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 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, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance, splice() has three scalar
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 elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate literal elements of the LIST.
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 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. Whitespace
37 between the function and left parenthesis doesn't count, so sometimes
38 you need to be careful:
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 scalar context by
59 returning the undefined value, and in 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 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 ("syscalls")
82 of the same name (like chown(2), fork(2), closedir(2), etc.) return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule include C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 Extension modules can also hook into the Perl parser to define new
90 kinds of keyword-headed expression. These may look like functions, but
91 may also look completely different. The syntax following the keyword
92 is defined entirely by the extension. If you are an implementor, see
93 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94 a module, see the module's documentation for details of the syntax that
97 =head2 Perl Functions by Category
100 Here are Perl's functions (including things that look like
101 functions, like some keywords and named operators)
102 arranged by category. Some functions appear in more
107 =item Functions for SCALARs or strings
108 X<scalar> X<string> X<character>
110 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
111 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
112 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
114 =item Regular expressions and pattern matching
115 X<regular expression> X<regex> X<regexp>
117 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
119 =item Numeric functions
120 X<numeric> X<number> X<trigonometric> X<trigonometry>
122 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123 C<sin>, C<sqrt>, C<srand>
125 =item Functions for real @ARRAYs
128 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
130 =item Functions for list data
133 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
135 =item Functions for real %HASHes
138 C<delete>, C<each>, C<exists>, C<keys>, C<values>
140 =item Input and output functions
141 X<I/O> X<input> X<output> X<dbm>
143 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
145 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
146 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
149 =item Functions for fixed-length data or records
151 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
153 =item Functions for filehandles, files, or directories
154 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
156 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
157 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
158 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159 C<umask>, C<unlink>, C<utime>
161 =item Keywords related to the control flow of your Perl program
164 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165 C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
166 C<redo>, C<return>, C<sub>, C<wantarray>,
168 =item Keywords related to the switch feature
170 C<break>, C<continue>, C<default>, C<given>, C<when>
172 Except for C<continue>, these are available only if you enable the
173 C<"switch"> feature or use the C<CORE::> prefix.
174 See L<feature> and L<perlsyn/"Switch statements">.
175 Alternately, include a C<use v5.10> or later to the current scope. In Perl
176 5.14 and earlier, C<continue> required the C<"switch"> feature, like the
179 =item Keywords related to scoping
181 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
183 C<state> is available only if the C<"state"> feature
184 is enabled or if it is prefixed with C<CORE::>. See
185 L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
187 =item Miscellaneous functions
189 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
190 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
192 =item Functions for processes and process groups
193 X<process> X<pid> X<process id>
195 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
196 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
197 C<times>, C<wait>, C<waitpid>
199 =item Keywords related to Perl modules
202 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
204 =item Keywords related to classes and object-orientation
205 X<object> X<class> X<package>
207 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
210 =item Low-level socket functions
213 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
214 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
215 C<socket>, C<socketpair>
217 =item System V interprocess communication functions
218 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
220 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
221 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
223 =item Fetching user and group info
224 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
226 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
227 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
228 C<getpwuid>, C<setgrent>, C<setpwent>
230 =item Fetching network info
231 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
233 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
234 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
235 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
236 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
237 C<setnetent>, C<setprotoent>, C<setservent>
239 =item Time-related functions
242 C<gmtime>, C<localtime>, C<time>, C<times>
244 =item Functions new in perl5
247 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
248 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
249 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
250 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
251 C<ucfirst>, C<untie>, C<use>, C<when>
253 * C<sub> was a keyword in Perl 4, but in Perl 5 it is an
254 operator, which can be used in expressions.
256 =item Functions obsoleted in perl5
258 C<dbmclose>, C<dbmopen>
263 X<portability> X<Unix> X<portable>
265 Perl was born in Unix and can therefore access all common Unix
266 system calls. In non-Unix environments, the functionality of some
267 Unix system calls may not be available or details of the available
268 functionality may differ slightly. The Perl functions affected
271 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
272 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
273 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
274 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
275 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
276 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
277 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
278 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
279 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
280 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
281 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
282 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
283 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
284 C<shmwrite>, C<socket>, C<socketpair>,
285 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
286 C<times>, C<truncate>, C<umask>, C<unlink>,
287 C<utime>, C<wait>, C<waitpid>
289 For more information about the portability of these functions, see
290 L<perlport> and other available platform-specific documentation.
292 =head2 Alphabetical Listing of Perl Functions
297 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>
298 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
306 A file test, where X is one of the letters listed below. This unary
307 operator takes one argument, either a filename, a filehandle, or a dirhandle,
308 and tests the associated file to see if something is true about it. If the
309 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
310 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
311 the undefined value if the file doesn't exist. Despite the funny
312 names, precedence is the same as any other named unary operator. The
313 operator may be any of:
315 -r File is readable by effective uid/gid.
316 -w File is writable by effective uid/gid.
317 -x File is executable by effective uid/gid.
318 -o File is owned by effective uid.
320 -R File is readable by real uid/gid.
321 -W File is writable by real uid/gid.
322 -X File is executable by real uid/gid.
323 -O File is owned by real uid.
326 -z File has zero size (is empty).
327 -s File has nonzero size (returns size in bytes).
329 -f File is a plain file.
330 -d File is a directory.
331 -l File is a symbolic link.
332 -p File is a named pipe (FIFO), or Filehandle is a pipe.
334 -b File is a block special file.
335 -c File is a character special file.
336 -t Filehandle is opened to a tty.
338 -u File has setuid bit set.
339 -g File has setgid bit set.
340 -k File has sticky bit set.
342 -T File is an ASCII text file (heuristic guess).
343 -B File is a "binary" file (opposite of -T).
345 -M Script start time minus file modification time, in days.
346 -A Same for access time.
347 -C Same for inode change time (Unix, may differ for other platforms)
353 next unless -f $_; # ignore specials
357 Note that C<-s/a/b/> does not do a negated substitution. Saying
358 C<-exp($foo)> still works as expected, however: only single letters
359 following a minus are interpreted as file tests.
361 These operators are exempt from the "looks like a function rule" described
362 above. That is, an opening parenthesis after the operator does not affect
363 how much of the following code constitutes the argument. Put the opening
364 parentheses before the operator to separate it from code that follows (this
365 applies only to operators with higher precedence than unary operators, of
368 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
369 (-s $file) + 1024 # correct
371 The interpretation of the file permission operators C<-r>, C<-R>,
372 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
373 of the file and the uids and gids of the user. There may be other
374 reasons you can't actually read, write, or execute the file: for
375 example network filesystem access controls, ACLs (access control lists),
376 read-only filesystems, and unrecognized executable formats. Note
377 that the use of these six specific operators to verify if some operation
378 is possible is usually a mistake, because it may be open to race
381 Also note that, for the superuser on the local filesystems, the C<-r>,
382 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
383 if any execute bit is set in the mode. Scripts run by the superuser
384 may thus need to do a stat() to determine the actual mode of the file,
385 or temporarily set their effective uid to something else.
387 If you are using ACLs, there is a pragma called C<filetest> that may
388 produce more accurate results than the bare stat() mode bits.
389 When under C<use filetest 'access'> the above-mentioned filetests
390 test whether the permission can(not) be granted using the
391 access(2) family of system calls. Also note that the C<-x> and C<-X> may
392 under this pragma return true even if there are no execute permission
393 bits set (nor any extra execute permission ACLs). This strangeness is
394 due to the underlying system calls' definitions. Note also that, due to
395 the implementation of C<use filetest 'access'>, the C<_> special
396 filehandle won't cache the results of the file tests when this pragma is
397 in effect. Read the documentation for the C<filetest> pragma for more
400 The C<-T> and C<-B> switches work as follows. The first block or so of the
401 file is examined for odd characters such as strange control codes or
402 characters with the high bit set. If too many strange characters (>30%)
403 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
404 containing a zero byte in the first block is considered a binary file. If C<-T>
405 or C<-B> is used on a filehandle, the current IO buffer is examined
406 rather than the first block. Both C<-T> and C<-B> return true on an empty
407 file, or a file at EOF when testing a filehandle. Because you have to
408 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
409 against the file first, as in C<next unless -f $file && -T $file>.
411 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
412 the special filehandle consisting of a solitary underline, then the stat
413 structure of the previous file test (or stat operator) is used, saving
414 a system call. (This doesn't work with C<-t>, and you need to remember
415 that lstat() and C<-l> leave values in the stat structure for the
416 symbolic link, not the real file.) (Also, if the stat buffer was filled by
417 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
420 print "Can do.\n" if -r $a || -w _ || -x _;
423 print "Readable\n" if -r _;
424 print "Writable\n" if -w _;
425 print "Executable\n" if -x _;
426 print "Setuid\n" if -u _;
427 print "Setgid\n" if -g _;
428 print "Sticky\n" if -k _;
429 print "Text\n" if -T _;
430 print "Binary\n" if -B _;
432 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
433 test operators, in a way that C<-f -w -x $file> is equivalent to
434 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
435 the return value of C<-f $file> as an argument to another filetest
436 operator, no special magic will happen.)
438 Portability issues: L<perlport/-X>.
445 Returns the absolute value of its argument.
446 If VALUE is omitted, uses C<$_>.
448 =item accept NEWSOCKET,GENERICSOCKET
451 Accepts an incoming socket connect, just as accept(2)
452 does. Returns the packed address if it succeeded, false otherwise.
453 See the example in L<perlipc/"Sockets: Client/Server Communication">.
455 On systems that support a close-on-exec flag on files, the flag will
456 be set for the newly opened file descriptor, as determined by the
457 value of $^F. See L<perlvar/$^F>.
466 Arranges to have a SIGALRM delivered to this process after the
467 specified number of wallclock seconds has elapsed. If SECONDS is not
468 specified, the value stored in C<$_> is used. (On some machines,
469 unfortunately, the elapsed time may be up to one second less or more
470 than you specified because of how seconds are counted, and process
471 scheduling may delay the delivery of the signal even further.)
473 Only one timer may be counting at once. Each call disables the
474 previous timer, and an argument of C<0> may be supplied to cancel the
475 previous timer without starting a new one. The returned value is the
476 amount of time remaining on the previous timer.
478 For delays of finer granularity than one second, the Time::HiRes module
479 (from CPAN, and starting from Perl 5.8 part of the standard
480 distribution) provides ualarm(). You may also use Perl's four-argument
481 version of select() leaving the first three arguments undefined, or you
482 might be able to use the C<syscall> interface to access setitimer(2) if
483 your system supports it. See L<perlfaq8> for details.
485 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
486 C<sleep> may be internally implemented on your system with C<alarm>.
488 If you want to use C<alarm> to time out a system call you need to use an
489 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
490 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
491 restart system calls on some systems. Using C<eval>/C<die> always works,
492 modulo the caveats given in L<perlipc/"Signals">.
495 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
497 $nread = sysread SOCKET, $buffer, $size;
501 die unless $@ eq "alarm\n"; # propagate unexpected errors
508 For more information see L<perlipc>.
510 Portability issues: L<perlport/alarm>.
513 X<atan2> X<arctangent> X<tan> X<tangent>
515 Returns the arctangent of Y/X in the range -PI to PI.
517 For the tangent operation, you may use the C<Math::Trig::tan>
518 function, or use the familiar relation:
520 sub tan { sin($_[0]) / cos($_[0]) }
522 The return value for C<atan2(0,0)> is implementation-defined; consult
523 your atan2(3) manpage for more information.
525 Portability issues: L<perlport/atan2>.
527 =item bind SOCKET,NAME
530 Binds a network address to a socket, just as bind(2)
531 does. Returns true if it succeeded, false otherwise. NAME should be a
532 packed address of the appropriate type for the socket. See the examples in
533 L<perlipc/"Sockets: Client/Server Communication">.
535 =item binmode FILEHANDLE, LAYER
536 X<binmode> X<binary> X<text> X<DOS> X<Windows>
538 =item binmode FILEHANDLE
540 Arranges for FILEHANDLE to be read or written in "binary" or "text"
541 mode on systems where the run-time libraries distinguish between
542 binary and text files. If FILEHANDLE is an expression, the value is
543 taken as the name of the filehandle. Returns true on success,
544 otherwise it returns C<undef> and sets C<$!> (errno).
546 On some systems (in general, DOS- and Windows-based systems) binmode()
547 is necessary when you're not working with a text file. For the sake
548 of portability it is a good idea always to use it when appropriate,
549 and never to use it when it isn't appropriate. Also, people can
550 set their I/O to be by default UTF8-encoded Unicode, not bytes.
552 In other words: regardless of platform, use binmode() on binary data,
553 like images, for example.
555 If LAYER is present it is a single string, but may contain multiple
556 directives. The directives alter the behaviour of the filehandle.
557 When LAYER is present, using binmode on a text file makes sense.
559 If LAYER is omitted or specified as C<:raw> the filehandle is made
560 suitable for passing binary data. This includes turning off possible CRLF
561 translation and marking it as bytes (as opposed to Unicode characters).
562 Note that, despite what may be implied in I<"Programming Perl"> (the
563 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
564 Other layers that would affect the binary nature of the stream are
565 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
566 PERLIO environment variable.
568 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
569 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
570 establish default I/O layers. See L<open>.
572 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
573 in "Programming Perl, 3rd Edition". However, since the publishing of this
574 book, by many known as "Camel III", the consensus of the naming of this
575 functionality has moved from "discipline" to "layer". All documentation
576 of this version of Perl therefore refers to "layers" rather than to
577 "disciplines". Now back to the regularly scheduled documentation...>
579 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
580 C<:utf8> just marks the data as UTF-8 without further checking,
581 while C<:encoding(UTF-8)> checks the data for actually being valid
582 UTF-8. More details can be found in L<PerlIO::encoding>.
584 In general, binmode() should be called after open() but before any I/O
585 is done on the filehandle. Calling binmode() normally flushes any
586 pending buffered output data (and perhaps pending input data) on the
587 handle. An exception to this is the C<:encoding> layer that
588 changes the default character encoding of the handle; see L</open>.
589 The C<:encoding> layer sometimes needs to be called in
590 mid-stream, and it doesn't flush the stream. The C<:encoding>
591 also implicitly pushes on top of itself the C<:utf8> layer because
592 internally Perl operates on UTF8-encoded Unicode characters.
594 The operating system, device drivers, C libraries, and Perl run-time
595 system all conspire to let the programmer treat a single
596 character (C<\n>) as the line terminator, irrespective of external
597 representation. On many operating systems, the native text file
598 representation matches the internal representation, but on some
599 platforms the external representation of C<\n> is made up of more than
602 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
603 a single character to end each line in the external representation of text
604 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
605 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
606 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
607 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
608 two characters C<\cM\cJ>. That means that if you don't use binmode() on
609 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
610 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
611 output. This is what you want for text files, but it can be disastrous for
614 Another consequence of using binmode() (on some systems) is that
615 special end-of-file markers will be seen as part of the data stream.
616 For systems from the Microsoft family this means that, if your binary
617 data contain C<\cZ>, the I/O subsystem will regard it as the end of
618 the file, unless you use binmode().
620 binmode() is important not only for readline() and print() operations,
621 but also when using read(), seek(), sysread(), syswrite() and tell()
622 (see L<perlport> for more details). See the C<$/> and C<$\> variables
623 in L<perlvar> for how to manually set your input and output
624 line-termination sequences.
626 Portability issues: L<perlport/binmode>.
628 =item bless REF,CLASSNAME
633 This function tells the thingy referenced by REF that it is now an object
634 in the CLASSNAME package. If CLASSNAME is omitted, the current package
635 is used. Because a C<bless> is often the last thing in a constructor,
636 it returns the reference for convenience. Always use the two-argument
637 version if a derived class might inherit the function doing the blessing.
638 SeeL<perlobj> for more about the blessing (and blessings) of objects.
640 Consider always blessing objects in CLASSNAMEs that are mixed case.
641 Namespaces with all lowercase names are considered reserved for
642 Perl pragmata. Builtin types have all uppercase names. To prevent
643 confusion, you may wish to avoid such package names as well. Make sure
644 that CLASSNAME is a true value.
646 See L<perlmod/"Perl Modules">.
650 Break out of a C<given()> block.
652 This keyword is enabled by the C<"switch"> feature: see
653 L<feature> for more information. You can also access it by
654 prefixing it with C<CORE::>. Alternately, include a C<use
655 v5.10> or later to the current scope.
658 X<caller> X<call stack> X<stack> X<stack trace>
662 Returns the context of the current subroutine call. In scalar context,
663 returns the caller's package name if there I<is> a caller (that is, if
664 we're in a subroutine or C<eval> or C<require>) and the undefined value
665 otherwise. In list context, returns
668 ($package, $filename, $line) = caller;
670 With EXPR, it returns some extra information that the debugger uses to
671 print a stack trace. The value of EXPR indicates how many call frames
672 to go back before the current one.
675 ($package, $filename, $line, $subroutine, $hasargs,
678 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
681 Here $subroutine may be C<(eval)> if the frame is not a subroutine
682 call, but an C<eval>. In such a case additional elements $evaltext and
683 C<$is_require> are set: C<$is_require> is true if the frame is created by a
684 C<require> or C<use> statement, $evaltext contains the text of the
685 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
686 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
687 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
688 frame.) $subroutine may also be C<(unknown)> if this particular
689 subroutine happens to have been deleted from the symbol table.
690 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
691 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
692 compiled with. The C<$hints> and C<$bitmask> values are subject to change
693 between versions of Perl, and are not meant for external use.
695 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
696 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
697 of this hash, as they are the actual values stored in the optree.
699 Furthermore, when called from within the DB package, caller returns more
700 detailed information: it sets the list variable C<@DB::args> to be the
701 arguments with which the subroutine was invoked.
703 Be aware that the optimizer might have optimized call frames away before
704 C<caller> had a chance to get the information. That means that C<caller(N)>
705 might not return information about the call frame you expect it to, for
706 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
707 previous time C<caller> was called.
709 Be aware that setting C<@DB::args> is I<best effort>, intended for
710 debugging or generating backtraces, and should not be relied upon. In
711 particular, as C<@_> contains aliases to the caller's arguments, Perl does
712 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
713 subroutine makes to C<@_> or its contents, not the original values at call
714 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
715 elements, so under certain cases its elements may have become freed and
716 reallocated for other variables or temporary values. Finally, a side effect
717 of the current implementation is that the effects of C<shift @_> can
718 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
719 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
720 elements), so C<@DB::args> is actually a hybrid of the current state and
721 initial state of C<@_>. Buyer beware.
728 =item chdir FILEHANDLE
730 =item chdir DIRHANDLE
734 Changes the working directory to EXPR, if possible. If EXPR is omitted,
735 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
736 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
737 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
738 neither is set, C<chdir> does nothing. It returns true on success,
739 false otherwise. See the example under C<die>.
741 On systems that support fchdir(2), you may pass a filehandle or
742 directory handle as the argument. On systems that don't support fchdir(2),
743 passing handles raises an exception.
746 X<chmod> X<permission> X<mode>
748 Changes the permissions of a list of files. The first element of the
749 list must be the numeric mode, which should probably be an octal
750 number, and which definitely should I<not> be a string of octal digits:
751 C<0644> is okay, but C<"0644"> is not. Returns the number of files
752 successfully changed. See also L</oct> if all you have is a string.
754 $cnt = chmod 0755, "foo", "bar";
755 chmod 0755, @executables;
756 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
758 $mode = "0644"; chmod oct($mode), "foo"; # this is better
759 $mode = 0644; chmod $mode, "foo"; # this is best
761 On systems that support fchmod(2), you may pass filehandles among the
762 files. On systems that don't support fchmod(2), passing filehandles raises
763 an exception. Filehandles must be passed as globs or glob references to be
764 recognized; barewords are considered filenames.
766 open(my $fh, "<", "foo");
767 my $perm = (stat $fh)[2] & 07777;
768 chmod($perm | 0600, $fh);
770 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
773 use Fcntl qw( :mode );
774 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
775 # Identical to the chmod 0755 of the example above.
777 Portability issues: L<perlport/chmod>.
780 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
786 This safer version of L</chop> removes any trailing string
787 that corresponds to the current value of C<$/> (also known as
788 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
789 number of characters removed from all its arguments. It's often used to
790 remove the newline from the end of an input record when you're worried
791 that the final record may be missing its newline. When in paragraph
792 mode (C<$/ = "">), it removes all trailing newlines from the string.
793 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
794 a reference to an integer or the like; see L<perlvar>) chomp() won't
796 If VARIABLE is omitted, it chomps C<$_>. Example:
799 chomp; # avoid \n on last field
804 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
806 You can actually chomp anything that's an lvalue, including an assignment:
809 chomp($answer = <STDIN>);
811 If you chomp a list, each element is chomped, and the total number of
812 characters removed is returned.
814 Note that parentheses are necessary when you're chomping anything
815 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
816 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
817 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
818 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
828 Chops off the last character of a string and returns the character
829 chopped. It is much more efficient than C<s/.$//s> because it neither
830 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
831 If VARIABLE is a hash, it chops the hash's values, but not its keys.
833 You can actually chop anything that's an lvalue, including an assignment.
835 If you chop a list, each element is chopped. Only the value of the
836 last C<chop> is returned.
838 Note that C<chop> returns the last character. To return all but the last
839 character, use C<substr($string, 0, -1)>.
844 X<chown> X<owner> X<user> X<group>
846 Changes the owner (and group) of a list of files. The first two
847 elements of the list must be the I<numeric> uid and gid, in that
848 order. A value of -1 in either position is interpreted by most
849 systems to leave that value unchanged. Returns the number of files
850 successfully changed.
852 $cnt = chown $uid, $gid, 'foo', 'bar';
853 chown $uid, $gid, @filenames;
855 On systems that support fchown(2), you may pass filehandles among the
856 files. On systems that don't support fchown(2), passing filehandles raises
857 an exception. Filehandles must be passed as globs or glob references to be
858 recognized; barewords are considered filenames.
860 Here's an example that looks up nonnumeric uids in the passwd file:
863 chomp($user = <STDIN>);
865 chomp($pattern = <STDIN>);
867 ($login,$pass,$uid,$gid) = getpwnam($user)
868 or die "$user not in passwd file";
870 @ary = glob($pattern); # expand filenames
871 chown $uid, $gid, @ary;
873 On most systems, you are not allowed to change the ownership of the
874 file unless you're the superuser, although you should be able to change
875 the group to any of your secondary groups. On insecure systems, these
876 restrictions may be relaxed, but this is not a portable assumption.
877 On POSIX systems, you can detect this condition this way:
879 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
880 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
882 Portability issues: L<perlport/chmod>.
885 X<chr> X<character> X<ASCII> X<Unicode>
889 Returns the character represented by that NUMBER in the character set.
890 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
891 chr(0x263a) is a Unicode smiley face.
893 Negative values give the Unicode replacement character (chr(0xfffd)),
894 except under the L<bytes> pragma, where the low eight bits of the value
895 (truncated to an integer) are used.
897 If NUMBER is omitted, uses C<$_>.
899 For the reverse, use L</ord>.
901 Note that characters from 128 to 255 (inclusive) are by default
902 internally not encoded as UTF-8 for backward compatibility reasons.
904 See L<perlunicode> for more about Unicode.
906 =item chroot FILENAME
911 This function works like the system call by the same name: it makes the
912 named directory the new root directory for all further pathnames that
913 begin with a C</> by your process and all its children. (It doesn't
914 change your current working directory, which is unaffected.) For security
915 reasons, this call is restricted to the superuser. If FILENAME is
916 omitted, does a C<chroot> to C<$_>.
918 Portability issues: L<perlport/chroot>.
920 =item close FILEHANDLE
925 Closes the file or pipe associated with the filehandle, flushes the IO
926 buffers, and closes the system file descriptor. Returns true if those
927 operations succeed and if no error was reported by any PerlIO
928 layer. Closes the currently selected filehandle if the argument is
931 You don't have to close FILEHANDLE if you are immediately going to do
932 another C<open> on it, because C<open> closes it for you. (See
933 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
934 counter (C<$.>), while the implicit close done by C<open> does not.
936 If the filehandle came from a piped open, C<close> returns false if one of
937 the other syscalls involved fails or if its program exits with non-zero
938 status. If the only problem was that the program exited non-zero, C<$!>
939 will be set to C<0>. Closing a pipe also waits for the process executing
940 on the pipe to exit--in case you wish to look at the output of the pipe
941 afterwards--and implicitly puts the exit status value of that command into
942 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
944 If there are multiple threads running, C<close> on a filehandle from a
945 piped open returns true without waiting for the child process to terminate,
946 if the filehandle is still open in another thread.
948 Closing the read end of a pipe before the process writing to it at the
949 other end is done writing results in the writer receiving a SIGPIPE. If
950 the other end can't handle that, be sure to read all the data before
955 open(OUTPUT, '|sort >foo') # pipe to sort
956 or die "Can't start sort: $!";
957 #... # print stuff to output
958 close OUTPUT # wait for sort to finish
959 or warn $! ? "Error closing sort pipe: $!"
960 : "Exit status $? from sort";
961 open(INPUT, 'foo') # get sort's results
962 or die "Can't open 'foo' for input: $!";
964 FILEHANDLE may be an expression whose value can be used as an indirect
965 filehandle, usually the real filehandle name or an autovivified handle.
967 =item closedir DIRHANDLE
970 Closes a directory opened by C<opendir> and returns the success of that
973 =item connect SOCKET,NAME
976 Attempts to connect to a remote socket, just like connect(2).
977 Returns true if it succeeded, false otherwise. NAME should be a
978 packed address of the appropriate type for the socket. See the examples in
979 L<perlipc/"Sockets: Client/Server Communication">.
986 When followed by a BLOCK, C<continue> is actually a
987 flow control statement rather than a function. If
988 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
989 C<foreach>), it is always executed just before the conditional is about to
990 be evaluated again, just like the third part of a C<for> loop in C. Thus
991 it can be used to increment a loop variable, even when the loop has been
992 continued via the C<next> statement (which is similar to the C C<continue>
995 C<last>, C<next>, or C<redo> may appear within a C<continue>
996 block; C<last> and C<redo> behave as if they had been executed within
997 the main block. So will C<next>, but since it will execute a C<continue>
998 block, it may be more entertaining.
1001 ### redo always comes here
1004 ### next always comes here
1006 # then back the top to re-check EXPR
1008 ### last always comes here
1010 Omitting the C<continue> section is equivalent to using an
1011 empty one, logically enough, so C<next> goes directly back
1012 to check the condition at the top of the loop.
1014 When there is no BLOCK, C<continue> is a function that
1015 falls through the current C<when> or C<default> block instead of iterating
1016 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1017 In Perl 5.14 and earlier, this form of C<continue> was
1018 only available when the C<"switch"> feature was enabled.
1019 See L<feature> and L<perlsyn/"Switch statements"> for more
1023 X<cos> X<cosine> X<acos> X<arccosine>
1027 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1028 takes the cosine of C<$_>.
1030 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1031 function, or use this relation:
1033 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1035 =item crypt PLAINTEXT,SALT
1036 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1037 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1039 Creates a digest string exactly like the crypt(3) function in the C
1040 library (assuming that you actually have a version there that has not
1041 been extirpated as a potential munition).
1043 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1044 into a short string, called a digest, which is returned. The same
1045 PLAINTEXT and SALT will always return the same string, but there is no
1046 (known) way to get the original PLAINTEXT from the hash. Small
1047 changes in the PLAINTEXT or SALT will result in large changes in the
1050 There is no decrypt function. This function isn't all that useful for
1051 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1052 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1053 primarily used to check if two pieces of text are the same without
1054 having to transmit or store the text itself. An example is checking
1055 if a correct password is given. The digest of the password is stored,
1056 not the password itself. The user types in a password that is
1057 crypt()'d with the same salt as the stored digest. If the two digests
1058 match, the password is correct.
1060 When verifying an existing digest string you should use the digest as
1061 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1062 to create the digest is visible as part of the digest. This ensures
1063 crypt() will hash the new string with the same salt as the digest.
1064 This allows your code to work with the standard L<crypt|/crypt> and
1065 with more exotic implementations. In other words, assume
1066 nothing about the returned string itself nor about how many bytes
1069 Traditionally the result is a string of 13 bytes: two first bytes of
1070 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1071 the first eight bytes of PLAINTEXT mattered. But alternative
1072 hashing schemes (like MD5), higher level security schemes (like C2),
1073 and implementations on non-Unix platforms may produce different
1076 When choosing a new salt create a random two character string whose
1077 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1078 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1079 characters is just a recommendation; the characters allowed in
1080 the salt depend solely on your system's crypt library, and Perl can't
1081 restrict what salts C<crypt()> accepts.
1083 Here's an example that makes sure that whoever runs this program knows
1086 $pwd = (getpwuid($<))[1];
1088 system "stty -echo";
1090 chomp($word = <STDIN>);
1094 if (crypt($word, $pwd) ne $pwd) {
1100 Of course, typing in your own password to whoever asks you
1103 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1104 of data, not least of all because you can't get the information
1105 back. Look at the L<Digest> module for more robust algorithms.
1107 If using crypt() on a Unicode string (which I<potentially> has
1108 characters with codepoints above 255), Perl tries to make sense
1109 of the situation by trying to downgrade (a copy of)
1110 the string back to an eight-bit byte string before calling crypt()
1111 (on that copy). If that works, good. If not, crypt() dies with
1112 C<Wide character in crypt>.
1114 Portability issues: L<perlport/crypt>.
1119 [This function has been largely superseded by the C<untie> function.]
1121 Breaks the binding between a DBM file and a hash.
1123 Portability issues: L<perlport/dbmclose>.
1125 =item dbmopen HASH,DBNAME,MASK
1126 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1128 [This function has been largely superseded by the
1129 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1131 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1132 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1133 argument is I<not> a filehandle, even though it looks like one). DBNAME
1134 is the name of the database (without the F<.dir> or F<.pag> extension if
1135 any). If the database does not exist, it is created with protection
1136 specified by MASK (as modified by the C<umask>). If your system supports
1137 only the older DBM functions, you may make only one C<dbmopen> call in your
1138 program. In older versions of Perl, if your system had neither DBM nor
1139 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1142 If you don't have write access to the DBM file, you can only read hash
1143 variables, not set them. If you want to test whether you can write,
1144 either use file tests or try setting a dummy hash entry inside an C<eval>
1147 Note that functions such as C<keys> and C<values> may return huge lists
1148 when used on large DBM files. You may prefer to use the C<each>
1149 function to iterate over large DBM files. Example:
1151 # print out history file offsets
1152 dbmopen(%HIST,'/usr/lib/news/history',0666);
1153 while (($key,$val) = each %HIST) {
1154 print $key, ' = ', unpack('L',$val), "\n";
1158 See also L<AnyDBM_File> for a more general description of the pros and
1159 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1160 rich implementation.
1162 You can control which DBM library you use by loading that library
1163 before you call dbmopen():
1166 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1167 or die "Can't open netscape history file: $!";
1169 Portability issues: L<perlport/dbmopen>.
1173 Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1174 that's always true. Only available after Perl 5.10, and only if the
1175 C<switch> feature has been requested or if the keyword is prefixed with
1176 C<CORE::>. See L</when>.
1179 X<defined> X<undef> X<undefined>
1183 Returns a Boolean value telling whether EXPR has a value other than
1184 the undefined value C<undef>. If EXPR is not present, C<$_> is
1187 Many operations return C<undef> to indicate failure, end of file,
1188 system error, uninitialized variable, and other exceptional
1189 conditions. This function allows you to distinguish C<undef> from
1190 other values. (A simple Boolean test will not distinguish among
1191 C<undef>, zero, the empty string, and C<"0">, which are all equally
1192 false.) Note that since C<undef> is a valid scalar, its presence
1193 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1194 returns C<undef> when its argument is an empty array, I<or> when the
1195 element to return happens to be C<undef>.
1197 You may also use C<defined(&func)> to check whether subroutine C<&func>
1198 has ever been defined. The return value is unaffected by any forward
1199 declarations of C<&func>. A subroutine that is not defined
1200 may still be callable: its package may have an C<AUTOLOAD> method that
1201 makes it spring into existence the first time that it is called; see
1204 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1205 used to report whether memory for that aggregate had ever been
1206 allocated. This behavior may disappear in future versions of Perl.
1207 You should instead use a simple test for size:
1209 if (@an_array) { print "has array elements\n" }
1210 if (%a_hash) { print "has hash members\n" }
1212 When used on a hash element, it tells you whether the value is defined,
1213 not whether the key exists in the hash. Use L</exists> for the latter
1218 print if defined $switch{D};
1219 print "$val\n" while defined($val = pop(@ary));
1220 die "Can't readlink $sym: $!"
1221 unless defined($value = readlink $sym);
1222 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1223 $debugging = 0 unless defined $debugging;
1225 Note: Many folks tend to overuse C<defined> and are then surprised to
1226 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1227 defined values. For example, if you say
1231 The pattern match succeeds and C<$1> is defined, although it
1232 matched "nothing". It didn't really fail to match anything. Rather, it
1233 matched something that happened to be zero characters long. This is all
1234 very above-board and honest. When a function returns an undefined value,
1235 it's an admission that it couldn't give you an honest answer. So you
1236 should use C<defined> only when questioning the integrity of what
1237 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1240 See also L</undef>, L</exists>, L</ref>.
1245 Given an expression that specifies an element or slice of a hash, C<delete>
1246 deletes the specified elements from that hash so that exists() on that element
1247 no longer returns true. Setting a hash element to the undefined value does
1248 not remove its key, but deleting it does; see L</exists>.
1250 In list context, returns the value or values deleted, or the last such
1251 element in scalar context. The return list's length always matches that of
1252 the argument list: deleting non-existent elements returns the undefined value
1253 in their corresponding positions.
1255 delete() may also be used on arrays and array slices, but its behavior is less
1256 straightforward. Although exists() will return false for deleted entries,
1257 deleting array elements never changes indices of existing values; use shift()
1258 or splice() for that. However, if all deleted elements fall at the end of an
1259 array, the array's size shrinks to the position of the highest element that
1260 still tests true for exists(), or to 0 if none do.
1262 B<WARNING:> Calling delete on array values is deprecated and likely to
1263 be removed in a future version of Perl.
1265 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1266 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1267 or array may not necessarily return anything; it depends on the implementation
1268 of the C<tied> package's DELETE method, which may do whatever it pleases.
1270 The C<delete local EXPR> construct localizes the deletion to the current
1271 block at run time. Until the block exits, elements locally deleted
1272 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1273 of composite types">.
1275 %hash = (foo => 11, bar => 22, baz => 33);
1276 $scalar = delete $hash{foo}; # $scalar is 11
1277 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1278 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1280 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1282 foreach $key (keys %HASH) {
1286 foreach $index (0 .. $#ARRAY) {
1287 delete $ARRAY[$index];
1292 delete @HASH{keys %HASH};
1294 delete @ARRAY[0 .. $#ARRAY];
1296 But both are slower than assigning the empty list
1297 or undefining %HASH or @ARRAY, which is the customary
1298 way to empty out an aggregate:
1300 %HASH = (); # completely empty %HASH
1301 undef %HASH; # forget %HASH ever existed
1303 @ARRAY = (); # completely empty @ARRAY
1304 undef @ARRAY; # forget @ARRAY ever existed
1306 The EXPR can be arbitrarily complicated provided its
1307 final operation is an element or slice of an aggregate:
1309 delete $ref->[$x][$y]{$key};
1310 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1312 delete $ref->[$x][$y][$index];
1313 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1316 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1318 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1319 into C<$@> and the C<eval> is terminated with the undefined value.
1320 If the exception is outside of all enclosing C<eval>s, then the uncaught
1321 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1322 need to exit the process with a specific exit code, see L</exit>.
1324 Equivalent examples:
1326 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1327 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1329 If the last element of LIST does not end in a newline, the current
1330 script line number and input line number (if any) are also printed,
1331 and a newline is supplied. Note that the "input line number" (also
1332 known as "chunk") is subject to whatever notion of "line" happens to
1333 be currently in effect, and is also available as the special variable
1334 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1336 Hint: sometimes appending C<", stopped"> to your message will cause it
1337 to make better sense when the string C<"at foo line 123"> is appended.
1338 Suppose you are running script "canasta".
1340 die "/etc/games is no good";
1341 die "/etc/games is no good, stopped";
1343 produce, respectively
1345 /etc/games is no good at canasta line 123.
1346 /etc/games is no good, stopped at canasta line 123.
1348 If the output is empty and C<$@> already contains a value (typically from a
1349 previous eval) that value is reused after appending C<"\t...propagated">.
1350 This is useful for propagating exceptions:
1353 die unless $@ =~ /Expected exception/;
1355 If the output is empty and C<$@> contains an object reference that has a
1356 C<PROPAGATE> method, that method will be called with additional file
1357 and line number parameters. The return value replaces the value in
1358 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1361 If C<$@> is empty then the string C<"Died"> is used.
1363 If an uncaught exception results in interpreter exit, the exit code is
1364 determined from the values of C<$!> and C<$?> with this pseudocode:
1366 exit $! if $!; # errno
1367 exit $? >> 8 if $? >> 8; # child exit status
1368 exit 255; # last resort
1370 The intent is to squeeze as much possible information about the likely cause
1371 into the limited space of the system exit code. However, as C<$!> is the value
1372 of C's C<errno>, which can be set by any system call, this means that the value
1373 of the exit code used by C<die> can be non-predictable, so should not be relied
1374 upon, other than to be non-zero.
1376 You can also call C<die> with a reference argument, and if this is trapped
1377 within an C<eval>, C<$@> contains that reference. This permits more
1378 elaborate exception handling using objects that maintain arbitrary state
1379 about the exception. Such a scheme is sometimes preferable to matching
1380 particular string values of C<$@> with regular expressions. Because C<$@>
1381 is a global variable and C<eval> may be used within object implementations,
1382 be careful that analyzing the error object doesn't replace the reference in
1383 the global variable. It's easiest to make a local copy of the reference
1384 before any manipulations. Here's an example:
1386 use Scalar::Util "blessed";
1388 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1389 if (my $ev_err = $@) {
1390 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1391 # handle Some::Module::Exception
1394 # handle all other possible exceptions
1398 Because Perl stringifies uncaught exception messages before display,
1399 you'll probably want to overload stringification operations on
1400 exception objects. See L<overload> for details about that.
1402 You can arrange for a callback to be run just before the C<die>
1403 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1404 handler is called with the error text and can change the error
1405 message, if it sees fit, by calling C<die> again. See
1406 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1407 L<"eval BLOCK"> for some examples. Although this feature was
1408 to be run only right before your program was to exit, this is not
1409 currently so: the C<$SIG{__DIE__}> hook is currently called
1410 even inside eval()ed blocks/strings! If one wants the hook to do
1411 nothing in such situations, put
1415 as the first line of the handler (see L<perlvar/$^S>). Because
1416 this promotes strange action at a distance, this counterintuitive
1417 behavior may be fixed in a future release.
1419 See also exit(), warn(), and the Carp module.
1424 Not really a function. Returns the value of the last command in the
1425 sequence of commands indicated by BLOCK. When modified by the C<while> or
1426 C<until> loop modifier, executes the BLOCK once before testing the loop
1427 condition. (On other statements the loop modifiers test the conditional
1430 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1431 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1432 See L<perlsyn> for alternative strategies.
1434 =item do SUBROUTINE(LIST)
1437 This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1438 a scalar variable or a subroutine beginning with C<&>.
1443 Uses the value of EXPR as a filename and executes the contents of the
1444 file as a Perl script.
1452 except that it's more efficient and concise, keeps track of the current
1453 filename for error messages, searches the C<@INC> directories, and updates
1454 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1455 these variables. It also differs in that code evaluated with C<do FILENAME>
1456 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1457 same, however, in that it does reparse the file every time you call it,
1458 so you probably don't want to do this inside a loop.
1460 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1461 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1462 and sets C<$!> to the error. Always check C<$@> first, as compilation
1463 could fail in a way that also sets C<$!>. If the file is successfully
1464 compiled, C<do> returns the value of the last expression evaluated.
1466 Inclusion of library modules is better done with the
1467 C<use> and C<require> operators, which also do automatic error checking
1468 and raise an exception if there's a problem.
1470 You might like to use C<do> to read in a program configuration
1471 file. Manual error checking can be done this way:
1473 # read in config files: system first, then user
1474 for $file ("/share/prog/defaults.rc",
1475 "$ENV{HOME}/.someprogrc")
1477 unless ($return = do $file) {
1478 warn "couldn't parse $file: $@" if $@;
1479 warn "couldn't do $file: $!" unless defined $return;
1480 warn "couldn't run $file" unless $return;
1485 X<dump> X<core> X<undump>
1489 This function causes an immediate core dump. See also the B<-u>
1490 command-line switch in L<perlrun>, which does the same thing.
1491 Primarily this is so that you can use the B<undump> program (not
1492 supplied) to turn your core dump into an executable binary after
1493 having initialized all your variables at the beginning of the
1494 program. When the new binary is executed it will begin by executing
1495 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1496 Think of it as a goto with an intervening core dump and reincarnation.
1497 If C<LABEL> is omitted, restarts the program from the top.
1499 B<WARNING>: Any files opened at the time of the dump will I<not>
1500 be open any more when the program is reincarnated, with possible
1501 resulting confusion by Perl.
1503 This function is now largely obsolete, mostly because it's very hard to
1504 convert a core file into an executable. That's why you should now invoke
1505 it as C<CORE::dump()>, if you don't want to be warned against a possible
1508 Portability issues: L<perlport/dump>.
1511 X<each> X<hash, iterator>
1518 When called in list context, returns a 2-element list consisting of the key
1519 and value for the next element of a hash, or the index and value for the
1520 next element of an array, so that you can iterate over it. When called in
1521 scalar context, returns only the key (not the value) in a hash, or the index
1524 Hash entries are returned in an apparently random order. The actual random
1525 order is subject to change in future versions of Perl, but it is
1526 guaranteed to be in the same order as either the C<keys> or C<values>
1527 function would produce on the same (unmodified) hash. Since Perl
1528 5.8.2 the ordering can be different even between different runs of Perl
1529 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1531 After C<each> has returned all entries from the hash or array, the next
1532 call to C<each> returns the empty list in list context and C<undef> in
1533 scalar context. The next call following that one restarts iteration. Each
1534 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1535 and C<values>. The iterator is implicitly reset when C<each> has reached
1536 the end as just described; it can be explicitly reset by calling C<keys> or
1537 C<values> on the hash or array. If you add or delete a hash's elements
1538 while iterating over it, entries may be skipped or duplicated--so don't do
1539 that. Exception: It is always safe to delete the item most recently
1540 returned by C<each()>, so the following code works properly:
1542 while (($key, $value) = each %hash) {
1544 delete $hash{$key}; # This is safe
1547 This prints out your environment like the printenv(1) program,
1548 but in a different order:
1550 while (($key,$value) = each %ENV) {
1551 print "$key=$value\n";
1554 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1555 reference to an unblessed hash or array. The argument will be dereferenced
1556 automatically. This aspect of C<each> is considered highly experimental.
1557 The exact behaviour may change in a future version of Perl.
1559 while (($key,$value) = each $hashref) { ... }
1561 See also C<keys>, C<values>, and C<sort>.
1563 =item eof FILEHANDLE
1572 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1573 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1574 gives the real filehandle. (Note that this function actually
1575 reads a character and then C<ungetc>s it, so isn't useful in an
1576 interactive context.) Do not read from a terminal file (or call
1577 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1578 as terminals may lose the end-of-file condition if you do.
1580 An C<eof> without an argument uses the last file read. Using C<eof()>
1581 with empty parentheses is different. It refers to the pseudo file
1582 formed from the files listed on the command line and accessed via the
1583 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1584 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1585 used will cause C<@ARGV> to be examined to determine if input is
1586 available. Similarly, an C<eof()> after C<< <> >> has returned
1587 end-of-file will assume you are processing another C<@ARGV> list,
1588 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1589 see L<perlop/"I/O Operators">.
1591 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1592 detect the end of each file, whereas C<eof()> will detect the end
1593 of the very last file only. Examples:
1595 # reset line numbering on each input file
1597 next if /^\s*#/; # skip comments
1600 close ARGV if eof; # Not eof()!
1603 # insert dashes just before last line of last file
1605 if (eof()) { # check for end of last file
1606 print "--------------\n";
1609 last if eof(); # needed if we're reading from a terminal
1612 Practical hint: you almost never need to use C<eof> in Perl, because the
1613 input operators typically return C<undef> when they run out of data or
1617 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1618 X<error, handling> X<exception, handling>
1624 In the first form, the return value of EXPR is parsed and executed as if it
1625 were a little Perl program. The value of the expression (which is itself
1626 determined within scalar context) is first parsed, and if there were no
1627 errors, executed as a block within the lexical context of the current Perl
1628 program. This means, that in particular, any outer lexical variables are
1629 visible to it, and any package variable settings or subroutine and format
1630 definitions remain afterwards.
1632 Note that the value is parsed every time the C<eval> executes.
1633 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1634 delay parsing and subsequent execution of the text of EXPR until run time.
1636 In the second form, the code within the BLOCK is parsed only once--at the
1637 same time the code surrounding the C<eval> itself was parsed--and executed
1638 within the context of the current Perl program. This form is typically
1639 used to trap exceptions more efficiently than the first (see below), while
1640 also providing the benefit of checking the code within BLOCK at compile
1643 The final semicolon, if any, may be omitted from the value of EXPR or within
1646 In both forms, the value returned is the value of the last expression
1647 evaluated inside the mini-program; a return statement may be also used, just
1648 as with subroutines. The expression providing the return value is evaluated
1649 in void, scalar, or list context, depending on the context of the C<eval>
1650 itself. See L</wantarray> for more on how the evaluation context can be
1653 If there is a syntax error or runtime error, or a C<die> statement is
1654 executed, C<eval> returns C<undef> in scalar context
1655 or an empty list--or, for syntax errors, a list containing a single
1656 undefined value--in list context, and C<$@> is set to the error
1657 message. The discrepancy in the return values in list context is
1658 considered a bug by some, and will probably be fixed in a future
1659 release. If there was no error, C<$@> is set to the empty string. A
1660 control flow operator like C<last> or C<goto> can bypass the setting of
1661 C<$@>. Beware that using C<eval> neither silences Perl from printing
1662 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1663 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1664 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1665 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1667 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1668 determining whether a particular feature (such as C<socket> or C<symlink>)
1669 is implemented. It is also Perl's exception-trapping mechanism, where
1670 the die operator is used to raise exceptions.
1672 If you want to trap errors when loading an XS module, some problems with
1673 the binary interface (such as Perl version skew) may be fatal even with
1674 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1676 If the code to be executed doesn't vary, you may use the eval-BLOCK
1677 form to trap run-time errors without incurring the penalty of
1678 recompiling each time. The error, if any, is still returned in C<$@>.
1681 # make divide-by-zero nonfatal
1682 eval { $answer = $a / $b; }; warn $@ if $@;
1684 # same thing, but less efficient
1685 eval '$answer = $a / $b'; warn $@ if $@;
1687 # a compile-time error
1688 eval { $answer = }; # WRONG
1691 eval '$answer ='; # sets $@
1693 Using the C<eval{}> form as an exception trap in libraries does have some
1694 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1695 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1696 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1697 as this example shows:
1699 # a private exception trap for divide-by-zero
1700 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1703 This is especially significant, given that C<__DIE__> hooks can call
1704 C<die> again, which has the effect of changing their error messages:
1706 # __DIE__ hooks may modify error messages
1708 local $SIG{'__DIE__'} =
1709 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1710 eval { die "foo lives here" };
1711 print $@ if $@; # prints "bar lives here"
1714 Because this promotes action at a distance, this counterintuitive behavior
1715 may be fixed in a future release.
1717 With an C<eval>, you should be especially careful to remember what's
1718 being looked at when:
1724 eval { $x }; # CASE 4
1726 eval "\$$x++"; # CASE 5
1729 Cases 1 and 2 above behave identically: they run the code contained in
1730 the variable $x. (Although case 2 has misleading double quotes making
1731 the reader wonder what else might be happening (nothing is).) Cases 3
1732 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1733 does nothing but return the value of $x. (Case 4 is preferred for
1734 purely visual reasons, but it also has the advantage of compiling at
1735 compile-time instead of at run-time.) Case 5 is a place where
1736 normally you I<would> like to use double quotes, except that in this
1737 particular situation, you can just use symbolic references instead, as
1740 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1741 of localised variables, which means that for your code to run on older
1742 versions, a temporary is required if you want to mask some but not all
1745 # alter $@ on nefarious repugnancy only
1749 local $@; # protect existing $@
1750 eval { test_repugnancy() };
1751 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1752 $@ =~ /nefarious/ and $e = $@;
1754 die $e if defined $e
1757 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1758 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1760 An C<eval ''> executed within the C<DB> package doesn't see the usual
1761 surrounding lexical scope, but rather the scope of the first non-DB piece
1762 of code that called it. You don't normally need to worry about this unless
1763 you are writing a Perl debugger.
1768 =item exec PROGRAM LIST
1770 The C<exec> function executes a system command I<and never returns>;
1771 use C<system> instead of C<exec> if you want it to return. It fails and
1772 returns false only if the command does not exist I<and> it is executed
1773 directly instead of via your system's command shell (see below).
1775 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1776 warns you if there is a following statement that isn't C<die>, C<warn>,
1777 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1778 I<really> want to follow an C<exec> with some other statement, you
1779 can use one of these styles to avoid the warning:
1781 exec ('foo') or print STDERR "couldn't exec foo: $!";
1782 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1784 If there is more than one argument in LIST, or if LIST is an array
1785 with more than one value, calls execvp(3) with the arguments in LIST.
1786 If there is only one scalar argument or an array with one element in it,
1787 the argument is checked for shell metacharacters, and if there are any,
1788 the entire argument is passed to the system's command shell for parsing
1789 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1790 If there are no shell metacharacters in the argument, it is split into
1791 words and passed directly to C<execvp>, which is more efficient.
1794 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1795 exec "sort $outfile | uniq";
1797 If you don't really want to execute the first argument, but want to lie
1798 to the program you are executing about its own name, you can specify
1799 the program you actually want to run as an "indirect object" (without a
1800 comma) in front of the LIST. (This always forces interpretation of the
1801 LIST as a multivalued list, even if there is only a single scalar in
1804 $shell = '/bin/csh';
1805 exec $shell '-sh'; # pretend it's a login shell
1809 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1811 When the arguments get executed via the system shell, results are
1812 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1815 Using an indirect object with C<exec> or C<system> is also more
1816 secure. This usage (which also works fine with system()) forces
1817 interpretation of the arguments as a multivalued list, even if the
1818 list had just one argument. That way you're safe from the shell
1819 expanding wildcards or splitting up words with whitespace in them.
1821 @args = ( "echo surprise" );
1823 exec @args; # subject to shell escapes
1825 exec { $args[0] } @args; # safe even with one-arg list
1827 The first version, the one without the indirect object, ran the I<echo>
1828 program, passing it C<"surprise"> an argument. The second version didn't;
1829 it tried to run a program named I<"echo surprise">, didn't find it, and set
1830 C<$?> to a non-zero value indicating failure.
1832 Beginning with v5.6.0, Perl attempts to flush all files opened for
1833 output before the exec, but this may not be supported on some platforms
1834 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1835 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1836 open handles to avoid lost output.
1838 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1839 C<DESTROY> methods on your objects.
1841 Portability issues: L<perlport/exec>.
1844 X<exists> X<autovivification>
1846 Given an expression that specifies an element of a hash, returns true if the
1847 specified element in the hash has ever been initialized, even if the
1848 corresponding value is undefined.
1850 print "Exists\n" if exists $hash{$key};
1851 print "Defined\n" if defined $hash{$key};
1852 print "True\n" if $hash{$key};
1854 exists may also be called on array elements, but its behavior is much less
1855 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1856 that calling exists on array values is deprecated and likely to be removed in
1857 a future version of Perl.
1859 print "Exists\n" if exists $array[$index];
1860 print "Defined\n" if defined $array[$index];
1861 print "True\n" if $array[$index];
1863 A hash or array element can be true only if it's defined and defined only if
1864 it exists, but the reverse doesn't necessarily hold true.
1866 Given an expression that specifies the name of a subroutine,
1867 returns true if the specified subroutine has ever been declared, even
1868 if it is undefined. Mentioning a subroutine name for exists or defined
1869 does not count as declaring it. Note that a subroutine that does not
1870 exist may still be callable: its package may have an C<AUTOLOAD>
1871 method that makes it spring into existence the first time that it is
1872 called; see L<perlsub>.
1874 print "Exists\n" if exists &subroutine;
1875 print "Defined\n" if defined &subroutine;
1877 Note that the EXPR can be arbitrarily complicated as long as the final
1878 operation is a hash or array key lookup or subroutine name:
1880 if (exists $ref->{A}->{B}->{$key}) { }
1881 if (exists $hash{A}{B}{$key}) { }
1883 if (exists $ref->{A}->{B}->[$ix]) { }
1884 if (exists $hash{A}{B}[$ix]) { }
1886 if (exists &{$ref->{A}{B}{$key}}) { }
1888 Although the mostly deeply nested array or hash will not spring into
1889 existence just because its existence was tested, any intervening ones will.
1890 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1891 into existence due to the existence test for the $key element above.
1892 This happens anywhere the arrow operator is used, including even here:
1895 if (exists $ref->{"Some key"}) { }
1896 print $ref; # prints HASH(0x80d3d5c)
1898 This surprising autovivification in what does not at first--or even
1899 second--glance appear to be an lvalue context may be fixed in a future
1902 Use of a subroutine call, rather than a subroutine name, as an argument
1903 to exists() is an error.
1906 exists &sub(); # Error
1909 X<exit> X<terminate> X<abort>
1913 Evaluates EXPR and exits immediately with that value. Example:
1916 exit 0 if $ans =~ /^[Xx]/;
1918 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1919 universally recognized values for EXPR are C<0> for success and C<1>
1920 for error; other values are subject to interpretation depending on the
1921 environment in which the Perl program is running. For example, exiting
1922 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1923 the mailer to return the item undelivered, but that's not true everywhere.
1925 Don't use C<exit> to abort a subroutine if there's any chance that
1926 someone might want to trap whatever error happened. Use C<die> instead,
1927 which can be trapped by an C<eval>.
1929 The exit() function does not always exit immediately. It calls any
1930 defined C<END> routines first, but these C<END> routines may not
1931 themselves abort the exit. Likewise any object destructors that need to
1932 be called are called before the real exit. C<END> routines and destructors
1933 can change the exit status by modifying C<$?>. If this is a problem, you
1934 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1935 See L<perlmod> for details.
1937 Portability issues: L<perlport/exit>.
1940 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1944 Returns I<e> (the natural logarithm base) to the power of EXPR.
1945 If EXPR is omitted, gives C<exp($_)>.
1947 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1950 Implements the fcntl(2) function. You'll probably have to say
1954 first to get the correct constant definitions. Argument processing and
1955 value returned work just like C<ioctl> below.
1959 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1960 or die "can't fcntl F_GETFL: $!";
1962 You don't have to check for C<defined> on the return from C<fcntl>.
1963 Like C<ioctl>, it maps a C<0> return from the system call into
1964 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1965 in numeric context. It is also exempt from the normal B<-w> warnings
1966 on improper numeric conversions.
1968 Note that C<fcntl> raises an exception if used on a machine that
1969 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1970 manpage to learn what functions are available on your system.
1972 Here's an example of setting a filehandle named C<REMOTE> to be
1973 non-blocking at the system level. You'll have to negotiate C<$|>
1974 on your own, though.
1976 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1978 $flags = fcntl(REMOTE, F_GETFL, 0)
1979 or die "Can't get flags for the socket: $!\n";
1981 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1982 or die "Can't set flags for the socket: $!\n";
1984 Portability issues: L<perlport/fcntl>.
1989 A special token that returns the name of the file in which it occurs.
1991 =item fileno FILEHANDLE
1994 Returns the file descriptor for a filehandle, or undefined if the
1995 filehandle is not open. If there is no real file descriptor at the OS
1996 level, as can happen with filehandles connected to memory objects via
1997 C<open> with a reference for the third argument, -1 is returned.
1999 This is mainly useful for constructing
2000 bitmaps for C<select> and low-level POSIX tty-handling operations.
2001 If FILEHANDLE is an expression, the value is taken as an indirect
2002 filehandle, generally its name.
2004 You can use this to find out whether two handles refer to the
2005 same underlying descriptor:
2007 if (fileno(THIS) == fileno(THAT)) {
2008 print "THIS and THAT are dups\n";
2011 =item flock FILEHANDLE,OPERATION
2012 X<flock> X<lock> X<locking>
2014 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2015 for success, false on failure. Produces a fatal error if used on a
2016 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2017 C<flock> is Perl's portable file-locking interface, although it locks
2018 entire files only, not records.
2020 Two potentially non-obvious but traditional C<flock> semantics are
2021 that it waits indefinitely until the lock is granted, and that its locks
2022 are B<merely advisory>. Such discretionary locks are more flexible, but
2023 offer fewer guarantees. This means that programs that do not also use
2024 C<flock> may modify files locked with C<flock>. See L<perlport>,
2025 your port's specific documentation, and your system-specific local manpages
2026 for details. It's best to assume traditional behavior if you're writing
2027 portable programs. (But if you're not, you should as always feel perfectly
2028 free to write for your own system's idiosyncrasies (sometimes called
2029 "features"). Slavish adherence to portability concerns shouldn't get
2030 in the way of your getting your job done.)
2032 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2033 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2034 you can use the symbolic names if you import them from the L<Fcntl> module,
2035 either individually, or as a group using the C<:flock> tag. LOCK_SH
2036 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2037 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2038 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2039 waiting for the lock; check the return status to see if you got it.
2041 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2042 before locking or unlocking it.
2044 Note that the emulation built with lockf(3) doesn't provide shared
2045 locks, and it requires that FILEHANDLE be open with write intent. These
2046 are the semantics that lockf(3) implements. Most if not all systems
2047 implement lockf(3) in terms of fcntl(2) locking, though, so the
2048 differing semantics shouldn't bite too many people.
2050 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2051 be open with read intent to use LOCK_SH and requires that it be open
2052 with write intent to use LOCK_EX.
2054 Note also that some versions of C<flock> cannot lock things over the
2055 network; you would need to use the more system-specific C<fcntl> for
2056 that. If you like you can force Perl to ignore your system's flock(2)
2057 function, and so provide its own fcntl(2)-based emulation, by passing
2058 the switch C<-Ud_flock> to the F<Configure> program when you configure
2059 and build a new Perl.
2061 Here's a mailbox appender for BSD systems.
2063 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
2067 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2069 # and, in case someone appended while we were waiting...
2070 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2075 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2078 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2079 or die "Can't open mailbox: $!";
2082 print $mbox $msg,"\n\n";
2085 On systems that support a real flock(2), locks are inherited across fork()
2086 calls, whereas those that must resort to the more capricious fcntl(2)
2087 function lose their locks, making it seriously harder to write servers.
2089 See also L<DB_File> for other flock() examples.
2091 Portability issues: L<perlport/flock>.
2094 X<fork> X<child> X<parent>
2096 Does a fork(2) system call to create a new process running the
2097 same program at the same point. It returns the child pid to the
2098 parent process, C<0> to the child process, or C<undef> if the fork is
2099 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2100 are shared, while everything else is copied. On most systems supporting
2101 fork(), great care has gone into making it extremely efficient (for
2102 example, using copy-on-write technology on data pages), making it the
2103 dominant paradigm for multitasking over the last few decades.
2105 Beginning with v5.6.0, Perl attempts to flush all files opened for
2106 output before forking the child process, but this may not be supported
2107 on some platforms (see L<perlport>). To be safe, you may need to set
2108 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2109 C<IO::Handle> on any open handles to avoid duplicate output.
2111 If you C<fork> without ever waiting on your children, you will
2112 accumulate zombies. On some systems, you can avoid this by setting
2113 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2114 forking and reaping moribund children.
2116 Note that if your forked child inherits system file descriptors like
2117 STDIN and STDOUT that are actually connected by a pipe or socket, even
2118 if you exit, then the remote server (such as, say, a CGI script or a
2119 backgrounded job launched from a remote shell) won't think you're done.
2120 You should reopen those to F</dev/null> if it's any issue.
2122 On some platforms such as Windows, where the fork() system call is not available,
2123 Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2124 at the level of the Perl program, be as compatible as possible with the "Unix" fork().
2125 However it has limitations that have to be considered in code intended to be portable.
2126 See L<perlfork> for more details.
2128 Portability issues: L<perlport/fork>.
2133 Declare a picture format for use by the C<write> function. For
2137 Test: @<<<<<<<< @||||| @>>>>>
2138 $str, $%, '$' . int($num)
2142 $num = $cost/$quantity;
2146 See L<perlform> for many details and examples.
2148 =item formline PICTURE,LIST
2151 This is an internal function used by C<format>s, though you may call it,
2152 too. It formats (see L<perlform>) a list of values according to the
2153 contents of PICTURE, placing the output into the format output
2154 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2155 Eventually, when a C<write> is done, the contents of
2156 C<$^A> are written to some filehandle. You could also read C<$^A>
2157 and then set C<$^A> back to C<"">. Note that a format typically
2158 does one C<formline> per line of form, but the C<formline> function itself
2159 doesn't care how many newlines are embedded in the PICTURE. This means
2160 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2161 You may therefore need to use multiple formlines to implement a single
2162 record format, just like the C<format> compiler.
2164 Be careful if you put double quotes around the picture, because an C<@>
2165 character may be taken to mean the beginning of an array name.
2166 C<formline> always returns true. See L<perlform> for other examples.
2168 If you are trying to use this instead of C<write> to capture the output,
2169 you may find it easier to open a filehandle to a scalar
2170 (C<< open $fh, ">", \$output >>) and write to that instead.
2172 =item getc FILEHANDLE
2173 X<getc> X<getchar> X<character> X<file, read>
2177 Returns the next character from the input file attached to FILEHANDLE,
2178 or the undefined value at end of file or if there was an error (in
2179 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2180 STDIN. This is not particularly efficient. However, it cannot be
2181 used by itself to fetch single characters without waiting for the user
2182 to hit enter. For that, try something more like:
2185 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2188 system "stty", '-icanon', 'eol', "\001";
2194 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2197 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2201 Determination of whether $BSD_STYLE should be set
2202 is left as an exercise to the reader.
2204 The C<POSIX::getattr> function can do this more portably on
2205 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2206 module from your nearest CPAN site; details on CPAN can be found under
2210 X<getlogin> X<login>
2212 This implements the C library function of the same name, which on most
2213 systems returns the current login from F</etc/utmp>, if any. If it
2214 returns the empty string, use C<getpwuid>.
2216 $login = getlogin || getpwuid($<) || "Kilroy";
2218 Do not consider C<getlogin> for authentication: it is not as
2219 secure as C<getpwuid>.
2221 Portability issues: L<perlport/getlogin>.
2223 =item getpeername SOCKET
2224 X<getpeername> X<peer>
2226 Returns the packed sockaddr address of the other end of the SOCKET
2230 $hersockaddr = getpeername(SOCK);
2231 ($port, $iaddr) = sockaddr_in($hersockaddr);
2232 $herhostname = gethostbyaddr($iaddr, AF_INET);
2233 $herstraddr = inet_ntoa($iaddr);
2238 Returns the current process group for the specified PID. Use
2239 a PID of C<0> to get the current process group for the
2240 current process. Will raise an exception if used on a machine that
2241 doesn't implement getpgrp(2). If PID is omitted, returns the process
2242 group of the current process. Note that the POSIX version of C<getpgrp>
2243 does not accept a PID argument, so only C<PID==0> is truly portable.
2245 Portability issues: L<perlport/getpgrp>.
2248 X<getppid> X<parent> X<pid>
2250 Returns the process id of the parent process.
2252 Note for Linux users: on Linux, the C functions C<getpid()> and
2253 C<getppid()> return different values from different threads. In order to
2254 be portable, this behavior is not reflected by the Perl-level function
2255 C<getppid()>, that returns a consistent value across threads. If you want
2256 to call the underlying C<getppid()>, you may use the CPAN module
2259 Portability issues: L<perlport/getppid>.
2261 =item getpriority WHICH,WHO
2262 X<getpriority> X<priority> X<nice>
2264 Returns the current priority for a process, a process group, or a user.
2265 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2266 machine that doesn't implement getpriority(2).
2268 Portability issues: L<perlport/getpriority>.
2271 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2272 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2273 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2274 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2275 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2276 X<endnetent> X<endprotoent> X<endservent>
2280 =item gethostbyname NAME
2282 =item getnetbyname NAME
2284 =item getprotobyname NAME
2290 =item getservbyname NAME,PROTO
2292 =item gethostbyaddr ADDR,ADDRTYPE
2294 =item getnetbyaddr ADDR,ADDRTYPE
2296 =item getprotobynumber NUMBER
2298 =item getservbyport PORT,PROTO
2316 =item sethostent STAYOPEN
2318 =item setnetent STAYOPEN
2320 =item setprotoent STAYOPEN
2322 =item setservent STAYOPEN
2336 These routines are the same as their counterparts in the
2337 system C library. In list context, the return values from the
2338 various get routines are as follows:
2340 ($name,$passwd,$uid,$gid,
2341 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2342 ($name,$passwd,$gid,$members) = getgr*
2343 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2344 ($name,$aliases,$addrtype,$net) = getnet*
2345 ($name,$aliases,$proto) = getproto*
2346 ($name,$aliases,$port,$proto) = getserv*
2348 (If the entry doesn't exist you get an empty list.)
2350 The exact meaning of the $gcos field varies but it usually contains
2351 the real name of the user (as opposed to the login name) and other
2352 information pertaining to the user. Beware, however, that in many
2353 system users are able to change this information and therefore it
2354 cannot be trusted and therefore the $gcos is tainted (see
2355 L<perlsec>). The $passwd and $shell, user's encrypted password and
2356 login shell, are also tainted, for the same reason.
2358 In scalar context, you get the name, unless the function was a
2359 lookup by name, in which case you get the other thing, whatever it is.
2360 (If the entry doesn't exist you get the undefined value.) For example:
2362 $uid = getpwnam($name);
2363 $name = getpwuid($num);
2365 $gid = getgrnam($name);
2366 $name = getgrgid($num);
2370 In I<getpw*()> the fields $quota, $comment, and $expire are special
2371 in that they are unsupported on many systems. If the
2372 $quota is unsupported, it is an empty scalar. If it is supported, it
2373 usually encodes the disk quota. If the $comment field is unsupported,
2374 it is an empty scalar. If it is supported it usually encodes some
2375 administrative comment about the user. In some systems the $quota
2376 field may be $change or $age, fields that have to do with password
2377 aging. In some systems the $comment field may be $class. The $expire
2378 field, if present, encodes the expiration period of the account or the
2379 password. For the availability and the exact meaning of these fields
2380 in your system, please consult getpwnam(3) and your system's
2381 F<pwd.h> file. You can also find out from within Perl what your
2382 $quota and $comment fields mean and whether you have the $expire field
2383 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2384 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2385 files are supported only if your vendor has implemented them in the
2386 intuitive fashion that calling the regular C library routines gets the
2387 shadow versions if you're running under privilege or if there exists
2388 the shadow(3) functions as found in System V (this includes Solaris
2389 and Linux). Those systems that implement a proprietary shadow password
2390 facility are unlikely to be supported.
2392 The $members value returned by I<getgr*()> is a space-separated list of
2393 the login names of the members of the group.
2395 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2396 C, it will be returned to you via C<$?> if the function call fails. The
2397 C<@addrs> value returned by a successful call is a list of raw
2398 addresses returned by the corresponding library call. In the
2399 Internet domain, each address is four bytes long; you can unpack it
2400 by saying something like:
2402 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2404 The Socket library makes this slightly easier:
2407 $iaddr = inet_aton("127.1"); # or whatever address
2408 $name = gethostbyaddr($iaddr, AF_INET);
2410 # or going the other way
2411 $straddr = inet_ntoa($iaddr);
2413 In the opposite way, to resolve a hostname to the IP address
2417 $packed_ip = gethostbyname("www.perl.org");
2418 if (defined $packed_ip) {
2419 $ip_address = inet_ntoa($packed_ip);
2422 Make sure C<gethostbyname()> is called in SCALAR context and that
2423 its return value is checked for definedness.
2425 The C<getprotobynumber> function, even though it only takes one argument,
2426 has the precedence of a list operator, so beware:
2428 getprotobynumber $number eq 'icmp' # WRONG
2429 getprotobynumber($number eq 'icmp') # actually means this
2430 getprotobynumber($number) eq 'icmp' # better this way
2432 If you get tired of remembering which element of the return list
2433 contains which return value, by-name interfaces are provided
2434 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2435 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2436 and C<User::grent>. These override the normal built-ins, supplying
2437 versions that return objects with the appropriate names
2438 for each field. For example:
2442 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2444 Even though it looks as though they're the same method calls (uid),
2445 they aren't, because a C<File::stat> object is different from
2446 a C<User::pwent> object.
2448 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2450 =item getsockname SOCKET
2453 Returns the packed sockaddr address of this end of the SOCKET connection,
2454 in case you don't know the address because you have several different
2455 IPs that the connection might have come in on.
2458 $mysockaddr = getsockname(SOCK);
2459 ($port, $myaddr) = sockaddr_in($mysockaddr);
2460 printf "Connect to %s [%s]\n",
2461 scalar gethostbyaddr($myaddr, AF_INET),
2464 =item getsockopt SOCKET,LEVEL,OPTNAME
2467 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2468 Options may exist at multiple protocol levels depending on the socket
2469 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2470 C<Socket> module) will exist. To query options at another level the
2471 protocol number of the appropriate protocol controlling the option
2472 should be supplied. For example, to indicate that an option is to be
2473 interpreted by the TCP protocol, LEVEL should be set to the protocol
2474 number of TCP, which you can get using C<getprotobyname>.
2476 The function returns a packed string representing the requested socket
2477 option, or C<undef> on error, with the reason for the error placed in
2478 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2479 consult getsockopt(2) for details. A common case is that the option is an
2480 integer, in which case the result is a packed integer, which you can decode
2481 using C<unpack> with the C<i> (or C<I>) format.
2483 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2485 use Socket qw(:all);
2487 defined(my $tcp = getprotobyname("tcp"))
2488 or die "Could not determine the protocol number for tcp";
2489 # my $tcp = IPPROTO_TCP; # Alternative
2490 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2491 or die "getsockopt TCP_NODELAY: $!";
2492 my $nodelay = unpack("I", $packed);
2493 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2495 Portability issues: L<perlport/getsockopt>.
2497 =item given EXPR BLOCK
2502 C<given> is analogous to the C<switch> keyword in other languages. C<given>
2503 and C<when> are used in Perl to implement C<switch>/C<case> like statements.
2504 Only available after Perl 5.10. For example:
2509 print "I like apples."
2512 print "I don't like oranges."
2515 print "I don't like anything"
2519 See L<perlsyn/"Switch statements"> for detailed information.
2522 X<glob> X<wildcard> X<filename, expansion> X<expand>
2526 In list context, returns a (possibly empty) list of filename expansions on
2527 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2528 scalar context, glob iterates through such filename expansions, returning
2529 undef when the list is exhausted. This is the internal function
2530 implementing the C<< <*.c> >> operator, but you can use it directly. If
2531 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2532 more detail in L<perlop/"I/O Operators">.
2534 Note that C<glob> splits its arguments on whitespace and treats
2535 each segment as separate pattern. As such, C<glob("*.c *.h")>
2536 matches all files with a F<.c> or F<.h> extension. The expression
2537 C<glob(".* *")> matches all files in the current working directory.
2539 If non-empty braces are the only wildcard characters used in the
2540 C<glob>, no filenames are matched, but potentially many strings
2541 are returned. For example, this produces nine strings, one for
2542 each pairing of fruits and colors:
2544 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2546 Beginning with v5.6.0, this operator is implemented using the standard
2547 C<File::Glob> extension. See L<File::Glob> for details, including
2548 C<bsd_glob> which does not treat whitespace as a pattern separator.
2550 Portability issues: L<perlport/glob>.
2553 X<gmtime> X<UTC> X<Greenwich>
2557 Works just like L</localtime> but the returned values are
2558 localized for the standard Greenwich time zone.
2560 Note: When called in list context, $isdst, the last value
2561 returned by gmtime, is always C<0>. There is no
2562 Daylight Saving Time in GMT.
2564 Portability issues: L<perlport/gmtime>.
2567 X<goto> X<jump> X<jmp>
2573 The C<goto-LABEL> form finds the statement labeled with LABEL and
2574 resumes execution there. It can't be used to get out of a block or
2575 subroutine given to C<sort>. It can be used to go almost anywhere
2576 else within the dynamic scope, including out of subroutines, but it's
2577 usually better to use some other construct such as C<last> or C<die>.
2578 The author of Perl has never felt the need to use this form of C<goto>
2579 (in Perl, that is; C is another matter). (The difference is that C
2580 does not offer named loops combined with loop control. Perl does, and
2581 this replaces most structured uses of C<goto> in other languages.)
2583 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2584 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2585 necessarily recommended if you're optimizing for maintainability:
2587 goto ("FOO", "BAR", "GLARCH")[$i];
2589 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2590 function" rule. A pair of parentheses following it does not (necessarily)
2591 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2593 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2594 deprecated and will issue a warning. Even then, it may not be used to
2595 go into any construct that requires initialization, such as a
2596 subroutine or a C<foreach> loop. It also can't be used to go into a
2597 construct that is optimized away.
2599 The C<goto-&NAME> form is quite different from the other forms of
2600 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2601 doesn't have the stigma associated with other gotos. Instead, it
2602 exits the current subroutine (losing any changes set by local()) and
2603 immediately calls in its place the named subroutine using the current
2604 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2605 load another subroutine and then pretend that the other subroutine had
2606 been called in the first place (except that any modifications to C<@_>
2607 in the current subroutine are propagated to the other subroutine.)
2608 After the C<goto>, not even C<caller> will be able to tell that this
2609 routine was called first.
2611 NAME needn't be the name of a subroutine; it can be a scalar variable
2612 containing a code reference or a block that evaluates to a code
2615 =item grep BLOCK LIST
2618 =item grep EXPR,LIST
2620 This is similar in spirit to, but not the same as, grep(1) and its
2621 relatives. In particular, it is not limited to using regular expressions.
2623 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2624 C<$_> to each element) and returns the list value consisting of those
2625 elements for which the expression evaluated to true. In scalar
2626 context, returns the number of times the expression was true.
2628 @foo = grep(!/^#/, @bar); # weed out comments
2632 @foo = grep {!/^#/} @bar; # weed out comments
2634 Note that C<$_> is an alias to the list value, so it can be used to
2635 modify the elements of the LIST. While this is useful and supported,
2636 it can cause bizarre results if the elements of LIST are not variables.
2637 Similarly, grep returns aliases into the original list, much as a for
2638 loop's index variable aliases the list elements. That is, modifying an
2639 element of a list returned by grep (for example, in a C<foreach>, C<map>
2640 or another C<grep>) actually modifies the element in the original list.
2641 This is usually something to be avoided when writing clear code.
2643 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2644 been declared with C<my $_>) then, in addition to being locally aliased to
2645 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2646 can't be seen from the outside, avoiding any potential side-effects.
2648 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2651 X<hex> X<hexadecimal>
2655 Interprets EXPR as a hex string and returns the corresponding value.
2656 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2657 L</oct>.) If EXPR is omitted, uses C<$_>.
2659 print hex '0xAf'; # prints '175'
2660 print hex 'aF'; # same
2662 Hex strings may only represent integers. Strings that would cause
2663 integer overflow trigger a warning. Leading whitespace is not stripped,
2664 unlike oct(). To present something as hex, look into L</printf>,
2665 L</sprintf>, and L</unpack>.
2670 There is no builtin C<import> function. It is just an ordinary
2671 method (subroutine) defined (or inherited) by modules that wish to export
2672 names to another module. The C<use> function calls the C<import> method
2673 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2675 =item index STR,SUBSTR,POSITION
2676 X<index> X<indexOf> X<InStr>
2678 =item index STR,SUBSTR
2680 The index function searches for one string within another, but without
2681 the wildcard-like behavior of a full regular-expression pattern match.
2682 It returns the position of the first occurrence of SUBSTR in STR at
2683 or after POSITION. If POSITION is omitted, starts searching from the
2684 beginning of the string. POSITION before the beginning of the string
2685 or after its end is treated as if it were the beginning or the end,
2686 respectively. POSITION and the return value are based at zero.
2687 If the substring is not found, C<index> returns -1.
2690 X<int> X<integer> X<truncate> X<trunc> X<floor>
2694 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2695 You should not use this function for rounding: one because it truncates
2696 towards C<0>, and two because machine representations of floating-point
2697 numbers can sometimes produce counterintuitive results. For example,
2698 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2699 because it's really more like -268.99999999999994315658 instead. Usually,
2700 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2701 functions will serve you better than will int().
2703 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2706 Implements the ioctl(2) function. You'll probably first have to say
2708 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2710 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2711 exist or doesn't have the correct definitions you'll have to roll your
2712 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2713 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2714 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2715 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2716 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2717 has no string value but does have a numeric value, that value will be
2718 passed rather than a pointer to the string value. To guarantee this to be
2719 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2720 functions may be needed to manipulate the values of structures used by
2723 The return value of C<ioctl> (and C<fcntl>) is as follows:
2725 if OS returns: then Perl returns:
2727 0 string "0 but true"
2728 anything else that number
2730 Thus Perl returns true on success and false on failure, yet you can
2731 still easily determine the actual value returned by the operating
2734 $retval = ioctl(...) || -1;
2735 printf "System returned %d\n", $retval;
2737 The special string C<"0 but true"> is exempt from B<-w> complaints
2738 about improper numeric conversions.
2740 Portability issues: L<perlport/ioctl>.
2742 =item join EXPR,LIST
2745 Joins the separate strings of LIST into a single string with fields
2746 separated by the value of EXPR, and returns that new string. Example:
2748 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2750 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2751 first argument. Compare L</split>.
2760 Returns a list consisting of all the keys of the named hash, or the indices
2761 of an array. (In scalar context, returns the number of keys or indices.)
2763 The keys of a hash are returned in an apparently random order. The actual
2764 random order is subject to change in future versions of Perl, but it
2765 is guaranteed to be the same order as either the C<values> or C<each>
2766 function produces (given that the hash has not been modified). Since
2767 Perl 5.8.1 the ordering can be different even between different runs of
2768 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2771 As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
2772 (see L</each>). In particular, calling keys() in void context resets
2773 the iterator with no other overhead.
2775 Here is yet another way to print your environment:
2778 @values = values %ENV;
2780 print pop(@keys), '=', pop(@values), "\n";
2783 or how about sorted by key:
2785 foreach $key (sort(keys %ENV)) {
2786 print $key, '=', $ENV{$key}, "\n";
2789 The returned values are copies of the original keys in the hash, so
2790 modifying them will not affect the original hash. Compare L</values>.
2792 To sort a hash by value, you'll need to use a C<sort> function.
2793 Here's a descending numeric sort of a hash by its values:
2795 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2796 printf "%4d %s\n", $hash{$key}, $key;
2799 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2800 allocated for the given hash. This can gain you a measure of efficiency if
2801 you know the hash is going to get big. (This is similar to pre-extending
2802 an array by assigning a larger number to $#array.) If you say
2806 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2807 in fact, since it rounds up to the next power of two. These
2808 buckets will be retained even if you do C<%hash = ()>, use C<undef
2809 %hash> if you want to free the storage while C<%hash> is still in scope.
2810 You can't shrink the number of buckets allocated for the hash using
2811 C<keys> in this way (but you needn't worry about doing this by accident,
2812 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2815 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2816 a reference to an unblessed hash or array. The argument will be
2817 dereferenced automatically. This aspect of C<keys> is considered highly
2818 experimental. The exact behaviour may change in a future version of Perl.
2820 for (keys $hashref) { ... }
2821 for (keys $obj->get_arrayref) { ... }
2823 See also C<each>, C<values>, and C<sort>.
2825 =item kill SIGNAL, LIST
2828 Sends a signal to a list of processes. Returns the number of
2829 processes successfully signaled (which is not necessarily the
2830 same as the number actually killed).
2832 $cnt = kill 1, $child1, $child2;
2835 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2836 checks whether it's I<possible> to send a signal to it (that
2837 means, to be brief, that the process is owned by the same user, or we are
2838 the super-user). This is useful to check that a child process is still
2839 alive (even if only as a zombie) and hasn't changed its UID. See
2840 L<perlport> for notes on the portability of this construct.
2842 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2843 of processes. That means you usually want to use positive not negative signals.
2844 You may also use a signal name in quotes.
2846 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2847 the operating system. For example, on POSIX-conforming systems, zero will
2848 signal the current process group and -1 will signal all processes.
2850 See L<perlipc/"Signals"> for more details.
2852 On some platforms such as Windows where the fork() system call is not available.
2853 Perl can be built to emulate fork() at the interpreter level.
2854 This emulation has limitations related to kill that have to be considered,
2855 for code running on Windows and in code intended to be portable.
2857 See L<perlfork> for more details.
2859 Portability issues: L<perlport/kill>.
2866 The C<last> command is like the C<break> statement in C (as used in
2867 loops); it immediately exits the loop in question. If the LABEL is
2868 omitted, the command refers to the innermost enclosing loop. The
2869 C<continue> block, if any, is not executed:
2871 LINE: while (<STDIN>) {
2872 last LINE if /^$/; # exit when done with header
2876 C<last> cannot be used to exit a block that returns a value such as
2877 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2878 a grep() or map() operation.
2880 Note that a block by itself is semantically identical to a loop
2881 that executes once. Thus C<last> can be used to effect an early
2882 exit out of such a block.
2884 See also L</continue> for an illustration of how C<last>, C<next>, and
2892 Returns a lowercased version of EXPR. This is the internal function
2893 implementing the C<\L> escape in double-quoted strings.
2895 If EXPR is omitted, uses C<$_>.
2897 What gets returned depends on several factors:
2901 =item If C<use bytes> is in effect:
2905 =item On EBCDIC platforms
2907 The results are what the C language system call C<tolower()> returns.
2909 =item On ASCII platforms
2911 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2916 =item Otherwise, If EXPR has the UTF8 flag set
2918 Unicode semantics are used for the case change.
2920 =item Otherwise, if C<use locale> is in effect
2922 Respects current LC_CTYPE locale. See L<perllocale>.
2924 =item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2926 Unicode semantics are used for the case change.
2932 =item On EBCDIC platforms
2934 The results are what the C language system call C<tolower()> returns.
2936 =item On ASCII platforms
2938 ASCII semantics are used for the case change. The lowercase of any character
2939 outside the ASCII range is the character itself.
2946 X<lcfirst> X<lowercase>
2950 Returns the value of EXPR with the first character lowercased. This
2951 is the internal function implementing the C<\l> escape in
2952 double-quoted strings.
2954 If EXPR is omitted, uses C<$_>.
2956 This function behaves the same way under various pragmata, such as in a locale,
2964 Returns the length in I<characters> of the value of EXPR. If EXPR is
2965 omitted, returns the length of C<$_>. If EXPR is undefined, returns
2968 This function cannot be used on an entire array or hash to find out how
2969 many elements these have. For that, use C<scalar @array> and C<scalar keys
2970 %hash>, respectively.
2972 Like all Perl character operations, length() normally deals in logical
2973 characters, not physical bytes. For how many bytes a string encoded as
2974 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2975 to C<use Encode> first). See L<Encode> and L<perlunicode>.
2980 A special token that compiles to the current line number.
2982 =item link OLDFILE,NEWFILE
2985 Creates a new filename linked to the old filename. Returns true for
2986 success, false otherwise.
2988 Portability issues: L<perlport/link>.
2990 =item listen SOCKET,QUEUESIZE
2993 Does the same thing that the listen(2) system call does. Returns true if
2994 it succeeded, false otherwise. See the example in
2995 L<perlipc/"Sockets: Client/Server Communication">.
3000 You really probably want to be using C<my> instead, because C<local> isn't
3001 what most people think of as "local". See
3002 L<perlsub/"Private Variables via my()"> for details.
3004 A local modifies the listed variables to be local to the enclosing
3005 block, file, or eval. If more than one value is listed, the list must
3006 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3007 for details, including issues with tied arrays and hashes.
3009 The C<delete local EXPR> construct can also be used to localize the deletion
3010 of array/hash elements to the current block.
3011 See L<perlsub/"Localized deletion of elements of composite types">.
3013 =item localtime EXPR
3014 X<localtime> X<ctime>
3018 Converts a time as returned by the time function to a 9-element list
3019 with the time analyzed for the local time zone. Typically used as
3023 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3026 All list elements are numeric and come straight out of the C `struct
3027 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3028 of the specified time.
3030 C<$mday> is the day of the month and C<$mon> the month in
3031 the range C<0..11>, with 0 indicating January and 11 indicating December.
3032 This makes it easy to get a month name from a list:
3034 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3035 print "$abbr[$mon] $mday";
3036 # $mon=9, $mday=18 gives "Oct 18"
3038 C<$year> is the number of years since 1900, B<not> just the last two digits
3039 of the year. That is, C<$year> is C<123> in year 2023. The proper way
3040 to get a 4-digit year is simply:
3044 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3045 to do that, would you?
3047 To get the last two digits of the year (e.g., "01" in 2001) do:
3049 $year = sprintf("%02d", $year % 100);
3051 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3052 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3053 (or C<0..365> in leap years.)
3055 C<$isdst> is true if the specified time occurs during Daylight Saving
3056 Time, false otherwise.
3058 If EXPR is omitted, C<localtime()> uses the current time (as returned
3061 In scalar context, C<localtime()> returns the ctime(3) value:
3063 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3065 This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
3066 instead of local time use the L</gmtime> builtin. See also the
3067 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3068 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3069 and mktime(3) functions.
3071 To get somewhat similar but locale-dependent date strings, set up your
3072 locale environment variables appropriately (please see L<perllocale>) and
3075 use POSIX qw(strftime);
3076 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3077 # or for GMT formatted appropriately for your locale:
3078 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3080 Note that the C<%a> and C<%b>, the short forms of the day of the week
3081 and the month of the year, may not necessarily be three characters wide.
3083 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3084 by-name access mechanism to the gmtime() and localtime() functions,
3087 For a comprehensive date and time representation look at the
3088 L<DateTime> module on CPAN.
3090 Portability issues: L<perlport/localtime>.
3095 This function places an advisory lock on a shared variable or referenced
3096 object contained in I<THING> until the lock goes out of scope.
3098 The value returned is the scalar itself, if the argument is a scalar, or a
3099 reference, if the argument is a hash, array or subroutine.
3101 lock() is a "weak keyword" : this means that if you've defined a function
3102 by this name (before any calls to it), that function will be called
3103 instead. If you are not under C<use threads::shared> this does nothing.
3104 See L<threads::shared>.
3107 X<log> X<logarithm> X<e> X<ln> X<base>
3111 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3112 returns the log of C<$_>. To get the
3113 log of another base, use basic algebra:
3114 The base-N log of a number is equal to the natural log of that number
3115 divided by the natural log of N. For example:
3119 return log($n)/log(10);
3122 See also L</exp> for the inverse operation.
3124 =item lstat FILEHANDLE
3129 =item lstat DIRHANDLE
3133 Does the same thing as the C<stat> function (including setting the
3134 special C<_> filehandle) but stats a symbolic link instead of the file
3135 the symbolic link points to. If symbolic links are unimplemented on
3136 your system, a normal C<stat> is done. For much more detailed
3137 information, please see the documentation for C<stat>.
3139 If EXPR is omitted, stats C<$_>.
3141 Portability issues: L<perlport/lstat>.
3145 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3147 =item map BLOCK LIST
3152 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3153 C<$_> to each element) and returns the list value composed of the
3154 results of each such evaluation. In scalar context, returns the
3155 total number of elements so generated. Evaluates BLOCK or EXPR in
3156 list context, so each element of LIST may produce zero, one, or
3157 more elements in the returned value.
3159 @chars = map(chr, @numbers);
3161 translates a list of numbers to the corresponding characters.
3163 my @squares = map { $_ * $_ } @numbers;
3165 translates a list of numbers to their squared values.
3167 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3169 shows that number of returned elements can differ from the number of
3170 input elements. To omit an element, return an empty list ().
3171 This could also be achieved by writing
3173 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3175 which makes the intention more clear.
3177 Map always returns a list, which can be
3178 assigned to a hash such that the elements
3179 become key/value pairs. See L<perldata> for more details.
3181 %hash = map { get_a_key_for($_) => $_ } @array;
3183 is just a funny way to write
3187 $hash{get_a_key_for($_)} = $_;
3190 Note that C<$_> is an alias to the list value, so it can be used to
3191 modify the elements of the LIST. While this is useful and supported,
3192 it can cause bizarre results if the elements of LIST are not variables.
3193 Using a regular C<foreach> loop for this purpose would be clearer in
3194 most cases. See also L</grep> for an array composed of those items of
3195 the original list for which the BLOCK or EXPR evaluates to true.
3197 If C<$_> is lexical in the scope where the C<map> appears (because it has
3198 been declared with C<my $_>), then, in addition to being locally aliased to
3199 the list elements, C<$_> keeps being lexical inside the block; that is, it
3200 can't be seen from the outside, avoiding any potential side-effects.
3202 C<{> starts both hash references and blocks, so C<map { ...> could be either
3203 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3204 ahead for the closing C<}> it has to take a guess at which it's dealing with
3205 based on what it finds just after the C<{>. Usually it gets it right, but if it
3206 doesn't it won't realize something is wrong until it gets to the C<}> and
3207 encounters the missing (or unexpected) comma. The syntax error will be
3208 reported close to the C<}>, but you'll need to change something near the C<{>
3209 such as using a unary C<+> to give Perl some help:
3211 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3212 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3213 %hash = map { ("\L$_" => 1) } @array # this also works
3214 %hash = map { lc($_) => 1 } @array # as does this.
3215 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3217 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3219 or to force an anon hash constructor use C<+{>:
3221 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
3223 to get a list of anonymous hashes each with only one entry apiece.
3225 =item mkdir FILENAME,MASK
3226 X<mkdir> X<md> X<directory, create>
3228 =item mkdir FILENAME
3232 Creates the directory specified by FILENAME, with permissions
3233 specified by MASK (as modified by C<umask>). If it succeeds it
3234 returns true; otherwise it returns false and sets C<$!> (errno).
3235 MASK defaults to 0777 if omitted, and FILENAME defaults
3236 to C<$_> if omitted.
3238 In general, it is better to create directories with a permissive MASK
3239 and let the user modify that with their C<umask> than it is to supply
3240 a restrictive MASK and give the user no way to be more permissive.
3241 The exceptions to this rule are when the file or directory should be
3242 kept private (mail files, for instance). The perlfunc(1) entry on
3243 C<umask> discusses the choice of MASK in more detail.
3245 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3246 number of trailing slashes. Some operating and filesystems do not get
3247 this right, so Perl automatically removes all trailing slashes to keep
3250 To recursively create a directory structure, look at
3251 the C<mkpath> function of the L<File::Path> module.
3253 =item msgctl ID,CMD,ARG
3256 Calls the System V IPC function msgctl(2). You'll probably have to say
3260 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3261 then ARG must be a variable that will hold the returned C<msqid_ds>
3262 structure. Returns like C<ioctl>: the undefined value for error,
3263 C<"0 but true"> for zero, or the actual return value otherwise. See also
3264 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3267 Portability issues: L<perlport/msgctl>.
3269 =item msgget KEY,FLAGS
3272 Calls the System V IPC function msgget(2). Returns the message queue
3273 id, or C<undef> on error. See also
3274 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3277 Portability issues: L<perlport/msgget>.
3279 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3282 Calls the System V IPC function msgrcv to receive a message from
3283 message queue ID into variable VAR with a maximum message size of
3284 SIZE. Note that when a message is received, the message type as a
3285 native long integer will be the first thing in VAR, followed by the
3286 actual message. This packing may be opened with C<unpack("l! a*")>.
3287 Taints the variable. Returns true if successful, false
3288 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3289 C<IPC::SysV> and C<IPC::SysV::Msg>.
3291 Portability issues: L<perlport/msgrcv>.
3293 =item msgsnd ID,MSG,FLAGS
3296 Calls the System V IPC function msgsnd to send the message MSG to the
3297 message queue ID. MSG must begin with the native long integer message
3298 type, be followed by the length of the actual message, and then finally
3299 the message itself. This kind of packing can be achieved with
3300 C<pack("l! a*", $type, $message)>. Returns true if successful,
3301 false on error. See also the C<IPC::SysV>
3302 and C<IPC::SysV::Msg> documentation.
3304 Portability issues: L<perlport/msgsnd>.
3311 =item my EXPR : ATTRS
3313 =item my TYPE EXPR : ATTRS
3315 A C<my> declares the listed variables to be local (lexically) to the
3316 enclosing block, file, or C<eval>. If more than one value is listed,
3317 the list must be placed in parentheses.
3319 The exact semantics and interface of TYPE and ATTRS are still
3320 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3321 and attributes are handled using the C<attributes> pragma, or starting
3322 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3323 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3324 L<attributes>, and L<Attribute::Handlers>.
3331 The C<next> command is like the C<continue> statement in C; it starts
3332 the next iteration of the loop:
3334 LINE: while (<STDIN>) {
3335 next LINE if /^#/; # discard comments
3339 Note that if there were a C<continue> block on the above, it would get
3340 executed even on discarded lines. If LABEL is omitted, the command
3341 refers to the innermost enclosing loop.
3343 C<next> cannot be used to exit a block which returns a value such as
3344 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3345 a grep() or map() operation.
3347 Note that a block by itself is semantically identical to a loop
3348 that executes once. Thus C<next> will exit such a block early.
3350 See also L</continue> for an illustration of how C<last>, C<next>, and
3353 =item no MODULE VERSION LIST
3357 =item no MODULE VERSION
3359 =item no MODULE LIST
3365 See the C<use> function, of which C<no> is the opposite.
3368 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3372 Interprets EXPR as an octal string and returns the corresponding
3373 value. (If EXPR happens to start off with C<0x>, interprets it as a
3374 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3375 binary string. Leading whitespace is ignored in all three cases.)
3376 The following will handle decimal, binary, octal, and hex in standard
3379 $val = oct($val) if $val =~ /^0/;
3381 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3382 in octal), use sprintf() or printf():
3384 $dec_perms = (stat("filename"))[2] & 07777;
3385 $oct_perm_str = sprintf "%o", $perms;
3387 The oct() function is commonly used when a string such as C<644> needs
3388 to be converted into a file mode, for example. Although Perl
3389 automatically converts strings into numbers as needed, this automatic
3390 conversion assumes base 10.
3392 Leading white space is ignored without warning, as too are any trailing
3393 non-digits, such as a decimal point (C<oct> only handles non-negative
3394 integers, not negative integers or floating point).
3396 =item open FILEHANDLE,EXPR
3397 X<open> X<pipe> X<file, open> X<fopen>
3399 =item open FILEHANDLE,MODE,EXPR
3401 =item open FILEHANDLE,MODE,EXPR,LIST
3403 =item open FILEHANDLE,MODE,REFERENCE
3405 =item open FILEHANDLE
3407 Opens the file whose filename is given by EXPR, and associates it with
3410 Simple examples to open a file for reading:
3412 open(my $fh, "<", "input.txt")
3413 or die "cannot open < input.txt: $!";
3417 open(my $fh, ">", "output.txt")
3418 or die "cannot open > output.txt: $!";
3420 (The following is a comprehensive reference to open(): for a gentler
3421 introduction you may consider L<perlopentut>.)
3423 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3424 new filehandle is autovivified, meaning that the variable is assigned a
3425 reference to a newly allocated anonymous filehandle. Otherwise if
3426 FILEHANDLE is an expression, its value is the real filehandle. (This is
3427 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3430 If EXPR is omitted, the global (package) scalar variable of the same
3431 name as the FILEHANDLE contains the filename. (Note that lexical
3432 variables--those declared with C<my> or C<state>--will not work for this
3433 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3436 If three (or more) arguments are specified, the open mode (including
3437 optional encoding) in the second argument are distinct from the filename in
3438 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3439 If MODE is C<< > >>, the file is opened for output, with existing files
3440 first being truncated ("clobbered") and nonexisting files newly created.
3441 If MODE is C<<< >> >>>, the file is opened for appending, again being
3442 created if necessary.
3444 You can put a C<+> in front of the C<< > >> or C<< < >> to
3445 indicate that you want both read and write access to the file; thus
3446 C<< +< >> is almost always preferred for read/write updates--the
3447 C<< +> >> mode would clobber the file first. You cant usually use
3448 either read-write mode for updating textfiles, since they have
3449 variable-length records. See the B<-i> switch in L<perlrun> for a
3450 better approach. The file is created with permissions of C<0666>
3451 modified by the process's C<umask> value.
3453 These various prefixes correspond to the fopen(3) modes of C<r>,
3454 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3456 In the one- and two-argument forms of the call, the mode and filename
3457 should be concatenated (in that order), preferably separated by white
3458 space. You can--but shouldn't--omit the mode in these forms when that mode
3459 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3460 the filename argument is a known literal.
3462 For three or more arguments if MODE is C<|->, the filename is
3463 interpreted as a command to which output is to be piped, and if MODE
3464 is C<-|>, the filename is interpreted as a command that pipes
3465 output to us. In the two-argument (and one-argument) form, one should
3466 replace dash (C<->) with the command.
3467 See L<perlipc/"Using open() for IPC"> for more examples of this.
3468 (You are not allowed to C<open> to a command that pipes both in I<and>
3469 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3470 L<perlipc/"Bidirectional Communication with Another Process"> for
3473 In the form of pipe opens taking three or more arguments, if LIST is specified
3474 (extra arguments after the command name) then LIST becomes arguments
3475 to the command invoked if the platform supports it. The meaning of
3476 C<open> with more than three arguments for non-pipe modes is not yet
3477 defined, but experimental "layers" may give extra LIST arguments
3480 In the two-argument (and one-argument) form, opening C<< <- >>
3481 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3483 You may (and usually should) use the three-argument form of open to specify
3484 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3485 that affect how the input and output are processed (see L<open> and
3486 L<PerlIO> for more details). For example:
3488 open(my $fh, "<:encoding(UTF-8)", "filename")
3489 || die "can't open UTF-8 encoded filename: $!";
3491 opens the UTF8-encoded file containing Unicode characters;
3492 see L<perluniintro>. Note that if layers are specified in the
3493 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3494 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3496 Open returns nonzero on success, the undefined value otherwise. If
3497 the C<open> involved a pipe, the return value happens to be the pid of
3500 If you're running Perl on a system that distinguishes between text
3501 files and binary files, then you should check out L</binmode> for tips
3502 for dealing with this. The key distinction between systems that need
3503 C<binmode> and those that don't is their text file formats. Systems
3504 like Unix, Mac OS, and Plan 9, that end lines with a single
3505 character and encode that character in C as C<"\n"> do not
3506 need C<binmode>. The rest need it.
3508 When opening a file, it's seldom a good idea to continue
3509 if the request failed, so C<open> is frequently used with
3510 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3511 where you want to format a suitable error message (but there are
3512 modules that can help with that problem)) always check
3513 the return value from opening a file.
3515 As a special case the three-argument form with a read/write mode and the third
3516 argument being C<undef>:
3518 open(my $tmp, "+>", undef) or die ...
3520 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3521 works for symmetry, but you really should consider writing something
3522 to the temporary file first. You will need to seek() to do the
3525 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3526 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3527 open filehandles directly to Perl scalars via:
3529 open($fh, ">", \$variable) || ..
3531 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3534 open(STDOUT, ">", \$variable)
3535 or die "Can't open STDOUT: $!";
3540 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3541 while (<ARTICLE>) {...
3543 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3544 # if the open fails, output is discarded
3546 open(my $dbase, "+<", "dbase.mine") # open for update
3547 or die "Can't open 'dbase.mine' for update: $!";
3549 open(my $dbase, "+<dbase.mine") # ditto
3550 or die "Can't open 'dbase.mine' for update: $!";
3552 open(ARTICLE, "-|", "caesar <$article") # decrypt article
3553 or die "Can't start caesar: $!";
3555 open(ARTICLE, "caesar <$article |") # ditto
3556 or die "Can't start caesar: $!";
3558 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3559 or die "Can't start sort: $!";
3562 open(MEMORY, ">", \$var)
3563 or die "Can't open memory file: $!";
3564 print MEMORY "foo!\n"; # output will appear in $var
3566 # process argument list of files along with any includes
3568 foreach $file (@ARGV) {
3569 process($file, "fh00");
3573 my($filename, $input) = @_;
3574 $input++; # this is a string increment
3575 unless (open($input, "<", $filename)) {
3576 print STDERR "Can't open $filename: $!\n";
3581 while (<$input>) { # note use of indirection
3582 if (/^#include "(.*)"/) {
3583 process($1, $input);
3590 See L<perliol> for detailed info on PerlIO.
3592 You may also, in the Bourne shell tradition, specify an EXPR beginning
3593 with C<< >& >>, in which case the rest of the string is interpreted
3594 as the name of a filehandle (or file descriptor, if numeric) to be
3595 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3596 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3597 The mode you specify should match the mode of the original filehandle.
3598 (Duping a filehandle does not take into account any existing contents
3599 of IO buffers.) If you use the three-argument form, then you can pass either a
3600 number, the name of a filehandle, or the normal "reference to a glob".
3602 Here is a script that saves, redirects, and restores C<STDOUT> and
3603 C<STDERR> using various methods:
3606 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
3607 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
3609 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
3610 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
3612 select STDERR; $| = 1; # make unbuffered
3613 select STDOUT; $| = 1; # make unbuffered
3615 print STDOUT "stdout 1\n"; # this works for
3616 print STDERR "stderr 1\n"; # subprocesses too
3618 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
3619 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
3621 print STDOUT "stdout 2\n";
3622 print STDERR "stderr 2\n";
3624 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3625 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3626 that file descriptor (and not call C<dup(2)>); this is more
3627 parsimonious of file descriptors. For example:
3629 # open for input, reusing the fileno of $fd
3630 open(FILEHANDLE, "<&=$fd")
3634 open(FILEHANDLE, "<&=", $fd)
3638 # open for append, using the fileno of OLDFH
3639 open(FH, ">>&=", OLDFH)
3643 open(FH, ">>&=OLDFH")
3645 Being parsimonious on filehandles is also useful (besides being
3646 parsimonious) for example when something is dependent on file
3647 descriptors, like for example locking using flock(). If you do just
3648 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
3649 descriptor as B, and therefore flock(A) will not flock(B) nor vice
3650 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
3651 the same underlying system file descriptor.
3653 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
3654 fdopen() to implement the C<=> functionality. On many Unix systems,
3655 fdopen() fails when file descriptors exceed a certain value, typically 255.
3656 For Perls 5.8.0 and later, PerlIO is (most often) the default.
3658 You can see whether your Perl was built with PerlIO by running C<perl -V>
3659 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
3660 have PerlIO; otherwise you don't.
3662 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
3663 with the one- or two-argument forms of C<open>),
3664 an implicit C<fork> is done, so C<open> returns twice: in the parent
3665 process it returns the pid
3666 of the child process, and in the child process it returns (a defined) C<0>.
3667 Use C<defined($pid)> or C<//> to determine whether the open was successful.
3669 For example, use either
3671 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
3674 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
3680 # either write TO_KID or else read FROM_KID
3684 # am the child; use STDIN/STDOUT normally
3689 The filehandle behaves normally for the parent, but I/O to that
3690 filehandle is piped from/to the STDOUT/STDIN of the child process.
3691 In the child process, the filehandle isn't opened--I/O happens from/to
3692 the new STDOUT/STDIN. Typically this is used like the normal
3693 piped open when you want to exercise more control over just how the
3694 pipe command gets executed, such as when running setuid and
3695 you don't want to have to scan shell commands for metacharacters.
3697 The following blocks are more or less equivalent:
3699 open(FOO, "|tr '[a-z]' '[A-Z]'");
3700 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
3701 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
3702 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
3704 open(FOO, "cat -n '$file'|");
3705 open(FOO, "-|", "cat -n '$file'");
3706 open(FOO, "-|") || exec "cat", "-n", $file;
3707 open(FOO, "-|", "cat", "-n", $file);
3709 The last two examples in each block show the pipe as "list form", which is
3710 not yet supported on all platforms. A good rule of thumb is that if
3711 your platform has a real C<fork()> (in other words, if your platform is
3712 Unix, including Linux and MacOS X), you can use the list form. You would
3713 want to use the list form of the pipe so you can pass literal arguments
3714 to the command without risk of the shell interpreting any shell metacharacters
3715 in them. However, this also bars you from opening pipes to commands
3716 that intentionally contain shell metacharacters, such as:
3718 open(FOO, "|cat -n | expand -4 | lpr")
3719 // die "Can't open pipeline to lpr: $!";
3721 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3723 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3724 output before any operation that may do a fork, but this may not be
3725 supported on some platforms (see L<perlport>). To be safe, you may need
3726 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3727 of C<IO::Handle> on any open handles.
3729 On systems that support a close-on-exec flag on files, the flag will
3730 be set for the newly opened file descriptor as determined by the value
3731 of C<$^F>. See L<perlvar/$^F>.
3733 Closing any piped filehandle causes the parent process to wait for the
3734 child to finish, then returns the status value in C<$?> and
3735 C<${^CHILD_ERROR_NATIVE}>.
3737 The filename passed to the one- and two-argument forms of open() will
3738 have leading and trailing whitespace deleted and normal
3739 redirection characters honored. This property, known as "magic open",
3740 can often be used to good effect. A user could specify a filename of
3741 F<"rsh cat file |">, or you could change certain filenames as needed:
3743 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3744 open(FH, $filename) or die "Can't open $filename: $!";
3746 Use the three-argument form to open a file with arbitrary weird characters in it,
3748 open(FOO, "<", $file)
3749 || die "can't open < $file: $!";
3751 otherwise it's necessary to protect any leading and trailing whitespace:
3753 $file =~ s#^(\s)#./$1#;
3754 open(FOO, "< $file\0")
3755 || die "open failed: $!";
3757 (this may not work on some bizarre filesystems). One should
3758 conscientiously choose between the I<magic> and I<three-argument> form
3761 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
3763 will allow the user to specify an argument of the form C<"rsh cat file |">,
3764 but will not work on a filename that happens to have a trailing space, while
3766 open(IN, "<", $ARGV[0])
3767 || die "can't open < $ARGV[0]: $!";
3769 will have exactly the opposite restrictions.
3771 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3772 should use the C<sysopen> function, which involves no such magic (but may
3773 use subtly different filemodes than Perl open(), which is mapped to C
3774 fopen()). This is another way to protect your filenames from
3775 interpretation. For example:
3778 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3779 or die "sysopen $path: $!";
3780 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3781 print HANDLE "stuff $$\n";
3783 print "File contains: ", <HANDLE>;
3785 Using the constructor from the C<IO::Handle> package (or one of its
3786 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3787 filehandles that have the scope of the variables used to hold them, then
3788 automatically (but silently) close once their reference counts become
3789 zero, typically at scope exit:
3793 sub read_myfile_munged {
3795 # or just leave it undef to autoviv
3796 my $handle = IO::File->new;
3797 open($handle, "<", "myfile") or die "myfile: $!";
3799 or return (); # Automatically closed here.
3800 mung($first) or die "mung failed"; # Or here.
3801 return (first, <$handle>) if $ALL; # Or here.
3802 return $first; # Or here.
3805 B<WARNING:> The previous example has a bug because the automatic
3806 close that happens when the refcount on C<handle> does not
3807 properly detect and report failures. I<Always> close the handle
3808 yourself and inspect the return value.
3811 || warn "close failed: $!";
3813 See L</seek> for some details about mixing reading and writing.
3815 Portability issues: L<perlport/open>.
3817 =item opendir DIRHANDLE,EXPR
3820 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3821 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3822 DIRHANDLE may be an expression whose value can be used as an indirect
3823 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3824 scalar variable (or array or hash element), the variable is assigned a
3825 reference to a new anonymous dirhandle; that is, it's autovivified.
3826 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3828 See the example at C<readdir>.
3835 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3836 or Unicode) value of the first character of EXPR.
3837 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
3838 (Note I<character>, not byte.)
3840 For the reverse, see L</chr>.
3841 See L<perlunicode> for more about Unicode.
3848 =item our EXPR : ATTRS
3850 =item our TYPE EXPR : ATTRS
3852 C<our> associates a simple name with a package variable in the current
3853 package for use within the current scope. When C<use strict 'vars'> is in
3854 effect, C<our> lets you use declared global variables without qualifying
3855 them with package names, within the lexical scope of the C<our> declaration.
3856 In this way C<our> differs from C<use vars>, which is package-scoped.
3858 Unlike C<my> or C<state>, which allocates storage for a variable and
3859 associates a simple name with that storage for use within the current
3860 scope, C<our> associates a simple name with a package (read: global)
3861 variable in the current package, for use within the current lexical scope.
3862 In other words, C<our> has the same scoping rules as C<my> or C<state>, but
3863 does not necessarily create a variable.
3865 If more than one value is listed, the list must be placed
3871 An C<our> declaration declares a global variable that will be visible
3872 across its entire lexical scope, even across package boundaries. The
3873 package in which the variable is entered is determined at the point
3874 of the declaration, not at the point of use. This means the following
3878 our $bar; # declares $Foo::bar for rest of lexical scope
3882 print $bar; # prints 20, as it refers to $Foo::bar
3884 Multiple C<our> declarations with the same name in the same lexical
3885 scope are allowed if they are in different packages. If they happen
3886 to be in the same package, Perl will emit warnings if you have asked
3887 for them, just like multiple C<my> declarations. Unlike a second
3888 C<my> declaration, which will bind the name to a fresh variable, a
3889 second C<our> declaration in the same package, in the same scope, is
3894 our $bar; # declares $Foo::bar for rest of lexical scope
3898 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3899 print $bar; # prints 30
3901 our $bar; # emits warning but has no other effect
3902 print $bar; # still prints 30
3904 An C<our> declaration may also have a list of attributes associated
3907 The exact semantics and interface of TYPE and ATTRS are still
3908 evolving. TYPE is currently bound to the use of C<fields> pragma,
3909 and attributes are handled using the C<attributes> pragma, or starting
3910 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3911 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3912 L<attributes>, and L<Attribute::Handlers>.
3914 =item pack TEMPLATE,LIST
3917 Takes a LIST of values and converts it into a string using the rules
3918 given by the TEMPLATE. The resulting string is the concatenation of
3919 the converted values. Typically, each converted value looks
3920 like its machine-level representation. For example, on 32-bit machines
3921 an integer may be represented by a sequence of 4 bytes, which will in
3922 Perl be presented as a string that's 4 characters long.
3924 See L<perlpacktut> for an introduction to this function.
3926 The TEMPLATE is a sequence of characters that give the order and type
3927 of values, as follows:
3929 a A string with arbitrary binary data, will be null padded.
3930 A A text (ASCII) string, will be space padded.
3931 Z A null-terminated (ASCIZ) string, will be null padded.
3933 b A bit string (ascending bit order inside each byte,
3935 B A bit string (descending bit order inside each byte).
3936 h A hex string (low nybble first).
3937 H A hex string (high nybble first).
3939 c A signed char (8-bit) value.
3940 C An unsigned char (octet) value.
3941 W An unsigned char value (can be greater than 255).
3943 s A signed short (16-bit) value.
3944 S An unsigned short value.
3946 l A signed long (32-bit) value.
3947 L An unsigned long value.
3949 q A signed quad (64-bit) value.
3950 Q An unsigned quad value.
3951 (Quads are available only if your system supports 64-bit
3952 integer values _and_ if Perl has been compiled to support
3953 those. Raises an exception otherwise.)
3955 i A signed integer value.
3956 I A unsigned integer value.
3957 (This 'integer' is _at_least_ 32 bits wide. Its exact
3958 size depends on what a local C compiler calls 'int'.)
3960 n An unsigned short (16-bit) in "network" (big-endian) order.
3961 N An unsigned long (32-bit) in "network" (big-endian) order.
3962 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3963 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3965 j A Perl internal signed integer value (IV).
3966 J A Perl internal unsigned integer value (UV).
3968 f A single-precision float in native format.
3969 d A double-precision float in native format.
3971 F A Perl internal floating-point value (NV) in native format
3972 D A float of long-double precision in native format.
3973 (Long doubles are available only if your system supports
3974 long double values _and_ if Perl has been compiled to
3975 support those. Raises an exception otherwise.)
3977 p A pointer to a null-terminated string.
3978 P A pointer to a structure (fixed-length string).
3980 u A uuencoded string.
3981 U A Unicode character number. Encodes to a character in char-
3982 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
3985 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
3986 for details). Its bytes represent an unsigned integer in
3987 base 128, most significant digit first, with as few digits
3988 as possible. Bit eight (the high bit) is set on each byte
3991 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
3993 @ Null-fill or truncate to absolute position, counted from the
3994 start of the innermost ()-group.
3995 . Null-fill or truncate to absolute position specified by
3997 ( Start of a ()-group.
3999 One or more modifiers below may optionally follow certain letters in the
4000 TEMPLATE (the second column lists letters for which the modifier is valid):
4002 ! sSlLiI Forces native (short, long, int) sizes instead
4003 of fixed (16-/32-bit) sizes.
4005 xX Make x and X act as alignment commands.
4007 nNvV Treat integers as signed instead of unsigned.
4009 @. Specify position as byte offset in the internal
4010 representation of the packed string. Efficient but
4013 > sSiIlLqQ Force big-endian byte-order on the type.
4014 jJfFdDpP (The "big end" touches the construct.)
4016 < sSiIlLqQ Force little-endian byte-order on the type.
4017 jJfFdDpP (The "little end" touches the construct.)
4019 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4020 to force a particular byte-order on all components in that group,
4021 including all its subgroups.
4023 The following rules apply:
4029 Each letter may optionally be followed by a number indicating the repeat
4030 count. A numeric repeat count may optionally be enclosed in brackets, as
4031 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4032 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4033 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4034 something else, described below. Supplying a C<*> for the repeat count
4035 instead of a number means to use however many items are left, except for:
4041 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4045 <.>, where it means relative to the start of the string.
4049 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4053 One can replace a numeric repeat count with a template letter enclosed in
4054 brackets to use the packed byte length of the bracketed template for the
4057 For example, the template C<x[L]> skips as many bytes as in a packed long,
4058 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4059 variable-expanded) unpacks. If the template in brackets contains alignment
4060 commands (such as C<x![d]>), its packed length is calculated as if the
4061 start of the template had the maximal possible alignment.
4063 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4064 trailing null byte, so the resulting string is always one byte longer than
4065 the byte length of the item itself.
4067 When used with C<@>, the repeat count represents an offset from the start
4068 of the innermost C<()> group.
4070 When used with C<.>, the repeat count determines the starting position to
4071 calculate the value offset as follows:
4077 If the repeat count is C<0>, it's relative to the current position.
4081 If the repeat count is C<*>, the offset is relative to the start of the
4086 And if it's an integer I<n>, the offset is relative to the start of the
4087 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4088 bigger then the group level.
4092 The repeat count for C<u> is interpreted as the maximal number of bytes
4093 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4094 count should not be more than 65.
4098 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4099 string of length count, padding with nulls or spaces as needed. When
4100 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4101 after the first null, and C<a> returns data with no stripping at all.
4103 If the value to pack is too long, the result is truncated. If it's too
4104 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4105 followed by a null byte. Thus C<Z> always packs a trailing null, except
4106 when the count is 0.
4110 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4111 Each such format generates 1 bit of the result. These are typically followed
4112 by a repeat count like C<B8> or C<B64>.
4114 Each result bit is based on the least-significant bit of the corresponding
4115 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4116 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4118 Starting from the beginning of the input string, each 8-tuple
4119 of characters is converted to 1 character of output. With format C<b>,
4120 the first character of the 8-tuple determines the least-significant bit of a
4121 character; with format C<B>, it determines the most-significant bit of
4124 If the length of the input string is not evenly divisible by 8, the
4125 remainder is packed as if the input string were padded by null characters
4126 at the end. Similarly during unpacking, "extra" bits are ignored.
4128 If the input string is longer than needed, remaining characters are ignored.
4130 A C<*> for the repeat count uses all characters of the input field.
4131 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4135 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4136 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4138 For each such format, pack() generates 4 bits of result.
4139 With non-alphabetical characters, the result is based on the 4 least-significant
4140 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4141 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4142 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4143 is compatible with the usual hexadecimal digits, so that C<"a"> and
4144 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4145 characters with this format.
4147 Starting from the beginning of the template to pack(), each pair
4148 of characters is converted to 1 character of output. With format C<h>, the
4149 first character of the pair determines the least-significant nybble of the
4150 output character; with format C<H>, it determines the most-significant
4153 If the length of the input string is not even, it behaves as if padded by
4154 a null character at the end. Similarly, "extra" nybbles are ignored during
4157 If the input string is longer than needed, extra characters are ignored.
4159 A C<*> for the repeat count uses all characters of the input field. For
4160 unpack(), nybbles are converted to a string of hexadecimal digits.
4164 The C<p> format packs a pointer to a null-terminated string. You are
4165 responsible for ensuring that the string is not a temporary value, as that
4166 could potentially get deallocated before you got around to using the packed
4167 result. The C<P> format packs a pointer to a structure of the size indicated
4168 by the length. A null pointer is created if the corresponding value for
4169 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4170 unpacks into C<undef>.
4172 If your system has a strange pointer size--meaning a pointer is neither as
4173 big as an int nor as big as a long--it may not be possible to pack or
4174 unpack pointers in big- or little-endian byte order. Attempting to do
4175 so raises an exception.
4179 The C</> template character allows packing and unpacking of a sequence of
4180 items where the packed structure contains a packed item count followed by
4181 the packed items themselves. This is useful when the structure you're
4182 unpacking has encoded the sizes or repeat counts for some of its fields
4183 within the structure itself as separate fields.
4185 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4186 I<length-item> describes how the length value is packed. Formats likely
4187 to be of most use are integer-packing ones like C<n> for Java strings,
4188 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4190 For C<pack>, I<sequence-item> may have a repeat count, in which case
4191 the minimum of that and the number of available items is used as the argument
4192 for I<length-item>. If it has no repeat count or uses a '*', the number
4193 of available items is used.
4195 For C<unpack>, an internal stack of integer arguments unpacked so far is
4196 used. You write C</>I<sequence-item> and the repeat count is obtained by
4197 popping off the last element from the stack. The I<sequence-item> must not
4198 have a repeat count.
4200 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4201 the I<length-item> is the string length, not the number of strings. With
4202 an explicit repeat count for pack, the packed string is adjusted to that
4203 length. For example:
4205 This code: gives this result:
4207 unpack("W/a", "\004Gurusamy") ("Guru")
4208 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4209 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4211 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4212 pack("a/W2", ord("a") .. ord("z")) "2ab"
4214 The I<length-item> is not returned explicitly from C<unpack>.
4216 Supplying a count to the I<length-item> format letter is only useful with
4217 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4218 introduce C<"\000"> characters, which Perl does not regard as legal in
4223 The integer types C<s>, C<S>, C<l>, and C<L> may be
4224 followed by a C<!> modifier to specify native shorts or
4225 longs. As shown in the example above, a bare C<l> means
4226 exactly 32 bits, although the native C<long> as seen by the local C compiler
4227 may be larger. This is mainly an issue on 64-bit platforms. You can
4228 see whether using C<!> makes any difference this way:
4230 printf "format s is %d, s! is %d\n",
4231 length pack("s"), length pack("s!");
4233 printf "format l is %d, l! is %d\n",
4234 length pack("l"), length pack("l!");
4237 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4238 they are identical to C<i> and C<I>.
4240 The actual sizes (in bytes) of native shorts, ints, longs, and long
4241 longs on the platform where Perl was built are also available from
4244 $ perl -V:{short,int,long{,long}}size
4250 or programmatically via the C<Config> module:
4253 print $Config{shortsize}, "\n";
4254 print $Config{intsize}, "\n";
4255 print $Config{longsize}, "\n";
4256 print $Config{longlongsize}, "\n";
4258 C<$Config{longlongsize}> is undefined on systems without
4263 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4264 inherently non-portable between processors and operating systems because
4265 they obey native byteorder and endianness. For example, a 4-byte integer
4266 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4267 handled by the CPU registers) into bytes as
4269 0x12 0x34 0x56 0x78 # big-endian
4270 0x78 0x56 0x34 0x12 # little-endian
4272 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4273 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4274 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4275 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4277 The names I<big-endian> and I<little-endian> are comic references to the
4278 egg-eating habits of the little-endian Lilliputians and the big-endian
4279 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4280 This entered computer lingo via the paper "On Holy Wars and a Plea for
4281 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4283 Some systems may have even weirder byte orders such as
4288 You can determine your system endianness with this incantation:
4290 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4292 The byteorder on the platform where Perl was built is also available
4296 print "$Config{byteorder}\n";
4298 or from the command line:
4302 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4303 and C<"87654321"> are big-endian.
4305 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4306 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4307 immediately below. See also L<perlport>.
4311 Starting with Perl 5.9.2, integer and floating-point formats, along with
4312 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4313 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4314 or little-endian byte-order. These modifiers are especially useful
4315 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4316 64-bit integers, or floating-point values.
4318 Here are some concerns to keep in mind when using an endianness modifier:
4324 Exchanging signed integers between different platforms works only
4325 when all platforms store them in the same format. Most platforms store
4326 signed integers in two's-complement notation, so usually this is not an issue.
4330 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4331 formats on big- or little-endian machines. Otherwise, attempting to
4332 use them raises an exception.
4336 Forcing big- or little-endian byte-order on floating-point values for
4337 data exchange can work only if all platforms use the same
4338 binary representation such as IEEE floating-point. Even if all
4339 platforms are using IEEE, there may still be subtle differences. Being able
4340 to use C<< > >> or C<< < >> on floating-point values can be useful,
4341 but also dangerous if you don't know exactly what you're doing.
4342 It is not a general way to portably store floating-point values.
4346 When using C<< > >> or C<< < >> on a C<()> group, this affects
4347 all types inside the group that accept byte-order modifiers,
4348 including all subgroups. It is silently ignored for all other
4349 types. You are not allowed to override the byte-order within a group
4350 that already has a byte-order modifier suffix.
4356 Real numbers (floats and doubles) are in native machine format only.
4357 Due to the multiplicity of floating-point formats and the lack of a
4358 standard "network" representation for them, no facility for interchange has been
4359 made. This means that packed floating-point data written on one machine
4360 may not be readable on another, even if both use IEEE floating-point
4361 arithmetic (because the endianness of the memory representation is not part
4362 of the IEEE spec). See also L<perlport>.
4364 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4365 modifiers to force big- or little-endian byte-order on floating-point values.
4367 Because Perl uses doubles (or long doubles, if configured) internally for
4368 all numeric calculation, converting from double into float and thence
4369 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4370 will not in general equal $foo.
4374 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4375 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4376 where the packed string is processed in its UTF-8-encoded Unicode form on
4377 a byte-by-byte basis. Character mode is the default unless the format string
4378 starts with C<U>. You can always switch mode mid-format with an explicit
4379 C<C0> or C<U0> in the format. This mode remains in effect until the next
4380 mode change, or until the end of the C<()> group it (directly) applies to.
4382 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4383 bytes is not necessarily obvious. Probably only the first of these
4386 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4387 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4389 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4390 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4392 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4393 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4395 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4396 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4397 C3.8E.C2.B1.C3.8F.C2.89
4399 Those examples also illustrate that you should not try to use
4400 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4404 You must yourself do any alignment or padding by inserting, for example,
4405 enough C<"x">es while packing. There is no way for pack() and unpack()
4406 to know where characters are going to or coming from, so they
4407 handle their output and input as flat sequences of characters.
4411 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4412 take a repeat count either as postfix, or for unpack(), also via the C</>
4413 template character. Within each repetition of a group, positioning with
4414 C<@> starts over at 0. Therefore, the result of
4416 pack("@1A((@2A)@3A)", qw[X Y Z])
4418 is the string C<"\0X\0\0YZ">.
4422 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4423 jump forward or back to the closest position aligned at a multiple of C<count>
4424 characters. For example, to pack() or unpack() a C structure like
4427 char c; /* one signed, 8-bit character */
4432 one may need to use the template C<c x![d] d c[2]>. This assumes that
4433 doubles must be aligned to the size of double.
4435 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4440 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4441 represent signed 16-/32-bit integers in big-/little-endian order.
4442 This is portable only when all platforms sharing packed data use the
4443 same binary representation for signed integers; for example, when all
4444 platforms use two's-complement representation.
4448 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4449 White space can separate pack codes from each other, but modifiers and
4450 repeat counts must follow immediately. Breaking complex templates into
4451 individual line-by-line components, suitably annotated, can do as much to
4452 improve legibility and maintainability of pack/unpack formats as C</x> can
4453 for complicated pattern matches.
4457 If TEMPLATE requires more arguments than pack() is given, pack()
4458 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4459 than given, extra arguments are ignored.
4465 $foo = pack("WWWW",65,66,67,68);
4467 $foo = pack("W4",65,66,67,68);
4469 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4470 # same thing with Unicode circled letters.
4471 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4472 # same thing with Unicode circled letters. You don't get the
4473 # UTF-8 bytes because the U at the start of the format caused
4474 # a switch to U0-mode, so the UTF-8 bytes get joined into
4476 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4477 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4478 # This is the UTF-8 encoding of the string in the
4481 $foo = pack("ccxxcc",65,66,67,68);
4484 # NOTE: The examples above featuring "W" and "c" are true
4485 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4486 # and UTF-8. On EBCDIC systems, the first example would be
4487 # $foo = pack("WWWW",193,194,195,196);
4489 $foo = pack("s2",1,2);
4490 # "\001\000\002\000" on little-endian
4491 # "\000\001\000\002" on big-endian
4493 $foo = pack("a4","abcd","x","y","z");
4496 $foo = pack("aaaa","abcd","x","y","z");
4499 $foo = pack("a14","abcdefg");
4500 # "abcdefg\0\0\0\0\0\0\0"
4502 $foo = pack("i9pl", gmtime);
4503 # a real struct tm (on my system anyway)
4505 $utmp_template = "Z8 Z8 Z16 L";
4506 $utmp = pack($utmp_template, @utmp1);
4507 # a struct utmp (BSDish)
4509 @utmp2 = unpack($utmp_template, $utmp);
4510 # "@utmp1" eq "@utmp2"
4513 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4516 $foo = pack('sx2l', 12, 34);
4517 # short 12, two zero bytes padding, long 34
4518 $bar = pack('s@4l', 12, 34);
4519 # short 12, zero fill to position 4, long 34
4521 $baz = pack('s.l', 12, 4, 34);
4522 # short 12, zero fill to position 4, long 34
4524 $foo = pack('nN', 42, 4711);
4525 # pack big-endian 16- and 32-bit unsigned integers
4526 $foo = pack('S>L>', 42, 4711);
4528 $foo = pack('s<l<', -42, 4711);
4529 # pack little-endian 16- and 32-bit signed integers
4530 $foo = pack('(sl)<', -42, 4711);
4533 The same template may generally also be used in unpack().
4535 =item package NAMESPACE
4537 =item package NAMESPACE VERSION
4538 X<package> X<module> X<namespace> X<version>
4540 =item package NAMESPACE BLOCK
4542 =item package NAMESPACE VERSION BLOCK
4543 X<package> X<module> X<namespace> X<version>
4545 Declares the BLOCK or the rest of the compilation unit as being in the
4546 given namespace. The scope of the package declaration is either the
4547 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4548 itself through the end of current scope (the enclosing block, file, or
4549 C<eval>). That is, the forms without a BLOCK are operative through the end
4550 of the current scope, just like the C<my>, C<state>, and C<our> operators.
4551 All unqualified dynamic identifiers in this scope will be in the given
4552 namespace, except where overridden by another C<package> declaration or
4553 when they're one of the special identifiers that qualify into C<main::>,
4554 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
4556 A package statement affects dynamic variables only, including those
4557 you've used C<local> on, but I<not> lexical variables, which are created
4558 with C<my>, C<state>, or C<our>. Typically it would be the first
4559 declaration in a file included by C<require> or C<use>. You can switch into a
4560 package in more than one place, since this only determines which default
4561 symbol table the compiler uses for the rest of that block. You can refer to
4562 identifiers in other packages than the current one by prefixing the identifier
4563 with the package name and a double colon, as in C<$SomePack::var>
4564 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4565 package as assumed. That is, C<$::sail> is equivalent to
4566 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4567 code, mostly from Perl 4).
4569 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4570 namespace to a L<version> object with the VERSION provided. VERSION must be a
4571 "strict" style version number as defined by the L<version> module: a positive
4572 decimal number (integer or decimal-fraction) without exponentiation or else a
4573 dotted-decimal v-string with a leading 'v' character and at least three
4574 components. You should set C<$VERSION> only once per package.
4576 See L<perlmod/"Packages"> for more information about packages, modules,
4577 and classes. See L<perlsub> for other scoping issues.
4579 =item pipe READHANDLE,WRITEHANDLE
4582 Opens a pair of connected pipes like the corresponding system call.
4583 Note that if you set up a loop of piped processes, deadlock can occur
4584 unless you are very careful. In addition, note that Perl's pipes use
4585 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4586 after each command, depending on the application.
4588 See L<IPC::Open2>, L<IPC::Open3>, and
4589 L<perlipc/"Bidirectional Communication with Another Process">
4590 for examples of such things.
4592 On systems that support a close-on-exec flag on files, that flag is set
4593 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4594 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4599 A special token that returns the name of the package in which it occurs.
4608 Pops and returns the last value of the array, shortening the array by
4611 Returns the undefined value if the array is empty, although this may also
4612 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4613 main program, but the C<@_> array in subroutines, just like C<shift>.
4615 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
4616 reference to an unblessed array. The argument will be dereferenced
4617 automatically. This aspect of C<pop> is considered highly experimental.
4618 The exact behaviour may change in a future version of Perl.
4621 X<pos> X<match, position>
4625 Returns the offset of where the last C<m//g> search left off for the
4626 variable in question (C<$_> is used when the variable is not
4627 specified). Note that 0 is a valid match offset. C<undef> indicates
4628 that the search position is reset (usually due to match failure, but
4629 can also be because no match has yet been run on the scalar).
4631 C<pos> directly accesses the location used by the regexp engine to
4632 store the offset, so assigning to C<pos> will change that offset, and
4633 so will also influence the C<\G> zero-width assertion in regular
4634 expressions. Both of these effects take place for the next match, so
4635 you can't affect the position with C<pos> during the current match,
4636 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
4638 Setting C<pos> also resets the I<matched with zero-length> flag, described
4639 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
4641 Because a failed C<m//gc> match doesn't reset the offset, the return
4642 from C<pos> won't change either in this case. See L<perlre> and
4645 =item print FILEHANDLE LIST
4648 =item print FILEHANDLE
4654 Prints a string or a list of strings. Returns true if successful.
4655 FILEHANDLE may be a scalar variable containing the name of or a reference
4656 to the filehandle, thus introducing one level of indirection. (NOTE: If
4657 FILEHANDLE is a variable and the next token is a term, it may be
4658 misinterpreted as an operator unless you interpose a C<+> or put
4659 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
4660 last selected (see L</select>) output handle. If LIST is omitted, prints
4661 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
4662 print the content of C<$_> to it, you must use a real filehandle like
4663 C<FH>, not an indirect one like C<$fh>. To set the default output handle
4664 to something other than STDOUT, use the select operation.
4666 The current value of C<$,> (if any) is printed between each LIST item. The
4667 current value of C<$\> (if any) is printed after the entire LIST has been
4668 printed. Because print takes a LIST, anything in the LIST is evaluated in
4669 list context, including any subroutines whose return lists you pass to
4670 C<print>. Be careful not to follow the print keyword with a left
4671 parenthesis unless you want the corresponding right parenthesis to
4672 terminate the arguments to the print; put parentheses around all arguments
4673 (or interpose a C<+>, but that doesn't look as good).
4675 If you're storing handles in an array or hash, or in general whenever
4676 you're using any expression more complex than a bareword handle or a plain,
4677 unsubscripted scalar variable to retrieve it, you will have to use a block
4678 returning the filehandle value instead, in which case the LIST may not be
4681 print { $files[$i] } "stuff\n";
4682 print { $OK ? STDOUT : STDERR } "stuff\n";
4684 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4685 L<perlipc> for more on signal handling.
4687 =item printf FILEHANDLE FORMAT, LIST
4690 =item printf FILEHANDLE
4692 =item printf FORMAT, LIST
4696 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4697 (the output record separator) is not appended. The first argument of the
4698 list will be interpreted as the C<printf> format. See
4699 L<sprintf|/sprintf FORMAT, LIST> for an
4700 explanation of the format argument. If you omit the LIST, C<$_> is used;
4701 to use FILEHANDLE without a LIST, you must use a real filehandle like
4702 C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and
4703 POSIX::setlocale() has been called, the character used for the decimal
4704 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4705 locale setting. See L<perllocale> and L<POSIX>.
4707 Don't fall into the trap of using a C<printf> when a simple
4708 C<print> would do. The C<print> is more efficient and less
4711 =item prototype FUNCTION
4714 Returns the prototype of a function as a string (or C<undef> if the
4715 function has no prototype). FUNCTION is a reference to, or the name of,
4716 the function whose prototype you want to retrieve.
4718 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4719 name for a Perl builtin. If the builtin is not I<overridable> (such as
4720 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4721 (such as C<system>), prototype() returns C<undef>, because the builtin
4722 does not really behave like a Perl function. Otherwise, the string
4723 describing the equivalent prototype is returned.
4725 =item push ARRAY,LIST
4728 =item push EXPR,LIST
4730 Treats ARRAY as a stack by appending the values of LIST to the end of
4731 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
4735 $ARRAY[++$#ARRAY] = $value;
4738 but is more efficient. Returns the number of elements in the array following
4739 the completed C<push>.
4741 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
4742 reference to an unblessed array. The argument will be dereferenced
4743 automatically. This aspect of C<push> is considered highly experimental.
4744 The exact behaviour may change in a future version of Perl.
4754 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4758 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4760 =item quotemeta EXPR
4761 X<quotemeta> X<metacharacter>
4765 Returns the value of EXPR with all non-"word"
4766 characters backslashed. (That is, all characters not matching
4767 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4768 returned string, regardless of any locale settings.)
4769 This is the internal function implementing
4770 the C<\Q> escape in double-quoted strings.
4772 If EXPR is omitted, uses C<$_>.
4774 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
4775 regular expressions, because by default an interpolated variable will be
4776 considered a mini-regular expression. For example:
4778 my $sentence = 'The quick brown fox jumped over the lazy dog';
4779 my $substring = 'quick.*?fox';
4780 $sentence =~ s{$substring}{big bad wolf};
4782 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
4786 my $sentence = 'The quick brown fox jumped over the lazy dog';
4787 my $substring = 'quick.*?fox';
4788 $sentence =~ s{\Q$substring\E}{big bad wolf};
4792 my $sentence = 'The quick brown fox jumped over the lazy dog';
4793 my $substring = 'quick.*?fox';
4794 my $quoted_substring = quotemeta($substring);
4795 $sentence =~ s{$quoted_substring}{big bad wolf};
4797 Will both leave the sentence as is. Normally, when accepting literal string
4798 input from the user, quotemeta() or C<\Q> must be used.
4800 In Perl 5.14, all characters whose code points are above 127 are not
4801 quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings.
4802 It is planned to change this behavior in 5.16, but the exact rules
4803 haven't been determined yet.
4810 Returns a random fractional number greater than or equal to C<0> and less
4811 than the value of EXPR. (EXPR should be positive.) If EXPR is
4812 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4813 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4814 and is subject to change in future versions of Perl). Automatically calls
4815 C<srand> unless C<srand> has already been called. See also C<srand>.
4817 Apply C<int()> to the value returned by C<rand()> if you want random
4818 integers instead of random fractional numbers. For example,
4822 returns a random integer between C<0> and C<9>, inclusive.
4824 (Note: If your rand function consistently returns numbers that are too
4825 large or too small, then your version of Perl was probably compiled
4826 with the wrong number of RANDBITS.)
4828 B<C<rand()> is not cryptographically secure. You should not rely
4829 on it in security-sensitive situations.> As of this writing, a
4830 number of third-party CPAN modules offer random number generators
4831 intended by their authors to be cryptographically secure,
4832 including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and
4833 L<Math::TrulyRandom>.
4835 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4836 X<read> X<file, read>
4838 =item read FILEHANDLE,SCALAR,LENGTH
4840 Attempts to read LENGTH I<characters> of data into variable SCALAR
4841 from the specified FILEHANDLE. Returns the number of characters
4842 actually read, C<0> at end of file, or undef if there was an error (in
4843 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4844 so that the last character actually read is the last character of the
4845 scalar after the read.
4847 An OFFSET may be specified to place the read data at some place in the
4848 string other than the beginning. A negative OFFSET specifies
4849 placement at that many characters counting backwards from the end of
4850 the string. A positive OFFSET greater than the length of SCALAR
4851 results in the string being padded to the required size with C<"\0">
4852 bytes before the result of the read is appended.
4854 The call is implemented in terms of either Perl's or your system's native
4855 fread(3) library function. To get a true read(2) system call, see
4856 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
4858 Note the I<characters>: depending on the status of the filehandle,
4859 either (8-bit) bytes or characters are read. By default, all
4860 filehandles operate on bytes, but for example if the filehandle has
4861 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4862 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
4863 characters, not bytes. Similarly for the C<:encoding> pragma:
4864 in that case pretty much any characters can be read.
4866 =item readdir DIRHANDLE
4869 Returns the next directory entry for a directory opened by C<opendir>.
4870 If used in list context, returns all the rest of the entries in the
4871 directory. If there are no more entries, returns the undefined value in
4872 scalar context and the empty list in list context.
4874 If you're planning to filetest the return values out of a C<readdir>, you'd
4875 better prepend the directory in question. Otherwise, because we didn't
4876 C<chdir> there, it would have been testing the wrong file.
4878 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4879 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4882 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4883 which will set C<$_> on every iteration.
4885 opendir(my $dh, $some_dir) || die;
4886 while(readdir $dh) {
4887 print "$some_dir/$_\n";
4894 X<readline> X<gets> X<fgets>
4896 Reads from the filehandle whose typeglob is contained in EXPR (or from
4897 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
4898 returns the next line until end-of-file is reached, whereupon the
4899 subsequent call returns C<undef>. In list context, reads until end-of-file
4900 is reached and returns a list of lines. Note that the notion of "line"
4901 used here is whatever you may have defined with C<$/> or
4902 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4904 When C<$/> is set to C<undef>, when C<readline> is in scalar
4905 context (i.e., file slurp mode), and when an empty file is read, it
4906 returns C<''> the first time, followed by C<undef> subsequently.
4908 This is the internal function implementing the C<< <EXPR> >>
4909 operator, but you can use it directly. The C<< <EXPR> >>
4910 operator is discussed in more detail in L<perlop/"I/O Operators">.
4913 $line = readline(*STDIN); # same thing
4915 If C<readline> encounters an operating system error, C<$!> will be set
4916 with the corresponding error message. It can be helpful to check
4917 C<$!> when you are reading from filehandles you don't trust, such as a
4918 tty or a socket. The following example uses the operator form of
4919 C<readline> and dies if the result is not defined.
4921 while ( ! eof($fh) ) {
4922 defined( $_ = <$fh> ) or die "readline failed: $!";
4926 Note that you have can't handle C<readline> errors that way with the
4927 C<ARGV> filehandle. In that case, you have to open each element of
4928 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4930 foreach my $arg (@ARGV) {
4931 open(my $fh, $arg) or warn "Can't open $arg: $!";
4933 while ( ! eof($fh) ) {
4934 defined( $_ = <$fh> )
4935 or die "readline failed for $arg: $!";
4945 Returns the value of a symbolic link, if symbolic links are
4946 implemented. If not, raises an exception. If there is a system
4947 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4948 omitted, uses C<$_>.
4950 Portability issues: L<perlport/readlink>.
4957 EXPR is executed as a system command.
4958 The collected standard output of the command is returned.
4959 In scalar context, it comes back as a single (potentially
4960 multi-line) string. In list context, returns a list of lines
4961 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4962 This is the internal function implementing the C<qx/EXPR/>
4963 operator, but you can use it directly. The C<qx/EXPR/>
4964 operator is discussed in more detail in L<perlop/"I/O Operators">.
4965 If EXPR is omitted, uses C<$_>.
4967 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4970 Receives a message on a socket. Attempts to receive LENGTH characters
4971 of data into variable SCALAR from the specified SOCKET filehandle.
4972 SCALAR will be grown or shrunk to the length actually read. Takes the
4973 same flags as the system call of the same name. Returns the address
4974 of the sender if SOCKET's protocol supports this; returns an empty
4975 string otherwise. If there's an error, returns the undefined value.
4976 This call is actually implemented in terms of recvfrom(2) system call.
4977 See L<perlipc/"UDP: Message Passing"> for examples.
4979 Note the I<characters>: depending on the status of the socket, either
4980 (8-bit) bytes or characters are received. By default all sockets
4981 operate on bytes, but for example if the socket has been changed using
4982 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4983 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
4984 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4985 case pretty much any characters can be read.
4992 The C<redo> command restarts the loop block without evaluating the
4993 conditional again. The C<continue> block, if any, is not executed. If
4994 the LABEL is omitted, the command refers to the innermost enclosing
4995 loop. Programs that want to lie to themselves about what was just input
4996 normally use this command:
4998 # a simpleminded Pascal comment stripper
4999 # (warning: assumes no { or } in strings)
5000 LINE: while (<STDIN>) {
5001 while (s|({.*}.*){.*}|$1 |) {}
5006 if (/}/) { # end of comment?
5015 C<redo> cannot be used to retry a block that returns a value such as
5016 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5017 a grep() or map() operation.
5019 Note that a block by itself is semantically identical to a loop
5020 that executes once. Thus C<redo> inside such a block will effectively
5021 turn it into a looping construct.
5023 See also L</continue> for an illustration of how C<last>, C<next>, and
5031 Returns a non-empty string if EXPR is a reference, the empty
5032 string otherwise. If EXPR
5033 is not specified, C<$_> will be used. The value returned depends on the
5034 type of thing the reference is a reference to.
5035 Builtin types include:
5049 If the referenced object has been blessed into a package, then that package
5050 name is returned instead. You can think of C<ref> as a C<typeof> operator.
5052 if (ref($r) eq "HASH") {
5053 print "r is a reference to a hash.\n";
5056 print "r is not a reference at all.\n";
5059 The return value C<LVALUE> indicates a reference to an lvalue that is not
5060 a variable. You get this from taking the reference of function calls like
5061 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5062 to a L<version string|perldata/"Version Strings">.
5064 The result C<Regexp> indicates that the argument is a regular expression
5065 resulting from C<qr//>.
5067 See also L<perlref>.
5069 =item rename OLDNAME,NEWNAME
5070 X<rename> X<move> X<mv> X<ren>
5072 Changes the name of a file; an existing file NEWNAME will be
5073 clobbered. Returns true for success, false otherwise.
5075 Behavior of this function varies wildly depending on your system
5076 implementation. For example, it will usually not work across file system
5077 boundaries, even though the system I<mv> command sometimes compensates
5078 for this. Other restrictions include whether it works on directories,
5079 open files, or pre-existing files. Check L<perlport> and either the
5080 rename(2) manpage or equivalent system documentation for details.
5082 For a platform independent C<move> function look at the L<File::Copy>
5085 Portability issues: L<perlport/rename>.
5087 =item require VERSION
5094 Demands a version of Perl specified by VERSION, or demands some semantics
5095 specified by EXPR or by C<$_> if EXPR is not supplied.
5097 VERSION may be either a numeric argument such as 5.006, which will be
5098 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5099 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5100 VERSION is greater than the version of the current Perl interpreter.
5101 Compare with L</use>, which can do a similar check at compile time.
5103 Specifying VERSION as a literal of the form v5.6.1 should generally be
5104 avoided, because it leads to misleading error messages under earlier
5105 versions of Perl that do not support this syntax. The equivalent numeric
5106 version should be used instead.
5108 require v5.6.1; # run time version check
5109 require 5.6.1; # ditto
5110 require 5.006_001; # ditto; preferred for backwards compatibility
5112 Otherwise, C<require> demands that a library file be included if it
5113 hasn't already been included. The file is included via the do-FILE
5114 mechanism, which is essentially just a variety of C<eval> with the
5115 caveat that lexical variables in the invoking script will be invisible
5116 to the included code. Has semantics similar to the following subroutine:
5119 my ($filename) = @_;
5120 if (exists $INC{$filename}) {
5121 return 1 if $INC{$filename};
5122 die "Compilation failed in require";
5124 my ($realfilename,$result);
5126 foreach $prefix (@INC) {
5127 $realfilename = "$prefix/$filename";
5128 if (-f $realfilename) {
5129 $INC{$filename} = $realfilename;
5130 $result = do $realfilename;
5134 die "Can't find $filename in \@INC";
5137 $INC{$filename} = undef;
5139 } elsif (!$result) {
5140 delete $INC{$filename};
5141 die "$filename did not return true value";
5147 Note that the file will not be included twice under the same specified
5150 The file must return true as the last statement to indicate
5151 successful execution of any initialization code, so it's customary to
5152 end such a file with C<1;> unless you're sure it'll return true
5153 otherwise. But it's better just to put the C<1;>, in case you add more
5156 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5157 replaces "F<::>" with "F</>" in the filename for you,
5158 to make it easy to load standard modules. This form of loading of
5159 modules does not risk altering your namespace.
5161 In other words, if you try this:
5163 require Foo::Bar; # a splendid bareword
5165 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5166 directories specified in the C<@INC> array.
5168 But if you try this:
5170 $class = 'Foo::Bar';
5171 require $class; # $class is not a bareword
5173 require "Foo::Bar"; # not a bareword because of the ""
5175 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5176 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5178 eval "require $class";
5180 Now that you understand how C<require> looks for files with a
5181 bareword argument, there is a little extra functionality going on behind
5182 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5183 first look for a similar filename with a "F<.pmc>" extension. If this file
5184 is found, it will be loaded in place of any file ending in a "F<.pm>"
5187 You can also insert hooks into the import facility by putting Perl code
5188 directly into the @INC array. There are three forms of hooks: subroutine
5189 references, array references, and blessed objects.
5191 Subroutine references are the simplest case. When the inclusion system
5192 walks through @INC and encounters a subroutine, this subroutine gets
5193 called with two parameters, the first a reference to itself, and the
5194 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5195 subroutine should return either nothing or else a list of up to three
5196 values in the following order:
5202 A filehandle, from which the file will be read.
5206 A reference to a subroutine. If there is no filehandle (previous item),
5207 then this subroutine is expected to generate one line of source code per
5208 call, writing the line into C<$_> and returning 1, then finally at end of
5209 file returning 0. If there is a filehandle, then the subroutine will be
5210 called to act as a simple source filter, with the line as read in C<$_>.
5211 Again, return 1 for each valid line, and 0 after all lines have been
5216 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5217 reference to the subroutine itself is passed in as C<$_[0]>.
5221 If an empty list, C<undef>, or nothing that matches the first 3 values above
5222 is returned, then C<require> looks at the remaining elements of @INC.
5223 Note that this filehandle must be a real filehandle (strictly a typeglob
5224 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5225 will be ignored and processing will stop there.
5227 If the hook is an array reference, its first element must be a subroutine
5228 reference. This subroutine is called as above, but the first parameter is
5229 the array reference. This lets you indirectly pass arguments to
5232 In other words, you can write:
5234 push @INC, \&my_sub;
5236 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5242 push @INC, [ \&my_sub, $x, $y, ... ];
5244 my ($arrayref, $filename) = @_;
5245 # Retrieve $x, $y, ...
5246 my @parameters = @$arrayref[1..$#$arrayref];
5250 If the hook is an object, it must provide an INC method that will be
5251 called as above, the first parameter being the object itself. (Note that
5252 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5253 into package C<main>.) Here is a typical code layout:
5259 my ($self, $filename) = @_;
5263 # In the main program
5264 push @INC, Foo->new(...);
5266 These hooks are also permitted to set the %INC entry
5267 corresponding to the files they have loaded. See L<perlvar/%INC>.
5269 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5276 Generally used in a C<continue> block at the end of a loop to clear
5277 variables and reset C<??> searches so that they work again. The
5278 expression is interpreted as a list of single characters (hyphens
5279 allowed for ranges). All variables and arrays beginning with one of
5280 those letters are reset to their pristine state. If the expression is
5281 omitted, one-match searches (C<?pattern?>) are reset to match again.
5282 Only resets variables or searches in the current package. Always returns
5285 reset 'X'; # reset all X variables
5286 reset 'a-z'; # reset lower case variables
5287 reset; # just reset ?one-time? searches
5289 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5290 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5291 variables; lexical variables are unaffected, but they clean themselves
5292 up on scope exit anyway, so you'll probably want to use them instead.
5300 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5301 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5302 context, depending on how the return value will be used, and the context
5303 may vary from one execution to the next (see L</wantarray>). If no EXPR
5304 is given, returns an empty list in list context, the undefined value in
5305 scalar context, and (of course) nothing at all in void context.
5307 (In the absence of an explicit C<return>, a subroutine, eval,
5308 or do FILE automatically returns the value of the last expression
5312 X<reverse> X<rev> X<invert>
5314 In list context, returns a list value consisting of the elements
5315 of LIST in the opposite order. In scalar context, concatenates the
5316 elements of LIST and returns a string value with all characters
5317 in the opposite order.
5319 print join(", ", reverse "world", "Hello"); # Hello, world
5321 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5323 Used without arguments in scalar context, reverse() reverses C<$_>.
5325 $_ = "dlrow ,olleH";
5326 print reverse; # No output, list context
5327 print scalar reverse; # Hello, world
5329 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5330 preserve non-existent elements whenever possible, i.e., for non magical
5331 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
5333 This operator is also handy for inverting a hash, although there are some
5334 caveats. If a value is duplicated in the original hash, only one of those
5335 can be represented as a key in the inverted hash. Also, this has to
5336 unwind one hash and build a whole new one, which may take some time
5337 on a large hash, such as from a DBM file.
5339 %by_name = reverse %by_address; # Invert the hash
5341 =item rewinddir DIRHANDLE
5344 Sets the current position to the beginning of the directory for the
5345 C<readdir> routine on DIRHANDLE.
5347 Portability issues: L<perlport/rewinddir>.
5349 =item rindex STR,SUBSTR,POSITION
5352 =item rindex STR,SUBSTR
5354 Works just like index() except that it returns the position of the I<last>
5355 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5356 last occurrence beginning at or before that position.
5358 =item rmdir FILENAME
5359 X<rmdir> X<rd> X<directory, remove>
5363 Deletes the directory specified by FILENAME if that directory is
5364 empty. If it succeeds it returns true; otherwise it returns false and
5365 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5367 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5368 the C<rmtree> function of the L<File::Path> module.
5372 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5374 =item say FILEHANDLE LIST
5377 =item say FILEHANDLE
5383 Just like C<print>, but implicitly appends a newline. C<say LIST> is
5384 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
5385 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
5386 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
5388 This keyword is available only when the C<"say"> feature
5389 is enabled, or when prefixed with C<CORE::>; see
5390 L<feature>. Alternately, include a C<use v5.10> or later to the current
5394 X<scalar> X<context>
5396 Forces EXPR to be interpreted in scalar context and returns the value
5399 @counts = ( scalar @a, scalar @b, scalar @c );
5401 There is no equivalent operator to force an expression to
5402 be interpolated in list context because in practice, this is never
5403 needed. If you really wanted to do so, however, you could use
5404 the construction C<@{[ (some expression) ]}>, but usually a simple
5405 C<(some expression)> suffices.
5407 Because C<scalar> is a unary operator, if you accidentally use a
5408 parenthesized list for the EXPR, this behaves as a scalar comma expression,
5409 evaluating all but the last element in void context and returning the final
5410 element evaluated in scalar context. This is seldom what you want.
5412 The following single statement:
5414 print uc(scalar(&foo,$bar)),$baz;
5416 is the moral equivalent of these two:
5419 print(uc($bar),$baz);
5421 See L<perlop> for more details on unary operators and the comma operator.
5423 =item seek FILEHANDLE,POSITION,WHENCE
5424 X<seek> X<fseek> X<filehandle, position>
5426 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5427 FILEHANDLE may be an expression whose value gives the name of the
5428 filehandle. The values for WHENCE are C<0> to set the new position
5429 I<in bytes> to POSITION; C<1> to set it to the current position plus
5430 POSITION; and C<2> to set it to EOF plus POSITION, typically
5431 negative. For WHENCE you may use the constants C<SEEK_SET>,
5432 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
5433 of the file) from the L<Fcntl> module. Returns C<1> on success, false
5436 Note the I<in bytes>: even if the filehandle has been set to
5437 operate on characters (for example by using the C<:encoding(utf8)> open
5438 layer), tell() will return byte offsets, not character offsets
5439 (because implementing that would render seek() and tell() rather slow).
5441 If you want to position the file for C<sysread> or C<syswrite>, don't use
5442 C<seek>, because buffering makes its effect on the file's read-write position
5443 unpredictable and non-portable. Use C<sysseek> instead.
5445 Due to the rules and rigors of ANSI C, on some systems you have to do a
5446 seek whenever you switch between reading and writing. Amongst other
5447 things, this may have the effect of calling stdio's clearerr(3).
5448 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
5452 This is also useful for applications emulating C<tail -f>. Once you hit
5453 EOF on your read and then sleep for a while, you (probably) have to stick in a
5454 dummy seek() to reset things. The C<seek> doesn't change the position,
5455 but it I<does> clear the end-of-file condition on the handle, so that the
5456 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
5458 If that doesn't work (some I/O implementations are particularly
5459 cantankerous), you might need something like this:
5462 for ($curpos = tell(FILE); $_ = <FILE>;
5463 $curpos = tell(FILE)) {
5464 # search for some stuff and put it into files
5466 sleep($for_a_while);
5467 seek(FILE, $curpos, 0);
5470 =item seekdir DIRHANDLE,POS
5473 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5474 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5475 about possible directory compaction as the corresponding system library
5478 =item select FILEHANDLE
5479 X<select> X<filehandle, default>
5483 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5484 sets the new current default filehandle for output. This has two
5485 effects: first, a C<write> or a C<print> without a filehandle
5486 default to this FILEHANDLE. Second, references to variables related to
5487 output will refer to this output channel.
5489 For example, to set the top-of-form format for more than one
5490 output channel, you might do the following:
5497 FILEHANDLE may be an expression whose value gives the name of the
5498 actual filehandle. Thus:
5500 $oldfh = select(STDERR); $| = 1; select($oldfh);
5502 Some programmers may prefer to think of filehandles as objects with
5503 methods, preferring to write the last example as:
5506 STDERR->autoflush(1);
5508 Portability issues: L<perlport/select>.
5510 =item select RBITS,WBITS,EBITS,TIMEOUT
5513 This calls the select(2) syscall with the bit masks specified, which
5514 can be constructed using C<fileno> and C<vec>, along these lines:
5516 $rin = $win = $ein = '';
5517 vec($rin, fileno(STDIN), 1) = 1;
5518 vec($win, fileno(STDOUT), 1) = 1;
5521 If you want to select on many filehandles, you may wish to write a
5522 subroutine like this:
5527 for my $fh (@fhlist) {
5528 vec($bits, fileno($fh), 1) = 1;
5532 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
5536 ($nfound,$timeleft) =
5537 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5539 or to block until something becomes ready just do this
5541 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5543 Most systems do not bother to return anything useful in $timeleft, so
5544 calling select() in scalar context just returns $nfound.
5546 Any of the bit masks can also be undef. The timeout, if specified, is
5547 in seconds, which may be fractional. Note: not all implementations are
5548 capable of returning the $timeleft. If not, they always return
5549 $timeleft equal to the supplied $timeout.
5551 You can effect a sleep of 250 milliseconds this way:
5553 select(undef, undef, undef, 0.25);
5555 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5556 is implementation-dependent. See also L<perlport> for notes on the
5557 portability of C<select>.
5559 On error, C<select> behaves just like select(2): it returns
5562 On some Unixes, select(2) may report a socket file descriptor as "ready for
5563 reading" even when no data is available, and thus any subsequent C<read>
5564 would block. This can be avoided if you always use O_NONBLOCK on the
5565 socket. See select(2) and fcntl(2) for further details.
5567 The standard C<IO::Select> module provides a user-friendlier interface
5568 to C<select>, mostly because it does all the bit-mask work for you.
5570 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5571 or <FH>) with C<select>, except as permitted by POSIX, and even
5572 then only on POSIX systems. You have to use C<sysread> instead.
5574 Portability issues: L<perlport/select>.
5576 =item semctl ID,SEMNUM,CMD,ARG
5579 Calls the System V IPC function semctl(2). You'll probably have to say
5583 first to get the correct constant definitions. If CMD is IPC_STAT or
5584 GETALL, then ARG must be a variable that will hold the returned
5585 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5586 the undefined value for error, "C<0 but true>" for zero, or the actual
5587 return value otherwise. The ARG must consist of a vector of native
5588 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5589 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5592 Portability issues: L<perlport/semctl>.
5594 =item semget KEY,NSEMS,FLAGS
5597 Calls the System V IPC function semget(2). Returns the semaphore id, or
5598 the undefined value on error. See also
5599 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5602 Portability issues: L<perlport/semget>.
5604 =item semop KEY,OPSTRING
5607 Calls the System V IPC function semop(2) for semaphore operations
5608 such as signalling and waiting. OPSTRING must be a packed array of
5609 semop structures. Each semop structure can be generated with
5610 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5611 implies the number of semaphore operations. Returns true if
5612 successful, false on error. As an example, the
5613 following code waits on semaphore $semnum of semaphore id $semid:
5615 $semop = pack("s!3", $semnum, -1, 0);
5616 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5618 To signal the semaphore, replace C<-1> with C<1>. See also
5619 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5622 Portability issues: L<perlport/semop>.
5624 =item send SOCKET,MSG,FLAGS,TO
5627 =item send SOCKET,MSG,FLAGS
5629 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5630 filehandle. Takes the same flags as the system call of the same name. On
5631 unconnected sockets, you must specify a destination to I<send to>, in which
5632 case it does a sendto(2) syscall. Returns the number of characters sent,
5633 or the undefined value on error. The sendmsg(2) syscall is currently
5634 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5636 Note the I<characters>: depending on the status of the socket, either
5637 (8-bit) bytes or characters are sent. By default all sockets operate
5638 on bytes, but for example if the socket has been changed using
5639 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5640 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5641 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5642 pragma: in that case pretty much any characters can be sent.
5644 =item setpgrp PID,PGRP
5647 Sets the current process group for the specified PID, C<0> for the current
5648 process. Raises an exception when used on a machine that doesn't
5649 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5650 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5651 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5654 Portability issues: L<perlport/setpgrp>.
5656 =item setpriority WHICH,WHO,PRIORITY
5657 X<setpriority> X<priority> X<nice> X<renice>
5659 Sets the current priority for a process, a process group, or a user.
5660 (See setpriority(2).) Raises an exception when used on a machine
5661 that doesn't implement setpriority(2).
5663 Portability issues: L<perlport/setpriority>.
5665 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5668 Sets the socket option requested. Returns C<undef> on error.
5669 Use integer constants provided by the C<Socket> module for
5670 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5671 getprotobyname. OPTVAL might either be a packed string or an integer.
5672 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5674 An example disabling Nagle's algorithm on a socket:
5676 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5677 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5679 Portability issues: L<perlport/setsockopt>.
5688 Shifts the first value of the array off and returns it, shortening the
5689 array by 1 and moving everything down. If there are no elements in the
5690 array, returns the undefined value. If ARRAY is omitted, shifts the
5691 C<@_> array within the lexical scope of subroutines and formats, and the
5692 C<@ARGV> array outside a subroutine and also within the lexical scopes
5693 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5694 C<UNITCHECK {}>, and C<END {}> constructs.
5696 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
5697 reference to an unblessed array. The argument will be dereferenced
5698 automatically. This aspect of C<shift> is considered highly experimental.
5699 The exact behaviour may change in a future version of Perl.
5701 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5702 same thing to the left end of an array that C<pop> and C<push> do to the
5705 =item shmctl ID,CMD,ARG
5708 Calls the System V IPC function shmctl. You'll probably have to say
5712 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5713 then ARG must be a variable that will hold the returned C<shmid_ds>
5714 structure. Returns like ioctl: C<undef> for error; "C<0> but
5715 true" for zero; and the actual return value otherwise.
5716 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5718 Portability issues: L<perlport/shmctl>.
5720 =item shmget KEY,SIZE,FLAGS
5723 Calls the System V IPC function shmget. Returns the shared memory
5724 segment id, or C<undef> on error.
5725 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5727 Portability issues: L<perlport/shmget>.
5729 =item shmread ID,VAR,POS,SIZE
5733 =item shmwrite ID,STRING,POS,SIZE
5735 Reads or writes the System V shared memory segment ID starting at
5736 position POS for size SIZE by attaching to it, copying in/out, and
5737 detaching from it. When reading, VAR must be a variable that will
5738 hold the data read. When writing, if STRING is too long, only SIZE
5739 bytes are used; if STRING is too short, nulls are written to fill out
5740 SIZE bytes. Return true if successful, false on error.
5741 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5742 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
5744 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
5746 =item shutdown SOCKET,HOW
5749 Shuts down a socket connection in the manner indicated by HOW, which
5750 has the same interpretation as in the syscall of the same name.
5752 shutdown(SOCKET, 0); # I/we have stopped reading data
5753 shutdown(SOCKET, 1); # I/we have stopped writing data
5754 shutdown(SOCKET, 2); # I/we have stopped using this socket
5756 This is useful with sockets when you want to tell the other
5757 side you're done writing but not done reading, or vice versa.
5758 It's also a more insistent form of close because it also
5759 disables the file descriptor in any forked copies in other
5762 Returns C<1> for success; on error, returns C<undef> if
5763 the first argument is not a valid filehandle, or returns C<0> and sets
5764 C<$!> for any other failure.
5767 X<sin> X<sine> X<asin> X<arcsine>
5771 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5772 returns sine of C<$_>.
5774 For the inverse sine operation, you may use the C<Math::Trig::asin>
5775 function, or use this relation:
5777 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5784 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5785 argument is given. Returns the integer number of seconds actually slept.
5787 May be interrupted if the process receives a signal such as C<SIGALRM>.
5790 local $SIG{ALARM} = sub { die "Alarm!\n" };
5793 die $@ unless $@ eq "Alarm!\n";
5795 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5796 is often implemented using C<alarm>.
5798 On some older systems, it may sleep up to a full second less than what
5799 you requested, depending on how it counts seconds. Most modern systems
5800 always sleep the full amount. They may appear to sleep longer than that,
5801 however, because your process might not be scheduled right away in a
5802 busy multitasking system.
5804 For delays of finer granularity than one second, the Time::HiRes module
5805 (from CPAN, and starting from Perl 5.8 part of the standard
5806 distribution) provides usleep(). You may also use Perl's four-argument
5807 version of select() leaving the first three arguments undefined, or you
5808 might be able to use the C<syscall> interface to access setitimer(2) if
5809 your system supports it. See L<perlfaq8> for details.
5811 See also the POSIX module's C<pause> function.
5813 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5816 Opens a socket of the specified kind and attaches it to filehandle
5817 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5818 the syscall of the same name. You should C<use Socket> first
5819 to get the proper definitions imported. See the examples in
5820 L<perlipc/"Sockets: Client/Server Communication">.
5822 On systems that support a close-on-exec flag on files, the flag will
5823 be set for the newly opened file descriptor, as determined by the
5824 value of $^F. See L<perlvar/$^F>.
5826 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5829 Creates an unnamed pair of sockets in the specified domain, of the
5830 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5831 for the syscall of the same name. If unimplemented, raises an exception.
5832 Returns true if successful.
5834 On systems that support a close-on-exec flag on files, the flag will
5835 be set for the newly opened file descriptors, as determined by the value
5836 of $^F. See L<perlvar/$^F>.
5838 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5839 to C<pipe(Rdr, Wtr)> is essentially:
5842 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5843 shutdown(Rdr, 1); # no more writing for reader
5844 shutdown(Wtr, 0); # no more reading for writer
5846 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5847 emulate socketpair using IP sockets to localhost if your system implements
5848 sockets but not socketpair.
5850 Portability issues: L<perlport/socketpair>.
5852 =item sort SUBNAME LIST
5853 X<sort> X<qsort> X<quicksort> X<mergesort>
5855 =item sort BLOCK LIST
5859 In list context, this sorts the LIST and returns the sorted list value.
5860 In scalar context, the behaviour of C<sort()> is undefined.
5862 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5863 order. If SUBNAME is specified, it gives the name of a subroutine
5864 that returns an integer less than, equal to, or greater than C<0>,
5865 depending on how the elements of the list are to be ordered. (The
5866 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5867 SUBNAME may be a scalar variable name (unsubscripted), in which case
5868 the value provides the name of (or a reference to) the actual
5869 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5870 an anonymous, in-line sort subroutine.
5872 If the subroutine's prototype is C<($$)>, the elements to be compared are
5873 passed by reference in C<@_>, as for a normal subroutine. This is slower
5874 than unprototyped subroutines, where the elements to be compared are passed
5875 into the subroutine as the package global variables $a and $b (see example
5876 below). Note that in the latter case, it is usually highly counter-productive
5877 to declare $a and $b as lexicals.
5879 The values to be compared are always passed by reference and should not
5882 You also cannot exit out of the sort block or subroutine using any of the
5883 loop control operators described in L<perlsyn> or with C<goto>.
5885 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5886 current collation locale. See L<perllocale>.
5888 sort() returns aliases into the original list, much as a for loop's index
5889 variable aliases the list elements. That is, modifying an element of a
5890 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5891 actually modifies the element in the original list. This is usually
5892 something to be avoided when writing clear code.
5894 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5895 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
5896 preserves the input order of elements that compare equal. Although
5897 quicksort's run time is O(NlogN) when averaged over all arrays of
5898 length N, the time can be O(N**2), I<quadratic> behavior, for some
5899 inputs.) In 5.7, the quicksort implementation was replaced with
5900 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5901 But benchmarks indicated that for some inputs, on some platforms,
5902 the original quicksort was faster. 5.8 has a sort pragma for
5903 limited control of the sort. Its rather blunt control of the
5904 underlying algorithm may not persist into future Perls, but the
5905 ability to characterize the input or output in implementation
5906 independent ways quite probably will. See L<the sort pragma|sort>.
5911 @articles = sort @files;
5913 # same thing, but with explicit sort routine
5914 @articles = sort {$a cmp $b} @files;
5916 # now case-insensitively
5917 @articles = sort {uc($a) cmp uc($b)} @files;
5919 # same thing in reversed order
5920 @articles = sort {$b cmp $a} @files;
5922 # sort numerically ascending
5923 @articles = sort {$a <=> $b} @files;
5925 # sort numerically descending
5926 @articles = sort {$b <=> $a} @files;
5928 # this sorts the %age hash by value instead of key
5929 # using an in-line function
5930 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5932 # sort using explicit subroutine name
5934 $age{$a} <=> $age{$b}; # presuming numeric
5936 @sortedclass = sort byage @class;
5938 sub backwards { $b cmp $a }
5939 @harry = qw(dog cat x Cain Abel);
5940 @george = qw(gone chased yz Punished Axed);
5942 # prints AbelCaincatdogx
5943 print sort backwards @harry;
5944 # prints xdogcatCainAbel
5945 print sort @george, 'to', @harry;
5946 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5948 # inefficiently sort by descending numeric compare using
5949 # the first integer after the first = sign, or the
5950 # whole record case-insensitively otherwise
5953 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5958 # same thing, but much more efficiently;
5959 # we'll build auxiliary indices instead
5961 my @nums = @caps = ();
5963 push @nums, ( /=(\d+)/ ? $1 : undef );
5967 my @new = @old[ sort {
5968 $nums[$b] <=> $nums[$a]
5970 $caps[$a] cmp $caps[$b]
5974 # same thing, but without any temps
5975 @new = map { $_->[0] }
5976 sort { $b->[1] <=> $a->[1]
5979 } map { [$_, /=(\d+)/, uc($_)] } @old;
5981 # using a prototype allows you to use any comparison subroutine
5982 # as a sort subroutine (including other package's subroutines)
5984 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5987 @new = sort other::backwards @old;
5989 # guarantee stability, regardless of algorithm
5991 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5993 # force use of mergesort (not portable outside Perl 5.8)
5994 use sort '_mergesort'; # note discouraging _
5995 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5997 Warning: syntactical care is required when sorting the list returned from
5998 a function. If you want to sort the list returned by the function call
5999 C<find_records(@key)>, you can use:
6001 @contact = sort { $a cmp $b } find_records @key;
6002 @contact = sort +find_records(@key);
6003 @contact = sort &find_records(@key);
6004 @contact = sort(find_records(@key));
6006 If instead you want to sort the array @key with the comparison routine
6007 C<find_records()> then you can use:
6009 @contact = sort { find_records() } @key;
6010 @contact = sort find_records(@key);
6011 @contact = sort(find_records @key);
6012 @contact = sort(find_records (@key));
6014 If you're using strict, you I<must not> declare $a
6015 and $b as lexicals. They are package globals. That means
6016 that if you're in the C<main> package and type
6018 @articles = sort {$b <=> $a} @files;
6020 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6021 but if you're in the C<FooPack> package, it's the same as typing
6023 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6025 The comparison function is required to behave. If it returns
6026 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6027 sometimes saying the opposite, for example) the results are not
6030 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6031 (not-a-number), and also because C<sort> raises an exception unless the
6032 result of a comparison is defined, be careful when sorting with a
6033 comparison function like C<< $a <=> $b >> any lists that might contain a
6034 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6035 eliminate any C<NaN>s from the input list.
6037 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6039 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6042 =item splice ARRAY or EXPR,OFFSET,LENGTH
6044 =item splice ARRAY or EXPR,OFFSET
6046 =item splice ARRAY or EXPR
6048 Removes the elements designated by OFFSET and LENGTH from an array, and
6049 replaces them with the elements of LIST, if any. In list context,
6050 returns the elements removed from the array. In scalar context,
6051 returns the last element removed, or C<undef> if no elements are
6052 removed. The array grows or shrinks as necessary.
6053 If OFFSET is negative then it starts that far from the end of the array.
6054 If LENGTH is omitted, removes everything from OFFSET onward.
6055 If LENGTH is negative, removes the elements from OFFSET onward
6056 except for -LENGTH elements at the end of the array.
6057 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6058 past the end of the array, Perl issues a warning, and splices at the
6061 The following equivalences hold (assuming C<< $#a >= $i >> )
6063 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6064 pop(@a) splice(@a,-1)
6065 shift(@a) splice(@a,0,1)
6066 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6067 $a[$i] = $y splice(@a,$i,1,$y)
6069 Example, assuming array lengths are passed before arrays:
6071 sub aeq { # compare two list values
6072 my(@a) = splice(@_,0,shift);
6073 my(@b) = splice(@_,0,shift);
6074 return 0 unless @a == @b; # same len?
6076 return 0 if pop(@a) ne pop(@b);
6080 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6082 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6083 reference to an unblessed array. The argument will be dereferenced
6084 automatically. This aspect of C<splice> is considered highly experimental.
6085 The exact behaviour may change in a future version of Perl.
6087 =item split /PATTERN/,EXPR,LIMIT
6090 =item split /PATTERN/,EXPR
6092 =item split /PATTERN/
6096 Splits the string EXPR into a list of strings and returns that list. By
6097 default, empty leading fields are preserved, and empty trailing ones are
6098 deleted. (If all fields are empty, they are considered to be trailing.)
6100 In scalar context, returns the number of fields found.
6102 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
6103 splits on whitespace (after skipping any leading whitespace). Anything
6104 matching PATTERN is taken to be a delimiter separating the fields. (Note
6105 that the delimiter may be longer than one character.)
6107 If LIMIT is specified and positive, it represents the maximum number
6108 of fields the EXPR will be split into, though the actual number of
6109 fields returned depends on the number of times PATTERN matches within
6110 EXPR. If LIMIT is unspecified or zero, trailing null fields are
6111 stripped (which potential users of C<pop> would do well to remember).
6112 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
6113 had been specified. Note that splitting an EXPR that evaluates to the
6114 empty string always returns the empty list, regardless of the LIMIT
6117 A pattern matching the empty string (not to be confused with
6118 an empty pattern C<//>, which is just one member of the set of patterns
6119 matching the empty string), splits EXPR into individual
6120 characters. For example:
6122 print join(':', split(/ */, 'hi there')), "\n";
6124 produces the output 'h:i:t:h:e:r:e'.
6126 As a special case for C<split>, the empty pattern C<//> specifically
6127 matches the empty string; this is not be confused with the normal use
6128 of an empty pattern to mean the last successful match. So to split
6129 a string into individual characters, the following:
6131 print join(':', split(//, 'hi there')), "\n";
6133 produces the output 'h:i: :t:h:e:r:e'.
6135 Empty leading fields are produced when there are positive-width matches at
6136 the beginning of the string; a zero-width match at the beginning of
6137 the string does not produce an empty field. For example:
6139 print join(':', split(/(?=\w)/, 'hi there!'));
6141 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
6142 hand, are produced when there is a match at the end of the string (and
6143 when LIMIT is given and is not 0), regardless of the length of the match.
6146 print join(':', split(//, 'hi there!', -1)), "\n";
6147 print join(':', split(/\W/, 'hi there!', -1)), "\n";
6149 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
6150 both with an empty trailing field.
6152 The LIMIT parameter can be used to split a line partially
6154 ($login, $passwd, $remainder) = split(/:/, $_, 3);
6156 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
6157 a LIMIT one larger than the number of variables in the list, to avoid
6158 unnecessary work. For the list above LIMIT would have been 4 by
6159 default. In time critical applications it behooves you not to split
6160 into more fields than you really need.
6162 If the PATTERN contains parentheses, additional list elements are
6163 created from each matching substring in the delimiter.
6165 split(/([,-])/, "1-10,20", 3);
6167 produces the list value
6169 (1, '-', 10, ',', 20)
6171 If you had the entire header of a normal Unix email message in $header,
6172 you could split it up into fields and their values this way:
6174 $header =~ s/\n(?=\s)//g; # fix continuation lines
6175 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
6177 The pattern C</PATTERN/> may be replaced with an expression to specify
6178 patterns that vary at runtime. (To do runtime compilation only once,
6179 use C</$variable/o>.)
6181 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
6182 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
6183 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
6184 will give you as many initial null fields (empty string) as there are leading spaces.
6185 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
6186 whitespace produces a null first field. A C<split> with no arguments
6187 really does a S<C<split(' ', $_)>> internally.
6189 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
6194 open(PASSWD, '/etc/passwd');
6197 ($login, $passwd, $uid, $gid,
6198 $gcos, $home, $shell) = split(/:/);
6202 As with regular pattern matching, any capturing parentheses that are not
6203 matched in a C<split()> will be set to C<undef> when returned:
6205 @fields = split /(A)|B/, "1A2B3";
6206 # @fields is (1, 'A', 2, undef, 3)
6208 =item sprintf FORMAT, LIST
6211 Returns a string formatted by the usual C<printf> conventions of the C
6212 library function C<sprintf>. See below for more details
6213 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
6214 the general principles.
6218 # Format number with up to 8 leading zeroes
6219 $result = sprintf("%08d", $number);
6221 # Round number to 3 digits after decimal point
6222 $rounded = sprintf("%.3f", $number);
6224 Perl does its own C<sprintf> formatting: it emulates the C
6225 function sprintf(3), but doesn't use it except for floating-point
6226 numbers, and even then only standard modifiers are allowed.
6227 Non-standard extensions in your local sprintf(3) are
6228 therefore unavailable from Perl.
6230 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
6231 pass it an array as your first argument. The array is given scalar context,
6232 and instead of using the 0th element of the array as the format, Perl will
6233 use the count of elements in the array as the format, which is almost never
6236 Perl's C<sprintf> permits the following universally-known conversions:
6239 %c a character with the given number
6241 %d a signed integer, in decimal
6242 %u an unsigned integer, in decimal
6243 %o an unsigned integer, in octal
6244 %x an unsigned integer, in hexadecimal
6245 %e a floating-point number, in scientific notation
6246 %f a floating-point number, in fixed decimal notation
6247 %g a floating-point number, in %e or %f notation
6249 In addition, Perl permits the following widely-supported conversions:
6251 %X like %x, but using upper-case letters
6252 %E like %e, but using an upper-case "E"
6253 %G like %g, but with an upper-case "E" (if applicable)
6254 %b an unsigned integer, in binary
6255 %B like %b, but using an upper-case "B" with the # flag
6256 %p a pointer (outputs the Perl value's address in hexadecimal)
6257 %n special: *stores* the number of characters output so far
6258 into the next variable in the parameter list
6260 Finally, for backward (and we do mean "backward") compatibility, Perl
6261 permits these unnecessary but widely-supported conversions:
6264 %D a synonym for %ld
6265 %U a synonym for %lu
6266 %O a synonym for %lo
6269 Note that the number of exponent digits in the scientific notation produced
6270 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
6271 exponent less than 100 is system-dependent: it may be three or less
6272 (zero-padded as necessary). In other words, 1.23 times ten to the
6273 99th may be either "1.23e99" or "1.23e099".
6275 Between the C<%> and the format letter, you may specify several
6276 additional attributes controlling the interpretation of the format.
6277 In order, these are:
6281 =item format parameter index
6283 An explicit format parameter index, such as C<2$>. By default sprintf
6284 will format the next unused argument in the list, but this allows you
6285 to take the arguments out of order:
6287 printf '%2$d %1$d', 12, 34; # prints "34 12"
6288 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
6294 space prefix non-negative number with a space
6295 + prefix non-negative number with a plus sign
6296 - left-justify within the field
6297 0 use zeros, not spaces, to right-justify
6298 # ensure the leading "0" for any octal,
6299 prefix non-zero hexadecimal with "0x" or "0X",
6300 prefix non-zero binary with "0b" or "0B"
6304 printf '<% d>', 12; # prints "< 12>"
6305 printf '<%+d>', 12; # prints "<+12>"
6306 printf '<%6s>', 12; # prints "< 12>"
6307 printf '<%-6s>', 12; # prints "<12 >"
6308 printf '<%06s>', 12; # prints "<000012>"
6309 printf '<%#o>', 12; # prints "<014>"
6310 printf '<%#x>', 12; # prints "<0xc>"
6311 printf '<%#X>', 12; # prints "<0XC>"
6312 printf '<%#b>', 12; # prints "<0b1100>"
6313 printf '<%#B>', 12; # prints "<0B1100>"
6315 When a space and a plus sign are given as the flags at once,
6316 a plus sign is used to prefix a positive number.
6318 printf '<%+ d>', 12; # prints "<+12>"
6319 printf '<% +d>', 12; # prints "<+12>"
6321 When the # flag and a precision are given in the %o conversion,
6322 the precision is incremented if it's necessary for the leading "0".
6324 printf '<%#.5o>', 012; # prints "<00012>"
6325 printf '<%#.5o>', 012345; # prints "<012345>"
6326 printf '<%#.0o>', 0; # prints "<0>"
6330 This flag tells Perl to interpret the supplied string as a vector of
6331 integers, one for each character in the string. Perl applies the format to
6332 each integer in turn, then joins the resulting strings with a separator (a
6333 dot C<.> by default). This can be useful for displaying ordinal values of
6334 characters in arbitrary strings:
6336 printf "%vd", "AB\x{100}"; # prints "65.66.256"
6337 printf "version is v%vd\n", $^V; # Perl's version
6339 Put an asterisk C<*> before the C<v> to override the string to
6340 use to separate the numbers:
6342 printf "address is %*vX\n", ":", $addr; # IPv6 address
6343 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
6345 You can also explicitly specify the argument number to use for
6346 the join string using something like C<*2$v>; for example:
6348 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
6350 =item (minimum) width
6352 Arguments are usually formatted to be only as wide as required to
6353 display the given value. You can override the width by putting
6354 a number here, or get the width from the next argument (with C<*>)
6355 or from a specified argument (e.g., with C<*2$>):
6357 printf "<%s>", "a"; # prints "<a>"
6358 printf "<%6s>", "a"; # prints "< a>"
6359 printf "<%*s>", 6, "a"; # prints "< a>"
6360 printf "<%*2$s>", "a", 6; # prints "< a>"
6361 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
6363 If a field width obtained through C<*> is negative, it has the same
6364 effect as the C<-> flag: left-justification.
6366 =item precision, or maximum width
6369 You can specify a precision (for numeric conversions) or a maximum
6370 width (for string conversions) by specifying a C<.> followed by a number.
6371 For floating-point formats except C<g> and C<G>, this specifies
6372 how many places right of the decimal point to show (the default being 6).
6375 # these examples are subject to system-specific variation
6376 printf '<%f>', 1; # prints "<1.000000>"
6377 printf '<%.1f>', 1; # prints "<1.0>"
6378 printf '<%.0f>', 1; # prints "<1>"
6379 printf '<%e>', 10; # prints "<1.000000e+01>"
6380 printf '<%.1e>', 10; # prints "<1.0e+01>"
6382 For "g" and "G", this specifies the maximum number of digits to show,
6383 including those prior to the decimal point and those after it; for
6386 # These examples are subject to system-specific variation.
6387 printf '<%g>', 1; # prints "<1>"
6388 printf '<%.10g>', 1; # prints "<1>"
6389 printf '<%g>', 100; # prints "<100>"
6390 printf '<%.1g>', 100; # prints "<1e+02>"
6391 printf '<%.2g>', 100.01; # prints "<1e+02>"
6392 printf '<%.5g>', 100.01; # prints "<100.01>"
6393 printf '<%.4g>', 100.01; # prints "<100>"
6395 For integer conversions, specifying a precision implies that the
6396 output of the number itself should be zero-padded to this width,
6397 where the 0 flag is ignored:
6399 printf '<%.6d>', 1; # prints "<000001>"
6400 printf '<%+.6d>', 1; # prints "<+000001>"
6401 printf '<%-10.6d>', 1; # prints "<000001 >"
6402 printf '<%10.6d>', 1; # prints "< 000001>"
6403 printf '<%010.6d>', 1; # prints "< 000001>"
6404 printf '<%+10.6d>', 1; # prints "< +000001>"
6406 printf '<%.6x>', 1; # prints "<000001>"
6407 printf '<%#.6x>', 1; # prints "<0x000001>"
6408 printf '<%-10.6x>', 1; # prints "<000001 >"
6409 printf '<%10.6x>', 1; # prints "< 000001>"
6410 printf '<%010.6x>', 1; # prints "< 000001>"
6411 printf '<%#10.6x>', 1; # prints "< 0x000001>"
6413 For string conversions, specifying a precision truncates the string
6414 to fit the specified width:
6416 printf '<%.5s>', "truncated"; # prints "<trunc>"
6417 printf '<%10.5s>', "truncated"; # prints "< trunc>"
6419 You can also get the precision from the next argument using C<.*>:
6421 printf '<%.6x>', 1; # prints "<000001>"
6422 printf '<%.*x>', 6, 1; # prints "<000001>"
6424 If a precision obtained through C<*> is negative, it counts
6425 as having no precision at all.
6427 printf '<%.*s>', 7, "string"; # prints "<string>"
6428 printf '<%.*s>', 3, "string"; # prints "<str>"
6429 printf '<%.*s>', 0, "string"; # prints "<>"
6430 printf '<%.*s>', -1, "string"; # prints "<string>"
6432 printf '<%.*d>', 1, 0; # prints "<0>"
6433 printf '<%.*d>', 0, 0; # prints "<>"
6434 printf '<%.*d>', -1, 0; # prints "<0>"
6436 You cannot currently get the precision from a specified number,
6437 but it is intended that this will be possible in the future, for
6438 example using C<.*2$>:
6440 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
6444 For numeric conversions, you can specify the size to interpret the
6445 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
6446 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
6447 whatever the default integer size is on your platform (usually 32 or 64
6448 bits), but you can override this to use instead one of the standard C types,
6449 as supported by the compiler used to build Perl:
6451 hh interpret integer as C type "char" or "unsigned char"
6452 on Perl 5.14 or later
6453 h interpret integer as C type "short" or "unsigned short"
6454 j interpret integer as C type "intmax_t" on Perl 5.14
6455 or later, and only with a C99 compiler (unportable)
6456 l interpret integer as C type "long" or "unsigned long"
6457 q, L, or ll interpret integer as C type "long long", "unsigned long long",
6458 or "quad" (typically 64-bit integers)
6459 t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
6460 z interpret integer as C type "size_t" on Perl 5.14 or later
6462 As of 5.14, none of these raises an exception if they are not supported on
6463 your platform. However, if warnings are enabled, a warning of the
6464 C<printf> warning class is issued on an unsupported conversion flag.
6465 Should you instead prefer an exception, do this:
6467 use warnings FATAL => "printf";
6469 If you would like to know about a version dependency before you
6470 start running the program, put something like this at its top:
6472 use 5.014; # for hh/j/t/z/ printf modifiers
6474 You can find out whether your Perl supports quads via L<Config>:
6477 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
6478 print "Nice quads!\n";
6481 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
6482 to be the default floating-point size on your platform (double or long double),
6483 but you can force "long double" with C<q>, C<L>, or C<ll> if your
6484 platform supports them. You can find out whether your Perl supports long
6485 doubles via L<Config>:
6488 print "long doubles\n" if $Config{d_longdbl} eq "define";
6490 You can find out whether Perl considers "long double" to be the default
6491 floating-point size to use on your platform via L<Config>:
6494 if ($Config{uselongdouble} eq "define") {
6495 print "long doubles by default\n";
6498 It can also be that long doubles and doubles are the same thing:
6501 ($Config{doublesize} == $Config{longdblsize}) &&
6502 print "doubles are long doubles\n";
6504 The size specifier C<V> has no effect for Perl code, but is supported for
6505 compatibility with XS code. It means "use the standard size for a Perl
6506 integer or floating-point number", which is the default.
6508 =item order of arguments
6510 Normally, sprintf() takes the next unused argument as the value to
6511 format for each format specification. If the format specification
6512 uses C<*> to require additional arguments, these are consumed from
6513 the argument list in the order they appear in the format
6514 specification I<before> the value to format. Where an argument is
6515 specified by an explicit index, this does not affect the normal
6516 order for the arguments, even when the explicitly specified index
6517 would have been the next argument.
6521 printf "<%*.*s>", $a, $b, $c;
6523 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6524 as the value to format; while:
6526 printf "<%*1$.*s>", $a, $b;
6528 would use C<$a> for the width and precision, and C<$b> as the
6531 Here are some more examples; be aware that when using an explicit
6532 index, the C<$> may need escaping:
6534 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6535 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6536 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6537 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6541 If C<use locale> is in effect and POSIX::setlocale() has been called,
6542 the character used for the decimal separator in formatted floating-point
6543 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6547 X<sqrt> X<root> X<square root>
6551 Return the positive square root of EXPR. If EXPR is omitted, uses
6552 C<$_>. Works only for non-negative operands unless you've
6553 loaded the C<Math::Complex> module.
6556 print sqrt(-4); # prints 2i
6559 X<srand> X<seed> X<randseed>
6563 Sets and returns the random number seed for the C<rand> operator.
6565 The point of the function is to "seed" the C<rand> function so that
6566 C<rand> can produce a different sequence each time you run your
6567 program. When called with a parameter, C<srand> uses that for the seed;
6568 otherwise it (semi-)randomly chooses a seed. In either case, starting with
6569 Perl 5.14, it returns the seed.
6571 If C<srand()> is not called explicitly, it is called implicitly without a
6572 parameter at the first use of the C<rand> operator. However, this was not true
6573 of versions of Perl before 5.004, so if your script will run under older
6574 Perl versions, it should call C<srand>; otherwise most programs won't call
6577 But there are a few situations in recent Perls where programs are likely to
6578 want to call C<srand>. One is for generating predictable results generally for
6579 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
6580 each time. Another other case is where you need a cryptographically-strong
6581 starting point rather than the generally acceptable default, which is based on
6582 time of day, process ID, and memory allocation, or the F</dev/urandom> device
6583 if available. And still another case is that you may want to call C<srand()>
6584 after a C<fork()> to avoid child processes sharing the same seed value as the
6585 parent (and consequently each other).
6587 Do B<not> call C<srand()> (i.e., without an argument) more than once per
6588 process. The internal state of the random number generator should
6589 contain more entropy than can be provided by any seed, so calling
6590 C<srand()> again actually I<loses> randomness.
6592 Most implementations of C<srand> take an integer and will silently
6593 truncate decimal numbers. This means C<srand(42)> will usually
6594 produce the same results as C<srand(42.1)>. To be safe, always pass
6595 C<srand> an integer.
6597 In versions of Perl prior to 5.004 the default seed was just the
6598 current C<time>. This isn't a particularly good seed, so many old
6599 programs supply their own seed value (often C<time ^ $$> or C<time ^
6600 ($$ + ($$ << 15))>), but that isn't necessary any more.
6602 For cryptographic purposes, however, you need something much more random
6603 than the default seed. Checksumming the compressed output of one or more
6604 rapidly changing operating system status programs is the usual method. For
6607 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6609 If you're particularly concerned with this, search the CPAN for
6610 random number generator modules instead of rolling out your own.
6612 Frequently called programs (like CGI scripts) that simply use
6616 for a seed can fall prey to the mathematical property that
6620 one-third of the time. So don't do that.
6622 A typical use of the returned seed is for a test program which has too many
6623 combinations to test comprehensively in the time available to it each run. It
6624 can test a random subset each time, and should there be a failure, log the seed
6625 used for that run so that it can later be used to reproduce the same results.
6627 =item stat FILEHANDLE
6628 X<stat> X<file, status> X<ctime>
6632 =item stat DIRHANDLE
6636 Returns a 13-element list giving the status info for a file, either
6637 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6638 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
6641 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6642 $atime,$mtime,$ctime,$blksize,$blocks)
6645 Not all fields are supported on all filesystem types. Here are the
6646 meanings of the fields:
6648 0 dev device number of filesystem
6650 2 mode file mode (type and permissions)
6651 3 nlink number of (hard) links to the file
6652 4 uid numeric user ID of file's owner
6653 5 gid numeric group ID of file's owner
6654 6 rdev the device identifier (special files only)
6655 7 size total size of file, in bytes
6656 8 atime last access time in seconds since the epoch
6657 9 mtime last modify time in seconds since the epoch
6658 10 ctime inode change time in seconds since the epoch (*)
6659 11 blksize preferred block size for file system I/O
6660 12 blocks actual number of blocks allocated
6662 (The epoch was at 00:00 January 1, 1970 GMT.)
6664 (*) Not all fields are supported on all filesystem types. Notably, the
6665 ctime field is non-portable. In particular, you cannot expect it to be a
6666 "creation time"; see L<perlport/"Files and Filesystems"> for details.
6668 If C<stat> is passed the special filehandle consisting of an underline, no
6669 stat is done, but the current contents of the stat structure from the
6670 last C<stat>, C<lstat>, or filetest are returned. Example:
6672 if (-x $file && (($d) = stat(_)) && $d < 0) {
6673 print "$file is executable NFS file\n";
6676 (This works on machines only for which the device number is negative
6679 Because the mode contains both the file type and its permissions, you
6680 should mask off the file type portion and (s)printf using a C<"%o">
6681 if you want to see the real permissions.
6683 $mode = (stat($filename))[2];
6684 printf "Permissions are %04o\n", $mode & 07777;
6686 In scalar context, C<stat> returns a boolean value indicating success
6687 or failure, and, if successful, sets the information associated with
6688 the special filehandle C<_>.
6690 The L<File::stat> module provides a convenient, by-name access mechanism:
6693 $sb = stat($filename);
6694 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6695 $filename, $sb->size, $sb->mode & 07777,
6696 scalar localtime $sb->mtime;
6698 You can import symbolic mode constants (C<S_IF*>) and functions
6699 (C<S_IS*>) from the Fcntl module:
6703 $mode = (stat($filename))[2];
6705 $user_rwx = ($mode & S_IRWXU) >> 6;
6706 $group_read = ($mode & S_IRGRP) >> 3;
6707 $other_execute = $mode & S_IXOTH;
6709 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6711 $is_setuid = $mode & S_ISUID;
6712 $is_directory = S_ISDIR($mode);
6714 You could write the last two using the C<-u> and C<-d> operators.
6715 Commonly available C<S_IF*> constants are:
6717 # Permissions: read, write, execute, for user, group, others.
6719 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6720 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6721 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6723 # Setuid/Setgid/Stickiness/SaveText.
6724 # Note that the exact meaning of these is system-dependent.
6726 S_ISUID S_ISGID S_ISVTX S_ISTXT
6728 # File types. Not all are necessarily available on
6731 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
6732 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6734 # The following are compatibility aliases for S_IRUSR,
6735 # S_IWUSR, and S_IXUSR.
6737 S_IREAD S_IWRITE S_IEXEC
6739 and the C<S_IF*> functions are
6741 S_IMODE($mode) the part of $mode containing the permission
6742 bits and the setuid/setgid/sticky bits
6744 S_IFMT($mode) the part of $mode containing the file type
6745 which can be bit-anded with (for example)
6746 S_IFREG or with the following functions
6748 # The operators -f, -d, -l, -b, -c, -p, and -S.
6750 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6751 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6753 # No direct -X operator counterpart, but for the first one
6754 # the -g operator is often equivalent. The ENFMT stands for
6755 # record flocking enforcement, a platform-dependent feature.
6757 S_ISENFMT($mode) S_ISWHT($mode)
6759 See your native chmod(2) and stat(2) documentation for more details
6760 about the C<S_*> constants. To get status info for a symbolic link
6761 instead of the target file behind the link, use the C<lstat> function.
6763 Portability issues: L<perlport/stat>.
6768 =item state TYPE EXPR
6770 =item state EXPR : ATTRS
6772 =item state TYPE EXPR : ATTRS
6774 C<state> declares a lexically scoped variable, just like C<my>.
6775 However, those variables will never be reinitialized, contrary to
6776 lexical variables that are reinitialized each time their enclosing block
6778 See L<perlsub/"Persistent Private Variables"> for details.
6780 C<state> variables are enabled only when the C<use feature "state"> pragma
6781 is in effect, unless the keyword is written as C<CORE::state>.
6782 See also L<feature>.
6789 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6790 doing many pattern matches on the string before it is next modified.
6791 This may or may not save time, depending on the nature and number of
6792 patterns you are searching and the distribution of character
6793 frequencies in the string to be searched; you probably want to compare
6794 run times with and without it to see which is faster. Those loops
6795 that scan for many short constant strings (including the constant
6796 parts of more complex patterns) will benefit most.
6797 (The way C<study> works is this: a linked list of every
6798 character in the string to be searched is made, so we know, for
6799 example, where all the C<'k'> characters are. From each search string,
6800 the rarest character is selected, based on some static frequency tables
6801 constructed from some C programs and English text. Only those places
6802 that contain this "rarest" character are examined.)
6804 For example, here is a loop that inserts index producing entries
6805 before any line containing a certain pattern:
6809 print ".IX foo\n" if /\bfoo\b/;
6810 print ".IX bar\n" if /\bbar\b/;
6811 print ".IX blurfl\n" if /\bblurfl\b/;
6816 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6817 will be looked at, because C<f> is rarer than C<o>. In general, this is
6818 a big win except in pathological cases. The only question is whether
6819 it saves you more time than it took to build the linked list in the
6822 Note that if you have to look for strings that you don't know till
6823 runtime, you can build an entire loop as a string and C<eval> that to
6824 avoid recompiling all your patterns all the time. Together with
6825 undefining C<$/> to input entire files as one record, this can be quite
6826 fast, often faster than specialized programs like fgrep(1). The following
6827 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6828 out the names of those files that contain a match:
6830 $search = 'while (<>) { study;';
6831 foreach $word (@words) {
6832 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6837 eval $search; # this screams
6838 $/ = "\n"; # put back to normal input delimiter
6839 foreach $file (sort keys(%seen)) {
6843 =item sub NAME BLOCK
6846 =item sub NAME (PROTO) BLOCK
6848 =item sub NAME : ATTRS BLOCK
6850 =item sub NAME (PROTO) : ATTRS BLOCK
6852 This is subroutine definition, not a real function I<per se>. Without a
6853 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
6854 function declaration, so does return a value: the CODE ref of the closure
6857 See L<perlsub> and L<perlref> for details about subroutines and
6858 references; see L<attributes> and L<Attribute::Handlers> for more
6859 information about attributes.
6861 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6862 X<substr> X<substring> X<mid> X<left> X<right>
6864 =item substr EXPR,OFFSET,LENGTH
6866 =item substr EXPR,OFFSET
6868 Extracts a substring out of EXPR and returns it. First character is at
6869 offset zero. If OFFSET is negative, starts
6870 that far back from the end of the string. If LENGTH is omitted, returns
6871 everything through the end of the string. If LENGTH is negative, leaves that
6872 many characters off the end of the string.
6874 my $s = "The black cat climbed the green tree";
6875 my $color = substr $s, 4, 5; # black
6876 my $middle = substr $s, 4, -11; # black cat climbed the
6877 my $end = substr $s, 14; # climbed the green tree
6878 my $tail = substr $s, -4; # tree
6879 my $z = substr $s, -4, 2; # tr
6881 You can use the substr() function as an lvalue, in which case EXPR
6882 must itself be an lvalue. If you assign something shorter than LENGTH,
6883 the string will shrink, and if you assign something longer than LENGTH,
6884 the string will grow to accommodate it. To keep the string the same
6885 length, you may need to pad or chop your value using C<sprintf>.
6887 If OFFSET and LENGTH specify a substring that is partly outside the
6888 string, only the part within the string is returned. If the substring
6889 is beyond either end of the string, substr() returns the undefined
6890 value and produces a warning. When used as an lvalue, specifying a
6891 substring that is entirely outside the string raises an exception.
6892 Here's an example showing the behavior for boundary cases:
6895 substr($name, 4) = 'dy'; # $name is now 'freddy'
6896 my $null = substr $name, 6, 2; # returns "" (no warning)
6897 my $oops = substr $name, 7; # returns undef, with warning
6898 substr($name, 7) = 'gap'; # raises an exception
6900 An alternative to using substr() as an lvalue is to specify the
6901 replacement string as the 4th argument. This allows you to replace
6902 parts of the EXPR and return what was there before in one operation,
6903 just as you can with splice().
6905 my $s = "The black cat climbed the green tree";
6906 my $z = substr $s, 14, 7, "jumped from"; # climbed
6907 # $s is now "The black cat jumped from the green tree"
6909 Note that the lvalue returned by the three-argument version of substr() acts as
6910 a 'magic bullet'; each time it is assigned to, it remembers which part
6911 of the original string is being modified; for example:
6914 for (substr($x,1,2)) {
6915 $_ = 'a'; print $x,"\n"; # prints 1a4
6916 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6918 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6921 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6924 =item symlink OLDFILE,NEWFILE
6925 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6927 Creates a new filename symbolically linked to the old filename.
6928 Returns C<1> for success, C<0> otherwise. On systems that don't support
6929 symbolic links, raises an exception. To check for that,
6932 $symlink_exists = eval { symlink("",""); 1 };
6934 Portability issues: L<perlport/symlink>.
6936 =item syscall NUMBER, LIST
6937 X<syscall> X<system call>
6939 Calls the system call specified as the first element of the list,
6940 passing the remaining elements as arguments to the system call. If
6941 unimplemented, raises an exception. The arguments are interpreted
6942 as follows: if a given argument is numeric, the argument is passed as
6943 an int. If not, the pointer to the string value is passed. You are
6944 responsible to make sure a string is pre-extended long enough to
6945 receive any result that might be written into a string. You can't use a
6946 string literal (or other read-only string) as an argument to C<syscall>
6947 because Perl has to assume that any string pointer might be written
6949 integer arguments are not literals and have never been interpreted in a
6950 numeric context, you may need to add C<0> to them to force them to look
6951 like numbers. This emulates the C<syswrite> function (or vice versa):
6953 require 'syscall.ph'; # may need to run h2ph
6955 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6957 Note that Perl supports passing of up to only 14 arguments to your syscall,
6958 which in practice should (usually) suffice.
6960 Syscall returns whatever value returned by the system call it calls.
6961 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6962 Note that some system calls I<can> legitimately return C<-1>. The proper
6963 way to handle such calls is to assign C<$!=0> before the call, then
6964 check the value of C<$!> if C<syscall> returns C<-1>.
6966 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6967 number of the read end of the pipe it creates, but there is no way
6968 to retrieve the file number of the other end. You can avoid this
6969 problem by using C<pipe> instead.
6971 Portability issues: L<perlport/syscall>.
6973 =item sysopen FILEHANDLE,FILENAME,MODE
6976 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6978 Opens the file whose filename is given by FILENAME, and associates it with
6979 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
6980 filehandle wanted; an undefined scalar will be suitably autovivified. This
6981 function calls the underlying operating system's I<open>(2) function with the
6982 parameters FILENAME, MODE, and PERMS.
6984 The possible values and flag bits of the MODE parameter are
6985 system-dependent; they are available via the standard module C<Fcntl>. See
6986 the documentation of your operating system's I<open>(2) syscall to see
6987 which values and flag bits are available. You may combine several flags
6988 using the C<|>-operator.
6990 Some of the most common values are C<O_RDONLY> for opening the file in
6991 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6992 and C<O_RDWR> for opening the file in read-write mode.
6993 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6995 For historical reasons, some values work on almost every system
6996 supported by Perl: 0 means read-only, 1 means write-only, and 2
6997 means read/write. We know that these values do I<not> work under
6998 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6999 use them in new code.
7001 If the file named by FILENAME does not exist and the C<open> call creates
7002 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7003 PERMS specifies the permissions of the newly created file. If you omit
7004 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7005 These permission values need to be in octal, and are modified by your
7006 process's current C<umask>.
7009 In many systems the C<O_EXCL> flag is available for opening files in
7010 exclusive mode. This is B<not> locking: exclusiveness means here that
7011 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7012 on network filesystems, and has no effect unless the C<O_CREAT> flag
7013 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7014 being opened if it is a symbolic link. It does not protect against
7015 symbolic links in the file's path.
7018 Sometimes you may want to truncate an already-existing file. This
7019 can be done using the C<O_TRUNC> flag. The behavior of
7020 C<O_TRUNC> with C<O_RDONLY> is undefined.
7023 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7024 that takes away the user's option to have a more permissive umask.
7025 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7028 Note that C<sysopen> depends on the fdopen() C library function.
7029 On many Unix systems, fdopen() is known to fail when file descriptors
7030 exceed a certain value, typically 255. If you need more file
7031 descriptors than that, consider rebuilding Perl to use the C<sfio>
7032 library, or perhaps using the POSIX::open() function.
7034 See L<perlopentut> for a kinder, gentler explanation of opening files.
7036 Portability issues: L<perlport/sysopen>.
7038 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7041 =item sysread FILEHANDLE,SCALAR,LENGTH
7043 Attempts to read LENGTH bytes of data into variable SCALAR from the
7044 specified FILEHANDLE, using the read(2). It bypasses
7045 buffered IO, so mixing this with other kinds of reads, C<print>,
7046 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7047 perlio or stdio layers usually buffers data. Returns the number of
7048 bytes actually read, C<0> at end of file, or undef if there was an
7049 error (in the latter case C<$!> is also set). SCALAR will be grown or
7050 shrunk so that the last byte actually read is the last byte of the
7051 scalar after the read.
7053 An OFFSET may be specified to place the read data at some place in the
7054 string other than the beginning. A negative OFFSET specifies
7055 placement at that many characters counting backwards from the end of
7056 the string. A positive OFFSET greater than the length of SCALAR
7057 results in the string being padded to the required size with C<"\0">
7058 bytes before the result of the read is appended.
7060 There is no syseof() function, which is ok, since eof() doesn't work
7061 well on device files (like ttys) anyway. Use sysread() and check
7062 for a return value for 0 to decide whether you're done.
7064 Note that if the filehandle has been marked as C<:utf8> Unicode
7065 characters are read instead of bytes (the LENGTH, OFFSET, and the
7066 return value of sysread() are in Unicode characters).
7067 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7068 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7070 =item sysseek FILEHANDLE,POSITION,WHENCE
7073 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7074 be an expression whose value gives the name of the filehandle. The values
7075 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7076 to the current position plus POSITION; and C<2> to set it to EOF plus
7077 POSITION, typically negative.
7079 Note the I<in bytes>: even if the filehandle has been set to operate
7080 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7081 tell() will return byte offsets, not character offsets (because
7082 implementing that would render sysseek() unacceptably slow).
7084 sysseek() bypasses normal buffered IO, so mixing it with reads other
7085 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7086 C<seek>, C<tell>, or C<eof> may cause confusion.
7088 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7089 and C<SEEK_END> (start of the file, current position, end of the file)
7090 from the Fcntl module. Use of the constants is also more portable
7091 than relying on 0, 1, and 2. For example to define a "systell" function:
7093 use Fcntl 'SEEK_CUR';
7094 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7096 Returns the new position, or the undefined value on failure. A position
7097 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7098 true on success and false on failure, yet you can still easily determine
7104 =item system PROGRAM LIST
7106 Does exactly the same thing as C<exec LIST>, except that a fork is
7107 done first and the parent process waits for the child process to
7108 exit. Note that argument processing varies depending on the
7109 number of arguments. If there is more than one argument in LIST,
7110 or if LIST is an array with more than one value, starts the program
7111 given by the first element of the list with arguments given by the
7112 rest of the list. If there is only one scalar argument, the argument
7113 is checked for shell metacharacters, and if there are any, the
7114 entire argument is passed to the system's command shell for parsing
7115 (this is C</bin/sh -c> on Unix platforms, but varies on other
7116 platforms). If there are no shell metacharacters in the argument,
7117 it is split into words and passed directly to C<execvp>, which is
7120 Beginning with v5.6.0, Perl will attempt to flush all files opened for
7121 output before any operation that may do a fork, but this may not be
7122 supported on some platforms (see L<perlport>). To be safe, you may need
7123 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7124 of C<IO::Handle> on any open handles.
7126 The return value is the exit status of the program as returned by the
7127 C<wait> call. To get the actual exit value, shift right by eight (see
7128 below). See also L</exec>. This is I<not> what you want to use to capture
7129 the output from a command; for that you should use merely backticks or
7130 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7131 indicates a failure to start the program or an error of the wait(2) system
7132 call (inspect $! for the reason).
7134 If you'd like to make C<system> (and many other bits of Perl) die on error,
7135 have a look at the L<autodie> pragma.
7137 Like C<exec>, C<system> allows you to lie to a program about its name if
7138 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
7140 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
7141 C<system>, if you expect your program to terminate on receipt of these
7142 signals you will need to arrange to do so yourself based on the return
7145 @args = ("command", "arg1", "arg2");
7147 or die "system @args failed: $?"
7149 If you'd like to manually inspect C<system>'s failure, you can check all
7150 possible failure modes by inspecting C<$?> like this:
7153 print "failed to execute: $!\n";
7156 printf "child died with signal %d, %s coredump\n",
7157 ($? & 127), ($? & 128) ? 'with' : 'without';
7160 printf "child exited with value %d\n", $? >> 8;
7163 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
7164 with the C<W*()> calls from the POSIX module.
7166 When C<system>'s arguments are executed indirectly by the shell,
7167 results and return codes are subject to its quirks.
7168 See L<perlop/"`STRING`"> and L</exec> for details.
7170 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
7171 handler. See L<perlipc> for details.
7173 Portability issues: L<perlport/system>.
7175 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7178 =item syswrite FILEHANDLE,SCALAR,LENGTH
7180 =item syswrite FILEHANDLE,SCALAR
7182 Attempts to write LENGTH bytes of data from variable SCALAR to the
7183 specified FILEHANDLE, using write(2). If LENGTH is
7184 not specified, writes whole SCALAR. It bypasses buffered IO, so
7185 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
7186 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
7187 stdio layers usually buffer data. Returns the number of bytes
7188 actually written, or C<undef> if there was an error (in this case the
7189 errno variable C<$!> is also set). If the LENGTH is greater than the
7190 data available in the SCALAR after the OFFSET, only as much data as is
7191 available will be written.
7193 An OFFSET may be specified to write the data from some part of the
7194 string other than the beginning. A negative OFFSET specifies writing
7195 that many characters counting backwards from the end of the string.
7196 If SCALAR is of length zero, you can only use an OFFSET of 0.
7198 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
7199 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
7200 return value of syswrite() are in (UTF8-encoded Unicode) characters.
7201 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7202 Alternately, if the handle is not marked with an encoding but you
7203 attempt to write characters with code points over 255, raises an exception.
7204 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7206 =item tell FILEHANDLE
7211 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
7212 error. FILEHANDLE may be an expression whose value gives the name of
7213 the actual filehandle. If FILEHANDLE is omitted, assumes the file
7216 Note the I<in bytes>: even if the filehandle has been set to
7217 operate on characters (for example by using the C<:encoding(utf8)> open
7218 layer), tell() will return byte offsets, not character offsets (because
7219 that would render seek() and tell() rather slow).
7221 The return value of tell() for the standard streams like the STDIN
7222 depends on the operating system: it may return -1 or something else.
7223 tell() on pipes, fifos, and sockets usually returns -1.
7225 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
7227 Do not use tell() (or other buffered I/O operations) on a filehandle
7228 that has been manipulated by sysread(), syswrite(), or sysseek().
7229 Those functions ignore the buffering, while tell() does not.
7231 =item telldir DIRHANDLE
7234 Returns the current position of the C<readdir> routines on DIRHANDLE.
7235 Value may be given to C<seekdir> to access a particular location in a
7236 directory. C<telldir> has the same caveats about possible directory
7237 compaction as the corresponding system library routine.
7239 =item tie VARIABLE,CLASSNAME,LIST
7242 This function binds a variable to a package class that will provide the
7243 implementation for the variable. VARIABLE is the name of the variable
7244 to be enchanted. CLASSNAME is the name of a class implementing objects
7245 of correct type. Any additional arguments are passed to the C<new>
7246 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
7247 or C<TIEHASH>). Typically these are arguments such as might be passed
7248 to the C<dbm_open()> function of C. The object returned by the C<new>
7249 method is also returned by the C<tie> function, which would be useful
7250 if you want to access other methods in CLASSNAME.
7252 Note that functions such as C<keys> and C<values> may return huge lists
7253 when used on large objects, like DBM files. You may prefer to use the
7254 C<each> function to iterate over such. Example:
7256 # print out history file offsets
7258 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
7259 while (($key,$val) = each %HIST) {
7260 print $key, ' = ', unpack('L',$val), "\n";
7264 A class implementing a hash should have the following methods:
7266 TIEHASH classname, LIST
7268 STORE this, key, value
7273 NEXTKEY this, lastkey
7278 A class implementing an ordinary array should have the following methods:
7280 TIEARRAY classname, LIST
7282 STORE this, key, value
7284 STORESIZE this, count
7290 SPLICE this, offset, length, LIST
7295 A class implementing a filehandle should have the following methods:
7297 TIEHANDLE classname, LIST
7298 READ this, scalar, length, offset
7301 WRITE this, scalar, length, offset
7303 PRINTF this, format, LIST
7307 SEEK this, position, whence
7309 OPEN this, mode, LIST
7314 A class implementing a scalar should have the following methods:
7316 TIESCALAR classname, LIST
7322 Not all methods indicated above need be implemented. See L<perltie>,
7323 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
7325 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
7326 for you; you need to do that explicitly yourself. See L<DB_File>
7327 or the F<Config> module for interesting C<tie> implementations.
7329 For further details see L<perltie>, L<"tied VARIABLE">.
7334 Returns a reference to the object underlying VARIABLE (the same value
7335 that was originally returned by the C<tie> call that bound the variable
7336 to a package.) Returns the undefined value if VARIABLE isn't tied to a
7342 Returns the number of non-leap seconds since whatever time the system
7343 considers to be the epoch, suitable for feeding to C<gmtime> and
7344 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
7345 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
7346 1904 in the current local time zone for its epoch.
7348 For measuring time in better granularity than one second, use the
7349 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
7350 if you have gettimeofday(2), you may be able to use the C<syscall>
7351 interface of Perl. See L<perlfaq8> for details.
7353 For date and time processing look at the many related modules on CPAN.
7354 For a comprehensive date and time representation look at the
7360 Returns a four-element list giving the user and system times in
7361 seconds for this process and any exited children of this process.
7363 ($user,$system,$cuser,$csystem) = times;
7365 In scalar context, C<times> returns C<$user>.
7367 Children's times are only included for terminated children.
7369 Portability issues: L<perlport/times>.
7373 The transliteration operator. Same as C<y///>. See
7374 L<perlop/"Quote and Quote-like Operators">.
7376 =item truncate FILEHANDLE,LENGTH
7379 =item truncate EXPR,LENGTH
7381 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
7382 specified length. Raises an exception if truncate isn't implemented
7383 on your system. Returns true if successful, C<undef> on error.
7385 The behavior is undefined if LENGTH is greater than the length of the
7388 The position in the file of FILEHANDLE is left unchanged. You may want to
7389 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
7391 Portability issues: L<perlport/truncate>.
7394 X<uc> X<uppercase> X<toupper>
7398 Returns an uppercased version of EXPR. This is the internal function
7399 implementing the C<\U> escape in double-quoted strings.
7400 It does not attempt to do titlecase mapping on initial letters. See
7401 L</ucfirst> for that.
7403 If EXPR is omitted, uses C<$_>.
7405 This function behaves the same way under various pragma, such as in a locale,
7409 X<ucfirst> X<uppercase>
7413 Returns the value of EXPR with the first character in uppercase
7414 (titlecase in Unicode). This is the internal function implementing
7415 the C<\u> escape in double-quoted strings.
7417 If EXPR is omitted, uses C<$_>.
7419 This function behaves the same way under various pragma, such as in a locale,
7427 Sets the umask for the process to EXPR and returns the previous value.
7428 If EXPR is omitted, merely returns the current umask.
7430 The Unix permission C<rwxr-x---> is represented as three sets of three
7431 bits, or three octal digits: C<0750> (the leading 0 indicates octal
7432 and isn't one of the digits). The C<umask> value is such a number
7433 representing disabled permissions bits. The permission (or "mode")
7434 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
7435 even if you tell C<sysopen> to create a file with permissions C<0777>,
7436 if your umask is C<0022>, then the file will actually be created with
7437 permissions C<0755>. If your C<umask> were C<0027> (group can't
7438 write; others can't read, write, or execute), then passing
7439 C<sysopen> C<0666> would create a file with mode C<0640> (because
7440 C<0666 &~ 027> is C<0640>).
7442 Here's some advice: supply a creation mode of C<0666> for regular
7443 files (in C<sysopen>) and one of C<0777> for directories (in
7444 C<mkdir>) and executable files. This gives users the freedom of
7445 choice: if they want protected files, they might choose process umasks
7446 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
7447 Programs should rarely if ever make policy decisions better left to
7448 the user. The exception to this is when writing files that should be
7449 kept private: mail files, web browser cookies, I<.rhosts> files, and
7452 If umask(2) is not implemented on your system and you are trying to
7453 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
7454 raises an exception. If umask(2) is not implemented and you are
7455 not trying to restrict access for yourself, returns C<undef>.
7457 Remember that a umask is a number, usually given in octal; it is I<not> a
7458 string of octal digits. See also L</oct>, if all you have is a string.
7460 Portability issues: L<perlport/umask>.
7463 X<undef> X<undefine>
7467 Undefines the value of EXPR, which must be an lvalue. Use only on a
7468 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
7469 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
7470 will probably not do what you expect on most predefined variables or
7471 DBM list values, so don't do that; see L</delete>. Always returns the
7472 undefined value. You can omit the EXPR, in which case nothing is
7473 undefined, but you still get an undefined value that you could, for
7474 instance, return from a subroutine, assign to a variable, or pass as a
7475 parameter. Examples:
7478 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
7482 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
7483 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7484 select undef, undef, undef, 0.25;
7485 ($a, $b, undef, $c) = &foo; # Ignore third value returned
7487 Note that this is a unary operator, not a list operator.
7490 X<unlink> X<delete> X<remove> X<rm> X<del>
7494 Deletes a list of files. On success, it returns the number of files
7495 it successfully deleted. On failure, it returns false and sets C<$!>
7498 my $unlinked = unlink 'a', 'b', 'c';
7500 unlink glob "*.bak";
7502 On error, C<unlink> will not tell you which files it could not remove.
7503 If you want to know which files you could not remove, try them one
7506 foreach my $file ( @goners ) {
7507 unlink $file or warn "Could not unlink $file: $!";
7510 Note: C<unlink> will not attempt to delete directories unless you are
7511 superuser and the B<-U> flag is supplied to Perl. Even if these
7512 conditions are met, be warned that unlinking a directory can inflict
7513 damage on your filesystem. Finally, using C<unlink> on directories is
7514 not supported on many operating systems. Use C<rmdir> instead.
7516 If LIST is omitted, C<unlink> uses C<$_>.
7518 =item unpack TEMPLATE,EXPR
7521 =item unpack TEMPLATE
7523 C<unpack> does the reverse of C<pack>: it takes a string
7524 and expands it out into a list of values.
7525 (In scalar context, it returns merely the first value produced.)
7527 If EXPR is omitted, unpacks the C<$_> string.
7528 See L<perlpacktut> for an introduction to this function.
7530 The string is broken into chunks described by the TEMPLATE. Each chunk
7531 is converted separately to a value. Typically, either the string is a result
7532 of C<pack>, or the characters of the string represent a C structure of some
7535 The TEMPLATE has the same format as in the C<pack> function.
7536 Here's a subroutine that does substring:
7539 my($what,$where,$howmuch) = @_;
7540 unpack("x$where a$howmuch", $what);
7545 sub ordinal { unpack("W",$_[0]); } # same as ord()
7547 In addition to fields allowed in pack(), you may prefix a field with
7548 a %<number> to indicate that
7549 you want a <number>-bit checksum of the items instead of the items
7550 themselves. Default is a 16-bit checksum. Checksum is calculated by
7551 summing numeric values of expanded values (for string fields the sum of
7552 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
7554 For example, the following
7555 computes the same number as the System V sum program:
7559 unpack("%32W*",<>) % 65535;
7562 The following efficiently counts the number of set bits in a bit vector:
7564 $setbits = unpack("%32b*", $selectmask);
7566 The C<p> and C<P> formats should be used with care. Since Perl
7567 has no way of checking whether the value passed to C<unpack()>
7568 corresponds to a valid memory location, passing a pointer value that's
7569 not known to be valid is likely to have disastrous consequences.
7571 If there are more pack codes or if the repeat count of a field or a group
7572 is larger than what the remainder of the input string allows, the result
7573 is not well defined: the repeat count may be decreased, or
7574 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7575 If the input string is longer than one described by the TEMPLATE,
7576 the remainder of that input string is ignored.
7578 See L</pack> for more examples and notes.
7580 =item untie VARIABLE
7583 Breaks the binding between a variable and a package.
7584 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
7585 Has no effect if the variable is not tied.
7587 =item unshift ARRAY,LIST
7590 =item unshift EXPR,LIST
7592 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7593 depending on how you look at it. Prepends list to the front of the
7594 array and returns the new number of elements in the array.
7596 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7598 Note the LIST is prepended whole, not one element at a time, so the
7599 prepended elements stay in the same order. Use C<reverse> to do the
7602 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
7603 a reference to an unblessed array. The argument will be dereferenced
7604 automatically. This aspect of C<unshift> is considered highly
7605 experimental. The exact behaviour may change in a future version of Perl.
7607 =item use Module VERSION LIST
7608 X<use> X<module> X<import>
7610 =item use Module VERSION
7612 =item use Module LIST
7618 Imports some semantics into the current package from the named module,
7619 generally by aliasing certain subroutine or variable names into your
7620 package. It is exactly equivalent to
7622 BEGIN { require Module; Module->import( LIST ); }
7624 except that Module I<must> be a bareword.
7625 The importation can be made conditional; see L<if>.
7627 In the peculiar C<use VERSION> form, VERSION may be either a positive
7628 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7629 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7630 exception is raised if VERSION is greater than the version of the
7631 current Perl interpreter; Perl will not attempt to parse the rest of the
7632 file. Compare with L</require>, which can do a similar check at run time.
7633 Symmetrically, C<no VERSION> allows you to specify that you want a version
7634 of Perl older than the specified one.
7636 Specifying VERSION as a literal of the form v5.6.1 should generally be
7637 avoided, because it leads to misleading error messages under earlier
7638 versions of Perl (that is, prior to 5.6.0) that do not support this
7639 syntax. The equivalent numeric version should be used instead.
7641 use v5.6.1; # compile time version check
7643 use 5.006_001; # ditto; preferred for backwards compatibility
7645 This is often useful if you need to check the current Perl version before
7646 C<use>ing library modules that won't work with older versions of Perl.
7647 (We try not to do this more than we have to.)
7649 Also, if the specified Perl version is greater than or equal to 5.9.5,
7650 C<use VERSION> will also load the C<feature> pragma and enable all
7651 features available in the requested version. See L<feature>.
7652 Similarly, if the specified Perl version is greater than or equal to
7653 5.11.0, strictures are enabled lexically as with C<use strict> (except
7654 that the F<strict.pm> file is not actually loaded).
7656 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7657 C<require> makes sure the module is loaded into memory if it hasn't been
7658 yet. The C<import> is not a builtin; it's just an ordinary static method
7659 call into the C<Module> package to tell the module to import the list of
7660 features back into the current package. The module can implement its
7661 C<import> method any way it likes, though most modules just choose to
7662 derive their C<import> method via inheritance from the C<Exporter> class that
7663 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7664 method can be found then the call is skipped, even if there is an AUTOLOAD
7667 If you do not want to call the package's C<import> method (for instance,
7668 to stop your namespace from being altered), explicitly supply the empty list:
7672 That is exactly equivalent to
7674 BEGIN { require Module }
7676 If the VERSION argument is present between Module and LIST, then the
7677 C<use> will call the VERSION method in class Module with the given
7678 version as an argument. The default VERSION method, inherited from
7679 the UNIVERSAL class, croaks if the given version is larger than the
7680 value of the variable C<$Module::VERSION>.
7682 Again, there is a distinction between omitting LIST (C<import> called
7683 with no arguments) and an explicit empty LIST C<()> (C<import> not
7684 called). Note that there is no comma after VERSION!
7686 Because this is a wide-open interface, pragmas (compiler directives)
7687 are also implemented this way. Currently implemented pragmas are:
7692 use sigtrap qw(SEGV BUS);
7693 use strict qw(subs vars refs);
7694 use subs qw(afunc blurfl);
7695 use warnings qw(all);
7696 use sort qw(stable _quicksort _mergesort);
7698 Some of these pseudo-modules import semantics into the current
7699 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7700 which import symbols into the current package (which are effective
7701 through the end of the file).
7703 Because C<use> takes effect at compile time, it doesn't respect the
7704 ordinary flow control of the code being compiled. In particular, putting
7705 a C<use> inside the false branch of a conditional doesn't prevent it
7706 from being processed. If a module or pragma only needs to be loaded
7707 conditionally, this can be done using the L<if> pragma:
7709 use if $] < 5.008, "utf8";
7710 use if WANT_WARNINGS, warnings => qw(all);
7712 There's a corresponding C<no> declaration that unimports meanings imported
7713 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7714 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7715 or no unimport method being found.
7721 Care should be taken when using the C<no VERSION> form of C<no>. It is
7722 I<only> meant to be used to assert that the running Perl is of a earlier
7723 version than its argument and I<not> to undo the feature-enabling side effects
7726 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7727 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7728 functionality from the command-line.
7733 Changes the access and modification times on each file of a list of
7734 files. The first two elements of the list must be the NUMERIC access
7735 and modification times, in that order. Returns the number of files
7736 successfully changed. The inode change time of each file is set
7737 to the current time. For example, this code has the same effect as the
7738 Unix touch(1) command when the files I<already exist> and belong to
7739 the user running the program:
7742 $atime = $mtime = time;
7743 utime $atime, $mtime, @ARGV;
7745 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7746 the utime(2) syscall from your C library is called with a null second
7747 argument. On most systems, this will set the file's access and
7748 modification times to the current time (i.e., equivalent to the example
7749 above) and will work even on files you don't own provided you have write
7753 utime(undef, undef, $file)
7754 || warn "couldn't touch $file: $!";
7757 Under NFS this will use the time of the NFS server, not the time of
7758 the local machine. If there is a time synchronization problem, the
7759 NFS server and local machine will have different times. The Unix
7760 touch(1) command will in fact normally use this form instead of the
7761 one shown in the first example.
7763 Passing only one of the first two elements as C<undef> is
7764 equivalent to passing a 0 and will not have the effect
7765 described when both are C<undef>. This also triggers an
7766 uninitialized warning.
7768 On systems that support futimes(2), you may pass filehandles among the
7769 files. On systems that don't support futimes(2), passing filehandles raises
7770 an exception. Filehandles must be passed as globs or glob references to be
7771 recognized; barewords are considered filenames.
7773 Portability issues: L<perlport/utime>.
7782 Returns a list consisting of all the values of the named hash, or the values
7783 of an array. (In scalar context, returns the number of values.)
7785 The values are returned in an apparently random order. The actual
7786 random order is subject to change in future versions of Perl, but it
7787 is guaranteed to be the same order as either the C<keys> or C<each>
7788 function would produce on the same (unmodified) hash. Since Perl
7789 5.8.1 the ordering is different even between different runs of Perl
7790 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7792 As a side effect, calling values() resets the HASH or ARRAY's internal
7794 see L</each>. (In particular, calling values() in void context resets
7795 the iterator with no other overhead. Apart from resetting the iterator,
7796 C<values @array> in list context is the same as plain C<@array>.
7797 We recommend that you use void context C<keys @array> for this, but reasoned
7798 that it taking C<values @array> out would require more documentation than
7801 Note that the values are not copied, which means modifying them will
7802 modify the contents of the hash:
7804 for (values %hash) { s/foo/bar/g } # modifies %hash values
7805 for (@hash{keys %hash}) { s/foo/bar/g } # same
7807 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
7808 a reference to an unblessed hash or array. The argument will be
7809 dereferenced automatically. This aspect of C<values> is considered highly
7810 experimental. The exact behaviour may change in a future version of Perl.
7812 for (values $hashref) { ... }
7813 for (values $obj->get_arrayref) { ... }
7815 See also C<keys>, C<each>, and C<sort>.
7817 =item vec EXPR,OFFSET,BITS
7818 X<vec> X<bit> X<bit vector>
7820 Treats the string in EXPR as a bit vector made up of elements of
7821 width BITS and returns the value of the element specified by OFFSET
7822 as an unsigned integer. BITS therefore specifies the number of bits
7823 that are reserved for each element in the bit vector. This must
7824 be a power of two from 1 to 32 (or 64, if your platform supports
7827 If BITS is 8, "elements" coincide with bytes of the input string.
7829 If BITS is 16 or more, bytes of the input string are grouped into chunks
7830 of size BITS/8, and each group is converted to a number as with
7831 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7832 for BITS==64). See L<"pack"> for details.
7834 If bits is 4 or less, the string is broken into bytes, then the bits
7835 of each byte are broken into 8/BITS groups. Bits of a byte are
7836 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7837 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7838 breaking the single input byte C<chr(0x36)> into two groups gives a list
7839 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7841 C<vec> may also be assigned to, in which case parentheses are needed
7842 to give the expression the correct precedence as in
7844 vec($image, $max_x * $x + $y, 8) = 3;
7846 If the selected element is outside the string, the value 0 is returned.
7847 If an element off the end of the string is written to, Perl will first
7848 extend the string with sufficiently many zero bytes. It is an error
7849 to try to write off the beginning of the string (i.e., negative OFFSET).
7851 If the string happens to be encoded as UTF-8 internally (and thus has
7852 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7853 internal byte string, not the conceptual character string, even if you
7854 only have characters with values less than 256.
7856 Strings created with C<vec> can also be manipulated with the logical
7857 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7858 vector operation is desired when both operands are strings.
7859 See L<perlop/"Bitwise String Operators">.
7861 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7862 The comments show the string after each step. Note that this code works
7863 in the same way on big-endian or little-endian machines.
7866 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7868 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7869 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7871 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7872 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7873 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7874 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7875 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7876 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7878 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7879 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7880 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7883 To transform a bit vector into a string or list of 0's and 1's, use these:
7885 $bits = unpack("b*", $vector);
7886 @bits = split(//, unpack("b*", $vector));
7888 If you know the exact length in bits, it can be used in place of the C<*>.
7890 Here is an example to illustrate how the bits actually fall in place:
7896 unpack("V",$_) 01234567890123456789012345678901
7897 ------------------------------------------------------------------
7902 for ($shift=0; $shift < $width; ++$shift) {
7903 for ($off=0; $off < 32/$width; ++$off) {
7904 $str = pack("B*", "0"x32);
7905 $bits = (1<<$shift);
7906 vec($str, $off, $width) = $bits;
7907 $res = unpack("b*",$str);
7908 $val = unpack("V", $str);
7915 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7916 $off, $width, $bits, $val, $res
7920 Regardless of the machine architecture on which it runs, the
7921 example above should print the following table:
7924 unpack("V",$_) 01234567890123456789012345678901
7925 ------------------------------------------------------------------
7926 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7927 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7928 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7929 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7930 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7931 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7932 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7933 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7934 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7935 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7936 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7937 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7938 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7939 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7940 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7941 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7942 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7943 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7944 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7945 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7946 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7947 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7948 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7949 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7950 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7951 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7952 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7953 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7954 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7955 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7956 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7957 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7958 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7959 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7960 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7961 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7962 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7963 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7964 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7965 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7966 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7967 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7968 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7969 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7970 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7971 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7972 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7973 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7974 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7975 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7976 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7977 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7978 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7979 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7980 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7981 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7982 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7983 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7984 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7985 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7986 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7987 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7988 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7989 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7990 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7991 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7992 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7993 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7994 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7995 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7996 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7997 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7998 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7999 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8000 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8001 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8002 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8003 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8004 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8005 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8006 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8007 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8008 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8009 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8010 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8011 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8012 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8013 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8014 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8015 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8016 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8017 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8018 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8019 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8020 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8021 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8022 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8023 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8024 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8025 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8026 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8027 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8028 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8029 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8030 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8031 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8032 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8033 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8034 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8035 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8036 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8037 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8038 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8039 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8040 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8041 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8042 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8043 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8044 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8045 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8046 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8047 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8048 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8049 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8050 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8051 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8052 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8053 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8058 Behaves like wait(2) on your system: it waits for a child
8059 process to terminate and returns the pid of the deceased process, or
8060 C<-1> if there are no child processes. The status is returned in C<$?>
8061 and C<${^CHILD_ERROR_NATIVE}>.
8062 Note that a return value of C<-1> could mean that child processes are
8063 being automatically reaped, as described in L<perlipc>.
8065 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8066 child created by qx() or system(). See L<perlipc> for details.
8068 Portability issues: L<perlport/wait>.
8070 =item waitpid PID,FLAGS
8073 Waits for a particular child process to terminate and returns the pid of
8074 the deceased process, or C<-1> if there is no such child process. On some
8075 systems, a value of 0 indicates that there are processes still running.
8076 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
8078 use POSIX ":sys_wait_h";
8081 $kid = waitpid(-1, WNOHANG);
8084 then you can do a non-blocking wait for all pending zombie processes.
8085 Non-blocking wait is available on machines supporting either the
8086 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
8087 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
8088 system call by remembering the status values of processes that have
8089 exited but have not been harvested by the Perl script yet.)
8091 Note that on some systems, a return value of C<-1> could mean that child
8092 processes are being automatically reaped. See L<perlipc> for details,
8093 and for other examples.
8095 Portability issues: L<perlport/waitpid>.
8098 X<wantarray> X<context>
8100 Returns true if the context of the currently executing subroutine or
8101 C<eval> is looking for a list value. Returns false if the context is
8102 looking for a scalar. Returns the undefined value if the context is
8103 looking for no value (void context).
8105 return unless defined wantarray; # don't bother doing more
8106 my @a = complex_calculation();
8107 return wantarray ? @a : "@a";
8109 C<wantarray()>'s result is unspecified in the top level of a file,
8110 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
8111 in a C<DESTROY> method.
8113 This function should have been named wantlist() instead.
8116 X<warn> X<warning> X<STDERR>
8118 Prints the value of LIST to STDERR. If the last element of LIST does
8119 not end in a newline, it appends the same file/line number text as C<die>
8122 If the output is empty and C<$@> already contains a value (typically from a
8123 previous eval) that value is used after appending C<"\t...caught">
8124 to C<$@>. This is useful for staying almost, but not entirely similar to
8127 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
8129 No message is printed if there is a C<$SIG{__WARN__}> handler
8130 installed. It is the handler's responsibility to deal with the message
8131 as it sees fit (like, for instance, converting it into a C<die>). Most
8132 handlers must therefore arrange to actually display the
8133 warnings that they are not prepared to deal with, by calling C<warn>
8134 again in the handler. Note that this is quite safe and will not
8135 produce an endless loop, since C<__WARN__> hooks are not called from
8138 You will find this behavior is slightly different from that of
8139 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
8140 instead call C<die> again to change it).
8142 Using a C<__WARN__> handler provides a powerful way to silence all
8143 warnings (even the so-called mandatory ones). An example:
8145 # wipe out *all* compile-time warnings
8146 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
8148 my $foo = 20; # no warning about duplicate my $foo,
8149 # but hey, you asked for it!
8150 # no compile-time or run-time warnings before here
8153 # run-time warnings enabled after here
8154 warn "\$foo is alive and $foo!"; # does show up
8156 See L<perlvar> for details on setting C<%SIG> entries and for more
8157 examples. See the Carp module for other kinds of warnings using its
8158 carp() and cluck() functions.
8160 =item when EXPR BLOCK
8165 C<when> is analogous to the C<case> keyword in other languages. Used with a
8166 C<foreach> loop or the experimental C<given> block, C<when> can be used in
8167 Perl to implement C<switch>/C<case> like statements. Available as a
8168 statement after Perl 5.10 and as a statement modifier after 5.14.
8169 Here are three examples:
8174 say "I like apples."
8177 say "I don't like oranges."
8180 say "I don't like anything"
8184 # require 5.14 for when as statement modifier
8187 say "I like apples." when /apples?/;
8188 say "I don't like oranges." when /oranges?;
8189 default { say "I don't like anything" }
8195 say "I like apples."
8198 say "I don't like oranges."
8201 say "I don't like anything"
8205 See L<perlsyn/"Switch statements"> for detailed information.
8207 =item write FILEHANDLE
8214 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
8215 using the format associated with that file. By default the format for
8216 a file is the one having the same name as the filehandle, but the
8217 format for the current output channel (see the C<select> function) may be set
8218 explicitly by assigning the name of the format to the C<$~> variable.
8220 Top of form processing is handled automatically: if there is insufficient
8221 room on the current page for the formatted record, the page is advanced by
8222 writing a form feed, a special top-of-page format is used to format the new
8223 page header before the record is written. By default, the top-of-page
8224 format is the name of the filehandle with "_TOP" appended. This would be a
8225 problem with autovivified filehandles, but it may be dynamically set to the
8226 format of your choice by assigning the name to the C<$^> variable while
8227 that filehandle is selected. The number of lines remaining on the current
8228 page is in variable C<$->, which can be set to C<0> to force a new page.
8230 If FILEHANDLE is unspecified, output goes to the current default output
8231 channel, which starts out as STDOUT but may be changed by the
8232 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
8233 is evaluated and the resulting string is used to look up the name of
8234 the FILEHANDLE at run time. For more on formats, see L<perlform>.
8236 Note that write is I<not> the opposite of C<read>. Unfortunately.
8240 The transliteration operator. Same as C<tr///>. See
8241 L<perlop/"Quote and Quote-like Operators">.