ab | 1237 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
2f9daede TP |
1238 | what you want. |
1239 | ||
dc848c6f | 1240 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 1241 | |
a0d0e21e | 1242 | =item delete EXPR |
d74e8afc | 1243 | X<delete> |
a0d0e21e | 1244 | |
d0a76353 RS |
1245 | Given an expression that specifies an element or slice of a hash, C<delete> |
1246 | deletes the specified elements from that hash so that exists() on that element | |
1247 | no longer returns true. Setting a hash element to the undefined value does | |
1248 | not remove its key, but deleting it does; see L</exists>. | |
80d38338 | 1249 | |
8f1da26d | 1250 | In list context, returns the value or values deleted, or the last such |
80d38338 | 1251 | element in scalar context. The return list's length always matches that of |
d0a76353 RS |
1252 | the argument list: deleting non-existent elements returns the undefined value |
1253 | in their corresponding positions. | |
80d38338 | 1254 | |
d0a76353 RS |
1255 | delete() may also be used on arrays and array slices, but its behavior is less |
1256 | straightforward. Although exists() will return false for deleted entries, | |
1257 | deleting array elements never changes indices of existing values; use shift() | |
1258 | or splice() for that. However, if all deleted elements fall at the end of an | |
1259 | array, the array's size shrinks to the position of the highest element that | |
1260 | still tests true for exists(), or to 0 if none do. | |
1261 | ||
8f1da26d | 1262 | B<WARNING:> Calling delete on array values is deprecated and likely to |
d0a76353 | 1263 | be removed in a future version of Perl. |
80d38338 TC |
1264 | |
1265 | Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to | |
1266 | a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash | |
1267 | or array may not necessarily return anything; it depends on the implementation | |
1268 | of the C<tied> package's DELETE method, which may do whatever it pleases. | |
a0d0e21e | 1269 | |
80d38338 TC |
1270 | The C<delete local EXPR> construct localizes the deletion to the current |
1271 | block at run time. Until the block exits, elements locally deleted | |
1272 | temporarily no longer exist. See L<perlsub/"Localized deletion of elements | |
1273 | of composite types">. | |
eba0920a EM |
1274 | |
1275 | %hash = (foo => 11, bar => 22, baz => 33); | |
1276 | $scalar = delete $hash{foo}; # $scalar is 11 | |
1277 | $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 | |
1278 | @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) | |
1279 | ||
01020589 | 1280 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
a0d0e21e | 1281 | |
5f05dabc | 1282 | foreach $key (keys %HASH) { |
a9a5a0dc | 1283 | delete $HASH{$key}; |
a0d0e21e LW |
1284 | } |
1285 | ||
01020589 | 1286 | foreach $index (0 .. $#ARRAY) { |
a9a5a0dc | 1287 | delete $ARRAY[$index]; |
01020589 GS |
1288 | } |
1289 | ||
1290 | And so do these: | |
5f05dabc | 1291 | |
01020589 GS |
1292 | delete @HASH{keys %HASH}; |
1293 | ||
9740c838 | 1294 | delete @ARRAY[0 .. $#ARRAY]; |
5f05dabc | 1295 | |
80d38338 TC |
1296 | But both are slower than assigning the empty list |
1297 | or undefining %HASH or @ARRAY, which is the customary | |
1298 | way to empty out an aggregate: | |
01020589 | 1299 | |
5ed4f2ec | 1300 | %HASH = (); # completely empty %HASH |
1301 | undef %HASH; # forget %HASH ever existed | |
2b5ab1e7 | 1302 | |
5ed4f2ec | 1303 | @ARRAY = (); # completely empty @ARRAY |
1304 | undef @ARRAY; # forget @ARRAY ever existed | |
2b5ab1e7 | 1305 | |
80d38338 TC |
1306 | The EXPR can be arbitrarily complicated provided its |
1307 | final operation is an element or slice of an aggregate: | |
a0d0e21e LW |
1308 | |
1309 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 1310 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e | 1311 | |
01020589 GS |
1312 | delete $ref->[$x][$y][$index]; |
1313 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1314 | ||
a0d0e21e | 1315 | =item die LIST |
d74e8afc | 1316 | X<die> X<throw> X<exception> X<raise> X<$@> X<abort> |
a0d0e21e | 1317 | |
4c050ad5 NC |
1318 | C<die> raises an exception. Inside an C<eval> the error message is stuffed |
1319 | into C<$@> and the C<eval> is terminated with the undefined value. | |
1320 | If the exception is outside of all enclosing C<eval>s, then the uncaught | |
1321 | exception prints LIST to C<STDERR> and exits with a non-zero value. If you | |
96090e4f | 1322 | need to exit the process with a specific exit code, see L</exit>. |
a0d0e21e LW |
1323 | |
1324 | Equivalent examples: | |
1325 | ||
1326 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 1327 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e | 1328 | |
ccac6780 | 1329 | If the last element of LIST does not end in a newline, the current |
df37ec69 WW |
1330 | script line number and input line number (if any) are also printed, |
1331 | and a newline is supplied. Note that the "input line number" (also | |
1332 | known as "chunk") is subject to whatever notion of "line" happens to | |
1333 | be currently in effect, and is also available as the special variable | |
1334 | C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1335 | ||
1336 | Hint: sometimes appending C<", stopped"> to your message will cause it | |
1337 | to make better sense when the string C<"at foo line 123"> is appended. | |
1338 | Suppose you are running script "canasta". | |
a0d0e21e LW |
1339 | |
1340 | die "/etc/games is no good"; | |
1341 | die "/etc/games is no good, stopped"; | |
1342 | ||
1343 | produce, respectively | |
1344 | ||
1345 | /etc/games is no good at canasta line 123. | |
1346 | /etc/games is no good, stopped at canasta line 123. | |
1347 | ||
a96d0188 | 1348 | If the output is empty and C<$@> already contains a value (typically from a |
7660c0ab | 1349 | previous eval) that value is reused after appending C<"\t...propagated">. |
fb73857a | 1350 | This is useful for propagating exceptions: |
1351 | ||
1352 | eval { ... }; | |
1353 | die unless $@ =~ /Expected exception/; | |
1354 | ||
a96d0188 | 1355 | If the output is empty and C<$@> contains an object reference that has a |
ad216e65 JH |
1356 | C<PROPAGATE> method, that method will be called with additional file |
1357 | and line number parameters. The return value replaces the value in | |
34169887 | 1358 | C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> |
ad216e65 JH |
1359 | were called. |
1360 | ||
7660c0ab | 1361 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 1362 | |
4c050ad5 NC |
1363 | If an uncaught exception results in interpreter exit, the exit code is |
1364 | determined from the values of C<$!> and C<$?> with this pseudocode: | |
1365 | ||
1366 | exit $! if $!; # errno | |
1367 | exit $? >> 8 if $? >> 8; # child exit status | |
1368 | exit 255; # last resort | |
1369 | ||
1370 | The intent is to squeeze as much possible information about the likely cause | |
1371 | into the limited space of the system exit code. However, as C<$!> is the value | |
1372 | of C's C<errno>, which can be set by any system call, this means that the value | |
1373 | of the exit code used by C<die> can be non-predictable, so should not be relied | |
1374 | upon, other than to be non-zero. | |
1375 | ||
80d38338 TC |
1376 | You can also call C<die> with a reference argument, and if this is trapped |
1377 | within an C<eval>, C<$@> contains that reference. This permits more | |
1378 | elaborate exception handling using objects that maintain arbitrary state | |
1379 | about the exception. Such a scheme is sometimes preferable to matching | |
1380 | particular string values of C<$@> with regular expressions. Because C<$@> | |
1381 | is a global variable and C<eval> may be used within object implementations, | |
1382 | be careful that analyzing the error object doesn't replace the reference in | |
1383 | the global variable. It's easiest to make a local copy of the reference | |
1384 | before any manipulations. Here's an example: | |
52531d10 | 1385 | |
80d38338 | 1386 | use Scalar::Util "blessed"; |
da279afe | 1387 | |
52531d10 | 1388 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; |
746d7dd7 GL |
1389 | if (my $ev_err = $@) { |
1390 | if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { | |
52531d10 GS |
1391 | # handle Some::Module::Exception |
1392 | } | |
1393 | else { | |
1394 | # handle all other possible exceptions | |
1395 | } | |
1396 | } | |
1397 | ||
3b10bc60 | 1398 | Because Perl stringifies uncaught exception messages before display, |
80d38338 | 1399 | you'll probably want to overload stringification operations on |
52531d10 GS |
1400 | exception objects. See L<overload> for details about that. |
1401 | ||
19799a22 GS |
1402 | You can arrange for a callback to be run just before the C<die> |
1403 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
3b10bc60 | 1404 | handler is called with the error text and can change the error |
19799a22 | 1405 | message, if it sees fit, by calling C<die> again. See |
96090e4f | 1406 | L<perlvar/%SIG> for details on setting C<%SIG> entries, and |
cf264981 | 1407 | L<"eval BLOCK"> for some examples. Although this feature was |
19799a22 | 1408 | to be run only right before your program was to exit, this is not |
3b10bc60 | 1409 | currently so: the C<$SIG{__DIE__}> hook is currently called |
19799a22 GS |
1410 | even inside eval()ed blocks/strings! If one wants the hook to do |
1411 | nothing in such situations, put | |
fb73857a | 1412 | |
5ed4f2ec | 1413 | die @_ if $^S; |
fb73857a | 1414 | |
19799a22 GS |
1415 | as the first line of the handler (see L<perlvar/$^S>). Because |
1416 | this promotes strange action at a distance, this counterintuitive | |
b76cc8ba | 1417 | behavior may be fixed in a future release. |
774d564b | 1418 | |
4c050ad5 NC |
1419 | See also exit(), warn(), and the Carp module. |
1420 | ||
a0d0e21e | 1421 | =item do BLOCK |
d74e8afc | 1422 | X<do> X<block> |
a0d0e21e LW |
1423 | |
1424 | Not really a function. Returns the value of the last command in the | |
6b275a1f RGS |
1425 | sequence of commands indicated by BLOCK. When modified by the C<while> or |
1426 | C<until> loop modifier, executes the BLOCK once before testing the loop | |
1427 | condition. (On other statements the loop modifiers test the conditional | |
1428 | first.) | |
a0d0e21e | 1429 | |
4968c1e4 | 1430 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 TC |
1431 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
1432 | See L<perlsyn> for alternative strategies. | |
4968c1e4 | 1433 | |
a0d0e21e | 1434 | =item do SUBROUTINE(LIST) |
d74e8afc | 1435 | X<do> |
a0d0e21e | 1436 | |
51124b83 FC |
1437 | This form of subroutine call is deprecated. SUBROUTINE can be a bareword, |
1438 | a scalar variable or a subroutine beginning with C<&>. | |
a0d0e21e LW |
1439 | |
1440 | =item do EXPR | |
d74e8afc | 1441 | X<do> |
a0d0e21e LW |
1442 | |
1443 | Uses the value of EXPR as a filename and executes the contents of the | |
ea63ef19 | 1444 | file as a Perl script. |
a0d0e21e LW |
1445 | |
1446 | do 'stat.pl'; | |
1447 | ||
1448 | is just like | |
1449 | ||
986b19de | 1450 | eval `cat stat.pl`; |
a0d0e21e | 1451 | |
2b5ab1e7 | 1452 | except that it's more efficient and concise, keeps track of the current |
96090e4f LB |
1453 | filename for error messages, searches the C<@INC> directories, and updates |
1454 | C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for | |
1455 | these variables. It also differs in that code evaluated with C<do FILENAME> | |
2b5ab1e7 TC |
1456 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the |
1457 | same, however, in that it does reparse the file every time you call it, | |
1458 | so you probably don't want to do this inside a loop. | |
a0d0e21e | 1459 | |
8f1da26d | 1460 | If C<do> can read the file but cannot compile it, it returns C<undef> and sets |
9dc513c5 DG |
1461 | an error message in C<$@>. If C<do> cannot read the file, it returns undef |
1462 | and sets C<$!> to the error. Always check C<$@> first, as compilation | |
1463 | could fail in a way that also sets C<$!>. If the file is successfully | |
1464 | compiled, C<do> returns the value of the last expression evaluated. | |
8e30cc93 | 1465 | |
80d38338 | 1466 | Inclusion of library modules is better done with the |
19799a22 | 1467 | C<use> and C<require> operators, which also do automatic error checking |
4633a7c4 | 1468 | and raise an exception if there's a problem. |
a0d0e21e | 1469 | |
5a964f20 TC |
1470 | You might like to use C<do> to read in a program configuration |
1471 | file. Manual error checking can be done this way: | |
1472 | ||
b76cc8ba | 1473 | # read in config files: system first, then user |
f86cebdf | 1474 | for $file ("/share/prog/defaults.rc", |
b76cc8ba | 1475 | "$ENV{HOME}/.someprogrc") |
a9a5a0dc VP |
1476 | { |
1477 | unless ($return = do $file) { | |
1478 | warn "couldn't parse $file: $@" if $@; | |
1479 | warn "couldn't do $file: $!" unless defined $return; | |
1480 | warn "couldn't run $file" unless $return; | |
1481 | } | |
5a964f20 TC |
1482 | } |
1483 | ||
a0d0e21e | 1484 | =item dump LABEL |
d74e8afc | 1485 | X<dump> X<core> X<undump> |
a0d0e21e | 1486 | |
1614b0e3 JD |
1487 | =item dump |
1488 | ||
19799a22 GS |
1489 | This function causes an immediate core dump. See also the B<-u> |
1490 | command-line switch in L<perlrun>, which does the same thing. | |
1491 | Primarily this is so that you can use the B<undump> program (not | |
1492 | supplied) to turn your core dump into an executable binary after | |
1493 | having initialized all your variables at the beginning of the | |
1494 | program. When the new binary is executed it will begin by executing | |
1495 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1496 | Think of it as a goto with an intervening core dump and reincarnation. | |
1497 | If C<LABEL> is omitted, restarts the program from the top. | |
1498 | ||
1499 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1500 | be open any more when the program is reincarnated, with possible | |
80d38338 | 1501 | resulting confusion by Perl. |
19799a22 | 1502 | |
59f521f4 RGS |
1503 | This function is now largely obsolete, mostly because it's very hard to |
1504 | convert a core file into an executable. That's why you should now invoke | |
1505 | it as C<CORE::dump()>, if you don't want to be warned against a possible | |
ac206dc8 | 1506 | typo. |
19799a22 | 1507 | |
ea9eb35a BJ |
1508 | Portability issues: L<perlport/dump>. |
1509 | ||
532eee96 | 1510 | =item each HASH |
d74e8afc | 1511 | X<each> X<hash, iterator> |
aa689395 | 1512 | |
532eee96 | 1513 | =item each ARRAY |
aeedbbed NC |
1514 | X<array, iterator> |
1515 | ||
f5a93a43 TC |
1516 | =item each EXPR |
1517 | ||
80d38338 TC |
1518 | When called in list context, returns a 2-element list consisting of the key |
1519 | and value for the next element of a hash, or the index and value for the | |
1520 | next element of an array, so that you can iterate over it. When called in | |
1521 | scalar context, returns only the key (not the value) in a hash, or the index | |
1522 | in an array. | |
2f9daede | 1523 | |
aeedbbed | 1524 | Hash entries are returned in an apparently random order. The actual random |
3b10bc60 | 1525 | order is subject to change in future versions of Perl, but it is |
504f80c1 | 1526 | guaranteed to be in the same order as either the C<keys> or C<values> |
4546b9e6 | 1527 | function would produce on the same (unmodified) hash. Since Perl |
22883ac5 | 1528 | 5.8.2 the ordering can be different even between different runs of Perl |
4546b9e6 | 1529 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). |
ab192400 | 1530 | |
80d38338 TC |
1531 | After C<each> has returned all entries from the hash or array, the next |
1532 | call to C<each> returns the empty list in list context and C<undef> in | |
1533 | scalar context. The next call following that one restarts iteration. Each | |
1534 | hash or array has its own internal iterator, accessed by C<each>, C<keys>, | |
1535 | and C<values>. The iterator is implicitly reset when C<each> has reached | |
1536 | the end as just described; it can be explicitly reset by calling C<keys> or | |
1537 | C<values> on the hash or array. If you add or delete a hash's elements | |
1538 | while iterating over it, entries may be skipped or duplicated--so don't do | |
1539 | that. Exception: It is always safe to delete the item most recently | |
3b10bc60 | 1540 | returned by C<each()>, so the following code works properly: |
74fc8b5f MJD |
1541 | |
1542 | while (($key, $value) = each %hash) { | |
1543 | print $key, "\n"; | |
1544 | delete $hash{$key}; # This is safe | |
1545 | } | |
aa689395 | 1546 | |
80d38338 | 1547 | This prints out your environment like the printenv(1) program, |
3b10bc60 | 1548 | but in a different order: |
a0d0e21e LW |
1549 | |
1550 | while (($key,$value) = each %ENV) { | |
a9a5a0dc | 1551 | print "$key=$value\n"; |
a0d0e21e LW |
1552 | } |
1553 | ||
f5a93a43 TC |
1554 | Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold |
1555 | reference to an unblessed hash or array. The argument will be dereferenced | |
1556 | automatically. This aspect of C<each> is considered highly experimental. | |
1557 | The exact behaviour may change in a future version of Perl. | |
cba5a3b0 DG |
1558 | |
1559 | while (($key,$value) = each $hashref) { ... } | |
1560 | ||
8f1da26d | 1561 | See also C<keys>, C<values>, and C<sort>. |
a0d0e21e LW |
1562 | |
1563 | =item eof FILEHANDLE | |
d74e8afc ITB |
1564 | X<eof> |
1565 | X<end of file> | |
1566 | X<end-of-file> | |
a0d0e21e | 1567 | |
4633a7c4 LW |
1568 | =item eof () |
1569 | ||
a0d0e21e LW |
1570 | =item eof |
1571 | ||
8f1da26d | 1572 | Returns 1 if the next read on FILEHANDLE will return end of file I<or> if |
a0d0e21e | 1573 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value |
5a964f20 | 1574 | gives the real filehandle. (Note that this function actually |
80d38338 | 1575 | reads a character and then C<ungetc>s it, so isn't useful in an |
748a9306 | 1576 | interactive context.) Do not read from a terminal file (or call |
19799a22 | 1577 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
748a9306 LW |
1578 | as terminals may lose the end-of-file condition if you do. |
1579 | ||
820475bd | 1580 | An C<eof> without an argument uses the last file read. Using C<eof()> |
80d38338 | 1581 | with empty parentheses is different. It refers to the pseudo file |
820475bd | 1582 | formed from the files listed on the command line and accessed via the |
61eff3bc JH |
1583 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, |
1584 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
820475bd | 1585 | used will cause C<@ARGV> to be examined to determine if input is |
67408cae | 1586 | available. Similarly, an C<eof()> after C<< <> >> has returned |
efdd0218 RB |
1587 | end-of-file will assume you are processing another C<@ARGV> list, |
1588 | and if you haven't set C<@ARGV>, will read input from C<STDIN>; | |
1589 | see L<perlop/"I/O Operators">. | |
820475bd | 1590 | |
61eff3bc | 1591 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to |
8f1da26d TC |
1592 | detect the end of each file, whereas C<eof()> will detect the end |
1593 | of the very last file only. Examples: | |
a0d0e21e | 1594 | |
748a9306 LW |
1595 | # reset line numbering on each input file |
1596 | while (<>) { | |
a9a5a0dc VP |
1597 | next if /^\s*#/; # skip comments |
1598 | print "$.\t$_"; | |
5a964f20 | 1599 | } continue { |
a9a5a0dc | 1600 | close ARGV if eof; # Not eof()! |
748a9306 LW |
1601 | } |
1602 | ||
a0d0e21e LW |
1603 | # insert dashes just before last line of last file |
1604 | while (<>) { | |
a9a5a0dc VP |
1605 | if (eof()) { # check for end of last file |
1606 | print "--------------\n"; | |
1607 | } | |
1608 | print; | |
4d0444a3 | 1609 | last if eof(); # needed if we're reading from a terminal |
a0d0e21e LW |
1610 | } |
1611 | ||
a0d0e21e | 1612 | Practical hint: you almost never need to use C<eof> in Perl, because the |
8f1da26d TC |
1613 | input operators typically return C<undef> when they run out of data or |
1614 | encounter an error. | |
a0d0e21e LW |
1615 | |
1616 | =item eval EXPR | |
d74e8afc | 1617 | X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> |
f723aae1 | 1618 | X<error, handling> X<exception, handling> |
a0d0e21e LW |
1619 | |
1620 | =item eval BLOCK | |
1621 | ||
ce2984c3 PF |
1622 | =item eval |
1623 | ||
c7cc6f1c GS |
1624 | In the first form, the return value of EXPR is parsed and executed as if it |
1625 | were a little Perl program. The value of the expression (which is itself | |
8f1da26d | 1626 | determined within scalar context) is first parsed, and if there were no |
2341804c DM |
1627 | errors, executed as a block within the lexical context of the current Perl |
1628 | program. This means, that in particular, any outer lexical variables are | |
1629 | visible to it, and any package variable settings or subroutine and format | |
1630 | definitions remain afterwards. | |
1631 | ||
1632 | Note that the value is parsed every time the C<eval> executes. | |
be3174d2 GS |
1633 | If EXPR is omitted, evaluates C<$_>. This form is typically used to |
1634 | delay parsing and subsequent execution of the text of EXPR until run time. | |
c7cc6f1c GS |
1635 | |
1636 | In the second form, the code within the BLOCK is parsed only once--at the | |
cf264981 | 1637 | same time the code surrounding the C<eval> itself was parsed--and executed |
c7cc6f1c GS |
1638 | within the context of the current Perl program. This form is typically |
1639 | used to trap exceptions more efficiently than the first (see below), while | |
1640 | also providing the benefit of checking the code within BLOCK at compile | |
1641 | time. | |
1642 | ||
1643 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1644 | the BLOCK. | |
1645 | ||
1646 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1647 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1648 | as with subroutines. The expression providing the return value is evaluated |
cf264981 SP |
1649 | in void, scalar, or list context, depending on the context of the C<eval> |
1650 | itself. See L</wantarray> for more on how the evaluation context can be | |
1651 | determined. | |
a0d0e21e | 1652 | |
19799a22 | 1653 | If there is a syntax error or runtime error, or a C<die> statement is |
8f1da26d | 1654 | executed, C<eval> returns C<undef> in scalar context |
1f0d8f98 FC |
1655 | or an empty list--or, for syntax errors, a list containing a single |
1656 | undefined value--in list context, and C<$@> is set to the error | |
1657 | message. The discrepancy in the return values in list context is | |
1658 | considered a bug by some, and will probably be fixed in a future | |
1659 | release. If there was no error, C<$@> is guaranteed to be the empty | |
3b10bc60 | 1660 | string. Beware that using C<eval> neither silences Perl from printing |
c7cc6f1c | 1661 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
d9984052 A |
1662 | To do either of those, you have to use the C<$SIG{__WARN__}> facility, or |
1663 | turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. | |
1664 | See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. | |
a0d0e21e | 1665 | |
19799a22 GS |
1666 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
1667 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
82bcec1b | 1668 | is implemented. It is also Perl's exception-trapping mechanism, where |
a0d0e21e LW |
1669 | the die operator is used to raise exceptions. |
1670 | ||
5f1da31c NT |
1671 | If you want to trap errors when loading an XS module, some problems with |
1672 | the binary interface (such as Perl version skew) may be fatal even with | |
1673 | C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>. | |
1674 | ||
a0d0e21e LW |
1675 | If the code to be executed doesn't vary, you may use the eval-BLOCK |
1676 | form to trap run-time errors without incurring the penalty of | |
1677 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1678 | Examples: | |
1679 | ||
54310121 | 1680 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1681 | eval { $answer = $a / $b; }; warn $@ if $@; |
1682 | ||
1683 | # same thing, but less efficient | |
1684 | eval '$answer = $a / $b'; warn $@ if $@; | |
1685 | ||
1686 | # a compile-time error | |
5ed4f2ec | 1687 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1688 | |
1689 | # a run-time error | |
5ed4f2ec | 1690 | eval '$answer ='; # sets $@ |
a0d0e21e | 1691 | |
cf264981 SP |
1692 | Using the C<eval{}> form as an exception trap in libraries does have some |
1693 | issues. Due to the current arguably broken state of C<__DIE__> hooks, you | |
1694 | may wish not to trigger any C<__DIE__> hooks that user code may have installed. | |
2b5ab1e7 | 1695 | You can use the C<local $SIG{__DIE__}> construct for this purpose, |
80d38338 | 1696 | as this example shows: |
774d564b | 1697 | |
80d38338 | 1698 | # a private exception trap for divide-by-zero |
f86cebdf GS |
1699 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
1700 | warn $@ if $@; | |
774d564b | 1701 | |
1702 | This is especially significant, given that C<__DIE__> hooks can call | |
19799a22 | 1703 | C<die> again, which has the effect of changing their error messages: |
774d564b | 1704 | |
1705 | # __DIE__ hooks may modify error messages | |
1706 | { | |
f86cebdf GS |
1707 | local $SIG{'__DIE__'} = |
1708 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1709 | eval { die "foo lives here" }; |
1710 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1711 | } |
1712 | ||
19799a22 | 1713 | Because this promotes action at a distance, this counterintuitive behavior |
2b5ab1e7 TC |
1714 | may be fixed in a future release. |
1715 | ||
19799a22 | 1716 | With an C<eval>, you should be especially careful to remember what's |
a0d0e21e LW |
1717 | being looked at when: |
1718 | ||
5ed4f2ec | 1719 | eval $x; # CASE 1 |
1720 | eval "$x"; # CASE 2 | |
a0d0e21e | 1721 | |
5ed4f2ec | 1722 | eval '$x'; # CASE 3 |
1723 | eval { $x }; # CASE 4 | |
a0d0e21e | 1724 | |
5ed4f2ec | 1725 | eval "\$$x++"; # CASE 5 |
1726 | $$x++; # CASE 6 | |
a0d0e21e | 1727 | |
2f9daede | 1728 | Cases 1 and 2 above behave identically: they run the code contained in |
19799a22 | 1729 | the variable $x. (Although case 2 has misleading double quotes making |
2f9daede | 1730 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1731 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
19799a22 | 1732 | does nothing but return the value of $x. (Case 4 is preferred for |
2f9daede TP |
1733 | purely visual reasons, but it also has the advantage of compiling at |
1734 | compile-time instead of at run-time.) Case 5 is a place where | |
19799a22 | 1735 | normally you I<would> like to use double quotes, except that in this |
2f9daede TP |
1736 | particular situation, you can just use symbolic references instead, as |
1737 | in case 6. | |
a0d0e21e | 1738 | |
b6538e4f TC |
1739 | Before Perl 5.14, the assignment to C<$@> occurred before restoration |
1740 | of localised variables, which means that for your code to run on older | |
b208c909 | 1741 | versions, a temporary is required if you want to mask some but not all |
8a5a710d DN |
1742 | errors: |
1743 | ||
1744 | # alter $@ on nefarious repugnancy only | |
1745 | { | |
1746 | my $e; | |
1747 | { | |
1748 | local $@; # protect existing $@ | |
1749 | eval { test_repugnancy() }; | |
b208c909 | 1750 | # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only |
8a5a710d DN |
1751 | $@ =~ /nefarious/ and $e = $@; |
1752 | } | |
1753 | die $e if defined $e | |
1754 | } | |
1755 | ||
4968c1e4 | 1756 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 | 1757 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
4968c1e4 | 1758 | |
3b10bc60 | 1759 | An C<eval ''> executed within the C<DB> package doesn't see the usual |
1760 | surrounding lexical scope, but rather the scope of the first non-DB piece | |
1761 | of code that called it. You don't normally need to worry about this unless | |
1762 | you are writing a Perl debugger. | |
d819b83a | 1763 | |
a0d0e21e | 1764 | =item exec LIST |
d74e8afc | 1765 | X<exec> X<execute> |
a0d0e21e | 1766 | |
8bf3b016 GS |
1767 | =item exec PROGRAM LIST |
1768 | ||
3b10bc60 | 1769 | The C<exec> function executes a system command I<and never returns>; |
19799a22 GS |
1770 | use C<system> instead of C<exec> if you want it to return. It fails and |
1771 | returns false only if the command does not exist I<and> it is executed | |
fb73857a | 1772 | directly instead of via your system's command shell (see below). |
a0d0e21e | 1773 | |
19799a22 | 1774 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
80d38338 | 1775 | warns you if there is a following statement that isn't C<die>, C<warn>, |
3b10bc60 | 1776 | or C<exit> (if C<-w> is set--but you always do that, right?). If you |
19799a22 | 1777 | I<really> want to follow an C<exec> with some other statement, you |
55d729e4 GS |
1778 | can use one of these styles to avoid the warning: |
1779 | ||
5a964f20 TC |
1780 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1781 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1782 | |
5a964f20 | 1783 | If there is more than one argument in LIST, or if LIST is an array |
f86cebdf | 1784 | with more than one value, calls execvp(3) with the arguments in LIST. |
5a964f20 TC |
1785 | If there is only one scalar argument or an array with one element in it, |
1786 | the argument is checked for shell metacharacters, and if there are any, | |
1787 | the entire argument is passed to the system's command shell for parsing | |
1788 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1789 | If there are no shell metacharacters in the argument, it is split into | |
b76cc8ba | 1790 | words and passed directly to C<execvp>, which is more efficient. |
19799a22 | 1791 | Examples: |
a0d0e21e | 1792 | |
19799a22 GS |
1793 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
1794 | exec "sort $outfile | uniq"; | |
a0d0e21e LW |
1795 | |
1796 | If you don't really want to execute the first argument, but want to lie | |
1797 | to the program you are executing about its own name, you can specify | |
1798 | the program you actually want to run as an "indirect object" (without a | |
1799 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1800 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1801 | the list.) Example: |
1802 | ||
1803 | $shell = '/bin/csh'; | |
5ed4f2ec | 1804 | exec $shell '-sh'; # pretend it's a login shell |
a0d0e21e LW |
1805 | |
1806 | or, more directly, | |
1807 | ||
5ed4f2ec | 1808 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell |
a0d0e21e | 1809 | |
3b10bc60 | 1810 | When the arguments get executed via the system shell, results are |
1811 | subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
bb32b41a GS |
1812 | for details. |
1813 | ||
19799a22 GS |
1814 | Using an indirect object with C<exec> or C<system> is also more |
1815 | secure. This usage (which also works fine with system()) forces | |
1816 | interpretation of the arguments as a multivalued list, even if the | |
1817 | list had just one argument. That way you're safe from the shell | |
1818 | expanding wildcards or splitting up words with whitespace in them. | |
5a964f20 TC |
1819 | |
1820 | @args = ( "echo surprise" ); | |
1821 | ||
2b5ab1e7 | 1822 | exec @args; # subject to shell escapes |
f86cebdf | 1823 | # if @args == 1 |
2b5ab1e7 | 1824 | exec { $args[0] } @args; # safe even with one-arg list |
5a964f20 TC |
1825 | |
1826 | The first version, the one without the indirect object, ran the I<echo> | |
80d38338 TC |
1827 | program, passing it C<"surprise"> an argument. The second version didn't; |
1828 | it tried to run a program named I<"echo surprise">, didn't find it, and set | |
1829 | C<$?> to a non-zero value indicating failure. | |
5a964f20 | 1830 | |
80d38338 | 1831 | Beginning with v5.6.0, Perl attempts to flush all files opened for |
0f897271 GS |
1832 | output before the exec, but this may not be supported on some platforms |
1833 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH | |
1834 | in English) or call the C<autoflush()> method of C<IO::Handle> on any | |
80d38338 | 1835 | open handles to avoid lost output. |
0f897271 | 1836 | |
80d38338 TC |
1837 | Note that C<exec> will not call your C<END> blocks, nor will it invoke |
1838 | C<DESTROY> methods on your objects. | |
7660c0ab | 1839 | |
ea9eb35a BJ |
1840 | Portability issues: L<perlport/exec>. |
1841 | ||
a0d0e21e | 1842 | =item exists EXPR |
d74e8afc | 1843 | X<exists> X<autovivification> |
a0d0e21e | 1844 | |
d0a76353 RS |
1845 | Given an expression that specifies an element of a hash, returns true if the |
1846 | specified element in the hash has ever been initialized, even if the | |
1847 | corresponding value is undefined. | |
a0d0e21e | 1848 | |
5ed4f2ec | 1849 | print "Exists\n" if exists $hash{$key}; |
1850 | print "Defined\n" if defined $hash{$key}; | |
01020589 GS |
1851 | print "True\n" if $hash{$key}; |
1852 | ||
d0a76353 | 1853 | exists may also be called on array elements, but its behavior is much less |
8f1da26d | 1854 | obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware> |
d0a76353 RS |
1855 | that calling exists on array values is deprecated and likely to be removed in |
1856 | a future version of Perl. | |
1857 | ||
5ed4f2ec | 1858 | print "Exists\n" if exists $array[$index]; |
1859 | print "Defined\n" if defined $array[$index]; | |
01020589 | 1860 | print "True\n" if $array[$index]; |
a0d0e21e | 1861 | |
8f1da26d | 1862 | A hash or array element can be true only if it's defined and defined only if |
a0d0e21e LW |
1863 | it exists, but the reverse doesn't necessarily hold true. |
1864 | ||
afebc493 GS |
1865 | Given an expression that specifies the name of a subroutine, |
1866 | returns true if the specified subroutine has ever been declared, even | |
1867 | if it is undefined. Mentioning a subroutine name for exists or defined | |
80d38338 | 1868 | does not count as declaring it. Note that a subroutine that does not |
847c7ebe DD |
1869 | exist may still be callable: its package may have an C<AUTOLOAD> |
1870 | method that makes it spring into existence the first time that it is | |
3b10bc60 | 1871 | called; see L<perlsub>. |
afebc493 | 1872 | |
5ed4f2ec | 1873 | print "Exists\n" if exists &subroutine; |
1874 | print "Defined\n" if defined &subroutine; | |
afebc493 | 1875 | |
a0d0e21e | 1876 | Note that the EXPR can be arbitrarily complicated as long as the final |
afebc493 | 1877 | operation is a hash or array key lookup or subroutine name: |
a0d0e21e | 1878 | |
5ed4f2ec | 1879 | if (exists $ref->{A}->{B}->{$key}) { } |
1880 | if (exists $hash{A}{B}{$key}) { } | |
2b5ab1e7 | 1881 | |
5ed4f2ec | 1882 | if (exists $ref->{A}->{B}->[$ix]) { } |
1883 | if (exists $hash{A}{B}[$ix]) { } | |
01020589 | 1884 | |
afebc493 GS |
1885 | if (exists &{$ref->{A}{B}{$key}}) { } |
1886 | ||
3b10bc60 | 1887 | Although the mostly deeply nested array or hash will not spring into |
1888 | existence just because its existence was tested, any intervening ones will. | |
61eff3bc | 1889 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring |
01020589 | 1890 | into existence due to the existence test for the $key element above. |
3b10bc60 | 1891 | This happens anywhere the arrow operator is used, including even here: |
5a964f20 | 1892 | |
2b5ab1e7 | 1893 | undef $ref; |
5ed4f2ec | 1894 | if (exists $ref->{"Some key"}) { } |
1895 | print $ref; # prints HASH(0x80d3d5c) | |
2b5ab1e7 TC |
1896 | |
1897 | This surprising autovivification in what does not at first--or even | |
1898 | second--glance appear to be an lvalue context may be fixed in a future | |
5a964f20 | 1899 | release. |
a0d0e21e | 1900 | |
afebc493 GS |
1901 | Use of a subroutine call, rather than a subroutine name, as an argument |
1902 | to exists() is an error. | |
1903 | ||
5ed4f2ec | 1904 | exists ⊂ # OK |
1905 | exists &sub(); # Error | |
afebc493 | 1906 | |
a0d0e21e | 1907 | =item exit EXPR |
d74e8afc | 1908 | X<exit> X<terminate> X<abort> |
a0d0e21e | 1909 | |
ce2984c3 PF |
1910 | =item exit |
1911 | ||
2b5ab1e7 | 1912 | Evaluates EXPR and exits immediately with that value. Example: |
a0d0e21e LW |
1913 | |
1914 | $ans = <STDIN>; | |
1915 | exit 0 if $ans =~ /^[Xx]/; | |
1916 | ||
19799a22 | 1917 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
2b5ab1e7 TC |
1918 | universally recognized values for EXPR are C<0> for success and C<1> |
1919 | for error; other values are subject to interpretation depending on the | |
1920 | environment in which the Perl program is running. For example, exiting | |
1921 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1922 | the mailer to return the item undelivered, but that's not true everywhere. | |
a0d0e21e | 1923 | |
19799a22 GS |
1924 | Don't use C<exit> to abort a subroutine if there's any chance that |
1925 | someone might want to trap whatever error happened. Use C<die> instead, | |
1926 | which can be trapped by an C<eval>. | |
28757baa | 1927 | |
19799a22 | 1928 | The exit() function does not always exit immediately. It calls any |
2b5ab1e7 | 1929 | defined C<END> routines first, but these C<END> routines may not |
19799a22 | 1930 | themselves abort the exit. Likewise any object destructors that need to |
60275626 FC |
1931 | be called are called before the real exit. C<END> routines and destructors |
1932 | can change the exit status by modifying C<$?>. If this is a problem, you | |
2b5ab1e7 | 1933 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. |
87275199 | 1934 | See L<perlmod> for details. |
5a964f20 | 1935 | |
ea9eb35a BJ |
1936 | Portability issues: L<perlport/exit>. |
1937 | ||
a0d0e21e | 1938 | =item exp EXPR |
d74e8afc | 1939 | X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> |
a0d0e21e | 1940 | |
54310121 | 1941 | =item exp |
bbce6d69 | 1942 | |
b76cc8ba | 1943 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1944 | If EXPR is omitted, gives C<exp($_)>. |
1945 | ||
1946 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
d74e8afc | 1947 | X<fcntl> |
a0d0e21e | 1948 | |
f86cebdf | 1949 | Implements the fcntl(2) function. You'll probably have to say |
a0d0e21e LW |
1950 | |
1951 | use Fcntl; | |
1952 | ||
0ade1984 | 1953 | first to get the correct constant definitions. Argument processing and |
3b10bc60 | 1954 | value returned work just like C<ioctl> below. |
a0d0e21e LW |
1955 | For example: |
1956 | ||
1957 | use Fcntl; | |
5a964f20 | 1958 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
a9a5a0dc | 1959 | or die "can't fcntl F_GETFL: $!"; |
5a964f20 | 1960 | |
554ad1fc | 1961 | You don't have to check for C<defined> on the return from C<fcntl>. |
951ba7fe GS |
1962 | Like C<ioctl>, it maps a C<0> return from the system call into |
1963 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
2b5ab1e7 TC |
1964 | in numeric context. It is also exempt from the normal B<-w> warnings |
1965 | on improper numeric conversions. | |
5a964f20 | 1966 | |
3b10bc60 | 1967 | Note that C<fcntl> raises an exception if used on a machine that |
2b5ab1e7 TC |
1968 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
1969 | manpage to learn what functions are available on your system. | |
a0d0e21e | 1970 | |
be2f7487 TH |
1971 | Here's an example of setting a filehandle named C<REMOTE> to be |
1972 | non-blocking at the system level. You'll have to negotiate C<$|> | |
1973 | on your own, though. | |
1974 | ||
1975 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
1976 | ||
1977 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
1978 | or die "Can't get flags for the socket: $!\n"; | |
1979 | ||
1980 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
1981 | or die "Can't set flags for the socket: $!\n"; | |
1982 | ||
ea9eb35a BJ |
1983 | Portability issues: L<perlport/fcntl>. |
1984 | ||
a0d0e21e | 1985 | =item fileno FILEHANDLE |
d74e8afc | 1986 | X<fileno> |
a0d0e21e | 1987 | |
2b5ab1e7 | 1988 | Returns the file descriptor for a filehandle, or undefined if the |
a7c1632d FC |
1989 | filehandle is not open. If there is no real file descriptor at the OS |
1990 | level, as can happen with filehandles connected to memory objects via | |
1991 | C<open> with a reference for the third argument, -1 is returned. | |
1992 | ||
1993 | This is mainly useful for constructing | |
19799a22 | 1994 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
2b5ab1e7 TC |
1995 | If FILEHANDLE is an expression, the value is taken as an indirect |
1996 | filehandle, generally its name. | |
5a964f20 | 1997 | |
b76cc8ba | 1998 | You can use this to find out whether two handles refer to the |
5a964f20 TC |
1999 | same underlying descriptor: |
2000 | ||
2001 | if (fileno(THIS) == fileno(THAT)) { | |
a9a5a0dc | 2002 | print "THIS and THAT are dups\n"; |
b76cc8ba NIS |
2003 | } |
2004 | ||
a0d0e21e | 2005 | =item flock FILEHANDLE,OPERATION |
d74e8afc | 2006 | X<flock> X<lock> X<locking> |
a0d0e21e | 2007 | |
19799a22 GS |
2008 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
2009 | for success, false on failure. Produces a fatal error if used on a | |
2b5ab1e7 | 2010 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
dbfe1e81 | 2011 | C<flock> is Perl's portable file-locking interface, although it locks |
3b10bc60 | 2012 | entire files only, not records. |
2b5ab1e7 TC |
2013 | |
2014 | Two potentially non-obvious but traditional C<flock> semantics are | |
2015 | that it waits indefinitely until the lock is granted, and that its locks | |
dbfe1e81 FC |
2016 | are B<merely advisory>. Such discretionary locks are more flexible, but |
2017 | offer fewer guarantees. This means that programs that do not also use | |
2018 | C<flock> may modify files locked with C<flock>. See L<perlport>, | |
8f1da26d | 2019 | your port's specific documentation, and your system-specific local manpages |
2b5ab1e7 TC |
2020 | for details. It's best to assume traditional behavior if you're writing |
2021 | portable programs. (But if you're not, you should as always feel perfectly | |
2022 | free to write for your own system's idiosyncrasies (sometimes called | |
2023 | "features"). Slavish adherence to portability concerns shouldn't get | |
2024 | in the way of your getting your job done.) | |
a3cb178b | 2025 | |
8ebc5c01 | 2026 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
2027 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
8f1da26d TC |
2028 | you can use the symbolic names if you import them from the L<Fcntl> module, |
2029 | either individually, or as a group using the C<:flock> tag. LOCK_SH | |
68dc0745 | 2030 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN |
ea3105be | 2031 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
8f1da26d | 2032 | LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking |
3b10bc60 | 2033 | waiting for the lock; check the return status to see if you got it. |
68dc0745 | 2034 | |
2b5ab1e7 TC |
2035 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
2036 | before locking or unlocking it. | |
8ebc5c01 | 2037 | |
f86cebdf | 2038 | Note that the emulation built with lockf(3) doesn't provide shared |
8ebc5c01 | 2039 | locks, and it requires that FILEHANDLE be open with write intent. These |
2b5ab1e7 | 2040 | are the semantics that lockf(3) implements. Most if not all systems |
f86cebdf | 2041 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
8ebc5c01 | 2042 | differing semantics shouldn't bite too many people. |
2043 | ||
becacb53 TM |
2044 | Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE |
2045 | be open with read intent to use LOCK_SH and requires that it be open | |
2046 | with write intent to use LOCK_EX. | |
2047 | ||
19799a22 GS |
2048 | Note also that some versions of C<flock> cannot lock things over the |
2049 | network; you would need to use the more system-specific C<fcntl> for | |
f86cebdf GS |
2050 | that. If you like you can force Perl to ignore your system's flock(2) |
2051 | function, and so provide its own fcntl(2)-based emulation, by passing | |
8ebc5c01 | 2052 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
8f1da26d | 2053 | and build a new Perl. |
4633a7c4 LW |
2054 | |
2055 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 2056 | |
7ed5353d | 2057 | use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants |
a0d0e21e LW |
2058 | |
2059 | sub lock { | |
a9a5a0dc VP |
2060 | my ($fh) = @_; |
2061 | flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; | |
7ed5353d | 2062 | |
a9a5a0dc VP |
2063 | # and, in case someone appended while we were waiting... |
2064 | seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; | |
a0d0e21e LW |
2065 | } |
2066 | ||
2067 | sub unlock { | |
a9a5a0dc VP |
2068 | my ($fh) = @_; |
2069 | flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; | |
a0d0e21e LW |
2070 | } |
2071 | ||
b0169937 | 2072 | open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") |
5ed4f2ec | 2073 | or die "Can't open mailbox: $!"; |
a0d0e21e | 2074 | |
7ed5353d | 2075 | lock($mbox); |
b0169937 | 2076 | print $mbox $msg,"\n\n"; |
7ed5353d | 2077 | unlock($mbox); |
a0d0e21e | 2078 | |
3b10bc60 | 2079 | On systems that support a real flock(2), locks are inherited across fork() |
2080 | calls, whereas those that must resort to the more capricious fcntl(2) | |
2081 | function lose their locks, making it seriously harder to write servers. | |
2b5ab1e7 | 2082 | |
cb1a09d0 | 2083 | See also L<DB_File> for other flock() examples. |
a0d0e21e | 2084 | |
ea9eb35a BJ |
2085 | Portability issues: L<perlport/flock>. |
2086 | ||
a0d0e21e | 2087 | =item fork |
d74e8afc | 2088 | X<fork> X<child> X<parent> |
a0d0e21e | 2089 | |
2b5ab1e7 TC |
2090 | Does a fork(2) system call to create a new process running the |
2091 | same program at the same point. It returns the child pid to the | |
2092 | parent process, C<0> to the child process, or C<undef> if the fork is | |
2093 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
2094 | are shared, while everything else is copied. On most systems supporting | |
2095 | fork(), great care has gone into making it extremely efficient (for | |
2096 | example, using copy-on-write technology on data pages), making it the | |
2097 | dominant paradigm for multitasking over the last few decades. | |
5a964f20 | 2098 | |
80d38338 | 2099 | Beginning with v5.6.0, Perl attempts to flush all files opened for |
0f897271 GS |
2100 | output before forking the child process, but this may not be supported |
2101 | on some platforms (see L<perlport>). To be safe, you may need to set | |
2102 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
80d38338 | 2103 | C<IO::Handle> on any open handles to avoid duplicate output. |
a0d0e21e | 2104 | |
19799a22 | 2105 | If you C<fork> without ever waiting on your children, you will |
2b5ab1e7 TC |
2106 | accumulate zombies. On some systems, you can avoid this by setting |
2107 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
2108 | forking and reaping moribund children. | |
cb1a09d0 | 2109 | |
28757baa | 2110 | Note that if your forked child inherits system file descriptors like |
2111 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2b5ab1e7 | 2112 | if you exit, then the remote server (such as, say, a CGI script or a |
19799a22 | 2113 | backgrounded job launched from a remote shell) won't think you're done. |
2b5ab1e7 | 2114 | You should reopen those to F</dev/null> if it's any issue. |
28757baa | 2115 | |
ea9eb35a BJ |
2116 | On some platforms such as Windows, where the fork() system call is not available, |
2117 | Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to, | |
2118 | at the level of the Perl program, be as compatible as possible with the "Unix" fork(). | |
6d17f725 | 2119 | However it has limitations that have to be considered in code intended to be portable. |
ea9eb35a BJ |
2120 | See L<perlfork> for more details. |
2121 | ||
2122 | Portability issues: L<perlport/fork>. | |
2123 | ||
cb1a09d0 | 2124 | =item format |
d74e8afc | 2125 | X<format> |
cb1a09d0 | 2126 | |
19799a22 | 2127 | Declare a picture format for use by the C<write> function. For |
cb1a09d0 AD |
2128 | example: |
2129 | ||
54310121 | 2130 | format Something = |
a9a5a0dc VP |
2131 | Test: @<<<<<<<< @||||| @>>>>> |
2132 | $str, $%, '$' . int($num) | |
cb1a09d0 AD |
2133 | . |
2134 | ||
2135 | $str = "widget"; | |
184e9718 | 2136 | $num = $cost/$quantity; |
cb1a09d0 AD |
2137 | $~ = 'Something'; |
2138 | write; | |
2139 | ||
2140 | See L<perlform> for many details and examples. | |
2141 | ||
8903cb82 | 2142 | =item formline PICTURE,LIST |
d74e8afc | 2143 | X<formline> |
a0d0e21e | 2144 | |
5a964f20 | 2145 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
2146 | too. It formats (see L<perlform>) a list of values according to the |
2147 | contents of PICTURE, placing the output into the format output | |
7660c0ab | 2148 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
19799a22 | 2149 | Eventually, when a C<write> is done, the contents of |
cf264981 SP |
2150 | C<$^A> are written to some filehandle. You could also read C<$^A> |
2151 | and then set C<$^A> back to C<"">. Note that a format typically | |
19799a22 | 2152 | does one C<formline> per line of form, but the C<formline> function itself |
748a9306 | 2153 | doesn't care how many newlines are embedded in the PICTURE. This means |
3b10bc60 | 2154 | that the C<~> and C<~~> tokens treat the entire PICTURE as a single line. |
748a9306 | 2155 | You may therefore need to use multiple formlines to implement a single |
3b10bc60 | 2156 | record format, just like the C<format> compiler. |
748a9306 | 2157 | |
19799a22 | 2158 | Be careful if you put double quotes around the picture, because an C<@> |
748a9306 | 2159 | character may be taken to mean the beginning of an array name. |
19799a22 | 2160 | C<formline> always returns true. See L<perlform> for other examples. |
a0d0e21e | 2161 | |
445b09e5 FC |
2162 | If you are trying to use this instead of C<write> to capture the output, |
2163 | you may find it easier to open a filehandle to a scalar | |
2164 | (C<< open $fh, ">", \$output >>) and write to that instead. | |
2165 | ||
a0d0e21e | 2166 | =item getc FILEHANDLE |
f723aae1 | 2167 | X<getc> X<getchar> X<character> X<file, read> |
a0d0e21e LW |
2168 | |
2169 | =item getc | |
2170 | ||
2171 | Returns the next character from the input file attached to FILEHANDLE, | |
3b10bc60 | 2172 | or the undefined value at end of file or if there was an error (in |
b5fe5ca2 SR |
2173 | the latter case C<$!> is set). If FILEHANDLE is omitted, reads from |
2174 | STDIN. This is not particularly efficient. However, it cannot be | |
2175 | used by itself to fetch single characters without waiting for the user | |
2176 | to hit enter. For that, try something more like: | |
4633a7c4 LW |
2177 | |
2178 | if ($BSD_STYLE) { | |
a9a5a0dc | 2179 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
4633a7c4 LW |
2180 | } |
2181 | else { | |
a9a5a0dc | 2182 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
2183 | } |
2184 | ||
2185 | $key = getc(STDIN); | |
2186 | ||
2187 | if ($BSD_STYLE) { | |
a9a5a0dc | 2188 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
4633a7c4 LW |
2189 | } |
2190 | else { | |
3b10bc60 | 2191 | system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL |
4633a7c4 LW |
2192 | } |
2193 | print "\n"; | |
2194 | ||
54310121 | 2195 | Determination of whether $BSD_STYLE should be set |
2196 | is left as an exercise to the reader. | |
cb1a09d0 | 2197 | |
19799a22 | 2198 | The C<POSIX::getattr> function can do this more portably on |
2b5ab1e7 | 2199 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
a3390c9f | 2200 | module from your nearest CPAN site; details on CPAN can be found under |
2b5ab1e7 | 2201 | L<perlmodlib/CPAN>. |
a0d0e21e LW |
2202 | |
2203 | =item getlogin | |
d74e8afc | 2204 | X<getlogin> X<login> |
a0d0e21e | 2205 | |
cf264981 | 2206 | This implements the C library function of the same name, which on most |
3b10bc60 | 2207 | systems returns the current login from F</etc/utmp>, if any. If it |
2208 | returns the empty string, use C<getpwuid>. | |
a0d0e21e | 2209 | |
f86702cc | 2210 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 2211 | |
19799a22 GS |
2212 | Do not consider C<getlogin> for authentication: it is not as |
2213 | secure as C<getpwuid>. | |
4633a7c4 | 2214 | |
ea9eb35a BJ |
2215 | Portability issues: L<perlport/getlogin>. |
2216 | ||
a0d0e21e | 2217 | =item getpeername SOCKET |
d74e8afc | 2218 | X<getpeername> X<peer> |
a0d0e21e | 2219 | |
a3390c9f FC |
2220 | Returns the packed sockaddr address of the other end of the SOCKET |
2221 | connection. | |
a0d0e21e | 2222 | |
4633a7c4 LW |
2223 | use Socket; |
2224 | $hersockaddr = getpeername(SOCK); | |
19799a22 | 2225 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
4633a7c4 LW |
2226 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
2227 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
2228 | |
2229 | =item getpgrp PID | |
d74e8afc | 2230 | X<getpgrp> X<group> |
a0d0e21e | 2231 | |
47e29363 | 2232 | Returns the current process group for the specified PID. Use |
7660c0ab | 2233 | a PID of C<0> to get the current process group for the |
4633a7c4 | 2234 | current process. Will raise an exception if used on a machine that |
a3390c9f FC |
2235 | doesn't implement getpgrp(2). If PID is omitted, returns the process |
2236 | group of the current process. Note that the POSIX version of C<getpgrp> | |
7660c0ab | 2237 | does not accept a PID argument, so only C<PID==0> is truly portable. |
a0d0e21e | 2238 | |
ea9eb35a BJ |
2239 | Portability issues: L<perlport/getpgrp>. |
2240 | ||
a0d0e21e | 2241 | =item getppid |
d74e8afc | 2242 | X<getppid> X<parent> X<pid> |
a0d0e21e LW |
2243 | |
2244 | Returns the process id of the parent process. | |
2245 | ||
4d76a344 RGS |
2246 | Note for Linux users: on Linux, the C functions C<getpid()> and |
2247 | C<getppid()> return different values from different threads. In order to | |
3b10bc60 | 2248 | be portable, this behavior is not reflected by the Perl-level function |
4d76a344 | 2249 | C<getppid()>, that returns a consistent value across threads. If you want |
e3256f86 RGS |
2250 | to call the underlying C<getppid()>, you may use the CPAN module |
2251 | C<Linux::Pid>. | |
4d76a344 | 2252 | |
ea9eb35a BJ |
2253 | Portability issues: L<perlport/getppid>. |
2254 | ||
a0d0e21e | 2255 | =item getpriority WHICH,WHO |
d74e8afc | 2256 | X<getpriority> X<priority> X<nice> |
a0d0e21e | 2257 | |
4633a7c4 | 2258 | Returns the current priority for a process, a process group, or a user. |
01aa884e | 2259 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a |
f86cebdf | 2260 | machine that doesn't implement getpriority(2). |
a0d0e21e | 2261 | |
ea9eb35a BJ |
2262 | Portability issues: L<perlport/getpriority>. |
2263 | ||
a0d0e21e | 2264 | =item getpwnam NAME |
d74e8afc ITB |
2265 | X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> |
2266 | X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> | |
2267 | X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> | |
2268 | X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> | |
2269 | X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> | |
2270 | X<endnetent> X<endprotoent> X<endservent> | |
a0d0e21e LW |
2271 | |
2272 | =item getgrnam NAME | |
2273 | ||
2274 | =item gethostbyname NAME | |
2275 | ||
2276 | =item getnetbyname NAME | |
2277 | ||
2278 | =item getprotobyname NAME | |
2279 | ||
2280 | =item getpwuid UID | |
2281 | ||
2282 | =item getgrgid GID | |
2283 | ||
2284 | =item getservbyname NAME,PROTO | |
2285 | ||
2286 | =item gethostbyaddr ADDR,ADDRTYPE | |
2287 | ||
2288 | =item getnetbyaddr ADDR,ADDRTYPE | |
2289 | ||
2290 | =item getprotobynumber NUMBER | |
2291 | ||
2292 | =item getservbyport PORT,PROTO | |
2293 | ||
2294 | =item getpwent | |
2295 | ||
2296 | =item getgrent | |
2297 | ||
2298 | =item gethostent | |
2299 | ||
2300 | =item getnetent | |
2301 | ||
2302 | =item getprotoent | |
2303 | ||
2304 | =item getservent | |
2305 | ||
2306 | =item setpwent | |
2307 | ||
2308 | =item setgrent | |
2309 | ||
2310 | =item sethostent STAYOPEN | |
2311 | ||
2312 | =item setnetent STAYOPEN | |
2313 | ||
2314 | =item setprotoent STAYOPEN | |
2315 | ||
2316 | =item setservent STAYOPEN | |
2317 | ||
2318 | =item endpwent | |
2319 | ||
2320 | =item endgrent | |
2321 | ||
2322 | =item endhostent | |
2323 | ||
2324 | =item endnetent | |
2325 | ||
2326 | =item endprotoent | |
2327 | ||
2328 | =item endservent | |
2329 | ||
80d38338 TC |
2330 | These routines are the same as their counterparts in the |
2331 | system C library. In list context, the return values from the | |
a0d0e21e LW |
2332 | various get routines are as follows: |
2333 | ||
2334 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 2335 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
2336 | ($name,$passwd,$gid,$members) = getgr* |
2337 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
2338 | ($name,$aliases,$addrtype,$net) = getnet* | |
2339 | ($name,$aliases,$proto) = getproto* | |
2340 | ($name,$aliases,$port,$proto) = getserv* | |
2341 | ||
3b10bc60 | 2342 | (If the entry doesn't exist you get an empty list.) |
a0d0e21e | 2343 | |
4602f195 JH |
2344 | The exact meaning of the $gcos field varies but it usually contains |
2345 | the real name of the user (as opposed to the login name) and other | |
2346 | information pertaining to the user. Beware, however, that in many | |
2347 | system users are able to change this information and therefore it | |
106325ad | 2348 | cannot be trusted and therefore the $gcos is tainted (see |
2959b6e3 | 2349 | L<perlsec>). The $passwd and $shell, user's encrypted password and |
a3390c9f | 2350 | login shell, are also tainted, for the same reason. |
4602f195 | 2351 | |
5a964f20 | 2352 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
2353 | lookup by name, in which case you get the other thing, whatever it is. |
2354 | (If the entry doesn't exist you get the undefined value.) For example: | |
2355 | ||
5a964f20 TC |
2356 | $uid = getpwnam($name); |
2357 | $name = getpwuid($num); | |
2358 | $name = getpwent(); | |
2359 | $gid = getgrnam($name); | |
08a33e13 | 2360 | $name = getgrgid($num); |
5a964f20 TC |
2361 | $name = getgrent(); |
2362 | #etc. | |
a0d0e21e | 2363 | |
4602f195 | 2364 | In I<getpw*()> the fields $quota, $comment, and $expire are special |
80d38338 | 2365 | in that they are unsupported on many systems. If the |
4602f195 JH |
2366 | $quota is unsupported, it is an empty scalar. If it is supported, it |
2367 | usually encodes the disk quota. If the $comment field is unsupported, | |
2368 | it is an empty scalar. If it is supported it usually encodes some | |
2369 | administrative comment about the user. In some systems the $quota | |
2370 | field may be $change or $age, fields that have to do with password | |
2371 | aging. In some systems the $comment field may be $class. The $expire | |
2372 | field, if present, encodes the expiration period of the account or the | |
2373 | password. For the availability and the exact meaning of these fields | |
8f1da26d | 2374 | in your system, please consult getpwnam(3) and your system's |
4602f195 JH |
2375 | F<pwd.h> file. You can also find out from within Perl what your |
2376 | $quota and $comment fields mean and whether you have the $expire field | |
2377 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
2378 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password | |
3b10bc60 | 2379 | files are supported only if your vendor has implemented them in the |
4602f195 | 2380 | intuitive fashion that calling the regular C library routines gets the |
5d3a0a3b | 2381 | shadow versions if you're running under privilege or if there exists |
cf264981 | 2382 | the shadow(3) functions as found in System V (this includes Solaris |
a3390c9f | 2383 | and Linux). Those systems that implement a proprietary shadow password |
5d3a0a3b | 2384 | facility are unlikely to be supported. |
6ee623d5 | 2385 | |
a3390c9f | 2386 | The $members value returned by I<getgr*()> is a space-separated list of |
a0d0e21e LW |
2387 | the login names of the members of the group. |
2388 | ||
2389 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
2390 | C, it will be returned to you via C<$?> if the function call fails. The | |
3b10bc60 | 2391 | C<@addrs> value returned by a successful call is a list of raw |
2392 | addresses returned by the corresponding library call. In the | |
2393 | Internet domain, each address is four bytes long; you can unpack it | |
a0d0e21e LW |
2394 | by saying something like: |
2395 | ||
f337b084 | 2396 | ($a,$b,$c,$d) = unpack('W4',$addr[0]); |
a0d0e21e | 2397 | |
2b5ab1e7 TC |
2398 | The Socket library makes this slightly easier: |
2399 | ||
2400 | use Socket; | |
2401 | $iaddr = inet_aton("127.1"); # or whatever address | |
2402 | $name = gethostbyaddr($iaddr, AF_INET); | |
2403 | ||
2404 | # or going the other way | |
19799a22 | 2405 | $straddr = inet_ntoa($iaddr); |
2b5ab1e7 | 2406 | |
d760c846 GS |
2407 | In the opposite way, to resolve a hostname to the IP address |
2408 | you can write this: | |
2409 | ||
2410 | use Socket; | |
2411 | $packed_ip = gethostbyname("www.perl.org"); | |
2412 | if (defined $packed_ip) { | |
2413 | $ip_address = inet_ntoa($packed_ip); | |
2414 | } | |
2415 | ||
2416 | Make sure <gethostbyname()> is called in SCALAR context and that | |
2417 | its return value is checked for definedness. | |
2418 | ||
19799a22 GS |
2419 | If you get tired of remembering which element of the return list |
2420 | contains which return value, by-name interfaces are provided | |
2421 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
2422 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
2423 | and C<User::grent>. These override the normal built-ins, supplying | |
2424 | versions that return objects with the appropriate names | |
2425 | for each field. For example: | |
5a964f20 TC |
2426 | |
2427 | use File::stat; | |
2428 | use User::pwent; | |
2429 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
2430 | ||
a3390c9f | 2431 | Even though it looks as though they're the same method calls (uid), |
b76cc8ba | 2432 | they aren't, because a C<File::stat> object is different from |
19799a22 | 2433 | a C<User::pwent> object. |
5a964f20 | 2434 | |
ea9eb35a BJ |
2435 | Portability issues: L<perlport/getpwnam> to L<perlport/endservent>. |
2436 | ||
a0d0e21e | 2437 | =item getsockname SOCKET |
d74e8afc | 2438 | X<getsockname> |
a0d0e21e | 2439 | |
19799a22 GS |
2440 | Returns the packed sockaddr address of this end of the SOCKET connection, |
2441 | in case you don't know the address because you have several different | |
2442 | IPs that the connection might have come in on. | |
a0d0e21e | 2443 | |
4633a7c4 LW |
2444 | use Socket; |
2445 | $mysockaddr = getsockname(SOCK); | |
19799a22 | 2446 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
b76cc8ba | 2447 | printf "Connect to %s [%s]\n", |
19799a22 GS |
2448 | scalar gethostbyaddr($myaddr, AF_INET), |
2449 | inet_ntoa($myaddr); | |
a0d0e21e LW |
2450 | |
2451 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
d74e8afc | 2452 | X<getsockopt> |
a0d0e21e | 2453 | |
636e6b1f TH |
2454 | Queries the option named OPTNAME associated with SOCKET at a given LEVEL. |
2455 | Options may exist at multiple protocol levels depending on the socket | |
2456 | type, but at least the uppermost socket level SOL_SOCKET (defined in the | |
2457 | C<Socket> module) will exist. To query options at another level the | |
2458 | protocol number of the appropriate protocol controlling the option | |
2459 | should be supplied. For example, to indicate that an option is to be | |
2460 | interpreted by the TCP protocol, LEVEL should be set to the protocol | |
80d38338 | 2461 | number of TCP, which you can get using C<getprotobyname>. |
636e6b1f | 2462 | |
80d38338 | 2463 | The function returns a packed string representing the requested socket |
3b10bc60 | 2464 | option, or C<undef> on error, with the reason for the error placed in |
a3390c9f | 2465 | C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME; |
80d38338 TC |
2466 | consult getsockopt(2) for details. A common case is that the option is an |
2467 | integer, in which case the result is a packed integer, which you can decode | |
2468 | using C<unpack> with the C<i> (or C<I>) format. | |
636e6b1f | 2469 | |
8f1da26d | 2470 | Here's an example to test whether Nagle's algorithm is enabled on a socket: |
636e6b1f | 2471 | |
4852725b | 2472 | use Socket qw(:all); |
636e6b1f TH |
2473 | |
2474 | defined(my $tcp = getprotobyname("tcp")) | |
a9a5a0dc | 2475 | or die "Could not determine the protocol number for tcp"; |
4852725b DD |
2476 | # my $tcp = IPPROTO_TCP; # Alternative |
2477 | my $packed = getsockopt($socket, $tcp, TCP_NODELAY) | |
80d38338 | 2478 | or die "getsockopt TCP_NODELAY: $!"; |
636e6b1f TH |
2479 | my $nodelay = unpack("I", $packed); |
2480 | print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n"; | |
2481 | ||
ea9eb35a | 2482 | Portability issues: L<perlport/getsockopt>. |
a0d0e21e | 2483 | |
15a348aa NC |
2484 | =item given EXPR BLOCK |
2485 | X<given> | |
2486 | ||
2487 | =item given BLOCK | |
2488 | ||
2489 | C<given> is analogous to the C<switch> keyword in other languages. C<given> | |
2490 | and C<when> are used in Perl to implement C<switch>/C<case> like statements. | |
8f1da26d | 2491 | Only available after Perl 5.10. For example: |
15a348aa | 2492 | |
8f1da26d | 2493 | use v5.10; |
15a348aa NC |
2494 | given ($fruit) { |
2495 | when (/apples?/) { | |
2496 | print "I like apples." | |
2497 | } | |
2498 | when (/oranges?/) { | |
2499 | print "I don't like oranges." | |
2500 | } | |
2501 | default { | |
2502 | print "I don't like anything" | |
2503 | } | |
2504 | } | |
2505 | ||
2506 | See L<perlsyn/"Switch statements"> for detailed information. | |
2507 | ||
a0d0e21e | 2508 | =item glob EXPR |
d74e8afc | 2509 | X<glob> X<wildcard> X<filename, expansion> X<expand> |
a0d0e21e | 2510 | |
0a753a76 | 2511 | =item glob |
2512 | ||
d9a9d457 JL |
2513 | In list context, returns a (possibly empty) list of filename expansions on |
2514 | the value of EXPR such as the standard Unix shell F</bin/csh> would do. In | |
2515 | scalar context, glob iterates through such filename expansions, returning | |
2516 | undef when the list is exhausted. This is the internal function | |
2517 | implementing the C<< <*.c> >> operator, but you can use it directly. If | |
2518 | EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in | |
2519 | more detail in L<perlop/"I/O Operators">. | |
a0d0e21e | 2520 | |
80d38338 TC |
2521 | Note that C<glob> splits its arguments on whitespace and treats |
2522 | each segment as separate pattern. As such, C<glob("*.c *.h")> | |
2523 | matches all files with a F<.c> or F<.h> extension. The expression | |
b474a1b1 | 2524 | C<glob(".* *")> matches all files in the current working directory. |
80d38338 TC |
2525 | |
2526 | If non-empty braces are the only wildcard characters used in the | |
2527 | C<glob>, no filenames are matched, but potentially many strings | |
2528 | are returned. For example, this produces nine strings, one for | |
2529 | each pairing of fruits and colors: | |
2530 | ||
2531 | @many = glob "{apple,tomato,cherry}={green,yellow,red}"; | |
5c0c9249 | 2532 | |
3a4b19e4 | 2533 | Beginning with v5.6.0, this operator is implemented using the standard |
5c0c9249 PF |
2534 | C<File::Glob> extension. See L<File::Glob> for details, including |
2535 | C<bsd_glob> which does not treat whitespace as a pattern separator. | |
3a4b19e4 | 2536 | |
ea9eb35a BJ |
2537 | Portability issues: L<perlport/glob>. |
2538 | ||
a0d0e21e | 2539 | =item gmtime EXPR |
d74e8afc | 2540 | X<gmtime> X<UTC> X<Greenwich> |
a0d0e21e | 2541 | |
ce2984c3 PF |
2542 | =item gmtime |
2543 | ||
4509d391 | 2544 | Works just like L</localtime> but the returned values are |
435fbc73 | 2545 | localized for the standard Greenwich time zone. |
a0d0e21e | 2546 | |
a3390c9f FC |
2547 | Note: When called in list context, $isdst, the last value |
2548 | returned by gmtime, is always C<0>. There is no | |
435fbc73 | 2549 | Daylight Saving Time in GMT. |
0a753a76 | 2550 | |
ea9eb35a | 2551 | Portability issues: L<perlport/gmtime>. |
62aa5637 | 2552 | |
a0d0e21e | 2553 | =item goto LABEL |
d74e8afc | 2554 | X<goto> X<jump> X<jmp> |
a0d0e21e | 2555 | |
748a9306 LW |
2556 | =item goto EXPR |
2557 | ||
a0d0e21e LW |
2558 | =item goto &NAME |
2559 | ||
b500e03b GG |
2560 | The C<goto-LABEL> form finds the statement labeled with LABEL and |
2561 | resumes execution there. It can't be used to get out of a block or | |
2562 | subroutine given to C<sort>. It can be used to go almost anywhere | |
2563 | else within the dynamic scope, including out of subroutines, but it's | |
2564 | usually better to use some other construct such as C<last> or C<die>. | |
2565 | The author of Perl has never felt the need to use this form of C<goto> | |
3b10bc60 | 2566 | (in Perl, that is; C is another matter). (The difference is that C |
b500e03b GG |
2567 | does not offer named loops combined with loop control. Perl does, and |
2568 | this replaces most structured uses of C<goto> in other languages.) | |
a0d0e21e | 2569 | |
7660c0ab A |
2570 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
2571 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
2572 | necessarily recommended if you're optimizing for maintainability: |
2573 | ||
2574 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2575 | ||
887d89fd FC |
2576 | As shown in this example, C<goto-EXPR> is exempt from the "looks like a |
2577 | function" rule. A pair of parentheses following it does not (necessarily) | |
2578 | delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>. | |
2579 | ||
b500e03b | 2580 | Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is |
0b98bec9 | 2581 | deprecated and will issue a warning. Even then, it may not be used to |
b500e03b GG |
2582 | go into any construct that requires initialization, such as a |
2583 | subroutine or a C<foreach> loop. It also can't be used to go into a | |
0b98bec9 | 2584 | construct that is optimized away. |
b500e03b | 2585 | |
1b6921cb BT |
2586 | The C<goto-&NAME> form is quite different from the other forms of |
2587 | C<goto>. In fact, it isn't a goto in the normal sense at all, and | |
2588 | doesn't have the stigma associated with other gotos. Instead, it | |
2589 | exits the current subroutine (losing any changes set by local()) and | |
2590 | immediately calls in its place the named subroutine using the current | |
2591 | value of @_. This is used by C<AUTOLOAD> subroutines that wish to | |
2592 | load another subroutine and then pretend that the other subroutine had | |
2593 | been called in the first place (except that any modifications to C<@_> | |
6cb9131c GS |
2594 | in the current subroutine are propagated to the other subroutine.) |
2595 | After the C<goto>, not even C<caller> will be able to tell that this | |
2596 | routine was called first. | |
2597 | ||
2598 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
8f1da26d | 2599 | containing a code reference or a block that evaluates to a code |
6cb9131c | 2600 | reference. |
a0d0e21e LW |
2601 | |
2602 | =item grep BLOCK LIST | |
d74e8afc | 2603 | X<grep> |
a0d0e21e LW |
2604 | |
2605 | =item grep EXPR,LIST | |
2606 | ||
2b5ab1e7 TC |
2607 | This is similar in spirit to, but not the same as, grep(1) and its |
2608 | relatives. In particular, it is not limited to using regular expressions. | |
2f9daede | 2609 | |
a0d0e21e | 2610 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 2611 | C<$_> to each element) and returns the list value consisting of those |
19799a22 GS |
2612 | elements for which the expression evaluated to true. In scalar |
2613 | context, returns the number of times the expression was true. | |
a0d0e21e LW |
2614 | |
2615 | @foo = grep(!/^#/, @bar); # weed out comments | |
2616 | ||
2617 | or equivalently, | |
2618 | ||
2619 | @foo = grep {!/^#/} @bar; # weed out comments | |
2620 | ||
be3174d2 GS |
2621 | Note that C<$_> is an alias to the list value, so it can be used to |
2622 | modify the elements of the LIST. While this is useful and supported, | |
2623 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
2624 | Similarly, grep returns aliases into the original list, much as a for |
2625 | loop's index variable aliases the list elements. That is, modifying an | |
19799a22 GS |
2626 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
2627 | or another C<grep>) actually modifies the element in the original list. | |
2b5ab1e7 | 2628 | This is usually something to be avoided when writing clear code. |
a0d0e21e | 2629 | |
a4fb8298 | 2630 | If C<$_> is lexical in the scope where the C<grep> appears (because it has |
cf264981 | 2631 | been declared with C<my $_>) then, in addition to being locally aliased to |
80d38338 | 2632 | the list elements, C<$_> keeps being lexical inside the block; i.e., it |
a4fb8298 RGS |
2633 | can't be seen from the outside, avoiding any potential side-effects. |
2634 | ||
19799a22 | 2635 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
38325410 | 2636 | |
a0d0e21e | 2637 | =item hex EXPR |
d74e8afc | 2638 | X<hex> X<hexadecimal> |
a0d0e21e | 2639 | |
54310121 | 2640 | =item hex |
bbce6d69 | 2641 | |
2b5ab1e7 | 2642 | Interprets EXPR as a hex string and returns the corresponding value. |
38366c11 | 2643 | (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see |
2b5ab1e7 | 2644 | L</oct>.) If EXPR is omitted, uses C<$_>. |
2f9daede TP |
2645 | |
2646 | print hex '0xAf'; # prints '175' | |
2647 | print hex 'aF'; # same | |
a0d0e21e | 2648 | |
19799a22 | 2649 | Hex strings may only represent integers. Strings that would cause |
53305cf1 | 2650 | integer overflow trigger a warning. Leading whitespace is not stripped, |
38366c11 | 2651 | unlike oct(). To present something as hex, look into L</printf>, |
8f1da26d | 2652 | L</sprintf>, and L</unpack>. |
19799a22 | 2653 | |
ce2984c3 | 2654 | =item import LIST |
d74e8afc | 2655 | X<import> |
a0d0e21e | 2656 | |
19799a22 | 2657 | There is no builtin C<import> function. It is just an ordinary |
4633a7c4 | 2658 | method (subroutine) defined (or inherited) by modules that wish to export |
19799a22 | 2659 | names to another module. The C<use> function calls the C<import> method |
cea6626f | 2660 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
2661 | |
2662 | =item index STR,SUBSTR,POSITION | |
d74e8afc | 2663 | X<index> X<indexOf> X<InStr> |
a0d0e21e LW |
2664 | |
2665 | =item index STR,SUBSTR | |
2666 | ||
2b5ab1e7 TC |
2667 | The index function searches for one string within another, but without |
2668 | the wildcard-like behavior of a full regular-expression pattern match. | |
2669 | It returns the position of the first occurrence of SUBSTR in STR at | |
2670 | or after POSITION. If POSITION is omitted, starts searching from the | |
26f149de YST |
2671 | beginning of the string. POSITION before the beginning of the string |
2672 | or after its end is treated as if it were the beginning or the end, | |
2673 | respectively. POSITION and the return value are based at C<0> (or whatever | |
2b5ab1e7 | 2674 | you've set the C<$[> variable to--but don't do that). If the substring |
cf264981 | 2675 | is not found, C<index> returns one less than the base, ordinarily C<-1>. |
a0d0e21e LW |
2676 | |
2677 | =item int EXPR | |
f723aae1 | 2678 | X<int> X<integer> X<truncate> X<trunc> X<floor> |
a0d0e21e | 2679 | |
54310121 | 2680 | =item int |
bbce6d69 | 2681 | |
7660c0ab | 2682 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
2b5ab1e7 | 2683 | You should not use this function for rounding: one because it truncates |
3b10bc60 | 2684 | towards C<0>, and two because machine representations of floating-point |
2b5ab1e7 TC |
2685 | numbers can sometimes produce counterintuitive results. For example, |
2686 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2687 | because it's really more like -268.99999999999994315658 instead. Usually, | |
19799a22 | 2688 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
2b5ab1e7 | 2689 | functions will serve you better than will int(). |
a0d0e21e LW |
2690 | |
2691 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
d74e8afc | 2692 | X<ioctl> |
a0d0e21e | 2693 | |
2b5ab1e7 | 2694 | Implements the ioctl(2) function. You'll probably first have to say |
a0d0e21e | 2695 | |
5ed4f2ec | 2696 | require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph |
a0d0e21e | 2697 | |
a11c483f | 2698 | to get the correct function definitions. If F<sys/ioctl.ph> doesn't |
a0d0e21e | 2699 | exist or doesn't have the correct definitions you'll have to roll your |
61eff3bc | 2700 | own, based on your C header files such as F<< <sys/ioctl.h> >>. |
5a964f20 | 2701 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 2702 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
3b10bc60 | 2703 | written depending on the FUNCTION; a C pointer to the string value of SCALAR |
19799a22 | 2704 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
4633a7c4 LW |
2705 | has no string value but does have a numeric value, that value will be |
2706 | passed rather than a pointer to the string value. To guarantee this to be | |
19799a22 GS |
2707 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
2708 | functions may be needed to manipulate the values of structures used by | |
b76cc8ba | 2709 | C<ioctl>. |
a0d0e21e | 2710 | |
19799a22 | 2711 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
a0d0e21e | 2712 | |
5ed4f2ec | 2713 | if OS returns: then Perl returns: |
2714 | -1 undefined value | |
2715 | 0 string "0 but true" | |
2716 | anything else that number | |
a0d0e21e | 2717 | |
19799a22 | 2718 | Thus Perl returns true on success and false on failure, yet you can |
a0d0e21e LW |
2719 | still easily determine the actual value returned by the operating |
2720 | system: | |
2721 | ||
2b5ab1e7 | 2722 | $retval = ioctl(...) || -1; |
a0d0e21e LW |
2723 | printf "System returned %d\n", $retval; |
2724 | ||
be2f7487 | 2725 | The special string C<"0 but true"> is exempt from B<-w> complaints |
5a964f20 TC |
2726 | about improper numeric conversions. |
2727 | ||
ea9eb35a BJ |
2728 | Portability issues: L<perlport/ioctl>. |
2729 | ||
a0d0e21e | 2730 | =item join EXPR,LIST |
d74e8afc | 2731 | X<join> |
a0d0e21e | 2732 | |
2b5ab1e7 TC |
2733 | Joins the separate strings of LIST into a single string with fields |
2734 | separated by the value of EXPR, and returns that new string. Example: | |
a0d0e21e | 2735 | |
2b5ab1e7 | 2736 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
a0d0e21e | 2737 | |
eb6e2d6f GS |
2738 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
2739 | first argument. Compare L</split>. | |
a0d0e21e | 2740 | |
532eee96 | 2741 | =item keys HASH |
d74e8afc | 2742 | X<keys> X<key> |
aa689395 | 2743 | |
532eee96 | 2744 | =item keys ARRAY |
aeedbbed | 2745 | |
f5a93a43 TC |
2746 | =item keys EXPR |
2747 | ||
aeedbbed NC |
2748 | Returns a list consisting of all the keys of the named hash, or the indices |
2749 | of an array. (In scalar context, returns the number of keys or indices.) | |
504f80c1 | 2750 | |
aeedbbed | 2751 | The keys of a hash are returned in an apparently random order. The actual |
3b10bc60 | 2752 | random order is subject to change in future versions of Perl, but it |
504f80c1 | 2753 | is guaranteed to be the same order as either the C<values> or C<each> |
4546b9e6 | 2754 | function produces (given that the hash has not been modified). Since |
c5f61d2f | 2755 | Perl 5.8.1 the ordering can be different even between different runs of |
4546b9e6 | 2756 | Perl for security reasons (see L<perlsec/"Algorithmic Complexity |
d6df3700 | 2757 | Attacks">). |
504f80c1 | 2758 | |
8f1da26d | 2759 | As a side effect, calling keys() resets the internal interator of the HASH or ARRAY |
cf264981 SP |
2760 | (see L</each>). In particular, calling keys() in void context resets |
2761 | the iterator with no other overhead. | |
a0d0e21e | 2762 | |
aa689395 | 2763 | Here is yet another way to print your environment: |
a0d0e21e LW |
2764 | |
2765 | @keys = keys %ENV; | |
2766 | @values = values %ENV; | |
b76cc8ba | 2767 | while (@keys) { |
a9a5a0dc | 2768 | print pop(@keys), '=', pop(@values), "\n"; |
a0d0e21e LW |
2769 | } |
2770 | ||
2771 | or how about sorted by key: | |
2772 | ||
2773 | foreach $key (sort(keys %ENV)) { | |
a9a5a0dc | 2774 | print $key, '=', $ENV{$key}, "\n"; |
a0d0e21e LW |
2775 | } |
2776 | ||
8ea1e5d4 GS |
2777 | The returned values are copies of the original keys in the hash, so |
2778 | modifying them will not affect the original hash. Compare L</values>. | |
2779 | ||
19799a22 | 2780 | To sort a hash by value, you'll need to use a C<sort> function. |
aa689395 | 2781 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 2782 | |
5a964f20 | 2783 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
a9a5a0dc | 2784 | printf "%4d %s\n", $hash{$key}, $key; |
4633a7c4 LW |
2785 | } |
2786 | ||
3b10bc60 | 2787 | Used as an lvalue, C<keys> allows you to increase the number of hash buckets |
aa689395 | 2788 | allocated for the given hash. This can gain you a measure of efficiency if |
2789 | you know the hash is going to get big. (This is similar to pre-extending | |
2790 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 2791 | |
2792 | keys %hash = 200; | |
2793 | ||
ab192400 GS |
2794 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
2795 | in fact, since it rounds up to the next power of two. These | |
55497cff | 2796 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
2797 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2798 | You can't shrink the number of buckets allocated for the hash using | |
19799a22 | 2799 | C<keys> in this way (but you needn't worry about doing this by accident, |
aeedbbed NC |
2800 | as trying has no effect). C<keys @array> in an lvalue context is a syntax |
2801 | error. | |
55497cff | 2802 | |
f5a93a43 TC |
2803 | Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain |
2804 | a reference to an unblessed hash or array. The argument will be | |
2805 | dereferenced automatically. This aspect of C<keys> is considered highly | |
2806 | experimental. The exact behaviour may change in a future version of Perl. | |
cba5a3b0 DG |
2807 | |
2808 | for (keys $hashref) { ... } | |
2809 | for (keys $obj->get_arrayref) { ... } | |
2810 | ||
8f1da26d | 2811 | See also C<each>, C<values>, and C<sort>. |
ab192400 | 2812 | |
b350dd2f | 2813 | =item kill SIGNAL, LIST |
d74e8afc | 2814 | X<kill> X<signal> |
a0d0e21e | 2815 | |
b350dd2f | 2816 | Sends a signal to a list of processes. Returns the number of |
517db077 GS |
2817 | processes successfully signaled (which is not necessarily the |
2818 | same as the number actually killed). | |
a0d0e21e LW |
2819 | |
2820 | $cnt = kill 1, $child1, $child2; | |
2821 | kill 9, @goners; | |
2822 | ||
3b10bc60 | 2823 | If SIGNAL is zero, no signal is sent to the process, but C<kill> |
2824 | checks whether it's I<possible> to send a signal to it (that | |
70fb64f6 | 2825 | means, to be brief, that the process is owned by the same user, or we are |
3b10bc60 | 2826 | the super-user). This is useful to check that a child process is still |
81fd35db DN |
2827 | alive (even if only as a zombie) and hasn't changed its UID. See |
2828 | L<perlport> for notes on the portability of this construct. | |
b350dd2f | 2829 | |
e2c0f81f DG |
2830 | Unlike in the shell, if SIGNAL is negative, it kills process groups instead |
2831 | of processes. That means you usually want to use positive not negative signals. | |
2832 | You may also use a signal name in quotes. | |
2833 | ||
2834 | The behavior of kill when a I<PROCESS> number is zero or negative depends on | |
2835 | the operating system. For example, on POSIX-conforming systems, zero will | |
2836 | signal the current process group and -1 will signal all processes. | |
1e9c1022 JL |
2837 | |
2838 | See L<perlipc/"Signals"> for more details. | |
a0d0e21e | 2839 | |
ea9eb35a BJ |
2840 | On some platforms such as Windows where the fork() system call is not available. |
2841 | Perl can be built to emulate fork() at the interpreter level. | |
6d17f725 | 2842 | This emulation has limitations related to kill that have to be considered, |
ea9eb35a BJ |
2843 | for code running on Windows and in code intended to be portable. |
2844 | ||
2845 | See L<perlfork> for more details. | |
2846 | ||
2847 | Portability issues: L<perlport/kill>. | |
2848 | ||
a0d0e21e | 2849 | =item last LABEL |
d74e8afc | 2850 | X<last> X<break> |
a0d0e21e LW |
2851 | |
2852 | =item last | |
2853 | ||
2854 | The C<last> command is like the C<break> statement in C (as used in | |
2855 | loops); it immediately exits the loop in question. If the LABEL is | |
2856 | omitted, the command refers to the innermost enclosing loop. The | |
2857 | C<continue> block, if any, is not executed: | |
2858 | ||
4633a7c4 | 2859 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
2860 | last LINE if /^$/; # exit when done with header |
2861 | #... | |
a0d0e21e LW |
2862 | } |
2863 | ||
80d38338 | 2864 | C<last> cannot be used to exit a block that returns a value such as |
8f1da26d | 2865 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit |
2b5ab1e7 | 2866 | a grep() or map() operation. |
4968c1e4 | 2867 | |
6c1372ed GS |
2868 | Note that a block by itself is semantically identical to a loop |
2869 | that executes once. Thus C<last> can be used to effect an early | |
2870 | exit out of such a block. | |
2871 | ||
98293880 JH |
2872 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2873 | C<redo> work. | |
1d2dff63 | 2874 | |
a0d0e21e | 2875 | =item lc EXPR |
d74e8afc | 2876 | X<lc> X<lowercase> |
a0d0e21e | 2877 | |
54310121 | 2878 | =item lc |
bbce6d69 | 2879 | |
d1be9408 | 2880 | Returns a lowercased version of EXPR. This is the internal function |
3980dc9c | 2881 | implementing the C<\L> escape in double-quoted strings. |
a0d0e21e | 2882 | |
7660c0ab | 2883 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2884 | |
3980dc9c KW |
2885 | What gets returned depends on several factors: |
2886 | ||
2887 | =over | |
2888 | ||
2889 | =item If C<use bytes> is in effect: | |
2890 | ||
2891 | =over | |
2892 | ||
2893 | =item On EBCDIC platforms | |
2894 | ||
2895 | The results are what the C language system call C<tolower()> returns. | |
2896 | ||
2897 | =item On ASCII platforms | |
2898 | ||
2899 | The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z> | |
2900 | respectively. | |
2901 | ||
2902 | =back | |
2903 | ||
2904 | =item Otherwise, If EXPR has the UTF8 flag set | |
2905 | ||
2906 | If the current package has a subroutine named C<ToLower>, it will be used to | |
96090e4f LB |
2907 | change the case |
2908 | (See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.) | |
3980dc9c KW |
2909 | Otherwise Unicode semantics are used for the case change. |
2910 | ||
2911 | =item Otherwise, if C<use locale> is in effect | |
2912 | ||
2913 | Respects current LC_CTYPE locale. See L<perllocale>. | |
2914 | ||
2915 | =item Otherwise, if C<use feature 'unicode_strings'> is in effect: | |
2916 | ||
2917 | Unicode semantics are used for the case change. Any subroutine named | |
15dbbbab | 2918 | C<ToLower> will be ignored. |
3980dc9c KW |
2919 | |
2920 | =item Otherwise: | |
2921 | ||
2922 | =over | |
2923 | ||
2924 | =item On EBCDIC platforms | |
2925 | ||
2926 | The results are what the C language system call C<tolower()> returns. | |
2927 | ||
2928 | =item On ASCII platforms | |
2929 | ||
2930 | ASCII semantics are used for the case change. The lowercase of any character | |
2931 | outside the ASCII range is the character itself. | |
2932 | ||
2933 | =back | |
2934 | ||
2935 | =back | |
2936 | ||
a0d0e21e | 2937 | =item lcfirst EXPR |
d74e8afc | 2938 | X<lcfirst> X<lowercase> |
a0d0e21e | 2939 | |
54310121 | 2940 | =item lcfirst |
bbce6d69 | 2941 | |
ad0029c4 JH |
2942 | Returns the value of EXPR with the first character lowercased. This |
2943 | is the internal function implementing the C<\l> escape in | |
3980dc9c | 2944 | double-quoted strings. |
a0d0e21e | 2945 | |
7660c0ab | 2946 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2947 | |
15dbbbab | 2948 | This function behaves the same way under various pragmata, such as in a locale, |
3980dc9c KW |
2949 | as L</lc> does. |
2950 | ||
a0d0e21e | 2951 | =item length EXPR |
d74e8afc | 2952 | X<length> X<size> |
a0d0e21e | 2953 | |
54310121 | 2954 | =item length |
bbce6d69 | 2955 | |
974da8e5 | 2956 | Returns the length in I<characters> of the value of EXPR. If EXPR is |
15dbbbab FC |
2957 | omitted, returns the length of C<$_>. If EXPR is undefined, returns |
2958 | C<undef>. | |
3b10bc60 | 2959 | |
2960 | This function cannot be used on an entire array or hash to find out how | |
2961 | many elements these have. For that, use C<scalar @array> and C<scalar keys | |
2962 | %hash>, respectively. | |
2963 | ||
2964 | Like all Perl character operations, length() normally deals in logical | |
2965 | characters, not physical bytes. For how many bytes a string encoded as | |
2966 | UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have | |
2967 | to C<use Encode> first). See L<Encode> and L<perlunicode>. | |
974da8e5 | 2968 | |
a0d0e21e | 2969 | =item link OLDFILE,NEWFILE |
d74e8afc | 2970 | X<link> |
a0d0e21e | 2971 | |
19799a22 | 2972 | Creates a new filename linked to the old filename. Returns true for |
b76cc8ba | 2973 | success, false otherwise. |
a0d0e21e | 2974 | |
ea9eb35a BJ |
2975 | Portability issues: L<perlport/link>. |
2976 | ||
a0d0e21e | 2977 | =item listen SOCKET,QUEUESIZE |
d74e8afc | 2978 | X<listen> |
a0d0e21e | 2979 | |
3b10bc60 | 2980 | Does the same thing that the listen(2) system call does. Returns true if |
b76cc8ba | 2981 | it succeeded, false otherwise. See the example in |
cea6626f | 2982 | L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
2983 | |
2984 | =item local EXPR | |
d74e8afc | 2985 | X<local> |
a0d0e21e | 2986 | |
19799a22 | 2987 | You really probably want to be using C<my> instead, because C<local> isn't |
b76cc8ba | 2988 | what most people think of as "local". See |
13a2d996 | 2989 | L<perlsub/"Private Variables via my()"> for details. |
2b5ab1e7 | 2990 | |
5a964f20 TC |
2991 | A local modifies the listed variables to be local to the enclosing |
2992 | block, file, or eval. If more than one value is listed, the list must | |
2993 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2994 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 2995 | |
d361fafa VP |
2996 | The C<delete local EXPR> construct can also be used to localize the deletion |
2997 | of array/hash elements to the current block. | |
2998 | See L<perlsub/"Localized deletion of elements of composite types">. | |
2999 | ||
a0d0e21e | 3000 | =item localtime EXPR |
435fbc73 | 3001 | X<localtime> X<ctime> |
a0d0e21e | 3002 | |
ba053783 AL |
3003 | =item localtime |
3004 | ||
19799a22 | 3005 | Converts a time as returned by the time function to a 9-element list |
5f05dabc | 3006 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
3007 | follows: |
3008 | ||
54310121 | 3009 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e | 3010 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
ba053783 | 3011 | localtime(time); |
a0d0e21e | 3012 | |
8f1da26d | 3013 | All list elements are numeric and come straight out of the C `struct |
ba053783 AL |
3014 | tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours |
3015 | of the specified time. | |
48a26b3a | 3016 | |
8f1da26d TC |
3017 | C<$mday> is the day of the month and C<$mon> the month in |
3018 | the range C<0..11>, with 0 indicating January and 11 indicating December. | |
ba053783 | 3019 | This makes it easy to get a month name from a list: |
54310121 | 3020 | |
ba053783 AL |
3021 | my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); |
3022 | print "$abbr[$mon] $mday"; | |
3023 | # $mon=9, $mday=18 gives "Oct 18" | |
abd75f24 | 3024 | |
8f1da26d | 3025 | C<$year> is the number of years since 1900, B<not> just the last two digits |
ba053783 | 3026 | of the year. That is, C<$year> is C<123> in year 2023. The proper way |
80d38338 | 3027 | to get a 4-digit year is simply: |
abd75f24 | 3028 | |
ba053783 | 3029 | $year += 1900; |
abd75f24 | 3030 | |
435fbc73 GS |
3031 | Otherwise you create non-Y2K-compliant programs--and you wouldn't want |
3032 | to do that, would you? | |
3033 | ||
8f1da26d | 3034 | To get the last two digits of the year (e.g., "01" in 2001) do: |
ba053783 AL |
3035 | |
3036 | $year = sprintf("%02d", $year % 100); | |
3037 | ||
3038 | C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating | |
3039 | Wednesday. C<$yday> is the day of the year, in the range C<0..364> | |
3040 | (or C<0..365> in leap years.) | |
3041 | ||
3042 | C<$isdst> is true if the specified time occurs during Daylight Saving | |
3043 | Time, false otherwise. | |
abd75f24 | 3044 | |
e1998452 | 3045 | If EXPR is omitted, C<localtime()> uses the current time (as returned |
e3176d09 | 3046 | by time(3)). |
a0d0e21e | 3047 | |
48a26b3a | 3048 | In scalar context, C<localtime()> returns the ctime(3) value: |
a0d0e21e | 3049 | |
5f05dabc | 3050 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 3051 | |
15dbbbab | 3052 | This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT |
fe86afc2 | 3053 | instead of local time use the L</gmtime> builtin. See also the |
8f1da26d | 3054 | C<Time::Local> module (for converting seconds, minutes, hours, and such back to |
fe86afc2 NC |
3055 | the integer value returned by time()), and the L<POSIX> module's strftime(3) |
3056 | and mktime(3) functions. | |
3057 | ||
15dbbbab | 3058 | To get somewhat similar but locale-dependent date strings, set up your |
fe86afc2 NC |
3059 | locale environment variables appropriately (please see L<perllocale>) and |
3060 | try for example: | |
a3cb178b | 3061 | |
5a964f20 | 3062 | use POSIX qw(strftime); |
2b5ab1e7 | 3063 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
fe86afc2 NC |
3064 | # or for GMT formatted appropriately for your locale: |
3065 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; | |
a3cb178b GS |
3066 | |
3067 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
3068 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e | 3069 | |
15dbbbab | 3070 | The L<Time::gmtime> and L<Time::localtime> modules provide a convenient, |
435fbc73 GS |
3071 | by-name access mechanism to the gmtime() and localtime() functions, |
3072 | respectively. | |
3073 | ||
3074 | For a comprehensive date and time representation look at the | |
3075 | L<DateTime> module on CPAN. | |
3076 | ||
ea9eb35a BJ |
3077 | Portability issues: L<perlport/localtime>. |
3078 | ||
07698885 | 3079 | =item lock THING |
d74e8afc | 3080 | X<lock> |
19799a22 | 3081 | |
15dbbbab | 3082 | This function places an advisory lock on a shared variable or referenced |
03730085 | 3083 | object contained in I<THING> until the lock goes out of scope. |
a6d5524e | 3084 | |
904028df FC |
3085 | The value returned is the scalar itself, if the argument is a scalar, or a |
3086 | reference, if the argument is a hash or array. | |
3087 | ||
f3a23afb | 3088 | lock() is a "weak keyword" : this means that if you've defined a function |
67408cae | 3089 | by this name (before any calls to it), that function will be called |
7b043ca5 RGS |
3090 | instead. If you are not under C<use threads::shared> this does nothing. |
3091 | See L<threads::shared>. | |
19799a22 | 3092 | |
a0d0e21e | 3093 | =item log EXPR |
d74e8afc | 3094 | X<log> X<logarithm> X<e> X<ln> X<base> |
a0d0e21e | 3095 | |
54310121 | 3096 | =item log |
bbce6d69 | 3097 | |
2b5ab1e7 | 3098 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
15dbbbab FC |
3099 | returns the log of C<$_>. To get the |
3100 | log of another base, use basic algebra: | |
19799a22 | 3101 | The base-N log of a number is equal to the natural log of that number |
2b5ab1e7 TC |
3102 | divided by the natural log of N. For example: |
3103 | ||
3104 | sub log10 { | |
a9a5a0dc VP |
3105 | my $n = shift; |
3106 | return log($n)/log(10); | |
b76cc8ba | 3107 | } |
2b5ab1e7 TC |
3108 | |
3109 | See also L</exp> for the inverse operation. | |
a0d0e21e | 3110 | |
a0d0e21e | 3111 | =item lstat EXPR |
d74e8afc | 3112 | X<lstat> |
a0d0e21e | 3113 | |
54310121 | 3114 | =item lstat |
bbce6d69 | 3115 | |
19799a22 | 3116 | Does the same thing as the C<stat> function (including setting the |
5a964f20 TC |
3117 | special C<_> filehandle) but stats a symbolic link instead of the file |
3118 | the symbolic link points to. If symbolic links are unimplemented on | |
c837d5b4 DP |
3119 | your system, a normal C<stat> is done. For much more detailed |
3120 | information, please see the documentation for C<stat>. | |
a0d0e21e | 3121 | |
7660c0ab | 3122 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 3123 | |
ea9eb35a BJ |
3124 | Portability issues: L<perlport/lstat>. |
3125 | ||
a0d0e21e LW |
3126 | =item m// |
3127 | ||
9f4b9cd0 | 3128 | The match operator. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e LW |
3129 | |
3130 | =item map BLOCK LIST | |
d74e8afc | 3131 | X<map> |
a0d0e21e LW |
3132 | |
3133 | =item map EXPR,LIST | |
3134 | ||
19799a22 GS |
3135 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
3136 | C<$_> to each element) and returns the list value composed of the | |
3137 | results of each such evaluation. In scalar context, returns the | |
3138 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
3139 | list context, so each element of LIST may produce zero, one, or | |
3140 | more elements in the returned value. | |
dd99ebda | 3141 | |
f9476272 | 3142 | @chars = map(chr, @numbers); |
a0d0e21e | 3143 | |
f9476272 AH |
3144 | translates a list of numbers to the corresponding characters. |
3145 | ||
3146 | my @squares = map { $_ * $_ } @numbers; | |
3147 | ||
3148 | translates a list of numbers to their squared values. | |
3149 | ||
3150 | my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers; | |
3151 | ||
3152 | shows that number of returned elements can differ from the number of | |
3153 | input elements. To omit an element, return an empty list (). | |
3154 | This could also be achieved by writing | |
3155 | ||
3156 | my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers; | |
3157 | ||
3158 | which makes the intention more clear. | |
3159 | ||
15dbbbab FC |
3160 | Map always returns a list, which can be |
3161 | assigned to a hash such that the elements | |
f9476272 | 3162 | become key/value pairs. See L<perldata> for more details. |
a0d0e21e | 3163 | |
d8216f19 | 3164 | %hash = map { get_a_key_for($_) => $_ } @array; |
a0d0e21e LW |
3165 | |
3166 | is just a funny way to write | |
3167 | ||
3168 | %hash = (); | |
d8216f19 | 3169 | foreach (@array) { |
a9a5a0dc | 3170 | $hash{get_a_key_for($_)} = $_; |
a0d0e21e LW |
3171 | } |
3172 | ||
be3174d2 GS |
3173 | Note that C<$_> is an alias to the list value, so it can be used to |
3174 | modify the elements of the LIST. While this is useful and supported, | |
3175 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
3176 | Using a regular C<foreach> loop for this purpose would be clearer in |
3177 | most cases. See also L</grep> for an array composed of those items of | |
3178 | the original list for which the BLOCK or EXPR evaluates to true. | |
fb73857a | 3179 | |
a4fb8298 | 3180 | If C<$_> is lexical in the scope where the C<map> appears (because it has |
d8216f19 RGS |
3181 | been declared with C<my $_>), then, in addition to being locally aliased to |
3182 | the list elements, C<$_> keeps being lexical inside the block; that is, it | |
a4fb8298 RGS |
3183 | can't be seen from the outside, avoiding any potential side-effects. |
3184 | ||
205fdb4d | 3185 | C<{> starts both hash references and blocks, so C<map { ...> could be either |
3b10bc60 | 3186 | the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look |
80d38338 TC |
3187 | ahead for the closing C<}> it has to take a guess at which it's dealing with |
3188 | based on what it finds just after the C<{>. Usually it gets it right, but if it | |
205fdb4d NC |
3189 | doesn't it won't realize something is wrong until it gets to the C<}> and |
3190 | encounters the missing (or unexpected) comma. The syntax error will be | |
80d38338 | 3191 | reported close to the C<}>, but you'll need to change something near the C<{> |
3b10bc60 | 3192 | such as using a unary C<+> to give Perl some help: |
205fdb4d | 3193 | |
3b10bc60 | 3194 | %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong |
3195 | %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right | |
3196 | %hash = map { ("\L$_" => 1) } @array # this also works | |
3197 | %hash = map { lc($_) => 1 } @array # as does this. | |
3198 | %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! | |
cea6626f | 3199 | |
3b10bc60 | 3200 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) |
205fdb4d | 3201 | |
d8216f19 | 3202 | or to force an anon hash constructor use C<+{>: |
205fdb4d | 3203 | |
3b10bc60 | 3204 | @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end |
205fdb4d | 3205 | |
3b10bc60 | 3206 | to get a list of anonymous hashes each with only one entry apiece. |
205fdb4d | 3207 | |
19799a22 | 3208 | =item mkdir FILENAME,MASK |
d74e8afc | 3209 | X<mkdir> X<md> X<directory, create> |
a0d0e21e | 3210 | |
5a211162 GS |
3211 | =item mkdir FILENAME |
3212 | ||
491873e5 RGS |
3213 | =item mkdir |
3214 | ||
0591cd52 | 3215 | Creates the directory specified by FILENAME, with permissions |
19799a22 | 3216 | specified by MASK (as modified by C<umask>). If it succeeds it |
8f1da26d TC |
3217 | returns true; otherwise it returns false and sets C<$!> (errno). |
3218 | MASK defaults to 0777 if omitted, and FILENAME defaults | |
3219 | to C<$_> if omitted. | |
0591cd52 | 3220 | |
8f1da26d TC |
3221 | In general, it is better to create directories with a permissive MASK |
3222 | and let the user modify that with their C<umask> than it is to supply | |
19799a22 | 3223 | a restrictive MASK and give the user no way to be more permissive. |
0591cd52 NT |
3224 | The exceptions to this rule are when the file or directory should be |
3225 | kept private (mail files, for instance). The perlfunc(1) entry on | |
19799a22 | 3226 | C<umask> discusses the choice of MASK in more detail. |
a0d0e21e | 3227 | |
cc1852e8 JH |
3228 | Note that according to the POSIX 1003.1-1996 the FILENAME may have any |
3229 | number of trailing slashes. Some operating and filesystems do not get | |
3230 | this right, so Perl automatically removes all trailing slashes to keep | |
3231 | everyone happy. | |
3232 | ||
80d38338 | 3233 | To recursively create a directory structure, look at |
dd184578 RGS |
3234 | the C<mkpath> function of the L<File::Path> module. |
3235 | ||
a0d0e21e | 3236 | =item msgctl ID,CMD,ARG |
d74e8afc | 3237 | X<msgctl> |
a0d0e21e | 3238 | |
f86cebdf | 3239 | Calls the System V IPC function msgctl(2). You'll probably have to say |
0ade1984 JH |
3240 | |
3241 | use IPC::SysV; | |
3242 | ||
7660c0ab | 3243 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
cf264981 | 3244 | then ARG must be a variable that will hold the returned C<msqid_ds> |
951ba7fe GS |
3245 | structure. Returns like C<ioctl>: the undefined value for error, |
3246 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
15dbbbab FC |
3247 | L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and |
3248 | C<IPC::Semaphore>. | |
a0d0e21e | 3249 | |
ea9eb35a BJ |
3250 | Portability issues: L<perlport/msgctl>. |
3251 | ||
a0d0e21e | 3252 | =item msgget KEY,FLAGS |
d74e8afc | 3253 | X<msgget> |
a0d0e21e | 3254 | |
f86cebdf | 3255 | Calls the System V IPC function msgget(2). Returns the message queue |
8f1da26d | 3256 | id, or C<undef> on error. See also |
15dbbbab FC |
3257 | L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and |
3258 | C<IPC::Msg>. | |
a0d0e21e | 3259 | |
ea9eb35a BJ |
3260 | Portability issues: L<perlport/msgget>. |
3261 | ||
a0d0e21e | 3262 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS |
d74e8afc | 3263 | X<msgrcv> |
a0d0e21e LW |
3264 | |
3265 | Calls the System V IPC function msgrcv to receive a message from | |
3266 | message queue ID into variable VAR with a maximum message size of | |
41d6edb2 JH |
3267 | SIZE. Note that when a message is received, the message type as a |
3268 | native long integer will be the first thing in VAR, followed by the | |
3269 | actual message. This packing may be opened with C<unpack("l! a*")>. | |
8f1da26d TC |
3270 | Taints the variable. Returns true if successful, false |
3271 | on error. See also L<perlipc/"SysV IPC"> and the documentation for | |
15dbbbab | 3272 | C<IPC::SysV> and C<IPC::SysV::Msg>. |
41d6edb2 | 3273 | |
ea9eb35a BJ |
3274 | Portability issues: L<perlport/msgrcv>. |
3275 | ||
41d6edb2 | 3276 | =item msgsnd ID,MSG,FLAGS |
d74e8afc | 3277 | X<msgsnd> |
41d6edb2 JH |
3278 | |
3279 | Calls the System V IPC function msgsnd to send the message MSG to the | |
3280 | message queue ID. MSG must begin with the native long integer message | |
8f1da26d | 3281 | type, be followed by the length of the actual message, and then finally |
41d6edb2 JH |
3282 | the message itself. This kind of packing can be achieved with |
3283 | C<pack("l! a*", $type, $message)>. Returns true if successful, | |
8f1da26d | 3284 | false on error. See also the C<IPC::SysV> |
41d6edb2 | 3285 | and C<IPC::SysV::Msg> documentation. |
a0d0e21e | 3286 | |
ea9eb35a BJ |
3287 | Portability issues: L<perlport/msgsnd>. |
3288 | ||
a0d0e21e | 3289 | =item my EXPR |
d74e8afc | 3290 | X<my> |
a0d0e21e | 3291 | |
307ea6df JH |
3292 | =item my TYPE EXPR |
3293 | ||
1d2de774 | 3294 | =item my EXPR : ATTRS |
09bef843 | 3295 | |
1d2de774 | 3296 | =item my TYPE EXPR : ATTRS |
307ea6df | 3297 | |
19799a22 | 3298 | A C<my> declares the listed variables to be local (lexically) to the |
1d2de774 JH |
3299 | enclosing block, file, or C<eval>. If more than one value is listed, |
3300 | the list must be placed in parentheses. | |
307ea6df | 3301 | |
1d2de774 | 3302 | The exact semantics and interface of TYPE and ATTRS are still |
15dbbbab | 3303 | evolving. TYPE is currently bound to the use of the C<fields> pragma, |
307ea6df JH |
3304 | and attributes are handled using the C<attributes> pragma, or starting |
3305 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3306 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3307 | L<attributes>, and L<Attribute::Handlers>. | |
4633a7c4 | 3308 | |
a0d0e21e | 3309 | =item next LABEL |
d74e8afc | 3310 | X<next> X<continue> |
a0d0e21e LW |
3311 | |
3312 | =item next | |
3313 | ||
3314 | The C<next> command is like the C<continue> statement in C; it starts | |
3315 | the next iteration of the loop: | |
3316 | ||
4633a7c4 | 3317 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
3318 | next LINE if /^#/; # discard comments |
3319 | #... | |
a0d0e21e LW |
3320 | } |
3321 | ||
3322 | Note that if there were a C<continue> block on the above, it would get | |
3b10bc60 | 3323 | executed even on discarded lines. If LABEL is omitted, the command |
a0d0e21e LW |
3324 | refers to the innermost enclosing loop. |
3325 | ||
4968c1e4 | 3326 | C<next> cannot be used to exit a block which returns a value such as |
8f1da26d | 3327 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit |
2b5ab1e7 | 3328 | a grep() or map() operation. |
4968c1e4 | 3329 | |
6c1372ed GS |
3330 | Note that a block by itself is semantically identical to a loop |
3331 | that executes once. Thus C<next> will exit such a block early. | |
3332 | ||
98293880 JH |
3333 | See also L</continue> for an illustration of how C<last>, C<next>, and |
3334 | C<redo> work. | |
1d2dff63 | 3335 | |
3b10bc60 | 3336 | =item no MODULE VERSION LIST |
3337 | X<no declarations> | |
3338 | X<unimporting> | |
4a66ea5a | 3339 | |
3b10bc60 | 3340 | =item no MODULE VERSION |
4a66ea5a | 3341 | |
3b10bc60 | 3342 | =item no MODULE LIST |
a0d0e21e | 3343 | |
3b10bc60 | 3344 | =item no MODULE |
4a66ea5a | 3345 | |
c986422f RGS |
3346 | =item no VERSION |
3347 | ||
593b9c14 | 3348 | See the C<use> function, of which C<no> is the opposite. |
a0d0e21e LW |
3349 | |
3350 | =item oct EXPR | |
d74e8afc | 3351 | X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> |
a0d0e21e | 3352 | |
54310121 | 3353 | =item oct |
bbce6d69 | 3354 | |
4633a7c4 | 3355 | Interprets EXPR as an octal string and returns the corresponding |
4f19785b WSI |
3356 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
3357 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
53305cf1 | 3358 | binary string. Leading whitespace is ignored in all three cases.) |
3b10bc60 | 3359 | The following will handle decimal, binary, octal, and hex in standard |
3360 | Perl notation: | |
a0d0e21e LW |
3361 | |
3362 | $val = oct($val) if $val =~ /^0/; | |
3363 | ||
19799a22 GS |
3364 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
3365 | in octal), use sprintf() or printf(): | |
3366 | ||
3b10bc60 | 3367 | $dec_perms = (stat("filename"))[2] & 07777; |
3368 | $oct_perm_str = sprintf "%o", $perms; | |
19799a22 GS |
3369 | |
3370 | The oct() function is commonly used when a string such as C<644> needs | |
3b10bc60 | 3371 | to be converted into a file mode, for example. Although Perl |
3372 | automatically converts strings into numbers as needed, this automatic | |
3373 | conversion assumes base 10. | |
3374 | ||
3375 | Leading white space is ignored without warning, as too are any trailing | |
3376 | non-digits, such as a decimal point (C<oct> only handles non-negative | |
3377 | integers, not negative integers or floating point). | |
a0d0e21e LW |
3378 | |
3379 | =item open FILEHANDLE,EXPR | |
d74e8afc | 3380 | X<open> X<pipe> X<file, open> X<fopen> |
a0d0e21e | 3381 | |
68bd7414 NIS |
3382 | =item open FILEHANDLE,MODE,EXPR |
3383 | ||
3384 | =item open FILEHANDLE,MODE,EXPR,LIST | |
3385 | ||
ba964c95 T |
3386 | =item open FILEHANDLE,MODE,REFERENCE |
3387 | ||
a0d0e21e LW |
3388 | =item open FILEHANDLE |
3389 | ||
3390 | Opens the file whose filename is given by EXPR, and associates it with | |
ed53a2bb JH |
3391 | FILEHANDLE. |
3392 | ||
460b70c2 GS |
3393 | Simple examples to open a file for reading: |
3394 | ||
8f1da26d TC |
3395 | open(my $fh, "<", "input.txt") |
3396 | or die "cannot open < input.txt: $!"; | |
460b70c2 GS |
3397 | |
3398 | and for writing: | |
3399 | ||
8f1da26d TC |
3400 | open(my $fh, ">", "output.txt") |
3401 | or die "cannot open > output.txt: $!"; | |
460b70c2 | 3402 | |
ed53a2bb JH |
3403 | (The following is a comprehensive reference to open(): for a gentler |
3404 | introduction you may consider L<perlopentut>.) | |
3405 | ||
8f1da26d TC |
3406 | If FILEHANDLE is an undefined scalar variable (or array or hash element), a |
3407 | new filehandle is autovivified, meaning that the variable is assigned a | |
3408 | reference to a newly allocated anonymous filehandle. Otherwise if | |
3409 | FILEHANDLE is an expression, its value is the real filehandle. (This is | |
3410 | considered a symbolic reference, so C<use strict "refs"> should I<not> be | |
3411 | in effect.) | |
3412 | ||
3413 | If EXPR is omitted, the global (package) scalar variable of the same | |
3414 | name as the FILEHANDLE contains the filename. (Note that lexical | |
3415 | variables--those declared with C<my> or C<state>--will not work for this | |
3416 | purpose; so if you're using C<my> or C<state>, specify EXPR in your | |
3417 | call to open.) | |
3418 | ||
3419 | If three (or more) arguments are specified, the open mode (including | |
3420 | optional encoding) in the second argument are distinct from the filename in | |
3421 | the third. If MODE is C<< < >> or nothing, the file is opened for input. | |
3422 | If MODE is C<< > >>, the file is opened for output, with existing files | |
3423 | first being truncated ("clobbered") and nonexisting files newly created. | |
3424 | If MODE is C<<< >> >>>, the file is opened for appending, again being | |
3425 | created if necessary. | |
3426 | ||
3427 | You can put a C<+> in front of the C<< > >> or C<< < >> to | |
ed53a2bb | 3428 | indicate that you want both read and write access to the file; thus |
8f1da26d TC |
3429 | C<< +< >> is almost always preferred for read/write updates--the |
3430 | C<< +> >> mode would clobber the file first. You cant usually use | |
ed53a2bb | 3431 | either read-write mode for updating textfiles, since they have |
bea6df1c | 3432 | variable-length records. See the B<-i> switch in L<perlrun> for a |
ed53a2bb | 3433 | better approach. The file is created with permissions of C<0666> |
e1020413 | 3434 | modified by the process's C<umask> value. |
ed53a2bb | 3435 | |
8f1da26d TC |
3436 | These various prefixes correspond to the fopen(3) modes of C<r>, |
3437 | C<r+>, C<w>, C<w+>, C<a>, and C<a+>. | |
5f05dabc | 3438 | |
8f1da26d TC |
3439 | In the one- and two-argument forms of the call, the mode and filename |
3440 | should be concatenated (in that order), preferably separated by white | |
3441 | space. You can--but shouldn't--omit the mode in these forms when that mode | |
3442 | is C<< < >>. It is always safe to use the two-argument form of C<open> if | |
3443 | the filename argument is a known literal. | |
6170680b | 3444 | |
8f1da26d | 3445 | For three or more arguments if MODE is C<|->, the filename is |
ed53a2bb | 3446 | interpreted as a command to which output is to be piped, and if MODE |
8f1da26d | 3447 | is C<-|>, the filename is interpreted as a command that pipes |
3b10bc60 | 3448 | output to us. In the two-argument (and one-argument) form, one should |
8f1da26d | 3449 | replace dash (C<->) with the command. |
ed53a2bb JH |
3450 | See L<perlipc/"Using open() for IPC"> for more examples of this. |
3451 | (You are not allowed to C<open> to a command that pipes both in I<and> | |
3452 | out, but see L<IPC::Open2>, L<IPC::Open3>, and | |
96090e4f LB |
3453 | L<perlipc/"Bidirectional Communication with Another Process"> for |
3454 | alternatives.) | |
ed53a2bb | 3455 | |
3b10bc60 | 3456 | In the form of pipe opens taking three or more arguments, if LIST is specified |
ed53a2bb JH |
3457 | (extra arguments after the command name) then LIST becomes arguments |
3458 | to the command invoked if the platform supports it. The meaning of | |
3459 | C<open> with more than three arguments for non-pipe modes is not yet | |
3b10bc60 | 3460 | defined, but experimental "layers" may give extra LIST arguments |
ed53a2bb | 3461 | meaning. |
6170680b | 3462 | |
8f1da26d TC |
3463 | In the two-argument (and one-argument) form, opening C<< <- >> |
3464 | or C<-> opens STDIN and opening C<< >- >> opens STDOUT. | |
6170680b | 3465 | |
8f1da26d TC |
3466 | You may (and usually should) use the three-argument form of open to specify |
3467 | I/O layers (sometimes referred to as "disciplines") to apply to the handle | |
fae2c0fb | 3468 | that affect how the input and output are processed (see L<open> and |
3b10bc60 | 3469 | L<PerlIO> for more details). For example: |
7207e29d | 3470 | |
3b10bc60 | 3471 | open(my $fh, "<:encoding(UTF-8)", "filename") |
3472 | || die "can't open UTF-8 encoded filename: $!"; | |
9124316e | 3473 | |
8f1da26d | 3474 | opens the UTF8-encoded file containing Unicode characters; |
6d5e88a0 | 3475 | see L<perluniintro>. Note that if layers are specified in the |
3b10bc60 | 3476 | three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>; |
6d5e88a0 | 3477 | usually set by the B<open> pragma or the switch B<-CioD>) are ignored. |
ed53a2bb | 3478 | |
80d38338 | 3479 | Open returns nonzero on success, the undefined value otherwise. If |
ed53a2bb JH |
3480 | the C<open> involved a pipe, the return value happens to be the pid of |
3481 | the subprocess. | |
cb1a09d0 | 3482 | |
ed53a2bb JH |
3483 | If you're running Perl on a system that distinguishes between text |
3484 | files and binary files, then you should check out L</binmode> for tips | |
3485 | for dealing with this. The key distinction between systems that need | |
3486 | C<binmode> and those that don't is their text file formats. Systems | |
80d38338 TC |
3487 | like Unix, Mac OS, and Plan 9, that end lines with a single |
3488 | character and encode that character in C as C<"\n"> do not | |
ed53a2bb | 3489 | need C<binmode>. The rest need it. |
cb1a09d0 | 3490 | |
80d38338 TC |
3491 | When opening a file, it's seldom a good idea to continue |
3492 | if the request failed, so C<open> is frequently used with | |
19799a22 | 3493 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, |
80d38338 TC |
3494 | where you want to format a suitable error message (but there are |
3495 | modules that can help with that problem)) always check | |
3496 | the return value from opening a file. | |
fb73857a | 3497 | |
8f1da26d | 3498 | As a special case the three-argument form with a read/write mode and the third |
ed53a2bb | 3499 | argument being C<undef>: |
b76cc8ba | 3500 | |
460b70c2 | 3501 | open(my $tmp, "+>", undef) or die ... |
b76cc8ba | 3502 | |
8f1da26d | 3503 | opens a filehandle to an anonymous temporary file. Also using C<< +< >> |
f253e835 JH |
3504 | works for symmetry, but you really should consider writing something |
3505 | to the temporary file first. You will need to seek() to do the | |
3506 | reading. | |
b76cc8ba | 3507 | |
3b10bc60 | 3508 | Since v5.8.0, Perl has built using PerlIO by default. Unless you've |
8f1da26d TC |
3509 | changed this (such as building Perl with C<Configure -Uuseperlio>), you can |
3510 | open filehandles directly to Perl scalars via: | |
ba964c95 | 3511 | |
8f1da26d | 3512 | open($fh, ">", \$variable) || .. |
b996200f | 3513 | |
3b10bc60 | 3514 | To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first: |
b996200f SB |
3515 | |
3516 | close STDOUT; | |
8f1da26d TC |
3517 | open(STDOUT, ">", \$variable) |
3518 | or die "Can't open STDOUT: $!"; | |
ba964c95 | 3519 | |
3b10bc60 | 3520 | General examples: |
a0d0e21e LW |
3521 | |
3522 | $ARTICLE = 100; | |
8f1da26d | 3523 | open(ARTICLE) or die "Can't find article $ARTICLE: $!\n"; |
a0d0e21e LW |
3524 | while (<ARTICLE>) {... |
3525 | ||
8f1da26d | 3526 | open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved) |
fb73857a | 3527 | # if the open fails, output is discarded |
a0d0e21e | 3528 | |
8f1da26d | 3529 | open(my $dbase, "+<", "dbase.mine") # open for update |
a9a5a0dc | 3530 | or die "Can't open 'dbase.mine' for update: $!"; |
cb1a09d0 | 3531 | |
8f1da26d | 3532 | open(my $dbase, "+<dbase.mine") # ditto |
a9a5a0dc | 3533 | or die "Can't open 'dbase.mine' for update: $!"; |
6170680b | 3534 | |
8f1da26d | 3535 | open(ARTICLE, "-|", "caesar <$article") # decrypt article |
a9a5a0dc | 3536 | or die "Can't start caesar: $!"; |
a0d0e21e | 3537 | |
5ed4f2ec | 3538 | open(ARTICLE, "caesar <$article |") # ditto |
a9a5a0dc | 3539 | or die "Can't start caesar: $!"; |
6170680b | 3540 | |
5ed4f2ec | 3541 | open(EXTRACT, "|sort >Tmp$$") # $$ is our process id |
a9a5a0dc | 3542 | or die "Can't start sort: $!"; |
a0d0e21e | 3543 | |
3b10bc60 | 3544 | # in-memory files |
8f1da26d | 3545 | open(MEMORY, ">", \$var) |
a9a5a0dc | 3546 | or die "Can't open memory file: $!"; |
80d38338 | 3547 | print MEMORY "foo!\n"; # output will appear in $var |
ba964c95 | 3548 | |
a0d0e21e LW |
3549 | # process argument list of files along with any includes |
3550 | ||
3551 | foreach $file (@ARGV) { | |
8f1da26d | 3552 | process($file, "fh00"); |
a0d0e21e LW |
3553 | } |
3554 | ||
3555 | sub process { | |
a9a5a0dc VP |
3556 | my($filename, $input) = @_; |
3557 | $input++; # this is a string increment | |
8f1da26d | 3558 | unless (open($input, "<", $filename)) { |
a9a5a0dc VP |
3559 | print STDERR "Can't open $filename: $!\n"; |
3560 | return; | |
3561 | } | |
5ed4f2ec | 3562 | |
a9a5a0dc VP |
3563 | local $_; |
3564 | while (<$input>) { # note use of indirection | |
3565 | if (/^#include "(.*)"/) { | |
3566 | process($1, $input); | |
3567 | next; | |
3568 | } | |
3569 | #... # whatever | |
5ed4f2ec | 3570 | } |
a0d0e21e LW |
3571 | } |
3572 | ||
ae4c5402 | 3573 | See L<perliol> for detailed info on PerlIO. |
2ce64696 | 3574 | |
a0d0e21e | 3575 | You may also, in the Bourne shell tradition, specify an EXPR beginning |
8f1da26d | 3576 | with C<< >& >>, in which case the rest of the string is interpreted |
00cafafa | 3577 | as the name of a filehandle (or file descriptor, if numeric) to be |
f4084e39 | 3578 | duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>, |
00cafafa JH |
3579 | C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. |
3580 | The mode you specify should match the mode of the original filehandle. | |
3581 | (Duping a filehandle does not take into account any existing contents | |
8f1da26d TC |
3582 | of IO buffers.) If you use the three-argument form, then you can pass either a |
3583 | number, the name of a filehandle, or the normal "reference to a glob". | |
6170680b | 3584 | |
eae1b76b SB |
3585 | Here is a script that saves, redirects, and restores C<STDOUT> and |
3586 | C<STDERR> using various methods: | |
a0d0e21e LW |
3587 | |
3588 | #!/usr/bin/perl | |
8f1da26d TC |
3589 | open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!"; |
3590 | open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!"; | |
818c4caa | 3591 | |
8f1da26d TC |
3592 | open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!"; |
3593 | open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!"; | |
a0d0e21e | 3594 | |
5ed4f2ec | 3595 | select STDERR; $| = 1; # make unbuffered |
3596 | select STDOUT; $| = 1; # make unbuffered | |
a0d0e21e | 3597 | |
5ed4f2ec | 3598 | print STDOUT "stdout 1\n"; # this works for |
3599 | print STDERR "stderr 1\n"; # subprocesses too | |
a0d0e21e | 3600 | |
8f1da26d TC |
3601 | open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!"; |
3602 | open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!"; | |
a0d0e21e LW |
3603 | |
3604 | print STDOUT "stdout 2\n"; | |
3605 | print STDERR "stderr 2\n"; | |
3606 | ||
ef8b303f JH |
3607 | If you specify C<< '<&=X' >>, where C<X> is a file descriptor number |
3608 | or a filehandle, then Perl will do an equivalent of C's C<fdopen> of | |
f4084e39 | 3609 | that file descriptor (and not call C<dup(2)>); this is more |
ef8b303f | 3610 | parsimonious of file descriptors. For example: |
a0d0e21e | 3611 | |
00cafafa | 3612 | # open for input, reusing the fileno of $fd |
a0d0e21e | 3613 | open(FILEHANDLE, "<&=$fd") |
df632fdf | 3614 | |
b76cc8ba | 3615 | or |
df632fdf | 3616 | |
b76cc8ba | 3617 | open(FILEHANDLE, "<&=", $fd) |
a0d0e21e | 3618 | |
00cafafa JH |
3619 | or |
3620 | ||
3621 | # open for append, using the fileno of OLDFH | |
3622 | open(FH, ">>&=", OLDFH) | |
3623 | ||
3624 | or | |
3625 | ||
3626 | open(FH, ">>&=OLDFH") | |
3627 | ||
ef8b303f JH |
3628 | Being parsimonious on filehandles is also useful (besides being |
3629 | parsimonious) for example when something is dependent on file | |
3630 | descriptors, like for example locking using flock(). If you do just | |
8f1da26d TC |
3631 | C<< open(A, ">>&B") >>, the filehandle A will not have the same file |
3632 | descriptor as B, and therefore flock(A) will not flock(B) nor vice | |
3633 | versa. But with C<< open(A, ">>&=B") >>, the filehandles will share | |
3634 | the same underlying system file descriptor. | |
3635 | ||
3636 | Note that under Perls older than 5.8.0, Perl uses the standard C library's' | |
3637 | fdopen() to implement the C<=> functionality. On many Unix systems, | |
3638 | fdopen() fails when file descriptors exceed a certain value, typically 255. | |
3639 | For Perls 5.8.0 and later, PerlIO is (most often) the default. | |
3640 | ||
3641 | You can see whether your Perl was built with PerlIO by running C<perl -V> | |
3642 | and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you | |
3643 | have PerlIO; otherwise you don't. | |
3644 | ||
3645 | If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|> | |
3646 | with the one- or two-argument forms of C<open>), | |
3647 | an implicit C<fork> is done, so C<open> returns twice: in the parent | |
3648 | process it returns the pid | |
3649 | of the child process, and in the child process it returns (a defined) C<0>. | |
3650 | Use C<defined($pid)> or C<//> to determine whether the open was successful. | |
3651 | ||
3652 | For example, use either | |
3653 | ||
5f64ea7a | 3654 | $child_pid = open(FROM_KID, "-|") // die "can't fork: $!"; |
8f1da26d TC |
3655 | |
3656 | or | |
3657 | $child_pid = open(TO_KID, "|-") // die "can't fork: $!"; | |
3658 | ||
3659 | followed by | |
3660 | ||
3661 | if ($child_pid) { | |
3662 | # am the parent: | |
3663 | # either write TO_KID or else read FROM_KID | |
3664 | ... | |
3665 | wait $child_pid; | |
3666 | } else { | |
3667 | # am the child; use STDIN/STDOUT normally | |
3668 | ... | |
3669 | exit; | |
3670 | } | |
3671 | ||
3b10bc60 | 3672 | The filehandle behaves normally for the parent, but I/O to that |
a0d0e21e | 3673 | filehandle is piped from/to the STDOUT/STDIN of the child process. |
3b10bc60 | 3674 | In the child process, the filehandle isn't opened--I/O happens from/to |
3675 | the new STDOUT/STDIN. Typically this is used like the normal | |
a0d0e21e | 3676 | piped open when you want to exercise more control over just how the |
3b10bc60 | 3677 | pipe command gets executed, such as when running setuid and |
3678 | you don't want to have to scan shell commands for metacharacters. | |
3679 | ||
5b867647 | 3680 | The following blocks are more or less equivalent: |
a0d0e21e LW |
3681 | |
3682 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
8f1da26d TC |
3683 | open(FOO, "|-", "tr '[a-z]' '[A-Z]'"); |
3684 | open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; | |
3685 | open(FOO, "|-", "tr", '[a-z]', '[A-Z]'); | |
a0d0e21e LW |
3686 | |
3687 | open(FOO, "cat -n '$file'|"); | |
8f1da26d TC |
3688 | open(FOO, "-|", "cat -n '$file'"); |
3689 | open(FOO, "-|") || exec "cat", "-n", $file; | |
3690 | open(FOO, "-|", "cat", "-n", $file); | |
b76cc8ba | 3691 | |
8f1da26d | 3692 | The last two examples in each block show the pipe as "list form", which is |
64da03b2 | 3693 | not yet supported on all platforms. A good rule of thumb is that if |
8f1da26d TC |
3694 | your platform has a real C<fork()> (in other words, if your platform is |
3695 | Unix, including Linux and MacOS X), you can use the list form. You would | |
3696 | want to use the list form of the pipe so you can pass literal arguments | |
3697 | to the command without risk of the shell interpreting any shell metacharacters | |
3698 | in them. However, this also bars you from opening pipes to commands | |
3699 | that intentionally contain shell metacharacters, such as: | |
3700 | ||
3701 | open(FOO, "|cat -n | expand -4 | lpr") | |
3702 | // die "Can't open pipeline to lpr: $!"; | |
a0d0e21e | 3703 | |
4633a7c4 LW |
3704 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
3705 | ||
0f897271 GS |
3706 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
3707 | output before any operation that may do a fork, but this may not be | |
3708 | supported on some platforms (see L<perlport>). To be safe, you may need | |
3709 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
3710 | of C<IO::Handle> on any open handles. | |
3711 | ||
ed53a2bb JH |
3712 | On systems that support a close-on-exec flag on files, the flag will |
3713 | be set for the newly opened file descriptor as determined by the value | |
8f1da26d | 3714 | of C<$^F>. See L<perlvar/$^F>. |
a0d0e21e | 3715 | |
0dccf244 | 3716 | Closing any piped filehandle causes the parent process to wait for the |
8f1da26d | 3717 | child to finish, then returns the status value in C<$?> and |
e5218da5 | 3718 | C<${^CHILD_ERROR_NATIVE}>. |
0dccf244 | 3719 | |
8f1da26d TC |
3720 | The filename passed to the one- and two-argument forms of open() will |
3721 | have leading and trailing whitespace deleted and normal | |
ed53a2bb | 3722 | redirection characters honored. This property, known as "magic open", |
5a964f20 | 3723 | can often be used to good effect. A user could specify a filename of |
7660c0ab | 3724 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
3725 | |
3726 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
3727 | open(FH, $filename) or die "Can't open $filename: $!"; | |
3728 | ||
8f1da26d | 3729 | Use the three-argument form to open a file with arbitrary weird characters in it, |
6170680b | 3730 | |
8f1da26d TC |
3731 | open(FOO, "<", $file) |
3732 | || die "can't open < $file: $!"; | |
6170680b IZ |
3733 | |
3734 | otherwise it's necessary to protect any leading and trailing whitespace: | |
5a964f20 TC |
3735 | |
3736 | $file =~ s#^(\s)#./$1#; | |
8f1da26d TC |
3737 | open(FOO, "< $file\0") |
3738 | || die "open failed: $!"; | |
5a964f20 | 3739 | |
a31a806a | 3740 | (this may not work on some bizarre filesystems). One should |
8f1da26d | 3741 | conscientiously choose between the I<magic> and I<three-argument> form |
6170680b IZ |
3742 | of open(): |
3743 | ||
8f1da26d | 3744 | open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!"; |
6170680b IZ |
3745 | |
3746 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
80d38338 | 3747 | but will not work on a filename that happens to have a trailing space, while |
6170680b | 3748 | |
8f1da26d TC |
3749 | open(IN, "<", $ARGV[0]) |
3750 | || die "can't open < $ARGV[0]: $!"; | |
6170680b IZ |
3751 | |
3752 | will have exactly the opposite restrictions. | |
3753 | ||
01aa884e | 3754 | If you want a "real" C C<open> (see L<open(2)> on your system), then you |
8f1da26d TC |
3755 | should use the C<sysopen> function, which involves no such magic (but may |
3756 | use subtly different filemodes than Perl open(), which is mapped to C | |
3757 | fopen()). This is another way to protect your filenames from | |
3758 | interpretation. For example: | |
5a964f20 TC |
3759 | |
3760 | use IO::Handle; | |
3761 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
a9a5a0dc | 3762 | or die "sysopen $path: $!"; |
5a964f20 | 3763 | $oldfh = select(HANDLE); $| = 1; select($oldfh); |
38762f02 | 3764 | print HANDLE "stuff $$\n"; |
5a964f20 TC |
3765 | seek(HANDLE, 0, 0); |
3766 | print "File contains: ", <HANDLE>; | |
3767 | ||
7660c0ab A |
3768 | Using the constructor from the C<IO::Handle> package (or one of its |
3769 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
8f1da26d TC |
3770 | filehandles that have the scope of the variables used to hold them, then |
3771 | automatically (but silently) close once their reference counts become | |
3772 | zero, typically at scope exit: | |
c07a80fd | 3773 | |
5f05dabc | 3774 | use IO::File; |
5a964f20 | 3775 | #... |
c07a80fd | 3776 | sub read_myfile_munged { |
a9a5a0dc | 3777 | my $ALL = shift; |
8f1da26d | 3778 | # or just leave it undef to autoviv |
a9a5a0dc | 3779 | my $handle = IO::File->new; |
8f1da26d | 3780 | open($handle, "<", "myfile") or die "myfile: $!"; |
a9a5a0dc VP |
3781 | $first = <$handle> |
3782 | or return (); # Automatically closed here. | |
8f1da26d TC |
3783 | mung($first) or die "mung failed"; # Or here. |
3784 | return (first, <$handle>) if $ALL; # Or here. | |
3785 | return $first; # Or here. | |
c07a80fd | 3786 | } |
3787 | ||
8f1da26d TC |
3788 | B<WARNING:> The previous example has a bug because the automatic |
3789 | close that happens when the refcount on C<handle> does not | |
3790 | properly detect and report failures. I<Always> close the handle | |
3791 | yourself and inspect the return value. | |
3792 | ||
3793 | close($handle) | |
3794 | || warn "close failed: $!"; | |
3795 | ||
b687b08b | 3796 | See L</seek> for some details about mixing reading and writing. |
a0d0e21e | 3797 | |
ea9eb35a BJ |
3798 | Portability issues: L<perlport/open>. |
3799 | ||
a0d0e21e | 3800 | =item opendir DIRHANDLE,EXPR |
d74e8afc | 3801 | X<opendir> |
a0d0e21e | 3802 | |
19799a22 GS |
3803 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
3804 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
a28cd5c9 NT |
3805 | DIRHANDLE may be an expression whose value can be used as an indirect |
3806 | dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined | |
3807 | scalar variable (or array or hash element), the variable is assigned a | |
8f1da26d | 3808 | reference to a new anonymous dirhandle; that is, it's autovivified. |
a0d0e21e LW |
3809 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
3810 | ||
bea6df1c | 3811 | See the example at C<readdir>. |
b0169937 | 3812 | |
a0d0e21e | 3813 | =item ord EXPR |
d74e8afc | 3814 | X<ord> X<encoding> |
a0d0e21e | 3815 | |
54310121 | 3816 | =item ord |
bbce6d69 | 3817 | |
121910a4 | 3818 | Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, |
8f1da26d TC |
3819 | or Unicode) value of the first character of EXPR. |
3820 | If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>. | |
3821 | (Note I<character>, not byte.) | |
121910a4 JH |
3822 | |
3823 | For the reverse, see L</chr>. | |
2575c402 | 3824 | See L<perlunicode> for more about Unicode. |
a0d0e21e | 3825 | |
77ca0c92 | 3826 | =item our EXPR |
d74e8afc | 3827 | X<our> X<global> |
77ca0c92 | 3828 | |
36fb85f3 | 3829 | =item our TYPE EXPR |
307ea6df | 3830 | |
1d2de774 | 3831 | =item our EXPR : ATTRS |
9969eac4 | 3832 | |
1d2de774 | 3833 | =item our TYPE EXPR : ATTRS |
307ea6df | 3834 | |
85d8b7d5 | 3835 | C<our> associates a simple name with a package variable in the current |
65c680eb MS |
3836 | package for use within the current scope. When C<use strict 'vars'> is in |
3837 | effect, C<our> lets you use declared global variables without qualifying | |
3838 | them with package names, within the lexical scope of the C<our> declaration. | |
bea6df1c | 3839 | In this way C<our> differs from C<use vars>, which is package-scoped. |
65c680eb | 3840 | |
8f1da26d TC |
3841 | Unlike C<my> or C<state>, which allocates storage for a variable and |
3842 | associates a simple name with that storage for use within the current | |
3843 | scope, C<our> associates a simple name with a package (read: global) | |
3844 | variable in the current package, for use within the current lexical scope. | |
3845 | In other words, C<our> has the same scoping rules as C<my> or C<state>, but | |
3846 | does not necessarily create a variable. | |
65c680eb MS |
3847 | |
3848 | If more than one value is listed, the list must be placed | |
3849 | in parentheses. | |
85d8b7d5 MS |
3850 | |
3851 | our $foo; | |
3852 | our($bar, $baz); | |
77ca0c92 | 3853 | |
f472eb5c GS |
3854 | An C<our> declaration declares a global variable that will be visible |
3855 | across its entire lexical scope, even across package boundaries. The | |
3856 | package in which the variable is entered is determined at the point | |
3857 | of the declaration, not at the point of use. This means the following | |
3858 | behavior holds: | |
3859 | ||
3860 | package Foo; | |
5ed4f2ec | 3861 | our $bar; # declares $Foo::bar for rest of lexical scope |
f472eb5c GS |
3862 | $bar = 20; |
3863 | ||
3864 | package Bar; | |
5ed4f2ec | 3865 | print $bar; # prints 20, as it refers to $Foo::bar |
f472eb5c | 3866 | |
65c680eb MS |
3867 | Multiple C<our> declarations with the same name in the same lexical |
3868 | scope are allowed if they are in different packages. If they happen | |
3869 | to be in the same package, Perl will emit warnings if you have asked | |
3870 | for them, just like multiple C<my> declarations. Unlike a second | |
3871 | C<my> declaration, which will bind the name to a fresh variable, a | |
3872 | second C<our> declaration in the same package, in the same scope, is | |
3873 | merely redundant. | |
f472eb5c GS |
3874 | |
3875 | use warnings; | |
3876 | package Foo; | |
5ed4f2ec | 3877 | our $bar; # declares $Foo::bar for rest of lexical scope |
f472eb5c GS |
3878 | $bar = 20; |
3879 | ||
3880 | package Bar; | |
5ed4f2ec | 3881 | our $bar = 30; # declares $Bar::bar for rest of lexical scope |
3882 | print $bar; # prints 30 | |
f472eb5c | 3883 | |
5ed4f2ec | 3884 | our $bar; # emits warning but has no other effect |
3885 | print $bar; # still prints 30 | |
f472eb5c | 3886 | |
9969eac4 | 3887 | An C<our> declaration may also have a list of attributes associated |
307ea6df JH |
3888 | with it. |
3889 | ||
1d2de774 JH |
3890 | The exact semantics and interface of TYPE and ATTRS are still |
3891 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
307ea6df JH |
3892 | and attributes are handled using the C<attributes> pragma, or starting |
3893 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3894 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3895 | L<attributes>, and L<Attribute::Handlers>. | |
3896 | ||
a0d0e21e | 3897 | =item pack TEMPLATE,LIST |
d74e8afc | 3898 | X<pack> |
a0d0e21e | 3899 | |
2b6c5635 GS |
3900 | Takes a LIST of values and converts it into a string using the rules |
3901 | given by the TEMPLATE. The resulting string is the concatenation of | |
3902 | the converted values. Typically, each converted value looks | |
3903 | like its machine-level representation. For example, on 32-bit machines | |
3980dc9c KW |
3904 | an integer may be represented by a sequence of 4 bytes, which will in |
3905 | Perl be presented as a string that's 4 characters long. | |
3906 | ||
3907 | See L<perlpacktut> for an introduction to this function. | |
e1b711da | 3908 | |
18529408 IZ |
3909 | The TEMPLATE is a sequence of characters that give the order and type |
3910 | of values, as follows: | |
a0d0e21e | 3911 | |
5ed4f2ec | 3912 | a A string with arbitrary binary data, will be null padded. |
3913 | A A text (ASCII) string, will be space padded. | |
3b10bc60 | 3914 | Z A null-terminated (ASCIZ) string, will be null padded. |
5a929a98 | 3915 | |
4d0444a3 FC |
3916 | b A bit string (ascending bit order inside each byte, |
3917 | like vec()). | |
5ed4f2ec | 3918 | B A bit string (descending bit order inside each byte). |
3919 | h A hex string (low nybble first). | |
3920 | H A hex string (high nybble first). | |
a0d0e21e | 3921 | |
5ed4f2ec | 3922 | c A signed char (8-bit) value. |
3923 | C An unsigned char (octet) value. | |
3b10bc60 | 3924 | W An unsigned char value (can be greater than 255). |
96e4d5b1 | 3925 | |
5ed4f2ec | 3926 | s A signed short (16-bit) value. |
3927 | S An unsigned short value. | |
96e4d5b1 | 3928 | |
5ed4f2ec | 3929 | l A signed long (32-bit) value. |
3930 | L An unsigned long value. | |
a0d0e21e | 3931 | |
5ed4f2ec | 3932 | q A signed quad (64-bit) value. |
3933 | Q An unsigned quad value. | |
4d0444a3 FC |
3934 | (Quads are available only if your system supports 64-bit |
3935 | integer values _and_ if Perl has been compiled to support | |
3936 | those. Raises an exception otherwise.) | |
dae0da7a | 3937 | |
5ed4f2ec | 3938 | i A signed integer value. |
3939 | I A unsigned integer value. | |
4d0444a3 FC |
3940 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
3941 | size depends on what a local C compiler calls 'int'.) | |
2b191d53 | 3942 | |
5ed4f2ec | 3943 | n An unsigned short (16-bit) in "network" (big-endian) order. |
3944 | N An unsigned long (32-bit) in "network" (big-endian) order. | |
3945 | v An unsigned short (16-bit) in "VAX" (little-endian) order. | |
3946 | V An unsigned long (32-bit) in "VAX" (little-endian) order. | |
1109a392 | 3947 | |
4d0444a3 FC |
3948 | j A Perl internal signed integer value (IV). |
3949 | J A Perl internal unsigned integer value (UV). | |
92d41999 | 3950 | |
3b10bc60 | 3951 | f A single-precision float in native format. |
3952 | d A double-precision float in native format. | |
a0d0e21e | 3953 | |
3b10bc60 | 3954 | F A Perl internal floating-point value (NV) in native format |
3955 | D A float of long-double precision in native format. | |
4d0444a3 FC |
3956 | (Long doubles are available only if your system supports |
3957 | long double values _and_ if Perl has been compiled to | |
3958 | support those. Raises an exception otherwise.) | |
92d41999 | 3959 | |
5ed4f2ec | 3960 | p A pointer to a null-terminated string. |
3961 | P A pointer to a structure (fixed-length string). | |
a0d0e21e | 3962 | |
5ed4f2ec | 3963 | u A uuencoded string. |
4d0444a3 FC |
3964 | U A Unicode character number. Encodes to a character in char- |
3965 | acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in | |
3966 | byte mode. | |
a0d0e21e | 3967 | |
4d0444a3 FC |
3968 | w A BER compressed integer (not an ASN.1 BER, see perlpacktut |
3969 | for details). Its bytes represent an unsigned integer in | |
3970 | base 128, most significant digit first, with as few digits | |
3971 | as possible. Bit eight (the high bit) is set on each byte | |
3972 | except the last. | |
def98dd4 | 3973 | |
3b10bc60 | 3974 | x A null byte (a.k.a ASCII NUL, "\000", chr(0)) |
5ed4f2ec | 3975 | X Back up a byte. |
3b10bc60 | 3976 | @ Null-fill or truncate to absolute position, counted from the |
3977 | start of the innermost ()-group. | |
4d0444a3 FC |
3978 | . Null-fill or truncate to absolute position specified by |
3979 | the value. | |
5ed4f2ec | 3980 | ( Start of a ()-group. |
a0d0e21e | 3981 | |
3b10bc60 | 3982 | One or more modifiers below may optionally follow certain letters in the |
3983 | TEMPLATE (the second column lists letters for which the modifier is valid): | |
1109a392 MHM |
3984 | |
3985 | ! sSlLiI Forces native (short, long, int) sizes instead | |
3986 | of fixed (16-/32-bit) sizes. | |
3987 | ||
3988 | xX Make x and X act as alignment commands. | |
3989 | ||
3990 | nNvV Treat integers as signed instead of unsigned. | |
3991 | ||
28be1210 TH |
3992 | @. Specify position as byte offset in the internal |
3993 | representation of the packed string. Efficient but | |
3994 | dangerous. | |
3995 | ||
1109a392 MHM |
3996 | > sSiIlLqQ Force big-endian byte-order on the type. |
3997 | jJfFdDpP (The "big end" touches the construct.) | |
3998 | ||
3999 | < sSiIlLqQ Force little-endian byte-order on the type. | |
4000 | jJfFdDpP (The "little end" touches the construct.) | |
4001 | ||
3b10bc60 | 4002 | The C<< > >> and C<< < >> modifiers can also be used on C<()> groups |
4003 | to force a particular byte-order on all components in that group, | |
4004 | including all its subgroups. | |
66c611c5 | 4005 | |
5a929a98 VU |
4006 | The following rules apply: |
4007 | ||
3b10bc60 | 4008 | =over |
5a929a98 VU |
4009 | |
4010 | =item * | |
4011 | ||
3b10bc60 | 4012 | Each letter may optionally be followed by a number indicating the repeat |
4013 | count. A numeric repeat count may optionally be enclosed in brackets, as | |
4014 | in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from | |
4015 | the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>, | |
4016 | C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means | |
4017 | something else, dscribed below. Supplying a C<*> for the repeat count | |
4018 | instead of a number means to use however many items are left, except for: | |
4019 | ||
4020 | =over | |
4021 | ||
4022 | =item * | |
4023 | ||
4024 | C<@>, C<x>, and C<X>, where it is equivalent to C<0>. | |
4025 | ||
4026 | =item * | |
4027 | ||
4028 | <.>, where it means relative to the start of the string. | |
4029 | ||
4030 | =item * | |
4031 | ||
4032 | C<u>, where it is equivalent to 1 (or 45, which here is equivalent). | |
4033 | ||
4034 | =back | |
4035 | ||
4036 | One can replace a numeric repeat count with a template letter enclosed in | |
4037 | brackets to use the packed byte length of the bracketed template for the | |
4038 | repeat count. | |
4039 | ||
4040 | For example, the template C<x[L]> skips as many bytes as in a packed long, | |
4041 | and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when | |
4042 | variable-expanded) unpacks. If the template in brackets contains alignment | |
4043 | commands (such as C<x![d]>), its packed length is calculated as if the | |
4044 | start of the template had the maximal possible alignment. | |
4045 | ||
4046 | When used with C<Z>, a C<*> as the repeat count is guaranteed to add a | |
4047 | trailing null byte, so the resulting string is always one byte longer than | |
4048 | the byte length of the item itself. | |
2b6c5635 | 4049 | |
28be1210 | 4050 | When used with C<@>, the repeat count represents an offset from the start |
3b10bc60 | 4051 | of the innermost C<()> group. |
4052 | ||
4053 | When used with C<.>, the repeat count determines the starting position to | |
4054 | calculate the value offset as follows: | |
4055 | ||
4056 | =over | |
4057 | ||
4058 | =item * | |
4059 | ||
4060 | If the repeat count is C<0>, it's relative to the current position. | |
28be1210 | 4061 | |
3b10bc60 | 4062 | =item * |
4063 | ||
4064 | If the repeat count is C<*>, the offset is relative to the start of the | |
4065 | packed string. | |
4066 | ||
4067 | =item * | |
4068 | ||
4069 | And if it's an integer I<n>, the offset is relative to the start of the | |
8f1da26d | 4070 | I<n>th innermost C<( )> group, or to the start of the string if I<n> is |
3b10bc60 | 4071 | bigger then the group level. |
4072 | ||
4073 | =back | |
28be1210 | 4074 | |
951ba7fe | 4075 | The repeat count for C<u> is interpreted as the maximal number of bytes |
f337b084 TH |
4076 | to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat |
4077 | count should not be more than 65. | |
5a929a98 VU |
4078 | |
4079 | =item * | |
4080 | ||
951ba7fe | 4081 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
3b10bc60 | 4082 | string of length count, padding with nulls or spaces as needed. When |
18bdf90a | 4083 | unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything |
8f1da26d | 4084 | after the first null, and C<a> returns data with no stripping at all. |
2b6c5635 | 4085 | |
3b10bc60 | 4086 | If the value to pack is too long, the result is truncated. If it's too |
4087 | long and an explicit count is provided, C<Z> packs only C<$count-1> bytes, | |
4088 | followed by a null byte. Thus C<Z> always packs a trailing null, except | |
8f1da26d | 4089 | when the count is 0. |
5a929a98 VU |
4090 | |
4091 | =item * | |
4092 | ||
3b10bc60 | 4093 | Likewise, the C<b> and C<B> formats pack a string that's that many bits long. |
8f1da26d TC |
4094 | Each such format generates 1 bit of the result. These are typically followed |
4095 | by a repeat count like C<B8> or C<B64>. | |
3b10bc60 | 4096 | |
c73032f5 | 4097 | Each result bit is based on the least-significant bit of the corresponding |
f337b084 | 4098 | input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> |
3b10bc60 | 4099 | and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">. |
c73032f5 | 4100 | |
3b10bc60 | 4101 | Starting from the beginning of the input string, each 8-tuple |
4102 | of characters is converted to 1 character of output. With format C<b>, | |
f337b084 | 4103 | the first character of the 8-tuple determines the least-significant bit of a |
3b10bc60 | 4104 | character; with format C<B>, it determines the most-significant bit of |
f337b084 | 4105 | a character. |
c73032f5 | 4106 | |
3b10bc60 | 4107 | If the length of the input string is not evenly divisible by 8, the |
f337b084 | 4108 | remainder is packed as if the input string were padded by null characters |
3b10bc60 | 4109 | at the end. Similarly during unpacking, "extra" bits are ignored. |
c73032f5 | 4110 | |
3b10bc60 | 4111 | If the input string is longer than needed, remaining characters are ignored. |
4112 | ||
4113 | A C<*> for the repeat count uses all characters of the input field. | |
8f1da26d | 4114 | On unpacking, bits are converted to a string of C<0>s and C<1>s. |
5a929a98 VU |
4115 | |
4116 | =item * | |
4117 | ||
3b10bc60 | 4118 | The C<h> and C<H> formats pack a string that many nybbles (4-bit groups, |
4119 | representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long. | |
5a929a98 | 4120 | |
8f1da26d | 4121 | For each such format, pack() generates 4 bits of result. |
3b10bc60 | 4122 | With non-alphabetical characters, the result is based on the 4 least-significant |
f337b084 TH |
4123 | bits of the input character, i.e., on C<ord($char)%16>. In particular, |
4124 | characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
ce7b6f06 | 4125 | C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result |
c73032f5 | 4126 | is compatible with the usual hexadecimal digits, so that C<"a"> and |
8f1da26d TC |
4127 | C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex |
4128 | characters with this format. | |
c73032f5 | 4129 | |
3b10bc60 | 4130 | Starting from the beginning of the template to pack(), each pair |
4131 | of characters is converted to 1 character of output. With format C<h>, the | |
f337b084 | 4132 | first character of the pair determines the least-significant nybble of the |
3b10bc60 | 4133 | output character; with format C<H>, it determines the most-significant |
c73032f5 IZ |
4134 | nybble. |
4135 | ||
3b10bc60 | 4136 | If the length of the input string is not even, it behaves as if padded by |
4137 | a null character at the end. Similarly, "extra" nybbles are ignored during | |
4138 | unpacking. | |
4139 | ||
4140 | If the input string is longer than needed, extra characters are ignored. | |
c73032f5 | 4141 | |
3b10bc60 | 4142 | A C<*> for the repeat count uses all characters of the input field. For |
4143 | unpack(), nybbles are converted to a string of hexadecimal digits. | |
c73032f5 | 4144 | |
5a929a98 VU |
4145 | =item * |
4146 | ||
3b10bc60 | 4147 | The C<p> format packs a pointer to a null-terminated string. You are |
4148 | responsible for ensuring that the string is not a temporary value, as that | |
4149 | could potentially get deallocated before you got around to using the packed | |
4150 | result. The C<P> format packs a pointer to a structure of the size indicated | |
4151 | by the length. A null pointer is created if the corresponding value for | |
4152 | C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer | |
4153 | unpacks into C<undef>. | |
5a929a98 | 4154 | |
3b10bc60 | 4155 | If your system has a strange pointer size--meaning a pointer is neither as |
4156 | big as an int nor as big as a long--it may not be possible to pack or | |
1109a392 | 4157 | unpack pointers in big- or little-endian byte order. Attempting to do |
3b10bc60 | 4158 | so raises an exception. |
1109a392 | 4159 | |
5a929a98 VU |
4160 | =item * |
4161 | ||
246f24af | 4162 | The C</> template character allows packing and unpacking of a sequence of |
3b10bc60 | 4163 | items where the packed structure contains a packed item count followed by |
4164 | the packed items themselves. This is useful when the structure you're | |
4165 | unpacking has encoded the sizes or repeat counts for some of its fields | |
4166 | within the structure itself as separate fields. | |
4167 | ||
4168 | For C<pack>, you write I<length-item>C</>I<sequence-item>, and the | |
4169 | I<length-item> describes how the length value is packed. Formats likely | |
4170 | to be of most use are integer-packing ones like C<n> for Java strings, | |
4171 | C<w> for ASN.1 or SNMP, and C<N> for Sun XDR. | |
4172 | ||
4173 | For C<pack>, I<sequence-item> may have a repeat count, in which case | |
4174 | the minimum of that and the number of available items is used as the argument | |
4175 | for I<length-item>. If it has no repeat count or uses a '*', the number | |
54f961c9 PD |
4176 | of available items is used. |
4177 | ||
3b10bc60 | 4178 | For C<unpack>, an internal stack of integer arguments unpacked so far is |
54f961c9 PD |
4179 | used. You write C</>I<sequence-item> and the repeat count is obtained by |
4180 | popping off the last element from the stack. The I<sequence-item> must not | |
4181 | have a repeat count. | |
246f24af | 4182 | |
3b10bc60 | 4183 | If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">), |
4184 | the I<length-item> is the string length, not the number of strings. With | |
4185 | an explicit repeat count for pack, the packed string is adjusted to that | |
4186 | length. For example: | |
246f24af | 4187 | |
4d0444a3 FC |
4188 | This code: gives this result: |
4189 | ||
4190 | unpack("W/a", "\004Gurusamy") ("Guru") | |
4191 | unpack("a3/A A*", "007 Bond J ") (" Bond", "J") | |
4192 | unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".") | |
3b10bc60 | 4193 | |
4d0444a3 FC |
4194 | pack("n/a* w/a","hello,","world") "\000\006hello,\005world" |
4195 | pack("a/W2", ord("a") .. ord("z")) "2ab" | |
43192e07 IP |
4196 | |
4197 | The I<length-item> is not returned explicitly from C<unpack>. | |
4198 | ||
3b10bc60 | 4199 | Supplying a count to the I<length-item> format letter is only useful with |
4200 | C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may | |
4201 | introduce C<"\000"> characters, which Perl does not regard as legal in | |
4202 | numeric strings. | |
43192e07 IP |
4203 | |
4204 | =item * | |
4205 | ||
951ba7fe | 4206 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
3b10bc60 | 4207 | followed by a C<!> modifier to specify native shorts or |
4208 | longs. As shown in the example above, a bare C<l> means | |
4209 | exactly 32 bits, although the native C<long> as seen by the local C compiler | |
4210 | may be larger. This is mainly an issue on 64-bit platforms. You can | |
4211 | see whether using C<!> makes any difference this way: | |
4212 | ||
4213 | printf "format s is %d, s! is %d\n", | |
4214 | length pack("s"), length pack("s!"); | |
726ea183 | 4215 | |
3b10bc60 | 4216 | printf "format l is %d, l! is %d\n", |
4217 | length pack("l"), length pack("l!"); | |
ef54e1a4 | 4218 | |
3b10bc60 | 4219 | |
4220 | C<i!> and C<I!> are also allowed, but only for completeness' sake: | |
951ba7fe | 4221 | they are identical to C<i> and C<I>. |
ef54e1a4 | 4222 | |
19799a22 | 4223 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
3b10bc60 | 4224 | longs on the platform where Perl was built are also available from |
4225 | the command line: | |
4226 | ||
4227 | $ perl -V:{short,int,long{,long}}size | |
4228 | shortsize='2'; | |
4229 | intsize='4'; | |
4230 | longsize='4'; | |
4231 | longlongsize='8'; | |
4232 | ||
4233 | or programmatically via the C<Config> module: | |
19799a22 GS |
4234 | |
4235 | use Config; | |
4236 | print $Config{shortsize}, "\n"; | |
4237 | print $Config{intsize}, "\n"; | |
4238 | print $Config{longsize}, "\n"; | |
4239 | print $Config{longlongsize}, "\n"; | |
ef54e1a4 | 4240 | |
3b10bc60 | 4241 | C<$Config{longlongsize}> is undefined on systems without |
4242 | long long support. | |
851646ae | 4243 | |
ef54e1a4 JH |
4244 | =item * |
4245 | ||
3b10bc60 | 4246 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are |
4247 | inherently non-portable between processors and operating systems because | |
4248 | they obey native byteorder and endianness. For example, a 4-byte integer | |
4249 | 0x12345678 (305419896 decimal) would be ordered natively (arranged in and | |
4250 | handled by the CPU registers) into bytes as | |
61eff3bc | 4251 | |
5ed4f2ec | 4252 | 0x12 0x34 0x56 0x78 # big-endian |
4253 | 0x78 0x56 0x34 0x12 # little-endian | |
61eff3bc | 4254 | |
3b10bc60 | 4255 | Basically, Intel and VAX CPUs are little-endian, while everybody else, |
4256 | including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are | |
8f1da26d TC |
4257 | big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) |
4258 | them in little-endian mode, but SGI/Cray uses them in big-endian mode. | |
719a3cf5 | 4259 | |
3b10bc60 | 4260 | The names I<big-endian> and I<little-endian> are comic references to the |
4261 | egg-eating habits of the little-endian Lilliputians and the big-endian | |
4262 | Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>. | |
4263 | This entered computer lingo via the paper "On Holy Wars and a Plea for | |
4264 | Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. | |
61eff3bc | 4265 | |
140cb37e | 4266 | Some systems may have even weirder byte orders such as |
61eff3bc | 4267 | |
5ed4f2ec | 4268 | 0x56 0x78 0x12 0x34 |
4269 | 0x34 0x12 0x78 0x56 | |
61eff3bc | 4270 | |
3b10bc60 | 4271 | You can determine your system endianness with this incantation: |
ef54e1a4 | 4272 | |
3b10bc60 | 4273 | printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); |
ef54e1a4 | 4274 | |
d99ad34e | 4275 | The byteorder on the platform where Perl was built is also available |
726ea183 | 4276 | via L<Config>: |
ef54e1a4 | 4277 | |
5ed4f2ec | 4278 | use Config; |
3b10bc60 | 4279 | print "$Config{byteorder}\n"; |
4280 | ||
4281 | or from the command line: | |
ef54e1a4 | 4282 | |
3b10bc60 | 4283 | $ perl -V:byteorder |
719a3cf5 | 4284 | |
3b10bc60 | 4285 | Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321"> |
4286 | and C<"87654321"> are big-endian. | |
4287 | ||
4288 | For portably packed integers, either use the formats C<n>, C<N>, C<v>, | |
4289 | and C<V> or else use the C<< > >> and C<< < >> modifiers described | |
4290 | immediately below. See also L<perlport>. | |
ef54e1a4 JH |
4291 | |
4292 | =item * | |
4293 | ||
3b10bc60 | 4294 | Starting with Perl 5.9.2, integer and floating-point formats, along with |
4295 | the C<p> and C<P> formats and C<()> groups, may all be followed by the | |
4296 | C<< > >> or C<< < >> endianness modifiers to respectively enforce big- | |
4297 | or little-endian byte-order. These modifiers are especially useful | |
8f1da26d | 4298 | given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers, |
3b10bc60 | 4299 | 64-bit integers, or floating-point values. |
4300 | ||
bea6df1c | 4301 | Here are some concerns to keep in mind when using an endianness modifier: |
3b10bc60 | 4302 | |
4303 | =over | |
4304 | ||
4305 | =item * | |
4306 | ||
4307 | Exchanging signed integers between different platforms works only | |
4308 | when all platforms store them in the same format. Most platforms store | |
4309 | signed integers in two's-complement notation, so usually this is not an issue. | |
1109a392 | 4310 | |
3b10bc60 | 4311 | =item * |
1109a392 | 4312 | |
3b10bc60 | 4313 | The C<< > >> or C<< < >> modifiers can only be used on floating-point |
1109a392 | 4314 | formats on big- or little-endian machines. Otherwise, attempting to |
3b10bc60 | 4315 | use them raises an exception. |
1109a392 | 4316 | |
3b10bc60 | 4317 | =item * |
4318 | ||
4319 | Forcing big- or little-endian byte-order on floating-point values for | |
4320 | data exchange can work only if all platforms use the same | |
4321 | binary representation such as IEEE floating-point. Even if all | |
4322 | platforms are using IEEE, there may still be subtle differences. Being able | |
4323 | to use C<< > >> or C<< < >> on floating-point values can be useful, | |
80d38338 | 4324 | but also dangerous if you don't know exactly what you're doing. |
3b10bc60 | 4325 | It is not a general way to portably store floating-point values. |
4326 | ||
4327 | =item * | |
1109a392 | 4328 | |
3b10bc60 | 4329 | When using C<< > >> or C<< < >> on a C<()> group, this affects |
4330 | all types inside the group that accept byte-order modifiers, | |
4331 | including all subgroups. It is silently ignored for all other | |
66c611c5 MHM |
4332 | types. You are not allowed to override the byte-order within a group |
4333 | that already has a byte-order modifier suffix. | |
4334 | ||
3b10bc60 | 4335 | =back |
4336 | ||
1109a392 MHM |
4337 | =item * |
4338 | ||
3b10bc60 | 4339 | Real numbers (floats and doubles) are in native machine format only. |
4340 | Due to the multiplicity of floating-point formats and the lack of a | |
4341 | standard "network" representation for them, no facility for interchange has been | |
4342 | made. This means that packed floating-point data written on one machine | |
4343 | may not be readable on another, even if both use IEEE floating-point | |
4344 | arithmetic (because the endianness of the memory representation is not part | |
851646ae | 4345 | of the IEEE spec). See also L<perlport>. |
5a929a98 | 4346 | |
3b10bc60 | 4347 | If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >> |
4348 | modifiers to force big- or little-endian byte-order on floating-point values. | |
1109a392 | 4349 | |
3b10bc60 | 4350 | Because Perl uses doubles (or long doubles, if configured) internally for |
4351 | all numeric calculation, converting from double into float and thence | |
4352 | to double again loses precision, so C<unpack("f", pack("f", $foo)>) | |
4353 | will not in general equal $foo. | |
5a929a98 | 4354 | |
851646ae JH |
4355 | =item * |
4356 | ||
3b10bc60 | 4357 | Pack and unpack can operate in two modes: character mode (C<C0> mode) where |
4358 | the packed string is processed per character, and UTF-8 mode (C<U0> mode) | |
f337b084 | 4359 | where the packed string is processed in its UTF-8-encoded Unicode form on |
3b10bc60 | 4360 | a byte-by-byte basis. Character mode is the default unless the format string |
4361 | starts with C<U>. You can always switch mode mid-format with an explicit | |
4362 | C<C0> or C<U0> in the format. This mode remains in effect until the next | |
4363 | mode change, or until the end of the C<()> group it (directly) applies to. | |
036b4402 | 4364 | |
8f1da26d TC |
4365 | Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode |
4366 | bytes is not necessarily obvious. Probably only the first of these | |
4367 | is what you want: | |
4368 | ||
4369 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | | |
4370 | perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)' | |
4371 | 03B1.03C9 | |
4372 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | | |
4373 | perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' | |
4374 | CE.B1.CF.89 | |
4375 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | | |
4376 | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)' | |
4377 | CE.B1.CF.89 | |
4378 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | | |
4379 | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' | |
4380 | C3.8E.C2.B1.C3.8F.C2.89 | |
4381 | ||
4382 | Those examples also illustrate that you should not try to use | |
4383 | C<pack>/C<unpack> as a substitute for the L<Encode> module. | |
4384 | ||
036b4402 GS |
4385 | =item * |
4386 | ||
3b10bc60 | 4387 | You must yourself do any alignment or padding by inserting, for example, |
4388 | enough C<"x">es while packing. There is no way for pack() and unpack() | |
4389 | to know where characters are going to or coming from, so they | |
4390 | handle their output and input as flat sequences of characters. | |
851646ae | 4391 | |
17f4a12d IZ |
4392 | =item * |
4393 | ||
3b10bc60 | 4394 | A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may |
4395 | take a repeat count either as postfix, or for unpack(), also via the C</> | |
4396 | template character. Within each repetition of a group, positioning with | |
4397 | C<@> starts over at 0. Therefore, the result of | |
49704364 | 4398 | |
3b10bc60 | 4399 | pack("@1A((@2A)@3A)", qw[X Y Z]) |
49704364 | 4400 | |
3b10bc60 | 4401 | is the string C<"\0X\0\0YZ">. |
49704364 | 4402 | |
18529408 IZ |
4403 | =item * |
4404 | ||
3b10bc60 | 4405 | C<x> and C<X> accept the C<!> modifier to act as alignment commands: they |
4406 | jump forward or back to the closest position aligned at a multiple of C<count> | |
4407 | characters. For example, to pack() or unpack() a C structure like | |
666f95b9 | 4408 | |
3b10bc60 | 4409 | struct { |
4410 | char c; /* one signed, 8-bit character */ | |
4411 | double d; | |
4412 | char cc[2]; | |
4413 | } | |
4414 | ||
4415 | one may need to use the template C<c x![d] d c[2]>. This assumes that | |
4416 | doubles must be aligned to the size of double. | |
4417 | ||
4418 | For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1; | |
4419 | both are no-ops. | |
666f95b9 | 4420 | |
62f95557 IZ |
4421 | =item * |
4422 | ||
3b10bc60 | 4423 | C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to |
4424 | represent signed 16-/32-bit integers in big-/little-endian order. | |
4425 | This is portable only when all platforms sharing packed data use the | |
4426 | same binary representation for signed integers; for example, when all | |
4427 | platforms use two's-complement representation. | |
068bd2e7 MHM |
4428 | |
4429 | =item * | |
4430 | ||
3b10bc60 | 4431 | Comments can be embedded in a TEMPLATE using C<#> through the end of line. |
4432 | White space can separate pack codes from each other, but modifiers and | |
4433 | repeat counts must follow immediately. Breaking complex templates into | |
4434 | individual line-by-line components, suitably annotated, can do as much to | |
4435 | improve legibility and maintainability of pack/unpack formats as C</x> can | |
4436 | for complicated pattern matches. | |
17f4a12d | 4437 | |
2b6c5635 GS |
4438 | =item * |
4439 | ||
bea6df1c | 4440 | If TEMPLATE requires more arguments than pack() is given, pack() |
cf264981 | 4441 | assumes additional C<""> arguments. If TEMPLATE requires fewer arguments |
3b10bc60 | 4442 | than given, extra arguments are ignored. |
2b6c5635 | 4443 | |
5a929a98 | 4444 | =back |
a0d0e21e LW |
4445 | |
4446 | Examples: | |
4447 | ||
f337b084 | 4448 | $foo = pack("WWWW",65,66,67,68); |
a0d0e21e | 4449 | # foo eq "ABCD" |
f337b084 | 4450 | $foo = pack("W4",65,66,67,68); |
a0d0e21e | 4451 | # same thing |
f337b084 TH |
4452 | $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); |
4453 | # same thing with Unicode circled letters. | |
a0ed51b3 | 4454 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
4d0444a3 FC |
4455 | # same thing with Unicode circled letters. You don't get the |
4456 | # UTF-8 bytes because the U at the start of the format caused | |
4457 | # a switch to U0-mode, so the UTF-8 bytes get joined into | |
4458 | # characters | |
f337b084 TH |
4459 | $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); |
4460 | # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" | |
4d0444a3 FC |
4461 | # This is the UTF-8 encoding of the string in the |
4462 | # previous example | |
a0d0e21e LW |
4463 | |
4464 | $foo = pack("ccxxcc",65,66,67,68); | |
4465 | # foo eq "AB\0\0CD" | |
4466 | ||
3b10bc60 | 4467 | # NOTE: The examples above featuring "W" and "c" are true |
9ccd05c0 | 4468 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 |
3b10bc60 | 4469 | # and UTF-8. On EBCDIC systems, the first example would be |
4470 | # $foo = pack("WWWW",193,194,195,196); | |
9ccd05c0 | 4471 | |
a0d0e21e | 4472 | $foo = pack("s2",1,2); |
ce7b6f06 KW |
4473 | # "\001\000\002\000" on little-endian |
4474 | # "\000\001\000\002" on big-endian | |
a0d0e21e LW |
4475 | |
4476 | $foo = pack("a4","abcd","x","y","z"); | |
4477 | # "abcd" | |
4478 | ||
4479 | $foo = pack("aaaa","abcd","x","y","z"); | |
4480 | # "axyz" | |
4481 | ||
4482 | $foo = pack("a14","abcdefg"); | |
4483 | # "abcdefg\0\0\0\0\0\0\0" | |
4484 | ||
4485 | $foo = pack("i9pl", gmtime); | |
4486 | # a real struct tm (on my system anyway) | |
4487 | ||
5a929a98 VU |
4488 | $utmp_template = "Z8 Z8 Z16 L"; |
4489 | $utmp = pack($utmp_template, @utmp1); | |
4490 | # a struct utmp (BSDish) | |
4491 | ||
4492 | @utmp2 = unpack($utmp_template, $utmp); | |
4493 | # "@utmp1" eq "@utmp2" | |
4494 | ||
a0d0e21e | 4495 | sub bintodec { |
a9a5a0dc | 4496 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); |
a0d0e21e LW |
4497 | } |
4498 | ||
851646ae JH |
4499 | $foo = pack('sx2l', 12, 34); |
4500 | # short 12, two zero bytes padding, long 34 | |
4501 | $bar = pack('s@4l', 12, 34); | |
4502 | # short 12, zero fill to position 4, long 34 | |
4503 | # $foo eq $bar | |
28be1210 TH |
4504 | $baz = pack('s.l', 12, 4, 34); |
4505 | # short 12, zero fill to position 4, long 34 | |
851646ae | 4506 | |
1109a392 MHM |
4507 | $foo = pack('nN', 42, 4711); |
4508 | # pack big-endian 16- and 32-bit unsigned integers | |
4509 | $foo = pack('S>L>', 42, 4711); | |
4510 | # exactly the same | |
4511 | $foo = pack('s<l<', -42, 4711); | |
4512 | # pack little-endian 16- and 32-bit signed integers | |
66c611c5 MHM |
4513 | $foo = pack('(sl)<', -42, 4711); |
4514 | # exactly the same | |
1109a392 | 4515 | |
5a929a98 | 4516 | The same template may generally also be used in unpack(). |
a0d0e21e | 4517 | |
8f1da26d TC |
4518 | =item package NAMESPACE |
4519 | ||
6fa4d285 DG |
4520 | =item package NAMESPACE VERSION |
4521 | X<package> X<module> X<namespace> X<version> | |
4522 | ||
8f1da26d | 4523 | =item package NAMESPACE BLOCK |
cb1a09d0 | 4524 | |
4e4da3ac Z |
4525 | =item package NAMESPACE VERSION BLOCK |
4526 | X<package> X<module> X<namespace> X<version> | |
4527 | ||
8f1da26d TC |
4528 | Declares the BLOCK or the rest of the compilation unit as being in the |
4529 | given namespace. The scope of the package declaration is either the | |
4e4da3ac | 4530 | supplied code BLOCK or, in the absence of a BLOCK, from the declaration |
8f1da26d TC |
4531 | itself through the end of current scope (the enclosing block, file, or |
4532 | C<eval>). That is, the forms without a BLOCK are operative through the end | |
4533 | of the current scope, just like the C<my>, C<state>, and C<our> operators. | |
4534 | All unqualified dynamic identifiers in this scope will be in the given | |
4535 | namespace, except where overridden by another C<package> declaration or | |
4536 | when they're one of the special identifiers that qualify into C<main::>, | |
4537 | like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables. | |
4e4da3ac | 4538 | |
3b10bc60 | 4539 | A package statement affects dynamic variables only, including those |
4540 | you've used C<local> on, but I<not> lexical variables, which are created | |
8f1da26d | 4541 | with C<my>, C<state>, or C<our>. Typically it would be the first |
3b10bc60 | 4542 | declaration in a file included by C<require> or C<use>. You can switch into a |
4543 | package in more than one place, since this only determines which default | |
4544 | symbol table the compiler uses for the rest of that block. You can refer to | |
4545 | identifiers in other packages than the current one by prefixing the identifier | |
4546 | with the package name and a double colon, as in C<$SomePack::var> | |
4547 | or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main> | |
4548 | package as assumed. That is, C<$::sail> is equivalent to | |
4549 | C<$main::sail> (as well as to C<$main'sail>, still seen in ancient | |
4550 | code, mostly from Perl 4). | |
4551 | ||
bd12309b | 4552 | If VERSION is provided, C<package> sets the C<$VERSION> variable in the given |
a2bff36e DG |
4553 | namespace to a L<version> object with the VERSION provided. VERSION must be a |
4554 | "strict" style version number as defined by the L<version> module: a positive | |
4555 | decimal number (integer or decimal-fraction) without exponentiation or else a | |
4556 | dotted-decimal v-string with a leading 'v' character and at least three | |
4557 | components. You should set C<$VERSION> only once per package. | |
6fa4d285 | 4558 | |
cb1a09d0 AD |
4559 | See L<perlmod/"Packages"> for more information about packages, modules, |
4560 | and classes. See L<perlsub> for other scoping issues. | |
4561 | ||
a0d0e21e | 4562 | =item pipe READHANDLE,WRITEHANDLE |
d74e8afc | 4563 | X<pipe> |
a0d0e21e LW |
4564 | |
4565 | Opens a pair of connected pipes like the corresponding system call. | |
4566 | Note that if you set up a loop of piped processes, deadlock can occur | |
4567 | unless you are very careful. In addition, note that Perl's pipes use | |
9124316e | 4568 | IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
4569 | after each command, depending on the application. |
4570 | ||
96090e4f LB |
4571 | See L<IPC::Open2>, L<IPC::Open3>, and |
4572 | L<perlipc/"Bidirectional Communication with Another Process"> | |
4633a7c4 LW |
4573 | for examples of such things. |
4574 | ||
3b10bc60 | 4575 | On systems that support a close-on-exec flag on files, that flag is set |
4576 | on all newly opened file descriptors whose C<fileno>s are I<higher> than | |
4577 | the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>. | |
4771b018 | 4578 | |
532eee96 | 4579 | =item pop ARRAY |
d74e8afc | 4580 | X<pop> X<stack> |
a0d0e21e | 4581 | |
f5a93a43 TC |
4582 | =item pop EXPR |
4583 | ||
54310121 | 4584 | =item pop |
28757baa | 4585 | |
a0d0e21e | 4586 | Pops and returns the last value of the array, shortening the array by |
cd7f9af7 | 4587 | one element. |
a0d0e21e | 4588 | |
3b10bc60 | 4589 | Returns the undefined value if the array is empty, although this may also |
4590 | happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the | |
4591 | main program, but the C<@_> array in subroutines, just like C<shift>. | |
a0d0e21e | 4592 | |
f5a93a43 TC |
4593 | Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a |
4594 | reference to an unblessed array. The argument will be dereferenced | |
4595 | automatically. This aspect of C<pop> is considered highly experimental. | |
4596 | The exact behaviour may change in a future version of Perl. | |
cba5a3b0 | 4597 | |
a0d0e21e | 4598 | =item pos SCALAR |
d74e8afc | 4599 | X<pos> X<match, position> |
a0d0e21e | 4600 | |
54310121 | 4601 | =item pos |
bbce6d69 | 4602 | |
7664c618 | 4603 | Returns the offset of where the last C<m//g> search left off for the |
4604 | variable in question (C<$_> is used when the variable is not | |
4605 | specified). Note that 0 is a valid match offset. C<undef> indicates | |
4606 | that the search position is reset (usually due to match failure, but | |
4607 | can also be because no match has yet been run on the scalar). | |
4608 | ||
4609 | C<pos> directly accesses the location used by the regexp engine to | |
4610 | store the offset, so assigning to C<pos> will change that offset, and | |
4611 | so will also influence the C<\G> zero-width assertion in regular | |
4612 | expressions. Both of these effects take place for the next match, so | |
4613 | you can't affect the position with C<pos> during the current match, | |
4614 | such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>. | |
4615 | ||
f9179917 FC |
4616 | Setting C<pos> also resets the I<matched with zero-length> flag, described |
4617 | under L<perlre/"Repeated Patterns Matching a Zero-length Substring">. | |
4618 | ||
7664c618 | 4619 | Because a failed C<m//gc> match doesn't reset the offset, the return |
4620 | from C<pos> won't change either in this case. See L<perlre> and | |
44a8e56a | 4621 | L<perlop>. |
a0d0e21e LW |
4622 | |
4623 | =item print FILEHANDLE LIST | |
d74e8afc | 4624 | X<print> |
a0d0e21e | 4625 | |
dee33c94 TC |
4626 | =item print FILEHANDLE |
4627 | ||
a0d0e21e LW |
4628 | =item print LIST |
4629 | ||
4630 | =item print | |
4631 | ||
19799a22 | 4632 | Prints a string or a list of strings. Returns true if successful. |
dee33c94 TC |
4633 | FILEHANDLE may be a scalar variable containing the name of or a reference |
4634 | to the filehandle, thus introducing one level of indirection. (NOTE: If | |
4635 | FILEHANDLE is a variable and the next token is a term, it may be | |
4636 | misinterpreted as an operator unless you interpose a C<+> or put | |
8f1da26d TC |
4637 | parentheses around the arguments.) If FILEHANDLE is omitted, prints to the |
4638 | last selected (see L</select>) output handle. If LIST is omitted, prints | |
4639 | C<$_> to the currently selected output handle. To use FILEHANDLE alone to | |
4640 | print the content of C<$_> to it, you must use a real filehandle like | |
4641 | C<FH>, not an indirect one like C<$fh>. To set the default output handle | |
4642 | to something other than STDOUT, use the select operation. | |
4643 | ||
4644 | The current value of C<$,> (if any) is printed between each LIST item. The | |
4645 | current value of C<$\> (if any) is printed after the entire LIST has been | |
4646 | printed. Because print takes a LIST, anything in the LIST is evaluated in | |
4647 | list context, including any subroutines whose return lists you pass to | |
4648 | C<print>. Be careful not to follow the print keyword with a left | |
4649 | parenthesis unless you want the corresponding right parenthesis to | |
4650 | terminate the arguments to the print; put parentheses around all arguments | |
4651 | (or interpose a C<+>, but that doesn't look as good). | |
4652 | ||
4653 | If you're storing handles in an array or hash, or in general whenever | |
4654 | you're using any expression more complex than a bareword handle or a plain, | |
4655 | unsubscripted scalar variable to retrieve it, you will have to use a block | |
4656 | returning the filehandle value instead, in which case the LIST may not be | |
4657 | omitted: | |
4633a7c4 LW |
4658 | |
4659 | print { $files[$i] } "stuff\n"; | |
4660 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
4661 | ||
785fd561 DG |
4662 | Printing to a closed pipe or socket will generate a SIGPIPE signal. See |
4663 | L<perlipc> for more on signal handling. | |
4664 | ||
5f05dabc | 4665 | =item printf FILEHANDLE FORMAT, LIST |
d74e8afc | 4666 | X<printf> |
a0d0e21e | 4667 | |
dee33c94 TC |
4668 | =item printf FILEHANDLE |
4669 | ||
5f05dabc | 4670 | =item printf FORMAT, LIST |
a0d0e21e | 4671 | |
dee33c94 TC |
4672 | =item printf |
4673 | ||
7660c0ab | 4674 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
dee33c94 | 4675 | (the output record separator) is not appended. The first argument of the |
01aa884e KW |
4676 | list will be interpreted as the C<printf> format. See |
4677 | L<sprintf|/sprintf FORMAT, LIST> for an | |
dee33c94 TC |
4678 | explanation of the format argument. If you omit the LIST, C<$_> is used; |
4679 | to use FILEHANDLE without a LIST, you must use a real filehandle like | |
4680 | C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and | |
4681 | POSIX::setlocale() has been called, the character used for the decimal | |
3b10bc60 | 4682 | separator in formatted floating-point numbers is affected by the LC_NUMERIC |
dee33c94 | 4683 | locale setting. See L<perllocale> and L<POSIX>. |
a0d0e21e | 4684 | |
19799a22 GS |
4685 | Don't fall into the trap of using a C<printf> when a simple |
4686 | C<print> would do. The C<print> is more efficient and less | |
28757baa | 4687 | error prone. |
4688 | ||
da0045b7 | 4689 | =item prototype FUNCTION |
d74e8afc | 4690 | X<prototype> |
da0045b7 | 4691 | |
4692 | Returns the prototype of a function as a string (or C<undef> if the | |
5f05dabc | 4693 | function has no prototype). FUNCTION is a reference to, or the name of, |
4694 | the function whose prototype you want to retrieve. | |
da0045b7 | 4695 | |
2b5ab1e7 | 4696 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a |
e1020413 | 4697 | name for a Perl builtin. If the builtin is not I<overridable> (such as |
0a2ca743 RGS |
4698 | C<qw//>) or if its arguments cannot be adequately expressed by a prototype |
4699 | (such as C<system>), prototype() returns C<undef>, because the builtin | |
4700 | does not really behave like a Perl function. Otherwise, the string | |
4701 | describing the equivalent prototype is returned. | |
b6c543e3 | 4702 | |
532eee96 | 4703 | =item push ARRAY,LIST |
1dc8ecb8 | 4704 | X<push> X<stack> |
a0d0e21e | 4705 | |
f5a93a43 TC |
4706 | =item push EXPR,LIST |
4707 | ||
8f1da26d TC |
4708 | Treats ARRAY as a stack by appending the values of LIST to the end of |
4709 | ARRAY. The length of ARRAY increases by the length of LIST. Has the same | |
4710 | effect as | |
a0d0e21e LW |
4711 | |
4712 | for $value (LIST) { | |
a9a5a0dc | 4713 | $ARRAY[++$#ARRAY] = $value; |
a0d0e21e LW |
4714 | } |
4715 | ||
cde9c211 SP |
4716 | but is more efficient. Returns the number of elements in the array following |
4717 | the completed C<push>. | |
a0d0e21e | 4718 | |
f5a93a43 TC |
4719 | Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a |
4720 | reference to an unblessed array. The argument will be dereferenced | |
4721 | automatically. This aspect of C<push> is considered highly experimental. | |
4722 | The exact behaviour may change in a future version of Perl. | |
cba5a3b0 | 4723 | |
a0d0e21e LW |
4724 | =item q/STRING/ |
4725 | ||
4726 | =item qq/STRING/ | |
4727 | ||
945c54fd | 4728 | =item qx/STRING/ |
a0d0e21e LW |
4729 | |
4730 | =item qw/STRING/ | |
4731 | ||
1d888ee3 MK |
4732 | Generalized quotes. See L<perlop/"Quote-Like Operators">. |
4733 | ||
4734 | =item qr/STRING/ | |
4735 | ||
4736 | Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">. | |
a0d0e21e LW |
4737 | |
4738 | =item quotemeta EXPR | |
d74e8afc | 4739 | X<quotemeta> X<metacharacter> |
a0d0e21e | 4740 | |
54310121 | 4741 | =item quotemeta |
bbce6d69 | 4742 | |
36bbe248 | 4743 | Returns the value of EXPR with all non-"word" |
a034a98d DD |
4744 | characters backslashed. (That is, all characters not matching |
4745 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
4746 | returned string, regardless of any locale settings.) | |
4747 | This is the internal function implementing | |
7660c0ab | 4748 | the C<\Q> escape in double-quoted strings. |
a0d0e21e | 4749 | |
7660c0ab | 4750 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 4751 | |
9702b155 RGS |
4752 | quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into |
4753 | regular expressions, because by default an interpolated variable will be | |
4754 | considered a mini-regular expression. For example: | |
4755 | ||
4756 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4757 | my $substring = 'quick.*?fox'; | |
4758 | $sentence =~ s{$substring}{big bad wolf}; | |
4759 | ||
4760 | Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>. | |
4761 | ||
4762 | On the other hand: | |
4763 | ||
4764 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4765 | my $substring = 'quick.*?fox'; | |
4766 | $sentence =~ s{\Q$substring\E}{big bad wolf}; | |
4767 | ||
4768 | Or: | |
4769 | ||
4770 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4771 | my $substring = 'quick.*?fox'; | |
4772 | my $quoted_substring = quotemeta($substring); | |
4773 | $sentence =~ s{$quoted_substring}{big bad wolf}; | |
4774 | ||
8f1da26d TC |
4775 | Will both leave the sentence as is. Normally, when accepting literal string |
4776 | input from the user, quotemeta() or C<\Q> must be used. | |
9702b155 | 4777 | |
b29c72cb | 4778 | In Perl 5.14, all characters whose code points are above 127 are not |
8f1da26d | 4779 | quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings. |
b29c72cb KW |
4780 | It is planned to change this behavior in 5.16, but the exact rules |
4781 | haven't been determined yet. | |
4782 | ||
a0d0e21e | 4783 | =item rand EXPR |
d74e8afc | 4784 | X<rand> X<random> |
a0d0e21e LW |
4785 | |
4786 | =item rand | |
4787 | ||
7660c0ab | 4788 | Returns a random fractional number greater than or equal to C<0> and less |
3e3baf6d | 4789 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
351f3254 | 4790 | omitted, the value C<1> is used. Currently EXPR with the value C<0> is |
3b10bc60 | 4791 | also special-cased as C<1> (this was undocumented before Perl 5.8.0 |
4792 | and is subject to change in future versions of Perl). Automatically calls | |
351f3254 | 4793 | C<srand> unless C<srand> has already been called. See also C<srand>. |
a0d0e21e | 4794 | |
6063ba18 WM |
4795 | Apply C<int()> to the value returned by C<rand()> if you want random |
4796 | integers instead of random fractional numbers. For example, | |
4797 | ||
4798 | int(rand(10)) | |
4799 | ||
4800 | returns a random integer between C<0> and C<9>, inclusive. | |
4801 | ||
2f9daede | 4802 | (Note: If your rand function consistently returns numbers that are too |
a0d0e21e | 4803 | large or too small, then your version of Perl was probably compiled |
2f9daede | 4804 | with the wrong number of RANDBITS.) |
a0d0e21e | 4805 | |
9700c45b JV |
4806 | B<C<rand()> is not cryptographically secure. You should not rely |
4807 | on it in security-sensitive situations.> As of this writing, a | |
4808 | number of third-party CPAN modules offer random number generators | |
4809 | intended by their authors to be cryptographically secure, | |
4810 | including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and | |
4811 | L<Math::TrulyRandom>. | |
4812 | ||
a0d0e21e | 4813 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET |
f723aae1 | 4814 | X<read> X<file, read> |
a0d0e21e LW |
4815 | |
4816 | =item read FILEHANDLE,SCALAR,LENGTH | |
4817 | ||
9124316e JH |
4818 | Attempts to read LENGTH I<characters> of data into variable SCALAR |
4819 | from the specified FILEHANDLE. Returns the number of characters | |
b5fe5ca2 | 4820 | actually read, C<0> at end of file, or undef if there was an error (in |
b49f3be6 SG |
4821 | the latter case C<$!> is also set). SCALAR will be grown or shrunk |
4822 | so that the last character actually read is the last character of the | |
4823 | scalar after the read. | |
4824 | ||
4825 | An OFFSET may be specified to place the read data at some place in the | |
4826 | string other than the beginning. A negative OFFSET specifies | |
4827 | placement at that many characters counting backwards from the end of | |
4828 | the string. A positive OFFSET greater than the length of SCALAR | |
4829 | results in the string being padded to the required size with C<"\0"> | |
4830 | bytes before the result of the read is appended. | |
4831 | ||
80d38338 | 4832 | The call is implemented in terms of either Perl's or your system's native |
01aa884e KW |
4833 | fread(3) library function. To get a true read(2) system call, see |
4834 | L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>. | |
9124316e JH |
4835 | |
4836 | Note the I<characters>: depending on the status of the filehandle, | |
8f1da26d | 4837 | either (8-bit) bytes or characters are read. By default, all |
9124316e | 4838 | filehandles operate on bytes, but for example if the filehandle has |
fae2c0fb | 4839 | been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> |
8f1da26d | 4840 | pragma, L<open>), the I/O will operate on UTF8-encoded Unicode |
1d714267 JH |
4841 | characters, not bytes. Similarly for the C<:encoding> pragma: |
4842 | in that case pretty much any characters can be read. | |
a0d0e21e LW |
4843 | |
4844 | =item readdir DIRHANDLE | |
d74e8afc | 4845 | X<readdir> |
a0d0e21e | 4846 | |
19799a22 | 4847 | Returns the next directory entry for a directory opened by C<opendir>. |
5a964f20 | 4848 | If used in list context, returns all the rest of the entries in the |
3b10bc60 | 4849 | directory. If there are no more entries, returns the undefined value in |
4850 | scalar context and the empty list in list context. | |
a0d0e21e | 4851 | |
19799a22 | 4852 | If you're planning to filetest the return values out of a C<readdir>, you'd |
5f05dabc | 4853 | better prepend the directory in question. Otherwise, because we didn't |
19799a22 | 4854 | C<chdir> there, it would have been testing the wrong file. |
cb1a09d0 | 4855 | |
b0169937 GS |
4856 | opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!"; |
4857 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); | |
4858 | closedir $dh; | |
cb1a09d0 | 4859 | |
114c60ec BG |
4860 | As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop, |
4861 | which will set C<$_> on every iteration. | |
4862 | ||
4863 | opendir(my $dh, $some_dir) || die; | |
4864 | while(readdir $dh) { | |
4865 | print "$some_dir/$_\n"; | |
4866 | } | |
4867 | closedir $dh; | |
4868 | ||
84902520 | 4869 | =item readline EXPR |
e4b7ebf3 RGS |
4870 | |
4871 | =item readline | |
d74e8afc | 4872 | X<readline> X<gets> X<fgets> |
84902520 | 4873 | |
e4b7ebf3 | 4874 | Reads from the filehandle whose typeglob is contained in EXPR (or from |
8f1da26d | 4875 | C<*ARGV> if EXPR is not provided). In scalar context, each call reads and |
80d38338 | 4876 | returns the next line until end-of-file is reached, whereupon the |
0f03d336 | 4877 | subsequent call returns C<undef>. In list context, reads until end-of-file |
e4b7ebf3 | 4878 | is reached and returns a list of lines. Note that the notion of "line" |
80d38338 | 4879 | used here is whatever you may have defined with C<$/> or |
e4b7ebf3 | 4880 | C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. |
fbad3eb5 | 4881 | |
0f03d336 | 4882 | When C<$/> is set to C<undef>, when C<readline> is in scalar |
80d38338 | 4883 | context (i.e., file slurp mode), and when an empty file is read, it |
449bc448 | 4884 | returns C<''> the first time, followed by C<undef> subsequently. |
fbad3eb5 | 4885 | |
61eff3bc JH |
4886 | This is the internal function implementing the C<< <EXPR> >> |
4887 | operator, but you can use it directly. The C<< <EXPR> >> | |
84902520 TB |
4888 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
4889 | ||
5a964f20 | 4890 | $line = <STDIN>; |
5ed4f2ec | 4891 | $line = readline(*STDIN); # same thing |
5a964f20 | 4892 | |
0f03d336 | 4893 | If C<readline> encounters an operating system error, C<$!> will be set |
4894 | with the corresponding error message. It can be helpful to check | |
4895 | C<$!> when you are reading from filehandles you don't trust, such as a | |
4896 | tty or a socket. The following example uses the operator form of | |
4897 | C<readline> and dies if the result is not defined. | |
4898 | ||
5ed4f2ec | 4899 | while ( ! eof($fh) ) { |
4900 | defined( $_ = <$fh> ) or die "readline failed: $!"; | |
4901 | ... | |
4902 | } | |
0f03d336 | 4903 | |
4904 | Note that you have can't handle C<readline> errors that way with the | |
4905 | C<ARGV> filehandle. In that case, you have to open each element of | |
4906 | C<@ARGV> yourself since C<eof> handles C<ARGV> differently. | |
4907 | ||
4908 | foreach my $arg (@ARGV) { | |
4909 | open(my $fh, $arg) or warn "Can't open $arg: $!"; | |
4910 | ||
4911 | while ( ! eof($fh) ) { | |
4912 | defined( $_ = <$fh> ) | |
4913 | or die "readline failed for $arg: $!"; | |
4914 | ... | |
00cb5da1 | 4915 | } |
00cb5da1 | 4916 | } |
e00e4ce9 | 4917 | |
a0d0e21e | 4918 | =item readlink EXPR |
d74e8afc | 4919 | X<readlink> |
a0d0e21e | 4920 | |
54310121 | 4921 | =item readlink |
bbce6d69 | 4922 | |
a0d0e21e | 4923 | Returns the value of a symbolic link, if symbolic links are |
3b10bc60 | 4924 | implemented. If not, raises an exception. If there is a system |
184e9718 | 4925 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
7660c0ab | 4926 | omitted, uses C<$_>. |
a0d0e21e | 4927 | |
ea9eb35a BJ |
4928 | Portability issues: L<perlport/readlink>. |
4929 | ||
84902520 | 4930 | =item readpipe EXPR |
8d7403e6 RGS |
4931 | |
4932 | =item readpipe | |
d74e8afc | 4933 | X<readpipe> |
84902520 | 4934 | |
5a964f20 | 4935 | EXPR is executed as a system command. |
84902520 TB |
4936 | The collected standard output of the command is returned. |
4937 | In scalar context, it comes back as a single (potentially | |
4938 | multi-line) string. In list context, returns a list of lines | |
7660c0ab | 4939 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
4940 | This is the internal function implementing the C<qx/EXPR/> |
4941 | operator, but you can use it directly. The C<qx/EXPR/> | |
4942 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
8d7403e6 | 4943 | If EXPR is omitted, uses C<$_>. |
84902520 | 4944 | |
399388f4 | 4945 | =item recv SOCKET,SCALAR,LENGTH,FLAGS |
d74e8afc | 4946 | X<recv> |
a0d0e21e | 4947 | |
9124316e JH |
4948 | Receives a message on a socket. Attempts to receive LENGTH characters |
4949 | of data into variable SCALAR from the specified SOCKET filehandle. | |
4950 | SCALAR will be grown or shrunk to the length actually read. Takes the | |
4951 | same flags as the system call of the same name. Returns the address | |
4952 | of the sender if SOCKET's protocol supports this; returns an empty | |
4953 | string otherwise. If there's an error, returns the undefined value. | |
4954 | This call is actually implemented in terms of recvfrom(2) system call. | |
4955 | See L<perlipc/"UDP: Message Passing"> for examples. | |
4956 | ||
4957 | Note the I<characters>: depending on the status of the socket, either | |
4958 | (8-bit) bytes or characters are received. By default all sockets | |
4959 | operate on bytes, but for example if the socket has been changed using | |
740d4bb2 | 4960 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see the |
8f1da26d | 4961 | C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode |
740d4bb2 JW |
4962 | characters, not bytes. Similarly for the C<:encoding> pragma: in that |
4963 | case pretty much any characters can be read. | |
a0d0e21e LW |
4964 | |
4965 | =item redo LABEL | |
d74e8afc | 4966 | X<redo> |
a0d0e21e LW |
4967 | |
4968 | =item redo | |
4969 | ||
4970 | The C<redo> command restarts the loop block without evaluating the | |
98293880 | 4971 | conditional again. The C<continue> block, if any, is not executed. If |
a0d0e21e | 4972 | the LABEL is omitted, the command refers to the innermost enclosing |
cf264981 SP |
4973 | loop. Programs that want to lie to themselves about what was just input |
4974 | normally use this command: | |
a0d0e21e LW |
4975 | |
4976 | # a simpleminded Pascal comment stripper | |
4977 | # (warning: assumes no { or } in strings) | |
4633a7c4 | 4978 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
4979 | while (s|({.*}.*){.*}|$1 |) {} |
4980 | s|{.*}| |; | |
4981 | if (s|{.*| |) { | |
4982 | $front = $_; | |
4983 | while (<STDIN>) { | |
4984 | if (/}/) { # end of comment? | |
4985 | s|^|$front\{|; | |
4986 | redo LINE; | |
4987 | } | |
4988 | } | |
5ed4f2ec | 4989 | } |
a9a5a0dc | 4990 | print; |
a0d0e21e LW |
4991 | } |
4992 | ||
80d38338 | 4993 | C<redo> cannot be used to retry a block that returns a value such as |
8f1da26d | 4994 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit |
2b5ab1e7 | 4995 | a grep() or map() operation. |
4968c1e4 | 4996 | |
6c1372ed GS |
4997 | Note that a block by itself is semantically identical to a loop |
4998 | that executes once. Thus C<redo> inside such a block will effectively | |
4999 | turn it into a looping construct. | |
5000 | ||
98293880 | 5001 | See also L</continue> for an illustration of how C<last>, C<next>, and |
1d2dff63 GS |
5002 | C<redo> work. |
5003 | ||
a0d0e21e | 5004 | =item ref EXPR |
d74e8afc | 5005 | X<ref> X<reference> |
a0d0e21e | 5006 | |
54310121 | 5007 | =item ref |
bbce6d69 | 5008 | |
8a2e0804 A |
5009 | Returns a non-empty string if EXPR is a reference, the empty |
5010 | string otherwise. If EXPR | |
7660c0ab | 5011 | is not specified, C<$_> will be used. The value returned depends on the |
bbce6d69 | 5012 | type of thing the reference is a reference to. |
a0d0e21e LW |
5013 | Builtin types include: |
5014 | ||
a0d0e21e LW |
5015 | SCALAR |
5016 | ARRAY | |
5017 | HASH | |
5018 | CODE | |
19799a22 | 5019 | REF |
a0d0e21e | 5020 | GLOB |
19799a22 | 5021 | LVALUE |
cc10766d RGS |
5022 | FORMAT |
5023 | IO | |
5024 | VSTRING | |
5025 | Regexp | |
a0d0e21e | 5026 | |
54310121 | 5027 | If the referenced object has been blessed into a package, then that package |
19799a22 | 5028 | name is returned instead. You can think of C<ref> as a C<typeof> operator. |
a0d0e21e LW |
5029 | |
5030 | if (ref($r) eq "HASH") { | |
a9a5a0dc | 5031 | print "r is a reference to a hash.\n"; |
54310121 | 5032 | } |
2b5ab1e7 | 5033 | unless (ref($r)) { |
a9a5a0dc | 5034 | print "r is not a reference at all.\n"; |
54310121 | 5035 | } |
a0d0e21e | 5036 | |
85dd5c8b WL |
5037 | The return value C<LVALUE> indicates a reference to an lvalue that is not |
5038 | a variable. You get this from taking the reference of function calls like | |
5039 | C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points | |
603c58be | 5040 | to a L<version string|perldata/"Version Strings">. |
85dd5c8b WL |
5041 | |
5042 | The result C<Regexp> indicates that the argument is a regular expression | |
5043 | resulting from C<qr//>. | |
5044 | ||
a0d0e21e LW |
5045 | See also L<perlref>. |
5046 | ||
5047 | =item rename OLDNAME,NEWNAME | |
d74e8afc | 5048 | X<rename> X<move> X<mv> X<ren> |
a0d0e21e | 5049 | |
19799a22 GS |
5050 | Changes the name of a file; an existing file NEWNAME will be |
5051 | clobbered. Returns true for success, false otherwise. | |
5052 | ||
2b5ab1e7 TC |
5053 | Behavior of this function varies wildly depending on your system |
5054 | implementation. For example, it will usually not work across file system | |
5055 | boundaries, even though the system I<mv> command sometimes compensates | |
5056 | for this. Other restrictions include whether it works on directories, | |
5057 | open files, or pre-existing files. Check L<perlport> and either the | |
5058 | rename(2) manpage or equivalent system documentation for details. | |
a0d0e21e | 5059 | |
dd184578 RGS |
5060 | For a platform independent C<move> function look at the L<File::Copy> |
5061 | module. | |
5062 | ||
ea9eb35a BJ |
5063 | Portability issues: L<perlport/rename>. |
5064 | ||
16070b82 | 5065 | =item require VERSION |
d74e8afc | 5066 | X<require> |
16070b82 | 5067 | |
a0d0e21e LW |
5068 | =item require EXPR |
5069 | ||
5070 | =item require | |
5071 | ||
3b825e41 RK |
5072 | Demands a version of Perl specified by VERSION, or demands some semantics |
5073 | specified by EXPR or by C<$_> if EXPR is not supplied. | |
44dcb63b | 5074 | |
3b825e41 RK |
5075 | VERSION may be either a numeric argument such as 5.006, which will be |
5076 | compared to C<$]>, or a literal of the form v5.6.1, which will be compared | |
3b10bc60 | 5077 | to C<$^V> (aka $PERL_VERSION). An exception is raised if |
3b825e41 RK |
5078 | VERSION is greater than the version of the current Perl interpreter. |
5079 | Compare with L</use>, which can do a similar check at compile time. | |
5080 | ||
5081 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
5082 | avoided, because it leads to misleading error messages under earlier | |
cf264981 | 5083 | versions of Perl that do not support this syntax. The equivalent numeric |
3b825e41 | 5084 | version should be used instead. |
44dcb63b | 5085 | |
5ed4f2ec | 5086 | require v5.6.1; # run time version check |
5087 | require 5.6.1; # ditto | |
5088 | require 5.006_001; # ditto; preferred for backwards compatibility | |
a0d0e21e | 5089 | |
362eead3 RGS |
5090 | Otherwise, C<require> demands that a library file be included if it |
5091 | hasn't already been included. The file is included via the do-FILE | |
73c71df6 CW |
5092 | mechanism, which is essentially just a variety of C<eval> with the |
5093 | caveat that lexical variables in the invoking script will be invisible | |
5094 | to the included code. Has semantics similar to the following subroutine: | |
a0d0e21e LW |
5095 | |
5096 | sub require { | |
20907158 AMS |
5097 | my ($filename) = @_; |
5098 | if (exists $INC{$filename}) { | |
5099 | return 1 if $INC{$filename}; | |
5100 | die "Compilation failed in require"; | |
5101 | } | |
5102 | my ($realfilename,$result); | |
5103 | ITER: { | |
5104 | foreach $prefix (@INC) { | |
5105 | $realfilename = "$prefix/$filename"; | |
5106 | if (-f $realfilename) { | |
5107 | $INC{$filename} = $realfilename; | |
5108 | $result = do $realfilename; | |
5109 | last ITER; | |
5110 | } | |
5111 | } | |
5112 | die "Can't find $filename in \@INC"; | |
5113 | } | |
5114 | if ($@) { | |
5115 | $INC{$filename} = undef; | |
5116 | die $@; | |
5117 | } elsif (!$result) { | |
5118 | delete $INC{$filename}; | |
5119 | die "$filename did not return true value"; | |
5120 | } else { | |
5121 | return $result; | |
5122 | } | |
a0d0e21e LW |
5123 | } |
5124 | ||
5125 | Note that the file will not be included twice under the same specified | |
a12755f0 SB |
5126 | name. |
5127 | ||
5128 | The file must return true as the last statement to indicate | |
a0d0e21e | 5129 | successful execution of any initialization code, so it's customary to |
19799a22 GS |
5130 | end such a file with C<1;> unless you're sure it'll return true |
5131 | otherwise. But it's better just to put the C<1;>, in case you add more | |
a0d0e21e LW |
5132 | statements. |
5133 | ||
54310121 | 5134 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
da0045b7 | 5135 | replaces "F<::>" with "F</>" in the filename for you, |
54310121 | 5136 | to make it easy to load standard modules. This form of loading of |
a0d0e21e LW |
5137 | modules does not risk altering your namespace. |
5138 | ||
ee580363 GS |
5139 | In other words, if you try this: |
5140 | ||
5ed4f2ec | 5141 | require Foo::Bar; # a splendid bareword |
ee580363 | 5142 | |
b76cc8ba | 5143 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
7660c0ab | 5144 | directories specified in the C<@INC> array. |
ee580363 | 5145 | |
5a964f20 | 5146 | But if you try this: |
ee580363 GS |
5147 | |
5148 | $class = 'Foo::Bar'; | |
5ed4f2ec | 5149 | require $class; # $class is not a bareword |
5a964f20 | 5150 | #or |
5ed4f2ec | 5151 | require "Foo::Bar"; # not a bareword because of the "" |
ee580363 | 5152 | |
b76cc8ba | 5153 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
19799a22 | 5154 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
ee580363 GS |
5155 | |
5156 | eval "require $class"; | |
5157 | ||
3b10bc60 | 5158 | Now that you understand how C<require> looks for files with a |
a91233bf RGS |
5159 | bareword argument, there is a little extra functionality going on behind |
5160 | the scenes. Before C<require> looks for a "F<.pm>" extension, it will | |
5161 | first look for a similar filename with a "F<.pmc>" extension. If this file | |
5162 | is found, it will be loaded in place of any file ending in a "F<.pm>" | |
5163 | extension. | |
662cc546 | 5164 | |
8f1da26d | 5165 | You can also insert hooks into the import facility by putting Perl code |
1c3d5054 | 5166 | directly into the @INC array. There are three forms of hooks: subroutine |
8f1da26d | 5167 | references, array references, and blessed objects. |
d54b56d5 RGS |
5168 | |
5169 | Subroutine references are the simplest case. When the inclusion system | |
5170 | walks through @INC and encounters a subroutine, this subroutine gets | |
3b10bc60 | 5171 | called with two parameters, the first a reference to itself, and the |
5172 | second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The | |
5173 | subroutine should return either nothing or else a list of up to three | |
5174 | values in the following order: | |
1f0bdf18 NC |
5175 | |
5176 | =over | |
5177 | ||
5178 | =item 1 | |
5179 | ||
1f0bdf18 NC |
5180 | A filehandle, from which the file will be read. |
5181 | ||
cec0e1a7 | 5182 | =item 2 |
1f0bdf18 | 5183 | |
60d352b3 RGS |
5184 | A reference to a subroutine. If there is no filehandle (previous item), |
5185 | then this subroutine is expected to generate one line of source code per | |
8f1da26d TC |
5186 | call, writing the line into C<$_> and returning 1, then finally at end of |
5187 | file returning 0. If there is a filehandle, then the subroutine will be | |
b8921b3e | 5188 | called to act as a simple source filter, with the line as read in C<$_>. |
60d352b3 RGS |
5189 | Again, return 1 for each valid line, and 0 after all lines have been |
5190 | returned. | |
1f0bdf18 | 5191 | |
cec0e1a7 | 5192 | =item 3 |
1f0bdf18 NC |
5193 | |
5194 | Optional state for the subroutine. The state is passed in as C<$_[1]>. A | |
5195 | reference to the subroutine itself is passed in as C<$_[0]>. | |
5196 | ||
5197 | =back | |
5198 | ||
5199 | If an empty list, C<undef>, or nothing that matches the first 3 values above | |
3b10bc60 | 5200 | is returned, then C<require> looks at the remaining elements of @INC. |
5201 | Note that this filehandle must be a real filehandle (strictly a typeglob | |
8f1da26d TC |
5202 | or reference to a typeglob, whether blessed or unblessed); tied filehandles |
5203 | will be ignored and processing will stop there. | |
d54b56d5 RGS |
5204 | |
5205 | If the hook is an array reference, its first element must be a subroutine | |
5206 | reference. This subroutine is called as above, but the first parameter is | |
3b10bc60 | 5207 | the array reference. This lets you indirectly pass arguments to |
d54b56d5 RGS |
5208 | the subroutine. |
5209 | ||
5210 | In other words, you can write: | |
5211 | ||
5212 | push @INC, \&my_sub; | |
5213 | sub my_sub { | |
a9a5a0dc VP |
5214 | my ($coderef, $filename) = @_; # $coderef is \&my_sub |
5215 | ... | |
d54b56d5 RGS |
5216 | } |
5217 | ||
5218 | or: | |
5219 | ||
5220 | push @INC, [ \&my_sub, $x, $y, ... ]; | |
5221 | sub my_sub { | |
a9a5a0dc VP |
5222 | my ($arrayref, $filename) = @_; |
5223 | # Retrieve $x, $y, ... | |
5224 | my @parameters = @$arrayref[1..$#$arrayref]; | |
5225 | ... | |
d54b56d5 RGS |
5226 | } |
5227 | ||
cf264981 | 5228 | If the hook is an object, it must provide an INC method that will be |
d54b56d5 | 5229 | called as above, the first parameter being the object itself. (Note that |
92c6daad NC |
5230 | you must fully qualify the sub's name, as unqualified C<INC> is always forced |
5231 | into package C<main>.) Here is a typical code layout: | |
d54b56d5 RGS |
5232 | |
5233 | # In Foo.pm | |
5234 | package Foo; | |
5235 | sub new { ... } | |
5236 | sub Foo::INC { | |
a9a5a0dc VP |
5237 | my ($self, $filename) = @_; |
5238 | ... | |
d54b56d5 RGS |
5239 | } |
5240 | ||
5241 | # In the main program | |
797f796a | 5242 | push @INC, Foo->new(...); |
d54b56d5 | 5243 | |
3b10bc60 | 5244 | These hooks are also permitted to set the %INC entry |
9ae8cd5b RGS |
5245 | corresponding to the files they have loaded. See L<perlvar/%INC>. |
5246 | ||
ee580363 | 5247 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. |
a0d0e21e LW |
5248 | |
5249 | =item reset EXPR | |
d74e8afc | 5250 | X<reset> |
a0d0e21e LW |
5251 | |
5252 | =item reset | |
5253 | ||
5254 | Generally used in a C<continue> block at the end of a loop to clear | |
7660c0ab | 5255 | variables and reset C<??> searches so that they work again. The |
a0d0e21e LW |
5256 | expression is interpreted as a list of single characters (hyphens |
5257 | allowed for ranges). All variables and arrays beginning with one of | |
5258 | those letters are reset to their pristine state. If the expression is | |
3b10bc60 | 5259 | omitted, one-match searches (C<?pattern?>) are reset to match again. |
5260 | Only resets variables or searches in the current package. Always returns | |
5261 | 1. Examples: | |
a0d0e21e | 5262 | |
5ed4f2ec | 5263 | reset 'X'; # reset all X variables |
5264 | reset 'a-z'; # reset lower case variables | |
5265 | reset; # just reset ?one-time? searches | |
a0d0e21e | 5266 | |
7660c0ab | 5267 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
2b5ab1e7 | 5268 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package |
3b10bc60 | 5269 | variables; lexical variables are unaffected, but they clean themselves |
2b5ab1e7 TC |
5270 | up on scope exit anyway, so you'll probably want to use them instead. |
5271 | See L</my>. | |
a0d0e21e | 5272 | |
54310121 | 5273 | =item return EXPR |
d74e8afc | 5274 | X<return> |
54310121 | 5275 | |
5276 | =item return | |
5277 | ||
b76cc8ba | 5278 | Returns from a subroutine, C<eval>, or C<do FILE> with the value |
5a964f20 | 5279 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
54310121 | 5280 | context, depending on how the return value will be used, and the context |
01aa884e | 5281 | may vary from one execution to the next (see L</wantarray>). If no EXPR |
2b5ab1e7 | 5282 | is given, returns an empty list in list context, the undefined value in |
3b10bc60 | 5283 | scalar context, and (of course) nothing at all in void context. |
a0d0e21e | 5284 | |
3b10bc60 | 5285 | (In the absence of an explicit C<return>, a subroutine, eval, |
5286 | or do FILE automatically returns the value of the last expression | |
2b5ab1e7 | 5287 | evaluated.) |
a0d0e21e LW |
5288 | |
5289 | =item reverse LIST | |
d74e8afc | 5290 | X<reverse> X<rev> X<invert> |
a0d0e21e | 5291 | |
5a964f20 TC |
5292 | In list context, returns a list value consisting of the elements |
5293 | of LIST in the opposite order. In scalar context, concatenates the | |
2b5ab1e7 | 5294 | elements of LIST and returns a string value with all characters |
a0ed51b3 | 5295 | in the opposite order. |
4633a7c4 | 5296 | |
9649ed94 | 5297 | print join(", ", reverse "world", "Hello"); # Hello, world |
4633a7c4 | 5298 | |
9649ed94 | 5299 | print scalar reverse "dlrow ,", "olleH"; # Hello, world |
2f9daede | 5300 | |
2d713cbd RGS |
5301 | Used without arguments in scalar context, reverse() reverses C<$_>. |
5302 | ||
9649ed94 B |
5303 | $_ = "dlrow ,olleH"; |
5304 | print reverse; # No output, list context | |
5305 | print scalar reverse; # Hello, world | |
5306 | ||
437d4214 | 5307 | Note that reversing an array to itself (as in C<@a = reverse @a>) will |
80d38338 | 5308 | preserve non-existent elements whenever possible, i.e., for non magical |
437d4214 VP |
5309 | arrays or tied arrays with C<EXISTS> and C<DELETE> methods. |
5310 | ||
2f9daede TP |
5311 | This operator is also handy for inverting a hash, although there are some |
5312 | caveats. If a value is duplicated in the original hash, only one of those | |
5313 | can be represented as a key in the inverted hash. Also, this has to | |
5314 | unwind one hash and build a whole new one, which may take some time | |
2b5ab1e7 | 5315 | on a large hash, such as from a DBM file. |
2f9daede | 5316 | |
5ed4f2ec | 5317 | %by_name = reverse %by_address; # Invert the hash |
a0d0e21e LW |
5318 | |
5319 | =item rewinddir DIRHANDLE | |
d74e8afc | 5320 | X<rewinddir> |
a0d0e21e LW |
5321 | |
5322 | Sets the current position to the beginning of the directory for the | |
19799a22 | 5323 | C<readdir> routine on DIRHANDLE. |
a0d0e21e | 5324 | |
ea9eb35a BJ |
5325 | Portability issues: L<perlport/rewinddir>. |
5326 | ||
a0d0e21e | 5327 | =item rindex STR,SUBSTR,POSITION |
d74e8afc | 5328 | X<rindex> |
a0d0e21e LW |
5329 | |
5330 | =item rindex STR,SUBSTR | |
5331 | ||
ff551661 | 5332 | Works just like index() except that it returns the position of the I<last> |
a0d0e21e | 5333 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
ff551661 | 5334 | last occurrence beginning at or before that position. |
a0d0e21e LW |
5335 | |
5336 | =item rmdir FILENAME | |
d74e8afc | 5337 | X<rmdir> X<rd> X<directory, remove> |
a0d0e21e | 5338 | |
54310121 | 5339 | =item rmdir |
bbce6d69 | 5340 | |
974da8e5 | 5341 | Deletes the directory specified by FILENAME if that directory is |
8f1da26d | 5342 | empty. If it succeeds it returns true; otherwise it returns false and |
974da8e5 | 5343 | sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. |
a0d0e21e | 5344 | |
e1020413 | 5345 | To remove a directory tree recursively (C<rm -rf> on Unix) look at |
dd184578 RGS |
5346 | the C<rmtree> function of the L<File::Path> module. |
5347 | ||
a0d0e21e LW |
5348 | =item s/// |
5349 | ||
9f4b9cd0 | 5350 | The substitution operator. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e | 5351 | |
0d863452 RH |
5352 | =item say FILEHANDLE LIST |
5353 | X<say> | |
5354 | ||
dee33c94 TC |
5355 | =item say FILEHANDLE |
5356 | ||
0d863452 RH |
5357 | =item say LIST |
5358 | ||
5359 | =item say | |
5360 | ||
dee33c94 TC |
5361 | Just like C<print>, but implicitly appends a newline. C<say LIST> is |
5362 | simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use | |
5363 | FILEHANDLE without a LIST to print the contents of C<$_> to it, you must | |
5364 | use a real filehandle like C<FH>, not an indirect one like C<$fh>. | |
f406c1e8 | 5365 | |
4a904372 FC |
5366 | This keyword is available only when the C<"say"> feature |
5367 | is enabled, or when prefixed with C<CORE::>; see | |
8f1da26d TC |
5368 | L<feature>. Alternately, include a C<use v5.10> or later to the current |
5369 | scope. | |
0d863452 | 5370 | |
a0d0e21e | 5371 | =item scalar EXPR |
d74e8afc | 5372 | X<scalar> X<context> |
a0d0e21e | 5373 | |
5a964f20 | 5374 | Forces EXPR to be interpreted in scalar context and returns the value |
54310121 | 5375 | of EXPR. |
cb1a09d0 AD |
5376 | |
5377 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
5378 | ||
54310121 | 5379 | There is no equivalent operator to force an expression to |
2b5ab1e7 | 5380 | be interpolated in list context because in practice, this is never |
cb1a09d0 AD |
5381 | needed. If you really wanted to do so, however, you could use |
5382 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
5383 | C<(some expression)> suffices. | |
a0d0e21e | 5384 | |
8f1da26d TC |
5385 | Because C<scalar> is a unary operator, if you accidentally use a |
5386 | parenthesized list for the EXPR, this behaves as a scalar comma expression, | |
5387 | evaluating all but the last element in void context and returning the final | |
5388 | element evaluated in scalar context. This is seldom what you want. | |
62c18ce2 GS |
5389 | |
5390 | The following single statement: | |
5391 | ||
5ed4f2ec | 5392 | print uc(scalar(&foo,$bar)),$baz; |
62c18ce2 GS |
5393 | |
5394 | is the moral equivalent of these two: | |
5395 | ||
5ed4f2ec | 5396 | &foo; |
5397 | print(uc($bar),$baz); | |
62c18ce2 GS |
5398 | |
5399 | See L<perlop> for more details on unary operators and the comma operator. | |
5400 | ||
a0d0e21e | 5401 | =item seek FILEHANDLE,POSITION,WHENCE |
d74e8afc | 5402 | X<seek> X<fseek> X<filehandle, position> |
a0d0e21e | 5403 | |
19799a22 | 5404 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. |
8903cb82 | 5405 | FILEHANDLE may be an expression whose value gives the name of the |
9124316e | 5406 | filehandle. The values for WHENCE are C<0> to set the new position |
8f1da26d TC |
5407 | I<in bytes> to POSITION; C<1> to set it to the current position plus |
5408 | POSITION; and C<2> to set it to EOF plus POSITION, typically | |
5409 | negative. For WHENCE you may use the constants C<SEEK_SET>, | |
9124316e | 5410 | C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end |
8f1da26d | 5411 | of the file) from the L<Fcntl> module. Returns C<1> on success, false |
9124316e JH |
5412 | otherwise. |
5413 | ||
5414 | Note the I<in bytes>: even if the filehandle has been set to | |
740d4bb2 | 5415 | operate on characters (for example by using the C<:encoding(utf8)> open |
fae2c0fb | 5416 | layer), tell() will return byte offsets, not character offsets |
9124316e | 5417 | (because implementing that would render seek() and tell() rather slow). |
8903cb82 | 5418 | |
3b10bc60 | 5419 | If you want to position the file for C<sysread> or C<syswrite>, don't use |
5420 | C<seek>, because buffering makes its effect on the file's read-write position | |
19799a22 | 5421 | unpredictable and non-portable. Use C<sysseek> instead. |
a0d0e21e | 5422 | |
2b5ab1e7 TC |
5423 | Due to the rules and rigors of ANSI C, on some systems you have to do a |
5424 | seek whenever you switch between reading and writing. Amongst other | |
5425 | things, this may have the effect of calling stdio's clearerr(3). | |
5426 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
cb1a09d0 AD |
5427 | |
5428 | seek(TEST,0,1); | |
5429 | ||
5430 | This is also useful for applications emulating C<tail -f>. Once you hit | |
3b10bc60 | 5431 | EOF on your read and then sleep for a while, you (probably) have to stick in a |
5432 | dummy seek() to reset things. The C<seek> doesn't change the position, | |
8903cb82 | 5433 | but it I<does> clear the end-of-file condition on the handle, so that the |
3b10bc60 | 5434 | next C<< <FILE> >> makes Perl try again to read something. (We hope.) |
cb1a09d0 | 5435 | |
3b10bc60 | 5436 | If that doesn't work (some I/O implementations are particularly |
5437 | cantankerous), you might need something like this: | |
cb1a09d0 AD |
5438 | |
5439 | for (;;) { | |
a9a5a0dc | 5440 | for ($curpos = tell(FILE); $_ = <FILE>; |
f86cebdf | 5441 | $curpos = tell(FILE)) { |
a9a5a0dc VP |
5442 | # search for some stuff and put it into files |
5443 | } | |
5444 | sleep($for_a_while); | |
5445 | seek(FILE, $curpos, 0); | |
cb1a09d0 AD |
5446 | } |
5447 | ||
a0d0e21e | 5448 | =item seekdir DIRHANDLE,POS |
d74e8afc | 5449 | X<seekdir> |
a0d0e21e | 5450 | |
19799a22 | 5451 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS |
cf264981 SP |
5452 | must be a value returned by C<telldir>. C<seekdir> also has the same caveats |
5453 | about possible directory compaction as the corresponding system library | |
a0d0e21e LW |
5454 | routine. |
5455 | ||
5456 | =item select FILEHANDLE | |
d74e8afc | 5457 | X<select> X<filehandle, default> |
a0d0e21e LW |
5458 | |
5459 | =item select | |
5460 | ||
b5dffda6 RGS |
5461 | Returns the currently selected filehandle. If FILEHANDLE is supplied, |
5462 | sets the new current default filehandle for output. This has two | |
8f1da26d | 5463 | effects: first, a C<write> or a C<print> without a filehandle |
a0d0e21e | 5464 | default to this FILEHANDLE. Second, references to variables related to |
8f1da26d TC |
5465 | output will refer to this output channel. |
5466 | ||
5467 | For example, to set the top-of-form format for more than one | |
5468 | output channel, you might do the following: | |
a0d0e21e LW |
5469 | |
5470 | select(REPORT1); | |
5471 | $^ = 'report1_top'; | |
5472 | select(REPORT2); | |
5473 | $^ = 'report2_top'; | |
5474 | ||
5475 | FILEHANDLE may be an expression whose value gives the name of the | |
5476 | actual filehandle. Thus: | |
5477 | ||
5478 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
5479 | ||
4633a7c4 LW |
5480 | Some programmers may prefer to think of filehandles as objects with |
5481 | methods, preferring to write the last example as: | |
a0d0e21e | 5482 | |
28757baa | 5483 | use IO::Handle; |
a0d0e21e LW |
5484 | STDERR->autoflush(1); |
5485 | ||
ea9eb35a BJ |
5486 | Portability issues: L<perlport/select>. |
5487 | ||
a0d0e21e | 5488 | =item select RBITS,WBITS,EBITS,TIMEOUT |
d74e8afc | 5489 | X<select> |
a0d0e21e | 5490 | |
3b10bc60 | 5491 | This calls the select(2) syscall with the bit masks specified, which |
19799a22 | 5492 | can be constructed using C<fileno> and C<vec>, along these lines: |
a0d0e21e LW |
5493 | |
5494 | $rin = $win = $ein = ''; | |
f0815dd4 TC |
5495 | vec($rin, fileno(STDIN), 1) = 1; |
5496 | vec($win, fileno(STDOUT), 1) = 1; | |
a0d0e21e LW |
5497 | $ein = $rin | $win; |
5498 | ||
3b10bc60 | 5499 | If you want to select on many filehandles, you may wish to write a |
5500 | subroutine like this: | |
a0d0e21e LW |
5501 | |
5502 | sub fhbits { | |
f0815dd4 TC |
5503 | my @fhlist = @_; |
5504 | my $bits = ""; | |
5505 | for my $fh (@fhlist) { | |
5506 | vec($bits, fileno($fh), 1) = 1; | |
a9a5a0dc | 5507 | } |
f0815dd4 | 5508 | return $bits; |
a0d0e21e | 5509 | } |
f0815dd4 | 5510 | $rin = fhbits(*STDIN, *TTY, *MYSOCK); |
a0d0e21e LW |
5511 | |
5512 | The usual idiom is: | |
5513 | ||
5514 | ($nfound,$timeleft) = | |
5515 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
5516 | ||
54310121 | 5517 | or to block until something becomes ready just do this |
a0d0e21e LW |
5518 | |
5519 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
5520 | ||
19799a22 GS |
5521 | Most systems do not bother to return anything useful in $timeleft, so |
5522 | calling select() in scalar context just returns $nfound. | |
c07a80fd | 5523 | |
5f05dabc | 5524 | Any of the bit masks can also be undef. The timeout, if specified, is |
a0d0e21e | 5525 | in seconds, which may be fractional. Note: not all implementations are |
be119125 | 5526 | capable of returning the $timeleft. If not, they always return |
19799a22 | 5527 | $timeleft equal to the supplied $timeout. |
a0d0e21e | 5528 | |
ff68c719 | 5529 | You can effect a sleep of 250 milliseconds this way: |
a0d0e21e LW |
5530 | |
5531 | select(undef, undef, undef, 0.25); | |
5532 | ||
b09fc1d8 | 5533 | Note that whether C<select> gets restarted after signals (say, SIGALRM) |
8b0ac1d7 MHM |
5534 | is implementation-dependent. See also L<perlport> for notes on the |
5535 | portability of C<select>. | |
40454f26 | 5536 | |
f0815dd4 | 5537 | On error, C<select> behaves just like select(2): it returns |
4189264e | 5538 | -1 and sets C<$!>. |
353e5636 | 5539 | |
8f1da26d TC |
5540 | On some Unixes, select(2) may report a socket file descriptor as "ready for |
5541 | reading" even when no data is available, and thus any subsequent C<read> | |
5542 | would block. This can be avoided if you always use O_NONBLOCK on the | |
5543 | socket. See select(2) and fcntl(2) for further details. | |
ec8ce15a | 5544 | |
f0815dd4 TC |
5545 | The standard C<IO::Select> module provides a user-friendlier interface |
5546 | to C<select>, mostly because it does all the bit-mask work for you. | |
5547 | ||
19799a22 | 5548 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> |
61eff3bc | 5549 | or <FH>) with C<select>, except as permitted by POSIX, and even |
19799a22 | 5550 | then only on POSIX systems. You have to use C<sysread> instead. |
a0d0e21e | 5551 | |
ea9eb35a BJ |
5552 | Portability issues: L<perlport/select>. |
5553 | ||
a0d0e21e | 5554 | =item semctl ID,SEMNUM,CMD,ARG |
d74e8afc | 5555 | X<semctl> |
a0d0e21e | 5556 | |
3b10bc60 | 5557 | Calls the System V IPC function semctl(2). You'll probably have to say |
0ade1984 JH |
5558 | |
5559 | use IPC::SysV; | |
5560 | ||
5561 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
cf264981 | 5562 | GETALL, then ARG must be a variable that will hold the returned |
e4038a1f MS |
5563 | semid_ds structure or semaphore value array. Returns like C<ioctl>: |
5564 | the undefined value for error, "C<0 but true>" for zero, or the actual | |
5565 | return value otherwise. The ARG must consist of a vector of native | |
106325ad | 5566 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. |
4755096e GS |
5567 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> |
5568 | documentation. | |
a0d0e21e | 5569 | |
ea9eb35a BJ |
5570 | Portability issues: L<perlport/semctl>. |
5571 | ||
a0d0e21e | 5572 | =item semget KEY,NSEMS,FLAGS |
d74e8afc | 5573 | X<semget> |
a0d0e21e | 5574 | |
3b10bc60 | 5575 | Calls the System V IPC function semget(2). Returns the semaphore id, or |
8f1da26d | 5576 | the undefined value on error. See also |
4755096e GS |
5577 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> |
5578 | documentation. | |
a0d0e21e | 5579 | |
ea9eb35a BJ |
5580 | Portability issues: L<perlport/semget>. |
5581 | ||
a0d0e21e | 5582 | =item semop KEY,OPSTRING |
d74e8afc | 5583 | X<semop> |
a0d0e21e | 5584 | |
80d38338 | 5585 | Calls the System V IPC function semop(2) for semaphore operations |
5354997a | 5586 | such as signalling and waiting. OPSTRING must be a packed array of |
a0d0e21e | 5587 | semop structures. Each semop structure can be generated with |
cf264981 SP |
5588 | C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING |
5589 | implies the number of semaphore operations. Returns true if | |
8f1da26d | 5590 | successful, false on error. As an example, the |
19799a22 | 5591 | following code waits on semaphore $semnum of semaphore id $semid: |
a0d0e21e | 5592 | |
f878ba33 | 5593 | $semop = pack("s!3", $semnum, -1, 0); |
a0d0e21e LW |
5594 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); |
5595 | ||
4755096e GS |
5596 | To signal the semaphore, replace C<-1> with C<1>. See also |
5597 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> | |
5598 | documentation. | |
a0d0e21e | 5599 | |
ea9eb35a BJ |
5600 | Portability issues: L<perlport/semop>. |
5601 | ||
a0d0e21e | 5602 | =item send SOCKET,MSG,FLAGS,TO |
d74e8afc | 5603 | X<send> |
a0d0e21e LW |
5604 | |
5605 | =item send SOCKET,MSG,FLAGS | |
5606 | ||
3b10bc60 | 5607 | Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET |
5608 | filehandle. Takes the same flags as the system call of the same name. On | |
5609 | unconnected sockets, you must specify a destination to I<send to>, in which | |
5610 | case it does a sendto(2) syscall. Returns the number of characters sent, | |
5611 | or the undefined value on error. The sendmsg(2) syscall is currently | |
5612 | unimplemented. See L<perlipc/"UDP: Message Passing"> for examples. | |
9124316e JH |
5613 | |
5614 | Note the I<characters>: depending on the status of the socket, either | |
5615 | (8-bit) bytes or characters are sent. By default all sockets operate | |
5616 | on bytes, but for example if the socket has been changed using | |
740d4bb2 JW |
5617 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see |
5618 | L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8 | |
5619 | encoded Unicode characters, not bytes. Similarly for the C<:encoding> | |
5620 | pragma: in that case pretty much any characters can be sent. | |
a0d0e21e LW |
5621 | |
5622 | =item setpgrp PID,PGRP | |
d74e8afc | 5623 | X<setpgrp> X<group> |
a0d0e21e | 5624 | |
7660c0ab | 5625 | Sets the current process group for the specified PID, C<0> for the current |
3b10bc60 | 5626 | process. Raises an exception when used on a machine that doesn't |
81777298 GS |
5627 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, |
5628 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
5629 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
5630 | C<POSIX::setsid()>. | |
a0d0e21e | 5631 | |
ea9eb35a BJ |
5632 | Portability issues: L<perlport/setpgrp>. |
5633 | ||
a0d0e21e | 5634 | =item setpriority WHICH,WHO,PRIORITY |
d74e8afc | 5635 | X<setpriority> X<priority> X<nice> X<renice> |
a0d0e21e LW |
5636 | |
5637 | Sets the current priority for a process, a process group, or a user. | |
3b10bc60 | 5638 | (See setpriority(2).) Raises an exception when used on a machine |
f86cebdf | 5639 | that doesn't implement setpriority(2). |
a0d0e21e | 5640 | |
ea9eb35a BJ |
5641 | Portability issues: L<perlport/setpriority>. |
5642 | ||
a0d0e21e | 5643 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL |
d74e8afc | 5644 | X<setsockopt> |
a0d0e21e | 5645 | |
8f1da26d TC |
5646 | Sets the socket option requested. Returns C<undef> on error. |
5647 | Use integer constants provided by the C<Socket> module for | |
23d0437f GA |
5648 | LEVEL and OPNAME. Values for LEVEL can also be obtained from |
5649 | getprotobyname. OPTVAL might either be a packed string or an integer. | |
5650 | An integer OPTVAL is shorthand for pack("i", OPTVAL). | |
5651 | ||
3b10bc60 | 5652 | An example disabling Nagle's algorithm on a socket: |
23d0437f GA |
5653 | |
5654 | use Socket qw(IPPROTO_TCP TCP_NODELAY); | |
5655 | setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); | |
a0d0e21e | 5656 | |
ea9eb35a BJ |
5657 | Portability issues: L<perlport/setsockopt>. |
5658 | ||
532eee96 | 5659 | =item shift ARRAY |
d74e8afc | 5660 | X<shift> |
a0d0e21e | 5661 | |
f5a93a43 TC |
5662 | =item shift EXPR |
5663 | ||
a0d0e21e LW |
5664 | =item shift |
5665 | ||
5666 | Shifts the first value of the array off and returns it, shortening the | |
5667 | array by 1 and moving everything down. If there are no elements in the | |
5668 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
7660c0ab | 5669 | C<@_> array within the lexical scope of subroutines and formats, and the |
80d38338 | 5670 | C<@ARGV> array outside a subroutine and also within the lexical scopes |
3c10abe3 | 5671 | established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, |
8f1da26d | 5672 | C<UNITCHECK {}>, and C<END {}> constructs. |
4f25aa18 | 5673 | |
f5a93a43 TC |
5674 | Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a |
5675 | reference to an unblessed array. The argument will be dereferenced | |
5676 | automatically. This aspect of C<shift> is considered highly experimental. | |
5677 | The exact behaviour may change in a future version of Perl. | |
cba5a3b0 | 5678 | |
a1b2c429 | 5679 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the |
19799a22 | 5680 | same thing to the left end of an array that C<pop> and C<push> do to the |
977336f5 | 5681 | right end. |
a0d0e21e LW |
5682 | |
5683 | =item shmctl ID,CMD,ARG | |
d74e8afc | 5684 | X<shmctl> |
a0d0e21e | 5685 | |
0ade1984 JH |
5686 | Calls the System V IPC function shmctl. You'll probably have to say |
5687 | ||
5688 | use IPC::SysV; | |
5689 | ||
7660c0ab | 5690 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
cf264981 | 5691 | then ARG must be a variable that will hold the returned C<shmid_ds> |
8f1da26d TC |
5692 | structure. Returns like ioctl: C<undef> for error; "C<0> but |
5693 | true" for zero; and the actual return value otherwise. | |
4755096e | 5694 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e | 5695 | |
ea9eb35a BJ |
5696 | Portability issues: L<perlport/shmctl>. |
5697 | ||
a0d0e21e | 5698 | =item shmget KEY,SIZE,FLAGS |
d74e8afc | 5699 | X<shmget> |
a0d0e21e LW |
5700 | |
5701 | Calls the System V IPC function shmget. Returns the shared memory | |
8f1da26d | 5702 | segment id, or C<undef> on error. |
4755096e | 5703 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e | 5704 | |
ea9eb35a BJ |
5705 | Portability issues: L<perlport/shmget>. |
5706 | ||
a0d0e21e | 5707 | =item shmread ID,VAR,POS,SIZE |
d74e8afc ITB |
5708 | X<shmread> |
5709 | X<shmwrite> | |
a0d0e21e LW |
5710 | |
5711 | =item shmwrite ID,STRING,POS,SIZE | |
5712 | ||
5713 | Reads or writes the System V shared memory segment ID starting at | |
5714 | position POS for size SIZE by attaching to it, copying in/out, and | |
5a964f20 | 5715 | detaching from it. When reading, VAR must be a variable that will |
a0d0e21e LW |
5716 | hold the data read. When writing, if STRING is too long, only SIZE |
5717 | bytes are used; if STRING is too short, nulls are written to fill out | |
8f1da26d | 5718 | SIZE bytes. Return true if successful, false on error. |
4755096e | 5719 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, |
8f1da26d | 5720 | C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN. |
a0d0e21e | 5721 | |
ea9eb35a BJ |
5722 | Portability issues: L<perlport/shmread> and L<perlport/shmwrite>. |
5723 | ||
a0d0e21e | 5724 | =item shutdown SOCKET,HOW |
d74e8afc | 5725 | X<shutdown> |
a0d0e21e LW |
5726 | |
5727 | Shuts down a socket connection in the manner indicated by HOW, which | |
3b10bc60 | 5728 | has the same interpretation as in the syscall of the same name. |
a0d0e21e | 5729 | |
f86cebdf GS |
5730 | shutdown(SOCKET, 0); # I/we have stopped reading data |
5731 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
5732 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
5a964f20 TC |
5733 | |
5734 | This is useful with sockets when you want to tell the other | |
5735 | side you're done writing but not done reading, or vice versa. | |
b76cc8ba | 5736 | It's also a more insistent form of close because it also |
19799a22 | 5737 | disables the file descriptor in any forked copies in other |
5a964f20 TC |
5738 | processes. |
5739 | ||
3b10bc60 | 5740 | Returns C<1> for success; on error, returns C<undef> if |
f126b98b PF |
5741 | the first argument is not a valid filehandle, or returns C<0> and sets |
5742 | C<$!> for any other failure. | |
5743 | ||
a0d0e21e | 5744 | =item sin EXPR |
d74e8afc | 5745 | X<sin> X<sine> X<asin> X<arcsine> |
a0d0e21e | 5746 | |
54310121 | 5747 | =item sin |
bbce6d69 | 5748 | |
a0d0e21e | 5749 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 5750 | returns sine of C<$_>. |
a0d0e21e | 5751 | |
ca6e1c26 | 5752 | For the inverse sine operation, you may use the C<Math::Trig::asin> |
28757baa | 5753 | function, or use this relation: |
5754 | ||
5755 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
5756 | ||
a0d0e21e | 5757 | =item sleep EXPR |
d74e8afc | 5758 | X<sleep> X<pause> |
a0d0e21e LW |
5759 | |
5760 | =item sleep | |
5761 | ||
80d38338 TC |
5762 | Causes the script to sleep for (integer) EXPR seconds, or forever if no |
5763 | argument is given. Returns the integer number of seconds actually slept. | |
b48653af | 5764 | |
7660c0ab | 5765 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
b48653af MS |
5766 | |
5767 | eval { | |
5768 | local $SIG{ALARM} = sub { die "Alarm!\n" }; | |
5769 | sleep; | |
5770 | }; | |
5771 | die $@ unless $@ eq "Alarm!\n"; | |
5772 | ||
5773 | You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep> | |
5774 | is often implemented using C<alarm>. | |
a0d0e21e LW |
5775 | |
5776 | On some older systems, it may sleep up to a full second less than what | |
5777 | you requested, depending on how it counts seconds. Most modern systems | |
5a964f20 TC |
5778 | always sleep the full amount. They may appear to sleep longer than that, |
5779 | however, because your process might not be scheduled right away in a | |
5780 | busy multitasking system. | |
a0d0e21e | 5781 | |
2bc69794 BS |
5782 | For delays of finer granularity than one second, the Time::HiRes module |
5783 | (from CPAN, and starting from Perl 5.8 part of the standard | |
5784 | distribution) provides usleep(). You may also use Perl's four-argument | |
5785 | version of select() leaving the first three arguments undefined, or you | |
5786 | might be able to use the C<syscall> interface to access setitimer(2) if | |
5787 | your system supports it. See L<perlfaq8> for details. | |
cb1a09d0 | 5788 | |
b6e2112e | 5789 | See also the POSIX module's C<pause> function. |
5f05dabc | 5790 | |
a0d0e21e | 5791 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
d74e8afc | 5792 | X<socket> |
a0d0e21e LW |
5793 | |
5794 | Opens a socket of the specified kind and attaches it to filehandle | |
19799a22 | 5795 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for |
3b10bc60 | 5796 | the syscall of the same name. You should C<use Socket> first |
19799a22 GS |
5797 | to get the proper definitions imported. See the examples in |
5798 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 5799 | |
8d2a6795 GS |
5800 | On systems that support a close-on-exec flag on files, the flag will |
5801 | be set for the newly opened file descriptor, as determined by the | |
5802 | value of $^F. See L<perlvar/$^F>. | |
5803 | ||
a0d0e21e | 5804 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
d74e8afc | 5805 | X<socketpair> |
a0d0e21e LW |
5806 | |
5807 | Creates an unnamed pair of sockets in the specified domain, of the | |
5f05dabc | 5808 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
3b10bc60 | 5809 | for the syscall of the same name. If unimplemented, raises an exception. |
5810 | Returns true if successful. | |
a0d0e21e | 5811 | |
8d2a6795 GS |
5812 | On systems that support a close-on-exec flag on files, the flag will |
5813 | be set for the newly opened file descriptors, as determined by the value | |
5814 | of $^F. See L<perlvar/$^F>. | |
5815 | ||
19799a22 | 5816 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call |
5a964f20 TC |
5817 | to C<pipe(Rdr, Wtr)> is essentially: |
5818 | ||
5819 | use Socket; | |
5820 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
5821 | shutdown(Rdr, 1); # no more writing for reader | |
5822 | shutdown(Wtr, 0); # no more reading for writer | |
5823 | ||
02fc2eee NC |
5824 | See L<perlipc> for an example of socketpair use. Perl 5.8 and later will |
5825 | emulate socketpair using IP sockets to localhost if your system implements | |
5826 | sockets but not socketpair. | |
5a964f20 | 5827 | |
ea9eb35a BJ |
5828 | Portability issues: L<perlport/socketpair>. |
5829 | ||
a0d0e21e | 5830 | =item sort SUBNAME LIST |
d74e8afc | 5831 | X<sort> X<qsort> X<quicksort> X<mergesort> |
a0d0e21e LW |
5832 | |
5833 | =item sort BLOCK LIST | |
5834 | ||
5835 | =item sort LIST | |
5836 | ||
41d39f30 | 5837 | In list context, this sorts the LIST and returns the sorted list value. |
9fdc1d08 | 5838 | In scalar context, the behaviour of C<sort()> is undefined. |
41d39f30 A |
5839 | |
5840 | If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison | |
5841 | order. If SUBNAME is specified, it gives the name of a subroutine | |
5842 | that returns an integer less than, equal to, or greater than C<0>, | |
3b10bc60 | 5843 | depending on how the elements of the list are to be ordered. (The |
5844 | C<< <=> >> and C<cmp> operators are extremely useful in such routines.) | |
41d39f30 A |
5845 | SUBNAME may be a scalar variable name (unsubscripted), in which case |
5846 | the value provides the name of (or a reference to) the actual | |
5847 | subroutine to use. In place of a SUBNAME, you can provide a BLOCK as | |
5848 | an anonymous, in-line sort subroutine. | |
a0d0e21e | 5849 | |
8f1da26d TC |
5850 | If the subroutine's prototype is C<($$)>, the elements to be compared are |
5851 | passed by reference in C<@_>, as for a normal subroutine. This is slower | |
5852 | than unprototyped subroutines, where the elements to be compared are passed | |
5853 | into the subroutine as the package global variables $a and $b (see example | |
5854 | below). Note that in the latter case, it is usually highly counter-productive | |
5855 | to declare $a and $b as lexicals. | |
43481408 | 5856 | |
c106e8bb RH |
5857 | The values to be compared are always passed by reference and should not |
5858 | be modified. | |
a0d0e21e | 5859 | |
0a753a76 | 5860 | You also cannot exit out of the sort block or subroutine using any of the |
19799a22 | 5861 | loop control operators described in L<perlsyn> or with C<goto>. |
0a753a76 | 5862 | |
a034a98d DD |
5863 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
5864 | current collation locale. See L<perllocale>. | |
5865 | ||
db5021a3 MS |
5866 | sort() returns aliases into the original list, much as a for loop's index |
5867 | variable aliases the list elements. That is, modifying an element of a | |
5868 | list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>) | |
5869 | actually modifies the element in the original list. This is usually | |
5870 | something to be avoided when writing clear code. | |
5871 | ||
58c7fc7c | 5872 | Perl 5.6 and earlier used a quicksort algorithm to implement sort. |
8f1da26d | 5873 | That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort |
58c7fc7c JH |
5874 | preserves the input order of elements that compare equal. Although |
5875 | quicksort's run time is O(NlogN) when averaged over all arrays of | |
5876 | length N, the time can be O(N**2), I<quadratic> behavior, for some | |
5877 | inputs.) In 5.7, the quicksort implementation was replaced with | |
cf264981 | 5878 | a stable mergesort algorithm whose worst-case behavior is O(NlogN). |
58c7fc7c JH |
5879 | But benchmarks indicated that for some inputs, on some platforms, |
5880 | the original quicksort was faster. 5.8 has a sort pragma for | |
5881 | limited control of the sort. Its rather blunt control of the | |
cf264981 | 5882 | underlying algorithm may not persist into future Perls, but the |
58c7fc7c | 5883 | ability to characterize the input or output in implementation |
c25fe68d | 5884 | independent ways quite probably will. See L<the sort pragma|sort>. |
c16425f1 | 5885 | |
a0d0e21e LW |
5886 | Examples: |
5887 | ||
5888 | # sort lexically | |
5889 | @articles = sort @files; | |
e1d16ab7 | 5890 | |
a0d0e21e LW |
5891 | # same thing, but with explicit sort routine |
5892 | @articles = sort {$a cmp $b} @files; | |
e1d16ab7 | 5893 | |
cb1a09d0 | 5894 | # now case-insensitively |
54310121 | 5895 | @articles = sort {uc($a) cmp uc($b)} @files; |
e1d16ab7 | 5896 | |
a0d0e21e LW |
5897 | # same thing in reversed order |
5898 | @articles = sort {$b cmp $a} @files; | |
e1d16ab7 | 5899 | |
a0d0e21e LW |
5900 | # sort numerically ascending |
5901 | @articles = sort {$a <=> $b} @files; | |
e1d16ab7 | 5902 | |
a0d0e21e LW |
5903 | # sort numerically descending |
5904 | @articles = sort {$b <=> $a} @files; | |
e1d16ab7 | 5905 | |
19799a22 GS |
5906 | # this sorts the %age hash by value instead of key |
5907 | # using an in-line function | |
5908 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
e1d16ab7 | 5909 | |
a0d0e21e LW |
5910 | # sort using explicit subroutine name |
5911 | sub byage { | |
4d0444a3 | 5912 | $age{$a} <=> $age{$b}; # presuming numeric |
a0d0e21e LW |
5913 | } |
5914 | @sortedclass = sort byage @class; | |
e1d16ab7 | 5915 | |
19799a22 GS |
5916 | sub backwards { $b cmp $a } |
5917 | @harry = qw(dog cat x Cain Abel); | |
5918 | @george = qw(gone chased yz Punished Axed); | |
a0d0e21e | 5919 | print sort @harry; |
e1d16ab7 | 5920 | # prints AbelCaincatdogx |
a0d0e21e | 5921 | print sort backwards @harry; |
e1d16ab7 | 5922 | # prints xdogcatCainAbel |
a0d0e21e | 5923 | print sort @george, 'to', @harry; |
e1d16ab7 | 5924 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz |
a0d0e21e | 5925 | |
54310121 | 5926 | # inefficiently sort by descending numeric compare using |
5927 | # the first integer after the first = sign, or the | |
cb1a09d0 AD |
5928 | # whole record case-insensitively otherwise |
5929 | ||
e1d16ab7 | 5930 | my @new = sort { |
5931 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
4d0444a3 FC |
5932 | || |
5933 | uc($a) cmp uc($b) | |
cb1a09d0 AD |
5934 | } @old; |
5935 | ||
5936 | # same thing, but much more efficiently; | |
5937 | # we'll build auxiliary indices instead | |
5938 | # for speed | |
e1d16ab7 | 5939 | my @nums = @caps = (); |
54310121 | 5940 | for (@old) { |
e1d16ab7 | 5941 | push @nums, ( /=(\d+)/ ? $1 : undef ); |
5942 | push @caps, uc($_); | |
54310121 | 5943 | } |
cb1a09d0 | 5944 | |
e1d16ab7 | 5945 | my @new = @old[ sort { |
4d0444a3 FC |
5946 | $nums[$b] <=> $nums[$a] |
5947 | || | |
5948 | $caps[$a] cmp $caps[$b] | |
5949 | } 0..$#old | |
5950 | ]; | |
cb1a09d0 | 5951 | |
19799a22 | 5952 | # same thing, but without any temps |
cb1a09d0 | 5953 | @new = map { $_->[0] } |
19799a22 | 5954 | sort { $b->[1] <=> $a->[1] |
4d0444a3 FC |
5955 | || |
5956 | $a->[2] cmp $b->[2] | |
5957 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
61eff3bc | 5958 | |
43481408 GS |
5959 | # using a prototype allows you to use any comparison subroutine |
5960 | # as a sort subroutine (including other package's subroutines) | |
5961 | package other; | |
5ed4f2ec | 5962 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here |
e1d16ab7 | 5963 | |
43481408 GS |
5964 | package main; |
5965 | @new = sort other::backwards @old; | |
e1d16ab7 | 5966 | |
58c7fc7c JH |
5967 | # guarantee stability, regardless of algorithm |
5968 | use sort 'stable'; | |
5969 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
e1d16ab7 | 5970 | |
268e9d79 JL |
5971 | # force use of mergesort (not portable outside Perl 5.8) |
5972 | use sort '_mergesort'; # note discouraging _ | |
58c7fc7c | 5973 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; |
58c7fc7c | 5974 | |
1cb246e8 RGS |
5975 | Warning: syntactical care is required when sorting the list returned from |
5976 | a function. If you want to sort the list returned by the function call | |
5977 | C<find_records(@key)>, you can use: | |
a9320c62 | 5978 | |
a9320c62 B |
5979 | @contact = sort { $a cmp $b } find_records @key; |
5980 | @contact = sort +find_records(@key); | |
5981 | @contact = sort &find_records(@key); | |
5982 | @contact = sort(find_records(@key)); | |
5983 | ||
5984 | If instead you want to sort the array @key with the comparison routine | |
1cb246e8 RGS |
5985 | C<find_records()> then you can use: |
5986 | ||
a9320c62 B |
5987 | @contact = sort { find_records() } @key; |
5988 | @contact = sort find_records(@key); | |
5989 | @contact = sort(find_records @key); | |
5990 | @contact = sort(find_records (@key)); | |
5991 | ||
19799a22 GS |
5992 | If you're using strict, you I<must not> declare $a |
5993 | and $b as lexicals. They are package globals. That means | |
1cb246e8 | 5994 | that if you're in the C<main> package and type |
13a2d996 | 5995 | |
47223a36 | 5996 | @articles = sort {$b <=> $a} @files; |
13a2d996 | 5997 | |
47223a36 JH |
5998 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), |
5999 | but if you're in the C<FooPack> package, it's the same as typing | |
cb1a09d0 AD |
6000 | |
6001 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
6002 | ||
55497cff | 6003 | The comparison function is required to behave. If it returns |
7660c0ab A |
6004 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
6005 | sometimes saying the opposite, for example) the results are not | |
6006 | well-defined. | |
55497cff | 6007 | |
03190201 | 6008 | Because C<< <=> >> returns C<undef> when either operand is C<NaN> |
8f1da26d TC |
6009 | (not-a-number), and laso because C<sort> raises an exception unless the |
6010 | result of a comparison is defined, be careful when sorting with a | |
6011 | comparison function like C<< $a <=> $b >> any lists that might contain a | |
6012 | C<NaN>. The following example takes advantage that C<NaN != NaN> to | |
3b10bc60 | 6013 | eliminate any C<NaN>s from the input list. |
03190201 JL |
6014 | |
6015 | @result = sort { $a <=> $b } grep { $_ == $_ } @input; | |
6016 | ||
f5a93a43 | 6017 | =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST |
d74e8afc | 6018 | X<splice> |
a0d0e21e | 6019 | |
f5a93a43 | 6020 | =item splice ARRAY or EXPR,OFFSET,LENGTH |
a0d0e21e | 6021 | |
f5a93a43 | 6022 | =item splice ARRAY or EXPR,OFFSET |
a0d0e21e | 6023 | |
f5a93a43 | 6024 | =item splice ARRAY or EXPR |
453f9044 | 6025 | |
a0d0e21e | 6026 | Removes the elements designated by OFFSET and LENGTH from an array, and |
5a964f20 TC |
6027 | replaces them with the elements of LIST, if any. In list context, |
6028 | returns the elements removed from the array. In scalar context, | |
43051805 | 6029 | returns the last element removed, or C<undef> if no elements are |
48cdf507 | 6030 | removed. The array grows or shrinks as necessary. |
19799a22 | 6031 | If OFFSET is negative then it starts that far from the end of the array. |
48cdf507 | 6032 | If LENGTH is omitted, removes everything from OFFSET onward. |
d0920e03 MJD |
6033 | If LENGTH is negative, removes the elements from OFFSET onward |
6034 | except for -LENGTH elements at the end of the array. | |
8cbc2e3b | 6035 | If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is |
3b10bc60 | 6036 | past the end of the array, Perl issues a warning, and splices at the |
8cbc2e3b | 6037 | end of the array. |
453f9044 | 6038 | |
3272a53d | 6039 | The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> ) |
a0d0e21e | 6040 | |
5ed4f2ec | 6041 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
6042 | pop(@a) splice(@a,-1) | |
6043 | shift(@a) splice(@a,0,1) | |
6044 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
6045 | $a[$i] = $y splice(@a,$i,1,$y) | |
a0d0e21e LW |
6046 | |
6047 | Example, assuming array lengths are passed before arrays: | |
6048 | ||
5ed4f2ec | 6049 | sub aeq { # compare two list values |
a9a5a0dc VP |
6050 | my(@a) = splice(@_,0,shift); |
6051 | my(@b) = splice(@_,0,shift); | |
6052 | return 0 unless @a == @b; # same len? | |
6053 | while (@a) { | |
6054 | return 0 if pop(@a) ne pop(@b); | |
6055 | } | |
6056 | return 1; | |
a0d0e21e LW |
6057 | } |
6058 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
6059 | ||
f5a93a43 TC |
6060 | Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a |
6061 | reference to an unblessed array. The argument will be dereferenced | |
6062 | automatically. This aspect of C<splice> is considered highly experimental. | |
6063 | The exact behaviour may change in a future version of Perl. | |
532eee96 | 6064 | |
a0d0e21e | 6065 | =item split /PATTERN/,EXPR,LIMIT |
d74e8afc | 6066 | X<split> |
a0d0e21e LW |
6067 | |
6068 | =item split /PATTERN/,EXPR | |
6069 | ||
6070 | =item split /PATTERN/ | |
6071 | ||
6072 | =item split | |
6073 | ||
b2e26e6e DJ |
6074 | Splits the string EXPR into a list of strings and returns that list. By |
6075 | default, empty leading fields are preserved, and empty trailing ones are | |
ab7ee80f | 6076 | deleted. (If all fields are empty, they are considered to be trailing.) |
a0d0e21e | 6077 | |
a6d8037e | 6078 | In scalar context, returns the number of fields found. |
a0d0e21e | 6079 | |
7660c0ab | 6080 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
4633a7c4 LW |
6081 | splits on whitespace (after skipping any leading whitespace). Anything |
6082 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
fb73857a | 6083 | that the delimiter may be longer than one character.) |
6084 | ||
836e0ee7 | 6085 | If LIMIT is specified and positive, it represents the maximum number |
e833de1e BS |
6086 | of fields the EXPR will be split into, though the actual number of |
6087 | fields returned depends on the number of times PATTERN matches within | |
6088 | EXPR. If LIMIT is unspecified or zero, trailing null fields are | |
6089 | stripped (which potential users of C<pop> would do well to remember). | |
6090 | If LIMIT is negative, it is treated as if an arbitrarily large LIMIT | |
6091 | had been specified. Note that splitting an EXPR that evaluates to the | |
6092 | empty string always returns the empty list, regardless of the LIMIT | |
6093 | specified. | |
a0d0e21e | 6094 | |
3b10bc60 | 6095 | A pattern matching the empty string (not to be confused with |
6096 | an empty pattern C<//>, which is just one member of the set of patterns | |
6097 | matching the epmty string), splits EXPR into individual | |
6098 | characters. For example: | |
a0d0e21e | 6099 | |
8241c1c0 | 6100 | print join(':', split(/ */, 'hi there')), "\n"; |
a0d0e21e LW |
6101 | |
6102 | produces the output 'h:i:t:h:e:r:e'. | |
6103 | ||
3b10bc60 | 6104 | As a special case for C<split>, the empty pattern C<//> specifically |
6105 | matches the empty string; this is not be confused with the normal use | |
6106 | of an empty pattern to mean the last successful match. So to split | |
6107 | a string into individual characters, the following: | |
6de67870 | 6108 | |
8241c1c0 | 6109 | print join(':', split(//, 'hi there')), "\n"; |
52ea55c9 | 6110 | |
de5763b0 | 6111 | produces the output 'h:i: :t:h:e:r:e'. |
52ea55c9 | 6112 | |
12977212 FC |
6113 | Empty leading fields are produced when there are positive-width matches at |
6114 | the beginning of the string; a zero-width match at the beginning of | |
6115 | the string does not produce an empty field. For example: | |
0156e0fd RB |
6116 | |
6117 | print join(':', split(/(?=\w)/, 'hi there!')); | |
6118 | ||
12977212 FC |
6119 | produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other |
6120 | hand, are produced when there is a match at the end of the string (and | |
6121 | when LIMIT is given and is not 0), regardless of the length of the match. | |
6122 | For example: | |
6123 | ||
8241c1c0 B |
6124 | print join(':', split(//, 'hi there!', -1)), "\n"; |
6125 | print join(':', split(/\W/, 'hi there!', -1)), "\n"; | |
12977212 FC |
6126 | |
6127 | produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively, | |
6128 | both with an empty trailing field. | |
0156e0fd | 6129 | |
5f05dabc | 6130 | The LIMIT parameter can be used to split a line partially |
a0d0e21e LW |
6131 | |
6132 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
6133 | ||
b5da07fd TB |
6134 | When assigning to a list, if LIMIT is omitted, or zero, Perl supplies |
6135 | a LIMIT one larger than the number of variables in the list, to avoid | |
a0d0e21e LW |
6136 | unnecessary work. For the list above LIMIT would have been 4 by |
6137 | default. In time critical applications it behooves you not to split | |
6138 | into more fields than you really need. | |
6139 | ||
19799a22 | 6140 | If the PATTERN contains parentheses, additional list elements are |
a0d0e21e LW |
6141 | created from each matching substring in the delimiter. |
6142 | ||
da0045b7 | 6143 | split(/([,-])/, "1-10,20", 3); |
a0d0e21e LW |
6144 | |
6145 | produces the list value | |
6146 | ||
6147 | (1, '-', 10, ',', 20) | |
6148 | ||
19799a22 | 6149 | If you had the entire header of a normal Unix email message in $header, |
4633a7c4 LW |
6150 | you could split it up into fields and their values this way: |
6151 | ||
9f4b9cd0 | 6152 | $header =~ s/\n(?=\s)//g; # fix continuation lines |
fb73857a | 6153 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
4633a7c4 | 6154 | |
a0d0e21e LW |
6155 | The pattern C</PATTERN/> may be replaced with an expression to specify |
6156 | patterns that vary at runtime. (To do runtime compilation only once, | |
748a9306 LW |
6157 | use C</$variable/o>.) |
6158 | ||
5da728e2 A |
6159 | As a special case, specifying a PATTERN of space (S<C<' '>>) will split on |
6160 | white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can | |
6161 | be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>> | |
3b10bc60 | 6162 | will give you as many initial null fields (empty string) as there are leading spaces. |
5da728e2 | 6163 | A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading |
19799a22 | 6164 | whitespace produces a null first field. A C<split> with no arguments |
5da728e2 | 6165 | really does a S<C<split(' ', $_)>> internally. |
a0d0e21e | 6166 | |
cc50a203 | 6167 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't |
1ec94568 MG |
6168 | much use otherwise. |
6169 | ||
a0d0e21e LW |
6170 | Example: |
6171 | ||
5a964f20 TC |
6172 | open(PASSWD, '/etc/passwd'); |
6173 | while (<PASSWD>) { | |
5b3eff12 MS |
6174 | chomp; |
6175 | ($login, $passwd, $uid, $gid, | |
f86cebdf | 6176 | $gcos, $home, $shell) = split(/:/); |
a9a5a0dc | 6177 | #... |
a0d0e21e LW |
6178 | } |
6179 | ||
6de67870 JP |
6180 | As with regular pattern matching, any capturing parentheses that are not |
6181 | matched in a C<split()> will be set to C<undef> when returned: | |
6182 | ||
6183 | @fields = split /(A)|B/, "1A2B3"; | |
6184 | # @fields is (1, 'A', 2, undef, 3) | |
a0d0e21e | 6185 | |
5f05dabc | 6186 | =item sprintf FORMAT, LIST |
d74e8afc | 6187 | X<sprintf> |
a0d0e21e | 6188 | |
6662521e GS |
6189 | Returns a string formatted by the usual C<printf> conventions of the C |
6190 | library function C<sprintf>. See below for more details | |
01aa884e | 6191 | and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of |
6662521e GS |
6192 | the general principles. |
6193 | ||
6194 | For example: | |
6195 | ||
6196 | # Format number with up to 8 leading zeroes | |
6197 | $result = sprintf("%08d", $number); | |
6198 | ||
6199 | # Round number to 3 digits after decimal point | |
6200 | $rounded = sprintf("%.3f", $number); | |
74a77017 | 6201 | |
3b10bc60 | 6202 | Perl does its own C<sprintf> formatting: it emulates the C |
6203 | function sprintf(3), but doesn't use it except for floating-point | |
6204 | numbers, and even then only standard modifiers are allowed. | |
6205 | Non-standard extensions in your local sprintf(3) are | |
6206 | therefore unavailable from Perl. | |
74a77017 | 6207 | |
194e7b38 DC |
6208 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you |
6209 | pass it an array as your first argument. The array is given scalar context, | |
6210 | and instead of using the 0th element of the array as the format, Perl will | |
6211 | use the count of elements in the array as the format, which is almost never | |
6212 | useful. | |
6213 | ||
19799a22 | 6214 | Perl's C<sprintf> permits the following universally-known conversions: |
74a77017 | 6215 | |
5ed4f2ec | 6216 | %% a percent sign |
6217 | %c a character with the given number | |
6218 | %s a string | |
6219 | %d a signed integer, in decimal | |
6220 | %u an unsigned integer, in decimal | |
6221 | %o an unsigned integer, in octal | |
6222 | %x an unsigned integer, in hexadecimal | |
6223 | %e a floating-point number, in scientific notation | |
6224 | %f a floating-point number, in fixed decimal notation | |
6225 | %g a floating-point number, in %e or %f notation | |
74a77017 | 6226 | |
1b3f7d21 | 6227 | In addition, Perl permits the following widely-supported conversions: |
74a77017 | 6228 | |
5ed4f2ec | 6229 | %X like %x, but using upper-case letters |
6230 | %E like %e, but using an upper-case "E" | |
6231 | %G like %g, but with an upper-case "E" (if applicable) | |
6232 | %b an unsigned integer, in binary | |
6233 | %B like %b, but using an upper-case "B" with the # flag | |
6234 | %p a pointer (outputs the Perl value's address in hexadecimal) | |
6235 | %n special: *stores* the number of characters output so far | |
4d0444a3 | 6236 | into the next variable in the parameter list |
74a77017 | 6237 | |
1b3f7d21 CS |
6238 | Finally, for backward (and we do mean "backward") compatibility, Perl |
6239 | permits these unnecessary but widely-supported conversions: | |
74a77017 | 6240 | |
5ed4f2ec | 6241 | %i a synonym for %d |
6242 | %D a synonym for %ld | |
6243 | %U a synonym for %lu | |
6244 | %O a synonym for %lo | |
6245 | %F a synonym for %f | |
74a77017 | 6246 | |
7b8dd722 HS |
6247 | Note that the number of exponent digits in the scientific notation produced |
6248 | by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the | |
b73fd64e JH |
6249 | exponent less than 100 is system-dependent: it may be three or less |
6250 | (zero-padded as necessary). In other words, 1.23 times ten to the | |
6251 | 99th may be either "1.23e99" or "1.23e099". | |
d764f01a | 6252 | |
80d38338 | 6253 | Between the C<%> and the format letter, you may specify several |
7b8dd722 HS |
6254 | additional attributes controlling the interpretation of the format. |
6255 | In order, these are: | |
74a77017 | 6256 | |
7b8dd722 HS |
6257 | =over 4 |
6258 | ||
6259 | =item format parameter index | |
6260 | ||
6261 | An explicit format parameter index, such as C<2$>. By default sprintf | |
6262 | will format the next unused argument in the list, but this allows you | |
3b10bc60 | 6263 | to take the arguments out of order: |
7b8dd722 HS |
6264 | |
6265 | printf '%2$d %1$d', 12, 34; # prints "34 12" | |
6266 | printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" | |
6267 | ||
6268 | =item flags | |
6269 | ||
6270 | one or more of: | |
e6bb52fd | 6271 | |
7a81c58e A |
6272 | space prefix non-negative number with a space |
6273 | + prefix non-negative number with a plus sign | |
74a77017 CS |
6274 | - left-justify within the field |
6275 | 0 use zeros, not spaces, to right-justify | |
e6bb52fd TS |
6276 | # ensure the leading "0" for any octal, |
6277 | prefix non-zero hexadecimal with "0x" or "0X", | |
6278 | prefix non-zero binary with "0b" or "0B" | |
7b8dd722 HS |
6279 | |
6280 | For example: | |
6281 | ||
e6bb52fd TS |
6282 | printf '<% d>', 12; # prints "< 12>" |
6283 | printf '<%+d>', 12; # prints "<+12>" | |
6284 | printf '<%6s>', 12; # prints "< 12>" | |
6285 | printf '<%-6s>', 12; # prints "<12 >" | |
6286 | printf '<%06s>', 12; # prints "<000012>" | |
6287 | printf '<%#o>', 12; # prints "<014>" | |
6288 | printf '<%#x>', 12; # prints "<0xc>" | |
6289 | printf '<%#X>', 12; # prints "<0XC>" | |
6290 | printf '<%#b>', 12; # prints "<0b1100>" | |
6291 | printf '<%#B>', 12; # prints "<0B1100>" | |
7b8dd722 | 6292 | |
9911cee9 TS |
6293 | When a space and a plus sign are given as the flags at once, |
6294 | a plus sign is used to prefix a positive number. | |
6295 | ||
6296 | printf '<%+ d>', 12; # prints "<+12>" | |
6297 | printf '<% +d>', 12; # prints "<+12>" | |
6298 | ||
e6bb52fd TS |
6299 | When the # flag and a precision are given in the %o conversion, |
6300 | the precision is incremented if it's necessary for the leading "0". | |
6301 | ||
6302 | printf '<%#.5o>', 012; # prints "<00012>" | |
6303 | printf '<%#.5o>', 012345; # prints "<012345>" | |
6304 | printf '<%#.0o>', 0; # prints "<0>" | |
6305 | ||
7b8dd722 HS |
6306 | =item vector flag |
6307 | ||
3b10bc60 | 6308 | This flag tells Perl to interpret the supplied string as a vector of |
920f3fa9 DM |
6309 | integers, one for each character in the string. Perl applies the format to |
6310 | each integer in turn, then joins the resulting strings with a separator (a | |
6311 | dot C<.> by default). This can be useful for displaying ordinal values of | |
6312 | characters in arbitrary strings: | |
7b8dd722 | 6313 | |
920f3fa9 | 6314 | printf "%vd", "AB\x{100}"; # prints "65.66.256" |
7b8dd722 HS |
6315 | printf "version is v%vd\n", $^V; # Perl's version |
6316 | ||
6317 | Put an asterisk C<*> before the C<v> to override the string to | |
6318 | use to separate the numbers: | |
6319 | ||
6320 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
6321 | printf "bits are %0*v8b\n", " ", $bits; # random bitstring | |
6322 | ||
6323 | You can also explicitly specify the argument number to use for | |
3b10bc60 | 6324 | the join string using something like C<*2$v>; for example: |
7b8dd722 HS |
6325 | |
6326 | printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses | |
6327 | ||
6328 | =item (minimum) width | |
6329 | ||
6330 | Arguments are usually formatted to be only as wide as required to | |
6331 | display the given value. You can override the width by putting | |
6332 | a number here, or get the width from the next argument (with C<*>) | |
3b10bc60 | 6333 | or from a specified argument (e.g., with C<*2$>): |
7b8dd722 | 6334 | |
8f1da26d TC |
6335 | printf "<%s>", "a"; # prints "<a>" |
6336 | printf "<%6s>", "a"; # prints "< a>" | |
6337 | printf "<%*s>", 6, "a"; # prints "< a>" | |
6338 | printf "<%*2$s>", "a", 6; # prints "< a>" | |
6339 | printf "<%2s>", "long"; # prints "<long>" (does not truncate) | |
7b8dd722 | 6340 | |
19799a22 GS |
6341 | If a field width obtained through C<*> is negative, it has the same |
6342 | effect as the C<-> flag: left-justification. | |
74a77017 | 6343 | |
7b8dd722 | 6344 | =item precision, or maximum width |
d74e8afc | 6345 | X<precision> |
7b8dd722 | 6346 | |
6c8c9a8e | 6347 | You can specify a precision (for numeric conversions) or a maximum |
7b8dd722 | 6348 | width (for string conversions) by specifying a C<.> followed by a number. |
8f1da26d | 6349 | For floating-point formats except C<g> and C<G>, this specifies |
3b10bc60 | 6350 | how many places right of the decimal point to show (the default being 6). |
6351 | For example: | |
7b8dd722 HS |
6352 | |
6353 | # these examples are subject to system-specific variation | |
6354 | printf '<%f>', 1; # prints "<1.000000>" | |
6355 | printf '<%.1f>', 1; # prints "<1.0>" | |
6356 | printf '<%.0f>', 1; # prints "<1>" | |
6357 | printf '<%e>', 10; # prints "<1.000000e+01>" | |
6358 | printf '<%.1e>', 10; # prints "<1.0e+01>" | |
6359 | ||
3b10bc60 | 6360 | For "g" and "G", this specifies the maximum number of digits to show, |
6361 | including thoe prior to the decimal point and those after it; for | |
6362 | example: | |
1ff2d182 | 6363 | |
3b10bc60 | 6364 | # These examples are subject to system-specific variation. |
1ff2d182 AS |
6365 | printf '<%g>', 1; # prints "<1>" |
6366 | printf '<%.10g>', 1; # prints "<1>" | |
6367 | printf '<%g>', 100; # prints "<100>" | |
6368 | printf '<%.1g>', 100; # prints "<1e+02>" | |
6369 | printf '<%.2g>', 100.01; # prints "<1e+02>" | |
6370 | printf '<%.5g>', 100.01; # prints "<100.01>" | |
6371 | printf '<%.4g>', 100.01; # prints "<100>" | |
6372 | ||
7b8dd722 | 6373 | For integer conversions, specifying a precision implies that the |
9911cee9 TS |
6374 | output of the number itself should be zero-padded to this width, |
6375 | where the 0 flag is ignored: | |
6376 | ||
6377 | printf '<%.6d>', 1; # prints "<000001>" | |
6378 | printf '<%+.6d>', 1; # prints "<+000001>" | |
6379 | printf '<%-10.6d>', 1; # prints "<000001 >" | |
6380 | printf '<%10.6d>', 1; # prints "< 000001>" | |
6381 | printf '<%010.6d>', 1; # prints "< 000001>" | |
6382 | printf '<%+10.6d>', 1; # prints "< +000001>" | |
7b8dd722 HS |
6383 | |
6384 | printf '<%.6x>', 1; # prints "<000001>" | |
6385 | printf '<%#.6x>', 1; # prints "<0x000001>" | |
6386 | printf '<%-10.6x>', 1; # prints "<000001 >" | |
9911cee9 TS |
6387 | printf '<%10.6x>', 1; # prints "< 000001>" |
6388 | printf '<%010.6x>', 1; # prints "< 000001>" | |
6389 | printf '<%#10.6x>', 1; # prints "< 0x000001>" | |
7b8dd722 HS |
6390 | |
6391 | For string conversions, specifying a precision truncates the string | |
3b10bc60 | 6392 | to fit the specified width: |
7b8dd722 HS |
6393 | |
6394 | printf '<%.5s>', "truncated"; # prints "<trunc>" | |
6395 | printf '<%10.5s>', "truncated"; # prints "< trunc>" | |
6396 | ||
6397 | You can also get the precision from the next argument using C<.*>: | |
b22c7a20 | 6398 | |
7b8dd722 HS |
6399 | printf '<%.6x>', 1; # prints "<000001>" |
6400 | printf '<%.*x>', 6, 1; # prints "<000001>" | |
6401 | ||
3b10bc60 | 6402 | If a precision obtained through C<*> is negative, it counts |
6403 | as having no precision at all. | |
9911cee9 TS |
6404 | |
6405 | printf '<%.*s>', 7, "string"; # prints "<string>" | |
6406 | printf '<%.*s>', 3, "string"; # prints "<str>" | |
6407 | printf '<%.*s>', 0, "string"; # prints "<>" | |
6408 | printf '<%.*s>', -1, "string"; # prints "<string>" | |
6409 | ||
6410 | printf '<%.*d>', 1, 0; # prints "<0>" | |
6411 | printf '<%.*d>', 0, 0; # prints "<>" | |
6412 | printf '<%.*d>', -1, 0; # prints "<0>" | |
6413 | ||
7b8dd722 | 6414 | You cannot currently get the precision from a specified number, |
3b10bc60 | 6415 | but it is intended that this will be possible in the future, for |
6416 | example using C<.*2$>: | |
7b8dd722 | 6417 | |
3b10bc60 | 6418 | printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>" |
7b8dd722 HS |
6419 | |
6420 | =item size | |
6421 | ||
6422 | For numeric conversions, you can specify the size to interpret the | |
1ff2d182 AS |
6423 | number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer |
6424 | conversions (C<d u o x X b i D U O>), numbers are usually assumed to be | |
6425 | whatever the default integer size is on your platform (usually 32 or 64 | |
6426 | bits), but you can override this to use instead one of the standard C types, | |
6427 | as supported by the compiler used to build Perl: | |
7b8dd722 | 6428 | |
3d21943e JV |
6429 | hh interpret integer as C type "char" or "unsigned char" |
6430 | on Perl 5.14 or later | |
7b8dd722 | 6431 | h interpret integer as C type "short" or "unsigned short" |
1c2e8cca | 6432 | j interpret integer as C type "intmax_t" on Perl 5.14 |
3d21943e JV |
6433 | or later, and only with a C99 compiler (unportable) |
6434 | l interpret integer as C type "long" or "unsigned long" | |
6435 | q, L, or ll interpret integer as C type "long long", "unsigned long long", | |
6436 | or "quad" (typically 64-bit integers) | |
1c2e8cca MG |
6437 | t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later |
6438 | z interpret integer as C type "size_t" on Perl 5.14 or later | |
3d21943e JV |
6439 | |
6440 | As of 5.14, none of these raises an exception if they are not supported on | |
6441 | your platform. However, if warnings are enabled, a warning of the | |
6442 | C<printf> warning class is issued on an unsupported conversion flag. | |
6443 | Should you instead prefer an exception, do this: | |
6444 | ||
6445 | use warnings FATAL => "printf"; | |
6446 | ||
6447 | If you would like to know about a version dependency before you | |
6448 | start running the program, put something like this at its top: | |
6449 | ||
6450 | use 5.014; # for hh/j/t/z/ printf modifiers | |
7b8dd722 | 6451 | |
3d21943e | 6452 | You can find out whether your Perl supports quads via L<Config>: |
7b8dd722 | 6453 | |
5ed4f2ec | 6454 | use Config; |
3b10bc60 | 6455 | if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) { |
6456 | print "Nice quads!\n"; | |
6457 | } | |
1ff2d182 | 6458 | |
3b10bc60 | 6459 | For floating-point conversions (C<e f g E F G>), numbers are usually assumed |
6460 | to be the default floating-point size on your platform (double or long double), | |
6461 | but you can force "long double" with C<q>, C<L>, or C<ll> if your | |
1ff2d182 AS |
6462 | platform supports them. You can find out whether your Perl supports long |
6463 | doubles via L<Config>: | |
6464 | ||
5ed4f2ec | 6465 | use Config; |
3b10bc60 | 6466 | print "long doubles\n" if $Config{d_longdbl} eq "define"; |
1ff2d182 | 6467 | |
3b10bc60 | 6468 | You can find out whether Perl considers "long double" to be the default |
6469 | floating-point size to use on your platform via L<Config>: | |
1ff2d182 | 6470 | |
3b10bc60 | 6471 | use Config; |
6472 | if ($Config{uselongdouble} eq "define") { | |
6473 | print "long doubles by default\n"; | |
6474 | } | |
1ff2d182 | 6475 | |
3b10bc60 | 6476 | It can also be that long doubles and doubles are the same thing: |
1ff2d182 AS |
6477 | |
6478 | use Config; | |
6479 | ($Config{doublesize} == $Config{longdblsize}) && | |
6480 | print "doubles are long doubles\n"; | |
6481 | ||
3b10bc60 | 6482 | The size specifier C<V> has no effect for Perl code, but is supported for |
6483 | compatibility with XS code. It means "use the standard size for a Perl | |
6484 | integer or floating-point number", which is the default. | |
7b8dd722 | 6485 | |
a472f209 HS |
6486 | =item order of arguments |
6487 | ||
3b10bc60 | 6488 | Normally, sprintf() takes the next unused argument as the value to |
a472f209 HS |
6489 | format for each format specification. If the format specification |
6490 | uses C<*> to require additional arguments, these are consumed from | |
3b10bc60 | 6491 | the argument list in the order they appear in the format |
6492 | specification I<before> the value to format. Where an argument is | |
6493 | specified by an explicit index, this does not affect the normal | |
6494 | order for the arguments, even when the explicitly specified index | |
6495 | would have been the next argument. | |
a472f209 HS |
6496 | |
6497 | So: | |
6498 | ||
3b10bc60 | 6499 | printf "<%*.*s>", $a, $b, $c; |
a472f209 | 6500 | |
3b10bc60 | 6501 | uses C<$a> for the width, C<$b> for the precision, and C<$c> |
6502 | as the value to format; while: | |
a472f209 | 6503 | |
3b10bc60 | 6504 | printf "<%*1$.*s>", $a, $b; |
a472f209 | 6505 | |
3b10bc60 | 6506 | would use C<$a> for the width and precision, and C<$b> as the |
a472f209 HS |
6507 | value to format. |
6508 | ||
3b10bc60 | 6509 | Here are some more examples; be aware that when using an explicit |
6510 | index, the C<$> may need escaping: | |
a472f209 | 6511 | |
5ed4f2ec | 6512 | printf "%2\$d %d\n", 12, 34; # will print "34 12\n" |
6513 | printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" | |
6514 | printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" | |
6515 | printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
a472f209 | 6516 | |
7b8dd722 | 6517 | =back |
b22c7a20 | 6518 | |
3b10bc60 | 6519 | If C<use locale> is in effect and POSIX::setlocale() has been called, |
6520 | the character used for the decimal separator in formatted floating-point | |
6521 | numbers is affected by the LC_NUMERIC locale. See L<perllocale> | |
7e4353e9 | 6522 | and L<POSIX>. |
a0d0e21e LW |
6523 | |
6524 | =item sqrt EXPR | |
d74e8afc | 6525 | X<sqrt> X<root> X<square root> |
a0d0e21e | 6526 | |
54310121 | 6527 | =item sqrt |
bbce6d69 | 6528 | |
3b10bc60 | 6529 | Return the positive square root of EXPR. If EXPR is omitted, uses |
6530 | C<$_>. Works only for non-negative operands unless you've | |
6531 | loaded the C<Math::Complex> module. | |
2b5ab1e7 TC |
6532 | |
6533 | use Math::Complex; | |
3b10bc60 | 6534 | print sqrt(-4); # prints 2i |
a0d0e21e LW |
6535 | |
6536 | =item srand EXPR | |
d74e8afc | 6537 | X<srand> X<seed> X<randseed> |
a0d0e21e | 6538 | |
93dc8474 CS |
6539 | =item srand |
6540 | ||
83832992 | 6541 | Sets and returns the random number seed for the C<rand> operator. |
0686c0b8 | 6542 | |
0686c0b8 JH |
6543 | The point of the function is to "seed" the C<rand> function so that |
6544 | C<rand> can produce a different sequence each time you run your | |
83832992 KW |
6545 | program. When called with a parameter, C<srand> uses that for the seed; |
6546 | otherwise it (semi-)randomly chooses a seed. In either case, starting with | |
6547 | Perl 5.14, it returns the seed. | |
6548 | ||
6549 | If C<srand()> is not called explicitly, it is called implicitly without a | |
6550 | parameter at the first use of the C<rand> operator. However, this was not true | |
6551 | of versions of Perl before 5.004, so if your script will run under older | |
6552 | Perl versions, it should call C<srand>; otherwise most programs won't call | |
6553 | C<srand()> at all. | |
6554 | ||
6555 | But there are a few situations in recent Perls where programs are likely to | |
6556 | want to call C<srand>. One is for generating predictable results generally for | |
6557 | testing or debugging. There, you use C<srand($seed)>, with the same C<$seed> | |
6558 | each time. Another other case is where you need a cryptographically-strong | |
6559 | starting point rather than the generally acceptable default, which is based on | |
6560 | time of day, process ID, and memory allocation, or the F</dev/urandom> device | |
6561 | if available. And still another case is that you may want to call C<srand()> | |
6562 | after a C<fork()> to avoid child processes sharing the same seed value as the | |
6563 | parent (and consequently each other). | |
6564 | ||
6565 | Do B<not> call C<srand()> (i.e., without an argument) more than once per | |
d460397b | 6566 | process. The internal state of the random number generator should |
0686c0b8 | 6567 | contain more entropy than can be provided by any seed, so calling |
83832992 | 6568 | C<srand()> again actually I<loses> randomness. |
0686c0b8 | 6569 | |
e0b236fe JH |
6570 | Most implementations of C<srand> take an integer and will silently |
6571 | truncate decimal numbers. This means C<srand(42)> will usually | |
6572 | produce the same results as C<srand(42.1)>. To be safe, always pass | |
6573 | C<srand> an integer. | |
0686c0b8 JH |
6574 | |
6575 | In versions of Perl prior to 5.004 the default seed was just the | |
6576 | current C<time>. This isn't a particularly good seed, so many old | |
6577 | programs supply their own seed value (often C<time ^ $$> or C<time ^ | |
6578 | ($$ + ($$ << 15))>), but that isn't necessary any more. | |
93dc8474 | 6579 | |
cf264981 SP |
6580 | For cryptographic purposes, however, you need something much more random |
6581 | than the default seed. Checksumming the compressed output of one or more | |
2f9daede TP |
6582 | rapidly changing operating system status programs is the usual method. For |
6583 | example: | |
28757baa | 6584 | |
784d6566 | 6585 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`); |
28757baa | 6586 | |
83918a83 RGS |
6587 | If you're particularly concerned with this, search the CPAN for |
6588 | random number generator modules instead of rolling out your own. | |
0078ec44 | 6589 | |
54310121 | 6590 | Frequently called programs (like CGI scripts) that simply use |
28757baa | 6591 | |
6592 | time ^ $$ | |
6593 | ||
54310121 | 6594 | for a seed can fall prey to the mathematical property that |
28757baa | 6595 | |
6596 | a^b == (a+1)^(b+1) | |
6597 | ||
0078ec44 | 6598 | one-third of the time. So don't do that. |
f86702cc | 6599 | |
83832992 KW |
6600 | A typical use of the returned seed is for a test program which has too many |
6601 | combinations to test comprehensively in the time available to it each run. It | |
6602 | can test a random subset each time, and should there be a failure, log the seed | |
8f1da26d | 6603 | used for that run so that it can later be used to reproduce the same results. |
83832992 | 6604 | |
a0d0e21e | 6605 | =item stat FILEHANDLE |
435fbc73 | 6606 | X<stat> X<file, status> X<ctime> |
a0d0e21e LW |
6607 | |
6608 | =item stat EXPR | |
6609 | ||
5228a96c SP |
6610 | =item stat DIRHANDLE |
6611 | ||
54310121 | 6612 | =item stat |
bbce6d69 | 6613 | |
1d2dff63 | 6614 | Returns a 13-element list giving the status info for a file, either |
5228a96c | 6615 | the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is |
8f1da26d | 6616 | omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically |
5228a96c | 6617 | used as follows: |
a0d0e21e LW |
6618 | |
6619 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
6620 | $atime,$mtime,$ctime,$blksize,$blocks) | |
6621 | = stat($filename); | |
6622 | ||
54310121 | 6623 | Not all fields are supported on all filesystem types. Here are the |
61967be2 | 6624 | meanings of the fields: |
c07a80fd | 6625 | |
54310121 | 6626 | 0 dev device number of filesystem |
6627 | 1 ino inode number | |
6628 | 2 mode file mode (type and permissions) | |
6629 | 3 nlink number of (hard) links to the file | |
6630 | 4 uid numeric user ID of file's owner | |
6631 | 5 gid numeric group ID of file's owner | |
6632 | 6 rdev the device identifier (special files only) | |
6633 | 7 size total size of file, in bytes | |
1c74f1bd GS |
6634 | 8 atime last access time in seconds since the epoch |
6635 | 9 mtime last modify time in seconds since the epoch | |
df2a7e48 | 6636 | 10 ctime inode change time in seconds since the epoch (*) |
54310121 | 6637 | 11 blksize preferred block size for file system I/O |
6638 | 12 blocks actual number of blocks allocated | |
c07a80fd | 6639 | |
6640 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
6641 | ||
3e2557b2 RGS |
6642 | (*) Not all fields are supported on all filesystem types. Notably, the |
6643 | ctime field is non-portable. In particular, you cannot expect it to be a | |
8f1da26d | 6644 | "creation time"; see L<perlport/"Files and Filesystems"> for details. |
df2a7e48 | 6645 | |
61967be2 | 6646 | If C<stat> is passed the special filehandle consisting of an underline, no |
a0d0e21e | 6647 | stat is done, but the current contents of the stat structure from the |
61967be2 | 6648 | last C<stat>, C<lstat>, or filetest are returned. Example: |
a0d0e21e LW |
6649 | |
6650 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
a9a5a0dc | 6651 | print "$file is executable NFS file\n"; |
a0d0e21e LW |
6652 | } |
6653 | ||
ca6e1c26 JH |
6654 | (This works on machines only for which the device number is negative |
6655 | under NFS.) | |
a0d0e21e | 6656 | |
2b5ab1e7 | 6657 | Because the mode contains both the file type and its permissions, you |
b76cc8ba | 6658 | should mask off the file type portion and (s)printf using a C<"%o"> |
2b5ab1e7 TC |
6659 | if you want to see the real permissions. |
6660 | ||
6661 | $mode = (stat($filename))[2]; | |
6662 | printf "Permissions are %04o\n", $mode & 07777; | |
6663 | ||
19799a22 | 6664 | In scalar context, C<stat> returns a boolean value indicating success |
1d2dff63 GS |
6665 | or failure, and, if successful, sets the information associated with |
6666 | the special filehandle C<_>. | |
6667 | ||
dd184578 | 6668 | The L<File::stat> module provides a convenient, by-name access mechanism: |
2b5ab1e7 TC |
6669 | |
6670 | use File::stat; | |
6671 | $sb = stat($filename); | |
b76cc8ba | 6672 | printf "File is %s, size is %s, perm %04o, mtime %s\n", |
a9a5a0dc VP |
6673 | $filename, $sb->size, $sb->mode & 07777, |
6674 | scalar localtime $sb->mtime; | |
2b5ab1e7 | 6675 | |
ca6e1c26 JH |
6676 | You can import symbolic mode constants (C<S_IF*>) and functions |
6677 | (C<S_IS*>) from the Fcntl module: | |
6678 | ||
6679 | use Fcntl ':mode'; | |
6680 | ||
6681 | $mode = (stat($filename))[2]; | |
6682 | ||
6683 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
6684 | $group_read = ($mode & S_IRGRP) >> 3; | |
6685 | $other_execute = $mode & S_IXOTH; | |
6686 | ||
3155e0b0 | 6687 | printf "Permissions are %04o\n", S_IMODE($mode), "\n"; |
ca6e1c26 JH |
6688 | |
6689 | $is_setuid = $mode & S_ISUID; | |
ad605d16 | 6690 | $is_directory = S_ISDIR($mode); |
ca6e1c26 JH |
6691 | |
6692 | You could write the last two using the C<-u> and C<-d> operators. | |
3b10bc60 | 6693 | Commonly available C<S_IF*> constants are: |
ca6e1c26 JH |
6694 | |
6695 | # Permissions: read, write, execute, for user, group, others. | |
6696 | ||
6697 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
6698 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
6699 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
61eff3bc | 6700 | |
3cee8101 | 6701 | # Setuid/Setgid/Stickiness/SaveText. |
7df0fd0b | 6702 | # Note that the exact meaning of these is system-dependent. |
ca6e1c26 JH |
6703 | |
6704 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
6705 | ||
7df0fd0b FC |
6706 | # File types. Not all are necessarily available on |
6707 | # your system. | |
ca6e1c26 | 6708 | |
7df0fd0b FC |
6709 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR |
6710 | S_IFIFO S_IFSOCK S_IFWHT S_ENFMT | |
ca6e1c26 | 6711 | |
7df0fd0b FC |
6712 | # The following are compatibility aliases for S_IRUSR, |
6713 | # S_IWUSR, and S_IXUSR. | |
ca6e1c26 JH |
6714 | |
6715 | S_IREAD S_IWRITE S_IEXEC | |
6716 | ||
61967be2 | 6717 | and the C<S_IF*> functions are |
ca6e1c26 | 6718 | |
7df0fd0b FC |
6719 | S_IMODE($mode) the part of $mode containing the permission |
6720 | bits and the setuid/setgid/sticky bits | |
ca6e1c26 | 6721 | |
7df0fd0b FC |
6722 | S_IFMT($mode) the part of $mode containing the file type |
6723 | which can be bit-anded with (for example) | |
6724 | S_IFREG or with the following functions | |
ca6e1c26 | 6725 | |
61967be2 | 6726 | # The operators -f, -d, -l, -b, -c, -p, and -S. |
ca6e1c26 JH |
6727 | |
6728 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
6729 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
6730 | ||
6731 | # No direct -X operator counterpart, but for the first one | |
6732 | # the -g operator is often equivalent. The ENFMT stands for | |
6733 | # record flocking enforcement, a platform-dependent feature. | |
6734 | ||
6735 | S_ISENFMT($mode) S_ISWHT($mode) | |
6736 | ||
6737 | See your native chmod(2) and stat(2) documentation for more details | |
61967be2 | 6738 | about the C<S_*> constants. To get status info for a symbolic link |
c837d5b4 | 6739 | instead of the target file behind the link, use the C<lstat> function. |
ca6e1c26 | 6740 | |
ea9eb35a BJ |
6741 | Portability issues: L<perlport/stat>. |
6742 | ||
36fb85f3 RGS |
6743 | =item state EXPR |
6744 | X<state> | |
6745 | ||
6746 | =item state TYPE EXPR | |
6747 | ||
6748 | =item state EXPR : ATTRS | |
6749 | ||
6750 | =item state TYPE EXPR : ATTRS | |
6751 | ||
4a904372 | 6752 | C<state> declares a lexically scoped variable, just like C<my>. |
b708784e | 6753 | However, those variables will never be reinitialized, contrary to |
36fb85f3 RGS |
6754 | lexical variables that are reinitialized each time their enclosing block |
6755 | is entered. | |
6756 | ||
3b10bc60 | 6757 | C<state> variables are enabled only when the C<use feature "state"> pragma |
4a904372 FC |
6758 | is in effect, unless the keyword is written as C<CORE::state>. |
6759 | See L<feature>. | |
36fb85f3 | 6760 | |
a0d0e21e | 6761 | =item study SCALAR |
d74e8afc | 6762 | X<study> |
a0d0e21e LW |
6763 | |
6764 | =item study | |
6765 | ||
184e9718 | 6766 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
a0d0e21e LW |
6767 | doing many pattern matches on the string before it is next modified. |
6768 | This may or may not save time, depending on the nature and number of | |
8f1da26d | 6769 | patterns you are searching and the distribution of character |
3b10bc60 | 6770 | frequencies in the string to be searched; you probably want to compare |
8f1da26d | 6771 | run times with and without it to see which is faster. Those loops |
cf264981 | 6772 | that scan for many short constant strings (including the constant |
4185c919 NC |
6773 | parts of more complex patterns) will benefit most. |
6774 | (The way C<study> works is this: a linked list of every | |
a0d0e21e | 6775 | character in the string to be searched is made, so we know, for |
7660c0ab | 6776 | example, where all the C<'k'> characters are. From each search string, |
a0d0e21e LW |
6777 | the rarest character is selected, based on some static frequency tables |
6778 | constructed from some C programs and English text. Only those places | |
6779 | that contain this "rarest" character are examined.) | |
6780 | ||
5a964f20 | 6781 | For example, here is a loop that inserts index producing entries |
a0d0e21e LW |
6782 | before any line containing a certain pattern: |
6783 | ||
6784 | while (<>) { | |
a9a5a0dc VP |
6785 | study; |
6786 | print ".IX foo\n" if /\bfoo\b/; | |
6787 | print ".IX bar\n" if /\bbar\b/; | |
6788 | print ".IX blurfl\n" if /\bblurfl\b/; | |
6789 | # ... | |
6790 | print; | |
a0d0e21e LW |
6791 | } |
6792 | ||
3b10bc60 | 6793 | In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f> |
951ba7fe | 6794 | will be looked at, because C<f> is rarer than C<o>. In general, this is |
a0d0e21e LW |
6795 | a big win except in pathological cases. The only question is whether |
6796 | it saves you more time than it took to build the linked list in the | |
6797 | first place. | |
6798 | ||
6799 | Note that if you have to look for strings that you don't know till | |
19799a22 | 6800 | runtime, you can build an entire loop as a string and C<eval> that to |
a0d0e21e | 6801 | avoid recompiling all your patterns all the time. Together with |
80d38338 | 6802 | undefining C<$/> to input entire files as one record, this can be quite |
f86cebdf | 6803 | fast, often faster than specialized programs like fgrep(1). The following |
184e9718 | 6804 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
a0d0e21e LW |
6805 | out the names of those files that contain a match: |
6806 | ||
6807 | $search = 'while (<>) { study;'; | |
6808 | foreach $word (@words) { | |
a9a5a0dc | 6809 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; |
a0d0e21e LW |
6810 | } |
6811 | $search .= "}"; | |
6812 | @ARGV = @files; | |
6813 | undef $/; | |
5ed4f2ec | 6814 | eval $search; # this screams |
6815 | $/ = "\n"; # put back to normal input delimiter | |
a0d0e21e | 6816 | foreach $file (sort keys(%seen)) { |
a9a5a0dc | 6817 | print $file, "\n"; |
a0d0e21e LW |
6818 | } |
6819 | ||
1d2de774 | 6820 | =item sub NAME BLOCK |
d74e8afc | 6821 | X<sub> |
cb1a09d0 | 6822 | |
1d2de774 | 6823 | =item sub NAME (PROTO) BLOCK |
cb1a09d0 | 6824 | |
1d2de774 JH |
6825 | =item sub NAME : ATTRS BLOCK |
6826 | ||
6827 | =item sub NAME (PROTO) : ATTRS BLOCK | |
6828 | ||
8f1da26d TC |
6829 | This is subroutine definition, not a real function I<per se>. Without a |
6830 | BLOCK it's just a forward declaration. Without a NAME, it's an anonymous | |
6831 | function declaration, so does return a value: the CODE ref of the closure | |
6832 | just created. | |
cb1a09d0 | 6833 | |
1d2de774 | 6834 | See L<perlsub> and L<perlref> for details about subroutines and |
8f1da26d | 6835 | references; see L<attributes> and L<Attribute::Handlers> for more |
1d2de774 | 6836 | information about attributes. |
cb1a09d0 | 6837 | |
87275199 | 6838 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT |
d74e8afc | 6839 | X<substr> X<substring> X<mid> X<left> X<right> |
7b8d334a | 6840 | |
87275199 | 6841 | =item substr EXPR,OFFSET,LENGTH |
a0d0e21e LW |
6842 | |
6843 | =item substr EXPR,OFFSET | |
6844 | ||
6845 | Extracts a substring out of EXPR and returns it. First character is at | |
8f1da26d | 6846 | offset C<0> (or whatever you've set C<$[> to (but B<<don't do that>)). |
84902520 | 6847 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
8f1da26d TC |
6848 | that far back from the end of the string. If LENGTH is omitted, returns |
6849 | everything through the end of the string. If LENGTH is negative, leaves that | |
748a9306 LW |
6850 | many characters off the end of the string. |
6851 | ||
e1de3ec0 | 6852 | my $s = "The black cat climbed the green tree"; |
5ed4f2ec | 6853 | my $color = substr $s, 4, 5; # black |
6854 | my $middle = substr $s, 4, -11; # black cat climbed the | |
6855 | my $end = substr $s, 14; # climbed the green tree | |
6856 | my $tail = substr $s, -4; # tree | |
6857 | my $z = substr $s, -4, 2; # tr | |
e1de3ec0 | 6858 | |
2b5ab1e7 | 6859 | You can use the substr() function as an lvalue, in which case EXPR |
87275199 GS |
6860 | must itself be an lvalue. If you assign something shorter than LENGTH, |
6861 | the string will shrink, and if you assign something longer than LENGTH, | |
2b5ab1e7 | 6862 | the string will grow to accommodate it. To keep the string the same |
3b10bc60 | 6863 | length, you may need to pad or chop your value using C<sprintf>. |
a0d0e21e | 6864 | |
87275199 GS |
6865 | If OFFSET and LENGTH specify a substring that is partly outside the |
6866 | string, only the part within the string is returned. If the substring | |
6867 | is beyond either end of the string, substr() returns the undefined | |
6868 | value and produces a warning. When used as an lvalue, specifying a | |
3b10bc60 | 6869 | substring that is entirely outside the string raises an exception. |
87275199 GS |
6870 | Here's an example showing the behavior for boundary cases: |
6871 | ||
6872 | my $name = 'fred'; | |
5ed4f2ec | 6873 | substr($name, 4) = 'dy'; # $name is now 'freddy' |
3b10bc60 | 6874 | my $null = substr $name, 6, 2; # returns "" (no warning) |
5ed4f2ec | 6875 | my $oops = substr $name, 7; # returns undef, with warning |
3b10bc60 | 6876 | substr($name, 7) = 'gap'; # raises an exception |
87275199 | 6877 | |
2b5ab1e7 | 6878 | An alternative to using substr() as an lvalue is to specify the |
7b8d334a | 6879 | replacement string as the 4th argument. This allows you to replace |
2b5ab1e7 TC |
6880 | parts of the EXPR and return what was there before in one operation, |
6881 | just as you can with splice(). | |
7b8d334a | 6882 | |
e1de3ec0 | 6883 | my $s = "The black cat climbed the green tree"; |
5ed4f2ec | 6884 | my $z = substr $s, 14, 7, "jumped from"; # climbed |
e1de3ec0 GS |
6885 | # $s is now "The black cat jumped from the green tree" |
6886 | ||
8f1da26d | 6887 | Note that the lvalue returned by the three-argument version of substr() acts as |
91f73676 DM |
6888 | a 'magic bullet'; each time it is assigned to, it remembers which part |
6889 | of the original string is being modified; for example: | |
6890 | ||
6891 | $x = '1234'; | |
6892 | for (substr($x,1,2)) { | |
5ed4f2ec | 6893 | $_ = 'a'; print $x,"\n"; # prints 1a4 |
6894 | $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 | |
91f73676 | 6895 | $x = '56789'; |
5ed4f2ec | 6896 | $_ = 'pq'; print $x,"\n"; # prints 5pq9 |
91f73676 DM |
6897 | } |
6898 | ||
91f73676 DM |
6899 | Prior to Perl version 5.9.1, the result of using an lvalue multiple times was |
6900 | unspecified. | |
c67bbae0 | 6901 | |
a0d0e21e | 6902 | =item symlink OLDFILE,NEWFILE |
d74e8afc | 6903 | X<symlink> X<link> X<symbolic link> X<link, symbolic> |
a0d0e21e LW |
6904 | |
6905 | Creates a new filename symbolically linked to the old filename. | |
7660c0ab | 6906 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
3b10bc60 | 6907 | symbolic links, raises an exception. To check for that, |
a0d0e21e LW |
6908 | use eval: |
6909 | ||
2b5ab1e7 | 6910 | $symlink_exists = eval { symlink("",""); 1 }; |
a0d0e21e | 6911 | |
ea9eb35a BJ |
6912 | Portability issues: L<perlport/symlink>. |
6913 | ||
5702da47 | 6914 | =item syscall NUMBER, LIST |
d74e8afc | 6915 | X<syscall> X<system call> |
a0d0e21e LW |
6916 | |
6917 | Calls the system call specified as the first element of the list, | |
6918 | passing the remaining elements as arguments to the system call. If | |
3b10bc60 | 6919 | unimplemented, raises an exception. The arguments are interpreted |
a0d0e21e LW |
6920 | as follows: if a given argument is numeric, the argument is passed as |
6921 | an int. If not, the pointer to the string value is passed. You are | |
6922 | responsible to make sure a string is pre-extended long enough to | |
a3cb178b | 6923 | receive any result that might be written into a string. You can't use a |
19799a22 | 6924 | string literal (or other read-only string) as an argument to C<syscall> |
a3cb178b GS |
6925 | because Perl has to assume that any string pointer might be written |
6926 | through. If your | |
a0d0e21e | 6927 | integer arguments are not literals and have never been interpreted in a |
7660c0ab | 6928 | numeric context, you may need to add C<0> to them to force them to look |
19799a22 | 6929 | like numbers. This emulates the C<syswrite> function (or vice versa): |
a0d0e21e | 6930 | |
5ed4f2ec | 6931 | require 'syscall.ph'; # may need to run h2ph |
a3cb178b GS |
6932 | $s = "hi there\n"; |
6933 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
a0d0e21e | 6934 | |
3b10bc60 | 6935 | Note that Perl supports passing of up to only 14 arguments to your syscall, |
6936 | which in practice should (usually) suffice. | |
a0d0e21e | 6937 | |
fb73857a | 6938 | Syscall returns whatever value returned by the system call it calls. |
19799a22 | 6939 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). |
8f1da26d TC |
6940 | Note that some system calls I<can> legitimately return C<-1>. The proper |
6941 | way to handle such calls is to assign C<$!=0> before the call, then | |
6942 | check the value of C<$!> if C<syscall> returns C<-1>. | |
fb73857a | 6943 | |
6944 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
8f1da26d | 6945 | number of the read end of the pipe it creates, but there is no way |
b76cc8ba | 6946 | to retrieve the file number of the other end. You can avoid this |
19799a22 | 6947 | problem by using C<pipe> instead. |
fb73857a | 6948 | |
ea9eb35a BJ |
6949 | Portability issues: L<perlport/syscall>. |
6950 | ||
c07a80fd | 6951 | =item sysopen FILEHANDLE,FILENAME,MODE |
d74e8afc | 6952 | X<sysopen> |
c07a80fd | 6953 | |
6954 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
6955 | ||
8f1da26d TC |
6956 | Opens the file whose filename is given by FILENAME, and associates it with |
6957 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real | |
6958 | filehandle wanted; an undefined scalar will be suitably autovivified. This | |
6959 | function calls the underlying operating system's I<open>(2) function with the | |
6960 | parameters FILENAME, MODE, and PERMS. | |
c07a80fd | 6961 | |
6962 | The possible values and flag bits of the MODE parameter are | |
8f1da26d TC |
6963 | system-dependent; they are available via the standard module C<Fcntl>. See |
6964 | the documentation of your operating system's I<open>(2) syscall to see | |
6965 | which values and flag bits are available. You may combine several flags | |
ea2b5ef6 JH |
6966 | using the C<|>-operator. |
6967 | ||
6968 | Some of the most common values are C<O_RDONLY> for opening the file in | |
6969 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
c188b257 | 6970 | and C<O_RDWR> for opening the file in read-write mode. |
d74e8afc | 6971 | X<O_RDONLY> X<O_RDWR> X<O_WRONLY> |
ea2b5ef6 | 6972 | |
adf5897a | 6973 | For historical reasons, some values work on almost every system |
3b10bc60 | 6974 | supported by Perl: 0 means read-only, 1 means write-only, and 2 |
adf5897a | 6975 | means read/write. We know that these values do I<not> work under |
7c5ffed3 | 6976 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
4af147f6 | 6977 | use them in new code. |
c07a80fd | 6978 | |
19799a22 | 6979 | If the file named by FILENAME does not exist and the C<open> call creates |
7660c0ab | 6980 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
5a964f20 | 6981 | PERMS specifies the permissions of the newly created file. If you omit |
19799a22 | 6982 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. |
5a964f20 | 6983 | These permission values need to be in octal, and are modified by your |
0591cd52 | 6984 | process's current C<umask>. |
d74e8afc | 6985 | X<O_CREAT> |
0591cd52 | 6986 | |
ea2b5ef6 JH |
6987 | In many systems the C<O_EXCL> flag is available for opening files in |
6988 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
c188b257 PF |
6989 | if the file already exists, sysopen() fails. C<O_EXCL> may not work |
6990 | on network filesystems, and has no effect unless the C<O_CREAT> flag | |
6991 | is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from | |
6992 | being opened if it is a symbolic link. It does not protect against | |
6993 | symbolic links in the file's path. | |
d74e8afc | 6994 | X<O_EXCL> |
c188b257 PF |
6995 | |
6996 | Sometimes you may want to truncate an already-existing file. This | |
6997 | can be done using the C<O_TRUNC> flag. The behavior of | |
6998 | C<O_TRUNC> with C<O_RDONLY> is undefined. | |
d74e8afc | 6999 | X<O_TRUNC> |
ea2b5ef6 | 7000 | |
19799a22 | 7001 | You should seldom if ever use C<0644> as argument to C<sysopen>, because |
2b5ab1e7 TC |
7002 | that takes away the user's option to have a more permissive umask. |
7003 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
7004 | on this. | |
c07a80fd | 7005 | |
4af147f6 | 7006 | Note that C<sysopen> depends on the fdopen() C library function. |
e1020413 | 7007 | On many Unix systems, fdopen() is known to fail when file descriptors |
4af147f6 CS |
7008 | exceed a certain value, typically 255. If you need more file |
7009 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
7010 | library, or perhaps using the POSIX::open() function. | |
7011 | ||
2b5ab1e7 | 7012 | See L<perlopentut> for a kinder, gentler explanation of opening files. |
28757baa | 7013 | |
ea9eb35a BJ |
7014 | Portability issues: L<perlport/sysopen>. |
7015 | ||
a0d0e21e | 7016 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
d74e8afc | 7017 | X<sysread> |
a0d0e21e LW |
7018 | |
7019 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
7020 | ||
3874323d | 7021 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
3b10bc60 | 7022 | specified FILEHANDLE, using the read(2). It bypasses |
3874323d JH |
7023 | buffered IO, so mixing this with other kinds of reads, C<print>, |
7024 | C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the | |
7025 | perlio or stdio layers usually buffers data. Returns the number of | |
7026 | bytes actually read, C<0> at end of file, or undef if there was an | |
7027 | error (in the latter case C<$!> is also set). SCALAR will be grown or | |
7028 | shrunk so that the last byte actually read is the last byte of the | |
7029 | scalar after the read. | |
ff68c719 | 7030 | |
7031 | An OFFSET may be specified to place the read data at some place in the | |
7032 | string other than the beginning. A negative OFFSET specifies | |
9124316e JH |
7033 | placement at that many characters counting backwards from the end of |
7034 | the string. A positive OFFSET greater than the length of SCALAR | |
7035 | results in the string being padded to the required size with C<"\0"> | |
7036 | bytes before the result of the read is appended. | |
a0d0e21e | 7037 | |
2b5ab1e7 | 7038 | There is no syseof() function, which is ok, since eof() doesn't work |
80d38338 | 7039 | well on device files (like ttys) anyway. Use sysread() and check |
19799a22 | 7040 | for a return value for 0 to decide whether you're done. |
2b5ab1e7 | 7041 | |
3874323d JH |
7042 | Note that if the filehandle has been marked as C<:utf8> Unicode |
7043 | characters are read instead of bytes (the LENGTH, OFFSET, and the | |
5eadf7c5 | 7044 | return value of sysread() are in Unicode characters). |
3874323d JH |
7045 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. |
7046 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. | |
7047 | ||
137443ea | 7048 | =item sysseek FILEHANDLE,POSITION,WHENCE |
d74e8afc | 7049 | X<sysseek> X<lseek> |
137443ea | 7050 | |
8f1da26d TC |
7051 | Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may |
7052 | be an expression whose value gives the name of the filehandle. The values | |
7053 | for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it | |
7054 | to the current position plus POSITION; and C<2> to set it to EOF plus | |
7055 | POSITION, typically negative. | |
9124316e JH |
7056 | |
7057 | Note the I<in bytes>: even if the filehandle has been set to operate | |
740d4bb2 JW |
7058 | on characters (for example by using the C<:encoding(utf8)> I/O layer), |
7059 | tell() will return byte offsets, not character offsets (because | |
80d38338 | 7060 | implementing that would render sysseek() unacceptably slow). |
9124316e | 7061 | |
8f1da26d TC |
7062 | sysseek() bypasses normal buffered IO, so mixing it with reads other |
7063 | than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>, | |
9124316e | 7064 | C<seek>, C<tell>, or C<eof> may cause confusion. |
86989e5d JH |
7065 | |
7066 | For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, | |
7067 | and C<SEEK_END> (start of the file, current position, end of the file) | |
7068 | from the Fcntl module. Use of the constants is also more portable | |
7069 | than relying on 0, 1, and 2. For example to define a "systell" function: | |
7070 | ||
5ed4f2ec | 7071 | use Fcntl 'SEEK_CUR'; |
7072 | sub systell { sysseek($_[0], 0, SEEK_CUR) } | |
8903cb82 | 7073 | |
7074 | Returns the new position, or the undefined value on failure. A position | |
19799a22 GS |
7075 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns |
7076 | true on success and false on failure, yet you can still easily determine | |
8903cb82 | 7077 | the new position. |
137443ea | 7078 | |
a0d0e21e | 7079 | =item system LIST |
d74e8afc | 7080 | X<system> X<shell> |
a0d0e21e | 7081 | |
8bf3b016 GS |
7082 | =item system PROGRAM LIST |
7083 | ||
19799a22 | 7084 | Does exactly the same thing as C<exec LIST>, except that a fork is |
8f1da26d | 7085 | done first and the parent process waits for the child process to |
80d38338 | 7086 | exit. Note that argument processing varies depending on the |
19799a22 GS |
7087 | number of arguments. If there is more than one argument in LIST, |
7088 | or if LIST is an array with more than one value, starts the program | |
7089 | given by the first element of the list with arguments given by the | |
7090 | rest of the list. If there is only one scalar argument, the argument | |
7091 | is checked for shell metacharacters, and if there are any, the | |
7092 | entire argument is passed to the system's command shell for parsing | |
7093 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
7094 | platforms). If there are no shell metacharacters in the argument, | |
7095 | it is split into words and passed directly to C<execvp>, which is | |
7096 | more efficient. | |
7097 | ||
0f897271 GS |
7098 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
7099 | output before any operation that may do a fork, but this may not be | |
7100 | supported on some platforms (see L<perlport>). To be safe, you may need | |
7101 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
7102 | of C<IO::Handle> on any open handles. | |
a2008d6d | 7103 | |
9d6eb86e | 7104 | The return value is the exit status of the program as returned by the |
25379e53 RGS |
7105 | C<wait> call. To get the actual exit value, shift right by eight (see |
7106 | below). See also L</exec>. This is I<not> what you want to use to capture | |
8f1da26d | 7107 | the output from a command; for that you should use merely backticks or |
d5a9bfb0 | 7108 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 |
25379e53 RGS |
7109 | indicates a failure to start the program or an error of the wait(2) system |
7110 | call (inspect $! for the reason). | |
a0d0e21e | 7111 | |
1af1c0d6 JV |
7112 | If you'd like to make C<system> (and many other bits of Perl) die on error, |
7113 | have a look at the L<autodie> pragma. | |
7114 | ||
19799a22 GS |
7115 | Like C<exec>, C<system> allows you to lie to a program about its name if |
7116 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
8bf3b016 | 7117 | |
4c2e8b59 BD |
7118 | Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of |
7119 | C<system>, if you expect your program to terminate on receipt of these | |
7120 | signals you will need to arrange to do so yourself based on the return | |
7121 | value. | |
28757baa | 7122 | |
7123 | @args = ("command", "arg1", "arg2"); | |
54310121 | 7124 | system(@args) == 0 |
a9a5a0dc | 7125 | or die "system @args failed: $?" |
28757baa | 7126 | |
95da743b | 7127 | If you'd like to manually inspect C<system>'s failure, you can check all |
1af1c0d6 | 7128 | possible failure modes by inspecting C<$?> like this: |
28757baa | 7129 | |
4ef107a6 | 7130 | if ($? == -1) { |
a9a5a0dc | 7131 | print "failed to execute: $!\n"; |
4ef107a6 DM |
7132 | } |
7133 | elsif ($? & 127) { | |
a9a5a0dc VP |
7134 | printf "child died with signal %d, %s coredump\n", |
7135 | ($? & 127), ($? & 128) ? 'with' : 'without'; | |
4ef107a6 DM |
7136 | } |
7137 | else { | |
a9a5a0dc | 7138 | printf "child exited with value %d\n", $? >> 8; |
4ef107a6 DM |
7139 | } |
7140 | ||
3b10bc60 | 7141 | Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}> |
7142 | with the C<W*()> calls from the POSIX module. | |
9d6eb86e | 7143 | |
3b10bc60 | 7144 | When C<system>'s arguments are executed indirectly by the shell, |
7145 | results and return codes are subject to its quirks. | |
c8db1d39 | 7146 | See L<perlop/"`STRING`"> and L</exec> for details. |
bb32b41a | 7147 | |
0a18a49b MH |
7148 | Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD> |
7149 | handler. See L<perlipc> for details. | |
7150 | ||
ea9eb35a BJ |
7151 | Portability issues: L<perlport/system>. |
7152 | ||
a0d0e21e | 7153 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
d74e8afc | 7154 | X<syswrite> |
a0d0e21e LW |
7155 | |
7156 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
7157 | ||
145d37e2 GA |
7158 | =item syswrite FILEHANDLE,SCALAR |
7159 | ||
3874323d | 7160 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
3b10bc60 | 7161 | specified FILEHANDLE, using write(2). If LENGTH is |
3874323d | 7162 | not specified, writes whole SCALAR. It bypasses buffered IO, so |
9124316e | 7163 | mixing this with reads (other than C<sysread())>, C<print>, C<write>, |
3874323d | 7164 | C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and |
8f1da26d | 7165 | stdio layers usually buffer data. Returns the number of bytes |
3874323d JH |
7166 | actually written, or C<undef> if there was an error (in this case the |
7167 | errno variable C<$!> is also set). If the LENGTH is greater than the | |
3b10bc60 | 7168 | data available in the SCALAR after the OFFSET, only as much data as is |
3874323d | 7169 | available will be written. |
ff68c719 | 7170 | |
7171 | An OFFSET may be specified to write the data from some part of the | |
7172 | string other than the beginning. A negative OFFSET specifies writing | |
9124316e | 7173 | that many characters counting backwards from the end of the string. |
3b10bc60 | 7174 | If SCALAR is of length zero, you can only use an OFFSET of 0. |
9124316e | 7175 | |
8f1da26d | 7176 | B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters |
3b10bc60 | 7177 | encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and |
8f1da26d | 7178 | return value of syswrite() are in (UTF8-encoded Unicode) characters. |
3874323d | 7179 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. |
8f1da26d TC |
7180 | Alternately, if the handle is not marked with an encoding but you |
7181 | attempt to write characters with code points over 255, raises an exception. | |
3874323d | 7182 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. |
a0d0e21e LW |
7183 | |
7184 | =item tell FILEHANDLE | |
d74e8afc | 7185 | X<tell> |
a0d0e21e LW |
7186 | |
7187 | =item tell | |
7188 | ||
9124316e JH |
7189 | Returns the current position I<in bytes> for FILEHANDLE, or -1 on |
7190 | error. FILEHANDLE may be an expression whose value gives the name of | |
7191 | the actual filehandle. If FILEHANDLE is omitted, assumes the file | |
7192 | last read. | |
7193 | ||
7194 | Note the I<in bytes>: even if the filehandle has been set to | |
740d4bb2 JW |
7195 | operate on characters (for example by using the C<:encoding(utf8)> open |
7196 | layer), tell() will return byte offsets, not character offsets (because | |
7197 | that would render seek() and tell() rather slow). | |
2b5ab1e7 | 7198 | |
cfd73201 JH |
7199 | The return value of tell() for the standard streams like the STDIN |
7200 | depends on the operating system: it may return -1 or something else. | |
7201 | tell() on pipes, fifos, and sockets usually returns -1. | |
7202 | ||
19799a22 | 7203 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. |
a0d0e21e | 7204 | |
3b10bc60 | 7205 | Do not use tell() (or other buffered I/O operations) on a filehandle |
8f1da26d | 7206 | that has been manipulated by sysread(), syswrite(), or sysseek(). |
59c9df15 | 7207 | Those functions ignore the buffering, while tell() does not. |
9124316e | 7208 | |
a0d0e21e | 7209 | =item telldir DIRHANDLE |
d74e8afc | 7210 | X<telldir> |
a0d0e21e | 7211 | |
19799a22 GS |
7212 | Returns the current position of the C<readdir> routines on DIRHANDLE. |
7213 | Value may be given to C<seekdir> to access a particular location in a | |
cf264981 SP |
7214 | directory. C<telldir> has the same caveats about possible directory |
7215 | compaction as the corresponding system library routine. | |
a0d0e21e | 7216 | |
4633a7c4 | 7217 | =item tie VARIABLE,CLASSNAME,LIST |
d74e8afc | 7218 | X<tie> |
a0d0e21e | 7219 | |
4633a7c4 LW |
7220 | This function binds a variable to a package class that will provide the |
7221 | implementation for the variable. VARIABLE is the name of the variable | |
7222 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
19799a22 | 7223 | of correct type. Any additional arguments are passed to the C<new> |
8a059744 GS |
7224 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
7225 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
19799a22 GS |
7226 | to the C<dbm_open()> function of C. The object returned by the C<new> |
7227 | method is also returned by the C<tie> function, which would be useful | |
8a059744 | 7228 | if you want to access other methods in CLASSNAME. |
a0d0e21e | 7229 | |
19799a22 | 7230 | Note that functions such as C<keys> and C<values> may return huge lists |
1d2dff63 | 7231 | when used on large objects, like DBM files. You may prefer to use the |
19799a22 | 7232 | C<each> function to iterate over such. Example: |
a0d0e21e LW |
7233 | |
7234 | # print out history file offsets | |
4633a7c4 | 7235 | use NDBM_File; |
da0045b7 | 7236 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
a0d0e21e | 7237 | while (($key,$val) = each %HIST) { |
a9a5a0dc | 7238 | print $key, ' = ', unpack('L',$val), "\n"; |
a0d0e21e LW |
7239 | } |
7240 | untie(%HIST); | |
7241 | ||
aa689395 | 7242 | A class implementing a hash should have the following methods: |
a0d0e21e | 7243 | |
4633a7c4 | 7244 | TIEHASH classname, LIST |
a0d0e21e LW |
7245 | FETCH this, key |
7246 | STORE this, key, value | |
7247 | DELETE this, key | |
8a059744 | 7248 | CLEAR this |
a0d0e21e LW |
7249 | EXISTS this, key |
7250 | FIRSTKEY this | |
7251 | NEXTKEY this, lastkey | |
a3bcc51e | 7252 | SCALAR this |
8a059744 | 7253 | DESTROY this |
d7da42b7 | 7254 | UNTIE this |
a0d0e21e | 7255 | |
4633a7c4 | 7256 | A class implementing an ordinary array should have the following methods: |
a0d0e21e | 7257 | |
4633a7c4 | 7258 | TIEARRAY classname, LIST |
a0d0e21e LW |
7259 | FETCH this, key |
7260 | STORE this, key, value | |
8a059744 GS |
7261 | FETCHSIZE this |
7262 | STORESIZE this, count | |
7263 | CLEAR this | |
7264 | PUSH this, LIST | |
7265 | POP this | |
7266 | SHIFT this | |
7267 | UNSHIFT this, LIST | |
7268 | SPLICE this, offset, length, LIST | |
7269 | EXTEND this, count | |
7270 | DESTROY this | |
d7da42b7 | 7271 | UNTIE this |
8a059744 | 7272 | |
3b10bc60 | 7273 | A class implementing a filehandle should have the following methods: |
8a059744 GS |
7274 | |
7275 | TIEHANDLE classname, LIST | |
7276 | READ this, scalar, length, offset | |
7277 | READLINE this | |
7278 | GETC this | |
7279 | WRITE this, scalar, length, offset | |
7280 | PRINT this, LIST | |
7281 | PRINTF this, format, LIST | |
e08f2115 GA |
7282 | BINMODE this |
7283 | EOF this | |
7284 | FILENO this | |
7285 | SEEK this, position, whence | |
7286 | TELL this | |
7287 | OPEN this, mode, LIST | |
8a059744 GS |
7288 | CLOSE this |
7289 | DESTROY this | |
d7da42b7 | 7290 | UNTIE this |
a0d0e21e | 7291 | |
4633a7c4 | 7292 | A class implementing a scalar should have the following methods: |
a0d0e21e | 7293 | |
4633a7c4 | 7294 | TIESCALAR classname, LIST |
54310121 | 7295 | FETCH this, |
a0d0e21e | 7296 | STORE this, value |
8a059744 | 7297 | DESTROY this |
d7da42b7 | 7298 | UNTIE this |
8a059744 GS |
7299 | |
7300 | Not all methods indicated above need be implemented. See L<perltie>, | |
2b5ab1e7 | 7301 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. |
a0d0e21e | 7302 | |
3b10bc60 | 7303 | Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module |
7304 | for you; you need to do that explicitly yourself. See L<DB_File> | |
19799a22 | 7305 | or the F<Config> module for interesting C<tie> implementations. |
4633a7c4 | 7306 | |
b687b08b | 7307 | For further details see L<perltie>, L<"tied VARIABLE">. |
cc6b7395 | 7308 | |
f3cbc334 | 7309 | =item tied VARIABLE |
d74e8afc | 7310 | X<tied> |
f3cbc334 RS |
7311 | |
7312 | Returns a reference to the object underlying VARIABLE (the same value | |
19799a22 | 7313 | that was originally returned by the C<tie> call that bound the variable |
f3cbc334 RS |
7314 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
7315 | package. | |
7316 | ||
a0d0e21e | 7317 | =item time |
d74e8afc | 7318 | X<time> X<epoch> |
a0d0e21e | 7319 | |
da0045b7 | 7320 | Returns the number of non-leap seconds since whatever time the system |
ef4d88db NC |
7321 | considers to be the epoch, suitable for feeding to C<gmtime> and |
7322 | C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970; | |
7323 | a prominent exception being Mac OS Classic which uses 00:00:00, January 1, | |
7324 | 1904 in the current local time zone for its epoch. | |
a0d0e21e | 7325 | |
8f1da26d TC |
7326 | For measuring time in better granularity than one second, use the |
7327 | L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or, | |
7328 | if you have gettimeofday(2), you may be able to use the C<syscall> | |
7329 | interface of Perl. See L<perlfaq8> for details. | |
68f8bed4 | 7330 | |
435fbc73 GS |
7331 | For date and time processing look at the many related modules on CPAN. |
7332 | For a comprehensive date and time representation look at the | |
7333 | L<DateTime> module. | |
7334 | ||
a0d0e21e | 7335 | =item times |
d74e8afc | 7336 | X<times> |
a0d0e21e | 7337 | |
8f1da26d TC |
7338 | Returns a four-element list giving the user and system times in |
7339 | seconds for this process and any exited children of this process. | |
a0d0e21e LW |
7340 | |
7341 | ($user,$system,$cuser,$csystem) = times; | |
7342 | ||
dc19f4fb MJD |
7343 | In scalar context, C<times> returns C<$user>. |
7344 | ||
3b10bc60 | 7345 | Children's times are only included for terminated children. |
2a958fe2 | 7346 | |
ea9eb35a BJ |
7347 | Portability issues: L<perlport/times>. |
7348 | ||
a0d0e21e LW |
7349 | =item tr/// |
7350 | ||
9f4b9cd0 SP |
7351 | The transliteration operator. Same as C<y///>. See |
7352 | L<perlop/"Quote and Quote-like Operators">. | |
a0d0e21e LW |
7353 | |
7354 | =item truncate FILEHANDLE,LENGTH | |
d74e8afc | 7355 | X<truncate> |
a0d0e21e LW |
7356 | |
7357 | =item truncate EXPR,LENGTH | |
7358 | ||
7359 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
3b10bc60 | 7360 | specified length. Raises an exception if truncate isn't implemented |
8f1da26d | 7361 | on your system. Returns true if successful, C<undef> on error. |
a0d0e21e | 7362 | |
90ddc76f MS |
7363 | The behavior is undefined if LENGTH is greater than the length of the |
7364 | file. | |
7365 | ||
8577f58c | 7366 | The position in the file of FILEHANDLE is left unchanged. You may want to |
96090e4f | 7367 | call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file. |
8577f58c | 7368 | |
ea9eb35a BJ |
7369 | Portability issues: L<perlport/truncate>. |
7370 | ||
a0d0e21e | 7371 | =item uc EXPR |
d74e8afc | 7372 | X<uc> X<uppercase> X<toupper> |
a0d0e21e | 7373 | |
54310121 | 7374 | =item uc |
bbce6d69 | 7375 | |
a0d0e21e | 7376 | Returns an uppercased version of EXPR. This is the internal function |
3980dc9c | 7377 | implementing the C<\U> escape in double-quoted strings. |
983ffd37 | 7378 | It does not attempt to do titlecase mapping on initial letters. See |
3980dc9c | 7379 | L</ucfirst> for that. |
a0d0e21e | 7380 | |
7660c0ab | 7381 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 7382 | |
3980dc9c KW |
7383 | This function behaves the same way under various pragma, such as in a locale, |
7384 | as L</lc> does. | |
7385 | ||
a0d0e21e | 7386 | =item ucfirst EXPR |
d74e8afc | 7387 | X<ucfirst> X<uppercase> |
a0d0e21e | 7388 | |
54310121 | 7389 | =item ucfirst |
bbce6d69 | 7390 | |
ad0029c4 JH |
7391 | Returns the value of EXPR with the first character in uppercase |
7392 | (titlecase in Unicode). This is the internal function implementing | |
3980dc9c | 7393 | the C<\u> escape in double-quoted strings. |
a0d0e21e | 7394 | |
7660c0ab | 7395 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 7396 | |
3980dc9c KW |
7397 | This function behaves the same way under various pragma, such as in a locale, |
7398 | as L</lc> does. | |
7399 | ||
a0d0e21e | 7400 | =item umask EXPR |
d74e8afc | 7401 | X<umask> |
a0d0e21e LW |
7402 | |
7403 | =item umask | |
7404 | ||
2f9daede | 7405 | Sets the umask for the process to EXPR and returns the previous value. |
eec2d3df GS |
7406 | If EXPR is omitted, merely returns the current umask. |
7407 | ||
0591cd52 NT |
7408 | The Unix permission C<rwxr-x---> is represented as three sets of three |
7409 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
b5a41e52 | 7410 | and isn't one of the digits). The C<umask> value is such a number |
0591cd52 NT |
7411 | representing disabled permissions bits. The permission (or "mode") |
7412 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
7413 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
8f1da26d | 7414 | if your umask is C<0022>, then the file will actually be created with |
0591cd52 NT |
7415 | permissions C<0755>. If your C<umask> were C<0027> (group can't |
7416 | write; others can't read, write, or execute), then passing | |
8f1da26d TC |
7417 | C<sysopen> C<0666> would create a file with mode C<0640> (because |
7418 | C<0666 &~ 027> is C<0640>). | |
0591cd52 NT |
7419 | |
7420 | Here's some advice: supply a creation mode of C<0666> for regular | |
19799a22 GS |
7421 | files (in C<sysopen>) and one of C<0777> for directories (in |
7422 | C<mkdir>) and executable files. This gives users the freedom of | |
0591cd52 NT |
7423 | choice: if they want protected files, they might choose process umasks |
7424 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
7425 | Programs should rarely if ever make policy decisions better left to | |
7426 | the user. The exception to this is when writing files that should be | |
7427 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
7428 | so on. | |
7429 | ||
f86cebdf | 7430 | If umask(2) is not implemented on your system and you are trying to |
3b10bc60 | 7431 | restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>), |
7432 | raises an exception. If umask(2) is not implemented and you are | |
eec2d3df GS |
7433 | not trying to restrict access for yourself, returns C<undef>. |
7434 | ||
7435 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
7436 | string of octal digits. See also L</oct>, if all you have is a string. | |
a0d0e21e | 7437 | |
ea9eb35a BJ |
7438 | Portability issues: L<perlport/umask>. |
7439 | ||
a0d0e21e | 7440 | =item undef EXPR |
d74e8afc | 7441 | X<undef> X<undefine> |
a0d0e21e LW |
7442 | |
7443 | =item undef | |
7444 | ||
54310121 | 7445 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
19799a22 | 7446 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine |
3b10bc60 | 7447 | (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}> |
20408e3c | 7448 | will probably not do what you expect on most predefined variables or |
4509d391 | 7449 | DBM list values, so don't do that; see L</delete>. Always returns the |
20408e3c GS |
7450 | undefined value. You can omit the EXPR, in which case nothing is |
7451 | undefined, but you still get an undefined value that you could, for | |
3b10bc60 | 7452 | instance, return from a subroutine, assign to a variable, or pass as a |
20408e3c | 7453 | parameter. Examples: |
a0d0e21e LW |
7454 | |
7455 | undef $foo; | |
f86cebdf | 7456 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
a0d0e21e | 7457 | undef @ary; |
aa689395 | 7458 | undef %hash; |
a0d0e21e | 7459 | undef &mysub; |
20408e3c | 7460 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
54310121 | 7461 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
2f9daede TP |
7462 | select undef, undef, undef, 0.25; |
7463 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
a0d0e21e | 7464 | |
5a964f20 TC |
7465 | Note that this is a unary operator, not a list operator. |
7466 | ||
a0d0e21e | 7467 | =item unlink LIST |
dd184578 | 7468 | X<unlink> X<delete> X<remove> X<rm> X<del> |
a0d0e21e | 7469 | |
54310121 | 7470 | =item unlink |
bbce6d69 | 7471 | |
40ea6f68 | 7472 | Deletes a list of files. On success, it returns the number of files |
7473 | it successfully deleted. On failure, it returns false and sets C<$!> | |
7474 | (errno): | |
a0d0e21e | 7475 | |
40ea6f68 | 7476 | my $unlinked = unlink 'a', 'b', 'c'; |
a0d0e21e | 7477 | unlink @goners; |
40ea6f68 | 7478 | unlink glob "*.bak"; |
a0d0e21e | 7479 | |
40ea6f68 | 7480 | On error, C<unlink> will not tell you which files it could not remove. |
734c9e01 | 7481 | If you want to know which files you could not remove, try them one |
40ea6f68 | 7482 | at a time: |
a0d0e21e | 7483 | |
40ea6f68 | 7484 | foreach my $file ( @goners ) { |
7485 | unlink $file or warn "Could not unlink $file: $!"; | |
3b10bc60 | 7486 | } |
40ea6f68 | 7487 | |
7488 | Note: C<unlink> will not attempt to delete directories unless you are | |
7489 | superuser and the B<-U> flag is supplied to Perl. Even if these | |
7490 | conditions are met, be warned that unlinking a directory can inflict | |
7491 | damage on your filesystem. Finally, using C<unlink> on directories is | |
7492 | not supported on many operating systems. Use C<rmdir> instead. | |
7493 | ||
7494 | If LIST is omitted, C<unlink> uses C<$_>. | |
bbce6d69 | 7495 | |
a0d0e21e | 7496 | =item unpack TEMPLATE,EXPR |
d74e8afc | 7497 | X<unpack> |
a0d0e21e | 7498 | |
13dcffc6 CS |
7499 | =item unpack TEMPLATE |
7500 | ||
19799a22 | 7501 | C<unpack> does the reverse of C<pack>: it takes a string |
2b6c5635 | 7502 | and expands it out into a list of values. |
19799a22 | 7503 | (In scalar context, it returns merely the first value produced.) |
2b6c5635 | 7504 | |
eae68503 | 7505 | If EXPR is omitted, unpacks the C<$_> string. |
3980dc9c | 7506 | See L<perlpacktut> for an introduction to this function. |
13dcffc6 | 7507 | |
2b6c5635 GS |
7508 | The string is broken into chunks described by the TEMPLATE. Each chunk |
7509 | is converted separately to a value. Typically, either the string is a result | |
f337b084 | 7510 | of C<pack>, or the characters of the string represent a C structure of some |
2b6c5635 GS |
7511 | kind. |
7512 | ||
19799a22 | 7513 | The TEMPLATE has the same format as in the C<pack> function. |
a0d0e21e LW |
7514 | Here's a subroutine that does substring: |
7515 | ||
7516 | sub substr { | |
5ed4f2ec | 7517 | my($what,$where,$howmuch) = @_; |
7518 | unpack("x$where a$howmuch", $what); | |
a0d0e21e LW |
7519 | } |
7520 | ||
7521 | and then there's | |
7522 | ||
f337b084 | 7523 | sub ordinal { unpack("W",$_[0]); } # same as ord() |
a0d0e21e | 7524 | |
2b6c5635 | 7525 | In addition to fields allowed in pack(), you may prefix a field with |
61eff3bc JH |
7526 | a %<number> to indicate that |
7527 | you want a <number>-bit checksum of the items instead of the items | |
2b6c5635 GS |
7528 | themselves. Default is a 16-bit checksum. Checksum is calculated by |
7529 | summing numeric values of expanded values (for string fields the sum of | |
8f1da26d | 7530 | C<ord($char)> is taken; for bit fields the sum of zeroes and ones). |
2b6c5635 GS |
7531 | |
7532 | For example, the following | |
a0d0e21e LW |
7533 | computes the same number as the System V sum program: |
7534 | ||
19799a22 | 7535 | $checksum = do { |
5ed4f2ec | 7536 | local $/; # slurp! |
7537 | unpack("%32W*",<>) % 65535; | |
19799a22 | 7538 | }; |
a0d0e21e LW |
7539 | |
7540 | The following efficiently counts the number of set bits in a bit vector: | |
7541 | ||
7542 | $setbits = unpack("%32b*", $selectmask); | |
7543 | ||
951ba7fe | 7544 | The C<p> and C<P> formats should be used with care. Since Perl |
3160c391 GS |
7545 | has no way of checking whether the value passed to C<unpack()> |
7546 | corresponds to a valid memory location, passing a pointer value that's | |
7547 | not known to be valid is likely to have disastrous consequences. | |
7548 | ||
49704364 WL |
7549 | If there are more pack codes or if the repeat count of a field or a group |
7550 | is larger than what the remainder of the input string allows, the result | |
3b10bc60 | 7551 | is not well defined: the repeat count may be decreased, or |
7552 | C<unpack()> may produce empty strings or zeros, or it may raise an exception. | |
7553 | If the input string is longer than one described by the TEMPLATE, | |
7554 | the remainder of that input string is ignored. | |
2b6c5635 | 7555 | |
851646ae | 7556 | See L</pack> for more examples and notes. |
5a929a98 | 7557 | |
98293880 | 7558 | =item untie VARIABLE |
d74e8afc | 7559 | X<untie> |
98293880 | 7560 | |
01aa884e KW |
7561 | Breaks the binding between a variable and a package. |
7562 | (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.) | |
1188453a | 7563 | Has no effect if the variable is not tied. |
98293880 | 7564 | |
532eee96 | 7565 | =item unshift ARRAY,LIST |
d74e8afc | 7566 | X<unshift> |
a0d0e21e | 7567 | |
f5a93a43 TC |
7568 | =item unshift EXPR,LIST |
7569 | ||
19799a22 | 7570 | Does the opposite of a C<shift>. Or the opposite of a C<push>, |
a0d0e21e | 7571 | depending on how you look at it. Prepends list to the front of the |
8f1da26d | 7572 | array and returns the new number of elements in the array. |
a0d0e21e | 7573 | |
76e4c2bb | 7574 | unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; |
a0d0e21e LW |
7575 | |
7576 | Note the LIST is prepended whole, not one element at a time, so the | |
19799a22 | 7577 | prepended elements stay in the same order. Use C<reverse> to do the |
a0d0e21e LW |
7578 | reverse. |
7579 | ||
f5a93a43 TC |
7580 | Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold |
7581 | a reference to an unblessed array. The argument will be dereferenced | |
7582 | automatically. This aspect of C<unshift> is considered highly | |
7583 | experimental. The exact behaviour may change in a future version of Perl. | |
cba5a3b0 | 7584 | |
f6c8478c | 7585 | =item use Module VERSION LIST |
d74e8afc | 7586 | X<use> X<module> X<import> |
f6c8478c GS |
7587 | |
7588 | =item use Module VERSION | |
7589 | ||
a0d0e21e LW |
7590 | =item use Module LIST |
7591 | ||
7592 | =item use Module | |
7593 | ||
da0045b7 | 7594 | =item use VERSION |
7595 | ||
a0d0e21e LW |
7596 | Imports some semantics into the current package from the named module, |
7597 | generally by aliasing certain subroutine or variable names into your | |
7598 | package. It is exactly equivalent to | |
7599 | ||
6d9d0573 | 7600 | BEGIN { require Module; Module->import( LIST ); } |
a0d0e21e | 7601 | |
54310121 | 7602 | except that Module I<must> be a bareword. |
8f1da26d | 7603 | The importation can be made conditional; see L<if>. |
da0045b7 | 7604 | |
bd12309b DG |
7605 | In the peculiar C<use VERSION> form, VERSION may be either a positive |
7606 | decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string | |
7607 | of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An | |
3b10bc60 | 7608 | exception is raised if VERSION is greater than the version of the |
c986422f RGS |
7609 | current Perl interpreter; Perl will not attempt to parse the rest of the |
7610 | file. Compare with L</require>, which can do a similar check at run time. | |
7611 | Symmetrically, C<no VERSION> allows you to specify that you want a version | |
3b10bc60 | 7612 | of Perl older than the specified one. |
3b825e41 RK |
7613 | |
7614 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
7615 | avoided, because it leads to misleading error messages under earlier | |
2e8342de RGS |
7616 | versions of Perl (that is, prior to 5.6.0) that do not support this |
7617 | syntax. The equivalent numeric version should be used instead. | |
fbc891ce | 7618 | |
5ed4f2ec | 7619 | use v5.6.1; # compile time version check |
7620 | use 5.6.1; # ditto | |
7621 | use 5.006_001; # ditto; preferred for backwards compatibility | |
16070b82 GS |
7622 | |
7623 | This is often useful if you need to check the current Perl version before | |
2e8342de RGS |
7624 | C<use>ing library modules that won't work with older versions of Perl. |
7625 | (We try not to do this more than we have to.) | |
da0045b7 | 7626 | |
3b10bc60 | 7627 | Also, if the specified Perl version is greater than or equal to 5.9.5, |
c986422f RGS |
7628 | C<use VERSION> will also load the C<feature> pragma and enable all |
7629 | features available in the requested version. See L<feature>. | |
3b10bc60 | 7630 | Similarly, if the specified Perl version is greater than or equal to |
5cc917d6 RGS |
7631 | 5.11.0, strictures are enabled lexically as with C<use strict> (except |
7632 | that the F<strict.pm> file is not actually loaded). | |
7dfde25d | 7633 | |
19799a22 | 7634 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The |
7660c0ab | 7635 | C<require> makes sure the module is loaded into memory if it hasn't been |
3b10bc60 | 7636 | yet. The C<import> is not a builtin; it's just an ordinary static method |
19799a22 | 7637 | call into the C<Module> package to tell the module to import the list of |
a0d0e21e | 7638 | features back into the current package. The module can implement its |
19799a22 GS |
7639 | C<import> method any way it likes, though most modules just choose to |
7640 | derive their C<import> method via inheritance from the C<Exporter> class that | |
7641 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
593b9c14 YST |
7642 | method can be found then the call is skipped, even if there is an AUTOLOAD |
7643 | method. | |
cb1a09d0 | 7644 | |
31686daf JP |
7645 | If you do not want to call the package's C<import> method (for instance, |
7646 | to stop your namespace from being altered), explicitly supply the empty list: | |
cb1a09d0 AD |
7647 | |
7648 | use Module (); | |
7649 | ||
7650 | That is exactly equivalent to | |
7651 | ||
5a964f20 | 7652 | BEGIN { require Module } |
a0d0e21e | 7653 | |
da0045b7 | 7654 | If the VERSION argument is present between Module and LIST, then the |
71be2cbc | 7655 | C<use> will call the VERSION method in class Module with the given |
7656 | version as an argument. The default VERSION method, inherited from | |
44dcb63b | 7657 | the UNIVERSAL class, croaks if the given version is larger than the |
b76cc8ba | 7658 | value of the variable C<$Module::VERSION>. |
f6c8478c GS |
7659 | |
7660 | Again, there is a distinction between omitting LIST (C<import> called | |
7661 | with no arguments) and an explicit empty LIST C<()> (C<import> not | |
7662 | called). Note that there is no comma after VERSION! | |
da0045b7 | 7663 | |
a0d0e21e LW |
7664 | Because this is a wide-open interface, pragmas (compiler directives) |
7665 | are also implemented this way. Currently implemented pragmas are: | |
7666 | ||
f3798619 | 7667 | use constant; |
4633a7c4 | 7668 | use diagnostics; |
f3798619 | 7669 | use integer; |
4438c4b7 JH |
7670 | use sigtrap qw(SEGV BUS); |
7671 | use strict qw(subs vars refs); | |
7672 | use subs qw(afunc blurfl); | |
7673 | use warnings qw(all); | |
58c7fc7c | 7674 | use sort qw(stable _quicksort _mergesort); |
a0d0e21e | 7675 | |
19799a22 | 7676 | Some of these pseudo-modules import semantics into the current |
5a964f20 TC |
7677 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
7678 | which import symbols into the current package (which are effective | |
7679 | through the end of the file). | |
a0d0e21e | 7680 | |
c362798e Z |
7681 | Because C<use> takes effect at compile time, it doesn't respect the |
7682 | ordinary flow control of the code being compiled. In particular, putting | |
7683 | a C<use> inside the false branch of a conditional doesn't prevent it | |
3b10bc60 | 7684 | from being processed. If a module or pragma only needs to be loaded |
c362798e Z |
7685 | conditionally, this can be done using the L<if> pragma: |
7686 | ||
7687 | use if $] < 5.008, "utf8"; | |
7688 | use if WANT_WARNINGS, warnings => qw(all); | |
7689 | ||
8f1da26d | 7690 | There's a corresponding C<no> declaration that unimports meanings imported |
19799a22 | 7691 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. |
80d38338 TC |
7692 | It behaves just as C<import> does with VERSION, an omitted or empty LIST, |
7693 | or no unimport method being found. | |
a0d0e21e LW |
7694 | |
7695 | no integer; | |
7696 | no strict 'refs'; | |
4438c4b7 | 7697 | no warnings; |
a0d0e21e | 7698 | |
e0de7c21 | 7699 | Care should be taken when using the C<no VERSION> form of C<no>. It is |
8f1da26d | 7700 | I<only> meant to be used to assert that the running Perl is of a earlier |
e0de7c21 RS |
7701 | version than its argument and I<not> to undo the feature-enabling side effects |
7702 | of C<use VERSION>. | |
7703 | ||
ac634a9a | 7704 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> |
3b10bc60 | 7705 | for the C<-M> and C<-m> command-line options to Perl that give C<use> |
31686daf | 7706 | functionality from the command-line. |
a0d0e21e LW |
7707 | |
7708 | =item utime LIST | |
d74e8afc | 7709 | X<utime> |
a0d0e21e LW |
7710 | |
7711 | Changes the access and modification times on each file of a list of | |
8f1da26d | 7712 | files. The first two elements of the list must be the NUMERIC access |
a0d0e21e | 7713 | and modification times, in that order. Returns the number of files |
46cdf678 | 7714 | successfully changed. The inode change time of each file is set |
4bc2a53d | 7715 | to the current time. For example, this code has the same effect as the |
a4142048 WL |
7716 | Unix touch(1) command when the files I<already exist> and belong to |
7717 | the user running the program: | |
a0d0e21e LW |
7718 | |
7719 | #!/usr/bin/perl | |
2c21a326 GA |
7720 | $atime = $mtime = time; |
7721 | utime $atime, $mtime, @ARGV; | |
4bc2a53d | 7722 | |
3b10bc60 | 7723 | Since Perl 5.7.2, if the first two elements of the list are C<undef>, |
7724 | the utime(2) syscall from your C library is called with a null second | |
4bc2a53d | 7725 | argument. On most systems, this will set the file's access and |
80d38338 | 7726 | modification times to the current time (i.e., equivalent to the example |
3b10bc60 | 7727 | above) and will work even on files you don't own provided you have write |
a4142048 | 7728 | permission: |
c6f7b413 | 7729 | |
3b10bc60 | 7730 | for $file (@ARGV) { |
7731 | utime(undef, undef, $file) | |
7732 | || warn "couldn't touch $file: $!"; | |
7733 | } | |
c6f7b413 | 7734 | |
2c21a326 GA |
7735 | Under NFS this will use the time of the NFS server, not the time of |
7736 | the local machine. If there is a time synchronization problem, the | |
7737 | NFS server and local machine will have different times. The Unix | |
7738 | touch(1) command will in fact normally use this form instead of the | |
7739 | one shown in the first example. | |
7740 | ||
3b10bc60 | 7741 | Passing only one of the first two elements as C<undef> is |
7742 | equivalent to passing a 0 and will not have the effect | |
7743 | described when both are C<undef>. This also triggers an | |
2c21a326 GA |
7744 | uninitialized warning. |
7745 | ||
3b10bc60 | 7746 | On systems that support futimes(2), you may pass filehandles among the |
7747 | files. On systems that don't support futimes(2), passing filehandles raises | |
7748 | an exception. Filehandles must be passed as globs or glob references to be | |
7749 | recognized; barewords are considered filenames. | |
e96b369d | 7750 | |
ea9eb35a BJ |
7751 | Portability issues: L<perlport/utime>. |
7752 | ||
532eee96 | 7753 | =item values HASH |
d74e8afc | 7754 | X<values> |
a0d0e21e | 7755 | |
532eee96 | 7756 | =item values ARRAY |
aeedbbed | 7757 | |
f5a93a43 TC |
7758 | =item values EXPR |
7759 | ||
aeedbbed | 7760 | Returns a list consisting of all the values of the named hash, or the values |
8f1da26d | 7761 | of an array. (In scalar context, returns the number of values.) |
504f80c1 JH |
7762 | |
7763 | The values are returned in an apparently random order. The actual | |
3b10bc60 | 7764 | random order is subject to change in future versions of Perl, but it |
504f80c1 | 7765 | is guaranteed to be the same order as either the C<keys> or C<each> |
4546b9e6 JH |
7766 | function would produce on the same (unmodified) hash. Since Perl |
7767 | 5.8.1 the ordering is different even between different runs of Perl | |
7768 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). | |
504f80c1 | 7769 | |
aeedbbed | 7770 | As a side effect, calling values() resets the HASH or ARRAY's internal |
8f1da26d | 7771 | iterator; |
2f65b2f0 | 7772 | see L</each>. (In particular, calling values() in void context resets |
aeedbbed | 7773 | the iterator with no other overhead. Apart from resetting the iterator, |
80d38338 | 7774 | C<values @array> in list context is the same as plain C<@array>. |
aeedbbed NC |
7775 | We recommend that you use void context C<keys @array> for this, but reasoned |
7776 | that it taking C<values @array> out would require more documentation than | |
7777 | leaving it in.) | |
7778 | ||
8ea1e5d4 GS |
7779 | Note that the values are not copied, which means modifying them will |
7780 | modify the contents of the hash: | |
2b5ab1e7 | 7781 | |
5ed4f2ec | 7782 | for (values %hash) { s/foo/bar/g } # modifies %hash values |
8ea1e5d4 | 7783 | for (@hash{keys %hash}) { s/foo/bar/g } # same |
2b5ab1e7 | 7784 | |
f5a93a43 TC |
7785 | Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold |
7786 | a reference to an unblessed hash or array. The argument will be | |
7787 | dereferenced automatically. This aspect of C<values> is considered highly | |
7788 | experimental. The exact behaviour may change in a future version of Perl. | |
cba5a3b0 DG |
7789 | |
7790 | for (values $hashref) { ... } | |
7791 | for (values $obj->get_arrayref) { ... } | |
7792 | ||
19799a22 | 7793 | See also C<keys>, C<each>, and C<sort>. |
a0d0e21e LW |
7794 | |
7795 | =item vec EXPR,OFFSET,BITS | |
d74e8afc | 7796 | X<vec> X<bit> X<bit vector> |
a0d0e21e | 7797 | |
e69129f1 | 7798 | Treats the string in EXPR as a bit vector made up of elements of |
8f1da26d | 7799 | width BITS and returns the value of the element specified by OFFSET |
e69129f1 GS |
7800 | as an unsigned integer. BITS therefore specifies the number of bits |
7801 | that are reserved for each element in the bit vector. This must | |
7802 | be a power of two from 1 to 32 (or 64, if your platform supports | |
7803 | that). | |
c5a0f51a | 7804 | |
b76cc8ba | 7805 | If BITS is 8, "elements" coincide with bytes of the input string. |
c73032f5 IZ |
7806 | |
7807 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
7808 | of size BITS/8, and each group is converted to a number as with | |
b1866b2d | 7809 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously |
c73032f5 IZ |
7810 | for BITS==64). See L<"pack"> for details. |
7811 | ||
7812 | If bits is 4 or less, the string is broken into bytes, then the bits | |
7813 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
7814 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
7815 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
7816 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
7817 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
7818 | ||
81e118e0 JH |
7819 | C<vec> may also be assigned to, in which case parentheses are needed |
7820 | to give the expression the correct precedence as in | |
22dc801b | 7821 | |
7822 | vec($image, $max_x * $x + $y, 8) = 3; | |
a0d0e21e | 7823 | |
fe58ced6 MG |
7824 | If the selected element is outside the string, the value 0 is returned. |
7825 | If an element off the end of the string is written to, Perl will first | |
7826 | extend the string with sufficiently many zero bytes. It is an error | |
80d38338 | 7827 | to try to write off the beginning of the string (i.e., negative OFFSET). |
fac70343 | 7828 | |
2575c402 JW |
7829 | If the string happens to be encoded as UTF-8 internally (and thus has |
7830 | the UTF8 flag set), this is ignored by C<vec>, and it operates on the | |
7831 | internal byte string, not the conceptual character string, even if you | |
7832 | only have characters with values less than 256. | |
246fae53 | 7833 | |
fac70343 GS |
7834 | Strings created with C<vec> can also be manipulated with the logical |
7835 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
7836 | vector operation is desired when both operands are strings. | |
c5a0f51a | 7837 | See L<perlop/"Bitwise String Operators">. |
a0d0e21e | 7838 | |
7660c0ab | 7839 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
19799a22 | 7840 | The comments show the string after each step. Note that this code works |
cca87523 GS |
7841 | in the same way on big-endian or little-endian machines. |
7842 | ||
7843 | my $foo = ''; | |
5ed4f2ec | 7844 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' |
e69129f1 GS |
7845 | |
7846 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
5ed4f2ec | 7847 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') |
7848 | ||
7849 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' | |
7850 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
7851 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
7852 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
7853 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
7854 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' | |
7855 | # 'r' is "\x72" | |
7856 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" | |
7857 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
7858 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' | |
7859 | # 'l' is "\x6c" | |
cca87523 | 7860 | |
19799a22 | 7861 | To transform a bit vector into a string or list of 0's and 1's, use these: |
a0d0e21e LW |
7862 | |
7863 | $bits = unpack("b*", $vector); | |
7864 | @bits = split(//, unpack("b*", $vector)); | |
7865 | ||
7660c0ab | 7866 | If you know the exact length in bits, it can be used in place of the C<*>. |
a0d0e21e | 7867 | |
e69129f1 GS |
7868 | Here is an example to illustrate how the bits actually fall in place: |
7869 | ||
7870 | #!/usr/bin/perl -wl | |
7871 | ||
7872 | print <<'EOT'; | |
b76cc8ba | 7873 | 0 1 2 3 |
e69129f1 GS |
7874 | unpack("V",$_) 01234567890123456789012345678901 |
7875 | ------------------------------------------------------------------ | |
7876 | EOT | |
7877 | ||
7878 | for $w (0..3) { | |
7879 | $width = 2**$w; | |
7880 | for ($shift=0; $shift < $width; ++$shift) { | |
7881 | for ($off=0; $off < 32/$width; ++$off) { | |
7882 | $str = pack("B*", "0"x32); | |
7883 | $bits = (1<<$shift); | |
7884 | vec($str, $off, $width) = $bits; | |
7885 | $res = unpack("b*",$str); | |
7886 | $val = unpack("V", $str); | |
7887 | write; | |
7888 | } | |
7889 | } | |
7890 | } | |
7891 | ||
7892 | format STDOUT = | |
7893 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
7894 | $off, $width, $bits, $val, $res | |
7895 | . | |
7896 | __END__ | |
7897 | ||
80d38338 TC |
7898 | Regardless of the machine architecture on which it runs, the |
7899 | example above should print the following table: | |
e69129f1 | 7900 | |
b76cc8ba | 7901 | 0 1 2 3 |
e69129f1 GS |
7902 | unpack("V",$_) 01234567890123456789012345678901 |
7903 | ------------------------------------------------------------------ | |
7904 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
7905 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
7906 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
7907 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
7908 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
7909 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
7910 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
7911 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
7912 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
7913 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
7914 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
7915 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
7916 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
7917 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
7918 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
7919 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
7920 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
7921 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
7922 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
7923 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
7924 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
7925 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
7926 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
7927 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
7928 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
7929 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
7930 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
7931 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
7932 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
7933 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
7934 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
7935 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
7936 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
7937 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
7938 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
7939 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
7940 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
7941 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
7942 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
7943 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
7944 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
7945 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
7946 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
7947 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
7948 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
7949 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
7950 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
7951 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
7952 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
7953 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
7954 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
7955 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
7956 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
7957 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
7958 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
7959 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
7960 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
7961 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
7962 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
7963 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
7964 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
7965 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
7966 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
7967 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
7968 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
7969 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
7970 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
7971 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
7972 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
7973 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
7974 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
7975 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
7976 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
7977 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
7978 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
7979 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
7980 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
7981 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
7982 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
7983 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
7984 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
7985 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
7986 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
7987 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
7988 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
7989 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
7990 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
7991 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
7992 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
7993 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
7994 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
7995 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
7996 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
7997 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
7998 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
7999 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
8000 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
8001 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
8002 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
8003 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
8004 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
8005 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
8006 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
8007 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
8008 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
8009 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
8010 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
8011 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
8012 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
8013 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
8014 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
8015 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
8016 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
8017 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
8018 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
8019 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
8020 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
8021 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
8022 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
8023 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
8024 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
8025 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
8026 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
8027 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
8028 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
8029 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
8030 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
8031 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
8032 | ||
a0d0e21e | 8033 | =item wait |
d74e8afc | 8034 | X<wait> |
a0d0e21e | 8035 | |
3b10bc60 | 8036 | Behaves like wait(2) on your system: it waits for a child |
2b5ab1e7 | 8037 | process to terminate and returns the pid of the deceased process, or |
e5218da5 | 8038 | C<-1> if there are no child processes. The status is returned in C<$?> |
ca8d723e | 8039 | and C<${^CHILD_ERROR_NATIVE}>. |
2b5ab1e7 TC |
8040 | Note that a return value of C<-1> could mean that child processes are |
8041 | being automatically reaped, as described in L<perlipc>. | |
a0d0e21e | 8042 | |
c69ca1d4 | 8043 | If you use wait in your handler for $SIG{CHLD} it may accidentally for the |
0a18a49b MH |
8044 | child created by qx() or system(). See L<perlipc> for details. |
8045 | ||
ea9eb35a BJ |
8046 | Portability issues: L<perlport/wait>. |
8047 | ||
a0d0e21e | 8048 | =item waitpid PID,FLAGS |
d74e8afc | 8049 | X<waitpid> |
a0d0e21e | 8050 | |
2b5ab1e7 TC |
8051 | Waits for a particular child process to terminate and returns the pid of |
8052 | the deceased process, or C<-1> if there is no such child process. On some | |
8053 | systems, a value of 0 indicates that there are processes still running. | |
ca8d723e | 8054 | The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say |
a0d0e21e | 8055 | |
5f05dabc | 8056 | use POSIX ":sys_wait_h"; |
5a964f20 | 8057 | #... |
b76cc8ba | 8058 | do { |
a9a5a0dc | 8059 | $kid = waitpid(-1, WNOHANG); |
84b74420 | 8060 | } while $kid > 0; |
a0d0e21e | 8061 | |
2b5ab1e7 TC |
8062 | then you can do a non-blocking wait for all pending zombie processes. |
8063 | Non-blocking wait is available on machines supporting either the | |
3b10bc60 | 8064 | waitpid(2) or wait4(2) syscalls. However, waiting for a particular |
2b5ab1e7 TC |
8065 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the |
8066 | system call by remembering the status values of processes that have | |
8067 | exited but have not been harvested by the Perl script yet.) | |
a0d0e21e | 8068 | |
2b5ab1e7 TC |
8069 | Note that on some systems, a return value of C<-1> could mean that child |
8070 | processes are being automatically reaped. See L<perlipc> for details, | |
8071 | and for other examples. | |
5a964f20 | 8072 | |
ea9eb35a BJ |
8073 | Portability issues: L<perlport/waitpid>. |
8074 | ||
a0d0e21e | 8075 | =item wantarray |
d74e8afc | 8076 | X<wantarray> X<context> |
a0d0e21e | 8077 | |
cc37eb0b | 8078 | Returns true if the context of the currently executing subroutine or |
20f13e4a | 8079 | C<eval> is looking for a list value. Returns false if the context is |
cc37eb0b RGS |
8080 | looking for a scalar. Returns the undefined value if the context is |
8081 | looking for no value (void context). | |
a0d0e21e | 8082 | |
5ed4f2ec | 8083 | return unless defined wantarray; # don't bother doing more |
54310121 | 8084 | my @a = complex_calculation(); |
8085 | return wantarray ? @a : "@a"; | |
a0d0e21e | 8086 | |
20f13e4a | 8087 | C<wantarray()>'s result is unspecified in the top level of a file, |
3c10abe3 AG |
8088 | in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or |
8089 | in a C<DESTROY> method. | |
20f13e4a | 8090 | |
19799a22 GS |
8091 | This function should have been named wantlist() instead. |
8092 | ||
a0d0e21e | 8093 | =item warn LIST |
d74e8afc | 8094 | X<warn> X<warning> X<STDERR> |
a0d0e21e | 8095 | |
2d6d0015 | 8096 | Prints the value of LIST to STDERR. If the last element of LIST does |
afd8c9c8 DM |
8097 | not end in a newline, it appends the same file/line number text as C<die> |
8098 | does. | |
774d564b | 8099 | |
a96d0188 | 8100 | If the output is empty and C<$@> already contains a value (typically from a |
7660c0ab | 8101 | previous eval) that value is used after appending C<"\t...caught"> |
19799a22 GS |
8102 | to C<$@>. This is useful for staying almost, but not entirely similar to |
8103 | C<die>. | |
43051805 | 8104 | |
7660c0ab | 8105 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
43051805 | 8106 | |
774d564b | 8107 | No message is printed if there is a C<$SIG{__WARN__}> handler |
8108 | installed. It is the handler's responsibility to deal with the message | |
19799a22 | 8109 | as it sees fit (like, for instance, converting it into a C<die>). Most |
80d38338 | 8110 | handlers must therefore arrange to actually display the |
19799a22 | 8111 | warnings that they are not prepared to deal with, by calling C<warn> |
774d564b | 8112 | again in the handler. Note that this is quite safe and will not |
8113 | produce an endless loop, since C<__WARN__> hooks are not called from | |
8114 | inside one. | |
8115 | ||
8116 | You will find this behavior is slightly different from that of | |
8117 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
19799a22 | 8118 | instead call C<die> again to change it). |
774d564b | 8119 | |
8120 | Using a C<__WARN__> handler provides a powerful way to silence all | |
8121 | warnings (even the so-called mandatory ones). An example: | |
8122 | ||
8123 | # wipe out *all* compile-time warnings | |
8124 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
8125 | my $foo = 10; | |
8126 | my $foo = 20; # no warning about duplicate my $foo, | |
8127 | # but hey, you asked for it! | |
8128 | # no compile-time or run-time warnings before here | |
8129 | $DOWARN = 1; | |
8130 | ||
8131 | # run-time warnings enabled after here | |
8132 | warn "\$foo is alive and $foo!"; # does show up | |
8133 | ||
8f1da26d | 8134 | See L<perlvar> for details on setting C<%SIG> entries and for more |
2b5ab1e7 TC |
8135 | examples. See the Carp module for other kinds of warnings using its |
8136 | carp() and cluck() functions. | |
a0d0e21e | 8137 | |
15a348aa NC |
8138 | =item when EXPR BLOCK |
8139 | X<when> | |
8140 | ||
8141 | =item when BLOCK | |
8142 | ||
8f1da26d TC |
8143 | C<when> is analogous to the C<case> keyword in other languages. Used with a |
8144 | C<foreach> loop or the experimental C<given> block, C<when> can be used in | |
8145 | Perl to implement C<switch>/C<case> like statements. Available as a | |
8146 | statement after Perl 5.10 and as a statement modifier after 5.14. | |
8147 | Here are three examples: | |
8148 | ||
8149 | use v5.10; | |
8150 | foreach (@fruits) { | |
8151 | when (/apples?/) { | |
8152 | say "I like apples." | |
8153 | } | |
8154 | when (/oranges?/) { | |
8155 | say "I don't like oranges." | |
8156 | } | |
8157 | default { | |
8158 | say "I don't like anything" | |
8159 | } | |
8160 | } | |
15a348aa | 8161 | |
8f1da26d TC |
8162 | # require 5.14 for when as statement modifier |
8163 | use v5.14; | |
8164 | foreach (@fruits) { | |
8165 | say "I like apples." when /apples?/; | |
8166 | say "I don't like oranges." when /oranges?; | |
8167 | default { say "I don't like anything" } | |
8168 | } | |
8169 | ||
8170 | use v5.10; | |
15a348aa NC |
8171 | given ($fruit) { |
8172 | when (/apples?/) { | |
8f1da26d | 8173 | say "I like apples." |
15a348aa NC |
8174 | } |
8175 | when (/oranges?/) { | |
8f1da26d | 8176 | say "I don't like oranges." |
15a348aa NC |
8177 | } |
8178 | default { | |
8f1da26d | 8179 | say "I don't like anything" |
15a348aa NC |
8180 | } |
8181 | } | |
8182 | ||
8183 | See L<perlsyn/"Switch statements"> for detailed information. | |
8184 | ||
a0d0e21e | 8185 | =item write FILEHANDLE |
d74e8afc | 8186 | X<write> |
a0d0e21e LW |
8187 | |
8188 | =item write EXPR | |
8189 | ||
8190 | =item write | |
8191 | ||
5a964f20 | 8192 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
a0d0e21e | 8193 | using the format associated with that file. By default the format for |
54310121 | 8194 | a file is the one having the same name as the filehandle, but the |
19799a22 | 8195 | format for the current output channel (see the C<select> function) may be set |
184e9718 | 8196 | explicitly by assigning the name of the format to the C<$~> variable. |
a0d0e21e | 8197 | |
8f1da26d TC |
8198 | Top of form processing is handled automatically: if there is insufficient |
8199 | room on the current page for the formatted record, the page is advanced by | |
8200 | writing a form feed, a special top-of-page format is used to format the new | |
8201 | page header before the record is written. By default, the top-of-page | |
8202 | format is the name of the filehandle with "_TOP" appended. This would be a | |
8203 | problem with autovivified filehandles, but it may be dynamically set to the | |
8204 | format of your choice by assigning the name to the C<$^> variable while | |
8205 | that filehandle is selected. The number of lines remaining on the current | |
8206 | page is in variable C<$->, which can be set to C<0> to force a new page. | |
a0d0e21e LW |
8207 | |
8208 | If FILEHANDLE is unspecified, output goes to the current default output | |
8209 | channel, which starts out as STDOUT but may be changed by the | |
19799a22 | 8210 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression |
a0d0e21e LW |
8211 | is evaluated and the resulting string is used to look up the name of |
8212 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
8213 | ||
19799a22 | 8214 | Note that write is I<not> the opposite of C<read>. Unfortunately. |
a0d0e21e LW |
8215 | |
8216 | =item y/// | |
8217 | ||
9f4b9cd0 SP |
8218 | The transliteration operator. Same as C<tr///>. See |
8219 | L<perlop/"Quote and Quote-like Operators">. | |
a0d0e21e LW |
8220 | |
8221 | =back | |
8f1da26d TC |
8222 | |
8223 | =cut |