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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlfunc - Perl builtin functions | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The functions in this section can serve as terms in an expression. | |
8 | They fall into two major categories: list operators and named unary | |
9 | operators. These differ in their precedence relationship with a | |
10 | following comma. (See the precedence table in L<perlop>.) List | |
11 | operators take more than one argument, while unary operators can never | |
12 | take more than one argument. Thus, a comma terminates the argument of | |
13 | a unary operator, but merely separates the arguments of a list | |
14 | operator. A unary operator generally provides a scalar context to its | |
2b5ab1e7 | 15 | argument, while a list operator may provide either scalar or list |
a0d0e21e | 16 | contexts for its arguments. If it does both, the scalar arguments will |
5f05dabc | 17 | be first, and the list argument will follow. (Note that there can ever |
0f31cffe | 18 | be only one such list argument.) For instance, splice() has three scalar |
2b5ab1e7 TC |
19 | arguments followed by a list, whereas gethostbyname() has four scalar |
20 | arguments. | |
a0d0e21e LW |
21 | |
22 | In the syntax descriptions that follow, list operators that expect a | |
23 | list (and provide list context for the elements of the list) are shown | |
24 | with LIST as an argument. Such a list may consist of any combination | |
25 | of scalar arguments or list values; the list values will be included | |
26 | in the list as if each individual element were interpolated at that | |
27 | point in the list, forming a longer single-dimensional list value. | |
28 | Elements of the LIST should be separated by commas. | |
29 | ||
30 | Any function in the list below may be used either with or without | |
31 | parentheses around its arguments. (The syntax descriptions omit the | |
5f05dabc | 32 | parentheses.) If you use the parentheses, the simple (but occasionally |
19799a22 | 33 | surprising) rule is this: It I<looks> like a function, therefore it I<is> a |
a0d0e21e LW |
34 | function, and precedence doesn't matter. Otherwise it's a list |
35 | operator or unary operator, and precedence does matter. And whitespace | |
36 | between the function and left parenthesis doesn't count--so you need to | |
37 | be careful sometimes: | |
38 | ||
68dc0745 | 39 | print 1+2+4; # Prints 7. |
40 | print(1+2) + 4; # Prints 3. | |
41 | print (1+2)+4; # Also prints 3! | |
42 | print +(1+2)+4; # Prints 7. | |
43 | print ((1+2)+4); # Prints 7. | |
a0d0e21e LW |
44 | |
45 | If you run Perl with the B<-w> switch it can warn you about this. For | |
46 | example, the third line above produces: | |
47 | ||
48 | print (...) interpreted as function at - line 1. | |
49 | Useless use of integer addition in void context at - line 1. | |
50 | ||
2b5ab1e7 TC |
51 | A few functions take no arguments at all, and therefore work as neither |
52 | unary nor list operators. These include such functions as C<time> | |
53 | and C<endpwent>. For example, C<time+86_400> always means | |
54 | C<time() + 86_400>. | |
55 | ||
a0d0e21e | 56 | For functions that can be used in either a scalar or list context, |
54310121 | 57 | nonabortive failure is generally indicated in a scalar context by |
a0d0e21e LW |
58 | returning the undefined value, and in a list context by returning the |
59 | null list. | |
60 | ||
5a964f20 TC |
61 | Remember the following important rule: There is B<no rule> that relates |
62 | the behavior of an expression in list context to its behavior in scalar | |
63 | context, or vice versa. It might do two totally different things. | |
a0d0e21e | 64 | Each operator and function decides which sort of value it would be most |
2b5ab1e7 | 65 | appropriate to return in scalar context. Some operators return the |
5a964f20 | 66 | length of the list that would have been returned in list context. Some |
a0d0e21e LW |
67 | operators return the first value in the list. Some operators return the |
68 | last value in the list. Some operators return a count of successful | |
69 | operations. In general, they do what you want, unless you want | |
70 | consistency. | |
71 | ||
5a964f20 TC |
72 | An named array in scalar context is quite different from what would at |
73 | first glance appear to be a list in scalar context. You can't get a list | |
74 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
75 | the context at compile time. It would generate the scalar comma operator | |
76 | there, not the list construction version of the comma. That means it | |
77 | was never a list to start with. | |
78 | ||
79 | In general, functions in Perl that serve as wrappers for system calls | |
f86cebdf | 80 | of the same name (like chown(2), fork(2), closedir(2), etc.) all return |
5a964f20 TC |
81 | true when they succeed and C<undef> otherwise, as is usually mentioned |
82 | in the descriptions below. This is different from the C interfaces, | |
19799a22 GS |
83 | which return C<-1> on failure. Exceptions to this rule are C<wait>, |
84 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> | |
5a964f20 TC |
85 | variable on failure. Other functions do not, except accidentally. |
86 | ||
cb1a09d0 AD |
87 | =head2 Perl Functions by Category |
88 | ||
89 | Here are Perl's functions (including things that look like | |
5a964f20 | 90 | functions, like some keywords and named operators) |
cb1a09d0 AD |
91 | arranged by category. Some functions appear in more |
92 | than one place. | |
93 | ||
13a2d996 | 94 | =over 4 |
cb1a09d0 AD |
95 | |
96 | =item Functions for SCALARs or strings | |
97 | ||
22fae026 | 98 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
945c54fd JH |
99 | C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, |
100 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> | |
cb1a09d0 AD |
101 | |
102 | =item Regular expressions and pattern matching | |
103 | ||
ab4f32c2 | 104 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
cb1a09d0 AD |
105 | |
106 | =item Numeric functions | |
107 | ||
22fae026 TM |
108 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, |
109 | C<sin>, C<sqrt>, C<srand> | |
cb1a09d0 AD |
110 | |
111 | =item Functions for real @ARRAYs | |
112 | ||
22fae026 | 113 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
cb1a09d0 AD |
114 | |
115 | =item Functions for list data | |
116 | ||
ab4f32c2 | 117 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> |
cb1a09d0 AD |
118 | |
119 | =item Functions for real %HASHes | |
120 | ||
22fae026 | 121 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
cb1a09d0 AD |
122 | |
123 | =item Input and output functions | |
124 | ||
22fae026 TM |
125 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, |
126 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
127 | C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, | |
128 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, | |
129 | C<warn>, C<write> | |
cb1a09d0 AD |
130 | |
131 | =item Functions for fixed length data or records | |
132 | ||
22fae026 | 133 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
cb1a09d0 AD |
134 | |
135 | =item Functions for filehandles, files, or directories | |
136 | ||
22fae026 | 137 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, |
5ff3f7a4 GS |
138 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, |
139 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, | |
140 | C<unlink>, C<utime> | |
cb1a09d0 AD |
141 | |
142 | =item Keywords related to the control flow of your perl program | |
143 | ||
98293880 JH |
144 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, |
145 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> | |
cb1a09d0 | 146 | |
54310121 | 147 | =item Keywords related to scoping |
cb1a09d0 | 148 | |
4375e838 | 149 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use> |
cb1a09d0 AD |
150 | |
151 | =item Miscellaneous functions | |
152 | ||
4375e838 | 153 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>, |
22fae026 | 154 | C<scalar>, C<undef>, C<wantarray> |
cb1a09d0 AD |
155 | |
156 | =item Functions for processes and process groups | |
157 | ||
22fae026 | 158 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
ab4f32c2 | 159 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
22fae026 | 160 | C<times>, C<wait>, C<waitpid> |
cb1a09d0 AD |
161 | |
162 | =item Keywords related to perl modules | |
163 | ||
22fae026 | 164 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
cb1a09d0 AD |
165 | |
166 | =item Keywords related to classes and object-orientedness | |
167 | ||
22fae026 TM |
168 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
169 | C<untie>, C<use> | |
cb1a09d0 AD |
170 | |
171 | =item Low-level socket functions | |
172 | ||
22fae026 TM |
173 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
174 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
80cbd5ad | 175 | C<sockatmark>, C<socket>, C<socketpair> |
cb1a09d0 AD |
176 | |
177 | =item System V interprocess communication functions | |
178 | ||
22fae026 TM |
179 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
180 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
cb1a09d0 AD |
181 | |
182 | =item Fetching user and group info | |
183 | ||
22fae026 TM |
184 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
185 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
186 | C<getpwuid>, C<setgrent>, C<setpwent> | |
cb1a09d0 AD |
187 | |
188 | =item Fetching network info | |
189 | ||
22fae026 TM |
190 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
191 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
192 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
193 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
194 | C<setnetent>, C<setprotoent>, C<setservent> | |
cb1a09d0 AD |
195 | |
196 | =item Time-related functions | |
197 | ||
22fae026 | 198 | C<gmtime>, C<localtime>, C<time>, C<times> |
cb1a09d0 | 199 | |
37798a01 | 200 | =item Functions new in perl5 |
201 | ||
22fae026 | 202 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, |
b76cc8ba | 203 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>, |
4375e838 | 204 | C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, |
22fae026 | 205 | C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> |
37798a01 | 206 | |
207 | * - C<sub> was a keyword in perl4, but in perl5 it is an | |
5a964f20 | 208 | operator, which can be used in expressions. |
37798a01 | 209 | |
210 | =item Functions obsoleted in perl5 | |
211 | ||
22fae026 | 212 | C<dbmclose>, C<dbmopen> |
37798a01 | 213 | |
cb1a09d0 AD |
214 | =back |
215 | ||
60f9f73c JH |
216 | =head2 Portability |
217 | ||
2b5ab1e7 TC |
218 | Perl was born in Unix and can therefore access all common Unix |
219 | system calls. In non-Unix environments, the functionality of some | |
220 | Unix system calls may not be available, or details of the available | |
221 | functionality may differ slightly. The Perl functions affected | |
60f9f73c JH |
222 | by this are: |
223 | ||
224 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
225 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
226 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
227 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, | |
228 | C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
229 | C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, | |
230 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, | |
231 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
232 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
2b5ab1e7 | 233 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, |
60f9f73c JH |
234 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
235 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
236 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
80cbd5ad JH |
237 | C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>, |
238 | C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, | |
239 | C<times>, C<truncate>, C<umask>, C<unlink>, | |
2b5ab1e7 | 240 | C<utime>, C<wait>, C<waitpid> |
60f9f73c JH |
241 | |
242 | For more information about the portability of these functions, see | |
243 | L<perlport> and other available platform-specific documentation. | |
244 | ||
cb1a09d0 AD |
245 | =head2 Alphabetical Listing of Perl Functions |
246 | ||
a0d0e21e LW |
247 | =over 8 |
248 | ||
22fae026 | 249 | =item I<-X> FILEHANDLE |
a0d0e21e | 250 | |
22fae026 | 251 | =item I<-X> EXPR |
a0d0e21e | 252 | |
22fae026 | 253 | =item I<-X> |
a0d0e21e LW |
254 | |
255 | A file test, where X is one of the letters listed below. This unary | |
256 | operator takes one argument, either a filename or a filehandle, and | |
257 | tests the associated file to see if something is true about it. If the | |
7660c0ab | 258 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
19799a22 | 259 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or |
a0d0e21e LW |
260 | the undefined value if the file doesn't exist. Despite the funny |
261 | names, precedence is the same as any other named unary operator, and | |
262 | the argument may be parenthesized like any other unary operator. The | |
263 | operator may be any of: | |
7e778d91 IZ |
264 | 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> |
265 | 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 LW |
266 | |
267 | -r File is readable by effective uid/gid. | |
268 | -w File is writable by effective uid/gid. | |
269 | -x File is executable by effective uid/gid. | |
270 | -o File is owned by effective uid. | |
271 | ||
272 | -R File is readable by real uid/gid. | |
273 | -W File is writable by real uid/gid. | |
274 | -X File is executable by real uid/gid. | |
275 | -O File is owned by real uid. | |
276 | ||
277 | -e File exists. | |
8e7e0aa8 MJD |
278 | -z File has zero size (is empty). |
279 | -s File has nonzero size (returns size in bytes). | |
a0d0e21e LW |
280 | |
281 | -f File is a plain file. | |
282 | -d File is a directory. | |
283 | -l File is a symbolic link. | |
9c4d0f16 | 284 | -p File is a named pipe (FIFO), or Filehandle is a pipe. |
a0d0e21e LW |
285 | -S File is a socket. |
286 | -b File is a block special file. | |
287 | -c File is a character special file. | |
288 | -t Filehandle is opened to a tty. | |
289 | ||
290 | -u File has setuid bit set. | |
291 | -g File has setgid bit set. | |
292 | -k File has sticky bit set. | |
293 | ||
2cdbc966 JD |
294 | -T File is an ASCII text file. |
295 | -B File is a "binary" file (opposite of -T). | |
a0d0e21e LW |
296 | |
297 | -M Age of file in days when script started. | |
298 | -A Same for access time. | |
299 | -C Same for inode change time. | |
300 | ||
a0d0e21e LW |
301 | Example: |
302 | ||
303 | while (<>) { | |
5b3eff12 | 304 | chomp; |
a0d0e21e | 305 | next unless -f $_; # ignore specials |
5a964f20 | 306 | #... |
a0d0e21e LW |
307 | } |
308 | ||
5ff3f7a4 GS |
309 | The interpretation of the file permission operators C<-r>, C<-R>, |
310 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
311 | of the file and the uids and gids of the user. There may be other | |
312 | reasons you can't actually read, write, or execute the file. Such | |
313 | reasons may be for example network filesystem access controls, ACLs | |
314 | (access control lists), read-only filesystems, and unrecognized | |
315 | executable formats. | |
316 | ||
2b5ab1e7 TC |
317 | Also note that, for the superuser on the local filesystems, the C<-r>, |
318 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
5ff3f7a4 GS |
319 | if any execute bit is set in the mode. Scripts run by the superuser |
320 | may thus need to do a stat() to determine the actual mode of the file, | |
2b5ab1e7 | 321 | or temporarily set their effective uid to something else. |
5ff3f7a4 GS |
322 | |
323 | If you are using ACLs, there is a pragma called C<filetest> that may | |
324 | produce more accurate results than the bare stat() mode bits. | |
5ff3f7a4 GS |
325 | When under the C<use filetest 'access'> the above-mentioned filetests |
326 | will test whether the permission can (not) be granted using the | |
468541a8 | 327 | access() family of system calls. Also note that the C<-x> and C<-X> may |
5ff3f7a4 GS |
328 | under this pragma return true even if there are no execute permission |
329 | bits set (nor any extra execute permission ACLs). This strangeness is | |
330 | due to the underlying system calls' definitions. Read the | |
331 | documentation for the C<filetest> pragma for more information. | |
332 | ||
a0d0e21e LW |
333 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
334 | C<-exp($foo)> still works as expected, however--only single letters | |
335 | following a minus are interpreted as file tests. | |
336 | ||
337 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
338 | file is examined for odd characters such as strange control codes or | |
61eff3bc | 339 | characters with the high bit set. If too many strange characters (>30%) |
a0d0e21e LW |
340 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file |
341 | containing null in the first block is considered a binary file. If C<-T> | |
342 | or C<-B> is used on a filehandle, the current stdio buffer is examined | |
19799a22 | 343 | rather than the first block. Both C<-T> and C<-B> return true on a null |
54310121 | 344 | file, or a file at EOF when testing a filehandle. Because you have to |
4633a7c4 LW |
345 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
346 | against the file first, as in C<next unless -f $file && -T $file>. | |
a0d0e21e | 347 | |
19799a22 | 348 | If any of the file tests (or either the C<stat> or C<lstat> operators) are given |
28757baa | 349 | the special filehandle consisting of a solitary underline, then the stat |
a0d0e21e LW |
350 | structure of the previous file test (or stat operator) is used, saving |
351 | a system call. (This doesn't work with C<-t>, and you need to remember | |
352 | that lstat() and C<-l> will leave values in the stat structure for the | |
353 | symbolic link, not the real file.) Example: | |
354 | ||
355 | print "Can do.\n" if -r $a || -w _ || -x _; | |
356 | ||
357 | stat($filename); | |
358 | print "Readable\n" if -r _; | |
359 | print "Writable\n" if -w _; | |
360 | print "Executable\n" if -x _; | |
361 | print "Setuid\n" if -u _; | |
362 | print "Setgid\n" if -g _; | |
363 | print "Sticky\n" if -k _; | |
364 | print "Text\n" if -T _; | |
365 | print "Binary\n" if -B _; | |
366 | ||
367 | =item abs VALUE | |
368 | ||
54310121 | 369 | =item abs |
bbce6d69 | 370 | |
a0d0e21e | 371 | Returns the absolute value of its argument. |
7660c0ab | 372 | If VALUE is omitted, uses C<$_>. |
a0d0e21e LW |
373 | |
374 | =item accept NEWSOCKET,GENERICSOCKET | |
375 | ||
f86cebdf | 376 | Accepts an incoming socket connect, just as the accept(2) system call |
19799a22 | 377 | does. Returns the packed address if it succeeded, false otherwise. |
2b5ab1e7 | 378 | See the example in L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e | 379 | |
8d2a6795 GS |
380 | On systems that support a close-on-exec flag on files, the flag will |
381 | be set for the newly opened file descriptor, as determined by the | |
382 | value of $^F. See L<perlvar/$^F>. | |
383 | ||
a0d0e21e LW |
384 | =item alarm SECONDS |
385 | ||
54310121 | 386 | =item alarm |
bbce6d69 | 387 | |
a0d0e21e | 388 | Arranges to have a SIGALRM delivered to this process after the |
bbce6d69 | 389 | specified number of seconds have elapsed. If SECONDS is not specified, |
7660c0ab | 390 | the value stored in C<$_> is used. (On some machines, |
a0d0e21e LW |
391 | unfortunately, the elapsed time may be up to one second less than you |
392 | specified because of how seconds are counted.) Only one timer may be | |
393 | counting at once. Each call disables the previous timer, and an | |
7660c0ab | 394 | argument of C<0> may be supplied to cancel the previous timer without |
a0d0e21e LW |
395 | starting a new one. The returned value is the amount of time remaining |
396 | on the previous timer. | |
397 | ||
4633a7c4 | 398 | For delays of finer granularity than one second, you may use Perl's |
19799a22 GS |
399 | four-argument version of select() leaving the first three arguments |
400 | undefined, or you might be able to use the C<syscall> interface to | |
2b5ab1e7 TC |
401 | access setitimer(2) if your system supports it. The Time::HiRes module |
402 | from CPAN may also prove useful. | |
403 | ||
68f8bed4 JH |
404 | It is usually a mistake to intermix C<alarm> and C<sleep> calls. |
405 | (C<sleep> may be internally implemented in your system with C<alarm>) | |
a0d0e21e | 406 | |
19799a22 GS |
407 | If you want to use C<alarm> to time out a system call you need to use an |
408 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
f86cebdf | 409 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
19799a22 | 410 | restart system calls on some systems. Using C<eval>/C<die> always works, |
5a964f20 | 411 | modulo the caveats given in L<perlipc/"Signals">. |
ff68c719 | 412 | |
413 | eval { | |
f86cebdf | 414 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
36477c24 | 415 | alarm $timeout; |
ff68c719 | 416 | $nread = sysread SOCKET, $buffer, $size; |
36477c24 | 417 | alarm 0; |
ff68c719 | 418 | }; |
ff68c719 | 419 | if ($@) { |
f86cebdf | 420 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
ff68c719 | 421 | # timed out |
422 | } | |
423 | else { | |
424 | # didn't | |
425 | } | |
426 | ||
a0d0e21e LW |
427 | =item atan2 Y,X |
428 | ||
429 | Returns the arctangent of Y/X in the range -PI to PI. | |
430 | ||
ca6e1c26 | 431 | For the tangent operation, you may use the C<Math::Trig::tan> |
28757baa | 432 | function, or use the familiar relation: |
433 | ||
434 | sub tan { sin($_[0]) / cos($_[0]) } | |
435 | ||
a0d0e21e LW |
436 | =item bind SOCKET,NAME |
437 | ||
438 | Binds a network address to a socket, just as the bind system call | |
19799a22 | 439 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
440 | packed address of the appropriate type for the socket. See the examples in |
441 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 442 | |
1c1fc3ea GS |
443 | =item binmode FILEHANDLE, DISCIPLINE |
444 | ||
a0d0e21e LW |
445 | =item binmode FILEHANDLE |
446 | ||
16fe6d59 GS |
447 | Arranges for FILEHANDLE to be read or written in "binary" or "text" mode |
448 | on systems where the run-time libraries distinguish between binary and | |
30168b04 | 449 | text files. If FILEHANDLE is an expression, the value is taken as the |
16fe6d59 GS |
450 | name of the filehandle. DISCIPLINE can be either of C<":raw"> for |
451 | binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is | |
452 | omitted, it defaults to C<":raw">. | |
30168b04 | 453 | |
16fe6d59 GS |
454 | binmode() should be called after open() but before any I/O is done on |
455 | the filehandle. | |
456 | ||
457 | On many systems binmode() currently has no effect, but in future, it | |
458 | will be extended to support user-defined input and output disciplines. | |
459 | On some systems binmode() is necessary when you're not working with a | |
460 | text file. For the sake of portability it is a good idea to always use | |
461 | it when appropriate, and to never use it when it isn't appropriate. | |
30168b04 GS |
462 | |
463 | In other words: Regardless of platform, use binmode() on binary | |
464 | files, and do not use binmode() on text files. | |
19799a22 | 465 | |
16fe6d59 GS |
466 | The C<open> pragma can be used to establish default disciplines. |
467 | See L<open>. | |
468 | ||
19799a22 | 469 | The operating system, device drivers, C libraries, and Perl run-time |
30168b04 GS |
470 | system all work together to let the programmer treat a single |
471 | character (C<\n>) as the line terminator, irrespective of the external | |
472 | representation. On many operating systems, the native text file | |
473 | representation matches the internal representation, but on some | |
474 | platforms the external representation of C<\n> is made up of more than | |
475 | one character. | |
476 | ||
477 | Mac OS and all variants of Unix use a single character to end each line | |
478 | in the external representation of text (even though that single | |
479 | character is not necessarily the same across these platforms). | |
480 | Consequently binmode() has no effect on these operating systems. In | |
481 | other systems like VMS, MS-DOS and the various flavors of MS-Windows | |
482 | your program sees a C<\n> as a simple C<\cJ>, but what's stored in text | |
483 | files are the two characters C<\cM\cJ>. That means that, if you don't | |
484 | use binmode() on these systems, C<\cM\cJ> sequences on disk will be | |
485 | converted to C<\n> on input, and any C<\n> in your program will be | |
486 | converted back to C<\cM\cJ> on output. This is what you want for text | |
487 | files, but it can be disastrous for binary files. | |
488 | ||
489 | Another consequence of using binmode() (on some systems) is that | |
490 | special end-of-file markers will be seen as part of the data stream. | |
491 | For systems from the Microsoft family this means that if your binary | |
4375e838 | 492 | data contains C<\cZ>, the I/O subsystem will regard it as the end of |
30168b04 GS |
493 | the file, unless you use binmode(). |
494 | ||
495 | binmode() is not only important for readline() and print() operations, | |
496 | but also when using read(), seek(), sysread(), syswrite() and tell() | |
497 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
498 | in L<perlvar> for how to manually set your input and output | |
499 | line-termination sequences. | |
a0d0e21e | 500 | |
4633a7c4 | 501 | =item bless REF,CLASSNAME |
a0d0e21e LW |
502 | |
503 | =item bless REF | |
504 | ||
2b5ab1e7 TC |
505 | This function tells the thingy referenced by REF that it is now an object |
506 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
19799a22 | 507 | is used. Because a C<bless> is often the last thing in a constructor, |
2b5ab1e7 TC |
508 | it returns the reference for convenience. Always use the two-argument |
509 | version if the function doing the blessing might be inherited by a | |
510 | derived class. See L<perltoot> and L<perlobj> for more about the blessing | |
511 | (and blessings) of objects. | |
a0d0e21e | 512 | |
57668c4d | 513 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
2b5ab1e7 TC |
514 | Namespaces with all lowercase names are considered reserved for |
515 | Perl pragmata. Builtin types have all uppercase names, so to prevent | |
516 | confusion, you may wish to avoid such package names as well. Make sure | |
517 | that CLASSNAME is a true value. | |
60ad88b8 GS |
518 | |
519 | See L<perlmod/"Perl Modules">. | |
520 | ||
a0d0e21e LW |
521 | =item caller EXPR |
522 | ||
523 | =item caller | |
524 | ||
5a964f20 | 525 | Returns the context of the current subroutine call. In scalar context, |
28757baa | 526 | returns the caller's package name if there is a caller, that is, if |
19799a22 | 527 | we're in a subroutine or C<eval> or C<require>, and the undefined value |
5a964f20 | 528 | otherwise. In list context, returns |
a0d0e21e | 529 | |
748a9306 | 530 | ($package, $filename, $line) = caller; |
a0d0e21e LW |
531 | |
532 | With EXPR, it returns some extra information that the debugger uses to | |
533 | print a stack trace. The value of EXPR indicates how many call frames | |
534 | to go back before the current one. | |
535 | ||
f3aa04c2 | 536 | ($package, $filename, $line, $subroutine, $hasargs, |
e476b1b5 | 537 | $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); |
e7ea3e70 | 538 | |
951ba7fe | 539 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
19799a22 | 540 | call, but an C<eval>. In such a case additional elements $evaltext and |
7660c0ab | 541 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
19799a22 | 542 | C<require> or C<use> statement, $evaltext contains the text of the |
277ddfaf | 543 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, |
951ba7fe | 544 | $filename is C<(eval)>, but $evaltext is undefined. (Note also that |
dc848c6f | 545 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
277ddfaf | 546 | frame. C<$hasargs> is true if a new instance of C<@_> was set up for the |
e476b1b5 GS |
547 | frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller |
548 | was compiled with. The C<$hints> and C<$bitmask> values are subject to | |
549 | change between versions of Perl, and are not meant for external use. | |
748a9306 LW |
550 | |
551 | Furthermore, when called from within the DB package, caller returns more | |
7660c0ab | 552 | detailed information: it sets the list variable C<@DB::args> to be the |
54310121 | 553 | arguments with which the subroutine was invoked. |
748a9306 | 554 | |
7660c0ab | 555 | Be aware that the optimizer might have optimized call frames away before |
19799a22 | 556 | C<caller> had a chance to get the information. That means that C<caller(N)> |
7660c0ab | 557 | might not return information about the call frame you expect it do, for |
b76cc8ba | 558 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the |
19799a22 | 559 | previous time C<caller> was called. |
7660c0ab | 560 | |
a0d0e21e LW |
561 | =item chdir EXPR |
562 | ||
2b5ab1e7 | 563 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
0bfc1ec4 GS |
564 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
565 | changes to the directory specified by C<$ENV{LOGDIR}>. If neither is | |
566 | set, C<chdir> does nothing. It returns true upon success, false | |
567 | otherwise. See the example under C<die>. | |
a0d0e21e LW |
568 | |
569 | =item chmod LIST | |
570 | ||
571 | Changes the permissions of a list of files. The first element of the | |
4633a7c4 | 572 | list must be the numerical mode, which should probably be an octal |
2f9daede TP |
573 | number, and which definitely should I<not> a string of octal digits: |
574 | C<0644> is okay, C<'0644'> is not. Returns the number of files | |
dc848c6f | 575 | successfully changed. See also L</oct>, if all you have is a string. |
a0d0e21e LW |
576 | |
577 | $cnt = chmod 0755, 'foo', 'bar'; | |
578 | chmod 0755, @executables; | |
f86cebdf GS |
579 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to |
580 | # --w----r-T | |
2f9daede TP |
581 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better |
582 | $mode = 0644; chmod $mode, 'foo'; # this is best | |
a0d0e21e | 583 | |
ca6e1c26 JH |
584 | You can also import the symbolic C<S_I*> constants from the Fcntl |
585 | module: | |
586 | ||
587 | use Fcntl ':mode'; | |
588 | ||
589 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; | |
590 | # This is identical to the chmod 0755 of the above example. | |
591 | ||
a0d0e21e LW |
592 | =item chomp VARIABLE |
593 | ||
594 | =item chomp LIST | |
595 | ||
596 | =item chomp | |
597 | ||
2b5ab1e7 TC |
598 | This safer version of L</chop> removes any trailing string |
599 | that corresponds to the current value of C<$/> (also known as | |
28757baa | 600 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
601 | number of characters removed from all its arguments. It's often used to | |
602 | remove the newline from the end of an input record when you're worried | |
2b5ab1e7 TC |
603 | that the final record may be missing its newline. When in paragraph |
604 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
4c5a6083 GS |
605 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
606 | a reference to an integer or the like, see L<perlvar>) chomp() won't | |
b76cc8ba | 607 | remove anything. |
19799a22 | 608 | If VARIABLE is omitted, it chomps C<$_>. Example: |
a0d0e21e LW |
609 | |
610 | while (<>) { | |
611 | chomp; # avoid \n on last field | |
612 | @array = split(/:/); | |
5a964f20 | 613 | # ... |
a0d0e21e LW |
614 | } |
615 | ||
4bf21a6d RD |
616 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. |
617 | ||
a0d0e21e LW |
618 | You can actually chomp anything that's an lvalue, including an assignment: |
619 | ||
620 | chomp($cwd = `pwd`); | |
621 | chomp($answer = <STDIN>); | |
622 | ||
623 | If you chomp a list, each element is chomped, and the total number of | |
624 | characters removed is returned. | |
625 | ||
626 | =item chop VARIABLE | |
627 | ||
628 | =item chop LIST | |
629 | ||
630 | =item chop | |
631 | ||
632 | Chops off the last character of a string and returns the character | |
5b3eff12 | 633 | chopped. It is much more efficient than C<s/.$//s> because it neither |
7660c0ab | 634 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
4bf21a6d RD |
635 | If VARIABLE is a hash, it chops the hash's values, but not its keys. |
636 | ||
5b3eff12 | 637 | You can actually chop anything that's an lvalue, including an assignment. |
a0d0e21e LW |
638 | |
639 | If you chop a list, each element is chopped. Only the value of the | |
19799a22 | 640 | last C<chop> is returned. |
a0d0e21e | 641 | |
19799a22 | 642 | Note that C<chop> returns the last character. To return all but the last |
748a9306 LW |
643 | character, use C<substr($string, 0, -1)>. |
644 | ||
a0d0e21e LW |
645 | =item chown LIST |
646 | ||
647 | Changes the owner (and group) of a list of files. The first two | |
19799a22 GS |
648 | elements of the list must be the I<numeric> uid and gid, in that |
649 | order. A value of -1 in either position is interpreted by most | |
650 | systems to leave that value unchanged. Returns the number of files | |
651 | successfully changed. | |
a0d0e21e LW |
652 | |
653 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
654 | chown $uid, $gid, @filenames; | |
655 | ||
54310121 | 656 | Here's an example that looks up nonnumeric uids in the passwd file: |
a0d0e21e LW |
657 | |
658 | print "User: "; | |
19799a22 | 659 | chomp($user = <STDIN>); |
5a964f20 | 660 | print "Files: "; |
19799a22 | 661 | chomp($pattern = <STDIN>); |
a0d0e21e LW |
662 | |
663 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
664 | or die "$user not in passwd file"; | |
665 | ||
5a964f20 | 666 | @ary = glob($pattern); # expand filenames |
a0d0e21e LW |
667 | chown $uid, $gid, @ary; |
668 | ||
54310121 | 669 | On most systems, you are not allowed to change the ownership of the |
4633a7c4 LW |
670 | file unless you're the superuser, although you should be able to change |
671 | the group to any of your secondary groups. On insecure systems, these | |
672 | restrictions may be relaxed, but this is not a portable assumption. | |
19799a22 GS |
673 | On POSIX systems, you can detect this condition this way: |
674 | ||
675 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
676 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
4633a7c4 | 677 | |
a0d0e21e LW |
678 | =item chr NUMBER |
679 | ||
54310121 | 680 | =item chr |
bbce6d69 | 681 | |
a0d0e21e | 682 | Returns the character represented by that NUMBER in the character set. |
a0ed51b3 | 683 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
b76cc8ba | 684 | chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>, |
aaa68c4a SC |
685 | characters higher than 127 are encoded in Unicode; if you don't want |
686 | this, temporarily C<use bytes> or use C<pack("C*",...)> | |
687 | ||
b76cc8ba | 688 | For the reverse, use L</ord>. |
2b5ab1e7 | 689 | See L<utf8> for more about Unicode. |
a0d0e21e | 690 | |
7660c0ab | 691 | If NUMBER is omitted, uses C<$_>. |
bbce6d69 | 692 | |
a0d0e21e LW |
693 | =item chroot FILENAME |
694 | ||
54310121 | 695 | =item chroot |
bbce6d69 | 696 | |
5a964f20 | 697 | This function works like the system call by the same name: it makes the |
4633a7c4 | 698 | named directory the new root directory for all further pathnames that |
951ba7fe | 699 | begin with a C</> by your process and all its children. (It doesn't |
28757baa | 700 | change your current working directory, which is unaffected.) For security |
4633a7c4 | 701 | reasons, this call is restricted to the superuser. If FILENAME is |
19799a22 | 702 | omitted, does a C<chroot> to C<$_>. |
a0d0e21e LW |
703 | |
704 | =item close FILEHANDLE | |
705 | ||
6a518fbc TP |
706 | =item close |
707 | ||
19799a22 | 708 | Closes the file or pipe associated with the file handle, returning true |
a0d0e21e | 709 | only if stdio successfully flushes buffers and closes the system file |
19799a22 | 710 | descriptor. Closes the currently selected filehandle if the argument |
6a518fbc | 711 | is omitted. |
fb73857a | 712 | |
713 | You don't have to close FILEHANDLE if you are immediately going to do | |
19799a22 GS |
714 | another C<open> on it, because C<open> will close it for you. (See |
715 | C<open>.) However, an explicit C<close> on an input file resets the line | |
716 | counter (C<$.>), while the implicit close done by C<open> does not. | |
fb73857a | 717 | |
19799a22 GS |
718 | If the file handle came from a piped open C<close> will additionally |
719 | return false if one of the other system calls involved fails or if the | |
fb73857a | 720 | program exits with non-zero status. (If the only problem was that the |
b76cc8ba | 721 | program exited non-zero C<$!> will be set to C<0>.) Closing a pipe |
2b5ab1e7 | 722 | also waits for the process executing on the pipe to complete, in case you |
b76cc8ba | 723 | want to look at the output of the pipe afterwards, and |
2b5ab1e7 | 724 | implicitly puts the exit status value of that command into C<$?>. |
5a964f20 | 725 | |
73689b13 GS |
726 | Prematurely closing the read end of a pipe (i.e. before the process |
727 | writing to it at the other end has closed it) will result in a | |
728 | SIGPIPE being delivered to the writer. If the other end can't | |
729 | handle that, be sure to read all the data before closing the pipe. | |
730 | ||
fb73857a | 731 | Example: |
a0d0e21e | 732 | |
fb73857a | 733 | open(OUTPUT, '|sort >foo') # pipe to sort |
734 | or die "Can't start sort: $!"; | |
5a964f20 | 735 | #... # print stuff to output |
fb73857a | 736 | close OUTPUT # wait for sort to finish |
737 | or warn $! ? "Error closing sort pipe: $!" | |
738 | : "Exit status $? from sort"; | |
739 | open(INPUT, 'foo') # get sort's results | |
740 | or die "Can't open 'foo' for input: $!"; | |
a0d0e21e | 741 | |
5a964f20 TC |
742 | FILEHANDLE may be an expression whose value can be used as an indirect |
743 | filehandle, usually the real filehandle name. | |
a0d0e21e LW |
744 | |
745 | =item closedir DIRHANDLE | |
746 | ||
19799a22 | 747 | Closes a directory opened by C<opendir> and returns the success of that |
5a964f20 TC |
748 | system call. |
749 | ||
750 | DIRHANDLE may be an expression whose value can be used as an indirect | |
751 | dirhandle, usually the real dirhandle name. | |
a0d0e21e LW |
752 | |
753 | =item connect SOCKET,NAME | |
754 | ||
755 | Attempts to connect to a remote socket, just as the connect system call | |
19799a22 | 756 | does. Returns true if it succeeded, false otherwise. NAME should be a |
4633a7c4 LW |
757 | packed address of the appropriate type for the socket. See the examples in |
758 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 759 | |
cb1a09d0 AD |
760 | =item continue BLOCK |
761 | ||
762 | Actually a flow control statement rather than a function. If there is a | |
98293880 JH |
763 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or |
764 | C<foreach>), it is always executed just before the conditional is about to | |
765 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
cb1a09d0 AD |
766 | it can be used to increment a loop variable, even when the loop has been |
767 | continued via the C<next> statement (which is similar to the C C<continue> | |
768 | statement). | |
769 | ||
98293880 | 770 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
19799a22 GS |
771 | block. C<last> and C<redo> will behave as if they had been executed within |
772 | the main block. So will C<next>, but since it will execute a C<continue> | |
1d2dff63 GS |
773 | block, it may be more entertaining. |
774 | ||
775 | while (EXPR) { | |
776 | ### redo always comes here | |
777 | do_something; | |
778 | } continue { | |
779 | ### next always comes here | |
780 | do_something_else; | |
781 | # then back the top to re-check EXPR | |
782 | } | |
783 | ### last always comes here | |
784 | ||
785 | Omitting the C<continue> section is semantically equivalent to using an | |
19799a22 | 786 | empty one, logically enough. In that case, C<next> goes directly back |
1d2dff63 GS |
787 | to check the condition at the top of the loop. |
788 | ||
a0d0e21e LW |
789 | =item cos EXPR |
790 | ||
d6217f1e GS |
791 | =item cos |
792 | ||
5a964f20 | 793 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 794 | takes cosine of C<$_>. |
a0d0e21e | 795 | |
ca6e1c26 | 796 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
28757baa | 797 | function, or use this relation: |
798 | ||
799 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
800 | ||
a0d0e21e LW |
801 | =item crypt PLAINTEXT,SALT |
802 | ||
f86cebdf | 803 | Encrypts a string exactly like the crypt(3) function in the C library |
4633a7c4 LW |
804 | (assuming that you actually have a version there that has not been |
805 | extirpated as a potential munition). This can prove useful for checking | |
806 | the password file for lousy passwords, amongst other things. Only the | |
807 | guys wearing white hats should do this. | |
a0d0e21e | 808 | |
19799a22 | 809 | Note that C<crypt> is intended to be a one-way function, much like breaking |
11155c91 CS |
810 | eggs to make an omelette. There is no (known) corresponding decrypt |
811 | function. As a result, this function isn't all that useful for | |
812 | cryptography. (For that, see your nearby CPAN mirror.) | |
2f9daede | 813 | |
e71965be RS |
814 | When verifying an existing encrypted string you should use the encrypted |
815 | text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This | |
19799a22 | 816 | allows your code to work with the standard C<crypt> and with more |
e71965be RS |
817 | exotic implementations. When choosing a new salt create a random two |
818 | character string whose characters come from the set C<[./0-9A-Za-z]> | |
819 | (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). | |
820 | ||
a0d0e21e LW |
821 | Here's an example that makes sure that whoever runs this program knows |
822 | their own password: | |
823 | ||
824 | $pwd = (getpwuid($<))[1]; | |
a0d0e21e LW |
825 | |
826 | system "stty -echo"; | |
827 | print "Password: "; | |
e71965be | 828 | chomp($word = <STDIN>); |
a0d0e21e LW |
829 | print "\n"; |
830 | system "stty echo"; | |
831 | ||
e71965be | 832 | if (crypt($word, $pwd) ne $pwd) { |
a0d0e21e LW |
833 | die "Sorry...\n"; |
834 | } else { | |
835 | print "ok\n"; | |
54310121 | 836 | } |
a0d0e21e | 837 | |
9f8f0c9d | 838 | Of course, typing in your own password to whoever asks you |
748a9306 | 839 | for it is unwise. |
a0d0e21e | 840 | |
19799a22 GS |
841 | The L<crypt> function is unsuitable for encrypting large quantities |
842 | of data, not least of all because you can't get the information | |
843 | back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories | |
844 | on your favorite CPAN mirror for a slew of potentially useful | |
845 | modules. | |
846 | ||
aa689395 | 847 | =item dbmclose HASH |
a0d0e21e | 848 | |
19799a22 | 849 | [This function has been largely superseded by the C<untie> function.] |
a0d0e21e | 850 | |
aa689395 | 851 | Breaks the binding between a DBM file and a hash. |
a0d0e21e | 852 | |
19799a22 | 853 | =item dbmopen HASH,DBNAME,MASK |
a0d0e21e | 854 | |
19799a22 | 855 | [This function has been largely superseded by the C<tie> function.] |
a0d0e21e | 856 | |
7b8d334a | 857 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
19799a22 GS |
858 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
859 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
aa689395 | 860 | is the name of the database (without the F<.dir> or F<.pag> extension if |
861 | any). If the database does not exist, it is created with protection | |
19799a22 GS |
862 | specified by MASK (as modified by the C<umask>). If your system supports |
863 | only the older DBM functions, you may perform only one C<dbmopen> in your | |
aa689395 | 864 | program. In older versions of Perl, if your system had neither DBM nor |
19799a22 | 865 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
aa689395 | 866 | sdbm(3). |
867 | ||
868 | If you don't have write access to the DBM file, you can only read hash | |
869 | variables, not set them. If you want to test whether you can write, | |
19799a22 | 870 | either use file tests or try setting a dummy hash entry inside an C<eval>, |
aa689395 | 871 | which will trap the error. |
a0d0e21e | 872 | |
19799a22 GS |
873 | Note that functions such as C<keys> and C<values> may return huge lists |
874 | when used on large DBM files. You may prefer to use the C<each> | |
a0d0e21e LW |
875 | function to iterate over large DBM files. Example: |
876 | ||
877 | # print out history file offsets | |
878 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
879 | while (($key,$val) = each %HIST) { | |
880 | print $key, ' = ', unpack('L',$val), "\n"; | |
881 | } | |
882 | dbmclose(%HIST); | |
883 | ||
cb1a09d0 | 884 | See also L<AnyDBM_File> for a more general description of the pros and |
184e9718 | 885 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
cb1a09d0 | 886 | rich implementation. |
4633a7c4 | 887 | |
2b5ab1e7 TC |
888 | You can control which DBM library you use by loading that library |
889 | before you call dbmopen(): | |
890 | ||
891 | use DB_File; | |
892 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
893 | or die "Can't open netscape history file: $!"; | |
894 | ||
a0d0e21e LW |
895 | =item defined EXPR |
896 | ||
54310121 | 897 | =item defined |
bbce6d69 | 898 | |
2f9daede TP |
899 | Returns a Boolean value telling whether EXPR has a value other than |
900 | the undefined value C<undef>. If EXPR is not present, C<$_> will be | |
901 | checked. | |
902 | ||
903 | Many operations return C<undef> to indicate failure, end of file, | |
904 | system error, uninitialized variable, and other exceptional | |
905 | conditions. This function allows you to distinguish C<undef> from | |
906 | other values. (A simple Boolean test will not distinguish among | |
7660c0ab | 907 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
2f9daede | 908 | false.) Note that since C<undef> is a valid scalar, its presence |
19799a22 | 909 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
2f9daede TP |
910 | returns C<undef> when its argument is an empty array, I<or> when the |
911 | element to return happens to be C<undef>. | |
912 | ||
f10b0346 GS |
913 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
914 | has ever been defined. The return value is unaffected by any forward | |
847c7ebe DD |
915 | declarations of C<&foo>. Note that a subroutine which is not defined |
916 | may still be callable: its package may have an C<AUTOLOAD> method that | |
917 | makes it spring into existence the first time that it is called -- see | |
918 | L<perlsub>. | |
f10b0346 GS |
919 | |
920 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
921 | used to report whether memory for that aggregate has ever been | |
922 | allocated. This behavior may disappear in future versions of Perl. | |
923 | You should instead use a simple test for size: | |
924 | ||
925 | if (@an_array) { print "has array elements\n" } | |
926 | if (%a_hash) { print "has hash members\n" } | |
2f9daede TP |
927 | |
928 | When used on a hash element, it tells you whether the value is defined, | |
dc848c6f | 929 | not whether the key exists in the hash. Use L</exists> for the latter |
2f9daede | 930 | purpose. |
a0d0e21e LW |
931 | |
932 | Examples: | |
933 | ||
934 | print if defined $switch{'D'}; | |
935 | print "$val\n" while defined($val = pop(@ary)); | |
936 | die "Can't readlink $sym: $!" | |
937 | unless defined($value = readlink $sym); | |
a0d0e21e | 938 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
2f9daede | 939 | $debugging = 0 unless defined $debugging; |
a0d0e21e | 940 | |
19799a22 | 941 | Note: Many folks tend to overuse C<defined>, and then are surprised to |
7660c0ab | 942 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
2f9daede | 943 | defined values. For example, if you say |
a5f75d66 AD |
944 | |
945 | "ab" =~ /a(.*)b/; | |
946 | ||
7660c0ab | 947 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
a5f75d66 | 948 | matched "nothing". But it didn't really match nothing--rather, it |
2b5ab1e7 | 949 | matched something that happened to be zero characters long. This is all |
a5f75d66 | 950 | very above-board and honest. When a function returns an undefined value, |
2f9daede | 951 | it's an admission that it couldn't give you an honest answer. So you |
19799a22 | 952 | should use C<defined> only when you're questioning the integrity of what |
7660c0ab | 953 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
2f9daede TP |
954 | what you want. |
955 | ||
dc848c6f | 956 | See also L</undef>, L</exists>, L</ref>. |
2f9daede | 957 | |
a0d0e21e LW |
958 | =item delete EXPR |
959 | ||
01020589 GS |
960 | Given an expression that specifies a hash element, array element, hash slice, |
961 | or array slice, deletes the specified element(s) from the hash or array. | |
8216c1fd | 962 | In the case of an array, if the array elements happen to be at the end, |
b76cc8ba | 963 | the size of the array will shrink to the highest element that tests |
8216c1fd | 964 | true for exists() (or 0 if no such element exists). |
a0d0e21e | 965 | |
01020589 GS |
966 | Returns each element so deleted or the undefined value if there was no such |
967 | element. Deleting from C<$ENV{}> modifies the environment. Deleting from | |
968 | a hash tied to a DBM file deletes the entry from the DBM file. Deleting | |
969 | from a C<tie>d hash or array may not necessarily return anything. | |
970 | ||
8ea97a1e GS |
971 | Deleting an array element effectively returns that position of the array |
972 | to its initial, uninitialized state. Subsequently testing for the same | |
8216c1fd GS |
973 | element with exists() will return false. Note that deleting array |
974 | elements in the middle of an array will not shift the index of the ones | |
975 | after them down--use splice() for that. See L</exists>. | |
8ea97a1e | 976 | |
01020589 | 977 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
a0d0e21e | 978 | |
5f05dabc | 979 | foreach $key (keys %HASH) { |
980 | delete $HASH{$key}; | |
a0d0e21e LW |
981 | } |
982 | ||
01020589 GS |
983 | foreach $index (0 .. $#ARRAY) { |
984 | delete $ARRAY[$index]; | |
985 | } | |
986 | ||
987 | And so do these: | |
5f05dabc | 988 | |
01020589 GS |
989 | delete @HASH{keys %HASH}; |
990 | ||
9740c838 | 991 | delete @ARRAY[0 .. $#ARRAY]; |
5f05dabc | 992 | |
2b5ab1e7 | 993 | But both of these are slower than just assigning the empty list |
01020589 GS |
994 | or undefining %HASH or @ARRAY: |
995 | ||
996 | %HASH = (); # completely empty %HASH | |
997 | undef %HASH; # forget %HASH ever existed | |
2b5ab1e7 | 998 | |
01020589 GS |
999 | @ARRAY = (); # completely empty @ARRAY |
1000 | undef @ARRAY; # forget @ARRAY ever existed | |
2b5ab1e7 TC |
1001 | |
1002 | Note that the EXPR can be arbitrarily complicated as long as the final | |
01020589 GS |
1003 | operation is a hash element, array element, hash slice, or array slice |
1004 | lookup: | |
a0d0e21e LW |
1005 | |
1006 | delete $ref->[$x][$y]{$key}; | |
5f05dabc | 1007 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
a0d0e21e | 1008 | |
01020589 GS |
1009 | delete $ref->[$x][$y][$index]; |
1010 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1011 | ||
a0d0e21e LW |
1012 | =item die LIST |
1013 | ||
19799a22 GS |
1014 | Outside an C<eval>, prints the value of LIST to C<STDERR> and |
1015 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, | |
61eff3bc JH |
1016 | exits with the value of C<<< ($? >> 8) >>> (backtick `command` |
1017 | status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside | |
19799a22 GS |
1018 | an C<eval(),> the error message is stuffed into C<$@> and the |
1019 | C<eval> is terminated with the undefined value. This makes | |
1020 | C<die> the way to raise an exception. | |
a0d0e21e LW |
1021 | |
1022 | Equivalent examples: | |
1023 | ||
1024 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
54310121 | 1025 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
a0d0e21e LW |
1026 | |
1027 | If the value of EXPR does not end in a newline, the current script line | |
1028 | number and input line number (if any) are also printed, and a newline | |
883faa13 GS |
1029 | is supplied. Note that the "input line number" (also known as "chunk") |
1030 | is subject to whatever notion of "line" happens to be currently in | |
1031 | effect, and is also available as the special variable C<$.>. | |
1032 | See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1033 | ||
1034 | Hint: sometimes appending C<", stopped"> to your message | |
7660c0ab | 1035 | will cause it to make better sense when the string C<"at foo line 123"> is |
a0d0e21e LW |
1036 | appended. Suppose you are running script "canasta". |
1037 | ||
1038 | die "/etc/games is no good"; | |
1039 | die "/etc/games is no good, stopped"; | |
1040 | ||
1041 | produce, respectively | |
1042 | ||
1043 | /etc/games is no good at canasta line 123. | |
1044 | /etc/games is no good, stopped at canasta line 123. | |
1045 | ||
2b5ab1e7 | 1046 | See also exit(), warn(), and the Carp module. |
a0d0e21e | 1047 | |
7660c0ab A |
1048 | If LIST is empty and C<$@> already contains a value (typically from a |
1049 | previous eval) that value is reused after appending C<"\t...propagated">. | |
fb73857a | 1050 | This is useful for propagating exceptions: |
1051 | ||
1052 | eval { ... }; | |
1053 | die unless $@ =~ /Expected exception/; | |
1054 | ||
7660c0ab | 1055 | If C<$@> is empty then the string C<"Died"> is used. |
fb73857a | 1056 | |
52531d10 GS |
1057 | die() can also be called with a reference argument. If this happens to be |
1058 | trapped within an eval(), $@ contains the reference. This behavior permits | |
1059 | a more elaborate exception handling implementation using objects that | |
4375e838 | 1060 | maintain arbitrary state about the nature of the exception. Such a scheme |
52531d10 GS |
1061 | is sometimes preferable to matching particular string values of $@ using |
1062 | regular expressions. Here's an example: | |
1063 | ||
1064 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; | |
1065 | if ($@) { | |
1066 | if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { | |
1067 | # handle Some::Module::Exception | |
1068 | } | |
1069 | else { | |
1070 | # handle all other possible exceptions | |
1071 | } | |
1072 | } | |
1073 | ||
19799a22 | 1074 | Because perl will stringify uncaught exception messages before displaying |
52531d10 GS |
1075 | them, you may want to overload stringification operations on such custom |
1076 | exception objects. See L<overload> for details about that. | |
1077 | ||
19799a22 GS |
1078 | You can arrange for a callback to be run just before the C<die> |
1079 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
1080 | handler will be called with the error text and can change the error | |
1081 | message, if it sees fit, by calling C<die> again. See | |
1082 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and | |
1083 | L<"eval BLOCK"> for some examples. Although this feature was meant | |
1084 | to be run only right before your program was to exit, this is not | |
1085 | currently the case--the C<$SIG{__DIE__}> hook is currently called | |
1086 | even inside eval()ed blocks/strings! If one wants the hook to do | |
1087 | nothing in such situations, put | |
fb73857a | 1088 | |
1089 | die @_ if $^S; | |
1090 | ||
19799a22 GS |
1091 | as the first line of the handler (see L<perlvar/$^S>). Because |
1092 | this promotes strange action at a distance, this counterintuitive | |
b76cc8ba | 1093 | behavior may be fixed in a future release. |
774d564b | 1094 | |
a0d0e21e LW |
1095 | =item do BLOCK |
1096 | ||
1097 | Not really a function. Returns the value of the last command in the | |
1098 | sequence of commands indicated by BLOCK. When modified by a loop | |
98293880 JH |
1099 | modifier, executes the BLOCK once before testing the loop condition. |
1100 | (On other statements the loop modifiers test the conditional first.) | |
a0d0e21e | 1101 | |
4968c1e4 | 1102 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 TC |
1103 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
1104 | See L<perlsyn> for alternative strategies. | |
4968c1e4 | 1105 | |
a0d0e21e LW |
1106 | =item do SUBROUTINE(LIST) |
1107 | ||
1108 | A deprecated form of subroutine call. See L<perlsub>. | |
1109 | ||
1110 | =item do EXPR | |
1111 | ||
1112 | Uses the value of EXPR as a filename and executes the contents of the | |
1113 | file as a Perl script. Its primary use is to include subroutines | |
1114 | from a Perl subroutine library. | |
1115 | ||
1116 | do 'stat.pl'; | |
1117 | ||
1118 | is just like | |
1119 | ||
986b19de | 1120 | eval `cat stat.pl`; |
a0d0e21e | 1121 | |
2b5ab1e7 TC |
1122 | except that it's more efficient and concise, keeps track of the current |
1123 | filename for error messages, searches the @INC libraries, and updates | |
1124 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these | |
1125 | variables. It also differs in that code evaluated with C<do FILENAME> | |
1126 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1127 | same, however, in that it does reparse the file every time you call it, | |
1128 | so you probably don't want to do this inside a loop. | |
a0d0e21e | 1129 | |
8e30cc93 | 1130 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
2b5ab1e7 | 1131 | error. If C<do> can read the file but cannot compile it, it |
8e30cc93 MG |
1132 | returns undef and sets an error message in C<$@>. If the file is |
1133 | successfully compiled, C<do> returns the value of the last expression | |
1134 | evaluated. | |
1135 | ||
a0d0e21e | 1136 | Note that inclusion of library modules is better done with the |
19799a22 | 1137 | C<use> and C<require> operators, which also do automatic error checking |
4633a7c4 | 1138 | and raise an exception if there's a problem. |
a0d0e21e | 1139 | |
5a964f20 TC |
1140 | You might like to use C<do> to read in a program configuration |
1141 | file. Manual error checking can be done this way: | |
1142 | ||
b76cc8ba | 1143 | # read in config files: system first, then user |
f86cebdf | 1144 | for $file ("/share/prog/defaults.rc", |
b76cc8ba | 1145 | "$ENV{HOME}/.someprogrc") |
2b5ab1e7 | 1146 | { |
5a964f20 | 1147 | unless ($return = do $file) { |
f86cebdf GS |
1148 | warn "couldn't parse $file: $@" if $@; |
1149 | warn "couldn't do $file: $!" unless defined $return; | |
1150 | warn "couldn't run $file" unless $return; | |
5a964f20 TC |
1151 | } |
1152 | } | |
1153 | ||
a0d0e21e LW |
1154 | =item dump LABEL |
1155 | ||
1614b0e3 JD |
1156 | =item dump |
1157 | ||
19799a22 GS |
1158 | This function causes an immediate core dump. See also the B<-u> |
1159 | command-line switch in L<perlrun>, which does the same thing. | |
1160 | Primarily this is so that you can use the B<undump> program (not | |
1161 | supplied) to turn your core dump into an executable binary after | |
1162 | having initialized all your variables at the beginning of the | |
1163 | program. When the new binary is executed it will begin by executing | |
1164 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1165 | Think of it as a goto with an intervening core dump and reincarnation. | |
1166 | If C<LABEL> is omitted, restarts the program from the top. | |
1167 | ||
1168 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1169 | be open any more when the program is reincarnated, with possible | |
b76cc8ba | 1170 | resulting confusion on the part of Perl. |
19799a22 GS |
1171 | |
1172 | This function is now largely obsolete, partly because it's very | |
1173 | hard to convert a core file into an executable, and because the | |
1174 | real compiler backends for generating portable bytecode and compilable | |
1175 | C code have superseded it. | |
1176 | ||
1177 | If you're looking to use L<dump> to speed up your program, consider | |
1178 | generating bytecode or native C code as described in L<perlcc>. If | |
1179 | you're just trying to accelerate a CGI script, consider using the | |
1180 | C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. | |
1181 | You might also consider autoloading or selfloading, which at least | |
b76cc8ba | 1182 | make your program I<appear> to run faster. |
5a964f20 | 1183 | |
aa689395 | 1184 | =item each HASH |
1185 | ||
5a964f20 | 1186 | When called in list context, returns a 2-element list consisting of the |
aa689395 | 1187 | key and value for the next element of a hash, so that you can iterate over |
74fc8b5f | 1188 | it. When called in scalar context, returns only the key for the next |
e902a979 | 1189 | element in the hash. |
2f9daede | 1190 | |
ab192400 GS |
1191 | Entries are returned in an apparently random order. The actual random |
1192 | order is subject to change in future versions of perl, but it is guaranteed | |
19799a22 | 1193 | to be in the same order as either the C<keys> or C<values> function |
ab192400 GS |
1194 | would produce on the same (unmodified) hash. |
1195 | ||
1196 | When the hash is entirely read, a null array is returned in list context | |
19799a22 GS |
1197 | (which when assigned produces a false (C<0>) value), and C<undef> in |
1198 | scalar context. The next call to C<each> after that will start iterating | |
1199 | again. There is a single iterator for each hash, shared by all C<each>, | |
1200 | C<keys>, and C<values> function calls in the program; it can be reset by | |
2f9daede TP |
1201 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
1202 | C<values HASH>. If you add or delete elements of a hash while you're | |
74fc8b5f MJD |
1203 | iterating over it, you may get entries skipped or duplicated, so |
1204 | don't. Exception: It is always safe to delete the item most recently | |
1205 | returned by C<each()>, which means that the following code will work: | |
1206 | ||
1207 | while (($key, $value) = each %hash) { | |
1208 | print $key, "\n"; | |
1209 | delete $hash{$key}; # This is safe | |
1210 | } | |
aa689395 | 1211 | |
f86cebdf | 1212 | The following prints out your environment like the printenv(1) program, |
aa689395 | 1213 | only in a different order: |
a0d0e21e LW |
1214 | |
1215 | while (($key,$value) = each %ENV) { | |
1216 | print "$key=$value\n"; | |
1217 | } | |
1218 | ||
19799a22 | 1219 | See also C<keys>, C<values> and C<sort>. |
a0d0e21e LW |
1220 | |
1221 | =item eof FILEHANDLE | |
1222 | ||
4633a7c4 LW |
1223 | =item eof () |
1224 | ||
a0d0e21e LW |
1225 | =item eof |
1226 | ||
1227 | Returns 1 if the next read on FILEHANDLE will return end of file, or if | |
1228 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
5a964f20 | 1229 | gives the real filehandle. (Note that this function actually |
19799a22 | 1230 | reads a character and then C<ungetc>s it, so isn't very useful in an |
748a9306 | 1231 | interactive context.) Do not read from a terminal file (or call |
19799a22 | 1232 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
748a9306 LW |
1233 | as terminals may lose the end-of-file condition if you do. |
1234 | ||
820475bd GS |
1235 | An C<eof> without an argument uses the last file read. Using C<eof()> |
1236 | with empty parentheses is very different. It refers to the pseudo file | |
1237 | formed from the files listed on the command line and accessed via the | |
61eff3bc JH |
1238 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, |
1239 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
820475bd GS |
1240 | used will cause C<@ARGV> to be examined to determine if input is |
1241 | available. | |
1242 | ||
61eff3bc | 1243 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to |
820475bd GS |
1244 | detect the end of each file, C<eof()> will only detect the end of the |
1245 | last file. Examples: | |
a0d0e21e | 1246 | |
748a9306 LW |
1247 | # reset line numbering on each input file |
1248 | while (<>) { | |
b76cc8ba | 1249 | next if /^\s*#/; # skip comments |
748a9306 | 1250 | print "$.\t$_"; |
5a964f20 TC |
1251 | } continue { |
1252 | close ARGV if eof; # Not eof()! | |
748a9306 LW |
1253 | } |
1254 | ||
a0d0e21e LW |
1255 | # insert dashes just before last line of last file |
1256 | while (<>) { | |
5a964f20 | 1257 | if (eof()) { # check for end of current file |
a0d0e21e | 1258 | print "--------------\n"; |
2b5ab1e7 | 1259 | close(ARGV); # close or last; is needed if we |
748a9306 | 1260 | # are reading from the terminal |
a0d0e21e LW |
1261 | } |
1262 | print; | |
1263 | } | |
1264 | ||
a0d0e21e | 1265 | Practical hint: you almost never need to use C<eof> in Perl, because the |
3ce0d271 GS |
1266 | input operators typically return C<undef> when they run out of data, or if |
1267 | there was an error. | |
a0d0e21e LW |
1268 | |
1269 | =item eval EXPR | |
1270 | ||
1271 | =item eval BLOCK | |
1272 | ||
c7cc6f1c GS |
1273 | In the first form, the return value of EXPR is parsed and executed as if it |
1274 | were a little Perl program. The value of the expression (which is itself | |
5a964f20 | 1275 | determined within scalar context) is first parsed, and if there weren't any |
be3174d2 GS |
1276 | errors, executed in the lexical context of the current Perl program, so |
1277 | that any variable settings or subroutine and format definitions remain | |
1278 | afterwards. Note that the value is parsed every time the eval executes. | |
1279 | If EXPR is omitted, evaluates C<$_>. This form is typically used to | |
1280 | delay parsing and subsequent execution of the text of EXPR until run time. | |
c7cc6f1c GS |
1281 | |
1282 | In the second form, the code within the BLOCK is parsed only once--at the | |
1283 | same time the code surrounding the eval itself was parsed--and executed | |
1284 | within the context of the current Perl program. This form is typically | |
1285 | used to trap exceptions more efficiently than the first (see below), while | |
1286 | also providing the benefit of checking the code within BLOCK at compile | |
1287 | time. | |
1288 | ||
1289 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1290 | the BLOCK. | |
1291 | ||
1292 | In both forms, the value returned is the value of the last expression | |
5a964f20 | 1293 | evaluated inside the mini-program; a return statement may be also used, just |
c7cc6f1c | 1294 | as with subroutines. The expression providing the return value is evaluated |
5a964f20 | 1295 | in void, scalar, or list context, depending on the context of the eval itself. |
c7cc6f1c | 1296 | See L</wantarray> for more on how the evaluation context can be determined. |
a0d0e21e | 1297 | |
19799a22 GS |
1298 | If there is a syntax error or runtime error, or a C<die> statement is |
1299 | executed, an undefined value is returned by C<eval>, and C<$@> is set to the | |
a0d0e21e | 1300 | error message. If there was no error, C<$@> is guaranteed to be a null |
19799a22 | 1301 | string. Beware that using C<eval> neither silences perl from printing |
c7cc6f1c GS |
1302 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
1303 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See | |
1304 | L</warn> and L<perlvar>. | |
a0d0e21e | 1305 | |
19799a22 GS |
1306 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
1307 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
a0d0e21e LW |
1308 | is implemented. It is also Perl's exception trapping mechanism, where |
1309 | the die operator is used to raise exceptions. | |
1310 | ||
1311 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1312 | form to trap run-time errors without incurring the penalty of | |
1313 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1314 | Examples: | |
1315 | ||
54310121 | 1316 | # make divide-by-zero nonfatal |
a0d0e21e LW |
1317 | eval { $answer = $a / $b; }; warn $@ if $@; |
1318 | ||
1319 | # same thing, but less efficient | |
1320 | eval '$answer = $a / $b'; warn $@ if $@; | |
1321 | ||
1322 | # a compile-time error | |
5a964f20 | 1323 | eval { $answer = }; # WRONG |
a0d0e21e LW |
1324 | |
1325 | # a run-time error | |
1326 | eval '$answer ='; # sets $@ | |
1327 | ||
2b5ab1e7 TC |
1328 | Due to the current arguably broken state of C<__DIE__> hooks, when using |
1329 | the C<eval{}> form as an exception trap in libraries, you may wish not | |
1330 | to trigger any C<__DIE__> hooks that user code may have installed. | |
1331 | You can use the C<local $SIG{__DIE__}> construct for this purpose, | |
1332 | as shown in this example: | |
774d564b | 1333 | |
1334 | # a very private exception trap for divide-by-zero | |
f86cebdf GS |
1335 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
1336 | warn $@ if $@; | |
774d564b | 1337 | |
1338 | This is especially significant, given that C<__DIE__> hooks can call | |
19799a22 | 1339 | C<die> again, which has the effect of changing their error messages: |
774d564b | 1340 | |
1341 | # __DIE__ hooks may modify error messages | |
1342 | { | |
f86cebdf GS |
1343 | local $SIG{'__DIE__'} = |
1344 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
c7cc6f1c GS |
1345 | eval { die "foo lives here" }; |
1346 | print $@ if $@; # prints "bar lives here" | |
774d564b | 1347 | } |
1348 | ||
19799a22 | 1349 | Because this promotes action at a distance, this counterintuitive behavior |
2b5ab1e7 TC |
1350 | may be fixed in a future release. |
1351 | ||
19799a22 | 1352 | With an C<eval>, you should be especially careful to remember what's |
a0d0e21e LW |
1353 | being looked at when: |
1354 | ||
1355 | eval $x; # CASE 1 | |
1356 | eval "$x"; # CASE 2 | |
1357 | ||
1358 | eval '$x'; # CASE 3 | |
1359 | eval { $x }; # CASE 4 | |
1360 | ||
5a964f20 | 1361 | eval "\$$x++"; # CASE 5 |
a0d0e21e LW |
1362 | $$x++; # CASE 6 |
1363 | ||
2f9daede | 1364 | Cases 1 and 2 above behave identically: they run the code contained in |
19799a22 | 1365 | the variable $x. (Although case 2 has misleading double quotes making |
2f9daede | 1366 | the reader wonder what else might be happening (nothing is).) Cases 3 |
7660c0ab | 1367 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
19799a22 | 1368 | does nothing but return the value of $x. (Case 4 is preferred for |
2f9daede TP |
1369 | purely visual reasons, but it also has the advantage of compiling at |
1370 | compile-time instead of at run-time.) Case 5 is a place where | |
19799a22 | 1371 | normally you I<would> like to use double quotes, except that in this |
2f9daede TP |
1372 | particular situation, you can just use symbolic references instead, as |
1373 | in case 6. | |
a0d0e21e | 1374 | |
4968c1e4 | 1375 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
2b5ab1e7 | 1376 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
4968c1e4 | 1377 | |
a0d0e21e LW |
1378 | =item exec LIST |
1379 | ||
8bf3b016 GS |
1380 | =item exec PROGRAM LIST |
1381 | ||
19799a22 GS |
1382 | The C<exec> function executes a system command I<and never returns>-- |
1383 | use C<system> instead of C<exec> if you want it to return. It fails and | |
1384 | returns false only if the command does not exist I<and> it is executed | |
fb73857a | 1385 | directly instead of via your system's command shell (see below). |
a0d0e21e | 1386 | |
19799a22 GS |
1387 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
1388 | warns you if there is a following statement which isn't C<die>, C<warn>, | |
1389 | or C<exit> (if C<-w> is set - but you always do that). If you | |
1390 | I<really> want to follow an C<exec> with some other statement, you | |
55d729e4 GS |
1391 | can use one of these styles to avoid the warning: |
1392 | ||
5a964f20 TC |
1393 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
1394 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
55d729e4 | 1395 | |
5a964f20 | 1396 | If there is more than one argument in LIST, or if LIST is an array |
f86cebdf | 1397 | with more than one value, calls execvp(3) with the arguments in LIST. |
5a964f20 TC |
1398 | If there is only one scalar argument or an array with one element in it, |
1399 | the argument is checked for shell metacharacters, and if there are any, | |
1400 | the entire argument is passed to the system's command shell for parsing | |
1401 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1402 | If there are no shell metacharacters in the argument, it is split into | |
b76cc8ba | 1403 | words and passed directly to C<execvp>, which is more efficient. |
19799a22 | 1404 | Examples: |
a0d0e21e | 1405 | |
19799a22 GS |
1406 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
1407 | exec "sort $outfile | uniq"; | |
a0d0e21e LW |
1408 | |
1409 | If you don't really want to execute the first argument, but want to lie | |
1410 | to the program you are executing about its own name, you can specify | |
1411 | the program you actually want to run as an "indirect object" (without a | |
1412 | comma) in front of the LIST. (This always forces interpretation of the | |
54310121 | 1413 | LIST as a multivalued list, even if there is only a single scalar in |
a0d0e21e LW |
1414 | the list.) Example: |
1415 | ||
1416 | $shell = '/bin/csh'; | |
1417 | exec $shell '-sh'; # pretend it's a login shell | |
1418 | ||
1419 | or, more directly, | |
1420 | ||
1421 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1422 | ||
bb32b41a GS |
1423 | When the arguments get executed via the system shell, results will |
1424 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1425 | for details. | |
1426 | ||
19799a22 GS |
1427 | Using an indirect object with C<exec> or C<system> is also more |
1428 | secure. This usage (which also works fine with system()) forces | |
1429 | interpretation of the arguments as a multivalued list, even if the | |
1430 | list had just one argument. That way you're safe from the shell | |
1431 | expanding wildcards or splitting up words with whitespace in them. | |
5a964f20 TC |
1432 | |
1433 | @args = ( "echo surprise" ); | |
1434 | ||
2b5ab1e7 | 1435 | exec @args; # subject to shell escapes |
f86cebdf | 1436 | # if @args == 1 |
2b5ab1e7 | 1437 | exec { $args[0] } @args; # safe even with one-arg list |
5a964f20 TC |
1438 | |
1439 | The first version, the one without the indirect object, ran the I<echo> | |
1440 | program, passing it C<"surprise"> an argument. The second version | |
1441 | didn't--it tried to run a program literally called I<"echo surprise">, | |
1442 | didn't find it, and set C<$?> to a non-zero value indicating failure. | |
1443 | ||
0f897271 GS |
1444 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1445 | output before the exec, but this may not be supported on some platforms | |
1446 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH | |
1447 | in English) or call the C<autoflush()> method of C<IO::Handle> on any | |
1448 | open handles in order to avoid lost output. | |
1449 | ||
19799a22 | 1450 | Note that C<exec> will not call your C<END> blocks, nor will it call |
7660c0ab A |
1451 | any C<DESTROY> methods in your objects. |
1452 | ||
a0d0e21e LW |
1453 | =item exists EXPR |
1454 | ||
01020589 | 1455 | Given an expression that specifies a hash element or array element, |
8ea97a1e GS |
1456 | returns true if the specified element in the hash or array has ever |
1457 | been initialized, even if the corresponding value is undefined. The | |
1458 | element is not autovivified if it doesn't exist. | |
a0d0e21e | 1459 | |
01020589 GS |
1460 | print "Exists\n" if exists $hash{$key}; |
1461 | print "Defined\n" if defined $hash{$key}; | |
1462 | print "True\n" if $hash{$key}; | |
1463 | ||
1464 | print "Exists\n" if exists $array[$index]; | |
1465 | print "Defined\n" if defined $array[$index]; | |
1466 | print "True\n" if $array[$index]; | |
a0d0e21e | 1467 | |
8ea97a1e | 1468 | A hash or array element can be true only if it's defined, and defined if |
a0d0e21e LW |
1469 | it exists, but the reverse doesn't necessarily hold true. |
1470 | ||
afebc493 GS |
1471 | Given an expression that specifies the name of a subroutine, |
1472 | returns true if the specified subroutine has ever been declared, even | |
1473 | if it is undefined. Mentioning a subroutine name for exists or defined | |
847c7ebe DD |
1474 | does not count as declaring it. Note that a subroutine which does not |
1475 | exist may still be callable: its package may have an C<AUTOLOAD> | |
1476 | method that makes it spring into existence the first time that it is | |
1477 | called -- see L<perlsub>. | |
afebc493 GS |
1478 | |
1479 | print "Exists\n" if exists &subroutine; | |
1480 | print "Defined\n" if defined &subroutine; | |
1481 | ||
a0d0e21e | 1482 | Note that the EXPR can be arbitrarily complicated as long as the final |
afebc493 | 1483 | operation is a hash or array key lookup or subroutine name: |
a0d0e21e | 1484 | |
2b5ab1e7 TC |
1485 | if (exists $ref->{A}->{B}->{$key}) { } |
1486 | if (exists $hash{A}{B}{$key}) { } | |
1487 | ||
01020589 GS |
1488 | if (exists $ref->{A}->{B}->[$ix]) { } |
1489 | if (exists $hash{A}{B}[$ix]) { } | |
1490 | ||
afebc493 GS |
1491 | if (exists &{$ref->{A}{B}{$key}}) { } |
1492 | ||
01020589 GS |
1493 | Although the deepest nested array or hash will not spring into existence |
1494 | just because its existence was tested, any intervening ones will. | |
61eff3bc | 1495 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring |
01020589 GS |
1496 | into existence due to the existence test for the $key element above. |
1497 | This happens anywhere the arrow operator is used, including even: | |
5a964f20 | 1498 | |
2b5ab1e7 TC |
1499 | undef $ref; |
1500 | if (exists $ref->{"Some key"}) { } | |
1501 | print $ref; # prints HASH(0x80d3d5c) | |
1502 | ||
1503 | This surprising autovivification in what does not at first--or even | |
1504 | second--glance appear to be an lvalue context may be fixed in a future | |
5a964f20 | 1505 | release. |
a0d0e21e | 1506 | |
479ba383 GS |
1507 | See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics |
1508 | on how exists() acts when used on a pseudo-hash. | |
e0478e5a | 1509 | |
afebc493 GS |
1510 | Use of a subroutine call, rather than a subroutine name, as an argument |
1511 | to exists() is an error. | |
1512 | ||
1513 | exists ⊂ # OK | |
1514 | exists &sub(); # Error | |
1515 | ||
a0d0e21e LW |
1516 | =item exit EXPR |
1517 | ||
2b5ab1e7 | 1518 | Evaluates EXPR and exits immediately with that value. Example: |
a0d0e21e LW |
1519 | |
1520 | $ans = <STDIN>; | |
1521 | exit 0 if $ans =~ /^[Xx]/; | |
1522 | ||
19799a22 | 1523 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
2b5ab1e7 TC |
1524 | universally recognized values for EXPR are C<0> for success and C<1> |
1525 | for error; other values are subject to interpretation depending on the | |
1526 | environment in which the Perl program is running. For example, exiting | |
1527 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1528 | the mailer to return the item undelivered, but that's not true everywhere. | |
a0d0e21e | 1529 | |
19799a22 GS |
1530 | Don't use C<exit> to abort a subroutine if there's any chance that |
1531 | someone might want to trap whatever error happened. Use C<die> instead, | |
1532 | which can be trapped by an C<eval>. | |
28757baa | 1533 | |
19799a22 | 1534 | The exit() function does not always exit immediately. It calls any |
2b5ab1e7 | 1535 | defined C<END> routines first, but these C<END> routines may not |
19799a22 | 1536 | themselves abort the exit. Likewise any object destructors that need to |
2b5ab1e7 TC |
1537 | be called are called before the real exit. If this is a problem, you |
1538 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
87275199 | 1539 | See L<perlmod> for details. |
5a964f20 | 1540 | |
a0d0e21e LW |
1541 | =item exp EXPR |
1542 | ||
54310121 | 1543 | =item exp |
bbce6d69 | 1544 | |
b76cc8ba | 1545 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
a0d0e21e LW |
1546 | If EXPR is omitted, gives C<exp($_)>. |
1547 | ||
1548 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1549 | ||
f86cebdf | 1550 | Implements the fcntl(2) function. You'll probably have to say |
a0d0e21e LW |
1551 | |
1552 | use Fcntl; | |
1553 | ||
0ade1984 | 1554 | first to get the correct constant definitions. Argument processing and |
b76cc8ba | 1555 | value return works just like C<ioctl> below. |
a0d0e21e LW |
1556 | For example: |
1557 | ||
1558 | use Fcntl; | |
5a964f20 TC |
1559 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
1560 | or die "can't fcntl F_GETFL: $!"; | |
1561 | ||
19799a22 | 1562 | You don't have to check for C<defined> on the return from C<fnctl>. |
951ba7fe GS |
1563 | Like C<ioctl>, it maps a C<0> return from the system call into |
1564 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
2b5ab1e7 TC |
1565 | in numeric context. It is also exempt from the normal B<-w> warnings |
1566 | on improper numeric conversions. | |
5a964f20 | 1567 | |
19799a22 | 1568 | Note that C<fcntl> will produce a fatal error if used on a machine that |
2b5ab1e7 TC |
1569 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
1570 | manpage to learn what functions are available on your system. | |
a0d0e21e LW |
1571 | |
1572 | =item fileno FILEHANDLE | |
1573 | ||
2b5ab1e7 TC |
1574 | Returns the file descriptor for a filehandle, or undefined if the |
1575 | filehandle is not open. This is mainly useful for constructing | |
19799a22 | 1576 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
2b5ab1e7 TC |
1577 | If FILEHANDLE is an expression, the value is taken as an indirect |
1578 | filehandle, generally its name. | |
5a964f20 | 1579 | |
b76cc8ba | 1580 | You can use this to find out whether two handles refer to the |
5a964f20 TC |
1581 | same underlying descriptor: |
1582 | ||
1583 | if (fileno(THIS) == fileno(THAT)) { | |
1584 | print "THIS and THAT are dups\n"; | |
b76cc8ba NIS |
1585 | } |
1586 | ||
1587 | (Filehandles connected to memory objects via new features of C<open> may | |
1588 | return undefined even though they are open.) | |
1589 | ||
a0d0e21e LW |
1590 | |
1591 | =item flock FILEHANDLE,OPERATION | |
1592 | ||
19799a22 GS |
1593 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
1594 | for success, false on failure. Produces a fatal error if used on a | |
2b5ab1e7 | 1595 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
19799a22 | 1596 | C<flock> is Perl's portable file locking interface, although it locks |
2b5ab1e7 TC |
1597 | only entire files, not records. |
1598 | ||
1599 | Two potentially non-obvious but traditional C<flock> semantics are | |
1600 | that it waits indefinitely until the lock is granted, and that its locks | |
1601 | B<merely advisory>. Such discretionary locks are more flexible, but offer | |
19799a22 GS |
1602 | fewer guarantees. This means that files locked with C<flock> may be |
1603 | modified by programs that do not also use C<flock>. See L<perlport>, | |
2b5ab1e7 TC |
1604 | your port's specific documentation, or your system-specific local manpages |
1605 | for details. It's best to assume traditional behavior if you're writing | |
1606 | portable programs. (But if you're not, you should as always feel perfectly | |
1607 | free to write for your own system's idiosyncrasies (sometimes called | |
1608 | "features"). Slavish adherence to portability concerns shouldn't get | |
1609 | in the way of your getting your job done.) | |
a3cb178b | 1610 | |
8ebc5c01 | 1611 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
1612 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
ea3105be | 1613 | you can use the symbolic names if you import them from the Fcntl module, |
68dc0745 | 1614 | either individually, or as a group using the ':flock' tag. LOCK_SH |
1615 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
ea3105be GS |
1616 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
1617 | LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking | |
68dc0745 | 1618 | waiting for the lock (check the return status to see if you got it). |
1619 | ||
2b5ab1e7 TC |
1620 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
1621 | before locking or unlocking it. | |
8ebc5c01 | 1622 | |
f86cebdf | 1623 | Note that the emulation built with lockf(3) doesn't provide shared |
8ebc5c01 | 1624 | locks, and it requires that FILEHANDLE be open with write intent. These |
2b5ab1e7 | 1625 | are the semantics that lockf(3) implements. Most if not all systems |
f86cebdf | 1626 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
8ebc5c01 | 1627 | differing semantics shouldn't bite too many people. |
1628 | ||
19799a22 GS |
1629 | Note also that some versions of C<flock> cannot lock things over the |
1630 | network; you would need to use the more system-specific C<fcntl> for | |
f86cebdf GS |
1631 | that. If you like you can force Perl to ignore your system's flock(2) |
1632 | function, and so provide its own fcntl(2)-based emulation, by passing | |
8ebc5c01 | 1633 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
1634 | perl. | |
4633a7c4 LW |
1635 | |
1636 | Here's a mailbox appender for BSD systems. | |
a0d0e21e | 1637 | |
7e1af8bc | 1638 | use Fcntl ':flock'; # import LOCK_* constants |
a0d0e21e LW |
1639 | |
1640 | sub lock { | |
7e1af8bc | 1641 | flock(MBOX,LOCK_EX); |
a0d0e21e LW |
1642 | # and, in case someone appended |
1643 | # while we were waiting... | |
1644 | seek(MBOX, 0, 2); | |
1645 | } | |
1646 | ||
1647 | sub unlock { | |
7e1af8bc | 1648 | flock(MBOX,LOCK_UN); |
a0d0e21e LW |
1649 | } |
1650 | ||
1651 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") | |
1652 | or die "Can't open mailbox: $!"; | |
1653 | ||
1654 | lock(); | |
1655 | print MBOX $msg,"\n\n"; | |
1656 | unlock(); | |
1657 | ||
2b5ab1e7 TC |
1658 | On systems that support a real flock(), locks are inherited across fork() |
1659 | calls, whereas those that must resort to the more capricious fcntl() | |
1660 | function lose the locks, making it harder to write servers. | |
1661 | ||
cb1a09d0 | 1662 | See also L<DB_File> for other flock() examples. |
a0d0e21e LW |
1663 | |
1664 | =item fork | |
1665 | ||
2b5ab1e7 TC |
1666 | Does a fork(2) system call to create a new process running the |
1667 | same program at the same point. It returns the child pid to the | |
1668 | parent process, C<0> to the child process, or C<undef> if the fork is | |
1669 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
1670 | are shared, while everything else is copied. On most systems supporting | |
1671 | fork(), great care has gone into making it extremely efficient (for | |
1672 | example, using copy-on-write technology on data pages), making it the | |
1673 | dominant paradigm for multitasking over the last few decades. | |
5a964f20 | 1674 | |
0f897271 GS |
1675 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
1676 | output before forking the child process, but this may not be supported | |
1677 | on some platforms (see L<perlport>). To be safe, you may need to set | |
1678 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
1679 | C<IO::Handle> on any open handles in order to avoid duplicate output. | |
a0d0e21e | 1680 | |
19799a22 | 1681 | If you C<fork> without ever waiting on your children, you will |
2b5ab1e7 TC |
1682 | accumulate zombies. On some systems, you can avoid this by setting |
1683 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
1684 | forking and reaping moribund children. | |
cb1a09d0 | 1685 | |
28757baa | 1686 | Note that if your forked child inherits system file descriptors like |
1687 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2b5ab1e7 | 1688 | if you exit, then the remote server (such as, say, a CGI script or a |
19799a22 | 1689 | backgrounded job launched from a remote shell) won't think you're done. |
2b5ab1e7 | 1690 | You should reopen those to F</dev/null> if it's any issue. |
28757baa | 1691 | |
cb1a09d0 AD |
1692 | =item format |
1693 | ||
19799a22 | 1694 | Declare a picture format for use by the C<write> function. For |
cb1a09d0 AD |
1695 | example: |
1696 | ||
54310121 | 1697 | format Something = |
cb1a09d0 AD |
1698 | Test: @<<<<<<<< @||||| @>>>>> |
1699 | $str, $%, '$' . int($num) | |
1700 | . | |
1701 | ||
1702 | $str = "widget"; | |
184e9718 | 1703 | $num = $cost/$quantity; |
cb1a09d0 AD |
1704 | $~ = 'Something'; |
1705 | write; | |
1706 | ||
1707 | See L<perlform> for many details and examples. | |
1708 | ||
8903cb82 | 1709 | =item formline PICTURE,LIST |
a0d0e21e | 1710 | |
5a964f20 | 1711 | This is an internal function used by C<format>s, though you may call it, |
a0d0e21e LW |
1712 | too. It formats (see L<perlform>) a list of values according to the |
1713 | contents of PICTURE, placing the output into the format output | |
7660c0ab | 1714 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
19799a22 | 1715 | Eventually, when a C<write> is done, the contents of |
a0d0e21e | 1716 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
7660c0ab | 1717 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
19799a22 | 1718 | does one C<formline> per line of form, but the C<formline> function itself |
748a9306 | 1719 | doesn't care how many newlines are embedded in the PICTURE. This means |
4633a7c4 | 1720 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
748a9306 LW |
1721 | You may therefore need to use multiple formlines to implement a single |
1722 | record format, just like the format compiler. | |
1723 | ||
19799a22 | 1724 | Be careful if you put double quotes around the picture, because an C<@> |
748a9306 | 1725 | character may be taken to mean the beginning of an array name. |
19799a22 | 1726 | C<formline> always returns true. See L<perlform> for other examples. |
a0d0e21e LW |
1727 | |
1728 | =item getc FILEHANDLE | |
1729 | ||
1730 | =item getc | |
1731 | ||
1732 | Returns the next character from the input file attached to FILEHANDLE, | |
2b5ab1e7 TC |
1733 | or the undefined value at end of file, or if there was an error. |
1734 | If FILEHANDLE is omitted, reads from STDIN. This is not particularly | |
1735 | efficient. However, it cannot be used by itself to fetch single | |
1736 | characters without waiting for the user to hit enter. For that, try | |
1737 | something more like: | |
4633a7c4 LW |
1738 | |
1739 | if ($BSD_STYLE) { | |
1740 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
1741 | } | |
1742 | else { | |
54310121 | 1743 | system "stty", '-icanon', 'eol', "\001"; |
4633a7c4 LW |
1744 | } |
1745 | ||
1746 | $key = getc(STDIN); | |
1747 | ||
1748 | if ($BSD_STYLE) { | |
1749 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
1750 | } | |
1751 | else { | |
5f05dabc | 1752 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
4633a7c4 LW |
1753 | } |
1754 | print "\n"; | |
1755 | ||
54310121 | 1756 | Determination of whether $BSD_STYLE should be set |
1757 | is left as an exercise to the reader. | |
cb1a09d0 | 1758 | |
19799a22 | 1759 | The C<POSIX::getattr> function can do this more portably on |
2b5ab1e7 TC |
1760 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
1761 | module from your nearest CPAN site; details on CPAN can be found on | |
1762 | L<perlmodlib/CPAN>. | |
a0d0e21e LW |
1763 | |
1764 | =item getlogin | |
1765 | ||
5a964f20 TC |
1766 | Implements the C library function of the same name, which on most |
1767 | systems returns the current login from F</etc/utmp>, if any. If null, | |
19799a22 | 1768 | use C<getpwuid>. |
a0d0e21e | 1769 | |
f86702cc | 1770 | $login = getlogin || getpwuid($<) || "Kilroy"; |
a0d0e21e | 1771 | |
19799a22 GS |
1772 | Do not consider C<getlogin> for authentication: it is not as |
1773 | secure as C<getpwuid>. | |
4633a7c4 | 1774 | |
a0d0e21e LW |
1775 | =item getpeername SOCKET |
1776 | ||
1777 | Returns the packed sockaddr address of other end of the SOCKET connection. | |
1778 | ||
4633a7c4 LW |
1779 | use Socket; |
1780 | $hersockaddr = getpeername(SOCK); | |
19799a22 | 1781 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
4633a7c4 LW |
1782 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
1783 | $herstraddr = inet_ntoa($iaddr); | |
a0d0e21e LW |
1784 | |
1785 | =item getpgrp PID | |
1786 | ||
47e29363 | 1787 | Returns the current process group for the specified PID. Use |
7660c0ab | 1788 | a PID of C<0> to get the current process group for the |
4633a7c4 | 1789 | current process. Will raise an exception if used on a machine that |
f86cebdf | 1790 | doesn't implement getpgrp(2). If PID is omitted, returns process |
19799a22 | 1791 | group of current process. Note that the POSIX version of C<getpgrp> |
7660c0ab | 1792 | does not accept a PID argument, so only C<PID==0> is truly portable. |
a0d0e21e LW |
1793 | |
1794 | =item getppid | |
1795 | ||
1796 | Returns the process id of the parent process. | |
1797 | ||
1798 | =item getpriority WHICH,WHO | |
1799 | ||
4633a7c4 LW |
1800 | Returns the current priority for a process, a process group, or a user. |
1801 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
f86cebdf | 1802 | machine that doesn't implement getpriority(2). |
a0d0e21e LW |
1803 | |
1804 | =item getpwnam NAME | |
1805 | ||
1806 | =item getgrnam NAME | |
1807 | ||
1808 | =item gethostbyname NAME | |
1809 | ||
1810 | =item getnetbyname NAME | |
1811 | ||
1812 | =item getprotobyname NAME | |
1813 | ||
1814 | =item getpwuid UID | |
1815 | ||
1816 | =item getgrgid GID | |
1817 | ||
1818 | =item getservbyname NAME,PROTO | |
1819 | ||
1820 | =item gethostbyaddr ADDR,ADDRTYPE | |
1821 | ||
1822 | =item getnetbyaddr ADDR,ADDRTYPE | |
1823 | ||
1824 | =item getprotobynumber NUMBER | |
1825 | ||
1826 | =item getservbyport PORT,PROTO | |
1827 | ||
1828 | =item getpwent | |
1829 | ||
1830 | =item getgrent | |
1831 | ||
1832 | =item gethostent | |
1833 | ||
1834 | =item getnetent | |
1835 | ||
1836 | =item getprotoent | |
1837 | ||
1838 | =item getservent | |
1839 | ||
1840 | =item setpwent | |
1841 | ||
1842 | =item setgrent | |
1843 | ||
1844 | =item sethostent STAYOPEN | |
1845 | ||
1846 | =item setnetent STAYOPEN | |
1847 | ||
1848 | =item setprotoent STAYOPEN | |
1849 | ||
1850 | =item setservent STAYOPEN | |
1851 | ||
1852 | =item endpwent | |
1853 | ||
1854 | =item endgrent | |
1855 | ||
1856 | =item endhostent | |
1857 | ||
1858 | =item endnetent | |
1859 | ||
1860 | =item endprotoent | |
1861 | ||
1862 | =item endservent | |
1863 | ||
1864 | These routines perform the same functions as their counterparts in the | |
5a964f20 | 1865 | system library. In list context, the return values from the |
a0d0e21e LW |
1866 | various get routines are as follows: |
1867 | ||
1868 | ($name,$passwd,$uid,$gid, | |
6ee623d5 | 1869 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
a0d0e21e LW |
1870 | ($name,$passwd,$gid,$members) = getgr* |
1871 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
1872 | ($name,$aliases,$addrtype,$net) = getnet* | |
1873 | ($name,$aliases,$proto) = getproto* | |
1874 | ($name,$aliases,$port,$proto) = getserv* | |
1875 | ||
1876 | (If the entry doesn't exist you get a null list.) | |
1877 | ||
4602f195 JH |
1878 | The exact meaning of the $gcos field varies but it usually contains |
1879 | the real name of the user (as opposed to the login name) and other | |
1880 | information pertaining to the user. Beware, however, that in many | |
1881 | system users are able to change this information and therefore it | |
106325ad | 1882 | cannot be trusted and therefore the $gcos is tainted (see |
2959b6e3 JH |
1883 | L<perlsec>). The $passwd and $shell, user's encrypted password and |
1884 | login shell, are also tainted, because of the same reason. | |
4602f195 | 1885 | |
5a964f20 | 1886 | In scalar context, you get the name, unless the function was a |
a0d0e21e LW |
1887 | lookup by name, in which case you get the other thing, whatever it is. |
1888 | (If the entry doesn't exist you get the undefined value.) For example: | |
1889 | ||
5a964f20 TC |
1890 | $uid = getpwnam($name); |
1891 | $name = getpwuid($num); | |
1892 | $name = getpwent(); | |
1893 | $gid = getgrnam($name); | |
1894 | $name = getgrgid($num; | |
1895 | $name = getgrent(); | |
1896 | #etc. | |
a0d0e21e | 1897 | |
4602f195 JH |
1898 | In I<getpw*()> the fields $quota, $comment, and $expire are special |
1899 | cases in the sense that in many systems they are unsupported. If the | |
1900 | $quota is unsupported, it is an empty scalar. If it is supported, it | |
1901 | usually encodes the disk quota. If the $comment field is unsupported, | |
1902 | it is an empty scalar. If it is supported it usually encodes some | |
1903 | administrative comment about the user. In some systems the $quota | |
1904 | field may be $change or $age, fields that have to do with password | |
1905 | aging. In some systems the $comment field may be $class. The $expire | |
1906 | field, if present, encodes the expiration period of the account or the | |
1907 | password. For the availability and the exact meaning of these fields | |
1908 | in your system, please consult your getpwnam(3) documentation and your | |
1909 | F<pwd.h> file. You can also find out from within Perl what your | |
1910 | $quota and $comment fields mean and whether you have the $expire field | |
1911 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
1912 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password | |
1913 | files are only supported if your vendor has implemented them in the | |
1914 | intuitive fashion that calling the regular C library routines gets the | |
5d3a0a3b GS |
1915 | shadow versions if you're running under privilege or if there exists |
1916 | the shadow(3) functions as found in System V ( this includes Solaris | |
1917 | and Linux.) Those systems which implement a proprietary shadow password | |
1918 | facility are unlikely to be supported. | |
6ee623d5 | 1919 | |
19799a22 | 1920 | The $members value returned by I<getgr*()> is a space separated list of |
a0d0e21e LW |
1921 | the login names of the members of the group. |
1922 | ||
1923 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
1924 | C, it will be returned to you via C<$?> if the function call fails. The | |
7660c0ab | 1925 | C<@addrs> value returned by a successful call is a list of the raw |
a0d0e21e LW |
1926 | addresses returned by the corresponding system library call. In the |
1927 | Internet domain, each address is four bytes long and you can unpack it | |
1928 | by saying something like: | |
1929 | ||
1930 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); | |
1931 | ||
2b5ab1e7 TC |
1932 | The Socket library makes this slightly easier: |
1933 | ||
1934 | use Socket; | |
1935 | $iaddr = inet_aton("127.1"); # or whatever address | |
1936 | $name = gethostbyaddr($iaddr, AF_INET); | |
1937 | ||
1938 | # or going the other way | |
19799a22 | 1939 | $straddr = inet_ntoa($iaddr); |
2b5ab1e7 | 1940 | |
19799a22 GS |
1941 | If you get tired of remembering which element of the return list |
1942 | contains which return value, by-name interfaces are provided | |
1943 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
1944 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
1945 | and C<User::grent>. These override the normal built-ins, supplying | |
1946 | versions that return objects with the appropriate names | |
1947 | for each field. For example: | |
5a964f20 TC |
1948 | |
1949 | use File::stat; | |
1950 | use User::pwent; | |
1951 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
1952 | ||
b76cc8ba NIS |
1953 | Even though it looks like they're the same method calls (uid), |
1954 | they aren't, because a C<File::stat> object is different from | |
19799a22 | 1955 | a C<User::pwent> object. |
5a964f20 | 1956 | |
a0d0e21e LW |
1957 | =item getsockname SOCKET |
1958 | ||
19799a22 GS |
1959 | Returns the packed sockaddr address of this end of the SOCKET connection, |
1960 | in case you don't know the address because you have several different | |
1961 | IPs that the connection might have come in on. | |
a0d0e21e | 1962 | |
4633a7c4 LW |
1963 | use Socket; |
1964 | $mysockaddr = getsockname(SOCK); | |
19799a22 | 1965 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
b76cc8ba | 1966 | printf "Connect to %s [%s]\n", |
19799a22 GS |
1967 | scalar gethostbyaddr($myaddr, AF_INET), |
1968 | inet_ntoa($myaddr); | |
a0d0e21e LW |
1969 | |
1970 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
1971 | ||
5a964f20 | 1972 | Returns the socket option requested, or undef if there is an error. |
a0d0e21e LW |
1973 | |
1974 | =item glob EXPR | |
1975 | ||
0a753a76 | 1976 | =item glob |
1977 | ||
2b5ab1e7 TC |
1978 | Returns the value of EXPR with filename expansions such as the |
1979 | standard Unix shell F</bin/csh> would do. This is the internal function | |
61eff3bc JH |
1980 | implementing the C<< <*.c> >> operator, but you can use it directly. |
1981 | If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is | |
2b5ab1e7 | 1982 | discussed in more detail in L<perlop/"I/O Operators">. |
a0d0e21e | 1983 | |
3a4b19e4 GS |
1984 | Beginning with v5.6.0, this operator is implemented using the standard |
1985 | C<File::Glob> extension. See L<File::Glob> for details. | |
1986 | ||
a0d0e21e LW |
1987 | =item gmtime EXPR |
1988 | ||
48a26b3a | 1989 | Converts a time as returned by the time function to a 8-element list |
54310121 | 1990 | with the time localized for the standard Greenwich time zone. |
4633a7c4 | 1991 | Typically used as follows: |
a0d0e21e | 1992 | |
b76cc8ba | 1993 | # 0 1 2 3 4 5 6 7 |
48a26b3a | 1994 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = |
a0d0e21e LW |
1995 | gmtime(time); |
1996 | ||
48a26b3a GS |
1997 | All list elements are numeric, and come straight out of the C `struct |
1998 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the | |
1999 | specified time. $mday is the day of the month, and $mon is the month | |
2000 | itself, in the range C<0..11> with 0 indicating January and 11 | |
2001 | indicating December. $year is the number of years since 1900. That | |
2002 | is, $year is C<123> in year 2023. $wday is the day of the week, with | |
2003 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of | |
b76cc8ba | 2004 | the year, in the range C<0..364> (or C<0..365> in leap years.) |
48a26b3a GS |
2005 | |
2006 | Note that the $year element is I<not> simply the last two digits of | |
2007 | the year. If you assume it is, then you create non-Y2K-compliant | |
2008 | programs--and you wouldn't want to do that, would you? | |
2f9daede | 2009 | |
abd75f24 GS |
2010 | The proper way to get a complete 4-digit year is simply: |
2011 | ||
2012 | $year += 1900; | |
2013 | ||
2014 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2015 | ||
2016 | $year = sprintf("%02d", $year % 100); | |
2017 | ||
48a26b3a | 2018 | If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). |
a0d0e21e | 2019 | |
48a26b3a | 2020 | In scalar context, C<gmtime()> returns the ctime(3) value: |
0a753a76 | 2021 | |
2022 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
2023 | ||
19799a22 | 2024 | Also see the C<timegm> function provided by the C<Time::Local> module, |
f86cebdf | 2025 | and the strftime(3) function available via the POSIX module. |
7660c0ab | 2026 | |
2b5ab1e7 TC |
2027 | This scalar value is B<not> locale dependent (see L<perllocale>), but |
2028 | is instead a Perl builtin. Also see the C<Time::Local> module, and the | |
2029 | strftime(3) and mktime(3) functions available via the POSIX module. To | |
7660c0ab A |
2030 | get somewhat similar but locale dependent date strings, set up your |
2031 | locale environment variables appropriately (please see L<perllocale>) | |
2032 | and try for example: | |
2033 | ||
2034 | use POSIX qw(strftime); | |
2b5ab1e7 | 2035 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; |
7660c0ab | 2036 | |
2b5ab1e7 TC |
2037 | Note that the C<%a> and C<%b> escapes, which represent the short forms |
2038 | of the day of the week and the month of the year, may not necessarily | |
2039 | be three characters wide in all locales. | |
0a753a76 | 2040 | |
a0d0e21e LW |
2041 | =item goto LABEL |
2042 | ||
748a9306 LW |
2043 | =item goto EXPR |
2044 | ||
a0d0e21e LW |
2045 | =item goto &NAME |
2046 | ||
7660c0ab | 2047 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
a0d0e21e | 2048 | execution there. It may not be used to go into any construct that |
7660c0ab | 2049 | requires initialization, such as a subroutine or a C<foreach> loop. It |
0a753a76 | 2050 | also can't be used to go into a construct that is optimized away, |
19799a22 | 2051 | or to get out of a block or subroutine given to C<sort>. |
0a753a76 | 2052 | It can be used to go almost anywhere else within the dynamic scope, |
a0d0e21e | 2053 | including out of subroutines, but it's usually better to use some other |
19799a22 | 2054 | construct such as C<last> or C<die>. The author of Perl has never felt the |
7660c0ab | 2055 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
a0d0e21e | 2056 | |
7660c0ab A |
2057 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
2058 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
748a9306 LW |
2059 | necessarily recommended if you're optimizing for maintainability: |
2060 | ||
2061 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2062 | ||
6cb9131c GS |
2063 | The C<goto-&NAME> form is quite different from the other forms of C<goto>. |
2064 | In fact, it isn't a goto in the normal sense at all, and doesn't have | |
2065 | the stigma associated with other gotos. Instead, it | |
2066 | substitutes a call to the named subroutine for the currently running | |
2067 | subroutine. This is used by C<AUTOLOAD> subroutines that wish to load | |
2068 | another subroutine and then pretend that the other subroutine had been | |
2069 | called in the first place (except that any modifications to C<@_> | |
2070 | in the current subroutine are propagated to the other subroutine.) | |
2071 | After the C<goto>, not even C<caller> will be able to tell that this | |
2072 | routine was called first. | |
2073 | ||
2074 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
2075 | containing a code reference, or a block which evaluates to a code | |
2076 | reference. | |
a0d0e21e LW |
2077 | |
2078 | =item grep BLOCK LIST | |
2079 | ||
2080 | =item grep EXPR,LIST | |
2081 | ||
2b5ab1e7 TC |
2082 | This is similar in spirit to, but not the same as, grep(1) and its |
2083 | relatives. In particular, it is not limited to using regular expressions. | |
2f9daede | 2084 | |
a0d0e21e | 2085 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
7660c0ab | 2086 | C<$_> to each element) and returns the list value consisting of those |
19799a22 GS |
2087 | elements for which the expression evaluated to true. In scalar |
2088 | context, returns the number of times the expression was true. | |
a0d0e21e LW |
2089 | |
2090 | @foo = grep(!/^#/, @bar); # weed out comments | |
2091 | ||
2092 | or equivalently, | |
2093 | ||
2094 | @foo = grep {!/^#/} @bar; # weed out comments | |
2095 | ||
be3174d2 GS |
2096 | Note that C<$_> is an alias to the list value, so it can be used to |
2097 | modify the elements of the LIST. While this is useful and supported, | |
2098 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
2099 | Similarly, grep returns aliases into the original list, much as a for |
2100 | loop's index variable aliases the list elements. That is, modifying an | |
19799a22 GS |
2101 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
2102 | or another C<grep>) actually modifies the element in the original list. | |
2b5ab1e7 | 2103 | This is usually something to be avoided when writing clear code. |
a0d0e21e | 2104 | |
19799a22 | 2105 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
38325410 | 2106 | |
a0d0e21e LW |
2107 | =item hex EXPR |
2108 | ||
54310121 | 2109 | =item hex |
bbce6d69 | 2110 | |
2b5ab1e7 TC |
2111 | Interprets EXPR as a hex string and returns the corresponding value. |
2112 | (To convert strings that might start with either 0, 0x, or 0b, see | |
2113 | L</oct>.) If EXPR is omitted, uses C<$_>. | |
2f9daede TP |
2114 | |
2115 | print hex '0xAf'; # prints '175' | |
2116 | print hex 'aF'; # same | |
a0d0e21e | 2117 | |
19799a22 | 2118 | Hex strings may only represent integers. Strings that would cause |
c6edd1b7 | 2119 | integer overflow trigger a warning. |
19799a22 | 2120 | |
a0d0e21e LW |
2121 | =item import |
2122 | ||
19799a22 | 2123 | There is no builtin C<import> function. It is just an ordinary |
4633a7c4 | 2124 | method (subroutine) defined (or inherited) by modules that wish to export |
19799a22 | 2125 | names to another module. The C<use> function calls the C<import> method |
cea6626f | 2126 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. |
a0d0e21e LW |
2127 | |
2128 | =item index STR,SUBSTR,POSITION | |
2129 | ||
2130 | =item index STR,SUBSTR | |
2131 | ||
2b5ab1e7 TC |
2132 | The index function searches for one string within another, but without |
2133 | the wildcard-like behavior of a full regular-expression pattern match. | |
2134 | It returns the position of the first occurrence of SUBSTR in STR at | |
2135 | or after POSITION. If POSITION is omitted, starts searching from the | |
2136 | beginning of the string. The return value is based at C<0> (or whatever | |
2137 | you've set the C<$[> variable to--but don't do that). If the substring | |
2138 | is not found, returns one less than the base, ordinarily C<-1>. | |
a0d0e21e LW |
2139 | |
2140 | =item int EXPR | |
2141 | ||
54310121 | 2142 | =item int |
bbce6d69 | 2143 | |
7660c0ab | 2144 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
2b5ab1e7 TC |
2145 | You should not use this function for rounding: one because it truncates |
2146 | towards C<0>, and two because machine representations of floating point | |
2147 | numbers can sometimes produce counterintuitive results. For example, | |
2148 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2149 | because it's really more like -268.99999999999994315658 instead. Usually, | |
19799a22 | 2150 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
2b5ab1e7 | 2151 | functions will serve you better than will int(). |
a0d0e21e LW |
2152 | |
2153 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
2154 | ||
2b5ab1e7 | 2155 | Implements the ioctl(2) function. You'll probably first have to say |
a0d0e21e | 2156 | |
4633a7c4 | 2157 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
a0d0e21e | 2158 | |
2b5ab1e7 | 2159 | to get the correct function definitions. If F<ioctl.ph> doesn't |
a0d0e21e | 2160 | exist or doesn't have the correct definitions you'll have to roll your |
61eff3bc | 2161 | own, based on your C header files such as F<< <sys/ioctl.h> >>. |
5a964f20 | 2162 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
54310121 | 2163 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
4633a7c4 | 2164 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
19799a22 | 2165 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
4633a7c4 LW |
2166 | has no string value but does have a numeric value, that value will be |
2167 | passed rather than a pointer to the string value. To guarantee this to be | |
19799a22 GS |
2168 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
2169 | functions may be needed to manipulate the values of structures used by | |
b76cc8ba | 2170 | C<ioctl>. |
a0d0e21e | 2171 | |
19799a22 | 2172 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
a0d0e21e LW |
2173 | |
2174 | if OS returns: then Perl returns: | |
2175 | -1 undefined value | |
2176 | 0 string "0 but true" | |
2177 | anything else that number | |
2178 | ||
19799a22 | 2179 | Thus Perl returns true on success and false on failure, yet you can |
a0d0e21e LW |
2180 | still easily determine the actual value returned by the operating |
2181 | system: | |
2182 | ||
2b5ab1e7 | 2183 | $retval = ioctl(...) || -1; |
a0d0e21e LW |
2184 | printf "System returned %d\n", $retval; |
2185 | ||
c2611fb3 | 2186 | The special string "C<0> but true" is exempt from B<-w> complaints |
5a964f20 TC |
2187 | about improper numeric conversions. |
2188 | ||
19799a22 GS |
2189 | Here's an example of setting a filehandle named C<REMOTE> to be |
2190 | non-blocking at the system level. You'll have to negotiate C<$|> | |
2191 | on your own, though. | |
2192 | ||
2193 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
2194 | ||
2195 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
2196 | or die "Can't get flags for the socket: $!\n"; | |
2197 | ||
2198 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
2199 | or die "Can't set flags for the socket: $!\n"; | |
2200 | ||
a0d0e21e LW |
2201 | =item join EXPR,LIST |
2202 | ||
2b5ab1e7 TC |
2203 | Joins the separate strings of LIST into a single string with fields |
2204 | separated by the value of EXPR, and returns that new string. Example: | |
a0d0e21e | 2205 | |
2b5ab1e7 | 2206 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
a0d0e21e | 2207 | |
eb6e2d6f GS |
2208 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
2209 | first argument. Compare L</split>. | |
a0d0e21e | 2210 | |
aa689395 | 2211 | =item keys HASH |
2212 | ||
19799a22 | 2213 | Returns a list consisting of all the keys of the named hash. (In |
1d2dff63 | 2214 | scalar context, returns the number of keys.) The keys are returned in |
ab192400 GS |
2215 | an apparently random order. The actual random order is subject to |
2216 | change in future versions of perl, but it is guaranteed to be the same | |
19799a22 | 2217 | order as either the C<values> or C<each> function produces (given |
ab192400 GS |
2218 | that the hash has not been modified). As a side effect, it resets |
2219 | HASH's iterator. | |
a0d0e21e | 2220 | |
aa689395 | 2221 | Here is yet another way to print your environment: |
a0d0e21e LW |
2222 | |
2223 | @keys = keys %ENV; | |
2224 | @values = values %ENV; | |
b76cc8ba | 2225 | while (@keys) { |
a0d0e21e LW |
2226 | print pop(@keys), '=', pop(@values), "\n"; |
2227 | } | |
2228 | ||
2229 | or how about sorted by key: | |
2230 | ||
2231 | foreach $key (sort(keys %ENV)) { | |
2232 | print $key, '=', $ENV{$key}, "\n"; | |
2233 | } | |
2234 | ||
8ea1e5d4 GS |
2235 | The returned values are copies of the original keys in the hash, so |
2236 | modifying them will not affect the original hash. Compare L</values>. | |
2237 | ||
19799a22 | 2238 | To sort a hash by value, you'll need to use a C<sort> function. |
aa689395 | 2239 | Here's a descending numeric sort of a hash by its values: |
4633a7c4 | 2240 | |
5a964f20 | 2241 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
4633a7c4 LW |
2242 | printf "%4d %s\n", $hash{$key}, $key; |
2243 | } | |
2244 | ||
19799a22 | 2245 | As an lvalue C<keys> allows you to increase the number of hash buckets |
aa689395 | 2246 | allocated for the given hash. This can gain you a measure of efficiency if |
2247 | you know the hash is going to get big. (This is similar to pre-extending | |
2248 | an array by assigning a larger number to $#array.) If you say | |
55497cff | 2249 | |
2250 | keys %hash = 200; | |
2251 | ||
ab192400 GS |
2252 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
2253 | in fact, since it rounds up to the next power of two. These | |
55497cff | 2254 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
2255 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2256 | You can't shrink the number of buckets allocated for the hash using | |
19799a22 | 2257 | C<keys> in this way (but you needn't worry about doing this by accident, |
55497cff | 2258 | as trying has no effect). |
2259 | ||
19799a22 | 2260 | See also C<each>, C<values> and C<sort>. |
ab192400 | 2261 | |
b350dd2f | 2262 | =item kill SIGNAL, LIST |
a0d0e21e | 2263 | |
b350dd2f | 2264 | Sends a signal to a list of processes. Returns the number of |
517db077 GS |
2265 | processes successfully signaled (which is not necessarily the |
2266 | same as the number actually killed). | |
a0d0e21e LW |
2267 | |
2268 | $cnt = kill 1, $child1, $child2; | |
2269 | kill 9, @goners; | |
2270 | ||
b350dd2f GS |
2271 | If SIGNAL is zero, no signal is sent to the process. This is a |
2272 | useful way to check that the process is alive and hasn't changed | |
2273 | its UID. See L<perlport> for notes on the portability of this | |
2274 | construct. | |
2275 | ||
2276 | Unlike in the shell, if SIGNAL is negative, it kills | |
4633a7c4 LW |
2277 | process groups instead of processes. (On System V, a negative I<PROCESS> |
2278 | number will also kill process groups, but that's not portable.) That | |
2279 | means you usually want to use positive not negative signals. You may also | |
da0045b7 | 2280 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
a0d0e21e LW |
2281 | |
2282 | =item last LABEL | |
2283 | ||
2284 | =item last | |
2285 | ||
2286 | The C<last> command is like the C<break> statement in C (as used in | |
2287 | loops); it immediately exits the loop in question. If the LABEL is | |
2288 | omitted, the command refers to the innermost enclosing loop. The | |
2289 | C<continue> block, if any, is not executed: | |
2290 | ||
4633a7c4 LW |
2291 | LINE: while (<STDIN>) { |
2292 | last LINE if /^$/; # exit when done with header | |
5a964f20 | 2293 | #... |
a0d0e21e LW |
2294 | } |
2295 | ||
4968c1e4 | 2296 | C<last> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2297 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2298 | a grep() or map() operation. | |
4968c1e4 | 2299 | |
6c1372ed GS |
2300 | Note that a block by itself is semantically identical to a loop |
2301 | that executes once. Thus C<last> can be used to effect an early | |
2302 | exit out of such a block. | |
2303 | ||
98293880 JH |
2304 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2305 | C<redo> work. | |
1d2dff63 | 2306 | |
a0d0e21e LW |
2307 | =item lc EXPR |
2308 | ||
54310121 | 2309 | =item lc |
bbce6d69 | 2310 | |
a0d0e21e | 2311 | Returns an lowercased version of EXPR. This is the internal function |
7660c0ab | 2312 | implementing the C<\L> escape in double-quoted strings. |
19799a22 GS |
2313 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
2314 | and L<utf8>. | |
a0d0e21e | 2315 | |
7660c0ab | 2316 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2317 | |
a0d0e21e LW |
2318 | =item lcfirst EXPR |
2319 | ||
54310121 | 2320 | =item lcfirst |
bbce6d69 | 2321 | |
a0d0e21e | 2322 | Returns the value of EXPR with the first character lowercased. This is |
7660c0ab | 2323 | the internal function implementing the C<\l> escape in double-quoted strings. |
a0ed51b3 | 2324 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0d0e21e | 2325 | |
7660c0ab | 2326 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 2327 | |
a0d0e21e LW |
2328 | =item length EXPR |
2329 | ||
54310121 | 2330 | =item length |
bbce6d69 | 2331 | |
a0ed51b3 | 2332 | Returns the length in characters of the value of EXPR. If EXPR is |
b76cc8ba | 2333 | omitted, returns length of C<$_>. Note that this cannot be used on |
2b5ab1e7 TC |
2334 | an entire array or hash to find out how many elements these have. |
2335 | For that, use C<scalar @array> and C<scalar keys %hash> respectively. | |
a0d0e21e LW |
2336 | |
2337 | =item link OLDFILE,NEWFILE | |
2338 | ||
19799a22 | 2339 | Creates a new filename linked to the old filename. Returns true for |
b76cc8ba | 2340 | success, false otherwise. |
a0d0e21e LW |
2341 | |
2342 | =item listen SOCKET,QUEUESIZE | |
2343 | ||
19799a22 | 2344 | Does the same thing that the listen system call does. Returns true if |
b76cc8ba | 2345 | it succeeded, false otherwise. See the example in |
cea6626f | 2346 | L<perlipc/"Sockets: Client/Server Communication">. |
a0d0e21e LW |
2347 | |
2348 | =item local EXPR | |
2349 | ||
19799a22 | 2350 | You really probably want to be using C<my> instead, because C<local> isn't |
b76cc8ba | 2351 | what most people think of as "local". See |
13a2d996 | 2352 | L<perlsub/"Private Variables via my()"> for details. |
2b5ab1e7 | 2353 | |
5a964f20 TC |
2354 | A local modifies the listed variables to be local to the enclosing |
2355 | block, file, or eval. If more than one value is listed, the list must | |
2356 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
2357 | for details, including issues with tied arrays and hashes. | |
a0d0e21e | 2358 | |
a0d0e21e LW |
2359 | =item localtime EXPR |
2360 | ||
19799a22 | 2361 | Converts a time as returned by the time function to a 9-element list |
5f05dabc | 2362 | with the time analyzed for the local time zone. Typically used as |
a0d0e21e LW |
2363 | follows: |
2364 | ||
54310121 | 2365 | # 0 1 2 3 4 5 6 7 8 |
a0d0e21e LW |
2366 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
2367 | localtime(time); | |
2368 | ||
48a26b3a GS |
2369 | All list elements are numeric, and come straight out of the C `struct |
2370 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the | |
2371 | specified time. $mday is the day of the month, and $mon is the month | |
2372 | itself, in the range C<0..11> with 0 indicating January and 11 | |
2373 | indicating December. $year is the number of years since 1900. That | |
2374 | is, $year is C<123> in year 2023. $wday is the day of the week, with | |
2375 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of | |
874b1813 | 2376 | the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst |
48a26b3a GS |
2377 | is true if the specified time occurs during daylight savings time, |
2378 | false otherwise. | |
2379 | ||
2380 | Note that the $year element is I<not> simply the last two digits of | |
2381 | the year. If you assume it is, then you create non-Y2K-compliant | |
2382 | programs--and you wouldn't want to do that, would you? | |
54310121 | 2383 | |
abd75f24 GS |
2384 | The proper way to get a complete 4-digit year is simply: |
2385 | ||
2386 | $year += 1900; | |
2387 | ||
2388 | And to get the last two digits of the year (e.g., '01' in 2001) do: | |
2389 | ||
2390 | $year = sprintf("%02d", $year % 100); | |
2391 | ||
48a26b3a | 2392 | If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). |
a0d0e21e | 2393 | |
48a26b3a | 2394 | In scalar context, C<localtime()> returns the ctime(3) value: |
a0d0e21e | 2395 | |
5f05dabc | 2396 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
a0d0e21e | 2397 | |
a3cb178b | 2398 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
68f8bed4 JH |
2399 | instead a Perl builtin. Also see the C<Time::Local> module |
2400 | (to convert the second, minutes, hours, ... back to seconds since the | |
2401 | stroke of midnight the 1st of January 1970, the value returned by | |
ca6e1c26 | 2402 | time()), and the strftime(3) and mktime(3) functions available via the |
68f8bed4 JH |
2403 | POSIX module. To get somewhat similar but locale dependent date |
2404 | strings, set up your locale environment variables appropriately | |
2405 | (please see L<perllocale>) and try for example: | |
a3cb178b | 2406 | |
5a964f20 | 2407 | use POSIX qw(strftime); |
2b5ab1e7 | 2408 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
a3cb178b GS |
2409 | |
2410 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
2411 | and the month of the year, may not necessarily be three characters wide. | |
a0d0e21e | 2412 | |
19799a22 GS |
2413 | =item lock |
2414 | ||
2415 | lock I<THING> | |
2416 | ||
2417 | This function places an advisory lock on a variable, subroutine, | |
2418 | or referenced object contained in I<THING> until the lock goes out | |
2419 | of scope. This is a built-in function only if your version of Perl | |
2420 | was built with threading enabled, and if you've said C<use Threads>. | |
2421 | Otherwise a user-defined function by this name will be called. See | |
2422 | L<Thread>. | |
2423 | ||
a0d0e21e LW |
2424 | =item log EXPR |
2425 | ||
54310121 | 2426 | =item log |
bbce6d69 | 2427 | |
2b5ab1e7 TC |
2428 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
2429 | returns log of C<$_>. To get the log of another base, use basic algebra: | |
19799a22 | 2430 | The base-N log of a number is equal to the natural log of that number |
2b5ab1e7 TC |
2431 | divided by the natural log of N. For example: |
2432 | ||
2433 | sub log10 { | |
2434 | my $n = shift; | |
2435 | return log($n)/log(10); | |
b76cc8ba | 2436 | } |
2b5ab1e7 TC |
2437 | |
2438 | See also L</exp> for the inverse operation. | |
a0d0e21e | 2439 | |
a0d0e21e LW |
2440 | =item lstat EXPR |
2441 | ||
54310121 | 2442 | =item lstat |
bbce6d69 | 2443 | |
19799a22 | 2444 | Does the same thing as the C<stat> function (including setting the |
5a964f20 TC |
2445 | special C<_> filehandle) but stats a symbolic link instead of the file |
2446 | the symbolic link points to. If symbolic links are unimplemented on | |
19799a22 | 2447 | your system, a normal C<stat> is done. |
a0d0e21e | 2448 | |
7660c0ab | 2449 | If EXPR is omitted, stats C<$_>. |
bbce6d69 | 2450 | |
a0d0e21e LW |
2451 | =item m// |
2452 | ||
2453 | The match operator. See L<perlop>. | |
2454 | ||
2455 | =item map BLOCK LIST | |
2456 | ||
2457 | =item map EXPR,LIST | |
2458 | ||
19799a22 GS |
2459 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
2460 | C<$_> to each element) and returns the list value composed of the | |
2461 | results of each such evaluation. In scalar context, returns the | |
2462 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
2463 | list context, so each element of LIST may produce zero, one, or | |
2464 | more elements in the returned value. | |
dd99ebda | 2465 | |
a0d0e21e LW |
2466 | @chars = map(chr, @nums); |
2467 | ||
2468 | translates a list of numbers to the corresponding characters. And | |
2469 | ||
4633a7c4 | 2470 | %hash = map { getkey($_) => $_ } @array; |
a0d0e21e LW |
2471 | |
2472 | is just a funny way to write | |
2473 | ||
2474 | %hash = (); | |
2475 | foreach $_ (@array) { | |
4633a7c4 | 2476 | $hash{getkey($_)} = $_; |
a0d0e21e LW |
2477 | } |
2478 | ||
be3174d2 GS |
2479 | Note that C<$_> is an alias to the list value, so it can be used to |
2480 | modify the elements of the LIST. While this is useful and supported, | |
2481 | it can cause bizarre results if the elements of LIST are not variables. | |
2b5ab1e7 TC |
2482 | Using a regular C<foreach> loop for this purpose would be clearer in |
2483 | most cases. See also L</grep> for an array composed of those items of | |
2484 | the original list for which the BLOCK or EXPR evaluates to true. | |
fb73857a | 2485 | |
205fdb4d NC |
2486 | C<{> starts both hash references and blocks, so C<map { ...> could be either |
2487 | the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look | |
2488 | ahead for the closing C<}> it has to take a guess at which its dealing with | |
2489 | based what it finds just after the C<{>. Usually it gets it right, but if it | |
2490 | doesn't it won't realize something is wrong until it gets to the C<}> and | |
2491 | encounters the missing (or unexpected) comma. The syntax error will be | |
2492 | reported close to the C<}> but you'll need to change something near the C<{> | |
2493 | such as using a unary C<+> to give perl some help: | |
2494 | ||
2495 | %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong | |
2496 | %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right | |
2497 | %hash = map { ("\L$_", 1) } @array # this also works | |
2498 | %hash = map { lc($_), 1 } @array # as does this. | |
2499 | %hash = map +( lc($_), 1 ), @array # this is EXPR and works! | |
cea6626f | 2500 | |
205fdb4d NC |
2501 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) |
2502 | ||
2503 | or to force an anon hash constructor use C<+{> | |
2504 | ||
2505 | @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end | |
2506 | ||
2507 | and you get list of anonymous hashes each with only 1 entry. | |
2508 | ||
19799a22 | 2509 | =item mkdir FILENAME,MASK |
a0d0e21e | 2510 | |
5a211162 GS |
2511 | =item mkdir FILENAME |
2512 | ||
0591cd52 | 2513 | Creates the directory specified by FILENAME, with permissions |
19799a22 GS |
2514 | specified by MASK (as modified by C<umask>). If it succeeds it |
2515 | returns true, otherwise it returns false and sets C<$!> (errno). | |
5a211162 | 2516 | If omitted, MASK defaults to 0777. |
0591cd52 | 2517 | |
19799a22 | 2518 | In general, it is better to create directories with permissive MASK, |
0591cd52 | 2519 | and let the user modify that with their C<umask>, than it is to supply |
19799a22 | 2520 | a restrictive MASK and give the user no way to be more permissive. |
0591cd52 NT |
2521 | The exceptions to this rule are when the file or directory should be |
2522 | kept private (mail files, for instance). The perlfunc(1) entry on | |
19799a22 | 2523 | C<umask> discusses the choice of MASK in more detail. |
a0d0e21e | 2524 | |
cc1852e8 JH |
2525 | Note that according to the POSIX 1003.1-1996 the FILENAME may have any |
2526 | number of trailing slashes. Some operating and filesystems do not get | |
2527 | this right, so Perl automatically removes all trailing slashes to keep | |
2528 | everyone happy. | |
2529 | ||
a0d0e21e LW |
2530 | =item msgctl ID,CMD,ARG |
2531 | ||
f86cebdf | 2532 | Calls the System V IPC function msgctl(2). You'll probably have to say |
0ade1984 JH |
2533 | |
2534 | use IPC::SysV; | |
2535 | ||
7660c0ab A |
2536 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
2537 | then ARG must be a variable which will hold the returned C<msqid_ds> | |
951ba7fe GS |
2538 | structure. Returns like C<ioctl>: the undefined value for error, |
2539 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
4755096e | 2540 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. |
a0d0e21e LW |
2541 | |
2542 | =item msgget KEY,FLAGS | |
2543 | ||
f86cebdf | 2544 | Calls the System V IPC function msgget(2). Returns the message queue |
4755096e GS |
2545 | id, or the undefined value if there is an error. See also |
2546 | L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. | |
a0d0e21e | 2547 | |
a0d0e21e LW |
2548 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS |
2549 | ||
2550 | Calls the System V IPC function msgrcv to receive a message from | |
2551 | message queue ID into variable VAR with a maximum message size of | |
41d6edb2 JH |
2552 | SIZE. Note that when a message is received, the message type as a |
2553 | native long integer will be the first thing in VAR, followed by the | |
2554 | actual message. This packing may be opened with C<unpack("l! a*")>. | |
2555 | Taints the variable. Returns true if successful, or false if there is | |
4755096e GS |
2556 | an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and |
2557 | C<IPC::SysV::Msg> documentation. | |
41d6edb2 JH |
2558 | |
2559 | =item msgsnd ID,MSG,FLAGS | |
2560 | ||
2561 | Calls the System V IPC function msgsnd to send the message MSG to the | |
2562 | message queue ID. MSG must begin with the native long integer message | |
2563 | type, and be followed by the length of the actual message, and finally | |
2564 | the message itself. This kind of packing can be achieved with | |
2565 | C<pack("l! a*", $type, $message)>. Returns true if successful, | |
2566 | or false if there is an error. See also C<IPC::SysV> | |
2567 | and C<IPC::SysV::Msg> documentation. | |
a0d0e21e LW |
2568 | |
2569 | =item my EXPR | |
2570 | ||
09bef843 SB |
2571 | =item my EXPR : ATTRIBUTES |
2572 | ||
19799a22 GS |
2573 | A C<my> declares the listed variables to be local (lexically) to the |
2574 | enclosing block, file, or C<eval>. If | |
5f05dabc | 2575 | more than one value is listed, the list must be placed in parentheses. See |
cb1a09d0 | 2576 | L<perlsub/"Private Variables via my()"> for details. |
4633a7c4 | 2577 | |
a0d0e21e LW |
2578 | =item next LABEL |
2579 | ||
2580 | =item next | |
2581 | ||
2582 | The C<next> command is like the C<continue> statement in C; it starts | |
2583 | the next iteration of the loop: | |
2584 | ||
4633a7c4 LW |
2585 | LINE: while (<STDIN>) { |
2586 | next LINE if /^#/; # discard comments | |
5a964f20 | 2587 | #... |
a0d0e21e LW |
2588 | } |
2589 | ||
2590 | Note that if there were a C<continue> block on the above, it would get | |
2591 | executed even on discarded lines. If the LABEL is omitted, the command | |
2592 | refers to the innermost enclosing loop. | |
2593 | ||
4968c1e4 | 2594 | C<next> cannot be used to exit a block which returns a value such as |
2b5ab1e7 TC |
2595 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
2596 | a grep() or map() operation. | |
4968c1e4 | 2597 | |
6c1372ed GS |
2598 | Note that a block by itself is semantically identical to a loop |
2599 | that executes once. Thus C<next> will exit such a block early. | |
2600 | ||
98293880 JH |
2601 | See also L</continue> for an illustration of how C<last>, C<next>, and |
2602 | C<redo> work. | |
1d2dff63 | 2603 | |
a0d0e21e LW |
2604 | =item no Module LIST |
2605 | ||
7660c0ab | 2606 | See the L</use> function, which C<no> is the opposite of. |
a0d0e21e LW |
2607 | |
2608 | =item oct EXPR | |
2609 | ||
54310121 | 2610 | =item oct |
bbce6d69 | 2611 | |
4633a7c4 | 2612 | Interprets EXPR as an octal string and returns the corresponding |
4f19785b WSI |
2613 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
2614 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
2615 | binary string.) The following will handle decimal, binary, octal, and | |
4633a7c4 | 2616 | hex in the standard Perl or C notation: |
a0d0e21e LW |
2617 | |
2618 | $val = oct($val) if $val =~ /^0/; | |
2619 | ||
19799a22 GS |
2620 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
2621 | in octal), use sprintf() or printf(): | |
2622 | ||
2623 | $perms = (stat("filename"))[2] & 07777; | |
2624 | $oct_perms = sprintf "%lo", $perms; | |
2625 | ||
2626 | The oct() function is commonly used when a string such as C<644> needs | |
2627 | to be converted into a file mode, for example. (Although perl will | |
2628 | automatically convert strings into numbers as needed, this automatic | |
2629 | conversion assumes base 10.) | |
a0d0e21e | 2630 | |
1c1fc3ea | 2631 | =item open FILEHANDLE,MODE,LIST |
6170680b | 2632 | |
a0d0e21e LW |
2633 | =item open FILEHANDLE,EXPR |
2634 | ||
2635 | =item open FILEHANDLE | |
2636 | ||
2637 | Opens the file whose filename is given by EXPR, and associates it with | |
5f05dabc | 2638 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
d6fd2b02 GS |
2639 | name of the real filehandle wanted. (This is considered a symbolic |
2640 | reference, so C<use strict 'refs'> should I<not> be in effect.) | |
2641 | ||
2642 | If EXPR is omitted, the scalar | |
5f05dabc | 2643 | variable of the same name as the FILEHANDLE contains the filename. |
19799a22 GS |
2644 | (Note that lexical variables--those declared with C<my>--will not work |
2645 | for this purpose; so if you're using C<my>, specify EXPR in your call | |
2b5ab1e7 TC |
2646 | to open.) See L<perlopentut> for a kinder, gentler explanation of opening |
2647 | files. | |
5f05dabc | 2648 | |
61eff3bc JH |
2649 | If MODE is C<< '<' >> or nothing, the file is opened for input. |
2650 | If MODE is C<< '>' >>, the file is truncated and opened for | |
2651 | output, being created if necessary. If MODE is C<<< '>>' >>>, | |
b76cc8ba | 2652 | the file is opened for appending, again being created if necessary. |
61eff3bc JH |
2653 | You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that |
2654 | you want both read and write access to the file; thus C<< '+<' >> is almost | |
2655 | always preferred for read/write updates--the C<< '+>' >> mode would clobber the | |
5a964f20 TC |
2656 | file first. You can't usually use either read-write mode for updating |
2657 | textfiles, since they have variable length records. See the B<-i> | |
0591cd52 NT |
2658 | switch in L<perlrun> for a better approach. The file is created with |
2659 | permissions of C<0666> modified by the process' C<umask> value. | |
5a964f20 | 2660 | |
61eff3bc JH |
2661 | These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, |
2662 | C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. | |
5f05dabc | 2663 | |
6170680b IZ |
2664 | In the 2-arguments (and 1-argument) form of the call the mode and |
2665 | filename should be concatenated (in this order), possibly separated by | |
61eff3bc | 2666 | spaces. It is possible to omit the mode if the mode is C<< '<' >>. |
6170680b | 2667 | |
7660c0ab | 2668 | If the filename begins with C<'|'>, the filename is interpreted as a |
5a964f20 | 2669 | command to which output is to be piped, and if the filename ends with a |
f244e06d GS |
2670 | C<'|'>, the filename is interpreted as a command which pipes output to |
2671 | us. See L<perlipc/"Using open() for IPC"> | |
19799a22 | 2672 | for more examples of this. (You are not allowed to C<open> to a command |
5a964f20 | 2673 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
4a4eefd0 GS |
2674 | and L<perlipc/"Bidirectional Communication with Another Process"> |
2675 | for alternatives.) | |
cb1a09d0 | 2676 | |
6170680b IZ |
2677 | If MODE is C<'|-'>, the filename is interpreted as a |
2678 | command to which output is to be piped, and if MODE is | |
2679 | C<'-|'>, the filename is interpreted as a command which pipes output to | |
2680 | us. In the 2-arguments (and 1-argument) form one should replace dash | |
2681 | (C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> | |
2682 | for more examples of this. (You are not allowed to C<open> to a command | |
2683 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, | |
2684 | and L<perlipc/"Bidirectional Communication"> for alternatives.) | |
2685 | ||
2686 | In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN | |
b76cc8ba | 2687 | and opening C<< '>-' >> opens STDOUT. |
6170680b IZ |
2688 | |
2689 | Open returns | |
19799a22 | 2690 | nonzero upon success, the undefined value otherwise. If the C<open> |
4633a7c4 | 2691 | involved a pipe, the return value happens to be the pid of the |
54310121 | 2692 | subprocess. |
cb1a09d0 AD |
2693 | |
2694 | If you're unfortunate enough to be running Perl on a system that | |
2695 | distinguishes between text files and binary files (modern operating | |
2696 | systems don't care), then you should check out L</binmode> for tips for | |
19799a22 | 2697 | dealing with this. The key distinction between systems that need C<binmode> |
5a964f20 TC |
2698 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
2699 | Plan9, which delimit lines with a single character, and which encode that | |
19799a22 | 2700 | character in C as C<"\n">, do not need C<binmode>. The rest need it. |
cb1a09d0 | 2701 | |
fb73857a | 2702 | When opening a file, it's usually a bad idea to continue normal execution |
19799a22 GS |
2703 | if the request failed, so C<open> is frequently used in connection with |
2704 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, | |
fb73857a | 2705 | where you want to make a nicely formatted error message (but there are |
5a964f20 | 2706 | modules that can help with that problem)) you should always check |
19799a22 | 2707 | the return value from opening a file. The infrequent exception is when |
fb73857a | 2708 | working with an unopened filehandle is actually what you want to do. |
2709 | ||
b76cc8ba NIS |
2710 | As a special case the 3 arg form with a read/write mode and the third argument |
2711 | being C<undef>: | |
2712 | ||
2713 | open(TMP, "+>", undef) or die ... | |
2714 | ||
2715 | opens a filehandle to an anonymous temporary file. | |
2716 | ||
cb1a09d0 | 2717 | Examples: |
a0d0e21e LW |
2718 | |
2719 | $ARTICLE = 100; | |
2720 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; | |
2721 | while (<ARTICLE>) {... | |
2722 | ||
6170680b | 2723 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
fb73857a | 2724 | # if the open fails, output is discarded |
a0d0e21e | 2725 | |
6170680b | 2726 | open(DBASE, '+<', 'dbase.mine') # open for update |
fb73857a | 2727 | or die "Can't open 'dbase.mine' for update: $!"; |
cb1a09d0 | 2728 | |
6170680b IZ |
2729 | open(DBASE, '+<dbase.mine') # ditto |
2730 | or die "Can't open 'dbase.mine' for update: $!"; | |
2731 | ||
2732 | open(ARTICLE, '-|', "caesar <$article") # decrypt article | |
fb73857a | 2733 | or die "Can't start caesar: $!"; |
a0d0e21e | 2734 | |
6170680b IZ |
2735 | open(ARTICLE, "caesar <$article |") # ditto |
2736 | or die "Can't start caesar: $!"; | |
2737 | ||
2738 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id | |
fb73857a | 2739 | or die "Can't start sort: $!"; |
a0d0e21e LW |
2740 | |
2741 | # process argument list of files along with any includes | |
2742 | ||
2743 | foreach $file (@ARGV) { | |
2744 | process($file, 'fh00'); | |
2745 | } | |
2746 | ||
2747 | sub process { | |
5a964f20 | 2748 | my($filename, $input) = @_; |
a0d0e21e LW |
2749 | $input++; # this is a string increment |
2750 | unless (open($input, $filename)) { | |
2751 | print STDERR "Can't open $filename: $!\n"; | |
2752 | return; | |
2753 | } | |
2754 | ||
5a964f20 | 2755 | local $_; |
a0d0e21e LW |
2756 | while (<$input>) { # note use of indirection |
2757 | if (/^#include "(.*)"/) { | |
2758 | process($1, $input); | |
2759 | next; | |
2760 | } | |
5a964f20 | 2761 | #... # whatever |
a0d0e21e LW |
2762 | } |
2763 | } | |
2764 | ||
2765 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
61eff3bc | 2766 | with C<< '>&' >>, in which case the rest of the string is interpreted as the |
5a964f20 | 2767 | name of a filehandle (or file descriptor, if numeric) to be |
61eff3bc JH |
2768 | duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, |
2769 | C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The | |
a0d0e21e | 2770 | mode you specify should match the mode of the original filehandle. |
184e9718 | 2771 | (Duping a filehandle does not take into account any existing contents of |
b76cc8ba NIS |
2772 | stdio buffers.) If you use the 3 arg form then you can pass either a number, |
2773 | the name of a filehandle or the normal "reference to a glob". | |
6170680b | 2774 | |
a0d0e21e LW |
2775 | Here is a script that saves, redirects, and restores STDOUT and |
2776 | STDERR: | |
2777 | ||
2778 | #!/usr/bin/perl | |
b76cc8ba | 2779 | open(my $oldout, ">&", \*STDOUT); |
5a964f20 | 2780 | open(OLDERR, ">&STDERR"); |
a0d0e21e | 2781 | |
6170680b IZ |
2782 | open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; |
2783 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; | |
a0d0e21e LW |
2784 | |
2785 | select(STDERR); $| = 1; # make unbuffered | |
2786 | select(STDOUT); $| = 1; # make unbuffered | |
2787 | ||
2788 | print STDOUT "stdout 1\n"; # this works for | |
2789 | print STDERR "stderr 1\n"; # subprocesses too | |
2790 | ||
2791 | close(STDOUT); | |
2792 | close(STDERR); | |
2793 | ||
5a964f20 TC |
2794 | open(STDOUT, ">&OLDOUT"); |
2795 | open(STDERR, ">&OLDERR"); | |
a0d0e21e LW |
2796 | |
2797 | print STDOUT "stdout 2\n"; | |
2798 | print STDERR "stderr 2\n"; | |
2799 | ||
61eff3bc | 2800 | If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an |
19799a22 | 2801 | equivalent of C's C<fdopen> of that file descriptor; this is more |
4633a7c4 | 2802 | parsimonious of file descriptors. For example: |
a0d0e21e LW |
2803 | |
2804 | open(FILEHANDLE, "<&=$fd") | |
b76cc8ba NIS |
2805 | or |
2806 | open(FILEHANDLE, "<&=", $fd) | |
a0d0e21e | 2807 | |
b76cc8ba NIS |
2808 | Note that if perl is using the standard C libaries fdopen() then on many UNIX systems, |
2809 | fdopen() is known to fail when file descriptors | |
4af147f6 | 2810 | exceed a certain value, typically 255. If you need more file |
b76cc8ba | 2811 | descriptors than that, consider rebuilding Perl to use the C<PerlIO>. |
4af147f6 | 2812 | |
6170680b IZ |
2813 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> |
2814 | with 2-arguments (or 1-argument) form of open(), then | |
a0d0e21e | 2815 | there is an implicit fork done, and the return value of open is the pid |
7660c0ab | 2816 | of the child within the parent process, and C<0> within the child |
184e9718 | 2817 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
a0d0e21e LW |
2818 | The filehandle behaves normally for the parent, but i/o to that |
2819 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
2820 | In the child process the filehandle isn't opened--i/o happens from/to | |
2821 | the new STDOUT or STDIN. Typically this is used like the normal | |
2822 | piped open when you want to exercise more control over just how the | |
2823 | pipe command gets executed, such as when you are running setuid, and | |
54310121 | 2824 | don't want to have to scan shell commands for metacharacters. |
6170680b | 2825 | The following triples are more or less equivalent: |
a0d0e21e LW |
2826 | |
2827 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
6170680b IZ |
2828 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); |
2829 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; | |
b76cc8ba | 2830 | open(FOO, '|-', "tr", '[a-z]', '[A-Z]'); |
a0d0e21e LW |
2831 | |
2832 | open(FOO, "cat -n '$file'|"); | |
6170680b IZ |
2833 | open(FOO, '-|', "cat -n '$file'"); |
2834 | open(FOO, '-|') || exec 'cat', '-n', $file; | |
b76cc8ba NIS |
2835 | open(FOO, '-|', "cat", '-n', $file); |
2836 | ||
2837 | The last example in each block shows the pipe as "list form", which is | |
2838 | not yet supported on all platforms. | |
a0d0e21e | 2839 | |
4633a7c4 LW |
2840 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
2841 | ||
0f897271 GS |
2842 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
2843 | output before any operation that may do a fork, but this may not be | |
2844 | supported on some platforms (see L<perlport>). To be safe, you may need | |
2845 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
2846 | of C<IO::Handle> on any open handles. | |
2847 | ||
2848 | On systems that support a | |
45bc9206 GS |
2849 | close-on-exec flag on files, the flag will be set for the newly opened |
2850 | file descriptor as determined by the value of $^F. See L<perlvar/$^F>. | |
a0d0e21e | 2851 | |
0dccf244 CS |
2852 | Closing any piped filehandle causes the parent process to wait for the |
2853 | child to finish, and returns the status value in C<$?>. | |
2854 | ||
6170680b IZ |
2855 | The filename passed to 2-argument (or 1-argument) form of open() |
2856 | will have leading and trailing | |
f86cebdf | 2857 | whitespace deleted, and the normal redirection characters |
b76cc8ba | 2858 | honored. This property, known as "magic open", |
5a964f20 | 2859 | can often be used to good effect. A user could specify a filename of |
7660c0ab | 2860 | F<"rsh cat file |">, or you could change certain filenames as needed: |
5a964f20 TC |
2861 | |
2862 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
2863 | open(FH, $filename) or die "Can't open $filename: $!"; | |
2864 | ||
6170680b IZ |
2865 | Use 3-argument form to open a file with arbitrary weird characters in it, |
2866 | ||
2867 | open(FOO, '<', $file); | |
2868 | ||
2869 | otherwise it's necessary to protect any leading and trailing whitespace: | |
5a964f20 TC |
2870 | |
2871 | $file =~ s#^(\s)#./$1#; | |
2872 | open(FOO, "< $file\0"); | |
2873 | ||
a31a806a | 2874 | (this may not work on some bizarre filesystems). One should |
106325ad | 2875 | conscientiously choose between the I<magic> and 3-arguments form |
6170680b IZ |
2876 | of open(): |
2877 | ||
2878 | open IN, $ARGV[0]; | |
2879 | ||
2880 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
2881 | but will not work on a filename which happens to have a trailing space, while | |
2882 | ||
2883 | open IN, '<', $ARGV[0]; | |
2884 | ||
2885 | will have exactly the opposite restrictions. | |
2886 | ||
19799a22 | 2887 | If you want a "real" C C<open> (see L<open(2)> on your system), then you |
6170680b IZ |
2888 | should use the C<sysopen> function, which involves no such magic (but |
2889 | may use subtly different filemodes than Perl open(), which is mapped | |
2890 | to C fopen()). This is | |
5a964f20 TC |
2891 | another way to protect your filenames from interpretation. For example: |
2892 | ||
2893 | use IO::Handle; | |
2894 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
2895 | or die "sysopen $path: $!"; | |
2896 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
38762f02 | 2897 | print HANDLE "stuff $$\n"; |
5a964f20 TC |
2898 | seek(HANDLE, 0, 0); |
2899 | print "File contains: ", <HANDLE>; | |
2900 | ||
7660c0ab A |
2901 | Using the constructor from the C<IO::Handle> package (or one of its |
2902 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
5a964f20 TC |
2903 | filehandles that have the scope of whatever variables hold references to |
2904 | them, and automatically close whenever and however you leave that scope: | |
c07a80fd | 2905 | |
5f05dabc | 2906 | use IO::File; |
5a964f20 | 2907 | #... |
c07a80fd | 2908 | sub read_myfile_munged { |
2909 | my $ALL = shift; | |
5f05dabc | 2910 | my $handle = new IO::File; |
c07a80fd | 2911 | open($handle, "myfile") or die "myfile: $!"; |
2912 | $first = <$handle> | |
2913 | or return (); # Automatically closed here. | |
2914 | mung $first or die "mung failed"; # Or here. | |
2915 | return $first, <$handle> if $ALL; # Or here. | |
2916 | $first; # Or here. | |
2917 | } | |
2918 | ||
b687b08b | 2919 | See L</seek> for some details about mixing reading and writing. |
a0d0e21e LW |
2920 | |
2921 | =item opendir DIRHANDLE,EXPR | |
2922 | ||
19799a22 GS |
2923 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
2924 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
a0d0e21e LW |
2925 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
2926 | ||
2927 | =item ord EXPR | |
2928 | ||
54310121 | 2929 | =item ord |
bbce6d69 | 2930 | |
a0ed51b3 | 2931 | Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If |
7660c0ab | 2932 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
2b5ab1e7 | 2933 | See L<utf8> for more about Unicode. |
a0d0e21e | 2934 | |
77ca0c92 LW |
2935 | =item our EXPR |
2936 | ||
2937 | An C<our> declares the listed variables to be valid globals within | |
2938 | the enclosing block, file, or C<eval>. That is, it has the same | |
2939 | scoping rules as a "my" declaration, but does not create a local | |
2940 | variable. If more than one value is listed, the list must be placed | |
2941 | in parentheses. The C<our> declaration has no semantic effect unless | |
2942 | "use strict vars" is in effect, in which case it lets you use the | |
2943 | declared global variable without qualifying it with a package name. | |
2944 | (But only within the lexical scope of the C<our> declaration. In this | |
2945 | it differs from "use vars", which is package scoped.) | |
2946 | ||
f472eb5c GS |
2947 | An C<our> declaration declares a global variable that will be visible |
2948 | across its entire lexical scope, even across package boundaries. The | |
2949 | package in which the variable is entered is determined at the point | |
2950 | of the declaration, not at the point of use. This means the following | |
2951 | behavior holds: | |
2952 | ||
2953 | package Foo; | |
2954 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2955 | $bar = 20; | |
2956 | ||
2957 | package Bar; | |
2958 | print $bar; # prints 20 | |
2959 | ||
2960 | Multiple C<our> declarations in the same lexical scope are allowed | |
2961 | if they are in different packages. If they happened to be in the same | |
2962 | package, Perl will emit warnings if you have asked for them. | |
2963 | ||
2964 | use warnings; | |
2965 | package Foo; | |
2966 | our $bar; # declares $Foo::bar for rest of lexical scope | |
2967 | $bar = 20; | |
2968 | ||
2969 | package Bar; | |
2970 | our $bar = 30; # declares $Bar::bar for rest of lexical scope | |
2971 | print $bar; # prints 30 | |
2972 | ||
2973 | our $bar; # emits warning | |
2974 | ||
a0d0e21e LW |
2975 | =item pack TEMPLATE,LIST |
2976 | ||
2b6c5635 GS |
2977 | Takes a LIST of values and converts it into a string using the rules |
2978 | given by the TEMPLATE. The resulting string is the concatenation of | |
2979 | the converted values. Typically, each converted value looks | |
2980 | like its machine-level representation. For example, on 32-bit machines | |
2981 | a converted integer may be represented by a sequence of 4 bytes. | |
2982 | ||
2983 | The TEMPLATE is a | |
a0d0e21e LW |
2984 | sequence of characters that give the order and type of values, as |
2985 | follows: | |
2986 | ||
5a929a98 | 2987 | a A string with arbitrary binary data, will be null padded. |
4375e838 | 2988 | A An ASCII string, will be space padded. |
5a929a98 VU |
2989 | Z A null terminated (asciz) string, will be null padded. |
2990 | ||
2b6c5635 GS |
2991 | b A bit string (ascending bit order inside each byte, like vec()). |
2992 | B A bit string (descending bit order inside each byte). | |
a0d0e21e LW |
2993 | h A hex string (low nybble first). |
2994 | H A hex string (high nybble first). | |
2995 | ||
2996 | c A signed char value. | |
a0ed51b3 | 2997 | C An unsigned char value. Only does bytes. See U for Unicode. |
96e4d5b1 | 2998 | |
a0d0e21e LW |
2999 | s A signed short value. |
3000 | S An unsigned short value. | |
96e4d5b1 | 3001 | (This 'short' is _exactly_ 16 bits, which may differ from |
851646ae JH |
3002 | what a local C compiler calls 'short'. If you want |
3003 | native-length shorts, use the '!' suffix.) | |
96e4d5b1 | 3004 | |
a0d0e21e LW |
3005 | i A signed integer value. |
3006 | I An unsigned integer value. | |
19799a22 | 3007 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
f86cebdf GS |
3008 | size depends on what a local C compiler calls 'int', |
3009 | and may even be larger than the 'long' described in | |
3010 | the next item.) | |
96e4d5b1 | 3011 | |
a0d0e21e LW |
3012 | l A signed long value. |
3013 | L An unsigned long value. | |
96e4d5b1 | 3014 | (This 'long' is _exactly_ 32 bits, which may differ from |
851646ae JH |
3015 | what a local C compiler calls 'long'. If you want |
3016 | native-length longs, use the '!' suffix.) | |
a0d0e21e | 3017 | |
5d11dd56 MG |
3018 | n An unsigned short in "network" (big-endian) order. |
3019 | N An unsigned long in "network" (big-endian) order. | |
3020 | v An unsigned short in "VAX" (little-endian) order. | |
3021 | V An unsigned long in "VAX" (little-endian) order. | |
96e4d5b1 | 3022 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
3023 | _exactly_ 32 bits, respectively.) | |
a0d0e21e | 3024 | |
dae0da7a JH |
3025 | q A signed quad (64-bit) value. |
3026 | Q An unsigned quad value. | |
851646ae JH |
3027 | (Quads are available only if your system supports 64-bit |
3028 | integer values _and_ if Perl has been compiled to support those. | |
dae0da7a JH |
3029 | Causes a fatal error otherwise.) |
3030 | ||
a0d0e21e LW |
3031 | f A single-precision float in the native format. |
3032 | d A double-precision float in the native format. | |
3033 | ||
3034 | p A pointer to a null-terminated string. | |
3035 | P A pointer to a structure (fixed-length string). | |
3036 | ||
3037 | u A uuencoded string. | |
a0ed51b3 LW |
3038 | U A Unicode character number. Encodes to UTF-8 internally. |
3039 | Works even if C<use utf8> is not in effect. | |
a0d0e21e | 3040 | |
96e4d5b1 | 3041 | w A BER compressed integer. Its bytes represent an unsigned |
f86cebdf GS |
3042 | integer in base 128, most significant digit first, with as |
3043 | few digits as possible. Bit eight (the high bit) is set | |
3044 | on each byte except the last. | |
def98dd4 | 3045 | |
a0d0e21e LW |
3046 | x A null byte. |
3047 | X Back up a byte. | |
3048 | @ Null fill to absolute position. | |
3049 | ||
5a929a98 VU |
3050 | The following rules apply: |
3051 | ||
3052 | =over 8 | |
3053 | ||
3054 | =item * | |
3055 | ||
5a964f20 | 3056 | Each letter may optionally be followed by a number giving a repeat |
951ba7fe GS |
3057 | count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, |
3058 | C<H>, and C<P> the pack function will gobble up that many values from | |
5a929a98 | 3059 | the LIST. A C<*> for the repeat count means to use however many items are |
951ba7fe GS |
3060 | left, except for C<@>, C<x>, C<X>, where it is equivalent |
3061 | to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the | |
2b6c5635 GS |
3062 | same). |
3063 | ||
951ba7fe | 3064 | When used with C<Z>, C<*> results in the addition of a trailing null |
2b6c5635 GS |
3065 | byte (so the packed result will be one longer than the byte C<length> |
3066 | of the item). | |
3067 | ||
951ba7fe | 3068 | The repeat count for C<u> is interpreted as the maximal number of bytes |
2b6c5635 | 3069 | to encode per line of output, with 0 and 1 replaced by 45. |
5a929a98 VU |
3070 | |
3071 | =item * | |
3072 | ||
951ba7fe | 3073 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
5a929a98 | 3074 | string of length count, padding with nulls or spaces as necessary. When |
951ba7fe GS |
3075 | unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything |
3076 | after the first null, and C<a> returns data verbatim. When packing, | |
3077 | C<a>, and C<Z> are equivalent. | |
2b6c5635 GS |
3078 | |
3079 | If the value-to-pack is too long, it is truncated. If too long and an | |
951ba7fe GS |
3080 | explicit count is provided, C<Z> packs only C<$count-1> bytes, followed |
3081 | by a null byte. Thus C<Z> always packs a trailing null byte under | |
2b6c5635 | 3082 | all circumstances. |
5a929a98 VU |
3083 | |
3084 | =item * | |
3085 | ||
951ba7fe | 3086 | Likewise, the C<b> and C<B> fields pack a string that many bits long. |
c73032f5 IZ |
3087 | Each byte of the input field of pack() generates 1 bit of the result. |
3088 | Each result bit is based on the least-significant bit of the corresponding | |
3089 | input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and | |
3090 | C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. | |
3091 | ||
3092 | Starting from the beginning of the input string of pack(), each 8-tuple | |
951ba7fe | 3093 | of bytes is converted to 1 byte of output. With format C<b> |
c73032f5 | 3094 | the first byte of the 8-tuple determines the least-significant bit of a |
951ba7fe | 3095 | byte, and with format C<B> it determines the most-significant bit of |
c73032f5 IZ |
3096 | a byte. |
3097 | ||
3098 | If the length of the input string is not exactly divisible by 8, the | |
3099 | remainder is packed as if the input string were padded by null bytes | |
3100 | at the end. Similarly, during unpack()ing the "extra" bits are ignored. | |
3101 | ||
3102 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
2b6c5635 GS |
3103 | A C<*> for the repeat count of pack() means to use all the bytes of |
3104 | the input field. On unpack()ing the bits are converted to a string | |
3105 | of C<"0">s and C<"1">s. | |
5a929a98 VU |
3106 | |
3107 | =item * | |
3108 | ||
951ba7fe | 3109 | The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, |
851646ae | 3110 | representable as hexadecimal digits, 0-9a-f) long. |
5a929a98 | 3111 | |
c73032f5 IZ |
3112 | Each byte of the input field of pack() generates 4 bits of the result. |
3113 | For non-alphabetical bytes the result is based on the 4 least-significant | |
3114 | bits of the input byte, i.e., on C<ord($byte)%16>. In particular, | |
3115 | bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
3116 | C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result | |
3117 | is compatible with the usual hexadecimal digits, so that C<"a"> and | |
3118 | C<"A"> both generate the nybble C<0xa==10>. The result for bytes | |
3119 | C<"g".."z"> and C<"G".."Z"> is not well-defined. | |
3120 | ||
3121 | Starting from the beginning of the input string of pack(), each pair | |
951ba7fe | 3122 | of bytes is converted to 1 byte of output. With format C<h> the |
c73032f5 | 3123 | first byte of the pair determines the least-significant nybble of the |
951ba7fe | 3124 | output byte, and with format C<H> it determines the most-significant |
c73032f5 IZ |
3125 | nybble. |
3126 | ||
3127 | If the length of the input string is not even, it behaves as if padded | |
3128 | by a null byte at the end. Similarly, during unpack()ing the "extra" | |
3129 | nybbles are ignored. | |
3130 | ||
3131 | If the input string of pack() is longer than needed, extra bytes are ignored. | |
3132 | A C<*> for the repeat count of pack() means to use all the bytes of | |
3133 | the input field. On unpack()ing the bits are converted to a string | |
3134 | of hexadecimal digits. | |
3135 | ||
5a929a98 VU |
3136 | =item * |
3137 | ||
951ba7fe | 3138 | The C<p> type packs a pointer to a null-terminated string. You are |
5a929a98 VU |
3139 | responsible for ensuring the string is not a temporary value (which can |
3140 | potentially get deallocated before you get around to using the packed result). | |
951ba7fe GS |
3141 | The C<P> type packs a pointer to a structure of the size indicated by the |
3142 | length. A NULL pointer is created if the corresponding value for C<p> or | |
3143 | C<P> is C<undef>, similarly for unpack(). | |
5a929a98 VU |
3144 | |
3145 | =item * | |
3146 | ||
951ba7fe GS |
3147 | The C</> template character allows packing and unpacking of strings where |
3148 | the packed structure contains a byte count followed by the string itself. | |
17f4a12d | 3149 | You write I<length-item>C</>I<string-item>. |
43192e07 IP |
3150 | |
3151 | The I<length-item> can be any C<pack> template letter, | |
3152 | and describes how the length value is packed. | |
3153 | The ones likely to be of most use are integer-packing ones like | |
951ba7fe GS |
3154 | C<n> (for Java strings), C<w> (for ASN.1 or SNMP) |
3155 | and C<N> (for Sun XDR). | |
43192e07 IP |
3156 | |
3157 | The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. | |
3158 | For C<unpack> the length of the string is obtained from the I<length-item>, | |
3159 | but if you put in the '*' it will be ignored. | |
3160 | ||
17f4a12d IZ |
3161 | unpack 'C/a', "\04Gurusamy"; gives 'Guru' |
3162 | unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') | |
3163 | pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" | |
43192e07 IP |
3164 | |
3165 | The I<length-item> is not returned explicitly from C<unpack>. | |
3166 | ||
951ba7fe GS |
3167 | Adding a count to the I<length-item> letter is unlikely to do anything |
3168 | useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a | |
3169 | I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, | |
43192e07 IP |
3170 | which Perl does not regard as legal in numeric strings. |
3171 | ||
3172 | =item * | |
3173 | ||
951ba7fe GS |
3174 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
3175 | immediately followed by a C<!> suffix to signify native shorts or | |
3176 | longs--as you can see from above for example a bare C<l> does mean | |
851646ae JH |
3177 | exactly 32 bits, the native C<long> (as seen by the local C compiler) |
3178 | may be larger. This is an issue mainly in 64-bit platforms. You can | |
951ba7fe | 3179 | see whether using C<!> makes any difference by |
726ea183 | 3180 | |
4d0c1c44 GS |
3181 | print length(pack("s")), " ", length(pack("s!")), "\n"; |
3182 | print length(pack("l")), " ", length(pack("l!")), "\n"; | |
ef54e1a4 | 3183 | |
951ba7fe GS |
3184 | C<i!> and C<I!> also work but only because of completeness; |
3185 | they are identical to C<i> and C<I>. | |
ef54e1a4 | 3186 | |
19799a22 GS |
3187 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
3188 | longs on the platform where Perl was built are also available via | |
3189 | L<Config>: | |
3190 | ||
3191 | use Config; | |
3192 | print $Config{shortsize}, "\n"; | |
3193 | print $Config{intsize}, "\n"; | |
3194 | print $Config{longsize}, "\n"; | |
3195 | print $Config{longlongsize}, "\n"; | |
ef54e1a4 | 3196 | |
5074e145 | 3197 | (The C<$Config{longlongsize}> will be undefine if your system does |
b76cc8ba | 3198 | not support long longs.) |
851646ae | 3199 | |
ef54e1a4 JH |
3200 | =item * |
3201 | ||
951ba7fe | 3202 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> |
ef54e1a4 JH |
3203 | are inherently non-portable between processors and operating systems |
3204 | because they obey the native byteorder and endianness. For example a | |
82e239e7 | 3205 | 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively |
ef54e1a4 | 3206 | (arranged in and handled by the CPU registers) into bytes as |
61eff3bc | 3207 | |
b35e152f JJ |
3208 | 0x12 0x34 0x56 0x78 # big-endian |
3209 | 0x78 0x56 0x34 0x12 # little-endian | |
61eff3bc | 3210 | |
b84d4f81 JH |
3211 | Basically, the Intel and VAX CPUs are little-endian, while everybody |
3212 | else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and | |
3213 | Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq | |
82e239e7 JH |
3214 | used/uses them in little-endian mode; SGI/Cray uses them in big-endian |
3215 | mode. | |
719a3cf5 | 3216 | |
19799a22 | 3217 | The names `big-endian' and `little-endian' are comic references to |
ef54e1a4 JH |
3218 | the classic "Gulliver's Travels" (via the paper "On Holy Wars and a |
3219 | Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and | |
19799a22 | 3220 | the egg-eating habits of the Lilliputians. |
61eff3bc | 3221 | |
140cb37e | 3222 | Some systems may have even weirder byte orders such as |
61eff3bc | 3223 | |
ef54e1a4 JH |
3224 | 0x56 0x78 0x12 0x34 |
3225 | 0x34 0x12 0x78 0x56 | |
61eff3bc | 3226 | |
ef54e1a4 JH |
3227 | You can see your system's preference with |
3228 | ||
3229 | print join(" ", map { sprintf "%#02x", $_ } | |
3230 | unpack("C*",pack("L",0x12345678))), "\n"; | |
3231 | ||
d99ad34e | 3232 | The byteorder on the platform where Perl was built is also available |
726ea183 | 3233 | via L<Config>: |
ef54e1a4 JH |
3234 | |
3235 | use Config; | |
3236 | print $Config{byteorder}, "\n"; | |
3237 | ||
d99ad34e JH |
3238 | Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> |
3239 | and C<'87654321'> are big-endian. | |
719a3cf5 | 3240 | |
951ba7fe | 3241 | If you want portable packed integers use the formats C<n>, C<N>, |
82e239e7 | 3242 | C<v>, and C<V>, their byte endianness and size are known. |
851646ae | 3243 | See also L<perlport>. |
ef54e1a4 JH |
3244 | |
3245 | =item * | |
3246 | ||
5a929a98 VU |
3247 | Real numbers (floats and doubles) are in the native machine format only; |
3248 | due to the multiplicity of floating formats around, and the lack of a | |
3249 | standard "network" representation, no facility for interchange has been | |
3250 | made. This means that packed floating point data written on one machine | |
3251 | may not be readable on another - even if both use IEEE floating point | |
3252 | arithmetic (as the endian-ness of the memory representation is not part | |
851646ae | 3253 | of the IEEE spec). See also L<perlport>. |
5a929a98 VU |
3254 | |
3255 | Note that Perl uses doubles internally for all numeric calculation, and | |
3256 | converting from double into float and thence back to double again will | |
3257 | lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general | |
19799a22 | 3258 | equal $foo). |
5a929a98 | 3259 | |
851646ae JH |
3260 | =item * |
3261 | ||
036b4402 GS |
3262 | If the pattern begins with a C<U>, the resulting string will be treated |
3263 | as Unicode-encoded. You can force UTF8 encoding on in a string with an | |
3264 | initial C<U0>, and the bytes that follow will be interpreted as Unicode | |
3265 | characters. If you don't want this to happen, you can begin your pattern | |
3266 | with C<C0> (or anything else) to force Perl not to UTF8 encode your | |
3267 | string, and then follow this with a C<U*> somewhere in your pattern. | |
3268 | ||
3269 | =item * | |
3270 | ||
851646ae | 3271 | You must yourself do any alignment or padding by inserting for example |
9ccd05c0 JH |
3272 | enough C<'x'>es while packing. There is no way to pack() and unpack() |
3273 | could know where the bytes are going to or coming from. Therefore | |
3274 | C<pack> (and C<unpack>) handle their output and input as flat | |
3275 | sequences of bytes. | |
851646ae | 3276 | |
17f4a12d IZ |
3277 | =item * |
3278 | ||
3279 | A comment in a TEMPLATE starts with C<#> and goes to the end of line. | |
3280 | ||
2b6c5635 GS |
3281 | =item * |
3282 | ||
3283 | If TEMPLATE requires more arguments to pack() than actually given, pack() | |
3284 | assumes additional C<""> arguments. If TEMPLATE requires less arguments | |
3285 | to pack() than actually given, extra arguments are ignored. | |
3286 | ||
5a929a98 | 3287 | =back |
a0d0e21e LW |
3288 | |
3289 | Examples: | |
3290 | ||
a0ed51b3 | 3291 | $foo = pack("CCCC",65,66,67,68); |
a0d0e21e | 3292 | # foo eq "ABCD" |
a0ed51b3 | 3293 | $foo = pack("C4",65,66,67,68); |
a0d0e21e | 3294 | # same thing |
a0ed51b3 LW |
3295 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
3296 | # same thing with Unicode circled letters | |
a0d0e21e LW |
3297 | |
3298 | $foo = pack("ccxxcc",65,66,67,68); | |
3299 | # foo eq "AB\0\0CD" | |
3300 | ||
9ccd05c0 JH |
3301 | # note: the above examples featuring "C" and "c" are true |
3302 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 | |
3303 | # and UTF-8. In EBCDIC the first example would be | |
3304 | # $foo = pack("CCCC",193,194,195,196); | |
3305 | ||
a0d0e21e LW |
3306 | $foo = pack("s2",1,2); |
3307 | # "\1\0\2\0" on little-endian | |
3308 | # "\0\1\0\2" on big-endian | |
3309 | ||
3310 | $foo = pack("a4","abcd","x","y","z"); | |
3311 | # "abcd" | |
3312 | ||
3313 | $foo = pack("aaaa","abcd","x","y","z"); | |
3314 | # "axyz" | |
3315 | ||
3316 | $foo = pack("a14","abcdefg"); | |
3317 | # "abcdefg\0\0\0\0\0\0\0" | |
3318 | ||
3319 | $foo = pack("i9pl", gmtime); | |
3320 | # a real struct tm (on my system anyway) | |
3321 | ||
5a929a98 VU |
3322 | $utmp_template = "Z8 Z8 Z16 L"; |
3323 | $utmp = pack($utmp_template, @utmp1); | |
3324 | # a struct utmp (BSDish) | |
3325 | ||
3326 | @utmp2 = unpack($utmp_template, $utmp); | |
3327 | # "@utmp1" eq "@utmp2" | |
3328 | ||
a0d0e21e LW |
3329 | sub bintodec { |
3330 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
3331 | } | |
3332 | ||
851646ae JH |
3333 | $foo = pack('sx2l', 12, 34); |
3334 | # short 12, two zero bytes padding, long 34 | |
3335 | $bar = pack('s@4l', 12, 34); | |
3336 | # short 12, zero fill to position 4, long 34 | |
3337 | # $foo eq $bar | |
3338 | ||
5a929a98 | 3339 | The same template may generally also be used in unpack(). |
a0d0e21e | 3340 | |
cb1a09d0 AD |
3341 | =item package NAMESPACE |
3342 | ||
b76cc8ba | 3343 | =item package |
d6217f1e | 3344 | |
cb1a09d0 | 3345 | Declares the compilation unit as being in the given namespace. The scope |
2b5ab1e7 | 3346 | of the package declaration is from the declaration itself through the end |
19799a22 | 3347 | of the enclosing block, file, or eval (the same as the C<my> operator). |
2b5ab1e7 TC |
3348 | All further unqualified dynamic identifiers will be in this namespace. |
3349 | A package statement affects only dynamic variables--including those | |
19799a22 GS |
3350 | you've used C<local> on--but I<not> lexical variables, which are created |
3351 | with C<my>. Typically it would be the first declaration in a file to | |
2b5ab1e7 TC |
3352 | be included by the C<require> or C<use> operator. You can switch into a |
3353 | package in more than one place; it merely influences which symbol table | |
3354 | is used by the compiler for the rest of that block. You can refer to | |
3355 | variables and filehandles in other packages by prefixing the identifier | |
3356 | with the package name and a double colon: C<$Package::Variable>. | |
3357 | If the package name is null, the C<main> package as assumed. That is, | |
3358 | C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, | |
3359 | still seen in older code). | |
cb1a09d0 | 3360 | |
5a964f20 TC |
3361 | If NAMESPACE is omitted, then there is no current package, and all |
3362 | identifiers must be fully qualified or lexicals. This is stricter | |
3363 | than C<use strict>, since it also extends to function names. | |
3364 | ||
cb1a09d0 AD |
3365 | See L<perlmod/"Packages"> for more information about packages, modules, |
3366 | and classes. See L<perlsub> for other scoping issues. | |
3367 | ||
a0d0e21e LW |
3368 | =item pipe READHANDLE,WRITEHANDLE |
3369 | ||
3370 | Opens a pair of connected pipes like the corresponding system call. | |
3371 | Note that if you set up a loop of piped processes, deadlock can occur | |
3372 | unless you are very careful. In addition, note that Perl's pipes use | |
184e9718 | 3373 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
a0d0e21e LW |
3374 | after each command, depending on the application. |
3375 | ||
7e1af8bc | 3376 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
4633a7c4 LW |
3377 | for examples of such things. |
3378 | ||
4771b018 GS |
3379 | On systems that support a close-on-exec flag on files, the flag will be set |
3380 | for the newly opened file descriptors as determined by the value of $^F. | |
3381 | See L<perlvar/$^F>. | |
3382 | ||
a0d0e21e LW |
3383 | =item pop ARRAY |
3384 | ||
54310121 | 3385 | =item pop |
28757baa | 3386 | |
a0d0e21e | 3387 | Pops and returns the last value of the array, shortening the array by |
19799a22 | 3388 | one element. Has an effect similar to |
a0d0e21e | 3389 | |
19799a22 | 3390 | $ARRAY[$#ARRAY--] |
a0d0e21e | 3391 | |
19799a22 GS |
3392 | If there are no elements in the array, returns the undefined value |
3393 | (although this may happen at other times as well). If ARRAY is | |
3394 | omitted, pops the C<@ARGV> array in the main program, and the C<@_> | |
3395 | array in subroutines, just like C<shift>. | |
a0d0e21e LW |
3396 | |
3397 | =item pos SCALAR | |
3398 | ||
54310121 | 3399 | =item pos |
bbce6d69 | 3400 | |
4633a7c4 | 3401 | Returns the offset of where the last C<m//g> search left off for the variable |
d6217f1e | 3402 | in question (C<$_> is used when the variable is not specified). May be |
44a8e56a | 3403 | modified to change that offset. Such modification will also influence |
3404 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and | |
3405 | L<perlop>. | |
a0d0e21e LW |
3406 | |
3407 | =item print FILEHANDLE LIST | |
3408 | ||
3409 | =item print LIST | |
3410 | ||
3411 | =item print | |
3412 | ||
19799a22 GS |
3413 | Prints a string or a list of strings. Returns true if successful. |
3414 | FILEHANDLE may be a scalar variable name, in which case the variable | |
3415 | contains the name of or a reference to the filehandle, thus introducing | |
3416 | one level of indirection. (NOTE: If FILEHANDLE is a variable and | |
3417 | the next token is a term, it may be misinterpreted as an operator | |
2b5ab1e7 | 3418 | unless you interpose a C<+> or put parentheses around the arguments.) |
19799a22 GS |
3419 | If FILEHANDLE is omitted, prints by default to standard output (or |
3420 | to the last selected output channel--see L</select>). If LIST is | |
3421 | also omitted, prints C<$_> to the currently selected output channel. | |
3422 | To set the default output channel to something other than STDOUT | |
3423 | use the select operation. The current value of C<$,> (if any) is | |
3424 | printed between each LIST item. The current value of C<$\> (if | |
3425 | any) is printed after the entire LIST has been printed. Because | |
3426 | print takes a LIST, anything in the LIST is evaluated in list | |
3427 | context, and any subroutine that you call will have one or more of | |
3428 | its expressions evaluated in list context. Also be careful not to | |
3429 | follow the print keyword with a left parenthesis unless you want | |
3430 | the corresponding right parenthesis to terminate the arguments to | |
3431 | the print--interpose a C<+> or put parentheses around all the | |
3432 | arguments. | |
a0d0e21e | 3433 | |
4633a7c4 | 3434 | Note that if you're storing FILEHANDLES in an array or other expression, |
da0045b7 | 3435 | you will have to use a block returning its value instead: |
4633a7c4 LW |
3436 | |
3437 | print { $files[$i] } "stuff\n"; | |
3438 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
3439 | ||
5f05dabc | 3440 | =item printf FILEHANDLE FORMAT, LIST |
a0d0e21e | 3441 | |
5f05dabc | 3442 | =item printf FORMAT, LIST |
a0d0e21e | 3443 | |
7660c0ab | 3444 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
a3cb178b | 3445 | (the output record separator) is not appended. The first argument |
19799a22 | 3446 | of the list will be interpreted as the C<printf> format. If C<use locale> is |
a034a98d DD |
3447 | in effect, the character used for the decimal point in formatted real numbers |
3448 | is affected by the LC_NUMERIC locale. See L<perllocale>. | |
a0d0e21e | 3449 | |
19799a22 GS |
3450 | Don't fall into the trap of using a C<printf> when a simple |
3451 | C<print> would do. The C<print> is more efficient and less | |
28757baa | 3452 | error prone. |
3453 | ||
da0045b7 | 3454 | =item prototype FUNCTION |
3455 | ||
3456 | Returns the prototype of a function as a string (or C<undef> if the | |
5f05dabc | 3457 | function has no prototype). FUNCTION is a reference to, or the name of, |
3458 | the function whose prototype you want to retrieve. | |
da0045b7 | 3459 | |
2b5ab1e7 TC |
3460 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a |
3461 | name for Perl builtin. If the builtin is not I<overridable> (such as | |
ab4f32c2 | 3462 | C<qw//>) or its arguments cannot be expressed by a prototype (such as |
19799a22 | 3463 | C<system>) returns C<undef> because the builtin does not really behave |
2b5ab1e7 TC |
3464 | like a Perl function. Otherwise, the string describing the equivalent |
3465 | prototype is returned. | |
b6c543e3 | 3466 | |
a0d0e21e LW |
3467 | =item push ARRAY,LIST |
3468 | ||
3469 | Treats ARRAY as a stack, and pushes the values of LIST | |
3470 | onto the end of ARRAY. The length of ARRAY increases by the length of | |
3471 | LIST. Has the same effect as | |
3472 | ||
3473 | for $value (LIST) { | |
3474 | $ARRAY[++$#ARRAY] = $value; | |
3475 | } | |
3476 | ||
3477 | but is more efficient. Returns the new number of elements in the array. | |
3478 | ||
3479 | =item q/STRING/ | |
3480 | ||
3481 | =item qq/STRING/ | |
3482 | ||
8782bef2 GB |
3483 | =item qr/STRING/ |
3484 | ||
945c54fd | 3485 | =item qx/STRING/ |
a0d0e21e LW |
3486 | |
3487 | =item qw/STRING/ | |
3488 | ||
4b6a7270 | 3489 | Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. |
a0d0e21e LW |
3490 | |
3491 | =item quotemeta EXPR | |
3492 | ||
54310121 | 3493 | =item quotemeta |
bbce6d69 | 3494 | |
36bbe248 | 3495 | Returns the value of EXPR with all non-"word" |
a034a98d DD |
3496 | characters backslashed. (That is, all characters not matching |
3497 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
3498 | returned string, regardless of any locale settings.) | |
3499 | This is the internal function implementing | |
7660c0ab | 3500 | the C<\Q> escape in double-quoted strings. |
a0d0e21e | 3501 | |
7660c0ab | 3502 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 3503 | |
a0d0e21e LW |
3504 | =item rand EXPR |
3505 | ||
3506 | =item rand | |
3507 | ||
7660c0ab | 3508 | Returns a random fractional number greater than or equal to C<0> and less |
3e3baf6d | 3509 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
19799a22 GS |
3510 | omitted, the value C<1> is used. Automatically calls C<srand> unless |
3511 | C<srand> has already been called. See also C<srand>. | |
a0d0e21e | 3512 | |
6063ba18 WM |
3513 | Apply C<int()> to the value returned by C<rand()> if you want random |
3514 | integers instead of random fractional numbers. For example, | |
3515 | ||
3516 | int(rand(10)) | |
3517 | ||
3518 | returns a random integer between C<0> and C<9>, inclusive. | |
3519 | ||
2f9daede | 3520 | (Note: If your rand function consistently returns numbers that are too |
a0d0e21e | 3521 | large or too small, then your version of Perl was probably compiled |
2f9daede | 3522 | with the wrong number of RANDBITS.) |
a0d0e21e LW |
3523 | |
3524 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
3525 | ||
3526 | =item read FILEHANDLE,SCALAR,LENGTH | |
3527 | ||
3528 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
081c6eb8 JH |
3529 | specified FILEHANDLE. Returns the number of bytes actually read, C<0> |
3530 | at end of file, or undef if there was an error. SCALAR will be grown | |
3531 | or shrunk to the length actually read. If SCALAR needs growing, the | |
3532 | new bytes will be zero bytes. An OFFSET may be specified to place | |
3533 | the read data into some other place in SCALAR than the beginning. | |
3534 | The call is actually implemented in terms of stdio's fread(3) call. | |
3535 | To get a true read(2) system call, see C<sysread>. | |
a0d0e21e LW |
3536 | |
3537 | =item readdir DIRHANDLE | |
3538 | ||
19799a22 | 3539 | Returns the next directory entry for a directory opened by C<opendir>. |
5a964f20 | 3540 | If used in list context, returns all the rest of the entries in the |
a0d0e21e | 3541 | directory. If there are no more entries, returns an undefined value in |
5a964f20 | 3542 | scalar context or a null list in list context. |
a0d0e21e | 3543 | |
19799a22 | 3544 | If you're planning to filetest the return values out of a C<readdir>, you'd |
5f05dabc | 3545 | better prepend the directory in question. Otherwise, because we didn't |
19799a22 | 3546 | C<chdir> there, it would have been testing the wrong file. |
cb1a09d0 AD |
3547 | |
3548 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; | |
3549 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); | |
3550 | closedir DIR; | |
3551 | ||
84902520 TB |
3552 | =item readline EXPR |
3553 | ||
fbad3eb5 GS |
3554 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar |
3555 | context, each call reads and returns the next line, until end-of-file is | |
3556 | reached, whereupon the subsequent call returns undef. In list context, | |
3557 | reads until end-of-file is reached and returns a list of lines. Note that | |
3558 | the notion of "line" used here is however you may have defined it | |
3559 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. | |
3560 | ||
2b5ab1e7 | 3561 | When C<$/> is set to C<undef>, when readline() is in scalar |
449bc448 GS |
3562 | context (i.e. file slurp mode), and when an empty file is read, it |
3563 | returns C<''> the first time, followed by C<undef> subsequently. | |
fbad3eb5 | 3564 | |
61eff3bc JH |
3565 | This is the internal function implementing the C<< <EXPR> >> |
3566 | operator, but you can use it directly. The C<< <EXPR> >> | |
84902520 TB |
3567 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
3568 | ||
5a964f20 TC |
3569 | $line = <STDIN>; |
3570 | $line = readline(*STDIN); # same thing | |
3571 | ||
a0d0e21e LW |
3572 | =item readlink EXPR |
3573 | ||
54310121 | 3574 | =item readlink |
bbce6d69 | 3575 | |
a0d0e21e LW |
3576 | Returns the value of a symbolic link, if symbolic links are |
3577 | implemented. If not, gives a fatal error. If there is some system | |
184e9718 | 3578 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
7660c0ab | 3579 | omitted, uses C<$_>. |
a0d0e21e | 3580 | |
84902520 TB |
3581 | =item readpipe EXPR |
3582 | ||
5a964f20 | 3583 | EXPR is executed as a system command. |
84902520 TB |
3584 | The collected standard output of the command is returned. |
3585 | In scalar context, it comes back as a single (potentially | |
3586 | multi-line) string. In list context, returns a list of lines | |
7660c0ab | 3587 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
84902520 TB |
3588 | This is the internal function implementing the C<qx/EXPR/> |
3589 | operator, but you can use it directly. The C<qx/EXPR/> | |
3590 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
3591 | ||
399388f4 | 3592 | =item recv SOCKET,SCALAR,LENGTH,FLAGS |
a0d0e21e LW |
3593 | |
3594 | Receives a message on a socket. Attempts to receive LENGTH bytes of | |
478234b4 GS |
3595 | data into variable SCALAR from the specified SOCKET filehandle. SCALAR |
3596 | will be grown or shrunk to the length actually read. Takes the same | |
3597 | flags as the system call of the same name. Returns the address of the | |
3598 | sender if SOCKET's protocol supports this; returns an empty string | |
3599 | otherwise. If there's an error, returns the undefined value. This call | |
3600 | is actually implemented in terms of recvfrom(2) system call. See | |
3601 | L<perlipc/"UDP: Message Passing"> for examples. | |
a0d0e21e LW |
3602 | |
3603 | =item redo LABEL | |
3604 | ||
3605 | =item redo | |
3606 | ||
3607 | The C<redo> command restarts the loop block without evaluating the | |
98293880 | 3608 | conditional again. The C<continue> block, if any, is not executed. If |
a0d0e21e LW |
3609 | the LABEL is omitted, the command refers to the innermost enclosing |
3610 | loop. This command is normally used by programs that want to lie to | |
3611 | themselves about what was just input: | |
3612 | ||
3613 | # a simpleminded Pascal comment stripper | |
3614 | # (warning: assumes no { or } in strings) | |
4633a7c4 | 3615 | LINE: while (<STDIN>) { |
a0d0e21e LW |
3616 | while (s|({.*}.*){.*}|$1 |) {} |
3617 | s|{.*}| |; | |
3618 | if (s|{.*| |) { | |
3619 | $front = $_; | |
3620 | while (<STDIN>) { | |
3621 | if (/}/) { # end of comment? | |
5a964f20 | 3622 | s|^|$front\{|; |
4633a7c4 | 3623 | redo LINE; |
a0d0e21e LW |
3624 | } |
3625 | } | |
3626 | } | |
3627 | print; | |
3628 | } | |
3629 | ||
4968c1e4 | 3630 | C<redo> cannot be used to retry a block which returns a value such as |
2b5ab1e7 TC |
3631 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
3632 | a grep() or map() operation. | |
4968c1e4 | 3633 | |
6c1372ed GS |
3634 | Note that a block by itself is semantically identical to a loop |
3635 | that executes once. Thus C<redo> inside such a block will effectively | |
3636 | turn it into a looping construct. | |
3637 | ||
98293880 | 3638 | See also L</continue> for an illustration of how C<last>, C<next>, and |
1d2dff63 GS |
3639 | C<redo> work. |
3640 | ||
a0d0e21e LW |
3641 | =item ref EXPR |
3642 | ||
54310121 | 3643 | =item ref |
bbce6d69 | 3644 | |
19799a22 | 3645 | Returns a true value if EXPR is a reference, false otherwise. If EXPR |
7660c0ab | 3646 | is not specified, C<$_> will be used. The value returned depends on the |
bbce6d69 | 3647 | type of thing the reference is a reference to. |
a0d0e21e LW |
3648 | Builtin types include: |
3649 | ||
a0d0e21e LW |
3650 | SCALAR |
3651 | ARRAY | |
3652 | HASH | |
3653 | CODE | |
19799a22 | 3654 | REF |
a0d0e21e | 3655 | GLOB |
19799a22 | 3656 | LVALUE |
a0d0e21e | 3657 | |
54310121 | 3658 | If the referenced object has been blessed into a package, then that package |
19799a22 | 3659 | name is returned instead. You can think of C<ref> as a C<typeof> operator. |
a0d0e21e LW |
3660 | |
3661 | if (ref($r) eq "HASH") { | |
aa689395 | 3662 | print "r is a reference to a hash.\n"; |
54310121 | 3663 | } |
2b5ab1e7 | 3664 | unless (ref($r)) { |
a0d0e21e | 3665 | print "r is not a reference at all.\n"; |
54310121 | 3666 | } |
2b5ab1e7 TC |
3667 | if (UNIVERSAL::isa($r, "HASH")) { # for subclassing |
3668 | print "r is a reference to something that isa hash.\n"; | |
b76cc8ba | 3669 | } |
a0d0e21e LW |
3670 | |
3671 | See also L<perlref>. | |
3672 | ||
3673 | =item rename OLDNAME,NEWNAME | |
3674 | ||
19799a22 GS |
3675 | Changes the name of a file; an existing file NEWNAME will be |
3676 | clobbered. Returns true for success, false otherwise. | |
3677 | ||
2b5ab1e7 TC |
3678 | Behavior of this function varies wildly depending on your system |
3679 | implementation. For example, it will usually not work across file system | |
3680 | boundaries, even though the system I<mv> command sometimes compensates | |
3681 | for this. Other restrictions include whether it works on directories, | |
3682 | open files, or pre-existing files. Check L<perlport> and either the | |
3683 | rename(2) manpage or equivalent system documentation for details. | |
a0d0e21e | 3684 | |
16070b82 GS |
3685 | =item require VERSION |
3686 | ||
a0d0e21e LW |
3687 | =item require EXPR |
3688 | ||
3689 | =item require | |
3690 | ||
7660c0ab | 3691 | Demands some semantics specified by EXPR, or by C<$_> if EXPR is not |
44dcb63b GS |
3692 | supplied. |
3693 | ||
dd629d5b | 3694 | If a VERSION is specified as a literal of the form v5.6.1, |
44dcb63b GS |
3695 | demands that the current version of Perl (C<$^V> or $PERL_VERSION) be |
3696 | at least as recent as that version, at run time. (For compatibility | |
3697 | with older versions of Perl, a numeric argument will also be interpreted | |
3698 | as VERSION.) Compare with L</use>, which can do a similar check at | |
3699 | compile time. | |
3700 | ||
dd629d5b GS |
3701 | require v5.6.1; # run time version check |
3702 | require 5.6.1; # ditto | |
3703 | require 5.005_03; # float version allowed for compatibility | |
a0d0e21e LW |
3704 | |
3705 | Otherwise, demands that a library file be included if it hasn't already | |
3706 | been included. The file is included via the do-FILE mechanism, which is | |
19799a22 | 3707 | essentially just a variety of C<eval>. Has semantics similar to the following |
a0d0e21e LW |
3708 | subroutine: |
3709 | ||
3710 | sub require { | |
5a964f20 | 3711 | my($filename) = @_; |
a0d0e21e | 3712 | return 1 if $INC{$filename}; |
5a964f20 | 3713 | my($realfilename,$result); |
a0d0e21e LW |
3714 | ITER: { |
3715 | foreach $prefix (@INC) { | |
3716 | $realfilename = "$prefix/$filename"; | |
3717 | if (-f $realfilename) { | |
f784dfa3 | 3718 | $INC{$filename} = $realfilename; |
a0d0e21e LW |
3719 | $result = do $realfilename; |
3720 | last ITER; | |
3721 | } | |
3722 | } | |
3723 | die "Can't find $filename in \@INC"; | |
3724 | } | |
f784dfa3 | 3725 | delete $INC{$filename} if $@ || !$result; |
a0d0e21e LW |
3726 | die $@ if $@; |
3727 | die "$filename did not return true value" unless $result; | |
5a964f20 | 3728 | return $result; |
a0d0e21e LW |
3729 | } |
3730 | ||
3731 | Note that the file will not be included twice under the same specified | |
19799a22 | 3732 | name. The file must return true as the last statement to indicate |
a0d0e21e | 3733 | successful execution of any initialization code, so it's customary to |
19799a22 GS |
3734 | end such a file with C<1;> unless you're sure it'll return true |
3735 | otherwise. But it's better just to put the C<1;>, in case you add more | |
a0d0e21e LW |
3736 | statements. |
3737 | ||
54310121 | 3738 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
da0045b7 | 3739 | replaces "F<::>" with "F</>" in the filename for you, |
54310121 | 3740 | to make it easy to load standard modules. This form of loading of |
a0d0e21e LW |
3741 | modules does not risk altering your namespace. |
3742 | ||
ee580363 GS |
3743 | In other words, if you try this: |
3744 | ||
b76cc8ba | 3745 | require Foo::Bar; # a splendid bareword |
ee580363 | 3746 | |
b76cc8ba | 3747 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
7660c0ab | 3748 | directories specified in the C<@INC> array. |
ee580363 | 3749 | |
5a964f20 | 3750 | But if you try this: |
ee580363 GS |
3751 | |
3752 | $class = 'Foo::Bar'; | |
f86cebdf | 3753 | require $class; # $class is not a bareword |
5a964f20 | 3754 | #or |
f86cebdf | 3755 | require "Foo::Bar"; # not a bareword because of the "" |
ee580363 | 3756 | |
b76cc8ba | 3757 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
19799a22 | 3758 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
ee580363 GS |
3759 | |
3760 | eval "require $class"; | |
3761 | ||
3762 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. | |
a0d0e21e LW |
3763 | |
3764 | =item reset EXPR | |
3765 | ||
3766 | =item reset | |
3767 | ||
3768 | Generally used in a C<continue> block at the end of a loop to clear | |
7660c0ab | 3769 | variables and reset C<??> searches so that they work again. The |
a0d0e21e LW |
3770 | expression is interpreted as a list of single characters (hyphens |
3771 | allowed for ranges). All variables and arrays beginning with one of | |
3772 | those letters are reset to their pristine state. If the expression is | |
7660c0ab | 3773 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets |
5f05dabc | 3774 | only variables or searches in the current package. Always returns |
a0d0e21e LW |
3775 | 1. Examples: |
3776 | ||
3777 | reset 'X'; # reset all X variables | |
3778 | reset 'a-z'; # reset lower case variables | |
2b5ab1e7 | 3779 | reset; # just reset ?one-time? searches |
a0d0e21e | 3780 | |
7660c0ab | 3781 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
2b5ab1e7 TC |
3782 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package |
3783 | variables--lexical variables are unaffected, but they clean themselves | |
3784 | up on scope exit anyway, so you'll probably want to use them instead. | |
3785 | See L</my>. | |
a0d0e21e | 3786 | |
54310121 | 3787 | =item return EXPR |
3788 | ||
3789 | =item return | |
3790 | ||
b76cc8ba | 3791 | Returns from a subroutine, C<eval>, or C<do FILE> with the value |
5a964f20 | 3792 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
54310121 | 3793 | context, depending on how the return value will be used, and the context |
19799a22 | 3794 | may vary from one execution to the next (see C<wantarray>). If no EXPR |
2b5ab1e7 TC |
3795 | is given, returns an empty list in list context, the undefined value in |
3796 | scalar context, and (of course) nothing at all in a void context. | |
a0d0e21e | 3797 | |
2b5ab1e7 TC |
3798 | (Note that in the absence of a explicit C<return>, a subroutine, eval, |
3799 | or do FILE will automatically return the value of the last expression | |
3800 | evaluated.) | |
a0d0e21e LW |
3801 | |
3802 | =item reverse LIST | |
3803 | ||
5a964f20 TC |
3804 | In list context, returns a list value consisting of the elements |
3805 | of LIST in the opposite order. In scalar context, concatenates the | |
2b5ab1e7 | 3806 | elements of LIST and returns a string value with all characters |
a0ed51b3 | 3807 | in the opposite order. |
4633a7c4 | 3808 | |
2f9daede | 3809 | print reverse <>; # line tac, last line first |
4633a7c4 | 3810 | |
2f9daede | 3811 | undef $/; # for efficiency of <> |
a0ed51b3 | 3812 | print scalar reverse <>; # character tac, last line tsrif |
2f9daede TP |
3813 | |
3814 | This operator is also handy for inverting a hash, although there are some | |
3815 | caveats. If a value is duplicated in the original hash, only one of those | |
3816 | can be represented as a key in the inverted hash. Also, this has to | |
3817 | unwind one hash and build a whole new one, which may take some time | |
2b5ab1e7 | 3818 | on a large hash, such as from a DBM file. |
2f9daede TP |
3819 | |
3820 | %by_name = reverse %by_address; # Invert the hash | |
a0d0e21e LW |
3821 | |
3822 | =item rewinddir DIRHANDLE | |
3823 | ||
3824 | Sets the current position to the beginning of the directory for the | |
19799a22 | 3825 | C<readdir> routine on DIRHANDLE. |
a0d0e21e LW |
3826 | |
3827 | =item rindex STR,SUBSTR,POSITION | |
3828 | ||
3829 | =item rindex STR,SUBSTR | |
3830 | ||
2b5ab1e7 | 3831 | Works just like index() except that it returns the position of the LAST |
a0d0e21e LW |
3832 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
3833 | last occurrence at or before that position. | |
3834 | ||
3835 | =item rmdir FILENAME | |
3836 | ||
54310121 | 3837 | =item rmdir |
bbce6d69 | 3838 | |
5a964f20 | 3839 | Deletes the directory specified by FILENAME if that directory is empty. If it |
19799a22 | 3840 | succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If |
7660c0ab | 3841 | FILENAME is omitted, uses C<$_>. |
a0d0e21e LW |
3842 | |
3843 | =item s/// | |
3844 | ||
3845 | The substitution operator. See L<perlop>. | |
3846 | ||
3847 | =item scalar EXPR | |
3848 | ||
5a964f20 | 3849 | Forces EXPR to be interpreted in scalar context and returns the value |
54310121 | 3850 | of EXPR. |
cb1a09d0 AD |
3851 | |
3852 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
3853 | ||
54310121 | 3854 | There is no equivalent operator to force an expression to |
2b5ab1e7 | 3855 | be interpolated in list context because in practice, this is never |
cb1a09d0 AD |
3856 | needed. If you really wanted to do so, however, you could use |
3857 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
3858 | C<(some expression)> suffices. | |
a0d0e21e | 3859 | |
19799a22 | 3860 | Because C<scalar> is unary operator, if you accidentally use for EXPR a |
2b5ab1e7 TC |
3861 | parenthesized list, this behaves as a scalar comma expression, evaluating |
3862 | all but the last element in void context and returning the final element | |
3863 | evaluated in scalar context. This is seldom what you want. | |
62c18ce2 GS |
3864 | |
3865 | The following single statement: | |
3866 | ||
3867 | print uc(scalar(&foo,$bar)),$baz; | |
3868 | ||
3869 | is the moral equivalent of these two: | |
3870 | ||
3871 | &foo; | |
3872 | print(uc($bar),$baz); | |
3873 | ||
3874 | See L<perlop> for more details on unary operators and the comma operator. | |
3875 | ||
a0d0e21e LW |
3876 | =item seek FILEHANDLE,POSITION,WHENCE |
3877 | ||
19799a22 | 3878 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. |
8903cb82 | 3879 | FILEHANDLE may be an expression whose value gives the name of the |
7660c0ab | 3880 | filehandle. The values for WHENCE are C<0> to set the new position to |
ac88732c JH |
3881 | POSITION, C<1> to set it to the current position plus POSITION, and |
3882 | C<2> to set it to EOF plus POSITION (typically negative). For WHENCE | |
3883 | you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> | |
ca6e1c26 JH |
3884 | (start of the file, current position, end of the file) from the Fcntl |
3885 | module. Returns C<1> upon success, C<0> otherwise. | |
8903cb82 | 3886 | |
19799a22 GS |
3887 | If you want to position file for C<sysread> or C<syswrite>, don't use |
3888 | C<seek>--buffering makes its effect on the file's system position | |
3889 | unpredictable and non-portable. Use C<sysseek> instead. | |
a0d0e21e | 3890 | |
2b5ab1e7 TC |
3891 | Due to the rules and rigors of ANSI C, on some systems you have to do a |
3892 | seek whenever you switch between reading and writing. Amongst other | |
3893 | things, this may have the effect of calling stdio's clearerr(3). | |
3894 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
cb1a09d0 AD |
3895 | |
3896 | seek(TEST,0,1); | |
3897 | ||
3898 | This is also useful for applications emulating C<tail -f>. Once you hit | |
3899 | EOF on your read, and then sleep for a while, you might have to stick in a | |
19799a22 | 3900 | seek() to reset things. The C<seek> doesn't change the current position, |
8903cb82 | 3901 | but it I<does> clear the end-of-file condition on the handle, so that the |
61eff3bc | 3902 | next C<< <FILE> >> makes Perl try again to read something. We hope. |
cb1a09d0 AD |
3903 | |
3904 | If that doesn't work (some stdios are particularly cantankerous), then | |
3905 | you may need something more like this: | |
3906 | ||
3907 | for (;;) { | |
f86cebdf GS |
3908 | for ($curpos = tell(FILE); $_ = <FILE>; |
3909 | $curpos = tell(FILE)) { | |
cb1a09d0 AD |
3910 | # search for some stuff and put it into files |
3911 | } | |
3912 | sleep($for_a_while); | |
3913 | seek(FILE, $curpos, 0); | |
3914 | } | |
3915 | ||
a0d0e21e LW |
3916 | =item seekdir DIRHANDLE,POS |
3917 | ||
19799a22 GS |
3918 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS |
3919 | must be a value returned by C<telldir>. Has the same caveats about | |
a0d0e21e LW |
3920 | possible directory compaction as the corresponding system library |
3921 | routine. | |
3922 | ||
3923 | =item select FILEHANDLE | |
3924 | ||
3925 | =item select | |
3926 | ||
3927 | Returns the currently selected filehandle. Sets the current default | |
3928 | filehandle for output, if FILEHANDLE is supplied. This has two | |
19799a22 | 3929 | effects: first, a C<write> or a C<print> without a filehandle will |
a0d0e21e LW |
3930 | default to this FILEHANDLE. Second, references to variables related to |
3931 | output will refer to this output channel. For example, if you have to | |
3932 | set the top of form format for more than one output channel, you might | |
3933 | do the following: | |
3934 | ||
3935 | select(REPORT1); | |
3936 | $^ = 'report1_top'; | |
3937 | select(REPORT2); | |
3938 | $^ = 'report2_top'; | |
3939 | ||
3940 | FILEHANDLE may be an expression whose value gives the name of the | |
3941 | actual filehandle. Thus: | |
3942 | ||
3943 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
3944 | ||
4633a7c4 LW |
3945 | Some programmers may prefer to think of filehandles as objects with |
3946 | methods, preferring to write the last example as: | |
a0d0e21e | 3947 | |
28757baa | 3948 | use IO::Handle; |
a0d0e21e LW |
3949 | STDERR->autoflush(1); |
3950 | ||
3951 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
3952 | ||
f86cebdf | 3953 | This calls the select(2) system call with the bit masks specified, which |
19799a22 | 3954 | can be constructed using C<fileno> and C<vec>, along these lines: |
a0d0e21e LW |
3955 | |
3956 | $rin = $win = $ein = ''; | |
3957 | vec($rin,fileno(STDIN),1) = 1; | |
3958 | vec($win,fileno(STDOUT),1) = 1; | |
3959 | $ein = $rin | $win; | |
3960 | ||
3961 | If you want to select on many filehandles you might wish to write a | |
3962 | subroutine: | |
3963 | ||
3964 | sub fhbits { | |
5a964f20 TC |
3965 | my(@fhlist) = split(' ',$_[0]); |
3966 | my($bits); | |
a0d0e21e LW |
3967 | for (@fhlist) { |
3968 | vec($bits,fileno($_),1) = 1; | |
3969 | } | |
3970 | $bits; | |
3971 | } | |
4633a7c4 | 3972 | $rin = fhbits('STDIN TTY SOCK'); |
a0d0e21e LW |
3973 | |
3974 | The usual idiom is: | |
3975 | ||
3976 | ($nfound,$timeleft) = | |
3977 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
3978 | ||
54310121 | 3979 | or to block until something becomes ready just do this |
a0d0e21e LW |
3980 | |
3981 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
3982 | ||
19799a22 GS |
3983 | Most systems do not bother to return anything useful in $timeleft, so |
3984 | calling select() in scalar context just returns $nfound. | |
c07a80fd | 3985 | |
5f05dabc | 3986 | Any of the bit masks can also be undef. The timeout, if specified, is |
a0d0e21e | 3987 | in seconds, which may be fractional. Note: not all implementations are |
19799a22 GS |
3988 | capable of returning the$timeleft. If not, they always return |
3989 | $timeleft equal to the supplied $timeout. | |
a0d0e21e | 3990 | |
ff68c719 | 3991 | You can effect a sleep of 250 milliseconds this way: |
a0d0e21e LW |
3992 | |
3993 | select(undef, undef, undef, 0.25); | |
3994 | ||
19799a22 | 3995 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> |
61eff3bc | 3996 | or <FH>) with C<select>, except as permitted by POSIX, and even |
19799a22 | 3997 | then only on POSIX systems. You have to use C<sysread> instead. |
a0d0e21e LW |
3998 | |
3999 | =item semctl ID,SEMNUM,CMD,ARG | |
4000 | ||
19799a22 | 4001 | Calls the System V IPC function C<semctl>. You'll probably have to say |
0ade1984 JH |
4002 | |
4003 | use IPC::SysV; | |
4004 | ||
4005 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
4006 | GETALL, then ARG must be a variable which will hold the returned | |
e4038a1f MS |
4007 | semid_ds structure or semaphore value array. Returns like C<ioctl>: |
4008 | the undefined value for error, "C<0 but true>" for zero, or the actual | |
4009 | return value otherwise. The ARG must consist of a vector of native | |
106325ad | 4010 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. |
4755096e GS |
4011 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> |
4012 | documentation. | |
a0d0e21e LW |
4013 | |
4014 | =item semget KEY,NSEMS,FLAGS | |
4015 | ||
4016 | Calls the System V IPC function semget. Returns the semaphore id, or | |
4755096e GS |
4017 | the undefined value if there is an error. See also |
4018 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> | |
4019 | documentation. | |
a0d0e21e LW |
4020 | |
4021 | =item semop KEY,OPSTRING | |
4022 | ||
4023 | Calls the System V IPC function semop to perform semaphore operations | |
4024 | such as signaling and waiting. OPSTRING must be a packed array of | |
4025 | semop structures. Each semop structure can be generated with | |
4026 | C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore | |
19799a22 GS |
4027 | operations is implied by the length of OPSTRING. Returns true if |
4028 | successful, or false if there is an error. As an example, the | |
4029 | following code waits on semaphore $semnum of semaphore id $semid: | |
a0d0e21e LW |
4030 | |
4031 | $semop = pack("sss", $semnum, -1, 0); | |
4032 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); | |
4033 | ||
4755096e GS |
4034 | To signal the semaphore, replace C<-1> with C<1>. See also |
4035 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> | |
4036 | documentation. | |
a0d0e21e LW |
4037 | |
4038 | =item send SOCKET,MSG,FLAGS,TO | |
4039 | ||
4040 | =item send SOCKET,MSG,FLAGS | |
4041 | ||
4042 | Sends a message on a socket. Takes the same flags as the system call | |
4043 | of the same name. On unconnected sockets you must specify a | |
19799a22 | 4044 | destination to send TO, in which case it does a C C<sendto>. Returns |
a0d0e21e | 4045 | the number of characters sent, or the undefined value if there is an |
2b5ab1e7 | 4046 | error. The C system call sendmsg(2) is currently unimplemented. |
4633a7c4 | 4047 | See L<perlipc/"UDP: Message Passing"> for examples. |
a0d0e21e LW |
4048 | |
4049 | =item setpgrp PID,PGRP | |
4050 | ||
7660c0ab | 4051 | Sets the current process group for the specified PID, C<0> for the current |
a0d0e21e | 4052 | process. Will produce a fatal error if used on a machine that doesn't |
81777298 GS |
4053 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, |
4054 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
4055 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
4056 | C<POSIX::setsid()>. | |
a0d0e21e LW |
4057 | |
4058 | =item setpriority WHICH,WHO,PRIORITY | |
4059 | ||
4060 | Sets the current priority for a process, a process group, or a user. | |
f86cebdf GS |
4061 | (See setpriority(2).) Will produce a fatal error if used on a machine |
4062 | that doesn't implement setpriority(2). | |
a0d0e21e LW |
4063 | |
4064 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
4065 | ||
4066 | Sets the socket option requested. Returns undefined if there is an | |
7660c0ab | 4067 | error. OPTVAL may be specified as C<undef> if you don't want to pass an |
a0d0e21e LW |
4068 | argument. |
4069 | ||
4070 | =item shift ARRAY | |
4071 | ||
4072 | =item shift | |
4073 | ||
4074 | Shifts the first value of the array off and returns it, shortening the | |
4075 | array by 1 and moving everything down. If there are no elements in the | |
4076 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
7660c0ab A |
4077 | C<@_> array within the lexical scope of subroutines and formats, and the |
4078 | C<@ARGV> array at file scopes or within the lexical scopes established by | |
7d30b5c4 | 4079 | the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> |
4f25aa18 GS |
4080 | constructs. |
4081 | ||
a1b2c429 | 4082 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the |
19799a22 | 4083 | same thing to the left end of an array that C<pop> and C<push> do to the |
977336f5 | 4084 | right end. |
a0d0e21e LW |
4085 | |
4086 | =item shmctl ID,CMD,ARG | |
4087 | ||
0ade1984 JH |
4088 | Calls the System V IPC function shmctl. You'll probably have to say |
4089 | ||
4090 | use IPC::SysV; | |
4091 | ||
7660c0ab A |
4092 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
4093 | then ARG must be a variable which will hold the returned C<shmid_ds> | |
4094 | structure. Returns like ioctl: the undefined value for error, "C<0> but | |
0ade1984 | 4095 | true" for zero, or the actual return value otherwise. |
4755096e | 4096 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e LW |
4097 | |
4098 | =item shmget KEY,SIZE,FLAGS | |
4099 | ||
4100 | Calls the System V IPC function shmget. Returns the shared memory | |
4101 | segment id, or the undefined value if there is an error. | |
4755096e | 4102 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
a0d0e21e LW |
4103 | |
4104 | =item shmread ID,VAR,POS,SIZE | |
4105 | ||
4106 | =item shmwrite ID,STRING,POS,SIZE | |
4107 | ||
4108 | Reads or writes the System V shared memory segment ID starting at | |
4109 | position POS for size SIZE by attaching to it, copying in/out, and | |
5a964f20 | 4110 | detaching from it. When reading, VAR must be a variable that will |
a0d0e21e LW |
4111 | hold the data read. When writing, if STRING is too long, only SIZE |
4112 | bytes are used; if STRING is too short, nulls are written to fill out | |
19799a22 | 4113 | SIZE bytes. Return true if successful, or false if there is an error. |
4755096e GS |
4114 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, |
4115 | C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. | |
a0d0e21e LW |
4116 | |
4117 | =item shutdown SOCKET,HOW | |
4118 | ||
4119 | Shuts down a socket connection in the manner indicated by HOW, which | |
4120 | has the same interpretation as in the system call of the same name. | |
4121 | ||
f86cebdf GS |
4122 | shutdown(SOCKET, 0); # I/we have stopped reading data |
4123 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
4124 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
5a964f20 TC |
4125 | |
4126 | This is useful with sockets when you want to tell the other | |
4127 | side you're done writing but not done reading, or vice versa. | |
b76cc8ba | 4128 | It's also a more insistent form of close because it also |
19799a22 | 4129 | disables the file descriptor in any forked copies in other |
5a964f20 TC |
4130 | processes. |
4131 | ||
a0d0e21e LW |
4132 | =item sin EXPR |
4133 | ||
54310121 | 4134 | =item sin |
bbce6d69 | 4135 | |
a0d0e21e | 4136 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
7660c0ab | 4137 | returns sine of C<$_>. |
a0d0e21e | 4138 | |
ca6e1c26 | 4139 | For the inverse sine operation, you may use the C<Math::Trig::asin> |
28757baa | 4140 | function, or use this relation: |
4141 | ||
4142 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
4143 | ||
a0d0e21e LW |
4144 | =item sleep EXPR |
4145 | ||
4146 | =item sleep | |
4147 | ||
4148 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. | |
7660c0ab | 4149 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
1d3434b8 | 4150 | Returns the number of seconds actually slept. You probably cannot |
19799a22 GS |
4151 | mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented |
4152 | using C<alarm>. | |
a0d0e21e LW |
4153 | |
4154 | On some older systems, it may sleep up to a full second less than what | |
4155 | you requested, depending on how it counts seconds. Most modern systems | |
5a964f20 TC |
4156 | always sleep the full amount. They may appear to sleep longer than that, |
4157 | however, because your process might not be scheduled right away in a | |
4158 | busy multitasking system. | |
a0d0e21e | 4159 | |
cb1a09d0 | 4160 | For delays of finer granularity than one second, you may use Perl's |
68f8bed4 JH |
4161 | C<syscall> interface to access setitimer(2) if your system supports |
4162 | it, or else see L</select> above. The Time::HiRes module from CPAN | |
4163 | may also help. | |
cb1a09d0 | 4164 | |
b6e2112e | 4165 | See also the POSIX module's C<pause> function. |
5f05dabc | 4166 | |
80cbd5ad JH |
4167 | =item sockatmark SOCKET |
4168 | ||
4169 | Returns true if the socket is positioned at the out-of-band mark | |
4170 | (also known as the urgent data mark), false otherwise. Use right | |
4171 | after reading from the socket. | |
4172 | ||
4173 | Not available directly, one has to import the function from | |
4174 | the IO::Socket extension | |
4175 | ||
4176 | use IO::Socket 'sockatmark'; | |
4177 | ||
4178 | Even this doesn't guarantee that sockatmark() really is available, | |
4179 | though, because sockatmark() is a relatively recent addition to | |
4180 | the family of socket functions. If it is unavailable, attempt to | |
4181 | use it will fail | |
4182 | ||
4183 | IO::Socket::atmark not implemented on this architecture ... | |
4184 | ||
4185 | See also L<IO::Socket>. | |
4186 | ||
a0d0e21e LW |
4187 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
4188 | ||
4189 | Opens a socket of the specified kind and attaches it to filehandle | |
19799a22 GS |
4190 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for |
4191 | the system call of the same name. You should C<use Socket> first | |
4192 | to get the proper definitions imported. See the examples in | |
4193 | L<perlipc/"Sockets: Client/Server Communication">. | |
a0d0e21e | 4194 | |
8d2a6795 GS |
4195 | On systems that support a close-on-exec flag on files, the flag will |
4196 | be set for the newly opened file descriptor, as determined by the | |
4197 | value of $^F. See L<perlvar/$^F>. | |
4198 | ||
a0d0e21e LW |
4199 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
4200 | ||
4201 | Creates an unnamed pair of sockets in the specified domain, of the | |
5f05dabc | 4202 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
a0d0e21e | 4203 | for the system call of the same name. If unimplemented, yields a fatal |
19799a22 | 4204 | error. Returns true if successful. |
a0d0e21e | 4205 | |
8d2a6795 GS |
4206 | On systems that support a close-on-exec flag on files, the flag will |
4207 | be set for the newly opened file descriptors, as determined by the value | |
4208 | of $^F. See L<perlvar/$^F>. | |
4209 | ||
19799a22 | 4210 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call |
5a964f20 TC |
4211 | to C<pipe(Rdr, Wtr)> is essentially: |
4212 | ||
4213 | use Socket; | |
4214 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
4215 | shutdown(Rdr, 1); # no more writing for reader | |
4216 | shutdown(Wtr, 0); # no more reading for writer | |
4217 | ||
4218 | See L<perlipc> for an example of socketpair use. | |
4219 | ||
a0d0e21e LW |
4220 | =item sort SUBNAME LIST |
4221 | ||
4222 | =item sort BLOCK LIST | |
4223 | ||
4224 | =item sort LIST | |
4225 | ||
2f9daede | 4226 | Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK |
19799a22 | 4227 | is omitted, C<sort>s in standard string comparison order. If SUBNAME is |
2f9daede | 4228 | specified, it gives the name of a subroutine that returns an integer |
7660c0ab | 4229 | less than, equal to, or greater than C<0>, depending on how the elements |
61eff3bc | 4230 | of the list are to be ordered. (The C<< <=> >> and C<cmp> |
2f9daede | 4231 | operators are extremely useful in such routines.) SUBNAME may be a |
1d3434b8 GS |
4232 | scalar variable name (unsubscripted), in which case the value provides |
4233 | the name of (or a reference to) the actual subroutine to use. In place | |
4234 | of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort | |
4235 | subroutine. | |
a0d0e21e | 4236 | |
43481408 | 4237 | If the subroutine's prototype is C<($$)>, the elements to be compared |
f9a36357 GS |
4238 | are passed by reference in C<@_>, as for a normal subroutine. This is |
4239 | slower than unprototyped subroutines, where the elements to be | |
4240 | compared are passed into the subroutine | |
43481408 GS |
4241 | as the package global variables $a and $b (see example below). Note that |
4242 | in the latter case, it is usually counter-productive to declare $a and | |
4243 | $b as lexicals. | |
4244 | ||
4245 | In either case, the subroutine may not be recursive. The values to be | |
4246 | compared are always passed by reference, so don't modify them. | |
a0d0e21e | 4247 | |
0a753a76 | 4248 | You also cannot exit out of the sort block or subroutine using any of the |
19799a22 | 4249 | loop control operators described in L<perlsyn> or with C<goto>. |
0a753a76 | 4250 | |
a034a98d DD |
4251 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
4252 | current collation locale. See L<perllocale>. | |
4253 | ||
a0d0e21e LW |
4254 | Examples: |
4255 | ||
4256 | # sort lexically | |
4257 | @articles = sort @files; | |
4258 | ||
4259 | # same thing, but with explicit sort routine | |
4260 | @articles = sort {$a cmp $b} @files; | |
4261 | ||
cb1a09d0 | 4262 | # now case-insensitively |
54310121 | 4263 | @articles = sort {uc($a) cmp uc($b)} @files; |
cb1a09d0 | 4264 | |
a0d0e21e LW |
4265 | # same thing in reversed order |
4266 | @articles = sort {$b cmp $a} @files; | |
4267 | ||
4268 | # sort numerically ascending | |
4269 | @articles = sort {$a <=> $b} @files; | |
4270 | ||
4271 | # sort numerically descending | |
4272 | @articles = sort {$b <=> $a} @files; | |
4273 | ||
19799a22 GS |
4274 | # this sorts the %age hash by value instead of key |
4275 | # using an in-line function | |
4276 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
4277 | ||
a0d0e21e LW |
4278 | # sort using explicit subroutine name |
4279 | sub byage { | |
2f9daede | 4280 | $age{$a} <=> $age{$b}; # presuming numeric |
a0d0e21e LW |
4281 | } |
4282 | @sortedclass = sort byage @class; | |
4283 | ||
19799a22 GS |
4284 | sub backwards { $b cmp $a } |
4285 | @harry = qw(dog cat x Cain Abel); | |
4286 | @george = qw(gone chased yz Punished Axed); | |
a0d0e21e LW |
4287 | print sort @harry; |
4288 | # prints AbelCaincatdogx | |
4289 | print sort backwards @harry; | |
4290 | # prints xdogcatCainAbel | |
4291 | print sort @george, 'to', @harry; | |
4292 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz | |
4293 | ||
54310121 | 4294 | # inefficiently sort by descending numeric compare using |
4295 | # the first integer after the first = sign, or the | |
cb1a09d0 AD |
4296 | # whole record case-insensitively otherwise |
4297 | ||
4298 | @new = sort { | |
4299 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
4300 | || | |
4301 | uc($a) cmp uc($b) | |
4302 | } @old; | |
4303 | ||
4304 | # same thing, but much more efficiently; | |
4305 | # we'll build auxiliary indices instead | |
4306 | # for speed | |
4307 | @nums = @caps = (); | |
54310121 | 4308 | for (@old) { |
cb1a09d0 AD |
4309 | push @nums, /=(\d+)/; |
4310 | push @caps, uc($_); | |
54310121 | 4311 | } |
cb1a09d0 AD |
4312 | |
4313 | @new = @old[ sort { | |
4314 | $nums[$b] <=> $nums[$a] | |
4315 | || | |
4316 | $caps[$a] cmp $caps[$b] | |
4317 | } 0..$#old | |
4318 | ]; | |
4319 | ||
19799a22 | 4320 | # same thing, but without any temps |
cb1a09d0 | 4321 | @new = map { $_->[0] } |
19799a22 GS |
4322 | sort { $b->[1] <=> $a->[1] |
4323 | || | |
4324 | $a->[2] cmp $b->[2] | |
4325 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
61eff3bc | 4326 | |
43481408 GS |
4327 | # using a prototype allows you to use any comparison subroutine |
4328 | # as a sort subroutine (including other package's subroutines) | |
4329 | package other; | |
4330 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here | |
4331 | ||
4332 | package main; | |
4333 | @new = sort other::backwards @old; | |
cb1a09d0 | 4334 | |
19799a22 GS |
4335 | If you're using strict, you I<must not> declare $a |
4336 | and $b as lexicals. They are package globals. That means | |
47223a36 | 4337 | if you're in the C<main> package and type |
13a2d996 | 4338 | |
47223a36 | 4339 | @articles = sort {$b <=> $a} @files; |
13a2d996 | 4340 | |
47223a36 JH |
4341 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), |
4342 | but if you're in the C<FooPack> package, it's the same as typing | |
cb1a09d0 AD |
4343 | |
4344 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
4345 | ||
55497cff | 4346 | The comparison function is required to behave. If it returns |
7660c0ab A |
4347 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
4348 | sometimes saying the opposite, for example) the results are not | |
4349 | well-defined. | |
55497cff | 4350 | |
a0d0e21e LW |
4351 | =item splice ARRAY,OFFSET,LENGTH,LIST |
4352 | ||
4353 | =item splice ARRAY,OFFSET,LENGTH | |
4354 | ||
4355 | =item splice ARRAY,OFFSET | |
4356 | ||
453f9044 GS |
4357 | =item splice ARRAY |
4358 | ||
a0d0e21e | 4359 | Removes the elements designated by OFFSET and LENGTH from an array, and |
5a964f20 TC |
4360 | replaces them with the elements of LIST, if any. In list context, |
4361 | returns the elements removed from the array. In scalar context, | |
43051805 | 4362 | returns the last element removed, or C<undef> if no elements are |
48cdf507 | 4363 | removed. The array grows or shrinks as necessary. |
19799a22 | 4364 | If OFFSET is negative then it starts that far from the end of the array. |
48cdf507 | 4365 | If LENGTH is omitted, removes everything from OFFSET onward. |
453f9044 GS |
4366 | If LENGTH is negative, leaves that many elements off the end of the array. |
4367 | If both OFFSET and LENGTH are omitted, removes everything. | |
4368 | ||
48cdf507 | 4369 | The following equivalences hold (assuming C<$[ == 0>): |
a0d0e21e | 4370 | |
48cdf507 | 4371 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
a0d0e21e LW |
4372 | pop(@a) splice(@a,-1) |
4373 | shift(@a) splice(@a,0,1) | |
4374 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
5a964f20 | 4375 | $a[$x] = $y splice(@a,$x,1,$y) |
a0d0e21e LW |
4376 | |
4377 | Example, assuming array lengths are passed before arrays: | |
4378 | ||
4379 | sub aeq { # compare two list values | |
5a964f20 TC |
4380 | my(@a) = splice(@_,0,shift); |
4381 | my(@b) = splice(@_,0,shift); | |
a0d0e21e LW |
4382 | return 0 unless @a == @b; # same len? |
4383 | while (@a) { | |
4384 | return 0 if pop(@a) ne pop(@b); | |
4385 | } | |
4386 | return 1; | |
4387 | } | |
4388 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
4389 | ||
4390 | =item split /PATTERN/,EXPR,LIMIT | |
4391 | ||
4392 | =item split /PATTERN/,EXPR | |
4393 | ||
4394 | =item split /PATTERN/ | |
4395 | ||
4396 | =item split | |
4397 | ||
19799a22 | 4398 | Splits a string into a list of strings and returns that list. By default, |
5a964f20 | 4399 | empty leading fields are preserved, and empty trailing ones are deleted. |
a0d0e21e | 4400 | |
46836f5c GS |
4401 | In scalar context, returns the number of fields found and splits into |
4402 | the C<@_> array. Use of split in scalar context is deprecated, however, | |
4403 | because it clobbers your subroutine arguments. | |
a0d0e21e | 4404 | |
7660c0ab | 4405 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
4633a7c4 LW |
4406 | splits on whitespace (after skipping any leading whitespace). Anything |
4407 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
fb73857a | 4408 | that the delimiter may be longer than one character.) |
4409 | ||
5a964f20 | 4410 | If LIMIT is specified and positive, splits into no more than that |
7b8d334a GS |
4411 | many fields (though it may split into fewer). If LIMIT is unspecified |
4412 | or zero, trailing null fields are stripped (which potential users | |
19799a22 | 4413 | of C<pop> would do well to remember). If LIMIT is negative, it is |
fb73857a | 4414 | treated as if an arbitrarily large LIMIT had been specified. |
a0d0e21e LW |
4415 | |
4416 | A pattern matching the null string (not to be confused with | |
748a9306 | 4417 | a null pattern C<//>, which is just one member of the set of patterns |
a0d0e21e LW |
4418 | matching a null string) will split the value of EXPR into separate |
4419 | characters at each point it matches that way. For example: | |
4420 | ||
4421 | print join(':', split(/ */, 'hi there')); | |
4422 | ||
4423 | produces the output 'h:i:t:h:e:r:e'. | |
4424 | ||
0156e0fd RB |
4425 | Empty leading (or trailing) fields are produced when there positive width |
4426 | matches at the beginning (or end) of the string; a zero-width match at the | |
4427 | beginning (or end) of the string does not produce an empty field. For | |
4428 | example: | |
4429 | ||
4430 | print join(':', split(/(?=\w)/, 'hi there!')); | |
4431 | ||
4432 | produces the output 'h:i :t:h:e:r:e!'. | |
4433 | ||
5f05dabc | 4434 | The LIMIT parameter can be used to split a line partially |
a0d0e21e LW |
4435 | |
4436 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
4437 | ||
4438 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT | |
4439 | one larger than the number of variables in the list, to avoid | |
4440 | unnecessary work. For the list above LIMIT would have been 4 by | |
4441 | default. In time critical applications it behooves you not to split | |
4442 | into more fields than you really need. | |
4443 | ||
19799a22 | 4444 | If the PATTERN contains parentheses, additional list elements are |
a0d0e21e LW |
4445 | created from each matching substring in the delimiter. |
4446 | ||
da0045b7 | 4447 | split(/([,-])/, "1-10,20", 3); |
a0d0e21e LW |
4448 | |
4449 | produces the list value | |
4450 | ||
4451 | (1, '-', 10, ',', 20) | |
4452 | ||
19799a22 | 4453 | If you had the entire header of a normal Unix email message in $header, |
4633a7c4 LW |
4454 | you could split it up into fields and their values this way: |
4455 | ||
4456 | $header =~ s/\n\s+/ /g; # fix continuation lines | |
fb73857a | 4457 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
4633a7c4 | 4458 | |
a0d0e21e LW |
4459 | The pattern C</PATTERN/> may be replaced with an expression to specify |
4460 | patterns that vary at runtime. (To do runtime compilation only once, | |
748a9306 LW |
4461 | use C</$variable/o>.) |
4462 | ||
4463 | As a special case, specifying a PATTERN of space (C<' '>) will split on | |
19799a22 | 4464 | white space just as C<split> with no arguments does. Thus, C<split(' ')> can |
748a9306 LW |
4465 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> |
4466 | will give you as many null initial fields as there are leading spaces. | |
19799a22 GS |
4467 | A C<split> on C</\s+/> is like a C<split(' ')> except that any leading |
4468 | whitespace produces a null first field. A C<split> with no arguments | |
748a9306 | 4469 | really does a C<split(' ', $_)> internally. |
a0d0e21e | 4470 | |
cc50a203 | 4471 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't |
1ec94568 MG |
4472 | much use otherwise. |
4473 | ||
a0d0e21e LW |
4474 | Example: |
4475 | ||
5a964f20 TC |
4476 | open(PASSWD, '/etc/passwd'); |
4477 | while (<PASSWD>) { | |
5b3eff12 MS |
4478 | chomp; |
4479 | ($login, $passwd, $uid, $gid, | |
f86cebdf | 4480 | $gcos, $home, $shell) = split(/:/); |
5a964f20 | 4481 | #... |
a0d0e21e LW |
4482 | } |
4483 | ||
a0d0e21e | 4484 | |
5f05dabc | 4485 | =item sprintf FORMAT, LIST |
a0d0e21e | 4486 | |
6662521e GS |
4487 | Returns a string formatted by the usual C<printf> conventions of the C |
4488 | library function C<sprintf>. See below for more details | |
4489 | and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of | |
4490 | the general principles. | |
4491 | ||
4492 | For example: | |
4493 | ||
4494 | # Format number with up to 8 leading zeroes | |
4495 | $result = sprintf("%08d", $number); | |
4496 | ||
4497 | # Round number to 3 digits after decimal point | |
4498 | $rounded = sprintf("%.3f", $number); | |
74a77017 | 4499 | |
19799a22 GS |
4500 | Perl does its own C<sprintf> formatting--it emulates the C |
4501 | function C<sprintf>, but it doesn't use it (except for floating-point | |
74a77017 | 4502 | numbers, and even then only the standard modifiers are allowed). As a |
19799a22 | 4503 | result, any non-standard extensions in your local C<sprintf> are not |
74a77017 CS |
4504 | available from Perl. |
4505 | ||
194e7b38 DC |
4506 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you |
4507 | pass it an array as your first argument. The array is given scalar context, | |
4508 | and instead of using the 0th element of the array as the format, Perl will | |
4509 | use the count of elements in the array as the format, which is almost never | |
4510 | useful. | |
4511 | ||
19799a22 | 4512 | Perl's C<sprintf> permits the following universally-known conversions: |
74a77017 CS |
4513 | |
4514 | %% a percent sign | |
4515 | %c a character with the given number | |
4516 | %s a string | |
4517 | %d a signed integer, in decimal | |
4518 | %u an unsigned integer, in decimal | |
4519 | %o an unsigned integer, in octal | |
4520 | %x an unsigned integer, in hexadecimal | |
4521 | %e a floating-point number, in scientific notation | |
4522 | %f a floating-point number, in fixed decimal notation | |
4523 | %g a floating-point number, in %e or %f notation | |
4524 | ||
1b3f7d21 | 4525 | In addition, Perl permits the following widely-supported conversions: |
74a77017 | 4526 | |
74a77017 CS |
4527 | %X like %x, but using upper-case letters |
4528 | %E like %e, but using an upper-case "E" | |
4529 | %G like %g, but with an upper-case "E" (if applicable) | |
4f19785b | 4530 | %b an unsigned integer, in binary |
74a77017 | 4531 | %p a pointer (outputs the Perl value's address in hexadecimal) |
1b3f7d21 | 4532 | %n special: *stores* the number of characters output so far |
b76cc8ba | 4533 | into the next variable in the parameter list |
74a77017 | 4534 | |
1b3f7d21 CS |
4535 | Finally, for backward (and we do mean "backward") compatibility, Perl |
4536 | permits these unnecessary but widely-supported conversions: | |
74a77017 | 4537 | |
1b3f7d21 | 4538 | %i a synonym for %d |
74a77017 CS |
4539 | %D a synonym for %ld |
4540 | %U a synonym for %lu | |
4541 | %O a synonym for %lo | |
4542 | %F a synonym for %f | |
4543 | ||
b73fd64e JH |
4544 | Note that the number of exponent digits in the scientific notation by |
4545 | C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the | |
4546 | exponent less than 100 is system-dependent: it may be three or less | |
4547 | (zero-padded as necessary). In other words, 1.23 times ten to the | |
4548 | 99th may be either "1.23e99" or "1.23e099". | |
d764f01a | 4549 | |
74a77017 CS |
4550 | Perl permits the following universally-known flags between the C<%> |
4551 | and the conversion letter: | |
4552 | ||
4553 | space prefix positive number with a space | |
4554 | + prefix positive number with a plus sign | |
4555 | - left-justify within the field | |
4556 | 0 use zeros, not spaces, to right-justify | |
a3cb178b | 4557 | # prefix non-zero octal with "0", non-zero hex with "0x" |
74a77017 | 4558 | number minimum field width |
f86cebdf GS |
4559 | .number "precision": digits after decimal point for |
4560 | floating-point, max length for string, minimum length | |
4561 | for integer | |
74a77017 | 4562 | l interpret integer as C type "long" or "unsigned long" |
74a77017 | 4563 | h interpret integer as C type "short" or "unsigned short" |
661cc6a6 | 4564 | If no flags, interpret integer as C type "int" or "unsigned" |
74a77017 | 4565 | |
eb3fce90 JH |
4566 | Perl supports parameter ordering, in other words, fetching the |
4567 | parameters in some explicitly specified "random" ordering as opposed | |
4568 | to the default implicit sequential ordering. The syntax is, instead | |
4569 | of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>, | |
4570 | where the I<digits> is the wanted index, from one upwards. For example: | |
4571 | ||
4572 | printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n" | |
4573 | printf "%*2\$d\n", 12, 3; # will print " 12\n" | |
4574 | ||
4575 | Note that using the reordering syntax does not interfere with the usual | |
4576 | implicit sequential fetching of the parameters: | |
4577 | ||
4578 | printf "%2\$d %d\n", 12, 34; # will print "34 12\n" | |
4579 | printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" | |
4580 | printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" | |
4581 | printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
4582 | printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
4583 | ||
4628e4f8 | 4584 | There are also two Perl-specific flags: |
74a77017 | 4585 | |
eb3fce90 JH |
4586 | V interpret integer as Perl's standard integer type |
4587 | v interpret string as a vector of integers, output as | |
4588 | numbers separated either by dots, or by an arbitrary | |
4589 | string received from the argument list when the flag | |
4590 | is preceded by C<*> | |
74a77017 | 4591 | |
19799a22 | 4592 | Where a number would appear in the flags, an asterisk (C<*>) may be |
74a77017 CS |
4593 | used instead, in which case Perl uses the next item in the parameter |
4594 | list as the given number (that is, as the field width or precision). | |
19799a22 GS |
4595 | If a field width obtained through C<*> is negative, it has the same |
4596 | effect as the C<-> flag: left-justification. | |
74a77017 | 4597 | |
b22c7a20 GS |
4598 | The C<v> flag is useful for displaying ordinal values of characters |
4599 | in arbitrary strings: | |
4600 | ||
4601 | printf "version is v%vd\n", $^V; # Perl's version | |
4602 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
dd629d5b | 4603 | printf "bits are %*vb\n", " ", $bits; # random bitstring |
b22c7a20 | 4604 | |
74a77017 CS |
4605 | If C<use locale> is in effect, the character used for the decimal |
4606 | point in formatted real numbers is affected by the LC_NUMERIC locale. | |
4607 | See L<perllocale>. | |
a0d0e21e | 4608 | |
07158430 | 4609 | If Perl understands "quads" (64-bit integers) (this requires |
a8764340 GS |
4610 | either that the platform natively support quads or that Perl |
4611 | be specifically compiled to support quads), the characters | |
07158430 JH |
4612 | |
4613 | d u o x X b i D U O | |
4614 | ||
4615 | print quads, and they may optionally be preceded by | |
4616 | ||
4617 | ll L q | |
4618 | ||
4619 | For example | |
4620 | ||
4621 | %lld %16LX %qo | |
4622 | ||
46465067 | 4623 | You can find out whether your Perl supports quads via L<Config>: |
07158430 JH |
4624 | |
4625 | use Config; | |
10cc9d2a | 4626 | ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && |
46465067 | 4627 | print "quads\n"; |
07158430 JH |
4628 | |
4629 | If Perl understands "long doubles" (this requires that the platform | |
a8764340 | 4630 | support long doubles), the flags |
07158430 JH |
4631 | |
4632 | e f g E F G | |
4633 | ||
4634 | may optionally be preceded by | |
4635 | ||
4636 | ll L | |
4637 | ||
4638 | For example | |
4639 | ||
4640 | %llf %Lg | |
4641 | ||
4642 | You can find out whether your Perl supports long doubles via L<Config>: | |
4643 | ||
4644 | use Config; | |
46465067 | 4645 | $Config{d_longdbl} eq 'define' && print "long doubles\n"; |
07158430 | 4646 | |
a0d0e21e LW |
4647 | =item sqrt EXPR |
4648 | ||
54310121 | 4649 | =item sqrt |
bbce6d69 | 4650 | |
a0d0e21e | 4651 | Return the square root of EXPR. If EXPR is omitted, returns square |
2b5ab1e7 TC |
4652 | root of C<$_>. Only works on non-negative operands, unless you've |
4653 | loaded the standard Math::Complex module. | |
4654 | ||
4655 | use Math::Complex; | |
4656 | print sqrt(-2); # prints 1.4142135623731i | |
a0d0e21e LW |
4657 | |
4658 | =item srand EXPR | |
4659 | ||
93dc8474 CS |
4660 | =item srand |
4661 | ||
19799a22 | 4662 | Sets the random number seed for the C<rand> operator. If EXPR is |
73c60299 RS |
4663 | omitted, uses a semi-random value supplied by the kernel (if it supports |
4664 | the F</dev/urandom> device) or based on the current time and process | |
93dc8474 | 4665 | ID, among other things. In versions of Perl prior to 5.004 the default |
19799a22 | 4666 | seed was just the current C<time>. This isn't a particularly good seed, |
93dc8474 | 4667 | so many old programs supply their own seed value (often C<time ^ $$> or |
61eff3bc | 4668 | C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more. |
93dc8474 | 4669 | |
19799a22 | 4670 | In fact, it's usually not necessary to call C<srand> at all, because if |
93dc8474 | 4671 | it is not called explicitly, it is called implicitly at the first use of |
19799a22 | 4672 | the C<rand> operator. However, this was not the case in version of Perl |
2f9daede | 4673 | before 5.004, so if your script will run under older Perl versions, it |
19799a22 | 4674 | should call C<srand>. |
93dc8474 | 4675 | |
2f9daede TP |
4676 | Note that you need something much more random than the default seed for |
4677 | cryptographic purposes. Checksumming the compressed output of one or more | |
4678 | rapidly changing operating system status programs is the usual method. For | |
4679 | example: | |
28757baa | 4680 | |
4681 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); | |
4682 | ||
7660c0ab | 4683 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
0078ec44 RS |
4684 | module in CPAN. |
4685 | ||
19799a22 | 4686 | Do I<not> call C<srand> multiple times in your program unless you know |
28757baa | 4687 | exactly what you're doing and why you're doing it. The point of the |
19799a22 | 4688 | function is to "seed" the C<rand> function so that C<rand> can produce |
28757baa | 4689 | a different sequence each time you run your program. Just do it once at the |
19799a22 | 4690 | top of your program, or you I<won't> get random numbers out of C<rand>! |
28757baa | 4691 | |
54310121 | 4692 | Frequently called programs (like CGI scripts) that simply use |
28757baa | 4693 | |
4694 | time ^ $$ | |
4695 | ||
54310121 | 4696 | for a seed can fall prey to the mathematical property that |
28757baa | 4697 | |
4698 | a^b == (a+1)^(b+1) | |
4699 | ||
0078ec44 | 4700 | one-third of the time. So don't do that. |
f86702cc | 4701 | |
a0d0e21e LW |
4702 | =item stat FILEHANDLE |
4703 | ||
4704 | =item stat EXPR | |
4705 | ||
54310121 | 4706 | =item stat |
bbce6d69 | 4707 | |
1d2dff63 GS |
4708 | Returns a 13-element list giving the status info for a file, either |
4709 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, | |
7660c0ab | 4710 | it stats C<$_>. Returns a null list if the stat fails. Typically used |
1d2dff63 | 4711 | as follows: |
a0d0e21e LW |
4712 | |
4713 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
4714 | $atime,$mtime,$ctime,$blksize,$blocks) | |
4715 | = stat($filename); | |
4716 | ||
54310121 | 4717 | Not all fields are supported on all filesystem types. Here are the |
c07a80fd | 4718 | meaning of the fields: |
4719 | ||
54310121 | 4720 | 0 dev device number of filesystem |
4721 | 1 ino inode number | |
4722 | 2 mode file mode (type and permissions) | |
4723 | 3 nlink number of (hard) links to the file | |
4724 | 4 uid numeric user ID of file's owner | |
4725 | 5 gid numeric group ID of file's owner | |
4726 | 6 rdev the device identifier (special files only) | |
4727 | 7 size total size of file, in bytes | |
1c74f1bd GS |
4728 | 8 atime last access time in seconds since the epoch |
4729 | 9 mtime last modify time in seconds since the epoch | |
4730 | 10 ctime inode change time (NOT creation time!) in seconds since the epoch | |
54310121 | 4731 | 11 blksize preferred block size for file system I/O |
4732 | 12 blocks actual number of blocks allocated | |
c07a80fd | 4733 | |
4734 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
4735 | ||
a0d0e21e LW |
4736 | If stat is passed the special filehandle consisting of an underline, no |
4737 | stat is done, but the current contents of the stat structure from the | |
4738 | last stat or filetest are returned. Example: | |
4739 | ||
4740 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
4741 | print "$file is executable NFS file\n"; | |
4742 | } | |
4743 | ||
ca6e1c26 JH |
4744 | (This works on machines only for which the device number is negative |
4745 | under NFS.) | |
a0d0e21e | 4746 | |
2b5ab1e7 | 4747 | Because the mode contains both the file type and its permissions, you |
b76cc8ba | 4748 | should mask off the file type portion and (s)printf using a C<"%o"> |
2b5ab1e7 TC |
4749 | if you want to see the real permissions. |
4750 | ||
4751 | $mode = (stat($filename))[2]; | |
4752 | printf "Permissions are %04o\n", $mode & 07777; | |
4753 | ||
19799a22 | 4754 | In scalar context, C<stat> returns a boolean value indicating success |
1d2dff63 GS |
4755 | or failure, and, if successful, sets the information associated with |
4756 | the special filehandle C<_>. | |
4757 | ||
2b5ab1e7 TC |
4758 | The File::stat module provides a convenient, by-name access mechanism: |
4759 | ||
4760 | use File::stat; | |
4761 | $sb = stat($filename); | |
b76cc8ba | 4762 | printf "File is %s, size is %s, perm %04o, mtime %s\n", |
2b5ab1e7 TC |
4763 | $filename, $sb->size, $sb->mode & 07777, |
4764 | scalar localtime $sb->mtime; | |
4765 | ||
ca6e1c26 JH |
4766 | You can import symbolic mode constants (C<S_IF*>) and functions |
4767 | (C<S_IS*>) from the Fcntl module: | |
4768 | ||
4769 | use Fcntl ':mode'; | |
4770 | ||
4771 | $mode = (stat($filename))[2]; | |
4772 | ||
4773 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
4774 | $group_read = ($mode & S_IRGRP) >> 3; | |
4775 | $other_execute = $mode & S_IXOTH; | |
4776 | ||
4777 | printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; | |
4778 | ||
4779 | $is_setuid = $mode & S_ISUID; | |
4780 | $is_setgid = S_ISDIR($mode); | |
4781 | ||
4782 | You could write the last two using the C<-u> and C<-d> operators. | |
4783 | The commonly available S_IF* constants are | |
4784 | ||
4785 | # Permissions: read, write, execute, for user, group, others. | |
4786 | ||
4787 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
4788 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
4789 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
61eff3bc | 4790 | |
ca6e1c26 JH |
4791 | # Setuid/Setgid/Stickiness. |
4792 | ||
4793 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
4794 | ||
4795 | # File types. Not necessarily all are available on your system. | |
4796 | ||
4797 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT | |
4798 | ||
4799 | # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. | |
4800 | ||
4801 | S_IREAD S_IWRITE S_IEXEC | |
4802 | ||
4803 | and the S_IF* functions are | |
4804 | ||
4375e838 | 4805 | S_IFMODE($mode) the part of $mode containing the permission bits |
ca6e1c26 JH |
4806 | and the setuid/setgid/sticky bits |
4807 | ||
4808 | S_IFMT($mode) the part of $mode containing the file type | |
b76cc8ba | 4809 | which can be bit-anded with e.g. S_IFREG |
ca6e1c26 JH |
4810 | or with the following functions |
4811 | ||
4812 | # The operators -f, -d, -l, -b, -c, -p, and -s. | |
4813 | ||
4814 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
4815 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
4816 | ||
4817 | # No direct -X operator counterpart, but for the first one | |
4818 | # the -g operator is often equivalent. The ENFMT stands for | |
4819 | # record flocking enforcement, a platform-dependent feature. | |
4820 | ||
4821 | S_ISENFMT($mode) S_ISWHT($mode) | |
4822 | ||
4823 | See your native chmod(2) and stat(2) documentation for more details | |
4824 | about the S_* constants. | |
4825 | ||
a0d0e21e LW |
4826 | =item study SCALAR |
4827 | ||
4828 | =item study | |
4829 | ||
184e9718 | 4830 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
a0d0e21e LW |
4831 | doing many pattern matches on the string before it is next modified. |
4832 | This may or may not save time, depending on the nature and number of | |
4833 | patterns you are searching on, and on the distribution of character | |
19799a22 | 4834 | frequencies in the string to be searched--you probably want to compare |
5f05dabc | 4835 | run times with and without it to see which runs faster. Those loops |
a0d0e21e LW |
4836 | which scan for many short constant strings (including the constant |
4837 | parts of more complex patterns) will benefit most. You may have only | |
19799a22 GS |
4838 | one C<study> active at a time--if you study a different scalar the first |
4839 | is "unstudied". (The way C<study> works is this: a linked list of every | |
a0d0e21e | 4840 | character in the string to be searched is made, so we know, for |
7660c0ab | 4841 | example, where all the C<'k'> characters are. From each search string, |
a0d0e21e LW |
4842 | the rarest character is selected, based on some static frequency tables |
4843 | constructed from some C programs and English text. Only those places | |
4844 | that contain this "rarest" character are examined.) | |
4845 | ||
5a964f20 | 4846 | For example, here is a loop that inserts index producing entries |
a0d0e21e LW |
4847 | before any line containing a certain pattern: |
4848 | ||
4849 | while (<>) { | |
4850 | study; | |
2b5ab1e7 TC |
4851 | print ".IX foo\n" if /\bfoo\b/; |
4852 | print ".IX bar\n" if /\bbar\b/; | |
4853 | print ".IX blurfl\n" if /\bblurfl\b/; | |
5a964f20 | 4854 | # ... |
a0d0e21e LW |
4855 | print; |
4856 | } | |
4857 | ||
951ba7fe GS |
4858 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> |
4859 | will be looked at, because C<f> is rarer than C<o>. In general, this is | |
a0d0e21e LW |
4860 | a big win except in pathological cases. The only question is whether |
4861 | it saves you more time than it took to build the linked list in the | |
4862 | first place. | |
4863 | ||
4864 | Note that if you have to look for strings that you don't know till | |
19799a22 | 4865 | runtime, you can build an entire loop as a string and C<eval> that to |
a0d0e21e | 4866 | avoid recompiling all your patterns all the time. Together with |
7660c0ab | 4867 | undefining C<$/> to input entire files as one record, this can be very |
f86cebdf | 4868 | fast, often faster than specialized programs like fgrep(1). The following |
184e9718 | 4869 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
a0d0e21e LW |
4870 | out the names of those files that contain a match: |
4871 | ||
4872 | $search = 'while (<>) { study;'; | |
4873 | foreach $word (@words) { | |
4874 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; | |
4875 | } | |
4876 | $search .= "}"; | |
4877 | @ARGV = @files; | |
4878 | undef $/; | |
4879 | eval $search; # this screams | |
5f05dabc | 4880 | $/ = "\n"; # put back to normal input delimiter |
a0d0e21e LW |
4881 | foreach $file (sort keys(%seen)) { |
4882 | print $file, "\n"; | |
4883 | } | |
4884 | ||
cb1a09d0 AD |
4885 | =item sub BLOCK |
4886 | ||
4887 | =item sub NAME | |
4888 | ||
4889 | =item sub NAME BLOCK | |
4890 | ||
4891 | This is subroutine definition, not a real function I<per se>. With just a | |
09bef843 SB |
4892 | NAME (and possibly prototypes or attributes), it's just a forward declaration. |
4893 | Without a NAME, it's an anonymous function declaration, and does actually | |
4894 | return a value: the CODE ref of the closure you just created. See L<perlsub> | |
4895 | and L<perlref> for details. | |
cb1a09d0 | 4896 | |
87275199 | 4897 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT |
7b8d334a | 4898 | |
87275199 | 4899 | =item substr EXPR,OFFSET,LENGTH |
a0d0e21e LW |
4900 | |
4901 | =item substr EXPR,OFFSET | |
4902 | ||
4903 | Extracts a substring out of EXPR and returns it. First character is at | |
7660c0ab | 4904 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
84902520 | 4905 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
87275199 GS |
4906 | that far from the end of the string. If LENGTH is omitted, returns |
4907 | everything to the end of the string. If LENGTH is negative, leaves that | |
748a9306 LW |
4908 | many characters off the end of the string. |
4909 | ||
2b5ab1e7 | 4910 | You can use the substr() function as an lvalue, in which case EXPR |
87275199 GS |
4911 | must itself be an lvalue. If you assign something shorter than LENGTH, |
4912 | the string will shrink, and if you assign something longer than LENGTH, | |
2b5ab1e7 | 4913 | the string will grow to accommodate it. To keep the string the same |
19799a22 | 4914 | length you may need to pad or chop your value using C<sprintf>. |
a0d0e21e | 4915 | |
87275199 GS |
4916 | If OFFSET and LENGTH specify a substring that is partly outside the |
4917 | string, only the part within the string is returned. If the substring | |
4918 | is beyond either end of the string, substr() returns the undefined | |
4919 | value and produces a warning. When used as an lvalue, specifying a | |
4920 | substring that is entirely outside the string is a fatal error. | |
4921 | Here's an example showing the behavior for boundary cases: | |
4922 | ||
4923 | my $name = 'fred'; | |
4924 | substr($name, 4) = 'dy'; # $name is now 'freddy' | |
4925 | my $null = substr $name, 6, 2; # returns '' (no warning) | |
4926 | my $oops = substr $name, 7; # returns undef, with warning | |
4927 | substr($name, 7) = 'gap'; # fatal error | |
4928 | ||
2b5ab1e7 | 4929 | An alternative to using substr() as an lvalue is to specify the |
7b8d334a | 4930 | replacement string as the 4th argument. This allows you to replace |
2b5ab1e7 TC |
4931 | parts of the EXPR and return what was there before in one operation, |
4932 | just as you can with splice(). | |
7b8d334a | 4933 | |
a0d0e21e LW |
4934 | =item symlink OLDFILE,NEWFILE |
4935 | ||
4936 | Creates a new filename symbolically linked to the old filename. | |
7660c0ab | 4937 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
a0d0e21e LW |
4938 | symbolic links, produces a fatal error at run time. To check for that, |
4939 | use eval: | |
4940 | ||
2b5ab1e7 | 4941 | $symlink_exists = eval { symlink("",""); 1 }; |
a0d0e21e LW |
4942 | |
4943 | =item syscall LIST | |
4944 | ||
4945 | Calls the system call specified as the first element of the list, | |
4946 | passing the remaining elements as arguments to the system call. If | |
4947 | unimplemented, produces a fatal error. The arguments are interpreted | |
4948 | as follows: if a given argument is numeric, the argument is passed as | |
4949 | an int. If not, the pointer to the string value is passed. You are | |
4950 | responsible to make sure a string is pre-extended long enough to | |
a3cb178b | 4951 | receive any result that might be written into a string. You can't use a |
19799a22 | 4952 | string literal (or other read-only string) as an argument to C<syscall> |
a3cb178b GS |
4953 | because Perl has to assume that any string pointer might be written |
4954 | through. If your | |
a0d0e21e | 4955 | integer arguments are not literals and have never been interpreted in a |
7660c0ab | 4956 | numeric context, you may need to add C<0> to them to force them to look |
19799a22 | 4957 | like numbers. This emulates the C<syswrite> function (or vice versa): |
a0d0e21e LW |
4958 | |
4959 | require 'syscall.ph'; # may need to run h2ph | |
a3cb178b GS |
4960 | $s = "hi there\n"; |
4961 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
a0d0e21e | 4962 | |
5f05dabc | 4963 | Note that Perl supports passing of up to only 14 arguments to your system call, |
a0d0e21e LW |
4964 | which in practice should usually suffice. |
4965 | ||
fb73857a | 4966 | Syscall returns whatever value returned by the system call it calls. |
19799a22 | 4967 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). |
7660c0ab | 4968 | Note that some system calls can legitimately return C<-1>. The proper |
fb73857a | 4969 | way to handle such calls is to assign C<$!=0;> before the call and |
7660c0ab | 4970 | check the value of C<$!> if syscall returns C<-1>. |
fb73857a | 4971 | |
4972 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
4973 | number of the read end of the pipe it creates. There is no way | |
b76cc8ba | 4974 | to retrieve the file number of the other end. You can avoid this |
19799a22 | 4975 | problem by using C<pipe> instead. |
fb73857a | 4976 | |
c07a80fd | 4977 | =item sysopen FILEHANDLE,FILENAME,MODE |
4978 | ||
4979 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
4980 | ||
4981 | Opens the file whose filename is given by FILENAME, and associates it | |
4982 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as | |
4983 | the name of the real filehandle wanted. This function calls the | |
19799a22 | 4984 | underlying operating system's C<open> function with the parameters |
c07a80fd | 4985 | FILENAME, MODE, PERMS. |
4986 | ||
4987 | The possible values and flag bits of the MODE parameter are | |
4988 | system-dependent; they are available via the standard module C<Fcntl>. | |
ea2b5ef6 JH |
4989 | See the documentation of your operating system's C<open> to see which |
4990 | values and flag bits are available. You may combine several flags | |
4991 | using the C<|>-operator. | |
4992 | ||
4993 | Some of the most common values are C<O_RDONLY> for opening the file in | |
4994 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
4995 | and C<O_RDWR> for opening the file in read-write mode, and. | |
4996 | ||
adf5897a DF |
4997 | For historical reasons, some values work on almost every system |
4998 | supported by perl: zero means read-only, one means write-only, and two | |
4999 | means read/write. We know that these values do I<not> work under | |
7c5ffed3 | 5000 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
4af147f6 | 5001 | use them in new code. |
c07a80fd | 5002 | |
19799a22 | 5003 | If the file named by FILENAME does not exist and the C<open> call creates |
7660c0ab | 5004 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
5a964f20 | 5005 | PERMS specifies the permissions of the newly created file. If you omit |
19799a22 | 5006 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. |
5a964f20 | 5007 | These permission values need to be in octal, and are modified by your |
0591cd52 NT |
5008 | process's current C<umask>. |
5009 | ||
ea2b5ef6 JH |
5010 | In many systems the C<O_EXCL> flag is available for opening files in |
5011 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
5012 | if the file already exists, sysopen() fails. The C<O_EXCL> wins | |
5013 | C<O_TRUNC>. | |
5014 | ||
5015 | Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. | |
5016 | ||
19799a22 | 5017 | You should seldom if ever use C<0644> as argument to C<sysopen>, because |
2b5ab1e7 TC |
5018 | that takes away the user's option to have a more permissive umask. |
5019 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
5020 | on this. | |
c07a80fd | 5021 | |
4af147f6 CS |
5022 | Note that C<sysopen> depends on the fdopen() C library function. |
5023 | On many UNIX systems, fdopen() is known to fail when file descriptors | |
5024 | exceed a certain value, typically 255. If you need more file | |
5025 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
5026 | library, or perhaps using the POSIX::open() function. | |
5027 | ||
2b5ab1e7 | 5028 | See L<perlopentut> for a kinder, gentler explanation of opening files. |
28757baa | 5029 | |
a0d0e21e LW |
5030 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
5031 | ||
5032 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
5033 | ||
5034 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
b43ceaf2 | 5035 | specified FILEHANDLE, using the system call read(2). It bypasses stdio, |
19799a22 GS |
5036 | so mixing this with other kinds of reads, C<print>, C<write>, |
5037 | C<seek>, C<tell>, or C<eof> can cause confusion because stdio | |
b43ceaf2 AB |
5038 | usually buffers data. Returns the number of bytes actually read, C<0> |
5039 | at end of file, or undef if there was an error. SCALAR will be grown or | |
5040 | shrunk so that the last byte actually read is the last byte of the | |
5041 | scalar after the read. | |
ff68c719 | 5042 | |
5043 | An OFFSET may be specified to place the read data at some place in the | |
5044 | string other than the beginning. A negative OFFSET specifies | |
5045 | placement at that many bytes counting backwards from the end of the | |
5046 | string. A positive OFFSET greater than the length of SCALAR results | |
7660c0ab | 5047 | in the string being padded to the required size with C<"\0"> bytes before |
ff68c719 | 5048 | the result of the read is appended. |
a0d0e21e | 5049 | |
2b5ab1e7 TC |
5050 | There is no syseof() function, which is ok, since eof() doesn't work |
5051 | very well on device files (like ttys) anyway. Use sysread() and check | |
19799a22 | 5052 | for a return value for 0 to decide whether you're done. |
2b5ab1e7 | 5053 | |
137443ea | 5054 | =item sysseek FILEHANDLE,POSITION,WHENCE |
5055 | ||
f86cebdf | 5056 | Sets FILEHANDLE's system position using the system call lseek(2). It |
19799a22 | 5057 | bypasses stdio, so mixing this with reads (other than C<sysread>), |
ac88732c JH |
5058 | C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. |
5059 | FILEHANDLE may be an expression whose value gives the name of the | |
5060 | filehandle. The values for WHENCE are C<0> to set the new position to | |
5061 | POSITION, C<1> to set the it to the current position plus POSITION, | |
5062 | and C<2> to set it to EOF plus POSITION (typically negative). For | |
5063 | WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and | |
5064 | C<SEEK_END> (start of the file, current position, end of the file) | |
ca6e1c26 | 5065 | from the Fcntl module. |
8903cb82 | 5066 | |
5067 | Returns the new position, or the undefined value on failure. A position | |
19799a22 GS |
5068 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns |
5069 | true on success and false on failure, yet you can still easily determine | |
8903cb82 | 5070 | the new position. |
137443ea | 5071 | |
a0d0e21e LW |
5072 | =item system LIST |
5073 | ||
8bf3b016 GS |
5074 | =item system PROGRAM LIST |
5075 | ||
19799a22 GS |
5076 | Does exactly the same thing as C<exec LIST>, except that a fork is |
5077 | done first, and the parent process waits for the child process to | |
5078 | complete. Note that argument processing varies depending on the | |
5079 | number of arguments. If there is more than one argument in LIST, | |
5080 | or if LIST is an array with more than one value, starts the program | |
5081 | given by the first element of the list with arguments given by the | |
5082 | rest of the list. If there is only one scalar argument, the argument | |
5083 | is checked for shell metacharacters, and if there are any, the | |
5084 | entire argument is passed to the system's command shell for parsing | |
5085 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
5086 | platforms). If there are no shell metacharacters in the argument, | |
5087 | it is split into words and passed directly to C<execvp>, which is | |
5088 | more efficient. | |
5089 | ||
0f897271 GS |
5090 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
5091 | output before any operation that may do a fork, but this may not be | |
5092 | supported on some platforms (see L<perlport>). To be safe, you may need | |
5093 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
5094 | of C<IO::Handle> on any open handles. | |
a2008d6d GS |
5095 | |
5096 | The return value is the exit status of the program as | |
19799a22 GS |
5097 | returned by the C<wait> call. To get the actual exit value divide by |
5098 | 256. See also L</exec>. This is I<not> what you want to use to capture | |
54310121 | 5099 | the output from a command, for that you should use merely backticks or |
d5a9bfb0 IZ |
5100 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 |
5101 | indicates a failure to start the program (inspect $! for the reason). | |
a0d0e21e | 5102 | |
19799a22 GS |
5103 | Like C<exec>, C<system> allows you to lie to a program about its name if |
5104 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
8bf3b016 | 5105 | |
19799a22 | 5106 | Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the |
28757baa | 5107 | program they're running doesn't actually interrupt your program. |
5108 | ||
5109 | @args = ("command", "arg1", "arg2"); | |
54310121 | 5110 | system(@args) == 0 |
5111 | or die "system @args failed: $?" | |
28757baa | 5112 | |
5a964f20 TC |
5113 | You can check all the failure possibilities by inspecting |
5114 | C<$?> like this: | |
28757baa | 5115 | |
5a964f20 TC |
5116 | $exit_value = $? >> 8; |
5117 | $signal_num = $? & 127; | |
5118 | $dumped_core = $? & 128; | |
f86702cc | 5119 | |
c8db1d39 TC |
5120 | When the arguments get executed via the system shell, results |
5121 | and return codes will be subject to its quirks and capabilities. | |
5122 | See L<perlop/"`STRING`"> and L</exec> for details. | |
bb32b41a | 5123 | |
a0d0e21e LW |
5124 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
5125 | ||
5126 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
5127 | ||
145d37e2 GA |
5128 | =item syswrite FILEHANDLE,SCALAR |
5129 | ||
a0d0e21e | 5130 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
19799a22 GS |
5131 | specified FILEHANDLE, using the system call write(2). If LENGTH |
5132 | is not specified, writes whole SCALAR. It bypasses stdio, so mixing | |
5133 | this with reads (other than C<sysread())>, C<print>, C<write>, | |
5134 | C<seek>, C<tell>, or C<eof> may cause confusion because stdio | |
5135 | usually buffers data. Returns the number of bytes actually written, | |
5136 | or C<undef> if there was an error. If the LENGTH is greater than | |
5137 | the available data in the SCALAR after the OFFSET, only as much | |
5138 | data as is available will be written. | |
ff68c719 | 5139 | |
5140 | An OFFSET may be specified to write the data from some part of the | |
5141 | string other than the beginning. A negative OFFSET specifies writing | |
fb73857a | 5142 | that many bytes counting backwards from the end of the string. In the |
5143 | case the SCALAR is empty you can use OFFSET but only zero offset. | |
a0d0e21e LW |
5144 | |
5145 | =item tell FILEHANDLE | |
5146 | ||
5147 | =item tell | |
5148 | ||
fc579abb NC |
5149 | Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE |
5150 | may be an expression whose value gives the name of the actual filehandle. | |
b76cc8ba | 5151 | If FILEHANDLE is omitted, assumes the file last read. |
2b5ab1e7 | 5152 | |
cfd73201 JH |
5153 | The return value of tell() for the standard streams like the STDIN |
5154 | depends on the operating system: it may return -1 or something else. | |
5155 | tell() on pipes, fifos, and sockets usually returns -1. | |
5156 | ||
19799a22 | 5157 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. |
a0d0e21e LW |
5158 | |
5159 | =item telldir DIRHANDLE | |
5160 | ||
19799a22 GS |
5161 | Returns the current position of the C<readdir> routines on DIRHANDLE. |
5162 | Value may be given to C<seekdir> to access a particular location in a | |
a0d0e21e LW |
5163 | directory. Has the same caveats about possible directory compaction as |
5164 | the corresponding system library routine. | |
5165 | ||
4633a7c4 | 5166 | =item tie VARIABLE,CLASSNAME,LIST |
a0d0e21e | 5167 | |
4633a7c4 LW |
5168 | This function binds a variable to a package class that will provide the |
5169 | implementation for the variable. VARIABLE is the name of the variable | |
5170 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
19799a22 | 5171 | of correct type. Any additional arguments are passed to the C<new> |
8a059744 GS |
5172 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
5173 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
19799a22 GS |
5174 | to the C<dbm_open()> function of C. The object returned by the C<new> |
5175 | method is also returned by the C<tie> function, which would be useful | |
8a059744 | 5176 | if you want to access other methods in CLASSNAME. |
a0d0e21e | 5177 | |
19799a22 | 5178 | Note that functions such as C<keys> and C<values> may return huge lists |
1d2dff63 | 5179 | when used on large objects, like DBM files. You may prefer to use the |
19799a22 | 5180 | C<each> function to iterate over such. Example: |
a0d0e21e LW |
5181 | |
5182 | # print out history file offsets | |
4633a7c4 | 5183 | use NDBM_File; |
da0045b7 | 5184 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
a0d0e21e LW |
5185 | while (($key,$val) = each %HIST) { |
5186 | print $key, ' = ', unpack('L',$val), "\n"; | |
5187 | } | |
5188 | untie(%HIST); | |
5189 | ||
aa689395 | 5190 | A class implementing a hash should have the following methods: |
a0d0e21e | 5191 | |
4633a7c4 | 5192 | TIEHASH classname, LIST |
a0d0e21e LW |
5193 | FETCH this, key |
5194 | STORE this, key, value | |
5195 | DELETE this, key | |
8a059744 | 5196 | CLEAR this |
a0d0e21e LW |
5197 | EXISTS this, key |
5198 | FIRSTKEY this | |
5199 | NEXTKEY this, lastkey | |
8a059744 | 5200 | DESTROY this |
d7da42b7 | 5201 | UNTIE this |
a0d0e21e | 5202 | |
4633a7c4 | 5203 | A class implementing an ordinary array should have the following methods: |
a0d0e21e | 5204 | |
4633a7c4 | 5205 | TIEARRAY classname, LIST |
a0d0e21e LW |
5206 | FETCH this, key |
5207 | STORE this, key, value | |
8a059744 GS |
5208 | FETCHSIZE this |
5209 | STORESIZE this, count | |
5210 | CLEAR this | |
5211 | PUSH this, LIST | |
5212 | POP this | |
5213 | SHIFT this | |
5214 | UNSHIFT this, LIST | |
5215 | SPLICE this, offset, length, LIST | |
5216 | EXTEND this, count | |
5217 | DESTROY this | |
d7da42b7 | 5218 | UNTIE this |
8a059744 GS |
5219 | |
5220 | A class implementing a file handle should have the following methods: | |
5221 | ||
5222 | TIEHANDLE classname, LIST | |
5223 | READ this, scalar, length, offset | |
5224 | READLINE this | |
5225 | GETC this | |
5226 | WRITE this, scalar, length, offset | |
5227 | PRINT this, LIST | |
5228 | PRINTF this, format, LIST | |
e08f2115 GA |
5229 | BINMODE this |
5230 | EOF this | |
5231 | FILENO this | |
5232 | SEEK this, position, whence | |
5233 | TELL this | |
5234 | OPEN this, mode, LIST | |
8a059744 GS |
5235 | CLOSE this |
5236 | DESTROY this | |
d7da42b7 | 5237 | UNTIE this |
a0d0e21e | 5238 | |
4633a7c4 | 5239 | A class implementing a scalar should have the following methods: |
a0d0e21e | 5240 | |
4633a7c4 | 5241 | TIESCALAR classname, LIST |
54310121 | 5242 | FETCH this, |
a0d0e21e | 5243 | STORE this, value |
8a059744 | 5244 | DESTROY this |
d7da42b7 | 5245 | UNTIE this |
8a059744 GS |
5246 | |
5247 | Not all methods indicated above need be implemented. See L<perltie>, | |
2b5ab1e7 | 5248 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. |
a0d0e21e | 5249 | |
19799a22 | 5250 | Unlike C<dbmopen>, the C<tie> function will not use or require a module |
4633a7c4 | 5251 | for you--you need to do that explicitly yourself. See L<DB_File> |
19799a22 | 5252 | or the F<Config> module for interesting C<tie> implementations. |
4633a7c4 | 5253 | |
b687b08b | 5254 | For further details see L<perltie>, L<"tied VARIABLE">. |
cc6b7395 | 5255 | |
f3cbc334 RS |
5256 | =item tied VARIABLE |
5257 | ||
5258 | Returns a reference to the object underlying VARIABLE (the same value | |
19799a22 | 5259 | that was originally returned by the C<tie> call that bound the variable |
f3cbc334 RS |
5260 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
5261 | package. | |
5262 | ||
a0d0e21e LW |
5263 | =item time |
5264 | ||
da0045b7 | 5265 | Returns the number of non-leap seconds since whatever time the system |
5266 | considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, | |
5267 | and 00:00:00 UTC, January 1, 1970 for most other systems). | |
19799a22 | 5268 | Suitable for feeding to C<gmtime> and C<localtime>. |
a0d0e21e | 5269 | |
68f8bed4 JH |
5270 | For measuring time in better granularity than one second, |
5271 | you may use either the Time::HiRes module from CPAN, or | |
5272 | if you have gettimeofday(2), you may be able to use the | |
5273 | C<syscall> interface of Perl, see L<perlfaq8> for details. | |
5274 | ||
a0d0e21e LW |
5275 | =item times |
5276 | ||
1d2dff63 | 5277 | Returns a four-element list giving the user and system times, in |
a0d0e21e LW |
5278 | seconds, for this process and the children of this process. |
5279 | ||
5280 | ($user,$system,$cuser,$csystem) = times; | |
5281 | ||
5282 | =item tr/// | |
5283 | ||
19799a22 | 5284 | The transliteration operator. Same as C<y///>. See L<perlop>. |
a0d0e21e LW |
5285 | |
5286 | =item truncate FILEHANDLE,LENGTH | |
5287 | ||
5288 | =item truncate EXPR,LENGTH | |
5289 | ||
5290 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
5291 | specified length. Produces a fatal error if truncate isn't implemented | |
19799a22 | 5292 | on your system. Returns true if successful, the undefined value |
a3cb178b | 5293 | otherwise. |
a0d0e21e LW |
5294 | |
5295 | =item uc EXPR | |
5296 | ||
54310121 | 5297 | =item uc |
bbce6d69 | 5298 | |
a0d0e21e | 5299 | Returns an uppercased version of EXPR. This is the internal function |
7660c0ab | 5300 | implementing the C<\U> escape in double-quoted strings. |
a034a98d | 5301 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
a0ed51b3 | 5302 | Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It |
19799a22 | 5303 | does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) |
a0d0e21e | 5304 | |
7660c0ab | 5305 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 5306 | |
a0d0e21e LW |
5307 | =item ucfirst EXPR |
5308 | ||
54310121 | 5309 | =item ucfirst |
bbce6d69 | 5310 | |
a0ed51b3 LW |
5311 | Returns the value of EXPR with the first character |
5312 | in uppercase (titlecase in Unicode). This is | |
7660c0ab | 5313 | the internal function implementing the C<\u> escape in double-quoted strings. |
2b5ab1e7 TC |
5314 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
5315 | and L<utf8>. | |
a0d0e21e | 5316 | |
7660c0ab | 5317 | If EXPR is omitted, uses C<$_>. |
bbce6d69 | 5318 | |
a0d0e21e LW |
5319 | =item umask EXPR |
5320 | ||
5321 | =item umask | |
5322 | ||
2f9daede | 5323 | Sets the umask for the process to EXPR and returns the previous value. |
eec2d3df GS |
5324 | If EXPR is omitted, merely returns the current umask. |
5325 | ||
0591cd52 NT |
5326 | The Unix permission C<rwxr-x---> is represented as three sets of three |
5327 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
b5a41e52 | 5328 | and isn't one of the digits). The C<umask> value is such a number |
0591cd52 NT |
5329 | representing disabled permissions bits. The permission (or "mode") |
5330 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
5331 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
5332 | if your umask is C<0022> then the file will actually be created with | |
5333 | permissions C<0755>. If your C<umask> were C<0027> (group can't | |
5334 | write; others can't read, write, or execute), then passing | |
19799a22 | 5335 | C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ |
0591cd52 NT |
5336 | 027> is C<0640>). |
5337 | ||
5338 | Here's some advice: supply a creation mode of C<0666> for regular | |
19799a22 GS |
5339 | files (in C<sysopen>) and one of C<0777> for directories (in |
5340 | C<mkdir>) and executable files. This gives users the freedom of | |
0591cd52 NT |
5341 | choice: if they want protected files, they might choose process umasks |
5342 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
5343 | Programs should rarely if ever make policy decisions better left to | |
5344 | the user. The exception to this is when writing files that should be | |
5345 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
5346 | so on. | |
5347 | ||
f86cebdf | 5348 | If umask(2) is not implemented on your system and you are trying to |
eec2d3df | 5349 | restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a |
f86cebdf | 5350 | fatal error at run time. If umask(2) is not implemented and you are |
eec2d3df GS |
5351 | not trying to restrict access for yourself, returns C<undef>. |
5352 | ||
5353 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
5354 | string of octal digits. See also L</oct>, if all you have is a string. | |
a0d0e21e LW |
5355 | |
5356 | =item undef EXPR | |
5357 | ||
5358 | =item undef | |
5359 | ||
54310121 | 5360 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
19799a22 GS |
5361 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine |
5362 | (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> | |
20408e3c GS |
5363 | will probably not do what you expect on most predefined variables or |
5364 | DBM list values, so don't do that; see L<delete>.) Always returns the | |
5365 | undefined value. You can omit the EXPR, in which case nothing is | |
5366 | undefined, but you still get an undefined value that you could, for | |
5367 | instance, return from a subroutine, assign to a variable or pass as a | |
5368 | parameter. Examples: | |
a0d0e21e LW |
5369 | |
5370 | undef $foo; | |
f86cebdf | 5371 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
a0d0e21e | 5372 | undef @ary; |
aa689395 | 5373 | undef %hash; |
a0d0e21e | 5374 | undef &mysub; |
20408e3c | 5375 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
54310121 | 5376 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
2f9daede TP |
5377 | select undef, undef, undef, 0.25; |
5378 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
a0d0e21e | 5379 | |
5a964f20 TC |
5380 | Note that this is a unary operator, not a list operator. |
5381 | ||
a0d0e21e LW |
5382 | =item unlink LIST |
5383 | ||
54310121 | 5384 | =item unlink |
bbce6d69 | 5385 | |
a0d0e21e LW |
5386 | Deletes a list of files. Returns the number of files successfully |
5387 | deleted. | |
5388 | ||
5389 | $cnt = unlink 'a', 'b', 'c'; | |
5390 | unlink @goners; | |
5391 | unlink <*.bak>; | |
5392 | ||
19799a22 | 5393 | Note: C<unlink> will not delete directories unless you are superuser and |
a0d0e21e LW |
5394 | the B<-U> flag is supplied to Perl. Even if these conditions are |
5395 | met, be warned that unlinking a directory can inflict damage on your | |
19799a22 | 5396 | filesystem. Use C<rmdir> instead. |
a0d0e21e | 5397 | |
7660c0ab | 5398 | If LIST is omitted, uses C<$_>. |
bbce6d69 | 5399 | |
a0d0e21e LW |
5400 | =item unpack TEMPLATE,EXPR |
5401 | ||
19799a22 | 5402 | C<unpack> does the reverse of C<pack>: it takes a string |
2b6c5635 | 5403 | and expands it out into a list of values. |
19799a22 | 5404 | (In scalar context, it returns merely the first value produced.) |
2b6c5635 GS |
5405 | |
5406 | The string is broken into chunks described by the TEMPLATE. Each chunk | |
5407 | is converted separately to a value. Typically, either the string is a result | |
5408 | of C<pack>, or the bytes of the string represent a C structure of some | |
5409 | kind. | |
5410 | ||
19799a22 | 5411 | The TEMPLATE has the same format as in the C<pack> function. |
a0d0e21e LW |
5412 | Here's a subroutine that does substring: |
5413 | ||
5414 | sub substr { | |
5a964f20 | 5415 | my($what,$where,$howmuch) = @_; |
a0d0e21e LW |
5416 | unpack("x$where a$howmuch", $what); |
5417 | } | |
5418 | ||
5419 | and then there's | |
5420 | ||
5421 | sub ordinal { unpack("c",$_[0]); } # same as ord() | |
5422 | ||
2b6c5635 | 5423 | In addition to fields allowed in pack(), you may prefix a field with |
61eff3bc JH |
5424 | a %<number> to indicate that |
5425 | you want a <number>-bit checksum of the items instead of the items | |
2b6c5635 GS |
5426 | themselves. Default is a 16-bit checksum. Checksum is calculated by |
5427 | summing numeric values of expanded values (for string fields the sum of | |
5428 | C<ord($char)> is taken, for bit fields the sum of zeroes and ones). | |
5429 | ||
5430 | For example, the following | |
a0d0e21e LW |
5431 | computes the same number as the System V sum program: |
5432 | ||
19799a22 GS |
5433 | $checksum = do { |
5434 | local $/; # slurp! | |
5435 | unpack("%32C*",<>) % 65535; | |
5436 | }; | |
a0d0e21e LW |
5437 | |
5438 | The following efficiently counts the number of set bits in a bit vector: | |
5439 | ||
5440 | $setbits = unpack("%32b*", $selectmask); | |
5441 | ||
951ba7fe | 5442 | The C<p> and C<P> formats should be used with care. Since Perl |
3160c391 GS |
5443 | has no way of checking whether the value passed to C<unpack()> |
5444 | corresponds to a valid memory location, passing a pointer value that's | |
5445 | not known to be valid is likely to have disastrous consequences. | |
5446 | ||
2b6c5635 GS |
5447 | If the repeat count of a field is larger than what the remainder of |
5448 | the input string allows, repeat count is decreased. If the input string | |
b76cc8ba | 5449 | is longer than one described by the TEMPLATE, the rest is ignored. |
2b6c5635 | 5450 | |
851646ae | 5451 | See L</pack> for more examples and notes. |
5a929a98 | 5452 | |
98293880 JH |
5453 | =item untie VARIABLE |
5454 | ||
19799a22 | 5455 | Breaks the binding between a variable and a package. (See C<tie>.) |
98293880 | 5456 | |
a0d0e21e LW |
5457 | =item unshift ARRAY,LIST |
5458 | ||
19799a22 | 5459 | Does the opposite of a C<shift>. Or the opposite of a C<push>, |
a0d0e21e LW |
5460 | depending on how you look at it. Prepends list to the front of the |
5461 | array, and returns the new number of elements in the array. | |
5462 | ||
5463 | unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; | |
5464 | ||
5465 | Note the LIST is prepended whole, not one element at a time, so the | |
19799a22 | 5466 | prepended elements stay in the same order. Use C<reverse> to do the |
a0d0e21e LW |
5467 | reverse. |
5468 | ||
f6c8478c GS |
5469 | =item use Module VERSION LIST |
5470 | ||
5471 | =item use Module VERSION | |
5472 | ||
a0d0e21e LW |
5473 | =item use Module LIST |
5474 | ||
5475 | =item use Module | |
5476 | ||
da0045b7 | 5477 | =item use VERSION |
5478 | ||
a0d0e21e LW |
5479 | Imports some semantics into the current package from the named module, |
5480 | generally by aliasing certain subroutine or variable names into your | |
5481 | package. It is exactly equivalent to | |
5482 | ||
5483 | BEGIN { require Module; import Module LIST; } | |
5484 | ||
54310121 | 5485 | except that Module I<must> be a bareword. |
da0045b7 | 5486 | |
dd629d5b | 5487 | VERSION, which can be specified as a literal of the form v5.6.1, demands |
44dcb63b GS |
5488 | that the current version of Perl (C<$^V> or $PERL_VERSION) be at least |
5489 | as recent as that version. (For compatibility with older versions of Perl, | |
5490 | a numeric literal will also be interpreted as VERSION.) If the version | |
5491 | of the running Perl interpreter is less than VERSION, then an error | |
5492 | message is printed and Perl exits immediately without attempting to | |
5493 | parse the rest of the file. Compare with L</require>, which can do a | |
5494 | similar check at run time. | |
16070b82 | 5495 | |
dd629d5b GS |
5496 | use v5.6.1; # compile time version check |
5497 | use 5.6.1; # ditto | |
5498 | use 5.005_03; # float version allowed for compatibility | |
16070b82 GS |
5499 | |
5500 | This is often useful if you need to check the current Perl version before | |
5501 | C<use>ing library modules that have changed in incompatible ways from | |
5502 | older versions of Perl. (We try not to do this more than we have to.) | |
da0045b7 | 5503 | |
19799a22 | 5504 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The |
7660c0ab | 5505 | C<require> makes sure the module is loaded into memory if it hasn't been |
19799a22 GS |
5506 | yet. The C<import> is not a builtin--it's just an ordinary static method |
5507 | call into the C<Module> package to tell the module to import the list of | |
a0d0e21e | 5508 | features back into the current package. The module can implement its |
19799a22 GS |
5509 | C<import> method any way it likes, though most modules just choose to |
5510 | derive their C<import> method via inheritance from the C<Exporter> class that | |
5511 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
10696ff6 | 5512 | method can be found then the call is skipped. |
cb1a09d0 | 5513 | |
31686daf JP |
5514 | If you do not want to call the package's C<import> method (for instance, |
5515 | to stop your namespace from being altered), explicitly supply the empty list: | |
cb1a09d0 AD |
5516 | |
5517 | use Module (); | |
5518 | ||
5519 | That is exactly equivalent to | |
5520 | ||
5a964f20 | 5521 | BEGIN { require Module } |
a0d0e21e | 5522 | |
da0045b7 | 5523 | If the VERSION argument is present between Module and LIST, then the |
71be2cbc | 5524 | C<use> will call the VERSION method in class Module with the given |
5525 | version as an argument. The default VERSION method, inherited from | |
44dcb63b | 5526 | the UNIVERSAL class, croaks if the given version is larger than the |
b76cc8ba | 5527 | value of the variable C<$Module::VERSION>. |
f6c8478c GS |
5528 | |
5529 | Again, there is a distinction between omitting LIST (C<import> called | |
5530 | with no arguments) and an explicit empty LIST C<()> (C<import> not | |
5531 | called). Note that there is no comma after VERSION! | |
da0045b7 | 5532 | |
a0d0e21e LW |
5533 | Because this is a wide-open interface, pragmas (compiler directives) |
5534 | are also implemented this way. Currently implemented pragmas are: | |
5535 | ||
f3798619 | 5536 | use constant; |
4633a7c4 | 5537 | use diagnostics; |
f3798619 | 5538 | use integer; |
4438c4b7 JH |
5539 | use sigtrap qw(SEGV BUS); |
5540 | use strict qw(subs vars refs); | |
5541 | use subs qw(afunc blurfl); | |
5542 | use warnings qw(all); | |
a0d0e21e | 5543 | |
19799a22 | 5544 | Some of these pseudo-modules import semantics into the current |
5a964f20 TC |
5545 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
5546 | which import symbols into the current package (which are effective | |
5547 | through the end of the file). | |
a0d0e21e | 5548 | |
19799a22 GS |
5549 | There's a corresponding C<no> command that unimports meanings imported |
5550 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. | |
a0d0e21e LW |
5551 | |
5552 | no integer; | |
5553 | no strict 'refs'; | |
4438c4b7 | 5554 | no warnings; |
a0d0e21e | 5555 | |
19799a22 | 5556 | If no C<unimport> method can be found the call fails with a fatal error. |
55497cff | 5557 | |
ac634a9a | 5558 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> |
31686daf JP |
5559 | for the C<-M> and C<-m> command-line options to perl that give C<use> |
5560 | functionality from the command-line. | |
a0d0e21e LW |
5561 | |
5562 | =item utime LIST | |
5563 | ||
5564 | Changes the access and modification times on each file of a list of | |
5565 | files. The first two elements of the list must be the NUMERICAL access | |
5566 | and modification times, in that order. Returns the number of files | |
46cdf678 | 5567 | successfully changed. The inode change time of each file is set |
19799a22 | 5568 | to the current time. This code has the same effect as the C<touch> |
a3cb178b | 5569 | command if the files already exist: |
a0d0e21e LW |
5570 | |
5571 | #!/usr/bin/perl | |
5572 | $now = time; | |
5573 | utime $now, $now, @ARGV; | |
5574 | ||
aa689395 | 5575 | =item values HASH |
a0d0e21e | 5576 | |
1d2dff63 GS |
5577 | Returns a list consisting of all the values of the named hash. (In a |
5578 | scalar context, returns the number of values.) The values are | |
ab192400 GS |
5579 | returned in an apparently random order. The actual random order is |
5580 | subject to change in future versions of perl, but it is guaranteed to | |
19799a22 | 5581 | be the same order as either the C<keys> or C<each> function would |
ab192400 GS |
5582 | produce on the same (unmodified) hash. |
5583 | ||
8ea1e5d4 GS |
5584 | Note that the values are not copied, which means modifying them will |
5585 | modify the contents of the hash: | |
2b5ab1e7 | 5586 | |
8ea1e5d4 GS |
5587 | for (values %hash) { s/foo/bar/g } # modifies %hash values |
5588 | for (@hash{keys %hash}) { s/foo/bar/g } # same | |
2b5ab1e7 TC |
5589 | |
5590 | As a side effect, calling values() resets the HASH's internal iterator. | |
19799a22 | 5591 | See also C<keys>, C<each>, and C<sort>. |
a0d0e21e LW |
5592 | |
5593 | =item vec EXPR,OFFSET,BITS | |
5594 | ||
e69129f1 GS |
5595 | Treats the string in EXPR as a bit vector made up of elements of |
5596 | width BITS, and returns the value of the element specified by OFFSET | |
5597 | as an unsigned integer. BITS therefore specifies the number of bits | |
5598 | that are reserved for each element in the bit vector. This must | |
5599 | be a power of two from 1 to 32 (or 64, if your platform supports | |
5600 | that). | |
c5a0f51a | 5601 | |
b76cc8ba | 5602 | If BITS is 8, "elements" coincide with bytes of the input string. |
c73032f5 IZ |
5603 | |
5604 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
5605 | of size BITS/8, and each group is converted to a number as with | |
b1866b2d | 5606 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously |
c73032f5 IZ |
5607 | for BITS==64). See L<"pack"> for details. |
5608 | ||
5609 | If bits is 4 or less, the string is broken into bytes, then the bits | |
5610 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
5611 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
5612 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
5613 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
5614 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
5615 | ||
81e118e0 JH |
5616 | C<vec> may also be assigned to, in which case parentheses are needed |
5617 | to give the expression the correct precedence as in | |
22dc801b | 5618 | |
5619 | vec($image, $max_x * $x + $y, 8) = 3; | |
a0d0e21e | 5620 | |
fe58ced6 MG |
5621 | If the selected element is outside the string, the value 0 is returned. |
5622 | If an element off the end of the string is written to, Perl will first | |
5623 | extend the string with sufficiently many zero bytes. It is an error | |
5624 | to try to write off the beginning of the string (i.e. negative OFFSET). | |
fac70343 | 5625 | |
33b45480 SB |
5626 | The string should not contain any character with the value > 255 (which |
5627 | can only happen if you're using UTF8 encoding). If it does, it will be | |
5628 | treated as something which is not UTF8 encoded. When the C<vec> was | |
5629 | assigned to, other parts of your program will also no longer consider the | |
5630 | string to be UTF8 encoded. In other words, if you do have such characters | |
5631 | in your string, vec() will operate on the actual byte string, and not the | |
5632 | conceptual character string. | |
246fae53 | 5633 | |
fac70343 GS |
5634 | Strings created with C<vec> can also be manipulated with the logical |
5635 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
5636 | vector operation is desired when both operands are strings. | |
c5a0f51a | 5637 | See L<perlop/"Bitwise String Operators">. |
a0d0e21e | 5638 | |
7660c0ab | 5639 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
19799a22 | 5640 | The comments show the string after each step. Note that this code works |
cca87523 GS |
5641 | in the same way on big-endian or little-endian machines. |
5642 | ||
5643 | my $foo = ''; | |
5644 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' | |
e69129f1 GS |
5645 | |
5646 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
5647 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') | |
5648 | ||
cca87523 GS |
5649 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' |
5650 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
5651 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
5652 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
5653 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
f86cebdf GS |
5654 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' |
5655 | # 'r' is "\x72" | |
cca87523 GS |
5656 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" |
5657 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
f86cebdf GS |
5658 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' |
5659 | # 'l' is "\x6c" | |
cca87523 | 5660 | |
19799a22 | 5661 | To transform a bit vector into a string or list of 0's and 1's, use these: |
a0d0e21e LW |
5662 | |
5663 | $bits = unpack("b*", $vector); | |
5664 | @bits = split(//, unpack("b*", $vector)); | |
5665 | ||
7660c0ab | 5666 | If you know the exact length in bits, it can be used in place of the C<*>. |
a0d0e21e | 5667 | |
e69129f1 GS |
5668 | Here is an example to illustrate how the bits actually fall in place: |
5669 | ||
5670 | #!/usr/bin/perl -wl | |
5671 | ||
5672 | print <<'EOT'; | |
b76cc8ba | 5673 | 0 1 2 3 |
e69129f1 GS |
5674 | unpack("V",$_) 01234567890123456789012345678901 |
5675 | ------------------------------------------------------------------ | |
5676 | EOT | |
5677 | ||
5678 | for $w (0..3) { | |
5679 | $width = 2**$w; | |
5680 | for ($shift=0; $shift < $width; ++$shift) { | |
5681 | for ($off=0; $off < 32/$width; ++$off) { | |
5682 | $str = pack("B*", "0"x32); | |
5683 | $bits = (1<<$shift); | |
5684 | vec($str, $off, $width) = $bits; | |
5685 | $res = unpack("b*",$str); | |
5686 | $val = unpack("V", $str); | |
5687 | write; | |
5688 | } | |
5689 | } | |
5690 | } | |
5691 | ||
5692 | format STDOUT = | |
5693 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
5694 | $off, $width, $bits, $val, $res | |
5695 | . | |
5696 | __END__ | |
5697 | ||
5698 | Regardless of the machine architecture on which it is run, the above | |
5699 | example should print the following table: | |
5700 | ||
b76cc8ba | 5701 | 0 1 2 3 |
e69129f1 GS |
5702 | unpack("V",$_) 01234567890123456789012345678901 |
5703 | ------------------------------------------------------------------ | |
5704 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
5705 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
5706 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
5707 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
5708 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
5709 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
5710 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
5711 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
5712 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
5713 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
5714 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
5715 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
5716 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
5717 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
5718 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
5719 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
5720 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
5721 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
5722 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
5723 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
5724 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
5725 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
5726 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
5727 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
5728 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
5729 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
5730 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
5731 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
5732 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
5733 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
5734 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
5735 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
5736 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
5737 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
5738 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
5739 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
5740 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
5741 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
5742 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
5743 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
5744 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
5745 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
5746 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
5747 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
5748 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
5749 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
5750 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
5751 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
5752 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
5753 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
5754 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
5755 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
5756 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
5757 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
5758 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
5759 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
5760 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
5761 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
5762 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
5763 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
5764 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
5765 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
5766 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
5767 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
5768 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
5769 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
5770 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
5771 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
5772 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
5773 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
5774 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
5775 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
5776 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
5777 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
5778 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
5779 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
5780 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
5781 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
5782 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
5783 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
5784 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
5785 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
5786 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
5787 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
5788 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
5789 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
5790 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
5791 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
5792 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
5793 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
5794 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
5795 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
5796 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
5797 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
5798 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
5799 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
5800 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
5801 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
5802 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
5803 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
5804 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
5805 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
5806 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
5807 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
5808 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
5809 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
5810 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
5811 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
5812 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
5813 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
5814 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
5815 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
5816 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
5817 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
5818 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
5819 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
5820 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
5821 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
5822 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
5823 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
5824 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
5825 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
5826 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
5827 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
5828 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
5829 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
5830 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
5831 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
5832 | ||
a0d0e21e LW |
5833 | =item wait |
5834 | ||
2b5ab1e7 TC |
5835 | Behaves like the wait(2) system call on your system: it waits for a child |
5836 | process to terminate and returns the pid of the deceased process, or | |
19799a22 | 5837 | C<-1> if there are no child processes. The status is returned in C<$?>. |
2b5ab1e7 TC |
5838 | Note that a return value of C<-1> could mean that child processes are |
5839 | being automatically reaped, as described in L<perlipc>. | |
a0d0e21e LW |
5840 | |
5841 | =item waitpid PID,FLAGS | |
5842 | ||
2b5ab1e7 TC |
5843 | Waits for a particular child process to terminate and returns the pid of |
5844 | the deceased process, or C<-1> if there is no such child process. On some | |
5845 | systems, a value of 0 indicates that there are processes still running. | |
5846 | The status is returned in C<$?>. If you say | |
a0d0e21e | 5847 | |
5f05dabc | 5848 | use POSIX ":sys_wait_h"; |
5a964f20 | 5849 | #... |
b76cc8ba | 5850 | do { |
2b5ab1e7 TC |
5851 | $kid = waitpid(-1,&WNOHANG); |
5852 | } until $kid == -1; | |
a0d0e21e | 5853 | |
2b5ab1e7 TC |
5854 | then you can do a non-blocking wait for all pending zombie processes. |
5855 | Non-blocking wait is available on machines supporting either the | |
5856 | waitpid(2) or wait4(2) system calls. However, waiting for a particular | |
5857 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the | |
5858 | system call by remembering the status values of processes that have | |
5859 | exited but have not been harvested by the Perl script yet.) | |
a0d0e21e | 5860 | |
2b5ab1e7 TC |
5861 | Note that on some systems, a return value of C<-1> could mean that child |
5862 | processes are being automatically reaped. See L<perlipc> for details, | |
5863 | and for other examples. | |
5a964f20 | 5864 | |
a0d0e21e LW |
5865 | =item wantarray |
5866 | ||
19799a22 GS |
5867 | Returns true if the context of the currently executing subroutine is |
5868 | looking for a list value. Returns false if the context is looking | |
54310121 | 5869 | for a scalar. Returns the undefined value if the context is looking |
5870 | for no value (void context). | |
a0d0e21e | 5871 | |
54310121 | 5872 | return unless defined wantarray; # don't bother doing more |
5873 | my @a = complex_calculation(); | |
5874 | return wantarray ? @a : "@a"; | |
a0d0e21e | 5875 | |
19799a22 GS |
5876 | This function should have been named wantlist() instead. |
5877 | ||
a0d0e21e LW |
5878 | =item warn LIST |
5879 | ||
19799a22 | 5880 | Produces a message on STDERR just like C<die>, but doesn't exit or throw |
774d564b | 5881 | an exception. |
5882 | ||
7660c0ab A |
5883 | If LIST is empty and C<$@> already contains a value (typically from a |
5884 | previous eval) that value is used after appending C<"\t...caught"> | |
19799a22 GS |
5885 | to C<$@>. This is useful for staying almost, but not entirely similar to |
5886 | C<die>. | |
43051805 | 5887 | |
7660c0ab | 5888 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
43051805 | 5889 | |
774d564b | 5890 | No message is printed if there is a C<$SIG{__WARN__}> handler |
5891 | installed. It is the handler's responsibility to deal with the message | |
19799a22 | 5892 | as it sees fit (like, for instance, converting it into a C<die>). Most |
774d564b | 5893 | handlers must therefore make arrangements to actually display the |
19799a22 | 5894 | warnings that they are not prepared to deal with, by calling C<warn> |
774d564b | 5895 | again in the handler. Note that this is quite safe and will not |
5896 | produce an endless loop, since C<__WARN__> hooks are not called from | |
5897 | inside one. | |
5898 | ||
5899 | You will find this behavior is slightly different from that of | |
5900 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
19799a22 | 5901 | instead call C<die> again to change it). |
774d564b | 5902 | |
5903 | Using a C<__WARN__> handler provides a powerful way to silence all | |
5904 | warnings (even the so-called mandatory ones). An example: | |
5905 | ||
5906 | # wipe out *all* compile-time warnings | |
5907 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
5908 | my $foo = 10; | |
5909 | my $foo = 20; # no warning about duplicate my $foo, | |
5910 | # but hey, you asked for it! | |
5911 | # no compile-time or run-time warnings before here | |
5912 | $DOWARN = 1; | |
5913 | ||
5914 | # run-time warnings enabled after here | |
5915 | warn "\$foo is alive and $foo!"; # does show up | |
5916 | ||
5917 | See L<perlvar> for details on setting C<%SIG> entries, and for more | |
2b5ab1e7 TC |
5918 | examples. See the Carp module for other kinds of warnings using its |
5919 | carp() and cluck() functions. | |
a0d0e21e LW |
5920 | |
5921 | =item write FILEHANDLE | |
5922 | ||
5923 | =item write EXPR | |
5924 | ||
5925 | =item write | |
5926 | ||
5a964f20 | 5927 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
a0d0e21e | 5928 | using the format associated with that file. By default the format for |
54310121 | 5929 | a file is the one having the same name as the filehandle, but the |
19799a22 | 5930 | format for the current output channel (see the C<select> function) may be set |
184e9718 | 5931 | explicitly by assigning the name of the format to the C<$~> variable. |
a0d0e21e LW |
5932 | |
5933 | Top of form processing is handled automatically: if there is | |
5934 | insufficient room on the current page for the formatted record, the | |
5935 | page is advanced by writing a form feed, a special top-of-page format | |
5936 | is used to format the new page header, and then the record is written. | |
5937 | By default the top-of-page format is the name of the filehandle with | |
5938 | "_TOP" appended, but it may be dynamically set to the format of your | |
184e9718 | 5939 | choice by assigning the name to the C<$^> variable while the filehandle is |
a0d0e21e | 5940 | selected. The number of lines remaining on the current page is in |
7660c0ab | 5941 | variable C<$->, which can be set to C<0> to force a new page. |
a0d0e21e LW |
5942 | |
5943 | If FILEHANDLE is unspecified, output goes to the current default output | |
5944 | channel, which starts out as STDOUT but may be changed by the | |
19799a22 | 5945 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression |
a0d0e21e LW |
5946 | is evaluated and the resulting string is used to look up the name of |
5947 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
5948 | ||
19799a22 | 5949 | Note that write is I<not> the opposite of C<read>. Unfortunately. |
a0d0e21e LW |
5950 | |
5951 | =item y/// | |
5952 | ||
7660c0ab | 5953 | The transliteration operator. Same as C<tr///>. See L<perlop>. |
a0d0e21e LW |
5954 | |
5955 | =back |