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>,
165 C<dump>, C<eval>, C<evalbytes> C<exit>,
166 C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
167 C<redo>, C<return>, C<sub>, C<wantarray>,
169 =item Keywords related to the switch feature
171 C<break>, C<continue>, C<default>, C<given>, C<when>
173 Except for C<continue>, these are available only if you enable the
174 C<"switch"> feature or use the C<CORE::> prefix.
175 See L<feature> and L<perlsyn/"Switch statements">.
176 Alternately, include a C<use v5.10> or later to the current scope. In Perl
177 5.14 and earlier, C<continue> required the C<"switch"> feature, like the
180 =item Keywords related to scoping
182 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
184 C<state> is available only if the C<"state"> feature
185 is enabled or if it is prefixed with C<CORE::>. See
186 L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
188 =item Miscellaneous functions
190 C<defined>, C<dump>, C<eval>, C<evalbytes>,
191 C<formline>, C<local>, C<my>, C<our>,
192 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
194 =item Functions for processes and process groups
195 X<process> X<pid> X<process id>
197 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
198 C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
199 C<setpriority>, C<sleep>, C<system>,
200 C<times>, C<wait>, C<waitpid>
202 =item Keywords related to Perl modules
205 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
207 =item Keywords related to classes and object-orientation
208 X<object> X<class> X<package>
210 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
213 =item Low-level socket functions
216 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
217 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
218 C<socket>, C<socketpair>
220 =item System V interprocess communication functions
221 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
223 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
224 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
226 =item Fetching user and group info
227 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
229 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
230 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
231 C<getpwuid>, C<setgrent>, C<setpwent>
233 =item Fetching network info
234 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
236 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
237 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
238 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
239 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
240 C<setnetent>, C<setprotoent>, C<setservent>
242 =item Time-related functions
245 C<gmtime>, C<localtime>, C<time>, C<times>
247 =item Functions new in perl5
250 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
251 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
252 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
253 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
254 C<ucfirst>, C<untie>, C<use>, C<when>
256 * C<sub> was a keyword in Perl 4, but in Perl 5 it is an
257 operator, which can be used in expressions.
259 =item Functions obsoleted in perl5
261 C<dbmclose>, C<dbmopen>
266 X<portability> X<Unix> X<portable>
268 Perl was born in Unix and can therefore access all common Unix
269 system calls. In non-Unix environments, the functionality of some
270 Unix system calls may not be available or details of the available
271 functionality may differ slightly. The Perl functions affected
274 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
275 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
276 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
277 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
278 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
279 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
280 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
281 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
282 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
283 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
284 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
285 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
286 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
287 C<shmwrite>, C<socket>, C<socketpair>,
288 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
289 C<times>, C<truncate>, C<umask>, C<unlink>,
290 C<utime>, C<wait>, C<waitpid>
292 For more information about the portability of these functions, see
293 L<perlport> and other available platform-specific documentation.
295 =head2 Alphabetical Listing of Perl Functions
300 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>
301 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
309 A file test, where X is one of the letters listed below. This unary
310 operator takes one argument, either a filename, a filehandle, or a dirhandle,
311 and tests the associated file to see if something is true about it. If the
312 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
313 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
314 the undefined value if the file doesn't exist. Despite the funny
315 names, precedence is the same as any other named unary operator. The
316 operator may be any of:
318 -r File is readable by effective uid/gid.
319 -w File is writable by effective uid/gid.
320 -x File is executable by effective uid/gid.
321 -o File is owned by effective uid.
323 -R File is readable by real uid/gid.
324 -W File is writable by real uid/gid.
325 -X File is executable by real uid/gid.
326 -O File is owned by real uid.
329 -z File has zero size (is empty).
330 -s File has nonzero size (returns size in bytes).
332 -f File is a plain file.
333 -d File is a directory.
334 -l File is a symbolic link.
335 -p File is a named pipe (FIFO), or Filehandle is a pipe.
337 -b File is a block special file.
338 -c File is a character special file.
339 -t Filehandle is opened to a tty.
341 -u File has setuid bit set.
342 -g File has setgid bit set.
343 -k File has sticky bit set.
345 -T File is an ASCII text file (heuristic guess).
346 -B File is a "binary" file (opposite of -T).
348 -M Script start time minus file modification time, in days.
349 -A Same for access time.
350 -C Same for inode change time (Unix, may differ for other platforms)
356 next unless -f $_; # ignore specials
360 Note that C<-s/a/b/> does not do a negated substitution. Saying
361 C<-exp($foo)> still works as expected, however: only single letters
362 following a minus are interpreted as file tests.
364 These operators are exempt from the "looks like a function rule" described
365 above. That is, an opening parenthesis after the operator does not affect
366 how much of the following code constitutes the argument. Put the opening
367 parentheses before the operator to separate it from code that follows (this
368 applies only to operators with higher precedence than unary operators, of
371 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
372 (-s $file) + 1024 # correct
374 The interpretation of the file permission operators C<-r>, C<-R>,
375 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
376 of the file and the uids and gids of the user. There may be other
377 reasons you can't actually read, write, or execute the file: for
378 example network filesystem access controls, ACLs (access control lists),
379 read-only filesystems, and unrecognized executable formats. Note
380 that the use of these six specific operators to verify if some operation
381 is possible is usually a mistake, because it may be open to race
384 Also note that, for the superuser on the local filesystems, the C<-r>,
385 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
386 if any execute bit is set in the mode. Scripts run by the superuser
387 may thus need to do a stat() to determine the actual mode of the file,
388 or temporarily set their effective uid to something else.
390 If you are using ACLs, there is a pragma called C<filetest> that may
391 produce more accurate results than the bare stat() mode bits.
392 When under C<use filetest 'access'> the above-mentioned filetests
393 test whether the permission can(not) be granted using the
394 access(2) family of system calls. Also note that the C<-x> and C<-X> may
395 under this pragma return true even if there are no execute permission
396 bits set (nor any extra execute permission ACLs). This strangeness is
397 due to the underlying system calls' definitions. Note also that, due to
398 the implementation of C<use filetest 'access'>, the C<_> special
399 filehandle won't cache the results of the file tests when this pragma is
400 in effect. Read the documentation for the C<filetest> pragma for more
403 The C<-T> and C<-B> switches work as follows. The first block or so of the
404 file is examined for odd characters such as strange control codes or
405 characters with the high bit set. If too many strange characters (>30%)
406 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
407 containing a zero byte in the first block is considered a binary file. If C<-T>
408 or C<-B> is used on a filehandle, the current IO buffer is examined
409 rather than the first block. Both C<-T> and C<-B> return true on an empty
410 file, or a file at EOF when testing a filehandle. Because you have to
411 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
412 against the file first, as in C<next unless -f $file && -T $file>.
414 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
415 the special filehandle consisting of a solitary underline, then the stat
416 structure of the previous file test (or stat operator) is used, saving
417 a system call. (This doesn't work with C<-t>, and you need to remember
418 that lstat() and C<-l> leave values in the stat structure for the
419 symbolic link, not the real file.) (Also, if the stat buffer was filled by
420 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
423 print "Can do.\n" if -r $a || -w _ || -x _;
426 print "Readable\n" if -r _;
427 print "Writable\n" if -w _;
428 print "Executable\n" if -x _;
429 print "Setuid\n" if -u _;
430 print "Setgid\n" if -g _;
431 print "Sticky\n" if -k _;
432 print "Text\n" if -T _;
433 print "Binary\n" if -B _;
435 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
436 test operators, in a way that C<-f -w -x $file> is equivalent to
437 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
438 the return value of C<-f $file> as an argument to another filetest
439 operator, no special magic will happen.)
441 Portability issues: L<perlport/-X>.
448 Returns the absolute value of its argument.
449 If VALUE is omitted, uses C<$_>.
451 =item accept NEWSOCKET,GENERICSOCKET
454 Accepts an incoming socket connect, just as accept(2)
455 does. Returns the packed address if it succeeded, false otherwise.
456 See the example in L<perlipc/"Sockets: Client/Server Communication">.
458 On systems that support a close-on-exec flag on files, the flag will
459 be set for the newly opened file descriptor, as determined by the
460 value of $^F. See L<perlvar/$^F>.
469 Arranges to have a SIGALRM delivered to this process after the
470 specified number of wallclock seconds has elapsed. If SECONDS is not
471 specified, the value stored in C<$_> is used. (On some machines,
472 unfortunately, the elapsed time may be up to one second less or more
473 than you specified because of how seconds are counted, and process
474 scheduling may delay the delivery of the signal even further.)
476 Only one timer may be counting at once. Each call disables the
477 previous timer, and an argument of C<0> may be supplied to cancel the
478 previous timer without starting a new one. The returned value is the
479 amount of time remaining on the previous timer.
481 For delays of finer granularity than one second, the Time::HiRes module
482 (from CPAN, and starting from Perl 5.8 part of the standard
483 distribution) provides ualarm(). You may also use Perl's four-argument
484 version of select() leaving the first three arguments undefined, or you
485 might be able to use the C<syscall> interface to access setitimer(2) if
486 your system supports it. See L<perlfaq8> for details.
488 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
489 C<sleep> may be internally implemented on your system with C<alarm>.
491 If you want to use C<alarm> to time out a system call you need to use an
492 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
493 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
494 restart system calls on some systems. Using C<eval>/C<die> always works,
495 modulo the caveats given in L<perlipc/"Signals">.
498 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
500 $nread = sysread SOCKET, $buffer, $size;
504 die unless $@ eq "alarm\n"; # propagate unexpected errors
511 For more information see L<perlipc>.
513 Portability issues: L<perlport/alarm>.
516 X<atan2> X<arctangent> X<tan> X<tangent>
518 Returns the arctangent of Y/X in the range -PI to PI.
520 For the tangent operation, you may use the C<Math::Trig::tan>
521 function, or use the familiar relation:
523 sub tan { sin($_[0]) / cos($_[0]) }
525 The return value for C<atan2(0,0)> is implementation-defined; consult
526 your atan2(3) manpage for more information.
528 Portability issues: L<perlport/atan2>.
530 =item bind SOCKET,NAME
533 Binds a network address to a socket, just as bind(2)
534 does. Returns true if it succeeded, false otherwise. NAME should be a
535 packed address of the appropriate type for the socket. See the examples in
536 L<perlipc/"Sockets: Client/Server Communication">.
538 =item binmode FILEHANDLE, LAYER
539 X<binmode> X<binary> X<text> X<DOS> X<Windows>
541 =item binmode FILEHANDLE
543 Arranges for FILEHANDLE to be read or written in "binary" or "text"
544 mode on systems where the run-time libraries distinguish between
545 binary and text files. If FILEHANDLE is an expression, the value is
546 taken as the name of the filehandle. Returns true on success,
547 otherwise it returns C<undef> and sets C<$!> (errno).
549 On some systems (in general, DOS- and Windows-based systems) binmode()
550 is necessary when you're not working with a text file. For the sake
551 of portability it is a good idea always to use it when appropriate,
552 and never to use it when it isn't appropriate. Also, people can
553 set their I/O to be by default UTF8-encoded Unicode, not bytes.
555 In other words: regardless of platform, use binmode() on binary data,
556 like images, for example.
558 If LAYER is present it is a single string, but may contain multiple
559 directives. The directives alter the behaviour of the filehandle.
560 When LAYER is present, using binmode on a text file makes sense.
562 If LAYER is omitted or specified as C<:raw> the filehandle is made
563 suitable for passing binary data. This includes turning off possible CRLF
564 translation and marking it as bytes (as opposed to Unicode characters).
565 Note that, despite what may be implied in I<"Programming Perl"> (the
566 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
567 Other layers that would affect the binary nature of the stream are
568 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
569 PERLIO environment variable.
571 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
572 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
573 establish default I/O layers. See L<open>.
575 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
576 in "Programming Perl, 3rd Edition". However, since the publishing of this
577 book, by many known as "Camel III", the consensus of the naming of this
578 functionality has moved from "discipline" to "layer". All documentation
579 of this version of Perl therefore refers to "layers" rather than to
580 "disciplines". Now back to the regularly scheduled documentation...>
582 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
583 C<:utf8> just marks the data as UTF-8 without further checking,
584 while C<:encoding(UTF-8)> checks the data for actually being valid
585 UTF-8. More details can be found in L<PerlIO::encoding>.
587 In general, binmode() should be called after open() but before any I/O
588 is done on the filehandle. Calling binmode() normally flushes any
589 pending buffered output data (and perhaps pending input data) on the
590 handle. An exception to this is the C<:encoding> layer that
591 changes the default character encoding of the handle; see L</open>.
592 The C<:encoding> layer sometimes needs to be called in
593 mid-stream, and it doesn't flush the stream. The C<:encoding>
594 also implicitly pushes on top of itself the C<:utf8> layer because
595 internally Perl operates on UTF8-encoded Unicode characters.
597 The operating system, device drivers, C libraries, and Perl run-time
598 system all conspire to let the programmer treat a single
599 character (C<\n>) as the line terminator, irrespective of external
600 representation. On many operating systems, the native text file
601 representation matches the internal representation, but on some
602 platforms the external representation of C<\n> is made up of more than
605 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
606 a single character to end each line in the external representation of text
607 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
608 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
609 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
610 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
611 two characters C<\cM\cJ>. That means that if you don't use binmode() on
612 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
613 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
614 output. This is what you want for text files, but it can be disastrous for
617 Another consequence of using binmode() (on some systems) is that
618 special end-of-file markers will be seen as part of the data stream.
619 For systems from the Microsoft family this means that, if your binary
620 data contain C<\cZ>, the I/O subsystem will regard it as the end of
621 the file, unless you use binmode().
623 binmode() is important not only for readline() and print() operations,
624 but also when using read(), seek(), sysread(), syswrite() and tell()
625 (see L<perlport> for more details). See the C<$/> and C<$\> variables
626 in L<perlvar> for how to manually set your input and output
627 line-termination sequences.
629 Portability issues: L<perlport/binmode>.
631 =item bless REF,CLASSNAME
636 This function tells the thingy referenced by REF that it is now an object
637 in the CLASSNAME package. If CLASSNAME is omitted, the current package
638 is used. Because a C<bless> is often the last thing in a constructor,
639 it returns the reference for convenience. Always use the two-argument
640 version if a derived class might inherit the function doing the blessing.
641 SeeL<perlobj> for more about the blessing (and blessings) of objects.
643 Consider always blessing objects in CLASSNAMEs that are mixed case.
644 Namespaces with all lowercase names are considered reserved for
645 Perl pragmata. Builtin types have all uppercase names. To prevent
646 confusion, you may wish to avoid such package names as well. Make sure
647 that CLASSNAME is a true value.
649 See L<perlmod/"Perl Modules">.
653 Break out of a C<given()> block.
655 This keyword is enabled by the C<"switch"> feature: see
656 L<feature> for more information. You can also access it by
657 prefixing it with C<CORE::>. Alternately, include a C<use
658 v5.10> or later to the current scope.
661 X<caller> X<call stack> X<stack> X<stack trace>
665 Returns the context of the current subroutine call. In scalar context,
666 returns the caller's package name if there I<is> a caller (that is, if
667 we're in a subroutine or C<eval> or C<require>) and the undefined value
668 otherwise. In list context, returns
671 ($package, $filename, $line) = caller;
673 With EXPR, it returns some extra information that the debugger uses to
674 print a stack trace. The value of EXPR indicates how many call frames
675 to go back before the current one.
678 ($package, $filename, $line, $subroutine, $hasargs,
681 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
684 Here $subroutine may be C<(eval)> if the frame is not a subroutine
685 call, but an C<eval>. In such a case additional elements $evaltext and
686 C<$is_require> are set: C<$is_require> is true if the frame is created by a
687 C<require> or C<use> statement, $evaltext contains the text of the
688 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
689 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
690 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
691 frame.) $subroutine may also be C<(unknown)> if this particular
692 subroutine happens to have been deleted from the symbol table.
693 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
694 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
695 compiled with. The C<$hints> and C<$bitmask> values are subject to change
696 between versions of Perl, and are not meant for external use.
698 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
699 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
700 of this hash, as they are the actual values stored in the optree.
702 Furthermore, when called from within the DB package, caller returns more
703 detailed information: it sets the list variable C<@DB::args> to be the
704 arguments with which the subroutine was invoked.
706 Be aware that the optimizer might have optimized call frames away before
707 C<caller> had a chance to get the information. That means that C<caller(N)>
708 might not return information about the call frame you expect it to, for
709 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
710 previous time C<caller> was called.
712 Be aware that setting C<@DB::args> is I<best effort>, intended for
713 debugging or generating backtraces, and should not be relied upon. In
714 particular, as C<@_> contains aliases to the caller's arguments, Perl does
715 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
716 subroutine makes to C<@_> or its contents, not the original values at call
717 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
718 elements, so under certain cases its elements may have become freed and
719 reallocated for other variables or temporary values. Finally, a side effect
720 of the current implementation is that the effects of C<shift @_> can
721 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
722 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
723 elements), so C<@DB::args> is actually a hybrid of the current state and
724 initial state of C<@_>. Buyer beware.
731 =item chdir FILEHANDLE
733 =item chdir DIRHANDLE
737 Changes the working directory to EXPR, if possible. If EXPR is omitted,
738 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
739 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
740 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
741 neither is set, C<chdir> does nothing. It returns true on success,
742 false otherwise. See the example under C<die>.
744 On systems that support fchdir(2), you may pass a filehandle or
745 directory handle as the argument. On systems that don't support fchdir(2),
746 passing handles raises an exception.
749 X<chmod> X<permission> X<mode>
751 Changes the permissions of a list of files. The first element of the
752 list must be the numeric mode, which should probably be an octal
753 number, and which definitely should I<not> be a string of octal digits:
754 C<0644> is okay, but C<"0644"> is not. Returns the number of files
755 successfully changed. See also L</oct> if all you have is a string.
757 $cnt = chmod 0755, "foo", "bar";
758 chmod 0755, @executables;
759 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
761 $mode = "0644"; chmod oct($mode), "foo"; # this is better
762 $mode = 0644; chmod $mode, "foo"; # this is best
764 On systems that support fchmod(2), you may pass filehandles among the
765 files. On systems that don't support fchmod(2), passing filehandles raises
766 an exception. Filehandles must be passed as globs or glob references to be
767 recognized; barewords are considered filenames.
769 open(my $fh, "<", "foo");
770 my $perm = (stat $fh)[2] & 07777;
771 chmod($perm | 0600, $fh);
773 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
776 use Fcntl qw( :mode );
777 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
778 # Identical to the chmod 0755 of the example above.
780 Portability issues: L<perlport/chmod>.
783 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
789 This safer version of L</chop> removes any trailing string
790 that corresponds to the current value of C<$/> (also known as
791 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
792 number of characters removed from all its arguments. It's often used to
793 remove the newline from the end of an input record when you're worried
794 that the final record may be missing its newline. When in paragraph
795 mode (C<$/ = "">), it removes all trailing newlines from the string.
796 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
797 a reference to an integer or the like; see L<perlvar>) chomp() won't
799 If VARIABLE is omitted, it chomps C<$_>. Example:
802 chomp; # avoid \n on last field
807 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
809 You can actually chomp anything that's an lvalue, including an assignment:
812 chomp($answer = <STDIN>);
814 If you chomp a list, each element is chomped, and the total number of
815 characters removed is returned.
817 Note that parentheses are necessary when you're chomping anything
818 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
819 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
820 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
821 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
831 Chops off the last character of a string and returns the character
832 chopped. It is much more efficient than C<s/.$//s> because it neither
833 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
834 If VARIABLE is a hash, it chops the hash's values, but not its keys.
836 You can actually chop anything that's an lvalue, including an assignment.
838 If you chop a list, each element is chopped. Only the value of the
839 last C<chop> is returned.
841 Note that C<chop> returns the last character. To return all but the last
842 character, use C<substr($string, 0, -1)>.
847 X<chown> X<owner> X<user> X<group>
849 Changes the owner (and group) of a list of files. The first two
850 elements of the list must be the I<numeric> uid and gid, in that
851 order. A value of -1 in either position is interpreted by most
852 systems to leave that value unchanged. Returns the number of files
853 successfully changed.
855 $cnt = chown $uid, $gid, 'foo', 'bar';
856 chown $uid, $gid, @filenames;
858 On systems that support fchown(2), you may pass filehandles among the
859 files. On systems that don't support fchown(2), passing filehandles raises
860 an exception. Filehandles must be passed as globs or glob references to be
861 recognized; barewords are considered filenames.
863 Here's an example that looks up nonnumeric uids in the passwd file:
866 chomp($user = <STDIN>);
868 chomp($pattern = <STDIN>);
870 ($login,$pass,$uid,$gid) = getpwnam($user)
871 or die "$user not in passwd file";
873 @ary = glob($pattern); # expand filenames
874 chown $uid, $gid, @ary;
876 On most systems, you are not allowed to change the ownership of the
877 file unless you're the superuser, although you should be able to change
878 the group to any of your secondary groups. On insecure systems, these
879 restrictions may be relaxed, but this is not a portable assumption.
880 On POSIX systems, you can detect this condition this way:
882 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
883 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
885 Portability issues: L<perlport/chmod>.
888 X<chr> X<character> X<ASCII> X<Unicode>
892 Returns the character represented by that NUMBER in the character set.
893 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
894 chr(0x263a) is a Unicode smiley face.
896 Negative values give the Unicode replacement character (chr(0xfffd)),
897 except under the L<bytes> pragma, where the low eight bits of the value
898 (truncated to an integer) are used.
900 If NUMBER is omitted, uses C<$_>.
902 For the reverse, use L</ord>.
904 Note that characters from 128 to 255 (inclusive) are by default
905 internally not encoded as UTF-8 for backward compatibility reasons.
907 See L<perlunicode> for more about Unicode.
909 =item chroot FILENAME
914 This function works like the system call by the same name: it makes the
915 named directory the new root directory for all further pathnames that
916 begin with a C</> by your process and all its children. (It doesn't
917 change your current working directory, which is unaffected.) For security
918 reasons, this call is restricted to the superuser. If FILENAME is
919 omitted, does a C<chroot> to C<$_>.
921 Portability issues: L<perlport/chroot>.
923 =item close FILEHANDLE
928 Closes the file or pipe associated with the filehandle, flushes the IO
929 buffers, and closes the system file descriptor. Returns true if those
930 operations succeed and if no error was reported by any PerlIO
931 layer. Closes the currently selected filehandle if the argument is
934 You don't have to close FILEHANDLE if you are immediately going to do
935 another C<open> on it, because C<open> closes it for you. (See
936 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
937 counter (C<$.>), while the implicit close done by C<open> does not.
939 If the filehandle came from a piped open, C<close> returns false if one of
940 the other syscalls involved fails or if its program exits with non-zero
941 status. If the only problem was that the program exited non-zero, C<$!>
942 will be set to C<0>. Closing a pipe also waits for the process executing
943 on the pipe to exit--in case you wish to look at the output of the pipe
944 afterwards--and implicitly puts the exit status value of that command into
945 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
947 If there are multiple threads running, C<close> on a filehandle from a
948 piped open returns true without waiting for the child process to terminate,
949 if the filehandle is still open in another thread.
951 Closing the read end of a pipe before the process writing to it at the
952 other end is done writing results in the writer receiving a SIGPIPE. If
953 the other end can't handle that, be sure to read all the data before
958 open(OUTPUT, '|sort >foo') # pipe to sort
959 or die "Can't start sort: $!";
960 #... # print stuff to output
961 close OUTPUT # wait for sort to finish
962 or warn $! ? "Error closing sort pipe: $!"
963 : "Exit status $? from sort";
964 open(INPUT, 'foo') # get sort's results
965 or die "Can't open 'foo' for input: $!";
967 FILEHANDLE may be an expression whose value can be used as an indirect
968 filehandle, usually the real filehandle name or an autovivified handle.
970 =item closedir DIRHANDLE
973 Closes a directory opened by C<opendir> and returns the success of that
976 =item connect SOCKET,NAME
979 Attempts to connect to a remote socket, just like connect(2).
980 Returns true if it succeeded, false otherwise. NAME should be a
981 packed address of the appropriate type for the socket. See the examples in
982 L<perlipc/"Sockets: Client/Server Communication">.
989 When followed by a BLOCK, C<continue> is actually a
990 flow control statement rather than a function. If
991 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
992 C<foreach>), it is always executed just before the conditional is about to
993 be evaluated again, just like the third part of a C<for> loop in C. Thus
994 it can be used to increment a loop variable, even when the loop has been
995 continued via the C<next> statement (which is similar to the C C<continue>
998 C<last>, C<next>, or C<redo> may appear within a C<continue>
999 block; C<last> and C<redo> behave as if they had been executed within
1000 the main block. So will C<next>, but since it will execute a C<continue>
1001 block, it may be more entertaining.
1004 ### redo always comes here
1007 ### next always comes here
1009 # then back the top to re-check EXPR
1011 ### last always comes here
1013 Omitting the C<continue> section is equivalent to using an
1014 empty one, logically enough, so C<next> goes directly back
1015 to check the condition at the top of the loop.
1017 When there is no BLOCK, C<continue> is a function that
1018 falls through the current C<when> or C<default> block instead of iterating
1019 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1020 In Perl 5.14 and earlier, this form of C<continue> was
1021 only available when the C<"switch"> feature was enabled.
1022 See L<feature> and L<perlsyn/"Switch statements"> for more
1026 X<cos> X<cosine> X<acos> X<arccosine>
1030 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1031 takes the cosine of C<$_>.
1033 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1034 function, or use this relation:
1036 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1038 =item crypt PLAINTEXT,SALT
1039 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1040 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1042 Creates a digest string exactly like the crypt(3) function in the C
1043 library (assuming that you actually have a version there that has not
1044 been extirpated as a potential munition).
1046 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1047 into a short string, called a digest, which is returned. The same
1048 PLAINTEXT and SALT will always return the same string, but there is no
1049 (known) way to get the original PLAINTEXT from the hash. Small
1050 changes in the PLAINTEXT or SALT will result in large changes in the
1053 There is no decrypt function. This function isn't all that useful for
1054 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1055 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1056 primarily used to check if two pieces of text are the same without
1057 having to transmit or store the text itself. An example is checking
1058 if a correct password is given. The digest of the password is stored,
1059 not the password itself. The user types in a password that is
1060 crypt()'d with the same salt as the stored digest. If the two digests
1061 match, the password is correct.
1063 When verifying an existing digest string you should use the digest as
1064 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1065 to create the digest is visible as part of the digest. This ensures
1066 crypt() will hash the new string with the same salt as the digest.
1067 This allows your code to work with the standard L<crypt|/crypt> and
1068 with more exotic implementations. In other words, assume
1069 nothing about the returned string itself nor about how many bytes
1072 Traditionally the result is a string of 13 bytes: two first bytes of
1073 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1074 the first eight bytes of PLAINTEXT mattered. But alternative
1075 hashing schemes (like MD5), higher level security schemes (like C2),
1076 and implementations on non-Unix platforms may produce different
1079 When choosing a new salt create a random two character string whose
1080 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1081 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1082 characters is just a recommendation; the characters allowed in
1083 the salt depend solely on your system's crypt library, and Perl can't
1084 restrict what salts C<crypt()> accepts.
1086 Here's an example that makes sure that whoever runs this program knows
1089 $pwd = (getpwuid($<))[1];
1091 system "stty -echo";
1093 chomp($word = <STDIN>);
1097 if (crypt($word, $pwd) ne $pwd) {
1103 Of course, typing in your own password to whoever asks you
1106 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1107 of data, not least of all because you can't get the information
1108 back. Look at the L<Digest> module for more robust algorithms.
1110 If using crypt() on a Unicode string (which I<potentially> has
1111 characters with codepoints above 255), Perl tries to make sense
1112 of the situation by trying to downgrade (a copy of)
1113 the string back to an eight-bit byte string before calling crypt()
1114 (on that copy). If that works, good. If not, crypt() dies with
1115 C<Wide character in crypt>.
1117 Portability issues: L<perlport/crypt>.
1122 [This function has been largely superseded by the C<untie> function.]
1124 Breaks the binding between a DBM file and a hash.
1126 Portability issues: L<perlport/dbmclose>.
1128 =item dbmopen HASH,DBNAME,MASK
1129 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1131 [This function has been largely superseded by the
1132 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1134 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1135 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1136 argument is I<not> a filehandle, even though it looks like one). DBNAME
1137 is the name of the database (without the F<.dir> or F<.pag> extension if
1138 any). If the database does not exist, it is created with protection
1139 specified by MASK (as modified by the C<umask>). If your system supports
1140 only the older DBM functions, you may make only one C<dbmopen> call in your
1141 program. In older versions of Perl, if your system had neither DBM nor
1142 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1145 If you don't have write access to the DBM file, you can only read hash
1146 variables, not set them. If you want to test whether you can write,
1147 either use file tests or try setting a dummy hash entry inside an C<eval>
1150 Note that functions such as C<keys> and C<values> may return huge lists
1151 when used on large DBM files. You may prefer to use the C<each>
1152 function to iterate over large DBM files. Example:
1154 # print out history file offsets
1155 dbmopen(%HIST,'/usr/lib/news/history',0666);
1156 while (($key,$val) = each %HIST) {
1157 print $key, ' = ', unpack('L',$val), "\n";
1161 See also L<AnyDBM_File> for a more general description of the pros and
1162 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1163 rich implementation.
1165 You can control which DBM library you use by loading that library
1166 before you call dbmopen():
1169 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1170 or die "Can't open netscape history file: $!";
1172 Portability issues: L<perlport/dbmopen>.
1176 Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1177 that's always true. Only available after Perl 5.10, and only if the
1178 C<switch> feature has been requested or if the keyword is prefixed with
1179 C<CORE::>. See L</when>.
1182 X<defined> X<undef> X<undefined>
1186 Returns a Boolean value telling whether EXPR has a value other than
1187 the undefined value C<undef>. If EXPR is not present, C<$_> is
1190 Many operations return C<undef> to indicate failure, end of file,
1191 system error, uninitialized variable, and other exceptional
1192 conditions. This function allows you to distinguish C<undef> from
1193 other values. (A simple Boolean test will not distinguish among
1194 C<undef>, zero, the empty string, and C<"0">, which are all equally
1195 false.) Note that since C<undef> is a valid scalar, its presence
1196 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1197 returns C<undef> when its argument is an empty array, I<or> when the
1198 element to return happens to be C<undef>.
1200 You may also use C<defined(&func)> to check whether subroutine C<&func>
1201 has ever been defined. The return value is unaffected by any forward
1202 declarations of C<&func>. A subroutine that is not defined
1203 may still be callable: its package may have an C<AUTOLOAD> method that
1204 makes it spring into existence the first time that it is called; see
1207 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1208 used to report whether memory for that aggregate had ever been
1209 allocated. This behavior may disappear in future versions of Perl.
1210 You should instead use a simple test for size:
1212 if (@an_array) { print "has array elements\n" }
1213 if (%a_hash) { print "has hash members\n" }
1215 When used on a hash element, it tells you whether the value is defined,
1216 not whether the key exists in the hash. Use L</exists> for the latter
1221 print if defined $switch{D};
1222 print "$val\n" while defined($val = pop(@ary));
1223 die "Can't readlink $sym: $!"
1224 unless defined($value = readlink $sym);
1225 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1226 $debugging = 0 unless defined $debugging;
1228 Note: Many folks tend to overuse C<defined> and are then surprised to
1229 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1230 defined values. For example, if you say
1234 The pattern match succeeds and C<$1> is defined, although it
1235 matched "nothing". It didn't really fail to match anything. Rather, it
1236 matched something that happened to be zero characters long. This is all
1237 very above-board and honest. When a function returns an undefined value,
1238 it's an admission that it couldn't give you an honest answer. So you
1239 should use C<defined> only when questioning the integrity of what
1240 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1243 See also L</undef>, L</exists>, L</ref>.
1248 Given an expression that specifies an element or slice of a hash, C<delete>
1249 deletes the specified elements from that hash so that exists() on that element
1250 no longer returns true. Setting a hash element to the undefined value does
1251 not remove its key, but deleting it does; see L</exists>.
1253 In list context, returns the value or values deleted, or the last such
1254 element in scalar context. The return list's length always matches that of
1255 the argument list: deleting non-existent elements returns the undefined value
1256 in their corresponding positions.
1258 delete() may also be used on arrays and array slices, but its behavior is less
1259 straightforward. Although exists() will return false for deleted entries,
1260 deleting array elements never changes indices of existing values; use shift()
1261 or splice() for that. However, if all deleted elements fall at the end of an
1262 array, the array's size shrinks to the position of the highest element that
1263 still tests true for exists(), or to 0 if none do.
1265 B<WARNING:> Calling delete on array values is deprecated and likely to
1266 be removed in a future version of Perl.
1268 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1269 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1270 or array may not necessarily return anything; it depends on the implementation
1271 of the C<tied> package's DELETE method, which may do whatever it pleases.
1273 The C<delete local EXPR> construct localizes the deletion to the current
1274 block at run time. Until the block exits, elements locally deleted
1275 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1276 of composite types">.
1278 %hash = (foo => 11, bar => 22, baz => 33);
1279 $scalar = delete $hash{foo}; # $scalar is 11
1280 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1281 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1283 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1285 foreach $key (keys %HASH) {
1289 foreach $index (0 .. $#ARRAY) {
1290 delete $ARRAY[$index];
1295 delete @HASH{keys %HASH};
1297 delete @ARRAY[0 .. $#ARRAY];
1299 But both are slower than assigning the empty list
1300 or undefining %HASH or @ARRAY, which is the customary
1301 way to empty out an aggregate:
1303 %HASH = (); # completely empty %HASH
1304 undef %HASH; # forget %HASH ever existed
1306 @ARRAY = (); # completely empty @ARRAY
1307 undef @ARRAY; # forget @ARRAY ever existed
1309 The EXPR can be arbitrarily complicated provided its
1310 final operation is an element or slice of an aggregate:
1312 delete $ref->[$x][$y]{$key};
1313 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1315 delete $ref->[$x][$y][$index];
1316 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1319 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1321 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1322 into C<$@> and the C<eval> is terminated with the undefined value.
1323 If the exception is outside of all enclosing C<eval>s, then the uncaught
1324 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1325 need to exit the process with a specific exit code, see L</exit>.
1327 Equivalent examples:
1329 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1330 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1332 If the last element of LIST does not end in a newline, the current
1333 script line number and input line number (if any) are also printed,
1334 and a newline is supplied. Note that the "input line number" (also
1335 known as "chunk") is subject to whatever notion of "line" happens to
1336 be currently in effect, and is also available as the special variable
1337 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1339 Hint: sometimes appending C<", stopped"> to your message will cause it
1340 to make better sense when the string C<"at foo line 123"> is appended.
1341 Suppose you are running script "canasta".
1343 die "/etc/games is no good";
1344 die "/etc/games is no good, stopped";
1346 produce, respectively
1348 /etc/games is no good at canasta line 123.
1349 /etc/games is no good, stopped at canasta line 123.
1351 If the output is empty and C<$@> already contains a value (typically from a
1352 previous eval) that value is reused after appending C<"\t...propagated">.
1353 This is useful for propagating exceptions:
1356 die unless $@ =~ /Expected exception/;
1358 If the output is empty and C<$@> contains an object reference that has a
1359 C<PROPAGATE> method, that method will be called with additional file
1360 and line number parameters. The return value replaces the value in
1361 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1364 If C<$@> is empty then the string C<"Died"> is used.
1366 If an uncaught exception results in interpreter exit, the exit code is
1367 determined from the values of C<$!> and C<$?> with this pseudocode:
1369 exit $! if $!; # errno
1370 exit $? >> 8 if $? >> 8; # child exit status
1371 exit 255; # last resort
1373 The intent is to squeeze as much possible information about the likely cause
1374 into the limited space of the system exit code. However, as C<$!> is the value
1375 of C's C<errno>, which can be set by any system call, this means that the value
1376 of the exit code used by C<die> can be non-predictable, so should not be relied
1377 upon, other than to be non-zero.
1379 You can also call C<die> with a reference argument, and if this is trapped
1380 within an C<eval>, C<$@> contains that reference. This permits more
1381 elaborate exception handling using objects that maintain arbitrary state
1382 about the exception. Such a scheme is sometimes preferable to matching
1383 particular string values of C<$@> with regular expressions. Because C<$@>
1384 is a global variable and C<eval> may be used within object implementations,
1385 be careful that analyzing the error object doesn't replace the reference in
1386 the global variable. It's easiest to make a local copy of the reference
1387 before any manipulations. Here's an example:
1389 use Scalar::Util "blessed";
1391 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1392 if (my $ev_err = $@) {
1393 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1394 # handle Some::Module::Exception
1397 # handle all other possible exceptions
1401 Because Perl stringifies uncaught exception messages before display,
1402 you'll probably want to overload stringification operations on
1403 exception objects. See L<overload> for details about that.
1405 You can arrange for a callback to be run just before the C<die>
1406 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1407 handler is called with the error text and can change the error
1408 message, if it sees fit, by calling C<die> again. See
1409 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1410 L<"eval BLOCK"> for some examples. Although this feature was
1411 to be run only right before your program was to exit, this is not
1412 currently so: the C<$SIG{__DIE__}> hook is currently called
1413 even inside eval()ed blocks/strings! If one wants the hook to do
1414 nothing in such situations, put
1418 as the first line of the handler (see L<perlvar/$^S>). Because
1419 this promotes strange action at a distance, this counterintuitive
1420 behavior may be fixed in a future release.
1422 See also exit(), warn(), and the Carp module.
1427 Not really a function. Returns the value of the last command in the
1428 sequence of commands indicated by BLOCK. When modified by the C<while> or
1429 C<until> loop modifier, executes the BLOCK once before testing the loop
1430 condition. (On other statements the loop modifiers test the conditional
1433 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1434 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1435 See L<perlsyn> for alternative strategies.
1437 =item do SUBROUTINE(LIST)
1440 This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1441 a scalar variable or a subroutine beginning with C<&>.
1446 Uses the value of EXPR as a filename and executes the contents of the
1447 file as a Perl script.
1455 except that it's more efficient and concise, keeps track of the current
1456 filename for error messages, searches the C<@INC> directories, and updates
1457 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1458 these variables. It also differs in that code evaluated with C<do FILENAME>
1459 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1460 same, however, in that it does reparse the file every time you call it,
1461 so you probably don't want to do this inside a loop.
1463 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1464 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1465 and sets C<$!> to the error. Always check C<$@> first, as compilation
1466 could fail in a way that also sets C<$!>. If the file is successfully
1467 compiled, C<do> returns the value of the last expression evaluated.
1469 Inclusion of library modules is better done with the
1470 C<use> and C<require> operators, which also do automatic error checking
1471 and raise an exception if there's a problem.
1473 You might like to use C<do> to read in a program configuration
1474 file. Manual error checking can be done this way:
1476 # read in config files: system first, then user
1477 for $file ("/share/prog/defaults.rc",
1478 "$ENV{HOME}/.someprogrc")
1480 unless ($return = do $file) {
1481 warn "couldn't parse $file: $@" if $@;
1482 warn "couldn't do $file: $!" unless defined $return;
1483 warn "couldn't run $file" unless $return;
1488 X<dump> X<core> X<undump>
1492 This function causes an immediate core dump. See also the B<-u>
1493 command-line switch in L<perlrun>, which does the same thing.
1494 Primarily this is so that you can use the B<undump> program (not
1495 supplied) to turn your core dump into an executable binary after
1496 having initialized all your variables at the beginning of the
1497 program. When the new binary is executed it will begin by executing
1498 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1499 Think of it as a goto with an intervening core dump and reincarnation.
1500 If C<LABEL> is omitted, restarts the program from the top.
1502 B<WARNING>: Any files opened at the time of the dump will I<not>
1503 be open any more when the program is reincarnated, with possible
1504 resulting confusion by Perl.
1506 This function is now largely obsolete, mostly because it's very hard to
1507 convert a core file into an executable. That's why you should now invoke
1508 it as C<CORE::dump()>, if you don't want to be warned against a possible
1511 Portability issues: L<perlport/dump>.
1514 X<each> X<hash, iterator>
1521 When called in list context, returns a 2-element list consisting of the key
1522 and value for the next element of a hash, or the index and value for the
1523 next element of an array, so that you can iterate over it. When called in
1524 scalar context, returns only the key (not the value) in a hash, or the index
1527 Hash entries are returned in an apparently random order. The actual random
1528 order is subject to change in future versions of Perl, but it is
1529 guaranteed to be in the same order as either the C<keys> or C<values>
1530 function would produce on the same (unmodified) hash. Since Perl
1531 5.8.2 the ordering can be different even between different runs of Perl
1532 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1534 After C<each> has returned all entries from the hash or array, the next
1535 call to C<each> returns the empty list in list context and C<undef> in
1536 scalar context. The next call following that one restarts iteration. Each
1537 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1538 and C<values>. The iterator is implicitly reset when C<each> has reached
1539 the end as just described; it can be explicitly reset by calling C<keys> or
1540 C<values> on the hash or array. If you add or delete a hash's elements
1541 while iterating over it, entries may be skipped or duplicated--so don't do
1542 that. Exception: It is always safe to delete the item most recently
1543 returned by C<each()>, so the following code works properly:
1545 while (($key, $value) = each %hash) {
1547 delete $hash{$key}; # This is safe
1550 This prints out your environment like the printenv(1) program,
1551 but in a different order:
1553 while (($key,$value) = each %ENV) {
1554 print "$key=$value\n";
1557 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1558 reference to an unblessed hash or array. The argument will be dereferenced
1559 automatically. This aspect of C<each> is considered highly experimental.
1560 The exact behaviour may change in a future version of Perl.
1562 while (($key,$value) = each $hashref) { ... }
1564 See also C<keys>, C<values>, and C<sort>.
1566 =item eof FILEHANDLE
1575 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1576 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1577 gives the real filehandle. (Note that this function actually
1578 reads a character and then C<ungetc>s it, so isn't useful in an
1579 interactive context.) Do not read from a terminal file (or call
1580 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1581 as terminals may lose the end-of-file condition if you do.
1583 An C<eof> without an argument uses the last file read. Using C<eof()>
1584 with empty parentheses is different. It refers to the pseudo file
1585 formed from the files listed on the command line and accessed via the
1586 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1587 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1588 used will cause C<@ARGV> to be examined to determine if input is
1589 available. Similarly, an C<eof()> after C<< <> >> has returned
1590 end-of-file will assume you are processing another C<@ARGV> list,
1591 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1592 see L<perlop/"I/O Operators">.
1594 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1595 detect the end of each file, whereas C<eof()> will detect the end
1596 of the very last file only. Examples:
1598 # reset line numbering on each input file
1600 next if /^\s*#/; # skip comments
1603 close ARGV if eof; # Not eof()!
1606 # insert dashes just before last line of last file
1608 if (eof()) { # check for end of last file
1609 print "--------------\n";
1612 last if eof(); # needed if we're reading from a terminal
1615 Practical hint: you almost never need to use C<eof> in Perl, because the
1616 input operators typically return C<undef> when they run out of data or
1620 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1621 X<error, handling> X<exception, handling>
1627 In the first form, the return value of EXPR is parsed and executed as if it
1628 were a little Perl program. The value of the expression (which is itself
1629 determined within scalar context) is first parsed, and if there were no
1630 errors, executed as a block within the lexical context of the current Perl
1631 program. This means, that in particular, any outer lexical variables are
1632 visible to it, and any package variable settings or subroutine and format
1633 definitions remain afterwards.
1635 Note that the value is parsed every time the C<eval> executes.
1636 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1637 delay parsing and subsequent execution of the text of EXPR until run time.
1639 If the C<unicode_eval> feature is enabled (which is the default under a
1640 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1641 characters, so C<use utf8> declarations have no effect, and source filters
1642 are forbidden. In the absence of the C<unicode_eval> feature, the string
1643 will sometimes be treated as characters and sometimes as bytes, depending
1644 on the internal encoding, and source filters activated within the C<eval>
1645 exhibit the erratic, but historical, behaviour of affecting some outer file
1646 scope that is still compiling. See also the L</evalbytes> keyword, which
1647 always treats its input as a byte stream and works properly with source
1648 filters, and the L<feature> pragma.
1650 In the second form, the code within the BLOCK is parsed only once--at the
1651 same time the code surrounding the C<eval> itself was parsed--and executed
1652 within the context of the current Perl program. This form is typically
1653 used to trap exceptions more efficiently than the first (see below), while
1654 also providing the benefit of checking the code within BLOCK at compile
1657 The final semicolon, if any, may be omitted from the value of EXPR or within
1660 In both forms, the value returned is the value of the last expression
1661 evaluated inside the mini-program; a return statement may be also used, just
1662 as with subroutines. The expression providing the return value is evaluated
1663 in void, scalar, or list context, depending on the context of the C<eval>
1664 itself. See L</wantarray> for more on how the evaluation context can be
1667 If there is a syntax error or runtime error, or a C<die> statement is
1668 executed, C<eval> returns C<undef> in scalar context
1669 or an empty list--or, for syntax errors, a list containing a single
1670 undefined value--in list context, and C<$@> is set to the error
1671 message. The discrepancy in the return values in list context is
1672 considered a bug by some, and will probably be fixed in a future
1673 release. If there was no error, C<$@> is set to the empty string. A
1674 control flow operator like C<last> or C<goto> can bypass the setting of
1675 C<$@>. Beware that using C<eval> neither silences Perl from printing
1676 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1677 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1678 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1679 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1681 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1682 determining whether a particular feature (such as C<socket> or C<symlink>)
1683 is implemented. It is also Perl's exception-trapping mechanism, where
1684 the die operator is used to raise exceptions.
1686 If you want to trap errors when loading an XS module, some problems with
1687 the binary interface (such as Perl version skew) may be fatal even with
1688 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1690 If the code to be executed doesn't vary, you may use the eval-BLOCK
1691 form to trap run-time errors without incurring the penalty of
1692 recompiling each time. The error, if any, is still returned in C<$@>.
1695 # make divide-by-zero nonfatal
1696 eval { $answer = $a / $b; }; warn $@ if $@;
1698 # same thing, but less efficient
1699 eval '$answer = $a / $b'; warn $@ if $@;
1701 # a compile-time error
1702 eval { $answer = }; # WRONG
1705 eval '$answer ='; # sets $@
1707 Using the C<eval{}> form as an exception trap in libraries does have some
1708 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1709 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1710 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1711 as this example shows:
1713 # a private exception trap for divide-by-zero
1714 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1717 This is especially significant, given that C<__DIE__> hooks can call
1718 C<die> again, which has the effect of changing their error messages:
1720 # __DIE__ hooks may modify error messages
1722 local $SIG{'__DIE__'} =
1723 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1724 eval { die "foo lives here" };
1725 print $@ if $@; # prints "bar lives here"
1728 Because this promotes action at a distance, this counterintuitive behavior
1729 may be fixed in a future release.
1731 With an C<eval>, you should be especially careful to remember what's
1732 being looked at when:
1738 eval { $x }; # CASE 4
1740 eval "\$$x++"; # CASE 5
1743 Cases 1 and 2 above behave identically: they run the code contained in
1744 the variable $x. (Although case 2 has misleading double quotes making
1745 the reader wonder what else might be happening (nothing is).) Cases 3
1746 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1747 does nothing but return the value of $x. (Case 4 is preferred for
1748 purely visual reasons, but it also has the advantage of compiling at
1749 compile-time instead of at run-time.) Case 5 is a place where
1750 normally you I<would> like to use double quotes, except that in this
1751 particular situation, you can just use symbolic references instead, as
1754 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1755 of localised variables, which means that for your code to run on older
1756 versions, a temporary is required if you want to mask some but not all
1759 # alter $@ on nefarious repugnancy only
1763 local $@; # protect existing $@
1764 eval { test_repugnancy() };
1765 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1766 $@ =~ /nefarious/ and $e = $@;
1768 die $e if defined $e
1771 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1772 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1774 An C<eval ''> executed within the C<DB> package doesn't see the usual
1775 surrounding lexical scope, but rather the scope of the first non-DB piece
1776 of code that called it. You don't normally need to worry about this unless
1777 you are writing a Perl debugger.
1779 =item evalbytes EXPR
1784 This function is like L</eval> with a string argument, except it always
1785 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1786 string containing characters whose ordinal value exceeds 255 results in an
1787 error. Source filters activated within the evaluated code apply to the
1790 This function is only available under the C<evalbytes> feature, a
1791 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1792 L<feature> for more information.
1797 =item exec PROGRAM LIST
1799 The C<exec> function executes a system command I<and never returns>;
1800 use C<system> instead of C<exec> if you want it to return. It fails and
1801 returns false only if the command does not exist I<and> it is executed
1802 directly instead of via your system's command shell (see below).
1804 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1805 warns you if there is a following statement that isn't C<die>, C<warn>,
1806 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1807 I<really> want to follow an C<exec> with some other statement, you
1808 can use one of these styles to avoid the warning:
1810 exec ('foo') or print STDERR "couldn't exec foo: $!";
1811 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1813 If there is more than one argument in LIST, or if LIST is an array
1814 with more than one value, calls execvp(3) with the arguments in LIST.
1815 If there is only one scalar argument or an array with one element in it,
1816 the argument is checked for shell metacharacters, and if there are any,
1817 the entire argument is passed to the system's command shell for parsing
1818 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1819 If there are no shell metacharacters in the argument, it is split into
1820 words and passed directly to C<execvp>, which is more efficient.
1823 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1824 exec "sort $outfile | uniq";
1826 If you don't really want to execute the first argument, but want to lie
1827 to the program you are executing about its own name, you can specify
1828 the program you actually want to run as an "indirect object" (without a
1829 comma) in front of the LIST. (This always forces interpretation of the
1830 LIST as a multivalued list, even if there is only a single scalar in
1833 $shell = '/bin/csh';
1834 exec $shell '-sh'; # pretend it's a login shell
1838 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1840 When the arguments get executed via the system shell, results are
1841 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1844 Using an indirect object with C<exec> or C<system> is also more
1845 secure. This usage (which also works fine with system()) forces
1846 interpretation of the arguments as a multivalued list, even if the
1847 list had just one argument. That way you're safe from the shell
1848 expanding wildcards or splitting up words with whitespace in them.
1850 @args = ( "echo surprise" );
1852 exec @args; # subject to shell escapes
1854 exec { $args[0] } @args; # safe even with one-arg list
1856 The first version, the one without the indirect object, ran the I<echo>
1857 program, passing it C<"surprise"> an argument. The second version didn't;
1858 it tried to run a program named I<"echo surprise">, didn't find it, and set
1859 C<$?> to a non-zero value indicating failure.
1861 Beginning with v5.6.0, Perl attempts to flush all files opened for
1862 output before the exec, but this may not be supported on some platforms
1863 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1864 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1865 open handles to avoid lost output.
1867 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1868 C<DESTROY> methods on your objects.
1870 Portability issues: L<perlport/exec>.
1873 X<exists> X<autovivification>
1875 Given an expression that specifies an element of a hash, returns true if the
1876 specified element in the hash has ever been initialized, even if the
1877 corresponding value is undefined.
1879 print "Exists\n" if exists $hash{$key};
1880 print "Defined\n" if defined $hash{$key};
1881 print "True\n" if $hash{$key};
1883 exists may also be called on array elements, but its behavior is much less
1884 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1885 that calling exists on array values is deprecated and likely to be removed in
1886 a future version of Perl.
1888 print "Exists\n" if exists $array[$index];
1889 print "Defined\n" if defined $array[$index];
1890 print "True\n" if $array[$index];
1892 A hash or array element can be true only if it's defined and defined only if
1893 it exists, but the reverse doesn't necessarily hold true.
1895 Given an expression that specifies the name of a subroutine,
1896 returns true if the specified subroutine has ever been declared, even
1897 if it is undefined. Mentioning a subroutine name for exists or defined
1898 does not count as declaring it. Note that a subroutine that does not
1899 exist may still be callable: its package may have an C<AUTOLOAD>
1900 method that makes it spring into existence the first time that it is
1901 called; see L<perlsub>.
1903 print "Exists\n" if exists &subroutine;
1904 print "Defined\n" if defined &subroutine;
1906 Note that the EXPR can be arbitrarily complicated as long as the final
1907 operation is a hash or array key lookup or subroutine name:
1909 if (exists $ref->{A}->{B}->{$key}) { }
1910 if (exists $hash{A}{B}{$key}) { }
1912 if (exists $ref->{A}->{B}->[$ix]) { }
1913 if (exists $hash{A}{B}[$ix]) { }
1915 if (exists &{$ref->{A}{B}{$key}}) { }
1917 Although the mostly deeply nested array or hash will not spring into
1918 existence just because its existence was tested, any intervening ones will.
1919 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1920 into existence due to the existence test for the $key element above.
1921 This happens anywhere the arrow operator is used, including even here:
1924 if (exists $ref->{"Some key"}) { }
1925 print $ref; # prints HASH(0x80d3d5c)
1927 This surprising autovivification in what does not at first--or even
1928 second--glance appear to be an lvalue context may be fixed in a future
1931 Use of a subroutine call, rather than a subroutine name, as an argument
1932 to exists() is an error.
1935 exists &sub(); # Error
1938 X<exit> X<terminate> X<abort>
1942 Evaluates EXPR and exits immediately with that value. Example:
1945 exit 0 if $ans =~ /^[Xx]/;
1947 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1948 universally recognized values for EXPR are C<0> for success and C<1>
1949 for error; other values are subject to interpretation depending on the
1950 environment in which the Perl program is running. For example, exiting
1951 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1952 the mailer to return the item undelivered, but that's not true everywhere.
1954 Don't use C<exit> to abort a subroutine if there's any chance that
1955 someone might want to trap whatever error happened. Use C<die> instead,
1956 which can be trapped by an C<eval>.
1958 The exit() function does not always exit immediately. It calls any
1959 defined C<END> routines first, but these C<END> routines may not
1960 themselves abort the exit. Likewise any object destructors that need to
1961 be called are called before the real exit. C<END> routines and destructors
1962 can change the exit status by modifying C<$?>. If this is a problem, you
1963 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1964 See L<perlmod> for details.
1966 Portability issues: L<perlport/exit>.
1969 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1973 Returns I<e> (the natural logarithm base) to the power of EXPR.
1974 If EXPR is omitted, gives C<exp($_)>.
1976 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1979 Implements the fcntl(2) function. You'll probably have to say
1983 first to get the correct constant definitions. Argument processing and
1984 value returned work just like C<ioctl> below.
1988 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1989 or die "can't fcntl F_GETFL: $!";
1991 You don't have to check for C<defined> on the return from C<fcntl>.
1992 Like C<ioctl>, it maps a C<0> return from the system call into
1993 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1994 in numeric context. It is also exempt from the normal B<-w> warnings
1995 on improper numeric conversions.
1997 Note that C<fcntl> raises an exception if used on a machine that
1998 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1999 manpage to learn what functions are available on your system.
2001 Here's an example of setting a filehandle named C<REMOTE> to be
2002 non-blocking at the system level. You'll have to negotiate C<$|>
2003 on your own, though.
2005 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2007 $flags = fcntl(REMOTE, F_GETFL, 0)
2008 or die "Can't get flags for the socket: $!\n";
2010 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2011 or die "Can't set flags for the socket: $!\n";
2013 Portability issues: L<perlport/fcntl>.
2018 A special token that returns the name of the file in which it occurs.
2020 =item fileno FILEHANDLE
2023 Returns the file descriptor for a filehandle, or undefined if the
2024 filehandle is not open. If there is no real file descriptor at the OS
2025 level, as can happen with filehandles connected to memory objects via
2026 C<open> with a reference for the third argument, -1 is returned.
2028 This is mainly useful for constructing
2029 bitmaps for C<select> and low-level POSIX tty-handling operations.
2030 If FILEHANDLE is an expression, the value is taken as an indirect
2031 filehandle, generally its name.
2033 You can use this to find out whether two handles refer to the
2034 same underlying descriptor:
2036 if (fileno(THIS) == fileno(THAT)) {
2037 print "THIS and THAT are dups\n";
2040 =item flock FILEHANDLE,OPERATION
2041 X<flock> X<lock> X<locking>
2043 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2044 for success, false on failure. Produces a fatal error if used on a
2045 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2046 C<flock> is Perl's portable file-locking interface, although it locks
2047 entire files only, not records.
2049 Two potentially non-obvious but traditional C<flock> semantics are
2050 that it waits indefinitely until the lock is granted, and that its locks
2051 are B<merely advisory>. Such discretionary locks are more flexible, but
2052 offer fewer guarantees. This means that programs that do not also use
2053 C<flock> may modify files locked with C<flock>. See L<perlport>,
2054 your port's specific documentation, and your system-specific local manpages
2055 for details. It's best to assume traditional behavior if you're writing
2056 portable programs. (But if you're not, you should as always feel perfectly
2057 free to write for your own system's idiosyncrasies (sometimes called
2058 "features"). Slavish adherence to portability concerns shouldn't get
2059 in the way of your getting your job done.)
2061 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2062 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2063 you can use the symbolic names if you import them from the L<Fcntl> module,
2064 either individually, or as a group using the C<:flock> tag. LOCK_SH
2065 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2066 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2067 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2068 waiting for the lock; check the return status to see if you got it.
2070 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2071 before locking or unlocking it.
2073 Note that the emulation built with lockf(3) doesn't provide shared
2074 locks, and it requires that FILEHANDLE be open with write intent. These
2075 are the semantics that lockf(3) implements. Most if not all systems
2076 implement lockf(3) in terms of fcntl(2) locking, though, so the
2077 differing semantics shouldn't bite too many people.
2079 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2080 be open with read intent to use LOCK_SH and requires that it be open
2081 with write intent to use LOCK_EX.
2083 Note also that some versions of C<flock> cannot lock things over the
2084 network; you would need to use the more system-specific C<fcntl> for
2085 that. If you like you can force Perl to ignore your system's flock(2)
2086 function, and so provide its own fcntl(2)-based emulation, by passing
2087 the switch C<-Ud_flock> to the F<Configure> program when you configure
2088 and build a new Perl.
2090 Here's a mailbox appender for BSD systems.
2092 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
2096 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2098 # and, in case someone appended while we were waiting...
2099 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2104 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2107 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2108 or die "Can't open mailbox: $!";
2111 print $mbox $msg,"\n\n";
2114 On systems that support a real flock(2), locks are inherited across fork()
2115 calls, whereas those that must resort to the more capricious fcntl(2)
2116 function lose their locks, making it seriously harder to write servers.
2118 See also L<DB_File> for other flock() examples.
2120 Portability issues: L<perlport/flock>.
2123 X<fork> X<child> X<parent>
2125 Does a fork(2) system call to create a new process running the
2126 same program at the same point. It returns the child pid to the
2127 parent process, C<0> to the child process, or C<undef> if the fork is
2128 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2129 are shared, while everything else is copied. On most systems supporting
2130 fork(), great care has gone into making it extremely efficient (for
2131 example, using copy-on-write technology on data pages), making it the
2132 dominant paradigm for multitasking over the last few decades.
2134 Beginning with v5.6.0, Perl attempts to flush all files opened for
2135 output before forking the child process, but this may not be supported
2136 on some platforms (see L<perlport>). To be safe, you may need to set
2137 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2138 C<IO::Handle> on any open handles to avoid duplicate output.
2140 If you C<fork> without ever waiting on your children, you will
2141 accumulate zombies. On some systems, you can avoid this by setting
2142 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2143 forking and reaping moribund children.
2145 Note that if your forked child inherits system file descriptors like
2146 STDIN and STDOUT that are actually connected by a pipe or socket, even
2147 if you exit, then the remote server (such as, say, a CGI script or a
2148 backgrounded job launched from a remote shell) won't think you're done.
2149 You should reopen those to F</dev/null> if it's any issue.
2151 On some platforms such as Windows, where the fork() system call is not available,
2152 Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2153 at the level of the Perl program, be as compatible as possible with the "Unix" fork().
2154 However it has limitations that have to be considered in code intended to be portable.
2155 See L<perlfork> for more details.
2157 Portability issues: L<perlport/fork>.
2162 Declare a picture format for use by the C<write> function. For
2166 Test: @<<<<<<<< @||||| @>>>>>
2167 $str, $%, '$' . int($num)
2171 $num = $cost/$quantity;
2175 See L<perlform> for many details and examples.
2177 =item formline PICTURE,LIST
2180 This is an internal function used by C<format>s, though you may call it,
2181 too. It formats (see L<perlform>) a list of values according to the
2182 contents of PICTURE, placing the output into the format output
2183 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2184 Eventually, when a C<write> is done, the contents of
2185 C<$^A> are written to some filehandle. You could also read C<$^A>
2186 and then set C<$^A> back to C<"">. Note that a format typically
2187 does one C<formline> per line of form, but the C<formline> function itself
2188 doesn't care how many newlines are embedded in the PICTURE. This means
2189 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2190 You may therefore need to use multiple formlines to implement a single
2191 record format, just like the C<format> compiler.
2193 Be careful if you put double quotes around the picture, because an C<@>
2194 character may be taken to mean the beginning of an array name.
2195 C<formline> always returns true. See L<perlform> for other examples.
2197 If you are trying to use this instead of C<write> to capture the output,
2198 you may find it easier to open a filehandle to a scalar
2199 (C<< open $fh, ">", \$output >>) and write to that instead.
2201 =item getc FILEHANDLE
2202 X<getc> X<getchar> X<character> X<file, read>
2206 Returns the next character from the input file attached to FILEHANDLE,
2207 or the undefined value at end of file or if there was an error (in
2208 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2209 STDIN. This is not particularly efficient. However, it cannot be
2210 used by itself to fetch single characters without waiting for the user
2211 to hit enter. For that, try something more like:
2214 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2217 system "stty", '-icanon', 'eol', "\001";
2223 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2226 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2230 Determination of whether $BSD_STYLE should be set
2231 is left as an exercise to the reader.
2233 The C<POSIX::getattr> function can do this more portably on
2234 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2235 module from your nearest CPAN site; details on CPAN can be found under
2239 X<getlogin> X<login>
2241 This implements the C library function of the same name, which on most
2242 systems returns the current login from F</etc/utmp>, if any. If it
2243 returns the empty string, use C<getpwuid>.
2245 $login = getlogin || getpwuid($<) || "Kilroy";
2247 Do not consider C<getlogin> for authentication: it is not as
2248 secure as C<getpwuid>.
2250 Portability issues: L<perlport/getlogin>.
2252 =item getpeername SOCKET
2253 X<getpeername> X<peer>
2255 Returns the packed sockaddr address of the other end of the SOCKET
2259 $hersockaddr = getpeername(SOCK);
2260 ($port, $iaddr) = sockaddr_in($hersockaddr);
2261 $herhostname = gethostbyaddr($iaddr, AF_INET);
2262 $herstraddr = inet_ntoa($iaddr);
2267 Returns the current process group for the specified PID. Use
2268 a PID of C<0> to get the current process group for the
2269 current process. Will raise an exception if used on a machine that
2270 doesn't implement getpgrp(2). If PID is omitted, returns the process
2271 group of the current process. Note that the POSIX version of C<getpgrp>
2272 does not accept a PID argument, so only C<PID==0> is truly portable.
2274 Portability issues: L<perlport/getpgrp>.
2277 X<getppid> X<parent> X<pid>
2279 Returns the process id of the parent process.
2281 Note for Linux users: on Linux, the C functions C<getpid()> and
2282 C<getppid()> return different values from different threads. In order to
2283 be portable, this behavior is not reflected by the Perl-level function
2284 C<getppid()>, that returns a consistent value across threads. If you want
2285 to call the underlying C<getppid()>, you may use the CPAN module
2288 Portability issues: L<perlport/getppid>.
2290 =item getpriority WHICH,WHO
2291 X<getpriority> X<priority> X<nice>
2293 Returns the current priority for a process, a process group, or a user.
2294 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2295 machine that doesn't implement getpriority(2).
2297 Portability issues: L<perlport/getpriority>.
2300 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2301 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2302 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2303 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2304 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2305 X<endnetent> X<endprotoent> X<endservent>
2309 =item gethostbyname NAME
2311 =item getnetbyname NAME
2313 =item getprotobyname NAME
2319 =item getservbyname NAME,PROTO
2321 =item gethostbyaddr ADDR,ADDRTYPE
2323 =item getnetbyaddr ADDR,ADDRTYPE
2325 =item getprotobynumber NUMBER
2327 =item getservbyport PORT,PROTO
2345 =item sethostent STAYOPEN
2347 =item setnetent STAYOPEN
2349 =item setprotoent STAYOPEN
2351 =item setservent STAYOPEN
2365 These routines are the same as their counterparts in the
2366 system C library. In list context, the return values from the
2367 various get routines are as follows:
2369 ($name,$passwd,$uid,$gid,
2370 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2371 ($name,$passwd,$gid,$members) = getgr*
2372 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2373 ($name,$aliases,$addrtype,$net) = getnet*
2374 ($name,$aliases,$proto) = getproto*
2375 ($name,$aliases,$port,$proto) = getserv*
2377 (If the entry doesn't exist you get an empty list.)
2379 The exact meaning of the $gcos field varies but it usually contains
2380 the real name of the user (as opposed to the login name) and other
2381 information pertaining to the user. Beware, however, that in many
2382 system users are able to change this information and therefore it
2383 cannot be trusted and therefore the $gcos is tainted (see
2384 L<perlsec>). The $passwd and $shell, user's encrypted password and
2385 login shell, are also tainted, for the same reason.
2387 In scalar context, you get the name, unless the function was a
2388 lookup by name, in which case you get the other thing, whatever it is.
2389 (If the entry doesn't exist you get the undefined value.) For example:
2391 $uid = getpwnam($name);
2392 $name = getpwuid($num);
2394 $gid = getgrnam($name);
2395 $name = getgrgid($num);
2399 In I<getpw*()> the fields $quota, $comment, and $expire are special
2400 in that they are unsupported on many systems. If the
2401 $quota is unsupported, it is an empty scalar. If it is supported, it
2402 usually encodes the disk quota. If the $comment field is unsupported,
2403 it is an empty scalar. If it is supported it usually encodes some
2404 administrative comment about the user. In some systems the $quota
2405 field may be $change or $age, fields that have to do with password
2406 aging. In some systems the $comment field may be $class. The $expire
2407 field, if present, encodes the expiration period of the account or the
2408 password. For the availability and the exact meaning of these fields
2409 in your system, please consult getpwnam(3) and your system's
2410 F<pwd.h> file. You can also find out from within Perl what your
2411 $quota and $comment fields mean and whether you have the $expire field
2412 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2413 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2414 files are supported only if your vendor has implemented them in the
2415 intuitive fashion that calling the regular C library routines gets the
2416 shadow versions if you're running under privilege or if there exists
2417 the shadow(3) functions as found in System V (this includes Solaris
2418 and Linux). Those systems that implement a proprietary shadow password
2419 facility are unlikely to be supported.
2421 The $members value returned by I<getgr*()> is a space-separated list of
2422 the login names of the members of the group.
2424 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2425 C, it will be returned to you via C<$?> if the function call fails. The
2426 C<@addrs> value returned by a successful call is a list of raw
2427 addresses returned by the corresponding library call. In the
2428 Internet domain, each address is four bytes long; you can unpack it
2429 by saying something like:
2431 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2433 The Socket library makes this slightly easier:
2436 $iaddr = inet_aton("127.1"); # or whatever address
2437 $name = gethostbyaddr($iaddr, AF_INET);
2439 # or going the other way
2440 $straddr = inet_ntoa($iaddr);
2442 In the opposite way, to resolve a hostname to the IP address
2446 $packed_ip = gethostbyname("www.perl.org");
2447 if (defined $packed_ip) {
2448 $ip_address = inet_ntoa($packed_ip);
2451 Make sure C<gethostbyname()> is called in SCALAR context and that
2452 its return value is checked for definedness.
2454 The C<getprotobynumber> function, even though it only takes one argument,
2455 has the precedence of a list operator, so beware:
2457 getprotobynumber $number eq 'icmp' # WRONG
2458 getprotobynumber($number eq 'icmp') # actually means this
2459 getprotobynumber($number) eq 'icmp' # better this way
2461 If you get tired of remembering which element of the return list
2462 contains which return value, by-name interfaces are provided
2463 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2464 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2465 and C<User::grent>. These override the normal built-ins, supplying
2466 versions that return objects with the appropriate names
2467 for each field. For example:
2471 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2473 Even though it looks as though they're the same method calls (uid),
2474 they aren't, because a C<File::stat> object is different from
2475 a C<User::pwent> object.
2477 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2479 =item getsockname SOCKET
2482 Returns the packed sockaddr address of this end of the SOCKET connection,
2483 in case you don't know the address because you have several different
2484 IPs that the connection might have come in on.
2487 $mysockaddr = getsockname(SOCK);
2488 ($port, $myaddr) = sockaddr_in($mysockaddr);
2489 printf "Connect to %s [%s]\n",
2490 scalar gethostbyaddr($myaddr, AF_INET),
2493 =item getsockopt SOCKET,LEVEL,OPTNAME
2496 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2497 Options may exist at multiple protocol levels depending on the socket
2498 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2499 C<Socket> module) will exist. To query options at another level the
2500 protocol number of the appropriate protocol controlling the option
2501 should be supplied. For example, to indicate that an option is to be
2502 interpreted by the TCP protocol, LEVEL should be set to the protocol
2503 number of TCP, which you can get using C<getprotobyname>.
2505 The function returns a packed string representing the requested socket
2506 option, or C<undef> on error, with the reason for the error placed in
2507 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2508 consult getsockopt(2) for details. A common case is that the option is an
2509 integer, in which case the result is a packed integer, which you can decode
2510 using C<unpack> with the C<i> (or C<I>) format.
2512 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2514 use Socket qw(:all);
2516 defined(my $tcp = getprotobyname("tcp"))
2517 or die "Could not determine the protocol number for tcp";
2518 # my $tcp = IPPROTO_TCP; # Alternative
2519 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2520 or die "getsockopt TCP_NODELAY: $!";
2521 my $nodelay = unpack("I", $packed);
2522 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2524 Portability issues: L<perlport/getsockopt>.
2526 =item given EXPR BLOCK
2531 C<given> is analogous to the C<switch> keyword in other languages. C<given>
2532 and C<when> are used in Perl to implement C<switch>/C<case> like statements.
2533 Only available after Perl 5.10. For example:
2538 print "I like apples."
2541 print "I don't like oranges."
2544 print "I don't like anything"
2548 See L<perlsyn/"Switch statements"> for detailed information.
2551 X<glob> X<wildcard> X<filename, expansion> X<expand>
2555 In list context, returns a (possibly empty) list of filename expansions on
2556 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2557 scalar context, glob iterates through such filename expansions, returning
2558 undef when the list is exhausted. This is the internal function
2559 implementing the C<< <*.c> >> operator, but you can use it directly. If
2560 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2561 more detail in L<perlop/"I/O Operators">.
2563 Note that C<glob> splits its arguments on whitespace and treats
2564 each segment as separate pattern. As such, C<glob("*.c *.h")>
2565 matches all files with a F<.c> or F<.h> extension. The expression
2566 C<glob(".* *")> matches all files in the current working directory.
2567 If you want to glob filenames that might contain whitespace, you'll
2568 have to use extra quotes around the spacey filename to protect it.
2569 For example, to glob filenames that have an C<e> followed by a space
2570 followed by an C<f>, use either of:
2572 @spacies = <"*e f*">;
2573 @spacies = glob '"*e f*"';
2574 @spacies = glob q("*e f*");
2576 If you had to get a variable through, you could do this:
2578 @spacies = glob "'*${var}e f*'";
2579 @spacies = glob qq("*${var}e f*");
2581 If non-empty braces are the only wildcard characters used in the
2582 C<glob>, no filenames are matched, but potentially many strings
2583 are returned. For example, this produces nine strings, one for
2584 each pairing of fruits and colors:
2586 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2588 Beginning with v5.6.0, this operator is implemented using the standard
2589 C<File::Glob> extension. See L<File::Glob> for details, including
2590 C<bsd_glob> which does not treat whitespace as a pattern separator.
2592 Portability issues: L<perlport/glob>.
2595 X<gmtime> X<UTC> X<Greenwich>
2599 Works just like L</localtime> but the returned values are
2600 localized for the standard Greenwich time zone.
2602 Note: When called in list context, $isdst, the last value
2603 returned by gmtime, is always C<0>. There is no
2604 Daylight Saving Time in GMT.
2606 Portability issues: L<perlport/gmtime>.
2609 X<goto> X<jump> X<jmp>
2615 The C<goto-LABEL> form finds the statement labeled with LABEL and
2616 resumes execution there. It can't be used to get out of a block or
2617 subroutine given to C<sort>. It can be used to go almost anywhere
2618 else within the dynamic scope, including out of subroutines, but it's
2619 usually better to use some other construct such as C<last> or C<die>.
2620 The author of Perl has never felt the need to use this form of C<goto>
2621 (in Perl, that is; C is another matter). (The difference is that C
2622 does not offer named loops combined with loop control. Perl does, and
2623 this replaces most structured uses of C<goto> in other languages.)
2625 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2626 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2627 necessarily recommended if you're optimizing for maintainability:
2629 goto ("FOO", "BAR", "GLARCH")[$i];
2631 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2632 function" rule. A pair of parentheses following it does not (necessarily)
2633 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2635 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2636 deprecated and will issue a warning. Even then, it may not be used to
2637 go into any construct that requires initialization, such as a
2638 subroutine or a C<foreach> loop. It also can't be used to go into a
2639 construct that is optimized away.
2641 The C<goto-&NAME> form is quite different from the other forms of
2642 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2643 doesn't have the stigma associated with other gotos. Instead, it
2644 exits the current subroutine (losing any changes set by local()) and
2645 immediately calls in its place the named subroutine using the current
2646 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2647 load another subroutine and then pretend that the other subroutine had
2648 been called in the first place (except that any modifications to C<@_>
2649 in the current subroutine are propagated to the other subroutine.)
2650 After the C<goto>, not even C<caller> will be able to tell that this
2651 routine was called first.
2653 NAME needn't be the name of a subroutine; it can be a scalar variable
2654 containing a code reference or a block that evaluates to a code
2657 =item grep BLOCK LIST
2660 =item grep EXPR,LIST
2662 This is similar in spirit to, but not the same as, grep(1) and its
2663 relatives. In particular, it is not limited to using regular expressions.
2665 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2666 C<$_> to each element) and returns the list value consisting of those
2667 elements for which the expression evaluated to true. In scalar
2668 context, returns the number of times the expression was true.
2670 @foo = grep(!/^#/, @bar); # weed out comments
2674 @foo = grep {!/^#/} @bar; # weed out comments
2676 Note that C<$_> is an alias to the list value, so it can be used to
2677 modify the elements of the LIST. While this is useful and supported,
2678 it can cause bizarre results if the elements of LIST are not variables.
2679 Similarly, grep returns aliases into the original list, much as a for
2680 loop's index variable aliases the list elements. That is, modifying an
2681 element of a list returned by grep (for example, in a C<foreach>, C<map>
2682 or another C<grep>) actually modifies the element in the original list.
2683 This is usually something to be avoided when writing clear code.
2685 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2686 been declared with C<my $_>) then, in addition to being locally aliased to
2687 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2688 can't be seen from the outside, avoiding any potential side-effects.
2690 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2693 X<hex> X<hexadecimal>
2697 Interprets EXPR as a hex string and returns the corresponding value.
2698 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2699 L</oct>.) If EXPR is omitted, uses C<$_>.
2701 print hex '0xAf'; # prints '175'
2702 print hex 'aF'; # same
2704 Hex strings may only represent integers. Strings that would cause
2705 integer overflow trigger a warning. Leading whitespace is not stripped,
2706 unlike oct(). To present something as hex, look into L</printf>,
2707 L</sprintf>, and L</unpack>.
2712 There is no builtin C<import> function. It is just an ordinary
2713 method (subroutine) defined (or inherited) by modules that wish to export
2714 names to another module. The C<use> function calls the C<import> method
2715 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2717 =item index STR,SUBSTR,POSITION
2718 X<index> X<indexOf> X<InStr>
2720 =item index STR,SUBSTR
2722 The index function searches for one string within another, but without
2723 the wildcard-like behavior of a full regular-expression pattern match.
2724 It returns the position of the first occurrence of SUBSTR in STR at
2725 or after POSITION. If POSITION is omitted, starts searching from the
2726 beginning of the string. POSITION before the beginning of the string
2727 or after its end is treated as if it were the beginning or the end,
2728 respectively. POSITION and the return value are based at zero.
2729 If the substring is not found, C<index> returns -1.
2732 X<int> X<integer> X<truncate> X<trunc> X<floor>
2736 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2737 You should not use this function for rounding: one because it truncates
2738 towards C<0>, and two because machine representations of floating-point
2739 numbers can sometimes produce counterintuitive results. For example,
2740 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2741 because it's really more like -268.99999999999994315658 instead. Usually,
2742 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2743 functions will serve you better than will int().
2745 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2748 Implements the ioctl(2) function. You'll probably first have to say
2750 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2752 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2753 exist or doesn't have the correct definitions you'll have to roll your
2754 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2755 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2756 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2757 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2758 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2759 has no string value but does have a numeric value, that value will be
2760 passed rather than a pointer to the string value. To guarantee this to be
2761 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2762 functions may be needed to manipulate the values of structures used by
2765 The return value of C<ioctl> (and C<fcntl>) is as follows:
2767 if OS returns: then Perl returns:
2769 0 string "0 but true"
2770 anything else that number
2772 Thus Perl returns true on success and false on failure, yet you can
2773 still easily determine the actual value returned by the operating
2776 $retval = ioctl(...) || -1;
2777 printf "System returned %d\n", $retval;
2779 The special string C<"0 but true"> is exempt from B<-w> complaints
2780 about improper numeric conversions.
2782 Portability issues: L<perlport/ioctl>.
2784 =item join EXPR,LIST
2787 Joins the separate strings of LIST into a single string with fields
2788 separated by the value of EXPR, and returns that new string. Example:
2790 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2792 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2793 first argument. Compare L</split>.
2802 Returns a list consisting of all the keys of the named hash, or the indices
2803 of an array. (In scalar context, returns the number of keys or indices.)
2805 The keys of a hash are returned in an apparently random order. The actual
2806 random order is subject to change in future versions of Perl, but it
2807 is guaranteed to be the same order as either the C<values> or C<each>
2808 function produces (given that the hash has not been modified). Since
2809 Perl 5.8.1 the ordering can be different even between different runs of
2810 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2813 As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
2814 (see L</each>). In particular, calling keys() in void context resets
2815 the iterator with no other overhead.
2817 Here is yet another way to print your environment:
2820 @values = values %ENV;
2822 print pop(@keys), '=', pop(@values), "\n";
2825 or how about sorted by key:
2827 foreach $key (sort(keys %ENV)) {
2828 print $key, '=', $ENV{$key}, "\n";
2831 The returned values are copies of the original keys in the hash, so
2832 modifying them will not affect the original hash. Compare L</values>.
2834 To sort a hash by value, you'll need to use a C<sort> function.
2835 Here's a descending numeric sort of a hash by its values:
2837 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2838 printf "%4d %s\n", $hash{$key}, $key;
2841 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2842 allocated for the given hash. This can gain you a measure of efficiency if
2843 you know the hash is going to get big. (This is similar to pre-extending
2844 an array by assigning a larger number to $#array.) If you say
2848 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2849 in fact, since it rounds up to the next power of two. These
2850 buckets will be retained even if you do C<%hash = ()>, use C<undef
2851 %hash> if you want to free the storage while C<%hash> is still in scope.
2852 You can't shrink the number of buckets allocated for the hash using
2853 C<keys> in this way (but you needn't worry about doing this by accident,
2854 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2857 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2858 a reference to an unblessed hash or array. The argument will be
2859 dereferenced automatically. This aspect of C<keys> is considered highly
2860 experimental. The exact behaviour may change in a future version of Perl.
2862 for (keys $hashref) { ... }
2863 for (keys $obj->get_arrayref) { ... }
2865 See also C<each>, C<values>, and C<sort>.
2867 =item kill SIGNAL, LIST
2870 Sends a signal to a list of processes. Returns the number of
2871 processes successfully signaled (which is not necessarily the
2872 same as the number actually killed).
2874 $cnt = kill 1, $child1, $child2;
2877 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2878 checks whether it's I<possible> to send a signal to it (that
2879 means, to be brief, that the process is owned by the same user, or we are
2880 the super-user). This is useful to check that a child process is still
2881 alive (even if only as a zombie) and hasn't changed its UID. See
2882 L<perlport> for notes on the portability of this construct.
2884 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2885 of processes. That means you usually want to use positive not negative signals.
2886 You may also use a signal name in quotes.
2888 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2889 the operating system. For example, on POSIX-conforming systems, zero will
2890 signal the current process group and -1 will signal all processes.
2892 See L<perlipc/"Signals"> for more details.
2894 On some platforms such as Windows where the fork() system call is not available.
2895 Perl can be built to emulate fork() at the interpreter level.
2896 This emulation has limitations related to kill that have to be considered,
2897 for code running on Windows and in code intended to be portable.
2899 See L<perlfork> for more details.
2901 Portability issues: L<perlport/kill>.
2908 The C<last> command is like the C<break> statement in C (as used in
2909 loops); it immediately exits the loop in question. If the LABEL is
2910 omitted, the command refers to the innermost enclosing loop. The
2911 C<continue> block, if any, is not executed:
2913 LINE: while (<STDIN>) {
2914 last LINE if /^$/; # exit when done with header
2918 C<last> cannot be used to exit a block that returns a value such as
2919 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2920 a grep() or map() operation.
2922 Note that a block by itself is semantically identical to a loop
2923 that executes once. Thus C<last> can be used to effect an early
2924 exit out of such a block.
2926 See also L</continue> for an illustration of how C<last>, C<next>, and
2934 Returns a lowercased version of EXPR. This is the internal function
2935 implementing the C<\L> escape in double-quoted strings.
2937 If EXPR is omitted, uses C<$_>.
2939 What gets returned depends on several factors:
2943 =item If C<use bytes> is in effect:
2947 =item On EBCDIC platforms
2949 The results are what the C language system call C<tolower()> returns.
2951 =item On ASCII platforms
2953 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2958 =item Otherwise, If EXPR has the UTF8 flag set
2960 Unicode semantics are used for the case change.
2962 =item Otherwise, if C<use locale> is in effect
2964 Respects current LC_CTYPE locale. See L<perllocale>.
2966 =item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2968 Unicode semantics are used for the case change.
2974 =item On EBCDIC platforms
2976 The results are what the C language system call C<tolower()> returns.
2978 =item On ASCII platforms
2980 ASCII semantics are used for the case change. The lowercase of any character
2981 outside the ASCII range is the character itself.
2988 X<lcfirst> X<lowercase>
2992 Returns the value of EXPR with the first character lowercased. This
2993 is the internal function implementing the C<\l> escape in
2994 double-quoted strings.
2996 If EXPR is omitted, uses C<$_>.
2998 This function behaves the same way under various pragmata, such as in a locale,
3006 Returns the length in I<characters> of the value of EXPR. If EXPR is
3007 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3010 This function cannot be used on an entire array or hash to find out how
3011 many elements these have. For that, use C<scalar @array> and C<scalar keys
3012 %hash>, respectively.
3014 Like all Perl character operations, length() normally deals in logical
3015 characters, not physical bytes. For how many bytes a string encoded as
3016 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3017 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3022 A special token that compiles to the current line number.
3024 =item link OLDFILE,NEWFILE
3027 Creates a new filename linked to the old filename. Returns true for
3028 success, false otherwise.
3030 Portability issues: L<perlport/link>.
3032 =item listen SOCKET,QUEUESIZE
3035 Does the same thing that the listen(2) system call does. Returns true if
3036 it succeeded, false otherwise. See the example in
3037 L<perlipc/"Sockets: Client/Server Communication">.
3042 You really probably want to be using C<my> instead, because C<local> isn't
3043 what most people think of as "local". See
3044 L<perlsub/"Private Variables via my()"> for details.
3046 A local modifies the listed variables to be local to the enclosing
3047 block, file, or eval. If more than one value is listed, the list must
3048 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3049 for details, including issues with tied arrays and hashes.
3051 The C<delete local EXPR> construct can also be used to localize the deletion
3052 of array/hash elements to the current block.
3053 See L<perlsub/"Localized deletion of elements of composite types">.
3055 =item localtime EXPR
3056 X<localtime> X<ctime>
3060 Converts a time as returned by the time function to a 9-element list
3061 with the time analyzed for the local time zone. Typically used as
3065 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3068 All list elements are numeric and come straight out of the C `struct
3069 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3070 of the specified time.
3072 C<$mday> is the day of the month and C<$mon> the month in
3073 the range C<0..11>, with 0 indicating January and 11 indicating December.
3074 This makes it easy to get a month name from a list:
3076 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3077 print "$abbr[$mon] $mday";
3078 # $mon=9, $mday=18 gives "Oct 18"
3080 C<$year> is the number of years since 1900, B<not> just the last two digits
3081 of the year. That is, C<$year> is C<123> in year 2023. The proper way
3082 to get a 4-digit year is simply:
3086 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3087 to do that, would you?
3089 To get the last two digits of the year (e.g., "01" in 2001) do:
3091 $year = sprintf("%02d", $year % 100);
3093 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3094 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3095 (or C<0..365> in leap years.)
3097 C<$isdst> is true if the specified time occurs during Daylight Saving
3098 Time, false otherwise.
3100 If EXPR is omitted, C<localtime()> uses the current time (as returned
3103 In scalar context, C<localtime()> returns the ctime(3) value:
3105 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3107 This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
3108 instead of local time use the L</gmtime> builtin. See also the
3109 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3110 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3111 and mktime(3) functions.
3113 To get somewhat similar but locale-dependent date strings, set up your
3114 locale environment variables appropriately (please see L<perllocale>) and
3117 use POSIX qw(strftime);
3118 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3119 # or for GMT formatted appropriately for your locale:
3120 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3122 Note that the C<%a> and C<%b>, the short forms of the day of the week
3123 and the month of the year, may not necessarily be three characters wide.
3125 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3126 by-name access mechanism to the gmtime() and localtime() functions,
3129 For a comprehensive date and time representation look at the
3130 L<DateTime> module on CPAN.
3132 Portability issues: L<perlport/localtime>.
3137 This function places an advisory lock on a shared variable or referenced
3138 object contained in I<THING> until the lock goes out of scope.
3140 The value returned is the scalar itself, if the argument is a scalar, or a
3141 reference, if the argument is a hash, array or subroutine.
3143 lock() is a "weak keyword" : this means that if you've defined a function
3144 by this name (before any calls to it), that function will be called
3145 instead. If you are not under C<use threads::shared> this does nothing.
3146 See L<threads::shared>.
3149 X<log> X<logarithm> X<e> X<ln> X<base>
3153 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3154 returns the log of C<$_>. To get the
3155 log of another base, use basic algebra:
3156 The base-N log of a number is equal to the natural log of that number
3157 divided by the natural log of N. For example:
3161 return log($n)/log(10);
3164 See also L</exp> for the inverse operation.
3166 =item lstat FILEHANDLE
3171 =item lstat DIRHANDLE
3175 Does the same thing as the C<stat> function (including setting the
3176 special C<_> filehandle) but stats a symbolic link instead of the file
3177 the symbolic link points to. If symbolic links are unimplemented on
3178 your system, a normal C<stat> is done. For much more detailed
3179 information, please see the documentation for C<stat>.
3181 If EXPR is omitted, stats C<$_>.
3183 Portability issues: L<perlport/lstat>.
3187 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3189 =item map BLOCK LIST
3194 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3195 C<$_> to each element) and returns the list value composed of the
3196 results of each such evaluation. In scalar context, returns the
3197 total number of elements so generated. Evaluates BLOCK or EXPR in
3198 list context, so each element of LIST may produce zero, one, or
3199 more elements in the returned value.
3201 @chars = map(chr, @numbers);
3203 translates a list of numbers to the corresponding characters.
3205 my @squares = map { $_ * $_ } @numbers;
3207 translates a list of numbers to their squared values.
3209 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3211 shows that number of returned elements can differ from the number of
3212 input elements. To omit an element, return an empty list ().
3213 This could also be achieved by writing
3215 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3217 which makes the intention more clear.
3219 Map always returns a list, which can be
3220 assigned to a hash such that the elements
3221 become key/value pairs. See L<perldata> for more details.
3223 %hash = map { get_a_key_for($_) => $_ } @array;
3225 is just a funny way to write
3229 $hash{get_a_key_for($_)} = $_;
3232 Note that C<$_> is an alias to the list value, so it can be used to
3233 modify the elements of the LIST. While this is useful and supported,
3234 it can cause bizarre results if the elements of LIST are not variables.
3235 Using a regular C<foreach> loop for this purpose would be clearer in
3236 most cases. See also L</grep> for an array composed of those items of
3237 the original list for which the BLOCK or EXPR evaluates to true.
3239 If C<$_> is lexical in the scope where the C<map> appears (because it has
3240 been declared with C<my $_>), then, in addition to being locally aliased to
3241 the list elements, C<$_> keeps being lexical inside the block; that is, it
3242 can't be seen from the outside, avoiding any potential side-effects.
3244 C<{> starts both hash references and blocks, so C<map { ...> could be either
3245 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3246 ahead for the closing C<}> it has to take a guess at which it's dealing with
3247 based on what it finds just after the C<{>. Usually it gets it right, but if it
3248 doesn't it won't realize something is wrong until it gets to the C<}> and
3249 encounters the missing (or unexpected) comma. The syntax error will be
3250 reported close to the C<}>, but you'll need to change something near the C<{>
3251 such as using a unary C<+> to give Perl some help:
3253 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3254 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3255 %hash = map { ("\L$_" => 1) } @array # this also works
3256 %hash = map { lc($_) => 1 } @array # as does this.
3257 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3259 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3261 or to force an anon hash constructor use C<+{>:
3263 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
3265 to get a list of anonymous hashes each with only one entry apiece.
3267 =item mkdir FILENAME,MASK
3268 X<mkdir> X<md> X<directory, create>
3270 =item mkdir FILENAME
3274 Creates the directory specified by FILENAME, with permissions
3275 specified by MASK (as modified by C<umask>). If it succeeds it
3276 returns true; otherwise it returns false and sets C<$!> (errno).
3277 MASK defaults to 0777 if omitted, and FILENAME defaults
3278 to C<$_> if omitted.
3280 In general, it is better to create directories with a permissive MASK
3281 and let the user modify that with their C<umask> than it is to supply
3282 a restrictive MASK and give the user no way to be more permissive.
3283 The exceptions to this rule are when the file or directory should be
3284 kept private (mail files, for instance). The perlfunc(1) entry on
3285 C<umask> discusses the choice of MASK in more detail.
3287 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3288 number of trailing slashes. Some operating and filesystems do not get
3289 this right, so Perl automatically removes all trailing slashes to keep
3292 To recursively create a directory structure, look at
3293 the C<mkpath> function of the L<File::Path> module.
3295 =item msgctl ID,CMD,ARG
3298 Calls the System V IPC function msgctl(2). You'll probably have to say
3302 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3303 then ARG must be a variable that will hold the returned C<msqid_ds>
3304 structure. Returns like C<ioctl>: the undefined value for error,
3305 C<"0 but true"> for zero, or the actual return value otherwise. See also
3306 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3309 Portability issues: L<perlport/msgctl>.
3311 =item msgget KEY,FLAGS
3314 Calls the System V IPC function msgget(2). Returns the message queue
3315 id, or C<undef> on error. See also
3316 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3319 Portability issues: L<perlport/msgget>.
3321 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3324 Calls the System V IPC function msgrcv to receive a message from
3325 message queue ID into variable VAR with a maximum message size of
3326 SIZE. Note that when a message is received, the message type as a
3327 native long integer will be the first thing in VAR, followed by the
3328 actual message. This packing may be opened with C<unpack("l! a*")>.
3329 Taints the variable. Returns true if successful, false
3330 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3331 C<IPC::SysV> and C<IPC::SysV::Msg>.
3333 Portability issues: L<perlport/msgrcv>.
3335 =item msgsnd ID,MSG,FLAGS
3338 Calls the System V IPC function msgsnd to send the message MSG to the
3339 message queue ID. MSG must begin with the native long integer message
3340 type, be followed by the length of the actual message, and then finally
3341 the message itself. This kind of packing can be achieved with
3342 C<pack("l! a*", $type, $message)>. Returns true if successful,
3343 false on error. See also the C<IPC::SysV>
3344 and C<IPC::SysV::Msg> documentation.
3346 Portability issues: L<perlport/msgsnd>.
3353 =item my EXPR : ATTRS
3355 =item my TYPE EXPR : ATTRS
3357 A C<my> declares the listed variables to be local (lexically) to the
3358 enclosing block, file, or C<eval>. If more than one value is listed,
3359 the list must be placed in parentheses.
3361 The exact semantics and interface of TYPE and ATTRS are still
3362 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3363 and attributes are handled using the C<attributes> pragma, or starting
3364 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3365 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3366 L<attributes>, and L<Attribute::Handlers>.
3373 The C<next> command is like the C<continue> statement in C; it starts
3374 the next iteration of the loop:
3376 LINE: while (<STDIN>) {
3377 next LINE if /^#/; # discard comments
3381 Note that if there were a C<continue> block on the above, it would get
3382 executed even on discarded lines. If LABEL is omitted, the command
3383 refers to the innermost enclosing loop.
3385 C<next> cannot be used to exit a block which returns a value such as
3386 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3387 a grep() or map() operation.
3389 Note that a block by itself is semantically identical to a loop
3390 that executes once. Thus C<next> will exit such a block early.
3392 See also L</continue> for an illustration of how C<last>, C<next>, and
3395 =item no MODULE VERSION LIST
3399 =item no MODULE VERSION
3401 =item no MODULE LIST
3407 See the C<use> function, of which C<no> is the opposite.
3410 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3414 Interprets EXPR as an octal string and returns the corresponding
3415 value. (If EXPR happens to start off with C<0x>, interprets it as a
3416 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3417 binary string. Leading whitespace is ignored in all three cases.)
3418 The following will handle decimal, binary, octal, and hex in standard
3421 $val = oct($val) if $val =~ /^0/;
3423 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3424 in octal), use sprintf() or printf():
3426 $dec_perms = (stat("filename"))[2] & 07777;
3427 $oct_perm_str = sprintf "%o", $perms;
3429 The oct() function is commonly used when a string such as C<644> needs
3430 to be converted into a file mode, for example. Although Perl
3431 automatically converts strings into numbers as needed, this automatic
3432 conversion assumes base 10.
3434 Leading white space is ignored without warning, as too are any trailing
3435 non-digits, such as a decimal point (C<oct> only handles non-negative
3436 integers, not negative integers or floating point).
3438 =item open FILEHANDLE,EXPR
3439 X<open> X<pipe> X<file, open> X<fopen>
3441 =item open FILEHANDLE,MODE,EXPR
3443 =item open FILEHANDLE,MODE,EXPR,LIST
3445 =item open FILEHANDLE,MODE,REFERENCE
3447 =item open FILEHANDLE
3449 Opens the file whose filename is given by EXPR, and associates it with
3452 Simple examples to open a file for reading:
3454 open(my $fh, "<", "input.txt")
3455 or die "cannot open < input.txt: $!";
3459 open(my $fh, ">", "output.txt")
3460 or die "cannot open > output.txt: $!";
3462 (The following is a comprehensive reference to open(): for a gentler
3463 introduction you may consider L<perlopentut>.)
3465 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3466 new filehandle is autovivified, meaning that the variable is assigned a
3467 reference to a newly allocated anonymous filehandle. Otherwise if
3468 FILEHANDLE is an expression, its value is the real filehandle. (This is
3469 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3472 If EXPR is omitted, the global (package) scalar variable of the same
3473 name as the FILEHANDLE contains the filename. (Note that lexical
3474 variables--those declared with C<my> or C<state>--will not work for this
3475 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3478 If three (or more) arguments are specified, the open mode (including
3479 optional encoding) in the second argument are distinct from the filename in
3480 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3481 If MODE is C<< > >>, the file is opened for output, with existing files
3482 first being truncated ("clobbered") and nonexisting files newly created.
3483 If MODE is C<<< >> >>>, the file is opened for appending, again being
3484 created if necessary.
3486 You can put a C<+> in front of the C<< > >> or C<< < >> to
3487 indicate that you want both read and write access to the file; thus
3488 C<< +< >> is almost always preferred for read/write updates--the
3489 C<< +> >> mode would clobber the file first. You cant usually use
3490 either read-write mode for updating textfiles, since they have
3491 variable-length records. See the B<-i> switch in L<perlrun> for a
3492 better approach. The file is created with permissions of C<0666>
3493 modified by the process's C<umask> value.
3495 These various prefixes correspond to the fopen(3) modes of C<r>,
3496 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3498 In the one- and two-argument forms of the call, the mode and filename
3499 should be concatenated (in that order), preferably separated by white
3500 space. You can--but shouldn't--omit the mode in these forms when that mode
3501 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3502 the filename argument is a known literal.
3504 For three or more arguments if MODE is C<|->, the filename is
3505 interpreted as a command to which output is to be piped, and if MODE
3506 is C<-|>, the filename is interpreted as a command that pipes
3507 output to us. In the two-argument (and one-argument) form, one should
3508 replace dash (C<->) with the command.
3509 See L<perlipc/"Using open() for IPC"> for more examples of this.
3510 (You are not allowed to C<open> to a command that pipes both in I<and>
3511 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3512 L<perlipc/"Bidirectional Communication with Another Process"> for
3515 In the form of pipe opens taking three or more arguments, if LIST is specified
3516 (extra arguments after the command name) then LIST becomes arguments
3517 to the command invoked if the platform supports it. The meaning of
3518 C<open> with more than three arguments for non-pipe modes is not yet
3519 defined, but experimental "layers" may give extra LIST arguments
3522 In the two-argument (and one-argument) form, opening C<< <- >>
3523 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3525 You may (and usually should) use the three-argument form of open to specify
3526 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3527 that affect how the input and output are processed (see L<open> and
3528 L<PerlIO> for more details). For example:
3530 open(my $fh, "<:encoding(UTF-8)", "filename")
3531 || die "can't open UTF-8 encoded filename: $!";
3533 opens the UTF8-encoded file containing Unicode characters;
3534 see L<perluniintro>. Note that if layers are specified in the
3535 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3536 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3537 Those layers will also be ignored if you specifying a colon with no name
3538 following it. In that case the default layer for the operating system
3539 (:raw on Unix, :crlf on Windows) is used.
3541 Open returns nonzero on success, the undefined value otherwise. If
3542 the C<open> involved a pipe, the return value happens to be the pid of
3545 If you're running Perl on a system that distinguishes between text
3546 files and binary files, then you should check out L</binmode> for tips
3547 for dealing with this. The key distinction between systems that need
3548 C<binmode> and those that don't is their text file formats. Systems
3549 like Unix, Mac OS, and Plan 9, that end lines with a single
3550 character and encode that character in C as C<"\n"> do not
3551 need C<binmode>. The rest need it.
3553 When opening a file, it's seldom a good idea to continue
3554 if the request failed, so C<open> is frequently used with
3555 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3556 where you want to format a suitable error message (but there are
3557 modules that can help with that problem)) always check
3558 the return value from opening a file.
3560 As a special case the three-argument form with a read/write mode and the third
3561 argument being C<undef>:
3563 open(my $tmp, "+>", undef) or die ...
3565 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3566 works for symmetry, but you really should consider writing something
3567 to the temporary file first. You will need to seek() to do the
3570 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3571 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3572 open filehandles directly to Perl scalars via:
3574 open($fh, ">", \$variable) || ..
3576 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3579 open(STDOUT, ">", \$variable)
3580 or die "Can't open STDOUT: $!";
3585 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3586 while (<ARTICLE>) {...
3588 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3589 # if the open fails, output is discarded
3591 open(my $dbase, "+<", "dbase.mine") # open for update
3592 or die "Can't open 'dbase.mine' for update: $!";
3594 open(my $dbase, "+<dbase.mine") # ditto
3595 or die "Can't open 'dbase.mine' for update: $!";
3597 open(ARTICLE, "-|", "caesar <$article") # decrypt article
3598 or die "Can't start caesar: $!";
3600 open(ARTICLE, "caesar <$article |") # ditto
3601 or die "Can't start caesar: $!";
3603 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3604 or die "Can't start sort: $!";
3607 open(MEMORY, ">", \$var)
3608 or die "Can't open memory file: $!";
3609 print MEMORY "foo!\n"; # output will appear in $var
3611 # process argument list of files along with any includes
3613 foreach $file (@ARGV) {
3614 process($file, "fh00");
3618 my($filename, $input) = @_;
3619 $input++; # this is a string increment
3620 unless (open($input, "<", $filename)) {
3621 print STDERR "Can't open $filename: $!\n";
3626 while (<$input>) { # note use of indirection
3627 if (/^#include "(.*)"/) {
3628 process($1, $input);
3635 See L<perliol> for detailed info on PerlIO.
3637 You may also, in the Bourne shell tradition, specify an EXPR beginning
3638 with C<< >& >>, in which case the rest of the string is interpreted
3639 as the name of a filehandle (or file descriptor, if numeric) to be
3640 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3641 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3642 The mode you specify should match the mode of the original filehandle.
3643 (Duping a filehandle does not take into account any existing contents
3644 of IO buffers.) If you use the three-argument form, then you can pass either a
3645 number, the name of a filehandle, or the normal "reference to a glob".
3647 Here is a script that saves, redirects, and restores C<STDOUT> and
3648 C<STDERR> using various methods:
3651 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
3652 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
3654 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
3655 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
3657 select STDERR; $| = 1; # make unbuffered
3658 select STDOUT; $| = 1; # make unbuffered
3660 print STDOUT "stdout 1\n"; # this works for
3661 print STDERR "stderr 1\n"; # subprocesses too
3663 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
3664 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
3666 print STDOUT "stdout 2\n";
3667 print STDERR "stderr 2\n";
3669 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3670 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3671 that file descriptor (and not call C<dup(2)>); this is more
3672 parsimonious of file descriptors. For example:
3674 # open for input, reusing the fileno of $fd
3675 open(FILEHANDLE, "<&=$fd")
3679 open(FILEHANDLE, "<&=", $fd)
3683 # open for append, using the fileno of OLDFH
3684 open(FH, ">>&=", OLDFH)
3688 open(FH, ">>&=OLDFH")
3690 Being parsimonious on filehandles is also useful (besides being
3691 parsimonious) for example when something is dependent on file
3692 descriptors, like for example locking using flock(). If you do just
3693 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
3694 descriptor as B, and therefore flock(A) will not flock(B) nor vice
3695 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
3696 the same underlying system file descriptor.
3698 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
3699 fdopen() to implement the C<=> functionality. On many Unix systems,
3700 fdopen() fails when file descriptors exceed a certain value, typically 255.
3701 For Perls 5.8.0 and later, PerlIO is (most often) the default.
3703 You can see whether your Perl was built with PerlIO by running C<perl -V>
3704 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
3705 have PerlIO; otherwise you don't.
3707 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
3708 with the one- or two-argument forms of C<open>),
3709 an implicit C<fork> is done, so C<open> returns twice: in the parent
3710 process it returns the pid
3711 of the child process, and in the child process it returns (a defined) C<0>.
3712 Use C<defined($pid)> or C<//> to determine whether the open was successful.
3714 For example, use either
3716 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
3719 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
3725 # either write TO_KID or else read FROM_KID
3729 # am the child; use STDIN/STDOUT normally
3734 The filehandle behaves normally for the parent, but I/O to that
3735 filehandle is piped from/to the STDOUT/STDIN of the child process.
3736 In the child process, the filehandle isn't opened--I/O happens from/to
3737 the new STDOUT/STDIN. Typically this is used like the normal
3738 piped open when you want to exercise more control over just how the
3739 pipe command gets executed, such as when running setuid and
3740 you don't want to have to scan shell commands for metacharacters.
3742 The following blocks are more or less equivalent:
3744 open(FOO, "|tr '[a-z]' '[A-Z]'");
3745 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
3746 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
3747 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
3749 open(FOO, "cat -n '$file'|");
3750 open(FOO, "-|", "cat -n '$file'");
3751 open(FOO, "-|") || exec "cat", "-n", $file;
3752 open(FOO, "-|", "cat", "-n", $file);
3754 The last two examples in each block show the pipe as "list form", which is
3755 not yet supported on all platforms. A good rule of thumb is that if
3756 your platform has a real C<fork()> (in other words, if your platform is
3757 Unix, including Linux and MacOS X), you can use the list form. You would
3758 want to use the list form of the pipe so you can pass literal arguments
3759 to the command without risk of the shell interpreting any shell metacharacters
3760 in them. However, this also bars you from opening pipes to commands
3761 that intentionally contain shell metacharacters, such as:
3763 open(FOO, "|cat -n | expand -4 | lpr")
3764 // die "Can't open pipeline to lpr: $!";
3766 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3768 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3769 output before any operation that may do a fork, but this may not be
3770 supported on some platforms (see L<perlport>). To be safe, you may need
3771 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3772 of C<IO::Handle> on any open handles.
3774 On systems that support a close-on-exec flag on files, the flag will
3775 be set for the newly opened file descriptor as determined by the value
3776 of C<$^F>. See L<perlvar/$^F>.
3778 Closing any piped filehandle causes the parent process to wait for the
3779 child to finish, then returns the status value in C<$?> and
3780 C<${^CHILD_ERROR_NATIVE}>.
3782 The filename passed to the one- and two-argument forms of open() will
3783 have leading and trailing whitespace deleted and normal
3784 redirection characters honored. This property, known as "magic open",
3785 can often be used to good effect. A user could specify a filename of
3786 F<"rsh cat file |">, or you could change certain filenames as needed:
3788 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3789 open(FH, $filename) or die "Can't open $filename: $!";
3791 Use the three-argument form to open a file with arbitrary weird characters in it,
3793 open(FOO, "<", $file)
3794 || die "can't open < $file: $!";
3796 otherwise it's necessary to protect any leading and trailing whitespace:
3798 $file =~ s#^(\s)#./$1#;
3799 open(FOO, "< $file\0")
3800 || die "open failed: $!";
3802 (this may not work on some bizarre filesystems). One should
3803 conscientiously choose between the I<magic> and I<three-argument> form
3806 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
3808 will allow the user to specify an argument of the form C<"rsh cat file |">,
3809 but will not work on a filename that happens to have a trailing space, while
3811 open(IN, "<", $ARGV[0])
3812 || die "can't open < $ARGV[0]: $!";
3814 will have exactly the opposite restrictions.
3816 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3817 should use the C<sysopen> function, which involves no such magic (but may
3818 use subtly different filemodes than Perl open(), which is mapped to C
3819 fopen()). This is another way to protect your filenames from
3820 interpretation. For example:
3823 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3824 or die "sysopen $path: $!";
3825 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3826 print HANDLE "stuff $$\n";
3828 print "File contains: ", <HANDLE>;
3830 Using the constructor from the C<IO::Handle> package (or one of its
3831 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3832 filehandles that have the scope of the variables used to hold them, then
3833 automatically (but silently) close once their reference counts become
3834 zero, typically at scope exit:
3838 sub read_myfile_munged {
3840 # or just leave it undef to autoviv
3841 my $handle = IO::File->new;
3842 open($handle, "<", "myfile") or die "myfile: $!";
3844 or return (); # Automatically closed here.
3845 mung($first) or die "mung failed"; # Or here.
3846 return (first, <$handle>) if $ALL; # Or here.
3847 return $first; # Or here.
3850 B<WARNING:> The previous example has a bug because the automatic
3851 close that happens when the refcount on C<handle> does not
3852 properly detect and report failures. I<Always> close the handle
3853 yourself and inspect the return value.
3856 || warn "close failed: $!";
3858 See L</seek> for some details about mixing reading and writing.
3860 Portability issues: L<perlport/open>.
3862 =item opendir DIRHANDLE,EXPR
3865 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3866 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3867 DIRHANDLE may be an expression whose value can be used as an indirect
3868 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3869 scalar variable (or array or hash element), the variable is assigned a
3870 reference to a new anonymous dirhandle; that is, it's autovivified.
3871 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3873 See the example at C<readdir>.
3880 Returns the numeric value of the first character of EXPR.
3881 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
3882 (Note I<character>, not byte.)
3884 For the reverse, see L</chr>.
3885 See L<perlunicode> for more about Unicode.
3892 =item our EXPR : ATTRS
3894 =item our TYPE EXPR : ATTRS
3896 C<our> associates a simple name with a package variable in the current
3897 package for use within the current scope. When C<use strict 'vars'> is in
3898 effect, C<our> lets you use declared global variables without qualifying
3899 them with package names, within the lexical scope of the C<our> declaration.
3900 In this way C<our> differs from C<use vars>, which is package-scoped.
3902 Unlike C<my> or C<state>, which allocates storage for a variable and
3903 associates a simple name with that storage for use within the current
3904 scope, C<our> associates a simple name with a package (read: global)
3905 variable in the current package, for use within the current lexical scope.
3906 In other words, C<our> has the same scoping rules as C<my> or C<state>, but
3907 does not necessarily create a variable.
3909 If more than one value is listed, the list must be placed
3915 An C<our> declaration declares a global variable that will be visible
3916 across its entire lexical scope, even across package boundaries. The
3917 package in which the variable is entered is determined at the point
3918 of the declaration, not at the point of use. This means the following
3922 our $bar; # declares $Foo::bar for rest of lexical scope
3926 print $bar; # prints 20, as it refers to $Foo::bar
3928 Multiple C<our> declarations with the same name in the same lexical
3929 scope are allowed if they are in different packages. If they happen
3930 to be in the same package, Perl will emit warnings if you have asked
3931 for them, just like multiple C<my> declarations. Unlike a second
3932 C<my> declaration, which will bind the name to a fresh variable, a
3933 second C<our> declaration in the same package, in the same scope, is
3938 our $bar; # declares $Foo::bar for rest of lexical scope
3942 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3943 print $bar; # prints 30
3945 our $bar; # emits warning but has no other effect
3946 print $bar; # still prints 30
3948 An C<our> declaration may also have a list of attributes associated
3951 The exact semantics and interface of TYPE and ATTRS are still
3952 evolving. TYPE is currently bound to the use of C<fields> pragma,
3953 and attributes are handled using the C<attributes> pragma, or starting
3954 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3955 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3956 L<attributes>, and L<Attribute::Handlers>.
3958 =item pack TEMPLATE,LIST
3961 Takes a LIST of values and converts it into a string using the rules
3962 given by the TEMPLATE. The resulting string is the concatenation of
3963 the converted values. Typically, each converted value looks
3964 like its machine-level representation. For example, on 32-bit machines
3965 an integer may be represented by a sequence of 4 bytes, which will in
3966 Perl be presented as a string that's 4 characters long.
3968 See L<perlpacktut> for an introduction to this function.
3970 The TEMPLATE is a sequence of characters that give the order and type
3971 of values, as follows:
3973 a A string with arbitrary binary data, will be null padded.
3974 A A text (ASCII) string, will be space padded.
3975 Z A null-terminated (ASCIZ) string, will be null padded.
3977 b A bit string (ascending bit order inside each byte,
3979 B A bit string (descending bit order inside each byte).
3980 h A hex string (low nybble first).
3981 H A hex string (high nybble first).
3983 c A signed char (8-bit) value.
3984 C An unsigned char (octet) value.
3985 W An unsigned char value (can be greater than 255).
3987 s A signed short (16-bit) value.
3988 S An unsigned short value.
3990 l A signed long (32-bit) value.
3991 L An unsigned long value.
3993 q A signed quad (64-bit) value.
3994 Q An unsigned quad value.
3995 (Quads are available only if your system supports 64-bit
3996 integer values _and_ if Perl has been compiled to support
3997 those. Raises an exception otherwise.)
3999 i A signed integer value.
4000 I A unsigned integer value.
4001 (This 'integer' is _at_least_ 32 bits wide. Its exact
4002 size depends on what a local C compiler calls 'int'.)
4004 n An unsigned short (16-bit) in "network" (big-endian) order.
4005 N An unsigned long (32-bit) in "network" (big-endian) order.
4006 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4007 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4009 j A Perl internal signed integer value (IV).
4010 J A Perl internal unsigned integer value (UV).
4012 f A single-precision float in native format.
4013 d A double-precision float in native format.
4015 F A Perl internal floating-point value (NV) in native format
4016 D A float of long-double precision in native format.
4017 (Long doubles are available only if your system supports
4018 long double values _and_ if Perl has been compiled to
4019 support those. Raises an exception otherwise.)
4021 p A pointer to a null-terminated string.
4022 P A pointer to a structure (fixed-length string).
4024 u A uuencoded string.
4025 U A Unicode character number. Encodes to a character in char-
4026 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4029 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4030 for details). Its bytes represent an unsigned integer in
4031 base 128, most significant digit first, with as few digits
4032 as possible. Bit eight (the high bit) is set on each byte
4035 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4037 @ Null-fill or truncate to absolute position, counted from the
4038 start of the innermost ()-group.
4039 . Null-fill or truncate to absolute position specified by
4041 ( Start of a ()-group.
4043 One or more modifiers below may optionally follow certain letters in the
4044 TEMPLATE (the second column lists letters for which the modifier is valid):
4046 ! sSlLiI Forces native (short, long, int) sizes instead
4047 of fixed (16-/32-bit) sizes.
4049 xX Make x and X act as alignment commands.
4051 nNvV Treat integers as signed instead of unsigned.
4053 @. Specify position as byte offset in the internal
4054 representation of the packed string. Efficient but
4057 > sSiIlLqQ Force big-endian byte-order on the type.
4058 jJfFdDpP (The "big end" touches the construct.)
4060 < sSiIlLqQ Force little-endian byte-order on the type.
4061 jJfFdDpP (The "little end" touches the construct.)
4063 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4064 to force a particular byte-order on all components in that group,
4065 including all its subgroups.
4067 The following rules apply:
4073 Each letter may optionally be followed by a number indicating the repeat
4074 count. A numeric repeat count may optionally be enclosed in brackets, as
4075 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4076 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4077 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4078 something else, described below. Supplying a C<*> for the repeat count
4079 instead of a number means to use however many items are left, except for:
4085 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4089 <.>, where it means relative to the start of the string.
4093 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4097 One can replace a numeric repeat count with a template letter enclosed in
4098 brackets to use the packed byte length of the bracketed template for the
4101 For example, the template C<x[L]> skips as many bytes as in a packed long,
4102 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4103 variable-expanded) unpacks. If the template in brackets contains alignment
4104 commands (such as C<x![d]>), its packed length is calculated as if the
4105 start of the template had the maximal possible alignment.
4107 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4108 trailing null byte, so the resulting string is always one byte longer than
4109 the byte length of the item itself.
4111 When used with C<@>, the repeat count represents an offset from the start
4112 of the innermost C<()> group.
4114 When used with C<.>, the repeat count determines the starting position to
4115 calculate the value offset as follows:
4121 If the repeat count is C<0>, it's relative to the current position.
4125 If the repeat count is C<*>, the offset is relative to the start of the
4130 And if it's an integer I<n>, the offset is relative to the start of the
4131 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4132 bigger then the group level.
4136 The repeat count for C<u> is interpreted as the maximal number of bytes
4137 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4138 count should not be more than 65.
4142 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4143 string of length count, padding with nulls or spaces as needed. When
4144 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4145 after the first null, and C<a> returns data with no stripping at all.
4147 If the value to pack is too long, the result is truncated. If it's too
4148 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4149 followed by a null byte. Thus C<Z> always packs a trailing null, except
4150 when the count is 0.
4154 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4155 Each such format generates 1 bit of the result. These are typically followed
4156 by a repeat count like C<B8> or C<B64>.
4158 Each result bit is based on the least-significant bit of the corresponding
4159 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4160 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4162 Starting from the beginning of the input string, each 8-tuple
4163 of characters is converted to 1 character of output. With format C<b>,
4164 the first character of the 8-tuple determines the least-significant bit of a
4165 character; with format C<B>, it determines the most-significant bit of
4168 If the length of the input string is not evenly divisible by 8, the
4169 remainder is packed as if the input string were padded by null characters
4170 at the end. Similarly during unpacking, "extra" bits are ignored.
4172 If the input string is longer than needed, remaining characters are ignored.
4174 A C<*> for the repeat count uses all characters of the input field.
4175 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4179 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4180 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4182 For each such format, pack() generates 4 bits of result.
4183 With non-alphabetical characters, the result is based on the 4 least-significant
4184 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4185 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4186 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4187 is compatible with the usual hexadecimal digits, so that C<"a"> and
4188 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4189 characters with this format.
4191 Starting from the beginning of the template to pack(), each pair
4192 of characters is converted to 1 character of output. With format C<h>, the
4193 first character of the pair determines the least-significant nybble of the
4194 output character; with format C<H>, it determines the most-significant
4197 If the length of the input string is not even, it behaves as if padded by
4198 a null character at the end. Similarly, "extra" nybbles are ignored during
4201 If the input string is longer than needed, extra characters are ignored.
4203 A C<*> for the repeat count uses all characters of the input field. For
4204 unpack(), nybbles are converted to a string of hexadecimal digits.
4208 The C<p> format packs a pointer to a null-terminated string. You are
4209 responsible for ensuring that the string is not a temporary value, as that
4210 could potentially get deallocated before you got around to using the packed
4211 result. The C<P> format packs a pointer to a structure of the size indicated
4212 by the length. A null pointer is created if the corresponding value for
4213 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4214 unpacks into C<undef>.
4216 If your system has a strange pointer size--meaning a pointer is neither as
4217 big as an int nor as big as a long--it may not be possible to pack or
4218 unpack pointers in big- or little-endian byte order. Attempting to do
4219 so raises an exception.
4223 The C</> template character allows packing and unpacking of a sequence of
4224 items where the packed structure contains a packed item count followed by
4225 the packed items themselves. This is useful when the structure you're
4226 unpacking has encoded the sizes or repeat counts for some of its fields
4227 within the structure itself as separate fields.
4229 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4230 I<length-item> describes how the length value is packed. Formats likely
4231 to be of most use are integer-packing ones like C<n> for Java strings,
4232 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4234 For C<pack>, I<sequence-item> may have a repeat count, in which case
4235 the minimum of that and the number of available items is used as the argument
4236 for I<length-item>. If it has no repeat count or uses a '*', the number
4237 of available items is used.
4239 For C<unpack>, an internal stack of integer arguments unpacked so far is
4240 used. You write C</>I<sequence-item> and the repeat count is obtained by
4241 popping off the last element from the stack. The I<sequence-item> must not
4242 have a repeat count.
4244 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4245 the I<length-item> is the string length, not the number of strings. With
4246 an explicit repeat count for pack, the packed string is adjusted to that
4247 length. For example:
4249 This code: gives this result:
4251 unpack("W/a", "\004Gurusamy") ("Guru")
4252 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4253 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4255 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4256 pack("a/W2", ord("a") .. ord("z")) "2ab"
4258 The I<length-item> is not returned explicitly from C<unpack>.
4260 Supplying a count to the I<length-item> format letter is only useful with
4261 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4262 introduce C<"\000"> characters, which Perl does not regard as legal in
4267 The integer types C<s>, C<S>, C<l>, and C<L> may be
4268 followed by a C<!> modifier to specify native shorts or
4269 longs. As shown in the example above, a bare C<l> means
4270 exactly 32 bits, although the native C<long> as seen by the local C compiler
4271 may be larger. This is mainly an issue on 64-bit platforms. You can
4272 see whether using C<!> makes any difference this way:
4274 printf "format s is %d, s! is %d\n",
4275 length pack("s"), length pack("s!");
4277 printf "format l is %d, l! is %d\n",
4278 length pack("l"), length pack("l!");
4281 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4282 they are identical to C<i> and C<I>.
4284 The actual sizes (in bytes) of native shorts, ints, longs, and long
4285 longs on the platform where Perl was built are also available from
4288 $ perl -V:{short,int,long{,long}}size
4294 or programmatically via the C<Config> module:
4297 print $Config{shortsize}, "\n";
4298 print $Config{intsize}, "\n";
4299 print $Config{longsize}, "\n";
4300 print $Config{longlongsize}, "\n";
4302 C<$Config{longlongsize}> is undefined on systems without
4307 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4308 inherently non-portable between processors and operating systems because
4309 they obey native byteorder and endianness. For example, a 4-byte integer
4310 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4311 handled by the CPU registers) into bytes as
4313 0x12 0x34 0x56 0x78 # big-endian
4314 0x78 0x56 0x34 0x12 # little-endian
4316 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4317 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4318 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4319 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4321 The names I<big-endian> and I<little-endian> are comic references to the
4322 egg-eating habits of the little-endian Lilliputians and the big-endian
4323 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4324 This entered computer lingo via the paper "On Holy Wars and a Plea for
4325 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4327 Some systems may have even weirder byte orders such as
4332 You can determine your system endianness with this incantation:
4334 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4336 The byteorder on the platform where Perl was built is also available
4340 print "$Config{byteorder}\n";
4342 or from the command line:
4346 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4347 and C<"87654321"> are big-endian.
4349 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4350 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4351 immediately below. See also L<perlport>.
4355 Starting with Perl 5.9.2, integer and floating-point formats, along with
4356 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4357 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4358 or little-endian byte-order. These modifiers are especially useful
4359 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4360 64-bit integers, or floating-point values.
4362 Here are some concerns to keep in mind when using an endianness modifier:
4368 Exchanging signed integers between different platforms works only
4369 when all platforms store them in the same format. Most platforms store
4370 signed integers in two's-complement notation, so usually this is not an issue.
4374 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4375 formats on big- or little-endian machines. Otherwise, attempting to
4376 use them raises an exception.
4380 Forcing big- or little-endian byte-order on floating-point values for
4381 data exchange can work only if all platforms use the same
4382 binary representation such as IEEE floating-point. Even if all
4383 platforms are using IEEE, there may still be subtle differences. Being able
4384 to use C<< > >> or C<< < >> on floating-point values can be useful,
4385 but also dangerous if you don't know exactly what you're doing.
4386 It is not a general way to portably store floating-point values.
4390 When using C<< > >> or C<< < >> on a C<()> group, this affects
4391 all types inside the group that accept byte-order modifiers,
4392 including all subgroups. It is silently ignored for all other
4393 types. You are not allowed to override the byte-order within a group
4394 that already has a byte-order modifier suffix.
4400 Real numbers (floats and doubles) are in native machine format only.
4401 Due to the multiplicity of floating-point formats and the lack of a
4402 standard "network" representation for them, no facility for interchange has been
4403 made. This means that packed floating-point data written on one machine
4404 may not be readable on another, even if both use IEEE floating-point
4405 arithmetic (because the endianness of the memory representation is not part
4406 of the IEEE spec). See also L<perlport>.
4408 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4409 modifiers to force big- or little-endian byte-order on floating-point values.
4411 Because Perl uses doubles (or long doubles, if configured) internally for
4412 all numeric calculation, converting from double into float and thence
4413 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4414 will not in general equal $foo.
4418 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4419 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4420 where the packed string is processed in its UTF-8-encoded Unicode form on
4421 a byte-by-byte basis. Character mode is the default unless the format string
4422 starts with C<U>. You can always switch mode mid-format with an explicit
4423 C<C0> or C<U0> in the format. This mode remains in effect until the next
4424 mode change, or until the end of the C<()> group it (directly) applies to.
4426 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4427 bytes is not necessarily obvious. Probably only the first of these
4430 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4431 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4433 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4434 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4436 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4437 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4439 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4440 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4441 C3.8E.C2.B1.C3.8F.C2.89
4443 Those examples also illustrate that you should not try to use
4444 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4448 You must yourself do any alignment or padding by inserting, for example,
4449 enough C<"x">es while packing. There is no way for pack() and unpack()
4450 to know where characters are going to or coming from, so they
4451 handle their output and input as flat sequences of characters.
4455 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4456 take a repeat count either as postfix, or for unpack(), also via the C</>
4457 template character. Within each repetition of a group, positioning with
4458 C<@> starts over at 0. Therefore, the result of
4460 pack("@1A((@2A)@3A)", qw[X Y Z])
4462 is the string C<"\0X\0\0YZ">.
4466 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4467 jump forward or back to the closest position aligned at a multiple of C<count>
4468 characters. For example, to pack() or unpack() a C structure like
4471 char c; /* one signed, 8-bit character */
4476 one may need to use the template C<c x![d] d c[2]>. This assumes that
4477 doubles must be aligned to the size of double.
4479 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4484 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4485 represent signed 16-/32-bit integers in big-/little-endian order.
4486 This is portable only when all platforms sharing packed data use the
4487 same binary representation for signed integers; for example, when all
4488 platforms use two's-complement representation.
4492 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4493 White space can separate pack codes from each other, but modifiers and
4494 repeat counts must follow immediately. Breaking complex templates into
4495 individual line-by-line components, suitably annotated, can do as much to
4496 improve legibility and maintainability of pack/unpack formats as C</x> can
4497 for complicated pattern matches.
4501 If TEMPLATE requires more arguments than pack() is given, pack()
4502 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4503 than given, extra arguments are ignored.
4509 $foo = pack("WWWW",65,66,67,68);
4511 $foo = pack("W4",65,66,67,68);
4513 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4514 # same thing with Unicode circled letters.
4515 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4516 # same thing with Unicode circled letters. You don't get the
4517 # UTF-8 bytes because the U at the start of the format caused
4518 # a switch to U0-mode, so the UTF-8 bytes get joined into
4520 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4521 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4522 # This is the UTF-8 encoding of the string in the
4525 $foo = pack("ccxxcc",65,66,67,68);
4528 # NOTE: The examples above featuring "W" and "c" are true
4529 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4530 # and UTF-8. On EBCDIC systems, the first example would be
4531 # $foo = pack("WWWW",193,194,195,196);
4533 $foo = pack("s2",1,2);
4534 # "\001\000\002\000" on little-endian
4535 # "\000\001\000\002" on big-endian
4537 $foo = pack("a4","abcd","x","y","z");
4540 $foo = pack("aaaa","abcd","x","y","z");
4543 $foo = pack("a14","abcdefg");
4544 # "abcdefg\0\0\0\0\0\0\0"
4546 $foo = pack("i9pl", gmtime);
4547 # a real struct tm (on my system anyway)
4549 $utmp_template = "Z8 Z8 Z16 L";
4550 $utmp = pack($utmp_template, @utmp1);
4551 # a struct utmp (BSDish)
4553 @utmp2 = unpack($utmp_template, $utmp);
4554 # "@utmp1" eq "@utmp2"
4557 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4560 $foo = pack('sx2l', 12, 34);
4561 # short 12, two zero bytes padding, long 34
4562 $bar = pack('s@4l', 12, 34);
4563 # short 12, zero fill to position 4, long 34
4565 $baz = pack('s.l', 12, 4, 34);
4566 # short 12, zero fill to position 4, long 34
4568 $foo = pack('nN', 42, 4711);
4569 # pack big-endian 16- and 32-bit unsigned integers
4570 $foo = pack('S>L>', 42, 4711);
4572 $foo = pack('s<l<', -42, 4711);
4573 # pack little-endian 16- and 32-bit signed integers
4574 $foo = pack('(sl)<', -42, 4711);
4577 The same template may generally also be used in unpack().
4579 =item package NAMESPACE
4581 =item package NAMESPACE VERSION
4582 X<package> X<module> X<namespace> X<version>
4584 =item package NAMESPACE BLOCK
4586 =item package NAMESPACE VERSION BLOCK
4587 X<package> X<module> X<namespace> X<version>
4589 Declares the BLOCK or the rest of the compilation unit as being in the
4590 given namespace. The scope of the package declaration is either the
4591 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4592 itself through the end of current scope (the enclosing block, file, or
4593 C<eval>). That is, the forms without a BLOCK are operative through the end
4594 of the current scope, just like the C<my>, C<state>, and C<our> operators.
4595 All unqualified dynamic identifiers in this scope will be in the given
4596 namespace, except where overridden by another C<package> declaration or
4597 when they're one of the special identifiers that qualify into C<main::>,
4598 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
4600 A package statement affects dynamic variables only, including those
4601 you've used C<local> on, but I<not> lexical variables, which are created
4602 with C<my>, C<state>, or C<our>. Typically it would be the first
4603 declaration in a file included by C<require> or C<use>. You can switch into a
4604 package in more than one place, since this only determines which default
4605 symbol table the compiler uses for the rest of that block. You can refer to
4606 identifiers in other packages than the current one by prefixing the identifier
4607 with the package name and a double colon, as in C<$SomePack::var>
4608 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4609 package as assumed. That is, C<$::sail> is equivalent to
4610 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4611 code, mostly from Perl 4).
4613 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4614 namespace to a L<version> object with the VERSION provided. VERSION must be a
4615 "strict" style version number as defined by the L<version> module: a positive
4616 decimal number (integer or decimal-fraction) without exponentiation or else a
4617 dotted-decimal v-string with a leading 'v' character and at least three
4618 components. You should set C<$VERSION> only once per package.
4620 See L<perlmod/"Packages"> for more information about packages, modules,
4621 and classes. See L<perlsub> for other scoping issues.
4623 =item pipe READHANDLE,WRITEHANDLE
4626 Opens a pair of connected pipes like the corresponding system call.
4627 Note that if you set up a loop of piped processes, deadlock can occur
4628 unless you are very careful. In addition, note that Perl's pipes use
4629 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4630 after each command, depending on the application.
4632 See L<IPC::Open2>, L<IPC::Open3>, and
4633 L<perlipc/"Bidirectional Communication with Another Process">
4634 for examples of such things.
4636 On systems that support a close-on-exec flag on files, that flag is set
4637 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4638 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4643 A special token that returns the name of the package in which it occurs.
4652 Pops and returns the last value of the array, shortening the array by
4655 Returns the undefined value if the array is empty, although this may also
4656 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4657 main program, but the C<@_> array in subroutines, just like C<shift>.
4659 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
4660 reference to an unblessed array. The argument will be dereferenced
4661 automatically. This aspect of C<pop> is considered highly experimental.
4662 The exact behaviour may change in a future version of Perl.
4665 X<pos> X<match, position>
4669 Returns the offset of where the last C<m//g> search left off for the
4670 variable in question (C<$_> is used when the variable is not
4671 specified). Note that 0 is a valid match offset. C<undef> indicates
4672 that the search position is reset (usually due to match failure, but
4673 can also be because no match has yet been run on the scalar).
4675 C<pos> directly accesses the location used by the regexp engine to
4676 store the offset, so assigning to C<pos> will change that offset, and
4677 so will also influence the C<\G> zero-width assertion in regular
4678 expressions. Both of these effects take place for the next match, so
4679 you can't affect the position with C<pos> during the current match,
4680 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
4682 Setting C<pos> also resets the I<matched with zero-length> flag, described
4683 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
4685 Because a failed C<m//gc> match doesn't reset the offset, the return
4686 from C<pos> won't change either in this case. See L<perlre> and
4689 =item print FILEHANDLE LIST
4692 =item print FILEHANDLE
4698 Prints a string or a list of strings. Returns true if successful.
4699 FILEHANDLE may be a scalar variable containing the name of or a reference
4700 to the filehandle, thus introducing one level of indirection. (NOTE: If
4701 FILEHANDLE is a variable and the next token is a term, it may be
4702 misinterpreted as an operator unless you interpose a C<+> or put
4703 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
4704 last selected (see L</select>) output handle. If LIST is omitted, prints
4705 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
4706 print the content of C<$_> to it, you must use a real filehandle like
4707 C<FH>, not an indirect one like C<$fh>. To set the default output handle
4708 to something other than STDOUT, use the select operation.
4710 The current value of C<$,> (if any) is printed between each LIST item. The
4711 current value of C<$\> (if any) is printed after the entire LIST has been
4712 printed. Because print takes a LIST, anything in the LIST is evaluated in
4713 list context, including any subroutines whose return lists you pass to
4714 C<print>. Be careful not to follow the print keyword with a left
4715 parenthesis unless you want the corresponding right parenthesis to
4716 terminate the arguments to the print; put parentheses around all arguments
4717 (or interpose a C<+>, but that doesn't look as good).
4719 If you're storing handles in an array or hash, or in general whenever
4720 you're using any expression more complex than a bareword handle or a plain,
4721 unsubscripted scalar variable to retrieve it, you will have to use a block
4722 returning the filehandle value instead, in which case the LIST may not be
4725 print { $files[$i] } "stuff\n";
4726 print { $OK ? STDOUT : STDERR } "stuff\n";
4728 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4729 L<perlipc> for more on signal handling.
4731 =item printf FILEHANDLE FORMAT, LIST
4734 =item printf FILEHANDLE
4736 =item printf FORMAT, LIST
4740 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4741 (the output record separator) is not appended. The first argument of the
4742 list will be interpreted as the C<printf> format. See
4743 L<sprintf|/sprintf FORMAT, LIST> for an
4744 explanation of the format argument. If you omit the LIST, C<$_> is used;
4745 to use FILEHANDLE without a LIST, you must use a real filehandle like
4746 C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and
4747 POSIX::setlocale() has been called, the character used for the decimal
4748 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4749 locale setting. See L<perllocale> and L<POSIX>.
4751 Don't fall into the trap of using a C<printf> when a simple
4752 C<print> would do. The C<print> is more efficient and less
4755 =item prototype FUNCTION
4758 Returns the prototype of a function as a string (or C<undef> if the
4759 function has no prototype). FUNCTION is a reference to, or the name of,
4760 the function whose prototype you want to retrieve.
4762 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4763 name for a Perl builtin. If the builtin is not I<overridable> (such as
4764 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4765 (such as C<system>), prototype() returns C<undef>, because the builtin
4766 does not really behave like a Perl function. Otherwise, the string
4767 describing the equivalent prototype is returned.
4769 =item push ARRAY,LIST
4772 =item push EXPR,LIST
4774 Treats ARRAY as a stack by appending the values of LIST to the end of
4775 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
4779 $ARRAY[++$#ARRAY] = $value;
4782 but is more efficient. Returns the number of elements in the array following
4783 the completed C<push>.
4785 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
4786 reference to an unblessed array. The argument will be dereferenced
4787 automatically. This aspect of C<push> is considered highly experimental.
4788 The exact behaviour may change in a future version of Perl.
4798 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4802 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4804 =item quotemeta EXPR
4805 X<quotemeta> X<metacharacter>
4809 Returns the value of EXPR with all non-"word"
4810 characters backslashed. (That is, all characters not matching
4811 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4812 returned string, regardless of any locale settings.)
4813 This is the internal function implementing
4814 the C<\Q> escape in double-quoted strings.
4816 If EXPR is omitted, uses C<$_>.
4818 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
4819 regular expressions, because by default an interpolated variable will be
4820 considered a mini-regular expression. For example:
4822 my $sentence = 'The quick brown fox jumped over the lazy dog';
4823 my $substring = 'quick.*?fox';
4824 $sentence =~ s{$substring}{big bad wolf};
4826 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
4830 my $sentence = 'The quick brown fox jumped over the lazy dog';
4831 my $substring = 'quick.*?fox';
4832 $sentence =~ s{\Q$substring\E}{big bad wolf};
4836 my $sentence = 'The quick brown fox jumped over the lazy dog';
4837 my $substring = 'quick.*?fox';
4838 my $quoted_substring = quotemeta($substring);
4839 $sentence =~ s{$quoted_substring}{big bad wolf};
4841 Will both leave the sentence as is. Normally, when accepting literal string
4842 input from the user, quotemeta() or C<\Q> must be used.
4844 In Perl 5.14, all characters whose code points are above 127 are not
4845 quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings.
4846 It is planned to change this behavior in 5.16, but the exact rules
4847 haven't been determined yet.
4854 Returns a random fractional number greater than or equal to C<0> and less
4855 than the value of EXPR. (EXPR should be positive.) If EXPR is
4856 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4857 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4858 and is subject to change in future versions of Perl). Automatically calls
4859 C<srand> unless C<srand> has already been called. See also C<srand>.
4861 Apply C<int()> to the value returned by C<rand()> if you want random
4862 integers instead of random fractional numbers. For example,
4866 returns a random integer between C<0> and C<9>, inclusive.
4868 (Note: If your rand function consistently returns numbers that are too
4869 large or too small, then your version of Perl was probably compiled
4870 with the wrong number of RANDBITS.)
4872 B<C<rand()> is not cryptographically secure. You should not rely
4873 on it in security-sensitive situations.> As of this writing, a
4874 number of third-party CPAN modules offer random number generators
4875 intended by their authors to be cryptographically secure,
4876 including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and
4877 L<Math::TrulyRandom>.
4879 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4880 X<read> X<file, read>
4882 =item read FILEHANDLE,SCALAR,LENGTH
4884 Attempts to read LENGTH I<characters> of data into variable SCALAR
4885 from the specified FILEHANDLE. Returns the number of characters
4886 actually read, C<0> at end of file, or undef if there was an error (in
4887 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4888 so that the last character actually read is the last character of the
4889 scalar after the read.
4891 An OFFSET may be specified to place the read data at some place in the
4892 string other than the beginning. A negative OFFSET specifies
4893 placement at that many characters counting backwards from the end of
4894 the string. A positive OFFSET greater than the length of SCALAR
4895 results in the string being padded to the required size with C<"\0">
4896 bytes before the result of the read is appended.
4898 The call is implemented in terms of either Perl's or your system's native
4899 fread(3) library function. To get a true read(2) system call, see
4900 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
4902 Note the I<characters>: depending on the status of the filehandle,
4903 either (8-bit) bytes or characters are read. By default, all
4904 filehandles operate on bytes, but for example if the filehandle has
4905 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4906 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
4907 characters, not bytes. Similarly for the C<:encoding> pragma:
4908 in that case pretty much any characters can be read.
4910 =item readdir DIRHANDLE
4913 Returns the next directory entry for a directory opened by C<opendir>.
4914 If used in list context, returns all the rest of the entries in the
4915 directory. If there are no more entries, returns the undefined value in
4916 scalar context and the empty list in list context.
4918 If you're planning to filetest the return values out of a C<readdir>, you'd
4919 better prepend the directory in question. Otherwise, because we didn't
4920 C<chdir> there, it would have been testing the wrong file.
4922 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4923 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4926 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4927 which will set C<$_> on every iteration.
4929 opendir(my $dh, $some_dir) || die;
4930 while(readdir $dh) {
4931 print "$some_dir/$_\n";
4938 X<readline> X<gets> X<fgets>
4940 Reads from the filehandle whose typeglob is contained in EXPR (or from
4941 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
4942 returns the next line until end-of-file is reached, whereupon the
4943 subsequent call returns C<undef>. In list context, reads until end-of-file
4944 is reached and returns a list of lines. Note that the notion of "line"
4945 used here is whatever you may have defined with C<$/> or
4946 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4948 When C<$/> is set to C<undef>, when C<readline> is in scalar
4949 context (i.e., file slurp mode), and when an empty file is read, it
4950 returns C<''> the first time, followed by C<undef> subsequently.
4952 This is the internal function implementing the C<< <EXPR> >>
4953 operator, but you can use it directly. The C<< <EXPR> >>
4954 operator is discussed in more detail in L<perlop/"I/O Operators">.
4957 $line = readline(*STDIN); # same thing
4959 If C<readline> encounters an operating system error, C<$!> will be set
4960 with the corresponding error message. It can be helpful to check
4961 C<$!> when you are reading from filehandles you don't trust, such as a
4962 tty or a socket. The following example uses the operator form of
4963 C<readline> and dies if the result is not defined.
4965 while ( ! eof($fh) ) {
4966 defined( $_ = <$fh> ) or die "readline failed: $!";
4970 Note that you have can't handle C<readline> errors that way with the
4971 C<ARGV> filehandle. In that case, you have to open each element of
4972 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4974 foreach my $arg (@ARGV) {
4975 open(my $fh, $arg) or warn "Can't open $arg: $!";
4977 while ( ! eof($fh) ) {
4978 defined( $_ = <$fh> )
4979 or die "readline failed for $arg: $!";
4989 Returns the value of a symbolic link, if symbolic links are
4990 implemented. If not, raises an exception. If there is a system
4991 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4992 omitted, uses C<$_>.
4994 Portability issues: L<perlport/readlink>.
5001 EXPR is executed as a system command.
5002 The collected standard output of the command is returned.
5003 In scalar context, it comes back as a single (potentially
5004 multi-line) string. In list context, returns a list of lines
5005 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5006 This is the internal function implementing the C<qx/EXPR/>
5007 operator, but you can use it directly. The C<qx/EXPR/>
5008 operator is discussed in more detail in L<perlop/"I/O Operators">.
5009 If EXPR is omitted, uses C<$_>.
5011 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5014 Receives a message on a socket. Attempts to receive LENGTH characters
5015 of data into variable SCALAR from the specified SOCKET filehandle.
5016 SCALAR will be grown or shrunk to the length actually read. Takes the
5017 same flags as the system call of the same name. Returns the address
5018 of the sender if SOCKET's protocol supports this; returns an empty
5019 string otherwise. If there's an error, returns the undefined value.
5020 This call is actually implemented in terms of recvfrom(2) system call.
5021 See L<perlipc/"UDP: Message Passing"> for examples.
5023 Note the I<characters>: depending on the status of the socket, either
5024 (8-bit) bytes or characters are received. By default all sockets
5025 operate on bytes, but for example if the socket has been changed using
5026 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5027 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5028 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5029 case pretty much any characters can be read.
5036 The C<redo> command restarts the loop block without evaluating the
5037 conditional again. The C<continue> block, if any, is not executed. If
5038 the LABEL is omitted, the command refers to the innermost enclosing
5039 loop. Programs that want to lie to themselves about what was just input
5040 normally use this command:
5042 # a simpleminded Pascal comment stripper
5043 # (warning: assumes no { or } in strings)
5044 LINE: while (<STDIN>) {
5045 while (s|({.*}.*){.*}|$1 |) {}
5050 if (/}/) { # end of comment?
5059 C<redo> cannot be used to retry a block that returns a value such as
5060 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5061 a grep() or map() operation.
5063 Note that a block by itself is semantically identical to a loop
5064 that executes once. Thus C<redo> inside such a block will effectively
5065 turn it into a looping construct.
5067 See also L</continue> for an illustration of how C<last>, C<next>, and
5075 Returns a non-empty string if EXPR is a reference, the empty
5076 string otherwise. If EXPR
5077 is not specified, C<$_> will be used. The value returned depends on the
5078 type of thing the reference is a reference to.
5079 Builtin types include:
5093 If the referenced object has been blessed into a package, then that package
5094 name is returned instead. You can think of C<ref> as a C<typeof> operator.
5096 if (ref($r) eq "HASH") {
5097 print "r is a reference to a hash.\n";
5100 print "r is not a reference at all.\n";
5103 The return value C<LVALUE> indicates a reference to an lvalue that is not
5104 a variable. You get this from taking the reference of function calls like
5105 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5106 to a L<version string|perldata/"Version Strings">.
5108 The result C<Regexp> indicates that the argument is a regular expression
5109 resulting from C<qr//>.
5111 See also L<perlref>.
5113 =item rename OLDNAME,NEWNAME
5114 X<rename> X<move> X<mv> X<ren>
5116 Changes the name of a file; an existing file NEWNAME will be
5117 clobbered. Returns true for success, false otherwise.
5119 Behavior of this function varies wildly depending on your system
5120 implementation. For example, it will usually not work across file system
5121 boundaries, even though the system I<mv> command sometimes compensates
5122 for this. Other restrictions include whether it works on directories,
5123 open files, or pre-existing files. Check L<perlport> and either the
5124 rename(2) manpage or equivalent system documentation for details.
5126 For a platform independent C<move> function look at the L<File::Copy>
5129 Portability issues: L<perlport/rename>.
5131 =item require VERSION
5138 Demands a version of Perl specified by VERSION, or demands some semantics
5139 specified by EXPR or by C<$_> if EXPR is not supplied.
5141 VERSION may be either a numeric argument such as 5.006, which will be
5142 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5143 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5144 VERSION is greater than the version of the current Perl interpreter.
5145 Compare with L</use>, which can do a similar check at compile time.
5147 Specifying VERSION as a literal of the form v5.6.1 should generally be
5148 avoided, because it leads to misleading error messages under earlier
5149 versions of Perl that do not support this syntax. The equivalent numeric
5150 version should be used instead.
5152 require v5.6.1; # run time version check
5153 require 5.6.1; # ditto
5154 require 5.006_001; # ditto; preferred for backwards compatibility
5156 Otherwise, C<require> demands that a library file be included if it
5157 hasn't already been included. The file is included via the do-FILE
5158 mechanism, which is essentially just a variety of C<eval> with the
5159 caveat that lexical variables in the invoking script will be invisible
5160 to the included code. Has semantics similar to the following subroutine:
5163 my ($filename) = @_;
5164 if (exists $INC{$filename}) {
5165 return 1 if $INC{$filename};
5166 die "Compilation failed in require";
5168 my ($realfilename,$result);
5170 foreach $prefix (@INC) {
5171 $realfilename = "$prefix/$filename";
5172 if (-f $realfilename) {
5173 $INC{$filename} = $realfilename;
5174 $result = do $realfilename;
5178 die "Can't find $filename in \@INC";
5181 $INC{$filename} = undef;
5183 } elsif (!$result) {
5184 delete $INC{$filename};
5185 die "$filename did not return true value";
5191 Note that the file will not be included twice under the same specified
5194 The file must return true as the last statement to indicate
5195 successful execution of any initialization code, so it's customary to
5196 end such a file with C<1;> unless you're sure it'll return true
5197 otherwise. But it's better just to put the C<1;>, in case you add more
5200 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5201 replaces "F<::>" with "F</>" in the filename for you,
5202 to make it easy to load standard modules. This form of loading of
5203 modules does not risk altering your namespace.
5205 In other words, if you try this:
5207 require Foo::Bar; # a splendid bareword
5209 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5210 directories specified in the C<@INC> array.
5212 But if you try this:
5214 $class = 'Foo::Bar';
5215 require $class; # $class is not a bareword
5217 require "Foo::Bar"; # not a bareword because of the ""
5219 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5220 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5222 eval "require $class";
5224 Now that you understand how C<require> looks for files with a
5225 bareword argument, there is a little extra functionality going on behind
5226 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5227 first look for a similar filename with a "F<.pmc>" extension. If this file
5228 is found, it will be loaded in place of any file ending in a "F<.pm>"
5231 You can also insert hooks into the import facility by putting Perl code
5232 directly into the @INC array. There are three forms of hooks: subroutine
5233 references, array references, and blessed objects.
5235 Subroutine references are the simplest case. When the inclusion system
5236 walks through @INC and encounters a subroutine, this subroutine gets
5237 called with two parameters, the first a reference to itself, and the
5238 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5239 subroutine should return either nothing or else a list of up to three
5240 values in the following order:
5246 A filehandle, from which the file will be read.
5250 A reference to a subroutine. If there is no filehandle (previous item),
5251 then this subroutine is expected to generate one line of source code per
5252 call, writing the line into C<$_> and returning 1, then finally at end of
5253 file returning 0. If there is a filehandle, then the subroutine will be
5254 called to act as a simple source filter, with the line as read in C<$_>.
5255 Again, return 1 for each valid line, and 0 after all lines have been
5260 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5261 reference to the subroutine itself is passed in as C<$_[0]>.
5265 If an empty list, C<undef>, or nothing that matches the first 3 values above
5266 is returned, then C<require> looks at the remaining elements of @INC.
5267 Note that this filehandle must be a real filehandle (strictly a typeglob
5268 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5269 will be ignored and processing will stop there.
5271 If the hook is an array reference, its first element must be a subroutine
5272 reference. This subroutine is called as above, but the first parameter is
5273 the array reference. This lets you indirectly pass arguments to
5276 In other words, you can write:
5278 push @INC, \&my_sub;
5280 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5286 push @INC, [ \&my_sub, $x, $y, ... ];
5288 my ($arrayref, $filename) = @_;
5289 # Retrieve $x, $y, ...
5290 my @parameters = @$arrayref[1..$#$arrayref];
5294 If the hook is an object, it must provide an INC method that will be
5295 called as above, the first parameter being the object itself. (Note that
5296 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5297 into package C<main>.) Here is a typical code layout:
5303 my ($self, $filename) = @_;
5307 # In the main program
5308 push @INC, Foo->new(...);
5310 These hooks are also permitted to set the %INC entry
5311 corresponding to the files they have loaded. See L<perlvar/%INC>.
5313 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5320 Generally used in a C<continue> block at the end of a loop to clear
5321 variables and reset C<??> searches so that they work again. The
5322 expression is interpreted as a list of single characters (hyphens
5323 allowed for ranges). All variables and arrays beginning with one of
5324 those letters are reset to their pristine state. If the expression is
5325 omitted, one-match searches (C<?pattern?>) are reset to match again.
5326 Only resets variables or searches in the current package. Always returns
5329 reset 'X'; # reset all X variables
5330 reset 'a-z'; # reset lower case variables
5331 reset; # just reset ?one-time? searches
5333 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5334 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5335 variables; lexical variables are unaffected, but they clean themselves
5336 up on scope exit anyway, so you'll probably want to use them instead.
5344 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5345 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5346 context, depending on how the return value will be used, and the context
5347 may vary from one execution to the next (see L</wantarray>). If no EXPR
5348 is given, returns an empty list in list context, the undefined value in
5349 scalar context, and (of course) nothing at all in void context.
5351 (In the absence of an explicit C<return>, a subroutine, eval,
5352 or do FILE automatically returns the value of the last expression
5356 X<reverse> X<rev> X<invert>
5358 In list context, returns a list value consisting of the elements
5359 of LIST in the opposite order. In scalar context, concatenates the
5360 elements of LIST and returns a string value with all characters
5361 in the opposite order.
5363 print join(", ", reverse "world", "Hello"); # Hello, world
5365 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5367 Used without arguments in scalar context, reverse() reverses C<$_>.
5369 $_ = "dlrow ,olleH";
5370 print reverse; # No output, list context
5371 print scalar reverse; # Hello, world
5373 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5374 preserve non-existent elements whenever possible, i.e., for non magical
5375 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
5377 This operator is also handy for inverting a hash, although there are some
5378 caveats. If a value is duplicated in the original hash, only one of those
5379 can be represented as a key in the inverted hash. Also, this has to
5380 unwind one hash and build a whole new one, which may take some time
5381 on a large hash, such as from a DBM file.
5383 %by_name = reverse %by_address; # Invert the hash
5385 =item rewinddir DIRHANDLE
5388 Sets the current position to the beginning of the directory for the
5389 C<readdir> routine on DIRHANDLE.
5391 Portability issues: L<perlport/rewinddir>.
5393 =item rindex STR,SUBSTR,POSITION
5396 =item rindex STR,SUBSTR
5398 Works just like index() except that it returns the position of the I<last>
5399 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5400 last occurrence beginning at or before that position.
5402 =item rmdir FILENAME
5403 X<rmdir> X<rd> X<directory, remove>
5407 Deletes the directory specified by FILENAME if that directory is
5408 empty. If it succeeds it returns true; otherwise it returns false and
5409 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5411 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5412 the C<rmtree> function of the L<File::Path> module.
5416 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5418 =item say FILEHANDLE LIST
5421 =item say FILEHANDLE
5427 Just like C<print>, but implicitly appends a newline. C<say LIST> is
5428 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
5429 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
5430 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
5432 This keyword is available only when the C<"say"> feature
5433 is enabled, or when prefixed with C<CORE::>; see
5434 L<feature>. Alternately, include a C<use v5.10> or later to the current
5438 X<scalar> X<context>
5440 Forces EXPR to be interpreted in scalar context and returns the value
5443 @counts = ( scalar @a, scalar @b, scalar @c );
5445 There is no equivalent operator to force an expression to
5446 be interpolated in list context because in practice, this is never
5447 needed. If you really wanted to do so, however, you could use
5448 the construction C<@{[ (some expression) ]}>, but usually a simple
5449 C<(some expression)> suffices.
5451 Because C<scalar> is a unary operator, if you accidentally use a
5452 parenthesized list for the EXPR, this behaves as a scalar comma expression,
5453 evaluating all but the last element in void context and returning the final
5454 element evaluated in scalar context. This is seldom what you want.
5456 The following single statement:
5458 print uc(scalar(&foo,$bar)),$baz;
5460 is the moral equivalent of these two:
5463 print(uc($bar),$baz);
5465 See L<perlop> for more details on unary operators and the comma operator.
5467 =item seek FILEHANDLE,POSITION,WHENCE
5468 X<seek> X<fseek> X<filehandle, position>
5470 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5471 FILEHANDLE may be an expression whose value gives the name of the
5472 filehandle. The values for WHENCE are C<0> to set the new position
5473 I<in bytes> to POSITION; C<1> to set it to the current position plus
5474 POSITION; and C<2> to set it to EOF plus POSITION, typically
5475 negative. For WHENCE you may use the constants C<SEEK_SET>,
5476 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
5477 of the file) from the L<Fcntl> module. Returns C<1> on success, false
5480 Note the I<in bytes>: even if the filehandle has been set to
5481 operate on characters (for example by using the C<:encoding(utf8)> open
5482 layer), tell() will return byte offsets, not character offsets
5483 (because implementing that would render seek() and tell() rather slow).
5485 If you want to position the file for C<sysread> or C<syswrite>, don't use
5486 C<seek>, because buffering makes its effect on the file's read-write position
5487 unpredictable and non-portable. Use C<sysseek> instead.
5489 Due to the rules and rigors of ANSI C, on some systems you have to do a
5490 seek whenever you switch between reading and writing. Amongst other
5491 things, this may have the effect of calling stdio's clearerr(3).
5492 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
5496 This is also useful for applications emulating C<tail -f>. Once you hit
5497 EOF on your read and then sleep for a while, you (probably) have to stick in a
5498 dummy seek() to reset things. The C<seek> doesn't change the position,
5499 but it I<does> clear the end-of-file condition on the handle, so that the
5500 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
5502 If that doesn't work (some I/O implementations are particularly
5503 cantankerous), you might need something like this:
5506 for ($curpos = tell(FILE); $_ = <FILE>;
5507 $curpos = tell(FILE)) {
5508 # search for some stuff and put it into files
5510 sleep($for_a_while);
5511 seek(FILE, $curpos, 0);
5514 =item seekdir DIRHANDLE,POS
5517 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5518 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5519 about possible directory compaction as the corresponding system library
5522 =item select FILEHANDLE
5523 X<select> X<filehandle, default>
5527 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5528 sets the new current default filehandle for output. This has two
5529 effects: first, a C<write> or a C<print> without a filehandle
5530 default to this FILEHANDLE. Second, references to variables related to
5531 output will refer to this output channel.
5533 For example, to set the top-of-form format for more than one
5534 output channel, you might do the following:
5541 FILEHANDLE may be an expression whose value gives the name of the
5542 actual filehandle. Thus:
5544 $oldfh = select(STDERR); $| = 1; select($oldfh);
5546 Some programmers may prefer to think of filehandles as objects with
5547 methods, preferring to write the last example as:
5550 STDERR->autoflush(1);
5552 Portability issues: L<perlport/select>.
5554 =item select RBITS,WBITS,EBITS,TIMEOUT
5557 This calls the select(2) syscall with the bit masks specified, which
5558 can be constructed using C<fileno> and C<vec>, along these lines:
5560 $rin = $win = $ein = '';
5561 vec($rin, fileno(STDIN), 1) = 1;
5562 vec($win, fileno(STDOUT), 1) = 1;
5565 If you want to select on many filehandles, you may wish to write a
5566 subroutine like this:
5571 for my $fh (@fhlist) {
5572 vec($bits, fileno($fh), 1) = 1;
5576 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
5580 ($nfound,$timeleft) =
5581 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5583 or to block until something becomes ready just do this
5585 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5587 Most systems do not bother to return anything useful in $timeleft, so
5588 calling select() in scalar context just returns $nfound.
5590 Any of the bit masks can also be undef. The timeout, if specified, is
5591 in seconds, which may be fractional. Note: not all implementations are
5592 capable of returning the $timeleft. If not, they always return
5593 $timeleft equal to the supplied $timeout.
5595 You can effect a sleep of 250 milliseconds this way:
5597 select(undef, undef, undef, 0.25);
5599 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5600 is implementation-dependent. See also L<perlport> for notes on the
5601 portability of C<select>.
5603 On error, C<select> behaves just like select(2): it returns
5606 On some Unixes, select(2) may report a socket file descriptor as "ready for
5607 reading" even when no data is available, and thus any subsequent C<read>
5608 would block. This can be avoided if you always use O_NONBLOCK on the
5609 socket. See select(2) and fcntl(2) for further details.
5611 The standard C<IO::Select> module provides a user-friendlier interface
5612 to C<select>, mostly because it does all the bit-mask work for you.
5614 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5615 or <FH>) with C<select>, except as permitted by POSIX, and even
5616 then only on POSIX systems. You have to use C<sysread> instead.
5618 Portability issues: L<perlport/select>.
5620 =item semctl ID,SEMNUM,CMD,ARG
5623 Calls the System V IPC function semctl(2). You'll probably have to say
5627 first to get the correct constant definitions. If CMD is IPC_STAT or
5628 GETALL, then ARG must be a variable that will hold the returned
5629 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5630 the undefined value for error, "C<0 but true>" for zero, or the actual
5631 return value otherwise. The ARG must consist of a vector of native
5632 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5633 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5636 Portability issues: L<perlport/semctl>.
5638 =item semget KEY,NSEMS,FLAGS
5641 Calls the System V IPC function semget(2). Returns the semaphore id, or
5642 the undefined value on error. See also
5643 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5646 Portability issues: L<perlport/semget>.
5648 =item semop KEY,OPSTRING
5651 Calls the System V IPC function semop(2) for semaphore operations
5652 such as signalling and waiting. OPSTRING must be a packed array of
5653 semop structures. Each semop structure can be generated with
5654 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5655 implies the number of semaphore operations. Returns true if
5656 successful, false on error. As an example, the
5657 following code waits on semaphore $semnum of semaphore id $semid:
5659 $semop = pack("s!3", $semnum, -1, 0);
5660 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5662 To signal the semaphore, replace C<-1> with C<1>. See also
5663 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5666 Portability issues: L<perlport/semop>.
5668 =item send SOCKET,MSG,FLAGS,TO
5671 =item send SOCKET,MSG,FLAGS
5673 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5674 filehandle. Takes the same flags as the system call of the same name. On
5675 unconnected sockets, you must specify a destination to I<send to>, in which
5676 case it does a sendto(2) syscall. Returns the number of characters sent,
5677 or the undefined value on error. The sendmsg(2) syscall is currently
5678 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5680 Note the I<characters>: depending on the status of the socket, either
5681 (8-bit) bytes or characters are sent. By default all sockets operate
5682 on bytes, but for example if the socket has been changed using
5683 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5684 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5685 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5686 pragma: in that case pretty much any characters can be sent.
5688 =item setpgrp PID,PGRP
5691 Sets the current process group for the specified PID, C<0> for the current
5692 process. Raises an exception when used on a machine that doesn't
5693 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5694 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5695 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5698 Portability issues: L<perlport/setpgrp>.
5700 =item setpriority WHICH,WHO,PRIORITY
5701 X<setpriority> X<priority> X<nice> X<renice>
5703 Sets the current priority for a process, a process group, or a user.
5704 (See setpriority(2).) Raises an exception when used on a machine
5705 that doesn't implement setpriority(2).
5707 Portability issues: L<perlport/setpriority>.
5709 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5712 Sets the socket option requested. Returns C<undef> on error.
5713 Use integer constants provided by the C<Socket> module for
5714 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5715 getprotobyname. OPTVAL might either be a packed string or an integer.
5716 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5718 An example disabling Nagle's algorithm on a socket:
5720 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5721 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5723 Portability issues: L<perlport/setsockopt>.
5732 Shifts the first value of the array off and returns it, shortening the
5733 array by 1 and moving everything down. If there are no elements in the
5734 array, returns the undefined value. If ARRAY is omitted, shifts the
5735 C<@_> array within the lexical scope of subroutines and formats, and the
5736 C<@ARGV> array outside a subroutine and also within the lexical scopes
5737 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5738 C<UNITCHECK {}>, and C<END {}> constructs.
5740 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
5741 reference to an unblessed array. The argument will be dereferenced
5742 automatically. This aspect of C<shift> is considered highly experimental.
5743 The exact behaviour may change in a future version of Perl.
5745 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5746 same thing to the left end of an array that C<pop> and C<push> do to the
5749 =item shmctl ID,CMD,ARG
5752 Calls the System V IPC function shmctl. You'll probably have to say
5756 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5757 then ARG must be a variable that will hold the returned C<shmid_ds>
5758 structure. Returns like ioctl: C<undef> for error; "C<0> but
5759 true" for zero; and the actual return value otherwise.
5760 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5762 Portability issues: L<perlport/shmctl>.
5764 =item shmget KEY,SIZE,FLAGS
5767 Calls the System V IPC function shmget. Returns the shared memory
5768 segment id, or C<undef> on error.
5769 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5771 Portability issues: L<perlport/shmget>.
5773 =item shmread ID,VAR,POS,SIZE
5777 =item shmwrite ID,STRING,POS,SIZE
5779 Reads or writes the System V shared memory segment ID starting at
5780 position POS for size SIZE by attaching to it, copying in/out, and
5781 detaching from it. When reading, VAR must be a variable that will
5782 hold the data read. When writing, if STRING is too long, only SIZE
5783 bytes are used; if STRING is too short, nulls are written to fill out
5784 SIZE bytes. Return true if successful, false on error.
5785 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5786 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
5788 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
5790 =item shutdown SOCKET,HOW
5793 Shuts down a socket connection in the manner indicated by HOW, which
5794 has the same interpretation as in the syscall of the same name.
5796 shutdown(SOCKET, 0); # I/we have stopped reading data
5797 shutdown(SOCKET, 1); # I/we have stopped writing data
5798 shutdown(SOCKET, 2); # I/we have stopped using this socket
5800 This is useful with sockets when you want to tell the other
5801 side you're done writing but not done reading, or vice versa.
5802 It's also a more insistent form of close because it also
5803 disables the file descriptor in any forked copies in other
5806 Returns C<1> for success; on error, returns C<undef> if
5807 the first argument is not a valid filehandle, or returns C<0> and sets
5808 C<$!> for any other failure.
5811 X<sin> X<sine> X<asin> X<arcsine>
5815 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5816 returns sine of C<$_>.
5818 For the inverse sine operation, you may use the C<Math::Trig::asin>
5819 function, or use this relation:
5821 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5828 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5829 argument is given. Returns the integer number of seconds actually slept.
5831 May be interrupted if the process receives a signal such as C<SIGALRM>.
5834 local $SIG{ALARM} = sub { die "Alarm!\n" };
5837 die $@ unless $@ eq "Alarm!\n";
5839 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5840 is often implemented using C<alarm>.
5842 On some older systems, it may sleep up to a full second less than what
5843 you requested, depending on how it counts seconds. Most modern systems
5844 always sleep the full amount. They may appear to sleep longer than that,
5845 however, because your process might not be scheduled right away in a
5846 busy multitasking system.
5848 For delays of finer granularity than one second, the Time::HiRes module
5849 (from CPAN, and starting from Perl 5.8 part of the standard
5850 distribution) provides usleep(). You may also use Perl's four-argument
5851 version of select() leaving the first three arguments undefined, or you
5852 might be able to use the C<syscall> interface to access setitimer(2) if
5853 your system supports it. See L<perlfaq8> for details.
5855 See also the POSIX module's C<pause> function.
5857 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5860 Opens a socket of the specified kind and attaches it to filehandle
5861 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5862 the syscall of the same name. You should C<use Socket> first
5863 to get the proper definitions imported. See the examples in
5864 L<perlipc/"Sockets: Client/Server Communication">.
5866 On systems that support a close-on-exec flag on files, the flag will
5867 be set for the newly opened file descriptor, as determined by the
5868 value of $^F. See L<perlvar/$^F>.
5870 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5873 Creates an unnamed pair of sockets in the specified domain, of the
5874 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5875 for the syscall of the same name. If unimplemented, raises an exception.
5876 Returns true if successful.
5878 On systems that support a close-on-exec flag on files, the flag will
5879 be set for the newly opened file descriptors, as determined by the value
5880 of $^F. See L<perlvar/$^F>.
5882 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5883 to C<pipe(Rdr, Wtr)> is essentially:
5886 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5887 shutdown(Rdr, 1); # no more writing for reader
5888 shutdown(Wtr, 0); # no more reading for writer
5890 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5891 emulate socketpair using IP sockets to localhost if your system implements
5892 sockets but not socketpair.
5894 Portability issues: L<perlport/socketpair>.
5896 =item sort SUBNAME LIST
5897 X<sort> X<qsort> X<quicksort> X<mergesort>
5899 =item sort BLOCK LIST
5903 In list context, this sorts the LIST and returns the sorted list value.
5904 In scalar context, the behaviour of C<sort()> is undefined.
5906 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5907 order. If SUBNAME is specified, it gives the name of a subroutine
5908 that returns an integer less than, equal to, or greater than C<0>,
5909 depending on how the elements of the list are to be ordered. (The
5910 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5911 SUBNAME may be a scalar variable name (unsubscripted), in which case
5912 the value provides the name of (or a reference to) the actual
5913 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5914 an anonymous, in-line sort subroutine.
5916 If the subroutine's prototype is C<($$)>, the elements to be compared are
5917 passed by reference in C<@_>, as for a normal subroutine. This is slower
5918 than unprototyped subroutines, where the elements to be compared are passed
5919 into the subroutine as the package global variables $a and $b (see example
5920 below). Note that in the latter case, it is usually highly counter-productive
5921 to declare $a and $b as lexicals.
5923 If the subroutine is an XSUB, the elements to be compared are pushed on to
5924 the stack, the way arguments are usually passed to XSUBs. $a and $b are
5927 The values to be compared are always passed by reference and should not
5930 You also cannot exit out of the sort block or subroutine using any of the
5931 loop control operators described in L<perlsyn> or with C<goto>.
5933 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5934 current collation locale. See L<perllocale>.
5936 sort() returns aliases into the original list, much as a for loop's index
5937 variable aliases the list elements. That is, modifying an element of a
5938 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5939 actually modifies the element in the original list. This is usually
5940 something to be avoided when writing clear code.
5942 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5943 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
5944 preserves the input order of elements that compare equal. Although
5945 quicksort's run time is O(NlogN) when averaged over all arrays of
5946 length N, the time can be O(N**2), I<quadratic> behavior, for some
5947 inputs.) In 5.7, the quicksort implementation was replaced with
5948 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5949 But benchmarks indicated that for some inputs, on some platforms,
5950 the original quicksort was faster. 5.8 has a sort pragma for
5951 limited control of the sort. Its rather blunt control of the
5952 underlying algorithm may not persist into future Perls, but the
5953 ability to characterize the input or output in implementation
5954 independent ways quite probably will. See L<the sort pragma|sort>.
5959 @articles = sort @files;
5961 # same thing, but with explicit sort routine
5962 @articles = sort {$a cmp $b} @files;
5964 # now case-insensitively
5965 @articles = sort {uc($a) cmp uc($b)} @files;
5967 # same thing in reversed order
5968 @articles = sort {$b cmp $a} @files;
5970 # sort numerically ascending
5971 @articles = sort {$a <=> $b} @files;
5973 # sort numerically descending
5974 @articles = sort {$b <=> $a} @files;
5976 # this sorts the %age hash by value instead of key
5977 # using an in-line function
5978 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5980 # sort using explicit subroutine name
5982 $age{$a} <=> $age{$b}; # presuming numeric
5984 @sortedclass = sort byage @class;
5986 sub backwards { $b cmp $a }
5987 @harry = qw(dog cat x Cain Abel);
5988 @george = qw(gone chased yz Punished Axed);
5990 # prints AbelCaincatdogx
5991 print sort backwards @harry;
5992 # prints xdogcatCainAbel
5993 print sort @george, 'to', @harry;
5994 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5996 # inefficiently sort by descending numeric compare using
5997 # the first integer after the first = sign, or the
5998 # whole record case-insensitively otherwise
6001 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6006 # same thing, but much more efficiently;
6007 # we'll build auxiliary indices instead
6009 my @nums = @caps = ();
6011 push @nums, ( /=(\d+)/ ? $1 : undef );
6015 my @new = @old[ sort {
6016 $nums[$b] <=> $nums[$a]
6018 $caps[$a] cmp $caps[$b]
6022 # same thing, but without any temps
6023 @new = map { $_->[0] }
6024 sort { $b->[1] <=> $a->[1]
6027 } map { [$_, /=(\d+)/, uc($_)] } @old;
6029 # using a prototype allows you to use any comparison subroutine
6030 # as a sort subroutine (including other package's subroutines)
6032 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
6035 @new = sort other::backwards @old;
6037 # guarantee stability, regardless of algorithm
6039 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6041 # force use of mergesort (not portable outside Perl 5.8)
6042 use sort '_mergesort'; # note discouraging _
6043 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6045 Warning: syntactical care is required when sorting the list returned from
6046 a function. If you want to sort the list returned by the function call
6047 C<find_records(@key)>, you can use:
6049 @contact = sort { $a cmp $b } find_records @key;
6050 @contact = sort +find_records(@key);
6051 @contact = sort &find_records(@key);
6052 @contact = sort(find_records(@key));
6054 If instead you want to sort the array @key with the comparison routine
6055 C<find_records()> then you can use:
6057 @contact = sort { find_records() } @key;
6058 @contact = sort find_records(@key);
6059 @contact = sort(find_records @key);
6060 @contact = sort(find_records (@key));
6062 If you're using strict, you I<must not> declare $a
6063 and $b as lexicals. They are package globals. That means
6064 that if you're in the C<main> package and type
6066 @articles = sort {$b <=> $a} @files;
6068 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6069 but if you're in the C<FooPack> package, it's the same as typing
6071 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6073 The comparison function is required to behave. If it returns
6074 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6075 sometimes saying the opposite, for example) the results are not
6078 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6079 (not-a-number), be careful when sorting with a
6080 comparison function like C<< $a <=> $b >> any lists that might contain a
6081 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6082 eliminate any C<NaN>s from the input list.
6084 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6086 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6089 =item splice ARRAY or EXPR,OFFSET,LENGTH
6091 =item splice ARRAY or EXPR,OFFSET
6093 =item splice ARRAY or EXPR
6095 Removes the elements designated by OFFSET and LENGTH from an array, and
6096 replaces them with the elements of LIST, if any. In list context,
6097 returns the elements removed from the array. In scalar context,
6098 returns the last element removed, or C<undef> if no elements are
6099 removed. The array grows or shrinks as necessary.
6100 If OFFSET is negative then it starts that far from the end of the array.
6101 If LENGTH is omitted, removes everything from OFFSET onward.
6102 If LENGTH is negative, removes the elements from OFFSET onward
6103 except for -LENGTH elements at the end of the array.
6104 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6105 past the end of the array, Perl issues a warning, and splices at the
6108 The following equivalences hold (assuming C<< $#a >= $i >> )
6110 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6111 pop(@a) splice(@a,-1)
6112 shift(@a) splice(@a,0,1)
6113 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6114 $a[$i] = $y splice(@a,$i,1,$y)
6116 Example, assuming array lengths are passed before arrays:
6118 sub aeq { # compare two list values
6119 my(@a) = splice(@_,0,shift);
6120 my(@b) = splice(@_,0,shift);
6121 return 0 unless @a == @b; # same len?
6123 return 0 if pop(@a) ne pop(@b);
6127 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6129 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6130 reference to an unblessed array. The argument will be dereferenced
6131 automatically. This aspect of C<splice> is considered highly experimental.
6132 The exact behaviour may change in a future version of Perl.
6134 =item split /PATTERN/,EXPR,LIMIT
6137 =item split /PATTERN/,EXPR
6139 =item split /PATTERN/
6143 Splits the string EXPR into a list of strings and returns that list. By
6144 default, empty leading fields are preserved, and empty trailing ones are
6145 deleted. (If all fields are empty, they are considered to be trailing.)
6147 In scalar context, returns the number of fields found.
6149 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
6150 splits on whitespace (after skipping any leading whitespace). Anything
6151 matching PATTERN is taken to be a delimiter separating the fields. (Note
6152 that the delimiter may be longer than one character.)
6154 If LIMIT is specified and positive, it represents the maximum number
6155 of fields the EXPR will be split into, though the actual number of
6156 fields returned depends on the number of times PATTERN matches within
6157 EXPR. If LIMIT is unspecified or zero, trailing null fields are
6158 stripped (which potential users of C<pop> would do well to remember).
6159 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
6160 had been specified. Note that splitting an EXPR that evaluates to the
6161 empty string always returns the empty list, regardless of the LIMIT
6164 A pattern matching the empty string (not to be confused with
6165 an empty pattern C<//>, which is just one member of the set of patterns
6166 matching the empty string), splits EXPR into individual
6167 characters. For example:
6169 print join(':', split(/ */, 'hi there')), "\n";
6171 produces the output 'h:i:t:h:e:r:e'.
6173 As a special case for C<split>, the empty pattern C<//> specifically
6174 matches the empty string; this is not be confused with the normal use
6175 of an empty pattern to mean the last successful match. So to split
6176 a string into individual characters, the following:
6178 print join(':', split(//, 'hi there')), "\n";
6180 produces the output 'h:i: :t:h:e:r:e'.
6182 Empty leading fields are produced when there are positive-width matches at
6183 the beginning of the string; a zero-width match at the beginning of
6184 the string does not produce an empty field. For example:
6186 print join(':', split(/(?=\w)/, 'hi there!'));
6188 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
6189 hand, are produced when there is a match at the end of the string (and
6190 when LIMIT is given and is not 0), regardless of the length of the match.
6193 print join(':', split(//, 'hi there!', -1)), "\n";
6194 print join(':', split(/\W/, 'hi there!', -1)), "\n";
6196 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
6197 both with an empty trailing field.
6199 The LIMIT parameter can be used to split a line partially
6201 ($login, $passwd, $remainder) = split(/:/, $_, 3);
6203 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
6204 a LIMIT one larger than the number of variables in the list, to avoid
6205 unnecessary work. For the list above LIMIT would have been 4 by
6206 default. In time critical applications it behooves you not to split
6207 into more fields than you really need.
6209 If the PATTERN contains parentheses, additional list elements are
6210 created from each matching substring in the delimiter.
6212 split(/([,-])/, "1-10,20", 3);
6214 produces the list value
6216 (1, '-', 10, ',', 20)
6218 If you had the entire header of a normal Unix email message in $header,
6219 you could split it up into fields and their values this way:
6221 $header =~ s/\n(?=\s)//g; # fix continuation lines
6222 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
6224 The pattern C</PATTERN/> may be replaced with an expression to specify
6225 patterns that vary at runtime. (To do runtime compilation only once,
6226 use C</$variable/o>.)
6228 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
6229 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
6230 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
6231 will give you as many initial null fields (empty string) as there are leading spaces.
6232 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
6233 whitespace produces a null first field. A C<split> with no arguments
6234 really does a S<C<split(' ', $_)>> internally.
6236 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
6241 open(PASSWD, '/etc/passwd');
6244 ($login, $passwd, $uid, $gid,
6245 $gcos, $home, $shell) = split(/:/);
6249 As with regular pattern matching, any capturing parentheses that are not
6250 matched in a C<split()> will be set to C<undef> when returned:
6252 @fields = split /(A)|B/, "1A2B3";
6253 # @fields is (1, 'A', 2, undef, 3)
6255 =item sprintf FORMAT, LIST
6258 Returns a string formatted by the usual C<printf> conventions of the C
6259 library function C<sprintf>. See below for more details
6260 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
6261 the general principles.
6265 # Format number with up to 8 leading zeroes
6266 $result = sprintf("%08d", $number);
6268 # Round number to 3 digits after decimal point
6269 $rounded = sprintf("%.3f", $number);
6271 Perl does its own C<sprintf> formatting: it emulates the C
6272 function sprintf(3), but doesn't use it except for floating-point
6273 numbers, and even then only standard modifiers are allowed.
6274 Non-standard extensions in your local sprintf(3) are
6275 therefore unavailable from Perl.
6277 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
6278 pass it an array as your first argument. The array is given scalar context,
6279 and instead of using the 0th element of the array as the format, Perl will
6280 use the count of elements in the array as the format, which is almost never
6283 Perl's C<sprintf> permits the following universally-known conversions:
6286 %c a character with the given number
6288 %d a signed integer, in decimal
6289 %u an unsigned integer, in decimal
6290 %o an unsigned integer, in octal
6291 %x an unsigned integer, in hexadecimal
6292 %e a floating-point number, in scientific notation
6293 %f a floating-point number, in fixed decimal notation
6294 %g a floating-point number, in %e or %f notation
6296 In addition, Perl permits the following widely-supported conversions:
6298 %X like %x, but using upper-case letters
6299 %E like %e, but using an upper-case "E"
6300 %G like %g, but with an upper-case "E" (if applicable)
6301 %b an unsigned integer, in binary
6302 %B like %b, but using an upper-case "B" with the # flag
6303 %p a pointer (outputs the Perl value's address in hexadecimal)
6304 %n special: *stores* the number of characters output so far
6305 into the next variable in the parameter list
6307 Finally, for backward (and we do mean "backward") compatibility, Perl
6308 permits these unnecessary but widely-supported conversions:
6311 %D a synonym for %ld
6312 %U a synonym for %lu
6313 %O a synonym for %lo
6316 Note that the number of exponent digits in the scientific notation produced
6317 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
6318 exponent less than 100 is system-dependent: it may be three or less
6319 (zero-padded as necessary). In other words, 1.23 times ten to the
6320 99th may be either "1.23e99" or "1.23e099".
6322 Between the C<%> and the format letter, you may specify several
6323 additional attributes controlling the interpretation of the format.
6324 In order, these are:
6328 =item format parameter index
6330 An explicit format parameter index, such as C<2$>. By default sprintf
6331 will format the next unused argument in the list, but this allows you
6332 to take the arguments out of order:
6334 printf '%2$d %1$d', 12, 34; # prints "34 12"
6335 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
6341 space prefix non-negative number with a space
6342 + prefix non-negative number with a plus sign
6343 - left-justify within the field
6344 0 use zeros, not spaces, to right-justify
6345 # ensure the leading "0" for any octal,
6346 prefix non-zero hexadecimal with "0x" or "0X",
6347 prefix non-zero binary with "0b" or "0B"
6351 printf '<% d>', 12; # prints "< 12>"
6352 printf '<%+d>', 12; # prints "<+12>"
6353 printf '<%6s>', 12; # prints "< 12>"
6354 printf '<%-6s>', 12; # prints "<12 >"
6355 printf '<%06s>', 12; # prints "<000012>"
6356 printf '<%#o>', 12; # prints "<014>"
6357 printf '<%#x>', 12; # prints "<0xc>"
6358 printf '<%#X>', 12; # prints "<0XC>"
6359 printf '<%#b>', 12; # prints "<0b1100>"
6360 printf '<%#B>', 12; # prints "<0B1100>"
6362 When a space and a plus sign are given as the flags at once,
6363 a plus sign is used to prefix a positive number.
6365 printf '<%+ d>', 12; # prints "<+12>"
6366 printf '<% +d>', 12; # prints "<+12>"
6368 When the # flag and a precision are given in the %o conversion,
6369 the precision is incremented if it's necessary for the leading "0".
6371 printf '<%#.5o>', 012; # prints "<00012>"
6372 printf '<%#.5o>', 012345; # prints "<012345>"
6373 printf '<%#.0o>', 0; # prints "<0>"
6377 This flag tells Perl to interpret the supplied string as a vector of
6378 integers, one for each character in the string. Perl applies the format to
6379 each integer in turn, then joins the resulting strings with a separator (a
6380 dot C<.> by default). This can be useful for displaying ordinal values of
6381 characters in arbitrary strings:
6383 printf "%vd", "AB\x{100}"; # prints "65.66.256"
6384 printf "version is v%vd\n", $^V; # Perl's version
6386 Put an asterisk C<*> before the C<v> to override the string to
6387 use to separate the numbers:
6389 printf "address is %*vX\n", ":", $addr; # IPv6 address
6390 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
6392 You can also explicitly specify the argument number to use for
6393 the join string using something like C<*2$v>; for example:
6395 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
6397 =item (minimum) width
6399 Arguments are usually formatted to be only as wide as required to
6400 display the given value. You can override the width by putting
6401 a number here, or get the width from the next argument (with C<*>)
6402 or from a specified argument (e.g., with C<*2$>):
6404 printf "<%s>", "a"; # prints "<a>"
6405 printf "<%6s>", "a"; # prints "< a>"
6406 printf "<%*s>", 6, "a"; # prints "< a>"
6407 printf "<%*2$s>", "a", 6; # prints "< a>"
6408 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
6410 If a field width obtained through C<*> is negative, it has the same
6411 effect as the C<-> flag: left-justification.
6413 =item precision, or maximum width
6416 You can specify a precision (for numeric conversions) or a maximum
6417 width (for string conversions) by specifying a C<.> followed by a number.
6418 For floating-point formats except C<g> and C<G>, this specifies
6419 how many places right of the decimal point to show (the default being 6).
6422 # these examples are subject to system-specific variation
6423 printf '<%f>', 1; # prints "<1.000000>"
6424 printf '<%.1f>', 1; # prints "<1.0>"
6425 printf '<%.0f>', 1; # prints "<1>"
6426 printf '<%e>', 10; # prints "<1.000000e+01>"
6427 printf '<%.1e>', 10; # prints "<1.0e+01>"
6429 For "g" and "G", this specifies the maximum number of digits to show,
6430 including those prior to the decimal point and those after it; for
6433 # These examples are subject to system-specific variation.
6434 printf '<%g>', 1; # prints "<1>"
6435 printf '<%.10g>', 1; # prints "<1>"
6436 printf '<%g>', 100; # prints "<100>"
6437 printf '<%.1g>', 100; # prints "<1e+02>"
6438 printf '<%.2g>', 100.01; # prints "<1e+02>"
6439 printf '<%.5g>', 100.01; # prints "<100.01>"
6440 printf '<%.4g>', 100.01; # prints "<100>"
6442 For integer conversions, specifying a precision implies that the
6443 output of the number itself should be zero-padded to this width,
6444 where the 0 flag is ignored:
6446 printf '<%.6d>', 1; # prints "<000001>"
6447 printf '<%+.6d>', 1; # prints "<+000001>"
6448 printf '<%-10.6d>', 1; # prints "<000001 >"
6449 printf '<%10.6d>', 1; # prints "< 000001>"
6450 printf '<%010.6d>', 1; # prints "< 000001>"
6451 printf '<%+10.6d>', 1; # prints "< +000001>"
6453 printf '<%.6x>', 1; # prints "<000001>"
6454 printf '<%#.6x>', 1; # prints "<0x000001>"
6455 printf '<%-10.6x>', 1; # prints "<000001 >"
6456 printf '<%10.6x>', 1; # prints "< 000001>"
6457 printf '<%010.6x>', 1; # prints "< 000001>"
6458 printf '<%#10.6x>', 1; # prints "< 0x000001>"
6460 For string conversions, specifying a precision truncates the string
6461 to fit the specified width:
6463 printf '<%.5s>', "truncated"; # prints "<trunc>"
6464 printf '<%10.5s>', "truncated"; # prints "< trunc>"
6466 You can also get the precision from the next argument using C<.*>:
6468 printf '<%.6x>', 1; # prints "<000001>"
6469 printf '<%.*x>', 6, 1; # prints "<000001>"
6471 If a precision obtained through C<*> is negative, it counts
6472 as having no precision at all.
6474 printf '<%.*s>', 7, "string"; # prints "<string>"
6475 printf '<%.*s>', 3, "string"; # prints "<str>"
6476 printf '<%.*s>', 0, "string"; # prints "<>"
6477 printf '<%.*s>', -1, "string"; # prints "<string>"
6479 printf '<%.*d>', 1, 0; # prints "<0>"
6480 printf '<%.*d>', 0, 0; # prints "<>"
6481 printf '<%.*d>', -1, 0; # prints "<0>"
6483 You cannot currently get the precision from a specified number,
6484 but it is intended that this will be possible in the future, for
6485 example using C<.*2$>:
6487 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
6491 For numeric conversions, you can specify the size to interpret the
6492 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
6493 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
6494 whatever the default integer size is on your platform (usually 32 or 64
6495 bits), but you can override this to use instead one of the standard C types,
6496 as supported by the compiler used to build Perl:
6498 hh interpret integer as C type "char" or "unsigned char"
6499 on Perl 5.14 or later
6500 h interpret integer as C type "short" or "unsigned short"
6501 j interpret integer as C type "intmax_t" on Perl 5.14
6502 or later, and only with a C99 compiler (unportable)
6503 l interpret integer as C type "long" or "unsigned long"
6504 q, L, or ll interpret integer as C type "long long", "unsigned long long",
6505 or "quad" (typically 64-bit integers)
6506 t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
6507 z interpret integer as C type "size_t" on Perl 5.14 or later
6509 As of 5.14, none of these raises an exception if they are not supported on
6510 your platform. However, if warnings are enabled, a warning of the
6511 C<printf> warning class is issued on an unsupported conversion flag.
6512 Should you instead prefer an exception, do this:
6514 use warnings FATAL => "printf";
6516 If you would like to know about a version dependency before you
6517 start running the program, put something like this at its top:
6519 use 5.014; # for hh/j/t/z/ printf modifiers
6521 You can find out whether your Perl supports quads via L<Config>:
6524 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
6525 print "Nice quads!\n";
6528 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
6529 to be the default floating-point size on your platform (double or long double),
6530 but you can force "long double" with C<q>, C<L>, or C<ll> if your
6531 platform supports them. You can find out whether your Perl supports long
6532 doubles via L<Config>:
6535 print "long doubles\n" if $Config{d_longdbl} eq "define";
6537 You can find out whether Perl considers "long double" to be the default
6538 floating-point size to use on your platform via L<Config>:
6541 if ($Config{uselongdouble} eq "define") {
6542 print "long doubles by default\n";
6545 It can also be that long doubles and doubles are the same thing:
6548 ($Config{doublesize} == $Config{longdblsize}) &&
6549 print "doubles are long doubles\n";
6551 The size specifier C<V> has no effect for Perl code, but is supported for
6552 compatibility with XS code. It means "use the standard size for a Perl
6553 integer or floating-point number", which is the default.
6555 =item order of arguments
6557 Normally, sprintf() takes the next unused argument as the value to
6558 format for each format specification. If the format specification
6559 uses C<*> to require additional arguments, these are consumed from
6560 the argument list in the order they appear in the format
6561 specification I<before> the value to format. Where an argument is
6562 specified by an explicit index, this does not affect the normal
6563 order for the arguments, even when the explicitly specified index
6564 would have been the next argument.
6568 printf "<%*.*s>", $a, $b, $c;
6570 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6571 as the value to format; while:
6573 printf "<%*1$.*s>", $a, $b;
6575 would use C<$a> for the width and precision, and C<$b> as the
6578 Here are some more examples; be aware that when using an explicit
6579 index, the C<$> may need escaping:
6581 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6582 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6583 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6584 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6588 If C<use locale> is in effect and POSIX::setlocale() has been called,
6589 the character used for the decimal separator in formatted floating-point
6590 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6594 X<sqrt> X<root> X<square root>
6598 Return the positive square root of EXPR. If EXPR is omitted, uses
6599 C<$_>. Works only for non-negative operands unless you've
6600 loaded the C<Math::Complex> module.
6603 print sqrt(-4); # prints 2i
6606 X<srand> X<seed> X<randseed>
6610 Sets and returns the random number seed for the C<rand> operator.
6612 The point of the function is to "seed" the C<rand> function so that
6613 C<rand> can produce a different sequence each time you run your
6614 program. When called with a parameter, C<srand> uses that for the seed;
6615 otherwise it (semi-)randomly chooses a seed. In either case, starting with
6616 Perl 5.14, it returns the seed.
6618 If C<srand()> is not called explicitly, it is called implicitly without a
6619 parameter at the first use of the C<rand> operator. However, this was not true
6620 of versions of Perl before 5.004, so if your script will run under older
6621 Perl versions, it should call C<srand>; otherwise most programs won't call
6624 But there are a few situations in recent Perls where programs are likely to
6625 want to call C<srand>. One is for generating predictable results generally for
6626 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
6627 each time. Another other case is where you need a cryptographically-strong
6628 starting point rather than the generally acceptable default, which is based on
6629 time of day, process ID, and memory allocation, or the F</dev/urandom> device
6630 if available. And still another case is that you may want to call C<srand()>
6631 after a C<fork()> to avoid child processes sharing the same seed value as the
6632 parent (and consequently each other).
6634 Do B<not> call C<srand()> (i.e., without an argument) more than once per
6635 process. The internal state of the random number generator should
6636 contain more entropy than can be provided by any seed, so calling
6637 C<srand()> again actually I<loses> randomness.
6639 Most implementations of C<srand> take an integer and will silently
6640 truncate decimal numbers. This means C<srand(42)> will usually
6641 produce the same results as C<srand(42.1)>. To be safe, always pass
6642 C<srand> an integer.
6644 In versions of Perl prior to 5.004 the default seed was just the
6645 current C<time>. This isn't a particularly good seed, so many old
6646 programs supply their own seed value (often C<time ^ $$> or C<time ^
6647 ($$ + ($$ << 15))>), but that isn't necessary any more.
6649 For cryptographic purposes, however, you need something much more random
6650 than the default seed. Checksumming the compressed output of one or more
6651 rapidly changing operating system status programs is the usual method. For
6654 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6656 If you're particularly concerned with this, search the CPAN for
6657 random number generator modules instead of rolling out your own.
6659 Frequently called programs (like CGI scripts) that simply use
6663 for a seed can fall prey to the mathematical property that
6667 one-third of the time. So don't do that.
6669 A typical use of the returned seed is for a test program which has too many
6670 combinations to test comprehensively in the time available to it each run. It
6671 can test a random subset each time, and should there be a failure, log the seed
6672 used for that run so that it can later be used to reproduce the same results.
6674 =item stat FILEHANDLE
6675 X<stat> X<file, status> X<ctime>
6679 =item stat DIRHANDLE
6683 Returns a 13-element list giving the status info for a file, either
6684 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6685 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
6688 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6689 $atime,$mtime,$ctime,$blksize,$blocks)
6692 Not all fields are supported on all filesystem types. Here are the
6693 meanings of the fields:
6695 0 dev device number of filesystem
6697 2 mode file mode (type and permissions)
6698 3 nlink number of (hard) links to the file
6699 4 uid numeric user ID of file's owner
6700 5 gid numeric group ID of file's owner
6701 6 rdev the device identifier (special files only)
6702 7 size total size of file, in bytes
6703 8 atime last access time in seconds since the epoch
6704 9 mtime last modify time in seconds since the epoch
6705 10 ctime inode change time in seconds since the epoch (*)
6706 11 blksize preferred block size for file system I/O
6707 12 blocks actual number of blocks allocated
6709 (The epoch was at 00:00 January 1, 1970 GMT.)
6711 (*) Not all fields are supported on all filesystem types. Notably, the
6712 ctime field is non-portable. In particular, you cannot expect it to be a
6713 "creation time"; see L<perlport/"Files and Filesystems"> for details.
6715 If C<stat> is passed the special filehandle consisting of an underline, no
6716 stat is done, but the current contents of the stat structure from the
6717 last C<stat>, C<lstat>, or filetest are returned. Example:
6719 if (-x $file && (($d) = stat(_)) && $d < 0) {
6720 print "$file is executable NFS file\n";
6723 (This works on machines only for which the device number is negative
6726 Because the mode contains both the file type and its permissions, you
6727 should mask off the file type portion and (s)printf using a C<"%o">
6728 if you want to see the real permissions.
6730 $mode = (stat($filename))[2];
6731 printf "Permissions are %04o\n", $mode & 07777;
6733 In scalar context, C<stat> returns a boolean value indicating success
6734 or failure, and, if successful, sets the information associated with
6735 the special filehandle C<_>.
6737 The L<File::stat> module provides a convenient, by-name access mechanism:
6740 $sb = stat($filename);
6741 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6742 $filename, $sb->size, $sb->mode & 07777,
6743 scalar localtime $sb->mtime;
6745 You can import symbolic mode constants (C<S_IF*>) and functions
6746 (C<S_IS*>) from the Fcntl module:
6750 $mode = (stat($filename))[2];
6752 $user_rwx = ($mode & S_IRWXU) >> 6;
6753 $group_read = ($mode & S_IRGRP) >> 3;
6754 $other_execute = $mode & S_IXOTH;
6756 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6758 $is_setuid = $mode & S_ISUID;
6759 $is_directory = S_ISDIR($mode);
6761 You could write the last two using the C<-u> and C<-d> operators.
6762 Commonly available C<S_IF*> constants are:
6764 # Permissions: read, write, execute, for user, group, others.
6766 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6767 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6768 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6770 # Setuid/Setgid/Stickiness/SaveText.
6771 # Note that the exact meaning of these is system-dependent.
6773 S_ISUID S_ISGID S_ISVTX S_ISTXT
6775 # File types. Not all are necessarily available on
6778 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
6779 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6781 # The following are compatibility aliases for S_IRUSR,
6782 # S_IWUSR, and S_IXUSR.
6784 S_IREAD S_IWRITE S_IEXEC
6786 and the C<S_IF*> functions are
6788 S_IMODE($mode) the part of $mode containing the permission
6789 bits and the setuid/setgid/sticky bits
6791 S_IFMT($mode) the part of $mode containing the file type
6792 which can be bit-anded with (for example)
6793 S_IFREG or with the following functions
6795 # The operators -f, -d, -l, -b, -c, -p, and -S.
6797 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6798 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6800 # No direct -X operator counterpart, but for the first one
6801 # the -g operator is often equivalent. The ENFMT stands for
6802 # record flocking enforcement, a platform-dependent feature.
6804 S_ISENFMT($mode) S_ISWHT($mode)
6806 See your native chmod(2) and stat(2) documentation for more details
6807 about the C<S_*> constants. To get status info for a symbolic link
6808 instead of the target file behind the link, use the C<lstat> function.
6810 Portability issues: L<perlport/stat>.
6815 =item state TYPE EXPR
6817 =item state EXPR : ATTRS
6819 =item state TYPE EXPR : ATTRS
6821 C<state> declares a lexically scoped variable, just like C<my>.
6822 However, those variables will never be reinitialized, contrary to
6823 lexical variables that are reinitialized each time their enclosing block
6825 See L<perlsub/"Persistent Private Variables"> for details.
6827 C<state> variables are enabled only when the C<use feature "state"> pragma
6828 is in effect, unless the keyword is written as C<CORE::state>.
6829 See also L<feature>.
6836 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6837 doing many pattern matches on the string before it is next modified.
6838 This may or may not save time, depending on the nature and number of
6839 patterns you are searching and the distribution of character
6840 frequencies in the string to be searched; you probably want to compare
6841 run times with and without it to see which is faster. Those loops
6842 that scan for many short constant strings (including the constant
6843 parts of more complex patterns) will benefit most.
6844 (The way C<study> works is this: a linked list of every
6845 character in the string to be searched is made, so we know, for
6846 example, where all the C<'k'> characters are. From each search string,
6847 the rarest character is selected, based on some static frequency tables
6848 constructed from some C programs and English text. Only those places
6849 that contain this "rarest" character are examined.)
6851 For example, here is a loop that inserts index producing entries
6852 before any line containing a certain pattern:
6856 print ".IX foo\n" if /\bfoo\b/;
6857 print ".IX bar\n" if /\bbar\b/;
6858 print ".IX blurfl\n" if /\bblurfl\b/;
6863 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6864 will be looked at, because C<f> is rarer than C<o>. In general, this is
6865 a big win except in pathological cases. The only question is whether
6866 it saves you more time than it took to build the linked list in the
6869 Note that if you have to look for strings that you don't know till
6870 runtime, you can build an entire loop as a string and C<eval> that to
6871 avoid recompiling all your patterns all the time. Together with
6872 undefining C<$/> to input entire files as one record, this can be quite
6873 fast, often faster than specialized programs like fgrep(1). The following
6874 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6875 out the names of those files that contain a match:
6877 $search = 'while (<>) { study;';
6878 foreach $word (@words) {
6879 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6884 eval $search; # this screams
6885 $/ = "\n"; # put back to normal input delimiter
6886 foreach $file (sort keys(%seen)) {
6890 =item sub NAME BLOCK
6893 =item sub NAME (PROTO) BLOCK
6895 =item sub NAME : ATTRS BLOCK
6897 =item sub NAME (PROTO) : ATTRS BLOCK
6899 This is subroutine definition, not a real function I<per se>. Without a
6900 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
6901 function declaration, so does return a value: the CODE ref of the closure
6904 See L<perlsub> and L<perlref> for details about subroutines and
6905 references; see L<attributes> and L<Attribute::Handlers> for more
6906 information about attributes.
6908 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6909 X<substr> X<substring> X<mid> X<left> X<right>
6911 =item substr EXPR,OFFSET,LENGTH
6913 =item substr EXPR,OFFSET
6915 Extracts a substring out of EXPR and returns it. First character is at
6916 offset zero. If OFFSET is negative, starts
6917 that far back from the end of the string. If LENGTH is omitted, returns
6918 everything through the end of the string. If LENGTH is negative, leaves that
6919 many characters off the end of the string.
6921 my $s = "The black cat climbed the green tree";
6922 my $color = substr $s, 4, 5; # black
6923 my $middle = substr $s, 4, -11; # black cat climbed the
6924 my $end = substr $s, 14; # climbed the green tree
6925 my $tail = substr $s, -4; # tree
6926 my $z = substr $s, -4, 2; # tr
6928 You can use the substr() function as an lvalue, in which case EXPR
6929 must itself be an lvalue. If you assign something shorter than LENGTH,
6930 the string will shrink, and if you assign something longer than LENGTH,
6931 the string will grow to accommodate it. To keep the string the same
6932 length, you may need to pad or chop your value using C<sprintf>.
6934 If OFFSET and LENGTH specify a substring that is partly outside the
6935 string, only the part within the string is returned. If the substring
6936 is beyond either end of the string, substr() returns the undefined
6937 value and produces a warning. When used as an lvalue, specifying a
6938 substring that is entirely outside the string raises an exception.
6939 Here's an example showing the behavior for boundary cases:
6942 substr($name, 4) = 'dy'; # $name is now 'freddy'
6943 my $null = substr $name, 6, 2; # returns "" (no warning)
6944 my $oops = substr $name, 7; # returns undef, with warning
6945 substr($name, 7) = 'gap'; # raises an exception
6947 An alternative to using substr() as an lvalue is to specify the
6948 replacement string as the 4th argument. This allows you to replace
6949 parts of the EXPR and return what was there before in one operation,
6950 just as you can with splice().
6952 my $s = "The black cat climbed the green tree";
6953 my $z = substr $s, 14, 7, "jumped from"; # climbed
6954 # $s is now "The black cat jumped from the green tree"
6956 Note that the lvalue returned by the three-argument version of substr() acts as
6957 a 'magic bullet'; each time it is assigned to, it remembers which part
6958 of the original string is being modified; for example:
6961 for (substr($x,1,2)) {
6962 $_ = 'a'; print $x,"\n"; # prints 1a4
6963 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6965 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6968 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6971 =item symlink OLDFILE,NEWFILE
6972 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6974 Creates a new filename symbolically linked to the old filename.
6975 Returns C<1> for success, C<0> otherwise. On systems that don't support
6976 symbolic links, raises an exception. To check for that,
6979 $symlink_exists = eval { symlink("",""); 1 };
6981 Portability issues: L<perlport/symlink>.
6983 =item syscall NUMBER, LIST
6984 X<syscall> X<system call>
6986 Calls the system call specified as the first element of the list,
6987 passing the remaining elements as arguments to the system call. If
6988 unimplemented, raises an exception. The arguments are interpreted
6989 as follows: if a given argument is numeric, the argument is passed as
6990 an int. If not, the pointer to the string value is passed. You are
6991 responsible to make sure a string is pre-extended long enough to
6992 receive any result that might be written into a string. You can't use a
6993 string literal (or other read-only string) as an argument to C<syscall>
6994 because Perl has to assume that any string pointer might be written
6996 integer arguments are not literals and have never been interpreted in a
6997 numeric context, you may need to add C<0> to them to force them to look
6998 like numbers. This emulates the C<syswrite> function (or vice versa):
7000 require 'syscall.ph'; # may need to run h2ph
7002 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7004 Note that Perl supports passing of up to only 14 arguments to your syscall,
7005 which in practice should (usually) suffice.
7007 Syscall returns whatever value returned by the system call it calls.
7008 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7009 Note that some system calls I<can> legitimately return C<-1>. The proper
7010 way to handle such calls is to assign C<$!=0> before the call, then
7011 check the value of C<$!> if C<syscall> returns C<-1>.
7013 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7014 number of the read end of the pipe it creates, but there is no way
7015 to retrieve the file number of the other end. You can avoid this
7016 problem by using C<pipe> instead.
7018 Portability issues: L<perlport/syscall>.
7020 =item sysopen FILEHANDLE,FILENAME,MODE
7023 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7025 Opens the file whose filename is given by FILENAME, and associates it with
7026 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7027 filehandle wanted; an undefined scalar will be suitably autovivified. This
7028 function calls the underlying operating system's I<open>(2) function with the
7029 parameters FILENAME, MODE, and PERMS.
7031 The possible values and flag bits of the MODE parameter are
7032 system-dependent; they are available via the standard module C<Fcntl>. See
7033 the documentation of your operating system's I<open>(2) syscall to see
7034 which values and flag bits are available. You may combine several flags
7035 using the C<|>-operator.
7037 Some of the most common values are C<O_RDONLY> for opening the file in
7038 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7039 and C<O_RDWR> for opening the file in read-write mode.
7040 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7042 For historical reasons, some values work on almost every system
7043 supported by Perl: 0 means read-only, 1 means write-only, and 2
7044 means read/write. We know that these values do I<not> work under
7045 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
7046 use them in new code.
7048 If the file named by FILENAME does not exist and the C<open> call creates
7049 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7050 PERMS specifies the permissions of the newly created file. If you omit
7051 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7052 These permission values need to be in octal, and are modified by your
7053 process's current C<umask>.
7056 In many systems the C<O_EXCL> flag is available for opening files in
7057 exclusive mode. This is B<not> locking: exclusiveness means here that
7058 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7059 on network filesystems, and has no effect unless the C<O_CREAT> flag
7060 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7061 being opened if it is a symbolic link. It does not protect against
7062 symbolic links in the file's path.
7065 Sometimes you may want to truncate an already-existing file. This
7066 can be done using the C<O_TRUNC> flag. The behavior of
7067 C<O_TRUNC> with C<O_RDONLY> is undefined.
7070 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7071 that takes away the user's option to have a more permissive umask.
7072 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7075 Note that C<sysopen> depends on the fdopen() C library function.
7076 On many Unix systems, fdopen() is known to fail when file descriptors
7077 exceed a certain value, typically 255. If you need more file
7078 descriptors than that, consider rebuilding Perl to use the C<sfio>
7079 library, or perhaps using the POSIX::open() function.
7081 See L<perlopentut> for a kinder, gentler explanation of opening files.
7083 Portability issues: L<perlport/sysopen>.
7085 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7088 =item sysread FILEHANDLE,SCALAR,LENGTH
7090 Attempts to read LENGTH bytes of data into variable SCALAR from the
7091 specified FILEHANDLE, using the read(2). It bypasses
7092 buffered IO, so mixing this with other kinds of reads, C<print>,
7093 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7094 perlio or stdio layers usually buffers data. Returns the number of
7095 bytes actually read, C<0> at end of file, or undef if there was an
7096 error (in the latter case C<$!> is also set). SCALAR will be grown or
7097 shrunk so that the last byte actually read is the last byte of the
7098 scalar after the read.
7100 An OFFSET may be specified to place the read data at some place in the
7101 string other than the beginning. A negative OFFSET specifies
7102 placement at that many characters counting backwards from the end of
7103 the string. A positive OFFSET greater than the length of SCALAR
7104 results in the string being padded to the required size with C<"\0">
7105 bytes before the result of the read is appended.
7107 There is no syseof() function, which is ok, since eof() doesn't work
7108 well on device files (like ttys) anyway. Use sysread() and check
7109 for a return value for 0 to decide whether you're done.
7111 Note that if the filehandle has been marked as C<:utf8> Unicode
7112 characters are read instead of bytes (the LENGTH, OFFSET, and the
7113 return value of sysread() are in Unicode characters).
7114 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7115 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7117 =item sysseek FILEHANDLE,POSITION,WHENCE
7120 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7121 be an expression whose value gives the name of the filehandle. The values
7122 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7123 to the current position plus POSITION; and C<2> to set it to EOF plus
7124 POSITION, typically negative.
7126 Note the I<in bytes>: even if the filehandle has been set to operate
7127 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7128 tell() will return byte offsets, not character offsets (because
7129 implementing that would render sysseek() unacceptably slow).
7131 sysseek() bypasses normal buffered IO, so mixing it with reads other
7132 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7133 C<seek>, C<tell>, or C<eof> may cause confusion.
7135 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7136 and C<SEEK_END> (start of the file, current position, end of the file)
7137 from the Fcntl module. Use of the constants is also more portable
7138 than relying on 0, 1, and 2. For example to define a "systell" function:
7140 use Fcntl 'SEEK_CUR';
7141 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7143 Returns the new position, or the undefined value on failure. A position
7144 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7145 true on success and false on failure, yet you can still easily determine
7151 =item system PROGRAM LIST
7153 Does exactly the same thing as C<exec LIST>, except that a fork is
7154 done first and the parent process waits for the child process to
7155 exit. Note that argument processing varies depending on the
7156 number of arguments. If there is more than one argument in LIST,
7157 or if LIST is an array with more than one value, starts the program
7158 given by the first element of the list with arguments given by the
7159 rest of the list. If there is only one scalar argument, the argument
7160 is checked for shell metacharacters, and if there are any, the
7161 entire argument is passed to the system's command shell for parsing
7162 (this is C</bin/sh -c> on Unix platforms, but varies on other
7163 platforms). If there are no shell metacharacters in the argument,
7164 it is split into words and passed directly to C<execvp>, which is
7167 Beginning with v5.6.0, Perl will attempt to flush all files opened for
7168 output before any operation that may do a fork, but this may not be
7169 supported on some platforms (see L<perlport>). To be safe, you may need
7170 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7171 of C<IO::Handle> on any open handles.
7173 The return value is the exit status of the program as returned by the
7174 C<wait> call. To get the actual exit value, shift right by eight (see
7175 below). See also L</exec>. This is I<not> what you want to use to capture
7176 the output from a command; for that you should use merely backticks or
7177 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7178 indicates a failure to start the program or an error of the wait(2) system
7179 call (inspect $! for the reason).
7181 If you'd like to make C<system> (and many other bits of Perl) die on error,
7182 have a look at the L<autodie> pragma.
7184 Like C<exec>, C<system> allows you to lie to a program about its name if
7185 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
7187 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
7188 C<system>, if you expect your program to terminate on receipt of these
7189 signals you will need to arrange to do so yourself based on the return
7192 @args = ("command", "arg1", "arg2");
7194 or die "system @args failed: $?"
7196 If you'd like to manually inspect C<system>'s failure, you can check all
7197 possible failure modes by inspecting C<$?> like this:
7200 print "failed to execute: $!\n";
7203 printf "child died with signal %d, %s coredump\n",
7204 ($? & 127), ($? & 128) ? 'with' : 'without';
7207 printf "child exited with value %d\n", $? >> 8;
7210 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
7211 with the C<W*()> calls from the POSIX module.
7213 When C<system>'s arguments are executed indirectly by the shell,
7214 results and return codes are subject to its quirks.
7215 See L<perlop/"`STRING`"> and L</exec> for details.
7217 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
7218 handler. See L<perlipc> for details.
7220 Portability issues: L<perlport/system>.
7222 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7225 =item syswrite FILEHANDLE,SCALAR,LENGTH
7227 =item syswrite FILEHANDLE,SCALAR
7229 Attempts to write LENGTH bytes of data from variable SCALAR to the
7230 specified FILEHANDLE, using write(2). If LENGTH is
7231 not specified, writes whole SCALAR. It bypasses buffered IO, so
7232 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
7233 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
7234 stdio layers usually buffer data. Returns the number of bytes
7235 actually written, or C<undef> if there was an error (in this case the
7236 errno variable C<$!> is also set). If the LENGTH is greater than the
7237 data available in the SCALAR after the OFFSET, only as much data as is
7238 available will be written.
7240 An OFFSET may be specified to write the data from some part of the
7241 string other than the beginning. A negative OFFSET specifies writing
7242 that many characters counting backwards from the end of the string.
7243 If SCALAR is of length zero, you can only use an OFFSET of 0.
7245 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
7246 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
7247 return value of syswrite() are in (UTF8-encoded Unicode) characters.
7248 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7249 Alternately, if the handle is not marked with an encoding but you
7250 attempt to write characters with code points over 255, raises an exception.
7251 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7253 =item tell FILEHANDLE
7258 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
7259 error. FILEHANDLE may be an expression whose value gives the name of
7260 the actual filehandle. If FILEHANDLE is omitted, assumes the file
7263 Note the I<in bytes>: even if the filehandle has been set to
7264 operate on characters (for example by using the C<:encoding(utf8)> open
7265 layer), tell() will return byte offsets, not character offsets (because
7266 that would render seek() and tell() rather slow).
7268 The return value of tell() for the standard streams like the STDIN
7269 depends on the operating system: it may return -1 or something else.
7270 tell() on pipes, fifos, and sockets usually returns -1.
7272 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
7274 Do not use tell() (or other buffered I/O operations) on a filehandle
7275 that has been manipulated by sysread(), syswrite(), or sysseek().
7276 Those functions ignore the buffering, while tell() does not.
7278 =item telldir DIRHANDLE
7281 Returns the current position of the C<readdir> routines on DIRHANDLE.
7282 Value may be given to C<seekdir> to access a particular location in a
7283 directory. C<telldir> has the same caveats about possible directory
7284 compaction as the corresponding system library routine.
7286 =item tie VARIABLE,CLASSNAME,LIST
7289 This function binds a variable to a package class that will provide the
7290 implementation for the variable. VARIABLE is the name of the variable
7291 to be enchanted. CLASSNAME is the name of a class implementing objects
7292 of correct type. Any additional arguments are passed to the C<new>
7293 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
7294 or C<TIEHASH>). Typically these are arguments such as might be passed
7295 to the C<dbm_open()> function of C. The object returned by the C<new>
7296 method is also returned by the C<tie> function, which would be useful
7297 if you want to access other methods in CLASSNAME.
7299 Note that functions such as C<keys> and C<values> may return huge lists
7300 when used on large objects, like DBM files. You may prefer to use the
7301 C<each> function to iterate over such. Example:
7303 # print out history file offsets
7305 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
7306 while (($key,$val) = each %HIST) {
7307 print $key, ' = ', unpack('L',$val), "\n";
7311 A class implementing a hash should have the following methods:
7313 TIEHASH classname, LIST
7315 STORE this, key, value
7320 NEXTKEY this, lastkey
7325 A class implementing an ordinary array should have the following methods:
7327 TIEARRAY classname, LIST
7329 STORE this, key, value
7331 STORESIZE this, count
7337 SPLICE this, offset, length, LIST
7342 A class implementing a filehandle should have the following methods:
7344 TIEHANDLE classname, LIST
7345 READ this, scalar, length, offset
7348 WRITE this, scalar, length, offset
7350 PRINTF this, format, LIST
7354 SEEK this, position, whence
7356 OPEN this, mode, LIST
7361 A class implementing a scalar should have the following methods:
7363 TIESCALAR classname, LIST
7369 Not all methods indicated above need be implemented. See L<perltie>,
7370 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
7372 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
7373 for you; you need to do that explicitly yourself. See L<DB_File>
7374 or the F<Config> module for interesting C<tie> implementations.
7376 For further details see L<perltie>, L<"tied VARIABLE">.
7381 Returns a reference to the object underlying VARIABLE (the same value
7382 that was originally returned by the C<tie> call that bound the variable
7383 to a package.) Returns the undefined value if VARIABLE isn't tied to a
7389 Returns the number of non-leap seconds since whatever time the system
7390 considers to be the epoch, suitable for feeding to C<gmtime> and
7391 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
7392 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
7393 1904 in the current local time zone for its epoch.
7395 For measuring time in better granularity than one second, use the
7396 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
7397 if you have gettimeofday(2), you may be able to use the C<syscall>
7398 interface of Perl. See L<perlfaq8> for details.
7400 For date and time processing look at the many related modules on CPAN.
7401 For a comprehensive date and time representation look at the
7407 Returns a four-element list giving the user and system times in
7408 seconds for this process and any exited children of this process.
7410 ($user,$system,$cuser,$csystem) = times;
7412 In scalar context, C<times> returns C<$user>.
7414 Children's times are only included for terminated children.
7416 Portability issues: L<perlport/times>.
7420 The transliteration operator. Same as C<y///>. See
7421 L<perlop/"Quote and Quote-like Operators">.
7423 =item truncate FILEHANDLE,LENGTH
7426 =item truncate EXPR,LENGTH
7428 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
7429 specified length. Raises an exception if truncate isn't implemented
7430 on your system. Returns true if successful, C<undef> on error.
7432 The behavior is undefined if LENGTH is greater than the length of the
7435 The position in the file of FILEHANDLE is left unchanged. You may want to
7436 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
7438 Portability issues: L<perlport/truncate>.
7441 X<uc> X<uppercase> X<toupper>
7445 Returns an uppercased version of EXPR. This is the internal function
7446 implementing the C<\U> escape in double-quoted strings.
7447 It does not attempt to do titlecase mapping on initial letters. See
7448 L</ucfirst> for that.
7450 If EXPR is omitted, uses C<$_>.
7452 This function behaves the same way under various pragma, such as in a locale,
7456 X<ucfirst> X<uppercase>
7460 Returns the value of EXPR with the first character in uppercase
7461 (titlecase in Unicode). This is the internal function implementing
7462 the C<\u> escape in double-quoted strings.
7464 If EXPR is omitted, uses C<$_>.
7466 This function behaves the same way under various pragma, such as in a locale,
7474 Sets the umask for the process to EXPR and returns the previous value.
7475 If EXPR is omitted, merely returns the current umask.
7477 The Unix permission C<rwxr-x---> is represented as three sets of three
7478 bits, or three octal digits: C<0750> (the leading 0 indicates octal
7479 and isn't one of the digits). The C<umask> value is such a number
7480 representing disabled permissions bits. The permission (or "mode")
7481 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
7482 even if you tell C<sysopen> to create a file with permissions C<0777>,
7483 if your umask is C<0022>, then the file will actually be created with
7484 permissions C<0755>. If your C<umask> were C<0027> (group can't
7485 write; others can't read, write, or execute), then passing
7486 C<sysopen> C<0666> would create a file with mode C<0640> (because
7487 C<0666 &~ 027> is C<0640>).
7489 Here's some advice: supply a creation mode of C<0666> for regular
7490 files (in C<sysopen>) and one of C<0777> for directories (in
7491 C<mkdir>) and executable files. This gives users the freedom of
7492 choice: if they want protected files, they might choose process umasks
7493 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
7494 Programs should rarely if ever make policy decisions better left to
7495 the user. The exception to this is when writing files that should be
7496 kept private: mail files, web browser cookies, I<.rhosts> files, and
7499 If umask(2) is not implemented on your system and you are trying to
7500 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
7501 raises an exception. If umask(2) is not implemented and you are
7502 not trying to restrict access for yourself, returns C<undef>.
7504 Remember that a umask is a number, usually given in octal; it is I<not> a
7505 string of octal digits. See also L</oct>, if all you have is a string.
7507 Portability issues: L<perlport/umask>.
7510 X<undef> X<undefine>
7514 Undefines the value of EXPR, which must be an lvalue. Use only on a
7515 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
7516 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
7517 will probably not do what you expect on most predefined variables or
7518 DBM list values, so don't do that; see L</delete>. Always returns the
7519 undefined value. You can omit the EXPR, in which case nothing is
7520 undefined, but you still get an undefined value that you could, for
7521 instance, return from a subroutine, assign to a variable, or pass as a
7522 parameter. Examples:
7525 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
7529 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
7530 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7531 select undef, undef, undef, 0.25;
7532 ($a, $b, undef, $c) = &foo; # Ignore third value returned
7534 Note that this is a unary operator, not a list operator.
7537 X<unlink> X<delete> X<remove> X<rm> X<del>
7541 Deletes a list of files. On success, it returns the number of files
7542 it successfully deleted. On failure, it returns false and sets C<$!>
7545 my $unlinked = unlink 'a', 'b', 'c';
7547 unlink glob "*.bak";
7549 On error, C<unlink> will not tell you which files it could not remove.
7550 If you want to know which files you could not remove, try them one
7553 foreach my $file ( @goners ) {
7554 unlink $file or warn "Could not unlink $file: $!";
7557 Note: C<unlink> will not attempt to delete directories unless you are
7558 superuser and the B<-U> flag is supplied to Perl. Even if these
7559 conditions are met, be warned that unlinking a directory can inflict
7560 damage on your filesystem. Finally, using C<unlink> on directories is
7561 not supported on many operating systems. Use C<rmdir> instead.
7563 If LIST is omitted, C<unlink> uses C<$_>.
7565 =item unpack TEMPLATE,EXPR
7568 =item unpack TEMPLATE
7570 C<unpack> does the reverse of C<pack>: it takes a string
7571 and expands it out into a list of values.
7572 (In scalar context, it returns merely the first value produced.)
7574 If EXPR is omitted, unpacks the C<$_> string.
7575 See L<perlpacktut> for an introduction to this function.
7577 The string is broken into chunks described by the TEMPLATE. Each chunk
7578 is converted separately to a value. Typically, either the string is a result
7579 of C<pack>, or the characters of the string represent a C structure of some
7582 The TEMPLATE has the same format as in the C<pack> function.
7583 Here's a subroutine that does substring:
7586 my($what,$where,$howmuch) = @_;
7587 unpack("x$where a$howmuch", $what);
7592 sub ordinal { unpack("W",$_[0]); } # same as ord()
7594 In addition to fields allowed in pack(), you may prefix a field with
7595 a %<number> to indicate that
7596 you want a <number>-bit checksum of the items instead of the items
7597 themselves. Default is a 16-bit checksum. Checksum is calculated by
7598 summing numeric values of expanded values (for string fields the sum of
7599 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
7601 For example, the following
7602 computes the same number as the System V sum program:
7606 unpack("%32W*",<>) % 65535;
7609 The following efficiently counts the number of set bits in a bit vector:
7611 $setbits = unpack("%32b*", $selectmask);
7613 The C<p> and C<P> formats should be used with care. Since Perl
7614 has no way of checking whether the value passed to C<unpack()>
7615 corresponds to a valid memory location, passing a pointer value that's
7616 not known to be valid is likely to have disastrous consequences.
7618 If there are more pack codes or if the repeat count of a field or a group
7619 is larger than what the remainder of the input string allows, the result
7620 is not well defined: the repeat count may be decreased, or
7621 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7622 If the input string is longer than one described by the TEMPLATE,
7623 the remainder of that input string is ignored.
7625 See L</pack> for more examples and notes.
7627 =item untie VARIABLE
7630 Breaks the binding between a variable and a package.
7631 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
7632 Has no effect if the variable is not tied.
7634 =item unshift ARRAY,LIST
7637 =item unshift EXPR,LIST
7639 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7640 depending on how you look at it. Prepends list to the front of the
7641 array and returns the new number of elements in the array.
7643 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7645 Note the LIST is prepended whole, not one element at a time, so the
7646 prepended elements stay in the same order. Use C<reverse> to do the
7649 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
7650 a reference to an unblessed array. The argument will be dereferenced
7651 automatically. This aspect of C<unshift> is considered highly
7652 experimental. The exact behaviour may change in a future version of Perl.
7654 =item use Module VERSION LIST
7655 X<use> X<module> X<import>
7657 =item use Module VERSION
7659 =item use Module LIST
7665 Imports some semantics into the current package from the named module,
7666 generally by aliasing certain subroutine or variable names into your
7667 package. It is exactly equivalent to
7669 BEGIN { require Module; Module->import( LIST ); }
7671 except that Module I<must> be a bareword.
7672 The importation can be made conditional; see L<if>.
7674 In the peculiar C<use VERSION> form, VERSION may be either a positive
7675 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7676 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7677 exception is raised if VERSION is greater than the version of the
7678 current Perl interpreter; Perl will not attempt to parse the rest of the
7679 file. Compare with L</require>, which can do a similar check at run time.
7680 Symmetrically, C<no VERSION> allows you to specify that you want a version
7681 of Perl older than the specified one.
7683 Specifying VERSION as a literal of the form v5.6.1 should generally be
7684 avoided, because it leads to misleading error messages under earlier
7685 versions of Perl (that is, prior to 5.6.0) that do not support this
7686 syntax. The equivalent numeric version should be used instead.
7688 use v5.6.1; # compile time version check
7690 use 5.006_001; # ditto; preferred for backwards compatibility
7692 This is often useful if you need to check the current Perl version before
7693 C<use>ing library modules that won't work with older versions of Perl.
7694 (We try not to do this more than we have to.)
7696 Also, if the specified Perl version is greater than or equal to 5.9.5,
7697 C<use VERSION> will also load the C<feature> pragma and enable all
7698 features available in the requested version. See L<feature>.
7699 Similarly, if the specified Perl version is greater than or equal to
7700 5.11.0, strictures are enabled lexically as with C<use strict> (except
7701 that the F<strict.pm> file is not actually loaded).
7703 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7704 C<require> makes sure the module is loaded into memory if it hasn't been
7705 yet. The C<import> is not a builtin; it's just an ordinary static method
7706 call into the C<Module> package to tell the module to import the list of
7707 features back into the current package. The module can implement its
7708 C<import> method any way it likes, though most modules just choose to
7709 derive their C<import> method via inheritance from the C<Exporter> class that
7710 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7711 method can be found then the call is skipped, even if there is an AUTOLOAD
7714 If you do not want to call the package's C<import> method (for instance,
7715 to stop your namespace from being altered), explicitly supply the empty list:
7719 That is exactly equivalent to
7721 BEGIN { require Module }
7723 If the VERSION argument is present between Module and LIST, then the
7724 C<use> will call the VERSION method in class Module with the given
7725 version as an argument. The default VERSION method, inherited from
7726 the UNIVERSAL class, croaks if the given version is larger than the
7727 value of the variable C<$Module::VERSION>.
7729 Again, there is a distinction between omitting LIST (C<import> called
7730 with no arguments) and an explicit empty LIST C<()> (C<import> not
7731 called). Note that there is no comma after VERSION!
7733 Because this is a wide-open interface, pragmas (compiler directives)
7734 are also implemented this way. Currently implemented pragmas are:
7739 use sigtrap qw(SEGV BUS);
7740 use strict qw(subs vars refs);
7741 use subs qw(afunc blurfl);
7742 use warnings qw(all);
7743 use sort qw(stable _quicksort _mergesort);
7745 Some of these pseudo-modules import semantics into the current
7746 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7747 which import symbols into the current package (which are effective
7748 through the end of the file).
7750 Because C<use> takes effect at compile time, it doesn't respect the
7751 ordinary flow control of the code being compiled. In particular, putting
7752 a C<use> inside the false branch of a conditional doesn't prevent it
7753 from being processed. If a module or pragma only needs to be loaded
7754 conditionally, this can be done using the L<if> pragma:
7756 use if $] < 5.008, "utf8";
7757 use if WANT_WARNINGS, warnings => qw(all);
7759 There's a corresponding C<no> declaration that unimports meanings imported
7760 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7761 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7762 or no unimport method being found.
7768 Care should be taken when using the C<no VERSION> form of C<no>. It is
7769 I<only> meant to be used to assert that the running Perl is of a earlier
7770 version than its argument and I<not> to undo the feature-enabling side effects
7773 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7774 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7775 functionality from the command-line.
7780 Changes the access and modification times on each file of a list of
7781 files. The first two elements of the list must be the NUMERIC access
7782 and modification times, in that order. Returns the number of files
7783 successfully changed. The inode change time of each file is set
7784 to the current time. For example, this code has the same effect as the
7785 Unix touch(1) command when the files I<already exist> and belong to
7786 the user running the program:
7789 $atime = $mtime = time;
7790 utime $atime, $mtime, @ARGV;
7792 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7793 the utime(2) syscall from your C library is called with a null second
7794 argument. On most systems, this will set the file's access and
7795 modification times to the current time (i.e., equivalent to the example
7796 above) and will work even on files you don't own provided you have write
7800 utime(undef, undef, $file)
7801 || warn "couldn't touch $file: $!";
7804 Under NFS this will use the time of the NFS server, not the time of
7805 the local machine. If there is a time synchronization problem, the
7806 NFS server and local machine will have different times. The Unix
7807 touch(1) command will in fact normally use this form instead of the
7808 one shown in the first example.
7810 Passing only one of the first two elements as C<undef> is
7811 equivalent to passing a 0 and will not have the effect
7812 described when both are C<undef>. This also triggers an
7813 uninitialized warning.
7815 On systems that support futimes(2), you may pass filehandles among the
7816 files. On systems that don't support futimes(2), passing filehandles raises
7817 an exception. Filehandles must be passed as globs or glob references to be
7818 recognized; barewords are considered filenames.
7820 Portability issues: L<perlport/utime>.
7829 Returns a list consisting of all the values of the named hash, or the values
7830 of an array. (In scalar context, returns the number of values.)
7832 The values are returned in an apparently random order. The actual
7833 random order is subject to change in future versions of Perl, but it
7834 is guaranteed to be the same order as either the C<keys> or C<each>
7835 function would produce on the same (unmodified) hash. Since Perl
7836 5.8.1 the ordering is different even between different runs of Perl
7837 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7839 As a side effect, calling values() resets the HASH or ARRAY's internal
7841 see L</each>. (In particular, calling values() in void context resets
7842 the iterator with no other overhead. Apart from resetting the iterator,
7843 C<values @array> in list context is the same as plain C<@array>.
7844 We recommend that you use void context C<keys @array> for this, but reasoned
7845 that it taking C<values @array> out would require more documentation than
7848 Note that the values are not copied, which means modifying them will
7849 modify the contents of the hash:
7851 for (values %hash) { s/foo/bar/g } # modifies %hash values
7852 for (@hash{keys %hash}) { s/foo/bar/g } # same
7854 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
7855 a reference to an unblessed hash or array. The argument will be
7856 dereferenced automatically. This aspect of C<values> is considered highly
7857 experimental. The exact behaviour may change in a future version of Perl.
7859 for (values $hashref) { ... }
7860 for (values $obj->get_arrayref) { ... }
7862 See also C<keys>, C<each>, and C<sort>.
7864 =item vec EXPR,OFFSET,BITS
7865 X<vec> X<bit> X<bit vector>
7867 Treats the string in EXPR as a bit vector made up of elements of
7868 width BITS and returns the value of the element specified by OFFSET
7869 as an unsigned integer. BITS therefore specifies the number of bits
7870 that are reserved for each element in the bit vector. This must
7871 be a power of two from 1 to 32 (or 64, if your platform supports
7874 If BITS is 8, "elements" coincide with bytes of the input string.
7876 If BITS is 16 or more, bytes of the input string are grouped into chunks
7877 of size BITS/8, and each group is converted to a number as with
7878 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7879 for BITS==64). See L<"pack"> for details.
7881 If bits is 4 or less, the string is broken into bytes, then the bits
7882 of each byte are broken into 8/BITS groups. Bits of a byte are
7883 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7884 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7885 breaking the single input byte C<chr(0x36)> into two groups gives a list
7886 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7888 C<vec> may also be assigned to, in which case parentheses are needed
7889 to give the expression the correct precedence as in
7891 vec($image, $max_x * $x + $y, 8) = 3;
7893 If the selected element is outside the string, the value 0 is returned.
7894 If an element off the end of the string is written to, Perl will first
7895 extend the string with sufficiently many zero bytes. It is an error
7896 to try to write off the beginning of the string (i.e., negative OFFSET).
7898 If the string happens to be encoded as UTF-8 internally (and thus has
7899 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7900 internal byte string, not the conceptual character string, even if you
7901 only have characters with values less than 256.
7903 Strings created with C<vec> can also be manipulated with the logical
7904 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7905 vector operation is desired when both operands are strings.
7906 See L<perlop/"Bitwise String Operators">.
7908 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7909 The comments show the string after each step. Note that this code works
7910 in the same way on big-endian or little-endian machines.
7913 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7915 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7916 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7918 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7919 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7920 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7921 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7922 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7923 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7925 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7926 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7927 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7930 To transform a bit vector into a string or list of 0's and 1's, use these:
7932 $bits = unpack("b*", $vector);
7933 @bits = split(//, unpack("b*", $vector));
7935 If you know the exact length in bits, it can be used in place of the C<*>.
7937 Here is an example to illustrate how the bits actually fall in place:
7943 unpack("V",$_) 01234567890123456789012345678901
7944 ------------------------------------------------------------------
7949 for ($shift=0; $shift < $width; ++$shift) {
7950 for ($off=0; $off < 32/$width; ++$off) {
7951 $str = pack("B*", "0"x32);
7952 $bits = (1<<$shift);
7953 vec($str, $off, $width) = $bits;
7954 $res = unpack("b*",$str);
7955 $val = unpack("V", $str);
7962 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7963 $off, $width, $bits, $val, $res
7967 Regardless of the machine architecture on which it runs, the
7968 example above should print the following table:
7971 unpack("V",$_) 01234567890123456789012345678901
7972 ------------------------------------------------------------------
7973 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7974 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7975 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7976 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7977 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7978 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7979 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7980 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7981 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7982 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7983 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7984 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7985 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7986 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7987 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7988 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7989 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7990 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7991 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7992 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7993 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7994 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7995 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7996 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7997 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7998 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7999 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8000 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8001 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8002 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8003 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8004 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8005 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8006 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8007 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8008 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8009 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8010 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8011 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8012 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8013 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8014 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8015 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8016 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8017 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8018 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8019 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8020 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8021 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8022 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8023 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8024 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8025 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8026 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8027 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8028 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8029 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8030 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8031 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8032 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8033 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8034 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8035 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8036 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8037 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8038 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8039 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8040 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8041 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8042 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8043 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8044 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8045 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8046 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8047 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8048 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8049 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8050 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8051 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8052 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8053 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8054 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8055 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8056 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8057 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8058 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8059 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8060 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8061 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8062 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8063 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8064 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8065 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8066 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8067 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8068 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8069 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8070 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8071 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8072 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8073 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8074 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8075 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8076 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8077 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8078 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8079 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8080 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8081 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8082 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8083 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8084 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8085 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8086 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8087 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8088 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8089 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8090 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8091 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8092 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8093 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8094 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8095 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8096 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8097 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8098 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8099 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8100 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8105 Behaves like wait(2) on your system: it waits for a child
8106 process to terminate and returns the pid of the deceased process, or
8107 C<-1> if there are no child processes. The status is returned in C<$?>
8108 and C<${^CHILD_ERROR_NATIVE}>.
8109 Note that a return value of C<-1> could mean that child processes are
8110 being automatically reaped, as described in L<perlipc>.
8112 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8113 child created by qx() or system(). See L<perlipc> for details.
8115 Portability issues: L<perlport/wait>.
8117 =item waitpid PID,FLAGS
8120 Waits for a particular child process to terminate and returns the pid of
8121 the deceased process, or C<-1> if there is no such child process. On some
8122 systems, a value of 0 indicates that there are processes still running.
8123 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
8125 use POSIX ":sys_wait_h";
8128 $kid = waitpid(-1, WNOHANG);
8131 then you can do a non-blocking wait for all pending zombie processes.
8132 Non-blocking wait is available on machines supporting either the
8133 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
8134 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
8135 system call by remembering the status values of processes that have
8136 exited but have not been harvested by the Perl script yet.)
8138 Note that on some systems, a return value of C<-1> could mean that child
8139 processes are being automatically reaped. See L<perlipc> for details,
8140 and for other examples.
8142 Portability issues: L<perlport/waitpid>.
8145 X<wantarray> X<context>
8147 Returns true if the context of the currently executing subroutine or
8148 C<eval> is looking for a list value. Returns false if the context is
8149 looking for a scalar. Returns the undefined value if the context is
8150 looking for no value (void context).
8152 return unless defined wantarray; # don't bother doing more
8153 my @a = complex_calculation();
8154 return wantarray ? @a : "@a";
8156 C<wantarray()>'s result is unspecified in the top level of a file,
8157 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
8158 in a C<DESTROY> method.
8160 This function should have been named wantlist() instead.
8163 X<warn> X<warning> X<STDERR>
8165 Prints the value of LIST to STDERR. If the last element of LIST does
8166 not end in a newline, it appends the same file/line number text as C<die>
8169 If the output is empty and C<$@> already contains a value (typically from a
8170 previous eval) that value is used after appending C<"\t...caught">
8171 to C<$@>. This is useful for staying almost, but not entirely similar to
8174 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
8176 No message is printed if there is a C<$SIG{__WARN__}> handler
8177 installed. It is the handler's responsibility to deal with the message
8178 as it sees fit (like, for instance, converting it into a C<die>). Most
8179 handlers must therefore arrange to actually display the
8180 warnings that they are not prepared to deal with, by calling C<warn>
8181 again in the handler. Note that this is quite safe and will not
8182 produce an endless loop, since C<__WARN__> hooks are not called from
8185 You will find this behavior is slightly different from that of
8186 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
8187 instead call C<die> again to change it).
8189 Using a C<__WARN__> handler provides a powerful way to silence all
8190 warnings (even the so-called mandatory ones). An example:
8192 # wipe out *all* compile-time warnings
8193 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
8195 my $foo = 20; # no warning about duplicate my $foo,
8196 # but hey, you asked for it!
8197 # no compile-time or run-time warnings before here
8200 # run-time warnings enabled after here
8201 warn "\$foo is alive and $foo!"; # does show up
8203 See L<perlvar> for details on setting C<%SIG> entries and for more
8204 examples. See the Carp module for other kinds of warnings using its
8205 carp() and cluck() functions.
8207 =item when EXPR BLOCK
8212 C<when> is analogous to the C<case> keyword in other languages. Used with a
8213 C<foreach> loop or the experimental C<given> block, C<when> can be used in
8214 Perl to implement C<switch>/C<case> like statements. Available as a
8215 statement after Perl 5.10 and as a statement modifier after 5.14.
8216 Here are three examples:
8221 say "I like apples."
8224 say "I don't like oranges."
8227 say "I don't like anything"
8231 # require 5.14 for when as statement modifier
8234 say "I like apples." when /apples?/;
8235 say "I don't like oranges." when /oranges?;
8236 default { say "I don't like anything" }
8242 say "I like apples."
8245 say "I don't like oranges."
8248 say "I don't like anything"
8252 See L<perlsyn/"Switch statements"> for detailed information.
8254 =item write FILEHANDLE
8261 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
8262 using the format associated with that file. By default the format for
8263 a file is the one having the same name as the filehandle, but the
8264 format for the current output channel (see the C<select> function) may be set
8265 explicitly by assigning the name of the format to the C<$~> variable.
8267 Top of form processing is handled automatically: if there is insufficient
8268 room on the current page for the formatted record, the page is advanced by
8269 writing a form feed, a special top-of-page format is used to format the new
8270 page header before the record is written. By default, the top-of-page
8271 format is the name of the filehandle with "_TOP" appended. This would be a
8272 problem with autovivified filehandles, but it may be dynamically set to the
8273 format of your choice by assigning the name to the C<$^> variable while
8274 that filehandle is selected. The number of lines remaining on the current
8275 page is in variable C<$->, which can be set to C<0> to force a new page.
8277 If FILEHANDLE is unspecified, output goes to the current default output
8278 channel, which starts out as STDOUT but may be changed by the
8279 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
8280 is evaluated and the resulting string is used to look up the name of
8281 the FILEHANDLE at run time. For more on formats, see L<perlform>.
8283 Note that write is I<not> the opposite of C<read>. Unfortunately.
8287 The transliteration operator. Same as C<tr///>. See
8288 L<perlop/"Quote and Quote-like Operators">.