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. 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>
1656 =for Pod::Functions retrieve the next key/value pair from a hash
1658 When called on a hash in list context, returns a 2-element list
1659 consisting of the key and value for the next element of a hash. In Perl
1660 5.12 and later only, it will also return the index and value for the next
1661 element of an array so that you can iterate over it; older Perls consider
1662 this a syntax error. When called in scalar context, returns only the key
1663 (not the value) in a hash, or the index in an array.
1665 Hash entries are returned in an apparently random order. The actual random
1666 order is specific to a given hash; the exact same series of operations
1667 on two hashes may result in a different order for each hash. Any insertion
1668 into the hash may change the order, as will any deletion, with the exception
1669 that the most recent key returned by C<each> or C<keys> may be deleted
1670 without changing the order. So long as a given hash is unmodified you may
1671 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1672 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1673 details on why hash order is randomized. Aside from the guarantees
1674 provided here the exact details of Perl's hash algorithm and the hash
1675 traversal order are subject to change in any release of Perl.
1677 After C<each> has returned all entries from the hash or array, the next
1678 call to C<each> returns the empty list in list context and C<undef> in
1679 scalar context; the next call following I<that> one restarts iteration.
1680 Each hash or array has its own internal iterator, accessed by C<each>,
1681 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1682 reached the end as just described; it can be explicitly reset by calling
1683 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1684 elements while iterating over it, the effect on the iterator is
1685 unspecified; for example, entries may be skipped or duplicated--so don't
1686 do that. Exception: It is always safe to delete the item most recently
1687 returned by C<each()>, so the following code works properly:
1689 while (($key, $value) = each %hash) {
1691 delete $hash{$key}; # This is safe
1694 Tied hashes may have a different ordering behaviour to perl's hash
1697 This prints out your environment like the printenv(1) program,
1698 but in a different order:
1700 while (($key,$value) = each %ENV) {
1701 print "$key=$value\n";
1704 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1705 reference to an unblessed hash or array. The argument will be dereferenced
1706 automatically. This aspect of C<each> is considered highly experimental.
1707 The exact behaviour may change in a future version of Perl.
1709 while (($key,$value) = each $hashref) { ... }
1711 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1712 which will set C<$_> on every iteration.
1715 print "$_=$ENV{$_}\n";
1718 To avoid confusing would-be users of your code who are running earlier
1719 versions of Perl with mysterious syntax errors, put this sort of thing at
1720 the top of your file to signal that your code will work I<only> on Perls of
1723 use 5.012; # so keys/values/each work on arrays
1724 use 5.014; # so keys/values/each work on scalars (experimental)
1725 use 5.018; # so each assigns to $_ in a lone while test
1727 See also C<keys>, C<values>, and C<sort>.
1729 =item eof FILEHANDLE
1738 =for Pod::Functions test a filehandle for its end
1740 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1741 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1742 gives the real filehandle. (Note that this function actually
1743 reads a character and then C<ungetc>s it, so isn't useful in an
1744 interactive context.) Do not read from a terminal file (or call
1745 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1746 as terminals may lose the end-of-file condition if you do.
1748 An C<eof> without an argument uses the last file read. Using C<eof()>
1749 with empty parentheses is different. It refers to the pseudo file
1750 formed from the files listed on the command line and accessed via the
1751 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1752 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1753 used will cause C<@ARGV> to be examined to determine if input is
1754 available. Similarly, an C<eof()> after C<< <> >> has returned
1755 end-of-file will assume you are processing another C<@ARGV> list,
1756 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1757 see L<perlop/"I/O Operators">.
1759 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1760 detect the end of each file, whereas C<eof()> will detect the end
1761 of the very last file only. Examples:
1763 # reset line numbering on each input file
1765 next if /^\s*#/; # skip comments
1768 close ARGV if eof; # Not eof()!
1771 # insert dashes just before last line of last file
1773 if (eof()) { # check for end of last file
1774 print "--------------\n";
1777 last if eof(); # needed if we're reading from a terminal
1780 Practical hint: you almost never need to use C<eof> in Perl, because the
1781 input operators typically return C<undef> when they run out of data or
1785 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1786 X<error, handling> X<exception, handling>
1792 =for Pod::Functions catch exceptions or compile and run code
1794 In the first form, often referred to as a "string eval", the return
1795 value of EXPR is parsed and executed as if it
1796 were a little Perl program. The value of the expression (which is itself
1797 determined within scalar context) is first parsed, and if there were no
1798 errors, executed as a block within the lexical context of the current Perl
1799 program. This means, that in particular, any outer lexical variables are
1800 visible to it, and any package variable settings or subroutine and format
1801 definitions remain afterwards.
1803 Note that the value is parsed every time the C<eval> executes.
1804 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1805 delay parsing and subsequent execution of the text of EXPR until run time.
1807 If the C<unicode_eval> feature is enabled (which is the default under a
1808 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1809 characters, so C<use utf8> declarations have no effect, and source filters
1810 are forbidden. In the absence of the C<unicode_eval> feature, the string
1811 will sometimes be treated as characters and sometimes as bytes, depending
1812 on the internal encoding, and source filters activated within the C<eval>
1813 exhibit the erratic, but historical, behaviour of affecting some outer file
1814 scope that is still compiling. See also the L</evalbytes> keyword, which
1815 always treats its input as a byte stream and works properly with source
1816 filters, and the L<feature> pragma.
1818 Problems can arise if the string expands a scalar containing a floating
1819 point number. That scalar can expand to letters, such as C<"NaN"> or
1820 C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1821 point character may be something other than a dot (such as a comma).
1822 None of these are likely to parse as you are likely expecting.
1824 In the second form, the code within the BLOCK is parsed only once--at the
1825 same time the code surrounding the C<eval> itself was parsed--and executed
1826 within the context of the current Perl program. This form is typically
1827 used to trap exceptions more efficiently than the first (see below), while
1828 also providing the benefit of checking the code within BLOCK at compile
1831 The final semicolon, if any, may be omitted from the value of EXPR or within
1834 In both forms, the value returned is the value of the last expression
1835 evaluated inside the mini-program; a return statement may be also used, just
1836 as with subroutines. The expression providing the return value is evaluated
1837 in void, scalar, or list context, depending on the context of the C<eval>
1838 itself. See L</wantarray> for more on how the evaluation context can be
1841 If there is a syntax error or runtime error, or a C<die> statement is
1842 executed, C<eval> returns C<undef> in scalar context
1843 or an empty list in list context, and C<$@> is set to the error
1844 message. (Prior to 5.16, a bug caused C<undef> to be returned
1845 in list context for syntax errors, but not for runtime errors.)
1846 If there was no error, C<$@> is set to the empty string. A
1847 control flow operator like C<last> or C<goto> can bypass the setting of
1848 C<$@>. Beware that using C<eval> neither silences Perl from printing
1849 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1850 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1851 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1852 See L</warn>, L<perlvar>, and L<warnings>.
1854 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1855 determining whether a particular feature (such as C<socket> or C<symlink>)
1856 is implemented. It is also Perl's exception-trapping mechanism, where
1857 the die operator is used to raise exceptions.
1859 If you want to trap errors when loading an XS module, some problems with
1860 the binary interface (such as Perl version skew) may be fatal even with
1861 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1863 If the code to be executed doesn't vary, you may use the eval-BLOCK
1864 form to trap run-time errors without incurring the penalty of
1865 recompiling each time. The error, if any, is still returned in C<$@>.
1868 # make divide-by-zero nonfatal
1869 eval { $answer = $a / $b; }; warn $@ if $@;
1871 # same thing, but less efficient
1872 eval '$answer = $a / $b'; warn $@ if $@;
1874 # a compile-time error
1875 eval { $answer = }; # WRONG
1878 eval '$answer ='; # sets $@
1880 Using the C<eval{}> form as an exception trap in libraries does have some
1881 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1882 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1883 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1884 as this example shows:
1886 # a private exception trap for divide-by-zero
1887 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1890 This is especially significant, given that C<__DIE__> hooks can call
1891 C<die> again, which has the effect of changing their error messages:
1893 # __DIE__ hooks may modify error messages
1895 local $SIG{'__DIE__'} =
1896 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1897 eval { die "foo lives here" };
1898 print $@ if $@; # prints "bar lives here"
1901 Because this promotes action at a distance, this counterintuitive behavior
1902 may be fixed in a future release.
1904 With an C<eval>, you should be especially careful to remember what's
1905 being looked at when:
1911 eval { $x }; # CASE 4
1913 eval "\$$x++"; # CASE 5
1916 Cases 1 and 2 above behave identically: they run the code contained in
1917 the variable $x. (Although case 2 has misleading double quotes making
1918 the reader wonder what else might be happening (nothing is).) Cases 3
1919 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1920 does nothing but return the value of $x. (Case 4 is preferred for
1921 purely visual reasons, but it also has the advantage of compiling at
1922 compile-time instead of at run-time.) Case 5 is a place where
1923 normally you I<would> like to use double quotes, except that in this
1924 particular situation, you can just use symbolic references instead, as
1927 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1928 of localized variables, which means that for your code to run on older
1929 versions, a temporary is required if you want to mask some but not all
1932 # alter $@ on nefarious repugnancy only
1936 local $@; # protect existing $@
1937 eval { test_repugnancy() };
1938 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1939 $@ =~ /nefarious/ and $e = $@;
1941 die $e if defined $e
1944 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1945 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1947 An C<eval ''> executed within a subroutine defined
1948 in the C<DB> package doesn't see the usual
1949 surrounding lexical scope, but rather the scope of the first non-DB piece
1950 of code that called it. You don't normally need to worry about this unless
1951 you are writing a Perl debugger.
1953 =item evalbytes EXPR
1958 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1960 This function is like L</eval> with a string argument, except it always
1961 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1962 string containing characters whose ordinal value exceeds 255 results in an
1963 error. Source filters activated within the evaluated code apply to the
1966 This function is only available under the C<evalbytes> feature, a
1967 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1968 L<feature> for more information.
1973 =item exec PROGRAM LIST
1975 =for Pod::Functions abandon this program to run another
1977 The C<exec> function executes a system command I<and never returns>;
1978 use C<system> instead of C<exec> if you want it to return. It fails and
1979 returns false only if the command does not exist I<and> it is executed
1980 directly instead of via your system's command shell (see below).
1982 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1983 warns you if C<exec> is called in void context and if there is a following
1984 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1985 you always do that, right?). If you I<really> want to follow an C<exec>
1986 with some other statement, you can use one of these styles to avoid the warning:
1988 exec ('foo') or print STDERR "couldn't exec foo: $!";
1989 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1991 If there is more than one argument in LIST, this calls execvp(3) with the
1992 arguments in LIST. If there is only one element in LIST, the argument is
1993 checked for shell metacharacters, and if there are any, the entire
1994 argument is passed to the system's command shell for parsing (this is
1995 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1996 there are no shell metacharacters in the argument, it is split into words
1997 and passed directly to C<execvp>, which is more efficient. Examples:
1999 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2000 exec "sort $outfile | uniq";
2002 If you don't really want to execute the first argument, but want to lie
2003 to the program you are executing about its own name, you can specify
2004 the program you actually want to run as an "indirect object" (without a
2005 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2006 forces interpretation of the LIST as a multivalued list, even if there
2007 is only a single scalar in the list.) Example:
2009 $shell = '/bin/csh';
2010 exec $shell '-sh'; # pretend it's a login shell
2014 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2016 When the arguments get executed via the system shell, results are
2017 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2020 Using an indirect object with C<exec> or C<system> is also more
2021 secure. This usage (which also works fine with system()) forces
2022 interpretation of the arguments as a multivalued list, even if the
2023 list had just one argument. That way you're safe from the shell
2024 expanding wildcards or splitting up words with whitespace in them.
2026 @args = ( "echo surprise" );
2028 exec @args; # subject to shell escapes
2030 exec { $args[0] } @args; # safe even with one-arg list
2032 The first version, the one without the indirect object, ran the I<echo>
2033 program, passing it C<"surprise"> an argument. The second version didn't;
2034 it tried to run a program named I<"echo surprise">, didn't find it, and set
2035 C<$?> to a non-zero value indicating failure.
2037 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2038 reliably avoid using the shell; C<exec LIST>, even with more than one
2039 element, will fall back to the shell if the first spawn fails.
2041 Perl attempts to flush all files opened for output before the exec,
2042 but this may not be supported on some platforms (see L<perlport>).
2043 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2044 call the C<autoflush()> method of C<IO::Handle> on any open handles
2045 to avoid lost output.
2047 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2048 C<DESTROY> methods on your objects.
2050 Portability issues: L<perlport/exec>.
2053 X<exists> X<autovivification>
2055 =for Pod::Functions test whether a hash key is present
2057 Given an expression that specifies an element of a hash, returns true if the
2058 specified element in the hash has ever been initialized, even if the
2059 corresponding value is undefined.
2061 print "Exists\n" if exists $hash{$key};
2062 print "Defined\n" if defined $hash{$key};
2063 print "True\n" if $hash{$key};
2065 exists may also be called on array elements, but its behavior is much less
2066 obvious and is strongly tied to the use of L</delete> on arrays.
2068 B<WARNING:> Calling C<exists> on array values is strongly discouraged. The
2069 notion of deleting or checking the existence of Perl array elements is not
2070 conceptually coherent, and can lead to surprising behavior.
2072 print "Exists\n" if exists $array[$index];
2073 print "Defined\n" if defined $array[$index];
2074 print "True\n" if $array[$index];
2076 A hash or array element can be true only if it's defined and defined only if
2077 it exists, but the reverse doesn't necessarily hold true.
2079 Given an expression that specifies the name of a subroutine,
2080 returns true if the specified subroutine has ever been declared, even
2081 if it is undefined. Mentioning a subroutine name for exists or defined
2082 does not count as declaring it. Note that a subroutine that does not
2083 exist may still be callable: its package may have an C<AUTOLOAD>
2084 method that makes it spring into existence the first time that it is
2085 called; see L<perlsub>.
2087 print "Exists\n" if exists &subroutine;
2088 print "Defined\n" if defined &subroutine;
2090 Note that the EXPR can be arbitrarily complicated as long as the final
2091 operation is a hash or array key lookup or subroutine name:
2093 if (exists $ref->{A}->{B}->{$key}) { }
2094 if (exists $hash{A}{B}{$key}) { }
2096 if (exists $ref->{A}->{B}->[$ix]) { }
2097 if (exists $hash{A}{B}[$ix]) { }
2099 if (exists &{$ref->{A}{B}{$key}}) { }
2101 Although the most deeply nested array or hash element will not spring into
2102 existence just because its existence was tested, any intervening ones will.
2103 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2104 into existence due to the existence test for the $key element above.
2105 This happens anywhere the arrow operator is used, including even here:
2108 if (exists $ref->{"Some key"}) { }
2109 print $ref; # prints HASH(0x80d3d5c)
2111 This surprising autovivification in what does not at first--or even
2112 second--glance appear to be an lvalue context may be fixed in a future
2115 Use of a subroutine call, rather than a subroutine name, as an argument
2116 to exists() is an error.
2119 exists &sub(); # Error
2122 X<exit> X<terminate> X<abort>
2126 =for Pod::Functions terminate this program
2128 Evaluates EXPR and exits immediately with that value. Example:
2131 exit 0 if $ans =~ /^[Xx]/;
2133 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2134 universally recognized values for EXPR are C<0> for success and C<1>
2135 for error; other values are subject to interpretation depending on the
2136 environment in which the Perl program is running. For example, exiting
2137 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2138 the mailer to return the item undelivered, but that's not true everywhere.
2140 Don't use C<exit> to abort a subroutine if there's any chance that
2141 someone might want to trap whatever error happened. Use C<die> instead,
2142 which can be trapped by an C<eval>.
2144 The exit() function does not always exit immediately. It calls any
2145 defined C<END> routines first, but these C<END> routines may not
2146 themselves abort the exit. Likewise any object destructors that need to
2147 be called are called before the real exit. C<END> routines and destructors
2148 can change the exit status by modifying C<$?>. If this is a problem, you
2149 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2150 See L<perlmod> for details.
2152 Portability issues: L<perlport/exit>.
2155 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2159 =for Pod::Functions raise I<e> to a power
2161 Returns I<e> (the natural logarithm base) to the power of EXPR.
2162 If EXPR is omitted, gives C<exp($_)>.
2165 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2169 =for Pod::Functions +fc return casefolded version of a string
2171 Returns the casefolded version of EXPR. This is the internal function
2172 implementing the C<\F> escape in double-quoted strings.
2174 Casefolding is the process of mapping strings to a form where case
2175 differences are erased; comparing two strings in their casefolded
2176 form is effectively a way of asking if two strings are equal,
2179 Roughly, if you ever found yourself writing this
2181 lc($this) eq lc($that) # Wrong!
2183 uc($this) eq uc($that) # Also wrong!
2185 $this =~ /^\Q$that\E\z/i # Right!
2189 fc($this) eq fc($that)
2191 And get the correct results.
2193 Perl only implements the full form of casefolding,
2194 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2195 L<Unicode::UCD/prop_invmap()>.
2196 For further information on casefolding, refer to
2197 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2198 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2199 available at L<http://www.unicode.org/versions/latest/>, as well as the
2200 Case Charts available at L<http://www.unicode.org/charts/case/>.
2202 If EXPR is omitted, uses C<$_>.
2204 This function behaves the same way under various pragma, such as within
2205 S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2206 exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2207 scope of S<C<use locale>>. The foldcase of this character would
2208 normally be C<"ss">, but as explained in the L</lc> section, case
2209 changes that cross the 255/256 boundary are problematic under locales,
2210 and are hence prohibited. Therefore, this function under locale returns
2211 instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2212 LONG S. Since that character itself folds to C<"s">, the string of two
2213 of them together should be equivalent to a single U+1E9E when foldcased.
2215 While the Unicode Standard defines two additional forms of casefolding,
2216 one for Turkic languages and one that never maps one character into multiple
2217 characters, these are not provided by the Perl core; However, the CPAN module
2218 C<Unicode::Casing> may be used to provide an implementation.
2220 This keyword is available only when the C<"fc"> feature is enabled,
2221 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2222 include a C<use v5.16> or later to the current scope.
2224 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2227 =for Pod::Functions file control system call
2229 Implements the fcntl(2) function. You'll probably have to say
2233 first to get the correct constant definitions. Argument processing and
2234 value returned work just like C<ioctl> below.
2238 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2239 or die "can't fcntl F_GETFL: $!";
2241 You don't have to check for C<defined> on the return from C<fcntl>.
2242 Like C<ioctl>, it maps a C<0> return from the system call into
2243 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2244 in numeric context. It is also exempt from the normal B<-w> warnings
2245 on improper numeric conversions.
2247 Note that C<fcntl> raises an exception if used on a machine that
2248 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2249 manpage to learn what functions are available on your system.
2251 Here's an example of setting a filehandle named C<REMOTE> to be
2252 non-blocking at the system level. You'll have to negotiate C<$|>
2253 on your own, though.
2255 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2257 $flags = fcntl(REMOTE, F_GETFL, 0)
2258 or die "Can't get flags for the socket: $!\n";
2260 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2261 or die "Can't set flags for the socket: $!\n";
2263 Portability issues: L<perlport/fcntl>.
2268 =for Pod::Functions the name of the current source file
2270 A special token that returns the name of the file in which it occurs.
2272 =item fileno FILEHANDLE
2275 =for Pod::Functions return file descriptor from filehandle
2277 Returns the file descriptor for a filehandle, or undefined if the
2278 filehandle is not open. If there is no real file descriptor at the OS
2279 level, as can happen with filehandles connected to memory objects via
2280 C<open> with a reference for the third argument, -1 is returned.
2282 This is mainly useful for constructing
2283 bitmaps for C<select> and low-level POSIX tty-handling operations.
2284 If FILEHANDLE is an expression, the value is taken as an indirect
2285 filehandle, generally its name.
2287 You can use this to find out whether two handles refer to the
2288 same underlying descriptor:
2290 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
2291 print "THIS and THAT are dups\n";
2292 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2293 print "THIS and THAT have different " .
2294 "underlying file descriptors\n";
2296 print "At least one of THIS and THAT does " .
2297 "not have a real file descriptor\n";
2300 The behavior of C<fileno> on a directory handle depends on the operating
2301 system. On a system with dirfd(3) or similar, C<fileno> on a directory
2302 handle returns the underlying file descriptor associated with the
2303 handle; on systems with no such support, it returns the undefined value,
2304 and sets C<$!> (errno).
2306 =item flock FILEHANDLE,OPERATION
2307 X<flock> X<lock> X<locking>
2309 =for Pod::Functions lock an entire file with an advisory lock
2311 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2312 for success, false on failure. Produces a fatal error if used on a
2313 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2314 C<flock> is Perl's portable file-locking interface, although it locks
2315 entire files only, not records.
2317 Two potentially non-obvious but traditional C<flock> semantics are
2318 that it waits indefinitely until the lock is granted, and that its locks
2319 are B<merely advisory>. Such discretionary locks are more flexible, but
2320 offer fewer guarantees. This means that programs that do not also use
2321 C<flock> may modify files locked with C<flock>. See L<perlport>,
2322 your port's specific documentation, and your system-specific local manpages
2323 for details. It's best to assume traditional behavior if you're writing
2324 portable programs. (But if you're not, you should as always feel perfectly
2325 free to write for your own system's idiosyncrasies (sometimes called
2326 "features"). Slavish adherence to portability concerns shouldn't get
2327 in the way of your getting your job done.)
2329 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2330 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2331 you can use the symbolic names if you import them from the L<Fcntl> module,
2332 either individually, or as a group using the C<:flock> tag. LOCK_SH
2333 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2334 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2335 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2336 waiting for the lock; check the return status to see if you got it.
2338 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2339 before locking or unlocking it.
2341 Note that the emulation built with lockf(3) doesn't provide shared
2342 locks, and it requires that FILEHANDLE be open with write intent. These
2343 are the semantics that lockf(3) implements. Most if not all systems
2344 implement lockf(3) in terms of fcntl(2) locking, though, so the
2345 differing semantics shouldn't bite too many people.
2347 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2348 be open with read intent to use LOCK_SH and requires that it be open
2349 with write intent to use LOCK_EX.
2351 Note also that some versions of C<flock> cannot lock things over the
2352 network; you would need to use the more system-specific C<fcntl> for
2353 that. If you like you can force Perl to ignore your system's flock(2)
2354 function, and so provide its own fcntl(2)-based emulation, by passing
2355 the switch C<-Ud_flock> to the F<Configure> program when you configure
2356 and build a new Perl.
2358 Here's a mailbox appender for BSD systems.
2360 # import LOCK_* and SEEK_END constants
2361 use Fcntl qw(:flock SEEK_END);
2365 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2367 # and, in case someone appended while we were waiting...
2368 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2373 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2376 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2377 or die "Can't open mailbox: $!";
2380 print $mbox $msg,"\n\n";
2383 On systems that support a real flock(2), locks are inherited across fork()
2384 calls, whereas those that must resort to the more capricious fcntl(2)
2385 function lose their locks, making it seriously harder to write servers.
2387 See also L<DB_File> for other flock() examples.
2389 Portability issues: L<perlport/flock>.
2392 X<fork> X<child> X<parent>
2394 =for Pod::Functions create a new process just like this one
2396 Does a fork(2) system call to create a new process running the
2397 same program at the same point. It returns the child pid to the
2398 parent process, C<0> to the child process, or C<undef> if the fork is
2399 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2400 are shared, while everything else is copied. On most systems supporting
2401 fork(), great care has gone into making it extremely efficient (for
2402 example, using copy-on-write technology on data pages), making it the
2403 dominant paradigm for multitasking over the last few decades.
2405 Perl attempts to flush all files opened for
2406 output before forking the child process, but this may not be supported
2407 on some platforms (see L<perlport>). To be safe, you may need to set
2408 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2409 C<IO::Handle> on any open handles to avoid duplicate output.
2411 If you C<fork> without ever waiting on your children, you will
2412 accumulate zombies. On some systems, you can avoid this by setting
2413 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2414 forking and reaping moribund children.
2416 Note that if your forked child inherits system file descriptors like
2417 STDIN and STDOUT that are actually connected by a pipe or socket, even
2418 if you exit, then the remote server (such as, say, a CGI script or a
2419 backgrounded job launched from a remote shell) won't think you're done.
2420 You should reopen those to F</dev/null> if it's any issue.
2422 On some platforms such as Windows, where the fork() system call is not available,
2423 Perl can be built to emulate fork() in the Perl interpreter.
2424 The emulation is designed, at the level of the Perl program,
2425 to be as compatible as possible with the "Unix" fork().
2426 However it has limitations that have to be considered in code intended to be portable.
2427 See L<perlfork> for more details.
2429 Portability issues: L<perlport/fork>.
2434 =for Pod::Functions declare a picture format with use by the write() function
2436 Declare a picture format for use by the C<write> function. For
2440 Test: @<<<<<<<< @||||| @>>>>>
2441 $str, $%, '$' . int($num)
2445 $num = $cost/$quantity;
2449 See L<perlform> for many details and examples.
2451 =item formline PICTURE,LIST
2454 =for Pod::Functions internal function used for formats
2456 This is an internal function used by C<format>s, though you may call it,
2457 too. It formats (see L<perlform>) a list of values according to the
2458 contents of PICTURE, placing the output into the format output
2459 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2460 Eventually, when a C<write> is done, the contents of
2461 C<$^A> are written to some filehandle. You could also read C<$^A>
2462 and then set C<$^A> back to C<"">. Note that a format typically
2463 does one C<formline> per line of form, but the C<formline> function itself
2464 doesn't care how many newlines are embedded in the PICTURE. This means
2465 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2466 You may therefore need to use multiple formlines to implement a single
2467 record format, just like the C<format> compiler.
2469 Be careful if you put double quotes around the picture, because an C<@>
2470 character may be taken to mean the beginning of an array name.
2471 C<formline> always returns true. See L<perlform> for other examples.
2473 If you are trying to use this instead of C<write> to capture the output,
2474 you may find it easier to open a filehandle to a scalar
2475 (C<< open $fh, ">", \$output >>) and write to that instead.
2477 =item getc FILEHANDLE
2478 X<getc> X<getchar> X<character> X<file, read>
2482 =for Pod::Functions get the next character from the filehandle
2484 Returns the next character from the input file attached to FILEHANDLE,
2485 or the undefined value at end of file or if there was an error (in
2486 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2487 STDIN. This is not particularly efficient. However, it cannot be
2488 used by itself to fetch single characters without waiting for the user
2489 to hit enter. For that, try something more like:
2492 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2495 system "stty", '-icanon', 'eol', "\001";
2501 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2504 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2508 Determination of whether $BSD_STYLE should be set
2509 is left as an exercise to the reader.
2511 The C<POSIX::getattr> function can do this more portably on
2512 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2513 module from your nearest L<CPAN|http://www.cpan.org> site.
2516 X<getlogin> X<login>
2518 =for Pod::Functions return who logged in at this tty
2520 This implements the C library function of the same name, which on most
2521 systems returns the current login from F</etc/utmp>, if any. If it
2522 returns the empty string, use C<getpwuid>.
2524 $login = getlogin || getpwuid($<) || "Kilroy";
2526 Do not consider C<getlogin> for authentication: it is not as
2527 secure as C<getpwuid>.
2529 Portability issues: L<perlport/getlogin>.
2531 =item getpeername SOCKET
2532 X<getpeername> X<peer>
2534 =for Pod::Functions find the other end of a socket connection
2536 Returns the packed sockaddr address of the other end of the SOCKET
2540 $hersockaddr = getpeername(SOCK);
2541 ($port, $iaddr) = sockaddr_in($hersockaddr);
2542 $herhostname = gethostbyaddr($iaddr, AF_INET);
2543 $herstraddr = inet_ntoa($iaddr);
2548 =for Pod::Functions get process group
2550 Returns the current process group for the specified PID. Use
2551 a PID of C<0> to get the current process group for the
2552 current process. Will raise an exception if used on a machine that
2553 doesn't implement getpgrp(2). If PID is omitted, returns the process
2554 group of the current process. Note that the POSIX version of C<getpgrp>
2555 does not accept a PID argument, so only C<PID==0> is truly portable.
2557 Portability issues: L<perlport/getpgrp>.
2560 X<getppid> X<parent> X<pid>
2562 =for Pod::Functions get parent process ID
2564 Returns the process id of the parent process.
2566 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2567 around non-POSIX thread semantics the minority of Linux systems (and
2568 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2569 has since been removed. See the documentation for L<$$|perlvar/$$> for
2572 Portability issues: L<perlport/getppid>.
2574 =item getpriority WHICH,WHO
2575 X<getpriority> X<priority> X<nice>
2577 =for Pod::Functions get current nice value
2579 Returns the current priority for a process, a process group, or a user.
2580 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2581 machine that doesn't implement getpriority(2).
2583 Portability issues: L<perlport/getpriority>.
2586 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2587 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2588 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2589 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2590 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2591 X<endnetent> X<endprotoent> X<endservent>
2593 =for Pod::Functions get passwd record given user login name
2597 =for Pod::Functions get group record given group name
2599 =item gethostbyname NAME
2601 =for Pod::Functions get host record given name
2603 =item getnetbyname NAME
2605 =for Pod::Functions get networks record given name
2607 =item getprotobyname NAME
2609 =for Pod::Functions get protocol record given name
2613 =for Pod::Functions get passwd record given user ID
2617 =for Pod::Functions get group record given group user ID
2619 =item getservbyname NAME,PROTO
2621 =for Pod::Functions get services record given its name
2623 =item gethostbyaddr ADDR,ADDRTYPE
2625 =for Pod::Functions get host record given its address
2627 =item getnetbyaddr ADDR,ADDRTYPE
2629 =for Pod::Functions get network record given its address
2631 =item getprotobynumber NUMBER
2633 =for Pod::Functions get protocol record numeric protocol
2635 =item getservbyport PORT,PROTO
2637 =for Pod::Functions get services record given numeric port
2641 =for Pod::Functions get next passwd record
2645 =for Pod::Functions get next group record
2649 =for Pod::Functions get next hosts record
2653 =for Pod::Functions get next networks record
2657 =for Pod::Functions get next protocols record
2661 =for Pod::Functions get next services record
2665 =for Pod::Functions prepare passwd file for use
2669 =for Pod::Functions prepare group file for use
2671 =item sethostent STAYOPEN
2673 =for Pod::Functions prepare hosts file for use
2675 =item setnetent STAYOPEN
2677 =for Pod::Functions prepare networks file for use
2679 =item setprotoent STAYOPEN
2681 =for Pod::Functions prepare protocols file for use
2683 =item setservent STAYOPEN
2685 =for Pod::Functions prepare services file for use
2689 =for Pod::Functions be done using passwd file
2693 =for Pod::Functions be done using group file
2697 =for Pod::Functions be done using hosts file
2701 =for Pod::Functions be done using networks file
2705 =for Pod::Functions be done using protocols file
2709 =for Pod::Functions be done using services file
2711 These routines are the same as their counterparts in the
2712 system C library. In list context, the return values from the
2713 various get routines are as follows:
2716 ( $name, $passwd, $gid, $members ) = getgr*
2717 ( $name, $aliases, $addrtype, $net ) = getnet*
2718 ( $name, $aliases, $port, $proto ) = getserv*
2719 ( $name, $aliases, $proto ) = getproto*
2720 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2721 ( $name, $passwd, $uid, $gid, $quota,
2722 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2725 (If the entry doesn't exist, the return value is a single meaningless true
2728 The exact meaning of the $gcos field varies but it usually contains
2729 the real name of the user (as opposed to the login name) and other
2730 information pertaining to the user. Beware, however, that in many
2731 system users are able to change this information and therefore it
2732 cannot be trusted and therefore the $gcos is tainted (see
2733 L<perlsec>). The $passwd and $shell, user's encrypted password and
2734 login shell, are also tainted, for the same reason.
2736 In scalar context, you get the name, unless the function was a
2737 lookup by name, in which case you get the other thing, whatever it is.
2738 (If the entry doesn't exist you get the undefined value.) For example:
2740 $uid = getpwnam($name);
2741 $name = getpwuid($num);
2743 $gid = getgrnam($name);
2744 $name = getgrgid($num);
2748 In I<getpw*()> the fields $quota, $comment, and $expire are special
2749 in that they are unsupported on many systems. If the
2750 $quota is unsupported, it is an empty scalar. If it is supported, it
2751 usually encodes the disk quota. If the $comment field is unsupported,
2752 it is an empty scalar. If it is supported it usually encodes some
2753 administrative comment about the user. In some systems the $quota
2754 field may be $change or $age, fields that have to do with password
2755 aging. In some systems the $comment field may be $class. The $expire
2756 field, if present, encodes the expiration period of the account or the
2757 password. For the availability and the exact meaning of these fields
2758 in your system, please consult getpwnam(3) and your system's
2759 F<pwd.h> file. You can also find out from within Perl what your
2760 $quota and $comment fields mean and whether you have the $expire field
2761 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2762 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2763 files are supported only if your vendor has implemented them in the
2764 intuitive fashion that calling the regular C library routines gets the
2765 shadow versions if you're running under privilege or if there exists
2766 the shadow(3) functions as found in System V (this includes Solaris
2767 and Linux). Those systems that implement a proprietary shadow password
2768 facility are unlikely to be supported.
2770 The $members value returned by I<getgr*()> is a space-separated list of
2771 the login names of the members of the group.
2773 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2774 C, it will be returned to you via C<$?> if the function call fails. The
2775 C<@addrs> value returned by a successful call is a list of raw
2776 addresses returned by the corresponding library call. In the
2777 Internet domain, each address is four bytes long; you can unpack it
2778 by saying something like:
2780 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2782 The Socket library makes this slightly easier:
2785 $iaddr = inet_aton("127.1"); # or whatever address
2786 $name = gethostbyaddr($iaddr, AF_INET);
2788 # or going the other way
2789 $straddr = inet_ntoa($iaddr);
2791 In the opposite way, to resolve a hostname to the IP address
2795 $packed_ip = gethostbyname("www.perl.org");
2796 if (defined $packed_ip) {
2797 $ip_address = inet_ntoa($packed_ip);
2800 Make sure C<gethostbyname()> is called in SCALAR context and that
2801 its return value is checked for definedness.
2803 The C<getprotobynumber> function, even though it only takes one argument,
2804 has the precedence of a list operator, so beware:
2806 getprotobynumber $number eq 'icmp' # WRONG
2807 getprotobynumber($number eq 'icmp') # actually means this
2808 getprotobynumber($number) eq 'icmp' # better this way
2810 If you get tired of remembering which element of the return list
2811 contains which return value, by-name interfaces are provided
2812 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2813 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2814 and C<User::grent>. These override the normal built-ins, supplying
2815 versions that return objects with the appropriate names
2816 for each field. For example:
2820 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2822 Even though it looks as though they're the same method calls (uid),
2823 they aren't, because a C<File::stat> object is different from
2824 a C<User::pwent> object.
2826 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2828 =item getsockname SOCKET
2831 =for Pod::Functions retrieve the sockaddr for a given socket
2833 Returns the packed sockaddr address of this end of the SOCKET connection,
2834 in case you don't know the address because you have several different
2835 IPs that the connection might have come in on.
2838 $mysockaddr = getsockname(SOCK);
2839 ($port, $myaddr) = sockaddr_in($mysockaddr);
2840 printf "Connect to %s [%s]\n",
2841 scalar gethostbyaddr($myaddr, AF_INET),
2844 =item getsockopt SOCKET,LEVEL,OPTNAME
2847 =for Pod::Functions get socket options on a given socket
2849 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2850 Options may exist at multiple protocol levels depending on the socket
2851 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2852 C<Socket> module) will exist. To query options at another level the
2853 protocol number of the appropriate protocol controlling the option
2854 should be supplied. For example, to indicate that an option is to be
2855 interpreted by the TCP protocol, LEVEL should be set to the protocol
2856 number of TCP, which you can get using C<getprotobyname>.
2858 The function returns a packed string representing the requested socket
2859 option, or C<undef> on error, with the reason for the error placed in
2860 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2861 consult getsockopt(2) for details. A common case is that the option is an
2862 integer, in which case the result is a packed integer, which you can decode
2863 using C<unpack> with the C<i> (or C<I>) format.
2865 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2867 use Socket qw(:all);
2869 defined(my $tcp = getprotobyname("tcp"))
2870 or die "Could not determine the protocol number for tcp";
2871 # my $tcp = IPPROTO_TCP; # Alternative
2872 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2873 or die "getsockopt TCP_NODELAY: $!";
2874 my $nodelay = unpack("I", $packed);
2875 print "Nagle's algorithm is turned ",
2876 $nodelay ? "off\n" : "on\n";
2878 Portability issues: L<perlport/getsockopt>.
2881 X<glob> X<wildcard> X<filename, expansion> X<expand>
2885 =for Pod::Functions expand filenames using wildcards
2887 In list context, returns a (possibly empty) list of filename expansions on
2888 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2889 scalar context, glob iterates through such filename expansions, returning
2890 undef when the list is exhausted. This is the internal function
2891 implementing the C<< <*.c> >> operator, but you can use it directly. If
2892 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2893 more detail in L<perlop/"I/O Operators">.
2895 Note that C<glob> splits its arguments on whitespace and treats
2896 each segment as separate pattern. As such, C<glob("*.c *.h")>
2897 matches all files with a F<.c> or F<.h> extension. The expression
2898 C<glob(".* *")> matches all files in the current working directory.
2899 If you want to glob filenames that might contain whitespace, you'll
2900 have to use extra quotes around the spacey filename to protect it.
2901 For example, to glob filenames that have an C<e> followed by a space
2902 followed by an C<f>, use either of:
2904 @spacies = <"*e f*">;
2905 @spacies = glob '"*e f*"';
2906 @spacies = glob q("*e f*");
2908 If you had to get a variable through, you could do this:
2910 @spacies = glob "'*${var}e f*'";
2911 @spacies = glob qq("*${var}e f*");
2913 If non-empty braces are the only wildcard characters used in the
2914 C<glob>, no filenames are matched, but potentially many strings
2915 are returned. For example, this produces nine strings, one for
2916 each pairing of fruits and colors:
2918 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2920 This operator is implemented using the standard
2921 C<File::Glob> extension. See L<File::Glob> for details, including
2922 C<bsd_glob> which does not treat whitespace as a pattern separator.
2924 Portability issues: L<perlport/glob>.
2927 X<gmtime> X<UTC> X<Greenwich>
2931 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2933 Works just like L</localtime> but the returned values are
2934 localized for the standard Greenwich time zone.
2936 Note: When called in list context, $isdst, the last value
2937 returned by gmtime, is always C<0>. There is no
2938 Daylight Saving Time in GMT.
2940 Portability issues: L<perlport/gmtime>.
2943 X<goto> X<jump> X<jmp>
2949 =for Pod::Functions create spaghetti code
2951 The C<goto LABEL> form finds the statement labeled with LABEL and
2952 resumes execution there. It can't be used to get out of a block or
2953 subroutine given to C<sort>. It can be used to go almost anywhere
2954 else within the dynamic scope, including out of subroutines, but it's
2955 usually better to use some other construct such as C<last> or C<die>.
2956 The author of Perl has never felt the need to use this form of C<goto>
2957 (in Perl, that is; C is another matter). (The difference is that C
2958 does not offer named loops combined with loop control. Perl does, and
2959 this replaces most structured uses of C<goto> in other languages.)
2961 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
2962 a label name. If it evaluates to a code reference, it will be handled
2963 like C<goto &NAME>, below. This is especially useful for implementing
2964 tail recursion via C<goto __SUB__>.
2966 If the expression evaluates to a label name, its scope will be resolved
2967 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2968 necessarily recommended if you're optimizing for maintainability:
2970 goto ("FOO", "BAR", "GLARCH")[$i];
2972 As shown in this example, C<goto EXPR> is exempt from the "looks like a
2973 function" rule. A pair of parentheses following it does not (necessarily)
2974 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2975 Also, unlike most named operators, this has the same precedence as
2978 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
2979 deprecated and will issue a warning. Even then, it may not be used to
2980 go into any construct that requires initialization, such as a
2981 subroutine or a C<foreach> loop. It also can't be used to go into a
2982 construct that is optimized away.
2984 The C<goto &NAME> form is quite different from the other forms of
2985 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2986 doesn't have the stigma associated with other gotos. Instead, it
2987 exits the current subroutine (losing any changes set by local()) and
2988 immediately calls in its place the named subroutine using the current
2989 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2990 load another subroutine and then pretend that the other subroutine had
2991 been called in the first place (except that any modifications to C<@_>
2992 in the current subroutine are propagated to the other subroutine.)
2993 After the C<goto>, not even C<caller> will be able to tell that this
2994 routine was called first.
2996 NAME needn't be the name of a subroutine; it can be a scalar variable
2997 containing a code reference or a block that evaluates to a code
3000 =item grep BLOCK LIST
3003 =item grep EXPR,LIST
3005 =for Pod::Functions locate elements in a list test true against a given criterion
3007 This is similar in spirit to, but not the same as, grep(1) and its
3008 relatives. In particular, it is not limited to using regular expressions.
3010 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3011 C<$_> to each element) and returns the list value consisting of those
3012 elements for which the expression evaluated to true. In scalar
3013 context, returns the number of times the expression was true.
3015 @foo = grep(!/^#/, @bar); # weed out comments
3019 @foo = grep {!/^#/} @bar; # weed out comments
3021 Note that C<$_> is an alias to the list value, so it can be used to
3022 modify the elements of the LIST. While this is useful and supported,
3023 it can cause bizarre results if the elements of LIST are not variables.
3024 Similarly, grep returns aliases into the original list, much as a for
3025 loop's index variable aliases the list elements. That is, modifying an
3026 element of a list returned by grep (for example, in a C<foreach>, C<map>
3027 or another C<grep>) actually modifies the element in the original list.
3028 This is usually something to be avoided when writing clear code.
3030 If C<$_> is lexical in the scope where the C<grep> appears (because it has
3031 been declared with the deprecated C<my $_> construct)
3032 then, in addition to being locally aliased to
3033 the list elements, C<$_> keeps being lexical inside the block; i.e., it
3034 can't be seen from the outside, avoiding any potential side-effects.
3036 See also L</map> for a list composed of the results of the BLOCK or EXPR.
3039 X<hex> X<hexadecimal>
3043 =for Pod::Functions convert a string to a hexadecimal number
3045 Interprets EXPR as a hex string and returns the corresponding value.
3046 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
3047 L</oct>.) If EXPR is omitted, uses C<$_>.
3049 print hex '0xAf'; # prints '175'
3050 print hex 'aF'; # same
3052 Hex strings may only represent integers. Strings that would cause
3053 integer overflow trigger a warning. Leading whitespace is not stripped,
3054 unlike oct(). To present something as hex, look into L</printf>,
3055 L</sprintf>, and L</unpack>.
3060 =for Pod::Functions patch a module's namespace into your own
3062 There is no builtin C<import> function. It is just an ordinary
3063 method (subroutine) defined (or inherited) by modules that wish to export
3064 names to another module. The C<use> function calls the C<import> method
3065 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3067 =item index STR,SUBSTR,POSITION
3068 X<index> X<indexOf> X<InStr>
3070 =item index STR,SUBSTR
3072 =for Pod::Functions find a substring within a string
3074 The index function searches for one string within another, but without
3075 the wildcard-like behavior of a full regular-expression pattern match.
3076 It returns the position of the first occurrence of SUBSTR in STR at
3077 or after POSITION. If POSITION is omitted, starts searching from the
3078 beginning of the string. POSITION before the beginning of the string
3079 or after its end is treated as if it were the beginning or the end,
3080 respectively. POSITION and the return value are based at zero.
3081 If the substring is not found, C<index> returns -1.
3084 X<int> X<integer> X<truncate> X<trunc> X<floor>
3088 =for Pod::Functions get the integer portion of a number
3090 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3091 You should not use this function for rounding: one because it truncates
3092 towards C<0>, and two because machine representations of floating-point
3093 numbers can sometimes produce counterintuitive results. For example,
3094 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3095 because it's really more like -268.99999999999994315658 instead. Usually,
3096 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3097 functions will serve you better than will int().
3099 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3102 =for Pod::Functions system-dependent device control system call
3104 Implements the ioctl(2) function. You'll probably first have to say
3106 require "sys/ioctl.ph"; # probably in
3107 # $Config{archlib}/sys/ioctl.ph
3109 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3110 exist or doesn't have the correct definitions you'll have to roll your
3111 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3112 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3113 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3114 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3115 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3116 has no string value but does have a numeric value, that value will be
3117 passed rather than a pointer to the string value. To guarantee this to be
3118 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3119 functions may be needed to manipulate the values of structures used by
3122 The return value of C<ioctl> (and C<fcntl>) is as follows:
3124 if OS returns: then Perl returns:
3126 0 string "0 but true"
3127 anything else that number
3129 Thus Perl returns true on success and false on failure, yet you can
3130 still easily determine the actual value returned by the operating
3133 $retval = ioctl(...) || -1;
3134 printf "System returned %d\n", $retval;
3136 The special string C<"0 but true"> is exempt from B<-w> complaints
3137 about improper numeric conversions.
3139 Portability issues: L<perlport/ioctl>.
3141 =item join EXPR,LIST
3144 =for Pod::Functions join a list into a string using a separator
3146 Joins the separate strings of LIST into a single string with fields
3147 separated by the value of EXPR, and returns that new string. Example:
3149 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3151 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3152 first argument. Compare L</split>.
3161 =for Pod::Functions retrieve list of indices from a hash
3163 Called in list context, returns a list consisting of all the keys of the
3164 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3165 releases prior to 5.12 will produce a syntax error if you try to use an
3166 array argument. In scalar context, returns the number of keys or indices.
3168 Hash entries are returned in an apparently random order. The actual random
3169 order is specific to a given hash; the exact same series of operations
3170 on two hashes may result in a different order for each hash. Any insertion
3171 into the hash may change the order, as will any deletion, with the exception
3172 that the most recent key returned by C<each> or C<keys> may be deleted
3173 without changing the order. So long as a given hash is unmodified you may
3174 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
3175 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3176 details on why hash order is randomized. Aside from the guarantees
3177 provided here the exact details of Perl's hash algorithm and the hash
3178 traversal order are subject to change in any release of Perl. Tied hashes
3179 may behave differently to Perl's hashes with respect to changes in order on
3180 insertion and deletion of items.
3182 As a side effect, calling keys() resets the internal iterator of the HASH or
3183 ARRAY (see L</each>). In particular, calling keys() in void context resets
3184 the iterator with no other overhead.
3186 Here is yet another way to print your environment:
3189 @values = values %ENV;
3191 print pop(@keys), '=', pop(@values), "\n";
3194 or how about sorted by key:
3196 foreach $key (sort(keys %ENV)) {
3197 print $key, '=', $ENV{$key}, "\n";
3200 The returned values are copies of the original keys in the hash, so
3201 modifying them will not affect the original hash. Compare L</values>.
3203 To sort a hash by value, you'll need to use a C<sort> function.
3204 Here's a descending numeric sort of a hash by its values:
3206 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3207 printf "%4d %s\n", $hash{$key}, $key;
3210 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3211 allocated for the given hash. This can gain you a measure of efficiency if
3212 you know the hash is going to get big. (This is similar to pre-extending
3213 an array by assigning a larger number to $#array.) If you say
3217 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3218 in fact, since it rounds up to the next power of two. These
3219 buckets will be retained even if you do C<%hash = ()>, use C<undef
3220 %hash> if you want to free the storage while C<%hash> is still in scope.
3221 You can't shrink the number of buckets allocated for the hash using
3222 C<keys> in this way (but you needn't worry about doing this by accident,
3223 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3226 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3227 a reference to an unblessed hash or array. The argument will be
3228 dereferenced automatically. This aspect of C<keys> is considered highly
3229 experimental. The exact behaviour may change in a future version of Perl.
3231 for (keys $hashref) { ... }
3232 for (keys $obj->get_arrayref) { ... }
3234 To avoid confusing would-be users of your code who are running earlier
3235 versions of Perl with mysterious syntax errors, put this sort of thing at
3236 the top of your file to signal that your code will work I<only> on Perls of
3239 use 5.012; # so keys/values/each work on arrays
3240 use 5.014; # so keys/values/each work on scalars (experimental)
3242 See also C<each>, C<values>, and C<sort>.
3244 =item kill SIGNAL, LIST
3249 =for Pod::Functions send a signal to a process or process group
3251 Sends a signal to a list of processes. Returns the number of arguments
3252 that were successfully used to signal (which is not necessarily the same
3253 as the number of processes actually killed, e.g. where a process group is
3256 $cnt = kill 'HUP', $child1, $child2;
3257 kill 'KILL', @goners;
3259 SIGNAL may be either a signal name (a string) or a signal number. A signal
3260 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3261 same signal. The string form of SIGNAL is recommended for portability because
3262 the same signal may have different numbers in different operating systems.
3264 A list of signal names supported by the current platform can be found in
3265 C<$Config{sig_name}>, which is provided by the C<Config> module. See L<Config>
3268 A negative signal name is the same as a negative signal number, killing process
3269 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3270 C<kill -9, $pgrp> will send C<SIGKILL> to
3271 the entire process group specified. That
3272 means you usually want to use positive not negative signals.
3274 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3275 no signal is sent to
3276 the process, but C<kill> checks whether it's I<possible> to send a signal to it
3277 (that means, to be brief, that the process is owned by the same user, or we are
3278 the super-user). This is useful to check that a child process is still
3279 alive (even if only as a zombie) and hasn't changed its UID. See
3280 L<perlport> for notes on the portability of this construct.
3282 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3283 the operating system. For example, on POSIX-conforming systems, zero will
3284 signal the current process group, -1 will signal all processes, and any
3285 other negative PROCESS number will act as a negative signal number and
3286 kill the entire process group specified.
3288 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3289 A warning may be produced in a future version.
3291 See L<perlipc/"Signals"> for more details.
3293 On some platforms such as Windows where the fork() system call is not
3294 available, Perl can be built to emulate fork() at the interpreter level.
3295 This emulation has limitations related to kill that have to be considered,
3296 for code running on Windows and in code intended to be portable.
3298 See L<perlfork> for more details.
3300 If there is no I<LIST> of processes, no signal is sent, and the return
3301 value is 0. This form is sometimes used, however, because it causes
3302 tainting checks to be run. But see
3303 L<perlsec/Laundering and Detecting Tainted Data>.
3305 Portability issues: L<perlport/kill>.
3314 =for Pod::Functions exit a block prematurely
3316 The C<last> command is like the C<break> statement in C (as used in
3317 loops); it immediately exits the loop in question. If the LABEL is
3318 omitted, the command refers to the innermost enclosing
3319 loop. The C<last EXPR> form, available starting in Perl
3320 5.18.0, allows a label name to be computed at run time,
3321 and is otherwise identical to C<last LABEL>. The
3322 C<continue> block, if any, is not executed:
3324 LINE: while (<STDIN>) {
3325 last LINE if /^$/; # exit when done with header
3329 C<last> cannot be used to exit a block that returns a value such as
3330 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3331 a grep() or map() operation.
3333 Note that a block by itself is semantically identical to a loop
3334 that executes once. Thus C<last> can be used to effect an early
3335 exit out of such a block.
3337 See also L</continue> for an illustration of how C<last>, C<next>, and
3340 Unlike most named operators, this has the same precedence as assignment.
3341 It is also exempt from the looks-like-a-function rule, so
3342 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3350 =for Pod::Functions return lower-case version of a string
3352 Returns a lowercased version of EXPR. This is the internal function
3353 implementing the C<\L> escape in double-quoted strings.
3355 If EXPR is omitted, uses C<$_>.
3357 What gets returned depends on several factors:
3361 =item If C<use bytes> is in effect:
3363 The results follow ASCII rules. Only the characters C<A-Z> change,
3364 to C<a-z> respectively.
3366 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3368 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3369 rules for the remaining code points (this last can only happen if
3370 the UTF8 flag is also set). See L<perllocale>.
3372 Starting in v5.20, Perl uses full Unicode rules if the locale is
3373 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3374 case changes that cross the 255/256
3375 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3376 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3377 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3378 locale), the lower case of U+1E9E is
3379 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3380 current locale, and Perl has no way of knowing if that character even
3381 exists in the locale, much less what code point it is. Perl returns
3382 a result that is above 255 (almost always the input character unchanged,
3383 for all instances (and there aren't many) where the 255/256 boundary
3384 would otherwise be crossed; and starting in v5.22, it raises a
3385 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3387 =item Otherwise, If EXPR has the UTF8 flag set:
3389 Unicode rules are used for the case change.
3391 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3393 Unicode rules are used for the case change.
3397 ASCII rules are used for the case change. The lowercase of any character
3398 outside the ASCII range is the character itself.
3403 X<lcfirst> X<lowercase>
3407 =for Pod::Functions return a string with just the next letter in lower case
3409 Returns the value of EXPR with the first character lowercased. This
3410 is the internal function implementing the C<\l> escape in
3411 double-quoted strings.
3413 If EXPR is omitted, uses C<$_>.
3415 This function behaves the same way under various pragmata, such as in a locale,
3423 =for Pod::Functions return the number of characters in a string
3425 Returns the length in I<characters> of the value of EXPR. If EXPR is
3426 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3429 This function cannot be used on an entire array or hash to find out how
3430 many elements these have. For that, use C<scalar @array> and C<scalar keys
3431 %hash>, respectively.
3433 Like all Perl character operations, length() normally deals in logical
3434 characters, not physical bytes. For how many bytes a string encoded as
3435 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3436 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3441 =for Pod::Functions the current source line number
3443 A special token that compiles to the current line number.
3445 =item link OLDFILE,NEWFILE
3448 =for Pod::Functions create a hard link in the filesystem
3450 Creates a new filename linked to the old filename. Returns true for
3451 success, false otherwise.
3453 Portability issues: L<perlport/link>.
3455 =item listen SOCKET,QUEUESIZE
3458 =for Pod::Functions register your socket as a server
3460 Does the same thing that the listen(2) system call does. Returns true if
3461 it succeeded, false otherwise. See the example in
3462 L<perlipc/"Sockets: Client/Server Communication">.
3467 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3469 You really probably want to be using C<my> instead, because C<local> isn't
3470 what most people think of as "local". See
3471 L<perlsub/"Private Variables via my()"> for details.
3473 A local modifies the listed variables to be local to the enclosing
3474 block, file, or eval. If more than one value is listed, the list must
3475 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3476 for details, including issues with tied arrays and hashes.
3478 The C<delete local EXPR> construct can also be used to localize the deletion
3479 of array/hash elements to the current block.
3480 See L<perlsub/"Localized deletion of elements of composite types">.
3482 =item localtime EXPR
3483 X<localtime> X<ctime>
3487 =for Pod::Functions convert UNIX time into record or string using local time
3489 Converts a time as returned by the time function to a 9-element list
3490 with the time analyzed for the local time zone. Typically used as
3494 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3497 All list elements are numeric and come straight out of the C `struct
3498 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3499 of the specified time.
3501 C<$mday> is the day of the month and C<$mon> the month in
3502 the range C<0..11>, with 0 indicating January and 11 indicating December.
3503 This makes it easy to get a month name from a list:
3505 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3506 print "$abbr[$mon] $mday";
3507 # $mon=9, $mday=18 gives "Oct 18"
3509 C<$year> contains the number of years since 1900. To get a 4-digit
3514 To get the last two digits of the year (e.g., "01" in 2001) do:
3516 $year = sprintf("%02d", $year % 100);
3518 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3519 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3520 (or C<0..365> in leap years.)
3522 C<$isdst> is true if the specified time occurs during Daylight Saving
3523 Time, false otherwise.
3525 If EXPR is omitted, C<localtime()> uses the current time (as returned
3528 In scalar context, C<localtime()> returns the ctime(3) value:
3530 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3532 The format of this scalar value is B<not> locale-dependent
3533 but built into Perl. For GMT instead of local
3534 time use the L</gmtime> builtin. See also the
3535 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3536 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3537 and mktime(3) functions.
3539 To get somewhat similar but locale-dependent date strings, set up your
3540 locale environment variables appropriately (please see L<perllocale>) and
3543 use POSIX qw(strftime);
3544 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3545 # or for GMT formatted appropriately for your locale:
3546 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3548 Note that the C<%a> and C<%b>, the short forms of the day of the week
3549 and the month of the year, may not necessarily be three characters wide.
3551 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3552 by-name access mechanism to the gmtime() and localtime() functions,
3555 For a comprehensive date and time representation look at the
3556 L<DateTime> module on CPAN.
3558 Portability issues: L<perlport/localtime>.
3563 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3565 This function places an advisory lock on a shared variable or referenced
3566 object contained in I<THING> until the lock goes out of scope.
3568 The value returned is the scalar itself, if the argument is a scalar, or a
3569 reference, if the argument is a hash, array or subroutine.
3571 lock() is a "weak keyword" : this means that if you've defined a function
3572 by this name (before any calls to it), that function will be called
3573 instead. If you are not under C<use threads::shared> this does nothing.
3574 See L<threads::shared>.
3577 X<log> X<logarithm> X<e> X<ln> X<base>
3581 =for Pod::Functions retrieve the natural logarithm for a number
3583 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3584 returns the log of C<$_>. To get the
3585 log of another base, use basic algebra:
3586 The base-N log of a number is equal to the natural log of that number
3587 divided by the natural log of N. For example:
3591 return log($n)/log(10);
3594 See also L</exp> for the inverse operation.
3596 =item lstat FILEHANDLE
3601 =item lstat DIRHANDLE
3605 =for Pod::Functions stat a symbolic link
3607 Does the same thing as the C<stat> function (including setting the
3608 special C<_> filehandle) but stats a symbolic link instead of the file
3609 the symbolic link points to. If symbolic links are unimplemented on
3610 your system, a normal C<stat> is done. For much more detailed
3611 information, please see the documentation for C<stat>.
3613 If EXPR is omitted, stats C<$_>.
3615 Portability issues: L<perlport/lstat>.
3619 =for Pod::Functions match a string with a regular expression pattern
3621 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3623 =item map BLOCK LIST
3628 =for Pod::Functions apply a change to a list to get back a new list with the changes
3630 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3631 C<$_> to each element) and returns the list value composed of the
3632 results of each such evaluation. In scalar context, returns the
3633 total number of elements so generated. Evaluates BLOCK or EXPR in
3634 list context, so each element of LIST may produce zero, one, or
3635 more elements in the returned value.
3637 @chars = map(chr, @numbers);
3639 translates a list of numbers to the corresponding characters.
3641 my @squares = map { $_ * $_ } @numbers;
3643 translates a list of numbers to their squared values.
3645 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3647 shows that number of returned elements can differ from the number of
3648 input elements. To omit an element, return an empty list ().
3649 This could also be achieved by writing
3651 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3653 which makes the intention more clear.
3655 Map always returns a list, which can be
3656 assigned to a hash such that the elements
3657 become key/value pairs. See L<perldata> for more details.
3659 %hash = map { get_a_key_for($_) => $_ } @array;
3661 is just a funny way to write
3665 $hash{get_a_key_for($_)} = $_;
3668 Note that C<$_> is an alias to the list value, so it can be used to
3669 modify the elements of the LIST. While this is useful and supported,
3670 it can cause bizarre results if the elements of LIST are not variables.
3671 Using a regular C<foreach> loop for this purpose would be clearer in
3672 most cases. See also L</grep> for an array composed of those items of
3673 the original list for which the BLOCK or EXPR evaluates to true.
3675 If C<$_> is lexical in the scope where the C<map> appears (because it has
3676 been declared with the deprecated C<my $_> construct),
3677 then, in addition to being locally aliased to
3678 the list elements, C<$_> keeps being lexical inside the block; that is, it
3679 can't be seen from the outside, avoiding any potential side-effects.
3681 C<{> starts both hash references and blocks, so C<map { ...> could be either
3682 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3683 ahead for the closing C<}> it has to take a guess at which it's dealing with
3684 based on what it finds just after the
3685 C<{>. Usually it gets it right, but if it
3686 doesn't it won't realize something is wrong until it gets to the C<}> and
3687 encounters the missing (or unexpected) comma. The syntax error will be
3688 reported close to the C<}>, but you'll need to change something near the C<{>
3689 such as using a unary C<+> or semicolon to give Perl some help:
3691 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3692 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3693 %hash = map {; "\L$_" => 1 } @array # this also works
3694 %hash = map { ("\L$_" => 1) } @array # as does this
3695 %hash = map { lc($_) => 1 } @array # and this.
3696 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3698 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3700 or to force an anon hash constructor use C<+{>:
3702 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3705 to get a list of anonymous hashes each with only one entry apiece.
3707 =item mkdir FILENAME,MASK
3708 X<mkdir> X<md> X<directory, create>
3710 =item mkdir FILENAME
3714 =for Pod::Functions create a directory
3716 Creates the directory specified by FILENAME, with permissions
3717 specified by MASK (as modified by C<umask>). If it succeeds it
3718 returns true; otherwise it returns false and sets C<$!> (errno).
3719 MASK defaults to 0777 if omitted, and FILENAME defaults
3720 to C<$_> if omitted.
3722 In general, it is better to create directories with a permissive MASK
3723 and let the user modify that with their C<umask> than it is to supply
3724 a restrictive MASK and give the user no way to be more permissive.
3725 The exceptions to this rule are when the file or directory should be
3726 kept private (mail files, for instance). The perlfunc(1) entry on
3727 C<umask> discusses the choice of MASK in more detail.
3729 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3730 number of trailing slashes. Some operating and filesystems do not get
3731 this right, so Perl automatically removes all trailing slashes to keep
3734 To recursively create a directory structure, look at
3735 the C<make_path> function of the L<File::Path> module.
3737 =item msgctl ID,CMD,ARG
3740 =for Pod::Functions SysV IPC message control operations
3742 Calls the System V IPC function msgctl(2). You'll probably have to say
3746 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3747 then ARG must be a variable that will hold the returned C<msqid_ds>
3748 structure. Returns like C<ioctl>: the undefined value for error,
3749 C<"0 but true"> for zero, or the actual return value otherwise. See also
3750 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3753 Portability issues: L<perlport/msgctl>.
3755 =item msgget KEY,FLAGS
3758 =for Pod::Functions get SysV IPC message queue
3760 Calls the System V IPC function msgget(2). Returns the message queue
3761 id, or C<undef> on error. See also
3762 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3765 Portability issues: L<perlport/msgget>.
3767 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3770 =for Pod::Functions receive a SysV IPC message from a message queue
3772 Calls the System V IPC function msgrcv to receive a message from
3773 message queue ID into variable VAR with a maximum message size of
3774 SIZE. Note that when a message is received, the message type as a
3775 native long integer will be the first thing in VAR, followed by the
3776 actual message. This packing may be opened with C<unpack("l! a*")>.
3777 Taints the variable. Returns true if successful, false
3778 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3779 C<IPC::SysV> and C<IPC::SysV::Msg>.
3781 Portability issues: L<perlport/msgrcv>.
3783 =item msgsnd ID,MSG,FLAGS
3786 =for Pod::Functions send a SysV IPC message to a message queue
3788 Calls the System V IPC function msgsnd to send the message MSG to the
3789 message queue ID. MSG must begin with the native long integer message
3790 type, be followed by the length of the actual message, and then finally
3791 the message itself. This kind of packing can be achieved with
3792 C<pack("l! a*", $type, $message)>. Returns true if successful,
3793 false on error. See also the C<IPC::SysV>
3794 and C<IPC::SysV::Msg> documentation.
3796 Portability issues: L<perlport/msgsnd>.
3801 =item my TYPE VARLIST
3803 =item my VARLIST : ATTRS
3805 =item my TYPE VARLIST : ATTRS
3807 =for Pod::Functions declare and assign a local variable (lexical scoping)
3809 A C<my> declares the listed variables to be local (lexically) to the
3810 enclosing block, file, or C<eval>. If more than one variable is listed,
3811 the list must be placed in parentheses.
3813 The exact semantics and interface of TYPE and ATTRS are still
3814 evolving. TYPE may be a bareword, a constant declared
3815 with C<use constant>, or C<__PACKAGE__>. It is
3816 currently bound to the use of the C<fields> pragma,
3817 and attributes are handled using the C<attributes> pragma, or starting
3818 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3819 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3820 L<attributes>, and L<Attribute::Handlers>.
3822 Note that with a parenthesised list, C<undef> can be used as a dummy
3823 placeholder, for example to skip assignment of initial values:
3825 my ( undef, $min, $hour ) = localtime;
3834 =for Pod::Functions iterate a block prematurely
3836 The C<next> command is like the C<continue> statement in C; it starts
3837 the next iteration of the loop:
3839 LINE: while (<STDIN>) {
3840 next LINE if /^#/; # discard comments
3844 Note that if there were a C<continue> block on the above, it would get
3845 executed even on discarded lines. If LABEL is omitted, the command
3846 refers to the innermost enclosing loop. The C<next EXPR> form, available
3847 as of Perl 5.18.0, allows a label name to be computed at run time, being
3848 otherwise identical to C<next LABEL>.
3850 C<next> cannot be used to exit a block which returns a value such as
3851 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3852 a grep() or map() operation.
3854 Note that a block by itself is semantically identical to a loop
3855 that executes once. Thus C<next> will exit such a block early.
3857 See also L</continue> for an illustration of how C<last>, C<next>, and
3860 Unlike most named operators, this has the same precedence as assignment.
3861 It is also exempt from the looks-like-a-function rule, so
3862 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3865 =item no MODULE VERSION LIST
3869 =item no MODULE VERSION
3871 =item no MODULE LIST
3877 =for Pod::Functions unimport some module symbols or semantics at compile time
3879 See the C<use> function, of which C<no> is the opposite.
3882 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3886 =for Pod::Functions convert a string to an octal number
3888 Interprets EXPR as an octal string and returns the corresponding
3889 value. (If EXPR happens to start off with C<0x>, interprets it as a
3890 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3891 binary string. Leading whitespace is ignored in all three cases.)
3892 The following will handle decimal, binary, octal, and hex in standard
3895 $val = oct($val) if $val =~ /^0/;
3897 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3898 in octal), use sprintf() or printf():
3900 $dec_perms = (stat("filename"))[2] & 07777;
3901 $oct_perm_str = sprintf "%o", $perms;
3903 The oct() function is commonly used when a string such as C<644> needs
3904 to be converted into a file mode, for example. Although Perl
3905 automatically converts strings into numbers as needed, this automatic
3906 conversion assumes base 10.
3908 Leading white space is ignored without warning, as too are any trailing
3909 non-digits, such as a decimal point (C<oct> only handles non-negative
3910 integers, not negative integers or floating point).
3912 =item open FILEHANDLE,EXPR
3913 X<open> X<pipe> X<file, open> X<fopen>
3915 =item open FILEHANDLE,MODE,EXPR
3917 =item open FILEHANDLE,MODE,EXPR,LIST
3919 =item open FILEHANDLE,MODE,REFERENCE
3921 =item open FILEHANDLE
3923 =for Pod::Functions open a file, pipe, or descriptor
3925 Opens the file whose filename is given by EXPR, and associates it with
3928 Simple examples to open a file for reading:
3930 open(my $fh, "<", "input.txt")
3931 or die "cannot open < input.txt: $!";
3935 open(my $fh, ">", "output.txt")
3936 or die "cannot open > output.txt: $!";
3938 (The following is a comprehensive reference to open(): for a gentler
3939 introduction you may consider L<perlopentut>.)
3941 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3942 new filehandle is autovivified, meaning that the variable is assigned a
3943 reference to a newly allocated anonymous filehandle. Otherwise if
3944 FILEHANDLE is an expression, its value is the real filehandle. (This is
3945 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3948 If three (or more) arguments are specified, the open mode (including
3949 optional encoding) in the second argument are distinct from the filename in
3950 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3951 If MODE is C<< > >>, the file is opened for output, with existing files
3952 first being truncated ("clobbered") and nonexisting files newly created.
3953 If MODE is C<<< >> >>>, the file is opened for appending, again being
3954 created if necessary.
3956 You can put a C<+> in front of the C<< > >> or C<< < >> to
3957 indicate that you want both read and write access to the file; thus
3958 C<< +< >> is almost always preferred for read/write updates--the
3959 C<< +> >> mode would clobber the file first. You can't usually use
3960 either read-write mode for updating textfiles, since they have
3961 variable-length records. See the B<-i> switch in L<perlrun> for a
3962 better approach. The file is created with permissions of C<0666>
3963 modified by the process's C<umask> value.
3965 These various prefixes correspond to the fopen(3) modes of C<r>,
3966 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3968 In the one- and two-argument forms of the call, the mode and filename
3969 should be concatenated (in that order), preferably separated by white
3970 space. You can--but shouldn't--omit the mode in these forms when that mode
3971 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3972 the filename argument is a known literal.
3974 For three or more arguments if MODE is C<|->, the filename is
3975 interpreted as a command to which output is to be piped, and if MODE
3976 is C<-|>, the filename is interpreted as a command that pipes
3977 output to us. In the two-argument (and one-argument) form, one should
3978 replace dash (C<->) with the command.
3979 See L<perlipc/"Using open() for IPC"> for more examples of this.
3980 (You are not allowed to C<open> to a command that pipes both in I<and>
3981 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3982 L<perlipc/"Bidirectional Communication with Another Process"> for
3985 In the form of pipe opens taking three or more arguments, if LIST is specified
3986 (extra arguments after the command name) then LIST becomes arguments
3987 to the command invoked if the platform supports it. The meaning of
3988 C<open> with more than three arguments for non-pipe modes is not yet
3989 defined, but experimental "layers" may give extra LIST arguments
3992 In the two-argument (and one-argument) form, opening C<< <- >>
3993 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3995 You may (and usually should) use the three-argument form of open to specify
3996 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3997 that affect how the input and output are processed (see L<open> and
3998 L<PerlIO> for more details). For example:
4000 open(my $fh, "<:encoding(UTF-8)", "filename")
4001 || die "can't open UTF-8 encoded filename: $!";
4003 opens the UTF8-encoded file containing Unicode characters;
4004 see L<perluniintro>. Note that if layers are specified in the
4005 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
4006 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
4007 Those layers will also be ignored if you specifying a colon with no name
4008 following it. In that case the default layer for the operating system
4009 (:raw on Unix, :crlf on Windows) is used.
4011 Open returns nonzero on success, the undefined value otherwise. If
4012 the C<open> involved a pipe, the return value happens to be the pid of
4015 If you're running Perl on a system that distinguishes between text
4016 files and binary files, then you should check out L</binmode> for tips
4017 for dealing with this. The key distinction between systems that need
4018 C<binmode> and those that don't is their text file formats. Systems
4019 like Unix, Mac OS, and Plan 9, that end lines with a single
4020 character and encode that character in C as C<"\n"> do not
4021 need C<binmode>. The rest need it.
4023 When opening a file, it's seldom a good idea to continue
4024 if the request failed, so C<open> is frequently used with
4025 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
4026 where you want to format a suitable error message (but there are
4027 modules that can help with that problem)) always check
4028 the return value from opening a file.
4030 The filehandle will be closed when its reference count reaches zero.
4031 If it is a lexically scoped variable declared with C<my>, that usually
4032 means the end of the enclosing scope. However, this automatic close
4033 does not check for errors, so it is better to explicitly close
4034 filehandles, especially those used for writing:
4037 || warn "close failed: $!";
4039 An older style is to use a bareword as the filehandle, as
4041 open(FH, "<", "input.txt")
4042 or die "cannot open < input.txt: $!";
4044 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4045 <FH> >> and so on. Note that it's a global variable, so this form is
4046 not recommended in new code.
4048 As a shortcut a one-argument call takes the filename from the global
4049 scalar variable of the same name as the filehandle:
4052 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4054 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4055 declared with C<my> or C<state>.
4057 As a special case the three-argument form with a read/write mode and the third
4058 argument being C<undef>:
4060 open(my $tmp, "+>", undef) or die ...
4062 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4063 works for symmetry, but you really should consider writing something
4064 to the temporary file first. You will need to seek() to do the
4067 Perl is built using PerlIO by default; Unless you've
4068 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4069 open filehandles directly to Perl scalars via:
4071 open($fh, ">", \$variable) || ..
4073 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4076 open(STDOUT, ">", \$variable)
4077 or die "Can't open STDOUT: $!";
4081 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
4082 # if the open fails, output is discarded
4084 open(my $dbase, "+<", "dbase.mine") # open for update
4085 or die "Can't open 'dbase.mine' for update: $!";
4087 open(my $dbase, "+<dbase.mine") # ditto
4088 or die "Can't open 'dbase.mine' for update: $!";
4090 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4091 or die "Can't start caesar: $!";
4093 open(ARTICLE, "caesar <$article |") # ditto
4094 or die "Can't start caesar: $!";
4096 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4097 or die "Can't start sort: $!";
4100 open(MEMORY, ">", \$var)
4101 or die "Can't open memory file: $!";
4102 print MEMORY "foo!\n"; # output will appear in $var
4104 # process argument list of files along with any includes
4106 foreach $file (@ARGV) {
4107 process($file, "fh00");
4111 my($filename, $input) = @_;
4112 $input++; # this is a string increment
4113 unless (open($input, "<", $filename)) {
4114 print STDERR "Can't open $filename: $!\n";
4119 while (<$input>) { # note use of indirection
4120 if (/^#include "(.*)"/) {
4121 process($1, $input);
4128 See L<perliol> for detailed info on PerlIO.
4130 You may also, in the Bourne shell tradition, specify an EXPR beginning
4131 with C<< >& >>, in which case the rest of the string is interpreted
4132 as the name of a filehandle (or file descriptor, if numeric) to be
4133 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4134 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4135 The mode you specify should match the mode of the original filehandle.
4136 (Duping a filehandle does not take into account any existing contents
4137 of IO buffers.) If you use the three-argument
4138 form, then you can pass either a
4139 number, the name of a filehandle, or the normal "reference to a glob".
4141 Here is a script that saves, redirects, and restores C<STDOUT> and
4142 C<STDERR> using various methods:
4145 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4146 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4148 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4149 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4151 select STDERR; $| = 1; # make unbuffered
4152 select STDOUT; $| = 1; # make unbuffered
4154 print STDOUT "stdout 1\n"; # this works for
4155 print STDERR "stderr 1\n"; # subprocesses too
4157 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4158 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4160 print STDOUT "stdout 2\n";
4161 print STDERR "stderr 2\n";
4163 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4164 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4165 that file descriptor (and not call C<dup(2)>); this is more
4166 parsimonious of file descriptors. For example:
4168 # open for input, reusing the fileno of $fd
4169 open(FILEHANDLE, "<&=$fd")
4173 open(FILEHANDLE, "<&=", $fd)
4177 # open for append, using the fileno of OLDFH
4178 open(FH, ">>&=", OLDFH)
4182 open(FH, ">>&=OLDFH")
4184 Being parsimonious on filehandles is also useful (besides being
4185 parsimonious) for example when something is dependent on file
4186 descriptors, like for example locking using flock(). If you do just
4187 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4188 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4189 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4190 the same underlying system file descriptor.
4192 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4193 fdopen() to implement the C<=> functionality. On many Unix systems,
4194 fdopen() fails when file descriptors exceed a certain value, typically 255.
4195 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4197 You can see whether your Perl was built with PerlIO by running C<perl -V>
4198 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4199 have PerlIO; otherwise you don't.
4201 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4202 with the one- or two-argument forms of C<open>),
4203 an implicit C<fork> is done, so C<open> returns twice: in the parent
4204 process it returns the pid
4205 of the child process, and in the child process it returns (a defined) C<0>.
4206 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4208 For example, use either
4210 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4214 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4220 # either write TO_KID or else read FROM_KID
4222 waitpid $child_pid, 0;
4224 # am the child; use STDIN/STDOUT normally
4229 The filehandle behaves normally for the parent, but I/O to that
4230 filehandle is piped from/to the STDOUT/STDIN of the child process.
4231 In the child process, the filehandle isn't opened--I/O happens from/to
4232 the new STDOUT/STDIN. Typically this is used like the normal
4233 piped open when you want to exercise more control over just how the
4234 pipe command gets executed, such as when running setuid and
4235 you don't want to have to scan shell commands for metacharacters.
4237 The following blocks are more or less equivalent:
4239 open(FOO, "|tr '[a-z]' '[A-Z]'");
4240 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4241 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4242 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4244 open(FOO, "cat -n '$file'|");
4245 open(FOO, "-|", "cat -n '$file'");
4246 open(FOO, "-|") || exec "cat", "-n", $file;
4247 open(FOO, "-|", "cat", "-n", $file);
4249 The last two examples in each block show the pipe as "list form", which is
4250 not yet supported on all platforms. A good rule of thumb is that if
4251 your platform has a real C<fork()> (in other words, if your platform is
4252 Unix, including Linux and MacOS X), you can use the list form. You would
4253 want to use the list form of the pipe so you can pass literal arguments
4254 to the command without risk of the shell interpreting any shell metacharacters
4255 in them. However, this also bars you from opening pipes to commands
4256 that intentionally contain shell metacharacters, such as:
4258 open(FOO, "|cat -n | expand -4 | lpr")
4259 // die "Can't open pipeline to lpr: $!";
4261 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4263 Perl will attempt to flush all files opened for
4264 output before any operation that may do a fork, but this may not be
4265 supported on some platforms (see L<perlport>). To be safe, you may need
4266 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4267 of C<IO::Handle> on any open handles.
4269 On systems that support a close-on-exec flag on files, the flag will
4270 be set for the newly opened file descriptor as determined by the value
4271 of C<$^F>. See L<perlvar/$^F>.
4273 Closing any piped filehandle causes the parent process to wait for the
4274 child to finish, then returns the status value in C<$?> and
4275 C<${^CHILD_ERROR_NATIVE}>.
4277 The filename passed to the one- and two-argument forms of open() will
4278 have leading and trailing whitespace deleted and normal
4279 redirection characters honored. This property, known as "magic open",
4280 can often be used to good effect. A user could specify a filename of
4281 F<"rsh cat file |">, or you could change certain filenames as needed:
4283 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4284 open(FH, $filename) or die "Can't open $filename: $!";
4286 Use the three-argument form to open a file with arbitrary weird characters in it,
4288 open(FOO, "<", $file)
4289 || die "can't open < $file: $!";
4291 otherwise it's necessary to protect any leading and trailing whitespace:
4293 $file =~ s#^(\s)#./$1#;
4294 open(FOO, "< $file\0")
4295 || die "open failed: $!";
4297 (this may not work on some bizarre filesystems). One should
4298 conscientiously choose between the I<magic> and I<three-argument> form
4301 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4303 will allow the user to specify an argument of the form C<"rsh cat file |">,
4304 but will not work on a filename that happens to have a trailing space, while
4306 open(IN, "<", $ARGV[0])
4307 || die "can't open < $ARGV[0]: $!";
4309 will have exactly the opposite restrictions.
4311 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4312 should use the C<sysopen> function, which involves no such magic (but may
4313 use subtly different filemodes than Perl open(), which is mapped to C
4314 fopen()). This is another way to protect your filenames from
4315 interpretation. For example:
4318 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4319 or die "sysopen $path: $!";
4320 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4321 print HANDLE "stuff $$\n";
4323 print "File contains: ", <HANDLE>;
4325 See L</seek> for some details about mixing reading and writing.
4327 Portability issues: L<perlport/open>.
4329 =item opendir DIRHANDLE,EXPR
4332 =for Pod::Functions open a directory
4334 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4335 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4336 DIRHANDLE may be an expression whose value can be used as an indirect
4337 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4338 scalar variable (or array or hash element), the variable is assigned a
4339 reference to a new anonymous dirhandle; that is, it's autovivified.
4340 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4342 See the example at C<readdir>.
4349 =for Pod::Functions find a character's numeric representation
4351 Returns the numeric value of the first character of EXPR.
4352 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4353 (Note I<character>, not byte.)
4355 For the reverse, see L</chr>.
4356 See L<perlunicode> for more about Unicode.
4361 =item our TYPE VARLIST
4363 =item our VARLIST : ATTRS
4365 =item our TYPE VARLIST : ATTRS
4367 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4369 C<our> makes a lexical alias to a package (i.e. global) variable of the
4370 same name in the current package for use within the current lexical scope.
4372 C<our> has the same scoping rules as C<my> or C<state>, meaning that it is
4373 only valid within a lexical scope. Unlike C<my> and C<state>, which both
4374 declare new (lexical) variables, C<our> only creates an alias to an
4375 existing variable: a package variable of the same name.
4377 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4378 a package variable without qualifying it with the package name, but only within
4379 the lexical scope of the C<our>
4380 declaration. This applies immediately--even
4381 within the same statement.
4389 our $foo; # alias to $Foo::foo
4390 print $foo; # prints 23
4393 print $Foo::foo; # prints 23
4395 print $foo; # ERROR: requires explicit package name
4397 This works even if the package variable has not been used before, as
4398 package variables spring into existence when first used.
4403 our $foo = 23; # just like $Foo::foo = 23
4405 print $Foo::foo; # prints 23
4407 Because the variable becomes legal immediately under C<use strict 'vars'>, so
4408 long as there is no variable with that name is already in scope, you can then
4409 reference the package variable again even within the same statement.
4414 my $foo = $foo; # error, undeclared $foo on right-hand side
4415 our $foo = $foo; # no errors
4417 If more than one variable is listed, the list must be placed
4422 An C<our> declaration declares an alias for a package variable that will be visible
4423 across its entire lexical scope, even across package boundaries. The
4424 package in which the variable is entered is determined at the point
4425 of the declaration, not at the point of use. This means the following
4429 our $bar; # declares $Foo::bar for rest of lexical scope
4433 print $bar; # prints 20, as it refers to $Foo::bar
4435 Multiple C<our> declarations with the same name in the same lexical
4436 scope are allowed if they are in different packages. If they happen
4437 to be in the same package, Perl will emit warnings if you have asked
4438 for them, just like multiple C<my> declarations. Unlike a second
4439 C<my> declaration, which will bind the name to a fresh variable, a
4440 second C<our> declaration in the same package, in the same scope, is
4445 our $bar; # declares $Foo::bar for rest of lexical scope
4449 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4450 print $bar; # prints 30
4452 our $bar; # emits warning but has no other effect
4453 print $bar; # still prints 30
4455 An C<our> declaration may also have a list of attributes associated
4458 The exact semantics and interface of TYPE and ATTRS are still
4459 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4460 and attributes are handled using the C<attributes> pragma, or, starting
4461 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4462 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4463 L<attributes>, and L<Attribute::Handlers>.
4465 Note that with a parenthesised list, C<undef> can be used as a dummy
4466 placeholder, for example to skip assignment of initial values:
4468 our ( undef, $min, $hour ) = localtime;
4470 C<our> differs from C<use vars>, which allows use of an unqualified name
4471 I<only> within the affected package, but across scopes.
4473 =item pack TEMPLATE,LIST
4476 =for Pod::Functions convert a list into a binary representation
4478 Takes a LIST of values and converts it into a string using the rules
4479 given by the TEMPLATE. The resulting string is the concatenation of
4480 the converted values. Typically, each converted value looks
4481 like its machine-level representation. For example, on 32-bit machines
4482 an integer may be represented by a sequence of 4 bytes, which will in
4483 Perl be presented as a string that's 4 characters long.
4485 See L<perlpacktut> for an introduction to this function.
4487 The TEMPLATE is a sequence of characters that give the order and type
4488 of values, as follows:
4490 a A string with arbitrary binary data, will be null padded.
4491 A A text (ASCII) string, will be space padded.
4492 Z A null-terminated (ASCIZ) string, will be null padded.
4494 b A bit string (ascending bit order inside each byte,
4496 B A bit string (descending bit order inside each byte).
4497 h A hex string (low nybble first).
4498 H A hex string (high nybble first).
4500 c A signed char (8-bit) value.
4501 C An unsigned char (octet) value.
4502 W An unsigned char value (can be greater than 255).
4504 s A signed short (16-bit) value.
4505 S An unsigned short value.
4507 l A signed long (32-bit) value.
4508 L An unsigned long value.
4510 q A signed quad (64-bit) value.
4511 Q An unsigned quad value.
4512 (Quads are available only if your system supports 64-bit
4513 integer values _and_ if Perl has been compiled to support
4514 those. Raises an exception otherwise.)
4516 i A signed integer value.
4517 I A unsigned integer value.
4518 (This 'integer' is _at_least_ 32 bits wide. Its exact
4519 size depends on what a local C compiler calls 'int'.)
4521 n An unsigned short (16-bit) in "network" (big-endian) order.
4522 N An unsigned long (32-bit) in "network" (big-endian) order.
4523 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4524 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4526 j A Perl internal signed integer value (IV).
4527 J A Perl internal unsigned integer value (UV).
4529 f A single-precision float in native format.
4530 d A double-precision float in native format.
4532 F A Perl internal floating-point value (NV) in native format
4533 D A float of long-double precision in native format.
4534 (Long doubles are available only if your system supports
4535 long double values _and_ if Perl has been compiled to
4536 support those. Raises an exception otherwise.
4537 Note that there are different long double formats.)
4539 p A pointer to a null-terminated string.
4540 P A pointer to a structure (fixed-length string).
4542 u A uuencoded string.
4543 U A Unicode character number. Encodes to a character in char-
4544 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4547 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4548 for details). Its bytes represent an unsigned integer in
4549 base 128, most significant digit first, with as few digits
4550 as possible. Bit eight (the high bit) is set on each byte
4553 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4555 @ Null-fill or truncate to absolute position, counted from the
4556 start of the innermost ()-group.
4557 . Null-fill or truncate to absolute position specified by
4559 ( Start of a ()-group.
4561 One or more modifiers below may optionally follow certain letters in the
4562 TEMPLATE (the second column lists letters for which the modifier is valid):
4564 ! sSlLiI Forces native (short, long, int) sizes instead
4565 of fixed (16-/32-bit) sizes.
4567 ! xX Make x and X act as alignment commands.
4569 ! nNvV Treat integers as signed instead of unsigned.
4571 ! @. Specify position as byte offset in the internal
4572 representation of the packed string. Efficient
4575 > sSiIlLqQ Force big-endian byte-order on the type.
4576 jJfFdDpP (The "big end" touches the construct.)
4578 < sSiIlLqQ Force little-endian byte-order on the type.
4579 jJfFdDpP (The "little end" touches the construct.)
4581 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4582 to force a particular byte-order on all components in that group,
4583 including all its subgroups.
4587 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4588 pack for processing data from NASA's Magellan probe. Magellan was in an
4589 elliptical orbit, using the antenna for the radar mapping when close to
4590 Venus and for communicating data back to Earth for the rest of the orbit.
4591 There were two transmission units, but one of these failed, and then the
4592 other developed a fault whereby it would randomly flip the sense of all the
4593 bits. It was easy to automatically detect complete records with the correct
4594 sense, and complete records with all the bits flipped. However, this didn't
4595 recover the records where the sense flipped midway. A colleague of Larry's
4596 was able to pretty much eyeball where the records flipped, so they wrote an
4597 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4598 manually correct the records and recover the data. For this purpose pack
4599 gained the hex and bit string format specifiers.
4601 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4602 27e2fb84680b9cc1), but the patch description makes no mention of their
4603 addition, let alone the story behind them.
4607 The following rules apply:
4613 Each letter may optionally be followed by a number indicating the repeat
4614 count. A numeric repeat count may optionally be enclosed in brackets, as
4615 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4616 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4617 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4618 something else, described below. Supplying a C<*> for the repeat count
4619 instead of a number means to use however many items are left, except for:
4625 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4629 <.>, where it means relative to the start of the string.
4633 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4637 One can replace a numeric repeat count with a template letter enclosed in
4638 brackets to use the packed byte length of the bracketed template for the
4641 For example, the template C<x[L]> skips as many bytes as in a packed long,
4642 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4643 variable-expanded) unpacks. If the template in brackets contains alignment
4644 commands (such as C<x![d]>), its packed length is calculated as if the
4645 start of the template had the maximal possible alignment.
4647 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4648 trailing null byte, so the resulting string is always one byte longer than
4649 the byte length of the item itself.
4651 When used with C<@>, the repeat count represents an offset from the start
4652 of the innermost C<()> group.
4654 When used with C<.>, the repeat count determines the starting position to
4655 calculate the value offset as follows:
4661 If the repeat count is C<0>, it's relative to the current position.
4665 If the repeat count is C<*>, the offset is relative to the start of the
4670 And if it's an integer I<n>, the offset is relative to the start of the
4671 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4672 bigger then the group level.
4676 The repeat count for C<u> is interpreted as the maximal number of bytes
4677 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4678 count should not be more than 65.
4682 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4683 string of length count, padding with nulls or spaces as needed. When
4684 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4685 after the first null, and C<a> returns data with no stripping at all.
4687 If the value to pack is too long, the result is truncated. If it's too
4688 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4689 followed by a null byte. Thus C<Z> always packs a trailing null, except
4690 when the count is 0.
4694 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4695 Each such format generates 1 bit of the result. These are typically followed
4696 by a repeat count like C<B8> or C<B64>.
4698 Each result bit is based on the least-significant bit of the corresponding
4699 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4700 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4702 Starting from the beginning of the input string, each 8-tuple
4703 of characters is converted to 1 character of output. With format C<b>,
4704 the first character of the 8-tuple determines the least-significant bit of a
4705 character; with format C<B>, it determines the most-significant bit of
4708 If the length of the input string is not evenly divisible by 8, the
4709 remainder is packed as if the input string were padded by null characters
4710 at the end. Similarly during unpacking, "extra" bits are ignored.
4712 If the input string is longer than needed, remaining characters are ignored.
4714 A C<*> for the repeat count uses all characters of the input field.
4715 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4719 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4720 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4722 For each such format, pack() generates 4 bits of result.
4723 With non-alphabetical characters, the result is based on the 4 least-significant
4724 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4725 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4726 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4727 is compatible with the usual hexadecimal digits, so that C<"a"> and
4728 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4729 characters with this format.
4731 Starting from the beginning of the template to pack(), each pair
4732 of characters is converted to 1 character of output. With format C<h>, the
4733 first character of the pair determines the least-significant nybble of the
4734 output character; with format C<H>, it determines the most-significant
4737 If the length of the input string is not even, it behaves as if padded by
4738 a null character at the end. Similarly, "extra" nybbles are ignored during
4741 If the input string is longer than needed, extra characters are ignored.
4743 A C<*> for the repeat count uses all characters of the input field. For
4744 unpack(), nybbles are converted to a string of hexadecimal digits.
4748 The C<p> format packs a pointer to a null-terminated string. You are
4749 responsible for ensuring that the string is not a temporary value, as that
4750 could potentially get deallocated before you got around to using the packed
4751 result. The C<P> format packs a pointer to a structure of the size indicated
4752 by the length. A null pointer is created if the corresponding value for
4753 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4754 unpacks into C<undef>.
4756 If your system has a strange pointer size--meaning a pointer is neither as
4757 big as an int nor as big as a long--it may not be possible to pack or
4758 unpack pointers in big- or little-endian byte order. Attempting to do
4759 so raises an exception.
4763 The C</> template character allows packing and unpacking of a sequence of
4764 items where the packed structure contains a packed item count followed by
4765 the packed items themselves. This is useful when the structure you're
4766 unpacking has encoded the sizes or repeat counts for some of its fields
4767 within the structure itself as separate fields.
4769 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4770 I<length-item> describes how the length value is packed. Formats likely
4771 to be of most use are integer-packing ones like C<n> for Java strings,
4772 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4774 For C<pack>, I<sequence-item> may have a repeat count, in which case
4775 the minimum of that and the number of available items is used as the argument
4776 for I<length-item>. If it has no repeat count or uses a '*', the number
4777 of available items is used.
4779 For C<unpack>, an internal stack of integer arguments unpacked so far is
4780 used. You write C</>I<sequence-item> and the repeat count is obtained by
4781 popping off the last element from the stack. The I<sequence-item> must not
4782 have a repeat count.
4784 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4785 the I<length-item> is the string length, not the number of strings. With
4786 an explicit repeat count for pack, the packed string is adjusted to that
4787 length. For example:
4789 This code: gives this result:
4791 unpack("W/a", "\004Gurusamy") ("Guru")
4792 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4793 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4795 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4796 pack("a/W2", ord("a") .. ord("z")) "2ab"
4798 The I<length-item> is not returned explicitly from C<unpack>.
4800 Supplying a count to the I<length-item> format letter is only useful with
4801 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4802 introduce C<"\000"> characters, which Perl does not regard as legal in
4807 The integer types C<s>, C<S>, C<l>, and C<L> may be
4808 followed by a C<!> modifier to specify native shorts or
4809 longs. As shown in the example above, a bare C<l> means
4810 exactly 32 bits, although the native C<long> as seen by the local C compiler
4811 may be larger. This is mainly an issue on 64-bit platforms. You can
4812 see whether using C<!> makes any difference this way:
4814 printf "format s is %d, s! is %d\n",
4815 length pack("s"), length pack("s!");
4817 printf "format l is %d, l! is %d\n",
4818 length pack("l"), length pack("l!");
4821 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4822 they are identical to C<i> and C<I>.
4824 The actual sizes (in bytes) of native shorts, ints, longs, and long
4825 longs on the platform where Perl was built are also available from
4828 $ perl -V:{short,int,long{,long}}size
4834 or programmatically via the C<Config> module:
4837 print $Config{shortsize}, "\n";
4838 print $Config{intsize}, "\n";
4839 print $Config{longsize}, "\n";
4840 print $Config{longlongsize}, "\n";
4842 C<$Config{longlongsize}> is undefined on systems without
4847 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4848 inherently non-portable between processors and operating systems because
4849 they obey native byteorder and endianness. For example, a 4-byte integer
4850 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4851 handled by the CPU registers) into bytes as
4853 0x12 0x34 0x56 0x78 # big-endian
4854 0x78 0x56 0x34 0x12 # little-endian
4856 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4857 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4858 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4859 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4861 The names I<big-endian> and I<little-endian> are comic references to the
4862 egg-eating habits of the little-endian Lilliputians and the big-endian
4863 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4864 This entered computer lingo via the paper "On Holy Wars and a Plea for
4865 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4867 Some systems may have even weirder byte orders such as
4872 These are called mid-endian, middle-endian, mixed-endian, or just weird.
4874 You can determine your system endianness with this incantation:
4876 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4878 The byteorder on the platform where Perl was built is also available
4882 print "$Config{byteorder}\n";
4884 or from the command line:
4888 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4889 and C<"87654321"> are big-endian. Systems with multiarchitecture binaries
4890 will have C<"ffff">, signifying that static information doesn't work,
4891 one must use runtime probing.
4893 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4894 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4895 immediately below. See also L<perlport>.
4899 Also floating point numbers have endianness. Usually (but not always)
4900 this agrees with the integer endianness. Even though most platforms
4901 these days use the IEEE 754 binary format, there are differences,
4902 especially if the long doubles are involved. You can see the
4903 C<Config> variables C<doublekind> and C<longdblkind> (also C<doublesize>,
4904 C<longdblsize>): the "kind" values are enums, unlike C<byteorder>.
4906 Portability-wise the best option is probably to keep to the IEEE 754
4907 64-bit doubles, and of agreed-upon endianness. Another possibility
4908 is the C<"%a">) format of C<printf>.
4912 Starting with Perl 5.10.0, integer and floating-point formats, along with
4913 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4914 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4915 or little-endian byte-order. These modifiers are especially useful
4916 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4917 64-bit integers, or floating-point values.
4919 Here are some concerns to keep in mind when using an endianness modifier:
4925 Exchanging signed integers between different platforms works only
4926 when all platforms store them in the same format. Most platforms store
4927 signed integers in two's-complement notation, so usually this is not an issue.
4931 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4932 formats on big- or little-endian machines. Otherwise, attempting to
4933 use them raises an exception.
4937 Forcing big- or little-endian byte-order on floating-point values for
4938 data exchange can work only if all platforms use the same
4939 binary representation such as IEEE floating-point. Even if all
4940 platforms are using IEEE, there may still be subtle differences. Being able
4941 to use C<< > >> or C<< < >> on floating-point values can be useful,
4942 but also dangerous if you don't know exactly what you're doing.
4943 It is not a general way to portably store floating-point values.
4947 When using C<< > >> or C<< < >> on a C<()> group, this affects
4948 all types inside the group that accept byte-order modifiers,
4949 including all subgroups. It is silently ignored for all other
4950 types. You are not allowed to override the byte-order within a group
4951 that already has a byte-order modifier suffix.
4957 Real numbers (floats and doubles) are in native machine format only.
4958 Due to the multiplicity of floating-point formats and the lack of a
4959 standard "network" representation for them, no facility for interchange has been
4960 made. This means that packed floating-point data written on one machine
4961 may not be readable on another, even if both use IEEE floating-point
4962 arithmetic (because the endianness of the memory representation is not part
4963 of the IEEE spec). See also L<perlport>.
4965 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4966 modifiers to force big- or little-endian byte-order on floating-point values.
4968 Because Perl uses doubles (or long doubles, if configured) internally for
4969 all numeric calculation, converting from double into float and thence
4970 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4971 will not in general equal $foo.
4975 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4976 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4977 where the packed string is processed in its UTF-8-encoded Unicode form on
4978 a byte-by-byte basis. Character mode is the default
4979 unless the format string starts with C<U>. You
4980 can always switch mode mid-format with an explicit
4981 C<C0> or C<U0> in the format. This mode remains in effect until the next
4982 mode change, or until the end of the C<()> group it (directly) applies to.
4984 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4985 bytes is not necessarily obvious. Probably only the first of these
4988 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4989 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4991 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4992 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4994 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4995 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4997 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4998 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4999 C3.8E.C2.B1.C3.8F.C2.89
5001 Those examples also illustrate that you should not try to use
5002 C<pack>/C<unpack> as a substitute for the L<Encode> module.
5006 You must yourself do any alignment or padding by inserting, for example,
5007 enough C<"x">es while packing. There is no way for pack() and unpack()
5008 to know where characters are going to or coming from, so they
5009 handle their output and input as flat sequences of characters.
5013 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
5014 take a repeat count either as postfix, or for unpack(), also via the C</>
5015 template character. Within each repetition of a group, positioning with
5016 C<@> starts over at 0. Therefore, the result of
5018 pack("@1A((@2A)@3A)", qw[X Y Z])
5020 is the string C<"\0X\0\0YZ">.
5024 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
5025 jump forward or back to the closest position aligned at a multiple of C<count>
5026 characters. For example, to pack() or unpack() a C structure like
5029 char c; /* one signed, 8-bit character */
5034 one may need to use the template C<c x![d] d c[2]>. This assumes that
5035 doubles must be aligned to the size of double.
5037 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
5042 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
5043 represent signed 16-/32-bit integers in big-/little-endian order.
5044 This is portable only when all platforms sharing packed data use the
5045 same binary representation for signed integers; for example, when all
5046 platforms use two's-complement representation.
5050 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
5051 White space can separate pack codes from each other, but modifiers and
5052 repeat counts must follow immediately. Breaking complex templates into
5053 individual line-by-line components, suitably annotated, can do as much to
5054 improve legibility and maintainability of pack/unpack formats as C</x> can
5055 for complicated pattern matches.
5059 If TEMPLATE requires more arguments than pack() is given, pack()
5060 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
5061 than given, extra arguments are ignored.
5065 Attempting to pack the special floating point values C<Inf> and C<NaN>
5066 (infinity, also in negative, and not-a-number) into packed integer values
5067 (like C<"L">) is a fatal error. The reason for this is that there simply
5068 isn't any sensible mapping for these special values into integers.
5074 $foo = pack("WWWW",65,66,67,68);
5076 $foo = pack("W4",65,66,67,68);
5078 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
5079 # same thing with Unicode circled letters.
5080 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
5081 # same thing with Unicode circled letters. You don't get the
5082 # UTF-8 bytes because the U at the start of the format caused
5083 # a switch to U0-mode, so the UTF-8 bytes get joined into
5085 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
5086 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
5087 # This is the UTF-8 encoding of the string in the
5090 $foo = pack("ccxxcc",65,66,67,68);
5093 # NOTE: The examples above featuring "W" and "c" are true
5094 # only on ASCII and ASCII-derived systems such as ISO Latin 1
5095 # and UTF-8. On EBCDIC systems, the first example would be
5096 # $foo = pack("WWWW",193,194,195,196);
5098 $foo = pack("s2",1,2);
5099 # "\001\000\002\000" on little-endian
5100 # "\000\001\000\002" on big-endian
5102 $foo = pack("a4","abcd","x","y","z");
5105 $foo = pack("aaaa","abcd","x","y","z");
5108 $foo = pack("a14","abcdefg");
5109 # "abcdefg\0\0\0\0\0\0\0"
5111 $foo = pack("i9pl", gmtime);
5112 # a real struct tm (on my system anyway)
5114 $utmp_template = "Z8 Z8 Z16 L";
5115 $utmp = pack($utmp_template, @utmp1);
5116 # a struct utmp (BSDish)
5118 @utmp2 = unpack($utmp_template, $utmp);
5119 # "@utmp1" eq "@utmp2"
5122 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5125 $foo = pack('sx2l', 12, 34);
5126 # short 12, two zero bytes padding, long 34
5127 $bar = pack('s@4l', 12, 34);
5128 # short 12, zero fill to position 4, long 34
5130 $baz = pack('s.l', 12, 4, 34);
5131 # short 12, zero fill to position 4, long 34
5133 $foo = pack('nN', 42, 4711);
5134 # pack big-endian 16- and 32-bit unsigned integers
5135 $foo = pack('S>L>', 42, 4711);
5137 $foo = pack('s<l<', -42, 4711);
5138 # pack little-endian 16- and 32-bit signed integers
5139 $foo = pack('(sl)<', -42, 4711);
5142 The same template may generally also be used in unpack().
5144 =item package NAMESPACE
5146 =item package NAMESPACE VERSION
5147 X<package> X<module> X<namespace> X<version>
5149 =item package NAMESPACE BLOCK
5151 =item package NAMESPACE VERSION BLOCK
5152 X<package> X<module> X<namespace> X<version>
5154 =for Pod::Functions declare a separate global namespace
5156 Declares the BLOCK or the rest of the compilation unit as being in the
5157 given namespace. The scope of the package declaration is either the
5158 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5159 itself through the end of current scope (the enclosing block, file, or
5160 C<eval>). That is, the forms without a BLOCK are operative through the end
5161 of the current scope, just like the C<my>, C<state>, and C<our> operators.
5162 All unqualified dynamic identifiers in this scope will be in the given
5163 namespace, except where overridden by another C<package> declaration or
5164 when they're one of the special identifiers that qualify into C<main::>,
5165 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5167 A package statement affects dynamic variables only, including those
5168 you've used C<local> on, but I<not> lexically-scoped variables, which are created
5169 with C<my>, C<state>, or C<our>. Typically it would be the first
5170 declaration in a file included by C<require> or C<use>. You can switch into a
5171 package in more than one place, since this only determines which default
5172 symbol table the compiler uses for the rest of that block. You can refer to
5173 identifiers in other packages than the current one by prefixing the identifier
5174 with the package name and a double colon, as in C<$SomePack::var>
5175 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5176 package as assumed. That is, C<$::sail> is equivalent to
5177 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5178 code, mostly from Perl 4).
5180 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
5181 namespace to a L<version> object with the VERSION provided. VERSION must be a
5182 "strict" style version number as defined by the L<version> module: a positive
5183 decimal number (integer or decimal-fraction) without exponentiation or else a
5184 dotted-decimal v-string with a leading 'v' character and at least three
5185 components. You should set C<$VERSION> only once per package.
5187 See L<perlmod/"Packages"> for more information about packages, modules,
5188 and classes. See L<perlsub> for other scoping issues.
5193 =for Pod::Functions +5.004 the current package
5195 A special token that returns the name of the package in which it occurs.
5197 =item pipe READHANDLE,WRITEHANDLE
5200 =for Pod::Functions open a pair of connected filehandles
5202 Opens a pair of connected pipes like the corresponding system call.
5203 Note that if you set up a loop of piped processes, deadlock can occur
5204 unless you are very careful. In addition, note that Perl's pipes use
5205 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5206 after each command, depending on the application.
5208 Returns true on success.
5210 See L<IPC::Open2>, L<IPC::Open3>, and
5211 L<perlipc/"Bidirectional Communication with Another Process">
5212 for examples of such things.
5214 On systems that support a close-on-exec flag on files, that flag is set
5215 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5216 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5225 =for Pod::Functions remove the last element from an array and return it
5227 Pops and returns the last value of the array, shortening the array by
5230 Returns the undefined value if the array is empty, although this may also
5231 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5232 main program, but the C<@_> array in subroutines, just like C<shift>.
5234 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
5235 reference to an unblessed array. The argument will be dereferenced
5236 automatically. This aspect of C<pop> is considered highly experimental.
5237 The exact behaviour may change in a future version of Perl.
5239 To avoid confusing would-be users of your code who are running earlier
5240 versions of Perl with mysterious syntax errors, put this sort of thing at
5241 the top of your file to signal that your code will work I<only> on Perls of
5244 use 5.014; # so push/pop/etc work on scalars (experimental)
5247 X<pos> X<match, position>
5251 =for Pod::Functions find or set the offset for the last/next m//g search
5253 Returns the offset of where the last C<m//g> search left off for the
5254 variable in question (C<$_> is used when the variable is not
5255 specified). Note that 0 is a valid match offset. C<undef> indicates
5256 that the search position is reset (usually due to match failure, but
5257 can also be because no match has yet been run on the scalar).
5259 C<pos> directly accesses the location used by the regexp engine to
5260 store the offset, so assigning to C<pos> will change that offset, and
5261 so will also influence the C<\G> zero-width assertion in regular
5262 expressions. Both of these effects take place for the next match, so
5263 you can't affect the position with C<pos> during the current match,
5264 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5266 Setting C<pos> also resets the I<matched with zero-length> flag, described
5267 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5269 Because a failed C<m//gc> match doesn't reset the offset, the return
5270 from C<pos> won't change either in this case. See L<perlre> and
5273 =item print FILEHANDLE LIST
5276 =item print FILEHANDLE
5282 =for Pod::Functions output a list to a filehandle
5284 Prints a string or a list of strings. Returns true if successful.
5285 FILEHANDLE may be a scalar variable containing the name of or a reference
5286 to the filehandle, thus introducing one level of indirection. (NOTE: If
5287 FILEHANDLE is a variable and the next token is a term, it may be
5288 misinterpreted as an operator unless you interpose a C<+> or put
5289 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5290 last selected (see L</select>) output handle. If LIST is omitted, prints
5291 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5292 print the content of C<$_> to it, you must use a real filehandle like
5293 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5294 to something other than STDOUT, use the select operation.
5296 The current value of C<$,> (if any) is printed between each LIST item. The
5297 current value of C<$\> (if any) is printed after the entire LIST has been
5298 printed. Because print takes a LIST, anything in the LIST is evaluated in
5299 list context, including any subroutines whose return lists you pass to
5300 C<print>. Be careful not to follow the print keyword with a left
5301 parenthesis unless you want the corresponding right parenthesis to
5302 terminate the arguments to the print; put parentheses around all arguments
5303 (or interpose a C<+>, but that doesn't look as good).
5305 If you're storing handles in an array or hash, or in general whenever
5306 you're using any expression more complex than a bareword handle or a plain,
5307 unsubscripted scalar variable to retrieve it, you will have to use a block
5308 returning the filehandle value instead, in which case the LIST may not be
5311 print { $files[$i] } "stuff\n";
5312 print { $OK ? STDOUT : STDERR } "stuff\n";
5314 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5315 L<perlipc> for more on signal handling.
5317 =item printf FILEHANDLE FORMAT, LIST
5320 =item printf FILEHANDLE
5322 =item printf FORMAT, LIST
5326 =for Pod::Functions output a formatted list to a filehandle
5328 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5329 (the output record separator) is not appended. The FORMAT and the
5330 LIST are actually parsed as a single list. The first argument
5331 of the list will be interpreted as the C<printf> format. This
5332 means that C<printf(@_)> will use C<$_[0]> as the format. See
5333 L<sprintf|/sprintf FORMAT, LIST> for an
5334 explanation of the format argument. If C<use locale> for C<LC_NUMERIC>
5335 Look for this throught pod
5337 POSIX::setlocale() has been called, the character used for the decimal
5338 separator in formatted floating-point numbers is affected by the C<LC_NUMERIC>
5339 locale setting. See L<perllocale> and L<POSIX>.
5341 For historical reasons, if you omit the list, C<$_> is used as the format;
5342 to use FILEHANDLE without a list, you must use a real filehandle like
5343 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5344 you want; if $_ contains formatting codes, they will be replaced with the
5345 empty string and a warning will be emitted if warnings are enabled. Just
5346 use C<print> if you want to print the contents of $_.
5348 Don't fall into the trap of using a C<printf> when a simple
5349 C<print> would do. The C<print> is more efficient and less
5352 =item prototype FUNCTION
5357 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5359 Returns the prototype of a function as a string (or C<undef> if the
5360 function has no prototype). FUNCTION is a reference to, or the name of,
5361 the function whose prototype you want to retrieve. If FUNCTION is omitted,
5364 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5365 name for a Perl builtin. If the builtin's arguments
5366 cannot be adequately expressed by a prototype
5367 (such as C<system>), prototype() returns C<undef>, because the builtin
5368 does not really behave like a Perl function. Otherwise, the string
5369 describing the equivalent prototype is returned.
5371 =item push ARRAY,LIST
5374 =item push EXPR,LIST
5376 =for Pod::Functions append one or more elements to an array
5378 Treats ARRAY as a stack by appending the values of LIST to the end of
5379 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5383 $ARRAY[++$#ARRAY] = $value;
5386 but is more efficient. Returns the number of elements in the array following
5387 the completed C<push>.
5389 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
5390 reference to an unblessed array. The argument will be dereferenced
5391 automatically. This aspect of C<push> is considered highly experimental.
5392 The exact behaviour may change in a future version of Perl.
5394 To avoid confusing would-be users of your code who are running earlier
5395 versions of Perl with mysterious syntax errors, put this sort of thing at
5396 the top of your file to signal that your code will work I<only> on Perls of
5399 use 5.014; # so push/pop/etc work on scalars (experimental)
5403 =for Pod::Functions singly quote a string
5407 =for Pod::Functions doubly quote a string
5411 =for Pod::Functions quote a list of words
5415 =for Pod::Functions backquote quote a string
5417 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5421 =for Pod::Functions +5.005 compile pattern
5423 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5425 =item quotemeta EXPR
5426 X<quotemeta> X<metacharacter>
5430 =for Pod::Functions quote regular expression magic characters
5432 Returns the value of EXPR with all the ASCII non-"word"
5433 characters backslashed. (That is, all ASCII characters not matching
5434 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5435 returned string, regardless of any locale settings.)
5436 This is the internal function implementing
5437 the C<\Q> escape in double-quoted strings.
5438 (See below for the behavior on non-ASCII code points.)
5440 If EXPR is omitted, uses C<$_>.
5442 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5443 regular expressions, because by default an interpolated variable will be
5444 considered a mini-regular expression. For example:
5446 my $sentence = 'The quick brown fox jumped over the lazy dog';
5447 my $substring = 'quick.*?fox';
5448 $sentence =~ s{$substring}{big bad wolf};
5450 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5454 my $sentence = 'The quick brown fox jumped over the lazy dog';
5455 my $substring = 'quick.*?fox';
5456 $sentence =~ s{\Q$substring\E}{big bad wolf};
5460 my $sentence = 'The quick brown fox jumped over the lazy dog';
5461 my $substring = 'quick.*?fox';
5462 my $quoted_substring = quotemeta($substring);
5463 $sentence =~ s{$quoted_substring}{big bad wolf};
5465 Will both leave the sentence as is.
5466 Normally, when accepting literal string
5467 input from the user, quotemeta() or C<\Q> must be used.
5469 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5470 strings, but not quoted in UTF-8 strings.
5472 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5473 quoting non-ASCII characters; the quoting of ASCII characters is
5476 Also unchanged is the quoting of non-UTF-8 strings when outside the
5477 scope of a C<use feature 'unicode_strings'>, which is to quote all
5478 characters in the upper Latin1 range. This provides complete backwards
5479 compatibility for old programs which do not use Unicode. (Note that
5480 C<unicode_strings> is automatically enabled within the scope of a
5481 S<C<use v5.12>> or greater.)
5483 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5484 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5485 above, locale does not affect the quoting of ASCII-range characters.
5486 This protects against those locales where characters such as C<"|"> are
5487 considered to be word characters.
5489 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5490 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5491 The only code points that are quoted are those that have any of the
5492 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5493 Default_Ignorable_Code_Point, or General_Category=Control.
5495 Of these properties, the two important ones are Pattern_Syntax and
5496 Pattern_White_Space. They have been set up by Unicode for exactly this
5497 purpose of deciding which characters in a regular expression pattern
5498 should be quoted. No character that can be in an identifier has these
5501 Perl promises, that if we ever add regular expression pattern
5502 metacharacters to the dozen already defined
5503 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5504 Pattern_Syntax property. Perl also promises, that if we ever add
5505 characters that are considered to be white space in regular expressions
5506 (currently mostly affected by C</x>), they will all have the
5507 Pattern_White_Space property.
5509 Unicode promises that the set of code points that have these two
5510 properties will never change, so something that is not quoted in v5.16
5511 will never need to be quoted in any future Perl release. (Not all the
5512 code points that match Pattern_Syntax have actually had characters
5513 assigned to them; so there is room to grow, but they are quoted
5514 whether assigned or not. Perl, of course, would never use an
5515 unassigned code point as an actual metacharacter.)
5517 Quoting characters that have the other 3 properties is done to enhance
5518 the readability of the regular expression and not because they actually
5519 need to be quoted for regular expression purposes (characters with the
5520 White_Space property are likely to be indistinguishable on the page or
5521 screen from those with the Pattern_White_Space property; and the other
5522 two properties contain non-printing characters).
5529 =for Pod::Functions retrieve the next pseudorandom number
5531 Returns a random fractional number greater than or equal to C<0> and less
5532 than the value of EXPR. (EXPR should be positive.) If EXPR is
5533 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5534 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5535 and is subject to change in future versions of Perl). Automatically calls
5536 C<srand> unless C<srand> has already been called. See also C<srand>.
5538 Apply C<int()> to the value returned by C<rand()> if you want random
5539 integers instead of random fractional numbers. For example,
5543 returns a random integer between C<0> and C<9>, inclusive.
5545 (Note: If your rand function consistently returns numbers that are too
5546 large or too small, then your version of Perl was probably compiled
5547 with the wrong number of RANDBITS.)
5549 B<C<rand()> is not cryptographically secure. You should not rely
5550 on it in security-sensitive situations.> As of this writing, a
5551 number of third-party CPAN modules offer random number generators
5552 intended by their authors to be cryptographically secure,
5553 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5554 and L<Math::TrulyRandom>.
5556 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5557 X<read> X<file, read>
5559 =item read FILEHANDLE,SCALAR,LENGTH
5561 =for Pod::Functions fixed-length buffered input from a filehandle
5563 Attempts to read LENGTH I<characters> of data into variable SCALAR
5564 from the specified FILEHANDLE. Returns the number of characters
5565 actually read, C<0> at end of file, or undef if there was an error (in
5566 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5567 so that the last character actually read is the last character of the
5568 scalar after the read.
5570 An OFFSET may be specified to place the read data at some place in the
5571 string other than the beginning. A negative OFFSET specifies
5572 placement at that many characters counting backwards from the end of
5573 the string. A positive OFFSET greater than the length of SCALAR
5574 results in the string being padded to the required size with C<"\0">
5575 bytes before the result of the read is appended.
5577 The call is implemented in terms of either Perl's or your system's native
5578 fread(3) library function. To get a true read(2) system call, see
5579 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5581 Note the I<characters>: depending on the status of the filehandle,
5582 either (8-bit) bytes or characters are read. By default, all
5583 filehandles operate on bytes, but for example if the filehandle has
5584 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5585 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5586 characters, not bytes. Similarly for the C<:encoding> pragma:
5587 in that case pretty much any characters can be read.
5589 =item readdir DIRHANDLE
5592 =for Pod::Functions get a directory from a directory handle
5594 Returns the next directory entry for a directory opened by C<opendir>.
5595 If used in list context, returns all the rest of the entries in the
5596 directory. If there are no more entries, returns the undefined value in
5597 scalar context and the empty list in list context.
5599 If you're planning to filetest the return values out of a C<readdir>, you'd
5600 better prepend the directory in question. Otherwise, because we didn't
5601 C<chdir> there, it would have been testing the wrong file.
5603 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5604 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5607 As of Perl 5.12 you can use a bare C<readdir> in a C<while> loop,
5608 which will set C<$_> on every iteration.
5610 opendir(my $dh, $some_dir) || die;
5611 while(readdir $dh) {
5612 print "$some_dir/$_\n";
5616 To avoid confusing would-be users of your code who are running earlier
5617 versions of Perl with mysterious failures, put this sort of thing at the
5618 top of your file to signal that your code will work I<only> on Perls of a
5621 use 5.012; # so readdir assigns to $_ in a lone while test
5626 X<readline> X<gets> X<fgets>
5628 =for Pod::Functions fetch a record from a file
5630 Reads from the filehandle whose typeglob is contained in EXPR (or from
5631 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5632 returns the next line until end-of-file is reached, whereupon the
5633 subsequent call returns C<undef>. In list context, reads until end-of-file
5634 is reached and returns a list of lines. Note that the notion of "line"
5635 used here is whatever you may have defined with C<$/> or
5636 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5638 When C<$/> is set to C<undef>, when C<readline> is in scalar
5639 context (i.e., file slurp mode), and when an empty file is read, it
5640 returns C<''> the first time, followed by C<undef> subsequently.
5642 This is the internal function implementing the C<< <EXPR> >>
5643 operator, but you can use it directly. The C<< <EXPR> >>
5644 operator is discussed in more detail in L<perlop/"I/O Operators">.
5647 $line = readline(*STDIN); # same thing
5649 If C<readline> encounters an operating system error, C<$!> will be set
5650 with the corresponding error message. It can be helpful to check
5651 C<$!> when you are reading from filehandles you don't trust, such as a
5652 tty or a socket. The following example uses the operator form of
5653 C<readline> and dies if the result is not defined.
5655 while ( ! eof($fh) ) {
5656 defined( $_ = <$fh> ) or die "readline failed: $!";
5660 Note that you have can't handle C<readline> errors that way with the
5661 C<ARGV> filehandle. In that case, you have to open each element of
5662 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5664 foreach my $arg (@ARGV) {
5665 open(my $fh, $arg) or warn "Can't open $arg: $!";
5667 while ( ! eof($fh) ) {
5668 defined( $_ = <$fh> )
5669 or die "readline failed for $arg: $!";
5679 =for Pod::Functions determine where a symbolic link is pointing
5681 Returns the value of a symbolic link, if symbolic links are
5682 implemented. If not, raises an exception. If there is a system
5683 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5684 omitted, uses C<$_>.
5686 Portability issues: L<perlport/readlink>.
5693 =for Pod::Functions execute a system command and collect standard output
5695 EXPR is executed as a system command.
5696 The collected standard output of the command is returned.
5697 In scalar context, it comes back as a single (potentially
5698 multi-line) string. In list context, returns a list of lines
5699 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5700 This is the internal function implementing the C<qx/EXPR/>
5701 operator, but you can use it directly. The C<qx/EXPR/>
5702 operator is discussed in more detail in L<perlop/"I/O Operators">.
5703 If EXPR is omitted, uses C<$_>.
5705 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5708 =for Pod::Functions receive a message over a Socket
5710 Receives a message on a socket. Attempts to receive LENGTH characters
5711 of data into variable SCALAR from the specified SOCKET filehandle.
5712 SCALAR will be grown or shrunk to the length actually read. Takes the
5713 same flags as the system call of the same name. Returns the address
5714 of the sender if SOCKET's protocol supports this; returns an empty
5715 string otherwise. If there's an error, returns the undefined value.
5716 This call is actually implemented in terms of recvfrom(2) system call.
5717 See L<perlipc/"UDP: Message Passing"> for examples.
5719 Note the I<characters>: depending on the status of the socket, either
5720 (8-bit) bytes or characters are received. By default all sockets
5721 operate on bytes, but for example if the socket has been changed using
5722 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5723 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5724 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5725 case pretty much any characters can be read.
5734 =for Pod::Functions start this loop iteration over again
5736 The C<redo> command restarts the loop block without evaluating the
5737 conditional again. The C<continue> block, if any, is not executed. If
5738 the LABEL is omitted, the command refers to the innermost enclosing
5739 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
5740 label name to be computed at run time, and is otherwise identical to C<redo
5741 LABEL>. Programs that want to lie to themselves about what was just input
5742 normally use this command:
5744 # a simpleminded Pascal comment stripper
5745 # (warning: assumes no { or } in strings)
5746 LINE: while (<STDIN>) {
5747 while (s|({.*}.*){.*}|$1 |) {}
5752 if (/}/) { # end of comment?
5761 C<redo> cannot be used to retry a block that returns a value such as
5762 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5763 a grep() or map() operation.
5765 Note that a block by itself is semantically identical to a loop
5766 that executes once. Thus C<redo> inside such a block will effectively
5767 turn it into a looping construct.
5769 See also L</continue> for an illustration of how C<last>, C<next>, and
5772 Unlike most named operators, this has the same precedence as assignment.
5773 It is also exempt from the looks-like-a-function rule, so
5774 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
5782 =for Pod::Functions find out the type of thing being referenced
5784 Returns a non-empty string if EXPR is a reference, the empty
5785 string otherwise. If EXPR is not specified, C<$_> will be used. The
5786 value returned depends on the type of thing the reference is a reference to.
5788 Builtin types include:
5802 You can think of C<ref> as a C<typeof> operator.
5804 if (ref($r) eq "HASH") {
5805 print "r is a reference to a hash.\n";
5808 print "r is not a reference at all.\n";
5811 The return value C<LVALUE> indicates a reference to an lvalue that is not
5812 a variable. You get this from taking the reference of function calls like
5813 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5814 to a L<version string|perldata/"Version Strings">.
5816 The result C<Regexp> indicates that the argument is a regular expression
5817 resulting from C<qr//>.
5819 If the referenced object has been blessed into a package, then that package
5820 name is returned instead. But don't use that, as it's now considered
5821 "bad practice". For one reason, an object could be using a class called
5822 C<Regexp> or C<IO>, or even C<HASH>. Also, C<ref> doesn't take into account
5823 subclasses, like C<isa> does.
5825 Instead, use C<blessed> (in the L<Scalar::Util> module) for boolean
5826 checks, C<isa> for specific class checks and C<reftype> (also from
5827 L<Scalar::Util>) for type checks. (See L<perlobj> for details and a
5828 C<blessed/isa> example.)
5830 See also L<perlref>.
5832 =item rename OLDNAME,NEWNAME
5833 X<rename> X<move> X<mv> X<ren>
5835 =for Pod::Functions change a filename
5837 Changes the name of a file; an existing file NEWNAME will be
5838 clobbered. Returns true for success, false otherwise.
5840 Behavior of this function varies wildly depending on your system
5841 implementation. For example, it will usually not work across file system
5842 boundaries, even though the system I<mv> command sometimes compensates
5843 for this. Other restrictions include whether it works on directories,
5844 open files, or pre-existing files. Check L<perlport> and either the
5845 rename(2) manpage or equivalent system documentation for details.
5847 For a platform independent C<move> function look at the L<File::Copy>
5850 Portability issues: L<perlport/rename>.
5852 =item require VERSION
5859 =for Pod::Functions load in external functions from a library at runtime
5861 Demands a version of Perl specified by VERSION, or demands some semantics
5862 specified by EXPR or by C<$_> if EXPR is not supplied.
5864 VERSION may be either a numeric argument such as 5.006, which will be
5865 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5866 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5867 VERSION is greater than the version of the current Perl interpreter.
5868 Compare with L</use>, which can do a similar check at compile time.
5870 Specifying VERSION as a literal of the form v5.6.1 should generally be
5871 avoided, because it leads to misleading error messages under earlier
5872 versions of Perl that do not support this syntax. The equivalent numeric
5873 version should be used instead.
5875 require v5.6.1; # run time version check
5876 require 5.6.1; # ditto
5877 require 5.006_001; # ditto; preferred for backwards
5880 Otherwise, C<require> demands that a library file be included if it
5881 hasn't already been included. The file is included via the do-FILE
5882 mechanism, which is essentially just a variety of C<eval> with the
5883 caveat that lexical variables in the invoking script will be invisible
5884 to the included code. If it were implemented in pure Perl, it
5885 would have semantics similar to the following:
5891 my ($filename) = @_;
5892 if ( my $version = eval { version->parse($filename) } ) {
5893 if ( $version > $^V ) {
5894 my $vn = $version->normal;
5895 croak "Perl $vn required--this is only $^V, stopped";
5900 if (exists $INC{$filename}) {
5901 return 1 if $INC{$filename};
5902 croak "Compilation failed in require";
5905 foreach $prefix (@INC) {
5907 #... do other stuff - see text below ....
5909 # (see text below about possible appending of .pmc
5910 # suffix to $filename)
5911 my $realfilename = "$prefix/$filename";
5912 next if ! -e $realfilename || -d _ || -b _;
5913 $INC{$filename} = $realfilename;
5914 my $result = do($realfilename);
5915 # but run in caller's namespace
5917 if (!defined $result) {
5918 $INC{$filename} = undef;
5919 croak $@ ? "$@Compilation failed in require"
5920 : "Can't locate $filename: $!\n";
5923 delete $INC{$filename};
5924 croak "$filename did not return true value";
5929 croak "Can't locate $filename in \@INC ...";
5932 Note that the file will not be included twice under the same specified
5935 The file must return true as the last statement to indicate
5936 successful execution of any initialization code, so it's customary to
5937 end such a file with C<1;> unless you're sure it'll return true
5938 otherwise. But it's better just to put the C<1;>, in case you add more
5941 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5942 replaces "F<::>" with "F</>" in the filename for you,
5943 to make it easy to load standard modules. This form of loading of
5944 modules does not risk altering your namespace.
5946 In other words, if you try this:
5948 require Foo::Bar; # a splendid bareword
5950 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5951 directories specified in the C<@INC> array.
5953 But if you try this:
5955 $class = 'Foo::Bar';
5956 require $class; # $class is not a bareword
5958 require "Foo::Bar"; # not a bareword because of the ""
5960 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5961 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5963 eval "require $class";
5965 Now that you understand how C<require> looks for files with a
5966 bareword argument, there is a little extra functionality going on behind
5967 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5968 first look for a similar filename with a "F<.pmc>" extension. If this file
5969 is found, it will be loaded in place of any file ending in a "F<.pm>"
5972 You can also insert hooks into the import facility by putting Perl code
5973 directly into the @INC array. There are three forms of hooks: subroutine
5974 references, array references, and blessed objects.
5976 Subroutine references are the simplest case. When the inclusion system
5977 walks through @INC and encounters a subroutine, this subroutine gets
5978 called with two parameters, the first a reference to itself, and the
5979 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5980 subroutine should return either nothing or else a list of up to four
5981 values in the following order:
5987 A reference to a scalar, containing any initial source code to prepend to
5988 the file or generator output.
5992 A filehandle, from which the file will be read.
5996 A reference to a subroutine. If there is no filehandle (previous item),
5997 then this subroutine is expected to generate one line of source code per
5998 call, writing the line into C<$_> and returning 1, then finally at end of
5999 file returning 0. If there is a filehandle, then the subroutine will be
6000 called to act as a simple source filter, with the line as read in C<$_>.
6001 Again, return 1 for each valid line, and 0 after all lines have been
6006 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
6007 reference to the subroutine itself is passed in as C<$_[0]>.
6011 If an empty list, C<undef>, or nothing that matches the first 3 values above
6012 is returned, then C<require> looks at the remaining elements of @INC.
6013 Note that this filehandle must be a real filehandle (strictly a typeglob
6014 or reference to a typeglob, whether blessed or unblessed); tied filehandles
6015 will be ignored and processing will stop there.
6017 If the hook is an array reference, its first element must be a subroutine
6018 reference. This subroutine is called as above, but the first parameter is
6019 the array reference. This lets you indirectly pass arguments to
6022 In other words, you can write:
6024 push @INC, \&my_sub;
6026 my ($coderef, $filename) = @_; # $coderef is \&my_sub
6032 push @INC, [ \&my_sub, $x, $y, ... ];
6034 my ($arrayref, $filename) = @_;
6035 # Retrieve $x, $y, ...
6036 my @parameters = @$arrayref[1..$#$arrayref];
6040 If the hook is an object, it must provide an INC method that will be
6041 called as above, the first parameter being the object itself. (Note that
6042 you must fully qualify the sub's name, as unqualified C<INC> is always forced
6043 into package C<main>.) Here is a typical code layout:
6049 my ($self, $filename) = @_;
6053 # In the main program
6054 push @INC, Foo->new(...);
6056 These hooks are also permitted to set the %INC entry
6057 corresponding to the files they have loaded. See L<perlvar/%INC>.
6059 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
6066 =for Pod::Functions clear all variables of a given name
6068 Generally used in a C<continue> block at the end of a loop to clear
6069 variables and reset C<??> searches so that they work again. The
6070 expression is interpreted as a list of single characters (hyphens
6071 allowed for ranges). All variables and arrays beginning with one of
6072 those letters are reset to their pristine state. If the expression is
6073 omitted, one-match searches (C<?pattern?>) are reset to match again.
6074 Only resets variables or searches in the current package. Always returns
6077 reset 'X'; # reset all X variables
6078 reset 'a-z'; # reset lower case variables
6079 reset; # just reset ?one-time? searches
6081 Resetting C<"A-Z"> is not recommended because you'll wipe out your
6082 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
6083 variables; lexical variables are unaffected, but they clean themselves
6084 up on scope exit anyway, so you'll probably want to use them instead.
6092 =for Pod::Functions get out of a function early
6094 Returns from a subroutine, C<eval>, or C<do FILE> with the value
6095 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
6096 context, depending on how the return value will be used, and the context
6097 may vary from one execution to the next (see L</wantarray>). If no EXPR
6098 is given, returns an empty list in list context, the undefined value in
6099 scalar context, and (of course) nothing at all in void context.
6101 (In the absence of an explicit C<return>, a subroutine, eval,
6102 or do FILE automatically returns the value of the last expression
6105 Unlike most named operators, this is also exempt from the
6106 looks-like-a-function rule, so C<return ("foo")."bar"> will
6107 cause "bar" to be part of the argument to C<return>.
6110 X<reverse> X<rev> X<invert>
6112 =for Pod::Functions flip a string or a list
6114 In list context, returns a list value consisting of the elements
6115 of LIST in the opposite order. In scalar context, concatenates the
6116 elements of LIST and returns a string value with all characters
6117 in the opposite order.
6119 print join(", ", reverse "world", "Hello"); # Hello, world
6121 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6123 Used without arguments in scalar context, reverse() reverses C<$_>.
6125 $_ = "dlrow ,olleH";
6126 print reverse; # No output, list context
6127 print scalar reverse; # Hello, world
6129 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6130 preserve non-existent elements whenever possible; i.e., for non-magical
6131 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6133 This operator is also handy for inverting a hash, although there are some
6134 caveats. If a value is duplicated in the original hash, only one of those
6135 can be represented as a key in the inverted hash. Also, this has to
6136 unwind one hash and build a whole new one, which may take some time
6137 on a large hash, such as from a DBM file.
6139 %by_name = reverse %by_address; # Invert the hash
6141 =item rewinddir DIRHANDLE
6144 =for Pod::Functions reset directory handle
6146 Sets the current position to the beginning of the directory for the
6147 C<readdir> routine on DIRHANDLE.
6149 Portability issues: L<perlport/rewinddir>.
6151 =item rindex STR,SUBSTR,POSITION
6154 =item rindex STR,SUBSTR
6156 =for Pod::Functions right-to-left substring search
6158 Works just like index() except that it returns the position of the I<last>
6159 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6160 last occurrence beginning at or before that position.
6162 =item rmdir FILENAME
6163 X<rmdir> X<rd> X<directory, remove>
6167 =for Pod::Functions remove a directory
6169 Deletes the directory specified by FILENAME if that directory is
6170 empty. If it succeeds it returns true; otherwise it returns false and
6171 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
6173 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6174 the C<rmtree> function of the L<File::Path> module.
6178 =for Pod::Functions replace a pattern with a string
6180 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6182 =item say FILEHANDLE LIST
6185 =item say FILEHANDLE
6191 =for Pod::Functions +say output a list to a filehandle, appending a newline
6193 Just like C<print>, but implicitly appends a newline. C<say LIST> is
6194 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
6195 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
6196 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
6198 This keyword is available only when the C<"say"> feature
6199 is enabled, or when prefixed with C<CORE::>; see
6200 L<feature>. Alternately, include a C<use v5.10> or later to the current
6204 X<scalar> X<context>
6206 =for Pod::Functions force a scalar context
6208 Forces EXPR to be interpreted in scalar context and returns the value
6211 @counts = ( scalar @a, scalar @b, scalar @c );
6213 There is no equivalent operator to force an expression to
6214 be interpolated in list context because in practice, this is never
6215 needed. If you really wanted to do so, however, you could use
6216 the construction C<@{[ (some expression) ]}>, but usually a simple
6217 C<(some expression)> suffices.
6219 Because C<scalar> is a unary operator, if you accidentally use a
6220 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6221 evaluating all but the last element in void context and returning the final
6222 element evaluated in scalar context. This is seldom what you want.
6224 The following single statement:
6226 print uc(scalar(&foo,$bar)),$baz;
6228 is the moral equivalent of these two:
6231 print(uc($bar),$baz);
6233 See L<perlop> for more details on unary operators and the comma operator.
6235 =item seek FILEHANDLE,POSITION,WHENCE
6236 X<seek> X<fseek> X<filehandle, position>
6238 =for Pod::Functions reposition file pointer for random-access I/O
6240 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
6241 FILEHANDLE may be an expression whose value gives the name of the
6242 filehandle. The values for WHENCE are C<0> to set the new position
6243 I<in bytes> to POSITION; C<1> to set it to the current position plus
6244 POSITION; and C<2> to set it to EOF plus POSITION, typically
6245 negative. For WHENCE you may use the constants C<SEEK_SET>,
6246 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6247 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6250 Note the I<in bytes>: even if the filehandle has been set to
6251 operate on characters (for example by using the C<:encoding(utf8)> open
6252 layer), tell() will return byte offsets, not character offsets
6253 (because implementing that would render seek() and tell() rather slow).
6255 If you want to position the file for C<sysread> or C<syswrite>, don't use
6256 C<seek>, because buffering makes its effect on the file's read-write position
6257 unpredictable and non-portable. Use C<sysseek> instead.
6259 Due to the rules and rigors of ANSI C, on some systems you have to do a
6260 seek whenever you switch between reading and writing. Amongst other
6261 things, this may have the effect of calling stdio's clearerr(3).
6262 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6266 This is also useful for applications emulating C<tail -f>. Once you hit
6267 EOF on your read and then sleep for a while, you (probably) have to stick in a
6268 dummy seek() to reset things. The C<seek> doesn't change the position,
6269 but it I<does> clear the end-of-file condition on the handle, so that the
6270 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6272 If that doesn't work (some I/O implementations are particularly
6273 cantankerous), you might need something like this:
6276 for ($curpos = tell(FILE); $_ = <FILE>;
6277 $curpos = tell(FILE)) {
6278 # search for some stuff and put it into files
6280 sleep($for_a_while);
6281 seek(FILE, $curpos, 0);
6284 =item seekdir DIRHANDLE,POS
6287 =for Pod::Functions reposition directory pointer
6289 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6290 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6291 about possible directory compaction as the corresponding system library
6294 =item select FILEHANDLE
6295 X<select> X<filehandle, default>
6299 =for Pod::Functions reset default output or do I/O multiplexing
6301 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6302 sets the new current default filehandle for output. This has two
6303 effects: first, a C<write> or a C<print> without a filehandle
6304 default to this FILEHANDLE. Second, references to variables related to
6305 output will refer to this output channel.
6307 For example, to set the top-of-form format for more than one
6308 output channel, you might do the following:
6315 FILEHANDLE may be an expression whose value gives the name of the
6316 actual filehandle. Thus:
6318 $oldfh = select(STDERR); $| = 1; select($oldfh);
6320 Some programmers may prefer to think of filehandles as objects with
6321 methods, preferring to write the last example as:
6324 STDERR->autoflush(1);
6326 Portability issues: L<perlport/select>.
6328 =item select RBITS,WBITS,EBITS,TIMEOUT
6331 This calls the select(2) syscall with the bit masks specified, which
6332 can be constructed using C<fileno> and C<vec>, along these lines:
6334 $rin = $win = $ein = '';
6335 vec($rin, fileno(STDIN), 1) = 1;
6336 vec($win, fileno(STDOUT), 1) = 1;
6339 If you want to select on many filehandles, you may wish to write a
6340 subroutine like this:
6345 for my $fh (@fhlist) {
6346 vec($bits, fileno($fh), 1) = 1;
6350 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6354 ($nfound,$timeleft) =
6355 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6357 or to block until something becomes ready just do this
6359 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6361 Most systems do not bother to return anything useful in $timeleft, so
6362 calling select() in scalar context just returns $nfound.
6364 Any of the bit masks can also be undef. The timeout, if specified, is
6365 in seconds, which may be fractional. Note: not all implementations are
6366 capable of returning the $timeleft. If not, they always return
6367 $timeleft equal to the supplied $timeout.
6369 You can effect a sleep of 250 milliseconds this way:
6371 select(undef, undef, undef, 0.25);
6373 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6374 is implementation-dependent. See also L<perlport> for notes on the
6375 portability of C<select>.
6377 On error, C<select> behaves just like select(2): it returns
6380 On some Unixes, select(2) may report a socket file descriptor as "ready for
6381 reading" even when no data is available, and thus any subsequent C<read>
6382 would block. This can be avoided if you always use O_NONBLOCK on the
6383 socket. See select(2) and fcntl(2) for further details.
6385 The standard C<IO::Select> module provides a user-friendlier interface
6386 to C<select>, mostly because it does all the bit-mask work for you.
6388 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6389 or <FH>) with C<select>, except as permitted by POSIX, and even
6390 then only on POSIX systems. You have to use C<sysread> instead.
6392 Portability issues: L<perlport/select>.
6394 =item semctl ID,SEMNUM,CMD,ARG
6397 =for Pod::Functions SysV semaphore control operations
6399 Calls the System V IPC function semctl(2). You'll probably have to say
6403 first to get the correct constant definitions. If CMD is IPC_STAT or
6404 GETALL, then ARG must be a variable that will hold the returned
6405 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6406 the undefined value for error, "C<0 but true>" for zero, or the actual
6407 return value otherwise. The ARG must consist of a vector of native
6408 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6409 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6412 Portability issues: L<perlport/semctl>.
6414 =item semget KEY,NSEMS,FLAGS
6417 =for Pod::Functions get set of SysV semaphores
6419 Calls the System V IPC function semget(2). Returns the semaphore id, or
6420 the undefined value on error. See also
6421 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6424 Portability issues: L<perlport/semget>.
6426 =item semop KEY,OPSTRING
6429 =for Pod::Functions SysV semaphore operations
6431 Calls the System V IPC function semop(2) for semaphore operations
6432 such as signalling and waiting. OPSTRING must be a packed array of
6433 semop structures. Each semop structure can be generated with
6434 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6435 implies the number of semaphore operations. Returns true if
6436 successful, false on error. As an example, the
6437 following code waits on semaphore $semnum of semaphore id $semid:
6439 $semop = pack("s!3", $semnum, -1, 0);
6440 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6442 To signal the semaphore, replace C<-1> with C<1>. See also
6443 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6446 Portability issues: L<perlport/semop>.
6448 =item send SOCKET,MSG,FLAGS,TO
6451 =item send SOCKET,MSG,FLAGS
6453 =for Pod::Functions send a message over a socket
6455 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6456 filehandle. Takes the same flags as the system call of the same name. On
6457 unconnected sockets, you must specify a destination to I<send to>, in which
6458 case it does a sendto(2) syscall. Returns the number of characters sent,
6459 or the undefined value on error. The sendmsg(2) syscall is currently
6460 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6462 Note the I<characters>: depending on the status of the socket, either
6463 (8-bit) bytes or characters are sent. By default all sockets operate
6464 on bytes, but for example if the socket has been changed using
6465 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6466 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6467 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6468 pragma: in that case pretty much any characters can be sent.
6470 =item setpgrp PID,PGRP
6473 =for Pod::Functions set the process group of a process
6475 Sets the current process group for the specified PID, C<0> for the current
6476 process. Raises an exception when used on a machine that doesn't
6477 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6478 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6479 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6482 Portability issues: L<perlport/setpgrp>.
6484 =item setpriority WHICH,WHO,PRIORITY
6485 X<setpriority> X<priority> X<nice> X<renice>
6487 =for Pod::Functions set a process's nice value
6489 Sets the current priority for a process, a process group, or a user.
6490 (See setpriority(2).) Raises an exception when used on a machine
6491 that doesn't implement setpriority(2).
6493 Portability issues: L<perlport/setpriority>.
6495 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6498 =for Pod::Functions set some socket options
6500 Sets the socket option requested. Returns C<undef> on error.
6501 Use integer constants provided by the C<Socket> module for
6502 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6503 getprotobyname. OPTVAL might either be a packed string or an integer.
6504 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6506 An example disabling Nagle's algorithm on a socket:
6508 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6509 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6511 Portability issues: L<perlport/setsockopt>.
6520 =for Pod::Functions remove the first element of an array, and return it
6522 Shifts the first value of the array off and returns it, shortening the
6523 array by 1 and moving everything down. If there are no elements in the
6524 array, returns the undefined value. If ARRAY is omitted, shifts the
6525 C<@_> array within the lexical scope of subroutines and formats, and the
6526 C<@ARGV> array outside a subroutine and also within the lexical scopes
6527 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6528 C<UNITCHECK {}>, and C<END {}> constructs.
6530 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
6531 reference to an unblessed array. The argument will be dereferenced
6532 automatically. This aspect of C<shift> is considered highly experimental.
6533 The exact behaviour may change in a future version of Perl.
6535 To avoid confusing would-be users of your code who are running earlier
6536 versions of Perl with mysterious syntax errors, put this sort of thing at
6537 the top of your file to signal that your code will work I<only> on Perls of
6540 use 5.014; # so push/pop/etc work on scalars (experimental)
6542 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6543 same thing to the left end of an array that C<pop> and C<push> do to the
6546 =item shmctl ID,CMD,ARG
6549 =for Pod::Functions SysV shared memory operations
6551 Calls the System V IPC function shmctl. You'll probably have to say
6555 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6556 then ARG must be a variable that will hold the returned C<shmid_ds>
6557 structure. Returns like ioctl: C<undef> for error; "C<0> but
6558 true" for zero; and the actual return value otherwise.
6559 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6561 Portability issues: L<perlport/shmctl>.
6563 =item shmget KEY,SIZE,FLAGS
6566 =for Pod::Functions get SysV shared memory segment identifier
6568 Calls the System V IPC function shmget. Returns the shared memory
6569 segment id, or C<undef> on error.
6570 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6572 Portability issues: L<perlport/shmget>.
6574 =item shmread ID,VAR,POS,SIZE
6578 =for Pod::Functions read SysV shared memory
6580 =item shmwrite ID,STRING,POS,SIZE
6582 =for Pod::Functions write SysV shared memory
6584 Reads or writes the System V shared memory segment ID starting at
6585 position POS for size SIZE by attaching to it, copying in/out, and
6586 detaching from it. When reading, VAR must be a variable that will
6587 hold the data read. When writing, if STRING is too long, only SIZE
6588 bytes are used; if STRING is too short, nulls are written to fill out
6589 SIZE bytes. Return true if successful, false on error.
6590 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6591 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6593 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6595 =item shutdown SOCKET,HOW
6598 =for Pod::Functions close down just half of a socket connection
6600 Shuts down a socket connection in the manner indicated by HOW, which
6601 has the same interpretation as in the syscall of the same name.
6603 shutdown(SOCKET, 0); # I/we have stopped reading data
6604 shutdown(SOCKET, 1); # I/we have stopped writing data
6605 shutdown(SOCKET, 2); # I/we have stopped using this socket
6607 This is useful with sockets when you want to tell the other
6608 side you're done writing but not done reading, or vice versa.
6609 It's also a more insistent form of close because it also
6610 disables the file descriptor in any forked copies in other
6613 Returns C<1> for success; on error, returns C<undef> if
6614 the first argument is not a valid filehandle, or returns C<0> and sets
6615 C<$!> for any other failure.
6618 X<sin> X<sine> X<asin> X<arcsine>
6622 =for Pod::Functions return the sine of a number
6624 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6625 returns sine of C<$_>.
6627 For the inverse sine operation, you may use the C<Math::Trig::asin>
6628 function, or use this relation:
6630 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6637 =for Pod::Functions block for some number of seconds
6639 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6640 argument is given. Returns the integer number of seconds actually slept.
6642 May be interrupted if the process receives a signal such as C<SIGALRM>.
6645 local $SIG{ALARM} = sub { die "Alarm!\n" };
6648 die $@ unless $@ eq "Alarm!\n";
6650 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6651 is often implemented using C<alarm>.
6653 On some older systems, it may sleep up to a full second less than what
6654 you requested, depending on how it counts seconds. Most modern systems
6655 always sleep the full amount. They may appear to sleep longer than that,
6656 however, because your process might not be scheduled right away in a
6657 busy multitasking system.
6659 For delays of finer granularity than one second, the Time::HiRes module
6660 (from CPAN, and starting from Perl 5.8 part of the standard
6661 distribution) provides usleep(). You may also use Perl's four-argument
6662 version of select() leaving the first three arguments undefined, or you
6663 might be able to use the C<syscall> interface to access setitimer(2) if
6664 your system supports it. See L<perlfaq8> for details.
6666 See also the POSIX module's C<pause> function.
6668 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6671 =for Pod::Functions create a socket
6673 Opens a socket of the specified kind and attaches it to filehandle
6674 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6675 the syscall of the same name. You should C<use Socket> first
6676 to get the proper definitions imported. See the examples in
6677 L<perlipc/"Sockets: Client/Server Communication">.
6679 On systems that support a close-on-exec flag on files, the flag will
6680 be set for the newly opened file descriptor, as determined by the
6681 value of $^F. See L<perlvar/$^F>.
6683 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6686 =for Pod::Functions create a pair of sockets
6688 Creates an unnamed pair of sockets in the specified domain, of the
6689 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6690 for the syscall of the same name. If unimplemented, raises an exception.
6691 Returns true if successful.
6693 On systems that support a close-on-exec flag on files, the flag will
6694 be set for the newly opened file descriptors, as determined by the value
6695 of $^F. See L<perlvar/$^F>.
6697 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6698 to C<pipe(Rdr, Wtr)> is essentially:
6701 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6702 shutdown(Rdr, 1); # no more writing for reader
6703 shutdown(Wtr, 0); # no more reading for writer
6705 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6706 emulate socketpair using IP sockets to localhost if your system implements
6707 sockets but not socketpair.
6709 Portability issues: L<perlport/socketpair>.
6711 =item sort SUBNAME LIST
6712 X<sort> X<qsort> X<quicksort> X<mergesort>
6714 =item sort BLOCK LIST
6718 =for Pod::Functions sort a list of values
6720 In list context, this sorts the LIST and returns the sorted list value.
6721 In scalar context, the behaviour of C<sort()> is undefined.
6723 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6724 order. If SUBNAME is specified, it gives the name of a subroutine
6725 that returns an integer less than, equal to, or greater than C<0>,
6726 depending on how the elements of the list are to be ordered. (The
6727 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6728 SUBNAME may be a scalar variable name (unsubscripted), in which case
6729 the value provides the name of (or a reference to) the actual
6730 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6731 an anonymous, in-line sort subroutine.
6733 If the subroutine's prototype is C<($$)>, the elements to be compared are
6734 passed by reference in C<@_>, as for a normal subroutine. This is slower
6735 than unprototyped subroutines, where the elements to be compared are passed
6736 into the subroutine as the package global variables $a and $b (see example
6737 below). Note that in the latter case, it is usually highly counter-productive
6738 to declare $a and $b as lexicals.
6740 If the subroutine is an XSUB, the elements to be compared are pushed on to
6741 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6744 The values to be compared are always passed by reference and should not
6747 You also cannot exit out of the sort block or subroutine using any of the
6748 loop control operators described in L<perlsyn> or with C<goto>.
6750 When C<use locale> (but not C<use locale 'not_characters'>) is in
6751 effect, C<sort LIST> sorts LIST according to the
6752 current collation locale. See L<perllocale>.
6754 sort() returns aliases into the original list, much as a for loop's index
6755 variable aliases the list elements. That is, modifying an element of a
6756 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6757 actually modifies the element in the original list. This is usually
6758 something to be avoided when writing clear code.
6760 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6761 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6762 preserves the input order of elements that compare equal. Although
6763 quicksort's run time is O(NlogN) when averaged over all arrays of
6764 length N, the time can be O(N**2), I<quadratic> behavior, for some
6765 inputs.) In 5.7, the quicksort implementation was replaced with
6766 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6767 But benchmarks indicated that for some inputs, on some platforms,
6768 the original quicksort was faster. 5.8 has a sort pragma for
6769 limited control of the sort. Its rather blunt control of the
6770 underlying algorithm may not persist into future Perls, but the
6771 ability to characterize the input or output in implementation
6772 independent ways quite probably will. See L<the sort pragma|sort>.
6777 @articles = sort @files;
6779 # same thing, but with explicit sort routine
6780 @articles = sort {$a cmp $b} @files;
6782 # now case-insensitively
6783 @articles = sort {fc($a) cmp fc($b)} @files;
6785 # same thing in reversed order
6786 @articles = sort {$b cmp $a} @files;
6788 # sort numerically ascending
6789 @articles = sort {$a <=> $b} @files;
6791 # sort numerically descending
6792 @articles = sort {$b <=> $a} @files;
6794 # this sorts the %age hash by value instead of key
6795 # using an in-line function
6796 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6798 # sort using explicit subroutine name
6800 $age{$a} <=> $age{$b}; # presuming numeric
6802 @sortedclass = sort byage @class;
6804 sub backwards { $b cmp $a }
6805 @harry = qw(dog cat x Cain Abel);
6806 @george = qw(gone chased yz Punished Axed);
6808 # prints AbelCaincatdogx
6809 print sort backwards @harry;
6810 # prints xdogcatCainAbel
6811 print sort @george, 'to', @harry;
6812 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6814 # inefficiently sort by descending numeric compare using
6815 # the first integer after the first = sign, or the
6816 # whole record case-insensitively otherwise
6819 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6824 # same thing, but much more efficiently;
6825 # we'll build auxiliary indices instead
6827 my @nums = @caps = ();
6829 push @nums, ( /=(\d+)/ ? $1 : undef );
6833 my @new = @old[ sort {
6834 $nums[$b] <=> $nums[$a]
6836 $caps[$a] cmp $caps[$b]
6840 # same thing, but without any temps
6841 @new = map { $_->[0] }
6842 sort { $b->[1] <=> $a->[1]
6845 } map { [$_, /=(\d+)/, fc($_)] } @old;
6847 # using a prototype allows you to use any comparison subroutine
6848 # as a sort subroutine (including other package's subroutines)
6850 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6853 @new = sort other::backwards @old;
6855 # guarantee stability, regardless of algorithm
6857 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6859 # force use of mergesort (not portable outside Perl 5.8)
6860 use sort '_mergesort'; # note discouraging _
6861 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6863 Warning: syntactical care is required when sorting the list returned from
6864 a function. If you want to sort the list returned by the function call
6865 C<find_records(@key)>, you can use:
6867 @contact = sort { $a cmp $b } find_records @key;
6868 @contact = sort +find_records(@key);
6869 @contact = sort &find_records(@key);
6870 @contact = sort(find_records(@key));
6872 If instead you want to sort the array @key with the comparison routine
6873 C<find_records()> then you can use:
6875 @contact = sort { find_records() } @key;
6876 @contact = sort find_records(@key);
6877 @contact = sort(find_records @key);
6878 @contact = sort(find_records (@key));
6880 If you're using strict, you I<must not> declare $a
6881 and $b as lexicals. They are package globals. That means
6882 that if you're in the C<main> package and type
6884 @articles = sort {$b <=> $a} @files;
6886 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6887 but if you're in the C<FooPack> package, it's the same as typing
6889 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6891 The comparison function is required to behave. If it returns
6892 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6893 sometimes saying the opposite, for example) the results are not
6896 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6897 (not-a-number), be careful when sorting with a
6898 comparison function like C<< $a <=> $b >> any lists that might contain a
6899 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6900 eliminate any C<NaN>s from the input list.
6902 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6904 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6907 =item splice ARRAY or EXPR,OFFSET,LENGTH
6909 =item splice ARRAY or EXPR,OFFSET
6911 =item splice ARRAY or EXPR
6913 =for Pod::Functions add or remove elements anywhere in an array
6915 Removes the elements designated by OFFSET and LENGTH from an array, and
6916 replaces them with the elements of LIST, if any. In list context,
6917 returns the elements removed from the array. In scalar context,
6918 returns the last element removed, or C<undef> if no elements are
6919 removed. The array grows or shrinks as necessary.
6920 If OFFSET is negative then it starts that far from the end of the array.
6921 If LENGTH is omitted, removes everything from OFFSET onward.
6922 If LENGTH is negative, removes the elements from OFFSET onward
6923 except for -LENGTH elements at the end of the array.
6924 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6925 past the end of the array and a LENGTH was provided, Perl issues a warning,
6926 and splices at the end of the array.
6928 The following equivalences hold (assuming C<< $#a >= $i >> )
6930 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6931 pop(@a) splice(@a,-1)
6932 shift(@a) splice(@a,0,1)
6933 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6934 $a[$i] = $y splice(@a,$i,1,$y)
6936 C<splice> can be used, for example, to implement n-ary queue processing:
6940 while (my @next_n = splice @_, 0, $n) {
6941 say join q{ -- }, @next_n;
6945 nary_print(3, qw(a b c d e f g h));
6951 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6952 reference to an unblessed array. The argument will be dereferenced
6953 automatically. This aspect of C<splice> is considered highly experimental.
6954 The exact behaviour may change in a future version of Perl.
6956 To avoid confusing would-be users of your code who are running earlier
6957 versions of Perl with mysterious syntax errors, put this sort of thing at
6958 the top of your file to signal that your code will work I<only> on Perls of
6961 use 5.014; # so push/pop/etc work on scalars (experimental)
6963 =item split /PATTERN/,EXPR,LIMIT
6966 =item split /PATTERN/,EXPR
6968 =item split /PATTERN/
6972 =for Pod::Functions split up a string using a regexp delimiter
6974 Splits the string EXPR into a list of strings and returns the
6975 list in list context, or the size of the list in scalar context.
6977 If only PATTERN is given, EXPR defaults to C<$_>.
6979 Anything in EXPR that matches PATTERN is taken to be a separator
6980 that separates the EXPR into substrings (called "I<fields>") that
6981 do B<not> include the separator. Note that a separator may be
6982 longer than one character or even have no characters at all (the
6983 empty string, which is a zero-width match).
6985 The PATTERN need not be constant; an expression may be used
6986 to specify a pattern that varies at runtime.
6988 If PATTERN matches the empty string, the EXPR is split at the match
6989 position (between characters). As an example, the following:
6991 print join(':', split('b', 'abc')), "\n";
6993 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6996 print join(':', split('', 'abc')), "\n";
6998 uses empty string matches as separators to produce the output
6999 'a:b:c'; thus, the empty string may be used to split EXPR into a
7000 list of its component characters.
7002 As a special case for C<split>, the empty pattern given in
7003 L<match operator|perlop/"m/PATTERN/msixpodualngc"> syntax (C<//>)
7004 specifically matches the empty string, which is contrary to its usual
7005 interpretation as the last successful match.
7007 If PATTERN is C</^/>, then it is treated as if it used the
7008 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
7009 isn't much use otherwise.
7011 As another special case, C<split> emulates the default behavior of the
7012 command line tool B<awk> when the PATTERN is either omitted or a I<literal
7013 string> composed of a single space character (such as S<C<' '>> or
7014 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
7015 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
7016 instead treated as if it were C</\s+/>; in particular, this means that
7017 I<any> contiguous whitespace (not just a single space character) is used as
7018 a separator. However, this special treatment can be avoided by specifying
7019 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
7020 only a single space character to be a separator. In earlier Perls this
7021 special case was restricted to the use of a plain S<C<" ">> as the
7022 pattern argument to split, in Perl 5.18.0 and later this special case is
7023 triggered by any expression which evaluates as the simple string S<C<" ">>.
7025 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
7026 the previously described I<awk> emulation.
7028 If LIMIT is specified and positive, it represents the maximum number
7029 of fields into which the EXPR may be split; in other words, LIMIT is
7030 one greater than the maximum number of times EXPR may be split. Thus,
7031 the LIMIT value C<1> means that EXPR may be split a maximum of zero
7032 times, producing a maximum of one field (namely, the entire value of
7033 EXPR). For instance:
7035 print join(':', split(//, 'abc', 1)), "\n";
7037 produces the output 'abc', and this:
7039 print join(':', split(//, 'abc', 2)), "\n";
7041 produces the output 'a:bc', and each of these:
7043 print join(':', split(//, 'abc', 3)), "\n";
7044 print join(':', split(//, 'abc', 4)), "\n";
7046 produces the output 'a:b:c'.
7048 If LIMIT is negative, it is treated as if it were instead arbitrarily
7049 large; as many fields as possible are produced.
7051 If LIMIT is omitted (or, equivalently, zero), then it is usually
7052 treated as if it were instead negative but with the exception that
7053 trailing empty fields are stripped (empty leading fields are always
7054 preserved); if all fields are empty, then all fields are considered to
7055 be trailing (and are thus stripped in this case). Thus, the following:
7057 print join(':', split(',', 'a,b,c,,,')), "\n";
7059 produces the output 'a:b:c', but the following:
7061 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
7063 produces the output 'a:b:c:::'.
7065 In time-critical applications, it is worthwhile to avoid splitting
7066 into more fields than necessary. Thus, when assigning to a list,
7067 if LIMIT is omitted (or zero), then LIMIT is treated as though it
7068 were one larger than the number of variables in the list; for the
7069 following, LIMIT is implicitly 3:
7071 ($login, $passwd) = split(/:/);
7073 Note that splitting an EXPR that evaluates to the empty string always
7074 produces zero fields, regardless of the LIMIT specified.
7076 An empty leading field is produced when there is a positive-width
7077 match at the beginning of EXPR. For instance:
7079 print join(':', split(/ /, ' abc')), "\n";
7081 produces the output ':abc'. However, a zero-width match at the
7082 beginning of EXPR never produces an empty field, so that:
7084 print join(':', split(//, ' abc'));
7086 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
7088 An empty trailing field, on the other hand, is produced when there is a
7089 match at the end of EXPR, regardless of the length of the match
7090 (of course, unless a non-zero LIMIT is given explicitly, such fields are
7091 removed, as in the last example). Thus:
7093 print join(':', split(//, ' abc', -1)), "\n";
7095 produces the output S<' :a:b:c:'>.
7097 If the PATTERN contains
7098 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
7099 then for each separator, an additional field is produced for each substring
7100 captured by a group (in the order in which the groups are specified,
7101 as per L<backreferences|perlretut/Backreferences>); if any group does not
7102 match, then it captures the C<undef> value instead of a substring. Also,
7103 note that any such additional field is produced whenever there is a
7104 separator (that is, whenever a split occurs), and such an additional field
7105 does B<not> count towards the LIMIT. Consider the following expressions
7106 evaluated in list context (each returned list is provided in the associated
7109 split(/-|,/, "1-10,20", 3)
7112 split(/(-|,)/, "1-10,20", 3)
7113 # ('1', '-', '10', ',', '20')
7115 split(/-|(,)/, "1-10,20", 3)
7116 # ('1', undef, '10', ',', '20')
7118 split(/(-)|,/, "1-10,20", 3)
7119 # ('1', '-', '10', undef, '20')
7121 split(/(-)|(,)/, "1-10,20", 3)
7122 # ('1', '-', undef, '10', undef, ',', '20')
7124 =item sprintf FORMAT, LIST
7127 =for Pod::Functions formatted print into a string
7129 Returns a string formatted by the usual C<printf> conventions of the C
7130 library function C<sprintf>. See below for more details
7131 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
7132 the general principles.
7136 # Format number with up to 8 leading zeroes
7137 $result = sprintf("%08d", $number);
7139 # Round number to 3 digits after decimal point
7140 $rounded = sprintf("%.3f", $number);
7142 Perl does its own C<sprintf> formatting: it emulates the C
7143 function sprintf(3), but doesn't use it except for floating-point
7144 numbers, and even then only standard modifiers are allowed.
7145 Non-standard extensions in your local sprintf(3) are
7146 therefore unavailable from Perl.
7148 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
7149 pass it an array as your first argument.
7150 The array is given scalar context,
7151 and instead of using the 0th element of the array as the format, Perl will
7152 use the count of elements in the array as the format, which is almost never
7155 Perl's C<sprintf> permits the following universally-known conversions:
7158 %c a character with the given number
7160 %d a signed integer, in decimal
7161 %u an unsigned integer, in decimal
7162 %o an unsigned integer, in octal
7163 %x an unsigned integer, in hexadecimal
7164 %e a floating-point number, in scientific notation
7165 %f a floating-point number, in fixed decimal notation
7166 %g a floating-point number, in %e or %f notation
7168 In addition, Perl permits the following widely-supported conversions:
7170 %X like %x, but using upper-case letters
7171 %E like %e, but using an upper-case "E"
7172 %G like %g, but with an upper-case "E" (if applicable)
7173 %b an unsigned integer, in binary
7174 %B like %b, but using an upper-case "B" with the # flag
7175 %p a pointer (outputs the Perl value's address in hexadecimal)
7176 %n special: *stores* the number of characters output so far
7177 into the next argument in the parameter list
7178 %a hexadecimal floating point
7179 %A like %a, but using upper-case letters
7181 Finally, for backward (and we do mean "backward") compatibility, Perl
7182 permits these unnecessary but widely-supported conversions:
7185 %D a synonym for %ld
7186 %U a synonym for %lu
7187 %O a synonym for %lo
7190 Note that the number of exponent digits in the scientific notation produced
7191 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7192 exponent less than 100 is system-dependent: it may be three or less
7193 (zero-padded as necessary). In other words, 1.23 times ten to the
7194 99th may be either "1.23e99" or "1.23e099". Similarly for C<%a> and C<%A>:
7195 the exponent or the hexadecimal digits may float: especially the
7196 "long doubles" Perl configuration option may cause surprises.
7198 Between the C<%> and the format letter, you may specify several
7199 additional attributes controlling the interpretation of the format.
7200 In order, these are:
7204 =item format parameter index
7206 An explicit format parameter index, such as C<2$>. By default sprintf
7207 will format the next unused argument in the list, but this allows you
7208 to take the arguments out of order:
7210 printf '%2$d %1$d', 12, 34; # prints "34 12"
7211 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7217 space prefix non-negative number with a space
7218 + prefix non-negative number with a plus sign
7219 - left-justify within the field
7220 0 use zeros, not spaces, to right-justify
7221 # ensure the leading "0" for any octal,
7222 prefix non-zero hexadecimal with "0x" or "0X",
7223 prefix non-zero binary with "0b" or "0B"
7227 printf '<% d>', 12; # prints "< 12>"
7228 printf '<%+d>', 12; # prints "<+12>"
7229 printf '<%6s>', 12; # prints "< 12>"
7230 printf '<%-6s>', 12; # prints "<12 >"
7231 printf '<%06s>', 12; # prints "<000012>"
7232 printf '<%#o>', 12; # prints "<014>"
7233 printf '<%#x>', 12; # prints "<0xc>"
7234 printf '<%#X>', 12; # prints "<0XC>"
7235 printf '<%#b>', 12; # prints "<0b1100>"
7236 printf '<%#B>', 12; # prints "<0B1100>"
7238 When a space and a plus sign are given as the flags at once,
7239 a plus sign is used to prefix a positive number.
7241 printf '<%+ d>', 12; # prints "<+12>"
7242 printf '<% +d>', 12; # prints "<+12>"
7244 When the # flag and a precision are given in the %o conversion,
7245 the precision is incremented if it's necessary for the leading "0".
7247 printf '<%#.5o>', 012; # prints "<00012>"
7248 printf '<%#.5o>', 012345; # prints "<012345>"
7249 printf '<%#.0o>', 0; # prints "<0>"
7253 This flag tells Perl to interpret the supplied string as a vector of
7254 integers, one for each character in the string. Perl applies the format to
7255 each integer in turn, then joins the resulting strings with a separator (a
7256 dot C<.> by default). This can be useful for displaying ordinal values of
7257 characters in arbitrary strings:
7259 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7260 printf "version is v%vd\n", $^V; # Perl's version
7262 Put an asterisk C<*> before the C<v> to override the string to
7263 use to separate the numbers:
7265 printf "address is %*vX\n", ":", $addr; # IPv6 address
7266 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7268 You can also explicitly specify the argument number to use for
7269 the join string using something like C<*2$v>; for example:
7271 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7274 =item (minimum) width
7276 Arguments are usually formatted to be only as wide as required to
7277 display the given value. You can override the width by putting
7278 a number here, or get the width from the next argument (with C<*>)
7279 or from a specified argument (e.g., with C<*2$>):
7281 printf "<%s>", "a"; # prints "<a>"
7282 printf "<%6s>", "a"; # prints "< a>"
7283 printf "<%*s>", 6, "a"; # prints "< a>"
7284 printf '<%*2$s>', "a", 6; # prints "< a>"
7285 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7287 If a field width obtained through C<*> is negative, it has the same
7288 effect as the C<-> flag: left-justification.
7290 =item precision, or maximum width
7293 You can specify a precision (for numeric conversions) or a maximum
7294 width (for string conversions) by specifying a C<.> followed by a number.
7295 For floating-point formats except C<g> and C<G>, this specifies
7296 how many places right of the decimal point to show (the default being 6).
7299 # these examples are subject to system-specific variation
7300 printf '<%f>', 1; # prints "<1.000000>"
7301 printf '<%.1f>', 1; # prints "<1.0>"
7302 printf '<%.0f>', 1; # prints "<1>"
7303 printf '<%e>', 10; # prints "<1.000000e+01>"
7304 printf '<%.1e>', 10; # prints "<1.0e+01>"
7306 For "g" and "G", this specifies the maximum number of digits to show,
7307 including those prior to the decimal point and those after it; for
7310 # These examples are subject to system-specific variation.
7311 printf '<%g>', 1; # prints "<1>"
7312 printf '<%.10g>', 1; # prints "<1>"
7313 printf '<%g>', 100; # prints "<100>"
7314 printf '<%.1g>', 100; # prints "<1e+02>"
7315 printf '<%.2g>', 100.01; # prints "<1e+02>"
7316 printf '<%.5g>', 100.01; # prints "<100.01>"
7317 printf '<%.4g>', 100.01; # prints "<100>"
7319 For integer conversions, specifying a precision implies that the
7320 output of the number itself should be zero-padded to this width,
7321 where the 0 flag is ignored:
7323 printf '<%.6d>', 1; # prints "<000001>"
7324 printf '<%+.6d>', 1; # prints "<+000001>"
7325 printf '<%-10.6d>', 1; # prints "<000001 >"
7326 printf '<%10.6d>', 1; # prints "< 000001>"
7327 printf '<%010.6d>', 1; # prints "< 000001>"
7328 printf '<%+10.6d>', 1; # prints "< +000001>"
7330 printf '<%.6x>', 1; # prints "<000001>"
7331 printf '<%#.6x>', 1; # prints "<0x000001>"
7332 printf '<%-10.6x>', 1; # prints "<000001 >"
7333 printf '<%10.6x>', 1; # prints "< 000001>"
7334 printf '<%010.6x>', 1; # prints "< 000001>"
7335 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7337 For string conversions, specifying a precision truncates the string
7338 to fit the specified width:
7340 printf '<%.5s>', "truncated"; # prints "<trunc>"
7341 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7343 You can also get the precision from the next argument using C<.*>:
7345 printf '<%.6x>', 1; # prints "<000001>"
7346 printf '<%.*x>', 6, 1; # prints "<000001>"
7348 If a precision obtained through C<*> is negative, it counts
7349 as having no precision at all.
7351 printf '<%.*s>', 7, "string"; # prints "<string>"
7352 printf '<%.*s>', 3, "string"; # prints "<str>"
7353 printf '<%.*s>', 0, "string"; # prints "<>"
7354 printf '<%.*s>', -1, "string"; # prints "<string>"
7356 printf '<%.*d>', 1, 0; # prints "<0>"
7357 printf '<%.*d>', 0, 0; # prints "<>"
7358 printf '<%.*d>', -1, 0; # prints "<0>"
7360 You cannot currently get the precision from a specified number,
7361 but it is intended that this will be possible in the future, for
7362 example using C<.*2$>:
7364 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print
7369 For numeric conversions, you can specify the size to interpret the
7370 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7371 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7372 whatever the default integer size is on your platform (usually 32 or 64
7373 bits), but you can override this to use instead one of the standard C types,
7374 as supported by the compiler used to build Perl:
7376 hh interpret integer as C type "char" or "unsigned
7377 char" on Perl 5.14 or later
7378 h interpret integer as C type "short" or
7380 j interpret integer as C type "intmax_t" on Perl
7381 5.14 or later, and only with a C99 compiler
7383 l interpret integer as C type "long" or
7385 q, L, or ll interpret integer as C type "long long",
7386 "unsigned long long", or "quad" (typically
7388 t interpret integer as C type "ptrdiff_t" on Perl
7390 z interpret integer as C type "size_t" on Perl 5.14
7393 As of 5.14, none of these raises an exception if they are not supported on
7394 your platform. However, if warnings are enabled, a warning of the
7395 C<printf> warning class is issued on an unsupported conversion flag.
7396 Should you instead prefer an exception, do this:
7398 use warnings FATAL => "printf";
7400 If you would like to know about a version dependency before you
7401 start running the program, put something like this at its top:
7403 use 5.014; # for hh/j/t/z/ printf modifiers
7405 You can find out whether your Perl supports quads via L<Config>:
7408 if ($Config{use64bitint} eq "define"
7409 || $Config{longsize} >= 8) {
7410 print "Nice quads!\n";
7413 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7414 to be the default floating-point size on your platform (double or long double),
7415 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7416 platform supports them. You can find out whether your Perl supports long
7417 doubles via L<Config>:
7420 print "long doubles\n" if $Config{d_longdbl} eq "define";
7422 You can find out whether Perl considers "long double" to be the default
7423 floating-point size to use on your platform via L<Config>:
7426 if ($Config{uselongdouble} eq "define") {
7427 print "long doubles by default\n";
7430 It can also be that long doubles and doubles are the same thing:
7433 ($Config{doublesize} == $Config{longdblsize}) &&
7434 print "doubles are long doubles\n";
7436 The size specifier C<V> has no effect for Perl code, but is supported for
7437 compatibility with XS code. It means "use the standard size for a Perl
7438 integer or floating-point number", which is the default.
7440 =item order of arguments
7442 Normally, sprintf() takes the next unused argument as the value to
7443 format for each format specification. If the format specification
7444 uses C<*> to require additional arguments, these are consumed from
7445 the argument list in the order they appear in the format
7446 specification I<before> the value to format. Where an argument is
7447 specified by an explicit index, this does not affect the normal
7448 order for the arguments, even when the explicitly specified index
7449 would have been the next argument.
7453 printf "<%*.*s>", $a, $b, $c;
7455 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7456 as the value to format; while:
7458 printf '<%*1$.*s>', $a, $b;
7460 would use C<$a> for the width and precision, and C<$b> as the
7463 Here are some more examples; be aware that when using an explicit
7464 index, the C<$> may need escaping:
7466 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7467 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7468 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7469 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7473 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7474 and POSIX::setlocale() has been called,
7475 the character used for the decimal separator in formatted floating-point
7476 numbers is affected by the C<LC_NUMERIC> locale. See L<perllocale>
7480 X<sqrt> X<root> X<square root>
7484 =for Pod::Functions square root function
7486 Return the positive square root of EXPR. If EXPR is omitted, uses
7487 C<$_>. Works only for non-negative operands unless you've
7488 loaded the C<Math::Complex> module.
7491 print sqrt(-4); # prints 2i
7494 X<srand> X<seed> X<randseed>
7498 =for Pod::Functions seed the random number generator
7500 Sets and returns the random number seed for the C<rand> operator.
7502 The point of the function is to "seed" the C<rand> function so that C<rand>
7503 can produce a different sequence each time you run your program. When
7504 called with a parameter, C<srand> uses that for the seed; otherwise it
7505 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7506 it returns the seed. To signal that your code will work I<only> on Perls
7507 of a recent vintage:
7509 use 5.014; # so srand returns the seed
7511 If C<srand()> is not called explicitly, it is called implicitly without a
7512 parameter at the first use of the C<rand> operator.
7513 However, there are a few situations where programs are likely to
7514 want to call C<srand>. One is for generating predictable results, generally for
7515 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7516 each time. Another case is that you may want to call C<srand()>
7517 after a C<fork()> to avoid child processes sharing the same seed value as the
7518 parent (and consequently each other).
7520 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7521 process. The internal state of the random number generator should
7522 contain more entropy than can be provided by any seed, so calling
7523 C<srand()> again actually I<loses> randomness.
7525 Most implementations of C<srand> take an integer and will silently
7526 truncate decimal numbers. This means C<srand(42)> will usually
7527 produce the same results as C<srand(42.1)>. To be safe, always pass
7528 C<srand> an integer.
7530 A typical use of the returned seed is for a test program which has too many
7531 combinations to test comprehensively in the time available to it each run. It
7532 can test a random subset each time, and should there be a failure, log the seed
7533 used for that run so that it can later be used to reproduce the same results.
7535 B<C<rand()> is not cryptographically secure. You should not rely
7536 on it in security-sensitive situations.> As of this writing, a
7537 number of third-party CPAN modules offer random number generators
7538 intended by their authors to be cryptographically secure,
7539 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7540 and L<Math::TrulyRandom>.
7542 =item stat FILEHANDLE
7543 X<stat> X<file, status> X<ctime>
7547 =item stat DIRHANDLE
7551 =for Pod::Functions get a file's status information
7553 Returns a 13-element list giving the status info for a file, either
7554 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7555 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7558 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7559 $atime,$mtime,$ctime,$blksize,$blocks)
7562 Not all fields are supported on all filesystem types. Here are the
7563 meanings of the fields:
7565 0 dev device number of filesystem
7567 2 mode file mode (type and permissions)
7568 3 nlink number of (hard) links to the file
7569 4 uid numeric user ID of file's owner
7570 5 gid numeric group ID of file's owner
7571 6 rdev the device identifier (special files only)
7572 7 size total size of file, in bytes
7573 8 atime last access time in seconds since the epoch
7574 9 mtime last modify time in seconds since the epoch
7575 10 ctime inode change time in seconds since the epoch (*)
7576 11 blksize preferred I/O size in bytes for interacting with the
7577 file (may vary from file to file)
7578 12 blocks actual number of system-specific blocks allocated
7579 on disk (often, but not always, 512 bytes each)
7581 (The epoch was at 00:00 January 1, 1970 GMT.)
7583 (*) Not all fields are supported on all filesystem types. Notably, the
7584 ctime field is non-portable. In particular, you cannot expect it to be a
7585 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7587 If C<stat> is passed the special filehandle consisting of an underline, no
7588 stat is done, but the current contents of the stat structure from the
7589 last C<stat>, C<lstat>, or filetest are returned. Example:
7591 if (-x $file && (($d) = stat(_)) && $d < 0) {
7592 print "$file is executable NFS file\n";
7595 (This works on machines only for which the device number is negative
7598 Because the mode contains both the file type and its permissions, you
7599 should mask off the file type portion and (s)printf using a C<"%o">
7600 if you want to see the real permissions.
7602 $mode = (stat($filename))[2];
7603 printf "Permissions are %04o\n", $mode & 07777;
7605 In scalar context, C<stat> returns a boolean value indicating success
7606 or failure, and, if successful, sets the information associated with
7607 the special filehandle C<_>.
7609 The L<File::stat> module provides a convenient, by-name access mechanism:
7612 $sb = stat($filename);
7613 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7614 $filename, $sb->size, $sb->mode & 07777,
7615 scalar localtime $sb->mtime;
7617 You can import symbolic mode constants (C<S_IF*>) and functions
7618 (C<S_IS*>) from the Fcntl module:
7622 $mode = (stat($filename))[2];
7624 $user_rwx = ($mode & S_IRWXU) >> 6;
7625 $group_read = ($mode & S_IRGRP) >> 3;
7626 $other_execute = $mode & S_IXOTH;
7628 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7630 $is_setuid = $mode & S_ISUID;
7631 $is_directory = S_ISDIR($mode);
7633 You could write the last two using the C<-u> and C<-d> operators.
7634 Commonly available C<S_IF*> constants are:
7636 # Permissions: read, write, execute, for user, group, others.
7638 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7639 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7640 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7642 # Setuid/Setgid/Stickiness/SaveText.
7643 # Note that the exact meaning of these is system-dependent.
7645 S_ISUID S_ISGID S_ISVTX S_ISTXT
7647 # File types. Not all are necessarily available on
7650 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7651 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7653 # The following are compatibility aliases for S_IRUSR,
7654 # S_IWUSR, and S_IXUSR.
7656 S_IREAD S_IWRITE S_IEXEC
7658 and the C<S_IF*> functions are
7660 S_IMODE($mode) the part of $mode containing the permission
7661 bits and the setuid/setgid/sticky bits
7663 S_IFMT($mode) the part of $mode containing the file type
7664 which can be bit-anded with (for example)
7665 S_IFREG or with the following functions
7667 # The operators -f, -d, -l, -b, -c, -p, and -S.
7669 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7670 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7672 # No direct -X operator counterpart, but for the first one
7673 # the -g operator is often equivalent. The ENFMT stands for
7674 # record flocking enforcement, a platform-dependent feature.
7676 S_ISENFMT($mode) S_ISWHT($mode)
7678 See your native chmod(2) and stat(2) documentation for more details
7679 about the C<S_*> constants. To get status info for a symbolic link
7680 instead of the target file behind the link, use the C<lstat> function.
7682 Portability issues: L<perlport/stat>.
7687 =item state TYPE VARLIST
7689 =item state VARLIST : ATTRS
7691 =item state TYPE VARLIST : ATTRS
7693 =for Pod::Functions +state declare and assign a persistent lexical variable
7695 C<state> declares a lexically scoped variable, just like C<my>.
7696 However, those variables will never be reinitialized, contrary to
7697 lexical variables that are reinitialized each time their enclosing block
7699 See L<perlsub/"Persistent Private Variables"> for details.
7701 If more than one variable is listed, the list must be placed in
7702 parentheses. With a parenthesised list, C<undef> can be used as a
7703 dummy placeholder. However, since initialization of state variables in
7704 list context is currently not possible this would serve no purpose.
7706 C<state> variables are enabled only when the C<use feature "state"> pragma
7707 is in effect, unless the keyword is written as C<CORE::state>.
7708 See also L<feature>. Alternately, include a C<use v5.10> or later to the
7716 =for Pod::Functions optimize input data for repeated searches
7718 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
7719 doing many pattern matches on the string before it is next modified.
7720 This may or may not save time, depending on the nature and number of
7721 patterns you are searching and the distribution of character
7722 frequencies in the string to be searched; you probably want to compare
7723 run times with and without it to see which is faster. Those loops
7724 that scan for many short constant strings (including the constant
7725 parts of more complex patterns) will benefit most.
7726 (The way C<study> works is this: a linked list of every
7727 character in the string to be searched is made, so we know, for
7728 example, where all the C<'k'> characters are. From each search string,
7729 the rarest character is selected, based on some static frequency tables
7730 constructed from some C programs and English text. Only those places
7731 that contain this "rarest" character are examined.)
7733 For example, here is a loop that inserts index producing entries
7734 before any line containing a certain pattern:
7738 print ".IX foo\n" if /\bfoo\b/;
7739 print ".IX bar\n" if /\bbar\b/;
7740 print ".IX blurfl\n" if /\bblurfl\b/;
7745 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7746 will be looked at, because C<f> is rarer than C<o>. In general, this is
7747 a big win except in pathological cases. The only question is whether
7748 it saves you more time than it took to build the linked list in the
7751 Note that if you have to look for strings that you don't know till
7752 runtime, you can build an entire loop as a string and C<eval> that to
7753 avoid recompiling all your patterns all the time. Together with
7754 undefining C<$/> to input entire files as one record, this can be quite
7755 fast, often faster than specialized programs like fgrep(1). The following
7756 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7757 out the names of those files that contain a match:
7759 $search = 'while (<>) { study;';
7760 foreach $word (@words) {
7761 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7766 eval $search; # this screams
7767 $/ = "\n"; # put back to normal input delimiter
7768 foreach $file (sort keys(%seen)) {
7772 =item sub NAME BLOCK
7775 =item sub NAME (PROTO) BLOCK
7777 =item sub NAME : ATTRS BLOCK
7779 =item sub NAME (PROTO) : ATTRS BLOCK
7781 =for Pod::Functions declare a subroutine, possibly anonymously
7783 This is subroutine definition, not a real function I<per se>. Without a
7784 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7785 function declaration, so does return a value: the CODE ref of the closure
7788 See L<perlsub> and L<perlref> for details about subroutines and
7789 references; see L<attributes> and L<Attribute::Handlers> for more
7790 information about attributes.
7795 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7797 A special token that returns a reference to the current subroutine, or
7798 C<undef> outside of a subroutine.
7800 The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>)
7801 is subject to change.
7803 This token is only available under C<use v5.16> or the "current_sub"
7804 feature. See L<feature>.
7806 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7807 X<substr> X<substring> X<mid> X<left> X<right>
7809 =item substr EXPR,OFFSET,LENGTH
7811 =item substr EXPR,OFFSET
7813 =for Pod::Functions get or alter a portion of a string
7815 Extracts a substring out of EXPR and returns it. First character is at
7816 offset zero. If OFFSET is negative, starts
7817 that far back from the end of the string. If LENGTH is omitted, returns
7818 everything through the end of the string. If LENGTH is negative, leaves that
7819 many characters off the end of the string.
7821 my $s = "The black cat climbed the green tree";
7822 my $color = substr $s, 4, 5; # black
7823 my $middle = substr $s, 4, -11; # black cat climbed the
7824 my $end = substr $s, 14; # climbed the green tree
7825 my $tail = substr $s, -4; # tree
7826 my $z = substr $s, -4, 2; # tr
7828 You can use the substr() function as an lvalue, in which case EXPR
7829 must itself be an lvalue. If you assign something shorter than LENGTH,
7830 the string will shrink, and if you assign something longer than LENGTH,
7831 the string will grow to accommodate it. To keep the string the same
7832 length, you may need to pad or chop your value using C<sprintf>.
7834 If OFFSET and LENGTH specify a substring that is partly outside the
7835 string, only the part within the string is returned. If the substring
7836 is beyond either end of the string, substr() returns the undefined
7837 value and produces a warning. When used as an lvalue, specifying a
7838 substring that is entirely outside the string raises an exception.
7839 Here's an example showing the behavior for boundary cases:
7842 substr($name, 4) = 'dy'; # $name is now 'freddy'
7843 my $null = substr $name, 6, 2; # returns "" (no warning)
7844 my $oops = substr $name, 7; # returns undef, with warning
7845 substr($name, 7) = 'gap'; # raises an exception
7847 An alternative to using substr() as an lvalue is to specify the
7848 replacement string as the 4th argument. This allows you to replace
7849 parts of the EXPR and return what was there before in one operation,
7850 just as you can with splice().
7852 my $s = "The black cat climbed the green tree";
7853 my $z = substr $s, 14, 7, "jumped from"; # climbed
7854 # $s is now "The black cat jumped from the green tree"
7856 Note that the lvalue returned by the three-argument version of substr() acts as
7857 a 'magic bullet'; each time it is assigned to, it remembers which part
7858 of the original string is being modified; for example:
7861 for (substr($x,1,2)) {
7862 $_ = 'a'; print $x,"\n"; # prints 1a4
7863 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7865 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7868 With negative offsets, it remembers its position from the end of the string
7869 when the target string is modified:
7872 for (substr($x, -3, 2)) {
7873 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7875 print $_,"\n"; # prints f
7878 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7879 unspecified. Prior to 5.16, the result with negative offsets was
7882 =item symlink OLDFILE,NEWFILE
7883 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7885 =for Pod::Functions create a symbolic link to a file
7887 Creates a new filename symbolically linked to the old filename.
7888 Returns C<1> for success, C<0> otherwise. On systems that don't support
7889 symbolic links, raises an exception. To check for that,
7892 $symlink_exists = eval { symlink("",""); 1 };
7894 Portability issues: L<perlport/symlink>.
7896 =item syscall NUMBER, LIST
7897 X<syscall> X<system call>
7899 =for Pod::Functions execute an arbitrary system call
7901 Calls the system call specified as the first element of the list,
7902 passing the remaining elements as arguments to the system call. If
7903 unimplemented, raises an exception. The arguments are interpreted
7904 as follows: if a given argument is numeric, the argument is passed as
7905 an int. If not, the pointer to the string value is passed. You are
7906 responsible to make sure a string is pre-extended long enough to
7907 receive any result that might be written into a string. You can't use a
7908 string literal (or other read-only string) as an argument to C<syscall>
7909 because Perl has to assume that any string pointer might be written
7911 integer arguments are not literals and have never been interpreted in a
7912 numeric context, you may need to add C<0> to them to force them to look
7913 like numbers. This emulates the C<syswrite> function (or vice versa):
7915 require 'syscall.ph'; # may need to run h2ph
7917 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7919 Note that Perl supports passing of up to only 14 arguments to your syscall,
7920 which in practice should (usually) suffice.
7922 Syscall returns whatever value returned by the system call it calls.
7923 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7924 Note that some system calls I<can> legitimately return C<-1>. The proper
7925 way to handle such calls is to assign C<$!=0> before the call, then
7926 check the value of C<$!> if C<syscall> returns C<-1>.
7928 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7929 number of the read end of the pipe it creates, but there is no way
7930 to retrieve the file number of the other end. You can avoid this
7931 problem by using C<pipe> instead.
7933 Portability issues: L<perlport/syscall>.
7935 =item sysopen FILEHANDLE,FILENAME,MODE
7938 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7940 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7942 Opens the file whose filename is given by FILENAME, and associates it with
7943 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7944 filehandle wanted; an undefined scalar will be suitably autovivified. This
7945 function calls the underlying operating system's I<open>(2) function with the
7946 parameters FILENAME, MODE, and PERMS.
7948 The possible values and flag bits of the MODE parameter are
7949 system-dependent; they are available via the standard module C<Fcntl>. See
7950 the documentation of your operating system's I<open>(2) syscall to see
7951 which values and flag bits are available. You may combine several flags
7952 using the C<|>-operator.
7954 Some of the most common values are C<O_RDONLY> for opening the file in
7955 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7956 and C<O_RDWR> for opening the file in read-write mode.
7957 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7959 For historical reasons, some values work on almost every system
7960 supported by Perl: 0 means read-only, 1 means write-only, and 2
7961 means read/write. We know that these values do I<not> work under
7962 OS/390 and on the Macintosh; you probably don't want to
7963 use them in new code.
7965 If the file named by FILENAME does not exist and the C<open> call creates
7966 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7967 PERMS specifies the permissions of the newly created file. If you omit
7968 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7969 These permission values need to be in octal, and are modified by your
7970 process's current C<umask>.
7973 In many systems the C<O_EXCL> flag is available for opening files in
7974 exclusive mode. This is B<not> locking: exclusiveness means here that
7975 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7976 on network filesystems, and has no effect unless the C<O_CREAT> flag
7977 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7978 being opened if it is a symbolic link. It does not protect against
7979 symbolic links in the file's path.
7982 Sometimes you may want to truncate an already-existing file. This
7983 can be done using the C<O_TRUNC> flag. The behavior of
7984 C<O_TRUNC> with C<O_RDONLY> is undefined.
7987 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7988 that takes away the user's option to have a more permissive umask.
7989 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7992 Note that C<sysopen> depends on the fdopen() C library function.
7993 On many Unix systems, fdopen() is known to fail when file descriptors
7994 exceed a certain value, typically 255. If you need more file
7995 descriptors than that, consider using the POSIX::open() function.
7997 See L<perlopentut> for a kinder, gentler explanation of opening files.
7999 Portability issues: L<perlport/sysopen>.
8001 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
8004 =item sysread FILEHANDLE,SCALAR,LENGTH
8006 =for Pod::Functions fixed-length unbuffered input from a filehandle
8008 Attempts to read LENGTH bytes of data into variable SCALAR from the
8009 specified FILEHANDLE, using the read(2). It bypasses
8010 buffered IO, so mixing this with other kinds of reads, C<print>,
8011 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
8012 perlio or stdio layers usually buffers data. Returns the number of
8013 bytes actually read, C<0> at end of file, or undef if there was an
8014 error (in the latter case C<$!> is also set). SCALAR will be grown or
8015 shrunk so that the last byte actually read is the last byte of the
8016 scalar after the read.
8018 An OFFSET may be specified to place the read data at some place in the
8019 string other than the beginning. A negative OFFSET specifies
8020 placement at that many characters counting backwards from the end of
8021 the string. A positive OFFSET greater than the length of SCALAR
8022 results in the string being padded to the required size with C<"\0">
8023 bytes before the result of the read is appended.
8025 There is no syseof() function, which is ok, since eof() doesn't work
8026 well on device files (like ttys) anyway. Use sysread() and check
8027 for a return value for 0 to decide whether you're done.
8029 Note that if the filehandle has been marked as C<:utf8> Unicode
8030 characters are read instead of bytes (the LENGTH, OFFSET, and the
8031 return value of sysread() are in Unicode characters).
8032 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8033 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8035 =item sysseek FILEHANDLE,POSITION,WHENCE
8038 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
8040 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
8041 be an expression whose value gives the name of the filehandle. The values
8042 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
8043 to the current position plus POSITION; and C<2> to set it to EOF plus
8044 POSITION, typically negative.
8046 Note the I<in bytes>: even if the filehandle has been set to operate
8047 on characters (for example by using the C<:encoding(utf8)> I/O layer),
8048 tell() will return byte offsets, not character offsets (because
8049 implementing that would render sysseek() unacceptably slow).
8051 sysseek() bypasses normal buffered IO, so mixing it with reads other
8052 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
8053 C<seek>, C<tell>, or C<eof> may cause confusion.
8055 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
8056 and C<SEEK_END> (start of the file, current position, end of the file)
8057 from the Fcntl module. Use of the constants is also more portable
8058 than relying on 0, 1, and 2. For example to define a "systell" function:
8060 use Fcntl 'SEEK_CUR';
8061 sub systell { sysseek($_[0], 0, SEEK_CUR) }
8063 Returns the new position, or the undefined value on failure. A position
8064 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
8065 true on success and false on failure, yet you can still easily determine
8071 =item system PROGRAM LIST
8073 =for Pod::Functions run a separate program
8075 Does exactly the same thing as C<exec LIST>, except that a fork is
8076 done first and the parent process waits for the child process to
8077 exit. Note that argument processing varies depending on the
8078 number of arguments. If there is more than one argument in LIST,
8079 or if LIST is an array with more than one value, starts the program
8080 given by the first element of the list with arguments given by the
8081 rest of the list. If there is only one scalar argument, the argument
8082 is checked for shell metacharacters, and if there are any, the
8083 entire argument is passed to the system's command shell for parsing
8084 (this is C</bin/sh -c> on Unix platforms, but varies on other
8085 platforms). If there are no shell metacharacters in the argument,
8086 it is split into words and passed directly to C<execvp>, which is
8087 more efficient. On Windows, only the C<system PROGRAM LIST> syntax will
8088 reliably avoid using the shell; C<system LIST>, even with more than one
8089 element, will fall back to the shell if the first spawn fails.
8091 Perl will attempt to flush all files opened for
8092 output before any operation that may do a fork, but this may not be
8093 supported on some platforms (see L<perlport>). To be safe, you may need
8094 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
8095 of C<IO::Handle> on any open handles.
8097 The return value is the exit status of the program as returned by the
8098 C<wait> call. To get the actual exit value, shift right by eight (see
8099 below). See also L</exec>. This is I<not> what you want to use to capture
8100 the output from a command; for that you should use merely backticks or
8101 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
8102 indicates a failure to start the program or an error of the wait(2) system
8103 call (inspect $! for the reason).
8105 If you'd like to make C<system> (and many other bits of Perl) die on error,
8106 have a look at the L<autodie> pragma.
8108 Like C<exec>, C<system> allows you to lie to a program about its name if
8109 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
8111 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8112 C<system>, if you expect your program to terminate on receipt of these
8113 signals you will need to arrange to do so yourself based on the return
8116 @args = ("command", "arg1", "arg2");
8118 or die "system @args failed: $?"
8120 If you'd like to manually inspect C<system>'s failure, you can check all
8121 possible failure modes by inspecting C<$?> like this:
8124 print "failed to execute: $!\n";
8127 printf "child died with signal %d, %s coredump\n",
8128 ($? & 127), ($? & 128) ? 'with' : 'without';
8131 printf "child exited with value %d\n", $? >> 8;
8134 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
8135 with the C<W*()> calls from the POSIX module.
8137 When C<system>'s arguments are executed indirectly by the shell,
8138 results and return codes are subject to its quirks.
8139 See L<perlop/"`STRING`"> and L</exec> for details.
8141 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
8142 handler. See L<perlipc> for details.
8144 Portability issues: L<perlport/system>.
8146 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8149 =item syswrite FILEHANDLE,SCALAR,LENGTH
8151 =item syswrite FILEHANDLE,SCALAR
8153 =for Pod::Functions fixed-length unbuffered output to a filehandle
8155 Attempts to write LENGTH bytes of data from variable SCALAR to the
8156 specified FILEHANDLE, using write(2). If LENGTH is
8157 not specified, writes whole SCALAR. It bypasses buffered IO, so
8158 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
8159 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
8160 stdio layers usually buffer data. Returns the number of bytes
8161 actually written, or C<undef> if there was an error (in this case the
8162 errno variable C<$!> is also set). If the LENGTH is greater than the
8163 data available in the SCALAR after the OFFSET, only as much data as is
8164 available will be written.
8166 An OFFSET may be specified to write the data from some part of the
8167 string other than the beginning. A negative OFFSET specifies writing
8168 that many characters counting backwards from the end of the string.
8169 If SCALAR is of length zero, you can only use an OFFSET of 0.
8171 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8172 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8173 return value of syswrite() are in (UTF8-encoded Unicode) characters.
8174 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8175 Alternately, if the handle is not marked with an encoding but you
8176 attempt to write characters with code points over 255, raises an exception.
8177 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8179 =item tell FILEHANDLE
8184 =for Pod::Functions get current seekpointer on a filehandle
8186 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8187 error. FILEHANDLE may be an expression whose value gives the name of
8188 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8191 Note the I<in bytes>: even if the filehandle has been set to
8192 operate on characters (for example by using the C<:encoding(utf8)> open
8193 layer), tell() will return byte offsets, not character offsets (because
8194 that would render seek() and tell() rather slow).
8196 The return value of tell() for the standard streams like the STDIN
8197 depends on the operating system: it may return -1 or something else.
8198 tell() on pipes, fifos, and sockets usually returns -1.
8200 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
8202 Do not use tell() (or other buffered I/O operations) on a filehandle
8203 that has been manipulated by sysread(), syswrite(), or sysseek().
8204 Those functions ignore the buffering, while tell() does not.
8206 =item telldir DIRHANDLE
8209 =for Pod::Functions get current seekpointer on a directory handle
8211 Returns the current position of the C<readdir> routines on DIRHANDLE.
8212 Value may be given to C<seekdir> to access a particular location in a
8213 directory. C<telldir> has the same caveats about possible directory
8214 compaction as the corresponding system library routine.
8216 =item tie VARIABLE,CLASSNAME,LIST
8219 =for Pod::Functions +5.002 bind a variable to an object class
8221 This function binds a variable to a package class that will provide the
8222 implementation for the variable. VARIABLE is the name of the variable
8223 to be enchanted. CLASSNAME is the name of a class implementing objects
8224 of correct type. Any additional arguments are passed to the
8225 appropriate constructor
8226 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8227 or C<TIEHASH>). Typically these are arguments such as might be passed
8228 to the C<dbm_open()> function of C. The object returned by the
8229 constructor is also returned by the C<tie> function, which would be useful
8230 if you want to access other methods in CLASSNAME.
8232 Note that functions such as C<keys> and C<values> may return huge lists
8233 when used on large objects, like DBM files. You may prefer to use the
8234 C<each> function to iterate over such. Example:
8236 # print out history file offsets
8238 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8239 while (($key,$val) = each %HIST) {
8240 print $key, ' = ', unpack('L',$val), "\n";
8244 A class implementing a hash should have the following methods:
8246 TIEHASH classname, LIST
8248 STORE this, key, value
8253 NEXTKEY this, lastkey
8258 A class implementing an ordinary array should have the following methods:
8260 TIEARRAY classname, LIST
8262 STORE this, key, value
8264 STORESIZE this, count
8270 SPLICE this, offset, length, LIST
8277 A class implementing a filehandle should have the following methods:
8279 TIEHANDLE classname, LIST
8280 READ this, scalar, length, offset
8283 WRITE this, scalar, length, offset
8285 PRINTF this, format, LIST
8289 SEEK this, position, whence
8291 OPEN this, mode, LIST
8296 A class implementing a scalar should have the following methods:
8298 TIESCALAR classname, LIST
8304 Not all methods indicated above need be implemented. See L<perltie>,
8305 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8307 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8308 for you; you need to do that explicitly yourself. See L<DB_File>
8309 or the F<Config> module for interesting C<tie> implementations.
8311 For further details see L<perltie>, L<"tied VARIABLE">.
8316 =for Pod::Functions get a reference to the object underlying a tied variable
8318 Returns a reference to the object underlying VARIABLE (the same value
8319 that was originally returned by the C<tie> call that bound the variable
8320 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8326 =for Pod::Functions return number of seconds since 1970
8328 Returns the number of non-leap seconds since whatever time the system
8329 considers to be the epoch, suitable for feeding to C<gmtime> and
8330 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8331 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8332 1904 in the current local time zone for its epoch.
8334 For measuring time in better granularity than one second, use the
8335 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8336 if you have gettimeofday(2), you may be able to use the C<syscall>
8337 interface of Perl. See L<perlfaq8> for details.
8339 For date and time processing look at the many related modules on CPAN.
8340 For a comprehensive date and time representation look at the
8346 =for Pod::Functions return elapsed time for self and child processes
8348 Returns a four-element list giving the user and system times in
8349 seconds for this process and any exited children of this process.
8351 ($user,$system,$cuser,$csystem) = times;
8353 In scalar context, C<times> returns C<$user>.
8355 Children's times are only included for terminated children.
8357 Portability issues: L<perlport/times>.
8361 =for Pod::Functions transliterate a string
8363 The transliteration operator. Same as C<y///>. See
8364 L<perlop/"Quote-Like Operators">.
8366 =item truncate FILEHANDLE,LENGTH
8369 =item truncate EXPR,LENGTH
8371 =for Pod::Functions shorten a file
8373 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8374 specified length. Raises an exception if truncate isn't implemented
8375 on your system. Returns true if successful, C<undef> on error.
8377 The behavior is undefined if LENGTH is greater than the length of the
8380 The position in the file of FILEHANDLE is left unchanged. You may want to
8381 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8383 Portability issues: L<perlport/truncate>.
8386 X<uc> X<uppercase> X<toupper>
8390 =for Pod::Functions return upper-case version of a string
8392 Returns an uppercased version of EXPR. This is the internal function
8393 implementing the C<\U> escape in double-quoted strings.
8394 It does not attempt to do titlecase mapping on initial letters. See
8395 L</ucfirst> for that.
8397 If EXPR is omitted, uses C<$_>.
8399 This function behaves the same way under various pragma, such as in a locale,
8403 X<ucfirst> X<uppercase>
8407 =for Pod::Functions return a string with just the next letter in upper case
8409 Returns the value of EXPR with the first character in uppercase
8410 (titlecase in Unicode). This is the internal function implementing
8411 the C<\u> escape in double-quoted strings.
8413 If EXPR is omitted, uses C<$_>.
8415 This function behaves the same way under various pragma, such as in a locale,
8423 =for Pod::Functions set file creation mode mask
8425 Sets the umask for the process to EXPR and returns the previous value.
8426 If EXPR is omitted, merely returns the current umask.
8428 The Unix permission C<rwxr-x---> is represented as three sets of three
8429 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8430 and isn't one of the digits). The C<umask> value is such a number
8431 representing disabled permissions bits. The permission (or "mode")
8432 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8433 even if you tell C<sysopen> to create a file with permissions C<0777>,
8434 if your umask is C<0022>, then the file will actually be created with
8435 permissions C<0755>. If your C<umask> were C<0027> (group can't
8436 write; others can't read, write, or execute), then passing
8437 C<sysopen> C<0666> would create a file with mode C<0640> (because
8438 C<0666 &~ 027> is C<0640>).
8440 Here's some advice: supply a creation mode of C<0666> for regular
8441 files (in C<sysopen>) and one of C<0777> for directories (in
8442 C<mkdir>) and executable files. This gives users the freedom of
8443 choice: if they want protected files, they might choose process umasks
8444 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8445 Programs should rarely if ever make policy decisions better left to
8446 the user. The exception to this is when writing files that should be
8447 kept private: mail files, web browser cookies, I<.rhosts> files, and
8450 If umask(2) is not implemented on your system and you are trying to
8451 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8452 raises an exception. If umask(2) is not implemented and you are
8453 not trying to restrict access for yourself, returns C<undef>.
8455 Remember that a umask is a number, usually given in octal; it is I<not> a
8456 string of octal digits. See also L</oct>, if all you have is a string.
8458 Portability issues: L<perlport/umask>.
8461 X<undef> X<undefine>
8465 =for Pod::Functions remove a variable or function definition
8467 Undefines the value of EXPR, which must be an lvalue. Use only on a
8468 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8469 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8470 will probably not do what you expect on most predefined variables or
8471 DBM list values, so don't do that; see L</delete>. Always returns the
8472 undefined value. You can omit the EXPR, in which case nothing is
8473 undefined, but you still get an undefined value that you could, for
8474 instance, return from a subroutine, assign to a variable, or pass as a
8475 parameter. Examples:
8478 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8482 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8483 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8484 select undef, undef, undef, 0.25;
8485 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8487 Note that this is a unary operator, not a list operator.
8490 X<unlink> X<delete> X<remove> X<rm> X<del>
8494 =for Pod::Functions remove one link to a file
8496 Deletes a list of files. On success, it returns the number of files
8497 it successfully deleted. On failure, it returns false and sets C<$!>
8500 my $unlinked = unlink 'a', 'b', 'c';
8502 unlink glob "*.bak";
8504 On error, C<unlink> will not tell you which files it could not remove.
8505 If you want to know which files you could not remove, try them one
8508 foreach my $file ( @goners ) {
8509 unlink $file or warn "Could not unlink $file: $!";
8512 Note: C<unlink> will not attempt to delete directories unless you are
8513 superuser and the B<-U> flag is supplied to Perl. Even if these
8514 conditions are met, be warned that unlinking a directory can inflict
8515 damage on your filesystem. Finally, using C<unlink> on directories is
8516 not supported on many operating systems. Use C<rmdir> instead.
8518 If LIST is omitted, C<unlink> uses C<$_>.
8520 =item unpack TEMPLATE,EXPR
8523 =item unpack TEMPLATE
8525 =for Pod::Functions convert binary structure into normal perl variables
8527 C<unpack> does the reverse of C<pack>: it takes a string
8528 and expands it out into a list of values.
8529 (In scalar context, it returns merely the first value produced.)
8531 If EXPR is omitted, unpacks the C<$_> string.
8532 See L<perlpacktut> for an introduction to this function.
8534 The string is broken into chunks described by the TEMPLATE. Each chunk
8535 is converted separately to a value. Typically, either the string is a result
8536 of C<pack>, or the characters of the string represent a C structure of some
8539 The TEMPLATE has the same format as in the C<pack> function.
8540 Here's a subroutine that does substring:
8543 my($what,$where,$howmuch) = @_;
8544 unpack("x$where a$howmuch", $what);
8549 sub ordinal { unpack("W",$_[0]); } # same as ord()
8551 In addition to fields allowed in pack(), you may prefix a field with
8552 a %<number> to indicate that
8553 you want a <number>-bit checksum of the items instead of the items
8554 themselves. Default is a 16-bit checksum. Checksum is calculated by
8555 summing numeric values of expanded values (for string fields the sum of
8556 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8558 For example, the following
8559 computes the same number as the System V sum program:
8563 unpack("%32W*",<>) % 65535;
8566 The following efficiently counts the number of set bits in a bit vector:
8568 $setbits = unpack("%32b*", $selectmask);
8570 The C<p> and C<P> formats should be used with care. Since Perl
8571 has no way of checking whether the value passed to C<unpack()>
8572 corresponds to a valid memory location, passing a pointer value that's
8573 not known to be valid is likely to have disastrous consequences.
8575 If there are more pack codes or if the repeat count of a field or a group
8576 is larger than what the remainder of the input string allows, the result
8577 is not well defined: the repeat count may be decreased, or
8578 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8579 If the input string is longer than one described by the TEMPLATE,
8580 the remainder of that input string is ignored.
8582 See L</pack> for more examples and notes.
8584 =item unshift ARRAY,LIST
8587 =item unshift EXPR,LIST
8589 =for Pod::Functions prepend more elements to the beginning of a list
8591 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8592 depending on how you look at it. Prepends list to the front of the
8593 array and returns the new number of elements in the array.
8595 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8597 Note the LIST is prepended whole, not one element at a time, so the
8598 prepended elements stay in the same order. Use C<reverse> to do the
8601 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
8602 a reference to an unblessed array. The argument will be dereferenced
8603 automatically. This aspect of C<unshift> is considered highly
8604 experimental. The exact behaviour may change in a future version of Perl.
8606 To avoid confusing would-be users of your code who are running earlier
8607 versions of Perl with mysterious syntax errors, put this sort of thing at
8608 the top of your file to signal that your code will work I<only> on Perls of
8611 use 5.014; # so push/pop/etc work on scalars (experimental)
8613 =item untie VARIABLE
8616 =for Pod::Functions break a tie binding to a variable
8618 Breaks the binding between a variable and a package.
8619 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8620 Has no effect if the variable is not tied.
8622 =item use Module VERSION LIST
8623 X<use> X<module> X<import>
8625 =item use Module VERSION
8627 =item use Module LIST
8633 =for Pod::Functions load in a module at compile time and import its namespace
8635 Imports some semantics into the current package from the named module,
8636 generally by aliasing certain subroutine or variable names into your
8637 package. It is exactly equivalent to
8639 BEGIN { require Module; Module->import( LIST ); }
8641 except that Module I<must> be a bareword.
8642 The importation can be made conditional by using the L<if> module.
8644 In the peculiar C<use VERSION> form, VERSION may be either a positive
8645 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8646 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8647 exception is raised if VERSION is greater than the version of the
8648 current Perl interpreter; Perl will not attempt to parse the rest of the
8649 file. Compare with L</require>, which can do a similar check at run time.
8650 Symmetrically, C<no VERSION> allows you to specify that you want a version
8651 of Perl older than the specified one.
8653 Specifying VERSION as a literal of the form v5.6.1 should generally be
8654 avoided, because it leads to misleading error messages under earlier
8655 versions of Perl (that is, prior to 5.6.0) that do not support this
8656 syntax. The equivalent numeric version should be used instead.
8658 use v5.6.1; # compile time version check
8660 use 5.006_001; # ditto; preferred for backwards compatibility
8662 This is often useful if you need to check the current Perl version before
8663 C<use>ing library modules that won't work with older versions of Perl.
8664 (We try not to do this more than we have to.)
8666 C<use VERSION> also lexically enables all features available in the requested
8667 version as defined by the C<feature> pragma, disabling any features
8668 not in the requested version's feature bundle. See L<feature>.
8669 Similarly, if the specified Perl version is greater than or equal to
8670 5.12.0, strictures are enabled lexically as
8671 with C<use strict>. Any explicit use of
8672 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8673 before it. Later use of C<use VERSION>
8674 will override all behavior of a previous
8675 C<use VERSION>, possibly removing the C<strict> and C<feature> added by
8676 C<use VERSION>. C<use VERSION> does not
8677 load the F<feature.pm> or F<strict.pm>
8680 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8681 C<require> makes sure the module is loaded into memory if it hasn't been
8682 yet. The C<import> is not a builtin; it's just an ordinary static method
8683 call into the C<Module> package to tell the module to import the list of
8684 features back into the current package. The module can implement its
8685 C<import> method any way it likes, though most modules just choose to
8686 derive their C<import> method via inheritance from the C<Exporter> class that
8687 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8688 method can be found then the call is skipped, even if there is an AUTOLOAD
8691 If you do not want to call the package's C<import> method (for instance,
8692 to stop your namespace from being altered), explicitly supply the empty list:
8696 That is exactly equivalent to
8698 BEGIN { require Module }
8700 If the VERSION argument is present between Module and LIST, then the
8701 C<use> will call the VERSION method in class Module with the given
8702 version as an argument. The default VERSION method, inherited from
8703 the UNIVERSAL class, croaks if the given version is larger than the
8704 value of the variable C<$Module::VERSION>.
8706 Again, there is a distinction between omitting LIST (C<import> called
8707 with no arguments) and an explicit empty LIST C<()> (C<import> not
8708 called). Note that there is no comma after VERSION!
8710 Because this is a wide-open interface, pragmas (compiler directives)
8711 are also implemented this way. Currently implemented pragmas are:
8716 use sigtrap qw(SEGV BUS);
8717 use strict qw(subs vars refs);
8718 use subs qw(afunc blurfl);
8719 use warnings qw(all);
8720 use sort qw(stable _quicksort _mergesort);
8722 Some of these pseudo-modules import semantics into the current
8723 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8724 which import symbols into the current package (which are effective
8725 through the end of the file).
8727 Because C<use> takes effect at compile time, it doesn't respect the
8728 ordinary flow control of the code being compiled. In particular, putting
8729 a C<use> inside the false branch of a conditional doesn't prevent it
8730 from being processed. If a module or pragma only needs to be loaded
8731 conditionally, this can be done using the L<if> pragma:
8733 use if $] < 5.008, "utf8";
8734 use if WANT_WARNINGS, warnings => qw(all);
8736 There's a corresponding C<no> declaration that unimports meanings imported
8737 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8738 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8739 or no unimport method being found.
8745 Care should be taken when using the C<no VERSION> form of C<no>. It is
8746 I<only> meant to be used to assert that the running Perl is of a earlier
8747 version than its argument and I<not> to undo the feature-enabling side effects
8750 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8751 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8752 functionality from the command-line.
8757 =for Pod::Functions set a file's last access and modify times
8759 Changes the access and modification times on each file of a list of
8760 files. The first two elements of the list must be the NUMERIC access
8761 and modification times, in that order. Returns the number of files
8762 successfully changed. The inode change time of each file is set
8763 to the current time. For example, this code has the same effect as the
8764 Unix touch(1) command when the files I<already exist> and belong to
8765 the user running the program:
8768 $atime = $mtime = time;
8769 utime $atime, $mtime, @ARGV;
8771 Since Perl 5.8.0, if the first two elements of the list are C<undef>,
8772 the utime(2) syscall from your C library is called with a null second
8773 argument. On most systems, this will set the file's access and
8774 modification times to the current time (i.e., equivalent to the example
8775 above) and will work even on files you don't own provided you have write
8779 utime(undef, undef, $file)
8780 || warn "couldn't touch $file: $!";
8783 Under NFS this will use the time of the NFS server, not the time of
8784 the local machine. If there is a time synchronization problem, the
8785 NFS server and local machine will have different times. The Unix
8786 touch(1) command will in fact normally use this form instead of the
8787 one shown in the first example.
8789 Passing only one of the first two elements as C<undef> is
8790 equivalent to passing a 0 and will not have the effect
8791 described when both are C<undef>. This also triggers an
8792 uninitialized warning.
8794 On systems that support futimes(2), you may pass filehandles among the
8795 files. On systems that don't support futimes(2), passing filehandles raises
8796 an exception. Filehandles must be passed as globs or glob references to be
8797 recognized; barewords are considered filenames.
8799 Portability issues: L<perlport/utime>.
8808 =for Pod::Functions return a list of the values in a hash
8810 In list context, returns a list consisting of all the values of the named
8811 hash. In Perl 5.12 or later only, will also return a list of the values of
8812 an array; prior to that release, attempting to use an array argument will
8813 produce a syntax error. In scalar context, returns the number of values.
8815 Hash entries are returned in an apparently random order. The actual random
8816 order is specific to a given hash; the exact same series of operations
8817 on two hashes may result in a different order for each hash. Any insertion
8818 into the hash may change the order, as will any deletion, with the exception
8819 that the most recent key returned by C<each> or C<keys> may be deleted
8820 without changing the order. So long as a given hash is unmodified you may
8821 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
8822 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
8823 details on why hash order is randomized. Aside from the guarantees
8824 provided here the exact details of Perl's hash algorithm and the hash
8825 traversal order are subject to change in any release of Perl. Tied hashes
8826 may behave differently to Perl's hashes with respect to changes in order on
8827 insertion and deletion of items.
8829 As a side effect, calling values() resets the HASH or ARRAY's internal
8830 iterator, see L</each>. (In particular, calling values() in void context
8831 resets the iterator with no other overhead. Apart from resetting the
8832 iterator, C<values @array> in list context is the same as plain C<@array>.
8833 (We recommend that you use void context C<keys @array> for this, but
8834 reasoned that taking C<values @array> out would require more
8835 documentation than leaving it in.)
8837 Note that the values are not copied, which means modifying them will
8838 modify the contents of the hash:
8840 for (values %hash) { s/foo/bar/g } # modifies %hash values
8841 for (@hash{keys %hash}) { s/foo/bar/g } # same
8843 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
8844 a reference to an unblessed hash or array. The argument will be
8845 dereferenced automatically. This aspect of C<values> is considered highly
8846 experimental. The exact behaviour may change in a future version of Perl.
8848 for (values $hashref) { ... }
8849 for (values $obj->get_arrayref) { ... }
8851 To avoid confusing would-be users of your code who are running earlier
8852 versions of Perl with mysterious syntax errors, put this sort of thing at
8853 the top of your file to signal that your code will work I<only> on Perls of
8856 use 5.012; # so keys/values/each work on arrays
8857 use 5.014; # so keys/values/each work on scalars (experimental)
8859 See also C<keys>, C<each>, and C<sort>.
8861 =item vec EXPR,OFFSET,BITS
8862 X<vec> X<bit> X<bit vector>
8864 =for Pod::Functions test or set particular bits in a string
8866 Treats the string in EXPR as a bit vector made up of elements of
8867 width BITS and returns the value of the element specified by OFFSET
8868 as an unsigned integer. BITS therefore specifies the number of bits
8869 that are reserved for each element in the bit vector. This must
8870 be a power of two from 1 to 32 (or 64, if your platform supports
8873 If BITS is 8, "elements" coincide with bytes of the input string.
8875 If BITS is 16 or more, bytes of the input string are grouped into chunks
8876 of size BITS/8, and each group is converted to a number as with
8877 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8878 for BITS==64). See L<"pack"> for details.
8880 If bits is 4 or less, the string is broken into bytes, then the bits
8881 of each byte are broken into 8/BITS groups. Bits of a byte are
8882 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8883 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8884 breaking the single input byte C<chr(0x36)> into two groups gives a list
8885 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8887 C<vec> may also be assigned to, in which case parentheses are needed
8888 to give the expression the correct precedence as in
8890 vec($image, $max_x * $x + $y, 8) = 3;
8892 If the selected element is outside the string, the value 0 is returned.
8893 If an element off the end of the string is written to, Perl will first
8894 extend the string with sufficiently many zero bytes. It is an error
8895 to try to write off the beginning of the string (i.e., negative OFFSET).
8897 If the string happens to be encoded as UTF-8 internally (and thus has
8898 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8899 internal byte string, not the conceptual character string, even if you
8900 only have characters with values less than 256.
8902 Strings created with C<vec> can also be manipulated with the logical
8903 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8904 vector operation is desired when both operands are strings.
8905 See L<perlop/"Bitwise String Operators">.
8907 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8908 The comments show the string after each step. Note that this code works
8909 in the same way on big-endian or little-endian machines.
8912 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8914 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8915 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8917 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8918 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8919 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8920 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8921 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8922 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8924 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8925 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8926 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8929 To transform a bit vector into a string or list of 0's and 1's, use these:
8931 $bits = unpack("b*", $vector);
8932 @bits = split(//, unpack("b*", $vector));
8934 If you know the exact length in bits, it can be used in place of the C<*>.
8936 Here is an example to illustrate how the bits actually fall in place:
8942 unpack("V",$_) 01234567890123456789012345678901
8943 ------------------------------------------------------------------
8948 for ($shift=0; $shift < $width; ++$shift) {
8949 for ($off=0; $off < 32/$width; ++$off) {
8950 $str = pack("B*", "0"x32);
8951 $bits = (1<<$shift);
8952 vec($str, $off, $width) = $bits;
8953 $res = unpack("b*",$str);
8954 $val = unpack("V", $str);
8961 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8962 $off, $width, $bits, $val, $res
8966 Regardless of the machine architecture on which it runs, the
8967 example above should print the following table:
8970 unpack("V",$_) 01234567890123456789012345678901
8971 ------------------------------------------------------------------
8972 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8973 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8974 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8975 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8976 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8977 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8978 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8979 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8980 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8981 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8982 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8983 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8984 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8985 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8986 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8987 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8988 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8989 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8990 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8991 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8992 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8993 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8994 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8995 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8996 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8997 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8998 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8999 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
9000 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
9001 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
9002 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
9003 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
9004 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
9005 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
9006 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
9007 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
9008 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
9009 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
9010 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
9011 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
9012 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
9013 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
9014 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
9015 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
9016 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
9017 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
9018 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
9019 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
9020 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
9021 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
9022 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
9023 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
9024 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
9025 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
9026 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
9027 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
9028 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
9029 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
9030 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
9031 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
9032 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
9033 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
9034 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
9035 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
9036 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
9037 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
9038 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
9039 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
9040 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
9041 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
9042 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
9043 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
9044 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
9045 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
9046 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
9047 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
9048 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
9049 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
9050 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
9051 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
9052 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
9053 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
9054 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
9055 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
9056 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
9057 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
9058 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
9059 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
9060 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
9061 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
9062 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
9063 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
9064 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
9065 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
9066 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
9067 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
9068 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
9069 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
9070 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
9071 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
9072 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
9073 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
9074 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
9075 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
9076 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
9077 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
9078 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
9079 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
9080 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
9081 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
9082 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
9083 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
9084 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
9085 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
9086 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
9087 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
9088 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
9089 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
9090 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
9091 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
9092 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
9093 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
9094 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
9095 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
9096 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
9097 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
9098 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
9099 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
9104 =for Pod::Functions wait for any child process to die
9106 Behaves like wait(2) on your system: it waits for a child
9107 process to terminate and returns the pid of the deceased process, or
9108 C<-1> if there are no child processes. The status is returned in C<$?>
9109 and C<${^CHILD_ERROR_NATIVE}>.
9110 Note that a return value of C<-1> could mean that child processes are
9111 being automatically reaped, as described in L<perlipc>.
9113 If you use C<wait> in your handler for $SIG{CHLD}, it may accidentally wait
9114 for the child created by qx() or system(). See L<perlipc> for details.
9116 Portability issues: L<perlport/wait>.
9118 =item waitpid PID,FLAGS
9121 =for Pod::Functions wait for a particular child process to die
9123 Waits for a particular child process to terminate and returns the pid of
9124 the deceased process, or C<-1> if there is no such child process. On some
9125 systems, a value of 0 indicates that there are processes still running.
9126 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
9128 use POSIX ":sys_wait_h";
9131 $kid = waitpid(-1, WNOHANG);
9134 then you can do a non-blocking wait for all pending zombie processes.
9135 Non-blocking wait is available on machines supporting either the
9136 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
9137 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9138 system call by remembering the status values of processes that have
9139 exited but have not been harvested by the Perl script yet.)
9141 Note that on some systems, a return value of C<-1> could mean that child
9142 processes are being automatically reaped. See L<perlipc> for details,
9143 and for other examples.
9145 Portability issues: L<perlport/waitpid>.
9148 X<wantarray> X<context>
9150 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9152 Returns true if the context of the currently executing subroutine or
9153 C<eval> is looking for a list value. Returns false if the context is
9154 looking for a scalar. Returns the undefined value if the context is
9155 looking for no value (void context).
9157 return unless defined wantarray; # don't bother doing more
9158 my @a = complex_calculation();
9159 return wantarray ? @a : "@a";
9161 C<wantarray()>'s result is unspecified in the top level of a file,
9162 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9163 in a C<DESTROY> method.
9165 This function should have been named wantlist() instead.
9168 X<warn> X<warning> X<STDERR>
9170 =for Pod::Functions print debugging info
9172 Prints the value of LIST to STDERR. If the last element of LIST does
9173 not end in a newline, it appends the same file/line number text as C<die>
9176 If the output is empty and C<$@> already contains a value (typically from a
9177 previous eval) that value is used after appending C<"\t...caught">
9178 to C<$@>. This is useful for staying almost, but not entirely similar to
9181 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
9183 No message is printed if there is a C<$SIG{__WARN__}> handler
9184 installed. It is the handler's responsibility to deal with the message
9185 as it sees fit (like, for instance, converting it into a C<die>). Most
9186 handlers must therefore arrange to actually display the
9187 warnings that they are not prepared to deal with, by calling C<warn>
9188 again in the handler. Note that this is quite safe and will not
9189 produce an endless loop, since C<__WARN__> hooks are not called from
9192 You will find this behavior is slightly different from that of
9193 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
9194 instead call C<die> again to change it).
9196 Using a C<__WARN__> handler provides a powerful way to silence all
9197 warnings (even the so-called mandatory ones). An example:
9199 # wipe out *all* compile-time warnings
9200 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9202 my $foo = 20; # no warning about duplicate my $foo,
9203 # but hey, you asked for it!
9204 # no compile-time or run-time warnings before here
9207 # run-time warnings enabled after here
9208 warn "\$foo is alive and $foo!"; # does show up
9210 See L<perlvar> for details on setting C<%SIG> entries and for more
9211 examples. See the Carp module for other kinds of warnings using its
9212 carp() and cluck() functions.
9214 =item write FILEHANDLE
9221 =for Pod::Functions print a picture record
9223 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9224 using the format associated with that file. By default the format for
9225 a file is the one having the same name as the filehandle, but the
9226 format for the current output channel (see the C<select> function) may be set
9227 explicitly by assigning the name of the format to the C<$~> variable.
9229 Top of form processing is handled automatically: if there is insufficient
9230 room on the current page for the formatted record, the page is advanced by
9231 writing a form feed and a special top-of-page
9232 format is used to format the new
9233 page header before the record is written. By default, the top-of-page
9234 format is the name of the filehandle with "_TOP" appended, or "top"
9235 in the current package if the former does not exist. This would be a
9236 problem with autovivified filehandles, but it may be dynamically set to the
9237 format of your choice by assigning the name to the C<$^> variable while
9238 that filehandle is selected. The number of lines remaining on the current
9239 page is in variable C<$->, which can be set to C<0> to force a new page.
9241 If FILEHANDLE is unspecified, output goes to the current default output
9242 channel, which starts out as STDOUT but may be changed by the
9243 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
9244 is evaluated and the resulting string is used to look up the name of
9245 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9247 Note that write is I<not> the opposite of C<read>. Unfortunately.
9251 =for Pod::Functions transliterate a string
9253 The transliteration operator. Same as C<tr///>. See
9254 L<perlop/"Quote-Like Operators">.
9258 =head2 Non-function Keywords by Cross-reference
9268 These keywords are documented in L<perldata/"Special Literals">.
9286 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9296 This method keyword is documented in L<perlobj/"Destructors">.
9328 These operators are documented in L<perlop>.
9338 This keyword is documented in L<perlsub/"Autoloading">.
9362 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9366 The "else if" keyword is spelled C<elsif> in Perl. There's no C<elif>
9367 or C<else if> either. It does parse C<elseif>, but only to warn you
9370 See the documentation for flow-control keywords in L<perlsyn/"Compound
9383 These flow-control keywords related to the experimental switch feature are
9384 documented in L<perlsyn/"Switch Statements">.