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<lock>, C<map>, C<my>, C<no>, C<our>,
232 C<prototype>, C<qr>, C<qw>, C<qx>, C<readline>, C<readpipe>, C<ref>,
233 C<sub>*, C<sysopen>, C<tie>, 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. The file handles must be passed
692 as globs or references to be recognized. Barewords are considered
695 open(my $fh, "<", "foo");
696 my $perm = (stat $fh)[2] & 07777;
697 chmod($perm | 0600, $fh);
699 You can also import the symbolic C<S_I*> constants from the Fcntl
704 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
705 # This is identical to the chmod 0755 of the above example.
708 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
714 This safer version of L</chop> removes any trailing string
715 that corresponds to the current value of C<$/> (also known as
716 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
717 number of characters removed from all its arguments. It's often used to
718 remove the newline from the end of an input record when you're worried
719 that the final record may be missing its newline. When in paragraph
720 mode (C<$/ = "">), it removes all trailing newlines from the string.
721 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
722 a reference to an integer or the like, see L<perlvar>) chomp() won't
724 If VARIABLE is omitted, it chomps C<$_>. Example:
727 chomp; # avoid \n on last field
732 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
734 You can actually chomp anything that's an lvalue, including an assignment:
737 chomp($answer = <STDIN>);
739 If you chomp a list, each element is chomped, and the total number of
740 characters removed is returned.
742 If the C<encoding> pragma is in scope then the lengths returned are
743 calculated from the length of C<$/> in Unicode characters, which is not
744 always the same as the length of C<$/> in the native encoding.
746 Note that parentheses are necessary when you're chomping anything
747 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
748 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
749 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
750 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
760 Chops off the last character of a string and returns the character
761 chopped. It is much more efficient than C<s/.$//s> because it neither
762 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
763 If VARIABLE is a hash, it chops the hash's values, but not its keys.
765 You can actually chop anything that's an lvalue, including an assignment.
767 If you chop a list, each element is chopped. Only the value of the
768 last C<chop> is returned.
770 Note that C<chop> returns the last character. To return all but the last
771 character, use C<substr($string, 0, -1)>.
776 X<chown> X<owner> X<user> X<group>
778 Changes the owner (and group) of a list of files. The first two
779 elements of the list must be the I<numeric> uid and gid, in that
780 order. A value of -1 in either position is interpreted by most
781 systems to leave that value unchanged. Returns the number of files
782 successfully changed.
784 $cnt = chown $uid, $gid, 'foo', 'bar';
785 chown $uid, $gid, @filenames;
787 On systems that support fchown, you might pass file handles among the
788 files. On systems that don't support fchown, passing file handles
789 produces a fatal error at run time. The file handles must be passed
790 as globs or references to be recognized. Barewords are considered
793 Here's an example that looks up nonnumeric uids in the passwd file:
796 chomp($user = <STDIN>);
798 chomp($pattern = <STDIN>);
800 ($login,$pass,$uid,$gid) = getpwnam($user)
801 or die "$user not in passwd file";
803 @ary = glob($pattern); # expand filenames
804 chown $uid, $gid, @ary;
806 On most systems, you are not allowed to change the ownership of the
807 file unless you're the superuser, although you should be able to change
808 the group to any of your secondary groups. On insecure systems, these
809 restrictions may be relaxed, but this is not a portable assumption.
810 On POSIX systems, you can detect this condition this way:
812 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
813 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
816 X<chr> X<character> X<ASCII> X<Unicode>
820 Returns the character represented by that NUMBER in the character set.
821 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
822 chr(0x263a) is a Unicode smiley face. Note that characters from 128
823 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for
824 backward compatibility reasons (but see L<encoding>).
826 Negative values give the Unicode replacement character (chr(0xfffd)),
827 except under the L<bytes> pragma, where low eight bits of the value
828 (truncated to an integer) are used.
830 If NUMBER is omitted, uses C<$_>.
832 For the reverse, use L</ord>.
834 Note that under the C<bytes> pragma the NUMBER is masked to
837 See L<perlunicode> and L<encoding> for more about Unicode.
839 =item chroot FILENAME
844 This function works like the system call by the same name: it makes the
845 named directory the new root directory for all further pathnames that
846 begin with a C</> by your process and all its children. (It doesn't
847 change your current working directory, which is unaffected.) For security
848 reasons, this call is restricted to the superuser. If FILENAME is
849 omitted, does a C<chroot> to C<$_>.
851 =item close FILEHANDLE
856 Closes the file or pipe associated with the file handle, returning
857 true only if IO buffers are successfully flushed and closes the system
858 file descriptor. Closes the currently selected filehandle if the
861 You don't have to close FILEHANDLE if you are immediately going to do
862 another C<open> on it, because C<open> will close it for you. (See
863 C<open>.) However, an explicit C<close> on an input file resets the line
864 counter (C<$.>), while the implicit close done by C<open> does not.
866 If the file handle came from a piped open, C<close> will additionally
867 return false if one of the other system calls involved fails, or if the
868 program exits with non-zero status. (If the only problem was that the
869 program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
870 also waits for the process executing on the pipe to complete, in case you
871 want to look at the output of the pipe afterwards, and
872 implicitly puts the exit status value of that command into C<$?> and
873 C<${^CHILD_ERROR_NATIVE}>.
875 Prematurely closing the read end of a pipe (i.e. before the process
876 writing to it at the other end has closed it) will result in a
877 SIGPIPE being delivered to the writer. If the other end can't
878 handle that, be sure to read all the data before closing the pipe.
882 open(OUTPUT, '|sort >foo') # pipe to sort
883 or die "Can't start sort: $!";
884 #... # print stuff to output
885 close OUTPUT # wait for sort to finish
886 or warn $! ? "Error closing sort pipe: $!"
887 : "Exit status $? from sort";
888 open(INPUT, 'foo') # get sort's results
889 or die "Can't open 'foo' for input: $!";
891 FILEHANDLE may be an expression whose value can be used as an indirect
892 filehandle, usually the real filehandle name.
894 =item closedir DIRHANDLE
897 Closes a directory opened by C<opendir> and returns the success of that
900 =item connect SOCKET,NAME
903 Attempts to connect to a remote socket, just as the connect system call
904 does. Returns true if it succeeded, false otherwise. NAME should be a
905 packed address of the appropriate type for the socket. See the examples in
906 L<perlipc/"Sockets: Client/Server Communication">.
913 C<continue> is actually a flow control statement rather than a function. If
914 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
915 C<foreach>), it is always executed just before the conditional is about to
916 be evaluated again, just like the third part of a C<for> loop in C. Thus
917 it can be used to increment a loop variable, even when the loop has been
918 continued via the C<next> statement (which is similar to the C C<continue>
921 C<last>, C<next>, or C<redo> may appear within a C<continue>
922 block. C<last> and C<redo> will behave as if they had been executed within
923 the main block. So will C<next>, but since it will execute a C<continue>
924 block, it may be more entertaining.
927 ### redo always comes here
930 ### next always comes here
932 # then back the top to re-check EXPR
934 ### last always comes here
936 Omitting the C<continue> section is semantically equivalent to using an
937 empty one, logically enough. In that case, C<next> goes directly back
938 to check the condition at the top of the loop.
940 If the "switch" feature is enabled, C<continue> is also a
941 function that will break out of the current C<when> or C<default>
942 block, and fall through to the next case. See L<feature> and
943 L<perlsyn/"Switch statements"> for more information.
947 X<cos> X<cosine> X<acos> X<arccosine>
951 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
952 takes cosine of C<$_>.
954 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
955 function, or use this relation:
957 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
959 =item crypt PLAINTEXT,SALT
960 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
961 X<decrypt> X<cryptography> X<passwd>
963 Creates a digest string exactly like the crypt(3) function in the C
964 library (assuming that you actually have a version there that has not
965 been extirpated as a potential munitions).
967 crypt() is a one-way hash function. The PLAINTEXT and SALT is turned
968 into a short string, called a digest, which is returned. The same
969 PLAINTEXT and SALT will always return the same string, but there is no
970 (known) way to get the original PLAINTEXT from the hash. Small
971 changes in the PLAINTEXT or SALT will result in large changes in the
974 There is no decrypt function. This function isn't all that useful for
975 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
976 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
977 primarily used to check if two pieces of text are the same without
978 having to transmit or store the text itself. An example is checking
979 if a correct password is given. The digest of the password is stored,
980 not the password itself. The user types in a password that is
981 crypt()'d with the same salt as the stored digest. If the two digests
982 match the password is correct.
984 When verifying an existing digest string you should use the digest as
985 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
986 to create the digest is visible as part of the digest. This ensures
987 crypt() will hash the new string with the same salt as the digest.
988 This allows your code to work with the standard L<crypt|/crypt> and
989 with more exotic implementations. In other words, do not assume
990 anything about the returned string itself, or how many bytes in the
993 Traditionally the result is a string of 13 bytes: two first bytes of
994 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
995 the first eight bytes of the digest string mattered, but alternative
996 hashing schemes (like MD5), higher level security schemes (like C2),
997 and implementations on non-UNIX platforms may produce different
1000 When choosing a new salt create a random two character string whose
1001 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1002 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1003 characters is just a recommendation; the characters allowed in
1004 the salt depend solely on your system's crypt library, and Perl can't
1005 restrict what salts C<crypt()> accepts.
1007 Here's an example that makes sure that whoever runs this program knows
1010 $pwd = (getpwuid($<))[1];
1012 system "stty -echo";
1014 chomp($word = <STDIN>);
1018 if (crypt($word, $pwd) ne $pwd) {
1024 Of course, typing in your own password to whoever asks you
1027 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1028 of data, not least of all because you can't get the information
1029 back. Look at the L<Digest> module for more robust algorithms.
1031 If using crypt() on a Unicode string (which I<potentially> has
1032 characters with codepoints above 255), Perl tries to make sense
1033 of the situation by trying to downgrade (a copy of the string)
1034 the string back to an eight-bit byte string before calling crypt()
1035 (on that copy). If that works, good. If not, crypt() dies with
1036 C<Wide character in crypt>.
1041 [This function has been largely superseded by the C<untie> function.]
1043 Breaks the binding between a DBM file and a hash.
1045 =item dbmopen HASH,DBNAME,MASK
1046 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1048 [This function has been largely superseded by the C<tie> function.]
1050 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1051 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1052 argument is I<not> a filehandle, even though it looks like one). DBNAME
1053 is the name of the database (without the F<.dir> or F<.pag> extension if
1054 any). If the database does not exist, it is created with protection
1055 specified by MASK (as modified by the C<umask>). If your system supports
1056 only the older DBM functions, you may perform only one C<dbmopen> in your
1057 program. In older versions of Perl, if your system had neither DBM nor
1058 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1061 If you don't have write access to the DBM file, you can only read hash
1062 variables, not set them. If you want to test whether you can write,
1063 either use file tests or try setting a dummy hash entry inside an C<eval>,
1064 which will trap the error.
1066 Note that functions such as C<keys> and C<values> may return huge lists
1067 when used on large DBM files. You may prefer to use the C<each>
1068 function to iterate over large DBM files. Example:
1070 # print out history file offsets
1071 dbmopen(%HIST,'/usr/lib/news/history',0666);
1072 while (($key,$val) = each %HIST) {
1073 print $key, ' = ', unpack('L',$val), "\n";
1077 See also L<AnyDBM_File> for a more general description of the pros and
1078 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1079 rich implementation.
1081 You can control which DBM library you use by loading that library
1082 before you call dbmopen():
1085 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1086 or die "Can't open netscape history file: $!";
1089 X<defined> X<undef> X<undefined>
1093 Returns a Boolean value telling whether EXPR has a value other than
1094 the undefined value C<undef>. If EXPR is not present, C<$_> will be
1097 Many operations return C<undef> to indicate failure, end of file,
1098 system error, uninitialized variable, and other exceptional
1099 conditions. This function allows you to distinguish C<undef> from
1100 other values. (A simple Boolean test will not distinguish among
1101 C<undef>, zero, the empty string, and C<"0">, which are all equally
1102 false.) Note that since C<undef> is a valid scalar, its presence
1103 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1104 returns C<undef> when its argument is an empty array, I<or> when the
1105 element to return happens to be C<undef>.
1107 You may also use C<defined(&func)> to check whether subroutine C<&func>
1108 has ever been defined. The return value is unaffected by any forward
1109 declarations of C<&func>. Note that a subroutine which is not defined
1110 may still be callable: its package may have an C<AUTOLOAD> method that
1111 makes it spring into existence the first time that it is called -- see
1114 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1115 used to report whether memory for that aggregate has ever been
1116 allocated. This behavior may disappear in future versions of Perl.
1117 You should instead use a simple test for size:
1119 if (@an_array) { print "has array elements\n" }
1120 if (%a_hash) { print "has hash members\n" }
1122 When used on a hash element, it tells you whether the value is defined,
1123 not whether the key exists in the hash. Use L</exists> for the latter
1128 print if defined $switch{'D'};
1129 print "$val\n" while defined($val = pop(@ary));
1130 die "Can't readlink $sym: $!"
1131 unless defined($value = readlink $sym);
1132 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1133 $debugging = 0 unless defined $debugging;
1135 Note: Many folks tend to overuse C<defined>, and then are surprised to
1136 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1137 defined values. For example, if you say
1141 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1142 matched "nothing". It didn't really fail to match anything. Rather, it
1143 matched something that happened to be zero characters long. This is all
1144 very above-board and honest. When a function returns an undefined value,
1145 it's an admission that it couldn't give you an honest answer. So you
1146 should use C<defined> only when you're questioning the integrity of what
1147 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1150 See also L</undef>, L</exists>, L</ref>.
1155 Given an expression that specifies a hash element, array element, hash slice,
1156 or array slice, deletes the specified element(s) from the hash or array.
1157 In the case of an array, if the array elements happen to be at the end,
1158 the size of the array will shrink to the highest element that tests
1159 true for exists() (or 0 if no such element exists).
1161 Returns a list with the same number of elements as the number of elements
1162 for which deletion was attempted. Each element of that list consists of
1163 either the value of the element deleted, or the undefined value. In scalar
1164 context, this means that you get the value of the last element deleted (or
1165 the undefined value if that element did not exist).
1167 %hash = (foo => 11, bar => 22, baz => 33);
1168 $scalar = delete $hash{foo}; # $scalar is 11
1169 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1170 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1172 Deleting from C<%ENV> modifies the environment. Deleting from
1173 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1174 from a C<tie>d hash or array may not necessarily return anything.
1176 Deleting an array element effectively returns that position of the array
1177 to its initial, uninitialized state. Subsequently testing for the same
1178 element with exists() will return false. Also, deleting array elements
1179 in the middle of an array will not shift the index of the elements
1180 after them down. Use splice() for that. See L</exists>.
1182 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1184 foreach $key (keys %HASH) {
1188 foreach $index (0 .. $#ARRAY) {
1189 delete $ARRAY[$index];
1194 delete @HASH{keys %HASH};
1196 delete @ARRAY[0 .. $#ARRAY];
1198 But both of these are slower than just assigning the empty list
1199 or undefining %HASH or @ARRAY:
1201 %HASH = (); # completely empty %HASH
1202 undef %HASH; # forget %HASH ever existed
1204 @ARRAY = (); # completely empty @ARRAY
1205 undef @ARRAY; # forget @ARRAY ever existed
1207 Note that the EXPR can be arbitrarily complicated as long as the final
1208 operation is a hash element, array element, hash slice, or array slice
1211 delete $ref->[$x][$y]{$key};
1212 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1214 delete $ref->[$x][$y][$index];
1215 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1218 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1220 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1221 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1222 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1223 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1224 an C<eval(),> the error message is stuffed into C<$@> and the
1225 C<eval> is terminated with the undefined value. This makes
1226 C<die> the way to raise an exception.
1228 Equivalent examples:
1230 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1231 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1233 If the last element of LIST does not end in a newline, the current
1234 script line number and input line number (if any) are also printed,
1235 and a newline is supplied. Note that the "input line number" (also
1236 known as "chunk") is subject to whatever notion of "line" happens to
1237 be currently in effect, and is also available as the special variable
1238 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1240 Hint: sometimes appending C<", stopped"> to your message will cause it
1241 to make better sense when the string C<"at foo line 123"> is appended.
1242 Suppose you are running script "canasta".
1244 die "/etc/games is no good";
1245 die "/etc/games is no good, stopped";
1247 produce, respectively
1249 /etc/games is no good at canasta line 123.
1250 /etc/games is no good, stopped at canasta line 123.
1252 See also exit(), warn(), and the Carp module.
1254 If LIST is empty and C<$@> already contains a value (typically from a
1255 previous eval) that value is reused after appending C<"\t...propagated">.
1256 This is useful for propagating exceptions:
1259 die unless $@ =~ /Expected exception/;
1261 If LIST is empty and C<$@> contains an object reference that has a
1262 C<PROPAGATE> method, that method will be called with additional file
1263 and line number parameters. The return value replaces the value in
1264 C<$@>. i.e. as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1267 If C<$@> is empty then the string C<"Died"> is used.
1269 die() can also be called with a reference argument. If this happens to be
1270 trapped within an eval(), $@ contains the reference. This behavior permits
1271 a more elaborate exception handling implementation using objects that
1272 maintain arbitrary state about the nature of the exception. Such a scheme
1273 is sometimes preferable to matching particular string values of $@ using
1274 regular expressions. Here's an example:
1276 use Scalar::Util 'blessed';
1278 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1280 if (blessed($@) && $@->isa("Some::Module::Exception")) {
1281 # handle Some::Module::Exception
1284 # handle all other possible exceptions
1288 Because perl will stringify uncaught exception messages before displaying
1289 them, you may want to overload stringification operations on such custom
1290 exception objects. See L<overload> for details about that.
1292 You can arrange for a callback to be run just before the C<die>
1293 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1294 handler will be called with the error text and can change the error
1295 message, if it sees fit, by calling C<die> again. See
1296 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1297 L<"eval BLOCK"> for some examples. Although this feature was
1298 to be run only right before your program was to exit, this is not
1299 currently the case--the C<$SIG{__DIE__}> hook is currently called
1300 even inside eval()ed blocks/strings! If one wants the hook to do
1301 nothing in such situations, put
1305 as the first line of the handler (see L<perlvar/$^S>). Because
1306 this promotes strange action at a distance, this counterintuitive
1307 behavior may be fixed in a future release.
1312 Not really a function. Returns the value of the last command in the
1313 sequence of commands indicated by BLOCK. When modified by the C<while> or
1314 C<until> loop modifier, executes the BLOCK once before testing the loop
1315 condition. (On other statements the loop modifiers test the conditional
1318 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1319 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1320 See L<perlsyn> for alternative strategies.
1322 =item do SUBROUTINE(LIST)
1325 This form of subroutine call is deprecated. See L<perlsub>.
1330 Uses the value of EXPR as a filename and executes the contents of the
1331 file as a Perl script.
1339 except that it's more efficient and concise, keeps track of the current
1340 filename for error messages, searches the @INC directories, and updates
1341 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1342 variables. It also differs in that code evaluated with C<do FILENAME>
1343 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1344 same, however, in that it does reparse the file every time you call it,
1345 so you probably don't want to do this inside a loop.
1347 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1348 error. If C<do> can read the file but cannot compile it, it
1349 returns undef and sets an error message in C<$@>. If the file is
1350 successfully compiled, C<do> returns the value of the last expression
1353 Note that inclusion of library modules is better done with the
1354 C<use> and C<require> operators, which also do automatic error checking
1355 and raise an exception if there's a problem.
1357 You might like to use C<do> to read in a program configuration
1358 file. Manual error checking can be done this way:
1360 # read in config files: system first, then user
1361 for $file ("/share/prog/defaults.rc",
1362 "$ENV{HOME}/.someprogrc")
1364 unless ($return = do $file) {
1365 warn "couldn't parse $file: $@" if $@;
1366 warn "couldn't do $file: $!" unless defined $return;
1367 warn "couldn't run $file" unless $return;
1372 X<dump> X<core> X<undump>
1376 This function causes an immediate core dump. See also the B<-u>
1377 command-line switch in L<perlrun>, which does the same thing.
1378 Primarily this is so that you can use the B<undump> program (not
1379 supplied) to turn your core dump into an executable binary after
1380 having initialized all your variables at the beginning of the
1381 program. When the new binary is executed it will begin by executing
1382 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1383 Think of it as a goto with an intervening core dump and reincarnation.
1384 If C<LABEL> is omitted, restarts the program from the top.
1386 B<WARNING>: Any files opened at the time of the dump will I<not>
1387 be open any more when the program is reincarnated, with possible
1388 resulting confusion on the part of Perl.
1390 This function is now largely obsolete, partly because it's very
1391 hard to convert a core file into an executable, and because the
1392 real compiler backends for generating portable bytecode and compilable
1393 C code have superseded it. That's why you should now invoke it as
1394 C<CORE::dump()>, if you don't want to be warned against a possible
1397 If you're looking to use L<dump> to speed up your program, consider
1398 generating bytecode or native C code as described in L<perlcc>. If
1399 you're just trying to accelerate a CGI script, consider using the
1400 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1401 You might also consider autoloading or selfloading, which at least
1402 make your program I<appear> to run faster.
1405 X<each> X<hash, iterator>
1407 When called in list context, returns a 2-element list consisting of the
1408 key and value for the next element of a hash, so that you can iterate over
1409 it. When called in scalar context, returns only the key for the next
1410 element in the hash.
1412 Entries are returned in an apparently random order. The actual random
1413 order is subject to change in future versions of perl, but it is
1414 guaranteed to be in the same order as either the C<keys> or C<values>
1415 function would produce on the same (unmodified) hash. Since Perl
1416 5.8.1 the ordering is different even between different runs of Perl
1417 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1419 When the hash is entirely read, a null array is returned in list context
1420 (which when assigned produces a false (C<0>) value), and C<undef> in
1421 scalar context. The next call to C<each> after that will start iterating
1422 again. There is a single iterator for each hash, shared by all C<each>,
1423 C<keys>, and C<values> function calls in the program; it can be reset by
1424 reading all the elements from the hash, or by evaluating C<keys HASH> or
1425 C<values HASH>. If you add or delete elements of a hash while you're
1426 iterating over it, you may get entries skipped or duplicated, so
1427 don't. Exception: It is always safe to delete the item most recently
1428 returned by C<each()>, which means that the following code will work:
1430 while (($key, $value) = each %hash) {
1432 delete $hash{$key}; # This is safe
1435 The following prints out your environment like the printenv(1) program,
1436 only in a different order:
1438 while (($key,$value) = each %ENV) {
1439 print "$key=$value\n";
1442 See also C<keys>, C<values> and C<sort>.
1444 =item eof FILEHANDLE
1453 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1454 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1455 gives the real filehandle. (Note that this function actually
1456 reads a character and then C<ungetc>s it, so isn't very useful in an
1457 interactive context.) Do not read from a terminal file (or call
1458 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1459 as terminals may lose the end-of-file condition if you do.
1461 An C<eof> without an argument uses the last file read. Using C<eof()>
1462 with empty parentheses is very different. It refers to the pseudo file
1463 formed from the files listed on the command line and accessed via the
1464 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1465 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1466 used will cause C<@ARGV> to be examined to determine if input is
1467 available. Similarly, an C<eof()> after C<< <> >> has returned
1468 end-of-file will assume you are processing another C<@ARGV> list,
1469 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1470 see L<perlop/"I/O Operators">.
1472 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1473 detect the end of each file, C<eof()> will only detect the end of the
1474 last file. Examples:
1476 # reset line numbering on each input file
1478 next if /^\s*#/; # skip comments
1481 close ARGV if eof; # Not eof()!
1484 # insert dashes just before last line of last file
1486 if (eof()) { # check for end of last file
1487 print "--------------\n";
1490 last if eof(); # needed if we're reading from a terminal
1493 Practical hint: you almost never need to use C<eof> in Perl, because the
1494 input operators typically return C<undef> when they run out of data, or if
1498 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1504 In the first form, the return value of EXPR is parsed and executed as if it
1505 were a little Perl program. The value of the expression (which is itself
1506 determined within scalar context) is first parsed, and if there weren't any
1507 errors, executed in the lexical context of the current Perl program, so
1508 that any variable settings or subroutine and format definitions remain
1509 afterwards. Note that the value is parsed every time the C<eval> executes.
1510 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1511 delay parsing and subsequent execution of the text of EXPR until run time.
1513 In the second form, the code within the BLOCK is parsed only once--at the
1514 same time the code surrounding the C<eval> itself was parsed--and executed
1515 within the context of the current Perl program. This form is typically
1516 used to trap exceptions more efficiently than the first (see below), while
1517 also providing the benefit of checking the code within BLOCK at compile
1520 The final semicolon, if any, may be omitted from the value of EXPR or within
1523 In both forms, the value returned is the value of the last expression
1524 evaluated inside the mini-program; a return statement may be also used, just
1525 as with subroutines. The expression providing the return value is evaluated
1526 in void, scalar, or list context, depending on the context of the C<eval>
1527 itself. See L</wantarray> for more on how the evaluation context can be
1530 If there is a syntax error or runtime error, or a C<die> statement is
1531 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1532 error message. If there was no error, C<$@> is guaranteed to be a null
1533 string. Beware that using C<eval> neither silences perl from printing
1534 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1535 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1536 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1537 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1539 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1540 determining whether a particular feature (such as C<socket> or C<symlink>)
1541 is implemented. It is also Perl's exception trapping mechanism, where
1542 the die operator is used to raise exceptions.
1544 If the code to be executed doesn't vary, you may use the eval-BLOCK
1545 form to trap run-time errors without incurring the penalty of
1546 recompiling each time. The error, if any, is still returned in C<$@>.
1549 # make divide-by-zero nonfatal
1550 eval { $answer = $a / $b; }; warn $@ if $@;
1552 # same thing, but less efficient
1553 eval '$answer = $a / $b'; warn $@ if $@;
1555 # a compile-time error
1556 eval { $answer = }; # WRONG
1559 eval '$answer ='; # sets $@
1561 Using the C<eval{}> form as an exception trap in libraries does have some
1562 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1563 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1564 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1565 as shown in this example:
1567 # a very private exception trap for divide-by-zero
1568 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1571 This is especially significant, given that C<__DIE__> hooks can call
1572 C<die> again, which has the effect of changing their error messages:
1574 # __DIE__ hooks may modify error messages
1576 local $SIG{'__DIE__'} =
1577 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1578 eval { die "foo lives here" };
1579 print $@ if $@; # prints "bar lives here"
1582 Because this promotes action at a distance, this counterintuitive behavior
1583 may be fixed in a future release.
1585 With an C<eval>, you should be especially careful to remember what's
1586 being looked at when:
1592 eval { $x }; # CASE 4
1594 eval "\$$x++"; # CASE 5
1597 Cases 1 and 2 above behave identically: they run the code contained in
1598 the variable $x. (Although case 2 has misleading double quotes making
1599 the reader wonder what else might be happening (nothing is).) Cases 3
1600 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1601 does nothing but return the value of $x. (Case 4 is preferred for
1602 purely visual reasons, but it also has the advantage of compiling at
1603 compile-time instead of at run-time.) Case 5 is a place where
1604 normally you I<would> like to use double quotes, except that in this
1605 particular situation, you can just use symbolic references instead, as
1608 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1609 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1611 Note that as a very special case, an C<eval ''> executed within the C<DB>
1612 package doesn't see the usual surrounding lexical scope, but rather the
1613 scope of the first non-DB piece of code that called it. You don't normally
1614 need to worry about this unless you are writing a Perl debugger.
1619 =item exec PROGRAM LIST
1621 The C<exec> function executes a system command I<and never returns>--
1622 use C<system> instead of C<exec> if you want it to return. It fails and
1623 returns false only if the command does not exist I<and> it is executed
1624 directly instead of via your system's command shell (see below).
1626 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1627 warns you if there is a following statement which isn't C<die>, C<warn>,
1628 or C<exit> (if C<-w> is set - but you always do that). If you
1629 I<really> want to follow an C<exec> with some other statement, you
1630 can use one of these styles to avoid the warning:
1632 exec ('foo') or print STDERR "couldn't exec foo: $!";
1633 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1635 If there is more than one argument in LIST, or if LIST is an array
1636 with more than one value, calls execvp(3) with the arguments in LIST.
1637 If there is only one scalar argument or an array with one element in it,
1638 the argument is checked for shell metacharacters, and if there are any,
1639 the entire argument is passed to the system's command shell for parsing
1640 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1641 If there are no shell metacharacters in the argument, it is split into
1642 words and passed directly to C<execvp>, which is more efficient.
1645 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1646 exec "sort $outfile | uniq";
1648 If you don't really want to execute the first argument, but want to lie
1649 to the program you are executing about its own name, you can specify
1650 the program you actually want to run as an "indirect object" (without a
1651 comma) in front of the LIST. (This always forces interpretation of the
1652 LIST as a multivalued list, even if there is only a single scalar in
1655 $shell = '/bin/csh';
1656 exec $shell '-sh'; # pretend it's a login shell
1660 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1662 When the arguments get executed via the system shell, results will
1663 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1666 Using an indirect object with C<exec> or C<system> is also more
1667 secure. This usage (which also works fine with system()) forces
1668 interpretation of the arguments as a multivalued list, even if the
1669 list had just one argument. That way you're safe from the shell
1670 expanding wildcards or splitting up words with whitespace in them.
1672 @args = ( "echo surprise" );
1674 exec @args; # subject to shell escapes
1676 exec { $args[0] } @args; # safe even with one-arg list
1678 The first version, the one without the indirect object, ran the I<echo>
1679 program, passing it C<"surprise"> an argument. The second version
1680 didn't--it tried to run a program literally called I<"echo surprise">,
1681 didn't find it, and set C<$?> to a non-zero value indicating failure.
1683 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1684 output before the exec, but this may not be supported on some platforms
1685 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1686 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1687 open handles in order to avoid lost output.
1689 Note that C<exec> will not call your C<END> blocks, nor will it call
1690 any C<DESTROY> methods in your objects.
1693 X<exists> X<autovivification>
1695 Given an expression that specifies a hash element or array element,
1696 returns true if the specified element in the hash or array has ever
1697 been initialized, even if the corresponding value is undefined. The
1698 element is not autovivified if it doesn't exist.
1700 print "Exists\n" if exists $hash{$key};
1701 print "Defined\n" if defined $hash{$key};
1702 print "True\n" if $hash{$key};
1704 print "Exists\n" if exists $array[$index];
1705 print "Defined\n" if defined $array[$index];
1706 print "True\n" if $array[$index];
1708 A hash or array element can be true only if it's defined, and defined if
1709 it exists, but the reverse doesn't necessarily hold true.
1711 Given an expression that specifies the name of a subroutine,
1712 returns true if the specified subroutine has ever been declared, even
1713 if it is undefined. Mentioning a subroutine name for exists or defined
1714 does not count as declaring it. Note that a subroutine which does not
1715 exist may still be callable: its package may have an C<AUTOLOAD>
1716 method that makes it spring into existence the first time that it is
1717 called -- see L<perlsub>.
1719 print "Exists\n" if exists &subroutine;
1720 print "Defined\n" if defined &subroutine;
1722 Note that the EXPR can be arbitrarily complicated as long as the final
1723 operation is a hash or array key lookup or subroutine name:
1725 if (exists $ref->{A}->{B}->{$key}) { }
1726 if (exists $hash{A}{B}{$key}) { }
1728 if (exists $ref->{A}->{B}->[$ix]) { }
1729 if (exists $hash{A}{B}[$ix]) { }
1731 if (exists &{$ref->{A}{B}{$key}}) { }
1733 Although the deepest nested array or hash will not spring into existence
1734 just because its existence was tested, any intervening ones will.
1735 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1736 into existence due to the existence test for the $key element above.
1737 This happens anywhere the arrow operator is used, including even:
1740 if (exists $ref->{"Some key"}) { }
1741 print $ref; # prints HASH(0x80d3d5c)
1743 This surprising autovivification in what does not at first--or even
1744 second--glance appear to be an lvalue context may be fixed in a future
1747 Use of a subroutine call, rather than a subroutine name, as an argument
1748 to exists() is an error.
1751 exists &sub(); # Error
1754 X<exit> X<terminate> X<abort>
1758 Evaluates EXPR and exits immediately with that value. Example:
1761 exit 0 if $ans =~ /^[Xx]/;
1763 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1764 universally recognized values for EXPR are C<0> for success and C<1>
1765 for error; other values are subject to interpretation depending on the
1766 environment in which the Perl program is running. For example, exiting
1767 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1768 the mailer to return the item undelivered, but that's not true everywhere.
1770 Don't use C<exit> to abort a subroutine if there's any chance that
1771 someone might want to trap whatever error happened. Use C<die> instead,
1772 which can be trapped by an C<eval>.
1774 The exit() function does not always exit immediately. It calls any
1775 defined C<END> routines first, but these C<END> routines may not
1776 themselves abort the exit. Likewise any object destructors that need to
1777 be called are called before the real exit. If this is a problem, you
1778 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1779 See L<perlmod> for details.
1782 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1786 Returns I<e> (the natural logarithm base) to the power of EXPR.
1787 If EXPR is omitted, gives C<exp($_)>.
1789 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1792 Implements the fcntl(2) function. You'll probably have to say
1796 first to get the correct constant definitions. Argument processing and
1797 value return works just like C<ioctl> below.
1801 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1802 or die "can't fcntl F_GETFL: $!";
1804 You don't have to check for C<defined> on the return from C<fcntl>.
1805 Like C<ioctl>, it maps a C<0> return from the system call into
1806 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1807 in numeric context. It is also exempt from the normal B<-w> warnings
1808 on improper numeric conversions.
1810 Note that C<fcntl> will produce a fatal error if used on a machine that
1811 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1812 manpage to learn what functions are available on your system.
1814 Here's an example of setting a filehandle named C<REMOTE> to be
1815 non-blocking at the system level. You'll have to negotiate C<$|>
1816 on your own, though.
1818 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1820 $flags = fcntl(REMOTE, F_GETFL, 0)
1821 or die "Can't get flags for the socket: $!\n";
1823 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1824 or die "Can't set flags for the socket: $!\n";
1826 =item fileno FILEHANDLE
1829 Returns the file descriptor for a filehandle, or undefined if the
1830 filehandle is not open. This is mainly useful for constructing
1831 bitmaps for C<select> and low-level POSIX tty-handling operations.
1832 If FILEHANDLE is an expression, the value is taken as an indirect
1833 filehandle, generally its name.
1835 You can use this to find out whether two handles refer to the
1836 same underlying descriptor:
1838 if (fileno(THIS) == fileno(THAT)) {
1839 print "THIS and THAT are dups\n";
1842 (Filehandles connected to memory objects via new features of C<open> may
1843 return undefined even though they are open.)
1846 =item flock FILEHANDLE,OPERATION
1847 X<flock> X<lock> X<locking>
1849 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1850 for success, false on failure. Produces a fatal error if used on a
1851 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1852 C<flock> is Perl's portable file locking interface, although it locks
1853 only entire files, not records.
1855 Two potentially non-obvious but traditional C<flock> semantics are
1856 that it waits indefinitely until the lock is granted, and that its locks
1857 B<merely advisory>. Such discretionary locks are more flexible, but offer
1858 fewer guarantees. This means that programs that do not also use C<flock>
1859 may modify files locked with C<flock>. See L<perlport>,
1860 your port's specific documentation, or your system-specific local manpages
1861 for details. It's best to assume traditional behavior if you're writing
1862 portable programs. (But if you're not, you should as always feel perfectly
1863 free to write for your own system's idiosyncrasies (sometimes called
1864 "features"). Slavish adherence to portability concerns shouldn't get
1865 in the way of your getting your job done.)
1867 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1868 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1869 you can use the symbolic names if you import them from the Fcntl module,
1870 either individually, or as a group using the ':flock' tag. LOCK_SH
1871 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1872 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1873 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1874 waiting for the lock (check the return status to see if you got it).
1876 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1877 before locking or unlocking it.
1879 Note that the emulation built with lockf(3) doesn't provide shared
1880 locks, and it requires that FILEHANDLE be open with write intent. These
1881 are the semantics that lockf(3) implements. Most if not all systems
1882 implement lockf(3) in terms of fcntl(2) locking, though, so the
1883 differing semantics shouldn't bite too many people.
1885 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1886 be open with read intent to use LOCK_SH and requires that it be open
1887 with write intent to use LOCK_EX.
1889 Note also that some versions of C<flock> cannot lock things over the
1890 network; you would need to use the more system-specific C<fcntl> for
1891 that. If you like you can force Perl to ignore your system's flock(2)
1892 function, and so provide its own fcntl(2)-based emulation, by passing
1893 the switch C<-Ud_flock> to the F<Configure> program when you configure
1896 Here's a mailbox appender for BSD systems.
1898 use Fcntl ':flock'; # import LOCK_* constants
1901 flock(MBOX,LOCK_EX);
1902 # and, in case someone appended
1903 # while we were waiting...
1908 flock(MBOX,LOCK_UN);
1911 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1912 or die "Can't open mailbox: $!";
1915 print MBOX $msg,"\n\n";
1918 On systems that support a real flock(), locks are inherited across fork()
1919 calls, whereas those that must resort to the more capricious fcntl()
1920 function lose the locks, making it harder to write servers.
1922 See also L<DB_File> for other flock() examples.
1925 X<fork> X<child> X<parent>
1927 Does a fork(2) system call to create a new process running the
1928 same program at the same point. It returns the child pid to the
1929 parent process, C<0> to the child process, or C<undef> if the fork is
1930 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1931 are shared, while everything else is copied. On most systems supporting
1932 fork(), great care has gone into making it extremely efficient (for
1933 example, using copy-on-write technology on data pages), making it the
1934 dominant paradigm for multitasking over the last few decades.
1936 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1937 output before forking the child process, but this may not be supported
1938 on some platforms (see L<perlport>). To be safe, you may need to set
1939 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1940 C<IO::Handle> on any open handles in order to avoid duplicate output.
1942 If you C<fork> without ever waiting on your children, you will
1943 accumulate zombies. On some systems, you can avoid this by setting
1944 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1945 forking and reaping moribund children.
1947 Note that if your forked child inherits system file descriptors like
1948 STDIN and STDOUT that are actually connected by a pipe or socket, even
1949 if you exit, then the remote server (such as, say, a CGI script or a
1950 backgrounded job launched from a remote shell) won't think you're done.
1951 You should reopen those to F</dev/null> if it's any issue.
1956 Declare a picture format for use by the C<write> function. For
1960 Test: @<<<<<<<< @||||| @>>>>>
1961 $str, $%, '$' . int($num)
1965 $num = $cost/$quantity;
1969 See L<perlform> for many details and examples.
1971 =item formline PICTURE,LIST
1974 This is an internal function used by C<format>s, though you may call it,
1975 too. It formats (see L<perlform>) a list of values according to the
1976 contents of PICTURE, placing the output into the format output
1977 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1978 Eventually, when a C<write> is done, the contents of
1979 C<$^A> are written to some filehandle. You could also read C<$^A>
1980 and then set C<$^A> back to C<"">. Note that a format typically
1981 does one C<formline> per line of form, but the C<formline> function itself
1982 doesn't care how many newlines are embedded in the PICTURE. This means
1983 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1984 You may therefore need to use multiple formlines to implement a single
1985 record format, just like the format compiler.
1987 Be careful if you put double quotes around the picture, because an C<@>
1988 character may be taken to mean the beginning of an array name.
1989 C<formline> always returns true. See L<perlform> for other examples.
1991 =item getc FILEHANDLE
1996 Returns the next character from the input file attached to FILEHANDLE,
1997 or the undefined value at end of file, or if there was an error (in
1998 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1999 STDIN. This is not particularly efficient. However, it cannot be
2000 used by itself to fetch single characters without waiting for the user
2001 to hit enter. For that, try something more like:
2004 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2007 system "stty", '-icanon', 'eol', "\001";
2013 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2016 system "stty", 'icanon', 'eol', '^@'; # ASCII null
2020 Determination of whether $BSD_STYLE should be set
2021 is left as an exercise to the reader.
2023 The C<POSIX::getattr> function can do this more portably on
2024 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2025 module from your nearest CPAN site; details on CPAN can be found on
2029 X<getlogin> X<login>
2031 This implements the C library function of the same name, which on most
2032 systems returns the current login from F</etc/utmp>, if any. If null,
2035 $login = getlogin || getpwuid($<) || "Kilroy";
2037 Do not consider C<getlogin> for authentication: it is not as
2038 secure as C<getpwuid>.
2040 =item getpeername SOCKET
2041 X<getpeername> X<peer>
2043 Returns the packed sockaddr address of other end of the SOCKET connection.
2046 $hersockaddr = getpeername(SOCK);
2047 ($port, $iaddr) = sockaddr_in($hersockaddr);
2048 $herhostname = gethostbyaddr($iaddr, AF_INET);
2049 $herstraddr = inet_ntoa($iaddr);
2054 Returns the current process group for the specified PID. Use
2055 a PID of C<0> to get the current process group for the
2056 current process. Will raise an exception if used on a machine that
2057 doesn't implement getpgrp(2). If PID is omitted, returns process
2058 group of current process. Note that the POSIX version of C<getpgrp>
2059 does not accept a PID argument, so only C<PID==0> is truly portable.
2062 X<getppid> X<parent> X<pid>
2064 Returns the process id of the parent process.
2066 Note for Linux users: on Linux, the C functions C<getpid()> and
2067 C<getppid()> return different values from different threads. In order to
2068 be portable, this behavior is not reflected by the perl-level function
2069 C<getppid()>, that returns a consistent value across threads. If you want
2070 to call the underlying C<getppid()>, you may use the CPAN module
2073 =item getpriority WHICH,WHO
2074 X<getpriority> X<priority> X<nice>
2076 Returns the current priority for a process, a process group, or a user.
2077 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2078 machine that doesn't implement getpriority(2).
2081 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2082 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2083 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2084 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2085 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2086 X<endnetent> X<endprotoent> X<endservent>
2090 =item gethostbyname NAME
2092 =item getnetbyname NAME
2094 =item getprotobyname NAME
2100 =item getservbyname NAME,PROTO
2102 =item gethostbyaddr ADDR,ADDRTYPE
2104 =item getnetbyaddr ADDR,ADDRTYPE
2106 =item getprotobynumber NUMBER
2108 =item getservbyport PORT,PROTO
2126 =item sethostent STAYOPEN
2128 =item setnetent STAYOPEN
2130 =item setprotoent STAYOPEN
2132 =item setservent STAYOPEN
2146 These routines perform the same functions as their counterparts in the
2147 system library. In list context, the return values from the
2148 various get routines are as follows:
2150 ($name,$passwd,$uid,$gid,
2151 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2152 ($name,$passwd,$gid,$members) = getgr*
2153 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2154 ($name,$aliases,$addrtype,$net) = getnet*
2155 ($name,$aliases,$proto) = getproto*
2156 ($name,$aliases,$port,$proto) = getserv*
2158 (If the entry doesn't exist you get a null list.)
2160 The exact meaning of the $gcos field varies but it usually contains
2161 the real name of the user (as opposed to the login name) and other
2162 information pertaining to the user. Beware, however, that in many
2163 system users are able to change this information and therefore it
2164 cannot be trusted and therefore the $gcos is tainted (see
2165 L<perlsec>). The $passwd and $shell, user's encrypted password and
2166 login shell, are also tainted, because of the same reason.
2168 In scalar context, you get the name, unless the function was a
2169 lookup by name, in which case you get the other thing, whatever it is.
2170 (If the entry doesn't exist you get the undefined value.) For example:
2172 $uid = getpwnam($name);
2173 $name = getpwuid($num);
2175 $gid = getgrnam($name);
2176 $name = getgrgid($num);
2180 In I<getpw*()> the fields $quota, $comment, and $expire are special
2181 cases in the sense that in many systems they are unsupported. If the
2182 $quota is unsupported, it is an empty scalar. If it is supported, it
2183 usually encodes the disk quota. If the $comment field is unsupported,
2184 it is an empty scalar. If it is supported it usually encodes some
2185 administrative comment about the user. In some systems the $quota
2186 field may be $change or $age, fields that have to do with password
2187 aging. In some systems the $comment field may be $class. The $expire
2188 field, if present, encodes the expiration period of the account or the
2189 password. For the availability and the exact meaning of these fields
2190 in your system, please consult your getpwnam(3) documentation and your
2191 F<pwd.h> file. You can also find out from within Perl what your
2192 $quota and $comment fields mean and whether you have the $expire field
2193 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2194 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2195 files are only supported if your vendor has implemented them in the
2196 intuitive fashion that calling the regular C library routines gets the
2197 shadow versions if you're running under privilege or if there exists
2198 the shadow(3) functions as found in System V (this includes Solaris
2199 and Linux.) Those systems that implement a proprietary shadow password
2200 facility are unlikely to be supported.
2202 The $members value returned by I<getgr*()> is a space separated list of
2203 the login names of the members of the group.
2205 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2206 C, it will be returned to you via C<$?> if the function call fails. The
2207 C<@addrs> value returned by a successful call is a list of the raw
2208 addresses returned by the corresponding system library call. In the
2209 Internet domain, each address is four bytes long and you can unpack it
2210 by saying something like:
2212 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2214 The Socket library makes this slightly easier:
2217 $iaddr = inet_aton("127.1"); # or whatever address
2218 $name = gethostbyaddr($iaddr, AF_INET);
2220 # or going the other way
2221 $straddr = inet_ntoa($iaddr);
2223 If you get tired of remembering which element of the return list
2224 contains which return value, by-name interfaces are provided
2225 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2226 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2227 and C<User::grent>. These override the normal built-ins, supplying
2228 versions that return objects with the appropriate names
2229 for each field. For example:
2233 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2235 Even though it looks like they're the same method calls (uid),
2236 they aren't, because a C<File::stat> object is different from
2237 a C<User::pwent> object.
2239 =item getsockname SOCKET
2242 Returns the packed sockaddr address of this end of the SOCKET connection,
2243 in case you don't know the address because you have several different
2244 IPs that the connection might have come in on.
2247 $mysockaddr = getsockname(SOCK);
2248 ($port, $myaddr) = sockaddr_in($mysockaddr);
2249 printf "Connect to %s [%s]\n",
2250 scalar gethostbyaddr($myaddr, AF_INET),
2253 =item getsockopt SOCKET,LEVEL,OPTNAME
2256 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2257 Options may exist at multiple protocol levels depending on the socket
2258 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2259 C<Socket> module) will exist. To query options at another level the
2260 protocol number of the appropriate protocol controlling the option
2261 should be supplied. For example, to indicate that an option is to be
2262 interpreted by the TCP protocol, LEVEL should be set to the protocol
2263 number of TCP, which you can get using getprotobyname.
2265 The call returns a packed string representing the requested socket option,
2266 or C<undef> if there is an error (the error reason will be in $!). What
2267 exactly is in the packed string depends in the LEVEL and OPTNAME, consult
2268 your system documentation for details. A very common case however is that
2269 the option is an integer, in which case the result will be a packed
2270 integer which you can decode using unpack with the C<i> (or C<I>) format.
2272 An example testing if Nagle's algorithm is turned on on a socket:
2274 use Socket qw(:all);
2276 defined(my $tcp = getprotobyname("tcp"))
2277 or die "Could not determine the protocol number for tcp";
2278 # my $tcp = IPPROTO_TCP; # Alternative
2279 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2280 or die "Could not query TCP_NODELAY socket option: $!";
2281 my $nodelay = unpack("I", $packed);
2282 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2286 X<glob> X<wildcard> X<filename, expansion> X<expand>
2290 In list context, returns a (possibly empty) list of filename expansions on
2291 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2292 scalar context, glob iterates through such filename expansions, returning
2293 undef when the list is exhausted. This is the internal function
2294 implementing the C<< <*.c> >> operator, but you can use it directly. If
2295 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2296 more detail in L<perlop/"I/O Operators">.
2298 Beginning with v5.6.0, this operator is implemented using the standard
2299 C<File::Glob> extension. See L<File::Glob> for details.
2302 X<gmtime> X<UTC> X<Greenwich>
2306 Converts a time as returned by the time function to an 9-element list
2307 with the time localized for the standard Greenwich time zone.
2308 Typically used as follows:
2311 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2314 All list elements are numeric, and come straight out of the C `struct
2315 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2316 specified time. $mday is the day of the month, and $mon is the month
2317 itself, in the range C<0..11> with 0 indicating January and 11
2318 indicating December. $year is the number of years since 1900. That
2319 is, $year is C<123> in year 2023. $wday is the day of the week, with
2320 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2321 the year, in the range C<0..364> (or C<0..365> in leap years). $isdst
2324 Note that the $year element is I<not> simply the last two digits of
2325 the year. If you assume it is then you create non-Y2K-compliant
2326 programs--and you wouldn't want to do that, would you?
2328 The proper way to get a complete 4-digit year is simply:
2332 And to get the last two digits of the year (e.g., '01' in 2001) do:
2334 $year = sprintf("%02d", $year % 100);
2336 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2338 In scalar context, C<gmtime()> returns the ctime(3) value:
2340 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2342 If you need local time instead of GMT use the L</localtime> builtin.
2343 See also the C<timegm> function provided by the C<Time::Local> module,
2344 and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
2346 This scalar value is B<not> locale dependent (see L<perllocale>), but is
2347 instead a Perl builtin. To get somewhat similar but locale dependent date
2348 strings, see the example in L</localtime>.
2350 See L<perlport/gmtime> for portability concerns.
2353 X<goto> X<jump> X<jmp>
2359 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2360 execution there. It may not be used to go into any construct that
2361 requires initialization, such as a subroutine or a C<foreach> loop. It
2362 also can't be used to go into a construct that is optimized away,
2363 or to get out of a block or subroutine given to C<sort>.
2364 It can be used to go almost anywhere else within the dynamic scope,
2365 including out of subroutines, but it's usually better to use some other
2366 construct such as C<last> or C<die>. The author of Perl has never felt the
2367 need to use this form of C<goto> (in Perl, that is--C is another matter).
2368 (The difference being that C does not offer named loops combined with
2369 loop control. Perl does, and this replaces most structured uses of C<goto>
2370 in other languages.)
2372 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2373 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2374 necessarily recommended if you're optimizing for maintainability:
2376 goto ("FOO", "BAR", "GLARCH")[$i];
2378 The C<goto-&NAME> form is quite different from the other forms of
2379 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2380 doesn't have the stigma associated with other gotos. Instead, it
2381 exits the current subroutine (losing any changes set by local()) and
2382 immediately calls in its place the named subroutine using the current
2383 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2384 load another subroutine and then pretend that the other subroutine had
2385 been called in the first place (except that any modifications to C<@_>
2386 in the current subroutine are propagated to the other subroutine.)
2387 After the C<goto>, not even C<caller> will be able to tell that this
2388 routine was called first.
2390 NAME needn't be the name of a subroutine; it can be a scalar variable
2391 containing a code reference, or a block that evaluates to a code
2394 =item grep BLOCK LIST
2397 =item grep EXPR,LIST
2399 This is similar in spirit to, but not the same as, grep(1) and its
2400 relatives. In particular, it is not limited to using regular expressions.
2402 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2403 C<$_> to each element) and returns the list value consisting of those
2404 elements for which the expression evaluated to true. In scalar
2405 context, returns the number of times the expression was true.
2407 @foo = grep(!/^#/, @bar); # weed out comments
2411 @foo = grep {!/^#/} @bar; # weed out comments
2413 Note that C<$_> is an alias to the list value, so it can be used to
2414 modify the elements of the LIST. While this is useful and supported,
2415 it can cause bizarre results if the elements of LIST are not variables.
2416 Similarly, grep returns aliases into the original list, much as a for
2417 loop's index variable aliases the list elements. That is, modifying an
2418 element of a list returned by grep (for example, in a C<foreach>, C<map>
2419 or another C<grep>) actually modifies the element in the original list.
2420 This is usually something to be avoided when writing clear code.
2422 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2423 been declared with C<my $_>) then, in addition to being locally aliased to
2424 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2425 can't be seen from the outside, avoiding any potential side-effects.
2427 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2430 X<hex> X<hexadecimal>
2434 Interprets EXPR as a hex string and returns the corresponding value.
2435 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2436 L</oct>.) If EXPR is omitted, uses C<$_>.
2438 print hex '0xAf'; # prints '175'
2439 print hex 'aF'; # same
2441 Hex strings may only represent integers. Strings that would cause
2442 integer overflow trigger a warning. Leading whitespace is not stripped,
2443 unlike oct(). To present something as hex, look into L</printf>,
2444 L</sprintf>, or L</unpack>.
2449 There is no builtin C<import> function. It is just an ordinary
2450 method (subroutine) defined (or inherited) by modules that wish to export
2451 names to another module. The C<use> function calls the C<import> method
2452 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2454 =item index STR,SUBSTR,POSITION
2455 X<index> X<indexOf> X<InStr>
2457 =item index STR,SUBSTR
2459 The index function searches for one string within another, but without
2460 the wildcard-like behavior of a full regular-expression pattern match.
2461 It returns the position of the first occurrence of SUBSTR in STR at
2462 or after POSITION. If POSITION is omitted, starts searching from the
2463 beginning of the string. POSITION before the beginning of the string
2464 or after its end is treated as if it were the beginning or the end,
2465 respectively. POSITION and the return value are based at C<0> (or whatever
2466 you've set the C<$[> variable to--but don't do that). If the substring
2467 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2470 X<int> X<integer> X<truncate> X<trunc>
2474 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2475 You should not use this function for rounding: one because it truncates
2476 towards C<0>, and two because machine representations of floating point
2477 numbers can sometimes produce counterintuitive results. For example,
2478 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2479 because it's really more like -268.99999999999994315658 instead. Usually,
2480 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2481 functions will serve you better than will int().
2483 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2486 Implements the ioctl(2) function. You'll probably first have to say
2488 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2490 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2491 exist or doesn't have the correct definitions you'll have to roll your
2492 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2493 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2494 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2495 written depending on the FUNCTION--a pointer to the string value of SCALAR
2496 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2497 has no string value but does have a numeric value, that value will be
2498 passed rather than a pointer to the string value. To guarantee this to be
2499 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2500 functions may be needed to manipulate the values of structures used by
2503 The return value of C<ioctl> (and C<fcntl>) is as follows:
2505 if OS returns: then Perl returns:
2507 0 string "0 but true"
2508 anything else that number
2510 Thus Perl returns true on success and false on failure, yet you can
2511 still easily determine the actual value returned by the operating
2514 $retval = ioctl(...) || -1;
2515 printf "System returned %d\n", $retval;
2517 The special string C<"0 but true"> is exempt from B<-w> complaints
2518 about improper numeric conversions.
2520 =item join EXPR,LIST
2523 Joins the separate strings of LIST into a single string with fields
2524 separated by the value of EXPR, and returns that new string. Example:
2526 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2528 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2529 first argument. Compare L</split>.
2534 Returns a list consisting of all the keys of the named hash.
2535 (In scalar context, returns the number of keys.)
2537 The keys are returned in an apparently random order. The actual
2538 random order is subject to change in future versions of perl, but it
2539 is guaranteed to be the same order as either the C<values> or C<each>
2540 function produces (given that the hash has not been modified). Since
2541 Perl 5.8.1 the ordering is different even between different runs of
2542 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2545 As a side effect, calling keys() resets the HASH's internal iterator
2546 (see L</each>). In particular, calling keys() in void context resets
2547 the iterator with no other overhead.
2549 Here is yet another way to print your environment:
2552 @values = values %ENV;
2554 print pop(@keys), '=', pop(@values), "\n";
2557 or how about sorted by key:
2559 foreach $key (sort(keys %ENV)) {
2560 print $key, '=', $ENV{$key}, "\n";
2563 The returned values are copies of the original keys in the hash, so
2564 modifying them will not affect the original hash. Compare L</values>.
2566 To sort a hash by value, you'll need to use a C<sort> function.
2567 Here's a descending numeric sort of a hash by its values:
2569 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2570 printf "%4d %s\n", $hash{$key}, $key;
2573 As an lvalue C<keys> allows you to increase the number of hash buckets
2574 allocated for the given hash. This can gain you a measure of efficiency if
2575 you know the hash is going to get big. (This is similar to pre-extending
2576 an array by assigning a larger number to $#array.) If you say
2580 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2581 in fact, since it rounds up to the next power of two. These
2582 buckets will be retained even if you do C<%hash = ()>, use C<undef
2583 %hash> if you want to free the storage while C<%hash> is still in scope.
2584 You can't shrink the number of buckets allocated for the hash using
2585 C<keys> in this way (but you needn't worry about doing this by accident,
2586 as trying has no effect).
2588 See also C<each>, C<values> and C<sort>.
2590 =item kill SIGNAL, LIST
2593 Sends a signal to a list of processes. Returns the number of
2594 processes successfully signaled (which is not necessarily the
2595 same as the number actually killed).
2597 $cnt = kill 1, $child1, $child2;
2600 If SIGNAL is zero, no signal is sent to the process, but the kill(2)
2601 system call will check whether it's possible to send a signal to it (that
2602 means, to be brief, that the process is owned by the same user, or we are
2603 the super-user). This is a useful way to check that a child process is
2604 alive and hasn't changed its UID. See L<perlport> for notes on the
2605 portability of this construct.
2607 Unlike in the shell, if SIGNAL is negative, it kills
2608 process groups instead of processes. (On System V, a negative I<PROCESS>
2609 number will also kill process groups, but that's not portable.) That
2610 means you usually want to use positive not negative signals. You may also
2611 use a signal name in quotes.
2613 See L<perlipc/"Signals"> for more details.
2620 The C<last> command is like the C<break> statement in C (as used in
2621 loops); it immediately exits the loop in question. If the LABEL is
2622 omitted, the command refers to the innermost enclosing loop. The
2623 C<continue> block, if any, is not executed:
2625 LINE: while (<STDIN>) {
2626 last LINE if /^$/; # exit when done with header
2630 C<last> cannot be used to exit a block which returns a value such as
2631 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2632 a grep() or map() operation.
2634 Note that a block by itself is semantically identical to a loop
2635 that executes once. Thus C<last> can be used to effect an early
2636 exit out of such a block.
2638 See also L</continue> for an illustration of how C<last>, C<next>, and
2646 Returns a lowercased version of EXPR. This is the internal function
2647 implementing the C<\L> escape in double-quoted strings. Respects
2648 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2649 and L<perlunicode> for more details about locale and Unicode support.
2651 If EXPR is omitted, uses C<$_>.
2654 X<lcfirst> X<lowercase>
2658 Returns the value of EXPR with the first character lowercased. This
2659 is the internal function implementing the C<\l> escape in
2660 double-quoted strings. Respects current LC_CTYPE locale if C<use
2661 locale> in force. See L<perllocale> and L<perlunicode> for more
2662 details about locale and Unicode support.
2664 If EXPR is omitted, uses C<$_>.
2671 Returns the length in I<characters> of the value of EXPR. If EXPR is
2672 omitted, returns length of C<$_>. Note that this cannot be used on
2673 an entire array or hash to find out how many elements these have.
2674 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2676 Note the I<characters>: if the EXPR is in Unicode, you will get the
2677 number of characters, not the number of bytes. To get the length
2678 in bytes, use C<do { use bytes; length(EXPR) }>, see L<bytes>.
2680 =item link OLDFILE,NEWFILE
2683 Creates a new filename linked to the old filename. Returns true for
2684 success, false otherwise.
2686 =item listen SOCKET,QUEUESIZE
2689 Does the same thing that the listen system call does. Returns true if
2690 it succeeded, false otherwise. See the example in
2691 L<perlipc/"Sockets: Client/Server Communication">.
2696 You really probably want to be using C<my> instead, because C<local> isn't
2697 what most people think of as "local". See
2698 L<perlsub/"Private Variables via my()"> for details.
2700 A local modifies the listed variables to be local to the enclosing
2701 block, file, or eval. If more than one value is listed, the list must
2702 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2703 for details, including issues with tied arrays and hashes.
2705 =item localtime EXPR
2710 Converts a time as returned by the time function to a 9-element list
2711 with the time analyzed for the local time zone. Typically used as
2715 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2718 All list elements are numeric, and come straight out of the C `struct
2719 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2720 of the specified time.
2722 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2723 the range C<0..11> with 0 indicating January and 11 indicating December.
2724 This makes it easy to get a month name from a list:
2726 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2727 print "$abbr[$mon] $mday";
2728 # $mon=9, $mday=18 gives "Oct 18"
2730 C<$year> is the number of years since 1900, not just the last two digits
2731 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2732 to get a complete 4-digit year is simply:
2736 To get the last two digits of the year (e.g., '01' in 2001) do:
2738 $year = sprintf("%02d", $year % 100);
2740 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2741 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2742 (or C<0..365> in leap years.)
2744 C<$isdst> is true if the specified time occurs during Daylight Saving
2745 Time, false otherwise.
2747 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2749 In scalar context, C<localtime()> returns the ctime(3) value:
2751 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2753 This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
2754 instead of local time use the L</gmtime> builtin. See also the
2755 C<Time::Local> module (to convert the second, minutes, hours, ... back to
2756 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2757 and mktime(3) functions.
2759 To get somewhat similar but locale dependent date strings, set up your
2760 locale environment variables appropriately (please see L<perllocale>) and
2763 use POSIX qw(strftime);
2764 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2765 # or for GMT formatted appropriately for your locale:
2766 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2768 Note that the C<%a> and C<%b>, the short forms of the day of the week
2769 and the month of the year, may not necessarily be three characters wide.
2771 See L<perlport/localtime> for portability concerns.
2776 This function places an advisory lock on a shared variable, or referenced
2777 object contained in I<THING> until the lock goes out of scope.
2779 lock() is a "weak keyword" : this means that if you've defined a function
2780 by this name (before any calls to it), that function will be called
2781 instead. (However, if you've said C<use threads>, lock() is always a
2782 keyword.) See L<threads>.
2785 X<log> X<logarithm> X<e> X<ln> X<base>
2789 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2790 returns log of C<$_>. To get the log of another base, use basic algebra:
2791 The base-N log of a number is equal to the natural log of that number
2792 divided by the natural log of N. For example:
2796 return log($n)/log(10);
2799 See also L</exp> for the inverse operation.
2806 Does the same thing as the C<stat> function (including setting the
2807 special C<_> filehandle) but stats a symbolic link instead of the file
2808 the symbolic link points to. If symbolic links are unimplemented on
2809 your system, a normal C<stat> is done. For much more detailed
2810 information, please see the documentation for C<stat>.
2812 If EXPR is omitted, stats C<$_>.
2816 The match operator. See L<perlop>.
2818 =item map BLOCK LIST
2823 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2824 C<$_> to each element) and returns the list value composed of the
2825 results of each such evaluation. In scalar context, returns the
2826 total number of elements so generated. Evaluates BLOCK or EXPR in
2827 list context, so each element of LIST may produce zero, one, or
2828 more elements in the returned value.
2830 @chars = map(chr, @nums);
2832 translates a list of numbers to the corresponding characters. And
2834 %hash = map { getkey($_) => $_ } @array;
2836 is just a funny way to write
2839 foreach $_ (@array) {
2840 $hash{getkey($_)} = $_;
2843 Note that C<$_> is an alias to the list value, so it can be used to
2844 modify the elements of the LIST. While this is useful and supported,
2845 it can cause bizarre results if the elements of LIST are not variables.
2846 Using a regular C<foreach> loop for this purpose would be clearer in
2847 most cases. See also L</grep> for an array composed of those items of
2848 the original list for which the BLOCK or EXPR evaluates to true.
2850 If C<$_> is lexical in the scope where the C<map> appears (because it has
2851 been declared with C<my $_>) then, in addition to being locally aliased to
2852 the list elements, C<$_> keeps being lexical inside the block; i.e. it
2853 can't be seen from the outside, avoiding any potential side-effects.
2855 C<{> starts both hash references and blocks, so C<map { ...> could be either
2856 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2857 ahead for the closing C<}> it has to take a guess at which its dealing with
2858 based what it finds just after the C<{>. Usually it gets it right, but if it
2859 doesn't it won't realize something is wrong until it gets to the C<}> and
2860 encounters the missing (or unexpected) comma. The syntax error will be
2861 reported close to the C<}> but you'll need to change something near the C<{>
2862 such as using a unary C<+> to give perl some help:
2864 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2865 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2866 %hash = map { ("\L$_", 1) } @array # this also works
2867 %hash = map { lc($_), 1 } @array # as does this.
2868 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2870 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2872 or to force an anon hash constructor use C<+{>
2874 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2876 and you get list of anonymous hashes each with only 1 entry.
2878 =item mkdir FILENAME,MASK
2879 X<mkdir> X<md> X<directory, create>
2881 =item mkdir FILENAME
2885 Creates the directory specified by FILENAME, with permissions
2886 specified by MASK (as modified by C<umask>). If it succeeds it
2887 returns true, otherwise it returns false and sets C<$!> (errno).
2888 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
2891 In general, it is better to create directories with permissive MASK,
2892 and let the user modify that with their C<umask>, than it is to supply
2893 a restrictive MASK and give the user no way to be more permissive.
2894 The exceptions to this rule are when the file or directory should be
2895 kept private (mail files, for instance). The perlfunc(1) entry on
2896 C<umask> discusses the choice of MASK in more detail.
2898 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2899 number of trailing slashes. Some operating and filesystems do not get
2900 this right, so Perl automatically removes all trailing slashes to keep
2903 =item msgctl ID,CMD,ARG
2906 Calls the System V IPC function msgctl(2). You'll probably have to say
2910 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2911 then ARG must be a variable that will hold the returned C<msqid_ds>
2912 structure. Returns like C<ioctl>: the undefined value for error,
2913 C<"0 but true"> for zero, or the actual return value otherwise. See also
2914 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2916 =item msgget KEY,FLAGS
2919 Calls the System V IPC function msgget(2). Returns the message queue
2920 id, or the undefined value if there is an error. See also
2921 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2923 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2926 Calls the System V IPC function msgrcv to receive a message from
2927 message queue ID into variable VAR with a maximum message size of
2928 SIZE. Note that when a message is received, the message type as a
2929 native long integer will be the first thing in VAR, followed by the
2930 actual message. This packing may be opened with C<unpack("l! a*")>.
2931 Taints the variable. Returns true if successful, or false if there is
2932 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2933 C<IPC::SysV::Msg> documentation.
2935 =item msgsnd ID,MSG,FLAGS
2938 Calls the System V IPC function msgsnd to send the message MSG to the
2939 message queue ID. MSG must begin with the native long integer message
2940 type, and be followed by the length of the actual message, and finally
2941 the message itself. This kind of packing can be achieved with
2942 C<pack("l! a*", $type, $message)>. Returns true if successful,
2943 or false if there is an error. See also C<IPC::SysV>
2944 and C<IPC::SysV::Msg> documentation.
2951 =item my EXPR : ATTRS
2953 =item my TYPE EXPR : ATTRS
2955 A C<my> declares the listed variables to be local (lexically) to the
2956 enclosing block, file, or C<eval>. If more than one value is listed,
2957 the list must be placed in parentheses.
2959 The exact semantics and interface of TYPE and ATTRS are still
2960 evolving. TYPE is currently bound to the use of C<fields> pragma,
2961 and attributes are handled using the C<attributes> pragma, or starting
2962 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2963 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2964 L<attributes>, and L<Attribute::Handlers>.
2971 The C<next> command is like the C<continue> statement in C; it starts
2972 the next iteration of the loop:
2974 LINE: while (<STDIN>) {
2975 next LINE if /^#/; # discard comments
2979 Note that if there were a C<continue> block on the above, it would get
2980 executed even on discarded lines. If the LABEL is omitted, the command
2981 refers to the innermost enclosing loop.
2983 C<next> cannot be used to exit a block which returns a value such as
2984 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2985 a grep() or map() operation.
2987 Note that a block by itself is semantically identical to a loop
2988 that executes once. Thus C<next> will exit such a block early.
2990 See also L</continue> for an illustration of how C<last>, C<next>, and
2993 =item no Module VERSION LIST
2996 =item no Module VERSION
2998 =item no Module LIST
3002 See the C<use> function, of which C<no> is the opposite.
3005 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3009 Interprets EXPR as an octal string and returns the corresponding
3010 value. (If EXPR happens to start off with C<0x>, interprets it as a
3011 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3012 binary string. Leading whitespace is ignored in all three cases.)
3013 The following will handle decimal, binary, octal, and hex in the standard
3016 $val = oct($val) if $val =~ /^0/;
3018 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3019 in octal), use sprintf() or printf():
3021 $perms = (stat("filename"))[2] & 07777;
3022 $oct_perms = sprintf "%lo", $perms;
3024 The oct() function is commonly used when a string such as C<644> needs
3025 to be converted into a file mode, for example. (Although perl will
3026 automatically convert strings into numbers as needed, this automatic
3027 conversion assumes base 10.)
3029 =item open FILEHANDLE,EXPR
3030 X<open> X<pipe> X<file, open> X<fopen>
3032 =item open FILEHANDLE,MODE,EXPR
3034 =item open FILEHANDLE,MODE,EXPR,LIST
3036 =item open FILEHANDLE,MODE,REFERENCE
3038 =item open FILEHANDLE
3040 Opens the file whose filename is given by EXPR, and associates it with
3043 (The following is a comprehensive reference to open(): for a gentler
3044 introduction you may consider L<perlopentut>.)
3046 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3047 the variable is assigned a reference to a new anonymous filehandle,
3048 otherwise if FILEHANDLE is an expression, its value is used as the name of
3049 the real filehandle wanted. (This is considered a symbolic reference, so
3050 C<use strict 'refs'> should I<not> be in effect.)
3052 If EXPR is omitted, the scalar variable of the same name as the
3053 FILEHANDLE contains the filename. (Note that lexical variables--those
3054 declared with C<my>--will not work for this purpose; so if you're
3055 using C<my>, specify EXPR in your call to open.)
3057 If three or more arguments are specified then the mode of opening and
3058 the file name are separate. If MODE is C<< '<' >> or nothing, the file
3059 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3060 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3061 the file is opened for appending, again being created if necessary.
3063 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3064 indicate that you want both read and write access to the file; thus
3065 C<< '+<' >> is almost always preferred for read/write updates--the C<<
3066 '+>' >> mode would clobber the file first. You can't usually use
3067 either read-write mode for updating textfiles, since they have
3068 variable length records. See the B<-i> switch in L<perlrun> for a
3069 better approach. The file is created with permissions of C<0666>
3070 modified by the process' C<umask> value.
3072 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3073 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3075 In the 2-arguments (and 1-argument) form of the call the mode and
3076 filename should be concatenated (in this order), possibly separated by
3077 spaces. It is possible to omit the mode in these forms if the mode is
3080 If the filename begins with C<'|'>, the filename is interpreted as a
3081 command to which output is to be piped, and if the filename ends with a
3082 C<'|'>, the filename is interpreted as a command which pipes output to
3083 us. See L<perlipc/"Using open() for IPC">
3084 for more examples of this. (You are not allowed to C<open> to a command
3085 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
3086 and L<perlipc/"Bidirectional Communication with Another Process">
3089 For three or more arguments if MODE is C<'|-'>, the filename is
3090 interpreted as a command to which output is to be piped, and if MODE
3091 is C<'-|'>, the filename is interpreted as a command which pipes
3092 output to us. In the 2-arguments (and 1-argument) form one should
3093 replace dash (C<'-'>) with the command.
3094 See L<perlipc/"Using open() for IPC"> for more examples of this.
3095 (You are not allowed to C<open> to a command that pipes both in I<and>
3096 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3097 L<perlipc/"Bidirectional Communication"> for alternatives.)
3099 In the three-or-more argument form of pipe opens, if LIST is specified
3100 (extra arguments after the command name) then LIST becomes arguments
3101 to the command invoked if the platform supports it. The meaning of
3102 C<open> with more than three arguments for non-pipe modes is not yet
3103 specified. Experimental "layers" may give extra LIST arguments
3106 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
3107 and opening C<< '>-' >> opens STDOUT.
3109 You may use the three-argument form of open to specify IO "layers"
3110 (sometimes also referred to as "disciplines") to be applied to the handle
3111 that affect how the input and output are processed (see L<open> and
3112 L<PerlIO> for more details). For example
3114 open(FH, "<:utf8", "file")
3116 will open the UTF-8 encoded file containing Unicode characters,
3117 see L<perluniintro>. Note that if layers are specified in the
3118 three-arg form then default layers stored in ${^OPEN} (see L<perlvar>;
3119 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3121 Open returns nonzero upon success, the undefined value otherwise. If
3122 the C<open> involved a pipe, the return value happens to be the pid of
3125 If you're running Perl on a system that distinguishes between text
3126 files and binary files, then you should check out L</binmode> for tips
3127 for dealing with this. The key distinction between systems that need
3128 C<binmode> and those that don't is their text file formats. Systems
3129 like Unix, Mac OS, and Plan 9, which delimit lines with a single
3130 character, and which encode that character in C as C<"\n">, do not
3131 need C<binmode>. The rest need it.
3133 When opening a file, it's usually a bad idea to continue normal execution
3134 if the request failed, so C<open> is frequently used in connection with
3135 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3136 where you want to make a nicely formatted error message (but there are
3137 modules that can help with that problem)) you should always check
3138 the return value from opening a file. The infrequent exception is when
3139 working with an unopened filehandle is actually what you want to do.
3141 As a special case the 3-arg form with a read/write mode and the third
3142 argument being C<undef>:
3144 open(TMP, "+>", undef) or die ...
3146 opens a filehandle to an anonymous temporary file. Also using "+<"
3147 works for symmetry, but you really should consider writing something
3148 to the temporary file first. You will need to seek() to do the
3151 Since v5.8.0, perl has built using PerlIO by default. Unless you've
3152 changed this (i.e. Configure -Uuseperlio), you can open file handles to
3153 "in memory" files held in Perl scalars via:
3155 open($fh, '>', \$variable) || ..
3157 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
3158 file, you have to close it first:
3161 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3166 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3167 while (<ARTICLE>) {...
3169 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3170 # if the open fails, output is discarded
3172 open(DBASE, '+<', 'dbase.mine') # open for update
3173 or die "Can't open 'dbase.mine' for update: $!";
3175 open(DBASE, '+<dbase.mine') # ditto
3176 or die "Can't open 'dbase.mine' for update: $!";
3178 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3179 or die "Can't start caesar: $!";
3181 open(ARTICLE, "caesar <$article |") # ditto
3182 or die "Can't start caesar: $!";
3184 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3185 or die "Can't start sort: $!";
3188 open(MEMORY,'>', \$var)
3189 or die "Can't open memory file: $!";
3190 print MEMORY "foo!\n"; # output will end up in $var
3192 # process argument list of files along with any includes
3194 foreach $file (@ARGV) {
3195 process($file, 'fh00');
3199 my($filename, $input) = @_;
3200 $input++; # this is a string increment
3201 unless (open($input, $filename)) {
3202 print STDERR "Can't open $filename: $!\n";
3207 while (<$input>) { # note use of indirection
3208 if (/^#include "(.*)"/) {
3209 process($1, $input);
3216 See L<perliol> for detailed info on PerlIO.
3218 You may also, in the Bourne shell tradition, specify an EXPR beginning
3219 with C<< '>&' >>, in which case the rest of the string is interpreted
3220 as the name of a filehandle (or file descriptor, if numeric) to be
3221 duped (as L<dup(2)>) and opened. You may use C<&> after C<< > >>,
3222 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3223 The mode you specify should match the mode of the original filehandle.
3224 (Duping a filehandle does not take into account any existing contents
3225 of IO buffers.) If you use the 3-arg form then you can pass either a
3226 number, the name of a filehandle or the normal "reference to a glob".
3228 Here is a script that saves, redirects, and restores C<STDOUT> and
3229 C<STDERR> using various methods:
3232 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3233 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3235 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3236 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3238 select STDERR; $| = 1; # make unbuffered
3239 select STDOUT; $| = 1; # make unbuffered
3241 print STDOUT "stdout 1\n"; # this works for
3242 print STDERR "stderr 1\n"; # subprocesses too
3244 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3245 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3247 print STDOUT "stdout 2\n";
3248 print STDERR "stderr 2\n";
3250 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3251 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3252 that file descriptor (and not call L<dup(2)>); this is more
3253 parsimonious of file descriptors. For example:
3255 # open for input, reusing the fileno of $fd
3256 open(FILEHANDLE, "<&=$fd")
3260 open(FILEHANDLE, "<&=", $fd)
3264 # open for append, using the fileno of OLDFH
3265 open(FH, ">>&=", OLDFH)
3269 open(FH, ">>&=OLDFH")
3271 Being parsimonious on filehandles is also useful (besides being
3272 parsimonious) for example when something is dependent on file
3273 descriptors, like for example locking using flock(). If you do just
3274 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3275 descriptor as B, and therefore flock(A) will not flock(B), and vice
3276 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3277 the same file descriptor.
3279 Note that if you are using Perls older than 5.8.0, Perl will be using
3280 the standard C libraries' fdopen() to implement the "=" functionality.
3281 On many UNIX systems fdopen() fails when file descriptors exceed a
3282 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3283 most often the default.
3285 You can see whether Perl has been compiled with PerlIO or not by
3286 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
3287 is C<define>, you have PerlIO, otherwise you don't.
3289 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3290 with 2-arguments (or 1-argument) form of open(), then
3291 there is an implicit fork done, and the return value of open is the pid
3292 of the child within the parent process, and C<0> within the child
3293 process. (Use C<defined($pid)> to determine whether the open was successful.)
3294 The filehandle behaves normally for the parent, but i/o to that
3295 filehandle is piped from/to the STDOUT/STDIN of the child process.
3296 In the child process the filehandle isn't opened--i/o happens from/to
3297 the new STDOUT or STDIN. Typically this is used like the normal
3298 piped open when you want to exercise more control over just how the
3299 pipe command gets executed, such as when you are running setuid, and
3300 don't want to have to scan shell commands for metacharacters.
3301 The following triples are more or less equivalent:
3303 open(FOO, "|tr '[a-z]' '[A-Z]'");
3304 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3305 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3306 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3308 open(FOO, "cat -n '$file'|");
3309 open(FOO, '-|', "cat -n '$file'");
3310 open(FOO, '-|') || exec 'cat', '-n', $file;
3311 open(FOO, '-|', "cat", '-n', $file);
3313 The last example in each block shows the pipe as "list form", which is
3314 not yet supported on all platforms. A good rule of thumb is that if
3315 your platform has true C<fork()> (in other words, if your platform is
3316 UNIX) you can use the list form.
3318 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3320 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3321 output before any operation that may do a fork, but this may not be
3322 supported on some platforms (see L<perlport>). To be safe, you may need
3323 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3324 of C<IO::Handle> on any open handles.
3326 On systems that support a close-on-exec flag on files, the flag will
3327 be set for the newly opened file descriptor as determined by the value
3328 of $^F. See L<perlvar/$^F>.
3330 Closing any piped filehandle causes the parent process to wait for the
3331 child to finish, and returns the status value in C<$?> and
3332 C<${^CHILD_ERROR_NATIVE}>.
3334 The filename passed to 2-argument (or 1-argument) form of open() will
3335 have leading and trailing whitespace deleted, and the normal
3336 redirection characters honored. This property, known as "magic open",
3337 can often be used to good effect. A user could specify a filename of
3338 F<"rsh cat file |">, or you could change certain filenames as needed:
3340 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3341 open(FH, $filename) or die "Can't open $filename: $!";
3343 Use 3-argument form to open a file with arbitrary weird characters in it,
3345 open(FOO, '<', $file);
3347 otherwise it's necessary to protect any leading and trailing whitespace:
3349 $file =~ s#^(\s)#./$1#;
3350 open(FOO, "< $file\0");
3352 (this may not work on some bizarre filesystems). One should
3353 conscientiously choose between the I<magic> and 3-arguments form
3358 will allow the user to specify an argument of the form C<"rsh cat file |">,
3359 but will not work on a filename which happens to have a trailing space, while
3361 open IN, '<', $ARGV[0];
3363 will have exactly the opposite restrictions.
3365 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3366 should use the C<sysopen> function, which involves no such magic (but
3367 may use subtly different filemodes than Perl open(), which is mapped
3368 to C fopen()). This is
3369 another way to protect your filenames from interpretation. For example:
3372 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3373 or die "sysopen $path: $!";
3374 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3375 print HANDLE "stuff $$\n";
3377 print "File contains: ", <HANDLE>;
3379 Using the constructor from the C<IO::Handle> package (or one of its
3380 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3381 filehandles that have the scope of whatever variables hold references to
3382 them, and automatically close whenever and however you leave that scope:
3386 sub read_myfile_munged {
3388 my $handle = new IO::File;
3389 open($handle, "myfile") or die "myfile: $!";
3391 or return (); # Automatically closed here.
3392 mung $first or die "mung failed"; # Or here.
3393 return $first, <$handle> if $ALL; # Or here.
3397 See L</seek> for some details about mixing reading and writing.
3399 =item opendir DIRHANDLE,EXPR
3402 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3403 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3404 DIRHANDLE may be an expression whose value can be used as an indirect
3405 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3406 scalar variable (or array or hash element), the variable is assigned a
3407 reference to a new anonymous dirhandle.
3408 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3415 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3416 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3419 For the reverse, see L</chr>.
3420 See L<perlunicode> and L<encoding> for more about Unicode.
3427 =item our EXPR : ATTRS
3429 =item our TYPE EXPR : ATTRS
3431 C<our> associates a simple name with a package variable in the current
3432 package for use within the current scope. When C<use strict 'vars'> is in
3433 effect, C<our> lets you use declared global variables without qualifying
3434 them with package names, within the lexical scope of the C<our> declaration.
3435 In this way C<our> differs from C<use vars>, which is package scoped.
3437 Unlike C<my>, which both allocates storage for a variable and associates
3438 a simple name with that storage for use within the current scope, C<our>
3439 associates a simple name with a package variable in the current package,
3440 for use within the current scope. In other words, C<our> has the same
3441 scoping rules as C<my>, but does not necessarily create a
3444 If more than one value is listed, the list must be placed
3450 An C<our> declaration declares a global variable that will be visible
3451 across its entire lexical scope, even across package boundaries. The
3452 package in which the variable is entered is determined at the point
3453 of the declaration, not at the point of use. This means the following
3457 our $bar; # declares $Foo::bar for rest of lexical scope
3461 print $bar; # prints 20, as it refers to $Foo::bar
3463 Multiple C<our> declarations with the same name in the same lexical
3464 scope are allowed if they are in different packages. If they happen
3465 to be in the same package, Perl will emit warnings if you have asked
3466 for them, just like multiple C<my> declarations. Unlike a second
3467 C<my> declaration, which will bind the name to a fresh variable, a
3468 second C<our> declaration in the same package, in the same scope, is
3473 our $bar; # declares $Foo::bar for rest of lexical scope
3477 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3478 print $bar; # prints 30
3480 our $bar; # emits warning but has no other effect
3481 print $bar; # still prints 30
3483 An C<our> declaration may also have a list of attributes associated
3486 The exact semantics and interface of TYPE and ATTRS are still
3487 evolving. TYPE is currently bound to the use of C<fields> pragma,
3488 and attributes are handled using the C<attributes> pragma, or starting
3489 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3490 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3491 L<attributes>, and L<Attribute::Handlers>.
3493 The only currently recognized C<our()> attribute is C<unique> which
3494 indicates that a single copy of the global is to be used by all
3495 interpreters should the program happen to be running in a
3496 multi-interpreter environment. (The default behaviour would be for
3497 each interpreter to have its own copy of the global.) Examples:
3499 our @EXPORT : unique = qw(foo);
3500 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3501 our $VERSION : unique = "1.00";
3503 Note that this attribute also has the effect of making the global
3504 readonly when the first new interpreter is cloned (for example,
3505 when the first new thread is created).
3507 Multi-interpreter environments can come to being either through the
3508 fork() emulation on Windows platforms, or by embedding perl in a
3509 multi-threaded application. The C<unique> attribute does nothing in
3510 all other environments.
3512 Warning: the current implementation of this attribute operates on the
3513 typeglob associated with the variable; this means that C<our $x : unique>
3514 also has the effect of C<our @x : unique; our %x : unique>. This may be
3517 =item pack TEMPLATE,LIST
3520 Takes a LIST of values and converts it into a string using the rules
3521 given by the TEMPLATE. The resulting string is the concatenation of
3522 the converted values. Typically, each converted value looks
3523 like its machine-level representation. For example, on 32-bit machines
3524 an integer may be represented by a sequence of 4 bytes that will be
3525 converted to a sequence of 4 characters.
3527 The TEMPLATE is a sequence of characters that give the order and type
3528 of values, as follows:
3530 a A string with arbitrary binary data, will be null padded.
3531 A A text (ASCII) string, will be space padded.
3532 Z A null terminated (ASCIZ) string, will be null padded.
3534 b A bit string (ascending bit order inside each byte, like vec()).
3535 B A bit string (descending bit order inside each byte).
3536 h A hex string (low nybble first).
3537 H A hex string (high nybble first).
3539 c A signed char (8-bit) value.
3540 C An unsigned C char (octet) even under Unicode. Should normally not
3541 be used. See U and W instead.
3542 W An unsigned char value (can be greater than 255).
3544 s A signed short (16-bit) value.
3545 S An unsigned short value.
3547 l A signed long (32-bit) value.
3548 L An unsigned long value.
3550 q A signed quad (64-bit) value.
3551 Q An unsigned quad value.
3552 (Quads are available only if your system supports 64-bit
3553 integer values _and_ if Perl has been compiled to support those.
3554 Causes a fatal error otherwise.)
3556 i A signed integer value.
3557 I A unsigned integer value.
3558 (This 'integer' is _at_least_ 32 bits wide. Its exact
3559 size depends on what a local C compiler calls 'int'.)
3561 n An unsigned short (16-bit) in "network" (big-endian) order.
3562 N An unsigned long (32-bit) in "network" (big-endian) order.
3563 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3564 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3566 j A Perl internal signed integer value (IV).
3567 J A Perl internal unsigned integer value (UV).
3569 f A single-precision float in the native format.
3570 d A double-precision float in the native format.
3572 F A Perl internal floating point value (NV) in the native format
3573 D A long double-precision float in the native format.
3574 (Long doubles are available only if your system supports long
3575 double values _and_ if Perl has been compiled to support those.
3576 Causes a fatal error otherwise.)
3578 p A pointer to a null-terminated string.
3579 P A pointer to a structure (fixed-length string).
3581 u A uuencoded string.
3582 U A Unicode character number. Encodes to UTF-8 internally
3583 (or UTF-EBCDIC in EBCDIC platforms).
3585 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3586 details). Its bytes represent an unsigned integer in base 128,
3587 most significant digit first, with as few digits as possible. Bit
3588 eight (the high bit) is set on each byte except the last.
3592 @ Null fill or truncate to absolute position, counted from the
3593 start of the innermost ()-group.
3594 . Null fill or truncate to absolute position specified by value.
3595 ( Start of a ()-group.
3597 One or more of the modifiers below may optionally follow some letters in the
3598 TEMPLATE (the second column lists the letters for which the modifier is
3601 ! sSlLiI Forces native (short, long, int) sizes instead
3602 of fixed (16-/32-bit) sizes.
3604 xX Make x and X act as alignment commands.
3606 nNvV Treat integers as signed instead of unsigned.
3608 @. Specify position as byte offset in the internal
3609 representation of the packed string. Efficient but
3612 > sSiIlLqQ Force big-endian byte-order on the type.
3613 jJfFdDpP (The "big end" touches the construct.)
3615 < sSiIlLqQ Force little-endian byte-order on the type.
3616 jJfFdDpP (The "little end" touches the construct.)
3618 The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
3619 in which case they force a certain byte-order on all components of
3620 that group, including subgroups.
3622 The following rules apply:
3628 Each letter may optionally be followed by a number giving a repeat
3629 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3630 C<H>, C<@>, C<.>, C<x>, C<X> and C<P> the pack function will gobble up
3631 that many values from the LIST. A C<*> for the repeat count means to
3632 use however many items are left, except for C<@>, C<x>, C<X>, where it
3633 is equivalent to C<0>, for <.> where it means relative to string start
3634 and C<u>, where it is equivalent to 1 (or 45, which is the same).
3635 A numeric repeat count may optionally be enclosed in brackets, as in
3636 C<pack 'C[80]', @arr>.
3638 One can replace the numeric repeat count by a template enclosed in brackets;
3639 then the packed length of this template in bytes is used as a count.
3640 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3641 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3642 If the template in brackets contains alignment commands (such as C<x![d]>),
3643 its packed length is calculated as if the start of the template has the maximal
3646 When used with C<Z>, C<*> results in the addition of a trailing null
3647 byte (so the packed result will be one longer than the byte C<length>
3650 When used with C<@>, the repeat count represents an offset from the start
3651 of the innermost () group.
3653 When used with C<.>, the repeat count is used to determine the starting
3654 position from where the value offset is calculated. If the repeat count
3655 is 0, it's relative to the current position. If the repeat count is C<*>,
3656 the offset is relative to the start of the packed string. And if its an
3657 integer C<n> the offset is relative to the start of the n-th innermost
3658 () group (or the start of the string if C<n> is bigger then the group
3661 The repeat count for C<u> is interpreted as the maximal number of bytes
3662 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3663 count should not be more than 65.
3667 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3668 string of length count, padding with nulls or spaces as necessary. When
3669 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3670 after the first null, and C<a> returns data verbatim.
3672 If the value-to-pack is too long, it is truncated. If too long and an
3673 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3674 by a null byte. Thus C<Z> always packs a trailing null (except when the
3679 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3680 Each character of the input field of pack() generates 1 bit of the result.
3681 Each result bit is based on the least-significant bit of the corresponding
3682 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3683 and C<"1"> generate bits 0 and 1, as do characters C<"\0"> and C<"\1">.
3685 Starting from the beginning of the input string of pack(), each 8-tuple
3686 of characters is converted to 1 character of output. With format C<b>
3687 the first character of the 8-tuple determines the least-significant bit of a
3688 character, and with format C<B> it determines the most-significant bit of
3691 If the length of the input string is not exactly divisible by 8, the
3692 remainder is packed as if the input string were padded by null characters
3693 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3695 If the input string of pack() is longer than needed, extra characters are
3696 ignored. A C<*> for the repeat count of pack() means to use all the
3697 characters of the input field. On unpack()ing the bits are converted to a
3698 string of C<"0">s and C<"1">s.
3702 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3703 representable as hexadecimal digits, 0-9a-f) long.
3705 Each character of the input field of pack() generates 4 bits of the result.
3706 For non-alphabetical characters the result is based on the 4 least-significant
3707 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3708 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3709 C<"\0"> and C<"\1">. For characters C<"a".."f"> and C<"A".."F"> the result
3710 is compatible with the usual hexadecimal digits, so that C<"a"> and
3711 C<"A"> both generate the nybble C<0xa==10>. The result for characters
3712 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3714 Starting from the beginning of the input string of pack(), each pair
3715 of characters is converted to 1 character of output. With format C<h> the
3716 first character of the pair determines the least-significant nybble of the
3717 output character, and with format C<H> it determines the most-significant
3720 If the length of the input string is not even, it behaves as if padded
3721 by a null character at the end. Similarly, during unpack()ing the "extra"
3722 nybbles are ignored.
3724 If the input string of pack() is longer than needed, extra characters are
3726 A C<*> for the repeat count of pack() means to use all the characters of
3727 the input field. On unpack()ing the nybbles are converted to a string
3728 of hexadecimal digits.
3732 The C<p> type packs a pointer to a null-terminated string. You are
3733 responsible for ensuring the string is not a temporary value (which can
3734 potentially get deallocated before you get around to using the packed result).
3735 The C<P> type packs a pointer to a structure of the size indicated by the
3736 length. A NULL pointer is created if the corresponding value for C<p> or
3737 C<P> is C<undef>, similarly for unpack().
3739 If your system has a strange pointer size (i.e. a pointer is neither as
3740 big as an int nor as big as a long), it may not be possible to pack or
3741 unpack pointers in big- or little-endian byte order. Attempting to do
3742 so will result in a fatal error.
3746 The C</> template character allows packing and unpacking of a sequence of
3747 items where the packed structure contains a packed item count followed by
3748 the packed items themselves.
3749 You write I<length-item>C</>I<sequence-item>.
3751 The I<length-item> can be any C<pack> template letter, and describes
3752 how the length value is packed. The ones likely to be of most use are
3753 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3754 SNMP) and C<N> (for Sun XDR).
3756 For C<pack>, the I<sequence-item> may have a repeat count, in which case
3757 the minimum of that and the number of available items is used as argument
3758 for the I<length-item>. If it has no repeat count or uses a '*', the number
3759 of available items is used. For C<unpack> the repeat count is always obtained
3760 by decoding the packed item count, and the I<sequence-item> must not have a
3763 If the I<sequence-item> refers to a string type (C<"A">, C<"a"> or C<"Z">),
3764 the I<length-item> is a string length, not a number of strings. If there is
3765 an explicit repeat count for pack, the packed string will be adjusted to that
3768 unpack 'W/a', "\04Gurusamy"; gives ('Guru')
3769 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond', 'J')
3770 pack 'n/a* w/a','hello,','world'; gives "\000\006hello,\005world"
3771 pack 'a/W2', ord('a') .. ord('z'); gives '2ab'
3773 The I<length-item> is not returned explicitly from C<unpack>.
3775 Adding a count to the I<length-item> letter is unlikely to do anything
3776 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3777 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3778 which Perl does not regard as legal in numeric strings.
3782 The integer types C<s>, C<S>, C<l>, and C<L> may be
3783 followed by a C<!> modifier to signify native shorts or
3784 longs--as you can see from above for example a bare C<l> does mean
3785 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3786 may be larger. This is an issue mainly in 64-bit platforms. You can
3787 see whether using C<!> makes any difference by
3789 print length(pack("s")), " ", length(pack("s!")), "\n";
3790 print length(pack("l")), " ", length(pack("l!")), "\n";
3792 C<i!> and C<I!> also work but only because of completeness;
3793 they are identical to C<i> and C<I>.
3795 The actual sizes (in bytes) of native shorts, ints, longs, and long
3796 longs on the platform where Perl was built are also available via
3800 print $Config{shortsize}, "\n";
3801 print $Config{intsize}, "\n";
3802 print $Config{longsize}, "\n";
3803 print $Config{longlongsize}, "\n";
3805 (The C<$Config{longlongsize}> will be undefined if your system does
3806 not support long longs.)
3810 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3811 are inherently non-portable between processors and operating systems
3812 because they obey the native byteorder and endianness. For example a
3813 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3814 (arranged in and handled by the CPU registers) into bytes as
3816 0x12 0x34 0x56 0x78 # big-endian
3817 0x78 0x56 0x34 0x12 # little-endian
3819 Basically, the Intel and VAX CPUs are little-endian, while everybody
3820 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3821 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3822 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3825 The names `big-endian' and `little-endian' are comic references to
3826 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3827 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3828 the egg-eating habits of the Lilliputians.
3830 Some systems may have even weirder byte orders such as
3835 You can see your system's preference with
3837 print join(" ", map { sprintf "%#02x", $_ }
3838 unpack("W*",pack("L",0x12345678))), "\n";
3840 The byteorder on the platform where Perl was built is also available
3844 print $Config{byteorder}, "\n";
3846 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3847 and C<'87654321'> are big-endian.
3849 If you want portable packed integers you can either use the formats
3850 C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
3851 modifiers. These modifiers are only available as of perl 5.9.2.
3852 See also L<perlport>.
3856 All integer and floating point formats as well as C<p> and C<P> and
3857 C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
3858 to force big- or little- endian byte-order, respectively.
3859 This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
3860 signed integers, 64-bit integers and floating point values. However,
3861 there are some things to keep in mind.
3863 Exchanging signed integers between different platforms only works
3864 if all platforms store them in the same format. Most platforms store
3865 signed integers in two's complement, so usually this is not an issue.
3867 The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
3868 formats on big- or little-endian machines. Otherwise, attempting to
3869 do so will result in a fatal error.
3871 Forcing big- or little-endian byte-order on floating point values for
3872 data exchange can only work if all platforms are using the same
3873 binary representation (e.g. IEEE floating point format). Even if all
3874 platforms are using IEEE, there may be subtle differences. Being able
3875 to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
3876 but also very dangerous if you don't know exactly what you're doing.
3877 It is definitely not a general way to portably store floating point
3880 When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
3881 all types inside the group that accept the byte-order modifiers,
3882 including all subgroups. It will silently be ignored for all other
3883 types. You are not allowed to override the byte-order within a group
3884 that already has a byte-order modifier suffix.
3888 Real numbers (floats and doubles) are in the native machine format only;
3889 due to the multiplicity of floating formats around, and the lack of a
3890 standard "network" representation, no facility for interchange has been
3891 made. This means that packed floating point data written on one machine
3892 may not be readable on another - even if both use IEEE floating point
3893 arithmetic (as the endian-ness of the memory representation is not part
3894 of the IEEE spec). See also L<perlport>.
3896 If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
3897 modifiers to force big- or little-endian byte-order on floating point values.
3899 Note that Perl uses doubles (or long doubles, if configured) internally for
3900 all numeric calculation, and converting from double into float and thence back
3901 to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
3902 will not in general equal $foo).
3906 Pack and unpack can operate in two modes, character mode (C<C0> mode) where
3907 the packed string is processed per character and UTF-8 mode (C<U0> mode)
3908 where the packed string is processed in its UTF-8-encoded Unicode form on
3909 a byte by byte basis. Character mode is the default unless the format string
3910 starts with an C<U>. You can switch mode at any moment with an explicit
3911 C<C0> or C<U0> in the format. A mode is in effect until the next mode switch
3912 or until the end of the ()-group in which it was entered.
3916 You must yourself do any alignment or padding by inserting for example
3917 enough C<'x'>es while packing. There is no way to pack() and unpack()
3918 could know where the characters are going to or coming from. Therefore
3919 C<pack> (and C<unpack>) handle their output and input as flat
3920 sequences of characters.
3924 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3925 take a repeat count, both as postfix, and for unpack() also via the C</>
3926 template character. Within each repetition of a group, positioning with
3927 C<@> starts again at 0. Therefore, the result of
3929 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3931 is the string "\0a\0\0bc".
3935 C<x> and C<X> accept C<!> modifier. In this case they act as
3936 alignment commands: they jump forward/back to the closest position
3937 aligned at a multiple of C<count> characters. For example, to pack() or
3938 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3939 use the template C<W x![d] d W[2]>; this assumes that doubles must be
3940 aligned on the double's size.
3942 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3943 both result in no-ops.
3947 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
3948 will represent signed 16-/32-bit integers in big-/little-endian order.
3949 This is only portable if all platforms sharing the packed data use the
3950 same binary representation for signed integers (e.g. all platforms are
3951 using two's complement representation).
3955 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3956 White space may be used to separate pack codes from each other, but
3957 modifiers and a repeat count must follow immediately.
3961 If TEMPLATE requires more arguments to pack() than actually given, pack()
3962 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
3963 to pack() than actually given, extra arguments are ignored.
3969 $foo = pack("WWWW",65,66,67,68);
3971 $foo = pack("W4",65,66,67,68);
3973 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
3974 # same thing with Unicode circled letters.
3975 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3976 # same thing with Unicode circled letters. You don't get the UTF-8
3977 # bytes because the U at the start of the format caused a switch to
3978 # U0-mode, so the UTF-8 bytes get joined into characters
3979 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
3980 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
3981 # This is the UTF-8 encoding of the string in the previous example
3983 $foo = pack("ccxxcc",65,66,67,68);
3986 # note: the above examples featuring "W" and "c" are true
3987 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3988 # and UTF-8. In EBCDIC the first example would be
3989 # $foo = pack("WWWW",193,194,195,196);
3991 $foo = pack("s2",1,2);
3992 # "\1\0\2\0" on little-endian
3993 # "\0\1\0\2" on big-endian
3995 $foo = pack("a4","abcd","x","y","z");
3998 $foo = pack("aaaa","abcd","x","y","z");
4001 $foo = pack("a14","abcdefg");
4002 # "abcdefg\0\0\0\0\0\0\0"
4004 $foo = pack("i9pl", gmtime);
4005 # a real struct tm (on my system anyway)
4007 $utmp_template = "Z8 Z8 Z16 L";
4008 $utmp = pack($utmp_template, @utmp1);
4009 # a struct utmp (BSDish)
4011 @utmp2 = unpack($utmp_template, $utmp);
4012 # "@utmp1" eq "@utmp2"
4015 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4018 $foo = pack('sx2l', 12, 34);
4019 # short 12, two zero bytes padding, long 34
4020 $bar = pack('s@4l', 12, 34);
4021 # short 12, zero fill to position 4, long 34
4023 $baz = pack('s.l', 12, 4, 34);
4024 # short 12, zero fill to position 4, long 34
4026 $foo = pack('nN', 42, 4711);
4027 # pack big-endian 16- and 32-bit unsigned integers
4028 $foo = pack('S>L>', 42, 4711);
4030 $foo = pack('s<l<', -42, 4711);
4031 # pack little-endian 16- and 32-bit signed integers
4032 $foo = pack('(sl)<', -42, 4711);
4035 The same template may generally also be used in unpack().
4037 =item package NAMESPACE
4038 X<package> X<module> X<namespace>
4042 Declares the compilation unit as being in the given namespace. The scope
4043 of the package declaration is from the declaration itself through the end
4044 of the enclosing block, file, or eval (the same as the C<my> operator).
4045 All further unqualified dynamic identifiers will be in this namespace.
4046 A package statement affects only dynamic variables--including those
4047 you've used C<local> on--but I<not> lexical variables, which are created
4048 with C<my>. Typically it would be the first declaration in a file to
4049 be included by the C<require> or C<use> operator. You can switch into a
4050 package in more than one place; it merely influences which symbol table
4051 is used by the compiler for the rest of that block. You can refer to
4052 variables and filehandles in other packages by prefixing the identifier
4053 with the package name and a double colon: C<$Package::Variable>.
4054 If the package name is null, the C<main> package as assumed. That is,
4055 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
4056 still seen in older code).
4058 If NAMESPACE is omitted, then there is no current package, and all
4059 identifiers must be fully qualified or lexicals. However, you are
4060 strongly advised not to make use of this feature. Its use can cause
4061 unexpected behaviour, even crashing some versions of Perl. It is
4062 deprecated, and will be removed from a future release.
4064 See L<perlmod/"Packages"> for more information about packages, modules,
4065 and classes. See L<perlsub> for other scoping issues.
4067 =item pipe READHANDLE,WRITEHANDLE
4070 Opens a pair of connected pipes like the corresponding system call.
4071 Note that if you set up a loop of piped processes, deadlock can occur
4072 unless you are very careful. In addition, note that Perl's pipes use
4073 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4074 after each command, depending on the application.
4076 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
4077 for examples of such things.
4079 On systems that support a close-on-exec flag on files, the flag will be set
4080 for the newly opened file descriptors as determined by the value of $^F.
4088 Pops and returns the last value of the array, shortening the array by
4089 one element. Has an effect similar to
4093 If there are no elements in the array, returns the undefined value
4094 (although this may happen at other times as well). If ARRAY is
4095 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
4096 array in subroutines, just like C<shift>.
4099 X<pos> X<match, position>
4103 Returns the offset of where the last C<m//g> search left off for the variable
4104 in question (C<$_> is used when the variable is not specified). Note that
4105 0 is a valid match offset. C<undef> indicates that the search position
4106 is reset (usually due to match failure, but can also be because no match has
4107 yet been performed on the scalar). C<pos> directly accesses the location used
4108 by the regexp engine to store the offset, so assigning to C<pos> will change
4109 that offset, and so will also influence the C<\G> zero-width assertion in
4110 regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
4111 the return from C<pos> won't change either in this case. See L<perlre> and
4114 =item print FILEHANDLE LIST
4121 Prints a string or a list of strings. Returns true if successful.
4122 FILEHANDLE may be a scalar variable name, in which case the variable
4123 contains the name of or a reference to the filehandle, thus introducing
4124 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4125 the next token is a term, it may be misinterpreted as an operator
4126 unless you interpose a C<+> or put parentheses around the arguments.)
4127 If FILEHANDLE is omitted, prints by default to standard output (or
4128 to the last selected output channel--see L</select>). If LIST is
4129 also omitted, prints C<$_> to the currently selected output channel.
4130 To set the default output channel to something other than STDOUT
4131 use the select operation. The current value of C<$,> (if any) is
4132 printed between each LIST item. The current value of C<$\> (if
4133 any) is printed after the entire LIST has been printed. Because
4134 print takes a LIST, anything in the LIST is evaluated in list
4135 context, and any subroutine that you call will have one or more of
4136 its expressions evaluated in list context. Also be careful not to
4137 follow the print keyword with a left parenthesis unless you want
4138 the corresponding right parenthesis to terminate the arguments to
4139 the print--interpose a C<+> or put parentheses around all the
4142 Note that if you're storing FILEHANDLEs in an array, or if you're using
4143 any other expression more complex than a scalar variable to retrieve it,
4144 you will have to use a block returning the filehandle value instead:
4146 print { $files[$i] } "stuff\n";
4147 print { $OK ? STDOUT : STDERR } "stuff\n";
4149 =item printf FILEHANDLE FORMAT, LIST
4152 =item printf FORMAT, LIST
4154 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4155 (the output record separator) is not appended. The first argument
4156 of the list will be interpreted as the C<printf> format. See C<sprintf>
4157 for an explanation of the format argument. If C<use locale> is in effect,
4158 the character used for the decimal point in formatted real numbers is
4159 affected by the LC_NUMERIC locale. See L<perllocale>.
4161 Don't fall into the trap of using a C<printf> when a simple
4162 C<print> would do. The C<print> is more efficient and less
4165 =item prototype FUNCTION
4168 Returns the prototype of a function as a string (or C<undef> if the
4169 function has no prototype). FUNCTION is a reference to, or the name of,
4170 the function whose prototype you want to retrieve.
4172 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4173 name for Perl builtin. If the builtin is not I<overridable> (such as
4174 C<qw//>) or its arguments cannot be expressed by a prototype (such as
4175 C<system>) returns C<undef> because the builtin does not really behave
4176 like a Perl function. Otherwise, the string describing the equivalent
4177 prototype is returned.
4179 =item push ARRAY,LIST
4182 Treats ARRAY as a stack, and pushes the values of LIST
4183 onto the end of ARRAY. The length of ARRAY increases by the length of
4184 LIST. Has the same effect as
4187 $ARRAY[++$#ARRAY] = $value;
4190 but is more efficient. Returns the number of elements in the array following
4191 the completed C<push>.
4203 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
4205 =item quotemeta EXPR
4206 X<quotemeta> X<metacharacter>
4210 Returns the value of EXPR with all non-"word"
4211 characters backslashed. (That is, all characters not matching
4212 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4213 returned string, regardless of any locale settings.)
4214 This is the internal function implementing
4215 the C<\Q> escape in double-quoted strings.
4217 If EXPR is omitted, uses C<$_>.
4224 Returns a random fractional number greater than or equal to C<0> and less
4225 than the value of EXPR. (EXPR should be positive.) If EXPR is
4226 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4227 also special-cased as C<1> - this has not been documented before perl 5.8.0
4228 and is subject to change in future versions of perl. Automatically calls
4229 C<srand> unless C<srand> has already been called. See also C<srand>.
4231 Apply C<int()> to the value returned by C<rand()> if you want random
4232 integers instead of random fractional numbers. For example,
4236 returns a random integer between C<0> and C<9>, inclusive.
4238 (Note: If your rand function consistently returns numbers that are too
4239 large or too small, then your version of Perl was probably compiled
4240 with the wrong number of RANDBITS.)
4242 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4245 =item read FILEHANDLE,SCALAR,LENGTH
4247 Attempts to read LENGTH I<characters> of data into variable SCALAR
4248 from the specified FILEHANDLE. Returns the number of characters
4249 actually read, C<0> at end of file, or undef if there was an error (in
4250 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4251 so that the last character actually read is the last character of the
4252 scalar after the read.
4254 An OFFSET may be specified to place the read data at some place in the
4255 string other than the beginning. A negative OFFSET specifies
4256 placement at that many characters counting backwards from the end of
4257 the string. A positive OFFSET greater than the length of SCALAR
4258 results in the string being padded to the required size with C<"\0">
4259 bytes before the result of the read is appended.
4261 The call is actually implemented in terms of either Perl's or system's
4262 fread() call. To get a true read(2) system call, see C<sysread>.
4264 Note the I<characters>: depending on the status of the filehandle,
4265 either (8-bit) bytes or characters are read. By default all
4266 filehandles operate on bytes, but for example if the filehandle has
4267 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4268 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4269 characters, not bytes. Similarly for the C<:encoding> pragma:
4270 in that case pretty much any characters can be read.
4272 =item readdir DIRHANDLE
4275 Returns the next directory entry for a directory opened by C<opendir>.
4276 If used in list context, returns all the rest of the entries in the
4277 directory. If there are no more entries, returns an undefined value in
4278 scalar context or a null list in list context.
4280 If you're planning to filetest the return values out of a C<readdir>, you'd
4281 better prepend the directory in question. Otherwise, because we didn't
4282 C<chdir> there, it would have been testing the wrong file.
4284 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
4285 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
4289 X<readline> X<gets> X<fgets>
4291 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
4292 context, each call reads and returns the next line, until end-of-file is
4293 reached, whereupon the subsequent call returns undef. In list context,
4294 reads until end-of-file is reached and returns a list of lines. Note that
4295 the notion of "line" used here is however you may have defined it
4296 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4298 When C<$/> is set to C<undef>, when readline() is in scalar
4299 context (i.e. file slurp mode), and when an empty file is read, it
4300 returns C<''> the first time, followed by C<undef> subsequently.
4302 This is the internal function implementing the C<< <EXPR> >>
4303 operator, but you can use it directly. The C<< <EXPR> >>
4304 operator is discussed in more detail in L<perlop/"I/O Operators">.
4307 $line = readline(*STDIN); # same thing
4309 If readline encounters an operating system error, C<$!> will be set with the
4310 corresponding error message. It can be helpful to check C<$!> when you are
4311 reading from filehandles you don't trust, such as a tty or a socket. The
4312 following example uses the operator form of C<readline>, and takes the necessary
4313 steps to ensure that C<readline> was successful.
4317 unless (defined( $line = <> )) {
4329 Returns the value of a symbolic link, if symbolic links are
4330 implemented. If not, gives a fatal error. If there is some system
4331 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4332 omitted, uses C<$_>.
4337 EXPR is executed as a system command.
4338 The collected standard output of the command is returned.
4339 In scalar context, it comes back as a single (potentially
4340 multi-line) string. In list context, returns a list of lines
4341 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4342 This is the internal function implementing the C<qx/EXPR/>
4343 operator, but you can use it directly. The C<qx/EXPR/>
4344 operator is discussed in more detail in L<perlop/"I/O Operators">.
4346 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4349 Receives a message on a socket. Attempts to receive LENGTH characters
4350 of data into variable SCALAR from the specified SOCKET filehandle.
4351 SCALAR will be grown or shrunk to the length actually read. Takes the
4352 same flags as the system call of the same name. Returns the address
4353 of the sender if SOCKET's protocol supports this; returns an empty
4354 string otherwise. If there's an error, returns the undefined value.
4355 This call is actually implemented in terms of recvfrom(2) system call.
4356 See L<perlipc/"UDP: Message Passing"> for examples.
4358 Note the I<characters>: depending on the status of the socket, either
4359 (8-bit) bytes or characters are received. By default all sockets
4360 operate on bytes, but for example if the socket has been changed using
4361 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
4362 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4363 characters, not bytes. Similarly for the C<:encoding> pragma:
4364 in that case pretty much any characters can be read.
4371 The C<redo> command restarts the loop block without evaluating the
4372 conditional again. The C<continue> block, if any, is not executed. If
4373 the LABEL is omitted, the command refers to the innermost enclosing
4374 loop. Programs that want to lie to themselves about what was just input
4375 normally use this command:
4377 # a simpleminded Pascal comment stripper
4378 # (warning: assumes no { or } in strings)
4379 LINE: while (<STDIN>) {
4380 while (s|({.*}.*){.*}|$1 |) {}
4385 if (/}/) { # end of comment?
4394 C<redo> cannot be used to retry a block which returns a value such as
4395 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4396 a grep() or map() operation.
4398 Note that a block by itself is semantically identical to a loop
4399 that executes once. Thus C<redo> inside such a block will effectively
4400 turn it into a looping construct.
4402 See also L</continue> for an illustration of how C<last>, C<next>, and
4410 Returns a non-empty string if EXPR is a reference, the empty
4411 string otherwise. If EXPR
4412 is not specified, C<$_> will be used. The value returned depends on the
4413 type of thing the reference is a reference to.
4414 Builtin types include:
4424 If the referenced object has been blessed into a package, then that package
4425 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4427 if (ref($r) eq "HASH") {
4428 print "r is a reference to a hash.\n";
4431 print "r is not a reference at all.\n";
4434 See also L<perlref>.
4436 =item rename OLDNAME,NEWNAME
4437 X<rename> X<move> X<mv> X<ren>
4439 Changes the name of a file; an existing file NEWNAME will be
4440 clobbered. Returns true for success, false otherwise.
4442 Behavior of this function varies wildly depending on your system
4443 implementation. For example, it will usually not work across file system
4444 boundaries, even though the system I<mv> command sometimes compensates
4445 for this. Other restrictions include whether it works on directories,
4446 open files, or pre-existing files. Check L<perlport> and either the
4447 rename(2) manpage or equivalent system documentation for details.
4449 =item require VERSION
4456 Demands a version of Perl specified by VERSION, or demands some semantics
4457 specified by EXPR or by C<$_> if EXPR is not supplied.
4459 VERSION may be either a numeric argument such as 5.006, which will be
4460 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4461 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
4462 VERSION is greater than the version of the current Perl interpreter.
4463 Compare with L</use>, which can do a similar check at compile time.
4465 Specifying VERSION as a literal of the form v5.6.1 should generally be
4466 avoided, because it leads to misleading error messages under earlier
4467 versions of Perl that do not support this syntax. The equivalent numeric
4468 version should be used instead.
4470 require v5.6.1; # run time version check
4471 require 5.6.1; # ditto
4472 require 5.006_001; # ditto; preferred for backwards compatibility
4474 Otherwise, C<require> demands that a library file be included if it
4475 hasn't already been included. The file is included via the do-FILE
4476 mechanism, which is essentially just a variety of C<eval>. Has
4477 semantics similar to the following subroutine:
4480 my ($filename) = @_;
4481 if (exists $INC{$filename}) {
4482 return 1 if $INC{$filename};
4483 die "Compilation failed in require";
4485 my ($realfilename,$result);
4487 foreach $prefix (@INC) {
4488 $realfilename = "$prefix/$filename";
4489 if (-f $realfilename) {
4490 $INC{$filename} = $realfilename;
4491 $result = do $realfilename;
4495 die "Can't find $filename in \@INC";
4498 $INC{$filename} = undef;
4500 } elsif (!$result) {
4501 delete $INC{$filename};
4502 die "$filename did not return true value";
4508 Note that the file will not be included twice under the same specified
4511 The file must return true as the last statement to indicate
4512 successful execution of any initialization code, so it's customary to
4513 end such a file with C<1;> unless you're sure it'll return true
4514 otherwise. But it's better just to put the C<1;>, in case you add more
4517 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4518 replaces "F<::>" with "F</>" in the filename for you,
4519 to make it easy to load standard modules. This form of loading of
4520 modules does not risk altering your namespace.
4522 In other words, if you try this:
4524 require Foo::Bar; # a splendid bareword
4526 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4527 directories specified in the C<@INC> array.
4529 But if you try this:
4531 $class = 'Foo::Bar';
4532 require $class; # $class is not a bareword
4534 require "Foo::Bar"; # not a bareword because of the ""
4536 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4537 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4539 eval "require $class";
4541 Now that you understand how C<require> looks for files in the case of a
4542 bareword argument, there is a little extra functionality going on behind
4543 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4544 first look for a similar filename with a "F<.pmc>" extension. If this file
4545 is found, it will be loaded in place of any file ending in a "F<.pm>"
4548 You can also insert hooks into the import facility, by putting directly
4549 Perl code into the @INC array. There are three forms of hooks: subroutine
4550 references, array references and blessed objects.
4552 Subroutine references are the simplest case. When the inclusion system
4553 walks through @INC and encounters a subroutine, this subroutine gets
4554 called with two parameters, the first being a reference to itself, and the
4555 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4556 subroutine should return C<undef> or a filehandle, from which the file to
4557 include will be read. If C<undef> is returned, C<require> will look at
4558 the remaining elements of @INC.
4560 If the hook is an array reference, its first element must be a subroutine
4561 reference. This subroutine is called as above, but the first parameter is
4562 the array reference. This enables to pass indirectly some arguments to
4565 In other words, you can write:
4567 push @INC, \&my_sub;
4569 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4575 push @INC, [ \&my_sub, $x, $y, ... ];
4577 my ($arrayref, $filename) = @_;
4578 # Retrieve $x, $y, ...
4579 my @parameters = @$arrayref[1..$#$arrayref];
4583 If the hook is an object, it must provide an INC method that will be
4584 called as above, the first parameter being the object itself. (Note that
4585 you must fully qualify the sub's name, as it is always forced into package
4586 C<main>.) Here is a typical code layout:
4592 my ($self, $filename) = @_;
4596 # In the main program
4597 push @INC, new Foo(...);
4599 Note that these hooks are also permitted to set the %INC entry
4600 corresponding to the files they have loaded. See L<perlvar/%INC>.
4602 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4609 Generally used in a C<continue> block at the end of a loop to clear
4610 variables and reset C<??> searches so that they work again. The
4611 expression is interpreted as a list of single characters (hyphens
4612 allowed for ranges). All variables and arrays beginning with one of
4613 those letters are reset to their pristine state. If the expression is
4614 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4615 only variables or searches in the current package. Always returns
4618 reset 'X'; # reset all X variables
4619 reset 'a-z'; # reset lower case variables
4620 reset; # just reset ?one-time? searches
4622 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4623 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4624 variables--lexical variables are unaffected, but they clean themselves
4625 up on scope exit anyway, so you'll probably want to use them instead.
4633 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4634 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4635 context, depending on how the return value will be used, and the context
4636 may vary from one execution to the next (see C<wantarray>). If no EXPR
4637 is given, returns an empty list in list context, the undefined value in
4638 scalar context, and (of course) nothing at all in a void context.
4640 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4641 or do FILE will automatically return the value of the last expression
4645 X<reverse> X<rev> X<invert>
4647 In list context, returns a list value consisting of the elements
4648 of LIST in the opposite order. In scalar context, concatenates the
4649 elements of LIST and returns a string value with all characters
4650 in the opposite order.
4652 print reverse <>; # line tac, last line first
4654 undef $/; # for efficiency of <>
4655 print scalar reverse <>; # character tac, last line tsrif
4657 Used without arguments in scalar context, reverse() reverses C<$_>.
4659 This operator is also handy for inverting a hash, although there are some
4660 caveats. If a value is duplicated in the original hash, only one of those
4661 can be represented as a key in the inverted hash. Also, this has to
4662 unwind one hash and build a whole new one, which may take some time
4663 on a large hash, such as from a DBM file.
4665 %by_name = reverse %by_address; # Invert the hash
4667 =item rewinddir DIRHANDLE
4670 Sets the current position to the beginning of the directory for the
4671 C<readdir> routine on DIRHANDLE.
4673 =item rindex STR,SUBSTR,POSITION
4676 =item rindex STR,SUBSTR
4678 Works just like index() except that it returns the position of the I<last>
4679 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4680 last occurrence beginning at or before that position.
4682 =item rmdir FILENAME
4683 X<rmdir> X<rd> X<directory, remove>
4687 Deletes the directory specified by FILENAME if that directory is
4688 empty. If it succeeds it returns true, otherwise it returns false and
4689 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
4693 The substitution operator. See L<perlop>.
4695 =item say FILEHANDLE LIST
4702 Just like C<print>, but implicitly appends a newline.
4703 C<say LIST> is simply an abbreviation for C<print LIST, "\n">,
4704 and C<say()> works just like C<print($_, "\n")>.
4706 That means that a call to say() appends any output record separator
4707 I<after> the added newline.
4709 This keyword is only available when the "say" feature is
4710 enabled: see L<feature>.
4713 X<scalar> X<context>
4715 Forces EXPR to be interpreted in scalar context and returns the value
4718 @counts = ( scalar @a, scalar @b, scalar @c );
4720 There is no equivalent operator to force an expression to
4721 be interpolated in list context because in practice, this is never
4722 needed. If you really wanted to do so, however, you could use
4723 the construction C<@{[ (some expression) ]}>, but usually a simple
4724 C<(some expression)> suffices.
4726 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4727 parenthesized list, this behaves as a scalar comma expression, evaluating
4728 all but the last element in void context and returning the final element
4729 evaluated in scalar context. This is seldom what you want.
4731 The following single statement:
4733 print uc(scalar(&foo,$bar)),$baz;
4735 is the moral equivalent of these two:
4738 print(uc($bar),$baz);
4740 See L<perlop> for more details on unary operators and the comma operator.
4742 =item seek FILEHANDLE,POSITION,WHENCE
4743 X<seek> X<fseek> X<filehandle, position>
4745 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4746 FILEHANDLE may be an expression whose value gives the name of the
4747 filehandle. The values for WHENCE are C<0> to set the new position
4748 I<in bytes> to POSITION, C<1> to set it to the current position plus
4749 POSITION, and C<2> to set it to EOF plus POSITION (typically
4750 negative). For WHENCE you may use the constants C<SEEK_SET>,
4751 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4752 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4755 Note the I<in bytes>: even if the filehandle has been set to
4756 operate on characters (for example by using the C<:utf8> open
4757 layer), tell() will return byte offsets, not character offsets
4758 (because implementing that would render seek() and tell() rather slow).
4760 If you want to position file for C<sysread> or C<syswrite>, don't use
4761 C<seek>--buffering makes its effect on the file's system position
4762 unpredictable and non-portable. Use C<sysseek> instead.
4764 Due to the rules and rigors of ANSI C, on some systems you have to do a
4765 seek whenever you switch between reading and writing. Amongst other
4766 things, this may have the effect of calling stdio's clearerr(3).
4767 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4771 This is also useful for applications emulating C<tail -f>. Once you hit
4772 EOF on your read, and then sleep for a while, you might have to stick in a
4773 seek() to reset things. The C<seek> doesn't change the current position,
4774 but it I<does> clear the end-of-file condition on the handle, so that the
4775 next C<< <FILE> >> makes Perl try again to read something. We hope.
4777 If that doesn't work (some IO implementations are particularly
4778 cantankerous), then you may need something more like this:
4781 for ($curpos = tell(FILE); $_ = <FILE>;
4782 $curpos = tell(FILE)) {
4783 # search for some stuff and put it into files
4785 sleep($for_a_while);
4786 seek(FILE, $curpos, 0);
4789 =item seekdir DIRHANDLE,POS
4792 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4793 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
4794 about possible directory compaction as the corresponding system library
4797 =item select FILEHANDLE
4798 X<select> X<filehandle, default>
4802 Returns the currently selected filehandle. Sets the current default
4803 filehandle for output, if FILEHANDLE is supplied. This has two
4804 effects: first, a C<write> or a C<print> without a filehandle will
4805 default to this FILEHANDLE. Second, references to variables related to
4806 output will refer to this output channel. For example, if you have to
4807 set the top of form format for more than one output channel, you might
4815 FILEHANDLE may be an expression whose value gives the name of the
4816 actual filehandle. Thus:
4818 $oldfh = select(STDERR); $| = 1; select($oldfh);
4820 Some programmers may prefer to think of filehandles as objects with
4821 methods, preferring to write the last example as:
4824 STDERR->autoflush(1);
4826 =item select RBITS,WBITS,EBITS,TIMEOUT
4829 This calls the select(2) system call with the bit masks specified, which
4830 can be constructed using C<fileno> and C<vec>, along these lines:
4832 $rin = $win = $ein = '';
4833 vec($rin,fileno(STDIN),1) = 1;
4834 vec($win,fileno(STDOUT),1) = 1;
4837 If you want to select on many filehandles you might wish to write a
4841 my(@fhlist) = split(' ',$_[0]);
4844 vec($bits,fileno($_),1) = 1;
4848 $rin = fhbits('STDIN TTY SOCK');
4852 ($nfound,$timeleft) =
4853 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4855 or to block until something becomes ready just do this
4857 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4859 Most systems do not bother to return anything useful in $timeleft, so
4860 calling select() in scalar context just returns $nfound.
4862 Any of the bit masks can also be undef. The timeout, if specified, is
4863 in seconds, which may be fractional. Note: not all implementations are
4864 capable of returning the $timeleft. If not, they always return
4865 $timeleft equal to the supplied $timeout.
4867 You can effect a sleep of 250 milliseconds this way:
4869 select(undef, undef, undef, 0.25);
4871 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4872 is implementation-dependent. See also L<perlport> for notes on the
4873 portability of C<select>.
4875 On error, C<select> behaves like the select(2) system call : it returns
4878 Note: on some Unixes, the select(2) system call may report a socket file
4879 descriptor as "ready for reading", when actually no data is available,
4880 thus a subsequent read blocks. It can be avoided using always the
4881 O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further
4884 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4885 or <FH>) with C<select>, except as permitted by POSIX, and even
4886 then only on POSIX systems. You have to use C<sysread> instead.
4888 =item semctl ID,SEMNUM,CMD,ARG
4891 Calls the System V IPC function C<semctl>. You'll probably have to say
4895 first to get the correct constant definitions. If CMD is IPC_STAT or
4896 GETALL, then ARG must be a variable that will hold the returned
4897 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4898 the undefined value for error, "C<0 but true>" for zero, or the actual
4899 return value otherwise. The ARG must consist of a vector of native
4900 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4901 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4904 =item semget KEY,NSEMS,FLAGS
4907 Calls the System V IPC function semget. Returns the semaphore id, or
4908 the undefined value if there is an error. See also
4909 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4912 =item semop KEY,OPSTRING
4915 Calls the System V IPC function semop to perform semaphore operations
4916 such as signalling and waiting. OPSTRING must be a packed array of
4917 semop structures. Each semop structure can be generated with
4918 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
4919 implies the number of semaphore operations. Returns true if
4920 successful, or false if there is an error. As an example, the
4921 following code waits on semaphore $semnum of semaphore id $semid:
4923 $semop = pack("s!3", $semnum, -1, 0);
4924 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4926 To signal the semaphore, replace C<-1> with C<1>. See also
4927 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4930 =item send SOCKET,MSG,FLAGS,TO
4933 =item send SOCKET,MSG,FLAGS
4935 Sends a message on a socket. Attempts to send the scalar MSG to the
4936 SOCKET filehandle. Takes the same flags as the system call of the
4937 same name. On unconnected sockets you must specify a destination to
4938 send TO, in which case it does a C C<sendto>. Returns the number of
4939 characters sent, or the undefined value if there is an error. The C
4940 system call sendmsg(2) is currently unimplemented. See
4941 L<perlipc/"UDP: Message Passing"> for examples.
4943 Note the I<characters>: depending on the status of the socket, either
4944 (8-bit) bytes or characters are sent. By default all sockets operate
4945 on bytes, but for example if the socket has been changed using
4946 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or the
4947 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded
4948 Unicode characters, not bytes. Similarly for the C<:encoding> pragma:
4949 in that case pretty much any characters can be sent.
4951 =item setpgrp PID,PGRP
4954 Sets the current process group for the specified PID, C<0> for the current
4955 process. Will produce a fatal error if used on a machine that doesn't
4956 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4957 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4958 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4961 =item setpriority WHICH,WHO,PRIORITY
4962 X<setpriority> X<priority> X<nice> X<renice>
4964 Sets the current priority for a process, a process group, or a user.
4965 (See setpriority(2).) Will produce a fatal error if used on a machine
4966 that doesn't implement setpriority(2).
4968 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4971 Sets the socket option requested. Returns undefined if there is an
4972 error. Use integer constants provided by the C<Socket> module for
4973 LEVEL and OPNAME. Values for LEVEL can also be obtained from
4974 getprotobyname. OPTVAL might either be a packed string or an integer.
4975 An integer OPTVAL is shorthand for pack("i", OPTVAL).
4977 An example disabling the Nagle's algorithm for a socket:
4979 use Socket qw(IPPROTO_TCP TCP_NODELAY);
4980 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
4987 Shifts the first value of the array off and returns it, shortening the
4988 array by 1 and moving everything down. If there are no elements in the
4989 array, returns the undefined value. If ARRAY is omitted, shifts the
4990 C<@_> array within the lexical scope of subroutines and formats, and the
4991 C<@ARGV> array outside of a subroutine and also within the lexical scopes
4992 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>
4993 and C<END {}> constructs.
4995 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4996 same thing to the left end of an array that C<pop> and C<push> do to the
4999 =item shmctl ID,CMD,ARG
5002 Calls the System V IPC function shmctl. You'll probably have to say
5006 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5007 then ARG must be a variable that will hold the returned C<shmid_ds>
5008 structure. Returns like ioctl: the undefined value for error, "C<0> but
5009 true" for zero, or the actual return value otherwise.
5010 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5012 =item shmget KEY,SIZE,FLAGS
5015 Calls the System V IPC function shmget. Returns the shared memory
5016 segment id, or the undefined value if there is an error.
5017 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5019 =item shmread ID,VAR,POS,SIZE
5023 =item shmwrite ID,STRING,POS,SIZE
5025 Reads or writes the System V shared memory segment ID starting at
5026 position POS for size SIZE by attaching to it, copying in/out, and
5027 detaching from it. When reading, VAR must be a variable that will
5028 hold the data read. When writing, if STRING is too long, only SIZE
5029 bytes are used; if STRING is too short, nulls are written to fill out
5030 SIZE bytes. Return true if successful, or false if there is an error.
5031 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5032 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5034 =item shutdown SOCKET,HOW
5037 Shuts down a socket connection in the manner indicated by HOW, which
5038 has the same interpretation as in the system call of the same name.
5040 shutdown(SOCKET, 0); # I/we have stopped reading data
5041 shutdown(SOCKET, 1); # I/we have stopped writing data
5042 shutdown(SOCKET, 2); # I/we have stopped using this socket
5044 This is useful with sockets when you want to tell the other
5045 side you're done writing but not done reading, or vice versa.
5046 It's also a more insistent form of close because it also
5047 disables the file descriptor in any forked copies in other
5051 X<sin> X<sine> X<asin> X<arcsine>
5055 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5056 returns sine of C<$_>.
5058 For the inverse sine operation, you may use the C<Math::Trig::asin>
5059 function, or use this relation:
5061 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5068 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
5069 May be interrupted if the process receives a signal such as C<SIGALRM>.
5070 Returns the number of seconds actually slept. You probably cannot
5071 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
5074 On some older systems, it may sleep up to a full second less than what
5075 you requested, depending on how it counts seconds. Most modern systems
5076 always sleep the full amount. They may appear to sleep longer than that,
5077 however, because your process might not be scheduled right away in a
5078 busy multitasking system.
5080 For delays of finer granularity than one second, you may use Perl's
5081 C<syscall> interface to access setitimer(2) if your system supports
5082 it, or else see L</select> above. The Time::HiRes module (from CPAN,
5083 and starting from Perl 5.8 part of the standard distribution) may also
5086 See also the POSIX module's C<pause> function.
5088 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5091 Opens a socket of the specified kind and attaches it to filehandle
5092 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5093 the system call of the same name. You should C<use Socket> first
5094 to get the proper definitions imported. See the examples in
5095 L<perlipc/"Sockets: Client/Server Communication">.
5097 On systems that support a close-on-exec flag on files, the flag will
5098 be set for the newly opened file descriptor, as determined by the
5099 value of $^F. See L<perlvar/$^F>.
5101 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5104 Creates an unnamed pair of sockets in the specified domain, of the
5105 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5106 for the system call of the same name. If unimplemented, yields a fatal
5107 error. Returns true if successful.
5109 On systems that support a close-on-exec flag on files, the flag will
5110 be set for the newly opened file descriptors, as determined by the value
5111 of $^F. See L<perlvar/$^F>.
5113 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5114 to C<pipe(Rdr, Wtr)> is essentially:
5117 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5118 shutdown(Rdr, 1); # no more writing for reader
5119 shutdown(Wtr, 0); # no more reading for writer
5121 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5122 emulate socketpair using IP sockets to localhost if your system implements
5123 sockets but not socketpair.
5125 =item sort SUBNAME LIST
5126 X<sort> X<qsort> X<quicksort> X<mergesort>
5128 =item sort BLOCK LIST
5132 In list context, this sorts the LIST and returns the sorted list value.
5133 In scalar context, the behaviour of C<sort()> is undefined.
5135 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5136 order. If SUBNAME is specified, it gives the name of a subroutine
5137 that returns an integer less than, equal to, or greater than C<0>,
5138 depending on how the elements of the list are to be ordered. (The C<<
5139 <=> >> and C<cmp> operators are extremely useful in such routines.)
5140 SUBNAME may be a scalar variable name (unsubscripted), in which case
5141 the value provides the name of (or a reference to) the actual
5142 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5143 an anonymous, in-line sort subroutine.
5145 If the subroutine's prototype is C<($$)>, the elements to be compared
5146 are passed by reference in C<@_>, as for a normal subroutine. This is
5147 slower than unprototyped subroutines, where the elements to be
5148 compared are passed into the subroutine
5149 as the package global variables $a and $b (see example below). Note that
5150 in the latter case, it is usually counter-productive to declare $a and
5153 The values to be compared are always passed by reference and should not
5156 You also cannot exit out of the sort block or subroutine using any of the
5157 loop control operators described in L<perlsyn> or with C<goto>.
5159 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5160 current collation locale. See L<perllocale>.
5162 sort() returns aliases into the original list, much as a for loop's index
5163 variable aliases the list elements. That is, modifying an element of a
5164 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5165 actually modifies the element in the original list. This is usually
5166 something to be avoided when writing clear code.
5168 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5169 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5170 preserves the input order of elements that compare equal. Although
5171 quicksort's run time is O(NlogN) when averaged over all arrays of
5172 length N, the time can be O(N**2), I<quadratic> behavior, for some
5173 inputs.) In 5.7, the quicksort implementation was replaced with
5174 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5175 But benchmarks indicated that for some inputs, on some platforms,
5176 the original quicksort was faster. 5.8 has a sort pragma for
5177 limited control of the sort. Its rather blunt control of the
5178 underlying algorithm may not persist into future Perls, but the
5179 ability to characterize the input or output in implementation
5180 independent ways quite probably will. See L<sort>.
5185 @articles = sort @files;
5187 # same thing, but with explicit sort routine
5188 @articles = sort {$a cmp $b} @files;
5190 # now case-insensitively
5191 @articles = sort {uc($a) cmp uc($b)} @files;
5193 # same thing in reversed order
5194 @articles = sort {$b cmp $a} @files;
5196 # sort numerically ascending
5197 @articles = sort {$a <=> $b} @files;
5199 # sort numerically descending
5200 @articles = sort {$b <=> $a} @files;
5202 # this sorts the %age hash by value instead of key
5203 # using an in-line function
5204 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5206 # sort using explicit subroutine name
5208 $age{$a} <=> $age{$b}; # presuming numeric
5210 @sortedclass = sort byage @class;
5212 sub backwards { $b cmp $a }
5213 @harry = qw(dog cat x Cain Abel);
5214 @george = qw(gone chased yz Punished Axed);
5216 # prints AbelCaincatdogx
5217 print sort backwards @harry;
5218 # prints xdogcatCainAbel
5219 print sort @george, 'to', @harry;
5220 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5222 # inefficiently sort by descending numeric compare using
5223 # the first integer after the first = sign, or the
5224 # whole record case-insensitively otherwise
5227 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5232 # same thing, but much more efficiently;
5233 # we'll build auxiliary indices instead
5237 push @nums, /=(\d+)/;
5242 $nums[$b] <=> $nums[$a]
5244 $caps[$a] cmp $caps[$b]
5248 # same thing, but without any temps
5249 @new = map { $_->[0] }
5250 sort { $b->[1] <=> $a->[1]
5253 } map { [$_, /=(\d+)/, uc($_)] } @old;
5255 # using a prototype allows you to use any comparison subroutine
5256 # as a sort subroutine (including other package's subroutines)
5258 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5261 @new = sort other::backwards @old;
5263 # guarantee stability, regardless of algorithm
5265 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5267 # force use of mergesort (not portable outside Perl 5.8)
5268 use sort '_mergesort'; # note discouraging _
5269 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5271 If you're using strict, you I<must not> declare $a
5272 and $b as lexicals. They are package globals. That means
5273 if you're in the C<main> package and type
5275 @articles = sort {$b <=> $a} @files;
5277 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5278 but if you're in the C<FooPack> package, it's the same as typing
5280 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5282 The comparison function is required to behave. If it returns
5283 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5284 sometimes saying the opposite, for example) the results are not
5287 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5288 (not-a-number), and because C<sort> will trigger a fatal error unless the
5289 result of a comparison is defined, when sorting with a comparison function
5290 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5291 The following example takes advantage of the fact that C<NaN != NaN> to
5292 eliminate any C<NaN>s from the input.
5294 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5296 =item splice ARRAY,OFFSET,LENGTH,LIST
5299 =item splice ARRAY,OFFSET,LENGTH
5301 =item splice ARRAY,OFFSET
5305 Removes the elements designated by OFFSET and LENGTH from an array, and
5306 replaces them with the elements of LIST, if any. In list context,
5307 returns the elements removed from the array. In scalar context,
5308 returns the last element removed, or C<undef> if no elements are
5309 removed. The array grows or shrinks as necessary.
5310 If OFFSET is negative then it starts that far from the end of the array.
5311 If LENGTH is omitted, removes everything from OFFSET onward.
5312 If LENGTH is negative, removes the elements from OFFSET onward
5313 except for -LENGTH elements at the end of the array.
5314 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5315 past the end of the array, perl issues a warning, and splices at the
5318 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5320 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5321 pop(@a) splice(@a,-1)
5322 shift(@a) splice(@a,0,1)
5323 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5324 $a[$i] = $y splice(@a,$i,1,$y)
5326 Example, assuming array lengths are passed before arrays:
5328 sub aeq { # compare two list values
5329 my(@a) = splice(@_,0,shift);
5330 my(@b) = splice(@_,0,shift);
5331 return 0 unless @a == @b; # same len?
5333 return 0 if pop(@a) ne pop(@b);
5337 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5339 =item split /PATTERN/,EXPR,LIMIT
5342 =item split /PATTERN/,EXPR
5344 =item split /PATTERN/
5348 Splits the string EXPR into a list of strings and returns that list. By
5349 default, empty leading fields are preserved, and empty trailing ones are
5350 deleted. (If all fields are empty, they are considered to be trailing.)
5352 In scalar context, returns the number of fields found and splits into
5353 the C<@_> array. Use of split in scalar context is deprecated, however,
5354 because it clobbers your subroutine arguments.
5356 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5357 splits on whitespace (after skipping any leading whitespace). Anything
5358 matching PATTERN is taken to be a delimiter separating the fields. (Note
5359 that the delimiter may be longer than one character.)
5361 If LIMIT is specified and positive, it represents the maximum number
5362 of fields the EXPR will be split into, though the actual number of
5363 fields returned depends on the number of times PATTERN matches within
5364 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5365 stripped (which potential users of C<pop> would do well to remember).
5366 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5367 had been specified. Note that splitting an EXPR that evaluates to the
5368 empty string always returns the empty list, regardless of the LIMIT
5371 A pattern matching the null string (not to be confused with
5372 a null pattern C<//>, which is just one member of the set of patterns
5373 matching a null string) will split the value of EXPR into separate
5374 characters at each point it matches that way. For example:
5376 print join(':', split(/ */, 'hi there'));
5378 produces the output 'h:i:t:h:e:r:e'.
5380 As a special case for C<split>, using the empty pattern C<//> specifically
5381 matches only the null string, and is not be confused with the regular use
5382 of C<//> to mean "the last successful pattern match". So, for C<split>,
5385 print join(':', split(//, 'hi there'));
5387 produces the output 'h:i: :t:h:e:r:e'.
5389 Empty leading (or trailing) fields are produced when there are positive
5390 width matches at the beginning (or end) of the string; a zero-width match
5391 at the beginning (or end) of the string does not produce an empty field.
5394 print join(':', split(/(?=\w)/, 'hi there!'));
5396 produces the output 'h:i :t:h:e:r:e!'.
5398 The LIMIT parameter can be used to split a line partially
5400 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5402 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5403 a LIMIT one larger than the number of variables in the list, to avoid
5404 unnecessary work. For the list above LIMIT would have been 4 by
5405 default. In time critical applications it behooves you not to split
5406 into more fields than you really need.
5408 If the PATTERN contains parentheses, additional list elements are
5409 created from each matching substring in the delimiter.
5411 split(/([,-])/, "1-10,20", 3);
5413 produces the list value
5415 (1, '-', 10, ',', 20)
5417 If you had the entire header of a normal Unix email message in $header,
5418 you could split it up into fields and their values this way:
5420 $header =~ s/\n\s+/ /g; # fix continuation lines
5421 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5423 The pattern C</PATTERN/> may be replaced with an expression to specify
5424 patterns that vary at runtime. (To do runtime compilation only once,
5425 use C</$variable/o>.)
5427 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5428 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5429 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5430 will give you as many null initial fields as there are leading spaces.
5431 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5432 whitespace produces a null first field. A C<split> with no arguments
5433 really does a S<C<split(' ', $_)>> internally.
5435 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5440 open(PASSWD, '/etc/passwd');
5443 ($login, $passwd, $uid, $gid,
5444 $gcos, $home, $shell) = split(/:/);
5448 As with regular pattern matching, any capturing parentheses that are not
5449 matched in a C<split()> will be set to C<undef> when returned:
5451 @fields = split /(A)|B/, "1A2B3";
5452 # @fields is (1, 'A', 2, undef, 3)
5454 =item sprintf FORMAT, LIST
5457 Returns a string formatted by the usual C<printf> conventions of the C
5458 library function C<sprintf>. See below for more details
5459 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
5460 the general principles.
5464 # Format number with up to 8 leading zeroes
5465 $result = sprintf("%08d", $number);
5467 # Round number to 3 digits after decimal point
5468 $rounded = sprintf("%.3f", $number);
5470 Perl does its own C<sprintf> formatting--it emulates the C
5471 function C<sprintf>, but it doesn't use it (except for floating-point
5472 numbers, and even then only the standard modifiers are allowed). As a
5473 result, any non-standard extensions in your local C<sprintf> are not
5474 available from Perl.
5476 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5477 pass it an array as your first argument. The array is given scalar context,
5478 and instead of using the 0th element of the array as the format, Perl will
5479 use the count of elements in the array as the format, which is almost never
5482 Perl's C<sprintf> permits the following universally-known conversions:
5485 %c a character with the given number
5487 %d a signed integer, in decimal
5488 %u an unsigned integer, in decimal
5489 %o an unsigned integer, in octal
5490 %x an unsigned integer, in hexadecimal
5491 %e a floating-point number, in scientific notation
5492 %f a floating-point number, in fixed decimal notation
5493 %g a floating-point number, in %e or %f notation
5495 In addition, Perl permits the following widely-supported conversions:
5497 %X like %x, but using upper-case letters
5498 %E like %e, but using an upper-case "E"
5499 %G like %g, but with an upper-case "E" (if applicable)
5500 %b an unsigned integer, in binary
5501 %p a pointer (outputs the Perl value's address in hexadecimal)
5502 %n special: *stores* the number of characters output so far
5503 into the next variable in the parameter list
5505 Finally, for backward (and we do mean "backward") compatibility, Perl
5506 permits these unnecessary but widely-supported conversions:
5509 %D a synonym for %ld
5510 %U a synonym for %lu
5511 %O a synonym for %lo
5514 Note that the number of exponent digits in the scientific notation produced
5515 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5516 exponent less than 100 is system-dependent: it may be three or less
5517 (zero-padded as necessary). In other words, 1.23 times ten to the
5518 99th may be either "1.23e99" or "1.23e099".
5520 Between the C<%> and the format letter, you may specify a number of
5521 additional attributes controlling the interpretation of the format.
5522 In order, these are:
5526 =item format parameter index
5528 An explicit format parameter index, such as C<2$>. By default sprintf
5529 will format the next unused argument in the list, but this allows you
5530 to take the arguments out of order, e.g.:
5532 printf '%2$d %1$d', 12, 34; # prints "34 12"
5533 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
5538 space prefix positive number with a space
5539 + prefix positive number with a plus sign
5540 - left-justify within the field
5541 0 use zeros, not spaces, to right-justify
5542 # prefix non-zero octal with "0", non-zero hex with "0x",
5543 non-zero binary with "0b"
5547 printf '<% d>', 12; # prints "< 12>"
5548 printf '<%+d>', 12; # prints "<+12>"
5549 printf '<%6s>', 12; # prints "< 12>"
5550 printf '<%-6s>', 12; # prints "<12 >"
5551 printf '<%06s>', 12; # prints "<000012>"
5552 printf '<%#x>', 12; # prints "<0xc>"
5556 This flag tells perl to interpret the supplied string as a vector of
5557 integers, one for each character in the string. Perl applies the format to
5558 each integer in turn, then joins the resulting strings with a separator (a
5559 dot C<.> by default). This can be useful for displaying ordinal values of
5560 characters in arbitrary strings:
5562 printf "%vd", "AB\x{100}"; # prints "65.66.256"
5563 printf "version is v%vd\n", $^V; # Perl's version
5565 Put an asterisk C<*> before the C<v> to override the string to
5566 use to separate the numbers:
5568 printf "address is %*vX\n", ":", $addr; # IPv6 address
5569 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
5571 You can also explicitly specify the argument number to use for
5572 the join string using e.g. C<*2$v>:
5574 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
5576 =item (minimum) width
5578 Arguments are usually formatted to be only as wide as required to
5579 display the given value. You can override the width by putting
5580 a number here, or get the width from the next argument (with C<*>)
5581 or from a specified argument (with e.g. C<*2$>):
5583 printf '<%s>', "a"; # prints "<a>"
5584 printf '<%6s>', "a"; # prints "< a>"
5585 printf '<%*s>', 6, "a"; # prints "< a>"
5586 printf '<%*2$s>', "a", 6; # prints "< a>"
5587 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5589 If a field width obtained through C<*> is negative, it has the same
5590 effect as the C<-> flag: left-justification.
5592 =item precision, or maximum width
5595 You can specify a precision (for numeric conversions) or a maximum
5596 width (for string conversions) by specifying a C<.> followed by a number.
5597 For floating point formats, with the exception of 'g' and 'G', this specifies
5598 the number of decimal places to show (the default being 6), e.g.:
5600 # these examples are subject to system-specific variation
5601 printf '<%f>', 1; # prints "<1.000000>"
5602 printf '<%.1f>', 1; # prints "<1.0>"
5603 printf '<%.0f>', 1; # prints "<1>"
5604 printf '<%e>', 10; # prints "<1.000000e+01>"
5605 printf '<%.1e>', 10; # prints "<1.0e+01>"
5607 For 'g' and 'G', this specifies the maximum number of digits to show,
5608 including prior to the decimal point as well as after it, e.g.:
5610 # these examples are subject to system-specific variation
5611 printf '<%g>', 1; # prints "<1>"
5612 printf '<%.10g>', 1; # prints "<1>"
5613 printf '<%g>', 100; # prints "<100>"
5614 printf '<%.1g>', 100; # prints "<1e+02>"
5615 printf '<%.2g>', 100.01; # prints "<1e+02>"
5616 printf '<%.5g>', 100.01; # prints "<100.01>"
5617 printf '<%.4g>', 100.01; # prints "<100>"
5619 For integer conversions, specifying a precision implies that the
5620 output of the number itself should be zero-padded to this width:
5622 printf '<%.6x>', 1; # prints "<000001>"
5623 printf '<%#.6x>', 1; # prints "<0x000001>"
5624 printf '<%-10.6x>', 1; # prints "<000001 >"
5626 For string conversions, specifying a precision truncates the string
5627 to fit in the specified width:
5629 printf '<%.5s>', "truncated"; # prints "<trunc>"
5630 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5632 You can also get the precision from the next argument using C<.*>:
5634 printf '<%.6x>', 1; # prints "<000001>"
5635 printf '<%.*x>', 6, 1; # prints "<000001>"
5637 You cannot currently get the precision from a specified number,
5638 but it is intended that this will be possible in the future using
5641 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5645 For numeric conversions, you can specify the size to interpret the
5646 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5647 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5648 whatever the default integer size is on your platform (usually 32 or 64
5649 bits), but you can override this to use instead one of the standard C types,
5650 as supported by the compiler used to build Perl:
5652 l interpret integer as C type "long" or "unsigned long"
5653 h interpret integer as C type "short" or "unsigned short"
5654 q, L or ll interpret integer as C type "long long", "unsigned long long".
5655 or "quads" (typically 64-bit integers)
5657 The last will produce errors if Perl does not understand "quads" in your
5658 installation. (This requires that either the platform natively supports quads
5659 or Perl was specifically compiled to support quads.) You can find out
5660 whether your Perl supports quads via L<Config>:
5663 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5666 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5667 to be the default floating point size on your platform (double or long double),
5668 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5669 platform supports them. You can find out whether your Perl supports long
5670 doubles via L<Config>:
5673 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5675 You can find out whether Perl considers 'long double' to be the default
5676 floating point size to use on your platform via L<Config>:
5679 ($Config{uselongdouble} eq 'define') &&
5680 print "long doubles by default\n";
5682 It can also be the case that long doubles and doubles are the same thing:
5685 ($Config{doublesize} == $Config{longdblsize}) &&
5686 print "doubles are long doubles\n";
5688 The size specifier C<V> has no effect for Perl code, but it is supported
5689 for compatibility with XS code; it means 'use the standard size for
5690 a Perl integer (or floating-point number)', which is already the
5691 default for Perl code.
5693 =item order of arguments
5695 Normally, sprintf takes the next unused argument as the value to
5696 format for each format specification. If the format specification
5697 uses C<*> to require additional arguments, these are consumed from
5698 the argument list in the order in which they appear in the format
5699 specification I<before> the value to format. Where an argument is
5700 specified using an explicit index, this does not affect the normal
5701 order for the arguments (even when the explicitly specified index
5702 would have been the next argument in any case).
5706 printf '<%*.*s>', $a, $b, $c;
5708 would use C<$a> for the width, C<$b> for the precision and C<$c>
5709 as the value to format, while:
5711 print '<%*1$.*s>', $a, $b;
5713 would use C<$a> for the width and the precision, and C<$b> as the
5716 Here are some more examples - beware that when using an explicit
5717 index, the C<$> may need to be escaped:
5719 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5720 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5721 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5722 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5726 If C<use locale> is in effect, the character used for the decimal
5727 point in formatted real numbers is affected by the LC_NUMERIC locale.
5731 X<sqrt> X<root> X<square root>
5735 Return the square root of EXPR. If EXPR is omitted, returns square
5736 root of C<$_>. Only works on non-negative operands, unless you've
5737 loaded the standard Math::Complex module.
5740 print sqrt(-2); # prints 1.4142135623731i
5743 X<srand> X<seed> X<randseed>
5747 Sets the random number seed for the C<rand> operator.
5749 The point of the function is to "seed" the C<rand> function so that
5750 C<rand> can produce a different sequence each time you run your
5753 If srand() is not called explicitly, it is called implicitly at the
5754 first use of the C<rand> operator. However, this was not the case in
5755 versions of Perl before 5.004, so if your script will run under older
5756 Perl versions, it should call C<srand>.
5758 Most programs won't even call srand() at all, except those that
5759 need a cryptographically-strong starting point rather than the
5760 generally acceptable default, which is based on time of day,
5761 process ID, and memory allocation, or the F</dev/urandom> device,
5764 You can call srand($seed) with the same $seed to reproduce the
5765 I<same> sequence from rand(), but this is usually reserved for
5766 generating predictable results for testing or debugging.
5767 Otherwise, don't call srand() more than once in your program.
5769 Do B<not> call srand() (i.e. without an argument) more than once in
5770 a script. The internal state of the random number generator should
5771 contain more entropy than can be provided by any seed, so calling
5772 srand() again actually I<loses> randomness.
5774 Most implementations of C<srand> take an integer and will silently
5775 truncate decimal numbers. This means C<srand(42)> will usually
5776 produce the same results as C<srand(42.1)>. To be safe, always pass
5777 C<srand> an integer.
5779 In versions of Perl prior to 5.004 the default seed was just the
5780 current C<time>. This isn't a particularly good seed, so many old
5781 programs supply their own seed value (often C<time ^ $$> or C<time ^
5782 ($$ + ($$ << 15))>), but that isn't necessary any more.
5784 For cryptographic purposes, however, you need something much more random
5785 than the default seed. Checksumming the compressed output of one or more
5786 rapidly changing operating system status programs is the usual method. For
5789 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5791 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5794 Frequently called programs (like CGI scripts) that simply use
5798 for a seed can fall prey to the mathematical property that
5802 one-third of the time. So don't do that.
5804 =item stat FILEHANDLE
5805 X<stat> X<file, status>
5811 Returns a 13-element list giving the status info for a file, either
5812 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5813 it stats C<$_>. Returns a null list if the stat fails. Typically used
5816 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5817 $atime,$mtime,$ctime,$blksize,$blocks)
5820 Not all fields are supported on all filesystem types. Here are the
5821 meanings of the fields:
5823 0 dev device number of filesystem
5825 2 mode file mode (type and permissions)
5826 3 nlink number of (hard) links to the file
5827 4 uid numeric user ID of file's owner
5828 5 gid numeric group ID of file's owner
5829 6 rdev the device identifier (special files only)
5830 7 size total size of file, in bytes
5831 8 atime last access time in seconds since the epoch
5832 9 mtime last modify time in seconds since the epoch
5833 10 ctime inode change time in seconds since the epoch (*)
5834 11 blksize preferred block size for file system I/O
5835 12 blocks actual number of blocks allocated
5837 (The epoch was at 00:00 January 1, 1970 GMT.)
5839 (*) Not all fields are supported on all filesystem types. Notably, the
5840 ctime field is non-portable. In particular, you cannot expect it to be a
5841 "creation time", see L<perlport/"Files and Filesystems"> for details.
5843 If C<stat> is passed the special filehandle consisting of an underline, no
5844 stat is done, but the current contents of the stat structure from the
5845 last C<stat>, C<lstat>, or filetest are returned. Example:
5847 if (-x $file && (($d) = stat(_)) && $d < 0) {
5848 print "$file is executable NFS file\n";
5851 (This works on machines only for which the device number is negative
5854 Because the mode contains both the file type and its permissions, you
5855 should mask off the file type portion and (s)printf using a C<"%o">
5856 if you want to see the real permissions.
5858 $mode = (stat($filename))[2];
5859 printf "Permissions are %04o\n", $mode & 07777;
5861 In scalar context, C<stat> returns a boolean value indicating success
5862 or failure, and, if successful, sets the information associated with
5863 the special filehandle C<_>.
5865 The File::stat module provides a convenient, by-name access mechanism:
5868 $sb = stat($filename);
5869 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5870 $filename, $sb->size, $sb->mode & 07777,
5871 scalar localtime $sb->mtime;
5873 You can import symbolic mode constants (C<S_IF*>) and functions
5874 (C<S_IS*>) from the Fcntl module:
5878 $mode = (stat($filename))[2];
5880 $user_rwx = ($mode & S_IRWXU) >> 6;
5881 $group_read = ($mode & S_IRGRP) >> 3;
5882 $other_execute = $mode & S_IXOTH;
5884 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5886 $is_setuid = $mode & S_ISUID;
5887 $is_setgid = S_ISDIR($mode);
5889 You could write the last two using the C<-u> and C<-d> operators.
5890 The commonly available C<S_IF*> constants are
5892 # Permissions: read, write, execute, for user, group, others.
5894 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5895 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5896 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5898 # Setuid/Setgid/Stickiness/SaveText.
5899 # Note that the exact meaning of these is system dependent.
5901 S_ISUID S_ISGID S_ISVTX S_ISTXT
5903 # File types. Not necessarily all are available on your system.
5905 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5907 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5909 S_IREAD S_IWRITE S_IEXEC
5911 and the C<S_IF*> functions are
5913 S_IMODE($mode) the part of $mode containing the permission bits
5914 and the setuid/setgid/sticky bits
5916 S_IFMT($mode) the part of $mode containing the file type
5917 which can be bit-anded with e.g. S_IFREG
5918 or with the following functions
5920 # The operators -f, -d, -l, -b, -c, -p, and -S.
5922 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5923 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5925 # No direct -X operator counterpart, but for the first one
5926 # the -g operator is often equivalent. The ENFMT stands for
5927 # record flocking enforcement, a platform-dependent feature.
5929 S_ISENFMT($mode) S_ISWHT($mode)
5931 See your native chmod(2) and stat(2) documentation for more details
5932 about the C<S_*> constants. To get status info for a symbolic link
5933 instead of the target file behind the link, use the C<lstat> function.
5940 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5941 doing many pattern matches on the string before it is next modified.
5942 This may or may not save time, depending on the nature and number of
5943 patterns you are searching on, and on the distribution of character
5944 frequencies in the string to be searched--you probably want to compare
5945 run times with and without it to see which runs faster. Those loops
5946 that scan for many short constant strings (including the constant
5947 parts of more complex patterns) will benefit most. You may have only
5948 one C<study> active at a time--if you study a different scalar the first
5949 is "unstudied". (The way C<study> works is this: a linked list of every
5950 character in the string to be searched is made, so we know, for
5951 example, where all the C<'k'> characters are. From each search string,
5952 the rarest character is selected, based on some static frequency tables
5953 constructed from some C programs and English text. Only those places
5954 that contain this "rarest" character are examined.)
5956 For example, here is a loop that inserts index producing entries
5957 before any line containing a certain pattern:
5961 print ".IX foo\n" if /\bfoo\b/;
5962 print ".IX bar\n" if /\bbar\b/;
5963 print ".IX blurfl\n" if /\bblurfl\b/;
5968 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5969 will be looked at, because C<f> is rarer than C<o>. In general, this is
5970 a big win except in pathological cases. The only question is whether
5971 it saves you more time than it took to build the linked list in the
5974 Note that if you have to look for strings that you don't know till
5975 runtime, you can build an entire loop as a string and C<eval> that to
5976 avoid recompiling all your patterns all the time. Together with
5977 undefining C<$/> to input entire files as one record, this can be very
5978 fast, often faster than specialized programs like fgrep(1). The following
5979 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5980 out the names of those files that contain a match:
5982 $search = 'while (<>) { study;';
5983 foreach $word (@words) {
5984 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5989 eval $search; # this screams
5990 $/ = "\n"; # put back to normal input delimiter
5991 foreach $file (sort keys(%seen)) {
5995 =item sub NAME BLOCK
5998 =item sub NAME (PROTO) BLOCK
6000 =item sub NAME : ATTRS BLOCK
6002 =item sub NAME (PROTO) : ATTRS BLOCK
6004 This is subroutine definition, not a real function I<per se>.
6005 Without a BLOCK it's just a forward declaration. Without a NAME,
6006 it's an anonymous function declaration, and does actually return
6007 a value: the CODE ref of the closure you just created.
6009 See L<perlsub> and L<perlref> for details about subroutines and
6010 references, and L<attributes> and L<Attribute::Handlers> for more
6011 information about attributes.
6013 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6014 X<substr> X<substring> X<mid> X<left> X<right>
6016 =item substr EXPR,OFFSET,LENGTH
6018 =item substr EXPR,OFFSET
6020 Extracts a substring out of EXPR and returns it. First character is at
6021 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6022 If OFFSET is negative (or more precisely, less than C<$[>), starts
6023 that far from the end of the string. If LENGTH is omitted, returns
6024 everything to the end of the string. If LENGTH is negative, leaves that
6025 many characters off the end of the string.
6027 You can use the substr() function as an lvalue, in which case EXPR
6028 must itself be an lvalue. If you assign something shorter than LENGTH,
6029 the string will shrink, and if you assign something longer than LENGTH,
6030 the string will grow to accommodate it. To keep the string the same
6031 length you may need to pad or chop your value using C<sprintf>.
6033 If OFFSET and LENGTH specify a substring that is partly outside the
6034 string, only the part within the string is returned. If the substring
6035 is beyond either end of the string, substr() returns the undefined
6036 value and produces a warning. When used as an lvalue, specifying a
6037 substring that is entirely outside the string is a fatal error.
6038 Here's an example showing the behavior for boundary cases:
6041 substr($name, 4) = 'dy'; # $name is now 'freddy'
6042 my $null = substr $name, 6, 2; # returns '' (no warning)
6043 my $oops = substr $name, 7; # returns undef, with warning
6044 substr($name, 7) = 'gap'; # fatal error
6046 An alternative to using substr() as an lvalue is to specify the
6047 replacement string as the 4th argument. This allows you to replace
6048 parts of the EXPR and return what was there before in one operation,
6049 just as you can with splice().
6051 Note that the lvalue returned by the 3-arg version of substr() acts as
6052 a 'magic bullet'; each time it is assigned to, it remembers which part
6053 of the original string is being modified; for example:
6056 for (substr($x,1,2)) {
6057 $_ = 'a'; print $x,"\n"; # prints 1a4
6058 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6060 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6064 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6067 =item symlink OLDFILE,NEWFILE
6068 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6070 Creates a new filename symbolically linked to the old filename.
6071 Returns C<1> for success, C<0> otherwise. On systems that don't support
6072 symbolic links, produces a fatal error at run time. To check for that,
6075 $symlink_exists = eval { symlink("",""); 1 };
6077 =item syscall NUMBER, LIST
6078 X<syscall> X<system call>
6080 Calls the system call specified as the first element of the list,
6081 passing the remaining elements as arguments to the system call. If
6082 unimplemented, produces a fatal error. The arguments are interpreted
6083 as follows: if a given argument is numeric, the argument is passed as
6084 an int. If not, the pointer to the string value is passed. You are
6085 responsible to make sure a string is pre-extended long enough to
6086 receive any result that might be written into a string. You can't use a
6087 string literal (or other read-only string) as an argument to C<syscall>
6088 because Perl has to assume that any string pointer might be written
6090 integer arguments are not literals and have never been interpreted in a
6091 numeric context, you may need to add C<0> to them to force them to look
6092 like numbers. This emulates the C<syswrite> function (or vice versa):
6094 require 'syscall.ph'; # may need to run h2ph
6096 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6098 Note that Perl supports passing of up to only 14 arguments to your system call,
6099 which in practice should usually suffice.
6101 Syscall returns whatever value returned by the system call it calls.
6102 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6103 Note that some system calls can legitimately return C<-1>. The proper
6104 way to handle such calls is to assign C<$!=0;> before the call and
6105 check the value of C<$!> if syscall returns C<-1>.
6107 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6108 number of the read end of the pipe it creates. There is no way
6109 to retrieve the file number of the other end. You can avoid this
6110 problem by using C<pipe> instead.
6112 =item sysopen FILEHANDLE,FILENAME,MODE
6115 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6117 Opens the file whose filename is given by FILENAME, and associates it
6118 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6119 the name of the real filehandle wanted. This function calls the
6120 underlying operating system's C<open> function with the parameters
6121 FILENAME, MODE, PERMS.
6123 The possible values and flag bits of the MODE parameter are
6124 system-dependent; they are available via the standard module C<Fcntl>.
6125 See the documentation of your operating system's C<open> to see which
6126 values and flag bits are available. You may combine several flags
6127 using the C<|>-operator.
6129 Some of the most common values are C<O_RDONLY> for opening the file in
6130 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6131 and C<O_RDWR> for opening the file in read-write mode.
6132 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6134 For historical reasons, some values work on almost every system
6135 supported by perl: zero means read-only, one means write-only, and two
6136 means read/write. We know that these values do I<not> work under
6137 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6138 use them in new code.
6140 If the file named by FILENAME does not exist and the C<open> call creates
6141 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6142 PERMS specifies the permissions of the newly created file. If you omit
6143 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6144 These permission values need to be in octal, and are modified by your
6145 process's current C<umask>.
6148 In many systems the C<O_EXCL> flag is available for opening files in
6149 exclusive mode. This is B<not> locking: exclusiveness means here that
6150 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6151 on network filesystems, and has no effect unless the C<O_CREAT> flag
6152 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6153 being opened if it is a symbolic link. It does not protect against
6154 symbolic links in the file's path.
6157 Sometimes you may want to truncate an already-existing file. This
6158 can be done using the C<O_TRUNC> flag. The behavior of
6159 C<O_TRUNC> with C<O_RDONLY> is undefined.
6162 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6163 that takes away the user's option to have a more permissive umask.
6164 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6167 Note that C<sysopen> depends on the fdopen() C library function.
6168 On many UNIX systems, fdopen() is known to fail when file descriptors
6169 exceed a certain value, typically 255. If you need more file
6170 descriptors than that, consider rebuilding Perl to use the C<sfio>
6171 library, or perhaps using the POSIX::open() function.
6173 See L<perlopentut> for a kinder, gentler explanation of opening files.
6175 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6178 =item sysread FILEHANDLE,SCALAR,LENGTH
6180 Attempts to read LENGTH bytes of data into variable SCALAR from the
6181 specified FILEHANDLE, using the system call read(2). It bypasses
6182 buffered IO, so mixing this with other kinds of reads, C<print>,
6183 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6184 perlio or stdio layers usually buffers data. Returns the number of
6185 bytes actually read, C<0> at end of file, or undef if there was an
6186 error (in the latter case C<$!> is also set). SCALAR will be grown or
6187 shrunk so that the last byte actually read is the last byte of the
6188 scalar after the read.
6190 An OFFSET may be specified to place the read data at some place in the
6191 string other than the beginning. A negative OFFSET specifies
6192 placement at that many characters counting backwards from the end of
6193 the string. A positive OFFSET greater than the length of SCALAR
6194 results in the string being padded to the required size with C<"\0">
6195 bytes before the result of the read is appended.
6197 There is no syseof() function, which is ok, since eof() doesn't work
6198 very well on device files (like ttys) anyway. Use sysread() and check
6199 for a return value for 0 to decide whether you're done.
6201 Note that if the filehandle has been marked as C<:utf8> Unicode
6202 characters are read instead of bytes (the LENGTH, OFFSET, and the
6203 return value of sysread() are in Unicode characters).
6204 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6205 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6207 =item sysseek FILEHANDLE,POSITION,WHENCE
6210 Sets FILEHANDLE's system position in bytes using the system call
6211 lseek(2). FILEHANDLE may be an expression whose value gives the name
6212 of the filehandle. The values for WHENCE are C<0> to set the new
6213 position to POSITION, C<1> to set the it to the current position plus
6214 POSITION, and C<2> to set it to EOF plus POSITION (typically
6217 Note the I<in bytes>: even if the filehandle has been set to operate
6218 on characters (for example by using the C<:utf8> I/O layer), tell()
6219 will return byte offsets, not character offsets (because implementing
6220 that would render sysseek() very slow).
6222 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6223 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6224 C<seek>, C<tell>, or C<eof> may cause confusion.
6226 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6227 and C<SEEK_END> (start of the file, current position, end of the file)
6228 from the Fcntl module. Use of the constants is also more portable
6229 than relying on 0, 1, and 2. For example to define a "systell" function:
6231 use Fcntl 'SEEK_CUR';
6232 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6234 Returns the new position, or the undefined value on failure. A position
6235 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6236 true on success and false on failure, yet you can still easily determine
6242 =item system PROGRAM LIST
6244 Does exactly the same thing as C<exec LIST>, except that a fork is
6245 done first, and the parent process waits for the child process to
6246 complete. Note that argument processing varies depending on the
6247 number of arguments. If there is more than one argument in LIST,
6248 or if LIST is an array with more than one value, starts the program
6249 given by the first element of the list with arguments given by the
6250 rest of the list. If there is only one scalar argument, the argument
6251 is checked for shell metacharacters, and if there are any, the
6252 entire argument is passed to the system's command shell for parsing
6253 (this is C</bin/sh -c> on Unix platforms, but varies on other
6254 platforms). If there are no shell metacharacters in the argument,
6255 it is split into words and passed directly to C<execvp>, which is
6258 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6259 output before any operation that may do a fork, but this may not be
6260 supported on some platforms (see L<perlport>). To be safe, you may need
6261 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6262 of C<IO::Handle> on any open handles.
6264 The return value is the exit status of the program as returned by the
6265 C<wait> call. To get the actual exit value, shift right by eight (see
6266 below). See also L</exec>. This is I<not> what you want to use to capture
6267 the output from a command, for that you should use merely backticks or
6268 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6269 indicates a failure to start the program or an error of the wait(2) system
6270 call (inspect $! for the reason).
6272 Like C<exec>, C<system> allows you to lie to a program about its name if
6273 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6275 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6276 C<system>, if you expect your program to terminate on receipt of these
6277 signals you will need to arrange to do so yourself based on the return
6280 @args = ("command", "arg1", "arg2");
6282 or die "system @args failed: $?"
6284 You can check all the failure possibilities by inspecting
6288 print "failed to execute: $!\n";
6291 printf "child died with signal %d, %s coredump\n",
6292 ($? & 127), ($? & 128) ? 'with' : 'without';
6295 printf "child exited with value %d\n", $? >> 8;
6298 Alternatively you might inspect the value of C<${^CHILD_ERROR_NATIVE}>
6299 with the W*() calls of the POSIX extension.
6301 When the arguments get executed via the system shell, results
6302 and return codes will be subject to its quirks and capabilities.
6303 See L<perlop/"`STRING`"> and L</exec> for details.
6305 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6308 =item syswrite FILEHANDLE,SCALAR,LENGTH
6310 =item syswrite FILEHANDLE,SCALAR
6312 Attempts to write LENGTH bytes of data from variable SCALAR to the
6313 specified FILEHANDLE, using the system call write(2). If LENGTH is
6314 not specified, writes whole SCALAR. It bypasses buffered IO, so
6315 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6316 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6317 stdio layers usually buffers data. Returns the number of bytes
6318 actually written, or C<undef> if there was an error (in this case the
6319 errno variable C<$!> is also set). If the LENGTH is greater than the
6320 available data in the SCALAR after the OFFSET, only as much data as is
6321 available will be written.
6323 An OFFSET may be specified to write the data from some part of the
6324 string other than the beginning. A negative OFFSET specifies writing
6325 that many characters counting backwards from the end of the string.
6326 In the case the SCALAR is empty you can use OFFSET but only zero offset.
6328 Note that if the filehandle has been marked as C<:utf8>, Unicode
6329 characters are written instead of bytes (the LENGTH, OFFSET, and the
6330 return value of syswrite() are in UTF-8 encoded Unicode characters).
6331 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6332 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6334 =item tell FILEHANDLE
6339 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6340 error. FILEHANDLE may be an expression whose value gives the name of
6341 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6344 Note the I<in bytes>: even if the filehandle has been set to
6345 operate on characters (for example by using the C<:utf8> open
6346 layer), tell() will return byte offsets, not character offsets
6347 (because that would render seek() and tell() rather slow).
6349 The return value of tell() for the standard streams like the STDIN
6350 depends on the operating system: it may return -1 or something else.
6351 tell() on pipes, fifos, and sockets usually returns -1.
6353 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6355 Do not use tell() (or other buffered I/O operations) on a file handle
6356 that has been manipulated by sysread(), syswrite() or sysseek().
6357 Those functions ignore the buffering, while tell() does not.
6359 =item telldir DIRHANDLE
6362 Returns the current position of the C<readdir> routines on DIRHANDLE.
6363 Value may be given to C<seekdir> to access a particular location in a
6364 directory. C<telldir> has the same caveats about possible directory
6365 compaction as the corresponding system library routine.
6367 =item tie VARIABLE,CLASSNAME,LIST
6370 This function binds a variable to a package class that will provide the
6371 implementation for the variable. VARIABLE is the name of the variable
6372 to be enchanted. CLASSNAME is the name of a class implementing objects
6373 of correct type. Any additional arguments are passed to the C<new>
6374 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6375 or C<TIEHASH>). Typically these are arguments such as might be passed
6376 to the C<dbm_open()> function of C. The object returned by the C<new>
6377 method is also returned by the C<tie> function, which would be useful
6378 if you want to access other methods in CLASSNAME.
6380 Note that functions such as C<keys> and C<values> may return huge lists
6381 when used on large objects, like DBM files. You may prefer to use the
6382 C<each> function to iterate over such. Example:
6384 # print out history file offsets
6386 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6387 while (($key,$val) = each %HIST) {
6388 print $key, ' = ', unpack('L',$val), "\n";
6392 A class implementing a hash should have the following methods:
6394 TIEHASH classname, LIST
6396 STORE this, key, value
6401 NEXTKEY this, lastkey
6406 A class implementing an ordinary array should have the following methods:
6408 TIEARRAY classname, LIST
6410 STORE this, key, value
6412 STORESIZE this, count
6418 SPLICE this, offset, length, LIST
6423 A class implementing a file handle should have the following methods:
6425 TIEHANDLE classname, LIST
6426 READ this, scalar, length, offset
6429 WRITE this, scalar, length, offset
6431 PRINTF this, format, LIST
6435 SEEK this, position, whence
6437 OPEN this, mode, LIST
6442 A class implementing a scalar should have the following methods:
6444 TIESCALAR classname, LIST
6450 Not all methods indicated above need be implemented. See L<perltie>,
6451 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
6453 Unlike C<dbmopen>, the C<tie> function will not use or require a module
6454 for you--you need to do that explicitly yourself. See L<DB_File>
6455 or the F<Config> module for interesting C<tie> implementations.
6457 For further details see L<perltie>, L<"tied VARIABLE">.
6462 Returns a reference to the object underlying VARIABLE (the same value
6463 that was originally returned by the C<tie> call that bound the variable
6464 to a package.) Returns the undefined value if VARIABLE isn't tied to a
6470 Returns the number of non-leap seconds since whatever time the system
6471 considers to be the epoch, suitable for feeding to C<gmtime> and
6472 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
6473 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
6474 1904 in the current local time zone for its epoch.
6476 For measuring time in better granularity than one second,
6477 you may use either the Time::HiRes module (from CPAN, and starting from
6478 Perl 5.8 part of the standard distribution), or if you have
6479 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
6480 See L<perlfaq8> for details.
6485 Returns a four-element list giving the user and system times, in
6486 seconds, for this process and the children of this process.
6488 ($user,$system,$cuser,$csystem) = times;
6490 In scalar context, C<times> returns C<$user>.
6494 The transliteration operator. Same as C<y///>. See L<perlop>.
6496 =item truncate FILEHANDLE,LENGTH
6499 =item truncate EXPR,LENGTH
6501 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
6502 specified length. Produces a fatal error if truncate isn't implemented
6503 on your system. Returns true if successful, the undefined value
6506 The behavior is undefined if LENGTH is greater than the length of the
6509 The position in the file of FILEHANDLE is left unchanged. You may want to
6510 call L<seek> before writing to the file.
6513 X<uc> X<uppercase> X<toupper>
6517 Returns an uppercased version of EXPR. This is the internal function
6518 implementing the C<\U> escape in double-quoted strings. Respects
6519 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
6520 and L<perlunicode> for more details about locale and Unicode support.
6521 It does not attempt to do titlecase mapping on initial letters. See
6522 C<ucfirst> for that.
6524 If EXPR is omitted, uses C<$_>.
6527 X<ucfirst> X<uppercase>
6531 Returns the value of EXPR with the first character in uppercase
6532 (titlecase in Unicode). This is the internal function implementing
6533 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
6534 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
6535 for more details about locale and Unicode support.
6537 If EXPR is omitted, uses C<$_>.
6544 Sets the umask for the process to EXPR and returns the previous value.
6545 If EXPR is omitted, merely returns the current umask.
6547 The Unix permission C<rwxr-x---> is represented as three sets of three
6548 bits, or three octal digits: C<0750> (the leading 0 indicates octal
6549 and isn't one of the digits). The C<umask> value is such a number
6550 representing disabled permissions bits. The permission (or "mode")
6551 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
6552 even if you tell C<sysopen> to create a file with permissions C<0777>,
6553 if your umask is C<0022> then the file will actually be created with
6554 permissions C<0755>. If your C<umask> were C<0027> (group can't
6555 write; others can't read, write, or execute), then passing
6556 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
6559 Here's some advice: supply a creation mode of C<0666> for regular
6560 files (in C<sysopen>) and one of C<0777> for directories (in
6561 C<mkdir>) and executable files. This gives users the freedom of
6562 choice: if they want protected files, they might choose process umasks
6563 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
6564 Programs should rarely if ever make policy decisions better left to
6565 the user. The exception to this is when writing files that should be
6566 kept private: mail files, web browser cookies, I<.rhosts> files, and
6569 If umask(2) is not implemented on your system and you are trying to
6570 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
6571 fatal error at run time. If umask(2) is not implemented and you are
6572 not trying to restrict access for yourself, returns C<undef>.
6574 Remember that a umask is a number, usually given in octal; it is I<not> a
6575 string of octal digits. See also L</oct>, if all you have is a string.
6578 X<undef> X<undefine>
6582 Undefines the value of EXPR, which must be an lvalue. Use only on a
6583 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
6584 (using C<&>), or a typeglob (using C<*>). (Saying C<undef $hash{$key}>
6585 will probably not do what you expect on most predefined variables or
6586 DBM list values, so don't do that; see L<delete>.) Always returns the
6587 undefined value. You can omit the EXPR, in which case nothing is
6588 undefined, but you still get an undefined value that you could, for
6589 instance, return from a subroutine, assign to a variable or pass as a
6590 parameter. Examples:
6593 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
6597 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
6598 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
6599 select undef, undef, undef, 0.25;
6600 ($a, $b, undef, $c) = &foo; # Ignore third value returned
6602 Note that this is a unary operator, not a list operator.
6605 X<unlink> X<delete> X<remove> X<rm>
6609 Deletes a list of files. Returns the number of files successfully
6612 $cnt = unlink 'a', 'b', 'c';
6616 Note: C<unlink> will not attempt to delete directories unless you are superuser
6617 and the B<-U> flag is supplied to Perl. Even if these conditions are
6618 met, be warned that unlinking a directory can inflict damage on your
6619 filesystem. Finally, using C<unlink> on directories is not supported on
6620 many operating systems. Use C<rmdir> instead.
6622 If LIST is omitted, uses C<$_>.
6624 =item unpack TEMPLATE,EXPR
6627 =item unpack TEMPLATE
6629 C<unpack> does the reverse of C<pack>: it takes a string
6630 and expands it out into a list of values.
6631 (In scalar context, it returns merely the first value produced.)
6633 If EXPR is omitted, unpacks the C<$_> string.
6635 The string is broken into chunks described by the TEMPLATE. Each chunk
6636 is converted separately to a value. Typically, either the string is a result
6637 of C<pack>, or the characters of the string represent a C structure of some
6640 The TEMPLATE has the same format as in the C<pack> function.
6641 Here's a subroutine that does substring:
6644 my($what,$where,$howmuch) = @_;
6645 unpack("x$where a$howmuch", $what);
6650 sub ordinal { unpack("W",$_[0]); } # same as ord()
6652 In addition to fields allowed in pack(), you may prefix a field with
6653 a %<number> to indicate that
6654 you want a <number>-bit checksum of the items instead of the items
6655 themselves. Default is a 16-bit checksum. Checksum is calculated by
6656 summing numeric values of expanded values (for string fields the sum of
6657 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6659 For example, the following
6660 computes the same number as the System V sum program:
6664 unpack("%32W*",<>) % 65535;
6667 The following efficiently counts the number of set bits in a bit vector:
6669 $setbits = unpack("%32b*", $selectmask);
6671 The C<p> and C<P> formats should be used with care. Since Perl
6672 has no way of checking whether the value passed to C<unpack()>
6673 corresponds to a valid memory location, passing a pointer value that's
6674 not known to be valid is likely to have disastrous consequences.
6676 If there are more pack codes or if the repeat count of a field or a group
6677 is larger than what the remainder of the input string allows, the result
6678 is not well defined: in some cases, the repeat count is decreased, or
6679 C<unpack()> will produce null strings or zeroes, or terminate with an
6680 error. If the input string is longer than one described by the TEMPLATE,
6681 the rest is ignored.
6683 See L</pack> for more examples and notes.
6685 =item untie VARIABLE
6688 Breaks the binding between a variable and a package. (See C<tie>.)
6689 Has no effect if the variable is not tied.
6691 =item unshift ARRAY,LIST
6694 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6695 depending on how you look at it. Prepends list to the front of the
6696 array, and returns the new number of elements in the array.
6698 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6700 Note the LIST is prepended whole, not one element at a time, so the
6701 prepended elements stay in the same order. Use C<reverse> to do the
6704 =item use Module VERSION LIST
6705 X<use> X<module> X<import>
6707 =item use Module VERSION
6709 =item use Module LIST
6715 Imports some semantics into the current package from the named module,
6716 generally by aliasing certain subroutine or variable names into your
6717 package. It is exactly equivalent to
6719 BEGIN { require Module; import Module LIST; }
6721 except that Module I<must> be a bareword.
6723 VERSION may be either a numeric argument such as 5.006, which will be
6724 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6725 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6726 greater than the version of the current Perl interpreter; Perl will not
6727 attempt to parse the rest of the file. Compare with L</require>, which can
6728 do a similar check at run time.
6730 Specifying VERSION as a literal of the form v5.6.1 should generally be
6731 avoided, because it leads to misleading error messages under earlier
6732 versions of Perl that do not support this syntax. The equivalent numeric
6733 version should be used instead.
6735 use v5.6.1; # compile time version check
6737 use 5.006_001; # ditto; preferred for backwards compatibility
6739 This is often useful if you need to check the current Perl version before
6740 C<use>ing library modules that have changed in incompatible ways from
6741 older versions of Perl. (We try not to do this more than we have to.)
6743 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6744 C<require> makes sure the module is loaded into memory if it hasn't been
6745 yet. The C<import> is not a builtin--it's just an ordinary static method
6746 call into the C<Module> package to tell the module to import the list of
6747 features back into the current package. The module can implement its
6748 C<import> method any way it likes, though most modules just choose to
6749 derive their C<import> method via inheritance from the C<Exporter> class that
6750 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6751 method can be found then the call is skipped, even if there is an AUTOLOAD
6754 If you do not want to call the package's C<import> method (for instance,
6755 to stop your namespace from being altered), explicitly supply the empty list:
6759 That is exactly equivalent to
6761 BEGIN { require Module }
6763 If the VERSION argument is present between Module and LIST, then the
6764 C<use> will call the VERSION method in class Module with the given
6765 version as an argument. The default VERSION method, inherited from
6766 the UNIVERSAL class, croaks if the given version is larger than the
6767 value of the variable C<$Module::VERSION>.
6769 Again, there is a distinction between omitting LIST (C<import> called
6770 with no arguments) and an explicit empty LIST C<()> (C<import> not
6771 called). Note that there is no comma after VERSION!
6773 Because this is a wide-open interface, pragmas (compiler directives)
6774 are also implemented this way. Currently implemented pragmas are:
6779 use sigtrap qw(SEGV BUS);
6780 use strict qw(subs vars refs);
6781 use subs qw(afunc blurfl);
6782 use warnings qw(all);
6783 use sort qw(stable _quicksort _mergesort);
6785 Some of these pseudo-modules import semantics into the current
6786 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6787 which import symbols into the current package (which are effective
6788 through the end of the file).
6790 There's a corresponding C<no> command that unimports meanings imported
6791 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6792 It behaves exactly as C<import> does with respect to VERSION, an
6793 omitted LIST, empty LIST, or no unimport method being found.
6799 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6800 for the C<-M> and C<-m> command-line options to perl that give C<use>
6801 functionality from the command-line.
6806 Changes the access and modification times on each file of a list of
6807 files. The first two elements of the list must be the NUMERICAL access
6808 and modification times, in that order. Returns the number of files
6809 successfully changed. The inode change time of each file is set
6810 to the current time. For example, this code has the same effect as the
6811 Unix touch(1) command when the files I<already exist> and belong to
6812 the user running the program:
6815 $atime = $mtime = time;
6816 utime $atime, $mtime, @ARGV;
6818 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6819 the utime(2) function in the C library will be called with a null second
6820 argument. On most systems, this will set the file's access and
6821 modification times to the current time (i.e. equivalent to the example
6822 above) and will even work on other users' files where you have write
6825 utime undef, undef, @ARGV;
6827 Under NFS this will use the time of the NFS server, not the time of
6828 the local machine. If there is a time synchronization problem, the
6829 NFS server and local machine will have different times. The Unix
6830 touch(1) command will in fact normally use this form instead of the
6831 one shown in the first example.
6833 Note that only passing one of the first two elements as C<undef> will
6834 be equivalent of passing it as 0 and will not have the same effect as
6835 described when they are both C<undef>. This case will also trigger an
6836 uninitialized warning.
6838 On systems that support futimes, you might pass file handles among the
6839 files. On systems that don't support futimes, passing file handles
6840 produces a fatal error at run time. The file handles must be passed
6841 as globs or references to be recognized. Barewords are considered
6847 Returns a list consisting of all the values of the named hash.
6848 (In a scalar context, returns the number of values.)
6850 The values are returned in an apparently random order. The actual
6851 random order is subject to change in future versions of perl, but it
6852 is guaranteed to be the same order as either the C<keys> or C<each>
6853 function would produce on the same (unmodified) hash. Since Perl
6854 5.8.1 the ordering is different even between different runs of Perl
6855 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
6857 As a side effect, calling values() resets the HASH's internal iterator,
6858 see L</each>. (In particular, calling values() in void context resets
6859 the iterator with no other overhead.)
6861 Note that the values are not copied, which means modifying them will
6862 modify the contents of the hash:
6864 for (values %hash) { s/foo/bar/g } # modifies %hash values
6865 for (@hash{keys %hash}) { s/foo/bar/g } # same
6867 See also C<keys>, C<each>, and C<sort>.
6869 =item vec EXPR,OFFSET,BITS
6870 X<vec> X<bit> X<bit vector>
6872 Treats the string in EXPR as a bit vector made up of elements of
6873 width BITS, and returns the value of the element specified by OFFSET
6874 as an unsigned integer. BITS therefore specifies the number of bits
6875 that are reserved for each element in the bit vector. This must
6876 be a power of two from 1 to 32 (or 64, if your platform supports
6879 If BITS is 8, "elements" coincide with bytes of the input string.
6881 If BITS is 16 or more, bytes of the input string are grouped into chunks
6882 of size BITS/8, and each group is converted to a number as with
6883 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6884 for BITS==64). See L<"pack"> for details.
6886 If bits is 4 or less, the string is broken into bytes, then the bits
6887 of each byte are broken into 8/BITS groups. Bits of a byte are
6888 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6889 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6890 breaking the single input byte C<chr(0x36)> into two groups gives a list
6891 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6893 C<vec> may also be assigned to, in which case parentheses are needed
6894 to give the expression the correct precedence as in
6896 vec($image, $max_x * $x + $y, 8) = 3;
6898 If the selected element is outside the string, the value 0 is returned.
6899 If an element off the end of the string is written to, Perl will first
6900 extend the string with sufficiently many zero bytes. It is an error
6901 to try to write off the beginning of the string (i.e. negative OFFSET).
6903 The string should not contain any character with the value > 255 (which
6904 can only happen if you're using UTF-8 encoding). If it does, it will be
6905 treated as something that is not UTF-8 encoded. When the C<vec> was
6906 assigned to, other parts of your program will also no longer consider the
6907 string to be UTF-8 encoded. In other words, if you do have such characters
6908 in your string, vec() will operate on the actual byte string, and not the
6909 conceptual character string.
6911 Strings created with C<vec> can also be manipulated with the logical
6912 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6913 vector operation is desired when both operands are strings.
6914 See L<perlop/"Bitwise String Operators">.
6916 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6917 The comments show the string after each step. Note that this code works
6918 in the same way on big-endian or little-endian machines.
6921 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6923 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6924 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6926 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6927 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6928 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6929 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6930 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6931 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6933 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6934 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6935 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6938 To transform a bit vector into a string or list of 0's and 1's, use these:
6940 $bits = unpack("b*", $vector);
6941 @bits = split(//, unpack("b*", $vector));
6943 If you know the exact length in bits, it can be used in place of the C<*>.
6945 Here is an example to illustrate how the bits actually fall in place:
6951 unpack("V",$_) 01234567890123456789012345678901
6952 ------------------------------------------------------------------
6957 for ($shift=0; $shift < $width; ++$shift) {
6958 for ($off=0; $off < 32/$width; ++$off) {
6959 $str = pack("B*", "0"x32);
6960 $bits = (1<<$shift);
6961 vec($str, $off, $width) = $bits;
6962 $res = unpack("b*",$str);
6963 $val = unpack("V", $str);
6970 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6971 $off, $width, $bits, $val, $res
6975 Regardless of the machine architecture on which it is run, the above
6976 example should print the following table:
6979 unpack("V",$_) 01234567890123456789012345678901
6980 ------------------------------------------------------------------
6981 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6982 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6983 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6984 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6985 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6986 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6987 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6988 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6989 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6990 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6991 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6992 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6993 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6994 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6995 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6996 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6997 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6998 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6999 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7000 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7001 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7002 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7003 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7004 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7005 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7006 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7007 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7008 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7009 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7010 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7011 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7012 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7013 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7014 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7015 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7016 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7017 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7018 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7019 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7020 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7021 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7022 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7023 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7024 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7025 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7026 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7027 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7028 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7029 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7030 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7031 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7032 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7033 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7034 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7035 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7036 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7037 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7038 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7039 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7040 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7041 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7042 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7043 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7044 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7045 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7046 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7047 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7048 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7049 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7050 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7051 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7052 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7053 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7054 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7055 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7056 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7057 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7058 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7059 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7060 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7061 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7062 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7063 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7064 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7065 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7066 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7067 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7068 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7069 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7070 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7071 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7072 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7073 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7074 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7075 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7076 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7077 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7078 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7079 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7080 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7081 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7082 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7083 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7084 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7085 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7086 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7087 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7088 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7089 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7090 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7091 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7092 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7093 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7094 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7095 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7096 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7097 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7098 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7099 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7100 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7101 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7102 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7103 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7104 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7105 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7106 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7107 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7108 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7113 Behaves like the wait(2) system call on your system: it waits for a child
7114 process to terminate and returns the pid of the deceased process, or
7115 C<-1> if there are no child processes. The status is returned in C<$?>
7116 and C<{^CHILD_ERROR_NATIVE}>.
7117 Note that a return value of C<-1> could mean that child processes are
7118 being automatically reaped, as described in L<perlipc>.
7120 =item waitpid PID,FLAGS
7123 Waits for a particular child process to terminate and returns the pid of
7124 the deceased process, or C<-1> if there is no such child process. On some
7125 systems, a value of 0 indicates that there are processes still running.
7126 The status is returned in C<$?> and C<{^CHILD_ERROR_NATIVE}>. If you say
7128 use POSIX ":sys_wait_h";
7131 $kid = waitpid(-1, WNOHANG);
7134 then you can do a non-blocking wait for all pending zombie processes.
7135 Non-blocking wait is available on machines supporting either the
7136 waitpid(2) or wait4(2) system calls. However, waiting for a particular
7137 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7138 system call by remembering the status values of processes that have
7139 exited but have not been harvested by the Perl script yet.)
7141 Note that on some systems, a return value of C<-1> could mean that child
7142 processes are being automatically reaped. See L<perlipc> for details,
7143 and for other examples.
7146 X<wantarray> X<context>
7148 Returns true if the context of the currently executing subroutine or
7149 C<eval> is looking for a list value. Returns false if the context is
7150 looking for a scalar. Returns the undefined value if the context is
7151 looking for no value (void context).
7153 return unless defined wantarray; # don't bother doing more
7154 my @a = complex_calculation();
7155 return wantarray ? @a : "@a";
7157 C<wantarray()>'s result is unspecified in the top level of a file,
7158 in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
7161 This function should have been named wantlist() instead.
7164 X<warn> X<warning> X<STDERR>
7166 Produces a message on STDERR just like C<die>, but doesn't exit or throw
7169 If LIST is empty and C<$@> already contains a value (typically from a
7170 previous eval) that value is used after appending C<"\t...caught">
7171 to C<$@>. This is useful for staying almost, but not entirely similar to
7174 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7176 No message is printed if there is a C<$SIG{__WARN__}> handler
7177 installed. It is the handler's responsibility to deal with the message
7178 as it sees fit (like, for instance, converting it into a C<die>). Most
7179 handlers must therefore make arrangements to actually display the
7180 warnings that they are not prepared to deal with, by calling C<warn>
7181 again in the handler. Note that this is quite safe and will not
7182 produce an endless loop, since C<__WARN__> hooks are not called from
7185 You will find this behavior is slightly different from that of
7186 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7187 instead call C<die> again to change it).
7189 Using a C<__WARN__> handler provides a powerful way to silence all
7190 warnings (even the so-called mandatory ones). An example:
7192 # wipe out *all* compile-time warnings
7193 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7195 my $foo = 20; # no warning about duplicate my $foo,
7196 # but hey, you asked for it!
7197 # no compile-time or run-time warnings before here
7200 # run-time warnings enabled after here
7201 warn "\$foo is alive and $foo!"; # does show up
7203 See L<perlvar> for details on setting C<%SIG> entries, and for more
7204 examples. See the Carp module for other kinds of warnings using its
7205 carp() and cluck() functions.
7207 =item write FILEHANDLE
7214 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7215 using the format associated with that file. By default the format for
7216 a file is the one having the same name as the filehandle, but the
7217 format for the current output channel (see the C<select> function) may be set
7218 explicitly by assigning the name of the format to the C<$~> variable.
7220 Top of form processing is handled automatically: if there is
7221 insufficient room on the current page for the formatted record, the
7222 page is advanced by writing a form feed, a special top-of-page format
7223 is used to format the new page header, and then the record is written.
7224 By default the top-of-page format is the name of the filehandle with
7225 "_TOP" appended, but it may be dynamically set to the format of your
7226 choice by assigning the name to the C<$^> variable while the filehandle is
7227 selected. The number of lines remaining on the current page is in
7228 variable C<$->, which can be set to C<0> to force a new page.
7230 If FILEHANDLE is unspecified, output goes to the current default output
7231 channel, which starts out as STDOUT but may be changed by the
7232 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7233 is evaluated and the resulting string is used to look up the name of
7234 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7236 Note that write is I<not> the opposite of C<read>. Unfortunately.
7240 The transliteration operator. Same as C<tr///>. See L<perlop>.