4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate literal elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use parentheses, the simple but occasionally
34 surprising rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. Whitespace
37 between the function and left parenthesis doesn't count, so sometimes
38 you need to be careful:
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in scalar context by
59 returning the undefined value, and in list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls ("syscalls")
82 of the same name (like chown(2), fork(2), closedir(2), etc.) return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule include C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 Extension modules can also hook into the Perl parser to define new
90 kinds of keyword-headed expression. These may look like functions, but
91 may also look completely different. The syntax following the keyword
92 is defined entirely by the extension. If you are an implementor, see
93 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94 a module, see the module's documentation for details of the syntax that
97 =head2 Perl Functions by Category
100 Here are Perl's functions (including things that look like
101 functions, like some keywords and named operators)
102 arranged by category. Some functions appear in more
107 =item Functions for SCALARs or strings
108 X<scalar> X<string> X<character>
110 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
111 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
112 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
114 =item Regular expressions and pattern matching
115 X<regular expression> X<regex> X<regexp>
117 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
119 =item Numeric functions
120 X<numeric> X<number> X<trigonometric> X<trigonometry>
122 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123 C<sin>, C<sqrt>, C<srand>
125 =item Functions for real @ARRAYs
128 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
130 =item Functions for list data
133 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
135 =item Functions for real %HASHes
138 C<delete>, C<each>, C<exists>, C<keys>, C<values>
140 =item Input and output functions
141 X<I/O> X<input> X<output> X<dbm>
143 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
145 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
146 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
149 =item Functions for fixed-length data or records
151 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
153 =item Functions for filehandles, files, or directories
154 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
156 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
157 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
158 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159 C<umask>, C<unlink>, C<utime>
161 =item Keywords related to the control flow of your Perl program
164 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
167 =item Keywords related to the switch feature
169 C<break>, C<continue>, C<default>, C<given>, C<when>
171 Except for C<continue>, these are available only if you enable the
172 C<"switch"> feature or use the C<CORE::> prefix.
173 See L<feature> and L<perlsyn/"Switch statements">.
174 Alternately, include a C<use v5.10> or later to the current scope. In Perl
175 5.14 and earlier, C<continue> required the C<"switch"> feature, like the
178 =item Keywords related to scoping
180 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
182 C<state> is available only if the C<"state"> feature
183 is enabled or if it is prefixed with C<CORE::>. See
184 L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
186 =item Miscellaneous functions
188 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
189 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
191 =item Functions for processes and process groups
192 X<process> X<pid> X<process id>
194 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
195 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
196 C<times>, C<wait>, C<waitpid>
198 =item Keywords related to Perl modules
201 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
203 =item Keywords related to classes and object-orientation
204 X<object> X<class> X<package>
206 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
209 =item Low-level socket functions
212 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
213 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
214 C<socket>, C<socketpair>
216 =item System V interprocess communication functions
217 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
219 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
220 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
222 =item Fetching user and group info
223 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
225 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
226 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
227 C<getpwuid>, C<setgrent>, C<setpwent>
229 =item Fetching network info
230 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
232 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
233 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
234 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
235 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
236 C<setnetent>, C<setprotoent>, C<setservent>
238 =item Time-related functions
241 C<gmtime>, C<localtime>, C<time>, C<times>
243 =item Functions new in perl5
246 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
247 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
248 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
249 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
250 C<ucfirst>, C<untie>, C<use>, C<when>
252 * C<sub> was a keyword in Perl 4, but in Perl 5 it is an
253 operator, which can be used in expressions.
255 =item Functions obsoleted in perl5
257 C<dbmclose>, C<dbmopen>
262 X<portability> X<Unix> X<portable>
264 Perl was born in Unix and can therefore access all common Unix
265 system calls. In non-Unix environments, the functionality of some
266 Unix system calls may not be available or details of the available
267 functionality may differ slightly. The Perl functions affected
270 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
271 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
272 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
273 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
274 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
275 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
276 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
277 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
278 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
279 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
280 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
281 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
282 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
283 C<shmwrite>, C<socket>, C<socketpair>,
284 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
285 C<times>, C<truncate>, C<umask>, C<unlink>,
286 C<utime>, C<wait>, C<waitpid>
288 For more information about the portability of these functions, see
289 L<perlport> and other available platform-specific documentation.
291 =head2 Alphabetical Listing of Perl Functions
296 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>
297 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
305 A file test, where X is one of the letters listed below. This unary
306 operator takes one argument, either a filename, a filehandle, or a dirhandle,
307 and tests the associated file to see if something is true about it. If the
308 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
309 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
310 the undefined value if the file doesn't exist. Despite the funny
311 names, precedence is the same as any other named unary operator. The
312 operator may be any of:
314 -r File is readable by effective uid/gid.
315 -w File is writable by effective uid/gid.
316 -x File is executable by effective uid/gid.
317 -o File is owned by effective uid.
319 -R File is readable by real uid/gid.
320 -W File is writable by real uid/gid.
321 -X File is executable by real uid/gid.
322 -O File is owned by real uid.
325 -z File has zero size (is empty).
326 -s File has nonzero size (returns size in bytes).
328 -f File is a plain file.
329 -d File is a directory.
330 -l File is a symbolic link.
331 -p File is a named pipe (FIFO), or Filehandle is a pipe.
333 -b File is a block special file.
334 -c File is a character special file.
335 -t Filehandle is opened to a tty.
337 -u File has setuid bit set.
338 -g File has setgid bit set.
339 -k File has sticky bit set.
341 -T File is an ASCII text file (heuristic guess).
342 -B File is a "binary" file (opposite of -T).
344 -M Script start time minus file modification time, in days.
345 -A Same for access time.
346 -C Same for inode change time (Unix, may differ for other platforms)
352 next unless -f $_; # ignore specials
356 Note that C<-s/a/b/> does not do a negated substitution. Saying
357 C<-exp($foo)> still works as expected, however: only single letters
358 following a minus are interpreted as file tests.
360 These operators are exempt from the "looks like a function rule" described
361 above. That is, an opening parenthesis after the operator does not affect
362 how much of the following code constitutes the argument. Put the opening
363 parentheses before the operator to separate it from code that follows (this
364 applies only to operators with higher precedence than unary operators, of
367 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
368 (-s $file) + 1024 # correct
370 The interpretation of the file permission operators C<-r>, C<-R>,
371 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
372 of the file and the uids and gids of the user. There may be other
373 reasons you can't actually read, write, or execute the file: for
374 example network filesystem access controls, ACLs (access control lists),
375 read-only filesystems, and unrecognized executable formats. Note
376 that the use of these six specific operators to verify if some operation
377 is possible is usually a mistake, because it may be open to race
380 Also note that, for the superuser on the local filesystems, the C<-r>,
381 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
382 if any execute bit is set in the mode. Scripts run by the superuser
383 may thus need to do a stat() to determine the actual mode of the file,
384 or temporarily set their effective uid to something else.
386 If you are using ACLs, there is a pragma called C<filetest> that may
387 produce more accurate results than the bare stat() mode bits.
388 When under C<use filetest 'access'> the above-mentioned filetests
389 test whether the permission can(not) be granted using the
390 access(2) family of system calls. Also note that the C<-x> and C<-X> may
391 under this pragma return true even if there are no execute permission
392 bits set (nor any extra execute permission ACLs). This strangeness is
393 due to the underlying system calls' definitions. Note also that, due to
394 the implementation of C<use filetest 'access'>, the C<_> special
395 filehandle won't cache the results of the file tests when this pragma is
396 in effect. Read the documentation for the C<filetest> pragma for more
399 The C<-T> and C<-B> switches work as follows. The first block or so of the
400 file is examined for odd characters such as strange control codes or
401 characters with the high bit set. If too many strange characters (>30%)
402 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
403 containing a zero byte in the first block is considered a binary file. If C<-T>
404 or C<-B> is used on a filehandle, the current IO buffer is examined
405 rather than the first block. Both C<-T> and C<-B> return true on an empty
406 file, or a file at EOF when testing a filehandle. Because you have to
407 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
408 against the file first, as in C<next unless -f $file && -T $file>.
410 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
411 the special filehandle consisting of a solitary underline, then the stat
412 structure of the previous file test (or stat operator) is used, saving
413 a system call. (This doesn't work with C<-t>, and you need to remember
414 that lstat() and C<-l> leave values in the stat structure for the
415 symbolic link, not the real file.) (Also, if the stat buffer was filled by
416 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
419 print "Can do.\n" if -r $a || -w _ || -x _;
422 print "Readable\n" if -r _;
423 print "Writable\n" if -w _;
424 print "Executable\n" if -x _;
425 print "Setuid\n" if -u _;
426 print "Setgid\n" if -g _;
427 print "Sticky\n" if -k _;
428 print "Text\n" if -T _;
429 print "Binary\n" if -B _;
431 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
432 test operators, in a way that C<-f -w -x $file> is equivalent to
433 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
434 the return value of C<-f $file> as an argument to another filetest
435 operator, no special magic will happen.)
437 Portability issues: L<perlport/-X>.
444 Returns the absolute value of its argument.
445 If VALUE is omitted, uses C<$_>.
447 =item accept NEWSOCKET,GENERICSOCKET
450 Accepts an incoming socket connect, just as accept(2)
451 does. Returns the packed address if it succeeded, false otherwise.
452 See the example in L<perlipc/"Sockets: Client/Server Communication">.
454 On systems that support a close-on-exec flag on files, the flag will
455 be set for the newly opened file descriptor, as determined by the
456 value of $^F. See L<perlvar/$^F>.
465 Arranges to have a SIGALRM delivered to this process after the
466 specified number of wallclock seconds has elapsed. If SECONDS is not
467 specified, the value stored in C<$_> is used. (On some machines,
468 unfortunately, the elapsed time may be up to one second less or more
469 than you specified because of how seconds are counted, and process
470 scheduling may delay the delivery of the signal even further.)
472 Only one timer may be counting at once. Each call disables the
473 previous timer, and an argument of C<0> may be supplied to cancel the
474 previous timer without starting a new one. The returned value is the
475 amount of time remaining on the previous timer.
477 For delays of finer granularity than one second, the Time::HiRes module
478 (from CPAN, and starting from Perl 5.8 part of the standard
479 distribution) provides ualarm(). You may also use Perl's four-argument
480 version of select() leaving the first three arguments undefined, or you
481 might be able to use the C<syscall> interface to access setitimer(2) if
482 your system supports it. See L<perlfaq8> for details.
484 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
485 C<sleep> may be internally implemented on your system with C<alarm>.
487 If you want to use C<alarm> to time out a system call you need to use an
488 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
489 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
490 restart system calls on some systems. Using C<eval>/C<die> always works,
491 modulo the caveats given in L<perlipc/"Signals">.
494 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
496 $nread = sysread SOCKET, $buffer, $size;
500 die unless $@ eq "alarm\n"; # propagate unexpected errors
507 For more information see L<perlipc>.
509 Portability issues: L<perlport/alarm>.
512 X<atan2> X<arctangent> X<tan> X<tangent>
514 Returns the arctangent of Y/X in the range -PI to PI.
516 For the tangent operation, you may use the C<Math::Trig::tan>
517 function, or use the familiar relation:
519 sub tan { sin($_[0]) / cos($_[0]) }
521 The return value for C<atan2(0,0)> is implementation-defined; consult
522 your atan2(3) manpage for more information.
524 Portability issues: L<perlport/atan2>.
526 =item bind SOCKET,NAME
529 Binds a network address to a socket, just as bind(2)
530 does. Returns true if it succeeded, false otherwise. NAME should be a
531 packed address of the appropriate type for the socket. See the examples in
532 L<perlipc/"Sockets: Client/Server Communication">.
534 =item binmode FILEHANDLE, LAYER
535 X<binmode> X<binary> X<text> X<DOS> X<Windows>
537 =item binmode FILEHANDLE
539 Arranges for FILEHANDLE to be read or written in "binary" or "text"
540 mode on systems where the run-time libraries distinguish between
541 binary and text files. If FILEHANDLE is an expression, the value is
542 taken as the name of the filehandle. Returns true on success,
543 otherwise it returns C<undef> and sets C<$!> (errno).
545 On some systems (in general, DOS- and Windows-based systems) binmode()
546 is necessary when you're not working with a text file. For the sake
547 of portability it is a good idea always to use it when appropriate,
548 and never to use it when it isn't appropriate. Also, people can
549 set their I/O to be by default UTF8-encoded Unicode, not bytes.
551 In other words: regardless of platform, use binmode() on binary data,
552 like images, for example.
554 If LAYER is present it is a single string, but may contain multiple
555 directives. The directives alter the behaviour of the filehandle.
556 When LAYER is present, using binmode on a text file makes sense.
558 If LAYER is omitted or specified as C<:raw> the filehandle is made
559 suitable for passing binary data. This includes turning off possible CRLF
560 translation and marking it as bytes (as opposed to Unicode characters).
561 Note that, despite what may be implied in I<"Programming Perl"> (the
562 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
563 Other layers that would affect the binary nature of the stream are
564 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
565 PERLIO environment variable.
567 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
568 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
569 establish default I/O layers. See L<open>.
571 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
572 in "Programming Perl, 3rd Edition". However, since the publishing of this
573 book, by many known as "Camel III", the consensus of the naming of this
574 functionality has moved from "discipline" to "layer". All documentation
575 of this version of Perl therefore refers to "layers" rather than to
576 "disciplines". Now back to the regularly scheduled documentation...>
578 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
579 C<:utf8> just marks the data as UTF-8 without further checking,
580 while C<:encoding(UTF-8)> checks the data for actually being valid
581 UTF-8. More details can be found in L<PerlIO::encoding>.
583 In general, binmode() should be called after open() but before any I/O
584 is done on the filehandle. Calling binmode() normally flushes any
585 pending buffered output data (and perhaps pending input data) on the
586 handle. An exception to this is the C<:encoding> layer that
587 changes the default character encoding of the handle; see L</open>.
588 The C<:encoding> layer sometimes needs to be called in
589 mid-stream, and it doesn't flush the stream. The C<:encoding>
590 also implicitly pushes on top of itself the C<:utf8> layer because
591 internally Perl operates on UTF8-encoded Unicode characters.
593 The operating system, device drivers, C libraries, and Perl run-time
594 system all conspire to let the programmer treat a single
595 character (C<\n>) as the line terminator, irrespective of external
596 representation. On many operating systems, the native text file
597 representation matches the internal representation, but on some
598 platforms the external representation of C<\n> is made up of more than
601 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
602 a single character to end each line in the external representation of text
603 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
604 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
605 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
606 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
607 two characters C<\cM\cJ>. That means that if you don't use binmode() on
608 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
609 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
610 output. This is what you want for text files, but it can be disastrous for
613 Another consequence of using binmode() (on some systems) is that
614 special end-of-file markers will be seen as part of the data stream.
615 For systems from the Microsoft family this means that, if your binary
616 data contain C<\cZ>, the I/O subsystem will regard it as the end of
617 the file, unless you use binmode().
619 binmode() is important not only for readline() and print() operations,
620 but also when using read(), seek(), sysread(), syswrite() and tell()
621 (see L<perlport> for more details). See the C<$/> and C<$\> variables
622 in L<perlvar> for how to manually set your input and output
623 line-termination sequences.
625 Portability issues: L<perlport/binmode>.
627 =item bless REF,CLASSNAME
632 This function tells the thingy referenced by REF that it is now an object
633 in the CLASSNAME package. If CLASSNAME is omitted, the current package
634 is used. Because a C<bless> is often the last thing in a constructor,
635 it returns the reference for convenience. Always use the two-argument
636 version if a derived class might inherit the function doing the blessing.
637 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
640 Consider always blessing objects in CLASSNAMEs that are mixed case.
641 Namespaces with all lowercase names are considered reserved for
642 Perl pragmata. Builtin types have all uppercase names. To prevent
643 confusion, you may wish to avoid such package names as well. Make sure
644 that CLASSNAME is a true value.
646 See L<perlmod/"Perl Modules">.
650 Break out of a C<given()> block.
652 This keyword is enabled by the C<"switch"> feature: see
653 L<feature> for more information. You can also access it by
654 prefixing it with C<CORE::>. Alternately, include a C<use
655 v5.10> or later to the current scope.
658 X<caller> X<call stack> X<stack> X<stack trace>
662 Returns the context of the current subroutine call. In scalar context,
663 returns the caller's package name if there I<is> a caller (that is, if
664 we're in a subroutine or C<eval> or C<require>) and the undefined value
665 otherwise. In list context, returns
668 ($package, $filename, $line) = caller;
670 With EXPR, it returns some extra information that the debugger uses to
671 print a stack trace. The value of EXPR indicates how many call frames
672 to go back before the current one.
675 ($package, $filename, $line, $subroutine, $hasargs,
678 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
681 Here $subroutine may be C<(eval)> if the frame is not a subroutine
682 call, but an C<eval>. In such a case additional elements $evaltext and
683 C<$is_require> are set: C<$is_require> is true if the frame is created by a
684 C<require> or C<use> statement, $evaltext contains the text of the
685 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
686 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
687 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
688 frame.) $subroutine may also be C<(unknown)> if this particular
689 subroutine happens to have been deleted from the symbol table.
690 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
691 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
692 compiled with. The C<$hints> and C<$bitmask> values are subject to change
693 between versions of Perl, and are not meant for external use.
695 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
696 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
697 of this hash, as they are the actual values stored in the optree.
699 Furthermore, when called from within the DB package, caller returns more
700 detailed information: it sets the list variable C<@DB::args> to be the
701 arguments with which the subroutine was invoked.
703 Be aware that the optimizer might have optimized call frames away before
704 C<caller> had a chance to get the information. That means that C<caller(N)>
705 might not return information about the call frame you expect it to, for
706 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
707 previous time C<caller> was called.
709 Be aware that setting C<@DB::args> is I<best effort>, intended for
710 debugging or generating backtraces, and should not be relied upon. In
711 particular, as C<@_> contains aliases to the caller's arguments, Perl does
712 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
713 subroutine makes to C<@_> or its contents, not the original values at call
714 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
715 elements, so under certain cases its elements may have become freed and
716 reallocated for other variables or temporary values. Finally, a side effect
717 of the current implementation is that the effects of C<shift @_> can
718 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
719 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
720 elements), so C<@DB::args> is actually a hybrid of the current state and
721 initial state of C<@_>. Buyer beware.
728 =item chdir FILEHANDLE
730 =item chdir DIRHANDLE
734 Changes the working directory to EXPR, if possible. If EXPR is omitted,
735 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
736 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
737 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
738 neither is set, C<chdir> does nothing. It returns true on success,
739 false otherwise. See the example under C<die>.
741 On systems that support fchdir(2), you may pass a filehandle or
742 directory handle as the argument. On systems that don't support fchdir(2),
743 passing handles raises an exception.
746 X<chmod> X<permission> X<mode>
748 Changes the permissions of a list of files. The first element of the
749 list must be the numeric mode, which should probably be an octal
750 number, and which definitely should I<not> be a string of octal digits:
751 C<0644> is okay, but C<"0644"> is not. Returns the number of files
752 successfully changed. See also L</oct> if all you have is a string.
754 $cnt = chmod 0755, "foo", "bar";
755 chmod 0755, @executables;
756 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
758 $mode = "0644"; chmod oct($mode), "foo"; # this is better
759 $mode = 0644; chmod $mode, "foo"; # this is best
761 On systems that support fchmod(2), you may pass filehandles among the
762 files. On systems that don't support fchmod(2), passing filehandles raises
763 an exception. Filehandles must be passed as globs or glob references to be
764 recognized; barewords are considered filenames.
766 open(my $fh, "<", "foo");
767 my $perm = (stat $fh)[2] & 07777;
768 chmod($perm | 0600, $fh);
770 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
773 use Fcntl qw( :mode );
774 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
775 # Identical to the chmod 0755 of the example above.
777 Portability issues: L<perlport/chmod>.
780 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
786 This safer version of L</chop> removes any trailing string
787 that corresponds to the current value of C<$/> (also known as
788 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
789 number of characters removed from all its arguments. It's often used to
790 remove the newline from the end of an input record when you're worried
791 that the final record may be missing its newline. When in paragraph
792 mode (C<$/ = "">), it removes all trailing newlines from the string.
793 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
794 a reference to an integer or the like; see L<perlvar>) chomp() won't
796 If VARIABLE is omitted, it chomps C<$_>. Example:
799 chomp; # avoid \n on last field
804 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
806 You can actually chomp anything that's an lvalue, including an assignment:
809 chomp($answer = <STDIN>);
811 If you chomp a list, each element is chomped, and the total number of
812 characters removed is returned.
814 Note that parentheses are necessary when you're chomping anything
815 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
816 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
817 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
818 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
828 Chops off the last character of a string and returns the character
829 chopped. It is much more efficient than C<s/.$//s> because it neither
830 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
831 If VARIABLE is a hash, it chops the hash's values, but not its keys.
833 You can actually chop anything that's an lvalue, including an assignment.
835 If you chop a list, each element is chopped. Only the value of the
836 last C<chop> is returned.
838 Note that C<chop> returns the last character. To return all but the last
839 character, use C<substr($string, 0, -1)>.
844 X<chown> X<owner> X<user> X<group>
846 Changes the owner (and group) of a list of files. The first two
847 elements of the list must be the I<numeric> uid and gid, in that
848 order. A value of -1 in either position is interpreted by most
849 systems to leave that value unchanged. Returns the number of files
850 successfully changed.
852 $cnt = chown $uid, $gid, 'foo', 'bar';
853 chown $uid, $gid, @filenames;
855 On systems that support fchown(2), you may pass filehandles among the
856 files. On systems that don't support fchown(2), passing filehandles raises
857 an exception. Filehandles must be passed as globs or glob references to be
858 recognized; barewords are considered filenames.
860 Here's an example that looks up nonnumeric uids in the passwd file:
863 chomp($user = <STDIN>);
865 chomp($pattern = <STDIN>);
867 ($login,$pass,$uid,$gid) = getpwnam($user)
868 or die "$user not in passwd file";
870 @ary = glob($pattern); # expand filenames
871 chown $uid, $gid, @ary;
873 On most systems, you are not allowed to change the ownership of the
874 file unless you're the superuser, although you should be able to change
875 the group to any of your secondary groups. On insecure systems, these
876 restrictions may be relaxed, but this is not a portable assumption.
877 On POSIX systems, you can detect this condition this way:
879 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
880 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
882 Portability issues: L<perlport/chmod>.
885 X<chr> X<character> X<ASCII> X<Unicode>
889 Returns the character represented by that NUMBER in the character set.
890 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
891 chr(0x263a) is a Unicode smiley face.
893 Negative values give the Unicode replacement character (chr(0xfffd)),
894 except under the L<bytes> pragma, where the low eight bits of the value
895 (truncated to an integer) are used.
897 If NUMBER is omitted, uses C<$_>.
899 For the reverse, use L</ord>.
901 Note that characters from 128 to 255 (inclusive) are by default
902 internally not encoded as UTF-8 for backward compatibility reasons.
904 See L<perlunicode> for more about Unicode.
906 =item chroot FILENAME
911 This function works like the system call by the same name: it makes the
912 named directory the new root directory for all further pathnames that
913 begin with a C</> by your process and all its children. (It doesn't
914 change your current working directory, which is unaffected.) For security
915 reasons, this call is restricted to the superuser. If FILENAME is
916 omitted, does a C<chroot> to C<$_>.
918 Portability issues: L<perlport/chroot>.
920 =item close FILEHANDLE
925 Closes the file or pipe associated with the filehandle, flushes the IO
926 buffers, and closes the system file descriptor. Returns true if those
927 operations succeed and if no error was reported by any PerlIO
928 layer. Closes the currently selected filehandle if the argument is
931 You don't have to close FILEHANDLE if you are immediately going to do
932 another C<open> on it, because C<open> closes it for you. (See
933 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
934 counter (C<$.>), while the implicit close done by C<open> does not.
936 If the filehandle came from a piped open, C<close> returns false if one of
937 the other syscalls involved fails or if its program exits with non-zero
938 status. If the only problem was that the program exited non-zero, C<$!>
939 will be set to C<0>. Closing a pipe also waits for the process executing
940 on the pipe to exit--in case you wish to look at the output of the pipe
941 afterwards--and implicitly puts the exit status value of that command into
942 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
944 If there are multiple threads running, C<close> on a filehandle from a
945 piped open returns true without waiting for the child process to terminate,
946 if the filehandle is still open in another thread.
948 Closing the read end of a pipe before the process writing to it at the
949 other end is done writing results in the writer receiving a SIGPIPE. If
950 the other end can't handle that, be sure to read all the data before
955 open(OUTPUT, '|sort >foo') # pipe to sort
956 or die "Can't start sort: $!";
957 #... # print stuff to output
958 close OUTPUT # wait for sort to finish
959 or warn $! ? "Error closing sort pipe: $!"
960 : "Exit status $? from sort";
961 open(INPUT, 'foo') # get sort's results
962 or die "Can't open 'foo' for input: $!";
964 FILEHANDLE may be an expression whose value can be used as an indirect
965 filehandle, usually the real filehandle name or an autovivified handle.
967 =item closedir DIRHANDLE
970 Closes a directory opened by C<opendir> and returns the success of that
973 =item connect SOCKET,NAME
976 Attempts to connect to a remote socket, just like connect(2).
977 Returns true if it succeeded, false otherwise. NAME should be a
978 packed address of the appropriate type for the socket. See the examples in
979 L<perlipc/"Sockets: Client/Server Communication">.
986 When followed by a BLOCK, C<continue> is actually a
987 flow control statement rather than a function. If
988 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
989 C<foreach>), it is always executed just before the conditional is about to
990 be evaluated again, just like the third part of a C<for> loop in C. Thus
991 it can be used to increment a loop variable, even when the loop has been
992 continued via the C<next> statement (which is similar to the C C<continue>
995 C<last>, C<next>, or C<redo> may appear within a C<continue>
996 block; C<last> and C<redo> behave as if they had been executed within
997 the main block. So will C<next>, but since it will execute a C<continue>
998 block, it may be more entertaining.
1001 ### redo always comes here
1004 ### next always comes here
1006 # then back the top to re-check EXPR
1008 ### last always comes here
1010 Omitting the C<continue> section is equivalent to using an
1011 empty one, logically enough, so C<next> goes directly back
1012 to check the condition at the top of the loop.
1014 When there is no BLOCK, C<continue> is a function that
1015 falls through the current C<when> or C<default> block instead of iterating
1016 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1017 In Perl 5.14 and earlier, this form of C<continue> was
1018 only available when the C<"switch"> feature was enabled.
1019 See L<feature> and L<perlsyn/"Switch statements"> for more
1023 X<cos> X<cosine> X<acos> X<arccosine>
1027 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1028 takes the cosine of C<$_>.
1030 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1031 function, or use this relation:
1033 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1035 =item crypt PLAINTEXT,SALT
1036 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1037 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1039 Creates a digest string exactly like the crypt(3) function in the C
1040 library (assuming that you actually have a version there that has not
1041 been extirpated as a potential munition).
1043 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1044 into a short string, called a digest, which is returned. The same
1045 PLAINTEXT and SALT will always return the same string, but there is no
1046 (known) way to get the original PLAINTEXT from the hash. Small
1047 changes in the PLAINTEXT or SALT will result in large changes in the
1050 There is no decrypt function. This function isn't all that useful for
1051 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1052 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1053 primarily used to check if two pieces of text are the same without
1054 having to transmit or store the text itself. An example is checking
1055 if a correct password is given. The digest of the password is stored,
1056 not the password itself. The user types in a password that is
1057 crypt()'d with the same salt as the stored digest. If the two digests
1058 match, the password is correct.
1060 When verifying an existing digest string you should use the digest as
1061 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1062 to create the digest is visible as part of the digest. This ensures
1063 crypt() will hash the new string with the same salt as the digest.
1064 This allows your code to work with the standard L<crypt|/crypt> and
1065 with more exotic implementations. In other words, assume
1066 nothing about the returned string itself nor about how many bytes
1069 Traditionally the result is a string of 13 bytes: two first bytes of
1070 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1071 the first eight bytes of PLAINTEXT mattered. But alternative
1072 hashing schemes (like MD5), higher level security schemes (like C2),
1073 and implementations on non-Unix platforms may produce different
1076 When choosing a new salt create a random two character string whose
1077 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1078 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1079 characters is just a recommendation; the characters allowed in
1080 the salt depend solely on your system's crypt library, and Perl can't
1081 restrict what salts C<crypt()> accepts.
1083 Here's an example that makes sure that whoever runs this program knows
1086 $pwd = (getpwuid($<))[1];
1088 system "stty -echo";
1090 chomp($word = <STDIN>);
1094 if (crypt($word, $pwd) ne $pwd) {
1100 Of course, typing in your own password to whoever asks you
1103 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1104 of data, not least of all because you can't get the information
1105 back. Look at the L<Digest> module for more robust algorithms.
1107 If using crypt() on a Unicode string (which I<potentially> has
1108 characters with codepoints above 255), Perl tries to make sense
1109 of the situation by trying to downgrade (a copy of)
1110 the string back to an eight-bit byte string before calling crypt()
1111 (on that copy). If that works, good. If not, crypt() dies with
1112 C<Wide character in crypt>.
1114 Portability issues: L<perlport/crypt>.
1119 [This function has been largely superseded by the C<untie> function.]
1121 Breaks the binding between a DBM file and a hash.
1123 Portability issues: L<perlport/dbmclose>.
1125 =item dbmopen HASH,DBNAME,MASK
1126 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1128 [This function has been largely superseded by the
1129 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1131 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1132 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1133 argument is I<not> a filehandle, even though it looks like one). DBNAME
1134 is the name of the database (without the F<.dir> or F<.pag> extension if
1135 any). If the database does not exist, it is created with protection
1136 specified by MASK (as modified by the C<umask>). If your system supports
1137 only the older DBM functions, you may make only one C<dbmopen> call in your
1138 program. In older versions of Perl, if your system had neither DBM nor
1139 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1142 If you don't have write access to the DBM file, you can only read hash
1143 variables, not set them. If you want to test whether you can write,
1144 either use file tests or try setting a dummy hash entry inside an C<eval>
1147 Note that functions such as C<keys> and C<values> may return huge lists
1148 when used on large DBM files. You may prefer to use the C<each>
1149 function to iterate over large DBM files. Example:
1151 # print out history file offsets
1152 dbmopen(%HIST,'/usr/lib/news/history',0666);
1153 while (($key,$val) = each %HIST) {
1154 print $key, ' = ', unpack('L',$val), "\n";
1158 See also L<AnyDBM_File> for a more general description of the pros and
1159 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1160 rich implementation.
1162 You can control which DBM library you use by loading that library
1163 before you call dbmopen():
1166 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1167 or die "Can't open netscape history file: $!";
1169 Portability issues: L<perlport/dbmopen>.
1173 Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
1174 that's always true. Only available after Perl 5.10, and only if the
1175 C<switch> feature has been requested or if the keyword is prefixed with
1176 C<CORE::>. See L</when>.
1179 X<defined> X<undef> X<undefined>
1183 Returns a Boolean value telling whether EXPR has a value other than
1184 the undefined value C<undef>. If EXPR is not present, C<$_> is
1187 Many operations return C<undef> to indicate failure, end of file,
1188 system error, uninitialized variable, and other exceptional
1189 conditions. This function allows you to distinguish C<undef> from
1190 other values. (A simple Boolean test will not distinguish among
1191 C<undef>, zero, the empty string, and C<"0">, which are all equally
1192 false.) Note that since C<undef> is a valid scalar, its presence
1193 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1194 returns C<undef> when its argument is an empty array, I<or> when the
1195 element to return happens to be C<undef>.
1197 You may also use C<defined(&func)> to check whether subroutine C<&func>
1198 has ever been defined. The return value is unaffected by any forward
1199 declarations of C<&func>. A subroutine that is not defined
1200 may still be callable: its package may have an C<AUTOLOAD> method that
1201 makes it spring into existence the first time that it is called; see
1204 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1205 used to report whether memory for that aggregate had ever been
1206 allocated. This behavior may disappear in future versions of Perl.
1207 You should instead use a simple test for size:
1209 if (@an_array) { print "has array elements\n" }
1210 if (%a_hash) { print "has hash members\n" }
1212 When used on a hash element, it tells you whether the value is defined,
1213 not whether the key exists in the hash. Use L</exists> for the latter
1218 print if defined $switch{D};
1219 print "$val\n" while defined($val = pop(@ary));
1220 die "Can't readlink $sym: $!"
1221 unless defined($value = readlink $sym);
1222 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1223 $debugging = 0 unless defined $debugging;
1225 Note: Many folks tend to overuse C<defined> and are then surprised to
1226 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1227 defined values. For example, if you say
1231 The pattern match succeeds and C<$1> is defined, although it
1232 matched "nothing". It didn't really fail to match anything. Rather, it
1233 matched something that happened to be zero characters long. This is all
1234 very above-board and honest. When a function returns an undefined value,
1235 it's an admission that it couldn't give you an honest answer. So you
1236 should use C<defined> only when questioning the integrity of what
1237 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1240 See also L</undef>, L</exists>, L</ref>.
1245 Given an expression that specifies an element or slice of a hash, C<delete>
1246 deletes the specified elements from that hash so that exists() on that element
1247 no longer returns true. Setting a hash element to the undefined value does
1248 not remove its key, but deleting it does; see L</exists>.
1250 In list context, returns the value or values deleted, or the last such
1251 element in scalar context. The return list's length always matches that of
1252 the argument list: deleting non-existent elements returns the undefined value
1253 in their corresponding positions.
1255 delete() may also be used on arrays and array slices, but its behavior is less
1256 straightforward. Although exists() will return false for deleted entries,
1257 deleting array elements never changes indices of existing values; use shift()
1258 or splice() for that. However, if all deleted elements fall at the end of an
1259 array, the array's size shrinks to the position of the highest element that
1260 still tests true for exists(), or to 0 if none do.
1262 B<WARNING:> Calling delete on array values is deprecated and likely to
1263 be removed in a future version of Perl.
1265 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1266 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1267 or array may not necessarily return anything; it depends on the implementation
1268 of the C<tied> package's DELETE method, which may do whatever it pleases.
1270 The C<delete local EXPR> construct localizes the deletion to the current
1271 block at run time. Until the block exits, elements locally deleted
1272 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1273 of composite types">.
1275 %hash = (foo => 11, bar => 22, baz => 33);
1276 $scalar = delete $hash{foo}; # $scalar is 11
1277 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1278 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1280 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1282 foreach $key (keys %HASH) {
1286 foreach $index (0 .. $#ARRAY) {
1287 delete $ARRAY[$index];
1292 delete @HASH{keys %HASH};
1294 delete @ARRAY[0 .. $#ARRAY];
1296 But both are slower than assigning the empty list
1297 or undefining %HASH or @ARRAY, which is the customary
1298 way to empty out an aggregate:
1300 %HASH = (); # completely empty %HASH
1301 undef %HASH; # forget %HASH ever existed
1303 @ARRAY = (); # completely empty @ARRAY
1304 undef @ARRAY; # forget @ARRAY ever existed
1306 The EXPR can be arbitrarily complicated provided its
1307 final operation is an element or slice of an aggregate:
1309 delete $ref->[$x][$y]{$key};
1310 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1312 delete $ref->[$x][$y][$index];
1313 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1316 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1318 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1319 into C<$@> and the C<eval> is terminated with the undefined value.
1320 If the exception is outside of all enclosing C<eval>s, then the uncaught
1321 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1322 need to exit the process with a specific exit code, see L</exit>.
1324 Equivalent examples:
1326 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1327 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1329 If the last element of LIST does not end in a newline, the current
1330 script line number and input line number (if any) are also printed,
1331 and a newline is supplied. Note that the "input line number" (also
1332 known as "chunk") is subject to whatever notion of "line" happens to
1333 be currently in effect, and is also available as the special variable
1334 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1336 Hint: sometimes appending C<", stopped"> to your message will cause it
1337 to make better sense when the string C<"at foo line 123"> is appended.
1338 Suppose you are running script "canasta".
1340 die "/etc/games is no good";
1341 die "/etc/games is no good, stopped";
1343 produce, respectively
1345 /etc/games is no good at canasta line 123.
1346 /etc/games is no good, stopped at canasta line 123.
1348 If the output is empty and C<$@> already contains a value (typically from a
1349 previous eval) that value is reused after appending C<"\t...propagated">.
1350 This is useful for propagating exceptions:
1353 die unless $@ =~ /Expected exception/;
1355 If the output is empty and C<$@> contains an object reference that has a
1356 C<PROPAGATE> method, that method will be called with additional file
1357 and line number parameters. The return value replaces the value in
1358 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1361 If C<$@> is empty then the string C<"Died"> is used.
1363 If an uncaught exception results in interpreter exit, the exit code is
1364 determined from the values of C<$!> and C<$?> with this pseudocode:
1366 exit $! if $!; # errno
1367 exit $? >> 8 if $? >> 8; # child exit status
1368 exit 255; # last resort
1370 The intent is to squeeze as much possible information about the likely cause
1371 into the limited space of the system exit code. However, as C<$!> is the value
1372 of C's C<errno>, which can be set by any system call, this means that the value
1373 of the exit code used by C<die> can be non-predictable, so should not be relied
1374 upon, other than to be non-zero.
1376 You can also call C<die> with a reference argument, and if this is trapped
1377 within an C<eval>, C<$@> contains that reference. This permits more
1378 elaborate exception handling using objects that maintain arbitrary state
1379 about the exception. Such a scheme is sometimes preferable to matching
1380 particular string values of C<$@> with regular expressions. Because C<$@>
1381 is a global variable and C<eval> may be used within object implementations,
1382 be careful that analyzing the error object doesn't replace the reference in
1383 the global variable. It's easiest to make a local copy of the reference
1384 before any manipulations. Here's an example:
1386 use Scalar::Util "blessed";
1388 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1389 if (my $ev_err = $@) {
1390 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1391 # handle Some::Module::Exception
1394 # handle all other possible exceptions
1398 Because Perl stringifies uncaught exception messages before display,
1399 you'll probably want to overload stringification operations on
1400 exception objects. See L<overload> for details about that.
1402 You can arrange for a callback to be run just before the C<die>
1403 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1404 handler is called with the error text and can change the error
1405 message, if it sees fit, by calling C<die> again. See
1406 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1407 L<"eval BLOCK"> for some examples. Although this feature was
1408 to be run only right before your program was to exit, this is not
1409 currently so: the C<$SIG{__DIE__}> hook is currently called
1410 even inside eval()ed blocks/strings! If one wants the hook to do
1411 nothing in such situations, put
1415 as the first line of the handler (see L<perlvar/$^S>). Because
1416 this promotes strange action at a distance, this counterintuitive
1417 behavior may be fixed in a future release.
1419 See also exit(), warn(), and the Carp module.
1424 Not really a function. Returns the value of the last command in the
1425 sequence of commands indicated by BLOCK. When modified by the C<while> or
1426 C<until> loop modifier, executes the BLOCK once before testing the loop
1427 condition. (On other statements the loop modifiers test the conditional
1430 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1431 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1432 See L<perlsyn> for alternative strategies.
1434 =item do SUBROUTINE(LIST)
1437 This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1438 a scalar variable or a subroutine beginning with C<&>.
1443 Uses the value of EXPR as a filename and executes the contents of the
1444 file as a Perl script.
1452 except that it's more efficient and concise, keeps track of the current
1453 filename for error messages, searches the C<@INC> directories, and updates
1454 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1455 these variables. It also differs in that code evaluated with C<do FILENAME>
1456 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1457 same, however, in that it does reparse the file every time you call it,
1458 so you probably don't want to do this inside a loop.
1460 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1461 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1462 and sets C<$!> to the error. Always check C<$@> first, as compilation
1463 could fail in a way that also sets C<$!>. If the file is successfully
1464 compiled, C<do> returns the value of the last expression evaluated.
1466 Inclusion of library modules is better done with the
1467 C<use> and C<require> operators, which also do automatic error checking
1468 and raise an exception if there's a problem.
1470 You might like to use C<do> to read in a program configuration
1471 file. Manual error checking can be done this way:
1473 # read in config files: system first, then user
1474 for $file ("/share/prog/defaults.rc",
1475 "$ENV{HOME}/.someprogrc")
1477 unless ($return = do $file) {
1478 warn "couldn't parse $file: $@" if $@;
1479 warn "couldn't do $file: $!" unless defined $return;
1480 warn "couldn't run $file" unless $return;
1485 X<dump> X<core> X<undump>
1489 This function causes an immediate core dump. See also the B<-u>
1490 command-line switch in L<perlrun>, which does the same thing.
1491 Primarily this is so that you can use the B<undump> program (not
1492 supplied) to turn your core dump into an executable binary after
1493 having initialized all your variables at the beginning of the
1494 program. When the new binary is executed it will begin by executing
1495 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1496 Think of it as a goto with an intervening core dump and reincarnation.
1497 If C<LABEL> is omitted, restarts the program from the top.
1499 B<WARNING>: Any files opened at the time of the dump will I<not>
1500 be open any more when the program is reincarnated, with possible
1501 resulting confusion by Perl.
1503 This function is now largely obsolete, mostly because it's very hard to
1504 convert a core file into an executable. That's why you should now invoke
1505 it as C<CORE::dump()>, if you don't want to be warned against a possible
1508 Portability issues: L<perlport/dump>.
1511 X<each> X<hash, iterator>
1518 When called in list context, returns a 2-element list consisting of the key
1519 and value for the next element of a hash, or the index and value for the
1520 next element of an array, so that you can iterate over it. When called in
1521 scalar context, returns only the key (not the value) in a hash, or the index
1524 Hash entries are returned in an apparently random order. The actual random
1525 order is subject to change in future versions of Perl, but it is
1526 guaranteed to be in the same order as either the C<keys> or C<values>
1527 function would produce on the same (unmodified) hash. Since Perl
1528 5.8.2 the ordering can be different even between different runs of Perl
1529 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1531 After C<each> has returned all entries from the hash or array, the next
1532 call to C<each> returns the empty list in list context and C<undef> in
1533 scalar context. The next call following that one restarts iteration. Each
1534 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1535 and C<values>. The iterator is implicitly reset when C<each> has reached
1536 the end as just described; it can be explicitly reset by calling C<keys> or
1537 C<values> on the hash or array. If you add or delete a hash's elements
1538 while iterating over it, entries may be skipped or duplicated--so don't do
1539 that. Exception: It is always safe to delete the item most recently
1540 returned by C<each()>, so the following code works properly:
1542 while (($key, $value) = each %hash) {
1544 delete $hash{$key}; # This is safe
1547 This prints out your environment like the printenv(1) program,
1548 but in a different order:
1550 while (($key,$value) = each %ENV) {
1551 print "$key=$value\n";
1554 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1555 reference to an unblessed hash or array. The argument will be dereferenced
1556 automatically. This aspect of C<each> is considered highly experimental.
1557 The exact behaviour may change in a future version of Perl.
1559 while (($key,$value) = each $hashref) { ... }
1561 See also C<keys>, C<values>, and C<sort>.
1563 =item eof FILEHANDLE
1572 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1573 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1574 gives the real filehandle. (Note that this function actually
1575 reads a character and then C<ungetc>s it, so isn't useful in an
1576 interactive context.) Do not read from a terminal file (or call
1577 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1578 as terminals may lose the end-of-file condition if you do.
1580 An C<eof> without an argument uses the last file read. Using C<eof()>
1581 with empty parentheses is different. It refers to the pseudo file
1582 formed from the files listed on the command line and accessed via the
1583 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1584 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1585 used will cause C<@ARGV> to be examined to determine if input is
1586 available. Similarly, an C<eof()> after C<< <> >> has returned
1587 end-of-file will assume you are processing another C<@ARGV> list,
1588 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1589 see L<perlop/"I/O Operators">.
1591 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1592 detect the end of each file, whereas C<eof()> will detect the end
1593 of the very last file only. Examples:
1595 # reset line numbering on each input file
1597 next if /^\s*#/; # skip comments
1600 close ARGV if eof; # Not eof()!
1603 # insert dashes just before last line of last file
1605 if (eof()) { # check for end of last file
1606 print "--------------\n";
1609 last if eof(); # needed if we're reading from a terminal
1612 Practical hint: you almost never need to use C<eof> in Perl, because the
1613 input operators typically return C<undef> when they run out of data or
1617 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1618 X<error, handling> X<exception, handling>
1624 In the first form, the return value of EXPR is parsed and executed as if it
1625 were a little Perl program. The value of the expression (which is itself
1626 determined within scalar context) is first parsed, and if there were no
1627 errors, executed as a block within the lexical context of the current Perl
1628 program. This means, that in particular, any outer lexical variables are
1629 visible to it, and any package variable settings or subroutine and format
1630 definitions remain afterwards.
1632 Note that the value is parsed every time the C<eval> executes.
1633 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1634 delay parsing and subsequent execution of the text of EXPR until run time.
1636 In the second form, the code within the BLOCK is parsed only once--at the
1637 same time the code surrounding the C<eval> itself was parsed--and executed
1638 within the context of the current Perl program. This form is typically
1639 used to trap exceptions more efficiently than the first (see below), while
1640 also providing the benefit of checking the code within BLOCK at compile
1643 The final semicolon, if any, may be omitted from the value of EXPR or within
1646 In both forms, the value returned is the value of the last expression
1647 evaluated inside the mini-program; a return statement may be also used, just
1648 as with subroutines. The expression providing the return value is evaluated
1649 in void, scalar, or list context, depending on the context of the C<eval>
1650 itself. See L</wantarray> for more on how the evaluation context can be
1653 If there is a syntax error or runtime error, or a C<die> statement is
1654 executed, C<eval> returns C<undef> in scalar context
1655 or an empty list--or, for syntax errors, a list containing a single
1656 undefined value--in list context, and C<$@> is set to the error
1657 message. The discrepancy in the return values in list context is
1658 considered a bug by some, and will probably be fixed in a future
1659 release. If there was no error, C<$@> is set to the empty string. A
1660 control flow operator like C<last> or C<goto> can bypass the setting of
1661 C<$@>. Beware that using C<eval> neither silences Perl from printing
1662 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1663 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1664 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1665 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1667 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1668 determining whether a particular feature (such as C<socket> or C<symlink>)
1669 is implemented. It is also Perl's exception-trapping mechanism, where
1670 the die operator is used to raise exceptions.
1672 If you want to trap errors when loading an XS module, some problems with
1673 the binary interface (such as Perl version skew) may be fatal even with
1674 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1676 If the code to be executed doesn't vary, you may use the eval-BLOCK
1677 form to trap run-time errors without incurring the penalty of
1678 recompiling each time. The error, if any, is still returned in C<$@>.
1681 # make divide-by-zero nonfatal
1682 eval { $answer = $a / $b; }; warn $@ if $@;
1684 # same thing, but less efficient
1685 eval '$answer = $a / $b'; warn $@ if $@;
1687 # a compile-time error
1688 eval { $answer = }; # WRONG
1691 eval '$answer ='; # sets $@
1693 Using the C<eval{}> form as an exception trap in libraries does have some
1694 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1695 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1696 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1697 as this example shows:
1699 # a private exception trap for divide-by-zero
1700 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1703 This is especially significant, given that C<__DIE__> hooks can call
1704 C<die> again, which has the effect of changing their error messages:
1706 # __DIE__ hooks may modify error messages
1708 local $SIG{'__DIE__'} =
1709 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1710 eval { die "foo lives here" };
1711 print $@ if $@; # prints "bar lives here"
1714 Because this promotes action at a distance, this counterintuitive behavior
1715 may be fixed in a future release.
1717 With an C<eval>, you should be especially careful to remember what's
1718 being looked at when:
1724 eval { $x }; # CASE 4
1726 eval "\$$x++"; # CASE 5
1729 Cases 1 and 2 above behave identically: they run the code contained in
1730 the variable $x. (Although case 2 has misleading double quotes making
1731 the reader wonder what else might be happening (nothing is).) Cases 3
1732 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1733 does nothing but return the value of $x. (Case 4 is preferred for
1734 purely visual reasons, but it also has the advantage of compiling at
1735 compile-time instead of at run-time.) Case 5 is a place where
1736 normally you I<would> like to use double quotes, except that in this
1737 particular situation, you can just use symbolic references instead, as
1740 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1741 of localised variables, which means that for your code to run on older
1742 versions, a temporary is required if you want to mask some but not all
1745 # alter $@ on nefarious repugnancy only
1749 local $@; # protect existing $@
1750 eval { test_repugnancy() };
1751 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1752 $@ =~ /nefarious/ and $e = $@;
1754 die $e if defined $e
1757 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1758 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1760 An C<eval ''> executed within the C<DB> package doesn't see the usual
1761 surrounding lexical scope, but rather the scope of the first non-DB piece
1762 of code that called it. You don't normally need to worry about this unless
1763 you are writing a Perl debugger.
1768 =item exec PROGRAM LIST
1770 The C<exec> function executes a system command I<and never returns>;
1771 use C<system> instead of C<exec> if you want it to return. It fails and
1772 returns false only if the command does not exist I<and> it is executed
1773 directly instead of via your system's command shell (see below).
1775 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1776 warns you if there is a following statement that isn't C<die>, C<warn>,
1777 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1778 I<really> want to follow an C<exec> with some other statement, you
1779 can use one of these styles to avoid the warning:
1781 exec ('foo') or print STDERR "couldn't exec foo: $!";
1782 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1784 If there is more than one argument in LIST, or if LIST is an array
1785 with more than one value, calls execvp(3) with the arguments in LIST.
1786 If there is only one scalar argument or an array with one element in it,
1787 the argument is checked for shell metacharacters, and if there are any,
1788 the entire argument is passed to the system's command shell for parsing
1789 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1790 If there are no shell metacharacters in the argument, it is split into
1791 words and passed directly to C<execvp>, which is more efficient.
1794 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1795 exec "sort $outfile | uniq";
1797 If you don't really want to execute the first argument, but want to lie
1798 to the program you are executing about its own name, you can specify
1799 the program you actually want to run as an "indirect object" (without a
1800 comma) in front of the LIST. (This always forces interpretation of the
1801 LIST as a multivalued list, even if there is only a single scalar in
1804 $shell = '/bin/csh';
1805 exec $shell '-sh'; # pretend it's a login shell
1809 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1811 When the arguments get executed via the system shell, results are
1812 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1815 Using an indirect object with C<exec> or C<system> is also more
1816 secure. This usage (which also works fine with system()) forces
1817 interpretation of the arguments as a multivalued list, even if the
1818 list had just one argument. That way you're safe from the shell
1819 expanding wildcards or splitting up words with whitespace in them.
1821 @args = ( "echo surprise" );
1823 exec @args; # subject to shell escapes
1825 exec { $args[0] } @args; # safe even with one-arg list
1827 The first version, the one without the indirect object, ran the I<echo>
1828 program, passing it C<"surprise"> an argument. The second version didn't;
1829 it tried to run a program named I<"echo surprise">, didn't find it, and set
1830 C<$?> to a non-zero value indicating failure.
1832 Beginning with v5.6.0, Perl attempts to flush all files opened for
1833 output before the exec, but this may not be supported on some platforms
1834 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1835 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1836 open handles to avoid lost output.
1838 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1839 C<DESTROY> methods on your objects.
1841 Portability issues: L<perlport/exec>.
1844 X<exists> X<autovivification>
1846 Given an expression that specifies an element of a hash, returns true if the
1847 specified element in the hash has ever been initialized, even if the
1848 corresponding value is undefined.
1850 print "Exists\n" if exists $hash{$key};
1851 print "Defined\n" if defined $hash{$key};
1852 print "True\n" if $hash{$key};
1854 exists may also be called on array elements, but its behavior is much less
1855 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1856 that calling exists on array values is deprecated and likely to be removed in
1857 a future version of Perl.
1859 print "Exists\n" if exists $array[$index];
1860 print "Defined\n" if defined $array[$index];
1861 print "True\n" if $array[$index];
1863 A hash or array element can be true only if it's defined and defined only if
1864 it exists, but the reverse doesn't necessarily hold true.
1866 Given an expression that specifies the name of a subroutine,
1867 returns true if the specified subroutine has ever been declared, even
1868 if it is undefined. Mentioning a subroutine name for exists or defined
1869 does not count as declaring it. Note that a subroutine that does not
1870 exist may still be callable: its package may have an C<AUTOLOAD>
1871 method that makes it spring into existence the first time that it is
1872 called; see L<perlsub>.
1874 print "Exists\n" if exists &subroutine;
1875 print "Defined\n" if defined &subroutine;
1877 Note that the EXPR can be arbitrarily complicated as long as the final
1878 operation is a hash or array key lookup or subroutine name:
1880 if (exists $ref->{A}->{B}->{$key}) { }
1881 if (exists $hash{A}{B}{$key}) { }
1883 if (exists $ref->{A}->{B}->[$ix]) { }
1884 if (exists $hash{A}{B}[$ix]) { }
1886 if (exists &{$ref->{A}{B}{$key}}) { }
1888 Although the mostly deeply nested array or hash will not spring into
1889 existence just because its existence was tested, any intervening ones will.
1890 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1891 into existence due to the existence test for the $key element above.
1892 This happens anywhere the arrow operator is used, including even here:
1895 if (exists $ref->{"Some key"}) { }
1896 print $ref; # prints HASH(0x80d3d5c)
1898 This surprising autovivification in what does not at first--or even
1899 second--glance appear to be an lvalue context may be fixed in a future
1902 Use of a subroutine call, rather than a subroutine name, as an argument
1903 to exists() is an error.
1906 exists &sub(); # Error
1909 X<exit> X<terminate> X<abort>
1913 Evaluates EXPR and exits immediately with that value. Example:
1916 exit 0 if $ans =~ /^[Xx]/;
1918 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1919 universally recognized values for EXPR are C<0> for success and C<1>
1920 for error; other values are subject to interpretation depending on the
1921 environment in which the Perl program is running. For example, exiting
1922 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1923 the mailer to return the item undelivered, but that's not true everywhere.
1925 Don't use C<exit> to abort a subroutine if there's any chance that
1926 someone might want to trap whatever error happened. Use C<die> instead,
1927 which can be trapped by an C<eval>.
1929 The exit() function does not always exit immediately. It calls any
1930 defined C<END> routines first, but these C<END> routines may not
1931 themselves abort the exit. Likewise any object destructors that need to
1932 be called are called before the real exit. C<END> routines and destructors
1933 can change the exit status by modifying C<$?>. If this is a problem, you
1934 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1935 See L<perlmod> for details.
1937 Portability issues: L<perlport/exit>.
1940 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1944 Returns I<e> (the natural logarithm base) to the power of EXPR.
1945 If EXPR is omitted, gives C<exp($_)>.
1947 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1950 Implements the fcntl(2) function. You'll probably have to say
1954 first to get the correct constant definitions. Argument processing and
1955 value returned work just like C<ioctl> below.
1959 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1960 or die "can't fcntl F_GETFL: $!";
1962 You don't have to check for C<defined> on the return from C<fcntl>.
1963 Like C<ioctl>, it maps a C<0> return from the system call into
1964 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1965 in numeric context. It is also exempt from the normal B<-w> warnings
1966 on improper numeric conversions.
1968 Note that C<fcntl> raises an exception if used on a machine that
1969 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1970 manpage to learn what functions are available on your system.
1972 Here's an example of setting a filehandle named C<REMOTE> to be
1973 non-blocking at the system level. You'll have to negotiate C<$|>
1974 on your own, though.
1976 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1978 $flags = fcntl(REMOTE, F_GETFL, 0)
1979 or die "Can't get flags for the socket: $!\n";
1981 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1982 or die "Can't set flags for the socket: $!\n";
1984 Portability issues: L<perlport/fcntl>.
1986 =item fileno FILEHANDLE
1989 Returns the file descriptor for a filehandle, or undefined if the
1990 filehandle is not open. If there is no real file descriptor at the OS
1991 level, as can happen with filehandles connected to memory objects via
1992 C<open> with a reference for the third argument, -1 is returned.
1994 This is mainly useful for constructing
1995 bitmaps for C<select> and low-level POSIX tty-handling operations.
1996 If FILEHANDLE is an expression, the value is taken as an indirect
1997 filehandle, generally its name.
1999 You can use this to find out whether two handles refer to the
2000 same underlying descriptor:
2002 if (fileno(THIS) == fileno(THAT)) {
2003 print "THIS and THAT are dups\n";
2006 =item flock FILEHANDLE,OPERATION
2007 X<flock> X<lock> X<locking>
2009 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2010 for success, false on failure. Produces a fatal error if used on a
2011 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2012 C<flock> is Perl's portable file-locking interface, although it locks
2013 entire files only, not records.
2015 Two potentially non-obvious but traditional C<flock> semantics are
2016 that it waits indefinitely until the lock is granted, and that its locks
2017 are B<merely advisory>. Such discretionary locks are more flexible, but
2018 offer fewer guarantees. This means that programs that do not also use
2019 C<flock> may modify files locked with C<flock>. See L<perlport>,
2020 your port's specific documentation, and your system-specific local manpages
2021 for details. It's best to assume traditional behavior if you're writing
2022 portable programs. (But if you're not, you should as always feel perfectly
2023 free to write for your own system's idiosyncrasies (sometimes called
2024 "features"). Slavish adherence to portability concerns shouldn't get
2025 in the way of your getting your job done.)
2027 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2028 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2029 you can use the symbolic names if you import them from the L<Fcntl> module,
2030 either individually, or as a group using the C<:flock> tag. LOCK_SH
2031 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2032 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2033 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2034 waiting for the lock; check the return status to see if you got it.
2036 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2037 before locking or unlocking it.
2039 Note that the emulation built with lockf(3) doesn't provide shared
2040 locks, and it requires that FILEHANDLE be open with write intent. These
2041 are the semantics that lockf(3) implements. Most if not all systems
2042 implement lockf(3) in terms of fcntl(2) locking, though, so the
2043 differing semantics shouldn't bite too many people.
2045 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2046 be open with read intent to use LOCK_SH and requires that it be open
2047 with write intent to use LOCK_EX.
2049 Note also that some versions of C<flock> cannot lock things over the
2050 network; you would need to use the more system-specific C<fcntl> for
2051 that. If you like you can force Perl to ignore your system's flock(2)
2052 function, and so provide its own fcntl(2)-based emulation, by passing
2053 the switch C<-Ud_flock> to the F<Configure> program when you configure
2054 and build a new Perl.
2056 Here's a mailbox appender for BSD systems.
2058 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
2062 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2064 # and, in case someone appended while we were waiting...
2065 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2070 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2073 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2074 or die "Can't open mailbox: $!";
2077 print $mbox $msg,"\n\n";
2080 On systems that support a real flock(2), locks are inherited across fork()
2081 calls, whereas those that must resort to the more capricious fcntl(2)
2082 function lose their locks, making it seriously harder to write servers.
2084 See also L<DB_File> for other flock() examples.
2086 Portability issues: L<perlport/flock>.
2089 X<fork> X<child> X<parent>
2091 Does a fork(2) system call to create a new process running the
2092 same program at the same point. It returns the child pid to the
2093 parent process, C<0> to the child process, or C<undef> if the fork is
2094 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2095 are shared, while everything else is copied. On most systems supporting
2096 fork(), great care has gone into making it extremely efficient (for
2097 example, using copy-on-write technology on data pages), making it the
2098 dominant paradigm for multitasking over the last few decades.
2100 Beginning with v5.6.0, Perl attempts to flush all files opened for
2101 output before forking the child process, but this may not be supported
2102 on some platforms (see L<perlport>). To be safe, you may need to set
2103 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2104 C<IO::Handle> on any open handles to avoid duplicate output.
2106 If you C<fork> without ever waiting on your children, you will
2107 accumulate zombies. On some systems, you can avoid this by setting
2108 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2109 forking and reaping moribund children.
2111 Note that if your forked child inherits system file descriptors like
2112 STDIN and STDOUT that are actually connected by a pipe or socket, even
2113 if you exit, then the remote server (such as, say, a CGI script or a
2114 backgrounded job launched from a remote shell) won't think you're done.
2115 You should reopen those to F</dev/null> if it's any issue.
2117 On some platforms such as Windows, where the fork() system call is not available,
2118 Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
2119 at the level of the Perl program, be as compatible as possible with the "Unix" fork().
2120 However it has limitations that have to be considered in code intended to be portable.
2121 See L<perlfork> for more details.
2123 Portability issues: L<perlport/fork>.
2128 Declare a picture format for use by the C<write> function. For
2132 Test: @<<<<<<<< @||||| @>>>>>
2133 $str, $%, '$' . int($num)
2137 $num = $cost/$quantity;
2141 See L<perlform> for many details and examples.
2143 =item formline PICTURE,LIST
2146 This is an internal function used by C<format>s, though you may call it,
2147 too. It formats (see L<perlform>) a list of values according to the
2148 contents of PICTURE, placing the output into the format output
2149 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2150 Eventually, when a C<write> is done, the contents of
2151 C<$^A> are written to some filehandle. You could also read C<$^A>
2152 and then set C<$^A> back to C<"">. Note that a format typically
2153 does one C<formline> per line of form, but the C<formline> function itself
2154 doesn't care how many newlines are embedded in the PICTURE. This means
2155 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2156 You may therefore need to use multiple formlines to implement a single
2157 record format, just like the C<format> compiler.
2159 Be careful if you put double quotes around the picture, because an C<@>
2160 character may be taken to mean the beginning of an array name.
2161 C<formline> always returns true. See L<perlform> for other examples.
2163 If you are trying to use this instead of C<write> to capture the output,
2164 you may find it easier to open a filehandle to a scalar
2165 (C<< open $fh, ">", \$output >>) and write to that instead.
2167 =item getc FILEHANDLE
2168 X<getc> X<getchar> X<character> X<file, read>
2172 Returns the next character from the input file attached to FILEHANDLE,
2173 or the undefined value at end of file or if there was an error (in
2174 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2175 STDIN. This is not particularly efficient. However, it cannot be
2176 used by itself to fetch single characters without waiting for the user
2177 to hit enter. For that, try something more like:
2180 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2183 system "stty", '-icanon', 'eol', "\001";
2189 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2192 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2196 Determination of whether $BSD_STYLE should be set
2197 is left as an exercise to the reader.
2199 The C<POSIX::getattr> function can do this more portably on
2200 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2201 module from your nearest CPAN site; details on CPAN can be found under
2205 X<getlogin> X<login>
2207 This implements the C library function of the same name, which on most
2208 systems returns the current login from F</etc/utmp>, if any. If it
2209 returns the empty string, use C<getpwuid>.
2211 $login = getlogin || getpwuid($<) || "Kilroy";
2213 Do not consider C<getlogin> for authentication: it is not as
2214 secure as C<getpwuid>.
2216 Portability issues: L<perlport/getlogin>.
2218 =item getpeername SOCKET
2219 X<getpeername> X<peer>
2221 Returns the packed sockaddr address of the other end of the SOCKET
2225 $hersockaddr = getpeername(SOCK);
2226 ($port, $iaddr) = sockaddr_in($hersockaddr);
2227 $herhostname = gethostbyaddr($iaddr, AF_INET);
2228 $herstraddr = inet_ntoa($iaddr);
2233 Returns the current process group for the specified PID. Use
2234 a PID of C<0> to get the current process group for the
2235 current process. Will raise an exception if used on a machine that
2236 doesn't implement getpgrp(2). If PID is omitted, returns the process
2237 group of the current process. Note that the POSIX version of C<getpgrp>
2238 does not accept a PID argument, so only C<PID==0> is truly portable.
2240 Portability issues: L<perlport/getpgrp>.
2243 X<getppid> X<parent> X<pid>
2245 Returns the process id of the parent process.
2247 Note for Linux users: on Linux, the C functions C<getpid()> and
2248 C<getppid()> return different values from different threads. In order to
2249 be portable, this behavior is not reflected by the Perl-level function
2250 C<getppid()>, that returns a consistent value across threads. If you want
2251 to call the underlying C<getppid()>, you may use the CPAN module
2254 Portability issues: L<perlport/getppid>.
2256 =item getpriority WHICH,WHO
2257 X<getpriority> X<priority> X<nice>
2259 Returns the current priority for a process, a process group, or a user.
2260 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2261 machine that doesn't implement getpriority(2).
2263 Portability issues: L<perlport/getpriority>.
2266 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2267 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2268 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2269 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2270 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2271 X<endnetent> X<endprotoent> X<endservent>
2275 =item gethostbyname NAME
2277 =item getnetbyname NAME
2279 =item getprotobyname NAME
2285 =item getservbyname NAME,PROTO
2287 =item gethostbyaddr ADDR,ADDRTYPE
2289 =item getnetbyaddr ADDR,ADDRTYPE
2291 =item getprotobynumber NUMBER
2293 =item getservbyport PORT,PROTO
2311 =item sethostent STAYOPEN
2313 =item setnetent STAYOPEN
2315 =item setprotoent STAYOPEN
2317 =item setservent STAYOPEN
2331 These routines are the same as their counterparts in the
2332 system C library. In list context, the return values from the
2333 various get routines are as follows:
2335 ($name,$passwd,$uid,$gid,
2336 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2337 ($name,$passwd,$gid,$members) = getgr*
2338 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2339 ($name,$aliases,$addrtype,$net) = getnet*
2340 ($name,$aliases,$proto) = getproto*
2341 ($name,$aliases,$port,$proto) = getserv*
2343 (If the entry doesn't exist you get an empty list.)
2345 The exact meaning of the $gcos field varies but it usually contains
2346 the real name of the user (as opposed to the login name) and other
2347 information pertaining to the user. Beware, however, that in many
2348 system users are able to change this information and therefore it
2349 cannot be trusted and therefore the $gcos is tainted (see
2350 L<perlsec>). The $passwd and $shell, user's encrypted password and
2351 login shell, are also tainted, for the same reason.
2353 In scalar context, you get the name, unless the function was a
2354 lookup by name, in which case you get the other thing, whatever it is.
2355 (If the entry doesn't exist you get the undefined value.) For example:
2357 $uid = getpwnam($name);
2358 $name = getpwuid($num);
2360 $gid = getgrnam($name);
2361 $name = getgrgid($num);
2365 In I<getpw*()> the fields $quota, $comment, and $expire are special
2366 in that they are unsupported on many systems. If the
2367 $quota is unsupported, it is an empty scalar. If it is supported, it
2368 usually encodes the disk quota. If the $comment field is unsupported,
2369 it is an empty scalar. If it is supported it usually encodes some
2370 administrative comment about the user. In some systems the $quota
2371 field may be $change or $age, fields that have to do with password
2372 aging. In some systems the $comment field may be $class. The $expire
2373 field, if present, encodes the expiration period of the account or the
2374 password. For the availability and the exact meaning of these fields
2375 in your system, please consult getpwnam(3) and your system's
2376 F<pwd.h> file. You can also find out from within Perl what your
2377 $quota and $comment fields mean and whether you have the $expire field
2378 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2379 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2380 files are supported only if your vendor has implemented them in the
2381 intuitive fashion that calling the regular C library routines gets the
2382 shadow versions if you're running under privilege or if there exists
2383 the shadow(3) functions as found in System V (this includes Solaris
2384 and Linux). Those systems that implement a proprietary shadow password
2385 facility are unlikely to be supported.
2387 The $members value returned by I<getgr*()> is a space-separated list of
2388 the login names of the members of the group.
2390 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2391 C, it will be returned to you via C<$?> if the function call fails. The
2392 C<@addrs> value returned by a successful call is a list of raw
2393 addresses returned by the corresponding library call. In the
2394 Internet domain, each address is four bytes long; you can unpack it
2395 by saying something like:
2397 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2399 The Socket library makes this slightly easier:
2402 $iaddr = inet_aton("127.1"); # or whatever address
2403 $name = gethostbyaddr($iaddr, AF_INET);
2405 # or going the other way
2406 $straddr = inet_ntoa($iaddr);
2408 In the opposite way, to resolve a hostname to the IP address
2412 $packed_ip = gethostbyname("www.perl.org");
2413 if (defined $packed_ip) {
2414 $ip_address = inet_ntoa($packed_ip);
2417 Make sure <gethostbyname()> is called in SCALAR context and that
2418 its return value is checked for definedness.
2420 If you get tired of remembering which element of the return list
2421 contains which return value, by-name interfaces are provided
2422 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2423 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2424 and C<User::grent>. These override the normal built-ins, supplying
2425 versions that return objects with the appropriate names
2426 for each field. For example:
2430 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2432 Even though it looks as though they're the same method calls (uid),
2433 they aren't, because a C<File::stat> object is different from
2434 a C<User::pwent> object.
2436 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2438 =item getsockname SOCKET
2441 Returns the packed sockaddr address of this end of the SOCKET connection,
2442 in case you don't know the address because you have several different
2443 IPs that the connection might have come in on.
2446 $mysockaddr = getsockname(SOCK);
2447 ($port, $myaddr) = sockaddr_in($mysockaddr);
2448 printf "Connect to %s [%s]\n",
2449 scalar gethostbyaddr($myaddr, AF_INET),
2452 =item getsockopt SOCKET,LEVEL,OPTNAME
2455 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2456 Options may exist at multiple protocol levels depending on the socket
2457 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2458 C<Socket> module) will exist. To query options at another level the
2459 protocol number of the appropriate protocol controlling the option
2460 should be supplied. For example, to indicate that an option is to be
2461 interpreted by the TCP protocol, LEVEL should be set to the protocol
2462 number of TCP, which you can get using C<getprotobyname>.
2464 The function returns a packed string representing the requested socket
2465 option, or C<undef> on error, with the reason for the error placed in
2466 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2467 consult getsockopt(2) for details. A common case is that the option is an
2468 integer, in which case the result is a packed integer, which you can decode
2469 using C<unpack> with the C<i> (or C<I>) format.
2471 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2473 use Socket qw(:all);
2475 defined(my $tcp = getprotobyname("tcp"))
2476 or die "Could not determine the protocol number for tcp";
2477 # my $tcp = IPPROTO_TCP; # Alternative
2478 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2479 or die "getsockopt TCP_NODELAY: $!";
2480 my $nodelay = unpack("I", $packed);
2481 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2483 Portability issues: L<perlport/getsockopt>.
2485 =item given EXPR BLOCK
2490 C<given> is analogous to the C<switch> keyword in other languages. C<given>
2491 and C<when> are used in Perl to implement C<switch>/C<case> like statements.
2492 Only available after Perl 5.10. For example:
2497 print "I like apples."
2500 print "I don't like oranges."
2503 print "I don't like anything"
2507 See L<perlsyn/"Switch statements"> for detailed information.
2510 X<glob> X<wildcard> X<filename, expansion> X<expand>
2514 In list context, returns a (possibly empty) list of filename expansions on
2515 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2516 scalar context, glob iterates through such filename expansions, returning
2517 undef when the list is exhausted. This is the internal function
2518 implementing the C<< <*.c> >> operator, but you can use it directly. If
2519 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2520 more detail in L<perlop/"I/O Operators">.
2522 Note that C<glob> splits its arguments on whitespace and treats
2523 each segment as separate pattern. As such, C<glob("*.c *.h")>
2524 matches all files with a F<.c> or F<.h> extension. The expression
2525 C<glob(".* *")> matches all files in the current working directory.
2527 If non-empty braces are the only wildcard characters used in the
2528 C<glob>, no filenames are matched, but potentially many strings
2529 are returned. For example, this produces nine strings, one for
2530 each pairing of fruits and colors:
2532 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2534 Beginning with v5.6.0, this operator is implemented using the standard
2535 C<File::Glob> extension. See L<File::Glob> for details, including
2536 C<bsd_glob> which does not treat whitespace as a pattern separator.
2538 Portability issues: L<perlport/glob>.
2541 X<gmtime> X<UTC> X<Greenwich>
2545 Works just like L</localtime> but the returned values are
2546 localized for the standard Greenwich time zone.
2548 Note: When called in list context, $isdst, the last value
2549 returned by gmtime, is always C<0>. There is no
2550 Daylight Saving Time in GMT.
2552 Portability issues: L<perlport/gmtime>.
2555 X<goto> X<jump> X<jmp>
2561 The C<goto-LABEL> form finds the statement labeled with LABEL and
2562 resumes execution there. It can't be used to get out of a block or
2563 subroutine given to C<sort>. It can be used to go almost anywhere
2564 else within the dynamic scope, including out of subroutines, but it's
2565 usually better to use some other construct such as C<last> or C<die>.
2566 The author of Perl has never felt the need to use this form of C<goto>
2567 (in Perl, that is; C is another matter). (The difference is that C
2568 does not offer named loops combined with loop control. Perl does, and
2569 this replaces most structured uses of C<goto> in other languages.)
2571 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2572 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2573 necessarily recommended if you're optimizing for maintainability:
2575 goto ("FOO", "BAR", "GLARCH")[$i];
2577 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2578 function" rule. A pair of parentheses following it does not (necessarily)
2579 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2581 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2582 deprecated and will issue a warning. Even then, it may not be used to
2583 go into any construct that requires initialization, such as a
2584 subroutine or a C<foreach> loop. It also can't be used to go into a
2585 construct that is optimized away.
2587 The C<goto-&NAME> form is quite different from the other forms of
2588 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2589 doesn't have the stigma associated with other gotos. Instead, it
2590 exits the current subroutine (losing any changes set by local()) and
2591 immediately calls in its place the named subroutine using the current
2592 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2593 load another subroutine and then pretend that the other subroutine had
2594 been called in the first place (except that any modifications to C<@_>
2595 in the current subroutine are propagated to the other subroutine.)
2596 After the C<goto>, not even C<caller> will be able to tell that this
2597 routine was called first.
2599 NAME needn't be the name of a subroutine; it can be a scalar variable
2600 containing a code reference or a block that evaluates to a code
2603 =item grep BLOCK LIST
2606 =item grep EXPR,LIST
2608 This is similar in spirit to, but not the same as, grep(1) and its
2609 relatives. In particular, it is not limited to using regular expressions.
2611 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2612 C<$_> to each element) and returns the list value consisting of those
2613 elements for which the expression evaluated to true. In scalar
2614 context, returns the number of times the expression was true.
2616 @foo = grep(!/^#/, @bar); # weed out comments
2620 @foo = grep {!/^#/} @bar; # weed out comments
2622 Note that C<$_> is an alias to the list value, so it can be used to
2623 modify the elements of the LIST. While this is useful and supported,
2624 it can cause bizarre results if the elements of LIST are not variables.
2625 Similarly, grep returns aliases into the original list, much as a for
2626 loop's index variable aliases the list elements. That is, modifying an
2627 element of a list returned by grep (for example, in a C<foreach>, C<map>
2628 or another C<grep>) actually modifies the element in the original list.
2629 This is usually something to be avoided when writing clear code.
2631 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2632 been declared with C<my $_>) then, in addition to being locally aliased to
2633 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2634 can't be seen from the outside, avoiding any potential side-effects.
2636 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2639 X<hex> X<hexadecimal>
2643 Interprets EXPR as a hex string and returns the corresponding value.
2644 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2645 L</oct>.) If EXPR is omitted, uses C<$_>.
2647 print hex '0xAf'; # prints '175'
2648 print hex 'aF'; # same
2650 Hex strings may only represent integers. Strings that would cause
2651 integer overflow trigger a warning. Leading whitespace is not stripped,
2652 unlike oct(). To present something as hex, look into L</printf>,
2653 L</sprintf>, and L</unpack>.
2658 There is no builtin C<import> function. It is just an ordinary
2659 method (subroutine) defined (or inherited) by modules that wish to export
2660 names to another module. The C<use> function calls the C<import> method
2661 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2663 =item index STR,SUBSTR,POSITION
2664 X<index> X<indexOf> X<InStr>
2666 =item index STR,SUBSTR
2668 The index function searches for one string within another, but without
2669 the wildcard-like behavior of a full regular-expression pattern match.
2670 It returns the position of the first occurrence of SUBSTR in STR at
2671 or after POSITION. If POSITION is omitted, starts searching from the
2672 beginning of the string. POSITION before the beginning of the string
2673 or after its end is treated as if it were the beginning or the end,
2674 respectively. POSITION and the return value are based at C<0> (or whatever
2675 you've set the C<$[> variable to--but don't do that). If the substring
2676 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2679 X<int> X<integer> X<truncate> X<trunc> X<floor>
2683 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2684 You should not use this function for rounding: one because it truncates
2685 towards C<0>, and two because machine representations of floating-point
2686 numbers can sometimes produce counterintuitive results. For example,
2687 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2688 because it's really more like -268.99999999999994315658 instead. Usually,
2689 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2690 functions will serve you better than will int().
2692 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2695 Implements the ioctl(2) function. You'll probably first have to say
2697 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2699 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2700 exist or doesn't have the correct definitions you'll have to roll your
2701 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2702 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2703 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2704 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2705 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2706 has no string value but does have a numeric value, that value will be
2707 passed rather than a pointer to the string value. To guarantee this to be
2708 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2709 functions may be needed to manipulate the values of structures used by
2712 The return value of C<ioctl> (and C<fcntl>) is as follows:
2714 if OS returns: then Perl returns:
2716 0 string "0 but true"
2717 anything else that number
2719 Thus Perl returns true on success and false on failure, yet you can
2720 still easily determine the actual value returned by the operating
2723 $retval = ioctl(...) || -1;
2724 printf "System returned %d\n", $retval;
2726 The special string C<"0 but true"> is exempt from B<-w> complaints
2727 about improper numeric conversions.
2729 Portability issues: L<perlport/ioctl>.
2731 =item join EXPR,LIST
2734 Joins the separate strings of LIST into a single string with fields
2735 separated by the value of EXPR, and returns that new string. Example:
2737 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2739 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2740 first argument. Compare L</split>.
2749 Returns a list consisting of all the keys of the named hash, or the indices
2750 of an array. (In scalar context, returns the number of keys or indices.)
2752 The keys of a hash are returned in an apparently random order. The actual
2753 random order is subject to change in future versions of Perl, but it
2754 is guaranteed to be the same order as either the C<values> or C<each>
2755 function produces (given that the hash has not been modified). Since
2756 Perl 5.8.1 the ordering can be different even between different runs of
2757 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2760 As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
2761 (see L</each>). In particular, calling keys() in void context resets
2762 the iterator with no other overhead.
2764 Here is yet another way to print your environment:
2767 @values = values %ENV;
2769 print pop(@keys), '=', pop(@values), "\n";
2772 or how about sorted by key:
2774 foreach $key (sort(keys %ENV)) {
2775 print $key, '=', $ENV{$key}, "\n";
2778 The returned values are copies of the original keys in the hash, so
2779 modifying them will not affect the original hash. Compare L</values>.
2781 To sort a hash by value, you'll need to use a C<sort> function.
2782 Here's a descending numeric sort of a hash by its values:
2784 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2785 printf "%4d %s\n", $hash{$key}, $key;
2788 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2789 allocated for the given hash. This can gain you a measure of efficiency if
2790 you know the hash is going to get big. (This is similar to pre-extending
2791 an array by assigning a larger number to $#array.) If you say
2795 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2796 in fact, since it rounds up to the next power of two. These
2797 buckets will be retained even if you do C<%hash = ()>, use C<undef
2798 %hash> if you want to free the storage while C<%hash> is still in scope.
2799 You can't shrink the number of buckets allocated for the hash using
2800 C<keys> in this way (but you needn't worry about doing this by accident,
2801 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2804 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
2805 a reference to an unblessed hash or array. The argument will be
2806 dereferenced automatically. This aspect of C<keys> is considered highly
2807 experimental. The exact behaviour may change in a future version of Perl.
2809 for (keys $hashref) { ... }
2810 for (keys $obj->get_arrayref) { ... }
2812 See also C<each>, C<values>, and C<sort>.
2814 =item kill SIGNAL, LIST
2817 Sends a signal to a list of processes. Returns the number of
2818 processes successfully signaled (which is not necessarily the
2819 same as the number actually killed).
2821 $cnt = kill 1, $child1, $child2;
2824 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2825 checks whether it's I<possible> to send a signal to it (that
2826 means, to be brief, that the process is owned by the same user, or we are
2827 the super-user). This is useful to check that a child process is still
2828 alive (even if only as a zombie) and hasn't changed its UID. See
2829 L<perlport> for notes on the portability of this construct.
2831 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2832 of processes. That means you usually want to use positive not negative signals.
2833 You may also use a signal name in quotes.
2835 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2836 the operating system. For example, on POSIX-conforming systems, zero will
2837 signal the current process group and -1 will signal all processes.
2839 See L<perlipc/"Signals"> for more details.
2841 On some platforms such as Windows where the fork() system call is not available.
2842 Perl can be built to emulate fork() at the interpreter level.
2843 This emulation has limitations related to kill that have to be considered,
2844 for code running on Windows and in code intended to be portable.
2846 See L<perlfork> for more details.
2848 Portability issues: L<perlport/kill>.
2855 The C<last> command is like the C<break> statement in C (as used in
2856 loops); it immediately exits the loop in question. If the LABEL is
2857 omitted, the command refers to the innermost enclosing loop. The
2858 C<continue> block, if any, is not executed:
2860 LINE: while (<STDIN>) {
2861 last LINE if /^$/; # exit when done with header
2865 C<last> cannot be used to exit a block that returns a value such as
2866 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
2867 a grep() or map() operation.
2869 Note that a block by itself is semantically identical to a loop
2870 that executes once. Thus C<last> can be used to effect an early
2871 exit out of such a block.
2873 See also L</continue> for an illustration of how C<last>, C<next>, and
2881 Returns a lowercased version of EXPR. This is the internal function
2882 implementing the C<\L> escape in double-quoted strings.
2884 If EXPR is omitted, uses C<$_>.
2886 What gets returned depends on several factors:
2890 =item If C<use bytes> is in effect:
2894 =item On EBCDIC platforms
2896 The results are what the C language system call C<tolower()> returns.
2898 =item On ASCII platforms
2900 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2905 =item Otherwise, If EXPR has the UTF8 flag set
2907 If the current package has a subroutine named C<ToLower>, it will be used to
2909 (See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
2910 Otherwise Unicode semantics are used for the case change.
2912 =item Otherwise, if C<use locale> is in effect
2914 Respects current LC_CTYPE locale. See L<perllocale>.
2916 =item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2918 Unicode semantics are used for the case change. Any subroutine named
2919 C<ToLower> will be ignored.
2925 =item On EBCDIC platforms
2927 The results are what the C language system call C<tolower()> returns.
2929 =item On ASCII platforms
2931 ASCII semantics are used for the case change. The lowercase of any character
2932 outside the ASCII range is the character itself.
2939 X<lcfirst> X<lowercase>
2943 Returns the value of EXPR with the first character lowercased. This
2944 is the internal function implementing the C<\l> escape in
2945 double-quoted strings.
2947 If EXPR is omitted, uses C<$_>.
2949 This function behaves the same way under various pragmata, such as in a locale,
2957 Returns the length in I<characters> of the value of EXPR. If EXPR is
2958 omitted, returns the length of C<$_>. If EXPR is undefined, returns
2961 This function cannot be used on an entire array or hash to find out how
2962 many elements these have. For that, use C<scalar @array> and C<scalar keys
2963 %hash>, respectively.
2965 Like all Perl character operations, length() normally deals in logical
2966 characters, not physical bytes. For how many bytes a string encoded as
2967 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2968 to C<use Encode> first). See L<Encode> and L<perlunicode>.
2970 =item link OLDFILE,NEWFILE
2973 Creates a new filename linked to the old filename. Returns true for
2974 success, false otherwise.
2976 Portability issues: L<perlport/link>.
2978 =item listen SOCKET,QUEUESIZE
2981 Does the same thing that the listen(2) system call does. Returns true if
2982 it succeeded, false otherwise. See the example in
2983 L<perlipc/"Sockets: Client/Server Communication">.
2988 You really probably want to be using C<my> instead, because C<local> isn't
2989 what most people think of as "local". See
2990 L<perlsub/"Private Variables via my()"> for details.
2992 A local modifies the listed variables to be local to the enclosing
2993 block, file, or eval. If more than one value is listed, the list must
2994 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2995 for details, including issues with tied arrays and hashes.
2997 The C<delete local EXPR> construct can also be used to localize the deletion
2998 of array/hash elements to the current block.
2999 See L<perlsub/"Localized deletion of elements of composite types">.
3001 =item localtime EXPR
3002 X<localtime> X<ctime>
3006 Converts a time as returned by the time function to a 9-element list
3007 with the time analyzed for the local time zone. Typically used as
3011 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3014 All list elements are numeric and come straight out of the C `struct
3015 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3016 of the specified time.
3018 C<$mday> is the day of the month and C<$mon> the month in
3019 the range C<0..11>, with 0 indicating January and 11 indicating December.
3020 This makes it easy to get a month name from a list:
3022 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
3023 print "$abbr[$mon] $mday";
3024 # $mon=9, $mday=18 gives "Oct 18"
3026 C<$year> is the number of years since 1900, B<not> just the last two digits
3027 of the year. That is, C<$year> is C<123> in year 2023. The proper way
3028 to get a 4-digit year is simply:
3032 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
3033 to do that, would you?
3035 To get the last two digits of the year (e.g., "01" in 2001) do:
3037 $year = sprintf("%02d", $year % 100);
3039 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3040 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3041 (or C<0..365> in leap years.)
3043 C<$isdst> is true if the specified time occurs during Daylight Saving
3044 Time, false otherwise.
3046 If EXPR is omitted, C<localtime()> uses the current time (as returned
3049 In scalar context, C<localtime()> returns the ctime(3) value:
3051 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3053 This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
3054 instead of local time use the L</gmtime> builtin. See also the
3055 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3056 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3057 and mktime(3) functions.
3059 To get somewhat similar but locale-dependent date strings, set up your
3060 locale environment variables appropriately (please see L<perllocale>) and
3063 use POSIX qw(strftime);
3064 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3065 # or for GMT formatted appropriately for your locale:
3066 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3068 Note that the C<%a> and C<%b>, the short forms of the day of the week
3069 and the month of the year, may not necessarily be three characters wide.
3071 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3072 by-name access mechanism to the gmtime() and localtime() functions,
3075 For a comprehensive date and time representation look at the
3076 L<DateTime> module on CPAN.
3078 Portability issues: L<perlport/localtime>.
3083 This function places an advisory lock on a shared variable or referenced
3084 object contained in I<THING> until the lock goes out of scope.
3086 The value returned is the scalar itself, if the argument is a scalar, or a
3087 reference, if the argument is a hash or array.
3089 lock() is a "weak keyword" : this means that if you've defined a function
3090 by this name (before any calls to it), that function will be called
3091 instead. If you are not under C<use threads::shared> this does nothing.
3092 See L<threads::shared>.
3095 X<log> X<logarithm> X<e> X<ln> X<base>
3099 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3100 returns the log of C<$_>. To get the
3101 log of another base, use basic algebra:
3102 The base-N log of a number is equal to the natural log of that number
3103 divided by the natural log of N. For example:
3107 return log($n)/log(10);
3110 See also L</exp> for the inverse operation.
3117 Does the same thing as the C<stat> function (including setting the
3118 special C<_> filehandle) but stats a symbolic link instead of the file
3119 the symbolic link points to. If symbolic links are unimplemented on
3120 your system, a normal C<stat> is done. For much more detailed
3121 information, please see the documentation for C<stat>.
3123 If EXPR is omitted, stats C<$_>.
3125 Portability issues: L<perlport/lstat>.
3129 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3131 =item map BLOCK LIST
3136 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3137 C<$_> to each element) and returns the list value composed of the
3138 results of each such evaluation. In scalar context, returns the
3139 total number of elements so generated. Evaluates BLOCK or EXPR in
3140 list context, so each element of LIST may produce zero, one, or
3141 more elements in the returned value.
3143 @chars = map(chr, @numbers);
3145 translates a list of numbers to the corresponding characters.
3147 my @squares = map { $_ * $_ } @numbers;
3149 translates a list of numbers to their squared values.
3151 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3153 shows that number of returned elements can differ from the number of
3154 input elements. To omit an element, return an empty list ().
3155 This could also be achieved by writing
3157 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3159 which makes the intention more clear.
3161 Map always returns a list, which can be
3162 assigned to a hash such that the elements
3163 become key/value pairs. See L<perldata> for more details.
3165 %hash = map { get_a_key_for($_) => $_ } @array;
3167 is just a funny way to write
3171 $hash{get_a_key_for($_)} = $_;
3174 Note that C<$_> is an alias to the list value, so it can be used to
3175 modify the elements of the LIST. While this is useful and supported,
3176 it can cause bizarre results if the elements of LIST are not variables.
3177 Using a regular C<foreach> loop for this purpose would be clearer in
3178 most cases. See also L</grep> for an array composed of those items of
3179 the original list for which the BLOCK or EXPR evaluates to true.
3181 If C<$_> is lexical in the scope where the C<map> appears (because it has
3182 been declared with C<my $_>), then, in addition to being locally aliased to
3183 the list elements, C<$_> keeps being lexical inside the block; that is, it
3184 can't be seen from the outside, avoiding any potential side-effects.
3186 C<{> starts both hash references and blocks, so C<map { ...> could be either
3187 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3188 ahead for the closing C<}> it has to take a guess at which it's dealing with
3189 based on what it finds just after the C<{>. Usually it gets it right, but if it
3190 doesn't it won't realize something is wrong until it gets to the C<}> and
3191 encounters the missing (or unexpected) comma. The syntax error will be
3192 reported close to the C<}>, but you'll need to change something near the C<{>
3193 such as using a unary C<+> to give Perl some help:
3195 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3196 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3197 %hash = map { ("\L$_" => 1) } @array # this also works
3198 %hash = map { lc($_) => 1 } @array # as does this.
3199 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3201 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3203 or to force an anon hash constructor use C<+{>:
3205 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
3207 to get a list of anonymous hashes each with only one entry apiece.
3209 =item mkdir FILENAME,MASK
3210 X<mkdir> X<md> X<directory, create>
3212 =item mkdir FILENAME
3216 Creates the directory specified by FILENAME, with permissions
3217 specified by MASK (as modified by C<umask>). If it succeeds it
3218 returns true; otherwise it returns false and sets C<$!> (errno).
3219 MASK defaults to 0777 if omitted, and FILENAME defaults
3220 to C<$_> if omitted.
3222 In general, it is better to create directories with a permissive MASK
3223 and let the user modify that with their C<umask> than it is to supply
3224 a restrictive MASK and give the user no way to be more permissive.
3225 The exceptions to this rule are when the file or directory should be
3226 kept private (mail files, for instance). The perlfunc(1) entry on
3227 C<umask> discusses the choice of MASK in more detail.
3229 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3230 number of trailing slashes. Some operating and filesystems do not get
3231 this right, so Perl automatically removes all trailing slashes to keep
3234 To recursively create a directory structure, look at
3235 the C<mkpath> function of the L<File::Path> module.
3237 =item msgctl ID,CMD,ARG
3240 Calls the System V IPC function msgctl(2). You'll probably have to say
3244 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3245 then ARG must be a variable that will hold the returned C<msqid_ds>
3246 structure. Returns like C<ioctl>: the undefined value for error,
3247 C<"0 but true"> for zero, or the actual return value otherwise. See also
3248 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3251 Portability issues: L<perlport/msgctl>.
3253 =item msgget KEY,FLAGS
3256 Calls the System V IPC function msgget(2). Returns the message queue
3257 id, or C<undef> on error. See also
3258 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3261 Portability issues: L<perlport/msgget>.
3263 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3266 Calls the System V IPC function msgrcv to receive a message from
3267 message queue ID into variable VAR with a maximum message size of
3268 SIZE. Note that when a message is received, the message type as a
3269 native long integer will be the first thing in VAR, followed by the
3270 actual message. This packing may be opened with C<unpack("l! a*")>.
3271 Taints the variable. Returns true if successful, false
3272 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3273 C<IPC::SysV> and C<IPC::SysV::Msg>.
3275 Portability issues: L<perlport/msgrcv>.
3277 =item msgsnd ID,MSG,FLAGS
3280 Calls the System V IPC function msgsnd to send the message MSG to the
3281 message queue ID. MSG must begin with the native long integer message
3282 type, be followed by the length of the actual message, and then finally
3283 the message itself. This kind of packing can be achieved with
3284 C<pack("l! a*", $type, $message)>. Returns true if successful,
3285 false on error. See also the C<IPC::SysV>
3286 and C<IPC::SysV::Msg> documentation.
3288 Portability issues: L<perlport/msgsnd>.
3295 =item my EXPR : ATTRS
3297 =item my TYPE EXPR : ATTRS
3299 A C<my> declares the listed variables to be local (lexically) to the
3300 enclosing block, file, or C<eval>. If more than one value is listed,
3301 the list must be placed in parentheses.
3303 The exact semantics and interface of TYPE and ATTRS are still
3304 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3305 and attributes are handled using the C<attributes> pragma, or starting
3306 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3307 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3308 L<attributes>, and L<Attribute::Handlers>.
3315 The C<next> command is like the C<continue> statement in C; it starts
3316 the next iteration of the loop:
3318 LINE: while (<STDIN>) {
3319 next LINE if /^#/; # discard comments
3323 Note that if there were a C<continue> block on the above, it would get
3324 executed even on discarded lines. If LABEL is omitted, the command
3325 refers to the innermost enclosing loop.
3327 C<next> cannot be used to exit a block which returns a value such as
3328 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3329 a grep() or map() operation.
3331 Note that a block by itself is semantically identical to a loop
3332 that executes once. Thus C<next> will exit such a block early.
3334 See also L</continue> for an illustration of how C<last>, C<next>, and
3337 =item no MODULE VERSION LIST
3341 =item no MODULE VERSION
3343 =item no MODULE LIST
3349 See the C<use> function, of which C<no> is the opposite.
3352 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3356 Interprets EXPR as an octal string and returns the corresponding
3357 value. (If EXPR happens to start off with C<0x>, interprets it as a
3358 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3359 binary string. Leading whitespace is ignored in all three cases.)
3360 The following will handle decimal, binary, octal, and hex in standard
3363 $val = oct($val) if $val =~ /^0/;
3365 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3366 in octal), use sprintf() or printf():
3368 $dec_perms = (stat("filename"))[2] & 07777;
3369 $oct_perm_str = sprintf "%o", $perms;
3371 The oct() function is commonly used when a string such as C<644> needs
3372 to be converted into a file mode, for example. Although Perl
3373 automatically converts strings into numbers as needed, this automatic
3374 conversion assumes base 10.
3376 Leading white space is ignored without warning, as too are any trailing
3377 non-digits, such as a decimal point (C<oct> only handles non-negative
3378 integers, not negative integers or floating point).
3380 =item open FILEHANDLE,EXPR
3381 X<open> X<pipe> X<file, open> X<fopen>
3383 =item open FILEHANDLE,MODE,EXPR
3385 =item open FILEHANDLE,MODE,EXPR,LIST
3387 =item open FILEHANDLE,MODE,REFERENCE
3389 =item open FILEHANDLE
3391 Opens the file whose filename is given by EXPR, and associates it with
3394 Simple examples to open a file for reading:
3396 open(my $fh, "<", "input.txt")
3397 or die "cannot open < input.txt: $!";
3401 open(my $fh, ">", "output.txt")
3402 or die "cannot open > output.txt: $!";
3404 (The following is a comprehensive reference to open(): for a gentler
3405 introduction you may consider L<perlopentut>.)
3407 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3408 new filehandle is autovivified, meaning that the variable is assigned a
3409 reference to a newly allocated anonymous filehandle. Otherwise if
3410 FILEHANDLE is an expression, its value is the real filehandle. (This is
3411 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3414 If EXPR is omitted, the global (package) scalar variable of the same
3415 name as the FILEHANDLE contains the filename. (Note that lexical
3416 variables--those declared with C<my> or C<state>--will not work for this
3417 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3420 If three (or more) arguments are specified, the open mode (including
3421 optional encoding) in the second argument are distinct from the filename in
3422 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3423 If MODE is C<< > >>, the file is opened for output, with existing files
3424 first being truncated ("clobbered") and nonexisting files newly created.
3425 If MODE is C<<< >> >>>, the file is opened for appending, again being
3426 created if necessary.
3428 You can put a C<+> in front of the C<< > >> or C<< < >> to
3429 indicate that you want both read and write access to the file; thus
3430 C<< +< >> is almost always preferred for read/write updates--the
3431 C<< +> >> mode would clobber the file first. You cant usually use
3432 either read-write mode for updating textfiles, since they have
3433 variable-length records. See the B<-i> switch in L<perlrun> for a
3434 better approach. The file is created with permissions of C<0666>
3435 modified by the process's C<umask> value.
3437 These various prefixes correspond to the fopen(3) modes of C<r>,
3438 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3440 In the one- and two-argument forms of the call, the mode and filename
3441 should be concatenated (in that order), preferably separated by white
3442 space. You can--but shouldn't--omit the mode in these forms when that mode
3443 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3444 the filename argument is a known literal.
3446 For three or more arguments if MODE is C<|->, the filename is
3447 interpreted as a command to which output is to be piped, and if MODE
3448 is C<-|>, the filename is interpreted as a command that pipes
3449 output to us. In the two-argument (and one-argument) form, one should
3450 replace dash (C<->) with the command.
3451 See L<perlipc/"Using open() for IPC"> for more examples of this.
3452 (You are not allowed to C<open> to a command that pipes both in I<and>
3453 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3454 L<perlipc/"Bidirectional Communication with Another Process"> for
3457 In the form of pipe opens taking three or more arguments, if LIST is specified
3458 (extra arguments after the command name) then LIST becomes arguments
3459 to the command invoked if the platform supports it. The meaning of
3460 C<open> with more than three arguments for non-pipe modes is not yet
3461 defined, but experimental "layers" may give extra LIST arguments
3464 In the two-argument (and one-argument) form, opening C<< <- >>
3465 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3467 You may (and usually should) use the three-argument form of open to specify
3468 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3469 that affect how the input and output are processed (see L<open> and
3470 L<PerlIO> for more details). For example:
3472 open(my $fh, "<:encoding(UTF-8)", "filename")
3473 || die "can't open UTF-8 encoded filename: $!";
3475 opens the UTF8-encoded file containing Unicode characters;
3476 see L<perluniintro>. Note that if layers are specified in the
3477 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3478 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3480 Open returns nonzero on success, the undefined value otherwise. If
3481 the C<open> involved a pipe, the return value happens to be the pid of
3484 If you're running Perl on a system that distinguishes between text
3485 files and binary files, then you should check out L</binmode> for tips
3486 for dealing with this. The key distinction between systems that need
3487 C<binmode> and those that don't is their text file formats. Systems
3488 like Unix, Mac OS, and Plan 9, that end lines with a single
3489 character and encode that character in C as C<"\n"> do not
3490 need C<binmode>. The rest need it.
3492 When opening a file, it's seldom a good idea to continue
3493 if the request failed, so C<open> is frequently used with
3494 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3495 where you want to format a suitable error message (but there are
3496 modules that can help with that problem)) always check
3497 the return value from opening a file.
3499 As a special case the three-argument form with a read/write mode and the third
3500 argument being C<undef>:
3502 open(my $tmp, "+>", undef) or die ...
3504 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3505 works for symmetry, but you really should consider writing something
3506 to the temporary file first. You will need to seek() to do the
3509 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3510 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3511 open filehandles directly to Perl scalars via:
3513 open($fh, ">", \$variable) || ..
3515 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3518 open(STDOUT, ">", \$variable)
3519 or die "Can't open STDOUT: $!";
3524 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3525 while (<ARTICLE>) {...
3527 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3528 # if the open fails, output is discarded
3530 open(my $dbase, "+<", "dbase.mine") # open for update
3531 or die "Can't open 'dbase.mine' for update: $!";
3533 open(my $dbase, "+<dbase.mine") # ditto
3534 or die "Can't open 'dbase.mine' for update: $!";
3536 open(ARTICLE, "-|", "caesar <$article") # decrypt article
3537 or die "Can't start caesar: $!";
3539 open(ARTICLE, "caesar <$article |") # ditto
3540 or die "Can't start caesar: $!";
3542 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3543 or die "Can't start sort: $!";
3546 open(MEMORY, ">", \$var)
3547 or die "Can't open memory file: $!";
3548 print MEMORY "foo!\n"; # output will appear in $var
3550 # process argument list of files along with any includes
3552 foreach $file (@ARGV) {
3553 process($file, "fh00");
3557 my($filename, $input) = @_;
3558 $input++; # this is a string increment
3559 unless (open($input, "<", $filename)) {
3560 print STDERR "Can't open $filename: $!\n";
3565 while (<$input>) { # note use of indirection
3566 if (/^#include "(.*)"/) {
3567 process($1, $input);
3574 See L<perliol> for detailed info on PerlIO.
3576 You may also, in the Bourne shell tradition, specify an EXPR beginning
3577 with C<< >& >>, in which case the rest of the string is interpreted
3578 as the name of a filehandle (or file descriptor, if numeric) to be
3579 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3580 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3581 The mode you specify should match the mode of the original filehandle.
3582 (Duping a filehandle does not take into account any existing contents
3583 of IO buffers.) If you use the three-argument form, then you can pass either a
3584 number, the name of a filehandle, or the normal "reference to a glob".
3586 Here is a script that saves, redirects, and restores C<STDOUT> and
3587 C<STDERR> using various methods:
3590 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
3591 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
3593 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
3594 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
3596 select STDERR; $| = 1; # make unbuffered
3597 select STDOUT; $| = 1; # make unbuffered
3599 print STDOUT "stdout 1\n"; # this works for
3600 print STDERR "stderr 1\n"; # subprocesses too
3602 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
3603 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
3605 print STDOUT "stdout 2\n";
3606 print STDERR "stderr 2\n";
3608 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3609 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3610 that file descriptor (and not call C<dup(2)>); this is more
3611 parsimonious of file descriptors. For example:
3613 # open for input, reusing the fileno of $fd
3614 open(FILEHANDLE, "<&=$fd")
3618 open(FILEHANDLE, "<&=", $fd)
3622 # open for append, using the fileno of OLDFH
3623 open(FH, ">>&=", OLDFH)
3627 open(FH, ">>&=OLDFH")
3629 Being parsimonious on filehandles is also useful (besides being
3630 parsimonious) for example when something is dependent on file
3631 descriptors, like for example locking using flock(). If you do just
3632 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
3633 descriptor as B, and therefore flock(A) will not flock(B) nor vice
3634 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
3635 the same underlying system file descriptor.
3637 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
3638 fdopen() to implement the C<=> functionality. On many Unix systems,
3639 fdopen() fails when file descriptors exceed a certain value, typically 255.
3640 For Perls 5.8.0 and later, PerlIO is (most often) the default.
3642 You can see whether your Perl was built with PerlIO by running C<perl -V>
3643 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
3644 have PerlIO; otherwise you don't.
3646 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
3647 with the one- or two-argument forms of C<open>),
3648 an implicit C<fork> is done, so C<open> returns twice: in the parent
3649 process it returns the pid
3650 of the child process, and in the child process it returns (a defined) C<0>.
3651 Use C<defined($pid)> or C<//> to determine whether the open was successful.
3653 For example, use either
3655 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
3658 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
3664 # either write TO_KID or else read FROM_KID
3668 # am the child; use STDIN/STDOUT normally
3673 The filehandle behaves normally for the parent, but I/O to that
3674 filehandle is piped from/to the STDOUT/STDIN of the child process.
3675 In the child process, the filehandle isn't opened--I/O happens from/to
3676 the new STDOUT/STDIN. Typically this is used like the normal
3677 piped open when you want to exercise more control over just how the
3678 pipe command gets executed, such as when running setuid and
3679 you don't want to have to scan shell commands for metacharacters.
3681 The following blocks are more or less equivalent:
3683 open(FOO, "|tr '[a-z]' '[A-Z]'");
3684 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
3685 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
3686 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
3688 open(FOO, "cat -n '$file'|");
3689 open(FOO, "-|", "cat -n '$file'");
3690 open(FOO, "-|") || exec "cat", "-n", $file;
3691 open(FOO, "-|", "cat", "-n", $file);
3693 The last two examples in each block show the pipe as "list form", which is
3694 not yet supported on all platforms. A good rule of thumb is that if
3695 your platform has a real C<fork()> (in other words, if your platform is
3696 Unix, including Linux and MacOS X), you can use the list form. You would
3697 want to use the list form of the pipe so you can pass literal arguments
3698 to the command without risk of the shell interpreting any shell metacharacters
3699 in them. However, this also bars you from opening pipes to commands
3700 that intentionally contain shell metacharacters, such as:
3702 open(FOO, "|cat -n | expand -4 | lpr")
3703 // die "Can't open pipeline to lpr: $!";
3705 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3707 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3708 output before any operation that may do a fork, but this may not be
3709 supported on some platforms (see L<perlport>). To be safe, you may need
3710 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3711 of C<IO::Handle> on any open handles.
3713 On systems that support a close-on-exec flag on files, the flag will
3714 be set for the newly opened file descriptor as determined by the value
3715 of C<$^F>. See L<perlvar/$^F>.
3717 Closing any piped filehandle causes the parent process to wait for the
3718 child to finish, then returns the status value in C<$?> and
3719 C<${^CHILD_ERROR_NATIVE}>.
3721 The filename passed to the one- and two-argument forms of open() will
3722 have leading and trailing whitespace deleted and normal
3723 redirection characters honored. This property, known as "magic open",
3724 can often be used to good effect. A user could specify a filename of
3725 F<"rsh cat file |">, or you could change certain filenames as needed:
3727 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3728 open(FH, $filename) or die "Can't open $filename: $!";
3730 Use the three-argument form to open a file with arbitrary weird characters in it,
3732 open(FOO, "<", $file)
3733 || die "can't open < $file: $!";
3735 otherwise it's necessary to protect any leading and trailing whitespace:
3737 $file =~ s#^(\s)#./$1#;
3738 open(FOO, "< $file\0")
3739 || die "open failed: $!";
3741 (this may not work on some bizarre filesystems). One should
3742 conscientiously choose between the I<magic> and I<three-argument> form
3745 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
3747 will allow the user to specify an argument of the form C<"rsh cat file |">,
3748 but will not work on a filename that happens to have a trailing space, while
3750 open(IN, "<", $ARGV[0])
3751 || die "can't open < $ARGV[0]: $!";
3753 will have exactly the opposite restrictions.
3755 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3756 should use the C<sysopen> function, which involves no such magic (but may
3757 use subtly different filemodes than Perl open(), which is mapped to C
3758 fopen()). This is another way to protect your filenames from
3759 interpretation. For example:
3762 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3763 or die "sysopen $path: $!";
3764 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3765 print HANDLE "stuff $$\n";
3767 print "File contains: ", <HANDLE>;
3769 Using the constructor from the C<IO::Handle> package (or one of its
3770 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3771 filehandles that have the scope of the variables used to hold them, then
3772 automatically (but silently) close once their reference counts become
3773 zero, typically at scope exit:
3777 sub read_myfile_munged {
3779 # or just leave it undef to autoviv
3780 my $handle = IO::File->new;
3781 open($handle, "<", "myfile") or die "myfile: $!";
3783 or return (); # Automatically closed here.
3784 mung($first) or die "mung failed"; # Or here.
3785 return (first, <$handle>) if $ALL; # Or here.
3786 return $first; # Or here.
3789 B<WARNING:> The previous example has a bug because the automatic
3790 close that happens when the refcount on C<handle> does not
3791 properly detect and report failures. I<Always> close the handle
3792 yourself and inspect the return value.
3795 || warn "close failed: $!";
3797 See L</seek> for some details about mixing reading and writing.
3799 Portability issues: L<perlport/open>.
3801 =item opendir DIRHANDLE,EXPR
3804 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3805 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3806 DIRHANDLE may be an expression whose value can be used as an indirect
3807 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3808 scalar variable (or array or hash element), the variable is assigned a
3809 reference to a new anonymous dirhandle; that is, it's autovivified.
3810 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3812 See the example at C<readdir>.
3819 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3820 or Unicode) value of the first character of EXPR.
3821 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
3822 (Note I<character>, not byte.)
3824 For the reverse, see L</chr>.
3825 See L<perlunicode> for more about Unicode.
3832 =item our EXPR : ATTRS
3834 =item our TYPE EXPR : ATTRS
3836 C<our> associates a simple name with a package variable in the current
3837 package for use within the current scope. When C<use strict 'vars'> is in
3838 effect, C<our> lets you use declared global variables without qualifying
3839 them with package names, within the lexical scope of the C<our> declaration.
3840 In this way C<our> differs from C<use vars>, which is package-scoped.
3842 Unlike C<my> or C<state>, which allocates storage for a variable and
3843 associates a simple name with that storage for use within the current
3844 scope, C<our> associates a simple name with a package (read: global)
3845 variable in the current package, for use within the current lexical scope.
3846 In other words, C<our> has the same scoping rules as C<my> or C<state>, but
3847 does not necessarily create a variable.
3849 If more than one value is listed, the list must be placed
3855 An C<our> declaration declares a global variable that will be visible
3856 across its entire lexical scope, even across package boundaries. The
3857 package in which the variable is entered is determined at the point
3858 of the declaration, not at the point of use. This means the following
3862 our $bar; # declares $Foo::bar for rest of lexical scope
3866 print $bar; # prints 20, as it refers to $Foo::bar
3868 Multiple C<our> declarations with the same name in the same lexical
3869 scope are allowed if they are in different packages. If they happen
3870 to be in the same package, Perl will emit warnings if you have asked
3871 for them, just like multiple C<my> declarations. Unlike a second
3872 C<my> declaration, which will bind the name to a fresh variable, a
3873 second C<our> declaration in the same package, in the same scope, is
3878 our $bar; # declares $Foo::bar for rest of lexical scope
3882 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3883 print $bar; # prints 30
3885 our $bar; # emits warning but has no other effect
3886 print $bar; # still prints 30
3888 An C<our> declaration may also have a list of attributes associated
3891 The exact semantics and interface of TYPE and ATTRS are still
3892 evolving. TYPE is currently bound to the use of C<fields> pragma,
3893 and attributes are handled using the C<attributes> pragma, or starting
3894 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3895 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3896 L<attributes>, and L<Attribute::Handlers>.
3898 =item pack TEMPLATE,LIST
3901 Takes a LIST of values and converts it into a string using the rules
3902 given by the TEMPLATE. The resulting string is the concatenation of
3903 the converted values. Typically, each converted value looks
3904 like its machine-level representation. For example, on 32-bit machines
3905 an integer may be represented by a sequence of 4 bytes, which will in
3906 Perl be presented as a string that's 4 characters long.
3908 See L<perlpacktut> for an introduction to this function.
3910 The TEMPLATE is a sequence of characters that give the order and type
3911 of values, as follows:
3913 a A string with arbitrary binary data, will be null padded.
3914 A A text (ASCII) string, will be space padded.
3915 Z A null-terminated (ASCIZ) string, will be null padded.
3917 b A bit string (ascending bit order inside each byte,
3919 B A bit string (descending bit order inside each byte).
3920 h A hex string (low nybble first).
3921 H A hex string (high nybble first).
3923 c A signed char (8-bit) value.
3924 C An unsigned char (octet) value.
3925 W An unsigned char value (can be greater than 255).
3927 s A signed short (16-bit) value.
3928 S An unsigned short value.
3930 l A signed long (32-bit) value.
3931 L An unsigned long value.
3933 q A signed quad (64-bit) value.
3934 Q An unsigned quad value.
3935 (Quads are available only if your system supports 64-bit
3936 integer values _and_ if Perl has been compiled to support
3937 those. Raises an exception otherwise.)
3939 i A signed integer value.
3940 I A unsigned integer value.
3941 (This 'integer' is _at_least_ 32 bits wide. Its exact
3942 size depends on what a local C compiler calls 'int'.)
3944 n An unsigned short (16-bit) in "network" (big-endian) order.
3945 N An unsigned long (32-bit) in "network" (big-endian) order.
3946 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3947 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3949 j A Perl internal signed integer value (IV).
3950 J A Perl internal unsigned integer value (UV).
3952 f A single-precision float in native format.
3953 d A double-precision float in native format.
3955 F A Perl internal floating-point value (NV) in native format
3956 D A float of long-double precision in native format.
3957 (Long doubles are available only if your system supports
3958 long double values _and_ if Perl has been compiled to
3959 support those. Raises an exception otherwise.)
3961 p A pointer to a null-terminated string.
3962 P A pointer to a structure (fixed-length string).
3964 u A uuencoded string.
3965 U A Unicode character number. Encodes to a character in char-
3966 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
3969 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
3970 for details). Its bytes represent an unsigned integer in
3971 base 128, most significant digit first, with as few digits
3972 as possible. Bit eight (the high bit) is set on each byte
3975 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
3977 @ Null-fill or truncate to absolute position, counted from the
3978 start of the innermost ()-group.
3979 . Null-fill or truncate to absolute position specified by
3981 ( Start of a ()-group.
3983 One or more modifiers below may optionally follow certain letters in the
3984 TEMPLATE (the second column lists letters for which the modifier is valid):
3986 ! sSlLiI Forces native (short, long, int) sizes instead
3987 of fixed (16-/32-bit) sizes.
3989 xX Make x and X act as alignment commands.
3991 nNvV Treat integers as signed instead of unsigned.
3993 @. Specify position as byte offset in the internal
3994 representation of the packed string. Efficient but
3997 > sSiIlLqQ Force big-endian byte-order on the type.
3998 jJfFdDpP (The "big end" touches the construct.)
4000 < sSiIlLqQ Force little-endian byte-order on the type.
4001 jJfFdDpP (The "little end" touches the construct.)
4003 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4004 to force a particular byte-order on all components in that group,
4005 including all its subgroups.
4007 The following rules apply:
4013 Each letter may optionally be followed by a number indicating the repeat
4014 count. A numeric repeat count may optionally be enclosed in brackets, as
4015 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4016 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4017 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4018 something else, described below. Supplying a C<*> for the repeat count
4019 instead of a number means to use however many items are left, except for:
4025 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4029 <.>, where it means relative to the start of the string.
4033 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4037 One can replace a numeric repeat count with a template letter enclosed in
4038 brackets to use the packed byte length of the bracketed template for the
4041 For example, the template C<x[L]> skips as many bytes as in a packed long,
4042 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4043 variable-expanded) unpacks. If the template in brackets contains alignment
4044 commands (such as C<x![d]>), its packed length is calculated as if the
4045 start of the template had the maximal possible alignment.
4047 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4048 trailing null byte, so the resulting string is always one byte longer than
4049 the byte length of the item itself.
4051 When used with C<@>, the repeat count represents an offset from the start
4052 of the innermost C<()> group.
4054 When used with C<.>, the repeat count determines the starting position to
4055 calculate the value offset as follows:
4061 If the repeat count is C<0>, it's relative to the current position.
4065 If the repeat count is C<*>, the offset is relative to the start of the
4070 And if it's an integer I<n>, the offset is relative to the start of the
4071 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4072 bigger then the group level.
4076 The repeat count for C<u> is interpreted as the maximal number of bytes
4077 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4078 count should not be more than 65.
4082 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4083 string of length count, padding with nulls or spaces as needed. When
4084 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4085 after the first null, and C<a> returns data with no stripping at all.
4087 If the value to pack is too long, the result is truncated. If it's too
4088 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4089 followed by a null byte. Thus C<Z> always packs a trailing null, except
4090 when the count is 0.
4094 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4095 Each such format generates 1 bit of the result. These are typically followed
4096 by a repeat count like C<B8> or C<B64>.
4098 Each result bit is based on the least-significant bit of the corresponding
4099 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4100 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4102 Starting from the beginning of the input string, each 8-tuple
4103 of characters is converted to 1 character of output. With format C<b>,
4104 the first character of the 8-tuple determines the least-significant bit of a
4105 character; with format C<B>, it determines the most-significant bit of
4108 If the length of the input string is not evenly divisible by 8, the
4109 remainder is packed as if the input string were padded by null characters
4110 at the end. Similarly during unpacking, "extra" bits are ignored.
4112 If the input string is longer than needed, remaining characters are ignored.
4114 A C<*> for the repeat count uses all characters of the input field.
4115 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4119 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4120 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4122 For each such format, pack() generates 4 bits of result.
4123 With non-alphabetical characters, the result is based on the 4 least-significant
4124 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4125 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4126 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4127 is compatible with the usual hexadecimal digits, so that C<"a"> and
4128 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4129 characters with this format.
4131 Starting from the beginning of the template to pack(), each pair
4132 of characters is converted to 1 character of output. With format C<h>, the
4133 first character of the pair determines the least-significant nybble of the
4134 output character; with format C<H>, it determines the most-significant
4137 If the length of the input string is not even, it behaves as if padded by
4138 a null character at the end. Similarly, "extra" nybbles are ignored during
4141 If the input string is longer than needed, extra characters are ignored.
4143 A C<*> for the repeat count uses all characters of the input field. For
4144 unpack(), nybbles are converted to a string of hexadecimal digits.
4148 The C<p> format packs a pointer to a null-terminated string. You are
4149 responsible for ensuring that the string is not a temporary value, as that
4150 could potentially get deallocated before you got around to using the packed
4151 result. The C<P> format packs a pointer to a structure of the size indicated
4152 by the length. A null pointer is created if the corresponding value for
4153 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4154 unpacks into C<undef>.
4156 If your system has a strange pointer size--meaning a pointer is neither as
4157 big as an int nor as big as a long--it may not be possible to pack or
4158 unpack pointers in big- or little-endian byte order. Attempting to do
4159 so raises an exception.
4163 The C</> template character allows packing and unpacking of a sequence of
4164 items where the packed structure contains a packed item count followed by
4165 the packed items themselves. This is useful when the structure you're
4166 unpacking has encoded the sizes or repeat counts for some of its fields
4167 within the structure itself as separate fields.
4169 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4170 I<length-item> describes how the length value is packed. Formats likely
4171 to be of most use are integer-packing ones like C<n> for Java strings,
4172 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4174 For C<pack>, I<sequence-item> may have a repeat count, in which case
4175 the minimum of that and the number of available items is used as the argument
4176 for I<length-item>. If it has no repeat count or uses a '*', the number
4177 of available items is used.
4179 For C<unpack>, an internal stack of integer arguments unpacked so far is
4180 used. You write C</>I<sequence-item> and the repeat count is obtained by
4181 popping off the last element from the stack. The I<sequence-item> must not
4182 have a repeat count.
4184 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4185 the I<length-item> is the string length, not the number of strings. With
4186 an explicit repeat count for pack, the packed string is adjusted to that
4187 length. For example:
4189 This code: gives this result:
4191 unpack("W/a", "\004Gurusamy") ("Guru")
4192 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4193 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4195 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4196 pack("a/W2", ord("a") .. ord("z")) "2ab"
4198 The I<length-item> is not returned explicitly from C<unpack>.
4200 Supplying a count to the I<length-item> format letter is only useful with
4201 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4202 introduce C<"\000"> characters, which Perl does not regard as legal in
4207 The integer types C<s>, C<S>, C<l>, and C<L> may be
4208 followed by a C<!> modifier to specify native shorts or
4209 longs. As shown in the example above, a bare C<l> means
4210 exactly 32 bits, although the native C<long> as seen by the local C compiler
4211 may be larger. This is mainly an issue on 64-bit platforms. You can
4212 see whether using C<!> makes any difference this way:
4214 printf "format s is %d, s! is %d\n",
4215 length pack("s"), length pack("s!");
4217 printf "format l is %d, l! is %d\n",
4218 length pack("l"), length pack("l!");
4221 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4222 they are identical to C<i> and C<I>.
4224 The actual sizes (in bytes) of native shorts, ints, longs, and long
4225 longs on the platform where Perl was built are also available from
4228 $ perl -V:{short,int,long{,long}}size
4234 or programmatically via the C<Config> module:
4237 print $Config{shortsize}, "\n";
4238 print $Config{intsize}, "\n";
4239 print $Config{longsize}, "\n";
4240 print $Config{longlongsize}, "\n";
4242 C<$Config{longlongsize}> is undefined on systems without
4247 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4248 inherently non-portable between processors and operating systems because
4249 they obey native byteorder and endianness. For example, a 4-byte integer
4250 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4251 handled by the CPU registers) into bytes as
4253 0x12 0x34 0x56 0x78 # big-endian
4254 0x78 0x56 0x34 0x12 # little-endian
4256 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4257 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4258 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4259 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4261 The names I<big-endian> and I<little-endian> are comic references to the
4262 egg-eating habits of the little-endian Lilliputians and the big-endian
4263 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4264 This entered computer lingo via the paper "On Holy Wars and a Plea for
4265 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4267 Some systems may have even weirder byte orders such as
4272 You can determine your system endianness with this incantation:
4274 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4276 The byteorder on the platform where Perl was built is also available
4280 print "$Config{byteorder}\n";
4282 or from the command line:
4286 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4287 and C<"87654321"> are big-endian.
4289 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4290 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4291 immediately below. See also L<perlport>.
4295 Starting with Perl 5.9.2, integer and floating-point formats, along with
4296 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4297 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4298 or little-endian byte-order. These modifiers are especially useful
4299 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4300 64-bit integers, or floating-point values.
4302 Here are some concerns to keep in mind when using an endianness modifier:
4308 Exchanging signed integers between different platforms works only
4309 when all platforms store them in the same format. Most platforms store
4310 signed integers in two's-complement notation, so usually this is not an issue.
4314 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4315 formats on big- or little-endian machines. Otherwise, attempting to
4316 use them raises an exception.
4320 Forcing big- or little-endian byte-order on floating-point values for
4321 data exchange can work only if all platforms use the same
4322 binary representation such as IEEE floating-point. Even if all
4323 platforms are using IEEE, there may still be subtle differences. Being able
4324 to use C<< > >> or C<< < >> on floating-point values can be useful,
4325 but also dangerous if you don't know exactly what you're doing.
4326 It is not a general way to portably store floating-point values.
4330 When using C<< > >> or C<< < >> on a C<()> group, this affects
4331 all types inside the group that accept byte-order modifiers,
4332 including all subgroups. It is silently ignored for all other
4333 types. You are not allowed to override the byte-order within a group
4334 that already has a byte-order modifier suffix.
4340 Real numbers (floats and doubles) are in native machine format only.
4341 Due to the multiplicity of floating-point formats and the lack of a
4342 standard "network" representation for them, no facility for interchange has been
4343 made. This means that packed floating-point data written on one machine
4344 may not be readable on another, even if both use IEEE floating-point
4345 arithmetic (because the endianness of the memory representation is not part
4346 of the IEEE spec). See also L<perlport>.
4348 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4349 modifiers to force big- or little-endian byte-order on floating-point values.
4351 Because Perl uses doubles (or long doubles, if configured) internally for
4352 all numeric calculation, converting from double into float and thence
4353 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4354 will not in general equal $foo.
4358 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4359 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4360 where the packed string is processed in its UTF-8-encoded Unicode form on
4361 a byte-by-byte basis. Character mode is the default unless the format string
4362 starts with C<U>. You can always switch mode mid-format with an explicit
4363 C<C0> or C<U0> in the format. This mode remains in effect until the next
4364 mode change, or until the end of the C<()> group it (directly) applies to.
4366 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4367 bytes is not necessarily obvious. Probably only the first of these
4370 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4371 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4373 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4374 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4376 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4377 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4379 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4380 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4381 C3.8E.C2.B1.C3.8F.C2.89
4383 Those examples also illustrate that you should not try to use
4384 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4388 You must yourself do any alignment or padding by inserting, for example,
4389 enough C<"x">es while packing. There is no way for pack() and unpack()
4390 to know where characters are going to or coming from, so they
4391 handle their output and input as flat sequences of characters.
4395 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4396 take a repeat count either as postfix, or for unpack(), also via the C</>
4397 template character. Within each repetition of a group, positioning with
4398 C<@> starts over at 0. Therefore, the result of
4400 pack("@1A((@2A)@3A)", qw[X Y Z])
4402 is the string C<"\0X\0\0YZ">.
4406 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4407 jump forward or back to the closest position aligned at a multiple of C<count>
4408 characters. For example, to pack() or unpack() a C structure like
4411 char c; /* one signed, 8-bit character */
4416 one may need to use the template C<c x![d] d c[2]>. This assumes that
4417 doubles must be aligned to the size of double.
4419 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4424 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4425 represent signed 16-/32-bit integers in big-/little-endian order.
4426 This is portable only when all platforms sharing packed data use the
4427 same binary representation for signed integers; for example, when all
4428 platforms use two's-complement representation.
4432 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4433 White space can separate pack codes from each other, but modifiers and
4434 repeat counts must follow immediately. Breaking complex templates into
4435 individual line-by-line components, suitably annotated, can do as much to
4436 improve legibility and maintainability of pack/unpack formats as C</x> can
4437 for complicated pattern matches.
4441 If TEMPLATE requires more arguments than pack() is given, pack()
4442 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4443 than given, extra arguments are ignored.
4449 $foo = pack("WWWW",65,66,67,68);
4451 $foo = pack("W4",65,66,67,68);
4453 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4454 # same thing with Unicode circled letters.
4455 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4456 # same thing with Unicode circled letters. You don't get the
4457 # UTF-8 bytes because the U at the start of the format caused
4458 # a switch to U0-mode, so the UTF-8 bytes get joined into
4460 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4461 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4462 # This is the UTF-8 encoding of the string in the
4465 $foo = pack("ccxxcc",65,66,67,68);
4468 # NOTE: The examples above featuring "W" and "c" are true
4469 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4470 # and UTF-8. On EBCDIC systems, the first example would be
4471 # $foo = pack("WWWW",193,194,195,196);
4473 $foo = pack("s2",1,2);
4474 # "\001\000\002\000" on little-endian
4475 # "\000\001\000\002" on big-endian
4477 $foo = pack("a4","abcd","x","y","z");
4480 $foo = pack("aaaa","abcd","x","y","z");
4483 $foo = pack("a14","abcdefg");
4484 # "abcdefg\0\0\0\0\0\0\0"
4486 $foo = pack("i9pl", gmtime);
4487 # a real struct tm (on my system anyway)
4489 $utmp_template = "Z8 Z8 Z16 L";
4490 $utmp = pack($utmp_template, @utmp1);
4491 # a struct utmp (BSDish)
4493 @utmp2 = unpack($utmp_template, $utmp);
4494 # "@utmp1" eq "@utmp2"
4497 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4500 $foo = pack('sx2l', 12, 34);
4501 # short 12, two zero bytes padding, long 34
4502 $bar = pack('s@4l', 12, 34);
4503 # short 12, zero fill to position 4, long 34
4505 $baz = pack('s.l', 12, 4, 34);
4506 # short 12, zero fill to position 4, long 34
4508 $foo = pack('nN', 42, 4711);
4509 # pack big-endian 16- and 32-bit unsigned integers
4510 $foo = pack('S>L>', 42, 4711);
4512 $foo = pack('s<l<', -42, 4711);
4513 # pack little-endian 16- and 32-bit signed integers
4514 $foo = pack('(sl)<', -42, 4711);
4517 The same template may generally also be used in unpack().
4519 =item package NAMESPACE
4521 =item package NAMESPACE VERSION
4522 X<package> X<module> X<namespace> X<version>
4524 =item package NAMESPACE BLOCK
4526 =item package NAMESPACE VERSION BLOCK
4527 X<package> X<module> X<namespace> X<version>
4529 Declares the BLOCK or the rest of the compilation unit as being in the
4530 given namespace. The scope of the package declaration is either the
4531 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4532 itself through the end of current scope (the enclosing block, file, or
4533 C<eval>). That is, the forms without a BLOCK are operative through the end
4534 of the current scope, just like the C<my>, C<state>, and C<our> operators.
4535 All unqualified dynamic identifiers in this scope will be in the given
4536 namespace, except where overridden by another C<package> declaration or
4537 when they're one of the special identifiers that qualify into C<main::>,
4538 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
4540 A package statement affects dynamic variables only, including those
4541 you've used C<local> on, but I<not> lexical variables, which are created
4542 with C<my>, C<state>, or C<our>. Typically it would be the first
4543 declaration in a file included by C<require> or C<use>. You can switch into a
4544 package in more than one place, since this only determines which default
4545 symbol table the compiler uses for the rest of that block. You can refer to
4546 identifiers in other packages than the current one by prefixing the identifier
4547 with the package name and a double colon, as in C<$SomePack::var>
4548 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4549 package as assumed. That is, C<$::sail> is equivalent to
4550 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4551 code, mostly from Perl 4).
4553 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4554 namespace to a L<version> object with the VERSION provided. VERSION must be a
4555 "strict" style version number as defined by the L<version> module: a positive
4556 decimal number (integer or decimal-fraction) without exponentiation or else a
4557 dotted-decimal v-string with a leading 'v' character and at least three
4558 components. You should set C<$VERSION> only once per package.
4560 See L<perlmod/"Packages"> for more information about packages, modules,
4561 and classes. See L<perlsub> for other scoping issues.
4563 =item pipe READHANDLE,WRITEHANDLE
4566 Opens a pair of connected pipes like the corresponding system call.
4567 Note that if you set up a loop of piped processes, deadlock can occur
4568 unless you are very careful. In addition, note that Perl's pipes use
4569 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4570 after each command, depending on the application.
4572 See L<IPC::Open2>, L<IPC::Open3>, and
4573 L<perlipc/"Bidirectional Communication with Another Process">
4574 for examples of such things.
4576 On systems that support a close-on-exec flag on files, that flag is set
4577 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4578 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4587 Pops and returns the last value of the array, shortening the array by
4590 Returns the undefined value if the array is empty, although this may also
4591 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4592 main program, but the C<@_> array in subroutines, just like C<shift>.
4594 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
4595 reference to an unblessed array. The argument will be dereferenced
4596 automatically. This aspect of C<pop> is considered highly experimental.
4597 The exact behaviour may change in a future version of Perl.
4600 X<pos> X<match, position>
4604 Returns the offset of where the last C<m//g> search left off for the
4605 variable in question (C<$_> is used when the variable is not
4606 specified). Note that 0 is a valid match offset. C<undef> indicates
4607 that the search position is reset (usually due to match failure, but
4608 can also be because no match has yet been run on the scalar).
4610 C<pos> directly accesses the location used by the regexp engine to
4611 store the offset, so assigning to C<pos> will change that offset, and
4612 so will also influence the C<\G> zero-width assertion in regular
4613 expressions. Both of these effects take place for the next match, so
4614 you can't affect the position with C<pos> during the current match,
4615 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
4617 Setting C<pos> also resets the I<matched with zero-length> flag, described
4618 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
4620 Because a failed C<m//gc> match doesn't reset the offset, the return
4621 from C<pos> won't change either in this case. See L<perlre> and
4624 =item print FILEHANDLE LIST
4627 =item print FILEHANDLE
4633 Prints a string or a list of strings. Returns true if successful.
4634 FILEHANDLE may be a scalar variable containing the name of or a reference
4635 to the filehandle, thus introducing one level of indirection. (NOTE: If
4636 FILEHANDLE is a variable and the next token is a term, it may be
4637 misinterpreted as an operator unless you interpose a C<+> or put
4638 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
4639 last selected (see L</select>) output handle. If LIST is omitted, prints
4640 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
4641 print the content of C<$_> to it, you must use a real filehandle like
4642 C<FH>, not an indirect one like C<$fh>. To set the default output handle
4643 to something other than STDOUT, use the select operation.
4645 The current value of C<$,> (if any) is printed between each LIST item. The
4646 current value of C<$\> (if any) is printed after the entire LIST has been
4647 printed. Because print takes a LIST, anything in the LIST is evaluated in
4648 list context, including any subroutines whose return lists you pass to
4649 C<print>. Be careful not to follow the print keyword with a left
4650 parenthesis unless you want the corresponding right parenthesis to
4651 terminate the arguments to the print; put parentheses around all arguments
4652 (or interpose a C<+>, but that doesn't look as good).
4654 If you're storing handles in an array or hash, or in general whenever
4655 you're using any expression more complex than a bareword handle or a plain,
4656 unsubscripted scalar variable to retrieve it, you will have to use a block
4657 returning the filehandle value instead, in which case the LIST may not be
4660 print { $files[$i] } "stuff\n";
4661 print { $OK ? STDOUT : STDERR } "stuff\n";
4663 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4664 L<perlipc> for more on signal handling.
4666 =item printf FILEHANDLE FORMAT, LIST
4669 =item printf FILEHANDLE
4671 =item printf FORMAT, LIST
4675 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4676 (the output record separator) is not appended. The first argument of the
4677 list will be interpreted as the C<printf> format. See
4678 L<sprintf|/sprintf FORMAT, LIST> for an
4679 explanation of the format argument. If you omit the LIST, C<$_> is used;
4680 to use FILEHANDLE without a LIST, you must use a real filehandle like
4681 C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and
4682 POSIX::setlocale() has been called, the character used for the decimal
4683 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4684 locale setting. See L<perllocale> and L<POSIX>.
4686 Don't fall into the trap of using a C<printf> when a simple
4687 C<print> would do. The C<print> is more efficient and less
4690 =item prototype FUNCTION
4693 Returns the prototype of a function as a string (or C<undef> if the
4694 function has no prototype). FUNCTION is a reference to, or the name of,
4695 the function whose prototype you want to retrieve.
4697 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4698 name for a Perl builtin. If the builtin is not I<overridable> (such as
4699 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4700 (such as C<system>), prototype() returns C<undef>, because the builtin
4701 does not really behave like a Perl function. Otherwise, the string
4702 describing the equivalent prototype is returned.
4704 =item push ARRAY,LIST
4707 =item push EXPR,LIST
4709 Treats ARRAY as a stack by appending the values of LIST to the end of
4710 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
4714 $ARRAY[++$#ARRAY] = $value;
4717 but is more efficient. Returns the number of elements in the array following
4718 the completed C<push>.
4720 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
4721 reference to an unblessed array. The argument will be dereferenced
4722 automatically. This aspect of C<push> is considered highly experimental.
4723 The exact behaviour may change in a future version of Perl.
4733 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4737 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4739 =item quotemeta EXPR
4740 X<quotemeta> X<metacharacter>
4744 Returns the value of EXPR with all non-"word"
4745 characters backslashed. (That is, all characters not matching
4746 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4747 returned string, regardless of any locale settings.)
4748 This is the internal function implementing
4749 the C<\Q> escape in double-quoted strings.
4751 If EXPR is omitted, uses C<$_>.
4753 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
4754 regular expressions, because by default an interpolated variable will be
4755 considered a mini-regular expression. For example:
4757 my $sentence = 'The quick brown fox jumped over the lazy dog';
4758 my $substring = 'quick.*?fox';
4759 $sentence =~ s{$substring}{big bad wolf};
4761 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
4765 my $sentence = 'The quick brown fox jumped over the lazy dog';
4766 my $substring = 'quick.*?fox';
4767 $sentence =~ s{\Q$substring\E}{big bad wolf};
4771 my $sentence = 'The quick brown fox jumped over the lazy dog';
4772 my $substring = 'quick.*?fox';
4773 my $quoted_substring = quotemeta($substring);
4774 $sentence =~ s{$quoted_substring}{big bad wolf};
4776 Will both leave the sentence as is. Normally, when accepting literal string
4777 input from the user, quotemeta() or C<\Q> must be used.
4779 In Perl 5.14, all characters whose code points are above 127 are not
4780 quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings.
4781 It is planned to change this behavior in 5.16, but the exact rules
4782 haven't been determined yet.
4789 Returns a random fractional number greater than or equal to C<0> and less
4790 than the value of EXPR. (EXPR should be positive.) If EXPR is
4791 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4792 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4793 and is subject to change in future versions of Perl). Automatically calls
4794 C<srand> unless C<srand> has already been called. See also C<srand>.
4796 Apply C<int()> to the value returned by C<rand()> if you want random
4797 integers instead of random fractional numbers. For example,
4801 returns a random integer between C<0> and C<9>, inclusive.
4803 (Note: If your rand function consistently returns numbers that are too
4804 large or too small, then your version of Perl was probably compiled
4805 with the wrong number of RANDBITS.)
4807 B<C<rand()> is not cryptographically secure. You should not rely
4808 on it in security-sensitive situations.> As of this writing, a
4809 number of third-party CPAN modules offer random number generators
4810 intended by their authors to be cryptographically secure,
4811 including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and
4812 L<Math::TrulyRandom>.
4814 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4815 X<read> X<file, read>
4817 =item read FILEHANDLE,SCALAR,LENGTH
4819 Attempts to read LENGTH I<characters> of data into variable SCALAR
4820 from the specified FILEHANDLE. Returns the number of characters
4821 actually read, C<0> at end of file, or undef if there was an error (in
4822 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4823 so that the last character actually read is the last character of the
4824 scalar after the read.
4826 An OFFSET may be specified to place the read data at some place in the
4827 string other than the beginning. A negative OFFSET specifies
4828 placement at that many characters counting backwards from the end of
4829 the string. A positive OFFSET greater than the length of SCALAR
4830 results in the string being padded to the required size with C<"\0">
4831 bytes before the result of the read is appended.
4833 The call is implemented in terms of either Perl's or your system's native
4834 fread(3) library function. To get a true read(2) system call, see
4835 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
4837 Note the I<characters>: depending on the status of the filehandle,
4838 either (8-bit) bytes or characters are read. By default, all
4839 filehandles operate on bytes, but for example if the filehandle has
4840 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4841 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
4842 characters, not bytes. Similarly for the C<:encoding> pragma:
4843 in that case pretty much any characters can be read.
4845 =item readdir DIRHANDLE
4848 Returns the next directory entry for a directory opened by C<opendir>.
4849 If used in list context, returns all the rest of the entries in the
4850 directory. If there are no more entries, returns the undefined value in
4851 scalar context and the empty list in list context.
4853 If you're planning to filetest the return values out of a C<readdir>, you'd
4854 better prepend the directory in question. Otherwise, because we didn't
4855 C<chdir> there, it would have been testing the wrong file.
4857 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4858 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4861 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4862 which will set C<$_> on every iteration.
4864 opendir(my $dh, $some_dir) || die;
4865 while(readdir $dh) {
4866 print "$some_dir/$_\n";
4873 X<readline> X<gets> X<fgets>
4875 Reads from the filehandle whose typeglob is contained in EXPR (or from
4876 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
4877 returns the next line until end-of-file is reached, whereupon the
4878 subsequent call returns C<undef>. In list context, reads until end-of-file
4879 is reached and returns a list of lines. Note that the notion of "line"
4880 used here is whatever you may have defined with C<$/> or
4881 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4883 When C<$/> is set to C<undef>, when C<readline> is in scalar
4884 context (i.e., file slurp mode), and when an empty file is read, it
4885 returns C<''> the first time, followed by C<undef> subsequently.
4887 This is the internal function implementing the C<< <EXPR> >>
4888 operator, but you can use it directly. The C<< <EXPR> >>
4889 operator is discussed in more detail in L<perlop/"I/O Operators">.
4892 $line = readline(*STDIN); # same thing
4894 If C<readline> encounters an operating system error, C<$!> will be set
4895 with the corresponding error message. It can be helpful to check
4896 C<$!> when you are reading from filehandles you don't trust, such as a
4897 tty or a socket. The following example uses the operator form of
4898 C<readline> and dies if the result is not defined.
4900 while ( ! eof($fh) ) {
4901 defined( $_ = <$fh> ) or die "readline failed: $!";
4905 Note that you have can't handle C<readline> errors that way with the
4906 C<ARGV> filehandle. In that case, you have to open each element of
4907 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4909 foreach my $arg (@ARGV) {
4910 open(my $fh, $arg) or warn "Can't open $arg: $!";
4912 while ( ! eof($fh) ) {
4913 defined( $_ = <$fh> )
4914 or die "readline failed for $arg: $!";
4924 Returns the value of a symbolic link, if symbolic links are
4925 implemented. If not, raises an exception. If there is a system
4926 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4927 omitted, uses C<$_>.
4929 Portability issues: L<perlport/readlink>.
4936 EXPR is executed as a system command.
4937 The collected standard output of the command is returned.
4938 In scalar context, it comes back as a single (potentially
4939 multi-line) string. In list context, returns a list of lines
4940 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4941 This is the internal function implementing the C<qx/EXPR/>
4942 operator, but you can use it directly. The C<qx/EXPR/>
4943 operator is discussed in more detail in L<perlop/"I/O Operators">.
4944 If EXPR is omitted, uses C<$_>.
4946 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4949 Receives a message on a socket. Attempts to receive LENGTH characters
4950 of data into variable SCALAR from the specified SOCKET filehandle.
4951 SCALAR will be grown or shrunk to the length actually read. Takes the
4952 same flags as the system call of the same name. Returns the address
4953 of the sender if SOCKET's protocol supports this; returns an empty
4954 string otherwise. If there's an error, returns the undefined value.
4955 This call is actually implemented in terms of recvfrom(2) system call.
4956 See L<perlipc/"UDP: Message Passing"> for examples.
4958 Note the I<characters>: depending on the status of the socket, either
4959 (8-bit) bytes or characters are received. By default all sockets
4960 operate on bytes, but for example if the socket has been changed using
4961 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4962 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
4963 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4964 case pretty much any characters can be read.
4971 The C<redo> command restarts the loop block without evaluating the
4972 conditional again. The C<continue> block, if any, is not executed. If
4973 the LABEL is omitted, the command refers to the innermost enclosing
4974 loop. Programs that want to lie to themselves about what was just input
4975 normally use this command:
4977 # a simpleminded Pascal comment stripper
4978 # (warning: assumes no { or } in strings)
4979 LINE: while (<STDIN>) {
4980 while (s|({.*}.*){.*}|$1 |) {}
4985 if (/}/) { # end of comment?
4994 C<redo> cannot be used to retry a block that returns a value such as
4995 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
4996 a grep() or map() operation.
4998 Note that a block by itself is semantically identical to a loop
4999 that executes once. Thus C<redo> inside such a block will effectively
5000 turn it into a looping construct.
5002 See also L</continue> for an illustration of how C<last>, C<next>, and
5010 Returns a non-empty string if EXPR is a reference, the empty
5011 string otherwise. If EXPR
5012 is not specified, C<$_> will be used. The value returned depends on the
5013 type of thing the reference is a reference to.
5014 Builtin types include:
5028 If the referenced object has been blessed into a package, then that package
5029 name is returned instead. You can think of C<ref> as a C<typeof> operator.
5031 if (ref($r) eq "HASH") {
5032 print "r is a reference to a hash.\n";
5035 print "r is not a reference at all.\n";
5038 The return value C<LVALUE> indicates a reference to an lvalue that is not
5039 a variable. You get this from taking the reference of function calls like
5040 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5041 to a L<version string|perldata/"Version Strings">.
5043 The result C<Regexp> indicates that the argument is a regular expression
5044 resulting from C<qr//>.
5046 See also L<perlref>.
5048 =item rename OLDNAME,NEWNAME
5049 X<rename> X<move> X<mv> X<ren>
5051 Changes the name of a file; an existing file NEWNAME will be
5052 clobbered. Returns true for success, false otherwise.
5054 Behavior of this function varies wildly depending on your system
5055 implementation. For example, it will usually not work across file system
5056 boundaries, even though the system I<mv> command sometimes compensates
5057 for this. Other restrictions include whether it works on directories,
5058 open files, or pre-existing files. Check L<perlport> and either the
5059 rename(2) manpage or equivalent system documentation for details.
5061 For a platform independent C<move> function look at the L<File::Copy>
5064 Portability issues: L<perlport/rename>.
5066 =item require VERSION
5073 Demands a version of Perl specified by VERSION, or demands some semantics
5074 specified by EXPR or by C<$_> if EXPR is not supplied.
5076 VERSION may be either a numeric argument such as 5.006, which will be
5077 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5078 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5079 VERSION is greater than the version of the current Perl interpreter.
5080 Compare with L</use>, which can do a similar check at compile time.
5082 Specifying VERSION as a literal of the form v5.6.1 should generally be
5083 avoided, because it leads to misleading error messages under earlier
5084 versions of Perl that do not support this syntax. The equivalent numeric
5085 version should be used instead.
5087 require v5.6.1; # run time version check
5088 require 5.6.1; # ditto
5089 require 5.006_001; # ditto; preferred for backwards compatibility
5091 Otherwise, C<require> demands that a library file be included if it
5092 hasn't already been included. The file is included via the do-FILE
5093 mechanism, which is essentially just a variety of C<eval> with the
5094 caveat that lexical variables in the invoking script will be invisible
5095 to the included code. Has semantics similar to the following subroutine:
5098 my ($filename) = @_;
5099 if (exists $INC{$filename}) {
5100 return 1 if $INC{$filename};
5101 die "Compilation failed in require";
5103 my ($realfilename,$result);
5105 foreach $prefix (@INC) {
5106 $realfilename = "$prefix/$filename";
5107 if (-f $realfilename) {
5108 $INC{$filename} = $realfilename;
5109 $result = do $realfilename;
5113 die "Can't find $filename in \@INC";
5116 $INC{$filename} = undef;
5118 } elsif (!$result) {
5119 delete $INC{$filename};
5120 die "$filename did not return true value";
5126 Note that the file will not be included twice under the same specified
5129 The file must return true as the last statement to indicate
5130 successful execution of any initialization code, so it's customary to
5131 end such a file with C<1;> unless you're sure it'll return true
5132 otherwise. But it's better just to put the C<1;>, in case you add more
5135 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5136 replaces "F<::>" with "F</>" in the filename for you,
5137 to make it easy to load standard modules. This form of loading of
5138 modules does not risk altering your namespace.
5140 In other words, if you try this:
5142 require Foo::Bar; # a splendid bareword
5144 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5145 directories specified in the C<@INC> array.
5147 But if you try this:
5149 $class = 'Foo::Bar';
5150 require $class; # $class is not a bareword
5152 require "Foo::Bar"; # not a bareword because of the ""
5154 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5155 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5157 eval "require $class";
5159 Now that you understand how C<require> looks for files with a
5160 bareword argument, there is a little extra functionality going on behind
5161 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5162 first look for a similar filename with a "F<.pmc>" extension. If this file
5163 is found, it will be loaded in place of any file ending in a "F<.pm>"
5166 You can also insert hooks into the import facility by putting Perl code
5167 directly into the @INC array. There are three forms of hooks: subroutine
5168 references, array references, and blessed objects.
5170 Subroutine references are the simplest case. When the inclusion system
5171 walks through @INC and encounters a subroutine, this subroutine gets
5172 called with two parameters, the first a reference to itself, and the
5173 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5174 subroutine should return either nothing or else a list of up to three
5175 values in the following order:
5181 A filehandle, from which the file will be read.
5185 A reference to a subroutine. If there is no filehandle (previous item),
5186 then this subroutine is expected to generate one line of source code per
5187 call, writing the line into C<$_> and returning 1, then finally at end of
5188 file returning 0. If there is a filehandle, then the subroutine will be
5189 called to act as a simple source filter, with the line as read in C<$_>.
5190 Again, return 1 for each valid line, and 0 after all lines have been
5195 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5196 reference to the subroutine itself is passed in as C<$_[0]>.
5200 If an empty list, C<undef>, or nothing that matches the first 3 values above
5201 is returned, then C<require> looks at the remaining elements of @INC.
5202 Note that this filehandle must be a real filehandle (strictly a typeglob
5203 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5204 will be ignored and processing will stop there.
5206 If the hook is an array reference, its first element must be a subroutine
5207 reference. This subroutine is called as above, but the first parameter is
5208 the array reference. This lets you indirectly pass arguments to
5211 In other words, you can write:
5213 push @INC, \&my_sub;
5215 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5221 push @INC, [ \&my_sub, $x, $y, ... ];
5223 my ($arrayref, $filename) = @_;
5224 # Retrieve $x, $y, ...
5225 my @parameters = @$arrayref[1..$#$arrayref];
5229 If the hook is an object, it must provide an INC method that will be
5230 called as above, the first parameter being the object itself. (Note that
5231 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5232 into package C<main>.) Here is a typical code layout:
5238 my ($self, $filename) = @_;
5242 # In the main program
5243 push @INC, Foo->new(...);
5245 These hooks are also permitted to set the %INC entry
5246 corresponding to the files they have loaded. See L<perlvar/%INC>.
5248 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5255 Generally used in a C<continue> block at the end of a loop to clear
5256 variables and reset C<??> searches so that they work again. The
5257 expression is interpreted as a list of single characters (hyphens
5258 allowed for ranges). All variables and arrays beginning with one of
5259 those letters are reset to their pristine state. If the expression is
5260 omitted, one-match searches (C<?pattern?>) are reset to match again.
5261 Only resets variables or searches in the current package. Always returns
5264 reset 'X'; # reset all X variables
5265 reset 'a-z'; # reset lower case variables
5266 reset; # just reset ?one-time? searches
5268 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5269 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5270 variables; lexical variables are unaffected, but they clean themselves
5271 up on scope exit anyway, so you'll probably want to use them instead.
5279 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5280 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5281 context, depending on how the return value will be used, and the context
5282 may vary from one execution to the next (see L</wantarray>). If no EXPR
5283 is given, returns an empty list in list context, the undefined value in
5284 scalar context, and (of course) nothing at all in void context.
5286 (In the absence of an explicit C<return>, a subroutine, eval,
5287 or do FILE automatically returns the value of the last expression
5291 X<reverse> X<rev> X<invert>
5293 In list context, returns a list value consisting of the elements
5294 of LIST in the opposite order. In scalar context, concatenates the
5295 elements of LIST and returns a string value with all characters
5296 in the opposite order.
5298 print join(", ", reverse "world", "Hello"); # Hello, world
5300 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5302 Used without arguments in scalar context, reverse() reverses C<$_>.
5304 $_ = "dlrow ,olleH";
5305 print reverse; # No output, list context
5306 print scalar reverse; # Hello, world
5308 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5309 preserve non-existent elements whenever possible, i.e., for non magical
5310 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
5312 This operator is also handy for inverting a hash, although there are some
5313 caveats. If a value is duplicated in the original hash, only one of those
5314 can be represented as a key in the inverted hash. Also, this has to
5315 unwind one hash and build a whole new one, which may take some time
5316 on a large hash, such as from a DBM file.
5318 %by_name = reverse %by_address; # Invert the hash
5320 =item rewinddir DIRHANDLE
5323 Sets the current position to the beginning of the directory for the
5324 C<readdir> routine on DIRHANDLE.
5326 Portability issues: L<perlport/rewinddir>.
5328 =item rindex STR,SUBSTR,POSITION
5331 =item rindex STR,SUBSTR
5333 Works just like index() except that it returns the position of the I<last>
5334 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5335 last occurrence beginning at or before that position.
5337 =item rmdir FILENAME
5338 X<rmdir> X<rd> X<directory, remove>
5342 Deletes the directory specified by FILENAME if that directory is
5343 empty. If it succeeds it returns true; otherwise it returns false and
5344 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5346 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5347 the C<rmtree> function of the L<File::Path> module.
5351 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5353 =item say FILEHANDLE LIST
5356 =item say FILEHANDLE
5362 Just like C<print>, but implicitly appends a newline. C<say LIST> is
5363 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
5364 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
5365 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
5367 This keyword is available only when the C<"say"> feature
5368 is enabled, or when prefixed with C<CORE::>; see
5369 L<feature>. Alternately, include a C<use v5.10> or later to the current
5373 X<scalar> X<context>
5375 Forces EXPR to be interpreted in scalar context and returns the value
5378 @counts = ( scalar @a, scalar @b, scalar @c );
5380 There is no equivalent operator to force an expression to
5381 be interpolated in list context because in practice, this is never
5382 needed. If you really wanted to do so, however, you could use
5383 the construction C<@{[ (some expression) ]}>, but usually a simple
5384 C<(some expression)> suffices.
5386 Because C<scalar> is a unary operator, if you accidentally use a
5387 parenthesized list for the EXPR, this behaves as a scalar comma expression,
5388 evaluating all but the last element in void context and returning the final
5389 element evaluated in scalar context. This is seldom what you want.
5391 The following single statement:
5393 print uc(scalar(&foo,$bar)),$baz;
5395 is the moral equivalent of these two:
5398 print(uc($bar),$baz);
5400 See L<perlop> for more details on unary operators and the comma operator.
5402 =item seek FILEHANDLE,POSITION,WHENCE
5403 X<seek> X<fseek> X<filehandle, position>
5405 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5406 FILEHANDLE may be an expression whose value gives the name of the
5407 filehandle. The values for WHENCE are C<0> to set the new position
5408 I<in bytes> to POSITION; C<1> to set it to the current position plus
5409 POSITION; and C<2> to set it to EOF plus POSITION, typically
5410 negative. For WHENCE you may use the constants C<SEEK_SET>,
5411 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
5412 of the file) from the L<Fcntl> module. Returns C<1> on success, false
5415 Note the I<in bytes>: even if the filehandle has been set to
5416 operate on characters (for example by using the C<:encoding(utf8)> open
5417 layer), tell() will return byte offsets, not character offsets
5418 (because implementing that would render seek() and tell() rather slow).
5420 If you want to position the file for C<sysread> or C<syswrite>, don't use
5421 C<seek>, because buffering makes its effect on the file's read-write position
5422 unpredictable and non-portable. Use C<sysseek> instead.
5424 Due to the rules and rigors of ANSI C, on some systems you have to do a
5425 seek whenever you switch between reading and writing. Amongst other
5426 things, this may have the effect of calling stdio's clearerr(3).
5427 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
5431 This is also useful for applications emulating C<tail -f>. Once you hit
5432 EOF on your read and then sleep for a while, you (probably) have to stick in a
5433 dummy seek() to reset things. The C<seek> doesn't change the position,
5434 but it I<does> clear the end-of-file condition on the handle, so that the
5435 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
5437 If that doesn't work (some I/O implementations are particularly
5438 cantankerous), you might need something like this:
5441 for ($curpos = tell(FILE); $_ = <FILE>;
5442 $curpos = tell(FILE)) {
5443 # search for some stuff and put it into files
5445 sleep($for_a_while);
5446 seek(FILE, $curpos, 0);
5449 =item seekdir DIRHANDLE,POS
5452 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5453 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5454 about possible directory compaction as the corresponding system library
5457 =item select FILEHANDLE
5458 X<select> X<filehandle, default>
5462 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5463 sets the new current default filehandle for output. This has two
5464 effects: first, a C<write> or a C<print> without a filehandle
5465 default to this FILEHANDLE. Second, references to variables related to
5466 output will refer to this output channel.
5468 For example, to set the top-of-form format for more than one
5469 output channel, you might do the following:
5476 FILEHANDLE may be an expression whose value gives the name of the
5477 actual filehandle. Thus:
5479 $oldfh = select(STDERR); $| = 1; select($oldfh);
5481 Some programmers may prefer to think of filehandles as objects with
5482 methods, preferring to write the last example as:
5485 STDERR->autoflush(1);
5487 Portability issues: L<perlport/select>.
5489 =item select RBITS,WBITS,EBITS,TIMEOUT
5492 This calls the select(2) syscall with the bit masks specified, which
5493 can be constructed using C<fileno> and C<vec>, along these lines:
5495 $rin = $win = $ein = '';
5496 vec($rin, fileno(STDIN), 1) = 1;
5497 vec($win, fileno(STDOUT), 1) = 1;
5500 If you want to select on many filehandles, you may wish to write a
5501 subroutine like this:
5506 for my $fh (@fhlist) {
5507 vec($bits, fileno($fh), 1) = 1;
5511 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
5515 ($nfound,$timeleft) =
5516 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5518 or to block until something becomes ready just do this
5520 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5522 Most systems do not bother to return anything useful in $timeleft, so
5523 calling select() in scalar context just returns $nfound.
5525 Any of the bit masks can also be undef. The timeout, if specified, is
5526 in seconds, which may be fractional. Note: not all implementations are
5527 capable of returning the $timeleft. If not, they always return
5528 $timeleft equal to the supplied $timeout.
5530 You can effect a sleep of 250 milliseconds this way:
5532 select(undef, undef, undef, 0.25);
5534 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5535 is implementation-dependent. See also L<perlport> for notes on the
5536 portability of C<select>.
5538 On error, C<select> behaves just like select(2): it returns
5541 On some Unixes, select(2) may report a socket file descriptor as "ready for
5542 reading" even when no data is available, and thus any subsequent C<read>
5543 would block. This can be avoided if you always use O_NONBLOCK on the
5544 socket. See select(2) and fcntl(2) for further details.
5546 The standard C<IO::Select> module provides a user-friendlier interface
5547 to C<select>, mostly because it does all the bit-mask work for you.
5549 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5550 or <FH>) with C<select>, except as permitted by POSIX, and even
5551 then only on POSIX systems. You have to use C<sysread> instead.
5553 Portability issues: L<perlport/select>.
5555 =item semctl ID,SEMNUM,CMD,ARG
5558 Calls the System V IPC function semctl(2). You'll probably have to say
5562 first to get the correct constant definitions. If CMD is IPC_STAT or
5563 GETALL, then ARG must be a variable that will hold the returned
5564 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5565 the undefined value for error, "C<0 but true>" for zero, or the actual
5566 return value otherwise. The ARG must consist of a vector of native
5567 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5568 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5571 Portability issues: L<perlport/semctl>.
5573 =item semget KEY,NSEMS,FLAGS
5576 Calls the System V IPC function semget(2). Returns the semaphore id, or
5577 the undefined value on error. See also
5578 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5581 Portability issues: L<perlport/semget>.
5583 =item semop KEY,OPSTRING
5586 Calls the System V IPC function semop(2) for semaphore operations
5587 such as signalling and waiting. OPSTRING must be a packed array of
5588 semop structures. Each semop structure can be generated with
5589 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5590 implies the number of semaphore operations. Returns true if
5591 successful, false on error. As an example, the
5592 following code waits on semaphore $semnum of semaphore id $semid:
5594 $semop = pack("s!3", $semnum, -1, 0);
5595 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5597 To signal the semaphore, replace C<-1> with C<1>. See also
5598 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5601 Portability issues: L<perlport/semop>.
5603 =item send SOCKET,MSG,FLAGS,TO
5606 =item send SOCKET,MSG,FLAGS
5608 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5609 filehandle. Takes the same flags as the system call of the same name. On
5610 unconnected sockets, you must specify a destination to I<send to>, in which
5611 case it does a sendto(2) syscall. Returns the number of characters sent,
5612 or the undefined value on error. The sendmsg(2) syscall is currently
5613 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5615 Note the I<characters>: depending on the status of the socket, either
5616 (8-bit) bytes or characters are sent. By default all sockets operate
5617 on bytes, but for example if the socket has been changed using
5618 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5619 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5620 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5621 pragma: in that case pretty much any characters can be sent.
5623 =item setpgrp PID,PGRP
5626 Sets the current process group for the specified PID, C<0> for the current
5627 process. Raises an exception when used on a machine that doesn't
5628 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5629 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5630 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5633 Portability issues: L<perlport/setpgrp>.
5635 =item setpriority WHICH,WHO,PRIORITY
5636 X<setpriority> X<priority> X<nice> X<renice>
5638 Sets the current priority for a process, a process group, or a user.
5639 (See setpriority(2).) Raises an exception when used on a machine
5640 that doesn't implement setpriority(2).
5642 Portability issues: L<perlport/setpriority>.
5644 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5647 Sets the socket option requested. Returns C<undef> on error.
5648 Use integer constants provided by the C<Socket> module for
5649 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5650 getprotobyname. OPTVAL might either be a packed string or an integer.
5651 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5653 An example disabling Nagle's algorithm on a socket:
5655 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5656 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5658 Portability issues: L<perlport/setsockopt>.
5667 Shifts the first value of the array off and returns it, shortening the
5668 array by 1 and moving everything down. If there are no elements in the
5669 array, returns the undefined value. If ARRAY is omitted, shifts the
5670 C<@_> array within the lexical scope of subroutines and formats, and the
5671 C<@ARGV> array outside a subroutine and also within the lexical scopes
5672 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5673 C<UNITCHECK {}>, and C<END {}> constructs.
5675 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
5676 reference to an unblessed array. The argument will be dereferenced
5677 automatically. This aspect of C<shift> is considered highly experimental.
5678 The exact behaviour may change in a future version of Perl.
5680 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5681 same thing to the left end of an array that C<pop> and C<push> do to the
5684 =item shmctl ID,CMD,ARG
5687 Calls the System V IPC function shmctl. You'll probably have to say
5691 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5692 then ARG must be a variable that will hold the returned C<shmid_ds>
5693 structure. Returns like ioctl: C<undef> for error; "C<0> but
5694 true" for zero; and the actual return value otherwise.
5695 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5697 Portability issues: L<perlport/shmctl>.
5699 =item shmget KEY,SIZE,FLAGS
5702 Calls the System V IPC function shmget. Returns the shared memory
5703 segment id, or C<undef> on error.
5704 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5706 Portability issues: L<perlport/shmget>.
5708 =item shmread ID,VAR,POS,SIZE
5712 =item shmwrite ID,STRING,POS,SIZE
5714 Reads or writes the System V shared memory segment ID starting at
5715 position POS for size SIZE by attaching to it, copying in/out, and
5716 detaching from it. When reading, VAR must be a variable that will
5717 hold the data read. When writing, if STRING is too long, only SIZE
5718 bytes are used; if STRING is too short, nulls are written to fill out
5719 SIZE bytes. Return true if successful, false on error.
5720 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5721 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
5723 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
5725 =item shutdown SOCKET,HOW
5728 Shuts down a socket connection in the manner indicated by HOW, which
5729 has the same interpretation as in the syscall of the same name.
5731 shutdown(SOCKET, 0); # I/we have stopped reading data
5732 shutdown(SOCKET, 1); # I/we have stopped writing data
5733 shutdown(SOCKET, 2); # I/we have stopped using this socket
5735 This is useful with sockets when you want to tell the other
5736 side you're done writing but not done reading, or vice versa.
5737 It's also a more insistent form of close because it also
5738 disables the file descriptor in any forked copies in other
5741 Returns C<1> for success; on error, returns C<undef> if
5742 the first argument is not a valid filehandle, or returns C<0> and sets
5743 C<$!> for any other failure.
5746 X<sin> X<sine> X<asin> X<arcsine>
5750 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5751 returns sine of C<$_>.
5753 For the inverse sine operation, you may use the C<Math::Trig::asin>
5754 function, or use this relation:
5756 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5763 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5764 argument is given. Returns the integer number of seconds actually slept.
5766 May be interrupted if the process receives a signal such as C<SIGALRM>.
5769 local $SIG{ALARM} = sub { die "Alarm!\n" };
5772 die $@ unless $@ eq "Alarm!\n";
5774 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5775 is often implemented using C<alarm>.
5777 On some older systems, it may sleep up to a full second less than what
5778 you requested, depending on how it counts seconds. Most modern systems
5779 always sleep the full amount. They may appear to sleep longer than that,
5780 however, because your process might not be scheduled right away in a
5781 busy multitasking system.
5783 For delays of finer granularity than one second, the Time::HiRes module
5784 (from CPAN, and starting from Perl 5.8 part of the standard
5785 distribution) provides usleep(). You may also use Perl's four-argument
5786 version of select() leaving the first three arguments undefined, or you
5787 might be able to use the C<syscall> interface to access setitimer(2) if
5788 your system supports it. See L<perlfaq8> for details.
5790 See also the POSIX module's C<pause> function.
5792 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5795 Opens a socket of the specified kind and attaches it to filehandle
5796 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5797 the syscall of the same name. You should C<use Socket> first
5798 to get the proper definitions imported. See the examples in
5799 L<perlipc/"Sockets: Client/Server Communication">.
5801 On systems that support a close-on-exec flag on files, the flag will
5802 be set for the newly opened file descriptor, as determined by the
5803 value of $^F. See L<perlvar/$^F>.
5805 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5808 Creates an unnamed pair of sockets in the specified domain, of the
5809 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5810 for the syscall of the same name. If unimplemented, raises an exception.
5811 Returns true if successful.
5813 On systems that support a close-on-exec flag on files, the flag will
5814 be set for the newly opened file descriptors, as determined by the value
5815 of $^F. See L<perlvar/$^F>.
5817 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5818 to C<pipe(Rdr, Wtr)> is essentially:
5821 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5822 shutdown(Rdr, 1); # no more writing for reader
5823 shutdown(Wtr, 0); # no more reading for writer
5825 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5826 emulate socketpair using IP sockets to localhost if your system implements
5827 sockets but not socketpair.
5829 Portability issues: L<perlport/socketpair>.
5831 =item sort SUBNAME LIST
5832 X<sort> X<qsort> X<quicksort> X<mergesort>
5834 =item sort BLOCK LIST
5838 In list context, this sorts the LIST and returns the sorted list value.
5839 In scalar context, the behaviour of C<sort()> is undefined.
5841 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5842 order. If SUBNAME is specified, it gives the name of a subroutine
5843 that returns an integer less than, equal to, or greater than C<0>,
5844 depending on how the elements of the list are to be ordered. (The
5845 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5846 SUBNAME may be a scalar variable name (unsubscripted), in which case
5847 the value provides the name of (or a reference to) the actual
5848 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5849 an anonymous, in-line sort subroutine.
5851 If the subroutine's prototype is C<($$)>, the elements to be compared are
5852 passed by reference in C<@_>, as for a normal subroutine. This is slower
5853 than unprototyped subroutines, where the elements to be compared are passed
5854 into the subroutine as the package global variables $a and $b (see example
5855 below). Note that in the latter case, it is usually highly counter-productive
5856 to declare $a and $b as lexicals.
5858 The values to be compared are always passed by reference and should not
5861 You also cannot exit out of the sort block or subroutine using any of the
5862 loop control operators described in L<perlsyn> or with C<goto>.
5864 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5865 current collation locale. See L<perllocale>.
5867 sort() returns aliases into the original list, much as a for loop's index
5868 variable aliases the list elements. That is, modifying an element of a
5869 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5870 actually modifies the element in the original list. This is usually
5871 something to be avoided when writing clear code.
5873 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5874 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
5875 preserves the input order of elements that compare equal. Although
5876 quicksort's run time is O(NlogN) when averaged over all arrays of
5877 length N, the time can be O(N**2), I<quadratic> behavior, for some
5878 inputs.) In 5.7, the quicksort implementation was replaced with
5879 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5880 But benchmarks indicated that for some inputs, on some platforms,
5881 the original quicksort was faster. 5.8 has a sort pragma for
5882 limited control of the sort. Its rather blunt control of the
5883 underlying algorithm may not persist into future Perls, but the
5884 ability to characterize the input or output in implementation
5885 independent ways quite probably will. See L<the sort pragma|sort>.
5890 @articles = sort @files;
5892 # same thing, but with explicit sort routine
5893 @articles = sort {$a cmp $b} @files;
5895 # now case-insensitively
5896 @articles = sort {uc($a) cmp uc($b)} @files;
5898 # same thing in reversed order
5899 @articles = sort {$b cmp $a} @files;
5901 # sort numerically ascending
5902 @articles = sort {$a <=> $b} @files;
5904 # sort numerically descending
5905 @articles = sort {$b <=> $a} @files;
5907 # this sorts the %age hash by value instead of key
5908 # using an in-line function
5909 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5911 # sort using explicit subroutine name
5913 $age{$a} <=> $age{$b}; # presuming numeric
5915 @sortedclass = sort byage @class;
5917 sub backwards { $b cmp $a }
5918 @harry = qw(dog cat x Cain Abel);
5919 @george = qw(gone chased yz Punished Axed);
5921 # prints AbelCaincatdogx
5922 print sort backwards @harry;
5923 # prints xdogcatCainAbel
5924 print sort @george, 'to', @harry;
5925 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5927 # inefficiently sort by descending numeric compare using
5928 # the first integer after the first = sign, or the
5929 # whole record case-insensitively otherwise
5932 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5937 # same thing, but much more efficiently;
5938 # we'll build auxiliary indices instead
5940 my @nums = @caps = ();
5942 push @nums, ( /=(\d+)/ ? $1 : undef );
5946 my @new = @old[ sort {
5947 $nums[$b] <=> $nums[$a]
5949 $caps[$a] cmp $caps[$b]
5953 # same thing, but without any temps
5954 @new = map { $_->[0] }
5955 sort { $b->[1] <=> $a->[1]
5958 } map { [$_, /=(\d+)/, uc($_)] } @old;
5960 # using a prototype allows you to use any comparison subroutine
5961 # as a sort subroutine (including other package's subroutines)
5963 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5966 @new = sort other::backwards @old;
5968 # guarantee stability, regardless of algorithm
5970 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5972 # force use of mergesort (not portable outside Perl 5.8)
5973 use sort '_mergesort'; # note discouraging _
5974 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5976 Warning: syntactical care is required when sorting the list returned from
5977 a function. If you want to sort the list returned by the function call
5978 C<find_records(@key)>, you can use:
5980 @contact = sort { $a cmp $b } find_records @key;
5981 @contact = sort +find_records(@key);
5982 @contact = sort &find_records(@key);
5983 @contact = sort(find_records(@key));
5985 If instead you want to sort the array @key with the comparison routine
5986 C<find_records()> then you can use:
5988 @contact = sort { find_records() } @key;
5989 @contact = sort find_records(@key);
5990 @contact = sort(find_records @key);
5991 @contact = sort(find_records (@key));
5993 If you're using strict, you I<must not> declare $a
5994 and $b as lexicals. They are package globals. That means
5995 that if you're in the C<main> package and type
5997 @articles = sort {$b <=> $a} @files;
5999 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6000 but if you're in the C<FooPack> package, it's the same as typing
6002 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6004 The comparison function is required to behave. If it returns
6005 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6006 sometimes saying the opposite, for example) the results are not
6009 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6010 (not-a-number), and also because C<sort> raises an exception unless the
6011 result of a comparison is defined, be careful when sorting with a
6012 comparison function like C<< $a <=> $b >> any lists that might contain a
6013 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6014 eliminate any C<NaN>s from the input list.
6016 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6018 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6021 =item splice ARRAY or EXPR,OFFSET,LENGTH
6023 =item splice ARRAY or EXPR,OFFSET
6025 =item splice ARRAY or EXPR
6027 Removes the elements designated by OFFSET and LENGTH from an array, and
6028 replaces them with the elements of LIST, if any. In list context,
6029 returns the elements removed from the array. In scalar context,
6030 returns the last element removed, or C<undef> if no elements are
6031 removed. The array grows or shrinks as necessary.
6032 If OFFSET is negative then it starts that far from the end of the array.
6033 If LENGTH is omitted, removes everything from OFFSET onward.
6034 If LENGTH is negative, removes the elements from OFFSET onward
6035 except for -LENGTH elements at the end of the array.
6036 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6037 past the end of the array, Perl issues a warning, and splices at the
6040 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
6042 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6043 pop(@a) splice(@a,-1)
6044 shift(@a) splice(@a,0,1)
6045 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6046 $a[$i] = $y splice(@a,$i,1,$y)
6048 Example, assuming array lengths are passed before arrays:
6050 sub aeq { # compare two list values
6051 my(@a) = splice(@_,0,shift);
6052 my(@b) = splice(@_,0,shift);
6053 return 0 unless @a == @b; # same len?
6055 return 0 if pop(@a) ne pop(@b);
6059 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6061 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6062 reference to an unblessed array. The argument will be dereferenced
6063 automatically. This aspect of C<splice> is considered highly experimental.
6064 The exact behaviour may change in a future version of Perl.
6066 =item split /PATTERN/,EXPR,LIMIT
6069 =item split /PATTERN/,EXPR
6071 =item split /PATTERN/
6075 Splits the string EXPR into a list of strings and returns that list. By
6076 default, empty leading fields are preserved, and empty trailing ones are
6077 deleted. (If all fields are empty, they are considered to be trailing.)
6079 In scalar context, returns the number of fields found.
6081 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
6082 splits on whitespace (after skipping any leading whitespace). Anything
6083 matching PATTERN is taken to be a delimiter separating the fields. (Note
6084 that the delimiter may be longer than one character.)
6086 If LIMIT is specified and positive, it represents the maximum number
6087 of fields the EXPR will be split into, though the actual number of
6088 fields returned depends on the number of times PATTERN matches within
6089 EXPR. If LIMIT is unspecified or zero, trailing null fields are
6090 stripped (which potential users of C<pop> would do well to remember).
6091 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
6092 had been specified. Note that splitting an EXPR that evaluates to the
6093 empty string always returns the empty list, regardless of the LIMIT
6096 A pattern matching the empty string (not to be confused with
6097 an empty pattern C<//>, which is just one member of the set of patterns
6098 matching the empty string), splits EXPR into individual
6099 characters. For example:
6101 print join(':', split(/ */, 'hi there')), "\n";
6103 produces the output 'h:i:t:h:e:r:e'.
6105 As a special case for C<split>, the empty pattern C<//> specifically
6106 matches the empty string; this is not be confused with the normal use
6107 of an empty pattern to mean the last successful match. So to split
6108 a string into individual characters, the following:
6110 print join(':', split(//, 'hi there')), "\n";
6112 produces the output 'h:i: :t:h:e:r:e'.
6114 Empty leading fields are produced when there are positive-width matches at
6115 the beginning of the string; a zero-width match at the beginning of
6116 the string does not produce an empty field. For example:
6118 print join(':', split(/(?=\w)/, 'hi there!'));
6120 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
6121 hand, are produced when there is a match at the end of the string (and
6122 when LIMIT is given and is not 0), regardless of the length of the match.
6125 print join(':', split(//, 'hi there!', -1)), "\n";
6126 print join(':', split(/\W/, 'hi there!', -1)), "\n";
6128 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
6129 both with an empty trailing field.
6131 The LIMIT parameter can be used to split a line partially
6133 ($login, $passwd, $remainder) = split(/:/, $_, 3);
6135 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
6136 a LIMIT one larger than the number of variables in the list, to avoid
6137 unnecessary work. For the list above LIMIT would have been 4 by
6138 default. In time critical applications it behooves you not to split
6139 into more fields than you really need.
6141 If the PATTERN contains parentheses, additional list elements are
6142 created from each matching substring in the delimiter.
6144 split(/([,-])/, "1-10,20", 3);
6146 produces the list value
6148 (1, '-', 10, ',', 20)
6150 If you had the entire header of a normal Unix email message in $header,
6151 you could split it up into fields and their values this way:
6153 $header =~ s/\n(?=\s)//g; # fix continuation lines
6154 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
6156 The pattern C</PATTERN/> may be replaced with an expression to specify
6157 patterns that vary at runtime. (To do runtime compilation only once,
6158 use C</$variable/o>.)
6160 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
6161 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
6162 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
6163 will give you as many initial null fields (empty string) as there are leading spaces.
6164 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
6165 whitespace produces a null first field. A C<split> with no arguments
6166 really does a S<C<split(' ', $_)>> internally.
6168 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
6173 open(PASSWD, '/etc/passwd');
6176 ($login, $passwd, $uid, $gid,
6177 $gcos, $home, $shell) = split(/:/);
6181 As with regular pattern matching, any capturing parentheses that are not
6182 matched in a C<split()> will be set to C<undef> when returned:
6184 @fields = split /(A)|B/, "1A2B3";
6185 # @fields is (1, 'A', 2, undef, 3)
6187 =item sprintf FORMAT, LIST
6190 Returns a string formatted by the usual C<printf> conventions of the C
6191 library function C<sprintf>. See below for more details
6192 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
6193 the general principles.
6197 # Format number with up to 8 leading zeroes
6198 $result = sprintf("%08d", $number);
6200 # Round number to 3 digits after decimal point
6201 $rounded = sprintf("%.3f", $number);
6203 Perl does its own C<sprintf> formatting: it emulates the C
6204 function sprintf(3), but doesn't use it except for floating-point
6205 numbers, and even then only standard modifiers are allowed.
6206 Non-standard extensions in your local sprintf(3) are
6207 therefore unavailable from Perl.
6209 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
6210 pass it an array as your first argument. The array is given scalar context,
6211 and instead of using the 0th element of the array as the format, Perl will
6212 use the count of elements in the array as the format, which is almost never
6215 Perl's C<sprintf> permits the following universally-known conversions:
6218 %c a character with the given number
6220 %d a signed integer, in decimal
6221 %u an unsigned integer, in decimal
6222 %o an unsigned integer, in octal
6223 %x an unsigned integer, in hexadecimal
6224 %e a floating-point number, in scientific notation
6225 %f a floating-point number, in fixed decimal notation
6226 %g a floating-point number, in %e or %f notation
6228 In addition, Perl permits the following widely-supported conversions:
6230 %X like %x, but using upper-case letters
6231 %E like %e, but using an upper-case "E"
6232 %G like %g, but with an upper-case "E" (if applicable)
6233 %b an unsigned integer, in binary
6234 %B like %b, but using an upper-case "B" with the # flag
6235 %p a pointer (outputs the Perl value's address in hexadecimal)
6236 %n special: *stores* the number of characters output so far
6237 into the next variable in the parameter list
6239 Finally, for backward (and we do mean "backward") compatibility, Perl
6240 permits these unnecessary but widely-supported conversions:
6243 %D a synonym for %ld
6244 %U a synonym for %lu
6245 %O a synonym for %lo
6248 Note that the number of exponent digits in the scientific notation produced
6249 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
6250 exponent less than 100 is system-dependent: it may be three or less
6251 (zero-padded as necessary). In other words, 1.23 times ten to the
6252 99th may be either "1.23e99" or "1.23e099".
6254 Between the C<%> and the format letter, you may specify several
6255 additional attributes controlling the interpretation of the format.
6256 In order, these are:
6260 =item format parameter index
6262 An explicit format parameter index, such as C<2$>. By default sprintf
6263 will format the next unused argument in the list, but this allows you
6264 to take the arguments out of order:
6266 printf '%2$d %1$d', 12, 34; # prints "34 12"
6267 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
6273 space prefix non-negative number with a space
6274 + prefix non-negative number with a plus sign
6275 - left-justify within the field
6276 0 use zeros, not spaces, to right-justify
6277 # ensure the leading "0" for any octal,
6278 prefix non-zero hexadecimal with "0x" or "0X",
6279 prefix non-zero binary with "0b" or "0B"
6283 printf '<% d>', 12; # prints "< 12>"
6284 printf '<%+d>', 12; # prints "<+12>"
6285 printf '<%6s>', 12; # prints "< 12>"
6286 printf '<%-6s>', 12; # prints "<12 >"
6287 printf '<%06s>', 12; # prints "<000012>"
6288 printf '<%#o>', 12; # prints "<014>"
6289 printf '<%#x>', 12; # prints "<0xc>"
6290 printf '<%#X>', 12; # prints "<0XC>"
6291 printf '<%#b>', 12; # prints "<0b1100>"
6292 printf '<%#B>', 12; # prints "<0B1100>"
6294 When a space and a plus sign are given as the flags at once,
6295 a plus sign is used to prefix a positive number.
6297 printf '<%+ d>', 12; # prints "<+12>"
6298 printf '<% +d>', 12; # prints "<+12>"
6300 When the # flag and a precision are given in the %o conversion,
6301 the precision is incremented if it's necessary for the leading "0".
6303 printf '<%#.5o>', 012; # prints "<00012>"
6304 printf '<%#.5o>', 012345; # prints "<012345>"
6305 printf '<%#.0o>', 0; # prints "<0>"
6309 This flag tells Perl to interpret the supplied string as a vector of
6310 integers, one for each character in the string. Perl applies the format to
6311 each integer in turn, then joins the resulting strings with a separator (a
6312 dot C<.> by default). This can be useful for displaying ordinal values of
6313 characters in arbitrary strings:
6315 printf "%vd", "AB\x{100}"; # prints "65.66.256"
6316 printf "version is v%vd\n", $^V; # Perl's version
6318 Put an asterisk C<*> before the C<v> to override the string to
6319 use to separate the numbers:
6321 printf "address is %*vX\n", ":", $addr; # IPv6 address
6322 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
6324 You can also explicitly specify the argument number to use for
6325 the join string using something like C<*2$v>; for example:
6327 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
6329 =item (minimum) width
6331 Arguments are usually formatted to be only as wide as required to
6332 display the given value. You can override the width by putting
6333 a number here, or get the width from the next argument (with C<*>)
6334 or from a specified argument (e.g., with C<*2$>):
6336 printf "<%s>", "a"; # prints "<a>"
6337 printf "<%6s>", "a"; # prints "< a>"
6338 printf "<%*s>", 6, "a"; # prints "< a>"
6339 printf "<%*2$s>", "a", 6; # prints "< a>"
6340 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
6342 If a field width obtained through C<*> is negative, it has the same
6343 effect as the C<-> flag: left-justification.
6345 =item precision, or maximum width
6348 You can specify a precision (for numeric conversions) or a maximum
6349 width (for string conversions) by specifying a C<.> followed by a number.
6350 For floating-point formats except C<g> and C<G>, this specifies
6351 how many places right of the decimal point to show (the default being 6).
6354 # these examples are subject to system-specific variation
6355 printf '<%f>', 1; # prints "<1.000000>"
6356 printf '<%.1f>', 1; # prints "<1.0>"
6357 printf '<%.0f>', 1; # prints "<1>"
6358 printf '<%e>', 10; # prints "<1.000000e+01>"
6359 printf '<%.1e>', 10; # prints "<1.0e+01>"
6361 For "g" and "G", this specifies the maximum number of digits to show,
6362 including those prior to the decimal point and those after it; for
6365 # These examples are subject to system-specific variation.
6366 printf '<%g>', 1; # prints "<1>"
6367 printf '<%.10g>', 1; # prints "<1>"
6368 printf '<%g>', 100; # prints "<100>"
6369 printf '<%.1g>', 100; # prints "<1e+02>"
6370 printf '<%.2g>', 100.01; # prints "<1e+02>"
6371 printf '<%.5g>', 100.01; # prints "<100.01>"
6372 printf '<%.4g>', 100.01; # prints "<100>"
6374 For integer conversions, specifying a precision implies that the
6375 output of the number itself should be zero-padded to this width,
6376 where the 0 flag is ignored:
6378 printf '<%.6d>', 1; # prints "<000001>"
6379 printf '<%+.6d>', 1; # prints "<+000001>"
6380 printf '<%-10.6d>', 1; # prints "<000001 >"
6381 printf '<%10.6d>', 1; # prints "< 000001>"
6382 printf '<%010.6d>', 1; # prints "< 000001>"
6383 printf '<%+10.6d>', 1; # prints "< +000001>"
6385 printf '<%.6x>', 1; # prints "<000001>"
6386 printf '<%#.6x>', 1; # prints "<0x000001>"
6387 printf '<%-10.6x>', 1; # prints "<000001 >"
6388 printf '<%10.6x>', 1; # prints "< 000001>"
6389 printf '<%010.6x>', 1; # prints "< 000001>"
6390 printf '<%#10.6x>', 1; # prints "< 0x000001>"
6392 For string conversions, specifying a precision truncates the string
6393 to fit the specified width:
6395 printf '<%.5s>', "truncated"; # prints "<trunc>"
6396 printf '<%10.5s>', "truncated"; # prints "< trunc>"
6398 You can also get the precision from the next argument using C<.*>:
6400 printf '<%.6x>', 1; # prints "<000001>"
6401 printf '<%.*x>', 6, 1; # prints "<000001>"
6403 If a precision obtained through C<*> is negative, it counts
6404 as having no precision at all.
6406 printf '<%.*s>', 7, "string"; # prints "<string>"
6407 printf '<%.*s>', 3, "string"; # prints "<str>"
6408 printf '<%.*s>', 0, "string"; # prints "<>"
6409 printf '<%.*s>', -1, "string"; # prints "<string>"
6411 printf '<%.*d>', 1, 0; # prints "<0>"
6412 printf '<%.*d>', 0, 0; # prints "<>"
6413 printf '<%.*d>', -1, 0; # prints "<0>"
6415 You cannot currently get the precision from a specified number,
6416 but it is intended that this will be possible in the future, for
6417 example using C<.*2$>:
6419 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
6423 For numeric conversions, you can specify the size to interpret the
6424 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
6425 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
6426 whatever the default integer size is on your platform (usually 32 or 64
6427 bits), but you can override this to use instead one of the standard C types,
6428 as supported by the compiler used to build Perl:
6430 hh interpret integer as C type "char" or "unsigned char"
6431 on Perl 5.14 or later
6432 h interpret integer as C type "short" or "unsigned short"
6433 j interpret integer as C type "intmax_t" on Perl 5.14
6434 or later, and only with a C99 compiler (unportable)
6435 l interpret integer as C type "long" or "unsigned long"
6436 q, L, or ll interpret integer as C type "long long", "unsigned long long",
6437 or "quad" (typically 64-bit integers)
6438 t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
6439 z interpret integer as C type "size_t" on Perl 5.14 or later
6441 As of 5.14, none of these raises an exception if they are not supported on
6442 your platform. However, if warnings are enabled, a warning of the
6443 C<printf> warning class is issued on an unsupported conversion flag.
6444 Should you instead prefer an exception, do this:
6446 use warnings FATAL => "printf";
6448 If you would like to know about a version dependency before you
6449 start running the program, put something like this at its top:
6451 use 5.014; # for hh/j/t/z/ printf modifiers
6453 You can find out whether your Perl supports quads via L<Config>:
6456 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
6457 print "Nice quads!\n";
6460 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
6461 to be the default floating-point size on your platform (double or long double),
6462 but you can force "long double" with C<q>, C<L>, or C<ll> if your
6463 platform supports them. You can find out whether your Perl supports long
6464 doubles via L<Config>:
6467 print "long doubles\n" if $Config{d_longdbl} eq "define";
6469 You can find out whether Perl considers "long double" to be the default
6470 floating-point size to use on your platform via L<Config>:
6473 if ($Config{uselongdouble} eq "define") {
6474 print "long doubles by default\n";
6477 It can also be that long doubles and doubles are the same thing:
6480 ($Config{doublesize} == $Config{longdblsize}) &&
6481 print "doubles are long doubles\n";
6483 The size specifier C<V> has no effect for Perl code, but is supported for
6484 compatibility with XS code. It means "use the standard size for a Perl
6485 integer or floating-point number", which is the default.
6487 =item order of arguments
6489 Normally, sprintf() takes the next unused argument as the value to
6490 format for each format specification. If the format specification
6491 uses C<*> to require additional arguments, these are consumed from
6492 the argument list in the order they appear in the format
6493 specification I<before> the value to format. Where an argument is
6494 specified by an explicit index, this does not affect the normal
6495 order for the arguments, even when the explicitly specified index
6496 would have been the next argument.
6500 printf "<%*.*s>", $a, $b, $c;
6502 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6503 as the value to format; while:
6505 printf "<%*1$.*s>", $a, $b;
6507 would use C<$a> for the width and precision, and C<$b> as the
6510 Here are some more examples; be aware that when using an explicit
6511 index, the C<$> may need escaping:
6513 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6514 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6515 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6516 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6520 If C<use locale> is in effect and POSIX::setlocale() has been called,
6521 the character used for the decimal separator in formatted floating-point
6522 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6526 X<sqrt> X<root> X<square root>
6530 Return the positive square root of EXPR. If EXPR is omitted, uses
6531 C<$_>. Works only for non-negative operands unless you've
6532 loaded the C<Math::Complex> module.
6535 print sqrt(-4); # prints 2i
6538 X<srand> X<seed> X<randseed>
6542 Sets and returns the random number seed for the C<rand> operator.
6544 The point of the function is to "seed" the C<rand> function so that
6545 C<rand> can produce a different sequence each time you run your
6546 program. When called with a parameter, C<srand> uses that for the seed;
6547 otherwise it (semi-)randomly chooses a seed. In either case, starting with
6548 Perl 5.14, it returns the seed.
6550 If C<srand()> is not called explicitly, it is called implicitly without a
6551 parameter at the first use of the C<rand> operator. However, this was not true
6552 of versions of Perl before 5.004, so if your script will run under older
6553 Perl versions, it should call C<srand>; otherwise most programs won't call
6556 But there are a few situations in recent Perls where programs are likely to
6557 want to call C<srand>. One is for generating predictable results generally for
6558 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
6559 each time. Another other case is where you need a cryptographically-strong
6560 starting point rather than the generally acceptable default, which is based on
6561 time of day, process ID, and memory allocation, or the F</dev/urandom> device
6562 if available. And still another case is that you may want to call C<srand()>
6563 after a C<fork()> to avoid child processes sharing the same seed value as the
6564 parent (and consequently each other).
6566 Do B<not> call C<srand()> (i.e., without an argument) more than once per
6567 process. The internal state of the random number generator should
6568 contain more entropy than can be provided by any seed, so calling
6569 C<srand()> again actually I<loses> randomness.
6571 Most implementations of C<srand> take an integer and will silently
6572 truncate decimal numbers. This means C<srand(42)> will usually
6573 produce the same results as C<srand(42.1)>. To be safe, always pass
6574 C<srand> an integer.
6576 In versions of Perl prior to 5.004 the default seed was just the
6577 current C<time>. This isn't a particularly good seed, so many old
6578 programs supply their own seed value (often C<time ^ $$> or C<time ^
6579 ($$ + ($$ << 15))>), but that isn't necessary any more.
6581 For cryptographic purposes, however, you need something much more random
6582 than the default seed. Checksumming the compressed output of one or more
6583 rapidly changing operating system status programs is the usual method. For
6586 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6588 If you're particularly concerned with this, search the CPAN for
6589 random number generator modules instead of rolling out your own.
6591 Frequently called programs (like CGI scripts) that simply use
6595 for a seed can fall prey to the mathematical property that
6599 one-third of the time. So don't do that.
6601 A typical use of the returned seed is for a test program which has too many
6602 combinations to test comprehensively in the time available to it each run. It
6603 can test a random subset each time, and should there be a failure, log the seed
6604 used for that run so that it can later be used to reproduce the same results.
6606 =item stat FILEHANDLE
6607 X<stat> X<file, status> X<ctime>
6611 =item stat DIRHANDLE
6615 Returns a 13-element list giving the status info for a file, either
6616 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6617 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
6620 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6621 $atime,$mtime,$ctime,$blksize,$blocks)
6624 Not all fields are supported on all filesystem types. Here are the
6625 meanings of the fields:
6627 0 dev device number of filesystem
6629 2 mode file mode (type and permissions)
6630 3 nlink number of (hard) links to the file
6631 4 uid numeric user ID of file's owner
6632 5 gid numeric group ID of file's owner
6633 6 rdev the device identifier (special files only)
6634 7 size total size of file, in bytes
6635 8 atime last access time in seconds since the epoch
6636 9 mtime last modify time in seconds since the epoch
6637 10 ctime inode change time in seconds since the epoch (*)
6638 11 blksize preferred block size for file system I/O
6639 12 blocks actual number of blocks allocated
6641 (The epoch was at 00:00 January 1, 1970 GMT.)
6643 (*) Not all fields are supported on all filesystem types. Notably, the
6644 ctime field is non-portable. In particular, you cannot expect it to be a
6645 "creation time"; see L<perlport/"Files and Filesystems"> for details.
6647 If C<stat> is passed the special filehandle consisting of an underline, no
6648 stat is done, but the current contents of the stat structure from the
6649 last C<stat>, C<lstat>, or filetest are returned. Example:
6651 if (-x $file && (($d) = stat(_)) && $d < 0) {
6652 print "$file is executable NFS file\n";
6655 (This works on machines only for which the device number is negative
6658 Because the mode contains both the file type and its permissions, you
6659 should mask off the file type portion and (s)printf using a C<"%o">
6660 if you want to see the real permissions.
6662 $mode = (stat($filename))[2];
6663 printf "Permissions are %04o\n", $mode & 07777;
6665 In scalar context, C<stat> returns a boolean value indicating success
6666 or failure, and, if successful, sets the information associated with
6667 the special filehandle C<_>.
6669 The L<File::stat> module provides a convenient, by-name access mechanism:
6672 $sb = stat($filename);
6673 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6674 $filename, $sb->size, $sb->mode & 07777,
6675 scalar localtime $sb->mtime;
6677 You can import symbolic mode constants (C<S_IF*>) and functions
6678 (C<S_IS*>) from the Fcntl module:
6682 $mode = (stat($filename))[2];
6684 $user_rwx = ($mode & S_IRWXU) >> 6;
6685 $group_read = ($mode & S_IRGRP) >> 3;
6686 $other_execute = $mode & S_IXOTH;
6688 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6690 $is_setuid = $mode & S_ISUID;
6691 $is_directory = S_ISDIR($mode);
6693 You could write the last two using the C<-u> and C<-d> operators.
6694 Commonly available C<S_IF*> constants are:
6696 # Permissions: read, write, execute, for user, group, others.
6698 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6699 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6700 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6702 # Setuid/Setgid/Stickiness/SaveText.
6703 # Note that the exact meaning of these is system-dependent.
6705 S_ISUID S_ISGID S_ISVTX S_ISTXT
6707 # File types. Not all are necessarily available on
6710 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
6711 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6713 # The following are compatibility aliases for S_IRUSR,
6714 # S_IWUSR, and S_IXUSR.
6716 S_IREAD S_IWRITE S_IEXEC
6718 and the C<S_IF*> functions are
6720 S_IMODE($mode) the part of $mode containing the permission
6721 bits and the setuid/setgid/sticky bits
6723 S_IFMT($mode) the part of $mode containing the file type
6724 which can be bit-anded with (for example)
6725 S_IFREG or with the following functions
6727 # The operators -f, -d, -l, -b, -c, -p, and -S.
6729 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6730 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6732 # No direct -X operator counterpart, but for the first one
6733 # the -g operator is often equivalent. The ENFMT stands for
6734 # record flocking enforcement, a platform-dependent feature.
6736 S_ISENFMT($mode) S_ISWHT($mode)
6738 See your native chmod(2) and stat(2) documentation for more details
6739 about the C<S_*> constants. To get status info for a symbolic link
6740 instead of the target file behind the link, use the C<lstat> function.
6742 Portability issues: L<perlport/stat>.
6747 =item state TYPE EXPR
6749 =item state EXPR : ATTRS
6751 =item state TYPE EXPR : ATTRS
6753 C<state> declares a lexically scoped variable, just like C<my>.
6754 However, those variables will never be reinitialized, contrary to
6755 lexical variables that are reinitialized each time their enclosing block
6758 C<state> variables are enabled only when the C<use feature "state"> pragma
6759 is in effect, unless the keyword is written as C<CORE::state>.
6767 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6768 doing many pattern matches on the string before it is next modified.
6769 This may or may not save time, depending on the nature and number of
6770 patterns you are searching and the distribution of character
6771 frequencies in the string to be searched; you probably want to compare
6772 run times with and without it to see which is faster. Those loops
6773 that scan for many short constant strings (including the constant
6774 parts of more complex patterns) will benefit most.
6775 (The way C<study> works is this: a linked list of every
6776 character in the string to be searched is made, so we know, for
6777 example, where all the C<'k'> characters are. From each search string,
6778 the rarest character is selected, based on some static frequency tables
6779 constructed from some C programs and English text. Only those places
6780 that contain this "rarest" character are examined.)
6782 For example, here is a loop that inserts index producing entries
6783 before any line containing a certain pattern:
6787 print ".IX foo\n" if /\bfoo\b/;
6788 print ".IX bar\n" if /\bbar\b/;
6789 print ".IX blurfl\n" if /\bblurfl\b/;
6794 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6795 will be looked at, because C<f> is rarer than C<o>. In general, this is
6796 a big win except in pathological cases. The only question is whether
6797 it saves you more time than it took to build the linked list in the
6800 Note that if you have to look for strings that you don't know till
6801 runtime, you can build an entire loop as a string and C<eval> that to
6802 avoid recompiling all your patterns all the time. Together with
6803 undefining C<$/> to input entire files as one record, this can be quite
6804 fast, often faster than specialized programs like fgrep(1). The following
6805 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6806 out the names of those files that contain a match:
6808 $search = 'while (<>) { study;';
6809 foreach $word (@words) {
6810 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6815 eval $search; # this screams
6816 $/ = "\n"; # put back to normal input delimiter
6817 foreach $file (sort keys(%seen)) {
6821 =item sub NAME BLOCK
6824 =item sub NAME (PROTO) BLOCK
6826 =item sub NAME : ATTRS BLOCK
6828 =item sub NAME (PROTO) : ATTRS BLOCK
6830 This is subroutine definition, not a real function I<per se>. Without a
6831 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
6832 function declaration, so does return a value: the CODE ref of the closure
6835 See L<perlsub> and L<perlref> for details about subroutines and
6836 references; see L<attributes> and L<Attribute::Handlers> for more
6837 information about attributes.
6839 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6840 X<substr> X<substring> X<mid> X<left> X<right>
6842 =item substr EXPR,OFFSET,LENGTH
6844 =item substr EXPR,OFFSET
6846 Extracts a substring out of EXPR and returns it. First character is at
6847 offset C<0> (or whatever you've set C<$[> to (but B<<don't do that>)).
6848 If OFFSET is negative (or more precisely, less than C<$[>), starts
6849 that far back from the end of the string. If LENGTH is omitted, returns
6850 everything through the end of the string. If LENGTH is negative, leaves that
6851 many characters off the end of the string.
6853 my $s = "The black cat climbed the green tree";
6854 my $color = substr $s, 4, 5; # black
6855 my $middle = substr $s, 4, -11; # black cat climbed the
6856 my $end = substr $s, 14; # climbed the green tree
6857 my $tail = substr $s, -4; # tree
6858 my $z = substr $s, -4, 2; # tr
6860 You can use the substr() function as an lvalue, in which case EXPR
6861 must itself be an lvalue. If you assign something shorter than LENGTH,
6862 the string will shrink, and if you assign something longer than LENGTH,
6863 the string will grow to accommodate it. To keep the string the same
6864 length, you may need to pad or chop your value using C<sprintf>.
6866 If OFFSET and LENGTH specify a substring that is partly outside the
6867 string, only the part within the string is returned. If the substring
6868 is beyond either end of the string, substr() returns the undefined
6869 value and produces a warning. When used as an lvalue, specifying a
6870 substring that is entirely outside the string raises an exception.
6871 Here's an example showing the behavior for boundary cases:
6874 substr($name, 4) = 'dy'; # $name is now 'freddy'
6875 my $null = substr $name, 6, 2; # returns "" (no warning)
6876 my $oops = substr $name, 7; # returns undef, with warning
6877 substr($name, 7) = 'gap'; # raises an exception
6879 An alternative to using substr() as an lvalue is to specify the
6880 replacement string as the 4th argument. This allows you to replace
6881 parts of the EXPR and return what was there before in one operation,
6882 just as you can with splice().
6884 my $s = "The black cat climbed the green tree";
6885 my $z = substr $s, 14, 7, "jumped from"; # climbed
6886 # $s is now "The black cat jumped from the green tree"
6888 Note that the lvalue returned by the three-argument version of substr() acts as
6889 a 'magic bullet'; each time it is assigned to, it remembers which part
6890 of the original string is being modified; for example:
6893 for (substr($x,1,2)) {
6894 $_ = 'a'; print $x,"\n"; # prints 1a4
6895 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6897 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6900 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6903 =item symlink OLDFILE,NEWFILE
6904 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6906 Creates a new filename symbolically linked to the old filename.
6907 Returns C<1> for success, C<0> otherwise. On systems that don't support
6908 symbolic links, raises an exception. To check for that,
6911 $symlink_exists = eval { symlink("",""); 1 };
6913 Portability issues: L<perlport/symlink>.
6915 =item syscall NUMBER, LIST
6916 X<syscall> X<system call>
6918 Calls the system call specified as the first element of the list,
6919 passing the remaining elements as arguments to the system call. If
6920 unimplemented, raises an exception. The arguments are interpreted
6921 as follows: if a given argument is numeric, the argument is passed as
6922 an int. If not, the pointer to the string value is passed. You are
6923 responsible to make sure a string is pre-extended long enough to
6924 receive any result that might be written into a string. You can't use a
6925 string literal (or other read-only string) as an argument to C<syscall>
6926 because Perl has to assume that any string pointer might be written
6928 integer arguments are not literals and have never been interpreted in a
6929 numeric context, you may need to add C<0> to them to force them to look
6930 like numbers. This emulates the C<syswrite> function (or vice versa):
6932 require 'syscall.ph'; # may need to run h2ph
6934 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6936 Note that Perl supports passing of up to only 14 arguments to your syscall,
6937 which in practice should (usually) suffice.
6939 Syscall returns whatever value returned by the system call it calls.
6940 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6941 Note that some system calls I<can> legitimately return C<-1>. The proper
6942 way to handle such calls is to assign C<$!=0> before the call, then
6943 check the value of C<$!> if C<syscall> returns C<-1>.
6945 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6946 number of the read end of the pipe it creates, but there is no way
6947 to retrieve the file number of the other end. You can avoid this
6948 problem by using C<pipe> instead.
6950 Portability issues: L<perlport/syscall>.
6952 =item sysopen FILEHANDLE,FILENAME,MODE
6955 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6957 Opens the file whose filename is given by FILENAME, and associates it with
6958 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
6959 filehandle wanted; an undefined scalar will be suitably autovivified. This
6960 function calls the underlying operating system's I<open>(2) function with the
6961 parameters FILENAME, MODE, and PERMS.
6963 The possible values and flag bits of the MODE parameter are
6964 system-dependent; they are available via the standard module C<Fcntl>. See
6965 the documentation of your operating system's I<open>(2) syscall to see
6966 which values and flag bits are available. You may combine several flags
6967 using the C<|>-operator.
6969 Some of the most common values are C<O_RDONLY> for opening the file in
6970 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6971 and C<O_RDWR> for opening the file in read-write mode.
6972 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6974 For historical reasons, some values work on almost every system
6975 supported by Perl: 0 means read-only, 1 means write-only, and 2
6976 means read/write. We know that these values do I<not> work under
6977 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6978 use them in new code.
6980 If the file named by FILENAME does not exist and the C<open> call creates
6981 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6982 PERMS specifies the permissions of the newly created file. If you omit
6983 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6984 These permission values need to be in octal, and are modified by your
6985 process's current C<umask>.
6988 In many systems the C<O_EXCL> flag is available for opening files in
6989 exclusive mode. This is B<not> locking: exclusiveness means here that
6990 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6991 on network filesystems, and has no effect unless the C<O_CREAT> flag
6992 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6993 being opened if it is a symbolic link. It does not protect against
6994 symbolic links in the file's path.
6997 Sometimes you may want to truncate an already-existing file. This
6998 can be done using the C<O_TRUNC> flag. The behavior of
6999 C<O_TRUNC> with C<O_RDONLY> is undefined.
7002 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7003 that takes away the user's option to have a more permissive umask.
7004 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7007 Note that C<sysopen> depends on the fdopen() C library function.
7008 On many Unix systems, fdopen() is known to fail when file descriptors
7009 exceed a certain value, typically 255. If you need more file
7010 descriptors than that, consider rebuilding Perl to use the C<sfio>
7011 library, or perhaps using the POSIX::open() function.
7013 See L<perlopentut> for a kinder, gentler explanation of opening files.
7015 Portability issues: L<perlport/sysopen>.
7017 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7020 =item sysread FILEHANDLE,SCALAR,LENGTH
7022 Attempts to read LENGTH bytes of data into variable SCALAR from the
7023 specified FILEHANDLE, using the read(2). It bypasses
7024 buffered IO, so mixing this with other kinds of reads, C<print>,
7025 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7026 perlio or stdio layers usually buffers data. Returns the number of
7027 bytes actually read, C<0> at end of file, or undef if there was an
7028 error (in the latter case C<$!> is also set). SCALAR will be grown or
7029 shrunk so that the last byte actually read is the last byte of the
7030 scalar after the read.
7032 An OFFSET may be specified to place the read data at some place in the
7033 string other than the beginning. A negative OFFSET specifies
7034 placement at that many characters counting backwards from the end of
7035 the string. A positive OFFSET greater than the length of SCALAR
7036 results in the string being padded to the required size with C<"\0">
7037 bytes before the result of the read is appended.
7039 There is no syseof() function, which is ok, since eof() doesn't work
7040 well on device files (like ttys) anyway. Use sysread() and check
7041 for a return value for 0 to decide whether you're done.
7043 Note that if the filehandle has been marked as C<:utf8> Unicode
7044 characters are read instead of bytes (the LENGTH, OFFSET, and the
7045 return value of sysread() are in Unicode characters).
7046 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7047 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7049 =item sysseek FILEHANDLE,POSITION,WHENCE
7052 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7053 be an expression whose value gives the name of the filehandle. The values
7054 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7055 to the current position plus POSITION; and C<2> to set it to EOF plus
7056 POSITION, typically negative.
7058 Note the I<in bytes>: even if the filehandle has been set to operate
7059 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7060 tell() will return byte offsets, not character offsets (because
7061 implementing that would render sysseek() unacceptably slow).
7063 sysseek() bypasses normal buffered IO, so mixing it with reads other
7064 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7065 C<seek>, C<tell>, or C<eof> may cause confusion.
7067 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7068 and C<SEEK_END> (start of the file, current position, end of the file)
7069 from the Fcntl module. Use of the constants is also more portable
7070 than relying on 0, 1, and 2. For example to define a "systell" function:
7072 use Fcntl 'SEEK_CUR';
7073 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7075 Returns the new position, or the undefined value on failure. A position
7076 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7077 true on success and false on failure, yet you can still easily determine
7083 =item system PROGRAM LIST
7085 Does exactly the same thing as C<exec LIST>, except that a fork is
7086 done first and the parent process waits for the child process to
7087 exit. Note that argument processing varies depending on the
7088 number of arguments. If there is more than one argument in LIST,
7089 or if LIST is an array with more than one value, starts the program
7090 given by the first element of the list with arguments given by the
7091 rest of the list. If there is only one scalar argument, the argument
7092 is checked for shell metacharacters, and if there are any, the
7093 entire argument is passed to the system's command shell for parsing
7094 (this is C</bin/sh -c> on Unix platforms, but varies on other
7095 platforms). If there are no shell metacharacters in the argument,
7096 it is split into words and passed directly to C<execvp>, which is
7099 Beginning with v5.6.0, Perl will attempt to flush all files opened for
7100 output before any operation that may do a fork, but this may not be
7101 supported on some platforms (see L<perlport>). To be safe, you may need
7102 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7103 of C<IO::Handle> on any open handles.
7105 The return value is the exit status of the program as returned by the
7106 C<wait> call. To get the actual exit value, shift right by eight (see
7107 below). See also L</exec>. This is I<not> what you want to use to capture
7108 the output from a command; for that you should use merely backticks or
7109 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7110 indicates a failure to start the program or an error of the wait(2) system
7111 call (inspect $! for the reason).
7113 If you'd like to make C<system> (and many other bits of Perl) die on error,
7114 have a look at the L<autodie> pragma.
7116 Like C<exec>, C<system> allows you to lie to a program about its name if
7117 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
7119 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
7120 C<system>, if you expect your program to terminate on receipt of these
7121 signals you will need to arrange to do so yourself based on the return
7124 @args = ("command", "arg1", "arg2");
7126 or die "system @args failed: $?"
7128 If you'd like to manually inspect C<system>'s failure, you can check all
7129 possible failure modes by inspecting C<$?> like this:
7132 print "failed to execute: $!\n";
7135 printf "child died with signal %d, %s coredump\n",
7136 ($? & 127), ($? & 128) ? 'with' : 'without';
7139 printf "child exited with value %d\n", $? >> 8;
7142 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
7143 with the C<W*()> calls from the POSIX module.
7145 When C<system>'s arguments are executed indirectly by the shell,
7146 results and return codes are subject to its quirks.
7147 See L<perlop/"`STRING`"> and L</exec> for details.
7149 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
7150 handler. See L<perlipc> for details.
7152 Portability issues: L<perlport/system>.
7154 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7157 =item syswrite FILEHANDLE,SCALAR,LENGTH
7159 =item syswrite FILEHANDLE,SCALAR
7161 Attempts to write LENGTH bytes of data from variable SCALAR to the
7162 specified FILEHANDLE, using write(2). If LENGTH is
7163 not specified, writes whole SCALAR. It bypasses buffered IO, so
7164 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
7165 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
7166 stdio layers usually buffer data. Returns the number of bytes
7167 actually written, or C<undef> if there was an error (in this case the
7168 errno variable C<$!> is also set). If the LENGTH is greater than the
7169 data available in the SCALAR after the OFFSET, only as much data as is
7170 available will be written.
7172 An OFFSET may be specified to write the data from some part of the
7173 string other than the beginning. A negative OFFSET specifies writing
7174 that many characters counting backwards from the end of the string.
7175 If SCALAR is of length zero, you can only use an OFFSET of 0.
7177 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
7178 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
7179 return value of syswrite() are in (UTF8-encoded Unicode) characters.
7180 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7181 Alternately, if the handle is not marked with an encoding but you
7182 attempt to write characters with code points over 255, raises an exception.
7183 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7185 =item tell FILEHANDLE
7190 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
7191 error. FILEHANDLE may be an expression whose value gives the name of
7192 the actual filehandle. If FILEHANDLE is omitted, assumes the file
7195 Note the I<in bytes>: even if the filehandle has been set to
7196 operate on characters (for example by using the C<:encoding(utf8)> open
7197 layer), tell() will return byte offsets, not character offsets (because
7198 that would render seek() and tell() rather slow).
7200 The return value of tell() for the standard streams like the STDIN
7201 depends on the operating system: it may return -1 or something else.
7202 tell() on pipes, fifos, and sockets usually returns -1.
7204 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
7206 Do not use tell() (or other buffered I/O operations) on a filehandle
7207 that has been manipulated by sysread(), syswrite(), or sysseek().
7208 Those functions ignore the buffering, while tell() does not.
7210 =item telldir DIRHANDLE
7213 Returns the current position of the C<readdir> routines on DIRHANDLE.
7214 Value may be given to C<seekdir> to access a particular location in a
7215 directory. C<telldir> has the same caveats about possible directory
7216 compaction as the corresponding system library routine.
7218 =item tie VARIABLE,CLASSNAME,LIST
7221 This function binds a variable to a package class that will provide the
7222 implementation for the variable. VARIABLE is the name of the variable
7223 to be enchanted. CLASSNAME is the name of a class implementing objects
7224 of correct type. Any additional arguments are passed to the C<new>
7225 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
7226 or C<TIEHASH>). Typically these are arguments such as might be passed
7227 to the C<dbm_open()> function of C. The object returned by the C<new>
7228 method is also returned by the C<tie> function, which would be useful
7229 if you want to access other methods in CLASSNAME.
7231 Note that functions such as C<keys> and C<values> may return huge lists
7232 when used on large objects, like DBM files. You may prefer to use the
7233 C<each> function to iterate over such. Example:
7235 # print out history file offsets
7237 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
7238 while (($key,$val) = each %HIST) {
7239 print $key, ' = ', unpack('L',$val), "\n";
7243 A class implementing a hash should have the following methods:
7245 TIEHASH classname, LIST
7247 STORE this, key, value
7252 NEXTKEY this, lastkey
7257 A class implementing an ordinary array should have the following methods:
7259 TIEARRAY classname, LIST
7261 STORE this, key, value
7263 STORESIZE this, count
7269 SPLICE this, offset, length, LIST
7274 A class implementing a filehandle should have the following methods:
7276 TIEHANDLE classname, LIST
7277 READ this, scalar, length, offset
7280 WRITE this, scalar, length, offset
7282 PRINTF this, format, LIST
7286 SEEK this, position, whence
7288 OPEN this, mode, LIST
7293 A class implementing a scalar should have the following methods:
7295 TIESCALAR classname, LIST
7301 Not all methods indicated above need be implemented. See L<perltie>,
7302 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
7304 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
7305 for you; you need to do that explicitly yourself. See L<DB_File>
7306 or the F<Config> module for interesting C<tie> implementations.
7308 For further details see L<perltie>, L<"tied VARIABLE">.
7313 Returns a reference to the object underlying VARIABLE (the same value
7314 that was originally returned by the C<tie> call that bound the variable
7315 to a package.) Returns the undefined value if VARIABLE isn't tied to a
7321 Returns the number of non-leap seconds since whatever time the system
7322 considers to be the epoch, suitable for feeding to C<gmtime> and
7323 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
7324 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
7325 1904 in the current local time zone for its epoch.
7327 For measuring time in better granularity than one second, use the
7328 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
7329 if you have gettimeofday(2), you may be able to use the C<syscall>
7330 interface of Perl. See L<perlfaq8> for details.
7332 For date and time processing look at the many related modules on CPAN.
7333 For a comprehensive date and time representation look at the
7339 Returns a four-element list giving the user and system times in
7340 seconds for this process and any exited children of this process.
7342 ($user,$system,$cuser,$csystem) = times;
7344 In scalar context, C<times> returns C<$user>.
7346 Children's times are only included for terminated children.
7348 Portability issues: L<perlport/times>.
7352 The transliteration operator. Same as C<y///>. See
7353 L<perlop/"Quote and Quote-like Operators">.
7355 =item truncate FILEHANDLE,LENGTH
7358 =item truncate EXPR,LENGTH
7360 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
7361 specified length. Raises an exception if truncate isn't implemented
7362 on your system. Returns true if successful, C<undef> on error.
7364 The behavior is undefined if LENGTH is greater than the length of the
7367 The position in the file of FILEHANDLE is left unchanged. You may want to
7368 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
7370 Portability issues: L<perlport/truncate>.
7373 X<uc> X<uppercase> X<toupper>
7377 Returns an uppercased version of EXPR. This is the internal function
7378 implementing the C<\U> escape in double-quoted strings.
7379 It does not attempt to do titlecase mapping on initial letters. See
7380 L</ucfirst> for that.
7382 If EXPR is omitted, uses C<$_>.
7384 This function behaves the same way under various pragma, such as in a locale,
7388 X<ucfirst> X<uppercase>
7392 Returns the value of EXPR with the first character in uppercase
7393 (titlecase in Unicode). This is the internal function implementing
7394 the C<\u> escape in double-quoted strings.
7396 If EXPR is omitted, uses C<$_>.
7398 This function behaves the same way under various pragma, such as in a locale,
7406 Sets the umask for the process to EXPR and returns the previous value.
7407 If EXPR is omitted, merely returns the current umask.
7409 The Unix permission C<rwxr-x---> is represented as three sets of three
7410 bits, or three octal digits: C<0750> (the leading 0 indicates octal
7411 and isn't one of the digits). The C<umask> value is such a number
7412 representing disabled permissions bits. The permission (or "mode")
7413 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
7414 even if you tell C<sysopen> to create a file with permissions C<0777>,
7415 if your umask is C<0022>, then the file will actually be created with
7416 permissions C<0755>. If your C<umask> were C<0027> (group can't
7417 write; others can't read, write, or execute), then passing
7418 C<sysopen> C<0666> would create a file with mode C<0640> (because
7419 C<0666 &~ 027> is C<0640>).
7421 Here's some advice: supply a creation mode of C<0666> for regular
7422 files (in C<sysopen>) and one of C<0777> for directories (in
7423 C<mkdir>) and executable files. This gives users the freedom of
7424 choice: if they want protected files, they might choose process umasks
7425 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
7426 Programs should rarely if ever make policy decisions better left to
7427 the user. The exception to this is when writing files that should be
7428 kept private: mail files, web browser cookies, I<.rhosts> files, and
7431 If umask(2) is not implemented on your system and you are trying to
7432 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
7433 raises an exception. If umask(2) is not implemented and you are
7434 not trying to restrict access for yourself, returns C<undef>.
7436 Remember that a umask is a number, usually given in octal; it is I<not> a
7437 string of octal digits. See also L</oct>, if all you have is a string.
7439 Portability issues: L<perlport/umask>.
7442 X<undef> X<undefine>
7446 Undefines the value of EXPR, which must be an lvalue. Use only on a
7447 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
7448 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
7449 will probably not do what you expect on most predefined variables or
7450 DBM list values, so don't do that; see L</delete>. Always returns the
7451 undefined value. You can omit the EXPR, in which case nothing is
7452 undefined, but you still get an undefined value that you could, for
7453 instance, return from a subroutine, assign to a variable, or pass as a
7454 parameter. Examples:
7457 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
7461 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
7462 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7463 select undef, undef, undef, 0.25;
7464 ($a, $b, undef, $c) = &foo; # Ignore third value returned
7466 Note that this is a unary operator, not a list operator.
7469 X<unlink> X<delete> X<remove> X<rm> X<del>
7473 Deletes a list of files. On success, it returns the number of files
7474 it successfully deleted. On failure, it returns false and sets C<$!>
7477 my $unlinked = unlink 'a', 'b', 'c';
7479 unlink glob "*.bak";
7481 On error, C<unlink> will not tell you which files it could not remove.
7482 If you want to know which files you could not remove, try them one
7485 foreach my $file ( @goners ) {
7486 unlink $file or warn "Could not unlink $file: $!";
7489 Note: C<unlink> will not attempt to delete directories unless you are
7490 superuser and the B<-U> flag is supplied to Perl. Even if these
7491 conditions are met, be warned that unlinking a directory can inflict
7492 damage on your filesystem. Finally, using C<unlink> on directories is
7493 not supported on many operating systems. Use C<rmdir> instead.
7495 If LIST is omitted, C<unlink> uses C<$_>.
7497 =item unpack TEMPLATE,EXPR
7500 =item unpack TEMPLATE
7502 C<unpack> does the reverse of C<pack>: it takes a string
7503 and expands it out into a list of values.
7504 (In scalar context, it returns merely the first value produced.)
7506 If EXPR is omitted, unpacks the C<$_> string.
7507 See L<perlpacktut> for an introduction to this function.
7509 The string is broken into chunks described by the TEMPLATE. Each chunk
7510 is converted separately to a value. Typically, either the string is a result
7511 of C<pack>, or the characters of the string represent a C structure of some
7514 The TEMPLATE has the same format as in the C<pack> function.
7515 Here's a subroutine that does substring:
7518 my($what,$where,$howmuch) = @_;
7519 unpack("x$where a$howmuch", $what);
7524 sub ordinal { unpack("W",$_[0]); } # same as ord()
7526 In addition to fields allowed in pack(), you may prefix a field with
7527 a %<number> to indicate that
7528 you want a <number>-bit checksum of the items instead of the items
7529 themselves. Default is a 16-bit checksum. Checksum is calculated by
7530 summing numeric values of expanded values (for string fields the sum of
7531 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
7533 For example, the following
7534 computes the same number as the System V sum program:
7538 unpack("%32W*",<>) % 65535;
7541 The following efficiently counts the number of set bits in a bit vector:
7543 $setbits = unpack("%32b*", $selectmask);
7545 The C<p> and C<P> formats should be used with care. Since Perl
7546 has no way of checking whether the value passed to C<unpack()>
7547 corresponds to a valid memory location, passing a pointer value that's
7548 not known to be valid is likely to have disastrous consequences.
7550 If there are more pack codes or if the repeat count of a field or a group
7551 is larger than what the remainder of the input string allows, the result
7552 is not well defined: the repeat count may be decreased, or
7553 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7554 If the input string is longer than one described by the TEMPLATE,
7555 the remainder of that input string is ignored.
7557 See L</pack> for more examples and notes.
7559 =item untie VARIABLE
7562 Breaks the binding between a variable and a package.
7563 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
7564 Has no effect if the variable is not tied.
7566 =item unshift ARRAY,LIST
7569 =item unshift EXPR,LIST
7571 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7572 depending on how you look at it. Prepends list to the front of the
7573 array and returns the new number of elements in the array.
7575 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7577 Note the LIST is prepended whole, not one element at a time, so the
7578 prepended elements stay in the same order. Use C<reverse> to do the
7581 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
7582 a reference to an unblessed array. The argument will be dereferenced
7583 automatically. This aspect of C<unshift> is considered highly
7584 experimental. The exact behaviour may change in a future version of Perl.
7586 =item use Module VERSION LIST
7587 X<use> X<module> X<import>
7589 =item use Module VERSION
7591 =item use Module LIST
7597 Imports some semantics into the current package from the named module,
7598 generally by aliasing certain subroutine or variable names into your
7599 package. It is exactly equivalent to
7601 BEGIN { require Module; Module->import( LIST ); }
7603 except that Module I<must> be a bareword.
7604 The importation can be made conditional; see L<if>.
7606 In the peculiar C<use VERSION> form, VERSION may be either a positive
7607 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7608 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7609 exception is raised if VERSION is greater than the version of the
7610 current Perl interpreter; Perl will not attempt to parse the rest of the
7611 file. Compare with L</require>, which can do a similar check at run time.
7612 Symmetrically, C<no VERSION> allows you to specify that you want a version
7613 of Perl older than the specified one.
7615 Specifying VERSION as a literal of the form v5.6.1 should generally be
7616 avoided, because it leads to misleading error messages under earlier
7617 versions of Perl (that is, prior to 5.6.0) that do not support this
7618 syntax. The equivalent numeric version should be used instead.
7620 use v5.6.1; # compile time version check
7622 use 5.006_001; # ditto; preferred for backwards compatibility
7624 This is often useful if you need to check the current Perl version before
7625 C<use>ing library modules that won't work with older versions of Perl.
7626 (We try not to do this more than we have to.)
7628 Also, if the specified Perl version is greater than or equal to 5.9.5,
7629 C<use VERSION> will also load the C<feature> pragma and enable all
7630 features available in the requested version. See L<feature>.
7631 Similarly, if the specified Perl version is greater than or equal to
7632 5.11.0, strictures are enabled lexically as with C<use strict> (except
7633 that the F<strict.pm> file is not actually loaded).
7635 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7636 C<require> makes sure the module is loaded into memory if it hasn't been
7637 yet. The C<import> is not a builtin; it's just an ordinary static method
7638 call into the C<Module> package to tell the module to import the list of
7639 features back into the current package. The module can implement its
7640 C<import> method any way it likes, though most modules just choose to
7641 derive their C<import> method via inheritance from the C<Exporter> class that
7642 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7643 method can be found then the call is skipped, even if there is an AUTOLOAD
7646 If you do not want to call the package's C<import> method (for instance,
7647 to stop your namespace from being altered), explicitly supply the empty list:
7651 That is exactly equivalent to
7653 BEGIN { require Module }
7655 If the VERSION argument is present between Module and LIST, then the
7656 C<use> will call the VERSION method in class Module with the given
7657 version as an argument. The default VERSION method, inherited from
7658 the UNIVERSAL class, croaks if the given version is larger than the
7659 value of the variable C<$Module::VERSION>.
7661 Again, there is a distinction between omitting LIST (C<import> called
7662 with no arguments) and an explicit empty LIST C<()> (C<import> not
7663 called). Note that there is no comma after VERSION!
7665 Because this is a wide-open interface, pragmas (compiler directives)
7666 are also implemented this way. Currently implemented pragmas are:
7671 use sigtrap qw(SEGV BUS);
7672 use strict qw(subs vars refs);
7673 use subs qw(afunc blurfl);
7674 use warnings qw(all);
7675 use sort qw(stable _quicksort _mergesort);
7677 Some of these pseudo-modules import semantics into the current
7678 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7679 which import symbols into the current package (which are effective
7680 through the end of the file).
7682 Because C<use> takes effect at compile time, it doesn't respect the
7683 ordinary flow control of the code being compiled. In particular, putting
7684 a C<use> inside the false branch of a conditional doesn't prevent it
7685 from being processed. If a module or pragma only needs to be loaded
7686 conditionally, this can be done using the L<if> pragma:
7688 use if $] < 5.008, "utf8";
7689 use if WANT_WARNINGS, warnings => qw(all);
7691 There's a corresponding C<no> declaration that unimports meanings imported
7692 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7693 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7694 or no unimport method being found.
7700 Care should be taken when using the C<no VERSION> form of C<no>. It is
7701 I<only> meant to be used to assert that the running Perl is of a earlier
7702 version than its argument and I<not> to undo the feature-enabling side effects
7705 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7706 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7707 functionality from the command-line.
7712 Changes the access and modification times on each file of a list of
7713 files. The first two elements of the list must be the NUMERIC access
7714 and modification times, in that order. Returns the number of files
7715 successfully changed. The inode change time of each file is set
7716 to the current time. For example, this code has the same effect as the
7717 Unix touch(1) command when the files I<already exist> and belong to
7718 the user running the program:
7721 $atime = $mtime = time;
7722 utime $atime, $mtime, @ARGV;
7724 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7725 the utime(2) syscall from your C library is called with a null second
7726 argument. On most systems, this will set the file's access and
7727 modification times to the current time (i.e., equivalent to the example
7728 above) and will work even on files you don't own provided you have write
7732 utime(undef, undef, $file)
7733 || warn "couldn't touch $file: $!";
7736 Under NFS this will use the time of the NFS server, not the time of
7737 the local machine. If there is a time synchronization problem, the
7738 NFS server and local machine will have different times. The Unix
7739 touch(1) command will in fact normally use this form instead of the
7740 one shown in the first example.
7742 Passing only one of the first two elements as C<undef> is
7743 equivalent to passing a 0 and will not have the effect
7744 described when both are C<undef>. This also triggers an
7745 uninitialized warning.
7747 On systems that support futimes(2), you may pass filehandles among the
7748 files. On systems that don't support futimes(2), passing filehandles raises
7749 an exception. Filehandles must be passed as globs or glob references to be
7750 recognized; barewords are considered filenames.
7752 Portability issues: L<perlport/utime>.
7761 Returns a list consisting of all the values of the named hash, or the values
7762 of an array. (In scalar context, returns the number of values.)
7764 The values are returned in an apparently random order. The actual
7765 random order is subject to change in future versions of Perl, but it
7766 is guaranteed to be the same order as either the C<keys> or C<each>
7767 function would produce on the same (unmodified) hash. Since Perl
7768 5.8.1 the ordering is different even between different runs of Perl
7769 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7771 As a side effect, calling values() resets the HASH or ARRAY's internal
7773 see L</each>. (In particular, calling values() in void context resets
7774 the iterator with no other overhead. Apart from resetting the iterator,
7775 C<values @array> in list context is the same as plain C<@array>.
7776 We recommend that you use void context C<keys @array> for this, but reasoned
7777 that it taking C<values @array> out would require more documentation than
7780 Note that the values are not copied, which means modifying them will
7781 modify the contents of the hash:
7783 for (values %hash) { s/foo/bar/g } # modifies %hash values
7784 for (@hash{keys %hash}) { s/foo/bar/g } # same
7786 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
7787 a reference to an unblessed hash or array. The argument will be
7788 dereferenced automatically. This aspect of C<values> is considered highly
7789 experimental. The exact behaviour may change in a future version of Perl.
7791 for (values $hashref) { ... }
7792 for (values $obj->get_arrayref) { ... }
7794 See also C<keys>, C<each>, and C<sort>.
7796 =item vec EXPR,OFFSET,BITS
7797 X<vec> X<bit> X<bit vector>
7799 Treats the string in EXPR as a bit vector made up of elements of
7800 width BITS and returns the value of the element specified by OFFSET
7801 as an unsigned integer. BITS therefore specifies the number of bits
7802 that are reserved for each element in the bit vector. This must
7803 be a power of two from 1 to 32 (or 64, if your platform supports
7806 If BITS is 8, "elements" coincide with bytes of the input string.
7808 If BITS is 16 or more, bytes of the input string are grouped into chunks
7809 of size BITS/8, and each group is converted to a number as with
7810 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7811 for BITS==64). See L<"pack"> for details.
7813 If bits is 4 or less, the string is broken into bytes, then the bits
7814 of each byte are broken into 8/BITS groups. Bits of a byte are
7815 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7816 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7817 breaking the single input byte C<chr(0x36)> into two groups gives a list
7818 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7820 C<vec> may also be assigned to, in which case parentheses are needed
7821 to give the expression the correct precedence as in
7823 vec($image, $max_x * $x + $y, 8) = 3;
7825 If the selected element is outside the string, the value 0 is returned.
7826 If an element off the end of the string is written to, Perl will first
7827 extend the string with sufficiently many zero bytes. It is an error
7828 to try to write off the beginning of the string (i.e., negative OFFSET).
7830 If the string happens to be encoded as UTF-8 internally (and thus has
7831 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7832 internal byte string, not the conceptual character string, even if you
7833 only have characters with values less than 256.
7835 Strings created with C<vec> can also be manipulated with the logical
7836 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7837 vector operation is desired when both operands are strings.
7838 See L<perlop/"Bitwise String Operators">.
7840 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7841 The comments show the string after each step. Note that this code works
7842 in the same way on big-endian or little-endian machines.
7845 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7847 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7848 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7850 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7851 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7852 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7853 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7854 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7855 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7857 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7858 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7859 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7862 To transform a bit vector into a string or list of 0's and 1's, use these:
7864 $bits = unpack("b*", $vector);
7865 @bits = split(//, unpack("b*", $vector));
7867 If you know the exact length in bits, it can be used in place of the C<*>.
7869 Here is an example to illustrate how the bits actually fall in place:
7875 unpack("V",$_) 01234567890123456789012345678901
7876 ------------------------------------------------------------------
7881 for ($shift=0; $shift < $width; ++$shift) {
7882 for ($off=0; $off < 32/$width; ++$off) {
7883 $str = pack("B*", "0"x32);
7884 $bits = (1<<$shift);
7885 vec($str, $off, $width) = $bits;
7886 $res = unpack("b*",$str);
7887 $val = unpack("V", $str);
7894 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7895 $off, $width, $bits, $val, $res
7899 Regardless of the machine architecture on which it runs, the
7900 example above should print the following table:
7903 unpack("V",$_) 01234567890123456789012345678901
7904 ------------------------------------------------------------------
7905 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7906 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7907 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7908 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7909 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7910 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7911 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7912 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7913 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7914 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7915 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7916 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7917 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7918 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7919 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7920 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7921 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7922 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7923 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7924 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7925 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7926 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7927 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7928 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7929 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7930 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7931 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7932 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7933 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7934 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7935 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7936 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7937 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7938 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7939 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7940 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7941 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7942 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7943 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7944 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7945 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7946 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7947 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7948 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7949 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7950 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7951 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7952 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7953 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7954 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7955 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7956 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7957 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7958 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7959 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7960 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7961 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7962 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7963 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7964 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7965 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7966 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7967 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7968 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7969 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7970 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7971 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7972 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7973 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7974 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7975 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7976 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7977 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7978 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7979 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7980 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7981 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7982 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7983 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7984 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7985 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7986 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7987 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7988 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7989 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7990 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7991 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7992 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7993 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7994 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7995 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7996 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7997 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7998 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7999 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8000 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8001 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8002 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8003 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8004 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8005 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8006 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8007 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8008 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8009 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8010 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8011 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8012 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8013 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8014 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8015 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8016 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8017 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8018 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8019 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8020 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8021 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8022 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8023 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8024 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8025 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8026 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8027 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8028 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8029 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8030 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8031 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8032 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8037 Behaves like wait(2) on your system: it waits for a child
8038 process to terminate and returns the pid of the deceased process, or
8039 C<-1> if there are no child processes. The status is returned in C<$?>
8040 and C<${^CHILD_ERROR_NATIVE}>.
8041 Note that a return value of C<-1> could mean that child processes are
8042 being automatically reaped, as described in L<perlipc>.
8044 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8045 child created by qx() or system(). See L<perlipc> for details.
8047 Portability issues: L<perlport/wait>.
8049 =item waitpid PID,FLAGS
8052 Waits for a particular child process to terminate and returns the pid of
8053 the deceased process, or C<-1> if there is no such child process. On some
8054 systems, a value of 0 indicates that there are processes still running.
8055 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
8057 use POSIX ":sys_wait_h";
8060 $kid = waitpid(-1, WNOHANG);
8063 then you can do a non-blocking wait for all pending zombie processes.
8064 Non-blocking wait is available on machines supporting either the
8065 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
8066 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
8067 system call by remembering the status values of processes that have
8068 exited but have not been harvested by the Perl script yet.)
8070 Note that on some systems, a return value of C<-1> could mean that child
8071 processes are being automatically reaped. See L<perlipc> for details,
8072 and for other examples.
8074 Portability issues: L<perlport/waitpid>.
8077 X<wantarray> X<context>
8079 Returns true if the context of the currently executing subroutine or
8080 C<eval> is looking for a list value. Returns false if the context is
8081 looking for a scalar. Returns the undefined value if the context is
8082 looking for no value (void context).
8084 return unless defined wantarray; # don't bother doing more
8085 my @a = complex_calculation();
8086 return wantarray ? @a : "@a";
8088 C<wantarray()>'s result is unspecified in the top level of a file,
8089 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
8090 in a C<DESTROY> method.
8092 This function should have been named wantlist() instead.
8095 X<warn> X<warning> X<STDERR>
8097 Prints the value of LIST to STDERR. If the last element of LIST does
8098 not end in a newline, it appends the same file/line number text as C<die>
8101 If the output is empty and C<$@> already contains a value (typically from a
8102 previous eval) that value is used after appending C<"\t...caught">
8103 to C<$@>. This is useful for staying almost, but not entirely similar to
8106 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
8108 No message is printed if there is a C<$SIG{__WARN__}> handler
8109 installed. It is the handler's responsibility to deal with the message
8110 as it sees fit (like, for instance, converting it into a C<die>). Most
8111 handlers must therefore arrange to actually display the
8112 warnings that they are not prepared to deal with, by calling C<warn>
8113 again in the handler. Note that this is quite safe and will not
8114 produce an endless loop, since C<__WARN__> hooks are not called from
8117 You will find this behavior is slightly different from that of
8118 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
8119 instead call C<die> again to change it).
8121 Using a C<__WARN__> handler provides a powerful way to silence all
8122 warnings (even the so-called mandatory ones). An example:
8124 # wipe out *all* compile-time warnings
8125 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
8127 my $foo = 20; # no warning about duplicate my $foo,
8128 # but hey, you asked for it!
8129 # no compile-time or run-time warnings before here
8132 # run-time warnings enabled after here
8133 warn "\$foo is alive and $foo!"; # does show up
8135 See L<perlvar> for details on setting C<%SIG> entries and for more
8136 examples. See the Carp module for other kinds of warnings using its
8137 carp() and cluck() functions.
8139 =item when EXPR BLOCK
8144 C<when> is analogous to the C<case> keyword in other languages. Used with a
8145 C<foreach> loop or the experimental C<given> block, C<when> can be used in
8146 Perl to implement C<switch>/C<case> like statements. Available as a
8147 statement after Perl 5.10 and as a statement modifier after 5.14.
8148 Here are three examples:
8153 say "I like apples."
8156 say "I don't like oranges."
8159 say "I don't like anything"
8163 # require 5.14 for when as statement modifier
8166 say "I like apples." when /apples?/;
8167 say "I don't like oranges." when /oranges?;
8168 default { say "I don't like anything" }
8174 say "I like apples."
8177 say "I don't like oranges."
8180 say "I don't like anything"
8184 See L<perlsyn/"Switch statements"> for detailed information.
8186 =item write FILEHANDLE
8193 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
8194 using the format associated with that file. By default the format for
8195 a file is the one having the same name as the filehandle, but the
8196 format for the current output channel (see the C<select> function) may be set
8197 explicitly by assigning the name of the format to the C<$~> variable.
8199 Top of form processing is handled automatically: if there is insufficient
8200 room on the current page for the formatted record, the page is advanced by
8201 writing a form feed, a special top-of-page format is used to format the new
8202 page header before the record is written. By default, the top-of-page
8203 format is the name of the filehandle with "_TOP" appended. This would be a
8204 problem with autovivified filehandles, but it may be dynamically set to the
8205 format of your choice by assigning the name to the C<$^> variable while
8206 that filehandle is selected. The number of lines remaining on the current
8207 page is in variable C<$->, which can be set to C<0> to force a new page.
8209 If FILEHANDLE is unspecified, output goes to the current default output
8210 channel, which starts out as STDOUT but may be changed by the
8211 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
8212 is evaluated and the resulting string is used to look up the name of
8213 the FILEHANDLE at run time. For more on formats, see L<perlform>.
8215 Note that write is I<not> the opposite of C<read>. Unfortunately.
8219 The transliteration operator. Same as C<tr///>. See
8220 L<perlop/"Quote and Quote-like Operators">.