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 | |
15 | operator. A unary operator generally provides a 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, |
54310121 | 58 | nonabortive failure is generally indicated in a scalar context by |
a0d0e21e | 59 | returning the undefined value, and in a 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") |
f86cebdf | 82 | of the same name (like chown(2), fork(2), closedir(2), etc.) all 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, | |
19799a22 GS |
85 | which return C<-1> on failure. Exceptions to this rule are C<wait>, |
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 | |
22fae026 | 128 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
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 AD |
148 | |
149 | =item Functions for fixed length data or records | |
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 | |
0d863452 RH |
167 | =item Keywords related to switch |
168 | ||
36fb85f3 | 169 | C<break>, C<continue>, C<given>, C<when>, C<default> |
0d863452 | 170 | |
3b10bc60 | 171 | (These are available only if you enable the C<"switch"> feature. |
0d863452 RH |
172 | See L<feature> and L<perlsyn/"Switch statements">.) |
173 | ||
54310121 | 174 | =item Keywords related to scoping |
cb1a09d0 | 175 | |
36fb85f3 RGS |
176 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>, |
177 | C<use> | |
178 | ||
3b10bc60 | 179 | (C<state> is available only if the C<"state"> feature is enabled. See |
36fb85f3 | 180 | L<feature>.) |
cb1a09d0 AD |
181 | |
182 | =item Miscellaneous functions | |
183 | ||
36fb85f3 | 184 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, |
834df1c5 | 185 | C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray> |
cb1a09d0 AD |
186 | |
187 | =item Functions for processes and process groups | |
d74e8afc | 188 | X<process> X<pid> X<process id> |
cb1a09d0 | 189 | |
22fae026 | 190 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
1dc8ecb8 | 191 | C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
22fae026 | 192 | C<times>, C<wait>, C<waitpid> |
cb1a09d0 | 193 | |
3b10bc60 | 194 | =item Keywords related to Perl modules |
d74e8afc | 195 | X<module> |
cb1a09d0 | 196 | |
22fae026 | 197 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
cb1a09d0 | 198 | |
353c6505 | 199 | =item Keywords related to classes and object-orientation |
d74e8afc | 200 | X<object> X<class> X<package> |
cb1a09d0 | 201 | |
22fae026 TM |
202 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
203 | C<untie>, C<use> | |
cb1a09d0 AD |
204 | |
205 | =item Low-level socket functions | |
d74e8afc | 206 | X<socket> X<sock> |
cb1a09d0 | 207 | |
22fae026 TM |
208 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
209 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
737dd4b4 | 210 | C<socket>, C<socketpair> |
cb1a09d0 AD |
211 | |
212 | =item System V interprocess communication functions | |
d74e8afc | 213 | X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> |
cb1a09d0 | 214 | |
22fae026 TM |
215 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
216 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
cb1a09d0 AD |
217 | |
218 | =item Fetching user and group info | |
d74e8afc | 219 | X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> |
cb1a09d0 | 220 | |
22fae026 TM |
221 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
222 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
223 | C<getpwuid>, C<setgrent>, C<setpwent> | |
cb1a09d0 AD |
224 | |
225 | =item Fetching network info | |
d74e8afc | 226 | X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> |
cb1a09d0 | 227 | |
22fae026 TM |
228 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
229 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
230 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
231 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
232 | C<setnetent>, C<setprotoent>, C<setservent> | |
cb1a09d0 AD |
233 | |
234 | =item Time-related functions | |
d74e8afc | 235 | X<time> X<date> |
cb1a09d0 | 236 | |
22fae026 | 237 | C<gmtime>, C<localtime>, C<time>, C<times> |
cb1a09d0 | 238 | |
37798a01 | 239 | =item Functions new in perl5 |
d74e8afc | 240 | X<perl5> |
37798a01 | 241 | |
834df1c5 SP |
242 | C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>, |
243 | C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>, | |
1dc8ecb8 | 244 | C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>, |
834df1c5 SP |
245 | C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, |
246 | C<ucfirst>, C<untie>, C<use>, C<when> | |
37798a01 | 247 | |
3b10bc60 | 248 | * C<sub> was a keyword in Perl 4, but in Perl 5 it is an |
5a964f20 | 249 | operator, which can be used in expressions. |
37798a01 | 250 | |
251 | =item Functions obsoleted in perl5 | |
252 | ||
22fae026 | 253 | C<dbmclose>, C<dbmopen> |
37798a01 | 254 | |
cb1a09d0 AD |
255 | =back |
256 | ||
60f9f73c | 257 | =head2 Portability |
d74e8afc | 258 | X<portability> X<Unix> X<portable> |
60f9f73c | 259 | |
2b5ab1e7 TC |
260 | Perl was born in Unix and can therefore access all common Unix |
261 | system calls. In non-Unix environments, the functionality of some | |
262 | Unix system calls may not be available, or details of the available | |
263 | functionality may differ slightly. The Perl functions affected | |
60f9f73c JH |
264 | by this are: |
265 | ||
266 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
267 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
268 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
ef5a6dd7 JH |
269 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>, |
270 | C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
54d7b083 | 271 | C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>, |
60f9f73c JH |
272 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, |
273 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
274 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
2b5ab1e7 | 275 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, |
60f9f73c JH |
276 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
277 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
278 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
737dd4b4 | 279 | C<shmwrite>, C<socket>, C<socketpair>, |
80cbd5ad JH |
280 | C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, |
281 | C<times>, C<truncate>, C<umask>, C<unlink>, | |
2b5ab1e7 | 282 | C<utime>, C<wait>, C<waitpid> |
60f9f73c JH |
283 | |
284 | For more information about the portability of these functions, see | |
285 | L<perlport> and other available platform-specific documentation. | |
286 | ||
cb1a09d0 AD |
287 | =head2 Alphabetical Listing of Perl Functions |
288 | ||
3b10bc60 | 289 | =over |
a0d0e21e | 290 | |
5b3c99c0 | 291 | =item -X FILEHANDLE |
d74e8afc ITB |
292 | 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> |
293 | 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 | 294 | |
5b3c99c0 | 295 | =item -X EXPR |
a0d0e21e | 296 | |
5228a96c SP |
297 | =item -X DIRHANDLE |
298 | ||
5b3c99c0 | 299 | =item -X |
a0d0e21e LW |
300 | |
301 | A file test, where X is one of the letters listed below. This unary | |
5228a96c SP |
302 | operator takes one argument, either a filename, a filehandle, or a dirhandle, |
303 | and tests the associated file to see if something is true about it. If the | |
7660c0ab | 304 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
19799a22 | 305 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or |
a0d0e21e | 306 | the undefined value if the file doesn't exist. Despite the funny |
d0821a6a | 307 | names, precedence is the same as any other named unary operator. The |
a0d0e21e LW |
308 | operator may be any of: |
309 | ||
5ed4f2ec | 310 | -r File is readable by effective uid/gid. |
311 | -w File is writable by effective uid/gid. | |
312 | -x File is executable by effective uid/gid. | |
313 | -o File is owned by effective uid. | |
a0d0e21e | 314 | |
5ed4f2ec | 315 | -R File is readable by real uid/gid. |
316 | -W File is writable by real uid/gid. | |
317 | -X File is executable by real uid/gid. | |
318 | -O File is owned by real uid. | |
a0d0e21e | 319 | |
5ed4f2ec | 320 | -e File exists. |
321 | -z File has zero size (is empty). | |
322 | -s File has nonzero size (returns size in bytes). | |
a0d0e21e | 323 | |
5ed4f2ec | 324 | -f File is a plain file. |
325 | -d File is a directory. | |
326 | -l File is a symbolic link. | |
327 | -p File is a named pipe (FIFO), or Filehandle is a pipe. | |
328 | -S File is a socket. | |
329 | -b File is a block special file. | |
330 | -c File is a character special file. | |
331 | -t Filehandle is opened to a tty. | |
a0d0e21e | 332 | |
5ed4f2ec | 333 | -u File has setuid bit set. |
334 | -g File has setgid bit set. | |
335 | -k File has sticky bit set. | |
a0d0e21e | 336 | |
5ed4f2ec | 337 | -T File is an ASCII text file (heuristic guess). |
338 | -B File is a "binary" file (opposite of -T). | |
a0d0e21e | 339 | |
5ed4f2ec | 340 | -M Script start time minus file modification time, in days. |
341 | -A Same for access time. | |
342 | -C Same for inode change time (Unix, may differ for other platforms) | |
a0d0e21e | 343 | |
a0d0e21e LW |
344 | Example: |
345 | ||
346 | while (<>) { | |
a9a5a0dc VP |
347 | chomp; |
348 | next unless -f $_; # ignore specials | |
349 | #... | |
a0d0e21e LW |
350 | } |
351 | ||
5ff3f7a4 GS |
352 | The interpretation of the file permission operators C<-r>, C<-R>, |
353 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
354 | of the file and the uids and gids of the user. There may be other | |
ecae030f MO |
355 | reasons you can't actually read, write, or execute the file: for |
356 | example network filesystem access controls, ACLs (access control lists), | |
357 | read-only filesystems, and unrecognized executable formats. Note | |
358 | that the use of these six specific operators to verify if some operation | |
359 | is possible is usually a mistake, because it may be open to race | |
360 | conditions. | |
5ff3f7a4 | 361 | |
2b5ab1e7 TC |
362 | Also note that, for the superuser on the local filesystems, the C<-r>, |
363 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
5ff3f7a4 GS |
364 | if any execute bit is set in the mode. Scripts run by the superuser |
365 | may thus need to do a stat() to determine the actual mode of the file, | |
2b5ab1e7 | 366 | or temporarily set their effective uid to something else. |
5ff3f7a4 GS |
367 | |
368 | If you are using ACLs, there is a pragma called C<filetest> that may | |
369 | produce more accurate results than the bare stat() mode bits. | |
5ff3f7a4 | 370 | When under the C<use filetest 'access'> the above-mentioned filetests |
3b10bc60 | 371 | test whether the permission can (not) be granted using the |
372 | access(2) family of system calls. Also note that the C<-x> and C<-X> may | |
5ff3f7a4 GS |
373 | under this pragma return true even if there are no execute permission |
374 | bits set (nor any extra execute permission ACLs). This strangeness is | |
ecae030f MO |
375 | due to the underlying system calls' definitions. Note also that, due to |
376 | the implementation of C<use filetest 'access'>, the C<_> special | |
377 | filehandle won't cache the results of the file tests when this pragma is | |
378 | in effect. Read the documentation for the C<filetest> pragma for more | |
379 | information. | |
5ff3f7a4 | 380 | |
a0d0e21e | 381 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
3b10bc60 | 382 | C<-exp($foo)> still works as expected, however: only single letters |
a0d0e21e LW |
383 | following a minus are interpreted as file tests. |
384 | ||
385 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
386 | file is examined for odd characters such as strange control codes or | |
61eff3bc | 387 | characters with the high bit set. If too many strange characters (>30%) |
cf264981 | 388 | are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file |
3b10bc60 | 389 | containing a zero byte in the first block is considered a binary file. If C<-T> |
9124316e | 390 | or C<-B> is used on a filehandle, the current IO buffer is examined |
3b10bc60 | 391 | rather than the first block. Both C<-T> and C<-B> return true on an empty |
54310121 | 392 | file, or a file at EOF when testing a filehandle. Because you have to |
4633a7c4 LW |
393 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
394 | against the file first, as in C<next unless -f $file && -T $file>. | |
a0d0e21e | 395 | |
19799a22 | 396 | If any of the file tests (or either the C<stat> or C<lstat> operators) are given |
28757baa | 397 | the special filehandle consisting of a solitary underline, then the stat |
a0d0e21e LW |
398 | structure of the previous file test (or stat operator) is used, saving |
399 | a system call. (This doesn't work with C<-t>, and you need to remember | |
3b10bc60 | 400 | that lstat() and C<-l> leave values in the stat structure for the |
5c9aa243 | 401 | symbolic link, not the real file.) (Also, if the stat buffer was filled by |
cf264981 | 402 | an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). |
5c9aa243 | 403 | Example: |
a0d0e21e LW |
404 | |
405 | print "Can do.\n" if -r $a || -w _ || -x _; | |
406 | ||
407 | stat($filename); | |
408 | print "Readable\n" if -r _; | |
409 | print "Writable\n" if -w _; | |
410 | print "Executable\n" if -x _; | |
411 | print "Setuid\n" if -u _; | |
412 | print "Setgid\n" if -g _; | |
413 | print "Sticky\n" if -k _; | |
414 | print "Text\n" if -T _; | |
415 | print "Binary\n" if -B _; | |
416 | ||
fbb0b3b3 RGS |
417 | As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file |
418 | test operators, in a way that C<-f -w -x $file> is equivalent to | |
3b10bc60 | 419 | C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use |
fbb0b3b3 RGS |
420 | the return value of C<-f $file> as an argument to another filetest |
421 | operator, no special magic will happen.) | |
422 | ||
a0d0e21e | 423 | =item abs VALUE |
d74e8afc | 424 | X<abs> X<absolute> |
a0d0e21e | 425 | |
54310121 | 426 | =item abs |
bbce6d69 | 427 | |
a0d0e21e | 428 | Returns the absolute value of its argument. |
7660c0ab | 429 | If VALUE is omitted, uses C<$_>. |
a0d0e21e LW |
430 | |
431 | =item accept NEWSOCKET,GENERICSOCKET | |
d74e8afc | 432 | X<accept> |
a0d0e21e | 433 | |
3b10bc60 | 434 | Accepts an incoming socket connect, just as accept(2) |
19799a22 | 435 | does. Returns the packed address if it succeeded, false otherwise. |
2b5ab1e7 | 436 | See the example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e | 437 | |
8d2a6795 GS |
438 | On systems that support a close-on-exec flag on files, the flag will |
439 | be set for the newly opened file descriptor, as determined by the | |
440 | value of $^F. See L<perlvar/$^F>. | |
441 | ||
a0d0e21e | 442 | =item alarm SECONDS |
d74e8afc ITB |
443 | X<alarm> |
444 | X<SIGALRM> | |
445 | X<timer> | |
a0d0e21e | 446 | |
54310121 | 447 | =item alarm |
bbce6d69 | 448 | |
a0d0e21e | 449 | Arranges to have a SIGALRM delivered to this process after the |
cf264981 | 450 | specified number of wallclock seconds has elapsed. If SECONDS is not |
d400eac8 JH |
451 | specified, the value stored in C<$_> is used. (On some machines, |
452 | unfortunately, the elapsed time may be up to one second less or more | |
453 | than you specified because of how seconds are counted, and process | |
454 | scheduling may delay the delivery of the signal even further.) | |
455 | ||
456 | Only one timer may be counting at once. Each call disables the | |
457 | previous timer, and an argument of C<0> may be supplied to cancel the | |
458 | previous timer without starting a new one. The returned value is the | |
459 | amount of time remaining on the previous timer. | |
a0d0e21e | 460 | |
2bc69794 BS |
461 | For delays of finer granularity than one second, the Time::HiRes module |
462 | (from CPAN, and starting from Perl 5.8 part of the standard | |
463 | distribution) provides ualarm(). You may also use Perl's four-argument | |
464 | version of select() leaving the first three arguments undefined, or you | |
465 | might be able to use the C<syscall> interface to access setitimer(2) if | |
466 | your system supports it. See L<perlfaq8> for details. | |
2b5ab1e7 | 467 | |
80d38338 TC |
468 | It is usually a mistake to intermix C<alarm> and C<sleep> calls, because |
469 | C<sleep> may be internally implemented on your system with C<alarm>. | |
a0d0e21e | 470 | |
19799a22 GS |
471 | If you want to use C<alarm> to time out a system call you need to use an |
472 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
f86cebdf | 473 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
19799a22 | 474 | restart system calls on some systems. Using C<eval>/C<die> always works, |
5a964f20 | 475 | modulo the caveats given in L<perlipc/"Signals">. |
ff68c719 | 476 | |
477 | eval { | |
a9a5a0dc VP |
478 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
479 | alarm $timeout; | |
480 | $nread = sysread SOCKET, $buffer, $size; | |
481 | alarm 0; | |
ff68c719 | 482 | }; |
ff68c719 | 483 | if ($@) { |
a9a5a0dc | 484 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
5ed4f2ec | 485 | # timed out |
ff68c719 | 486 | } |
487 | else { | |
5ed4f2ec | 488 | # didn't |
ff68c719 | 489 | } |
490 | ||
91d81acc JH |
491 | For more information see L<perlipc>. |
492 | ||
a0d0e21e | 493 | =item atan2 Y,X |
d74e8afc | 494 | X<atan2> X<arctangent> X<tan> X<tangent> |
a0d0e21e LW |
495 | |
496 | Returns the arctangent of Y/X in the range -PI to PI. | |
497 | ||
ca6e1c26 | 498 | For the tangent operation, you may use the C<Math::Trig::tan> |
28757baa | 499 | function, or use the familiar relation: |
500 | ||
501 | sub tan { sin($_[0]) / cos($_[0]) } | |
502 | ||
a1021d57 RGS |
503 | The return value for C<atan2(0,0)> is implementation-defined; consult |
504 | your atan2(3) manpage for more information. | |
bf5f1b4c | 505 | |
a0d0e21e | 506 | =item bind SOCKET,NAME |
d74e8afc | 507 | X<bind> |
a0d0e21e | 508 | |
3b10bc60 | 509 | Binds a network address to a socket, just as bind(2) |
19799a22 | 510 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
511 | packed address of the appropriate type for the socket. See the examples in |
512 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 513 | |
fae2c0fb | 514 | =item binmode FILEHANDLE, LAYER |
d74e8afc | 515 | X<binmode> X<binary> X<text> X<DOS> X<Windows> |
1c1fc3ea | 516 | |
a0d0e21e LW |
517 | =item binmode FILEHANDLE |
518 | ||
1cbfc93d NIS |
519 | Arranges for FILEHANDLE to be read or written in "binary" or "text" |
520 | mode on systems where the run-time libraries distinguish between | |
521 | binary and text files. If FILEHANDLE is an expression, the value is | |
522 | taken as the name of the filehandle. Returns true on success, | |
b5fe5ca2 | 523 | otherwise it returns C<undef> and sets C<$!> (errno). |
1cbfc93d | 524 | |
d807c6f4 JH |
525 | On some systems (in general, DOS and Windows-based systems) binmode() |
526 | is necessary when you're not working with a text file. For the sake | |
527 | of portability it is a good idea to always use it when appropriate, | |
528 | and to never use it when it isn't appropriate. Also, people can | |
529 | set their I/O to be by default UTF-8 encoded Unicode, not bytes. | |
530 | ||
531 | In other words: regardless of platform, use binmode() on binary data, | |
532 | like for example images. | |
533 | ||
534 | If LAYER is present it is a single string, but may contain multiple | |
3b10bc60 | 535 | directives. The directives alter the behaviour of the filehandle. |
920f5fe1 | 536 | When LAYER is present using binmode on a text file makes sense. |
d807c6f4 | 537 | |
fae2c0fb | 538 | If LAYER is omitted or specified as C<:raw> the filehandle is made |
0226bbdb NIS |
539 | suitable for passing binary data. This includes turning off possible CRLF |
540 | translation and marking it as bytes (as opposed to Unicode characters). | |
749683d2 | 541 | Note that, despite what may be implied in I<"Programming Perl"> (the |
3b10bc60 | 542 | Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>. |
543 | Other layers that would affect the binary nature of the stream are | |
544 | I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the | |
0226bbdb | 545 | PERLIO environment variable. |
01e6739c | 546 | |
3b10bc60 | 547 | The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the |
d807c6f4 JH |
548 | form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to |
549 | establish default I/O layers. See L<open>. | |
550 | ||
fae2c0fb RGS |
551 | I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" |
552 | in "Programming Perl, 3rd Edition". However, since the publishing of this | |
553 | book, by many known as "Camel III", the consensus of the naming of this | |
554 | functionality has moved from "discipline" to "layer". All documentation | |
555 | of this version of Perl therefore refers to "layers" rather than to | |
556 | "disciplines". Now back to the regularly scheduled documentation...> | |
557 | ||
6902c96a T |
558 | To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>. |
559 | C<:utf8> just marks the data as UTF-8 without further checking, | |
560 | while C<:encoding(utf8)> checks the data for actually being valid | |
561 | UTF-8. More details can be found in L<PerlIO::encoding>. | |
1cbfc93d | 562 | |
ed53a2bb | 563 | In general, binmode() should be called after open() but before any I/O |
3b10bc60 | 564 | is done on the filehandle. Calling binmode() normally flushes any |
01e6739c | 565 | pending buffered output data (and perhaps pending input data) on the |
fae2c0fb | 566 | handle. An exception to this is the C<:encoding> layer that |
01e6739c | 567 | changes the default character encoding of the handle, see L<open>. |
fae2c0fb | 568 | The C<:encoding> layer sometimes needs to be called in |
3874323d JH |
569 | mid-stream, and it doesn't flush the stream. The C<:encoding> |
570 | also implicitly pushes on top of itself the C<:utf8> layer because | |
3b10bc60 | 571 | internally Perl operates on UTF8-encoded Unicode characters. |
16fe6d59 | 572 | |
19799a22 | 573 | The operating system, device drivers, C libraries, and Perl run-time |
30168b04 GS |
574 | system all work together to let the programmer treat a single |
575 | character (C<\n>) as the line terminator, irrespective of the external | |
576 | representation. On many operating systems, the native text file | |
577 | representation matches the internal representation, but on some | |
578 | platforms the external representation of C<\n> is made up of more than | |
579 | one character. | |
580 | ||
68bd7414 NIS |
581 | Mac OS, all variants of Unix, and Stream_LF files on VMS use a single |
582 | character to end each line in the external representation of text (even | |
5e12dbfa | 583 | though that single character is CARRIAGE RETURN on Mac OS and LINE FEED |
01e6739c NIS |
584 | on Unix and most VMS files). In other systems like OS/2, DOS and the |
585 | various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, | |
586 | but what's stored in text files are the two characters C<\cM\cJ>. That | |
587 | means that, if you don't use binmode() on these systems, C<\cM\cJ> | |
588 | sequences on disk will be converted to C<\n> on input, and any C<\n> in | |
589 | your program will be converted back to C<\cM\cJ> on output. This is what | |
590 | you want for text files, but it can be disastrous for binary files. | |
30168b04 GS |
591 | |
592 | Another consequence of using binmode() (on some systems) is that | |
593 | special end-of-file markers will be seen as part of the data stream. | |
594 | For systems from the Microsoft family this means that if your binary | |
4375e838 | 595 | data contains C<\cZ>, the I/O subsystem will regard it as the end of |
30168b04 GS |
596 | the file, unless you use binmode(). |
597 | ||
3b10bc60 | 598 | binmode() is important not only for readline() and print() operations, |
30168b04 GS |
599 | but also when using read(), seek(), sysread(), syswrite() and tell() |
600 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
601 | in L<perlvar> for how to manually set your input and output | |
602 | line-termination sequences. | |
a0d0e21e | 603 | |
4633a7c4 | 604 | =item bless REF,CLASSNAME |
d74e8afc | 605 | X<bless> |
a0d0e21e LW |
606 | |
607 | =item bless REF | |
608 | ||
2b5ab1e7 TC |
609 | This function tells the thingy referenced by REF that it is now an object |
610 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
19799a22 | 611 | is used. Because a C<bless> is often the last thing in a constructor, |
2b5ab1e7 | 612 | it returns the reference for convenience. Always use the two-argument |
cf264981 SP |
613 | version if a derived class might inherit the function doing the blessing. |
614 | See L<perltoot> and L<perlobj> for more about the blessing (and blessings) | |
615 | of objects. | |
a0d0e21e | 616 | |
57668c4d | 617 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
2b5ab1e7 | 618 | Namespaces with all lowercase names are considered reserved for |
cf264981 | 619 | Perl pragmata. Builtin types have all uppercase names. To prevent |
2b5ab1e7 TC |
620 | confusion, you may wish to avoid such package names as well. Make sure |
621 | that CLASSNAME is a true value. | |
60ad88b8 GS |
622 | |
623 | See L<perlmod/"Perl Modules">. | |
624 | ||
0d863452 RH |
625 | =item break |
626 | ||
627 | Break out of a C<given()> block. | |
628 | ||
3b10bc60 | 629 | This keyword is enabled by the C<"switch"> feature: see L<feature> |
0d863452 RH |
630 | for more information. |
631 | ||
a0d0e21e | 632 | =item caller EXPR |
d74e8afc | 633 | X<caller> X<call stack> X<stack> X<stack trace> |
a0d0e21e LW |
634 | |
635 | =item caller | |
636 | ||
5a964f20 | 637 | Returns the context of the current subroutine call. In scalar context, |
80d38338 TC |
638 | returns the caller's package name if there I<is> a caller (that is, if |
639 | we're in a subroutine or C<eval> or C<require>) and the undefined value | |
5a964f20 | 640 | otherwise. In list context, returns |
a0d0e21e | 641 | |
ee6b43cc | 642 | # 0 1 2 |
748a9306 | 643 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
644 | |
645 | With EXPR, it returns some extra information that the debugger uses to | |
646 | print a stack trace. The value of EXPR indicates how many call frames | |
647 | to go back before the current one. | |
648 | ||
ee6b43cc | 649 | # 0 1 2 3 4 |
f3aa04c2 | 650 | ($package, $filename, $line, $subroutine, $hasargs, |
ee6b43cc | 651 | |
652 | # 5 6 7 8 9 10 | |
b3ca2e83 | 653 | $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) |
ee6b43cc | 654 | = caller($i); |
e7ea3e70 | 655 | |
951ba7fe | 656 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
19799a22 | 657 | call, but an C<eval>. In such a case additional elements $evaltext and |
7660c0ab | 658 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
19799a22 | 659 | C<require> or C<use> statement, $evaltext contains the text of the |
277ddfaf | 660 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, |
cc1c2e42 | 661 | $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that |
0fc9dec4 RGS |
662 | each C<use> statement creates a C<require> frame inside an C<eval EXPR> |
663 | frame.) $subroutine may also be C<(unknown)> if this particular | |
664 | subroutine happens to have been deleted from the symbol table. | |
665 | C<$hasargs> is true if a new instance of C<@_> was set up for the frame. | |
666 | C<$hints> and C<$bitmask> contain pragmatic hints that the caller was | |
667 | compiled with. The C<$hints> and C<$bitmask> values are subject to change | |
668 | between versions of Perl, and are not meant for external use. | |
748a9306 | 669 | |
b3ca2e83 NC |
670 | C<$hinthash> is a reference to a hash containing the value of C<%^H> when the |
671 | caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values | |
672 | of this hash, as they are the actual values stored in the optree. | |
673 | ||
748a9306 | 674 | Furthermore, when called from within the DB package, caller returns more |
7660c0ab | 675 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 676 | arguments with which the subroutine was invoked. |
748a9306 | 677 | |
7660c0ab | 678 | Be aware that the optimizer might have optimized call frames away before |
19799a22 | 679 | C<caller> had a chance to get the information. That means that C<caller(N)> |
80d38338 | 680 | might not return information about the call frame you expect it to, for |
b76cc8ba | 681 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the |
19799a22 | 682 | previous time C<caller> was called. |
7660c0ab | 683 | |
ca9f0cb5 NC |
684 | Also be aware that setting C<@DB::args> is I<best effort>, intended for |
685 | debugging or generating backtraces, and should not be relied upon. In | |
686 | particular, as C<@_> contains aliases to the caller's arguments, Perl does | |
687 | not take a copy of C<@_>, so C<@DB::args> will contain modifications the | |
688 | subroutine makes to C<@_> or its contents, not the original values at call | |
689 | time. C<@DB::args>, like C<@_>, does not hold explicit references to its | |
690 | elements, so under certain cases its elements may have become freed and | |
691 | reallocated for other variables or temporary values. Finally, a side effect | |
692 | of the current implementation means that the effects of C<shift @_> can | |
693 | I<normally> be undone (but not C<pop @_> or other splicing, and not if a | |
694 | reference to C<@_> has been taken, and subject to the caveat about reallocated | |
695 | elements), so C<@DB::args> is actually a hybrid of the current state and | |
696 | initial state of C<@_>. Buyer beware. | |
697 | ||
a0d0e21e | 698 | =item chdir EXPR |
d74e8afc ITB |
699 | X<chdir> |
700 | X<cd> | |
f723aae1 | 701 | X<directory, change> |
a0d0e21e | 702 | |
c4aca7d0 GA |
703 | =item chdir FILEHANDLE |
704 | ||
705 | =item chdir DIRHANDLE | |
706 | ||
ce2984c3 PF |
707 | =item chdir |
708 | ||
ffce7b87 | 709 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
0bfc1ec4 | 710 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
ffce7b87 | 711 | changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the |
b4ad75f0 | 712 | variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If |
80d38338 | 713 | neither is set, C<chdir> does nothing. It returns true on success, |
b4ad75f0 | 714 | false otherwise. See the example under C<die>. |
a0d0e21e | 715 | |
3b10bc60 | 716 | On systems that support fchdir(2), you may pass a filehandle or |
717 | directory handle as argument. On systems that don't support fchdir(2), | |
718 | passing handles raises an exception. | |
c4aca7d0 | 719 | |
a0d0e21e | 720 | =item chmod LIST |
d74e8afc | 721 | X<chmod> X<permission> X<mode> |
a0d0e21e LW |
722 | |
723 | Changes the permissions of a list of files. The first element of the | |
4633a7c4 | 724 | list must be the numerical mode, which should probably be an octal |
4ad40acf | 725 | number, and which definitely should I<not> be a string of octal digits: |
3b10bc60 | 726 | C<0644> is okay, but C<"0644"> is not. Returns the number of files |
dc848c6f | 727 | successfully changed. See also L</oct>, if all you have is a string. |
a0d0e21e | 728 | |
3b10bc60 | 729 | $cnt = chmod 0755, "foo", "bar"; |
a0d0e21e | 730 | chmod 0755, @executables; |
3b10bc60 | 731 | $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to |
f86cebdf | 732 | # --w----r-T |
3b10bc60 | 733 | $mode = "0644"; chmod oct($mode), "foo"; # this is better |
734 | $mode = 0644; chmod $mode, "foo"; # this is best | |
a0d0e21e | 735 | |
3b10bc60 | 736 | On systems that support fchmod(2), you may pass filehandles among the |
737 | files. On systems that don't support fchmod(2), passing filehandles raises | |
738 | an exception. Filehandles must be passed as globs or glob references to be | |
739 | recognized; barewords are considered filenames. | |
c4aca7d0 GA |
740 | |
741 | open(my $fh, "<", "foo"); | |
742 | my $perm = (stat $fh)[2] & 07777; | |
743 | chmod($perm | 0600, $fh); | |
744 | ||
3b10bc60 | 745 | You can also import the symbolic C<S_I*> constants from the C<Fcntl> |
ca6e1c26 JH |
746 | module: |
747 | ||
3b10bc60 | 748 | use Fcntl qw( :mode ); |
ca6e1c26 | 749 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; |
3b10bc60 | 750 | # Identical to the chmod 0755 of the example above. |
ca6e1c26 | 751 | |
a0d0e21e | 752 | =item chomp VARIABLE |
d74e8afc | 753 | X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> |
a0d0e21e | 754 | |
313c9f5c | 755 | =item chomp( LIST ) |
a0d0e21e LW |
756 | |
757 | =item chomp | |
758 | ||
2b5ab1e7 TC |
759 | This safer version of L</chop> removes any trailing string |
760 | that corresponds to the current value of C<$/> (also known as | |
28757baa | 761 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
762 | number of characters removed from all its arguments. It's often used to | |
763 | remove the newline from the end of an input record when you're worried | |
2b5ab1e7 TC |
764 | that the final record may be missing its newline. When in paragraph |
765 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
4c5a6083 GS |
766 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
767 | a reference to an integer or the like, see L<perlvar>) chomp() won't | |
b76cc8ba | 768 | remove anything. |
19799a22 | 769 | If VARIABLE is omitted, it chomps C<$_>. Example: |
a0d0e21e LW |
770 | |
771 | while (<>) { | |
a9a5a0dc VP |
772 | chomp; # avoid \n on last field |
773 | @array = split(/:/); | |
774 | # ... | |
a0d0e21e LW |
775 | } |
776 | ||
4bf21a6d RD |
777 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. |
778 | ||
a0d0e21e LW |
779 | You can actually chomp anything that's an lvalue, including an assignment: |
780 | ||
781 | chomp($cwd = `pwd`); | |
782 | chomp($answer = <STDIN>); | |
783 | ||
784 | If you chomp a list, each element is chomped, and the total number of | |
785 | characters removed is returned. | |
786 | ||
15e44fd8 RGS |
787 | Note that parentheses are necessary when you're chomping anything |
788 | that is not a simple variable. This is because C<chomp $cwd = `pwd`;> | |
789 | is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as | |
790 | C<chomp( $cwd = `pwd` )> which you might expect. Similarly, | |
791 | C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than | |
792 | as C<chomp($a, $b)>. | |
793 | ||
a0d0e21e | 794 | =item chop VARIABLE |
d74e8afc | 795 | X<chop> |
a0d0e21e | 796 | |
313c9f5c | 797 | =item chop( LIST ) |
a0d0e21e LW |
798 | |
799 | =item chop | |
800 | ||
801 | Chops off the last character of a string and returns the character | |
5b3eff12 | 802 | chopped. It is much more efficient than C<s/.$//s> because it neither |
7660c0ab | 803 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
4bf21a6d RD |
804 | If VARIABLE is a hash, it chops the hash's values, but not its keys. |
805 | ||
5b3eff12 | 806 | You can actually chop anything that's an lvalue, including an assignment. |
a0d0e21e LW |
807 | |
808 | If you chop a list, each element is chopped. Only the value of the | |
19799a22 | 809 | last C<chop> is returned. |
a0d0e21e | 810 | |
19799a22 | 811 | Note that C<chop> returns the last character. To return all but the last |
748a9306 LW |
812 | character, use C<substr($string, 0, -1)>. |
813 | ||
15e44fd8 RGS |
814 | See also L</chomp>. |
815 | ||
a0d0e21e | 816 | =item chown LIST |
d74e8afc | 817 | X<chown> X<owner> X<user> X<group> |
a0d0e21e LW |
818 | |
819 | Changes the owner (and group) of a list of files. The first two | |
19799a22 GS |
820 | elements of the list must be the I<numeric> uid and gid, in that |
821 | order. A value of -1 in either position is interpreted by most | |
822 | systems to leave that value unchanged. Returns the number of files | |
823 | successfully changed. | |
a0d0e21e LW |
824 | |
825 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
826 | chown $uid, $gid, @filenames; | |
827 | ||
3b10bc60 | 828 | On systems that support fchown(2), you may pass filehandles among the |
829 | files. On systems that don't support fchown(2), passing filehandles raises | |
830 | an exception. Filehandles must be passed as globs or glob references to be | |
831 | recognized; barewords are considered filenames. | |
c4aca7d0 | 832 | |
54310121 | 833 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
834 | |
835 | print "User: "; | |
19799a22 | 836 | chomp($user = <STDIN>); |
5a964f20 | 837 | print "Files: "; |
19799a22 | 838 | chomp($pattern = <STDIN>); |
a0d0e21e LW |
839 | |
840 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
a9a5a0dc | 841 | or die "$user not in passwd file"; |
a0d0e21e | 842 | |
5ed4f2ec | 843 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
844 | chown $uid, $gid, @ary; |
845 | ||
54310121 | 846 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
847 | file unless you're the superuser, although you should be able to change |
848 | the group to any of your secondary groups. On insecure systems, these | |
849 | restrictions may be relaxed, but this is not a portable assumption. | |
19799a22 GS |
850 | On POSIX systems, you can detect this condition this way: |
851 | ||
852 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
853 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
4633a7c4 | 854 | |
a0d0e21e | 855 | =item chr NUMBER |
d74e8afc | 856 | X<chr> X<character> X<ASCII> X<Unicode> |
a0d0e21e | 857 | |
54310121 | 858 | =item chr |
bbce6d69 | 859 | |
a0d0e21e | 860 | Returns the character represented by that NUMBER in the character set. |
a0ed51b3 | 861 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
2575c402 | 862 | chr(0x263a) is a Unicode smiley face. |
aaa68c4a | 863 | |
8a064bd6 | 864 | Negative values give the Unicode replacement character (chr(0xfffd)), |
80d38338 | 865 | except under the L<bytes> pragma, where the low eight bits of the value |
8a064bd6 JH |
866 | (truncated to an integer) are used. |
867 | ||
974da8e5 JH |
868 | If NUMBER is omitted, uses C<$_>. |
869 | ||
b76cc8ba | 870 | For the reverse, use L</ord>. |
a0d0e21e | 871 | |
2575c402 JW |
872 | Note that characters from 128 to 255 (inclusive) are by default |
873 | internally not encoded as UTF-8 for backward compatibility reasons. | |
974da8e5 | 874 | |
2575c402 | 875 | See L<perlunicode> for more about Unicode. |
bbce6d69 | 876 | |
a0d0e21e | 877 | =item chroot FILENAME |
d74e8afc | 878 | X<chroot> X<root> |
a0d0e21e | 879 | |
54310121 | 880 | =item chroot |
bbce6d69 | 881 | |
5a964f20 | 882 | This function works like the system call by the same name: it makes the |
4633a7c4 | 883 | named directory the new root directory for all further pathnames that |
951ba7fe | 884 | begin with a C</> by your process and all its children. (It doesn't |
28757baa | 885 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 886 | reasons, this call is restricted to the superuser. If FILENAME is |
19799a22 | 887 | omitted, does a C<chroot> to C<$_>. |
a0d0e21e LW |
888 | |
889 | =item close FILEHANDLE | |
d74e8afc | 890 | X<close> |
a0d0e21e | 891 | |
6a518fbc TP |
892 | =item close |
893 | ||
3b10bc60 | 894 | Closes the file or pipe associated with the filehandle, flushes the IO |
e0f13c26 RGS |
895 | buffers, and closes the system file descriptor. Returns true if those |
896 | operations have succeeded and if no error was reported by any PerlIO | |
897 | layer. Closes the currently selected filehandle if the argument is | |
898 | omitted. | |
fb73857a | 899 | |
900 | You don't have to close FILEHANDLE if you are immediately going to do | |
3b10bc60 | 901 | another C<open> on it, because C<open> closes it for you. (See |
19799a22 GS |
902 | C<open>.) However, an explicit C<close> on an input file resets the line |
903 | counter (C<$.>), while the implicit close done by C<open> does not. | |
fb73857a | 904 | |
3b10bc60 | 905 | If the filehandle came from a piped open, C<close> returns false if one of |
906 | the other syscalls involved fails or if its program exits with non-zero | |
907 | status. If the only problem was that the program exited non-zero, C<$!> | |
908 | will be set to C<0>. Closing a pipe also waits for the process executing | |
909 | on the pipe to exit--in case you wish to look at the output of the pipe | |
910 | afterwards--and implicitly puts the exit status value of that command into | |
911 | C<$?> and C<${^CHILD_ERROR_NATIVE}>. | |
5a964f20 | 912 | |
80d38338 TC |
913 | Closing the read end of a pipe before the process writing to it at the |
914 | other end is done writing results in the writer receiving a SIGPIPE. If | |
915 | the other end can't handle that, be sure to read all the data before | |
916 | closing the pipe. | |
73689b13 | 917 | |
fb73857a | 918 | Example: |
a0d0e21e | 919 | |
fb73857a | 920 | open(OUTPUT, '|sort >foo') # pipe to sort |
921 | or die "Can't start sort: $!"; | |
5ed4f2ec | 922 | #... # print stuff to output |
923 | close OUTPUT # wait for sort to finish | |
fb73857a | 924 | or warn $! ? "Error closing sort pipe: $!" |
925 | : "Exit status $? from sort"; | |
5ed4f2ec | 926 | open(INPUT, 'foo') # get sort's results |
fb73857a | 927 | or die "Can't open 'foo' for input: $!"; |
a0d0e21e | 928 | |
5a964f20 TC |
929 | FILEHANDLE may be an expression whose value can be used as an indirect |
930 | filehandle, usually the real filehandle name. | |
a0d0e21e LW |
931 | |
932 | =item closedir DIRHANDLE | |
d74e8afc | 933 | X<closedir> |
a0d0e21e | 934 | |
19799a22 | 935 | Closes a directory opened by C<opendir> and returns the success of that |
5a964f20 TC |
936 | system call. |
937 | ||
a0d0e21e | 938 | =item connect SOCKET,NAME |
d74e8afc | 939 | X<connect> |
a0d0e21e | 940 | |
80d38338 TC |
941 | Attempts to connect to a remote socket, just like connect(2). |
942 | Returns true if it succeeded, false otherwise. NAME should be a | |
4633a7c4 LW |
943 | packed address of the appropriate type for the socket. See the examples in |
944 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 945 | |
cb1a09d0 | 946 | =item continue BLOCK |
d74e8afc | 947 | X<continue> |
cb1a09d0 | 948 | |
0d863452 RH |
949 | =item continue |
950 | ||
cf264981 SP |
951 | C<continue> is actually a flow control statement rather than a function. If |
952 | there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or | |
98293880 JH |
953 | C<foreach>), it is always executed just before the conditional is about to |
954 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
cb1a09d0 AD |
955 | it can be used to increment a loop variable, even when the loop has been |
956 | continued via the C<next> statement (which is similar to the C C<continue> | |
957 | statement). | |
958 | ||
98293880 | 959 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
3b10bc60 | 960 | block; C<last> and C<redo> behave as if they had been executed within |
19799a22 | 961 | the main block. So will C<next>, but since it will execute a C<continue> |
1d2dff63 GS |
962 | block, it may be more entertaining. |
963 | ||
964 | while (EXPR) { | |
a9a5a0dc VP |
965 | ### redo always comes here |
966 | do_something; | |
1d2dff63 | 967 | } continue { |
a9a5a0dc VP |
968 | ### next always comes here |
969 | do_something_else; | |
970 | # then back the top to re-check EXPR | |
1d2dff63 GS |
971 | } |
972 | ### last always comes here | |
973 | ||
3b10bc60 | 974 | Omitting the C<continue> section is equivalent to using an |
975 | empty one, logically enough, so C<next> goes directly back | |
1d2dff63 GS |
976 | to check the condition at the top of the loop. |
977 | ||
3b10bc60 | 978 | If the C<"switch"> feature is enabled, C<continue> is also a |
979 | function that exits the current C<when> (or C<default>) block and | |
980 | falls through to the next one. See L<feature> and | |
0d863452 RH |
981 | L<perlsyn/"Switch statements"> for more information. |
982 | ||
983 | ||
a0d0e21e | 984 | =item cos EXPR |
d74e8afc | 985 | X<cos> X<cosine> X<acos> X<arccosine> |
a0d0e21e | 986 | |
d6217f1e GS |
987 | =item cos |
988 | ||
5a964f20 | 989 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 990 | takes cosine of C<$_>. |
a0d0e21e | 991 | |
ca6e1c26 | 992 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
28757baa | 993 | function, or use this relation: |
994 | ||
995 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
996 | ||
a0d0e21e | 997 | =item crypt PLAINTEXT,SALT |
d74e8afc | 998 | X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> |
f723aae1 | 999 | X<decrypt> X<cryptography> X<passwd> X<encrypt> |
a0d0e21e | 1000 | |
ef2e6798 MS |
1001 | Creates a digest string exactly like the crypt(3) function in the C |
1002 | library (assuming that you actually have a version there that has not | |
bb23f8d1 | 1003 | been extirpated as a potential munition). |
ef2e6798 MS |
1004 | |
1005 | crypt() is a one-way hash function. The PLAINTEXT and SALT is turned | |
1006 | into a short string, called a digest, which is returned. The same | |
1007 | PLAINTEXT and SALT will always return the same string, but there is no | |
1008 | (known) way to get the original PLAINTEXT from the hash. Small | |
1009 | changes in the PLAINTEXT or SALT will result in large changes in the | |
1010 | digest. | |
1011 | ||
1012 | There is no decrypt function. This function isn't all that useful for | |
1013 | cryptography (for that, look for F<Crypt> modules on your nearby CPAN | |
1014 | mirror) and the name "crypt" is a bit of a misnomer. Instead it is | |
1015 | primarily used to check if two pieces of text are the same without | |
1016 | having to transmit or store the text itself. An example is checking | |
1017 | if a correct password is given. The digest of the password is stored, | |
cf264981 | 1018 | not the password itself. The user types in a password that is |
ef2e6798 MS |
1019 | crypt()'d with the same salt as the stored digest. If the two digests |
1020 | match the password is correct. | |
1021 | ||
1022 | When verifying an existing digest string you should use the digest as | |
1023 | the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used | |
cf264981 | 1024 | to create the digest is visible as part of the digest. This ensures |
ef2e6798 MS |
1025 | crypt() will hash the new string with the same salt as the digest. |
1026 | This allows your code to work with the standard L<crypt|/crypt> and | |
1027 | with more exotic implementations. In other words, do not assume | |
1028 | anything about the returned string itself, or how many bytes in the | |
1029 | digest matter. | |
85c16d83 JH |
1030 | |
1031 | Traditionally the result is a string of 13 bytes: two first bytes of | |
1032 | the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only | |
bb23f8d1 | 1033 | the first eight bytes of PLAINTEXT mattered. But alternative |
ef2e6798 | 1034 | hashing schemes (like MD5), higher level security schemes (like C2), |
e1020413 | 1035 | and implementations on non-Unix platforms may produce different |
ef2e6798 | 1036 | strings. |
85c16d83 JH |
1037 | |
1038 | When choosing a new salt create a random two character string whose | |
1039 | characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', | |
d3989d75 CW |
1040 | '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of |
1041 | characters is just a recommendation; the characters allowed in | |
1042 | the salt depend solely on your system's crypt library, and Perl can't | |
1043 | restrict what salts C<crypt()> accepts. | |
e71965be | 1044 | |
a0d0e21e | 1045 | Here's an example that makes sure that whoever runs this program knows |
cf264981 | 1046 | their password: |
a0d0e21e LW |
1047 | |
1048 | $pwd = (getpwuid($<))[1]; | |
a0d0e21e LW |
1049 | |
1050 | system "stty -echo"; | |
1051 | print "Password: "; | |
e71965be | 1052 | chomp($word = <STDIN>); |
a0d0e21e LW |
1053 | print "\n"; |
1054 | system "stty echo"; | |
1055 | ||
e71965be | 1056 | if (crypt($word, $pwd) ne $pwd) { |
a9a5a0dc | 1057 | die "Sorry...\n"; |
a0d0e21e | 1058 | } else { |
a9a5a0dc | 1059 | print "ok\n"; |
54310121 | 1060 | } |
a0d0e21e | 1061 | |
9f8f0c9d | 1062 | Of course, typing in your own password to whoever asks you |
748a9306 | 1063 | for it is unwise. |
a0d0e21e | 1064 | |
ef2e6798 | 1065 | The L<crypt|/crypt> function is unsuitable for hashing large quantities |
19799a22 | 1066 | of data, not least of all because you can't get the information |
ef2e6798 | 1067 | back. Look at the L<Digest> module for more robust algorithms. |
19799a22 | 1068 | |
f2791508 JH |
1069 | If using crypt() on a Unicode string (which I<potentially> has |
1070 | characters with codepoints above 255), Perl tries to make sense | |
1071 | of the situation by trying to downgrade (a copy of the string) | |
1072 | the string back to an eight-bit byte string before calling crypt() | |
1073 | (on that copy). If that works, good. If not, crypt() dies with | |
1074 | C<Wide character in crypt>. | |
85c16d83 | 1075 | |
aa689395 | 1076 | =item dbmclose HASH |
d74e8afc | 1077 | X<dbmclose> |
a0d0e21e | 1078 | |
19799a22 | 1079 | [This function has been largely superseded by the C<untie> function.] |
a0d0e21e | 1080 | |
aa689395 | 1081 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 1082 | |
19799a22 | 1083 | =item dbmopen HASH,DBNAME,MASK |
d74e8afc | 1084 | X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> |
a0d0e21e | 1085 | |
19799a22 | 1086 | [This function has been largely superseded by the C<tie> function.] |
a0d0e21e | 1087 | |
7b8d334a | 1088 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
19799a22 GS |
1089 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
1090 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
aa689395 | 1091 | is the name of the database (without the F<.dir> or F<.pag> extension if |
1092 | any). If the database does not exist, it is created with protection | |
19799a22 | 1093 | specified by MASK (as modified by the C<umask>). If your system supports |
80d38338 | 1094 | only the older DBM functions, you may make only one C<dbmopen> call in your |
aa689395 | 1095 | program. In older versions of Perl, if your system had neither DBM nor |
19799a22 | 1096 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
aa689395 | 1097 | sdbm(3). |
1098 | ||
1099 | If you don't have write access to the DBM file, you can only read hash | |
1100 | variables, not set them. If you want to test whether you can write, | |
3b10bc60 | 1101 | either use file tests or try setting a dummy hash entry inside an C<eval> |
1102 | to trap the error. | |
a0d0e21e | 1103 | |
19799a22 GS |
1104 | Note that functions such as C<keys> and C<values> may return huge lists |
1105 | when used on large DBM files. You may prefer to use the C<each> | |
a0d0e21e LW |
1106 | function to iterate over large DBM files. Example: |
1107 | ||
1108 | # print out history file offsets | |
1109 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
1110 | while (($key,$val) = each %HIST) { | |
a9a5a0dc | 1111 | print $key, ' = ', unpack('L',$val), "\n"; |
a0d0e21e LW |
1112 | } |
1113 | dbmclose(%HIST); | |
1114 | ||
cb1a09d0 | 1115 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 1116 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 1117 | rich implementation. |
4633a7c4 | 1118 | |
2b5ab1e7 TC |
1119 | You can control which DBM library you use by loading that library |
1120 | before you call dbmopen(): | |
1121 | ||
1122 | use DB_File; | |
1123 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
a9a5a0dc | 1124 | or die "Can't open netscape history file: $!"; |
2b5ab1e7 | 1125 | |
a0d0e21e | 1126 | =item defined EXPR |
d74e8afc | 1127 | X<defined> X<undef> X<undefined> |
a0d0e21e | 1128 | |
54310121 | 1129 | =item defined |
bbce6d69 | 1130 | |
2f9daede | 1131 | Returns a Boolean value telling whether EXPR has a value other than |
3b10bc60 | 1132 | the undefined value C<undef>. If EXPR is not present, C<$_> is |
2f9daede TP |
1133 | checked. |
1134 | ||
1135 | Many operations return C<undef> to indicate failure, end of file, | |
1136 | system error, uninitialized variable, and other exceptional | |
1137 | conditions. This function allows you to distinguish C<undef> from | |
1138 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 1139 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 1140 | false.) Note that since C<undef> is a valid scalar, its presence |
19799a22 | 1141 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
2f9daede TP |
1142 | returns C<undef> when its argument is an empty array, I<or> when the |
1143 | element to return happens to be C<undef>. | |
1144 | ||
f10b0346 GS |
1145 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
1146 | has ever been defined. The return value is unaffected by any forward | |
80d38338 | 1147 | declarations of C<&func>. A subroutine that is not defined |
847c7ebe | 1148 | may still be callable: its package may have an C<AUTOLOAD> method that |
3b10bc60 | 1149 | makes it spring into existence the first time that it is called; see |
847c7ebe | 1150 | L<perlsub>. |
f10b0346 GS |
1151 | |
1152 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
1153 | used to report whether memory for that aggregate has ever been | |
1154 | allocated. This behavior may disappear in future versions of Perl. | |
1155 | You should instead use a simple test for size: | |
1156 | ||
1157 | if (@an_array) { print "has array elements\n" } | |
1158 | if (%a_hash) { print "has hash members\n" } | |
2f9daede TP |
1159 | |
1160 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 1161 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 1162 | purpose. |
a0d0e21e LW |
1163 | |
1164 | Examples: | |
1165 | ||
1166 | print if defined $switch{'D'}; | |
1167 | print "$val\n" while defined($val = pop(@ary)); | |
1168 | die "Can't readlink $sym: $!" | |
a9a5a0dc | 1169 | unless defined($value = readlink $sym); |
a0d0e21e | 1170 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 1171 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 1172 | |
19799a22 | 1173 | Note: Many folks tend to overuse C<defined>, and then are surprised to |
7660c0ab | 1174 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
2f9daede | 1175 | defined values. For example, if you say |
a5f75d66 AD |
1176 | |
1177 | "ab" =~ /a(.*)b/; | |
1178 | ||
80d38338 | 1179 | The pattern match succeeds and C<$1> is defined, although it |
cf264981 | 1180 | matched "nothing". It didn't really fail to match anything. Rather, it |
2b5ab1e7 | 1181 | matched something that happened to be zero characters long. This is all |
a5f75d66 | 1182 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 1183 | it's an admission that it couldn't give you an honest answer. So you |
3b10bc60 | 1184 | should use C<defined> only when questioning the integrity of what |
7660c0ab | 1185 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
2f9daede TP |
1186 | what you want. |
1187 | ||
dc848c6f | 1188 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 1189 | |
a0d0e21e | 1190 | =item delete EXPR |
d74e8afc | 1191 | X<delete> |
a0d0e21e | 1192 | |
d0a76353 RS |
1193 | Given an expression that specifies an element or slice of a hash, C<delete> |
1194 | deletes the specified elements from that hash so that exists() on that element | |
1195 | no longer returns true. Setting a hash element to the undefined value does | |
1196 | not remove its key, but deleting it does; see L</exists>. | |
80d38338 | 1197 | |
d0a76353 | 1198 | It returns the value or values deleted in list context, or the last such |
80d38338 | 1199 | element in scalar context. The return list's length always matches that of |
d0a76353 RS |
1200 | the argument list: deleting non-existent elements returns the undefined value |
1201 | in their corresponding positions. | |
80d38338 | 1202 | |
d0a76353 RS |
1203 | delete() may also be used on arrays and array slices, but its behavior is less |
1204 | straightforward. Although exists() will return false for deleted entries, | |
1205 | deleting array elements never changes indices of existing values; use shift() | |
1206 | or splice() for that. However, if all deleted elements fall at the end of an | |
1207 | array, the array's size shrinks to the position of the highest element that | |
1208 | still tests true for exists(), or to 0 if none do. | |
1209 | ||
1210 | B<Be aware> that calling delete on array values is deprecated and likely to | |
1211 | be removed in a future version of Perl. | |
80d38338 TC |
1212 | |
1213 | Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to | |
1214 | a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash | |
1215 | or array may not necessarily return anything; it depends on the implementation | |
1216 | of the C<tied> package's DELETE method, which may do whatever it pleases. | |
a0d0e21e | 1217 | |
80d38338 TC |
1218 | The C<delete local EXPR> construct localizes the deletion to the current |
1219 | block at run time. Until the block exits, elements locally deleted | |
1220 | temporarily no longer exist. See L<perlsub/"Localized deletion of elements | |
1221 | of composite types">. | |
eba0920a EM |
1222 | |
1223 | %hash = (foo => 11, bar => 22, baz => 33); | |
1224 | $scalar = delete $hash{foo}; # $scalar is 11 | |
1225 | $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 | |
1226 | @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) | |
1227 | ||
01020589 | 1228 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
a0d0e21e | 1229 | |
5f05dabc | 1230 | foreach $key (keys %HASH) { |
a9a5a0dc | 1231 | delete $HASH{$key}; |
a0d0e21e LW |
1232 | } |
1233 | ||
01020589 | 1234 | foreach $index (0 .. $#ARRAY) { |
a9a5a0dc | 1235 | delete $ARRAY[$index]; |
01020589 GS |
1236 | } |
1237 | ||
1238 | And so do these: | |
5f05dabc | 1239 | |
01020589 GS |
1240 | delete @HASH{keys %HASH}; |
1241 | ||
9740c838 | 1242 | delete @ARRAY[0 .. $#ARRAY]; |
5f05dabc | 1243 | |
80d38338 TC |
1244 | But both are slower than assigning the empty list |
1245 | or undefining %HASH or @ARRAY, which is the customary | |
1246 | way to empty out an aggregate: | |
01020589 | 1247 | |
5ed4f2ec | 1248 | %HASH = (); # completely empty %HASH |
1249 | undef %HASH; # forget %HASH ever existed | |
2b5ab1e7 | 1250 | |
5ed4f2ec | 1251 | @ARRAY = (); # completely empty @ARRAY |
1252 | undef @ARRAY; # forget @ARRAY ever existed | |
2b5ab1e7 | 1253 | |
80d38338 TC |
1254 | The EXPR can be arbitrarily complicated provided its |
1255 | final operation is an element or slice of an aggregate: | |
a0d0e21e LW |
1256 | |
1257 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 1258 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e | 1259 | |
01020589 GS |
1260 | delete $ref->[$x][$y][$index]; |
1261 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1262 | ||
a0d0e21e | 1263 | =item die LIST |
d74e8afc | 1264 | X<die> X<throw> X<exception> X<raise> X<$@> X<abort> |
a0d0e21e | 1265 | |
19799a22 GS |
1266 | Outside an C<eval>, prints the value of LIST to C<STDERR> and |
1267 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, | |
61eff3bc JH |
1268 | exits with the value of C<<< ($? >> 8) >>> (backtick `command` |
1269 | status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside | |
19799a22 GS |
1270 | an C<eval(),> the error message is stuffed into C<$@> and the |
1271 | C<eval> is terminated with the undefined value. This makes | |
1272 | C<die> the way to raise an exception. | |
a0d0e21e LW |
1273 | |
1274 | Equivalent examples: | |
1275 | ||
1276 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 1277 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e | 1278 | |
ccac6780 | 1279 | If the last element of LIST does not end in a newline, the current |
df37ec69 WW |
1280 | script line number and input line number (if any) are also printed, |
1281 | and a newline is supplied. Note that the "input line number" (also | |
1282 | known as "chunk") is subject to whatever notion of "line" happens to | |
1283 | be currently in effect, and is also available as the special variable | |
1284 | C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1285 | ||
1286 | Hint: sometimes appending C<", stopped"> to your message will cause it | |
1287 | to make better sense when the string C<"at foo line 123"> is appended. | |
1288 | Suppose you are running script "canasta". | |
a0d0e21e LW |
1289 | |
1290 | die "/etc/games is no good"; | |
1291 | die "/etc/games is no good, stopped"; | |
1292 | ||
1293 | produce, respectively | |
1294 | ||
1295 | /etc/games is no good at canasta line 123. | |
1296 | /etc/games is no good, stopped at canasta line 123. | |
1297 | ||
2b5ab1e7 | 1298 | See also exit(), warn(), and the Carp module. |
a0d0e21e | 1299 | |
a96d0188 | 1300 | If the output is empty and C<$@> already contains a value (typically from a |
7660c0ab | 1301 | previous eval) that value is reused after appending C<"\t...propagated">. |
fb73857a | 1302 | This is useful for propagating exceptions: |
1303 | ||
1304 | eval { ... }; | |
1305 | die unless $@ =~ /Expected exception/; | |
1306 | ||
a96d0188 | 1307 | If the output is empty and C<$@> contains an object reference that has a |
ad216e65 JH |
1308 | C<PROPAGATE> method, that method will be called with additional file |
1309 | and line number parameters. The return value replaces the value in | |
80d38338 | 1310 | C<$@>. i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> |
ad216e65 JH |
1311 | were called. |
1312 | ||
7660c0ab | 1313 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 1314 | |
80d38338 TC |
1315 | You can also call C<die> with a reference argument, and if this is trapped |
1316 | within an C<eval>, C<$@> contains that reference. This permits more | |
1317 | elaborate exception handling using objects that maintain arbitrary state | |
1318 | about the exception. Such a scheme is sometimes preferable to matching | |
1319 | particular string values of C<$@> with regular expressions. Because C<$@> | |
1320 | is a global variable and C<eval> may be used within object implementations, | |
1321 | be careful that analyzing the error object doesn't replace the reference in | |
1322 | the global variable. It's easiest to make a local copy of the reference | |
1323 | before any manipulations. Here's an example: | |
52531d10 | 1324 | |
80d38338 | 1325 | use Scalar::Util "blessed"; |
da279afe | 1326 | |
52531d10 | 1327 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; |
746d7dd7 GL |
1328 | if (my $ev_err = $@) { |
1329 | if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { | |
52531d10 GS |
1330 | # handle Some::Module::Exception |
1331 | } | |
1332 | else { | |
1333 | # handle all other possible exceptions | |
1334 | } | |
1335 | } | |
1336 | ||
3b10bc60 | 1337 | Because Perl stringifies uncaught exception messages before display, |
80d38338 | 1338 | you'll probably want to overload stringification operations on |
52531d10 GS |
1339 | exception objects. See L<overload> for details about that. |
1340 | ||
19799a22 GS |
1341 | You can arrange for a callback to be run just before the C<die> |
1342 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
3b10bc60 | 1343 | handler is called with the error text and can change the error |
19799a22 GS |
1344 | message, if it sees fit, by calling C<die> again. See |
1345 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and | |
cf264981 | 1346 | L<"eval BLOCK"> for some examples. Although this feature was |
19799a22 | 1347 | to be run only right before your program was to exit, this is not |
3b10bc60 | 1348 | currently so: the C<$SIG{__DIE__}> hook is currently called |
19799a22 GS |
1349 | even inside eval()ed blocks/strings! If one wants the hook to do |
1350 | nothing in such situations, put | |
fb73857a | 1351 | |
5ed4f2ec | 1352 | die @_ if $^S; |
fb73857a | 1353 | |
19799a22 GS |
1354 | as the first line of the handler (see L<perlvar/$^S>). Because |
1355 | this promotes strange action at a distance, this counterintuitive | |
b76cc8ba | 1356 | behavior may be fixed in a future release. |
774d564b | 1357 | |
a0d0e21e | 1358 | =item do BLOCK |
d74e8afc | 1359 | X<do> X<block> |
a0d0e21e LW |
1360 | |
1361 | Not really a function. Returns the value of the last command in the | |
6b275a1f RGS |
1362 | sequence of commands indicated by BLOCK. When modified by the C<while> or |
1363 | C<until> loop modifier, executes the BLOCK once before testing the loop | |
1364 | condition. (On other statements the loop modifiers test the conditional | |
1365 | first.) | |
a0d0e21e | 1366 | |
4968c1e4 | 1367 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 TC |
1368 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
1369 | See L<perlsyn> for alternative strategies. | |
4968c1e4 | 1370 | |
a0d0e21e | 1371 | =item do SUBROUTINE(LIST) |
d74e8afc | 1372 | X<do> |
a0d0e21e | 1373 | |
cf264981 | 1374 | This form of subroutine call is deprecated. See L<perlsub>. |
a0d0e21e LW |
1375 | |
1376 | =item do EXPR | |
d74e8afc | 1377 | X<do> |
a0d0e21e LW |
1378 | |
1379 | Uses the value of EXPR as a filename and executes the contents of the | |
ea63ef19 | 1380 | file as a Perl script. |
a0d0e21e LW |
1381 | |
1382 | do 'stat.pl'; | |
1383 | ||
1384 | is just like | |
1385 | ||
986b19de | 1386 | eval `cat stat.pl`; |
a0d0e21e | 1387 | |
2b5ab1e7 | 1388 | except that it's more efficient and concise, keeps track of the current |
ea63ef19 | 1389 | filename for error messages, searches the @INC directories, and updates |
2b5ab1e7 TC |
1390 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these |
1391 | variables. It also differs in that code evaluated with C<do FILENAME> | |
1392 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1393 | same, however, in that it does reparse the file every time you call it, | |
1394 | so you probably don't want to do this inside a loop. | |
a0d0e21e | 1395 | |
8e30cc93 | 1396 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
2b5ab1e7 | 1397 | error. If C<do> can read the file but cannot compile it, it |
8e30cc93 MG |
1398 | returns undef and sets an error message in C<$@>. If the file is |
1399 | successfully compiled, C<do> returns the value of the last expression | |
1400 | evaluated. | |
1401 | ||
80d38338 | 1402 | Inclusion of library modules is better done with the |
19799a22 | 1403 | C<use> and C<require> operators, which also do automatic error checking |
4633a7c4 | 1404 | and raise an exception if there's a problem. |
a0d0e21e | 1405 | |
5a964f20 TC |
1406 | You might like to use C<do> to read in a program configuration |
1407 | file. Manual error checking can be done this way: | |
1408 | ||
b76cc8ba | 1409 | # read in config files: system first, then user |
f86cebdf | 1410 | for $file ("/share/prog/defaults.rc", |
b76cc8ba | 1411 | "$ENV{HOME}/.someprogrc") |
a9a5a0dc VP |
1412 | { |
1413 | unless ($return = do $file) { | |
1414 | warn "couldn't parse $file: $@" if $@; | |
1415 | warn "couldn't do $file: $!" unless defined $return; | |
1416 | warn "couldn't run $file" unless $return; | |
1417 | } | |
5a964f20 TC |
1418 | } |
1419 | ||
a0d0e21e | 1420 | =item dump LABEL |
d74e8afc | 1421 | X<dump> X<core> X<undump> |
a0d0e21e | 1422 | |
1614b0e3 JD |
1423 | =item dump |
1424 | ||
19799a22 GS |
1425 | This function causes an immediate core dump. See also the B<-u> |
1426 | command-line switch in L<perlrun>, which does the same thing. | |
1427 | Primarily this is so that you can use the B<undump> program (not | |
1428 | supplied) to turn your core dump into an executable binary after | |
1429 | having initialized all your variables at the beginning of the | |
1430 | program. When the new binary is executed it will begin by executing | |
1431 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1432 | Think of it as a goto with an intervening core dump and reincarnation. | |
1433 | If C<LABEL> is omitted, restarts the program from the top. | |
1434 | ||
1435 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1436 | be open any more when the program is reincarnated, with possible | |
80d38338 | 1437 | resulting confusion by Perl. |
19799a22 | 1438 | |
59f521f4 RGS |
1439 | This function is now largely obsolete, mostly because it's very hard to |
1440 | convert a core file into an executable. That's why you should now invoke | |
1441 | it as C<CORE::dump()>, if you don't want to be warned against a possible | |
ac206dc8 | 1442 | typo. |
19799a22 | 1443 | |
aa689395 | 1444 | =item each HASH |
d74e8afc | 1445 | X<each> X<hash, iterator> |
aa689395 | 1446 | |
aeedbbed NC |
1447 | =item each ARRAY |
1448 | X<array, iterator> | |
1449 | ||
80d38338 TC |
1450 | When called in list context, returns a 2-element list consisting of the key |
1451 | and value for the next element of a hash, or the index and value for the | |
1452 | next element of an array, so that you can iterate over it. When called in | |
1453 | scalar context, returns only the key (not the value) in a hash, or the index | |
1454 | in an array. | |
2f9daede | 1455 | |
aeedbbed | 1456 | Hash entries are returned in an apparently random order. The actual random |
3b10bc60 | 1457 | order is subject to change in future versions of Perl, but it is |
504f80c1 | 1458 | guaranteed to be in the same order as either the C<keys> or C<values> |
4546b9e6 | 1459 | function would produce on the same (unmodified) hash. Since Perl |
22883ac5 | 1460 | 5.8.2 the ordering can be different even between different runs of Perl |
4546b9e6 | 1461 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). |
ab192400 | 1462 | |
80d38338 TC |
1463 | After C<each> has returned all entries from the hash or array, the next |
1464 | call to C<each> returns the empty list in list context and C<undef> in | |
1465 | scalar context. The next call following that one restarts iteration. Each | |
1466 | hash or array has its own internal iterator, accessed by C<each>, C<keys>, | |
1467 | and C<values>. The iterator is implicitly reset when C<each> has reached | |
1468 | the end as just described; it can be explicitly reset by calling C<keys> or | |
1469 | C<values> on the hash or array. If you add or delete a hash's elements | |
1470 | while iterating over it, entries may be skipped or duplicated--so don't do | |
1471 | that. Exception: It is always safe to delete the item most recently | |
3b10bc60 | 1472 | returned by C<each()>, so the following code works properly: |
74fc8b5f MJD |
1473 | |
1474 | while (($key, $value) = each %hash) { | |
1475 | print $key, "\n"; | |
1476 | delete $hash{$key}; # This is safe | |
1477 | } | |
aa689395 | 1478 | |
80d38338 | 1479 | This prints out your environment like the printenv(1) program, |
3b10bc60 | 1480 | but in a different order: |
a0d0e21e LW |
1481 | |
1482 | while (($key,$value) = each %ENV) { | |
a9a5a0dc | 1483 | print "$key=$value\n"; |
a0d0e21e LW |
1484 | } |
1485 | ||
19799a22 | 1486 | See also C<keys>, C<values> and C<sort>. |
a0d0e21e LW |
1487 | |
1488 | =item eof FILEHANDLE | |
d74e8afc ITB |
1489 | X<eof> |
1490 | X<end of file> | |
1491 | X<end-of-file> | |
a0d0e21e | 1492 | |
4633a7c4 LW |
1493 | =item eof () |
1494 | ||
a0d0e21e LW |
1495 | =item eof |
1496 | ||
1497 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1498 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
5a964f20 | 1499 | gives the real filehandle. (Note that this function actually |
80d38338 | 1500 | reads a character and then C<ungetc>s it, so isn't useful in an |
748a9306 | 1501 | interactive context.) Do not read from a terminal file (or call |
19799a22 | 1502 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
748a9306 LW |
1503 | as terminals may lose the end-of-file condition if you do. |
1504 | ||
820475bd | 1505 | An C<eof> without an argument uses the last file read. Using C<eof()> |
80d38338 | 1506 | with empty parentheses is different. It refers to the pseudo file |
820475bd | 1507 | formed from the files listed on the command line and accessed via the |
61eff3bc JH |
1508 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, |
1509 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
820475bd | 1510 | used will cause C<@ARGV> to be examined to determine if input is |
67408cae | 1511 | available. Similarly, an C<eof()> after C<< <> >> has returned |
efdd0218 RB |
1512 | end-of-file will assume you are processing another C<@ARGV> list, |
1513 | and if you haven't set C<@ARGV>, will read input from C<STDIN>; | |
1514 | see L<perlop/"I/O Operators">. | |
820475bd | 1515 | |
61eff3bc | 1516 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to |
3b10bc60 | 1517 | detect the end of each file, C<eof()> will detect the end of only the |
820475bd | 1518 | last file. Examples: |
a0d0e21e | 1519 | |
748a9306 LW |
1520 | # reset line numbering on each input file |
1521 | while (<>) { | |
a9a5a0dc VP |
1522 | next if /^\s*#/; # skip comments |
1523 | print "$.\t$_"; | |
5a964f20 | 1524 | } continue { |
a9a5a0dc | 1525 | close ARGV if eof; # Not eof()! |
748a9306 LW |
1526 | } |
1527 | ||
a0d0e21e LW |
1528 | # insert dashes just before last line of last file |
1529 | while (<>) { | |
a9a5a0dc VP |
1530 | if (eof()) { # check for end of last file |
1531 | print "--------------\n"; | |
1532 | } | |
1533 | print; | |
1534 | last if eof(); # needed if we're reading from a terminal | |
a0d0e21e LW |
1535 | } |
1536 | ||
a0d0e21e | 1537 | Practical hint: you almost never need to use C<eof> in Perl, because the |
3ce0d271 GS |
1538 | input operators typically return C<undef> when they run out of data, or if |
1539 | there was an error. | |
a0d0e21e LW |
1540 | |
1541 | =item eval EXPR | |
d74e8afc | 1542 | X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> |
f723aae1 | 1543 | X<error, handling> X<exception, handling> |
a0d0e21e LW |
1544 | |
1545 | =item eval BLOCK | |
1546 | ||
ce2984c3 PF |
1547 | =item eval |
1548 | ||
c7cc6f1c GS |
1549 | In the first form, the return value of EXPR is parsed and executed as if it |
1550 | were a little Perl program. The value of the expression (which is itself | |
5a964f20 | 1551 | determined within scalar context) is first parsed, and if there weren't any |
be3174d2 GS |
1552 | errors, executed in the lexical context of the current Perl program, so |
1553 | that any variable settings or subroutine and format definitions remain | |
cf264981 | 1554 | afterwards. Note that the value is parsed every time the C<eval> executes. |
be3174d2 GS |
1555 | If EXPR is omitted, evaluates C<$_>. This form is typically used to |
1556 | delay parsing and subsequent execution of the text of EXPR until run time. | |
c7cc6f1c GS |
1557 | |
1558 | In the second form, the code within the BLOCK is parsed only once--at the | |
cf264981 | 1559 | same time the code surrounding the C<eval> itself was parsed--and executed |
c7cc6f1c GS |
1560 | within the context of the current Perl program. This form is typically |
1561 | used to trap exceptions more efficiently than the first (see below), while | |
1562 | also providing the benefit of checking the code within BLOCK at compile | |
1563 | time. | |
1564 | ||
1565 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1566 | the BLOCK. | |
1567 | ||
1568 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1569 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1570 | as with subroutines. The expression providing the return value is evaluated |
cf264981 SP |
1571 | in void, scalar, or list context, depending on the context of the C<eval> |
1572 | itself. See L</wantarray> for more on how the evaluation context can be | |
1573 | determined. | |
a0d0e21e | 1574 | |
19799a22 | 1575 | If there is a syntax error or runtime error, or a C<die> statement is |
bbead3ca BL |
1576 | executed, C<eval> returns an undefined value in scalar context |
1577 | or an empty list in list context, and C<$@> is set to the | |
3b10bc60 | 1578 | error message. If there was no error, C<$@> is guaranteed to be the empty |
1579 | string. Beware that using C<eval> neither silences Perl from printing | |
c7cc6f1c | 1580 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
d9984052 A |
1581 | To do either of those, you have to use the C<$SIG{__WARN__}> facility, or |
1582 | turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. | |
1583 | See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. | |
a0d0e21e | 1584 | |
19799a22 GS |
1585 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
1586 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
a0d0e21e LW |
1587 | is implemented. It is also Perl's exception trapping mechanism, where |
1588 | the die operator is used to raise exceptions. | |
1589 | ||
5f1da31c NT |
1590 | If you want to trap errors when loading an XS module, some problems with |
1591 | the binary interface (such as Perl version skew) may be fatal even with | |
1592 | C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>. | |
1593 | ||
a0d0e21e LW |
1594 | If the code to be executed doesn't vary, you may use the eval-BLOCK |
1595 | form to trap run-time errors without incurring the penalty of | |
1596 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1597 | Examples: | |
1598 | ||
54310121 | 1599 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1600 | eval { $answer = $a / $b; }; warn $@ if $@; |
1601 | ||
1602 | # same thing, but less efficient | |
1603 | eval '$answer = $a / $b'; warn $@ if $@; | |
1604 | ||
1605 | # a compile-time error | |
5ed4f2ec | 1606 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1607 | |
1608 | # a run-time error | |
5ed4f2ec | 1609 | eval '$answer ='; # sets $@ |
a0d0e21e | 1610 | |
cf264981 SP |
1611 | Using the C<eval{}> form as an exception trap in libraries does have some |
1612 | issues. Due to the current arguably broken state of C<__DIE__> hooks, you | |
1613 | may wish not to trigger any C<__DIE__> hooks that user code may have installed. | |
2b5ab1e7 | 1614 | You can use the C<local $SIG{__DIE__}> construct for this purpose, |
80d38338 | 1615 | as this example shows: |
774d564b | 1616 | |
80d38338 | 1617 | # a private exception trap for divide-by-zero |
f86cebdf GS |
1618 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
1619 | warn $@ if $@; | |
774d564b | 1620 | |
1621 | This is especially significant, given that C<__DIE__> hooks can call | |
19799a22 | 1622 | C<die> again, which has the effect of changing their error messages: |
774d564b | 1623 | |
1624 | # __DIE__ hooks may modify error messages | |
1625 | { | |
f86cebdf GS |
1626 | local $SIG{'__DIE__'} = |
1627 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1628 | eval { die "foo lives here" }; |
1629 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1630 | } |
1631 | ||
19799a22 | 1632 | Because this promotes action at a distance, this counterintuitive behavior |
2b5ab1e7 TC |
1633 | may be fixed in a future release. |
1634 | ||
19799a22 | 1635 | With an C<eval>, you should be especially careful to remember what's |
a0d0e21e LW |
1636 | being looked at when: |
1637 | ||
5ed4f2ec | 1638 | eval $x; # CASE 1 |
1639 | eval "$x"; # CASE 2 | |
a0d0e21e | 1640 | |
5ed4f2ec | 1641 | eval '$x'; # CASE 3 |
1642 | eval { $x }; # CASE 4 | |
a0d0e21e | 1643 | |
5ed4f2ec | 1644 | eval "\$$x++"; # CASE 5 |
1645 | $$x++; # CASE 6 | |
a0d0e21e | 1646 | |
2f9daede | 1647 | Cases 1 and 2 above behave identically: they run the code contained in |
19799a22 | 1648 | the variable $x. (Although case 2 has misleading double quotes making |
2f9daede | 1649 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1650 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
19799a22 | 1651 | does nothing but return the value of $x. (Case 4 is preferred for |
2f9daede TP |
1652 | purely visual reasons, but it also has the advantage of compiling at |
1653 | compile-time instead of at run-time.) Case 5 is a place where | |
19799a22 | 1654 | normally you I<would> like to use double quotes, except that in this |
2f9daede TP |
1655 | particular situation, you can just use symbolic references instead, as |
1656 | in case 6. | |
a0d0e21e | 1657 | |
8a5a710d DN |
1658 | The assignment to C<$@> occurs before restoration of localised variables, |
1659 | which means a temporary is required if you want to mask some but not all | |
1660 | errors: | |
1661 | ||
1662 | # alter $@ on nefarious repugnancy only | |
1663 | { | |
1664 | my $e; | |
1665 | { | |
1666 | local $@; # protect existing $@ | |
1667 | eval { test_repugnancy() }; | |
1668 | # $@ =~ /nefarious/ and die $@; # DOES NOT WORK | |
1669 | $@ =~ /nefarious/ and $e = $@; | |
1670 | } | |
1671 | die $e if defined $e | |
1672 | } | |
1673 | ||
4968c1e4 | 1674 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 | 1675 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
4968c1e4 | 1676 | |
3b10bc60 | 1677 | An C<eval ''> executed within the C<DB> package doesn't see the usual |
1678 | surrounding lexical scope, but rather the scope of the first non-DB piece | |
1679 | of code that called it. You don't normally need to worry about this unless | |
1680 | you are writing a Perl debugger. | |
d819b83a | 1681 | |
a0d0e21e | 1682 | =item exec LIST |
d74e8afc | 1683 | X<exec> X<execute> |
a0d0e21e | 1684 | |
8bf3b016 GS |
1685 | =item exec PROGRAM LIST |
1686 | ||
3b10bc60 | 1687 | The C<exec> function executes a system command I<and never returns>; |
19799a22 GS |
1688 | use C<system> instead of C<exec> if you want it to return. It fails and |
1689 | returns false only if the command does not exist I<and> it is executed | |
fb73857a | 1690 | directly instead of via your system's command shell (see below). |
a0d0e21e | 1691 | |
19799a22 | 1692 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
80d38338 | 1693 | warns you if there is a following statement that isn't C<die>, C<warn>, |
3b10bc60 | 1694 | or C<exit> (if C<-w> is set--but you always do that, right?). If you |
19799a22 | 1695 | I<really> want to follow an C<exec> with some other statement, you |
55d729e4 GS |
1696 | can use one of these styles to avoid the warning: |
1697 | ||
5a964f20 TC |
1698 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1699 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1700 | |
5a964f20 | 1701 | If there is more than one argument in LIST, or if LIST is an array |
f86cebdf | 1702 | with more than one value, calls execvp(3) with the arguments in LIST. |
5a964f20 TC |
1703 | If there is only one scalar argument or an array with one element in it, |
1704 | the argument is checked for shell metacharacters, and if there are any, | |
1705 | the entire argument is passed to the system's command shell for parsing | |
1706 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1707 | If there are no shell metacharacters in the argument, it is split into | |
b76cc8ba | 1708 | words and passed directly to C<execvp>, which is more efficient. |
19799a22 | 1709 | Examples: |
a0d0e21e | 1710 | |
19799a22 GS |
1711 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
1712 | exec "sort $outfile | uniq"; | |
a0d0e21e LW |
1713 | |
1714 | If you don't really want to execute the first argument, but want to lie | |
1715 | to the program you are executing about its own name, you can specify | |
1716 | the program you actually want to run as an "indirect object" (without a | |
1717 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1718 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1719 | the list.) Example: |
1720 | ||
1721 | $shell = '/bin/csh'; | |
5ed4f2ec | 1722 | exec $shell '-sh'; # pretend it's a login shell |
a0d0e21e LW |
1723 | |
1724 | or, more directly, | |
1725 | ||
5ed4f2ec | 1726 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell |
a0d0e21e | 1727 | |
3b10bc60 | 1728 | When the arguments get executed via the system shell, results are |
1729 | subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
bb32b41a GS |
1730 | for details. |
1731 | ||
19799a22 GS |
1732 | Using an indirect object with C<exec> or C<system> is also more |
1733 | secure. This usage (which also works fine with system()) forces | |
1734 | interpretation of the arguments as a multivalued list, even if the | |
1735 | list had just one argument. That way you're safe from the shell | |
1736 | expanding wildcards or splitting up words with whitespace in them. | |
5a964f20 TC |
1737 | |
1738 | @args = ( "echo surprise" ); | |
1739 | ||
2b5ab1e7 | 1740 | exec @args; # subject to shell escapes |
f86cebdf | 1741 | # if @args == 1 |
2b5ab1e7 | 1742 | exec { $args[0] } @args; # safe even with one-arg list |
5a964f20 TC |
1743 | |
1744 | The first version, the one without the indirect object, ran the I<echo> | |
80d38338 TC |
1745 | program, passing it C<"surprise"> an argument. The second version didn't; |
1746 | it tried to run a program named I<"echo surprise">, didn't find it, and set | |
1747 | C<$?> to a non-zero value indicating failure. | |
5a964f20 | 1748 | |
80d38338 | 1749 | Beginning with v5.6.0, Perl attempts to flush all files opened for |
0f897271 GS |
1750 | output before the exec, but this may not be supported on some platforms |
1751 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH | |
1752 | in English) or call the C<autoflush()> method of C<IO::Handle> on any | |
80d38338 | 1753 | open handles to avoid lost output. |
0f897271 | 1754 | |
80d38338 TC |
1755 | Note that C<exec> will not call your C<END> blocks, nor will it invoke |
1756 | C<DESTROY> methods on your objects. | |
7660c0ab | 1757 | |
a0d0e21e | 1758 | =item exists EXPR |
d74e8afc | 1759 | X<exists> X<autovivification> |
a0d0e21e | 1760 | |
d0a76353 RS |
1761 | Given an expression that specifies an element of a hash, returns true if the |
1762 | specified element in the hash has ever been initialized, even if the | |
1763 | corresponding value is undefined. | |
a0d0e21e | 1764 | |
5ed4f2ec | 1765 | print "Exists\n" if exists $hash{$key}; |
1766 | print "Defined\n" if defined $hash{$key}; | |
01020589 GS |
1767 | print "True\n" if $hash{$key}; |
1768 | ||
d0a76353 RS |
1769 | exists may also be called on array elements, but its behavior is much less |
1770 | obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware> | |
1771 | that calling exists on array values is deprecated and likely to be removed in | |
1772 | a future version of Perl. | |
1773 | ||
5ed4f2ec | 1774 | print "Exists\n" if exists $array[$index]; |
1775 | print "Defined\n" if defined $array[$index]; | |
01020589 | 1776 | print "True\n" if $array[$index]; |
a0d0e21e | 1777 | |
8ea97a1e | 1778 | A hash or array element can be true only if it's defined, and defined if |
a0d0e21e LW |
1779 | it exists, but the reverse doesn't necessarily hold true. |
1780 | ||
afebc493 GS |
1781 | Given an expression that specifies the name of a subroutine, |
1782 | returns true if the specified subroutine has ever been declared, even | |
1783 | if it is undefined. Mentioning a subroutine name for exists or defined | |
80d38338 | 1784 | does not count as declaring it. Note that a subroutine that does not |
847c7ebe DD |
1785 | exist may still be callable: its package may have an C<AUTOLOAD> |
1786 | method that makes it spring into existence the first time that it is | |
3b10bc60 | 1787 | called; see L<perlsub>. |
afebc493 | 1788 | |
5ed4f2ec | 1789 | print "Exists\n" if exists &subroutine; |
1790 | print "Defined\n" if defined &subroutine; | |
afebc493 | 1791 | |
a0d0e21e | 1792 | Note that the EXPR can be arbitrarily complicated as long as the final |
afebc493 | 1793 | operation is a hash or array key lookup or subroutine name: |
a0d0e21e | 1794 | |
5ed4f2ec | 1795 | if (exists $ref->{A}->{B}->{$key}) { } |
1796 | if (exists $hash{A}{B}{$key}) { } | |
2b5ab1e7 | 1797 | |
5ed4f2ec | 1798 | if (exists $ref->{A}->{B}->[$ix]) { } |
1799 | if (exists $hash{A}{B}[$ix]) { } | |
01020589 | 1800 | |
afebc493 GS |
1801 | if (exists &{$ref->{A}{B}{$key}}) { } |
1802 | ||
3b10bc60 | 1803 | Although the mostly deeply nested array or hash will not spring into |
1804 | existence just because its existence was tested, any intervening ones will. | |
61eff3bc | 1805 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring |
01020589 | 1806 | into existence due to the existence test for the $key element above. |
3b10bc60 | 1807 | This happens anywhere the arrow operator is used, including even here: |
5a964f20 | 1808 | |
2b5ab1e7 | 1809 | undef $ref; |
5ed4f2ec | 1810 | if (exists $ref->{"Some key"}) { } |
1811 | print $ref; # prints HASH(0x80d3d5c) | |
2b5ab1e7 TC |
1812 | |
1813 | This surprising autovivification in what does not at first--or even | |
1814 | second--glance appear to be an lvalue context may be fixed in a future | |
5a964f20 | 1815 | release. |
a0d0e21e | 1816 | |
afebc493 GS |
1817 | Use of a subroutine call, rather than a subroutine name, as an argument |
1818 | to exists() is an error. | |
1819 | ||
5ed4f2ec | 1820 | exists ⊂ # OK |
1821 | exists &sub(); # Error | |
afebc493 | 1822 | |
a0d0e21e | 1823 | =item exit EXPR |
d74e8afc | 1824 | X<exit> X<terminate> X<abort> |
a0d0e21e | 1825 | |
ce2984c3 PF |
1826 | =item exit |
1827 | ||
2b5ab1e7 | 1828 | Evaluates EXPR and exits immediately with that value. Example: |
a0d0e21e LW |
1829 | |
1830 | $ans = <STDIN>; | |
1831 | exit 0 if $ans =~ /^[Xx]/; | |
1832 | ||
19799a22 | 1833 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
2b5ab1e7 TC |
1834 | universally recognized values for EXPR are C<0> for success and C<1> |
1835 | for error; other values are subject to interpretation depending on the | |
1836 | environment in which the Perl program is running. For example, exiting | |
1837 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1838 | the mailer to return the item undelivered, but that's not true everywhere. | |
a0d0e21e | 1839 | |
19799a22 GS |
1840 | Don't use C<exit> to abort a subroutine if there's any chance that |
1841 | someone might want to trap whatever error happened. Use C<die> instead, | |
1842 | which can be trapped by an C<eval>. | |
28757baa | 1843 | |
19799a22 | 1844 | The exit() function does not always exit immediately. It calls any |
2b5ab1e7 | 1845 | defined C<END> routines first, but these C<END> routines may not |
19799a22 | 1846 | themselves abort the exit. Likewise any object destructors that need to |
2b5ab1e7 TC |
1847 | be called are called before the real exit. If this is a problem, you |
1848 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
87275199 | 1849 | See L<perlmod> for details. |
5a964f20 | 1850 | |
a0d0e21e | 1851 | =item exp EXPR |
d74e8afc | 1852 | X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> |
a0d0e21e | 1853 | |
54310121 | 1854 | =item exp |
bbce6d69 | 1855 | |
b76cc8ba | 1856 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1857 | If EXPR is omitted, gives C<exp($_)>. |
1858 | ||
1859 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
d74e8afc | 1860 | X<fcntl> |
a0d0e21e | 1861 | |
f86cebdf | 1862 | Implements the fcntl(2) function. You'll probably have to say |
a0d0e21e LW |
1863 | |
1864 | use Fcntl; | |
1865 | ||
0ade1984 | 1866 | first to get the correct constant definitions. Argument processing and |
3b10bc60 | 1867 | value returned work just like C<ioctl> below. |
a0d0e21e LW |
1868 | For example: |
1869 | ||
1870 | use Fcntl; | |
5a964f20 | 1871 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
a9a5a0dc | 1872 | or die "can't fcntl F_GETFL: $!"; |
5a964f20 | 1873 | |
554ad1fc | 1874 | You don't have to check for C<defined> on the return from C<fcntl>. |
951ba7fe GS |
1875 | Like C<ioctl>, it maps a C<0> return from the system call into |
1876 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
2b5ab1e7 TC |
1877 | in numeric context. It is also exempt from the normal B<-w> warnings |
1878 | on improper numeric conversions. | |
5a964f20 | 1879 | |
3b10bc60 | 1880 | Note that C<fcntl> raises an exception if used on a machine that |
2b5ab1e7 TC |
1881 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
1882 | manpage to learn what functions are available on your system. | |
a0d0e21e | 1883 | |
be2f7487 TH |
1884 | Here's an example of setting a filehandle named C<REMOTE> to be |
1885 | non-blocking at the system level. You'll have to negotiate C<$|> | |
1886 | on your own, though. | |
1887 | ||
1888 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
1889 | ||
1890 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
1891 | or die "Can't get flags for the socket: $!\n"; | |
1892 | ||
1893 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
1894 | or die "Can't set flags for the socket: $!\n"; | |
1895 | ||
a0d0e21e | 1896 | =item fileno FILEHANDLE |
d74e8afc | 1897 | X<fileno> |
a0d0e21e | 1898 | |
2b5ab1e7 TC |
1899 | Returns the file descriptor for a filehandle, or undefined if the |
1900 | filehandle is not open. This is mainly useful for constructing | |
19799a22 | 1901 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
2b5ab1e7 TC |
1902 | If FILEHANDLE is an expression, the value is taken as an indirect |
1903 | filehandle, generally its name. | |
5a964f20 | 1904 | |
b76cc8ba | 1905 | You can use this to find out whether two handles refer to the |
5a964f20 TC |
1906 | same underlying descriptor: |
1907 | ||
1908 | if (fileno(THIS) == fileno(THAT)) { | |
a9a5a0dc | 1909 | print "THIS and THAT are dups\n"; |
b76cc8ba NIS |
1910 | } |
1911 | ||
1912 | (Filehandles connected to memory objects via new features of C<open> may | |
1913 | return undefined even though they are open.) | |
1914 | ||
a0d0e21e LW |
1915 | |
1916 | =item flock FILEHANDLE,OPERATION | |
d74e8afc | 1917 | X<flock> X<lock> X<locking> |
a0d0e21e | 1918 | |
19799a22 GS |
1919 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
1920 | for success, false on failure. Produces a fatal error if used on a | |
2b5ab1e7 | 1921 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
19799a22 | 1922 | C<flock> is Perl's portable file locking interface, although it locks |
3b10bc60 | 1923 | entire files only, not records. |
2b5ab1e7 TC |
1924 | |
1925 | Two potentially non-obvious but traditional C<flock> semantics are | |
1926 | that it waits indefinitely until the lock is granted, and that its locks | |
1927 | B<merely advisory>. Such discretionary locks are more flexible, but offer | |
cf264981 SP |
1928 | fewer guarantees. This means that programs that do not also use C<flock> |
1929 | may modify files locked with C<flock>. See L<perlport>, | |
2b5ab1e7 TC |
1930 | your port's specific documentation, or your system-specific local manpages |
1931 | for details. It's best to assume traditional behavior if you're writing | |
1932 | portable programs. (But if you're not, you should as always feel perfectly | |
1933 | free to write for your own system's idiosyncrasies (sometimes called | |
1934 | "features"). Slavish adherence to portability concerns shouldn't get | |
1935 | in the way of your getting your job done.) | |
a3cb178b | 1936 | |
8ebc5c01 | 1937 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
1938 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
ea3105be | 1939 | you can use the symbolic names if you import them from the Fcntl module, |
68dc0745 | 1940 | either individually, or as a group using the ':flock' tag. LOCK_SH |
1941 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
ea3105be | 1942 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
3b10bc60 | 1943 | LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking |
1944 | waiting for the lock; check the return status to see if you got it. | |
68dc0745 | 1945 | |
2b5ab1e7 TC |
1946 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
1947 | before locking or unlocking it. | |
8ebc5c01 | 1948 | |
f86cebdf | 1949 | Note that the emulation built with lockf(3) doesn't provide shared |
8ebc5c01 | 1950 | locks, and it requires that FILEHANDLE be open with write intent. These |
2b5ab1e7 | 1951 | are the semantics that lockf(3) implements. Most if not all systems |
f86cebdf | 1952 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
8ebc5c01 | 1953 | differing semantics shouldn't bite too many people. |
1954 | ||
becacb53 TM |
1955 | Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE |
1956 | be open with read intent to use LOCK_SH and requires that it be open | |
1957 | with write intent to use LOCK_EX. | |
1958 | ||
19799a22 GS |
1959 | Note also that some versions of C<flock> cannot lock things over the |
1960 | network; you would need to use the more system-specific C<fcntl> for | |
f86cebdf GS |
1961 | that. If you like you can force Perl to ignore your system's flock(2) |
1962 | function, and so provide its own fcntl(2)-based emulation, by passing | |
8ebc5c01 | 1963 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
3b10bc60 | 1964 | Perl. |
4633a7c4 LW |
1965 | |
1966 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 1967 | |
7ed5353d | 1968 | use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants |
a0d0e21e LW |
1969 | |
1970 | sub lock { | |
a9a5a0dc VP |
1971 | my ($fh) = @_; |
1972 | flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; | |
7ed5353d | 1973 | |
a9a5a0dc VP |
1974 | # and, in case someone appended while we were waiting... |
1975 | seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; | |
a0d0e21e LW |
1976 | } |
1977 | ||
1978 | sub unlock { | |
a9a5a0dc VP |
1979 | my ($fh) = @_; |
1980 | flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; | |
a0d0e21e LW |
1981 | } |
1982 | ||
b0169937 | 1983 | open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") |
5ed4f2ec | 1984 | or die "Can't open mailbox: $!"; |
a0d0e21e | 1985 | |
7ed5353d | 1986 | lock($mbox); |
b0169937 | 1987 | print $mbox $msg,"\n\n"; |
7ed5353d | 1988 | unlock($mbox); |
a0d0e21e | 1989 | |
3b10bc60 | 1990 | On systems that support a real flock(2), locks are inherited across fork() |
1991 | calls, whereas those that must resort to the more capricious fcntl(2) | |
1992 | function lose their locks, making it seriously harder to write servers. | |
2b5ab1e7 | 1993 | |
cb1a09d0 | 1994 | See also L<DB_File> for other flock() examples. |
a0d0e21e LW |
1995 | |
1996 | =item fork | |
d74e8afc | 1997 | X<fork> X<child> X<parent> |
a0d0e21e | 1998 | |
2b5ab1e7 TC |
1999 | Does a fork(2) system call to create a new process running the |
2000 | same program at the same point. It returns the child pid to the | |
2001 | parent process, C<0> to the child process, or C<undef> if the fork is | |
2002 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
2003 | are shared, while everything else is copied. On most systems supporting | |
2004 | fork(), great care has gone into making it extremely efficient (for | |
2005 | example, using copy-on-write technology on data pages), making it the | |
2006 | dominant paradigm for multitasking over the last few decades. | |
5a964f20 | 2007 | |
80d38338 | 2008 | Beginning with v5.6.0, Perl attempts to flush all files opened for |
0f897271 GS |
2009 | output before forking the child process, but this may not be supported |
2010 | on some platforms (see L<perlport>). To be safe, you may need to set | |
2011 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
80d38338 | 2012 | C<IO::Handle> on any open handles to avoid duplicate output. |
a0d0e21e | 2013 | |
19799a22 | 2014 | If you C<fork> without ever waiting on your children, you will |
2b5ab1e7 TC |
2015 | accumulate zombies. On some systems, you can avoid this by setting |
2016 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
2017 | forking and reaping moribund children. | |
cb1a09d0 | 2018 | |
28757baa | 2019 | Note that if your forked child inherits system file descriptors like |
2020 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2b5ab1e7 | 2021 | if you exit, then the remote server (such as, say, a CGI script or a |
19799a22 | 2022 | backgrounded job launched from a remote shell) won't think you're done. |
2b5ab1e7 | 2023 | You should reopen those to F</dev/null> if it's any issue. |
28757baa | 2024 | |
cb1a09d0 | 2025 | =item format |
d74e8afc | 2026 | X<format> |
cb1a09d0 | 2027 | |
19799a22 | 2028 | Declare a picture format for use by the C<write> function. For |
cb1a09d0 AD |
2029 | example: |
2030 | ||
54310121 | 2031 | format Something = |
a9a5a0dc VP |
2032 | Test: @<<<<<<<< @||||| @>>>>> |
2033 | $str, $%, '$' . int($num) | |
cb1a09d0 AD |
2034 | . |
2035 | ||
2036 | $str = "widget"; | |
184e9718 | 2037 | $num = $cost/$quantity; |
cb1a09d0 AD |
2038 | $~ = 'Something'; |
2039 | write; | |
2040 | ||
2041 | See L<perlform> for many details and examples. | |
2042 | ||
8903cb82 | 2043 | =item formline PICTURE,LIST |
d74e8afc | 2044 | X<formline> |
a0d0e21e | 2045 | |
5a964f20 | 2046 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
2047 | too. It formats (see L<perlform>) a list of values according to the |
2048 | contents of PICTURE, placing the output into the format output | |
7660c0ab | 2049 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
19799a22 | 2050 | Eventually, when a C<write> is done, the contents of |
cf264981 SP |
2051 | C<$^A> are written to some filehandle. You could also read C<$^A> |
2052 | and then set C<$^A> back to C<"">. Note that a format typically | |
19799a22 | 2053 | does one C<formline> per line of form, but the C<formline> function itself |
748a9306 | 2054 | doesn't care how many newlines are embedded in the PICTURE. This means |
3b10bc60 | 2055 | that the C<~> and C<~~> tokens treat the entire PICTURE as a single line. |
748a9306 | 2056 | You may therefore need to use multiple formlines to implement a single |
3b10bc60 | 2057 | record format, just like the C<format> compiler. |
748a9306 | 2058 | |
19799a22 | 2059 | Be careful if you put double quotes around the picture, because an C<@> |
748a9306 | 2060 | character may be taken to mean the beginning of an array name. |
19799a22 | 2061 | C<formline> always returns true. See L<perlform> for other examples. |
a0d0e21e LW |
2062 | |
2063 | =item getc FILEHANDLE | |
f723aae1 | 2064 | X<getc> X<getchar> X<character> X<file, read> |
a0d0e21e LW |
2065 | |
2066 | =item getc | |
2067 | ||
2068 | Returns the next character from the input file attached to FILEHANDLE, | |
3b10bc60 | 2069 | or the undefined value at end of file or if there was an error (in |
b5fe5ca2 SR |
2070 | the latter case C<$!> is set). If FILEHANDLE is omitted, reads from |
2071 | STDIN. This is not particularly efficient. However, it cannot be | |
2072 | used by itself to fetch single characters without waiting for the user | |
2073 | to hit enter. For that, try something more like: | |
4633a7c4 LW |
2074 | |
2075 | if ($BSD_STYLE) { | |
a9a5a0dc | 2076 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
4633a7c4 LW |
2077 | } |
2078 | else { | |
a9a5a0dc | 2079 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
2080 | } |
2081 | ||
2082 | $key = getc(STDIN); | |
2083 | ||
2084 | if ($BSD_STYLE) { | |
a9a5a0dc | 2085 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
4633a7c4 LW |
2086 | } |
2087 | else { | |
3b10bc60 | 2088 | system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL |
4633a7c4 LW |
2089 | } |
2090 | print "\n"; | |
2091 | ||
54310121 | 2092 | Determination of whether $BSD_STYLE should be set |
2093 | is left as an exercise to the reader. | |
cb1a09d0 | 2094 | |
19799a22 | 2095 | The C<POSIX::getattr> function can do this more portably on |
2b5ab1e7 TC |
2096 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
2097 | module from your nearest CPAN site; details on CPAN can be found on | |
2098 | L<perlmodlib/CPAN>. | |
a0d0e21e LW |
2099 | |
2100 | =item getlogin | |
d74e8afc | 2101 | X<getlogin> X<login> |
a0d0e21e | 2102 | |
cf264981 | 2103 | This implements the C library function of the same name, which on most |
3b10bc60 | 2104 | systems returns the current login from F</etc/utmp>, if any. If it |
2105 | returns the empty string, use C<getpwuid>. | |
a0d0e21e | 2106 | |
f86702cc | 2107 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 2108 | |
19799a22 GS |
2109 | Do not consider C<getlogin> for authentication: it is not as |
2110 | secure as C<getpwuid>. | |
4633a7c4 | 2111 | |
a0d0e21e | 2112 | =item getpeername SOCKET |
d74e8afc | 2113 | X<getpeername> X<peer> |
a0d0e21e LW |
2114 | |
2115 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
2116 | ||
4633a7c4 LW |
2117 | use Socket; |
2118 | $hersockaddr = getpeername(SOCK); | |
19799a22 | 2119 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
4633a7c4 LW |
2120 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
2121 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
2122 | |
2123 | =item getpgrp PID | |
d74e8afc | 2124 | X<getpgrp> X<group> |
a0d0e21e | 2125 | |
47e29363 | 2126 | Returns the current process group for the specified PID. Use |
7660c0ab | 2127 | a PID of C<0> to get the current process group for the |
4633a7c4 | 2128 | current process. Will raise an exception if used on a machine that |
f86cebdf | 2129 | doesn't implement getpgrp(2). If PID is omitted, returns process |
19799a22 | 2130 | group of current process. Note that the POSIX version of C<getpgrp> |
7660c0ab | 2131 | does not accept a PID argument, so only C<PID==0> is truly portable. |
a0d0e21e LW |
2132 | |
2133 | =item getppid | |
d74e8afc | 2134 | X<getppid> X<parent> X<pid> |
a0d0e21e LW |
2135 | |
2136 | Returns the process id of the parent process. | |
2137 | ||
4d76a344 RGS |
2138 | Note for Linux users: on Linux, the C functions C<getpid()> and |
2139 | C<getppid()> return different values from different threads. In order to | |
3b10bc60 | 2140 | be portable, this behavior is not reflected by the Perl-level function |
4d76a344 | 2141 | C<getppid()>, that returns a consistent value across threads. If you want |
e3256f86 RGS |
2142 | to call the underlying C<getppid()>, you may use the CPAN module |
2143 | C<Linux::Pid>. | |
4d76a344 | 2144 | |
a0d0e21e | 2145 | =item getpriority WHICH,WHO |
d74e8afc | 2146 | X<getpriority> X<priority> X<nice> |
a0d0e21e | 2147 | |
4633a7c4 | 2148 | Returns the current priority for a process, a process group, or a user. |
f4084e39 | 2149 | (See C<getpriority(2)>.) Will raise a fatal exception if used on a |
f86cebdf | 2150 | machine that doesn't implement getpriority(2). |
a0d0e21e LW |
2151 | |
2152 | =item getpwnam NAME | |
d74e8afc ITB |
2153 | X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> |
2154 | X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> | |
2155 | X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> | |
2156 | X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> | |
2157 | X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> | |
2158 | X<endnetent> X<endprotoent> X<endservent> | |
a0d0e21e LW |
2159 | |
2160 | =item getgrnam NAME | |
2161 | ||
2162 | =item gethostbyname NAME | |
2163 | ||
2164 | =item getnetbyname NAME | |
2165 | ||
2166 | =item getprotobyname NAME | |
2167 | ||
2168 | =item getpwuid UID | |
2169 | ||
2170 | =item getgrgid GID | |
2171 | ||
2172 | =item getservbyname NAME,PROTO | |
2173 | ||
2174 | =item gethostbyaddr ADDR,ADDRTYPE | |
2175 | ||
2176 | =item getnetbyaddr ADDR,ADDRTYPE | |
2177 | ||
2178 | =item getprotobynumber NUMBER | |
2179 | ||
2180 | =item getservbyport PORT,PROTO | |
2181 | ||
2182 | =item getpwent | |
2183 | ||
2184 | =item getgrent | |
2185 | ||
2186 | =item gethostent | |
2187 | ||
2188 | =item getnetent | |
2189 | ||
2190 | =item getprotoent | |
2191 | ||
2192 | =item getservent | |
2193 | ||
2194 | =item setpwent | |
2195 | ||
2196 | =item setgrent | |
2197 | ||
2198 | =item sethostent STAYOPEN | |
2199 | ||
2200 | =item setnetent STAYOPEN | |
2201 | ||
2202 | =item setprotoent STAYOPEN | |
2203 | ||
2204 | =item setservent STAYOPEN | |
2205 | ||
2206 | =item endpwent | |
2207 | ||
2208 | =item endgrent | |
2209 | ||
2210 | =item endhostent | |
2211 | ||
2212 | =item endnetent | |
2213 | ||
2214 | =item endprotoent | |
2215 | ||
2216 | =item endservent | |
2217 | ||
80d38338 TC |
2218 | These routines are the same as their counterparts in the |
2219 | system C library. In list context, the return values from the | |
a0d0e21e LW |
2220 | various get routines are as follows: |
2221 | ||
2222 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 2223 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
2224 | ($name,$passwd,$gid,$members) = getgr* |
2225 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
2226 | ($name,$aliases,$addrtype,$net) = getnet* | |
2227 | ($name,$aliases,$proto) = getproto* | |
2228 | ($name,$aliases,$port,$proto) = getserv* | |
2229 | ||
3b10bc60 | 2230 | (If the entry doesn't exist you get an empty list.) |
a0d0e21e | 2231 | |
4602f195 JH |
2232 | The exact meaning of the $gcos field varies but it usually contains |
2233 | the real name of the user (as opposed to the login name) and other | |
2234 | information pertaining to the user. Beware, however, that in many | |
2235 | system users are able to change this information and therefore it | |
106325ad | 2236 | cannot be trusted and therefore the $gcos is tainted (see |
2959b6e3 JH |
2237 | L<perlsec>). The $passwd and $shell, user's encrypted password and |
2238 | login shell, are also tainted, because of the same reason. | |
4602f195 | 2239 | |
5a964f20 | 2240 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
2241 | lookup by name, in which case you get the other thing, whatever it is. |
2242 | (If the entry doesn't exist you get the undefined value.) For example: | |
2243 | ||
5a964f20 TC |
2244 | $uid = getpwnam($name); |
2245 | $name = getpwuid($num); | |
2246 | $name = getpwent(); | |
2247 | $gid = getgrnam($name); | |
08a33e13 | 2248 | $name = getgrgid($num); |
5a964f20 TC |
2249 | $name = getgrent(); |
2250 | #etc. | |
a0d0e21e | 2251 | |
4602f195 | 2252 | In I<getpw*()> the fields $quota, $comment, and $expire are special |
80d38338 | 2253 | in that they are unsupported on many systems. If the |
4602f195 JH |
2254 | $quota is unsupported, it is an empty scalar. If it is supported, it |
2255 | usually encodes the disk quota. If the $comment field is unsupported, | |
2256 | it is an empty scalar. If it is supported it usually encodes some | |
2257 | administrative comment about the user. In some systems the $quota | |
2258 | field may be $change or $age, fields that have to do with password | |
2259 | aging. In some systems the $comment field may be $class. The $expire | |
2260 | field, if present, encodes the expiration period of the account or the | |
2261 | password. For the availability and the exact meaning of these fields | |
2262 | in your system, please consult your getpwnam(3) documentation and your | |
2263 | F<pwd.h> file. You can also find out from within Perl what your | |
2264 | $quota and $comment fields mean and whether you have the $expire field | |
2265 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
2266 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password | |
3b10bc60 | 2267 | files are supported only if your vendor has implemented them in the |
4602f195 | 2268 | intuitive fashion that calling the regular C library routines gets the |
5d3a0a3b | 2269 | shadow versions if you're running under privilege or if there exists |
cf264981 SP |
2270 | the shadow(3) functions as found in System V (this includes Solaris |
2271 | and Linux.) Those systems that implement a proprietary shadow password | |
5d3a0a3b | 2272 | facility are unlikely to be supported. |
6ee623d5 | 2273 | |
19799a22 | 2274 | The $members value returned by I<getgr*()> is a space separated list of |
a0d0e21e LW |
2275 | the login names of the members of the group. |
2276 | ||
2277 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
2278 | C, it will be returned to you via C<$?> if the function call fails. The | |
3b10bc60 | 2279 | C<@addrs> value returned by a successful call is a list of raw |
2280 | addresses returned by the corresponding library call. In the | |
2281 | Internet domain, each address is four bytes long; you can unpack it | |
a0d0e21e LW |
2282 | by saying something like: |
2283 | ||
f337b084 | 2284 | ($a,$b,$c,$d) = unpack('W4',$addr[0]); |
a0d0e21e | 2285 | |
2b5ab1e7 TC |
2286 | The Socket library makes this slightly easier: |
2287 | ||
2288 | use Socket; | |
2289 | $iaddr = inet_aton("127.1"); # or whatever address | |
2290 | $name = gethostbyaddr($iaddr, AF_INET); | |
2291 | ||
2292 | # or going the other way | |
19799a22 | 2293 | $straddr = inet_ntoa($iaddr); |
2b5ab1e7 | 2294 | |
d760c846 GS |
2295 | In the opposite way, to resolve a hostname to the IP address |
2296 | you can write this: | |
2297 | ||
2298 | use Socket; | |
2299 | $packed_ip = gethostbyname("www.perl.org"); | |
2300 | if (defined $packed_ip) { | |
2301 | $ip_address = inet_ntoa($packed_ip); | |
2302 | } | |
2303 | ||
2304 | Make sure <gethostbyname()> is called in SCALAR context and that | |
2305 | its return value is checked for definedness. | |
2306 | ||
19799a22 GS |
2307 | If you get tired of remembering which element of the return list |
2308 | contains which return value, by-name interfaces are provided | |
2309 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
2310 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
2311 | and C<User::grent>. These override the normal built-ins, supplying | |
2312 | versions that return objects with the appropriate names | |
2313 | for each field. For example: | |
5a964f20 TC |
2314 | |
2315 | use File::stat; | |
2316 | use User::pwent; | |
2317 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
2318 | ||
b76cc8ba NIS |
2319 | Even though it looks like they're the same method calls (uid), |
2320 | they aren't, because a C<File::stat> object is different from | |
19799a22 | 2321 | a C<User::pwent> object. |
5a964f20 | 2322 | |
a0d0e21e | 2323 | =item getsockname SOCKET |
d74e8afc | 2324 | X<getsockname> |
a0d0e21e | 2325 | |
19799a22 GS |
2326 | Returns the packed sockaddr address of this end of the SOCKET connection, |
2327 | in case you don't know the address because you have several different | |
2328 | IPs that the connection might have come in on. | |
a0d0e21e | 2329 | |
4633a7c4 LW |
2330 | use Socket; |
2331 | $mysockaddr = getsockname(SOCK); | |
19799a22 | 2332 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
b76cc8ba | 2333 | printf "Connect to %s [%s]\n", |
19799a22 GS |
2334 | scalar gethostbyaddr($myaddr, AF_INET), |
2335 | inet_ntoa($myaddr); | |
a0d0e21e LW |
2336 | |
2337 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
d74e8afc | 2338 | X<getsockopt> |
a0d0e21e | 2339 | |
636e6b1f TH |
2340 | Queries the option named OPTNAME associated with SOCKET at a given LEVEL. |
2341 | Options may exist at multiple protocol levels depending on the socket | |
2342 | type, but at least the uppermost socket level SOL_SOCKET (defined in the | |
2343 | C<Socket> module) will exist. To query options at another level the | |
2344 | protocol number of the appropriate protocol controlling the option | |
2345 | should be supplied. For example, to indicate that an option is to be | |
2346 | interpreted by the TCP protocol, LEVEL should be set to the protocol | |
80d38338 | 2347 | number of TCP, which you can get using C<getprotobyname>. |
636e6b1f | 2348 | |
80d38338 | 2349 | The function returns a packed string representing the requested socket |
3b10bc60 | 2350 | option, or C<undef> on error, with the reason for the error placed in |
2351 | C<$!>). Just what is in the packed string depends on LEVEL and OPTNAME; | |
80d38338 TC |
2352 | consult getsockopt(2) for details. A common case is that the option is an |
2353 | integer, in which case the result is a packed integer, which you can decode | |
2354 | using C<unpack> with the C<i> (or C<I>) format. | |
636e6b1f | 2355 | |
80d38338 | 2356 | An example to test whether Nagle's algorithm is turned on on a socket: |
636e6b1f | 2357 | |
4852725b | 2358 | use Socket qw(:all); |
636e6b1f TH |
2359 | |
2360 | defined(my $tcp = getprotobyname("tcp")) | |
a9a5a0dc | 2361 | or die "Could not determine the protocol number for tcp"; |
4852725b DD |
2362 | # my $tcp = IPPROTO_TCP; # Alternative |
2363 | my $packed = getsockopt($socket, $tcp, TCP_NODELAY) | |
80d38338 | 2364 | or die "getsockopt TCP_NODELAY: $!"; |
636e6b1f TH |
2365 | my $nodelay = unpack("I", $packed); |
2366 | print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n"; | |
2367 | ||
a0d0e21e LW |
2368 | |
2369 | =item glob EXPR | |
d74e8afc | 2370 | X<glob> X<wildcard> X<filename, expansion> X<expand> |
a0d0e21e | 2371 | |
0a753a76 | 2372 | =item glob |
2373 | ||
d9a9d457 JL |
2374 | In list context, returns a (possibly empty) list of filename expansions on |
2375 | the value of EXPR such as the standard Unix shell F</bin/csh> would do. In | |
2376 | scalar context, glob iterates through such filename expansions, returning | |
2377 | undef when the list is exhausted. This is the internal function | |
2378 | implementing the C<< <*.c> >> operator, but you can use it directly. If | |
2379 | EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in | |
2380 | more detail in L<perlop/"I/O Operators">. | |
a0d0e21e | 2381 | |
80d38338 TC |
2382 | Note that C<glob> splits its arguments on whitespace and treats |
2383 | each segment as separate pattern. As such, C<glob("*.c *.h")> | |
2384 | matches all files with a F<.c> or F<.h> extension. The expression | |
2385 | C<glob(".* *")> matchs all files in the current working directory. | |
2386 | ||
2387 | If non-empty braces are the only wildcard characters used in the | |
2388 | C<glob>, no filenames are matched, but potentially many strings | |
2389 | are returned. For example, this produces nine strings, one for | |
2390 | each pairing of fruits and colors: | |
2391 | ||
2392 | @many = glob "{apple,tomato,cherry}={green,yellow,red}"; | |
5c0c9249 | 2393 | |
3a4b19e4 | 2394 | Beginning with v5.6.0, this operator is implemented using the standard |
5c0c9249 PF |
2395 | C<File::Glob> extension. See L<File::Glob> for details, including |
2396 | C<bsd_glob> which does not treat whitespace as a pattern separator. | |
3a4b19e4 | 2397 | |
a0d0e21e | 2398 | =item gmtime EXPR |
d74e8afc | 2399 | X<gmtime> X<UTC> X<Greenwich> |
a0d0e21e | 2400 | |
ce2984c3 PF |
2401 | =item gmtime |
2402 | ||
435fbc73 GS |
2403 | Works just like L<localtime> but the returned values are |
2404 | localized for the standard Greenwich time zone. | |
a0d0e21e | 2405 | |
435fbc73 GS |
2406 | Note: when called in list context, $isdst, the last value |
2407 | returned by gmtime is always C<0>. There is no | |
2408 | Daylight Saving Time in GMT. | |
0a753a76 | 2409 | |
62aa5637 MS |
2410 | See L<perlport/gmtime> for portability concerns. |
2411 | ||
a0d0e21e | 2412 | =item goto LABEL |
d74e8afc | 2413 | X<goto> X<jump> X<jmp> |
a0d0e21e | 2414 | |
748a9306 LW |
2415 | =item goto EXPR |
2416 | ||
a0d0e21e LW |
2417 | =item goto &NAME |
2418 | ||
b500e03b GG |
2419 | The C<goto-LABEL> form finds the statement labeled with LABEL and |
2420 | resumes execution there. It can't be used to get out of a block or | |
2421 | subroutine given to C<sort>. It can be used to go almost anywhere | |
2422 | else within the dynamic scope, including out of subroutines, but it's | |
2423 | usually better to use some other construct such as C<last> or C<die>. | |
2424 | The author of Perl has never felt the need to use this form of C<goto> | |
3b10bc60 | 2425 | (in Perl, that is; C is another matter). (The difference is that C |
b500e03b GG |
2426 | does not offer named loops combined with loop control. Perl does, and |
2427 | this replaces most structured uses of C<goto> in other languages.) | |
a0d0e21e | 2428 | |
7660c0ab A |
2429 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
2430 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
2431 | necessarily recommended if you're optimizing for maintainability: |
2432 | ||
2433 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2434 | ||
b500e03b | 2435 | Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is |
0b98bec9 | 2436 | deprecated and will issue a warning. Even then, it may not be used to |
b500e03b GG |
2437 | go into any construct that requires initialization, such as a |
2438 | subroutine or a C<foreach> loop. It also can't be used to go into a | |
0b98bec9 | 2439 | construct that is optimized away. |
b500e03b | 2440 | |
1b6921cb BT |
2441 | The C<goto-&NAME> form is quite different from the other forms of |
2442 | C<goto>. In fact, it isn't a goto in the normal sense at all, and | |
2443 | doesn't have the stigma associated with other gotos. Instead, it | |
2444 | exits the current subroutine (losing any changes set by local()) and | |
2445 | immediately calls in its place the named subroutine using the current | |
2446 | value of @_. This is used by C<AUTOLOAD> subroutines that wish to | |
2447 | load another subroutine and then pretend that the other subroutine had | |
2448 | been called in the first place (except that any modifications to C<@_> | |
6cb9131c GS |
2449 | in the current subroutine are propagated to the other subroutine.) |
2450 | After the C<goto>, not even C<caller> will be able to tell that this | |
2451 | routine was called first. | |
2452 | ||
2453 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
cf264981 | 2454 | containing a code reference, or a block that evaluates to a code |
6cb9131c | 2455 | reference. |
a0d0e21e LW |
2456 | |
2457 | =item grep BLOCK LIST | |
d74e8afc | 2458 | X<grep> |
a0d0e21e LW |
2459 | |
2460 | =item grep EXPR,LIST | |
2461 | ||
2b5ab1e7 TC |
2462 | This is similar in spirit to, but not the same as, grep(1) and its |
2463 | relatives. In particular, it is not limited to using regular expressions. | |
2f9daede | 2464 | |
a0d0e21e | 2465 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 2466 | C<$_> to each element) and returns the list value consisting of those |
19799a22 GS |
2467 | elements for which the expression evaluated to true. In scalar |
2468 | context, returns the number of times the expression was true. | |
a0d0e21e LW |
2469 | |
2470 | @foo = grep(!/^#/, @bar); # weed out comments | |
2471 | ||
2472 | or equivalently, | |
2473 | ||
2474 | @foo = grep {!/^#/} @bar; # weed out comments | |
2475 | ||
be3174d2 GS |
2476 | Note that C<$_> is an alias to the list value, so it can be used to |
2477 | modify the elements of the LIST. While this is useful and supported, | |
2478 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
2479 | Similarly, grep returns aliases into the original list, much as a for |
2480 | loop's index variable aliases the list elements. That is, modifying an | |
19799a22 GS |
2481 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
2482 | or another C<grep>) actually modifies the element in the original list. | |
2b5ab1e7 | 2483 | This is usually something to be avoided when writing clear code. |
a0d0e21e | 2484 | |
a4fb8298 | 2485 | If C<$_> is lexical in the scope where the C<grep> appears (because it has |
cf264981 | 2486 | been declared with C<my $_>) then, in addition to being locally aliased to |
80d38338 | 2487 | the list elements, C<$_> keeps being lexical inside the block; i.e., it |
a4fb8298 RGS |
2488 | can't be seen from the outside, avoiding any potential side-effects. |
2489 | ||
19799a22 | 2490 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
38325410 | 2491 | |
a0d0e21e | 2492 | =item hex EXPR |
d74e8afc | 2493 | X<hex> X<hexadecimal> |
a0d0e21e | 2494 | |
54310121 | 2495 | =item hex |
bbce6d69 | 2496 | |
2b5ab1e7 | 2497 | Interprets EXPR as a hex string and returns the corresponding value. |
38366c11 | 2498 | (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see |
2b5ab1e7 | 2499 | L</oct>.) If EXPR is omitted, uses C<$_>. |
2f9daede TP |
2500 | |
2501 | print hex '0xAf'; # prints '175' | |
2502 | print hex 'aF'; # same | |
a0d0e21e | 2503 | |
19799a22 | 2504 | Hex strings may only represent integers. Strings that would cause |
53305cf1 | 2505 | integer overflow trigger a warning. Leading whitespace is not stripped, |
38366c11 DN |
2506 | unlike oct(). To present something as hex, look into L</printf>, |
2507 | L</sprintf>, or L</unpack>. | |
19799a22 | 2508 | |
ce2984c3 | 2509 | =item import LIST |
d74e8afc | 2510 | X<import> |
a0d0e21e | 2511 | |
19799a22 | 2512 | There is no builtin C<import> function. It is just an ordinary |
4633a7c4 | 2513 | method (subroutine) defined (or inherited) by modules that wish to export |
19799a22 | 2514 | names to another module. The C<use> function calls the C<import> method |
cea6626f | 2515 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
2516 | |
2517 | =item index STR,SUBSTR,POSITION | |
d74e8afc | 2518 | X<index> X<indexOf> X<InStr> |
a0d0e21e LW |
2519 | |
2520 | =item index STR,SUBSTR | |
2521 | ||
2b5ab1e7 TC |
2522 | The index function searches for one string within another, but without |
2523 | the wildcard-like behavior of a full regular-expression pattern match. | |
2524 | It returns the position of the first occurrence of SUBSTR in STR at | |
2525 | or after POSITION. If POSITION is omitted, starts searching from the | |
26f149de YST |
2526 | beginning of the string. POSITION before the beginning of the string |
2527 | or after its end is treated as if it were the beginning or the end, | |
2528 | respectively. POSITION and the return value are based at C<0> (or whatever | |
2b5ab1e7 | 2529 | you've set the C<$[> variable to--but don't do that). If the substring |
cf264981 | 2530 | is not found, C<index> returns one less than the base, ordinarily C<-1>. |
a0d0e21e LW |
2531 | |
2532 | =item int EXPR | |
f723aae1 | 2533 | X<int> X<integer> X<truncate> X<trunc> X<floor> |
a0d0e21e | 2534 | |
54310121 | 2535 | =item int |
bbce6d69 | 2536 | |
7660c0ab | 2537 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
2b5ab1e7 | 2538 | You should not use this function for rounding: one because it truncates |
3b10bc60 | 2539 | towards C<0>, and two because machine representations of floating-point |
2b5ab1e7 TC |
2540 | numbers can sometimes produce counterintuitive results. For example, |
2541 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2542 | because it's really more like -268.99999999999994315658 instead. Usually, | |
19799a22 | 2543 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
2b5ab1e7 | 2544 | functions will serve you better than will int(). |
a0d0e21e LW |
2545 | |
2546 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
d74e8afc | 2547 | X<ioctl> |
a0d0e21e | 2548 | |
2b5ab1e7 | 2549 | Implements the ioctl(2) function. You'll probably first have to say |
a0d0e21e | 2550 | |
5ed4f2ec | 2551 | require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph |
a0d0e21e | 2552 | |
a11c483f | 2553 | to get the correct function definitions. If F<sys/ioctl.ph> doesn't |
a0d0e21e | 2554 | exist or doesn't have the correct definitions you'll have to roll your |
61eff3bc | 2555 | own, based on your C header files such as F<< <sys/ioctl.h> >>. |
5a964f20 | 2556 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 2557 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
3b10bc60 | 2558 | written depending on the FUNCTION; a C pointer to the string value of SCALAR |
19799a22 | 2559 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
4633a7c4 LW |
2560 | has no string value but does have a numeric value, that value will be |
2561 | passed rather than a pointer to the string value. To guarantee this to be | |
19799a22 GS |
2562 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
2563 | functions may be needed to manipulate the values of structures used by | |
b76cc8ba | 2564 | C<ioctl>. |
a0d0e21e | 2565 | |
19799a22 | 2566 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
a0d0e21e | 2567 | |
5ed4f2ec | 2568 | if OS returns: then Perl returns: |
2569 | -1 undefined value | |
2570 | 0 string "0 but true" | |
2571 | anything else that number | |
a0d0e21e | 2572 | |
19799a22 | 2573 | Thus Perl returns true on success and false on failure, yet you can |
a0d0e21e LW |
2574 | still easily determine the actual value returned by the operating |
2575 | system: | |
2576 | ||
2b5ab1e7 | 2577 | $retval = ioctl(...) || -1; |
a0d0e21e LW |
2578 | printf "System returned %d\n", $retval; |
2579 | ||
be2f7487 | 2580 | The special string C<"0 but true"> is exempt from B<-w> complaints |
5a964f20 TC |
2581 | about improper numeric conversions. |
2582 | ||
a0d0e21e | 2583 | =item join EXPR,LIST |
d74e8afc | 2584 | X<join> |
a0d0e21e | 2585 | |
2b5ab1e7 TC |
2586 | Joins the separate strings of LIST into a single string with fields |
2587 | separated by the value of EXPR, and returns that new string. Example: | |
a0d0e21e | 2588 | |
2b5ab1e7 | 2589 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
a0d0e21e | 2590 | |
eb6e2d6f GS |
2591 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
2592 | first argument. Compare L</split>. | |
a0d0e21e | 2593 | |
aa689395 | 2594 | =item keys HASH |
d74e8afc | 2595 | X<keys> X<key> |
aa689395 | 2596 | |
aeedbbed NC |
2597 | =item keys ARRAY |
2598 | ||
2599 | Returns a list consisting of all the keys of the named hash, or the indices | |
2600 | of an array. (In scalar context, returns the number of keys or indices.) | |
504f80c1 | 2601 | |
aeedbbed | 2602 | The keys of a hash are returned in an apparently random order. The actual |
3b10bc60 | 2603 | random order is subject to change in future versions of Perl, but it |
504f80c1 | 2604 | is guaranteed to be the same order as either the C<values> or C<each> |
4546b9e6 JH |
2605 | function produces (given that the hash has not been modified). Since |
2606 | Perl 5.8.1 the ordering is different even between different runs of | |
2607 | Perl for security reasons (see L<perlsec/"Algorithmic Complexity | |
d6df3700 | 2608 | Attacks">). |
504f80c1 | 2609 | |
aeedbbed | 2610 | As a side effect, calling keys() resets the HASH or ARRAY's internal iterator |
cf264981 SP |
2611 | (see L</each>). In particular, calling keys() in void context resets |
2612 | the iterator with no other overhead. | |
a0d0e21e | 2613 | |
aa689395 | 2614 | Here is yet another way to print your environment: |
a0d0e21e LW |
2615 | |
2616 | @keys = keys %ENV; | |
2617 | @values = values %ENV; | |
b76cc8ba | 2618 | while (@keys) { |
a9a5a0dc | 2619 | print pop(@keys), '=', pop(@values), "\n"; |
a0d0e21e LW |
2620 | } |
2621 | ||
2622 | or how about sorted by key: | |
2623 | ||
2624 | foreach $key (sort(keys %ENV)) { | |
a9a5a0dc | 2625 | print $key, '=', $ENV{$key}, "\n"; |
a0d0e21e LW |
2626 | } |
2627 | ||
8ea1e5d4 GS |
2628 | The returned values are copies of the original keys in the hash, so |
2629 | modifying them will not affect the original hash. Compare L</values>. | |
2630 | ||
19799a22 | 2631 | To sort a hash by value, you'll need to use a C<sort> function. |
aa689395 | 2632 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 2633 | |
5a964f20 | 2634 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
a9a5a0dc | 2635 | printf "%4d %s\n", $hash{$key}, $key; |
4633a7c4 LW |
2636 | } |
2637 | ||
3b10bc60 | 2638 | Used as an lvalue, C<keys> allows you to increase the number of hash buckets |
aa689395 | 2639 | allocated for the given hash. This can gain you a measure of efficiency if |
2640 | you know the hash is going to get big. (This is similar to pre-extending | |
2641 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 2642 | |
2643 | keys %hash = 200; | |
2644 | ||
ab192400 GS |
2645 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
2646 | in fact, since it rounds up to the next power of two. These | |
55497cff | 2647 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
2648 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2649 | You can't shrink the number of buckets allocated for the hash using | |
19799a22 | 2650 | C<keys> in this way (but you needn't worry about doing this by accident, |
aeedbbed NC |
2651 | as trying has no effect). C<keys @array> in an lvalue context is a syntax |
2652 | error. | |
55497cff | 2653 | |
19799a22 | 2654 | See also C<each>, C<values> and C<sort>. |
ab192400 | 2655 | |
b350dd2f | 2656 | =item kill SIGNAL, LIST |
d74e8afc | 2657 | X<kill> X<signal> |
a0d0e21e | 2658 | |
b350dd2f | 2659 | Sends a signal to a list of processes. Returns the number of |
517db077 GS |
2660 | processes successfully signaled (which is not necessarily the |
2661 | same as the number actually killed). | |
a0d0e21e LW |
2662 | |
2663 | $cnt = kill 1, $child1, $child2; | |
2664 | kill 9, @goners; | |
2665 | ||
3b10bc60 | 2666 | If SIGNAL is zero, no signal is sent to the process, but C<kill> |
2667 | checks whether it's I<possible> to send a signal to it (that | |
70fb64f6 | 2668 | means, to be brief, that the process is owned by the same user, or we are |
3b10bc60 | 2669 | the super-user). This is useful to check that a child process is still |
81fd35db DN |
2670 | alive (even if only as a zombie) and hasn't changed its UID. See |
2671 | L<perlport> for notes on the portability of this construct. | |
b350dd2f | 2672 | |
e2c0f81f DG |
2673 | Unlike in the shell, if SIGNAL is negative, it kills process groups instead |
2674 | of processes. That means you usually want to use positive not negative signals. | |
2675 | You may also use a signal name in quotes. | |
2676 | ||
2677 | The behavior of kill when a I<PROCESS> number is zero or negative depends on | |
2678 | the operating system. For example, on POSIX-conforming systems, zero will | |
2679 | signal the current process group and -1 will signal all processes. | |
1e9c1022 JL |
2680 | |
2681 | See L<perlipc/"Signals"> for more details. | |
a0d0e21e LW |
2682 | |
2683 | =item last LABEL | |
d74e8afc | 2684 | X<last> X<break> |
a0d0e21e LW |
2685 | |
2686 | =item last | |
2687 | ||
2688 | The C<last> command is like the C<break> statement in C (as used in | |
2689 | loops); it immediately exits the loop in question. If the LABEL is | |
2690 | omitted, the command refers to the innermost enclosing loop. The | |
2691 | C<continue> block, if any, is not executed: | |
2692 | ||
4633a7c4 | 2693 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
2694 | last LINE if /^$/; # exit when done with header |
2695 | #... | |
a0d0e21e LW |
2696 | } |
2697 | ||
80d38338 | 2698 | C<last> cannot be used to exit a block that returns a value such as |
2b5ab1e7 TC |
2699 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2700 | a grep() or map() operation. | |
4968c1e4 | 2701 | |
6c1372ed GS |
2702 | Note that a block by itself is semantically identical to a loop |
2703 | that executes once. Thus C<last> can be used to effect an early | |
2704 | exit out of such a block. | |
2705 | ||
98293880 JH |
2706 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2707 | C<redo> work. | |
1d2dff63 | 2708 | |
a0d0e21e | 2709 | =item lc EXPR |
d74e8afc | 2710 | X<lc> X<lowercase> |
a0d0e21e | 2711 | |
54310121 | 2712 | =item lc |
bbce6d69 | 2713 | |
d1be9408 | 2714 | Returns a lowercased version of EXPR. This is the internal function |
3980dc9c | 2715 | implementing the C<\L> escape in double-quoted strings. |
a0d0e21e | 2716 | |
7660c0ab | 2717 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2718 | |
3980dc9c KW |
2719 | What gets returned depends on several factors: |
2720 | ||
2721 | =over | |
2722 | ||
2723 | =item If C<use bytes> is in effect: | |
2724 | ||
2725 | =over | |
2726 | ||
2727 | =item On EBCDIC platforms | |
2728 | ||
2729 | The results are what the C language system call C<tolower()> returns. | |
2730 | ||
2731 | =item On ASCII platforms | |
2732 | ||
2733 | The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z> | |
2734 | respectively. | |
2735 | ||
2736 | =back | |
2737 | ||
2738 | =item Otherwise, If EXPR has the UTF8 flag set | |
2739 | ||
2740 | If the current package has a subroutine named C<ToLower>, it will be used to | |
2741 | change the case (See L<perlunicode/User-Defined Case Mappings>.) | |
2742 | Otherwise Unicode semantics are used for the case change. | |
2743 | ||
2744 | =item Otherwise, if C<use locale> is in effect | |
2745 | ||
2746 | Respects current LC_CTYPE locale. See L<perllocale>. | |
2747 | ||
2748 | =item Otherwise, if C<use feature 'unicode_strings'> is in effect: | |
2749 | ||
2750 | Unicode semantics are used for the case change. Any subroutine named | |
2751 | C<ToLower> will not be used. | |
2752 | ||
2753 | =item Otherwise: | |
2754 | ||
2755 | =over | |
2756 | ||
2757 | =item On EBCDIC platforms | |
2758 | ||
2759 | The results are what the C language system call C<tolower()> returns. | |
2760 | ||
2761 | =item On ASCII platforms | |
2762 | ||
2763 | ASCII semantics are used for the case change. The lowercase of any character | |
2764 | outside the ASCII range is the character itself. | |
2765 | ||
2766 | =back | |
2767 | ||
2768 | =back | |
2769 | ||
a0d0e21e | 2770 | =item lcfirst EXPR |
d74e8afc | 2771 | X<lcfirst> X<lowercase> |
a0d0e21e | 2772 | |
54310121 | 2773 | =item lcfirst |
bbce6d69 | 2774 | |
ad0029c4 JH |
2775 | Returns the value of EXPR with the first character lowercased. This |
2776 | is the internal function implementing the C<\l> escape in | |
3980dc9c | 2777 | double-quoted strings. |
a0d0e21e | 2778 | |
7660c0ab | 2779 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2780 | |
3980dc9c KW |
2781 | This function behaves the same way under various pragma, such as in a locale, |
2782 | as L</lc> does. | |
2783 | ||
a0d0e21e | 2784 | =item length EXPR |
d74e8afc | 2785 | X<length> X<size> |
a0d0e21e | 2786 | |
54310121 | 2787 | =item length |
bbce6d69 | 2788 | |
974da8e5 | 2789 | Returns the length in I<characters> of the value of EXPR. If EXPR is |
9f621bb0 | 2790 | omitted, returns length of C<$_>. If EXPR is undefined, returns C<undef>. |
3b10bc60 | 2791 | |
2792 | This function cannot be used on an entire array or hash to find out how | |
2793 | many elements these have. For that, use C<scalar @array> and C<scalar keys | |
2794 | %hash>, respectively. | |
2795 | ||
2796 | Like all Perl character operations, length() normally deals in logical | |
2797 | characters, not physical bytes. For how many bytes a string encoded as | |
2798 | UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have | |
2799 | to C<use Encode> first). See L<Encode> and L<perlunicode>. | |
974da8e5 | 2800 | |
a0d0e21e | 2801 | =item link OLDFILE,NEWFILE |
d74e8afc | 2802 | X<link> |
a0d0e21e | 2803 | |
19799a22 | 2804 | Creates a new filename linked to the old filename. Returns true for |
b76cc8ba | 2805 | success, false otherwise. |
a0d0e21e LW |
2806 | |
2807 | =item listen SOCKET,QUEUESIZE | |
d74e8afc | 2808 | X<listen> |
a0d0e21e | 2809 | |
3b10bc60 | 2810 | Does the same thing that the listen(2) system call does. Returns true if |
b76cc8ba | 2811 | it succeeded, false otherwise. See the example in |
cea6626f | 2812 | L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
2813 | |
2814 | =item local EXPR | |
d74e8afc | 2815 | X<local> |
a0d0e21e | 2816 | |
19799a22 | 2817 | You really probably want to be using C<my> instead, because C<local> isn't |
b76cc8ba | 2818 | what most people think of as "local". See |
13a2d996 | 2819 | L<perlsub/"Private Variables via my()"> for details. |
2b5ab1e7 | 2820 | |
5a964f20 TC |
2821 | A local modifies the listed variables to be local to the enclosing |
2822 | block, file, or eval. If more than one value is listed, the list must | |
2823 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2824 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 2825 | |
d361fafa VP |
2826 | The C<delete local EXPR> construct can also be used to localize the deletion |
2827 | of array/hash elements to the current block. | |
2828 | See L<perlsub/"Localized deletion of elements of composite types">. | |
2829 | ||
a0d0e21e | 2830 | =item localtime EXPR |
435fbc73 | 2831 | X<localtime> X<ctime> |
a0d0e21e | 2832 | |
ba053783 AL |
2833 | =item localtime |
2834 | ||
19799a22 | 2835 | Converts a time as returned by the time function to a 9-element list |
5f05dabc | 2836 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
2837 | follows: |
2838 | ||
54310121 | 2839 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e | 2840 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
ba053783 | 2841 | localtime(time); |
a0d0e21e | 2842 | |
48a26b3a | 2843 | All list elements are numeric, and come straight out of the C `struct |
ba053783 AL |
2844 | tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours |
2845 | of the specified time. | |
48a26b3a | 2846 | |
ba053783 AL |
2847 | C<$mday> is the day of the month, and C<$mon> is the month itself, in |
2848 | the range C<0..11> with 0 indicating January and 11 indicating December. | |
2849 | This makes it easy to get a month name from a list: | |
54310121 | 2850 | |
ba053783 AL |
2851 | my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); |
2852 | print "$abbr[$mon] $mday"; | |
2853 | # $mon=9, $mday=18 gives "Oct 18" | |
abd75f24 | 2854 | |
ba053783 AL |
2855 | C<$year> is the number of years since 1900, not just the last two digits |
2856 | of the year. That is, C<$year> is C<123> in year 2023. The proper way | |
80d38338 | 2857 | to get a 4-digit year is simply: |
abd75f24 | 2858 | |
ba053783 | 2859 | $year += 1900; |
abd75f24 | 2860 | |
435fbc73 GS |
2861 | Otherwise you create non-Y2K-compliant programs--and you wouldn't want |
2862 | to do that, would you? | |
2863 | ||
ba053783 AL |
2864 | To get the last two digits of the year (e.g., '01' in 2001) do: |
2865 | ||
2866 | $year = sprintf("%02d", $year % 100); | |
2867 | ||
2868 | C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating | |
2869 | Wednesday. C<$yday> is the day of the year, in the range C<0..364> | |
2870 | (or C<0..365> in leap years.) | |
2871 | ||
2872 | C<$isdst> is true if the specified time occurs during Daylight Saving | |
2873 | Time, false otherwise. | |
abd75f24 | 2874 | |
e1998452 | 2875 | If EXPR is omitted, C<localtime()> uses the current time (as returned |
e3176d09 | 2876 | by time(3)). |
a0d0e21e | 2877 | |
48a26b3a | 2878 | In scalar context, C<localtime()> returns the ctime(3) value: |
a0d0e21e | 2879 | |
5f05dabc | 2880 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 2881 | |
fe86afc2 NC |
2882 | This scalar value is B<not> locale dependent but is a Perl builtin. For GMT |
2883 | instead of local time use the L</gmtime> builtin. See also the | |
2884 | C<Time::Local> module (to convert the second, minutes, hours, ... back to | |
2885 | the integer value returned by time()), and the L<POSIX> module's strftime(3) | |
2886 | and mktime(3) functions. | |
2887 | ||
2888 | To get somewhat similar but locale dependent date strings, set up your | |
2889 | locale environment variables appropriately (please see L<perllocale>) and | |
2890 | try for example: | |
a3cb178b | 2891 | |
5a964f20 | 2892 | use POSIX qw(strftime); |
2b5ab1e7 | 2893 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
fe86afc2 NC |
2894 | # or for GMT formatted appropriately for your locale: |
2895 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; | |
a3cb178b GS |
2896 | |
2897 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2898 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e | 2899 | |
62aa5637 MS |
2900 | See L<perlport/localtime> for portability concerns. |
2901 | ||
435fbc73 GS |
2902 | The L<Time::gmtime> and L<Time::localtime> modules provides a convenient, |
2903 | by-name access mechanism to the gmtime() and localtime() functions, | |
2904 | respectively. | |
2905 | ||
2906 | For a comprehensive date and time representation look at the | |
2907 | L<DateTime> module on CPAN. | |
2908 | ||
07698885 | 2909 | =item lock THING |
d74e8afc | 2910 | X<lock> |
19799a22 | 2911 | |
01e6739c | 2912 | This function places an advisory lock on a shared variable, or referenced |
03730085 | 2913 | object contained in I<THING> until the lock goes out of scope. |
a6d5524e | 2914 | |
f3a23afb | 2915 | lock() is a "weak keyword" : this means that if you've defined a function |
67408cae | 2916 | by this name (before any calls to it), that function will be called |
7b043ca5 RGS |
2917 | instead. If you are not under C<use threads::shared> this does nothing. |
2918 | See L<threads::shared>. | |
19799a22 | 2919 | |
a0d0e21e | 2920 | =item log EXPR |
d74e8afc | 2921 | X<log> X<logarithm> X<e> X<ln> X<base> |
a0d0e21e | 2922 | |
54310121 | 2923 | =item log |
bbce6d69 | 2924 | |
2b5ab1e7 TC |
2925 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
2926 | returns log of C<$_>. To get the log of another base, use basic algebra: | |
19799a22 | 2927 | The base-N log of a number is equal to the natural log of that number |
2b5ab1e7 TC |
2928 | divided by the natural log of N. For example: |
2929 | ||
2930 | sub log10 { | |
a9a5a0dc VP |
2931 | my $n = shift; |
2932 | return log($n)/log(10); | |
b76cc8ba | 2933 | } |
2b5ab1e7 TC |
2934 | |
2935 | See also L</exp> for the inverse operation. | |
a0d0e21e | 2936 | |
a0d0e21e | 2937 | =item lstat EXPR |
d74e8afc | 2938 | X<lstat> |
a0d0e21e | 2939 | |
54310121 | 2940 | =item lstat |
bbce6d69 | 2941 | |
19799a22 | 2942 | Does the same thing as the C<stat> function (including setting the |
5a964f20 TC |
2943 | special C<_> filehandle) but stats a symbolic link instead of the file |
2944 | the symbolic link points to. If symbolic links are unimplemented on | |
c837d5b4 DP |
2945 | your system, a normal C<stat> is done. For much more detailed |
2946 | information, please see the documentation for C<stat>. | |
a0d0e21e | 2947 | |
7660c0ab | 2948 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 2949 | |
a0d0e21e LW |
2950 | =item m// |
2951 | ||
9f4b9cd0 | 2952 | The match operator. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e LW |
2953 | |
2954 | =item map BLOCK LIST | |
d74e8afc | 2955 | X<map> |
a0d0e21e LW |
2956 | |
2957 | =item map EXPR,LIST | |
2958 | ||
19799a22 GS |
2959 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
2960 | C<$_> to each element) and returns the list value composed of the | |
2961 | results of each such evaluation. In scalar context, returns the | |
2962 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
2963 | list context, so each element of LIST may produce zero, one, or | |
2964 | more elements in the returned value. | |
dd99ebda | 2965 | |
a0d0e21e LW |
2966 | @chars = map(chr, @nums); |
2967 | ||
2968 | translates a list of numbers to the corresponding characters. And | |
2969 | ||
d8216f19 | 2970 | %hash = map { get_a_key_for($_) => $_ } @array; |
a0d0e21e LW |
2971 | |
2972 | is just a funny way to write | |
2973 | ||
2974 | %hash = (); | |
d8216f19 | 2975 | foreach (@array) { |
a9a5a0dc | 2976 | $hash{get_a_key_for($_)} = $_; |
a0d0e21e LW |
2977 | } |
2978 | ||
be3174d2 GS |
2979 | Note that C<$_> is an alias to the list value, so it can be used to |
2980 | modify the elements of the LIST. While this is useful and supported, | |
2981 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
2982 | Using a regular C<foreach> loop for this purpose would be clearer in |
2983 | most cases. See also L</grep> for an array composed of those items of | |
2984 | the original list for which the BLOCK or EXPR evaluates to true. | |
fb73857a | 2985 | |
a4fb8298 | 2986 | If C<$_> is lexical in the scope where the C<map> appears (because it has |
d8216f19 RGS |
2987 | been declared with C<my $_>), then, in addition to being locally aliased to |
2988 | the list elements, C<$_> keeps being lexical inside the block; that is, it | |
a4fb8298 RGS |
2989 | can't be seen from the outside, avoiding any potential side-effects. |
2990 | ||
205fdb4d | 2991 | C<{> starts both hash references and blocks, so C<map { ...> could be either |
3b10bc60 | 2992 | the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look |
80d38338 TC |
2993 | ahead for the closing C<}> it has to take a guess at which it's dealing with |
2994 | based on what it finds just after the C<{>. Usually it gets it right, but if it | |
205fdb4d NC |
2995 | doesn't it won't realize something is wrong until it gets to the C<}> and |
2996 | encounters the missing (or unexpected) comma. The syntax error will be | |
80d38338 | 2997 | reported close to the C<}>, but you'll need to change something near the C<{> |
3b10bc60 | 2998 | such as using a unary C<+> to give Perl some help: |
205fdb4d | 2999 | |
3b10bc60 | 3000 | %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong |
3001 | %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right | |
3002 | %hash = map { ("\L$_" => 1) } @array # this also works | |
3003 | %hash = map { lc($_) => 1 } @array # as does this. | |
3004 | %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! | |
cea6626f | 3005 | |
3b10bc60 | 3006 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) |
205fdb4d | 3007 | |
d8216f19 | 3008 | or to force an anon hash constructor use C<+{>: |
205fdb4d | 3009 | |
3b10bc60 | 3010 | @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end |
205fdb4d | 3011 | |
3b10bc60 | 3012 | to get a list of anonymous hashes each with only one entry apiece. |
205fdb4d | 3013 | |
19799a22 | 3014 | =item mkdir FILENAME,MASK |
d74e8afc | 3015 | X<mkdir> X<md> X<directory, create> |
a0d0e21e | 3016 | |
5a211162 GS |
3017 | =item mkdir FILENAME |
3018 | ||
491873e5 RGS |
3019 | =item mkdir |
3020 | ||
0591cd52 | 3021 | Creates the directory specified by FILENAME, with permissions |
19799a22 GS |
3022 | specified by MASK (as modified by C<umask>). If it succeeds it |
3023 | returns true, otherwise it returns false and sets C<$!> (errno). | |
491873e5 RGS |
3024 | If omitted, MASK defaults to 0777. If omitted, FILENAME defaults |
3025 | to C<$_>. | |
0591cd52 | 3026 | |
19799a22 | 3027 | In general, it is better to create directories with permissive MASK, |
0591cd52 | 3028 | and let the user modify that with their C<umask>, than it is to supply |
19799a22 | 3029 | a restrictive MASK and give the user no way to be more permissive. |
0591cd52 NT |
3030 | The exceptions to this rule are when the file or directory should be |
3031 | kept private (mail files, for instance). The perlfunc(1) entry on | |
19799a22 | 3032 | C<umask> discusses the choice of MASK in more detail. |
a0d0e21e | 3033 | |
cc1852e8 JH |
3034 | Note that according to the POSIX 1003.1-1996 the FILENAME may have any |
3035 | number of trailing slashes. Some operating and filesystems do not get | |
3036 | this right, so Perl automatically removes all trailing slashes to keep | |
3037 | everyone happy. | |
3038 | ||
80d38338 | 3039 | To recursively create a directory structure, look at |
dd184578 RGS |
3040 | the C<mkpath> function of the L<File::Path> module. |
3041 | ||
a0d0e21e | 3042 | =item msgctl ID,CMD,ARG |
d74e8afc | 3043 | X<msgctl> |
a0d0e21e | 3044 | |
f86cebdf | 3045 | Calls the System V IPC function msgctl(2). You'll probably have to say |
0ade1984 JH |
3046 | |
3047 | use IPC::SysV; | |
3048 | ||
7660c0ab | 3049 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
cf264981 | 3050 | then ARG must be a variable that will hold the returned C<msqid_ds> |
951ba7fe GS |
3051 | structure. Returns like C<ioctl>: the undefined value for error, |
3052 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
4755096e | 3053 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. |
a0d0e21e LW |
3054 | |
3055 | =item msgget KEY,FLAGS | |
d74e8afc | 3056 | X<msgget> |
a0d0e21e | 3057 | |
f86cebdf | 3058 | Calls the System V IPC function msgget(2). Returns the message queue |
4755096e GS |
3059 | id, or the undefined value if there is an error. See also |
3060 | L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. | |
a0d0e21e | 3061 | |
a0d0e21e | 3062 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS |
d74e8afc | 3063 | X<msgrcv> |
a0d0e21e LW |
3064 | |
3065 | Calls the System V IPC function msgrcv to receive a message from | |
3066 | message queue ID into variable VAR with a maximum message size of | |
41d6edb2 JH |
3067 | SIZE. Note that when a message is received, the message type as a |
3068 | native long integer will be the first thing in VAR, followed by the | |
3069 | actual message. This packing may be opened with C<unpack("l! a*")>. | |
3070 | Taints the variable. Returns true if successful, or false if there is | |
4755096e GS |
3071 | an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and |
3072 | C<IPC::SysV::Msg> documentation. | |
41d6edb2 JH |
3073 | |
3074 | =item msgsnd ID,MSG,FLAGS | |
d74e8afc | 3075 | X<msgsnd> |
41d6edb2 JH |
3076 | |
3077 | Calls the System V IPC function msgsnd to send the message MSG to the | |
3078 | message queue ID. MSG must begin with the native long integer message | |
3079 | type, and be followed by the length of the actual message, and finally | |
3080 | the message itself. This kind of packing can be achieved with | |
3081 | C<pack("l! a*", $type, $message)>. Returns true if successful, | |
3082 | or false if there is an error. See also C<IPC::SysV> | |
3083 | and C<IPC::SysV::Msg> documentation. | |
a0d0e21e LW |
3084 | |
3085 | =item my EXPR | |
d74e8afc | 3086 | X<my> |
a0d0e21e | 3087 | |
307ea6df JH |
3088 | =item my TYPE EXPR |
3089 | ||
1d2de774 | 3090 | =item my EXPR : ATTRS |
09bef843 | 3091 | |
1d2de774 | 3092 | =item my TYPE EXPR : ATTRS |
307ea6df | 3093 | |
19799a22 | 3094 | A C<my> declares the listed variables to be local (lexically) to the |
1d2de774 JH |
3095 | enclosing block, file, or C<eval>. If more than one value is listed, |
3096 | the list must be placed in parentheses. | |
307ea6df | 3097 | |
1d2de774 JH |
3098 | The exact semantics and interface of TYPE and ATTRS are still |
3099 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
307ea6df JH |
3100 | and attributes are handled using the C<attributes> pragma, or starting |
3101 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3102 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3103 | L<attributes>, and L<Attribute::Handlers>. | |
4633a7c4 | 3104 | |
a0d0e21e | 3105 | =item next LABEL |
d74e8afc | 3106 | X<next> X<continue> |
a0d0e21e LW |
3107 | |
3108 | =item next | |
3109 | ||
3110 | The C<next> command is like the C<continue> statement in C; it starts | |
3111 | the next iteration of the loop: | |
3112 | ||
4633a7c4 | 3113 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
3114 | next LINE if /^#/; # discard comments |
3115 | #... | |
a0d0e21e LW |
3116 | } |
3117 | ||
3118 | Note that if there were a C<continue> block on the above, it would get | |
3b10bc60 | 3119 | executed even on discarded lines. If LABEL is omitted, the command |
a0d0e21e LW |
3120 | refers to the innermost enclosing loop. |
3121 | ||
4968c1e4 | 3122 | C<next> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
3123 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
3124 | a grep() or map() operation. | |
4968c1e4 | 3125 | |
6c1372ed GS |
3126 | Note that a block by itself is semantically identical to a loop |
3127 | that executes once. Thus C<next> will exit such a block early. | |
3128 | ||
98293880 JH |
3129 | See also L</continue> for an illustration of how C<last>, C<next>, and |
3130 | C<redo> work. | |
1d2dff63 | 3131 | |
3b10bc60 | 3132 | =item no MODULE VERSION LIST |
3133 | X<no declarations> | |
3134 | X<unimporting> | |
4a66ea5a | 3135 | |
3b10bc60 | 3136 | =item no MODULE VERSION |
4a66ea5a | 3137 | |
3b10bc60 | 3138 | =item no MODULE LIST |
a0d0e21e | 3139 | |
3b10bc60 | 3140 | =item no MODULE |
4a66ea5a | 3141 | |
c986422f RGS |
3142 | =item no VERSION |
3143 | ||
593b9c14 | 3144 | See the C<use> function, of which C<no> is the opposite. |
a0d0e21e LW |
3145 | |
3146 | =item oct EXPR | |
d74e8afc | 3147 | X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> |
a0d0e21e | 3148 | |
54310121 | 3149 | =item oct |
bbce6d69 | 3150 | |
4633a7c4 | 3151 | Interprets EXPR as an octal string and returns the corresponding |
4f19785b WSI |
3152 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
3153 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
53305cf1 | 3154 | binary string. Leading whitespace is ignored in all three cases.) |
3b10bc60 | 3155 | The following will handle decimal, binary, octal, and hex in standard |
3156 | Perl notation: | |
a0d0e21e LW |
3157 | |
3158 | $val = oct($val) if $val =~ /^0/; | |
3159 | ||
19799a22 GS |
3160 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
3161 | in octal), use sprintf() or printf(): | |
3162 | ||
3b10bc60 | 3163 | $dec_perms = (stat("filename"))[2] & 07777; |
3164 | $oct_perm_str = sprintf "%o", $perms; | |
19799a22 GS |
3165 | |
3166 | The oct() function is commonly used when a string such as C<644> needs | |
3b10bc60 | 3167 | to be converted into a file mode, for example. Although Perl |
3168 | automatically converts strings into numbers as needed, this automatic | |
3169 | conversion assumes base 10. | |
3170 | ||
3171 | Leading white space is ignored without warning, as too are any trailing | |
3172 | non-digits, such as a decimal point (C<oct> only handles non-negative | |
3173 | integers, not negative integers or floating point). | |
a0d0e21e LW |
3174 | |
3175 | =item open FILEHANDLE,EXPR | |
d74e8afc | 3176 | X<open> X<pipe> X<file, open> X<fopen> |
a0d0e21e | 3177 | |
68bd7414 NIS |
3178 | =item open FILEHANDLE,MODE,EXPR |
3179 | ||
3180 | =item open FILEHANDLE,MODE,EXPR,LIST | |
3181 | ||
ba964c95 T |
3182 | =item open FILEHANDLE,MODE,REFERENCE |
3183 | ||
a0d0e21e LW |
3184 | =item open FILEHANDLE |
3185 | ||
3186 | Opens the file whose filename is given by EXPR, and associates it with | |
ed53a2bb JH |
3187 | FILEHANDLE. |
3188 | ||
460b70c2 GS |
3189 | Simple examples to open a file for reading: |
3190 | ||
3191 | open(my $fh, '<', "input.txt") or die $!; | |
3192 | ||
3193 | and for writing: | |
3194 | ||
3195 | open(my $fh, '>', "output.txt") or die $!; | |
3196 | ||
ed53a2bb JH |
3197 | (The following is a comprehensive reference to open(): for a gentler |
3198 | introduction you may consider L<perlopentut>.) | |
3199 | ||
a28cd5c9 NT |
3200 | If FILEHANDLE is an undefined scalar variable (or array or hash element) |
3201 | the variable is assigned a reference to a new anonymous filehandle, | |
3202 | otherwise if FILEHANDLE is an expression, its value is used as the name of | |
3203 | the real filehandle wanted. (This is considered a symbolic reference, so | |
3204 | C<use strict 'refs'> should I<not> be in effect.) | |
ed53a2bb JH |
3205 | |
3206 | If EXPR is omitted, the scalar variable of the same name as the | |
3207 | FILEHANDLE contains the filename. (Note that lexical variables--those | |
3208 | declared with C<my>--will not work for this purpose; so if you're | |
67408cae | 3209 | using C<my>, specify EXPR in your call to open.) |
ed53a2bb JH |
3210 | |
3211 | If three or more arguments are specified then the mode of opening and | |
3b10bc60 | 3212 | the filename are separate. If MODE is C<< '<' >> or nothing, the file |
ed53a2bb JH |
3213 | is opened for input. If MODE is C<< '>' >>, the file is truncated and |
3214 | opened for output, being created if necessary. If MODE is C<<< '>>' >>>, | |
b76cc8ba | 3215 | the file is opened for appending, again being created if necessary. |
5a964f20 | 3216 | |
ed53a2bb JH |
3217 | You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to |
3218 | indicate that you want both read and write access to the file; thus | |
3b10bc60 | 3219 | C<< '+<' >> is almost always preferred for read/write updates--the |
3220 | C<< '+>' >> mode would clobber the file first. You can't usually use | |
ed53a2bb JH |
3221 | either read-write mode for updating textfiles, since they have |
3222 | variable length records. See the B<-i> switch in L<perlrun> for a | |
3223 | better approach. The file is created with permissions of C<0666> | |
e1020413 | 3224 | modified by the process's C<umask> value. |
ed53a2bb JH |
3225 | |
3226 | These various prefixes correspond to the fopen(3) modes of C<'r'>, | |
3227 | C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. | |
5f05dabc | 3228 | |
3b10bc60 | 3229 | In the two-argument (and one-argument) form of the call, the mode and |
3230 | filename should be concatenated (in that order), possibly separated by | |
3231 | spaces. You may omit the mode in these forms when that mode is | |
68bd7414 | 3232 | C<< '<' >>. |
6170680b | 3233 | |
7660c0ab | 3234 | If the filename begins with C<'|'>, the filename is interpreted as a |
5a964f20 | 3235 | command to which output is to be piped, and if the filename ends with a |
80d38338 | 3236 | C<'|'>, the filename is interpreted as a command that pipes output to |
f244e06d | 3237 | us. See L<perlipc/"Using open() for IPC"> |
19799a22 | 3238 | for more examples of this. (You are not allowed to C<open> to a command |
5a964f20 | 3239 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
4a4eefd0 GS |
3240 | and L<perlipc/"Bidirectional Communication with Another Process"> |
3241 | for alternatives.) | |
cb1a09d0 | 3242 | |
ed53a2bb JH |
3243 | For three or more arguments if MODE is C<'|-'>, the filename is |
3244 | interpreted as a command to which output is to be piped, and if MODE | |
80d38338 | 3245 | is C<'-|'>, the filename is interpreted as a command that pipes |
3b10bc60 | 3246 | output to us. In the two-argument (and one-argument) form, one should |
ed53a2bb JH |
3247 | replace dash (C<'-'>) with the command. |
3248 | See L<perlipc/"Using open() for IPC"> for more examples of this. | |
3249 | (You are not allowed to C<open> to a command that pipes both in I<and> | |
3250 | out, but see L<IPC::Open2>, L<IPC::Open3>, and | |
3251 | L<perlipc/"Bidirectional Communication"> for alternatives.) | |
3252 | ||
3b10bc60 | 3253 | In the form of pipe opens taking three or more arguments, if LIST is specified |
ed53a2bb JH |
3254 | (extra arguments after the command name) then LIST becomes arguments |
3255 | to the command invoked if the platform supports it. The meaning of | |
3256 | C<open> with more than three arguments for non-pipe modes is not yet | |
3b10bc60 | 3257 | defined, but experimental "layers" may give extra LIST arguments |
ed53a2bb | 3258 | meaning. |
6170680b | 3259 | |
3b10bc60 | 3260 | In the two-argument (and one-argument) form, opening C<< '<-' >> |
3261 | or C<'-'> opens STDIN and opening C<< '>-' >> opens STDOUT. | |
6170680b | 3262 | |
3b10bc60 | 3263 | You may use the three-argument form of open to specify I/O layers |
3264 | (sometimes referred to as "disciplines") to apply to the handle | |
fae2c0fb | 3265 | that affect how the input and output are processed (see L<open> and |
3b10bc60 | 3266 | L<PerlIO> for more details). For example: |
7207e29d | 3267 | |
3b10bc60 | 3268 | open(my $fh, "<:encoding(UTF-8)", "filename") |
3269 | || die "can't open UTF-8 encoded filename: $!"; | |
9124316e | 3270 | |
3b10bc60 | 3271 | opens the UTF-8 encoded file containing Unicode characters; |
6d5e88a0 | 3272 | see L<perluniintro>. Note that if layers are specified in the |
3b10bc60 | 3273 | three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>; |
6d5e88a0 | 3274 | usually set by the B<open> pragma or the switch B<-CioD>) are ignored. |
ed53a2bb | 3275 | |
80d38338 | 3276 | Open returns nonzero on success, the undefined value otherwise. If |
ed53a2bb JH |
3277 | the C<open> involved a pipe, the return value happens to be the pid of |
3278 | the subprocess. | |
cb1a09d0 | 3279 | |
ed53a2bb JH |
3280 | If you're running Perl on a system that distinguishes between text |
3281 | files and binary files, then you should check out L</binmode> for tips | |
3282 | for dealing with this. The key distinction between systems that need | |
3283 | C<binmode> and those that don't is their text file formats. Systems | |
80d38338 TC |
3284 | like Unix, Mac OS, and Plan 9, that end lines with a single |
3285 | character and encode that character in C as C<"\n"> do not | |
ed53a2bb | 3286 | need C<binmode>. The rest need it. |
cb1a09d0 | 3287 | |
80d38338 TC |
3288 | When opening a file, it's seldom a good idea to continue |
3289 | if the request failed, so C<open> is frequently used with | |
19799a22 | 3290 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, |
80d38338 TC |
3291 | where you want to format a suitable error message (but there are |
3292 | modules that can help with that problem)) always check | |
3293 | the return value from opening a file. | |
fb73857a | 3294 | |
cf264981 | 3295 | As a special case the 3-arg form with a read/write mode and the third |
ed53a2bb | 3296 | argument being C<undef>: |
b76cc8ba | 3297 | |
460b70c2 | 3298 | open(my $tmp, "+>", undef) or die ... |
b76cc8ba | 3299 | |
f253e835 JH |
3300 | opens a filehandle to an anonymous temporary file. Also using "+<" |
3301 | works for symmetry, but you really should consider writing something | |
3302 | to the temporary file first. You will need to seek() to do the | |
3303 | reading. | |
b76cc8ba | 3304 | |
3b10bc60 | 3305 | Since v5.8.0, Perl has built using PerlIO by default. Unless you've |
3306 | changed this (i.e., Configure -Uuseperlio), you can open filehandles | |
3307 | directly to Perl scalars via: | |
ba964c95 | 3308 | |
b996200f SB |
3309 | open($fh, '>', \$variable) || .. |
3310 | ||
3b10bc60 | 3311 | To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first: |
b996200f SB |
3312 | |
3313 | close STDOUT; | |
3314 | open STDOUT, '>', \$variable or die "Can't open STDOUT: $!"; | |
ba964c95 | 3315 | |
3b10bc60 | 3316 | General examples: |
a0d0e21e LW |
3317 | |
3318 | $ARTICLE = 100; | |
3319 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
3320 | while (<ARTICLE>) {... | |
3321 | ||
5ed4f2ec | 3322 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
fb73857a | 3323 | # if the open fails, output is discarded |
a0d0e21e | 3324 | |
5ed4f2ec | 3325 | open(my $dbase, '+<', 'dbase.mine') # open for update |
a9a5a0dc | 3326 | or die "Can't open 'dbase.mine' for update: $!"; |
cb1a09d0 | 3327 | |
5ed4f2ec | 3328 | open(my $dbase, '+<dbase.mine') # ditto |
a9a5a0dc | 3329 | or die "Can't open 'dbase.mine' for update: $!"; |
6170680b | 3330 | |
5ed4f2ec | 3331 | open(ARTICLE, '-|', "caesar <$article") # decrypt article |
a9a5a0dc | 3332 | or die "Can't start caesar: $!"; |
a0d0e21e | 3333 | |
5ed4f2ec | 3334 | open(ARTICLE, "caesar <$article |") # ditto |
a9a5a0dc | 3335 | or die "Can't start caesar: $!"; |
6170680b | 3336 | |
5ed4f2ec | 3337 | open(EXTRACT, "|sort >Tmp$$") # $$ is our process id |
a9a5a0dc | 3338 | or die "Can't start sort: $!"; |
a0d0e21e | 3339 | |
3b10bc60 | 3340 | # in-memory files |
ba964c95 | 3341 | open(MEMORY,'>', \$var) |
a9a5a0dc | 3342 | or die "Can't open memory file: $!"; |
80d38338 | 3343 | print MEMORY "foo!\n"; # output will appear in $var |
ba964c95 | 3344 | |
a0d0e21e LW |
3345 | # process argument list of files along with any includes |
3346 | ||
3347 | foreach $file (@ARGV) { | |
a9a5a0dc | 3348 | process($file, 'fh00'); |
a0d0e21e LW |
3349 | } |
3350 | ||
3351 | sub process { | |
a9a5a0dc VP |
3352 | my($filename, $input) = @_; |
3353 | $input++; # this is a string increment | |
3354 | unless (open($input, $filename)) { | |
3355 | print STDERR "Can't open $filename: $!\n"; | |
3356 | return; | |
3357 | } | |
5ed4f2ec | 3358 | |
a9a5a0dc VP |
3359 | local $_; |
3360 | while (<$input>) { # note use of indirection | |
3361 | if (/^#include "(.*)"/) { | |
3362 | process($1, $input); | |
3363 | next; | |
3364 | } | |
3365 | #... # whatever | |
5ed4f2ec | 3366 | } |
a0d0e21e LW |
3367 | } |
3368 | ||
ae4c5402 | 3369 | See L<perliol> for detailed info on PerlIO. |
2ce64696 | 3370 | |
a0d0e21e | 3371 | You may also, in the Bourne shell tradition, specify an EXPR beginning |
00cafafa JH |
3372 | with C<< '>&' >>, in which case the rest of the string is interpreted |
3373 | as the name of a filehandle (or file descriptor, if numeric) to be | |
f4084e39 | 3374 | duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>, |
00cafafa JH |
3375 | C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. |
3376 | The mode you specify should match the mode of the original filehandle. | |
3377 | (Duping a filehandle does not take into account any existing contents | |
cf264981 | 3378 | of IO buffers.) If you use the 3-arg form then you can pass either a |
00cafafa | 3379 | number, the name of a filehandle or the normal "reference to a glob". |
6170680b | 3380 | |
eae1b76b SB |
3381 | Here is a script that saves, redirects, and restores C<STDOUT> and |
3382 | C<STDERR> using various methods: | |
a0d0e21e LW |
3383 | |
3384 | #!/usr/bin/perl | |
eae1b76b SB |
3385 | open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!"; |
3386 | open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!"; | |
818c4caa | 3387 | |
eae1b76b SB |
3388 | open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!"; |
3389 | open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!"; | |
a0d0e21e | 3390 | |
5ed4f2ec | 3391 | select STDERR; $| = 1; # make unbuffered |
3392 | select STDOUT; $| = 1; # make unbuffered | |
a0d0e21e | 3393 | |
5ed4f2ec | 3394 | print STDOUT "stdout 1\n"; # this works for |
3395 | print STDERR "stderr 1\n"; # subprocesses too | |
a0d0e21e | 3396 | |
eae1b76b SB |
3397 | open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!"; |
3398 | open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!"; | |
a0d0e21e LW |
3399 | |
3400 | print STDOUT "stdout 2\n"; | |
3401 | print STDERR "stderr 2\n"; | |
3402 | ||
ef8b303f JH |
3403 | If you specify C<< '<&=X' >>, where C<X> is a file descriptor number |
3404 | or a filehandle, then Perl will do an equivalent of C's C<fdopen> of | |
f4084e39 | 3405 | that file descriptor (and not call C<dup(2)>); this is more |
ef8b303f | 3406 | parsimonious of file descriptors. For example: |
a0d0e21e | 3407 | |
00cafafa | 3408 | # open for input, reusing the fileno of $fd |
a0d0e21e | 3409 | open(FILEHANDLE, "<&=$fd") |
df632fdf | 3410 | |
b76cc8ba | 3411 | or |
df632fdf | 3412 | |
b76cc8ba | 3413 | open(FILEHANDLE, "<&=", $fd) |
a0d0e21e | 3414 | |
00cafafa JH |
3415 | or |
3416 | ||
3417 | # open for append, using the fileno of OLDFH | |
3418 | open(FH, ">>&=", OLDFH) | |
3419 | ||
3420 | or | |
3421 | ||
3422 | open(FH, ">>&=OLDFH") | |
3423 | ||
ef8b303f JH |
3424 | Being parsimonious on filehandles is also useful (besides being |
3425 | parsimonious) for example when something is dependent on file | |
3426 | descriptors, like for example locking using flock(). If you do just | |
3427 | C<< open(A, '>>&B') >>, the filehandle A will not have the same file | |
3428 | descriptor as B, and therefore flock(A) will not flock(B), and vice | |
3429 | versa. But with C<< open(A, '>>&=B') >> the filehandles will share | |
3430 | the same file descriptor. | |
3431 | ||
3432 | Note that if you are using Perls older than 5.8.0, Perl will be using | |
3433 | the standard C libraries' fdopen() to implement the "=" functionality. | |
e1020413 | 3434 | On many Unix systems fdopen() fails when file descriptors exceed a |
ef8b303f JH |
3435 | certain value, typically 255. For Perls 5.8.0 and later, PerlIO is |
3436 | most often the default. | |
4af147f6 | 3437 | |
df632fdf JH |
3438 | You can see whether Perl has been compiled with PerlIO or not by |
3439 | running C<perl -V> and looking for C<useperlio=> line. If C<useperlio> | |
3440 | is C<define>, you have PerlIO, otherwise you don't. | |
3441 | ||
6170680b IZ |
3442 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> |
3443 | with 2-arguments (or 1-argument) form of open(), then | |
a0d0e21e | 3444 | there is an implicit fork done, and the return value of open is the pid |
7660c0ab | 3445 | of the child within the parent process, and C<0> within the child |
184e9718 | 3446 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
3b10bc60 | 3447 | The filehandle behaves normally for the parent, but I/O to that |
a0d0e21e | 3448 | filehandle is piped from/to the STDOUT/STDIN of the child process. |
3b10bc60 | 3449 | In the child process, the filehandle isn't opened--I/O happens from/to |
3450 | the new STDOUT/STDIN. Typically this is used like the normal | |
a0d0e21e | 3451 | piped open when you want to exercise more control over just how the |
3b10bc60 | 3452 | pipe command gets executed, such as when running setuid and |
3453 | you don't want to have to scan shell commands for metacharacters. | |
3454 | ||
6170680b | 3455 | The following triples are more or less equivalent: |
a0d0e21e LW |
3456 | |
3457 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
6170680b IZ |
3458 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); |
3459 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; | |
b76cc8ba | 3460 | open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); |
a0d0e21e LW |
3461 | |
3462 | open(FOO, "cat -n '$file'|"); | |
6170680b IZ |
3463 | open(FOO, '-|', "cat -n '$file'"); |
3464 | open(FOO, '-|') || exec 'cat', '-n', $file; | |
b76cc8ba NIS |
3465 | open(FOO, '-|', "cat", '-n', $file); |
3466 | ||
3467 | The last example in each block shows the pipe as "list form", which is | |
64da03b2 JH |
3468 | not yet supported on all platforms. A good rule of thumb is that if |
3469 | your platform has true C<fork()> (in other words, if your platform is | |
e1020413 | 3470 | Unix) you can use the list form. |
a0d0e21e | 3471 | |
4633a7c4 LW |
3472 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
3473 | ||
0f897271 GS |
3474 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
3475 | output before any operation that may do a fork, but this may not be | |
3476 | supported on some platforms (see L<perlport>). To be safe, you may need | |
3477 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
3478 | of C<IO::Handle> on any open handles. | |
3479 | ||
ed53a2bb JH |
3480 | On systems that support a close-on-exec flag on files, the flag will |
3481 | be set for the newly opened file descriptor as determined by the value | |
3482 | of $^F. See L<perlvar/$^F>. | |
a0d0e21e | 3483 | |
0dccf244 | 3484 | Closing any piped filehandle causes the parent process to wait for the |
e5218da5 GA |
3485 | child to finish, and returns the status value in C<$?> and |
3486 | C<${^CHILD_ERROR_NATIVE}>. | |
0dccf244 | 3487 | |
ed53a2bb JH |
3488 | The filename passed to 2-argument (or 1-argument) form of open() will |
3489 | have leading and trailing whitespace deleted, and the normal | |
3490 | redirection characters honored. This property, known as "magic open", | |
5a964f20 | 3491 | can often be used to good effect. A user could specify a filename of |
7660c0ab | 3492 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
3493 | |
3494 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
3495 | open(FH, $filename) or die "Can't open $filename: $!"; | |
3496 | ||
6170680b IZ |
3497 | Use 3-argument form to open a file with arbitrary weird characters in it, |
3498 | ||
3499 | open(FOO, '<', $file); | |
3500 | ||
3501 | otherwise it's necessary to protect any leading and trailing whitespace: | |
5a964f20 TC |
3502 | |
3503 | $file =~ s#^(\s)#./$1#; | |
3504 | open(FOO, "< $file\0"); | |
3505 | ||
a31a806a | 3506 | (this may not work on some bizarre filesystems). One should |
106325ad | 3507 | conscientiously choose between the I<magic> and 3-arguments form |
6170680b IZ |
3508 | of open(): |
3509 | ||
3510 | open IN, $ARGV[0]; | |
3511 | ||
3512 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
80d38338 | 3513 | but will not work on a filename that happens to have a trailing space, while |
6170680b IZ |
3514 | |
3515 | open IN, '<', $ARGV[0]; | |
3516 | ||
3517 | will have exactly the opposite restrictions. | |
3518 | ||
f4084e39 | 3519 | If you want a "real" C C<open> (see C<open(2)> on your system), then you |
6170680b IZ |
3520 | should use the C<sysopen> function, which involves no such magic (but |
3521 | may use subtly different filemodes than Perl open(), which is mapped | |
3522 | to C fopen()). This is | |
5a964f20 TC |
3523 | another way to protect your filenames from interpretation. For example: |
3524 | ||
3525 | use IO::Handle; | |
3526 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
a9a5a0dc | 3527 | or die "sysopen $path: $!"; |
5a964f20 | 3528 | $oldfh = select(HANDLE); $| = 1; select($oldfh); |
38762f02 | 3529 | print HANDLE "stuff $$\n"; |
5a964f20 TC |
3530 | seek(HANDLE, 0, 0); |
3531 | print "File contains: ", <HANDLE>; | |
3532 | ||
7660c0ab A |
3533 | Using the constructor from the C<IO::Handle> package (or one of its |
3534 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
5a964f20 TC |
3535 | filehandles that have the scope of whatever variables hold references to |
3536 | them, and automatically close whenever and however you leave that scope: | |
c07a80fd | 3537 | |
5f05dabc | 3538 | use IO::File; |
5a964f20 | 3539 | #... |
c07a80fd | 3540 | sub read_myfile_munged { |
a9a5a0dc VP |
3541 | my $ALL = shift; |
3542 | my $handle = IO::File->new; | |
3543 | open($handle, "myfile") or die "myfile: $!"; | |
3544 | $first = <$handle> | |
3545 | or return (); # Automatically closed here. | |
3546 | mung $first or die "mung failed"; # Or here. | |
3547 | return $first, <$handle> if $ALL; # Or here. | |
3548 | $first; # Or here. | |
c07a80fd | 3549 | } |
3550 | ||
b687b08b | 3551 | See L</seek> for some details about mixing reading and writing. |
a0d0e21e LW |
3552 | |
3553 | =item opendir DIRHANDLE,EXPR | |
d74e8afc | 3554 | X<opendir> |
a0d0e21e | 3555 | |
19799a22 GS |
3556 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
3557 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
a28cd5c9 NT |
3558 | DIRHANDLE may be an expression whose value can be used as an indirect |
3559 | dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined | |
3560 | scalar variable (or array or hash element), the variable is assigned a | |
3561 | reference to a new anonymous dirhandle. | |
a0d0e21e LW |
3562 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
3563 | ||
b0169937 GS |
3564 | See example at C<readdir>. |
3565 | ||
a0d0e21e | 3566 | =item ord EXPR |
d74e8afc | 3567 | X<ord> X<encoding> |
a0d0e21e | 3568 | |
54310121 | 3569 | =item ord |
bbce6d69 | 3570 | |
121910a4 JH |
3571 | Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, |
3572 | or Unicode) value of the first character of EXPR. If EXPR is omitted, | |
3573 | uses C<$_>. | |
3574 | ||
3575 | For the reverse, see L</chr>. | |
2575c402 | 3576 | See L<perlunicode> for more about Unicode. |
a0d0e21e | 3577 | |
77ca0c92 | 3578 | =item our EXPR |
d74e8afc | 3579 | X<our> X<global> |
77ca0c92 | 3580 | |
36fb85f3 | 3581 | =item our TYPE EXPR |
307ea6df | 3582 | |
1d2de774 | 3583 | =item our EXPR : ATTRS |
9969eac4 | 3584 | |
1d2de774 | 3585 | =item our TYPE EXPR : ATTRS |
307ea6df | 3586 | |
85d8b7d5 | 3587 | C<our> associates a simple name with a package variable in the current |
65c680eb MS |
3588 | package for use within the current scope. When C<use strict 'vars'> is in |
3589 | effect, C<our> lets you use declared global variables without qualifying | |
3590 | them with package names, within the lexical scope of the C<our> declaration. | |
3591 | In this way C<our> differs from C<use vars>, which is package scoped. | |
3592 | ||
cf264981 | 3593 | Unlike C<my>, which both allocates storage for a variable and associates |
65c680eb MS |
3594 | a simple name with that storage for use within the current scope, C<our> |
3595 | associates a simple name with a package variable in the current package, | |
3596 | for use within the current scope. In other words, C<our> has the same | |
3597 | scoping rules as C<my>, but does not necessarily create a | |
3598 | variable. | |
3599 | ||
3600 | If more than one value is listed, the list must be placed | |
3601 | in parentheses. | |
85d8b7d5 MS |
3602 | |
3603 | our $foo; | |
3604 | our($bar, $baz); | |
77ca0c92 | 3605 | |
f472eb5c GS |
3606 | An C<our> declaration declares a global variable that will be visible |
3607 | across its entire lexical scope, even across package boundaries. The | |
3608 | package in which the variable is entered is determined at the point | |
3609 | of the declaration, not at the point of use. This means the following | |
3610 | behavior holds: | |
3611 | ||
3612 | package Foo; | |
5ed4f2ec | 3613 | our $bar; # declares $Foo::bar for rest of lexical scope |
f472eb5c GS |
3614 | $bar = 20; |
3615 | ||
3616 | package Bar; | |
5ed4f2ec | 3617 | print $bar; # prints 20, as it refers to $Foo::bar |
f472eb5c | 3618 | |
65c680eb MS |
3619 | Multiple C<our> declarations with the same name in the same lexical |
3620 | scope are allowed if they are in different packages. If they happen | |
3621 | to be in the same package, Perl will emit warnings if you have asked | |
3622 | for them, just like multiple C<my> declarations. Unlike a second | |
3623 | C<my> declaration, which will bind the name to a fresh variable, a | |
3624 | second C<our> declaration in the same package, in the same scope, is | |
3625 | merely redundant. | |
f472eb5c GS |
3626 | |
3627 | use warnings; | |
3628 | package Foo; | |
5ed4f2ec | 3629 | our $bar; # declares $Foo::bar for rest of lexical scope |
f472eb5c GS |
3630 | $bar = 20; |
3631 | ||
3632 | package Bar; | |
5ed4f2ec | 3633 | our $bar = 30; # declares $Bar::bar for rest of lexical scope |
3634 | print $bar; # prints 30 | |
f472eb5c | 3635 | |
5ed4f2ec | 3636 | our $bar; # emits warning but has no other effect |
3637 | print $bar; # still prints 30 | |
f472eb5c | 3638 | |
9969eac4 | 3639 | An C<our> declaration may also have a list of attributes associated |
307ea6df JH |
3640 | with it. |
3641 | ||
1d2de774 JH |
3642 | The exact semantics and interface of TYPE and ATTRS are still |
3643 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
307ea6df JH |
3644 | and attributes are handled using the C<attributes> pragma, or starting |
3645 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3646 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3647 | L<attributes>, and L<Attribute::Handlers>. | |
3648 | ||
a0d0e21e | 3649 | =item pack TEMPLATE,LIST |
d74e8afc | 3650 | X<pack> |
a0d0e21e | 3651 | |
2b6c5635 GS |
3652 | Takes a LIST of values and converts it into a string using the rules |
3653 | given by the TEMPLATE. The resulting string is the concatenation of | |
3654 | the converted values. Typically, each converted value looks | |
3655 | like its machine-level representation. For example, on 32-bit machines | |
3980dc9c KW |
3656 | an integer may be represented by a sequence of 4 bytes, which will in |
3657 | Perl be presented as a string that's 4 characters long. | |
3658 | ||
3659 | See L<perlpacktut> for an introduction to this function. | |
e1b711da | 3660 | |
18529408 IZ |
3661 | The TEMPLATE is a sequence of characters that give the order and type |
3662 | of values, as follows: | |
a0d0e21e | 3663 | |
5ed4f2ec | 3664 | a A string with arbitrary binary data, will be null padded. |
3665 | A A text (ASCII) string, will be space padded. | |
3b10bc60 | 3666 | Z A null-terminated (ASCIZ) string, will be null padded. |
5a929a98 | 3667 | |
5ed4f2ec | 3668 | b A bit string (ascending bit order inside each byte, like vec()). |
3669 | B A bit string (descending bit order inside each byte). | |
3670 | h A hex string (low nybble first). | |
3671 | H A hex string (high nybble first). | |
a0d0e21e | 3672 | |
5ed4f2ec | 3673 | c A signed char (8-bit) value. |
3674 | C An unsigned char (octet) value. | |
3b10bc60 | 3675 | W An unsigned char value (can be greater than 255). |
96e4d5b1 | 3676 | |
5ed4f2ec | 3677 | s A signed short (16-bit) value. |
3678 | S An unsigned short value. | |
96e4d5b1 | 3679 | |
5ed4f2ec | 3680 | l A signed long (32-bit) value. |
3681 | L An unsigned long value. | |
a0d0e21e | 3682 | |
5ed4f2ec | 3683 | q A signed quad (64-bit) value. |
3684 | Q An unsigned quad value. | |
3685 | (Quads are available only if your system supports 64-bit | |
3686 | integer values _and_ if Perl has been compiled to support those. | |
3b10bc60 | 3687 | Raises an exception otherwise.) |
dae0da7a | 3688 | |
5ed4f2ec | 3689 | i A signed integer value. |
3690 | I A unsigned integer value. | |
3691 | (This 'integer' is _at_least_ 32 bits wide. Its exact | |
1109a392 | 3692 | size depends on what a local C compiler calls 'int'.) |
2b191d53 | 3693 | |
5ed4f2ec | 3694 | n An unsigned short (16-bit) in "network" (big-endian) order. |
3695 | N An unsigned long (32-bit) in "network" (big-endian) order. | |
3696 | v An unsigned short (16-bit) in "VAX" (little-endian) order. | |
3697 | V An unsigned long (32-bit) in "VAX" (little-endian) order. | |
1109a392 MHM |
3698 | |
3699 | j A Perl internal signed integer value (IV). | |
3700 | J A Perl internal unsigned integer value (UV). | |
92d41999 | 3701 | |
3b10bc60 | 3702 | f A single-precision float in native format. |
3703 | d A double-precision float in native format. | |
a0d0e21e | 3704 | |
3b10bc60 | 3705 | F A Perl internal floating-point value (NV) in native format |
3706 | D A float of long-double precision in native format. | |
5ed4f2ec | 3707 | (Long doubles are available only if your system supports long |
3708 | double values _and_ if Perl has been compiled to support those. | |
3b10bc60 | 3709 | Raises an exception otherwise.) |
92d41999 | 3710 | |
5ed4f2ec | 3711 | p A pointer to a null-terminated string. |
3712 | P A pointer to a structure (fixed-length string). | |
a0d0e21e | 3713 | |
5ed4f2ec | 3714 | u A uuencoded string. |
3715 | U A Unicode character number. Encodes to a character in character mode | |
1651fc44 | 3716 | and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode. |
a0d0e21e | 3717 | |
5ed4f2ec | 3718 | w A BER compressed integer (not an ASN.1 BER, see perlpacktut for |
3b10bc60 | 3719 | details). Its bytes represent an unsigned integer in base 128, |
3720 | most significant digit first, with as few digits as possible. Bit | |
3721 | eight (the high bit) is set on each byte except the last. | |
def98dd4 | 3722 | |
3b10bc60 | 3723 | x A null byte (a.k.a ASCII NUL, "\000", chr(0)) |
5ed4f2ec | 3724 | X Back up a byte. |
3b10bc60 | 3725 | @ Null-fill or truncate to absolute position, counted from the |
3726 | start of the innermost ()-group. | |
3727 | . Null-fill or truncate to absolute position specified by the value. | |
5ed4f2ec | 3728 | ( Start of a ()-group. |
a0d0e21e | 3729 | |
3b10bc60 | 3730 | One or more modifiers below may optionally follow certain letters in the |
3731 | TEMPLATE (the second column lists letters for which the modifier is valid): | |
1109a392 MHM |
3732 | |
3733 | ! sSlLiI Forces native (short, long, int) sizes instead | |
3734 | of fixed (16-/32-bit) sizes. | |
3735 | ||
3736 | xX Make x and X act as alignment commands. | |
3737 | ||
3738 | nNvV Treat integers as signed instead of unsigned. | |
3739 | ||
28be1210 TH |
3740 | @. Specify position as byte offset in the internal |
3741 | representation of the packed string. Efficient but | |
3742 | dangerous. | |
3743 | ||
1109a392 MHM |
3744 | > sSiIlLqQ Force big-endian byte-order on the type. |
3745 | jJfFdDpP (The "big end" touches the construct.) | |
3746 | ||
3747 | < sSiIlLqQ Force little-endian byte-order on the type. | |
3748 | jJfFdDpP (The "little end" touches the construct.) | |
3749 | ||
3b10bc60 | 3750 | The C<< > >> and C<< < >> modifiers can also be used on C<()> groups |
3751 | to force a particular byte-order on all components in that group, | |
3752 | including all its subgroups. | |
66c611c5 | 3753 | |
5a929a98 VU |
3754 | The following rules apply: |
3755 | ||
3b10bc60 | 3756 | =over |
5a929a98 VU |
3757 | |
3758 | =item * | |
3759 | ||
3b10bc60 | 3760 | Each letter may optionally be followed by a number indicating the repeat |
3761 | count. A numeric repeat count may optionally be enclosed in brackets, as | |
3762 | in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from | |
3763 | the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>, | |
3764 | C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means | |
3765 | something else, dscribed below. Supplying a C<*> for the repeat count | |
3766 | instead of a number means to use however many items are left, except for: | |
3767 | ||
3768 | =over | |
3769 | ||
3770 | =item * | |
3771 | ||
3772 | C<@>, C<x>, and C<X>, where it is equivalent to C<0>. | |
3773 | ||
3774 | =item * | |
3775 | ||
3776 | <.>, where it means relative to the start of the string. | |
3777 | ||
3778 | =item * | |
3779 | ||
3780 | C<u>, where it is equivalent to 1 (or 45, which here is equivalent). | |
3781 | ||
3782 | =back | |
3783 | ||
3784 | One can replace a numeric repeat count with a template letter enclosed in | |
3785 | brackets to use the packed byte length of the bracketed template for the | |
3786 | repeat count. | |
3787 | ||
3788 | For example, the template C<x[L]> skips as many bytes as in a packed long, | |
3789 | and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when | |
3790 | variable-expanded) unpacks. If the template in brackets contains alignment | |
3791 | commands (such as C<x![d]>), its packed length is calculated as if the | |
3792 | start of the template had the maximal possible alignment. | |
3793 | ||
3794 | When used with C<Z>, a C<*> as the repeat count is guaranteed to add a | |
3795 | trailing null byte, so the resulting string is always one byte longer than | |
3796 | the byte length of the item itself. | |
2b6c5635 | 3797 | |
28be1210 | 3798 | When used with C<@>, the repeat count represents an offset from the start |
3b10bc60 | 3799 | of the innermost C<()> group. |
3800 | ||
3801 | When used with C<.>, the repeat count determines the starting position to | |
3802 | calculate the value offset as follows: | |
3803 | ||
3804 | =over | |
3805 | ||
3806 | =item * | |
3807 | ||
3808 | If the repeat count is C<0>, it's relative to the current position. | |
28be1210 | 3809 | |
3b10bc60 | 3810 | =item * |
3811 | ||
3812 | If the repeat count is C<*>, the offset is relative to the start of the | |
3813 | packed string. | |
3814 | ||
3815 | =item * | |
3816 | ||
3817 | And if it's an integer I<n>, the offset is relative to the start of the | |
3818 | I<n>th innermost C<()> group, or to the start of the string if I<n> is | |
3819 | bigger then the group level. | |
3820 | ||
3821 | =back | |
28be1210 | 3822 | |
951ba7fe | 3823 | The repeat count for C<u> is interpreted as the maximal number of bytes |
f337b084 TH |
3824 | to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat |
3825 | count should not be more than 65. | |
5a929a98 VU |
3826 | |
3827 | =item * | |
3828 | ||
951ba7fe | 3829 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
3b10bc60 | 3830 | string of length count, padding with nulls or spaces as needed. When |
18bdf90a | 3831 | unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything |
3b10bc60 | 3832 | after the first null, and C<a> returns data without any sort of trimming. |
2b6c5635 | 3833 | |
3b10bc60 | 3834 | If the value to pack is too long, the result is truncated. If it's too |
3835 | long and an explicit count is provided, C<Z> packs only C<$count-1> bytes, | |
3836 | followed by a null byte. Thus C<Z> always packs a trailing null, except | |
3837 | for when the count is 0. | |
5a929a98 VU |
3838 | |
3839 | =item * | |
3840 | ||
3b10bc60 | 3841 | Likewise, the C<b> and C<B> formats pack a string that's that many bits long. |
3842 | Each such format generates 1 bit of the result. | |
3843 | ||
c73032f5 | 3844 | Each result bit is based on the least-significant bit of the corresponding |
f337b084 | 3845 | input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> |
3b10bc60 | 3846 | and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">. |
c73032f5 | 3847 | |
3b10bc60 | 3848 | Starting from the beginning of the input string, each 8-tuple |
3849 | of characters is converted to 1 character of output. With format C<b>, | |
f337b084 | 3850 | the first character of the 8-tuple determines the least-significant bit of a |
3b10bc60 | 3851 | character; with format C<B>, it determines the most-significant bit of |
f337b084 | 3852 | a character. |
c73032f5 | 3853 | |
3b10bc60 | 3854 | If the length of the input string is not evenly divisible by 8, the |
f337b084 | 3855 | remainder is packed as if the input string were padded by null characters |
3b10bc60 | 3856 | at the end. Similarly during unpacking, "extra" bits are ignored. |
c73032f5 | 3857 | |
3b10bc60 | 3858 | If the input string is longer than needed, remaining characters are ignored. |
3859 | ||
3860 | A C<*> for the repeat count uses all characters of the input field. | |
3861 | On unpacking, bits are converted to a string of C<"0">s and C<"1">s. | |
5a929a98 VU |
3862 | |
3863 | =item * | |
3864 | ||
3b10bc60 | 3865 | The C<h> and C<H> formats pack a string that many nybbles (4-bit groups, |
3866 | representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long. | |
5a929a98 | 3867 | |
3b10bc60 | 3868 | For each such format, pack() generates 4 bits of the result. |
3869 | With non-alphabetical characters, the result is based on the 4 least-significant | |
f337b084 TH |
3870 | bits of the input character, i.e., on C<ord($char)%16>. In particular, |
3871 | characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
3b10bc60 | 3872 | C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F">, the result |
c73032f5 | 3873 | is compatible with the usual hexadecimal digits, so that C<"a"> and |
3b10bc60 | 3874 | C<"A"> both generate the nybble C<0xa==10>. Do not use any characters |
3875 | but these with this format. | |
c73032f5 | 3876 | |
3b10bc60 | 3877 | Starting from the beginning of the template to pack(), each pair |
3878 | of characters is converted to 1 character of output. With format C<h>, the | |
f337b084 | 3879 | first character of the pair determines the least-significant nybble of the |
3b10bc60 | 3880 | output character; with format C<H>, it determines the most-significant |
c73032f5 IZ |
3881 | nybble. |
3882 | ||
3b10bc60 | 3883 | If the length of the input string is not even, it behaves as if padded by |
3884 | a null character at the end. Similarly, "extra" nybbles are ignored during | |
3885 | unpacking. | |
3886 | ||
3887 | If the input string is longer than needed, extra characters are ignored. | |
c73032f5 | 3888 | |
3b10bc60 | 3889 | A C<*> for the repeat count uses all characters of the input field. For |
3890 | unpack(), nybbles are converted to a string of hexadecimal digits. | |
c73032f5 | 3891 | |
5a929a98 VU |
3892 | =item * |
3893 | ||
3b10bc60 | 3894 | The C<p> format packs a pointer to a null-terminated string. You are |
3895 | responsible for ensuring that the string is not a temporary value, as that | |
3896 | could potentially get deallocated before you got around to using the packed | |
3897 | result. The C<P> format packs a pointer to a structure of the size indicated | |
3898 | by the length. A null pointer is created if the corresponding value for | |
3899 | C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer | |
3900 | unpacks into C<undef>. | |
5a929a98 | 3901 | |
3b10bc60 | 3902 | If your system has a strange pointer size--meaning a pointer is neither as |
3903 | big as an int nor as big as a long--it may not be possible to pack or | |
1109a392 | 3904 | unpack pointers in big- or little-endian byte order. Attempting to do |
3b10bc60 | 3905 | so raises an exception. |
1109a392 | 3906 | |
5a929a98 VU |
3907 | =item * |
3908 | ||
246f24af | 3909 | The C</> template character allows packing and unpacking of a sequence of |
3b10bc60 | 3910 | items where the packed structure contains a packed item count followed by |
3911 | the packed items themselves. This is useful when the structure you're | |
3912 | unpacking has encoded the sizes or repeat counts for some of its fields | |
3913 | within the structure itself as separate fields. | |
3914 | ||
3915 | For C<pack>, you write I<length-item>C</>I<sequence-item>, and the | |
3916 | I<length-item> describes how the length value is packed. Formats likely | |
3917 | to be of most use are integer-packing ones like C<n> for Java strings, | |
3918 | C<w> for ASN.1 or SNMP, and C<N> for Sun XDR. | |
3919 | ||
3920 | For C<pack>, I<sequence-item> may have a repeat count, in which case | |
3921 | the minimum of that and the number of available items is used as the argument | |
3922 | for I<length-item>. If it has no repeat count or uses a '*', the number | |
54f961c9 PD |
3923 | of available items is used. |
3924 | ||
3b10bc60 | 3925 | For C<unpack>, an internal stack of integer arguments unpacked so far is |
54f961c9 PD |
3926 | used. You write C</>I<sequence-item> and the repeat count is obtained by |
3927 | popping off the last element from the stack. The I<sequence-item> must not | |
3928 | have a repeat count. | |
246f24af | 3929 | |
3b10bc60 | 3930 | If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">), |
3931 | the I<length-item> is the string length, not the number of strings. With | |
3932 | an explicit repeat count for pack, the packed string is adjusted to that | |
3933 | length. For example: | |
246f24af | 3934 | |
3b10bc60 | 3935 | unpack("W/a", "\04Gurusamy") gives ("Guru") |
3936 | unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J") | |
3937 | unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".") | |
3938 | ||
3939 | pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world" | |
3940 | pack("a/W2", ord("a") .. ord("z")) gives "2ab" | |
43192e07 IP |
3941 | |
3942 | The I<length-item> is not returned explicitly from C<unpack>. | |
3943 | ||
3b10bc60 | 3944 | Supplying a count to the I<length-item> format letter is only useful with |
3945 | C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may | |
3946 | introduce C<"\000"> characters, which Perl does not regard as legal in | |
3947 | numeric strings. | |
43192e07 IP |
3948 | |
3949 | =item * | |
3950 | ||
951ba7fe | 3951 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
3b10bc60 | 3952 | followed by a C<!> modifier to specify native shorts or |
3953 | longs. As shown in the example above, a bare C<l> means | |
3954 | exactly 32 bits, although the native C<long> as seen by the local C compiler | |
3955 | may be larger. This is mainly an issue on 64-bit platforms. You can | |
3956 | see whether using C<!> makes any difference this way: | |
3957 | ||
3958 | printf "format s is %d, s! is %d\n", | |
3959 | length pack("s"), length pack("s!"); | |
726ea183 | 3960 | |
3b10bc60 | 3961 | printf "format l is %d, l! is %d\n", |
3962 | length pack("l"), length pack("l!"); | |
ef54e1a4 | 3963 | |
3b10bc60 | 3964 | |
3965 | C<i!> and C<I!> are also allowed, but only for completeness' sake: | |
951ba7fe | 3966 | they are identical to C<i> and C<I>. |
ef54e1a4 | 3967 | |
19799a22 | 3968 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
3b10bc60 | 3969 | longs on the platform where Perl was built are also available from |
3970 | the command line: | |
3971 | ||
3972 | $ perl -V:{short,int,long{,long}}size | |
3973 | shortsize='2'; | |
3974 | intsize='4'; | |
3975 | longsize='4'; | |
3976 | longlongsize='8'; | |
3977 | ||
3978 | or programmatically via the C<Config> module: | |
19799a22 GS |
3979 | |
3980 | use Config; | |
3981 | print $Config{shortsize}, "\n"; | |
3982 | print $Config{intsize}, "\n"; | |
3983 | print $Config{longsize}, "\n"; | |
3984 | print $Config{longlongsize}, "\n"; | |
ef54e1a4 | 3985 | |
3b10bc60 | 3986 | C<$Config{longlongsize}> is undefined on systems without |
3987 | long long support. | |
851646ae | 3988 | |
ef54e1a4 JH |
3989 | =item * |
3990 | ||
3b10bc60 | 3991 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are |
3992 | inherently non-portable between processors and operating systems because | |
3993 | they obey native byteorder and endianness. For example, a 4-byte integer | |
3994 | 0x12345678 (305419896 decimal) would be ordered natively (arranged in and | |
3995 | handled by the CPU registers) into bytes as | |
61eff3bc | 3996 | |
5ed4f2ec | 3997 | 0x12 0x34 0x56 0x78 # big-endian |
3998 | 0x78 0x56 0x34 0x12 # little-endian | |
61eff3bc | 3999 | |
3b10bc60 | 4000 | Basically, Intel and VAX CPUs are little-endian, while everybody else, |
4001 | including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are | |
4002 | big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in | |
4003 | little-endian mode, but SGI/Cray uses them in big-endian mode. | |
719a3cf5 | 4004 | |
3b10bc60 | 4005 | The names I<big-endian> and I<little-endian> are comic references to the |
4006 | egg-eating habits of the little-endian Lilliputians and the big-endian | |
4007 | Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>. | |
4008 | This entered computer lingo via the paper "On Holy Wars and a Plea for | |
4009 | Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. | |
61eff3bc | 4010 | |
140cb37e | 4011 | Some systems may have even weirder byte orders such as |
61eff3bc | 4012 | |
5ed4f2ec | 4013 | 0x56 0x78 0x12 0x34 |
4014 | 0x34 0x12 0x78 0x56 | |
61eff3bc | 4015 | |
3b10bc60 | 4016 | You can determine your system endianness with this incantation: |
ef54e1a4 | 4017 | |
3b10bc60 | 4018 | printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); |
ef54e1a4 | 4019 | |
d99ad34e | 4020 | The byteorder on the platform where Perl was built is also available |
726ea183 | 4021 | via L<Config>: |
ef54e1a4 | 4022 | |
5ed4f2ec | 4023 | use Config; |
3b10bc60 | 4024 | print "$Config{byteorder}\n"; |
4025 | ||
4026 | or from the command line: | |
ef54e1a4 | 4027 | |
3b10bc60 | 4028 | $ perl -V:byteorder |
719a3cf5 | 4029 | |
3b10bc60 | 4030 | Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321"> |
4031 | and C<"87654321"> are big-endian. | |
4032 | ||
4033 | For portably packed integers, either use the formats C<n>, C<N>, C<v>, | |
4034 | and C<V> or else use the C<< > >> and C<< < >> modifiers described | |
4035 | immediately below. See also L<perlport>. | |
ef54e1a4 JH |
4036 | |
4037 | =item * | |
4038 | ||
3b10bc60 | 4039 | Starting with Perl 5.9.2, integer and floating-point formats, along with |
4040 | the C<p> and C<P> formats and C<()> groups, may all be followed by the | |
4041 | C<< > >> or C<< < >> endianness modifiers to respectively enforce big- | |
4042 | or little-endian byte-order. These modifiers are especially useful | |
4043 | given how C<n>, C<N>, C<v> and C<V> don't cover signed integers, | |
4044 | 64-bit integers, or floating-point values. | |
4045 | ||
4046 | Here are some concerns to keep in mind when using endianness modifier: | |
4047 | ||
4048 | =over | |
4049 | ||
4050 | =item * | |
4051 | ||
4052 | Exchanging signed integers between different platforms works only | |
4053 | when all platforms store them in the same format. Most platforms store | |
4054 | signed integers in two's-complement notation, so usually this is not an issue. | |
1109a392 | 4055 | |
3b10bc60 | 4056 | =item * |
1109a392 | 4057 | |
3b10bc60 | 4058 | The C<< > >> or C<< < >> modifiers can only be used on floating-point |
1109a392 | 4059 | formats on big- or little-endian machines. Otherwise, attempting to |
3b10bc60 | 4060 | use them raises an exception. |
1109a392 | 4061 | |
3b10bc60 | 4062 | =item * |
4063 | ||
4064 | Forcing big- or little-endian byte-order on floating-point values for | |
4065 | data exchange can work only if all platforms use the same | |
4066 | binary representation such as IEEE floating-point. Even if all | |
4067 | platforms are using IEEE, there may still be subtle differences. Being able | |
4068 | to use C<< > >> or C<< < >> on floating-point values can be useful, | |
80d38338 | 4069 | but also dangerous if you don't know exactly what you're doing. |
3b10bc60 | 4070 | It is not a general way to portably store floating-point values. |
4071 | ||
4072 | =item * | |
1109a392 | 4073 | |
3b10bc60 | 4074 | When using C<< > >> or C<< < >> on a C<()> group, this affects |
4075 | all types inside the group that accept byte-order modifiers, | |
4076 | including all subgroups. It is silently ignored for all other | |
66c611c5 MHM |
4077 | types. You are not allowed to override the byte-order within a group |
4078 | that already has a byte-order modifier suffix. | |
4079 | ||
3b10bc60 | 4080 | =back |
4081 | ||
1109a392 MHM |
4082 | =item * |
4083 | ||
3b10bc60 | 4084 | Real numbers (floats and doubles) are in native machine format only. |
4085 | Due to the multiplicity of floating-point formats and the lack of a | |
4086 | standard "network" representation for them, no facility for interchange has been | |
4087 | made. This means that packed floating-point data written on one machine | |
4088 | may not be readable on another, even if both use IEEE floating-point | |
4089 | arithmetic (because the endianness of the memory representation is not part | |
851646ae | 4090 | of the IEEE spec). See also L<perlport>. |
5a929a98 | 4091 | |
3b10bc60 | 4092 | If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >> |
4093 | modifiers to force big- or little-endian byte-order on floating-point values. | |
1109a392 | 4094 | |
3b10bc60 | 4095 | Because Perl uses doubles (or long doubles, if configured) internally for |
4096 | all numeric calculation, converting from double into float and thence | |
4097 | to double again loses precision, so C<unpack("f", pack("f", $foo)>) | |
4098 | will not in general equal $foo. | |
5a929a98 | 4099 | |
851646ae JH |
4100 | =item * |
4101 | ||
3b10bc60 | 4102 | Pack and unpack can operate in two modes: character mode (C<C0> mode) where |
4103 | the packed string is processed per character, and UTF-8 mode (C<U0> mode) | |
f337b084 | 4104 | where the packed string is processed in its UTF-8-encoded Unicode form on |
3b10bc60 | 4105 | a byte-by-byte basis. Character mode is the default unless the format string |
4106 | starts with C<U>. You can always switch mode mid-format with an explicit | |
4107 | C<C0> or C<U0> in the format. This mode remains in effect until the next | |
4108 | mode change, or until the end of the C<()> group it (directly) applies to. | |
036b4402 GS |
4109 | |
4110 | =item * | |
4111 | ||
3b10bc60 | 4112 | You must yourself do any alignment or padding by inserting, for example, |
4113 | enough C<"x">es while packing. There is no way for pack() and unpack() | |
4114 | to know where characters are going to or coming from, so they | |
4115 | handle their output and input as flat sequences of characters. | |
851646ae | 4116 | |
17f4a12d IZ |
4117 | =item * |
4118 | ||
3b10bc60 | 4119 | A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may |
4120 | take a repeat count either as postfix, or for unpack(), also via the C</> | |
4121 | template character. Within each repetition of a group, positioning with | |
4122 | C<@> starts over at 0. Therefore, the result of | |
49704364 | 4123 | |
3b10bc60 | 4124 | pack("@1A((@2A)@3A)", qw[X Y Z]) |
49704364 | 4125 | |
3b10bc60 | 4126 | is the string C<"\0X\0\0YZ">. |
49704364 | 4127 | |
18529408 IZ |
4128 | =item * |
4129 | ||
3b10bc60 | 4130 | C<x> and C<X> accept the C<!> modifier to act as alignment commands: they |
4131 | jump forward or back to the closest position aligned at a multiple of C<count> | |
4132 | characters. For example, to pack() or unpack() a C structure like | |
666f95b9 | 4133 | |
3b10bc60 | 4134 | struct { |
4135 | char c; /* one signed, 8-bit character */ | |
4136 | double d; | |
4137 | char cc[2]; | |
4138 | } | |
4139 | ||
4140 | one may need to use the template C<c x![d] d c[2]>. This assumes that | |
4141 | doubles must be aligned to the size of double. | |
4142 | ||
4143 | For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1; | |
4144 | both are no-ops. | |
666f95b9 | 4145 | |
62f95557 IZ |
4146 | =item * |
4147 | ||
3b10bc60 | 4148 | C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to |
4149 | represent signed 16-/32-bit integers in big-/little-endian order. | |
4150 | This is portable only when all platforms sharing packed data use the | |
4151 | same binary representation for signed integers; for example, when all | |
4152 | platforms use two's-complement representation. | |
068bd2e7 MHM |
4153 | |
4154 | =item * | |
4155 | ||
3b10bc60 | 4156 | Comments can be embedded in a TEMPLATE using C<#> through the end of line. |
4157 | White space can separate pack codes from each other, but modifiers and | |
4158 | repeat counts must follow immediately. Breaking complex templates into | |
4159 | individual line-by-line components, suitably annotated, can do as much to | |
4160 | improve legibility and maintainability of pack/unpack formats as C</x> can | |
4161 | for complicated pattern matches. | |
17f4a12d | 4162 | |
2b6c5635 GS |
4163 | =item * |
4164 | ||
3b10bc60 | 4165 | If TEMPLATE requires more arguments that pack() is given, pack() |
cf264981 | 4166 | assumes additional C<""> arguments. If TEMPLATE requires fewer arguments |
3b10bc60 | 4167 | than given, extra arguments are ignored. |
2b6c5635 | 4168 | |
5a929a98 | 4169 | =back |
a0d0e21e LW |
4170 | |
4171 | Examples: | |
4172 | ||
f337b084 | 4173 | $foo = pack("WWWW",65,66,67,68); |
a0d0e21e | 4174 | # foo eq "ABCD" |
f337b084 | 4175 | $foo = pack("W4",65,66,67,68); |
a0d0e21e | 4176 | # same thing |
f337b084 TH |
4177 | $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); |
4178 | # same thing with Unicode circled letters. | |
a0ed51b3 | 4179 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
f337b084 TH |
4180 | # same thing with Unicode circled letters. You don't get the UTF-8 |
4181 | # bytes because the U at the start of the format caused a switch to | |
4182 | # U0-mode, so the UTF-8 bytes get joined into characters | |
4183 | $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); | |
4184 | # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" | |
4185 | # This is the UTF-8 encoding of the string in the previous example | |
a0d0e21e LW |
4186 | |
4187 | $foo = pack("ccxxcc",65,66,67,68); | |
4188 | # foo eq "AB\0\0CD" | |
4189 | ||
3b10bc60 | 4190 | # NOTE: The examples above featuring "W" and "c" are true |
9ccd05c0 | 4191 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 |
3b10bc60 | 4192 | # and UTF-8. On EBCDIC systems, the first example would be |
4193 | # $foo = pack("WWWW",193,194,195,196); | |
9ccd05c0 | 4194 | |
a0d0e21e LW |
4195 | $foo = pack("s2",1,2); |
4196 | # "\1\0\2\0" on little-endian | |
4197 | # "\0\1\0\2" on big-endian | |
4198 | ||
4199 | $foo = pack("a4","abcd","x","y","z"); | |
4200 | # "abcd" | |
4201 | ||
4202 | $foo = pack("aaaa","abcd","x","y","z"); | |
4203 | # "axyz" | |
4204 | ||
4205 | $foo = pack("a14","abcdefg"); | |
4206 | # "abcdefg\0\0\0\0\0\0\0" | |
4207 | ||
4208 | $foo = pack("i9pl", gmtime); | |
4209 | # a real struct tm (on my system anyway) | |
4210 | ||
5a929a98 VU |
4211 | $utmp_template = "Z8 Z8 Z16 L"; |
4212 | $utmp = pack($utmp_template, @utmp1); | |
4213 | # a struct utmp (BSDish) | |
4214 | ||
4215 | @utmp2 = unpack($utmp_template, $utmp); | |
4216 | # "@utmp1" eq "@utmp2" | |
4217 | ||
a0d0e21e | 4218 | sub bintodec { |
a9a5a0dc | 4219 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); |
a0d0e21e LW |
4220 | } |
4221 | ||
851646ae JH |
4222 | $foo = pack('sx2l', 12, 34); |
4223 | # short 12, two zero bytes padding, long 34 | |
4224 | $bar = pack('s@4l', 12, 34); | |
4225 | # short 12, zero fill to position 4, long 34 | |
4226 | # $foo eq $bar | |
28be1210 TH |
4227 | $baz = pack('s.l', 12, 4, 34); |
4228 | # short 12, zero fill to position 4, long 34 | |
851646ae | 4229 | |
1109a392 MHM |
4230 | $foo = pack('nN', 42, 4711); |
4231 | # pack big-endian 16- and 32-bit unsigned integers | |
4232 | $foo = pack('S>L>', 42, 4711); | |
4233 | # exactly the same | |
4234 | $foo = pack('s<l<', -42, 4711); | |
4235 | # pack little-endian 16- and 32-bit signed integers | |
66c611c5 MHM |
4236 | $foo = pack('(sl)<', -42, 4711); |
4237 | # exactly the same | |
1109a392 | 4238 | |
5a929a98 | 4239 | The same template may generally also be used in unpack(). |
a0d0e21e | 4240 | |
6fa4d285 DG |
4241 | =item package NAMESPACE VERSION |
4242 | X<package> X<module> X<namespace> X<version> | |
4243 | ||
cb1a09d0 AD |
4244 | =item package NAMESPACE |
4245 | ||
4e4da3ac Z |
4246 | =item package NAMESPACE VERSION BLOCK |
4247 | X<package> X<module> X<namespace> X<version> | |
4248 | ||
4249 | =item package NAMESPACE BLOCK | |
4250 | ||
4251 | Declares the BLOCK, or the rest of the compilation unit, as being in | |
4252 | the given namespace. The scope of the package declaration is either the | |
4253 | supplied code BLOCK or, in the absence of a BLOCK, from the declaration | |
4254 | itself through the end of the enclosing block, file, or eval (the same | |
4255 | as the C<my> operator). All unqualified dynamic identifiers in this | |
4256 | scope will be in the given namespace, except where overridden by another | |
4257 | C<package> declaration. | |
4258 | ||
3b10bc60 | 4259 | A package statement affects dynamic variables only, including those |
4260 | you've used C<local> on, but I<not> lexical variables, which are created | |
4261 | with C<my> (or C<our> (or C<state>)). Typically it would be the first | |
4262 | declaration in a file included by C<require> or C<use>. You can switch into a | |
4263 | package in more than one place, since this only determines which default | |
4264 | symbol table the compiler uses for the rest of that block. You can refer to | |
4265 | identifiers in other packages than the current one by prefixing the identifier | |
4266 | with the package name and a double colon, as in C<$SomePack::var> | |
4267 | or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main> | |
4268 | package as assumed. That is, C<$::sail> is equivalent to | |
4269 | C<$main::sail> (as well as to C<$main'sail>, still seen in ancient | |
4270 | code, mostly from Perl 4). | |
4271 | ||
bd12309b | 4272 | If VERSION is provided, C<package> sets the C<$VERSION> variable in the given |
a2bff36e DG |
4273 | namespace to a L<version> object with the VERSION provided. VERSION must be a |
4274 | "strict" style version number as defined by the L<version> module: a positive | |
4275 | decimal number (integer or decimal-fraction) without exponentiation or else a | |
4276 | dotted-decimal v-string with a leading 'v' character and at least three | |
4277 | components. You should set C<$VERSION> only once per package. | |
6fa4d285 | 4278 | |
cb1a09d0 AD |
4279 | See L<perlmod/"Packages"> for more information about packages, modules, |
4280 | and classes. See L<perlsub> for other scoping issues. | |
4281 | ||
a0d0e21e | 4282 | =item pipe READHANDLE,WRITEHANDLE |
d74e8afc | 4283 | X<pipe> |
a0d0e21e LW |
4284 | |
4285 | Opens a pair of connected pipes like the corresponding system call. | |
4286 | Note that if you set up a loop of piped processes, deadlock can occur | |
4287 | unless you are very careful. In addition, note that Perl's pipes use | |
9124316e | 4288 | IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
4289 | after each command, depending on the application. |
4290 | ||
7e1af8bc | 4291 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
4633a7c4 LW |
4292 | for examples of such things. |
4293 | ||
3b10bc60 | 4294 | On systems that support a close-on-exec flag on files, that flag is set |
4295 | on all newly opened file descriptors whose C<fileno>s are I<higher> than | |
4296 | the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>. | |
4771b018 | 4297 | |
a0d0e21e | 4298 | =item pop ARRAY |
d74e8afc | 4299 | X<pop> X<stack> |
a0d0e21e | 4300 | |
54310121 | 4301 | =item pop |
28757baa | 4302 | |
a0d0e21e | 4303 | Pops and returns the last value of the array, shortening the array by |
cd7f9af7 | 4304 | one element. |
a0d0e21e | 4305 | |
3b10bc60 | 4306 | Returns the undefined value if the array is empty, although this may also |
4307 | happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the | |
4308 | main program, but the C<@_> array in subroutines, just like C<shift>. | |
a0d0e21e LW |
4309 | |
4310 | =item pos SCALAR | |
d74e8afc | 4311 | X<pos> X<match, position> |
a0d0e21e | 4312 | |
54310121 | 4313 | =item pos |
bbce6d69 | 4314 | |
4633a7c4 | 4315 | Returns the offset of where the last C<m//g> search left off for the variable |
b17c04f3 | 4316 | in question (C<$_> is used when the variable is not specified). Note that |
cf264981 | 4317 | 0 is a valid match offset. C<undef> indicates that the search position |
b17c04f3 | 4318 | is reset (usually due to match failure, but can also be because no match has |
80d38338 | 4319 | yet been run on the scalar). C<pos> directly accesses the location used |
b17c04f3 NC |
4320 | by the regexp engine to store the offset, so assigning to C<pos> will change |
4321 | that offset, and so will also influence the C<\G> zero-width assertion in | |
4322 | regular expressions. Because a failed C<m//gc> match doesn't reset the offset, | |
4323 | the return from C<pos> won't change either in this case. See L<perlre> and | |
44a8e56a | 4324 | L<perlop>. |
a0d0e21e LW |
4325 | |
4326 | =item print FILEHANDLE LIST | |
d74e8afc | 4327 | X<print> |
a0d0e21e LW |
4328 | |
4329 | =item print LIST | |
4330 | ||
4331 | =item print | |
4332 | ||
19799a22 | 4333 | Prints a string or a list of strings. Returns true if successful. |
3b10bc60 | 4334 | FILEHANDLE may be a scalar variable containing |
4335 | the name of or a reference to the filehandle, thus introducing | |
19799a22 GS |
4336 | one level of indirection. (NOTE: If FILEHANDLE is a variable and |
4337 | the next token is a term, it may be misinterpreted as an operator | |
2b5ab1e7 | 4338 | unless you interpose a C<+> or put parentheses around the arguments.) |
3b10bc60 | 4339 | If FILEHANDLE is omitted, prints to standard output by default, or |
4340 | to the last selected output channel; see L</select>. If LIST is | |
4341 | also omitted, prints C<$_> to the currently selected output handle. | |
4342 | To set the default output handle to something other than STDOUT | |
19799a22 GS |
4343 | use the select operation. The current value of C<$,> (if any) is |
4344 | printed between each LIST item. The current value of C<$\> (if | |
4345 | any) is printed after the entire LIST has been printed. Because | |
4346 | print takes a LIST, anything in the LIST is evaluated in list | |
4347 | context, and any subroutine that you call will have one or more of | |
4348 | its expressions evaluated in list context. Also be careful not to | |
4349 | follow the print keyword with a left parenthesis unless you want | |
4350 | the corresponding right parenthesis to terminate the arguments to | |
3b10bc60 | 4351 | the print; put parentheses around all the arguments |
4352 | (or interpose a C<+>, but that doesn't look as good). | |
a0d0e21e | 4353 | |
39c9c9cd RGS |
4354 | Note that if you're storing FILEHANDLEs in an array, or if you're using |
4355 | any other expression more complex than a scalar variable to retrieve it, | |
4356 | you will have to use a block returning the filehandle value instead: | |
4633a7c4 LW |
4357 | |
4358 | print { $files[$i] } "stuff\n"; | |
4359 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
4360 | ||
785fd561 DG |
4361 | Printing to a closed pipe or socket will generate a SIGPIPE signal. See |
4362 | L<perlipc> for more on signal handling. | |
4363 | ||
5f05dabc | 4364 | =item printf FILEHANDLE FORMAT, LIST |
d74e8afc | 4365 | X<printf> |
a0d0e21e | 4366 | |
5f05dabc | 4367 | =item printf FORMAT, LIST |
a0d0e21e | 4368 | |
7660c0ab | 4369 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
a3cb178b | 4370 | (the output record separator) is not appended. The first argument |
f39758bf | 4371 | of the list will be interpreted as the C<printf> format. See C<sprintf> |
7e4353e9 RGS |
4372 | for an explanation of the format argument. If C<use locale> is in effect, |
4373 | and POSIX::setlocale() has been called, the character used for the decimal | |
3b10bc60 | 4374 | separator in formatted floating-point numbers is affected by the LC_NUMERIC |
7e4353e9 | 4375 | locale. See L<perllocale> and L<POSIX>. |
a0d0e21e | 4376 | |
19799a22 GS |
4377 | Don't fall into the trap of using a C<printf> when a simple |
4378 | C<print> would do. The C<print> is more efficient and less | |
28757baa | 4379 | error prone. |
4380 | ||
da0045b7 | 4381 | =item prototype FUNCTION |
d74e8afc | 4382 | X<prototype> |
da0045b7 | 4383 | |
4384 | Returns the prototype of a function as a string (or C<undef> if the | |
5f05dabc | 4385 | function has no prototype). FUNCTION is a reference to, or the name of, |
4386 | the function whose prototype you want to retrieve. | |
da0045b7 | 4387 | |
2b5ab1e7 | 4388 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a |
e1020413 | 4389 | name for a Perl builtin. If the builtin is not I<overridable> (such as |
0a2ca743 RGS |
4390 | C<qw//>) or if its arguments cannot be adequately expressed by a prototype |
4391 | (such as C<system>), prototype() returns C<undef>, because the builtin | |
4392 | does not really behave like a Perl function. Otherwise, the string | |
4393 | describing the equivalent prototype is returned. | |
b6c543e3 | 4394 | |
a0d0e21e | 4395 | =item push ARRAY,LIST |
1dc8ecb8 | 4396 | X<push> X<stack> |
a0d0e21e LW |
4397 | |
4398 | Treats ARRAY as a stack, and pushes the values of LIST | |
4399 | onto the end of ARRAY. The length of ARRAY increases by the length of | |
4400 | LIST. Has the same effect as | |
4401 | ||
4402 | for $value (LIST) { | |
a9a5a0dc | 4403 | $ARRAY[++$#ARRAY] = $value; |
a0d0e21e LW |
4404 | } |
4405 | ||
cde9c211 SP |
4406 | but is more efficient. Returns the number of elements in the array following |
4407 | the completed C<push>. | |
a0d0e21e LW |
4408 | |
4409 | =item q/STRING/ | |
4410 | ||
4411 | =item qq/STRING/ | |
4412 | ||
945c54fd | 4413 | =item qx/STRING/ |
a0d0e21e LW |
4414 | |
4415 | =item qw/STRING/ | |
4416 | ||
1d888ee3 MK |
4417 | Generalized quotes. See L<perlop/"Quote-Like Operators">. |
4418 | ||
4419 | =item qr/STRING/ | |
4420 | ||
4421 | Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">. | |
a0d0e21e LW |
4422 | |
4423 | =item quotemeta EXPR | |
d74e8afc | 4424 | X<quotemeta> X<metacharacter> |
a0d0e21e | 4425 | |
54310121 | 4426 | =item quotemeta |
bbce6d69 | 4427 | |
36bbe248 | 4428 | Returns the value of EXPR with all non-"word" |
a034a98d DD |
4429 | characters backslashed. (That is, all characters not matching |
4430 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
4431 | returned string, regardless of any locale settings.) | |
4432 | This is the internal function implementing | |
7660c0ab | 4433 | the C<\Q> escape in double-quoted strings. |
a0d0e21e | 4434 | |
7660c0ab | 4435 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 4436 | |
9702b155 RGS |
4437 | quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into |
4438 | regular expressions, because by default an interpolated variable will be | |
4439 | considered a mini-regular expression. For example: | |
4440 | ||
4441 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4442 | my $substring = 'quick.*?fox'; | |
4443 | $sentence =~ s{$substring}{big bad wolf}; | |
4444 | ||
4445 | Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>. | |
4446 | ||
4447 | On the other hand: | |
4448 | ||
4449 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4450 | my $substring = 'quick.*?fox'; | |
4451 | $sentence =~ s{\Q$substring\E}{big bad wolf}; | |
4452 | ||
4453 | Or: | |
4454 | ||
4455 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4456 | my $substring = 'quick.*?fox'; | |
4457 | my $quoted_substring = quotemeta($substring); | |
4458 | $sentence =~ s{$quoted_substring}{big bad wolf}; | |
4459 | ||
4460 | Will both leave the sentence as is. Normally, when accepting string input from | |
4461 | the user, quotemeta() or C<\Q> must be used. | |
4462 | ||
a0d0e21e | 4463 | =item rand EXPR |
d74e8afc | 4464 | X<rand> X<random> |
a0d0e21e LW |
4465 | |
4466 | =item rand | |
4467 | ||
7660c0ab | 4468 | Returns a random fractional number greater than or equal to C<0> and less |
3e3baf6d | 4469 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
351f3254 | 4470 | omitted, the value C<1> is used. Currently EXPR with the value C<0> is |
3b10bc60 | 4471 | also special-cased as C<1> (this was undocumented before Perl 5.8.0 |
4472 | and is subject to change in future versions of Perl). Automatically calls | |
351f3254 | 4473 | C<srand> unless C<srand> has already been called. See also C<srand>. |
a0d0e21e | 4474 | |
6063ba18 WM |
4475 | Apply C<int()> to the value returned by C<rand()> if you want random |
4476 | integers instead of random fractional numbers. For example, | |
4477 | ||
4478 | int(rand(10)) | |
4479 | ||
4480 | returns a random integer between C<0> and C<9>, inclusive. | |
4481 | ||
2f9daede | 4482 | (Note: If your rand function consistently returns numbers that are too |
a0d0e21e | 4483 | large or too small, then your version of Perl was probably compiled |
2f9daede | 4484 | with the wrong number of RANDBITS.) |
a0d0e21e LW |
4485 | |
4486 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
f723aae1 | 4487 | X<read> X<file, read> |
a0d0e21e LW |
4488 | |
4489 | =item read FILEHANDLE,SCALAR,LENGTH | |
4490 | ||
9124316e JH |
4491 | Attempts to read LENGTH I<characters> of data into variable SCALAR |
4492 | from the specified FILEHANDLE. Returns the number of characters | |
b5fe5ca2 | 4493 | actually read, C<0> at end of file, or undef if there was an error (in |
b49f3be6 SG |
4494 | the latter case C<$!> is also set). SCALAR will be grown or shrunk |
4495 | so that the last character actually read is the last character of the | |
4496 | scalar after the read. | |
4497 | ||
4498 | An OFFSET may be specified to place the read data at some place in the | |
4499 | string other than the beginning. A negative OFFSET specifies | |
4500 | placement at that many characters counting backwards from the end of | |
4501 | the string. A positive OFFSET greater than the length of SCALAR | |
4502 | results in the string being padded to the required size with C<"\0"> | |
4503 | bytes before the result of the read is appended. | |
4504 | ||
80d38338 TC |
4505 | The call is implemented in terms of either Perl's or your system's native |
4506 | fread(3) library function. To get a true read(2) system call, see C<sysread>. | |
9124316e JH |
4507 | |
4508 | Note the I<characters>: depending on the status of the filehandle, | |
4509 | either (8-bit) bytes or characters are read. By default all | |
4510 | filehandles operate on bytes, but for example if the filehandle has | |
fae2c0fb | 4511 | been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> |
1d714267 JH |
4512 | pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode |
4513 | characters, not bytes. Similarly for the C<:encoding> pragma: | |
4514 | in that case pretty much any characters can be read. | |
a0d0e21e LW |
4515 | |
4516 | =item readdir DIRHANDLE | |
d74e8afc | 4517 | X<readdir> |
a0d0e21e | 4518 | |
19799a22 | 4519 | Returns the next directory entry for a directory opened by C<opendir>. |
5a964f20 | 4520 | If used in list context, returns all the rest of the entries in the |
3b10bc60 | 4521 | directory. If there are no more entries, returns the undefined value in |
4522 | scalar context and the empty list in list context. | |
a0d0e21e | 4523 | |
19799a22 | 4524 | If you're planning to filetest the return values out of a C<readdir>, you'd |
5f05dabc | 4525 | better prepend the directory in question. Otherwise, because we didn't |
19799a22 | 4526 | C<chdir> there, it would have been testing the wrong file. |
cb1a09d0 | 4527 | |
b0169937 GS |
4528 | opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!"; |
4529 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); | |
4530 | closedir $dh; | |
cb1a09d0 | 4531 | |
114c60ec BG |
4532 | As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop, |
4533 | which will set C<$_> on every iteration. | |
4534 | ||
4535 | opendir(my $dh, $some_dir) || die; | |
4536 | while(readdir $dh) { | |
4537 | print "$some_dir/$_\n"; | |
4538 | } | |
4539 | closedir $dh; | |
4540 | ||
84902520 | 4541 | =item readline EXPR |
e4b7ebf3 RGS |
4542 | |
4543 | =item readline | |
d74e8afc | 4544 | X<readline> X<gets> X<fgets> |
84902520 | 4545 | |
e4b7ebf3 RGS |
4546 | Reads from the filehandle whose typeglob is contained in EXPR (or from |
4547 | *ARGV if EXPR is not provided). In scalar context, each call reads and | |
80d38338 | 4548 | returns the next line until end-of-file is reached, whereupon the |
0f03d336 | 4549 | subsequent call returns C<undef>. In list context, reads until end-of-file |
e4b7ebf3 | 4550 | is reached and returns a list of lines. Note that the notion of "line" |
80d38338 | 4551 | used here is whatever you may have defined with C<$/> or |
e4b7ebf3 | 4552 | C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. |
fbad3eb5 | 4553 | |
0f03d336 | 4554 | When C<$/> is set to C<undef>, when C<readline> is in scalar |
80d38338 | 4555 | context (i.e., file slurp mode), and when an empty file is read, it |
449bc448 | 4556 | returns C<''> the first time, followed by C<undef> subsequently. |
fbad3eb5 | 4557 | |
61eff3bc JH |
4558 | This is the internal function implementing the C<< <EXPR> >> |
4559 | operator, but you can use it directly. The C<< <EXPR> >> | |
84902520 TB |
4560 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
4561 | ||
5a964f20 | 4562 | $line = <STDIN>; |
5ed4f2ec | 4563 | $line = readline(*STDIN); # same thing |
5a964f20 | 4564 | |
0f03d336 | 4565 | If C<readline> encounters an operating system error, C<$!> will be set |
4566 | with the corresponding error message. It can be helpful to check | |
4567 | C<$!> when you are reading from filehandles you don't trust, such as a | |
4568 | tty or a socket. The following example uses the operator form of | |
4569 | C<readline> and dies if the result is not defined. | |
4570 | ||
5ed4f2ec | 4571 | while ( ! eof($fh) ) { |
4572 | defined( $_ = <$fh> ) or die "readline failed: $!"; | |
4573 | ... | |
4574 | } | |
0f03d336 | 4575 | |
4576 | Note that you have can't handle C<readline> errors that way with the | |
4577 | C<ARGV> filehandle. In that case, you have to open each element of | |
4578 | C<@ARGV> yourself since C<eof> handles C<ARGV> differently. | |
4579 | ||
4580 | foreach my $arg (@ARGV) { | |
4581 | open(my $fh, $arg) or warn "Can't open $arg: $!"; | |
4582 | ||
4583 | while ( ! eof($fh) ) { | |
4584 | defined( $_ = <$fh> ) | |
4585 | or die "readline failed for $arg: $!"; | |
4586 | ... | |
00cb5da1 | 4587 | } |
00cb5da1 | 4588 | } |
e00e4ce9 | 4589 | |
a0d0e21e | 4590 | =item readlink EXPR |
d74e8afc | 4591 | X<readlink> |
a0d0e21e | 4592 | |
54310121 | 4593 | =item readlink |
bbce6d69 | 4594 | |
a0d0e21e | 4595 | Returns the value of a symbolic link, if symbolic links are |
3b10bc60 | 4596 | implemented. If not, raises an exception. If there is a system |
184e9718 | 4597 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
7660c0ab | 4598 | omitted, uses C<$_>. |
a0d0e21e | 4599 | |
84902520 | 4600 | =item readpipe EXPR |
8d7403e6 RGS |
4601 | |
4602 | =item readpipe | |
d74e8afc | 4603 | X<readpipe> |
84902520 | 4604 | |
5a964f20 | 4605 | EXPR is executed as a system command. |
84902520 TB |
4606 | The collected standard output of the command is returned. |
4607 | In scalar context, it comes back as a single (potentially | |
4608 | multi-line) string. In list context, returns a list of lines | |
7660c0ab | 4609 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
4610 | This is the internal function implementing the C<qx/EXPR/> |
4611 | operator, but you can use it directly. The C<qx/EXPR/> | |
4612 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
8d7403e6 | 4613 | If EXPR is omitted, uses C<$_>. |
84902520 | 4614 | |
399388f4 | 4615 | =item recv SOCKET,SCALAR,LENGTH,FLAGS |
d74e8afc | 4616 | X<recv> |
a0d0e21e | 4617 | |
9124316e JH |
4618 | Receives a message on a socket. Attempts to receive LENGTH characters |
4619 | of data into variable SCALAR from the specified SOCKET filehandle. | |
4620 | SCALAR will be grown or shrunk to the length actually read. Takes the | |
4621 | same flags as the system call of the same name. Returns the address | |
4622 | of the sender if SOCKET's protocol supports this; returns an empty | |
4623 | string otherwise. If there's an error, returns the undefined value. | |
4624 | This call is actually implemented in terms of recvfrom(2) system call. | |
4625 | See L<perlipc/"UDP: Message Passing"> for examples. | |
4626 | ||
4627 | Note the I<characters>: depending on the status of the socket, either | |
4628 | (8-bit) bytes or characters are received. By default all sockets | |
4629 | operate on bytes, but for example if the socket has been changed using | |
740d4bb2 JW |
4630 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see the |
4631 | C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode | |
4632 | characters, not bytes. Similarly for the C<:encoding> pragma: in that | |
4633 | case pretty much any characters can be read. | |
a0d0e21e LW |
4634 | |
4635 | =item redo LABEL | |
d74e8afc | 4636 | X<redo> |
a0d0e21e LW |
4637 | |
4638 | =item redo | |
4639 | ||
4640 | The C<redo> command restarts the loop block without evaluating the | |
98293880 | 4641 | conditional again. The C<continue> block, if any, is not executed. If |
a0d0e21e | 4642 | the LABEL is omitted, the command refers to the innermost enclosing |
cf264981 SP |
4643 | loop. Programs that want to lie to themselves about what was just input |
4644 | normally use this command: | |
a0d0e21e LW |
4645 | |
4646 | # a simpleminded Pascal comment stripper | |
4647 | # (warning: assumes no { or } in strings) | |
4633a7c4 | 4648 | LINE: while (<STDIN>) { |
a9a5a0dc VP |
4649 | while (s|({.*}.*){.*}|$1 |) {} |
4650 | s|{.*}| |; | |
4651 | if (s|{.*| |) { | |
4652 | $front = $_; | |
4653 | while (<STDIN>) { | |
4654 | if (/}/) { # end of comment? | |
4655 | s|^|$front\{|; | |
4656 | redo LINE; | |
4657 | } | |
4658 | } | |
5ed4f2ec | 4659 | } |
a9a5a0dc | 4660 | print; |
a0d0e21e LW |
4661 | } |
4662 | ||
80d38338 | 4663 | C<redo> cannot be used to retry a block that returns a value such as |
2b5ab1e7 TC |
4664 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
4665 | a grep() or map() operation. | |
4968c1e4 | 4666 | |
6c1372ed GS |
4667 | Note that a block by itself is semantically identical to a loop |
4668 | that executes once. Thus C<redo> inside such a block will effectively | |
4669 | turn it into a looping construct. | |
4670 | ||
98293880 | 4671 | See also L</continue> for an illustration of how C<last>, C<next>, and |
1d2dff63 GS |
4672 | C<redo> work. |
4673 | ||
a0d0e21e | 4674 | =item ref EXPR |
d74e8afc | 4675 | X<ref> X<reference> |
a0d0e21e | 4676 | |
54310121 | 4677 | =item ref |
bbce6d69 | 4678 | |
8a2e0804 A |
4679 | Returns a non-empty string if EXPR is a reference, the empty |
4680 | string otherwise. If EXPR | |
7660c0ab | 4681 | is not specified, C<$_> will be used. The value returned depends on the |
bbce6d69 | 4682 | type of thing the reference is a reference to. |
a0d0e21e LW |
4683 | Builtin types include: |
4684 | ||
a0d0e21e LW |
4685 | SCALAR |
4686 | ARRAY | |
4687 | HASH | |
4688 | CODE | |
19799a22 | 4689 | REF |
a0d0e21e | 4690 | GLOB |
19799a22 | 4691 | LVALUE |
cc10766d RGS |
4692 | FORMAT |
4693 | IO | |
4694 | VSTRING | |
4695 | Regexp | |
a0d0e21e | 4696 | |
54310121 | 4697 | If the referenced object has been blessed into a package, then that package |
19799a22 | 4698 | name is returned instead. You can think of C<ref> as a C<typeof> operator. |
a0d0e21e LW |
4699 | |
4700 | if (ref($r) eq "HASH") { | |
a9a5a0dc | 4701 | print "r is a reference to a hash.\n"; |
54310121 | 4702 | } |
2b5ab1e7 | 4703 | unless (ref($r)) { |
a9a5a0dc | 4704 | print "r is not a reference at all.\n"; |
54310121 | 4705 | } |
a0d0e21e | 4706 | |
85dd5c8b WL |
4707 | The return value C<LVALUE> indicates a reference to an lvalue that is not |
4708 | a variable. You get this from taking the reference of function calls like | |
4709 | C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points | |
603c58be | 4710 | to a L<version string|perldata/"Version Strings">. |
85dd5c8b WL |
4711 | |
4712 | The result C<Regexp> indicates that the argument is a regular expression | |
4713 | resulting from C<qr//>. | |
4714 | ||
a0d0e21e LW |
4715 | See also L<perlref>. |
4716 | ||
4717 | =item rename OLDNAME,NEWNAME | |
d74e8afc | 4718 | X<rename> X<move> X<mv> X<ren> |
a0d0e21e | 4719 | |
19799a22 GS |
4720 | Changes the name of a file; an existing file NEWNAME will be |
4721 | clobbered. Returns true for success, false otherwise. | |
4722 | ||
2b5ab1e7 TC |
4723 | Behavior of this function varies wildly depending on your system |
4724 | implementation. For example, it will usually not work across file system | |
4725 | boundaries, even though the system I<mv> command sometimes compensates | |
4726 | for this. Other restrictions include whether it works on directories, | |
4727 | open files, or pre-existing files. Check L<perlport> and either the | |
4728 | rename(2) manpage or equivalent system documentation for details. | |
a0d0e21e | 4729 | |
dd184578 RGS |
4730 | For a platform independent C<move> function look at the L<File::Copy> |
4731 | module. | |
4732 | ||
16070b82 | 4733 | =item require VERSION |
d74e8afc | 4734 | X<require> |
16070b82 | 4735 | |
a0d0e21e LW |
4736 | =item require EXPR |
4737 | ||
4738 | =item require | |
4739 | ||
3b825e41 RK |
4740 | Demands a version of Perl specified by VERSION, or demands some semantics |
4741 | specified by EXPR or by C<$_> if EXPR is not supplied. | |
44dcb63b | 4742 | |
3b825e41 RK |
4743 | VERSION may be either a numeric argument such as 5.006, which will be |
4744 | compared to C<$]>, or a literal of the form v5.6.1, which will be compared | |
3b10bc60 | 4745 | to C<$^V> (aka $PERL_VERSION). An exception is raised if |
3b825e41 RK |
4746 | VERSION is greater than the version of the current Perl interpreter. |
4747 | Compare with L</use>, which can do a similar check at compile time. | |
4748 | ||
4749 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
4750 | avoided, because it leads to misleading error messages under earlier | |
cf264981 | 4751 | versions of Perl that do not support this syntax. The equivalent numeric |
3b825e41 | 4752 | version should be used instead. |
44dcb63b | 4753 | |
5ed4f2ec | 4754 | require v5.6.1; # run time version check |
4755 | require 5.6.1; # ditto | |
4756 | require 5.006_001; # ditto; preferred for backwards compatibility | |
a0d0e21e | 4757 | |
362eead3 RGS |
4758 | Otherwise, C<require> demands that a library file be included if it |
4759 | hasn't already been included. The file is included via the do-FILE | |
73c71df6 CW |
4760 | mechanism, which is essentially just a variety of C<eval> with the |
4761 | caveat that lexical variables in the invoking script will be invisible | |
4762 | to the included code. Has semantics similar to the following subroutine: | |
a0d0e21e LW |
4763 | |
4764 | sub require { | |
20907158 AMS |
4765 | my ($filename) = @_; |
4766 | if (exists $INC{$filename}) { | |
4767 | return 1 if $INC{$filename}; | |
4768 | die "Compilation failed in require"; | |
4769 | } | |
4770 | my ($realfilename,$result); | |
4771 | ITER: { | |
4772 | foreach $prefix (@INC) { | |
4773 | $realfilename = "$prefix/$filename"; | |
4774 | if (-f $realfilename) { | |
4775 | $INC{$filename} = $realfilename; | |
4776 | $result = do $realfilename; | |
4777 | last ITER; | |
4778 | } | |
4779 | } | |
4780 | die "Can't find $filename in \@INC"; | |
4781 | } | |
4782 | if ($@) { | |
4783 | $INC{$filename} = undef; | |
4784 | die $@; | |
4785 | } elsif (!$result) { | |
4786 | delete $INC{$filename}; | |
4787 | die "$filename did not return true value"; | |
4788 | } else { | |
4789 | return $result; | |
4790 | } | |
a0d0e21e LW |
4791 | } |
4792 | ||
4793 | Note that the file will not be included twice under the same specified | |
a12755f0 SB |
4794 | name. |
4795 | ||
4796 | The file must return true as the last statement to indicate | |
a0d0e21e | 4797 | successful execution of any initialization code, so it's customary to |
19799a22 GS |
4798 | end such a file with C<1;> unless you're sure it'll return true |
4799 | otherwise. But it's better just to put the C<1;>, in case you add more | |
a0d0e21e LW |
4800 | statements. |
4801 | ||
54310121 | 4802 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
da0045b7 | 4803 | replaces "F<::>" with "F</>" in the filename for you, |
54310121 | 4804 | to make it easy to load standard modules. This form of loading of |
a0d0e21e LW |
4805 | modules does not risk altering your namespace. |
4806 | ||
ee580363 GS |
4807 | In other words, if you try this: |
4808 | ||
5ed4f2ec | 4809 | require Foo::Bar; # a splendid bareword |
ee580363 | 4810 | |
b76cc8ba | 4811 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
7660c0ab | 4812 | directories specified in the C<@INC> array. |
ee580363 | 4813 | |
5a964f20 | 4814 | But if you try this: |
ee580363 GS |
4815 | |
4816 | $class = 'Foo::Bar'; | |
5ed4f2ec | 4817 | require $class; # $class is not a bareword |
5a964f20 | 4818 | #or |
5ed4f2ec | 4819 | require "Foo::Bar"; # not a bareword because of the "" |
ee580363 | 4820 | |
b76cc8ba | 4821 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
19799a22 | 4822 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
ee580363 GS |
4823 | |
4824 | eval "require $class"; | |
4825 | ||
3b10bc60 | 4826 | Now that you understand how C<require> looks for files with a |
a91233bf RGS |
4827 | bareword argument, there is a little extra functionality going on behind |
4828 | the scenes. Before C<require> looks for a "F<.pm>" extension, it will | |
4829 | first look for a similar filename with a "F<.pmc>" extension. If this file | |
4830 | is found, it will be loaded in place of any file ending in a "F<.pm>" | |
4831 | extension. | |
662cc546 | 4832 | |
1c3d5054 CBW |
4833 | You can also insert hooks into the import facility, by putting Perl code |
4834 | directly into the @INC array. There are three forms of hooks: subroutine | |
d54b56d5 RGS |
4835 | references, array references and blessed objects. |
4836 | ||
4837 | Subroutine references are the simplest case. When the inclusion system | |
4838 | walks through @INC and encounters a subroutine, this subroutine gets | |
3b10bc60 | 4839 | called with two parameters, the first a reference to itself, and the |
4840 | second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The | |
4841 | subroutine should return either nothing or else a list of up to three | |
4842 | values in the following order: | |
1f0bdf18 NC |
4843 | |
4844 | =over | |
4845 | ||
4846 | =item 1 | |
4847 | ||
1f0bdf18 NC |
4848 | A filehandle, from which the file will be read. |
4849 | ||
cec0e1a7 | 4850 | =item 2 |
1f0bdf18 | 4851 | |
60d352b3 RGS |
4852 | A reference to a subroutine. If there is no filehandle (previous item), |
4853 | then this subroutine is expected to generate one line of source code per | |
4854 | call, writing the line into C<$_> and returning 1, then returning 0 at | |
3b10bc60 | 4855 | end of file. If there is a filehandle, then the subroutine will be |
b8921b3e | 4856 | called to act as a simple source filter, with the line as read in C<$_>. |
60d352b3 RGS |
4857 | Again, return 1 for each valid line, and 0 after all lines have been |
4858 | returned. | |
1f0bdf18 | 4859 | |
cec0e1a7 | 4860 | =item 3 |
1f0bdf18 NC |
4861 | |
4862 | Optional state for the subroutine. The state is passed in as C<$_[1]>. A | |
4863 | reference to the subroutine itself is passed in as C<$_[0]>. | |
4864 | ||
4865 | =back | |
4866 | ||
4867 | If an empty list, C<undef>, or nothing that matches the first 3 values above | |
3b10bc60 | 4868 | is returned, then C<require> looks at the remaining elements of @INC. |
4869 | Note that this filehandle must be a real filehandle (strictly a typeglob | |
4870 | or reference to a typeglob, blessed or unblessed); tied filehandles will be | |
1f0bdf18 | 4871 | ignored and return value processing will stop there. |
d54b56d5 RGS |
4872 | |
4873 | If the hook is an array reference, its first element must be a subroutine | |
4874 | reference. This subroutine is called as above, but the first parameter is | |
3b10bc60 | 4875 | the array reference. This lets you indirectly pass arguments to |
d54b56d5 RGS |
4876 | the subroutine. |
4877 | ||
4878 | In other words, you can write: | |
4879 | ||
4880 | push @INC, \&my_sub; | |
4881 | sub my_sub { | |
a9a5a0dc VP |
4882 | my ($coderef, $filename) = @_; # $coderef is \&my_sub |
4883 | ... | |
d54b56d5 RGS |
4884 | } |
4885 | ||
4886 | or: | |
4887 | ||
4888 | push @INC, [ \&my_sub, $x, $y, ... ]; | |
4889 | sub my_sub { | |
a9a5a0dc VP |
4890 | my ($arrayref, $filename) = @_; |
4891 | # Retrieve $x, $y, ... | |
4892 | my @parameters = @$arrayref[1..$#$arrayref]; | |
4893 | ... | |
d54b56d5 RGS |
4894 | } |
4895 | ||
cf264981 | 4896 | If the hook is an object, it must provide an INC method that will be |
d54b56d5 | 4897 | called as above, the first parameter being the object itself. (Note that |
92c6daad NC |
4898 | you must fully qualify the sub's name, as unqualified C<INC> is always forced |
4899 | into package C<main>.) Here is a typical code layout: | |
d54b56d5 RGS |
4900 | |
4901 | # In Foo.pm | |
4902 | package Foo; | |
4903 | sub new { ... } | |
4904 | sub Foo::INC { | |
a9a5a0dc VP |
4905 | my ($self, $filename) = @_; |
4906 | ... | |
d54b56d5 RGS |
4907 | } |
4908 | ||
4909 | # In the main program | |
797f796a | 4910 | push @INC, Foo->new(...); |
d54b56d5 | 4911 | |
3b10bc60 | 4912 | These hooks are also permitted to set the %INC entry |
9ae8cd5b RGS |
4913 | corresponding to the files they have loaded. See L<perlvar/%INC>. |
4914 | ||
ee580363 | 4915 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. |
a0d0e21e LW |
4916 | |
4917 | =item reset EXPR | |
d74e8afc | 4918 | X<reset> |
a0d0e21e LW |
4919 | |
4920 | =item reset | |
4921 | ||
4922 | Generally used in a C<continue> block at the end of a loop to clear | |
7660c0ab | 4923 | variables and reset C<??> searches so that they work again. The |
a0d0e21e LW |
4924 | expression is interpreted as a list of single characters (hyphens |
4925 | allowed for ranges). All variables and arrays beginning with one of | |
4926 | those letters are reset to their pristine state. If the expression is | |
3b10bc60 | 4927 | omitted, one-match searches (C<?pattern?>) are reset to match again. |
4928 | Only resets variables or searches in the current package. Always returns | |
4929 | 1. Examples: | |
a0d0e21e | 4930 | |
5ed4f2ec | 4931 | reset 'X'; # reset all X variables |
4932 | reset 'a-z'; # reset lower case variables | |
4933 | reset; # just reset ?one-time? searches | |
a0d0e21e | 4934 | |
7660c0ab | 4935 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
2b5ab1e7 | 4936 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package |
3b10bc60 | 4937 | variables; lexical variables are unaffected, but they clean themselves |
2b5ab1e7 TC |
4938 | up on scope exit anyway, so you'll probably want to use them instead. |
4939 | See L</my>. | |
a0d0e21e | 4940 | |
54310121 | 4941 | =item return EXPR |
d74e8afc | 4942 | X<return> |
54310121 | 4943 | |
4944 | =item return | |
4945 | ||
b76cc8ba | 4946 | Returns from a subroutine, C<eval>, or C<do FILE> with the value |
5a964f20 | 4947 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
54310121 | 4948 | context, depending on how the return value will be used, and the context |
19799a22 | 4949 | may vary from one execution to the next (see C<wantarray>). If no EXPR |
2b5ab1e7 | 4950 | is given, returns an empty list in list context, the undefined value in |
3b10bc60 | 4951 | scalar context, and (of course) nothing at all in void context. |
a0d0e21e | 4952 | |
3b10bc60 | 4953 | (In the absence of an explicit C<return>, a subroutine, eval, |
4954 | or do FILE automatically returns the value of the last expression | |
2b5ab1e7 | 4955 | evaluated.) |
a0d0e21e LW |
4956 | |
4957 | =item reverse LIST | |
d74e8afc | 4958 | X<reverse> X<rev> X<invert> |
a0d0e21e | 4959 | |
5a964f20 TC |
4960 | In list context, returns a list value consisting of the elements |
4961 | of LIST in the opposite order. In scalar context, concatenates the | |
2b5ab1e7 | 4962 | elements of LIST and returns a string value with all characters |
a0ed51b3 | 4963 | in the opposite order. |
4633a7c4 | 4964 | |
9649ed94 | 4965 | print join(", ", reverse "world", "Hello"); # Hello, world |
4633a7c4 | 4966 | |
9649ed94 | 4967 | print scalar reverse "dlrow ,", "olleH"; # Hello, world |
2f9daede | 4968 | |
2d713cbd RGS |
4969 | Used without arguments in scalar context, reverse() reverses C<$_>. |
4970 | ||
9649ed94 B |
4971 | $_ = "dlrow ,olleH"; |
4972 | print reverse; # No output, list context | |
4973 | print scalar reverse; # Hello, world | |
4974 | ||
437d4214 | 4975 | Note that reversing an array to itself (as in C<@a = reverse @a>) will |
80d38338 | 4976 | preserve non-existent elements whenever possible, i.e., for non magical |
437d4214 VP |
4977 | arrays or tied arrays with C<EXISTS> and C<DELETE> methods. |
4978 | ||
2f9daede TP |
4979 | This operator is also handy for inverting a hash, although there are some |
4980 | caveats. If a value is duplicated in the original hash, only one of those | |
4981 | can be represented as a key in the inverted hash. Also, this has to | |
4982 | unwind one hash and build a whole new one, which may take some time | |
2b5ab1e7 | 4983 | on a large hash, such as from a DBM file. |
2f9daede | 4984 | |
5ed4f2ec | 4985 | %by_name = reverse %by_address; # Invert the hash |
a0d0e21e LW |
4986 | |
4987 | =item rewinddir DIRHANDLE | |
d74e8afc | 4988 | X<rewinddir> |
a0d0e21e LW |
4989 | |
4990 | Sets the current position to the beginning of the directory for the | |
19799a22 | 4991 | C<readdir> routine on DIRHANDLE. |
a0d0e21e LW |
4992 | |
4993 | =item rindex STR,SUBSTR,POSITION | |
d74e8afc | 4994 | X<rindex> |
a0d0e21e LW |
4995 | |
4996 | =item rindex STR,SUBSTR | |
4997 | ||
ff551661 | 4998 | Works just like index() except that it returns the position of the I<last> |
a0d0e21e | 4999 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
ff551661 | 5000 | last occurrence beginning at or before that position. |
a0d0e21e LW |
5001 | |
5002 | =item rmdir FILENAME | |
d74e8afc | 5003 | X<rmdir> X<rd> X<directory, remove> |
a0d0e21e | 5004 | |
54310121 | 5005 | =item rmdir |
bbce6d69 | 5006 | |
974da8e5 JH |
5007 | Deletes the directory specified by FILENAME if that directory is |
5008 | empty. If it succeeds it returns true, otherwise it returns false and | |
5009 | sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. | |
a0d0e21e | 5010 | |
e1020413 | 5011 | To remove a directory tree recursively (C<rm -rf> on Unix) look at |
dd184578 RGS |
5012 | the C<rmtree> function of the L<File::Path> module. |
5013 | ||
a0d0e21e LW |
5014 | =item s/// |
5015 | ||
9f4b9cd0 | 5016 | The substitution operator. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e | 5017 | |
0d863452 RH |
5018 | =item say FILEHANDLE LIST |
5019 | X<say> | |
5020 | ||
5021 | =item say LIST | |
5022 | ||
5023 | =item say | |
5024 | ||
5025 | Just like C<print>, but implicitly appends a newline. | |
187a5aa6 | 5026 | C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print |
cfc4a7da | 5027 | LIST }>. |
f406c1e8 | 5028 | |
3b10bc60 | 5029 | This keyword is available only when the "say" feature is |
0d863452 RH |
5030 | enabled: see L<feature>. |
5031 | ||
a0d0e21e | 5032 | =item scalar EXPR |
d74e8afc | 5033 | X<scalar> X<context> |
a0d0e21e | 5034 | |
5a964f20 | 5035 | Forces EXPR to be interpreted in scalar context and returns the value |
54310121 | 5036 | of EXPR. |
cb1a09d0 AD |
5037 | |
5038 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
5039 | ||
54310121 | 5040 | There is no equivalent operator to force an expression to |
2b5ab1e7 | 5041 | be interpolated in list context because in practice, this is never |
cb1a09d0 AD |
5042 | needed. If you really wanted to do so, however, you could use |
5043 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
5044 | C<(some expression)> suffices. | |
a0d0e21e | 5045 | |
3b10bc60 | 5046 | Because C<scalar> is a unary operator, if you accidentally use for EXPR a |
2b5ab1e7 TC |
5047 | parenthesized list, this behaves as a scalar comma expression, evaluating |
5048 | all but the last element in void context and returning the final element | |
5049 | evaluated in scalar context. This is seldom what you want. | |
62c18ce2 GS |
5050 | |
5051 | The following single statement: | |
5052 | ||
5ed4f2ec | 5053 | print uc(scalar(&foo,$bar)),$baz; |
62c18ce2 GS |
5054 | |
5055 | is the moral equivalent of these two: | |
5056 | ||
5ed4f2ec | 5057 | &foo; |
5058 | print(uc($bar),$baz); | |
62c18ce2 GS |
5059 | |
5060 | See L<perlop> for more details on unary operators and the comma operator. | |
5061 | ||
a0d0e21e | 5062 | =item seek FILEHANDLE,POSITION,WHENCE |
d74e8afc | 5063 | X<seek> X<fseek> X<filehandle, position> |
a0d0e21e | 5064 | |
19799a22 | 5065 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. |
8903cb82 | 5066 | FILEHANDLE may be an expression whose value gives the name of the |
9124316e JH |
5067 | filehandle. The values for WHENCE are C<0> to set the new position |
5068 | I<in bytes> to POSITION, C<1> to set it to the current position plus | |
5069 | POSITION, and C<2> to set it to EOF plus POSITION (typically | |
5070 | negative). For WHENCE you may use the constants C<SEEK_SET>, | |
5071 | C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end | |
80d38338 | 5072 | of the file) from the Fcntl module. Returns C<1> on success, C<0> |
9124316e JH |
5073 | otherwise. |
5074 | ||
5075 | Note the I<in bytes>: even if the filehandle has been set to | |
740d4bb2 | 5076 | operate on characters (for example by using the C<:encoding(utf8)> open |
fae2c0fb | 5077 | layer), tell() will return byte offsets, not character offsets |
9124316e | 5078 | (because implementing that would render seek() and tell() rather slow). |
8903cb82 | 5079 | |
3b10bc60 | 5080 | If you want to position the file for C<sysread> or C<syswrite>, don't use |
5081 | C<seek>, because buffering makes its effect on the file's read-write position | |
19799a22 | 5082 | unpredictable and non-portable. Use C<sysseek> instead. |
a0d0e21e | 5083 | |
2b5ab1e7 TC |
5084 | Due to the rules and rigors of ANSI C, on some systems you have to do a |
5085 | seek whenever you switch between reading and writing. Amongst other | |
5086 | things, this may have the effect of calling stdio's clearerr(3). | |
5087 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
cb1a09d0 AD |
5088 | |
5089 | seek(TEST,0,1); | |
5090 | ||
5091 | This is also useful for applications emulating C<tail -f>. Once you hit | |
3b10bc60 | 5092 | EOF on your read and then sleep for a while, you (probably) have to stick in a |
5093 | dummy seek() to reset things. The C<seek> doesn't change the position, | |
8903cb82 | 5094 | but it I<does> clear the end-of-file condition on the handle, so that the |
3b10bc60 | 5095 | next C<< <FILE> >> makes Perl try again to read something. (We hope.) |
cb1a09d0 | 5096 | |
3b10bc60 | 5097 | If that doesn't work (some I/O implementations are particularly |
5098 | cantankerous), you might need something like this: | |
cb1a09d0 AD |
5099 | |
5100 | for (;;) { | |
a9a5a0dc | 5101 | for ($curpos = tell(FILE); $_ = <FILE>; |
f86cebdf | 5102 | $curpos = tell(FILE)) { |
a9a5a0dc VP |
5103 | # search for some stuff and put it into files |
5104 | } | |
5105 | sleep($for_a_while); | |
5106 | seek(FILE, $curpos, 0); | |
cb1a09d0 AD |
5107 | } |
5108 | ||
a0d0e21e | 5109 | =item seekdir DIRHANDLE,POS |
d74e8afc | 5110 | X<seekdir> |
a0d0e21e | 5111 | |
19799a22 | 5112 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS |
cf264981 SP |
5113 | must be a value returned by C<telldir>. C<seekdir> also has the same caveats |
5114 | about possible directory compaction as the corresponding system library | |
a0d0e21e LW |
5115 | routine. |
5116 | ||
5117 | =item select FILEHANDLE | |
d74e8afc | 5118 | X<select> X<filehandle, default> |
a0d0e21e LW |
5119 | |
5120 | =item select | |
5121 | ||
b5dffda6 RGS |
5122 | Returns the currently selected filehandle. If FILEHANDLE is supplied, |
5123 | sets the new current default filehandle for output. This has two | |
19799a22 | 5124 | effects: first, a C<write> or a C<print> without a filehandle will |
a0d0e21e LW |
5125 | default to this FILEHANDLE. Second, references to variables related to |
5126 | output will refer to this output channel. For example, if you have to | |
5127 | set the top of form format for more than one output channel, you might | |
5128 | do the following: | |
5129 | ||
5130 | select(REPORT1); | |
5131 | $^ = 'report1_top'; | |
5132 | select(REPORT2); | |
5133 | $^ = 'report2_top'; | |
5134 | ||
5135 | FILEHANDLE may be an expression whose value gives the name of the | |
5136 | actual filehandle. Thus: | |
5137 | ||
5138 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
5139 | ||
4633a7c4 LW |
5140 | Some programmers may prefer to think of filehandles as objects with |
5141 | methods, preferring to write the last example as: | |
a0d0e21e | 5142 | |
28757baa | 5143 | use IO::Handle; |
a0d0e21e LW |
5144 | STDERR->autoflush(1); |
5145 | ||
5146 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
d74e8afc | 5147 | X<select> |
a0d0e21e | 5148 | |
3b10bc60 | 5149 | This calls the select(2) syscall with the bit masks specified, which |
19799a22 | 5150 | can be constructed using C<fileno> and C<vec>, along these lines: |
a0d0e21e LW |
5151 | |
5152 | $rin = $win = $ein = ''; | |
5153 | vec($rin,fileno(STDIN),1) = 1; | |
5154 | vec($win,fileno(STDOUT),1) = 1; | |
5155 | $ein = $rin | $win; | |
5156 | ||
3b10bc60 | 5157 | If you want to select on many filehandles, you may wish to write a |
5158 | subroutine like this: | |
a0d0e21e LW |
5159 | |
5160 | sub fhbits { | |
a9a5a0dc VP |
5161 | my(@fhlist) = split(' ',$_[0]); |
5162 | my($bits); | |
5163 | for (@fhlist) { | |
5164 | vec($bits,fileno($_),1) = 1; | |
5165 | } | |
5166 | $bits; | |
a0d0e21e | 5167 | } |
4633a7c4 | 5168 | $rin = fhbits('STDIN TTY SOCK'); |
a0d0e21e LW |
5169 | |
5170 | The usual idiom is: | |
5171 | ||
5172 | ($nfound,$timeleft) = | |
5173 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
5174 | ||
54310121 | 5175 | or to block until something becomes ready just do this |
a0d0e21e LW |
5176 | |
5177 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
5178 | ||
19799a22 GS |
5179 | Most systems do not bother to return anything useful in $timeleft, so |
5180 | calling select() in scalar context just returns $nfound. | |
c07a80fd | 5181 | |
5f05dabc | 5182 | Any of the bit masks can also be undef. The timeout, if specified, is |
a0d0e21e | 5183 | in seconds, which may be fractional. Note: not all implementations are |
be119125 | 5184 | capable of returning the $timeleft. If not, they always return |
19799a22 | 5185 | $timeleft equal to the supplied $timeout. |
a0d0e21e | 5186 | |
ff68c719 | 5187 | You can effect a sleep of 250 milliseconds this way: |
a0d0e21e LW |
5188 | |
5189 | select(undef, undef, undef, 0.25); | |
5190 | ||
b09fc1d8 | 5191 | Note that whether C<select> gets restarted after signals (say, SIGALRM) |
8b0ac1d7 MHM |
5192 | is implementation-dependent. See also L<perlport> for notes on the |
5193 | portability of C<select>. | |
40454f26 | 5194 | |
3b10bc60 | 5195 | On error, C<select> behaves like select(2): it returns |
4189264e | 5196 | -1 and sets C<$!>. |
353e5636 | 5197 | |
3b10bc60 | 5198 | On some Unixes, select(2) may report a socket file |
5199 | descriptor as "ready for reading" when no data is available, and | |
5200 | thus a subsequent read blocks. This can be avoided if you always use | |
5201 | O_NONBLOCK on the socket. See select(2) and fcntl(2) for further | |
ec8ce15a HPM |
5202 | details. |
5203 | ||
19799a22 | 5204 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> |
61eff3bc | 5205 | or <FH>) with C<select>, except as permitted by POSIX, and even |
19799a22 | 5206 | then only on POSIX systems. You have to use C<sysread> instead. |
a0d0e21e LW |
5207 | |
5208 | =item semctl ID,SEMNUM,CMD,ARG | |
d74e8afc | 5209 | X<semctl> |
a0d0e21e | 5210 | |
3b10bc60 | 5211 | Calls the System V IPC function semctl(2). You'll probably have to say |
0ade1984 JH |
5212 | |
5213 | use IPC::SysV; | |
5214 | ||
5215 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
cf264981 | 5216 | GETALL, then ARG must be a variable that will hold the returned |
e4038a1f MS |
5217 | semid_ds structure or semaphore value array. Returns like C<ioctl>: |
5218 | the undefined value for error, "C<0 but true>" for zero, or the actual | |
5219 | return value otherwise. The ARG must consist of a vector of native | |
106325ad | 5220 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. |
4755096e GS |
5221 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> |
5222 | documentation. | |
a0d0e21e LW |
5223 | |
5224 | =item semget KEY,NSEMS,FLAGS | |
d74e8afc | 5225 | X<semget> |
a0d0e21e | 5226 | |
3b10bc60 | 5227 | Calls the System V IPC function semget(2). Returns the semaphore id, or |
4755096e GS |
5228 | the undefined value if there is an error. See also |
5229 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> | |
5230 | documentation. | |
a0d0e21e LW |
5231 | |
5232 | =item semop KEY,OPSTRING | |
d74e8afc | 5233 | X<semop> |
a0d0e21e | 5234 | |
80d38338 | 5235 | Calls the System V IPC function semop(2) for semaphore operations |
5354997a | 5236 | such as signalling and waiting. OPSTRING must be a packed array of |
a0d0e21e | 5237 | semop structures. Each semop structure can be generated with |
cf264981 SP |
5238 | C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING |
5239 | implies the number of semaphore operations. Returns true if | |
19799a22 GS |
5240 | successful, or false if there is an error. As an example, the |
5241 | following code waits on semaphore $semnum of semaphore id $semid: | |
a0d0e21e | 5242 | |
f878ba33 | 5243 | $semop = pack("s!3", $semnum, -1, 0); |
a0d0e21e LW |
5244 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); |
5245 | ||
4755096e GS |
5246 | To signal the semaphore, replace C<-1> with C<1>. See also |
5247 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> | |
5248 | documentation. | |
a0d0e21e LW |
5249 | |
5250 | =item send SOCKET,MSG,FLAGS,TO | |
d74e8afc | 5251 | X<send> |
a0d0e21e LW |
5252 | |
5253 | =item send SOCKET,MSG,FLAGS | |
5254 | ||
3b10bc60 | 5255 | Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET |
5256 | filehandle. Takes the same flags as the system call of the same name. On | |
5257 | unconnected sockets, you must specify a destination to I<send to>, in which | |
5258 | case it does a sendto(2) syscall. Returns the number of characters sent, | |
5259 | or the undefined value on error. The sendmsg(2) syscall is currently | |
5260 | unimplemented. See L<perlipc/"UDP: Message Passing"> for examples. | |
9124316e JH |
5261 | |
5262 | Note the I<characters>: depending on the status of the socket, either | |
5263 | (8-bit) bytes or characters are sent. By default all sockets operate | |
5264 | on bytes, but for example if the socket has been changed using | |
740d4bb2 JW |
5265 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see |
5266 | L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8 | |
5267 | encoded Unicode characters, not bytes. Similarly for the C<:encoding> | |
5268 | pragma: in that case pretty much any characters can be sent. | |
a0d0e21e LW |
5269 | |
5270 | =item setpgrp PID,PGRP | |
d74e8afc | 5271 | X<setpgrp> X<group> |
a0d0e21e | 5272 | |
7660c0ab | 5273 | Sets the current process group for the specified PID, C<0> for the current |
3b10bc60 | 5274 | process. Raises an exception when used on a machine that doesn't |
81777298 GS |
5275 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, |
5276 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
5277 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
5278 | C<POSIX::setsid()>. | |
a0d0e21e LW |
5279 | |
5280 | =item setpriority WHICH,WHO,PRIORITY | |
d74e8afc | 5281 | X<setpriority> X<priority> X<nice> X<renice> |
a0d0e21e LW |
5282 | |
5283 | Sets the current priority for a process, a process group, or a user. | |
3b10bc60 | 5284 | (See setpriority(2).) Raises an exception when used on a machine |
f86cebdf | 5285 | that doesn't implement setpriority(2). |
a0d0e21e LW |
5286 | |
5287 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
d74e8afc | 5288 | X<setsockopt> |
a0d0e21e LW |
5289 | |
5290 | Sets the socket option requested. Returns undefined if there is an | |
23d0437f GA |
5291 | error. Use integer constants provided by the C<Socket> module for |
5292 | LEVEL and OPNAME. Values for LEVEL can also be obtained from | |
5293 | getprotobyname. OPTVAL might either be a packed string or an integer. | |
5294 | An integer OPTVAL is shorthand for pack("i", OPTVAL). | |
5295 | ||
3b10bc60 | 5296 | An example disabling Nagle's algorithm on a socket: |
23d0437f GA |
5297 | |
5298 | use Socket qw(IPPROTO_TCP TCP_NODELAY); | |
5299 | setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); | |
a0d0e21e LW |
5300 | |
5301 | =item shift ARRAY | |
d74e8afc | 5302 | X<shift> |
a0d0e21e LW |
5303 | |
5304 | =item shift | |
5305 | ||
5306 | Shifts the first value of the array off and returns it, shortening the | |
5307 | array by 1 and moving everything down. If there are no elements in the | |
5308 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
7660c0ab | 5309 | C<@_> array within the lexical scope of subroutines and formats, and the |
80d38338 | 5310 | C<@ARGV> array outside a subroutine and also within the lexical scopes |
3c10abe3 AG |
5311 | established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, |
5312 | C<UNITCHECK {}> and C<END {}> constructs. | |
4f25aa18 | 5313 | |
a1b2c429 | 5314 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the |
19799a22 | 5315 | same thing to the left end of an array that C<pop> and C<push> do to the |
977336f5 | 5316 | right end. |
a0d0e21e LW |
5317 | |
5318 | =item shmctl ID,CMD,ARG | |
d74e8afc | 5319 | X<shmctl> |
a0d0e21e | 5320 | |
0ade1984 JH |
5321 | Calls the System V IPC function shmctl. You'll probably have to say |
5322 | ||
5323 | use IPC::SysV; | |
5324 | ||
7660c0ab | 5325 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
cf264981 | 5326 | then ARG must be a variable that will hold the returned C<shmid_ds> |
7660c0ab | 5327 | structure. Returns like ioctl: the undefined value for error, "C<0> but |
0ade1984 | 5328 | true" for zero, or the actual return value otherwise. |
4755096e | 5329 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e LW |
5330 | |
5331 | =item shmget KEY,SIZE,FLAGS | |
d74e8afc | 5332 | X<shmget> |
a0d0e21e LW |
5333 | |
5334 | Calls the System V IPC function shmget. Returns the shared memory | |
5335 | segment id, or the undefined value if there is an error. | |
4755096e | 5336 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e LW |
5337 | |
5338 | =item shmread ID,VAR,POS,SIZE | |
d74e8afc ITB |
5339 | X<shmread> |
5340 | X<shmwrite> | |
a0d0e21e LW |
5341 | |
5342 | =item shmwrite ID,STRING,POS,SIZE | |
5343 | ||
5344 | Reads or writes the System V shared memory segment ID starting at | |
5345 | position POS for size SIZE by attaching to it, copying in/out, and | |
5a964f20 | 5346 | detaching from it. When reading, VAR must be a variable that will |
a0d0e21e LW |
5347 | hold the data read. When writing, if STRING is too long, only SIZE |
5348 | bytes are used; if STRING is too short, nulls are written to fill out | |
19799a22 | 5349 | SIZE bytes. Return true if successful, or false if there is an error. |
4755096e GS |
5350 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, |
5351 | C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. | |
a0d0e21e LW |
5352 | |
5353 | =item shutdown SOCKET,HOW | |
d74e8afc | 5354 | X<shutdown> |
a0d0e21e LW |
5355 | |
5356 | Shuts down a socket connection in the manner indicated by HOW, which | |
3b10bc60 | 5357 | has the same interpretation as in the syscall of the same name. |
a0d0e21e | 5358 | |
f86cebdf GS |
5359 | shutdown(SOCKET, 0); # I/we have stopped reading data |
5360 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
5361 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
5a964f20 TC |
5362 | |
5363 | This is useful with sockets when you want to tell the other | |
5364 | side you're done writing but not done reading, or vice versa. | |
b76cc8ba | 5365 | It's also a more insistent form of close because it also |
19799a22 | 5366 | disables the file descriptor in any forked copies in other |
5a964f20 TC |
5367 | processes. |
5368 | ||
3b10bc60 | 5369 | Returns C<1> for success; on error, returns C<undef> if |
f126b98b PF |
5370 | the first argument is not a valid filehandle, or returns C<0> and sets |
5371 | C<$!> for any other failure. | |
5372 | ||
a0d0e21e | 5373 | =item sin EXPR |
d74e8afc | 5374 | X<sin> X<sine> X<asin> X<arcsine> |
a0d0e21e | 5375 | |
54310121 | 5376 | =item sin |
bbce6d69 | 5377 | |
a0d0e21e | 5378 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 5379 | returns sine of C<$_>. |
a0d0e21e | 5380 | |
ca6e1c26 | 5381 | For the inverse sine operation, you may use the C<Math::Trig::asin> |
28757baa | 5382 | function, or use this relation: |
5383 | ||
5384 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
5385 | ||
a0d0e21e | 5386 | =item sleep EXPR |
d74e8afc | 5387 | X<sleep> X<pause> |
a0d0e21e LW |
5388 | |
5389 | =item sleep | |
5390 | ||
80d38338 TC |
5391 | Causes the script to sleep for (integer) EXPR seconds, or forever if no |
5392 | argument is given. Returns the integer number of seconds actually slept. | |
b48653af | 5393 | |
7660c0ab | 5394 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
b48653af MS |
5395 | |
5396 | eval { | |
5397 | local $SIG{ALARM} = sub { die "Alarm!\n" }; | |
5398 | sleep; | |
5399 | }; | |
5400 | die $@ unless $@ eq "Alarm!\n"; | |
5401 | ||
5402 | You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep> | |
5403 | is often implemented using C<alarm>. | |
a0d0e21e LW |
5404 | |
5405 | On some older systems, it may sleep up to a full second less than what | |
5406 | you requested, depending on how it counts seconds. Most modern systems | |
5a964f20 TC |
5407 | always sleep the full amount. They may appear to sleep longer than that, |
5408 | however, because your process might not be scheduled right away in a | |
5409 | busy multitasking system. | |
a0d0e21e | 5410 | |
2bc69794 BS |
5411 | For delays of finer granularity than one second, the Time::HiRes module |
5412 | (from CPAN, and starting from Perl 5.8 part of the standard | |
5413 | distribution) provides usleep(). You may also use Perl's four-argument | |
5414 | version of select() leaving the first three arguments undefined, or you | |
5415 | might be able to use the C<syscall> interface to access setitimer(2) if | |
5416 | your system supports it. See L<perlfaq8> for details. | |
cb1a09d0 | 5417 | |
b6e2112e | 5418 | See also the POSIX module's C<pause> function. |
5f05dabc | 5419 | |
a0d0e21e | 5420 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
d74e8afc | 5421 | X<socket> |
a0d0e21e LW |
5422 | |
5423 | Opens a socket of the specified kind and attaches it to filehandle | |
19799a22 | 5424 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for |
3b10bc60 | 5425 | the syscall of the same name. You should C<use Socket> first |
19799a22 GS |
5426 | to get the proper definitions imported. See the examples in |
5427 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 5428 | |
8d2a6795 GS |
5429 | On systems that support a close-on-exec flag on files, the flag will |
5430 | be set for the newly opened file descriptor, as determined by the | |
5431 | value of $^F. See L<perlvar/$^F>. | |
5432 | ||
a0d0e21e | 5433 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
d74e8afc | 5434 | X<socketpair> |
a0d0e21e LW |
5435 | |
5436 | Creates an unnamed pair of sockets in the specified domain, of the | |
5f05dabc | 5437 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
3b10bc60 | 5438 | for the syscall of the same name. If unimplemented, raises an exception. |
5439 | Returns true if successful. | |
a0d0e21e | 5440 | |
8d2a6795 GS |
5441 | On systems that support a close-on-exec flag on files, the flag will |
5442 | be set for the newly opened file descriptors, as determined by the value | |
5443 | of $^F. See L<perlvar/$^F>. | |
5444 | ||
19799a22 | 5445 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call |
5a964f20 TC |
5446 | to C<pipe(Rdr, Wtr)> is essentially: |
5447 | ||
5448 | use Socket; | |
5449 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
5450 | shutdown(Rdr, 1); # no more writing for reader | |
5451 | shutdown(Wtr, 0); # no more reading for writer | |
5452 | ||
02fc2eee NC |
5453 | See L<perlipc> for an example of socketpair use. Perl 5.8 and later will |
5454 | emulate socketpair using IP sockets to localhost if your system implements | |
5455 | sockets but not socketpair. | |
5a964f20 | 5456 | |
a0d0e21e | 5457 | =item sort SUBNAME LIST |
d74e8afc | 5458 | X<sort> X<qsort> X<quicksort> X<mergesort> |
a0d0e21e LW |
5459 | |
5460 | =item sort BLOCK LIST | |
5461 | ||
5462 | =item sort LIST | |
5463 | ||
41d39f30 | 5464 | In list context, this sorts the LIST and returns the sorted list value. |
9fdc1d08 | 5465 | In scalar context, the behaviour of C<sort()> is undefined. |
41d39f30 A |
5466 | |
5467 | If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison | |
5468 | order. If SUBNAME is specified, it gives the name of a subroutine | |
5469 | that returns an integer less than, equal to, or greater than C<0>, | |
3b10bc60 | 5470 | depending on how the elements of the list are to be ordered. (The |
5471 | C<< <=> >> and C<cmp> operators are extremely useful in such routines.) | |
41d39f30 A |
5472 | SUBNAME may be a scalar variable name (unsubscripted), in which case |
5473 | the value provides the name of (or a reference to) the actual | |
5474 | subroutine to use. In place of a SUBNAME, you can provide a BLOCK as | |
5475 | an anonymous, in-line sort subroutine. | |
a0d0e21e | 5476 | |
43481408 | 5477 | If the subroutine's prototype is C<($$)>, the elements to be compared |
f9a36357 GS |
5478 | are passed by reference in C<@_>, as for a normal subroutine. This is |
5479 | slower than unprototyped subroutines, where the elements to be | |
5480 | compared are passed into the subroutine | |
43481408 GS |
5481 | as the package global variables $a and $b (see example below). Note that |
5482 | in the latter case, it is usually counter-productive to declare $a and | |
5483 | $b as lexicals. | |
5484 | ||
c106e8bb RH |
5485 | The values to be compared are always passed by reference and should not |
5486 | be modified. | |
a0d0e21e | 5487 | |
0a753a76 | 5488 | You also cannot exit out of the sort block or subroutine using any of the |
19799a22 | 5489 | loop control operators described in L<perlsyn> or with C<goto>. |
0a753a76 | 5490 | |
a034a98d DD |
5491 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
5492 | current collation locale. See L<perllocale>. | |
5493 | ||
db5021a3 MS |
5494 | sort() returns aliases into the original list, much as a for loop's index |
5495 | variable aliases the list elements. That is, modifying an element of a | |
5496 | list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>) | |
5497 | actually modifies the element in the original list. This is usually | |
5498 | something to be avoided when writing clear code. | |
5499 | ||
58c7fc7c JH |
5500 | Perl 5.6 and earlier used a quicksort algorithm to implement sort. |
5501 | That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort | |
5502 | preserves the input order of elements that compare equal. Although | |
5503 | quicksort's run time is O(NlogN) when averaged over all arrays of | |
5504 | length N, the time can be O(N**2), I<quadratic> behavior, for some | |
5505 | inputs.) In 5.7, the quicksort implementation was replaced with | |
cf264981 | 5506 | a stable mergesort algorithm whose worst-case behavior is O(NlogN). |
58c7fc7c JH |
5507 | But benchmarks indicated that for some inputs, on some platforms, |
5508 | the original quicksort was faster. 5.8 has a sort pragma for | |
5509 | limited control of the sort. Its rather blunt control of the | |
cf264981 | 5510 | underlying algorithm may not persist into future Perls, but the |
58c7fc7c | 5511 | ability to characterize the input or output in implementation |
c25fe68d | 5512 | independent ways quite probably will. See L<the sort pragma|sort>. |
c16425f1 | 5513 | |
a0d0e21e LW |
5514 | Examples: |
5515 | ||
5516 | # sort lexically | |
5517 | @articles = sort @files; | |
e1d16ab7 | 5518 | |
a0d0e21e LW |
5519 | # same thing, but with explicit sort routine |
5520 | @articles = sort {$a cmp $b} @files; | |
e1d16ab7 | 5521 | |
cb1a09d0 | 5522 | # now case-insensitively |
54310121 | 5523 | @articles = sort {uc($a) cmp uc($b)} @files; |
e1d16ab7 | 5524 | |
a0d0e21e LW |
5525 | # same thing in reversed order |
5526 | @articles = sort {$b cmp $a} @files; | |
e1d16ab7 | 5527 | |
a0d0e21e LW |
5528 | # sort numerically ascending |
5529 | @articles = sort {$a <=> $b} @files; | |
e1d16ab7 | 5530 | |
a0d0e21e LW |
5531 | # sort numerically descending |
5532 | @articles = sort {$b <=> $a} @files; | |
e1d16ab7 | 5533 | |
19799a22 GS |
5534 | # this sorts the %age hash by value instead of key |
5535 | # using an in-line function | |
5536 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
e1d16ab7 | 5537 | |
a0d0e21e LW |
5538 | # sort using explicit subroutine name |
5539 | sub byage { | |
5ed4f2ec | 5540 | $age{$a} <=> $age{$b}; # presuming numeric |
a0d0e21e LW |
5541 | } |
5542 | @sortedclass = sort byage @class; | |
e1d16ab7 | 5543 | |
19799a22 GS |
5544 | sub backwards { $b cmp $a } |
5545 | @harry = qw(dog cat x Cain Abel); | |
5546 | @george = qw(gone chased yz Punished Axed); | |
a0d0e21e | 5547 | print sort @harry; |
e1d16ab7 | 5548 | # prints AbelCaincatdogx |
a0d0e21e | 5549 | print sort backwards @harry; |
e1d16ab7 | 5550 | # prints xdogcatCainAbel |
a0d0e21e | 5551 | print sort @george, 'to', @harry; |
e1d16ab7 | 5552 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz |
a0d0e21e | 5553 | |
54310121 | 5554 | # inefficiently sort by descending numeric compare using |
5555 | # the first integer after the first = sign, or the | |
cb1a09d0 AD |
5556 | # whole record case-insensitively otherwise |
5557 | ||
e1d16ab7 | 5558 | my @new = sort { |
5559 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
5560 | || | |
5561 | uc($a) cmp uc($b) | |
cb1a09d0 AD |
5562 | } @old; |
5563 | ||
5564 | # same thing, but much more efficiently; | |
5565 | # we'll build auxiliary indices instead | |
5566 | # for speed | |
e1d16ab7 | 5567 | my @nums = @caps = (); |
54310121 | 5568 | for (@old) { |
e1d16ab7 | 5569 | push @nums, ( /=(\d+)/ ? $1 : undef ); |
5570 | push @caps, uc($_); | |
54310121 | 5571 | } |
cb1a09d0 | 5572 | |
e1d16ab7 | 5573 | my @new = @old[ sort { |
5574 | $nums[$b] <=> $nums[$a] | |
5575 | || | |
5576 | $caps[$a] cmp $caps[$b] | |
5577 | } 0..$#old | |
5578 | ]; | |
cb1a09d0 | 5579 | |
19799a22 | 5580 | # same thing, but without any temps |
cb1a09d0 | 5581 | @new = map { $_->[0] } |
19799a22 | 5582 | sort { $b->[1] <=> $a->[1] |
e1d16ab7 | 5583 | || |
5584 | $a->[2] cmp $b->[2] | |
5585 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
61eff3bc | 5586 | |
43481408 GS |
5587 | # using a prototype allows you to use any comparison subroutine |
5588 | # as a sort subroutine (including other package's subroutines) | |
5589 | package other; | |
5ed4f2ec | 5590 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here |
e1d16ab7 | 5591 | |
43481408 GS |
5592 | package main; |
5593 | @new = sort other::backwards @old; | |
e1d16ab7 | 5594 | |
58c7fc7c JH |
5595 | # guarantee stability, regardless of algorithm |
5596 | use sort 'stable'; | |
5597 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
e1d16ab7 | 5598 | |
268e9d79 JL |
5599 | # force use of mergesort (not portable outside Perl 5.8) |
5600 | use sort '_mergesort'; # note discouraging _ | |
58c7fc7c | 5601 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; |
58c7fc7c | 5602 | |
1cb246e8 RGS |
5603 | Warning: syntactical care is required when sorting the list returned from |
5604 | a function. If you want to sort the list returned by the function call | |
5605 | C<find_records(@key)>, you can use: | |
a9320c62 | 5606 | |
a9320c62 B |
5607 | @contact = sort { $a cmp $b } find_records @key; |
5608 | @contact = sort +find_records(@key); | |
5609 | @contact = sort &find_records(@key); | |
5610 | @contact = sort(find_records(@key)); | |
5611 | ||
5612 | If instead you want to sort the array @key with the comparison routine | |
1cb246e8 RGS |
5613 | C<find_records()> then you can use: |
5614 | ||
a9320c62 B |
5615 | @contact = sort { find_records() } @key; |
5616 | @contact = sort find_records(@key); | |
5617 | @contact = sort(find_records @key); | |
5618 | @contact = sort(find_records (@key)); | |
5619 | ||
19799a22 GS |
5620 | If you're using strict, you I<must not> declare $a |
5621 | and $b as lexicals. They are package globals. That means | |
1cb246e8 | 5622 | that if you're in the C<main> package and type |
13a2d996 | 5623 | |
47223a36 | 5624 | @articles = sort {$b <=> $a} @files; |
13a2d996 | 5625 | |
47223a36 JH |
5626 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), |
5627 | but if you're in the C<FooPack> package, it's the same as typing | |
cb1a09d0 AD |
5628 | |
5629 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
5630 | ||
55497cff | 5631 | The comparison function is required to behave. If it returns |
7660c0ab A |
5632 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
5633 | sometimes saying the opposite, for example) the results are not | |
5634 | well-defined. | |
55497cff | 5635 | |
03190201 | 5636 | Because C<< <=> >> returns C<undef> when either operand is C<NaN> |
3b10bc60 | 5637 | (not-a-number), and because C<sort> raises an exception unless the |
03190201 JL |
5638 | result of a comparison is defined, when sorting with a comparison function |
5639 | like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>. | |
80d38338 | 5640 | The following example takes advantage that C<NaN != NaN> to |
3b10bc60 | 5641 | eliminate any C<NaN>s from the input list. |
03190201 JL |
5642 | |
5643 | @result = sort { $a <=> $b } grep { $_ == $_ } @input; | |
5644 | ||
a0d0e21e | 5645 | =item splice ARRAY,OFFSET,LENGTH,LIST |
d74e8afc | 5646 | X<splice> |
a0d0e21e LW |
5647 | |
5648 | =item splice ARRAY,OFFSET,LENGTH | |
5649 | ||
5650 | =item splice ARRAY,OFFSET | |
5651 | ||
453f9044 GS |
5652 | =item splice ARRAY |
5653 | ||
a0d0e21e | 5654 | Removes the elements designated by OFFSET and LENGTH from an array, and |
5a964f20 TC |
5655 | replaces them with the elements of LIST, if any. In list context, |
5656 | returns the elements removed from the array. In scalar context, | |
43051805 | 5657 | returns the last element removed, or C<undef> if no elements are |
48cdf507 | 5658 | removed. The array grows or shrinks as necessary. |
19799a22 | 5659 | If OFFSET is negative then it starts that far from the end of the array. |
48cdf507 | 5660 | If LENGTH is omitted, removes everything from OFFSET onward. |
d0920e03 MJD |
5661 | If LENGTH is negative, removes the elements from OFFSET onward |
5662 | except for -LENGTH elements at the end of the array. | |
8cbc2e3b | 5663 | If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is |
3b10bc60 | 5664 | past the end of the array, Perl issues a warning, and splices at the |
8cbc2e3b | 5665 | end of the array. |
453f9044 | 5666 | |
3272a53d | 5667 | The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> ) |
a0d0e21e | 5668 | |
5ed4f2ec | 5669 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
5670 | pop(@a) splice(@a,-1) | |
5671 | shift(@a) splice(@a,0,1) | |
5672 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
5673 | $a[$i] = $y splice(@a,$i,1,$y) | |
a0d0e21e LW |
5674 | |
5675 | Example, assuming array lengths are passed before arrays: | |
5676 | ||
5ed4f2ec | 5677 | sub aeq { # compare two list values |
a9a5a0dc VP |
5678 | my(@a) = splice(@_,0,shift); |
5679 | my(@b) = splice(@_,0,shift); | |
5680 | return 0 unless @a == @b; # same len? | |
5681 | while (@a) { | |
5682 | return 0 if pop(@a) ne pop(@b); | |
5683 | } | |
5684 | return 1; | |
a0d0e21e LW |
5685 | } |
5686 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
5687 | ||
5688 | =item split /PATTERN/,EXPR,LIMIT | |
d74e8afc | 5689 | X<split> |
a0d0e21e LW |
5690 | |
5691 | =item split /PATTERN/,EXPR | |
5692 | ||
5693 | =item split /PATTERN/ | |
5694 | ||
5695 | =item split | |
5696 | ||
b2e26e6e DJ |
5697 | Splits the string EXPR into a list of strings and returns that list. By |
5698 | default, empty leading fields are preserved, and empty trailing ones are | |
ab7ee80f | 5699 | deleted. (If all fields are empty, they are considered to be trailing.) |
a0d0e21e | 5700 | |
a6d8037e | 5701 | In scalar context, returns the number of fields found. |
a0d0e21e | 5702 | |
7660c0ab | 5703 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
4633a7c4 LW |
5704 | splits on whitespace (after skipping any leading whitespace). Anything |
5705 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
fb73857a | 5706 | that the delimiter may be longer than one character.) |
5707 | ||
836e0ee7 | 5708 | If LIMIT is specified and positive, it represents the maximum number |
e833de1e BS |
5709 | of fields the EXPR will be split into, though the actual number of |
5710 | fields returned depends on the number of times PATTERN matches within | |
5711 | EXPR. If LIMIT is unspecified or zero, trailing null fields are | |
5712 | stripped (which potential users of C<pop> would do well to remember). | |
5713 | If LIMIT is negative, it is treated as if an arbitrarily large LIMIT | |
5714 | had been specified. Note that splitting an EXPR that evaluates to the | |
5715 | empty string always returns the empty list, regardless of the LIMIT | |
5716 | specified. | |
a0d0e21e | 5717 | |
3b10bc60 | 5718 | A pattern matching the empty string (not to be confused with |
5719 | an empty pattern C<//>, which is just one member of the set of patterns | |
5720 | matching the epmty string), splits EXPR into individual | |
5721 | characters. For example: | |
a0d0e21e | 5722 | |
8241c1c0 | 5723 | print join(':', split(/ */, 'hi there')), "\n"; |
a0d0e21e LW |
5724 | |
5725 | produces the output 'h:i:t:h:e:r:e'. | |
5726 | ||
3b10bc60 | 5727 | As a special case for C<split>, the empty pattern C<//> specifically |
5728 | matches the empty string; this is not be confused with the normal use | |
5729 | of an empty pattern to mean the last successful match. So to split | |
5730 | a string into individual characters, the following: | |
6de67870 | 5731 | |
8241c1c0 | 5732 | print join(':', split(//, 'hi there')), "\n"; |
52ea55c9 | 5733 | |
de5763b0 | 5734 | produces the output 'h:i: :t:h:e:r:e'. |
52ea55c9 | 5735 | |
12977212 FC |
5736 | Empty leading fields are produced when there are positive-width matches at |
5737 | the beginning of the string; a zero-width match at the beginning of | |
5738 | the string does not produce an empty field. For example: | |
0156e0fd RB |
5739 | |
5740 | print join(':', split(/(?=\w)/, 'hi there!')); | |
5741 | ||
12977212 FC |
5742 | produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other |
5743 | hand, are produced when there is a match at the end of the string (and | |
5744 | when LIMIT is given and is not 0), regardless of the length of the match. | |
5745 | For example: | |
5746 | ||
8241c1c0 B |
5747 | print join(':', split(//, 'hi there!', -1)), "\n"; |
5748 | print join(':', split(/\W/, 'hi there!', -1)), "\n"; | |
12977212 FC |
5749 | |
5750 | produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively, | |
5751 | both with an empty trailing field. | |
0156e0fd | 5752 | |
5f05dabc | 5753 | The LIMIT parameter can be used to split a line partially |
a0d0e21e LW |
5754 | |
5755 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
5756 | ||
b5da07fd TB |
5757 | When assigning to a list, if LIMIT is omitted, or zero, Perl supplies |
5758 | a LIMIT one larger than the number of variables in the list, to avoid | |
a0d0e21e LW |
5759 | unnecessary work. For the list above LIMIT would have been 4 by |
5760 | default. In time critical applications it behooves you not to split | |
5761 | into more fields than you really need. | |
5762 | ||
19799a22 | 5763 | If the PATTERN contains parentheses, additional list elements are |
a0d0e21e LW |
5764 | created from each matching substring in the delimiter. |
5765 | ||
da0045b7 | 5766 | split(/([,-])/, "1-10,20", 3); |
a0d0e21e LW |
5767 | |
5768 | produces the list value | |
5769 | ||
5770 | (1, '-', 10, ',', 20) | |
5771 | ||
19799a22 | 5772 | If you had the entire header of a normal Unix email message in $header, |
4633a7c4 LW |
5773 | you could split it up into fields and their values this way: |
5774 | ||
9f4b9cd0 | 5775 | $header =~ s/\n(?=\s)//g; # fix continuation lines |
fb73857a | 5776 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
4633a7c4 | 5777 | |
a0d0e21e LW |
5778 | The pattern C</PATTERN/> may be replaced with an expression to specify |
5779 | patterns that vary at runtime. (To do runtime compilation only once, | |
748a9306 LW |
5780 | use C</$variable/o>.) |
5781 | ||
5da728e2 A |
5782 | As a special case, specifying a PATTERN of space (S<C<' '>>) will split on |
5783 | white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can | |
5784 | be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>> | |
3b10bc60 | 5785 | will give you as many initial null fields (empty string) as there are leading spaces. |
5da728e2 | 5786 | A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading |
19799a22 | 5787 | whitespace produces a null first field. A C<split> with no arguments |
5da728e2 | 5788 | really does a S<C<split(' ', $_)>> internally. |
a0d0e21e | 5789 | |
cc50a203 | 5790 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't |
1ec94568 MG |
5791 | much use otherwise. |
5792 | ||
a0d0e21e LW |
5793 | Example: |
5794 | ||
5a964f20 TC |
5795 | open(PASSWD, '/etc/passwd'); |
5796 | while (<PASSWD>) { | |
5b3eff12 MS |
5797 | chomp; |
5798 | ($login, $passwd, $uid, $gid, | |
f86cebdf | 5799 | $gcos, $home, $shell) = split(/:/); |
a9a5a0dc | 5800 | #... |
a0d0e21e LW |
5801 | } |
5802 | ||
6de67870 JP |
5803 | As with regular pattern matching, any capturing parentheses that are not |
5804 | matched in a C<split()> will be set to C<undef> when returned: | |
5805 | ||
5806 | @fields = split /(A)|B/, "1A2B3"; | |
5807 | # @fields is (1, 'A', 2, undef, 3) | |
a0d0e21e | 5808 | |
5f05dabc | 5809 | =item sprintf FORMAT, LIST |
d74e8afc | 5810 | X<sprintf> |
a0d0e21e | 5811 | |
6662521e GS |
5812 | Returns a string formatted by the usual C<printf> conventions of the C |
5813 | library function C<sprintf>. See below for more details | |
f4084e39 | 5814 | and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of |
6662521e GS |
5815 | the general principles. |
5816 | ||
5817 | For example: | |
5818 | ||
5819 | # Format number with up to 8 leading zeroes | |
5820 | $result = sprintf("%08d", $number); | |
5821 | ||
5822 | # Round number to 3 digits after decimal point | |
5823 | $rounded = sprintf("%.3f", $number); | |
74a77017 | 5824 | |
3b10bc60 | 5825 | Perl does its own C<sprintf> formatting: it emulates the C |
5826 | function sprintf(3), but doesn't use it except for floating-point | |
5827 | numbers, and even then only standard modifiers are allowed. | |
5828 | Non-standard extensions in your local sprintf(3) are | |
5829 | therefore unavailable from Perl. | |
74a77017 | 5830 | |
194e7b38 DC |
5831 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you |
5832 | pass it an array as your first argument. The array is given scalar context, | |
5833 | and instead of using the 0th element of the array as the format, Perl will | |
5834 | use the count of elements in the array as the format, which is almost never | |
5835 | useful. | |
5836 | ||
19799a22 | 5837 | Perl's C<sprintf> permits the following universally-known conversions: |
74a77017 | 5838 | |
5ed4f2ec | 5839 | %% a percent sign |
5840 | %c a character with the given number | |
5841 | %s a string | |
5842 | %d a signed integer, in decimal | |
5843 | %u an unsigned integer, in decimal | |
5844 | %o an unsigned integer, in octal | |
5845 | %x an unsigned integer, in hexadecimal | |
5846 | %e a floating-point number, in scientific notation | |
5847 | %f a floating-point number, in fixed decimal notation | |
5848 | %g a floating-point number, in %e or %f notation | |
74a77017 | 5849 | |
1b3f7d21 | 5850 | In addition, Perl permits the following widely-supported conversions: |
74a77017 | 5851 | |
5ed4f2ec | 5852 | %X like %x, but using upper-case letters |
5853 | %E like %e, but using an upper-case "E" | |
5854 | %G like %g, but with an upper-case "E" (if applicable) | |
5855 | %b an unsigned integer, in binary | |
5856 | %B like %b, but using an upper-case "B" with the # flag | |
5857 | %p a pointer (outputs the Perl value's address in hexadecimal) | |
5858 | %n special: *stores* the number of characters output so far | |
b76cc8ba | 5859 | into the next variable in the parameter list |
74a77017 | 5860 | |
1b3f7d21 CS |
5861 | Finally, for backward (and we do mean "backward") compatibility, Perl |
5862 | permits these unnecessary but widely-supported conversions: | |
74a77017 | 5863 | |
5ed4f2ec | 5864 | %i a synonym for %d |
5865 | %D a synonym for %ld | |
5866 | %U a synonym for %lu | |
5867 | %O a synonym for %lo | |
5868 | %F a synonym for %f | |
74a77017 | 5869 | |
7b8dd722 HS |
5870 | Note that the number of exponent digits in the scientific notation produced |
5871 | by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the | |
b73fd64e JH |
5872 | exponent less than 100 is system-dependent: it may be three or less |
5873 | (zero-padded as necessary). In other words, 1.23 times ten to the | |
5874 | 99th may be either "1.23e99" or "1.23e099". | |
d764f01a | 5875 | |
80d38338 | 5876 | Between the C<%> and the format letter, you may specify several |
7b8dd722 HS |
5877 | additional attributes controlling the interpretation of the format. |
5878 | In order, these are: | |
74a77017 | 5879 | |
7b8dd722 HS |
5880 | =over 4 |
5881 | ||
5882 | =item format parameter index | |
5883 | ||
5884 | An explicit format parameter index, such as C<2$>. By default sprintf | |
5885 | will format the next unused argument in the list, but this allows you | |
3b10bc60 | 5886 | to take the arguments out of order: |
7b8dd722 HS |
5887 | |
5888 | printf '%2$d %1$d', 12, 34; # prints "34 12" | |
5889 | printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" | |
5890 | ||
5891 | =item flags | |
5892 | ||
5893 | one or more of: | |
e6bb52fd | 5894 | |
7a81c58e A |
5895 | space prefix non-negative number with a space |
5896 | + prefix non-negative number with a plus sign | |
74a77017 CS |
5897 | - left-justify within the field |
5898 | 0 use zeros, not spaces, to right-justify | |
e6bb52fd TS |
5899 | # ensure the leading "0" for any octal, |
5900 | prefix non-zero hexadecimal with "0x" or "0X", | |
5901 | prefix non-zero binary with "0b" or "0B" | |
7b8dd722 HS |
5902 | |
5903 | For example: | |
5904 | ||
e6bb52fd TS |
5905 | printf '<% d>', 12; # prints "< 12>" |
5906 | printf '<%+d>', 12; # prints "<+12>" | |
5907 | printf '<%6s>', 12; # prints "< 12>" | |
5908 | printf '<%-6s>', 12; # prints "<12 >" | |
5909 | printf '<%06s>', 12; # prints "<000012>" | |
5910 | printf '<%#o>', 12; # prints "<014>" | |
5911 | printf '<%#x>', 12; # prints "<0xc>" | |
5912 | printf '<%#X>', 12; # prints "<0XC>" | |
5913 | printf '<%#b>', 12; # prints "<0b1100>" | |
5914 | printf '<%#B>', 12; # prints "<0B1100>" | |
7b8dd722 | 5915 | |
9911cee9 TS |
5916 | When a space and a plus sign are given as the flags at once, |
5917 | a plus sign is used to prefix a positive number. | |
5918 | ||
5919 | printf '<%+ d>', 12; # prints "<+12>" | |
5920 | printf '<% +d>', 12; # prints "<+12>" | |
5921 | ||
e6bb52fd TS |
5922 | When the # flag and a precision are given in the %o conversion, |
5923 | the precision is incremented if it's necessary for the leading "0". | |
5924 | ||
5925 | printf '<%#.5o>', 012; # prints "<00012>" | |
5926 | printf '<%#.5o>', 012345; # prints "<012345>" | |
5927 | printf '<%#.0o>', 0; # prints "<0>" | |
5928 | ||
7b8dd722 HS |
5929 | =item vector flag |
5930 | ||
3b10bc60 | 5931 | This flag tells Perl to interpret the supplied string as a vector of |
920f3fa9 DM |
5932 | integers, one for each character in the string. Perl applies the format to |
5933 | each integer in turn, then joins the resulting strings with a separator (a | |
5934 | dot C<.> by default). This can be useful for displaying ordinal values of | |
5935 | characters in arbitrary strings: | |
7b8dd722 | 5936 | |
920f3fa9 | 5937 | printf "%vd", "AB\x{100}"; # prints "65.66.256" |
7b8dd722 HS |
5938 | printf "version is v%vd\n", $^V; # Perl's version |
5939 | ||
5940 | Put an asterisk C<*> before the C<v> to override the string to | |
5941 | use to separate the numbers: | |
5942 | ||
5943 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
5944 | printf "bits are %0*v8b\n", " ", $bits; # random bitstring | |
5945 | ||
5946 | You can also explicitly specify the argument number to use for | |
3b10bc60 | 5947 | the join string using something like C<*2$v>; for example: |
7b8dd722 HS |
5948 | |
5949 | printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses | |
5950 | ||
5951 | =item (minimum) width | |
5952 | ||
5953 | Arguments are usually formatted to be only as wide as required to | |
5954 | display the given value. You can override the width by putting | |
5955 | a number here, or get the width from the next argument (with C<*>) | |
3b10bc60 | 5956 | or from a specified argument (e.g., with C<*2$>): |
7b8dd722 HS |
5957 | |
5958 | printf '<%s>', "a"; # prints "<a>" | |
5959 | printf '<%6s>', "a"; # prints "< a>" | |
5960 | printf '<%*s>', 6, "a"; # prints "< a>" | |
5961 | printf '<%*2$s>', "a", 6; # prints "< a>" | |
5962 | printf '<%2s>', "long"; # prints "<long>" (does not truncate) | |
5963 | ||
19799a22 GS |
5964 | If a field width obtained through C<*> is negative, it has the same |
5965 | effect as the C<-> flag: left-justification. | |
74a77017 | 5966 | |
7b8dd722 | 5967 | =item precision, or maximum width |
d74e8afc | 5968 | X<precision> |
7b8dd722 | 5969 | |
6c8c9a8e | 5970 | You can specify a precision (for numeric conversions) or a maximum |
7b8dd722 | 5971 | width (for string conversions) by specifying a C<.> followed by a number. |
3b10bc60 | 5972 | For floating-point formats except 'g' and 'G', this specifies |
5973 | how many places right of the decimal point to show (the default being 6). | |
5974 | For example: | |
7b8dd722 HS |
5975 | |
5976 | # these examples are subject to system-specific variation | |
5977 | printf '<%f>', 1; # prints "<1.000000>" | |
5978 | printf '<%.1f>', 1; # prints "<1.0>" | |
5979 | printf '<%.0f>', 1; # prints "<1>" | |
5980 | printf '<%e>', 10; # prints "<1.000000e+01>" | |
5981 | printf '<%.1e>', 10; # prints "<1.0e+01>" | |
5982 | ||
3b10bc60 | 5983 | For "g" and "G", this specifies the maximum number of digits to show, |
5984 | including thoe prior to the decimal point and those after it; for | |
5985 | example: | |
1ff2d182 | 5986 | |
3b10bc60 | 5987 | # These examples are subject to system-specific variation. |
1ff2d182 AS |
5988 | printf '<%g>', 1; # prints "<1>" |
5989 | printf '<%.10g>', 1; # prints "<1>" | |
5990 | printf '<%g>', 100; # prints "<100>" | |
5991 | printf '<%.1g>', 100; # prints "<1e+02>" | |
5992 | printf '<%.2g>', 100.01; # prints "<1e+02>" | |
5993 | printf '<%.5g>', 100.01; # prints "<100.01>" | |
5994 | printf '<%.4g>', 100.01; # prints "<100>" | |
5995 | ||
7b8dd722 | 5996 | For integer conversions, specifying a precision implies that the |
9911cee9 TS |
5997 | output of the number itself should be zero-padded to this width, |
5998 | where the 0 flag is ignored: | |
5999 | ||
6000 | printf '<%.6d>', 1; # prints "<000001>" | |
6001 | printf '<%+.6d>', 1; # prints "<+000001>" | |
6002 | printf '<%-10.6d>', 1; # prints "<000001 >" | |
6003 | printf '<%10.6d>', 1; # prints "< 000001>" | |
6004 | printf '<%010.6d>', 1; # prints "< 000001>" | |
6005 | printf '<%+10.6d>', 1; # prints "< +000001>" | |
7b8dd722 HS |
6006 | |
6007 | printf '<%.6x>', 1; # prints "<000001>" | |
6008 | printf '<%#.6x>', 1; # prints "<0x000001>" | |
6009 | printf '<%-10.6x>', 1; # prints "<000001 >" | |
9911cee9 TS |
6010 | printf '<%10.6x>', 1; # prints "< 000001>" |
6011 | printf '<%010.6x>', 1; # prints "< 000001>" | |
6012 | printf '<%#10.6x>', 1; # prints "< 0x000001>" | |
7b8dd722 HS |
6013 | |
6014 | For string conversions, specifying a precision truncates the string | |
3b10bc60 | 6015 | to fit the specified width: |
7b8dd722 HS |
6016 | |
6017 | printf '<%.5s>', "truncated"; # prints "<trunc>" | |
6018 | printf '<%10.5s>', "truncated"; # prints "< trunc>" | |
6019 | ||
6020 | You can also get the precision from the next argument using C<.*>: | |
b22c7a20 | 6021 | |
7b8dd722 HS |
6022 | printf '<%.6x>', 1; # prints "<000001>" |
6023 | printf '<%.*x>', 6, 1; # prints "<000001>" | |
6024 | ||
3b10bc60 | 6025 | If a precision obtained through C<*> is negative, it counts |
6026 | as having no precision at all. | |
9911cee9 TS |
6027 | |
6028 | printf '<%.*s>', 7, "string"; # prints "<string>" | |
6029 | printf '<%.*s>', 3, "string"; # prints "<str>" | |
6030 | printf '<%.*s>', 0, "string"; # prints "<>" | |
6031 | printf '<%.*s>', -1, "string"; # prints "<string>" | |
6032 | ||
6033 | printf '<%.*d>', 1, 0; # prints "<0>" | |
6034 | printf '<%.*d>', 0, 0; # prints "<>" | |
6035 | printf '<%.*d>', -1, 0; # prints "<0>" | |
6036 | ||
7b8dd722 | 6037 | You cannot currently get the precision from a specified number, |
3b10bc60 | 6038 | but it is intended that this will be possible in the future, for |
6039 | example using C<.*2$>: | |
7b8dd722 | 6040 | |
3b10bc60 | 6041 | printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>" |
7b8dd722 HS |
6042 | |
6043 | =item size | |
6044 | ||
6045 | For numeric conversions, you can specify the size to interpret the | |
1ff2d182 AS |
6046 | number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer |
6047 | conversions (C<d u o x X b i D U O>), numbers are usually assumed to be | |
6048 | whatever the default integer size is on your platform (usually 32 or 64 | |
6049 | bits), but you can override this to use instead one of the standard C types, | |
6050 | as supported by the compiler used to build Perl: | |
7b8dd722 HS |
6051 | |
6052 | l interpret integer as C type "long" or "unsigned long" | |
6053 | h interpret integer as C type "short" or "unsigned short" | |
1ff2d182 AS |
6054 | q, L or ll interpret integer as C type "long long", "unsigned long long". |
6055 | or "quads" (typically 64-bit integers) | |
7b8dd722 | 6056 | |
3b10bc60 | 6057 | The last will raise an exception if Perl does not understand "quads" in your |
6058 | installation. (This requires either that the platform natively support quads, | |
6059 | or that Perl were specifically compiled to support quads.) You can find out | |
1ff2d182 | 6060 | whether your Perl supports quads via L<Config>: |
7b8dd722 | 6061 | |
5ed4f2ec | 6062 | use Config; |
3b10bc60 | 6063 | if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) { |
6064 | print "Nice quads!\n"; | |
6065 | } | |
1ff2d182 | 6066 | |
3b10bc60 | 6067 | For floating-point conversions (C<e f g E F G>), numbers are usually assumed |
6068 | to be the default floating-point size on your platform (double or long double), | |
6069 | but you can force "long double" with C<q>, C<L>, or C<ll> if your | |
1ff2d182 AS |
6070 | platform supports them. You can find out whether your Perl supports long |
6071 | doubles via L<Config>: | |
6072 | ||
5ed4f2ec | 6073 | use Config; |
3b10bc60 | 6074 | print "long doubles\n" if $Config{d_longdbl} eq "define"; |
1ff2d182 | 6075 | |
3b10bc60 | 6076 | You can find out whether Perl considers "long double" to be the default |
6077 | floating-point size to use on your platform via L<Config>: | |
1ff2d182 | 6078 | |
3b10bc60 | 6079 | use Config; |
6080 | if ($Config{uselongdouble} eq "define") { | |
6081 | print "long doubles by default\n"; | |
6082 | } | |
1ff2d182 | 6083 | |
3b10bc60 | 6084 | It can also be that long doubles and doubles are the same thing: |
1ff2d182 AS |
6085 | |
6086 | use Config; | |
6087 | ($Config{doublesize} == $Config{longdblsize}) && | |
6088 | print "doubles are long doubles\n"; | |
6089 | ||
3b10bc60 | 6090 | The size specifier C<V> has no effect for Perl code, but is supported for |
6091 | compatibility with XS code. It means "use the standard size for a Perl | |
6092 | integer or floating-point number", which is the default. | |
7b8dd722 | 6093 | |
a472f209 HS |
6094 | =item order of arguments |
6095 | ||
3b10bc60 | 6096 | Normally, sprintf() takes the next unused argument as the value to |
a472f209 HS |
6097 | format for each format specification. If the format specification |
6098 | uses C<*> to require additional arguments, these are consumed from | |
3b10bc60 | 6099 | the argument list in the order they appear in the format |
6100 | specification I<before> the value to format. Where an argument is | |
6101 | specified by an explicit index, this does not affect the normal | |
6102 | order for the arguments, even when the explicitly specified index | |
6103 | would have been the next argument. | |
a472f209 HS |
6104 | |
6105 | So: | |
6106 | ||
3b10bc60 | 6107 | printf "<%*.*s>", $a, $b, $c; |
a472f209 | 6108 | |
3b10bc60 | 6109 | uses C<$a> for the width, C<$b> for the precision, and C<$c> |
6110 | as the value to format; while: | |
a472f209 | 6111 | |
3b10bc60 | 6112 | printf "<%*1$.*s>", $a, $b; |
a472f209 | 6113 | |
3b10bc60 | 6114 | would use C<$a> for the width and precision, and C<$b> as the |
a472f209 HS |
6115 | value to format. |
6116 | ||
3b10bc60 | 6117 | Here are some more examples; be aware that when using an explicit |
6118 | index, the C<$> may need escaping: | |
a472f209 | 6119 | |
5ed4f2ec | 6120 | printf "%2\$d %d\n", 12, 34; # will print "34 12\n" |
6121 | printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" | |
6122 | printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" | |
6123 | printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
a472f209 | 6124 | |
7b8dd722 | 6125 | =back |
b22c7a20 | 6126 | |
3b10bc60 | 6127 | If C<use locale> is in effect and POSIX::setlocale() has been called, |
6128 | the character used for the decimal separator in formatted floating-point | |
6129 | numbers is affected by the LC_NUMERIC locale. See L<perllocale> | |
7e4353e9 | 6130 | and L<POSIX>. |
a0d0e21e LW |
6131 | |
6132 | =item sqrt EXPR | |
d74e8afc | 6133 | X<sqrt> X<root> X<square root> |
a0d0e21e | 6134 | |
54310121 | 6135 | =item sqrt |
bbce6d69 | 6136 | |
3b10bc60 | 6137 | Return the positive square root of EXPR. If EXPR is omitted, uses |
6138 | C<$_>. Works only for non-negative operands unless you've | |
6139 | loaded the C<Math::Complex> module. | |
2b5ab1e7 TC |
6140 | |
6141 | use Math::Complex; | |
3b10bc60 | 6142 | print sqrt(-4); # prints 2i |
a0d0e21e LW |
6143 | |
6144 | =item srand EXPR | |
d74e8afc | 6145 | X<srand> X<seed> X<randseed> |
a0d0e21e | 6146 | |
93dc8474 CS |
6147 | =item srand |
6148 | ||
0686c0b8 JH |
6149 | Sets the random number seed for the C<rand> operator. |
6150 | ||
0686c0b8 JH |
6151 | The point of the function is to "seed" the C<rand> function so that |
6152 | C<rand> can produce a different sequence each time you run your | |
e0b236fe | 6153 | program. |
0686c0b8 | 6154 | |
e0b236fe | 6155 | If srand() is not called explicitly, it is called implicitly at the |
3b10bc60 | 6156 | first use of the C<rand> operator. However, this was not true of |
e0b236fe JH |
6157 | versions of Perl before 5.004, so if your script will run under older |
6158 | Perl versions, it should call C<srand>. | |
93dc8474 | 6159 | |
e0b236fe JH |
6160 | Most programs won't even call srand() at all, except those that |
6161 | need a cryptographically-strong starting point rather than the | |
6162 | generally acceptable default, which is based on time of day, | |
3b10bc60 | 6163 | process ID, and memory allocation, or the F</dev/urandom> device |
67408cae | 6164 | if available. |
9be67dbc | 6165 | |
e0b236fe JH |
6166 | You can call srand($seed) with the same $seed to reproduce the |
6167 | I<same> sequence from rand(), but this is usually reserved for | |
6168 | generating predictable results for testing or debugging. | |
6169 | Otherwise, don't call srand() more than once in your program. | |
0686c0b8 | 6170 | |
80d38338 | 6171 | Do B<not> call srand() (i.e., without an argument) more than once in |
3a3e71eb | 6172 | a script. The internal state of the random number generator should |
0686c0b8 | 6173 | contain more entropy than can be provided by any seed, so calling |
e0b236fe | 6174 | srand() again actually I<loses> randomness. |
0686c0b8 | 6175 | |
e0b236fe JH |
6176 | Most implementations of C<srand> take an integer and will silently |
6177 | truncate decimal numbers. This means C<srand(42)> will usually | |
6178 | produce the same results as C<srand(42.1)>. To be safe, always pass | |
6179 | C<srand> an integer. | |
0686c0b8 JH |
6180 | |
6181 | In versions of Perl prior to 5.004 the default seed was just the | |
6182 | current C<time>. This isn't a particularly good seed, so many old | |
6183 | programs supply their own seed value (often C<time ^ $$> or C<time ^ | |
6184 | ($$ + ($$ << 15))>), but that isn't necessary any more. | |
93dc8474 | 6185 | |
cf264981 SP |
6186 | For cryptographic purposes, however, you need something much more random |
6187 | than the default seed. Checksumming the compressed output of one or more | |
2f9daede TP |
6188 | rapidly changing operating system status programs is the usual method. For |
6189 | example: | |
28757baa | 6190 | |
784d6566 | 6191 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`); |
28757baa | 6192 | |
7660c0ab | 6193 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
0078ec44 RS |
6194 | module in CPAN. |
6195 | ||
54310121 | 6196 | Frequently called programs (like CGI scripts) that simply use |
28757baa | 6197 | |
6198 | time ^ $$ | |
6199 | ||
54310121 | 6200 | for a seed can fall prey to the mathematical property that |
28757baa | 6201 | |
6202 | a^b == (a+1)^(b+1) | |
6203 | ||
0078ec44 | 6204 | one-third of the time. So don't do that. |
f86702cc | 6205 | |
a0d0e21e | 6206 | =item stat FILEHANDLE |
435fbc73 | 6207 | X<stat> X<file, status> X<ctime> |
a0d0e21e LW |
6208 | |
6209 | =item stat EXPR | |
6210 | ||
5228a96c SP |
6211 | =item stat DIRHANDLE |
6212 | ||
54310121 | 6213 | =item stat |
bbce6d69 | 6214 | |
1d2dff63 | 6215 | Returns a 13-element list giving the status info for a file, either |
5228a96c | 6216 | the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is |
3b10bc60 | 6217 | omitted, it stats C<$_>. Returns the empty list if C<stat> fails. Typically |
5228a96c | 6218 | used as follows: |
a0d0e21e LW |
6219 | |
6220 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
6221 | $atime,$mtime,$ctime,$blksize,$blocks) | |
6222 | = stat($filename); | |
6223 | ||
54310121 | 6224 | Not all fields are supported on all filesystem types. Here are the |
61967be2 | 6225 | meanings of the fields: |
c07a80fd | 6226 | |
54310121 | 6227 | 0 dev device number of filesystem |
6228 | 1 ino inode number | |
6229 | 2 mode file mode (type and permissions) | |
6230 | 3 nlink number of (hard) links to the file | |
6231 | 4 uid numeric user ID of file's owner | |
6232 | 5 gid numeric group ID of file's owner | |
6233 | 6 rdev the device identifier (special files only) | |
6234 | 7 size total size of file, in bytes | |
1c74f1bd GS |
6235 | 8 atime last access time in seconds since the epoch |
6236 | 9 mtime last modify time in seconds since the epoch | |
df2a7e48 | 6237 | 10 ctime inode change time in seconds since the epoch (*) |
54310121 | 6238 | 11 blksize preferred block size for file system I/O |
6239 | 12 blocks actual number of blocks allocated | |
c07a80fd | 6240 | |
6241 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
6242 | ||
3e2557b2 RGS |
6243 | (*) Not all fields are supported on all filesystem types. Notably, the |
6244 | ctime field is non-portable. In particular, you cannot expect it to be a | |
6245 | "creation time", see L<perlport/"Files and Filesystems"> for details. | |
df2a7e48 | 6246 | |
61967be2 | 6247 | If C<stat> is passed the special filehandle consisting of an underline, no |
a0d0e21e | 6248 | stat is done, but the current contents of the stat structure from the |
61967be2 | 6249 | last C<stat>, C<lstat>, or filetest are returned. Example: |
a0d0e21e LW |
6250 | |
6251 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
a9a5a0dc | 6252 | print "$file is executable NFS file\n"; |
a0d0e21e LW |
6253 | } |
6254 | ||
ca6e1c26 JH |
6255 | (This works on machines only for which the device number is negative |
6256 | under NFS.) | |
a0d0e21e | 6257 | |
2b5ab1e7 | 6258 | Because the mode contains both the file type and its permissions, you |
b76cc8ba | 6259 | should mask off the file type portion and (s)printf using a C<"%o"> |
2b5ab1e7 TC |
6260 | if you want to see the real permissions. |
6261 | ||
6262 | $mode = (stat($filename))[2]; | |
6263 | printf "Permissions are %04o\n", $mode & 07777; | |
6264 | ||
19799a22 | 6265 | In scalar context, C<stat> returns a boolean value indicating success |
1d2dff63 GS |
6266 | or failure, and, if successful, sets the information associated with |
6267 | the special filehandle C<_>. | |
6268 | ||
dd184578 | 6269 | The L<File::stat> module provides a convenient, by-name access mechanism: |
2b5ab1e7 TC |
6270 | |
6271 | use File::stat; | |
6272 | $sb = stat($filename); | |
b76cc8ba | 6273 | printf "File is %s, size is %s, perm %04o, mtime %s\n", |
a9a5a0dc VP |
6274 | $filename, $sb->size, $sb->mode & 07777, |
6275 | scalar localtime $sb->mtime; | |
2b5ab1e7 | 6276 | |
ca6e1c26 JH |
6277 | You can import symbolic mode constants (C<S_IF*>) and functions |
6278 | (C<S_IS*>) from the Fcntl module: | |
6279 | ||
6280 | use Fcntl ':mode'; | |
6281 | ||
6282 | $mode = (stat($filename))[2]; | |
6283 | ||
6284 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
6285 | $group_read = ($mode & S_IRGRP) >> 3; | |
6286 | $other_execute = $mode & S_IXOTH; | |
6287 | ||
3155e0b0 | 6288 | printf "Permissions are %04o\n", S_IMODE($mode), "\n"; |
ca6e1c26 JH |
6289 | |
6290 | $is_setuid = $mode & S_ISUID; | |
ad605d16 | 6291 | $is_directory = S_ISDIR($mode); |
ca6e1c26 JH |
6292 | |
6293 | You could write the last two using the C<-u> and C<-d> operators. | |
3b10bc60 | 6294 | Commonly available C<S_IF*> constants are: |
ca6e1c26 JH |
6295 | |
6296 | # Permissions: read, write, execute, for user, group, others. | |
6297 | ||
6298 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
6299 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
6300 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
61eff3bc | 6301 | |
3cee8101 RGS |
6302 | # Setuid/Setgid/Stickiness/SaveText. |
6303 | # Note that the exact meaning of these is system dependent. | |
ca6e1c26 JH |
6304 | |
6305 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
6306 | ||
6307 | # File types. Not necessarily all are available on your system. | |
6308 | ||
135ed46b | 6309 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT |
ca6e1c26 JH |
6310 | |
6311 | # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. | |
6312 | ||
6313 | S_IREAD S_IWRITE S_IEXEC | |
6314 | ||
61967be2 | 6315 | and the C<S_IF*> functions are |
ca6e1c26 | 6316 | |
5ed4f2ec | 6317 | S_IMODE($mode) the part of $mode containing the permission bits |
6318 | and the setuid/setgid/sticky bits | |
ca6e1c26 | 6319 | |
5ed4f2ec | 6320 | S_IFMT($mode) the part of $mode containing the file type |
3b10bc60 | 6321 | which can be bit-anded with (for example) S_IFREG |
ca6e1c26 JH |
6322 | or with the following functions |
6323 | ||
61967be2 | 6324 | # The operators -f, -d, -l, -b, -c, -p, and -S. |
ca6e1c26 JH |
6325 | |
6326 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
6327 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
6328 | ||
6329 | # No direct -X operator counterpart, but for the first one | |
6330 | # the -g operator is often equivalent. The ENFMT stands for | |
6331 | # record flocking enforcement, a platform-dependent feature. | |
6332 | ||
6333 | S_ISENFMT($mode) S_ISWHT($mode) | |
6334 | ||
6335 | See your native chmod(2) and stat(2) documentation for more details | |
61967be2 | 6336 | about the C<S_*> constants. To get status info for a symbolic link |
c837d5b4 | 6337 | instead of the target file behind the link, use the C<lstat> function. |
ca6e1c26 | 6338 | |
36fb85f3 RGS |
6339 | =item state EXPR |
6340 | X<state> | |
6341 | ||
6342 | =item state TYPE EXPR | |
6343 | ||
6344 | =item state EXPR : ATTRS | |
6345 | ||
6346 | =item state TYPE EXPR : ATTRS | |
6347 | ||
6348 | C<state> declares a lexically scoped variable, just like C<my> does. | |
b708784e | 6349 | However, those variables will never be reinitialized, contrary to |
36fb85f3 RGS |
6350 | lexical variables that are reinitialized each time their enclosing block |
6351 | is entered. | |
6352 | ||
3b10bc60 | 6353 | C<state> variables are enabled only when the C<use feature "state"> pragma |
6354 | is in effect. See L<feature>. | |
36fb85f3 | 6355 | |
a0d0e21e | 6356 | =item study SCALAR |
d74e8afc | 6357 | X<study> |
a0d0e21e LW |
6358 | |
6359 | =item study | |
6360 | ||
184e9718 | 6361 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
a0d0e21e LW |
6362 | doing many pattern matches on the string before it is next modified. |
6363 | This may or may not save time, depending on the nature and number of | |
6364 | patterns you are searching on, and on the distribution of character | |
3b10bc60 | 6365 | frequencies in the string to be searched; you probably want to compare |
5f05dabc | 6366 | run times with and without it to see which runs faster. Those loops |
cf264981 | 6367 | that scan for many short constant strings (including the constant |
a0d0e21e | 6368 | parts of more complex patterns) will benefit most. You may have only |
3b10bc60 | 6369 | one C<study> active at a time: if you study a different scalar the first |
19799a22 | 6370 | is "unstudied". (The way C<study> works is this: a linked list of every |
a0d0e21e | 6371 | character in the string to be searched is made, so we know, for |
7660c0ab | 6372 | example, where all the C<'k'> characters are. From each search string, |
a0d0e21e LW |
6373 | the rarest character is selected, based on some static frequency tables |
6374 | constructed from some C programs and English text. Only those places | |
6375 | that contain this "rarest" character are examined.) | |
6376 | ||
5a964f20 | 6377 | For example, here is a loop that inserts index producing entries |
a0d0e21e LW |
6378 | before any line containing a certain pattern: |
6379 | ||
6380 | while (<>) { | |
a9a5a0dc VP |
6381 | study; |
6382 | print ".IX foo\n" if /\bfoo\b/; | |
6383 | print ".IX bar\n" if /\bbar\b/; | |
6384 | print ".IX blurfl\n" if /\bblurfl\b/; | |
6385 | # ... | |
6386 | print; | |
a0d0e21e LW |
6387 | } |
6388 | ||
3b10bc60 | 6389 | In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f> |
951ba7fe | 6390 | will be looked at, because C<f> is rarer than C<o>. In general, this is |
a0d0e21e LW |
6391 | a big win except in pathological cases. The only question is whether |
6392 | it saves you more time than it took to build the linked list in the | |
6393 | first place. | |
6394 | ||
6395 | Note that if you have to look for strings that you don't know till | |
19799a22 | 6396 | runtime, you can build an entire loop as a string and C<eval> that to |
a0d0e21e | 6397 | avoid recompiling all your patterns all the time. Together with |
80d38338 | 6398 | undefining C<$/> to input entire files as one record, this can be quite |
f86cebdf | 6399 | fast, often faster than specialized programs like fgrep(1). The following |
184e9718 | 6400 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
a0d0e21e LW |
6401 | out the names of those files that contain a match: |
6402 | ||
6403 | $search = 'while (<>) { study;'; | |
6404 | foreach $word (@words) { | |
a9a5a0dc | 6405 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; |
a0d0e21e LW |
6406 | } |
6407 | $search .= "}"; | |
6408 | @ARGV = @files; | |
6409 | undef $/; | |
5ed4f2ec | 6410 | eval $search; # this screams |
6411 | $/ = "\n"; # put back to normal input delimiter | |
a0d0e21e | 6412 | foreach $file (sort keys(%seen)) { |
a9a5a0dc | 6413 | print $file, "\n"; |
a0d0e21e LW |
6414 | } |
6415 | ||
1d2de774 | 6416 | =item sub NAME BLOCK |
d74e8afc | 6417 | X<sub> |
cb1a09d0 | 6418 | |
1d2de774 | 6419 | =item sub NAME (PROTO) BLOCK |
cb1a09d0 | 6420 | |
1d2de774 JH |
6421 | =item sub NAME : ATTRS BLOCK |
6422 | ||
6423 | =item sub NAME (PROTO) : ATTRS BLOCK | |
6424 | ||
6425 | This is subroutine definition, not a real function I<per se>. | |
6426 | Without a BLOCK it's just a forward declaration. Without a NAME, | |
6427 | it's an anonymous function declaration, and does actually return | |
6428 | a value: the CODE ref of the closure you just created. | |
cb1a09d0 | 6429 | |
1d2de774 | 6430 | See L<perlsub> and L<perlref> for details about subroutines and |
0795dc2b | 6431 | references, and L<attributes> and L<Attribute::Handlers> for more |
1d2de774 | 6432 | information about attributes. |
cb1a09d0 | 6433 | |
87275199 | 6434 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT |
d74e8afc | 6435 | X<substr> X<substring> X<mid> X<left> X<right> |
7b8d334a | 6436 | |
87275199 | 6437 | =item substr EXPR,OFFSET,LENGTH |
a0d0e21e LW |
6438 | |
6439 | =item substr EXPR,OFFSET | |
6440 | ||
6441 | Extracts a substring out of EXPR and returns it. First character is at | |
7660c0ab | 6442 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
84902520 | 6443 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
87275199 GS |
6444 | that far from the end of the string. If LENGTH is omitted, returns |
6445 | everything to the end of the string. If LENGTH is negative, leaves that | |
748a9306 LW |
6446 | many characters off the end of the string. |
6447 | ||
e1de3ec0 | 6448 | my $s = "The black cat climbed the green tree"; |
5ed4f2ec | 6449 | my $color = substr $s, 4, 5; # black |
6450 | my $middle = substr $s, 4, -11; # black cat climbed the | |
6451 | my $end = substr $s, 14; # climbed the green tree | |
6452 | my $tail = substr $s, -4; # tree | |
6453 | my $z = substr $s, -4, 2; # tr | |
e1de3ec0 | 6454 | |
2b5ab1e7 | 6455 | You can use the substr() function as an lvalue, in which case EXPR |
87275199 GS |
6456 | must itself be an lvalue. If you assign something shorter than LENGTH, |
6457 | the string will shrink, and if you assign something longer than LENGTH, | |
2b5ab1e7 | 6458 | the string will grow to accommodate it. To keep the string the same |
3b10bc60 | 6459 | length, you may need to pad or chop your value using C<sprintf>. |
a0d0e21e | 6460 | |
87275199 GS |
6461 | If OFFSET and LENGTH specify a substring that is partly outside the |
6462 | string, only the part within the string is returned. If the substring | |
6463 | is beyond either end of the string, substr() returns the undefined | |
6464 | value and produces a warning. When used as an lvalue, specifying a | |
3b10bc60 | 6465 | substring that is entirely outside the string raises an exception. |
87275199 GS |
6466 | Here's an example showing the behavior for boundary cases: |
6467 | ||
6468 | my $name = 'fred'; | |
5ed4f2ec | 6469 | substr($name, 4) = 'dy'; # $name is now 'freddy' |
3b10bc60 | 6470 | my $null = substr $name, 6, 2; # returns "" (no warning) |
5ed4f2ec | 6471 | my $oops = substr $name, 7; # returns undef, with warning |
3b10bc60 | 6472 | substr($name, 7) = 'gap'; # raises an exception |
87275199 | 6473 | |
2b5ab1e7 | 6474 | An alternative to using substr() as an lvalue is to specify the |
7b8d334a | 6475 | replacement string as the 4th argument. This allows you to replace |
2b5ab1e7 TC |
6476 | parts of the EXPR and return what was there before in one operation, |
6477 | just as you can with splice(). | |
7b8d334a | 6478 | |
e1de3ec0 | 6479 | my $s = "The black cat climbed the green tree"; |
5ed4f2ec | 6480 | my $z = substr $s, 14, 7, "jumped from"; # climbed |
e1de3ec0 GS |
6481 | # $s is now "The black cat jumped from the green tree" |
6482 | ||
cf264981 | 6483 | Note that the lvalue returned by the 3-arg version of substr() acts as |
91f73676 DM |
6484 | a 'magic bullet'; each time it is assigned to, it remembers which part |
6485 | of the original string is being modified; for example: | |
6486 | ||
6487 | $x = '1234'; | |
6488 | for (substr($x,1,2)) { | |
5ed4f2ec | 6489 | $_ = 'a'; print $x,"\n"; # prints 1a4 |
6490 | $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 | |
91f73676 | 6491 | $x = '56789'; |
5ed4f2ec | 6492 | $_ = 'pq'; print $x,"\n"; # prints 5pq9 |
91f73676 DM |
6493 | } |
6494 | ||
91f73676 DM |
6495 | Prior to Perl version 5.9.1, the result of using an lvalue multiple times was |
6496 | unspecified. | |
c67bbae0 | 6497 | |
a0d0e21e | 6498 | =item symlink OLDFILE,NEWFILE |
d74e8afc | 6499 | X<symlink> X<link> X<symbolic link> X<link, symbolic> |
a0d0e21e LW |
6500 | |
6501 | Creates a new filename symbolically linked to the old filename. | |
7660c0ab | 6502 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
3b10bc60 | 6503 | symbolic links, raises an exception. To check for that, |
a0d0e21e LW |
6504 | use eval: |
6505 | ||
2b5ab1e7 | 6506 | $symlink_exists = eval { symlink("",""); 1 }; |
a0d0e21e | 6507 | |
5702da47 | 6508 | =item syscall NUMBER, LIST |
d74e8afc | 6509 | X<syscall> X<system call> |
a0d0e21e LW |
6510 | |
6511 | Calls the system call specified as the first element of the list, | |
6512 | passing the remaining elements as arguments to the system call. If | |
3b10bc60 | 6513 | unimplemented, raises an exception. The arguments are interpreted |
a0d0e21e LW |
6514 | as follows: if a given argument is numeric, the argument is passed as |
6515 | an int. If not, the pointer to the string value is passed. You are | |
6516 | responsible to make sure a string is pre-extended long enough to | |
a3cb178b | 6517 | receive any result that might be written into a string. You can't use a |
19799a22 | 6518 | string literal (or other read-only string) as an argument to C<syscall> |
a3cb178b GS |
6519 | because Perl has to assume that any string pointer might be written |
6520 | through. If your | |
a0d0e21e | 6521 | integer arguments are not literals and have never been interpreted in a |
7660c0ab | 6522 | numeric context, you may need to add C<0> to them to force them to look |
19799a22 | 6523 | like numbers. This emulates the C<syswrite> function (or vice versa): |
a0d0e21e | 6524 | |
5ed4f2ec | 6525 | require 'syscall.ph'; # may need to run h2ph |
a3cb178b GS |
6526 | $s = "hi there\n"; |
6527 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
a0d0e21e | 6528 | |
3b10bc60 | 6529 | Note that Perl supports passing of up to only 14 arguments to your syscall, |
6530 | which in practice should (usually) suffice. | |
a0d0e21e | 6531 | |
fb73857a | 6532 | Syscall returns whatever value returned by the system call it calls. |
19799a22 | 6533 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). |
7660c0ab | 6534 | Note that some system calls can legitimately return C<-1>. The proper |
fb73857a | 6535 | way to handle such calls is to assign C<$!=0;> before the call and |
7660c0ab | 6536 | check the value of C<$!> if syscall returns C<-1>. |
fb73857a | 6537 | |
6538 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
6539 | number of the read end of the pipe it creates. There is no way | |
b76cc8ba | 6540 | to retrieve the file number of the other end. You can avoid this |
19799a22 | 6541 | problem by using C<pipe> instead. |
fb73857a | 6542 | |
c07a80fd | 6543 | =item sysopen FILEHANDLE,FILENAME,MODE |
d74e8afc | 6544 | X<sysopen> |
c07a80fd | 6545 | |
6546 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
6547 | ||
6548 | Opens the file whose filename is given by FILENAME, and associates it | |
6549 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as | |
6550 | the name of the real filehandle wanted. This function calls the | |
19799a22 | 6551 | underlying operating system's C<open> function with the parameters |
c07a80fd | 6552 | FILENAME, MODE, PERMS. |
6553 | ||
6554 | The possible values and flag bits of the MODE parameter are | |
6555 | system-dependent; they are available via the standard module C<Fcntl>. | |
ea2b5ef6 JH |
6556 | See the documentation of your operating system's C<open> to see which |
6557 | values and flag bits are available. You may combine several flags | |
6558 | using the C<|>-operator. | |
6559 | ||
6560 | Some of the most common values are C<O_RDONLY> for opening the file in | |
6561 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
c188b257 | 6562 | and C<O_RDWR> for opening the file in read-write mode. |
d74e8afc | 6563 | X<O_RDONLY> X<O_RDWR> X<O_WRONLY> |
ea2b5ef6 | 6564 | |
adf5897a | 6565 | For historical reasons, some values work on almost every system |
3b10bc60 | 6566 | supported by Perl: 0 means read-only, 1 means write-only, and 2 |
adf5897a | 6567 | means read/write. We know that these values do I<not> work under |
7c5ffed3 | 6568 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
4af147f6 | 6569 | use them in new code. |
c07a80fd | 6570 | |
19799a22 | 6571 | If the file named by FILENAME does not exist and the C<open> call creates |
7660c0ab | 6572 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
5a964f20 | 6573 | PERMS specifies the permissions of the newly created file. If you omit |
19799a22 | 6574 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. |
5a964f20 | 6575 | These permission values need to be in octal, and are modified by your |
0591cd52 | 6576 | process's current C<umask>. |
d74e8afc | 6577 | X<O_CREAT> |
0591cd52 | 6578 | |
ea2b5ef6 JH |
6579 | In many systems the C<O_EXCL> flag is available for opening files in |
6580 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
c188b257 PF |
6581 | if the file already exists, sysopen() fails. C<O_EXCL> may not work |
6582 | on network filesystems, and has no effect unless the C<O_CREAT> flag | |
6583 | is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from | |
6584 | being opened if it is a symbolic link. It does not protect against | |
6585 | symbolic links in the file's path. | |
d74e8afc | 6586 | X<O_EXCL> |
c188b257 PF |
6587 | |
6588 | Sometimes you may want to truncate an already-existing file. This | |
6589 | can be done using the C<O_TRUNC> flag. The behavior of | |
6590 | C<O_TRUNC> with C<O_RDONLY> is undefined. | |
d74e8afc | 6591 | X<O_TRUNC> |
ea2b5ef6 | 6592 | |
19799a22 | 6593 | You should seldom if ever use C<0644> as argument to C<sysopen>, because |
2b5ab1e7 TC |
6594 | that takes away the user's option to have a more permissive umask. |
6595 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
6596 | on this. | |
c07a80fd | 6597 | |
4af147f6 | 6598 | Note that C<sysopen> depends on the fdopen() C library function. |
e1020413 | 6599 | On many Unix systems, fdopen() is known to fail when file descriptors |
4af147f6 CS |
6600 | exceed a certain value, typically 255. If you need more file |
6601 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
6602 | library, or perhaps using the POSIX::open() function. | |
6603 | ||
2b5ab1e7 | 6604 | See L<perlopentut> for a kinder, gentler explanation of opening files. |
28757baa | 6605 | |
a0d0e21e | 6606 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
d74e8afc | 6607 | X<sysread> |
a0d0e21e LW |
6608 | |
6609 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
6610 | ||
3874323d | 6611 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
3b10bc60 | 6612 | specified FILEHANDLE, using the read(2). It bypasses |
3874323d JH |
6613 | buffered IO, so mixing this with other kinds of reads, C<print>, |
6614 | C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the | |
6615 | perlio or stdio layers usually buffers data. Returns the number of | |
6616 | bytes actually read, C<0> at end of file, or undef if there was an | |
6617 | error (in the latter case C<$!> is also set). SCALAR will be grown or | |
6618 | shrunk so that the last byte actually read is the last byte of the | |
6619 | scalar after the read. | |
ff68c719 | 6620 | |
6621 | An OFFSET may be specified to place the read data at some place in the | |
6622 | string other than the beginning. A negative OFFSET specifies | |
9124316e JH |
6623 | placement at that many characters counting backwards from the end of |
6624 | the string. A positive OFFSET greater than the length of SCALAR | |
6625 | results in the string being padded to the required size with C<"\0"> | |
6626 | bytes before the result of the read is appended. | |
a0d0e21e | 6627 | |
2b5ab1e7 | 6628 | There is no syseof() function, which is ok, since eof() doesn't work |
80d38338 | 6629 | well on device files (like ttys) anyway. Use sysread() and check |
19799a22 | 6630 | for a return value for 0 to decide whether you're done. |
2b5ab1e7 | 6631 | |
3874323d JH |
6632 | Note that if the filehandle has been marked as C<:utf8> Unicode |
6633 | characters are read instead of bytes (the LENGTH, OFFSET, and the | |
5eadf7c5 | 6634 | return value of sysread() are in Unicode characters). |
3874323d JH |
6635 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. |
6636 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. | |
6637 | ||
137443ea | 6638 | =item sysseek FILEHANDLE,POSITION,WHENCE |
d74e8afc | 6639 | X<sysseek> X<lseek> |
137443ea | 6640 | |
3b10bc60 | 6641 | Sets FILEHANDLE's system position in bytes using |
9124316e JH |
6642 | lseek(2). FILEHANDLE may be an expression whose value gives the name |
6643 | of the filehandle. The values for WHENCE are C<0> to set the new | |
6644 | position to POSITION, C<1> to set the it to the current position plus | |
6645 | POSITION, and C<2> to set it to EOF plus POSITION (typically | |
6646 | negative). | |
6647 | ||
6648 | Note the I<in bytes>: even if the filehandle has been set to operate | |
740d4bb2 JW |
6649 | on characters (for example by using the C<:encoding(utf8)> I/O layer), |
6650 | tell() will return byte offsets, not character offsets (because | |
80d38338 | 6651 | implementing that would render sysseek() unacceptably slow). |
9124316e | 6652 | |
3874323d | 6653 | sysseek() bypasses normal buffered IO, so mixing this with reads (other |
aaa270e5 | 6654 | than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>, |
9124316e | 6655 | C<seek>, C<tell>, or C<eof> may cause confusion. |
86989e5d JH |
6656 | |
6657 | For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, | |
6658 | and C<SEEK_END> (start of the file, current position, end of the file) | |
6659 | from the Fcntl module. Use of the constants is also more portable | |
6660 | than relying on 0, 1, and 2. For example to define a "systell" function: | |
6661 | ||
5ed4f2ec | 6662 | use Fcntl 'SEEK_CUR'; |
6663 | sub systell { sysseek($_[0], 0, SEEK_CUR) } | |
8903cb82 | 6664 | |
6665 | Returns the new position, or the undefined value on failure. A position | |
19799a22 GS |
6666 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns |
6667 | true on success and false on failure, yet you can still easily determine | |
8903cb82 | 6668 | the new position. |
137443ea | 6669 | |
a0d0e21e | 6670 | =item system LIST |
d74e8afc | 6671 | X<system> X<shell> |
a0d0e21e | 6672 | |
8bf3b016 GS |
6673 | =item system PROGRAM LIST |
6674 | ||
19799a22 GS |
6675 | Does exactly the same thing as C<exec LIST>, except that a fork is |
6676 | done first, and the parent process waits for the child process to | |
80d38338 | 6677 | exit. Note that argument processing varies depending on the |
19799a22 GS |
6678 | number of arguments. If there is more than one argument in LIST, |
6679 | or if LIST is an array with more than one value, starts the program | |
6680 | given by the first element of the list with arguments given by the | |
6681 | rest of the list. If there is only one scalar argument, the argument | |
6682 | is checked for shell metacharacters, and if there are any, the | |
6683 | entire argument is passed to the system's command shell for parsing | |
6684 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
6685 | platforms). If there are no shell metacharacters in the argument, | |
6686 | it is split into words and passed directly to C<execvp>, which is | |
6687 | more efficient. | |
6688 | ||
0f897271 GS |
6689 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
6690 | output before any operation that may do a fork, but this may not be | |
6691 | supported on some platforms (see L<perlport>). To be safe, you may need | |
6692 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
6693 | of C<IO::Handle> on any open handles. | |
a2008d6d | 6694 | |
9d6eb86e | 6695 | The return value is the exit status of the program as returned by the |
25379e53 RGS |
6696 | C<wait> call. To get the actual exit value, shift right by eight (see |
6697 | below). See also L</exec>. This is I<not> what you want to use to capture | |
54310121 | 6698 | the output from a command, for that you should use merely backticks or |
d5a9bfb0 | 6699 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 |
25379e53 RGS |
6700 | indicates a failure to start the program or an error of the wait(2) system |
6701 | call (inspect $! for the reason). | |
a0d0e21e | 6702 | |
1af1c0d6 JV |
6703 | If you'd like to make C<system> (and many other bits of Perl) die on error, |
6704 | have a look at the L<autodie> pragma. | |
6705 | ||
19799a22 GS |
6706 | Like C<exec>, C<system> allows you to lie to a program about its name if |
6707 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
8bf3b016 | 6708 | |
4c2e8b59 BD |
6709 | Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of |
6710 | C<system>, if you expect your program to terminate on receipt of these | |
6711 | signals you will need to arrange to do so yourself based on the return | |
6712 | value. | |
28757baa | 6713 | |
6714 | @args = ("command", "arg1", "arg2"); | |
54310121 | 6715 | system(@args) == 0 |
a9a5a0dc | 6716 | or die "system @args failed: $?" |
28757baa | 6717 | |
95da743b | 6718 | If you'd like to manually inspect C<system>'s failure, you can check all |
1af1c0d6 | 6719 | possible failure modes by inspecting C<$?> like this: |
28757baa | 6720 | |
4ef107a6 | 6721 | if ($? == -1) { |
a9a5a0dc | 6722 | print "failed to execute: $!\n"; |
4ef107a6 DM |
6723 | } |
6724 | elsif ($? & 127) { | |
a9a5a0dc VP |
6725 | printf "child died with signal %d, %s coredump\n", |
6726 | ($? & 127), ($? & 128) ? 'with' : 'without'; | |
4ef107a6 DM |
6727 | } |
6728 | else { | |
a9a5a0dc | 6729 | printf "child exited with value %d\n", $? >> 8; |
4ef107a6 DM |
6730 | } |
6731 | ||
3b10bc60 | 6732 | Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}> |
6733 | with the C<W*()> calls from the POSIX module. | |
9d6eb86e | 6734 | |
3b10bc60 | 6735 | When C<system>'s arguments are executed indirectly by the shell, |
6736 | results and return codes are subject to its quirks. | |
c8db1d39 | 6737 | See L<perlop/"`STRING`"> and L</exec> for details. |
bb32b41a | 6738 | |
a0d0e21e | 6739 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
d74e8afc | 6740 | X<syswrite> |
a0d0e21e LW |
6741 | |
6742 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
6743 | ||
145d37e2 GA |
6744 | =item syswrite FILEHANDLE,SCALAR |
6745 | ||
3874323d | 6746 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
3b10bc60 | 6747 | specified FILEHANDLE, using write(2). If LENGTH is |
3874323d | 6748 | not specified, writes whole SCALAR. It bypasses buffered IO, so |
9124316e | 6749 | mixing this with reads (other than C<sysread())>, C<print>, C<write>, |
3874323d JH |
6750 | C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and |
6751 | stdio layers usually buffers data. Returns the number of bytes | |
6752 | actually written, or C<undef> if there was an error (in this case the | |
6753 | errno variable C<$!> is also set). If the LENGTH is greater than the | |
3b10bc60 | 6754 | data available in the SCALAR after the OFFSET, only as much data as is |
3874323d | 6755 | available will be written. |
ff68c719 | 6756 | |
6757 | An OFFSET may be specified to write the data from some part of the | |
6758 | string other than the beginning. A negative OFFSET specifies writing | |
9124316e | 6759 | that many characters counting backwards from the end of the string. |
3b10bc60 | 6760 | If SCALAR is of length zero, you can only use an OFFSET of 0. |
9124316e | 6761 | |
3b10bc60 | 6762 | B<Warning>: If the filehandle is marked C<:utf8>, Unicode characters |
6763 | encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and | |
6764 | return value of syswrite() are in (UTF-8 encoded Unicode) characters. | |
3874323d JH |
6765 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. |
6766 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. | |
a0d0e21e LW |
6767 | |
6768 | =item tell FILEHANDLE | |
d74e8afc | 6769 | X<tell> |
a0d0e21e LW |
6770 | |
6771 | =item tell | |
6772 | ||
9124316e JH |
6773 | Returns the current position I<in bytes> for FILEHANDLE, or -1 on |
6774 | error. FILEHANDLE may be an expression whose value gives the name of | |
6775 | the actual filehandle. If FILEHANDLE is omitted, assumes the file | |
6776 | last read. | |
6777 | ||
6778 | Note the I<in bytes>: even if the filehandle has been set to | |
740d4bb2 JW |
6779 | operate on characters (for example by using the C<:encoding(utf8)> open |
6780 | layer), tell() will return byte offsets, not character offsets (because | |
6781 | that would render seek() and tell() rather slow). | |
2b5ab1e7 | 6782 | |
cfd73201 JH |
6783 | The return value of tell() for the standard streams like the STDIN |
6784 | depends on the operating system: it may return -1 or something else. | |
6785 | tell() on pipes, fifos, and sockets usually returns -1. | |
6786 | ||
19799a22 | 6787 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. |
a0d0e21e | 6788 | |
3b10bc60 | 6789 | Do not use tell() (or other buffered I/O operations) on a filehandle |
59c9df15 NIS |
6790 | that has been manipulated by sysread(), syswrite() or sysseek(). |
6791 | Those functions ignore the buffering, while tell() does not. | |
9124316e | 6792 | |
a0d0e21e | 6793 | =item telldir DIRHANDLE |
d74e8afc | 6794 | X<telldir> |
a0d0e21e | 6795 | |
19799a22 GS |
6796 | Returns the current position of the C<readdir> routines on DIRHANDLE. |
6797 | Value may be given to C<seekdir> to access a particular location in a | |
cf264981 SP |
6798 | directory. C<telldir> has the same caveats about possible directory |
6799 | compaction as the corresponding system library routine. | |
a0d0e21e | 6800 | |
4633a7c4 | 6801 | =item tie VARIABLE,CLASSNAME,LIST |
d74e8afc | 6802 | X<tie> |
a0d0e21e | 6803 | |
4633a7c4 LW |
6804 | This function binds a variable to a package class that will provide the |
6805 | implementation for the variable. VARIABLE is the name of the variable | |
6806 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
19799a22 | 6807 | of correct type. Any additional arguments are passed to the C<new> |
8a059744 GS |
6808 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
6809 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
19799a22 GS |
6810 | to the C<dbm_open()> function of C. The object returned by the C<new> |
6811 | method is also returned by the C<tie> function, which would be useful | |
8a059744 | 6812 | if you want to access other methods in CLASSNAME. |
a0d0e21e | 6813 | |
19799a22 | 6814 | Note that functions such as C<keys> and C<values> may return huge lists |
1d2dff63 | 6815 | when used on large objects, like DBM files. You may prefer to use the |
19799a22 | 6816 | C<each> function to iterate over such. Example: |
a0d0e21e LW |
6817 | |
6818 | # print out history file offsets | |
4633a7c4 | 6819 | use NDBM_File; |
da0045b7 | 6820 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
a0d0e21e | 6821 | while (($key,$val) = each %HIST) { |
a9a5a0dc | 6822 | print $key, ' = ', unpack('L',$val), "\n"; |
a0d0e21e LW |
6823 | } |
6824 | untie(%HIST); | |
6825 | ||
aa689395 | 6826 | A class implementing a hash should have the following methods: |
a0d0e21e | 6827 | |
4633a7c4 | 6828 | TIEHASH classname, LIST |
a0d0e21e LW |
6829 | FETCH this, key |
6830 | STORE this, key, value | |
6831 | DELETE this, key | |
8a059744 | 6832 | CLEAR this |
a0d0e21e LW |
6833 | EXISTS this, key |
6834 | FIRSTKEY this | |
6835 | NEXTKEY this, lastkey | |
a3bcc51e | 6836 | SCALAR this |
8a059744 | 6837 | DESTROY this |
d7da42b7 | 6838 | UNTIE this |
a0d0e21e | 6839 | |
4633a7c4 | 6840 | A class implementing an ordinary array should have the following methods: |
a0d0e21e | 6841 | |
4633a7c4 | 6842 | TIEARRAY classname, LIST |
a0d0e21e LW |
6843 | FETCH this, key |
6844 | STORE this, key, value | |
8a059744 GS |
6845 | FETCHSIZE this |
6846 | STORESIZE this, count | |
6847 | CLEAR this | |
6848 | PUSH this, LIST | |
6849 | POP this | |
6850 | SHIFT this | |
6851 | UNSHIFT this, LIST | |
6852 | SPLICE this, offset, length, LIST | |
6853 | EXTEND this, count | |
6854 | DESTROY this | |
d7da42b7 | 6855 | UNTIE this |
8a059744 | 6856 | |
3b10bc60 | 6857 | A class implementing a filehandle should have the following methods: |
8a059744 GS |
6858 | |
6859 | TIEHANDLE classname, LIST | |
6860 | READ this, scalar, length, offset | |
6861 | READLINE this | |
6862 | GETC this | |
6863 | WRITE this, scalar, length, offset | |
6864 | PRINT this, LIST | |
6865 | PRINTF this, format, LIST | |
e08f2115 GA |
6866 | BINMODE this |
6867 | EOF this | |
6868 | FILENO this | |
6869 | SEEK this, position, whence | |
6870 | TELL this | |
6871 | OPEN this, mode, LIST | |
8a059744 GS |
6872 | CLOSE this |
6873 | DESTROY this | |
d7da42b7 | 6874 | UNTIE this |
a0d0e21e | 6875 | |
4633a7c4 | 6876 | A class implementing a scalar should have the following methods: |
a0d0e21e | 6877 | |
4633a7c4 | 6878 | TIESCALAR classname, LIST |
54310121 | 6879 | FETCH this, |
a0d0e21e | 6880 | STORE this, value |
8a059744 | 6881 | DESTROY this |
d7da42b7 | 6882 | UNTIE this |
8a059744 GS |
6883 | |
6884 | Not all methods indicated above need be implemented. See L<perltie>, | |
2b5ab1e7 | 6885 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. |
a0d0e21e | 6886 | |
3b10bc60 | 6887 | Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module |
6888 | for you; you need to do that explicitly yourself. See L<DB_File> | |
19799a22 | 6889 | or the F<Config> module for interesting C<tie> implementations. |
4633a7c4 | 6890 | |
b687b08b | 6891 | For further details see L<perltie>, L<"tied VARIABLE">. |
cc6b7395 | 6892 | |
f3cbc334 | 6893 | =item tied VARIABLE |
d74e8afc | 6894 | X<tied> |
f3cbc334 RS |
6895 | |
6896 | Returns a reference to the object underlying VARIABLE (the same value | |
19799a22 | 6897 | that was originally returned by the C<tie> call that bound the variable |
f3cbc334 RS |
6898 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
6899 | package. | |
6900 | ||
a0d0e21e | 6901 | =item time |
d74e8afc | 6902 | X<time> X<epoch> |
a0d0e21e | 6903 | |
da0045b7 | 6904 | Returns the number of non-leap seconds since whatever time the system |
ef4d88db NC |
6905 | considers to be the epoch, suitable for feeding to C<gmtime> and |
6906 | C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970; | |
6907 | a prominent exception being Mac OS Classic which uses 00:00:00, January 1, | |
6908 | 1904 in the current local time zone for its epoch. | |
a0d0e21e | 6909 | |
68f8bed4 | 6910 | For measuring time in better granularity than one second, |
435fbc73 | 6911 | you may use either the L<Time::HiRes> module (from CPAN, and starting from |
c5f9c75a RGS |
6912 | Perl 5.8 part of the standard distribution), or if you have |
6913 | gettimeofday(2), you may be able to use the C<syscall> interface of Perl. | |
6914 | See L<perlfaq8> for details. | |
68f8bed4 | 6915 | |
435fbc73 GS |
6916 | For date and time processing look at the many related modules on CPAN. |
6917 | For a comprehensive date and time representation look at the | |
6918 | L<DateTime> module. | |
6919 | ||
a0d0e21e | 6920 | =item times |
d74e8afc | 6921 | X<times> |
a0d0e21e | 6922 | |
1d2dff63 | 6923 | Returns a four-element list giving the user and system times, in |
a0d0e21e LW |
6924 | seconds, for this process and the children of this process. |
6925 | ||
6926 | ($user,$system,$cuser,$csystem) = times; | |
6927 | ||
dc19f4fb MJD |
6928 | In scalar context, C<times> returns C<$user>. |
6929 | ||
3b10bc60 | 6930 | Children's times are only included for terminated children. |
2a958fe2 | 6931 | |
a0d0e21e LW |
6932 | =item tr/// |
6933 | ||
9f4b9cd0 SP |
6934 | The transliteration operator. Same as C<y///>. See |
6935 | L<perlop/"Quote and Quote-like Operators">. | |
a0d0e21e LW |
6936 | |
6937 | =item truncate FILEHANDLE,LENGTH | |
d74e8afc | 6938 | X<truncate> |
a0d0e21e LW |
6939 | |
6940 | =item truncate EXPR,LENGTH | |
6941 | ||
6942 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
3b10bc60 | 6943 | specified length. Raises an exception if truncate isn't implemented |
19799a22 | 6944 | on your system. Returns true if successful, the undefined value |
a3cb178b | 6945 | otherwise. |
a0d0e21e | 6946 | |
90ddc76f MS |
6947 | The behavior is undefined if LENGTH is greater than the length of the |
6948 | file. | |
6949 | ||
8577f58c RK |
6950 | The position in the file of FILEHANDLE is left unchanged. You may want to |
6951 | call L<seek> before writing to the file. | |
6952 | ||
a0d0e21e | 6953 | =item uc EXPR |
d74e8afc | 6954 | X<uc> X<uppercase> X<toupper> |
a0d0e21e | 6955 | |
54310121 | 6956 | =item uc |
bbce6d69 | 6957 | |
a0d0e21e | 6958 | Returns an uppercased version of EXPR. This is the internal function |
3980dc9c | 6959 | implementing the C<\U> escape in double-quoted strings. |
983ffd37 | 6960 | It does not attempt to do titlecase mapping on initial letters. See |
3980dc9c | 6961 | L</ucfirst> for that. |
a0d0e21e | 6962 | |
7660c0ab | 6963 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 6964 | |
3980dc9c KW |
6965 | This function behaves the same way under various pragma, such as in a locale, |
6966 | as L</lc> does. | |
6967 | ||
a0d0e21e | 6968 | =item ucfirst EXPR |
d74e8afc | 6969 | X<ucfirst> X<uppercase> |
a0d0e21e | 6970 | |
54310121 | 6971 | =item ucfirst |
bbce6d69 | 6972 | |
ad0029c4 JH |
6973 | Returns the value of EXPR with the first character in uppercase |
6974 | (titlecase in Unicode). This is the internal function implementing | |
3980dc9c | 6975 | the C<\u> escape in double-quoted strings. |
a0d0e21e | 6976 | |
7660c0ab | 6977 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 6978 | |
3980dc9c KW |
6979 | This function behaves the same way under various pragma, such as in a locale, |
6980 | as L</lc> does. | |
6981 | ||
a0d0e21e | 6982 | =item umask EXPR |
d74e8afc | 6983 | X<umask> |
a0d0e21e LW |
6984 | |
6985 | =item umask | |
6986 | ||
2f9daede | 6987 | Sets the umask for the process to EXPR and returns the previous value. |
eec2d3df GS |
6988 | If EXPR is omitted, merely returns the current umask. |
6989 | ||
0591cd52 NT |
6990 | The Unix permission C<rwxr-x---> is represented as three sets of three |
6991 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
b5a41e52 | 6992 | and isn't one of the digits). The C<umask> value is such a number |
0591cd52 NT |
6993 | representing disabled permissions bits. The permission (or "mode") |
6994 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
6995 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
6996 | if your umask is C<0022> then the file will actually be created with | |
6997 | permissions C<0755>. If your C<umask> were C<0027> (group can't | |
6998 | write; others can't read, write, or execute), then passing | |
19799a22 | 6999 | C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ |
0591cd52 NT |
7000 | 027> is C<0640>). |
7001 | ||
7002 | Here's some advice: supply a creation mode of C<0666> for regular | |
19799a22 GS |
7003 | files (in C<sysopen>) and one of C<0777> for directories (in |
7004 | C<mkdir>) and executable files. This gives users the freedom of | |
0591cd52 NT |
7005 | choice: if they want protected files, they might choose process umasks |
7006 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
7007 | Programs should rarely if ever make policy decisions better left to | |
7008 | the user. The exception to this is when writing files that should be | |
7009 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
7010 | so on. | |
7011 | ||
f86cebdf | 7012 | If umask(2) is not implemented on your system and you are trying to |
3b10bc60 | 7013 | restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>), |
7014 | raises an exception. If umask(2) is not implemented and you are | |
eec2d3df GS |
7015 | not trying to restrict access for yourself, returns C<undef>. |
7016 | ||
7017 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
7018 | string of octal digits. See also L</oct>, if all you have is a string. | |
a0d0e21e LW |
7019 | |
7020 | =item undef EXPR | |
d74e8afc | 7021 | X<undef> X<undefine> |
a0d0e21e LW |
7022 | |
7023 | =item undef | |
7024 | ||
54310121 | 7025 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
19799a22 | 7026 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine |
3b10bc60 | 7027 | (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}> |
20408e3c | 7028 | will probably not do what you expect on most predefined variables or |
3b10bc60 | 7029 | DBM list values, so don't do that; see L<delete>. Always returns the |
20408e3c GS |
7030 | undefined value. You can omit the EXPR, in which case nothing is |
7031 | undefined, but you still get an undefined value that you could, for | |
3b10bc60 | 7032 | instance, return from a subroutine, assign to a variable, or pass as a |
20408e3c | 7033 | parameter. Examples: |
a0d0e21e LW |
7034 | |
7035 | undef $foo; | |
f86cebdf | 7036 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
a0d0e21e | 7037 | undef @ary; |
aa689395 | 7038 | undef %hash; |
a0d0e21e | 7039 | undef &mysub; |
20408e3c | 7040 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
54310121 | 7041 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
2f9daede TP |
7042 | select undef, undef, undef, 0.25; |
7043 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
a0d0e21e | 7044 | |
5a964f20 TC |
7045 | Note that this is a unary operator, not a list operator. |
7046 | ||
a0d0e21e | 7047 | =item unlink LIST |
dd184578 | 7048 | X<unlink> X<delete> X<remove> X<rm> X<del> |
a0d0e21e | 7049 | |
54310121 | 7050 | =item unlink |
bbce6d69 | 7051 | |
40ea6f68 | 7052 | Deletes a list of files. On success, it returns the number of files |
7053 | it successfully deleted. On failure, it returns false and sets C<$!> | |
7054 | (errno): | |
a0d0e21e | 7055 | |
40ea6f68 | 7056 | my $unlinked = unlink 'a', 'b', 'c'; |
a0d0e21e | 7057 | unlink @goners; |
40ea6f68 | 7058 | unlink glob "*.bak"; |
a0d0e21e | 7059 | |
40ea6f68 | 7060 | On error, C<unlink> will not tell you which files it could not remove. |
734c9e01 | 7061 | If you want to know which files you could not remove, try them one |
40ea6f68 | 7062 | at a time: |
a0d0e21e | 7063 | |
40ea6f68 | 7064 | foreach my $file ( @goners ) { |
7065 | unlink $file or warn "Could not unlink $file: $!"; | |
3b10bc60 | 7066 | } |
40ea6f68 | 7067 | |
7068 | Note: C<unlink> will not attempt to delete directories unless you are | |
7069 | superuser and the B<-U> flag is supplied to Perl. Even if these | |
7070 | conditions are met, be warned that unlinking a directory can inflict | |
7071 | damage on your filesystem. Finally, using C<unlink> on directories is | |
7072 | not supported on many operating systems. Use C<rmdir> instead. | |
7073 | ||
7074 | If LIST is omitted, C<unlink> uses C<$_>. | |
bbce6d69 | 7075 | |
a0d0e21e | 7076 | =item unpack TEMPLATE,EXPR |
d74e8afc | 7077 | X<unpack> |
a0d0e21e | 7078 | |
13dcffc6 CS |
7079 | =item unpack TEMPLATE |
7080 | ||
19799a22 | 7081 | C<unpack> does the reverse of C<pack>: it takes a string |
2b6c5635 | 7082 | and expands it out into a list of values. |
19799a22 | 7083 | (In scalar context, it returns merely the first value produced.) |
2b6c5635 | 7084 | |
eae68503 | 7085 | If EXPR is omitted, unpacks the C<$_> string. |
3980dc9c | 7086 | See L<perlpacktut> for an introduction to this function. |
13dcffc6 | 7087 | |
2b6c5635 GS |
7088 | The string is broken into chunks described by the TEMPLATE. Each chunk |
7089 | is converted separately to a value. Typically, either the string is a result | |
f337b084 | 7090 | of C<pack>, or the characters of the string represent a C structure of some |
2b6c5635 GS |
7091 | kind. |
7092 | ||
19799a22 | 7093 | The TEMPLATE has the same format as in the C<pack> function. |
a0d0e21e LW |
7094 | Here's a subroutine that does substring: |
7095 | ||
7096 | sub substr { | |
5ed4f2ec | 7097 | my($what,$where,$howmuch) = @_; |
7098 | unpack("x$where a$howmuch", $what); | |
a0d0e21e LW |
7099 | } |
7100 | ||
7101 | and then there's | |
7102 | ||
f337b084 | 7103 | sub ordinal { unpack("W",$_[0]); } # same as ord() |
a0d0e21e | 7104 | |
2b6c5635 | 7105 | In addition to fields allowed in pack(), you may prefix a field with |
61eff3bc JH |
7106 | a %<number> to indicate that |
7107 | you want a <number>-bit checksum of the items instead of the items | |
2b6c5635 GS |
7108 | themselves. Default is a 16-bit checksum. Checksum is calculated by |
7109 | summing numeric values of expanded values (for string fields the sum of | |
7110 | C<ord($char)> is taken, for bit fields the sum of zeroes and ones). | |
7111 | ||
7112 | For example, the following | |
a0d0e21e LW |
7113 | computes the same number as the System V sum program: |
7114 | ||
19799a22 | 7115 | $checksum = do { |
5ed4f2ec | 7116 | local $/; # slurp! |
7117 | unpack("%32W*",<>) % 65535; | |
19799a22 | 7118 | }; |
a0d0e21e LW |
7119 | |
7120 | The following efficiently counts the number of set bits in a bit vector: | |
7121 | ||
7122 | $setbits = unpack("%32b*", $selectmask); | |
7123 | ||
951ba7fe | 7124 | The C<p> and C<P> formats should be used with care. Since Perl |
3160c391 GS |
7125 | has no way of checking whether the value passed to C<unpack()> |
7126 | corresponds to a valid memory location, passing a pointer value that's | |
7127 | not known to be valid is likely to have disastrous consequences. | |
7128 | ||
49704364 WL |
7129 | If there are more pack codes or if the repeat count of a field or a group |
7130 | is larger than what the remainder of the input string allows, the result | |
3b10bc60 | 7131 | is not well defined: the repeat count may be decreased, or |
7132 | C<unpack()> may produce empty strings or zeros, or it may raise an exception. | |
7133 | If the input string is longer than one described by the TEMPLATE, | |
7134 | the remainder of that input string is ignored. | |
2b6c5635 | 7135 | |
851646ae | 7136 | See L</pack> for more examples and notes. |
5a929a98 | 7137 | |
98293880 | 7138 | =item untie VARIABLE |
d74e8afc | 7139 | X<untie> |
98293880 | 7140 | |
19799a22 | 7141 | Breaks the binding between a variable and a package. (See C<tie>.) |
1188453a | 7142 | Has no effect if the variable is not tied. |
98293880 | 7143 | |
a0d0e21e | 7144 | =item unshift ARRAY,LIST |
d74e8afc | 7145 | X<unshift> |
a0d0e21e | 7146 | |
19799a22 | 7147 | Does the opposite of a C<shift>. Or the opposite of a C<push>, |
a0d0e21e LW |
7148 | depending on how you look at it. Prepends list to the front of the |
7149 | array, and returns the new number of elements in the array. | |
7150 | ||
76e4c2bb | 7151 | unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; |
a0d0e21e LW |
7152 | |
7153 | Note the LIST is prepended whole, not one element at a time, so the | |
19799a22 | 7154 | prepended elements stay in the same order. Use C<reverse> to do the |
a0d0e21e LW |
7155 | reverse. |
7156 | ||
f6c8478c | 7157 | =item use Module VERSION LIST |
d74e8afc | 7158 | X<use> X<module> X<import> |
f6c8478c GS |
7159 | |
7160 | =item use Module VERSION | |
7161 | ||
a0d0e21e LW |
7162 | =item use Module LIST |
7163 | ||
7164 | =item use Module | |
7165 | ||
da0045b7 | 7166 | =item use VERSION |
7167 | ||
a0d0e21e LW |
7168 | Imports some semantics into the current package from the named module, |
7169 | generally by aliasing certain subroutine or variable names into your | |
7170 | package. It is exactly equivalent to | |
7171 | ||
6d9d0573 | 7172 | BEGIN { require Module; Module->import( LIST ); } |
a0d0e21e | 7173 | |
54310121 | 7174 | except that Module I<must> be a bareword. |
da0045b7 | 7175 | |
bd12309b DG |
7176 | In the peculiar C<use VERSION> form, VERSION may be either a positive |
7177 | decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string | |
7178 | of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An | |
3b10bc60 | 7179 | exception is raised if VERSION is greater than the version of the |
c986422f RGS |
7180 | current Perl interpreter; Perl will not attempt to parse the rest of the |
7181 | file. Compare with L</require>, which can do a similar check at run time. | |
7182 | Symmetrically, C<no VERSION> allows you to specify that you want a version | |
3b10bc60 | 7183 | of Perl older than the specified one. |
3b825e41 RK |
7184 | |
7185 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
7186 | avoided, because it leads to misleading error messages under earlier | |
2e8342de RGS |
7187 | versions of Perl (that is, prior to 5.6.0) that do not support this |
7188 | syntax. The equivalent numeric version should be used instead. | |
fbc891ce | 7189 | |
5ed4f2ec | 7190 | use v5.6.1; # compile time version check |
7191 | use 5.6.1; # ditto | |
7192 | use 5.006_001; # ditto; preferred for backwards compatibility | |
16070b82 GS |
7193 | |
7194 | This is often useful if you need to check the current Perl version before | |
2e8342de RGS |
7195 | C<use>ing library modules that won't work with older versions of Perl. |
7196 | (We try not to do this more than we have to.) | |
da0045b7 | 7197 | |
3b10bc60 | 7198 | Also, if the specified Perl version is greater than or equal to 5.9.5, |
c986422f RGS |
7199 | C<use VERSION> will also load the C<feature> pragma and enable all |
7200 | features available in the requested version. See L<feature>. | |
3b10bc60 | 7201 | Similarly, if the specified Perl version is greater than or equal to |
5cc917d6 RGS |
7202 | 5.11.0, strictures are enabled lexically as with C<use strict> (except |
7203 | that the F<strict.pm> file is not actually loaded). | |
7dfde25d | 7204 | |
19799a22 | 7205 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The |
7660c0ab | 7206 | C<require> makes sure the module is loaded into memory if it hasn't been |
3b10bc60 | 7207 | yet. The C<import> is not a builtin; it's just an ordinary static method |
19799a22 | 7208 | call into the C<Module> package to tell the module to import the list of |
a0d0e21e | 7209 | features back into the current package. The module can implement its |
19799a22 GS |
7210 | C<import> method any way it likes, though most modules just choose to |
7211 | derive their C<import> method via inheritance from the C<Exporter> class that | |
7212 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
593b9c14 YST |
7213 | method can be found then the call is skipped, even if there is an AUTOLOAD |
7214 | method. | |
cb1a09d0 | 7215 | |
31686daf JP |
7216 | If you do not want to call the package's C<import> method (for instance, |
7217 | to stop your namespace from being altered), explicitly supply the empty list: | |
cb1a09d0 AD |
7218 | |
7219 | use Module (); | |
7220 | ||
7221 | That is exactly equivalent to | |
7222 | ||
5a964f20 | 7223 | BEGIN { require Module } |
a0d0e21e | 7224 | |
da0045b7 | 7225 | If the VERSION argument is present between Module and LIST, then the |
71be2cbc | 7226 | C<use> will call the VERSION method in class Module with the given |
7227 | version as an argument. The default VERSION method, inherited from | |
44dcb63b | 7228 | the UNIVERSAL class, croaks if the given version is larger than the |
b76cc8ba | 7229 | value of the variable C<$Module::VERSION>. |
f6c8478c GS |
7230 | |
7231 | Again, there is a distinction between omitting LIST (C<import> called | |
7232 | with no arguments) and an explicit empty LIST C<()> (C<import> not | |
7233 | called). Note that there is no comma after VERSION! | |
da0045b7 | 7234 | |
a0d0e21e LW |
7235 | Because this is a wide-open interface, pragmas (compiler directives) |
7236 | are also implemented this way. Currently implemented pragmas are: | |
7237 | ||
f3798619 | 7238 | use constant; |
4633a7c4 | 7239 | use diagnostics; |
f3798619 | 7240 | use integer; |
4438c4b7 JH |
7241 | use sigtrap qw(SEGV BUS); |
7242 | use strict qw(subs vars refs); | |
7243 | use subs qw(afunc blurfl); | |
7244 | use warnings qw(all); | |
58c7fc7c | 7245 | use sort qw(stable _quicksort _mergesort); |
a0d0e21e | 7246 | |
19799a22 | 7247 | Some of these pseudo-modules import semantics into the current |
5a964f20 TC |
7248 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
7249 | which import symbols into the current package (which are effective | |
7250 | through the end of the file). | |
a0d0e21e | 7251 | |
c362798e Z |
7252 | Because C<use> takes effect at compile time, it doesn't respect the |
7253 | ordinary flow control of the code being compiled. In particular, putting | |
7254 | a C<use> inside the false branch of a conditional doesn't prevent it | |
3b10bc60 | 7255 | from being processed. If a module or pragma only needs to be loaded |
c362798e Z |
7256 | conditionally, this can be done using the L<if> pragma: |
7257 | ||
7258 | use if $] < 5.008, "utf8"; | |
7259 | use if WANT_WARNINGS, warnings => qw(all); | |
7260 | ||
19799a22 GS |
7261 | There's a corresponding C<no> command that unimports meanings imported |
7262 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. | |
80d38338 TC |
7263 | It behaves just as C<import> does with VERSION, an omitted or empty LIST, |
7264 | or no unimport method being found. | |
a0d0e21e LW |
7265 | |
7266 | no integer; | |
7267 | no strict 'refs'; | |
4438c4b7 | 7268 | no warnings; |
a0d0e21e | 7269 | |
ac634a9a | 7270 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> |
3b10bc60 | 7271 | for the C<-M> and C<-m> command-line options to Perl that give C<use> |
31686daf | 7272 | functionality from the command-line. |
a0d0e21e LW |
7273 | |
7274 | =item utime LIST | |
d74e8afc | 7275 | X<utime> |
a0d0e21e LW |
7276 | |
7277 | Changes the access and modification times on each file of a list of | |
7278 | files. The first two elements of the list must be the NUMERICAL access | |
7279 | and modification times, in that order. Returns the number of files | |
46cdf678 | 7280 | successfully changed. The inode change time of each file is set |
4bc2a53d | 7281 | to the current time. For example, this code has the same effect as the |
a4142048 WL |
7282 | Unix touch(1) command when the files I<already exist> and belong to |
7283 | the user running the program: | |
a0d0e21e LW |
7284 | |
7285 | #!/usr/bin/perl | |
2c21a326 GA |
7286 | $atime = $mtime = time; |
7287 | utime $atime, $mtime, @ARGV; | |
4bc2a53d | 7288 | |
3b10bc60 | 7289 | Since Perl 5.7.2, if the first two elements of the list are C<undef>, |
7290 | the utime(2) syscall from your C library is called with a null second | |
4bc2a53d | 7291 | argument. On most systems, this will set the file's access and |
80d38338 | 7292 | modification times to the current time (i.e., equivalent to the example |
3b10bc60 | 7293 | above) and will work even on files you don't own provided you have write |
a4142048 | 7294 | permission: |
c6f7b413 | 7295 | |
3b10bc60 | 7296 | for $file (@ARGV) { |
7297 | utime(undef, undef, $file) | |
7298 | || warn "couldn't touch $file: $!"; | |
7299 | } | |
c6f7b413 | 7300 | |
2c21a326 GA |
7301 | Under NFS this will use the time of the NFS server, not the time of |
7302 | the local machine. If there is a time synchronization problem, the | |
7303 | NFS server and local machine will have different times. The Unix | |
7304 | touch(1) command will in fact normally use this form instead of the | |
7305 | one shown in the first example. | |
7306 | ||
3b10bc60 | 7307 | Passing only one of the first two elements as C<undef> is |
7308 | equivalent to passing a 0 and will not have the effect | |
7309 | described when both are C<undef>. This also triggers an | |
2c21a326 GA |
7310 | uninitialized warning. |
7311 | ||
3b10bc60 | 7312 | On systems that support futimes(2), you may pass filehandles among the |
7313 | files. On systems that don't support futimes(2), passing filehandles raises | |
7314 | an exception. Filehandles must be passed as globs or glob references to be | |
7315 | recognized; barewords are considered filenames. | |
e96b369d | 7316 | |
aa689395 | 7317 | =item values HASH |
d74e8afc | 7318 | X<values> |
a0d0e21e | 7319 | |
aeedbbed NC |
7320 | =item values ARRAY |
7321 | ||
7322 | Returns a list consisting of all the values of the named hash, or the values | |
7323 | of an array. (In a scalar context, returns the number of values.) | |
504f80c1 JH |
7324 | |
7325 | The values are returned in an apparently random order. The actual | |
3b10bc60 | 7326 | random order is subject to change in future versions of Perl, but it |
504f80c1 | 7327 | is guaranteed to be the same order as either the C<keys> or C<each> |
4546b9e6 JH |
7328 | function would produce on the same (unmodified) hash. Since Perl |
7329 | 5.8.1 the ordering is different even between different runs of Perl | |
7330 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). | |
504f80c1 | 7331 | |
aeedbbed NC |
7332 | As a side effect, calling values() resets the HASH or ARRAY's internal |
7333 | iterator, | |
2f65b2f0 | 7334 | see L</each>. (In particular, calling values() in void context resets |
aeedbbed | 7335 | the iterator with no other overhead. Apart from resetting the iterator, |
80d38338 | 7336 | C<values @array> in list context is the same as plain C<@array>. |
aeedbbed NC |
7337 | We recommend that you use void context C<keys @array> for this, but reasoned |
7338 | that it taking C<values @array> out would require more documentation than | |
7339 | leaving it in.) | |
7340 | ||
ab192400 | 7341 | |
8ea1e5d4 GS |
7342 | Note that the values are not copied, which means modifying them will |
7343 | modify the contents of the hash: | |
2b5ab1e7 | 7344 | |
5ed4f2ec | 7345 | for (values %hash) { s/foo/bar/g } # modifies %hash values |
8ea1e5d4 | 7346 | for (@hash{keys %hash}) { s/foo/bar/g } # same |
2b5ab1e7 | 7347 | |
19799a22 | 7348 | See also C<keys>, C<each>, and C<sort>. |
a0d0e21e LW |
7349 | |
7350 | =item vec EXPR,OFFSET,BITS | |
d74e8afc | 7351 | X<vec> X<bit> X<bit vector> |
a0d0e21e | 7352 | |
e69129f1 GS |
7353 | Treats the string in EXPR as a bit vector made up of elements of |
7354 | width BITS, and returns the value of the element specified by OFFSET | |
7355 | as an unsigned integer. BITS therefore specifies the number of bits | |
7356 | that are reserved for each element in the bit vector. This must | |
7357 | be a power of two from 1 to 32 (or 64, if your platform supports | |
7358 | that). | |
c5a0f51a | 7359 | |
b76cc8ba | 7360 | If BITS is 8, "elements" coincide with bytes of the input string. |
c73032f5 IZ |
7361 | |
7362 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
7363 | of size BITS/8, and each group is converted to a number as with | |
b1866b2d | 7364 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously |
c73032f5 IZ |
7365 | for BITS==64). See L<"pack"> for details. |
7366 | ||
7367 | If bits is 4 or less, the string is broken into bytes, then the bits | |
7368 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
7369 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
7370 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
7371 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
7372 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
7373 | ||
81e118e0 JH |
7374 | C<vec> may also be assigned to, in which case parentheses are needed |
7375 | to give the expression the correct precedence as in | |
22dc801b | 7376 | |
7377 | vec($image, $max_x * $x + $y, 8) = 3; | |
a0d0e21e | 7378 | |
fe58ced6 MG |
7379 | If the selected element is outside the string, the value 0 is returned. |
7380 | If an element off the end of the string is written to, Perl will first | |
7381 | extend the string with sufficiently many zero bytes. It is an error | |
80d38338 | 7382 | to try to write off the beginning of the string (i.e., negative OFFSET). |
fac70343 | 7383 | |
2575c402 JW |
7384 | If the string happens to be encoded as UTF-8 internally (and thus has |
7385 | the UTF8 flag set), this is ignored by C<vec>, and it operates on the | |
7386 | internal byte string, not the conceptual character string, even if you | |
7387 | only have characters with values less than 256. | |
246fae53 | 7388 | |
fac70343 GS |
7389 | Strings created with C<vec> can also be manipulated with the logical |
7390 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
7391 | vector operation is desired when both operands are strings. | |
c5a0f51a | 7392 | See L<perlop/"Bitwise String Operators">. |
a0d0e21e | 7393 | |
7660c0ab | 7394 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
19799a22 | 7395 | The comments show the string after each step. Note that this code works |
cca87523 GS |
7396 | in the same way on big-endian or little-endian machines. |
7397 | ||
7398 | my $foo = ''; | |
5ed4f2ec | 7399 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' |
e69129f1 GS |
7400 | |
7401 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
5ed4f2ec | 7402 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') |
7403 | ||
7404 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' | |
7405 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
7406 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
7407 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
7408 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
7409 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' | |
7410 | # 'r' is "\x72" | |
7411 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" | |
7412 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
7413 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' | |
7414 | # 'l' is "\x6c" | |
cca87523 | 7415 | |
19799a22 | 7416 | To transform a bit vector into a string or list of 0's and 1's, use these: |
a0d0e21e LW |
7417 | |
7418 | $bits = unpack("b*", $vector); | |
7419 | @bits = split(//, unpack("b*", $vector)); | |
7420 | ||
7660c0ab | 7421 | If you know the exact length in bits, it can be used in place of the C<*>. |
a0d0e21e | 7422 | |
e69129f1 GS |
7423 | Here is an example to illustrate how the bits actually fall in place: |
7424 | ||
7425 | #!/usr/bin/perl -wl | |
7426 | ||
7427 | print <<'EOT'; | |
b76cc8ba | 7428 | 0 1 2 3 |
e69129f1 GS |
7429 | unpack("V",$_) 01234567890123456789012345678901 |
7430 | ------------------------------------------------------------------ | |
7431 | EOT | |
7432 | ||
7433 | for $w (0..3) { | |
7434 | $width = 2**$w; | |
7435 | for ($shift=0; $shift < $width; ++$shift) { | |
7436 | for ($off=0; $off < 32/$width; ++$off) { | |
7437 | $str = pack("B*", "0"x32); | |
7438 | $bits = (1<<$shift); | |
7439 | vec($str, $off, $width) = $bits; | |
7440 | $res = unpack("b*",$str); | |
7441 | $val = unpack("V", $str); | |
7442 | write; | |
7443 | } | |
7444 | } | |
7445 | } | |
7446 | ||
7447 | format STDOUT = | |
7448 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
7449 | $off, $width, $bits, $val, $res | |
7450 | . | |
7451 | __END__ | |
7452 | ||
80d38338 TC |
7453 | Regardless of the machine architecture on which it runs, the |
7454 | example above should print the following table: | |
e69129f1 | 7455 | |
b76cc8ba | 7456 | 0 1 2 3 |
e69129f1 GS |
7457 | unpack("V",$_) 01234567890123456789012345678901 |
7458 | ------------------------------------------------------------------ | |
7459 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
7460 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
7461 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
7462 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
7463 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
7464 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
7465 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
7466 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
7467 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
7468 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
7469 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
7470 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
7471 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
7472 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
7473 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
7474 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
7475 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
7476 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
7477 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
7478 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
7479 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
7480 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
7481 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
7482 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
7483 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
7484 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
7485 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
7486 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
7487 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
7488 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
7489 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
7490 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
7491 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
7492 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
7493 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
7494 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
7495 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
7496 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
7497 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
7498 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
7499 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
7500 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
7501 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
7502 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
7503 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
7504 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
7505 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
7506 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
7507 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
7508 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
7509 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
7510 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
7511 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
7512 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
7513 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
7514 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
7515 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
7516 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
7517 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
7518 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
7519 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
7520 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
7521 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
7522 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
7523 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
7524 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
7525 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
7526 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
7527 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
7528 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
7529 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
7530 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
7531 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
7532 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
7533 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
7534 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
7535 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
7536 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
7537 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
7538 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
7539 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
7540 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
7541 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
7542 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
7543 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
7544 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
7545 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
7546 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
7547 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
7548 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
7549 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
7550 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
7551 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
7552 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
7553 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
7554 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
7555 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
7556 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
7557 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
7558 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
7559 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
7560 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
7561 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
7562 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
7563 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
7564 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
7565 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
7566 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
7567 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
7568 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
7569 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
7570 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
7571 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
7572 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
7573 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
7574 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
7575 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
7576 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
7577 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
7578 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
7579 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
7580 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
7581 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
7582 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
7583 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
7584 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
7585 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
7586 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
7587 | ||
a0d0e21e | 7588 | =item wait |
d74e8afc | 7589 | X<wait> |
a0d0e21e | 7590 | |
3b10bc60 | 7591 | Behaves like wait(2) on your system: it waits for a child |
2b5ab1e7 | 7592 | process to terminate and returns the pid of the deceased process, or |
e5218da5 | 7593 | C<-1> if there are no child processes. The status is returned in C<$?> |
ca8d723e | 7594 | and C<${^CHILD_ERROR_NATIVE}>. |
2b5ab1e7 TC |
7595 | Note that a return value of C<-1> could mean that child processes are |
7596 | being automatically reaped, as described in L<perlipc>. | |
a0d0e21e LW |
7597 | |
7598 | =item waitpid PID,FLAGS | |
d74e8afc | 7599 | X<waitpid> |
a0d0e21e | 7600 | |
2b5ab1e7 TC |
7601 | Waits for a particular child process to terminate and returns the pid of |
7602 | the deceased process, or C<-1> if there is no such child process. On some | |
7603 | systems, a value of 0 indicates that there are processes still running. | |
ca8d723e | 7604 | The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say |
a0d0e21e | 7605 | |
5f05dabc | 7606 | use POSIX ":sys_wait_h"; |
5a964f20 | 7607 | #... |
b76cc8ba | 7608 | do { |
a9a5a0dc | 7609 | $kid = waitpid(-1, WNOHANG); |
84b74420 | 7610 | } while $kid > 0; |
a0d0e21e | 7611 | |
2b5ab1e7 TC |
7612 | then you can do a non-blocking wait for all pending zombie processes. |
7613 | Non-blocking wait is available on machines supporting either the | |
3b10bc60 | 7614 | waitpid(2) or wait4(2) syscalls. However, waiting for a particular |
2b5ab1e7 TC |
7615 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the |
7616 | system call by remembering the status values of processes that have | |
7617 | exited but have not been harvested by the Perl script yet.) | |
a0d0e21e | 7618 | |
2b5ab1e7 TC |
7619 | Note that on some systems, a return value of C<-1> could mean that child |
7620 | processes are being automatically reaped. See L<perlipc> for details, | |
7621 | and for other examples. | |
5a964f20 | 7622 | |
a0d0e21e | 7623 | =item wantarray |
d74e8afc | 7624 | X<wantarray> X<context> |
a0d0e21e | 7625 | |
cc37eb0b | 7626 | Returns true if the context of the currently executing subroutine or |
20f13e4a | 7627 | C<eval> is looking for a list value. Returns false if the context is |
cc37eb0b RGS |
7628 | looking for a scalar. Returns the undefined value if the context is |
7629 | looking for no value (void context). | |
a0d0e21e | 7630 | |
5ed4f2ec | 7631 | return unless defined wantarray; # don't bother doing more |
54310121 | 7632 | my @a = complex_calculation(); |
7633 | return wantarray ? @a : "@a"; | |
a0d0e21e | 7634 | |
20f13e4a | 7635 | C<wantarray()>'s result is unspecified in the top level of a file, |
3c10abe3 AG |
7636 | in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or |
7637 | in a C<DESTROY> method. | |
20f13e4a | 7638 | |
19799a22 GS |
7639 | This function should have been named wantlist() instead. |
7640 | ||
a0d0e21e | 7641 | =item warn LIST |
d74e8afc | 7642 | X<warn> X<warning> X<STDERR> |
a0d0e21e | 7643 | |
2d6d0015 | 7644 | Prints the value of LIST to STDERR. If the last element of LIST does |
afd8c9c8 DM |
7645 | not end in a newline, it appends the same file/line number text as C<die> |
7646 | does. | |
774d564b | 7647 | |
a96d0188 | 7648 | If the output is empty and C<$@> already contains a value (typically from a |
7660c0ab | 7649 | previous eval) that value is used after appending C<"\t...caught"> |
19799a22 GS |
7650 | to C<$@>. This is useful for staying almost, but not entirely similar to |
7651 | C<die>. | |
43051805 | 7652 | |
7660c0ab | 7653 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
43051805 | 7654 | |
774d564b | 7655 | No message is printed if there is a C<$SIG{__WARN__}> handler |
7656 | installed. It is the handler's responsibility to deal with the message | |
19799a22 | 7657 | as it sees fit (like, for instance, converting it into a C<die>). Most |
80d38338 | 7658 | handlers must therefore arrange to actually display the |
19799a22 | 7659 | warnings that they are not prepared to deal with, by calling C<warn> |
774d564b | 7660 | again in the handler. Note that this is quite safe and will not |
7661 | produce an endless loop, since C<__WARN__> hooks are not called from | |
7662 | inside one. | |
7663 | ||
7664 | You will find this behavior is slightly different from that of | |
7665 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
19799a22 | 7666 | instead call C<die> again to change it). |
774d564b | 7667 | |
7668 | Using a C<__WARN__> handler provides a powerful way to silence all | |
7669 | warnings (even the so-called mandatory ones). An example: | |
7670 | ||
7671 | # wipe out *all* compile-time warnings | |
7672 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
7673 | my $foo = 10; | |
7674 | my $foo = 20; # no warning about duplicate my $foo, | |
7675 | # but hey, you asked for it! | |
7676 | # no compile-time or run-time warnings before here | |
7677 | $DOWARN = 1; | |
7678 | ||
7679 | # run-time warnings enabled after here | |
7680 | warn "\$foo is alive and $foo!"; # does show up | |
7681 | ||
7682 | See L<perlvar> for details on setting C<%SIG> entries, and for more | |
2b5ab1e7 TC |
7683 | examples. See the Carp module for other kinds of warnings using its |
7684 | carp() and cluck() functions. | |
a0d0e21e LW |
7685 | |
7686 | =item write FILEHANDLE | |
d74e8afc | 7687 | X<write> |
a0d0e21e LW |
7688 | |
7689 | =item write EXPR | |
7690 | ||
7691 | =item write | |
7692 | ||
5a964f20 | 7693 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
a0d0e21e | 7694 | using the format associated with that file. By default the format for |
54310121 | 7695 | a file is the one having the same name as the filehandle, but the |
19799a22 | 7696 | format for the current output channel (see the C<select> function) may be set |
184e9718 | 7697 | explicitly by assigning the name of the format to the C<$~> variable. |
a0d0e21e LW |
7698 | |
7699 | Top of form processing is handled automatically: if there is | |
7700 | insufficient room on the current page for the formatted record, the | |
7701 | page is advanced by writing a form feed, a special top-of-page format | |
7702 | is used to format the new page header, and then the record is written. | |
7703 | By default the top-of-page format is the name of the filehandle with | |
7704 | "_TOP" appended, but it may be dynamically set to the format of your | |
184e9718 | 7705 | choice by assigning the name to the C<$^> variable while the filehandle is |
a0d0e21e | 7706 | selected. The number of lines remaining on the current page is in |
7660c0ab | 7707 | variable C<$->, which can be set to C<0> to force a new page. |
a0d0e21e LW |
7708 | |
7709 | If FILEHANDLE is unspecified, output goes to the current default output | |
7710 | channel, which starts out as STDOUT but may be changed by the | |
19799a22 | 7711 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression |
a0d0e21e LW |
7712 | is evaluated and the resulting string is used to look up the name of |
7713 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
7714 | ||
19799a22 | 7715 | Note that write is I<not> the opposite of C<read>. Unfortunately. |
a0d0e21e LW |
7716 | |
7717 | =item y/// | |
7718 | ||
9f4b9cd0 SP |
7719 | The transliteration operator. Same as C<tr///>. See |
7720 | L<perlop/"Quote and Quote-like Operators">. | |
a0d0e21e LW |
7721 | |
7722 | =back |