4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides a scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, the scalar arguments will
18 be first, and the list argument will follow. (Note that there can ever
19 be only one such list argument.) For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for the 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 elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use the parentheses, the simple (but occasionally
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. And whitespace
37 between the function and left parenthesis doesn't count--so you need to
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.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
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
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in a scalar context by
59 returning the undefined value, and in a list context by returning the
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 it 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
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.
81 In general, functions in Perl that serve as wrappers for system calls
82 of the same name (like chown(2), fork(2), closedir(2), etc.) all 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 are 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.
89 =head2 Perl Functions by Category
92 Here are Perl's functions (including things that look like
93 functions, like some keywords and named operators)
94 arranged by category. Some functions appear in more
99 =item Functions for SCALARs or strings
100 X<scalar> X<string> X<character>
102 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
103 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
104 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
106 =item Regular expressions and pattern matching
107 X<regular expression> X<regex> X<regexp>
109 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
111 =item Numeric functions
112 X<numeric> X<number> X<trigonometric> X<trigonometry>
114 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
115 C<sin>, C<sqrt>, C<srand>
117 =item Functions for real @ARRAYs
120 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
122 =item Functions for list data
125 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
127 =item Functions for real %HASHes
130 C<delete>, C<each>, C<exists>, C<keys>, C<values>
132 =item Input and output functions
133 X<I/O> X<input> X<output> X<dbm>
135 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
136 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
137 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
138 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
141 =item Functions for fixed length data or records
143 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
145 =item Functions for filehandles, files, or directories
146 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
148 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
149 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
150 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
151 C<umask>, C<unlink>, C<utime>
153 =item Keywords related to the control flow of your Perl program
156 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
157 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
159 =item Keywords related to switch
161 C<break>, C<continue>
163 (These are only available if you enable the "switch" feature.
164 See L<feature> and L<perlsyn/"Switch statements">.)
166 =item Keywords related to scoping
168 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
170 =item Miscellaneous functions
172 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
173 C<scalar>, C<undef>, C<wantarray>
175 =item Functions for processes and process groups
176 X<process> X<pid> X<process id>
178 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
179 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
180 C<times>, C<wait>, C<waitpid>
182 =item Keywords related to perl modules
185 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
187 =item Keywords related to classes and object-orientedness
188 X<object> X<class> X<package>
190 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
193 =item Low-level socket functions
196 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
197 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
198 C<socket>, C<socketpair>
200 =item System V interprocess communication functions
201 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
203 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
204 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
206 =item Fetching user and group info
207 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
209 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
210 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
211 C<getpwuid>, C<setgrent>, C<setpwent>
213 =item Fetching network info
214 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
216 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
217 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
218 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
219 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
220 C<setnetent>, C<setprotoent>, C<setservent>
222 =item Time-related functions
225 C<gmtime>, C<localtime>, C<time>, C<times>
227 =item Functions new in perl5
230 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
231 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
232 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
233 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
235 * - C<sub> was a keyword in perl4, but in perl5 it is an
236 operator, which can be used in expressions.
238 =item Functions obsoleted in perl5
240 C<dbmclose>, C<dbmopen>
245 X<portability> X<Unix> X<portable>
247 Perl was born in Unix and can therefore access all common Unix
248 system calls. In non-Unix environments, the functionality of some
249 Unix system calls may not be available, or details of the available
250 functionality may differ slightly. The Perl functions affected
253 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
254 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
255 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
256 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
257 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
258 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
259 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
260 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
261 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
262 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
263 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
264 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
265 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
266 C<shmwrite>, C<socket>, C<socketpair>,
267 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
268 C<times>, C<truncate>, C<umask>, C<unlink>,
269 C<utime>, C<wait>, C<waitpid>
271 For more information about the portability of these functions, see
272 L<perlport> and other available platform-specific documentation.
274 =head2 Alphabetical Listing of Perl Functions
279 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>
280 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
286 A file test, where X is one of the letters listed below. This unary
287 operator takes one argument, either a filename or a filehandle, and
288 tests the associated file to see if something is true about it. If the
289 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
290 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
291 the undefined value if the file doesn't exist. Despite the funny
292 names, precedence is the same as any other named unary operator, and
293 the argument may be parenthesized like any other unary operator. The
294 operator may be any of:
296 -r File is readable by effective uid/gid.
297 -w File is writable by effective uid/gid.
298 -x File is executable by effective uid/gid.
299 -o File is owned by effective uid.
301 -R File is readable by real uid/gid.
302 -W File is writable by real uid/gid.
303 -X File is executable by real uid/gid.
304 -O File is owned by real uid.
307 -z File has zero size (is empty).
308 -s File has nonzero size (returns size in bytes).
310 -f File is a plain file.
311 -d File is a directory.
312 -l File is a symbolic link.
313 -p File is a named pipe (FIFO), or Filehandle is a pipe.
315 -b File is a block special file.
316 -c File is a character special file.
317 -t Filehandle is opened to a tty.
319 -u File has setuid bit set.
320 -g File has setgid bit set.
321 -k File has sticky bit set.
323 -T File is an ASCII text file (heuristic guess).
324 -B File is a "binary" file (opposite of -T).
326 -M Script start time minus file modification time, in days.
327 -A Same for access time.
328 -C Same for inode change time (Unix, may differ for other platforms)
334 next unless -f $_; # ignore specials
338 The interpretation of the file permission operators C<-r>, C<-R>,
339 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
340 of the file and the uids and gids of the user. There may be other
341 reasons you can't actually read, write, or execute the file. Such
342 reasons may be for example network filesystem access controls, ACLs
343 (access control lists), read-only filesystems, and unrecognized
346 Also note that, for the superuser on the local filesystems, the C<-r>,
347 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
348 if any execute bit is set in the mode. Scripts run by the superuser
349 may thus need to do a stat() to determine the actual mode of the file,
350 or temporarily set their effective uid to something else.
352 If you are using ACLs, there is a pragma called C<filetest> that may
353 produce more accurate results than the bare stat() mode bits.
354 When under the C<use filetest 'access'> the above-mentioned filetests
355 will test whether the permission can (not) be granted using the
356 access() family of system calls. Also note that the C<-x> and C<-X> may
357 under this pragma return true even if there are no execute permission
358 bits set (nor any extra execute permission ACLs). This strangeness is
359 due to the underlying system calls' definitions. Read the
360 documentation for the C<filetest> pragma for more information.
362 Note that C<-s/a/b/> does not do a negated substitution. Saying
363 C<-exp($foo)> still works as expected, however--only single letters
364 following a minus are interpreted as file tests.
366 The C<-T> and C<-B> switches work as follows. The first block or so of the
367 file is examined for odd characters such as strange control codes or
368 characters with the high bit set. If too many strange characters (>30%)
369 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
370 containing null in the first block is considered a binary file. If C<-T>
371 or C<-B> is used on a filehandle, the current IO buffer is examined
372 rather than the first block. Both C<-T> and C<-B> return true on a null
373 file, or a file at EOF when testing a filehandle. Because you have to
374 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
375 against the file first, as in C<next unless -f $file && -T $file>.
377 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
378 the special filehandle consisting of a solitary underline, then the stat
379 structure of the previous file test (or stat operator) is used, saving
380 a system call. (This doesn't work with C<-t>, and you need to remember
381 that lstat() and C<-l> will leave values in the stat structure for the
382 symbolic link, not the real file.) (Also, if the stat buffer was filled by
383 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
386 print "Can do.\n" if -r $a || -w _ || -x _;
389 print "Readable\n" if -r _;
390 print "Writable\n" if -w _;
391 print "Executable\n" if -x _;
392 print "Setuid\n" if -u _;
393 print "Setgid\n" if -g _;
394 print "Sticky\n" if -k _;
395 print "Text\n" if -T _;
396 print "Binary\n" if -B _;
398 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
399 test operators, in a way that C<-f -w -x $file> is equivalent to
400 C<-x $file && -w _ && -f _>. (This is only syntax fancy: if you use
401 the return value of C<-f $file> as an argument to another filetest
402 operator, no special magic will happen.)
409 Returns the absolute value of its argument.
410 If VALUE is omitted, uses C<$_>.
412 =item accept NEWSOCKET,GENERICSOCKET
415 Accepts an incoming socket connect, just as the accept(2) system call
416 does. Returns the packed address if it succeeded, false otherwise.
417 See the example in L<perlipc/"Sockets: Client/Server Communication">.
419 On systems that support a close-on-exec flag on files, the flag will
420 be set for the newly opened file descriptor, as determined by the
421 value of $^F. See L<perlvar/$^F>.
430 Arranges to have a SIGALRM delivered to this process after the
431 specified number of wallclock seconds has elapsed. If SECONDS is not
432 specified, the value stored in C<$_> is used. (On some machines,
433 unfortunately, the elapsed time may be up to one second less or more
434 than you specified because of how seconds are counted, and process
435 scheduling may delay the delivery of the signal even further.)
437 Only one timer may be counting at once. Each call disables the
438 previous timer, and an argument of C<0> may be supplied to cancel the
439 previous timer without starting a new one. The returned value is the
440 amount of time remaining on the previous timer.
442 For delays of finer granularity than one second, you may use Perl's
443 four-argument version of select() leaving the first three arguments
444 undefined, or you might be able to use the C<syscall> interface to
445 access setitimer(2) if your system supports it. The Time::HiRes
446 module (from CPAN, and starting from Perl 5.8 part of the standard
447 distribution) may also prove useful.
449 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
450 (C<sleep> may be internally implemented in your system with C<alarm>)
452 If you want to use C<alarm> to time out a system call you need to use an
453 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
454 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
455 restart system calls on some systems. Using C<eval>/C<die> always works,
456 modulo the caveats given in L<perlipc/"Signals">.
459 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
461 $nread = sysread SOCKET, $buffer, $size;
465 die unless $@ eq "alarm\n"; # propagate unexpected errors
472 For more information see L<perlipc>.
475 X<atan2> X<arctangent> X<tan> X<tangent>
477 Returns the arctangent of Y/X in the range -PI to PI.
479 For the tangent operation, you may use the C<Math::Trig::tan>
480 function, or use the familiar relation:
482 sub tan { sin($_[0]) / cos($_[0]) }
484 Note that atan2(0, 0) is not well-defined.
486 =item bind SOCKET,NAME
489 Binds a network address to a socket, just as the bind system call
490 does. Returns true if it succeeded, false otherwise. NAME should be a
491 packed address of the appropriate type for the socket. See the examples in
492 L<perlipc/"Sockets: Client/Server Communication">.
494 =item binmode FILEHANDLE, LAYER
495 X<binmode> X<binary> X<text> X<DOS> X<Windows>
497 =item binmode FILEHANDLE
499 Arranges for FILEHANDLE to be read or written in "binary" or "text"
500 mode on systems where the run-time libraries distinguish between
501 binary and text files. If FILEHANDLE is an expression, the value is
502 taken as the name of the filehandle. Returns true on success,
503 otherwise it returns C<undef> and sets C<$!> (errno).
505 On some systems (in general, DOS and Windows-based systems) binmode()
506 is necessary when you're not working with a text file. For the sake
507 of portability it is a good idea to always use it when appropriate,
508 and to never use it when it isn't appropriate. Also, people can
509 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
511 In other words: regardless of platform, use binmode() on binary data,
512 like for example images.
514 If LAYER is present it is a single string, but may contain multiple
515 directives. The directives alter the behaviour of the file handle.
516 When LAYER is present using binmode on text file makes sense.
518 If LAYER is omitted or specified as C<:raw> the filehandle is made
519 suitable for passing binary data. This includes turning off possible CRLF
520 translation and marking it as bytes (as opposed to Unicode characters).
521 Note that, despite what may be implied in I<"Programming Perl"> (the
522 Camel) or elsewhere, C<:raw> is I<not> the simply inverse of C<:crlf>
523 -- other layers which would affect binary nature of the stream are
524 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
525 PERLIO environment variable.
527 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
528 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
529 establish default I/O layers. See L<open>.
531 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
532 in "Programming Perl, 3rd Edition". However, since the publishing of this
533 book, by many known as "Camel III", the consensus of the naming of this
534 functionality has moved from "discipline" to "layer". All documentation
535 of this version of Perl therefore refers to "layers" rather than to
536 "disciplines". Now back to the regularly scheduled documentation...>
538 To mark FILEHANDLE as UTF-8, use C<:utf8>.
540 In general, binmode() should be called after open() but before any I/O
541 is done on the filehandle. Calling binmode() will normally flush any
542 pending buffered output data (and perhaps pending input data) on the
543 handle. An exception to this is the C<:encoding> layer that
544 changes the default character encoding of the handle, see L<open>.
545 The C<:encoding> layer sometimes needs to be called in
546 mid-stream, and it doesn't flush the stream. The C<:encoding>
547 also implicitly pushes on top of itself the C<:utf8> layer because
548 internally Perl will operate on UTF-8 encoded Unicode characters.
550 The operating system, device drivers, C libraries, and Perl run-time
551 system all work together to let the programmer treat a single
552 character (C<\n>) as the line terminator, irrespective of the external
553 representation. On many operating systems, the native text file
554 representation matches the internal representation, but on some
555 platforms the external representation of C<\n> is made up of more than
558 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
559 character to end each line in the external representation of text (even
560 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
561 on Unix and most VMS files). In other systems like OS/2, DOS and the
562 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
563 but what's stored in text files are the two characters C<\cM\cJ>. That
564 means that, if you don't use binmode() on these systems, C<\cM\cJ>
565 sequences on disk will be converted to C<\n> on input, and any C<\n> in
566 your program will be converted back to C<\cM\cJ> on output. This is what
567 you want for text files, but it can be disastrous for binary files.
569 Another consequence of using binmode() (on some systems) is that
570 special end-of-file markers will be seen as part of the data stream.
571 For systems from the Microsoft family this means that if your binary
572 data contains C<\cZ>, the I/O subsystem will regard it as the end of
573 the file, unless you use binmode().
575 binmode() is not only important for readline() and print() operations,
576 but also when using read(), seek(), sysread(), syswrite() and tell()
577 (see L<perlport> for more details). See the C<$/> and C<$\> variables
578 in L<perlvar> for how to manually set your input and output
579 line-termination sequences.
581 =item bless REF,CLASSNAME
586 This function tells the thingy referenced by REF that it is now an object
587 in the CLASSNAME package. If CLASSNAME is omitted, the current package
588 is used. Because a C<bless> is often the last thing in a constructor,
589 it returns the reference for convenience. Always use the two-argument
590 version if a derived class might inherit the function doing the blessing.
591 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
594 Consider always blessing objects in CLASSNAMEs that are mixed case.
595 Namespaces with all lowercase names are considered reserved for
596 Perl pragmata. Builtin types have all uppercase names. To prevent
597 confusion, you may wish to avoid such package names as well. Make sure
598 that CLASSNAME is a true value.
600 See L<perlmod/"Perl Modules">.
604 Break out of a C<given()> block.
606 This keyword is enabled by the "switch" feature: see L<feature>
607 for more information.
610 X<caller> X<call stack> X<stack> X<stack trace>
614 Returns the context of the current subroutine call. In scalar context,
615 returns the caller's package name if there is a caller, that is, if
616 we're in a subroutine or C<eval> or C<require>, and the undefined value
617 otherwise. In list context, returns
619 ($package, $filename, $line) = caller;
621 With EXPR, it returns some extra information that the debugger uses to
622 print a stack trace. The value of EXPR indicates how many call frames
623 to go back before the current one.
625 ($package, $filename, $line, $subroutine, $hasargs,
626 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
628 Here $subroutine may be C<(eval)> if the frame is not a subroutine
629 call, but an C<eval>. In such a case additional elements $evaltext and
630 C<$is_require> are set: C<$is_require> is true if the frame is created by a
631 C<require> or C<use> statement, $evaltext contains the text of the
632 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
633 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
634 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
635 frame.) $subroutine may also be C<(unknown)> if this particular
636 subroutine happens to have been deleted from the symbol table.
637 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
638 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
639 compiled with. The C<$hints> and C<$bitmask> values are subject to change
640 between versions of Perl, and are not meant for external use.
642 Furthermore, when called from within the DB package, caller returns more
643 detailed information: it sets the list variable C<@DB::args> to be the
644 arguments with which the subroutine was invoked.
646 Be aware that the optimizer might have optimized call frames away before
647 C<caller> had a chance to get the information. That means that C<caller(N)>
648 might not return information about the call frame you expect it do, for
649 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
650 previous time C<caller> was called.
656 =item chdir FILEHANDLE
658 =item chdir DIRHANDLE
662 Changes the working directory to EXPR, if possible. If EXPR is omitted,
663 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
664 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
665 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
666 neither is set, C<chdir> does nothing. It returns true upon success,
667 false otherwise. See the example under C<die>.
669 On systems that support fchdir, you might pass a file handle or
670 directory handle as argument. On systems that don't support fchdir,
671 passing handles produces a fatal error at run time.
674 X<chmod> X<permission> X<mode>
676 Changes the permissions of a list of files. The first element of the
677 list must be the numerical mode, which should probably be an octal
678 number, and which definitely should I<not> be a string of octal digits:
679 C<0644> is okay, C<'0644'> is not. Returns the number of files
680 successfully changed. See also L</oct>, if all you have is a string.
682 $cnt = chmod 0755, 'foo', 'bar';
683 chmod 0755, @executables;
684 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
686 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
687 $mode = 0644; chmod $mode, 'foo'; # this is best
689 On systems that support fchmod, you might pass file handles among the
690 files. On systems that don't support fchmod, passing file handles
691 produces a fatal error at run time.
693 open(my $fh, "<", "foo");
694 my $perm = (stat $fh)[2] & 07777;
695 chmod($perm | 0600, $fh);
697 You can also import the symbolic C<S_I*> constants from the Fcntl
702 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
703 # This is identical to the chmod 0755 of the above example.
706 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
712 This safer version of L</chop> removes any trailing string
713 that corresponds to the current value of C<$/> (also known as
714 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
715 number of characters removed from all its arguments. It's often used to
716 remove the newline from the end of an input record when you're worried
717 that the final record may be missing its newline. When in paragraph
718 mode (C<$/ = "">), it removes all trailing newlines from the string.
719 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
720 a reference to an integer or the like, see L<perlvar>) chomp() won't
722 If VARIABLE is omitted, it chomps C<$_>. Example:
725 chomp; # avoid \n on last field
730 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
732 You can actually chomp anything that's an lvalue, including an assignment:
735 chomp($answer = <STDIN>);
737 If you chomp a list, each element is chomped, and the total number of
738 characters removed is returned.
740 If the C<encoding> pragma is in scope then the lengths returned are
741 calculated from the length of C<$/> in Unicode characters, which is not
742 always the same as the length of C<$/> in the native encoding.
744 Note that parentheses are necessary when you're chomping anything
745 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
746 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
747 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
748 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
758 Chops off the last character of a string and returns the character
759 chopped. It is much more efficient than C<s/.$//s> because it neither
760 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
761 If VARIABLE is a hash, it chops the hash's values, but not its keys.
763 You can actually chop anything that's an lvalue, including an assignment.
765 If you chop a list, each element is chopped. Only the value of the
766 last C<chop> is returned.
768 Note that C<chop> returns the last character. To return all but the last
769 character, use C<substr($string, 0, -1)>.
774 X<chown> X<owner> X<user> X<group>
776 Changes the owner (and group) of a list of files. The first two
777 elements of the list must be the I<numeric> uid and gid, in that
778 order. A value of -1 in either position is interpreted by most
779 systems to leave that value unchanged. Returns the number of files
780 successfully changed.
782 $cnt = chown $uid, $gid, 'foo', 'bar';
783 chown $uid, $gid, @filenames;
785 On systems that support fchown, you might pass file handles among the
786 files. On systems that don't support fchown, passing file handles
787 produces a fatal error at run time.
789 Here's an example that looks up nonnumeric uids in the passwd file:
792 chomp($user = <STDIN>);
794 chomp($pattern = <STDIN>);
796 ($login,$pass,$uid,$gid) = getpwnam($user)
797 or die "$user not in passwd file";
799 @ary = glob($pattern); # expand filenames
800 chown $uid, $gid, @ary;
802 On most systems, you are not allowed to change the ownership of the
803 file unless you're the superuser, although you should be able to change
804 the group to any of your secondary groups. On insecure systems, these
805 restrictions may be relaxed, but this is not a portable assumption.
806 On POSIX systems, you can detect this condition this way:
808 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
809 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
812 X<chr> X<character> X<ASCII> X<Unicode>
816 Returns the character represented by that NUMBER in the character set.
817 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
818 chr(0x263a) is a Unicode smiley face. Note that characters from 128
819 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
820 backward compatibility reasons (but see L<encoding>).
822 Negative values give the Unicode replacement character (chr(0xfffd)),
823 except under the L<bytes> pragma, where low eight bits of the value
824 (truncated to an integer) are used.
826 If NUMBER is omitted, uses C<$_>.
828 For the reverse, use L</ord>.
830 Note that under the C<bytes> pragma the NUMBER is masked to
833 See L<perlunicode> and L<encoding> for more about Unicode.
835 =item chroot FILENAME
840 This function works like the system call by the same name: it makes the
841 named directory the new root directory for all further pathnames that
842 begin with a C</> by your process and all its children. (It doesn't
843 change your current working directory, which is unaffected.) For security
844 reasons, this call is restricted to the superuser. If FILENAME is
845 omitted, does a C<chroot> to C<$_>.
847 =item close FILEHANDLE
852 Closes the file or pipe associated with the file handle, returning
853 true only if IO buffers are successfully flushed and closes the system
854 file descriptor. Closes the currently selected filehandle if the
857 You don't have to close FILEHANDLE if you are immediately going to do
858 another C<open> on it, because C<open> will close it for you. (See
859 C<open>.) However, an explicit C<close> on an input file resets the line
860 counter (C<$.>), while the implicit close done by C<open> does not.
862 If the file handle came from a piped open, C<close> will additionally
863 return false if one of the other system calls involved fails, or if the
864 program exits with non-zero status. (If the only problem was that the
865 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
866 also waits for the process executing on the pipe to complete, in case you
867 want to look at the output of the pipe afterwards, and
868 implicitly puts the exit status value of that command into C<$?> and
869 C<${^CHILD_ERROR_NATIVE}>.
871 Prematurely closing the read end of a pipe (i.e. before the process
872 writing to it at the other end has closed it) will result in a
873 SIGPIPE being delivered to the writer. If the other end can't
874 handle that, be sure to read all the data before closing the pipe.
878 open(OUTPUT, '|sort >foo') # pipe to sort
879 or die "Can't start sort: $!";
880 #... # print stuff to output
881 close OUTPUT # wait for sort to finish
882 or warn $! ? "Error closing sort pipe: $!"
883 : "Exit status $? from sort";
884 open(INPUT, 'foo') # get sort's results
885 or die "Can't open 'foo' for input: $!";
887 FILEHANDLE may be an expression whose value can be used as an indirect
888 filehandle, usually the real filehandle name.
890 =item closedir DIRHANDLE
893 Closes a directory opened by C<opendir> and returns the success of that
896 =item connect SOCKET,NAME
899 Attempts to connect to a remote socket, just as the connect system call
900 does. Returns true if it succeeded, false otherwise. NAME should be a
901 packed address of the appropriate type for the socket. See the examples in
902 L<perlipc/"Sockets: Client/Server Communication">.
909 C<continue> is actually a flow control statement rather than a function. If
910 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
911 C<foreach>), it is always executed just before the conditional is about to
912 be evaluated again, just like the third part of a C<for> loop in C. Thus
913 it can be used to increment a loop variable, even when the loop has been
914 continued via the C<next> statement (which is similar to the C C<continue>
917 C<last>, C<next>, or C<redo> may appear within a C<continue>
918 block. C<last> and C<redo> will behave as if they had been executed within
919 the main block. So will C<next>, but since it will execute a C<continue>
920 block, it may be more entertaining.
923 ### redo always comes here
926 ### next always comes here
928 # then back the top to re-check EXPR
930 ### last always comes here
932 Omitting the C<continue> section is semantically equivalent to using an
933 empty one, logically enough. In that case, C<next> goes directly back
934 to check the condition at the top of the loop.
936 If the "switch" feature is enabled, C<continue> is also a
937 function that will break out of the current C<when> or C<default>
938 block, and fall through to the next case. See L<feature> and
939 L<perlsyn/"Switch statements"> for more information.
943 X<cos> X<cosine> X<acos> X<arccosine>
947 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
948 takes cosine of C<$_>.
950 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
951 function, or use this relation:
953 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
955 =item crypt PLAINTEXT,SALT
956 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
957 X<decrypt> X<cryptography> X<passwd>
959 Creates a digest string exactly like the crypt(3) function in the C
960 library (assuming that you actually have a version there that has not
961 been extirpated as a potential munitions).
963 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
964 into a short string, called a digest, which is returned. The same
965 PLAINTEXT and SALT will always return the same string, but there is no
966 (known) way to get the original PLAINTEXT from the hash. Small
967 changes in the PLAINTEXT or SALT will result in large changes in the
970 There is no decrypt function. This function isn't all that useful for
971 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
972 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
973 primarily used to check if two pieces of text are the same without
974 having to transmit or store the text itself. An example is checking
975 if a correct password is given. The digest of the password is stored,
976 not the password itself. The user types in a password that is
977 crypt()'d with the same salt as the stored digest. If the two digests
978 match the password is correct.
980 When verifying an existing digest string you should use the digest as
981 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
982 to create the digest is visible as part of the digest. This ensures
983 crypt() will hash the new string with the same salt as the digest.
984 This allows your code to work with the standard L<crypt|/crypt> and
985 with more exotic implementations. In other words, do not assume
986 anything about the returned string itself, or how many bytes in the
989 Traditionally the result is a string of 13 bytes: two first bytes of
990 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
991 the first eight bytes of the digest string mattered, but alternative
992 hashing schemes (like MD5), higher level security schemes (like C2),
993 and implementations on non-UNIX platforms may produce different
996 When choosing a new salt create a random two character string whose
997 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
998 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
999 characters is just a recommendation; the characters allowed in
1000 the salt depend solely on your system's crypt library, and Perl can't
1001 restrict what salts C<crypt()> accepts.
1003 Here's an example that makes sure that whoever runs this program knows
1006 $pwd = (getpwuid($<))[1];
1008 system "stty -echo";
1010 chomp($word = <STDIN>);
1014 if (crypt($word, $pwd) ne $pwd) {
1020 Of course, typing in your own password to whoever asks you
1023 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1024 of data, not least of all because you can't get the information
1025 back. Look at the L<Digest> module for more robust algorithms.
1027 If using crypt() on a Unicode string (which I<potentially> has
1028 characters with codepoints above 255), Perl tries to make sense
1029 of the situation by trying to downgrade (a copy of the string)
1030 the string back to an eight-bit byte string before calling crypt()
1031 (on that copy). If that works, good. If not, crypt() dies with
1032 C<Wide character in crypt>.
1037 [This function has been largely superseded by the C<untie> function.]
1039 Breaks the binding between a DBM file and a hash.
1041 =item dbmopen HASH,DBNAME,MASK
1042 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1044 [This function has been largely superseded by the C<tie> function.]
1046 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1047 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1048 argument is I<not> a filehandle, even though it looks like one). DBNAME
1049 is the name of the database (without the F<.dir> or F<.pag> extension if
1050 any). If the database does not exist, it is created with protection
1051 specified by MASK (as modified by the C<umask>). If your system supports
1052 only the older DBM functions, you may perform only one C<dbmopen> in your
1053 program. In older versions of Perl, if your system had neither DBM nor
1054 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1057 If you don't have write access to the DBM file, you can only read hash
1058 variables, not set them. If you want to test whether you can write,
1059 either use file tests or try setting a dummy hash entry inside an C<eval>,
1060 which will trap the error.
1062 Note that functions such as C<keys> and C<values> may return huge lists
1063 when used on large DBM files. You may prefer to use the C<each>
1064 function to iterate over large DBM files. Example:
1066 # print out history file offsets
1067 dbmopen(%HIST,'/usr/lib/news/history',0666);
1068 while (($key,$val) = each %HIST) {
1069 print $key, ' = ', unpack('L',$val), "\n";
1073 See also L<AnyDBM_File> for a more general description of the pros and
1074 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1075 rich implementation.
1077 You can control which DBM library you use by loading that library
1078 before you call dbmopen():
1081 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1082 or die "Can't open netscape history file: $!";
1085 X<defined> X<undef> X<undefined>
1089 Returns a Boolean value telling whether EXPR has a value other than
1090 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1093 Many operations return C<undef> to indicate failure, end of file,
1094 system error, uninitialized variable, and other exceptional
1095 conditions. This function allows you to distinguish C<undef> from
1096 other values. (A simple Boolean test will not distinguish among
1097 C<undef>, zero, the empty string, and C<"0">, which are all equally
1098 false.) Note that since C<undef> is a valid scalar, its presence
1099 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1100 returns C<undef> when its argument is an empty array, I<or> when the
1101 element to return happens to be C<undef>.
1103 You may also use C<defined(&func)> to check whether subroutine C<&func>
1104 has ever been defined. The return value is unaffected by any forward
1105 declarations of C<&func>. Note that a subroutine which is not defined
1106 may still be callable: its package may have an C<AUTOLOAD> method that
1107 makes it spring into existence the first time that it is called -- see
1110 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1111 used to report whether memory for that aggregate has ever been
1112 allocated. This behavior may disappear in future versions of Perl.
1113 You should instead use a simple test for size:
1115 if (@an_array) { print "has array elements\n" }
1116 if (%a_hash) { print "has hash members\n" }
1118 When used on a hash element, it tells you whether the value is defined,
1119 not whether the key exists in the hash. Use L</exists> for the latter
1124 print if defined $switch{'D'};
1125 print "$val\n" while defined($val = pop(@ary));
1126 die "Can't readlink $sym: $!"
1127 unless defined($value = readlink $sym);
1128 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1129 $debugging = 0 unless defined $debugging;
1131 Note: Many folks tend to overuse C<defined>, and then are surprised to
1132 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1133 defined values. For example, if you say
1137 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1138 matched "nothing". It didn't really fail to match anything. Rather, it
1139 matched something that happened to be zero characters long. This is all
1140 very above-board and honest. When a function returns an undefined value,
1141 it's an admission that it couldn't give you an honest answer. So you
1142 should use C<defined> only when you're questioning the integrity of what
1143 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1146 See also L</undef>, L</exists>, L</ref>.
1151 Given an expression that specifies a hash element, array element, hash slice,
1152 or array slice, deletes the specified element(s) from the hash or array.
1153 In the case of an array, if the array elements happen to be at the end,
1154 the size of the array will shrink to the highest element that tests
1155 true for exists() (or 0 if no such element exists).
1157 Returns a list with the same number of elements as the number of elements
1158 for which deletion was attempted. Each element of that list consists of
1159 either the value of the element deleted, or the undefined value. In scalar
1160 context, this means that you get the value of the last element deleted (or
1161 the undefined value if that element did not exist).
1163 %hash = (foo => 11, bar => 22, baz => 33);
1164 $scalar = delete $hash{foo}; # $scalar is 11
1165 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1166 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1168 Deleting from C<%ENV> modifies the environment. Deleting from
1169 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1170 from a C<tie>d hash or array may not necessarily return anything.
1172 Deleting an array element effectively returns that position of the array
1173 to its initial, uninitialized state. Subsequently testing for the same
1174 element with exists() will return false. Also, deleting array elements
1175 in the middle of an array will not shift the index of the elements
1176 after them down. Use splice() for that. See L</exists>.
1178 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1180 foreach $key (keys %HASH) {
1184 foreach $index (0 .. $#ARRAY) {
1185 delete $ARRAY[$index];
1190 delete @HASH{keys %HASH};
1192 delete @ARRAY[0 .. $#ARRAY];
1194 But both of these are slower than just assigning the empty list
1195 or undefining %HASH or @ARRAY:
1197 %HASH = (); # completely empty %HASH
1198 undef %HASH; # forget %HASH ever existed
1200 @ARRAY = (); # completely empty @ARRAY
1201 undef @ARRAY; # forget @ARRAY ever existed
1203 Note that the EXPR can be arbitrarily complicated as long as the final
1204 operation is a hash element, array element, hash slice, or array slice
1207 delete $ref->[$x][$y]{$key};
1208 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1210 delete $ref->[$x][$y][$index];
1211 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1214 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1216 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1217 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1218 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1219 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1220 an C<eval(),> the error message is stuffed into C<$@> and the
1221 C<eval> is terminated with the undefined value. This makes
1222 C<die> the way to raise an exception.
1224 Equivalent examples:
1226 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1227 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1229 If the last element of LIST does not end in a newline, the current
1230 script line number and input line number (if any) are also printed,
1231 and a newline is supplied. Note that the "input line number" (also
1232 known as "chunk") is subject to whatever notion of "line" happens to
1233 be currently in effect, and is also available as the special variable
1234 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1236 Hint: sometimes appending C<", stopped"> to your message will cause it
1237 to make better sense when the string C<"at foo line 123"> is appended.
1238 Suppose you are running script "canasta".
1240 die "/etc/games is no good";
1241 die "/etc/games is no good, stopped";
1243 produce, respectively
1245 /etc/games is no good at canasta line 123.
1246 /etc/games is no good, stopped at canasta line 123.
1248 See also exit(), warn(), and the Carp module.
1250 If LIST is empty and C<$@> already contains a value (typically from a
1251 previous eval) that value is reused after appending C<"\t...propagated">.
1252 This is useful for propagating exceptions:
1255 die unless $@ =~ /Expected exception/;
1257 If LIST is empty and C<$@> contains an object reference that has a
1258 C<PROPAGATE> method, that method will be called with additional file
1259 and line number parameters. The return value replaces the value in
1260 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1263 If C<$@> is empty then the string C<"Died"> is used.
1265 die() can also be called with a reference argument. If this happens to be
1266 trapped within an eval(), $@ contains the reference. This behavior permits
1267 a more elaborate exception handling implementation using objects that
1268 maintain arbitrary state about the nature of the exception. Such a scheme
1269 is sometimes preferable to matching particular string values of $@ using
1270 regular expressions. Here's an example:
1272 use Scalar::Util 'blessed';
1274 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1276 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1277 # handle Some::Module::Exception
1280 # handle all other possible exceptions
1284 Because perl will stringify uncaught exception messages before displaying
1285 them, you may want to overload stringification operations on such custom
1286 exception objects. See L<overload> for details about that.
1288 You can arrange for a callback to be run just before the C<die>
1289 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1290 handler will be called with the error text and can change the error
1291 message, if it sees fit, by calling C<die> again. See
1292 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1293 L<"eval BLOCK"> for some examples. Although this feature was
1294 to be run only right before your program was to exit, this is not
1295 currently the case--the C<$SIG{__DIE__}> hook is currently called
1296 even inside eval()ed blocks/strings! If one wants the hook to do
1297 nothing in such situations, put
1301 as the first line of the handler (see L<perlvar/$^S>). Because
1302 this promotes strange action at a distance, this counterintuitive
1303 behavior may be fixed in a future release.
1308 Not really a function. Returns the value of the last command in the
1309 sequence of commands indicated by BLOCK. When modified by the C<while> or
1310 C<until> loop modifier, executes the BLOCK once before testing the loop
1311 condition. (On other statements the loop modifiers test the conditional
1314 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1315 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1316 See L<perlsyn> for alternative strategies.
1318 =item do SUBROUTINE(LIST)
1321 This form of subroutine call is deprecated. See L<perlsub>.
1326 Uses the value of EXPR as a filename and executes the contents of the
1327 file as a Perl script.
1335 except that it's more efficient and concise, keeps track of the current
1336 filename for error messages, searches the @INC directories, and updates
1337 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1338 variables. It also differs in that code evaluated with C<do FILENAME>
1339 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1340 same, however, in that it does reparse the file every time you call it,
1341 so you probably don't want to do this inside a loop.
1343 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1344 error. If C<do> can read the file but cannot compile it, it
1345 returns undef and sets an error message in C<$@>. If the file is
1346 successfully compiled, C<do> returns the value of the last expression
1349 Note that inclusion of library modules is better done with the
1350 C<use> and C<require> operators, which also do automatic error checking
1351 and raise an exception if there's a problem.
1353 You might like to use C<do> to read in a program configuration
1354 file. Manual error checking can be done this way:
1356 # read in config files: system first, then user
1357 for $file ("/share/prog/defaults.rc",
1358 "$ENV{HOME}/.someprogrc")
1360 unless ($return = do $file) {
1361 warn "couldn't parse $file: $@" if $@;
1362 warn "couldn't do $file: $!" unless defined $return;
1363 warn "couldn't run $file" unless $return;
1368 X<dump> X<core> X<undump>
1372 This function causes an immediate core dump. See also the B<-u>
1373 command-line switch in L<perlrun>, which does the same thing.
1374 Primarily this is so that you can use the B<undump> program (not
1375 supplied) to turn your core dump into an executable binary after
1376 having initialized all your variables at the beginning of the
1377 program. When the new binary is executed it will begin by executing
1378 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1379 Think of it as a goto with an intervening core dump and reincarnation.
1380 If C<LABEL> is omitted, restarts the program from the top.
1382 B<WARNING>: Any files opened at the time of the dump will I<not>
1383 be open any more when the program is reincarnated, with possible
1384 resulting confusion on the part of Perl.
1386 This function is now largely obsolete, partly because it's very
1387 hard to convert a core file into an executable, and because the
1388 real compiler backends for generating portable bytecode and compilable
1389 C code have superseded it. That's why you should now invoke it as
1390 C<CORE::dump()>, if you don't want to be warned against a possible
1393 If you're looking to use L<dump> to speed up your program, consider
1394 generating bytecode or native C code as described in L<perlcc>. If
1395 you're just trying to accelerate a CGI script, consider using the
1396 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1397 You might also consider autoloading or selfloading, which at least
1398 make your program I<appear> to run faster.
1401 X<each> X<hash, iterator>
1403 When called in list context, returns a 2-element list consisting of the
1404 key and value for the next element of a hash, so that you can iterate over
1405 it. When called in scalar context, returns only the key for the next
1406 element in the hash.
1408 Entries are returned in an apparently random order. The actual random
1409 order is subject to change in future versions of perl, but it is
1410 guaranteed to be in the same order as either the C<keys> or C<values>
1411 function would produce on the same (unmodified) hash. Since Perl
1412 5.8.1 the ordering is different even between different runs of Perl
1413 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1415 When the hash is entirely read, a null array is returned in list context
1416 (which when assigned produces a false (C<0>) value), and C<undef> in
1417 scalar context. The next call to C<each> after that will start iterating
1418 again. There is a single iterator for each hash, shared by all C<each>,
1419 C<keys>, and C<values> function calls in the program; it can be reset by
1420 reading all the elements from the hash, or by evaluating C<keys HASH> or
1421 C<values HASH>. If you add or delete elements of a hash while you're
1422 iterating over it, you may get entries skipped or duplicated, so
1423 don't. Exception: It is always safe to delete the item most recently
1424 returned by C<each()>, which means that the following code will work:
1426 while (($key, $value) = each %hash) {
1428 delete $hash{$key}; # This is safe
1431 The following prints out your environment like the printenv(1) program,
1432 only in a different order:
1434 while (($key,$value) = each %ENV) {
1435 print "$key=$value\n";
1438 See also C<keys>, C<values> and C<sort>.
1440 =item eof FILEHANDLE
1449 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1450 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1451 gives the real filehandle. (Note that this function actually
1452 reads a character and then C<ungetc>s it, so isn't very useful in an
1453 interactive context.) Do not read from a terminal file (or call
1454 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1455 as terminals may lose the end-of-file condition if you do.
1457 An C<eof> without an argument uses the last file read. Using C<eof()>
1458 with empty parentheses is very different. It refers to the pseudo file
1459 formed from the files listed on the command line and accessed via the
1460 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1461 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1462 used will cause C<@ARGV> to be examined to determine if input is
1463 available. Similarly, an C<eof()> after C<< <> >> has returned
1464 end-of-file will assume you are processing another C<@ARGV> list,
1465 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1466 see L<perlop/"I/O Operators">.
1468 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1469 detect the end of each file, C<eof()> will only detect the end of the
1470 last file. Examples:
1472 # reset line numbering on each input file
1474 next if /^\s*#/; # skip comments
1477 close ARGV if eof; # Not eof()!
1480 # insert dashes just before last line of last file
1482 if (eof()) { # check for end of last file
1483 print "--------------\n";
1486 last if eof(); # needed if we're reading from a terminal
1489 Practical hint: you almost never need to use C<eof> in Perl, because the
1490 input operators typically return C<undef> when they run out of data, or if
1494 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1500 In the first form, the return value of EXPR is parsed and executed as if it
1501 were a little Perl program. The value of the expression (which is itself
1502 determined within scalar context) is first parsed, and if there weren't any
1503 errors, executed in the lexical context of the current Perl program, so
1504 that any variable settings or subroutine and format definitions remain
1505 afterwards. Note that the value is parsed every time the C<eval> executes.
1506 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1507 delay parsing and subsequent execution of the text of EXPR until run time.
1509 In the second form, the code within the BLOCK is parsed only once--at the
1510 same time the code surrounding the C<eval> itself was parsed--and executed
1511 within the context of the current Perl program. This form is typically
1512 used to trap exceptions more efficiently than the first (see below), while
1513 also providing the benefit of checking the code within BLOCK at compile
1516 The final semicolon, if any, may be omitted from the value of EXPR or within
1519 In both forms, the value returned is the value of the last expression
1520 evaluated inside the mini-program; a return statement may be also used, just
1521 as with subroutines. The expression providing the return value is evaluated
1522 in void, scalar, or list context, depending on the context of the C<eval>
1523 itself. See L</wantarray> for more on how the evaluation context can be
1526 If there is a syntax error or runtime error, or a C<die> statement is
1527 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1528 error message. If there was no error, C<$@> is guaranteed to be a null
1529 string. Beware that using C<eval> neither silences perl from printing
1530 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1531 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1532 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1533 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1535 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1536 determining whether a particular feature (such as C<socket> or C<symlink>)
1537 is implemented. It is also Perl's exception trapping mechanism, where
1538 the die operator is used to raise exceptions.
1540 If the code to be executed doesn't vary, you may use the eval-BLOCK
1541 form to trap run-time errors without incurring the penalty of
1542 recompiling each time. The error, if any, is still returned in C<$@>.
1545 # make divide-by-zero nonfatal
1546 eval { $answer = $a / $b; }; warn $@ if $@;
1548 # same thing, but less efficient
1549 eval '$answer = $a / $b'; warn $@ if $@;
1551 # a compile-time error
1552 eval { $answer = }; # WRONG
1555 eval '$answer ='; # sets $@
1557 Using the C<eval{}> form as an exception trap in libraries does have some
1558 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1559 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1560 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1561 as shown in this example:
1563 # a very private exception trap for divide-by-zero
1564 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1567 This is especially significant, given that C<__DIE__> hooks can call
1568 C<die> again, which has the effect of changing their error messages:
1570 # __DIE__ hooks may modify error messages
1572 local $SIG{'__DIE__'} =
1573 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1574 eval { die "foo lives here" };
1575 print $@ if $@; # prints "bar lives here"
1578 Because this promotes action at a distance, this counterintuitive behavior
1579 may be fixed in a future release.
1581 With an C<eval>, you should be especially careful to remember what's
1582 being looked at when:
1588 eval { $x }; # CASE 4
1590 eval "\$$x++"; # CASE 5
1593 Cases 1 and 2 above behave identically: they run the code contained in
1594 the variable $x. (Although case 2 has misleading double quotes making
1595 the reader wonder what else might be happening (nothing is).) Cases 3
1596 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1597 does nothing but return the value of $x. (Case 4 is preferred for
1598 purely visual reasons, but it also has the advantage of compiling at
1599 compile-time instead of at run-time.) Case 5 is a place where
1600 normally you I<would> like to use double quotes, except that in this
1601 particular situation, you can just use symbolic references instead, as
1604 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1605 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1607 Note that as a very special case, an C<eval ''> executed within the C<DB>
1608 package doesn't see the usual surrounding lexical scope, but rather the
1609 scope of the first non-DB piece of code that called it. You don't normally
1610 need to worry about this unless you are writing a Perl debugger.
1615 =item exec PROGRAM LIST
1617 The C<exec> function executes a system command I<and never returns>--
1618 use C<system> instead of C<exec> if you want it to return. It fails and
1619 returns false only if the command does not exist I<and> it is executed
1620 directly instead of via your system's command shell (see below).
1622 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1623 warns you if there is a following statement which isn't C<die>, C<warn>,
1624 or C<exit> (if C<-w> is set - but you always do that). If you
1625 I<really> want to follow an C<exec> with some other statement, you
1626 can use one of these styles to avoid the warning:
1628 exec ('foo') or print STDERR "couldn't exec foo: $!";
1629 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1631 If there is more than one argument in LIST, or if LIST is an array
1632 with more than one value, calls execvp(3) with the arguments in LIST.
1633 If there is only one scalar argument or an array with one element in it,
1634 the argument is checked for shell metacharacters, and if there are any,
1635 the entire argument is passed to the system's command shell for parsing
1636 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1637 If there are no shell metacharacters in the argument, it is split into
1638 words and passed directly to C<execvp>, which is more efficient.
1641 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1642 exec "sort $outfile | uniq";
1644 If you don't really want to execute the first argument, but want to lie
1645 to the program you are executing about its own name, you can specify
1646 the program you actually want to run as an "indirect object" (without a
1647 comma) in front of the LIST. (This always forces interpretation of the
1648 LIST as a multivalued list, even if there is only a single scalar in
1651 $shell = '/bin/csh';
1652 exec $shell '-sh'; # pretend it's a login shell
1656 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1658 When the arguments get executed via the system shell, results will
1659 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1662 Using an indirect object with C<exec> or C<system> is also more
1663 secure. This usage (which also works fine with system()) forces
1664 interpretation of the arguments as a multivalued list, even if the
1665 list had just one argument. That way you're safe from the shell
1666 expanding wildcards or splitting up words with whitespace in them.
1668 @args = ( "echo surprise" );
1670 exec @args; # subject to shell escapes
1672 exec { $args[0] } @args; # safe even with one-arg list
1674 The first version, the one without the indirect object, ran the I<echo>
1675 program, passing it C<"surprise"> an argument. The second version
1676 didn't--it tried to run a program literally called I<"echo surprise">,
1677 didn't find it, and set C<$?> to a non-zero value indicating failure.
1679 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1680 output before the exec, but this may not be supported on some platforms
1681 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1682 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1683 open handles in order to avoid lost output.
1685 Note that C<exec> will not call your C<END> blocks, nor will it call
1686 any C<DESTROY> methods in your objects.
1689 X<exists> X<autovivification>
1691 Given an expression that specifies a hash element or array element,
1692 returns true if the specified element in the hash or array has ever
1693 been initialized, even if the corresponding value is undefined. The
1694 element is not autovivified if it doesn't exist.
1696 print "Exists\n" if exists $hash{$key};
1697 print "Defined\n" if defined $hash{$key};
1698 print "True\n" if $hash{$key};
1700 print "Exists\n" if exists $array[$index];
1701 print "Defined\n" if defined $array[$index];
1702 print "True\n" if $array[$index];
1704 A hash or array element can be true only if it's defined, and defined if
1705 it exists, but the reverse doesn't necessarily hold true.
1707 Given an expression that specifies the name of a subroutine,
1708 returns true if the specified subroutine has ever been declared, even
1709 if it is undefined. Mentioning a subroutine name for exists or defined
1710 does not count as declaring it. Note that a subroutine which does not
1711 exist may still be callable: its package may have an C<AUTOLOAD>
1712 method that makes it spring into existence the first time that it is
1713 called -- see L<perlsub>.
1715 print "Exists\n" if exists &subroutine;
1716 print "Defined\n" if defined &subroutine;
1718 Note that the EXPR can be arbitrarily complicated as long as the final
1719 operation is a hash or array key lookup or subroutine name:
1721 if (exists $ref->{A}->{B}->{$key}) { }
1722 if (exists $hash{A}{B}{$key}) { }
1724 if (exists $ref->{A}->{B}->[$ix]) { }
1725 if (exists $hash{A}{B}[$ix]) { }
1727 if (exists &{$ref->{A}{B}{$key}}) { }
1729 Although the deepest nested array or hash will not spring into existence
1730 just because its existence was tested, any intervening ones will.
1731 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1732 into existence due to the existence test for the $key element above.
1733 This happens anywhere the arrow operator is used, including even:
1736 if (exists $ref->{"Some key"}) { }
1737 print $ref; # prints HASH(0x80d3d5c)
1739 This surprising autovivification in what does not at first--or even
1740 second--glance appear to be an lvalue context may be fixed in a future
1743 Use of a subroutine call, rather than a subroutine name, as an argument
1744 to exists() is an error.
1747 exists &sub(); # Error
1750 X<exit> X<terminate> X<abort>
1754 Evaluates EXPR and exits immediately with that value. Example:
1757 exit 0 if $ans =~ /^[Xx]/;
1759 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1760 universally recognized values for EXPR are C<0> for success and C<1>
1761 for error; other values are subject to interpretation depending on the
1762 environment in which the Perl program is running. For example, exiting
1763 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1764 the mailer to return the item undelivered, but that's not true everywhere.
1766 Don't use C<exit> to abort a subroutine if there's any chance that
1767 someone might want to trap whatever error happened. Use C<die> instead,
1768 which can be trapped by an C<eval>.
1770 The exit() function does not always exit immediately. It calls any
1771 defined C<END> routines first, but these C<END> routines may not
1772 themselves abort the exit. Likewise any object destructors that need to
1773 be called are called before the real exit. If this is a problem, you
1774 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1775 See L<perlmod> for details.
1778 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1782 Returns I<e> (the natural logarithm base) to the power of EXPR.
1783 If EXPR is omitted, gives C<exp($_)>.
1785 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1788 Implements the fcntl(2) function. You'll probably have to say
1792 first to get the correct constant definitions. Argument processing and
1793 value return works just like C<ioctl> below.
1797 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1798 or die "can't fcntl F_GETFL: $!";
1800 You don't have to check for C<defined> on the return from C<fcntl>.
1801 Like C<ioctl>, it maps a C<0> return from the system call into
1802 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1803 in numeric context. It is also exempt from the normal B<-w> warnings
1804 on improper numeric conversions.
1806 Note that C<fcntl> will produce a fatal error if used on a machine that
1807 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1808 manpage to learn what functions are available on your system.
1810 Here's an example of setting a filehandle named C<REMOTE> to be
1811 non-blocking at the system level. You'll have to negotiate C<$|>
1812 on your own, though.
1814 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1816 $flags = fcntl(REMOTE, F_GETFL, 0)
1817 or die "Can't get flags for the socket: $!\n";
1819 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1820 or die "Can't set flags for the socket: $!\n";
1822 =item fileno FILEHANDLE
1825 Returns the file descriptor for a filehandle, or undefined if the
1826 filehandle is not open. This is mainly useful for constructing
1827 bitmaps for C<select> and low-level POSIX tty-handling operations.
1828 If FILEHANDLE is an expression, the value is taken as an indirect
1829 filehandle, generally its name.
1831 You can use this to find out whether two handles refer to the
1832 same underlying descriptor:
1834 if (fileno(THIS) == fileno(THAT)) {
1835 print "THIS and THAT are dups\n";
1838 (Filehandles connected to memory objects via new features of C<open> may
1839 return undefined even though they are open.)
1842 =item flock FILEHANDLE,OPERATION
1843 X<flock> X<lock> X<locking>
1845 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1846 for success, false on failure. Produces a fatal error if used on a
1847 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1848 C<flock> is Perl's portable file locking interface, although it locks
1849 only entire files, not records.
1851 Two potentially non-obvious but traditional C<flock> semantics are
1852 that it waits indefinitely until the lock is granted, and that its locks
1853 B<merely advisory>. Such discretionary locks are more flexible, but offer
1854 fewer guarantees. This means that programs that do not also use C<flock>
1855 may modify files locked with C<flock>. See L<perlport>,
1856 your port's specific documentation, or your system-specific local manpages
1857 for details. It's best to assume traditional behavior if you're writing
1858 portable programs. (But if you're not, you should as always feel perfectly
1859 free to write for your own system's idiosyncrasies (sometimes called
1860 "features"). Slavish adherence to portability concerns shouldn't get
1861 in the way of your getting your job done.)
1863 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1864 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1865 you can use the symbolic names if you import them from the Fcntl module,
1866 either individually, or as a group using the ':flock' tag. LOCK_SH
1867 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1868 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1869 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1870 waiting for the lock (check the return status to see if you got it).
1872 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1873 before locking or unlocking it.
1875 Note that the emulation built with lockf(3) doesn't provide shared
1876 locks, and it requires that FILEHANDLE be open with write intent. These
1877 are the semantics that lockf(3) implements. Most if not all systems
1878 implement lockf(3) in terms of fcntl(2) locking, though, so the
1879 differing semantics shouldn't bite too many people.
1881 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1882 be open with read intent to use LOCK_SH and requires that it be open
1883 with write intent to use LOCK_EX.
1885 Note also that some versions of C<flock> cannot lock things over the
1886 network; you would need to use the more system-specific C<fcntl> for
1887 that. If you like you can force Perl to ignore your system's flock(2)
1888 function, and so provide its own fcntl(2)-based emulation, by passing
1889 the switch C<-Ud_flock> to the F<Configure> program when you configure
1892 Here's a mailbox appender for BSD systems.
1894 use Fcntl ':flock'; # import LOCK_* constants
1897 flock(MBOX,LOCK_EX);
1898 # and, in case someone appended
1899 # while we were waiting...
1904 flock(MBOX,LOCK_UN);
1907 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1908 or die "Can't open mailbox: $!";
1911 print MBOX $msg,"\n\n";
1914 On systems that support a real flock(), locks are inherited across fork()
1915 calls, whereas those that must resort to the more capricious fcntl()
1916 function lose the locks, making it harder to write servers.
1918 See also L<DB_File> for other flock() examples.
1921 X<fork> X<child> X<parent>
1923 Does a fork(2) system call to create a new process running the
1924 same program at the same point. It returns the child pid to the
1925 parent process, C<0> to the child process, or C<undef> if the fork is
1926 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1927 are shared, while everything else is copied. On most systems supporting
1928 fork(), great care has gone into making it extremely efficient (for
1929 example, using copy-on-write technology on data pages), making it the
1930 dominant paradigm for multitasking over the last few decades.
1932 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1933 output before forking the child process, but this may not be supported
1934 on some platforms (see L<perlport>). To be safe, you may need to set
1935 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1936 C<IO::Handle> on any open handles in order to avoid duplicate output.
1938 If you C<fork> without ever waiting on your children, you will
1939 accumulate zombies. On some systems, you can avoid this by setting
1940 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1941 forking and reaping moribund children.
1943 Note that if your forked child inherits system file descriptors like
1944 STDIN and STDOUT that are actually connected by a pipe or socket, even
1945 if you exit, then the remote server (such as, say, a CGI script or a
1946 backgrounded job launched from a remote shell) won't think you're done.
1947 You should reopen those to F</dev/null> if it's any issue.
1952 Declare a picture format for use by the C<write> function. For
1956 Test: @<<<<<<<< @||||| @>>>>>
1957 $str, $%, '$' . int($num)
1961 $num = $cost/$quantity;
1965 See L<perlform> for many details and examples.
1967 =item formline PICTURE,LIST
1970 This is an internal function used by C<format>s, though you may call it,
1971 too. It formats (see L<perlform>) a list of values according to the
1972 contents of PICTURE, placing the output into the format output
1973 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1974 Eventually, when a C<write> is done, the contents of
1975 C<$^A> are written to some filehandle. You could also read C<$^A>
1976 and then set C<$^A> back to C<"">. Note that a format typically
1977 does one C<formline> per line of form, but the C<formline> function itself
1978 doesn't care how many newlines are embedded in the PICTURE. This means
1979 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1980 You may therefore need to use multiple formlines to implement a single
1981 record format, just like the format compiler.
1983 Be careful if you put double quotes around the picture, because an C<@>
1984 character may be taken to mean the beginning of an array name.
1985 C<formline> always returns true. See L<perlform> for other examples.
1987 =item getc FILEHANDLE
1992 Returns the next character from the input file attached to FILEHANDLE,
1993 or the undefined value at end of file, or if there was an error (in
1994 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1995 STDIN. This is not particularly efficient. However, it cannot be
1996 used by itself to fetch single characters without waiting for the user
1997 to hit enter. For that, try something more like:
2000 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2003 system "stty", '-icanon', 'eol', "\001";
2009 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2012 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2016 Determination of whether $BSD_STYLE should be set
2017 is left as an exercise to the reader.
2019 The C<POSIX::getattr> function can do this more portably on
2020 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2021 module from your nearest CPAN site; details on CPAN can be found on
2025 X<getlogin> X<login>
2027 This implements the C library function of the same name, which on most
2028 systems returns the current login from F</etc/utmp>, if any. If null,
2031 $login = getlogin || getpwuid($<) || "Kilroy";
2033 Do not consider C<getlogin> for authentication: it is not as
2034 secure as C<getpwuid>.
2036 =item getpeername SOCKET
2037 X<getpeername> X<peer>
2039 Returns the packed sockaddr address of other end of the SOCKET connection.
2042 $hersockaddr = getpeername(SOCK);
2043 ($port, $iaddr) = sockaddr_in($hersockaddr);
2044 $herhostname = gethostbyaddr($iaddr, AF_INET);
2045 $herstraddr = inet_ntoa($iaddr);
2050 Returns the current process group for the specified PID. Use
2051 a PID of C<0> to get the current process group for the
2052 current process. Will raise an exception if used on a machine that
2053 doesn't implement getpgrp(2). If PID is omitted, returns process
2054 group of current process. Note that the POSIX version of C<getpgrp>
2055 does not accept a PID argument, so only C<PID==0> is truly portable.
2058 X<getppid> X<parent> X<pid>
2060 Returns the process id of the parent process.
2062 Note for Linux users: on Linux, the C functions C<getpid()> and
2063 C<getppid()> return different values from different threads. In order to
2064 be portable, this behavior is not reflected by the perl-level function
2065 C<getppid()>, that returns a consistent value across threads. If you want
2066 to call the underlying C<getppid()>, you may use the CPAN module
2069 =item getpriority WHICH,WHO
2070 X<getpriority> X<priority> X<nice>
2072 Returns the current priority for a process, a process group, or a user.
2073 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2074 machine that doesn't implement getpriority(2).
2077 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2078 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2079 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2080 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2081 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2082 X<endnetent> X<endprotoent> X<endservent>
2086 =item gethostbyname NAME
2088 =item getnetbyname NAME
2090 =item getprotobyname NAME
2096 =item getservbyname NAME,PROTO
2098 =item gethostbyaddr ADDR,ADDRTYPE
2100 =item getnetbyaddr ADDR,ADDRTYPE
2102 =item getprotobynumber NUMBER
2104 =item getservbyport PORT,PROTO
2122 =item sethostent STAYOPEN
2124 =item setnetent STAYOPEN
2126 =item setprotoent STAYOPEN
2128 =item setservent STAYOPEN
2142 These routines perform the same functions as their counterparts in the
2143 system library. In list context, the return values from the
2144 various get routines are as follows:
2146 ($name,$passwd,$uid,$gid,
2147 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2148 ($name,$passwd,$gid,$members) = getgr*
2149 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2150 ($name,$aliases,$addrtype,$net) = getnet*
2151 ($name,$aliases,$proto) = getproto*
2152 ($name,$aliases,$port,$proto) = getserv*
2154 (If the entry doesn't exist you get a null list.)
2156 The exact meaning of the $gcos field varies but it usually contains
2157 the real name of the user (as opposed to the login name) and other
2158 information pertaining to the user. Beware, however, that in many
2159 system users are able to change this information and therefore it
2160 cannot be trusted and therefore the $gcos is tainted (see
2161 L<perlsec>). The $passwd and $shell, user's encrypted password and
2162 login shell, are also tainted, because of the same reason.
2164 In scalar context, you get the name, unless the function was a
2165 lookup by name, in which case you get the other thing, whatever it is.
2166 (If the entry doesn't exist you get the undefined value.) For example:
2168 $uid = getpwnam($name);
2169 $name = getpwuid($num);
2171 $gid = getgrnam($name);
2172 $name = getgrgid($num);
2176 In I<getpw*()> the fields $quota, $comment, and $expire are special
2177 cases in the sense that in many systems they are unsupported. If the
2178 $quota is unsupported, it is an empty scalar. If it is supported, it
2179 usually encodes the disk quota. If the $comment field is unsupported,
2180 it is an empty scalar. If it is supported it usually encodes some
2181 administrative comment about the user. In some systems the $quota
2182 field may be $change or $age, fields that have to do with password
2183 aging. In some systems the $comment field may be $class. The $expire
2184 field, if present, encodes the expiration period of the account or the
2185 password. For the availability and the exact meaning of these fields
2186 in your system, please consult your getpwnam(3) documentation and your
2187 F<pwd.h> file. You can also find out from within Perl what your
2188 $quota and $comment fields mean and whether you have the $expire field
2189 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2190 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2191 files are only supported if your vendor has implemented them in the
2192 intuitive fashion that calling the regular C library routines gets the
2193 shadow versions if you're running under privilege or if there exists
2194 the shadow(3) functions as found in System V (this includes Solaris
2195 and Linux.) Those systems that implement a proprietary shadow password
2196 facility are unlikely to be supported.
2198 The $members value returned by I<getgr*()> is a space separated list of
2199 the login names of the members of the group.
2201 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2202 C, it will be returned to you via C<$?> if the function call fails. The
2203 C<@addrs> value returned by a successful call is a list of the raw
2204 addresses returned by the corresponding system library call. In the
2205 Internet domain, each address is four bytes long and you can unpack it
2206 by saying something like:
2208 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2210 The Socket library makes this slightly easier:
2213 $iaddr = inet_aton("127.1"); # or whatever address
2214 $name = gethostbyaddr($iaddr, AF_INET);
2216 # or going the other way
2217 $straddr = inet_ntoa($iaddr);
2219 If you get tired of remembering which element of the return list
2220 contains which return value, by-name interfaces are provided
2221 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2222 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2223 and C<User::grent>. These override the normal built-ins, supplying
2224 versions that return objects with the appropriate names
2225 for each field. For example:
2229 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2231 Even though it looks like they're the same method calls (uid),
2232 they aren't, because a C<File::stat> object is different from
2233 a C<User::pwent> object.
2235 =item getsockname SOCKET
2238 Returns the packed sockaddr address of this end of the SOCKET connection,
2239 in case you don't know the address because you have several different
2240 IPs that the connection might have come in on.
2243 $mysockaddr = getsockname(SOCK);
2244 ($port, $myaddr) = sockaddr_in($mysockaddr);
2245 printf "Connect to %s [%s]\n",
2246 scalar gethostbyaddr($myaddr, AF_INET),
2249 =item getsockopt SOCKET,LEVEL,OPTNAME
2252 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2253 Options may exist at multiple protocol levels depending on the socket
2254 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2255 C<Socket> module) will exist. To query options at another level the
2256 protocol number of the appropriate protocol controlling the option
2257 should be supplied. For example, to indicate that an option is to be
2258 interpreted by the TCP protocol, LEVEL should be set to the protocol
2259 number of TCP, which you can get using getprotobyname.
2261 The call returns a packed string representing the requested socket option,
2262 or C<undef> if there is an error (the error reason will be in $!). What
2263 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2264 your system documentation for details. A very common case however is that
2265 the option is an integer, in which case the result will be a packed
2266 integer which you can decode using unpack with the C<i> (or C<I>) format.
2268 An example testing if Nagle's algorithm is turned on on a socket:
2270 use Socket qw(:all);
2272 defined(my $tcp = getprotobyname("tcp"))
2273 or die "Could not determine the protocol number for tcp";
2274 # my $tcp = IPPROTO_TCP; # Alternative
2275 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2276 or die "Could not query TCP_NODELAY socket option: $!";
2277 my $nodelay = unpack("I", $packed);
2278 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2282 X<glob> X<wildcard> X<filename, expansion> X<expand>
2286 In list context, returns a (possibly empty) list of filename expansions on
2287 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2288 scalar context, glob iterates through such filename expansions, returning
2289 undef when the list is exhausted. This is the internal function
2290 implementing the C<< <*.c> >> operator, but you can use it directly. If
2291 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2292 more detail in L<perlop/"I/O Operators">.
2294 Beginning with v5.6.0, this operator is implemented using the standard
2295 C<File::Glob> extension. See L<File::Glob> for details.
2298 X<gmtime> X<UTC> X<Greenwich>
2302 Converts a time as returned by the time function to an 9-element list
2303 with the time localized for the standard Greenwich time zone.
2304 Typically used as follows:
2307 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2310 All list elements are numeric, and come straight out of the C `struct
2311 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2312 specified time. $mday is the day of the month, and $mon is the month
2313 itself, in the range C<0..11> with 0 indicating January and 11
2314 indicating December. $year is the number of years since 1900. That
2315 is, $year is C<123> in year 2023. $wday is the day of the week, with
2316 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2317 the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2320 Note that the $year element is I<not> simply the last two digits of
2321 the year. If you assume it is then you create non-Y2K-compliant
2322 programs--and you wouldn't want to do that, would you?
2324 The proper way to get a complete 4-digit year is simply:
2328 And to get the last two digits of the year (e.g., '01' in 2001) do:
2330 $year = sprintf("%02d", $year % 100);
2332 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2334 In scalar context, C<gmtime()> returns the ctime(3) value:
2336 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2338 If you need local time instead of GMT use the L</localtime> builtin.
2339 See also the C<timegm> function provided by the C<Time::Local> module,
2340 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2342 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2343 instead a Perl builtin. To get somewhat similar but locale dependent date
2344 strings, see the example in L</localtime>.
2346 See L<perlport/gmtime> for portability concerns.
2349 X<goto> X<jump> X<jmp>
2355 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2356 execution there. It may not be used to go into any construct that
2357 requires initialization, such as a subroutine or a C<foreach> loop. It
2358 also can't be used to go into a construct that is optimized away,
2359 or to get out of a block or subroutine given to C<sort>.
2360 It can be used to go almost anywhere else within the dynamic scope,
2361 including out of subroutines, but it's usually better to use some other
2362 construct such as C<last> or C<die>. The author of Perl has never felt the
2363 need to use this form of C<goto> (in Perl, that is--C is another matter).
2364 (The difference being that C does not offer named loops combined with
2365 loop control. Perl does, and this replaces most structured uses of C<goto>
2366 in other languages.)
2368 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2369 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2370 necessarily recommended if you're optimizing for maintainability:
2372 goto ("FOO", "BAR", "GLARCH")[$i];
2374 The C<goto-&NAME> form is quite different from the other forms of
2375 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2376 doesn't have the stigma associated with other gotos. Instead, it
2377 exits the current subroutine (losing any changes set by local()) and
2378 immediately calls in its place the named subroutine using the current
2379 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2380 load another subroutine and then pretend that the other subroutine had
2381 been called in the first place (except that any modifications to C<@_>
2382 in the current subroutine are propagated to the other subroutine.)
2383 After the C<goto>, not even C<caller> will be able to tell that this
2384 routine was called first.
2386 NAME needn't be the name of a subroutine; it can be a scalar variable
2387 containing a code reference, or a block that evaluates to a code
2390 =item grep BLOCK LIST
2393 =item grep EXPR,LIST
2395 This is similar in spirit to, but not the same as, grep(1) and its
2396 relatives. In particular, it is not limited to using regular expressions.
2398 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2399 C<$_> to each element) and returns the list value consisting of those
2400 elements for which the expression evaluated to true. In scalar
2401 context, returns the number of times the expression was true.
2403 @foo = grep(!/^#/, @bar); # weed out comments
2407 @foo = grep {!/^#/} @bar; # weed out comments
2409 Note that C<$_> is an alias to the list value, so it can be used to
2410 modify the elements of the LIST. While this is useful and supported,
2411 it can cause bizarre results if the elements of LIST are not variables.
2412 Similarly, grep returns aliases into the original list, much as a for
2413 loop's index variable aliases the list elements. That is, modifying an
2414 element of a list returned by grep (for example, in a C<foreach>, C<map>
2415 or another C<grep>) actually modifies the element in the original list.
2416 This is usually something to be avoided when writing clear code.
2418 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2419 been declared with C<my $_>) then, in addition to being locally aliased to
2420 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2421 can't be seen from the outside, avoiding any potential side-effects.
2423 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2426 X<hex> X<hexadecimal>
2430 Interprets EXPR as a hex string and returns the corresponding value.
2431 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2432 L</oct>.) If EXPR is omitted, uses C<$_>.
2434 print hex '0xAf'; # prints '175'
2435 print hex 'aF'; # same
2437 Hex strings may only represent integers. Strings that would cause
2438 integer overflow trigger a warning. Leading whitespace is not stripped,
2439 unlike oct(). To present something as hex, look into L</printf>,
2440 L</sprintf>, or L</unpack>.
2445 There is no builtin C<import> function. It is just an ordinary
2446 method (subroutine) defined (or inherited) by modules that wish to export
2447 names to another module. The C<use> function calls the C<import> method
2448 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2450 =item index STR,SUBSTR,POSITION
2451 X<index> X<indexOf> X<InStr>
2453 =item index STR,SUBSTR
2455 The index function searches for one string within another, but without
2456 the wildcard-like behavior of a full regular-expression pattern match.
2457 It returns the position of the first occurrence of SUBSTR in STR at
2458 or after POSITION. If POSITION is omitted, starts searching from the
2459 beginning of the string. POSITION before the beginning of the string
2460 or after its end is treated as if it were the beginning or the end,
2461 respectively. POSITION and the return value are based at C<0> (or whatever
2462 you've set the C<$[> variable to--but don't do that). If the substring
2463 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2466 X<int> X<integer> X<truncate> X<trunc>
2470 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2471 You should not use this function for rounding: one because it truncates
2472 towards C<0>, and two because machine representations of floating point
2473 numbers can sometimes produce counterintuitive results. For example,
2474 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2475 because it's really more like -268.99999999999994315658 instead. Usually,
2476 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2477 functions will serve you better than will int().
2479 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2482 Implements the ioctl(2) function. You'll probably first have to say
2484 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2486 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2487 exist or doesn't have the correct definitions you'll have to roll your
2488 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2489 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2490 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2491 written depending on the FUNCTION--a pointer to the string value of SCALAR
2492 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2493 has no string value but does have a numeric value, that value will be
2494 passed rather than a pointer to the string value. To guarantee this to be
2495 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2496 functions may be needed to manipulate the values of structures used by
2499 The return value of C<ioctl> (and C<fcntl>) is as follows:
2501 if OS returns: then Perl returns:
2503 0 string "0 but true"
2504 anything else that number
2506 Thus Perl returns true on success and false on failure, yet you can
2507 still easily determine the actual value returned by the operating
2510 $retval = ioctl(...) || -1;
2511 printf "System returned %d\n", $retval;
2513 The special string C<"0 but true"> is exempt from B<-w> complaints
2514 about improper numeric conversions.
2516 =item join EXPR,LIST
2519 Joins the separate strings of LIST into a single string with fields
2520 separated by the value of EXPR, and returns that new string. Example:
2522 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2524 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2525 first argument. Compare L</split>.
2530 Returns a list consisting of all the keys of the named hash.
2531 (In scalar context, returns the number of keys.)
2533 The keys are returned in an apparently random order. The actual
2534 random order is subject to change in future versions of perl, but it
2535 is guaranteed to be the same order as either the C<values> or C<each>
2536 function produces (given that the hash has not been modified). Since
2537 Perl 5.8.1 the ordering is different even between different runs of
2538 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2541 As a side effect, calling keys() resets the HASH's internal iterator
2542 (see L</each>). In particular, calling keys() in void context resets
2543 the iterator with no other overhead.
2545 Here is yet another way to print your environment:
2548 @values = values %ENV;
2550 print pop(@keys), '=', pop(@values), "\n";
2553 or how about sorted by key:
2555 foreach $key (sort(keys %ENV)) {
2556 print $key, '=', $ENV{$key}, "\n";
2559 The returned values are copies of the original keys in the hash, so
2560 modifying them will not affect the original hash. Compare L</values>.
2562 To sort a hash by value, you'll need to use a C<sort> function.
2563 Here's a descending numeric sort of a hash by its values:
2565 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2566 printf "%4d %s\n", $hash{$key}, $key;
2569 As an lvalue C<keys> allows you to increase the number of hash buckets
2570 allocated for the given hash. This can gain you a measure of efficiency if
2571 you know the hash is going to get big. (This is similar to pre-extending
2572 an array by assigning a larger number to $#array.) If you say
2576 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2577 in fact, since it rounds up to the next power of two. These
2578 buckets will be retained even if you do C<%hash = ()>, use C<undef
2579 %hash> if you want to free the storage while C<%hash> is still in scope.
2580 You can't shrink the number of buckets allocated for the hash using
2581 C<keys> in this way (but you needn't worry about doing this by accident,
2582 as trying has no effect).
2584 See also C<each>, C<values> and C<sort>.
2586 =item kill SIGNAL, LIST
2589 Sends a signal to a list of processes. Returns the number of
2590 processes successfully signaled (which is not necessarily the
2591 same as the number actually killed).
2593 $cnt = kill 1, $child1, $child2;
2596 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2597 system call will check whether it's possible to send a signal to it (that
2598 means, to be brief, that the process is owned by the same user, or we are
2599 the super-user). This is a useful way to check that a child process is
2600 alive and hasn't changed its UID. See L<perlport> for notes on the
2601 portability of this construct.
2603 Unlike in the shell, if SIGNAL is negative, it kills
2604 process groups instead of processes. (On System V, a negative I<PROCESS>
2605 number will also kill process groups, but that's not portable.) That
2606 means you usually want to use positive not negative signals. You may also
2607 use a signal name in quotes.
2609 See L<perlipc/"Signals"> for more details.
2616 The C<last> command is like the C<break> statement in C (as used in
2617 loops); it immediately exits the loop in question. If the LABEL is
2618 omitted, the command refers to the innermost enclosing loop. The
2619 C<continue> block, if any, is not executed:
2621 LINE: while (<STDIN>) {
2622 last LINE if /^$/; # exit when done with header
2626 C<last> cannot be used to exit a block which returns a value such as
2627 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2628 a grep() or map() operation.
2630 Note that a block by itself is semantically identical to a loop
2631 that executes once. Thus C<last> can be used to effect an early
2632 exit out of such a block.
2634 See also L</continue> for an illustration of how C<last>, C<next>, and
2642 Returns a lowercased version of EXPR. This is the internal function
2643 implementing the C<\L> escape in double-quoted strings. Respects
2644 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2645 and L<perlunicode> for more details about locale and Unicode support.
2647 If EXPR is omitted, uses C<$_>.
2650 X<lcfirst> X<lowercase>
2654 Returns the value of EXPR with the first character lowercased. This
2655 is the internal function implementing the C<\l> escape in
2656 double-quoted strings. Respects current LC_CTYPE locale if C<use
2657 locale> in force. See L<perllocale> and L<perlunicode> for more
2658 details about locale and Unicode support.
2660 If EXPR is omitted, uses C<$_>.
2667 Returns the length in I<characters> of the value of EXPR. If EXPR is
2668 omitted, returns length of C<$_>. Note that this cannot be used on
2669 an entire array or hash to find out how many elements these have.
2670 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2672 Note the I<characters>: if the EXPR is in Unicode, you will get the
2673 number of characters, not the number of bytes. To get the length
2674 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2676 =item link OLDFILE,NEWFILE
2679 Creates a new filename linked to the old filename. Returns true for
2680 success, false otherwise.
2682 =item listen SOCKET,QUEUESIZE
2685 Does the same thing that the listen system call does. Returns true if
2686 it succeeded, false otherwise. See the example in
2687 L<perlipc/"Sockets: Client/Server Communication">.
2692 You really probably want to be using C<my> instead, because C<local> isn't
2693 what most people think of as "local". See
2694 L<perlsub/"Private Variables via my()"> for details.
2696 A local modifies the listed variables to be local to the enclosing
2697 block, file, or eval. If more than one value is listed, the list must
2698 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2699 for details, including issues with tied arrays and hashes.
2701 =item localtime EXPR
2706 Converts a time as returned by the time function to a 9-element list
2707 with the time analyzed for the local time zone. Typically used as
2711 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2714 All list elements are numeric, and come straight out of the C `struct
2715 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2716 of the specified time.
2718 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2719 the range C<0..11> with 0 indicating January and 11 indicating December.
2720 This makes it easy to get a month name from a list:
2722 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2723 print "$abbr[$mon] $mday";
2724 # $mon=9, $mday=18 gives "Oct 18"
2726 C<$year> is the number of years since 1900, not just the last two digits
2727 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2728 to get a complete 4-digit year is simply:
2732 To get the last two digits of the year (e.g., '01' in 2001) do:
2734 $year = sprintf("%02d", $year % 100);
2736 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2737 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2738 (or C<0..365> in leap years.)
2740 C<$isdst> is true if the specified time occurs during Daylight Saving
2741 Time, false otherwise.
2743 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2745 In scalar context, C<localtime()> returns the ctime(3) value:
2747 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2749 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2750 instead of local time use the L</gmtime> builtin. See also the
2751 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2752 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2753 and mktime(3) functions.
2755 To get somewhat similar but locale dependent date strings, set up your
2756 locale environment variables appropriately (please see L<perllocale>) and
2759 use POSIX qw(strftime);
2760 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2761 # or for GMT formatted appropriately for your locale:
2762 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2764 Note that the C<%a> and C<%b>, the short forms of the day of the week
2765 and the month of the year, may not necessarily be three characters wide.
2767 See L<perlport/localtime> for portability concerns.
2772 This function places an advisory lock on a shared variable, or referenced
2773 object contained in I<THING> until the lock goes out of scope.
2775 lock() is a "weak keyword" : this means that if you've defined a function
2776 by this name (before any calls to it), that function will be called
2777 instead. (However, if you've said C<use threads>, lock() is always a
2778 keyword.) See L<threads>.
2781 X<log> X<logarithm> X<e> X<ln> X<base>
2785 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2786 returns log of C<$_>. To get the log of another base, use basic algebra:
2787 The base-N log of a number is equal to the natural log of that number
2788 divided by the natural log of N. For example:
2792 return log($n)/log(10);
2795 See also L</exp> for the inverse operation.
2802 Does the same thing as the C<stat> function (including setting the
2803 special C<_> filehandle) but stats a symbolic link instead of the file
2804 the symbolic link points to. If symbolic links are unimplemented on
2805 your system, a normal C<stat> is done. For much more detailed
2806 information, please see the documentation for C<stat>.
2808 If EXPR is omitted, stats C<$_>.
2812 The match operator. See L<perlop>.
2814 =item map BLOCK LIST
2819 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2820 C<$_> to each element) and returns the list value composed of the
2821 results of each such evaluation. In scalar context, returns the
2822 total number of elements so generated. Evaluates BLOCK or EXPR in
2823 list context, so each element of LIST may produce zero, one, or
2824 more elements in the returned value.
2826 @chars = map(chr, @nums);
2828 translates a list of numbers to the corresponding characters. And
2830 %hash = map { getkey($_) => $_ } @array;
2832 is just a funny way to write
2835 foreach $_ (@array) {
2836 $hash{getkey($_)} = $_;
2839 Note that C<$_> is an alias to the list value, so it can be used to
2840 modify the elements of the LIST. While this is useful and supported,
2841 it can cause bizarre results if the elements of LIST are not variables.
2842 Using a regular C<foreach> loop for this purpose would be clearer in
2843 most cases. See also L</grep> for an array composed of those items of
2844 the original list for which the BLOCK or EXPR evaluates to true.
2846 If C<$_> is lexical in the scope where the C<map> appears (because it has
2847 been declared with C<my $_>) then, in addition to being locally aliased to
2848 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2849 can't be seen from the outside, avoiding any potential side-effects.
2851 C<{> starts both hash references and blocks, so C<map { ...> could be either
2852 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2853 ahead for the closing C<}> it has to take a guess at which its dealing with
2854 based what it finds just after the C<{>. Usually it gets it right, but if it
2855 doesn't it won't realize something is wrong until it gets to the C<}> and
2856 encounters the missing (or unexpected) comma. The syntax error will be
2857 reported close to the C<}> but you'll need to change something near the C<{>
2858 such as using a unary C<+> to give perl some help:
2860 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2861 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2862 %hash = map { ("\L$_", 1) } @array # this also works
2863 %hash = map { lc($_), 1 } @array # as does this.
2864 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2866 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2868 or to force an anon hash constructor use C<+{>
2870 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2872 and you get list of anonymous hashes each with only 1 entry.
2874 =item mkdir FILENAME,MASK
2875 X<mkdir> X<md> X<directory, create>
2877 =item mkdir FILENAME
2881 Creates the directory specified by FILENAME, with permissions
2882 specified by MASK (as modified by C<umask>). If it succeeds it
2883 returns true, otherwise it returns false and sets C<$!> (errno).
2884 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2887 In general, it is better to create directories with permissive MASK,
2888 and let the user modify that with their C<umask>, than it is to supply
2889 a restrictive MASK and give the user no way to be more permissive.
2890 The exceptions to this rule are when the file or directory should be
2891 kept private (mail files, for instance). The perlfunc(1) entry on
2892 C<umask> discusses the choice of MASK in more detail.
2894 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2895 number of trailing slashes. Some operating and filesystems do not get
2896 this right, so Perl automatically removes all trailing slashes to keep
2899 =item msgctl ID,CMD,ARG
2902 Calls the System V IPC function msgctl(2). You'll probably have to say
2906 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2907 then ARG must be a variable that will hold the returned C<msqid_ds>
2908 structure. Returns like C<ioctl>: the undefined value for error,
2909 C<"0 but true"> for zero, or the actual return value otherwise. See also
2910 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2912 =item msgget KEY,FLAGS
2915 Calls the System V IPC function msgget(2). Returns the message queue
2916 id, or the undefined value if there is an error. See also
2917 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2919 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2922 Calls the System V IPC function msgrcv to receive a message from
2923 message queue ID into variable VAR with a maximum message size of
2924 SIZE. Note that when a message is received, the message type as a
2925 native long integer will be the first thing in VAR, followed by the
2926 actual message. This packing may be opened with C<unpack("l! a*")>.
2927 Taints the variable. Returns true if successful, or false if there is
2928 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2929 C<IPC::SysV::Msg> documentation.
2931 =item msgsnd ID,MSG,FLAGS
2934 Calls the System V IPC function msgsnd to send the message MSG to the
2935 message queue ID. MSG must begin with the native long integer message
2936 type, and be followed by the length of the actual message, and finally
2937 the message itself. This kind of packing can be achieved with
2938 C<pack("l! a*", $type, $message)>. Returns true if successful,
2939 or false if there is an error. See also C<IPC::SysV>
2940 and C<IPC::SysV::Msg> documentation.
2947 =item my EXPR : ATTRS
2949 =item my TYPE EXPR : ATTRS
2951 A C<my> declares the listed variables to be local (lexically) to the
2952 enclosing block, file, or C<eval>. If more than one value is listed,
2953 the list must be placed in parentheses.
2955 The exact semantics and interface of TYPE and ATTRS are still
2956 evolving. TYPE is currently bound to the use of C<fields> pragma,
2957 and attributes are handled using the C<attributes> pragma, or starting
2958 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2959 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2960 L<attributes>, and L<Attribute::Handlers>.
2967 The C<next> command is like the C<continue> statement in C; it starts
2968 the next iteration of the loop:
2970 LINE: while (<STDIN>) {
2971 next LINE if /^#/; # discard comments
2975 Note that if there were a C<continue> block on the above, it would get
2976 executed even on discarded lines. If the LABEL is omitted, the command
2977 refers to the innermost enclosing loop.
2979 C<next> cannot be used to exit a block which returns a value such as
2980 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2981 a grep() or map() operation.
2983 Note that a block by itself is semantically identical to a loop
2984 that executes once. Thus C<next> will exit such a block early.
2986 See also L</continue> for an illustration of how C<last>, C<next>, and
2989 =item no Module VERSION LIST
2992 =item no Module VERSION
2994 =item no Module LIST
2998 See the C<use> function, of which C<no> is the opposite.
3001 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3005 Interprets EXPR as an octal string and returns the corresponding
3006 value. (If EXPR happens to start off with C<0x>, interprets it as a
3007 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3008 binary string. Leading whitespace is ignored in all three cases.)
3009 The following will handle decimal, binary, octal, and hex in the standard
3012 $val = oct($val) if $val =~ /^0/;
3014 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3015 in octal), use sprintf() or printf():
3017 $perms = (stat("filename"))[2] & 07777;
3018 $oct_perms = sprintf "%lo", $perms;
3020 The oct() function is commonly used when a string such as C<644> needs
3021 to be converted into a file mode, for example. (Although perl will
3022 automatically convert strings into numbers as needed, this automatic
3023 conversion assumes base 10.)
3025 =item open FILEHANDLE,EXPR
3026 X<open> X<pipe> X<file, open> X<fopen>
3028 =item open FILEHANDLE,MODE,EXPR
3030 =item open FILEHANDLE,MODE,EXPR,LIST
3032 =item open FILEHANDLE,MODE,REFERENCE
3034 =item open FILEHANDLE
3036 Opens the file whose filename is given by EXPR, and associates it with
3039 (The following is a comprehensive reference to open(): for a gentler
3040 introduction you may consider L<perlopentut>.)
3042 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3043 the variable is assigned a reference to a new anonymous filehandle,
3044 otherwise if FILEHANDLE is an expression, its value is used as the name of
3045 the real filehandle wanted. (This is considered a symbolic reference, so
3046 C<use strict 'refs'> should I<not> be in effect.)
3048 If EXPR is omitted, the scalar variable of the same name as the
3049 FILEHANDLE contains the filename. (Note that lexical variables--those
3050 declared with C<my>--will not work for this purpose; so if you're
3051 using C<my>, specify EXPR in your call to open.)
3053 If three or more arguments are specified then the mode of opening and
3054 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3055 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3056 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3057 the file is opened for appending, again being created if necessary.
3059 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3060 indicate that you want both read and write access to the file; thus
3061 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3062 '+>' >> mode would clobber the file first. You can't usually use
3063 either read-write mode for updating textfiles, since they have
3064 variable length records. See the B<-i> switch in L<perlrun> for a
3065 better approach. The file is created with permissions of C<0666>
3066 modified by the process' C<umask> value.
3068 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3069 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3071 In the 2-arguments (and 1-argument) form of the call the mode and
3072 filename should be concatenated (in this order), possibly separated by
3073 spaces. It is possible to omit the mode in these forms if the mode is
3076 If the filename begins with C<'|'>, the filename is interpreted as a
3077 command to which output is to be piped, and if the filename ends with a
3078 C<'|'>, the filename is interpreted as a command which pipes output to
3079 us. See L<perlipc/"Using open() for IPC">
3080 for more examples of this. (You are not allowed to C<open> to a command
3081 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3082 and L<perlipc/"Bidirectional Communication with Another Process">
3085 For three or more arguments if MODE is C<'|-'>, the filename is
3086 interpreted as a command to which output is to be piped, and if MODE
3087 is C<'-|'>, the filename is interpreted as a command which pipes
3088 output to us. In the 2-arguments (and 1-argument) form one should
3089 replace dash (C<'-'>) with the command.
3090 See L<perlipc/"Using open() for IPC"> for more examples of this.
3091 (You are not allowed to C<open> to a command that pipes both in I<and>
3092 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3093 L<perlipc/"Bidirectional Communication"> for alternatives.)
3095 In the three-or-more argument form of pipe opens, if LIST is specified
3096 (extra arguments after the command name) then LIST becomes arguments
3097 to the command invoked if the platform supports it. The meaning of
3098 C<open> with more than three arguments for non-pipe modes is not yet
3099 specified. Experimental "layers" may give extra LIST arguments
3102 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3103 and opening C<< '>-' >> opens STDOUT.
3105 You may use the three-argument form of open to specify IO "layers"
3106 (sometimes also referred to as "disciplines") to be applied to the handle
3107 that affect how the input and output are processed (see L<open> and
3108 L<PerlIO> for more details). For example
3110 open(FH, "<:utf8", "file")
3112 will open the UTF-8 encoded file containing Unicode characters,
3113 see L<perluniintro>. Note that if layers are specified in the
3114 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3115 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3117 Open returns nonzero upon success, the undefined value otherwise. If
3118 the C<open> involved a pipe, the return value happens to be the pid of
3121 If you're running Perl on a system that distinguishes between text
3122 files and binary files, then you should check out L</binmode> for tips
3123 for dealing with this. The key distinction between systems that need
3124 C<binmode> and those that don't is their text file formats. Systems
3125 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3126 character, and which encode that character in C as C<"\n">, do not
3127 need C<binmode>. The rest need it.
3129 When opening a file, it's usually a bad idea to continue normal execution
3130 if the request failed, so C<open> is frequently used in connection with
3131 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3132 where you want to make a nicely formatted error message (but there are
3133 modules that can help with that problem)) you should always check
3134 the return value from opening a file. The infrequent exception is when
3135 working with an unopened filehandle is actually what you want to do.
3137 As a special case the 3-arg form with a read/write mode and the third
3138 argument being C<undef>:
3140 open(TMP, "+>", undef) or die ...
3142 opens a filehandle to an anonymous temporary file. Also using "+<"
3143 works for symmetry, but you really should consider writing something
3144 to the temporary file first. You will need to seek() to do the
3147 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3148 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3149 "in memory" files held in Perl scalars via:
3151 open($fh, '>', \$variable) || ..
3153 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3154 file, you have to close it first:
3157 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3162 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3163 while (<ARTICLE>) {...
3165 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3166 # if the open fails, output is discarded
3168 open(DBASE, '+<', 'dbase.mine') # open for update
3169 or die "Can't open 'dbase.mine' for update: $!";
3171 open(DBASE, '+<dbase.mine') # ditto
3172 or die "Can't open 'dbase.mine' for update: $!";
3174 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3175 or die "Can't start caesar: $!";
3177 open(ARTICLE, "caesar <$article |") # ditto
3178 or die "Can't start caesar: $!";
3180 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3181 or die "Can't start sort: $!";
3184 open(MEMORY,'>', \$var)
3185 or die "Can't open memory file: $!";
3186 print MEMORY "foo!\n"; # output will end up in $var
3188 # process argument list of files along with any includes
3190 foreach $file (@ARGV) {
3191 process($file, 'fh00');
3195 my($filename, $input) = @_;
3196 $input++; # this is a string increment
3197 unless (open($input, $filename)) {
3198 print STDERR "Can't open $filename: $!\n";
3203 while (<$input>) { # note use of indirection
3204 if (/^#include "(.*)"/) {
3205 process($1, $input);
3212 See L<perliol> for detailed info on PerlIO.
3214 You may also, in the Bourne shell tradition, specify an EXPR beginning
3215 with C<< '>&' >>, in which case the rest of the string is interpreted
3216 as the name of a filehandle (or file descriptor, if numeric) to be
3217 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3218 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3219 The mode you specify should match the mode of the original filehandle.
3220 (Duping a filehandle does not take into account any existing contents
3221 of IO buffers.) If you use the 3-arg form then you can pass either a
3222 number, the name of a filehandle or the normal "reference to a glob".
3224 Here is a script that saves, redirects, and restores C<STDOUT> and
3225 C<STDERR> using various methods:
3228 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3229 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3231 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3232 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3234 select STDERR; $| = 1; # make unbuffered
3235 select STDOUT; $| = 1; # make unbuffered
3237 print STDOUT "stdout 1\n"; # this works for
3238 print STDERR "stderr 1\n"; # subprocesses too
3240 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3241 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3243 print STDOUT "stdout 2\n";
3244 print STDERR "stderr 2\n";
3246 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3247 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3248 that file descriptor (and not call L<dup(2)>); this is more
3249 parsimonious of file descriptors. For example:
3251 # open for input, reusing the fileno of $fd
3252 open(FILEHANDLE, "<&=$fd")
3256 open(FILEHANDLE, "<&=", $fd)
3260 # open for append, using the fileno of OLDFH
3261 open(FH, ">>&=", OLDFH)
3265 open(FH, ">>&=OLDFH")
3267 Being parsimonious on filehandles is also useful (besides being
3268 parsimonious) for example when something is dependent on file
3269 descriptors, like for example locking using flock(). If you do just
3270 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3271 descriptor as B, and therefore flock(A) will not flock(B), and vice
3272 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3273 the same file descriptor.
3275 Note that if you are using Perls older than 5.8.0, Perl will be using
3276 the standard C libraries' fdopen() to implement the "=" functionality.
3277 On many UNIX systems fdopen() fails when file descriptors exceed a
3278 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3279 most often the default.
3281 You can see whether Perl has been compiled with PerlIO or not by
3282 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3283 is C<define>, you have PerlIO, otherwise you don't.
3285 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3286 with 2-arguments (or 1-argument) form of open(), then
3287 there is an implicit fork done, and the return value of open is the pid
3288 of the child within the parent process, and C<0> within the child
3289 process. (Use C<defined($pid)> to determine whether the open was successful.)
3290 The filehandle behaves normally for the parent, but i/o to that
3291 filehandle is piped from/to the STDOUT/STDIN of the child process.
3292 In the child process the filehandle isn't opened--i/o happens from/to
3293 the new STDOUT or STDIN. Typically this is used like the normal
3294 piped open when you want to exercise more control over just how the
3295 pipe command gets executed, such as when you are running setuid, and
3296 don't want to have to scan shell commands for metacharacters.
3297 The following triples are more or less equivalent:
3299 open(FOO, "|tr '[a-z]' '[A-Z]'");
3300 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3301 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3302 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3304 open(FOO, "cat -n '$file'|");
3305 open(FOO, '-|', "cat -n '$file'");
3306 open(FOO, '-|') || exec 'cat', '-n', $file;
3307 open(FOO, '-|', "cat", '-n', $file);
3309 The last example in each block shows the pipe as "list form", which is
3310 not yet supported on all platforms. A good rule of thumb is that if
3311 your platform has true C<fork()> (in other words, if your platform is
3312 UNIX) you can use the list form.
3314 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3316 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3317 output before any operation that may do a fork, but this may not be
3318 supported on some platforms (see L<perlport>). To be safe, you may need
3319 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3320 of C<IO::Handle> on any open handles.
3322 On systems that support a close-on-exec flag on files, the flag will
3323 be set for the newly opened file descriptor as determined by the value
3324 of $^F. See L<perlvar/$^F>.
3326 Closing any piped filehandle causes the parent process to wait for the
3327 child to finish, and returns the status value in C<$?> and
3328 C<${^CHILD_ERROR_NATIVE}>.
3330 The filename passed to 2-argument (or 1-argument) form of open() will
3331 have leading and trailing whitespace deleted, and the normal
3332 redirection characters honored. This property, known as "magic open",
3333 can often be used to good effect. A user could specify a filename of
3334 F<"rsh cat file |">, or you could change certain filenames as needed:
3336 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3337 open(FH, $filename) or die "Can't open $filename: $!";
3339 Use 3-argument form to open a file with arbitrary weird characters in it,
3341 open(FOO, '<', $file);
3343 otherwise it's necessary to protect any leading and trailing whitespace:
3345 $file =~ s#^(\s)#./$1#;
3346 open(FOO, "< $file\0");
3348 (this may not work on some bizarre filesystems). One should
3349 conscientiously choose between the I<magic> and 3-arguments form
3354 will allow the user to specify an argument of the form C<"rsh cat file |">,
3355 but will not work on a filename which happens to have a trailing space, while
3357 open IN, '<', $ARGV[0];
3359 will have exactly the opposite restrictions.
3361 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3362 should use the C<sysopen> function, which involves no such magic (but
3363 may use subtly different filemodes than Perl open(), which is mapped
3364 to C fopen()). This is
3365 another way to protect your filenames from interpretation. For example:
3368 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3369 or die "sysopen $path: $!";
3370 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3371 print HANDLE "stuff $$\n";
3373 print "File contains: ", <HANDLE>;
3375 Using the constructor from the C<IO::Handle> package (or one of its
3376 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3377 filehandles that have the scope of whatever variables hold references to
3378 them, and automatically close whenever and however you leave that scope:
3382 sub read_myfile_munged {
3384 my $handle = new IO::File;
3385 open($handle, "myfile") or die "myfile: $!";
3387 or return (); # Automatically closed here.
3388 mung $first or die "mung failed"; # Or here.
3389 return $first, <$handle> if $ALL; # Or here.
3393 See L</seek> for some details about mixing reading and writing.
3395 =item opendir DIRHANDLE,EXPR
3398 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3399 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3400 DIRHANDLE may be an expression whose value can be used as an indirect
3401 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3402 scalar variable (or array or hash element), the variable is assigned a
3403 reference to a new anonymous dirhandle.
3404 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3411 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3412 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3415 For the reverse, see L</chr>.
3416 See L<perlunicode> and L<encoding> for more about Unicode.
3423 =item our EXPR : ATTRS
3425 =item our TYPE EXPR : ATTRS
3427 C<our> associates a simple name with a package variable in the current
3428 package for use within the current scope. When C<use strict 'vars'> is in
3429 effect, C<our> lets you use declared global variables without qualifying
3430 them with package names, within the lexical scope of the C<our> declaration.
3431 In this way C<our> differs from C<use vars>, which is package scoped.
3433 Unlike C<my>, which both allocates storage for a variable and associates
3434 a simple name with that storage for use within the current scope, C<our>
3435 associates a simple name with a package variable in the current package,
3436 for use within the current scope. In other words, C<our> has the same
3437 scoping rules as C<my>, but does not necessarily create a
3440 If more than one value is listed, the list must be placed
3446 An C<our> declaration declares a global variable that will be visible
3447 across its entire lexical scope, even across package boundaries. The
3448 package in which the variable is entered is determined at the point
3449 of the declaration, not at the point of use. This means the following
3453 our $bar; # declares $Foo::bar for rest of lexical scope
3457 print $bar; # prints 20, as it refers to $Foo::bar
3459 Multiple C<our> declarations with the same name in the same lexical
3460 scope are allowed if they are in different packages. If they happen
3461 to be in the same package, Perl will emit warnings if you have asked
3462 for them, just like multiple C<my> declarations. Unlike a second
3463 C<my> declaration, which will bind the name to a fresh variable, a
3464 second C<our> declaration in the same package, in the same scope, is
3469 our $bar; # declares $Foo::bar for rest of lexical scope
3473 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3474 print $bar; # prints 30
3476 our $bar; # emits warning but has no other effect
3477 print $bar; # still prints 30
3479 An C<our> declaration may also have a list of attributes associated
3482 The exact semantics and interface of TYPE and ATTRS are still
3483 evolving. TYPE is currently bound to the use of C<fields> pragma,
3484 and attributes are handled using the C<attributes> pragma, or starting
3485 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3486 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3487 L<attributes>, and L<Attribute::Handlers>.
3489 The only currently recognized C<our()> attribute is C<unique> which
3490 indicates that a single copy of the global is to be used by all
3491 interpreters should the program happen to be running in a
3492 multi-interpreter environment. (The default behaviour would be for
3493 each interpreter to have its own copy of the global.) Examples:
3495 our @EXPORT : unique = qw(foo);
3496 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3497 our $VERSION : unique = "1.00";
3499 Note that this attribute also has the effect of making the global
3500 readonly when the first new interpreter is cloned (for example,
3501 when the first new thread is created).
3503 Multi-interpreter environments can come to being either through the
3504 fork() emulation on Windows platforms, or by embedding perl in a
3505 multi-threaded application. The C<unique> attribute does nothing in
3506 all other environments.
3508 Warning: the current implementation of this attribute operates on the
3509 typeglob associated with the variable; this means that C<our $x : unique>
3510 also has the effect of C<our @x : unique; our %x : unique>. This may be
3513 =item pack TEMPLATE,LIST
3516 Takes a LIST of values and converts it into a string using the rules
3517 given by the TEMPLATE. The resulting string is the concatenation of
3518 the converted values. Typically, each converted value looks
3519 like its machine-level representation. For example, on 32-bit machines
3520 an integer may be represented by a sequence of 4 bytes that will be
3521 converted to a sequence of 4 characters.
3523 The TEMPLATE is a sequence of characters that give the order and type
3524 of values, as follows:
3526 a A string with arbitrary binary data, will be null padded.
3527 A A text (ASCII) string, will be space padded.
3528 Z A null terminated (ASCIZ) string, will be null padded.
3530 b A bit string (ascending bit order inside each byte, like vec()).
3531 B A bit string (descending bit order inside each byte).
3532 h A hex string (low nybble first).
3533 H A hex string (high nybble first).
3535 c A signed char (8-bit) value.
3536 C An unsigned C char (octet) even under Unicode. Should normally not
3537 be used. See U and W instead.
3538 W An unsigned char value (can be greater than 255).
3540 s A signed short (16-bit) value.
3541 S An unsigned short value.
3543 l A signed long (32-bit) value.
3544 L An unsigned long value.
3546 q A signed quad (64-bit) value.
3547 Q An unsigned quad value.
3548 (Quads are available only if your system supports 64-bit
3549 integer values _and_ if Perl has been compiled to support those.
3550 Causes a fatal error otherwise.)
3552 i A signed integer value.
3553 I A unsigned integer value.
3554 (This 'integer' is _at_least_ 32 bits wide. Its exact
3555 size depends on what a local C compiler calls 'int'.)
3557 n An unsigned short (16-bit) in "network" (big-endian) order.
3558 N An unsigned long (32-bit) in "network" (big-endian) order.
3559 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3560 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3562 j A Perl internal signed integer value (IV).
3563 J A Perl internal unsigned integer value (UV).
3565 f A single-precision float in the native format.
3566 d A double-precision float in the native format.
3568 F A Perl internal floating point value (NV) in the native format
3569 D A long double-precision float in the native format.
3570 (Long doubles are available only if your system supports long
3571 double values _and_ if Perl has been compiled to support those.
3572 Causes a fatal error otherwise.)
3574 p A pointer to a null-terminated string.
3575 P A pointer to a structure (fixed-length string).
3577 u A uuencoded string.
3578 U A Unicode character number. Encodes to UTF-8 internally
3579 (or UTF-EBCDIC in EBCDIC platforms).
3581 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3582 details). Its bytes represent an unsigned integer in base 128,
3583 most significant digit first, with as few digits as possible. Bit
3584 eight (the high bit) is set on each byte except the last.
3588 @ Null fill or truncate to absolute position, counted from the
3589 start of the innermost ()-group.
3590 . Null fill or truncate to absolute position specified by value.
3591 ( Start of a ()-group.
3593 One or more of the modifiers below may optionally follow some letters in the
3594 TEMPLATE (the second column lists the letters for which the modifier is
3597 ! sSlLiI Forces native (short, long, int) sizes instead
3598 of fixed (16-/32-bit) sizes.
3600 xX Make x and X act as alignment commands.
3602 nNvV Treat integers as signed instead of unsigned.
3604 @. Specify position as byte offset in the internal
3605 representation of the packed string. Efficient but
3608 > sSiIlLqQ Force big-endian byte-order on the type.
3609 jJfFdDpP (The "big end" touches the construct.)
3611 < sSiIlLqQ Force little-endian byte-order on the type.
3612 jJfFdDpP (The "little end" touches the construct.)
3614 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3615 in which case they force a certain byte-order on all components of
3616 that group, including subgroups.
3618 The following rules apply:
3624 Each letter may optionally be followed by a number giving a repeat
3625 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3626 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3627 that many values from the LIST. A C<*> for the repeat count means to
3628 use however many items are left, except for C<@>, C<x>, C<X>, where it
3629 is equivalent to C<0>, for <.> where it means relative to string start
3630 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3631 A numeric repeat count may optionally be enclosed in brackets, as in
3632 C<pack 'C[80]', @arr>.
3634 One can replace the numeric repeat count by a template enclosed in brackets;
3635 then the packed length of this template in bytes is used as a count.
3636 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3637 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3638 If the template in brackets contains alignment commands (such as C<x![d]>),
3639 its packed length is calculated as if the start of the template has the maximal
3642 When used with C<Z>, C<*> results in the addition of a trailing null
3643 byte (so the packed result will be one longer than the byte C<length>
3646 When used with C<@>, the repeat count represents an offset from the start
3647 of the innermost () group.
3649 When used with C<.>, the repeat count is used to determine the starting
3650 position from where the value offset is calculated. If the repeat count
3651 is 0, it's relative to the current position. If the repeat count is C<*>,
3652 the offset is relative to the start of the packed string. And if its an
3653 integer C<n> the offset is relative to the start of the n-th innermost
3654 () group (or the start of the string if C<n> is bigger then the group
3657 The repeat count for C<u> is interpreted as the maximal number of bytes
3658 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3659 count should not be more than 65.
3663 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3664 string of length count, padding with nulls or spaces as necessary. When
3665 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3666 after the first null, and C<a> returns data verbatim.
3668 If the value-to-pack is too long, it is truncated. If too long and an
3669 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3670 by a null byte. Thus C<Z> always packs a trailing null (except when the
3675 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3676 Each character of the input field of pack() generates 1 bit of the result.
3677 Each result bit is based on the least-significant bit of the corresponding
3678 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3679 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3681 Starting from the beginning of the input string of pack(), each 8-tuple
3682 of characters is converted to 1 character of output. With format C<b>
3683 the first character of the 8-tuple determines the least-significant bit of a
3684 character, and with format C<B> it determines the most-significant bit of
3687 If the length of the input string is not exactly divisible by 8, the
3688 remainder is packed as if the input string were padded by null characters
3689 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3691 If the input string of pack() is longer than needed, extra characters are
3692 ignored. A C<*> for the repeat count of pack() means to use all the
3693 characters of the input field. On unpack()ing the bits are converted to a
3694 string of C<"0">s and C<"1">s.
3698 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3699 representable as hexadecimal digits, 0-9a-f) long.
3701 Each character of the input field of pack() generates 4 bits of the result.
3702 For non-alphabetical characters the result is based on the 4 least-significant
3703 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3704 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3705 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3706 is compatible with the usual hexadecimal digits, so that C<"a"> and
3707 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3708 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3710 Starting from the beginning of the input string of pack(), each pair
3711 of characters is converted to 1 character of output. With format C<h> the
3712 first character of the pair determines the least-significant nybble of the
3713 output character, and with format C<H> it determines the most-significant
3716 If the length of the input string is not even, it behaves as if padded
3717 by a null character at the end. Similarly, during unpack()ing the "extra"
3718 nybbles are ignored.
3720 If the input string of pack() is longer than needed, extra characters are
3722 A C<*> for the repeat count of pack() means to use all the characters of
3723 the input field. On unpack()ing the nybbles are converted to a string
3724 of hexadecimal digits.
3728 The C<p> type packs a pointer to a null-terminated string. You are
3729 responsible for ensuring the string is not a temporary value (which can
3730 potentially get deallocated before you get around to using the packed result).
3731 The C<P> type packs a pointer to a structure of the size indicated by the
3732 length. A NULL pointer is created if the corresponding value for C<p> or
3733 C<P> is C<undef>, similarly for unpack().
3735 If your system has a strange pointer size (i.e. a pointer is neither as
3736 big as an int nor as big as a long), it may not be possible to pack or
3737 unpack pointers in big- or little-endian byte order. Attempting to do
3738 so will result in a fatal error.
3742 The C</> template character allows packing and unpacking of a sequence of
3743 items where the packed structure contains a packed item count followed by
3744 the packed items themselves.
3745 You write I<length-item>C</>I<sequence-item>.
3747 The I<length-item> can be any C<pack> template letter, and describes
3748 how the length value is packed. The ones likely to be of most use are
3749 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3750 SNMP) and C<N> (for Sun XDR).
3752 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3753 the minimum of that and the number of available items is used as argument
3754 for the I<length-item>. If it has no repeat count or uses a '*', the number
3755 of available items is used. For C<unpack> the repeat count is always obtained
3756 by decoding the packed item count, and the I<sequence-item> must not have a
3759 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3760 the I<length-item> is a string length, not a number of strings. If there is
3761 an explicit repeat count for pack, the packed string will be adjusted to that
3764 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3765 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3766 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3767 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3769 The I<length-item> is not returned explicitly from C<unpack>.
3771 Adding a count to the I<length-item> letter is unlikely to do anything
3772 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3773 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3774 which Perl does not regard as legal in numeric strings.
3778 The integer types C<s>, C<S>, C<l>, and C<L> may be
3779 followed by a C<!> modifier to signify native shorts or
3780 longs--as you can see from above for example a bare C<l> does mean
3781 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3782 may be larger. This is an issue mainly in 64-bit platforms. You can
3783 see whether using C<!> makes any difference by
3785 print length(pack("s")), " ", length(pack("s!")), "\n";
3786 print length(pack("l")), " ", length(pack("l!")), "\n";
3788 C<i!> and C<I!> also work but only because of completeness;
3789 they are identical to C<i> and C<I>.
3791 The actual sizes (in bytes) of native shorts, ints, longs, and long
3792 longs on the platform where Perl was built are also available via
3796 print $Config{shortsize}, "\n";
3797 print $Config{intsize}, "\n";
3798 print $Config{longsize}, "\n";
3799 print $Config{longlongsize}, "\n";
3801 (The C<$Config{longlongsize}> will be undefined if your system does
3802 not support long longs.)
3806 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3807 are inherently non-portable between processors and operating systems
3808 because they obey the native byteorder and endianness. For example a
3809 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3810 (arranged in and handled by the CPU registers) into bytes as
3812 0x12 0x34 0x56 0x78 # big-endian
3813 0x78 0x56 0x34 0x12 # little-endian
3815 Basically, the Intel and VAX CPUs are little-endian, while everybody
3816 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3817 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3818 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3821 The names `big-endian' and `little-endian' are comic references to
3822 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3823 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3824 the egg-eating habits of the Lilliputians.
3826 Some systems may have even weirder byte orders such as
3831 You can see your system's preference with
3833 print join(" ", map { sprintf "%#02x", $_ }
3834 unpack("W*",pack("L",0x12345678))), "\n";
3836 The byteorder on the platform where Perl was built is also available
3840 print $Config{byteorder}, "\n";
3842 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3843 and C<'87654321'> are big-endian.
3845 If you want portable packed integers you can either use the formats
3846 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3847 modifiers. These modifiers are only available as of perl 5.9.2.
3848 See also L<perlport>.
3852 All integer and floating point formats as well as C<p> and C<P> and
3853 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3854 to force big- or little- endian byte-order, respectively.
3855 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3856 signed integers, 64-bit integers and floating point values. However,
3857 there are some things to keep in mind.
3859 Exchanging signed integers between different platforms only works
3860 if all platforms store them in the same format. Most platforms store
3861 signed integers in two's complement, so usually this is not an issue.
3863 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3864 formats on big- or little-endian machines. Otherwise, attempting to
3865 do so will result in a fatal error.
3867 Forcing big- or little-endian byte-order on floating point values for
3868 data exchange can only work if all platforms are using the same
3869 binary representation (e.g. IEEE floating point format). Even if all
3870 platforms are using IEEE, there may be subtle differences. Being able
3871 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3872 but also very dangerous if you don't know exactly what you're doing.
3873 It is definitely not a general way to portably store floating point
3876 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3877 all types inside the group that accept the byte-order modifiers,
3878 including all subgroups. It will silently be ignored for all other
3879 types. You are not allowed to override the byte-order within a group
3880 that already has a byte-order modifier suffix.
3884 Real numbers (floats and doubles) are in the native machine format only;
3885 due to the multiplicity of floating formats around, and the lack of a
3886 standard "network" representation, no facility for interchange has been
3887 made. This means that packed floating point data written on one machine
3888 may not be readable on another - even if both use IEEE floating point
3889 arithmetic (as the endian-ness of the memory representation is not part
3890 of the IEEE spec). See also L<perlport>.
3892 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3893 modifiers to force big- or little-endian byte-order on floating point values.
3895 Note that Perl uses doubles (or long doubles, if configured) internally for
3896 all numeric calculation, and converting from double into float and thence back
3897 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3898 will not in general equal $foo).
3902 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3903 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3904 where the packed string is processed in its UTF-8-encoded Unicode form on
3905 a byte by byte basis. Character mode is the default unless the format string
3906 starts with an C<U>. You can switch mode at any moment with an explicit
3907 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3908 or until the end of the ()-group in which it was entered.
3912 You must yourself do any alignment or padding by inserting for example
3913 enough C<'x'>es while packing. There is no way to pack() and unpack()
3914 could know where the characters are going to or coming from. Therefore
3915 C<pack> (and C<unpack>) handle their output and input as flat
3916 sequences of characters.
3920 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3921 take a repeat count, both as postfix, and for unpack() also via the C</>
3922 template character. Within each repetition of a group, positioning with
3923 C<@> starts again at 0. Therefore, the result of
3925 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3927 is the string "\0a\0\0bc".
3931 C<x> and C<X> accept C<!> modifier. In this case they act as
3932 alignment commands: they jump forward/back to the closest position
3933 aligned at a multiple of C<count> characters. For example, to pack() or
3934 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3935 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3936 aligned on the double's size.
3938 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3939 both result in no-ops.
3943 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3944 will represent signed 16-/32-bit integers in big-/little-endian order.
3945 This is only portable if all platforms sharing the packed data use the
3946 same binary representation for signed integers (e.g. all platforms are
3947 using two's complement representation).
3951 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3952 White space may be used to separate pack codes from each other, but
3953 modifiers and a repeat count must follow immediately.
3957 If TEMPLATE requires more arguments to pack() than actually given, pack()
3958 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3959 to pack() than actually given, extra arguments are ignored.
3965 $foo = pack("WWWW",65,66,67,68);
3967 $foo = pack("W4",65,66,67,68);
3969 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3970 # same thing with Unicode circled letters.
3971 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3972 # same thing with Unicode circled letters. You don't get the UTF-8
3973 # bytes because the U at the start of the format caused a switch to
3974 # U0-mode, so the UTF-8 bytes get joined into characters
3975 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3976 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3977 # This is the UTF-8 encoding of the string in the previous example
3979 $foo = pack("ccxxcc",65,66,67,68);
3982 # note: the above examples featuring "W" and "c" are true
3983 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3984 # and UTF-8. In EBCDIC the first example would be
3985 # $foo = pack("WWWW",193,194,195,196);
3987 $foo = pack("s2",1,2);
3988 # "\1\0\2\0" on little-endian
3989 # "\0\1\0\2" on big-endian
3991 $foo = pack("a4","abcd","x","y","z");
3994 $foo = pack("aaaa","abcd","x","y","z");
3997 $foo = pack("a14","abcdefg");
3998 # "abcdefg\0\0\0\0\0\0\0"
4000 $foo = pack("i9pl", gmtime);
4001 # a real struct tm (on my system anyway)
4003 $utmp_template = "Z8 Z8 Z16 L";
4004 $utmp = pack($utmp_template, @utmp1);
4005 # a struct utmp (BSDish)
4007 @utmp2 = unpack($utmp_template, $utmp);
4008 # "@utmp1" eq "@utmp2"
4011 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4014 $foo = pack('sx2l', 12, 34);
4015 # short 12, two zero bytes padding, long 34
4016 $bar = pack('s@4l', 12, 34);
4017 # short 12, zero fill to position 4, long 34
4019 $baz = pack('s.l', 12, 4, 34);
4020 # short 12, zero fill to position 4, long 34
4022 $foo = pack('nN', 42, 4711);
4023 # pack big-endian 16- and 32-bit unsigned integers
4024 $foo = pack('S>L>', 42, 4711);
4026 $foo = pack('s<l<', -42, 4711);
4027 # pack little-endian 16- and 32-bit signed integers
4028 $foo = pack('(sl)<', -42, 4711);
4031 The same template may generally also be used in unpack().
4033 =item package NAMESPACE
4034 X<package> X<module> X<namespace>
4038 Declares the compilation unit as being in the given namespace. The scope
4039 of the package declaration is from the declaration itself through the end
4040 of the enclosing block, file, or eval (the same as the C<my> operator).
4041 All further unqualified dynamic identifiers will be in this namespace.
4042 A package statement affects only dynamic variables--including those
4043 you've used C<local> on--but I<not> lexical variables, which are created
4044 with C<my>. Typically it would be the first declaration in a file to
4045 be included by the C<require> or C<use> operator. You can switch into a
4046 package in more than one place; it merely influences which symbol table
4047 is used by the compiler for the rest of that block. You can refer to
4048 variables and filehandles in other packages by prefixing the identifier
4049 with the package name and a double colon: C<$Package::Variable>.
4050 If the package name is null, the C<main> package as assumed. That is,
4051 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4052 still seen in older code).
4054 If NAMESPACE is omitted, then there is no current package, and all
4055 identifiers must be fully qualified or lexicals. However, you are
4056 strongly advised not to make use of this feature. Its use can cause
4057 unexpected behaviour, even crashing some versions of Perl. It is
4058 deprecated, and will be removed from a future release.
4060 See L<perlmod/"Packages"> for more information about packages, modules,
4061 and classes. See L<perlsub> for other scoping issues.
4063 =item pipe READHANDLE,WRITEHANDLE
4066 Opens a pair of connected pipes like the corresponding system call.
4067 Note that if you set up a loop of piped processes, deadlock can occur
4068 unless you are very careful. In addition, note that Perl's pipes use
4069 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4070 after each command, depending on the application.
4072 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4073 for examples of such things.
4075 On systems that support a close-on-exec flag on files, the flag will be set
4076 for the newly opened file descriptors as determined by the value of $^F.
4084 Pops and returns the last value of the array, shortening the array by
4085 one element. Has an effect similar to
4089 If there are no elements in the array, returns the undefined value
4090 (although this may happen at other times as well). If ARRAY is
4091 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4092 array in subroutines, just like C<shift>.
4095 X<pos> X<match, position>
4099 Returns the offset of where the last C<m//g> search left off for the variable
4100 in question (C<$_> is used when the variable is not specified). Note that
4101 0 is a valid match offset. C<undef> indicates that the search position
4102 is reset (usually due to match failure, but can also be because no match has
4103 yet been performed on the scalar). C<pos> directly accesses the location used
4104 by the regexp engine to store the offset, so assigning to C<pos> will change
4105 that offset, and so will also influence the C<\G> zero-width assertion in
4106 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4107 the return from C<pos> won't change either in this case. See L<perlre> and
4110 =item print FILEHANDLE LIST
4117 Prints a string or a list of strings. Returns true if successful.
4118 FILEHANDLE may be a scalar variable name, in which case the variable
4119 contains the name of or a reference to the filehandle, thus introducing
4120 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4121 the next token is a term, it may be misinterpreted as an operator
4122 unless you interpose a C<+> or put parentheses around the arguments.)
4123 If FILEHANDLE is omitted, prints by default to standard output (or
4124 to the last selected output channel--see L</select>). If LIST is
4125 also omitted, prints C<$_> to the currently selected output channel.
4126 To set the default output channel to something other than STDOUT
4127 use the select operation. The current value of C<$,> (if any) is
4128 printed between each LIST item. The current value of C<$\> (if
4129 any) is printed after the entire LIST has been printed. Because
4130 print takes a LIST, anything in the LIST is evaluated in list
4131 context, and any subroutine that you call will have one or more of
4132 its expressions evaluated in list context. Also be careful not to
4133 follow the print keyword with a left parenthesis unless you want
4134 the corresponding right parenthesis to terminate the arguments to
4135 the print--interpose a C<+> or put parentheses around all the
4138 Note that if you're storing FILEHANDLEs in an array, or if you're using
4139 any other expression more complex than a scalar variable to retrieve it,
4140 you will have to use a block returning the filehandle value instead:
4142 print { $files[$i] } "stuff\n";
4143 print { $OK ? STDOUT : STDERR } "stuff\n";
4145 =item printf FILEHANDLE FORMAT, LIST
4148 =item printf FORMAT, LIST
4150 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4151 (the output record separator) is not appended. The first argument
4152 of the list will be interpreted as the C<printf> format. See C<sprintf>
4153 for an explanation of the format argument. If C<use locale> is in effect,
4154 the character used for the decimal point in formatted real numbers is
4155 affected by the LC_NUMERIC locale. See L<perllocale>.
4157 Don't fall into the trap of using a C<printf> when a simple
4158 C<print> would do. The C<print> is more efficient and less
4161 =item prototype FUNCTION
4164 Returns the prototype of a function as a string (or C<undef> if the
4165 function has no prototype). FUNCTION is a reference to, or the name of,
4166 the function whose prototype you want to retrieve.
4168 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4169 name for Perl builtin. If the builtin is not I<overridable> (such as
4170 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4171 C<system>) returns C<undef> because the builtin does not really behave
4172 like a Perl function. Otherwise, the string describing the equivalent
4173 prototype is returned.
4175 =item push ARRAY,LIST
4178 Treats ARRAY as a stack, and pushes the values of LIST
4179 onto the end of ARRAY. The length of ARRAY increases by the length of
4180 LIST. Has the same effect as
4183 $ARRAY[++$#ARRAY] = $value;
4186 but is more efficient. Returns the number of elements in the array following
4187 the completed C<push>.
4199 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4201 =item quotemeta EXPR
4202 X<quotemeta> X<metacharacter>
4206 Returns the value of EXPR with all non-"word"
4207 characters backslashed. (That is, all characters not matching
4208 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4209 returned string, regardless of any locale settings.)
4210 This is the internal function implementing
4211 the C<\Q> escape in double-quoted strings.
4213 If EXPR is omitted, uses C<$_>.
4220 Returns a random fractional number greater than or equal to C<0> and less
4221 than the value of EXPR. (EXPR should be positive.) If EXPR is
4222 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4223 also special-cased as C<1> - this has not been documented before perl 5.8.0
4224 and is subject to change in future versions of perl. Automatically calls
4225 C<srand> unless C<srand> has already been called. See also C<srand>.
4227 Apply C<int()> to the value returned by C<rand()> if you want random
4228 integers instead of random fractional numbers. For example,
4232 returns a random integer between C<0> and C<9>, inclusive.
4234 (Note: If your rand function consistently returns numbers that are too
4235 large or too small, then your version of Perl was probably compiled
4236 with the wrong number of RANDBITS.)
4238 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4241 =item read FILEHANDLE,SCALAR,LENGTH
4243 Attempts to read LENGTH I<characters> of data into variable SCALAR
4244 from the specified FILEHANDLE. Returns the number of characters
4245 actually read, C<0> at end of file, or undef if there was an error (in
4246 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4247 so that the last character actually read is the last character of the
4248 scalar after the read.
4250 An OFFSET may be specified to place the read data at some place in the
4251 string other than the beginning. A negative OFFSET specifies
4252 placement at that many characters counting backwards from the end of
4253 the string. A positive OFFSET greater than the length of SCALAR
4254 results in the string being padded to the required size with C<"\0">
4255 bytes before the result of the read is appended.
4257 The call is actually implemented in terms of either Perl's or system's
4258 fread() call. To get a true read(2) system call, see C<sysread>.
4260 Note the I<characters>: depending on the status of the filehandle,
4261 either (8-bit) bytes or characters are read. By default all
4262 filehandles operate on bytes, but for example if the filehandle has
4263 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4264 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4265 characters, not bytes. Similarly for the C<:encoding> pragma:
4266 in that case pretty much any characters can be read.
4268 =item readdir DIRHANDLE
4271 Returns the next directory entry for a directory opened by C<opendir>.
4272 If used in list context, returns all the rest of the entries in the
4273 directory. If there are no more entries, returns an undefined value in
4274 scalar context or a null list in list context.
4276 If you're planning to filetest the return values out of a C<readdir>, you'd
4277 better prepend the directory in question. Otherwise, because we didn't
4278 C<chdir> there, it would have been testing the wrong file.
4280 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4281 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4285 X<readline> X<gets> X<fgets>
4287 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4288 context, each call reads and returns the next line, until end-of-file is
4289 reached, whereupon the subsequent call returns undef. In list context,
4290 reads until end-of-file is reached and returns a list of lines. Note that
4291 the notion of "line" used here is however you may have defined it
4292 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4294 When C<$/> is set to C<undef>, when readline() is in scalar
4295 context (i.e. file slurp mode), and when an empty file is read, it
4296 returns C<''> the first time, followed by C<undef> subsequently.
4298 This is the internal function implementing the C<< <EXPR> >>
4299 operator, but you can use it directly. The C<< <EXPR> >>
4300 operator is discussed in more detail in L<perlop/"I/O Operators">.
4303 $line = readline(*STDIN); # same thing
4305 If readline encounters an operating system error, C<$!> will be set with the
4306 corresponding error message. It can be helpful to check C<$!> when you are
4307 reading from filehandles you don't trust, such as a tty or a socket. The
4308 following example uses the operator form of C<readline>, and takes the necessary
4309 steps to ensure that C<readline> was successful.
4313 unless (defined( $line = <> )) {
4325 Returns the value of a symbolic link, if symbolic links are
4326 implemented. If not, gives a fatal error. If there is some system
4327 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4328 omitted, uses C<$_>.
4333 EXPR is executed as a system command.
4334 The collected standard output of the command is returned.
4335 In scalar context, it comes back as a single (potentially
4336 multi-line) string. In list context, returns a list of lines
4337 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4338 This is the internal function implementing the C<qx/EXPR/>
4339 operator, but you can use it directly. The C<qx/EXPR/>
4340 operator is discussed in more detail in L<perlop/"I/O Operators">.
4342 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4345 Receives a message on a socket. Attempts to receive LENGTH characters
4346 of data into variable SCALAR from the specified SOCKET filehandle.
4347 SCALAR will be grown or shrunk to the length actually read. Takes the
4348 same flags as the system call of the same name. Returns the address
4349 of the sender if SOCKET's protocol supports this; returns an empty
4350 string otherwise. If there's an error, returns the undefined value.
4351 This call is actually implemented in terms of recvfrom(2) system call.
4352 See L<perlipc/"UDP: Message Passing"> for examples.
4354 Note the I<characters>: depending on the status of the socket, either
4355 (8-bit) bytes or characters are received. By default all sockets
4356 operate on bytes, but for example if the socket has been changed using
4357 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4358 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4359 characters, not bytes. Similarly for the C<:encoding> pragma:
4360 in that case pretty much any characters can be read.
4367 The C<redo> command restarts the loop block without evaluating the
4368 conditional again. The C<continue> block, if any, is not executed. If
4369 the LABEL is omitted, the command refers to the innermost enclosing
4370 loop. Programs that want to lie to themselves about what was just input
4371 normally use this command:
4373 # a simpleminded Pascal comment stripper
4374 # (warning: assumes no { or } in strings)
4375 LINE: while (<STDIN>) {
4376 while (s|({.*}.*){.*}|$1 |) {}
4381 if (/}/) { # end of comment?
4390 C<redo> cannot be used to retry a block which returns a value such as
4391 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4392 a grep() or map() operation.
4394 Note that a block by itself is semantically identical to a loop
4395 that executes once. Thus C<redo> inside such a block will effectively
4396 turn it into a looping construct.
4398 See also L</continue> for an illustration of how C<last>, C<next>, and
4406 Returns a non-empty string if EXPR is a reference, the empty
4407 string otherwise. If EXPR
4408 is not specified, C<$_> will be used. The value returned depends on the
4409 type of thing the reference is a reference to.
4410 Builtin types include:
4420 If the referenced object has been blessed into a package, then that package
4421 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4423 if (ref($r) eq "HASH") {
4424 print "r is a reference to a hash.\n";
4427 print "r is not a reference at all.\n";
4430 See also L<perlref>.
4432 =item rename OLDNAME,NEWNAME
4433 X<rename> X<move> X<mv> X<ren>
4435 Changes the name of a file; an existing file NEWNAME will be
4436 clobbered. Returns true for success, false otherwise.
4438 Behavior of this function varies wildly depending on your system
4439 implementation. For example, it will usually not work across file system
4440 boundaries, even though the system I<mv> command sometimes compensates
4441 for this. Other restrictions include whether it works on directories,
4442 open files, or pre-existing files. Check L<perlport> and either the
4443 rename(2) manpage or equivalent system documentation for details.
4445 =item require VERSION
4452 Demands a version of Perl specified by VERSION, or demands some semantics
4453 specified by EXPR or by C<$_> if EXPR is not supplied.
4455 VERSION may be either a numeric argument such as 5.006, which will be
4456 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4457 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4458 VERSION is greater than the version of the current Perl interpreter.
4459 Compare with L</use>, which can do a similar check at compile time.
4461 Specifying VERSION as a literal of the form v5.6.1 should generally be
4462 avoided, because it leads to misleading error messages under earlier
4463 versions of Perl that do not support this syntax. The equivalent numeric
4464 version should be used instead.
4466 require v5.6.1; # run time version check
4467 require 5.6.1; # ditto
4468 require 5.006_001; # ditto; preferred for backwards compatibility
4470 Otherwise, C<require> demands that a library file be included if it
4471 hasn't already been included. The file is included via the do-FILE
4472 mechanism, which is essentially just a variety of C<eval>. Has
4473 semantics similar to the following subroutine:
4476 my ($filename) = @_;
4477 if (exists $INC{$filename}) {
4478 return 1 if $INC{$filename};
4479 die "Compilation failed in require";
4481 my ($realfilename,$result);
4483 foreach $prefix (@INC) {
4484 $realfilename = "$prefix/$filename";
4485 if (-f $realfilename) {
4486 $INC{$filename} = $realfilename;
4487 $result = do $realfilename;
4491 die "Can't find $filename in \@INC";
4494 $INC{$filename} = undef;
4496 } elsif (!$result) {
4497 delete $INC{$filename};
4498 die "$filename did not return true value";
4504 Note that the file will not be included twice under the same specified
4507 The file must return true as the last statement to indicate
4508 successful execution of any initialization code, so it's customary to
4509 end such a file with C<1;> unless you're sure it'll return true
4510 otherwise. But it's better just to put the C<1;>, in case you add more
4513 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4514 replaces "F<::>" with "F</>" in the filename for you,
4515 to make it easy to load standard modules. This form of loading of
4516 modules does not risk altering your namespace.
4518 In other words, if you try this:
4520 require Foo::Bar; # a splendid bareword
4522 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4523 directories specified in the C<@INC> array.
4525 But if you try this:
4527 $class = 'Foo::Bar';
4528 require $class; # $class is not a bareword
4530 require "Foo::Bar"; # not a bareword because of the ""
4532 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4533 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4535 eval "require $class";
4537 Now that you understand how C<require> looks for files in the case of
4538 a bareword argument, there is a little extra functionality going on
4539 behind the scenes. Before C<require> looks for a "F<.pm>" extension,
4540 it will first look for a filename with a "F<.pmc>" extension. A file
4541 with this extension is assumed to be Perl bytecode generated by
4542 L<B::Bytecode|B::Bytecode>. If this file is found, and its modification
4543 time is newer than a coinciding "F<.pm>" non-compiled file, it will be
4544 loaded in place of that non-compiled file ending in a "F<.pm>" extension.
4546 You can also insert hooks into the import facility, by putting directly
4547 Perl code into the @INC array. There are three forms of hooks: subroutine
4548 references, array references and blessed objects.
4550 Subroutine references are the simplest case. When the inclusion system
4551 walks through @INC and encounters a subroutine, this subroutine gets
4552 called with two parameters, the first being a reference to itself, and the
4553 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4554 subroutine should return C<undef> or a filehandle, from which the file to
4555 include will be read. If C<undef> is returned, C<require> will look at
4556 the remaining elements of @INC.
4558 If the hook is an array reference, its first element must be a subroutine
4559 reference. This subroutine is called as above, but the first parameter is
4560 the array reference. This enables to pass indirectly some arguments to
4563 In other words, you can write:
4565 push @INC, \&my_sub;
4567 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4573 push @INC, [ \&my_sub, $x, $y, ... ];
4575 my ($arrayref, $filename) = @_;
4576 # Retrieve $x, $y, ...
4577 my @parameters = @$arrayref[1..$#$arrayref];
4581 If the hook is an object, it must provide an INC method that will be
4582 called as above, the first parameter being the object itself. (Note that
4583 you must fully qualify the sub's name, as it is always forced into package
4584 C<main>.) Here is a typical code layout:
4590 my ($self, $filename) = @_;
4594 # In the main program
4595 push @INC, new Foo(...);
4597 Note that these hooks are also permitted to set the %INC entry
4598 corresponding to the files they have loaded. See L<perlvar/%INC>.
4600 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4607 Generally used in a C<continue> block at the end of a loop to clear
4608 variables and reset C<??> searches so that they work again. The
4609 expression is interpreted as a list of single characters (hyphens
4610 allowed for ranges). All variables and arrays beginning with one of
4611 those letters are reset to their pristine state. If the expression is
4612 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4613 only variables or searches in the current package. Always returns
4616 reset 'X'; # reset all X variables
4617 reset 'a-z'; # reset lower case variables
4618 reset; # just reset ?one-time? searches
4620 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4621 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4622 variables--lexical variables are unaffected, but they clean themselves
4623 up on scope exit anyway, so you'll probably want to use them instead.
4631 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4632 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4633 context, depending on how the return value will be used, and the context
4634 may vary from one execution to the next (see C<wantarray>). If no EXPR
4635 is given, returns an empty list in list context, the undefined value in
4636 scalar context, and (of course) nothing at all in a void context.
4638 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4639 or do FILE will automatically return the value of the last expression
4643 X<reverse> X<rev> X<invert>
4645 In list context, returns a list value consisting of the elements
4646 of LIST in the opposite order. In scalar context, concatenates the
4647 elements of LIST and returns a string value with all characters
4648 in the opposite order.
4650 print reverse <>; # line tac, last line first
4652 undef $/; # for efficiency of <>
4653 print scalar reverse <>; # character tac, last line tsrif
4655 Used without arguments in scalar context, reverse() reverses C<$_>.
4657 This operator is also handy for inverting a hash, although there are some
4658 caveats. If a value is duplicated in the original hash, only one of those
4659 can be represented as a key in the inverted hash. Also, this has to
4660 unwind one hash and build a whole new one, which may take some time
4661 on a large hash, such as from a DBM file.
4663 %by_name = reverse %by_address; # Invert the hash
4665 =item rewinddir DIRHANDLE
4668 Sets the current position to the beginning of the directory for the
4669 C<readdir> routine on DIRHANDLE.
4671 =item rindex STR,SUBSTR,POSITION
4674 =item rindex STR,SUBSTR
4676 Works just like index() except that it returns the position of the I<last>
4677 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4678 last occurrence beginning at or before that position.
4680 =item rmdir FILENAME
4681 X<rmdir> X<rd> X<directory, remove>
4685 Deletes the directory specified by FILENAME if that directory is
4686 empty. If it succeeds it returns true, otherwise it returns false and
4687 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4691 The substitution operator. See L<perlop>.
4693 =item say FILEHANDLE LIST
4700 Just like C<print>, but implicitly appends a newline.
4701 C<say LIST> is simply an abbreviation for C<print LIST, "\n">,
4702 and C<say()> works just like C<print($_, "\n")>.
4704 That means that a call to say() appends any output record separator
4705 I<after> the added newline.
4707 This keyword is only available when the "say" feature is
4708 enabled: see L<feature>.
4711 X<scalar> X<context>
4713 Forces EXPR to be interpreted in scalar context and returns the value
4716 @counts = ( scalar @a, scalar @b, scalar @c );
4718 There is no equivalent operator to force an expression to
4719 be interpolated in list context because in practice, this is never
4720 needed. If you really wanted to do so, however, you could use
4721 the construction C<@{[ (some expression) ]}>, but usually a simple
4722 C<(some expression)> suffices.
4724 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4725 parenthesized list, this behaves as a scalar comma expression, evaluating
4726 all but the last element in void context and returning the final element
4727 evaluated in scalar context. This is seldom what you want.
4729 The following single statement:
4731 print uc(scalar(&foo,$bar)),$baz;
4733 is the moral equivalent of these two:
4736 print(uc($bar),$baz);
4738 See L<perlop> for more details on unary operators and the comma operator.
4740 =item seek FILEHANDLE,POSITION,WHENCE
4741 X<seek> X<fseek> X<filehandle, position>
4743 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4744 FILEHANDLE may be an expression whose value gives the name of the
4745 filehandle. The values for WHENCE are C<0> to set the new position
4746 I<in bytes> to POSITION, C<1> to set it to the current position plus
4747 POSITION, and C<2> to set it to EOF plus POSITION (typically
4748 negative). For WHENCE you may use the constants C<SEEK_SET>,
4749 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4750 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4753 Note the I<in bytes>: even if the filehandle has been set to
4754 operate on characters (for example by using the C<:utf8> open
4755 layer), tell() will return byte offsets, not character offsets
4756 (because implementing that would render seek() and tell() rather slow).
4758 If you want to position file for C<sysread> or C<syswrite>, don't use
4759 C<seek>--buffering makes its effect on the file's system position
4760 unpredictable and non-portable. Use C<sysseek> instead.
4762 Due to the rules and rigors of ANSI C, on some systems you have to do a
4763 seek whenever you switch between reading and writing. Amongst other
4764 things, this may have the effect of calling stdio's clearerr(3).
4765 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4769 This is also useful for applications emulating C<tail -f>. Once you hit
4770 EOF on your read, and then sleep for a while, you might have to stick in a
4771 seek() to reset things. The C<seek> doesn't change the current position,
4772 but it I<does> clear the end-of-file condition on the handle, so that the
4773 next C<< <FILE> >> makes Perl try again to read something. We hope.
4775 If that doesn't work (some IO implementations are particularly
4776 cantankerous), then you may need something more like this:
4779 for ($curpos = tell(FILE); $_ = <FILE>;
4780 $curpos = tell(FILE)) {
4781 # search for some stuff and put it into files
4783 sleep($for_a_while);
4784 seek(FILE, $curpos, 0);
4787 =item seekdir DIRHANDLE,POS
4790 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4791 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4792 about possible directory compaction as the corresponding system library
4795 =item select FILEHANDLE
4796 X<select> X<filehandle, default>
4800 Returns the currently selected filehandle. Sets the current default
4801 filehandle for output, if FILEHANDLE is supplied. This has two
4802 effects: first, a C<write> or a C<print> without a filehandle will
4803 default to this FILEHANDLE. Second, references to variables related to
4804 output will refer to this output channel. For example, if you have to
4805 set the top of form format for more than one output channel, you might
4813 FILEHANDLE may be an expression whose value gives the name of the
4814 actual filehandle. Thus:
4816 $oldfh = select(STDERR); $| = 1; select($oldfh);
4818 Some programmers may prefer to think of filehandles as objects with
4819 methods, preferring to write the last example as:
4822 STDERR->autoflush(1);
4824 =item select RBITS,WBITS,EBITS,TIMEOUT
4827 This calls the select(2) system call with the bit masks specified, which
4828 can be constructed using C<fileno> and C<vec>, along these lines:
4830 $rin = $win = $ein = '';
4831 vec($rin,fileno(STDIN),1) = 1;
4832 vec($win,fileno(STDOUT),1) = 1;
4835 If you want to select on many filehandles you might wish to write a
4839 my(@fhlist) = split(' ',$_[0]);
4842 vec($bits,fileno($_),1) = 1;
4846 $rin = fhbits('STDIN TTY SOCK');
4850 ($nfound,$timeleft) =
4851 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4853 or to block until something becomes ready just do this
4855 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4857 Most systems do not bother to return anything useful in $timeleft, so
4858 calling select() in scalar context just returns $nfound.
4860 Any of the bit masks can also be undef. The timeout, if specified, is
4861 in seconds, which may be fractional. Note: not all implementations are
4862 capable of returning the $timeleft. If not, they always return
4863 $timeleft equal to the supplied $timeout.
4865 You can effect a sleep of 250 milliseconds this way:
4867 select(undef, undef, undef, 0.25);
4869 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4870 is implementation-dependent. See also L<perlport> for notes on the
4871 portability of C<select>.
4873 On error, C<select> behaves like the select(2) system call : it returns
4876 Note: on some Unixes, the select(2) system call may report a socket file
4877 descriptor as "ready for reading", when actually no data is available,
4878 thus a subsequent read blocks. It can be avoided using always the
4879 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4882 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4883 or <FH>) with C<select>, except as permitted by POSIX, and even
4884 then only on POSIX systems. You have to use C<sysread> instead.
4886 =item semctl ID,SEMNUM,CMD,ARG
4889 Calls the System V IPC function C<semctl>. You'll probably have to say
4893 first to get the correct constant definitions. If CMD is IPC_STAT or
4894 GETALL, then ARG must be a variable that will hold the returned
4895 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4896 the undefined value for error, "C<0 but true>" for zero, or the actual
4897 return value otherwise. The ARG must consist of a vector of native
4898 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4899 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4902 =item semget KEY,NSEMS,FLAGS
4905 Calls the System V IPC function semget. Returns the semaphore id, or
4906 the undefined value if there is an error. See also
4907 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4910 =item semop KEY,OPSTRING
4913 Calls the System V IPC function semop to perform semaphore operations
4914 such as signalling and waiting. OPSTRING must be a packed array of
4915 semop structures. Each semop structure can be generated with
4916 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4917 implies the number of semaphore operations. Returns true if
4918 successful, or false if there is an error. As an example, the
4919 following code waits on semaphore $semnum of semaphore id $semid:
4921 $semop = pack("s!3", $semnum, -1, 0);
4922 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4924 To signal the semaphore, replace C<-1> with C<1>. See also
4925 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4928 =item send SOCKET,MSG,FLAGS,TO
4931 =item send SOCKET,MSG,FLAGS
4933 Sends a message on a socket. Attempts to send the scalar MSG to the
4934 SOCKET filehandle. Takes the same flags as the system call of the
4935 same name. On unconnected sockets you must specify a destination to
4936 send TO, in which case it does a C C<sendto>. Returns the number of
4937 characters sent, or the undefined value if there is an error. The C
4938 system call sendmsg(2) is currently unimplemented. See
4939 L<perlipc/"UDP: Message Passing"> for examples.
4941 Note the I<characters>: depending on the status of the socket, either
4942 (8-bit) bytes or characters are sent. By default all sockets operate
4943 on bytes, but for example if the socket has been changed using
4944 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4945 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4946 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4947 in that case pretty much any characters can be sent.
4949 =item setpgrp PID,PGRP
4952 Sets the current process group for the specified PID, C<0> for the current
4953 process. Will produce a fatal error if used on a machine that doesn't
4954 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4955 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4956 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4959 =item setpriority WHICH,WHO,PRIORITY
4960 X<setpriority> X<priority> X<nice> X<renice>
4962 Sets the current priority for a process, a process group, or a user.
4963 (See setpriority(2).) Will produce a fatal error if used on a machine
4964 that doesn't implement setpriority(2).
4966 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4969 Sets the socket option requested. Returns undefined if there is an
4970 error. Use integer constants provided by the C<Socket> module for
4971 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4972 getprotobyname. OPTVAL might either be a packed string or an integer.
4973 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4975 An example disabling the Nagle's algorithm for a socket:
4977 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4978 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
4985 Shifts the first value of the array off and returns it, shortening the
4986 array by 1 and moving everything down. If there are no elements in the
4987 array, returns the undefined value. If ARRAY is omitted, shifts the
4988 C<@_> array within the lexical scope of subroutines and formats, and the
4989 C<@ARGV> array outside of a subroutine and also within the lexical scopes
4990 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
4991 and C<END {}> constructs.
4993 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4994 same thing to the left end of an array that C<pop> and C<push> do to the
4997 =item shmctl ID,CMD,ARG
5000 Calls the System V IPC function shmctl. You'll probably have to say
5004 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5005 then ARG must be a variable that will hold the returned C<shmid_ds>
5006 structure. Returns like ioctl: the undefined value for error, "C<0> but
5007 true" for zero, or the actual return value otherwise.
5008 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5010 =item shmget KEY,SIZE,FLAGS
5013 Calls the System V IPC function shmget. Returns the shared memory
5014 segment id, or the undefined value if there is an error.
5015 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5017 =item shmread ID,VAR,POS,SIZE
5021 =item shmwrite ID,STRING,POS,SIZE
5023 Reads or writes the System V shared memory segment ID starting at
5024 position POS for size SIZE by attaching to it, copying in/out, and
5025 detaching from it. When reading, VAR must be a variable that will
5026 hold the data read. When writing, if STRING is too long, only SIZE
5027 bytes are used; if STRING is too short, nulls are written to fill out
5028 SIZE bytes. Return true if successful, or false if there is an error.
5029 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5030 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5032 =item shutdown SOCKET,HOW
5035 Shuts down a socket connection in the manner indicated by HOW, which
5036 has the same interpretation as in the system call of the same name.
5038 shutdown(SOCKET, 0); # I/we have stopped reading data
5039 shutdown(SOCKET, 1); # I/we have stopped writing data
5040 shutdown(SOCKET, 2); # I/we have stopped using this socket
5042 This is useful with sockets when you want to tell the other
5043 side you're done writing but not done reading, or vice versa.
5044 It's also a more insistent form of close because it also
5045 disables the file descriptor in any forked copies in other
5049 X<sin> X<sine> X<asin> X<arcsine>
5053 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5054 returns sine of C<$_>.
5056 For the inverse sine operation, you may use the C<Math::Trig::asin>
5057 function, or use this relation:
5059 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5066 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5067 May be interrupted if the process receives a signal such as C<SIGALRM>.
5068 Returns the number of seconds actually slept. You probably cannot
5069 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5072 On some older systems, it may sleep up to a full second less than what
5073 you requested, depending on how it counts seconds. Most modern systems
5074 always sleep the full amount. They may appear to sleep longer than that,
5075 however, because your process might not be scheduled right away in a
5076 busy multitasking system.
5078 For delays of finer granularity than one second, you may use Perl's
5079 C<syscall> interface to access setitimer(2) if your system supports
5080 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5081 and starting from Perl 5.8 part of the standard distribution) may also
5084 See also the POSIX module's C<pause> function.
5086 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5089 Opens a socket of the specified kind and attaches it to filehandle
5090 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5091 the system call of the same name. You should C<use Socket> first
5092 to get the proper definitions imported. See the examples in
5093 L<perlipc/"Sockets: Client/Server Communication">.
5095 On systems that support a close-on-exec flag on files, the flag will
5096 be set for the newly opened file descriptor, as determined by the
5097 value of $^F. See L<perlvar/$^F>.
5099 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5102 Creates an unnamed pair of sockets in the specified domain, of the
5103 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5104 for the system call of the same name. If unimplemented, yields a fatal
5105 error. Returns true if successful.
5107 On systems that support a close-on-exec flag on files, the flag will
5108 be set for the newly opened file descriptors, as determined by the value
5109 of $^F. See L<perlvar/$^F>.
5111 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5112 to C<pipe(Rdr, Wtr)> is essentially:
5115 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5116 shutdown(Rdr, 1); # no more writing for reader
5117 shutdown(Wtr, 0); # no more reading for writer
5119 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5120 emulate socketpair using IP sockets to localhost if your system implements
5121 sockets but not socketpair.
5123 =item sort SUBNAME LIST
5124 X<sort> X<qsort> X<quicksort> X<mergesort>
5126 =item sort BLOCK LIST
5130 In list context, this sorts the LIST and returns the sorted list value.
5131 In scalar context, the behaviour of C<sort()> is undefined.
5133 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5134 order. If SUBNAME is specified, it gives the name of a subroutine
5135 that returns an integer less than, equal to, or greater than C<0>,
5136 depending on how the elements of the list are to be ordered. (The C<<
5137 <=> >> and C<cmp> operators are extremely useful in such routines.)
5138 SUBNAME may be a scalar variable name (unsubscripted), in which case
5139 the value provides the name of (or a reference to) the actual
5140 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5141 an anonymous, in-line sort subroutine.
5143 If the subroutine's prototype is C<($$)>, the elements to be compared
5144 are passed by reference in C<@_>, as for a normal subroutine. This is
5145 slower than unprototyped subroutines, where the elements to be
5146 compared are passed into the subroutine
5147 as the package global variables $a and $b (see example below). Note that
5148 in the latter case, it is usually counter-productive to declare $a and
5151 The values to be compared are always passed by reference and should not
5154 You also cannot exit out of the sort block or subroutine using any of the
5155 loop control operators described in L<perlsyn> or with C<goto>.
5157 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5158 current collation locale. See L<perllocale>.
5160 sort() returns aliases into the original list, much as a for loop's index
5161 variable aliases the list elements. That is, modifying an element of a
5162 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5163 actually modifies the element in the original list. This is usually
5164 something to be avoided when writing clear code.
5166 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5167 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5168 preserves the input order of elements that compare equal. Although
5169 quicksort's run time is O(NlogN) when averaged over all arrays of
5170 length N, the time can be O(N**2), I<quadratic> behavior, for some
5171 inputs.) In 5.7, the quicksort implementation was replaced with
5172 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5173 But benchmarks indicated that for some inputs, on some platforms,
5174 the original quicksort was faster. 5.8 has a sort pragma for
5175 limited control of the sort. Its rather blunt control of the
5176 underlying algorithm may not persist into future Perls, but the
5177 ability to characterize the input or output in implementation
5178 independent ways quite probably will. See L<sort>.
5183 @articles = sort @files;
5185 # same thing, but with explicit sort routine
5186 @articles = sort {$a cmp $b} @files;
5188 # now case-insensitively
5189 @articles = sort {uc($a) cmp uc($b)} @files;
5191 # same thing in reversed order
5192 @articles = sort {$b cmp $a} @files;
5194 # sort numerically ascending
5195 @articles = sort {$a <=> $b} @files;
5197 # sort numerically descending
5198 @articles = sort {$b <=> $a} @files;
5200 # this sorts the %age hash by value instead of key
5201 # using an in-line function
5202 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5204 # sort using explicit subroutine name
5206 $age{$a} <=> $age{$b}; # presuming numeric
5208 @sortedclass = sort byage @class;
5210 sub backwards { $b cmp $a }
5211 @harry = qw(dog cat x Cain Abel);
5212 @george = qw(gone chased yz Punished Axed);
5214 # prints AbelCaincatdogx
5215 print sort backwards @harry;
5216 # prints xdogcatCainAbel
5217 print sort @george, 'to', @harry;
5218 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5220 # inefficiently sort by descending numeric compare using
5221 # the first integer after the first = sign, or the
5222 # whole record case-insensitively otherwise
5225 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5230 # same thing, but much more efficiently;
5231 # we'll build auxiliary indices instead
5235 push @nums, /=(\d+)/;
5240 $nums[$b] <=> $nums[$a]
5242 $caps[$a] cmp $caps[$b]
5246 # same thing, but without any temps
5247 @new = map { $_->[0] }
5248 sort { $b->[1] <=> $a->[1]
5251 } map { [$_, /=(\d+)/, uc($_)] } @old;
5253 # using a prototype allows you to use any comparison subroutine
5254 # as a sort subroutine (including other package's subroutines)
5256 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5259 @new = sort other::backwards @old;
5261 # guarantee stability, regardless of algorithm
5263 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5265 # force use of mergesort (not portable outside Perl 5.8)
5266 use sort '_mergesort'; # note discouraging _
5267 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5269 If you're using strict, you I<must not> declare $a
5270 and $b as lexicals. They are package globals. That means
5271 if you're in the C<main> package and type
5273 @articles = sort {$b <=> $a} @files;
5275 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5276 but if you're in the C<FooPack> package, it's the same as typing
5278 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5280 The comparison function is required to behave. If it returns
5281 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5282 sometimes saying the opposite, for example) the results are not
5285 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5286 (not-a-number), and because C<sort> will trigger a fatal error unless the
5287 result of a comparison is defined, when sorting with a comparison function
5288 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5289 The following example takes advantage of the fact that C<NaN != NaN> to
5290 eliminate any C<NaN>s from the input.
5292 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5294 =item splice ARRAY,OFFSET,LENGTH,LIST
5297 =item splice ARRAY,OFFSET,LENGTH
5299 =item splice ARRAY,OFFSET
5303 Removes the elements designated by OFFSET and LENGTH from an array, and
5304 replaces them with the elements of LIST, if any. In list context,
5305 returns the elements removed from the array. In scalar context,
5306 returns the last element removed, or C<undef> if no elements are
5307 removed. The array grows or shrinks as necessary.
5308 If OFFSET is negative then it starts that far from the end of the array.
5309 If LENGTH is omitted, removes everything from OFFSET onward.
5310 If LENGTH is negative, removes the elements from OFFSET onward
5311 except for -LENGTH elements at the end of the array.
5312 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5313 past the end of the array, perl issues a warning, and splices at the
5316 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5318 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5319 pop(@a) splice(@a,-1)
5320 shift(@a) splice(@a,0,1)
5321 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5322 $a[$i] = $y splice(@a,$i,1,$y)
5324 Example, assuming array lengths are passed before arrays:
5326 sub aeq { # compare two list values
5327 my(@a) = splice(@_,0,shift);
5328 my(@b) = splice(@_,0,shift);
5329 return 0 unless @a == @b; # same len?
5331 return 0 if pop(@a) ne pop(@b);
5335 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5337 =item split /PATTERN/,EXPR,LIMIT
5340 =item split /PATTERN/,EXPR
5342 =item split /PATTERN/
5346 Splits the string EXPR into a list of strings and returns that list. By
5347 default, empty leading fields are preserved, and empty trailing ones are
5348 deleted. (If all fields are empty, they are considered to be trailing.)
5350 In scalar context, returns the number of fields found and splits into
5351 the C<@_> array. Use of split in scalar context is deprecated, however,
5352 because it clobbers your subroutine arguments.
5354 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5355 splits on whitespace (after skipping any leading whitespace). Anything
5356 matching PATTERN is taken to be a delimiter separating the fields. (Note
5357 that the delimiter may be longer than one character.)
5359 If LIMIT is specified and positive, it represents the maximum number
5360 of fields the EXPR will be split into, though the actual number of
5361 fields returned depends on the number of times PATTERN matches within
5362 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5363 stripped (which potential users of C<pop> would do well to remember).
5364 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5365 had been specified. Note that splitting an EXPR that evaluates to the
5366 empty string always returns the empty list, regardless of the LIMIT
5369 A pattern matching the null string (not to be confused with
5370 a null pattern C<//>, which is just one member of the set of patterns
5371 matching a null string) will split the value of EXPR into separate
5372 characters at each point it matches that way. For example:
5374 print join(':', split(/ */, 'hi there'));
5376 produces the output 'h:i:t:h:e:r:e'.
5378 As a special case for C<split>, using the empty pattern C<//> specifically
5379 matches only the null string, and is not be confused with the regular use
5380 of C<//> to mean "the last successful pattern match". So, for C<split>,
5383 print join(':', split(//, 'hi there'));
5385 produces the output 'h:i: :t:h:e:r:e'.
5387 Empty leading (or trailing) fields are produced when there are positive
5388 width matches at the beginning (or end) of the string; a zero-width match
5389 at the beginning (or end) of the string does not produce an empty field.
5392 print join(':', split(/(?=\w)/, 'hi there!'));
5394 produces the output 'h:i :t:h:e:r:e!'.
5396 The LIMIT parameter can be used to split a line partially
5398 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5400 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5401 a LIMIT one larger than the number of variables in the list, to avoid
5402 unnecessary work. For the list above LIMIT would have been 4 by
5403 default. In time critical applications it behooves you not to split
5404 into more fields than you really need.
5406 If the PATTERN contains parentheses, additional list elements are
5407 created from each matching substring in the delimiter.
5409 split(/([,-])/, "1-10,20", 3);
5411 produces the list value
5413 (1, '-', 10, ',', 20)
5415 If you had the entire header of a normal Unix email message in $header,
5416 you could split it up into fields and their values this way:
5418 $header =~ s/\n\s+/ /g; # fix continuation lines
5419 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5421 The pattern C</PATTERN/> may be replaced with an expression to specify
5422 patterns that vary at runtime. (To do runtime compilation only once,
5423 use C</$variable/o>.)
5425 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5426 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5427 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5428 will give you as many null initial fields as there are leading spaces.
5429 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5430 whitespace produces a null first field. A C<split> with no arguments
5431 really does a S<C<split(' ', $_)>> internally.
5433 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5438 open(PASSWD, '/etc/passwd');
5441 ($login, $passwd, $uid, $gid,
5442 $gcos, $home, $shell) = split(/:/);
5446 As with regular pattern matching, any capturing parentheses that are not
5447 matched in a C<split()> will be set to C<undef> when returned:
5449 @fields = split /(A)|B/, "1A2B3";
5450 # @fields is (1, 'A', 2, undef, 3)
5452 =item sprintf FORMAT, LIST
5455 Returns a string formatted by the usual C<printf> conventions of the C
5456 library function C<sprintf>. See below for more details
5457 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5458 the general principles.
5462 # Format number with up to 8 leading zeroes
5463 $result = sprintf("%08d", $number);
5465 # Round number to 3 digits after decimal point
5466 $rounded = sprintf("%.3f", $number);
5468 Perl does its own C<sprintf> formatting--it emulates the C
5469 function C<sprintf>, but it doesn't use it (except for floating-point
5470 numbers, and even then only the standard modifiers are allowed). As a
5471 result, any non-standard extensions in your local C<sprintf> are not
5472 available from Perl.
5474 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5475 pass it an array as your first argument. The array is given scalar context,
5476 and instead of using the 0th element of the array as the format, Perl will
5477 use the count of elements in the array as the format, which is almost never
5480 Perl's C<sprintf> permits the following universally-known conversions:
5483 %c a character with the given number
5485 %d a signed integer, in decimal
5486 %u an unsigned integer, in decimal
5487 %o an unsigned integer, in octal
5488 %x an unsigned integer, in hexadecimal
5489 %e a floating-point number, in scientific notation
5490 %f a floating-point number, in fixed decimal notation
5491 %g a floating-point number, in %e or %f notation
5493 In addition, Perl permits the following widely-supported conversions:
5495 %X like %x, but using upper-case letters
5496 %E like %e, but using an upper-case "E"
5497 %G like %g, but with an upper-case "E" (if applicable)
5498 %b an unsigned integer, in binary
5499 %p a pointer (outputs the Perl value's address in hexadecimal)
5500 %n special: *stores* the number of characters output so far
5501 into the next variable in the parameter list
5503 Finally, for backward (and we do mean "backward") compatibility, Perl
5504 permits these unnecessary but widely-supported conversions:
5507 %D a synonym for %ld
5508 %U a synonym for %lu
5509 %O a synonym for %lo
5512 Note that the number of exponent digits in the scientific notation produced
5513 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5514 exponent less than 100 is system-dependent: it may be three or less
5515 (zero-padded as necessary). In other words, 1.23 times ten to the
5516 99th may be either "1.23e99" or "1.23e099".
5518 Between the C<%> and the format letter, you may specify a number of
5519 additional attributes controlling the interpretation of the format.
5520 In order, these are:
5524 =item format parameter index
5526 An explicit format parameter index, such as C<2$>. By default sprintf
5527 will format the next unused argument in the list, but this allows you
5528 to take the arguments out of order, e.g.:
5530 printf '%2$d %1$d', 12, 34; # prints "34 12"
5531 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5536 space prefix positive number with a space
5537 + prefix positive number with a plus sign
5538 - left-justify within the field
5539 0 use zeros, not spaces, to right-justify
5540 # prefix non-zero octal with "0", non-zero hex with "0x",
5541 non-zero binary with "0b"
5545 printf '<% d>', 12; # prints "< 12>"
5546 printf '<%+d>', 12; # prints "<+12>"
5547 printf '<%6s>', 12; # prints "< 12>"
5548 printf '<%-6s>', 12; # prints "<12 >"
5549 printf '<%06s>', 12; # prints "<000012>"
5550 printf '<%#x>', 12; # prints "<0xc>"
5554 This flag tells perl to interpret the supplied string as a vector of
5555 integers, one for each character in the string. Perl applies the format to
5556 each integer in turn, then joins the resulting strings with a separator (a
5557 dot C<.> by default). This can be useful for displaying ordinal values of
5558 characters in arbitrary strings:
5560 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5561 printf "version is v%vd\n", $^V; # Perl's version
5563 Put an asterisk C<*> before the C<v> to override the string to
5564 use to separate the numbers:
5566 printf "address is %*vX\n", ":", $addr; # IPv6 address
5567 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5569 You can also explicitly specify the argument number to use for
5570 the join string using e.g. C<*2$v>:
5572 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5574 =item (minimum) width
5576 Arguments are usually formatted to be only as wide as required to
5577 display the given value. You can override the width by putting
5578 a number here, or get the width from the next argument (with C<*>)
5579 or from a specified argument (with e.g. C<*2$>):
5581 printf '<%s>', "a"; # prints "<a>"
5582 printf '<%6s>', "a"; # prints "< a>"
5583 printf '<%*s>', 6, "a"; # prints "< a>"
5584 printf '<%*2$s>', "a", 6; # prints "< a>"
5585 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5587 If a field width obtained through C<*> is negative, it has the same
5588 effect as the C<-> flag: left-justification.
5590 =item precision, or maximum width
5593 You can specify a precision (for numeric conversions) or a maximum
5594 width (for string conversions) by specifying a C<.> followed by a number.
5595 For floating point formats, with the exception of 'g' and 'G', this specifies
5596 the number of decimal places to show (the default being 6), e.g.:
5598 # these examples are subject to system-specific variation
5599 printf '<%f>', 1; # prints "<1.000000>"
5600 printf '<%.1f>', 1; # prints "<1.0>"
5601 printf '<%.0f>', 1; # prints "<1>"
5602 printf '<%e>', 10; # prints "<1.000000e+01>"
5603 printf '<%.1e>', 10; # prints "<1.0e+01>"
5605 For 'g' and 'G', this specifies the maximum number of digits to show,
5606 including prior to the decimal point as well as after it, e.g.:
5608 # these examples are subject to system-specific variation
5609 printf '<%g>', 1; # prints "<1>"
5610 printf '<%.10g>', 1; # prints "<1>"
5611 printf '<%g>', 100; # prints "<100>"
5612 printf '<%.1g>', 100; # prints "<1e+02>"
5613 printf '<%.2g>', 100.01; # prints "<1e+02>"
5614 printf '<%.5g>', 100.01; # prints "<100.01>"
5615 printf '<%.4g>', 100.01; # prints "<100>"
5617 For integer conversions, specifying a precision implies that the
5618 output of the number itself should be zero-padded to this width:
5620 printf '<%.6x>', 1; # prints "<000001>"
5621 printf '<%#.6x>', 1; # prints "<0x000001>"
5622 printf '<%-10.6x>', 1; # prints "<000001 >"
5624 For string conversions, specifying a precision truncates the string
5625 to fit in the specified width:
5627 printf '<%.5s>', "truncated"; # prints "<trunc>"
5628 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5630 You can also get the precision from the next argument using C<.*>:
5632 printf '<%.6x>', 1; # prints "<000001>"
5633 printf '<%.*x>', 6, 1; # prints "<000001>"
5635 You cannot currently get the precision from a specified number,
5636 but it is intended that this will be possible in the future using
5639 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5643 For numeric conversions, you can specify the size to interpret the
5644 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5645 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5646 whatever the default integer size is on your platform (usually 32 or 64
5647 bits), but you can override this to use instead one of the standard C types,
5648 as supported by the compiler used to build Perl:
5650 l interpret integer as C type "long" or "unsigned long"
5651 h interpret integer as C type "short" or "unsigned short"
5652 q, L or ll interpret integer as C type "long long", "unsigned long long".
5653 or "quads" (typically 64-bit integers)
5655 The last will produce errors if Perl does not understand "quads" in your
5656 installation. (This requires that either the platform natively supports quads
5657 or Perl was specifically compiled to support quads.) You can find out
5658 whether your Perl supports quads via L<Config>:
5661 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5664 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5665 to be the default floating point size on your platform (double or long double),
5666 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5667 platform supports them. You can find out whether your Perl supports long
5668 doubles via L<Config>:
5671 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5673 You can find out whether Perl considers 'long double' to be the default
5674 floating point size to use on your platform via L<Config>:
5677 ($Config{uselongdouble} eq 'define') &&
5678 print "long doubles by default\n";
5680 It can also be the case that long doubles and doubles are the same thing:
5683 ($Config{doublesize} == $Config{longdblsize}) &&
5684 print "doubles are long doubles\n";
5686 The size specifier C<V> has no effect for Perl code, but it is supported
5687 for compatibility with XS code; it means 'use the standard size for
5688 a Perl integer (or floating-point number)', which is already the
5689 default for Perl code.
5691 =item order of arguments
5693 Normally, sprintf takes the next unused argument as the value to
5694 format for each format specification. If the format specification
5695 uses C<*> to require additional arguments, these are consumed from
5696 the argument list in the order in which they appear in the format
5697 specification I<before> the value to format. Where an argument is
5698 specified using an explicit index, this does not affect the normal
5699 order for the arguments (even when the explicitly specified index
5700 would have been the next argument in any case).
5704 printf '<%*.*s>', $a, $b, $c;
5706 would use C<$a> for the width, C<$b> for the precision and C<$c>
5707 as the value to format, while:
5709 print '<%*1$.*s>', $a, $b;
5711 would use C<$a> for the width and the precision, and C<$b> as the
5714 Here are some more examples - beware that when using an explicit
5715 index, the C<$> may need to be escaped:
5717 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5718 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5719 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5720 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5724 If C<use locale> is in effect, the character used for the decimal
5725 point in formatted real numbers is affected by the LC_NUMERIC locale.
5729 X<sqrt> X<root> X<square root>
5733 Return the square root of EXPR. If EXPR is omitted, returns square
5734 root of C<$_>. Only works on non-negative operands, unless you've
5735 loaded the standard Math::Complex module.
5738 print sqrt(-2); # prints 1.4142135623731i
5741 X<srand> X<seed> X<randseed>
5745 Sets the random number seed for the C<rand> operator.
5747 The point of the function is to "seed" the C<rand> function so that
5748 C<rand> can produce a different sequence each time you run your
5751 If srand() is not called explicitly, it is called implicitly at the
5752 first use of the C<rand> operator. However, this was not the case in
5753 versions of Perl before 5.004, so if your script will run under older
5754 Perl versions, it should call C<srand>.
5756 Most programs won't even call srand() at all, except those that
5757 need a cryptographically-strong starting point rather than the
5758 generally acceptable default, which is based on time of day,
5759 process ID, and memory allocation, or the F</dev/urandom> device,
5762 You can call srand($seed) with the same $seed to reproduce the
5763 I<same> sequence from rand(), but this is usually reserved for
5764 generating predictable results for testing or debugging.
5765 Otherwise, don't call srand() more than once in your program.
5767 Do B<not> call srand() (i.e. without an argument) more than once in
5768 a script. The internal state of the random number generator should
5769 contain more entropy than can be provided by any seed, so calling
5770 srand() again actually I<loses> randomness.
5772 Most implementations of C<srand> take an integer and will silently
5773 truncate decimal numbers. This means C<srand(42)> will usually
5774 produce the same results as C<srand(42.1)>. To be safe, always pass
5775 C<srand> an integer.
5777 In versions of Perl prior to 5.004 the default seed was just the
5778 current C<time>. This isn't a particularly good seed, so many old
5779 programs supply their own seed value (often C<time ^ $$> or C<time ^
5780 ($$ + ($$ << 15))>), but that isn't necessary any more.
5782 For cryptographic purposes, however, you need something much more random
5783 than the default seed. Checksumming the compressed output of one or more
5784 rapidly changing operating system status programs is the usual method. For
5787 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5789 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5792 Frequently called programs (like CGI scripts) that simply use
5796 for a seed can fall prey to the mathematical property that
5800 one-third of the time. So don't do that.
5802 =item stat FILEHANDLE
5803 X<stat> X<file, status>
5809 Returns a 13-element list giving the status info for a file, either
5810 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5811 it stats C<$_>. Returns a null list if the stat fails. Typically used
5814 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5815 $atime,$mtime,$ctime,$blksize,$blocks)
5818 Not all fields are supported on all filesystem types. Here are the
5819 meanings of the fields:
5821 0 dev device number of filesystem
5823 2 mode file mode (type and permissions)
5824 3 nlink number of (hard) links to the file
5825 4 uid numeric user ID of file's owner
5826 5 gid numeric group ID of file's owner
5827 6 rdev the device identifier (special files only)
5828 7 size total size of file, in bytes
5829 8 atime last access time in seconds since the epoch
5830 9 mtime last modify time in seconds since the epoch
5831 10 ctime inode change time in seconds since the epoch (*)
5832 11 blksize preferred block size for file system I/O
5833 12 blocks actual number of blocks allocated
5835 (The epoch was at 00:00 January 1, 1970 GMT.)
5837 (*) Not all fields are supported on all filesystem types. Notably, the
5838 ctime field is non-portable. In particular, you cannot expect it to be a
5839 "creation time", see L<perlport/"Files and Filesystems"> for details.
5841 If C<stat> is passed the special filehandle consisting of an underline, no
5842 stat is done, but the current contents of the stat structure from the
5843 last C<stat>, C<lstat>, or filetest are returned. Example:
5845 if (-x $file && (($d) = stat(_)) && $d < 0) {
5846 print "$file is executable NFS file\n";
5849 (This works on machines only for which the device number is negative
5852 Because the mode contains both the file type and its permissions, you
5853 should mask off the file type portion and (s)printf using a C<"%o">
5854 if you want to see the real permissions.
5856 $mode = (stat($filename))[2];
5857 printf "Permissions are %04o\n", $mode & 07777;
5859 In scalar context, C<stat> returns a boolean value indicating success
5860 or failure, and, if successful, sets the information associated with
5861 the special filehandle C<_>.
5863 The File::stat module provides a convenient, by-name access mechanism:
5866 $sb = stat($filename);
5867 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5868 $filename, $sb->size, $sb->mode & 07777,
5869 scalar localtime $sb->mtime;
5871 You can import symbolic mode constants (C<S_IF*>) and functions
5872 (C<S_IS*>) from the Fcntl module:
5876 $mode = (stat($filename))[2];
5878 $user_rwx = ($mode & S_IRWXU) >> 6;
5879 $group_read = ($mode & S_IRGRP) >> 3;
5880 $other_execute = $mode & S_IXOTH;
5882 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5884 $is_setuid = $mode & S_ISUID;
5885 $is_setgid = S_ISDIR($mode);
5887 You could write the last two using the C<-u> and C<-d> operators.
5888 The commonly available C<S_IF*> constants are
5890 # Permissions: read, write, execute, for user, group, others.
5892 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5893 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5894 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5896 # Setuid/Setgid/Stickiness/SaveText.
5897 # Note that the exact meaning of these is system dependent.
5899 S_ISUID S_ISGID S_ISVTX S_ISTXT
5901 # File types. Not necessarily all are available on your system.
5903 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5905 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5907 S_IREAD S_IWRITE S_IEXEC
5909 and the C<S_IF*> functions are
5911 S_IMODE($mode) the part of $mode containing the permission bits
5912 and the setuid/setgid/sticky bits
5914 S_IFMT($mode) the part of $mode containing the file type
5915 which can be bit-anded with e.g. S_IFREG
5916 or with the following functions
5918 # The operators -f, -d, -l, -b, -c, -p, and -S.
5920 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5921 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5923 # No direct -X operator counterpart, but for the first one
5924 # the -g operator is often equivalent. The ENFMT stands for
5925 # record flocking enforcement, a platform-dependent feature.
5927 S_ISENFMT($mode) S_ISWHT($mode)
5929 See your native chmod(2) and stat(2) documentation for more details
5930 about the C<S_*> constants. To get status info for a symbolic link
5931 instead of the target file behind the link, use the C<lstat> function.
5938 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5939 doing many pattern matches on the string before it is next modified.
5940 This may or may not save time, depending on the nature and number of
5941 patterns you are searching on, and on the distribution of character
5942 frequencies in the string to be searched--you probably want to compare
5943 run times with and without it to see which runs faster. Those loops
5944 that scan for many short constant strings (including the constant
5945 parts of more complex patterns) will benefit most. You may have only
5946 one C<study> active at a time--if you study a different scalar the first
5947 is "unstudied". (The way C<study> works is this: a linked list of every
5948 character in the string to be searched is made, so we know, for
5949 example, where all the C<'k'> characters are. From each search string,
5950 the rarest character is selected, based on some static frequency tables
5951 constructed from some C programs and English text. Only those places
5952 that contain this "rarest" character are examined.)
5954 For example, here is a loop that inserts index producing entries
5955 before any line containing a certain pattern:
5959 print ".IX foo\n" if /\bfoo\b/;
5960 print ".IX bar\n" if /\bbar\b/;
5961 print ".IX blurfl\n" if /\bblurfl\b/;
5966 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5967 will be looked at, because C<f> is rarer than C<o>. In general, this is
5968 a big win except in pathological cases. The only question is whether
5969 it saves you more time than it took to build the linked list in the
5972 Note that if you have to look for strings that you don't know till
5973 runtime, you can build an entire loop as a string and C<eval> that to
5974 avoid recompiling all your patterns all the time. Together with
5975 undefining C<$/> to input entire files as one record, this can be very
5976 fast, often faster than specialized programs like fgrep(1). The following
5977 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5978 out the names of those files that contain a match:
5980 $search = 'while (<>) { study;';
5981 foreach $word (@words) {
5982 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5987 eval $search; # this screams
5988 $/ = "\n"; # put back to normal input delimiter
5989 foreach $file (sort keys(%seen)) {
5993 =item sub NAME BLOCK
5996 =item sub NAME (PROTO) BLOCK
5998 =item sub NAME : ATTRS BLOCK
6000 =item sub NAME (PROTO) : ATTRS BLOCK
6002 This is subroutine definition, not a real function I<per se>.
6003 Without a BLOCK it's just a forward declaration. Without a NAME,
6004 it's an anonymous function declaration, and does actually return
6005 a value: the CODE ref of the closure you just created.
6007 See L<perlsub> and L<perlref> for details about subroutines and
6008 references, and L<attributes> and L<Attribute::Handlers> for more
6009 information about attributes.
6011 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6012 X<substr> X<substring> X<mid> X<left> X<right>
6014 =item substr EXPR,OFFSET,LENGTH
6016 =item substr EXPR,OFFSET
6018 Extracts a substring out of EXPR and returns it. First character is at
6019 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6020 If OFFSET is negative (or more precisely, less than C<$[>), starts
6021 that far from the end of the string. If LENGTH is omitted, returns
6022 everything to the end of the string. If LENGTH is negative, leaves that
6023 many characters off the end of the string.
6025 You can use the substr() function as an lvalue, in which case EXPR
6026 must itself be an lvalue. If you assign something shorter than LENGTH,
6027 the string will shrink, and if you assign something longer than LENGTH,
6028 the string will grow to accommodate it. To keep the string the same
6029 length you may need to pad or chop your value using C<sprintf>.
6031 If OFFSET and LENGTH specify a substring that is partly outside the
6032 string, only the part within the string is returned. If the substring
6033 is beyond either end of the string, substr() returns the undefined
6034 value and produces a warning. When used as an lvalue, specifying a
6035 substring that is entirely outside the string is a fatal error.
6036 Here's an example showing the behavior for boundary cases:
6039 substr($name, 4) = 'dy'; # $name is now 'freddy'
6040 my $null = substr $name, 6, 2; # returns '' (no warning)
6041 my $oops = substr $name, 7; # returns undef, with warning
6042 substr($name, 7) = 'gap'; # fatal error
6044 An alternative to using substr() as an lvalue is to specify the
6045 replacement string as the 4th argument. This allows you to replace
6046 parts of the EXPR and return what was there before in one operation,
6047 just as you can with splice().
6049 Note that the lvalue returned by the 3-arg version of substr() acts as
6050 a 'magic bullet'; each time it is assigned to, it remembers which part
6051 of the original string is being modified; for example:
6054 for (substr($x,1,2)) {
6055 $_ = 'a'; print $x,"\n"; # prints 1a4
6056 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6058 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6062 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6065 =item symlink OLDFILE,NEWFILE
6066 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6068 Creates a new filename symbolically linked to the old filename.
6069 Returns C<1> for success, C<0> otherwise. On systems that don't support
6070 symbolic links, produces a fatal error at run time. To check for that,
6073 $symlink_exists = eval { symlink("",""); 1 };
6075 =item syscall NUMBER, LIST
6076 X<syscall> X<system call>
6078 Calls the system call specified as the first element of the list,
6079 passing the remaining elements as arguments to the system call. If
6080 unimplemented, produces a fatal error. The arguments are interpreted
6081 as follows: if a given argument is numeric, the argument is passed as
6082 an int. If not, the pointer to the string value is passed. You are
6083 responsible to make sure a string is pre-extended long enough to
6084 receive any result that might be written into a string. You can't use a
6085 string literal (or other read-only string) as an argument to C<syscall>
6086 because Perl has to assume that any string pointer might be written
6088 integer arguments are not literals and have never been interpreted in a
6089 numeric context, you may need to add C<0> to them to force them to look
6090 like numbers. This emulates the C<syswrite> function (or vice versa):
6092 require 'syscall.ph'; # may need to run h2ph
6094 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6096 Note that Perl supports passing of up to only 14 arguments to your system call,
6097 which in practice should usually suffice.
6099 Syscall returns whatever value returned by the system call it calls.
6100 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6101 Note that some system calls can legitimately return C<-1>. The proper
6102 way to handle such calls is to assign C<$!=0;> before the call and
6103 check the value of C<$!> if syscall returns C<-1>.
6105 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6106 number of the read end of the pipe it creates. There is no way
6107 to retrieve the file number of the other end. You can avoid this
6108 problem by using C<pipe> instead.
6110 =item sysopen FILEHANDLE,FILENAME,MODE
6113 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6115 Opens the file whose filename is given by FILENAME, and associates it
6116 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6117 the name of the real filehandle wanted. This function calls the
6118 underlying operating system's C<open> function with the parameters
6119 FILENAME, MODE, PERMS.
6121 The possible values and flag bits of the MODE parameter are
6122 system-dependent; they are available via the standard module C<Fcntl>.
6123 See the documentation of your operating system's C<open> to see which
6124 values and flag bits are available. You may combine several flags
6125 using the C<|>-operator.
6127 Some of the most common values are C<O_RDONLY> for opening the file in
6128 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6129 and C<O_RDWR> for opening the file in read-write mode.
6130 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6132 For historical reasons, some values work on almost every system
6133 supported by perl: zero means read-only, one means write-only, and two
6134 means read/write. We know that these values do I<not> work under
6135 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6136 use them in new code.
6138 If the file named by FILENAME does not exist and the C<open> call creates
6139 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6140 PERMS specifies the permissions of the newly created file. If you omit
6141 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6142 These permission values need to be in octal, and are modified by your
6143 process's current C<umask>.
6146 In many systems the C<O_EXCL> flag is available for opening files in
6147 exclusive mode. This is B<not> locking: exclusiveness means here that
6148 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6149 on network filesystems, and has no effect unless the C<O_CREAT> flag
6150 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6151 being opened if it is a symbolic link. It does not protect against
6152 symbolic links in the file's path.
6155 Sometimes you may want to truncate an already-existing file. This
6156 can be done using the C<O_TRUNC> flag. The behavior of
6157 C<O_TRUNC> with C<O_RDONLY> is undefined.
6160 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6161 that takes away the user's option to have a more permissive umask.
6162 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6165 Note that C<sysopen> depends on the fdopen() C library function.
6166 On many UNIX systems, fdopen() is known to fail when file descriptors
6167 exceed a certain value, typically 255. If you need more file
6168 descriptors than that, consider rebuilding Perl to use the C<sfio>
6169 library, or perhaps using the POSIX::open() function.
6171 See L<perlopentut> for a kinder, gentler explanation of opening files.
6173 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6176 =item sysread FILEHANDLE,SCALAR,LENGTH
6178 Attempts to read LENGTH bytes of data into variable SCALAR from the
6179 specified FILEHANDLE, using the system call read(2). It bypasses
6180 buffered IO, so mixing this with other kinds of reads, C<print>,
6181 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6182 perlio or stdio layers usually buffers data. Returns the number of
6183 bytes actually read, C<0> at end of file, or undef if there was an
6184 error (in the latter case C<$!> is also set). SCALAR will be grown or
6185 shrunk so that the last byte actually read is the last byte of the
6186 scalar after the read.
6188 An OFFSET may be specified to place the read data at some place in the
6189 string other than the beginning. A negative OFFSET specifies
6190 placement at that many characters counting backwards from the end of
6191 the string. A positive OFFSET greater than the length of SCALAR
6192 results in the string being padded to the required size with C<"\0">
6193 bytes before the result of the read is appended.
6195 There is no syseof() function, which is ok, since eof() doesn't work
6196 very well on device files (like ttys) anyway. Use sysread() and check
6197 for a return value for 0 to decide whether you're done.
6199 Note that if the filehandle has been marked as C<:utf8> Unicode
6200 characters are read instead of bytes (the LENGTH, OFFSET, and the
6201 return value of sysread() are in Unicode characters).
6202 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6203 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6205 =item sysseek FILEHANDLE,POSITION,WHENCE
6208 Sets FILEHANDLE's system position in bytes using the system call
6209 lseek(2). FILEHANDLE may be an expression whose value gives the name
6210 of the filehandle. The values for WHENCE are C<0> to set the new
6211 position to POSITION, C<1> to set the it to the current position plus
6212 POSITION, and C<2> to set it to EOF plus POSITION (typically
6215 Note the I<in bytes>: even if the filehandle has been set to operate
6216 on characters (for example by using the C<:utf8> I/O layer), tell()
6217 will return byte offsets, not character offsets (because implementing
6218 that would render sysseek() very slow).
6220 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6221 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6222 C<seek>, C<tell>, or C<eof> may cause confusion.
6224 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6225 and C<SEEK_END> (start of the file, current position, end of the file)
6226 from the Fcntl module. Use of the constants is also more portable
6227 than relying on 0, 1, and 2. For example to define a "systell" function:
6229 use Fcntl 'SEEK_CUR';
6230 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6232 Returns the new position, or the undefined value on failure. A position
6233 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6234 true on success and false on failure, yet you can still easily determine
6240 =item system PROGRAM LIST
6242 Does exactly the same thing as C<exec LIST>, except that a fork is
6243 done first, and the parent process waits for the child process to
6244 complete. Note that argument processing varies depending on the
6245 number of arguments. If there is more than one argument in LIST,
6246 or if LIST is an array with more than one value, starts the program
6247 given by the first element of the list with arguments given by the
6248 rest of the list. If there is only one scalar argument, the argument
6249 is checked for shell metacharacters, and if there are any, the
6250 entire argument is passed to the system's command shell for parsing
6251 (this is C</bin/sh -c> on Unix platforms, but varies on other
6252 platforms). If there are no shell metacharacters in the argument,
6253 it is split into words and passed directly to C<execvp>, which is
6256 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6257 output before any operation that may do a fork, but this may not be
6258 supported on some platforms (see L<perlport>). To be safe, you may need
6259 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6260 of C<IO::Handle> on any open handles.
6262 The return value is the exit status of the program as returned by the
6263 C<wait> call. To get the actual exit value, shift right by eight (see
6264 below). See also L</exec>. This is I<not> what you want to use to capture
6265 the output from a command, for that you should use merely backticks or
6266 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6267 indicates a failure to start the program or an error of the wait(2) system
6268 call (inspect $! for the reason).
6270 Like C<exec>, C<system> allows you to lie to a program about its name if
6271 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6273 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6274 C<system>, if you expect your program to terminate on receipt of these
6275 signals you will need to arrange to do so yourself based on the return
6278 @args = ("command", "arg1", "arg2");
6280 or die "system @args failed: $?"
6282 You can check all the failure possibilities by inspecting
6286 print "failed to execute: $!\n";
6289 printf "child died with signal %d, %s coredump\n",
6290 ($? & 127), ($? & 128) ? 'with' : 'without';
6293 printf "child exited with value %d\n", $? >> 8;
6296 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6297 with the W*() calls of the POSIX extension.
6299 When the arguments get executed via the system shell, results
6300 and return codes will be subject to its quirks and capabilities.
6301 See L<perlop/"`STRING`"> and L</exec> for details.
6303 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6306 =item syswrite FILEHANDLE,SCALAR,LENGTH
6308 =item syswrite FILEHANDLE,SCALAR
6310 Attempts to write LENGTH bytes of data from variable SCALAR to the
6311 specified FILEHANDLE, using the system call write(2). If LENGTH is
6312 not specified, writes whole SCALAR. It bypasses buffered IO, so
6313 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6314 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6315 stdio layers usually buffers data. Returns the number of bytes
6316 actually written, or C<undef> if there was an error (in this case the
6317 errno variable C<$!> is also set). If the LENGTH is greater than the
6318 available data in the SCALAR after the OFFSET, only as much data as is
6319 available will be written.
6321 An OFFSET may be specified to write the data from some part of the
6322 string other than the beginning. A negative OFFSET specifies writing
6323 that many characters counting backwards from the end of the string.
6324 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6326 Note that if the filehandle has been marked as C<:utf8>, Unicode
6327 characters are written instead of bytes (the LENGTH, OFFSET, and the
6328 return value of syswrite() are in UTF-8 encoded Unicode characters).
6329 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6330 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6332 =item tell FILEHANDLE
6337 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6338 error. FILEHANDLE may be an expression whose value gives the name of
6339 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6342 Note the I<in bytes>: even if the filehandle has been set to
6343 operate on characters (for example by using the C<:utf8> open
6344 layer), tell() will return byte offsets, not character offsets
6345 (because that would render seek() and tell() rather slow).
6347 The return value of tell() for the standard streams like the STDIN
6348 depends on the operating system: it may return -1 or something else.
6349 tell() on pipes, fifos, and sockets usually returns -1.
6351 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6353 Do not use tell() (or other buffered I/O operations) on a file handle
6354 that has been manipulated by sysread(), syswrite() or sysseek().
6355 Those functions ignore the buffering, while tell() does not.
6357 =item telldir DIRHANDLE
6360 Returns the current position of the C<readdir> routines on DIRHANDLE.
6361 Value may be given to C<seekdir> to access a particular location in a
6362 directory. C<telldir> has the same caveats about possible directory
6363 compaction as the corresponding system library routine.
6365 =item tie VARIABLE,CLASSNAME,LIST
6368 This function binds a variable to a package class that will provide the
6369 implementation for the variable. VARIABLE is the name of the variable
6370 to be enchanted. CLASSNAME is the name of a class implementing objects
6371 of correct type. Any additional arguments are passed to the C<new>
6372 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6373 or C<TIEHASH>). Typically these are arguments such as might be passed
6374 to the C<dbm_open()> function of C. The object returned by the C<new>
6375 method is also returned by the C<tie> function, which would be useful
6376 if you want to access other methods in CLASSNAME.
6378 Note that functions such as C<keys> and C<values> may return huge lists
6379 when used on large objects, like DBM files. You may prefer to use the
6380 C<each> function to iterate over such. Example:
6382 # print out history file offsets
6384 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6385 while (($key,$val) = each %HIST) {
6386 print $key, ' = ', unpack('L',$val), "\n";
6390 A class implementing a hash should have the following methods:
6392 TIEHASH classname, LIST
6394 STORE this, key, value
6399 NEXTKEY this, lastkey
6404 A class implementing an ordinary array should have the following methods:
6406 TIEARRAY classname, LIST
6408 STORE this, key, value
6410 STORESIZE this, count
6416 SPLICE this, offset, length, LIST
6421 A class implementing a file handle should have the following methods:
6423 TIEHANDLE classname, LIST
6424 READ this, scalar, length, offset
6427 WRITE this, scalar, length, offset
6429 PRINTF this, format, LIST
6433 SEEK this, position, whence
6435 OPEN this, mode, LIST
6440 A class implementing a scalar should have the following methods:
6442 TIESCALAR classname, LIST
6448 Not all methods indicated above need be implemented. See L<perltie>,
6449 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6451 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6452 for you--you need to do that explicitly yourself. See L<DB_File>
6453 or the F<Config> module for interesting C<tie> implementations.
6455 For further details see L<perltie>, L<"tied VARIABLE">.
6460 Returns a reference to the object underlying VARIABLE (the same value
6461 that was originally returned by the C<tie> call that bound the variable
6462 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6468 Returns the number of non-leap seconds since whatever time the system
6469 considers to be the epoch, suitable for feeding to C<gmtime> and
6470 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6471 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6472 1904 in the current local time zone for its epoch.
6474 For measuring time in better granularity than one second,
6475 you may use either the Time::HiRes module (from CPAN, and starting from
6476 Perl 5.8 part of the standard distribution), or if you have
6477 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6478 See L<perlfaq8> for details.
6483 Returns a four-element list giving the user and system times, in
6484 seconds, for this process and the children of this process.
6486 ($user,$system,$cuser,$csystem) = times;
6488 In scalar context, C<times> returns C<$user>.
6492 The transliteration operator. Same as C<y///>. See L<perlop>.
6494 =item truncate FILEHANDLE,LENGTH
6497 =item truncate EXPR,LENGTH
6499 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6500 specified length. Produces a fatal error if truncate isn't implemented
6501 on your system. Returns true if successful, the undefined value
6504 The behavior is undefined if LENGTH is greater than the length of the
6508 X<uc> X<uppercase> X<toupper>
6512 Returns an uppercased version of EXPR. This is the internal function
6513 implementing the C<\U> escape in double-quoted strings. Respects
6514 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6515 and L<perlunicode> for more details about locale and Unicode support.
6516 It does not attempt to do titlecase mapping on initial letters. See
6517 C<ucfirst> for that.
6519 If EXPR is omitted, uses C<$_>.
6522 X<ucfirst> X<uppercase>
6526 Returns the value of EXPR with the first character in uppercase
6527 (titlecase in Unicode). This is the internal function implementing
6528 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6529 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6530 for more details about locale and Unicode support.
6532 If EXPR is omitted, uses C<$_>.
6539 Sets the umask for the process to EXPR and returns the previous value.
6540 If EXPR is omitted, merely returns the current umask.
6542 The Unix permission C<rwxr-x---> is represented as three sets of three
6543 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6544 and isn't one of the digits). The C<umask> value is such a number
6545 representing disabled permissions bits. The permission (or "mode")
6546 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6547 even if you tell C<sysopen> to create a file with permissions C<0777>,
6548 if your umask is C<0022> then the file will actually be created with
6549 permissions C<0755>. If your C<umask> were C<0027> (group can't
6550 write; others can't read, write, or execute), then passing
6551 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6554 Here's some advice: supply a creation mode of C<0666> for regular
6555 files (in C<sysopen>) and one of C<0777> for directories (in
6556 C<mkdir>) and executable files. This gives users the freedom of
6557 choice: if they want protected files, they might choose process umasks
6558 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6559 Programs should rarely if ever make policy decisions better left to
6560 the user. The exception to this is when writing files that should be
6561 kept private: mail files, web browser cookies, I<.rhosts> files, and
6564 If umask(2) is not implemented on your system and you are trying to
6565 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6566 fatal error at run time. If umask(2) is not implemented and you are
6567 not trying to restrict access for yourself, returns C<undef>.
6569 Remember that a umask is a number, usually given in octal; it is I<not> a
6570 string of octal digits. See also L</oct>, if all you have is a string.
6573 X<undef> X<undefine>
6577 Undefines the value of EXPR, which must be an lvalue. Use only on a
6578 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6579 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6580 will probably not do what you expect on most predefined variables or
6581 DBM list values, so don't do that; see L<delete>.) Always returns the
6582 undefined value. You can omit the EXPR, in which case nothing is
6583 undefined, but you still get an undefined value that you could, for
6584 instance, return from a subroutine, assign to a variable or pass as a
6585 parameter. Examples:
6588 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6592 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6593 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6594 select undef, undef, undef, 0.25;
6595 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6597 Note that this is a unary operator, not a list operator.
6600 X<unlink> X<delete> X<remove> X<rm>
6604 Deletes a list of files. Returns the number of files successfully
6607 $cnt = unlink 'a', 'b', 'c';
6611 Note: C<unlink> will not attempt to delete directories unless you are superuser
6612 and the B<-U> flag is supplied to Perl. Even if these conditions are
6613 met, be warned that unlinking a directory can inflict damage on your
6614 filesystem. Finally, using C<unlink> on directories is not supported on
6615 many operating systems. Use C<rmdir> instead.
6617 If LIST is omitted, uses C<$_>.
6619 =item unpack TEMPLATE,EXPR
6622 =item unpack TEMPLATE
6624 C<unpack> does the reverse of C<pack>: it takes a string
6625 and expands it out into a list of values.
6626 (In scalar context, it returns merely the first value produced.)
6628 If EXPR is omitted, unpacks the C<$_> string.
6630 The string is broken into chunks described by the TEMPLATE. Each chunk
6631 is converted separately to a value. Typically, either the string is a result
6632 of C<pack>, or the characters of the string represent a C structure of some
6635 The TEMPLATE has the same format as in the C<pack> function.
6636 Here's a subroutine that does substring:
6639 my($what,$where,$howmuch) = @_;
6640 unpack("x$where a$howmuch", $what);
6645 sub ordinal { unpack("W",$_[0]); } # same as ord()
6647 In addition to fields allowed in pack(), you may prefix a field with
6648 a %<number> to indicate that
6649 you want a <number>-bit checksum of the items instead of the items
6650 themselves. Default is a 16-bit checksum. Checksum is calculated by
6651 summing numeric values of expanded values (for string fields the sum of
6652 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6654 For example, the following
6655 computes the same number as the System V sum program:
6659 unpack("%32W*",<>) % 65535;
6662 The following efficiently counts the number of set bits in a bit vector:
6664 $setbits = unpack("%32b*", $selectmask);
6666 The C<p> and C<P> formats should be used with care. Since Perl
6667 has no way of checking whether the value passed to C<unpack()>
6668 corresponds to a valid memory location, passing a pointer value that's
6669 not known to be valid is likely to have disastrous consequences.
6671 If there are more pack codes or if the repeat count of a field or a group
6672 is larger than what the remainder of the input string allows, the result
6673 is not well defined: in some cases, the repeat count is decreased, or
6674 C<unpack()> will produce null strings or zeroes, or terminate with an
6675 error. If the input string is longer than one described by the TEMPLATE,
6676 the rest is ignored.
6678 See L</pack> for more examples and notes.
6680 =item untie VARIABLE
6683 Breaks the binding between a variable and a package. (See C<tie>.)
6684 Has no effect if the variable is not tied.
6686 =item unshift ARRAY,LIST
6689 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6690 depending on how you look at it. Prepends list to the front of the
6691 array, and returns the new number of elements in the array.
6693 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6695 Note the LIST is prepended whole, not one element at a time, so the
6696 prepended elements stay in the same order. Use C<reverse> to do the
6699 =item use Module VERSION LIST
6700 X<use> X<module> X<import>
6702 =item use Module VERSION
6704 =item use Module LIST
6710 Imports some semantics into the current package from the named module,
6711 generally by aliasing certain subroutine or variable names into your
6712 package. It is exactly equivalent to
6714 BEGIN { require Module; import Module LIST; }
6716 except that Module I<must> be a bareword.
6718 VERSION may be either a numeric argument such as 5.006, which will be
6719 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6720 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6721 greater than the version of the current Perl interpreter; Perl will not
6722 attempt to parse the rest of the file. Compare with L</require>, which can
6723 do a similar check at run time.
6725 Specifying VERSION as a literal of the form v5.6.1 should generally be
6726 avoided, because it leads to misleading error messages under earlier
6727 versions of Perl that do not support this syntax. The equivalent numeric
6728 version should be used instead.
6730 use v5.6.1; # compile time version check
6732 use 5.006_001; # ditto; preferred for backwards compatibility
6734 This is often useful if you need to check the current Perl version before
6735 C<use>ing library modules that have changed in incompatible ways from
6736 older versions of Perl. (We try not to do this more than we have to.)
6738 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6739 C<require> makes sure the module is loaded into memory if it hasn't been
6740 yet. The C<import> is not a builtin--it's just an ordinary static method
6741 call into the C<Module> package to tell the module to import the list of
6742 features back into the current package. The module can implement its
6743 C<import> method any way it likes, though most modules just choose to
6744 derive their C<import> method via inheritance from the C<Exporter> class that
6745 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6746 method can be found then the call is skipped, even if there is an AUTOLOAD
6749 If you do not want to call the package's C<import> method (for instance,
6750 to stop your namespace from being altered), explicitly supply the empty list:
6754 That is exactly equivalent to
6756 BEGIN { require Module }
6758 If the VERSION argument is present between Module and LIST, then the
6759 C<use> will call the VERSION method in class Module with the given
6760 version as an argument. The default VERSION method, inherited from
6761 the UNIVERSAL class, croaks if the given version is larger than the
6762 value of the variable C<$Module::VERSION>.
6764 Again, there is a distinction between omitting LIST (C<import> called
6765 with no arguments) and an explicit empty LIST C<()> (C<import> not
6766 called). Note that there is no comma after VERSION!
6768 Because this is a wide-open interface, pragmas (compiler directives)
6769 are also implemented this way. Currently implemented pragmas are:
6774 use sigtrap qw(SEGV BUS);
6775 use strict qw(subs vars refs);
6776 use subs qw(afunc blurfl);
6777 use warnings qw(all);
6778 use sort qw(stable _quicksort _mergesort);
6780 Some of these pseudo-modules import semantics into the current
6781 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6782 which import symbols into the current package (which are effective
6783 through the end of the file).
6785 There's a corresponding C<no> command that unimports meanings imported
6786 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6787 It behaves exactly as C<import> does with respect to VERSION, an
6788 omitted LIST, empty LIST, or no unimport method being found.
6794 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6795 for the C<-M> and C<-m> command-line options to perl that give C<use>
6796 functionality from the command-line.
6801 Changes the access and modification times on each file of a list of
6802 files. The first two elements of the list must be the NUMERICAL access
6803 and modification times, in that order. Returns the number of files
6804 successfully changed. The inode change time of each file is set
6805 to the current time. For example, this code has the same effect as the
6806 Unix touch(1) command when the files I<already exist> and belong to
6807 the user running the program:
6810 $atime = $mtime = time;
6811 utime $atime, $mtime, @ARGV;
6813 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6814 the utime(2) function in the C library will be called with a null second
6815 argument. On most systems, this will set the file's access and
6816 modification times to the current time (i.e. equivalent to the example
6817 above) and will even work on other users' files where you have write
6820 utime undef, undef, @ARGV;
6822 Under NFS this will use the time of the NFS server, not the time of
6823 the local machine. If there is a time synchronization problem, the
6824 NFS server and local machine will have different times. The Unix
6825 touch(1) command will in fact normally use this form instead of the
6826 one shown in the first example.
6828 Note that only passing one of the first two elements as C<undef> will
6829 be equivalent of passing it as 0 and will not have the same effect as
6830 described when they are both C<undef>. This case will also trigger an
6831 uninitialized warning.
6833 On systems that support futimes, you might pass file handles among the
6834 files. On systems that don't support futimes, passing file handles
6835 produces a fatal error at run time.
6840 Returns a list consisting of all the values of the named hash.
6841 (In a scalar context, returns the number of values.)
6843 The values are returned in an apparently random order. The actual
6844 random order is subject to change in future versions of perl, but it
6845 is guaranteed to be the same order as either the C<keys> or C<each>
6846 function would produce on the same (unmodified) hash. Since Perl
6847 5.8.1 the ordering is different even between different runs of Perl
6848 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6850 As a side effect, calling values() resets the HASH's internal iterator,
6851 see L</each>. (In particular, calling values() in void context resets
6852 the iterator with no other overhead.)
6854 Note that the values are not copied, which means modifying them will
6855 modify the contents of the hash:
6857 for (values %hash) { s/foo/bar/g } # modifies %hash values
6858 for (@hash{keys %hash}) { s/foo/bar/g } # same
6860 See also C<keys>, C<each>, and C<sort>.
6862 =item vec EXPR,OFFSET,BITS
6863 X<vec> X<bit> X<bit vector>
6865 Treats the string in EXPR as a bit vector made up of elements of
6866 width BITS, and returns the value of the element specified by OFFSET
6867 as an unsigned integer. BITS therefore specifies the number of bits
6868 that are reserved for each element in the bit vector. This must
6869 be a power of two from 1 to 32 (or 64, if your platform supports
6872 If BITS is 8, "elements" coincide with bytes of the input string.
6874 If BITS is 16 or more, bytes of the input string are grouped into chunks
6875 of size BITS/8, and each group is converted to a number as with
6876 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6877 for BITS==64). See L<"pack"> for details.
6879 If bits is 4 or less, the string is broken into bytes, then the bits
6880 of each byte are broken into 8/BITS groups. Bits of a byte are
6881 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6882 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6883 breaking the single input byte C<chr(0x36)> into two groups gives a list
6884 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6886 C<vec> may also be assigned to, in which case parentheses are needed
6887 to give the expression the correct precedence as in
6889 vec($image, $max_x * $x + $y, 8) = 3;
6891 If the selected element is outside the string, the value 0 is returned.
6892 If an element off the end of the string is written to, Perl will first
6893 extend the string with sufficiently many zero bytes. It is an error
6894 to try to write off the beginning of the string (i.e. negative OFFSET).
6896 The string should not contain any character with the value > 255 (which
6897 can only happen if you're using UTF-8 encoding). If it does, it will be
6898 treated as something that is not UTF-8 encoded. When the C<vec> was
6899 assigned to, other parts of your program will also no longer consider the
6900 string to be UTF-8 encoded. In other words, if you do have such characters
6901 in your string, vec() will operate on the actual byte string, and not the
6902 conceptual character string.
6904 Strings created with C<vec> can also be manipulated with the logical
6905 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6906 vector operation is desired when both operands are strings.
6907 See L<perlop/"Bitwise String Operators">.
6909 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6910 The comments show the string after each step. Note that this code works
6911 in the same way on big-endian or little-endian machines.
6914 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6916 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6917 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6919 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6920 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6921 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6922 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6923 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6924 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6926 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6927 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6928 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6931 To transform a bit vector into a string or list of 0's and 1's, use these:
6933 $bits = unpack("b*", $vector);
6934 @bits = split(//, unpack("b*", $vector));
6936 If you know the exact length in bits, it can be used in place of the C<*>.
6938 Here is an example to illustrate how the bits actually fall in place:
6944 unpack("V",$_) 01234567890123456789012345678901
6945 ------------------------------------------------------------------
6950 for ($shift=0; $shift < $width; ++$shift) {
6951 for ($off=0; $off < 32/$width; ++$off) {
6952 $str = pack("B*", "0"x32);
6953 $bits = (1<<$shift);
6954 vec($str, $off, $width) = $bits;
6955 $res = unpack("b*",$str);
6956 $val = unpack("V", $str);
6963 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6964 $off, $width, $bits, $val, $res
6968 Regardless of the machine architecture on which it is run, the above
6969 example should print the following table:
6972 unpack("V",$_) 01234567890123456789012345678901
6973 ------------------------------------------------------------------
6974 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6975 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6976 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6977 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6978 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6979 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6980 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6981 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6982 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6983 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6984 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6985 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6986 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6987 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6988 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6989 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6990 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6991 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6992 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6993 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6994 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6995 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6996 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6997 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6998 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6999 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7000 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7001 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7002 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7003 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7004 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7005 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7006 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7007 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7008 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7009 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7010 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7011 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7012 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7013 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7014 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7015 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7016 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7017 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7018 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7019 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7020 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7021 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7022 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7023 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7024 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7025 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7026 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7027 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7028 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7029 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7030 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7031 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7032 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7033 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7034 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7035 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7036 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7037 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7038 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7039 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7040 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7041 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7042 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7043 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7044 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7045 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7046 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7047 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7048 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7049 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7050 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7051 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7052 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7053 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7054 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7055 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7056 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7057 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7058 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7059 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7060 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7061 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7062 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7063 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7064 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7065 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7066 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7067 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7068 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7069 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7070 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7071 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7072 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7073 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7074 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7075 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7076 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7077 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7078 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7079 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7080 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7081 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7082 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7083 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7084 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7085 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7086 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7087 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7088 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7089 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7090 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7091 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7092 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7093 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7094 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7095 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7096 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7097 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7098 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7099 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7100 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7101 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7106 Behaves like the wait(2) system call on your system: it waits for a child
7107 process to terminate and returns the pid of the deceased process, or
7108 C<-1> if there are no child processes. The status is returned in C<$?>
7109 and C<{^CHILD_ERROR_NATIVE}>.
7110 Note that a return value of C<-1> could mean that child processes are
7111 being automatically reaped, as described in L<perlipc>.
7113 =item waitpid PID,FLAGS
7116 Waits for a particular child process to terminate and returns the pid of
7117 the deceased process, or C<-1> if there is no such child process. On some
7118 systems, a value of 0 indicates that there are processes still running.
7119 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7121 use POSIX ":sys_wait_h";
7124 $kid = waitpid(-1, WNOHANG);
7127 then you can do a non-blocking wait for all pending zombie processes.
7128 Non-blocking wait is available on machines supporting either the
7129 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7130 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7131 system call by remembering the status values of processes that have
7132 exited but have not been harvested by the Perl script yet.)
7134 Note that on some systems, a return value of C<-1> could mean that child
7135 processes are being automatically reaped. See L<perlipc> for details,
7136 and for other examples.
7139 X<wantarray> X<context>
7141 Returns true if the context of the currently executing subroutine or
7142 C<eval> is looking for a list value. Returns false if the context is
7143 looking for a scalar. Returns the undefined value if the context is
7144 looking for no value (void context).
7146 return unless defined wantarray; # don't bother doing more
7147 my @a = complex_calculation();
7148 return wantarray ? @a : "@a";
7150 C<wantarray()>'s result is unspecified in the top level of a file,
7151 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
7154 This function should have been named wantlist() instead.
7157 X<warn> X<warning> X<STDERR>
7159 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7162 If LIST is empty and C<$@> already contains a value (typically from a
7163 previous eval) that value is used after appending C<"\t...caught">
7164 to C<$@>. This is useful for staying almost, but not entirely similar to
7167 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7169 No message is printed if there is a C<$SIG{__WARN__}> handler
7170 installed. It is the handler's responsibility to deal with the message
7171 as it sees fit (like, for instance, converting it into a C<die>). Most
7172 handlers must therefore make arrangements to actually display the
7173 warnings that they are not prepared to deal with, by calling C<warn>
7174 again in the handler. Note that this is quite safe and will not
7175 produce an endless loop, since C<__WARN__> hooks are not called from
7178 You will find this behavior is slightly different from that of
7179 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7180 instead call C<die> again to change it).
7182 Using a C<__WARN__> handler provides a powerful way to silence all
7183 warnings (even the so-called mandatory ones). An example:
7185 # wipe out *all* compile-time warnings
7186 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7188 my $foo = 20; # no warning about duplicate my $foo,
7189 # but hey, you asked for it!
7190 # no compile-time or run-time warnings before here
7193 # run-time warnings enabled after here
7194 warn "\$foo is alive and $foo!"; # does show up
7196 See L<perlvar> for details on setting C<%SIG> entries, and for more
7197 examples. See the Carp module for other kinds of warnings using its
7198 carp() and cluck() functions.
7200 =item write FILEHANDLE
7207 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7208 using the format associated with that file. By default the format for
7209 a file is the one having the same name as the filehandle, but the
7210 format for the current output channel (see the C<select> function) may be set
7211 explicitly by assigning the name of the format to the C<$~> variable.
7213 Top of form processing is handled automatically: if there is
7214 insufficient room on the current page for the formatted record, the
7215 page is advanced by writing a form feed, a special top-of-page format
7216 is used to format the new page header, and then the record is written.
7217 By default the top-of-page format is the name of the filehandle with
7218 "_TOP" appended, but it may be dynamically set to the format of your
7219 choice by assigning the name to the C<$^> variable while the filehandle is
7220 selected. The number of lines remaining on the current page is in
7221 variable C<$->, which can be set to C<0> to force a new page.
7223 If FILEHANDLE is unspecified, output goes to the current default output
7224 channel, which starts out as STDOUT but may be changed by the
7225 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7226 is evaluated and the resulting string is used to look up the name of
7227 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7229 Note that write is I<not> the opposite of C<read>. Unfortunately.
7233 The transliteration operator. Same as C<tr///>. See L<perlop>.