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1 | =head1 NAME |
2 | X<function> | |
3 | ||
4 | perlfunc - Perl builtin functions | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
8 | The functions in this section can serve as terms in an expression. | |
9 | They fall into two major categories: list operators and named unary | |
10 | operators. These differ in their precedence relationship with a | |
11 | following comma. (See the precedence table in L<perlop>.) List | |
12 | operators take more than one argument, while unary operators can never | |
13 | take more than one argument. Thus, a comma terminates the argument of | |
14 | a unary operator, but merely separates the arguments of a list | |
15 | operator. A unary operator generally provides scalar context to its | |
16 | argument, while a list operator may provide either scalar or list | |
a6b91202 | 17 | contexts for its arguments. If it does both, scalar arguments |
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18 | come first and list argument follow, and there can only ever |
19 | be one such list argument. For instance, splice() has three scalar | |
20 | arguments followed by a list, whereas gethostbyname() has four scalar | |
21 | arguments. | |
22 | ||
23 | In the syntax descriptions that follow, list operators that expect a | |
24 | list (and provide list context for elements of the list) are shown | |
25 | with LIST as an argument. Such a list may consist of any combination | |
26 | of scalar arguments or list values; the list values will be included | |
27 | in the list as if each individual element were interpolated at that | |
28 | point in the list, forming a longer single-dimensional list value. | |
29 | Commas should separate literal elements of the LIST. | |
30 | ||
31 | Any function in the list below may be used either with or without | |
32 | parentheses around its arguments. (The syntax descriptions omit the | |
a6b91202 | 33 | parentheses.) If you use parentheses, the simple but occasionally |
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34 | surprising rule is this: It I<looks> like a function, therefore it I<is> a |
35 | function, and precedence doesn't matter. Otherwise it's a list | |
36 | operator or unary operator, and precedence does matter. Whitespace | |
37 | between the function and left parenthesis doesn't count, so sometimes | |
38 | you need to be careful: | |
39 | ||
40 | print 1+2+4; # Prints 7. | |
41 | print(1+2) + 4; # Prints 3. | |
42 | print (1+2)+4; # Also prints 3! | |
43 | print +(1+2)+4; # Prints 7. | |
44 | print ((1+2)+4); # Prints 7. | |
45 | ||
46 | If you run Perl with the B<-w> switch it can warn you about this. For | |
47 | example, the third line above produces: | |
48 | ||
49 | print (...) interpreted as function at - line 1. | |
50 | Useless use of integer addition in void context at - line 1. | |
51 | ||
52 | A few functions take no arguments at all, and therefore work as neither | |
53 | unary nor list operators. These include such functions as C<time> | |
54 | and C<endpwent>. For example, C<time+86_400> always means | |
55 | C<time() + 86_400>. | |
56 | ||
57 | For functions that can be used in either a scalar or list context, | |
58 | nonabortive failure is generally indicated in scalar context by | |
59 | returning the undefined value, and in list context by returning the | |
60 | empty list. | |
61 | ||
62 | Remember the following important rule: There is B<no rule> that relates | |
63 | the behavior of an expression in list context to its behavior in scalar | |
64 | context, or vice versa. It might do two totally different things. | |
65 | Each operator and function decides which sort of value would be most | |
66 | appropriate to return in scalar context. Some operators return the | |
67 | length of the list that would have been returned in list context. Some | |
68 | operators return the first value in the list. Some operators return the | |
69 | last value in the list. Some operators return a count of successful | |
70 | operations. In general, they do what you want, unless you want | |
71 | consistency. | |
72 | X<context> | |
73 | ||
74 | A named array in scalar context is quite different from what would at | |
75 | first glance appear to be a list in scalar context. You can't get a list | |
76 | like C<(1,2,3)> into being in scalar context, because the compiler knows | |
77 | the context at compile time. It would generate the scalar comma operator | |
78 | there, not the list construction version of the comma. That means it | |
79 | was never a list to start with. | |
80 | ||
81 | In general, functions in Perl that serve as wrappers for system calls ("syscalls") | |
82 | of the same name (like chown(2), fork(2), closedir(2), etc.) return | |
83 | true when they succeed and C<undef> otherwise, as is usually mentioned | |
84 | in the descriptions below. This is different from the C interfaces, | |
85 | which return C<-1> on failure. Exceptions to this rule include C<wait>, | |
86 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> | |
87 | variable on failure. Other functions do not, except accidentally. | |
88 | ||
89 | Extension modules can also hook into the Perl parser to define new | |
90 | kinds of keyword-headed expression. These may look like functions, but | |
91 | may also look completely different. The syntax following the keyword | |
92 | is defined entirely by the extension. If you are an implementor, see | |
93 | L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such | |
94 | a module, see the module's documentation for details of the syntax that | |
95 | it defines. | |
96 | ||
97 | =head2 Perl Functions by Category | |
98 | X<function> | |
99 | ||
100 | Here are Perl's functions (including things that look like | |
101 | functions, like some keywords and named operators) | |
102 | arranged by category. Some functions appear in more | |
103 | than one place. | |
104 | ||
105 | =over 4 | |
106 | ||
107 | =item Functions for SCALARs or strings | |
108 | X<scalar> X<string> X<character> | |
109 | ||
110 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, | |
111 | C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>, | |
112 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> | |
113 | ||
114 | =item Regular expressions and pattern matching | |
115 | X<regular expression> X<regex> X<regexp> | |
116 | ||
117 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> | |
118 | ||
119 | =item Numeric functions | |
120 | X<numeric> X<number> X<trigonometric> X<trigonometry> | |
121 | ||
122 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, | |
123 | C<sin>, C<sqrt>, C<srand> | |
124 | ||
125 | =item Functions for real @ARRAYs | |
126 | X<array> | |
127 | ||
128 | C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values> | |
129 | ||
130 | =item Functions for list data | |
131 | X<list> | |
132 | ||
133 | C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack> | |
134 | ||
135 | =item Functions for real %HASHes | |
136 | X<hash> | |
137 | ||
138 | C<delete>, C<each>, C<exists>, C<keys>, C<values> | |
139 | ||
140 | =item Input and output functions | |
141 | X<I/O> X<input> X<output> X<dbm> | |
142 | ||
143 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, | |
144 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, | |
145 | C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>, | |
146 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, | |
147 | C<warn>, C<write> | |
148 | ||
149 | =item Functions for fixed-length data or records | |
150 | ||
151 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> | |
152 | ||
153 | =item Functions for filehandles, files, or directories | |
154 | X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink> | |
155 | ||
156 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, | |
157 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, | |
158 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>, | |
159 | C<umask>, C<unlink>, C<utime> | |
160 | ||
161 | =item Keywords related to the control flow of your Perl program | |
162 | X<control flow> | |
163 | ||
164 | C<caller>, C<continue>, C<die>, C<do>, | |
165 | C<dump>, C<eval>, C<evalbytes> C<exit>, | |
166 | C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>, | |
167 | C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray> | |
168 | ||
169 | C<__SUB__> is only available with a C<use v5.16> (or higher) declaration or | |
170 | with the C<"current_sub"> feature (see L<feature>). | |
171 | ||
172 | =item Keywords related to the switch feature | |
173 | ||
174 | C<break>, C<continue>, C<default>, C<given>, C<when> | |
175 | ||
176 | Except for C<continue>, these are available only if you enable the | |
177 | C<"switch"> feature or use the C<CORE::> prefix. | |
a6b91202 | 178 | See L<feature> and L<perlsyn/"Switch statements">. |
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179 | Alternately, include a C<use v5.10> or later to the current scope. In Perl |
180 | 5.14 and earlier, C<continue> required the C<"switch"> feature, like the | |
181 | other keywords. | |
182 | ||
183 | =item Keywords related to scoping | |
184 | ||
185 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use> | |
186 | ||
187 | C<state> is available only if the C<"state"> feature | |
188 | is enabled or if it is prefixed with C<CORE::>. See | |
189 | L<feature>. Alternately, include a C<use v5.10> or later to the current scope. | |
190 | ||
191 | =item Miscellaneous functions | |
192 | ||
193 | C<defined>, C<dump>, C<eval>, C<evalbytes>, | |
194 | C<formline>, C<local>, C<my>, C<our>, | |
195 | C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray> | |
196 | ||
197 | =item Functions for processes and process groups | |
198 | X<process> X<pid> X<process id> | |
199 | ||
200 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, | |
201 | C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>, | |
202 | C<setpriority>, C<sleep>, C<system>, | |
203 | C<times>, C<wait>, C<waitpid> | |
204 | ||
205 | =item Keywords related to Perl modules | |
206 | X<module> | |
207 | ||
208 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> | |
209 | ||
210 | =item Keywords related to classes and object-orientation | |
211 | X<object> X<class> X<package> | |
212 | ||
213 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, | |
214 | C<untie>, C<use> | |
215 | ||
216 | =item Low-level socket functions | |
217 | X<socket> X<sock> | |
218 | ||
219 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, | |
220 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, | |
221 | C<socket>, C<socketpair> | |
222 | ||
223 | =item System V interprocess communication functions | |
224 | X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> | |
225 | ||
226 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, | |
227 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> | |
228 | ||
229 | =item Fetching user and group info | |
230 | X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> | |
231 | ||
232 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, | |
233 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, | |
234 | C<getpwuid>, C<setgrent>, C<setpwent> | |
235 | ||
236 | =item Fetching network info | |
237 | X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> | |
238 | ||
239 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, | |
240 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
241 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, | |
242 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, | |
243 | C<setnetent>, C<setprotoent>, C<setservent> | |
244 | ||
245 | =item Time-related functions | |
246 | X<time> X<date> | |
247 | ||
248 | C<gmtime>, C<localtime>, C<time>, C<times> | |
249 | ||
250 | =item Functions new in perl5 | |
251 | X<perl5> | |
252 | ||
a6b91202 | 253 | C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>, |
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254 | C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>, |
255 | C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>, | |
256 | C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>, | |
257 | C<ucfirst>, C<untie>, C<use>, C<when> | |
258 | ||
259 | * C<sub> was a keyword in Perl 4, but in Perl 5 it is an | |
260 | operator, which can be used in expressions. | |
261 | ||
262 | =item Functions obsoleted in perl5 | |
263 | ||
264 | C<dbmclose>, C<dbmopen> | |
265 | ||
266 | =back | |
267 | ||
268 | =head2 Portability | |
269 | X<portability> X<Unix> X<portable> | |
270 | ||
271 | Perl was born in Unix and can therefore access all common Unix | |
272 | system calls. In non-Unix environments, the functionality of some | |
273 | Unix system calls may not be available or details of the available | |
274 | functionality may differ slightly. The Perl functions affected | |
275 | by this are: | |
276 | ||
277 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, | |
278 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, | |
279 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, | |
280 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>, | |
281 | C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, | |
282 | C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>, | |
283 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, | |
284 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, | |
285 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, | |
286 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, | |
287 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, | |
288 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, | |
289 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, | |
290 | C<shmwrite>, C<socket>, C<socketpair>, | |
291 | C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, | |
292 | C<times>, C<truncate>, C<umask>, C<unlink>, | |
293 | C<utime>, C<wait>, C<waitpid> | |
294 | ||
295 | For more information about the portability of these functions, see | |
296 | L<perlport> and other available platform-specific documentation. | |
297 | ||
298 | =head2 Alphabetical Listing of Perl Functions | |
299 | ||
a6b91202 | 300 | =over |
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301 | |
302 | =item -X FILEHANDLE | |
303 | 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> | |
304 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> | |
305 | ||
306 | =item -X EXPR | |
307 | ||
308 | =item -X DIRHANDLE | |
309 | ||
310 | =item -X | |
311 | ||
312 | A file test, where X is one of the letters listed below. This unary | |
a6b91202 | 313 | operator takes one argument, either a filename, a filehandle, or a dirhandle, |
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314 | and tests the associated file to see if something is true about it. If the |
315 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. | |
316 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or | |
317 | the undefined value if the file doesn't exist. Despite the funny | |
318 | names, precedence is the same as any other named unary operator. The | |
319 | operator may be any of: | |
320 | ||
321 | -r File is readable by effective uid/gid. | |
322 | -w File is writable by effective uid/gid. | |
323 | -x File is executable by effective uid/gid. | |
324 | -o File is owned by effective uid. | |
325 | ||
326 | -R File is readable by real uid/gid. | |
327 | -W File is writable by real uid/gid. | |
328 | -X File is executable by real uid/gid. | |
329 | -O File is owned by real uid. | |
330 | ||
331 | -e File exists. | |
332 | -z File has zero size (is empty). | |
333 | -s File has nonzero size (returns size in bytes). | |
334 | ||
335 | -f File is a plain file. | |
336 | -d File is a directory. | |
337 | -l File is a symbolic link. | |
338 | -p File is a named pipe (FIFO), or Filehandle is a pipe. | |
339 | -S File is a socket. | |
340 | -b File is a block special file. | |
341 | -c File is a character special file. | |
342 | -t Filehandle is opened to a tty. | |
343 | ||
344 | -u File has setuid bit set. | |
345 | -g File has setgid bit set. | |
346 | -k File has sticky bit set. | |
347 | ||
348 | -T File is an ASCII text file (heuristic guess). | |
349 | -B File is a "binary" file (opposite of -T). | |
350 | ||
351 | -M Script start time minus file modification time, in days. | |
352 | -A Same for access time. | |
353 | -C Same for inode change time (Unix, may differ for other platforms) | |
354 | ||
355 | Example: | |
356 | ||
357 | while (<>) { | |
358 | chomp; | |
359 | next unless -f $_; # ignore specials | |
360 | #... | |
361 | } | |
362 | ||
363 | Note that C<-s/a/b/> does not do a negated substitution. Saying | |
364 | C<-exp($foo)> still works as expected, however: only single letters | |
365 | following a minus are interpreted as file tests. | |
366 | ||
367 | These operators are exempt from the "looks like a function rule" described | |
368 | above. That is, an opening parenthesis after the operator does not affect | |
369 | how much of the following code constitutes the argument. Put the opening | |
370 | parentheses before the operator to separate it from code that follows (this | |
371 | applies only to operators with higher precedence than unary operators, of | |
372 | course): | |
373 | ||
374 | -s($file) + 1024 # probably wrong; same as -s($file + 1024) | |
375 | (-s $file) + 1024 # correct | |
376 | ||
377 | The interpretation of the file permission operators C<-r>, C<-R>, | |
378 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode | |
379 | of the file and the uids and gids of the user. There may be other | |
380 | reasons you can't actually read, write, or execute the file: for | |
381 | example network filesystem access controls, ACLs (access control lists), | |
382 | read-only filesystems, and unrecognized executable formats. Note | |
383 | that the use of these six specific operators to verify if some operation | |
384 | is possible is usually a mistake, because it may be open to race | |
385 | conditions. | |
386 | ||
387 | Also note that, for the superuser on the local filesystems, the C<-r>, | |
388 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 | |
389 | if any execute bit is set in the mode. Scripts run by the superuser | |
390 | may thus need to do a stat() to determine the actual mode of the file, | |
391 | or temporarily set their effective uid to something else. | |
392 | ||
393 | If you are using ACLs, there is a pragma called C<filetest> that may | |
394 | produce more accurate results than the bare stat() mode bits. | |
395 | When under C<use filetest 'access'> the above-mentioned filetests | |
396 | test whether the permission can(not) be granted using the | |
397 | access(2) family of system calls. Also note that the C<-x> and C<-X> may | |
398 | under this pragma return true even if there are no execute permission | |
399 | bits set (nor any extra execute permission ACLs). This strangeness is | |
400 | due to the underlying system calls' definitions. Note also that, due to | |
401 | the implementation of C<use filetest 'access'>, the C<_> special | |
402 | filehandle won't cache the results of the file tests when this pragma is | |
403 | in effect. Read the documentation for the C<filetest> pragma for more | |
404 | information. | |
405 | ||
406 | The C<-T> and C<-B> switches work as follows. The first block or so of the | |
407 | file is examined for odd characters such as strange control codes or | |
408 | characters with the high bit set. If too many strange characters (>30%) | |
409 | are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file | |
410 | containing a zero byte in the first block is considered a binary file. If C<-T> | |
411 | or C<-B> is used on a filehandle, the current IO buffer is examined | |
412 | rather than the first block. Both C<-T> and C<-B> return true on an empty | |
413 | file, or a file at EOF when testing a filehandle. Because you have to | |
414 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> | |
415 | against the file first, as in C<next unless -f $file && -T $file>. | |
416 | ||
417 | If any of the file tests (or either the C<stat> or C<lstat> operator) is given | |
418 | the special filehandle consisting of a solitary underline, then the stat | |
419 | structure of the previous file test (or stat operator) is used, saving | |
420 | a system call. (This doesn't work with C<-t>, and you need to remember | |
421 | that lstat() and C<-l> leave values in the stat structure for the | |
422 | symbolic link, not the real file.) (Also, if the stat buffer was filled by | |
423 | an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). | |
424 | Example: | |
425 | ||
426 | print "Can do.\n" if -r $a || -w _ || -x _; | |
427 | ||
428 | stat($filename); | |
429 | print "Readable\n" if -r _; | |
430 | print "Writable\n" if -w _; | |
431 | print "Executable\n" if -x _; | |
432 | print "Setuid\n" if -u _; | |
433 | print "Setgid\n" if -g _; | |
434 | print "Sticky\n" if -k _; | |
435 | print "Text\n" if -T _; | |
436 | print "Binary\n" if -B _; | |
437 | ||
438 | As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file | |
439 | test operators, in a way that C<-f -w -x $file> is equivalent to | |
440 | C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use | |
441 | the return value of C<-f $file> as an argument to another filetest | |
442 | operator, no special magic will happen.) | |
443 | ||
444 | Portability issues: L<perlport/-X>. | |
445 | ||
446 | =item abs VALUE | |
447 | X<abs> X<absolute> | |
448 | ||
449 | =item abs | |
450 | ||
451 | Returns the absolute value of its argument. | |
452 | If VALUE is omitted, uses C<$_>. | |
453 | ||
454 | =item accept NEWSOCKET,GENERICSOCKET | |
455 | X<accept> | |
456 | ||
a6b91202 | 457 | Accepts an incoming socket connect, just as accept(2) |
0909e3f8 RS |
458 | does. Returns the packed address if it succeeded, false otherwise. |
459 | See the example in L<perlipc/"Sockets: Client/Server Communication">. | |
460 | ||
461 | On systems that support a close-on-exec flag on files, the flag will | |
462 | be set for the newly opened file descriptor, as determined by the | |
463 | value of $^F. See L<perlvar/$^F>. | |
464 | ||
465 | =item alarm SECONDS | |
466 | X<alarm> | |
467 | X<SIGALRM> | |
468 | X<timer> | |
469 | ||
470 | =item alarm | |
471 | ||
472 | Arranges to have a SIGALRM delivered to this process after the | |
473 | specified number of wallclock seconds has elapsed. If SECONDS is not | |
474 | specified, the value stored in C<$_> is used. (On some machines, | |
475 | unfortunately, the elapsed time may be up to one second less or more | |
476 | than you specified because of how seconds are counted, and process | |
477 | scheduling may delay the delivery of the signal even further.) | |
478 | ||
479 | Only one timer may be counting at once. Each call disables the | |
480 | previous timer, and an argument of C<0> may be supplied to cancel the | |
481 | previous timer without starting a new one. The returned value is the | |
482 | amount of time remaining on the previous timer. | |
483 | ||
484 | For delays of finer granularity than one second, the Time::HiRes module | |
485 | (from CPAN, and starting from Perl 5.8 part of the standard | |
486 | distribution) provides ualarm(). You may also use Perl's four-argument | |
487 | version of select() leaving the first three arguments undefined, or you | |
488 | might be able to use the C<syscall> interface to access setitimer(2) if | |
489 | your system supports it. See L<perlfaq8> for details. | |
490 | ||
491 | It is usually a mistake to intermix C<alarm> and C<sleep> calls, because | |
492 | C<sleep> may be internally implemented on your system with C<alarm>. | |
493 | ||
494 | If you want to use C<alarm> to time out a system call you need to use an | |
495 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to | |
496 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to | |
497 | restart system calls on some systems. Using C<eval>/C<die> always works, | |
498 | modulo the caveats given in L<perlipc/"Signals">. | |
499 | ||
500 | eval { | |
501 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required | |
502 | alarm $timeout; | |
503 | $nread = sysread SOCKET, $buffer, $size; | |
504 | alarm 0; | |
505 | }; | |
506 | if ($@) { | |
507 | die unless $@ eq "alarm\n"; # propagate unexpected errors | |
508 | # timed out | |
509 | } | |
510 | else { | |
511 | # didn't | |
512 | } | |
513 | ||
514 | For more information see L<perlipc>. | |
515 | ||
516 | Portability issues: L<perlport/alarm>. | |
517 | ||
518 | =item atan2 Y,X | |
519 | X<atan2> X<arctangent> X<tan> X<tangent> | |
520 | ||
521 | Returns the arctangent of Y/X in the range -PI to PI. | |
522 | ||
523 | For the tangent operation, you may use the C<Math::Trig::tan> | |
524 | function, or use the familiar relation: | |
525 | ||
526 | sub tan { sin($_[0]) / cos($_[0]) } | |
527 | ||
528 | The return value for C<atan2(0,0)> is implementation-defined; consult | |
529 | your atan2(3) manpage for more information. | |
530 | ||
531 | Portability issues: L<perlport/atan2>. | |
532 | ||
533 | =item bind SOCKET,NAME | |
534 | X<bind> | |
535 | ||
536 | Binds a network address to a socket, just as bind(2) | |
537 | does. Returns true if it succeeded, false otherwise. NAME should be a | |
538 | packed address of the appropriate type for the socket. See the examples in | |
539 | L<perlipc/"Sockets: Client/Server Communication">. | |
540 | ||
541 | =item binmode FILEHANDLE, LAYER | |
542 | X<binmode> X<binary> X<text> X<DOS> X<Windows> | |
543 | ||
544 | =item binmode FILEHANDLE | |
545 | ||
546 | Arranges for FILEHANDLE to be read or written in "binary" or "text" | |
547 | mode on systems where the run-time libraries distinguish between | |
548 | binary and text files. If FILEHANDLE is an expression, the value is | |
549 | taken as the name of the filehandle. Returns true on success, | |
550 | otherwise it returns C<undef> and sets C<$!> (errno). | |
551 | ||
552 | On some systems (in general, DOS- and Windows-based systems) binmode() | |
553 | is necessary when you're not working with a text file. For the sake | |
554 | of portability it is a good idea always to use it when appropriate, | |
555 | and never to use it when it isn't appropriate. Also, people can | |
556 | set their I/O to be by default UTF8-encoded Unicode, not bytes. | |
557 | ||
558 | In other words: regardless of platform, use binmode() on binary data, | |
559 | like images, for example. | |
560 | ||
561 | If LAYER is present it is a single string, but may contain multiple | |
562 | directives. The directives alter the behaviour of the filehandle. | |
563 | When LAYER is present, using binmode on a text file makes sense. | |
564 | ||
565 | If LAYER is omitted or specified as C<:raw> the filehandle is made | |
566 | suitable for passing binary data. This includes turning off possible CRLF | |
567 | translation and marking it as bytes (as opposed to Unicode characters). | |
568 | Note that, despite what may be implied in I<"Programming Perl"> (the | |
569 | Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>. | |
570 | Other layers that would affect the binary nature of the stream are | |
571 | I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the | |
572 | PERLIO environment variable. | |
573 | ||
574 | The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the | |
575 | form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to | |
576 | establish default I/O layers. See L<open>. | |
577 | ||
578 | I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" | |
579 | in "Programming Perl, 3rd Edition". However, since the publishing of this | |
580 | book, by many known as "Camel III", the consensus of the naming of this | |
581 | functionality has moved from "discipline" to "layer". All documentation | |
582 | of this version of Perl therefore refers to "layers" rather than to | |
583 | "disciplines". Now back to the regularly scheduled documentation...> | |
584 | ||
585 | To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>. | |
586 | C<:utf8> just marks the data as UTF-8 without further checking, | |
587 | while C<:encoding(UTF-8)> checks the data for actually being valid | |
588 | UTF-8. More details can be found in L<PerlIO::encoding>. | |
589 | ||
590 | In general, binmode() should be called after open() but before any I/O | |
591 | is done on the filehandle. Calling binmode() normally flushes any | |
592 | pending buffered output data (and perhaps pending input data) on the | |
593 | handle. An exception to this is the C<:encoding> layer that | |
594 | changes the default character encoding of the handle; see L</open>. | |
595 | The C<:encoding> layer sometimes needs to be called in | |
596 | mid-stream, and it doesn't flush the stream. The C<:encoding> | |
597 | also implicitly pushes on top of itself the C<:utf8> layer because | |
598 | internally Perl operates on UTF8-encoded Unicode characters. | |
599 | ||
600 | The operating system, device drivers, C libraries, and Perl run-time | |
601 | system all conspire to let the programmer treat a single | |
602 | character (C<\n>) as the line terminator, irrespective of external | |
603 | representation. On many operating systems, the native text file | |
604 | representation matches the internal representation, but on some | |
605 | platforms the external representation of C<\n> is made up of more than | |
606 | one character. | |
607 | ||
608 | All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use | |
609 | a single character to end each line in the external representation of text | |
610 | (even though that single character is CARRIAGE RETURN on old, pre-Darwin | |
611 | flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other | |
612 | systems like OS/2, DOS, and the various flavors of MS-Windows, your program | |
613 | sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the | |
614 | two characters C<\cM\cJ>. That means that if you don't use binmode() on | |
615 | these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on | |
616 | input, and any C<\n> in your program will be converted back to C<\cM\cJ> on | |
617 | output. This is what you want for text files, but it can be disastrous for | |
618 | binary files. | |
619 | ||
620 | Another consequence of using binmode() (on some systems) is that | |
621 | special end-of-file markers will be seen as part of the data stream. | |
622 | For systems from the Microsoft family this means that, if your binary | |
623 | data contain C<\cZ>, the I/O subsystem will regard it as the end of | |
624 | the file, unless you use binmode(). | |
625 | ||
626 | binmode() is important not only for readline() and print() operations, | |
627 | but also when using read(), seek(), sysread(), syswrite() and tell() | |
628 | (see L<perlport> for more details). See the C<$/> and C<$\> variables | |
629 | in L<perlvar> for how to manually set your input and output | |
630 | line-termination sequences. | |
631 | ||
632 | Portability issues: L<perlport/binmode>. | |
633 | ||
634 | =item bless REF,CLASSNAME | |
635 | X<bless> | |
636 | ||
637 | =item bless REF | |
638 | ||
639 | This function tells the thingy referenced by REF that it is now an object | |
640 | in the CLASSNAME package. If CLASSNAME is omitted, the current package | |
641 | is used. Because a C<bless> is often the last thing in a constructor, | |
642 | it returns the reference for convenience. Always use the two-argument | |
643 | version if a derived class might inherit the function doing the blessing. | |
644 | SeeL<perlobj> for more about the blessing (and blessings) of objects. | |
645 | ||
646 | Consider always blessing objects in CLASSNAMEs that are mixed case. | |
647 | Namespaces with all lowercase names are considered reserved for | |
648 | Perl pragmata. Builtin types have all uppercase names. To prevent | |
649 | confusion, you may wish to avoid such package names as well. Make sure | |
650 | that CLASSNAME is a true value. | |
651 | ||
652 | See L<perlmod/"Perl Modules">. | |
653 | ||
654 | =item break | |
655 | ||
656 | Break out of a C<given()> block. | |
657 | ||
658 | This keyword is enabled by the C<"switch"> feature: see | |
659 | L<feature> for more information. You can also access it by | |
660 | prefixing it with C<CORE::>. Alternately, include a C<use | |
661 | v5.10> or later to the current scope. | |
662 | ||
663 | =item caller EXPR | |
664 | X<caller> X<call stack> X<stack> X<stack trace> | |
665 | ||
666 | =item caller | |
667 | ||
668 | Returns the context of the current subroutine call. In scalar context, | |
669 | returns the caller's package name if there I<is> a caller (that is, if | |
670 | we're in a subroutine or C<eval> or C<require>) and the undefined value | |
671 | otherwise. In list context, returns | |
672 | ||
673 | # 0 1 2 | |
674 | ($package, $filename, $line) = caller; | |
675 | ||
676 | With EXPR, it returns some extra information that the debugger uses to | |
677 | print a stack trace. The value of EXPR indicates how many call frames | |
678 | to go back before the current one. | |
679 | ||
680 | # 0 1 2 3 4 | |
681 | ($package, $filename, $line, $subroutine, $hasargs, | |
682 | ||
683 | # 5 6 7 8 9 10 | |
684 | $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) | |
685 | = caller($i); | |
686 | ||
687 | Here $subroutine may be C<(eval)> if the frame is not a subroutine | |
688 | call, but an C<eval>. In such a case additional elements $evaltext and | |
689 | C<$is_require> are set: C<$is_require> is true if the frame is created by a | |
690 | C<require> or C<use> statement, $evaltext contains the text of the | |
691 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, | |
692 | $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that | |
693 | each C<use> statement creates a C<require> frame inside an C<eval EXPR> | |
694 | frame.) $subroutine may also be C<(unknown)> if this particular | |
695 | subroutine happens to have been deleted from the symbol table. | |
696 | C<$hasargs> is true if a new instance of C<@_> was set up for the frame. | |
697 | C<$hints> and C<$bitmask> contain pragmatic hints that the caller was | |
698 | compiled with. The C<$hints> and C<$bitmask> values are subject to change | |
699 | between versions of Perl, and are not meant for external use. | |
700 | ||
701 | C<$hinthash> is a reference to a hash containing the value of C<%^H> when the | |
702 | caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values | |
703 | of this hash, as they are the actual values stored in the optree. | |
704 | ||
705 | Furthermore, when called from within the DB package, caller returns more | |
706 | detailed information: it sets the list variable C<@DB::args> to be the | |
707 | arguments with which the subroutine was invoked. | |
708 | ||
709 | Be aware that the optimizer might have optimized call frames away before | |
710 | C<caller> had a chance to get the information. That means that C<caller(N)> | |
711 | might not return information about the call frame you expect it to, for | |
712 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the | |
713 | previous time C<caller> was called. | |
714 | ||
715 | Be aware that setting C<@DB::args> is I<best effort>, intended for | |
716 | debugging or generating backtraces, and should not be relied upon. In | |
717 | particular, as C<@_> contains aliases to the caller's arguments, Perl does | |
718 | not take a copy of C<@_>, so C<@DB::args> will contain modifications the | |
719 | subroutine makes to C<@_> or its contents, not the original values at call | |
720 | time. C<@DB::args>, like C<@_>, does not hold explicit references to its | |
721 | elements, so under certain cases its elements may have become freed and | |
722 | reallocated for other variables or temporary values. Finally, a side effect | |
723 | of the current implementation is that the effects of C<shift @_> can | |
724 | I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a | |
725 | reference to C<@_> has been taken, I<and> subject to the caveat about reallocated | |
726 | elements), so C<@DB::args> is actually a hybrid of the current state and | |
727 | initial state of C<@_>. Buyer beware. | |
728 | ||
729 | =item chdir EXPR | |
730 | X<chdir> | |
731 | X<cd> | |
732 | X<directory, change> | |
733 | ||
734 | =item chdir FILEHANDLE | |
735 | ||
736 | =item chdir DIRHANDLE | |
737 | ||
738 | =item chdir | |
739 | ||
740 | Changes the working directory to EXPR, if possible. If EXPR is omitted, | |
741 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, | |
742 | changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the | |
743 | variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If | |
744 | neither is set, C<chdir> does nothing. It returns true on success, | |
745 | false otherwise. See the example under C<die>. | |
746 | ||
747 | On systems that support fchdir(2), you may pass a filehandle or | |
748 | directory handle as the argument. On systems that don't support fchdir(2), | |
749 | passing handles raises an exception. | |
750 | ||
751 | =item chmod LIST | |
752 | X<chmod> X<permission> X<mode> | |
753 | ||
754 | Changes the permissions of a list of files. The first element of the | |
755 | list must be the numeric mode, which should probably be an octal | |
756 | number, and which definitely should I<not> be a string of octal digits: | |
757 | C<0644> is okay, but C<"0644"> is not. Returns the number of files | |
758 | successfully changed. See also L</oct> if all you have is a string. | |
759 | ||
760 | $cnt = chmod 0755, "foo", "bar"; | |
761 | chmod 0755, @executables; | |
762 | $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to | |
763 | # --w----r-T | |
764 | $mode = "0644"; chmod oct($mode), "foo"; # this is better | |
765 | $mode = 0644; chmod $mode, "foo"; # this is best | |
766 | ||
767 | On systems that support fchmod(2), you may pass filehandles among the | |
768 | files. On systems that don't support fchmod(2), passing filehandles raises | |
769 | an exception. Filehandles must be passed as globs or glob references to be | |
770 | recognized; barewords are considered filenames. | |
771 | ||
772 | open(my $fh, "<", "foo"); | |
773 | my $perm = (stat $fh)[2] & 07777; | |
774 | chmod($perm | 0600, $fh); | |
775 | ||
776 | You can also import the symbolic C<S_I*> constants from the C<Fcntl> | |
777 | module: | |
778 | ||
779 | use Fcntl qw( :mode ); | |
780 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; | |
781 | # Identical to the chmod 0755 of the example above. | |
782 | ||
783 | Portability issues: L<perlport/chmod>. | |
784 | ||
785 | =item chomp VARIABLE | |
786 | X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> | |
787 | ||
788 | =item chomp( LIST ) | |
789 | ||
790 | =item chomp | |
791 | ||
792 | This safer version of L</chop> removes any trailing string | |
793 | that corresponds to the current value of C<$/> (also known as | |
794 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total | |
795 | number of characters removed from all its arguments. It's often used to | |
796 | remove the newline from the end of an input record when you're worried | |
797 | that the final record may be missing its newline. When in paragraph | |
798 | mode (C<$/ = "">), it removes all trailing newlines from the string. | |
799 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is | |
800 | a reference to an integer or the like; see L<perlvar>) chomp() won't | |
801 | remove anything. | |
802 | If VARIABLE is omitted, it chomps C<$_>. Example: | |
803 | ||
804 | while (<>) { | |
805 | chomp; # avoid \n on last field | |
806 | @array = split(/:/); | |
807 | # ... | |
808 | } | |
809 | ||
810 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. | |
811 | ||
812 | You can actually chomp anything that's an lvalue, including an assignment: | |
813 | ||
814 | chomp($cwd = `pwd`); | |
815 | chomp($answer = <STDIN>); | |
816 | ||
817 | If you chomp a list, each element is chomped, and the total number of | |
818 | characters removed is returned. | |
819 | ||
820 | Note that parentheses are necessary when you're chomping anything | |
821 | that is not a simple variable. This is because C<chomp $cwd = `pwd`;> | |
822 | is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as | |
823 | C<chomp( $cwd = `pwd` )> which you might expect. Similarly, | |
824 | C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than | |
825 | as C<chomp($a, $b)>. | |
826 | ||
827 | =item chop VARIABLE | |
828 | X<chop> | |
829 | ||
830 | =item chop( LIST ) | |
831 | ||
832 | =item chop | |
833 | ||
834 | Chops off the last character of a string and returns the character | |
835 | chopped. It is much more efficient than C<s/.$//s> because it neither | |
836 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. | |
837 | If VARIABLE is a hash, it chops the hash's values, but not its keys. | |
838 | ||
839 | You can actually chop anything that's an lvalue, including an assignment. | |
840 | ||
841 | If you chop a list, each element is chopped. Only the value of the | |
842 | last C<chop> is returned. | |
843 | ||
844 | Note that C<chop> returns the last character. To return all but the last | |
845 | character, use C<substr($string, 0, -1)>. | |
846 | ||
847 | See also L</chomp>. | |
848 | ||
849 | =item chown LIST | |
850 | X<chown> X<owner> X<user> X<group> | |
851 | ||
852 | Changes the owner (and group) of a list of files. The first two | |
853 | elements of the list must be the I<numeric> uid and gid, in that | |
854 | order. A value of -1 in either position is interpreted by most | |
855 | systems to leave that value unchanged. Returns the number of files | |
856 | successfully changed. | |
857 | ||
858 | $cnt = chown $uid, $gid, 'foo', 'bar'; | |
859 | chown $uid, $gid, @filenames; | |
860 | ||
861 | On systems that support fchown(2), you may pass filehandles among the | |
862 | files. On systems that don't support fchown(2), passing filehandles raises | |
863 | an exception. Filehandles must be passed as globs or glob references to be | |
864 | recognized; barewords are considered filenames. | |
865 | ||
866 | Here's an example that looks up nonnumeric uids in the passwd file: | |
867 | ||
868 | print "User: "; | |
869 | chomp($user = <STDIN>); | |
870 | print "Files: "; | |
871 | chomp($pattern = <STDIN>); | |
872 | ||
873 | ($login,$pass,$uid,$gid) = getpwnam($user) | |
874 | or die "$user not in passwd file"; | |
875 | ||
876 | @ary = glob($pattern); # expand filenames | |
877 | chown $uid, $gid, @ary; | |
878 | ||
879 | On most systems, you are not allowed to change the ownership of the | |
880 | file unless you're the superuser, although you should be able to change | |
881 | the group to any of your secondary groups. On insecure systems, these | |
882 | restrictions may be relaxed, but this is not a portable assumption. | |
883 | On POSIX systems, you can detect this condition this way: | |
884 | ||
885 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); | |
886 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); | |
887 | ||
888 | Portability issues: L<perlport/chmod>. | |
889 | ||
890 | =item chr NUMBER | |
891 | X<chr> X<character> X<ASCII> X<Unicode> | |
892 | ||
893 | =item chr | |
894 | ||
895 | Returns the character represented by that NUMBER in the character set. | |
896 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and | |
a6b91202 | 897 | chr(0x263a) is a Unicode smiley face. |
0909e3f8 RS |
898 | |
899 | Negative values give the Unicode replacement character (chr(0xfffd)), | |
900 | except under the L<bytes> pragma, where the low eight bits of the value | |
901 | (truncated to an integer) are used. | |
902 | ||
903 | If NUMBER is omitted, uses C<$_>. | |
904 | ||
905 | For the reverse, use L</ord>. | |
906 | ||
907 | Note that characters from 128 to 255 (inclusive) are by default | |
908 | internally not encoded as UTF-8 for backward compatibility reasons. | |
909 | ||
910 | See L<perlunicode> for more about Unicode. | |
911 | ||
912 | =item chroot FILENAME | |
913 | X<chroot> X<root> | |
914 | ||
915 | =item chroot | |
916 | ||
917 | This function works like the system call by the same name: it makes the | |
918 | named directory the new root directory for all further pathnames that | |
919 | begin with a C</> by your process and all its children. (It doesn't | |
920 | change your current working directory, which is unaffected.) For security | |
921 | reasons, this call is restricted to the superuser. If FILENAME is | |
922 | omitted, does a C<chroot> to C<$_>. | |
923 | ||
924 | Portability issues: L<perlport/chroot>. | |
925 | ||
926 | =item close FILEHANDLE | |
927 | X<close> | |
928 | ||
929 | =item close | |
930 | ||
931 | Closes the file or pipe associated with the filehandle, flushes the IO | |
932 | buffers, and closes the system file descriptor. Returns true if those | |
933 | operations succeed and if no error was reported by any PerlIO | |
934 | layer. Closes the currently selected filehandle if the argument is | |
935 | omitted. | |
936 | ||
937 | You don't have to close FILEHANDLE if you are immediately going to do | |
938 | another C<open> on it, because C<open> closes it for you. (See | |
939 | L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line | |
940 | counter (C<$.>), while the implicit close done by C<open> does not. | |
941 | ||
942 | If the filehandle came from a piped open, C<close> returns false if one of | |
943 | the other syscalls involved fails or if its program exits with non-zero | |
944 | status. If the only problem was that the program exited non-zero, C<$!> | |
945 | will be set to C<0>. Closing a pipe also waits for the process executing | |
946 | on the pipe to exit--in case you wish to look at the output of the pipe | |
947 | afterwards--and implicitly puts the exit status value of that command into | |
948 | C<$?> and C<${^CHILD_ERROR_NATIVE}>. | |
949 | ||
950 | If there are multiple threads running, C<close> on a filehandle from a | |
951 | piped open returns true without waiting for the child process to terminate, | |
952 | if the filehandle is still open in another thread. | |
953 | ||
954 | Closing the read end of a pipe before the process writing to it at the | |
955 | other end is done writing results in the writer receiving a SIGPIPE. If | |
956 | the other end can't handle that, be sure to read all the data before | |
957 | closing the pipe. | |
958 | ||
959 | Example: | |
960 | ||
961 | open(OUTPUT, '|sort >foo') # pipe to sort | |
962 | or die "Can't start sort: $!"; | |
963 | #... # print stuff to output | |
964 | close OUTPUT # wait for sort to finish | |
965 | or warn $! ? "Error closing sort pipe: $!" | |
966 | : "Exit status $? from sort"; | |
967 | open(INPUT, 'foo') # get sort's results | |
968 | or die "Can't open 'foo' for input: $!"; | |
969 | ||
970 | FILEHANDLE may be an expression whose value can be used as an indirect | |
971 | filehandle, usually the real filehandle name or an autovivified handle. | |
972 | ||
973 | =item closedir DIRHANDLE | |
974 | X<closedir> | |
975 | ||
976 | Closes a directory opened by C<opendir> and returns the success of that | |
977 | system call. | |
978 | ||
979 | =item connect SOCKET,NAME | |
980 | X<connect> | |
981 | ||
982 | Attempts to connect to a remote socket, just like connect(2). | |
983 | Returns true if it succeeded, false otherwise. NAME should be a | |
984 | packed address of the appropriate type for the socket. See the examples in | |
985 | L<perlipc/"Sockets: Client/Server Communication">. | |
986 | ||
987 | =item continue BLOCK | |
988 | X<continue> | |
989 | ||
990 | =item continue | |
991 | ||
992 | When followed by a BLOCK, C<continue> is actually a | |
993 | flow control statement rather than a function. If | |
994 | there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or | |
995 | C<foreach>), it is always executed just before the conditional is about to | |
996 | be evaluated again, just like the third part of a C<for> loop in C. Thus | |
997 | it can be used to increment a loop variable, even when the loop has been | |
998 | continued via the C<next> statement (which is similar to the C C<continue> | |
999 | statement). | |
1000 | ||
1001 | C<last>, C<next>, or C<redo> may appear within a C<continue> | |
1002 | block; C<last> and C<redo> behave as if they had been executed within | |
1003 | the main block. So will C<next>, but since it will execute a C<continue> | |
1004 | block, it may be more entertaining. | |
1005 | ||
1006 | while (EXPR) { | |
1007 | ### redo always comes here | |
1008 | do_something; | |
1009 | } continue { | |
1010 | ### next always comes here | |
1011 | do_something_else; | |
1012 | # then back the top to re-check EXPR | |
1013 | } | |
1014 | ### last always comes here | |
1015 | ||
1016 | Omitting the C<continue> section is equivalent to using an | |
1017 | empty one, logically enough, so C<next> goes directly back | |
1018 | to check the condition at the top of the loop. | |
1019 | ||
1020 | When there is no BLOCK, C<continue> is a function that | |
1021 | falls through the current C<when> or C<default> block instead of iterating | |
1022 | a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>. | |
1023 | In Perl 5.14 and earlier, this form of C<continue> was | |
1024 | only available when the C<"switch"> feature was enabled. | |
1025 | See L<feature> and L<perlsyn/"Switch statements"> for more | |
1026 | information. | |
1027 | ||
1028 | =item cos EXPR | |
1029 | X<cos> X<cosine> X<acos> X<arccosine> | |
1030 | ||
1031 | =item cos | |
1032 | ||
1033 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, | |
1034 | takes the cosine of C<$_>. | |
1035 | ||
1036 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> | |
1037 | function, or use this relation: | |
1038 | ||
1039 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } | |
1040 | ||
1041 | =item crypt PLAINTEXT,SALT | |
1042 | X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> | |
1043 | X<decrypt> X<cryptography> X<passwd> X<encrypt> | |
1044 | ||
1045 | Creates a digest string exactly like the crypt(3) function in the C | |
1046 | library (assuming that you actually have a version there that has not | |
1047 | been extirpated as a potential munition). | |
1048 | ||
1049 | crypt() is a one-way hash function. The PLAINTEXT and SALT are turned | |
1050 | into a short string, called a digest, which is returned. The same | |
1051 | PLAINTEXT and SALT will always return the same string, but there is no | |
1052 | (known) way to get the original PLAINTEXT from the hash. Small | |
1053 | changes in the PLAINTEXT or SALT will result in large changes in the | |
1054 | digest. | |
1055 | ||
1056 | There is no decrypt function. This function isn't all that useful for | |
1057 | cryptography (for that, look for F<Crypt> modules on your nearby CPAN | |
1058 | mirror) and the name "crypt" is a bit of a misnomer. Instead it is | |
1059 | primarily used to check if two pieces of text are the same without | |
1060 | having to transmit or store the text itself. An example is checking | |
1061 | if a correct password is given. The digest of the password is stored, | |
1062 | not the password itself. The user types in a password that is | |
1063 | crypt()'d with the same salt as the stored digest. If the two digests | |
1064 | match, the password is correct. | |
1065 | ||
1066 | When verifying an existing digest string you should use the digest as | |
1067 | the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used | |
1068 | to create the digest is visible as part of the digest. This ensures | |
1069 | crypt() will hash the new string with the same salt as the digest. | |
1070 | This allows your code to work with the standard L<crypt|/crypt> and | |
1071 | with more exotic implementations. In other words, assume | |
a6b91202 | 1072 | nothing about the returned string itself nor about how many bytes |
0909e3f8 RS |
1073 | of SALT may matter. |
1074 | ||
1075 | Traditionally the result is a string of 13 bytes: two first bytes of | |
1076 | the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only | |
1077 | the first eight bytes of PLAINTEXT mattered. But alternative | |
1078 | hashing schemes (like MD5), higher level security schemes (like C2), | |
1079 | and implementations on non-Unix platforms may produce different | |
1080 | strings. | |
1081 | ||
1082 | When choosing a new salt create a random two character string whose | |
1083 | characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', | |
1084 | '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of | |
1085 | characters is just a recommendation; the characters allowed in | |
1086 | the salt depend solely on your system's crypt library, and Perl can't | |
1087 | restrict what salts C<crypt()> accepts. | |
1088 | ||
1089 | Here's an example that makes sure that whoever runs this program knows | |
1090 | their password: | |
1091 | ||
1092 | $pwd = (getpwuid($<))[1]; | |
1093 | ||
1094 | system "stty -echo"; | |
1095 | print "Password: "; | |
1096 | chomp($word = <STDIN>); | |
1097 | print "\n"; | |
1098 | system "stty echo"; | |
1099 | ||
1100 | if (crypt($word, $pwd) ne $pwd) { | |
1101 | die "Sorry...\n"; | |
1102 | } else { | |
1103 | print "ok\n"; | |
1104 | } | |
1105 | ||
1106 | Of course, typing in your own password to whoever asks you | |
1107 | for it is unwise. | |
1108 | ||
1109 | The L<crypt|/crypt> function is unsuitable for hashing large quantities | |
1110 | of data, not least of all because you can't get the information | |
1111 | back. Look at the L<Digest> module for more robust algorithms. | |
1112 | ||
1113 | If using crypt() on a Unicode string (which I<potentially> has | |
1114 | characters with codepoints above 255), Perl tries to make sense | |
1115 | of the situation by trying to downgrade (a copy of) | |
1116 | the string back to an eight-bit byte string before calling crypt() | |
1117 | (on that copy). If that works, good. If not, crypt() dies with | |
1118 | C<Wide character in crypt>. | |
1119 | ||
1120 | Portability issues: L<perlport/crypt>. | |
1121 | ||
1122 | =item dbmclose HASH | |
1123 | X<dbmclose> | |
1124 | ||
1125 | [This function has been largely superseded by the C<untie> function.] | |
1126 | ||
1127 | Breaks the binding between a DBM file and a hash. | |
1128 | ||
1129 | Portability issues: L<perlport/dbmclose>. | |
1130 | ||
1131 | =item dbmopen HASH,DBNAME,MASK | |
1132 | X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> | |
1133 | ||
1134 | [This function has been largely superseded by the | |
1135 | L<tie|/tie VARIABLE,CLASSNAME,LIST> function.] | |
1136 | ||
1137 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a | |
1138 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first | |
1139 | argument is I<not> a filehandle, even though it looks like one). DBNAME | |
1140 | is the name of the database (without the F<.dir> or F<.pag> extension if | |
1141 | any). If the database does not exist, it is created with protection | |
1142 | specified by MASK (as modified by the C<umask>). If your system supports | |
1143 | only the older DBM functions, you may make only one C<dbmopen> call in your | |
1144 | program. In older versions of Perl, if your system had neither DBM nor | |
1145 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to | |
1146 | sdbm(3). | |
1147 | ||
1148 | If you don't have write access to the DBM file, you can only read hash | |
1149 | variables, not set them. If you want to test whether you can write, | |
a6b91202 | 1150 | either use file tests or try setting a dummy hash entry inside an C<eval> |
0909e3f8 RS |
1151 | to trap the error. |
1152 | ||
1153 | Note that functions such as C<keys> and C<values> may return huge lists | |
1154 | when used on large DBM files. You may prefer to use the C<each> | |
1155 | function to iterate over large DBM files. Example: | |
1156 | ||
1157 | # print out history file offsets | |
1158 | dbmopen(%HIST,'/usr/lib/news/history',0666); | |
1159 | while (($key,$val) = each %HIST) { | |
1160 | print $key, ' = ', unpack('L',$val), "\n"; | |
1161 | } | |
1162 | dbmclose(%HIST); | |
1163 | ||
1164 | See also L<AnyDBM_File> for a more general description of the pros and | |
1165 | cons of the various dbm approaches, as well as L<DB_File> for a particularly | |
1166 | rich implementation. | |
1167 | ||
1168 | You can control which DBM library you use by loading that library | |
1169 | before you call dbmopen(): | |
1170 | ||
1171 | use DB_File; | |
1172 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") | |
1173 | or die "Can't open netscape history file: $!"; | |
1174 | ||
1175 | Portability issues: L<perlport/dbmopen>. | |
1176 | ||
1177 | =item default BLOCK | |
1178 | ||
1179 | Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when> | |
1180 | that's always true. Only available after Perl 5.10, and only if the | |
1181 | C<switch> feature has been requested or if the keyword is prefixed with | |
1182 | C<CORE::>. See L</when>. | |
1183 | ||
1184 | =item defined EXPR | |
1185 | X<defined> X<undef> X<undefined> | |
1186 | ||
1187 | =item defined | |
1188 | ||
1189 | Returns a Boolean value telling whether EXPR has a value other than | |
1190 | the undefined value C<undef>. If EXPR is not present, C<$_> is | |
1191 | checked. | |
1192 | ||
1193 | Many operations return C<undef> to indicate failure, end of file, | |
1194 | system error, uninitialized variable, and other exceptional | |
1195 | conditions. This function allows you to distinguish C<undef> from | |
1196 | other values. (A simple Boolean test will not distinguish among | |
1197 | C<undef>, zero, the empty string, and C<"0">, which are all equally | |
1198 | false.) Note that since C<undef> is a valid scalar, its presence | |
1199 | doesn't I<necessarily> indicate an exceptional condition: C<pop> | |
1200 | returns C<undef> when its argument is an empty array, I<or> when the | |
1201 | element to return happens to be C<undef>. | |
1202 | ||
1203 | You may also use C<defined(&func)> to check whether subroutine C<&func> | |
1204 | has ever been defined. The return value is unaffected by any forward | |
1205 | declarations of C<&func>. A subroutine that is not defined | |
1206 | may still be callable: its package may have an C<AUTOLOAD> method that | |
1207 | makes it spring into existence the first time that it is called; see | |
1208 | L<perlsub>. | |
1209 | ||
1210 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It | |
1211 | used to report whether memory for that aggregate had ever been | |
1212 | allocated. This behavior may disappear in future versions of Perl. | |
1213 | You should instead use a simple test for size: | |
1214 | ||
1215 | if (@an_array) { print "has array elements\n" } | |
1216 | if (%a_hash) { print "has hash members\n" } | |
1217 | ||
1218 | When used on a hash element, it tells you whether the value is defined, | |
1219 | not whether the key exists in the hash. Use L</exists> for the latter | |
1220 | purpose. | |
1221 | ||
1222 | Examples: | |
1223 | ||
1224 | print if defined $switch{D}; | |
1225 | print "$val\n" while defined($val = pop(@ary)); | |
1226 | die "Can't readlink $sym: $!" | |
1227 | unless defined($value = readlink $sym); | |
1228 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } | |
1229 | $debugging = 0 unless defined $debugging; | |
1230 | ||
1231 | Note: Many folks tend to overuse C<defined> and are then surprised to | |
1232 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, | |
1233 | defined values. For example, if you say | |
1234 | ||
1235 | "ab" =~ /a(.*)b/; | |
1236 | ||
1237 | The pattern match succeeds and C<$1> is defined, although it | |
1238 | matched "nothing". It didn't really fail to match anything. Rather, it | |
1239 | matched something that happened to be zero characters long. This is all | |
1240 | very above-board and honest. When a function returns an undefined value, | |
1241 | it's an admission that it couldn't give you an honest answer. So you | |
1242 | should use C<defined> only when questioning the integrity of what | |
1243 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is | |
1244 | what you want. | |
1245 | ||
1246 | See also L</undef>, L</exists>, L</ref>. | |
1247 | ||
1248 | =item delete EXPR | |
1249 | X<delete> | |
1250 | ||
1251 | Given an expression that specifies an element or slice of a hash, C<delete> | |
1252 | deletes the specified elements from that hash so that exists() on that element | |
1253 | no longer returns true. Setting a hash element to the undefined value does | |
1254 | not remove its key, but deleting it does; see L</exists>. | |
1255 | ||
1256 | In list context, returns the value or values deleted, or the last such | |
1257 | element in scalar context. The return list's length always matches that of | |
1258 | the argument list: deleting non-existent elements returns the undefined value | |
1259 | in their corresponding positions. | |
1260 | ||
1261 | delete() may also be used on arrays and array slices, but its behavior is less | |
1262 | straightforward. Although exists() will return false for deleted entries, | |
1263 | deleting array elements never changes indices of existing values; use shift() | |
1264 | or splice() for that. However, if all deleted elements fall at the end of an | |
1265 | array, the array's size shrinks to the position of the highest element that | |
1266 | still tests true for exists(), or to 0 if none do. | |
1267 | ||
1268 | B<WARNING:> Calling delete on array values is deprecated and likely to | |
1269 | be removed in a future version of Perl. | |
1270 | ||
1271 | Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to | |
1272 | a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash | |
1273 | or array may not necessarily return anything; it depends on the implementation | |
1274 | of the C<tied> package's DELETE method, which may do whatever it pleases. | |
1275 | ||
1276 | The C<delete local EXPR> construct localizes the deletion to the current | |
1277 | block at run time. Until the block exits, elements locally deleted | |
1278 | temporarily no longer exist. See L<perlsub/"Localized deletion of elements | |
1279 | of composite types">. | |
1280 | ||
1281 | %hash = (foo => 11, bar => 22, baz => 33); | |
1282 | $scalar = delete $hash{foo}; # $scalar is 11 | |
1283 | $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 | |
1284 | @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) | |
1285 | ||
1286 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: | |
1287 | ||
1288 | foreach $key (keys %HASH) { | |
1289 | delete $HASH{$key}; | |
1290 | } | |
1291 | ||
1292 | foreach $index (0 .. $#ARRAY) { | |
1293 | delete $ARRAY[$index]; | |
1294 | } | |
1295 | ||
1296 | And so do these: | |
1297 | ||
1298 | delete @HASH{keys %HASH}; | |
1299 | ||
1300 | delete @ARRAY[0 .. $#ARRAY]; | |
1301 | ||
1302 | But both are slower than assigning the empty list | |
a6b91202 | 1303 | or undefining %HASH or @ARRAY, which is the customary |
0909e3f8 RS |
1304 | way to empty out an aggregate: |
1305 | ||
1306 | %HASH = (); # completely empty %HASH | |
1307 | undef %HASH; # forget %HASH ever existed | |
1308 | ||
1309 | @ARRAY = (); # completely empty @ARRAY | |
1310 | undef @ARRAY; # forget @ARRAY ever existed | |
1311 | ||
1312 | The EXPR can be arbitrarily complicated provided its | |
1313 | final operation is an element or slice of an aggregate: | |
1314 | ||
1315 | delete $ref->[$x][$y]{$key}; | |
1316 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; | |
1317 | ||
1318 | delete $ref->[$x][$y][$index]; | |
1319 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; | |
1320 | ||
1321 | =item die LIST | |
1322 | X<die> X<throw> X<exception> X<raise> X<$@> X<abort> | |
1323 | ||
1324 | C<die> raises an exception. Inside an C<eval> the error message is stuffed | |
1325 | into C<$@> and the C<eval> is terminated with the undefined value. | |
1326 | If the exception is outside of all enclosing C<eval>s, then the uncaught | |
1327 | exception prints LIST to C<STDERR> and exits with a non-zero value. If you | |
1328 | need to exit the process with a specific exit code, see L</exit>. | |
1329 | ||
1330 | Equivalent examples: | |
1331 | ||
1332 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; | |
1333 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" | |
1334 | ||
1335 | If the last element of LIST does not end in a newline, the current | |
1336 | script line number and input line number (if any) are also printed, | |
1337 | and a newline is supplied. Note that the "input line number" (also | |
1338 | known as "chunk") is subject to whatever notion of "line" happens to | |
1339 | be currently in effect, and is also available as the special variable | |
1340 | C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. | |
1341 | ||
1342 | Hint: sometimes appending C<", stopped"> to your message will cause it | |
1343 | to make better sense when the string C<"at foo line 123"> is appended. | |
1344 | Suppose you are running script "canasta". | |
1345 | ||
1346 | die "/etc/games is no good"; | |
1347 | die "/etc/games is no good, stopped"; | |
1348 | ||
1349 | produce, respectively | |
1350 | ||
1351 | /etc/games is no good at canasta line 123. | |
1352 | /etc/games is no good, stopped at canasta line 123. | |
1353 | ||
1354 | If the output is empty and C<$@> already contains a value (typically from a | |
1355 | previous eval) that value is reused after appending C<"\t...propagated">. | |
1356 | This is useful for propagating exceptions: | |
1357 | ||
1358 | eval { ... }; | |
1359 | die unless $@ =~ /Expected exception/; | |
1360 | ||
1361 | If the output is empty and C<$@> contains an object reference that has a | |
1362 | C<PROPAGATE> method, that method will be called with additional file | |
1363 | and line number parameters. The return value replaces the value in | |
1364 | C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> | |
1365 | were called. | |
1366 | ||
1367 | If C<$@> is empty then the string C<"Died"> is used. | |
1368 | ||
1369 | If an uncaught exception results in interpreter exit, the exit code is | |
1370 | determined from the values of C<$!> and C<$?> with this pseudocode: | |
1371 | ||
1372 | exit $! if $!; # errno | |
1373 | exit $? >> 8 if $? >> 8; # child exit status | |
1374 | exit 255; # last resort | |
1375 | ||
1376 | The intent is to squeeze as much possible information about the likely cause | |
1377 | into the limited space of the system exit code. However, as C<$!> is the value | |
1378 | of C's C<errno>, which can be set by any system call, this means that the value | |
1379 | of the exit code used by C<die> can be non-predictable, so should not be relied | |
1380 | upon, other than to be non-zero. | |
1381 | ||
1382 | You can also call C<die> with a reference argument, and if this is trapped | |
1383 | within an C<eval>, C<$@> contains that reference. This permits more | |
1384 | elaborate exception handling using objects that maintain arbitrary state | |
1385 | about the exception. Such a scheme is sometimes preferable to matching | |
a6b91202 | 1386 | particular string values of C<$@> with regular expressions. Because C<$@> |
0909e3f8 RS |
1387 | is a global variable and C<eval> may be used within object implementations, |
1388 | be careful that analyzing the error object doesn't replace the reference in | |
1389 | the global variable. It's easiest to make a local copy of the reference | |
1390 | before any manipulations. Here's an example: | |
1391 | ||
1392 | use Scalar::Util "blessed"; | |
1393 | ||
1394 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; | |
1395 | if (my $ev_err = $@) { | |
1396 | if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { | |
1397 | # handle Some::Module::Exception | |
1398 | } | |
1399 | else { | |
1400 | # handle all other possible exceptions | |
1401 | } | |
1402 | } | |
1403 | ||
1404 | Because Perl stringifies uncaught exception messages before display, | |
1405 | you'll probably want to overload stringification operations on | |
1406 | exception objects. See L<overload> for details about that. | |
1407 | ||
1408 | You can arrange for a callback to be run just before the C<die> | |
1409 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated | |
1410 | handler is called with the error text and can change the error | |
1411 | message, if it sees fit, by calling C<die> again. See | |
1412 | L<perlvar/%SIG> for details on setting C<%SIG> entries, and | |
a6b91202 | 1413 | L<"eval BLOCK"> for some examples. Although this feature was |
0909e3f8 RS |
1414 | to be run only right before your program was to exit, this is not |
1415 | currently so: the C<$SIG{__DIE__}> hook is currently called | |
1416 | even inside eval()ed blocks/strings! If one wants the hook to do | |
1417 | nothing in such situations, put | |
1418 | ||
1419 | die @_ if $^S; | |
1420 | ||
1421 | as the first line of the handler (see L<perlvar/$^S>). Because | |
1422 | this promotes strange action at a distance, this counterintuitive | |
1423 | behavior may be fixed in a future release. | |
1424 | ||
1425 | See also exit(), warn(), and the Carp module. | |
1426 | ||
1427 | =item do BLOCK | |
1428 | X<do> X<block> | |
1429 | ||
1430 | Not really a function. Returns the value of the last command in the | |
1431 | sequence of commands indicated by BLOCK. When modified by the C<while> or | |
1432 | C<until> loop modifier, executes the BLOCK once before testing the loop | |
1433 | condition. (On other statements the loop modifiers test the conditional | |
1434 | first.) | |
1435 | ||
1436 | C<do BLOCK> does I<not> count as a loop, so the loop control statements | |
1437 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. | |
1438 | See L<perlsyn> for alternative strategies. | |
1439 | ||
1440 | =item do SUBROUTINE(LIST) | |
1441 | X<do> | |
1442 | ||
1443 | This form of subroutine call is deprecated. SUBROUTINE can be a bareword, | |
1444 | a scalar variable or a subroutine beginning with C<&>. | |
1445 | ||
1446 | =item do EXPR | |
1447 | X<do> | |
1448 | ||
1449 | Uses the value of EXPR as a filename and executes the contents of the | |
1450 | file as a Perl script. | |
1451 | ||
1452 | do 'stat.pl'; | |
1453 | ||
1454 | is just like | |
1455 | ||
1456 | eval `cat stat.pl`; | |
1457 | ||
1458 | except that it's more efficient and concise, keeps track of the current | |
1459 | filename for error messages, searches the C<@INC> directories, and updates | |
1460 | C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for | |
1461 | these variables. It also differs in that code evaluated with C<do FILENAME> | |
1462 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the | |
1463 | same, however, in that it does reparse the file every time you call it, | |
1464 | so you probably don't want to do this inside a loop. | |
1465 | ||
1466 | If C<do> can read the file but cannot compile it, it returns C<undef> and sets | |
1467 | an error message in C<$@>. If C<do> cannot read the file, it returns undef | |
1468 | and sets C<$!> to the error. Always check C<$@> first, as compilation | |
1469 | could fail in a way that also sets C<$!>. If the file is successfully | |
1470 | compiled, C<do> returns the value of the last expression evaluated. | |
1471 | ||
1472 | Inclusion of library modules is better done with the | |
1473 | C<use> and C<require> operators, which also do automatic error checking | |
1474 | and raise an exception if there's a problem. | |
1475 | ||
1476 | You might like to use C<do> to read in a program configuration | |
1477 | file. Manual error checking can be done this way: | |
1478 | ||
1479 | # read in config files: system first, then user | |
1480 | for $file ("/share/prog/defaults.rc", | |
1481 | "$ENV{HOME}/.someprogrc") | |
1482 | { | |
1483 | unless ($return = do $file) { | |
1484 | warn "couldn't parse $file: $@" if $@; | |
1485 | warn "couldn't do $file: $!" unless defined $return; | |
1486 | warn "couldn't run $file" unless $return; | |
1487 | } | |
1488 | } | |
1489 | ||
1490 | =item dump LABEL | |
1491 | X<dump> X<core> X<undump> | |
1492 | ||
1493 | =item dump | |
1494 | ||
1495 | This function causes an immediate core dump. See also the B<-u> | |
1496 | command-line switch in L<perlrun>, which does the same thing. | |
1497 | Primarily this is so that you can use the B<undump> program (not | |
1498 | supplied) to turn your core dump into an executable binary after | |
1499 | having initialized all your variables at the beginning of the | |
1500 | program. When the new binary is executed it will begin by executing | |
1501 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). | |
1502 | Think of it as a goto with an intervening core dump and reincarnation. | |
1503 | If C<LABEL> is omitted, restarts the program from the top. | |
1504 | ||
1505 | B<WARNING>: Any files opened at the time of the dump will I<not> | |
1506 | be open any more when the program is reincarnated, with possible | |
1507 | resulting confusion by Perl. | |
1508 | ||
1509 | This function is now largely obsolete, mostly because it's very hard to | |
1510 | convert a core file into an executable. That's why you should now invoke | |
1511 | it as C<CORE::dump()>, if you don't want to be warned against a possible | |
1512 | typo. | |
1513 | ||
1514 | Portability issues: L<perlport/dump>. | |
1515 | ||
1516 | =item each HASH | |
1517 | X<each> X<hash, iterator> | |
1518 | ||
1519 | =item each ARRAY | |
1520 | X<array, iterator> | |
1521 | ||
1522 | =item each EXPR | |
1523 | ||
1524 | When called in list context, returns a 2-element list consisting of the key | |
1525 | and value for the next element of a hash, or the index and value for the | |
1526 | next element of an array, so that you can iterate over it. When called in | |
1527 | scalar context, returns only the key (not the value) in a hash, or the index | |
1528 | in an array. | |
1529 | ||
1530 | Hash entries are returned in an apparently random order. The actual random | |
1531 | order is subject to change in future versions of Perl, but it is | |
1532 | guaranteed to be in the same order as either the C<keys> or C<values> | |
1533 | function would produce on the same (unmodified) hash. Since Perl | |
1534 | 5.8.2 the ordering can be different even between different runs of Perl | |
1535 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). | |
1536 | ||
1537 | After C<each> has returned all entries from the hash or array, the next | |
1538 | call to C<each> returns the empty list in list context and C<undef> in | |
1539 | scalar context. The next call following that one restarts iteration. Each | |
1540 | hash or array has its own internal iterator, accessed by C<each>, C<keys>, | |
1541 | and C<values>. The iterator is implicitly reset when C<each> has reached | |
1542 | the end as just described; it can be explicitly reset by calling C<keys> or | |
1543 | C<values> on the hash or array. If you add or delete a hash's elements | |
1544 | while iterating over it, entries may be skipped or duplicated--so don't do | |
1545 | that. Exception: It is always safe to delete the item most recently | |
1546 | returned by C<each()>, so the following code works properly: | |
1547 | ||
1548 | while (($key, $value) = each %hash) { | |
1549 | print $key, "\n"; | |
1550 | delete $hash{$key}; # This is safe | |
1551 | } | |
1552 | ||
1553 | This prints out your environment like the printenv(1) program, | |
1554 | but in a different order: | |
1555 | ||
1556 | while (($key,$value) = each %ENV) { | |
1557 | print "$key=$value\n"; | |
1558 | } | |
1559 | ||
1560 | Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold | |
1561 | reference to an unblessed hash or array. The argument will be dereferenced | |
1562 | automatically. This aspect of C<each> is considered highly experimental. | |
1563 | The exact behaviour may change in a future version of Perl. | |
1564 | ||
1565 | while (($key,$value) = each $hashref) { ... } | |
1566 | ||
1567 | See also C<keys>, C<values>, and C<sort>. | |
1568 | ||
1569 | =item eof FILEHANDLE | |
1570 | X<eof> | |
1571 | X<end of file> | |
1572 | X<end-of-file> | |
1573 | ||
1574 | =item eof () | |
1575 | ||
1576 | =item eof | |
1577 | ||
1578 | Returns 1 if the next read on FILEHANDLE will return end of file I<or> if | |
1579 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value | |
1580 | gives the real filehandle. (Note that this function actually | |
1581 | reads a character and then C<ungetc>s it, so isn't useful in an | |
1582 | interactive context.) Do not read from a terminal file (or call | |
1583 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such | |
1584 | as terminals may lose the end-of-file condition if you do. | |
1585 | ||
1586 | An C<eof> without an argument uses the last file read. Using C<eof()> | |
1587 | with empty parentheses is different. It refers to the pseudo file | |
1588 | formed from the files listed on the command line and accessed via the | |
1589 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, | |
1590 | as a normal filehandle is, an C<eof()> before C<< <> >> has been | |
1591 | used will cause C<@ARGV> to be examined to determine if input is | |
1592 | available. Similarly, an C<eof()> after C<< <> >> has returned | |
1593 | end-of-file will assume you are processing another C<@ARGV> list, | |
1594 | and if you haven't set C<@ARGV>, will read input from C<STDIN>; | |
1595 | see L<perlop/"I/O Operators">. | |
1596 | ||
1597 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to | |
a6b91202 | 1598 | detect the end of each file, whereas C<eof()> will detect the end |
0909e3f8 RS |
1599 | of the very last file only. Examples: |
1600 | ||
1601 | # reset line numbering on each input file | |
1602 | while (<>) { | |
1603 | next if /^\s*#/; # skip comments | |
1604 | print "$.\t$_"; | |
1605 | } continue { | |
1606 | close ARGV if eof; # Not eof()! | |
1607 | } | |
1608 | ||
1609 | # insert dashes just before last line of last file | |
1610 | while (<>) { | |
1611 | if (eof()) { # check for end of last file | |
1612 | print "--------------\n"; | |
1613 | } | |
1614 | print; | |
1615 | last if eof(); # needed if we're reading from a terminal | |
1616 | } | |
1617 | ||
1618 | Practical hint: you almost never need to use C<eof> in Perl, because the | |
a6b91202 | 1619 | input operators typically return C<undef> when they run out of data or |
0909e3f8 RS |
1620 | encounter an error. |
1621 | ||
1622 | =item eval EXPR | |
1623 | X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> | |
1624 | X<error, handling> X<exception, handling> | |
1625 | ||
1626 | =item eval BLOCK | |
1627 | ||
1628 | =item eval | |
1629 | ||
1630 | In the first form, the return value of EXPR is parsed and executed as if it | |
1631 | were a little Perl program. The value of the expression (which is itself | |
1632 | determined within scalar context) is first parsed, and if there were no | |
1633 | errors, executed as a block within the lexical context of the current Perl | |
1634 | program. This means, that in particular, any outer lexical variables are | |
1635 | visible to it, and any package variable settings or subroutine and format | |
1636 | definitions remain afterwards. | |
1637 | ||
1638 | Note that the value is parsed every time the C<eval> executes. | |
1639 | If EXPR is omitted, evaluates C<$_>. This form is typically used to | |
1640 | delay parsing and subsequent execution of the text of EXPR until run time. | |
1641 | ||
1642 | If the C<unicode_eval> feature is enabled (which is the default under a | |
1643 | C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of | |
1644 | characters, so C<use utf8> declarations have no effect, and source filters | |
1645 | are forbidden. In the absence of the C<unicode_eval> feature, the string | |
1646 | will sometimes be treated as characters and sometimes as bytes, depending | |
1647 | on the internal encoding, and source filters activated within the C<eval> | |
1648 | exhibit the erratic, but historical, behaviour of affecting some outer file | |
1649 | scope that is still compiling. See also the L</evalbytes> keyword, which | |
1650 | always treats its input as a byte stream and works properly with source | |
1651 | filters, and the L<feature> pragma. | |
1652 | ||
1653 | In the second form, the code within the BLOCK is parsed only once--at the | |
1654 | same time the code surrounding the C<eval> itself was parsed--and executed | |
1655 | within the context of the current Perl program. This form is typically | |
1656 | used to trap exceptions more efficiently than the first (see below), while | |
1657 | also providing the benefit of checking the code within BLOCK at compile | |
1658 | time. | |
1659 | ||
1660 | The final semicolon, if any, may be omitted from the value of EXPR or within | |
1661 | the BLOCK. | |
1662 | ||
1663 | In both forms, the value returned is the value of the last expression | |
1664 | evaluated inside the mini-program; a return statement may be also used, just | |
1665 | as with subroutines. The expression providing the return value is evaluated | |
a6b91202 A |
1666 | in void, scalar, or list context, depending on the context of the C<eval> |
1667 | itself. See L</wantarray> for more on how the evaluation context can be | |
0909e3f8 RS |
1668 | determined. |
1669 | ||
1670 | If there is a syntax error or runtime error, or a C<die> statement is | |
1671 | executed, C<eval> returns C<undef> in scalar context | |
1672 | or an empty list--or, for syntax errors, a list containing a single | |
1673 | undefined value--in list context, and C<$@> is set to the error | |
1674 | message. The discrepancy in the return values in list context is | |
1675 | considered a bug by some, and will probably be fixed in a future | |
1676 | release. If there was no error, C<$@> is set to the empty string. A | |
1677 | control flow operator like C<last> or C<goto> can bypass the setting of | |
1678 | C<$@>. Beware that using C<eval> neither silences Perl from printing | |
1679 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. | |
1680 | To do either of those, you have to use the C<$SIG{__WARN__}> facility, or | |
1681 | turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. | |
1682 | See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. | |
1683 | ||
1684 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for | |
1685 | determining whether a particular feature (such as C<socket> or C<symlink>) | |
1686 | is implemented. It is also Perl's exception-trapping mechanism, where | |
1687 | the die operator is used to raise exceptions. | |
1688 | ||
1689 | If you want to trap errors when loading an XS module, some problems with | |
1690 | the binary interface (such as Perl version skew) may be fatal even with | |
1691 | C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>. | |
1692 | ||
1693 | If the code to be executed doesn't vary, you may use the eval-BLOCK | |
1694 | form to trap run-time errors without incurring the penalty of | |
1695 | recompiling each time. The error, if any, is still returned in C<$@>. | |
1696 | Examples: | |
1697 | ||
1698 | # make divide-by-zero nonfatal | |
1699 | eval { $answer = $a / $b; }; warn $@ if $@; | |
1700 | ||
1701 | # same thing, but less efficient | |
1702 | eval '$answer = $a / $b'; warn $@ if $@; | |
1703 | ||
1704 | # a compile-time error | |
1705 | eval { $answer = }; # WRONG | |
1706 | ||
1707 | # a run-time error | |
1708 | eval '$answer ='; # sets $@ | |
1709 | ||
1710 | Using the C<eval{}> form as an exception trap in libraries does have some | |
1711 | issues. Due to the current arguably broken state of C<__DIE__> hooks, you | |
1712 | may wish not to trigger any C<__DIE__> hooks that user code may have installed. | |
1713 | You can use the C<local $SIG{__DIE__}> construct for this purpose, | |
1714 | as this example shows: | |
1715 | ||
1716 | # a private exception trap for divide-by-zero | |
1717 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; | |
1718 | warn $@ if $@; | |
1719 | ||
1720 | This is especially significant, given that C<__DIE__> hooks can call | |
1721 | C<die> again, which has the effect of changing their error messages: | |
1722 | ||
1723 | # __DIE__ hooks may modify error messages | |
1724 | { | |
1725 | local $SIG{'__DIE__'} = | |
1726 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; | |
1727 | eval { die "foo lives here" }; | |
1728 | print $@ if $@; # prints "bar lives here" | |
1729 | } | |
1730 | ||
1731 | Because this promotes action at a distance, this counterintuitive behavior | |
1732 | may be fixed in a future release. | |
1733 | ||
1734 | With an C<eval>, you should be especially careful to remember what's | |
1735 | being looked at when: | |
1736 | ||
1737 | eval $x; # CASE 1 | |
1738 | eval "$x"; # CASE 2 | |
1739 | ||
1740 | eval '$x'; # CASE 3 | |
1741 | eval { $x }; # CASE 4 | |
1742 | ||
1743 | eval "\$$x++"; # CASE 5 | |
1744 | $$x++; # CASE 6 | |
1745 | ||
1746 | Cases 1 and 2 above behave identically: they run the code contained in | |
1747 | the variable $x. (Although case 2 has misleading double quotes making | |
1748 | the reader wonder what else might be happening (nothing is).) Cases 3 | |
1749 | and 4 likewise behave in the same way: they run the code C<'$x'>, which | |
1750 | does nothing but return the value of $x. (Case 4 is preferred for | |
1751 | purely visual reasons, but it also has the advantage of compiling at | |
1752 | compile-time instead of at run-time.) Case 5 is a place where | |
1753 | normally you I<would> like to use double quotes, except that in this | |
1754 | particular situation, you can just use symbolic references instead, as | |
1755 | in case 6. | |
1756 | ||
a6b91202 | 1757 | Before Perl 5.14, the assignment to C<$@> occurred before restoration |
0909e3f8 RS |
1758 | of localised variables, which means that for your code to run on older |
1759 | versions, a temporary is required if you want to mask some but not all | |
1760 | errors: | |
1761 | ||
1762 | # alter $@ on nefarious repugnancy only | |
1763 | { | |
1764 | my $e; | |
1765 | { | |
1766 | local $@; # protect existing $@ | |
1767 | eval { test_repugnancy() }; | |
1768 | # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only | |
1769 | $@ =~ /nefarious/ and $e = $@; | |
1770 | } | |
1771 | die $e if defined $e | |
1772 | } | |
1773 | ||
1774 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements | |
1775 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. | |
1776 | ||
1777 | An C<eval ''> executed within the C<DB> package doesn't see the usual | |
1778 | surrounding lexical scope, but rather the scope of the first non-DB piece | |
1779 | of code that called it. You don't normally need to worry about this unless | |
1780 | you are writing a Perl debugger. | |
1781 | ||
1782 | =item evalbytes EXPR | |
1783 | X<evalbytes> | |
1784 | ||
1785 | =item evalbytes | |
1786 | ||
1787 | This function is like L</eval> with a string argument, except it always | |
1788 | parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A | |
1789 | string containing characters whose ordinal value exceeds 255 results in an | |
1790 | error. Source filters activated within the evaluated code apply to the | |
1791 | code itself. | |
1792 | ||
1793 | This function is only available under the C<evalbytes> feature, a | |
1794 | C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See | |
1795 | L<feature> for more information. | |
1796 | ||
1797 | =item exec LIST | |
1798 | X<exec> X<execute> | |
1799 | ||
1800 | =item exec PROGRAM LIST | |
1801 | ||
1802 | The C<exec> function executes a system command I<and never returns>; | |
1803 | use C<system> instead of C<exec> if you want it to return. It fails and | |
1804 | returns false only if the command does not exist I<and> it is executed | |
1805 | directly instead of via your system's command shell (see below). | |
1806 | ||
1807 | Since it's a common mistake to use C<exec> instead of C<system>, Perl | |
1808 | warns you if there is a following statement that isn't C<die>, C<warn>, | |
1809 | or C<exit> (if C<-w> is set--but you always do that, right?). If you | |
1810 | I<really> want to follow an C<exec> with some other statement, you | |
1811 | can use one of these styles to avoid the warning: | |
1812 | ||
1813 | exec ('foo') or print STDERR "couldn't exec foo: $!"; | |
1814 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; | |
1815 | ||
1816 | If there is more than one argument in LIST, or if LIST is an array | |
1817 | with more than one value, calls execvp(3) with the arguments in LIST. | |
1818 | If there is only one scalar argument or an array with one element in it, | |
1819 | the argument is checked for shell metacharacters, and if there are any, | |
1820 | the entire argument is passed to the system's command shell for parsing | |
1821 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). | |
1822 | If there are no shell metacharacters in the argument, it is split into | |
1823 | words and passed directly to C<execvp>, which is more efficient. | |
1824 | Examples: | |
1825 | ||
1826 | exec '/bin/echo', 'Your arguments are: ', @ARGV; | |
1827 | exec "sort $outfile | uniq"; | |
1828 | ||
1829 | If you don't really want to execute the first argument, but want to lie | |
1830 | to the program you are executing about its own name, you can specify | |
1831 | the program you actually want to run as an "indirect object" (without a | |
1832 | comma) in front of the LIST. (This always forces interpretation of the | |
1833 | LIST as a multivalued list, even if there is only a single scalar in | |
1834 | the list.) Example: | |
1835 | ||
1836 | $shell = '/bin/csh'; | |
1837 | exec $shell '-sh'; # pretend it's a login shell | |
1838 | ||
1839 | or, more directly, | |
1840 | ||
1841 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell | |
1842 | ||
1843 | When the arguments get executed via the system shell, results are | |
1844 | subject to its quirks and capabilities. See L<perlop/"`STRING`"> | |
1845 | for details. | |
1846 | ||
1847 | Using an indirect object with C<exec> or C<system> is also more | |
1848 | secure. This usage (which also works fine with system()) forces | |
1849 | interpretation of the arguments as a multivalued list, even if the | |
1850 | list had just one argument. That way you're safe from the shell | |
1851 | expanding wildcards or splitting up words with whitespace in them. | |
1852 | ||
1853 | @args = ( "echo surprise" ); | |
1854 | ||
1855 | exec @args; # subject to shell escapes | |
1856 | # if @args == 1 | |
1857 | exec { $args[0] } @args; # safe even with one-arg list | |
1858 | ||
1859 | The first version, the one without the indirect object, ran the I<echo> | |
1860 | program, passing it C<"surprise"> an argument. The second version didn't; | |
1861 | it tried to run a program named I<"echo surprise">, didn't find it, and set | |
1862 | C<$?> to a non-zero value indicating failure. | |
1863 | ||
1864 | Beginning with v5.6.0, Perl attempts to flush all files opened for | |
1865 | output before the exec, but this may not be supported on some platforms | |
1866 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH | |
1867 | in English) or call the C<autoflush()> method of C<IO::Handle> on any | |
1868 | open handles to avoid lost output. | |
1869 | ||
1870 | Note that C<exec> will not call your C<END> blocks, nor will it invoke | |
1871 | C<DESTROY> methods on your objects. | |
1872 | ||
1873 | Portability issues: L<perlport/exec>. | |
1874 | ||
1875 | =item exists EXPR | |
1876 | X<exists> X<autovivification> | |
1877 | ||
1878 | Given an expression that specifies an element of a hash, returns true if the | |
1879 | specified element in the hash has ever been initialized, even if the | |
1880 | corresponding value is undefined. | |
1881 | ||
1882 | print "Exists\n" if exists $hash{$key}; | |
1883 | print "Defined\n" if defined $hash{$key}; | |
1884 | print "True\n" if $hash{$key}; | |
1885 | ||
1886 | exists may also be called on array elements, but its behavior is much less | |
1887 | obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware> | |
1888 | that calling exists on array values is deprecated and likely to be removed in | |
1889 | a future version of Perl. | |
1890 | ||
1891 | print "Exists\n" if exists $array[$index]; | |
1892 | print "Defined\n" if defined $array[$index]; | |
1893 | print "True\n" if $array[$index]; | |
1894 | ||
1895 | A hash or array element can be true only if it's defined and defined only if | |
1896 | it exists, but the reverse doesn't necessarily hold true. | |
1897 | ||
1898 | Given an expression that specifies the name of a subroutine, | |
1899 | returns true if the specified subroutine has ever been declared, even | |
1900 | if it is undefined. Mentioning a subroutine name for exists or defined | |
1901 | does not count as declaring it. Note that a subroutine that does not | |
1902 | exist may still be callable: its package may have an C<AUTOLOAD> | |
1903 | method that makes it spring into existence the first time that it is | |
1904 | called; see L<perlsub>. | |
1905 | ||
1906 | print "Exists\n" if exists &subroutine; | |
1907 | print "Defined\n" if defined &subroutine; | |
1908 | ||
1909 | Note that the EXPR can be arbitrarily complicated as long as the final | |
1910 | operation is a hash or array key lookup or subroutine name: | |
1911 | ||
1912 | if (exists $ref->{A}->{B}->{$key}) { } | |
1913 | if (exists $hash{A}{B}{$key}) { } | |
1914 | ||
1915 | if (exists $ref->{A}->{B}->[$ix]) { } | |
1916 | if (exists $hash{A}{B}[$ix]) { } | |
1917 | ||
1918 | if (exists &{$ref->{A}{B}{$key}}) { } | |
1919 | ||
1920 | Although the mostly deeply nested array or hash will not spring into | |
1921 | existence just because its existence was tested, any intervening ones will. | |
1922 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring | |
1923 | into existence due to the existence test for the $key element above. | |
1924 | This happens anywhere the arrow operator is used, including even here: | |
1925 | ||
1926 | undef $ref; | |
1927 | if (exists $ref->{"Some key"}) { } | |
1928 | print $ref; # prints HASH(0x80d3d5c) | |
1929 | ||
1930 | This surprising autovivification in what does not at first--or even | |
1931 | second--glance appear to be an lvalue context may be fixed in a future | |
1932 | release. | |
1933 | ||
1934 | Use of a subroutine call, rather than a subroutine name, as an argument | |
1935 | to exists() is an error. | |
1936 | ||
1937 | exists ⊂ # OK | |
1938 | exists &sub(); # Error | |
1939 | ||
1940 | =item exit EXPR | |
1941 | X<exit> X<terminate> X<abort> | |
1942 | ||
1943 | =item exit | |
1944 | ||
1945 | Evaluates EXPR and exits immediately with that value. Example: | |
1946 | ||
1947 | $ans = <STDIN>; | |
1948 | exit 0 if $ans =~ /^[Xx]/; | |
1949 | ||
1950 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only | |
1951 | universally recognized values for EXPR are C<0> for success and C<1> | |
1952 | for error; other values are subject to interpretation depending on the | |
1953 | environment in which the Perl program is running. For example, exiting | |
1954 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause | |
1955 | the mailer to return the item undelivered, but that's not true everywhere. | |
1956 | ||
1957 | Don't use C<exit> to abort a subroutine if there's any chance that | |
1958 | someone might want to trap whatever error happened. Use C<die> instead, | |
1959 | which can be trapped by an C<eval>. | |
1960 | ||
1961 | The exit() function does not always exit immediately. It calls any | |
1962 | defined C<END> routines first, but these C<END> routines may not | |
1963 | themselves abort the exit. Likewise any object destructors that need to | |
1964 | be called are called before the real exit. C<END> routines and destructors | |
1965 | can change the exit status by modifying C<$?>. If this is a problem, you | |
1966 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. | |
1967 | See L<perlmod> for details. | |
1968 | ||
1969 | Portability issues: L<perlport/exit>. | |
1970 | ||
1971 | =item exp EXPR | |
1972 | X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> | |
1973 | ||
1974 | =item exp | |
1975 | ||
1976 | Returns I<e> (the natural logarithm base) to the power of EXPR. | |
1977 | If EXPR is omitted, gives C<exp($_)>. | |
1978 | ||
1979 | =item fcntl FILEHANDLE,FUNCTION,SCALAR | |
1980 | X<fcntl> | |
1981 | ||
1982 | Implements the fcntl(2) function. You'll probably have to say | |
1983 | ||
1984 | use Fcntl; | |
1985 | ||
1986 | first to get the correct constant definitions. Argument processing and | |
1987 | value returned work just like C<ioctl> below. | |
1988 | For example: | |
1989 | ||
1990 | use Fcntl; | |
1991 | fcntl($filehandle, F_GETFL, $packed_return_buffer) | |
1992 | or die "can't fcntl F_GETFL: $!"; | |
1993 | ||
1994 | You don't have to check for C<defined> on the return from C<fcntl>. | |
1995 | Like C<ioctl>, it maps a C<0> return from the system call into | |
1996 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> | |
1997 | in numeric context. It is also exempt from the normal B<-w> warnings | |
1998 | on improper numeric conversions. | |
1999 | ||
2000 | Note that C<fcntl> raises an exception if used on a machine that | |
2001 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) | |
2002 | manpage to learn what functions are available on your system. | |
2003 | ||
2004 | Here's an example of setting a filehandle named C<REMOTE> to be | |
2005 | non-blocking at the system level. You'll have to negotiate C<$|> | |
2006 | on your own, though. | |
2007 | ||
2008 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); | |
2009 | ||
2010 | $flags = fcntl(REMOTE, F_GETFL, 0) | |
2011 | or die "Can't get flags for the socket: $!\n"; | |
2012 | ||
2013 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) | |
2014 | or die "Can't set flags for the socket: $!\n"; | |
2015 | ||
2016 | Portability issues: L<perlport/fcntl>. | |
2017 | ||
2018 | =item __FILE__ | |
2019 | X<__FILE__> | |
2020 | ||
2021 | A special token that returns the name of the file in which it occurs. | |
2022 | ||
2023 | =item fileno FILEHANDLE | |
2024 | X<fileno> | |
2025 | ||
2026 | Returns the file descriptor for a filehandle, or undefined if the | |
2027 | filehandle is not open. If there is no real file descriptor at the OS | |
2028 | level, as can happen with filehandles connected to memory objects via | |
2029 | C<open> with a reference for the third argument, -1 is returned. | |
2030 | ||
2031 | This is mainly useful for constructing | |
2032 | bitmaps for C<select> and low-level POSIX tty-handling operations. | |
2033 | If FILEHANDLE is an expression, the value is taken as an indirect | |
2034 | filehandle, generally its name. | |
2035 | ||
2036 | You can use this to find out whether two handles refer to the | |
2037 | same underlying descriptor: | |
2038 | ||
2039 | if (fileno(THIS) == fileno(THAT)) { | |
2040 | print "THIS and THAT are dups\n"; | |
2041 | } | |
2042 | ||
2043 | =item flock FILEHANDLE,OPERATION | |
2044 | X<flock> X<lock> X<locking> | |
2045 | ||
2046 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true | |
2047 | for success, false on failure. Produces a fatal error if used on a | |
2048 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). | |
2049 | C<flock> is Perl's portable file-locking interface, although it locks | |
2050 | entire files only, not records. | |
2051 | ||
2052 | Two potentially non-obvious but traditional C<flock> semantics are | |
2053 | that it waits indefinitely until the lock is granted, and that its locks | |
2054 | are B<merely advisory>. Such discretionary locks are more flexible, but | |
2055 | offer fewer guarantees. This means that programs that do not also use | |
a6b91202 | 2056 | C<flock> may modify files locked with C<flock>. See L<perlport>, |
0909e3f8 RS |
2057 | your port's specific documentation, and your system-specific local manpages |
2058 | for details. It's best to assume traditional behavior if you're writing | |
2059 | portable programs. (But if you're not, you should as always feel perfectly | |
2060 | free to write for your own system's idiosyncrasies (sometimes called | |
2061 | "features"). Slavish adherence to portability concerns shouldn't get | |
2062 | in the way of your getting your job done.) | |
2063 | ||
2064 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with | |
2065 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but | |
2066 | you can use the symbolic names if you import them from the L<Fcntl> module, | |
2067 | either individually, or as a group using the C<:flock> tag. LOCK_SH | |
2068 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN | |
2069 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with | |
2070 | LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking | |
2071 | waiting for the lock; check the return status to see if you got it. | |
2072 | ||
2073 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE | |
2074 | before locking or unlocking it. | |
2075 | ||
2076 | Note that the emulation built with lockf(3) doesn't provide shared | |
2077 | locks, and it requires that FILEHANDLE be open with write intent. These | |
2078 | are the semantics that lockf(3) implements. Most if not all systems | |
2079 | implement lockf(3) in terms of fcntl(2) locking, though, so the | |
2080 | differing semantics shouldn't bite too many people. | |
2081 | ||
2082 | Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE | |
2083 | be open with read intent to use LOCK_SH and requires that it be open | |
2084 | with write intent to use LOCK_EX. | |
2085 | ||
2086 | Note also that some versions of C<flock> cannot lock things over the | |
2087 | network; you would need to use the more system-specific C<fcntl> for | |
2088 | that. If you like you can force Perl to ignore your system's flock(2) | |
2089 | function, and so provide its own fcntl(2)-based emulation, by passing | |
2090 | the switch C<-Ud_flock> to the F<Configure> program when you configure | |
2091 | and build a new Perl. | |
2092 | ||
2093 | Here's a mailbox appender for BSD systems. | |
2094 | ||
2095 | use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants | |
2096 | ||
2097 | sub lock { | |
2098 | my ($fh) = @_; | |
2099 | flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; | |
2100 | ||
2101 | # and, in case someone appended while we were waiting... | |
2102 | seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; | |
2103 | } | |
2104 | ||
2105 | sub unlock { | |
2106 | my ($fh) = @_; | |
2107 | flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; | |
2108 | } | |
2109 | ||
2110 | open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") | |
2111 | or die "Can't open mailbox: $!"; | |
2112 | ||
2113 | lock($mbox); | |
2114 | print $mbox $msg,"\n\n"; | |
2115 | unlock($mbox); | |
2116 | ||
2117 | On systems that support a real flock(2), locks are inherited across fork() | |
2118 | calls, whereas those that must resort to the more capricious fcntl(2) | |
2119 | function lose their locks, making it seriously harder to write servers. | |
2120 | ||
2121 | See also L<DB_File> for other flock() examples. | |
2122 | ||
2123 | Portability issues: L<perlport/flock>. | |
2124 | ||
2125 | =item fork | |
2126 | X<fork> X<child> X<parent> | |
2127 | ||
2128 | Does a fork(2) system call to create a new process running the | |
2129 | same program at the same point. It returns the child pid to the | |
2130 | parent process, C<0> to the child process, or C<undef> if the fork is | |
2131 | unsuccessful. File descriptors (and sometimes locks on those descriptors) | |
2132 | are shared, while everything else is copied. On most systems supporting | |
2133 | fork(), great care has gone into making it extremely efficient (for | |
2134 | example, using copy-on-write technology on data pages), making it the | |
2135 | dominant paradigm for multitasking over the last few decades. | |
2136 | ||
2137 | Beginning with v5.6.0, Perl attempts to flush all files opened for | |
2138 | output before forking the child process, but this may not be supported | |
2139 | on some platforms (see L<perlport>). To be safe, you may need to set | |
2140 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of | |
2141 | C<IO::Handle> on any open handles to avoid duplicate output. | |
2142 | ||
2143 | If you C<fork> without ever waiting on your children, you will | |
2144 | accumulate zombies. On some systems, you can avoid this by setting | |
2145 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of | |
2146 | forking and reaping moribund children. | |
2147 | ||
2148 | Note that if your forked child inherits system file descriptors like | |
2149 | STDIN and STDOUT that are actually connected by a pipe or socket, even | |
2150 | if you exit, then the remote server (such as, say, a CGI script or a | |
2151 | backgrounded job launched from a remote shell) won't think you're done. | |
2152 | You should reopen those to F</dev/null> if it's any issue. | |
2153 | ||
2154 | On some platforms such as Windows, where the fork() system call is not available, | |
2155 | Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to, | |
2156 | at the level of the Perl program, be as compatible as possible with the "Unix" fork(). | |
2157 | However it has limitations that have to be considered in code intended to be portable. | |
2158 | See L<perlfork> for more details. | |
2159 | ||
2160 | Portability issues: L<perlport/fork>. | |
2161 | ||
2162 | =item format | |
2163 | X<format> | |
2164 | ||
2165 | Declare a picture format for use by the C<write> function. For | |
2166 | example: | |
2167 | ||
2168 | format Something = | |
2169 | Test: @<<<<<<<< @||||| @>>>>> | |
2170 | $str, $%, '$' . int($num) | |
2171 | . | |
2172 | ||
2173 | $str = "widget"; | |
2174 | $num = $cost/$quantity; | |
2175 | $~ = 'Something'; | |
2176 | write; | |
2177 | ||
2178 | See L<perlform> for many details and examples. | |
2179 | ||
2180 | =item formline PICTURE,LIST | |
2181 | X<formline> | |
2182 | ||
2183 | This is an internal function used by C<format>s, though you may call it, | |
2184 | too. It formats (see L<perlform>) a list of values according to the | |
2185 | contents of PICTURE, placing the output into the format output | |
2186 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). | |
2187 | Eventually, when a C<write> is done, the contents of | |
2188 | C<$^A> are written to some filehandle. You could also read C<$^A> | |
2189 | and then set C<$^A> back to C<"">. Note that a format typically | |
2190 | does one C<formline> per line of form, but the C<formline> function itself | |
2191 | doesn't care how many newlines are embedded in the PICTURE. This means | |
2192 | that the C<~> and C<~~> tokens treat the entire PICTURE as a single line. | |
2193 | You may therefore need to use multiple formlines to implement a single | |
2194 | record format, just like the C<format> compiler. | |
2195 | ||
2196 | Be careful if you put double quotes around the picture, because an C<@> | |
2197 | character may be taken to mean the beginning of an array name. | |
2198 | C<formline> always returns true. See L<perlform> for other examples. | |
2199 | ||
2200 | If you are trying to use this instead of C<write> to capture the output, | |
2201 | you may find it easier to open a filehandle to a scalar | |
2202 | (C<< open $fh, ">", \$output >>) and write to that instead. | |
2203 | ||
2204 | =item getc FILEHANDLE | |
2205 | X<getc> X<getchar> X<character> X<file, read> | |
2206 | ||
2207 | =item getc | |
2208 | ||
2209 | Returns the next character from the input file attached to FILEHANDLE, | |
2210 | or the undefined value at end of file or if there was an error (in | |
2211 | the latter case C<$!> is set). If FILEHANDLE is omitted, reads from | |
2212 | STDIN. This is not particularly efficient. However, it cannot be | |
2213 | used by itself to fetch single characters without waiting for the user | |
2214 | to hit enter. For that, try something more like: | |
2215 | ||
2216 | if ($BSD_STYLE) { | |
2217 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; | |
2218 | } | |
2219 | else { | |
2220 | system "stty", '-icanon', 'eol', "\001"; | |
2221 | } | |
2222 | ||
2223 | $key = getc(STDIN); | |
2224 | ||
2225 | if ($BSD_STYLE) { | |
2226 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; | |
2227 | } | |
2228 | else { | |
2229 | system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL | |
2230 | } | |
2231 | print "\n"; | |
2232 | ||
2233 | Determination of whether $BSD_STYLE should be set | |
2234 | is left as an exercise to the reader. | |
2235 | ||
2236 | The C<POSIX::getattr> function can do this more portably on | |
2237 | systems purporting POSIX compliance. See also the C<Term::ReadKey> | |
2238 | module from your nearest CPAN site; details on CPAN can be found under | |
2239 | L<perlmodlib/CPAN>. | |
2240 | ||
2241 | =item getlogin | |
2242 | X<getlogin> X<login> | |
2243 | ||
2244 | This implements the C library function of the same name, which on most | |
2245 | systems returns the current login from F</etc/utmp>, if any. If it | |
2246 | returns the empty string, use C<getpwuid>. | |
2247 | ||
2248 | $login = getlogin || getpwuid($<) || "Kilroy"; | |
2249 | ||
2250 | Do not consider C<getlogin> for authentication: it is not as | |
2251 | secure as C<getpwuid>. | |
2252 | ||
2253 | Portability issues: L<perlport/getlogin>. | |
2254 | ||
2255 | =item getpeername SOCKET | |
2256 | X<getpeername> X<peer> | |
2257 | ||
2258 | Returns the packed sockaddr address of the other end of the SOCKET | |
2259 | connection. | |
2260 | ||
2261 | use Socket; | |
2262 | $hersockaddr = getpeername(SOCK); | |
2263 | ($port, $iaddr) = sockaddr_in($hersockaddr); | |
2264 | $herhostname = gethostbyaddr($iaddr, AF_INET); | |
2265 | $herstraddr = inet_ntoa($iaddr); | |
2266 | ||
2267 | =item getpgrp PID | |
2268 | X<getpgrp> X<group> | |
2269 | ||
2270 | Returns the current process group for the specified PID. Use | |
2271 | a PID of C<0> to get the current process group for the | |
2272 | current process. Will raise an exception if used on a machine that | |
2273 | doesn't implement getpgrp(2). If PID is omitted, returns the process | |
2274 | group of the current process. Note that the POSIX version of C<getpgrp> | |
2275 | does not accept a PID argument, so only C<PID==0> is truly portable. | |
2276 | ||
2277 | Portability issues: L<perlport/getpgrp>. | |
2278 | ||
2279 | =item getppid | |
2280 | X<getppid> X<parent> X<pid> | |
2281 | ||
2282 | Returns the process id of the parent process. | |
2283 | ||
2284 | Note for Linux users: on Linux, the C functions C<getpid()> and | |
2285 | C<getppid()> return different values from different threads. In order to | |
2286 | be portable, this behavior is not reflected by the Perl-level function | |
2287 | C<getppid()>, that returns a consistent value across threads. If you want | |
2288 | to call the underlying C<getppid()>, you may use the CPAN module | |
2289 | C<Linux::Pid>. | |
2290 | ||
2291 | Portability issues: L<perlport/getppid>. | |
2292 | ||
2293 | =item getpriority WHICH,WHO | |
2294 | X<getpriority> X<priority> X<nice> | |
2295 | ||
2296 | Returns the current priority for a process, a process group, or a user. | |
2297 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a | |
2298 | machine that doesn't implement getpriority(2). | |
2299 | ||
2300 | Portability issues: L<perlport/getpriority>. | |
2301 | ||
2302 | =item getpwnam NAME | |
2303 | X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> | |
2304 | X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> | |
2305 | X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> | |
2306 | X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> | |
2307 | X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> | |
a6b91202 | 2308 | X<endnetent> X<endprotoent> X<endservent> |
0909e3f8 RS |
2309 | |
2310 | =item getgrnam NAME | |
2311 | ||
2312 | =item gethostbyname NAME | |
2313 | ||
2314 | =item getnetbyname NAME | |
2315 | ||
2316 | =item getprotobyname NAME | |
2317 | ||
2318 | =item getpwuid UID | |
2319 | ||
2320 | =item getgrgid GID | |
2321 | ||
2322 | =item getservbyname NAME,PROTO | |
2323 | ||
2324 | =item gethostbyaddr ADDR,ADDRTYPE | |
2325 | ||
2326 | =item getnetbyaddr ADDR,ADDRTYPE | |
2327 | ||
2328 | =item getprotobynumber NUMBER | |
2329 | ||
2330 | =item getservbyport PORT,PROTO | |
2331 | ||
2332 | =item getpwent | |
2333 | ||
2334 | =item getgrent | |
2335 | ||
2336 | =item gethostent | |
2337 | ||
2338 | =item getnetent | |
2339 | ||
2340 | =item getprotoent | |
2341 | ||
2342 | =item getservent | |
2343 | ||
2344 | =item setpwent | |
2345 | ||
2346 | =item setgrent | |
2347 | ||
2348 | =item sethostent STAYOPEN | |
2349 | ||
2350 | =item setnetent STAYOPEN | |
2351 | ||
2352 | =item setprotoent STAYOPEN | |
2353 | ||
2354 | =item setservent STAYOPEN | |
2355 | ||
2356 | =item endpwent | |
2357 | ||
2358 | =item endgrent | |
2359 | ||
2360 | =item endhostent | |
2361 | ||
2362 | =item endnetent | |
2363 | ||
2364 | =item endprotoent | |
2365 | ||
2366 | =item endservent | |
2367 | ||
2368 | These routines are the same as their counterparts in the | |
2369 | system C library. In list context, the return values from the | |
2370 | various get routines are as follows: | |
2371 | ||
2372 | ($name,$passwd,$uid,$gid, | |
2373 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* | |
2374 | ($name,$passwd,$gid,$members) = getgr* | |
2375 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* | |
2376 | ($name,$aliases,$addrtype,$net) = getnet* | |
2377 | ($name,$aliases,$proto) = getproto* | |
2378 | ($name,$aliases,$port,$proto) = getserv* | |
2379 | ||
2380 | (If the entry doesn't exist you get an empty list.) | |
2381 | ||
2382 | The exact meaning of the $gcos field varies but it usually contains | |
2383 | the real name of the user (as opposed to the login name) and other | |
2384 | information pertaining to the user. Beware, however, that in many | |
2385 | system users are able to change this information and therefore it | |
2386 | cannot be trusted and therefore the $gcos is tainted (see | |
2387 | L<perlsec>). The $passwd and $shell, user's encrypted password and | |
2388 | login shell, are also tainted, for the same reason. | |
2389 | ||
2390 | In scalar context, you get the name, unless the function was a | |
2391 | lookup by name, in which case you get the other thing, whatever it is. | |
2392 | (If the entry doesn't exist you get the undefined value.) For example: | |
2393 | ||
2394 | $uid = getpwnam($name); | |
2395 | $name = getpwuid($num); | |
2396 | $name = getpwent(); | |
2397 | $gid = getgrnam($name); | |
2398 | $name = getgrgid($num); | |
2399 | $name = getgrent(); | |
2400 | #etc. | |
2401 | ||
2402 | In I<getpw*()> the fields $quota, $comment, and $expire are special | |
2403 | in that they are unsupported on many systems. If the | |
2404 | $quota is unsupported, it is an empty scalar. If it is supported, it | |
2405 | usually encodes the disk quota. If the $comment field is unsupported, | |
2406 | it is an empty scalar. If it is supported it usually encodes some | |
2407 | administrative comment about the user. In some systems the $quota | |
2408 | field may be $change or $age, fields that have to do with password | |
2409 | aging. In some systems the $comment field may be $class. The $expire | |
2410 | field, if present, encodes the expiration period of the account or the | |
2411 | password. For the availability and the exact meaning of these fields | |
a6b91202 | 2412 | in your system, please consult getpwnam(3) and your system's |
0909e3f8 RS |
2413 | F<pwd.h> file. You can also find out from within Perl what your |
2414 | $quota and $comment fields mean and whether you have the $expire field | |
2415 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, | |
2416 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password | |
2417 | files are supported only if your vendor has implemented them in the | |
2418 | intuitive fashion that calling the regular C library routines gets the | |
2419 | shadow versions if you're running under privilege or if there exists | |
2420 | the shadow(3) functions as found in System V (this includes Solaris | |
2421 | and Linux). Those systems that implement a proprietary shadow password | |
2422 | facility are unlikely to be supported. | |
2423 | ||
2424 | The $members value returned by I<getgr*()> is a space-separated list of | |
2425 | the login names of the members of the group. | |
2426 | ||
2427 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in | |
2428 | C, it will be returned to you via C<$?> if the function call fails. The | |
2429 | C<@addrs> value returned by a successful call is a list of raw | |
2430 | addresses returned by the corresponding library call. In the | |
2431 | Internet domain, each address is four bytes long; you can unpack it | |
2432 | by saying something like: | |
2433 | ||
2434 | ($a,$b,$c,$d) = unpack('W4',$addr[0]); | |
2435 | ||
2436 | The Socket library makes this slightly easier: | |
2437 | ||
2438 | use Socket; | |
2439 | $iaddr = inet_aton("127.1"); # or whatever address | |
2440 | $name = gethostbyaddr($iaddr, AF_INET); | |
2441 | ||
2442 | # or going the other way | |
2443 | $straddr = inet_ntoa($iaddr); | |
2444 | ||
2445 | In the opposite way, to resolve a hostname to the IP address | |
2446 | you can write this: | |
2447 | ||
2448 | use Socket; | |
2449 | $packed_ip = gethostbyname("www.perl.org"); | |
2450 | if (defined $packed_ip) { | |
2451 | $ip_address = inet_ntoa($packed_ip); | |
2452 | } | |
2453 | ||
2454 | Make sure C<gethostbyname()> is called in SCALAR context and that | |
2455 | its return value is checked for definedness. | |
2456 | ||
2457 | The C<getprotobynumber> function, even though it only takes one argument, | |
2458 | has the precedence of a list operator, so beware: | |
2459 | ||
2460 | getprotobynumber $number eq 'icmp' # WRONG | |
2461 | getprotobynumber($number eq 'icmp') # actually means this | |
2462 | getprotobynumber($number) eq 'icmp' # better this way | |
2463 | ||
2464 | If you get tired of remembering which element of the return list | |
2465 | contains which return value, by-name interfaces are provided | |
2466 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, | |
2467 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, | |
2468 | and C<User::grent>. These override the normal built-ins, supplying | |
2469 | versions that return objects with the appropriate names | |
2470 | for each field. For example: | |
2471 | ||
2472 | use File::stat; | |
2473 | use User::pwent; | |
2474 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); | |
2475 | ||
2476 | Even though it looks as though they're the same method calls (uid), | |
2477 | they aren't, because a C<File::stat> object is different from | |
2478 | a C<User::pwent> object. | |
2479 | ||
2480 | Portability issues: L<perlport/getpwnam> to L<perlport/endservent>. | |
2481 | ||
2482 | =item getsockname SOCKET | |
2483 | X<getsockname> | |
2484 | ||
2485 | Returns the packed sockaddr address of this end of the SOCKET connection, | |
2486 | in case you don't know the address because you have several different | |
2487 | IPs that the connection might have come in on. | |
2488 | ||
2489 | use Socket; | |
2490 | $mysockaddr = getsockname(SOCK); | |
2491 | ($port, $myaddr) = sockaddr_in($mysockaddr); | |
2492 | printf "Connect to %s [%s]\n", | |
2493 | scalar gethostbyaddr($myaddr, AF_INET), | |
2494 | inet_ntoa($myaddr); | |
2495 | ||
2496 | =item getsockopt SOCKET,LEVEL,OPTNAME | |
2497 | X<getsockopt> | |
2498 | ||
2499 | Queries the option named OPTNAME associated with SOCKET at a given LEVEL. | |
2500 | Options may exist at multiple protocol levels depending on the socket | |
2501 | type, but at least the uppermost socket level SOL_SOCKET (defined in the | |
2502 | C<Socket> module) will exist. To query options at another level the | |
2503 | protocol number of the appropriate protocol controlling the option | |
2504 | should be supplied. For example, to indicate that an option is to be | |
2505 | interpreted by the TCP protocol, LEVEL should be set to the protocol | |
2506 | number of TCP, which you can get using C<getprotobyname>. | |
2507 | ||
2508 | The function returns a packed string representing the requested socket | |
2509 | option, or C<undef> on error, with the reason for the error placed in | |
2510 | C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME; | |
2511 | consult getsockopt(2) for details. A common case is that the option is an | |
2512 | integer, in which case the result is a packed integer, which you can decode | |
2513 | using C<unpack> with the C<i> (or C<I>) format. | |
2514 | ||
2515 | Here's an example to test whether Nagle's algorithm is enabled on a socket: | |
2516 | ||
2517 | use Socket qw(:all); | |
2518 | ||
2519 | defined(my $tcp = getprotobyname("tcp")) | |
2520 | or die "Could not determine the protocol number for tcp"; | |
2521 | # my $tcp = IPPROTO_TCP; # Alternative | |
2522 | my $packed = getsockopt($socket, $tcp, TCP_NODELAY) | |
2523 | or die "getsockopt TCP_NODELAY: $!"; | |
2524 | my $nodelay = unpack("I", $packed); | |
2525 | print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n"; | |
2526 | ||
2527 | Portability issues: L<perlport/getsockopt>. | |
2528 | ||
2529 | =item given EXPR BLOCK | |
2530 | X<given> | |
2531 | ||
2532 | =item given BLOCK | |
2533 | ||
2534 | C<given> is analogous to the C<switch> keyword in other languages. C<given> | |
2535 | and C<when> are used in Perl to implement C<switch>/C<case> like statements. | |
2536 | Only available after Perl 5.10. For example: | |
2537 | ||
2538 | use v5.10; | |
2539 | given ($fruit) { | |
2540 | when (/apples?/) { | |
2541 | print "I like apples." | |
2542 | } | |
2543 | when (/oranges?/) { | |
2544 | print "I don't like oranges." | |
2545 | } | |
2546 | default { | |
2547 | print "I don't like anything" | |
2548 | } | |
2549 | } | |
2550 | ||
2551 | See L<perlsyn/"Switch statements"> for detailed information. | |
2552 | ||
2553 | =item glob EXPR | |
2554 | X<glob> X<wildcard> X<filename, expansion> X<expand> | |
2555 | ||
2556 | =item glob | |
2557 | ||
2558 | In list context, returns a (possibly empty) list of filename expansions on | |
2559 | the value of EXPR such as the standard Unix shell F</bin/csh> would do. In | |
2560 | scalar context, glob iterates through such filename expansions, returning | |
2561 | undef when the list is exhausted. This is the internal function | |
2562 | implementing the C<< <*.c> >> operator, but you can use it directly. If | |
2563 | EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in | |
2564 | more detail in L<perlop/"I/O Operators">. | |
2565 | ||
2566 | Note that C<glob> splits its arguments on whitespace and treats | |
a6b91202 | 2567 | each segment as separate pattern. As such, C<glob("*.c *.h")> |
0909e3f8 RS |
2568 | matches all files with a F<.c> or F<.h> extension. The expression |
2569 | C<glob(".* *")> matches all files in the current working directory. | |
2570 | If you want to glob filenames that might contain whitespace, you'll | |
2571 | have to use extra quotes around the spacey filename to protect it. | |
2572 | For example, to glob filenames that have an C<e> followed by a space | |
2573 | followed by an C<f>, use either of: | |
2574 | ||
2575 | @spacies = <"*e f*">; | |
2576 | @spacies = glob '"*e f*"'; | |
2577 | @spacies = glob q("*e f*"); | |
2578 | ||
2579 | If you had to get a variable through, you could do this: | |
2580 | ||
2581 | @spacies = glob "'*${var}e f*'"; | |
2582 | @spacies = glob qq("*${var}e f*"); | |
2583 | ||
2584 | If non-empty braces are the only wildcard characters used in the | |
2585 | C<glob>, no filenames are matched, but potentially many strings | |
2586 | are returned. For example, this produces nine strings, one for | |
2587 | each pairing of fruits and colors: | |
2588 | ||
2589 | @many = glob "{apple,tomato,cherry}={green,yellow,red}"; | |
2590 | ||
2591 | Beginning with v5.6.0, this operator is implemented using the standard | |
2592 | C<File::Glob> extension. See L<File::Glob> for details, including | |
2593 | C<bsd_glob> which does not treat whitespace as a pattern separator. | |
2594 | ||
2595 | Portability issues: L<perlport/glob>. | |
2596 | ||
2597 | =item gmtime EXPR | |
2598 | X<gmtime> X<UTC> X<Greenwich> | |
2599 | ||
2600 | =item gmtime | |
2601 | ||
2602 | Works just like L</localtime> but the returned values are | |
2603 | localized for the standard Greenwich time zone. | |
2604 | ||
2605 | Note: When called in list context, $isdst, the last value | |
2606 | returned by gmtime, is always C<0>. There is no | |
2607 | Daylight Saving Time in GMT. | |
2608 | ||
2609 | Portability issues: L<perlport/gmtime>. | |
2610 | ||
2611 | =item goto LABEL | |
2612 | X<goto> X<jump> X<jmp> | |
2613 | ||
2614 | =item goto EXPR | |
2615 | ||
2616 | =item goto &NAME | |
2617 | ||
2618 | The C<goto-LABEL> form finds the statement labeled with LABEL and | |
2619 | resumes execution there. It can't be used to get out of a block or | |
2620 | subroutine given to C<sort>. It can be used to go almost anywhere | |
2621 | else within the dynamic scope, including out of subroutines, but it's | |
2622 | usually better to use some other construct such as C<last> or C<die>. | |
2623 | The author of Perl has never felt the need to use this form of C<goto> | |
2624 | (in Perl, that is; C is another matter). (The difference is that C | |
2625 | does not offer named loops combined with loop control. Perl does, and | |
2626 | this replaces most structured uses of C<goto> in other languages.) | |
2627 | ||
2628 | The C<goto-EXPR> form expects a label name, whose scope will be resolved | |
2629 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
2630 | necessarily recommended if you're optimizing for maintainability: | |
2631 | ||
2632 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
2633 | ||
2634 | As shown in this example, C<goto-EXPR> is exempt from the "looks like a | |
2635 | function" rule. A pair of parentheses following it does not (necessarily) | |
2636 | delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>. | |
2637 | ||
2638 | Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is | |
2639 | deprecated and will issue a warning. Even then, it may not be used to | |
2640 | go into any construct that requires initialization, such as a | |
2641 | subroutine or a C<foreach> loop. It also can't be used to go into a | |
2642 | construct that is optimized away. | |
2643 | ||
2644 | The C<goto-&NAME> form is quite different from the other forms of | |
2645 | C<goto>. In fact, it isn't a goto in the normal sense at all, and | |
2646 | doesn't have the stigma associated with other gotos. Instead, it | |
2647 | exits the current subroutine (losing any changes set by local()) and | |
2648 | immediately calls in its place the named subroutine using the current | |
2649 | value of @_. This is used by C<AUTOLOAD> subroutines that wish to | |
2650 | load another subroutine and then pretend that the other subroutine had | |
2651 | been called in the first place (except that any modifications to C<@_> | |
2652 | in the current subroutine are propagated to the other subroutine.) | |
2653 | After the C<goto>, not even C<caller> will be able to tell that this | |
2654 | routine was called first. | |
2655 | ||
2656 | NAME needn't be the name of a subroutine; it can be a scalar variable | |
2657 | containing a code reference or a block that evaluates to a code | |
2658 | reference. | |
2659 | ||
2660 | =item grep BLOCK LIST | |
2661 | X<grep> | |
2662 | ||
2663 | =item grep EXPR,LIST | |
2664 | ||
2665 | This is similar in spirit to, but not the same as, grep(1) and its | |
2666 | relatives. In particular, it is not limited to using regular expressions. | |
2667 | ||
2668 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting | |
2669 | C<$_> to each element) and returns the list value consisting of those | |
2670 | elements for which the expression evaluated to true. In scalar | |
2671 | context, returns the number of times the expression was true. | |
2672 | ||
2673 | @foo = grep(!/^#/, @bar); # weed out comments | |
2674 | ||
2675 | or equivalently, | |
2676 | ||
2677 | @foo = grep {!/^#/} @bar; # weed out comments | |
2678 | ||
2679 | Note that C<$_> is an alias to the list value, so it can be used to | |
2680 | modify the elements of the LIST. While this is useful and supported, | |
2681 | it can cause bizarre results if the elements of LIST are not variables. | |
2682 | Similarly, grep returns aliases into the original list, much as a for | |
2683 | loop's index variable aliases the list elements. That is, modifying an | |
2684 | element of a list returned by grep (for example, in a C<foreach>, C<map> | |
2685 | or another C<grep>) actually modifies the element in the original list. | |
2686 | This is usually something to be avoided when writing clear code. | |
2687 | ||
2688 | If C<$_> is lexical in the scope where the C<grep> appears (because it has | |
2689 | been declared with C<my $_>) then, in addition to being locally aliased to | |
2690 | the list elements, C<$_> keeps being lexical inside the block; i.e., it | |
2691 | can't be seen from the outside, avoiding any potential side-effects. | |
2692 | ||
2693 | See also L</map> for a list composed of the results of the BLOCK or EXPR. | |
2694 | ||
2695 | =item hex EXPR | |
2696 | X<hex> X<hexadecimal> | |
2697 | ||
2698 | =item hex | |
2699 | ||
2700 | Interprets EXPR as a hex string and returns the corresponding value. | |
2701 | (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see | |
2702 | L</oct>.) If EXPR is omitted, uses C<$_>. | |
2703 | ||
2704 | print hex '0xAf'; # prints '175' | |
2705 | print hex 'aF'; # same | |
2706 | ||
2707 | Hex strings may only represent integers. Strings that would cause | |
2708 | integer overflow trigger a warning. Leading whitespace is not stripped, | |
2709 | unlike oct(). To present something as hex, look into L</printf>, | |
2710 | L</sprintf>, and L</unpack>. | |
2711 | ||
2712 | =item import LIST | |
2713 | X<import> | |
2714 | ||
2715 | There is no builtin C<import> function. It is just an ordinary | |
2716 | method (subroutine) defined (or inherited) by modules that wish to export | |
2717 | names to another module. The C<use> function calls the C<import> method | |
2718 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. | |
2719 | ||
2720 | =item index STR,SUBSTR,POSITION | |
2721 | X<index> X<indexOf> X<InStr> | |
2722 | ||
2723 | =item index STR,SUBSTR | |
2724 | ||
2725 | The index function searches for one string within another, but without | |
2726 | the wildcard-like behavior of a full regular-expression pattern match. | |
2727 | It returns the position of the first occurrence of SUBSTR in STR at | |
2728 | or after POSITION. If POSITION is omitted, starts searching from the | |
2729 | beginning of the string. POSITION before the beginning of the string | |
2730 | or after its end is treated as if it were the beginning or the end, | |
2731 | respectively. POSITION and the return value are based at zero. | |
2732 | If the substring is not found, C<index> returns -1. | |
2733 | ||
2734 | =item int EXPR | |
2735 | X<int> X<integer> X<truncate> X<trunc> X<floor> | |
2736 | ||
2737 | =item int | |
2738 | ||
2739 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. | |
2740 | You should not use this function for rounding: one because it truncates | |
2741 | towards C<0>, and two because machine representations of floating-point | |
2742 | numbers can sometimes produce counterintuitive results. For example, | |
2743 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's | |
2744 | because it's really more like -268.99999999999994315658 instead. Usually, | |
2745 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> | |
2746 | functions will serve you better than will int(). | |
2747 | ||
2748 | =item ioctl FILEHANDLE,FUNCTION,SCALAR | |
2749 | X<ioctl> | |
2750 | ||
2751 | Implements the ioctl(2) function. You'll probably first have to say | |
2752 | ||
2753 | require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph | |
2754 | ||
2755 | to get the correct function definitions. If F<sys/ioctl.ph> doesn't | |
2756 | exist or doesn't have the correct definitions you'll have to roll your | |
2757 | own, based on your C header files such as F<< <sys/ioctl.h> >>. | |
2758 | (There is a Perl script called B<h2ph> that comes with the Perl kit that | |
2759 | may help you in this, but it's nontrivial.) SCALAR will be read and/or | |
2760 | written depending on the FUNCTION; a C pointer to the string value of SCALAR | |
2761 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR | |
2762 | has no string value but does have a numeric value, that value will be | |
2763 | passed rather than a pointer to the string value. To guarantee this to be | |
2764 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> | |
2765 | functions may be needed to manipulate the values of structures used by | |
2766 | C<ioctl>. | |
2767 | ||
2768 | The return value of C<ioctl> (and C<fcntl>) is as follows: | |
2769 | ||
2770 | if OS returns: then Perl returns: | |
2771 | -1 undefined value | |
2772 | 0 string "0 but true" | |
2773 | anything else that number | |
2774 | ||
2775 | Thus Perl returns true on success and false on failure, yet you can | |
2776 | still easily determine the actual value returned by the operating | |
2777 | system: | |
2778 | ||
2779 | $retval = ioctl(...) || -1; | |
2780 | printf "System returned %d\n", $retval; | |
2781 | ||
2782 | The special string C<"0 but true"> is exempt from B<-w> complaints | |
2783 | about improper numeric conversions. | |
2784 | ||
2785 | Portability issues: L<perlport/ioctl>. | |
2786 | ||
2787 | =item join EXPR,LIST | |
2788 | X<join> | |
2789 | ||
2790 | Joins the separate strings of LIST into a single string with fields | |
2791 | separated by the value of EXPR, and returns that new string. Example: | |
2792 | ||
2793 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); | |
2794 | ||
2795 | Beware that unlike C<split>, C<join> doesn't take a pattern as its | |
2796 | first argument. Compare L</split>. | |
2797 | ||
2798 | =item keys HASH | |
2799 | X<keys> X<key> | |
2800 | ||
2801 | =item keys ARRAY | |
2802 | ||
2803 | =item keys EXPR | |
2804 | ||
2805 | Returns a list consisting of all the keys of the named hash, or the indices | |
2806 | of an array. (In scalar context, returns the number of keys or indices.) | |
2807 | ||
2808 | The keys of a hash are returned in an apparently random order. The actual | |
2809 | random order is subject to change in future versions of Perl, but it | |
2810 | is guaranteed to be the same order as either the C<values> or C<each> | |
2811 | function produces (given that the hash has not been modified). Since | |
2812 | Perl 5.8.1 the ordering can be different even between different runs of | |
2813 | Perl for security reasons (see L<perlsec/"Algorithmic Complexity | |
2814 | Attacks">). | |
2815 | ||
2816 | As a side effect, calling keys() resets the internal interator of the HASH or ARRAY | |
2817 | (see L</each>). In particular, calling keys() in void context resets | |
2818 | the iterator with no other overhead. | |
2819 | ||
2820 | Here is yet another way to print your environment: | |
2821 | ||
2822 | @keys = keys %ENV; | |
2823 | @values = values %ENV; | |
2824 | while (@keys) { | |
2825 | print pop(@keys), '=', pop(@values), "\n"; | |
2826 | } | |
2827 | ||
2828 | or how about sorted by key: | |
2829 | ||
2830 | foreach $key (sort(keys %ENV)) { | |
2831 | print $key, '=', $ENV{$key}, "\n"; | |
2832 | } | |
2833 | ||
2834 | The returned values are copies of the original keys in the hash, so | |
2835 | modifying them will not affect the original hash. Compare L</values>. | |
2836 | ||
2837 | To sort a hash by value, you'll need to use a C<sort> function. | |
2838 | Here's a descending numeric sort of a hash by its values: | |
2839 | ||
2840 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { | |
2841 | printf "%4d %s\n", $hash{$key}, $key; | |
2842 | } | |
2843 | ||
2844 | Used as an lvalue, C<keys> allows you to increase the number of hash buckets | |
2845 | allocated for the given hash. This can gain you a measure of efficiency if | |
2846 | you know the hash is going to get big. (This is similar to pre-extending | |
2847 | an array by assigning a larger number to $#array.) If you say | |
2848 | ||
2849 | keys %hash = 200; | |
2850 | ||
2851 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, | |
2852 | in fact, since it rounds up to the next power of two. These | |
2853 | buckets will be retained even if you do C<%hash = ()>, use C<undef | |
2854 | %hash> if you want to free the storage while C<%hash> is still in scope. | |
2855 | You can't shrink the number of buckets allocated for the hash using | |
2856 | C<keys> in this way (but you needn't worry about doing this by accident, | |
2857 | as trying has no effect). C<keys @array> in an lvalue context is a syntax | |
2858 | error. | |
2859 | ||
2860 | Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain | |
2861 | a reference to an unblessed hash or array. The argument will be | |
2862 | dereferenced automatically. This aspect of C<keys> is considered highly | |
2863 | experimental. The exact behaviour may change in a future version of Perl. | |
2864 | ||
2865 | for (keys $hashref) { ... } | |
2866 | for (keys $obj->get_arrayref) { ... } | |
2867 | ||
2868 | See also C<each>, C<values>, and C<sort>. | |
2869 | ||
2870 | =item kill SIGNAL, LIST | |
2871 | X<kill> X<signal> | |
2872 | ||
2873 | Sends a signal to a list of processes. Returns the number of | |
2874 | processes successfully signaled (which is not necessarily the | |
2875 | same as the number actually killed). | |
2876 | ||
2877 | $cnt = kill 1, $child1, $child2; | |
2878 | kill 9, @goners; | |
2879 | ||
2880 | If SIGNAL is zero, no signal is sent to the process, but C<kill> | |
2881 | checks whether it's I<possible> to send a signal to it (that | |
2882 | means, to be brief, that the process is owned by the same user, or we are | |
2883 | the super-user). This is useful to check that a child process is still | |
2884 | alive (even if only as a zombie) and hasn't changed its UID. See | |
2885 | L<perlport> for notes on the portability of this construct. | |
2886 | ||
2887 | Unlike in the shell, if SIGNAL is negative, it kills process groups instead | |
2888 | of processes. That means you usually want to use positive not negative signals. | |
2889 | You may also use a signal name in quotes. | |
2890 | ||
2891 | The behavior of kill when a I<PROCESS> number is zero or negative depends on | |
2892 | the operating system. For example, on POSIX-conforming systems, zero will | |
2893 | signal the current process group and -1 will signal all processes. | |
2894 | ||
2895 | See L<perlipc/"Signals"> for more details. | |
2896 | ||
2897 | On some platforms such as Windows where the fork() system call is not available. | |
2898 | Perl can be built to emulate fork() at the interpreter level. | |
2899 | This emulation has limitations related to kill that have to be considered, | |
2900 | for code running on Windows and in code intended to be portable. | |
2901 | ||
2902 | See L<perlfork> for more details. | |
2903 | ||
2904 | Portability issues: L<perlport/kill>. | |
2905 | ||
2906 | =item last LABEL | |
2907 | X<last> X<break> | |
2908 | ||
2909 | =item last | |
2910 | ||
2911 | The C<last> command is like the C<break> statement in C (as used in | |
2912 | loops); it immediately exits the loop in question. If the LABEL is | |
2913 | omitted, the command refers to the innermost enclosing loop. The | |
2914 | C<continue> block, if any, is not executed: | |
2915 | ||
2916 | LINE: while (<STDIN>) { | |
2917 | last LINE if /^$/; # exit when done with header | |
2918 | #... | |
2919 | } | |
2920 | ||
2921 | C<last> cannot be used to exit a block that returns a value such as | |
2922 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit | |
2923 | a grep() or map() operation. | |
2924 | ||
2925 | Note that a block by itself is semantically identical to a loop | |
2926 | that executes once. Thus C<last> can be used to effect an early | |
2927 | exit out of such a block. | |
2928 | ||
2929 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
2930 | C<redo> work. | |
2931 | ||
2932 | =item lc EXPR | |
2933 | X<lc> X<lowercase> | |
2934 | ||
2935 | =item lc | |
2936 | ||
2937 | Returns a lowercased version of EXPR. This is the internal function | |
2938 | implementing the C<\L> escape in double-quoted strings. | |
2939 | ||
2940 | If EXPR is omitted, uses C<$_>. | |
2941 | ||
2942 | What gets returned depends on several factors: | |
2943 | ||
2944 | =over | |
2945 | ||
2946 | =item If C<use bytes> is in effect: | |
2947 | ||
2948 | =over | |
2949 | ||
2950 | =item On EBCDIC platforms | |
2951 | ||
2952 | The results are what the C language system call C<tolower()> returns. | |
2953 | ||
2954 | =item On ASCII platforms | |
2955 | ||
2956 | The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z> | |
2957 | respectively. | |
2958 | ||
2959 | =back | |
2960 | ||
2961 | =item Otherwise, If EXPR has the UTF8 flag set | |
2962 | ||
2963 | Unicode semantics are used for the case change. | |
2964 | ||
2965 | =item Otherwise, if C<use locale> is in effect | |
2966 | ||
2967 | Respects current LC_CTYPE locale. See L<perllocale>. | |
2968 | ||
2969 | =item Otherwise, if C<use feature 'unicode_strings'> is in effect: | |
2970 | ||
2971 | Unicode semantics are used for the case change. | |
2972 | ||
2973 | =item Otherwise: | |
2974 | ||
2975 | =over | |
2976 | ||
2977 | =item On EBCDIC platforms | |
2978 | ||
2979 | The results are what the C language system call C<tolower()> returns. | |
2980 | ||
2981 | =item On ASCII platforms | |
2982 | ||
2983 | ASCII semantics are used for the case change. The lowercase of any character | |
2984 | outside the ASCII range is the character itself. | |
2985 | ||
2986 | =back | |
2987 | ||
2988 | =back | |
2989 | ||
2990 | =item lcfirst EXPR | |
2991 | X<lcfirst> X<lowercase> | |
2992 | ||
2993 | =item lcfirst | |
2994 | ||
2995 | Returns the value of EXPR with the first character lowercased. This | |
2996 | is the internal function implementing the C<\l> escape in | |
2997 | double-quoted strings. | |
2998 | ||
2999 | If EXPR is omitted, uses C<$_>. | |
3000 | ||
3001 | This function behaves the same way under various pragmata, such as in a locale, | |
3002 | as L</lc> does. | |
3003 | ||
3004 | =item length EXPR | |
3005 | X<length> X<size> | |
3006 | ||
3007 | =item length | |
3008 | ||
3009 | Returns the length in I<characters> of the value of EXPR. If EXPR is | |
3010 | omitted, returns the length of C<$_>. If EXPR is undefined, returns | |
3011 | C<undef>. | |
3012 | ||
3013 | This function cannot be used on an entire array or hash to find out how | |
3014 | many elements these have. For that, use C<scalar @array> and C<scalar keys | |
3015 | %hash>, respectively. | |
3016 | ||
3017 | Like all Perl character operations, length() normally deals in logical | |
3018 | characters, not physical bytes. For how many bytes a string encoded as | |
3019 | UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have | |
3020 | to C<use Encode> first). See L<Encode> and L<perlunicode>. | |
3021 | ||
3022 | =item __LINE__ | |
3023 | X<__LINE__> | |
3024 | ||
3025 | A special token that compiles to the current line number. | |
3026 | ||
3027 | =item link OLDFILE,NEWFILE | |
3028 | X<link> | |
3029 | ||
3030 | Creates a new filename linked to the old filename. Returns true for | |
3031 | success, false otherwise. | |
3032 | ||
3033 | Portability issues: L<perlport/link>. | |
3034 | ||
3035 | =item listen SOCKET,QUEUESIZE | |
3036 | X<listen> | |
3037 | ||
3038 | Does the same thing that the listen(2) system call does. Returns true if | |
3039 | it succeeded, false otherwise. See the example in | |
3040 | L<perlipc/"Sockets: Client/Server Communication">. | |
3041 | ||
3042 | =item local EXPR | |
3043 | X<local> | |
3044 | ||
3045 | You really probably want to be using C<my> instead, because C<local> isn't | |
3046 | what most people think of as "local". See | |
3047 | L<perlsub/"Private Variables via my()"> for details. | |
3048 | ||
3049 | A local modifies the listed variables to be local to the enclosing | |
3050 | block, file, or eval. If more than one value is listed, the list must | |
3051 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> | |
3052 | for details, including issues with tied arrays and hashes. | |
3053 | ||
3054 | The C<delete local EXPR> construct can also be used to localize the deletion | |
3055 | of array/hash elements to the current block. | |
3056 | See L<perlsub/"Localized deletion of elements of composite types">. | |
3057 | ||
3058 | =item localtime EXPR | |
3059 | X<localtime> X<ctime> | |
3060 | ||
3061 | =item localtime | |
3062 | ||
3063 | Converts a time as returned by the time function to a 9-element list | |
3064 | with the time analyzed for the local time zone. Typically used as | |
3065 | follows: | |
3066 | ||
3067 | # 0 1 2 3 4 5 6 7 8 | |
3068 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = | |
3069 | localtime(time); | |
3070 | ||
3071 | All list elements are numeric and come straight out of the C `struct | |
3072 | tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours | |
3073 | of the specified time. | |
3074 | ||
3075 | C<$mday> is the day of the month and C<$mon> the month in | |
3076 | the range C<0..11>, with 0 indicating January and 11 indicating December. | |
3077 | This makes it easy to get a month name from a list: | |
3078 | ||
3079 | my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); | |
3080 | print "$abbr[$mon] $mday"; | |
3081 | # $mon=9, $mday=18 gives "Oct 18" | |
3082 | ||
3083 | C<$year> is the number of years since 1900, B<not> just the last two digits | |
3084 | of the year. That is, C<$year> is C<123> in year 2023. The proper way | |
3085 | to get a 4-digit year is simply: | |
3086 | ||
3087 | $year += 1900; | |
3088 | ||
3089 | Otherwise you create non-Y2K-compliant programs--and you wouldn't want | |
3090 | to do that, would you? | |
3091 | ||
3092 | To get the last two digits of the year (e.g., "01" in 2001) do: | |
3093 | ||
3094 | $year = sprintf("%02d", $year % 100); | |
3095 | ||
3096 | C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating | |
3097 | Wednesday. C<$yday> is the day of the year, in the range C<0..364> | |
3098 | (or C<0..365> in leap years.) | |
3099 | ||
3100 | C<$isdst> is true if the specified time occurs during Daylight Saving | |
3101 | Time, false otherwise. | |
3102 | ||
3103 | If EXPR is omitted, C<localtime()> uses the current time (as returned | |
3104 | by time(3)). | |
3105 | ||
3106 | In scalar context, C<localtime()> returns the ctime(3) value: | |
3107 | ||
3108 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" | |
3109 | ||
3110 | This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT | |
3111 | instead of local time use the L</gmtime> builtin. See also the | |
3112 | C<Time::Local> module (for converting seconds, minutes, hours, and such back to | |
3113 | the integer value returned by time()), and the L<POSIX> module's strftime(3) | |
3114 | and mktime(3) functions. | |
3115 | ||
3116 | To get somewhat similar but locale-dependent date strings, set up your | |
3117 | locale environment variables appropriately (please see L<perllocale>) and | |
3118 | try for example: | |
3119 | ||
3120 | use POSIX qw(strftime); | |
3121 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; | |
3122 | # or for GMT formatted appropriately for your locale: | |
3123 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; | |
3124 | ||
3125 | Note that the C<%a> and C<%b>, the short forms of the day of the week | |
3126 | and the month of the year, may not necessarily be three characters wide. | |
3127 | ||
3128 | The L<Time::gmtime> and L<Time::localtime> modules provide a convenient, | |
3129 | by-name access mechanism to the gmtime() and localtime() functions, | |
3130 | respectively. | |
3131 | ||
3132 | For a comprehensive date and time representation look at the | |
3133 | L<DateTime> module on CPAN. | |
3134 | ||
3135 | Portability issues: L<perlport/localtime>. | |
3136 | ||
3137 | =item lock THING | |
3138 | X<lock> | |
3139 | ||
3140 | This function places an advisory lock on a shared variable or referenced | |
3141 | object contained in I<THING> until the lock goes out of scope. | |
3142 | ||
3143 | The value returned is the scalar itself, if the argument is a scalar, or a | |
3144 | reference, if the argument is a hash, array or subroutine. | |
3145 | ||
3146 | lock() is a "weak keyword" : this means that if you've defined a function | |
3147 | by this name (before any calls to it), that function will be called | |
3148 | instead. If you are not under C<use threads::shared> this does nothing. | |
3149 | See L<threads::shared>. | |
3150 | ||
3151 | =item log EXPR | |
3152 | X<log> X<logarithm> X<e> X<ln> X<base> | |
3153 | ||
3154 | =item log | |
3155 | ||
3156 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, | |
3157 | returns the log of C<$_>. To get the | |
3158 | log of another base, use basic algebra: | |
3159 | The base-N log of a number is equal to the natural log of that number | |
3160 | divided by the natural log of N. For example: | |
3161 | ||
3162 | sub log10 { | |
3163 | my $n = shift; | |
3164 | return log($n)/log(10); | |
3165 | } | |
3166 | ||
3167 | See also L</exp> for the inverse operation. | |
3168 | ||
3169 | =item lstat FILEHANDLE | |
3170 | X<lstat> | |
3171 | ||
3172 | =item lstat EXPR | |
3173 | ||
3174 | =item lstat DIRHANDLE | |
3175 | ||
3176 | =item lstat | |
3177 | ||
3178 | Does the same thing as the C<stat> function (including setting the | |
3179 | special C<_> filehandle) but stats a symbolic link instead of the file | |
3180 | the symbolic link points to. If symbolic links are unimplemented on | |
3181 | your system, a normal C<stat> is done. For much more detailed | |
3182 | information, please see the documentation for C<stat>. | |
3183 | ||
3184 | If EXPR is omitted, stats C<$_>. | |
3185 | ||
3186 | Portability issues: L<perlport/lstat>. | |
3187 | ||
3188 | =item m// | |
3189 | ||
3190 | The match operator. See L<perlop/"Regexp Quote-Like Operators">. | |
3191 | ||
3192 | =item map BLOCK LIST | |
3193 | X<map> | |
3194 | ||
3195 | =item map EXPR,LIST | |
3196 | ||
3197 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting | |
3198 | C<$_> to each element) and returns the list value composed of the | |
3199 | results of each such evaluation. In scalar context, returns the | |
3200 | total number of elements so generated. Evaluates BLOCK or EXPR in | |
3201 | list context, so each element of LIST may produce zero, one, or | |
3202 | more elements in the returned value. | |
3203 | ||
3204 | @chars = map(chr, @numbers); | |
3205 | ||
3206 | translates a list of numbers to the corresponding characters. | |
3207 | ||
3208 | my @squares = map { $_ * $_ } @numbers; | |
3209 | ||
3210 | translates a list of numbers to their squared values. | |
3211 | ||
3212 | my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers; | |
3213 | ||
3214 | shows that number of returned elements can differ from the number of | |
3215 | input elements. To omit an element, return an empty list (). | |
3216 | This could also be achieved by writing | |
3217 | ||
3218 | my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers; | |
3219 | ||
3220 | which makes the intention more clear. | |
3221 | ||
3222 | Map always returns a list, which can be | |
3223 | assigned to a hash such that the elements | |
3224 | become key/value pairs. See L<perldata> for more details. | |
3225 | ||
3226 | %hash = map { get_a_key_for($_) => $_ } @array; | |
3227 | ||
3228 | is just a funny way to write | |
3229 | ||
3230 | %hash = (); | |
3231 | foreach (@array) { | |
3232 | $hash{get_a_key_for($_)} = $_; | |
3233 | } | |
3234 | ||
3235 | Note that C<$_> is an alias to the list value, so it can be used to | |
3236 | modify the elements of the LIST. While this is useful and supported, | |
3237 | it can cause bizarre results if the elements of LIST are not variables. | |
3238 | Using a regular C<foreach> loop for this purpose would be clearer in | |
3239 | most cases. See also L</grep> for an array composed of those items of | |
3240 | the original list for which the BLOCK or EXPR evaluates to true. | |
3241 | ||
3242 | If C<$_> is lexical in the scope where the C<map> appears (because it has | |
3243 | been declared with C<my $_>), then, in addition to being locally aliased to | |
3244 | the list elements, C<$_> keeps being lexical inside the block; that is, it | |
3245 | can't be seen from the outside, avoiding any potential side-effects. | |
3246 | ||
3247 | C<{> starts both hash references and blocks, so C<map { ...> could be either | |
3248 | the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look | |
3249 | ahead for the closing C<}> it has to take a guess at which it's dealing with | |
3250 | based on what it finds just after the C<{>. Usually it gets it right, but if it | |
3251 | doesn't it won't realize something is wrong until it gets to the C<}> and | |
3252 | encounters the missing (or unexpected) comma. The syntax error will be | |
3253 | reported close to the C<}>, but you'll need to change something near the C<{> | |
3254 | such as using a unary C<+> to give Perl some help: | |
3255 | ||
3256 | %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong | |
3257 | %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right | |
3258 | %hash = map { ("\L$_" => 1) } @array # this also works | |
3259 | %hash = map { lc($_) => 1 } @array # as does this. | |
3260 | %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! | |
3261 | ||
3262 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) | |
3263 | ||
3264 | or to force an anon hash constructor use C<+{>: | |
3265 | ||
3266 | @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end | |
3267 | ||
3268 | to get a list of anonymous hashes each with only one entry apiece. | |
3269 | ||
3270 | =item mkdir FILENAME,MASK | |
3271 | X<mkdir> X<md> X<directory, create> | |
3272 | ||
3273 | =item mkdir FILENAME | |
3274 | ||
3275 | =item mkdir | |
3276 | ||
3277 | Creates the directory specified by FILENAME, with permissions | |
3278 | specified by MASK (as modified by C<umask>). If it succeeds it | |
3279 | returns true; otherwise it returns false and sets C<$!> (errno). | |
3280 | MASK defaults to 0777 if omitted, and FILENAME defaults | |
3281 | to C<$_> if omitted. | |
3282 | ||
3283 | In general, it is better to create directories with a permissive MASK | |
3284 | and let the user modify that with their C<umask> than it is to supply | |
3285 | a restrictive MASK and give the user no way to be more permissive. | |
3286 | The exceptions to this rule are when the file or directory should be | |
3287 | kept private (mail files, for instance). The perlfunc(1) entry on | |
3288 | C<umask> discusses the choice of MASK in more detail. | |
3289 | ||
3290 | Note that according to the POSIX 1003.1-1996 the FILENAME may have any | |
3291 | number of trailing slashes. Some operating and filesystems do not get | |
3292 | this right, so Perl automatically removes all trailing slashes to keep | |
3293 | everyone happy. | |
3294 | ||
3295 | To recursively create a directory structure, look at | |
3296 | the C<mkpath> function of the L<File::Path> module. | |
3297 | ||
3298 | =item msgctl ID,CMD,ARG | |
3299 | X<msgctl> | |
3300 | ||
3301 | Calls the System V IPC function msgctl(2). You'll probably have to say | |
3302 | ||
3303 | use IPC::SysV; | |
3304 | ||
3305 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, | |
3306 | then ARG must be a variable that will hold the returned C<msqid_ds> | |
3307 | structure. Returns like C<ioctl>: the undefined value for error, | |
3308 | C<"0 but true"> for zero, or the actual return value otherwise. See also | |
3309 | L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and | |
3310 | C<IPC::Semaphore>. | |
3311 | ||
3312 | Portability issues: L<perlport/msgctl>. | |
3313 | ||
3314 | =item msgget KEY,FLAGS | |
3315 | X<msgget> | |
3316 | ||
3317 | Calls the System V IPC function msgget(2). Returns the message queue | |
3318 | id, or C<undef> on error. See also | |
3319 | L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and | |
3320 | C<IPC::Msg>. | |
3321 | ||
3322 | Portability issues: L<perlport/msgget>. | |
3323 | ||
3324 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS | |
3325 | X<msgrcv> | |
3326 | ||
3327 | Calls the System V IPC function msgrcv to receive a message from | |
3328 | message queue ID into variable VAR with a maximum message size of | |
3329 | SIZE. Note that when a message is received, the message type as a | |
3330 | native long integer will be the first thing in VAR, followed by the | |
3331 | actual message. This packing may be opened with C<unpack("l! a*")>. | |
a6b91202 | 3332 | Taints the variable. Returns true if successful, false |
0909e3f8 RS |
3333 | on error. See also L<perlipc/"SysV IPC"> and the documentation for |
3334 | C<IPC::SysV> and C<IPC::SysV::Msg>. | |
3335 | ||
3336 | Portability issues: L<perlport/msgrcv>. | |
3337 | ||
3338 | =item msgsnd ID,MSG,FLAGS | |
3339 | X<msgsnd> | |
3340 | ||
3341 | Calls the System V IPC function msgsnd to send the message MSG to the | |
3342 | message queue ID. MSG must begin with the native long integer message | |
3343 | type, be followed by the length of the actual message, and then finally | |
3344 | the message itself. This kind of packing can be achieved with | |
3345 | C<pack("l! a*", $type, $message)>. Returns true if successful, | |
3346 | false on error. See also the C<IPC::SysV> | |
3347 | and C<IPC::SysV::Msg> documentation. | |
3348 | ||
3349 | Portability issues: L<perlport/msgsnd>. | |
3350 | ||
3351 | =item my EXPR | |
3352 | X<my> | |
3353 | ||
3354 | =item my TYPE EXPR | |
3355 | ||
3356 | =item my EXPR : ATTRS | |
3357 | ||
3358 | =item my TYPE EXPR : ATTRS | |
3359 | ||
3360 | A C<my> declares the listed variables to be local (lexically) to the | |
3361 | enclosing block, file, or C<eval>. If more than one value is listed, | |
3362 | the list must be placed in parentheses. | |
3363 | ||
3364 | The exact semantics and interface of TYPE and ATTRS are still | |
3365 | evolving. TYPE is currently bound to the use of the C<fields> pragma, | |
3366 | and attributes are handled using the C<attributes> pragma, or starting | |
3367 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3368 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3369 | L<attributes>, and L<Attribute::Handlers>. | |
3370 | ||
3371 | =item next LABEL | |
3372 | X<next> X<continue> | |
3373 | ||
3374 | =item next | |
3375 | ||
3376 | The C<next> command is like the C<continue> statement in C; it starts | |
3377 | the next iteration of the loop: | |
3378 | ||
3379 | LINE: while (<STDIN>) { | |
3380 | next LINE if /^#/; # discard comments | |
3381 | #... | |
3382 | } | |
3383 | ||
3384 | Note that if there were a C<continue> block on the above, it would get | |
3385 | executed even on discarded lines. If LABEL is omitted, the command | |
3386 | refers to the innermost enclosing loop. | |
3387 | ||
3388 | C<next> cannot be used to exit a block which returns a value such as | |
3389 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit | |
3390 | a grep() or map() operation. | |
3391 | ||
3392 | Note that a block by itself is semantically identical to a loop | |
3393 | that executes once. Thus C<next> will exit such a block early. | |
3394 | ||
3395 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
3396 | C<redo> work. | |
3397 | ||
3398 | =item no MODULE VERSION LIST | |
3399 | X<no declarations> | |
3400 | X<unimporting> | |
3401 | ||
3402 | =item no MODULE VERSION | |
3403 | ||
3404 | =item no MODULE LIST | |
3405 | ||
3406 | =item no MODULE | |
3407 | ||
3408 | =item no VERSION | |
3409 | ||
3410 | See the C<use> function, of which C<no> is the opposite. | |
3411 | ||
3412 | =item oct EXPR | |
3413 | X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> | |
3414 | ||
3415 | =item oct | |
3416 | ||
3417 | Interprets EXPR as an octal string and returns the corresponding | |
3418 | value. (If EXPR happens to start off with C<0x>, interprets it as a | |
3419 | hex string. If EXPR starts off with C<0b>, it is interpreted as a | |
3420 | binary string. Leading whitespace is ignored in all three cases.) | |
3421 | The following will handle decimal, binary, octal, and hex in standard | |
3422 | Perl notation: | |
3423 | ||
3424 | $val = oct($val) if $val =~ /^0/; | |
3425 | ||
3426 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number | |
3427 | in octal), use sprintf() or printf(): | |
3428 | ||
3429 | $dec_perms = (stat("filename"))[2] & 07777; | |
3430 | $oct_perm_str = sprintf "%o", $perms; | |
3431 | ||
3432 | The oct() function is commonly used when a string such as C<644> needs | |
a6b91202 | 3433 | to be converted into a file mode, for example. Although Perl |
0909e3f8 RS |
3434 | automatically converts strings into numbers as needed, this automatic |
3435 | conversion assumes base 10. | |
3436 | ||
a6b91202 | 3437 | Leading white space is ignored without warning, as too are any trailing |
0909e3f8 RS |
3438 | non-digits, such as a decimal point (C<oct> only handles non-negative |
3439 | integers, not negative integers or floating point). | |
3440 | ||
3441 | =item open FILEHANDLE,EXPR | |
3442 | X<open> X<pipe> X<file, open> X<fopen> | |
3443 | ||
3444 | =item open FILEHANDLE,MODE,EXPR | |
3445 | ||
3446 | =item open FILEHANDLE,MODE,EXPR,LIST | |
3447 | ||
3448 | =item open FILEHANDLE,MODE,REFERENCE | |
3449 | ||
3450 | =item open FILEHANDLE | |
3451 | ||
3452 | Opens the file whose filename is given by EXPR, and associates it with | |
3453 | FILEHANDLE. | |
3454 | ||
3455 | Simple examples to open a file for reading: | |
3456 | ||
a6b91202 | 3457 | open(my $fh, "<", "input.txt") |
0909e3f8 RS |
3458 | or die "cannot open < input.txt: $!"; |
3459 | ||
3460 | and for writing: | |
3461 | ||
a6b91202 | 3462 | open(my $fh, ">", "output.txt") |
0909e3f8 RS |
3463 | or die "cannot open > output.txt: $!"; |
3464 | ||
3465 | (The following is a comprehensive reference to open(): for a gentler | |
3466 | introduction you may consider L<perlopentut>.) | |
3467 | ||
3468 | If FILEHANDLE is an undefined scalar variable (or array or hash element), a | |
3469 | new filehandle is autovivified, meaning that the variable is assigned a | |
3470 | reference to a newly allocated anonymous filehandle. Otherwise if | |
3471 | FILEHANDLE is an expression, its value is the real filehandle. (This is | |
3472 | considered a symbolic reference, so C<use strict "refs"> should I<not> be | |
3473 | in effect.) | |
3474 | ||
3475 | If EXPR is omitted, the global (package) scalar variable of the same | |
a6b91202 | 3476 | name as the FILEHANDLE contains the filename. (Note that lexical |
0909e3f8 RS |
3477 | variables--those declared with C<my> or C<state>--will not work for this |
3478 | purpose; so if you're using C<my> or C<state>, specify EXPR in your | |
3479 | call to open.) | |
3480 | ||
3481 | If three (or more) arguments are specified, the open mode (including | |
3482 | optional encoding) in the second argument are distinct from the filename in | |
3483 | the third. If MODE is C<< < >> or nothing, the file is opened for input. | |
3484 | If MODE is C<< > >>, the file is opened for output, with existing files | |
3485 | first being truncated ("clobbered") and nonexisting files newly created. | |
3486 | If MODE is C<<< >> >>>, the file is opened for appending, again being | |
3487 | created if necessary. | |
3488 | ||
3489 | You can put a C<+> in front of the C<< > >> or C<< < >> to | |
3490 | indicate that you want both read and write access to the file; thus | |
a6b91202 | 3491 | C<< +< >> is almost always preferred for read/write updates--the |
0909e3f8 RS |
3492 | C<< +> >> mode would clobber the file first. You cant usually use |
3493 | either read-write mode for updating textfiles, since they have | |
3494 | variable-length records. See the B<-i> switch in L<perlrun> for a | |
3495 | better approach. The file is created with permissions of C<0666> | |
3496 | modified by the process's C<umask> value. | |
3497 | ||
3498 | These various prefixes correspond to the fopen(3) modes of C<r>, | |
3499 | C<r+>, C<w>, C<w+>, C<a>, and C<a+>. | |
3500 | ||
3501 | In the one- and two-argument forms of the call, the mode and filename | |
3502 | should be concatenated (in that order), preferably separated by white | |
3503 | space. You can--but shouldn't--omit the mode in these forms when that mode | |
3504 | is C<< < >>. It is always safe to use the two-argument form of C<open> if | |
3505 | the filename argument is a known literal. | |
3506 | ||
3507 | For three or more arguments if MODE is C<|->, the filename is | |
3508 | interpreted as a command to which output is to be piped, and if MODE | |
3509 | is C<-|>, the filename is interpreted as a command that pipes | |
3510 | output to us. In the two-argument (and one-argument) form, one should | |
3511 | replace dash (C<->) with the command. | |
3512 | See L<perlipc/"Using open() for IPC"> for more examples of this. | |
3513 | (You are not allowed to C<open> to a command that pipes both in I<and> | |
3514 | out, but see L<IPC::Open2>, L<IPC::Open3>, and | |
3515 | L<perlipc/"Bidirectional Communication with Another Process"> for | |
3516 | alternatives.) | |
3517 | ||
3518 | In the form of pipe opens taking three or more arguments, if LIST is specified | |
3519 | (extra arguments after the command name) then LIST becomes arguments | |
3520 | to the command invoked if the platform supports it. The meaning of | |
3521 | C<open> with more than three arguments for non-pipe modes is not yet | |
3522 | defined, but experimental "layers" may give extra LIST arguments | |
3523 | meaning. | |
3524 | ||
a6b91202 | 3525 | In the two-argument (and one-argument) form, opening C<< <- >> |
0909e3f8 RS |
3526 | or C<-> opens STDIN and opening C<< >- >> opens STDOUT. |
3527 | ||
3528 | You may (and usually should) use the three-argument form of open to specify | |
3529 | I/O layers (sometimes referred to as "disciplines") to apply to the handle | |
3530 | that affect how the input and output are processed (see L<open> and | |
3531 | L<PerlIO> for more details). For example: | |
3532 | ||
3533 | open(my $fh, "<:encoding(UTF-8)", "filename") | |
3534 | || die "can't open UTF-8 encoded filename: $!"; | |
3535 | ||
3536 | opens the UTF8-encoded file containing Unicode characters; | |
3537 | see L<perluniintro>. Note that if layers are specified in the | |
3538 | three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>; | |
3539 | usually set by the B<open> pragma or the switch B<-CioD>) are ignored. | |
3540 | Those layers will also be ignored if you specifying a colon with no name | |
3541 | following it. In that case the default layer for the operating system | |
3542 | (:raw on Unix, :crlf on Windows) is used. | |
3543 | ||
3544 | Open returns nonzero on success, the undefined value otherwise. If | |
3545 | the C<open> involved a pipe, the return value happens to be the pid of | |
3546 | the subprocess. | |
3547 | ||
3548 | If you're running Perl on a system that distinguishes between text | |
3549 | files and binary files, then you should check out L</binmode> for tips | |
3550 | for dealing with this. The key distinction between systems that need | |
3551 | C<binmode> and those that don't is their text file formats. Systems | |
3552 | like Unix, Mac OS, and Plan 9, that end lines with a single | |
3553 | character and encode that character in C as C<"\n"> do not | |
3554 | need C<binmode>. The rest need it. | |
3555 | ||
a6b91202 | 3556 | When opening a file, it's seldom a good idea to continue |
0909e3f8 RS |
3557 | if the request failed, so C<open> is frequently used with |
3558 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, | |
3559 | where you want to format a suitable error message (but there are | |
3560 | modules that can help with that problem)) always check | |
a6b91202 | 3561 | the return value from opening a file. |
0909e3f8 RS |
3562 | |
3563 | As a special case the three-argument form with a read/write mode and the third | |
3564 | argument being C<undef>: | |
3565 | ||
3566 | open(my $tmp, "+>", undef) or die ... | |
3567 | ||
3568 | opens a filehandle to an anonymous temporary file. Also using C<< +< >> | |
3569 | works for symmetry, but you really should consider writing something | |
3570 | to the temporary file first. You will need to seek() to do the | |
3571 | reading. | |
3572 | ||
3573 | Since v5.8.0, Perl has built using PerlIO by default. Unless you've | |
3574 | changed this (such as building Perl with C<Configure -Uuseperlio>), you can | |
3575 | open filehandles directly to Perl scalars via: | |
3576 | ||
3577 | open($fh, ">", \$variable) || .. | |
3578 | ||
3579 | To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first: | |
3580 | ||
3581 | close STDOUT; | |
3582 | open(STDOUT, ">", \$variable) | |
3583 | or die "Can't open STDOUT: $!"; | |
3584 | ||
3585 | General examples: | |
3586 | ||
3587 | $ARTICLE = 100; | |
3588 | open(ARTICLE) or die "Can't find article $ARTICLE: $!\n"; | |
3589 | while (<ARTICLE>) {... | |
3590 | ||
3591 | open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved) | |
3592 | # if the open fails, output is discarded | |
3593 | ||
3594 | open(my $dbase, "+<", "dbase.mine") # open for update | |
3595 | or die "Can't open 'dbase.mine' for update: $!"; | |
3596 | ||
3597 | open(my $dbase, "+<dbase.mine") # ditto | |
3598 | or die "Can't open 'dbase.mine' for update: $!"; | |
3599 | ||
3600 | open(ARTICLE, "-|", "caesar <$article") # decrypt article | |
3601 | or die "Can't start caesar: $!"; | |
3602 | ||
3603 | open(ARTICLE, "caesar <$article |") # ditto | |
3604 | or die "Can't start caesar: $!"; | |
3605 | ||
3606 | open(EXTRACT, "|sort >Tmp$$") # $$ is our process id | |
3607 | or die "Can't start sort: $!"; | |
3608 | ||
3609 | # in-memory files | |
3610 | open(MEMORY, ">", \$var) | |
3611 | or die "Can't open memory file: $!"; | |
3612 | print MEMORY "foo!\n"; # output will appear in $var | |
3613 | ||
3614 | # process argument list of files along with any includes | |
3615 | ||
3616 | foreach $file (@ARGV) { | |
3617 | process($file, "fh00"); | |
3618 | } | |
3619 | ||
3620 | sub process { | |
3621 | my($filename, $input) = @_; | |
3622 | $input++; # this is a string increment | |
3623 | unless (open($input, "<", $filename)) { | |
3624 | print STDERR "Can't open $filename: $!\n"; | |
3625 | return; | |
3626 | } | |
3627 | ||
3628 | local $_; | |
3629 | while (<$input>) { # note use of indirection | |
3630 | if (/^#include "(.*)"/) { | |
3631 | process($1, $input); | |
3632 | next; | |
3633 | } | |
3634 | #... # whatever | |
3635 | } | |
3636 | } | |
3637 | ||
3638 | See L<perliol> for detailed info on PerlIO. | |
3639 | ||
3640 | You may also, in the Bourne shell tradition, specify an EXPR beginning | |
3641 | with C<< >& >>, in which case the rest of the string is interpreted | |
3642 | as the name of a filehandle (or file descriptor, if numeric) to be | |
3643 | duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>, | |
3644 | C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. | |
3645 | The mode you specify should match the mode of the original filehandle. | |
3646 | (Duping a filehandle does not take into account any existing contents | |
3647 | of IO buffers.) If you use the three-argument form, then you can pass either a | |
3648 | number, the name of a filehandle, or the normal "reference to a glob". | |
3649 | ||
3650 | Here is a script that saves, redirects, and restores C<STDOUT> and | |
3651 | C<STDERR> using various methods: | |
3652 | ||
3653 | #!/usr/bin/perl | |
3654 | open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!"; | |
3655 | open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!"; | |
3656 | ||
3657 | open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!"; | |
3658 | open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!"; | |
3659 | ||
3660 | select STDERR; $| = 1; # make unbuffered | |
3661 | select STDOUT; $| = 1; # make unbuffered | |
3662 | ||
3663 | print STDOUT "stdout 1\n"; # this works for | |
3664 | print STDERR "stderr 1\n"; # subprocesses too | |
3665 | ||
3666 | open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!"; | |
3667 | open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!"; | |
3668 | ||
3669 | print STDOUT "stdout 2\n"; | |
3670 | print STDERR "stderr 2\n"; | |
3671 | ||
3672 | If you specify C<< '<&=X' >>, where C<X> is a file descriptor number | |
3673 | or a filehandle, then Perl will do an equivalent of C's C<fdopen> of | |
3674 | that file descriptor (and not call C<dup(2)>); this is more | |
3675 | parsimonious of file descriptors. For example: | |
3676 | ||
3677 | # open for input, reusing the fileno of $fd | |
3678 | open(FILEHANDLE, "<&=$fd") | |
3679 | ||
3680 | or | |
3681 | ||
3682 | open(FILEHANDLE, "<&=", $fd) | |
3683 | ||
3684 | or | |
3685 | ||
3686 | # open for append, using the fileno of OLDFH | |
3687 | open(FH, ">>&=", OLDFH) | |
3688 | ||
3689 | or | |
3690 | ||
3691 | open(FH, ">>&=OLDFH") | |
3692 | ||
3693 | Being parsimonious on filehandles is also useful (besides being | |
3694 | parsimonious) for example when something is dependent on file | |
3695 | descriptors, like for example locking using flock(). If you do just | |
3696 | C<< open(A, ">>&B") >>, the filehandle A will not have the same file | |
3697 | descriptor as B, and therefore flock(A) will not flock(B) nor vice | |
3698 | versa. But with C<< open(A, ">>&=B") >>, the filehandles will share | |
3699 | the same underlying system file descriptor. | |
3700 | ||
3701 | Note that under Perls older than 5.8.0, Perl uses the standard C library's' | |
3702 | fdopen() to implement the C<=> functionality. On many Unix systems, | |
3703 | fdopen() fails when file descriptors exceed a certain value, typically 255. | |
3704 | For Perls 5.8.0 and later, PerlIO is (most often) the default. | |
3705 | ||
3706 | You can see whether your Perl was built with PerlIO by running C<perl -V> | |
3707 | and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you | |
3708 | have PerlIO; otherwise you don't. | |
3709 | ||
3710 | If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|> | |
a6b91202 | 3711 | with the one- or two-argument forms of C<open>), |
0909e3f8 RS |
3712 | an implicit C<fork> is done, so C<open> returns twice: in the parent |
3713 | process it returns the pid | |
3714 | of the child process, and in the child process it returns (a defined) C<0>. | |
3715 | Use C<defined($pid)> or C<//> to determine whether the open was successful. | |
3716 | ||
3717 | For example, use either | |
3718 | ||
3719 | $child_pid = open(FROM_KID, "-|") // die "can't fork: $!"; | |
3720 | ||
3721 | or | |
3722 | $child_pid = open(TO_KID, "|-") // die "can't fork: $!"; | |
3723 | ||
a6b91202 | 3724 | followed by |
0909e3f8 RS |
3725 | |
3726 | if ($child_pid) { | |
3727 | # am the parent: | |
3728 | # either write TO_KID or else read FROM_KID | |
3729 | ... | |
3730 | wait $child_pid; | |
3731 | } else { | |
3732 | # am the child; use STDIN/STDOUT normally | |
3733 | ... | |
3734 | exit; | |
a6b91202 | 3735 | } |
0909e3f8 RS |
3736 | |
3737 | The filehandle behaves normally for the parent, but I/O to that | |
3738 | filehandle is piped from/to the STDOUT/STDIN of the child process. | |
3739 | In the child process, the filehandle isn't opened--I/O happens from/to | |
3740 | the new STDOUT/STDIN. Typically this is used like the normal | |
3741 | piped open when you want to exercise more control over just how the | |
3742 | pipe command gets executed, such as when running setuid and | |
3743 | you don't want to have to scan shell commands for metacharacters. | |
3744 | ||
3745 | The following blocks are more or less equivalent: | |
3746 | ||
3747 | open(FOO, "|tr '[a-z]' '[A-Z]'"); | |
3748 | open(FOO, "|-", "tr '[a-z]' '[A-Z]'"); | |
3749 | open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; | |
3750 | open(FOO, "|-", "tr", '[a-z]', '[A-Z]'); | |
3751 | ||
3752 | open(FOO, "cat -n '$file'|"); | |
3753 | open(FOO, "-|", "cat -n '$file'"); | |
3754 | open(FOO, "-|") || exec "cat", "-n", $file; | |
3755 | open(FOO, "-|", "cat", "-n", $file); | |
3756 | ||
3757 | The last two examples in each block show the pipe as "list form", which is | |
3758 | not yet supported on all platforms. A good rule of thumb is that if | |
3759 | your platform has a real C<fork()> (in other words, if your platform is | |
a6b91202 | 3760 | Unix, including Linux and MacOS X), you can use the list form. You would |
0909e3f8 RS |
3761 | want to use the list form of the pipe so you can pass literal arguments |
3762 | to the command without risk of the shell interpreting any shell metacharacters | |
3763 | in them. However, this also bars you from opening pipes to commands | |
3764 | that intentionally contain shell metacharacters, such as: | |
3765 | ||
3766 | open(FOO, "|cat -n | expand -4 | lpr") | |
3767 | // die "Can't open pipeline to lpr: $!"; | |
3768 | ||
3769 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. | |
3770 | ||
3771 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
3772 | output before any operation that may do a fork, but this may not be | |
3773 | supported on some platforms (see L<perlport>). To be safe, you may need | |
3774 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
3775 | of C<IO::Handle> on any open handles. | |
3776 | ||
3777 | On systems that support a close-on-exec flag on files, the flag will | |
3778 | be set for the newly opened file descriptor as determined by the value | |
3779 | of C<$^F>. See L<perlvar/$^F>. | |
3780 | ||
3781 | Closing any piped filehandle causes the parent process to wait for the | |
3782 | child to finish, then returns the status value in C<$?> and | |
3783 | C<${^CHILD_ERROR_NATIVE}>. | |
3784 | ||
3785 | The filename passed to the one- and two-argument forms of open() will | |
3786 | have leading and trailing whitespace deleted and normal | |
3787 | redirection characters honored. This property, known as "magic open", | |
3788 | can often be used to good effect. A user could specify a filename of | |
3789 | F<"rsh cat file |">, or you could change certain filenames as needed: | |
3790 | ||
3791 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; | |
3792 | open(FH, $filename) or die "Can't open $filename: $!"; | |
3793 | ||
3794 | Use the three-argument form to open a file with arbitrary weird characters in it, | |
3795 | ||
3796 | open(FOO, "<", $file) | |
3797 | || die "can't open < $file: $!"; | |
3798 | ||
3799 | otherwise it's necessary to protect any leading and trailing whitespace: | |
3800 | ||
3801 | $file =~ s#^(\s)#./$1#; | |
3802 | open(FOO, "< $file\0") | |
3803 | || die "open failed: $!"; | |
3804 | ||
3805 | (this may not work on some bizarre filesystems). One should | |
3806 | conscientiously choose between the I<magic> and I<three-argument> form | |
3807 | of open(): | |
3808 | ||
3809 | open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!"; | |
3810 | ||
3811 | will allow the user to specify an argument of the form C<"rsh cat file |">, | |
3812 | but will not work on a filename that happens to have a trailing space, while | |
3813 | ||
3814 | open(IN, "<", $ARGV[0]) | |
3815 | || die "can't open < $ARGV[0]: $!"; | |
3816 | ||
3817 | will have exactly the opposite restrictions. | |
3818 | ||
3819 | If you want a "real" C C<open> (see L<open(2)> on your system), then you | |
3820 | should use the C<sysopen> function, which involves no such magic (but may | |
3821 | use subtly different filemodes than Perl open(), which is mapped to C | |
3822 | fopen()). This is another way to protect your filenames from | |
3823 | interpretation. For example: | |
3824 | ||
3825 | use IO::Handle; | |
3826 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) | |
3827 | or die "sysopen $path: $!"; | |
3828 | $oldfh = select(HANDLE); $| = 1; select($oldfh); | |
3829 | print HANDLE "stuff $$\n"; | |
3830 | seek(HANDLE, 0, 0); | |
3831 | print "File contains: ", <HANDLE>; | |
3832 | ||
3833 | Using the constructor from the C<IO::Handle> package (or one of its | |
3834 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous | |
3835 | filehandles that have the scope of the variables used to hold them, then | |
3836 | automatically (but silently) close once their reference counts become | |
3837 | zero, typically at scope exit: | |
3838 | ||
3839 | use IO::File; | |
3840 | #... | |
3841 | sub read_myfile_munged { | |
3842 | my $ALL = shift; | |
3843 | # or just leave it undef to autoviv | |
3844 | my $handle = IO::File->new; | |
3845 | open($handle, "<", "myfile") or die "myfile: $!"; | |
3846 | $first = <$handle> | |
3847 | or return (); # Automatically closed here. | |
3848 | mung($first) or die "mung failed"; # Or here. | |
3849 | return (first, <$handle>) if $ALL; # Or here. | |
3850 | return $first; # Or here. | |
3851 | } | |
3852 | ||
3853 | B<WARNING:> The previous example has a bug because the automatic | |
3854 | close that happens when the refcount on C<handle> does not | |
3855 | properly detect and report failures. I<Always> close the handle | |
3856 | yourself and inspect the return value. | |
3857 | ||
a6b91202 | 3858 | close($handle) |
0909e3f8 RS |
3859 | || warn "close failed: $!"; |
3860 | ||
3861 | See L</seek> for some details about mixing reading and writing. | |
3862 | ||
3863 | Portability issues: L<perlport/open>. | |
3864 | ||
3865 | =item opendir DIRHANDLE,EXPR | |
3866 | X<opendir> | |
3867 | ||
3868 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, | |
3869 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. | |
3870 | DIRHANDLE may be an expression whose value can be used as an indirect | |
3871 | dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined | |
3872 | scalar variable (or array or hash element), the variable is assigned a | |
3873 | reference to a new anonymous dirhandle; that is, it's autovivified. | |
3874 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. | |
3875 | ||
3876 | See the example at C<readdir>. | |
3877 | ||
3878 | =item ord EXPR | |
3879 | X<ord> X<encoding> | |
3880 | ||
3881 | =item ord | |
3882 | ||
3883 | Returns the numeric value of the first character of EXPR. | |
3884 | If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>. | |
3885 | (Note I<character>, not byte.) | |
3886 | ||
3887 | For the reverse, see L</chr>. | |
3888 | See L<perlunicode> for more about Unicode. | |
3889 | ||
3890 | =item our EXPR | |
3891 | X<our> X<global> | |
3892 | ||
3893 | =item our TYPE EXPR | |
3894 | ||
3895 | =item our EXPR : ATTRS | |
3896 | ||
3897 | =item our TYPE EXPR : ATTRS | |
3898 | ||
3899 | C<our> associates a simple name with a package variable in the current | |
3900 | package for use within the current scope. When C<use strict 'vars'> is in | |
3901 | effect, C<our> lets you use declared global variables without qualifying | |
3902 | them with package names, within the lexical scope of the C<our> declaration. | |
3903 | In this way C<our> differs from C<use vars>, which is package-scoped. | |
3904 | ||
3905 | Unlike C<my> or C<state>, which allocates storage for a variable and | |
3906 | associates a simple name with that storage for use within the current | |
3907 | scope, C<our> associates a simple name with a package (read: global) | |
3908 | variable in the current package, for use within the current lexical scope. | |
3909 | In other words, C<our> has the same scoping rules as C<my> or C<state>, but | |
3910 | does not necessarily create a variable. | |
3911 | ||
3912 | If more than one value is listed, the list must be placed | |
3913 | in parentheses. | |
3914 | ||
3915 | our $foo; | |
3916 | our($bar, $baz); | |
3917 | ||
3918 | An C<our> declaration declares a global variable that will be visible | |
3919 | across its entire lexical scope, even across package boundaries. The | |
3920 | package in which the variable is entered is determined at the point | |
3921 | of the declaration, not at the point of use. This means the following | |
3922 | behavior holds: | |
3923 | ||
3924 | package Foo; | |
3925 | our $bar; # declares $Foo::bar for rest of lexical scope | |
3926 | $bar = 20; | |
3927 | ||
3928 | package Bar; | |
3929 | print $bar; # prints 20, as it refers to $Foo::bar | |
3930 | ||
3931 | Multiple C<our> declarations with the same name in the same lexical | |
3932 | scope are allowed if they are in different packages. If they happen | |
3933 | to be in the same package, Perl will emit warnings if you have asked | |
3934 | for them, just like multiple C<my> declarations. Unlike a second | |
3935 | C<my> declaration, which will bind the name to a fresh variable, a | |
3936 | second C<our> declaration in the same package, in the same scope, is | |
3937 | merely redundant. | |
3938 | ||
3939 | use warnings; | |
3940 | package Foo; | |
3941 | our $bar; # declares $Foo::bar for rest of lexical scope | |
3942 | $bar = 20; | |
3943 | ||
3944 | package Bar; | |
3945 | our $bar = 30; # declares $Bar::bar for rest of lexical scope | |
3946 | print $bar; # prints 30 | |
3947 | ||
3948 | our $bar; # emits warning but has no other effect | |
3949 | print $bar; # still prints 30 | |
3950 | ||
3951 | An C<our> declaration may also have a list of attributes associated | |
3952 | with it. | |
3953 | ||
3954 | The exact semantics and interface of TYPE and ATTRS are still | |
3955 | evolving. TYPE is currently bound to the use of C<fields> pragma, | |
3956 | and attributes are handled using the C<attributes> pragma, or starting | |
3957 | from Perl 5.8.0 also via the C<Attribute::Handlers> module. See | |
3958 | L<perlsub/"Private Variables via my()"> for details, and L<fields>, | |
3959 | L<attributes>, and L<Attribute::Handlers>. | |
3960 | ||
3961 | =item pack TEMPLATE,LIST | |
3962 | X<pack> | |
3963 | ||
3964 | Takes a LIST of values and converts it into a string using the rules | |
3965 | given by the TEMPLATE. The resulting string is the concatenation of | |
3966 | the converted values. Typically, each converted value looks | |
3967 | like its machine-level representation. For example, on 32-bit machines | |
3968 | an integer may be represented by a sequence of 4 bytes, which will in | |
a6b91202 | 3969 | Perl be presented as a string that's 4 characters long. |
0909e3f8 RS |
3970 | |
3971 | See L<perlpacktut> for an introduction to this function. | |
3972 | ||
3973 | The TEMPLATE is a sequence of characters that give the order and type | |
3974 | of values, as follows: | |
3975 | ||
3976 | a A string with arbitrary binary data, will be null padded. | |
3977 | A A text (ASCII) string, will be space padded. | |
3978 | Z A null-terminated (ASCIZ) string, will be null padded. | |
3979 | ||
3980 | b A bit string (ascending bit order inside each byte, | |
3981 | like vec()). | |
3982 | B A bit string (descending bit order inside each byte). | |
3983 | h A hex string (low nybble first). | |
3984 | H A hex string (high nybble first). | |
3985 | ||
3986 | c A signed char (8-bit) value. | |
3987 | C An unsigned char (octet) value. | |
3988 | W An unsigned char value (can be greater than 255). | |
3989 | ||
3990 | s A signed short (16-bit) value. | |
3991 | S An unsigned short value. | |
3992 | ||
3993 | l A signed long (32-bit) value. | |
3994 | L An unsigned long value. | |
3995 | ||
3996 | q A signed quad (64-bit) value. | |
3997 | Q An unsigned quad value. | |
3998 | (Quads are available only if your system supports 64-bit | |
3999 | integer values _and_ if Perl has been compiled to support | |
4000 | those. Raises an exception otherwise.) | |
4001 | ||
4002 | i A signed integer value. | |
4003 | I A unsigned integer value. | |
4004 | (This 'integer' is _at_least_ 32 bits wide. Its exact | |
4005 | size depends on what a local C compiler calls 'int'.) | |
4006 | ||
4007 | n An unsigned short (16-bit) in "network" (big-endian) order. | |
4008 | N An unsigned long (32-bit) in "network" (big-endian) order. | |
4009 | v An unsigned short (16-bit) in "VAX" (little-endian) order. | |
4010 | V An unsigned long (32-bit) in "VAX" (little-endian) order. | |
4011 | ||
4012 | j A Perl internal signed integer value (IV). | |
4013 | J A Perl internal unsigned integer value (UV). | |
4014 | ||
4015 | f A single-precision float in native format. | |
4016 | d A double-precision float in native format. | |
4017 | ||
4018 | F A Perl internal floating-point value (NV) in native format | |
4019 | D A float of long-double precision in native format. | |
4020 | (Long doubles are available only if your system supports | |
4021 | long double values _and_ if Perl has been compiled to | |
4022 | support those. Raises an exception otherwise.) | |
4023 | ||
4024 | p A pointer to a null-terminated string. | |
4025 | P A pointer to a structure (fixed-length string). | |
4026 | ||
4027 | u A uuencoded string. | |
4028 | U A Unicode character number. Encodes to a character in char- | |
4029 | acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in | |
4030 | byte mode. | |
4031 | ||
4032 | w A BER compressed integer (not an ASN.1 BER, see perlpacktut | |
4033 | for details). Its bytes represent an unsigned integer in | |
4034 | base 128, most significant digit first, with as few digits | |
4035 | as possible. Bit eight (the high bit) is set on each byte | |
4036 | except the last. | |
4037 | ||
4038 | x A null byte (a.k.a ASCII NUL, "\000", chr(0)) | |
4039 | X Back up a byte. | |
4040 | @ Null-fill or truncate to absolute position, counted from the | |
4041 | start of the innermost ()-group. | |
4042 | . Null-fill or truncate to absolute position specified by | |
4043 | the value. | |
4044 | ( Start of a ()-group. | |
4045 | ||
4046 | One or more modifiers below may optionally follow certain letters in the | |
4047 | TEMPLATE (the second column lists letters for which the modifier is valid): | |
4048 | ||
4049 | ! sSlLiI Forces native (short, long, int) sizes instead | |
4050 | of fixed (16-/32-bit) sizes. | |
4051 | ||
4052 | xX Make x and X act as alignment commands. | |
4053 | ||
4054 | nNvV Treat integers as signed instead of unsigned. | |
4055 | ||
4056 | @. Specify position as byte offset in the internal | |
4057 | representation of the packed string. Efficient but | |
4058 | dangerous. | |
4059 | ||
4060 | > sSiIlLqQ Force big-endian byte-order on the type. | |
4061 | jJfFdDpP (The "big end" touches the construct.) | |
4062 | ||
4063 | < sSiIlLqQ Force little-endian byte-order on the type. | |
4064 | jJfFdDpP (The "little end" touches the construct.) | |
4065 | ||
a6b91202 A |
4066 | The C<< > >> and C<< < >> modifiers can also be used on C<()> groups |
4067 | to force a particular byte-order on all components in that group, | |
0909e3f8 RS |
4068 | including all its subgroups. |
4069 | ||
4070 | The following rules apply: | |
4071 | ||
a6b91202 | 4072 | =over |
0909e3f8 RS |
4073 | |
4074 | =item * | |
4075 | ||
4076 | Each letter may optionally be followed by a number indicating the repeat | |
4077 | count. A numeric repeat count may optionally be enclosed in brackets, as | |
4078 | in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from | |
4079 | the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>, | |
4080 | C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means | |
4081 | something else, described below. Supplying a C<*> for the repeat count | |
4082 | instead of a number means to use however many items are left, except for: | |
4083 | ||
a6b91202 | 4084 | =over |
0909e3f8 | 4085 | |
a6b91202 | 4086 | =item * |
0909e3f8 RS |
4087 | |
4088 | C<@>, C<x>, and C<X>, where it is equivalent to C<0>. | |
4089 | ||
a6b91202 | 4090 | =item * |
0909e3f8 RS |
4091 | |
4092 | <.>, where it means relative to the start of the string. | |
4093 | ||
a6b91202 | 4094 | =item * |
0909e3f8 RS |
4095 | |
4096 | C<u>, where it is equivalent to 1 (or 45, which here is equivalent). | |
4097 | ||
a6b91202 | 4098 | =back |
0909e3f8 RS |
4099 | |
4100 | One can replace a numeric repeat count with a template letter enclosed in | |
4101 | brackets to use the packed byte length of the bracketed template for the | |
4102 | repeat count. | |
4103 | ||
4104 | For example, the template C<x[L]> skips as many bytes as in a packed long, | |
4105 | and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when | |
4106 | variable-expanded) unpacks. If the template in brackets contains alignment | |
4107 | commands (such as C<x![d]>), its packed length is calculated as if the | |
4108 | start of the template had the maximal possible alignment. | |
4109 | ||
4110 | When used with C<Z>, a C<*> as the repeat count is guaranteed to add a | |
4111 | trailing null byte, so the resulting string is always one byte longer than | |
4112 | the byte length of the item itself. | |
4113 | ||
4114 | When used with C<@>, the repeat count represents an offset from the start | |
4115 | of the innermost C<()> group. | |
4116 | ||
4117 | When used with C<.>, the repeat count determines the starting position to | |
4118 | calculate the value offset as follows: | |
4119 | ||
a6b91202 | 4120 | =over |
0909e3f8 RS |
4121 | |
4122 | =item * | |
4123 | ||
4124 | If the repeat count is C<0>, it's relative to the current position. | |
4125 | ||
4126 | =item * | |
4127 | ||
4128 | If the repeat count is C<*>, the offset is relative to the start of the | |
4129 | packed string. | |
4130 | ||
4131 | =item * | |
4132 | ||
4133 | And if it's an integer I<n>, the offset is relative to the start of the | |
4134 | I<n>th innermost C<( )> group, or to the start of the string if I<n> is | |
4135 | bigger then the group level. | |
4136 | ||
4137 | =back | |
4138 | ||
4139 | The repeat count for C<u> is interpreted as the maximal number of bytes | |
a6b91202 | 4140 | to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat |
0909e3f8 RS |
4141 | count should not be more than 65. |
4142 | ||
4143 | =item * | |
4144 | ||
4145 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a | |
4146 | string of length count, padding with nulls or spaces as needed. When | |
4147 | unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything | |
4148 | after the first null, and C<a> returns data with no stripping at all. | |
4149 | ||
4150 | If the value to pack is too long, the result is truncated. If it's too | |
4151 | long and an explicit count is provided, C<Z> packs only C<$count-1> bytes, | |
4152 | followed by a null byte. Thus C<Z> always packs a trailing null, except | |
4153 | when the count is 0. | |
4154 | ||
4155 | =item * | |
4156 | ||
4157 | Likewise, the C<b> and C<B> formats pack a string that's that many bits long. | |
4158 | Each such format generates 1 bit of the result. These are typically followed | |
4159 | by a repeat count like C<B8> or C<B64>. | |
4160 | ||
4161 | Each result bit is based on the least-significant bit of the corresponding | |
4162 | input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> | |
4163 | and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">. | |
4164 | ||
4165 | Starting from the beginning of the input string, each 8-tuple | |
4166 | of characters is converted to 1 character of output. With format C<b>, | |
4167 | the first character of the 8-tuple determines the least-significant bit of a | |
4168 | character; with format C<B>, it determines the most-significant bit of | |
4169 | a character. | |
4170 | ||
4171 | If the length of the input string is not evenly divisible by 8, the | |
4172 | remainder is packed as if the input string were padded by null characters | |
4173 | at the end. Similarly during unpacking, "extra" bits are ignored. | |
4174 | ||
4175 | If the input string is longer than needed, remaining characters are ignored. | |
4176 | ||
a6b91202 | 4177 | A C<*> for the repeat count uses all characters of the input field. |
0909e3f8 RS |
4178 | On unpacking, bits are converted to a string of C<0>s and C<1>s. |
4179 | ||
4180 | =item * | |
4181 | ||
4182 | The C<h> and C<H> formats pack a string that many nybbles (4-bit groups, | |
4183 | representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long. | |
4184 | ||
4185 | For each such format, pack() generates 4 bits of result. | |
4186 | With non-alphabetical characters, the result is based on the 4 least-significant | |
4187 | bits of the input character, i.e., on C<ord($char)%16>. In particular, | |
4188 | characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes | |
4189 | C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result | |
4190 | is compatible with the usual hexadecimal digits, so that C<"a"> and | |
a6b91202 | 4191 | C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex |
0909e3f8 RS |
4192 | characters with this format. |
4193 | ||
4194 | Starting from the beginning of the template to pack(), each pair | |
4195 | of characters is converted to 1 character of output. With format C<h>, the | |
4196 | first character of the pair determines the least-significant nybble of the | |
4197 | output character; with format C<H>, it determines the most-significant | |
4198 | nybble. | |
4199 | ||
4200 | If the length of the input string is not even, it behaves as if padded by | |
4201 | a null character at the end. Similarly, "extra" nybbles are ignored during | |
4202 | unpacking. | |
4203 | ||
4204 | If the input string is longer than needed, extra characters are ignored. | |
4205 | ||
4206 | A C<*> for the repeat count uses all characters of the input field. For | |
4207 | unpack(), nybbles are converted to a string of hexadecimal digits. | |
4208 | ||
4209 | =item * | |
4210 | ||
4211 | The C<p> format packs a pointer to a null-terminated string. You are | |
4212 | responsible for ensuring that the string is not a temporary value, as that | |
4213 | could potentially get deallocated before you got around to using the packed | |
4214 | result. The C<P> format packs a pointer to a structure of the size indicated | |
4215 | by the length. A null pointer is created if the corresponding value for | |
4216 | C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer | |
4217 | unpacks into C<undef>. | |
4218 | ||
4219 | If your system has a strange pointer size--meaning a pointer is neither as | |
4220 | big as an int nor as big as a long--it may not be possible to pack or | |
4221 | unpack pointers in big- or little-endian byte order. Attempting to do | |
4222 | so raises an exception. | |
4223 | ||
4224 | =item * | |
4225 | ||
4226 | The C</> template character allows packing and unpacking of a sequence of | |
4227 | items where the packed structure contains a packed item count followed by | |
4228 | the packed items themselves. This is useful when the structure you're | |
4229 | unpacking has encoded the sizes or repeat counts for some of its fields | |
4230 | within the structure itself as separate fields. | |
4231 | ||
4232 | For C<pack>, you write I<length-item>C</>I<sequence-item>, and the | |
4233 | I<length-item> describes how the length value is packed. Formats likely | |
4234 | to be of most use are integer-packing ones like C<n> for Java strings, | |
4235 | C<w> for ASN.1 or SNMP, and C<N> for Sun XDR. | |
4236 | ||
4237 | For C<pack>, I<sequence-item> may have a repeat count, in which case | |
4238 | the minimum of that and the number of available items is used as the argument | |
4239 | for I<length-item>. If it has no repeat count or uses a '*', the number | |
4240 | of available items is used. | |
4241 | ||
4242 | For C<unpack>, an internal stack of integer arguments unpacked so far is | |
4243 | used. You write C</>I<sequence-item> and the repeat count is obtained by | |
4244 | popping off the last element from the stack. The I<sequence-item> must not | |
4245 | have a repeat count. | |
4246 | ||
4247 | If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">), | |
4248 | the I<length-item> is the string length, not the number of strings. With | |
4249 | an explicit repeat count for pack, the packed string is adjusted to that | |
4250 | length. For example: | |
4251 | ||
4252 | This code: gives this result: | |
a6b91202 | 4253 | |
0909e3f8 RS |
4254 | unpack("W/a", "\004Gurusamy") ("Guru") |
4255 | unpack("a3/A A*", "007 Bond J ") (" Bond", "J") | |
4256 | unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".") | |
4257 | ||
4258 | pack("n/a* w/a","hello,","world") "\000\006hello,\005world" | |
4259 | pack("a/W2", ord("a") .. ord("z")) "2ab" | |
4260 | ||
4261 | The I<length-item> is not returned explicitly from C<unpack>. | |
4262 | ||
4263 | Supplying a count to the I<length-item> format letter is only useful with | |
4264 | C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may | |
4265 | introduce C<"\000"> characters, which Perl does not regard as legal in | |
4266 | numeric strings. | |
4267 | ||
4268 | =item * | |
4269 | ||
4270 | The integer types C<s>, C<S>, C<l>, and C<L> may be | |
4271 | followed by a C<!> modifier to specify native shorts or | |
4272 | longs. As shown in the example above, a bare C<l> means | |
4273 | exactly 32 bits, although the native C<long> as seen by the local C compiler | |
4274 | may be larger. This is mainly an issue on 64-bit platforms. You can | |
4275 | see whether using C<!> makes any difference this way: | |
4276 | ||
a6b91202 | 4277 | printf "format s is %d, s! is %d\n", |
0909e3f8 RS |
4278 | length pack("s"), length pack("s!"); |
4279 | ||
a6b91202 | 4280 | printf "format l is %d, l! is %d\n", |
0909e3f8 RS |
4281 | length pack("l"), length pack("l!"); |
4282 | ||
4283 | ||
4284 | C<i!> and C<I!> are also allowed, but only for completeness' sake: | |
4285 | they are identical to C<i> and C<I>. | |
4286 | ||
4287 | The actual sizes (in bytes) of native shorts, ints, longs, and long | |
4288 | longs on the platform where Perl was built are also available from | |
4289 | the command line: | |
4290 | ||
4291 | $ perl -V:{short,int,long{,long}}size | |
4292 | shortsize='2'; | |
4293 | intsize='4'; | |
4294 | longsize='4'; | |
4295 | longlongsize='8'; | |
4296 | ||
4297 | or programmatically via the C<Config> module: | |
4298 | ||
4299 | use Config; | |
4300 | print $Config{shortsize}, "\n"; | |
4301 | print $Config{intsize}, "\n"; | |
4302 | print $Config{longsize}, "\n"; | |
4303 | print $Config{longlongsize}, "\n"; | |
4304 | ||
a6b91202 | 4305 | C<$Config{longlongsize}> is undefined on systems without |
0909e3f8 RS |
4306 | long long support. |
4307 | ||
4308 | =item * | |
4309 | ||
4310 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are | |
4311 | inherently non-portable between processors and operating systems because | |
4312 | they obey native byteorder and endianness. For example, a 4-byte integer | |
4313 | 0x12345678 (305419896 decimal) would be ordered natively (arranged in and | |
4314 | handled by the CPU registers) into bytes as | |
4315 | ||
4316 | 0x12 0x34 0x56 0x78 # big-endian | |
4317 | 0x78 0x56 0x34 0x12 # little-endian | |
4318 | ||
4319 | Basically, Intel and VAX CPUs are little-endian, while everybody else, | |
4320 | including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are | |
a6b91202 | 4321 | big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) |
0909e3f8 RS |
4322 | them in little-endian mode, but SGI/Cray uses them in big-endian mode. |
4323 | ||
4324 | The names I<big-endian> and I<little-endian> are comic references to the | |
4325 | egg-eating habits of the little-endian Lilliputians and the big-endian | |
4326 | Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>. | |
4327 | This entered computer lingo via the paper "On Holy Wars and a Plea for | |
4328 | Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. | |
4329 | ||
4330 | Some systems may have even weirder byte orders such as | |
4331 | ||
4332 | 0x56 0x78 0x12 0x34 | |
4333 | 0x34 0x12 0x78 0x56 | |
4334 | ||
4335 | You can determine your system endianness with this incantation: | |
4336 | ||
a6b91202 | 4337 | printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); |
0909e3f8 RS |
4338 | |
4339 | The byteorder on the platform where Perl was built is also available | |
4340 | via L<Config>: | |
4341 | ||
4342 | use Config; | |
4343 | print "$Config{byteorder}\n"; | |
4344 | ||
4345 | or from the command line: | |
4346 | ||
4347 | $ perl -V:byteorder | |
4348 | ||
4349 | Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321"> | |
4350 | and C<"87654321"> are big-endian. | |
4351 | ||
a6b91202 | 4352 | For portably packed integers, either use the formats C<n>, C<N>, C<v>, |
0909e3f8 RS |
4353 | and C<V> or else use the C<< > >> and C<< < >> modifiers described |
4354 | immediately below. See also L<perlport>. | |
4355 | ||
4356 | =item * | |
4357 | ||
4358 | Starting with Perl 5.9.2, integer and floating-point formats, along with | |
a6b91202 | 4359 | the C<p> and C<P> formats and C<()> groups, may all be followed by the |
0909e3f8 | 4360 | C<< > >> or C<< < >> endianness modifiers to respectively enforce big- |
a6b91202 A |
4361 | or little-endian byte-order. These modifiers are especially useful |
4362 | given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers, | |
0909e3f8 RS |
4363 | 64-bit integers, or floating-point values. |
4364 | ||
4365 | Here are some concerns to keep in mind when using an endianness modifier: | |
4366 | ||
4367 | =over | |
4368 | ||
a6b91202 | 4369 | =item * |
0909e3f8 | 4370 | |
a6b91202 | 4371 | Exchanging signed integers between different platforms works only |
0909e3f8 RS |
4372 | when all platforms store them in the same format. Most platforms store |
4373 | signed integers in two's-complement notation, so usually this is not an issue. | |
4374 | ||
a6b91202 | 4375 | =item * |
0909e3f8 RS |
4376 | |
4377 | The C<< > >> or C<< < >> modifiers can only be used on floating-point | |
4378 | formats on big- or little-endian machines. Otherwise, attempting to | |
4379 | use them raises an exception. | |
4380 | ||
a6b91202 | 4381 | =item * |
0909e3f8 RS |
4382 | |
4383 | Forcing big- or little-endian byte-order on floating-point values for | |
4384 | data exchange can work only if all platforms use the same | |
4385 | binary representation such as IEEE floating-point. Even if all | |
4386 | platforms are using IEEE, there may still be subtle differences. Being able | |
4387 | to use C<< > >> or C<< < >> on floating-point values can be useful, | |
4388 | but also dangerous if you don't know exactly what you're doing. | |
4389 | It is not a general way to portably store floating-point values. | |
4390 | ||
a6b91202 | 4391 | =item * |
0909e3f8 RS |
4392 | |
4393 | When using C<< > >> or C<< < >> on a C<()> group, this affects | |
4394 | all types inside the group that accept byte-order modifiers, | |
4395 | including all subgroups. It is silently ignored for all other | |
4396 | types. You are not allowed to override the byte-order within a group | |
4397 | that already has a byte-order modifier suffix. | |
4398 | ||
4399 | =back | |
4400 | ||
4401 | =item * | |
4402 | ||
4403 | Real numbers (floats and doubles) are in native machine format only. | |
4404 | Due to the multiplicity of floating-point formats and the lack of a | |
4405 | standard "network" representation for them, no facility for interchange has been | |
4406 | made. This means that packed floating-point data written on one machine | |
4407 | may not be readable on another, even if both use IEEE floating-point | |
4408 | arithmetic (because the endianness of the memory representation is not part | |
4409 | of the IEEE spec). See also L<perlport>. | |
4410 | ||
4411 | If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >> | |
4412 | modifiers to force big- or little-endian byte-order on floating-point values. | |
4413 | ||
4414 | Because Perl uses doubles (or long doubles, if configured) internally for | |
a6b91202 | 4415 | all numeric calculation, converting from double into float and thence |
0909e3f8 RS |
4416 | to double again loses precision, so C<unpack("f", pack("f", $foo)>) |
4417 | will not in general equal $foo. | |
4418 | ||
4419 | =item * | |
4420 | ||
4421 | Pack and unpack can operate in two modes: character mode (C<C0> mode) where | |
4422 | the packed string is processed per character, and UTF-8 mode (C<U0> mode) | |
4423 | where the packed string is processed in its UTF-8-encoded Unicode form on | |
a6b91202 A |
4424 | a byte-by-byte basis. Character mode is the default unless the format string |
4425 | starts with C<U>. You can always switch mode mid-format with an explicit | |
4426 | C<C0> or C<U0> in the format. This mode remains in effect until the next | |
0909e3f8 RS |
4427 | mode change, or until the end of the C<()> group it (directly) applies to. |
4428 | ||
a6b91202 | 4429 | Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode |
0909e3f8 RS |
4430 | bytes is not necessarily obvious. Probably only the first of these |
4431 | is what you want: | |
4432 | ||
a6b91202 | 4433 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | |
0909e3f8 RS |
4434 | perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)' |
4435 | 03B1.03C9 | |
a6b91202 | 4436 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | |
0909e3f8 RS |
4437 | perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' |
4438 | CE.B1.CF.89 | |
a6b91202 | 4439 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | |
0909e3f8 RS |
4440 | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)' |
4441 | CE.B1.CF.89 | |
a6b91202 | 4442 | $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | |
0909e3f8 RS |
4443 | perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' |
4444 | C3.8E.C2.B1.C3.8F.C2.89 | |
4445 | ||
4446 | Those examples also illustrate that you should not try to use | |
4447 | C<pack>/C<unpack> as a substitute for the L<Encode> module. | |
4448 | ||
4449 | =item * | |
4450 | ||
4451 | You must yourself do any alignment or padding by inserting, for example, | |
4452 | enough C<"x">es while packing. There is no way for pack() and unpack() | |
a6b91202 | 4453 | to know where characters are going to or coming from, so they |
0909e3f8 RS |
4454 | handle their output and input as flat sequences of characters. |
4455 | ||
4456 | =item * | |
4457 | ||
4458 | A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may | |
4459 | take a repeat count either as postfix, or for unpack(), also via the C</> | |
4460 | template character. Within each repetition of a group, positioning with | |
4461 | C<@> starts over at 0. Therefore, the result of | |
4462 | ||
4463 | pack("@1A((@2A)@3A)", qw[X Y Z]) | |
4464 | ||
4465 | is the string C<"\0X\0\0YZ">. | |
4466 | ||
4467 | =item * | |
4468 | ||
4469 | C<x> and C<X> accept the C<!> modifier to act as alignment commands: they | |
4470 | jump forward or back to the closest position aligned at a multiple of C<count> | |
4471 | characters. For example, to pack() or unpack() a C structure like | |
4472 | ||
4473 | struct { | |
4474 | char c; /* one signed, 8-bit character */ | |
a6b91202 | 4475 | double d; |
0909e3f8 RS |
4476 | char cc[2]; |
4477 | } | |
4478 | ||
4479 | one may need to use the template C<c x![d] d c[2]>. This assumes that | |
4480 | doubles must be aligned to the size of double. | |
4481 | ||
4482 | For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1; | |
4483 | both are no-ops. | |
4484 | ||
4485 | =item * | |
4486 | ||
4487 | C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to | |
4488 | represent signed 16-/32-bit integers in big-/little-endian order. | |
4489 | This is portable only when all platforms sharing packed data use the | |
4490 | same binary representation for signed integers; for example, when all | |
4491 | platforms use two's-complement representation. | |
4492 | ||
4493 | =item * | |
4494 | ||
4495 | Comments can be embedded in a TEMPLATE using C<#> through the end of line. | |
4496 | White space can separate pack codes from each other, but modifiers and | |
4497 | repeat counts must follow immediately. Breaking complex templates into | |
4498 | individual line-by-line components, suitably annotated, can do as much to | |
4499 | improve legibility and maintainability of pack/unpack formats as C</x> can | |
4500 | for complicated pattern matches. | |
4501 | ||
4502 | =item * | |
4503 | ||
4504 | If TEMPLATE requires more arguments than pack() is given, pack() | |
4505 | assumes additional C<""> arguments. If TEMPLATE requires fewer arguments | |
4506 | than given, extra arguments are ignored. | |
4507 | ||
4508 | =back | |
4509 | ||
4510 | Examples: | |
4511 | ||
4512 | $foo = pack("WWWW",65,66,67,68); | |
4513 | # foo eq "ABCD" | |
4514 | $foo = pack("W4",65,66,67,68); | |
4515 | # same thing | |
4516 | $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); | |
4517 | # same thing with Unicode circled letters. | |
4518 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); | |
4519 | # same thing with Unicode circled letters. You don't get the | |
4520 | # UTF-8 bytes because the U at the start of the format caused | |
4521 | # a switch to U0-mode, so the UTF-8 bytes get joined into | |
4522 | # characters | |
4523 | $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); | |
4524 | # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" | |
4525 | # This is the UTF-8 encoding of the string in the | |
4526 | # previous example | |
4527 | ||
4528 | $foo = pack("ccxxcc",65,66,67,68); | |
4529 | # foo eq "AB\0\0CD" | |
4530 | ||
4531 | # NOTE: The examples above featuring "W" and "c" are true | |
4532 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 | |
4533 | # and UTF-8. On EBCDIC systems, the first example would be | |
4534 | # $foo = pack("WWWW",193,194,195,196); | |
4535 | ||
4536 | $foo = pack("s2",1,2); | |
4537 | # "\001\000\002\000" on little-endian | |
4538 | # "\000\001\000\002" on big-endian | |
4539 | ||
4540 | $foo = pack("a4","abcd","x","y","z"); | |
4541 | # "abcd" | |
4542 | ||
4543 | $foo = pack("aaaa","abcd","x","y","z"); | |
4544 | # "axyz" | |
4545 | ||
4546 | $foo = pack("a14","abcdefg"); | |
4547 | # "abcdefg\0\0\0\0\0\0\0" | |
4548 | ||
4549 | $foo = pack("i9pl", gmtime); | |
4550 | # a real struct tm (on my system anyway) | |
4551 | ||
4552 | $utmp_template = "Z8 Z8 Z16 L"; | |
4553 | $utmp = pack($utmp_template, @utmp1); | |
4554 | # a struct utmp (BSDish) | |
4555 | ||
4556 | @utmp2 = unpack($utmp_template, $utmp); | |
4557 | # "@utmp1" eq "@utmp2" | |
4558 | ||
4559 | sub bintodec { | |
4560 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); | |
4561 | } | |
4562 | ||
4563 | $foo = pack('sx2l', 12, 34); | |
4564 | # short 12, two zero bytes padding, long 34 | |
4565 | $bar = pack('s@4l', 12, 34); | |
4566 | # short 12, zero fill to position 4, long 34 | |
4567 | # $foo eq $bar | |
4568 | $baz = pack('s.l', 12, 4, 34); | |
4569 | # short 12, zero fill to position 4, long 34 | |
4570 | ||
4571 | $foo = pack('nN', 42, 4711); | |
4572 | # pack big-endian 16- and 32-bit unsigned integers | |
4573 | $foo = pack('S>L>', 42, 4711); | |
4574 | # exactly the same | |
4575 | $foo = pack('s<l<', -42, 4711); | |
4576 | # pack little-endian 16- and 32-bit signed integers | |
4577 | $foo = pack('(sl)<', -42, 4711); | |
4578 | # exactly the same | |
4579 | ||
4580 | The same template may generally also be used in unpack(). | |
4581 | ||
4582 | =item package NAMESPACE | |
4583 | ||
4584 | =item package NAMESPACE VERSION | |
4585 | X<package> X<module> X<namespace> X<version> | |
4586 | ||
4587 | =item package NAMESPACE BLOCK | |
4588 | ||
4589 | =item package NAMESPACE VERSION BLOCK | |
4590 | X<package> X<module> X<namespace> X<version> | |
4591 | ||
4592 | Declares the BLOCK or the rest of the compilation unit as being in the | |
4593 | given namespace. The scope of the package declaration is either the | |
4594 | supplied code BLOCK or, in the absence of a BLOCK, from the declaration | |
4595 | itself through the end of current scope (the enclosing block, file, or | |
4596 | C<eval>). That is, the forms without a BLOCK are operative through the end | |
4597 | of the current scope, just like the C<my>, C<state>, and C<our> operators. | |
4598 | All unqualified dynamic identifiers in this scope will be in the given | |
4599 | namespace, except where overridden by another C<package> declaration or | |
4600 | when they're one of the special identifiers that qualify into C<main::>, | |
4601 | like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables. | |
4602 | ||
4603 | A package statement affects dynamic variables only, including those | |
4604 | you've used C<local> on, but I<not> lexical variables, which are created | |
a6b91202 | 4605 | with C<my>, C<state>, or C<our>. Typically it would be the first |
0909e3f8 | 4606 | declaration in a file included by C<require> or C<use>. You can switch into a |
a6b91202 | 4607 | package in more than one place, since this only determines which default |
0909e3f8 RS |
4608 | symbol table the compiler uses for the rest of that block. You can refer to |
4609 | identifiers in other packages than the current one by prefixing the identifier | |
4610 | with the package name and a double colon, as in C<$SomePack::var> | |
4611 | or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main> | |
4612 | package as assumed. That is, C<$::sail> is equivalent to | |
4613 | C<$main::sail> (as well as to C<$main'sail>, still seen in ancient | |
4614 | code, mostly from Perl 4). | |
4615 | ||
4616 | If VERSION is provided, C<package> sets the C<$VERSION> variable in the given | |
4617 | namespace to a L<version> object with the VERSION provided. VERSION must be a | |
4618 | "strict" style version number as defined by the L<version> module: a positive | |
4619 | decimal number (integer or decimal-fraction) without exponentiation or else a | |
4620 | dotted-decimal v-string with a leading 'v' character and at least three | |
4621 | components. You should set C<$VERSION> only once per package. | |
4622 | ||
4623 | See L<perlmod/"Packages"> for more information about packages, modules, | |
4624 | and classes. See L<perlsub> for other scoping issues. | |
4625 | ||
4626 | =item pipe READHANDLE,WRITEHANDLE | |
4627 | X<pipe> | |
4628 | ||
4629 | Opens a pair of connected pipes like the corresponding system call. | |
4630 | Note that if you set up a loop of piped processes, deadlock can occur | |
4631 | unless you are very careful. In addition, note that Perl's pipes use | |
4632 | IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE | |
4633 | after each command, depending on the application. | |
4634 | ||
4635 | See L<IPC::Open2>, L<IPC::Open3>, and | |
4636 | L<perlipc/"Bidirectional Communication with Another Process"> | |
4637 | for examples of such things. | |
4638 | ||
4639 | On systems that support a close-on-exec flag on files, that flag is set | |
a6b91202 | 4640 | on all newly opened file descriptors whose C<fileno>s are I<higher> than |
0909e3f8 RS |
4641 | the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>. |
4642 | ||
4643 | =item __PACKAGE__ | |
4644 | X<__PACKAGE__> | |
4645 | ||
4646 | A special token that returns the name of the package in which it occurs. | |
4647 | ||
4648 | =item pop ARRAY | |
4649 | X<pop> X<stack> | |
4650 | ||
4651 | =item pop EXPR | |
4652 | ||
4653 | =item pop | |
4654 | ||
4655 | Pops and returns the last value of the array, shortening the array by | |
4656 | one element. | |
4657 | ||
4658 | Returns the undefined value if the array is empty, although this may also | |
4659 | happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the | |
4660 | main program, but the C<@_> array in subroutines, just like C<shift>. | |
4661 | ||
4662 | Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a | |
4663 | reference to an unblessed array. The argument will be dereferenced | |
4664 | automatically. This aspect of C<pop> is considered highly experimental. | |
4665 | The exact behaviour may change in a future version of Perl. | |
4666 | ||
4667 | =item pos SCALAR | |
4668 | X<pos> X<match, position> | |
4669 | ||
4670 | =item pos | |
4671 | ||
4672 | Returns the offset of where the last C<m//g> search left off for the | |
4673 | variable in question (C<$_> is used when the variable is not | |
4674 | specified). Note that 0 is a valid match offset. C<undef> indicates | |
4675 | that the search position is reset (usually due to match failure, but | |
4676 | can also be because no match has yet been run on the scalar). | |
4677 | ||
4678 | C<pos> directly accesses the location used by the regexp engine to | |
4679 | store the offset, so assigning to C<pos> will change that offset, and | |
4680 | so will also influence the C<\G> zero-width assertion in regular | |
4681 | expressions. Both of these effects take place for the next match, so | |
4682 | you can't affect the position with C<pos> during the current match, | |
4683 | such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>. | |
4684 | ||
4685 | Setting C<pos> also resets the I<matched with zero-length> flag, described | |
4686 | under L<perlre/"Repeated Patterns Matching a Zero-length Substring">. | |
4687 | ||
4688 | Because a failed C<m//gc> match doesn't reset the offset, the return | |
4689 | from C<pos> won't change either in this case. See L<perlre> and | |
4690 | L<perlop>. | |
4691 | ||
4692 | =item print FILEHANDLE LIST | |
4693 | X<print> | |
4694 | ||
4695 | =item print FILEHANDLE | |
4696 | ||
4697 | =item print LIST | |
4698 | ||
4699 | =item print | |
4700 | ||
4701 | Prints a string or a list of strings. Returns true if successful. | |
4702 | FILEHANDLE may be a scalar variable containing the name of or a reference | |
4703 | to the filehandle, thus introducing one level of indirection. (NOTE: If | |
4704 | FILEHANDLE is a variable and the next token is a term, it may be | |
4705 | misinterpreted as an operator unless you interpose a C<+> or put | |
4706 | parentheses around the arguments.) If FILEHANDLE is omitted, prints to the | |
4707 | last selected (see L</select>) output handle. If LIST is omitted, prints | |
4708 | C<$_> to the currently selected output handle. To use FILEHANDLE alone to | |
4709 | print the content of C<$_> to it, you must use a real filehandle like | |
4710 | C<FH>, not an indirect one like C<$fh>. To set the default output handle | |
4711 | to something other than STDOUT, use the select operation. | |
4712 | ||
4713 | The current value of C<$,> (if any) is printed between each LIST item. The | |
4714 | current value of C<$\> (if any) is printed after the entire LIST has been | |
4715 | printed. Because print takes a LIST, anything in the LIST is evaluated in | |
4716 | list context, including any subroutines whose return lists you pass to | |
4717 | C<print>. Be careful not to follow the print keyword with a left | |
4718 | parenthesis unless you want the corresponding right parenthesis to | |
4719 | terminate the arguments to the print; put parentheses around all arguments | |
4720 | (or interpose a C<+>, but that doesn't look as good). | |
4721 | ||
4722 | If you're storing handles in an array or hash, or in general whenever | |
4723 | you're using any expression more complex than a bareword handle or a plain, | |
4724 | unsubscripted scalar variable to retrieve it, you will have to use a block | |
4725 | returning the filehandle value instead, in which case the LIST may not be | |
4726 | omitted: | |
4727 | ||
4728 | print { $files[$i] } "stuff\n"; | |
4729 | print { $OK ? STDOUT : STDERR } "stuff\n"; | |
4730 | ||
4731 | Printing to a closed pipe or socket will generate a SIGPIPE signal. See | |
4732 | L<perlipc> for more on signal handling. | |
4733 | ||
4734 | =item printf FILEHANDLE FORMAT, LIST | |
4735 | X<printf> | |
4736 | ||
4737 | =item printf FILEHANDLE | |
4738 | ||
4739 | =item printf FORMAT, LIST | |
4740 | ||
4741 | =item printf | |
4742 | ||
4743 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> | |
4744 | (the output record separator) is not appended. The first argument of the | |
4745 | list will be interpreted as the C<printf> format. See | |
4746 | L<sprintf|/sprintf FORMAT, LIST> for an | |
4747 | explanation of the format argument. If you omit the LIST, C<$_> is used; | |
4748 | to use FILEHANDLE without a LIST, you must use a real filehandle like | |
4749 | C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and | |
4750 | POSIX::setlocale() has been called, the character used for the decimal | |
4751 | separator in formatted floating-point numbers is affected by the LC_NUMERIC | |
4752 | locale setting. See L<perllocale> and L<POSIX>. | |
4753 | ||
4754 | Don't fall into the trap of using a C<printf> when a simple | |
4755 | C<print> would do. The C<print> is more efficient and less | |
4756 | error prone. | |
4757 | ||
4758 | =item prototype FUNCTION | |
4759 | X<prototype> | |
4760 | ||
4761 | Returns the prototype of a function as a string (or C<undef> if the | |
4762 | function has no prototype). FUNCTION is a reference to, or the name of, | |
4763 | the function whose prototype you want to retrieve. | |
4764 | ||
4765 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a | |
4766 | name for a Perl builtin. If the builtin is not I<overridable> (such as | |
4767 | C<qw//>) or if its arguments cannot be adequately expressed by a prototype | |
4768 | (such as C<system>), prototype() returns C<undef>, because the builtin | |
4769 | does not really behave like a Perl function. Otherwise, the string | |
4770 | describing the equivalent prototype is returned. | |
4771 | ||
4772 | =item push ARRAY,LIST | |
4773 | X<push> X<stack> | |
4774 | ||
4775 | =item push EXPR,LIST | |
4776 | ||
4777 | Treats ARRAY as a stack by appending the values of LIST to the end of | |
4778 | ARRAY. The length of ARRAY increases by the length of LIST. Has the same | |
4779 | effect as | |
4780 | ||
4781 | for $value (LIST) { | |
4782 | $ARRAY[++$#ARRAY] = $value; | |
4783 | } | |
4784 | ||
4785 | but is more efficient. Returns the number of elements in the array following | |
4786 | the completed C<push>. | |
4787 | ||
4788 | Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a | |
4789 | reference to an unblessed array. The argument will be dereferenced | |
4790 | automatically. This aspect of C<push> is considered highly experimental. | |
4791 | The exact behaviour may change in a future version of Perl. | |
4792 | ||
4793 | =item q/STRING/ | |
4794 | ||
4795 | =item qq/STRING/ | |
4796 | ||
4797 | =item qx/STRING/ | |
4798 | ||
4799 | =item qw/STRING/ | |
4800 | ||
4801 | Generalized quotes. See L<perlop/"Quote-Like Operators">. | |
4802 | ||
4803 | =item qr/STRING/ | |
4804 | ||
4805 | Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">. | |
4806 | ||
4807 | =item quotemeta EXPR | |
4808 | X<quotemeta> X<metacharacter> | |
4809 | ||
4810 | =item quotemeta | |
4811 | ||
4812 | Returns the value of EXPR with all non-"word" | |
4813 | characters backslashed. (That is, all characters not matching | |
4814 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the | |
4815 | returned string, regardless of any locale settings.) | |
4816 | This is the internal function implementing | |
4817 | the C<\Q> escape in double-quoted strings. | |
4818 | ||
4819 | If EXPR is omitted, uses C<$_>. | |
4820 | ||
4821 | quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into | |
4822 | regular expressions, because by default an interpolated variable will be | |
4823 | considered a mini-regular expression. For example: | |
4824 | ||
4825 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4826 | my $substring = 'quick.*?fox'; | |
4827 | $sentence =~ s{$substring}{big bad wolf}; | |
4828 | ||
4829 | Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>. | |
4830 | ||
4831 | On the other hand: | |
4832 | ||
4833 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4834 | my $substring = 'quick.*?fox'; | |
4835 | $sentence =~ s{\Q$substring\E}{big bad wolf}; | |
4836 | ||
4837 | Or: | |
4838 | ||
4839 | my $sentence = 'The quick brown fox jumped over the lazy dog'; | |
4840 | my $substring = 'quick.*?fox'; | |
4841 | my $quoted_substring = quotemeta($substring); | |
4842 | $sentence =~ s{$quoted_substring}{big bad wolf}; | |
4843 | ||
4844 | Will both leave the sentence as is. Normally, when accepting literal string | |
4845 | input from the user, quotemeta() or C<\Q> must be used. | |
4846 | ||
4847 | In Perl 5.14, all characters whose code points are above 127 are not | |
4848 | quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings. | |
4849 | It is planned to change this behavior in 5.16, but the exact rules | |
4850 | haven't been determined yet. | |
4851 | ||
4852 | =item rand EXPR | |
4853 | X<rand> X<random> | |
4854 | ||
4855 | =item rand | |
4856 | ||
4857 | Returns a random fractional number greater than or equal to C<0> and less | |
4858 | than the value of EXPR. (EXPR should be positive.) If EXPR is | |
4859 | omitted, the value C<1> is used. Currently EXPR with the value C<0> is | |
4860 | also special-cased as C<1> (this was undocumented before Perl 5.8.0 | |
4861 | and is subject to change in future versions of Perl). Automatically calls | |
4862 | C<srand> unless C<srand> has already been called. See also C<srand>. | |
4863 | ||
4864 | Apply C<int()> to the value returned by C<rand()> if you want random | |
4865 | integers instead of random fractional numbers. For example, | |
4866 | ||
4867 | int(rand(10)) | |
4868 | ||
4869 | returns a random integer between C<0> and C<9>, inclusive. | |
4870 | ||
4871 | (Note: If your rand function consistently returns numbers that are too | |
4872 | large or too small, then your version of Perl was probably compiled | |
4873 | with the wrong number of RANDBITS.) | |
4874 | ||
4875 | B<C<rand()> is not cryptographically secure. You should not rely | |
4876 | on it in security-sensitive situations.> As of this writing, a | |
4877 | number of third-party CPAN modules offer random number generators | |
4878 | intended by their authors to be cryptographically secure, | |
4879 | including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, | |
4880 | and L<Math::TrulyRandom>. | |
4881 | ||
4882 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET | |
4883 | X<read> X<file, read> | |
4884 | ||
4885 | =item read FILEHANDLE,SCALAR,LENGTH | |
4886 | ||
4887 | Attempts to read LENGTH I<characters> of data into variable SCALAR | |
4888 | from the specified FILEHANDLE. Returns the number of characters | |
4889 | actually read, C<0> at end of file, or undef if there was an error (in | |
a6b91202 | 4890 | the latter case C<$!> is also set). SCALAR will be grown or shrunk |
0909e3f8 RS |
4891 | so that the last character actually read is the last character of the |
4892 | scalar after the read. | |
4893 | ||
4894 | An OFFSET may be specified to place the read data at some place in the | |
4895 | string other than the beginning. A negative OFFSET specifies | |
4896 | placement at that many characters counting backwards from the end of | |
4897 | the string. A positive OFFSET greater than the length of SCALAR | |
4898 | results in the string being padded to the required size with C<"\0"> | |
4899 | bytes before the result of the read is appended. | |
4900 | ||
4901 | The call is implemented in terms of either Perl's or your system's native | |
4902 | fread(3) library function. To get a true read(2) system call, see | |
4903 | L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>. | |
4904 | ||
4905 | Note the I<characters>: depending on the status of the filehandle, | |
4906 | either (8-bit) bytes or characters are read. By default, all | |
4907 | filehandles operate on bytes, but for example if the filehandle has | |
4908 | been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> | |
4909 | pragma, L<open>), the I/O will operate on UTF8-encoded Unicode | |
4910 | characters, not bytes. Similarly for the C<:encoding> pragma: | |
4911 | in that case pretty much any characters can be read. | |
4912 | ||
4913 | =item readdir DIRHANDLE | |
4914 | X<readdir> | |
4915 | ||
4916 | Returns the next directory entry for a directory opened by C<opendir>. | |
4917 | If used in list context, returns all the rest of the entries in the | |
4918 | directory. If there are no more entries, returns the undefined value in | |
4919 | scalar context and the empty list in list context. | |
4920 | ||
4921 | If you're planning to filetest the return values out of a C<readdir>, you'd | |
4922 | better prepend the directory in question. Otherwise, because we didn't | |
4923 | C<chdir> there, it would have been testing the wrong file. | |
4924 | ||
4925 | opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!"; | |
4926 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); | |
4927 | closedir $dh; | |
4928 | ||
4929 | As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop, | |
4930 | which will set C<$_> on every iteration. | |
4931 | ||
4932 | opendir(my $dh, $some_dir) || die; | |
4933 | while(readdir $dh) { | |
4934 | print "$some_dir/$_\n"; | |
4935 | } | |
4936 | closedir $dh; | |
4937 | ||
4938 | =item readline EXPR | |
4939 | ||
4940 | =item readline | |
4941 | X<readline> X<gets> X<fgets> | |
4942 | ||
4943 | Reads from the filehandle whose typeglob is contained in EXPR (or from | |
4944 | C<*ARGV> if EXPR is not provided). In scalar context, each call reads and | |
4945 | returns the next line until end-of-file is reached, whereupon the | |
4946 | subsequent call returns C<undef>. In list context, reads until end-of-file | |
4947 | is reached and returns a list of lines. Note that the notion of "line" | |
4948 | used here is whatever you may have defined with C<$/> or | |
4949 | C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. | |
4950 | ||
4951 | When C<$/> is set to C<undef>, when C<readline> is in scalar | |
4952 | context (i.e., file slurp mode), and when an empty file is read, it | |
4953 | returns C<''> the first time, followed by C<undef> subsequently. | |
4954 | ||
4955 | This is the internal function implementing the C<< <EXPR> >> | |
4956 | operator, but you can use it directly. The C<< <EXPR> >> | |
4957 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
4958 | ||
4959 | $line = <STDIN>; | |
4960 | $line = readline(*STDIN); # same thing | |
4961 | ||
4962 | If C<readline> encounters an operating system error, C<$!> will be set | |
4963 | with the corresponding error message. It can be helpful to check | |
4964 | C<$!> when you are reading from filehandles you don't trust, such as a | |
4965 | tty or a socket. The following example uses the operator form of | |
4966 | C<readline> and dies if the result is not defined. | |
4967 | ||
4968 | while ( ! eof($fh) ) { | |
4969 | defined( $_ = <$fh> ) or die "readline failed: $!"; | |
4970 | ... | |
4971 | } | |
4972 | ||
4973 | Note that you have can't handle C<readline> errors that way with the | |
4974 | C<ARGV> filehandle. In that case, you have to open each element of | |
4975 | C<@ARGV> yourself since C<eof> handles C<ARGV> differently. | |
4976 | ||
4977 | foreach my $arg (@ARGV) { | |
4978 | open(my $fh, $arg) or warn "Can't open $arg: $!"; | |
4979 | ||
4980 | while ( ! eof($fh) ) { | |
4981 | defined( $_ = <$fh> ) | |
4982 | or die "readline failed for $arg: $!"; | |
4983 | ... | |
4984 | } | |
4985 | } | |
4986 | ||
4987 | =item readlink EXPR | |
4988 | X<readlink> | |
4989 | ||
4990 | =item readlink | |
4991 | ||
4992 | Returns the value of a symbolic link, if symbolic links are | |
4993 | implemented. If not, raises an exception. If there is a system | |
4994 | error, returns the undefined value and sets C<$!> (errno). If EXPR is | |
4995 | omitted, uses C<$_>. | |
4996 | ||
4997 | Portability issues: L<perlport/readlink>. | |
4998 | ||
4999 | =item readpipe EXPR | |
5000 | ||
5001 | =item readpipe | |
5002 | X<readpipe> | |
5003 | ||
5004 | EXPR is executed as a system command. | |
5005 | The collected standard output of the command is returned. | |
5006 | In scalar context, it comes back as a single (potentially | |
5007 | multi-line) string. In list context, returns a list of lines | |
5008 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). | |
5009 | This is the internal function implementing the C<qx/EXPR/> | |
5010 | operator, but you can use it directly. The C<qx/EXPR/> | |
5011 | operator is discussed in more detail in L<perlop/"I/O Operators">. | |
5012 | If EXPR is omitted, uses C<$_>. | |
5013 | ||
5014 | =item recv SOCKET,SCALAR,LENGTH,FLAGS | |
5015 | X<recv> | |
5016 | ||
5017 | Receives a message on a socket. Attempts to receive LENGTH characters | |
5018 | of data into variable SCALAR from the specified SOCKET filehandle. | |
5019 | SCALAR will be grown or shrunk to the length actually read. Takes the | |
5020 | same flags as the system call of the same name. Returns the address | |
5021 | of the sender if SOCKET's protocol supports this; returns an empty | |
5022 | string otherwise. If there's an error, returns the undefined value. | |
5023 | This call is actually implemented in terms of recvfrom(2) system call. | |
5024 | See L<perlipc/"UDP: Message Passing"> for examples. | |
5025 | ||
5026 | Note the I<characters>: depending on the status of the socket, either | |
5027 | (8-bit) bytes or characters are received. By default all sockets | |
5028 | operate on bytes, but for example if the socket has been changed using | |
5029 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see the | |
5030 | C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode | |
5031 | characters, not bytes. Similarly for the C<:encoding> pragma: in that | |
5032 | case pretty much any characters can be read. | |
5033 | ||
5034 | =item redo LABEL | |
5035 | X<redo> | |
5036 | ||
5037 | =item redo | |
5038 | ||
5039 | The C<redo> command restarts the loop block without evaluating the | |
5040 | conditional again. The C<continue> block, if any, is not executed. If | |
5041 | the LABEL is omitted, the command refers to the innermost enclosing | |
a6b91202 | 5042 | loop. Programs that want to lie to themselves about what was just input |
0909e3f8 RS |
5043 | normally use this command: |
5044 | ||
5045 | # a simpleminded Pascal comment stripper | |
5046 | # (warning: assumes no { or } in strings) | |
5047 | LINE: while (<STDIN>) { | |
5048 | while (s|({.*}.*){.*}|$1 |) {} | |
5049 | s|{.*}| |; | |
5050 | if (s|{.*| |) { | |
5051 | $front = $_; | |
5052 | while (<STDIN>) { | |
5053 | if (/}/) { # end of comment? | |
5054 | s|^|$front\{|; | |
5055 | redo LINE; | |
5056 | } | |
5057 | } | |
5058 | } | |
5059 | print; | |
5060 | } | |
5061 | ||
5062 | C<redo> cannot be used to retry a block that returns a value such as | |
5063 | C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit | |
5064 | a grep() or map() operation. | |
5065 | ||
5066 | Note that a block by itself is semantically identical to a loop | |
5067 | that executes once. Thus C<redo> inside such a block will effectively | |
5068 | turn it into a looping construct. | |
5069 | ||
5070 | See also L</continue> for an illustration of how C<last>, C<next>, and | |
5071 | C<redo> work. | |
5072 | ||
5073 | =item ref EXPR | |
5074 | X<ref> X<reference> | |
5075 | ||
5076 | =item ref | |
5077 | ||
5078 | Returns a non-empty string if EXPR is a reference, the empty | |
5079 | string otherwise. If EXPR | |
5080 | is not specified, C<$_> will be used. The value returned depends on the | |
5081 | type of thing the reference is a reference to. | |
5082 | Builtin types include: | |
5083 | ||
5084 | SCALAR | |
5085 | ARRAY | |
5086 | HASH | |
5087 | CODE | |
5088 | REF | |
5089 | GLOB | |
5090 | LVALUE | |
5091 | FORMAT | |
5092 | IO | |
5093 | VSTRING | |
5094 | Regexp | |
5095 | ||
5096 | If the referenced object has been blessed into a package, then that package | |
5097 | name is returned instead. You can think of C<ref> as a C<typeof> operator. | |
5098 | ||
5099 | if (ref($r) eq "HASH") { | |
5100 | print "r is a reference to a hash.\n"; | |
5101 | } | |
5102 | unless (ref($r)) { | |
5103 | print "r is not a reference at all.\n"; | |
5104 | } | |
5105 | ||
5106 | The return value C<LVALUE> indicates a reference to an lvalue that is not | |
5107 | a variable. You get this from taking the reference of function calls like | |
5108 | C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points | |
5109 | to a L<version string|perldata/"Version Strings">. | |
5110 | ||
5111 | The result C<Regexp> indicates that the argument is a regular expression | |
5112 | resulting from C<qr//>. | |
5113 | ||
5114 | See also L<perlref>. | |
5115 | ||
5116 | =item rename OLDNAME,NEWNAME | |
5117 | X<rename> X<move> X<mv> X<ren> | |
5118 | ||
5119 | Changes the name of a file; an existing file NEWNAME will be | |
5120 | clobbered. Returns true for success, false otherwise. | |
5121 | ||
5122 | Behavior of this function varies wildly depending on your system | |
5123 | implementation. For example, it will usually not work across file system | |
5124 | boundaries, even though the system I<mv> command sometimes compensates | |
5125 | for this. Other restrictions include whether it works on directories, | |
5126 | open files, or pre-existing files. Check L<perlport> and either the | |
5127 | rename(2) manpage or equivalent system documentation for details. | |
5128 | ||
5129 | For a platform independent C<move> function look at the L<File::Copy> | |
5130 | module. | |
5131 | ||
5132 | Portability issues: L<perlport/rename>. | |
5133 | ||
5134 | =item require VERSION | |
5135 | X<require> | |
5136 | ||
5137 | =item require EXPR | |
5138 | ||
5139 | =item require | |
5140 | ||
5141 | Demands a version of Perl specified by VERSION, or demands some semantics | |
5142 | specified by EXPR or by C<$_> if EXPR is not supplied. | |
5143 | ||
5144 | VERSION may be either a numeric argument such as 5.006, which will be | |
5145 | compared to C<$]>, or a literal of the form v5.6.1, which will be compared | |
5146 | to C<$^V> (aka $PERL_VERSION). An exception is raised if | |
5147 | VERSION is greater than the version of the current Perl interpreter. | |
5148 | Compare with L</use>, which can do a similar check at compile time. | |
5149 | ||
5150 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
5151 | avoided, because it leads to misleading error messages under earlier | |
5152 | versions of Perl that do not support this syntax. The equivalent numeric | |
5153 | version should be used instead. | |
5154 | ||
5155 | require v5.6.1; # run time version check | |
5156 | require 5.6.1; # ditto | |
5157 | require 5.006_001; # ditto; preferred for backwards compatibility | |
5158 | ||
5159 | Otherwise, C<require> demands that a library file be included if it | |
5160 | hasn't already been included. The file is included via the do-FILE | |
5161 | mechanism, which is essentially just a variety of C<eval> with the | |
5162 | caveat that lexical variables in the invoking script will be invisible | |
5163 | to the included code. Has semantics similar to the following subroutine: | |
5164 | ||
5165 | sub require { | |
5166 | my ($filename) = @_; | |
5167 | if (exists $INC{$filename}) { | |
5168 | return 1 if $INC{$filename}; | |
5169 | die "Compilation failed in require"; | |
5170 | } | |
5171 | my ($realfilename,$result); | |
5172 | ITER: { | |
5173 | foreach $prefix (@INC) { | |
5174 | $realfilename = "$prefix/$filename"; | |
5175 | if (-f $realfilename) { | |
5176 | $INC{$filename} = $realfilename; | |
5177 | $result = do $realfilename; | |
5178 | last ITER; | |
5179 | } | |
5180 | } | |
5181 | die "Can't find $filename in \@INC"; | |
5182 | } | |
5183 | if ($@) { | |
5184 | $INC{$filename} = undef; | |
5185 | die $@; | |
5186 | } elsif (!$result) { | |
5187 | delete $INC{$filename}; | |
5188 | die "$filename did not return true value"; | |
5189 | } else { | |
5190 | return $result; | |
5191 | } | |
5192 | } | |
5193 | ||
5194 | Note that the file will not be included twice under the same specified | |
5195 | name. | |
5196 | ||
5197 | The file must return true as the last statement to indicate | |
5198 | successful execution of any initialization code, so it's customary to | |
5199 | end such a file with C<1;> unless you're sure it'll return true | |
5200 | otherwise. But it's better just to put the C<1;>, in case you add more | |
5201 | statements. | |
5202 | ||
5203 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and | |
5204 | replaces "F<::>" with "F</>" in the filename for you, | |
5205 | to make it easy to load standard modules. This form of loading of | |
5206 | modules does not risk altering your namespace. | |
5207 | ||
5208 | In other words, if you try this: | |
5209 | ||
5210 | require Foo::Bar; # a splendid bareword | |
5211 | ||
5212 | The require function will actually look for the "F<Foo/Bar.pm>" file in the | |
5213 | directories specified in the C<@INC> array. | |
5214 | ||
5215 | But if you try this: | |
5216 | ||
5217 | $class = 'Foo::Bar'; | |
5218 | require $class; # $class is not a bareword | |
5219 | #or | |
5220 | require "Foo::Bar"; # not a bareword because of the "" | |
5221 | ||
5222 | The require function will look for the "F<Foo::Bar>" file in the @INC array and | |
5223 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: | |
5224 | ||
5225 | eval "require $class"; | |
5226 | ||
5227 | Now that you understand how C<require> looks for files with a | |
5228 | bareword argument, there is a little extra functionality going on behind | |
5229 | the scenes. Before C<require> looks for a "F<.pm>" extension, it will | |
5230 | first look for a similar filename with a "F<.pmc>" extension. If this file | |
5231 | is found, it will be loaded in place of any file ending in a "F<.pm>" | |
5232 | extension. | |
5233 | ||
5234 | You can also insert hooks into the import facility by putting Perl code | |
5235 | directly into the @INC array. There are three forms of hooks: subroutine | |
5236 | references, array references, and blessed objects. | |
5237 | ||
5238 | Subroutine references are the simplest case. When the inclusion system | |
5239 | walks through @INC and encounters a subroutine, this subroutine gets | |
5240 | called with two parameters, the first a reference to itself, and the | |
5241 | second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The | |
a6b91202 | 5242 | subroutine should return either nothing or else a list of up to three |
0909e3f8 RS |
5243 | values in the following order: |
5244 | ||
5245 | =over | |
5246 | ||
5247 | =item 1 | |
5248 | ||
a6b91202 | 5249 | A filehandle, from which the file will be read. |
0909e3f8 RS |
5250 | |
5251 | =item 2 | |
5252 | ||
5253 | A reference to a subroutine. If there is no filehandle (previous item), | |
5254 | then this subroutine is expected to generate one line of source code per | |
5255 | call, writing the line into C<$_> and returning 1, then finally at end of | |
5256 | file returning 0. If there is a filehandle, then the subroutine will be | |
5257 | called to act as a simple source filter, with the line as read in C<$_>. | |
5258 | Again, return 1 for each valid line, and 0 after all lines have been | |
5259 | returned. | |
5260 | ||
5261 | =item 3 | |
5262 | ||
5263 | Optional state for the subroutine. The state is passed in as C<$_[1]>. A | |
5264 | reference to the subroutine itself is passed in as C<$_[0]>. | |
5265 | ||
5266 | =back | |
5267 | ||
5268 | If an empty list, C<undef>, or nothing that matches the first 3 values above | |
5269 | is returned, then C<require> looks at the remaining elements of @INC. | |
5270 | Note that this filehandle must be a real filehandle (strictly a typeglob | |
a6b91202 | 5271 | or reference to a typeglob, whether blessed or unblessed); tied filehandles |
0909e3f8 RS |
5272 | will be ignored and processing will stop there. |
5273 | ||
5274 | If the hook is an array reference, its first element must be a subroutine | |
5275 | reference. This subroutine is called as above, but the first parameter is | |
5276 | the array reference. This lets you indirectly pass arguments to | |
5277 | the subroutine. | |
5278 | ||
5279 | In other words, you can write: | |
5280 | ||
5281 | push @INC, \&my_sub; | |
5282 | sub my_sub { | |
5283 | my ($coderef, $filename) = @_; # $coderef is \&my_sub | |
5284 | ... | |
5285 | } | |
5286 | ||
5287 | or: | |
5288 | ||
5289 | push @INC, [ \&my_sub, $x, $y, ... ]; | |
5290 | sub my_sub { | |
5291 | my ($arrayref, $filename) = @_; | |
5292 | # Retrieve $x, $y, ... | |
5293 | my @parameters = @$arrayref[1..$#$arrayref]; | |
5294 | ... | |
5295 | } | |
5296 | ||
5297 | If the hook is an object, it must provide an INC method that will be | |
5298 | called as above, the first parameter being the object itself. (Note that | |
5299 | you must fully qualify the sub's name, as unqualified C<INC> is always forced | |
5300 | into package C<main>.) Here is a typical code layout: | |
5301 | ||
5302 | # In Foo.pm | |
5303 | package Foo; | |
5304 | sub new { ... } | |
5305 | sub Foo::INC { | |
5306 | my ($self, $filename) = @_; | |
5307 | ... | |
5308 | } | |
5309 | ||
5310 | # In the main program | |
5311 | push @INC, Foo->new(...); | |
5312 | ||
5313 | These hooks are also permitted to set the %INC entry | |
5314 | corresponding to the files they have loaded. See L<perlvar/%INC>. | |
5315 | ||
5316 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. | |
5317 | ||
5318 | =item reset EXPR | |
5319 | X<reset> | |
5320 | ||
5321 | =item reset | |
5322 | ||
5323 | Generally used in a C<continue> block at the end of a loop to clear | |
5324 | variables and reset C<??> searches so that they work again. The | |
5325 | expression is interpreted as a list of single characters (hyphens | |
5326 | allowed for ranges). All variables and arrays beginning with one of | |
5327 | those letters are reset to their pristine state. If the expression is | |
a6b91202 | 5328 | omitted, one-match searches (C<?pattern?>) are reset to match again. |
0909e3f8 RS |
5329 | Only resets variables or searches in the current package. Always returns |
5330 | 1. Examples: | |
5331 | ||
5332 | reset 'X'; # reset all X variables | |
5333 | reset 'a-z'; # reset lower case variables | |
5334 | reset; # just reset ?one-time? searches | |
5335 | ||
5336 | Resetting C<"A-Z"> is not recommended because you'll wipe out your | |
5337 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package | |
5338 | variables; lexical variables are unaffected, but they clean themselves | |
5339 | up on scope exit anyway, so you'll probably want to use them instead. | |
5340 | See L</my>. | |
5341 | ||
5342 | =item return EXPR | |
5343 | X<return> | |
5344 | ||
5345 | =item return | |
5346 | ||
5347 | Returns from a subroutine, C<eval>, or C<do FILE> with the value | |
5348 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void | |
5349 | context, depending on how the return value will be used, and the context | |
5350 | may vary from one execution to the next (see L</wantarray>). If no EXPR | |
5351 | is given, returns an empty list in list context, the undefined value in | |
5352 | scalar context, and (of course) nothing at all in void context. | |
5353 | ||
5354 | (In the absence of an explicit C<return>, a subroutine, eval, | |
5355 | or do FILE automatically returns the value of the last expression | |
5356 | evaluated.) | |
5357 | ||
5358 | =item reverse LIST | |
5359 | X<reverse> X<rev> X<invert> | |
5360 | ||
5361 | In list context, returns a list value consisting of the elements | |
5362 | of LIST in the opposite order. In scalar context, concatenates the | |
5363 | elements of LIST and returns a string value with all characters | |
5364 | in the opposite order. | |
5365 | ||
5366 | print join(", ", reverse "world", "Hello"); # Hello, world | |
5367 | ||
5368 | print scalar reverse "dlrow ,", "olleH"; # Hello, world | |
5369 | ||
5370 | Used without arguments in scalar context, reverse() reverses C<$_>. | |
5371 | ||
5372 | $_ = "dlrow ,olleH"; | |
5373 | print reverse; # No output, list context | |
5374 | print scalar reverse; # Hello, world | |
5375 | ||
5376 | Note that reversing an array to itself (as in C<@a = reverse @a>) will | |
5377 | preserve non-existent elements whenever possible, i.e., for non magical | |
5378 | arrays or tied arrays with C<EXISTS> and C<DELETE> methods. | |
5379 | ||
5380 | This operator is also handy for inverting a hash, although there are some | |
5381 | caveats. If a value is duplicated in the original hash, only one of those | |
5382 | can be represented as a key in the inverted hash. Also, this has to | |
5383 | unwind one hash and build a whole new one, which may take some time | |
5384 | on a large hash, such as from a DBM file. | |
5385 | ||
5386 | %by_name = reverse %by_address; # Invert the hash | |
5387 | ||
5388 | =item rewinddir DIRHANDLE | |
5389 | X<rewinddir> | |
5390 | ||
5391 | Sets the current position to the beginning of the directory for the | |
5392 | C<readdir> routine on DIRHANDLE. | |
5393 | ||
5394 | Portability issues: L<perlport/rewinddir>. | |
5395 | ||
5396 | =item rindex STR,SUBSTR,POSITION | |
5397 | X<rindex> | |
5398 | ||
5399 | =item rindex STR,SUBSTR | |
5400 | ||
5401 | Works just like index() except that it returns the position of the I<last> | |
5402 | occurrence of SUBSTR in STR. If POSITION is specified, returns the | |
5403 | last occurrence beginning at or before that position. | |
5404 | ||
5405 | =item rmdir FILENAME | |
5406 | X<rmdir> X<rd> X<directory, remove> | |
5407 | ||
5408 | =item rmdir | |
5409 | ||
5410 | Deletes the directory specified by FILENAME if that directory is | |
5411 | empty. If it succeeds it returns true; otherwise it returns false and | |
5412 | sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. | |
5413 | ||
5414 | To remove a directory tree recursively (C<rm -rf> on Unix) look at | |
5415 | the C<rmtree> function of the L<File::Path> module. | |
5416 | ||
5417 | =item s/// | |
5418 | ||
5419 | The substitution operator. See L<perlop/"Regexp Quote-Like Operators">. | |
5420 | ||
5421 | =item say FILEHANDLE LIST | |
5422 | X<say> | |
5423 | ||
5424 | =item say FILEHANDLE | |
5425 | ||
5426 | =item say LIST | |
5427 | ||
5428 | =item say | |
5429 | ||
5430 | Just like C<print>, but implicitly appends a newline. C<say LIST> is | |
5431 | simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use | |
5432 | FILEHANDLE without a LIST to print the contents of C<$_> to it, you must | |
5433 | use a real filehandle like C<FH>, not an indirect one like C<$fh>. | |
5434 | ||
5435 | This keyword is available only when the C<"say"> feature | |
5436 | is enabled, or when prefixed with C<CORE::>; see | |
5437 | L<feature>. Alternately, include a C<use v5.10> or later to the current | |
5438 | scope. | |
5439 | ||
5440 | =item scalar EXPR | |
5441 | X<scalar> X<context> | |
5442 | ||
5443 | Forces EXPR to be interpreted in scalar context and returns the value | |
5444 | of EXPR. | |
5445 | ||
5446 | @counts = ( scalar @a, scalar @b, scalar @c ); | |
5447 | ||
5448 | There is no equivalent operator to force an expression to | |
5449 | be interpolated in list context because in practice, this is never | |
5450 | needed. If you really wanted to do so, however, you could use | |
5451 | the construction C<@{[ (some expression) ]}>, but usually a simple | |
5452 | C<(some expression)> suffices. | |
5453 | ||
5454 | Because C<scalar> is a unary operator, if you accidentally use a | |
5455 | parenthesized list for the EXPR, this behaves as a scalar comma expression, | |
5456 | evaluating all but the last element in void context and returning the final | |
5457 | element evaluated in scalar context. This is seldom what you want. | |
5458 | ||
5459 | The following single statement: | |
5460 | ||
5461 | print uc(scalar(&foo,$bar)),$baz; | |
5462 | ||
5463 | is the moral equivalent of these two: | |
5464 | ||
5465 | &foo; | |
5466 | print(uc($bar),$baz); | |
5467 | ||
5468 | See L<perlop> for more details on unary operators and the comma operator. | |
5469 | ||
5470 | =item seek FILEHANDLE,POSITION,WHENCE | |
5471 | X<seek> X<fseek> X<filehandle, position> | |
5472 | ||
5473 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. | |
5474 | FILEHANDLE may be an expression whose value gives the name of the | |
5475 | filehandle. The values for WHENCE are C<0> to set the new position | |
5476 | I<in bytes> to POSITION; C<1> to set it to the current position plus | |
5477 | POSITION; and C<2> to set it to EOF plus POSITION, typically | |
5478 | negative. For WHENCE you may use the constants C<SEEK_SET>, | |
5479 | C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end | |
5480 | of the file) from the L<Fcntl> module. Returns C<1> on success, false | |
5481 | otherwise. | |
5482 | ||
5483 | Note the I<in bytes>: even if the filehandle has been set to | |
5484 | operate on characters (for example by using the C<:encoding(utf8)> open | |
5485 | layer), tell() will return byte offsets, not character offsets | |
5486 | (because implementing that would render seek() and tell() rather slow). | |
5487 | ||
5488 | If you want to position the file for C<sysread> or C<syswrite>, don't use | |
5489 | C<seek>, because buffering makes its effect on the file's read-write position | |
5490 | unpredictable and non-portable. Use C<sysseek> instead. | |
5491 | ||
5492 | Due to the rules and rigors of ANSI C, on some systems you have to do a | |
5493 | seek whenever you switch between reading and writing. Amongst other | |
5494 | things, this may have the effect of calling stdio's clearerr(3). | |
5495 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: | |
5496 | ||
5497 | seek(TEST,0,1); | |
5498 | ||
5499 | This is also useful for applications emulating C<tail -f>. Once you hit | |
5500 | EOF on your read and then sleep for a while, you (probably) have to stick in a | |
5501 | dummy seek() to reset things. The C<seek> doesn't change the position, | |
5502 | but it I<does> clear the end-of-file condition on the handle, so that the | |
5503 | next C<< <FILE> >> makes Perl try again to read something. (We hope.) | |
5504 | ||
5505 | If that doesn't work (some I/O implementations are particularly | |
5506 | cantankerous), you might need something like this: | |
5507 | ||
5508 | for (;;) { | |
5509 | for ($curpos = tell(FILE); $_ = <FILE>; | |
5510 | $curpos = tell(FILE)) { | |
5511 | # search for some stuff and put it into files | |
5512 | } | |
5513 | sleep($for_a_while); | |
5514 | seek(FILE, $curpos, 0); | |
5515 | } | |
5516 | ||
5517 | =item seekdir DIRHANDLE,POS | |
5518 | X<seekdir> | |
5519 | ||
5520 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS | |
5521 | must be a value returned by C<telldir>. C<seekdir> also has the same caveats | |
5522 | about possible directory compaction as the corresponding system library | |
5523 | routine. | |
5524 | ||
5525 | =item select FILEHANDLE | |
5526 | X<select> X<filehandle, default> | |
5527 | ||
5528 | =item select | |
5529 | ||
5530 | Returns the currently selected filehandle. If FILEHANDLE is supplied, | |
5531 | sets the new current default filehandle for output. This has two | |
a6b91202 | 5532 | effects: first, a C<write> or a C<print> without a filehandle |
0909e3f8 | 5533 | default to this FILEHANDLE. Second, references to variables related to |
a6b91202 | 5534 | output will refer to this output channel. |
0909e3f8 RS |
5535 | |
5536 | For example, to set the top-of-form format for more than one | |
5537 | output channel, you might do the following: | |
5538 | ||
5539 | select(REPORT1); | |
5540 | $^ = 'report1_top'; | |
5541 | select(REPORT2); | |
5542 | $^ = 'report2_top'; | |
5543 | ||
5544 | FILEHANDLE may be an expression whose value gives the name of the | |
5545 | actual filehandle. Thus: | |
5546 | ||
5547 | $oldfh = select(STDERR); $| = 1; select($oldfh); | |
5548 | ||
5549 | Some programmers may prefer to think of filehandles as objects with | |
5550 | methods, preferring to write the last example as: | |
5551 | ||
5552 | use IO::Handle; | |
5553 | STDERR->autoflush(1); | |
5554 | ||
5555 | Portability issues: L<perlport/select>. | |
5556 | ||
5557 | =item select RBITS,WBITS,EBITS,TIMEOUT | |
5558 | X<select> | |
5559 | ||
5560 | This calls the select(2) syscall with the bit masks specified, which | |
5561 | can be constructed using C<fileno> and C<vec>, along these lines: | |
5562 | ||
5563 | $rin = $win = $ein = ''; | |
5564 | vec($rin, fileno(STDIN), 1) = 1; | |
5565 | vec($win, fileno(STDOUT), 1) = 1; | |
5566 | $ein = $rin | $win; | |
5567 | ||
5568 | If you want to select on many filehandles, you may wish to write a | |
5569 | subroutine like this: | |
5570 | ||
5571 | sub fhbits { | |
5572 | my @fhlist = @_; | |
5573 | my $bits = ""; | |
5574 | for my $fh (@fhlist) { | |
5575 | vec($bits, fileno($fh), 1) = 1; | |
5576 | } | |
5577 | return $bits; | |
5578 | } | |
5579 | $rin = fhbits(*STDIN, *TTY, *MYSOCK); | |
5580 | ||
5581 | The usual idiom is: | |
5582 | ||
5583 | ($nfound,$timeleft) = | |
5584 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); | |
5585 | ||
5586 | or to block until something becomes ready just do this | |
5587 | ||
5588 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); | |
5589 | ||
5590 | Most systems do not bother to return anything useful in $timeleft, so | |
5591 | calling select() in scalar context just returns $nfound. | |
5592 | ||
5593 | Any of the bit masks can also be undef. The timeout, if specified, is | |
5594 | in seconds, which may be fractional. Note: not all implementations are | |
5595 | capable of returning the $timeleft. If not, they always return | |
5596 | $timeleft equal to the supplied $timeout. | |
5597 | ||
5598 | You can effect a sleep of 250 milliseconds this way: | |
5599 | ||
5600 | select(undef, undef, undef, 0.25); | |
5601 | ||
5602 | Note that whether C<select> gets restarted after signals (say, SIGALRM) | |
5603 | is implementation-dependent. See also L<perlport> for notes on the | |
5604 | portability of C<select>. | |
5605 | ||
5606 | On error, C<select> behaves just like select(2): it returns | |
5607 | -1 and sets C<$!>. | |
5608 | ||
5609 | On some Unixes, select(2) may report a socket file descriptor as "ready for | |
5610 | reading" even when no data is available, and thus any subsequent C<read> | |
5611 | would block. This can be avoided if you always use O_NONBLOCK on the | |
5612 | socket. See select(2) and fcntl(2) for further details. | |
5613 | ||
5614 | The standard C<IO::Select> module provides a user-friendlier interface | |
5615 | to C<select>, mostly because it does all the bit-mask work for you. | |
5616 | ||
5617 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> | |
5618 | or <FH>) with C<select>, except as permitted by POSIX, and even | |
5619 | then only on POSIX systems. You have to use C<sysread> instead. | |
5620 | ||
5621 | Portability issues: L<perlport/select>. | |
5622 | ||
5623 | =item semctl ID,SEMNUM,CMD,ARG | |
5624 | X<semctl> | |
5625 | ||
5626 | Calls the System V IPC function semctl(2). You'll probably have to say | |
5627 | ||
5628 | use IPC::SysV; | |
5629 | ||
5630 | first to get the correct constant definitions. If CMD is IPC_STAT or | |
5631 | GETALL, then ARG must be a variable that will hold the returned | |
5632 | semid_ds structure or semaphore value array. Returns like C<ioctl>: | |
5633 | the undefined value for error, "C<0 but true>" for zero, or the actual | |
5634 | return value otherwise. The ARG must consist of a vector of native | |
5635 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. | |
5636 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> | |
5637 | documentation. | |
5638 | ||
5639 | Portability issues: L<perlport/semctl>. | |
5640 | ||
5641 | =item semget KEY,NSEMS,FLAGS | |
5642 | X<semget> | |
5643 | ||
5644 | Calls the System V IPC function semget(2). Returns the semaphore id, or | |
5645 | the undefined value on error. See also | |
5646 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> | |
5647 | documentation. | |
5648 | ||
5649 | Portability issues: L<perlport/semget>. | |
5650 | ||
5651 | =item semop KEY,OPSTRING | |
5652 | X<semop> | |
5653 | ||
5654 | Calls the System V IPC function semop(2) for semaphore operations | |
5655 | such as signalling and waiting. OPSTRING must be a packed array of | |
5656 | semop structures. Each semop structure can be generated with | |
a6b91202 | 5657 | C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING |
0909e3f8 RS |
5658 | implies the number of semaphore operations. Returns true if |
5659 | successful, false on error. As an example, the | |
5660 | following code waits on semaphore $semnum of semaphore id $semid: | |
5661 | ||
5662 | $semop = pack("s!3", $semnum, -1, 0); | |
5663 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); | |
5664 | ||
5665 | To signal the semaphore, replace C<-1> with C<1>. See also | |
5666 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> | |
5667 | documentation. | |
5668 | ||
5669 | Portability issues: L<perlport/semop>. | |
5670 | ||
5671 | =item send SOCKET,MSG,FLAGS,TO | |
5672 | X<send> | |
5673 | ||
5674 | =item send SOCKET,MSG,FLAGS | |
5675 | ||
5676 | Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET | |
5677 | filehandle. Takes the same flags as the system call of the same name. On | |
5678 | unconnected sockets, you must specify a destination to I<send to>, in which | |
5679 | case it does a sendto(2) syscall. Returns the number of characters sent, | |
5680 | or the undefined value on error. The sendmsg(2) syscall is currently | |
5681 | unimplemented. See L<perlipc/"UDP: Message Passing"> for examples. | |
5682 | ||
5683 | Note the I<characters>: depending on the status of the socket, either | |
5684 | (8-bit) bytes or characters are sent. By default all sockets operate | |
5685 | on bytes, but for example if the socket has been changed using | |
5686 | binmode() to operate with the C<:encoding(utf8)> I/O layer (see | |
5687 | L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8 | |
5688 | encoded Unicode characters, not bytes. Similarly for the C<:encoding> | |
5689 | pragma: in that case pretty much any characters can be sent. | |
5690 | ||
5691 | =item setpgrp PID,PGRP | |
5692 | X<setpgrp> X<group> | |
5693 | ||
5694 | Sets the current process group for the specified PID, C<0> for the current | |
5695 | process. Raises an exception when used on a machine that doesn't | |
5696 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, | |
5697 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not | |
5698 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also | |
5699 | C<POSIX::setsid()>. | |
5700 | ||
5701 | Portability issues: L<perlport/setpgrp>. | |
5702 | ||
5703 | =item setpriority WHICH,WHO,PRIORITY | |
5704 | X<setpriority> X<priority> X<nice> X<renice> | |
5705 | ||
5706 | Sets the current priority for a process, a process group, or a user. | |
5707 | (See setpriority(2).) Raises an exception when used on a machine | |
5708 | that doesn't implement setpriority(2). | |
5709 | ||
5710 | Portability issues: L<perlport/setpriority>. | |
5711 | ||
5712 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL | |
5713 | X<setsockopt> | |
5714 | ||
5715 | Sets the socket option requested. Returns C<undef> on error. | |
5716 | Use integer constants provided by the C<Socket> module for | |
5717 | LEVEL and OPNAME. Values for LEVEL can also be obtained from | |
5718 | getprotobyname. OPTVAL might either be a packed string or an integer. | |
5719 | An integer OPTVAL is shorthand for pack("i", OPTVAL). | |
5720 | ||
5721 | An example disabling Nagle's algorithm on a socket: | |
5722 | ||
5723 | use Socket qw(IPPROTO_TCP TCP_NODELAY); | |
5724 | setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); | |
5725 | ||
5726 | Portability issues: L<perlport/setsockopt>. | |
5727 | ||
5728 | =item shift ARRAY | |
5729 | X<shift> | |
5730 | ||
5731 | =item shift EXPR | |
5732 | ||
5733 | =item shift | |
5734 | ||
5735 | Shifts the first value of the array off and returns it, shortening the | |
5736 | array by 1 and moving everything down. If there are no elements in the | |
5737 | array, returns the undefined value. If ARRAY is omitted, shifts the | |
5738 | C<@_> array within the lexical scope of subroutines and formats, and the | |
5739 | C<@ARGV> array outside a subroutine and also within the lexical scopes | |
5740 | established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, | |
5741 | C<UNITCHECK {}>, and C<END {}> constructs. | |
5742 | ||
5743 | Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a | |
5744 | reference to an unblessed array. The argument will be dereferenced | |
5745 | automatically. This aspect of C<shift> is considered highly experimental. | |
5746 | The exact behaviour may change in a future version of Perl. | |
5747 | ||
5748 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the | |
5749 | same thing to the left end of an array that C<pop> and C<push> do to the | |
5750 | right end. | |
5751 | ||
5752 | =item shmctl ID,CMD,ARG | |
5753 | X<shmctl> | |
5754 | ||
5755 | Calls the System V IPC function shmctl. You'll probably have to say | |
5756 | ||
5757 | use IPC::SysV; | |
5758 | ||
5759 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, | |
5760 | then ARG must be a variable that will hold the returned C<shmid_ds> | |
5761 | structure. Returns like ioctl: C<undef> for error; "C<0> but | |
5762 | true" for zero; and the actual return value otherwise. | |
5763 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. | |
5764 | ||
5765 | Portability issues: L<perlport/shmctl>. | |
5766 | ||
5767 | =item shmget KEY,SIZE,FLAGS | |
5768 | X<shmget> | |
5769 | ||
5770 | Calls the System V IPC function shmget. Returns the shared memory | |
5771 | segment id, or C<undef> on error. | |
5772 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. | |
5773 | ||
5774 | Portability issues: L<perlport/shmget>. | |
5775 | ||
5776 | =item shmread ID,VAR,POS,SIZE | |
5777 | X<shmread> | |
5778 | X<shmwrite> | |
5779 | ||
5780 | =item shmwrite ID,STRING,POS,SIZE | |
5781 | ||
5782 | Reads or writes the System V shared memory segment ID starting at | |
5783 | position POS for size SIZE by attaching to it, copying in/out, and | |
5784 | detaching from it. When reading, VAR must be a variable that will | |
5785 | hold the data read. When writing, if STRING is too long, only SIZE | |
5786 | bytes are used; if STRING is too short, nulls are written to fill out | |
5787 | SIZE bytes. Return true if successful, false on error. | |
5788 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, | |
5789 | C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN. | |
5790 | ||
5791 | Portability issues: L<perlport/shmread> and L<perlport/shmwrite>. | |
5792 | ||
5793 | =item shutdown SOCKET,HOW | |
5794 | X<shutdown> | |
5795 | ||
5796 | Shuts down a socket connection in the manner indicated by HOW, which | |
5797 | has the same interpretation as in the syscall of the same name. | |
5798 | ||
5799 | shutdown(SOCKET, 0); # I/we have stopped reading data | |
5800 | shutdown(SOCKET, 1); # I/we have stopped writing data | |
5801 | shutdown(SOCKET, 2); # I/we have stopped using this socket | |
5802 | ||
5803 | This is useful with sockets when you want to tell the other | |
5804 | side you're done writing but not done reading, or vice versa. | |
5805 | It's also a more insistent form of close because it also | |
5806 | disables the file descriptor in any forked copies in other | |
5807 | processes. | |
5808 | ||
5809 | Returns C<1> for success; on error, returns C<undef> if | |
5810 | the first argument is not a valid filehandle, or returns C<0> and sets | |
5811 | C<$!> for any other failure. | |
5812 | ||
5813 | =item sin EXPR | |
5814 | X<sin> X<sine> X<asin> X<arcsine> | |
5815 | ||
5816 | =item sin | |
5817 | ||
5818 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, | |
5819 | returns sine of C<$_>. | |
5820 | ||
5821 | For the inverse sine operation, you may use the C<Math::Trig::asin> | |
5822 | function, or use this relation: | |
5823 | ||
5824 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } | |
5825 | ||
5826 | =item sleep EXPR | |
5827 | X<sleep> X<pause> | |
5828 | ||
5829 | =item sleep | |
5830 | ||
a6b91202 A |
5831 | Causes the script to sleep for (integer) EXPR seconds, or forever if no |
5832 | argument is given. Returns the integer number of seconds actually slept. | |
0909e3f8 RS |
5833 | |
5834 | May be interrupted if the process receives a signal such as C<SIGALRM>. | |
5835 | ||
5836 | eval { | |
5837 | local $SIG{ALARM} = sub { die "Alarm!\n" }; | |
5838 | sleep; | |
5839 | }; | |
5840 | die $@ unless $@ eq "Alarm!\n"; | |
5841 | ||
5842 | You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep> | |
5843 | is often implemented using C<alarm>. | |
5844 | ||
5845 | On some older systems, it may sleep up to a full second less than what | |
5846 | you requested, depending on how it counts seconds. Most modern systems | |
5847 | always sleep the full amount. They may appear to sleep longer than that, | |
5848 | however, because your process might not be scheduled right away in a | |
5849 | busy multitasking system. | |
5850 | ||
5851 | For delays of finer granularity than one second, the Time::HiRes module | |
5852 | (from CPAN, and starting from Perl 5.8 part of the standard | |
5853 | distribution) provides usleep(). You may also use Perl's four-argument | |
5854 | version of select() leaving the first three arguments undefined, or you | |
5855 | might be able to use the C<syscall> interface to access setitimer(2) if | |
5856 | your system supports it. See L<perlfaq8> for details. | |
5857 | ||
5858 | See also the POSIX module's C<pause> function. | |
5859 | ||
5860 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL | |
5861 | X<socket> | |
5862 | ||
5863 | Opens a socket of the specified kind and attaches it to filehandle | |
5864 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for | |
5865 | the syscall of the same name. You should C<use Socket> first | |
5866 | to get the proper definitions imported. See the examples in | |
5867 | L<perlipc/"Sockets: Client/Server Communication">. | |
5868 | ||
5869 | On systems that support a close-on-exec flag on files, the flag will | |
5870 | be set for the newly opened file descriptor, as determined by the | |
5871 | value of $^F. See L<perlvar/$^F>. | |
5872 | ||
5873 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL | |
5874 | X<socketpair> | |
5875 | ||
5876 | Creates an unnamed pair of sockets in the specified domain, of the | |
5877 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as | |
5878 | for the syscall of the same name. If unimplemented, raises an exception. | |
5879 | Returns true if successful. | |
5880 | ||
5881 | On systems that support a close-on-exec flag on files, the flag will | |
5882 | be set for the newly opened file descriptors, as determined by the value | |
5883 | of $^F. See L<perlvar/$^F>. | |
5884 | ||
5885 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call | |
5886 | to C<pipe(Rdr, Wtr)> is essentially: | |
5887 | ||
5888 | use Socket; | |
5889 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); | |
5890 | shutdown(Rdr, 1); # no more writing for reader | |
5891 | shutdown(Wtr, 0); # no more reading for writer | |
5892 | ||
5893 | See L<perlipc> for an example of socketpair use. Perl 5.8 and later will | |
5894 | emulate socketpair using IP sockets to localhost if your system implements | |
5895 | sockets but not socketpair. | |
5896 | ||
5897 | Portability issues: L<perlport/socketpair>. | |
5898 | ||
5899 | =item sort SUBNAME LIST | |
5900 | X<sort> X<qsort> X<quicksort> X<mergesort> | |
5901 | ||
5902 | =item sort BLOCK LIST | |
5903 | ||
5904 | =item sort LIST | |
5905 | ||
5906 | In list context, this sorts the LIST and returns the sorted list value. | |
5907 | In scalar context, the behaviour of C<sort()> is undefined. | |
5908 | ||
5909 | If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison | |
5910 | order. If SUBNAME is specified, it gives the name of a subroutine | |
5911 | that returns an integer less than, equal to, or greater than C<0>, | |
a6b91202 | 5912 | depending on how the elements of the list are to be ordered. (The |
0909e3f8 RS |
5913 | C<< <=> >> and C<cmp> operators are extremely useful in such routines.) |
5914 | SUBNAME may be a scalar variable name (unsubscripted), in which case | |
5915 | the value provides the name of (or a reference to) the actual | |
5916 | subroutine to use. In place of a SUBNAME, you can provide a BLOCK as | |
5917 | an anonymous, in-line sort subroutine. | |
5918 | ||
5919 | If the subroutine's prototype is C<($$)>, the elements to be compared are | |
5920 | passed by reference in C<@_>, as for a normal subroutine. This is slower | |
5921 | than unprototyped subroutines, where the elements to be compared are passed | |
5922 | into the subroutine as the package global variables $a and $b (see example | |
5923 | below). Note that in the latter case, it is usually highly counter-productive | |
5924 | to declare $a and $b as lexicals. | |
5925 | ||
5926 | If the subroutine is an XSUB, the elements to be compared are pushed on to | |
5927 | the stack, the way arguments are usually passed to XSUBs. $a and $b are | |
5928 | not set. | |
5929 | ||
5930 | The values to be compared are always passed by reference and should not | |
5931 | be modified. | |
5932 | ||
5933 | You also cannot exit out of the sort block or subroutine using any of the | |
5934 | loop control operators described in L<perlsyn> or with C<goto>. | |
5935 | ||
5936 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the | |
5937 | current collation locale. See L<perllocale>. | |
5938 | ||
5939 | sort() returns aliases into the original list, much as a for loop's index | |
5940 | variable aliases the list elements. That is, modifying an element of a | |
5941 | list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>) | |
5942 | actually modifies the element in the original list. This is usually | |
5943 | something to be avoided when writing clear code. | |
5944 | ||
5945 | Perl 5.6 and earlier used a quicksort algorithm to implement sort. | |
5946 | That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort | |
5947 | preserves the input order of elements that compare equal. Although | |
5948 | quicksort's run time is O(NlogN) when averaged over all arrays of | |
5949 | length N, the time can be O(N**2), I<quadratic> behavior, for some | |
5950 | inputs.) In 5.7, the quicksort implementation was replaced with | |
5951 | a stable mergesort algorithm whose worst-case behavior is O(NlogN). | |
5952 | But benchmarks indicated that for some inputs, on some platforms, | |
5953 | the original quicksort was faster. 5.8 has a sort pragma for | |
5954 | limited control of the sort. Its rather blunt control of the | |
5955 | underlying algorithm may not persist into future Perls, but the | |
5956 | ability to characterize the input or output in implementation | |
5957 | independent ways quite probably will. See L<the sort pragma|sort>. | |
5958 | ||
5959 | Examples: | |
5960 | ||
5961 | # sort lexically | |
5962 | @articles = sort @files; | |
a6b91202 | 5963 | |
0909e3f8 RS |
5964 | # same thing, but with explicit sort routine |
5965 | @articles = sort {$a cmp $b} @files; | |
a6b91202 | 5966 | |
0909e3f8 RS |
5967 | # now case-insensitively |
5968 | @articles = sort {uc($a) cmp uc($b)} @files; | |
a6b91202 | 5969 | |
0909e3f8 RS |
5970 | # same thing in reversed order |
5971 | @articles = sort {$b cmp $a} @files; | |
a6b91202 | 5972 | |
0909e3f8 RS |
5973 | # sort numerically ascending |
5974 | @articles = sort {$a <=> $b} @files; | |
a6b91202 | 5975 | |
0909e3f8 RS |
5976 | # sort numerically descending |
5977 | @articles = sort {$b <=> $a} @files; | |
a6b91202 | 5978 | |
0909e3f8 RS |
5979 | # this sorts the %age hash by value instead of key |
5980 | # using an in-line function | |
5981 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; | |
a6b91202 | 5982 | |
0909e3f8 RS |
5983 | # sort using explicit subroutine name |
5984 | sub byage { | |
5985 | $age{$a} <=> $age{$b}; # presuming numeric | |
5986 | } | |
5987 | @sortedclass = sort byage @class; | |
a6b91202 | 5988 | |
0909e3f8 RS |
5989 | sub backwards { $b cmp $a } |
5990 | @harry = qw(dog cat x Cain Abel); | |
5991 | @george = qw(gone chased yz Punished Axed); | |
5992 | print sort @harry; | |
5993 | # prints AbelCaincatdogx | |
5994 | print sort backwards @harry; | |
5995 | # prints xdogcatCainAbel | |
5996 | print sort @george, 'to', @harry; | |
5997 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz | |
5998 | ||
5999 | # inefficiently sort by descending numeric compare using | |
6000 | # the first integer after the first = sign, or the | |
6001 | # whole record case-insensitively otherwise | |
6002 | ||
6003 | my @new = sort { | |
6004 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] | |
6005 | || | |
6006 | uc($a) cmp uc($b) | |
6007 | } @old; | |
6008 | ||
6009 | # same thing, but much more efficiently; | |
6010 | # we'll build auxiliary indices instead | |
6011 | # for speed | |
6012 | my @nums = @caps = (); | |
6013 | for (@old) { | |
6014 | push @nums, ( /=(\d+)/ ? $1 : undef ); | |
6015 | push @caps, uc($_); | |
6016 | } | |
6017 | ||
6018 | my @new = @old[ sort { | |
6019 | $nums[$b] <=> $nums[$a] | |
6020 | || | |
6021 | $caps[$a] cmp $caps[$b] | |
6022 | } 0..$#old | |
6023 | ]; | |
6024 | ||
6025 | # same thing, but without any temps | |
6026 | @new = map { $_->[0] } | |
6027 | sort { $b->[1] <=> $a->[1] | |
6028 | || | |
6029 | $a->[2] cmp $b->[2] | |
6030 | } map { [$_, /=(\d+)/, uc($_)] } @old; | |
6031 | ||
6032 | # using a prototype allows you to use any comparison subroutine | |
6033 | # as a sort subroutine (including other package's subroutines) | |
6034 | package other; | |
6035 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here | |
a6b91202 | 6036 | |
0909e3f8 RS |
6037 | package main; |
6038 | @new = sort other::backwards @old; | |
a6b91202 | 6039 | |
0909e3f8 RS |
6040 | # guarantee stability, regardless of algorithm |
6041 | use sort 'stable'; | |
6042 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
a6b91202 | 6043 | |
0909e3f8 RS |
6044 | # force use of mergesort (not portable outside Perl 5.8) |
6045 | use sort '_mergesort'; # note discouraging _ | |
6046 | @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; | |
6047 | ||
6048 | Warning: syntactical care is required when sorting the list returned from | |
6049 | a function. If you want to sort the list returned by the function call | |
6050 | C<find_records(@key)>, you can use: | |
6051 | ||
6052 | @contact = sort { $a cmp $b } find_records @key; | |
6053 | @contact = sort +find_records(@key); | |
6054 | @contact = sort &find_records(@key); | |
6055 | @contact = sort(find_records(@key)); | |
6056 | ||
6057 | If instead you want to sort the array @key with the comparison routine | |
6058 | C<find_records()> then you can use: | |
6059 | ||
6060 | @contact = sort { find_records() } @key; | |
6061 | @contact = sort find_records(@key); | |
6062 | @contact = sort(find_records @key); | |
6063 | @contact = sort(find_records (@key)); | |
6064 | ||
6065 | If you're using strict, you I<must not> declare $a | |
6066 | and $b as lexicals. They are package globals. That means | |
6067 | that if you're in the C<main> package and type | |
6068 | ||
6069 | @articles = sort {$b <=> $a} @files; | |
6070 | ||
6071 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), | |
6072 | but if you're in the C<FooPack> package, it's the same as typing | |
6073 | ||
6074 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; | |
6075 | ||
6076 | The comparison function is required to behave. If it returns | |
6077 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and | |
6078 | sometimes saying the opposite, for example) the results are not | |
6079 | well-defined. | |
6080 | ||
6081 | Because C<< <=> >> returns C<undef> when either operand is C<NaN> | |
6082 | (not-a-number), be careful when sorting with a | |
6083 | comparison function like C<< $a <=> $b >> any lists that might contain a | |
6084 | C<NaN>. The following example takes advantage that C<NaN != NaN> to | |
6085 | eliminate any C<NaN>s from the input list. | |
6086 | ||
6087 | @result = sort { $a <=> $b } grep { $_ == $_ } @input; | |
6088 | ||
6089 | =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST | |
6090 | X<splice> | |
6091 | ||
6092 | =item splice ARRAY or EXPR,OFFSET,LENGTH | |
6093 | ||
6094 | =item splice ARRAY or EXPR,OFFSET | |
6095 | ||
6096 | =item splice ARRAY or EXPR | |
6097 | ||
6098 | Removes the elements designated by OFFSET and LENGTH from an array, and | |
6099 | replaces them with the elements of LIST, if any. In list context, | |
6100 | returns the elements removed from the array. In scalar context, | |
6101 | returns the last element removed, or C<undef> if no elements are | |
6102 | removed. The array grows or shrinks as necessary. | |
6103 | If OFFSET is negative then it starts that far from the end of the array. | |
6104 | If LENGTH is omitted, removes everything from OFFSET onward. | |
6105 | If LENGTH is negative, removes the elements from OFFSET onward | |
6106 | except for -LENGTH elements at the end of the array. | |
6107 | If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is | |
6108 | past the end of the array, Perl issues a warning, and splices at the | |
6109 | end of the array. | |
6110 | ||
6111 | The following equivalences hold (assuming C<< $#a >= $i >> ) | |
6112 | ||
6113 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) | |
6114 | pop(@a) splice(@a,-1) | |
6115 | shift(@a) splice(@a,0,1) | |
6116 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) | |
6117 | $a[$i] = $y splice(@a,$i,1,$y) | |
6118 | ||
6119 | Example, assuming array lengths are passed before arrays: | |
6120 | ||
6121 | sub aeq { # compare two list values | |
6122 | my(@a) = splice(@_,0,shift); | |
6123 | my(@b) = splice(@_,0,shift); | |
6124 | return 0 unless @a == @b; # same len? | |
6125 | while (@a) { | |
6126 | return 0 if pop(@a) ne pop(@b); | |
6127 | } | |
6128 | return 1; | |
6129 | } | |
6130 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } | |
6131 | ||
6132 | Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a | |
6133 | reference to an unblessed array. The argument will be dereferenced | |
6134 | automatically. This aspect of C<splice> is considered highly experimental. | |
6135 | The exact behaviour may change in a future version of Perl. | |
6136 | ||
6137 | =item split /PATTERN/,EXPR,LIMIT | |
6138 | X<split> | |
6139 | ||
6140 | =item split /PATTERN/,EXPR | |
6141 | ||
6142 | =item split /PATTERN/ | |
6143 | ||
6144 | =item split | |
6145 | ||
6146 | Splits the string EXPR into a list of strings and returns that list. By | |
6147 | default, empty leading fields are preserved, and empty trailing ones are | |
6148 | deleted. (If all fields are empty, they are considered to be trailing.) | |
6149 | ||
6150 | In scalar context, returns the number of fields found. | |
6151 | ||
6152 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, | |
6153 | splits on whitespace (after skipping any leading whitespace). Anything | |
6154 | matching PATTERN is taken to be a delimiter separating the fields. (Note | |
6155 | that the delimiter may be longer than one character.) | |
6156 | ||
6157 | If LIMIT is specified and positive, it represents the maximum number | |
6158 | of fields the EXPR will be split into, though the actual number of | |
6159 | fields returned depends on the number of times PATTERN matches within | |
6160 | EXPR. If LIMIT is unspecified or zero, trailing null fields are | |
6161 | stripped (which potential users of C<pop> would do well to remember). | |
6162 | If LIMIT is negative, it is treated as if an arbitrarily large LIMIT | |
6163 | had been specified. Note that splitting an EXPR that evaluates to the | |
6164 | empty string always returns the empty list, regardless of the LIMIT | |
6165 | specified. | |
6166 | ||
6167 | A pattern matching the empty string (not to be confused with | |
6168 | an empty pattern C<//>, which is just one member of the set of patterns | |
6169 | matching the empty string), splits EXPR into individual | |
6170 | characters. For example: | |
6171 | ||
6172 | print join(':', split(/ */, 'hi there')), "\n"; | |
6173 | ||
6174 | produces the output 'h:i:t:h:e:r:e'. | |
6175 | ||
6176 | As a special case for C<split>, the empty pattern C<//> specifically | |
6177 | matches the empty string; this is not be confused with the normal use | |
6178 | of an empty pattern to mean the last successful match. So to split | |
6179 | a string into individual characters, the following: | |
6180 | ||
6181 | print join(':', split(//, 'hi there')), "\n"; | |
6182 | ||
6183 | produces the output 'h:i: :t:h:e:r:e'. | |
6184 | ||
6185 | Empty leading fields are produced when there are positive-width matches at | |
6186 | the beginning of the string; a zero-width match at the beginning of | |
6187 | the string does not produce an empty field. For example: | |
6188 | ||
6189 | print join(':', split(/(?=\w)/, 'hi there!')); | |
6190 | ||
6191 | produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other | |
6192 | hand, are produced when there is a match at the end of the string (and | |
6193 | when LIMIT is given and is not 0), regardless of the length of the match. | |
6194 | For example: | |
6195 | ||
6196 | print join(':', split(//, 'hi there!', -1)), "\n"; | |
6197 | print join(':', split(/\W/, 'hi there!', -1)), "\n"; | |
6198 | ||
6199 | produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively, | |
6200 | both with an empty trailing field. | |
6201 | ||
6202 | The LIMIT parameter can be used to split a line partially | |
6203 | ||
6204 | ($login, $passwd, $remainder) = split(/:/, $_, 3); | |
6205 | ||
6206 | When assigning to a list, if LIMIT is omitted, or zero, Perl supplies | |
6207 | a LIMIT one larger than the number of variables in the list, to avoid | |
6208 | unnecessary work. For the list above LIMIT would have been 4 by | |
6209 | default. In time critical applications it behooves you not to split | |
6210 | into more fields than you really need. | |
6211 | ||
6212 | If the PATTERN contains parentheses, additional list elements are | |
6213 | created from each matching substring in the delimiter. | |
6214 | ||
6215 | split(/([,-])/, "1-10,20", 3); | |
6216 | ||
6217 | produces the list value | |
6218 | ||
6219 | (1, '-', 10, ',', 20) | |
6220 | ||
6221 | If you had the entire header of a normal Unix email message in $header, | |
6222 | you could split it up into fields and their values this way: | |
6223 | ||
6224 | $header =~ s/\n(?=\s)//g; # fix continuation lines | |
6225 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); | |
6226 | ||
6227 | The pattern C</PATTERN/> may be replaced with an expression to specify | |
6228 | patterns that vary at runtime. (To do runtime compilation only once, | |
6229 | use C</$variable/o>.) | |
6230 | ||
6231 | As a special case, specifying a PATTERN of space (S<C<' '>>) will split on | |
6232 | white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can | |
6233 | be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>> | |
6234 | will give you as many initial null fields (empty string) as there are leading spaces. | |
6235 | A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading | |
6236 | whitespace produces a null first field. A C<split> with no arguments | |
6237 | really does a S<C<split(' ', $_)>> internally. | |
6238 | ||
6239 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't | |
6240 | much use otherwise. | |
6241 | ||
6242 | Example: | |
6243 | ||
6244 | open(PASSWD, '/etc/passwd'); | |
6245 | while (<PASSWD>) { | |
6246 | chomp; | |
6247 | ($login, $passwd, $uid, $gid, | |
6248 | $gcos, $home, $shell) = split(/:/); | |
6249 | #... | |
6250 | } | |
6251 | ||
6252 | As with regular pattern matching, any capturing parentheses that are not | |
6253 | matched in a C<split()> will be set to C<undef> when returned: | |
6254 | ||
6255 | @fields = split /(A)|B/, "1A2B3"; | |
6256 | # @fields is (1, 'A', 2, undef, 3) | |
6257 | ||
6258 | =item sprintf FORMAT, LIST | |
6259 | X<sprintf> | |
6260 | ||
6261 | Returns a string formatted by the usual C<printf> conventions of the C | |
6262 | library function C<sprintf>. See below for more details | |
6263 | and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of | |
6264 | the general principles. | |
6265 | ||
6266 | For example: | |
6267 | ||
6268 | # Format number with up to 8 leading zeroes | |
6269 | $result = sprintf("%08d", $number); | |
6270 | ||
6271 | # Round number to 3 digits after decimal point | |
6272 | $rounded = sprintf("%.3f", $number); | |
6273 | ||
6274 | Perl does its own C<sprintf> formatting: it emulates the C | |
6275 | function sprintf(3), but doesn't use it except for floating-point | |
a6b91202 A |
6276 | numbers, and even then only standard modifiers are allowed. |
6277 | Non-standard extensions in your local sprintf(3) are | |
0909e3f8 RS |
6278 | therefore unavailable from Perl. |
6279 | ||
6280 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you | |
6281 | pass it an array as your first argument. The array is given scalar context, | |
6282 | and instead of using the 0th element of the array as the format, Perl will | |
6283 | use the count of elements in the array as the format, which is almost never | |
6284 | useful. | |
6285 | ||
6286 | Perl's C<sprintf> permits the following universally-known conversions: | |
6287 | ||
6288 | %% a percent sign | |
6289 | %c a character with the given number | |
6290 | %s a string | |
6291 | %d a signed integer, in decimal | |
6292 | %u an unsigned integer, in decimal | |
6293 | %o an unsigned integer, in octal | |
6294 | %x an unsigned integer, in hexadecimal | |
6295 | %e a floating-point number, in scientific notation | |
6296 | %f a floating-point number, in fixed decimal notation | |
6297 | %g a floating-point number, in %e or %f notation | |
6298 | ||
6299 | In addition, Perl permits the following widely-supported conversions: | |
6300 | ||
6301 | %X like %x, but using upper-case letters | |
6302 | %E like %e, but using an upper-case "E" | |
6303 | %G like %g, but with an upper-case "E" (if applicable) | |
6304 | %b an unsigned integer, in binary | |
6305 | %B like %b, but using an upper-case "B" with the # flag | |
6306 | %p a pointer (outputs the Perl value's address in hexadecimal) | |
6307 | %n special: *stores* the number of characters output so far | |
6308 | into the next variable in the parameter list | |
6309 | ||
6310 | Finally, for backward (and we do mean "backward") compatibility, Perl | |
6311 | permits these unnecessary but widely-supported conversions: | |
6312 | ||
6313 | %i a synonym for %d | |
6314 | %D a synonym for %ld | |
6315 | %U a synonym for %lu | |
6316 | %O a synonym for %lo | |
6317 | %F a synonym for %f | |
6318 | ||
6319 | Note that the number of exponent digits in the scientific notation produced | |
6320 | by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the | |
6321 | exponent less than 100 is system-dependent: it may be three or less | |
6322 | (zero-padded as necessary). In other words, 1.23 times ten to the | |
6323 | 99th may be either "1.23e99" or "1.23e099". | |
6324 | ||
6325 | Between the C<%> and the format letter, you may specify several | |
6326 | additional attributes controlling the interpretation of the format. | |
6327 | In order, these are: | |
6328 | ||
6329 | =over 4 | |
6330 | ||
6331 | =item format parameter index | |
6332 | ||
6333 | An explicit format parameter index, such as C<2$>. By default sprintf | |
6334 | will format the next unused argument in the list, but this allows you | |
6335 | to take the arguments out of order: | |
6336 | ||
6337 | printf '%2$d %1$d', 12, 34; # prints "34 12" | |
6338 | printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" | |
6339 | ||
6340 | =item flags | |
6341 | ||
6342 | one or more of: | |
6343 | ||
6344 | space prefix non-negative number with a space | |
6345 | + prefix non-negative number with a plus sign | |
6346 | - left-justify within the field | |
6347 | 0 use zeros, not spaces, to right-justify | |
6348 | # ensure the leading "0" for any octal, | |
6349 | prefix non-zero hexadecimal with "0x" or "0X", | |
6350 | prefix non-zero binary with "0b" or "0B" | |
6351 | ||
6352 | For example: | |
6353 | ||
6354 | printf '<% d>', 12; # prints "< 12>" | |
6355 | printf '<%+d>', 12; # prints "<+12>" | |
6356 | printf '<%6s>', 12; # prints "< 12>" | |
6357 | printf '<%-6s>', 12; # prints "<12 >" | |
6358 | printf '<%06s>', 12; # prints "<000012>" | |
6359 | printf '<%#o>', 12; # prints "<014>" | |
6360 | printf '<%#x>', 12; # prints "<0xc>" | |
6361 | printf '<%#X>', 12; # prints "<0XC>" | |
6362 | printf '<%#b>', 12; # prints "<0b1100>" | |
6363 | printf '<%#B>', 12; # prints "<0B1100>" | |
6364 | ||
6365 | When a space and a plus sign are given as the flags at once, | |
6366 | a plus sign is used to prefix a positive number. | |
6367 | ||
6368 | printf '<%+ d>', 12; # prints "<+12>" | |
6369 | printf '<% +d>', 12; # prints "<+12>" | |
6370 | ||
6371 | When the # flag and a precision are given in the %o conversion, | |
6372 | the precision is incremented if it's necessary for the leading "0". | |
6373 | ||
6374 | printf '<%#.5o>', 012; # prints "<00012>" | |
6375 | printf '<%#.5o>', 012345; # prints "<012345>" | |
6376 | printf '<%#.0o>', 0; # prints "<0>" | |
6377 | ||
6378 | =item vector flag | |
6379 | ||
6380 | This flag tells Perl to interpret the supplied string as a vector of | |
6381 | integers, one for each character in the string. Perl applies the format to | |
6382 | each integer in turn, then joins the resulting strings with a separator (a | |
6383 | dot C<.> by default). This can be useful for displaying ordinal values of | |
6384 | characters in arbitrary strings: | |
6385 | ||
6386 | printf "%vd", "AB\x{100}"; # prints "65.66.256" | |
6387 | printf "version is v%vd\n", $^V; # Perl's version | |
6388 | ||
6389 | Put an asterisk C<*> before the C<v> to override the string to | |
6390 | use to separate the numbers: | |
6391 | ||
6392 | printf "address is %*vX\n", ":", $addr; # IPv6 address | |
6393 | printf "bits are %0*v8b\n", " ", $bits; # random bitstring | |
6394 | ||
6395 | You can also explicitly specify the argument number to use for | |
6396 | the join string using something like C<*2$v>; for example: | |
6397 | ||
6398 | printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses | |
6399 | ||
6400 | =item (minimum) width | |
6401 | ||
6402 | Arguments are usually formatted to be only as wide as required to | |
6403 | display the given value. You can override the width by putting | |
6404 | a number here, or get the width from the next argument (with C<*>) | |
6405 | or from a specified argument (e.g., with C<*2$>): | |
6406 | ||
6407 | printf "<%s>", "a"; # prints "<a>" | |
6408 | printf "<%6s>", "a"; # prints "< a>" | |
6409 | printf "<%*s>", 6, "a"; # prints "< a>" | |
6410 | printf "<%*2$s>", "a", 6; # prints "< a>" | |
6411 | printf "<%2s>", "long"; # prints "<long>" (does not truncate) | |
6412 | ||
6413 | If a field width obtained through C<*> is negative, it has the same | |
6414 | effect as the C<-> flag: left-justification. | |
6415 | ||
6416 | =item precision, or maximum width | |
6417 | X<precision> | |
6418 | ||
6419 | You can specify a precision (for numeric conversions) or a maximum | |
6420 | width (for string conversions) by specifying a C<.> followed by a number. | |
6421 | For floating-point formats except C<g> and C<G>, this specifies | |
6422 | how many places right of the decimal point to show (the default being 6). | |
6423 | For example: | |
6424 | ||
6425 | # these examples are subject to system-specific variation | |
6426 | printf '<%f>', 1; # prints "<1.000000>" | |
6427 | printf '<%.1f>', 1; # prints "<1.0>" | |
6428 | printf '<%.0f>', 1; # prints "<1>" | |
6429 | printf '<%e>', 10; # prints "<1.000000e+01>" | |
6430 | printf '<%.1e>', 10; # prints "<1.0e+01>" | |
6431 | ||
6432 | For "g" and "G", this specifies the maximum number of digits to show, | |
a6b91202 | 6433 | including those prior to the decimal point and those after it; for |
0909e3f8 RS |
6434 | example: |
6435 | ||
6436 | # These examples are subject to system-specific variation. | |
6437 | printf '<%g>', 1; # prints "<1>" | |
6438 | printf '<%.10g>', 1; # prints "<1>" | |
6439 | printf '<%g>', 100; # prints "<100>" | |
6440 | printf '<%.1g>', 100; # prints "<1e+02>" | |
6441 | printf '<%.2g>', 100.01; # prints "<1e+02>" | |
6442 | printf '<%.5g>', 100.01; # prints "<100.01>" | |
6443 | printf '<%.4g>', 100.01; # prints "<100>" | |
6444 | ||
6445 | For integer conversions, specifying a precision implies that the | |
6446 | output of the number itself should be zero-padded to this width, | |
6447 | where the 0 flag is ignored: | |
6448 | ||
6449 | printf '<%.6d>', 1; # prints "<000001>" | |
6450 | printf '<%+.6d>', 1; # prints "<+000001>" | |
6451 | printf '<%-10.6d>', 1; # prints "<000001 >" | |
6452 | printf '<%10.6d>', 1; # prints "< 000001>" | |
6453 | printf '<%010.6d>', 1; # prints "< 000001>" | |
6454 | printf '<%+10.6d>', 1; # prints "< +000001>" | |
6455 | ||
6456 | printf '<%.6x>', 1; # prints "<000001>" | |
6457 | printf '<%#.6x>', 1; # prints "<0x000001>" | |
6458 | printf '<%-10.6x>', 1; # prints "<000001 >" | |
6459 | printf '<%10.6x>', 1; # prints "< 000001>" | |
6460 | printf '<%010.6x>', 1; # prints "< 000001>" | |
6461 | printf '<%#10.6x>', 1; # prints "< 0x000001>" | |
6462 | ||
6463 | For string conversions, specifying a precision truncates the string | |
6464 | to fit the specified width: | |
6465 | ||
6466 | printf '<%.5s>', "truncated"; # prints "<trunc>" | |
6467 | printf '<%10.5s>', "truncated"; # prints "< trunc>" | |
6468 | ||
6469 | You can also get the precision from the next argument using C<.*>: | |
6470 | ||
6471 | printf '<%.6x>', 1; # prints "<000001>" | |
6472 | printf '<%.*x>', 6, 1; # prints "<000001>" | |
6473 | ||
6474 | If a precision obtained through C<*> is negative, it counts | |
6475 | as having no precision at all. | |
6476 | ||
6477 | printf '<%.*s>', 7, "string"; # prints "<string>" | |
6478 | printf '<%.*s>', 3, "string"; # prints "<str>" | |
6479 | printf '<%.*s>', 0, "string"; # prints "<>" | |
6480 | printf '<%.*s>', -1, "string"; # prints "<string>" | |
6481 | ||
6482 | printf '<%.*d>', 1, 0; # prints "<0>" | |
6483 | printf '<%.*d>', 0, 0; # prints "<>" | |
6484 | printf '<%.*d>', -1, 0; # prints "<0>" | |
6485 | ||
6486 | You cannot currently get the precision from a specified number, | |
6487 | but it is intended that this will be possible in the future, for | |
6488 | example using C<.*2$>: | |
6489 | ||
6490 | printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>" | |
6491 | ||
6492 | =item size | |
6493 | ||
6494 | For numeric conversions, you can specify the size to interpret the | |
6495 | number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer | |
6496 | conversions (C<d u o x X b i D U O>), numbers are usually assumed to be | |
6497 | whatever the default integer size is on your platform (usually 32 or 64 | |
6498 | bits), but you can override this to use instead one of the standard C types, | |
6499 | as supported by the compiler used to build Perl: | |
6500 | ||
6501 | hh interpret integer as C type "char" or "unsigned char" | |
6502 | on Perl 5.14 or later | |
6503 | h interpret integer as C type "short" or "unsigned short" | |
a6b91202 | 6504 | j interpret integer as C type "intmax_t" on Perl 5.14 |
0909e3f8 RS |
6505 | or later, and only with a C99 compiler (unportable) |
6506 | l interpret integer as C type "long" or "unsigned long" | |
6507 | q, L, or ll interpret integer as C type "long long", "unsigned long long", | |
6508 | or "quad" (typically 64-bit integers) | |
6509 | t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later | |
6510 | z interpret integer as C type "size_t" on Perl 5.14 or later | |
6511 | ||
6512 | As of 5.14, none of these raises an exception if they are not supported on | |
6513 | your platform. However, if warnings are enabled, a warning of the | |
a6b91202 | 6514 | C<printf> warning class is issued on an unsupported conversion flag. |
0909e3f8 RS |
6515 | Should you instead prefer an exception, do this: |
6516 | ||
6517 | use warnings FATAL => "printf"; | |
6518 | ||
6519 | If you would like to know about a version dependency before you | |
6520 | start running the program, put something like this at its top: | |
6521 | ||
6522 | use 5.014; # for hh/j/t/z/ printf modifiers | |
6523 | ||
6524 | You can find out whether your Perl supports quads via L<Config>: | |
6525 | ||
6526 | use Config; | |
6527 | if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) { | |
6528 | print "Nice quads!\n"; | |
6529 | } | |
6530 | ||
6531 | For floating-point conversions (C<e f g E F G>), numbers are usually assumed | |
6532 | to be the default floating-point size on your platform (double or long double), | |
6533 | but you can force "long double" with C<q>, C<L>, or C<ll> if your | |
6534 | platform supports them. You can find out whether your Perl supports long | |
6535 | doubles via L<Config>: | |
6536 | ||
6537 | use Config; | |
6538 | print "long doubles\n" if $Config{d_longdbl} eq "define"; | |
6539 | ||
6540 | You can find out whether Perl considers "long double" to be the default | |
6541 | floating-point size to use on your platform via L<Config>: | |
6542 | ||
6543 | use Config; | |
6544 | if ($Config{uselongdouble} eq "define") { | |
6545 | print "long doubles by default\n"; | |
6546 | } | |
6547 | ||
6548 | It can also be that long doubles and doubles are the same thing: | |
6549 | ||
6550 | use Config; | |
6551 | ($Config{doublesize} == $Config{longdblsize}) && | |
6552 | print "doubles are long doubles\n"; | |
6553 | ||
6554 | The size specifier C<V> has no effect for Perl code, but is supported for | |
6555 | compatibility with XS code. It means "use the standard size for a Perl | |
6556 | integer or floating-point number", which is the default. | |
6557 | ||
6558 | =item order of arguments | |
6559 | ||
6560 | Normally, sprintf() takes the next unused argument as the value to | |
6561 | format for each format specification. If the format specification | |
6562 | uses C<*> to require additional arguments, these are consumed from | |
6563 | the argument list in the order they appear in the format | |
6564 | specification I<before> the value to format. Where an argument is | |
6565 | specified by an explicit index, this does not affect the normal | |
6566 | order for the arguments, even when the explicitly specified index | |
6567 | would have been the next argument. | |
6568 | ||
6569 | So: | |
6570 | ||
6571 | printf "<%*.*s>", $a, $b, $c; | |
6572 | ||
6573 | uses C<$a> for the width, C<$b> for the precision, and C<$c> | |
6574 | as the value to format; while: | |
6575 | ||
6576 | printf "<%*1$.*s>", $a, $b; | |
6577 | ||
6578 | would use C<$a> for the width and precision, and C<$b> as the | |
6579 | value to format. | |
6580 | ||
6581 | Here are some more examples; be aware that when using an explicit | |
6582 | index, the C<$> may need escaping: | |
6583 | ||
6584 | printf "%2\$d %d\n", 12, 34; # will print "34 12\n" | |
6585 | printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" | |
6586 | printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" | |
6587 | printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" | |
6588 | ||
6589 | =back | |
6590 | ||
6591 | If C<use locale> is in effect and POSIX::setlocale() has been called, | |
6592 | the character used for the decimal separator in formatted floating-point | |
6593 | numbers is affected by the LC_NUMERIC locale. See L<perllocale> | |
6594 | and L<POSIX>. | |
6595 | ||
6596 | =item sqrt EXPR | |
6597 | X<sqrt> X<root> X<square root> | |
6598 | ||
6599 | =item sqrt | |
6600 | ||
6601 | Return the positive square root of EXPR. If EXPR is omitted, uses | |
6602 | C<$_>. Works only for non-negative operands unless you've | |
6603 | loaded the C<Math::Complex> module. | |
6604 | ||
6605 | use Math::Complex; | |
6606 | print sqrt(-4); # prints 2i | |
6607 | ||
6608 | =item srand EXPR | |
6609 | X<srand> X<seed> X<randseed> | |
6610 | ||
6611 | =item srand | |
6612 | ||
6613 | Sets and returns the random number seed for the C<rand> operator. | |
6614 | ||
6615 | The point of the function is to "seed" the C<rand> function so that | |
6616 | C<rand> can produce a different sequence each time you run your | |
6617 | program. When called with a parameter, C<srand> uses that for the seed; | |
6618 | otherwise it (semi-)randomly chooses a seed. In either case, starting with | |
6619 | Perl 5.14, it returns the seed. | |
6620 | ||
6621 | If C<srand()> is not called explicitly, it is called implicitly without a | |
6622 | parameter at the first use of the C<rand> operator. However, this was not true | |
6623 | of versions of Perl before 5.004, so if your script will run under older | |
6624 | Perl versions, it should call C<srand>; otherwise most programs won't call | |
6625 | C<srand()> at all. | |
6626 | ||
6627 | But there are a few situations in recent Perls where programs are likely to | |
6628 | want to call C<srand>. One is for generating predictable results generally for | |
6629 | testing or debugging. There, you use C<srand($seed)>, with the same C<$seed> | |
6630 | each time. Another case is that you may want to call C<srand()> | |
6631 | after a C<fork()> to avoid child processes sharing the same seed value as the | |
6632 | parent (and consequently each other). | |
6633 | ||
6634 | Do B<not> call C<srand()> (i.e., without an argument) more than once per | |
6635 | process. The internal state of the random number generator should | |
6636 | contain more entropy than can be provided by any seed, so calling | |
6637 | C<srand()> again actually I<loses> randomness. | |
6638 | ||
6639 | Most implementations of C<srand> take an integer and will silently | |
6640 | truncate decimal numbers. This means C<srand(42)> will usually | |
6641 | produce the same results as C<srand(42.1)>. To be safe, always pass | |
6642 | C<srand> an integer. | |
6643 | ||
6644 | In versions of Perl prior to 5.004 the default seed was just the | |
6645 | current C<time>. This isn't a particularly good seed, so many old | |
6646 | programs supply their own seed value (often C<time ^ $$> or C<time ^ | |
6647 | ($$ + ($$ << 15))>), but that isn't necessary any more. | |
6648 | ||
6649 | Frequently called programs (like CGI scripts) that simply use | |
6650 | ||
6651 | time ^ $$ | |
6652 | ||
6653 | for a seed can fall prey to the mathematical property that | |
6654 | ||
6655 | a^b == (a+1)^(b+1) | |
6656 | ||
6657 | one-third of the time. So don't do that. | |
6658 | ||
6659 | A typical use of the returned seed is for a test program which has too many | |
6660 | combinations to test comprehensively in the time available to it each run. It | |
6661 | can test a random subset each time, and should there be a failure, log the seed | |
6662 | used for that run so that it can later be used to reproduce the same results. | |
6663 | ||
6664 | B<C<rand()> is not cryptographically secure. You should not rely | |
6665 | on it in security-sensitive situations.> As of this writing, a | |
6666 | number of third-party CPAN modules offer random number generators | |
6667 | intended by their authors to be cryptographically secure, | |
6668 | including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, | |
6669 | and L<Math::TrulyRandom>. | |
6670 | ||
6671 | =item stat FILEHANDLE | |
6672 | X<stat> X<file, status> X<ctime> | |
6673 | ||
6674 | =item stat EXPR | |
6675 | ||
6676 | =item stat DIRHANDLE | |
6677 | ||
6678 | =item stat | |
6679 | ||
6680 | Returns a 13-element list giving the status info for a file, either | |
a6b91202 | 6681 | the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is |
0909e3f8 RS |
6682 | omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically |
6683 | used as follows: | |
6684 | ||
6685 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, | |
6686 | $atime,$mtime,$ctime,$blksize,$blocks) | |
6687 | = stat($filename); | |
6688 | ||
6689 | Not all fields are supported on all filesystem types. Here are the | |
6690 | meanings of the fields: | |
6691 | ||
6692 | 0 dev device number of filesystem | |
6693 | 1 ino inode number | |
6694 | 2 mode file mode (type and permissions) | |
6695 | 3 nlink number of (hard) links to the file | |
6696 | 4 uid numeric user ID of file's owner | |
6697 | 5 gid numeric group ID of file's owner | |
6698 | 6 rdev the device identifier (special files only) | |
6699 | 7 size total size of file, in bytes | |
6700 | 8 atime last access time in seconds since the epoch | |
6701 | 9 mtime last modify time in seconds since the epoch | |
6702 | 10 ctime inode change time in seconds since the epoch (*) | |
6703 | 11 blksize preferred block size for file system I/O | |
6704 | 12 blocks actual number of blocks allocated | |
6705 | ||
6706 | (The epoch was at 00:00 January 1, 1970 GMT.) | |
6707 | ||
6708 | (*) Not all fields are supported on all filesystem types. Notably, the | |
6709 | ctime field is non-portable. In particular, you cannot expect it to be a | |
6710 | "creation time"; see L<perlport/"Files and Filesystems"> for details. | |
6711 | ||
6712 | If C<stat> is passed the special filehandle consisting of an underline, no | |
6713 | stat is done, but the current contents of the stat structure from the | |
6714 | last C<stat>, C<lstat>, or filetest are returned. Example: | |
6715 | ||
6716 | if (-x $file && (($d) = stat(_)) && $d < 0) { | |
6717 | print "$file is executable NFS file\n"; | |
6718 | } | |
6719 | ||
6720 | (This works on machines only for which the device number is negative | |
6721 | under NFS.) | |
6722 | ||
6723 | Because the mode contains both the file type and its permissions, you | |
6724 | should mask off the file type portion and (s)printf using a C<"%o"> | |
6725 | if you want to see the real permissions. | |
6726 | ||
6727 | $mode = (stat($filename))[2]; | |
6728 | printf "Permissions are %04o\n", $mode & 07777; | |
6729 | ||
6730 | In scalar context, C<stat> returns a boolean value indicating success | |
6731 | or failure, and, if successful, sets the information associated with | |
6732 | the special filehandle C<_>. | |
6733 | ||
6734 | The L<File::stat> module provides a convenient, by-name access mechanism: | |
6735 | ||
6736 | use File::stat; | |
6737 | $sb = stat($filename); | |
6738 | printf "File is %s, size is %s, perm %04o, mtime %s\n", | |
6739 | $filename, $sb->size, $sb->mode & 07777, | |
6740 | scalar localtime $sb->mtime; | |
6741 | ||
6742 | You can import symbolic mode constants (C<S_IF*>) and functions | |
6743 | (C<S_IS*>) from the Fcntl module: | |
6744 | ||
6745 | use Fcntl ':mode'; | |
6746 | ||
6747 | $mode = (stat($filename))[2]; | |
6748 | ||
6749 | $user_rwx = ($mode & S_IRWXU) >> 6; | |
6750 | $group_read = ($mode & S_IRGRP) >> 3; | |
6751 | $other_execute = $mode & S_IXOTH; | |
6752 | ||
6753 | printf "Permissions are %04o\n", S_IMODE($mode), "\n"; | |
6754 | ||
6755 | $is_setuid = $mode & S_ISUID; | |
6756 | $is_directory = S_ISDIR($mode); | |
6757 | ||
6758 | You could write the last two using the C<-u> and C<-d> operators. | |
6759 | Commonly available C<S_IF*> constants are: | |
6760 | ||
6761 | # Permissions: read, write, execute, for user, group, others. | |
6762 | ||
6763 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR | |
6764 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP | |
6765 | S_IRWXO S_IROTH S_IWOTH S_IXOTH | |
6766 | ||
6767 | # Setuid/Setgid/Stickiness/SaveText. | |
6768 | # Note that the exact meaning of these is system-dependent. | |
6769 | ||
6770 | S_ISUID S_ISGID S_ISVTX S_ISTXT | |
6771 | ||
6772 | # File types. Not all are necessarily available on | |
6773 | # your system. | |
6774 | ||
6775 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR | |
6776 | S_IFIFO S_IFSOCK S_IFWHT S_ENFMT | |
6777 | ||
6778 | # The following are compatibility aliases for S_IRUSR, | |
6779 | # S_IWUSR, and S_IXUSR. | |
6780 | ||
6781 | S_IREAD S_IWRITE S_IEXEC | |
6782 | ||
6783 | and the C<S_IF*> functions are | |
6784 | ||
6785 | S_IMODE($mode) the part of $mode containing the permission | |
6786 | bits and the setuid/setgid/sticky bits | |
6787 | ||
6788 | S_IFMT($mode) the part of $mode containing the file type | |
6789 | which can be bit-anded with (for example) | |
6790 | S_IFREG or with the following functions | |
6791 | ||
6792 | # The operators -f, -d, -l, -b, -c, -p, and -S. | |
6793 | ||
6794 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) | |
6795 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) | |
6796 | ||
6797 | # No direct -X operator counterpart, but for the first one | |
6798 | # the -g operator is often equivalent. The ENFMT stands for | |
6799 | # record flocking enforcement, a platform-dependent feature. | |
6800 | ||
6801 | S_ISENFMT($mode) S_ISWHT($mode) | |
6802 | ||
6803 | See your native chmod(2) and stat(2) documentation for more details | |
6804 | about the C<S_*> constants. To get status info for a symbolic link | |
6805 | instead of the target file behind the link, use the C<lstat> function. | |
6806 | ||
6807 | Portability issues: L<perlport/stat>. | |
6808 | ||
6809 | =item state EXPR | |
6810 | X<state> | |
6811 | ||
6812 | =item state TYPE EXPR | |
6813 | ||
6814 | =item state EXPR : ATTRS | |
6815 | ||
6816 | =item state TYPE EXPR : ATTRS | |
6817 | ||
6818 | C<state> declares a lexically scoped variable, just like C<my>. | |
6819 | However, those variables will never be reinitialized, contrary to | |
6820 | lexical variables that are reinitialized each time their enclosing block | |
6821 | is entered. | |
6822 | See L<perlsub/"Persistent Private Variables"> for details. | |
6823 | ||
a6b91202 | 6824 | C<state> variables are enabled only when the C<use feature "state"> pragma |
0909e3f8 RS |
6825 | is in effect, unless the keyword is written as C<CORE::state>. |
6826 | See also L<feature>. | |
6827 | ||
6828 | =item study SCALAR | |
6829 | X<study> | |
6830 | ||
6831 | =item study | |
6832 | ||
6833 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of | |
6834 | doing many pattern matches on the string before it is next modified. | |
6835 | This may or may not save time, depending on the nature and number of | |
6836 | patterns you are searching and the distribution of character | |
6837 | frequencies in the string to be searched; you probably want to compare | |
6838 | run times with and without it to see which is faster. Those loops | |
6839 | that scan for many short constant strings (including the constant | |
6840 | parts of more complex patterns) will benefit most. | |
6841 | (The way C<study> works is this: a linked list of every | |
6842 | character in the string to be searched is made, so we know, for | |
6843 | example, where all the C<'k'> characters are. From each search string, | |
6844 | the rarest character is selected, based on some static frequency tables | |
6845 | constructed from some C programs and English text. Only those places | |
6846 | that contain this "rarest" character are examined.) | |
6847 | ||
6848 | For example, here is a loop that inserts index producing entries | |
6849 | before any line containing a certain pattern: | |
6850 | ||
6851 | while (<>) { | |
6852 | study; | |
6853 | print ".IX foo\n" if /\bfoo\b/; | |
6854 | print ".IX bar\n" if /\bbar\b/; | |
6855 | print ".IX blurfl\n" if /\bblurfl\b/; | |
6856 | # ... | |
6857 | print; | |
6858 | } | |
6859 | ||
6860 | In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f> | |
6861 | will be looked at, because C<f> is rarer than C<o>. In general, this is | |
6862 | a big win except in pathological cases. The only question is whether | |
6863 | it saves you more time than it took to build the linked list in the | |
6864 | first place. | |
6865 | ||
6866 | Note that if you have to look for strings that you don't know till | |
6867 | runtime, you can build an entire loop as a string and C<eval> that to | |
6868 | avoid recompiling all your patterns all the time. Together with | |
6869 | undefining C<$/> to input entire files as one record, this can be quite | |
6870 | fast, often faster than specialized programs like fgrep(1). The following | |
6871 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints | |
6872 | out the names of those files that contain a match: | |
6873 | ||
6874 | $search = 'while (<>) { study;'; | |
6875 | foreach $word (@words) { | |
6876 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; | |
6877 | } | |
6878 | $search .= "}"; | |
6879 | @ARGV = @files; | |
6880 | undef $/; | |
6881 | eval $search; # this screams | |
6882 | $/ = "\n"; # put back to normal input delimiter | |
6883 | foreach $file (sort keys(%seen)) { | |
6884 | print $file, "\n"; | |
6885 | } | |
6886 | ||
6887 | =item sub NAME BLOCK | |
6888 | X<sub> | |
6889 | ||
6890 | =item sub NAME (PROTO) BLOCK | |
6891 | ||
6892 | =item sub NAME : ATTRS BLOCK | |
6893 | ||
6894 | =item sub NAME (PROTO) : ATTRS BLOCK | |
6895 | ||
6896 | This is subroutine definition, not a real function I<per se>. Without a | |
6897 | BLOCK it's just a forward declaration. Without a NAME, it's an anonymous | |
6898 | function declaration, so does return a value: the CODE ref of the closure | |
6899 | just created. | |
6900 | ||
6901 | See L<perlsub> and L<perlref> for details about subroutines and | |
6902 | references; see L<attributes> and L<Attribute::Handlers> for more | |
6903 | information about attributes. | |
6904 | ||
6905 | =item __SUB__ | |
6906 | X<__SUB__> | |
6907 | ||
6908 | A special token that returns the a reference to the current subroutine, or | |
6909 | C<undef> outside of a subroutine. | |
6910 | ||
6911 | This token is only available under C<use v5.16> or the "current_sub" | |
6912 | feature. See L<feature>. | |
6913 | ||
6914 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT | |
6915 | X<substr> X<substring> X<mid> X<left> X<right> | |
6916 | ||
6917 | =item substr EXPR,OFFSET,LENGTH | |
6918 | ||
6919 | =item substr EXPR,OFFSET | |
6920 | ||
6921 | Extracts a substring out of EXPR and returns it. First character is at | |
6922 | offset zero. If OFFSET is negative, starts | |
6923 | that far back from the end of the string. If LENGTH is omitted, returns | |
6924 | everything through the end of the string. If LENGTH is negative, leaves that | |
6925 | many characters off the end of the string. | |
6926 | ||
6927 | my $s = "The black cat climbed the green tree"; | |
6928 | my $color = substr $s, 4, 5; # black | |
6929 | my $middle = substr $s, 4, -11; # black cat climbed the | |
6930 | my $end = substr $s, 14; # climbed the green tree | |
6931 | my $tail = substr $s, -4; # tree | |
6932 | my $z = substr $s, -4, 2; # tr | |
6933 | ||
6934 | You can use the substr() function as an lvalue, in which case EXPR | |
6935 | must itself be an lvalue. If you assign something shorter than LENGTH, | |
6936 | the string will shrink, and if you assign something longer than LENGTH, | |
6937 | the string will grow to accommodate it. To keep the string the same | |
6938 | length, you may need to pad or chop your value using C<sprintf>. | |
6939 | ||
6940 | If OFFSET and LENGTH specify a substring that is partly outside the | |
6941 | string, only the part within the string is returned. If the substring | |
6942 | is beyond either end of the string, substr() returns the undefined | |
6943 | value and produces a warning. When used as an lvalue, specifying a | |
6944 | substring that is entirely outside the string raises an exception. | |
6945 | Here's an example showing the behavior for boundary cases: | |
6946 | ||
6947 | my $name = 'fred'; | |
6948 | substr($name, 4) = 'dy'; # $name is now 'freddy' | |
6949 | my $null = substr $name, 6, 2; # returns "" (no warning) | |
6950 | my $oops = substr $name, 7; # returns undef, with warning | |
6951 | substr($name, 7) = 'gap'; # raises an exception | |
6952 | ||
6953 | An alternative to using substr() as an lvalue is to specify the | |
6954 | replacement string as the 4th argument. This allows you to replace | |
6955 | parts of the EXPR and return what was there before in one operation, | |
6956 | just as you can with splice(). | |
6957 | ||
6958 | my $s = "The black cat climbed the green tree"; | |
6959 | my $z = substr $s, 14, 7, "jumped from"; # climbed | |
6960 | # $s is now "The black cat jumped from the green tree" | |
6961 | ||
6962 | Note that the lvalue returned by the three-argument version of substr() acts as | |
6963 | a 'magic bullet'; each time it is assigned to, it remembers which part | |
6964 | of the original string is being modified; for example: | |
6965 | ||
6966 | $x = '1234'; | |
6967 | for (substr($x,1,2)) { | |
6968 | $_ = 'a'; print $x,"\n"; # prints 1a4 | |
6969 | $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 | |
6970 | $x = '56789'; | |
6971 | $_ = 'pq'; print $x,"\n"; # prints 5pq9 | |
6972 | } | |
6973 | ||
6974 | Prior to Perl version 5.9.1, the result of using an lvalue multiple times was | |
6975 | unspecified. | |
6976 | ||
6977 | =item symlink OLDFILE,NEWFILE | |
6978 | X<symlink> X<link> X<symbolic link> X<link, symbolic> | |
6979 | ||
6980 | Creates a new filename symbolically linked to the old filename. | |
6981 | Returns C<1> for success, C<0> otherwise. On systems that don't support | |
6982 | symbolic links, raises an exception. To check for that, | |
6983 | use eval: | |
6984 | ||
6985 | $symlink_exists = eval { symlink("",""); 1 }; | |
6986 | ||
6987 | Portability issues: L<perlport/symlink>. | |
6988 | ||
6989 | =item syscall NUMBER, LIST | |
6990 | X<syscall> X<system call> | |
6991 | ||
6992 | Calls the system call specified as the first element of the list, | |
6993 | passing the remaining elements as arguments to the system call. If | |
6994 | unimplemented, raises an exception. The arguments are interpreted | |
6995 | as follows: if a given argument is numeric, the argument is passed as | |
6996 | an int. If not, the pointer to the string value is passed. You are | |
6997 | responsible to make sure a string is pre-extended long enough to | |
6998 | receive any result that might be written into a string. You can't use a | |
6999 | string literal (or other read-only string) as an argument to C<syscall> | |
7000 | because Perl has to assume that any string pointer might be written | |
7001 | through. If your | |
7002 | integer arguments are not literals and have never been interpreted in a | |
7003 | numeric context, you may need to add C<0> to them to force them to look | |
7004 | like numbers. This emulates the C<syswrite> function (or vice versa): | |
7005 | ||
7006 | require 'syscall.ph'; # may need to run h2ph | |
7007 | $s = "hi there\n"; | |
7008 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); | |
7009 | ||
7010 | Note that Perl supports passing of up to only 14 arguments to your syscall, | |
7011 | which in practice should (usually) suffice. | |
7012 | ||
7013 | Syscall returns whatever value returned by the system call it calls. | |
7014 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). | |
7015 | Note that some system calls I<can> legitimately return C<-1>. The proper | |
7016 | way to handle such calls is to assign C<$!=0> before the call, then | |
7017 | check the value of C<$!> if C<syscall> returns C<-1>. | |
7018 | ||
7019 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file | |
7020 | number of the read end of the pipe it creates, but there is no way | |
7021 | to retrieve the file number of the other end. You can avoid this | |
7022 | problem by using C<pipe> instead. | |
7023 | ||
7024 | Portability issues: L<perlport/syscall>. | |
7025 | ||
7026 | =item sysopen FILEHANDLE,FILENAME,MODE | |
7027 | X<sysopen> | |
7028 | ||
7029 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS | |
7030 | ||
7031 | Opens the file whose filename is given by FILENAME, and associates it with | |
7032 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real | |
7033 | filehandle wanted; an undefined scalar will be suitably autovivified. This | |
7034 | function calls the underlying operating system's I<open>(2) function with the | |
7035 | parameters FILENAME, MODE, and PERMS. | |
7036 | ||
7037 | The possible values and flag bits of the MODE parameter are | |
7038 | system-dependent; they are available via the standard module C<Fcntl>. See | |
7039 | the documentation of your operating system's I<open>(2) syscall to see | |
7040 | which values and flag bits are available. You may combine several flags | |
7041 | using the C<|>-operator. | |
7042 | ||
7043 | Some of the most common values are C<O_RDONLY> for opening the file in | |
7044 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, | |
7045 | and C<O_RDWR> for opening the file in read-write mode. | |
7046 | X<O_RDONLY> X<O_RDWR> X<O_WRONLY> | |
7047 | ||
7048 | For historical reasons, some values work on almost every system | |
7049 | supported by Perl: 0 means read-only, 1 means write-only, and 2 | |
7050 | means read/write. We know that these values do I<not> work under | |
7051 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to | |
7052 | use them in new code. | |
7053 | ||
7054 | If the file named by FILENAME does not exist and the C<open> call creates | |
7055 | it (typically because MODE includes the C<O_CREAT> flag), then the value of | |
7056 | PERMS specifies the permissions of the newly created file. If you omit | |
7057 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. | |
7058 | These permission values need to be in octal, and are modified by your | |
7059 | process's current C<umask>. | |
7060 | X<O_CREAT> | |
7061 | ||
7062 | In many systems the C<O_EXCL> flag is available for opening files in | |
7063 | exclusive mode. This is B<not> locking: exclusiveness means here that | |
7064 | if the file already exists, sysopen() fails. C<O_EXCL> may not work | |
7065 | on network filesystems, and has no effect unless the C<O_CREAT> flag | |
7066 | is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from | |
7067 | being opened if it is a symbolic link. It does not protect against | |
7068 | symbolic links in the file's path. | |
7069 | X<O_EXCL> | |
7070 | ||
7071 | Sometimes you may want to truncate an already-existing file. This | |
7072 | can be done using the C<O_TRUNC> flag. The behavior of | |
7073 | C<O_TRUNC> with C<O_RDONLY> is undefined. | |
7074 | X<O_TRUNC> | |
7075 | ||
7076 | You should seldom if ever use C<0644> as argument to C<sysopen>, because | |
7077 | that takes away the user's option to have a more permissive umask. | |
7078 | Better to omit it. See the perlfunc(1) entry on C<umask> for more | |
7079 | on this. | |
7080 | ||
7081 | Note that C<sysopen> depends on the fdopen() C library function. | |
7082 | On many Unix systems, fdopen() is known to fail when file descriptors | |
7083 | exceed a certain value, typically 255. If you need more file | |
7084 | descriptors than that, consider rebuilding Perl to use the C<sfio> | |
7085 | library, or perhaps using the POSIX::open() function. | |
7086 | ||
7087 | See L<perlopentut> for a kinder, gentler explanation of opening files. | |
7088 | ||
7089 | Portability issues: L<perlport/sysopen>. | |
7090 | ||
7091 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET | |
7092 | X<sysread> | |
7093 | ||
7094 | =item sysread FILEHANDLE,SCALAR,LENGTH | |
7095 | ||
7096 | Attempts to read LENGTH bytes of data into variable SCALAR from the | |
7097 | specified FILEHANDLE, using the read(2). It bypasses | |
7098 | buffered IO, so mixing this with other kinds of reads, C<print>, | |
7099 | C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the | |
7100 | perlio or stdio layers usually buffers data. Returns the number of | |
7101 | bytes actually read, C<0> at end of file, or undef if there was an | |
7102 | error (in the latter case C<$!> is also set). SCALAR will be grown or | |
7103 | shrunk so that the last byte actually read is the last byte of the | |
7104 | scalar after the read. | |
7105 | ||
7106 | An OFFSET may be specified to place the read data at some place in the | |
7107 | string other than the beginning. A negative OFFSET specifies | |
7108 | placement at that many characters counting backwards from the end of | |
7109 | the string. A positive OFFSET greater than the length of SCALAR | |
7110 | results in the string being padded to the required size with C<"\0"> | |
7111 | bytes before the result of the read is appended. | |
7112 | ||
7113 | There is no syseof() function, which is ok, since eof() doesn't work | |
7114 | well on device files (like ttys) anyway. Use sysread() and check | |
7115 | for a return value for 0 to decide whether you're done. | |
7116 | ||
7117 | Note that if the filehandle has been marked as C<:utf8> Unicode | |
7118 | characters are read instead of bytes (the LENGTH, OFFSET, and the | |
7119 | return value of sysread() are in Unicode characters). | |
7120 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. | |
7121 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. | |
7122 | ||
7123 | =item sysseek FILEHANDLE,POSITION,WHENCE | |
7124 | X<sysseek> X<lseek> | |
7125 | ||
7126 | Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may | |
7127 | be an expression whose value gives the name of the filehandle. The values | |
7128 | for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it | |
7129 | to the current position plus POSITION; and C<2> to set it to EOF plus | |
7130 | POSITION, typically negative. | |
7131 | ||
7132 | Note the I<in bytes>: even if the filehandle has been set to operate | |
7133 | on characters (for example by using the C<:encoding(utf8)> I/O layer), | |
7134 | tell() will return byte offsets, not character offsets (because | |
7135 | implementing that would render sysseek() unacceptably slow). | |
7136 | ||
7137 | sysseek() bypasses normal buffered IO, so mixing it with reads other | |
7138 | than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>, | |
7139 | C<seek>, C<tell>, or C<eof> may cause confusion. | |
7140 | ||
7141 | For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, | |
7142 | and C<SEEK_END> (start of the file, current position, end of the file) | |
7143 | from the Fcntl module. Use of the constants is also more portable | |
7144 | than relying on 0, 1, and 2. For example to define a "systell" function: | |
7145 | ||
7146 | use Fcntl 'SEEK_CUR'; | |
7147 | sub systell { sysseek($_[0], 0, SEEK_CUR) } | |
7148 | ||
7149 | Returns the new position, or the undefined value on failure. A position | |
7150 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns | |
7151 | true on success and false on failure, yet you can still easily determine | |
7152 | the new position. | |
7153 | ||
7154 | =item system LIST | |
7155 | X<system> X<shell> | |
7156 | ||
7157 | =item system PROGRAM LIST | |
7158 | ||
7159 | Does exactly the same thing as C<exec LIST>, except that a fork is | |
7160 | done first and the parent process waits for the child process to | |
7161 | exit. Note that argument processing varies depending on the | |
7162 | number of arguments. If there is more than one argument in LIST, | |
7163 | or if LIST is an array with more than one value, starts the program | |
7164 | given by the first element of the list with arguments given by the | |
7165 | rest of the list. If there is only one scalar argument, the argument | |
7166 | is checked for shell metacharacters, and if there are any, the | |
7167 | entire argument is passed to the system's command shell for parsing | |
7168 | (this is C</bin/sh -c> on Unix platforms, but varies on other | |
7169 | platforms). If there are no shell metacharacters in the argument, | |
7170 | it is split into words and passed directly to C<execvp>, which is | |
7171 | more efficient. | |
7172 | ||
7173 | Beginning with v5.6.0, Perl will attempt to flush all files opened for | |
7174 | output before any operation that may do a fork, but this may not be | |
7175 | supported on some platforms (see L<perlport>). To be safe, you may need | |
7176 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method | |
7177 | of C<IO::Handle> on any open handles. | |
7178 | ||
7179 | The return value is the exit status of the program as returned by the | |
7180 | C<wait> call. To get the actual exit value, shift right by eight (see | |
7181 | below). See also L</exec>. This is I<not> what you want to use to capture | |
7182 | the output from a command; for that you should use merely backticks or | |
7183 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 | |
7184 | indicates a failure to start the program or an error of the wait(2) system | |
7185 | call (inspect $! for the reason). | |
7186 | ||
7187 | If you'd like to make C<system> (and many other bits of Perl) die on error, | |
7188 | have a look at the L<autodie> pragma. | |
7189 | ||
7190 | Like C<exec>, C<system> allows you to lie to a program about its name if | |
7191 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. | |
7192 | ||
7193 | Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of | |
7194 | C<system>, if you expect your program to terminate on receipt of these | |
7195 | signals you will need to arrange to do so yourself based on the return | |
7196 | value. | |
7197 | ||
7198 | @args = ("command", "arg1", "arg2"); | |
7199 | system(@args) == 0 | |
7200 | or die "system @args failed: $?" | |
7201 | ||
7202 | If you'd like to manually inspect C<system>'s failure, you can check all | |
7203 | possible failure modes by inspecting C<$?> like this: | |
7204 | ||
7205 | if ($? == -1) { | |
7206 | print "failed to execute: $!\n"; | |
7207 | } | |
7208 | elsif ($? & 127) { | |
7209 | printf "child died with signal %d, %s coredump\n", | |
7210 | ($? & 127), ($? & 128) ? 'with' : 'without'; | |
7211 | } | |
7212 | else { | |
7213 | printf "child exited with value %d\n", $? >> 8; | |
7214 | } | |
7215 | ||
7216 | Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}> | |
7217 | with the C<W*()> calls from the POSIX module. | |
7218 | ||
a6b91202 | 7219 | When C<system>'s arguments are executed indirectly by the shell, |
0909e3f8 RS |
7220 | results and return codes are subject to its quirks. |
7221 | See L<perlop/"`STRING`"> and L</exec> for details. | |
7222 | ||
7223 | Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD> | |
7224 | handler. See L<perlipc> for details. | |
7225 | ||
7226 | Portability issues: L<perlport/system>. | |
7227 | ||
7228 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET | |
7229 | X<syswrite> | |
7230 | ||
7231 | =item syswrite FILEHANDLE,SCALAR,LENGTH | |
7232 | ||
7233 | =item syswrite FILEHANDLE,SCALAR | |
7234 | ||
7235 | Attempts to write LENGTH bytes of data from variable SCALAR to the | |
7236 | specified FILEHANDLE, using write(2). If LENGTH is | |
7237 | not specified, writes whole SCALAR. It bypasses buffered IO, so | |
7238 | mixing this with reads (other than C<sysread())>, C<print>, C<write>, | |
7239 | C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and | |
7240 | stdio layers usually buffer data. Returns the number of bytes | |
7241 | actually written, or C<undef> if there was an error (in this case the | |
7242 | errno variable C<$!> is also set). If the LENGTH is greater than the | |
7243 | data available in the SCALAR after the OFFSET, only as much data as is | |
7244 | available will be written. | |
7245 | ||
7246 | An OFFSET may be specified to write the data from some part of the | |
7247 | string other than the beginning. A negative OFFSET specifies writing | |
7248 | that many characters counting backwards from the end of the string. | |
7249 | If SCALAR is of length zero, you can only use an OFFSET of 0. | |
7250 | ||
7251 | B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters | |
7252 | encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and | |
7253 | return value of syswrite() are in (UTF8-encoded Unicode) characters. | |
7254 | The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. | |
7255 | Alternately, if the handle is not marked with an encoding but you | |
7256 | attempt to write characters with code points over 255, raises an exception. | |
7257 | See L</binmode>, L</open>, and the C<open> pragma, L<open>. | |
7258 | ||
7259 | =item tell FILEHANDLE | |
7260 | X<tell> | |
7261 | ||
7262 | =item tell | |
7263 | ||
7264 | Returns the current position I<in bytes> for FILEHANDLE, or -1 on | |
7265 | error. FILEHANDLE may be an expression whose value gives the name of | |
7266 | the actual filehandle. If FILEHANDLE is omitted, assumes the file | |
7267 | last read. | |
7268 | ||
7269 | Note the I<in bytes>: even if the filehandle has been set to | |
7270 | operate on characters (for example by using the C<:encoding(utf8)> open | |
7271 | layer), tell() will return byte offsets, not character offsets (because | |
7272 | that would render seek() and tell() rather slow). | |
7273 | ||
7274 | The return value of tell() for the standard streams like the STDIN | |
7275 | depends on the operating system: it may return -1 or something else. | |
7276 | tell() on pipes, fifos, and sockets usually returns -1. | |
7277 | ||
7278 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. | |
7279 | ||
7280 | Do not use tell() (or other buffered I/O operations) on a filehandle | |
7281 | that has been manipulated by sysread(), syswrite(), or sysseek(). | |
7282 | Those functions ignore the buffering, while tell() does not. | |
7283 | ||
7284 | =item telldir DIRHANDLE | |
7285 | X<telldir> | |
7286 | ||
7287 | Returns the current position of the C<readdir> routines on DIRHANDLE. | |
7288 | Value may be given to C<seekdir> to access a particular location in a | |
7289 | directory. C<telldir> has the same caveats about possible directory | |
7290 | compaction as the corresponding system library routine. | |
7291 | ||
7292 | =item tie VARIABLE,CLASSNAME,LIST | |
7293 | X<tie> | |
7294 | ||
7295 | This function binds a variable to a package class that will provide the | |
7296 | implementation for the variable. VARIABLE is the name of the variable | |
7297 | to be enchanted. CLASSNAME is the name of a class implementing objects | |
7298 | of correct type. Any additional arguments are passed to the C<new> | |
7299 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, | |
7300 | or C<TIEHASH>). Typically these are arguments such as might be passed | |
7301 | to the C<dbm_open()> function of C. The object returned by the C<new> | |
7302 | method is also returned by the C<tie> function, which would be useful | |
7303 | if you want to access other methods in CLASSNAME. | |
7304 | ||
7305 | Note that functions such as C<keys> and C<values> may return huge lists | |
7306 | when used on large objects, like DBM files. You may prefer to use the | |
7307 | C<each> function to iterate over such. Example: | |
7308 | ||
7309 | # print out history file offsets | |
7310 | use NDBM_File; | |
7311 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); | |
7312 | while (($key,$val) = each %HIST) { | |
7313 | print $key, ' = ', unpack('L',$val), "\n"; | |
7314 | } | |
7315 | untie(%HIST); | |
7316 | ||
7317 | A class implementing a hash should have the following methods: | |
7318 | ||
7319 | TIEHASH classname, LIST | |
7320 | FETCH this, key | |
7321 | STORE this, key, value | |
7322 | DELETE this, key | |
7323 | CLEAR this | |
7324 | EXISTS this, key | |
7325 | FIRSTKEY this | |
7326 | NEXTKEY this, lastkey | |
7327 | SCALAR this | |
7328 | DESTROY this | |
7329 | UNTIE this | |
7330 | ||
7331 | A class implementing an ordinary array should have the following methods: | |
7332 | ||
7333 | TIEARRAY classname, LIST | |
7334 | FETCH this, key | |
7335 | STORE this, key, value | |
7336 | FETCHSIZE this | |
7337 | STORESIZE this, count | |
7338 | CLEAR this | |
7339 | PUSH this, LIST | |
7340 | POP this | |
7341 | SHIFT this | |
7342 | UNSHIFT this, LIST | |
7343 | SPLICE this, offset, length, LIST | |
7344 | EXTEND this, count | |
7345 | DESTROY this | |
7346 | UNTIE this | |
7347 | ||
7348 | A class implementing a filehandle should have the following methods: | |
7349 | ||
7350 | TIEHANDLE classname, LIST | |
7351 | READ this, scalar, length, offset | |
7352 | READLINE this | |
7353 | GETC this | |
7354 | WRITE this, scalar, length, offset | |
7355 | PRINT this, LIST | |
7356 | PRINTF this, format, LIST | |
7357 | BINMODE this | |
7358 | EOF this | |
7359 | FILENO this | |
7360 | SEEK this, position, whence | |
7361 | TELL this | |
7362 | OPEN this, mode, LIST | |
7363 | CLOSE this | |
7364 | DESTROY this | |
7365 | UNTIE this | |
7366 | ||
7367 | A class implementing a scalar should have the following methods: | |
7368 | ||
7369 | TIESCALAR classname, LIST | |
7370 | FETCH this, | |
7371 | STORE this, value | |
7372 | DESTROY this | |
7373 | UNTIE this | |
7374 | ||
7375 | Not all methods indicated above need be implemented. See L<perltie>, | |
7376 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. | |
7377 | ||
7378 | Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module | |
7379 | for you; you need to do that explicitly yourself. See L<DB_File> | |
7380 | or the F<Config> module for interesting C<tie> implementations. | |
7381 | ||
7382 | For further details see L<perltie>, L<"tied VARIABLE">. | |
7383 | ||
7384 | =item tied VARIABLE | |
7385 | X<tied> | |
7386 | ||
7387 | Returns a reference to the object underlying VARIABLE (the same value | |
7388 | that was originally returned by the C<tie> call that bound the variable | |
7389 | to a package.) Returns the undefined value if VARIABLE isn't tied to a | |
7390 | package. | |
7391 | ||
7392 | =item time | |
7393 | X<time> X<epoch> | |
7394 | ||
7395 | Returns the number of non-leap seconds since whatever time the system | |
7396 | considers to be the epoch, suitable for feeding to C<gmtime> and | |
7397 | C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970; | |
7398 | a prominent exception being Mac OS Classic which uses 00:00:00, January 1, | |
7399 | 1904 in the current local time zone for its epoch. | |
7400 | ||
7401 | For measuring time in better granularity than one second, use the | |
7402 | L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or, | |
7403 | if you have gettimeofday(2), you may be able to use the C<syscall> | |
7404 | interface of Perl. See L<perlfaq8> for details. | |
7405 | ||
7406 | For date and time processing look at the many related modules on CPAN. | |
7407 | For a comprehensive date and time representation look at the | |
7408 | L<DateTime> module. | |
7409 | ||
7410 | =item times | |
7411 | X<times> | |
7412 | ||
7413 | Returns a four-element list giving the user and system times in | |
7414 | seconds for this process and any exited children of this process. | |
7415 | ||
7416 | ($user,$system,$cuser,$csystem) = times; | |
7417 | ||
7418 | In scalar context, C<times> returns C<$user>. | |
7419 | ||
7420 | Children's times are only included for terminated children. | |
7421 | ||
7422 | Portability issues: L<perlport/times>. | |
7423 | ||
7424 | =item tr/// | |
7425 | ||
7426 | The transliteration operator. Same as C<y///>. See | |
7427 | L<perlop/"Quote and Quote-like Operators">. | |
7428 | ||
7429 | =item truncate FILEHANDLE,LENGTH | |
7430 | X<truncate> | |
7431 | ||
7432 | =item truncate EXPR,LENGTH | |
7433 | ||
7434 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the | |
7435 | specified length. Raises an exception if truncate isn't implemented | |
7436 | on your system. Returns true if successful, C<undef> on error. | |
7437 | ||
7438 | The behavior is undefined if LENGTH is greater than the length of the | |
7439 | file. | |
7440 | ||
7441 | The position in the file of FILEHANDLE is left unchanged. You may want to | |
7442 | call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file. | |
7443 | ||
7444 | Portability issues: L<perlport/truncate>. | |
7445 | ||
7446 | =item uc EXPR | |
7447 | X<uc> X<uppercase> X<toupper> | |
7448 | ||
7449 | =item uc | |
7450 | ||
7451 | Returns an uppercased version of EXPR. This is the internal function | |
7452 | implementing the C<\U> escape in double-quoted strings. | |
7453 | It does not attempt to do titlecase mapping on initial letters. See | |
7454 | L</ucfirst> for that. | |
7455 | ||
7456 | If EXPR is omitted, uses C<$_>. | |
7457 | ||
7458 | This function behaves the same way under various pragma, such as in a locale, | |
7459 | as L</lc> does. | |
7460 | ||
7461 | =item ucfirst EXPR | |
7462 | X<ucfirst> X<uppercase> | |
7463 | ||
7464 | =item ucfirst | |
7465 | ||
7466 | Returns the value of EXPR with the first character in uppercase | |
7467 | (titlecase in Unicode). This is the internal function implementing | |
7468 | the C<\u> escape in double-quoted strings. | |
7469 | ||
7470 | If EXPR is omitted, uses C<$_>. | |
7471 | ||
7472 | This function behaves the same way under various pragma, such as in a locale, | |
7473 | as L</lc> does. | |
7474 | ||
7475 | =item umask EXPR | |
7476 | X<umask> | |
7477 | ||
7478 | =item umask | |
7479 | ||
7480 | Sets the umask for the process to EXPR and returns the previous value. | |
7481 | If EXPR is omitted, merely returns the current umask. | |
7482 | ||
7483 | The Unix permission C<rwxr-x---> is represented as three sets of three | |
7484 | bits, or three octal digits: C<0750> (the leading 0 indicates octal | |
7485 | and isn't one of the digits). The C<umask> value is such a number | |
7486 | representing disabled permissions bits. The permission (or "mode") | |
7487 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so | |
7488 | even if you tell C<sysopen> to create a file with permissions C<0777>, | |
7489 | if your umask is C<0022>, then the file will actually be created with | |
7490 | permissions C<0755>. If your C<umask> were C<0027> (group can't | |
7491 | write; others can't read, write, or execute), then passing | |
a6b91202 | 7492 | C<sysopen> C<0666> would create a file with mode C<0640> (because |
0909e3f8 RS |
7493 | C<0666 &~ 027> is C<0640>). |
7494 | ||
7495 | Here's some advice: supply a creation mode of C<0666> for regular | |
7496 | files (in C<sysopen>) and one of C<0777> for directories (in | |
7497 | C<mkdir>) and executable files. This gives users the freedom of | |
7498 | choice: if they want protected files, they might choose process umasks | |
7499 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. | |
7500 | Programs should rarely if ever make policy decisions better left to | |
7501 | the user. The exception to this is when writing files that should be | |
7502 | kept private: mail files, web browser cookies, I<.rhosts> files, and | |
7503 | so on. | |
7504 | ||
7505 | If umask(2) is not implemented on your system and you are trying to | |
a6b91202 | 7506 | restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>), |
0909e3f8 RS |
7507 | raises an exception. If umask(2) is not implemented and you are |
7508 | not trying to restrict access for yourself, returns C<undef>. | |
7509 | ||
7510 | Remember that a umask is a number, usually given in octal; it is I<not> a | |
7511 | string of octal digits. See also L</oct>, if all you have is a string. | |
7512 | ||
7513 | Portability issues: L<perlport/umask>. | |
7514 | ||
7515 | =item undef EXPR | |
7516 | X<undef> X<undefine> | |
7517 | ||
7518 | =item undef | |
7519 | ||
7520 | Undefines the value of EXPR, which must be an lvalue. Use only on a | |
7521 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine | |
7522 | (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}> | |
7523 | will probably not do what you expect on most predefined variables or | |
7524 | DBM list values, so don't do that; see L</delete>. Always returns the | |
7525 | undefined value. You can omit the EXPR, in which case nothing is | |
7526 | undefined, but you still get an undefined value that you could, for | |
7527 | instance, return from a subroutine, assign to a variable, or pass as a | |
7528 | parameter. Examples: | |
7529 | ||
7530 | undef $foo; | |
7531 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; | |
7532 | undef @ary; | |
7533 | undef %hash; | |
7534 | undef &mysub; | |
7535 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. | |
7536 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; | |
7537 | select undef, undef, undef, 0.25; | |
7538 | ($a, $b, undef, $c) = &foo; # Ignore third value returned | |
7539 | ||
7540 | Note that this is a unary operator, not a list operator. | |
7541 | ||
7542 | =item unlink LIST | |
7543 | X<unlink> X<delete> X<remove> X<rm> X<del> | |
7544 | ||
7545 | =item unlink | |
7546 | ||
7547 | Deletes a list of files. On success, it returns the number of files | |
7548 | it successfully deleted. On failure, it returns false and sets C<$!> | |
7549 | (errno): | |
7550 | ||
7551 | my $unlinked = unlink 'a', 'b', 'c'; | |
7552 | unlink @goners; | |
7553 | unlink glob "*.bak"; | |
7554 | ||
7555 | On error, C<unlink> will not tell you which files it could not remove. | |
7556 | If you want to know which files you could not remove, try them one | |
7557 | at a time: | |
7558 | ||
7559 | foreach my $file ( @goners ) { | |
7560 | unlink $file or warn "Could not unlink $file: $!"; | |
7561 | } | |
7562 | ||
7563 | Note: C<unlink> will not attempt to delete directories unless you are | |
7564 | superuser and the B<-U> flag is supplied to Perl. Even if these | |
7565 | conditions are met, be warned that unlinking a directory can inflict | |
7566 | damage on your filesystem. Finally, using C<unlink> on directories is | |
7567 | not supported on many operating systems. Use C<rmdir> instead. | |
7568 | ||
7569 | If LIST is omitted, C<unlink> uses C<$_>. | |
7570 | ||
7571 | =item unpack TEMPLATE,EXPR | |
7572 | X<unpack> | |
7573 | ||
7574 | =item unpack TEMPLATE | |
7575 | ||
7576 | C<unpack> does the reverse of C<pack>: it takes a string | |
7577 | and expands it out into a list of values. | |
7578 | (In scalar context, it returns merely the first value produced.) | |
7579 | ||
7580 | If EXPR is omitted, unpacks the C<$_> string. | |
7581 | See L<perlpacktut> for an introduction to this function. | |
7582 | ||
7583 | The string is broken into chunks described by the TEMPLATE. Each chunk | |
7584 | is converted separately to a value. Typically, either the string is a result | |
7585 | of C<pack>, or the characters of the string represent a C structure of some | |
7586 | kind. | |
7587 | ||
7588 | The TEMPLATE has the same format as in the C<pack> function. | |
7589 | Here's a subroutine that does substring: | |
7590 | ||
7591 | sub substr { | |
7592 | my($what,$where,$howmuch) = @_; | |
7593 | unpack("x$where a$howmuch", $what); | |
7594 | } | |
7595 | ||
7596 | and then there's | |
7597 | ||
7598 | sub ordinal { unpack("W",$_[0]); } # same as ord() | |
7599 | ||
7600 | In addition to fields allowed in pack(), you may prefix a field with | |
7601 | a %<number> to indicate that | |
7602 | you want a <number>-bit checksum of the items instead of the items | |
7603 | themselves. Default is a 16-bit checksum. Checksum is calculated by | |
7604 | summing numeric values of expanded values (for string fields the sum of | |
7605 | C<ord($char)> is taken; for bit fields the sum of zeroes and ones). | |
7606 | ||
7607 | For example, the following | |
7608 | computes the same number as the System V sum program: | |
7609 | ||
7610 | $checksum = do { | |
7611 | local $/; # slurp! | |
7612 | unpack("%32W*",<>) % 65535; | |
7613 | }; | |
7614 | ||
7615 | The following efficiently counts the number of set bits in a bit vector: | |
7616 | ||
7617 | $setbits = unpack("%32b*", $selectmask); | |
7618 | ||
7619 | The C<p> and C<P> formats should be used with care. Since Perl | |
7620 | has no way of checking whether the value passed to C<unpack()> | |
7621 | corresponds to a valid memory location, passing a pointer value that's | |
7622 | not known to be valid is likely to have disastrous consequences. | |
7623 | ||
7624 | If there are more pack codes or if the repeat count of a field or a group | |
7625 | is larger than what the remainder of the input string allows, the result | |
7626 | is not well defined: the repeat count may be decreased, or | |
7627 | C<unpack()> may produce empty strings or zeros, or it may raise an exception. | |
7628 | If the input string is longer than one described by the TEMPLATE, | |
7629 | the remainder of that input string is ignored. | |
7630 | ||
7631 | See L</pack> for more examples and notes. | |
7632 | ||
7633 | =item untie VARIABLE | |
7634 | X<untie> | |
7635 | ||
7636 | Breaks the binding between a variable and a package. | |
7637 | (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.) | |
7638 | Has no effect if the variable is not tied. | |
7639 | ||
7640 | =item unshift ARRAY,LIST | |
7641 | X<unshift> | |
7642 | ||
7643 | =item unshift EXPR,LIST | |
7644 | ||
7645 | Does the opposite of a C<shift>. Or the opposite of a C<push>, | |
7646 | depending on how you look at it. Prepends list to the front of the | |
7647 | array and returns the new number of elements in the array. | |
7648 | ||
7649 | unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; | |
7650 | ||
7651 | Note the LIST is prepended whole, not one element at a time, so the | |
7652 | prepended elements stay in the same order. Use C<reverse> to do the | |
7653 | reverse. | |
7654 | ||
7655 | Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold | |
7656 | a reference to an unblessed array. The argument will be dereferenced | |
7657 | automatically. This aspect of C<unshift> is considered highly | |
7658 | experimental. The exact behaviour may change in a future version of Perl. | |
7659 | ||
7660 | =item use Module VERSION LIST | |
7661 | X<use> X<module> X<import> | |
7662 | ||
7663 | =item use Module VERSION | |
7664 | ||
7665 | =item use Module LIST | |
7666 | ||
7667 | =item use Module | |
7668 | ||
7669 | =item use VERSION | |
7670 | ||
7671 | Imports some semantics into the current package from the named module, | |
7672 | generally by aliasing certain subroutine or variable names into your | |
7673 | package. It is exactly equivalent to | |
7674 | ||
7675 | BEGIN { require Module; Module->import( LIST ); } | |
7676 | ||
7677 | except that Module I<must> be a bareword. | |
7678 | The importation can be made conditional; see L<if>. | |
7679 | ||
7680 | In the peculiar C<use VERSION> form, VERSION may be either a positive | |
7681 | decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string | |
7682 | of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An | |
7683 | exception is raised if VERSION is greater than the version of the | |
7684 | current Perl interpreter; Perl will not attempt to parse the rest of the | |
7685 | file. Compare with L</require>, which can do a similar check at run time. | |
7686 | Symmetrically, C<no VERSION> allows you to specify that you want a version | |
7687 | of Perl older than the specified one. | |
7688 | ||
7689 | Specifying VERSION as a literal of the form v5.6.1 should generally be | |
7690 | avoided, because it leads to misleading error messages under earlier | |
7691 | versions of Perl (that is, prior to 5.6.0) that do not support this | |
7692 | syntax. The equivalent numeric version should be used instead. | |
7693 | ||
7694 | use v5.6.1; # compile time version check | |
7695 | use 5.6.1; # ditto | |
7696 | use 5.006_001; # ditto; preferred for backwards compatibility | |
7697 | ||
7698 | This is often useful if you need to check the current Perl version before | |
7699 | C<use>ing library modules that won't work with older versions of Perl. | |
7700 | (We try not to do this more than we have to.) | |
7701 | ||
7702 | Also, if the specified Perl version is greater than or equal to 5.9.5, | |
7703 | C<use VERSION> will also load the C<feature> pragma and enable all | |
7704 | features available in the requested version. See L<feature>. | |
7705 | Similarly, if the specified Perl version is greater than or equal to | |
7706 | 5.11.0, strictures are enabled lexically as with C<use strict> (except | |
7707 | that the F<strict.pm> file is not actually loaded). | |
7708 | ||
7709 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The | |
7710 | C<require> makes sure the module is loaded into memory if it hasn't been | |
7711 | yet. The C<import> is not a builtin; it's just an ordinary static method | |
7712 | call into the C<Module> package to tell the module to import the list of | |
7713 | features back into the current package. The module can implement its | |
7714 | C<import> method any way it likes, though most modules just choose to | |
7715 | derive their C<import> method via inheritance from the C<Exporter> class that | |
7716 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> | |
7717 | method can be found then the call is skipped, even if there is an AUTOLOAD | |
7718 | method. | |
7719 | ||
7720 | If you do not want to call the package's C<import> method (for instance, | |
7721 | to stop your namespace from being altered), explicitly supply the empty list: | |
7722 | ||
7723 | use Module (); | |
7724 | ||
7725 | That is exactly equivalent to | |
7726 | ||
7727 | BEGIN { require Module } | |
7728 | ||
7729 | If the VERSION argument is present between Module and LIST, then the | |
7730 | C<use> will call the VERSION method in class Module with the given | |
7731 | version as an argument. The default VERSION method, inherited from | |
7732 | the UNIVERSAL class, croaks if the given version is larger than the | |
7733 | value of the variable C<$Module::VERSION>. | |
7734 | ||
7735 | Again, there is a distinction between omitting LIST (C<import> called | |
7736 | with no arguments) and an explicit empty LIST C<()> (C<import> not | |
7737 | called). Note that there is no comma after VERSION! | |
7738 | ||
7739 | Because this is a wide-open interface, pragmas (compiler directives) | |
7740 | are also implemented this way. Currently implemented pragmas are: | |
7741 | ||
7742 | use constant; | |
7743 | use diagnostics; | |
7744 | use integer; | |
7745 | use sigtrap qw(SEGV BUS); | |
7746 | use strict qw(subs vars refs); | |
7747 | use subs qw(afunc blurfl); | |
7748 | use warnings qw(all); | |
7749 | use sort qw(stable _quicksort _mergesort); | |
7750 | ||
7751 | Some of these pseudo-modules import semantics into the current | |
7752 | block scope (like C<strict> or C<integer>, unlike ordinary modules, | |
7753 | which import symbols into the current package (which are effective | |
7754 | through the end of the file). | |
7755 | ||
7756 | Because C<use> takes effect at compile time, it doesn't respect the | |
7757 | ordinary flow control of the code being compiled. In particular, putting | |
7758 | a C<use> inside the false branch of a conditional doesn't prevent it | |
a6b91202 | 7759 | from being processed. If a module or pragma only needs to be loaded |
0909e3f8 RS |
7760 | conditionally, this can be done using the L<if> pragma: |
7761 | ||
7762 | use if $] < 5.008, "utf8"; | |
7763 | use if WANT_WARNINGS, warnings => qw(all); | |
7764 | ||
7765 | There's a corresponding C<no> declaration that unimports meanings imported | |
7766 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. | |
a6b91202 | 7767 | It behaves just as C<import> does with VERSION, an omitted or empty LIST, |
0909e3f8 RS |
7768 | or no unimport method being found. |
7769 | ||
7770 | no integer; | |
7771 | no strict 'refs'; | |
7772 | no warnings; | |
7773 | ||
7774 | Care should be taken when using the C<no VERSION> form of C<no>. It is | |
7775 | I<only> meant to be used to assert that the running Perl is of a earlier | |
7776 | version than its argument and I<not> to undo the feature-enabling side effects | |
7777 | of C<use VERSION>. | |
7778 | ||
7779 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> | |
7780 | for the C<-M> and C<-m> command-line options to Perl that give C<use> | |
7781 | functionality from the command-line. | |
7782 | ||
7783 | =item utime LIST | |
7784 | X<utime> | |
7785 | ||
7786 | Changes the access and modification times on each file of a list of | |
7787 | files. The first two elements of the list must be the NUMERIC access | |
7788 | and modification times, in that order. Returns the number of files | |
7789 | successfully changed. The inode change time of each file is set | |
7790 | to the current time. For example, this code has the same effect as the | |
7791 | Unix touch(1) command when the files I<already exist> and belong to | |
7792 | the user running the program: | |
7793 | ||
7794 | #!/usr/bin/perl | |
7795 | $atime = $mtime = time; | |
7796 | utime $atime, $mtime, @ARGV; | |
7797 | ||
a6b91202 | 7798 | Since Perl 5.7.2, if the first two elements of the list are C<undef>, |
0909e3f8 RS |
7799 | the utime(2) syscall from your C library is called with a null second |
7800 | argument. On most systems, this will set the file's access and | |
7801 | modification times to the current time (i.e., equivalent to the example | |
7802 | above) and will work even on files you don't own provided you have write | |
7803 | permission: | |
7804 | ||
7805 | for $file (@ARGV) { | |
a6b91202 | 7806 | utime(undef, undef, $file) |
0909e3f8 | 7807 | || warn "couldn't touch $file: $!"; |
a6b91202 | 7808 | } |
0909e3f8 RS |
7809 | |
7810 | Under NFS this will use the time of the NFS server, not the time of | |
7811 | the local machine. If there is a time synchronization problem, the | |
7812 | NFS server and local machine will have different times. The Unix | |
7813 | touch(1) command will in fact normally use this form instead of the | |
7814 | one shown in the first example. | |
7815 | ||
7816 | Passing only one of the first two elements as C<undef> is | |
a6b91202 | 7817 | equivalent to passing a 0 and will not have the effect |
0909e3f8 RS |
7818 | described when both are C<undef>. This also triggers an |
7819 | uninitialized warning. | |
7820 | ||
7821 | On systems that support futimes(2), you may pass filehandles among the | |
7822 | files. On systems that don't support futimes(2), passing filehandles raises | |
7823 | an exception. Filehandles must be passed as globs or glob references to be | |
7824 | recognized; barewords are considered filenames. | |
7825 | ||
7826 | Portability issues: L<perlport/utime>. | |
7827 | ||
7828 | =item values HASH | |
7829 | X<values> | |
7830 | ||
7831 | =item values ARRAY | |
7832 | ||
7833 | =item values EXPR | |
7834 | ||
7835 | Returns a list consisting of all the values of the named hash, or the values | |
7836 | of an array. (In scalar context, returns the number of values.) | |
7837 | ||
7838 | The values are returned in an apparently random order. The actual | |
7839 | random order is subject to change in future versions of Perl, but it | |
7840 | is guaranteed to be the same order as either the C<keys> or C<each> | |
7841 | function would produce on the same (unmodified) hash. Since Perl | |
7842 | 5.8.1 the ordering is different even between different runs of Perl | |
7843 | for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). | |
7844 | ||
7845 | As a side effect, calling values() resets the HASH or ARRAY's internal | |
7846 | iterator; | |
7847 | see L</each>. (In particular, calling values() in void context resets | |
7848 | the iterator with no other overhead. Apart from resetting the iterator, | |
7849 | C<values @array> in list context is the same as plain C<@array>. | |
7850 | We recommend that you use void context C<keys @array> for this, but reasoned | |
7851 | that it taking C<values @array> out would require more documentation than | |
7852 | leaving it in.) | |
7853 | ||
7854 | Note that the values are not copied, which means modifying them will | |
7855 | modify the contents of the hash: | |
7856 | ||
7857 | for (values %hash) { s/foo/bar/g } # modifies %hash values | |
7858 | for (@hash{keys %hash}) { s/foo/bar/g } # same | |
7859 | ||
7860 | Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold | |
7861 | a reference to an unblessed hash or array. The argument will be | |
7862 | dereferenced automatically. This aspect of C<values> is considered highly | |
7863 | experimental. The exact behaviour may change in a future version of Perl. | |
7864 | ||
7865 | for (values $hashref) { ... } | |
7866 | for (values $obj->get_arrayref) { ... } | |
7867 | ||
7868 | See also C<keys>, C<each>, and C<sort>. | |
7869 | ||
7870 | =item vec EXPR,OFFSET,BITS | |
7871 | X<vec> X<bit> X<bit vector> | |
7872 | ||
7873 | Treats the string in EXPR as a bit vector made up of elements of | |
7874 | width BITS and returns the value of the element specified by OFFSET | |
7875 | as an unsigned integer. BITS therefore specifies the number of bits | |
7876 | that are reserved for each element in the bit vector. This must | |
7877 | be a power of two from 1 to 32 (or 64, if your platform supports | |
7878 | that). | |
7879 | ||
7880 | If BITS is 8, "elements" coincide with bytes of the input string. | |
7881 | ||
7882 | If BITS is 16 or more, bytes of the input string are grouped into chunks | |
7883 | of size BITS/8, and each group is converted to a number as with | |
7884 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously | |
7885 | for BITS==64). See L<"pack"> for details. | |
7886 | ||
7887 | If bits is 4 or less, the string is broken into bytes, then the bits | |
7888 | of each byte are broken into 8/BITS groups. Bits of a byte are | |
7889 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, | |
7890 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, | |
7891 | breaking the single input byte C<chr(0x36)> into two groups gives a list | |
7892 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. | |
7893 | ||
7894 | C<vec> may also be assigned to, in which case parentheses are needed | |
7895 | to give the expression the correct precedence as in | |
7896 | ||
7897 | vec($image, $max_x * $x + $y, 8) = 3; | |
7898 | ||
7899 | If the selected element is outside the string, the value 0 is returned. | |
7900 | If an element off the end of the string is written to, Perl will first | |
7901 | extend the string with sufficiently many zero bytes. It is an error | |
7902 | to try to write off the beginning of the string (i.e., negative OFFSET). | |
7903 | ||
7904 | If the string happens to be encoded as UTF-8 internally (and thus has | |
7905 | the UTF8 flag set), this is ignored by C<vec>, and it operates on the | |
7906 | internal byte string, not the conceptual character string, even if you | |
a6b91202 | 7907 | only have characters with values less than 256. |
0909e3f8 RS |
7908 | |
7909 | Strings created with C<vec> can also be manipulated with the logical | |
7910 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit | |
7911 | vector operation is desired when both operands are strings. | |
7912 | See L<perlop/"Bitwise String Operators">. | |
7913 | ||
7914 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. | |
7915 | The comments show the string after each step. Note that this code works | |
7916 | in the same way on big-endian or little-endian machines. | |
7917 | ||
7918 | my $foo = ''; | |
7919 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' | |
7920 | ||
7921 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits | |
7922 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') | |
7923 | ||
7924 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' | |
7925 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' | |
7926 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' | |
7927 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' | |
7928 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" | |
7929 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' | |
7930 | # 'r' is "\x72" | |
7931 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" | |
7932 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" | |
7933 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' | |
7934 | # 'l' is "\x6c" | |
7935 | ||
7936 | To transform a bit vector into a string or list of 0's and 1's, use these: | |
7937 | ||
7938 | $bits = unpack("b*", $vector); | |
7939 | @bits = split(//, unpack("b*", $vector)); | |
7940 | ||
7941 | If you know the exact length in bits, it can be used in place of the C<*>. | |
7942 | ||
7943 | Here is an example to illustrate how the bits actually fall in place: | |
7944 | ||
7945 | #!/usr/bin/perl -wl | |
7946 | ||
7947 | print <<'EOT'; | |
7948 | 0 1 2 3 | |
7949 | unpack("V",$_) 01234567890123456789012345678901 | |
7950 | ------------------------------------------------------------------ | |
7951 | EOT | |
7952 | ||
7953 | for $w (0..3) { | |
7954 | $width = 2**$w; | |
7955 | for ($shift=0; $shift < $width; ++$shift) { | |
7956 | for ($off=0; $off < 32/$width; ++$off) { | |
7957 | $str = pack("B*", "0"x32); | |
7958 | $bits = (1<<$shift); | |
7959 | vec($str, $off, $width) = $bits; | |
7960 | $res = unpack("b*",$str); | |
7961 | $val = unpack("V", $str); | |
7962 | write; | |
7963 | } | |
7964 | } | |
7965 | } | |
7966 | ||
7967 | format STDOUT = | |
7968 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> | |
7969 | $off, $width, $bits, $val, $res | |
7970 | . | |
7971 | __END__ | |
7972 | ||
a6b91202 | 7973 | Regardless of the machine architecture on which it runs, the |
0909e3f8 RS |
7974 | example above should print the following table: |
7975 | ||
7976 | 0 1 2 3 | |
7977 | unpack("V",$_) 01234567890123456789012345678901 | |
7978 | ------------------------------------------------------------------ | |
7979 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 | |
7980 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 | |
7981 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 | |
7982 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 | |
7983 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 | |
7984 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 | |
7985 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 | |
7986 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 | |
7987 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 | |
7988 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 | |
7989 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 | |
7990 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 | |
7991 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 | |
7992 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 | |
7993 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 | |
7994 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 | |
7995 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 | |
7996 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 | |
7997 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 | |
7998 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 | |
7999 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 | |
8000 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 | |
8001 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 | |
8002 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 | |
8003 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 | |
8004 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 | |
8005 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 | |
8006 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 | |
8007 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 | |
8008 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 | |
8009 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 | |
8010 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 | |
8011 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 | |
8012 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 | |
8013 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 | |
8014 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 | |
8015 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 | |
8016 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 | |
8017 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 | |
8018 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 | |
8019 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 | |
8020 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 | |
8021 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 | |
8022 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 | |
8023 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 | |
8024 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 | |
8025 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 | |
8026 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 | |
8027 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 | |
8028 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 | |
8029 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 | |
8030 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 | |
8031 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 | |
8032 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 | |
8033 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 | |
8034 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 | |
8035 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 | |
8036 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 | |
8037 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 | |
8038 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 | |
8039 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 | |
8040 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 | |
8041 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 | |
8042 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 | |
8043 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 | |
8044 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 | |
8045 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 | |
8046 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 | |
8047 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 | |
8048 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 | |
8049 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 | |
8050 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 | |
8051 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 | |
8052 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 | |
8053 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 | |
8054 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 | |
8055 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 | |
8056 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 | |
8057 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 | |
8058 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 | |
8059 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 | |
8060 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 | |
8061 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 | |
8062 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 | |
8063 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 | |
8064 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 | |
8065 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 | |
8066 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 | |
8067 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 | |
8068 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 | |
8069 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 | |
8070 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 | |
8071 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 | |
8072 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 | |
8073 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 | |
8074 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 | |
8075 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 | |
8076 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 | |
8077 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 | |
8078 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 | |
8079 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 | |
8080 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 | |
8081 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 | |
8082 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 | |
8083 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 | |
8084 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 | |
8085 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 | |
8086 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 | |
8087 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 | |
8088 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 | |
8089 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 | |
8090 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 | |
8091 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 | |
8092 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 | |
8093 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 | |
8094 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 | |
8095 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 | |
8096 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 | |
8097 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 | |
8098 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 | |
8099 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 | |
8100 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 | |
8101 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 | |
8102 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 | |
8103 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 | |
8104 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 | |
8105 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 | |
8106 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 | |
8107 | ||
8108 | =item wait | |
8109 | X<wait> | |
8110 | ||
8111 | Behaves like wait(2) on your system: it waits for a child | |
8112 | process to terminate and returns the pid of the deceased process, or | |
8113 | C<-1> if there are no child processes. The status is returned in C<$?> | |
8114 | and C<${^CHILD_ERROR_NATIVE}>. | |
8115 | Note that a return value of C<-1> could mean that child processes are | |
8116 | being automatically reaped, as described in L<perlipc>. | |
8117 | ||
8118 | If you use wait in your handler for $SIG{CHLD} it may accidentally for the | |
8119 | child created by qx() or system(). See L<perlipc> for details. | |
8120 | ||
8121 | Portability issues: L<perlport/wait>. | |
8122 | ||
8123 | =item waitpid PID,FLAGS | |
8124 | X<waitpid> | |
8125 | ||
8126 | Waits for a particular child process to terminate and returns the pid of | |
8127 | the deceased process, or C<-1> if there is no such child process. On some | |
8128 | systems, a value of 0 indicates that there are processes still running. | |
8129 | The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say | |
8130 | ||
8131 | use POSIX ":sys_wait_h"; | |
8132 | #... | |
8133 | do { | |
8134 | $kid = waitpid(-1, WNOHANG); | |
8135 | } while $kid > 0; | |
8136 | ||
8137 | then you can do a non-blocking wait for all pending zombie processes. | |
8138 | Non-blocking wait is available on machines supporting either the | |
8139 | waitpid(2) or wait4(2) syscalls. However, waiting for a particular | |
8140 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the | |
8141 | system call by remembering the status values of processes that have | |
8142 | exited but have not been harvested by the Perl script yet.) | |
8143 | ||
8144 | Note that on some systems, a return value of C<-1> could mean that child | |
8145 | processes are being automatically reaped. See L<perlipc> for details, | |
8146 | and for other examples. | |
8147 | ||
8148 | Portability issues: L<perlport/waitpid>. | |
8149 | ||
8150 | =item wantarray | |
8151 | X<wantarray> X<context> | |
8152 | ||
8153 | Returns true if the context of the currently executing subroutine or | |
8154 | C<eval> is looking for a list value. Returns false if the context is | |
8155 | looking for a scalar. Returns the undefined value if the context is | |
8156 | looking for no value (void context). | |
8157 | ||
8158 | return unless defined wantarray; # don't bother doing more | |
8159 | my @a = complex_calculation(); | |
8160 | return wantarray ? @a : "@a"; | |
8161 | ||
8162 | C<wantarray()>'s result is unspecified in the top level of a file, | |
8163 | in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or | |
8164 | in a C<DESTROY> method. | |
8165 | ||
8166 | This function should have been named wantlist() instead. | |
8167 | ||
8168 | =item warn LIST | |
8169 | X<warn> X<warning> X<STDERR> | |
8170 | ||
8171 | Prints the value of LIST to STDERR. If the last element of LIST does | |
8172 | not end in a newline, it appends the same file/line number text as C<die> | |
8173 | does. | |
8174 | ||
8175 | If the output is empty and C<$@> already contains a value (typically from a | |
8176 | previous eval) that value is used after appending C<"\t...caught"> | |
8177 | to C<$@>. This is useful for staying almost, but not entirely similar to | |
8178 | C<die>. | |
8179 | ||
8180 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. | |
8181 | ||
8182 | No message is printed if there is a C<$SIG{__WARN__}> handler | |
8183 | installed. It is the handler's responsibility to deal with the message | |
8184 | as it sees fit (like, for instance, converting it into a C<die>). Most | |
8185 | handlers must therefore arrange to actually display the | |
8186 | warnings that they are not prepared to deal with, by calling C<warn> | |
8187 | again in the handler. Note that this is quite safe and will not | |
8188 | produce an endless loop, since C<__WARN__> hooks are not called from | |
8189 | inside one. | |
8190 | ||
8191 | You will find this behavior is slightly different from that of | |
8192 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can | |
8193 | instead call C<die> again to change it). | |
8194 | ||
8195 | Using a C<__WARN__> handler provides a powerful way to silence all | |
8196 | warnings (even the so-called mandatory ones). An example: | |
8197 | ||
8198 | # wipe out *all* compile-time warnings | |
8199 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } | |
8200 | my $foo = 10; | |
8201 | my $foo = 20; # no warning about duplicate my $foo, | |
8202 | # but hey, you asked for it! | |
8203 | # no compile-time or run-time warnings before here | |
8204 | $DOWARN = 1; | |
8205 | ||
8206 | # run-time warnings enabled after here | |
8207 | warn "\$foo is alive and $foo!"; # does show up | |
8208 | ||
8209 | See L<perlvar> for details on setting C<%SIG> entries and for more | |
8210 | examples. See the Carp module for other kinds of warnings using its | |
8211 | carp() and cluck() functions. | |
8212 | ||
8213 | =item when EXPR BLOCK | |
8214 | X<when> | |
8215 | ||
8216 | =item when BLOCK | |
8217 | ||
8218 | C<when> is analogous to the C<case> keyword in other languages. Used with a | |
8219 | C<foreach> loop or the experimental C<given> block, C<when> can be used in | |
8220 | Perl to implement C<switch>/C<case> like statements. Available as a | |
a6b91202 | 8221 | statement after Perl 5.10 and as a statement modifier after 5.14. |
0909e3f8 RS |
8222 | Here are three examples: |
8223 | ||
8224 | use v5.10; | |
8225 | foreach (@fruits) { | |
8226 | when (/apples?/) { | |
8227 | say "I like apples." | |
8228 | } | |
8229 | when (/oranges?/) { | |
8230 | say "I don't like oranges." | |
8231 | } | |
8232 | default { | |
8233 | say "I don't like anything" | |
8234 | } | |
8235 | } | |
8236 | ||
8237 | # require 5.14 for when as statement modifier | |
8238 | use v5.14; | |
8239 | foreach (@fruits) { | |
a6b91202 | 8240 | say "I like apples." when /apples?/; |
0909e3f8 RS |
8241 | say "I don't like oranges." when /oranges?; |
8242 | default { say "I don't like anything" } | |
8243 | } | |
8244 | ||
8245 | use v5.10; | |
8246 | given ($fruit) { | |
8247 | when (/apples?/) { | |
8248 | say "I like apples." | |
8249 | } | |
8250 | when (/oranges?/) { | |
8251 | say "I don't like oranges." | |
8252 | } | |
8253 | default { | |
8254 | say "I don't like anything" | |
8255 | } | |
8256 | } | |
8257 | ||
8258 | See L<perlsyn/"Switch statements"> for detailed information. | |
8259 | ||
8260 | =item write FILEHANDLE | |
8261 | X<write> | |
8262 | ||
8263 | =item write EXPR | |
8264 | ||
8265 | =item write | |
8266 | ||
8267 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, | |
8268 | using the format associated with that file. By default the format for | |
8269 | a file is the one having the same name as the filehandle, but the | |
8270 | format for the current output channel (see the C<select> function) may be set | |
8271 | explicitly by assigning the name of the format to the C<$~> variable. | |
8272 | ||
8273 | Top of form processing is handled automatically: if there is insufficient | |
8274 | room on the current page for the formatted record, the page is advanced by | |
8275 | writing a form feed, a special top-of-page format is used to format the new | |
8276 | page header before the record is written. By default, the top-of-page | |
8277 | format is the name of the filehandle with "_TOP" appended. This would be a | |
8278 | problem with autovivified filehandles, but it may be dynamically set to the | |
8279 | format of your choice by assigning the name to the C<$^> variable while | |
8280 | that filehandle is selected. The number of lines remaining on the current | |
8281 | page is in variable C<$->, which can be set to C<0> to force a new page. | |
8282 | ||
8283 | If FILEHANDLE is unspecified, output goes to the current default output | |
8284 | channel, which starts out as STDOUT but may be changed by the | |
8285 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression | |
8286 | is evaluated and the resulting string is used to look up the name of | |
8287 | the FILEHANDLE at run time. For more on formats, see L<perlform>. | |
8288 | ||
8289 | Note that write is I<not> the opposite of C<read>. Unfortunately. | |
8290 | ||
8291 | =item y/// | |
8292 | ||
8293 | The transliteration operator. Same as C<tr///>. See | |
8294 | L<perlop/"Quote and Quote-like Operators">. | |
8295 | ||
8296 | =back | |
8297 | ||
8298 | =cut |