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 =for Pod::Functions =String
112 C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113 C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
114 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
116 C<fc> is available only if the C<"fc"> feature is enabled or if it is
117 prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
118 with a C<use v5.16> (or higher) declaration in the current scope.
121 =item Regular expressions and pattern matching
122 X<regular expression> X<regex> X<regexp>
124 =for Pod::Functions =Regexp
126 C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
128 =item Numeric functions
129 X<numeric> X<number> X<trigonometric> X<trigonometry>
131 =for Pod::Functions =Math
133 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134 C<sin>, C<sqrt>, C<srand>
136 =item Functions for real @ARRAYs
139 =for Pod::Functions =ARRAY
141 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
143 =item Functions for list data
146 =for Pod::Functions =LIST
148 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
150 =item Functions for real %HASHes
153 =for Pod::Functions =HASH
155 C<delete>, C<each>, C<exists>, C<keys>, C<values>
157 =item Input and output functions
158 X<I/O> X<input> X<output> X<dbm>
160 =for Pod::Functions =I/O
162 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
164 C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165 C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166 C<truncate>, C<warn>, C<write>
168 C<say> is available only if the C<"say"> feature is enabled or if it is
169 prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170 with a C<use v5.10> (or higher) declaration in the current scope.
172 =item Functions for fixed-length data or records
174 =for Pod::Functions =Binary
176 C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
179 =item Functions for filehandles, files, or directories
180 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
182 =for Pod::Functions =File
184 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
185 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
186 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187 C<umask>, C<unlink>, C<utime>
189 =item Keywords related to the control flow of your Perl program
192 =for Pod::Functions =Flow
194 C<break>, C<caller>, C<continue>, C<die>, C<do>,
195 C<dump>, C<eval>, C<evalbytes> C<exit>,
196 C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
197 C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
199 C<break> is available only if you enable the experimental C<"switch">
200 feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201 the C<default>, C<given> and C<when> statements, which are documented in
202 L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203 automatically with a C<use v5.10> (or higher) declaration in the current
204 scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205 feature, like the other keywords.
207 C<evalbytes> is only available with the C<"evalbytes"> feature (see
208 L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
209 with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
210 the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211 with a C<use v5.16> (or higher) declaration in the current scope.
213 =item Keywords related to scoping
215 =for Pod::Functions =Namespace
217 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
219 C<state> is available only if the C<"state"> feature is enabled or if it is
220 prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221 with a C<use v5.10> (or higher) declaration in the current scope.
223 =item Miscellaneous functions
225 =for Pod::Functions =Misc
227 C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
229 =item Functions for processes and process groups
230 X<process> X<pid> X<process id>
232 =for Pod::Functions =Process
234 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
235 C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236 C<setpriority>, C<sleep>, C<system>,
237 C<times>, C<wait>, C<waitpid>
239 =item Keywords related to Perl modules
242 =for Pod::Functions =Modules
244 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
246 =item Keywords related to classes and object-orientation
247 X<object> X<class> X<package>
249 =for Pod::Functions =Objects
251 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
254 =item Low-level socket functions
257 =for Pod::Functions =Socket
259 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
261 C<socket>, C<socketpair>
263 =item System V interprocess communication functions
264 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
266 =for Pod::Functions =SysV
268 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
271 =item Fetching user and group info
272 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
274 =for Pod::Functions =User
276 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278 C<getpwuid>, C<setgrent>, C<setpwent>
280 =item Fetching network info
281 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
283 =for Pod::Functions =Network
285 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289 C<setnetent>, C<setprotoent>, C<setservent>
291 =item Time-related functions
294 =for Pod::Functions =Time
296 C<gmtime>, C<localtime>, C<time>, C<times>
298 =item Non-function keywords
300 =for Pod::Functions =!Non-functions
302 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
303 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306 C<while>, C<x>, C<xor>
311 X<portability> X<Unix> X<portable>
313 Perl was born in Unix and can therefore access all common Unix
314 system calls. In non-Unix environments, the functionality of some
315 Unix system calls may not be available or details of the available
316 functionality may differ slightly. The Perl functions affected
319 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
322 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
324 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
325 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
328 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
329 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
332 C<shmwrite>, C<socket>, C<socketpair>,
333 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334 C<times>, C<truncate>, C<umask>, C<unlink>,
335 C<utime>, C<wait>, C<waitpid>
337 For more information about the portability of these functions, see
338 L<perlport> and other available platform-specific documentation.
340 =head2 Alphabetical Listing of Perl Functions
345 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>
346 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
354 =for Pod::Functions a file test (-r, -x, etc)
356 A file test, where X is one of the letters listed below. This unary
357 operator takes one argument, either a filename, a filehandle, or a dirhandle,
358 and tests the associated file to see if something is true about it. If the
359 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
360 Unless otherwise documented, it returns C<1> for true and C<''> for false.
361 If the file doesn't exist or can't be examined, it returns C<undef> and
362 sets C<$!> (errno). Despite the funny names, precedence is the same as any
363 other named unary operator. The operator may be any of:
365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link (false if symlinks aren't
382 supported by the file system).
383 -p File is a named pipe (FIFO), or Filehandle is a pipe.
385 -b File is a block special file.
386 -c File is a character special file.
387 -t Filehandle is opened to a tty.
389 -u File has setuid bit set.
390 -g File has setgid bit set.
391 -k File has sticky bit set.
393 -T File is an ASCII text file (heuristic guess).
394 -B File is a "binary" file (opposite of -T).
396 -M Script start time minus file modification time, in days.
397 -A Same for access time.
398 -C Same for inode change time (Unix, may differ for other
405 next unless -f $_; # ignore specials
409 Note that C<-s/a/b/> does not do a negated substitution. Saying
410 C<-exp($foo)> still works as expected, however: only single letters
411 following a minus are interpreted as file tests.
413 These operators are exempt from the "looks like a function rule" described
414 above. That is, an opening parenthesis after the operator does not affect
415 how much of the following code constitutes the argument. Put the opening
416 parentheses before the operator to separate it from code that follows (this
417 applies only to operators with higher precedence than unary operators, of
420 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
421 (-s $file) + 1024 # correct
423 The interpretation of the file permission operators C<-r>, C<-R>,
424 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
425 of the file and the uids and gids of the user. There may be other
426 reasons you can't actually read, write, or execute the file: for
427 example network filesystem access controls, ACLs (access control lists),
428 read-only filesystems, and unrecognized executable formats. Note
429 that the use of these six specific operators to verify if some operation
430 is possible is usually a mistake, because it may be open to race
433 Also note that, for the superuser on the local filesystems, the C<-r>,
434 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
435 if any execute bit is set in the mode. Scripts run by the superuser
436 may thus need to do a stat() to determine the actual mode of the file,
437 or temporarily set their effective uid to something else.
439 If you are using ACLs, there is a pragma called C<filetest> that may
440 produce more accurate results than the bare stat() mode bits.
441 When under C<use filetest 'access'> the above-mentioned filetests
442 test whether the permission can(not) be granted using the
443 access(2) family of system calls. Also note that the C<-x> and C<-X> may
444 under this pragma return true even if there are no execute permission
445 bits set (nor any extra execute permission ACLs). This strangeness is
446 due to the underlying system calls' definitions. Note also that, due to
447 the implementation of C<use filetest 'access'>, the C<_> special
448 filehandle won't cache the results of the file tests when this pragma is
449 in effect. Read the documentation for the C<filetest> pragma for more
452 The C<-T> and C<-B> switches work as follows. The first block or so of the
453 file is examined for odd characters such as strange control codes or
454 characters with the high bit set. If too many strange characters (>30%)
455 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
456 containing a zero byte in the first block is considered a binary file. If C<-T>
457 or C<-B> is used on a filehandle, the current IO buffer is examined
458 rather than the first block. Both C<-T> and C<-B> return true on an empty
459 file, or a file at EOF when testing a filehandle. Because you have to
460 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
461 against the file first, as in C<next unless -f $file && -T $file>.
463 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
464 the special filehandle consisting of a solitary underline, then the stat
465 structure of the previous file test (or stat operator) is used, saving
466 a system call. (This doesn't work with C<-t>, and you need to remember
467 that lstat() and C<-l> leave values in the stat structure for the
468 symbolic link, not the real file.) (Also, if the stat buffer was filled by
469 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
472 print "Can do.\n" if -r $a || -w _ || -x _;
475 print "Readable\n" if -r _;
476 print "Writable\n" if -w _;
477 print "Executable\n" if -x _;
478 print "Setuid\n" if -u _;
479 print "Setgid\n" if -g _;
480 print "Sticky\n" if -k _;
481 print "Text\n" if -T _;
482 print "Binary\n" if -B _;
484 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
485 test operators, in a way that C<-f -w -x $file> is equivalent to
486 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
487 the return value of C<-f $file> as an argument to another filetest
488 operator, no special magic will happen.)
490 Portability issues: L<perlport/-X>.
492 To avoid confusing would-be users of your code with mysterious
493 syntax errors, put something like this at the top of your script:
495 use 5.010; # so filetest ops can stack
502 =for Pod::Functions absolute value function
504 Returns the absolute value of its argument.
505 If VALUE is omitted, uses C<$_>.
507 =item accept NEWSOCKET,GENERICSOCKET
510 =for Pod::Functions accept an incoming socket connect
512 Accepts an incoming socket connect, just as accept(2)
513 does. Returns the packed address if it succeeded, false otherwise.
514 See the example in L<perlipc/"Sockets: Client/Server Communication">.
516 On systems that support a close-on-exec flag on files, the flag will
517 be set for the newly opened file descriptor, as determined by the
518 value of $^F. See L<perlvar/$^F>.
527 =for Pod::Functions schedule a SIGALRM
529 Arranges to have a SIGALRM delivered to this process after the
530 specified number of wallclock seconds has elapsed. If SECONDS is not
531 specified, the value stored in C<$_> is used. (On some machines,
532 unfortunately, the elapsed time may be up to one second less or more
533 than you specified because of how seconds are counted, and process
534 scheduling may delay the delivery of the signal even further.)
536 Only one timer may be counting at once. Each call disables the
537 previous timer, and an argument of C<0> may be supplied to cancel the
538 previous timer without starting a new one. The returned value is the
539 amount of time remaining on the previous timer.
541 For delays of finer granularity than one second, the Time::HiRes module
542 (from CPAN, and starting from Perl 5.8 part of the standard
543 distribution) provides ualarm(). You may also use Perl's four-argument
544 version of select() leaving the first three arguments undefined, or you
545 might be able to use the C<syscall> interface to access setitimer(2) if
546 your system supports it. See L<perlfaq8> for details.
548 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
549 C<sleep> may be internally implemented on your system with C<alarm>.
551 If you want to use C<alarm> to time out a system call you need to use an
552 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
553 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
554 restart system calls on some systems. Using C<eval>/C<die> always works,
555 modulo the caveats given in L<perlipc/"Signals">.
558 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
560 $nread = sysread SOCKET, $buffer, $size;
564 die unless $@ eq "alarm\n"; # propagate unexpected errors
571 For more information see L<perlipc>.
573 Portability issues: L<perlport/alarm>.
576 X<atan2> X<arctangent> X<tan> X<tangent>
578 =for Pod::Functions arctangent of Y/X in the range -PI to PI
580 Returns the arctangent of Y/X in the range -PI to PI.
582 For the tangent operation, you may use the C<Math::Trig::tan>
583 function, or use the familiar relation:
585 sub tan { sin($_[0]) / cos($_[0]) }
587 The return value for C<atan2(0,0)> is implementation-defined; consult
588 your atan2(3) manpage for more information.
590 Portability issues: L<perlport/atan2>.
592 =item bind SOCKET,NAME
595 =for Pod::Functions binds an address to a socket
597 Binds a network address to a socket, just as bind(2)
598 does. Returns true if it succeeded, false otherwise. NAME should be a
599 packed address of the appropriate type for the socket. See the examples in
600 L<perlipc/"Sockets: Client/Server Communication">.
602 =item binmode FILEHANDLE, LAYER
603 X<binmode> X<binary> X<text> X<DOS> X<Windows>
605 =item binmode FILEHANDLE
607 =for Pod::Functions prepare binary files for I/O
609 Arranges for FILEHANDLE to be read or written in "binary" or "text"
610 mode on systems where the run-time libraries distinguish between
611 binary and text files. If FILEHANDLE is an expression, the value is
612 taken as the name of the filehandle. Returns true on success,
613 otherwise it returns C<undef> and sets C<$!> (errno).
615 On some systems (in general, DOS- and Windows-based systems) binmode()
616 is necessary when you're not working with a text file. For the sake
617 of portability it is a good idea always to use it when appropriate,
618 and never to use it when it isn't appropriate. Also, people can
619 set their I/O to be by default UTF8-encoded Unicode, not bytes.
621 In other words: regardless of platform, use binmode() on binary data,
622 like images, for example.
624 If LAYER is present it is a single string, but may contain multiple
625 directives. The directives alter the behaviour of the filehandle.
626 When LAYER is present, using binmode on a text file makes sense.
628 If LAYER is omitted or specified as C<:raw> the filehandle is made
629 suitable for passing binary data. This includes turning off possible CRLF
630 translation and marking it as bytes (as opposed to Unicode characters).
631 Note that, despite what may be implied in I<"Programming Perl"> (the
632 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
633 Other layers that would affect the binary nature of the stream are
634 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
635 PERLIO environment variable.
637 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
638 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
639 establish default I/O layers. See L<open>.
641 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
642 in "Programming Perl, 3rd Edition". However, since the publishing of this
643 book, by many known as "Camel III", the consensus of the naming of this
644 functionality has moved from "discipline" to "layer". All documentation
645 of this version of Perl therefore refers to "layers" rather than to
646 "disciplines". Now back to the regularly scheduled documentation...>
648 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
649 C<:utf8> just marks the data as UTF-8 without further checking,
650 while C<:encoding(UTF-8)> checks the data for actually being valid
651 UTF-8. More details can be found in L<PerlIO::encoding>.
653 In general, binmode() should be called after open() but before any I/O
654 is done on the filehandle. Calling binmode() normally flushes any
655 pending buffered output data (and perhaps pending input data) on the
656 handle. An exception to this is the C<:encoding> layer that
657 changes the default character encoding of the handle; see L</open>.
658 The C<:encoding> layer sometimes needs to be called in
659 mid-stream, and it doesn't flush the stream. The C<:encoding>
660 also implicitly pushes on top of itself the C<:utf8> layer because
661 internally Perl operates on UTF8-encoded Unicode characters.
663 The operating system, device drivers, C libraries, and Perl run-time
664 system all conspire to let the programmer treat a single
665 character (C<\n>) as the line terminator, irrespective of external
666 representation. On many operating systems, the native text file
667 representation matches the internal representation, but on some
668 platforms the external representation of C<\n> is made up of more than
671 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
672 a single character to end each line in the external representation of text
673 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
674 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
675 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
676 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
677 two characters C<\cM\cJ>. That means that if you don't use binmode() on
678 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
679 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
680 output. This is what you want for text files, but it can be disastrous for
683 Another consequence of using binmode() (on some systems) is that
684 special end-of-file markers will be seen as part of the data stream.
685 For systems from the Microsoft family this means that, if your binary
686 data contain C<\cZ>, the I/O subsystem will regard it as the end of
687 the file, unless you use binmode().
689 binmode() is important not only for readline() and print() operations,
690 but also when using read(), seek(), sysread(), syswrite() and tell()
691 (see L<perlport> for more details). See the C<$/> and C<$\> variables
692 in L<perlvar> for how to manually set your input and output
693 line-termination sequences.
695 Portability issues: L<perlport/binmode>.
697 =item bless REF,CLASSNAME
702 =for Pod::Functions create an object
704 This function tells the thingy referenced by REF that it is now an object
705 in the CLASSNAME package. If CLASSNAME is omitted, the current package
706 is used. Because a C<bless> is often the last thing in a constructor,
707 it returns the reference for convenience. Always use the two-argument
708 version if a derived class might inherit the function doing the blessing.
709 See L<perlobj> for more about the blessing (and blessings) of objects.
711 Consider always blessing objects in CLASSNAMEs that are mixed case.
712 Namespaces with all lowercase names are considered reserved for
713 Perl pragmata. Builtin types have all uppercase names. To prevent
714 confusion, you may wish to avoid such package names as well. Make sure
715 that CLASSNAME is a true value.
717 See L<perlmod/"Perl Modules">.
721 =for Pod::Functions +switch break out of a C<given> block
723 Break out of a C<given()> block.
725 This keyword is enabled by the C<"switch"> feature; see L<feature> for
726 more information on C<"switch">. You can also access it by prefixing it
727 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
731 X<caller> X<call stack> X<stack> X<stack trace>
735 =for Pod::Functions get context of the current subroutine call
737 Returns the context of the current pure perl subroutine call. In scalar
738 context, returns the caller's package name if there I<is> a caller (that is, if
739 we're in a subroutine or C<eval> or C<require>) and the undefined value
740 otherwise. caller never returns XS subs and they are skipped. The next pure
741 perl sub will appear instead of the XS sub in caller's return values. In list
742 context, caller returns
745 ($package, $filename, $line) = caller;
747 With EXPR, it returns some extra information that the debugger uses to
748 print a stack trace. The value of EXPR indicates how many call frames
749 to go back before the current one.
752 ($package, $filename, $line, $subroutine, $hasargs,
755 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
758 Here, $subroutine is the function that the caller called (rather than the
759 function containing the caller). Note that $subroutine may be C<(eval)> if
760 the frame is not a subroutine call, but an C<eval>. In such a case
761 additional elements $evaltext and
762 C<$is_require> are set: C<$is_require> is true if the frame is created by a
763 C<require> or C<use> statement, $evaltext contains the text of the
764 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
765 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
766 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
767 frame.) $subroutine may also be C<(unknown)> if this particular
768 subroutine happens to have been deleted from the symbol table.
769 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
770 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
771 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
772 corresponds to C<${^WARNING_BITS}>. The
773 C<$hints> and C<$bitmask> values are subject
774 to change between versions of Perl, and are not meant for external use.
776 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
777 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
778 of this hash, as they are the actual values stored in the optree.
780 Furthermore, when called from within the DB package in
781 list context, and with an argument, caller returns more
782 detailed information: it sets the list variable C<@DB::args> to be the
783 arguments with which the subroutine was invoked.
785 Be aware that the optimizer might have optimized call frames away before
786 C<caller> had a chance to get the information. That means that C<caller(N)>
787 might not return information about the call frame you expect it to, for
788 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
789 previous time C<caller> was called.
791 Be aware that setting C<@DB::args> is I<best effort>, intended for
792 debugging or generating backtraces, and should not be relied upon. In
793 particular, as C<@_> contains aliases to the caller's arguments, Perl does
794 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
795 subroutine makes to C<@_> or its contents, not the original values at call
796 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
797 elements, so under certain cases its elements may have become freed and
798 reallocated for other variables or temporary values. Finally, a side effect
799 of the current implementation is that the effects of C<shift @_> can
800 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
801 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
802 elements), so C<@DB::args> is actually a hybrid of the current state and
803 initial state of C<@_>. Buyer beware.
810 =item chdir FILEHANDLE
812 =item chdir DIRHANDLE
816 =for Pod::Functions change your current working directory
818 Changes the working directory to EXPR, if possible. If EXPR is omitted,
819 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
820 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
821 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
822 neither is set, C<chdir> does nothing. It returns true on success,
823 false otherwise. See the example under C<die>.
825 On systems that support fchdir(2), you may pass a filehandle or
826 directory handle as the argument. On systems that don't support fchdir(2),
827 passing handles raises an exception.
830 X<chmod> X<permission> X<mode>
832 =for Pod::Functions changes the permissions on a list of files
834 Changes the permissions of a list of files. The first element of the
835 list must be the numeric mode, which should probably be an octal
836 number, and which definitely should I<not> be a string of octal digits:
837 C<0644> is okay, but C<"0644"> is not. Returns the number of files
838 successfully changed. See also L</oct> if all you have is a string.
840 $cnt = chmod 0755, "foo", "bar";
841 chmod 0755, @executables;
842 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
844 $mode = "0644"; chmod oct($mode), "foo"; # this is better
845 $mode = 0644; chmod $mode, "foo"; # this is best
847 On systems that support fchmod(2), you may pass filehandles among the
848 files. On systems that don't support fchmod(2), passing filehandles raises
849 an exception. Filehandles must be passed as globs or glob references to be
850 recognized; barewords are considered filenames.
852 open(my $fh, "<", "foo");
853 my $perm = (stat $fh)[2] & 07777;
854 chmod($perm | 0600, $fh);
856 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
859 use Fcntl qw( :mode );
860 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
861 # Identical to the chmod 0755 of the example above.
863 Portability issues: L<perlport/chmod>.
866 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
872 =for Pod::Functions remove a trailing record separator from a string
874 This safer version of L</chop> removes any trailing string
875 that corresponds to the current value of C<$/> (also known as
876 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
877 number of characters removed from all its arguments. It's often used to
878 remove the newline from the end of an input record when you're worried
879 that the final record may be missing its newline. When in paragraph
880 mode (C<$/ = "">), it removes all trailing newlines from the string.
881 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
882 a reference to an integer or the like; see L<perlvar>) chomp() won't
884 If VARIABLE is omitted, it chomps C<$_>. Example:
887 chomp; # avoid \n on last field
892 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
893 resetting the C<each> iterator in the process.
895 You can actually chomp anything that's an lvalue, including an assignment:
898 chomp($answer = <STDIN>);
900 If you chomp a list, each element is chomped, and the total number of
901 characters removed is returned.
903 Note that parentheses are necessary when you're chomping anything
904 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
905 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
906 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
907 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
917 =for Pod::Functions remove the last character from a string
919 Chops off the last character of a string and returns the character
920 chopped. It is much more efficient than C<s/.$//s> because it neither
921 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
922 If VARIABLE is a hash, it chops the hash's values, but not its keys,
923 resetting the C<each> iterator in the process.
925 You can actually chop anything that's an lvalue, including an assignment.
927 If you chop a list, each element is chopped. Only the value of the
928 last C<chop> is returned.
930 Note that C<chop> returns the last character. To return all but the last
931 character, use C<substr($string, 0, -1)>.
936 X<chown> X<owner> X<user> X<group>
938 =for Pod::Functions change the ownership on a list of files
940 Changes the owner (and group) of a list of files. The first two
941 elements of the list must be the I<numeric> uid and gid, in that
942 order. A value of -1 in either position is interpreted by most
943 systems to leave that value unchanged. Returns the number of files
944 successfully changed.
946 $cnt = chown $uid, $gid, 'foo', 'bar';
947 chown $uid, $gid, @filenames;
949 On systems that support fchown(2), you may pass filehandles among the
950 files. On systems that don't support fchown(2), passing filehandles raises
951 an exception. Filehandles must be passed as globs or glob references to be
952 recognized; barewords are considered filenames.
954 Here's an example that looks up nonnumeric uids in the passwd file:
957 chomp($user = <STDIN>);
959 chomp($pattern = <STDIN>);
961 ($login,$pass,$uid,$gid) = getpwnam($user)
962 or die "$user not in passwd file";
964 @ary = glob($pattern); # expand filenames
965 chown $uid, $gid, @ary;
967 On most systems, you are not allowed to change the ownership of the
968 file unless you're the superuser, although you should be able to change
969 the group to any of your secondary groups. On insecure systems, these
970 restrictions may be relaxed, but this is not a portable assumption.
971 On POSIX systems, you can detect this condition this way:
973 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
974 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
976 Portability issues: L<perlport/chown>.
979 X<chr> X<character> X<ASCII> X<Unicode>
983 =for Pod::Functions get character this number represents
985 Returns the character represented by that NUMBER in the character set.
986 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
987 chr(0x263a) is a Unicode smiley face.
989 Negative values give the Unicode replacement character (chr(0xfffd)),
990 except under the L<bytes> pragma, where the low eight bits of the value
991 (truncated to an integer) are used.
993 If NUMBER is omitted, uses C<$_>.
995 For the reverse, use L</ord>.
997 Note that characters from 128 to 255 (inclusive) are by default
998 internally not encoded as UTF-8 for backward compatibility reasons.
1000 See L<perlunicode> for more about Unicode.
1002 =item chroot FILENAME
1007 =for Pod::Functions make directory new root for path lookups
1009 This function works like the system call by the same name: it makes the
1010 named directory the new root directory for all further pathnames that
1011 begin with a C</> by your process and all its children. (It doesn't
1012 change your current working directory, which is unaffected.) For security
1013 reasons, this call is restricted to the superuser. If FILENAME is
1014 omitted, does a C<chroot> to C<$_>.
1016 B<NOTE:> It is good security practice to do C<chdir("/")> (to the root
1017 directory) immediately after a C<chroot()>.
1019 Portability issues: L<perlport/chroot>.
1021 =item close FILEHANDLE
1026 =for Pod::Functions close file (or pipe or socket) handle
1028 Closes the file or pipe associated with the filehandle, flushes the IO
1029 buffers, and closes the system file descriptor. Returns true if those
1030 operations succeed and if no error was reported by any PerlIO
1031 layer. Closes the currently selected filehandle if the argument is
1034 You don't have to close FILEHANDLE if you are immediately going to do
1035 another C<open> on it, because C<open> closes it for you. (See
1036 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1037 counter (C<$.>), while the implicit close done by C<open> does not.
1039 If the filehandle came from a piped open, C<close> returns false if one of
1040 the other syscalls involved fails or if its program exits with non-zero
1041 status. If the only problem was that the program exited non-zero, C<$!>
1042 will be set to C<0>. Closing a pipe also waits for the process executing
1043 on the pipe to exit--in case you wish to look at the output of the pipe
1044 afterwards--and implicitly puts the exit status value of that command into
1045 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1047 If there are multiple threads running, C<close> on a filehandle from a
1048 piped open returns true without waiting for the child process to terminate,
1049 if the filehandle is still open in another thread.
1051 Closing the read end of a pipe before the process writing to it at the
1052 other end is done writing results in the writer receiving a SIGPIPE. If
1053 the other end can't handle that, be sure to read all the data before
1058 open(OUTPUT, '|sort >foo') # pipe to sort
1059 or die "Can't start sort: $!";
1060 #... # print stuff to output
1061 close OUTPUT # wait for sort to finish
1062 or warn $! ? "Error closing sort pipe: $!"
1063 : "Exit status $? from sort";
1064 open(INPUT, 'foo') # get sort's results
1065 or die "Can't open 'foo' for input: $!";
1067 FILEHANDLE may be an expression whose value can be used as an indirect
1068 filehandle, usually the real filehandle name or an autovivified handle.
1070 =item closedir DIRHANDLE
1073 =for Pod::Functions close directory handle
1075 Closes a directory opened by C<opendir> and returns the success of that
1078 =item connect SOCKET,NAME
1081 =for Pod::Functions connect to a remote socket
1083 Attempts to connect to a remote socket, just like connect(2).
1084 Returns true if it succeeded, false otherwise. NAME should be a
1085 packed address of the appropriate type for the socket. See the examples in
1086 L<perlipc/"Sockets: Client/Server Communication">.
1088 =item continue BLOCK
1093 =for Pod::Functions optional trailing block in a while or foreach
1095 When followed by a BLOCK, C<continue> is actually a
1096 flow control statement rather than a function. If
1097 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1098 C<foreach>), it is always executed just before the conditional is about to
1099 be evaluated again, just like the third part of a C<for> loop in C. Thus
1100 it can be used to increment a loop variable, even when the loop has been
1101 continued via the C<next> statement (which is similar to the C C<continue>
1104 C<last>, C<next>, or C<redo> may appear within a C<continue>
1105 block; C<last> and C<redo> behave as if they had been executed within
1106 the main block. So will C<next>, but since it will execute a C<continue>
1107 block, it may be more entertaining.
1110 ### redo always comes here
1113 ### next always comes here
1115 # then back the top to re-check EXPR
1117 ### last always comes here
1119 Omitting the C<continue> section is equivalent to using an
1120 empty one, logically enough, so C<next> goes directly back
1121 to check the condition at the top of the loop.
1123 When there is no BLOCK, C<continue> is a function that
1124 falls through the current C<when> or C<default> block instead of iterating
1125 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1126 In Perl 5.14 and earlier, this form of C<continue> was
1127 only available when the C<"switch"> feature was enabled.
1128 See L<feature> and L<perlsyn/"Switch Statements"> for more
1132 X<cos> X<cosine> X<acos> X<arccosine>
1136 =for Pod::Functions cosine function
1138 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1139 takes the cosine of C<$_>.
1141 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1142 function, or use this relation:
1144 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1146 =item crypt PLAINTEXT,SALT
1147 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1148 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1150 =for Pod::Functions one-way passwd-style encryption
1152 Creates a digest string exactly like the crypt(3) function in the C
1153 library (assuming that you actually have a version there that has not
1154 been extirpated as a potential munition).
1156 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1157 into a short string, called a digest, which is returned. The same
1158 PLAINTEXT and SALT will always return the same string, but there is no
1159 (known) way to get the original PLAINTEXT from the hash. Small
1160 changes in the PLAINTEXT or SALT will result in large changes in the
1163 There is no decrypt function. This function isn't all that useful for
1164 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1165 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1166 primarily used to check if two pieces of text are the same without
1167 having to transmit or store the text itself. An example is checking
1168 if a correct password is given. The digest of the password is stored,
1169 not the password itself. The user types in a password that is
1170 crypt()'d with the same salt as the stored digest. If the two digests
1171 match, the password is correct.
1173 When verifying an existing digest string you should use the digest as
1174 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1175 to create the digest is visible as part of the digest. This ensures
1176 crypt() will hash the new string with the same salt as the digest.
1177 This allows your code to work with the standard L<crypt|/crypt> and
1178 with more exotic implementations. In other words, assume
1179 nothing about the returned string itself nor about how many bytes
1182 Traditionally the result is a string of 13 bytes: two first bytes of
1183 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1184 the first eight bytes of PLAINTEXT mattered. But alternative
1185 hashing schemes (like MD5), higher level security schemes (like C2),
1186 and implementations on non-Unix platforms may produce different
1189 When choosing a new salt create a random two character string whose
1190 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1191 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1192 characters is just a recommendation; the characters allowed in
1193 the salt depend solely on your system's crypt library, and Perl can't
1194 restrict what salts C<crypt()> accepts.
1196 Here's an example that makes sure that whoever runs this program knows
1199 $pwd = (getpwuid($<))[1];
1201 system "stty -echo";
1203 chomp($word = <STDIN>);
1207 if (crypt($word, $pwd) ne $pwd) {
1213 Of course, typing in your own password to whoever asks you
1216 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1217 of data, not least of all because you can't get the information
1218 back. Look at the L<Digest> module for more robust algorithms.
1220 If using crypt() on a Unicode string (which I<potentially> has
1221 characters with codepoints above 255), Perl tries to make sense
1222 of the situation by trying to downgrade (a copy of)
1223 the string back to an eight-bit byte string before calling crypt()
1224 (on that copy). If that works, good. If not, crypt() dies with
1225 C<Wide character in crypt>.
1227 Portability issues: L<perlport/crypt>.
1232 =for Pod::Functions breaks binding on a tied dbm file
1234 [This function has been largely superseded by the C<untie> function.]
1236 Breaks the binding between a DBM file and a hash.
1238 Portability issues: L<perlport/dbmclose>.
1240 =item dbmopen HASH,DBNAME,MASK
1241 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1243 =for Pod::Functions create binding on a tied dbm file
1245 [This function has been largely superseded by the
1246 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1248 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1249 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1250 argument is I<not> a filehandle, even though it looks like one). DBNAME
1251 is the name of the database (without the F<.dir> or F<.pag> extension if
1252 any). If the database does not exist, it is created with protection
1253 specified by MASK (as modified by the C<umask>). To prevent creation of
1254 the database if it doesn't exist, you may specify a MODE
1255 of 0, and the function will return a false value if it
1256 can't find an existing database. If your system supports
1257 only the older DBM functions, you may make only one C<dbmopen> call in your
1258 program. In older versions of Perl, if your system had neither DBM nor
1259 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1262 If you don't have write access to the DBM file, you can only read hash
1263 variables, not set them. If you want to test whether you can write,
1264 either use file tests or try setting a dummy hash entry inside an C<eval>
1267 Note that functions such as C<keys> and C<values> may return huge lists
1268 when used on large DBM files. You may prefer to use the C<each>
1269 function to iterate over large DBM files. Example:
1271 # print out history file offsets
1272 dbmopen(%HIST,'/usr/lib/news/history',0666);
1273 while (($key,$val) = each %HIST) {
1274 print $key, ' = ', unpack('L',$val), "\n";
1278 See also L<AnyDBM_File> for a more general description of the pros and
1279 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1280 rich implementation.
1282 You can control which DBM library you use by loading that library
1283 before you call dbmopen():
1286 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1287 or die "Can't open netscape history file: $!";
1289 Portability issues: L<perlport/dbmopen>.
1292 X<defined> X<undef> X<undefined>
1296 =for Pod::Functions test whether a value, variable, or function is defined
1298 Returns a Boolean value telling whether EXPR has a value other than
1299 the undefined value C<undef>. If EXPR is not present, C<$_> is
1302 Many operations return C<undef> to indicate failure, end of file,
1303 system error, uninitialized variable, and other exceptional
1304 conditions. This function allows you to distinguish C<undef> from
1305 other values. (A simple Boolean test will not distinguish among
1306 C<undef>, zero, the empty string, and C<"0">, which are all equally
1307 false.) Note that since C<undef> is a valid scalar, its presence
1308 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1309 returns C<undef> when its argument is an empty array, I<or> when the
1310 element to return happens to be C<undef>.
1312 You may also use C<defined(&func)> to check whether subroutine C<&func>
1313 has ever been defined. The return value is unaffected by any forward
1314 declarations of C<&func>. A subroutine that is not defined
1315 may still be callable: its package may have an C<AUTOLOAD> method that
1316 makes it spring into existence the first time that it is called; see
1319 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1320 used to report whether memory for that aggregate had ever been
1321 allocated. This behavior may disappear in future versions of Perl.
1322 You should instead use a simple test for size:
1324 if (@an_array) { print "has array elements\n" }
1325 if (%a_hash) { print "has hash members\n" }
1327 When used on a hash element, it tells you whether the value is defined,
1328 not whether the key exists in the hash. Use L</exists> for the latter
1333 print if defined $switch{D};
1334 print "$val\n" while defined($val = pop(@ary));
1335 die "Can't readlink $sym: $!"
1336 unless defined($value = readlink $sym);
1337 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1338 $debugging = 0 unless defined $debugging;
1340 Note: Many folks tend to overuse C<defined> and are then surprised to
1341 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1342 defined values. For example, if you say
1346 The pattern match succeeds and C<$1> is defined, although it
1347 matched "nothing". It didn't really fail to match anything. Rather, it
1348 matched something that happened to be zero characters long. This is all
1349 very above-board and honest. When a function returns an undefined value,
1350 it's an admission that it couldn't give you an honest answer. So you
1351 should use C<defined> only when questioning the integrity of what
1352 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1355 See also L</undef>, L</exists>, L</ref>.
1360 =for Pod::Functions deletes a value from a hash
1362 Given an expression that specifies an element or slice of a hash, C<delete>
1363 deletes the specified elements from that hash so that exists() on that element
1364 no longer returns true. Setting a hash element to the undefined value does
1365 not remove its key, but deleting it does; see L</exists>.
1367 In list context, returns the value or values deleted, or the last such
1368 element in scalar context. The return list's length always matches that of
1369 the argument list: deleting non-existent elements returns the undefined value
1370 in their corresponding positions.
1372 delete() may also be used on arrays and array slices, but its behavior is less
1373 straightforward. Although exists() will return false for deleted entries,
1374 deleting array elements never changes indices of existing values; use shift()
1375 or splice() for that. However, if any deleted elements fall at the end of an
1376 array, the array's size shrinks to the position of the highest element that
1377 still tests true for exists(), or to 0 if none do. In other words, an
1378 array won't have trailing nonexistent elements after a delete.
1380 B<WARNING:> Calling delete on array values is deprecated and likely to
1381 be removed in a future version of Perl.
1383 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1384 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1385 or array may not necessarily return anything; it depends on the implementation
1386 of the C<tied> package's DELETE method, which may do whatever it pleases.
1388 The C<delete local EXPR> construct localizes the deletion to the current
1389 block at run time. Until the block exits, elements locally deleted
1390 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1391 of composite types">.
1393 %hash = (foo => 11, bar => 22, baz => 33);
1394 $scalar = delete $hash{foo}; # $scalar is 11
1395 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1396 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1398 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1400 foreach $key (keys %HASH) {
1404 foreach $index (0 .. $#ARRAY) {
1405 delete $ARRAY[$index];
1410 delete @HASH{keys %HASH};
1412 delete @ARRAY[0 .. $#ARRAY];
1414 But both are slower than assigning the empty list
1415 or undefining %HASH or @ARRAY, which is the customary
1416 way to empty out an aggregate:
1418 %HASH = (); # completely empty %HASH
1419 undef %HASH; # forget %HASH ever existed
1421 @ARRAY = (); # completely empty @ARRAY
1422 undef @ARRAY; # forget @ARRAY ever existed
1424 The EXPR can be arbitrarily complicated provided its
1425 final operation is an element or slice of an aggregate:
1427 delete $ref->[$x][$y]{$key};
1428 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1430 delete $ref->[$x][$y][$index];
1431 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1434 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1436 =for Pod::Functions raise an exception or bail out
1438 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1439 into C<$@> and the C<eval> is terminated with the undefined value.
1440 If the exception is outside of all enclosing C<eval>s, then the uncaught
1441 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1442 need to exit the process with a specific exit code, see L</exit>.
1444 Equivalent examples:
1446 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1447 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1449 If the last element of LIST does not end in a newline, the current
1450 script line number and input line number (if any) are also printed,
1451 and a newline is supplied. Note that the "input line number" (also
1452 known as "chunk") is subject to whatever notion of "line" happens to
1453 be currently in effect, and is also available as the special variable
1454 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1456 Hint: sometimes appending C<", stopped"> to your message will cause it
1457 to make better sense when the string C<"at foo line 123"> is appended.
1458 Suppose you are running script "canasta".
1460 die "/etc/games is no good";
1461 die "/etc/games is no good, stopped";
1463 produce, respectively
1465 /etc/games is no good at canasta line 123.
1466 /etc/games is no good, stopped at canasta line 123.
1468 If the output is empty and C<$@> already contains a value (typically from a
1469 previous eval) that value is reused after appending C<"\t...propagated">.
1470 This is useful for propagating exceptions:
1473 die unless $@ =~ /Expected exception/;
1475 If the output is empty and C<$@> contains an object reference that has a
1476 C<PROPAGATE> method, that method will be called with additional file
1477 and line number parameters. The return value replaces the value in
1478 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1481 If C<$@> is empty then the string C<"Died"> is used.
1483 If an uncaught exception results in interpreter exit, the exit code is
1484 determined from the values of C<$!> and C<$?> with this pseudocode:
1486 exit $! if $!; # errno
1487 exit $? >> 8 if $? >> 8; # child exit status
1488 exit 255; # last resort
1490 The intent is to squeeze as much possible information about the likely cause
1491 into the limited space of the system exit
1492 code. However, as C<$!> is the value
1493 of C's C<errno>, which can be set by any system call, this means that the value
1494 of the exit code used by C<die> can be non-predictable, so should not be relied
1495 upon, other than to be non-zero.
1497 You can also call C<die> with a reference argument, and if this is trapped
1498 within an C<eval>, C<$@> contains that reference. This permits more
1499 elaborate exception handling using objects that maintain arbitrary state
1500 about the exception. Such a scheme is sometimes preferable to matching
1501 particular string values of C<$@> with regular expressions. Because C<$@>
1502 is a global variable and C<eval> may be used within object implementations,
1503 be careful that analyzing the error object doesn't replace the reference in
1504 the global variable. It's easiest to make a local copy of the reference
1505 before any manipulations. Here's an example:
1507 use Scalar::Util "blessed";
1509 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1510 if (my $ev_err = $@) {
1511 if (blessed($ev_err)
1512 && $ev_err->isa("Some::Module::Exception")) {
1513 # handle Some::Module::Exception
1516 # handle all other possible exceptions
1520 Because Perl stringifies uncaught exception messages before display,
1521 you'll probably want to overload stringification operations on
1522 exception objects. See L<overload> for details about that.
1524 You can arrange for a callback to be run just before the C<die>
1525 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1526 handler is called with the error text and can change the error
1527 message, if it sees fit, by calling C<die> again. See
1528 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1529 L<"eval BLOCK"> for some examples. Although this feature was
1530 to be run only right before your program was to exit, this is not
1531 currently so: the C<$SIG{__DIE__}> hook is currently called
1532 even inside eval()ed blocks/strings! If one wants the hook to do
1533 nothing in such situations, put
1537 as the first line of the handler (see L<perlvar/$^S>). Because
1538 this promotes strange action at a distance, this counterintuitive
1539 behavior may be fixed in a future release.
1541 See also exit(), warn(), and the Carp module.
1546 =for Pod::Functions turn a BLOCK into a TERM
1548 Not really a function. Returns the value of the last command in the
1549 sequence of commands indicated by BLOCK. When modified by the C<while> or
1550 C<until> loop modifier, executes the BLOCK once before testing the loop
1551 condition. (On other statements the loop modifiers test the conditional
1554 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1555 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1556 See L<perlsyn> for alternative strategies.
1561 Uses the value of EXPR as a filename and executes the contents of the
1562 file as a Perl script.
1570 except that it's more concise, runs no external processes, keeps track of
1572 filename for error messages, searches the C<@INC> directories, and updates
1573 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1574 these variables. It also differs in that code evaluated with C<do FILENAME>
1575 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1576 same, however, in that it does reparse the file every time you call it,
1577 so you probably don't want to do this inside a loop.
1579 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1580 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1581 and sets C<$!> to the error. Always check C<$@> first, as compilation
1582 could fail in a way that also sets C<$!>. If the file is successfully
1583 compiled, C<do> returns the value of the last expression evaluated.
1585 Inclusion of library modules is better done with the
1586 C<use> and C<require> operators, which also do automatic error checking
1587 and raise an exception if there's a problem.
1589 You might like to use C<do> to read in a program configuration
1590 file. Manual error checking can be done this way:
1592 # read in config files: system first, then user
1593 for $file ("/share/prog/defaults.rc",
1594 "$ENV{HOME}/.someprogrc")
1596 unless ($return = do $file) {
1597 warn "couldn't parse $file: $@" if $@;
1598 warn "couldn't do $file: $!" unless defined $return;
1599 warn "couldn't run $file" unless $return;
1604 X<dump> X<core> X<undump>
1610 =for Pod::Functions create an immediate core dump
1612 This function causes an immediate core dump. See also the B<-u>
1613 command-line switch in L<perlrun>, which does the same thing.
1614 Primarily this is so that you can use the B<undump> program (not
1615 supplied) to turn your core dump into an executable binary after
1616 having initialized all your variables at the beginning of the
1617 program. When the new binary is executed it will begin by executing
1618 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1619 Think of it as a goto with an intervening core dump and reincarnation.
1620 If C<LABEL> is omitted, restarts the program from the top. The
1621 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1622 computed at run time, being otherwise identical to C<dump LABEL>.
1624 B<WARNING>: Any files opened at the time of the dump will I<not>
1625 be open any more when the program is reincarnated, with possible
1626 resulting confusion by Perl.
1628 This function is now largely obsolete, mostly because it's very hard to
1629 convert a core file into an executable. That's why you should now invoke
1630 it as C<CORE::dump()>, if you don't want to be warned against a possible
1633 Unlike most named operators, this has the same precedence as assignment.
1634 It is also exempt from the looks-like-a-function rule, so
1635 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1638 Portability issues: L<perlport/dump>.
1641 X<each> X<hash, iterator>
1648 =for Pod::Functions retrieve the next key/value pair from a hash
1650 When called on a hash in list context, returns a 2-element list
1651 consisting of the key and value for the next element of a hash. In Perl
1652 5.12 and later only, it will also return the index and value for the next
1653 element of an array so that you can iterate over it; older Perls consider
1654 this a syntax error. When called in scalar context, returns only the key
1655 (not the value) in a hash, or the index in an array.
1657 Hash entries are returned in an apparently random order. The actual random
1658 order is specific to a given hash; the exact same series of operations
1659 on two hashes may result in a different order for each hash. Any insertion
1660 into the hash may change the order, as will any deletion, with the exception
1661 that the most recent key returned by C<each> or C<keys> may be deleted
1662 without changing the order. So long as a given hash is unmodified you may
1663 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1664 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1665 details on why hash order is randomized. Aside from the guarantees
1666 provided here the exact details of Perl's hash algorithm and the hash
1667 traversal order are subject to change in any release of Perl.
1669 After C<each> has returned all entries from the hash or array, the next
1670 call to C<each> returns the empty list in list context and C<undef> in
1671 scalar context; the next call following I<that> one restarts iteration.
1672 Each hash or array has its own internal iterator, accessed by C<each>,
1673 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1674 reached the end as just described; it can be explicitly reset by calling
1675 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1676 elements while iterating over it, the effect on the iterator is
1677 unspecified; for example, entries may be skipped or duplicated--so don't
1678 do that. Exception: It is always safe to delete the item most recently
1679 returned by C<each()>, so the following code works properly:
1681 while (($key, $value) = each %hash) {
1683 delete $hash{$key}; # This is safe
1686 This prints out your environment like the printenv(1) program,
1687 but in a different order:
1689 while (($key,$value) = each %ENV) {
1690 print "$key=$value\n";
1693 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1694 reference to an unblessed hash or array. The argument will be dereferenced
1695 automatically. This aspect of C<each> is considered highly experimental.
1696 The exact behaviour may change in a future version of Perl.
1698 while (($key,$value) = each $hashref) { ... }
1700 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1701 which will set C<$_> on every iteration.
1704 print "$_=$ENV{$_}\n";
1707 To avoid confusing would-be users of your code who are running earlier
1708 versions of Perl with mysterious syntax errors, put this sort of thing at
1709 the top of your file to signal that your code will work I<only> on Perls of
1712 use 5.012; # so keys/values/each work on arrays
1713 use 5.014; # so keys/values/each work on scalars (experimental)
1714 use 5.018; # so each assigns to $_ in a lone while test
1716 See also C<keys>, C<values>, and C<sort>.
1718 =item eof FILEHANDLE
1727 =for Pod::Functions test a filehandle for its end
1729 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1730 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1731 gives the real filehandle. (Note that this function actually
1732 reads a character and then C<ungetc>s it, so isn't useful in an
1733 interactive context.) Do not read from a terminal file (or call
1734 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1735 as terminals may lose the end-of-file condition if you do.
1737 An C<eof> without an argument uses the last file read. Using C<eof()>
1738 with empty parentheses is different. It refers to the pseudo file
1739 formed from the files listed on the command line and accessed via the
1740 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1741 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1742 used will cause C<@ARGV> to be examined to determine if input is
1743 available. Similarly, an C<eof()> after C<< <> >> has returned
1744 end-of-file will assume you are processing another C<@ARGV> list,
1745 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1746 see L<perlop/"I/O Operators">.
1748 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1749 detect the end of each file, whereas C<eof()> will detect the end
1750 of the very last file only. Examples:
1752 # reset line numbering on each input file
1754 next if /^\s*#/; # skip comments
1757 close ARGV if eof; # Not eof()!
1760 # insert dashes just before last line of last file
1762 if (eof()) { # check for end of last file
1763 print "--------------\n";
1766 last if eof(); # needed if we're reading from a terminal
1769 Practical hint: you almost never need to use C<eof> in Perl, because the
1770 input operators typically return C<undef> when they run out of data or
1774 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1775 X<error, handling> X<exception, handling>
1781 =for Pod::Functions catch exceptions or compile and run code
1783 In the first form, often referred to as a "string eval", the return
1784 value of EXPR is parsed and executed as if it
1785 were a little Perl program. The value of the expression (which is itself
1786 determined within scalar context) is first parsed, and if there were no
1787 errors, executed as a block within the lexical context of the current Perl
1788 program. This means, that in particular, any outer lexical variables are
1789 visible to it, and any package variable settings or subroutine and format
1790 definitions remain afterwards.
1792 Note that the value is parsed every time the C<eval> executes.
1793 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1794 delay parsing and subsequent execution of the text of EXPR until run time.
1796 If the C<unicode_eval> feature is enabled (which is the default under a
1797 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1798 characters, so C<use utf8> declarations have no effect, and source filters
1799 are forbidden. In the absence of the C<unicode_eval> feature, the string
1800 will sometimes be treated as characters and sometimes as bytes, depending
1801 on the internal encoding, and source filters activated within the C<eval>
1802 exhibit the erratic, but historical, behaviour of affecting some outer file
1803 scope that is still compiling. See also the L</evalbytes> keyword, which
1804 always treats its input as a byte stream and works properly with source
1805 filters, and the L<feature> pragma.
1807 Problems can arise if the string expands a scalar containing a floating
1808 point number. That scalar can expand to letters, such as C<"NaN"> or
1809 C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1810 point character may be something other than a dot (such as a comma).
1811 None of these are likely to parse as you are likely expecting.
1813 In the second form, the code within the BLOCK is parsed only once--at the
1814 same time the code surrounding the C<eval> itself was parsed--and executed
1815 within the context of the current Perl program. This form is typically
1816 used to trap exceptions more efficiently than the first (see below), while
1817 also providing the benefit of checking the code within BLOCK at compile
1820 The final semicolon, if any, may be omitted from the value of EXPR or within
1823 In both forms, the value returned is the value of the last expression
1824 evaluated inside the mini-program; a return statement may be also used, just
1825 as with subroutines. The expression providing the return value is evaluated
1826 in void, scalar, or list context, depending on the context of the C<eval>
1827 itself. See L</wantarray> for more on how the evaluation context can be
1830 If there is a syntax error or runtime error, or a C<die> statement is
1831 executed, C<eval> returns C<undef> in scalar context
1832 or an empty list in list context, and C<$@> is set to the error
1833 message. (Prior to 5.16, a bug caused C<undef> to be returned
1834 in list context for syntax errors, but not for runtime errors.)
1835 If there was no error, C<$@> is set to the empty string. A
1836 control flow operator like C<last> or C<goto> can bypass the setting of
1837 C<$@>. Beware that using C<eval> neither silences Perl from printing
1838 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1839 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1840 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1841 See L</warn>, L<perlvar>, and L<warnings>.
1843 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1844 determining whether a particular feature (such as C<socket> or C<symlink>)
1845 is implemented. It is also Perl's exception-trapping mechanism, where
1846 the die operator is used to raise exceptions.
1848 If you want to trap errors when loading an XS module, some problems with
1849 the binary interface (such as Perl version skew) may be fatal even with
1850 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1852 If the code to be executed doesn't vary, you may use the eval-BLOCK
1853 form to trap run-time errors without incurring the penalty of
1854 recompiling each time. The error, if any, is still returned in C<$@>.
1857 # make divide-by-zero nonfatal
1858 eval { $answer = $a / $b; }; warn $@ if $@;
1860 # same thing, but less efficient
1861 eval '$answer = $a / $b'; warn $@ if $@;
1863 # a compile-time error
1864 eval { $answer = }; # WRONG
1867 eval '$answer ='; # sets $@
1869 Using the C<eval{}> form as an exception trap in libraries does have some
1870 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1871 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1872 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1873 as this example shows:
1875 # a private exception trap for divide-by-zero
1876 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1879 This is especially significant, given that C<__DIE__> hooks can call
1880 C<die> again, which has the effect of changing their error messages:
1882 # __DIE__ hooks may modify error messages
1884 local $SIG{'__DIE__'} =
1885 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1886 eval { die "foo lives here" };
1887 print $@ if $@; # prints "bar lives here"
1890 Because this promotes action at a distance, this counterintuitive behavior
1891 may be fixed in a future release.
1893 With an C<eval>, you should be especially careful to remember what's
1894 being looked at when:
1900 eval { $x }; # CASE 4
1902 eval "\$$x++"; # CASE 5
1905 Cases 1 and 2 above behave identically: they run the code contained in
1906 the variable $x. (Although case 2 has misleading double quotes making
1907 the reader wonder what else might be happening (nothing is).) Cases 3
1908 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1909 does nothing but return the value of $x. (Case 4 is preferred for
1910 purely visual reasons, but it also has the advantage of compiling at
1911 compile-time instead of at run-time.) Case 5 is a place where
1912 normally you I<would> like to use double quotes, except that in this
1913 particular situation, you can just use symbolic references instead, as
1916 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1917 of localized variables, which means that for your code to run on older
1918 versions, a temporary is required if you want to mask some but not all
1921 # alter $@ on nefarious repugnancy only
1925 local $@; # protect existing $@
1926 eval { test_repugnancy() };
1927 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1928 $@ =~ /nefarious/ and $e = $@;
1930 die $e if defined $e
1933 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1934 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1936 An C<eval ''> executed within a subroutine defined
1937 in the C<DB> package doesn't see the usual
1938 surrounding lexical scope, but rather the scope of the first non-DB piece
1939 of code that called it. You don't normally need to worry about this unless
1940 you are writing a Perl debugger.
1942 =item evalbytes EXPR
1947 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1949 This function is like L</eval> with a string argument, except it always
1950 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1951 string containing characters whose ordinal value exceeds 255 results in an
1952 error. Source filters activated within the evaluated code apply to the
1955 This function is only available under the C<evalbytes> feature, a
1956 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1957 L<feature> for more information.
1962 =item exec PROGRAM LIST
1964 =for Pod::Functions abandon this program to run another
1966 The C<exec> function executes a system command I<and never returns>;
1967 use C<system> instead of C<exec> if you want it to return. It fails and
1968 returns false only if the command does not exist I<and> it is executed
1969 directly instead of via your system's command shell (see below).
1971 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1972 warns you if C<exec> is called in void context and if there is a following
1973 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1974 you always do that, right?). If you I<really> want to follow an C<exec>
1975 with some other statement, you can use one of these styles to avoid the warning:
1977 exec ('foo') or print STDERR "couldn't exec foo: $!";
1978 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1980 If there is more than one argument in LIST, this calls execvp(3) with the
1981 arguments in LIST. If there is only one element in LIST, the argument is
1982 checked for shell metacharacters, and if there are any, the entire
1983 argument is passed to the system's command shell for parsing (this is
1984 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1985 there are no shell metacharacters in the argument, it is split into words
1986 and passed directly to C<execvp>, which is more efficient. Examples:
1988 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1989 exec "sort $outfile | uniq";
1991 If you don't really want to execute the first argument, but want to lie
1992 to the program you are executing about its own name, you can specify
1993 the program you actually want to run as an "indirect object" (without a
1994 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
1995 forces interpretation of the LIST as a multivalued list, even if there
1996 is only a single scalar in the list.) Example:
1998 $shell = '/bin/csh';
1999 exec $shell '-sh'; # pretend it's a login shell
2003 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2005 When the arguments get executed via the system shell, results are
2006 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2009 Using an indirect object with C<exec> or C<system> is also more
2010 secure. This usage (which also works fine with system()) forces
2011 interpretation of the arguments as a multivalued list, even if the
2012 list had just one argument. That way you're safe from the shell
2013 expanding wildcards or splitting up words with whitespace in them.
2015 @args = ( "echo surprise" );
2017 exec @args; # subject to shell escapes
2019 exec { $args[0] } @args; # safe even with one-arg list
2021 The first version, the one without the indirect object, ran the I<echo>
2022 program, passing it C<"surprise"> an argument. The second version didn't;
2023 it tried to run a program named I<"echo surprise">, didn't find it, and set
2024 C<$?> to a non-zero value indicating failure.
2026 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2027 reliably avoid using the shell; C<exec LIST>, even with more than one
2028 element, will fall back to the shell if the first spawn fails.
2030 Perl attempts to flush all files opened for output before the exec,
2031 but this may not be supported on some platforms (see L<perlport>).
2032 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2033 call the C<autoflush()> method of C<IO::Handle> on any open handles
2034 to avoid lost output.
2036 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2037 C<DESTROY> methods on your objects.
2039 Portability issues: L<perlport/exec>.
2042 X<exists> X<autovivification>
2044 =for Pod::Functions test whether a hash key is present
2046 Given an expression that specifies an element of a hash, returns true if the
2047 specified element in the hash has ever been initialized, even if the
2048 corresponding value is undefined.
2050 print "Exists\n" if exists $hash{$key};
2051 print "Defined\n" if defined $hash{$key};
2052 print "True\n" if $hash{$key};
2054 exists may also be called on array elements, but its behavior is much less
2055 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2056 that calling exists on array values is deprecated and likely to be removed in
2057 a future version of Perl.
2059 print "Exists\n" if exists $array[$index];
2060 print "Defined\n" if defined $array[$index];
2061 print "True\n" if $array[$index];
2063 A hash or array element can be true only if it's defined and defined only if
2064 it exists, but the reverse doesn't necessarily hold true.
2066 Given an expression that specifies the name of a subroutine,
2067 returns true if the specified subroutine has ever been declared, even
2068 if it is undefined. Mentioning a subroutine name for exists or defined
2069 does not count as declaring it. Note that a subroutine that does not
2070 exist may still be callable: its package may have an C<AUTOLOAD>
2071 method that makes it spring into existence the first time that it is
2072 called; see L<perlsub>.
2074 print "Exists\n" if exists &subroutine;
2075 print "Defined\n" if defined &subroutine;
2077 Note that the EXPR can be arbitrarily complicated as long as the final
2078 operation is a hash or array key lookup or subroutine name:
2080 if (exists $ref->{A}->{B}->{$key}) { }
2081 if (exists $hash{A}{B}{$key}) { }
2083 if (exists $ref->{A}->{B}->[$ix]) { }
2084 if (exists $hash{A}{B}[$ix]) { }
2086 if (exists &{$ref->{A}{B}{$key}}) { }
2088 Although the most deeply nested array or hash element will not spring into
2089 existence just because its existence was tested, any intervening ones will.
2090 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2091 into existence due to the existence test for the $key element above.
2092 This happens anywhere the arrow operator is used, including even here:
2095 if (exists $ref->{"Some key"}) { }
2096 print $ref; # prints HASH(0x80d3d5c)
2098 This surprising autovivification in what does not at first--or even
2099 second--glance appear to be an lvalue context may be fixed in a future
2102 Use of a subroutine call, rather than a subroutine name, as an argument
2103 to exists() is an error.
2106 exists &sub(); # Error
2109 X<exit> X<terminate> X<abort>
2113 =for Pod::Functions terminate this program
2115 Evaluates EXPR and exits immediately with that value. Example:
2118 exit 0 if $ans =~ /^[Xx]/;
2120 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2121 universally recognized values for EXPR are C<0> for success and C<1>
2122 for error; other values are subject to interpretation depending on the
2123 environment in which the Perl program is running. For example, exiting
2124 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2125 the mailer to return the item undelivered, but that's not true everywhere.
2127 Don't use C<exit> to abort a subroutine if there's any chance that
2128 someone might want to trap whatever error happened. Use C<die> instead,
2129 which can be trapped by an C<eval>.
2131 The exit() function does not always exit immediately. It calls any
2132 defined C<END> routines first, but these C<END> routines may not
2133 themselves abort the exit. Likewise any object destructors that need to
2134 be called are called before the real exit. C<END> routines and destructors
2135 can change the exit status by modifying C<$?>. If this is a problem, you
2136 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2137 See L<perlmod> for details.
2139 Portability issues: L<perlport/exit>.
2142 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2146 =for Pod::Functions raise I<e> to a power
2148 Returns I<e> (the natural logarithm base) to the power of EXPR.
2149 If EXPR is omitted, gives C<exp($_)>.
2152 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2156 =for Pod::Functions +fc return casefolded version of a string
2158 Returns the casefolded version of EXPR. This is the internal function
2159 implementing the C<\F> escape in double-quoted strings.
2161 Casefolding is the process of mapping strings to a form where case
2162 differences are erased; comparing two strings in their casefolded
2163 form is effectively a way of asking if two strings are equal,
2166 Roughly, if you ever found yourself writing this
2168 lc($this) eq lc($that) # Wrong!
2170 uc($this) eq uc($that) # Also wrong!
2172 $this =~ /^\Q$that\E\z/i # Right!
2176 fc($this) eq fc($that)
2178 And get the correct results.
2180 Perl only implements the full form of casefolding,
2181 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2182 L<Unicode::UCD/prop_invmap()>.
2183 For further information on casefolding, refer to
2184 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2185 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2186 available at L<http://www.unicode.org/versions/latest/>, as well as the
2187 Case Charts available at L<http://www.unicode.org/charts/case/>.
2189 If EXPR is omitted, uses C<$_>.
2191 This function behaves the same way under various pragma, such as within
2192 S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2193 exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2194 scope of S<C<use locale>>. The foldcase of this character would
2195 normally be C<"ss">, but as explained in the L</lc> section, case
2196 changes that cross the 255/256 boundary are problematic under locales,
2197 and are hence prohibited. Therefore, this function under locale returns
2198 instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2199 LONG S. Since that character itself folds to C<"s">, the string of two
2200 of them together should be equivalent to a single U+1E9E when foldcased.
2202 While the Unicode Standard defines two additional forms of casefolding,
2203 one for Turkic languages and one that never maps one character into multiple
2204 characters, these are not provided by the Perl core; However, the CPAN module
2205 C<Unicode::Casing> may be used to provide an implementation.
2207 This keyword is available only when the C<"fc"> feature is enabled,
2208 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2209 include a C<use v5.16> or later to the current scope.
2211 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2214 =for Pod::Functions file control system call
2216 Implements the fcntl(2) function. You'll probably have to say
2220 first to get the correct constant definitions. Argument processing and
2221 value returned work just like C<ioctl> below.
2225 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2226 or die "can't fcntl F_GETFL: $!";
2228 You don't have to check for C<defined> on the return from C<fcntl>.
2229 Like C<ioctl>, it maps a C<0> return from the system call into
2230 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2231 in numeric context. It is also exempt from the normal B<-w> warnings
2232 on improper numeric conversions.
2234 Note that C<fcntl> raises an exception if used on a machine that
2235 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2236 manpage to learn what functions are available on your system.
2238 Here's an example of setting a filehandle named C<REMOTE> to be
2239 non-blocking at the system level. You'll have to negotiate C<$|>
2240 on your own, though.
2242 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2244 $flags = fcntl(REMOTE, F_GETFL, 0)
2245 or die "Can't get flags for the socket: $!\n";
2247 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2248 or die "Can't set flags for the socket: $!\n";
2250 Portability issues: L<perlport/fcntl>.
2255 =for Pod::Functions the name of the current source file
2257 A special token that returns the name of the file in which it occurs.
2259 =item fileno FILEHANDLE
2262 =for Pod::Functions return file descriptor from filehandle
2264 Returns the file descriptor for a filehandle, or undefined if the
2265 filehandle is not open. If there is no real file descriptor at the OS
2266 level, as can happen with filehandles connected to memory objects via
2267 C<open> with a reference for the third argument, -1 is returned.
2269 This is mainly useful for constructing
2270 bitmaps for C<select> and low-level POSIX tty-handling operations.
2271 If FILEHANDLE is an expression, the value is taken as an indirect
2272 filehandle, generally its name.
2274 You can use this to find out whether two handles refer to the
2275 same underlying descriptor:
2277 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
2278 print "THIS and THAT are dups\n";
2279 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2280 print "THIS and THAT have different " .
2281 "underlying file descriptors\n";
2283 print "At least one of THIS and THAT does " .
2284 "not have a real file descriptor\n";
2287 =item flock FILEHANDLE,OPERATION
2288 X<flock> X<lock> X<locking>
2290 =for Pod::Functions lock an entire file with an advisory lock
2292 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2293 for success, false on failure. Produces a fatal error if used on a
2294 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2295 C<flock> is Perl's portable file-locking interface, although it locks
2296 entire files only, not records.
2298 Two potentially non-obvious but traditional C<flock> semantics are
2299 that it waits indefinitely until the lock is granted, and that its locks
2300 are B<merely advisory>. Such discretionary locks are more flexible, but
2301 offer fewer guarantees. This means that programs that do not also use
2302 C<flock> may modify files locked with C<flock>. See L<perlport>,
2303 your port's specific documentation, and your system-specific local manpages
2304 for details. It's best to assume traditional behavior if you're writing
2305 portable programs. (But if you're not, you should as always feel perfectly
2306 free to write for your own system's idiosyncrasies (sometimes called
2307 "features"). Slavish adherence to portability concerns shouldn't get
2308 in the way of your getting your job done.)
2310 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2311 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2312 you can use the symbolic names if you import them from the L<Fcntl> module,
2313 either individually, or as a group using the C<:flock> tag. LOCK_SH
2314 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2315 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2316 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2317 waiting for the lock; check the return status to see if you got it.
2319 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2320 before locking or unlocking it.
2322 Note that the emulation built with lockf(3) doesn't provide shared
2323 locks, and it requires that FILEHANDLE be open with write intent. These
2324 are the semantics that lockf(3) implements. Most if not all systems
2325 implement lockf(3) in terms of fcntl(2) locking, though, so the
2326 differing semantics shouldn't bite too many people.
2328 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2329 be open with read intent to use LOCK_SH and requires that it be open
2330 with write intent to use LOCK_EX.
2332 Note also that some versions of C<flock> cannot lock things over the
2333 network; you would need to use the more system-specific C<fcntl> for
2334 that. If you like you can force Perl to ignore your system's flock(2)
2335 function, and so provide its own fcntl(2)-based emulation, by passing
2336 the switch C<-Ud_flock> to the F<Configure> program when you configure
2337 and build a new Perl.
2339 Here's a mailbox appender for BSD systems.
2341 # import LOCK_* and SEEK_END constants
2342 use Fcntl qw(:flock SEEK_END);
2346 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2348 # and, in case someone appended while we were waiting...
2349 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2354 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2357 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2358 or die "Can't open mailbox: $!";
2361 print $mbox $msg,"\n\n";
2364 On systems that support a real flock(2), locks are inherited across fork()
2365 calls, whereas those that must resort to the more capricious fcntl(2)
2366 function lose their locks, making it seriously harder to write servers.
2368 See also L<DB_File> for other flock() examples.
2370 Portability issues: L<perlport/flock>.
2373 X<fork> X<child> X<parent>
2375 =for Pod::Functions create a new process just like this one
2377 Does a fork(2) system call to create a new process running the
2378 same program at the same point. It returns the child pid to the
2379 parent process, C<0> to the child process, or C<undef> if the fork is
2380 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2381 are shared, while everything else is copied. On most systems supporting
2382 fork(), great care has gone into making it extremely efficient (for
2383 example, using copy-on-write technology on data pages), making it the
2384 dominant paradigm for multitasking over the last few decades.
2386 Perl attempts to flush all files opened for
2387 output before forking the child process, but this may not be supported
2388 on some platforms (see L<perlport>). To be safe, you may need to set
2389 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2390 C<IO::Handle> on any open handles to avoid duplicate output.
2392 If you C<fork> without ever waiting on your children, you will
2393 accumulate zombies. On some systems, you can avoid this by setting
2394 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2395 forking and reaping moribund children.
2397 Note that if your forked child inherits system file descriptors like
2398 STDIN and STDOUT that are actually connected by a pipe or socket, even
2399 if you exit, then the remote server (such as, say, a CGI script or a
2400 backgrounded job launched from a remote shell) won't think you're done.
2401 You should reopen those to F</dev/null> if it's any issue.
2403 On some platforms such as Windows, where the fork() system call is not available,
2404 Perl can be built to emulate fork() in the Perl interpreter.
2405 The emulation is designed, at the level of the Perl program,
2406 to be as compatible as possible with the "Unix" fork().
2407 However it has limitations that have to be considered in code intended to be portable.
2408 See L<perlfork> for more details.
2410 Portability issues: L<perlport/fork>.
2415 =for Pod::Functions declare a picture format with use by the write() function
2417 Declare a picture format for use by the C<write> function. For
2421 Test: @<<<<<<<< @||||| @>>>>>
2422 $str, $%, '$' . int($num)
2426 $num = $cost/$quantity;
2430 See L<perlform> for many details and examples.
2432 =item formline PICTURE,LIST
2435 =for Pod::Functions internal function used for formats
2437 This is an internal function used by C<format>s, though you may call it,
2438 too. It formats (see L<perlform>) a list of values according to the
2439 contents of PICTURE, placing the output into the format output
2440 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2441 Eventually, when a C<write> is done, the contents of
2442 C<$^A> are written to some filehandle. You could also read C<$^A>
2443 and then set C<$^A> back to C<"">. Note that a format typically
2444 does one C<formline> per line of form, but the C<formline> function itself
2445 doesn't care how many newlines are embedded in the PICTURE. This means
2446 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2447 You may therefore need to use multiple formlines to implement a single
2448 record format, just like the C<format> compiler.
2450 Be careful if you put double quotes around the picture, because an C<@>
2451 character may be taken to mean the beginning of an array name.
2452 C<formline> always returns true. See L<perlform> for other examples.
2454 If you are trying to use this instead of C<write> to capture the output,
2455 you may find it easier to open a filehandle to a scalar
2456 (C<< open $fh, ">", \$output >>) and write to that instead.
2458 =item getc FILEHANDLE
2459 X<getc> X<getchar> X<character> X<file, read>
2463 =for Pod::Functions get the next character from the filehandle
2465 Returns the next character from the input file attached to FILEHANDLE,
2466 or the undefined value at end of file or if there was an error (in
2467 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2468 STDIN. This is not particularly efficient. However, it cannot be
2469 used by itself to fetch single characters without waiting for the user
2470 to hit enter. For that, try something more like:
2473 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2476 system "stty", '-icanon', 'eol', "\001";
2482 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2485 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2489 Determination of whether $BSD_STYLE should be set
2490 is left as an exercise to the reader.
2492 The C<POSIX::getattr> function can do this more portably on
2493 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2494 module from your nearest L<CPAN|http://www.cpan.org> site.
2497 X<getlogin> X<login>
2499 =for Pod::Functions return who logged in at this tty
2501 This implements the C library function of the same name, which on most
2502 systems returns the current login from F</etc/utmp>, if any. If it
2503 returns the empty string, use C<getpwuid>.
2505 $login = getlogin || getpwuid($<) || "Kilroy";
2507 Do not consider C<getlogin> for authentication: it is not as
2508 secure as C<getpwuid>.
2510 Portability issues: L<perlport/getlogin>.
2512 =item getpeername SOCKET
2513 X<getpeername> X<peer>
2515 =for Pod::Functions find the other end of a socket connection
2517 Returns the packed sockaddr address of the other end of the SOCKET
2521 $hersockaddr = getpeername(SOCK);
2522 ($port, $iaddr) = sockaddr_in($hersockaddr);
2523 $herhostname = gethostbyaddr($iaddr, AF_INET);
2524 $herstraddr = inet_ntoa($iaddr);
2529 =for Pod::Functions get process group
2531 Returns the current process group for the specified PID. Use
2532 a PID of C<0> to get the current process group for the
2533 current process. Will raise an exception if used on a machine that
2534 doesn't implement getpgrp(2). If PID is omitted, returns the process
2535 group of the current process. Note that the POSIX version of C<getpgrp>
2536 does not accept a PID argument, so only C<PID==0> is truly portable.
2538 Portability issues: L<perlport/getpgrp>.
2541 X<getppid> X<parent> X<pid>
2543 =for Pod::Functions get parent process ID
2545 Returns the process id of the parent process.
2547 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2548 around non-POSIX thread semantics the minority of Linux systems (and
2549 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2550 has since been removed. See the documentation for L<$$|perlvar/$$> for
2553 Portability issues: L<perlport/getppid>.
2555 =item getpriority WHICH,WHO
2556 X<getpriority> X<priority> X<nice>
2558 =for Pod::Functions get current nice value
2560 Returns the current priority for a process, a process group, or a user.
2561 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2562 machine that doesn't implement getpriority(2).
2564 Portability issues: L<perlport/getpriority>.
2567 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2568 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2569 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2570 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2571 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2572 X<endnetent> X<endprotoent> X<endservent>
2574 =for Pod::Functions get passwd record given user login name
2578 =for Pod::Functions get group record given group name
2580 =item gethostbyname NAME
2582 =for Pod::Functions get host record given name
2584 =item getnetbyname NAME
2586 =for Pod::Functions get networks record given name
2588 =item getprotobyname NAME
2590 =for Pod::Functions get protocol record given name
2594 =for Pod::Functions get passwd record given user ID
2598 =for Pod::Functions get group record given group user ID
2600 =item getservbyname NAME,PROTO
2602 =for Pod::Functions get services record given its name
2604 =item gethostbyaddr ADDR,ADDRTYPE
2606 =for Pod::Functions get host record given its address
2608 =item getnetbyaddr ADDR,ADDRTYPE
2610 =for Pod::Functions get network record given its address
2612 =item getprotobynumber NUMBER
2614 =for Pod::Functions get protocol record numeric protocol
2616 =item getservbyport PORT,PROTO
2618 =for Pod::Functions get services record given numeric port
2622 =for Pod::Functions get next passwd record
2626 =for Pod::Functions get next group record
2630 =for Pod::Functions get next hosts record
2634 =for Pod::Functions get next networks record
2638 =for Pod::Functions get next protocols record
2642 =for Pod::Functions get next services record
2646 =for Pod::Functions prepare passwd file for use
2650 =for Pod::Functions prepare group file for use
2652 =item sethostent STAYOPEN
2654 =for Pod::Functions prepare hosts file for use
2656 =item setnetent STAYOPEN
2658 =for Pod::Functions prepare networks file for use
2660 =item setprotoent STAYOPEN
2662 =for Pod::Functions prepare protocols file for use
2664 =item setservent STAYOPEN
2666 =for Pod::Functions prepare services file for use
2670 =for Pod::Functions be done using passwd file
2674 =for Pod::Functions be done using group file
2678 =for Pod::Functions be done using hosts file
2682 =for Pod::Functions be done using networks file
2686 =for Pod::Functions be done using protocols file
2690 =for Pod::Functions be done using services file
2692 These routines are the same as their counterparts in the
2693 system C library. In list context, the return values from the
2694 various get routines are as follows:
2697 ( $name, $passwd, $gid, $members ) = getgr*
2698 ( $name, $aliases, $addrtype, $net ) = getnet*
2699 ( $name, $aliases, $port, $proto ) = getserv*
2700 ( $name, $aliases, $proto ) = getproto*
2701 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2702 ( $name, $passwd, $uid, $gid, $quota,
2703 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2706 (If the entry doesn't exist you get an empty list.)
2708 The exact meaning of the $gcos field varies but it usually contains
2709 the real name of the user (as opposed to the login name) and other
2710 information pertaining to the user. Beware, however, that in many
2711 system users are able to change this information and therefore it
2712 cannot be trusted and therefore the $gcos is tainted (see
2713 L<perlsec>). The $passwd and $shell, user's encrypted password and
2714 login shell, are also tainted, for the same reason.
2716 In scalar context, you get the name, unless the function was a
2717 lookup by name, in which case you get the other thing, whatever it is.
2718 (If the entry doesn't exist you get the undefined value.) For example:
2720 $uid = getpwnam($name);
2721 $name = getpwuid($num);
2723 $gid = getgrnam($name);
2724 $name = getgrgid($num);
2728 In I<getpw*()> the fields $quota, $comment, and $expire are special
2729 in that they are unsupported on many systems. If the
2730 $quota is unsupported, it is an empty scalar. If it is supported, it
2731 usually encodes the disk quota. If the $comment field is unsupported,
2732 it is an empty scalar. If it is supported it usually encodes some
2733 administrative comment about the user. In some systems the $quota
2734 field may be $change or $age, fields that have to do with password
2735 aging. In some systems the $comment field may be $class. The $expire
2736 field, if present, encodes the expiration period of the account or the
2737 password. For the availability and the exact meaning of these fields
2738 in your system, please consult getpwnam(3) and your system's
2739 F<pwd.h> file. You can also find out from within Perl what your
2740 $quota and $comment fields mean and whether you have the $expire field
2741 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2742 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2743 files are supported only if your vendor has implemented them in the
2744 intuitive fashion that calling the regular C library routines gets the
2745 shadow versions if you're running under privilege or if there exists
2746 the shadow(3) functions as found in System V (this includes Solaris
2747 and Linux). Those systems that implement a proprietary shadow password
2748 facility are unlikely to be supported.
2750 The $members value returned by I<getgr*()> is a space-separated list of
2751 the login names of the members of the group.
2753 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2754 C, it will be returned to you via C<$?> if the function call fails. The
2755 C<@addrs> value returned by a successful call is a list of raw
2756 addresses returned by the corresponding library call. In the
2757 Internet domain, each address is four bytes long; you can unpack it
2758 by saying something like:
2760 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2762 The Socket library makes this slightly easier:
2765 $iaddr = inet_aton("127.1"); # or whatever address
2766 $name = gethostbyaddr($iaddr, AF_INET);
2768 # or going the other way
2769 $straddr = inet_ntoa($iaddr);
2771 In the opposite way, to resolve a hostname to the IP address
2775 $packed_ip = gethostbyname("www.perl.org");
2776 if (defined $packed_ip) {
2777 $ip_address = inet_ntoa($packed_ip);
2780 Make sure C<gethostbyname()> is called in SCALAR context and that
2781 its return value is checked for definedness.
2783 The C<getprotobynumber> function, even though it only takes one argument,
2784 has the precedence of a list operator, so beware:
2786 getprotobynumber $number eq 'icmp' # WRONG
2787 getprotobynumber($number eq 'icmp') # actually means this
2788 getprotobynumber($number) eq 'icmp' # better this way
2790 If you get tired of remembering which element of the return list
2791 contains which return value, by-name interfaces are provided
2792 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2793 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2794 and C<User::grent>. These override the normal built-ins, supplying
2795 versions that return objects with the appropriate names
2796 for each field. For example:
2800 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2802 Even though it looks as though they're the same method calls (uid),
2803 they aren't, because a C<File::stat> object is different from
2804 a C<User::pwent> object.
2806 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2808 =item getsockname SOCKET
2811 =for Pod::Functions retrieve the sockaddr for a given socket
2813 Returns the packed sockaddr address of this end of the SOCKET connection,
2814 in case you don't know the address because you have several different
2815 IPs that the connection might have come in on.
2818 $mysockaddr = getsockname(SOCK);
2819 ($port, $myaddr) = sockaddr_in($mysockaddr);
2820 printf "Connect to %s [%s]\n",
2821 scalar gethostbyaddr($myaddr, AF_INET),
2824 =item getsockopt SOCKET,LEVEL,OPTNAME
2827 =for Pod::Functions get socket options on a given socket
2829 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2830 Options may exist at multiple protocol levels depending on the socket
2831 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2832 C<Socket> module) will exist. To query options at another level the
2833 protocol number of the appropriate protocol controlling the option
2834 should be supplied. For example, to indicate that an option is to be
2835 interpreted by the TCP protocol, LEVEL should be set to the protocol
2836 number of TCP, which you can get using C<getprotobyname>.
2838 The function returns a packed string representing the requested socket
2839 option, or C<undef> on error, with the reason for the error placed in
2840 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2841 consult getsockopt(2) for details. A common case is that the option is an
2842 integer, in which case the result is a packed integer, which you can decode
2843 using C<unpack> with the C<i> (or C<I>) format.
2845 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2847 use Socket qw(:all);
2849 defined(my $tcp = getprotobyname("tcp"))
2850 or die "Could not determine the protocol number for tcp";
2851 # my $tcp = IPPROTO_TCP; # Alternative
2852 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2853 or die "getsockopt TCP_NODELAY: $!";
2854 my $nodelay = unpack("I", $packed);
2855 print "Nagle's algorithm is turned ",
2856 $nodelay ? "off\n" : "on\n";
2858 Portability issues: L<perlport/getsockopt>.
2861 X<glob> X<wildcard> X<filename, expansion> X<expand>
2865 =for Pod::Functions expand filenames using wildcards
2867 In list context, returns a (possibly empty) list of filename expansions on
2868 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2869 scalar context, glob iterates through such filename expansions, returning
2870 undef when the list is exhausted. This is the internal function
2871 implementing the C<< <*.c> >> operator, but you can use it directly. If
2872 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2873 more detail in L<perlop/"I/O Operators">.
2875 Note that C<glob> splits its arguments on whitespace and treats
2876 each segment as separate pattern. As such, C<glob("*.c *.h")>
2877 matches all files with a F<.c> or F<.h> extension. The expression
2878 C<glob(".* *")> matches all files in the current working directory.
2879 If you want to glob filenames that might contain whitespace, you'll
2880 have to use extra quotes around the spacey filename to protect it.
2881 For example, to glob filenames that have an C<e> followed by a space
2882 followed by an C<f>, use either of:
2884 @spacies = <"*e f*">;
2885 @spacies = glob '"*e f*"';
2886 @spacies = glob q("*e f*");
2888 If you had to get a variable through, you could do this:
2890 @spacies = glob "'*${var}e f*'";
2891 @spacies = glob qq("*${var}e f*");
2893 If non-empty braces are the only wildcard characters used in the
2894 C<glob>, no filenames are matched, but potentially many strings
2895 are returned. For example, this produces nine strings, one for
2896 each pairing of fruits and colors:
2898 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2900 This operator is implemented using the standard
2901 C<File::Glob> extension. See L<File::Glob> for details, including
2902 C<bsd_glob> which does not treat whitespace as a pattern separator.
2904 Portability issues: L<perlport/glob>.
2907 X<gmtime> X<UTC> X<Greenwich>
2911 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2913 Works just like L</localtime> but the returned values are
2914 localized for the standard Greenwich time zone.
2916 Note: When called in list context, $isdst, the last value
2917 returned by gmtime, is always C<0>. There is no
2918 Daylight Saving Time in GMT.
2920 Portability issues: L<perlport/gmtime>.
2923 X<goto> X<jump> X<jmp>
2929 =for Pod::Functions create spaghetti code
2931 The C<goto LABEL> form finds the statement labeled with LABEL and
2932 resumes execution there. It can't be used to get out of a block or
2933 subroutine given to C<sort>. It can be used to go almost anywhere
2934 else within the dynamic scope, including out of subroutines, but it's
2935 usually better to use some other construct such as C<last> or C<die>.
2936 The author of Perl has never felt the need to use this form of C<goto>
2937 (in Perl, that is; C is another matter). (The difference is that C
2938 does not offer named loops combined with loop control. Perl does, and
2939 this replaces most structured uses of C<goto> in other languages.)
2941 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
2942 a label name. If it evaluates to a code reference, it will be handled
2943 like C<goto &NAME>, below. This is especially useful for implementing
2944 tail recursion via C<goto __SUB__>.
2946 If the expression evaluates to a label name, its scope will be resolved
2947 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2948 necessarily recommended if you're optimizing for maintainability:
2950 goto ("FOO", "BAR", "GLARCH")[$i];
2952 As shown in this example, C<goto EXPR> is exempt from the "looks like a
2953 function" rule. A pair of parentheses following it does not (necessarily)
2954 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2955 Also, unlike most named operators, this has the same precedence as
2958 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
2959 deprecated and will issue a warning. Even then, it may not be used to
2960 go into any construct that requires initialization, such as a
2961 subroutine or a C<foreach> loop. It also can't be used to go into a
2962 construct that is optimized away.
2964 The C<goto &NAME> form is quite different from the other forms of
2965 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2966 doesn't have the stigma associated with other gotos. Instead, it
2967 exits the current subroutine (losing any changes set by local()) and
2968 immediately calls in its place the named subroutine using the current
2969 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2970 load another subroutine and then pretend that the other subroutine had
2971 been called in the first place (except that any modifications to C<@_>
2972 in the current subroutine are propagated to the other subroutine.)
2973 After the C<goto>, not even C<caller> will be able to tell that this
2974 routine was called first.
2976 NAME needn't be the name of a subroutine; it can be a scalar variable
2977 containing a code reference or a block that evaluates to a code
2980 =item grep BLOCK LIST
2983 =item grep EXPR,LIST
2985 =for Pod::Functions locate elements in a list test true against a given criterion
2987 This is similar in spirit to, but not the same as, grep(1) and its
2988 relatives. In particular, it is not limited to using regular expressions.
2990 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2991 C<$_> to each element) and returns the list value consisting of those
2992 elements for which the expression evaluated to true. In scalar
2993 context, returns the number of times the expression was true.
2995 @foo = grep(!/^#/, @bar); # weed out comments
2999 @foo = grep {!/^#/} @bar; # weed out comments
3001 Note that C<$_> is an alias to the list value, so it can be used to
3002 modify the elements of the LIST. While this is useful and supported,
3003 it can cause bizarre results if the elements of LIST are not variables.
3004 Similarly, grep returns aliases into the original list, much as a for
3005 loop's index variable aliases the list elements. That is, modifying an
3006 element of a list returned by grep (for example, in a C<foreach>, C<map>
3007 or another C<grep>) actually modifies the element in the original list.
3008 This is usually something to be avoided when writing clear code.
3010 If C<$_> is lexical in the scope where the C<grep> appears (because it has
3011 been declared with the deprecated C<my $_> construct)
3012 then, in addition to being locally aliased to
3013 the list elements, C<$_> keeps being lexical inside the block; i.e., it
3014 can't be seen from the outside, avoiding any potential side-effects.
3016 See also L</map> for a list composed of the results of the BLOCK or EXPR.
3019 X<hex> X<hexadecimal>
3023 =for Pod::Functions convert a string to a hexadecimal number
3025 Interprets EXPR as a hex string and returns the corresponding value.
3026 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
3027 L</oct>.) If EXPR is omitted, uses C<$_>.
3029 print hex '0xAf'; # prints '175'
3030 print hex 'aF'; # same
3032 Hex strings may only represent integers. Strings that would cause
3033 integer overflow trigger a warning. Leading whitespace is not stripped,
3034 unlike oct(). To present something as hex, look into L</printf>,
3035 L</sprintf>, and L</unpack>.
3040 =for Pod::Functions patch a module's namespace into your own
3042 There is no builtin C<import> function. It is just an ordinary
3043 method (subroutine) defined (or inherited) by modules that wish to export
3044 names to another module. The C<use> function calls the C<import> method
3045 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3047 =item index STR,SUBSTR,POSITION
3048 X<index> X<indexOf> X<InStr>
3050 =item index STR,SUBSTR
3052 =for Pod::Functions find a substring within a string
3054 The index function searches for one string within another, but without
3055 the wildcard-like behavior of a full regular-expression pattern match.
3056 It returns the position of the first occurrence of SUBSTR in STR at
3057 or after POSITION. If POSITION is omitted, starts searching from the
3058 beginning of the string. POSITION before the beginning of the string
3059 or after its end is treated as if it were the beginning or the end,
3060 respectively. POSITION and the return value are based at zero.
3061 If the substring is not found, C<index> returns -1.
3064 X<int> X<integer> X<truncate> X<trunc> X<floor>
3068 =for Pod::Functions get the integer portion of a number
3070 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3071 You should not use this function for rounding: one because it truncates
3072 towards C<0>, and two because machine representations of floating-point
3073 numbers can sometimes produce counterintuitive results. For example,
3074 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3075 because it's really more like -268.99999999999994315658 instead. Usually,
3076 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3077 functions will serve you better than will int().
3079 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3082 =for Pod::Functions system-dependent device control system call
3084 Implements the ioctl(2) function. You'll probably first have to say
3086 require "sys/ioctl.ph"; # probably in
3087 # $Config{archlib}/sys/ioctl.ph
3089 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3090 exist or doesn't have the correct definitions you'll have to roll your
3091 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3092 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3093 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3094 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3095 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3096 has no string value but does have a numeric value, that value will be
3097 passed rather than a pointer to the string value. To guarantee this to be
3098 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3099 functions may be needed to manipulate the values of structures used by
3102 The return value of C<ioctl> (and C<fcntl>) is as follows:
3104 if OS returns: then Perl returns:
3106 0 string "0 but true"
3107 anything else that number
3109 Thus Perl returns true on success and false on failure, yet you can
3110 still easily determine the actual value returned by the operating
3113 $retval = ioctl(...) || -1;
3114 printf "System returned %d\n", $retval;
3116 The special string C<"0 but true"> is exempt from B<-w> complaints
3117 about improper numeric conversions.
3119 Portability issues: L<perlport/ioctl>.
3121 =item join EXPR,LIST
3124 =for Pod::Functions join a list into a string using a separator
3126 Joins the separate strings of LIST into a single string with fields
3127 separated by the value of EXPR, and returns that new string. Example:
3129 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3131 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3132 first argument. Compare L</split>.
3141 =for Pod::Functions retrieve list of indices from a hash
3143 Called in list context, returns a list consisting of all the keys of the
3144 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3145 releases prior to 5.12 will produce a syntax error if you try to use an
3146 array argument. In scalar context, returns the number of keys or indices.
3148 Hash entries are returned in an apparently random order. The actual random
3149 order is specific to a given hash; the exact same series of operations
3150 on two hashes may result in a different order for each hash. Any insertion
3151 into the hash may change the order, as will any deletion, with the exception
3152 that the most recent key returned by C<each> or C<keys> may be deleted
3153 without changing the order. So long as a given hash is unmodified you may
3154 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
3155 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3156 details on why hash order is randomized. Aside from the guarantees
3157 provided here the exact details of Perl's hash algorithm and the hash
3158 traversal order are subject to change in any release of Perl.
3160 As a side effect, calling keys() resets the internal iterator of the HASH or
3161 ARRAY (see L</each>). In particular, calling keys() in void context resets
3162 the iterator with no other overhead.
3164 Here is yet another way to print your environment:
3167 @values = values %ENV;
3169 print pop(@keys), '=', pop(@values), "\n";
3172 or how about sorted by key:
3174 foreach $key (sort(keys %ENV)) {
3175 print $key, '=', $ENV{$key}, "\n";
3178 The returned values are copies of the original keys in the hash, so
3179 modifying them will not affect the original hash. Compare L</values>.
3181 To sort a hash by value, you'll need to use a C<sort> function.
3182 Here's a descending numeric sort of a hash by its values:
3184 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3185 printf "%4d %s\n", $hash{$key}, $key;
3188 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3189 allocated for the given hash. This can gain you a measure of efficiency if
3190 you know the hash is going to get big. (This is similar to pre-extending
3191 an array by assigning a larger number to $#array.) If you say
3195 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3196 in fact, since it rounds up to the next power of two. These
3197 buckets will be retained even if you do C<%hash = ()>, use C<undef
3198 %hash> if you want to free the storage while C<%hash> is still in scope.
3199 You can't shrink the number of buckets allocated for the hash using
3200 C<keys> in this way (but you needn't worry about doing this by accident,
3201 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3204 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3205 a reference to an unblessed hash or array. The argument will be
3206 dereferenced automatically. This aspect of C<keys> is considered highly
3207 experimental. The exact behaviour may change in a future version of Perl.
3209 for (keys $hashref) { ... }
3210 for (keys $obj->get_arrayref) { ... }
3212 To avoid confusing would-be users of your code who are running earlier
3213 versions of Perl with mysterious syntax errors, put this sort of thing at
3214 the top of your file to signal that your code will work I<only> on Perls of
3217 use 5.012; # so keys/values/each work on arrays
3218 use 5.014; # so keys/values/each work on scalars (experimental)
3220 See also C<each>, C<values>, and C<sort>.
3222 =item kill SIGNAL, LIST
3227 =for Pod::Functions send a signal to a process or process group
3229 Sends a signal to a list of processes. Returns the number of arguments
3230 that were successfully used to signal (which is not necessarily the same
3231 as the number of processes actually killed, e.g. where a process group is
3234 $cnt = kill 'HUP', $child1, $child2;
3235 kill 'KILL', @goners;
3237 SIGNAL may be either a signal name (a string) or a signal number. A signal
3238 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3239 same signal. The string form of SIGNAL is recommended for portability because
3240 the same signal may have different numbers in different operating systems.
3242 A list of signal names supported by the current platform can be found in
3243 C<$Config{sig_name}>, which is provided by the C<Config> module. See L<Config>
3246 A negative signal name is the same as a negative signal number, killing process
3247 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3248 C<kill -9, $pgrp> will send C<SIGKILL> to
3249 the entire process group specified. That
3250 means you usually want to use positive not negative signals.
3252 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3253 no signal is sent to
3254 the process, but C<kill> checks whether it's I<possible> to send a signal to it
3255 (that means, to be brief, that the process is owned by the same user, or we are
3256 the super-user). This is useful to check that a child process is still
3257 alive (even if only as a zombie) and hasn't changed its UID. See
3258 L<perlport> for notes on the portability of this construct.
3260 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3261 the operating system. For example, on POSIX-conforming systems, zero will
3262 signal the current process group, -1 will signal all processes, and any
3263 other negative PROCESS number will act as a negative signal number and
3264 kill the entire process group specified.
3266 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3267 A warning may be produced in a future version.
3269 See L<perlipc/"Signals"> for more details.
3271 On some platforms such as Windows where the fork() system call is not
3272 available, Perl can be built to emulate fork() at the interpreter level.
3273 This emulation has limitations related to kill that have to be considered,
3274 for code running on Windows and in code intended to be portable.
3276 See L<perlfork> for more details.
3278 If there is no I<LIST> of processes, no signal is sent, and the return
3279 value is 0. This form is sometimes used, however, because it causes
3280 tainting checks to be run. But see
3281 L<perlsec/Laundering and Detecting Tainted Data>.
3283 Portability issues: L<perlport/kill>.
3292 =for Pod::Functions exit a block prematurely
3294 The C<last> command is like the C<break> statement in C (as used in
3295 loops); it immediately exits the loop in question. If the LABEL is
3296 omitted, the command refers to the innermost enclosing
3297 loop. The C<last EXPR> form, available starting in Perl
3298 5.18.0, allows a label name to be computed at run time,
3299 and is otherwise identical to C<last LABEL>. The
3300 C<continue> block, if any, is not executed:
3302 LINE: while (<STDIN>) {
3303 last LINE if /^$/; # exit when done with header
3307 C<last> cannot be used to exit a block that returns a value such as
3308 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3309 a grep() or map() operation.
3311 Note that a block by itself is semantically identical to a loop
3312 that executes once. Thus C<last> can be used to effect an early
3313 exit out of such a block.
3315 See also L</continue> for an illustration of how C<last>, C<next>, and
3318 Unlike most named operators, this has the same precedence as assignment.
3319 It is also exempt from the looks-like-a-function rule, so
3320 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3328 =for Pod::Functions return lower-case version of a string
3330 Returns a lowercased version of EXPR. This is the internal function
3331 implementing the C<\L> escape in double-quoted strings.
3333 If EXPR is omitted, uses C<$_>.
3335 What gets returned depends on several factors:
3339 =item If C<use bytes> is in effect:
3341 The results follow ASCII rules. Only the characters C<A-Z> change,
3342 to C<a-z> respectively.
3344 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3346 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3347 rules for the remaining code points (this last can only happen if
3348 the UTF8 flag is also set). See L<perllocale>.
3350 Starting in v5.20, Perl wil use full Unicode rules if the locale is
3351 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3352 case changes that cross the 255/256
3353 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3354 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3355 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3356 locale), the lower case of U+1E9E is
3357 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3358 current locale, and Perl has no way of knowing if that character even
3359 exists in the locale, much less what code point it is. Perl returns
3360 the input character unchanged, for all instances (and there aren't
3361 many) where the 255/256 boundary would otherwise be crossed.
3363 =item Otherwise, If EXPR has the UTF8 flag set:
3365 Unicode rules are used for the case change.
3367 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3369 Unicode rules are used for the case change.
3373 ASCII rules are used for the case change. The lowercase of any character
3374 outside the ASCII range is the character itself.
3379 X<lcfirst> X<lowercase>
3383 =for Pod::Functions return a string with just the next letter in lower case
3385 Returns the value of EXPR with the first character lowercased. This
3386 is the internal function implementing the C<\l> escape in
3387 double-quoted strings.
3389 If EXPR is omitted, uses C<$_>.
3391 This function behaves the same way under various pragmata, such as in a locale,
3399 =for Pod::Functions return the number of characters in a string
3401 Returns the length in I<characters> of the value of EXPR. If EXPR is
3402 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3405 This function cannot be used on an entire array or hash to find out how
3406 many elements these have. For that, use C<scalar @array> and C<scalar keys
3407 %hash>, respectively.
3409 Like all Perl character operations, length() normally deals in logical
3410 characters, not physical bytes. For how many bytes a string encoded as
3411 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3412 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3417 =for Pod::Functions the current source line number
3419 A special token that compiles to the current line number.
3421 =item link OLDFILE,NEWFILE
3424 =for Pod::Functions create a hard link in the filesystem
3426 Creates a new filename linked to the old filename. Returns true for
3427 success, false otherwise.
3429 Portability issues: L<perlport/link>.
3431 =item listen SOCKET,QUEUESIZE
3434 =for Pod::Functions register your socket as a server
3436 Does the same thing that the listen(2) system call does. Returns true if
3437 it succeeded, false otherwise. See the example in
3438 L<perlipc/"Sockets: Client/Server Communication">.
3443 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3445 You really probably want to be using C<my> instead, because C<local> isn't
3446 what most people think of as "local". See
3447 L<perlsub/"Private Variables via my()"> for details.
3449 A local modifies the listed variables to be local to the enclosing
3450 block, file, or eval. If more than one value is listed, the list must
3451 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3452 for details, including issues with tied arrays and hashes.
3454 The C<delete local EXPR> construct can also be used to localize the deletion
3455 of array/hash elements to the current block.
3456 See L<perlsub/"Localized deletion of elements of composite types">.
3458 =item localtime EXPR
3459 X<localtime> X<ctime>
3463 =for Pod::Functions convert UNIX time into record or string using local time
3465 Converts a time as returned by the time function to a 9-element list
3466 with the time analyzed for the local time zone. Typically used as
3470 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3473 All list elements are numeric and come straight out of the C `struct
3474 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3475 of the specified time.
3477 C<$mday> is the day of the month and C<$mon> the month in
3478 the range C<0..11>, with 0 indicating January and 11 indicating December.
3479 This makes it easy to get a month name from a list:
3481 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3482 print "$abbr[$mon] $mday";
3483 # $mon=9, $mday=18 gives "Oct 18"
3485 C<$year> contains the number of years since 1900. To get a 4-digit
3490 To get the last two digits of the year (e.g., "01" in 2001) do:
3492 $year = sprintf("%02d", $year % 100);
3494 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3495 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3496 (or C<0..365> in leap years.)
3498 C<$isdst> is true if the specified time occurs during Daylight Saving
3499 Time, false otherwise.
3501 If EXPR is omitted, C<localtime()> uses the current time (as returned
3504 In scalar context, C<localtime()> returns the ctime(3) value:
3506 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3508 The format of this scalar value is B<not> locale-dependent
3509 but built into Perl. For GMT instead of local
3510 time use the L</gmtime> builtin. See also the
3511 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3512 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3513 and mktime(3) functions.
3515 To get somewhat similar but locale-dependent date strings, set up your
3516 locale environment variables appropriately (please see L<perllocale>) and
3519 use POSIX qw(strftime);
3520 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3521 # or for GMT formatted appropriately for your locale:
3522 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3524 Note that the C<%a> and C<%b>, the short forms of the day of the week
3525 and the month of the year, may not necessarily be three characters wide.
3527 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3528 by-name access mechanism to the gmtime() and localtime() functions,
3531 For a comprehensive date and time representation look at the
3532 L<DateTime> module on CPAN.
3534 Portability issues: L<perlport/localtime>.
3539 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3541 This function places an advisory lock on a shared variable or referenced
3542 object contained in I<THING> until the lock goes out of scope.
3544 The value returned is the scalar itself, if the argument is a scalar, or a
3545 reference, if the argument is a hash, array or subroutine.
3547 lock() is a "weak keyword" : this means that if you've defined a function
3548 by this name (before any calls to it), that function will be called
3549 instead. If you are not under C<use threads::shared> this does nothing.
3550 See L<threads::shared>.
3553 X<log> X<logarithm> X<e> X<ln> X<base>
3557 =for Pod::Functions retrieve the natural logarithm for a number
3559 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3560 returns the log of C<$_>. To get the
3561 log of another base, use basic algebra:
3562 The base-N log of a number is equal to the natural log of that number
3563 divided by the natural log of N. For example:
3567 return log($n)/log(10);
3570 See also L</exp> for the inverse operation.
3572 =item lstat FILEHANDLE
3577 =item lstat DIRHANDLE
3581 =for Pod::Functions stat a symbolic link
3583 Does the same thing as the C<stat> function (including setting the
3584 special C<_> filehandle) but stats a symbolic link instead of the file
3585 the symbolic link points to. If symbolic links are unimplemented on
3586 your system, a normal C<stat> is done. For much more detailed
3587 information, please see the documentation for C<stat>.
3589 If EXPR is omitted, stats C<$_>.
3591 Portability issues: L<perlport/lstat>.
3595 =for Pod::Functions match a string with a regular expression pattern
3597 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3599 =item map BLOCK LIST
3604 =for Pod::Functions apply a change to a list to get back a new list with the changes
3606 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3607 C<$_> to each element) and returns the list value composed of the
3608 results of each such evaluation. In scalar context, returns the
3609 total number of elements so generated. Evaluates BLOCK or EXPR in
3610 list context, so each element of LIST may produce zero, one, or
3611 more elements in the returned value.
3613 @chars = map(chr, @numbers);
3615 translates a list of numbers to the corresponding characters.
3617 my @squares = map { $_ * $_ } @numbers;
3619 translates a list of numbers to their squared values.
3621 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3623 shows that number of returned elements can differ from the number of
3624 input elements. To omit an element, return an empty list ().
3625 This could also be achieved by writing
3627 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3629 which makes the intention more clear.
3631 Map always returns a list, which can be
3632 assigned to a hash such that the elements
3633 become key/value pairs. See L<perldata> for more details.
3635 %hash = map { get_a_key_for($_) => $_ } @array;
3637 is just a funny way to write
3641 $hash{get_a_key_for($_)} = $_;
3644 Note that C<$_> is an alias to the list value, so it can be used to
3645 modify the elements of the LIST. While this is useful and supported,
3646 it can cause bizarre results if the elements of LIST are not variables.
3647 Using a regular C<foreach> loop for this purpose would be clearer in
3648 most cases. See also L</grep> for an array composed of those items of
3649 the original list for which the BLOCK or EXPR evaluates to true.
3651 If C<$_> is lexical in the scope where the C<map> appears (because it has
3652 been declared with the deprecated C<my $_> construct),
3653 then, in addition to being locally aliased to
3654 the list elements, C<$_> keeps being lexical inside the block; that is, it
3655 can't be seen from the outside, avoiding any potential side-effects.
3657 C<{> starts both hash references and blocks, so C<map { ...> could be either
3658 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3659 ahead for the closing C<}> it has to take a guess at which it's dealing with
3660 based on what it finds just after the
3661 C<{>. Usually it gets it right, but if it
3662 doesn't it won't realize something is wrong until it gets to the C<}> and
3663 encounters the missing (or unexpected) comma. The syntax error will be
3664 reported close to the C<}>, but you'll need to change something near the C<{>
3665 such as using a unary C<+> to give Perl some help:
3667 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3668 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3669 %hash = map { ("\L$_" => 1) } @array # this also works
3670 %hash = map { lc($_) => 1 } @array # as does this.
3671 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3673 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3675 or to force an anon hash constructor use C<+{>:
3677 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3680 to get a list of anonymous hashes each with only one entry apiece.
3682 =item mkdir FILENAME,MASK
3683 X<mkdir> X<md> X<directory, create>
3685 =item mkdir FILENAME
3689 =for Pod::Functions create a directory
3691 Creates the directory specified by FILENAME, with permissions
3692 specified by MASK (as modified by C<umask>). If it succeeds it
3693 returns true; otherwise it returns false and sets C<$!> (errno).
3694 MASK defaults to 0777 if omitted, and FILENAME defaults
3695 to C<$_> if omitted.
3697 In general, it is better to create directories with a permissive MASK
3698 and let the user modify that with their C<umask> than it is to supply
3699 a restrictive MASK and give the user no way to be more permissive.
3700 The exceptions to this rule are when the file or directory should be
3701 kept private (mail files, for instance). The perlfunc(1) entry on
3702 C<umask> discusses the choice of MASK in more detail.
3704 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3705 number of trailing slashes. Some operating and filesystems do not get
3706 this right, so Perl automatically removes all trailing slashes to keep
3709 To recursively create a directory structure, look at
3710 the C<make_path> function of the L<File::Path> module.
3712 =item msgctl ID,CMD,ARG
3715 =for Pod::Functions SysV IPC message control operations
3717 Calls the System V IPC function msgctl(2). You'll probably have to say
3721 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3722 then ARG must be a variable that will hold the returned C<msqid_ds>
3723 structure. Returns like C<ioctl>: the undefined value for error,
3724 C<"0 but true"> for zero, or the actual return value otherwise. See also
3725 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3728 Portability issues: L<perlport/msgctl>.
3730 =item msgget KEY,FLAGS
3733 =for Pod::Functions get SysV IPC message queue
3735 Calls the System V IPC function msgget(2). Returns the message queue
3736 id, or C<undef> on error. See also
3737 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3740 Portability issues: L<perlport/msgget>.
3742 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3745 =for Pod::Functions receive a SysV IPC message from a message queue
3747 Calls the System V IPC function msgrcv to receive a message from
3748 message queue ID into variable VAR with a maximum message size of
3749 SIZE. Note that when a message is received, the message type as a
3750 native long integer will be the first thing in VAR, followed by the
3751 actual message. This packing may be opened with C<unpack("l! a*")>.
3752 Taints the variable. Returns true if successful, false
3753 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3754 C<IPC::SysV> and C<IPC::SysV::Msg>.
3756 Portability issues: L<perlport/msgrcv>.
3758 =item msgsnd ID,MSG,FLAGS
3761 =for Pod::Functions send a SysV IPC message to a message queue
3763 Calls the System V IPC function msgsnd to send the message MSG to the
3764 message queue ID. MSG must begin with the native long integer message
3765 type, be followed by the length of the actual message, and then finally
3766 the message itself. This kind of packing can be achieved with
3767 C<pack("l! a*", $type, $message)>. Returns true if successful,
3768 false on error. See also the C<IPC::SysV>
3769 and C<IPC::SysV::Msg> documentation.
3771 Portability issues: L<perlport/msgsnd>.
3776 =item my TYPE VARLIST
3778 =item my VARLIST : ATTRS
3780 =item my TYPE VARLIST : ATTRS
3782 =for Pod::Functions declare and assign a local variable (lexical scoping)
3784 A C<my> declares the listed variables to be local (lexically) to the
3785 enclosing block, file, or C<eval>. If more than one variable is listed,
3786 the list must be placed in parentheses.
3788 The exact semantics and interface of TYPE and ATTRS are still
3789 evolving. TYPE may be a bareword, a constant declared
3790 with C<use constant>, or C<__PACKAGE__>. It is
3791 currently bound to the use of the C<fields> pragma,
3792 and attributes are handled using the C<attributes> pragma, or starting
3793 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3794 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3795 L<attributes>, and L<Attribute::Handlers>.
3797 Note that with a parenthesised list, C<undef> can be used as a dummy
3798 placeholder, for example to skip assignment of initial values:
3800 my ( undef, $min, $hour ) = localtime;
3809 =for Pod::Functions iterate a block prematurely
3811 The C<next> command is like the C<continue> statement in C; it starts
3812 the next iteration of the loop:
3814 LINE: while (<STDIN>) {
3815 next LINE if /^#/; # discard comments
3819 Note that if there were a C<continue> block on the above, it would get
3820 executed even on discarded lines. If LABEL is omitted, the command
3821 refers to the innermost enclosing loop. The C<next EXPR> form, available
3822 as of Perl 5.18.0, allows a label name to be computed at run time, being
3823 otherwise identical to C<next LABEL>.
3825 C<next> cannot be used to exit a block which returns a value such as
3826 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3827 a grep() or map() operation.
3829 Note that a block by itself is semantically identical to a loop
3830 that executes once. Thus C<next> will exit such a block early.
3832 See also L</continue> for an illustration of how C<last>, C<next>, and
3835 Unlike most named operators, this has the same precedence as assignment.
3836 It is also exempt from the looks-like-a-function rule, so
3837 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3840 =item no MODULE VERSION LIST
3844 =item no MODULE VERSION
3846 =item no MODULE LIST
3852 =for Pod::Functions unimport some module symbols or semantics at compile time
3854 See the C<use> function, of which C<no> is the opposite.
3857 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3861 =for Pod::Functions convert a string to an octal number
3863 Interprets EXPR as an octal string and returns the corresponding
3864 value. (If EXPR happens to start off with C<0x>, interprets it as a
3865 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3866 binary string. Leading whitespace is ignored in all three cases.)
3867 The following will handle decimal, binary, octal, and hex in standard
3870 $val = oct($val) if $val =~ /^0/;
3872 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3873 in octal), use sprintf() or printf():
3875 $dec_perms = (stat("filename"))[2] & 07777;
3876 $oct_perm_str = sprintf "%o", $perms;
3878 The oct() function is commonly used when a string such as C<644> needs
3879 to be converted into a file mode, for example. Although Perl
3880 automatically converts strings into numbers as needed, this automatic
3881 conversion assumes base 10.
3883 Leading white space is ignored without warning, as too are any trailing
3884 non-digits, such as a decimal point (C<oct> only handles non-negative
3885 integers, not negative integers or floating point).
3887 =item open FILEHANDLE,EXPR
3888 X<open> X<pipe> X<file, open> X<fopen>
3890 =item open FILEHANDLE,MODE,EXPR
3892 =item open FILEHANDLE,MODE,EXPR,LIST
3894 =item open FILEHANDLE,MODE,REFERENCE
3896 =item open FILEHANDLE
3898 =for Pod::Functions open a file, pipe, or descriptor
3900 Opens the file whose filename is given by EXPR, and associates it with
3903 Simple examples to open a file for reading:
3905 open(my $fh, "<", "input.txt")
3906 or die "cannot open < input.txt: $!";
3910 open(my $fh, ">", "output.txt")
3911 or die "cannot open > output.txt: $!";
3913 (The following is a comprehensive reference to open(): for a gentler
3914 introduction you may consider L<perlopentut>.)
3916 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3917 new filehandle is autovivified, meaning that the variable is assigned a
3918 reference to a newly allocated anonymous filehandle. Otherwise if
3919 FILEHANDLE is an expression, its value is the real filehandle. (This is
3920 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3923 If three (or more) arguments are specified, the open mode (including
3924 optional encoding) in the second argument are distinct from the filename in
3925 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3926 If MODE is C<< > >>, the file is opened for output, with existing files
3927 first being truncated ("clobbered") and nonexisting files newly created.
3928 If MODE is C<<< >> >>>, the file is opened for appending, again being
3929 created if necessary.
3931 You can put a C<+> in front of the C<< > >> or C<< < >> to
3932 indicate that you want both read and write access to the file; thus
3933 C<< +< >> is almost always preferred for read/write updates--the
3934 C<< +> >> mode would clobber the file first. You can't usually use
3935 either read-write mode for updating textfiles, since they have
3936 variable-length records. See the B<-i> switch in L<perlrun> for a
3937 better approach. The file is created with permissions of C<0666>
3938 modified by the process's C<umask> value.
3940 These various prefixes correspond to the fopen(3) modes of C<r>,
3941 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3943 In the one- and two-argument forms of the call, the mode and filename
3944 should be concatenated (in that order), preferably separated by white
3945 space. You can--but shouldn't--omit the mode in these forms when that mode
3946 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3947 the filename argument is a known literal.
3949 For three or more arguments if MODE is C<|->, the filename is
3950 interpreted as a command to which output is to be piped, and if MODE
3951 is C<-|>, the filename is interpreted as a command that pipes
3952 output to us. In the two-argument (and one-argument) form, one should
3953 replace dash (C<->) with the command.
3954 See L<perlipc/"Using open() for IPC"> for more examples of this.
3955 (You are not allowed to C<open> to a command that pipes both in I<and>
3956 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3957 L<perlipc/"Bidirectional Communication with Another Process"> for
3960 In the form of pipe opens taking three or more arguments, if LIST is specified
3961 (extra arguments after the command name) then LIST becomes arguments
3962 to the command invoked if the platform supports it. The meaning of
3963 C<open> with more than three arguments for non-pipe modes is not yet
3964 defined, but experimental "layers" may give extra LIST arguments
3967 In the two-argument (and one-argument) form, opening C<< <- >>
3968 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3970 You may (and usually should) use the three-argument form of open to specify
3971 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3972 that affect how the input and output are processed (see L<open> and
3973 L<PerlIO> for more details). For example:
3975 open(my $fh, "<:encoding(UTF-8)", "filename")
3976 || die "can't open UTF-8 encoded filename: $!";
3978 opens the UTF8-encoded file containing Unicode characters;
3979 see L<perluniintro>. Note that if layers are specified in the
3980 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3981 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3982 Those layers will also be ignored if you specifying a colon with no name
3983 following it. In that case the default layer for the operating system
3984 (:raw on Unix, :crlf on Windows) is used.
3986 Open returns nonzero on success, the undefined value otherwise. If
3987 the C<open> involved a pipe, the return value happens to be the pid of
3990 If you're running Perl on a system that distinguishes between text
3991 files and binary files, then you should check out L</binmode> for tips
3992 for dealing with this. The key distinction between systems that need
3993 C<binmode> and those that don't is their text file formats. Systems
3994 like Unix, Mac OS, and Plan 9, that end lines with a single
3995 character and encode that character in C as C<"\n"> do not
3996 need C<binmode>. The rest need it.
3998 When opening a file, it's seldom a good idea to continue
3999 if the request failed, so C<open> is frequently used with
4000 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
4001 where you want to format a suitable error message (but there are
4002 modules that can help with that problem)) always check
4003 the return value from opening a file.
4005 The filehandle will be closed when its reference count reaches zero.
4006 If it is a lexically scoped variable declared with C<my>, that usually
4007 means the end of the enclosing scope. However, this automatic close
4008 does not check for errors, so it is better to explicitly close
4009 filehandles, especially those used for writing:
4012 || warn "close failed: $!";
4014 An older style is to use a bareword as the filehandle, as
4016 open(FH, "<", "input.txt")
4017 or die "cannot open < input.txt: $!";
4019 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4020 <FH> >> and so on. Note that it's a global variable, so this form is
4021 not recommended in new code.
4023 As a shortcut a one-argument call takes the filename from the global
4024 scalar variable of the same name as the filehandle:
4027 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4029 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4030 declared with C<my> or C<state>.
4032 As a special case the three-argument form with a read/write mode and the third
4033 argument being C<undef>:
4035 open(my $tmp, "+>", undef) or die ...
4037 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4038 works for symmetry, but you really should consider writing something
4039 to the temporary file first. You will need to seek() to do the
4042 Perl is built using PerlIO by default; Unless you've
4043 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4044 open filehandles directly to Perl scalars via:
4046 open($fh, ">", \$variable) || ..
4048 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4051 open(STDOUT, ">", \$variable)
4052 or die "Can't open STDOUT: $!";
4056 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
4057 # if the open fails, output is discarded
4059 open(my $dbase, "+<", "dbase.mine") # open for update
4060 or die "Can't open 'dbase.mine' for update: $!";
4062 open(my $dbase, "+<dbase.mine") # ditto
4063 or die "Can't open 'dbase.mine' for update: $!";
4065 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4066 or die "Can't start caesar: $!";
4068 open(ARTICLE, "caesar <$article |") # ditto
4069 or die "Can't start caesar: $!";
4071 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4072 or die "Can't start sort: $!";
4075 open(MEMORY, ">", \$var)
4076 or die "Can't open memory file: $!";
4077 print MEMORY "foo!\n"; # output will appear in $var
4079 # process argument list of files along with any includes
4081 foreach $file (@ARGV) {
4082 process($file, "fh00");
4086 my($filename, $input) = @_;
4087 $input++; # this is a string increment
4088 unless (open($input, "<", $filename)) {
4089 print STDERR "Can't open $filename: $!\n";
4094 while (<$input>) { # note use of indirection
4095 if (/^#include "(.*)"/) {
4096 process($1, $input);
4103 See L<perliol> for detailed info on PerlIO.
4105 You may also, in the Bourne shell tradition, specify an EXPR beginning
4106 with C<< >& >>, in which case the rest of the string is interpreted
4107 as the name of a filehandle (or file descriptor, if numeric) to be
4108 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4109 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4110 The mode you specify should match the mode of the original filehandle.
4111 (Duping a filehandle does not take into account any existing contents
4112 of IO buffers.) If you use the three-argument
4113 form, then you can pass either a
4114 number, the name of a filehandle, or the normal "reference to a glob".
4116 Here is a script that saves, redirects, and restores C<STDOUT> and
4117 C<STDERR> using various methods:
4120 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4121 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4123 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4124 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4126 select STDERR; $| = 1; # make unbuffered
4127 select STDOUT; $| = 1; # make unbuffered
4129 print STDOUT "stdout 1\n"; # this works for
4130 print STDERR "stderr 1\n"; # subprocesses too
4132 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4133 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4135 print STDOUT "stdout 2\n";
4136 print STDERR "stderr 2\n";
4138 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4139 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4140 that file descriptor (and not call C<dup(2)>); this is more
4141 parsimonious of file descriptors. For example:
4143 # open for input, reusing the fileno of $fd
4144 open(FILEHANDLE, "<&=$fd")
4148 open(FILEHANDLE, "<&=", $fd)
4152 # open for append, using the fileno of OLDFH
4153 open(FH, ">>&=", OLDFH)
4157 open(FH, ">>&=OLDFH")
4159 Being parsimonious on filehandles is also useful (besides being
4160 parsimonious) for example when something is dependent on file
4161 descriptors, like for example locking using flock(). If you do just
4162 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4163 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4164 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4165 the same underlying system file descriptor.
4167 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4168 fdopen() to implement the C<=> functionality. On many Unix systems,
4169 fdopen() fails when file descriptors exceed a certain value, typically 255.
4170 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4172 You can see whether your Perl was built with PerlIO by running C<perl -V>
4173 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4174 have PerlIO; otherwise you don't.
4176 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4177 with the one- or two-argument forms of C<open>),
4178 an implicit C<fork> is done, so C<open> returns twice: in the parent
4179 process it returns the pid
4180 of the child process, and in the child process it returns (a defined) C<0>.
4181 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4183 For example, use either
4185 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4189 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4195 # either write TO_KID or else read FROM_KID
4197 waitpid $child_pid, 0;
4199 # am the child; use STDIN/STDOUT normally
4204 The filehandle behaves normally for the parent, but I/O to that
4205 filehandle is piped from/to the STDOUT/STDIN of the child process.
4206 In the child process, the filehandle isn't opened--I/O happens from/to
4207 the new STDOUT/STDIN. Typically this is used like the normal
4208 piped open when you want to exercise more control over just how the
4209 pipe command gets executed, such as when running setuid and
4210 you don't want to have to scan shell commands for metacharacters.
4212 The following blocks are more or less equivalent:
4214 open(FOO, "|tr '[a-z]' '[A-Z]'");
4215 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4216 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4217 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4219 open(FOO, "cat -n '$file'|");
4220 open(FOO, "-|", "cat -n '$file'");
4221 open(FOO, "-|") || exec "cat", "-n", $file;
4222 open(FOO, "-|", "cat", "-n", $file);
4224 The last two examples in each block show the pipe as "list form", which is
4225 not yet supported on all platforms. A good rule of thumb is that if
4226 your platform has a real C<fork()> (in other words, if your platform is
4227 Unix, including Linux and MacOS X), you can use the list form. You would
4228 want to use the list form of the pipe so you can pass literal arguments
4229 to the command without risk of the shell interpreting any shell metacharacters
4230 in them. However, this also bars you from opening pipes to commands
4231 that intentionally contain shell metacharacters, such as:
4233 open(FOO, "|cat -n | expand -4 | lpr")
4234 // die "Can't open pipeline to lpr: $!";
4236 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4238 Perl will attempt to flush all files opened for
4239 output before any operation that may do a fork, but this may not be
4240 supported on some platforms (see L<perlport>). To be safe, you may need
4241 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4242 of C<IO::Handle> on any open handles.
4244 On systems that support a close-on-exec flag on files, the flag will
4245 be set for the newly opened file descriptor as determined by the value
4246 of C<$^F>. See L<perlvar/$^F>.
4248 Closing any piped filehandle causes the parent process to wait for the
4249 child to finish, then returns the status value in C<$?> and
4250 C<${^CHILD_ERROR_NATIVE}>.
4252 The filename passed to the one- and two-argument forms of open() will
4253 have leading and trailing whitespace deleted and normal
4254 redirection characters honored. This property, known as "magic open",
4255 can often be used to good effect. A user could specify a filename of
4256 F<"rsh cat file |">, or you could change certain filenames as needed:
4258 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4259 open(FH, $filename) or die "Can't open $filename: $!";
4261 Use the three-argument form to open a file with arbitrary weird characters in it,
4263 open(FOO, "<", $file)
4264 || die "can't open < $file: $!";
4266 otherwise it's necessary to protect any leading and trailing whitespace:
4268 $file =~ s#^(\s)#./$1#;
4269 open(FOO, "< $file\0")
4270 || die "open failed: $!";
4272 (this may not work on some bizarre filesystems). One should
4273 conscientiously choose between the I<magic> and I<three-argument> form
4276 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4278 will allow the user to specify an argument of the form C<"rsh cat file |">,
4279 but will not work on a filename that happens to have a trailing space, while
4281 open(IN, "<", $ARGV[0])
4282 || die "can't open < $ARGV[0]: $!";
4284 will have exactly the opposite restrictions.
4286 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4287 should use the C<sysopen> function, which involves no such magic (but may
4288 use subtly different filemodes than Perl open(), which is mapped to C
4289 fopen()). This is another way to protect your filenames from
4290 interpretation. For example:
4293 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4294 or die "sysopen $path: $!";
4295 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4296 print HANDLE "stuff $$\n";
4298 print "File contains: ", <HANDLE>;
4300 See L</seek> for some details about mixing reading and writing.
4302 Portability issues: L<perlport/open>.
4304 =item opendir DIRHANDLE,EXPR
4307 =for Pod::Functions open a directory
4309 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4310 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4311 DIRHANDLE may be an expression whose value can be used as an indirect
4312 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4313 scalar variable (or array or hash element), the variable is assigned a
4314 reference to a new anonymous dirhandle; that is, it's autovivified.
4315 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4317 See the example at C<readdir>.
4324 =for Pod::Functions find a character's numeric representation
4326 Returns the numeric value of the first character of EXPR.
4327 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4328 (Note I<character>, not byte.)
4330 For the reverse, see L</chr>.
4331 See L<perlunicode> for more about Unicode.
4336 =item our TYPE VARLIST
4338 =item our VARLIST : ATTRS
4340 =item our TYPE VARLIST : ATTRS
4342 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4344 C<our> makes a lexical alias to a package variable of the same name in the current
4345 package for use within the current lexical scope.
4347 C<our> has the same scoping rules as C<my> or C<state>, but C<our> only
4348 declares an alias, whereas C<my> or C<state> both declare a variable name and
4349 allocate storage for that name within the current scope.
4351 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4352 a package variable without qualifying it with the package name, but only within
4353 the lexical scope of the C<our> declaration. In this way, C<our> differs from
4354 C<use vars>, which allows use of an unqualified name I<only> within the
4355 affected package, but across scopes.
4357 If more than one variable is listed, the list must be placed
4363 An C<our> declaration declares an alias for a package variable that will be visible
4364 across its entire lexical scope, even across package boundaries. The
4365 package in which the variable is entered is determined at the point
4366 of the declaration, not at the point of use. This means the following
4370 our $bar; # declares $Foo::bar for rest of lexical scope
4374 print $bar; # prints 20, as it refers to $Foo::bar
4376 Multiple C<our> declarations with the same name in the same lexical
4377 scope are allowed if they are in different packages. If they happen
4378 to be in the same package, Perl will emit warnings if you have asked
4379 for them, just like multiple C<my> declarations. Unlike a second
4380 C<my> declaration, which will bind the name to a fresh variable, a
4381 second C<our> declaration in the same package, in the same scope, is
4386 our $bar; # declares $Foo::bar for rest of lexical scope
4390 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4391 print $bar; # prints 30
4393 our $bar; # emits warning but has no other effect
4394 print $bar; # still prints 30
4396 An C<our> declaration may also have a list of attributes associated
4399 The exact semantics and interface of TYPE and ATTRS are still
4400 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4401 and attributes are handled using the C<attributes> pragma, or, starting
4402 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4403 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4404 L<attributes>, and L<Attribute::Handlers>.
4406 Note that with a parenthesised list, C<undef> can be used as a dummy
4407 placeholder, for example to skip assignment of initial values:
4409 our ( undef, $min, $hour ) = localtime;
4411 =item pack TEMPLATE,LIST
4414 =for Pod::Functions convert a list into a binary representation
4416 Takes a LIST of values and converts it into a string using the rules
4417 given by the TEMPLATE. The resulting string is the concatenation of
4418 the converted values. Typically, each converted value looks
4419 like its machine-level representation. For example, on 32-bit machines
4420 an integer may be represented by a sequence of 4 bytes, which will in
4421 Perl be presented as a string that's 4 characters long.
4423 See L<perlpacktut> for an introduction to this function.
4425 The TEMPLATE is a sequence of characters that give the order and type
4426 of values, as follows:
4428 a A string with arbitrary binary data, will be null padded.
4429 A A text (ASCII) string, will be space padded.
4430 Z A null-terminated (ASCIZ) string, will be null padded.
4432 b A bit string (ascending bit order inside each byte,
4434 B A bit string (descending bit order inside each byte).
4435 h A hex string (low nybble first).
4436 H A hex string (high nybble first).
4438 c A signed char (8-bit) value.
4439 C An unsigned char (octet) value.
4440 W An unsigned char value (can be greater than 255).
4442 s A signed short (16-bit) value.
4443 S An unsigned short value.
4445 l A signed long (32-bit) value.
4446 L An unsigned long value.
4448 q A signed quad (64-bit) value.
4449 Q An unsigned quad value.
4450 (Quads are available only if your system supports 64-bit
4451 integer values _and_ if Perl has been compiled to support
4452 those. Raises an exception otherwise.)
4454 i A signed integer value.
4455 I A unsigned integer value.
4456 (This 'integer' is _at_least_ 32 bits wide. Its exact
4457 size depends on what a local C compiler calls 'int'.)
4459 n An unsigned short (16-bit) in "network" (big-endian) order.
4460 N An unsigned long (32-bit) in "network" (big-endian) order.
4461 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4462 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4464 j A Perl internal signed integer value (IV).
4465 J A Perl internal unsigned integer value (UV).
4467 f A single-precision float in native format.
4468 d A double-precision float in native format.
4470 F A Perl internal floating-point value (NV) in native format
4471 D A float of long-double precision in native format.
4472 (Long doubles are available only if your system supports
4473 long double values _and_ if Perl has been compiled to
4474 support those. Raises an exception otherwise.)
4476 p A pointer to a null-terminated string.
4477 P A pointer to a structure (fixed-length string).
4479 u A uuencoded string.
4480 U A Unicode character number. Encodes to a character in char-
4481 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4484 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4485 for details). Its bytes represent an unsigned integer in
4486 base 128, most significant digit first, with as few digits
4487 as possible. Bit eight (the high bit) is set on each byte
4490 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4492 @ Null-fill or truncate to absolute position, counted from the
4493 start of the innermost ()-group.
4494 . Null-fill or truncate to absolute position specified by
4496 ( Start of a ()-group.
4498 One or more modifiers below may optionally follow certain letters in the
4499 TEMPLATE (the second column lists letters for which the modifier is valid):
4501 ! sSlLiI Forces native (short, long, int) sizes instead
4502 of fixed (16-/32-bit) sizes.
4504 ! xX Make x and X act as alignment commands.
4506 ! nNvV Treat integers as signed instead of unsigned.
4508 ! @. Specify position as byte offset in the internal
4509 representation of the packed string. Efficient
4512 > sSiIlLqQ Force big-endian byte-order on the type.
4513 jJfFdDpP (The "big end" touches the construct.)
4515 < sSiIlLqQ Force little-endian byte-order on the type.
4516 jJfFdDpP (The "little end" touches the construct.)
4518 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4519 to force a particular byte-order on all components in that group,
4520 including all its subgroups.
4524 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4525 pack for processing data from NASA's Magellan probe. Magellan was in an
4526 elliptical orbit, using the antenna for the radar mapping when close to
4527 Venus and for communicating data back to Earth for the rest of the orbit.
4528 There were two transmission units, but one of these failed, and then the
4529 other developed a fault whereby it would randomly flip the sense of all the
4530 bits. It was easy to automatically detect complete records with the correct
4531 sense, and complete records with all the bits flipped. However, this didn't
4532 recover the records where the sense flipped midway. A colleague of Larry's
4533 was able to pretty much eyeball where the records flipped, so they wrote an
4534 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4535 manually correct the records and recover the data. For this purpose pack
4536 gained the hex and bit string format specifiers.
4538 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4539 27e2fb84680b9cc1), but the patch description makes no mention of their
4540 addition, let alone the story behind them.
4544 The following rules apply:
4550 Each letter may optionally be followed by a number indicating the repeat
4551 count. A numeric repeat count may optionally be enclosed in brackets, as
4552 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4553 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4554 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4555 something else, described below. Supplying a C<*> for the repeat count
4556 instead of a number means to use however many items are left, except for:
4562 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4566 <.>, where it means relative to the start of the string.
4570 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4574 One can replace a numeric repeat count with a template letter enclosed in
4575 brackets to use the packed byte length of the bracketed template for the
4578 For example, the template C<x[L]> skips as many bytes as in a packed long,
4579 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4580 variable-expanded) unpacks. If the template in brackets contains alignment
4581 commands (such as C<x![d]>), its packed length is calculated as if the
4582 start of the template had the maximal possible alignment.
4584 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4585 trailing null byte, so the resulting string is always one byte longer than
4586 the byte length of the item itself.
4588 When used with C<@>, the repeat count represents an offset from the start
4589 of the innermost C<()> group.
4591 When used with C<.>, the repeat count determines the starting position to
4592 calculate the value offset as follows:
4598 If the repeat count is C<0>, it's relative to the current position.
4602 If the repeat count is C<*>, the offset is relative to the start of the
4607 And if it's an integer I<n>, the offset is relative to the start of the
4608 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4609 bigger then the group level.
4613 The repeat count for C<u> is interpreted as the maximal number of bytes
4614 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4615 count should not be more than 65.
4619 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4620 string of length count, padding with nulls or spaces as needed. When
4621 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4622 after the first null, and C<a> returns data with no stripping at all.
4624 If the value to pack is too long, the result is truncated. If it's too
4625 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4626 followed by a null byte. Thus C<Z> always packs a trailing null, except
4627 when the count is 0.
4631 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4632 Each such format generates 1 bit of the result. These are typically followed
4633 by a repeat count like C<B8> or C<B64>.
4635 Each result bit is based on the least-significant bit of the corresponding
4636 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4637 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4639 Starting from the beginning of the input string, each 8-tuple
4640 of characters is converted to 1 character of output. With format C<b>,
4641 the first character of the 8-tuple determines the least-significant bit of a
4642 character; with format C<B>, it determines the most-significant bit of
4645 If the length of the input string is not evenly divisible by 8, the
4646 remainder is packed as if the input string were padded by null characters
4647 at the end. Similarly during unpacking, "extra" bits are ignored.
4649 If the input string is longer than needed, remaining characters are ignored.
4651 A C<*> for the repeat count uses all characters of the input field.
4652 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4656 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4657 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4659 For each such format, pack() generates 4 bits of result.
4660 With non-alphabetical characters, the result is based on the 4 least-significant
4661 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4662 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4663 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4664 is compatible with the usual hexadecimal digits, so that C<"a"> and
4665 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4666 characters with this format.
4668 Starting from the beginning of the template to pack(), each pair
4669 of characters is converted to 1 character of output. With format C<h>, the
4670 first character of the pair determines the least-significant nybble of the
4671 output character; with format C<H>, it determines the most-significant
4674 If the length of the input string is not even, it behaves as if padded by
4675 a null character at the end. Similarly, "extra" nybbles are ignored during
4678 If the input string is longer than needed, extra characters are ignored.
4680 A C<*> for the repeat count uses all characters of the input field. For
4681 unpack(), nybbles are converted to a string of hexadecimal digits.
4685 The C<p> format packs a pointer to a null-terminated string. You are
4686 responsible for ensuring that the string is not a temporary value, as that
4687 could potentially get deallocated before you got around to using the packed
4688 result. The C<P> format packs a pointer to a structure of the size indicated
4689 by the length. A null pointer is created if the corresponding value for
4690 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4691 unpacks into C<undef>.
4693 If your system has a strange pointer size--meaning a pointer is neither as
4694 big as an int nor as big as a long--it may not be possible to pack or
4695 unpack pointers in big- or little-endian byte order. Attempting to do
4696 so raises an exception.
4700 The C</> template character allows packing and unpacking of a sequence of
4701 items where the packed structure contains a packed item count followed by
4702 the packed items themselves. This is useful when the structure you're
4703 unpacking has encoded the sizes or repeat counts for some of its fields
4704 within the structure itself as separate fields.
4706 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4707 I<length-item> describes how the length value is packed. Formats likely
4708 to be of most use are integer-packing ones like C<n> for Java strings,
4709 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4711 For C<pack>, I<sequence-item> may have a repeat count, in which case
4712 the minimum of that and the number of available items is used as the argument
4713 for I<length-item>. If it has no repeat count or uses a '*', the number
4714 of available items is used.
4716 For C<unpack>, an internal stack of integer arguments unpacked so far is
4717 used. You write C</>I<sequence-item> and the repeat count is obtained by
4718 popping off the last element from the stack. The I<sequence-item> must not
4719 have a repeat count.
4721 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4722 the I<length-item> is the string length, not the number of strings. With
4723 an explicit repeat count for pack, the packed string is adjusted to that
4724 length. For example:
4726 This code: gives this result:
4728 unpack("W/a", "\004Gurusamy") ("Guru")
4729 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4730 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4732 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4733 pack("a/W2", ord("a") .. ord("z")) "2ab"
4735 The I<length-item> is not returned explicitly from C<unpack>.
4737 Supplying a count to the I<length-item> format letter is only useful with
4738 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4739 introduce C<"\000"> characters, which Perl does not regard as legal in
4744 The integer types C<s>, C<S>, C<l>, and C<L> may be
4745 followed by a C<!> modifier to specify native shorts or
4746 longs. As shown in the example above, a bare C<l> means
4747 exactly 32 bits, although the native C<long> as seen by the local C compiler
4748 may be larger. This is mainly an issue on 64-bit platforms. You can
4749 see whether using C<!> makes any difference this way:
4751 printf "format s is %d, s! is %d\n",
4752 length pack("s"), length pack("s!");
4754 printf "format l is %d, l! is %d\n",
4755 length pack("l"), length pack("l!");
4758 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4759 they are identical to C<i> and C<I>.
4761 The actual sizes (in bytes) of native shorts, ints, longs, and long
4762 longs on the platform where Perl was built are also available from
4765 $ perl -V:{short,int,long{,long}}size
4771 or programmatically via the C<Config> module:
4774 print $Config{shortsize}, "\n";
4775 print $Config{intsize}, "\n";
4776 print $Config{longsize}, "\n";
4777 print $Config{longlongsize}, "\n";
4779 C<$Config{longlongsize}> is undefined on systems without
4784 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4785 inherently non-portable between processors and operating systems because
4786 they obey native byteorder and endianness. For example, a 4-byte integer
4787 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4788 handled by the CPU registers) into bytes as
4790 0x12 0x34 0x56 0x78 # big-endian
4791 0x78 0x56 0x34 0x12 # little-endian
4793 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4794 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4795 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4796 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4798 The names I<big-endian> and I<little-endian> are comic references to the
4799 egg-eating habits of the little-endian Lilliputians and the big-endian
4800 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4801 This entered computer lingo via the paper "On Holy Wars and a Plea for
4802 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4804 Some systems may have even weirder byte orders such as
4809 You can determine your system endianness with this incantation:
4811 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4813 The byteorder on the platform where Perl was built is also available
4817 print "$Config{byteorder}\n";
4819 or from the command line:
4823 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4824 and C<"87654321"> are big-endian.
4826 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4827 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4828 immediately below. See also L<perlport>.
4832 Starting with Perl 5.10.0, integer and floating-point formats, along with
4833 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4834 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4835 or little-endian byte-order. These modifiers are especially useful
4836 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4837 64-bit integers, or floating-point values.
4839 Here are some concerns to keep in mind when using an endianness modifier:
4845 Exchanging signed integers between different platforms works only
4846 when all platforms store them in the same format. Most platforms store
4847 signed integers in two's-complement notation, so usually this is not an issue.
4851 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4852 formats on big- or little-endian machines. Otherwise, attempting to
4853 use them raises an exception.
4857 Forcing big- or little-endian byte-order on floating-point values for
4858 data exchange can work only if all platforms use the same
4859 binary representation such as IEEE floating-point. Even if all
4860 platforms are using IEEE, there may still be subtle differences. Being able
4861 to use C<< > >> or C<< < >> on floating-point values can be useful,
4862 but also dangerous if you don't know exactly what you're doing.
4863 It is not a general way to portably store floating-point values.
4867 When using C<< > >> or C<< < >> on a C<()> group, this affects
4868 all types inside the group that accept byte-order modifiers,
4869 including all subgroups. It is silently ignored for all other
4870 types. You are not allowed to override the byte-order within a group
4871 that already has a byte-order modifier suffix.
4877 Real numbers (floats and doubles) are in native machine format only.
4878 Due to the multiplicity of floating-point formats and the lack of a
4879 standard "network" representation for them, no facility for interchange has been
4880 made. This means that packed floating-point data written on one machine
4881 may not be readable on another, even if both use IEEE floating-point
4882 arithmetic (because the endianness of the memory representation is not part
4883 of the IEEE spec). See also L<perlport>.
4885 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4886 modifiers to force big- or little-endian byte-order on floating-point values.
4888 Because Perl uses doubles (or long doubles, if configured) internally for
4889 all numeric calculation, converting from double into float and thence
4890 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4891 will not in general equal $foo.
4895 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4896 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4897 where the packed string is processed in its UTF-8-encoded Unicode form on
4898 a byte-by-byte basis. Character mode is the default
4899 unless the format string starts with C<U>. You
4900 can always switch mode mid-format with an explicit
4901 C<C0> or C<U0> in the format. This mode remains in effect until the next
4902 mode change, or until the end of the C<()> group it (directly) applies to.
4904 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4905 bytes is not necessarily obvious. Probably only the first of these
4908 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4909 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4911 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4912 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4914 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4915 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4917 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4918 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4919 C3.8E.C2.B1.C3.8F.C2.89
4921 Those examples also illustrate that you should not try to use
4922 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4926 You must yourself do any alignment or padding by inserting, for example,
4927 enough C<"x">es while packing. There is no way for pack() and unpack()
4928 to know where characters are going to or coming from, so they
4929 handle their output and input as flat sequences of characters.
4933 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4934 take a repeat count either as postfix, or for unpack(), also via the C</>
4935 template character. Within each repetition of a group, positioning with
4936 C<@> starts over at 0. Therefore, the result of
4938 pack("@1A((@2A)@3A)", qw[X Y Z])
4940 is the string C<"\0X\0\0YZ">.
4944 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4945 jump forward or back to the closest position aligned at a multiple of C<count>
4946 characters. For example, to pack() or unpack() a C structure like
4949 char c; /* one signed, 8-bit character */
4954 one may need to use the template C<c x![d] d c[2]>. This assumes that
4955 doubles must be aligned to the size of double.
4957 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4962 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4963 represent signed 16-/32-bit integers in big-/little-endian order.
4964 This is portable only when all platforms sharing packed data use the
4965 same binary representation for signed integers; for example, when all
4966 platforms use two's-complement representation.
4970 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4971 White space can separate pack codes from each other, but modifiers and
4972 repeat counts must follow immediately. Breaking complex templates into
4973 individual line-by-line components, suitably annotated, can do as much to
4974 improve legibility and maintainability of pack/unpack formats as C</x> can
4975 for complicated pattern matches.
4979 If TEMPLATE requires more arguments than pack() is given, pack()
4980 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4981 than given, extra arguments are ignored.
4987 $foo = pack("WWWW",65,66,67,68);
4989 $foo = pack("W4",65,66,67,68);
4991 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4992 # same thing with Unicode circled letters.
4993 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4994 # same thing with Unicode circled letters. You don't get the
4995 # UTF-8 bytes because the U at the start of the format caused
4996 # a switch to U0-mode, so the UTF-8 bytes get joined into
4998 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4999 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5000 # This is the UTF-8 encoding of the string in the
5003 $foo = pack("ccxxcc",65,66,67,68);
5006 # NOTE: The examples above featuring "W" and "c" are true
5007 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5008 # and UTF-8. On EBCDIC systems, the first example would be
5009 # $foo = pack("WWWW",193,194,195,196);
5011 $foo = pack("s2",1,2);
5012 # "\001\000\002\000" on little-endian
5013 # "\000\001\000\002" on big-endian
5015 $foo = pack("a4","abcd","x","y","z");
5018 $foo = pack("aaaa","abcd","x","y","z");
5021 $foo = pack("a14","abcdefg");
5022 # "abcdefg\0\0\0\0\0\0\0"
5024 $foo = pack("i9pl", gmtime);
5025 # a real struct tm (on my system anyway)
5027 $utmp_template = "Z8 Z8 Z16 L";
5028 $utmp = pack($utmp_template, @utmp1);
5029 # a struct utmp (BSDish)
5031 @utmp2 = unpack($utmp_template, $utmp);
5032 # "@utmp1" eq "@utmp2"
5035 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5038 $foo = pack('sx2l', 12, 34);
5039 # short 12, two zero bytes padding, long 34
5040 $bar = pack('s@4l', 12, 34);
5041 # short 12, zero fill to position 4, long 34
5043 $baz = pack('s.l', 12, 4, 34);
5044 # short 12, zero fill to position 4, long 34
5046 $foo = pack('nN', 42, 4711);
5047 # pack big-endian 16- and 32-bit unsigned integers
5048 $foo = pack('S>L>', 42, 4711);
5050 $foo = pack('s<l<', -42, 4711);
5051 # pack little-endian 16- and 32-bit signed integers
5052 $foo = pack('(sl)<', -42, 4711);
5055 The same template may generally also be used in unpack().
5057 =item package NAMESPACE
5059 =item package NAMESPACE VERSION
5060 X<package> X<module> X<namespace> X<version>
5062 =item package NAMESPACE BLOCK
5064 =item package NAMESPACE VERSION BLOCK
5065 X<package> X<module> X<namespace> X<version>
5067 =for Pod::Functions declare a separate global namespace
5069 Declares the BLOCK or the rest of the compilation unit as being in the
5070 given namespace. The scope of the package declaration is either the
5071 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5072 itself through the end of current scope (the enclosing block, file, or
5073 C<eval>). That is, the forms without a BLOCK are operative through the end
5074 of the current scope, just like the C<my>, C<state>, and C<our> operators.
5075 All unqualified dynamic identifiers in this scope will be in the given
5076 namespace, except where overridden by another C<package> declaration or
5077 when they're one of the special identifiers that qualify into C<main::>,
5078 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5080 A package statement affects dynamic variables only, including those
5081 you've used C<local> on, but I<not> lexically-scoped variables, which are created
5082 with C<my>, C<state>, or C<our>. Typically it would be the first
5083 declaration in a file included by C<require> or C<use>. You can switch into a
5084 package in more than one place, since this only determines which default
5085 symbol table the compiler uses for the rest of that block. You can refer to
5086 identifiers in other packages than the current one by prefixing the identifier
5087 with the package name and a double colon, as in C<$SomePack::var>
5088 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5089 package as assumed. That is, C<$::sail> is equivalent to
5090 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5091 code, mostly from Perl 4).
5093 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
5094 namespace to a L<version> object with the VERSION provided. VERSION must be a
5095 "strict" style version number as defined by the L<version> module: a positive
5096 decimal number (integer or decimal-fraction) without exponentiation or else a
5097 dotted-decimal v-string with a leading 'v' character and at least three
5098 components. You should set C<$VERSION> only once per package.
5100 See L<perlmod/"Packages"> for more information about packages, modules,
5101 and classes. See L<perlsub> for other scoping issues.
5106 =for Pod::Functions +5.004 the current package
5108 A special token that returns the name of the package in which it occurs.
5110 =item pipe READHANDLE,WRITEHANDLE
5113 =for Pod::Functions open a pair of connected filehandles
5115 Opens a pair of connected pipes like the corresponding system call.
5116 Note that if you set up a loop of piped processes, deadlock can occur
5117 unless you are very careful. In addition, note that Perl's pipes use
5118 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5119 after each command, depending on the application.
5121 Returns true on success.
5123 See L<IPC::Open2>, L<IPC::Open3>, and
5124 L<perlipc/"Bidirectional Communication with Another Process">
5125 for examples of such things.
5127 On systems that support a close-on-exec flag on files, that flag is set
5128 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5129 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5138 =for Pod::Functions remove the last element from an array and return it
5140 Pops and returns the last value of the array, shortening the array by
5143 Returns the undefined value if the array is empty, although this may also
5144 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5145 main program, but the C<@_> array in subroutines, just like C<shift>.
5147 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
5148 reference to an unblessed array. The argument will be dereferenced
5149 automatically. This aspect of C<pop> is considered highly experimental.
5150 The exact behaviour may change in a future version of Perl.
5152 To avoid confusing would-be users of your code who are running earlier
5153 versions of Perl with mysterious syntax errors, put this sort of thing at
5154 the top of your file to signal that your code will work I<only> on Perls of
5157 use 5.014; # so push/pop/etc work on scalars (experimental)
5160 X<pos> X<match, position>
5164 =for Pod::Functions find or set the offset for the last/next m//g search
5166 Returns the offset of where the last C<m//g> search left off for the
5167 variable in question (C<$_> is used when the variable is not
5168 specified). Note that 0 is a valid match offset. C<undef> indicates
5169 that the search position is reset (usually due to match failure, but
5170 can also be because no match has yet been run on the scalar).
5172 C<pos> directly accesses the location used by the regexp engine to
5173 store the offset, so assigning to C<pos> will change that offset, and
5174 so will also influence the C<\G> zero-width assertion in regular
5175 expressions. Both of these effects take place for the next match, so
5176 you can't affect the position with C<pos> during the current match,
5177 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5179 Setting C<pos> also resets the I<matched with zero-length> flag, described
5180 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5182 Because a failed C<m//gc> match doesn't reset the offset, the return
5183 from C<pos> won't change either in this case. See L<perlre> and
5186 =item print FILEHANDLE LIST
5189 =item print FILEHANDLE
5195 =for Pod::Functions output a list to a filehandle
5197 Prints a string or a list of strings. Returns true if successful.
5198 FILEHANDLE may be a scalar variable containing the name of or a reference
5199 to the filehandle, thus introducing one level of indirection. (NOTE: If
5200 FILEHANDLE is a variable and the next token is a term, it may be
5201 misinterpreted as an operator unless you interpose a C<+> or put
5202 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5203 last selected (see L</select>) output handle. If LIST is omitted, prints
5204 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5205 print the content of C<$_> to it, you must use a real filehandle like
5206 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5207 to something other than STDOUT, use the select operation.
5209 The current value of C<$,> (if any) is printed between each LIST item. The
5210 current value of C<$\> (if any) is printed after the entire LIST has been
5211 printed. Because print takes a LIST, anything in the LIST is evaluated in
5212 list context, including any subroutines whose return lists you pass to
5213 C<print>. Be careful not to follow the print keyword with a left
5214 parenthesis unless you want the corresponding right parenthesis to
5215 terminate the arguments to the print; put parentheses around all arguments
5216 (or interpose a C<+>, but that doesn't look as good).
5218 If you're storing handles in an array or hash, or in general whenever
5219 you're using any expression more complex than a bareword handle or a plain,
5220 unsubscripted scalar variable to retrieve it, you will have to use a block
5221 returning the filehandle value instead, in which case the LIST may not be
5224 print { $files[$i] } "stuff\n";
5225 print { $OK ? STDOUT : STDERR } "stuff\n";
5227 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5228 L<perlipc> for more on signal handling.
5230 =item printf FILEHANDLE FORMAT, LIST
5233 =item printf FILEHANDLE
5235 =item printf FORMAT, LIST
5239 =for Pod::Functions output a formatted list to a filehandle
5241 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5242 (the output record separator) is not appended. The FORMAT and the
5243 LIST are actually parsed as a single list. The first argument
5244 of the list will be interpreted as the C<printf> format. This
5245 means that C<printf(@_)> will use C<$_[0]> as the format. See
5246 L<sprintf|/sprintf FORMAT, LIST> for an
5247 explanation of the format argument. If C<use locale> for C<LC_NUMERIC>
5248 Look for this throught pod
5250 POSIX::setlocale() has been called, the character used for the decimal
5251 separator in formatted floating-point numbers is affected by the C<LC_NUMERIC>
5252 locale setting. See L<perllocale> and L<POSIX>.
5254 For historical reasons, if you omit the list, C<$_> is used as the format;
5255 to use FILEHANDLE without a list, you must use a real filehandle like
5256 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5257 you want; if $_ contains formatting codes, they will be replaced with the
5258 empty string and a warning will be emitted if warnings are enabled. Just
5259 use C<print> if you want to print the contents of $_.
5261 Don't fall into the trap of using a C<printf> when a simple
5262 C<print> would do. The C<print> is more efficient and less
5265 =item prototype FUNCTION
5268 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5270 Returns the prototype of a function as a string (or C<undef> if the
5271 function has no prototype). FUNCTION is a reference to, or the name of,
5272 the function whose prototype you want to retrieve.
5274 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5275 name for a Perl builtin. If the builtin's arguments
5276 cannot be adequately expressed by a prototype
5277 (such as C<system>), prototype() returns C<undef>, because the builtin
5278 does not really behave like a Perl function. Otherwise, the string
5279 describing the equivalent prototype is returned.
5281 =item push ARRAY,LIST
5284 =item push EXPR,LIST
5286 =for Pod::Functions append one or more elements to an array
5288 Treats ARRAY as a stack by appending the values of LIST to the end of
5289 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5293 $ARRAY[++$#ARRAY] = $value;
5296 but is more efficient. Returns the number of elements in the array following
5297 the completed C<push>.
5299 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
5300 reference to an unblessed array. The argument will be dereferenced
5301 automatically. This aspect of C<push> is considered highly experimental.
5302 The exact behaviour may change in a future version of Perl.
5304 To avoid confusing would-be users of your code who are running earlier
5305 versions of Perl with mysterious syntax errors, put this sort of thing at
5306 the top of your file to signal that your code will work I<only> on Perls of
5309 use 5.014; # so push/pop/etc work on scalars (experimental)
5313 =for Pod::Functions singly quote a string
5317 =for Pod::Functions doubly quote a string
5321 =for Pod::Functions quote a list of words
5325 =for Pod::Functions backquote quote a string
5327 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5331 =for Pod::Functions +5.005 compile pattern
5333 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5335 =item quotemeta EXPR
5336 X<quotemeta> X<metacharacter>
5340 =for Pod::Functions quote regular expression magic characters
5342 Returns the value of EXPR with all the ASCII non-"word"
5343 characters backslashed. (That is, all ASCII characters not matching
5344 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5345 returned string, regardless of any locale settings.)
5346 This is the internal function implementing
5347 the C<\Q> escape in double-quoted strings.
5348 (See below for the behavior on non-ASCII code points.)
5350 If EXPR is omitted, uses C<$_>.
5352 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5353 regular expressions, because by default an interpolated variable will be
5354 considered a mini-regular expression. For example:
5356 my $sentence = 'The quick brown fox jumped over the lazy dog';
5357 my $substring = 'quick.*?fox';
5358 $sentence =~ s{$substring}{big bad wolf};
5360 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5364 my $sentence = 'The quick brown fox jumped over the lazy dog';
5365 my $substring = 'quick.*?fox';
5366 $sentence =~ s{\Q$substring\E}{big bad wolf};
5370 my $sentence = 'The quick brown fox jumped over the lazy dog';
5371 my $substring = 'quick.*?fox';
5372 my $quoted_substring = quotemeta($substring);
5373 $sentence =~ s{$quoted_substring}{big bad wolf};
5375 Will both leave the sentence as is.
5376 Normally, when accepting literal string
5377 input from the user, quotemeta() or C<\Q> must be used.
5379 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5380 strings, but not quoted in UTF-8 strings.
5382 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5383 quoting non-ASCII characters; the quoting of ASCII characters is
5386 Also unchanged is the quoting of non-UTF-8 strings when outside the
5387 scope of a C<use feature 'unicode_strings'>, which is to quote all
5388 characters in the upper Latin1 range. This provides complete backwards
5389 compatibility for old programs which do not use Unicode. (Note that
5390 C<unicode_strings> is automatically enabled within the scope of a
5391 S<C<use v5.12>> or greater.)
5393 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5394 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5395 above, locale does not affect the quoting of ASCII-range characters.
5396 This protects against those locales where characters such as C<"|"> are
5397 considered to be word characters.
5399 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5400 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5401 The only code points that are quoted are those that have any of the
5402 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5403 Default_Ignorable_Code_Point, or General_Category=Control.
5405 Of these properties, the two important ones are Pattern_Syntax and
5406 Pattern_White_Space. They have been set up by Unicode for exactly this
5407 purpose of deciding which characters in a regular expression pattern
5408 should be quoted. No character that can be in an identifier has these
5411 Perl promises, that if we ever add regular expression pattern
5412 metacharacters to the dozen already defined
5413 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5414 Pattern_Syntax property. Perl also promises, that if we ever add
5415 characters that are considered to be white space in regular expressions
5416 (currently mostly affected by C</x>), they will all have the
5417 Pattern_White_Space property.
5419 Unicode promises that the set of code points that have these two
5420 properties will never change, so something that is not quoted in v5.16
5421 will never need to be quoted in any future Perl release. (Not all the
5422 code points that match Pattern_Syntax have actually had characters
5423 assigned to them; so there is room to grow, but they are quoted
5424 whether assigned or not. Perl, of course, would never use an
5425 unassigned code point as an actual metacharacter.)
5427 Quoting characters that have the other 3 properties is done to enhance
5428 the readability of the regular expression and not because they actually
5429 need to be quoted for regular expression purposes (characters with the
5430 White_Space property are likely to be indistinguishable on the page or
5431 screen from those with the Pattern_White_Space property; and the other
5432 two properties contain non-printing characters).
5439 =for Pod::Functions retrieve the next pseudorandom number
5441 Returns a random fractional number greater than or equal to C<0> and less
5442 than the value of EXPR. (EXPR should be positive.) If EXPR is
5443 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5444 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5445 and is subject to change in future versions of Perl). Automatically calls
5446 C<srand> unless C<srand> has already been called. See also C<srand>.
5448 Apply C<int()> to the value returned by C<rand()> if you want random
5449 integers instead of random fractional numbers. For example,
5453 returns a random integer between C<0> and C<9>, inclusive.
5455 (Note: If your rand function consistently returns numbers that are too
5456 large or too small, then your version of Perl was probably compiled
5457 with the wrong number of RANDBITS.)
5459 B<C<rand()> is not cryptographically secure. You should not rely
5460 on it in security-sensitive situations.> As of this writing, a
5461 number of third-party CPAN modules offer random number generators
5462 intended by their authors to be cryptographically secure,
5463 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5464 and L<Math::TrulyRandom>.
5466 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5467 X<read> X<file, read>
5469 =item read FILEHANDLE,SCALAR,LENGTH
5471 =for Pod::Functions fixed-length buffered input from a filehandle
5473 Attempts to read LENGTH I<characters> of data into variable SCALAR
5474 from the specified FILEHANDLE. Returns the number of characters
5475 actually read, C<0> at end of file, or undef if there was an error (in
5476 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5477 so that the last character actually read is the last character of the
5478 scalar after the read.
5480 An OFFSET may be specified to place the read data at some place in the
5481 string other than the beginning. A negative OFFSET specifies
5482 placement at that many characters counting backwards from the end of
5483 the string. A positive OFFSET greater than the length of SCALAR
5484 results in the string being padded to the required size with C<"\0">
5485 bytes before the result of the read is appended.
5487 The call is implemented in terms of either Perl's or your system's native
5488 fread(3) library function. To get a true read(2) system call, see
5489 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5491 Note the I<characters>: depending on the status of the filehandle,
5492 either (8-bit) bytes or characters are read. By default, all
5493 filehandles operate on bytes, but for example if the filehandle has
5494 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5495 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5496 characters, not bytes. Similarly for the C<:encoding> pragma:
5497 in that case pretty much any characters can be read.
5499 =item readdir DIRHANDLE
5502 =for Pod::Functions get a directory from a directory handle
5504 Returns the next directory entry for a directory opened by C<opendir>.
5505 If used in list context, returns all the rest of the entries in the
5506 directory. If there are no more entries, returns the undefined value in
5507 scalar context and the empty list in list context.
5509 If you're planning to filetest the return values out of a C<readdir>, you'd
5510 better prepend the directory in question. Otherwise, because we didn't
5511 C<chdir> there, it would have been testing the wrong file.
5513 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5514 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5517 As of Perl 5.12 you can use a bare C<readdir> in a C<while> loop,
5518 which will set C<$_> on every iteration.
5520 opendir(my $dh, $some_dir) || die;
5521 while(readdir $dh) {
5522 print "$some_dir/$_\n";
5526 To avoid confusing would-be users of your code who are running earlier
5527 versions of Perl with mysterious failures, put this sort of thing at the
5528 top of your file to signal that your code will work I<only> on Perls of a
5531 use 5.012; # so readdir assigns to $_ in a lone while test
5536 X<readline> X<gets> X<fgets>
5538 =for Pod::Functions fetch a record from a file
5540 Reads from the filehandle whose typeglob is contained in EXPR (or from
5541 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5542 returns the next line until end-of-file is reached, whereupon the
5543 subsequent call returns C<undef>. In list context, reads until end-of-file
5544 is reached and returns a list of lines. Note that the notion of "line"
5545 used here is whatever you may have defined with C<$/> or
5546 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5548 When C<$/> is set to C<undef>, when C<readline> is in scalar
5549 context (i.e., file slurp mode), and when an empty file is read, it
5550 returns C<''> the first time, followed by C<undef> subsequently.
5552 This is the internal function implementing the C<< <EXPR> >>
5553 operator, but you can use it directly. The C<< <EXPR> >>
5554 operator is discussed in more detail in L<perlop/"I/O Operators">.
5557 $line = readline(*STDIN); # same thing
5559 If C<readline> encounters an operating system error, C<$!> will be set
5560 with the corresponding error message. It can be helpful to check
5561 C<$!> when you are reading from filehandles you don't trust, such as a
5562 tty or a socket. The following example uses the operator form of
5563 C<readline> and dies if the result is not defined.
5565 while ( ! eof($fh) ) {
5566 defined( $_ = <$fh> ) or die "readline failed: $!";
5570 Note that you have can't handle C<readline> errors that way with the
5571 C<ARGV> filehandle. In that case, you have to open each element of
5572 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5574 foreach my $arg (@ARGV) {
5575 open(my $fh, $arg) or warn "Can't open $arg: $!";
5577 while ( ! eof($fh) ) {
5578 defined( $_ = <$fh> )
5579 or die "readline failed for $arg: $!";
5589 =for Pod::Functions determine where a symbolic link is pointing
5591 Returns the value of a symbolic link, if symbolic links are
5592 implemented. If not, raises an exception. If there is a system
5593 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5594 omitted, uses C<$_>.
5596 Portability issues: L<perlport/readlink>.
5603 =for Pod::Functions execute a system command and collect standard output
5605 EXPR is executed as a system command.
5606 The collected standard output of the command is returned.
5607 In scalar context, it comes back as a single (potentially
5608 multi-line) string. In list context, returns a list of lines
5609 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5610 This is the internal function implementing the C<qx/EXPR/>
5611 operator, but you can use it directly. The C<qx/EXPR/>
5612 operator is discussed in more detail in L<perlop/"I/O Operators">.
5613 If EXPR is omitted, uses C<$_>.
5615 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5618 =for Pod::Functions receive a message over a Socket
5620 Receives a message on a socket. Attempts to receive LENGTH characters
5621 of data into variable SCALAR from the specified SOCKET filehandle.
5622 SCALAR will be grown or shrunk to the length actually read. Takes the
5623 same flags as the system call of the same name. Returns the address
5624 of the sender if SOCKET's protocol supports this; returns an empty
5625 string otherwise. If there's an error, returns the undefined value.
5626 This call is actually implemented in terms of recvfrom(2) system call.
5627 See L<perlipc/"UDP: Message Passing"> for examples.
5629 Note the I<characters>: depending on the status of the socket, either
5630 (8-bit) bytes or characters are received. By default all sockets
5631 operate on bytes, but for example if the socket has been changed using
5632 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5633 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5634 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5635 case pretty much any characters can be read.
5644 =for Pod::Functions start this loop iteration over again
5646 The C<redo> command restarts the loop block without evaluating the
5647 conditional again. The C<continue> block, if any, is not executed. If
5648 the LABEL is omitted, the command refers to the innermost enclosing
5649 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
5650 label name to be computed at run time, and is otherwise identical to C<redo
5651 LABEL>. Programs that want to lie to themselves about what was just input
5652 normally use this command:
5654 # a simpleminded Pascal comment stripper
5655 # (warning: assumes no { or } in strings)
5656 LINE: while (<STDIN>) {
5657 while (s|({.*}.*){.*}|$1 |) {}
5662 if (/}/) { # end of comment?
5671 C<redo> cannot be used to retry a block that returns a value such as
5672 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5673 a grep() or map() operation.
5675 Note that a block by itself is semantically identical to a loop
5676 that executes once. Thus C<redo> inside such a block will effectively
5677 turn it into a looping construct.
5679 See also L</continue> for an illustration of how C<last>, C<next>, and
5682 Unlike most named operators, this has the same precedence as assignment.
5683 It is also exempt from the looks-like-a-function rule, so
5684 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
5692 =for Pod::Functions find out the type of thing being referenced
5694 Returns a non-empty string if EXPR is a reference, the empty
5695 string otherwise. If EXPR is not specified, C<$_> will be used. The
5696 value returned depends on the type of thing the reference is a reference to.
5698 Builtin types include:
5712 You can think of C<ref> as a C<typeof> operator.
5714 if (ref($r) eq "HASH") {
5715 print "r is a reference to a hash.\n";
5718 print "r is not a reference at all.\n";
5721 The return value C<LVALUE> indicates a reference to an lvalue that is not
5722 a variable. You get this from taking the reference of function calls like
5723 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5724 to a L<version string|perldata/"Version Strings">.
5726 The result C<Regexp> indicates that the argument is a regular expression
5727 resulting from C<qr//>.
5729 If the referenced object has been blessed into a package, then that package
5730 name is returned instead. But don't use that, as it's now considered
5731 "bad practice". For one reason, an object could be using a class called
5732 C<Regexp> or C<IO>, or even C<HASH>. Also, C<ref> doesn't take into account
5733 subclasses, like C<isa> does.
5735 Instead, use C<blessed> (in the L<Scalar::Util> module) for boolean
5736 checks, C<isa> for specific class checks and C<reftype> (also from
5737 L<Scalar::Util>) for type checks. (See L<perlobj> for details and a
5738 C<blessed/isa> example.)
5740 See also L<perlref>.
5742 =item rename OLDNAME,NEWNAME
5743 X<rename> X<move> X<mv> X<ren>
5745 =for Pod::Functions change a filename
5747 Changes the name of a file; an existing file NEWNAME will be
5748 clobbered. Returns true for success, false otherwise.
5750 Behavior of this function varies wildly depending on your system
5751 implementation. For example, it will usually not work across file system
5752 boundaries, even though the system I<mv> command sometimes compensates
5753 for this. Other restrictions include whether it works on directories,
5754 open files, or pre-existing files. Check L<perlport> and either the
5755 rename(2) manpage or equivalent system documentation for details.
5757 For a platform independent C<move> function look at the L<File::Copy>
5760 Portability issues: L<perlport/rename>.
5762 =item require VERSION
5769 =for Pod::Functions load in external functions from a library at runtime
5771 Demands a version of Perl specified by VERSION, or demands some semantics
5772 specified by EXPR or by C<$_> if EXPR is not supplied.
5774 VERSION may be either a numeric argument such as 5.006, which will be
5775 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5776 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5777 VERSION is greater than the version of the current Perl interpreter.
5778 Compare with L</use>, which can do a similar check at compile time.
5780 Specifying VERSION as a literal of the form v5.6.1 should generally be
5781 avoided, because it leads to misleading error messages under earlier
5782 versions of Perl that do not support this syntax. The equivalent numeric
5783 version should be used instead.
5785 require v5.6.1; # run time version check
5786 require 5.6.1; # ditto
5787 require 5.006_001; # ditto; preferred for backwards
5790 Otherwise, C<require> demands that a library file be included if it
5791 hasn't already been included. The file is included via the do-FILE
5792 mechanism, which is essentially just a variety of C<eval> with the
5793 caveat that lexical variables in the invoking script will be invisible
5794 to the included code. If it were implemented in pure Perl, it
5795 would have semantics similar to the following:
5801 my ($filename) = @_;
5802 if ( my $version = eval { version->parse($filename) } ) {
5803 if ( $version > $^V ) {
5804 my $vn = $version->normal;
5805 croak "Perl $vn required--this is only $^V, stopped";
5810 if (exists $INC{$filename}) {
5811 return 1 if $INC{$filename};
5812 croak "Compilation failed in require";
5815 foreach $prefix (@INC) {
5817 #... do other stuff - see text below ....
5819 # (see text below about possible appending of .pmc
5820 # suffix to $filename)
5821 my $realfilename = "$prefix/$filename";
5822 next if ! -e $realfilename || -d _ || -b _;
5823 $INC{$filename} = $realfilename;
5824 my $result = do($realfilename);
5825 # but run in caller's namespace
5827 if (!defined $result) {
5828 $INC{$filename} = undef;
5829 croak $@ ? "$@Compilation failed in require"
5830 : "Can't locate $filename: $!\n";
5833 delete $INC{$filename};
5834 croak "$filename did not return true value";
5839 croak "Can't locate $filename in \@INC ...";
5842 Note that the file will not be included twice under the same specified
5845 The file must return true as the last statement to indicate
5846 successful execution of any initialization code, so it's customary to
5847 end such a file with C<1;> unless you're sure it'll return true
5848 otherwise. But it's better just to put the C<1;>, in case you add more
5851 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5852 replaces "F<::>" with "F</>" in the filename for you,
5853 to make it easy to load standard modules. This form of loading of
5854 modules does not risk altering your namespace.
5856 In other words, if you try this:
5858 require Foo::Bar; # a splendid bareword
5860 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5861 directories specified in the C<@INC> array.
5863 But if you try this:
5865 $class = 'Foo::Bar';
5866 require $class; # $class is not a bareword
5868 require "Foo::Bar"; # not a bareword because of the ""
5870 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5871 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5873 eval "require $class";
5875 Now that you understand how C<require> looks for files with a
5876 bareword argument, there is a little extra functionality going on behind
5877 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5878 first look for a similar filename with a "F<.pmc>" extension. If this file
5879 is found, it will be loaded in place of any file ending in a "F<.pm>"
5882 You can also insert hooks into the import facility by putting Perl code
5883 directly into the @INC array. There are three forms of hooks: subroutine
5884 references, array references, and blessed objects.
5886 Subroutine references are the simplest case. When the inclusion system
5887 walks through @INC and encounters a subroutine, this subroutine gets
5888 called with two parameters, the first a reference to itself, and the
5889 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5890 subroutine should return either nothing or else a list of up to four
5891 values in the following order:
5897 A reference to a scalar, containing any initial source code to prepend to
5898 the file or generator output.
5902 A filehandle, from which the file will be read.
5906 A reference to a subroutine. If there is no filehandle (previous item),
5907 then this subroutine is expected to generate one line of source code per
5908 call, writing the line into C<$_> and returning 1, then finally at end of
5909 file returning 0. If there is a filehandle, then the subroutine will be
5910 called to act as a simple source filter, with the line as read in C<$_>.
5911 Again, return 1 for each valid line, and 0 after all lines have been
5916 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5917 reference to the subroutine itself is passed in as C<$_[0]>.
5921 If an empty list, C<undef>, or nothing that matches the first 3 values above
5922 is returned, then C<require> looks at the remaining elements of @INC.
5923 Note that this filehandle must be a real filehandle (strictly a typeglob
5924 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5925 will be ignored and processing will stop there.
5927 If the hook is an array reference, its first element must be a subroutine
5928 reference. This subroutine is called as above, but the first parameter is
5929 the array reference. This lets you indirectly pass arguments to
5932 In other words, you can write:
5934 push @INC, \&my_sub;
5936 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5942 push @INC, [ \&my_sub, $x, $y, ... ];
5944 my ($arrayref, $filename) = @_;
5945 # Retrieve $x, $y, ...
5946 my @parameters = @$arrayref[1..$#$arrayref];
5950 If the hook is an object, it must provide an INC method that will be
5951 called as above, the first parameter being the object itself. (Note that
5952 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5953 into package C<main>.) Here is a typical code layout:
5959 my ($self, $filename) = @_;
5963 # In the main program
5964 push @INC, Foo->new(...);
5966 These hooks are also permitted to set the %INC entry
5967 corresponding to the files they have loaded. See L<perlvar/%INC>.
5969 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5976 =for Pod::Functions clear all variables of a given name
5978 Generally used in a C<continue> block at the end of a loop to clear
5979 variables and reset C<??> searches so that they work again. The
5980 expression is interpreted as a list of single characters (hyphens
5981 allowed for ranges). All variables and arrays beginning with one of
5982 those letters are reset to their pristine state. If the expression is
5983 omitted, one-match searches (C<?pattern?>) are reset to match again.
5984 Only resets variables or searches in the current package. Always returns
5987 reset 'X'; # reset all X variables
5988 reset 'a-z'; # reset lower case variables
5989 reset; # just reset ?one-time? searches
5991 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5992 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5993 variables; lexical variables are unaffected, but they clean themselves
5994 up on scope exit anyway, so you'll probably want to use them instead.
6002 =for Pod::Functions get out of a function early
6004 Returns from a subroutine, C<eval>, or C<do FILE> with the value
6005 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6006 context, depending on how the return value will be used, and the context
6007 may vary from one execution to the next (see L</wantarray>). If no EXPR
6008 is given, returns an empty list in list context, the undefined value in
6009 scalar context, and (of course) nothing at all in void context.
6011 (In the absence of an explicit C<return>, a subroutine, eval,
6012 or do FILE automatically returns the value of the last expression
6015 Unlike most named operators, this is also exempt from the
6016 looks-like-a-function rule, so C<return ("foo")."bar"> will
6017 cause "bar" to be part of the argument to C<return>.
6020 X<reverse> X<rev> X<invert>
6022 =for Pod::Functions flip a string or a list
6024 In list context, returns a list value consisting of the elements
6025 of LIST in the opposite order. In scalar context, concatenates the
6026 elements of LIST and returns a string value with all characters
6027 in the opposite order.
6029 print join(", ", reverse "world", "Hello"); # Hello, world
6031 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6033 Used without arguments in scalar context, reverse() reverses C<$_>.
6035 $_ = "dlrow ,olleH";
6036 print reverse; # No output, list context
6037 print scalar reverse; # Hello, world
6039 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6040 preserve non-existent elements whenever possible; i.e., for non-magical
6041 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6043 This operator is also handy for inverting a hash, although there are some
6044 caveats. If a value is duplicated in the original hash, only one of those
6045 can be represented as a key in the inverted hash. Also, this has to
6046 unwind one hash and build a whole new one, which may take some time
6047 on a large hash, such as from a DBM file.
6049 %by_name = reverse %by_address; # Invert the hash
6051 =item rewinddir DIRHANDLE
6054 =for Pod::Functions reset directory handle
6056 Sets the current position to the beginning of the directory for the
6057 C<readdir> routine on DIRHANDLE.
6059 Portability issues: L<perlport/rewinddir>.
6061 =item rindex STR,SUBSTR,POSITION
6064 =item rindex STR,SUBSTR
6066 =for Pod::Functions right-to-left substring search
6068 Works just like index() except that it returns the position of the I<last>
6069 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6070 last occurrence beginning at or before that position.
6072 =item rmdir FILENAME
6073 X<rmdir> X<rd> X<directory, remove>
6077 =for Pod::Functions remove a directory
6079 Deletes the directory specified by FILENAME if that directory is
6080 empty. If it succeeds it returns true; otherwise it returns false and
6081 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
6083 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6084 the C<rmtree> function of the L<File::Path> module.
6088 =for Pod::Functions replace a pattern with a string
6090 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6092 =item say FILEHANDLE LIST
6095 =item say FILEHANDLE
6101 =for Pod::Functions +say output a list to a filehandle, appending a newline
6103 Just like C<print>, but implicitly appends a newline. C<say LIST> is
6104 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
6105 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
6106 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
6108 This keyword is available only when the C<"say"> feature
6109 is enabled, or when prefixed with C<CORE::>; see
6110 L<feature>. Alternately, include a C<use v5.10> or later to the current
6114 X<scalar> X<context>
6116 =for Pod::Functions force a scalar context
6118 Forces EXPR to be interpreted in scalar context and returns the value
6121 @counts = ( scalar @a, scalar @b, scalar @c );
6123 There is no equivalent operator to force an expression to
6124 be interpolated in list context because in practice, this is never
6125 needed. If you really wanted to do so, however, you could use
6126 the construction C<@{[ (some expression) ]}>, but usually a simple
6127 C<(some expression)> suffices.
6129 Because C<scalar> is a unary operator, if you accidentally use a
6130 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6131 evaluating all but the last element in void context and returning the final
6132 element evaluated in scalar context. This is seldom what you want.
6134 The following single statement:
6136 print uc(scalar(&foo,$bar)),$baz;
6138 is the moral equivalent of these two:
6141 print(uc($bar),$baz);
6143 See L<perlop> for more details on unary operators and the comma operator.
6145 =item seek FILEHANDLE,POSITION,WHENCE
6146 X<seek> X<fseek> X<filehandle, position>
6148 =for Pod::Functions reposition file pointer for random-access I/O
6150 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
6151 FILEHANDLE may be an expression whose value gives the name of the
6152 filehandle. The values for WHENCE are C<0> to set the new position
6153 I<in bytes> to POSITION; C<1> to set it to the current position plus
6154 POSITION; and C<2> to set it to EOF plus POSITION, typically
6155 negative. For WHENCE you may use the constants C<SEEK_SET>,
6156 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6157 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6160 Note the I<in bytes>: even if the filehandle has been set to
6161 operate on characters (for example by using the C<:encoding(utf8)> open
6162 layer), tell() will return byte offsets, not character offsets
6163 (because implementing that would render seek() and tell() rather slow).
6165 If you want to position the file for C<sysread> or C<syswrite>, don't use
6166 C<seek>, because buffering makes its effect on the file's read-write position
6167 unpredictable and non-portable. Use C<sysseek> instead.
6169 Due to the rules and rigors of ANSI C, on some systems you have to do a
6170 seek whenever you switch between reading and writing. Amongst other
6171 things, this may have the effect of calling stdio's clearerr(3).
6172 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6176 This is also useful for applications emulating C<tail -f>. Once you hit
6177 EOF on your read and then sleep for a while, you (probably) have to stick in a
6178 dummy seek() to reset things. The C<seek> doesn't change the position,
6179 but it I<does> clear the end-of-file condition on the handle, so that the
6180 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6182 If that doesn't work (some I/O implementations are particularly
6183 cantankerous), you might need something like this:
6186 for ($curpos = tell(FILE); $_ = <FILE>;
6187 $curpos = tell(FILE)) {
6188 # search for some stuff and put it into files
6190 sleep($for_a_while);
6191 seek(FILE, $curpos, 0);
6194 =item seekdir DIRHANDLE,POS
6197 =for Pod::Functions reposition directory pointer
6199 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6200 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6201 about possible directory compaction as the corresponding system library
6204 =item select FILEHANDLE
6205 X<select> X<filehandle, default>
6209 =for Pod::Functions reset default output or do I/O multiplexing
6211 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6212 sets the new current default filehandle for output. This has two
6213 effects: first, a C<write> or a C<print> without a filehandle
6214 default to this FILEHANDLE. Second, references to variables related to
6215 output will refer to this output channel.
6217 For example, to set the top-of-form format for more than one
6218 output channel, you might do the following:
6225 FILEHANDLE may be an expression whose value gives the name of the
6226 actual filehandle. Thus:
6228 $oldfh = select(STDERR); $| = 1; select($oldfh);
6230 Some programmers may prefer to think of filehandles as objects with
6231 methods, preferring to write the last example as:
6234 STDERR->autoflush(1);
6236 Portability issues: L<perlport/select>.
6238 =item select RBITS,WBITS,EBITS,TIMEOUT
6241 This calls the select(2) syscall with the bit masks specified, which
6242 can be constructed using C<fileno> and C<vec>, along these lines:
6244 $rin = $win = $ein = '';
6245 vec($rin, fileno(STDIN), 1) = 1;
6246 vec($win, fileno(STDOUT), 1) = 1;
6249 If you want to select on many filehandles, you may wish to write a
6250 subroutine like this:
6255 for my $fh (@fhlist) {
6256 vec($bits, fileno($fh), 1) = 1;
6260 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6264 ($nfound,$timeleft) =
6265 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6267 or to block until something becomes ready just do this
6269 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6271 Most systems do not bother to return anything useful in $timeleft, so
6272 calling select() in scalar context just returns $nfound.
6274 Any of the bit masks can also be undef. The timeout, if specified, is
6275 in seconds, which may be fractional. Note: not all implementations are
6276 capable of returning the $timeleft. If not, they always return
6277 $timeleft equal to the supplied $timeout.
6279 You can effect a sleep of 250 milliseconds this way:
6281 select(undef, undef, undef, 0.25);
6283 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6284 is implementation-dependent. See also L<perlport> for notes on the
6285 portability of C<select>.
6287 On error, C<select> behaves just like select(2): it returns
6290 On some Unixes, select(2) may report a socket file descriptor as "ready for
6291 reading" even when no data is available, and thus any subsequent C<read>
6292 would block. This can be avoided if you always use O_NONBLOCK on the
6293 socket. See select(2) and fcntl(2) for further details.
6295 The standard C<IO::Select> module provides a user-friendlier interface
6296 to C<select>, mostly because it does all the bit-mask work for you.
6298 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6299 or <FH>) with C<select>, except as permitted by POSIX, and even
6300 then only on POSIX systems. You have to use C<sysread> instead.
6302 Portability issues: L<perlport/select>.
6304 =item semctl ID,SEMNUM,CMD,ARG
6307 =for Pod::Functions SysV semaphore control operations
6309 Calls the System V IPC function semctl(2). You'll probably have to say
6313 first to get the correct constant definitions. If CMD is IPC_STAT or
6314 GETALL, then ARG must be a variable that will hold the returned
6315 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6316 the undefined value for error, "C<0 but true>" for zero, or the actual
6317 return value otherwise. The ARG must consist of a vector of native
6318 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6319 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6322 Portability issues: L<perlport/semctl>.
6324 =item semget KEY,NSEMS,FLAGS
6327 =for Pod::Functions get set of SysV semaphores
6329 Calls the System V IPC function semget(2). Returns the semaphore id, or
6330 the undefined value on error. See also
6331 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6334 Portability issues: L<perlport/semget>.
6336 =item semop KEY,OPSTRING
6339 =for Pod::Functions SysV semaphore operations
6341 Calls the System V IPC function semop(2) for semaphore operations
6342 such as signalling and waiting. OPSTRING must be a packed array of
6343 semop structures. Each semop structure can be generated with
6344 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6345 implies the number of semaphore operations. Returns true if
6346 successful, false on error. As an example, the
6347 following code waits on semaphore $semnum of semaphore id $semid:
6349 $semop = pack("s!3", $semnum, -1, 0);
6350 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6352 To signal the semaphore, replace C<-1> with C<1>. See also
6353 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6356 Portability issues: L<perlport/semop>.
6358 =item send SOCKET,MSG,FLAGS,TO
6361 =item send SOCKET,MSG,FLAGS
6363 =for Pod::Functions send a message over a socket
6365 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6366 filehandle. Takes the same flags as the system call of the same name. On
6367 unconnected sockets, you must specify a destination to I<send to>, in which
6368 case it does a sendto(2) syscall. Returns the number of characters sent,
6369 or the undefined value on error. The sendmsg(2) syscall is currently
6370 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6372 Note the I<characters>: depending on the status of the socket, either
6373 (8-bit) bytes or characters are sent. By default all sockets operate
6374 on bytes, but for example if the socket has been changed using
6375 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6376 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6377 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6378 pragma: in that case pretty much any characters can be sent.
6380 =item setpgrp PID,PGRP
6383 =for Pod::Functions set the process group of a process
6385 Sets the current process group for the specified PID, C<0> for the current
6386 process. Raises an exception when used on a machine that doesn't
6387 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6388 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6389 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6392 Portability issues: L<perlport/setpgrp>.
6394 =item setpriority WHICH,WHO,PRIORITY
6395 X<setpriority> X<priority> X<nice> X<renice>
6397 =for Pod::Functions set a process's nice value
6399 Sets the current priority for a process, a process group, or a user.
6400 (See setpriority(2).) Raises an exception when used on a machine
6401 that doesn't implement setpriority(2).
6403 Portability issues: L<perlport/setpriority>.
6405 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6408 =for Pod::Functions set some socket options
6410 Sets the socket option requested. Returns C<undef> on error.
6411 Use integer constants provided by the C<Socket> module for
6412 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6413 getprotobyname. OPTVAL might either be a packed string or an integer.
6414 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6416 An example disabling Nagle's algorithm on a socket:
6418 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6419 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6421 Portability issues: L<perlport/setsockopt>.
6430 =for Pod::Functions remove the first element of an array, and return it
6432 Shifts the first value of the array off and returns it, shortening the
6433 array by 1 and moving everything down. If there are no elements in the
6434 array, returns the undefined value. If ARRAY is omitted, shifts the
6435 C<@_> array within the lexical scope of subroutines and formats, and the
6436 C<@ARGV> array outside a subroutine and also within the lexical scopes
6437 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6438 C<UNITCHECK {}>, and C<END {}> constructs.
6440 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
6441 reference to an unblessed array. The argument will be dereferenced
6442 automatically. This aspect of C<shift> is considered highly experimental.
6443 The exact behaviour may change in a future version of Perl.
6445 To avoid confusing would-be users of your code who are running earlier
6446 versions of Perl with mysterious syntax errors, put this sort of thing at
6447 the top of your file to signal that your code will work I<only> on Perls of
6450 use 5.014; # so push/pop/etc work on scalars (experimental)
6452 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6453 same thing to the left end of an array that C<pop> and C<push> do to the
6456 =item shmctl ID,CMD,ARG
6459 =for Pod::Functions SysV shared memory operations
6461 Calls the System V IPC function shmctl. You'll probably have to say
6465 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6466 then ARG must be a variable that will hold the returned C<shmid_ds>
6467 structure. Returns like ioctl: C<undef> for error; "C<0> but
6468 true" for zero; and the actual return value otherwise.
6469 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6471 Portability issues: L<perlport/shmctl>.
6473 =item shmget KEY,SIZE,FLAGS
6476 =for Pod::Functions get SysV shared memory segment identifier
6478 Calls the System V IPC function shmget. Returns the shared memory
6479 segment id, or C<undef> on error.
6480 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6482 Portability issues: L<perlport/shmget>.
6484 =item shmread ID,VAR,POS,SIZE
6488 =for Pod::Functions read SysV shared memory
6490 =item shmwrite ID,STRING,POS,SIZE
6492 =for Pod::Functions write SysV shared memory
6494 Reads or writes the System V shared memory segment ID starting at
6495 position POS for size SIZE by attaching to it, copying in/out, and
6496 detaching from it. When reading, VAR must be a variable that will
6497 hold the data read. When writing, if STRING is too long, only SIZE
6498 bytes are used; if STRING is too short, nulls are written to fill out
6499 SIZE bytes. Return true if successful, false on error.
6500 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6501 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6503 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6505 =item shutdown SOCKET,HOW
6508 =for Pod::Functions close down just half of a socket connection
6510 Shuts down a socket connection in the manner indicated by HOW, which
6511 has the same interpretation as in the syscall of the same name.
6513 shutdown(SOCKET, 0); # I/we have stopped reading data
6514 shutdown(SOCKET, 1); # I/we have stopped writing data
6515 shutdown(SOCKET, 2); # I/we have stopped using this socket
6517 This is useful with sockets when you want to tell the other
6518 side you're done writing but not done reading, or vice versa.
6519 It's also a more insistent form of close because it also
6520 disables the file descriptor in any forked copies in other
6523 Returns C<1> for success; on error, returns C<undef> if
6524 the first argument is not a valid filehandle, or returns C<0> and sets
6525 C<$!> for any other failure.
6528 X<sin> X<sine> X<asin> X<arcsine>
6532 =for Pod::Functions return the sine of a number
6534 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6535 returns sine of C<$_>.
6537 For the inverse sine operation, you may use the C<Math::Trig::asin>
6538 function, or use this relation:
6540 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6547 =for Pod::Functions block for some number of seconds
6549 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6550 argument is given. Returns the integer number of seconds actually slept.
6552 May be interrupted if the process receives a signal such as C<SIGALRM>.
6555 local $SIG{ALARM} = sub { die "Alarm!\n" };
6558 die $@ unless $@ eq "Alarm!\n";
6560 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6561 is often implemented using C<alarm>.
6563 On some older systems, it may sleep up to a full second less than what
6564 you requested, depending on how it counts seconds. Most modern systems
6565 always sleep the full amount. They may appear to sleep longer than that,
6566 however, because your process might not be scheduled right away in a
6567 busy multitasking system.
6569 For delays of finer granularity than one second, the Time::HiRes module
6570 (from CPAN, and starting from Perl 5.8 part of the standard
6571 distribution) provides usleep(). You may also use Perl's four-argument
6572 version of select() leaving the first three arguments undefined, or you
6573 might be able to use the C<syscall> interface to access setitimer(2) if
6574 your system supports it. See L<perlfaq8> for details.
6576 See also the POSIX module's C<pause> function.
6578 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6581 =for Pod::Functions create a socket
6583 Opens a socket of the specified kind and attaches it to filehandle
6584 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6585 the syscall of the same name. You should C<use Socket> first
6586 to get the proper definitions imported. See the examples in
6587 L<perlipc/"Sockets: Client/Server Communication">.
6589 On systems that support a close-on-exec flag on files, the flag will
6590 be set for the newly opened file descriptor, as determined by the
6591 value of $^F. See L<perlvar/$^F>.
6593 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6596 =for Pod::Functions create a pair of sockets
6598 Creates an unnamed pair of sockets in the specified domain, of the
6599 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6600 for the syscall of the same name. If unimplemented, raises an exception.
6601 Returns true if successful.
6603 On systems that support a close-on-exec flag on files, the flag will
6604 be set for the newly opened file descriptors, as determined by the value
6605 of $^F. See L<perlvar/$^F>.
6607 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6608 to C<pipe(Rdr, Wtr)> is essentially:
6611 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6612 shutdown(Rdr, 1); # no more writing for reader
6613 shutdown(Wtr, 0); # no more reading for writer
6615 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6616 emulate socketpair using IP sockets to localhost if your system implements
6617 sockets but not socketpair.
6619 Portability issues: L<perlport/socketpair>.
6621 =item sort SUBNAME LIST
6622 X<sort> X<qsort> X<quicksort> X<mergesort>
6624 =item sort BLOCK LIST
6628 =for Pod::Functions sort a list of values
6630 In list context, this sorts the LIST and returns the sorted list value.
6631 In scalar context, the behaviour of C<sort()> is undefined.
6633 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6634 order. If SUBNAME is specified, it gives the name of a subroutine
6635 that returns an integer less than, equal to, or greater than C<0>,
6636 depending on how the elements of the list are to be ordered. (The
6637 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6638 SUBNAME may be a scalar variable name (unsubscripted), in which case
6639 the value provides the name of (or a reference to) the actual
6640 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6641 an anonymous, in-line sort subroutine.
6643 If the subroutine's prototype is C<($$)>, the elements to be compared are
6644 passed by reference in C<@_>, as for a normal subroutine. This is slower
6645 than unprototyped subroutines, where the elements to be compared are passed
6646 into the subroutine as the package global variables $a and $b (see example
6647 below). Note that in the latter case, it is usually highly counter-productive
6648 to declare $a and $b as lexicals.
6650 If the subroutine is an XSUB, the elements to be compared are pushed on to
6651 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6654 The values to be compared are always passed by reference and should not
6657 You also cannot exit out of the sort block or subroutine using any of the
6658 loop control operators described in L<perlsyn> or with C<goto>.
6660 When C<use locale> (but not C<use locale 'not_characters'>) is in
6661 effect, C<sort LIST> sorts LIST according to the
6662 current collation locale. See L<perllocale>.
6664 sort() returns aliases into the original list, much as a for loop's index
6665 variable aliases the list elements. That is, modifying an element of a
6666 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6667 actually modifies the element in the original list. This is usually
6668 something to be avoided when writing clear code.
6670 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6671 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6672 preserves the input order of elements that compare equal. Although
6673 quicksort's run time is O(NlogN) when averaged over all arrays of
6674 length N, the time can be O(N**2), I<quadratic> behavior, for some
6675 inputs.) In 5.7, the quicksort implementation was replaced with
6676 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6677 But benchmarks indicated that for some inputs, on some platforms,
6678 the original quicksort was faster. 5.8 has a sort pragma for
6679 limited control of the sort. Its rather blunt control of the
6680 underlying algorithm may not persist into future Perls, but the
6681 ability to characterize the input or output in implementation
6682 independent ways quite probably will. See L<the sort pragma|sort>.
6687 @articles = sort @files;
6689 # same thing, but with explicit sort routine
6690 @articles = sort {$a cmp $b} @files;
6692 # now case-insensitively
6693 @articles = sort {fc($a) cmp fc($b)} @files;
6695 # same thing in reversed order
6696 @articles = sort {$b cmp $a} @files;
6698 # sort numerically ascending
6699 @articles = sort {$a <=> $b} @files;
6701 # sort numerically descending
6702 @articles = sort {$b <=> $a} @files;
6704 # this sorts the %age hash by value instead of key
6705 # using an in-line function
6706 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6708 # sort using explicit subroutine name
6710 $age{$a} <=> $age{$b}; # presuming numeric
6712 @sortedclass = sort byage @class;
6714 sub backwards { $b cmp $a }
6715 @harry = qw(dog cat x Cain Abel);
6716 @george = qw(gone chased yz Punished Axed);
6718 # prints AbelCaincatdogx
6719 print sort backwards @harry;
6720 # prints xdogcatCainAbel
6721 print sort @george, 'to', @harry;
6722 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6724 # inefficiently sort by descending numeric compare using
6725 # the first integer after the first = sign, or the
6726 # whole record case-insensitively otherwise
6729 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6734 # same thing, but much more efficiently;
6735 # we'll build auxiliary indices instead
6737 my @nums = @caps = ();
6739 push @nums, ( /=(\d+)/ ? $1 : undef );
6743 my @new = @old[ sort {
6744 $nums[$b] <=> $nums[$a]
6746 $caps[$a] cmp $caps[$b]
6750 # same thing, but without any temps
6751 @new = map { $_->[0] }
6752 sort { $b->[1] <=> $a->[1]
6755 } map { [$_, /=(\d+)/, fc($_)] } @old;
6757 # using a prototype allows you to use any comparison subroutine
6758 # as a sort subroutine (including other package's subroutines)
6760 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6763 @new = sort other::backwards @old;
6765 # guarantee stability, regardless of algorithm
6767 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6769 # force use of mergesort (not portable outside Perl 5.8)
6770 use sort '_mergesort'; # note discouraging _
6771 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6773 Warning: syntactical care is required when sorting the list returned from
6774 a function. If you want to sort the list returned by the function call
6775 C<find_records(@key)>, you can use:
6777 @contact = sort { $a cmp $b } find_records @key;
6778 @contact = sort +find_records(@key);
6779 @contact = sort &find_records(@key);
6780 @contact = sort(find_records(@key));
6782 If instead you want to sort the array @key with the comparison routine
6783 C<find_records()> then you can use:
6785 @contact = sort { find_records() } @key;
6786 @contact = sort find_records(@key);
6787 @contact = sort(find_records @key);
6788 @contact = sort(find_records (@key));
6790 If you're using strict, you I<must not> declare $a
6791 and $b as lexicals. They are package globals. That means
6792 that if you're in the C<main> package and type
6794 @articles = sort {$b <=> $a} @files;
6796 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6797 but if you're in the C<FooPack> package, it's the same as typing
6799 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6801 The comparison function is required to behave. If it returns
6802 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6803 sometimes saying the opposite, for example) the results are not
6806 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6807 (not-a-number), be careful when sorting with a
6808 comparison function like C<< $a <=> $b >> any lists that might contain a
6809 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6810 eliminate any C<NaN>s from the input list.
6812 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6814 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6817 =item splice ARRAY or EXPR,OFFSET,LENGTH
6819 =item splice ARRAY or EXPR,OFFSET
6821 =item splice ARRAY or EXPR
6823 =for Pod::Functions add or remove elements anywhere in an array
6825 Removes the elements designated by OFFSET and LENGTH from an array, and
6826 replaces them with the elements of LIST, if any. In list context,
6827 returns the elements removed from the array. In scalar context,
6828 returns the last element removed, or C<undef> if no elements are
6829 removed. The array grows or shrinks as necessary.
6830 If OFFSET is negative then it starts that far from the end of the array.
6831 If LENGTH is omitted, removes everything from OFFSET onward.
6832 If LENGTH is negative, removes the elements from OFFSET onward
6833 except for -LENGTH elements at the end of the array.
6834 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6835 past the end of the array and a LENGTH was provided, Perl issues a warning,
6836 and splices at the end of the array.
6838 The following equivalences hold (assuming C<< $#a >= $i >> )
6840 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6841 pop(@a) splice(@a,-1)
6842 shift(@a) splice(@a,0,1)
6843 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6844 $a[$i] = $y splice(@a,$i,1,$y)
6846 C<splice> can be used, for example, to implement n-ary queue processing:
6850 while (my @next_n = splice @_, 0, $n) {
6851 say join q{ -- }, @next_n;
6855 nary_print(3, qw(a b c d e f g h));
6861 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6862 reference to an unblessed array. The argument will be dereferenced
6863 automatically. This aspect of C<splice> is considered highly experimental.
6864 The exact behaviour may change in a future version of Perl.
6866 To avoid confusing would-be users of your code who are running earlier
6867 versions of Perl with mysterious syntax errors, put this sort of thing at
6868 the top of your file to signal that your code will work I<only> on Perls of
6871 use 5.014; # so push/pop/etc work on scalars (experimental)
6873 =item split /PATTERN/,EXPR,LIMIT
6876 =item split /PATTERN/,EXPR
6878 =item split /PATTERN/
6882 =for Pod::Functions split up a string using a regexp delimiter
6884 Splits the string EXPR into a list of strings and returns the
6885 list in list context, or the size of the list in scalar context.
6887 If only PATTERN is given, EXPR defaults to C<$_>.
6889 Anything in EXPR that matches PATTERN is taken to be a separator
6890 that separates the EXPR into substrings (called "I<fields>") that
6891 do B<not> include the separator. Note that a separator may be
6892 longer than one character or even have no characters at all (the
6893 empty string, which is a zero-width match).
6895 The PATTERN need not be constant; an expression may be used
6896 to specify a pattern that varies at runtime.
6898 If PATTERN matches the empty string, the EXPR is split at the match
6899 position (between characters). As an example, the following:
6901 print join(':', split('b', 'abc')), "\n";
6903 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6906 print join(':', split('', 'abc')), "\n";
6908 uses empty string matches as separators to produce the output
6909 'a:b:c'; thus, the empty string may be used to split EXPR into a
6910 list of its component characters.
6912 As a special case for C<split>, the empty pattern given in
6913 L<match operator|perlop/"m/PATTERN/msixpodualgc"> syntax (C<//>) specifically matches the empty string, which is contrary to its usual
6914 interpretation as the last successful match.
6916 If PATTERN is C</^/>, then it is treated as if it used the
6917 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
6918 isn't much use otherwise.
6920 As another special case, C<split> emulates the default behavior of the
6921 command line tool B<awk> when the PATTERN is either omitted or a I<literal
6922 string> composed of a single space character (such as S<C<' '>> or
6923 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
6924 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
6925 instead treated as if it were C</\s+/>; in particular, this means that
6926 I<any> contiguous whitespace (not just a single space character) is used as
6927 a separator. However, this special treatment can be avoided by specifying
6928 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
6929 only a single space character to be a separator. In earlier Perls this
6930 special case was restricted to the use of a plain S<C<" ">> as the
6931 pattern argument to split, in Perl 5.18.0 and later this special case is
6932 triggered by any expression which evaluates as the simple string S<C<" ">>.
6934 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
6935 the previously described I<awk> emulation.
6937 If LIMIT is specified and positive, it represents the maximum number
6938 of fields into which the EXPR may be split; in other words, LIMIT is
6939 one greater than the maximum number of times EXPR may be split. Thus,
6940 the LIMIT value C<1> means that EXPR may be split a maximum of zero
6941 times, producing a maximum of one field (namely, the entire value of
6942 EXPR). For instance:
6944 print join(':', split(//, 'abc', 1)), "\n";
6946 produces the output 'abc', and this:
6948 print join(':', split(//, 'abc', 2)), "\n";
6950 produces the output 'a:bc', and each of these:
6952 print join(':', split(//, 'abc', 3)), "\n";
6953 print join(':', split(//, 'abc', 4)), "\n";
6955 produces the output 'a:b:c'.
6957 If LIMIT is negative, it is treated as if it were instead arbitrarily
6958 large; as many fields as possible are produced.
6960 If LIMIT is omitted (or, equivalently, zero), then it is usually
6961 treated as if it were instead negative but with the exception that
6962 trailing empty fields are stripped (empty leading fields are always
6963 preserved); if all fields are empty, then all fields are considered to
6964 be trailing (and are thus stripped in this case). Thus, the following:
6966 print join(':', split(',', 'a,b,c,,,')), "\n";
6968 produces the output 'a:b:c', but the following:
6970 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
6972 produces the output 'a:b:c:::'.
6974 In time-critical applications, it is worthwhile to avoid splitting
6975 into more fields than necessary. Thus, when assigning to a list,
6976 if LIMIT is omitted (or zero), then LIMIT is treated as though it
6977 were one larger than the number of variables in the list; for the
6978 following, LIMIT is implicitly 3:
6980 ($login, $passwd) = split(/:/);
6982 Note that splitting an EXPR that evaluates to the empty string always
6983 produces zero fields, regardless of the LIMIT specified.
6985 An empty leading field is produced when there is a positive-width
6986 match at the beginning of EXPR. For instance:
6988 print join(':', split(/ /, ' abc')), "\n";
6990 produces the output ':abc'. However, a zero-width match at the
6991 beginning of EXPR never produces an empty field, so that:
6993 print join(':', split(//, ' abc'));
6995 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
6997 An empty trailing field, on the other hand, is produced when there is a
6998 match at the end of EXPR, regardless of the length of the match
6999 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7000 removed, as in the last example). Thus:
7002 print join(':', split(//, ' abc', -1)), "\n";
7004 produces the output S<' :a:b:c:'>.
7006 If the PATTERN contains
7007 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7008 then for each separator, an additional field is produced for each substring
7009 captured by a group (in the order in which the groups are specified,
7010 as per L<backreferences|perlretut/Backreferences>); if any group does not
7011 match, then it captures the C<undef> value instead of a substring. Also,
7012 note that any such additional field is produced whenever there is a
7013 separator (that is, whenever a split occurs), and such an additional field
7014 does B<not> count towards the LIMIT. Consider the following expressions
7015 evaluated in list context (each returned list is provided in the associated
7018 split(/-|,/, "1-10,20", 3)
7021 split(/(-|,)/, "1-10,20", 3)
7022 # ('1', '-', '10', ',', '20')
7024 split(/-|(,)/, "1-10,20", 3)
7025 # ('1', undef, '10', ',', '20')
7027 split(/(-)|,/, "1-10,20", 3)
7028 # ('1', '-', '10', undef, '20')
7030 split(/(-)|(,)/, "1-10,20", 3)
7031 # ('1', '-', undef, '10', undef, ',', '20')
7033 =item sprintf FORMAT, LIST
7036 =for Pod::Functions formatted print into a string
7038 Returns a string formatted by the usual C<printf> conventions of the C
7039 library function C<sprintf>. See below for more details
7040 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
7041 the general principles.
7045 # Format number with up to 8 leading zeroes
7046 $result = sprintf("%08d", $number);
7048 # Round number to 3 digits after decimal point
7049 $rounded = sprintf("%.3f", $number);
7051 Perl does its own C<sprintf> formatting: it emulates the C
7052 function sprintf(3), but doesn't use it except for floating-point
7053 numbers, and even then only standard modifiers are allowed.
7054 Non-standard extensions in your local sprintf(3) are
7055 therefore unavailable from Perl.
7057 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
7058 pass it an array as your first argument.
7059 The array is given scalar context,
7060 and instead of using the 0th element of the array as the format, Perl will
7061 use the count of elements in the array as the format, which is almost never
7064 Perl's C<sprintf> permits the following universally-known conversions:
7067 %c a character with the given number
7069 %d a signed integer, in decimal
7070 %u an unsigned integer, in decimal
7071 %o an unsigned integer, in octal
7072 %x an unsigned integer, in hexadecimal
7073 %e a floating-point number, in scientific notation
7074 %f a floating-point number, in fixed decimal notation
7075 %g a floating-point number, in %e or %f notation
7077 In addition, Perl permits the following widely-supported conversions:
7079 %X like %x, but using upper-case letters
7080 %E like %e, but using an upper-case "E"
7081 %G like %g, but with an upper-case "E" (if applicable)
7082 %b an unsigned integer, in binary
7083 %B like %b, but using an upper-case "B" with the # flag
7084 %p a pointer (outputs the Perl value's address in hexadecimal)
7085 %n special: *stores* the number of characters output so far
7086 into the next argument in the parameter list
7088 Finally, for backward (and we do mean "backward") compatibility, Perl
7089 permits these unnecessary but widely-supported conversions:
7092 %D a synonym for %ld
7093 %U a synonym for %lu
7094 %O a synonym for %lo
7097 Note that the number of exponent digits in the scientific notation produced
7098 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7099 exponent less than 100 is system-dependent: it may be three or less
7100 (zero-padded as necessary). In other words, 1.23 times ten to the
7101 99th may be either "1.23e99" or "1.23e099".
7103 Between the C<%> and the format letter, you may specify several
7104 additional attributes controlling the interpretation of the format.
7105 In order, these are:
7109 =item format parameter index
7111 An explicit format parameter index, such as C<2$>. By default sprintf
7112 will format the next unused argument in the list, but this allows you
7113 to take the arguments out of order:
7115 printf '%2$d %1$d', 12, 34; # prints "34 12"
7116 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7122 space prefix non-negative number with a space
7123 + prefix non-negative number with a plus sign
7124 - left-justify within the field
7125 0 use zeros, not spaces, to right-justify
7126 # ensure the leading "0" for any octal,
7127 prefix non-zero hexadecimal with "0x" or "0X",
7128 prefix non-zero binary with "0b" or "0B"
7132 printf '<% d>', 12; # prints "< 12>"
7133 printf '<%+d>', 12; # prints "<+12>"
7134 printf '<%6s>', 12; # prints "< 12>"
7135 printf '<%-6s>', 12; # prints "<12 >"
7136 printf '<%06s>', 12; # prints "<000012>"
7137 printf '<%#o>', 12; # prints "<014>"
7138 printf '<%#x>', 12; # prints "<0xc>"
7139 printf '<%#X>', 12; # prints "<0XC>"
7140 printf '<%#b>', 12; # prints "<0b1100>"
7141 printf '<%#B>', 12; # prints "<0B1100>"
7143 When a space and a plus sign are given as the flags at once,
7144 a plus sign is used to prefix a positive number.
7146 printf '<%+ d>', 12; # prints "<+12>"
7147 printf '<% +d>', 12; # prints "<+12>"
7149 When the # flag and a precision are given in the %o conversion,
7150 the precision is incremented if it's necessary for the leading "0".
7152 printf '<%#.5o>', 012; # prints "<00012>"
7153 printf '<%#.5o>', 012345; # prints "<012345>"
7154 printf '<%#.0o>', 0; # prints "<0>"
7158 This flag tells Perl to interpret the supplied string as a vector of
7159 integers, one for each character in the string. Perl applies the format to
7160 each integer in turn, then joins the resulting strings with a separator (a
7161 dot C<.> by default). This can be useful for displaying ordinal values of
7162 characters in arbitrary strings:
7164 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7165 printf "version is v%vd\n", $^V; # Perl's version
7167 Put an asterisk C<*> before the C<v> to override the string to
7168 use to separate the numbers:
7170 printf "address is %*vX\n", ":", $addr; # IPv6 address
7171 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7173 You can also explicitly specify the argument number to use for
7174 the join string using something like C<*2$v>; for example:
7176 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7179 =item (minimum) width
7181 Arguments are usually formatted to be only as wide as required to
7182 display the given value. You can override the width by putting
7183 a number here, or get the width from the next argument (with C<*>)
7184 or from a specified argument (e.g., with C<*2$>):
7186 printf "<%s>", "a"; # prints "<a>"
7187 printf "<%6s>", "a"; # prints "< a>"
7188 printf "<%*s>", 6, "a"; # prints "< a>"
7189 printf '<%*2$s>', "a", 6; # prints "< a>"
7190 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7192 If a field width obtained through C<*> is negative, it has the same
7193 effect as the C<-> flag: left-justification.
7195 =item precision, or maximum width
7198 You can specify a precision (for numeric conversions) or a maximum
7199 width (for string conversions) by specifying a C<.> followed by a number.
7200 For floating-point formats except C<g> and C<G>, this specifies
7201 how many places right of the decimal point to show (the default being 6).
7204 # these examples are subject to system-specific variation
7205 printf '<%f>', 1; # prints "<1.000000>"
7206 printf '<%.1f>', 1; # prints "<1.0>"
7207 printf '<%.0f>', 1; # prints "<1>"
7208 printf '<%e>', 10; # prints "<1.000000e+01>"
7209 printf '<%.1e>', 10; # prints "<1.0e+01>"
7211 For "g" and "G", this specifies the maximum number of digits to show,
7212 including those prior to the decimal point and those after it; for
7215 # These examples are subject to system-specific variation.
7216 printf '<%g>', 1; # prints "<1>"
7217 printf '<%.10g>', 1; # prints "<1>"
7218 printf '<%g>', 100; # prints "<100>"
7219 printf '<%.1g>', 100; # prints "<1e+02>"
7220 printf '<%.2g>', 100.01; # prints "<1e+02>"
7221 printf '<%.5g>', 100.01; # prints "<100.01>"
7222 printf '<%.4g>', 100.01; # prints "<100>"
7224 For integer conversions, specifying a precision implies that the
7225 output of the number itself should be zero-padded to this width,
7226 where the 0 flag is ignored:
7228 printf '<%.6d>', 1; # prints "<000001>"
7229 printf '<%+.6d>', 1; # prints "<+000001>"
7230 printf '<%-10.6d>', 1; # prints "<000001 >"
7231 printf '<%10.6d>', 1; # prints "< 000001>"
7232 printf '<%010.6d>', 1; # prints "< 000001>"
7233 printf '<%+10.6d>', 1; # prints "< +000001>"
7235 printf '<%.6x>', 1; # prints "<000001>"
7236 printf '<%#.6x>', 1; # prints "<0x000001>"
7237 printf '<%-10.6x>', 1; # prints "<000001 >"
7238 printf '<%10.6x>', 1; # prints "< 000001>"
7239 printf '<%010.6x>', 1; # prints "< 000001>"
7240 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7242 For string conversions, specifying a precision truncates the string
7243 to fit the specified width:
7245 printf '<%.5s>', "truncated"; # prints "<trunc>"
7246 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7248 You can also get the precision from the next argument using C<.*>:
7250 printf '<%.6x>', 1; # prints "<000001>"
7251 printf '<%.*x>', 6, 1; # prints "<000001>"
7253 If a precision obtained through C<*> is negative, it counts
7254 as having no precision at all.
7256 printf '<%.*s>', 7, "string"; # prints "<string>"
7257 printf '<%.*s>', 3, "string"; # prints "<str>"
7258 printf '<%.*s>', 0, "string"; # prints "<>"
7259 printf '<%.*s>', -1, "string"; # prints "<string>"
7261 printf '<%.*d>', 1, 0; # prints "<0>"
7262 printf '<%.*d>', 0, 0; # prints "<>"
7263 printf '<%.*d>', -1, 0; # prints "<0>"
7265 You cannot currently get the precision from a specified number,
7266 but it is intended that this will be possible in the future, for
7267 example using C<.*2$>:
7269 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print
7274 For numeric conversions, you can specify the size to interpret the
7275 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7276 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7277 whatever the default integer size is on your platform (usually 32 or 64
7278 bits), but you can override this to use instead one of the standard C types,
7279 as supported by the compiler used to build Perl:
7281 hh interpret integer as C type "char" or "unsigned
7282 char" on Perl 5.14 or later
7283 h interpret integer as C type "short" or
7285 j interpret integer as C type "intmax_t" on Perl
7286 5.14 or later, and only with a C99 compiler
7288 l interpret integer as C type "long" or
7290 q, L, or ll interpret integer as C type "long long",
7291 "unsigned long long", or "quad" (typically
7293 t interpret integer as C type "ptrdiff_t" on Perl
7295 z interpret integer as C type "size_t" on Perl 5.14
7298 As of 5.14, none of these raises an exception if they are not supported on
7299 your platform. However, if warnings are enabled, a warning of the
7300 C<printf> warning class is issued on an unsupported conversion flag.
7301 Should you instead prefer an exception, do this:
7303 use warnings FATAL => "printf";
7305 If you would like to know about a version dependency before you
7306 start running the program, put something like this at its top:
7308 use 5.014; # for hh/j/t/z/ printf modifiers
7310 You can find out whether your Perl supports quads via L<Config>:
7313 if ($Config{use64bitint} eq "define"
7314 || $Config{longsize} >= 8) {
7315 print "Nice quads!\n";
7318 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7319 to be the default floating-point size on your platform (double or long double),
7320 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7321 platform supports them. You can find out whether your Perl supports long
7322 doubles via L<Config>:
7325 print "long doubles\n" if $Config{d_longdbl} eq "define";
7327 You can find out whether Perl considers "long double" to be the default
7328 floating-point size to use on your platform via L<Config>:
7331 if ($Config{uselongdouble} eq "define") {
7332 print "long doubles by default\n";
7335 It can also be that long doubles and doubles are the same thing:
7338 ($Config{doublesize} == $Config{longdblsize}) &&
7339 print "doubles are long doubles\n";
7341 The size specifier C<V> has no effect for Perl code, but is supported for
7342 compatibility with XS code. It means "use the standard size for a Perl
7343 integer or floating-point number", which is the default.
7345 =item order of arguments
7347 Normally, sprintf() takes the next unused argument as the value to
7348 format for each format specification. If the format specification
7349 uses C<*> to require additional arguments, these are consumed from
7350 the argument list in the order they appear in the format
7351 specification I<before> the value to format. Where an argument is
7352 specified by an explicit index, this does not affect the normal
7353 order for the arguments, even when the explicitly specified index
7354 would have been the next argument.
7358 printf "<%*.*s>", $a, $b, $c;
7360 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7361 as the value to format; while:
7363 printf '<%*1$.*s>', $a, $b;
7365 would use C<$a> for the width and precision, and C<$b> as the
7368 Here are some more examples; be aware that when using an explicit
7369 index, the C<$> may need escaping:
7371 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7372 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7373 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7374 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7378 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7379 and POSIX::setlocale() has been called,
7380 the character used for the decimal separator in formatted floating-point
7381 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
7385 X<sqrt> X<root> X<square root>
7389 =for Pod::Functions square root function
7391 Return the positive square root of EXPR. If EXPR is omitted, uses
7392 C<$_>. Works only for non-negative operands unless you've
7393 loaded the C<Math::Complex> module.
7396 print sqrt(-4); # prints 2i
7399 X<srand> X<seed> X<randseed>
7403 =for Pod::Functions seed the random number generator
7405 Sets and returns the random number seed for the C<rand> operator.
7407 The point of the function is to "seed" the C<rand> function so that C<rand>
7408 can produce a different sequence each time you run your program. When
7409 called with a parameter, C<srand> uses that for the seed; otherwise it
7410 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7411 it returns the seed. To signal that your code will work I<only> on Perls
7412 of a recent vintage:
7414 use 5.014; # so srand returns the seed
7416 If C<srand()> is not called explicitly, it is called implicitly without a
7417 parameter at the first use of the C<rand> operator.
7418 However, there are a few situations where programs are likely to
7419 want to call C<srand>. One is for generating predictable results, generally for
7420 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7421 each time. Another case is that you may want to call C<srand()>
7422 after a C<fork()> to avoid child processes sharing the same seed value as the
7423 parent (and consequently each other).
7425 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7426 process. The internal state of the random number generator should
7427 contain more entropy than can be provided by any seed, so calling
7428 C<srand()> again actually I<loses> randomness.
7430 Most implementations of C<srand> take an integer and will silently
7431 truncate decimal numbers. This means C<srand(42)> will usually
7432 produce the same results as C<srand(42.1)>. To be safe, always pass
7433 C<srand> an integer.
7435 A typical use of the returned seed is for a test program which has too many
7436 combinations to test comprehensively in the time available to it each run. It
7437 can test a random subset each time, and should there be a failure, log the seed
7438 used for that run so that it can later be used to reproduce the same results.
7440 B<C<rand()> is not cryptographically secure. You should not rely
7441 on it in security-sensitive situations.> As of this writing, a
7442 number of third-party CPAN modules offer random number generators
7443 intended by their authors to be cryptographically secure,
7444 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7445 and L<Math::TrulyRandom>.
7447 =item stat FILEHANDLE
7448 X<stat> X<file, status> X<ctime>
7452 =item stat DIRHANDLE
7456 =for Pod::Functions get a file's status information
7458 Returns a 13-element list giving the status info for a file, either
7459 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7460 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7463 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7464 $atime,$mtime,$ctime,$blksize,$blocks)
7467 Not all fields are supported on all filesystem types. Here are the
7468 meanings of the fields:
7470 0 dev device number of filesystem
7472 2 mode file mode (type and permissions)
7473 3 nlink number of (hard) links to the file
7474 4 uid numeric user ID of file's owner
7475 5 gid numeric group ID of file's owner
7476 6 rdev the device identifier (special files only)
7477 7 size total size of file, in bytes
7478 8 atime last access time in seconds since the epoch
7479 9 mtime last modify time in seconds since the epoch
7480 10 ctime inode change time in seconds since the epoch (*)
7481 11 blksize preferred I/O size in bytes for interacting with the
7482 file (may vary from file to file)
7483 12 blocks actual number of system-specific blocks allocated
7484 on disk (often, but not always, 512 bytes each)
7486 (The epoch was at 00:00 January 1, 1970 GMT.)
7488 (*) Not all fields are supported on all filesystem types. Notably, the
7489 ctime field is non-portable. In particular, you cannot expect it to be a
7490 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7492 If C<stat> is passed the special filehandle consisting of an underline, no
7493 stat is done, but the current contents of the stat structure from the
7494 last C<stat>, C<lstat>, or filetest are returned. Example:
7496 if (-x $file && (($d) = stat(_)) && $d < 0) {
7497 print "$file is executable NFS file\n";
7500 (This works on machines only for which the device number is negative
7503 Because the mode contains both the file type and its permissions, you
7504 should mask off the file type portion and (s)printf using a C<"%o">
7505 if you want to see the real permissions.
7507 $mode = (stat($filename))[2];
7508 printf "Permissions are %04o\n", $mode & 07777;
7510 In scalar context, C<stat> returns a boolean value indicating success
7511 or failure, and, if successful, sets the information associated with
7512 the special filehandle C<_>.
7514 The L<File::stat> module provides a convenient, by-name access mechanism:
7517 $sb = stat($filename);
7518 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7519 $filename, $sb->size, $sb->mode & 07777,
7520 scalar localtime $sb->mtime;
7522 You can import symbolic mode constants (C<S_IF*>) and functions
7523 (C<S_IS*>) from the Fcntl module:
7527 $mode = (stat($filename))[2];
7529 $user_rwx = ($mode & S_IRWXU) >> 6;
7530 $group_read = ($mode & S_IRGRP) >> 3;
7531 $other_execute = $mode & S_IXOTH;
7533 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7535 $is_setuid = $mode & S_ISUID;
7536 $is_directory = S_ISDIR($mode);
7538 You could write the last two using the C<-u> and C<-d> operators.
7539 Commonly available C<S_IF*> constants are:
7541 # Permissions: read, write, execute, for user, group, others.
7543 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7544 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7545 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7547 # Setuid/Setgid/Stickiness/SaveText.
7548 # Note that the exact meaning of these is system-dependent.
7550 S_ISUID S_ISGID S_ISVTX S_ISTXT
7552 # File types. Not all are necessarily available on
7555 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7556 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7558 # The following are compatibility aliases for S_IRUSR,
7559 # S_IWUSR, and S_IXUSR.
7561 S_IREAD S_IWRITE S_IEXEC
7563 and the C<S_IF*> functions are
7565 S_IMODE($mode) the part of $mode containing the permission
7566 bits and the setuid/setgid/sticky bits
7568 S_IFMT($mode) the part of $mode containing the file type
7569 which can be bit-anded with (for example)
7570 S_IFREG or with the following functions
7572 # The operators -f, -d, -l, -b, -c, -p, and -S.
7574 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7575 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7577 # No direct -X operator counterpart, but for the first one
7578 # the -g operator is often equivalent. The ENFMT stands for
7579 # record flocking enforcement, a platform-dependent feature.
7581 S_ISENFMT($mode) S_ISWHT($mode)
7583 See your native chmod(2) and stat(2) documentation for more details
7584 about the C<S_*> constants. To get status info for a symbolic link
7585 instead of the target file behind the link, use the C<lstat> function.
7587 Portability issues: L<perlport/stat>.
7592 =item state TYPE VARLIST
7594 =item state VARLIST : ATTRS
7596 =item state TYPE VARLIST : ATTRS
7598 =for Pod::Functions +state declare and assign a persistent lexical variable
7600 C<state> declares a lexically scoped variable, just like C<my>.
7601 However, those variables will never be reinitialized, contrary to
7602 lexical variables that are reinitialized each time their enclosing block
7604 See L<perlsub/"Persistent Private Variables"> for details.
7606 If more than one variable is listed, the list must be placed in
7607 parentheses. With a parenthesised list, C<undef> can be used as a
7608 dummy placeholder. However, since initialization of state variables in
7609 list context is currently not possible this would serve no purpose.
7611 C<state> variables are enabled only when the C<use feature "state"> pragma
7612 is in effect, unless the keyword is written as C<CORE::state>.
7613 See also L<feature>.
7620 =for Pod::Functions optimize input data for repeated searches
7622 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
7623 doing many pattern matches on the string before it is next modified.
7624 This may or may not save time, depending on the nature and number of
7625 patterns you are searching and the distribution of character
7626 frequencies in the string to be searched; you probably want to compare
7627 run times with and without it to see which is faster. Those loops
7628 that scan for many short constant strings (including the constant
7629 parts of more complex patterns) will benefit most.
7630 (The way C<study> works is this: a linked list of every
7631 character in the string to be searched is made, so we know, for
7632 example, where all the C<'k'> characters are. From each search string,
7633 the rarest character is selected, based on some static frequency tables
7634 constructed from some C programs and English text. Only those places
7635 that contain this "rarest" character are examined.)
7637 For example, here is a loop that inserts index producing entries
7638 before any line containing a certain pattern:
7642 print ".IX foo\n" if /\bfoo\b/;
7643 print ".IX bar\n" if /\bbar\b/;
7644 print ".IX blurfl\n" if /\bblurfl\b/;
7649 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7650 will be looked at, because C<f> is rarer than C<o>. In general, this is
7651 a big win except in pathological cases. The only question is whether
7652 it saves you more time than it took to build the linked list in the
7655 Note that if you have to look for strings that you don't know till
7656 runtime, you can build an entire loop as a string and C<eval> that to
7657 avoid recompiling all your patterns all the time. Together with
7658 undefining C<$/> to input entire files as one record, this can be quite
7659 fast, often faster than specialized programs like fgrep(1). The following
7660 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7661 out the names of those files that contain a match:
7663 $search = 'while (<>) { study;';
7664 foreach $word (@words) {
7665 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7670 eval $search; # this screams
7671 $/ = "\n"; # put back to normal input delimiter
7672 foreach $file (sort keys(%seen)) {
7676 =item sub NAME BLOCK
7679 =item sub NAME (PROTO) BLOCK
7681 =item sub NAME : ATTRS BLOCK
7683 =item sub NAME (PROTO) : ATTRS BLOCK
7685 =for Pod::Functions declare a subroutine, possibly anonymously
7687 This is subroutine definition, not a real function I<per se>. Without a
7688 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7689 function declaration, so does return a value: the CODE ref of the closure
7692 See L<perlsub> and L<perlref> for details about subroutines and
7693 references; see L<attributes> and L<Attribute::Handlers> for more
7694 information about attributes.
7699 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7701 A special token that returns a reference to the current subroutine, or
7702 C<undef> outside of a subroutine.
7704 The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>)
7705 is subject to change.
7707 This token is only available under C<use v5.16> or the "current_sub"
7708 feature. See L<feature>.
7710 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7711 X<substr> X<substring> X<mid> X<left> X<right>
7713 =item substr EXPR,OFFSET,LENGTH
7715 =item substr EXPR,OFFSET
7717 =for Pod::Functions get or alter a portion of a string
7719 Extracts a substring out of EXPR and returns it. First character is at
7720 offset zero. If OFFSET is negative, starts
7721 that far back from the end of the string. If LENGTH is omitted, returns
7722 everything through the end of the string. If LENGTH is negative, leaves that
7723 many characters off the end of the string.
7725 my $s = "The black cat climbed the green tree";
7726 my $color = substr $s, 4, 5; # black
7727 my $middle = substr $s, 4, -11; # black cat climbed the
7728 my $end = substr $s, 14; # climbed the green tree
7729 my $tail = substr $s, -4; # tree
7730 my $z = substr $s, -4, 2; # tr
7732 You can use the substr() function as an lvalue, in which case EXPR
7733 must itself be an lvalue. If you assign something shorter than LENGTH,
7734 the string will shrink, and if you assign something longer than LENGTH,
7735 the string will grow to accommodate it. To keep the string the same
7736 length, you may need to pad or chop your value using C<sprintf>.
7738 If OFFSET and LENGTH specify a substring that is partly outside the
7739 string, only the part within the string is returned. If the substring
7740 is beyond either end of the string, substr() returns the undefined
7741 value and produces a warning. When used as an lvalue, specifying a
7742 substring that is entirely outside the string raises an exception.
7743 Here's an example showing the behavior for boundary cases:
7746 substr($name, 4) = 'dy'; # $name is now 'freddy'
7747 my $null = substr $name, 6, 2; # returns "" (no warning)
7748 my $oops = substr $name, 7; # returns undef, with warning
7749 substr($name, 7) = 'gap'; # raises an exception
7751 An alternative to using substr() as an lvalue is to specify the
7752 replacement string as the 4th argument. This allows you to replace
7753 parts of the EXPR and return what was there before in one operation,
7754 just as you can with splice().
7756 my $s = "The black cat climbed the green tree";
7757 my $z = substr $s, 14, 7, "jumped from"; # climbed
7758 # $s is now "The black cat jumped from the green tree"
7760 Note that the lvalue returned by the three-argument version of substr() acts as
7761 a 'magic bullet'; each time it is assigned to, it remembers which part
7762 of the original string is being modified; for example:
7765 for (substr($x,1,2)) {
7766 $_ = 'a'; print $x,"\n"; # prints 1a4
7767 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7769 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7772 With negative offsets, it remembers its position from the end of the string
7773 when the target string is modified:
7776 for (substr($x, -3, 2)) {
7777 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7779 print $_,"\n"; # prints f
7782 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7783 unspecified. Prior to 5.16, the result with negative offsets was
7786 =item symlink OLDFILE,NEWFILE
7787 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7789 =for Pod::Functions create a symbolic link to a file
7791 Creates a new filename symbolically linked to the old filename.
7792 Returns C<1> for success, C<0> otherwise. On systems that don't support
7793 symbolic links, raises an exception. To check for that,
7796 $symlink_exists = eval { symlink("",""); 1 };
7798 Portability issues: L<perlport/symlink>.
7800 =item syscall NUMBER, LIST
7801 X<syscall> X<system call>
7803 =for Pod::Functions execute an arbitrary system call
7805 Calls the system call specified as the first element of the list,
7806 passing the remaining elements as arguments to the system call. If
7807 unimplemented, raises an exception. The arguments are interpreted
7808 as follows: if a given argument is numeric, the argument is passed as
7809 an int. If not, the pointer to the string value is passed. You are
7810 responsible to make sure a string is pre-extended long enough to
7811 receive any result that might be written into a string. You can't use a
7812 string literal (or other read-only string) as an argument to C<syscall>
7813 because Perl has to assume that any string pointer might be written
7815 integer arguments are not literals and have never been interpreted in a
7816 numeric context, you may need to add C<0> to them to force them to look
7817 like numbers. This emulates the C<syswrite> function (or vice versa):
7819 require 'syscall.ph'; # may need to run h2ph
7821 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7823 Note that Perl supports passing of up to only 14 arguments to your syscall,
7824 which in practice should (usually) suffice.
7826 Syscall returns whatever value returned by the system call it calls.
7827 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7828 Note that some system calls I<can> legitimately return C<-1>. The proper
7829 way to handle such calls is to assign C<$!=0> before the call, then
7830 check the value of C<$!> if C<syscall> returns C<-1>.
7832 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7833 number of the read end of the pipe it creates, but there is no way
7834 to retrieve the file number of the other end. You can avoid this
7835 problem by using C<pipe> instead.
7837 Portability issues: L<perlport/syscall>.
7839 =item sysopen FILEHANDLE,FILENAME,MODE
7842 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7844 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7846 Opens the file whose filename is given by FILENAME, and associates it with
7847 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7848 filehandle wanted; an undefined scalar will be suitably autovivified. This
7849 function calls the underlying operating system's I<open>(2) function with the
7850 parameters FILENAME, MODE, and PERMS.
7852 The possible values and flag bits of the MODE parameter are
7853 system-dependent; they are available via the standard module C<Fcntl>. See
7854 the documentation of your operating system's I<open>(2) syscall to see
7855 which values and flag bits are available. You may combine several flags
7856 using the C<|>-operator.
7858 Some of the most common values are C<O_RDONLY> for opening the file in
7859 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7860 and C<O_RDWR> for opening the file in read-write mode.
7861 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7863 For historical reasons, some values work on almost every system
7864 supported by Perl: 0 means read-only, 1 means write-only, and 2
7865 means read/write. We know that these values do I<not> work under
7866 OS/390 and on the Macintosh; you probably don't want to
7867 use them in new code.
7869 If the file named by FILENAME does not exist and the C<open> call creates
7870 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7871 PERMS specifies the permissions of the newly created file. If you omit
7872 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7873 These permission values need to be in octal, and are modified by your
7874 process's current C<umask>.
7877 In many systems the C<O_EXCL> flag is available for opening files in
7878 exclusive mode. This is B<not> locking: exclusiveness means here that
7879 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7880 on network filesystems, and has no effect unless the C<O_CREAT> flag
7881 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7882 being opened if it is a symbolic link. It does not protect against
7883 symbolic links in the file's path.
7886 Sometimes you may want to truncate an already-existing file. This
7887 can be done using the C<O_TRUNC> flag. The behavior of
7888 C<O_TRUNC> with C<O_RDONLY> is undefined.
7891 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7892 that takes away the user's option to have a more permissive umask.
7893 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7896 Note that C<sysopen> depends on the fdopen() C library function.
7897 On many Unix systems, fdopen() is known to fail when file descriptors
7898 exceed a certain value, typically 255. If you need more file
7899 descriptors than that, consider using the POSIX::open() function.
7901 See L<perlopentut> for a kinder, gentler explanation of opening files.
7903 Portability issues: L<perlport/sysopen>.
7905 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7908 =item sysread FILEHANDLE,SCALAR,LENGTH
7910 =for Pod::Functions fixed-length unbuffered input from a filehandle
7912 Attempts to read LENGTH bytes of data into variable SCALAR from the
7913 specified FILEHANDLE, using the read(2). It bypasses
7914 buffered IO, so mixing this with other kinds of reads, C<print>,
7915 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7916 perlio or stdio layers usually buffers data. Returns the number of
7917 bytes actually read, C<0> at end of file, or undef if there was an
7918 error (in the latter case C<$!> is also set). SCALAR will be grown or
7919 shrunk so that the last byte actually read is the last byte of the
7920 scalar after the read.
7922 An OFFSET may be specified to place the read data at some place in the
7923 string other than the beginning. A negative OFFSET specifies
7924 placement at that many characters counting backwards from the end of
7925 the string. A positive OFFSET greater than the length of SCALAR
7926 results in the string being padded to the required size with C<"\0">
7927 bytes before the result of the read is appended.
7929 There is no syseof() function, which is ok, since eof() doesn't work
7930 well on device files (like ttys) anyway. Use sysread() and check
7931 for a return value for 0 to decide whether you're done.
7933 Note that if the filehandle has been marked as C<:utf8> Unicode
7934 characters are read instead of bytes (the LENGTH, OFFSET, and the
7935 return value of sysread() are in Unicode characters).
7936 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7937 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7939 =item sysseek FILEHANDLE,POSITION,WHENCE
7942 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
7944 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7945 be an expression whose value gives the name of the filehandle. The values
7946 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7947 to the current position plus POSITION; and C<2> to set it to EOF plus
7948 POSITION, typically negative.
7950 Note the I<in bytes>: even if the filehandle has been set to operate
7951 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7952 tell() will return byte offsets, not character offsets (because
7953 implementing that would render sysseek() unacceptably slow).
7955 sysseek() bypasses normal buffered IO, so mixing it with reads other
7956 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7957 C<seek>, C<tell>, or C<eof> may cause confusion.
7959 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7960 and C<SEEK_END> (start of the file, current position, end of the file)
7961 from the Fcntl module. Use of the constants is also more portable
7962 than relying on 0, 1, and 2. For example to define a "systell" function:
7964 use Fcntl 'SEEK_CUR';
7965 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7967 Returns the new position, or the undefined value on failure. A position
7968 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7969 true on success and false on failure, yet you can still easily determine
7975 =item system PROGRAM LIST
7977 =for Pod::Functions run a separate program
7979 Does exactly the same thing as C<exec LIST>, except that a fork is
7980 done first and the parent process waits for the child process to
7981 exit. Note that argument processing varies depending on the
7982 number of arguments. If there is more than one argument in LIST,
7983 or if LIST is an array with more than one value, starts the program
7984 given by the first element of the list with arguments given by the
7985 rest of the list. If there is only one scalar argument, the argument
7986 is checked for shell metacharacters, and if there are any, the
7987 entire argument is passed to the system's command shell for parsing
7988 (this is C</bin/sh -c> on Unix platforms, but varies on other
7989 platforms). If there are no shell metacharacters in the argument,
7990 it is split into words and passed directly to C<execvp>, which is
7991 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
7992 reliably avoid using the shell; C<system LIST>, even with more than one
7993 element, will fall back to the shell if the first spawn fails.
7995 Perl will attempt to flush all files opened for
7996 output before any operation that may do a fork, but this may not be
7997 supported on some platforms (see L<perlport>). To be safe, you may need
7998 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7999 of C<IO::Handle> on any open handles.
8001 The return value is the exit status of the program as returned by the
8002 C<wait> call. To get the actual exit value, shift right by eight (see
8003 below). See also L</exec>. This is I<not> what you want to use to capture
8004 the output from a command; for that you should use merely backticks or
8005 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
8006 indicates a failure to start the program or an error of the wait(2) system
8007 call (inspect $! for the reason).
8009 If you'd like to make C<system> (and many other bits of Perl) die on error,
8010 have a look at the L<autodie> pragma.
8012 Like C<exec>, C<system> allows you to lie to a program about its name if
8013 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
8015 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8016 C<system>, if you expect your program to terminate on receipt of these
8017 signals you will need to arrange to do so yourself based on the return
8020 @args = ("command", "arg1", "arg2");
8022 or die "system @args failed: $?"
8024 If you'd like to manually inspect C<system>'s failure, you can check all
8025 possible failure modes by inspecting C<$?> like this:
8028 print "failed to execute: $!\n";
8031 printf "child died with signal %d, %s coredump\n",
8032 ($? & 127), ($? & 128) ? 'with' : 'without';
8035 printf "child exited with value %d\n", $? >> 8;
8038 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
8039 with the C<W*()> calls from the POSIX module.
8041 When C<system>'s arguments are executed indirectly by the shell,
8042 results and return codes are subject to its quirks.
8043 See L<perlop/"`STRING`"> and L</exec> for details.
8045 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
8046 handler. See L<perlipc> for details.
8048 Portability issues: L<perlport/system>.
8050 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8053 =item syswrite FILEHANDLE,SCALAR,LENGTH
8055 =item syswrite FILEHANDLE,SCALAR
8057 =for Pod::Functions fixed-length unbuffered output to a filehandle
8059 Attempts to write LENGTH bytes of data from variable SCALAR to the
8060 specified FILEHANDLE, using write(2). If LENGTH is
8061 not specified, writes whole SCALAR. It bypasses buffered IO, so
8062 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
8063 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
8064 stdio layers usually buffer data. Returns the number of bytes
8065 actually written, or C<undef> if there was an error (in this case the
8066 errno variable C<$!> is also set). If the LENGTH is greater than the
8067 data available in the SCALAR after the OFFSET, only as much data as is
8068 available will be written.
8070 An OFFSET may be specified to write the data from some part of the
8071 string other than the beginning. A negative OFFSET specifies writing
8072 that many characters counting backwards from the end of the string.
8073 If SCALAR is of length zero, you can only use an OFFSET of 0.
8075 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8076 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8077 return value of syswrite() are in (UTF8-encoded Unicode) characters.
8078 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8079 Alternately, if the handle is not marked with an encoding but you
8080 attempt to write characters with code points over 255, raises an exception.
8081 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8083 =item tell FILEHANDLE
8088 =for Pod::Functions get current seekpointer on a filehandle
8090 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8091 error. FILEHANDLE may be an expression whose value gives the name of
8092 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8095 Note the I<in bytes>: even if the filehandle has been set to
8096 operate on characters (for example by using the C<:encoding(utf8)> open
8097 layer), tell() will return byte offsets, not character offsets (because
8098 that would render seek() and tell() rather slow).
8100 The return value of tell() for the standard streams like the STDIN
8101 depends on the operating system: it may return -1 or something else.
8102 tell() on pipes, fifos, and sockets usually returns -1.
8104 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
8106 Do not use tell() (or other buffered I/O operations) on a filehandle
8107 that has been manipulated by sysread(), syswrite(), or sysseek().
8108 Those functions ignore the buffering, while tell() does not.
8110 =item telldir DIRHANDLE
8113 =for Pod::Functions get current seekpointer on a directory handle
8115 Returns the current position of the C<readdir> routines on DIRHANDLE.
8116 Value may be given to C<seekdir> to access a particular location in a
8117 directory. C<telldir> has the same caveats about possible directory
8118 compaction as the corresponding system library routine.
8120 =item tie VARIABLE,CLASSNAME,LIST
8123 =for Pod::Functions +5.002 bind a variable to an object class
8125 This function binds a variable to a package class that will provide the
8126 implementation for the variable. VARIABLE is the name of the variable
8127 to be enchanted. CLASSNAME is the name of a class implementing objects
8128 of correct type. Any additional arguments are passed to the
8129 appropriate constructor
8130 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8131 or C<TIEHASH>). Typically these are arguments such as might be passed
8132 to the C<dbm_open()> function of C. The object returned by the
8133 constructor is also returned by the C<tie> function, which would be useful
8134 if you want to access other methods in CLASSNAME.
8136 Note that functions such as C<keys> and C<values> may return huge lists
8137 when used on large objects, like DBM files. You may prefer to use the
8138 C<each> function to iterate over such. Example:
8140 # print out history file offsets
8142 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8143 while (($key,$val) = each %HIST) {
8144 print $key, ' = ', unpack('L',$val), "\n";
8148 A class implementing a hash should have the following methods:
8150 TIEHASH classname, LIST
8152 STORE this, key, value
8157 NEXTKEY this, lastkey
8162 A class implementing an ordinary array should have the following methods:
8164 TIEARRAY classname, LIST
8166 STORE this, key, value
8168 STORESIZE this, count
8174 SPLICE this, offset, length, LIST
8181 A class implementing a filehandle should have the following methods:
8183 TIEHANDLE classname, LIST
8184 READ this, scalar, length, offset
8187 WRITE this, scalar, length, offset
8189 PRINTF this, format, LIST
8193 SEEK this, position, whence
8195 OPEN this, mode, LIST
8200 A class implementing a scalar should have the following methods:
8202 TIESCALAR classname, LIST
8208 Not all methods indicated above need be implemented. See L<perltie>,
8209 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8211 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8212 for you; you need to do that explicitly yourself. See L<DB_File>
8213 or the F<Config> module for interesting C<tie> implementations.
8215 For further details see L<perltie>, L<"tied VARIABLE">.
8220 =for Pod::Functions get a reference to the object underlying a tied variable
8222 Returns a reference to the object underlying VARIABLE (the same value
8223 that was originally returned by the C<tie> call that bound the variable
8224 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8230 =for Pod::Functions return number of seconds since 1970
8232 Returns the number of non-leap seconds since whatever time the system
8233 considers to be the epoch, suitable for feeding to C<gmtime> and
8234 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8235 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8236 1904 in the current local time zone for its epoch.
8238 For measuring time in better granularity than one second, use the
8239 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8240 if you have gettimeofday(2), you may be able to use the C<syscall>
8241 interface of Perl. See L<perlfaq8> for details.
8243 For date and time processing look at the many related modules on CPAN.
8244 For a comprehensive date and time representation look at the
8250 =for Pod::Functions return elapsed time for self and child processes
8252 Returns a four-element list giving the user and system times in
8253 seconds for this process and any exited children of this process.
8255 ($user,$system,$cuser,$csystem) = times;
8257 In scalar context, C<times> returns C<$user>.
8259 Children's times are only included for terminated children.
8261 Portability issues: L<perlport/times>.
8265 =for Pod::Functions transliterate a string
8267 The transliteration operator. Same as C<y///>. See
8268 L<perlop/"Quote-Like Operators">.
8270 =item truncate FILEHANDLE,LENGTH
8273 =item truncate EXPR,LENGTH
8275 =for Pod::Functions shorten a file
8277 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8278 specified length. Raises an exception if truncate isn't implemented
8279 on your system. Returns true if successful, C<undef> on error.
8281 The behavior is undefined if LENGTH is greater than the length of the
8284 The position in the file of FILEHANDLE is left unchanged. You may want to
8285 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8287 Portability issues: L<perlport/truncate>.
8290 X<uc> X<uppercase> X<toupper>
8294 =for Pod::Functions return upper-case version of a string
8296 Returns an uppercased version of EXPR. This is the internal function
8297 implementing the C<\U> escape in double-quoted strings.
8298 It does not attempt to do titlecase mapping on initial letters. See
8299 L</ucfirst> for that.
8301 If EXPR is omitted, uses C<$_>.
8303 This function behaves the same way under various pragma, such as in a locale,
8307 X<ucfirst> X<uppercase>
8311 =for Pod::Functions return a string with just the next letter in upper case
8313 Returns the value of EXPR with the first character in uppercase
8314 (titlecase in Unicode). This is the internal function implementing
8315 the C<\u> escape in double-quoted strings.
8317 If EXPR is omitted, uses C<$_>.
8319 This function behaves the same way under various pragma, such as in a locale,
8327 =for Pod::Functions set file creation mode mask
8329 Sets the umask for the process to EXPR and returns the previous value.
8330 If EXPR is omitted, merely returns the current umask.
8332 The Unix permission C<rwxr-x---> is represented as three sets of three
8333 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8334 and isn't one of the digits). The C<umask> value is such a number
8335 representing disabled permissions bits. The permission (or "mode")
8336 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8337 even if you tell C<sysopen> to create a file with permissions C<0777>,
8338 if your umask is C<0022>, then the file will actually be created with
8339 permissions C<0755>. If your C<umask> were C<0027> (group can't
8340 write; others can't read, write, or execute), then passing
8341 C<sysopen> C<0666> would create a file with mode C<0640> (because
8342 C<0666 &~ 027> is C<0640>).
8344 Here's some advice: supply a creation mode of C<0666> for regular
8345 files (in C<sysopen>) and one of C<0777> for directories (in
8346 C<mkdir>) and executable files. This gives users the freedom of
8347 choice: if they want protected files, they might choose process umasks
8348 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8349 Programs should rarely if ever make policy decisions better left to
8350 the user. The exception to this is when writing files that should be
8351 kept private: mail files, web browser cookies, I<.rhosts> files, and
8354 If umask(2) is not implemented on your system and you are trying to
8355 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8356 raises an exception. If umask(2) is not implemented and you are
8357 not trying to restrict access for yourself, returns C<undef>.
8359 Remember that a umask is a number, usually given in octal; it is I<not> a
8360 string of octal digits. See also L</oct>, if all you have is a string.
8362 Portability issues: L<perlport/umask>.
8365 X<undef> X<undefine>
8369 =for Pod::Functions remove a variable or function definition
8371 Undefines the value of EXPR, which must be an lvalue. Use only on a
8372 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8373 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8374 will probably not do what you expect on most predefined variables or
8375 DBM list values, so don't do that; see L</delete>. Always returns the
8376 undefined value. You can omit the EXPR, in which case nothing is
8377 undefined, but you still get an undefined value that you could, for
8378 instance, return from a subroutine, assign to a variable, or pass as a
8379 parameter. Examples:
8382 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8386 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8387 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8388 select undef, undef, undef, 0.25;
8389 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8391 Note that this is a unary operator, not a list operator.
8394 X<unlink> X<delete> X<remove> X<rm> X<del>
8398 =for Pod::Functions remove one link to a file
8400 Deletes a list of files. On success, it returns the number of files
8401 it successfully deleted. On failure, it returns false and sets C<$!>
8404 my $unlinked = unlink 'a', 'b', 'c';
8406 unlink glob "*.bak";
8408 On error, C<unlink> will not tell you which files it could not remove.
8409 If you want to know which files you could not remove, try them one
8412 foreach my $file ( @goners ) {
8413 unlink $file or warn "Could not unlink $file: $!";
8416 Note: C<unlink> will not attempt to delete directories unless you are
8417 superuser and the B<-U> flag is supplied to Perl. Even if these
8418 conditions are met, be warned that unlinking a directory can inflict
8419 damage on your filesystem. Finally, using C<unlink> on directories is
8420 not supported on many operating systems. Use C<rmdir> instead.
8422 If LIST is omitted, C<unlink> uses C<$_>.
8424 =item unpack TEMPLATE,EXPR
8427 =item unpack TEMPLATE
8429 =for Pod::Functions convert binary structure into normal perl variables
8431 C<unpack> does the reverse of C<pack>: it takes a string
8432 and expands it out into a list of values.
8433 (In scalar context, it returns merely the first value produced.)
8435 If EXPR is omitted, unpacks the C<$_> string.
8436 See L<perlpacktut> for an introduction to this function.
8438 The string is broken into chunks described by the TEMPLATE. Each chunk
8439 is converted separately to a value. Typically, either the string is a result
8440 of C<pack>, or the characters of the string represent a C structure of some
8443 The TEMPLATE has the same format as in the C<pack> function.
8444 Here's a subroutine that does substring:
8447 my($what,$where,$howmuch) = @_;
8448 unpack("x$where a$howmuch", $what);
8453 sub ordinal { unpack("W",$_[0]); } # same as ord()
8455 In addition to fields allowed in pack(), you may prefix a field with
8456 a %<number> to indicate that
8457 you want a <number>-bit checksum of the items instead of the items
8458 themselves. Default is a 16-bit checksum. Checksum is calculated by
8459 summing numeric values of expanded values (for string fields the sum of
8460 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8462 For example, the following
8463 computes the same number as the System V sum program:
8467 unpack("%32W*",<>) % 65535;
8470 The following efficiently counts the number of set bits in a bit vector:
8472 $setbits = unpack("%32b*", $selectmask);
8474 The C<p> and C<P> formats should be used with care. Since Perl
8475 has no way of checking whether the value passed to C<unpack()>
8476 corresponds to a valid memory location, passing a pointer value that's
8477 not known to be valid is likely to have disastrous consequences.
8479 If there are more pack codes or if the repeat count of a field or a group
8480 is larger than what the remainder of the input string allows, the result
8481 is not well defined: the repeat count may be decreased, or
8482 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8483 If the input string is longer than one described by the TEMPLATE,
8484 the remainder of that input string is ignored.
8486 See L</pack> for more examples and notes.
8488 =item unshift ARRAY,LIST
8491 =item unshift EXPR,LIST
8493 =for Pod::Functions prepend more elements to the beginning of a list
8495 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8496 depending on how you look at it. Prepends list to the front of the
8497 array and returns the new number of elements in the array.
8499 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8501 Note the LIST is prepended whole, not one element at a time, so the
8502 prepended elements stay in the same order. Use C<reverse> to do the
8505 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
8506 a reference to an unblessed array. The argument will be dereferenced
8507 automatically. This aspect of C<unshift> is considered highly
8508 experimental. The exact behaviour may change in a future version of Perl.
8510 To avoid confusing would-be users of your code who are running earlier
8511 versions of Perl with mysterious syntax errors, put this sort of thing at
8512 the top of your file to signal that your code will work I<only> on Perls of
8515 use 5.014; # so push/pop/etc work on scalars (experimental)
8517 =item untie VARIABLE
8520 =for Pod::Functions break a tie binding to a variable
8522 Breaks the binding between a variable and a package.
8523 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8524 Has no effect if the variable is not tied.
8526 =item use Module VERSION LIST
8527 X<use> X<module> X<import>
8529 =item use Module VERSION
8531 =item use Module LIST
8537 =for Pod::Functions load in a module at compile time and import its namespace
8539 Imports some semantics into the current package from the named module,
8540 generally by aliasing certain subroutine or variable names into your
8541 package. It is exactly equivalent to
8543 BEGIN { require Module; Module->import( LIST ); }
8545 except that Module I<must> be a bareword.
8546 The importation can be made conditional by using the L<if> module.
8548 In the peculiar C<use VERSION> form, VERSION may be either a positive
8549 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8550 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8551 exception is raised if VERSION is greater than the version of the
8552 current Perl interpreter; Perl will not attempt to parse the rest of the
8553 file. Compare with L</require>, which can do a similar check at run time.
8554 Symmetrically, C<no VERSION> allows you to specify that you want a version
8555 of Perl older than the specified one.
8557 Specifying VERSION as a literal of the form v5.6.1 should generally be
8558 avoided, because it leads to misleading error messages under earlier
8559 versions of Perl (that is, prior to 5.6.0) that do not support this
8560 syntax. The equivalent numeric version should be used instead.
8562 use v5.6.1; # compile time version check
8564 use 5.006_001; # ditto; preferred for backwards compatibility
8566 This is often useful if you need to check the current Perl version before
8567 C<use>ing library modules that won't work with older versions of Perl.
8568 (We try not to do this more than we have to.)
8570 C<use VERSION> also enables all features available in the requested
8571 version as defined by the C<feature> pragma, disabling any features
8572 not in the requested version's feature bundle. See L<feature>.
8573 Similarly, if the specified Perl version is greater than or equal to
8574 5.12.0, strictures are enabled lexically as
8575 with C<use strict>. Any explicit use of
8576 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8577 before it. In both cases, the F<feature.pm> and F<strict.pm> files are
8578 not actually loaded.
8580 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8581 C<require> makes sure the module is loaded into memory if it hasn't been
8582 yet. The C<import> is not a builtin; it's just an ordinary static method
8583 call into the C<Module> package to tell the module to import the list of
8584 features back into the current package. The module can implement its
8585 C<import> method any way it likes, though most modules just choose to
8586 derive their C<import> method via inheritance from the C<Exporter> class that
8587 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8588 method can be found then the call is skipped, even if there is an AUTOLOAD
8591 If you do not want to call the package's C<import> method (for instance,
8592 to stop your namespace from being altered), explicitly supply the empty list:
8596 That is exactly equivalent to
8598 BEGIN { require Module }
8600 If the VERSION argument is present between Module and LIST, then the
8601 C<use> will call the VERSION method in class Module with the given
8602 version as an argument. The default VERSION method, inherited from
8603 the UNIVERSAL class, croaks if the given version is larger than the
8604 value of the variable C<$Module::VERSION>.
8606 Again, there is a distinction between omitting LIST (C<import> called
8607 with no arguments) and an explicit empty LIST C<()> (C<import> not
8608 called). Note that there is no comma after VERSION!
8610 Because this is a wide-open interface, pragmas (compiler directives)
8611 are also implemented this way. Currently implemented pragmas are:
8616 use sigtrap qw(SEGV BUS);
8617 use strict qw(subs vars refs);
8618 use subs qw(afunc blurfl);
8619 use warnings qw(all);
8620 use sort qw(stable _quicksort _mergesort);
8622 Some of these pseudo-modules import semantics into the current
8623 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8624 which import symbols into the current package (which are effective
8625 through the end of the file).
8627 Because C<use> takes effect at compile time, it doesn't respect the
8628 ordinary flow control of the code being compiled. In particular, putting
8629 a C<use> inside the false branch of a conditional doesn't prevent it
8630 from being processed. If a module or pragma only needs to be loaded
8631 conditionally, this can be done using the L<if> pragma:
8633 use if $] < 5.008, "utf8";
8634 use if WANT_WARNINGS, warnings => qw(all);
8636 There's a corresponding C<no> declaration that unimports meanings imported
8637 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8638 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8639 or no unimport method being found.
8645 Care should be taken when using the C<no VERSION> form of C<no>. It is
8646 I<only> meant to be used to assert that the running Perl is of a earlier
8647 version than its argument and I<not> to undo the feature-enabling side effects
8650 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8651 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8652 functionality from the command-line.
8657 =for Pod::Functions set a file's last access and modify times
8659 Changes the access and modification times on each file of a list of
8660 files. The first two elements of the list must be the NUMERIC access
8661 and modification times, in that order. Returns the number of files
8662 successfully changed. The inode change time of each file is set
8663 to the current time. For example, this code has the same effect as the
8664 Unix touch(1) command when the files I<already exist> and belong to
8665 the user running the program:
8668 $atime = $mtime = time;
8669 utime $atime, $mtime, @ARGV;
8671 Since Perl 5.8.0, if the first two elements of the list are C<undef>,
8672 the utime(2) syscall from your C library is called with a null second
8673 argument. On most systems, this will set the file's access and
8674 modification times to the current time (i.e., equivalent to the example
8675 above) and will work even on files you don't own provided you have write
8679 utime(undef, undef, $file)
8680 || warn "couldn't touch $file: $!";
8683 Under NFS this will use the time of the NFS server, not the time of
8684 the local machine. If there is a time synchronization problem, the
8685 NFS server and local machine will have different times. The Unix
8686 touch(1) command will in fact normally use this form instead of the
8687 one shown in the first example.
8689 Passing only one of the first two elements as C<undef> is
8690 equivalent to passing a 0 and will not have the effect
8691 described when both are C<undef>. This also triggers an
8692 uninitialized warning.
8694 On systems that support futimes(2), you may pass filehandles among the
8695 files. On systems that don't support futimes(2), passing filehandles raises
8696 an exception. Filehandles must be passed as globs or glob references to be
8697 recognized; barewords are considered filenames.
8699 Portability issues: L<perlport/utime>.
8708 =for Pod::Functions return a list of the values in a hash
8710 In list context, returns a list consisting of all the values of the named
8711 hash. In Perl 5.12 or later only, will also return a list of the values of
8712 an array; prior to that release, attempting to use an array argument will
8713 produce a syntax error. In scalar context, returns the number of values.
8715 Hash entries are returned in an apparently random order. The actual random
8716 order is specific to a given hash; the exact same series of operations
8717 on two hashes may result in a different order for each hash. Any insertion
8718 into the hash may change the order, as will any deletion, with the exception
8719 that the most recent key returned by C<each> or C<keys> may be deleted
8720 without changing the order. So long as a given hash is unmodified you may
8721 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
8722 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
8723 details on why hash order is randomized. Aside from the guarantees
8724 provided here the exact details of Perl's hash algorithm and the hash
8725 traversal order are subject to change in any release of Perl.
8727 As a side effect, calling values() resets the HASH or ARRAY's internal
8728 iterator, see L</each>. (In particular, calling values() in void context
8729 resets the iterator with no other overhead. Apart from resetting the
8730 iterator, C<values @array> in list context is the same as plain C<@array>.
8731 (We recommend that you use void context C<keys @array> for this, but
8732 reasoned that taking C<values @array> out would require more
8733 documentation than leaving it in.)
8735 Note that the values are not copied, which means modifying them will
8736 modify the contents of the hash:
8738 for (values %hash) { s/foo/bar/g } # modifies %hash values
8739 for (@hash{keys %hash}) { s/foo/bar/g } # same
8741 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
8742 a reference to an unblessed hash or array. The argument will be
8743 dereferenced automatically. This aspect of C<values> is considered highly
8744 experimental. The exact behaviour may change in a future version of Perl.
8746 for (values $hashref) { ... }
8747 for (values $obj->get_arrayref) { ... }
8749 To avoid confusing would-be users of your code who are running earlier
8750 versions of Perl with mysterious syntax errors, put this sort of thing at
8751 the top of your file to signal that your code will work I<only> on Perls of
8754 use 5.012; # so keys/values/each work on arrays
8755 use 5.014; # so keys/values/each work on scalars (experimental)
8757 See also C<keys>, C<each>, and C<sort>.
8759 =item vec EXPR,OFFSET,BITS
8760 X<vec> X<bit> X<bit vector>
8762 =for Pod::Functions test or set particular bits in a string
8764 Treats the string in EXPR as a bit vector made up of elements of
8765 width BITS and returns the value of the element specified by OFFSET
8766 as an unsigned integer. BITS therefore specifies the number of bits
8767 that are reserved for each element in the bit vector. This must
8768 be a power of two from 1 to 32 (or 64, if your platform supports
8771 If BITS is 8, "elements" coincide with bytes of the input string.
8773 If BITS is 16 or more, bytes of the input string are grouped into chunks
8774 of size BITS/8, and each group is converted to a number as with
8775 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8776 for BITS==64). See L<"pack"> for details.
8778 If bits is 4 or less, the string is broken into bytes, then the bits
8779 of each byte are broken into 8/BITS groups. Bits of a byte are
8780 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8781 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8782 breaking the single input byte C<chr(0x36)> into two groups gives a list
8783 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8785 C<vec> may also be assigned to, in which case parentheses are needed
8786 to give the expression the correct precedence as in
8788 vec($image, $max_x * $x + $y, 8) = 3;
8790 If the selected element is outside the string, the value 0 is returned.
8791 If an element off the end of the string is written to, Perl will first
8792 extend the string with sufficiently many zero bytes. It is an error
8793 to try to write off the beginning of the string (i.e., negative OFFSET).
8795 If the string happens to be encoded as UTF-8 internally (and thus has
8796 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8797 internal byte string, not the conceptual character string, even if you
8798 only have characters with values less than 256.
8800 Strings created with C<vec> can also be manipulated with the logical
8801 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8802 vector operation is desired when both operands are strings.
8803 See L<perlop/"Bitwise String Operators">.
8805 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8806 The comments show the string after each step. Note that this code works
8807 in the same way on big-endian or little-endian machines.
8810 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8812 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8813 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8815 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8816 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8817 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8818 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8819 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8820 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8822 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8823 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8824 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8827 To transform a bit vector into a string or list of 0's and 1's, use these:
8829 $bits = unpack("b*", $vector);
8830 @bits = split(//, unpack("b*", $vector));
8832 If you know the exact length in bits, it can be used in place of the C<*>.
8834 Here is an example to illustrate how the bits actually fall in place:
8840 unpack("V",$_) 01234567890123456789012345678901
8841 ------------------------------------------------------------------
8846 for ($shift=0; $shift < $width; ++$shift) {
8847 for ($off=0; $off < 32/$width; ++$off) {
8848 $str = pack("B*", "0"x32);
8849 $bits = (1<<$shift);
8850 vec($str, $off, $width) = $bits;
8851 $res = unpack("b*",$str);
8852 $val = unpack("V", $str);
8859 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8860 $off, $width, $bits, $val, $res
8864 Regardless of the machine architecture on which it runs, the
8865 example above should print the following table:
8868 unpack("V",$_) 01234567890123456789012345678901
8869 ------------------------------------------------------------------
8870 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8871 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8872 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8873 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8874 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8875 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8876 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8877 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8878 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8879 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8880 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8881 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8882 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8883 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8884 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8885 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8886 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8887 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8888 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8889 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8890 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8891 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8892 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8893 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8894 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8895 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8896 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8897 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8898 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8899 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8900 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8901 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8902 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8903 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8904 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8905 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8906 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8907 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8908 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8909 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8910 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8911 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8912 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8913 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8914 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8915 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8916 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8917 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8918 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8919 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8920 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8921 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8922 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8923 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8924 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8925 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8926 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8927 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8928 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8929 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8930 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8931 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8932 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8933 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8934 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8935 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8936 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8937 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8938 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8939 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8940 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8941 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8942 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8943 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8944 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8945 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8946 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8947 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8948 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8949 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8950 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8951 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8952 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8953 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8954 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8955 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8956 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8957 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8958 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8959 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8960 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8961 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8962 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8963 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8964 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8965 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8966 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8967 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8968 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8969 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8970 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8971 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8972 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8973 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8974 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8975 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8976 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8977 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8978 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8979 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8980 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8981 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8982 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8983 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8984 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8985 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8986 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8987 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8988 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8989 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8990 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8991 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8992 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8993 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8994 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8995 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8996 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8997 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9002 =for Pod::Functions wait for any child process to die
9004 Behaves like wait(2) on your system: it waits for a child
9005 process to terminate and returns the pid of the deceased process, or
9006 C<-1> if there are no child processes. The status is returned in C<$?>
9007 and C<${^CHILD_ERROR_NATIVE}>.
9008 Note that a return value of C<-1> could mean that child processes are
9009 being automatically reaped, as described in L<perlipc>.
9011 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
9012 child created by qx() or system(). See L<perlipc> for details.
9014 Portability issues: L<perlport/wait>.
9016 =item waitpid PID,FLAGS
9019 =for Pod::Functions wait for a particular child process to die
9021 Waits for a particular child process to terminate and returns the pid of
9022 the deceased process, or C<-1> if there is no such child process. On some
9023 systems, a value of 0 indicates that there are processes still running.
9024 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
9026 use POSIX ":sys_wait_h";
9029 $kid = waitpid(-1, WNOHANG);
9032 then you can do a non-blocking wait for all pending zombie processes.
9033 Non-blocking wait is available on machines supporting either the
9034 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
9035 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9036 system call by remembering the status values of processes that have
9037 exited but have not been harvested by the Perl script yet.)
9039 Note that on some systems, a return value of C<-1> could mean that child
9040 processes are being automatically reaped. See L<perlipc> for details,
9041 and for other examples.
9043 Portability issues: L<perlport/waitpid>.
9046 X<wantarray> X<context>
9048 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9050 Returns true if the context of the currently executing subroutine or
9051 C<eval> is looking for a list value. Returns false if the context is
9052 looking for a scalar. Returns the undefined value if the context is
9053 looking for no value (void context).
9055 return unless defined wantarray; # don't bother doing more
9056 my @a = complex_calculation();
9057 return wantarray ? @a : "@a";
9059 C<wantarray()>'s result is unspecified in the top level of a file,
9060 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9061 in a C<DESTROY> method.
9063 This function should have been named wantlist() instead.
9066 X<warn> X<warning> X<STDERR>
9068 =for Pod::Functions print debugging info
9070 Prints the value of LIST to STDERR. If the last element of LIST does
9071 not end in a newline, it appends the same file/line number text as C<die>
9074 If the output is empty and C<$@> already contains a value (typically from a
9075 previous eval) that value is used after appending C<"\t...caught">
9076 to C<$@>. This is useful for staying almost, but not entirely similar to
9079 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
9081 No message is printed if there is a C<$SIG{__WARN__}> handler
9082 installed. It is the handler's responsibility to deal with the message
9083 as it sees fit (like, for instance, converting it into a C<die>). Most
9084 handlers must therefore arrange to actually display the
9085 warnings that they are not prepared to deal with, by calling C<warn>
9086 again in the handler. Note that this is quite safe and will not
9087 produce an endless loop, since C<__WARN__> hooks are not called from
9090 You will find this behavior is slightly different from that of
9091 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
9092 instead call C<die> again to change it).
9094 Using a C<__WARN__> handler provides a powerful way to silence all
9095 warnings (even the so-called mandatory ones). An example:
9097 # wipe out *all* compile-time warnings
9098 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9100 my $foo = 20; # no warning about duplicate my $foo,
9101 # but hey, you asked for it!
9102 # no compile-time or run-time warnings before here
9105 # run-time warnings enabled after here
9106 warn "\$foo is alive and $foo!"; # does show up
9108 See L<perlvar> for details on setting C<%SIG> entries and for more
9109 examples. See the Carp module for other kinds of warnings using its
9110 carp() and cluck() functions.
9112 =item write FILEHANDLE
9119 =for Pod::Functions print a picture record
9121 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9122 using the format associated with that file. By default the format for
9123 a file is the one having the same name as the filehandle, but the
9124 format for the current output channel (see the C<select> function) may be set
9125 explicitly by assigning the name of the format to the C<$~> variable.
9127 Top of form processing is handled automatically: if there is insufficient
9128 room on the current page for the formatted record, the page is advanced by
9129 writing a form feed and a special top-of-page
9130 format is used to format the new
9131 page header before the record is written. By default, the top-of-page
9132 format is the name of the filehandle with "_TOP" appended, or "top"
9133 in the current package if the former does not exist. This would be a
9134 problem with autovivified filehandles, but it may be dynamically set to the
9135 format of your choice by assigning the name to the C<$^> variable while
9136 that filehandle is selected. The number of lines remaining on the current
9137 page is in variable C<$->, which can be set to C<0> to force a new page.
9139 If FILEHANDLE is unspecified, output goes to the current default output
9140 channel, which starts out as STDOUT but may be changed by the
9141 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
9142 is evaluated and the resulting string is used to look up the name of
9143 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9145 Note that write is I<not> the opposite of C<read>. Unfortunately.
9149 =for Pod::Functions transliterate a string
9151 The transliteration operator. Same as C<tr///>. See
9152 L<perlop/"Quote-Like Operators">.
9156 =head2 Non-function Keywords by Cross-reference
9166 These keywords are documented in L<perldata/"Special Literals">.
9184 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9194 This method keyword is documented in L<perlobj/"Destructors">.
9226 These operators are documented in L<perlop>.
9236 This keyword is documented in L<perlsub/"Autoloading">.
9262 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9274 These flow-control keywords related to the experimental switch feature are
9275 documented in L<perlsyn/"Switch Statements">.