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 or UTF-8 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
453 the file is examined to see if it is valid UTF-8 that includes non-ASCII
454 characters. If, so it's a C<-T> file. Otherwise, that same portion of
455 the file is examined for odd characters such as strange control codes or
456 characters with the high bit set. If more than a third of the
457 characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
458 Also, any file containing a zero byte in the examined portion is
459 considered a binary file. (If executed within the scope of a L<S<use
460 locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
461 anything that isn't a printable nor space in the current locale.) If
462 C<-T> or C<-B> is used on a filehandle, the current IO buffer is
464 rather than the first block. Both C<-T> and C<-B> return true on an empty
465 file, or a file at EOF when testing a filehandle. Because you have to
466 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
467 against the file first, as in C<next unless -f $file && -T $file>.
469 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
470 the special filehandle consisting of a solitary underline, then the stat
471 structure of the previous file test (or stat operator) is used, saving
472 a system call. (This doesn't work with C<-t>, and you need to remember
473 that lstat() and C<-l> leave values in the stat structure for the
474 symbolic link, not the real file.) (Also, if the stat buffer was filled by
475 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
478 print "Can do.\n" if -r $a || -w _ || -x _;
481 print "Readable\n" if -r _;
482 print "Writable\n" if -w _;
483 print "Executable\n" if -x _;
484 print "Setuid\n" if -u _;
485 print "Setgid\n" if -g _;
486 print "Sticky\n" if -k _;
487 print "Text\n" if -T _;
488 print "Binary\n" if -B _;
490 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
491 test operators, in a way that C<-f -w -x $file> is equivalent to
492 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
493 the return value of C<-f $file> as an argument to another filetest
494 operator, no special magic will happen.)
496 Portability issues: L<perlport/-X>.
498 To avoid confusing would-be users of your code with mysterious
499 syntax errors, put something like this at the top of your script:
501 use 5.010; # so filetest ops can stack
508 =for Pod::Functions absolute value function
510 Returns the absolute value of its argument.
511 If VALUE is omitted, uses C<$_>.
513 =item accept NEWSOCKET,GENERICSOCKET
516 =for Pod::Functions accept an incoming socket connect
518 Accepts an incoming socket connect, just as accept(2)
519 does. Returns the packed address if it succeeded, false otherwise.
520 See the example in L<perlipc/"Sockets: Client/Server Communication">.
522 On systems that support a close-on-exec flag on files, the flag will
523 be set for the newly opened file descriptor, as determined by the
524 value of $^F. See L<perlvar/$^F>.
533 =for Pod::Functions schedule a SIGALRM
535 Arranges to have a SIGALRM delivered to this process after the
536 specified number of wallclock seconds has elapsed. If SECONDS is not
537 specified, the value stored in C<$_> is used. (On some machines,
538 unfortunately, the elapsed time may be up to one second less or more
539 than you specified because of how seconds are counted, and process
540 scheduling may delay the delivery of the signal even further.)
542 Only one timer may be counting at once. Each call disables the
543 previous timer, and an argument of C<0> may be supplied to cancel the
544 previous timer without starting a new one. The returned value is the
545 amount of time remaining on the previous timer.
547 For delays of finer granularity than one second, the Time::HiRes module
548 (from CPAN, and starting from Perl 5.8 part of the standard
549 distribution) provides ualarm(). You may also use Perl's four-argument
550 version of select() leaving the first three arguments undefined, or you
551 might be able to use the C<syscall> interface to access setitimer(2) if
552 your system supports it. See L<perlfaq8> for details.
554 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
555 C<sleep> may be internally implemented on your system with C<alarm>.
557 If you want to use C<alarm> to time out a system call you need to use an
558 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
559 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
560 restart system calls on some systems. Using C<eval>/C<die> always works,
561 modulo the caveats given in L<perlipc/"Signals">.
564 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
566 $nread = sysread SOCKET, $buffer, $size;
570 die unless $@ eq "alarm\n"; # propagate unexpected errors
577 For more information see L<perlipc>.
579 Portability issues: L<perlport/alarm>.
582 X<atan2> X<arctangent> X<tan> X<tangent>
584 =for Pod::Functions arctangent of Y/X in the range -PI to PI
586 Returns the arctangent of Y/X in the range -PI to PI.
588 For the tangent operation, you may use the C<Math::Trig::tan>
589 function, or use the familiar relation:
591 sub tan { sin($_[0]) / cos($_[0]) }
593 The return value for C<atan2(0,0)> is implementation-defined; consult
594 your atan2(3) manpage for more information.
596 Portability issues: L<perlport/atan2>.
598 =item bind SOCKET,NAME
601 =for Pod::Functions binds an address to a socket
603 Binds a network address to a socket, just as bind(2)
604 does. Returns true if it succeeded, false otherwise. NAME should be a
605 packed address of the appropriate type for the socket. See the examples in
606 L<perlipc/"Sockets: Client/Server Communication">.
608 =item binmode FILEHANDLE, LAYER
609 X<binmode> X<binary> X<text> X<DOS> X<Windows>
611 =item binmode FILEHANDLE
613 =for Pod::Functions prepare binary files for I/O
615 Arranges for FILEHANDLE to be read or written in "binary" or "text"
616 mode on systems where the run-time libraries distinguish between
617 binary and text files. If FILEHANDLE is an expression, the value is
618 taken as the name of the filehandle. Returns true on success,
619 otherwise it returns C<undef> and sets C<$!> (errno).
621 On some systems (in general, DOS- and Windows-based systems) binmode()
622 is necessary when you're not working with a text file. For the sake
623 of portability it is a good idea always to use it when appropriate,
624 and never to use it when it isn't appropriate. Also, people can
625 set their I/O to be by default UTF8-encoded Unicode, not bytes.
627 In other words: regardless of platform, use binmode() on binary data,
628 like images, for example.
630 If LAYER is present it is a single string, but may contain multiple
631 directives. The directives alter the behaviour of the filehandle.
632 When LAYER is present, using binmode on a text file makes sense.
634 If LAYER is omitted or specified as C<:raw> the filehandle is made
635 suitable for passing binary data. This includes turning off possible CRLF
636 translation and marking it as bytes (as opposed to Unicode characters).
637 Note that, despite what may be implied in I<"Programming Perl"> (the
638 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
639 Other layers that would affect the binary nature of the stream are
640 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
641 PERLIO environment variable.
643 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
644 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
645 establish default I/O layers. See L<open>.
647 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
648 in "Programming Perl, 3rd Edition". However, since the publishing of this
649 book, by many known as "Camel III", the consensus of the naming of this
650 functionality has moved from "discipline" to "layer". All documentation
651 of this version of Perl therefore refers to "layers" rather than to
652 "disciplines". Now back to the regularly scheduled documentation...>
654 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
655 C<:utf8> just marks the data as UTF-8 without further checking,
656 while C<:encoding(UTF-8)> checks the data for actually being valid
657 UTF-8. More details can be found in L<PerlIO::encoding>.
659 In general, binmode() should be called after open() but before any I/O
660 is done on the filehandle. Calling binmode() normally flushes any
661 pending buffered output data (and perhaps pending input data) on the
662 handle. An exception to this is the C<:encoding> layer that
663 changes the default character encoding of the handle; see L</open>.
664 The C<:encoding> layer sometimes needs to be called in
665 mid-stream, and it doesn't flush the stream. The C<:encoding>
666 also implicitly pushes on top of itself the C<:utf8> layer because
667 internally Perl operates on UTF8-encoded Unicode characters.
669 The operating system, device drivers, C libraries, and Perl run-time
670 system all conspire to let the programmer treat a single
671 character (C<\n>) as the line terminator, irrespective of external
672 representation. On many operating systems, the native text file
673 representation matches the internal representation, but on some
674 platforms the external representation of C<\n> is made up of more than
677 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
678 a single character to end each line in the external representation of text
679 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
680 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
681 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
682 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
683 two characters C<\cM\cJ>. That means that if you don't use binmode() on
684 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
685 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
686 output. This is what you want for text files, but it can be disastrous for
689 Another consequence of using binmode() (on some systems) is that
690 special end-of-file markers will be seen as part of the data stream.
691 For systems from the Microsoft family this means that, if your binary
692 data contain C<\cZ>, the I/O subsystem will regard it as the end of
693 the file, unless you use binmode().
695 binmode() is important not only for readline() and print() operations,
696 but also when using read(), seek(), sysread(), syswrite() and tell()
697 (see L<perlport> for more details). See the C<$/> and C<$\> variables
698 in L<perlvar> for how to manually set your input and output
699 line-termination sequences.
701 Portability issues: L<perlport/binmode>.
703 =item bless REF,CLASSNAME
708 =for Pod::Functions create an object
710 This function tells the thingy referenced by REF that it is now an object
711 in the CLASSNAME package. If CLASSNAME is omitted, the current package
712 is used. Because a C<bless> is often the last thing in a constructor,
713 it returns the reference for convenience. Always use the two-argument
714 version if a derived class might inherit the function doing the blessing.
715 See L<perlobj> for more about the blessing (and blessings) of objects.
717 Consider always blessing objects in CLASSNAMEs that are mixed case.
718 Namespaces with all lowercase names are considered reserved for
719 Perl pragmata. Builtin types have all uppercase names. To prevent
720 confusion, you may wish to avoid such package names as well. Make sure
721 that CLASSNAME is a true value.
723 See L<perlmod/"Perl Modules">.
727 =for Pod::Functions +switch break out of a C<given> block
729 Break out of a C<given()> block.
731 This keyword is enabled by the C<"switch"> feature; see L<feature> for
732 more information on C<"switch">. You can also access it by prefixing it
733 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
737 X<caller> X<call stack> X<stack> X<stack trace>
741 =for Pod::Functions get context of the current subroutine call
743 Returns the context of the current pure perl subroutine call. In scalar
744 context, returns the caller's package name if there I<is> a caller (that is, if
745 we're in a subroutine or C<eval> or C<require>) and the undefined value
746 otherwise. caller never returns XS subs and they are skipped. The next pure
747 perl sub will appear instead of the XS
748 sub in caller's return values. In list
749 context, caller returns
752 ($package, $filename, $line) = caller;
754 With EXPR, it returns some extra information that the debugger uses to
755 print a stack trace. The value of EXPR indicates how many call frames
756 to go back before the current one.
759 ($package, $filename, $line, $subroutine, $hasargs,
762 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
765 Here, $subroutine is the function that the caller called (rather than the
766 function containing the caller). Note that $subroutine may be C<(eval)> if
767 the frame is not a subroutine call, but an C<eval>. In such a case
768 additional elements $evaltext and
769 C<$is_require> are set: C<$is_require> is true if the frame is created by a
770 C<require> or C<use> statement, $evaltext contains the text of the
771 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
772 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
773 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
774 frame.) $subroutine may also be C<(unknown)> if this particular
775 subroutine happens to have been deleted from the symbol table.
776 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
777 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
778 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
779 corresponds to C<${^WARNING_BITS}>. The
780 C<$hints> and C<$bitmask> values are subject
781 to change between versions of Perl, and are not meant for external use.
783 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
784 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
785 of this hash, as they are the actual values stored in the optree.
787 Furthermore, when called from within the DB package in
788 list context, and with an argument, caller returns more
789 detailed information: it sets the list variable C<@DB::args> to be the
790 arguments with which the subroutine was invoked.
792 Be aware that the optimizer might have optimized call frames away before
793 C<caller> had a chance to get the information. That means that C<caller(N)>
794 might not return information about the call frame you expect it to, for
795 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
796 previous time C<caller> was called.
798 Be aware that setting C<@DB::args> is I<best effort>, intended for
799 debugging or generating backtraces, and should not be relied upon. In
800 particular, as C<@_> contains aliases to the caller's arguments, Perl does
801 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
802 subroutine makes to C<@_> or its contents, not the original values at call
803 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
804 elements, so under certain cases its elements may have become freed and
805 reallocated for other variables or temporary values. Finally, a side effect
806 of the current implementation is that the effects of C<shift @_> can
807 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
808 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
809 elements), so C<@DB::args> is actually a hybrid of the current state and
810 initial state of C<@_>. Buyer beware.
817 =item chdir FILEHANDLE
819 =item chdir DIRHANDLE
823 =for Pod::Functions change your current working directory
825 Changes the working directory to EXPR, if possible. If EXPR is omitted,
826 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
827 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
828 variable C<$ENV{'SYS$LOGIN'}> is also checked, and used if it is set.) If
829 neither is set, C<chdir> does nothing and fails. It returns true on success,
830 false otherwise. See the example under C<die>.
832 On systems that support fchdir(2), you may pass a filehandle or
833 directory handle as the argument. On systems that don't support fchdir(2),
834 passing handles raises an exception.
837 X<chmod> X<permission> X<mode>
839 =for Pod::Functions changes the permissions on a list of files
841 Changes the permissions of a list of files. The first element of the
842 list must be the numeric mode, which should probably be an octal
843 number, and which definitely should I<not> be a string of octal digits:
844 C<0644> is okay, but C<"0644"> is not. Returns the number of files
845 successfully changed. See also L</oct> if all you have is a string.
847 $cnt = chmod 0755, "foo", "bar";
848 chmod 0755, @executables;
849 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
851 $mode = "0644"; chmod oct($mode), "foo"; # this is better
852 $mode = 0644; chmod $mode, "foo"; # this is best
854 On systems that support fchmod(2), you may pass filehandles among the
855 files. On systems that don't support fchmod(2), passing filehandles raises
856 an exception. Filehandles must be passed as globs or glob references to be
857 recognized; barewords are considered filenames.
859 open(my $fh, "<", "foo");
860 my $perm = (stat $fh)[2] & 07777;
861 chmod($perm | 0600, $fh);
863 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
866 use Fcntl qw( :mode );
867 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
868 # Identical to the chmod 0755 of the example above.
870 Portability issues: L<perlport/chmod>.
873 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
879 =for Pod::Functions remove a trailing record separator from a string
881 This safer version of L</chop> removes any trailing string
882 that corresponds to the current value of C<$/> (also known as
883 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
884 number of characters removed from all its arguments. It's often used to
885 remove the newline from the end of an input record when you're worried
886 that the final record may be missing its newline. When in paragraph
887 mode (C<$/ = ''>), it removes all trailing newlines from the string.
888 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
889 a reference to an integer or the like; see L<perlvar>) chomp() won't
891 If VARIABLE is omitted, it chomps C<$_>. Example:
894 chomp; # avoid \n on last field
899 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
900 resetting the C<each> iterator in the process.
902 You can actually chomp anything that's an lvalue, including an assignment:
905 chomp($answer = <STDIN>);
907 If you chomp a list, each element is chomped, and the total number of
908 characters removed is returned.
910 Note that parentheses are necessary when you're chomping anything
911 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
912 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
913 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
914 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
924 =for Pod::Functions remove the last character from a string
926 Chops off the last character of a string and returns the character
927 chopped. It is much more efficient than C<s/.$//s> because it neither
928 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
929 If VARIABLE is a hash, it chops the hash's values, but not its keys,
930 resetting the C<each> iterator in the process.
932 You can actually chop anything that's an lvalue, including an assignment.
934 If you chop a list, each element is chopped. Only the value of the
935 last C<chop> is returned.
937 Note that C<chop> returns the last character. To return all but the last
938 character, use C<substr($string, 0, -1)>.
943 X<chown> X<owner> X<user> X<group>
945 =for Pod::Functions change the ownership on a list of files
947 Changes the owner (and group) of a list of files. The first two
948 elements of the list must be the I<numeric> uid and gid, in that
949 order. A value of -1 in either position is interpreted by most
950 systems to leave that value unchanged. Returns the number of files
951 successfully changed.
953 $cnt = chown $uid, $gid, 'foo', 'bar';
954 chown $uid, $gid, @filenames;
956 On systems that support fchown(2), you may pass filehandles among the
957 files. On systems that don't support fchown(2), passing filehandles raises
958 an exception. Filehandles must be passed as globs or glob references to be
959 recognized; barewords are considered filenames.
961 Here's an example that looks up nonnumeric uids in the passwd file:
964 chomp($user = <STDIN>);
966 chomp($pattern = <STDIN>);
968 ($login,$pass,$uid,$gid) = getpwnam($user)
969 or die "$user not in passwd file";
971 @ary = glob($pattern); # expand filenames
972 chown $uid, $gid, @ary;
974 On most systems, you are not allowed to change the ownership of the
975 file unless you're the superuser, although you should be able to change
976 the group to any of your secondary groups. On insecure systems, these
977 restrictions may be relaxed, but this is not a portable assumption.
978 On POSIX systems, you can detect this condition this way:
980 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
981 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
983 Portability issues: L<perlport/chown>.
986 X<chr> X<character> X<ASCII> X<Unicode>
990 =for Pod::Functions get character this number represents
992 Returns the character represented by that NUMBER in the character set.
993 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
994 chr(0x263a) is a Unicode smiley face.
996 Negative values give the Unicode replacement character (chr(0xfffd)),
997 except under the L<bytes> pragma, where the low eight bits of the value
998 (truncated to an integer) are used.
1000 If NUMBER is omitted, uses C<$_>.
1002 For the reverse, use L</ord>.
1004 Note that characters from 128 to 255 (inclusive) are by default
1005 internally not encoded as UTF-8 for backward compatibility reasons.
1007 See L<perlunicode> for more about Unicode.
1009 =item chroot FILENAME
1014 =for Pod::Functions make directory new root for path lookups
1016 This function works like the system call by the same name: it makes the
1017 named directory the new root directory for all further pathnames that
1018 begin with a C</> by your process and all its children. (It doesn't
1019 change your current working directory, which is unaffected.) For security
1020 reasons, this call is restricted to the superuser. If FILENAME is
1021 omitted, does a C<chroot> to C<$_>.
1023 B<NOTE:> It is good security practice to do C<chdir("/")> (to the root
1024 directory) immediately after a C<chroot()>.
1026 Portability issues: L<perlport/chroot>.
1028 =item close FILEHANDLE
1033 =for Pod::Functions close file (or pipe or socket) handle
1035 Closes the file or pipe associated with the filehandle, flushes the IO
1036 buffers, and closes the system file descriptor. Returns true if those
1037 operations succeed and if no error was reported by any PerlIO
1038 layer. Closes the currently selected filehandle if the argument is
1041 You don't have to close FILEHANDLE if you are immediately going to do
1042 another C<open> on it, because C<open> closes it for you. (See
1043 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1044 counter (C<$.>), while the implicit close done by C<open> does not.
1046 If the filehandle came from a piped open, C<close> returns false if one of
1047 the other syscalls involved fails or if its program exits with non-zero
1048 status. If the only problem was that the program exited non-zero, C<$!>
1049 will be set to C<0>. Closing a pipe also waits for the process executing
1050 on the pipe to exit--in case you wish to look at the output of the pipe
1051 afterwards--and implicitly puts the exit status value of that command into
1052 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1054 If there are multiple threads running, C<close> on a filehandle from a
1055 piped open returns true without waiting for the child process to terminate,
1056 if the filehandle is still open in another thread.
1058 Closing the read end of a pipe before the process writing to it at the
1059 other end is done writing results in the writer receiving a SIGPIPE. If
1060 the other end can't handle that, be sure to read all the data before
1065 open(OUTPUT, '|sort >foo') # pipe to sort
1066 or die "Can't start sort: $!";
1067 #... # print stuff to output
1068 close OUTPUT # wait for sort to finish
1069 or warn $! ? "Error closing sort pipe: $!"
1070 : "Exit status $? from sort";
1071 open(INPUT, 'foo') # get sort's results
1072 or die "Can't open 'foo' for input: $!";
1074 FILEHANDLE may be an expression whose value can be used as an indirect
1075 filehandle, usually the real filehandle name or an autovivified handle.
1077 =item closedir DIRHANDLE
1080 =for Pod::Functions close directory handle
1082 Closes a directory opened by C<opendir> and returns the success of that
1085 =item connect SOCKET,NAME
1088 =for Pod::Functions connect to a remote socket
1090 Attempts to connect to a remote socket, just like connect(2).
1091 Returns true if it succeeded, false otherwise. NAME should be a
1092 packed address of the appropriate type for the socket. See the examples in
1093 L<perlipc/"Sockets: Client/Server Communication">.
1095 =item continue BLOCK
1100 =for Pod::Functions optional trailing block in a while or foreach
1102 When followed by a BLOCK, C<continue> is actually a
1103 flow control statement rather than a function. If
1104 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1105 C<foreach>), it is always executed just before the conditional is about to
1106 be evaluated again, just like the third part of a C<for> loop in C. Thus
1107 it can be used to increment a loop variable, even when the loop has been
1108 continued via the C<next> statement (which is similar to the C C<continue>
1111 C<last>, C<next>, or C<redo> may appear within a C<continue>
1112 block; C<last> and C<redo> behave as if they had been executed within
1113 the main block. So will C<next>, but since it will execute a C<continue>
1114 block, it may be more entertaining.
1117 ### redo always comes here
1120 ### next always comes here
1122 # then back the top to re-check EXPR
1124 ### last always comes here
1126 Omitting the C<continue> section is equivalent to using an
1127 empty one, logically enough, so C<next> goes directly back
1128 to check the condition at the top of the loop.
1130 When there is no BLOCK, C<continue> is a function that
1131 falls through the current C<when> or C<default> block instead of iterating
1132 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1133 In Perl 5.14 and earlier, this form of C<continue> was
1134 only available when the C<"switch"> feature was enabled.
1135 See L<feature> and L<perlsyn/"Switch Statements"> for more
1139 X<cos> X<cosine> X<acos> X<arccosine>
1143 =for Pod::Functions cosine function
1145 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1146 takes the cosine of C<$_>.
1148 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1149 function, or use this relation:
1151 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1153 =item crypt PLAINTEXT,SALT
1154 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1155 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1157 =for Pod::Functions one-way passwd-style encryption
1159 Creates a digest string exactly like the crypt(3) function in the C
1160 library (assuming that you actually have a version there that has not
1161 been extirpated as a potential munition).
1163 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1164 into a short string, called a digest, which is returned. The same
1165 PLAINTEXT and SALT will always return the same string, but there is no
1166 (known) way to get the original PLAINTEXT from the hash. Small
1167 changes in the PLAINTEXT or SALT will result in large changes in the
1170 There is no decrypt function. This function isn't all that useful for
1171 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1172 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1173 primarily used to check if two pieces of text are the same without
1174 having to transmit or store the text itself. An example is checking
1175 if a correct password is given. The digest of the password is stored,
1176 not the password itself. The user types in a password that is
1177 crypt()'d with the same salt as the stored digest. If the two digests
1178 match, the password is correct.
1180 When verifying an existing digest string you should use the digest as
1181 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1182 to create the digest is visible as part of the digest. This ensures
1183 crypt() will hash the new string with the same salt as the digest.
1184 This allows your code to work with the standard L<crypt|/crypt> and
1185 with more exotic implementations. In other words, assume
1186 nothing about the returned string itself nor about how many bytes
1189 Traditionally the result is a string of 13 bytes: two first bytes of
1190 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1191 the first eight bytes of PLAINTEXT mattered. But alternative
1192 hashing schemes (like MD5), higher level security schemes (like C2),
1193 and implementations on non-Unix platforms may produce different
1196 When choosing a new salt create a random two character string whose
1197 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1198 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1199 characters is just a recommendation; the characters allowed in
1200 the salt depend solely on your system's crypt library, and Perl can't
1201 restrict what salts C<crypt()> accepts.
1203 Here's an example that makes sure that whoever runs this program knows
1206 $pwd = (getpwuid($<))[1];
1208 system "stty -echo";
1210 chomp($word = <STDIN>);
1214 if (crypt($word, $pwd) ne $pwd) {
1220 Of course, typing in your own password to whoever asks you
1223 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1224 of data, not least of all because you can't get the information
1225 back. Look at the L<Digest> module for more robust algorithms.
1227 If using crypt() on a Unicode string (which I<potentially> has
1228 characters with codepoints above 255), Perl tries to make sense
1229 of the situation by trying to downgrade (a copy of)
1230 the string back to an eight-bit byte string before calling crypt()
1231 (on that copy). If that works, good. If not, crypt() dies with
1232 C<Wide character in crypt>.
1234 Portability issues: L<perlport/crypt>.
1239 =for Pod::Functions breaks binding on a tied dbm file
1241 [This function has been largely superseded by the C<untie> function.]
1243 Breaks the binding between a DBM file and a hash.
1245 Portability issues: L<perlport/dbmclose>.
1247 =item dbmopen HASH,DBNAME,MASK
1248 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1250 =for Pod::Functions create binding on a tied dbm file
1252 [This function has been largely superseded by the
1253 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1255 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1256 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1257 argument is I<not> a filehandle, even though it looks like one). DBNAME
1258 is the name of the database (without the F<.dir> or F<.pag> extension if
1259 any). If the database does not exist, it is created with protection
1260 specified by MASK (as modified by the C<umask>). To prevent creation of
1261 the database if it doesn't exist, you may specify a MODE
1262 of 0, and the function will return a false value if it
1263 can't find an existing database. If your system supports
1264 only the older DBM functions, you may make only one C<dbmopen> call in your
1265 program. In older versions of Perl, if your system had neither DBM nor
1266 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1269 If you don't have write access to the DBM file, you can only read hash
1270 variables, not set them. If you want to test whether you can write,
1271 either use file tests or try setting a dummy hash entry inside an C<eval>
1274 Note that functions such as C<keys> and C<values> may return huge lists
1275 when used on large DBM files. You may prefer to use the C<each>
1276 function to iterate over large DBM files. Example:
1278 # print out history file offsets
1279 dbmopen(%HIST,'/usr/lib/news/history',0666);
1280 while (($key,$val) = each %HIST) {
1281 print $key, ' = ', unpack('L',$val), "\n";
1285 See also L<AnyDBM_File> for a more general description of the pros and
1286 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1287 rich implementation.
1289 You can control which DBM library you use by loading that library
1290 before you call dbmopen():
1293 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1294 or die "Can't open netscape history file: $!";
1296 Portability issues: L<perlport/dbmopen>.
1299 X<defined> X<undef> X<undefined>
1303 =for Pod::Functions test whether a value, variable, or function is defined
1305 Returns a Boolean value telling whether EXPR has a value other than
1306 the undefined value C<undef>. If EXPR is not present, C<$_> is
1309 Many operations return C<undef> to indicate failure, end of file,
1310 system error, uninitialized variable, and other exceptional
1311 conditions. This function allows you to distinguish C<undef> from
1312 other values. (A simple Boolean test will not distinguish among
1313 C<undef>, zero, the empty string, and C<"0">, which are all equally
1314 false.) Note that since C<undef> is a valid scalar, its presence
1315 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1316 returns C<undef> when its argument is an empty array, I<or> when the
1317 element to return happens to be C<undef>.
1319 You may also use C<defined(&func)> to check whether subroutine C<&func>
1320 has ever been defined. The return value is unaffected by any forward
1321 declarations of C<&func>. A subroutine that is not defined
1322 may still be callable: its package may have an C<AUTOLOAD> method that
1323 makes it spring into existence the first time that it is called; see
1326 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1327 used to report whether memory for that aggregate had ever been
1328 allocated. This behavior may disappear in future versions of Perl.
1329 You should instead use a simple test for size:
1331 if (@an_array) { print "has array elements\n" }
1332 if (%a_hash) { print "has hash members\n" }
1334 When used on a hash element, it tells you whether the value is defined,
1335 not whether the key exists in the hash. Use L</exists> for the latter
1340 print if defined $switch{D};
1341 print "$val\n" while defined($val = pop(@ary));
1342 die "Can't readlink $sym: $!"
1343 unless defined($value = readlink $sym);
1344 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1345 $debugging = 0 unless defined $debugging;
1347 Note: Many folks tend to overuse C<defined> and are then surprised to
1348 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1349 defined values. For example, if you say
1353 The pattern match succeeds and C<$1> is defined, although it
1354 matched "nothing". It didn't really fail to match anything. Rather, it
1355 matched something that happened to be zero characters long. This is all
1356 very above-board and honest. When a function returns an undefined value,
1357 it's an admission that it couldn't give you an honest answer. So you
1358 should use C<defined> only when questioning the integrity of what
1359 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1362 See also L</undef>, L</exists>, L</ref>.
1367 =for Pod::Functions deletes a value from a hash
1369 Given an expression that specifies an element or slice of a hash, C<delete>
1370 deletes the specified elements from that hash so that exists() on that element
1371 no longer returns true. Setting a hash element to the undefined value does
1372 not remove its key, but deleting it does; see L</exists>.
1374 In list context, returns the value or values deleted, or the last such
1375 element in scalar context. The return list's length always matches that of
1376 the argument list: deleting non-existent elements returns the undefined value
1377 in their corresponding positions.
1379 delete() may also be used on arrays and array slices, but its behavior is less
1380 straightforward. Although exists() will return false for deleted entries,
1381 deleting array elements never changes indices of existing values; use shift()
1382 or splice() for that. However, if any deleted elements fall at the end of an
1383 array, the array's size shrinks to the position of the highest element that
1384 still tests true for exists(), or to 0 if none do. In other words, an
1385 array won't have trailing nonexistent elements after a delete.
1387 B<WARNING:> Calling C<delete> on array values is strongly discouraged. The
1388 notion of deleting or checking the existence of Perl array elements is not
1389 conceptually coherent, and can lead to surprising behavior.
1391 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1392 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1393 or array may not necessarily return anything; it depends on the implementation
1394 of the C<tied> package's DELETE method, which may do whatever it pleases.
1396 The C<delete local EXPR> construct localizes the deletion to the current
1397 block at run time. Until the block exits, elements locally deleted
1398 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1399 of composite types">.
1401 %hash = (foo => 11, bar => 22, baz => 33);
1402 $scalar = delete $hash{foo}; # $scalar is 11
1403 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1404 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1406 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1408 foreach $key (keys %HASH) {
1412 foreach $index (0 .. $#ARRAY) {
1413 delete $ARRAY[$index];
1418 delete @HASH{keys %HASH};
1420 delete @ARRAY[0 .. $#ARRAY];
1422 But both are slower than assigning the empty list
1423 or undefining %HASH or @ARRAY, which is the customary
1424 way to empty out an aggregate:
1426 %HASH = (); # completely empty %HASH
1427 undef %HASH; # forget %HASH ever existed
1429 @ARRAY = (); # completely empty @ARRAY
1430 undef @ARRAY; # forget @ARRAY ever existed
1432 The EXPR can be arbitrarily complicated provided its
1433 final operation is an element or slice of an aggregate:
1435 delete $ref->[$x][$y]{$key};
1436 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1438 delete $ref->[$x][$y][$index];
1439 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1442 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1444 =for Pod::Functions raise an exception or bail out
1446 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1447 into C<$@> and the C<eval> is terminated with the undefined value.
1448 If the exception is outside of all enclosing C<eval>s, then the uncaught
1449 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1450 need to exit the process with a specific exit code, see L</exit>.
1452 Equivalent examples:
1454 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1455 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1457 If the last element of LIST does not end in a newline, the current
1458 script line number and input line number (if any) are also printed,
1459 and a newline is supplied. Note that the "input line number" (also
1460 known as "chunk") is subject to whatever notion of "line" happens to
1461 be currently in effect, and is also available as the special variable
1462 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1464 Hint: sometimes appending C<", stopped"> to your message will cause it
1465 to make better sense when the string C<"at foo line 123"> is appended.
1466 Suppose you are running script "canasta".
1468 die "/etc/games is no good";
1469 die "/etc/games is no good, stopped";
1471 produce, respectively
1473 /etc/games is no good at canasta line 123.
1474 /etc/games is no good, stopped at canasta line 123.
1476 If the output is empty and C<$@> already contains a value (typically from a
1477 previous eval) that value is reused after appending C<"\t...propagated">.
1478 This is useful for propagating exceptions:
1481 die unless $@ =~ /Expected exception/;
1483 If the output is empty and C<$@> contains an object reference that has a
1484 C<PROPAGATE> method, that method will be called with additional file
1485 and line number parameters. The return value replaces the value in
1486 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1489 If C<$@> is empty then the string C<"Died"> is used.
1491 If an uncaught exception results in interpreter exit, the exit code is
1492 determined from the values of C<$!> and C<$?> with this pseudocode:
1494 exit $! if $!; # errno
1495 exit $? >> 8 if $? >> 8; # child exit status
1496 exit 255; # last resort
1498 The intent is to squeeze as much possible information about the likely cause
1499 into the limited space of the system exit
1500 code. However, as C<$!> is the value
1501 of C's C<errno>, which can be set by any system call, this means that the value
1502 of the exit code used by C<die> can be non-predictable, so should not be relied
1503 upon, other than to be non-zero.
1505 You can also call C<die> with a reference argument, and if this is trapped
1506 within an C<eval>, C<$@> contains that reference. This permits more
1507 elaborate exception handling using objects that maintain arbitrary state
1508 about the exception. Such a scheme is sometimes preferable to matching
1509 particular string values of C<$@> with regular expressions. Because C<$@>
1510 is a global variable and C<eval> may be used within object implementations,
1511 be careful that analyzing the error object doesn't replace the reference in
1512 the global variable. It's easiest to make a local copy of the reference
1513 before any manipulations. Here's an example:
1515 use Scalar::Util "blessed";
1517 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1518 if (my $ev_err = $@) {
1519 if (blessed($ev_err)
1520 && $ev_err->isa("Some::Module::Exception")) {
1521 # handle Some::Module::Exception
1524 # handle all other possible exceptions
1528 Because Perl stringifies uncaught exception messages before display,
1529 you'll probably want to overload stringification operations on
1530 exception objects. See L<overload> for details about that.
1532 You can arrange for a callback to be run just before the C<die>
1533 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1534 handler is called with the error text and can change the error
1535 message, if it sees fit, by calling C<die> again. See
1536 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1537 L<"eval BLOCK"> for some examples. Although this feature was
1538 to be run only right before your program was to exit, this is not
1539 currently so: the C<$SIG{__DIE__}> hook is currently called
1540 even inside eval()ed blocks/strings! If one wants the hook to do
1541 nothing in such situations, put
1545 as the first line of the handler (see L<perlvar/$^S>). Because
1546 this promotes strange action at a distance, this counterintuitive
1547 behavior may be fixed in a future release.
1549 See also exit(), warn(), and the Carp module.
1554 =for Pod::Functions turn a BLOCK into a TERM
1556 Not really a function. Returns the value of the last command in the
1557 sequence of commands indicated by BLOCK. When modified by the C<while> or
1558 C<until> loop modifier, executes the BLOCK once before testing the loop
1559 condition. (On other statements the loop modifiers test the conditional
1562 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1563 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1564 See L<perlsyn> for alternative strategies.
1569 Uses the value of EXPR as a filename and executes the contents of the
1570 file as a Perl script.
1578 except that it's more concise, runs no external processes, keeps track of
1580 filename for error messages, searches the C<@INC> directories, and updates
1581 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1582 these variables. It also differs in that code evaluated with C<do FILENAME>
1583 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1584 same, however, in that it does reparse the file every time you call it,
1585 so you probably don't want to do this inside a loop.
1587 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1588 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1589 and sets C<$!> to the error. Always check C<$@> first, as compilation
1590 could fail in a way that also sets C<$!>. If the file is successfully
1591 compiled, C<do> returns the value of the last expression evaluated.
1593 Inclusion of library modules is better done with the
1594 C<use> and C<require> operators, which also do automatic error checking
1595 and raise an exception if there's a problem.
1597 You might like to use C<do> to read in a program configuration
1598 file. Manual error checking can be done this way:
1600 # read in config files: system first, then user
1601 for $file ("/share/prog/defaults.rc",
1602 "$ENV{HOME}/.someprogrc")
1604 unless ($return = do $file) {
1605 warn "couldn't parse $file: $@" if $@;
1606 warn "couldn't do $file: $!" unless defined $return;
1607 warn "couldn't run $file" unless $return;
1612 X<dump> X<core> X<undump>
1618 =for Pod::Functions create an immediate core dump
1620 This function causes an immediate core dump. See also the B<-u>
1621 command-line switch in L<perlrun>, which does the same thing.
1622 Primarily this is so that you can use the B<undump> program (not
1623 supplied) to turn your core dump into an executable binary after
1624 having initialized all your variables at the beginning of the
1625 program. When the new binary is executed it will begin by executing
1626 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1627 Think of it as a goto with an intervening core dump and reincarnation.
1628 If C<LABEL> is omitted, restarts the program from the top. The
1629 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1630 computed at run time, being otherwise identical to C<dump LABEL>.
1632 B<WARNING>: Any files opened at the time of the dump will I<not>
1633 be open any more when the program is reincarnated, with possible
1634 resulting confusion by Perl.
1636 This function is now largely obsolete, mostly because it's very hard to
1637 convert a core file into an executable. That's why you should now invoke
1638 it as C<CORE::dump()>, if you don't want to be warned against a possible
1641 Unlike most named operators, this has the same precedence as assignment.
1642 It is also exempt from the looks-like-a-function rule, so
1643 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1646 Portability issues: L<perlport/dump>.
1649 X<each> X<hash, iterator>
1654 =for Pod::Functions retrieve the next key/value pair from a hash
1656 When called on a hash in list context, returns a 2-element list
1657 consisting of the key and value for the next element of a hash. In Perl
1658 5.12 and later only, it will also return the index and value for the next
1659 element of an array so that you can iterate over it; older Perls consider
1660 this a syntax error. When called in scalar context, returns only the key
1661 (not the value) in a hash, or the index in an array.
1663 Hash entries are returned in an apparently random order. The actual random
1664 order is specific to a given hash; the exact same series of operations
1665 on two hashes may result in a different order for each hash. Any insertion
1666 into the hash may change the order, as will any deletion, with the exception
1667 that the most recent key returned by C<each> or C<keys> may be deleted
1668 without changing the order. So long as a given hash is unmodified you may
1669 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1670 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1671 details on why hash order is randomized. Aside from the guarantees
1672 provided here the exact details of Perl's hash algorithm and the hash
1673 traversal order are subject to change in any release of Perl.
1675 After C<each> has returned all entries from the hash or array, the next
1676 call to C<each> returns the empty list in list context and C<undef> in
1677 scalar context; the next call following I<that> one restarts iteration.
1678 Each hash or array has its own internal iterator, accessed by C<each>,
1679 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1680 reached the end as just described; it can be explicitly reset by calling
1681 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1682 elements while iterating over it, the effect on the iterator is
1683 unspecified; for example, entries may be skipped or duplicated--so don't
1684 do that. Exception: It is always safe to delete the item most recently
1685 returned by C<each()>, so the following code works properly:
1687 while (($key, $value) = each %hash) {
1689 delete $hash{$key}; # This is safe
1692 Tied hashes may have a different ordering behaviour to perl's hash
1695 This prints out your environment like the printenv(1) program,
1696 but in a different order:
1698 while (($key,$value) = each %ENV) {
1699 print "$key=$value\n";
1702 Starting with Perl 5.14, an experimental feature allowed C<each> to take a
1703 scalar expression. This experiment has been deemed unsuccessful, and was
1704 removed as of Perl 5.24.
1706 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1707 which will set C<$_> on every iteration.
1710 print "$_=$ENV{$_}\n";
1713 To avoid confusing would-be users of your code who are running earlier
1714 versions of Perl with mysterious syntax errors, put this sort of thing at
1715 the top of your file to signal that your code will work I<only> on Perls of
1718 use 5.012; # so keys/values/each work on arrays
1719 use 5.018; # so each assigns to $_ in a lone while test
1721 See also C<keys>, C<values>, and C<sort>.
1723 =item eof FILEHANDLE
1732 =for Pod::Functions test a filehandle for its end
1734 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1735 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1736 gives the real filehandle. (Note that this function actually
1737 reads a character and then C<ungetc>s it, so isn't useful in an
1738 interactive context.) Do not read from a terminal file (or call
1739 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1740 as terminals may lose the end-of-file condition if you do.
1742 An C<eof> without an argument uses the last file read. Using C<eof()>
1743 with empty parentheses is different. It refers to the pseudo file
1744 formed from the files listed on the command line and accessed via the
1745 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1746 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1747 used will cause C<@ARGV> to be examined to determine if input is
1748 available. Similarly, an C<eof()> after C<< <> >> has returned
1749 end-of-file will assume you are processing another C<@ARGV> list,
1750 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1751 see L<perlop/"I/O Operators">.
1753 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1754 detect the end of each file, whereas C<eof()> will detect the end
1755 of the very last file only. Examples:
1757 # reset line numbering on each input file
1759 next if /^\s*#/; # skip comments
1762 close ARGV if eof; # Not eof()!
1765 # insert dashes just before last line of last file
1767 if (eof()) { # check for end of last file
1768 print "--------------\n";
1771 last if eof(); # needed if we're reading from a terminal
1774 Practical hint: you almost never need to use C<eof> in Perl, because the
1775 input operators typically return C<undef> when they run out of data or
1779 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1780 X<error, handling> X<exception, handling>
1786 =for Pod::Functions catch exceptions or compile and run code
1788 In the first form, often referred to as a "string eval", the return
1789 value of EXPR is parsed and executed as if it
1790 were a little Perl program. The value of the expression (which is itself
1791 determined within scalar context) is first parsed, and if there were no
1792 errors, executed as a block within the lexical context of the current Perl
1793 program. This means, that in particular, any outer lexical variables are
1794 visible to it, and any package variable settings or subroutine and format
1795 definitions remain afterwards.
1797 Note that the value is parsed every time the C<eval> executes.
1798 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1799 delay parsing and subsequent execution of the text of EXPR until run time.
1801 If the C<unicode_eval> feature is enabled (which is the default under a
1802 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1803 characters, so C<use utf8> declarations have no effect, and source filters
1804 are forbidden. In the absence of the C<unicode_eval> feature, the string
1805 will sometimes be treated as characters and sometimes as bytes, depending
1806 on the internal encoding, and source filters activated within the C<eval>
1807 exhibit the erratic, but historical, behaviour of affecting some outer file
1808 scope that is still compiling. See also the L</evalbytes> keyword, which
1809 always treats its input as a byte stream and works properly with source
1810 filters, and the L<feature> pragma.
1812 Problems can arise if the string expands a scalar containing a floating
1813 point number. That scalar can expand to letters, such as C<"NaN"> or
1814 C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1815 point character may be something other than a dot (such as a comma).
1816 None of these are likely to parse as you are likely expecting.
1818 In the second form, the code within the BLOCK is parsed only once--at the
1819 same time the code surrounding the C<eval> itself was parsed--and executed
1820 within the context of the current Perl program. This form is typically
1821 used to trap exceptions more efficiently than the first (see below), while
1822 also providing the benefit of checking the code within BLOCK at compile
1825 The final semicolon, if any, may be omitted from the value of EXPR or within
1828 In both forms, the value returned is the value of the last expression
1829 evaluated inside the mini-program; a return statement may be also used, just
1830 as with subroutines. The expression providing the return value is evaluated
1831 in void, scalar, or list context, depending on the context of the C<eval>
1832 itself. See L</wantarray> for more on how the evaluation context can be
1835 If there is a syntax error or runtime error, or a C<die> statement is
1836 executed, C<eval> returns C<undef> in scalar context
1837 or an empty list in list context, and C<$@> is set to the error
1838 message. (Prior to 5.16, a bug caused C<undef> to be returned
1839 in list context for syntax errors, but not for runtime errors.)
1840 If there was no error, C<$@> is set to the empty string. A
1841 control flow operator like C<last> or C<goto> can bypass the setting of
1842 C<$@>. Beware that using C<eval> neither silences Perl from printing
1843 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1844 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1845 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1846 See L</warn>, L<perlvar>, and L<warnings>.
1848 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1849 determining whether a particular feature (such as C<socket> or C<symlink>)
1850 is implemented. It is also Perl's exception-trapping mechanism, where
1851 the die operator is used to raise exceptions.
1853 If you want to trap errors when loading an XS module, some problems with
1854 the binary interface (such as Perl version skew) may be fatal even with
1855 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1857 If the code to be executed doesn't vary, you may use the eval-BLOCK
1858 form to trap run-time errors without incurring the penalty of
1859 recompiling each time. The error, if any, is still returned in C<$@>.
1862 # make divide-by-zero nonfatal
1863 eval { $answer = $a / $b; }; warn $@ if $@;
1865 # same thing, but less efficient
1866 eval '$answer = $a / $b'; warn $@ if $@;
1868 # a compile-time error
1869 eval { $answer = }; # WRONG
1872 eval '$answer ='; # sets $@
1874 Using the C<eval{}> form as an exception trap in libraries does have some
1875 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1876 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1877 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1878 as this example shows:
1880 # a private exception trap for divide-by-zero
1881 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1884 This is especially significant, given that C<__DIE__> hooks can call
1885 C<die> again, which has the effect of changing their error messages:
1887 # __DIE__ hooks may modify error messages
1889 local $SIG{'__DIE__'} =
1890 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1891 eval { die "foo lives here" };
1892 print $@ if $@; # prints "bar lives here"
1895 Because this promotes action at a distance, this counterintuitive behavior
1896 may be fixed in a future release.
1898 With an C<eval>, you should be especially careful to remember what's
1899 being looked at when:
1905 eval { $x }; # CASE 4
1907 eval "\$$x++"; # CASE 5
1910 Cases 1 and 2 above behave identically: they run the code contained in
1911 the variable $x. (Although case 2 has misleading double quotes making
1912 the reader wonder what else might be happening (nothing is).) Cases 3
1913 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1914 does nothing but return the value of $x. (Case 4 is preferred for
1915 purely visual reasons, but it also has the advantage of compiling at
1916 compile-time instead of at run-time.) Case 5 is a place where
1917 normally you I<would> like to use double quotes, except that in this
1918 particular situation, you can just use symbolic references instead, as
1921 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1922 of localized variables, which means that for your code to run on older
1923 versions, a temporary is required if you want to mask some but not all
1926 # alter $@ on nefarious repugnancy only
1930 local $@; # protect existing $@
1931 eval { test_repugnancy() };
1932 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1933 $@ =~ /nefarious/ and $e = $@;
1935 die $e if defined $e
1938 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1939 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1941 An C<eval ''> executed within a subroutine defined
1942 in the C<DB> package doesn't see the usual
1943 surrounding lexical scope, but rather the scope of the first non-DB piece
1944 of code that called it. You don't normally need to worry about this unless
1945 you are writing a Perl debugger.
1947 =item evalbytes EXPR
1952 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1954 This function is like L</eval> with a string argument, except it always
1955 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1956 string containing characters whose ordinal value exceeds 255 results in an
1957 error. Source filters activated within the evaluated code apply to the
1960 This function is only available under the C<evalbytes> feature, a
1961 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1962 L<feature> for more information.
1967 =item exec PROGRAM LIST
1969 =for Pod::Functions abandon this program to run another
1971 The C<exec> function executes a system command I<and never returns>;
1972 use C<system> instead of C<exec> if you want it to return. It fails and
1973 returns false only if the command does not exist I<and> it is executed
1974 directly instead of via your system's command shell (see below).
1976 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1977 warns you if C<exec> is called in void context and if there is a following
1978 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1979 you always do that, right?). If you I<really> want to follow an C<exec>
1980 with some other statement, you can use one of these styles to avoid the warning:
1982 exec ('foo') or print STDERR "couldn't exec foo: $!";
1983 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1985 If there is more than one argument in LIST, this calls execvp(3) with the
1986 arguments in LIST. If there is only one element in LIST, the argument is
1987 checked for shell metacharacters, and if there are any, the entire
1988 argument is passed to the system's command shell for parsing (this is
1989 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1990 there are no shell metacharacters in the argument, it is split into words
1991 and passed directly to C<execvp>, which is more efficient. Examples:
1993 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1994 exec "sort $outfile | uniq";
1996 If you don't really want to execute the first argument, but want to lie
1997 to the program you are executing about its own name, you can specify
1998 the program you actually want to run as an "indirect object" (without a
1999 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2000 forces interpretation of the LIST as a multivalued list, even if there
2001 is only a single scalar in the list.) Example:
2003 $shell = '/bin/csh';
2004 exec $shell '-sh'; # pretend it's a login shell
2008 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2010 When the arguments get executed via the system shell, results are
2011 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2014 Using an indirect object with C<exec> or C<system> is also more
2015 secure. This usage (which also works fine with system()) forces
2016 interpretation of the arguments as a multivalued list, even if the
2017 list had just one argument. That way you're safe from the shell
2018 expanding wildcards or splitting up words with whitespace in them.
2020 @args = ( "echo surprise" );
2022 exec @args; # subject to shell escapes
2024 exec { $args[0] } @args; # safe even with one-arg list
2026 The first version, the one without the indirect object, ran the I<echo>
2027 program, passing it C<"surprise"> an argument. The second version didn't;
2028 it tried to run a program named I<"echo surprise">, didn't find it, and set
2029 C<$?> to a non-zero value indicating failure.
2031 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2032 reliably avoid using the shell; C<exec LIST>, even with more than one
2033 element, will fall back to the shell if the first spawn fails.
2035 Perl attempts to flush all files opened for output before the exec,
2036 but this may not be supported on some platforms (see L<perlport>).
2037 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2038 call the C<autoflush()> method of C<IO::Handle> on any open handles
2039 to avoid lost output.
2041 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2042 C<DESTROY> methods on your objects.
2044 Portability issues: L<perlport/exec>.
2047 X<exists> X<autovivification>
2049 =for Pod::Functions test whether a hash key is present
2051 Given an expression that specifies an element of a hash, returns true if the
2052 specified element in the hash has ever been initialized, even if the
2053 corresponding value is undefined.
2055 print "Exists\n" if exists $hash{$key};
2056 print "Defined\n" if defined $hash{$key};
2057 print "True\n" if $hash{$key};
2059 exists may also be called on array elements, but its behavior is much less
2060 obvious and is strongly tied to the use of L</delete> on arrays.
2062 B<WARNING:> Calling C<exists> on array values is strongly discouraged. The
2063 notion of deleting or checking the existence of Perl array elements is not
2064 conceptually coherent, and can lead to surprising behavior.
2066 print "Exists\n" if exists $array[$index];
2067 print "Defined\n" if defined $array[$index];
2068 print "True\n" if $array[$index];
2070 A hash or array element can be true only if it's defined and defined only if
2071 it exists, but the reverse doesn't necessarily hold true.
2073 Given an expression that specifies the name of a subroutine,
2074 returns true if the specified subroutine has ever been declared, even
2075 if it is undefined. Mentioning a subroutine name for exists or defined
2076 does not count as declaring it. Note that a subroutine that does not
2077 exist may still be callable: its package may have an C<AUTOLOAD>
2078 method that makes it spring into existence the first time that it is
2079 called; see L<perlsub>.
2081 print "Exists\n" if exists &subroutine;
2082 print "Defined\n" if defined &subroutine;
2084 Note that the EXPR can be arbitrarily complicated as long as the final
2085 operation is a hash or array key lookup or subroutine name:
2087 if (exists $ref->{A}->{B}->{$key}) { }
2088 if (exists $hash{A}{B}{$key}) { }
2090 if (exists $ref->{A}->{B}->[$ix]) { }
2091 if (exists $hash{A}{B}[$ix]) { }
2093 if (exists &{$ref->{A}{B}{$key}}) { }
2095 Although the most deeply nested array or hash element will not spring into
2096 existence just because its existence was tested, any intervening ones will.
2097 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2098 into existence due to the existence test for the $key element above.
2099 This happens anywhere the arrow operator is used, including even here:
2102 if (exists $ref->{"Some key"}) { }
2103 print $ref; # prints HASH(0x80d3d5c)
2105 This surprising autovivification in what does not at first--or even
2106 second--glance appear to be an lvalue context may be fixed in a future
2109 Use of a subroutine call, rather than a subroutine name, as an argument
2110 to exists() is an error.
2113 exists &sub(); # Error
2116 X<exit> X<terminate> X<abort>
2120 =for Pod::Functions terminate this program
2122 Evaluates EXPR and exits immediately with that value. Example:
2125 exit 0 if $ans =~ /^[Xx]/;
2127 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2128 universally recognized values for EXPR are C<0> for success and C<1>
2129 for error; other values are subject to interpretation depending on the
2130 environment in which the Perl program is running. For example, exiting
2131 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2132 the mailer to return the item undelivered, but that's not true everywhere.
2134 Don't use C<exit> to abort a subroutine if there's any chance that
2135 someone might want to trap whatever error happened. Use C<die> instead,
2136 which can be trapped by an C<eval>.
2138 The exit() function does not always exit immediately. It calls any
2139 defined C<END> routines first, but these C<END> routines may not
2140 themselves abort the exit. Likewise any object destructors that need to
2141 be called are called before the real exit. C<END> routines and destructors
2142 can change the exit status by modifying C<$?>. If this is a problem, you
2143 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2144 See L<perlmod> for details.
2146 Portability issues: L<perlport/exit>.
2149 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2153 =for Pod::Functions raise I<e> to a power
2155 Returns I<e> (the natural logarithm base) to the power of EXPR.
2156 If EXPR is omitted, gives C<exp($_)>.
2159 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2163 =for Pod::Functions +fc return casefolded version of a string
2165 Returns the casefolded version of EXPR. This is the internal function
2166 implementing the C<\F> escape in double-quoted strings.
2168 Casefolding is the process of mapping strings to a form where case
2169 differences are erased; comparing two strings in their casefolded
2170 form is effectively a way of asking if two strings are equal,
2173 Roughly, if you ever found yourself writing this
2175 lc($this) eq lc($that) # Wrong!
2177 uc($this) eq uc($that) # Also wrong!
2179 $this =~ /^\Q$that\E\z/i # Right!
2183 fc($this) eq fc($that)
2185 And get the correct results.
2187 Perl only implements the full form of casefolding,
2188 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2189 L<Unicode::UCD/prop_invmap()>.
2190 For further information on casefolding, refer to
2191 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2192 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2193 available at L<http://www.unicode.org/versions/latest/>, as well as the
2194 Case Charts available at L<http://www.unicode.org/charts/case/>.
2196 If EXPR is omitted, uses C<$_>.
2198 This function behaves the same way under various pragma, such as within
2199 S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2200 exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2201 scope of S<C<use locale>>. The foldcase of this character would
2202 normally be C<"ss">, but as explained in the L</lc> section, case
2203 changes that cross the 255/256 boundary are problematic under locales,
2204 and are hence prohibited. Therefore, this function under locale returns
2205 instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2206 LONG S. Since that character itself folds to C<"s">, the string of two
2207 of them together should be equivalent to a single U+1E9E when foldcased.
2209 While the Unicode Standard defines two additional forms of casefolding,
2210 one for Turkic languages and one that never maps one character into multiple
2211 characters, these are not provided by the Perl core; However, the CPAN module
2212 C<Unicode::Casing> may be used to provide an implementation.
2214 This keyword is available only when the C<"fc"> feature is enabled,
2215 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2216 include a C<use v5.16> or later to the current scope.
2218 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2221 =for Pod::Functions file control system call
2223 Implements the fcntl(2) function. You'll probably have to say
2227 first to get the correct constant definitions. Argument processing and
2228 value returned work just like C<ioctl> below.
2232 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2233 or die "can't fcntl F_GETFL: $!";
2235 You don't have to check for C<defined> on the return from C<fcntl>.
2236 Like C<ioctl>, it maps a C<0> return from the system call into
2237 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2238 in numeric context. It is also exempt from the normal B<-w> warnings
2239 on improper numeric conversions.
2241 Note that C<fcntl> raises an exception if used on a machine that
2242 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2243 manpage to learn what functions are available on your system.
2245 Here's an example of setting a filehandle named C<REMOTE> to be
2246 non-blocking at the system level. You'll have to negotiate C<$|>
2247 on your own, though.
2249 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2251 $flags = fcntl(REMOTE, F_GETFL, 0)
2252 or die "Can't get flags for the socket: $!\n";
2254 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2255 or die "Can't set flags for the socket: $!\n";
2257 Portability issues: L<perlport/fcntl>.
2262 =for Pod::Functions the name of the current source file
2264 A special token that returns the name of the file in which it occurs.
2266 =item fileno FILEHANDLE
2269 =for Pod::Functions return file descriptor from filehandle
2271 Returns the file descriptor for a filehandle, or undefined if the
2272 filehandle is not open. If there is no real file descriptor at the OS
2273 level, as can happen with filehandles connected to memory objects via
2274 C<open> with a reference for the third argument, -1 is returned.
2276 This is mainly useful for constructing
2277 bitmaps for C<select> and low-level POSIX tty-handling operations.
2278 If FILEHANDLE is an expression, the value is taken as an indirect
2279 filehandle, generally its name.
2281 You can use this to find out whether two handles refer to the
2282 same underlying descriptor:
2284 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
2285 print "THIS and THAT are dups\n";
2286 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2287 print "THIS and THAT have different " .
2288 "underlying file descriptors\n";
2290 print "At least one of THIS and THAT does " .
2291 "not have a real file descriptor\n";
2294 The behavior of C<fileno> on a directory handle depends on the operating
2295 system. On a system with dirfd(3) or similar, C<fileno> on a directory
2296 handle returns the underlying file descriptor associated with the
2297 handle; on systems with no such support, it returns the undefined value,
2298 and sets C<$!> (errno).
2300 =item flock FILEHANDLE,OPERATION
2301 X<flock> X<lock> X<locking>
2303 =for Pod::Functions lock an entire file with an advisory lock
2305 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2306 for success, false on failure. Produces a fatal error if used on a
2307 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2308 C<flock> is Perl's portable file-locking interface, although it locks
2309 entire files only, not records.
2311 Two potentially non-obvious but traditional C<flock> semantics are
2312 that it waits indefinitely until the lock is granted, and that its locks
2313 are B<merely advisory>. Such discretionary locks are more flexible, but
2314 offer fewer guarantees. This means that programs that do not also use
2315 C<flock> may modify files locked with C<flock>. See L<perlport>,
2316 your port's specific documentation, and your system-specific local manpages
2317 for details. It's best to assume traditional behavior if you're writing
2318 portable programs. (But if you're not, you should as always feel perfectly
2319 free to write for your own system's idiosyncrasies (sometimes called
2320 "features"). Slavish adherence to portability concerns shouldn't get
2321 in the way of your getting your job done.)
2323 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2324 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2325 you can use the symbolic names if you import them from the L<Fcntl> module,
2326 either individually, or as a group using the C<:flock> tag. LOCK_SH
2327 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2328 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2329 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2330 waiting for the lock; check the return status to see if you got it.
2332 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2333 before locking or unlocking it.
2335 Note that the emulation built with lockf(3) doesn't provide shared
2336 locks, and it requires that FILEHANDLE be open with write intent. These
2337 are the semantics that lockf(3) implements. Most if not all systems
2338 implement lockf(3) in terms of fcntl(2) locking, though, so the
2339 differing semantics shouldn't bite too many people.
2341 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2342 be open with read intent to use LOCK_SH and requires that it be open
2343 with write intent to use LOCK_EX.
2345 Note also that some versions of C<flock> cannot lock things over the
2346 network; you would need to use the more system-specific C<fcntl> for
2347 that. If you like you can force Perl to ignore your system's flock(2)
2348 function, and so provide its own fcntl(2)-based emulation, by passing
2349 the switch C<-Ud_flock> to the F<Configure> program when you configure
2350 and build a new Perl.
2352 Here's a mailbox appender for BSD systems.
2354 # import LOCK_* and SEEK_END constants
2355 use Fcntl qw(:flock SEEK_END);
2359 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2361 # and, in case someone appended while we were waiting...
2362 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2367 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2370 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2371 or die "Can't open mailbox: $!";
2374 print $mbox $msg,"\n\n";
2377 On systems that support a real flock(2), locks are inherited across fork()
2378 calls, whereas those that must resort to the more capricious fcntl(2)
2379 function lose their locks, making it seriously harder to write servers.
2381 See also L<DB_File> for other flock() examples.
2383 Portability issues: L<perlport/flock>.
2386 X<fork> X<child> X<parent>
2388 =for Pod::Functions create a new process just like this one
2390 Does a fork(2) system call to create a new process running the
2391 same program at the same point. It returns the child pid to the
2392 parent process, C<0> to the child process, or C<undef> if the fork is
2393 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2394 are shared, while everything else is copied. On most systems supporting
2395 fork(), great care has gone into making it extremely efficient (for
2396 example, using copy-on-write technology on data pages), making it the
2397 dominant paradigm for multitasking over the last few decades.
2399 Perl attempts to flush all files opened for
2400 output before forking the child process, but this may not be supported
2401 on some platforms (see L<perlport>). To be safe, you may need to set
2402 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2403 C<IO::Handle> on any open handles to avoid duplicate output.
2405 If you C<fork> without ever waiting on your children, you will
2406 accumulate zombies. On some systems, you can avoid this by setting
2407 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2408 forking and reaping moribund children.
2410 Note that if your forked child inherits system file descriptors like
2411 STDIN and STDOUT that are actually connected by a pipe or socket, even
2412 if you exit, then the remote server (such as, say, a CGI script or a
2413 backgrounded job launched from a remote shell) won't think you're done.
2414 You should reopen those to F</dev/null> if it's any issue.
2416 On some platforms such as Windows, where the fork() system call is not available,
2417 Perl can be built to emulate fork() in the Perl interpreter.
2418 The emulation is designed, at the level of the Perl program,
2419 to be as compatible as possible with the "Unix" fork().
2420 However it has limitations that have to be considered in code intended to be portable.
2421 See L<perlfork> for more details.
2423 Portability issues: L<perlport/fork>.
2428 =for Pod::Functions declare a picture format with use by the write() function
2430 Declare a picture format for use by the C<write> function. For
2434 Test: @<<<<<<<< @||||| @>>>>>
2435 $str, $%, '$' . int($num)
2439 $num = $cost/$quantity;
2443 See L<perlform> for many details and examples.
2445 =item formline PICTURE,LIST
2448 =for Pod::Functions internal function used for formats
2450 This is an internal function used by C<format>s, though you may call it,
2451 too. It formats (see L<perlform>) a list of values according to the
2452 contents of PICTURE, placing the output into the format output
2453 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2454 Eventually, when a C<write> is done, the contents of
2455 C<$^A> are written to some filehandle. You could also read C<$^A>
2456 and then set C<$^A> back to C<"">. Note that a format typically
2457 does one C<formline> per line of form, but the C<formline> function itself
2458 doesn't care how many newlines are embedded in the PICTURE. This means
2459 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2460 You may therefore need to use multiple formlines to implement a single
2461 record format, just like the C<format> compiler.
2463 Be careful if you put double quotes around the picture, because an C<@>
2464 character may be taken to mean the beginning of an array name.
2465 C<formline> always returns true. See L<perlform> for other examples.
2467 If you are trying to use this instead of C<write> to capture the output,
2468 you may find it easier to open a filehandle to a scalar
2469 (C<< open $fh, ">", \$output >>) and write to that instead.
2471 =item getc FILEHANDLE
2472 X<getc> X<getchar> X<character> X<file, read>
2476 =for Pod::Functions get the next character from the filehandle
2478 Returns the next character from the input file attached to FILEHANDLE,
2479 or the undefined value at end of file or if there was an error (in
2480 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2481 STDIN. This is not particularly efficient. However, it cannot be
2482 used by itself to fetch single characters without waiting for the user
2483 to hit enter. For that, try something more like:
2486 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2489 system "stty", '-icanon', 'eol', "\001";
2495 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2498 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2502 Determination of whether $BSD_STYLE should be set
2503 is left as an exercise to the reader.
2505 The C<POSIX::getattr> function can do this more portably on
2506 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2507 module from your nearest L<CPAN|http://www.cpan.org> site.
2510 X<getlogin> X<login>
2512 =for Pod::Functions return who logged in at this tty
2514 This implements the C library function of the same name, which on most
2515 systems returns the current login from F</etc/utmp>, if any. If it
2516 returns the empty string, use C<getpwuid>.
2518 $login = getlogin || getpwuid($<) || "Kilroy";
2520 Do not consider C<getlogin> for authentication: it is not as
2521 secure as C<getpwuid>.
2523 Portability issues: L<perlport/getlogin>.
2525 =item getpeername SOCKET
2526 X<getpeername> X<peer>
2528 =for Pod::Functions find the other end of a socket connection
2530 Returns the packed sockaddr address of the other end of the SOCKET
2534 $hersockaddr = getpeername(SOCK);
2535 ($port, $iaddr) = sockaddr_in($hersockaddr);
2536 $herhostname = gethostbyaddr($iaddr, AF_INET);
2537 $herstraddr = inet_ntoa($iaddr);
2542 =for Pod::Functions get process group
2544 Returns the current process group for the specified PID. Use
2545 a PID of C<0> to get the current process group for the
2546 current process. Will raise an exception if used on a machine that
2547 doesn't implement getpgrp(2). If PID is omitted, returns the process
2548 group of the current process. Note that the POSIX version of C<getpgrp>
2549 does not accept a PID argument, so only C<PID==0> is truly portable.
2551 Portability issues: L<perlport/getpgrp>.
2554 X<getppid> X<parent> X<pid>
2556 =for Pod::Functions get parent process ID
2558 Returns the process id of the parent process.
2560 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2561 around non-POSIX thread semantics the minority of Linux systems (and
2562 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2563 has since been removed. See the documentation for L<$$|perlvar/$$> for
2566 Portability issues: L<perlport/getppid>.
2568 =item getpriority WHICH,WHO
2569 X<getpriority> X<priority> X<nice>
2571 =for Pod::Functions get current nice value
2573 Returns the current priority for a process, a process group, or a user.
2574 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2575 machine that doesn't implement getpriority(2).
2577 Portability issues: L<perlport/getpriority>.
2580 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2581 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2582 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2583 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2584 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2585 X<endnetent> X<endprotoent> X<endservent>
2587 =for Pod::Functions get passwd record given user login name
2591 =for Pod::Functions get group record given group name
2593 =item gethostbyname NAME
2595 =for Pod::Functions get host record given name
2597 =item getnetbyname NAME
2599 =for Pod::Functions get networks record given name
2601 =item getprotobyname NAME
2603 =for Pod::Functions get protocol record given name
2607 =for Pod::Functions get passwd record given user ID
2611 =for Pod::Functions get group record given group user ID
2613 =item getservbyname NAME,PROTO
2615 =for Pod::Functions get services record given its name
2617 =item gethostbyaddr ADDR,ADDRTYPE
2619 =for Pod::Functions get host record given its address
2621 =item getnetbyaddr ADDR,ADDRTYPE
2623 =for Pod::Functions get network record given its address
2625 =item getprotobynumber NUMBER
2627 =for Pod::Functions get protocol record numeric protocol
2629 =item getservbyport PORT,PROTO
2631 =for Pod::Functions get services record given numeric port
2635 =for Pod::Functions get next passwd record
2639 =for Pod::Functions get next group record
2643 =for Pod::Functions get next hosts record
2647 =for Pod::Functions get next networks record
2651 =for Pod::Functions get next protocols record
2655 =for Pod::Functions get next services record
2659 =for Pod::Functions prepare passwd file for use
2663 =for Pod::Functions prepare group file for use
2665 =item sethostent STAYOPEN
2667 =for Pod::Functions prepare hosts file for use
2669 =item setnetent STAYOPEN
2671 =for Pod::Functions prepare networks file for use
2673 =item setprotoent STAYOPEN
2675 =for Pod::Functions prepare protocols file for use
2677 =item setservent STAYOPEN
2679 =for Pod::Functions prepare services file for use
2683 =for Pod::Functions be done using passwd file
2687 =for Pod::Functions be done using group file
2691 =for Pod::Functions be done using hosts file
2695 =for Pod::Functions be done using networks file
2699 =for Pod::Functions be done using protocols file
2703 =for Pod::Functions be done using services file
2705 These routines are the same as their counterparts in the
2706 system C library. In list context, the return values from the
2707 various get routines are as follows:
2710 ( $name, $passwd, $gid, $members ) = getgr*
2711 ( $name, $aliases, $addrtype, $net ) = getnet*
2712 ( $name, $aliases, $port, $proto ) = getserv*
2713 ( $name, $aliases, $proto ) = getproto*
2714 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2715 ( $name, $passwd, $uid, $gid, $quota,
2716 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2719 (If the entry doesn't exist, the return value is a single meaningless true
2722 The exact meaning of the $gcos field varies but it usually contains
2723 the real name of the user (as opposed to the login name) and other
2724 information pertaining to the user. Beware, however, that in many
2725 system users are able to change this information and therefore it
2726 cannot be trusted and therefore the $gcos is tainted (see
2727 L<perlsec>). The $passwd and $shell, user's encrypted password and
2728 login shell, are also tainted, for the same reason.
2730 In scalar context, you get the name, unless the function was a
2731 lookup by name, in which case you get the other thing, whatever it is.
2732 (If the entry doesn't exist you get the undefined value.) For example:
2734 $uid = getpwnam($name);
2735 $name = getpwuid($num);
2737 $gid = getgrnam($name);
2738 $name = getgrgid($num);
2742 In I<getpw*()> the fields $quota, $comment, and $expire are special
2743 in that they are unsupported on many systems. If the
2744 $quota is unsupported, it is an empty scalar. If it is supported, it
2745 usually encodes the disk quota. If the $comment field is unsupported,
2746 it is an empty scalar. If it is supported it usually encodes some
2747 administrative comment about the user. In some systems the $quota
2748 field may be $change or $age, fields that have to do with password
2749 aging. In some systems the $comment field may be $class. The $expire
2750 field, if present, encodes the expiration period of the account or the
2751 password. For the availability and the exact meaning of these fields
2752 in your system, please consult getpwnam(3) and your system's
2753 F<pwd.h> file. You can also find out from within Perl what your
2754 $quota and $comment fields mean and whether you have the $expire field
2755 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2756 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2757 files are supported only if your vendor has implemented them in the
2758 intuitive fashion that calling the regular C library routines gets the
2759 shadow versions if you're running under privilege or if there exists
2760 the shadow(3) functions as found in System V (this includes Solaris
2761 and Linux). Those systems that implement a proprietary shadow password
2762 facility are unlikely to be supported.
2764 The $members value returned by I<getgr*()> is a space-separated list of
2765 the login names of the members of the group.
2767 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2768 C, it will be returned to you via C<$?> if the function call fails. The
2769 C<@addrs> value returned by a successful call is a list of raw
2770 addresses returned by the corresponding library call. In the
2771 Internet domain, each address is four bytes long; you can unpack it
2772 by saying something like:
2774 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2776 The Socket library makes this slightly easier:
2779 $iaddr = inet_aton("127.1"); # or whatever address
2780 $name = gethostbyaddr($iaddr, AF_INET);
2782 # or going the other way
2783 $straddr = inet_ntoa($iaddr);
2785 In the opposite way, to resolve a hostname to the IP address
2789 $packed_ip = gethostbyname("www.perl.org");
2790 if (defined $packed_ip) {
2791 $ip_address = inet_ntoa($packed_ip);
2794 Make sure C<gethostbyname()> is called in SCALAR context and that
2795 its return value is checked for definedness.
2797 The C<getprotobynumber> function, even though it only takes one argument,
2798 has the precedence of a list operator, so beware:
2800 getprotobynumber $number eq 'icmp' # WRONG
2801 getprotobynumber($number eq 'icmp') # actually means this
2802 getprotobynumber($number) eq 'icmp' # better this way
2804 If you get tired of remembering which element of the return list
2805 contains which return value, by-name interfaces are provided
2806 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2807 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2808 and C<User::grent>. These override the normal built-ins, supplying
2809 versions that return objects with the appropriate names
2810 for each field. For example:
2814 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2816 Even though it looks as though they're the same method calls (uid),
2817 they aren't, because a C<File::stat> object is different from
2818 a C<User::pwent> object.
2820 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2822 =item getsockname SOCKET
2825 =for Pod::Functions retrieve the sockaddr for a given socket
2827 Returns the packed sockaddr address of this end of the SOCKET connection,
2828 in case you don't know the address because you have several different
2829 IPs that the connection might have come in on.
2832 $mysockaddr = getsockname(SOCK);
2833 ($port, $myaddr) = sockaddr_in($mysockaddr);
2834 printf "Connect to %s [%s]\n",
2835 scalar gethostbyaddr($myaddr, AF_INET),
2838 =item getsockopt SOCKET,LEVEL,OPTNAME
2841 =for Pod::Functions get socket options on a given socket
2843 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2844 Options may exist at multiple protocol levels depending on the socket
2845 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2846 C<Socket> module) will exist. To query options at another level the
2847 protocol number of the appropriate protocol controlling the option
2848 should be supplied. For example, to indicate that an option is to be
2849 interpreted by the TCP protocol, LEVEL should be set to the protocol
2850 number of TCP, which you can get using C<getprotobyname>.
2852 The function returns a packed string representing the requested socket
2853 option, or C<undef> on error, with the reason for the error placed in
2854 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2855 consult getsockopt(2) for details. A common case is that the option is an
2856 integer, in which case the result is a packed integer, which you can decode
2857 using C<unpack> with the C<i> (or C<I>) format.
2859 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2861 use Socket qw(:all);
2863 defined(my $tcp = getprotobyname("tcp"))
2864 or die "Could not determine the protocol number for tcp";
2865 # my $tcp = IPPROTO_TCP; # Alternative
2866 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2867 or die "getsockopt TCP_NODELAY: $!";
2868 my $nodelay = unpack("I", $packed);
2869 print "Nagle's algorithm is turned ",
2870 $nodelay ? "off\n" : "on\n";
2872 Portability issues: L<perlport/getsockopt>.
2875 X<glob> X<wildcard> X<filename, expansion> X<expand>
2879 =for Pod::Functions expand filenames using wildcards
2881 In list context, returns a (possibly empty) list of filename expansions on
2882 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2883 scalar context, glob iterates through such filename expansions, returning
2884 undef when the list is exhausted. This is the internal function
2885 implementing the C<< <*.c> >> operator, but you can use it directly. If
2886 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2887 more detail in L<perlop/"I/O Operators">.
2889 Note that C<glob> splits its arguments on whitespace and treats
2890 each segment as separate pattern. As such, C<glob("*.c *.h")>
2891 matches all files with a F<.c> or F<.h> extension. The expression
2892 C<glob(".* *")> matches all files in the current working directory.
2893 If you want to glob filenames that might contain whitespace, you'll
2894 have to use extra quotes around the spacey filename to protect it.
2895 For example, to glob filenames that have an C<e> followed by a space
2896 followed by an C<f>, use either of:
2898 @spacies = <"*e f*">;
2899 @spacies = glob '"*e f*"';
2900 @spacies = glob q("*e f*");
2902 If you had to get a variable through, you could do this:
2904 @spacies = glob "'*${var}e f*'";
2905 @spacies = glob qq("*${var}e f*");
2907 If non-empty braces are the only wildcard characters used in the
2908 C<glob>, no filenames are matched, but potentially many strings
2909 are returned. For example, this produces nine strings, one for
2910 each pairing of fruits and colors:
2912 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2914 This operator is implemented using the standard
2915 C<File::Glob> extension. See L<File::Glob> for details, including
2916 C<bsd_glob> which does not treat whitespace as a pattern separator.
2918 Portability issues: L<perlport/glob>.
2921 X<gmtime> X<UTC> X<Greenwich>
2925 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2927 Works just like L</localtime> but the returned values are
2928 localized for the standard Greenwich time zone.
2930 Note: When called in list context, $isdst, the last value
2931 returned by gmtime, is always C<0>. There is no
2932 Daylight Saving Time in GMT.
2934 Portability issues: L<perlport/gmtime>.
2937 X<goto> X<jump> X<jmp>
2943 =for Pod::Functions create spaghetti code
2945 The C<goto LABEL> form finds the statement labeled with LABEL and
2946 resumes execution there. It can't be used to get out of a block or
2947 subroutine given to C<sort>. It can be used to go almost anywhere
2948 else within the dynamic scope, including out of subroutines, but it's
2949 usually better to use some other construct such as C<last> or C<die>.
2950 The author of Perl has never felt the need to use this form of C<goto>
2951 (in Perl, that is; C is another matter). (The difference is that C
2952 does not offer named loops combined with loop control. Perl does, and
2953 this replaces most structured uses of C<goto> in other languages.)
2955 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
2956 a label name. If it evaluates to a code reference, it will be handled
2957 like C<goto &NAME>, below. This is especially useful for implementing
2958 tail recursion via C<goto __SUB__>.
2960 If the expression evaluates to a label name, its scope will be resolved
2961 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2962 necessarily recommended if you're optimizing for maintainability:
2964 goto ("FOO", "BAR", "GLARCH")[$i];
2966 As shown in this example, C<goto EXPR> is exempt from the "looks like a
2967 function" rule. A pair of parentheses following it does not (necessarily)
2968 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2969 Also, unlike most named operators, this has the same precedence as
2972 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
2973 deprecated and will issue a warning. Even then, it may not be used to
2974 go into any construct that requires initialization, such as a
2975 subroutine or a C<foreach> loop. It also can't be used to go into a
2976 construct that is optimized away.
2978 The C<goto &NAME> form is quite different from the other forms of
2979 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2980 doesn't have the stigma associated with other gotos. Instead, it
2981 exits the current subroutine (losing any changes set by local()) and
2982 immediately calls in its place the named subroutine using the current
2983 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2984 load another subroutine and then pretend that the other subroutine had
2985 been called in the first place (except that any modifications to C<@_>
2986 in the current subroutine are propagated to the other subroutine.)
2987 After the C<goto>, not even C<caller> will be able to tell that this
2988 routine was called first.
2990 NAME needn't be the name of a subroutine; it can be a scalar variable
2991 containing a code reference or a block that evaluates to a code
2994 =item grep BLOCK LIST
2997 =item grep EXPR,LIST
2999 =for Pod::Functions locate elements in a list test true against a given criterion
3001 This is similar in spirit to, but not the same as, grep(1) and its
3002 relatives. In particular, it is not limited to using regular expressions.
3004 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3005 C<$_> to each element) and returns the list value consisting of those
3006 elements for which the expression evaluated to true. In scalar
3007 context, returns the number of times the expression was true.
3009 @foo = grep(!/^#/, @bar); # weed out comments
3013 @foo = grep {!/^#/} @bar; # weed out comments
3015 Note that C<$_> is an alias to the list value, so it can be used to
3016 modify the elements of the LIST. While this is useful and supported,
3017 it can cause bizarre results if the elements of LIST are not variables.
3018 Similarly, grep returns aliases into the original list, much as a for
3019 loop's index variable aliases the list elements. That is, modifying an
3020 element of a list returned by grep (for example, in a C<foreach>, C<map>
3021 or another C<grep>) actually modifies the element in the original list.
3022 This is usually something to be avoided when writing clear code.
3024 If C<$_> is lexical in the scope where the C<grep> appears (because it has
3025 been declared with the deprecated C<my $_> construct)
3026 then, in addition to being locally aliased to
3027 the list elements, C<$_> keeps being lexical inside the block; i.e., it
3028 can't be seen from the outside, avoiding any potential side-effects.
3030 See also L</map> for a list composed of the results of the BLOCK or EXPR.
3033 X<hex> X<hexadecimal>
3037 =for Pod::Functions convert a string to a hexadecimal number
3039 Interprets EXPR as a hex string and returns the corresponding value.
3040 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
3041 L</oct>.) If EXPR is omitted, uses C<$_>.
3043 print hex '0xAf'; # prints '175'
3044 print hex 'aF'; # same
3046 Hex strings may only represent integers. Strings that would cause
3047 integer overflow trigger a warning. Leading whitespace is not stripped,
3048 unlike oct(). To present something as hex, look into L</printf>,
3049 L</sprintf>, and L</unpack>.
3054 =for Pod::Functions patch a module's namespace into your own
3056 There is no builtin C<import> function. It is just an ordinary
3057 method (subroutine) defined (or inherited) by modules that wish to export
3058 names to another module. The C<use> function calls the C<import> method
3059 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3061 =item index STR,SUBSTR,POSITION
3062 X<index> X<indexOf> X<InStr>
3064 =item index STR,SUBSTR
3066 =for Pod::Functions find a substring within a string
3068 The index function searches for one string within another, but without
3069 the wildcard-like behavior of a full regular-expression pattern match.
3070 It returns the position of the first occurrence of SUBSTR in STR at
3071 or after POSITION. If POSITION is omitted, starts searching from the
3072 beginning of the string. POSITION before the beginning of the string
3073 or after its end is treated as if it were the beginning or the end,
3074 respectively. POSITION and the return value are based at zero.
3075 If the substring is not found, C<index> returns -1.
3078 X<int> X<integer> X<truncate> X<trunc> X<floor>
3082 =for Pod::Functions get the integer portion of a number
3084 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3085 You should not use this function for rounding: one because it truncates
3086 towards C<0>, and two because machine representations of floating-point
3087 numbers can sometimes produce counterintuitive results. For example,
3088 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3089 because it's really more like -268.99999999999994315658 instead. Usually,
3090 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3091 functions will serve you better than will int().
3093 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3096 =for Pod::Functions system-dependent device control system call
3098 Implements the ioctl(2) function. You'll probably first have to say
3100 require "sys/ioctl.ph"; # probably in
3101 # $Config{archlib}/sys/ioctl.ph
3103 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3104 exist or doesn't have the correct definitions you'll have to roll your
3105 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3106 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3107 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3108 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3109 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3110 has no string value but does have a numeric value, that value will be
3111 passed rather than a pointer to the string value. To guarantee this to be
3112 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3113 functions may be needed to manipulate the values of structures used by
3116 The return value of C<ioctl> (and C<fcntl>) is as follows:
3118 if OS returns: then Perl returns:
3120 0 string "0 but true"
3121 anything else that number
3123 Thus Perl returns true on success and false on failure, yet you can
3124 still easily determine the actual value returned by the operating
3127 $retval = ioctl(...) || -1;
3128 printf "System returned %d\n", $retval;
3130 The special string C<"0 but true"> is exempt from B<-w> complaints
3131 about improper numeric conversions.
3133 Portability issues: L<perlport/ioctl>.
3135 =item join EXPR,LIST
3138 =for Pod::Functions join a list into a string using a separator
3140 Joins the separate strings of LIST into a single string with fields
3141 separated by the value of EXPR, and returns that new string. Example:
3143 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3145 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3146 first argument. Compare L</split>.
3153 =for Pod::Functions retrieve list of indices from a hash
3155 Called in list context, returns a list consisting of all the keys of the
3156 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3157 releases prior to 5.12 will produce a syntax error if you try to use an
3158 array argument. In scalar context, returns the number of keys or indices.
3160 Hash entries are returned in an apparently random order. The actual random
3161 order is specific to a given hash; the exact same series of operations
3162 on two hashes may result in a different order for each hash. Any insertion
3163 into the hash may change the order, as will any deletion, with the exception
3164 that the most recent key returned by C<each> or C<keys> may be deleted
3165 without changing the order. So long as a given hash is unmodified you may
3166 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
3167 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3168 details on why hash order is randomized. Aside from the guarantees
3169 provided here the exact details of Perl's hash algorithm and the hash
3170 traversal order are subject to change in any release of Perl. Tied hashes
3171 may behave differently to Perl's hashes with respect to changes in order on
3172 insertion and deletion of items.
3174 As a side effect, calling keys() resets the internal iterator of the HASH or
3175 ARRAY (see L</each>). In particular, calling keys() in void context resets
3176 the iterator with no other overhead.
3178 Here is yet another way to print your environment:
3181 @values = values %ENV;
3183 print pop(@keys), '=', pop(@values), "\n";
3186 or how about sorted by key:
3188 foreach $key (sort(keys %ENV)) {
3189 print $key, '=', $ENV{$key}, "\n";
3192 The returned values are copies of the original keys in the hash, so
3193 modifying them will not affect the original hash. Compare L</values>.
3195 To sort a hash by value, you'll need to use a C<sort> function.
3196 Here's a descending numeric sort of a hash by its values:
3198 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3199 printf "%4d %s\n", $hash{$key}, $key;
3202 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3203 allocated for the given hash. This can gain you a measure of efficiency if
3204 you know the hash is going to get big. (This is similar to pre-extending
3205 an array by assigning a larger number to $#array.) If you say
3209 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3210 in fact, since it rounds up to the next power of two. These
3211 buckets will be retained even if you do C<%hash = ()>, use C<undef
3212 %hash> if you want to free the storage while C<%hash> is still in scope.
3213 You can't shrink the number of buckets allocated for the hash using
3214 C<keys> in this way (but you needn't worry about doing this by accident,
3215 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3218 Starting with Perl 5.14, an experimental feature allowed C<keys> to take a
3219 scalar expression. This experiment has been deemed unsuccessful, and was
3220 removed as of Perl 5.24.
3222 To avoid confusing would-be users of your code who are running earlier
3223 versions of Perl with mysterious syntax errors, put this sort of thing at
3224 the top of your file to signal that your code will work I<only> on Perls of
3227 use 5.012; # so keys/values/each work on arrays
3229 See also C<each>, C<values>, and C<sort>.
3231 =item kill SIGNAL, LIST
3236 =for Pod::Functions send a signal to a process or process group
3238 Sends a signal to a list of processes. Returns the number of arguments
3239 that were successfully used to signal (which is not necessarily the same
3240 as the number of processes actually killed, e.g. where a process group is
3243 $cnt = kill 'HUP', $child1, $child2;
3244 kill 'KILL', @goners;
3246 SIGNAL may be either a signal name (a string) or a signal number. A signal
3247 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3248 same signal. The string form of SIGNAL is recommended for portability because
3249 the same signal may have different numbers in different operating systems.
3251 A list of signal names supported by the current platform can be found in
3252 C<$Config{sig_name}>, which is provided by the C<Config> module. See L<Config>
3255 A negative signal name is the same as a negative signal number, killing process
3256 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3257 C<kill -9, $pgrp> will send C<SIGKILL> to
3258 the entire process group specified. That
3259 means you usually want to use positive not negative signals.
3261 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3262 no signal is sent to
3263 the process, but C<kill> checks whether it's I<possible> to send a signal to it
3264 (that means, to be brief, that the process is owned by the same user, or we are
3265 the super-user). This is useful to check that a child process is still
3266 alive (even if only as a zombie) and hasn't changed its UID. See
3267 L<perlport> for notes on the portability of this construct.
3269 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3270 the operating system. For example, on POSIX-conforming systems, zero will
3271 signal the current process group, -1 will signal all processes, and any
3272 other negative PROCESS number will act as a negative signal number and
3273 kill the entire process group specified.
3275 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3276 A warning may be produced in a future version.
3278 See L<perlipc/"Signals"> for more details.
3280 On some platforms such as Windows where the fork() system call is not
3281 available, Perl can be built to emulate fork() at the interpreter level.
3282 This emulation has limitations related to kill that have to be considered,
3283 for code running on Windows and in code intended to be portable.
3285 See L<perlfork> for more details.
3287 If there is no I<LIST> of processes, no signal is sent, and the return
3288 value is 0. This form is sometimes used, however, because it causes
3289 tainting checks to be run. But see
3290 L<perlsec/Laundering and Detecting Tainted Data>.
3292 Portability issues: L<perlport/kill>.
3301 =for Pod::Functions exit a block prematurely
3303 The C<last> command is like the C<break> statement in C (as used in
3304 loops); it immediately exits the loop in question. If the LABEL is
3305 omitted, the command refers to the innermost enclosing
3306 loop. The C<last EXPR> form, available starting in Perl
3307 5.18.0, allows a label name to be computed at run time,
3308 and is otherwise identical to C<last LABEL>. The
3309 C<continue> block, if any, is not executed:
3311 LINE: while (<STDIN>) {
3312 last LINE if /^$/; # exit when done with header
3316 C<last> cannot be used to exit a block that returns a value such as
3317 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3318 a grep() or map() operation.
3320 Note that a block by itself is semantically identical to a loop
3321 that executes once. Thus C<last> can be used to effect an early
3322 exit out of such a block.
3324 See also L</continue> for an illustration of how C<last>, C<next>, and
3327 Unlike most named operators, this has the same precedence as assignment.
3328 It is also exempt from the looks-like-a-function rule, so
3329 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3337 =for Pod::Functions return lower-case version of a string
3339 Returns a lowercased version of EXPR. This is the internal function
3340 implementing the C<\L> escape in double-quoted strings.
3342 If EXPR is omitted, uses C<$_>.
3344 What gets returned depends on several factors:
3348 =item If C<use bytes> is in effect:
3350 The results follow ASCII rules. Only the characters C<A-Z> change,
3351 to C<a-z> respectively.
3353 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3355 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3356 rules for the remaining code points (this last can only happen if
3357 the UTF8 flag is also set). See L<perllocale>.
3359 Starting in v5.20, Perl uses full Unicode rules if the locale is
3360 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3361 case changes that cross the 255/256
3362 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3363 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3364 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3365 locale), the lower case of U+1E9E is
3366 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3367 current locale, and Perl has no way of knowing if that character even
3368 exists in the locale, much less what code point it is. Perl returns
3369 a result that is above 255 (almost always the input character unchanged,
3370 for all instances (and there aren't many) where the 255/256 boundary
3371 would otherwise be crossed; and starting in v5.22, it raises a
3372 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3374 =item Otherwise, If EXPR has the UTF8 flag set:
3376 Unicode rules are used for the case change.
3378 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3380 Unicode rules are used for the case change.
3384 ASCII rules are used for the case change. The lowercase of any character
3385 outside the ASCII range is the character itself.
3390 X<lcfirst> X<lowercase>
3394 =for Pod::Functions return a string with just the next letter in lower case
3396 Returns the value of EXPR with the first character lowercased. This
3397 is the internal function implementing the C<\l> escape in
3398 double-quoted strings.
3400 If EXPR is omitted, uses C<$_>.
3402 This function behaves the same way under various pragmata, such as in a locale,
3410 =for Pod::Functions return the number of characters in a string
3412 Returns the length in I<characters> of the value of EXPR. If EXPR is
3413 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3416 This function cannot be used on an entire array or hash to find out how
3417 many elements these have. For that, use C<scalar @array> and C<scalar keys
3418 %hash>, respectively.
3420 Like all Perl character operations, length() normally deals in logical
3421 characters, not physical bytes. For how many bytes a string encoded as
3422 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3423 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3428 =for Pod::Functions the current source line number
3430 A special token that compiles to the current line number.
3432 =item link OLDFILE,NEWFILE
3435 =for Pod::Functions create a hard link in the filesystem
3437 Creates a new filename linked to the old filename. Returns true for
3438 success, false otherwise.
3440 Portability issues: L<perlport/link>.
3442 =item listen SOCKET,QUEUESIZE
3445 =for Pod::Functions register your socket as a server
3447 Does the same thing that the listen(2) system call does. Returns true if
3448 it succeeded, false otherwise. See the example in
3449 L<perlipc/"Sockets: Client/Server Communication">.
3454 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3456 You really probably want to be using C<my> instead, because C<local> isn't
3457 what most people think of as "local". See
3458 L<perlsub/"Private Variables via my()"> for details.
3460 A local modifies the listed variables to be local to the enclosing
3461 block, file, or eval. If more than one value is listed, the list must
3462 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3463 for details, including issues with tied arrays and hashes.
3465 The C<delete local EXPR> construct can also be used to localize the deletion
3466 of array/hash elements to the current block.
3467 See L<perlsub/"Localized deletion of elements of composite types">.
3469 =item localtime EXPR
3470 X<localtime> X<ctime>
3474 =for Pod::Functions convert UNIX time into record or string using local time
3476 Converts a time as returned by the time function to a 9-element list
3477 with the time analyzed for the local time zone. Typically used as
3481 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3484 All list elements are numeric and come straight out of the C `struct
3485 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3486 of the specified time.
3488 C<$mday> is the day of the month and C<$mon> the month in
3489 the range C<0..11>, with 0 indicating January and 11 indicating December.
3490 This makes it easy to get a month name from a list:
3492 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3493 print "$abbr[$mon] $mday";
3494 # $mon=9, $mday=18 gives "Oct 18"
3496 C<$year> contains the number of years since 1900. To get a 4-digit
3501 To get the last two digits of the year (e.g., "01" in 2001) do:
3503 $year = sprintf("%02d", $year % 100);
3505 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3506 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3507 (or C<0..365> in leap years.)
3509 C<$isdst> is true if the specified time occurs during Daylight Saving
3510 Time, false otherwise.
3512 If EXPR is omitted, C<localtime()> uses the current time (as returned
3515 In scalar context, C<localtime()> returns the ctime(3) value:
3517 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3519 The format of this scalar value is B<not> locale-dependent
3520 but built into Perl. For GMT instead of local
3521 time use the L</gmtime> builtin. See also the
3522 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3523 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3524 and mktime(3) functions.
3526 To get somewhat similar but locale-dependent date strings, set up your
3527 locale environment variables appropriately (please see L<perllocale>) and
3530 use POSIX qw(strftime);
3531 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3532 # or for GMT formatted appropriately for your locale:
3533 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3535 Note that the C<%a> and C<%b>, the short forms of the day of the week
3536 and the month of the year, may not necessarily be three characters wide.
3538 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3539 by-name access mechanism to the gmtime() and localtime() functions,
3542 For a comprehensive date and time representation look at the
3543 L<DateTime> module on CPAN.
3545 Portability issues: L<perlport/localtime>.
3550 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3552 This function places an advisory lock on a shared variable or referenced
3553 object contained in I<THING> until the lock goes out of scope.
3555 The value returned is the scalar itself, if the argument is a scalar, or a
3556 reference, if the argument is a hash, array or subroutine.
3558 lock() is a "weak keyword" : this means that if you've defined a function
3559 by this name (before any calls to it), that function will be called
3560 instead. If you are not under C<use threads::shared> this does nothing.
3561 See L<threads::shared>.
3564 X<log> X<logarithm> X<e> X<ln> X<base>
3568 =for Pod::Functions retrieve the natural logarithm for a number
3570 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3571 returns the log of C<$_>. To get the
3572 log of another base, use basic algebra:
3573 The base-N log of a number is equal to the natural log of that number
3574 divided by the natural log of N. For example:
3578 return log($n)/log(10);
3581 See also L</exp> for the inverse operation.
3583 =item lstat FILEHANDLE
3588 =item lstat DIRHANDLE
3592 =for Pod::Functions stat a symbolic link
3594 Does the same thing as the C<stat> function (including setting the
3595 special C<_> filehandle) but stats a symbolic link instead of the file
3596 the symbolic link points to. If symbolic links are unimplemented on
3597 your system, a normal C<stat> is done. For much more detailed
3598 information, please see the documentation for C<stat>.
3600 If EXPR is omitted, stats C<$_>.
3602 Portability issues: L<perlport/lstat>.
3606 =for Pod::Functions match a string with a regular expression pattern
3608 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3610 =item map BLOCK LIST
3615 =for Pod::Functions apply a change to a list to get back a new list with the changes
3617 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3618 C<$_> to each element) and returns the list value composed of the
3619 results of each such evaluation. In scalar context, returns the
3620 total number of elements so generated. Evaluates BLOCK or EXPR in
3621 list context, so each element of LIST may produce zero, one, or
3622 more elements in the returned value.
3624 @chars = map(chr, @numbers);
3626 translates a list of numbers to the corresponding characters.
3628 my @squares = map { $_ * $_ } @numbers;
3630 translates a list of numbers to their squared values.
3632 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3634 shows that number of returned elements can differ from the number of
3635 input elements. To omit an element, return an empty list ().
3636 This could also be achieved by writing
3638 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3640 which makes the intention more clear.
3642 Map always returns a list, which can be
3643 assigned to a hash such that the elements
3644 become key/value pairs. See L<perldata> for more details.
3646 %hash = map { get_a_key_for($_) => $_ } @array;
3648 is just a funny way to write
3652 $hash{get_a_key_for($_)} = $_;
3655 Note that C<$_> is an alias to the list value, so it can be used to
3656 modify the elements of the LIST. While this is useful and supported,
3657 it can cause bizarre results if the elements of LIST are not variables.
3658 Using a regular C<foreach> loop for this purpose would be clearer in
3659 most cases. See also L</grep> for an array composed of those items of
3660 the original list for which the BLOCK or EXPR evaluates to true.
3662 If C<$_> is lexical in the scope where the C<map> appears (because it has
3663 been declared with the deprecated C<my $_> construct),
3664 then, in addition to being locally aliased to
3665 the list elements, C<$_> keeps being lexical inside the block; that is, it
3666 can't be seen from the outside, avoiding any potential side-effects.
3668 C<{> starts both hash references and blocks, so C<map { ...> could be either
3669 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3670 ahead for the closing C<}> it has to take a guess at which it's dealing with
3671 based on what it finds just after the
3672 C<{>. Usually it gets it right, but if it
3673 doesn't it won't realize something is wrong until it gets to the C<}> and
3674 encounters the missing (or unexpected) comma. The syntax error will be
3675 reported close to the C<}>, but you'll need to change something near the C<{>
3676 such as using a unary C<+> or semicolon to give Perl some help:
3678 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3679 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3680 %hash = map {; "\L$_" => 1 } @array # this also works
3681 %hash = map { ("\L$_" => 1) } @array # as does this
3682 %hash = map { lc($_) => 1 } @array # and this.
3683 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3685 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3687 or to force an anon hash constructor use C<+{>:
3689 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3692 to get a list of anonymous hashes each with only one entry apiece.
3694 =item mkdir FILENAME,MASK
3695 X<mkdir> X<md> X<directory, create>
3697 =item mkdir FILENAME
3701 =for Pod::Functions create a directory
3703 Creates the directory specified by FILENAME, with permissions
3704 specified by MASK (as modified by C<umask>). If it succeeds it
3705 returns true; otherwise it returns false and sets C<$!> (errno).
3706 MASK defaults to 0777 if omitted, and FILENAME defaults
3707 to C<$_> if omitted.
3709 In general, it is better to create directories with a permissive MASK
3710 and let the user modify that with their C<umask> than it is to supply
3711 a restrictive MASK and give the user no way to be more permissive.
3712 The exceptions to this rule are when the file or directory should be
3713 kept private (mail files, for instance). The perlfunc(1) entry on
3714 C<umask> discusses the choice of MASK in more detail.
3716 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3717 number of trailing slashes. Some operating and filesystems do not get
3718 this right, so Perl automatically removes all trailing slashes to keep
3721 To recursively create a directory structure, look at
3722 the C<make_path> function of the L<File::Path> module.
3724 =item msgctl ID,CMD,ARG
3727 =for Pod::Functions SysV IPC message control operations
3729 Calls the System V IPC function msgctl(2). You'll probably have to say
3733 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3734 then ARG must be a variable that will hold the returned C<msqid_ds>
3735 structure. Returns like C<ioctl>: the undefined value for error,
3736 C<"0 but true"> for zero, or the actual return value otherwise. See also
3737 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3740 Portability issues: L<perlport/msgctl>.
3742 =item msgget KEY,FLAGS
3745 =for Pod::Functions get SysV IPC message queue
3747 Calls the System V IPC function msgget(2). Returns the message queue
3748 id, or C<undef> on error. See also
3749 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3752 Portability issues: L<perlport/msgget>.
3754 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3757 =for Pod::Functions receive a SysV IPC message from a message queue
3759 Calls the System V IPC function msgrcv to receive a message from
3760 message queue ID into variable VAR with a maximum message size of
3761 SIZE. Note that when a message is received, the message type as a
3762 native long integer will be the first thing in VAR, followed by the
3763 actual message. This packing may be opened with C<unpack("l! a*")>.
3764 Taints the variable. Returns true if successful, false
3765 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3766 C<IPC::SysV> and C<IPC::SysV::Msg>.
3768 Portability issues: L<perlport/msgrcv>.
3770 =item msgsnd ID,MSG,FLAGS
3773 =for Pod::Functions send a SysV IPC message to a message queue
3775 Calls the System V IPC function msgsnd to send the message MSG to the
3776 message queue ID. MSG must begin with the native long integer message
3777 type, be followed by the length of the actual message, and then finally
3778 the message itself. This kind of packing can be achieved with
3779 C<pack("l! a*", $type, $message)>. Returns true if successful,
3780 false on error. See also the C<IPC::SysV>
3781 and C<IPC::SysV::Msg> documentation.
3783 Portability issues: L<perlport/msgsnd>.
3788 =item my TYPE VARLIST
3790 =item my VARLIST : ATTRS
3792 =item my TYPE VARLIST : ATTRS
3794 =for Pod::Functions declare and assign a local variable (lexical scoping)
3796 A C<my> declares the listed variables to be local (lexically) to the
3797 enclosing block, file, or C<eval>. If more than one variable is listed,
3798 the list must be placed in parentheses.
3800 The exact semantics and interface of TYPE and ATTRS are still
3801 evolving. TYPE may be a bareword, a constant declared
3802 with C<use constant>, or C<__PACKAGE__>. It is
3803 currently bound to the use of the C<fields> pragma,
3804 and attributes are handled using the C<attributes> pragma, or starting
3805 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3806 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3807 L<attributes>, and L<Attribute::Handlers>.
3809 Note that with a parenthesised list, C<undef> can be used as a dummy
3810 placeholder, for example to skip assignment of initial values:
3812 my ( undef, $min, $hour ) = localtime;
3821 =for Pod::Functions iterate a block prematurely
3823 The C<next> command is like the C<continue> statement in C; it starts
3824 the next iteration of the loop:
3826 LINE: while (<STDIN>) {
3827 next LINE if /^#/; # discard comments
3831 Note that if there were a C<continue> block on the above, it would get
3832 executed even on discarded lines. If LABEL is omitted, the command
3833 refers to the innermost enclosing loop. The C<next EXPR> form, available
3834 as of Perl 5.18.0, allows a label name to be computed at run time, being
3835 otherwise identical to C<next LABEL>.
3837 C<next> cannot be used to exit a block which returns a value such as
3838 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3839 a grep() or map() operation.
3841 Note that a block by itself is semantically identical to a loop
3842 that executes once. Thus C<next> will exit such a block early.
3844 See also L</continue> for an illustration of how C<last>, C<next>, and
3847 Unlike most named operators, this has the same precedence as assignment.
3848 It is also exempt from the looks-like-a-function rule, so
3849 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3852 =item no MODULE VERSION LIST
3856 =item no MODULE VERSION
3858 =item no MODULE LIST
3864 =for Pod::Functions unimport some module symbols or semantics at compile time
3866 See the C<use> function, of which C<no> is the opposite.
3869 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3873 =for Pod::Functions convert a string to an octal number
3875 Interprets EXPR as an octal string and returns the corresponding
3876 value. (If EXPR happens to start off with C<0x>, interprets it as a
3877 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3878 binary string. Leading whitespace is ignored in all three cases.)
3879 The following will handle decimal, binary, octal, and hex in standard
3882 $val = oct($val) if $val =~ /^0/;
3884 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3885 in octal), use sprintf() or printf():
3887 $dec_perms = (stat("filename"))[2] & 07777;
3888 $oct_perm_str = sprintf "%o", $perms;
3890 The oct() function is commonly used when a string such as C<644> needs
3891 to be converted into a file mode, for example. Although Perl
3892 automatically converts strings into numbers as needed, this automatic
3893 conversion assumes base 10.
3895 Leading white space is ignored without warning, as too are any trailing
3896 non-digits, such as a decimal point (C<oct> only handles non-negative
3897 integers, not negative integers or floating point).
3899 =item open FILEHANDLE,EXPR
3900 X<open> X<pipe> X<file, open> X<fopen>
3902 =item open FILEHANDLE,MODE,EXPR
3904 =item open FILEHANDLE,MODE,EXPR,LIST
3906 =item open FILEHANDLE,MODE,REFERENCE
3908 =item open FILEHANDLE
3910 =for Pod::Functions open a file, pipe, or descriptor
3912 Opens the file whose filename is given by EXPR, and associates it with
3915 Simple examples to open a file for reading:
3917 open(my $fh, "<", "input.txt")
3918 or die "cannot open < input.txt: $!";
3922 open(my $fh, ">", "output.txt")
3923 or die "cannot open > output.txt: $!";
3925 (The following is a comprehensive reference to open(): for a gentler
3926 introduction you may consider L<perlopentut>.)
3928 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3929 new filehandle is autovivified, meaning that the variable is assigned a
3930 reference to a newly allocated anonymous filehandle. Otherwise if
3931 FILEHANDLE is an expression, its value is the real filehandle. (This is
3932 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3935 If three (or more) arguments are specified, the open mode (including
3936 optional encoding) in the second argument are distinct from the filename in
3937 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3938 If MODE is C<< > >>, the file is opened for output, with existing files
3939 first being truncated ("clobbered") and nonexisting files newly created.
3940 If MODE is C<<< >> >>>, the file is opened for appending, again being
3941 created if necessary.
3943 You can put a C<+> in front of the C<< > >> or C<< < >> to
3944 indicate that you want both read and write access to the file; thus
3945 C<< +< >> is almost always preferred for read/write updates--the
3946 C<< +> >> mode would clobber the file first. You can't usually use
3947 either read-write mode for updating textfiles, since they have
3948 variable-length records. See the B<-i> switch in L<perlrun> for a
3949 better approach. The file is created with permissions of C<0666>
3950 modified by the process's C<umask> value.
3952 These various prefixes correspond to the fopen(3) modes of C<r>,
3953 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3955 In the one- and two-argument forms of the call, the mode and filename
3956 should be concatenated (in that order), preferably separated by white
3957 space. You can--but shouldn't--omit the mode in these forms when that mode
3958 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3959 the filename argument is a known literal.
3961 For three or more arguments if MODE is C<|->, the filename is
3962 interpreted as a command to which output is to be piped, and if MODE
3963 is C<-|>, the filename is interpreted as a command that pipes
3964 output to us. In the two-argument (and one-argument) form, one should
3965 replace dash (C<->) with the command.
3966 See L<perlipc/"Using open() for IPC"> for more examples of this.
3967 (You are not allowed to C<open> to a command that pipes both in I<and>
3968 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3969 L<perlipc/"Bidirectional Communication with Another Process"> for
3972 In the form of pipe opens taking three or more arguments, if LIST is specified
3973 (extra arguments after the command name) then LIST becomes arguments
3974 to the command invoked if the platform supports it. The meaning of
3975 C<open> with more than three arguments for non-pipe modes is not yet
3976 defined, but experimental "layers" may give extra LIST arguments
3979 In the two-argument (and one-argument) form, opening C<< <- >>
3980 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3982 You may (and usually should) use the three-argument form of open to specify
3983 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3984 that affect how the input and output are processed (see L<open> and
3985 L<PerlIO> for more details). For example:
3987 open(my $fh, "<:encoding(UTF-8)", "filename")
3988 || die "can't open UTF-8 encoded filename: $!";
3990 opens the UTF8-encoded file containing Unicode characters;
3991 see L<perluniintro>. Note that if layers are specified in the
3992 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3993 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3994 Those layers will also be ignored if you specifying a colon with no name
3995 following it. In that case the default layer for the operating system
3996 (:raw on Unix, :crlf on Windows) is used.
3998 Open returns nonzero on success, the undefined value otherwise. If
3999 the C<open> involved a pipe, the return value happens to be the pid of
4002 If you're running Perl on a system that distinguishes between text
4003 files and binary files, then you should check out L</binmode> for tips
4004 for dealing with this. The key distinction between systems that need
4005 C<binmode> and those that don't is their text file formats. Systems
4006 like Unix, Mac OS, and Plan 9, that end lines with a single
4007 character and encode that character in C as C<"\n"> do not
4008 need C<binmode>. The rest need it.
4010 When opening a file, it's seldom a good idea to continue
4011 if the request failed, so C<open> is frequently used with
4012 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
4013 where you want to format a suitable error message (but there are
4014 modules that can help with that problem)) always check
4015 the return value from opening a file.
4017 The filehandle will be closed when its reference count reaches zero.
4018 If it is a lexically scoped variable declared with C<my>, that usually
4019 means the end of the enclosing scope. However, this automatic close
4020 does not check for errors, so it is better to explicitly close
4021 filehandles, especially those used for writing:
4024 || warn "close failed: $!";
4026 An older style is to use a bareword as the filehandle, as
4028 open(FH, "<", "input.txt")
4029 or die "cannot open < input.txt: $!";
4031 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4032 <FH> >> and so on. Note that it's a global variable, so this form is
4033 not recommended in new code.
4035 As a shortcut a one-argument call takes the filename from the global
4036 scalar variable of the same name as the filehandle:
4039 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4041 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4042 declared with C<my> or C<state>.
4044 As a special case the three-argument form with a read/write mode and the third
4045 argument being C<undef>:
4047 open(my $tmp, "+>", undef) or die ...
4049 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4050 works for symmetry, but you really should consider writing something
4051 to the temporary file first. You will need to seek() to do the
4054 Perl is built using PerlIO by default; Unless you've
4055 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4056 open filehandles directly to Perl scalars via:
4058 open($fh, ">", \$variable) || ..
4060 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4063 open(STDOUT, ">", \$variable)
4064 or die "Can't open STDOUT: $!";
4068 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
4069 # if the open fails, output is discarded
4071 open(my $dbase, "+<", "dbase.mine") # open for update
4072 or die "Can't open 'dbase.mine' for update: $!";
4074 open(my $dbase, "+<dbase.mine") # ditto
4075 or die "Can't open 'dbase.mine' for update: $!";
4077 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4078 or die "Can't start caesar: $!";
4080 open(ARTICLE, "caesar <$article |") # ditto
4081 or die "Can't start caesar: $!";
4083 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4084 or die "Can't start sort: $!";
4087 open(MEMORY, ">", \$var)
4088 or die "Can't open memory file: $!";
4089 print MEMORY "foo!\n"; # output will appear in $var
4091 # process argument list of files along with any includes
4093 foreach $file (@ARGV) {
4094 process($file, "fh00");
4098 my($filename, $input) = @_;
4099 $input++; # this is a string increment
4100 unless (open($input, "<", $filename)) {
4101 print STDERR "Can't open $filename: $!\n";
4106 while (<$input>) { # note use of indirection
4107 if (/^#include "(.*)"/) {
4108 process($1, $input);
4115 See L<perliol> for detailed info on PerlIO.
4117 You may also, in the Bourne shell tradition, specify an EXPR beginning
4118 with C<< >& >>, in which case the rest of the string is interpreted
4119 as the name of a filehandle (or file descriptor, if numeric) to be
4120 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4121 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4122 The mode you specify should match the mode of the original filehandle.
4123 (Duping a filehandle does not take into account any existing contents
4124 of IO buffers.) If you use the three-argument
4125 form, then you can pass either a
4126 number, the name of a filehandle, or the normal "reference to a glob".
4128 Here is a script that saves, redirects, and restores C<STDOUT> and
4129 C<STDERR> using various methods:
4132 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4133 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4135 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4136 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4138 select STDERR; $| = 1; # make unbuffered
4139 select STDOUT; $| = 1; # make unbuffered
4141 print STDOUT "stdout 1\n"; # this works for
4142 print STDERR "stderr 1\n"; # subprocesses too
4144 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4145 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4147 print STDOUT "stdout 2\n";
4148 print STDERR "stderr 2\n";
4150 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4151 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4152 that file descriptor (and not call C<dup(2)>); this is more
4153 parsimonious of file descriptors. For example:
4155 # open for input, reusing the fileno of $fd
4156 open(FILEHANDLE, "<&=$fd")
4160 open(FILEHANDLE, "<&=", $fd)
4164 # open for append, using the fileno of OLDFH
4165 open(FH, ">>&=", OLDFH)
4169 open(FH, ">>&=OLDFH")
4171 Being parsimonious on filehandles is also useful (besides being
4172 parsimonious) for example when something is dependent on file
4173 descriptors, like for example locking using flock(). If you do just
4174 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4175 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4176 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4177 the same underlying system file descriptor.
4179 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4180 fdopen() to implement the C<=> functionality. On many Unix systems,
4181 fdopen() fails when file descriptors exceed a certain value, typically 255.
4182 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4184 You can see whether your Perl was built with PerlIO by running C<perl -V>
4185 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4186 have PerlIO; otherwise you don't.
4188 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4189 with the one- or two-argument forms of C<open>),
4190 an implicit C<fork> is done, so C<open> returns twice: in the parent
4191 process it returns the pid
4192 of the child process, and in the child process it returns (a defined) C<0>.
4193 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4195 For example, use either
4197 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4201 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4207 # either write TO_KID or else read FROM_KID
4209 waitpid $child_pid, 0;
4211 # am the child; use STDIN/STDOUT normally
4216 The filehandle behaves normally for the parent, but I/O to that
4217 filehandle is piped from/to the STDOUT/STDIN of the child process.
4218 In the child process, the filehandle isn't opened--I/O happens from/to
4219 the new STDOUT/STDIN. Typically this is used like the normal
4220 piped open when you want to exercise more control over just how the
4221 pipe command gets executed, such as when running setuid and
4222 you don't want to have to scan shell commands for metacharacters.
4224 The following blocks are more or less equivalent:
4226 open(FOO, "|tr '[a-z]' '[A-Z]'");
4227 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4228 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4229 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4231 open(FOO, "cat -n '$file'|");
4232 open(FOO, "-|", "cat -n '$file'");
4233 open(FOO, "-|") || exec "cat", "-n", $file;
4234 open(FOO, "-|", "cat", "-n", $file);
4236 The last two examples in each block show the pipe as "list form", which is
4237 not yet supported on all platforms. A good rule of thumb is that if
4238 your platform has a real C<fork()> (in other words, if your platform is
4239 Unix, including Linux and MacOS X), you can use the list form. You would
4240 want to use the list form of the pipe so you can pass literal arguments
4241 to the command without risk of the shell interpreting any shell metacharacters
4242 in them. However, this also bars you from opening pipes to commands
4243 that intentionally contain shell metacharacters, such as:
4245 open(FOO, "|cat -n | expand -4 | lpr")
4246 // die "Can't open pipeline to lpr: $!";
4248 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4250 Perl will attempt to flush all files opened for
4251 output before any operation that may do a fork, but this may not be
4252 supported on some platforms (see L<perlport>). To be safe, you may need
4253 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4254 of C<IO::Handle> on any open handles.
4256 On systems that support a close-on-exec flag on files, the flag will
4257 be set for the newly opened file descriptor as determined by the value
4258 of C<$^F>. See L<perlvar/$^F>.
4260 Closing any piped filehandle causes the parent process to wait for the
4261 child to finish, then returns the status value in C<$?> and
4262 C<${^CHILD_ERROR_NATIVE}>.
4264 The filename passed to the one- and two-argument forms of open() will
4265 have leading and trailing whitespace deleted and normal
4266 redirection characters honored. This property, known as "magic open",
4267 can often be used to good effect. A user could specify a filename of
4268 F<"rsh cat file |">, or you could change certain filenames as needed:
4270 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4271 open(FH, $filename) or die "Can't open $filename: $!";
4273 Use the three-argument form to open a file with arbitrary weird characters in it,
4275 open(FOO, "<", $file)
4276 || die "can't open < $file: $!";
4278 otherwise it's necessary to protect any leading and trailing whitespace:
4280 $file =~ s#^(\s)#./$1#;
4281 open(FOO, "< $file\0")
4282 || die "open failed: $!";
4284 (this may not work on some bizarre filesystems). One should
4285 conscientiously choose between the I<magic> and I<three-argument> form
4288 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4290 will allow the user to specify an argument of the form C<"rsh cat file |">,
4291 but will not work on a filename that happens to have a trailing space, while
4293 open(IN, "<", $ARGV[0])
4294 || die "can't open < $ARGV[0]: $!";
4296 will have exactly the opposite restrictions.
4298 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4299 should use the C<sysopen> function, which involves no such magic (but may
4300 use subtly different filemodes than Perl open(), which is mapped to C
4301 fopen()). This is another way to protect your filenames from
4302 interpretation. For example:
4305 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4306 or die "sysopen $path: $!";
4307 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4308 print HANDLE "stuff $$\n";
4310 print "File contains: ", <HANDLE>;
4312 See L</seek> for some details about mixing reading and writing.
4314 Portability issues: L<perlport/open>.
4316 =item opendir DIRHANDLE,EXPR
4319 =for Pod::Functions open a directory
4321 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4322 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4323 DIRHANDLE may be an expression whose value can be used as an indirect
4324 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4325 scalar variable (or array or hash element), the variable is assigned a
4326 reference to a new anonymous dirhandle; that is, it's autovivified.
4327 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4329 See the example at C<readdir>.
4336 =for Pod::Functions find a character's numeric representation
4338 Returns the numeric value of the first character of EXPR.
4339 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4340 (Note I<character>, not byte.)
4342 For the reverse, see L</chr>.
4343 See L<perlunicode> for more about Unicode.
4348 =item our TYPE VARLIST
4350 =item our VARLIST : ATTRS
4352 =item our TYPE VARLIST : ATTRS
4354 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4356 C<our> makes a lexical alias to a package (i.e. global) variable of the
4357 same name in the current package for use within the current lexical scope.
4359 C<our> has the same scoping rules as C<my> or C<state>, meaning that it is
4360 only valid within a lexical scope. Unlike C<my> and C<state>, which both
4361 declare new (lexical) variables, C<our> only creates an alias to an
4362 existing variable: a package variable of the same name.
4364 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4365 a package variable without qualifying it with the package name, but only within
4366 the lexical scope of the C<our>
4367 declaration. This applies immediately--even
4368 within the same statement.
4376 our $foo; # alias to $Foo::foo
4377 print $foo; # prints 23
4380 print $Foo::foo; # prints 23
4382 print $foo; # ERROR: requires explicit package name
4384 This works even if the package variable has not been used before, as
4385 package variables spring into existence when first used.
4390 our $foo = 23; # just like $Foo::foo = 23
4392 print $Foo::foo; # prints 23
4394 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4395 long as there is no variable with that name is already in scope, you can then
4396 reference the package variable again even within the same statement.
4401 my $foo = $foo; # error, undeclared $foo on right-hand side
4402 our $foo = $foo; # no errors
4404 If more than one variable is listed, the list must be placed
4409 An C<our> declaration declares an alias for a package variable that will be visible
4410 across its entire lexical scope, even across package boundaries. The
4411 package in which the variable is entered is determined at the point
4412 of the declaration, not at the point of use. This means the following
4416 our $bar; # declares $Foo::bar for rest of lexical scope
4420 print $bar; # prints 20, as it refers to $Foo::bar
4422 Multiple C<our> declarations with the same name in the same lexical
4423 scope are allowed if they are in different packages. If they happen
4424 to be in the same package, Perl will emit warnings if you have asked
4425 for them, just like multiple C<my> declarations. Unlike a second
4426 C<my> declaration, which will bind the name to a fresh variable, a
4427 second C<our> declaration in the same package, in the same scope, is
4432 our $bar; # declares $Foo::bar for rest of lexical scope
4436 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4437 print $bar; # prints 30
4439 our $bar; # emits warning but has no other effect
4440 print $bar; # still prints 30
4442 An C<our> declaration may also have a list of attributes associated
4445 The exact semantics and interface of TYPE and ATTRS are still
4446 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4447 and attributes are handled using the C<attributes> pragma, or, starting
4448 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4449 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4450 L<attributes>, and L<Attribute::Handlers>.
4452 Note that with a parenthesised list, C<undef> can be used as a dummy
4453 placeholder, for example to skip assignment of initial values:
4455 our ( undef, $min, $hour ) = localtime;
4457 C<our> differs from C<use vars>, which allows use of an unqualified name
4458 I<only> within the affected package, but across scopes.
4460 =item pack TEMPLATE,LIST
4463 =for Pod::Functions convert a list into a binary representation
4465 Takes a LIST of values and converts it into a string using the rules
4466 given by the TEMPLATE. The resulting string is the concatenation of
4467 the converted values. Typically, each converted value looks
4468 like its machine-level representation. For example, on 32-bit machines
4469 an integer may be represented by a sequence of 4 bytes, which will in
4470 Perl be presented as a string that's 4 characters long.
4472 See L<perlpacktut> for an introduction to this function.
4474 The TEMPLATE is a sequence of characters that give the order and type
4475 of values, as follows:
4477 a A string with arbitrary binary data, will be null padded.
4478 A A text (ASCII) string, will be space padded.
4479 Z A null-terminated (ASCIZ) string, will be null padded.
4481 b A bit string (ascending bit order inside each byte,
4483 B A bit string (descending bit order inside each byte).
4484 h A hex string (low nybble first).
4485 H A hex string (high nybble first).
4487 c A signed char (8-bit) value.
4488 C An unsigned char (octet) value.
4489 W An unsigned char value (can be greater than 255).
4491 s A signed short (16-bit) value.
4492 S An unsigned short value.
4494 l A signed long (32-bit) value.
4495 L An unsigned long value.
4497 q A signed quad (64-bit) value.
4498 Q An unsigned quad value.
4499 (Quads are available only if your system supports 64-bit
4500 integer values _and_ if Perl has been compiled to support
4501 those. Raises an exception otherwise.)
4503 i A signed integer value.
4504 I A unsigned integer value.
4505 (This 'integer' is _at_least_ 32 bits wide. Its exact
4506 size depends on what a local C compiler calls 'int'.)
4508 n An unsigned short (16-bit) in "network" (big-endian) order.
4509 N An unsigned long (32-bit) in "network" (big-endian) order.
4510 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4511 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4513 j A Perl internal signed integer value (IV).
4514 J A Perl internal unsigned integer value (UV).
4516 f A single-precision float in native format.
4517 d A double-precision float in native format.
4519 F A Perl internal floating-point value (NV) in native format
4520 D A float of long-double precision in native format.
4521 (Long doubles are available only if your system supports
4522 long double values _and_ if Perl has been compiled to
4523 support those. Raises an exception otherwise.
4524 Note that there are different long double formats.)
4526 p A pointer to a null-terminated string.
4527 P A pointer to a structure (fixed-length string).
4529 u A uuencoded string.
4530 U A Unicode character number. Encodes to a character in char-
4531 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4534 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4535 for details). Its bytes represent an unsigned integer in
4536 base 128, most significant digit first, with as few digits
4537 as possible. Bit eight (the high bit) is set on each byte
4540 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4542 @ Null-fill or truncate to absolute position, counted from the
4543 start of the innermost ()-group.
4544 . Null-fill or truncate to absolute position specified by
4546 ( Start of a ()-group.
4548 One or more modifiers below may optionally follow certain letters in the
4549 TEMPLATE (the second column lists letters for which the modifier is valid):
4551 ! sSlLiI Forces native (short, long, int) sizes instead
4552 of fixed (16-/32-bit) sizes.
4554 ! xX Make x and X act as alignment commands.
4556 ! nNvV Treat integers as signed instead of unsigned.
4558 ! @. Specify position as byte offset in the internal
4559 representation of the packed string. Efficient
4562 > sSiIlLqQ Force big-endian byte-order on the type.
4563 jJfFdDpP (The "big end" touches the construct.)
4565 < sSiIlLqQ Force little-endian byte-order on the type.
4566 jJfFdDpP (The "little end" touches the construct.)
4568 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4569 to force a particular byte-order on all components in that group,
4570 including all its subgroups.
4574 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4575 pack for processing data from NASA's Magellan probe. Magellan was in an
4576 elliptical orbit, using the antenna for the radar mapping when close to
4577 Venus and for communicating data back to Earth for the rest of the orbit.
4578 There were two transmission units, but one of these failed, and then the
4579 other developed a fault whereby it would randomly flip the sense of all the
4580 bits. It was easy to automatically detect complete records with the correct
4581 sense, and complete records with all the bits flipped. However, this didn't
4582 recover the records where the sense flipped midway. A colleague of Larry's
4583 was able to pretty much eyeball where the records flipped, so they wrote an
4584 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4585 manually correct the records and recover the data. For this purpose pack
4586 gained the hex and bit string format specifiers.
4588 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4589 27e2fb84680b9cc1), but the patch description makes no mention of their
4590 addition, let alone the story behind them.
4594 The following rules apply:
4600 Each letter may optionally be followed by a number indicating the repeat
4601 count. A numeric repeat count may optionally be enclosed in brackets, as
4602 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4603 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4604 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4605 something else, described below. Supplying a C<*> for the repeat count
4606 instead of a number means to use however many items are left, except for:
4612 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4616 <.>, where it means relative to the start of the string.
4620 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4624 One can replace a numeric repeat count with a template letter enclosed in
4625 brackets to use the packed byte length of the bracketed template for the
4628 For example, the template C<x[L]> skips as many bytes as in a packed long,
4629 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4630 variable-expanded) unpacks. If the template in brackets contains alignment
4631 commands (such as C<x![d]>), its packed length is calculated as if the
4632 start of the template had the maximal possible alignment.
4634 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4635 trailing null byte, so the resulting string is always one byte longer than
4636 the byte length of the item itself.
4638 When used with C<@>, the repeat count represents an offset from the start
4639 of the innermost C<()> group.
4641 When used with C<.>, the repeat count determines the starting position to
4642 calculate the value offset as follows:
4648 If the repeat count is C<0>, it's relative to the current position.
4652 If the repeat count is C<*>, the offset is relative to the start of the
4657 And if it's an integer I<n>, the offset is relative to the start of the
4658 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4659 bigger then the group level.
4663 The repeat count for C<u> is interpreted as the maximal number of bytes
4664 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4665 count should not be more than 65.
4669 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4670 string of length count, padding with nulls or spaces as needed. When
4671 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4672 after the first null, and C<a> returns data with no stripping at all.
4674 If the value to pack is too long, the result is truncated. If it's too
4675 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4676 followed by a null byte. Thus C<Z> always packs a trailing null, except
4677 when the count is 0.
4681 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4682 Each such format generates 1 bit of the result. These are typically followed
4683 by a repeat count like C<B8> or C<B64>.
4685 Each result bit is based on the least-significant bit of the corresponding
4686 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4687 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4689 Starting from the beginning of the input string, each 8-tuple
4690 of characters is converted to 1 character of output. With format C<b>,
4691 the first character of the 8-tuple determines the least-significant bit of a
4692 character; with format C<B>, it determines the most-significant bit of
4695 If the length of the input string is not evenly divisible by 8, the
4696 remainder is packed as if the input string were padded by null characters
4697 at the end. Similarly during unpacking, "extra" bits are ignored.
4699 If the input string is longer than needed, remaining characters are ignored.
4701 A C<*> for the repeat count uses all characters of the input field.
4702 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4706 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4707 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4709 For each such format, pack() generates 4 bits of result.
4710 With non-alphabetical characters, the result is based on the 4 least-significant
4711 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4712 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4713 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4714 is compatible with the usual hexadecimal digits, so that C<"a"> and
4715 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4716 characters with this format.
4718 Starting from the beginning of the template to pack(), each pair
4719 of characters is converted to 1 character of output. With format C<h>, the
4720 first character of the pair determines the least-significant nybble of the
4721 output character; with format C<H>, it determines the most-significant
4724 If the length of the input string is not even, it behaves as if padded by
4725 a null character at the end. Similarly, "extra" nybbles are ignored during
4728 If the input string is longer than needed, extra characters are ignored.
4730 A C<*> for the repeat count uses all characters of the input field. For
4731 unpack(), nybbles are converted to a string of hexadecimal digits.
4735 The C<p> format packs a pointer to a null-terminated string. You are
4736 responsible for ensuring that the string is not a temporary value, as that
4737 could potentially get deallocated before you got around to using the packed
4738 result. The C<P> format packs a pointer to a structure of the size indicated
4739 by the length. A null pointer is created if the corresponding value for
4740 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4741 unpacks into C<undef>.
4743 If your system has a strange pointer size--meaning a pointer is neither as
4744 big as an int nor as big as a long--it may not be possible to pack or
4745 unpack pointers in big- or little-endian byte order. Attempting to do
4746 so raises an exception.
4750 The C</> template character allows packing and unpacking of a sequence of
4751 items where the packed structure contains a packed item count followed by
4752 the packed items themselves. This is useful when the structure you're
4753 unpacking has encoded the sizes or repeat counts for some of its fields
4754 within the structure itself as separate fields.
4756 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4757 I<length-item> describes how the length value is packed. Formats likely
4758 to be of most use are integer-packing ones like C<n> for Java strings,
4759 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4761 For C<pack>, I<sequence-item> may have a repeat count, in which case
4762 the minimum of that and the number of available items is used as the argument
4763 for I<length-item>. If it has no repeat count or uses a '*', the number
4764 of available items is used.
4766 For C<unpack>, an internal stack of integer arguments unpacked so far is
4767 used. You write C</>I<sequence-item> and the repeat count is obtained by
4768 popping off the last element from the stack. The I<sequence-item> must not
4769 have a repeat count.
4771 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4772 the I<length-item> is the string length, not the number of strings. With
4773 an explicit repeat count for pack, the packed string is adjusted to that
4774 length. For example:
4776 This code: gives this result:
4778 unpack("W/a", "\004Gurusamy") ("Guru")
4779 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4780 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4782 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4783 pack("a/W2", ord("a") .. ord("z")) "2ab"
4785 The I<length-item> is not returned explicitly from C<unpack>.
4787 Supplying a count to the I<length-item> format letter is only useful with
4788 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4789 introduce C<"\000"> characters, which Perl does not regard as legal in
4794 The integer types C<s>, C<S>, C<l>, and C<L> may be
4795 followed by a C<!> modifier to specify native shorts or
4796 longs. As shown in the example above, a bare C<l> means
4797 exactly 32 bits, although the native C<long> as seen by the local C compiler
4798 may be larger. This is mainly an issue on 64-bit platforms. You can
4799 see whether using C<!> makes any difference this way:
4801 printf "format s is %d, s! is %d\n",
4802 length pack("s"), length pack("s!");
4804 printf "format l is %d, l! is %d\n",
4805 length pack("l"), length pack("l!");
4808 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4809 they are identical to C<i> and C<I>.
4811 The actual sizes (in bytes) of native shorts, ints, longs, and long
4812 longs on the platform where Perl was built are also available from
4815 $ perl -V:{short,int,long{,long}}size
4821 or programmatically via the C<Config> module:
4824 print $Config{shortsize}, "\n";
4825 print $Config{intsize}, "\n";
4826 print $Config{longsize}, "\n";
4827 print $Config{longlongsize}, "\n";
4829 C<$Config{longlongsize}> is undefined on systems without
4834 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4835 inherently non-portable between processors and operating systems because
4836 they obey native byteorder and endianness. For example, a 4-byte integer
4837 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4838 handled by the CPU registers) into bytes as
4840 0x12 0x34 0x56 0x78 # big-endian
4841 0x78 0x56 0x34 0x12 # little-endian
4843 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4844 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4845 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4846 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4848 The names I<big-endian> and I<little-endian> are comic references to the
4849 egg-eating habits of the little-endian Lilliputians and the big-endian
4850 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4851 This entered computer lingo via the paper "On Holy Wars and a Plea for
4852 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4854 Some systems may have even weirder byte orders such as
4859 These are called mid-endian, middle-endian, mixed-endian, or just weird.
4861 You can determine your system endianness with this incantation:
4863 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4865 The byteorder on the platform where Perl was built is also available
4869 print "$Config{byteorder}\n";
4871 or from the command line:
4875 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4876 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
4877 will have C<"ffff">, signifying that static information doesn't work,
4878 one must use runtime probing.
4880 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4881 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4882 immediately below. See also L<perlport>.
4886 Also floating point numbers have endianness. Usually (but not always)
4887 this agrees with the integer endianness. Even though most platforms
4888 these days use the IEEE 754 binary format, there are differences,
4889 especially if the long doubles are involved. You can see the
4890 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
4891 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
4893 Portability-wise the best option is probably to keep to the IEEE 754
4894 64-bit doubles, and of agreed-upon endianness. Another possibility
4895 is the C<"%a">) format of C<printf>.
4899 Starting with Perl 5.10.0, integer and floating-point formats, along with
4900 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4901 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4902 or little-endian byte-order. These modifiers are especially useful
4903 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4904 64-bit integers, or floating-point values.
4906 Here are some concerns to keep in mind when using an endianness modifier:
4912 Exchanging signed integers between different platforms works only
4913 when all platforms store them in the same format. Most platforms store
4914 signed integers in two's-complement notation, so usually this is not an issue.
4918 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4919 formats on big- or little-endian machines. Otherwise, attempting to
4920 use them raises an exception.
4924 Forcing big- or little-endian byte-order on floating-point values for
4925 data exchange can work only if all platforms use the same
4926 binary representation such as IEEE floating-point. Even if all
4927 platforms are using IEEE, there may still be subtle differences. Being able
4928 to use C<< > >> or C<< < >> on floating-point values can be useful,
4929 but also dangerous if you don't know exactly what you're doing.
4930 It is not a general way to portably store floating-point values.
4934 When using C<< > >> or C<< < >> on a C<()> group, this affects
4935 all types inside the group that accept byte-order modifiers,
4936 including all subgroups. It is silently ignored for all other
4937 types. You are not allowed to override the byte-order within a group
4938 that already has a byte-order modifier suffix.
4944 Real numbers (floats and doubles) are in native machine format only.
4945 Due to the multiplicity of floating-point formats and the lack of a
4946 standard "network" representation for them, no facility for interchange has been
4947 made. This means that packed floating-point data written on one machine
4948 may not be readable on another, even if both use IEEE floating-point
4949 arithmetic (because the endianness of the memory representation is not part
4950 of the IEEE spec). See also L<perlport>.
4952 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4953 modifiers to force big- or little-endian byte-order on floating-point values.
4955 Because Perl uses doubles (or long doubles, if configured) internally for
4956 all numeric calculation, converting from double into float and thence
4957 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4958 will not in general equal $foo.
4962 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4963 the packed string is processed per character, and UTF-8 byte mode (C<U0> mode)
4964 where the packed string is processed in its UTF-8-encoded Unicode form on
4965 a byte-by-byte basis. Character mode is the default
4966 unless the format string starts with C<U>. You
4967 can always switch mode mid-format with an explicit
4968 C<C0> or C<U0> in the format. This mode remains in effect until the next
4969 mode change, or until the end of the C<()> group it (directly) applies to.
4971 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4972 bytes is not necessarily obvious. Probably only the first of these
4975 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4976 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4978 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4979 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4981 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4982 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4984 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4985 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4986 C3.8E.C2.B1.C3.8F.C2.89
4988 Those examples also illustrate that you should not try to use
4989 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4993 You must yourself do any alignment or padding by inserting, for example,
4994 enough C<"x">es while packing. There is no way for pack() and unpack()
4995 to know where characters are going to or coming from, so they
4996 handle their output and input as flat sequences of characters.
5000 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5001 take a repeat count either as postfix, or for unpack(), also via the C</>
5002 template character. Within each repetition of a group, positioning with
5003 C<@> starts over at 0. Therefore, the result of
5005 pack("@1A((@2A)@3A)", qw[X Y Z])
5007 is the string C<"\0X\0\0YZ">.
5011 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5012 jump forward or back to the closest position aligned at a multiple of C<count>
5013 characters. For example, to pack() or unpack() a C structure like
5016 char c; /* one signed, 8-bit character */
5021 one may need to use the template C<c x![d] d c[2]>. This assumes that
5022 doubles must be aligned to the size of double.
5024 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5029 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5030 represent signed 16-/32-bit integers in big-/little-endian order.
5031 This is portable only when all platforms sharing packed data use the
5032 same binary representation for signed integers; for example, when all
5033 platforms use two's-complement representation.
5037 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5038 White space can separate pack codes from each other, but modifiers and
5039 repeat counts must follow immediately. Breaking complex templates into
5040 individual line-by-line components, suitably annotated, can do as much to
5041 improve legibility and maintainability of pack/unpack formats as C</x> can
5042 for complicated pattern matches.
5046 If TEMPLATE requires more arguments than pack() is given, pack()
5047 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5048 than given, extra arguments are ignored.
5052 Attempting to pack the special floating point values C<Inf> and C<NaN>
5053 (infinity, also in negative, and not-a-number) into packed integer values
5054 (like C<"L">) is a fatal error. The reason for this is that there simply
5055 isn't any sensible mapping for these special values into integers.
5061 $foo = pack("WWWW",65,66,67,68);
5063 $foo = pack("W4",65,66,67,68);
5065 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5066 # same thing with Unicode circled letters.
5067 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5068 # same thing with Unicode circled letters. You don't get the
5069 # UTF-8 bytes because the U at the start of the format caused
5070 # a switch to U0-mode, so the UTF-8 bytes get joined into
5072 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5073 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5074 # This is the UTF-8 encoding of the string in the
5077 $foo = pack("ccxxcc",65,66,67,68);
5080 # NOTE: The examples above featuring "W" and "c" are true
5081 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5082 # and UTF-8. On EBCDIC systems, the first example would be
5083 # $foo = pack("WWWW",193,194,195,196);
5085 $foo = pack("s2",1,2);
5086 # "\001\000\002\000" on little-endian
5087 # "\000\001\000\002" on big-endian
5089 $foo = pack("a4","abcd","x","y","z");
5092 $foo = pack("aaaa","abcd","x","y","z");
5095 $foo = pack("a14","abcdefg");
5096 # "abcdefg\0\0\0\0\0\0\0"
5098 $foo = pack("i9pl", gmtime);
5099 # a real struct tm (on my system anyway)
5101 $utmp_template = "Z8 Z8 Z16 L";
5102 $utmp = pack($utmp_template, @utmp1);
5103 # a struct utmp (BSDish)
5105 @utmp2 = unpack($utmp_template, $utmp);
5106 # "@utmp1" eq "@utmp2"
5109 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5112 $foo = pack('sx2l', 12, 34);
5113 # short 12, two zero bytes padding, long 34
5114 $bar = pack('s@4l', 12, 34);
5115 # short 12, zero fill to position 4, long 34
5117 $baz = pack('s.l', 12, 4, 34);
5118 # short 12, zero fill to position 4, long 34
5120 $foo = pack('nN', 42, 4711);
5121 # pack big-endian 16- and 32-bit unsigned integers
5122 $foo = pack('S>L>', 42, 4711);
5124 $foo = pack('s<l<', -42, 4711);
5125 # pack little-endian 16- and 32-bit signed integers
5126 $foo = pack('(sl)<', -42, 4711);
5129 The same template may generally also be used in unpack().
5131 =item package NAMESPACE
5133 =item package NAMESPACE VERSION
5134 X<package> X<module> X<namespace> X<version>
5136 =item package NAMESPACE BLOCK
5138 =item package NAMESPACE VERSION BLOCK
5139 X<package> X<module> X<namespace> X<version>
5141 =for Pod::Functions declare a separate global namespace
5143 Declares the BLOCK or the rest of the compilation unit as being in the
5144 given namespace. The scope of the package declaration is either the
5145 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5146 itself through the end of current scope (the enclosing block, file, or
5147 C<eval>). That is, the forms without a BLOCK are operative through the end
5148 of the current scope, just like the C<my>, C<state>, and C<our> operators.
5149 All unqualified dynamic identifiers in this scope will be in the given
5150 namespace, except where overridden by another C<package> declaration or
5151 when they're one of the special identifiers that qualify into C<main::>,
5152 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5154 A package statement affects dynamic variables only, including those
5155 you've used C<local> on, but I<not> lexically-scoped variables, which are created
5156 with C<my>, C<state>, or C<our>. Typically it would be the first
5157 declaration in a file included by C<require> or C<use>. You can switch into a
5158 package in more than one place, since this only determines which default
5159 symbol table the compiler uses for the rest of that block. You can refer to
5160 identifiers in other packages than the current one by prefixing the identifier
5161 with the package name and a double colon, as in C<$SomePack::var>
5162 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5163 package as assumed. That is, C<$::sail> is equivalent to
5164 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5165 code, mostly from Perl 4).
5167 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
5168 namespace to a L<version> object with the VERSION provided. VERSION must be a
5169 "strict" style version number as defined by the L<version> module: a positive
5170 decimal number (integer or decimal-fraction) without exponentiation or else a
5171 dotted-decimal v-string with a leading 'v' character and at least three
5172 components. You should set C<$VERSION> only once per package.
5174 See L<perlmod/"Packages"> for more information about packages, modules,
5175 and classes. See L<perlsub> for other scoping issues.
5180 =for Pod::Functions +5.004 the current package
5182 A special token that returns the name of the package in which it occurs.
5184 =item pipe READHANDLE,WRITEHANDLE
5187 =for Pod::Functions open a pair of connected filehandles
5189 Opens a pair of connected pipes like the corresponding system call.
5190 Note that if you set up a loop of piped processes, deadlock can occur
5191 unless you are very careful. In addition, note that Perl's pipes use
5192 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5193 after each command, depending on the application.
5195 Returns true on success.
5197 See L<IPC::Open2>, L<IPC::Open3>, and
5198 L<perlipc/"Bidirectional Communication with Another Process">
5199 for examples of such things.
5201 On systems that support a close-on-exec flag on files, that flag is set
5202 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5203 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5210 =for Pod::Functions remove the last element from an array and return it
5212 Pops and returns the last value of the array, shortening the array by
5215 Returns the undefined value if the array is empty, although this may also
5216 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5217 main program, but the C<@_> array in subroutines, just like C<shift>.
5219 Starting with Perl 5.14, an experimental feature allowed C<pop> to take a
5220 scalar expression. This experiment has been deemed unsuccessful, and was
5221 removed as of Perl 5.24.
5224 X<pos> X<match, position>
5228 =for Pod::Functions find or set the offset for the last/next m//g search
5230 Returns the offset of where the last C<m//g> search left off for the
5231 variable in question (C<$_> is used when the variable is not
5232 specified). Note that 0 is a valid match offset. C<undef> indicates
5233 that the search position is reset (usually due to match failure, but
5234 can also be because no match has yet been run on the scalar).
5236 C<pos> directly accesses the location used by the regexp engine to
5237 store the offset, so assigning to C<pos> will change that offset, and
5238 so will also influence the C<\G> zero-width assertion in regular
5239 expressions. Both of these effects take place for the next match, so
5240 you can't affect the position with C<pos> during the current match,
5241 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5243 Setting C<pos> also resets the I<matched with zero-length> flag, described
5244 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5246 Because a failed C<m//gc> match doesn't reset the offset, the return
5247 from C<pos> won't change either in this case. See L<perlre> and
5250 =item print FILEHANDLE LIST
5253 =item print FILEHANDLE
5259 =for Pod::Functions output a list to a filehandle
5261 Prints a string or a list of strings. Returns true if successful.
5262 FILEHANDLE may be a scalar variable containing the name of or a reference
5263 to the filehandle, thus introducing one level of indirection. (NOTE: If
5264 FILEHANDLE is a variable and the next token is a term, it may be
5265 misinterpreted as an operator unless you interpose a C<+> or put
5266 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5267 last selected (see L</select>) output handle. If LIST is omitted, prints
5268 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5269 print the content of C<$_> to it, you must use a real filehandle like
5270 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5271 to something other than STDOUT, use the select operation.
5273 The current value of C<$,> (if any) is printed between each LIST item. The
5274 current value of C<$\> (if any) is printed after the entire LIST has been
5275 printed. Because print takes a LIST, anything in the LIST is evaluated in
5276 list context, including any subroutines whose return lists you pass to
5277 C<print>. Be careful not to follow the print keyword with a left
5278 parenthesis unless you want the corresponding right parenthesis to
5279 terminate the arguments to the print; put parentheses around all arguments
5280 (or interpose a C<+>, but that doesn't look as good).
5282 If you're storing handles in an array or hash, or in general whenever
5283 you're using any expression more complex than a bareword handle or a plain,
5284 unsubscripted scalar variable to retrieve it, you will have to use a block
5285 returning the filehandle value instead, in which case the LIST may not be
5288 print { $files[$i] } "stuff\n";
5289 print { $OK ? STDOUT : STDERR } "stuff\n";
5291 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5292 L<perlipc> for more on signal handling.
5294 =item printf FILEHANDLE FORMAT, LIST
5297 =item printf FILEHANDLE
5299 =item printf FORMAT, LIST
5303 =for Pod::Functions output a formatted list to a filehandle
5305 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5306 (the output record separator) is not appended. The FORMAT and the
5307 LIST are actually parsed as a single list. The first argument
5308 of the list will be interpreted as the C<printf> format. This
5309 means that C<printf(@_)> will use C<$_[0]> as the format. See
5310 L<sprintf|/sprintf FORMAT, LIST> for an
5311 explanation of the format argument. If C<use locale> for C<LC_NUMERIC>
5312 Look for this throught pod
5314 POSIX::setlocale() has been called, the character used for the decimal
5315 separator in formatted floating-point numbers is affected by the C<LC_NUMERIC>
5316 locale setting. See L<perllocale> and L<POSIX>.
5318 For historical reasons, if you omit the list, C<$_> is used as the format;
5319 to use FILEHANDLE without a list, you must use a real filehandle like
5320 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5321 you want; if $_ contains formatting codes, they will be replaced with the
5322 empty string and a warning will be emitted if warnings are enabled. Just
5323 use C<print> if you want to print the contents of $_.
5325 Don't fall into the trap of using a C<printf> when a simple
5326 C<print> would do. The C<print> is more efficient and less
5329 =item prototype FUNCTION
5334 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5336 Returns the prototype of a function as a string (or C<undef> if the
5337 function has no prototype). FUNCTION is a reference to, or the name of,
5338 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5341 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5342 name for a Perl builtin. If the builtin's arguments
5343 cannot be adequately expressed by a prototype
5344 (such as C<system>), prototype() returns C<undef>, because the builtin
5345 does not really behave like a Perl function. Otherwise, the string
5346 describing the equivalent prototype is returned.
5348 =item push ARRAY,LIST
5351 =for Pod::Functions append one or more elements to an array
5353 Treats ARRAY as a stack by appending the values of LIST to the end of
5354 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5358 $ARRAY[++$#ARRAY] = $value;
5361 but is more efficient. Returns the number of elements in the array following
5362 the completed C<push>.
5364 Starting with Perl 5.14, an experimental feature allowed C<push> to take a
5365 scalar expression. This experiment has been deemed unsuccessful, and was
5366 removed as of Perl 5.24.
5370 =for Pod::Functions singly quote a string
5374 =for Pod::Functions doubly quote a string
5378 =for Pod::Functions quote a list of words
5382 =for Pod::Functions backquote quote a string
5384 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5388 =for Pod::Functions +5.005 compile pattern
5390 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5392 =item quotemeta EXPR
5393 X<quotemeta> X<metacharacter>
5397 =for Pod::Functions quote regular expression magic characters
5399 Returns the value of EXPR with all the ASCII non-"word"
5400 characters backslashed. (That is, all ASCII characters not matching
5401 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5402 returned string, regardless of any locale settings.)
5403 This is the internal function implementing
5404 the C<\Q> escape in double-quoted strings.
5405 (See below for the behavior on non-ASCII code points.)
5407 If EXPR is omitted, uses C<$_>.
5409 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5410 regular expressions, because by default an interpolated variable will be
5411 considered a mini-regular expression. For example:
5413 my $sentence = 'The quick brown fox jumped over the lazy dog';
5414 my $substring = 'quick.*?fox';
5415 $sentence =~ s{$substring}{big bad wolf};
5417 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5421 my $sentence = 'The quick brown fox jumped over the lazy dog';
5422 my $substring = 'quick.*?fox';
5423 $sentence =~ s{\Q$substring\E}{big bad wolf};
5427 my $sentence = 'The quick brown fox jumped over the lazy dog';
5428 my $substring = 'quick.*?fox';
5429 my $quoted_substring = quotemeta($substring);
5430 $sentence =~ s{$quoted_substring}{big bad wolf};
5432 Will both leave the sentence as is.
5433 Normally, when accepting literal string
5434 input from the user, quotemeta() or C<\Q> must be used.
5436 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5437 strings, but not quoted in UTF-8 strings.
5439 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5440 quoting non-ASCII characters; the quoting of ASCII characters is
5443 Also unchanged is the quoting of non-UTF-8 strings when outside the
5444 scope of a C<use feature 'unicode_strings'>, which is to quote all
5445 characters in the upper Latin1 range. This provides complete backwards
5446 compatibility for old programs which do not use Unicode. (Note that
5447 C<unicode_strings> is automatically enabled within the scope of a
5448 S<C<use v5.12>> or greater.)
5450 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5451 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5452 above, locale does not affect the quoting of ASCII-range characters.
5453 This protects against those locales where characters such as C<"|"> are
5454 considered to be word characters.
5456 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5457 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5458 The only code points that are quoted are those that have any of the
5459 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5460 Default_Ignorable_Code_Point, or General_Category=Control.
5462 Of these properties, the two important ones are Pattern_Syntax and
5463 Pattern_White_Space. They have been set up by Unicode for exactly this
5464 purpose of deciding which characters in a regular expression pattern
5465 should be quoted. No character that can be in an identifier has these
5468 Perl promises, that if we ever add regular expression pattern
5469 metacharacters to the dozen already defined
5470 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5471 Pattern_Syntax property. Perl also promises, that if we ever add
5472 characters that are considered to be white space in regular expressions
5473 (currently mostly affected by C</x>), they will all have the
5474 Pattern_White_Space property.
5476 Unicode promises that the set of code points that have these two
5477 properties will never change, so something that is not quoted in v5.16
5478 will never need to be quoted in any future Perl release. (Not all the
5479 code points that match Pattern_Syntax have actually had characters
5480 assigned to them; so there is room to grow, but they are quoted
5481 whether assigned or not. Perl, of course, would never use an
5482 unassigned code point as an actual metacharacter.)
5484 Quoting characters that have the other 3 properties is done to enhance
5485 the readability of the regular expression and not because they actually
5486 need to be quoted for regular expression purposes (characters with the
5487 White_Space property are likely to be indistinguishable on the page or
5488 screen from those with the Pattern_White_Space property; and the other
5489 two properties contain non-printing characters).
5496 =for Pod::Functions retrieve the next pseudorandom number
5498 Returns a random fractional number greater than or equal to C<0> and less
5499 than the value of EXPR. (EXPR should be positive.) If EXPR is
5500 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5501 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5502 and is subject to change in future versions of Perl). Automatically calls
5503 C<srand> unless C<srand> has already been called. See also C<srand>.
5505 Apply C<int()> to the value returned by C<rand()> if you want random
5506 integers instead of random fractional numbers. For example,
5510 returns a random integer between C<0> and C<9>, inclusive.
5512 (Note: If your rand function consistently returns numbers that are too
5513 large or too small, then your version of Perl was probably compiled
5514 with the wrong number of RANDBITS.)
5516 B<C<rand()> is not cryptographically secure. You should not rely
5517 on it in security-sensitive situations.> As of this writing, a
5518 number of third-party CPAN modules offer random number generators
5519 intended by their authors to be cryptographically secure,
5520 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5521 and L<Math::TrulyRandom>.
5523 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5524 X<read> X<file, read>
5526 =item read FILEHANDLE,SCALAR,LENGTH
5528 =for Pod::Functions fixed-length buffered input from a filehandle
5530 Attempts to read LENGTH I<characters> of data into variable SCALAR
5531 from the specified FILEHANDLE. Returns the number of characters
5532 actually read, C<0> at end of file, or undef if there was an error (in
5533 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5534 so that the last character actually read is the last character of the
5535 scalar after the read.
5537 An OFFSET may be specified to place the read data at some place in the
5538 string other than the beginning. A negative OFFSET specifies
5539 placement at that many characters counting backwards from the end of
5540 the string. A positive OFFSET greater than the length of SCALAR
5541 results in the string being padded to the required size with C<"\0">
5542 bytes before the result of the read is appended.
5544 The call is implemented in terms of either Perl's or your system's native
5545 fread(3) library function. To get a true read(2) system call, see
5546 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5548 Note the I<characters>: depending on the status of the filehandle,
5549 either (8-bit) bytes or characters are read. By default, all
5550 filehandles operate on bytes, but for example if the filehandle has
5551 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5552 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5553 characters, not bytes. Similarly for the C<:encoding> pragma:
5554 in that case pretty much any characters can be read.
5556 =item readdir DIRHANDLE
5559 =for Pod::Functions get a directory from a directory handle
5561 Returns the next directory entry for a directory opened by C<opendir>.
5562 If used in list context, returns all the rest of the entries in the
5563 directory. If there are no more entries, returns the undefined value in
5564 scalar context and the empty list in list context.
5566 If you're planning to filetest the return values out of a C<readdir>, you'd
5567 better prepend the directory in question. Otherwise, because we didn't
5568 C<chdir> there, it would have been testing the wrong file.
5570 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5571 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5574 As of Perl 5.12 you can use a bare C<readdir> in a C<while> loop,
5575 which will set C<$_> on every iteration.
5577 opendir(my $dh, $some_dir) || die;
5578 while(readdir $dh) {
5579 print "$some_dir/$_\n";
5583 To avoid confusing would-be users of your code who are running earlier
5584 versions of Perl with mysterious failures, put this sort of thing at the
5585 top of your file to signal that your code will work I<only> on Perls of a
5588 use 5.012; # so readdir assigns to $_ in a lone while test
5593 X<readline> X<gets> X<fgets>
5595 =for Pod::Functions fetch a record from a file
5597 Reads from the filehandle whose typeglob is contained in EXPR (or from
5598 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5599 returns the next line until end-of-file is reached, whereupon the
5600 subsequent call returns C<undef>. In list context, reads until end-of-file
5601 is reached and returns a list of lines. Note that the notion of "line"
5602 used here is whatever you may have defined with C<$/> or
5603 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5605 When C<$/> is set to C<undef>, when C<readline> is in scalar
5606 context (i.e., file slurp mode), and when an empty file is read, it
5607 returns C<''> the first time, followed by C<undef> subsequently.
5609 This is the internal function implementing the C<< <EXPR> >>
5610 operator, but you can use it directly. The C<< <EXPR> >>
5611 operator is discussed in more detail in L<perlop/"I/O Operators">.
5614 $line = readline(*STDIN); # same thing
5616 If C<readline> encounters an operating system error, C<$!> will be set
5617 with the corresponding error message. It can be helpful to check
5618 C<$!> when you are reading from filehandles you don't trust, such as a
5619 tty or a socket. The following example uses the operator form of
5620 C<readline> and dies if the result is not defined.
5622 while ( ! eof($fh) ) {
5623 defined( $_ = <$fh> ) or die "readline failed: $!";
5627 Note that you have can't handle C<readline> errors that way with the
5628 C<ARGV> filehandle. In that case, you have to open each element of
5629 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5631 foreach my $arg (@ARGV) {
5632 open(my $fh, $arg) or warn "Can't open $arg: $!";
5634 while ( ! eof($fh) ) {
5635 defined( $_ = <$fh> )
5636 or die "readline failed for $arg: $!";
5646 =for Pod::Functions determine where a symbolic link is pointing
5648 Returns the value of a symbolic link, if symbolic links are
5649 implemented. If not, raises an exception. If there is a system
5650 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5651 omitted, uses C<$_>.
5653 Portability issues: L<perlport/readlink>.
5660 =for Pod::Functions execute a system command and collect standard output
5662 EXPR is executed as a system command.
5663 The collected standard output of the command is returned.
5664 In scalar context, it comes back as a single (potentially
5665 multi-line) string. In list context, returns a list of lines
5666 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5667 This is the internal function implementing the C<qx/EXPR/>
5668 operator, but you can use it directly. The C<qx/EXPR/>
5669 operator is discussed in more detail in L<perlop/"I/O Operators">.
5670 If EXPR is omitted, uses C<$_>.
5672 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5675 =for Pod::Functions receive a message over a Socket
5677 Receives a message on a socket. Attempts to receive LENGTH characters
5678 of data into variable SCALAR from the specified SOCKET filehandle.
5679 SCALAR will be grown or shrunk to the length actually read. Takes the
5680 same flags as the system call of the same name. Returns the address
5681 of the sender if SOCKET's protocol supports this; returns an empty
5682 string otherwise. If there's an error, returns the undefined value.
5683 This call is actually implemented in terms of recvfrom(2) system call.
5684 See L<perlipc/"UDP: Message Passing"> for examples.
5686 Note the I<characters>: depending on the status of the socket, either
5687 (8-bit) bytes or characters are received. By default all sockets
5688 operate on bytes, but for example if the socket has been changed using
5689 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5690 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5691 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5692 case pretty much any characters can be read.
5701 =for Pod::Functions start this loop iteration over again
5703 The C<redo> command restarts the loop block without evaluating the
5704 conditional again. The C<continue> block, if any, is not executed. If
5705 the LABEL is omitted, the command refers to the innermost enclosing
5706 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
5707 label name to be computed at run time, and is otherwise identical to C<redo
5708 LABEL>. Programs that want to lie to themselves about what was just input
5709 normally use this command:
5711 # a simpleminded Pascal comment stripper
5712 # (warning: assumes no { or } in strings)
5713 LINE: while (<STDIN>) {
5714 while (s|({.*}.*){.*}|$1 |) {}
5719 if (/}/) { # end of comment?
5728 C<redo> cannot be used to retry a block that returns a value such as
5729 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5730 a grep() or map() operation.
5732 Note that a block by itself is semantically identical to a loop
5733 that executes once. Thus C<redo> inside such a block will effectively
5734 turn it into a looping construct.
5736 See also L</continue> for an illustration of how C<last>, C<next>, and
5739 Unlike most named operators, this has the same precedence as assignment.
5740 It is also exempt from the looks-like-a-function rule, so
5741 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
5749 =for Pod::Functions find out the type of thing being referenced
5751 Returns a non-empty string if EXPR is a reference, the empty
5752 string otherwise. If EXPR is not specified, C<$_> will be used. The
5753 value returned depends on the type of thing the reference is a reference to.
5755 Builtin types include:
5769 You can think of C<ref> as a C<typeof> operator.
5771 if (ref($r) eq "HASH") {
5772 print "r is a reference to a hash.\n";
5775 print "r is not a reference at all.\n";
5778 The return value C<LVALUE> indicates a reference to an lvalue that is not
5779 a variable. You get this from taking the reference of function calls like
5780 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5781 to a L<version string|perldata/"Version Strings">.
5783 The result C<Regexp> indicates that the argument is a regular expression
5784 resulting from C<qr//>.
5786 If the referenced object has been blessed into a package, then that package
5787 name is returned instead. But don't use that, as it's now considered
5788 "bad practice". For one reason, an object could be using a class called
5789 C<Regexp> or C<IO>, or even C<HASH>. Also, C<ref> doesn't take into account
5790 subclasses, like C<isa> does.
5792 Instead, use C<blessed> (in the L<Scalar::Util> module) for boolean
5793 checks, C<isa> for specific class checks and C<reftype> (also from
5794 L<Scalar::Util>) for type checks. (See L<perlobj> for details and a
5795 C<blessed>/C<isa> example.)
5797 See also L<perlref>.
5799 =item rename OLDNAME,NEWNAME
5800 X<rename> X<move> X<mv> X<ren>
5802 =for Pod::Functions change a filename
5804 Changes the name of a file; an existing file NEWNAME will be
5805 clobbered. Returns true for success, false otherwise.
5807 Behavior of this function varies wildly depending on your system
5808 implementation. For example, it will usually not work across file system
5809 boundaries, even though the system I<mv> command sometimes compensates
5810 for this. Other restrictions include whether it works on directories,
5811 open files, or pre-existing files. Check L<perlport> and either the
5812 rename(2) manpage or equivalent system documentation for details.
5814 For a platform independent C<move> function look at the L<File::Copy>
5817 Portability issues: L<perlport/rename>.
5819 =item require VERSION
5826 =for Pod::Functions load in external functions from a library at runtime
5828 Demands a version of Perl specified by VERSION, or demands some semantics
5829 specified by EXPR or by C<$_> if EXPR is not supplied.
5831 VERSION may be either a numeric argument such as 5.006, which will be
5832 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5833 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5834 VERSION is greater than the version of the current Perl interpreter.
5835 Compare with L</use>, which can do a similar check at compile time.
5837 Specifying VERSION as a literal of the form v5.6.1 should generally be
5838 avoided, because it leads to misleading error messages under earlier
5839 versions of Perl that do not support this syntax. The equivalent numeric
5840 version should be used instead.
5842 require v5.6.1; # run time version check
5843 require 5.6.1; # ditto
5844 require 5.006_001; # ditto; preferred for backwards
5847 Otherwise, C<require> demands that a library file be included if it
5848 hasn't already been included. The file is included via the do-FILE
5849 mechanism, which is essentially just a variety of C<eval> with the
5850 caveat that lexical variables in the invoking script will be invisible
5851 to the included code. If it were implemented in pure Perl, it
5852 would have semantics similar to the following:
5858 my ($filename) = @_;
5859 if ( my $version = eval { version->parse($filename) } ) {
5860 if ( $version > $^V ) {
5861 my $vn = $version->normal;
5862 croak "Perl $vn required--this is only $^V, stopped";
5867 if (exists $INC{$filename}) {
5868 return 1 if $INC{$filename};
5869 croak "Compilation failed in require";
5872 foreach $prefix (@INC) {
5874 #... do other stuff - see text below ....
5876 # (see text below about possible appending of .pmc
5877 # suffix to $filename)
5878 my $realfilename = "$prefix/$filename";
5879 next if ! -e $realfilename || -d _ || -b _;
5880 $INC{$filename} = $realfilename;
5881 my $result = do($realfilename);
5882 # but run in caller's namespace
5884 if (!defined $result) {
5885 $INC{$filename} = undef;
5886 croak $@ ? "$@Compilation failed in require"
5887 : "Can't locate $filename: $!\n";
5890 delete $INC{$filename};
5891 croak "$filename did not return true value";
5896 croak "Can't locate $filename in \@INC ...";
5899 Note that the file will not be included twice under the same specified
5902 The file must return true as the last statement to indicate
5903 successful execution of any initialization code, so it's customary to
5904 end such a file with C<1;> unless you're sure it'll return true
5905 otherwise. But it's better just to put the C<1;>, in case you add more
5908 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5909 replaces "F<::>" with "F</>" in the filename for you,
5910 to make it easy to load standard modules. This form of loading of
5911 modules does not risk altering your namespace.
5913 In other words, if you try this:
5915 require Foo::Bar; # a splendid bareword
5917 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5918 directories specified in the C<@INC> array.
5920 But if you try this:
5922 $class = 'Foo::Bar';
5923 require $class; # $class is not a bareword
5925 require "Foo::Bar"; # not a bareword because of the ""
5927 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5928 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5930 eval "require $class";
5932 Now that you understand how C<require> looks for files with a
5933 bareword argument, there is a little extra functionality going on behind
5934 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5935 first look for a similar filename with a "F<.pmc>" extension. If this file
5936 is found, it will be loaded in place of any file ending in a "F<.pm>"
5939 You can also insert hooks into the import facility by putting Perl code
5940 directly into the @INC array. There are three forms of hooks: subroutine
5941 references, array references, and blessed objects.
5943 Subroutine references are the simplest case. When the inclusion system
5944 walks through @INC and encounters a subroutine, this subroutine gets
5945 called with two parameters, the first a reference to itself, and the
5946 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5947 subroutine should return either nothing or else a list of up to four
5948 values in the following order:
5954 A reference to a scalar, containing any initial source code to prepend to
5955 the file or generator output.
5959 A filehandle, from which the file will be read.
5963 A reference to a subroutine. If there is no filehandle (previous item),
5964 then this subroutine is expected to generate one line of source code per
5965 call, writing the line into C<$_> and returning 1, then finally at end of
5966 file returning 0. If there is a filehandle, then the subroutine will be
5967 called to act as a simple source filter, with the line as read in C<$_>.
5968 Again, return 1 for each valid line, and 0 after all lines have been
5973 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5974 reference to the subroutine itself is passed in as C<$_[0]>.
5978 If an empty list, C<undef>, or nothing that matches the first 3 values above
5979 is returned, then C<require> looks at the remaining elements of @INC.
5980 Note that this filehandle must be a real filehandle (strictly a typeglob
5981 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5982 will be ignored and processing will stop there.
5984 If the hook is an array reference, its first element must be a subroutine
5985 reference. This subroutine is called as above, but the first parameter is
5986 the array reference. This lets you indirectly pass arguments to
5989 In other words, you can write:
5991 push @INC, \&my_sub;
5993 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5999 push @INC, [ \&my_sub, $x, $y, ... ];
6001 my ($arrayref, $filename) = @_;
6002 # Retrieve $x, $y, ...
6003 my @parameters = @$arrayref[1..$#$arrayref];
6007 If the hook is an object, it must provide an INC method that will be
6008 called as above, the first parameter being the object itself. (Note that
6009 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6010 into package C<main>.) Here is a typical code layout:
6016 my ($self, $filename) = @_;
6020 # In the main program
6021 push @INC, Foo->new(...);
6023 These hooks are also permitted to set the %INC entry
6024 corresponding to the files they have loaded. See L<perlvar/%INC>.
6026 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
6033 =for Pod::Functions clear all variables of a given name
6035 Generally used in a C<continue> block at the end of a loop to clear
6036 variables and reset C<??> searches so that they work again. The
6037 expression is interpreted as a list of single characters (hyphens
6038 allowed for ranges). All variables and arrays beginning with one of
6039 those letters are reset to their pristine state. If the expression is
6040 omitted, one-match searches (C<?pattern?>) are reset to match again.
6041 Only resets variables or searches in the current package. Always returns
6044 reset 'X'; # reset all X variables
6045 reset 'a-z'; # reset lower case variables
6046 reset; # just reset ?one-time? searches
6048 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6049 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
6050 variables; lexical variables are unaffected, but they clean themselves
6051 up on scope exit anyway, so you'll probably want to use them instead.
6059 =for Pod::Functions get out of a function early
6061 Returns from a subroutine, C<eval>, or C<do FILE> with the value
6062 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6063 context, depending on how the return value will be used, and the context
6064 may vary from one execution to the next (see L</wantarray>). If no EXPR
6065 is given, returns an empty list in list context, the undefined value in
6066 scalar context, and (of course) nothing at all in void context.
6068 (In the absence of an explicit C<return>, a subroutine, eval,
6069 or do FILE automatically returns the value of the last expression
6072 Unlike most named operators, this is also exempt from the
6073 looks-like-a-function rule, so C<return ("foo")."bar"> will
6074 cause "bar" to be part of the argument to C<return>.
6077 X<reverse> X<rev> X<invert>
6079 =for Pod::Functions flip a string or a list
6081 In list context, returns a list value consisting of the elements
6082 of LIST in the opposite order. In scalar context, concatenates the
6083 elements of LIST and returns a string value with all characters
6084 in the opposite order.
6086 print join(", ", reverse "world", "Hello"); # Hello, world
6088 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6090 Used without arguments in scalar context, reverse() reverses C<$_>.
6092 $_ = "dlrow ,olleH";
6093 print reverse; # No output, list context
6094 print scalar reverse; # Hello, world
6096 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6097 preserve non-existent elements whenever possible; i.e., for non-magical
6098 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6100 This operator is also handy for inverting a hash, although there are some
6101 caveats. If a value is duplicated in the original hash, only one of those
6102 can be represented as a key in the inverted hash. Also, this has to
6103 unwind one hash and build a whole new one, which may take some time
6104 on a large hash, such as from a DBM file.
6106 %by_name = reverse %by_address; # Invert the hash
6108 =item rewinddir DIRHANDLE
6111 =for Pod::Functions reset directory handle
6113 Sets the current position to the beginning of the directory for the
6114 C<readdir> routine on DIRHANDLE.
6116 Portability issues: L<perlport/rewinddir>.
6118 =item rindex STR,SUBSTR,POSITION
6121 =item rindex STR,SUBSTR
6123 =for Pod::Functions right-to-left substring search
6125 Works just like index() except that it returns the position of the I<last>
6126 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6127 last occurrence beginning at or before that position.
6129 =item rmdir FILENAME
6130 X<rmdir> X<rd> X<directory, remove>
6134 =for Pod::Functions remove a directory
6136 Deletes the directory specified by FILENAME if that directory is
6137 empty. If it succeeds it returns true; otherwise it returns false and
6138 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
6140 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6141 the C<rmtree> function of the L<File::Path> module.
6145 =for Pod::Functions replace a pattern with a string
6147 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6149 =item say FILEHANDLE LIST
6152 =item say FILEHANDLE
6158 =for Pod::Functions +say output a list to a filehandle, appending a newline
6160 Just like C<print>, but implicitly appends a newline. C<say LIST> is
6161 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
6162 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
6163 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
6165 This keyword is available only when the C<"say"> feature
6166 is enabled, or when prefixed with C<CORE::>; see
6167 L<feature>. Alternately, include a C<use v5.10> or later to the current
6171 X<scalar> X<context>
6173 =for Pod::Functions force a scalar context
6175 Forces EXPR to be interpreted in scalar context and returns the value
6178 @counts = ( scalar @a, scalar @b, scalar @c );
6180 There is no equivalent operator to force an expression to
6181 be interpolated in list context because in practice, this is never
6182 needed. If you really wanted to do so, however, you could use
6183 the construction C<@{[ (some expression) ]}>, but usually a simple
6184 C<(some expression)> suffices.
6186 Because C<scalar> is a unary operator, if you accidentally use a
6187 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6188 evaluating all but the last element in void context and returning the final
6189 element evaluated in scalar context. This is seldom what you want.
6191 The following single statement:
6193 print uc(scalar(&foo,$bar)),$baz;
6195 is the moral equivalent of these two:
6198 print(uc($bar),$baz);
6200 See L<perlop> for more details on unary operators and the comma operator.
6202 =item seek FILEHANDLE,POSITION,WHENCE
6203 X<seek> X<fseek> X<filehandle, position>
6205 =for Pod::Functions reposition file pointer for random-access I/O
6207 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
6208 FILEHANDLE may be an expression whose value gives the name of the
6209 filehandle. The values for WHENCE are C<0> to set the new position
6210 I<in bytes> to POSITION; C<1> to set it to the current position plus
6211 POSITION; and C<2> to set it to EOF plus POSITION, typically
6212 negative. For WHENCE you may use the constants C<SEEK_SET>,
6213 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6214 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6217 Note the I<in bytes>: even if the filehandle has been set to
6218 operate on characters (for example by using the C<:encoding(utf8)> open
6219 layer), tell() will return byte offsets, not character offsets
6220 (because implementing that would render seek() and tell() rather slow).
6222 If you want to position the file for C<sysread> or C<syswrite>, don't use
6223 C<seek>, because buffering makes its effect on the file's read-write position
6224 unpredictable and non-portable. Use C<sysseek> instead.
6226 Due to the rules and rigors of ANSI C, on some systems you have to do a
6227 seek whenever you switch between reading and writing. Amongst other
6228 things, this may have the effect of calling stdio's clearerr(3).
6229 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6233 This is also useful for applications emulating C<tail -f>. Once you hit
6234 EOF on your read and then sleep for a while, you (probably) have to stick in a
6235 dummy seek() to reset things. The C<seek> doesn't change the position,
6236 but it I<does> clear the end-of-file condition on the handle, so that the
6237 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6239 If that doesn't work (some I/O implementations are particularly
6240 cantankerous), you might need something like this:
6243 for ($curpos = tell(FILE); $_ = <FILE>;
6244 $curpos = tell(FILE)) {
6245 # search for some stuff and put it into files
6247 sleep($for_a_while);
6248 seek(FILE, $curpos, 0);
6251 =item seekdir DIRHANDLE,POS
6254 =for Pod::Functions reposition directory pointer
6256 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6257 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6258 about possible directory compaction as the corresponding system library
6261 =item select FILEHANDLE
6262 X<select> X<filehandle, default>
6266 =for Pod::Functions reset default output or do I/O multiplexing
6268 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6269 sets the new current default filehandle for output. This has two
6270 effects: first, a C<write> or a C<print> without a filehandle
6271 default to this FILEHANDLE. Second, references to variables related to
6272 output will refer to this output channel.
6274 For example, to set the top-of-form format for more than one
6275 output channel, you might do the following:
6282 FILEHANDLE may be an expression whose value gives the name of the
6283 actual filehandle. Thus:
6285 $oldfh = select(STDERR); $| = 1; select($oldfh);
6287 Some programmers may prefer to think of filehandles as objects with
6288 methods, preferring to write the last example as:
6291 STDERR->autoflush(1);
6293 Portability issues: L<perlport/select>.
6295 =item select RBITS,WBITS,EBITS,TIMEOUT
6298 This calls the select(2) syscall with the bit masks specified, which
6299 can be constructed using C<fileno> and C<vec>, along these lines:
6301 $rin = $win = $ein = '';
6302 vec($rin, fileno(STDIN), 1) = 1;
6303 vec($win, fileno(STDOUT), 1) = 1;
6306 If you want to select on many filehandles, you may wish to write a
6307 subroutine like this:
6312 for my $fh (@fhlist) {
6313 vec($bits, fileno($fh), 1) = 1;
6317 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6321 ($nfound,$timeleft) =
6322 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6324 or to block until something becomes ready just do this
6326 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6328 Most systems do not bother to return anything useful in $timeleft, so
6329 calling select() in scalar context just returns $nfound.
6331 Any of the bit masks can also be undef. The timeout, if specified, is
6332 in seconds, which may be fractional. Note: not all implementations are
6333 capable of returning the $timeleft. If not, they always return
6334 $timeleft equal to the supplied $timeout.
6336 You can effect a sleep of 250 milliseconds this way:
6338 select(undef, undef, undef, 0.25);
6340 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6341 is implementation-dependent. See also L<perlport> for notes on the
6342 portability of C<select>.
6344 On error, C<select> behaves just like select(2): it returns
6347 On some Unixes, select(2) may report a socket file descriptor as "ready for
6348 reading" even when no data is available, and thus any subsequent C<read>
6349 would block. This can be avoided if you always use O_NONBLOCK on the
6350 socket. See select(2) and fcntl(2) for further details.
6352 The standard C<IO::Select> module provides a user-friendlier interface
6353 to C<select>, mostly because it does all the bit-mask work for you.
6355 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6356 or <FH>) with C<select>, except as permitted by POSIX, and even
6357 then only on POSIX systems. You have to use C<sysread> instead.
6359 Portability issues: L<perlport/select>.
6361 =item semctl ID,SEMNUM,CMD,ARG
6364 =for Pod::Functions SysV semaphore control operations
6366 Calls the System V IPC function semctl(2). You'll probably have to say
6370 first to get the correct constant definitions. If CMD is IPC_STAT or
6371 GETALL, then ARG must be a variable that will hold the returned
6372 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6373 the undefined value for error, "C<0 but true>" for zero, or the actual
6374 return value otherwise. The ARG must consist of a vector of native
6375 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6376 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6379 Portability issues: L<perlport/semctl>.
6381 =item semget KEY,NSEMS,FLAGS
6384 =for Pod::Functions get set of SysV semaphores
6386 Calls the System V IPC function semget(2). Returns the semaphore id, or
6387 the undefined value on error. See also
6388 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6391 Portability issues: L<perlport/semget>.
6393 =item semop KEY,OPSTRING
6396 =for Pod::Functions SysV semaphore operations
6398 Calls the System V IPC function semop(2) for semaphore operations
6399 such as signalling and waiting. OPSTRING must be a packed array of
6400 semop structures. Each semop structure can be generated with
6401 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6402 implies the number of semaphore operations. Returns true if
6403 successful, false on error. As an example, the
6404 following code waits on semaphore $semnum of semaphore id $semid:
6406 $semop = pack("s!3", $semnum, -1, 0);
6407 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6409 To signal the semaphore, replace C<-1> with C<1>. See also
6410 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6413 Portability issues: L<perlport/semop>.
6415 =item send SOCKET,MSG,FLAGS,TO
6418 =item send SOCKET,MSG,FLAGS
6420 =for Pod::Functions send a message over a socket
6422 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6423 filehandle. Takes the same flags as the system call of the same name. On
6424 unconnected sockets, you must specify a destination to I<send to>, in which
6425 case it does a sendto(2) syscall. Returns the number of characters sent,
6426 or the undefined value on error. The sendmsg(2) syscall is currently
6427 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6429 Note the I<characters>: depending on the status of the socket, either
6430 (8-bit) bytes or characters are sent. By default all sockets operate
6431 on bytes, but for example if the socket has been changed using
6432 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6433 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6434 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6435 pragma: in that case pretty much any characters can be sent.
6437 =item setpgrp PID,PGRP
6440 =for Pod::Functions set the process group of a process
6442 Sets the current process group for the specified PID, C<0> for the current
6443 process. Raises an exception when used on a machine that doesn't
6444 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6445 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6446 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6449 Portability issues: L<perlport/setpgrp>.
6451 =item setpriority WHICH,WHO,PRIORITY
6452 X<setpriority> X<priority> X<nice> X<renice>
6454 =for Pod::Functions set a process's nice value
6456 Sets the current priority for a process, a process group, or a user.
6457 (See setpriority(2).) Raises an exception when used on a machine
6458 that doesn't implement setpriority(2).
6460 Portability issues: L<perlport/setpriority>.
6462 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6465 =for Pod::Functions set some socket options
6467 Sets the socket option requested. Returns C<undef> on error.
6468 Use integer constants provided by the C<Socket> module for
6469 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6470 getprotobyname. OPTVAL might either be a packed string or an integer.
6471 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6473 An example disabling Nagle's algorithm on a socket:
6475 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6476 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6478 Portability issues: L<perlport/setsockopt>.
6485 =for Pod::Functions remove the first element of an array, and return it
6487 Shifts the first value of the array off and returns it, shortening the
6488 array by 1 and moving everything down. If there are no elements in the
6489 array, returns the undefined value. If ARRAY is omitted, shifts the
6490 C<@_> array within the lexical scope of subroutines and formats, and the
6491 C<@ARGV> array outside a subroutine and also within the lexical scopes
6492 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6493 C<UNITCHECK {}>, and C<END {}> constructs.
6495 Starting with Perl 5.14, an experimental feature allowed C<shift> to take a
6496 scalar expression. This experiment has been deemed unsuccessful, and was
6497 removed as of Perl 5.24.
6499 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6500 same thing to the left end of an array that C<pop> and C<push> do to the
6503 =item shmctl ID,CMD,ARG
6506 =for Pod::Functions SysV shared memory operations
6508 Calls the System V IPC function shmctl. You'll probably have to say
6512 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6513 then ARG must be a variable that will hold the returned C<shmid_ds>
6514 structure. Returns like ioctl: C<undef> for error; "C<0> but
6515 true" for zero; and the actual return value otherwise.
6516 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6518 Portability issues: L<perlport/shmctl>.
6520 =item shmget KEY,SIZE,FLAGS
6523 =for Pod::Functions get SysV shared memory segment identifier
6525 Calls the System V IPC function shmget. Returns the shared memory
6526 segment id, or C<undef> on error.
6527 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6529 Portability issues: L<perlport/shmget>.
6531 =item shmread ID,VAR,POS,SIZE
6535 =for Pod::Functions read SysV shared memory
6537 =item shmwrite ID,STRING,POS,SIZE
6539 =for Pod::Functions write SysV shared memory
6541 Reads or writes the System V shared memory segment ID starting at
6542 position POS for size SIZE by attaching to it, copying in/out, and
6543 detaching from it. When reading, VAR must be a variable that will
6544 hold the data read. When writing, if STRING is too long, only SIZE
6545 bytes are used; if STRING is too short, nulls are written to fill out
6546 SIZE bytes. Return true if successful, false on error.
6547 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6548 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6550 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6552 =item shutdown SOCKET,HOW
6555 =for Pod::Functions close down just half of a socket connection
6557 Shuts down a socket connection in the manner indicated by HOW, which
6558 has the same interpretation as in the syscall of the same name.
6560 shutdown(SOCKET, 0); # I/we have stopped reading data
6561 shutdown(SOCKET, 1); # I/we have stopped writing data
6562 shutdown(SOCKET, 2); # I/we have stopped using this socket
6564 This is useful with sockets when you want to tell the other
6565 side you're done writing but not done reading, or vice versa.
6566 It's also a more insistent form of close because it also
6567 disables the file descriptor in any forked copies in other
6570 Returns C<1> for success; on error, returns C<undef> if
6571 the first argument is not a valid filehandle, or returns C<0> and sets
6572 C<$!> for any other failure.
6575 X<sin> X<sine> X<asin> X<arcsine>
6579 =for Pod::Functions return the sine of a number
6581 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6582 returns sine of C<$_>.
6584 For the inverse sine operation, you may use the C<Math::Trig::asin>
6585 function, or use this relation:
6587 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6594 =for Pod::Functions block for some number of seconds
6596 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6597 argument is given. Returns the integer number of seconds actually slept.
6599 May be interrupted if the process receives a signal such as C<SIGALRM>.
6602 local $SIG{ALARM} = sub { die "Alarm!\n" };
6605 die $@ unless $@ eq "Alarm!\n";
6607 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6608 is often implemented using C<alarm>.
6610 On some older systems, it may sleep up to a full second less than what
6611 you requested, depending on how it counts seconds. Most modern systems
6612 always sleep the full amount. They may appear to sleep longer than that,
6613 however, because your process might not be scheduled right away in a
6614 busy multitasking system.
6616 For delays of finer granularity than one second, the Time::HiRes module
6617 (from CPAN, and starting from Perl 5.8 part of the standard
6618 distribution) provides usleep(). You may also use Perl's four-argument
6619 version of select() leaving the first three arguments undefined, or you
6620 might be able to use the C<syscall> interface to access setitimer(2) if
6621 your system supports it. See L<perlfaq8> for details.
6623 See also the POSIX module's C<pause> function.
6625 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6628 =for Pod::Functions create a socket
6630 Opens a socket of the specified kind and attaches it to filehandle
6631 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6632 the syscall of the same name. You should C<use Socket> first
6633 to get the proper definitions imported. See the examples in
6634 L<perlipc/"Sockets: Client/Server Communication">.
6636 On systems that support a close-on-exec flag on files, the flag will
6637 be set for the newly opened file descriptor, as determined by the
6638 value of $^F. See L<perlvar/$^F>.
6640 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6643 =for Pod::Functions create a pair of sockets
6645 Creates an unnamed pair of sockets in the specified domain, of the
6646 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6647 for the syscall of the same name. If unimplemented, raises an exception.
6648 Returns true if successful.
6650 On systems that support a close-on-exec flag on files, the flag will
6651 be set for the newly opened file descriptors, as determined by the value
6652 of $^F. See L<perlvar/$^F>.
6654 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6655 to C<pipe(Rdr, Wtr)> is essentially:
6658 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6659 shutdown(Rdr, 1); # no more writing for reader
6660 shutdown(Wtr, 0); # no more reading for writer
6662 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6663 emulate socketpair using IP sockets to localhost if your system implements
6664 sockets but not socketpair.
6666 Portability issues: L<perlport/socketpair>.
6668 =item sort SUBNAME LIST
6669 X<sort> X<qsort> X<quicksort> X<mergesort>
6671 =item sort BLOCK LIST
6675 =for Pod::Functions sort a list of values
6677 In list context, this sorts the LIST and returns the sorted list value.
6678 In scalar context, the behaviour of C<sort()> is undefined.
6680 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6681 order. If SUBNAME is specified, it gives the name of a subroutine
6682 that returns an integer less than, equal to, or greater than C<0>,
6683 depending on how the elements of the list are to be ordered. (The
6684 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6685 SUBNAME may be a scalar variable name (unsubscripted), in which case
6686 the value provides the name of (or a reference to) the actual
6687 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6688 an anonymous, in-line sort subroutine.
6690 If the subroutine's prototype is C<($$)>, the elements to be compared are
6691 passed by reference in C<@_>, as for a normal subroutine. This is slower
6692 than unprototyped subroutines, where the elements to be compared are passed
6693 into the subroutine as the package global variables $a and $b (see example
6694 below). Note that in the latter case, it is usually highly counter-productive
6695 to declare $a and $b as lexicals.
6697 If the subroutine is an XSUB, the elements to be compared are pushed on to
6698 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6701 The values to be compared are always passed by reference and should not
6704 You also cannot exit out of the sort block or subroutine using any of the
6705 loop control operators described in L<perlsyn> or with C<goto>.
6707 When C<use locale> (but not C<use locale 'not_characters'>) is in
6708 effect, C<sort LIST> sorts LIST according to the
6709 current collation locale. See L<perllocale>.
6711 sort() returns aliases into the original list, much as a for loop's index
6712 variable aliases the list elements. That is, modifying an element of a
6713 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6714 actually modifies the element in the original list. This is usually
6715 something to be avoided when writing clear code.
6717 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6718 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6719 preserves the input order of elements that compare equal. Although
6720 quicksort's run time is O(NlogN) when averaged over all arrays of
6721 length N, the time can be O(N**2), I<quadratic> behavior, for some
6722 inputs.) In 5.7, the quicksort implementation was replaced with
6723 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6724 But benchmarks indicated that for some inputs, on some platforms,
6725 the original quicksort was faster. 5.8 has a sort pragma for
6726 limited control of the sort. Its rather blunt control of the
6727 underlying algorithm may not persist into future Perls, but the
6728 ability to characterize the input or output in implementation
6729 independent ways quite probably will. See L<the sort pragma|sort>.
6734 @articles = sort @files;
6736 # same thing, but with explicit sort routine
6737 @articles = sort {$a cmp $b} @files;
6739 # now case-insensitively
6740 @articles = sort {fc($a) cmp fc($b)} @files;
6742 # same thing in reversed order
6743 @articles = sort {$b cmp $a} @files;
6745 # sort numerically ascending
6746 @articles = sort {$a <=> $b} @files;
6748 # sort numerically descending
6749 @articles = sort {$b <=> $a} @files;
6751 # this sorts the %age hash by value instead of key
6752 # using an in-line function
6753 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6755 # sort using explicit subroutine name
6757 $age{$a} <=> $age{$b}; # presuming numeric
6759 @sortedclass = sort byage @class;
6761 sub backwards { $b cmp $a }
6762 @harry = qw(dog cat x Cain Abel);
6763 @george = qw(gone chased yz Punished Axed);
6765 # prints AbelCaincatdogx
6766 print sort backwards @harry;
6767 # prints xdogcatCainAbel
6768 print sort @george, 'to', @harry;
6769 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6771 # inefficiently sort by descending numeric compare using
6772 # the first integer after the first = sign, or the
6773 # whole record case-insensitively otherwise
6776 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6781 # same thing, but much more efficiently;
6782 # we'll build auxiliary indices instead
6786 push @nums, ( /=(\d+)/ ? $1 : undef );
6790 my @new = @old[ sort {
6791 $nums[$b] <=> $nums[$a]
6793 $caps[$a] cmp $caps[$b]
6797 # same thing, but without any temps
6798 @new = map { $_->[0] }
6799 sort { $b->[1] <=> $a->[1]
6802 } map { [$_, /=(\d+)/, fc($_)] } @old;
6804 # using a prototype allows you to use any comparison subroutine
6805 # as a sort subroutine (including other package's subroutines)
6807 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6810 @new = sort other::backwards @old;
6812 # guarantee stability, regardless of algorithm
6814 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6816 # force use of mergesort (not portable outside Perl 5.8)
6817 use sort '_mergesort'; # note discouraging _
6818 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6820 Warning: syntactical care is required when sorting the list returned from
6821 a function. If you want to sort the list returned by the function call
6822 C<find_records(@key)>, you can use:
6824 @contact = sort { $a cmp $b } find_records @key;
6825 @contact = sort +find_records(@key);
6826 @contact = sort &find_records(@key);
6827 @contact = sort(find_records(@key));
6829 If instead you want to sort the array @key with the comparison routine
6830 C<find_records()> then you can use:
6832 @contact = sort { find_records() } @key;
6833 @contact = sort find_records(@key);
6834 @contact = sort(find_records @key);
6835 @contact = sort(find_records (@key));
6837 If you're using strict, you I<must not> declare $a
6838 and $b as lexicals. They are package globals. That means
6839 that if you're in the C<main> package and type
6841 @articles = sort {$b <=> $a} @files;
6843 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6844 but if you're in the C<FooPack> package, it's the same as typing
6846 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6848 The comparison function is required to behave. If it returns
6849 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6850 sometimes saying the opposite, for example) the results are not
6853 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6854 (not-a-number), be careful when sorting with a
6855 comparison function like C<< $a <=> $b >> any lists that might contain a
6856 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6857 eliminate any C<NaN>s from the input list.
6859 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6861 =item splice ARRAY,OFFSET,LENGTH,LIST
6864 =item splice ARRAY,OFFSET,LENGTH
6866 =item splice ARRAY,OFFSET
6870 =for Pod::Functions add or remove elements anywhere in an array
6872 Removes the elements designated by OFFSET and LENGTH from an array, and
6873 replaces them with the elements of LIST, if any. In list context,
6874 returns the elements removed from the array. In scalar context,
6875 returns the last element removed, or C<undef> if no elements are
6876 removed. The array grows or shrinks as necessary.
6877 If OFFSET is negative then it starts that far from the end of the array.
6878 If LENGTH is omitted, removes everything from OFFSET onward.
6879 If LENGTH is negative, removes the elements from OFFSET onward
6880 except for -LENGTH elements at the end of the array.
6881 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6882 past the end of the array and a LENGTH was provided, Perl issues a warning,
6883 and splices at the end of the array.
6885 The following equivalences hold (assuming C<< $#a >= $i >> )
6887 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6888 pop(@a) splice(@a,-1)
6889 shift(@a) splice(@a,0,1)
6890 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6891 $a[$i] = $y splice(@a,$i,1,$y)
6893 C<splice> can be used, for example, to implement n-ary queue processing:
6897 while (my @next_n = splice @_, 0, $n) {
6898 say join q{ -- }, @next_n;
6902 nary_print(3, qw(a b c d e f g h));
6908 Starting with Perl 5.14, an experimental feature allowed C<splice> to take a
6909 scalar expression. This experiment has been deemed unsuccessful, and was
6910 removed as of Perl 5.24.
6912 =item split /PATTERN/,EXPR,LIMIT
6915 =item split /PATTERN/,EXPR
6917 =item split /PATTERN/
6921 =for Pod::Functions split up a string using a regexp delimiter
6923 Splits the string EXPR into a list of strings and returns the
6924 list in list context, or the size of the list in scalar context.
6926 If only PATTERN is given, EXPR defaults to C<$_>.
6928 Anything in EXPR that matches PATTERN is taken to be a separator
6929 that separates the EXPR into substrings (called "I<fields>") that
6930 do B<not> include the separator. Note that a separator may be
6931 longer than one character or even have no characters at all (the
6932 empty string, which is a zero-width match).
6934 The PATTERN need not be constant; an expression may be used
6935 to specify a pattern that varies at runtime.
6937 If PATTERN matches the empty string, the EXPR is split at the match
6938 position (between characters). As an example, the following:
6940 print join(':', split('b', 'abc')), "\n";
6942 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6945 print join(':', split('', 'abc')), "\n";
6947 uses empty string matches as separators to produce the output
6948 'a:b:c'; thus, the empty string may be used to split EXPR into a
6949 list of its component characters.
6951 As a special case for C<split>, the empty pattern given in
6952 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
6953 specifically matches the empty string, which is contrary to its usual
6954 interpretation as the last successful match.
6956 If PATTERN is C</^/>, then it is treated as if it used the
6957 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
6958 isn't much use otherwise.
6960 As another special case, C<split> emulates the default behavior of the
6961 command line tool B<awk> when the PATTERN is either omitted or a I<literal
6962 string> composed of a single space character (such as S<C<' '>> or
6963 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
6964 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
6965 instead treated as if it were C</\s+/>; in particular, this means that
6966 I<any> contiguous whitespace (not just a single space character) is used as
6967 a separator. However, this special treatment can be avoided by specifying
6968 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
6969 only a single space character to be a separator. In earlier Perls this
6970 special case was restricted to the use of a plain S<C<" ">> as the
6971 pattern argument to split, in Perl 5.18.0 and later this special case is
6972 triggered by any expression which evaluates as the simple string S<C<" ">>.
6974 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
6975 the previously described I<awk> emulation.
6977 If LIMIT is specified and positive, it represents the maximum number
6978 of fields into which the EXPR may be split; in other words, LIMIT is
6979 one greater than the maximum number of times EXPR may be split. Thus,
6980 the LIMIT value C<1> means that EXPR may be split a maximum of zero
6981 times, producing a maximum of one field (namely, the entire value of
6982 EXPR). For instance:
6984 print join(':', split(//, 'abc', 1)), "\n";
6986 produces the output 'abc', and this:
6988 print join(':', split(//, 'abc', 2)), "\n";
6990 produces the output 'a:bc', and each of these:
6992 print join(':', split(//, 'abc', 3)), "\n";
6993 print join(':', split(//, 'abc', 4)), "\n";
6995 produces the output 'a:b:c'.
6997 If LIMIT is negative, it is treated as if it were instead arbitrarily
6998 large; as many fields as possible are produced.
7000 If LIMIT is omitted (or, equivalently, zero), then it is usually
7001 treated as if it were instead negative but with the exception that
7002 trailing empty fields are stripped (empty leading fields are always
7003 preserved); if all fields are empty, then all fields are considered to
7004 be trailing (and are thus stripped in this case). Thus, the following:
7006 print join(':', split(',', 'a,b,c,,,')), "\n";
7008 produces the output 'a:b:c', but the following:
7010 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
7012 produces the output 'a:b:c:::'.
7014 In time-critical applications, it is worthwhile to avoid splitting
7015 into more fields than necessary. Thus, when assigning to a list,
7016 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7017 were one larger than the number of variables in the list; for the
7018 following, LIMIT is implicitly 3:
7020 ($login, $passwd) = split(/:/);
7022 Note that splitting an EXPR that evaluates to the empty string always
7023 produces zero fields, regardless of the LIMIT specified.
7025 An empty leading field is produced when there is a positive-width
7026 match at the beginning of EXPR. For instance:
7028 print join(':', split(/ /, ' abc')), "\n";
7030 produces the output ':abc'. However, a zero-width match at the
7031 beginning of EXPR never produces an empty field, so that:
7033 print join(':', split(//, ' abc'));
7035 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
7037 An empty trailing field, on the other hand, is produced when there is a
7038 match at the end of EXPR, regardless of the length of the match
7039 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7040 removed, as in the last example). Thus:
7042 print join(':', split(//, ' abc', -1)), "\n";
7044 produces the output S<' :a:b:c:'>.
7046 If the PATTERN contains
7047 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7048 then for each separator, an additional field is produced for each substring
7049 captured by a group (in the order in which the groups are specified,
7050 as per L<backreferences|perlretut/Backreferences>); if any group does not
7051 match, then it captures the C<undef> value instead of a substring. Also,
7052 note that any such additional field is produced whenever there is a
7053 separator (that is, whenever a split occurs), and such an additional field
7054 does B<not> count towards the LIMIT. Consider the following expressions
7055 evaluated in list context (each returned list is provided in the associated
7058 split(/-|,/, "1-10,20", 3)
7061 split(/(-|,)/, "1-10,20", 3)
7062 # ('1', '-', '10', ',', '20')
7064 split(/-|(,)/, "1-10,20", 3)
7065 # ('1', undef, '10', ',', '20')
7067 split(/(-)|,/, "1-10,20", 3)
7068 # ('1', '-', '10', undef, '20')
7070 split(/(-)|(,)/, "1-10,20", 3)
7071 # ('1', '-', undef, '10', undef, ',', '20')
7073 =item sprintf FORMAT, LIST
7076 =for Pod::Functions formatted print into a string
7078 Returns a string formatted by the usual C<printf> conventions of the C
7079 library function C<sprintf>. See below for more details
7080 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
7081 the general principles.
7085 # Format number with up to 8 leading zeroes
7086 $result = sprintf("%08d", $number);
7088 # Round number to 3 digits after decimal point
7089 $rounded = sprintf("%.3f", $number);
7091 Perl does its own C<sprintf> formatting: it emulates the C
7092 function sprintf(3), but doesn't use it except for floating-point
7093 numbers, and even then only standard modifiers are allowed.
7094 Non-standard extensions in your local sprintf(3) are
7095 therefore unavailable from Perl.
7097 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
7098 pass it an array as your first argument.
7099 The array is given scalar context,
7100 and instead of using the 0th element of the array as the format, Perl will
7101 use the count of elements in the array as the format, which is almost never
7104 Perl's C<sprintf> permits the following universally-known conversions:
7107 %c a character with the given number
7109 %d a signed integer, in decimal
7110 %u an unsigned integer, in decimal
7111 %o an unsigned integer, in octal
7112 %x an unsigned integer, in hexadecimal
7113 %e a floating-point number, in scientific notation
7114 %f a floating-point number, in fixed decimal notation
7115 %g a floating-point number, in %e or %f notation
7117 In addition, Perl permits the following widely-supported conversions:
7119 %X like %x, but using upper-case letters
7120 %E like %e, but using an upper-case "E"
7121 %G like %g, but with an upper-case "E" (if applicable)
7122 %b an unsigned integer, in binary
7123 %B like %b, but using an upper-case "B" with the # flag
7124 %p a pointer (outputs the Perl value's address in hexadecimal)
7125 %n special: *stores* the number of characters output so far
7126 into the next argument in the parameter list
7127 %a hexadecimal floating point
7128 %A like %a, but using upper-case letters
7130 Finally, for backward (and we do mean "backward") compatibility, Perl
7131 permits these unnecessary but widely-supported conversions:
7134 %D a synonym for %ld
7135 %U a synonym for %lu
7136 %O a synonym for %lo
7139 Note that the number of exponent digits in the scientific notation produced
7140 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7141 exponent less than 100 is system-dependent: it may be three or less
7142 (zero-padded as necessary). In other words, 1.23 times ten to the
7143 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7144 the exponent or the hexadecimal digits may float: especially the
7145 "long doubles" Perl configuration option may cause surprises.
7147 Between the C<%> and the format letter, you may specify several
7148 additional attributes controlling the interpretation of the format.
7149 In order, these are:
7153 =item format parameter index
7155 An explicit format parameter index, such as C<2$>. By default sprintf
7156 will format the next unused argument in the list, but this allows you
7157 to take the arguments out of order:
7159 printf '%2$d %1$d', 12, 34; # prints "34 12"
7160 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7166 space prefix non-negative number with a space
7167 + prefix non-negative number with a plus sign
7168 - left-justify within the field
7169 0 use zeros, not spaces, to right-justify
7170 # ensure the leading "0" for any octal,
7171 prefix non-zero hexadecimal with "0x" or "0X",
7172 prefix non-zero binary with "0b" or "0B"
7176 printf '<% d>', 12; # prints "< 12>"
7177 printf '<%+d>', 12; # prints "<+12>"
7178 printf '<%6s>', 12; # prints "< 12>"
7179 printf '<%-6s>', 12; # prints "<12 >"
7180 printf '<%06s>', 12; # prints "<000012>"
7181 printf '<%#o>', 12; # prints "<014>"
7182 printf '<%#x>', 12; # prints "<0xc>"
7183 printf '<%#X>', 12; # prints "<0XC>"
7184 printf '<%#b>', 12; # prints "<0b1100>"
7185 printf '<%#B>', 12; # prints "<0B1100>"
7187 When a space and a plus sign are given as the flags at once,
7188 a plus sign is used to prefix a positive number.
7190 printf '<%+ d>', 12; # prints "<+12>"
7191 printf '<% +d>', 12; # prints "<+12>"
7193 When the # flag and a precision are given in the %o conversion,
7194 the precision is incremented if it's necessary for the leading "0".
7196 printf '<%#.5o>', 012; # prints "<00012>"
7197 printf '<%#.5o>', 012345; # prints "<012345>"
7198 printf '<%#.0o>', 0; # prints "<0>"
7202 This flag tells Perl to interpret the supplied string as a vector of
7203 integers, one for each character in the string. Perl applies the format to
7204 each integer in turn, then joins the resulting strings with a separator (a
7205 dot C<.> by default). This can be useful for displaying ordinal values of
7206 characters in arbitrary strings:
7208 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7209 printf "version is v%vd\n", $^V; # Perl's version
7211 Put an asterisk C<*> before the C<v> to override the string to
7212 use to separate the numbers:
7214 printf "address is %*vX\n", ":", $addr; # IPv6 address
7215 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7217 You can also explicitly specify the argument number to use for
7218 the join string using something like C<*2$v>; for example:
7220 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7223 =item (minimum) width
7225 Arguments are usually formatted to be only as wide as required to
7226 display the given value. You can override the width by putting
7227 a number here, or get the width from the next argument (with C<*>)
7228 or from a specified argument (e.g., with C<*2$>):
7230 printf "<%s>", "a"; # prints "<a>"
7231 printf "<%6s>", "a"; # prints "< a>"
7232 printf "<%*s>", 6, "a"; # prints "< a>"
7233 printf '<%*2$s>', "a", 6; # prints "< a>"
7234 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7236 If a field width obtained through C<*> is negative, it has the same
7237 effect as the C<-> flag: left-justification.
7239 =item precision, or maximum width
7242 You can specify a precision (for numeric conversions) or a maximum
7243 width (for string conversions) by specifying a C<.> followed by a number.
7244 For floating-point formats except C<g> and C<G>, this specifies
7245 how many places right of the decimal point to show (the default being 6).
7248 # these examples are subject to system-specific variation
7249 printf '<%f>', 1; # prints "<1.000000>"
7250 printf '<%.1f>', 1; # prints "<1.0>"
7251 printf '<%.0f>', 1; # prints "<1>"
7252 printf '<%e>', 10; # prints "<1.000000e+01>"
7253 printf '<%.1e>', 10; # prints "<1.0e+01>"
7255 For "g" and "G", this specifies the maximum number of digits to show,
7256 including those prior to the decimal point and those after it; for
7259 # These examples are subject to system-specific variation.
7260 printf '<%g>', 1; # prints "<1>"
7261 printf '<%.10g>', 1; # prints "<1>"
7262 printf '<%g>', 100; # prints "<100>"
7263 printf '<%.1g>', 100; # prints "<1e+02>"
7264 printf '<%.2g>', 100.01; # prints "<1e+02>"
7265 printf '<%.5g>', 100.01; # prints "<100.01>"
7266 printf '<%.4g>', 100.01; # prints "<100>"
7268 For integer conversions, specifying a precision implies that the
7269 output of the number itself should be zero-padded to this width,
7270 where the 0 flag is ignored:
7272 printf '<%.6d>', 1; # prints "<000001>"
7273 printf '<%+.6d>', 1; # prints "<+000001>"
7274 printf '<%-10.6d>', 1; # prints "<000001 >"
7275 printf '<%10.6d>', 1; # prints "< 000001>"
7276 printf '<%010.6d>', 1; # prints "< 000001>"
7277 printf '<%+10.6d>', 1; # prints "< +000001>"
7279 printf '<%.6x>', 1; # prints "<000001>"
7280 printf '<%#.6x>', 1; # prints "<0x000001>"
7281 printf '<%-10.6x>', 1; # prints "<000001 >"
7282 printf '<%10.6x>', 1; # prints "< 000001>"
7283 printf '<%010.6x>', 1; # prints "< 000001>"
7284 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7286 For string conversions, specifying a precision truncates the string
7287 to fit the specified width:
7289 printf '<%.5s>', "truncated"; # prints "<trunc>"
7290 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7292 You can also get the precision from the next argument using C<.*>, or from a
7293 specified argument (e.g., with C<.*2$>):
7295 printf '<%.6x>', 1; # prints "<000001>"
7296 printf '<%.*x>', 6, 1; # prints "<000001>"
7298 printf '<%.*2$x>', 1, 6; # prints "<000001>"
7300 printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
7302 If a precision obtained through C<*> is negative, it counts
7303 as having no precision at all.
7305 printf '<%.*s>', 7, "string"; # prints "<string>"
7306 printf '<%.*s>', 3, "string"; # prints "<str>"
7307 printf '<%.*s>', 0, "string"; # prints "<>"
7308 printf '<%.*s>', -1, "string"; # prints "<string>"
7310 printf '<%.*d>', 1, 0; # prints "<0>"
7311 printf '<%.*d>', 0, 0; # prints "<>"
7312 printf '<%.*d>', -1, 0; # prints "<0>"
7316 For numeric conversions, you can specify the size to interpret the
7317 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7318 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7319 whatever the default integer size is on your platform (usually 32 or 64
7320 bits), but you can override this to use instead one of the standard C types,
7321 as supported by the compiler used to build Perl:
7323 hh interpret integer as C type "char" or "unsigned
7324 char" on Perl 5.14 or later
7325 h interpret integer as C type "short" or
7327 j interpret integer as C type "intmax_t" on Perl
7328 5.14 or later, and only with a C99 compiler
7330 l interpret integer as C type "long" or
7332 q, L, or ll interpret integer as C type "long long",
7333 "unsigned long long", or "quad" (typically
7335 t interpret integer as C type "ptrdiff_t" on Perl
7337 z interpret integer as C type "size_t" on Perl 5.14
7340 As of 5.14, none of these raises an exception if they are not supported on
7341 your platform. However, if warnings are enabled, a warning of the
7342 C<printf> warning class is issued on an unsupported conversion flag.
7343 Should you instead prefer an exception, do this:
7345 use warnings FATAL => "printf";
7347 If you would like to know about a version dependency before you
7348 start running the program, put something like this at its top:
7350 use 5.014; # for hh/j/t/z/ printf modifiers
7352 You can find out whether your Perl supports quads via L<Config>:
7355 if ($Config{use64bitint} eq "define"
7356 || $Config{longsize} >= 8) {
7357 print "Nice quads!\n";
7360 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7361 to be the default floating-point size on your platform (double or long double),
7362 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7363 platform supports them. You can find out whether your Perl supports long
7364 doubles via L<Config>:
7367 print "long doubles\n" if $Config{d_longdbl} eq "define";
7369 You can find out whether Perl considers "long double" to be the default
7370 floating-point size to use on your platform via L<Config>:
7373 if ($Config{uselongdouble} eq "define") {
7374 print "long doubles by default\n";
7377 It can also be that long doubles and doubles are the same thing:
7380 ($Config{doublesize} == $Config{longdblsize}) &&
7381 print "doubles are long doubles\n";
7383 The size specifier C<V> has no effect for Perl code, but is supported for
7384 compatibility with XS code. It means "use the standard size for a Perl
7385 integer or floating-point number", which is the default.
7387 =item order of arguments
7389 Normally, sprintf() takes the next unused argument as the value to
7390 format for each format specification. If the format specification
7391 uses C<*> to require additional arguments, these are consumed from
7392 the argument list in the order they appear in the format
7393 specification I<before> the value to format. Where an argument is
7394 specified by an explicit index, this does not affect the normal
7395 order for the arguments, even when the explicitly specified index
7396 would have been the next argument.
7400 printf "<%*.*s>", $a, $b, $c;
7402 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7403 as the value to format; while:
7405 printf '<%*1$.*s>', $a, $b;
7407 would use C<$a> for the width and precision, and C<$b> as the
7410 Here are some more examples; be aware that when using an explicit
7411 index, the C<$> may need escaping:
7413 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7414 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7415 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7416 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7417 printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
7421 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7422 and POSIX::setlocale() has been called,
7423 the character used for the decimal separator in formatted floating-point
7424 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
7428 X<sqrt> X<root> X<square root>
7432 =for Pod::Functions square root function
7434 Return the positive square root of EXPR. If EXPR is omitted, uses
7435 C<$_>. Works only for non-negative operands unless you've
7436 loaded the C<Math::Complex> module.
7439 print sqrt(-4); # prints 2i
7442 X<srand> X<seed> X<randseed>
7446 =for Pod::Functions seed the random number generator
7448 Sets and returns the random number seed for the C<rand> operator.
7450 The point of the function is to "seed" the C<rand> function so that C<rand>
7451 can produce a different sequence each time you run your program. When
7452 called with a parameter, C<srand> uses that for the seed; otherwise it
7453 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7454 it returns the seed. To signal that your code will work I<only> on Perls
7455 of a recent vintage:
7457 use 5.014; # so srand returns the seed
7459 If C<srand()> is not called explicitly, it is called implicitly without a
7460 parameter at the first use of the C<rand> operator.
7461 However, there are a few situations where programs are likely to
7462 want to call C<srand>. One is for generating predictable results, generally for
7463 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7464 each time. Another case is that you may want to call C<srand()>
7465 after a C<fork()> to avoid child processes sharing the same seed value as the
7466 parent (and consequently each other).
7468 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7469 process. The internal state of the random number generator should
7470 contain more entropy than can be provided by any seed, so calling
7471 C<srand()> again actually I<loses> randomness.
7473 Most implementations of C<srand> take an integer and will silently
7474 truncate decimal numbers. This means C<srand(42)> will usually
7475 produce the same results as C<srand(42.1)>. To be safe, always pass
7476 C<srand> an integer.
7478 A typical use of the returned seed is for a test program which has too many
7479 combinations to test comprehensively in the time available to it each run. It
7480 can test a random subset each time, and should there be a failure, log the seed
7481 used for that run so that it can later be used to reproduce the same results.
7483 B<C<rand()> is not cryptographically secure. You should not rely
7484 on it in security-sensitive situations.> As of this writing, a
7485 number of third-party CPAN modules offer random number generators
7486 intended by their authors to be cryptographically secure,
7487 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7488 and L<Math::TrulyRandom>.
7490 =item stat FILEHANDLE
7491 X<stat> X<file, status> X<ctime>
7495 =item stat DIRHANDLE
7499 =for Pod::Functions get a file's status information
7501 Returns a 13-element list giving the status info for a file, either
7502 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7503 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7506 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7507 $atime,$mtime,$ctime,$blksize,$blocks)
7510 Not all fields are supported on all filesystem types. Here are the
7511 meanings of the fields:
7513 0 dev device number of filesystem
7515 2 mode file mode (type and permissions)
7516 3 nlink number of (hard) links to the file
7517 4 uid numeric user ID of file's owner
7518 5 gid numeric group ID of file's owner
7519 6 rdev the device identifier (special files only)
7520 7 size total size of file, in bytes
7521 8 atime last access time in seconds since the epoch
7522 9 mtime last modify time in seconds since the epoch
7523 10 ctime inode change time in seconds since the epoch (*)
7524 11 blksize preferred I/O size in bytes for interacting with the
7525 file (may vary from file to file)
7526 12 blocks actual number of system-specific blocks allocated
7527 on disk (often, but not always, 512 bytes each)
7529 (The epoch was at 00:00 January 1, 1970 GMT.)
7531 (*) Not all fields are supported on all filesystem types. Notably, the
7532 ctime field is non-portable. In particular, you cannot expect it to be a
7533 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7535 If C<stat> is passed the special filehandle consisting of an underline, no
7536 stat is done, but the current contents of the stat structure from the
7537 last C<stat>, C<lstat>, or filetest are returned. Example:
7539 if (-x $file && (($d) = stat(_)) && $d < 0) {
7540 print "$file is executable NFS file\n";
7543 (This works on machines only for which the device number is negative
7546 Because the mode contains both the file type and its permissions, you
7547 should mask off the file type portion and (s)printf using a C<"%o">
7548 if you want to see the real permissions.
7550 $mode = (stat($filename))[2];
7551 printf "Permissions are %04o\n", $mode & 07777;
7553 In scalar context, C<stat> returns a boolean value indicating success
7554 or failure, and, if successful, sets the information associated with
7555 the special filehandle C<_>.
7557 The L<File::stat> module provides a convenient, by-name access mechanism:
7560 $sb = stat($filename);
7561 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7562 $filename, $sb->size, $sb->mode & 07777,
7563 scalar localtime $sb->mtime;
7565 You can import symbolic mode constants (C<S_IF*>) and functions
7566 (C<S_IS*>) from the Fcntl module:
7570 $mode = (stat($filename))[2];
7572 $user_rwx = ($mode & S_IRWXU) >> 6;
7573 $group_read = ($mode & S_IRGRP) >> 3;
7574 $other_execute = $mode & S_IXOTH;
7576 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7578 $is_setuid = $mode & S_ISUID;
7579 $is_directory = S_ISDIR($mode);
7581 You could write the last two using the C<-u> and C<-d> operators.
7582 Commonly available C<S_IF*> constants are:
7584 # Permissions: read, write, execute, for user, group, others.
7586 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7587 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7588 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7590 # Setuid/Setgid/Stickiness/SaveText.
7591 # Note that the exact meaning of these is system-dependent.
7593 S_ISUID S_ISGID S_ISVTX S_ISTXT
7595 # File types. Not all are necessarily available on
7598 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7599 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7601 # The following are compatibility aliases for S_IRUSR,
7602 # S_IWUSR, and S_IXUSR.
7604 S_IREAD S_IWRITE S_IEXEC
7606 and the C<S_IF*> functions are
7608 S_IMODE($mode) the part of $mode containing the permission
7609 bits and the setuid/setgid/sticky bits
7611 S_IFMT($mode) the part of $mode containing the file type
7612 which can be bit-anded with (for example)
7613 S_IFREG or with the following functions
7615 # The operators -f, -d, -l, -b, -c, -p, and -S.
7617 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7618 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7620 # No direct -X operator counterpart, but for the first one
7621 # the -g operator is often equivalent. The ENFMT stands for
7622 # record flocking enforcement, a platform-dependent feature.
7624 S_ISENFMT($mode) S_ISWHT($mode)
7626 See your native chmod(2) and stat(2) documentation for more details
7627 about the C<S_*> constants. To get status info for a symbolic link
7628 instead of the target file behind the link, use the C<lstat> function.
7630 Portability issues: L<perlport/stat>.
7635 =item state TYPE VARLIST
7637 =item state VARLIST : ATTRS
7639 =item state TYPE VARLIST : ATTRS
7641 =for Pod::Functions +state declare and assign a persistent lexical variable
7643 C<state> declares a lexically scoped variable, just like C<my>.
7644 However, those variables will never be reinitialized, contrary to
7645 lexical variables that are reinitialized each time their enclosing block
7647 See L<perlsub/"Persistent Private Variables"> for details.
7649 If more than one variable is listed, the list must be placed in
7650 parentheses. With a parenthesised list, C<undef> can be used as a
7651 dummy placeholder. However, since initialization of state variables in
7652 list context is currently not possible this would serve no purpose.
7654 C<state> variables are enabled only when the C<use feature "state"> pragma
7655 is in effect, unless the keyword is written as C<CORE::state>.
7656 See also L<feature>. Alternately, include a C<use v5.10> or later to the
7664 =for Pod::Functions optimize input data for repeated searches
7666 May take extra time to study SCALAR (C<$_> if unspecified) in anticipation
7667 of doing many pattern matches on the string before it is next modified.
7668 This may or may not save time, depending on the nature and number of
7669 patterns you are searching and the distribution of character
7670 frequencies in the string to be searched; you probably want to compare
7671 run times with and without it to see which is faster. Those loops
7672 that scan for many short constant strings (including the constant
7673 parts of more complex patterns) will benefit most.
7675 Note that since Perl version 5.16 this function has been a no-op, but
7676 this might change in a future release.
7678 (The way C<study> works is this: a linked list of every
7679 character in the string to be searched is made, so we know, for
7680 example, where all the C<'k'> characters are. From each search string,
7681 the rarest character is selected, based on some static frequency tables
7682 constructed from some C programs and English text. Only those places
7683 that contain this "rarest" character are examined.)
7685 For example, here is a loop that inserts index producing entries
7686 before any line containing a certain pattern:
7690 print ".IX foo\n" if /\bfoo\b/;
7691 print ".IX bar\n" if /\bbar\b/;
7692 print ".IX blurfl\n" if /\bblurfl\b/;
7697 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7698 will be looked at, because C<f> is rarer than C<o>. In general, this is
7699 a big win except in pathological cases. The only question is whether
7700 it saves you more time than it took to build the linked list in the
7703 Note that if you have to look for strings that you don't know till
7704 runtime, you can build an entire loop as a string and C<eval> that to
7705 avoid recompiling all your patterns all the time. Together with
7706 undefining C<$/> to input entire files as one record, this can be quite
7707 fast, often faster than specialized programs like fgrep(1). The following
7708 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7709 out the names of those files that contain a match:
7711 $search = 'while (<>) { study;';
7712 foreach $word (@words) {
7713 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7718 eval $search; # this screams
7719 $/ = "\n"; # put back to normal input delimiter
7720 foreach $file (sort keys(%seen)) {
7724 =item sub NAME BLOCK
7727 =item sub NAME (PROTO) BLOCK
7729 =item sub NAME : ATTRS BLOCK
7731 =item sub NAME (PROTO) : ATTRS BLOCK
7733 =for Pod::Functions declare a subroutine, possibly anonymously
7735 This is subroutine definition, not a real function I<per se>. Without a
7736 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7737 function declaration, so does return a value: the CODE ref of the closure
7740 See L<perlsub> and L<perlref> for details about subroutines and
7741 references; see L<attributes> and L<Attribute::Handlers> for more
7742 information about attributes.
7747 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7749 A special token that returns a reference to the current subroutine, or
7750 C<undef> outside of a subroutine.
7752 The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>)
7753 is subject to change.
7755 This token is only available under C<use v5.16> or the "current_sub"
7756 feature. See L<feature>.
7758 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7759 X<substr> X<substring> X<mid> X<left> X<right>
7761 =item substr EXPR,OFFSET,LENGTH
7763 =item substr EXPR,OFFSET
7765 =for Pod::Functions get or alter a portion of a string
7767 Extracts a substring out of EXPR and returns it. First character is at
7768 offset zero. If OFFSET is negative, starts
7769 that far back from the end of the string. If LENGTH is omitted, returns
7770 everything through the end of the string. If LENGTH is negative, leaves that
7771 many characters off the end of the string.
7773 my $s = "The black cat climbed the green tree";
7774 my $color = substr $s, 4, 5; # black
7775 my $middle = substr $s, 4, -11; # black cat climbed the
7776 my $end = substr $s, 14; # climbed the green tree
7777 my $tail = substr $s, -4; # tree
7778 my $z = substr $s, -4, 2; # tr
7780 You can use the substr() function as an lvalue, in which case EXPR
7781 must itself be an lvalue. If you assign something shorter than LENGTH,
7782 the string will shrink, and if you assign something longer than LENGTH,
7783 the string will grow to accommodate it. To keep the string the same
7784 length, you may need to pad or chop your value using C<sprintf>.
7786 If OFFSET and LENGTH specify a substring that is partly outside the
7787 string, only the part within the string is returned. If the substring
7788 is beyond either end of the string, substr() returns the undefined
7789 value and produces a warning. When used as an lvalue, specifying a
7790 substring that is entirely outside the string raises an exception.
7791 Here's an example showing the behavior for boundary cases:
7794 substr($name, 4) = 'dy'; # $name is now 'freddy'
7795 my $null = substr $name, 6, 2; # returns "" (no warning)
7796 my $oops = substr $name, 7; # returns undef, with warning
7797 substr($name, 7) = 'gap'; # raises an exception
7799 An alternative to using substr() as an lvalue is to specify the
7800 replacement string as the 4th argument. This allows you to replace
7801 parts of the EXPR and return what was there before in one operation,
7802 just as you can with splice().
7804 my $s = "The black cat climbed the green tree";
7805 my $z = substr $s, 14, 7, "jumped from"; # climbed
7806 # $s is now "The black cat jumped from the green tree"
7808 Note that the lvalue returned by the three-argument version of substr() acts as
7809 a 'magic bullet'; each time it is assigned to, it remembers which part
7810 of the original string is being modified; for example:
7813 for (substr($x,1,2)) {
7814 $_ = 'a'; print $x,"\n"; # prints 1a4
7815 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7817 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7820 With negative offsets, it remembers its position from the end of the string
7821 when the target string is modified:
7824 for (substr($x, -3, 2)) {
7825 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7827 print $_,"\n"; # prints f
7830 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7831 unspecified. Prior to 5.16, the result with negative offsets was
7834 =item symlink OLDFILE,NEWFILE
7835 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7837 =for Pod::Functions create a symbolic link to a file
7839 Creates a new filename symbolically linked to the old filename.
7840 Returns C<1> for success, C<0> otherwise. On systems that don't support
7841 symbolic links, raises an exception. To check for that,
7844 $symlink_exists = eval { symlink("",""); 1 };
7846 Portability issues: L<perlport/symlink>.
7848 =item syscall NUMBER, LIST
7849 X<syscall> X<system call>
7851 =for Pod::Functions execute an arbitrary system call
7853 Calls the system call specified as the first element of the list,
7854 passing the remaining elements as arguments to the system call. If
7855 unimplemented, raises an exception. The arguments are interpreted
7856 as follows: if a given argument is numeric, the argument is passed as
7857 an int. If not, the pointer to the string value is passed. You are
7858 responsible to make sure a string is pre-extended long enough to
7859 receive any result that might be written into a string. You can't use a
7860 string literal (or other read-only string) as an argument to C<syscall>
7861 because Perl has to assume that any string pointer might be written
7863 integer arguments are not literals and have never been interpreted in a
7864 numeric context, you may need to add C<0> to them to force them to look
7865 like numbers. This emulates the C<syswrite> function (or vice versa):
7867 require 'syscall.ph'; # may need to run h2ph
7869 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7871 Note that Perl supports passing of up to only 14 arguments to your syscall,
7872 which in practice should (usually) suffice.
7874 Syscall returns whatever value returned by the system call it calls.
7875 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7876 Note that some system calls I<can> legitimately return C<-1>. The proper
7877 way to handle such calls is to assign C<$!=0> before the call, then
7878 check the value of C<$!> if C<syscall> returns C<-1>.
7880 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7881 number of the read end of the pipe it creates, but there is no way
7882 to retrieve the file number of the other end. You can avoid this
7883 problem by using C<pipe> instead.
7885 Portability issues: L<perlport/syscall>.
7887 =item sysopen FILEHANDLE,FILENAME,MODE
7890 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7892 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7894 Opens the file whose filename is given by FILENAME, and associates it with
7895 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7896 filehandle wanted; an undefined scalar will be suitably autovivified. This
7897 function calls the underlying operating system's I<open>(2) function with the
7898 parameters FILENAME, MODE, and PERMS.
7900 Returns true on success and C<undef> otherwise.
7902 The possible values and flag bits of the MODE parameter are
7903 system-dependent; they are available via the standard module C<Fcntl>. See
7904 the documentation of your operating system's I<open>(2) syscall to see
7905 which values and flag bits are available. You may combine several flags
7906 using the C<|>-operator.
7908 Some of the most common values are C<O_RDONLY> for opening the file in
7909 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7910 and C<O_RDWR> for opening the file in read-write mode.
7911 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7913 For historical reasons, some values work on almost every system
7914 supported by Perl: 0 means read-only, 1 means write-only, and 2
7915 means read/write. We know that these values do I<not> work under
7916 OS/390 and on the Macintosh; you probably don't want to
7917 use them in new code.
7919 If the file named by FILENAME does not exist and the C<open> call creates
7920 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7921 PERMS specifies the permissions of the newly created file. If you omit
7922 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7923 These permission values need to be in octal, and are modified by your
7924 process's current C<umask>.
7927 In many systems the C<O_EXCL> flag is available for opening files in
7928 exclusive mode. This is B<not> locking: exclusiveness means here that
7929 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7930 on network filesystems, and has no effect unless the C<O_CREAT> flag
7931 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7932 being opened if it is a symbolic link. It does not protect against
7933 symbolic links in the file's path.
7936 Sometimes you may want to truncate an already-existing file. This
7937 can be done using the C<O_TRUNC> flag. The behavior of
7938 C<O_TRUNC> with C<O_RDONLY> is undefined.
7941 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7942 that takes away the user's option to have a more permissive umask.
7943 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7946 Note that C<sysopen> depends on the fdopen() C library function.
7947 On many Unix systems, fdopen() is known to fail when file descriptors
7948 exceed a certain value, typically 255. If you need more file
7949 descriptors than that, consider using the POSIX::open() function.
7951 See L<perlopentut> for a kinder, gentler explanation of opening files.
7953 Portability issues: L<perlport/sysopen>.
7955 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7958 =item sysread FILEHANDLE,SCALAR,LENGTH
7960 =for Pod::Functions fixed-length unbuffered input from a filehandle
7962 Attempts to read LENGTH bytes of data into variable SCALAR from the
7963 specified FILEHANDLE, using the read(2). It bypasses
7964 buffered IO, so mixing this with other kinds of reads, C<print>,
7965 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7966 perlio or stdio layers usually buffers data. Returns the number of
7967 bytes actually read, C<0> at end of file, or undef if there was an
7968 error (in the latter case C<$!> is also set). SCALAR will be grown or
7969 shrunk so that the last byte actually read is the last byte of the
7970 scalar after the read.
7972 An OFFSET may be specified to place the read data at some place in the
7973 string other than the beginning. A negative OFFSET specifies
7974 placement at that many characters counting backwards from the end of
7975 the string. A positive OFFSET greater than the length of SCALAR
7976 results in the string being padded to the required size with C<"\0">
7977 bytes before the result of the read is appended.
7979 There is no syseof() function, which is ok, since eof() doesn't work
7980 well on device files (like ttys) anyway. Use sysread() and check
7981 for a return value for 0 to decide whether you're done.
7983 Note that if the filehandle has been marked as C<:utf8> Unicode
7984 characters are read instead of bytes (the LENGTH, OFFSET, and the
7985 return value of sysread() are in Unicode characters).
7986 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7987 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7989 =item sysseek FILEHANDLE,POSITION,WHENCE
7992 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
7994 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7995 be an expression whose value gives the name of the filehandle. The values
7996 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7997 to the current position plus POSITION; and C<2> to set it to EOF plus
7998 POSITION, typically negative.
8000 Note the I<in bytes>: even if the filehandle has been set to operate
8001 on characters (for example by using the C<:encoding(utf8)> I/O layer),
8002 tell() will return byte offsets, not character offsets (because
8003 implementing that would render sysseek() unacceptably slow).
8005 sysseek() bypasses normal buffered IO, so mixing it with reads other
8006 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
8007 C<seek>, C<tell>, or C<eof> may cause confusion.
8009 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8010 and C<SEEK_END> (start of the file, current position, end of the file)
8011 from the Fcntl module. Use of the constants is also more portable
8012 than relying on 0, 1, and 2. For example to define a "systell" function:
8014 use Fcntl 'SEEK_CUR';
8015 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8017 Returns the new position, or the undefined value on failure. A position
8018 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
8019 true on success and false on failure, yet you can still easily determine
8025 =item system PROGRAM LIST
8027 =for Pod::Functions run a separate program
8029 Does exactly the same thing as C<exec LIST>, except that a fork is
8030 done first and the parent process waits for the child process to
8031 exit. Note that argument processing varies depending on the
8032 number of arguments. If there is more than one argument in LIST,
8033 or if LIST is an array with more than one value, starts the program
8034 given by the first element of the list with arguments given by the
8035 rest of the list. If there is only one scalar argument, the argument
8036 is checked for shell metacharacters, and if there are any, the
8037 entire argument is passed to the system's command shell for parsing
8038 (this is C</bin/sh -c> on Unix platforms, but varies on other
8039 platforms). If there are no shell metacharacters in the argument,
8040 it is split into words and passed directly to C<execvp>, which is
8041 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8042 reliably avoid using the shell; C<system LIST>, even with more than one
8043 element, will fall back to the shell if the first spawn fails.
8045 Perl will attempt to flush all files opened for
8046 output before any operation that may do a fork, but this may not be
8047 supported on some platforms (see L<perlport>). To be safe, you may need
8048 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
8049 of C<IO::Handle> on any open handles.
8051 The return value is the exit status of the program as returned by the
8052 C<wait> call. To get the actual exit value, shift right by eight (see
8053 below). See also L</exec>. This is I<not> what you want to use to capture
8054 the output from a command; for that you should use merely backticks or
8055 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
8056 indicates a failure to start the program or an error of the wait(2) system
8057 call (inspect $! for the reason).
8059 If you'd like to make C<system> (and many other bits of Perl) die on error,
8060 have a look at the L<autodie> pragma.
8062 Like C<exec>, C<system> allows you to lie to a program about its name if
8063 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
8065 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8066 C<system>, if you expect your program to terminate on receipt of these
8067 signals you will need to arrange to do so yourself based on the return
8070 @args = ("command", "arg1", "arg2");
8072 or die "system @args failed: $?"
8074 If you'd like to manually inspect C<system>'s failure, you can check all
8075 possible failure modes by inspecting C<$?> like this:
8078 print "failed to execute: $!\n";
8081 printf "child died with signal %d, %s coredump\n",
8082 ($? & 127), ($? & 128) ? 'with' : 'without';
8085 printf "child exited with value %d\n", $? >> 8;
8088 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
8089 with the C<W*()> calls from the POSIX module.
8091 When C<system>'s arguments are executed indirectly by the shell,
8092 results and return codes are subject to its quirks.
8093 See L<perlop/"`STRING`"> and L</exec> for details.
8095 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
8096 handler. See L<perlipc> for details.
8098 Portability issues: L<perlport/system>.
8100 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8103 =item syswrite FILEHANDLE,SCALAR,LENGTH
8105 =item syswrite FILEHANDLE,SCALAR
8107 =for Pod::Functions fixed-length unbuffered output to a filehandle
8109 Attempts to write LENGTH bytes of data from variable SCALAR to the
8110 specified FILEHANDLE, using write(2). If LENGTH is
8111 not specified, writes whole SCALAR. It bypasses buffered IO, so
8112 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
8113 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
8114 stdio layers usually buffer data. Returns the number of bytes
8115 actually written, or C<undef> if there was an error (in this case the
8116 errno variable C<$!> is also set). If the LENGTH is greater than the
8117 data available in the SCALAR after the OFFSET, only as much data as is
8118 available will be written.
8120 An OFFSET may be specified to write the data from some part of the
8121 string other than the beginning. A negative OFFSET specifies writing
8122 that many characters counting backwards from the end of the string.
8123 If SCALAR is of length zero, you can only use an OFFSET of 0.
8125 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8126 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8127 return value of syswrite() are in (UTF8-encoded Unicode) characters.
8128 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8129 Alternately, if the handle is not marked with an encoding but you
8130 attempt to write characters with code points over 255, raises an exception.
8131 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8133 =item tell FILEHANDLE
8138 =for Pod::Functions get current seekpointer on a filehandle
8140 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8141 error. FILEHANDLE may be an expression whose value gives the name of
8142 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8145 Note the I<in bytes>: even if the filehandle has been set to
8146 operate on characters (for example by using the C<:encoding(utf8)> open
8147 layer), tell() will return byte offsets, not character offsets (because
8148 that would render seek() and tell() rather slow).
8150 The return value of tell() for the standard streams like the STDIN
8151 depends on the operating system: it may return -1 or something else.
8152 tell() on pipes, fifos, and sockets usually returns -1.
8154 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
8156 Do not use tell() (or other buffered I/O operations) on a filehandle
8157 that has been manipulated by sysread(), syswrite(), or sysseek().
8158 Those functions ignore the buffering, while tell() does not.
8160 =item telldir DIRHANDLE
8163 =for Pod::Functions get current seekpointer on a directory handle
8165 Returns the current position of the C<readdir> routines on DIRHANDLE.
8166 Value may be given to C<seekdir> to access a particular location in a
8167 directory. C<telldir> has the same caveats about possible directory
8168 compaction as the corresponding system library routine.
8170 =item tie VARIABLE,CLASSNAME,LIST
8173 =for Pod::Functions +5.002 bind a variable to an object class
8175 This function binds a variable to a package class that will provide the
8176 implementation for the variable. VARIABLE is the name of the variable
8177 to be enchanted. CLASSNAME is the name of a class implementing objects
8178 of correct type. Any additional arguments are passed to the
8179 appropriate constructor
8180 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8181 or C<TIEHASH>). Typically these are arguments such as might be passed
8182 to the C<dbm_open()> function of C. The object returned by the
8183 constructor is also returned by the C<tie> function, which would be useful
8184 if you want to access other methods in CLASSNAME.
8186 Note that functions such as C<keys> and C<values> may return huge lists
8187 when used on large objects, like DBM files. You may prefer to use the
8188 C<each> function to iterate over such. Example:
8190 # print out history file offsets
8192 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8193 while (($key,$val) = each %HIST) {
8194 print $key, ' = ', unpack('L',$val), "\n";
8198 A class implementing a hash should have the following methods:
8200 TIEHASH classname, LIST
8202 STORE this, key, value
8207 NEXTKEY this, lastkey
8212 A class implementing an ordinary array should have the following methods:
8214 TIEARRAY classname, LIST
8216 STORE this, key, value
8218 STORESIZE this, count
8224 SPLICE this, offset, length, LIST
8231 A class implementing a filehandle should have the following methods:
8233 TIEHANDLE classname, LIST
8234 READ this, scalar, length, offset
8237 WRITE this, scalar, length, offset
8239 PRINTF this, format, LIST
8243 SEEK this, position, whence
8245 OPEN this, mode, LIST
8250 A class implementing a scalar should have the following methods:
8252 TIESCALAR classname, LIST
8258 Not all methods indicated above need be implemented. See L<perltie>,
8259 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8261 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8262 for you; you need to do that explicitly yourself. See L<DB_File>
8263 or the F<Config> module for interesting C<tie> implementations.
8265 For further details see L<perltie>, L<"tied VARIABLE">.
8270 =for Pod::Functions get a reference to the object underlying a tied variable
8272 Returns a reference to the object underlying VARIABLE (the same value
8273 that was originally returned by the C<tie> call that bound the variable
8274 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8280 =for Pod::Functions return number of seconds since 1970
8282 Returns the number of non-leap seconds since whatever time the system
8283 considers to be the epoch, suitable for feeding to C<gmtime> and
8284 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8285 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8286 1904 in the current local time zone for its epoch.
8288 For measuring time in better granularity than one second, use the
8289 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8290 if you have gettimeofday(2), you may be able to use the C<syscall>
8291 interface of Perl. See L<perlfaq8> for details.
8293 For date and time processing look at the many related modules on CPAN.
8294 For a comprehensive date and time representation look at the
8300 =for Pod::Functions return elapsed time for self and child processes
8302 Returns a four-element list giving the user and system times in
8303 seconds for this process and any exited children of this process.
8305 ($user,$system,$cuser,$csystem) = times;
8307 In scalar context, C<times> returns C<$user>.
8309 Children's times are only included for terminated children.
8311 Portability issues: L<perlport/times>.
8315 =for Pod::Functions transliterate a string
8317 The transliteration operator. Same as C<y///>. See
8318 L<perlop/"Quote-Like Operators">.
8320 =item truncate FILEHANDLE,LENGTH
8323 =item truncate EXPR,LENGTH
8325 =for Pod::Functions shorten a file
8327 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8328 specified length. Raises an exception if truncate isn't implemented
8329 on your system. Returns true if successful, C<undef> on error.
8331 The behavior is undefined if LENGTH is greater than the length of the
8334 The position in the file of FILEHANDLE is left unchanged. You may want to
8335 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8337 Portability issues: L<perlport/truncate>.
8340 X<uc> X<uppercase> X<toupper>
8344 =for Pod::Functions return upper-case version of a string
8346 Returns an uppercased version of EXPR. This is the internal function
8347 implementing the C<\U> escape in double-quoted strings.
8348 It does not attempt to do titlecase mapping on initial letters. See
8349 L</ucfirst> for that.
8351 If EXPR is omitted, uses C<$_>.
8353 This function behaves the same way under various pragma, such as in a locale,
8357 X<ucfirst> X<uppercase>
8361 =for Pod::Functions return a string with just the next letter in upper case
8363 Returns the value of EXPR with the first character in uppercase
8364 (titlecase in Unicode). This is the internal function implementing
8365 the C<\u> escape in double-quoted strings.
8367 If EXPR is omitted, uses C<$_>.
8369 This function behaves the same way under various pragma, such as in a locale,
8377 =for Pod::Functions set file creation mode mask
8379 Sets the umask for the process to EXPR and returns the previous value.
8380 If EXPR is omitted, merely returns the current umask.
8382 The Unix permission C<rwxr-x---> is represented as three sets of three
8383 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8384 and isn't one of the digits). The C<umask> value is such a number
8385 representing disabled permissions bits. The permission (or "mode")
8386 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8387 even if you tell C<sysopen> to create a file with permissions C<0777>,
8388 if your umask is C<0022>, then the file will actually be created with
8389 permissions C<0755>. If your C<umask> were C<0027> (group can't
8390 write; others can't read, write, or execute), then passing
8391 C<sysopen> C<0666> would create a file with mode C<0640> (because
8392 C<0666 &~ 027> is C<0640>).
8394 Here's some advice: supply a creation mode of C<0666> for regular
8395 files (in C<sysopen>) and one of C<0777> for directories (in
8396 C<mkdir>) and executable files. This gives users the freedom of
8397 choice: if they want protected files, they might choose process umasks
8398 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8399 Programs should rarely if ever make policy decisions better left to
8400 the user. The exception to this is when writing files that should be
8401 kept private: mail files, web browser cookies, I<.rhosts> files, and
8404 If umask(2) is not implemented on your system and you are trying to
8405 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8406 raises an exception. If umask(2) is not implemented and you are
8407 not trying to restrict access for yourself, returns C<undef>.
8409 Remember that a umask is a number, usually given in octal; it is I<not> a
8410 string of octal digits. See also L</oct>, if all you have is a string.
8412 Portability issues: L<perlport/umask>.
8415 X<undef> X<undefine>
8419 =for Pod::Functions remove a variable or function definition
8421 Undefines the value of EXPR, which must be an lvalue. Use only on a
8422 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8423 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8424 will probably not do what you expect on most predefined variables or
8425 DBM list values, so don't do that; see L</delete>. Always returns the
8426 undefined value. You can omit the EXPR, in which case nothing is
8427 undefined, but you still get an undefined value that you could, for
8428 instance, return from a subroutine, assign to a variable, or pass as a
8429 parameter. Examples:
8432 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8436 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8437 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8438 select undef, undef, undef, 0.25;
8439 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8441 Note that this is a unary operator, not a list operator.
8444 X<unlink> X<delete> X<remove> X<rm> X<del>
8448 =for Pod::Functions remove one link to a file
8450 Deletes a list of files. On success, it returns the number of files
8451 it successfully deleted. On failure, it returns false and sets C<$!>
8454 my $unlinked = unlink 'a', 'b', 'c';
8456 unlink glob "*.bak";
8458 On error, C<unlink> will not tell you which files it could not remove.
8459 If you want to know which files you could not remove, try them one
8462 foreach my $file ( @goners ) {
8463 unlink $file or warn "Could not unlink $file: $!";
8466 Note: C<unlink> will not attempt to delete directories unless you are
8467 superuser and the B<-U> flag is supplied to Perl. Even if these
8468 conditions are met, be warned that unlinking a directory can inflict
8469 damage on your filesystem. Finally, using C<unlink> on directories is
8470 not supported on many operating systems. Use C<rmdir> instead.
8472 If LIST is omitted, C<unlink> uses C<$_>.
8474 =item unpack TEMPLATE,EXPR
8477 =item unpack TEMPLATE
8479 =for Pod::Functions convert binary structure into normal perl variables
8481 C<unpack> does the reverse of C<pack>: it takes a string
8482 and expands it out into a list of values.
8483 (In scalar context, it returns merely the first value produced.)
8485 If EXPR is omitted, unpacks the C<$_> string.
8486 See L<perlpacktut> for an introduction to this function.
8488 The string is broken into chunks described by the TEMPLATE. Each chunk
8489 is converted separately to a value. Typically, either the string is a result
8490 of C<pack>, or the characters of the string represent a C structure of some
8493 The TEMPLATE has the same format as in the C<pack> function.
8494 Here's a subroutine that does substring:
8497 my($what,$where,$howmuch) = @_;
8498 unpack("x$where a$howmuch", $what);
8503 sub ordinal { unpack("W",$_[0]); } # same as ord()
8505 In addition to fields allowed in pack(), you may prefix a field with
8506 a %<number> to indicate that
8507 you want a <number>-bit checksum of the items instead of the items
8508 themselves. Default is a 16-bit checksum. Checksum is calculated by
8509 summing numeric values of expanded values (for string fields the sum of
8510 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8512 For example, the following
8513 computes the same number as the System V sum program:
8517 unpack("%32W*",<>) % 65535;
8520 The following efficiently counts the number of set bits in a bit vector:
8522 $setbits = unpack("%32b*", $selectmask);
8524 The C<p> and C<P> formats should be used with care. Since Perl
8525 has no way of checking whether the value passed to C<unpack()>
8526 corresponds to a valid memory location, passing a pointer value that's
8527 not known to be valid is likely to have disastrous consequences.
8529 If there are more pack codes or if the repeat count of a field or a group
8530 is larger than what the remainder of the input string allows, the result
8531 is not well defined: the repeat count may be decreased, or
8532 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8533 If the input string is longer than one described by the TEMPLATE,
8534 the remainder of that input string is ignored.
8536 See L</pack> for more examples and notes.
8538 =item unshift ARRAY,LIST
8541 =for Pod::Functions prepend more elements to the beginning of a list
8543 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8544 depending on how you look at it. Prepends list to the front of the
8545 array and returns the new number of elements in the array.
8547 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8549 Note the LIST is prepended whole, not one element at a time, so the
8550 prepended elements stay in the same order. Use C<reverse> to do the
8553 Starting with Perl 5.14, an experimental feature allowed C<unshift> to take
8554 a scalar expression. This experiment has been deemed unsuccessful, and was
8555 removed as of Perl 5.24.
8557 =item untie VARIABLE
8560 =for Pod::Functions break a tie binding to a variable
8562 Breaks the binding between a variable and a package.
8563 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8564 Has no effect if the variable is not tied.
8566 =item use Module VERSION LIST
8567 X<use> X<module> X<import>
8569 =item use Module VERSION
8571 =item use Module LIST
8577 =for Pod::Functions load in a module at compile time and import its namespace
8579 Imports some semantics into the current package from the named module,
8580 generally by aliasing certain subroutine or variable names into your
8581 package. It is exactly equivalent to
8583 BEGIN { require Module; Module->import( LIST ); }
8585 except that Module I<must> be a bareword.
8586 The importation can be made conditional by using the L<if> module.
8588 In the peculiar C<use VERSION> form, VERSION may be either a positive
8589 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8590 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8591 exception is raised if VERSION is greater than the version of the
8592 current Perl interpreter; Perl will not attempt to parse the rest of the
8593 file. Compare with L</require>, which can do a similar check at run time.
8594 Symmetrically, C<no VERSION> allows you to specify that you want a version
8595 of Perl older than the specified one.
8597 Specifying VERSION as a literal of the form v5.6.1 should generally be
8598 avoided, because it leads to misleading error messages under earlier
8599 versions of Perl (that is, prior to 5.6.0) that do not support this
8600 syntax. The equivalent numeric version should be used instead.
8602 use v5.6.1; # compile time version check
8604 use 5.006_001; # ditto; preferred for backwards compatibility
8606 This is often useful if you need to check the current Perl version before
8607 C<use>ing library modules that won't work with older versions of Perl.
8608 (We try not to do this more than we have to.)
8610 C<use VERSION> also lexically enables all features available in the requested
8611 version as defined by the C<feature> pragma, disabling any features
8612 not in the requested version's feature bundle. See L<feature>.
8613 Similarly, if the specified Perl version is greater than or equal to
8614 5.12.0, strictures are enabled lexically as
8615 with C<use strict>. Any explicit use of
8616 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8617 before it. Later use of C<use VERSION>
8618 will override all behavior of a previous
8619 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
8620 C<use VERSION>. C<use VERSION> does not
8621 load the F<feature.pm> or F<strict.pm>
8624 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8625 C<require> makes sure the module is loaded into memory if it hasn't been
8626 yet. The C<import> is not a builtin; it's just an ordinary static method
8627 call into the C<Module> package to tell the module to import the list of
8628 features back into the current package. The module can implement its
8629 C<import> method any way it likes, though most modules just choose to
8630 derive their C<import> method via inheritance from the C<Exporter> class that
8631 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8632 method can be found then the call is skipped, even if there is an AUTOLOAD
8635 If you do not want to call the package's C<import> method (for instance,
8636 to stop your namespace from being altered), explicitly supply the empty list:
8640 That is exactly equivalent to
8642 BEGIN { require Module }
8644 If the VERSION argument is present between Module and LIST, then the
8645 C<use> will call the VERSION method in class Module with the given
8646 version as an argument. The default VERSION method, inherited from
8647 the UNIVERSAL class, croaks if the given version is larger than the
8648 value of the variable C<$Module::VERSION>.
8650 Again, there is a distinction between omitting LIST (C<import> called
8651 with no arguments) and an explicit empty LIST C<()> (C<import> not
8652 called). Note that there is no comma after VERSION!
8654 Because this is a wide-open interface, pragmas (compiler directives)
8655 are also implemented this way. Currently implemented pragmas are:
8660 use sigtrap qw(SEGV BUS);
8661 use strict qw(subs vars refs);
8662 use subs qw(afunc blurfl);
8663 use warnings qw(all);
8664 use sort qw(stable _quicksort _mergesort);
8666 Some of these pseudo-modules import semantics into the current
8667 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8668 which import symbols into the current package (which are effective
8669 through the end of the file).
8671 Because C<use> takes effect at compile time, it doesn't respect the
8672 ordinary flow control of the code being compiled. In particular, putting
8673 a C<use> inside the false branch of a conditional doesn't prevent it
8674 from being processed. If a module or pragma only needs to be loaded
8675 conditionally, this can be done using the L<if> pragma:
8677 use if $] < 5.008, "utf8";
8678 use if WANT_WARNINGS, warnings => qw(all);
8680 There's a corresponding C<no> declaration that unimports meanings imported
8681 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8682 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8683 or no unimport method being found.
8689 Care should be taken when using the C<no VERSION> form of C<no>. It is
8690 I<only> meant to be used to assert that the running Perl is of a earlier
8691 version than its argument and I<not> to undo the feature-enabling side effects
8694 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8695 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8696 functionality from the command-line.
8701 =for Pod::Functions set a file's last access and modify times
8703 Changes the access and modification times on each file of a list of
8704 files. The first two elements of the list must be the NUMERIC access
8705 and modification times, in that order. Returns the number of files
8706 successfully changed. The inode change time of each file is set
8707 to the current time. For example, this code has the same effect as the
8708 Unix touch(1) command when the files I<already exist> and belong to
8709 the user running the program:
8712 $atime = $mtime = time;
8713 utime $atime, $mtime, @ARGV;
8715 Since Perl 5.8.0, if the first two elements of the list are C<undef>,
8716 the utime(2) syscall from your C library is called with a null second
8717 argument. On most systems, this will set the file's access and
8718 modification times to the current time (i.e., equivalent to the example
8719 above) and will work even on files you don't own provided you have write
8723 utime(undef, undef, $file)
8724 || warn "couldn't touch $file: $!";
8727 Under NFS this will use the time of the NFS server, not the time of
8728 the local machine. If there is a time synchronization problem, the
8729 NFS server and local machine will have different times. The Unix
8730 touch(1) command will in fact normally use this form instead of the
8731 one shown in the first example.
8733 Passing only one of the first two elements as C<undef> is
8734 equivalent to passing a 0 and will not have the effect
8735 described when both are C<undef>. This also triggers an
8736 uninitialized warning.
8738 On systems that support futimes(2), you may pass filehandles among the
8739 files. On systems that don't support futimes(2), passing filehandles raises
8740 an exception. Filehandles must be passed as globs or glob references to be
8741 recognized; barewords are considered filenames.
8743 Portability issues: L<perlport/utime>.
8750 =for Pod::Functions return a list of the values in a hash
8752 In list context, returns a list consisting of all the values of the named
8753 hash. In Perl 5.12 or later only, will also return a list of the values of
8754 an array; prior to that release, attempting to use an array argument will
8755 produce a syntax error. In scalar context, returns the number of values.
8757 Hash entries are returned in an apparently random order. The actual random
8758 order is specific to a given hash; the exact same series of operations
8759 on two hashes may result in a different order for each hash. Any insertion
8760 into the hash may change the order, as will any deletion, with the exception
8761 that the most recent key returned by C<each> or C<keys> may be deleted
8762 without changing the order. So long as a given hash is unmodified you may
8763 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
8764 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
8765 details on why hash order is randomized. Aside from the guarantees
8766 provided here the exact details of Perl's hash algorithm and the hash
8767 traversal order are subject to change in any release of Perl. Tied hashes
8768 may behave differently to Perl's hashes with respect to changes in order on
8769 insertion and deletion of items.
8771 As a side effect, calling values() resets the HASH or ARRAY's internal
8772 iterator, see L</each>. (In particular, calling values() in void context
8773 resets the iterator with no other overhead. Apart from resetting the
8774 iterator, C<values @array> in list context is the same as plain C<@array>.
8775 (We recommend that you use void context C<keys @array> for this, but
8776 reasoned that taking C<values @array> out would require more
8777 documentation than leaving it in.)
8779 Note that the values are not copied, which means modifying them will
8780 modify the contents of the hash:
8782 for (values %hash) { s/foo/bar/g } # modifies %hash values
8783 for (@hash{keys %hash}) { s/foo/bar/g } # same
8785 Starting with Perl 5.14, an experimental feature allowed C<values> to take a
8786 scalar expression. This experiment has been deemed unsuccessful, and was
8787 removed as of Perl 5.24.
8789 To avoid confusing would-be users of your code who are running earlier
8790 versions of Perl with mysterious syntax errors, put this sort of thing at
8791 the top of your file to signal that your code will work I<only> on Perls of
8794 use 5.012; # so keys/values/each work on arrays
8796 See also C<keys>, C<each>, and C<sort>.
8798 =item vec EXPR,OFFSET,BITS
8799 X<vec> X<bit> X<bit vector>
8801 =for Pod::Functions test or set particular bits in a string
8803 Treats the string in EXPR as a bit vector made up of elements of
8804 width BITS and returns the value of the element specified by OFFSET
8805 as an unsigned integer. BITS therefore specifies the number of bits
8806 that are reserved for each element in the bit vector. This must
8807 be a power of two from 1 to 32 (or 64, if your platform supports
8810 If BITS is 8, "elements" coincide with bytes of the input string.
8812 If BITS is 16 or more, bytes of the input string are grouped into chunks
8813 of size BITS/8, and each group is converted to a number as with
8814 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8815 for BITS==64). See L<"pack"> for details.
8817 If bits is 4 or less, the string is broken into bytes, then the bits
8818 of each byte are broken into 8/BITS groups. Bits of a byte are
8819 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8820 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8821 breaking the single input byte C<chr(0x36)> into two groups gives a list
8822 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8824 C<vec> may also be assigned to, in which case parentheses are needed
8825 to give the expression the correct precedence as in
8827 vec($image, $max_x * $x + $y, 8) = 3;
8829 If the selected element is outside the string, the value 0 is returned.
8830 If an element off the end of the string is written to, Perl will first
8831 extend the string with sufficiently many zero bytes. It is an error
8832 to try to write off the beginning of the string (i.e., negative OFFSET).
8834 If the string happens to be encoded as UTF-8 internally (and thus has
8835 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8836 internal byte string, not the conceptual character string, even if you
8837 only have characters with values less than 256.
8839 Strings created with C<vec> can also be manipulated with the logical
8840 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8841 vector operation is desired when both operands are strings.
8842 See L<perlop/"Bitwise String Operators">.
8844 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8845 The comments show the string after each step. Note that this code works
8846 in the same way on big-endian or little-endian machines.
8849 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8851 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8852 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8854 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8855 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8856 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8857 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8858 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8859 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8861 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8862 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8863 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8866 To transform a bit vector into a string or list of 0's and 1's, use these:
8868 $bits = unpack("b*", $vector);
8869 @bits = split(//, unpack("b*", $vector));
8871 If you know the exact length in bits, it can be used in place of the C<*>.
8873 Here is an example to illustrate how the bits actually fall in place:
8879 unpack("V",$_) 01234567890123456789012345678901
8880 ------------------------------------------------------------------
8885 for ($shift=0; $shift < $width; ++$shift) {
8886 for ($off=0; $off < 32/$width; ++$off) {
8887 $str = pack("B*", "0"x32);
8888 $bits = (1<<$shift);
8889 vec($str, $off, $width) = $bits;
8890 $res = unpack("b*",$str);
8891 $val = unpack("V", $str);
8898 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8899 $off, $width, $bits, $val, $res
8903 Regardless of the machine architecture on which it runs, the
8904 example above should print the following table:
8907 unpack("V",$_) 01234567890123456789012345678901
8908 ------------------------------------------------------------------
8909 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8910 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8911 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8912 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8913 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8914 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8915 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8916 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8917 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8918 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8919 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8920 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8921 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8922 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8923 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8924 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8925 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8926 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8927 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8928 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8929 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8930 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8931 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8932 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8933 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8934 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8935 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8936 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8937 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8938 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8939 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8940 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8941 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8942 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8943 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8944 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8945 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8946 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8947 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8948 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8949 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8950 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8951 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8952 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8953 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8954 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8955 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8956 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8957 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8958 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8959 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8960 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8961 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8962 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8963 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8964 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8965 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8966 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8967 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8968 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8969 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8970 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8971 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8972 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8973 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8974 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8975 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8976 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8977 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8978 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8979 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8980 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8981 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8982 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8983 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8984 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8985 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8986 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8987 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8988 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8989 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8990 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8991 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8992 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8993 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8994 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8995 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8996 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8997 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8998 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8999 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9000 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9001 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9002 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9003 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9004 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9005 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9006 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9007 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9008 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9009 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9010 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9011 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9012 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9013 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9014 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9015 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9016 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9017 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9018 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9019 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9020 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9021 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9022 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9023 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9024 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9025 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9026 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9027 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9028 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9029 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9030 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9031 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9032 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9033 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9034 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9035 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9036 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9041 =for Pod::Functions wait for any child process to die
9043 Behaves like wait(2) on your system: it waits for a child
9044 process to terminate and returns the pid of the deceased process, or
9045 C<-1> if there are no child processes. The status is returned in C<$?>
9046 and C<${^CHILD_ERROR_NATIVE}>.
9047 Note that a return value of C<-1> could mean that child processes are
9048 being automatically reaped, as described in L<perlipc>.
9050 If you use C<wait> in your handler for $SIG{CHLD}, it may accidentally wait
9051 for the child created by qx() or system(). See L<perlipc> for details.
9053 Portability issues: L<perlport/wait>.
9055 =item waitpid PID,FLAGS
9058 =for Pod::Functions wait for a particular child process to die
9060 Waits for a particular child process to terminate and returns the pid of
9061 the deceased process, or C<-1> if there is no such child process. On some
9062 systems, a value of 0 indicates that there are processes still running.
9063 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
9065 use POSIX ":sys_wait_h";
9068 $kid = waitpid(-1, WNOHANG);
9071 then you can do a non-blocking wait for all pending zombie processes.
9072 Non-blocking wait is available on machines supporting either the
9073 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
9074 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9075 system call by remembering the status values of processes that have
9076 exited but have not been harvested by the Perl script yet.)
9078 Note that on some systems, a return value of C<-1> could mean that child
9079 processes are being automatically reaped. See L<perlipc> for details,
9080 and for other examples.
9082 Portability issues: L<perlport/waitpid>.
9085 X<wantarray> X<context>
9087 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9089 Returns true if the context of the currently executing subroutine or
9090 C<eval> is looking for a list value. Returns false if the context is
9091 looking for a scalar. Returns the undefined value if the context is
9092 looking for no value (void context).
9094 return unless defined wantarray; # don't bother doing more
9095 my @a = complex_calculation();
9096 return wantarray ? @a : "@a";
9098 C<wantarray()>'s result is unspecified in the top level of a file,
9099 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9100 in a C<DESTROY> method.
9102 This function should have been named wantlist() instead.
9105 X<warn> X<warning> X<STDERR>
9107 =for Pod::Functions print debugging info
9109 Prints the value of LIST to STDERR. If the last element of LIST does
9110 not end in a newline, it appends the same file/line number text as C<die>
9113 If the output is empty and C<$@> already contains a value (typically from a
9114 previous eval) that value is used after appending C<"\t...caught">
9115 to C<$@>. This is useful for staying almost, but not entirely similar to
9118 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
9120 No message is printed if there is a C<$SIG{__WARN__}> handler
9121 installed. It is the handler's responsibility to deal with the message
9122 as it sees fit (like, for instance, converting it into a C<die>). Most
9123 handlers must therefore arrange to actually display the
9124 warnings that they are not prepared to deal with, by calling C<warn>
9125 again in the handler. Note that this is quite safe and will not
9126 produce an endless loop, since C<__WARN__> hooks are not called from
9129 You will find this behavior is slightly different from that of
9130 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
9131 instead call C<die> again to change it).
9133 Using a C<__WARN__> handler provides a powerful way to silence all
9134 warnings (even the so-called mandatory ones). An example:
9136 # wipe out *all* compile-time warnings
9137 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9139 my $foo = 20; # no warning about duplicate my $foo,
9140 # but hey, you asked for it!
9141 # no compile-time or run-time warnings before here
9144 # run-time warnings enabled after here
9145 warn "\$foo is alive and $foo!"; # does show up
9147 See L<perlvar> for details on setting C<%SIG> entries and for more
9148 examples. See the Carp module for other kinds of warnings using its
9149 carp() and cluck() functions.
9151 =item write FILEHANDLE
9158 =for Pod::Functions print a picture record
9160 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9161 using the format associated with that file. By default the format for
9162 a file is the one having the same name as the filehandle, but the
9163 format for the current output channel (see the C<select> function) may be set
9164 explicitly by assigning the name of the format to the C<$~> variable.
9166 Top of form processing is handled automatically: if there is insufficient
9167 room on the current page for the formatted record, the page is advanced by
9168 writing a form feed and a special top-of-page
9169 format is used to format the new
9170 page header before the record is written. By default, the top-of-page
9171 format is the name of the filehandle with "_TOP" appended, or "top"
9172 in the current package if the former does not exist. This would be a
9173 problem with autovivified filehandles, but it may be dynamically set to the
9174 format of your choice by assigning the name to the C<$^> variable while
9175 that filehandle is selected. The number of lines remaining on the current
9176 page is in variable C<$->, which can be set to C<0> to force a new page.
9178 If FILEHANDLE is unspecified, output goes to the current default output
9179 channel, which starts out as STDOUT but may be changed by the
9180 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
9181 is evaluated and the resulting string is used to look up the name of
9182 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9184 Note that write is I<not> the opposite of C<read>. Unfortunately.
9188 =for Pod::Functions transliterate a string
9190 The transliteration operator. Same as C<tr///>. See
9191 L<perlop/"Quote-Like Operators">.
9195 =head2 Non-function Keywords by Cross-reference
9205 These keywords are documented in L<perldata/"Special Literals">.
9223 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9233 This method keyword is documented in L<perlobj/"Destructors">.
9265 These operators are documented in L<perlop>.
9275 This keyword is documented in L<perlsub/"Autoloading">.
9299 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9303 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
9304 or C<else if> either. It does parse C<elseif>, but only to warn you
9307 See the documentation for flow-control keywords in L<perlsyn/"Compound
9320 These flow-control keywords related to the experimental switch feature are
9321 documented in L<perlsyn/"Switch Statements">.