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, or
361 the undefined value if the file doesn't exist. Despite the funny
362 names, precedence is the same as any other named unary operator. The
363 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.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
397 -C Same for inode change time (Unix, may differ for other
404 next unless -f $_; # ignore specials
408 Note that C<-s/a/b/> does not do a negated substitution. Saying
409 C<-exp($foo)> still works as expected, however: only single letters
410 following a minus are interpreted as file tests.
412 These operators are exempt from the "looks like a function rule" described
413 above. That is, an opening parenthesis after the operator does not affect
414 how much of the following code constitutes the argument. Put the opening
415 parentheses before the operator to separate it from code that follows (this
416 applies only to operators with higher precedence than unary operators, of
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
422 The interpretation of the file permission operators C<-r>, C<-R>,
423 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424 of the file and the uids and gids of the user. There may be other
425 reasons you can't actually read, write, or execute the file: for
426 example network filesystem access controls, ACLs (access control lists),
427 read-only filesystems, and unrecognized executable formats. Note
428 that the use of these six specific operators to verify if some operation
429 is possible is usually a mistake, because it may be open to race
432 Also note that, for the superuser on the local filesystems, the C<-r>,
433 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
434 if any execute bit is set in the mode. Scripts run by the superuser
435 may thus need to do a stat() to determine the actual mode of the file,
436 or temporarily set their effective uid to something else.
438 If you are using ACLs, there is a pragma called C<filetest> that may
439 produce more accurate results than the bare stat() mode bits.
440 When under C<use filetest 'access'> the above-mentioned filetests
441 test whether the permission can(not) be granted using the
442 access(2) family of system calls. Also note that the C<-x> and C<-X> may
443 under this pragma return true even if there are no execute permission
444 bits set (nor any extra execute permission ACLs). This strangeness is
445 due to the underlying system calls' definitions. Note also that, due to
446 the implementation of C<use filetest 'access'>, the C<_> special
447 filehandle won't cache the results of the file tests when this pragma is
448 in effect. Read the documentation for the C<filetest> pragma for more
451 The C<-T> and C<-B> switches work as follows. The first block or so of the
452 file is examined for odd characters such as strange control codes or
453 characters with the high bit set. If too many strange characters (>30%)
454 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
455 containing a zero byte in the first block is considered a binary file. If C<-T>
456 or C<-B> is used on a filehandle, the current IO buffer is examined
457 rather than the first block. Both C<-T> and C<-B> return true on an empty
458 file, or a file at EOF when testing a filehandle. Because you have to
459 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460 against the file first, as in C<next unless -f $file && -T $file>.
462 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
463 the special filehandle consisting of a solitary underline, then the stat
464 structure of the previous file test (or stat operator) is used, saving
465 a system call. (This doesn't work with C<-t>, and you need to remember
466 that lstat() and C<-l> leave values in the stat structure for the
467 symbolic link, not the real file.) (Also, if the stat buffer was filled by
468 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
471 print "Can do.\n" if -r $a || -w _ || -x _;
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
483 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
484 test operators, in a way that C<-f -w -x $file> is equivalent to
485 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
486 the return value of C<-f $file> as an argument to another filetest
487 operator, no special magic will happen.)
489 Portability issues: L<perlport/-X>.
491 To avoid confusing would-be users of your code with mysterious
492 syntax errors, put something like this at the top of your script:
494 use 5.010; # so filetest ops can stack
501 =for Pod::Functions absolute value function
503 Returns the absolute value of its argument.
504 If VALUE is omitted, uses C<$_>.
506 =item accept NEWSOCKET,GENERICSOCKET
509 =for Pod::Functions accept an incoming socket connect
511 Accepts an incoming socket connect, just as accept(2)
512 does. Returns the packed address if it succeeded, false otherwise.
513 See the example in L<perlipc/"Sockets: Client/Server Communication">.
515 On systems that support a close-on-exec flag on files, the flag will
516 be set for the newly opened file descriptor, as determined by the
517 value of $^F. See L<perlvar/$^F>.
526 =for Pod::Functions schedule a SIGALRM
528 Arranges to have a SIGALRM delivered to this process after the
529 specified number of wallclock seconds has elapsed. If SECONDS is not
530 specified, the value stored in C<$_> is used. (On some machines,
531 unfortunately, the elapsed time may be up to one second less or more
532 than you specified because of how seconds are counted, and process
533 scheduling may delay the delivery of the signal even further.)
535 Only one timer may be counting at once. Each call disables the
536 previous timer, and an argument of C<0> may be supplied to cancel the
537 previous timer without starting a new one. The returned value is the
538 amount of time remaining on the previous timer.
540 For delays of finer granularity than one second, the Time::HiRes module
541 (from CPAN, and starting from Perl 5.8 part of the standard
542 distribution) provides ualarm(). You may also use Perl's four-argument
543 version of select() leaving the first three arguments undefined, or you
544 might be able to use the C<syscall> interface to access setitimer(2) if
545 your system supports it. See L<perlfaq8> for details.
547 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548 C<sleep> may be internally implemented on your system with C<alarm>.
550 If you want to use C<alarm> to time out a system call you need to use an
551 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
552 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
553 restart system calls on some systems. Using C<eval>/C<die> always works,
554 modulo the caveats given in L<perlipc/"Signals">.
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
559 $nread = sysread SOCKET, $buffer, $size;
563 die unless $@ eq "alarm\n"; # propagate unexpected errors
570 For more information see L<perlipc>.
572 Portability issues: L<perlport/alarm>.
575 X<atan2> X<arctangent> X<tan> X<tangent>
577 =for Pod::Functions arctangent of Y/X in the range -PI to PI
579 Returns the arctangent of Y/X in the range -PI to PI.
581 For the tangent operation, you may use the C<Math::Trig::tan>
582 function, or use the familiar relation:
584 sub tan { sin($_[0]) / cos($_[0]) }
586 The return value for C<atan2(0,0)> is implementation-defined; consult
587 your atan2(3) manpage for more information.
589 Portability issues: L<perlport/atan2>.
591 =item bind SOCKET,NAME
594 =for Pod::Functions binds an address to a socket
596 Binds a network address to a socket, just as bind(2)
597 does. Returns true if it succeeded, false otherwise. NAME should be a
598 packed address of the appropriate type for the socket. See the examples in
599 L<perlipc/"Sockets: Client/Server Communication">.
601 =item binmode FILEHANDLE, LAYER
602 X<binmode> X<binary> X<text> X<DOS> X<Windows>
604 =item binmode FILEHANDLE
606 =for Pod::Functions prepare binary files for I/O
608 Arranges for FILEHANDLE to be read or written in "binary" or "text"
609 mode on systems where the run-time libraries distinguish between
610 binary and text files. If FILEHANDLE is an expression, the value is
611 taken as the name of the filehandle. Returns true on success,
612 otherwise it returns C<undef> and sets C<$!> (errno).
614 On some systems (in general, DOS- and Windows-based systems) binmode()
615 is necessary when you're not working with a text file. For the sake
616 of portability it is a good idea always to use it when appropriate,
617 and never to use it when it isn't appropriate. Also, people can
618 set their I/O to be by default UTF8-encoded Unicode, not bytes.
620 In other words: regardless of platform, use binmode() on binary data,
621 like images, for example.
623 If LAYER is present it is a single string, but may contain multiple
624 directives. The directives alter the behaviour of the filehandle.
625 When LAYER is present, using binmode on a text file makes sense.
627 If LAYER is omitted or specified as C<:raw> the filehandle is made
628 suitable for passing binary data. This includes turning off possible CRLF
629 translation and marking it as bytes (as opposed to Unicode characters).
630 Note that, despite what may be implied in I<"Programming Perl"> (the
631 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632 Other layers that would affect the binary nature of the stream are
633 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
634 PERLIO environment variable.
636 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
637 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638 establish default I/O layers. See L<open>.
640 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641 in "Programming Perl, 3rd Edition". However, since the publishing of this
642 book, by many known as "Camel III", the consensus of the naming of this
643 functionality has moved from "discipline" to "layer". All documentation
644 of this version of Perl therefore refers to "layers" rather than to
645 "disciplines". Now back to the regularly scheduled documentation...>
647 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
648 C<:utf8> just marks the data as UTF-8 without further checking,
649 while C<:encoding(UTF-8)> checks the data for actually being valid
650 UTF-8. More details can be found in L<PerlIO::encoding>.
652 In general, binmode() should be called after open() but before any I/O
653 is done on the filehandle. Calling binmode() normally flushes any
654 pending buffered output data (and perhaps pending input data) on the
655 handle. An exception to this is the C<:encoding> layer that
656 changes the default character encoding of the handle; see L</open>.
657 The C<:encoding> layer sometimes needs to be called in
658 mid-stream, and it doesn't flush the stream. The C<:encoding>
659 also implicitly pushes on top of itself the C<:utf8> layer because
660 internally Perl operates on UTF8-encoded Unicode characters.
662 The operating system, device drivers, C libraries, and Perl run-time
663 system all conspire to let the programmer treat a single
664 character (C<\n>) as the line terminator, irrespective of external
665 representation. On many operating systems, the native text file
666 representation matches the internal representation, but on some
667 platforms the external representation of C<\n> is made up of more than
670 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671 a single character to end each line in the external representation of text
672 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
673 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
674 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676 two characters C<\cM\cJ>. That means that if you don't use binmode() on
677 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679 output. This is what you want for text files, but it can be disastrous for
682 Another consequence of using binmode() (on some systems) is that
683 special end-of-file markers will be seen as part of the data stream.
684 For systems from the Microsoft family this means that, if your binary
685 data contain C<\cZ>, the I/O subsystem will regard it as the end of
686 the file, unless you use binmode().
688 binmode() is important not only for readline() and print() operations,
689 but also when using read(), seek(), sysread(), syswrite() and tell()
690 (see L<perlport> for more details). See the C<$/> and C<$\> variables
691 in L<perlvar> for how to manually set your input and output
692 line-termination sequences.
694 Portability issues: L<perlport/binmode>.
696 =item bless REF,CLASSNAME
701 =for Pod::Functions create an object
703 This function tells the thingy referenced by REF that it is now an object
704 in the CLASSNAME package. If CLASSNAME is omitted, the current package
705 is used. Because a C<bless> is often the last thing in a constructor,
706 it returns the reference for convenience. Always use the two-argument
707 version if a derived class might inherit the function doing the blessing.
708 See L<perlobj> for more about the blessing (and blessings) of objects.
710 Consider always blessing objects in CLASSNAMEs that are mixed case.
711 Namespaces with all lowercase names are considered reserved for
712 Perl pragmata. Builtin types have all uppercase names. To prevent
713 confusion, you may wish to avoid such package names as well. Make sure
714 that CLASSNAME is a true value.
716 See L<perlmod/"Perl Modules">.
720 =for Pod::Functions +switch break out of a C<given> block
722 Break out of a C<given()> block.
724 This keyword is enabled by the C<"switch"> feature; see L<feature> for
725 more information on C<"switch">. You can also access it by prefixing it
726 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
730 X<caller> X<call stack> X<stack> X<stack trace>
734 =for Pod::Functions get context of the current subroutine call
736 Returns the context of the current subroutine call. In scalar context,
737 returns the caller's package name if there I<is> a caller (that is, if
738 we're in a subroutine or C<eval> or C<require>) and the undefined value
739 otherwise. In list context, returns
742 ($package, $filename, $line) = caller;
744 With EXPR, it returns some extra information that the debugger uses to
745 print a stack trace. The value of EXPR indicates how many call frames
746 to go back before the current one.
749 ($package, $filename, $line, $subroutine, $hasargs,
752 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
755 Here $subroutine may be C<(eval)> if the frame is not a subroutine
756 call, but an C<eval>. In such a case additional elements $evaltext and
757 C<$is_require> are set: C<$is_require> is true if the frame is created by a
758 C<require> or C<use> statement, $evaltext contains the text of the
759 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
760 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
761 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
762 frame.) $subroutine may also be C<(unknown)> if this particular
763 subroutine happens to have been deleted from the symbol table.
764 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
765 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
766 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
767 corresponds to C<${^WARNING_BITS}>. The
768 C<$hints> and C<$bitmask> values are subject
769 to change between versions of Perl, and are not meant for external use.
771 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
772 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
773 of this hash, as they are the actual values stored in the optree.
775 Furthermore, when called from within the DB package in
776 list context, and with an argument, caller returns more
777 detailed information: it sets the list variable C<@DB::args> to be the
778 arguments with which the subroutine was invoked.
780 Be aware that the optimizer might have optimized call frames away before
781 C<caller> had a chance to get the information. That means that C<caller(N)>
782 might not return information about the call frame you expect it to, for
783 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
784 previous time C<caller> was called.
786 Be aware that setting C<@DB::args> is I<best effort>, intended for
787 debugging or generating backtraces, and should not be relied upon. In
788 particular, as C<@_> contains aliases to the caller's arguments, Perl does
789 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790 subroutine makes to C<@_> or its contents, not the original values at call
791 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
792 elements, so under certain cases its elements may have become freed and
793 reallocated for other variables or temporary values. Finally, a side effect
794 of the current implementation is that the effects of C<shift @_> can
795 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
797 elements), so C<@DB::args> is actually a hybrid of the current state and
798 initial state of C<@_>. Buyer beware.
805 =item chdir FILEHANDLE
807 =item chdir DIRHANDLE
811 =for Pod::Functions change your current working directory
813 Changes the working directory to EXPR, if possible. If EXPR is omitted,
814 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
815 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817 neither is set, C<chdir> does nothing. It returns true on success,
818 false otherwise. See the example under C<die>.
820 On systems that support fchdir(2), you may pass a filehandle or
821 directory handle as the argument. On systems that don't support fchdir(2),
822 passing handles raises an exception.
825 X<chmod> X<permission> X<mode>
827 =for Pod::Functions changes the permissions on a list of files
829 Changes the permissions of a list of files. The first element of the
830 list must be the numeric mode, which should probably be an octal
831 number, and which definitely should I<not> be a string of octal digits:
832 C<0644> is okay, but C<"0644"> is not. Returns the number of files
833 successfully changed. See also L</oct> if all you have is a string.
835 $cnt = chmod 0755, "foo", "bar";
836 chmod 0755, @executables;
837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
842 On systems that support fchmod(2), you may pass filehandles among the
843 files. On systems that don't support fchmod(2), passing filehandles raises
844 an exception. Filehandles must be passed as globs or glob references to be
845 recognized; barewords are considered filenames.
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
851 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
854 use Fcntl qw( :mode );
855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
856 # Identical to the chmod 0755 of the example above.
858 Portability issues: L<perlport/chmod>.
861 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
867 =for Pod::Functions remove a trailing record separator from a string
869 This safer version of L</chop> removes any trailing string
870 that corresponds to the current value of C<$/> (also known as
871 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872 number of characters removed from all its arguments. It's often used to
873 remove the newline from the end of an input record when you're worried
874 that the final record may be missing its newline. When in paragraph
875 mode (C<$/ = "">), it removes all trailing newlines from the string.
876 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
877 a reference to an integer or the like; see L<perlvar>) chomp() won't
879 If VARIABLE is omitted, it chomps C<$_>. Example:
882 chomp; # avoid \n on last field
887 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
889 You can actually chomp anything that's an lvalue, including an assignment:
892 chomp($answer = <STDIN>);
894 If you chomp a list, each element is chomped, and the total number of
895 characters removed is returned.
897 Note that parentheses are necessary when you're chomping anything
898 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
899 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
900 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
901 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
911 =for Pod::Functions remove the last character from a string
913 Chops off the last character of a string and returns the character
914 chopped. It is much more efficient than C<s/.$//s> because it neither
915 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
916 If VARIABLE is a hash, it chops the hash's values, but not its keys.
918 You can actually chop anything that's an lvalue, including an assignment.
920 If you chop a list, each element is chopped. Only the value of the
921 last C<chop> is returned.
923 Note that C<chop> returns the last character. To return all but the last
924 character, use C<substr($string, 0, -1)>.
929 X<chown> X<owner> X<user> X<group>
931 =for Pod::Functions change the ownership on a list of files
933 Changes the owner (and group) of a list of files. The first two
934 elements of the list must be the I<numeric> uid and gid, in that
935 order. A value of -1 in either position is interpreted by most
936 systems to leave that value unchanged. Returns the number of files
937 successfully changed.
939 $cnt = chown $uid, $gid, 'foo', 'bar';
940 chown $uid, $gid, @filenames;
942 On systems that support fchown(2), you may pass filehandles among the
943 files. On systems that don't support fchown(2), passing filehandles raises
944 an exception. Filehandles must be passed as globs or glob references to be
945 recognized; barewords are considered filenames.
947 Here's an example that looks up nonnumeric uids in the passwd file:
950 chomp($user = <STDIN>);
952 chomp($pattern = <STDIN>);
954 ($login,$pass,$uid,$gid) = getpwnam($user)
955 or die "$user not in passwd file";
957 @ary = glob($pattern); # expand filenames
958 chown $uid, $gid, @ary;
960 On most systems, you are not allowed to change the ownership of the
961 file unless you're the superuser, although you should be able to change
962 the group to any of your secondary groups. On insecure systems, these
963 restrictions may be relaxed, but this is not a portable assumption.
964 On POSIX systems, you can detect this condition this way:
966 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
967 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
969 Portability issues: L<perlport/chmod>.
972 X<chr> X<character> X<ASCII> X<Unicode>
976 =for Pod::Functions get character this number represents
978 Returns the character represented by that NUMBER in the character set.
979 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
980 chr(0x263a) is a Unicode smiley face.
982 Negative values give the Unicode replacement character (chr(0xfffd)),
983 except under the L<bytes> pragma, where the low eight bits of the value
984 (truncated to an integer) are used.
986 If NUMBER is omitted, uses C<$_>.
988 For the reverse, use L</ord>.
990 Note that characters from 128 to 255 (inclusive) are by default
991 internally not encoded as UTF-8 for backward compatibility reasons.
993 See L<perlunicode> for more about Unicode.
995 =item chroot FILENAME
1000 =for Pod::Functions make directory new root for path lookups
1002 This function works like the system call by the same name: it makes the
1003 named directory the new root directory for all further pathnames that
1004 begin with a C</> by your process and all its children. (It doesn't
1005 change your current working directory, which is unaffected.) For security
1006 reasons, this call is restricted to the superuser. If FILENAME is
1007 omitted, does a C<chroot> to C<$_>.
1009 Portability issues: L<perlport/chroot>.
1011 =item close FILEHANDLE
1016 =for Pod::Functions close file (or pipe or socket) handle
1018 Closes the file or pipe associated with the filehandle, flushes the IO
1019 buffers, and closes the system file descriptor. Returns true if those
1020 operations succeed and if no error was reported by any PerlIO
1021 layer. Closes the currently selected filehandle if the argument is
1024 You don't have to close FILEHANDLE if you are immediately going to do
1025 another C<open> on it, because C<open> closes it for you. (See
1026 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1027 counter (C<$.>), while the implicit close done by C<open> does not.
1029 If the filehandle came from a piped open, C<close> returns false if one of
1030 the other syscalls involved fails or if its program exits with non-zero
1031 status. If the only problem was that the program exited non-zero, C<$!>
1032 will be set to C<0>. Closing a pipe also waits for the process executing
1033 on the pipe to exit--in case you wish to look at the output of the pipe
1034 afterwards--and implicitly puts the exit status value of that command into
1035 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1037 If there are multiple threads running, C<close> on a filehandle from a
1038 piped open returns true without waiting for the child process to terminate,
1039 if the filehandle is still open in another thread.
1041 Closing the read end of a pipe before the process writing to it at the
1042 other end is done writing results in the writer receiving a SIGPIPE. If
1043 the other end can't handle that, be sure to read all the data before
1048 open(OUTPUT, '|sort >foo') # pipe to sort
1049 or die "Can't start sort: $!";
1050 #... # print stuff to output
1051 close OUTPUT # wait for sort to finish
1052 or warn $! ? "Error closing sort pipe: $!"
1053 : "Exit status $? from sort";
1054 open(INPUT, 'foo') # get sort's results
1055 or die "Can't open 'foo' for input: $!";
1057 FILEHANDLE may be an expression whose value can be used as an indirect
1058 filehandle, usually the real filehandle name or an autovivified handle.
1060 =item closedir DIRHANDLE
1063 =for Pod::Functions close directory handle
1065 Closes a directory opened by C<opendir> and returns the success of that
1068 =item connect SOCKET,NAME
1071 =for Pod::Functions connect to a remote socket
1073 Attempts to connect to a remote socket, just like connect(2).
1074 Returns true if it succeeded, false otherwise. NAME should be a
1075 packed address of the appropriate type for the socket. See the examples in
1076 L<perlipc/"Sockets: Client/Server Communication">.
1078 =item continue BLOCK
1083 =for Pod::Functions optional trailing block in a while or foreach
1085 When followed by a BLOCK, C<continue> is actually a
1086 flow control statement rather than a function. If
1087 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1088 C<foreach>), it is always executed just before the conditional is about to
1089 be evaluated again, just like the third part of a C<for> loop in C. Thus
1090 it can be used to increment a loop variable, even when the loop has been
1091 continued via the C<next> statement (which is similar to the C C<continue>
1094 C<last>, C<next>, or C<redo> may appear within a C<continue>
1095 block; C<last> and C<redo> behave as if they had been executed within
1096 the main block. So will C<next>, but since it will execute a C<continue>
1097 block, it may be more entertaining.
1100 ### redo always comes here
1103 ### next always comes here
1105 # then back the top to re-check EXPR
1107 ### last always comes here
1109 Omitting the C<continue> section is equivalent to using an
1110 empty one, logically enough, so C<next> goes directly back
1111 to check the condition at the top of the loop.
1113 When there is no BLOCK, C<continue> is a function that
1114 falls through the current C<when> or C<default> block instead of iterating
1115 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1116 In Perl 5.14 and earlier, this form of C<continue> was
1117 only available when the C<"switch"> feature was enabled.
1118 See L<feature> and L<perlsyn/"Switch Statements"> for more
1122 X<cos> X<cosine> X<acos> X<arccosine>
1126 =for Pod::Functions cosine function
1128 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1129 takes the cosine of C<$_>.
1131 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1132 function, or use this relation:
1134 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1136 =item crypt PLAINTEXT,SALT
1137 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1138 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1140 =for Pod::Functions one-way passwd-style encryption
1142 Creates a digest string exactly like the crypt(3) function in the C
1143 library (assuming that you actually have a version there that has not
1144 been extirpated as a potential munition).
1146 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1147 into a short string, called a digest, which is returned. The same
1148 PLAINTEXT and SALT will always return the same string, but there is no
1149 (known) way to get the original PLAINTEXT from the hash. Small
1150 changes in the PLAINTEXT or SALT will result in large changes in the
1153 There is no decrypt function. This function isn't all that useful for
1154 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1155 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1156 primarily used to check if two pieces of text are the same without
1157 having to transmit or store the text itself. An example is checking
1158 if a correct password is given. The digest of the password is stored,
1159 not the password itself. The user types in a password that is
1160 crypt()'d with the same salt as the stored digest. If the two digests
1161 match, the password is correct.
1163 When verifying an existing digest string you should use the digest as
1164 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1165 to create the digest is visible as part of the digest. This ensures
1166 crypt() will hash the new string with the same salt as the digest.
1167 This allows your code to work with the standard L<crypt|/crypt> and
1168 with more exotic implementations. In other words, assume
1169 nothing about the returned string itself nor about how many bytes
1172 Traditionally the result is a string of 13 bytes: two first bytes of
1173 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1174 the first eight bytes of PLAINTEXT mattered. But alternative
1175 hashing schemes (like MD5), higher level security schemes (like C2),
1176 and implementations on non-Unix platforms may produce different
1179 When choosing a new salt create a random two character string whose
1180 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1181 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1182 characters is just a recommendation; the characters allowed in
1183 the salt depend solely on your system's crypt library, and Perl can't
1184 restrict what salts C<crypt()> accepts.
1186 Here's an example that makes sure that whoever runs this program knows
1189 $pwd = (getpwuid($<))[1];
1191 system "stty -echo";
1193 chomp($word = <STDIN>);
1197 if (crypt($word, $pwd) ne $pwd) {
1203 Of course, typing in your own password to whoever asks you
1206 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1207 of data, not least of all because you can't get the information
1208 back. Look at the L<Digest> module for more robust algorithms.
1210 If using crypt() on a Unicode string (which I<potentially> has
1211 characters with codepoints above 255), Perl tries to make sense
1212 of the situation by trying to downgrade (a copy of)
1213 the string back to an eight-bit byte string before calling crypt()
1214 (on that copy). If that works, good. If not, crypt() dies with
1215 C<Wide character in crypt>.
1217 Portability issues: L<perlport/crypt>.
1222 =for Pod::Functions breaks binding on a tied dbm file
1224 [This function has been largely superseded by the C<untie> function.]
1226 Breaks the binding between a DBM file and a hash.
1228 Portability issues: L<perlport/dbmclose>.
1230 =item dbmopen HASH,DBNAME,MASK
1231 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1233 =for Pod::Functions create binding on a tied dbm file
1235 [This function has been largely superseded by the
1236 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1238 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1239 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1240 argument is I<not> a filehandle, even though it looks like one). DBNAME
1241 is the name of the database (without the F<.dir> or F<.pag> extension if
1242 any). If the database does not exist, it is created with protection
1243 specified by MASK (as modified by the C<umask>). To prevent creation of
1244 the database if it doesn't exist, you may specify a MODE
1245 of 0, and the function will return a false value if it
1246 can't find an existing database. If your system supports
1247 only the older DBM functions, you may make only one C<dbmopen> call in your
1248 program. In older versions of Perl, if your system had neither DBM nor
1249 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1252 If you don't have write access to the DBM file, you can only read hash
1253 variables, not set them. If you want to test whether you can write,
1254 either use file tests or try setting a dummy hash entry inside an C<eval>
1257 Note that functions such as C<keys> and C<values> may return huge lists
1258 when used on large DBM files. You may prefer to use the C<each>
1259 function to iterate over large DBM files. Example:
1261 # print out history file offsets
1262 dbmopen(%HIST,'/usr/lib/news/history',0666);
1263 while (($key,$val) = each %HIST) {
1264 print $key, ' = ', unpack('L',$val), "\n";
1268 See also L<AnyDBM_File> for a more general description of the pros and
1269 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1270 rich implementation.
1272 You can control which DBM library you use by loading that library
1273 before you call dbmopen():
1276 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1277 or die "Can't open netscape history file: $!";
1279 Portability issues: L<perlport/dbmopen>.
1282 X<defined> X<undef> X<undefined>
1286 =for Pod::Functions test whether a value, variable, or function is defined
1288 Returns a Boolean value telling whether EXPR has a value other than
1289 the undefined value C<undef>. If EXPR is not present, C<$_> is
1292 Many operations return C<undef> to indicate failure, end of file,
1293 system error, uninitialized variable, and other exceptional
1294 conditions. This function allows you to distinguish C<undef> from
1295 other values. (A simple Boolean test will not distinguish among
1296 C<undef>, zero, the empty string, and C<"0">, which are all equally
1297 false.) Note that since C<undef> is a valid scalar, its presence
1298 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1299 returns C<undef> when its argument is an empty array, I<or> when the
1300 element to return happens to be C<undef>.
1302 You may also use C<defined(&func)> to check whether subroutine C<&func>
1303 has ever been defined. The return value is unaffected by any forward
1304 declarations of C<&func>. A subroutine that is not defined
1305 may still be callable: its package may have an C<AUTOLOAD> method that
1306 makes it spring into existence the first time that it is called; see
1309 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1310 used to report whether memory for that aggregate had ever been
1311 allocated. This behavior may disappear in future versions of Perl.
1312 You should instead use a simple test for size:
1314 if (@an_array) { print "has array elements\n" }
1315 if (%a_hash) { print "has hash members\n" }
1317 When used on a hash element, it tells you whether the value is defined,
1318 not whether the key exists in the hash. Use L</exists> for the latter
1323 print if defined $switch{D};
1324 print "$val\n" while defined($val = pop(@ary));
1325 die "Can't readlink $sym: $!"
1326 unless defined($value = readlink $sym);
1327 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1328 $debugging = 0 unless defined $debugging;
1330 Note: Many folks tend to overuse C<defined> and are then surprised to
1331 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1332 defined values. For example, if you say
1336 The pattern match succeeds and C<$1> is defined, although it
1337 matched "nothing". It didn't really fail to match anything. Rather, it
1338 matched something that happened to be zero characters long. This is all
1339 very above-board and honest. When a function returns an undefined value,
1340 it's an admission that it couldn't give you an honest answer. So you
1341 should use C<defined> only when questioning the integrity of what
1342 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1345 See also L</undef>, L</exists>, L</ref>.
1350 =for Pod::Functions deletes a value from a hash
1352 Given an expression that specifies an element or slice of a hash, C<delete>
1353 deletes the specified elements from that hash so that exists() on that element
1354 no longer returns true. Setting a hash element to the undefined value does
1355 not remove its key, but deleting it does; see L</exists>.
1357 In list context, returns the value or values deleted, or the last such
1358 element in scalar context. The return list's length always matches that of
1359 the argument list: deleting non-existent elements returns the undefined value
1360 in their corresponding positions.
1362 delete() may also be used on arrays and array slices, but its behavior is less
1363 straightforward. Although exists() will return false for deleted entries,
1364 deleting array elements never changes indices of existing values; use shift()
1365 or splice() for that. However, if all deleted elements fall at the end of an
1366 array, the array's size shrinks to the position of the highest element that
1367 still tests true for exists(), or to 0 if none do.
1369 B<WARNING:> Calling delete on array values is deprecated and likely to
1370 be removed in a future version of Perl.
1372 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1373 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1374 or array may not necessarily return anything; it depends on the implementation
1375 of the C<tied> package's DELETE method, which may do whatever it pleases.
1377 The C<delete local EXPR> construct localizes the deletion to the current
1378 block at run time. Until the block exits, elements locally deleted
1379 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1380 of composite types">.
1382 %hash = (foo => 11, bar => 22, baz => 33);
1383 $scalar = delete $hash{foo}; # $scalar is 11
1384 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1385 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1387 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1389 foreach $key (keys %HASH) {
1393 foreach $index (0 .. $#ARRAY) {
1394 delete $ARRAY[$index];
1399 delete @HASH{keys %HASH};
1401 delete @ARRAY[0 .. $#ARRAY];
1403 But both are slower than assigning the empty list
1404 or undefining %HASH or @ARRAY, which is the customary
1405 way to empty out an aggregate:
1407 %HASH = (); # completely empty %HASH
1408 undef %HASH; # forget %HASH ever existed
1410 @ARRAY = (); # completely empty @ARRAY
1411 undef @ARRAY; # forget @ARRAY ever existed
1413 The EXPR can be arbitrarily complicated provided its
1414 final operation is an element or slice of an aggregate:
1416 delete $ref->[$x][$y]{$key};
1417 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1419 delete $ref->[$x][$y][$index];
1420 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1423 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1425 =for Pod::Functions raise an exception or bail out
1427 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1428 into C<$@> and the C<eval> is terminated with the undefined value.
1429 If the exception is outside of all enclosing C<eval>s, then the uncaught
1430 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1431 need to exit the process with a specific exit code, see L</exit>.
1433 Equivalent examples:
1435 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1436 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1438 If the last element of LIST does not end in a newline, the current
1439 script line number and input line number (if any) are also printed,
1440 and a newline is supplied. Note that the "input line number" (also
1441 known as "chunk") is subject to whatever notion of "line" happens to
1442 be currently in effect, and is also available as the special variable
1443 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1445 Hint: sometimes appending C<", stopped"> to your message will cause it
1446 to make better sense when the string C<"at foo line 123"> is appended.
1447 Suppose you are running script "canasta".
1449 die "/etc/games is no good";
1450 die "/etc/games is no good, stopped";
1452 produce, respectively
1454 /etc/games is no good at canasta line 123.
1455 /etc/games is no good, stopped at canasta line 123.
1457 If the output is empty and C<$@> already contains a value (typically from a
1458 previous eval) that value is reused after appending C<"\t...propagated">.
1459 This is useful for propagating exceptions:
1462 die unless $@ =~ /Expected exception/;
1464 If the output is empty and C<$@> contains an object reference that has a
1465 C<PROPAGATE> method, that method will be called with additional file
1466 and line number parameters. The return value replaces the value in
1467 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1470 If C<$@> is empty then the string C<"Died"> is used.
1472 If an uncaught exception results in interpreter exit, the exit code is
1473 determined from the values of C<$!> and C<$?> with this pseudocode:
1475 exit $! if $!; # errno
1476 exit $? >> 8 if $? >> 8; # child exit status
1477 exit 255; # last resort
1479 The intent is to squeeze as much possible information about the likely cause
1480 into the limited space of the system exit
1481 code. However, as C<$!> is the value
1482 of C's C<errno>, which can be set by any system call, this means that the value
1483 of the exit code used by C<die> can be non-predictable, so should not be relied
1484 upon, other than to be non-zero.
1486 You can also call C<die> with a reference argument, and if this is trapped
1487 within an C<eval>, C<$@> contains that reference. This permits more
1488 elaborate exception handling using objects that maintain arbitrary state
1489 about the exception. Such a scheme is sometimes preferable to matching
1490 particular string values of C<$@> with regular expressions. Because C<$@>
1491 is a global variable and C<eval> may be used within object implementations,
1492 be careful that analyzing the error object doesn't replace the reference in
1493 the global variable. It's easiest to make a local copy of the reference
1494 before any manipulations. Here's an example:
1496 use Scalar::Util "blessed";
1498 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1499 if (my $ev_err = $@) {
1500 if (blessed($ev_err)
1501 && $ev_err->isa("Some::Module::Exception")) {
1502 # handle Some::Module::Exception
1505 # handle all other possible exceptions
1509 Because Perl stringifies uncaught exception messages before display,
1510 you'll probably want to overload stringification operations on
1511 exception objects. See L<overload> for details about that.
1513 You can arrange for a callback to be run just before the C<die>
1514 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1515 handler is called with the error text and can change the error
1516 message, if it sees fit, by calling C<die> again. See
1517 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1518 L<"eval BLOCK"> for some examples. Although this feature was
1519 to be run only right before your program was to exit, this is not
1520 currently so: the C<$SIG{__DIE__}> hook is currently called
1521 even inside eval()ed blocks/strings! If one wants the hook to do
1522 nothing in such situations, put
1526 as the first line of the handler (see L<perlvar/$^S>). Because
1527 this promotes strange action at a distance, this counterintuitive
1528 behavior may be fixed in a future release.
1530 See also exit(), warn(), and the Carp module.
1535 =for Pod::Functions turn a BLOCK into a TERM
1537 Not really a function. Returns the value of the last command in the
1538 sequence of commands indicated by BLOCK. When modified by the C<while> or
1539 C<until> loop modifier, executes the BLOCK once before testing the loop
1540 condition. (On other statements the loop modifiers test the conditional
1543 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1544 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1545 See L<perlsyn> for alternative strategies.
1547 =item do SUBROUTINE(LIST)
1550 This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1556 Uses the value of EXPR as a filename and executes the contents of the
1557 file as a Perl script.
1565 except that it's more efficient and concise, keeps track of the current
1566 filename for error messages, searches the C<@INC> directories, and updates
1567 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1568 these variables. It also differs in that code evaluated with C<do FILENAME>
1569 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1570 same, however, in that it does reparse the file every time you call it,
1571 so you probably don't want to do this inside a loop.
1573 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1574 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1575 and sets C<$!> to the error. Always check C<$@> first, as compilation
1576 could fail in a way that also sets C<$!>. If the file is successfully
1577 compiled, C<do> returns the value of the last expression evaluated.
1579 Inclusion of library modules is better done with the
1580 C<use> and C<require> operators, which also do automatic error checking
1581 and raise an exception if there's a problem.
1583 You might like to use C<do> to read in a program configuration
1584 file. Manual error checking can be done this way:
1586 # read in config files: system first, then user
1587 for $file ("/share/prog/defaults.rc",
1588 "$ENV{HOME}/.someprogrc")
1590 unless ($return = do $file) {
1591 warn "couldn't parse $file: $@" if $@;
1592 warn "couldn't do $file: $!" unless defined $return;
1593 warn "couldn't run $file" unless $return;
1598 X<dump> X<core> X<undump>
1604 =for Pod::Functions create an immediate core dump
1606 This function causes an immediate core dump. See also the B<-u>
1607 command-line switch in L<perlrun>, which does the same thing.
1608 Primarily this is so that you can use the B<undump> program (not
1609 supplied) to turn your core dump into an executable binary after
1610 having initialized all your variables at the beginning of the
1611 program. When the new binary is executed it will begin by executing
1612 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1613 Think of it as a goto with an intervening core dump and reincarnation.
1614 If C<LABEL> is omitted, restarts the program from the top. The
1615 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1616 computed at run time, being otherwise identical to C<dump LABEL>.
1618 B<WARNING>: Any files opened at the time of the dump will I<not>
1619 be open any more when the program is reincarnated, with possible
1620 resulting confusion by Perl.
1622 This function is now largely obsolete, mostly because it's very hard to
1623 convert a core file into an executable. That's why you should now invoke
1624 it as C<CORE::dump()>, if you don't want to be warned against a possible
1627 Unlike most named operators, this has the same precedence as assignment.
1628 It is also exempt from the looks-like-a-function rule, so
1629 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1632 Portability issues: L<perlport/dump>.
1635 X<each> X<hash, iterator>
1642 =for Pod::Functions retrieve the next key/value pair from a hash
1644 When called on a hash in list context, returns a 2-element list
1645 consisting of the key and value for the next element of a hash. In Perl
1646 5.12 and later only, it will also return the index and value for the next
1647 element of an array so that you can iterate over it; older Perls consider
1648 this a syntax error. When called in scalar context, returns only the key
1649 (not the value) in a hash, or the index in an array.
1651 Hash entries are returned in an apparently random order. The actual random
1652 order is subject to change in future versions of Perl, but it is
1653 guaranteed to be in the same order as either the C<keys> or C<values>
1654 function would produce on the same (unmodified) hash. Since Perl
1655 5.8.2 the ordering can be different even between different runs of Perl
1656 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1658 After C<each> has returned all entries from the hash or array, the next
1659 call to C<each> returns the empty list in list context and C<undef> in
1660 scalar context; the next call following I<that> one restarts iteration.
1661 Each hash or array has its own internal iterator, accessed by C<each>,
1662 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1663 reached the end as just described; it can be explicitly reset by calling
1664 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1665 elements while iterating over it, entries may be skipped or duplicated--so
1666 don't do that. Exception: In the current implementation, it is always safe
1667 to delete the item most recently returned by C<each()>, so the following
1668 code works properly:
1670 while (($key, $value) = each %hash) {
1672 delete $hash{$key}; # This is safe
1675 This prints out your environment like the printenv(1) program,
1676 but in a different order:
1678 while (($key,$value) = each %ENV) {
1679 print "$key=$value\n";
1682 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1683 reference to an unblessed hash or array. The argument will be dereferenced
1684 automatically. This aspect of C<each> is considered highly experimental.
1685 The exact behaviour may change in a future version of Perl.
1687 while (($key,$value) = each $hashref) { ... }
1689 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1690 which will set C<$_> on every iteration.
1693 print "$_=$ENV{$_}\n";
1696 To avoid confusing would-be users of your code who are running earlier
1697 versions of Perl with mysterious syntax errors, put this sort of thing at
1698 the top of your file to signal that your code will work I<only> on Perls of
1701 use 5.012; # so keys/values/each work on arrays
1702 use 5.014; # so keys/values/each work on scalars (experimental)
1703 use 5.018; # so each assigns to $_ in a lone while test
1705 See also C<keys>, C<values>, and C<sort>.
1707 =item eof FILEHANDLE
1716 =for Pod::Functions test a filehandle for its end
1718 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1719 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1720 gives the real filehandle. (Note that this function actually
1721 reads a character and then C<ungetc>s it, so isn't useful in an
1722 interactive context.) Do not read from a terminal file (or call
1723 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1724 as terminals may lose the end-of-file condition if you do.
1726 An C<eof> without an argument uses the last file read. Using C<eof()>
1727 with empty parentheses is different. It refers to the pseudo file
1728 formed from the files listed on the command line and accessed via the
1729 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1730 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1731 used will cause C<@ARGV> to be examined to determine if input is
1732 available. Similarly, an C<eof()> after C<< <> >> has returned
1733 end-of-file will assume you are processing another C<@ARGV> list,
1734 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1735 see L<perlop/"I/O Operators">.
1737 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1738 detect the end of each file, whereas C<eof()> will detect the end
1739 of the very last file only. Examples:
1741 # reset line numbering on each input file
1743 next if /^\s*#/; # skip comments
1746 close ARGV if eof; # Not eof()!
1749 # insert dashes just before last line of last file
1751 if (eof()) { # check for end of last file
1752 print "--------------\n";
1755 last if eof(); # needed if we're reading from a terminal
1758 Practical hint: you almost never need to use C<eof> in Perl, because the
1759 input operators typically return C<undef> when they run out of data or
1763 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1764 X<error, handling> X<exception, handling>
1770 =for Pod::Functions catch exceptions or compile and run code
1772 In the first form, the return value of EXPR is parsed and executed as if it
1773 were a little Perl program. The value of the expression (which is itself
1774 determined within scalar context) is first parsed, and if there were no
1775 errors, executed as a block within the lexical context of the current Perl
1776 program. This means, that in particular, any outer lexical variables are
1777 visible to it, and any package variable settings or subroutine and format
1778 definitions remain afterwards.
1780 Note that the value is parsed every time the C<eval> executes.
1781 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1782 delay parsing and subsequent execution of the text of EXPR until run time.
1784 If the C<unicode_eval> feature is enabled (which is the default under a
1785 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1786 characters, so C<use utf8> declarations have no effect, and source filters
1787 are forbidden. In the absence of the C<unicode_eval> feature, the string
1788 will sometimes be treated as characters and sometimes as bytes, depending
1789 on the internal encoding, and source filters activated within the C<eval>
1790 exhibit the erratic, but historical, behaviour of affecting some outer file
1791 scope that is still compiling. See also the L</evalbytes> keyword, which
1792 always treats its input as a byte stream and works properly with source
1793 filters, and the L<feature> pragma.
1795 In the second form, the code within the BLOCK is parsed only once--at the
1796 same time the code surrounding the C<eval> itself was parsed--and executed
1797 within the context of the current Perl program. This form is typically
1798 used to trap exceptions more efficiently than the first (see below), while
1799 also providing the benefit of checking the code within BLOCK at compile
1802 The final semicolon, if any, may be omitted from the value of EXPR or within
1805 In both forms, the value returned is the value of the last expression
1806 evaluated inside the mini-program; a return statement may be also used, just
1807 as with subroutines. The expression providing the return value is evaluated
1808 in void, scalar, or list context, depending on the context of the C<eval>
1809 itself. See L</wantarray> for more on how the evaluation context can be
1812 If there is a syntax error or runtime error, or a C<die> statement is
1813 executed, C<eval> returns C<undef> in scalar context
1814 or an empty list in list context, and C<$@> is set to the error
1815 message. (Prior to 5.16, a bug caused C<undef> to be returned
1816 in list context for syntax errors, but not for runtime errors.)
1817 If there was no error, C<$@> is set to the empty string. A
1818 control flow operator like C<last> or C<goto> can bypass the setting of
1819 C<$@>. Beware that using C<eval> neither silences Perl from printing
1820 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1821 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1822 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1823 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1825 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1826 determining whether a particular feature (such as C<socket> or C<symlink>)
1827 is implemented. It is also Perl's exception-trapping mechanism, where
1828 the die operator is used to raise exceptions.
1830 If you want to trap errors when loading an XS module, some problems with
1831 the binary interface (such as Perl version skew) may be fatal even with
1832 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1834 If the code to be executed doesn't vary, you may use the eval-BLOCK
1835 form to trap run-time errors without incurring the penalty of
1836 recompiling each time. The error, if any, is still returned in C<$@>.
1839 # make divide-by-zero nonfatal
1840 eval { $answer = $a / $b; }; warn $@ if $@;
1842 # same thing, but less efficient
1843 eval '$answer = $a / $b'; warn $@ if $@;
1845 # a compile-time error
1846 eval { $answer = }; # WRONG
1849 eval '$answer ='; # sets $@
1851 Using the C<eval{}> form as an exception trap in libraries does have some
1852 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1853 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1854 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1855 as this example shows:
1857 # a private exception trap for divide-by-zero
1858 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1861 This is especially significant, given that C<__DIE__> hooks can call
1862 C<die> again, which has the effect of changing their error messages:
1864 # __DIE__ hooks may modify error messages
1866 local $SIG{'__DIE__'} =
1867 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1868 eval { die "foo lives here" };
1869 print $@ if $@; # prints "bar lives here"
1872 Because this promotes action at a distance, this counterintuitive behavior
1873 may be fixed in a future release.
1875 With an C<eval>, you should be especially careful to remember what's
1876 being looked at when:
1882 eval { $x }; # CASE 4
1884 eval "\$$x++"; # CASE 5
1887 Cases 1 and 2 above behave identically: they run the code contained in
1888 the variable $x. (Although case 2 has misleading double quotes making
1889 the reader wonder what else might be happening (nothing is).) Cases 3
1890 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1891 does nothing but return the value of $x. (Case 4 is preferred for
1892 purely visual reasons, but it also has the advantage of compiling at
1893 compile-time instead of at run-time.) Case 5 is a place where
1894 normally you I<would> like to use double quotes, except that in this
1895 particular situation, you can just use symbolic references instead, as
1898 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1899 of localized variables, which means that for your code to run on older
1900 versions, a temporary is required if you want to mask some but not all
1903 # alter $@ on nefarious repugnancy only
1907 local $@; # protect existing $@
1908 eval { test_repugnancy() };
1909 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1910 $@ =~ /nefarious/ and $e = $@;
1912 die $e if defined $e
1915 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1916 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1918 An C<eval ''> executed within a subroutine defined
1919 in the C<DB> package doesn't see the usual
1920 surrounding lexical scope, but rather the scope of the first non-DB piece
1921 of code that called it. You don't normally need to worry about this unless
1922 you are writing a Perl debugger.
1924 =item evalbytes EXPR
1929 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1931 This function is like L</eval> with a string argument, except it always
1932 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1933 string containing characters whose ordinal value exceeds 255 results in an
1934 error. Source filters activated within the evaluated code apply to the
1937 This function is only available under the C<evalbytes> feature, a
1938 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1939 L<feature> for more information.
1944 =item exec PROGRAM LIST
1946 =for Pod::Functions abandon this program to run another
1948 The C<exec> function executes a system command I<and never returns>;
1949 use C<system> instead of C<exec> if you want it to return. It fails and
1950 returns false only if the command does not exist I<and> it is executed
1951 directly instead of via your system's command shell (see below).
1953 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1954 warns you if C<exec> is called in void context and if there is a following
1955 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1956 you always do that, right?). If you I<really> want to follow an C<exec>
1957 with some other statement, you can use one of these styles to avoid the warning:
1959 exec ('foo') or print STDERR "couldn't exec foo: $!";
1960 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1962 If there is more than one argument in LIST, or if LIST is an array
1963 with more than one value, calls execvp(3) with the arguments in LIST.
1964 If there is only one scalar argument or an array with one element in it,
1965 the argument is checked for shell metacharacters, and if there are any,
1966 the entire argument is passed to the system's command shell for parsing
1967 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1968 If there are no shell metacharacters in the argument, it is split into
1969 words and passed directly to C<execvp>, which is more efficient.
1972 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1973 exec "sort $outfile | uniq";
1975 If you don't really want to execute the first argument, but want to lie
1976 to the program you are executing about its own name, you can specify
1977 the program you actually want to run as an "indirect object" (without a
1978 comma) in front of the LIST. (This always forces interpretation of the
1979 LIST as a multivalued list, even if there is only a single scalar in
1982 $shell = '/bin/csh';
1983 exec $shell '-sh'; # pretend it's a login shell
1987 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1989 When the arguments get executed via the system shell, results are
1990 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1993 Using an indirect object with C<exec> or C<system> is also more
1994 secure. This usage (which also works fine with system()) forces
1995 interpretation of the arguments as a multivalued list, even if the
1996 list had just one argument. That way you're safe from the shell
1997 expanding wildcards or splitting up words with whitespace in them.
1999 @args = ( "echo surprise" );
2001 exec @args; # subject to shell escapes
2003 exec { $args[0] } @args; # safe even with one-arg list
2005 The first version, the one without the indirect object, ran the I<echo>
2006 program, passing it C<"surprise"> an argument. The second version didn't;
2007 it tried to run a program named I<"echo surprise">, didn't find it, and set
2008 C<$?> to a non-zero value indicating failure.
2010 Perl attempts to flush all files opened for output before the exec,
2011 but this may not be supported on some platforms (see L<perlport>).
2012 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2013 call the C<autoflush()> method of C<IO::Handle> on any open handles
2014 to avoid lost output.
2016 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2017 C<DESTROY> methods on your objects.
2019 Portability issues: L<perlport/exec>.
2022 X<exists> X<autovivification>
2024 =for Pod::Functions test whether a hash key is present
2026 Given an expression that specifies an element of a hash, returns true if the
2027 specified element in the hash has ever been initialized, even if the
2028 corresponding value is undefined.
2030 print "Exists\n" if exists $hash{$key};
2031 print "Defined\n" if defined $hash{$key};
2032 print "True\n" if $hash{$key};
2034 exists may also be called on array elements, but its behavior is much less
2035 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2036 that calling exists on array values is deprecated and likely to be removed in
2037 a future version of Perl.
2039 print "Exists\n" if exists $array[$index];
2040 print "Defined\n" if defined $array[$index];
2041 print "True\n" if $array[$index];
2043 A hash or array element can be true only if it's defined and defined only if
2044 it exists, but the reverse doesn't necessarily hold true.
2046 Given an expression that specifies the name of a subroutine,
2047 returns true if the specified subroutine has ever been declared, even
2048 if it is undefined. Mentioning a subroutine name for exists or defined
2049 does not count as declaring it. Note that a subroutine that does not
2050 exist may still be callable: its package may have an C<AUTOLOAD>
2051 method that makes it spring into existence the first time that it is
2052 called; see L<perlsub>.
2054 print "Exists\n" if exists &subroutine;
2055 print "Defined\n" if defined &subroutine;
2057 Note that the EXPR can be arbitrarily complicated as long as the final
2058 operation is a hash or array key lookup or subroutine name:
2060 if (exists $ref->{A}->{B}->{$key}) { }
2061 if (exists $hash{A}{B}{$key}) { }
2063 if (exists $ref->{A}->{B}->[$ix]) { }
2064 if (exists $hash{A}{B}[$ix]) { }
2066 if (exists &{$ref->{A}{B}{$key}}) { }
2068 Although the most deeply nested array or hash element will not spring into
2069 existence just because its existence was tested, any intervening ones will.
2070 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2071 into existence due to the existence test for the $key element above.
2072 This happens anywhere the arrow operator is used, including even here:
2075 if (exists $ref->{"Some key"}) { }
2076 print $ref; # prints HASH(0x80d3d5c)
2078 This surprising autovivification in what does not at first--or even
2079 second--glance appear to be an lvalue context may be fixed in a future
2082 Use of a subroutine call, rather than a subroutine name, as an argument
2083 to exists() is an error.
2086 exists &sub(); # Error
2089 X<exit> X<terminate> X<abort>
2093 =for Pod::Functions terminate this program
2095 Evaluates EXPR and exits immediately with that value. Example:
2098 exit 0 if $ans =~ /^[Xx]/;
2100 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2101 universally recognized values for EXPR are C<0> for success and C<1>
2102 for error; other values are subject to interpretation depending on the
2103 environment in which the Perl program is running. For example, exiting
2104 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2105 the mailer to return the item undelivered, but that's not true everywhere.
2107 Don't use C<exit> to abort a subroutine if there's any chance that
2108 someone might want to trap whatever error happened. Use C<die> instead,
2109 which can be trapped by an C<eval>.
2111 The exit() function does not always exit immediately. It calls any
2112 defined C<END> routines first, but these C<END> routines may not
2113 themselves abort the exit. Likewise any object destructors that need to
2114 be called are called before the real exit. C<END> routines and destructors
2115 can change the exit status by modifying C<$?>. If this is a problem, you
2116 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2117 See L<perlmod> for details.
2119 Portability issues: L<perlport/exit>.
2122 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2126 =for Pod::Functions raise I<e> to a power
2128 Returns I<e> (the natural logarithm base) to the power of EXPR.
2129 If EXPR is omitted, gives C<exp($_)>.
2132 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2136 =for Pod::Functions +fc return casefolded version of a string
2138 Returns the casefolded version of EXPR. This is the internal function
2139 implementing the C<\F> escape in double-quoted strings.
2141 Casefolding is the process of mapping strings to a form where case
2142 differences are erased; comparing two strings in their casefolded
2143 form is effectively a way of asking if two strings are equal,
2146 Roughly, if you ever found yourself writing this
2148 lc($this) eq lc($that) # Wrong!
2150 uc($this) eq uc($that) # Also wrong!
2152 $this =~ /^\Q$that\E\z/i # Right!
2156 fc($this) eq fc($that)
2158 And get the correct results.
2160 Perl only implements the full form of casefolding,
2161 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2162 L<Unicode::UCD/prop_invmap()>.
2163 For further information on casefolding, refer to
2164 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2165 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2166 available at L<http://www.unicode.org/versions/latest/>, as well as the
2167 Case Charts available at L<http://www.unicode.org/charts/case/>.
2169 If EXPR is omitted, uses C<$_>.
2171 This function behaves the same way under various pragma, such as in a locale,
2174 While the Unicode Standard defines two additional forms of casefolding,
2175 one for Turkic languages and one that never maps one character into multiple
2176 characters, these are not provided by the Perl core; However, the CPAN module
2177 C<Unicode::Casing> may be used to provide an implementation.
2179 This keyword is available only when the C<"fc"> feature is enabled,
2180 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2181 include a C<use v5.16> or later to the current scope.
2183 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2186 =for Pod::Functions file control system call
2188 Implements the fcntl(2) function. You'll probably have to say
2192 first to get the correct constant definitions. Argument processing and
2193 value returned work just like C<ioctl> below.
2197 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2198 or die "can't fcntl F_GETFL: $!";
2200 You don't have to check for C<defined> on the return from C<fcntl>.
2201 Like C<ioctl>, it maps a C<0> return from the system call into
2202 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2203 in numeric context. It is also exempt from the normal B<-w> warnings
2204 on improper numeric conversions.
2206 Note that C<fcntl> raises an exception if used on a machine that
2207 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2208 manpage to learn what functions are available on your system.
2210 Here's an example of setting a filehandle named C<REMOTE> to be
2211 non-blocking at the system level. You'll have to negotiate C<$|>
2212 on your own, though.
2214 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2216 $flags = fcntl(REMOTE, F_GETFL, 0)
2217 or die "Can't get flags for the socket: $!\n";
2219 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2220 or die "Can't set flags for the socket: $!\n";
2222 Portability issues: L<perlport/fcntl>.
2227 =for Pod::Functions the name of the current source file
2229 A special token that returns the name of the file in which it occurs.
2231 =item fileno FILEHANDLE
2234 =for Pod::Functions return file descriptor from filehandle
2236 Returns the file descriptor for a filehandle, or undefined if the
2237 filehandle is not open. If there is no real file descriptor at the OS
2238 level, as can happen with filehandles connected to memory objects via
2239 C<open> with a reference for the third argument, -1 is returned.
2241 This is mainly useful for constructing
2242 bitmaps for C<select> and low-level POSIX tty-handling operations.
2243 If FILEHANDLE is an expression, the value is taken as an indirect
2244 filehandle, generally its name.
2246 You can use this to find out whether two handles refer to the
2247 same underlying descriptor:
2249 if (fileno(THIS) == fileno(THAT)) {
2250 print "THIS and THAT are dups\n";
2253 =item flock FILEHANDLE,OPERATION
2254 X<flock> X<lock> X<locking>
2256 =for Pod::Functions lock an entire file with an advisory lock
2258 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2259 for success, false on failure. Produces a fatal error if used on a
2260 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2261 C<flock> is Perl's portable file-locking interface, although it locks
2262 entire files only, not records.
2264 Two potentially non-obvious but traditional C<flock> semantics are
2265 that it waits indefinitely until the lock is granted, and that its locks
2266 are B<merely advisory>. Such discretionary locks are more flexible, but
2267 offer fewer guarantees. This means that programs that do not also use
2268 C<flock> may modify files locked with C<flock>. See L<perlport>,
2269 your port's specific documentation, and your system-specific local manpages
2270 for details. It's best to assume traditional behavior if you're writing
2271 portable programs. (But if you're not, you should as always feel perfectly
2272 free to write for your own system's idiosyncrasies (sometimes called
2273 "features"). Slavish adherence to portability concerns shouldn't get
2274 in the way of your getting your job done.)
2276 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2277 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2278 you can use the symbolic names if you import them from the L<Fcntl> module,
2279 either individually, or as a group using the C<:flock> tag. LOCK_SH
2280 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2281 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2282 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2283 waiting for the lock; check the return status to see if you got it.
2285 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2286 before locking or unlocking it.
2288 Note that the emulation built with lockf(3) doesn't provide shared
2289 locks, and it requires that FILEHANDLE be open with write intent. These
2290 are the semantics that lockf(3) implements. Most if not all systems
2291 implement lockf(3) in terms of fcntl(2) locking, though, so the
2292 differing semantics shouldn't bite too many people.
2294 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2295 be open with read intent to use LOCK_SH and requires that it be open
2296 with write intent to use LOCK_EX.
2298 Note also that some versions of C<flock> cannot lock things over the
2299 network; you would need to use the more system-specific C<fcntl> for
2300 that. If you like you can force Perl to ignore your system's flock(2)
2301 function, and so provide its own fcntl(2)-based emulation, by passing
2302 the switch C<-Ud_flock> to the F<Configure> program when you configure
2303 and build a new Perl.
2305 Here's a mailbox appender for BSD systems.
2307 # import LOCK_* and SEEK_END constants
2308 use Fcntl qw(:flock SEEK_END);
2312 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2314 # and, in case someone appended while we were waiting...
2315 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2320 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2323 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2324 or die "Can't open mailbox: $!";
2327 print $mbox $msg,"\n\n";
2330 On systems that support a real flock(2), locks are inherited across fork()
2331 calls, whereas those that must resort to the more capricious fcntl(2)
2332 function lose their locks, making it seriously harder to write servers.
2334 See also L<DB_File> for other flock() examples.
2336 Portability issues: L<perlport/flock>.
2339 X<fork> X<child> X<parent>
2341 =for Pod::Functions create a new process just like this one
2343 Does a fork(2) system call to create a new process running the
2344 same program at the same point. It returns the child pid to the
2345 parent process, C<0> to the child process, or C<undef> if the fork is
2346 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2347 are shared, while everything else is copied. On most systems supporting
2348 fork(), great care has gone into making it extremely efficient (for
2349 example, using copy-on-write technology on data pages), making it the
2350 dominant paradigm for multitasking over the last few decades.
2352 Perl attempts to flush all files opened for
2353 output before forking the child process, but this may not be supported
2354 on some platforms (see L<perlport>). To be safe, you may need to set
2355 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2356 C<IO::Handle> on any open handles to avoid duplicate output.
2358 If you C<fork> without ever waiting on your children, you will
2359 accumulate zombies. On some systems, you can avoid this by setting
2360 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2361 forking and reaping moribund children.
2363 Note that if your forked child inherits system file descriptors like
2364 STDIN and STDOUT that are actually connected by a pipe or socket, even
2365 if you exit, then the remote server (such as, say, a CGI script or a
2366 backgrounded job launched from a remote shell) won't think you're done.
2367 You should reopen those to F</dev/null> if it's any issue.
2369 On some platforms such as Windows, where the fork() system call is not available,
2370 Perl can be built to emulate fork() in the Perl interpreter.
2371 The emulation is designed, at the level of the Perl program,
2372 to be as compatible as possible with the "Unix" fork().
2373 However it has limitations that have to be considered in code intended to be portable.
2374 See L<perlfork> for more details.
2376 Portability issues: L<perlport/fork>.
2381 =for Pod::Functions declare a picture format with use by the write() function
2383 Declare a picture format for use by the C<write> function. For
2387 Test: @<<<<<<<< @||||| @>>>>>
2388 $str, $%, '$' . int($num)
2392 $num = $cost/$quantity;
2396 See L<perlform> for many details and examples.
2398 =item formline PICTURE,LIST
2401 =for Pod::Functions internal function used for formats
2403 This is an internal function used by C<format>s, though you may call it,
2404 too. It formats (see L<perlform>) a list of values according to the
2405 contents of PICTURE, placing the output into the format output
2406 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2407 Eventually, when a C<write> is done, the contents of
2408 C<$^A> are written to some filehandle. You could also read C<$^A>
2409 and then set C<$^A> back to C<"">. Note that a format typically
2410 does one C<formline> per line of form, but the C<formline> function itself
2411 doesn't care how many newlines are embedded in the PICTURE. This means
2412 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2413 You may therefore need to use multiple formlines to implement a single
2414 record format, just like the C<format> compiler.
2416 Be careful if you put double quotes around the picture, because an C<@>
2417 character may be taken to mean the beginning of an array name.
2418 C<formline> always returns true. See L<perlform> for other examples.
2420 If you are trying to use this instead of C<write> to capture the output,
2421 you may find it easier to open a filehandle to a scalar
2422 (C<< open $fh, ">", \$output >>) and write to that instead.
2424 =item getc FILEHANDLE
2425 X<getc> X<getchar> X<character> X<file, read>
2429 =for Pod::Functions get the next character from the filehandle
2431 Returns the next character from the input file attached to FILEHANDLE,
2432 or the undefined value at end of file or if there was an error (in
2433 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2434 STDIN. This is not particularly efficient. However, it cannot be
2435 used by itself to fetch single characters without waiting for the user
2436 to hit enter. For that, try something more like:
2439 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2442 system "stty", '-icanon', 'eol', "\001";
2448 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2451 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2455 Determination of whether $BSD_STYLE should be set
2456 is left as an exercise to the reader.
2458 The C<POSIX::getattr> function can do this more portably on
2459 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2460 module from your nearest CPAN site; details on CPAN can be found under
2464 X<getlogin> X<login>
2466 =for Pod::Functions return who logged in at this tty
2468 This implements the C library function of the same name, which on most
2469 systems returns the current login from F</etc/utmp>, if any. If it
2470 returns the empty string, use C<getpwuid>.
2472 $login = getlogin || getpwuid($<) || "Kilroy";
2474 Do not consider C<getlogin> for authentication: it is not as
2475 secure as C<getpwuid>.
2477 Portability issues: L<perlport/getlogin>.
2479 =item getpeername SOCKET
2480 X<getpeername> X<peer>
2482 =for Pod::Functions find the other end of a socket connection
2484 Returns the packed sockaddr address of the other end of the SOCKET
2488 $hersockaddr = getpeername(SOCK);
2489 ($port, $iaddr) = sockaddr_in($hersockaddr);
2490 $herhostname = gethostbyaddr($iaddr, AF_INET);
2491 $herstraddr = inet_ntoa($iaddr);
2496 =for Pod::Functions get process group
2498 Returns the current process group for the specified PID. Use
2499 a PID of C<0> to get the current process group for the
2500 current process. Will raise an exception if used on a machine that
2501 doesn't implement getpgrp(2). If PID is omitted, returns the process
2502 group of the current process. Note that the POSIX version of C<getpgrp>
2503 does not accept a PID argument, so only C<PID==0> is truly portable.
2505 Portability issues: L<perlport/getpgrp>.
2508 X<getppid> X<parent> X<pid>
2510 =for Pod::Functions get parent process ID
2512 Returns the process id of the parent process.
2514 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2515 around non-POSIX thread semantics the minority of Linux systems (and
2516 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2517 has since been removed. See the documentation for L<$$|perlvar/$$> for
2520 Portability issues: L<perlport/getppid>.
2522 =item getpriority WHICH,WHO
2523 X<getpriority> X<priority> X<nice>
2525 =for Pod::Functions get current nice value
2527 Returns the current priority for a process, a process group, or a user.
2528 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2529 machine that doesn't implement getpriority(2).
2531 Portability issues: L<perlport/getpriority>.
2534 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2535 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2536 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2537 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2538 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2539 X<endnetent> X<endprotoent> X<endservent>
2541 =for Pod::Functions get passwd record given user login name
2545 =for Pod::Functions get group record given group name
2547 =item gethostbyname NAME
2549 =for Pod::Functions get host record given name
2551 =item getnetbyname NAME
2553 =for Pod::Functions get networks record given name
2555 =item getprotobyname NAME
2557 =for Pod::Functions get protocol record given name
2561 =for Pod::Functions get passwd record given user ID
2565 =for Pod::Functions get group record given group user ID
2567 =item getservbyname NAME,PROTO
2569 =for Pod::Functions get services record given its name
2571 =item gethostbyaddr ADDR,ADDRTYPE
2573 =for Pod::Functions get host record given its address
2575 =item getnetbyaddr ADDR,ADDRTYPE
2577 =for Pod::Functions get network record given its address
2579 =item getprotobynumber NUMBER
2581 =for Pod::Functions get protocol record numeric protocol
2583 =item getservbyport PORT,PROTO
2585 =for Pod::Functions get services record given numeric port
2589 =for Pod::Functions get next passwd record
2593 =for Pod::Functions get next group record
2597 =for Pod::Functions get next hosts record
2601 =for Pod::Functions get next networks record
2605 =for Pod::Functions get next protocols record
2609 =for Pod::Functions get next services record
2613 =for Pod::Functions prepare passwd file for use
2617 =for Pod::Functions prepare group file for use
2619 =item sethostent STAYOPEN
2621 =for Pod::Functions prepare hosts file for use
2623 =item setnetent STAYOPEN
2625 =for Pod::Functions prepare networks file for use
2627 =item setprotoent STAYOPEN
2629 =for Pod::Functions prepare protocols file for use
2631 =item setservent STAYOPEN
2633 =for Pod::Functions prepare services file for use
2637 =for Pod::Functions be done using passwd file
2641 =for Pod::Functions be done using group file
2645 =for Pod::Functions be done using hosts file
2649 =for Pod::Functions be done using networks file
2653 =for Pod::Functions be done using protocols file
2657 =for Pod::Functions be done using services file
2659 These routines are the same as their counterparts in the
2660 system C library. In list context, the return values from the
2661 various get routines are as follows:
2663 ($name,$passwd,$uid,$gid,
2664 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2665 ($name,$passwd,$gid,$members) = getgr*
2666 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2667 ($name,$aliases,$addrtype,$net) = getnet*
2668 ($name,$aliases,$proto) = getproto*
2669 ($name,$aliases,$port,$proto) = getserv*
2671 (If the entry doesn't exist you get an empty list.)
2673 The exact meaning of the $gcos field varies but it usually contains
2674 the real name of the user (as opposed to the login name) and other
2675 information pertaining to the user. Beware, however, that in many
2676 system users are able to change this information and therefore it
2677 cannot be trusted and therefore the $gcos is tainted (see
2678 L<perlsec>). The $passwd and $shell, user's encrypted password and
2679 login shell, are also tainted, for the same reason.
2681 In scalar context, you get the name, unless the function was a
2682 lookup by name, in which case you get the other thing, whatever it is.
2683 (If the entry doesn't exist you get the undefined value.) For example:
2685 $uid = getpwnam($name);
2686 $name = getpwuid($num);
2688 $gid = getgrnam($name);
2689 $name = getgrgid($num);
2693 In I<getpw*()> the fields $quota, $comment, and $expire are special
2694 in that they are unsupported on many systems. If the
2695 $quota is unsupported, it is an empty scalar. If it is supported, it
2696 usually encodes the disk quota. If the $comment field is unsupported,
2697 it is an empty scalar. If it is supported it usually encodes some
2698 administrative comment about the user. In some systems the $quota
2699 field may be $change or $age, fields that have to do with password
2700 aging. In some systems the $comment field may be $class. The $expire
2701 field, if present, encodes the expiration period of the account or the
2702 password. For the availability and the exact meaning of these fields
2703 in your system, please consult getpwnam(3) and your system's
2704 F<pwd.h> file. You can also find out from within Perl what your
2705 $quota and $comment fields mean and whether you have the $expire field
2706 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2707 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2708 files are supported only if your vendor has implemented them in the
2709 intuitive fashion that calling the regular C library routines gets the
2710 shadow versions if you're running under privilege or if there exists
2711 the shadow(3) functions as found in System V (this includes Solaris
2712 and Linux). Those systems that implement a proprietary shadow password
2713 facility are unlikely to be supported.
2715 The $members value returned by I<getgr*()> is a space-separated list of
2716 the login names of the members of the group.
2718 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2719 C, it will be returned to you via C<$?> if the function call fails. The
2720 C<@addrs> value returned by a successful call is a list of raw
2721 addresses returned by the corresponding library call. In the
2722 Internet domain, each address is four bytes long; you can unpack it
2723 by saying something like:
2725 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2727 The Socket library makes this slightly easier:
2730 $iaddr = inet_aton("127.1"); # or whatever address
2731 $name = gethostbyaddr($iaddr, AF_INET);
2733 # or going the other way
2734 $straddr = inet_ntoa($iaddr);
2736 In the opposite way, to resolve a hostname to the IP address
2740 $packed_ip = gethostbyname("www.perl.org");
2741 if (defined $packed_ip) {
2742 $ip_address = inet_ntoa($packed_ip);
2745 Make sure C<gethostbyname()> is called in SCALAR context and that
2746 its return value is checked for definedness.
2748 The C<getprotobynumber> function, even though it only takes one argument,
2749 has the precedence of a list operator, so beware:
2751 getprotobynumber $number eq 'icmp' # WRONG
2752 getprotobynumber($number eq 'icmp') # actually means this
2753 getprotobynumber($number) eq 'icmp' # better this way
2755 If you get tired of remembering which element of the return list
2756 contains which return value, by-name interfaces are provided
2757 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2758 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2759 and C<User::grent>. These override the normal built-ins, supplying
2760 versions that return objects with the appropriate names
2761 for each field. For example:
2765 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2767 Even though it looks as though they're the same method calls (uid),
2768 they aren't, because a C<File::stat> object is different from
2769 a C<User::pwent> object.
2771 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2773 =item getsockname SOCKET
2776 =for Pod::Functions retrieve the sockaddr for a given socket
2778 Returns the packed sockaddr address of this end of the SOCKET connection,
2779 in case you don't know the address because you have several different
2780 IPs that the connection might have come in on.
2783 $mysockaddr = getsockname(SOCK);
2784 ($port, $myaddr) = sockaddr_in($mysockaddr);
2785 printf "Connect to %s [%s]\n",
2786 scalar gethostbyaddr($myaddr, AF_INET),
2789 =item getsockopt SOCKET,LEVEL,OPTNAME
2792 =for Pod::Functions get socket options on a given socket
2794 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2795 Options may exist at multiple protocol levels depending on the socket
2796 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2797 C<Socket> module) will exist. To query options at another level the
2798 protocol number of the appropriate protocol controlling the option
2799 should be supplied. For example, to indicate that an option is to be
2800 interpreted by the TCP protocol, LEVEL should be set to the protocol
2801 number of TCP, which you can get using C<getprotobyname>.
2803 The function returns a packed string representing the requested socket
2804 option, or C<undef> on error, with the reason for the error placed in
2805 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2806 consult getsockopt(2) for details. A common case is that the option is an
2807 integer, in which case the result is a packed integer, which you can decode
2808 using C<unpack> with the C<i> (or C<I>) format.
2810 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2812 use Socket qw(:all);
2814 defined(my $tcp = getprotobyname("tcp"))
2815 or die "Could not determine the protocol number for tcp";
2816 # my $tcp = IPPROTO_TCP; # Alternative
2817 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2818 or die "getsockopt TCP_NODELAY: $!";
2819 my $nodelay = unpack("I", $packed);
2820 print "Nagle's algorithm is turned ",
2821 $nodelay ? "off\n" : "on\n";
2823 Portability issues: L<perlport/getsockopt>.
2826 X<glob> X<wildcard> X<filename, expansion> X<expand>
2830 =for Pod::Functions expand filenames using wildcards
2832 In list context, returns a (possibly empty) list of filename expansions on
2833 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2834 scalar context, glob iterates through such filename expansions, returning
2835 undef when the list is exhausted. This is the internal function
2836 implementing the C<< <*.c> >> operator, but you can use it directly. If
2837 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2838 more detail in L<perlop/"I/O Operators">.
2840 Note that C<glob> splits its arguments on whitespace and treats
2841 each segment as separate pattern. As such, C<glob("*.c *.h")>
2842 matches all files with a F<.c> or F<.h> extension. The expression
2843 C<glob(".* *")> matches all files in the current working directory.
2844 If you want to glob filenames that might contain whitespace, you'll
2845 have to use extra quotes around the spacey filename to protect it.
2846 For example, to glob filenames that have an C<e> followed by a space
2847 followed by an C<f>, use either of:
2849 @spacies = <"*e f*">;
2850 @spacies = glob '"*e f*"';
2851 @spacies = glob q("*e f*");
2853 If you had to get a variable through, you could do this:
2855 @spacies = glob "'*${var}e f*'";
2856 @spacies = glob qq("*${var}e f*");
2858 If non-empty braces are the only wildcard characters used in the
2859 C<glob>, no filenames are matched, but potentially many strings
2860 are returned. For example, this produces nine strings, one for
2861 each pairing of fruits and colors:
2863 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2865 This operator is implemented using the standard
2866 C<File::Glob> extension. See L<File::Glob> for details, including
2867 C<bsd_glob> which does not treat whitespace as a pattern separator.
2869 Portability issues: L<perlport/glob>.
2872 X<gmtime> X<UTC> X<Greenwich>
2876 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2878 Works just like L</localtime> but the returned values are
2879 localized for the standard Greenwich time zone.
2881 Note: When called in list context, $isdst, the last value
2882 returned by gmtime, is always C<0>. There is no
2883 Daylight Saving Time in GMT.
2885 Portability issues: L<perlport/gmtime>.
2888 X<goto> X<jump> X<jmp>
2894 =for Pod::Functions create spaghetti code
2896 The C<goto-LABEL> form finds the statement labeled with LABEL and
2897 resumes execution there. It can't be used to get out of a block or
2898 subroutine given to C<sort>. It can be used to go almost anywhere
2899 else within the dynamic scope, including out of subroutines, but it's
2900 usually better to use some other construct such as C<last> or C<die>.
2901 The author of Perl has never felt the need to use this form of C<goto>
2902 (in Perl, that is; C is another matter). (The difference is that C
2903 does not offer named loops combined with loop control. Perl does, and
2904 this replaces most structured uses of C<goto> in other languages.)
2906 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2907 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2908 necessarily recommended if you're optimizing for maintainability:
2910 goto ("FOO", "BAR", "GLARCH")[$i];
2912 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2913 function" rule. A pair of parentheses following it does not (necessarily)
2914 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2915 Also, unlike most named operators, this has the same precedence as
2918 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2919 deprecated and will issue a warning. Even then, it may not be used to
2920 go into any construct that requires initialization, such as a
2921 subroutine or a C<foreach> loop. It also can't be used to go into a
2922 construct that is optimized away.
2924 The C<goto-&NAME> form is quite different from the other forms of
2925 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2926 doesn't have the stigma associated with other gotos. Instead, it
2927 exits the current subroutine (losing any changes set by local()) and
2928 immediately calls in its place the named subroutine using the current
2929 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2930 load another subroutine and then pretend that the other subroutine had
2931 been called in the first place (except that any modifications to C<@_>
2932 in the current subroutine are propagated to the other subroutine.)
2933 After the C<goto>, not even C<caller> will be able to tell that this
2934 routine was called first.
2936 NAME needn't be the name of a subroutine; it can be a scalar variable
2937 containing a code reference or a block that evaluates to a code
2940 =item grep BLOCK LIST
2943 =item grep EXPR,LIST
2945 =for Pod::Functions locate elements in a list test true against a given criterion
2947 This is similar in spirit to, but not the same as, grep(1) and its
2948 relatives. In particular, it is not limited to using regular expressions.
2950 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2951 C<$_> to each element) and returns the list value consisting of those
2952 elements for which the expression evaluated to true. In scalar
2953 context, returns the number of times the expression was true.
2955 @foo = grep(!/^#/, @bar); # weed out comments
2959 @foo = grep {!/^#/} @bar; # weed out comments
2961 Note that C<$_> is an alias to the list value, so it can be used to
2962 modify the elements of the LIST. While this is useful and supported,
2963 it can cause bizarre results if the elements of LIST are not variables.
2964 Similarly, grep returns aliases into the original list, much as a for
2965 loop's index variable aliases the list elements. That is, modifying an
2966 element of a list returned by grep (for example, in a C<foreach>, C<map>
2967 or another C<grep>) actually modifies the element in the original list.
2968 This is usually something to be avoided when writing clear code.
2970 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2971 been declared with the deprecated C<my $_> construct)
2972 then, in addition to being locally aliased to
2973 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2974 can't be seen from the outside, avoiding any potential side-effects.
2976 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2979 X<hex> X<hexadecimal>
2983 =for Pod::Functions convert a string to a hexadecimal number
2985 Interprets EXPR as a hex string and returns the corresponding value.
2986 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2987 L</oct>.) If EXPR is omitted, uses C<$_>.
2989 print hex '0xAf'; # prints '175'
2990 print hex 'aF'; # same
2992 Hex strings may only represent integers. Strings that would cause
2993 integer overflow trigger a warning. Leading whitespace is not stripped,
2994 unlike oct(). To present something as hex, look into L</printf>,
2995 L</sprintf>, and L</unpack>.
3000 =for Pod::Functions patch a module's namespace into your own
3002 There is no builtin C<import> function. It is just an ordinary
3003 method (subroutine) defined (or inherited) by modules that wish to export
3004 names to another module. The C<use> function calls the C<import> method
3005 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3007 =item index STR,SUBSTR,POSITION
3008 X<index> X<indexOf> X<InStr>
3010 =item index STR,SUBSTR
3012 =for Pod::Functions find a substring within a string
3014 The index function searches for one string within another, but without
3015 the wildcard-like behavior of a full regular-expression pattern match.
3016 It returns the position of the first occurrence of SUBSTR in STR at
3017 or after POSITION. If POSITION is omitted, starts searching from the
3018 beginning of the string. POSITION before the beginning of the string
3019 or after its end is treated as if it were the beginning or the end,
3020 respectively. POSITION and the return value are based at zero.
3021 If the substring is not found, C<index> returns -1.
3024 X<int> X<integer> X<truncate> X<trunc> X<floor>
3028 =for Pod::Functions get the integer portion of a number
3030 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3031 You should not use this function for rounding: one because it truncates
3032 towards C<0>, and two because machine representations of floating-point
3033 numbers can sometimes produce counterintuitive results. For example,
3034 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3035 because it's really more like -268.99999999999994315658 instead. Usually,
3036 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3037 functions will serve you better than will int().
3039 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3042 =for Pod::Functions system-dependent device control system call
3044 Implements the ioctl(2) function. You'll probably first have to say
3046 require "sys/ioctl.ph"; # probably in
3047 # $Config{archlib}/sys/ioctl.ph
3049 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3050 exist or doesn't have the correct definitions you'll have to roll your
3051 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3052 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3053 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3054 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3055 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3056 has no string value but does have a numeric value, that value will be
3057 passed rather than a pointer to the string value. To guarantee this to be
3058 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3059 functions may be needed to manipulate the values of structures used by
3062 The return value of C<ioctl> (and C<fcntl>) is as follows:
3064 if OS returns: then Perl returns:
3066 0 string "0 but true"
3067 anything else that number
3069 Thus Perl returns true on success and false on failure, yet you can
3070 still easily determine the actual value returned by the operating
3073 $retval = ioctl(...) || -1;
3074 printf "System returned %d\n", $retval;
3076 The special string C<"0 but true"> is exempt from B<-w> complaints
3077 about improper numeric conversions.
3079 Portability issues: L<perlport/ioctl>.
3081 =item join EXPR,LIST
3084 =for Pod::Functions join a list into a string using a separator
3086 Joins the separate strings of LIST into a single string with fields
3087 separated by the value of EXPR, and returns that new string. Example:
3089 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3091 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3092 first argument. Compare L</split>.
3101 =for Pod::Functions retrieve list of indices from a hash
3103 Called in list context, returns a list consisting of all the keys of the
3104 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3105 releases prior to 5.12 will produce a syntax error if you try to use an
3106 array argument. In scalar context, returns the number of keys or indices.
3108 The keys of a hash are returned in an apparently random order. The actual
3109 random order is subject to change in future versions of Perl, but it
3110 is guaranteed to be the same order as either the C<values> or C<each>
3111 function produces (given that the hash has not been modified). Since
3112 Perl 5.8.1 the ordering can be different even between different runs of
3113 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
3116 As a side effect, calling keys() resets the internal iterator of the HASH or
3117 ARRAY (see L</each>). In particular, calling keys() in void context resets
3118 the iterator with no other overhead.
3120 Here is yet another way to print your environment:
3123 @values = values %ENV;
3125 print pop(@keys), '=', pop(@values), "\n";
3128 or how about sorted by key:
3130 foreach $key (sort(keys %ENV)) {
3131 print $key, '=', $ENV{$key}, "\n";
3134 The returned values are copies of the original keys in the hash, so
3135 modifying them will not affect the original hash. Compare L</values>.
3137 To sort a hash by value, you'll need to use a C<sort> function.
3138 Here's a descending numeric sort of a hash by its values:
3140 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3141 printf "%4d %s\n", $hash{$key}, $key;
3144 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3145 allocated for the given hash. This can gain you a measure of efficiency if
3146 you know the hash is going to get big. (This is similar to pre-extending
3147 an array by assigning a larger number to $#array.) If you say
3151 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3152 in fact, since it rounds up to the next power of two. These
3153 buckets will be retained even if you do C<%hash = ()>, use C<undef
3154 %hash> if you want to free the storage while C<%hash> is still in scope.
3155 You can't shrink the number of buckets allocated for the hash using
3156 C<keys> in this way (but you needn't worry about doing this by accident,
3157 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3160 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3161 a reference to an unblessed hash or array. The argument will be
3162 dereferenced automatically. This aspect of C<keys> is considered highly
3163 experimental. The exact behaviour may change in a future version of Perl.
3165 for (keys $hashref) { ... }
3166 for (keys $obj->get_arrayref) { ... }
3168 To avoid confusing would-be users of your code who are running earlier
3169 versions of Perl with mysterious syntax errors, put this sort of thing at
3170 the top of your file to signal that your code will work I<only> on Perls of
3173 use 5.012; # so keys/values/each work on arrays
3174 use 5.014; # so keys/values/each work on scalars (experimental)
3176 See also C<each>, C<values>, and C<sort>.
3178 =item kill SIGNAL, LIST
3183 =for Pod::Functions send a signal to a process or process group
3185 Sends a signal to a list of processes. Returns the number of
3186 processes successfully signaled (which is not necessarily the
3187 same as the number actually killed).
3189 $cnt = kill 1, $child1, $child2;
3192 If SIGNAL is zero, no signal is sent to the process, but C<kill>
3193 checks whether it's I<possible> to send a signal to it (that
3194 means, to be brief, that the process is owned by the same user, or we are
3195 the super-user). This is useful to check that a child process is still
3196 alive (even if only as a zombie) and hasn't changed its UID. See
3197 L<perlport> for notes on the portability of this construct.
3199 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
3200 of processes. That means you usually
3201 want to use positive not negative signals.
3203 You may also use a signal name in quotes. A negative signal name is the
3204 same as a negative signal number, killing process groups instead of processes.
3205 For example, C<kill -KILL, $pgrp> will send C<SIGKILL> to the entire process
3208 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3209 the operating system. For example, on POSIX-conforming systems, zero will
3210 signal the current process group, -1 will signal all processes, and any
3211 other negative PROCESS number will act as a negative signal number and
3212 kill the entire process group specified.
3214 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3215 A warning may be produced in a future version.
3217 See L<perlipc/"Signals"> for more details.
3219 On some platforms such as Windows where the fork() system call is not available.
3220 Perl can be built to emulate fork() at the interpreter level.
3221 This emulation has limitations related to kill that have to be considered,
3222 for code running on Windows and in code intended to be portable.
3224 See L<perlfork> for more details.
3226 If there is no I<LIST> of processes, no signal is sent, and the return
3227 value is 0. This form is sometimes used, however, because it causes
3228 tainting checks to be run. But see
3229 L<perlsec/Laundering and Detecting Tainted Data>.
3231 Portability issues: L<perlport/kill>.
3240 =for Pod::Functions exit a block prematurely
3242 The C<last> command is like the C<break> statement in C (as used in
3243 loops); it immediately exits the loop in question. If the LABEL is
3244 omitted, the command refers to the innermost enclosing
3245 loop. The C<last EXPR> form, available starting in Perl
3246 5.18.0, allows a label name to be computed at run time,
3247 and is otherwise identical to C<last LABEL>. The
3248 C<continue> block, if any, is not executed:
3250 LINE: while (<STDIN>) {
3251 last LINE if /^$/; # exit when done with header
3255 C<last> cannot be used to exit a block that returns a value such as
3256 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3257 a grep() or map() operation.
3259 Note that a block by itself is semantically identical to a loop
3260 that executes once. Thus C<last> can be used to effect an early
3261 exit out of such a block.
3263 See also L</continue> for an illustration of how C<last>, C<next>, and
3266 Unlike most named operators, this has the same precedence as assignment.
3267 It is also exempt from the looks-like-a-function rule, so
3268 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3276 =for Pod::Functions return lower-case version of a string
3278 Returns a lowercased version of EXPR. This is the internal function
3279 implementing the C<\L> escape in double-quoted strings.
3281 If EXPR is omitted, uses C<$_>.
3283 What gets returned depends on several factors:
3287 =item If C<use bytes> is in effect:
3291 =item On EBCDIC platforms
3293 The results are what the C language system call C<tolower()> returns.
3295 =item On ASCII platforms
3297 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3302 =item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3304 Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3305 semantics for the remaining code points (this last can only happen if
3306 the UTF8 flag is also set). See L<perllocale>.
3308 A deficiency in this is that case changes that cross the 255/256
3309 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3310 LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3311 platforms). But under C<use locale>, the lower case of U+1E9E is
3312 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3313 current locale, and Perl has no way of knowing if that character even
3314 exists in the locale, much less what code point it is. Perl returns
3315 the input character unchanged, for all instances (and there aren't
3316 many) where the 255/256 boundary would otherwise be crossed.
3318 =item Otherwise, If EXPR has the UTF8 flag set:
3320 Unicode semantics are used for the case change.
3322 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect:
3324 Unicode semantics are used for the case change.
3330 =item On EBCDIC platforms
3332 The results are what the C language system call C<tolower()> returns.
3334 =item On ASCII platforms
3336 ASCII semantics are used for the case change. The lowercase of any character
3337 outside the ASCII range is the character itself.
3344 X<lcfirst> X<lowercase>
3348 =for Pod::Functions return a string with just the next letter in lower case
3350 Returns the value of EXPR with the first character lowercased. This
3351 is the internal function implementing the C<\l> escape in
3352 double-quoted strings.
3354 If EXPR is omitted, uses C<$_>.
3356 This function behaves the same way under various pragmata, such as in a locale,
3364 =for Pod::Functions return the number of characters in a string
3366 Returns the length in I<characters> of the value of EXPR. If EXPR is
3367 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3370 This function cannot be used on an entire array or hash to find out how
3371 many elements these have. For that, use C<scalar @array> and C<scalar keys
3372 %hash>, respectively.
3374 Like all Perl character operations, length() normally deals in logical
3375 characters, not physical bytes. For how many bytes a string encoded as
3376 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3377 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3382 =for Pod::Functions the current source line number
3384 A special token that compiles to the current line number.
3386 =item link OLDFILE,NEWFILE
3389 =for Pod::Functions create a hard link in the filesystem
3391 Creates a new filename linked to the old filename. Returns true for
3392 success, false otherwise.
3394 Portability issues: L<perlport/link>.
3396 =item listen SOCKET,QUEUESIZE
3399 =for Pod::Functions register your socket as a server
3401 Does the same thing that the listen(2) system call does. Returns true if
3402 it succeeded, false otherwise. See the example in
3403 L<perlipc/"Sockets: Client/Server Communication">.
3408 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3410 You really probably want to be using C<my> instead, because C<local> isn't
3411 what most people think of as "local". See
3412 L<perlsub/"Private Variables via my()"> for details.
3414 A local modifies the listed variables to be local to the enclosing
3415 block, file, or eval. If more than one value is listed, the list must
3416 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3417 for details, including issues with tied arrays and hashes.
3419 The C<delete local EXPR> construct can also be used to localize the deletion
3420 of array/hash elements to the current block.
3421 See L<perlsub/"Localized deletion of elements of composite types">.
3423 =item localtime EXPR
3424 X<localtime> X<ctime>
3428 =for Pod::Functions convert UNIX time into record or string using local time
3430 Converts a time as returned by the time function to a 9-element list
3431 with the time analyzed for the local time zone. Typically used as
3435 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3438 All list elements are numeric and come straight out of the C `struct
3439 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3440 of the specified time.
3442 C<$mday> is the day of the month and C<$mon> the month in
3443 the range C<0..11>, with 0 indicating January and 11 indicating December.
3444 This makes it easy to get a month name from a list:
3446 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3447 print "$abbr[$mon] $mday";
3448 # $mon=9, $mday=18 gives "Oct 18"
3450 C<$year> contains the number of years since 1900. To get a 4-digit
3455 To get the last two digits of the year (e.g., "01" in 2001) do:
3457 $year = sprintf("%02d", $year % 100);
3459 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3460 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3461 (or C<0..365> in leap years.)
3463 C<$isdst> is true if the specified time occurs during Daylight Saving
3464 Time, false otherwise.
3466 If EXPR is omitted, C<localtime()> uses the current time (as returned
3469 In scalar context, C<localtime()> returns the ctime(3) value:
3471 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3473 The format of this scalar value is B<not> locale-dependent
3474 but built into Perl. For GMT instead of local
3475 time use the L</gmtime> builtin. See also the
3476 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3477 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3478 and mktime(3) functions.
3480 To get somewhat similar but locale-dependent date strings, set up your
3481 locale environment variables appropriately (please see L<perllocale>) and
3484 use POSIX qw(strftime);
3485 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3486 # or for GMT formatted appropriately for your locale:
3487 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3489 Note that the C<%a> and C<%b>, the short forms of the day of the week
3490 and the month of the year, may not necessarily be three characters wide.
3492 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3493 by-name access mechanism to the gmtime() and localtime() functions,
3496 For a comprehensive date and time representation look at the
3497 L<DateTime> module on CPAN.
3499 Portability issues: L<perlport/localtime>.
3504 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3506 This function places an advisory lock on a shared variable or referenced
3507 object contained in I<THING> until the lock goes out of scope.
3509 The value returned is the scalar itself, if the argument is a scalar, or a
3510 reference, if the argument is a hash, array or subroutine.
3512 lock() is a "weak keyword" : this means that if you've defined a function
3513 by this name (before any calls to it), that function will be called
3514 instead. If you are not under C<use threads::shared> this does nothing.
3515 See L<threads::shared>.
3518 X<log> X<logarithm> X<e> X<ln> X<base>
3522 =for Pod::Functions retrieve the natural logarithm for a number
3524 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3525 returns the log of C<$_>. To get the
3526 log of another base, use basic algebra:
3527 The base-N log of a number is equal to the natural log of that number
3528 divided by the natural log of N. For example:
3532 return log($n)/log(10);
3535 See also L</exp> for the inverse operation.
3537 =item lstat FILEHANDLE
3542 =item lstat DIRHANDLE
3546 =for Pod::Functions stat a symbolic link
3548 Does the same thing as the C<stat> function (including setting the
3549 special C<_> filehandle) but stats a symbolic link instead of the file
3550 the symbolic link points to. If symbolic links are unimplemented on
3551 your system, a normal C<stat> is done. For much more detailed
3552 information, please see the documentation for C<stat>.
3554 If EXPR is omitted, stats C<$_>.
3556 Portability issues: L<perlport/lstat>.
3560 =for Pod::Functions match a string with a regular expression pattern
3562 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3564 =item map BLOCK LIST
3569 =for Pod::Functions apply a change to a list to get back a new list with the changes
3571 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3572 C<$_> to each element) and returns the list value composed of the
3573 results of each such evaluation. In scalar context, returns the
3574 total number of elements so generated. Evaluates BLOCK or EXPR in
3575 list context, so each element of LIST may produce zero, one, or
3576 more elements in the returned value.
3578 @chars = map(chr, @numbers);
3580 translates a list of numbers to the corresponding characters.
3582 my @squares = map { $_ * $_ } @numbers;
3584 translates a list of numbers to their squared values.
3586 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3588 shows that number of returned elements can differ from the number of
3589 input elements. To omit an element, return an empty list ().
3590 This could also be achieved by writing
3592 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3594 which makes the intention more clear.
3596 Map always returns a list, which can be
3597 assigned to a hash such that the elements
3598 become key/value pairs. See L<perldata> for more details.
3600 %hash = map { get_a_key_for($_) => $_ } @array;
3602 is just a funny way to write
3606 $hash{get_a_key_for($_)} = $_;
3609 Note that C<$_> is an alias to the list value, so it can be used to
3610 modify the elements of the LIST. While this is useful and supported,
3611 it can cause bizarre results if the elements of LIST are not variables.
3612 Using a regular C<foreach> loop for this purpose would be clearer in
3613 most cases. See also L</grep> for an array composed of those items of
3614 the original list for which the BLOCK or EXPR evaluates to true.
3616 If C<$_> is lexical in the scope where the C<map> appears (because it has
3617 been declared with the deprecated C<my $_> construct),
3618 then, in addition to being locally aliased to
3619 the list elements, C<$_> keeps being lexical inside the block; that is, it
3620 can't be seen from the outside, avoiding any potential side-effects.
3622 C<{> starts both hash references and blocks, so C<map { ...> could be either
3623 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3624 ahead for the closing C<}> it has to take a guess at which it's dealing with
3625 based on what it finds just after the
3626 C<{>. Usually it gets it right, but if it
3627 doesn't it won't realize something is wrong until it gets to the C<}> and
3628 encounters the missing (or unexpected) comma. The syntax error will be
3629 reported close to the C<}>, but you'll need to change something near the C<{>
3630 such as using a unary C<+> to give Perl some help:
3632 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3633 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3634 %hash = map { ("\L$_" => 1) } @array # this also works
3635 %hash = map { lc($_) => 1 } @array # as does this.
3636 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3638 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3640 or to force an anon hash constructor use C<+{>:
3642 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3645 to get a list of anonymous hashes each with only one entry apiece.
3647 =item mkdir FILENAME,MASK
3648 X<mkdir> X<md> X<directory, create>
3650 =item mkdir FILENAME
3654 =for Pod::Functions create a directory
3656 Creates the directory specified by FILENAME, with permissions
3657 specified by MASK (as modified by C<umask>). If it succeeds it
3658 returns true; otherwise it returns false and sets C<$!> (errno).
3659 MASK defaults to 0777 if omitted, and FILENAME defaults
3660 to C<$_> if omitted.
3662 In general, it is better to create directories with a permissive MASK
3663 and let the user modify that with their C<umask> than it is to supply
3664 a restrictive MASK and give the user no way to be more permissive.
3665 The exceptions to this rule are when the file or directory should be
3666 kept private (mail files, for instance). The perlfunc(1) entry on
3667 C<umask> discusses the choice of MASK in more detail.
3669 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3670 number of trailing slashes. Some operating and filesystems do not get
3671 this right, so Perl automatically removes all trailing slashes to keep
3674 To recursively create a directory structure, look at
3675 the C<mkpath> function of the L<File::Path> module.
3677 =item msgctl ID,CMD,ARG
3680 =for Pod::Functions SysV IPC message control operations
3682 Calls the System V IPC function msgctl(2). You'll probably have to say
3686 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3687 then ARG must be a variable that will hold the returned C<msqid_ds>
3688 structure. Returns like C<ioctl>: the undefined value for error,
3689 C<"0 but true"> for zero, or the actual return value otherwise. See also
3690 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3693 Portability issues: L<perlport/msgctl>.
3695 =item msgget KEY,FLAGS
3698 =for Pod::Functions get SysV IPC message queue
3700 Calls the System V IPC function msgget(2). Returns the message queue
3701 id, or C<undef> on error. See also
3702 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3705 Portability issues: L<perlport/msgget>.
3707 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3710 =for Pod::Functions receive a SysV IPC message from a message queue
3712 Calls the System V IPC function msgrcv to receive a message from
3713 message queue ID into variable VAR with a maximum message size of
3714 SIZE. Note that when a message is received, the message type as a
3715 native long integer will be the first thing in VAR, followed by the
3716 actual message. This packing may be opened with C<unpack("l! a*")>.
3717 Taints the variable. Returns true if successful, false
3718 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3719 C<IPC::SysV> and C<IPC::SysV::Msg>.
3721 Portability issues: L<perlport/msgrcv>.
3723 =item msgsnd ID,MSG,FLAGS
3726 =for Pod::Functions send a SysV IPC message to a message queue
3728 Calls the System V IPC function msgsnd to send the message MSG to the
3729 message queue ID. MSG must begin with the native long integer message
3730 type, be followed by the length of the actual message, and then finally
3731 the message itself. This kind of packing can be achieved with
3732 C<pack("l! a*", $type, $message)>. Returns true if successful,
3733 false on error. See also the C<IPC::SysV>
3734 and C<IPC::SysV::Msg> documentation.
3736 Portability issues: L<perlport/msgsnd>.
3743 =item my EXPR : ATTRS
3745 =item my TYPE EXPR : ATTRS
3747 =for Pod::Functions declare and assign a local variable (lexical scoping)
3749 A C<my> declares the listed variables to be local (lexically) to the
3750 enclosing block, file, or C<eval>. If more than one value is listed,
3751 the list must be placed in parentheses.
3753 The exact semantics and interface of TYPE and ATTRS are still
3754 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3755 and attributes are handled using the C<attributes> pragma, or starting
3756 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3757 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3758 L<attributes>, and L<Attribute::Handlers>.
3767 =for Pod::Functions iterate a block prematurely
3769 The C<next> command is like the C<continue> statement in C; it starts
3770 the next iteration of the loop:
3772 LINE: while (<STDIN>) {
3773 next LINE if /^#/; # discard comments
3777 Note that if there were a C<continue> block on the above, it would get
3778 executed even on discarded lines. If LABEL is omitted, the command
3779 refers to the innermost enclosing loop. The C<next EXPR> form, available
3780 as of Perl 5.18.0, allows a label name to be computed at run time, being
3781 otherwise identical to C<next LABEL>.
3783 C<next> cannot be used to exit a block which returns a value such as
3784 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3785 a grep() or map() operation.
3787 Note that a block by itself is semantically identical to a loop
3788 that executes once. Thus C<next> will exit such a block early.
3790 See also L</continue> for an illustration of how C<last>, C<next>, and
3793 Unlike most named operators, this has the same precedence as assignment.
3794 It is also exempt from the looks-like-a-function rule, so
3795 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3798 =item no MODULE VERSION LIST
3802 =item no MODULE VERSION
3804 =item no MODULE LIST
3810 =for Pod::Functions unimport some module symbols or semantics at compile time
3812 See the C<use> function, of which C<no> is the opposite.
3815 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3819 =for Pod::Functions convert a string to an octal number
3821 Interprets EXPR as an octal string and returns the corresponding
3822 value. (If EXPR happens to start off with C<0x>, interprets it as a
3823 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3824 binary string. Leading whitespace is ignored in all three cases.)
3825 The following will handle decimal, binary, octal, and hex in standard
3828 $val = oct($val) if $val =~ /^0/;
3830 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3831 in octal), use sprintf() or printf():
3833 $dec_perms = (stat("filename"))[2] & 07777;
3834 $oct_perm_str = sprintf "%o", $perms;
3836 The oct() function is commonly used when a string such as C<644> needs
3837 to be converted into a file mode, for example. Although Perl
3838 automatically converts strings into numbers as needed, this automatic
3839 conversion assumes base 10.
3841 Leading white space is ignored without warning, as too are any trailing
3842 non-digits, such as a decimal point (C<oct> only handles non-negative
3843 integers, not negative integers or floating point).
3845 =item open FILEHANDLE,EXPR
3846 X<open> X<pipe> X<file, open> X<fopen>
3848 =item open FILEHANDLE,MODE,EXPR
3850 =item open FILEHANDLE,MODE,EXPR,LIST
3852 =item open FILEHANDLE,MODE,REFERENCE
3854 =item open FILEHANDLE
3856 =for Pod::Functions open a file, pipe, or descriptor
3858 Opens the file whose filename is given by EXPR, and associates it with
3861 Simple examples to open a file for reading:
3863 open(my $fh, "<", "input.txt")
3864 or die "cannot open < input.txt: $!";
3868 open(my $fh, ">", "output.txt")
3869 or die "cannot open > output.txt: $!";
3871 (The following is a comprehensive reference to open(): for a gentler
3872 introduction you may consider L<perlopentut>.)
3874 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3875 new filehandle is autovivified, meaning that the variable is assigned a
3876 reference to a newly allocated anonymous filehandle. Otherwise if
3877 FILEHANDLE is an expression, its value is the real filehandle. (This is
3878 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3881 If EXPR is omitted, the global (package) scalar variable of the same
3882 name as the FILEHANDLE contains the filename. (Note that lexical
3883 variables--those declared with C<my> or C<state>--will not work for this
3884 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3887 If three (or more) arguments are specified, the open mode (including
3888 optional encoding) in the second argument are distinct from the filename in
3889 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3890 If MODE is C<< > >>, the file is opened for output, with existing files
3891 first being truncated ("clobbered") and nonexisting files newly created.
3892 If MODE is C<<< >> >>>, the file is opened for appending, again being
3893 created if necessary.
3895 You can put a C<+> in front of the C<< > >> or C<< < >> to
3896 indicate that you want both read and write access to the file; thus
3897 C<< +< >> is almost always preferred for read/write updates--the
3898 C<< +> >> mode would clobber the file first. You can't usually use
3899 either read-write mode for updating textfiles, since they have
3900 variable-length records. See the B<-i> switch in L<perlrun> for a
3901 better approach. The file is created with permissions of C<0666>
3902 modified by the process's C<umask> value.
3904 These various prefixes correspond to the fopen(3) modes of C<r>,
3905 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3907 In the one- and two-argument forms of the call, the mode and filename
3908 should be concatenated (in that order), preferably separated by white
3909 space. You can--but shouldn't--omit the mode in these forms when that mode
3910 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3911 the filename argument is a known literal.
3913 For three or more arguments if MODE is C<|->, the filename is
3914 interpreted as a command to which output is to be piped, and if MODE
3915 is C<-|>, the filename is interpreted as a command that pipes
3916 output to us. In the two-argument (and one-argument) form, one should
3917 replace dash (C<->) with the command.
3918 See L<perlipc/"Using open() for IPC"> for more examples of this.
3919 (You are not allowed to C<open> to a command that pipes both in I<and>
3920 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3921 L<perlipc/"Bidirectional Communication with Another Process"> for
3924 In the form of pipe opens taking three or more arguments, if LIST is specified
3925 (extra arguments after the command name) then LIST becomes arguments
3926 to the command invoked if the platform supports it. The meaning of
3927 C<open> with more than three arguments for non-pipe modes is not yet
3928 defined, but experimental "layers" may give extra LIST arguments
3931 In the two-argument (and one-argument) form, opening C<< <- >>
3932 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3934 You may (and usually should) use the three-argument form of open to specify
3935 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3936 that affect how the input and output are processed (see L<open> and
3937 L<PerlIO> for more details). For example:
3939 open(my $fh, "<:encoding(UTF-8)", "filename")
3940 || die "can't open UTF-8 encoded filename: $!";
3942 opens the UTF8-encoded file containing Unicode characters;
3943 see L<perluniintro>. Note that if layers are specified in the
3944 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3945 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3946 Those layers will also be ignored if you specifying a colon with no name
3947 following it. In that case the default layer for the operating system
3948 (:raw on Unix, :crlf on Windows) is used.
3950 Open returns nonzero on success, the undefined value otherwise. If
3951 the C<open> involved a pipe, the return value happens to be the pid of
3954 If you're running Perl on a system that distinguishes between text
3955 files and binary files, then you should check out L</binmode> for tips
3956 for dealing with this. The key distinction between systems that need
3957 C<binmode> and those that don't is their text file formats. Systems
3958 like Unix, Mac OS, and Plan 9, that end lines with a single
3959 character and encode that character in C as C<"\n"> do not
3960 need C<binmode>. The rest need it.
3962 When opening a file, it's seldom a good idea to continue
3963 if the request failed, so C<open> is frequently used with
3964 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3965 where you want to format a suitable error message (but there are
3966 modules that can help with that problem)) always check
3967 the return value from opening a file.
3969 As a special case the three-argument form with a read/write mode and the third
3970 argument being C<undef>:
3972 open(my $tmp, "+>", undef) or die ...
3974 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3975 works for symmetry, but you really should consider writing something
3976 to the temporary file first. You will need to seek() to do the
3979 Perl is built using PerlIO by default; Unless you've
3980 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3981 open filehandles directly to Perl scalars via:
3983 open($fh, ">", \$variable) || ..
3985 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3988 open(STDOUT, ">", \$variable)
3989 or die "Can't open STDOUT: $!";
3994 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3995 while (<ARTICLE>) {...
3997 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3998 # if the open fails, output is discarded
4000 open(my $dbase, "+<", "dbase.mine") # open for update
4001 or die "Can't open 'dbase.mine' for update: $!";
4003 open(my $dbase, "+<dbase.mine") # ditto
4004 or die "Can't open 'dbase.mine' for update: $!";
4006 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4007 or die "Can't start caesar: $!";
4009 open(ARTICLE, "caesar <$article |") # ditto
4010 or die "Can't start caesar: $!";
4012 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4013 or die "Can't start sort: $!";
4016 open(MEMORY, ">", \$var)
4017 or die "Can't open memory file: $!";
4018 print MEMORY "foo!\n"; # output will appear in $var
4020 # process argument list of files along with any includes
4022 foreach $file (@ARGV) {
4023 process($file, "fh00");
4027 my($filename, $input) = @_;
4028 $input++; # this is a string increment
4029 unless (open($input, "<", $filename)) {
4030 print STDERR "Can't open $filename: $!\n";
4035 while (<$input>) { # note use of indirection
4036 if (/^#include "(.*)"/) {
4037 process($1, $input);
4044 See L<perliol> for detailed info on PerlIO.
4046 You may also, in the Bourne shell tradition, specify an EXPR beginning
4047 with C<< >& >>, in which case the rest of the string is interpreted
4048 as the name of a filehandle (or file descriptor, if numeric) to be
4049 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4050 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4051 The mode you specify should match the mode of the original filehandle.
4052 (Duping a filehandle does not take into account any existing contents
4053 of IO buffers.) If you use the three-argument
4054 form, then you can pass either a
4055 number, the name of a filehandle, or the normal "reference to a glob".
4057 Here is a script that saves, redirects, and restores C<STDOUT> and
4058 C<STDERR> using various methods:
4061 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4062 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4064 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4065 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4067 select STDERR; $| = 1; # make unbuffered
4068 select STDOUT; $| = 1; # make unbuffered
4070 print STDOUT "stdout 1\n"; # this works for
4071 print STDERR "stderr 1\n"; # subprocesses too
4073 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4074 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4076 print STDOUT "stdout 2\n";
4077 print STDERR "stderr 2\n";
4079 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4080 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4081 that file descriptor (and not call C<dup(2)>); this is more
4082 parsimonious of file descriptors. For example:
4084 # open for input, reusing the fileno of $fd
4085 open(FILEHANDLE, "<&=$fd")
4089 open(FILEHANDLE, "<&=", $fd)
4093 # open for append, using the fileno of OLDFH
4094 open(FH, ">>&=", OLDFH)
4098 open(FH, ">>&=OLDFH")
4100 Being parsimonious on filehandles is also useful (besides being
4101 parsimonious) for example when something is dependent on file
4102 descriptors, like for example locking using flock(). If you do just
4103 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4104 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4105 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4106 the same underlying system file descriptor.
4108 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4109 fdopen() to implement the C<=> functionality. On many Unix systems,
4110 fdopen() fails when file descriptors exceed a certain value, typically 255.
4111 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4113 You can see whether your Perl was built with PerlIO by running C<perl -V>
4114 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4115 have PerlIO; otherwise you don't.
4117 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4118 with the one- or two-argument forms of C<open>),
4119 an implicit C<fork> is done, so C<open> returns twice: in the parent
4120 process it returns the pid
4121 of the child process, and in the child process it returns (a defined) C<0>.
4122 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4124 For example, use either
4126 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4130 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4136 # either write TO_KID or else read FROM_KID
4138 waitpid $child_pid, 0;
4140 # am the child; use STDIN/STDOUT normally
4145 The filehandle behaves normally for the parent, but I/O to that
4146 filehandle is piped from/to the STDOUT/STDIN of the child process.
4147 In the child process, the filehandle isn't opened--I/O happens from/to
4148 the new STDOUT/STDIN. Typically this is used like the normal
4149 piped open when you want to exercise more control over just how the
4150 pipe command gets executed, such as when running setuid and
4151 you don't want to have to scan shell commands for metacharacters.
4153 The following blocks are more or less equivalent:
4155 open(FOO, "|tr '[a-z]' '[A-Z]'");
4156 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4157 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4158 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4160 open(FOO, "cat -n '$file'|");
4161 open(FOO, "-|", "cat -n '$file'");
4162 open(FOO, "-|") || exec "cat", "-n", $file;
4163 open(FOO, "-|", "cat", "-n", $file);
4165 The last two examples in each block show the pipe as "list form", which is
4166 not yet supported on all platforms. A good rule of thumb is that if
4167 your platform has a real C<fork()> (in other words, if your platform is
4168 Unix, including Linux and MacOS X), you can use the list form. You would
4169 want to use the list form of the pipe so you can pass literal arguments
4170 to the command without risk of the shell interpreting any shell metacharacters
4171 in them. However, this also bars you from opening pipes to commands
4172 that intentionally contain shell metacharacters, such as:
4174 open(FOO, "|cat -n | expand -4 | lpr")
4175 // die "Can't open pipeline to lpr: $!";
4177 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4179 Perl will attempt to flush all files opened for
4180 output before any operation that may do a fork, but this may not be
4181 supported on some platforms (see L<perlport>). To be safe, you may need
4182 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4183 of C<IO::Handle> on any open handles.
4185 On systems that support a close-on-exec flag on files, the flag will
4186 be set for the newly opened file descriptor as determined by the value
4187 of C<$^F>. See L<perlvar/$^F>.
4189 Closing any piped filehandle causes the parent process to wait for the
4190 child to finish, then returns the status value in C<$?> and
4191 C<${^CHILD_ERROR_NATIVE}>.
4193 The filename passed to the one- and two-argument forms of open() will
4194 have leading and trailing whitespace deleted and normal
4195 redirection characters honored. This property, known as "magic open",
4196 can often be used to good effect. A user could specify a filename of
4197 F<"rsh cat file |">, or you could change certain filenames as needed:
4199 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4200 open(FH, $filename) or die "Can't open $filename: $!";
4202 Use the three-argument form to open a file with arbitrary weird characters in it,
4204 open(FOO, "<", $file)
4205 || die "can't open < $file: $!";
4207 otherwise it's necessary to protect any leading and trailing whitespace:
4209 $file =~ s#^(\s)#./$1#;
4210 open(FOO, "< $file\0")
4211 || die "open failed: $!";
4213 (this may not work on some bizarre filesystems). One should
4214 conscientiously choose between the I<magic> and I<three-argument> form
4217 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4219 will allow the user to specify an argument of the form C<"rsh cat file |">,
4220 but will not work on a filename that happens to have a trailing space, while
4222 open(IN, "<", $ARGV[0])
4223 || die "can't open < $ARGV[0]: $!";
4225 will have exactly the opposite restrictions.
4227 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4228 should use the C<sysopen> function, which involves no such magic (but may
4229 use subtly different filemodes than Perl open(), which is mapped to C
4230 fopen()). This is another way to protect your filenames from
4231 interpretation. For example:
4234 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4235 or die "sysopen $path: $!";
4236 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4237 print HANDLE "stuff $$\n";
4239 print "File contains: ", <HANDLE>;
4241 Using the constructor from the C<IO::Handle> package (or one of its
4242 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
4243 filehandles that have the scope of the variables used to hold them, then
4244 automatically (but silently) close once their reference counts become
4245 zero, typically at scope exit:
4249 sub read_myfile_munged {
4251 # or just leave it undef to autoviv
4252 my $handle = IO::File->new;
4253 open($handle, "<", "myfile") or die "myfile: $!";
4255 or return (); # Automatically closed here.
4256 mung($first) or die "mung failed"; # Or here.
4257 return (first, <$handle>) if $ALL; # Or here.
4258 return $first; # Or here.
4261 B<WARNING:> The previous example has a bug because the automatic
4262 close that happens when the refcount on C<handle> reaches zero does not
4263 properly detect and report failures. I<Always> close the handle
4264 yourself and inspect the return value.
4267 || warn "close failed: $!";
4269 See L</seek> for some details about mixing reading and writing.
4271 Portability issues: L<perlport/open>.
4273 =item opendir DIRHANDLE,EXPR
4276 =for Pod::Functions open a directory
4278 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4279 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4280 DIRHANDLE may be an expression whose value can be used as an indirect
4281 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4282 scalar variable (or array or hash element), the variable is assigned a
4283 reference to a new anonymous dirhandle; that is, it's autovivified.
4284 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4286 See the example at C<readdir>.
4293 =for Pod::Functions find a character's numeric representation
4295 Returns the numeric value of the first character of EXPR.
4296 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4297 (Note I<character>, not byte.)
4299 For the reverse, see L</chr>.
4300 See L<perlunicode> for more about Unicode.
4307 =item our EXPR : ATTRS
4309 =item our TYPE EXPR : ATTRS
4311 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4313 C<our> makes a lexical alias to a package variable of the same name in the current
4314 package for use within the current lexical scope.
4316 C<our> has the same scoping rules as C<my> or C<state>, but C<our> only
4317 declares an alias, whereas C<my> or C<state> both declare a variable name and
4318 allocate storage for that name within the current scope.
4320 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4321 a package variable without qualifying it with the package name, but only within
4322 the lexical scope of the C<our> declaration. In this way, C<our> differs from
4323 C<use vars>, which allows use of an unqualified name I<only> within the
4324 affected package, but across scopes.
4326 If more than one value is listed, the list must be placed
4332 An C<our> declaration declares an alias for a package variable that will be visible
4333 across its entire lexical scope, even across package boundaries. The
4334 package in which the variable is entered is determined at the point
4335 of the declaration, not at the point of use. This means the following
4339 our $bar; # declares $Foo::bar for rest of lexical scope
4343 print $bar; # prints 20, as it refers to $Foo::bar
4345 Multiple C<our> declarations with the same name in the same lexical
4346 scope are allowed if they are in different packages. If they happen
4347 to be in the same package, Perl will emit warnings if you have asked
4348 for them, just like multiple C<my> declarations. Unlike a second
4349 C<my> declaration, which will bind the name to a fresh variable, a
4350 second C<our> declaration in the same package, in the same scope, is
4355 our $bar; # declares $Foo::bar for rest of lexical scope
4359 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4360 print $bar; # prints 30
4362 our $bar; # emits warning but has no other effect
4363 print $bar; # still prints 30
4365 An C<our> declaration may also have a list of attributes associated
4368 The exact semantics and interface of TYPE and ATTRS are still
4369 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4370 and attributes are handled using the C<attributes> pragma, or, starting
4371 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4372 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4373 L<attributes>, and L<Attribute::Handlers>.
4375 =item pack TEMPLATE,LIST
4378 =for Pod::Functions convert a list into a binary representation
4380 Takes a LIST of values and converts it into a string using the rules
4381 given by the TEMPLATE. The resulting string is the concatenation of
4382 the converted values. Typically, each converted value looks
4383 like its machine-level representation. For example, on 32-bit machines
4384 an integer may be represented by a sequence of 4 bytes, which will in
4385 Perl be presented as a string that's 4 characters long.
4387 See L<perlpacktut> for an introduction to this function.
4389 The TEMPLATE is a sequence of characters that give the order and type
4390 of values, as follows:
4392 a A string with arbitrary binary data, will be null padded.
4393 A A text (ASCII) string, will be space padded.
4394 Z A null-terminated (ASCIZ) string, will be null padded.
4396 b A bit string (ascending bit order inside each byte,
4398 B A bit string (descending bit order inside each byte).
4399 h A hex string (low nybble first).
4400 H A hex string (high nybble first).
4402 c A signed char (8-bit) value.
4403 C An unsigned char (octet) value.
4404 W An unsigned char value (can be greater than 255).
4406 s A signed short (16-bit) value.
4407 S An unsigned short value.
4409 l A signed long (32-bit) value.
4410 L An unsigned long value.
4412 q A signed quad (64-bit) value.
4413 Q An unsigned quad value.
4414 (Quads are available only if your system supports 64-bit
4415 integer values _and_ if Perl has been compiled to support
4416 those. Raises an exception otherwise.)
4418 i A signed integer value.
4419 I A unsigned integer value.
4420 (This 'integer' is _at_least_ 32 bits wide. Its exact
4421 size depends on what a local C compiler calls 'int'.)
4423 n An unsigned short (16-bit) in "network" (big-endian) order.
4424 N An unsigned long (32-bit) in "network" (big-endian) order.
4425 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4426 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4428 j A Perl internal signed integer value (IV).
4429 J A Perl internal unsigned integer value (UV).
4431 f A single-precision float in native format.
4432 d A double-precision float in native format.
4434 F A Perl internal floating-point value (NV) in native format
4435 D A float of long-double precision in native format.
4436 (Long doubles are available only if your system supports
4437 long double values _and_ if Perl has been compiled to
4438 support those. Raises an exception otherwise.)
4440 p A pointer to a null-terminated string.
4441 P A pointer to a structure (fixed-length string).
4443 u A uuencoded string.
4444 U A Unicode character number. Encodes to a character in char-
4445 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4448 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4449 for details). Its bytes represent an unsigned integer in
4450 base 128, most significant digit first, with as few digits
4451 as possible. Bit eight (the high bit) is set on each byte
4454 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4456 @ Null-fill or truncate to absolute position, counted from the
4457 start of the innermost ()-group.
4458 . Null-fill or truncate to absolute position specified by
4460 ( Start of a ()-group.
4462 One or more modifiers below may optionally follow certain letters in the
4463 TEMPLATE (the second column lists letters for which the modifier is valid):
4465 ! sSlLiI Forces native (short, long, int) sizes instead
4466 of fixed (16-/32-bit) sizes.
4468 xX Make x and X act as alignment commands.
4470 nNvV Treat integers as signed instead of unsigned.
4472 @. Specify position as byte offset in the internal
4473 representation of the packed string. Efficient
4476 > sSiIlLqQ Force big-endian byte-order on the type.
4477 jJfFdDpP (The "big end" touches the construct.)
4479 < sSiIlLqQ Force little-endian byte-order on the type.
4480 jJfFdDpP (The "little end" touches the construct.)
4482 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4483 to force a particular byte-order on all components in that group,
4484 including all its subgroups.
4488 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4489 pack for processing data from NASA's Magellan probe. Magellan was in an
4490 elliptical orbit, using the antenna for the radar mapping when close to
4491 Venus and for communicating data back to Earth for the rest of the orbit.
4492 There were two transmission units, but one of these failed, and then the
4493 other developed a fault whereby it would randomly flip the sense of all the
4494 bits. It was easy to automatically detect complete records with the correct
4495 sense, and complete records with all the bits flipped. However, this didn't
4496 recover the records where the sense flipped midway. A colleague of Larry's
4497 was able to pretty much eyeball where the records flipped, so they wrote an
4498 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4499 manually correct the records and recover the data. For this purpose pack
4500 gained the hex and bit string format specifiers.
4502 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4503 27e2fb84680b9cc1), but the patch description makes no mention of their
4504 addition, let alone the story behind them.
4508 The following rules apply:
4514 Each letter may optionally be followed by a number indicating the repeat
4515 count. A numeric repeat count may optionally be enclosed in brackets, as
4516 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4517 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4518 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4519 something else, described below. Supplying a C<*> for the repeat count
4520 instead of a number means to use however many items are left, except for:
4526 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4530 <.>, where it means relative to the start of the string.
4534 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4538 One can replace a numeric repeat count with a template letter enclosed in
4539 brackets to use the packed byte length of the bracketed template for the
4542 For example, the template C<x[L]> skips as many bytes as in a packed long,
4543 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4544 variable-expanded) unpacks. If the template in brackets contains alignment
4545 commands (such as C<x![d]>), its packed length is calculated as if the
4546 start of the template had the maximal possible alignment.
4548 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4549 trailing null byte, so the resulting string is always one byte longer than
4550 the byte length of the item itself.
4552 When used with C<@>, the repeat count represents an offset from the start
4553 of the innermost C<()> group.
4555 When used with C<.>, the repeat count determines the starting position to
4556 calculate the value offset as follows:
4562 If the repeat count is C<0>, it's relative to the current position.
4566 If the repeat count is C<*>, the offset is relative to the start of the
4571 And if it's an integer I<n>, the offset is relative to the start of the
4572 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4573 bigger then the group level.
4577 The repeat count for C<u> is interpreted as the maximal number of bytes
4578 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4579 count should not be more than 65.
4583 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4584 string of length count, padding with nulls or spaces as needed. When
4585 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4586 after the first null, and C<a> returns data with no stripping at all.
4588 If the value to pack is too long, the result is truncated. If it's too
4589 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4590 followed by a null byte. Thus C<Z> always packs a trailing null, except
4591 when the count is 0.
4595 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4596 Each such format generates 1 bit of the result. These are typically followed
4597 by a repeat count like C<B8> or C<B64>.
4599 Each result bit is based on the least-significant bit of the corresponding
4600 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4601 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4603 Starting from the beginning of the input string, each 8-tuple
4604 of characters is converted to 1 character of output. With format C<b>,
4605 the first character of the 8-tuple determines the least-significant bit of a
4606 character; with format C<B>, it determines the most-significant bit of
4609 If the length of the input string is not evenly divisible by 8, the
4610 remainder is packed as if the input string were padded by null characters
4611 at the end. Similarly during unpacking, "extra" bits are ignored.
4613 If the input string is longer than needed, remaining characters are ignored.
4615 A C<*> for the repeat count uses all characters of the input field.
4616 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4620 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4621 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4623 For each such format, pack() generates 4 bits of result.
4624 With non-alphabetical characters, the result is based on the 4 least-significant
4625 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4626 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4627 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4628 is compatible with the usual hexadecimal digits, so that C<"a"> and
4629 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4630 characters with this format.
4632 Starting from the beginning of the template to pack(), each pair
4633 of characters is converted to 1 character of output. With format C<h>, the
4634 first character of the pair determines the least-significant nybble of the
4635 output character; with format C<H>, it determines the most-significant
4638 If the length of the input string is not even, it behaves as if padded by
4639 a null character at the end. Similarly, "extra" nybbles are ignored during
4642 If the input string is longer than needed, extra characters are ignored.
4644 A C<*> for the repeat count uses all characters of the input field. For
4645 unpack(), nybbles are converted to a string of hexadecimal digits.
4649 The C<p> format packs a pointer to a null-terminated string. You are
4650 responsible for ensuring that the string is not a temporary value, as that
4651 could potentially get deallocated before you got around to using the packed
4652 result. The C<P> format packs a pointer to a structure of the size indicated
4653 by the length. A null pointer is created if the corresponding value for
4654 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4655 unpacks into C<undef>.
4657 If your system has a strange pointer size--meaning a pointer is neither as
4658 big as an int nor as big as a long--it may not be possible to pack or
4659 unpack pointers in big- or little-endian byte order. Attempting to do
4660 so raises an exception.
4664 The C</> template character allows packing and unpacking of a sequence of
4665 items where the packed structure contains a packed item count followed by
4666 the packed items themselves. This is useful when the structure you're
4667 unpacking has encoded the sizes or repeat counts for some of its fields
4668 within the structure itself as separate fields.
4670 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4671 I<length-item> describes how the length value is packed. Formats likely
4672 to be of most use are integer-packing ones like C<n> for Java strings,
4673 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4675 For C<pack>, I<sequence-item> may have a repeat count, in which case
4676 the minimum of that and the number of available items is used as the argument
4677 for I<length-item>. If it has no repeat count or uses a '*', the number
4678 of available items is used.
4680 For C<unpack>, an internal stack of integer arguments unpacked so far is
4681 used. You write C</>I<sequence-item> and the repeat count is obtained by
4682 popping off the last element from the stack. The I<sequence-item> must not
4683 have a repeat count.
4685 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4686 the I<length-item> is the string length, not the number of strings. With
4687 an explicit repeat count for pack, the packed string is adjusted to that
4688 length. For example:
4690 This code: gives this result:
4692 unpack("W/a", "\004Gurusamy") ("Guru")
4693 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4694 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4696 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4697 pack("a/W2", ord("a") .. ord("z")) "2ab"
4699 The I<length-item> is not returned explicitly from C<unpack>.
4701 Supplying a count to the I<length-item> format letter is only useful with
4702 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4703 introduce C<"\000"> characters, which Perl does not regard as legal in
4708 The integer types C<s>, C<S>, C<l>, and C<L> may be
4709 followed by a C<!> modifier to specify native shorts or
4710 longs. As shown in the example above, a bare C<l> means
4711 exactly 32 bits, although the native C<long> as seen by the local C compiler
4712 may be larger. This is mainly an issue on 64-bit platforms. You can
4713 see whether using C<!> makes any difference this way:
4715 printf "format s is %d, s! is %d\n",
4716 length pack("s"), length pack("s!");
4718 printf "format l is %d, l! is %d\n",
4719 length pack("l"), length pack("l!");
4722 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4723 they are identical to C<i> and C<I>.
4725 The actual sizes (in bytes) of native shorts, ints, longs, and long
4726 longs on the platform where Perl was built are also available from
4729 $ perl -V:{short,int,long{,long}}size
4735 or programmatically via the C<Config> module:
4738 print $Config{shortsize}, "\n";
4739 print $Config{intsize}, "\n";
4740 print $Config{longsize}, "\n";
4741 print $Config{longlongsize}, "\n";
4743 C<$Config{longlongsize}> is undefined on systems without
4748 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4749 inherently non-portable between processors and operating systems because
4750 they obey native byteorder and endianness. For example, a 4-byte integer
4751 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4752 handled by the CPU registers) into bytes as
4754 0x12 0x34 0x56 0x78 # big-endian
4755 0x78 0x56 0x34 0x12 # little-endian
4757 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4758 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4759 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4760 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4762 The names I<big-endian> and I<little-endian> are comic references to the
4763 egg-eating habits of the little-endian Lilliputians and the big-endian
4764 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4765 This entered computer lingo via the paper "On Holy Wars and a Plea for
4766 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4768 Some systems may have even weirder byte orders such as
4773 You can determine your system endianness with this incantation:
4775 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4777 The byteorder on the platform where Perl was built is also available
4781 print "$Config{byteorder}\n";
4783 or from the command line:
4787 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4788 and C<"87654321"> are big-endian.
4790 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4791 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4792 immediately below. See also L<perlport>.
4796 Starting with Perl 5.10.0, integer and floating-point formats, along with
4797 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4798 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4799 or little-endian byte-order. These modifiers are especially useful
4800 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4801 64-bit integers, or floating-point values.
4803 Here are some concerns to keep in mind when using an endianness modifier:
4809 Exchanging signed integers between different platforms works only
4810 when all platforms store them in the same format. Most platforms store
4811 signed integers in two's-complement notation, so usually this is not an issue.
4815 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4816 formats on big- or little-endian machines. Otherwise, attempting to
4817 use them raises an exception.
4821 Forcing big- or little-endian byte-order on floating-point values for
4822 data exchange can work only if all platforms use the same
4823 binary representation such as IEEE floating-point. Even if all
4824 platforms are using IEEE, there may still be subtle differences. Being able
4825 to use C<< > >> or C<< < >> on floating-point values can be useful,
4826 but also dangerous if you don't know exactly what you're doing.
4827 It is not a general way to portably store floating-point values.
4831 When using C<< > >> or C<< < >> on a C<()> group, this affects
4832 all types inside the group that accept byte-order modifiers,
4833 including all subgroups. It is silently ignored for all other
4834 types. You are not allowed to override the byte-order within a group
4835 that already has a byte-order modifier suffix.
4841 Real numbers (floats and doubles) are in native machine format only.
4842 Due to the multiplicity of floating-point formats and the lack of a
4843 standard "network" representation for them, no facility for interchange has been
4844 made. This means that packed floating-point data written on one machine
4845 may not be readable on another, even if both use IEEE floating-point
4846 arithmetic (because the endianness of the memory representation is not part
4847 of the IEEE spec). See also L<perlport>.
4849 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4850 modifiers to force big- or little-endian byte-order on floating-point values.
4852 Because Perl uses doubles (or long doubles, if configured) internally for
4853 all numeric calculation, converting from double into float and thence
4854 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4855 will not in general equal $foo.
4859 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4860 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4861 where the packed string is processed in its UTF-8-encoded Unicode form on
4862 a byte-by-byte basis. Character mode is the default
4863 unless the format string starts with C<U>. You
4864 can always switch mode mid-format with an explicit
4865 C<C0> or C<U0> in the format. This mode remains in effect until the next
4866 mode change, or until the end of the C<()> group it (directly) applies to.
4868 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4869 bytes is not necessarily obvious. Probably only the first of these
4872 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4873 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4875 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4876 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4878 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4879 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4881 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4882 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4883 C3.8E.C2.B1.C3.8F.C2.89
4885 Those examples also illustrate that you should not try to use
4886 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4890 You must yourself do any alignment or padding by inserting, for example,
4891 enough C<"x">es while packing. There is no way for pack() and unpack()
4892 to know where characters are going to or coming from, so they
4893 handle their output and input as flat sequences of characters.
4897 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4898 take a repeat count either as postfix, or for unpack(), also via the C</>
4899 template character. Within each repetition of a group, positioning with
4900 C<@> starts over at 0. Therefore, the result of
4902 pack("@1A((@2A)@3A)", qw[X Y Z])
4904 is the string C<"\0X\0\0YZ">.
4908 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4909 jump forward or back to the closest position aligned at a multiple of C<count>
4910 characters. For example, to pack() or unpack() a C structure like
4913 char c; /* one signed, 8-bit character */
4918 one may need to use the template C<c x![d] d c[2]>. This assumes that
4919 doubles must be aligned to the size of double.
4921 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4926 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4927 represent signed 16-/32-bit integers in big-/little-endian order.
4928 This is portable only when all platforms sharing packed data use the
4929 same binary representation for signed integers; for example, when all
4930 platforms use two's-complement representation.
4934 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4935 White space can separate pack codes from each other, but modifiers and
4936 repeat counts must follow immediately. Breaking complex templates into
4937 individual line-by-line components, suitably annotated, can do as much to
4938 improve legibility and maintainability of pack/unpack formats as C</x> can
4939 for complicated pattern matches.
4943 If TEMPLATE requires more arguments than pack() is given, pack()
4944 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4945 than given, extra arguments are ignored.
4951 $foo = pack("WWWW",65,66,67,68);
4953 $foo = pack("W4",65,66,67,68);
4955 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4956 # same thing with Unicode circled letters.
4957 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4958 # same thing with Unicode circled letters. You don't get the
4959 # UTF-8 bytes because the U at the start of the format caused
4960 # a switch to U0-mode, so the UTF-8 bytes get joined into
4962 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4963 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4964 # This is the UTF-8 encoding of the string in the
4967 $foo = pack("ccxxcc",65,66,67,68);
4970 # NOTE: The examples above featuring "W" and "c" are true
4971 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4972 # and UTF-8. On EBCDIC systems, the first example would be
4973 # $foo = pack("WWWW",193,194,195,196);
4975 $foo = pack("s2",1,2);
4976 # "\001\000\002\000" on little-endian
4977 # "\000\001\000\002" on big-endian
4979 $foo = pack("a4","abcd","x","y","z");
4982 $foo = pack("aaaa","abcd","x","y","z");
4985 $foo = pack("a14","abcdefg");
4986 # "abcdefg\0\0\0\0\0\0\0"
4988 $foo = pack("i9pl", gmtime);
4989 # a real struct tm (on my system anyway)
4991 $utmp_template = "Z8 Z8 Z16 L";
4992 $utmp = pack($utmp_template, @utmp1);
4993 # a struct utmp (BSDish)
4995 @utmp2 = unpack($utmp_template, $utmp);
4996 # "@utmp1" eq "@utmp2"
4999 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5002 $foo = pack('sx2l', 12, 34);
5003 # short 12, two zero bytes padding, long 34
5004 $bar = pack('s@4l', 12, 34);
5005 # short 12, zero fill to position 4, long 34
5007 $baz = pack('s.l', 12, 4, 34);
5008 # short 12, zero fill to position 4, long 34
5010 $foo = pack('nN', 42, 4711);
5011 # pack big-endian 16- and 32-bit unsigned integers
5012 $foo = pack('S>L>', 42, 4711);
5014 $foo = pack('s<l<', -42, 4711);
5015 # pack little-endian 16- and 32-bit signed integers
5016 $foo = pack('(sl)<', -42, 4711);
5019 The same template may generally also be used in unpack().
5021 =item package NAMESPACE
5023 =item package NAMESPACE VERSION
5024 X<package> X<module> X<namespace> X<version>
5026 =item package NAMESPACE BLOCK
5028 =item package NAMESPACE VERSION BLOCK
5029 X<package> X<module> X<namespace> X<version>
5031 =for Pod::Functions declare a separate global namespace
5033 Declares the BLOCK or the rest of the compilation unit as being in the
5034 given namespace. The scope of the package declaration is either the
5035 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5036 itself through the end of current scope (the enclosing block, file, or
5037 C<eval>). That is, the forms without a BLOCK are operative through the end
5038 of the current scope, just like the C<my>, C<state>, and C<our> operators.
5039 All unqualified dynamic identifiers in this scope will be in the given
5040 namespace, except where overridden by another C<package> declaration or
5041 when they're one of the special identifiers that qualify into C<main::>,
5042 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5044 A package statement affects dynamic variables only, including those
5045 you've used C<local> on, but I<not> lexically-scoped variables, which are created
5046 with C<my>, C<state>, or C<our>. Typically it would be the first
5047 declaration in a file included by C<require> or C<use>. You can switch into a
5048 package in more than one place, since this only determines which default
5049 symbol table the compiler uses for the rest of that block. You can refer to
5050 identifiers in other packages than the current one by prefixing the identifier
5051 with the package name and a double colon, as in C<$SomePack::var>
5052 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5053 package as assumed. That is, C<$::sail> is equivalent to
5054 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5055 code, mostly from Perl 4).
5057 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
5058 namespace to a L<version> object with the VERSION provided. VERSION must be a
5059 "strict" style version number as defined by the L<version> module: a positive
5060 decimal number (integer or decimal-fraction) without exponentiation or else a
5061 dotted-decimal v-string with a leading 'v' character and at least three
5062 components. You should set C<$VERSION> only once per package.
5064 See L<perlmod/"Packages"> for more information about packages, modules,
5065 and classes. See L<perlsub> for other scoping issues.
5070 =for Pod::Functions +5.004 the current package
5072 A special token that returns the name of the package in which it occurs.
5074 =item pipe READHANDLE,WRITEHANDLE
5077 =for Pod::Functions open a pair of connected filehandles
5079 Opens a pair of connected pipes like the corresponding system call.
5080 Note that if you set up a loop of piped processes, deadlock can occur
5081 unless you are very careful. In addition, note that Perl's pipes use
5082 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5083 after each command, depending on the application.
5085 Returns true on success.
5087 See L<IPC::Open2>, L<IPC::Open3>, and
5088 L<perlipc/"Bidirectional Communication with Another Process">
5089 for examples of such things.
5091 On systems that support a close-on-exec flag on files, that flag is set
5092 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5093 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5102 =for Pod::Functions remove the last element from an array and return it
5104 Pops and returns the last value of the array, shortening the array by
5107 Returns the undefined value if the array is empty, although this may also
5108 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5109 main program, but the C<@_> array in subroutines, just like C<shift>.
5111 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
5112 reference to an unblessed array. The argument will be dereferenced
5113 automatically. This aspect of C<pop> is considered highly experimental.
5114 The exact behaviour may change in a future version of Perl.
5116 To avoid confusing would-be users of your code who are running earlier
5117 versions of Perl with mysterious syntax errors, put this sort of thing at
5118 the top of your file to signal that your code will work I<only> on Perls of
5121 use 5.014; # so push/pop/etc work on scalars (experimental)
5124 X<pos> X<match, position>
5128 =for Pod::Functions find or set the offset for the last/next m//g search
5130 Returns the offset of where the last C<m//g> search left off for the
5131 variable in question (C<$_> is used when the variable is not
5132 specified). Note that 0 is a valid match offset. C<undef> indicates
5133 that the search position is reset (usually due to match failure, but
5134 can also be because no match has yet been run on the scalar).
5136 C<pos> directly accesses the location used by the regexp engine to
5137 store the offset, so assigning to C<pos> will change that offset, and
5138 so will also influence the C<\G> zero-width assertion in regular
5139 expressions. Both of these effects take place for the next match, so
5140 you can't affect the position with C<pos> during the current match,
5141 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5143 Setting C<pos> also resets the I<matched with zero-length> flag, described
5144 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5146 Because a failed C<m//gc> match doesn't reset the offset, the return
5147 from C<pos> won't change either in this case. See L<perlre> and
5150 =item print FILEHANDLE LIST
5153 =item print FILEHANDLE
5159 =for Pod::Functions output a list to a filehandle
5161 Prints a string or a list of strings. Returns true if successful.
5162 FILEHANDLE may be a scalar variable containing the name of or a reference
5163 to the filehandle, thus introducing one level of indirection. (NOTE: If
5164 FILEHANDLE is a variable and the next token is a term, it may be
5165 misinterpreted as an operator unless you interpose a C<+> or put
5166 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5167 last selected (see L</select>) output handle. If LIST is omitted, prints
5168 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5169 print the content of C<$_> to it, you must use a real filehandle like
5170 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5171 to something other than STDOUT, use the select operation.
5173 The current value of C<$,> (if any) is printed between each LIST item. The
5174 current value of C<$\> (if any) is printed after the entire LIST has been
5175 printed. Because print takes a LIST, anything in the LIST is evaluated in
5176 list context, including any subroutines whose return lists you pass to
5177 C<print>. Be careful not to follow the print keyword with a left
5178 parenthesis unless you want the corresponding right parenthesis to
5179 terminate the arguments to the print; put parentheses around all arguments
5180 (or interpose a C<+>, but that doesn't look as good).
5182 If you're storing handles in an array or hash, or in general whenever
5183 you're using any expression more complex than a bareword handle or a plain,
5184 unsubscripted scalar variable to retrieve it, you will have to use a block
5185 returning the filehandle value instead, in which case the LIST may not be
5188 print { $files[$i] } "stuff\n";
5189 print { $OK ? STDOUT : STDERR } "stuff\n";
5191 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5192 L<perlipc> for more on signal handling.
5194 =item printf FILEHANDLE FORMAT, LIST
5197 =item printf FILEHANDLE
5199 =item printf FORMAT, LIST
5203 =for Pod::Functions output a formatted list to a filehandle
5205 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5206 (the output record separator) is not appended. The FORMAT and the
5207 LIST are actually parsed as a single list. The first argument
5208 of the list will be interpreted as the C<printf> format. This
5209 means that C<printf(@_)> will use C<$_[0]> as the format. See
5210 L<sprintf|/sprintf FORMAT, LIST> for an
5211 explanation of the format argument. If C<use locale> (including
5212 C<use locale ':not_characters'>) is in effect and
5213 POSIX::setlocale() has been called, the character used for the decimal
5214 separator in formatted floating-point numbers is affected by the LC_NUMERIC
5215 locale setting. See L<perllocale> and L<POSIX>.
5217 For historical reasons, if you omit the list, C<$_> is used as the format;
5218 to use FILEHANDLE without a list, you must use a real filehandle like
5219 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5220 you want; if $_ contains formatting codes, they will be replaced with the
5221 empty string and a warning will be emitted if warnings are enabled. Just
5222 use C<print> if you want to print the contents of $_.
5224 Don't fall into the trap of using a C<printf> when a simple
5225 C<print> would do. The C<print> is more efficient and less
5228 =item prototype FUNCTION
5231 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5233 Returns the prototype of a function as a string (or C<undef> if the
5234 function has no prototype). FUNCTION is a reference to, or the name of,
5235 the function whose prototype you want to retrieve.
5237 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5238 name for a Perl builtin. If the builtin's arguments
5239 cannot be adequately expressed by a prototype
5240 (such as C<system>), prototype() returns C<undef>, because the builtin
5241 does not really behave like a Perl function. Otherwise, the string
5242 describing the equivalent prototype is returned.
5244 =item push ARRAY,LIST
5247 =item push EXPR,LIST
5249 =for Pod::Functions append one or more elements to an array
5251 Treats ARRAY as a stack by appending the values of LIST to the end of
5252 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5256 $ARRAY[++$#ARRAY] = $value;
5259 but is more efficient. Returns the number of elements in the array following
5260 the completed C<push>.
5262 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
5263 reference to an unblessed array. The argument will be dereferenced
5264 automatically. This aspect of C<push> is considered highly experimental.
5265 The exact behaviour may change in a future version of Perl.
5267 To avoid confusing would-be users of your code who are running earlier
5268 versions of Perl with mysterious syntax errors, put this sort of thing at
5269 the top of your file to signal that your code will work I<only> on Perls of
5272 use 5.014; # so push/pop/etc work on scalars (experimental)
5276 =for Pod::Functions singly quote a string
5280 =for Pod::Functions doubly quote a string
5284 =for Pod::Functions quote a list of words
5288 =for Pod::Functions backquote quote a string
5290 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5294 =for Pod::Functions +5.005 compile pattern
5296 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5298 =item quotemeta EXPR
5299 X<quotemeta> X<metacharacter>
5303 =for Pod::Functions quote regular expression magic characters
5305 Returns the value of EXPR with all the ASCII non-"word"
5306 characters backslashed. (That is, all ASCII characters not matching
5307 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5308 returned string, regardless of any locale settings.)
5309 This is the internal function implementing
5310 the C<\Q> escape in double-quoted strings.
5311 (See below for the behavior on non-ASCII code points.)
5313 If EXPR is omitted, uses C<$_>.
5315 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5316 regular expressions, because by default an interpolated variable will be
5317 considered a mini-regular expression. For example:
5319 my $sentence = 'The quick brown fox jumped over the lazy dog';
5320 my $substring = 'quick.*?fox';
5321 $sentence =~ s{$substring}{big bad wolf};
5323 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5327 my $sentence = 'The quick brown fox jumped over the lazy dog';
5328 my $substring = 'quick.*?fox';
5329 $sentence =~ s{\Q$substring\E}{big bad wolf};
5333 my $sentence = 'The quick brown fox jumped over the lazy dog';
5334 my $substring = 'quick.*?fox';
5335 my $quoted_substring = quotemeta($substring);
5336 $sentence =~ s{$quoted_substring}{big bad wolf};
5338 Will both leave the sentence as is.
5339 Normally, when accepting literal string
5340 input from the user, quotemeta() or C<\Q> must be used.
5342 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5343 strings, but not quoted in UTF-8 strings.
5345 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5346 quoting non-ASCII characters; the quoting of ASCII characters is
5349 Also unchanged is the quoting of non-UTF-8 strings when outside the
5350 scope of a C<use feature 'unicode_strings'>, which is to quote all
5351 characters in the upper Latin1 range. This provides complete backwards
5352 compatibility for old programs which do not use Unicode. (Note that
5353 C<unicode_strings> is automatically enabled within the scope of a
5354 S<C<use v5.12>> or greater.)
5356 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5357 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5358 above, locale does not affect the quoting of ASCII-range characters.
5359 This protects against those locales where characters such as C<"|"> are
5360 considered to be word characters.
5362 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5363 Unicode (see L<http://www.unicode.org/reports/tr31/>.)
5364 The only code points that are quoted are those that have any of the
5365 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5366 Default_Ignorable_Code_Point, or General_Category=Control.
5368 Of these properties, the two important ones are Pattern_Syntax and
5369 Pattern_White_Space. They have been set up by Unicode for exactly this
5370 purpose of deciding which characters in a regular expression pattern
5371 should be quoted. No character that can be in an identifier has these
5374 Perl promises, that if we ever add regular expression pattern
5375 metacharacters to the dozen already defined
5376 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5377 Pattern_Syntax property. Perl also promises, that if we ever add
5378 characters that are considered to be white space in regular expressions
5379 (currently mostly affected by C</x>), they will all have the
5380 Pattern_White_Space property.
5382 Unicode promises that the set of code points that have these two
5383 properties will never change, so something that is not quoted in v5.16
5384 will never need to be quoted in any future Perl release. (Not all the
5385 code points that match Pattern_Syntax have actually had characters
5386 assigned to them; so there is room to grow, but they are quoted
5387 whether assigned or not. Perl, of course, would never use an
5388 unassigned code point as an actual metacharacter.)
5390 Quoting characters that have the other 3 properties is done to enhance
5391 the readability of the regular expression and not because they actually
5392 need to be quoted for regular expression purposes (characters with the
5393 White_Space property are likely to be indistinguishable on the page or
5394 screen from those with the Pattern_White_Space property; and the other
5395 two properties contain non-printing characters).
5402 =for Pod::Functions retrieve the next pseudorandom number
5404 Returns a random fractional number greater than or equal to C<0> and less
5405 than the value of EXPR. (EXPR should be positive.) If EXPR is
5406 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5407 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5408 and is subject to change in future versions of Perl). Automatically calls
5409 C<srand> unless C<srand> has already been called. See also C<srand>.
5411 Apply C<int()> to the value returned by C<rand()> if you want random
5412 integers instead of random fractional numbers. For example,
5416 returns a random integer between C<0> and C<9>, inclusive.
5418 (Note: If your rand function consistently returns numbers that are too
5419 large or too small, then your version of Perl was probably compiled
5420 with the wrong number of RANDBITS.)
5422 B<C<rand()> is not cryptographically secure. You should not rely
5423 on it in security-sensitive situations.> As of this writing, a
5424 number of third-party CPAN modules offer random number generators
5425 intended by their authors to be cryptographically secure,
5426 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5427 and L<Math::TrulyRandom>.
5429 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5430 X<read> X<file, read>
5432 =item read FILEHANDLE,SCALAR,LENGTH
5434 =for Pod::Functions fixed-length buffered input from a filehandle
5436 Attempts to read LENGTH I<characters> of data into variable SCALAR
5437 from the specified FILEHANDLE. Returns the number of characters
5438 actually read, C<0> at end of file, or undef if there was an error (in
5439 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5440 so that the last character actually read is the last character of the
5441 scalar after the read.
5443 An OFFSET may be specified to place the read data at some place in the
5444 string other than the beginning. A negative OFFSET specifies
5445 placement at that many characters counting backwards from the end of
5446 the string. A positive OFFSET greater than the length of SCALAR
5447 results in the string being padded to the required size with C<"\0">
5448 bytes before the result of the read is appended.
5450 The call is implemented in terms of either Perl's or your system's native
5451 fread(3) library function. To get a true read(2) system call, see
5452 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5454 Note the I<characters>: depending on the status of the filehandle,
5455 either (8-bit) bytes or characters are read. By default, all
5456 filehandles operate on bytes, but for example if the filehandle has
5457 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5458 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5459 characters, not bytes. Similarly for the C<:encoding> pragma:
5460 in that case pretty much any characters can be read.
5462 =item readdir DIRHANDLE
5465 =for Pod::Functions get a directory from a directory handle
5467 Returns the next directory entry for a directory opened by C<opendir>.
5468 If used in list context, returns all the rest of the entries in the
5469 directory. If there are no more entries, returns the undefined value in
5470 scalar context and the empty list in list context.
5472 If you're planning to filetest the return values out of a C<readdir>, you'd
5473 better prepend the directory in question. Otherwise, because we didn't
5474 C<chdir> there, it would have been testing the wrong file.
5476 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5477 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5480 As of Perl 5.12 you can use a bare C<readdir> in a C<while> loop,
5481 which will set C<$_> on every iteration.
5483 opendir(my $dh, $some_dir) || die;
5484 while(readdir $dh) {
5485 print "$some_dir/$_\n";
5489 To avoid confusing would-be users of your code who are running earlier
5490 versions of Perl with mysterious failures, put this sort of thing at the
5491 top of your file to signal that your code will work I<only> on Perls of a
5494 use 5.012; # so readdir assigns to $_ in a lone while test
5499 X<readline> X<gets> X<fgets>
5501 =for Pod::Functions fetch a record from a file
5503 Reads from the filehandle whose typeglob is contained in EXPR (or from
5504 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5505 returns the next line until end-of-file is reached, whereupon the
5506 subsequent call returns C<undef>. In list context, reads until end-of-file
5507 is reached and returns a list of lines. Note that the notion of "line"
5508 used here is whatever you may have defined with C<$/> or
5509 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5511 When C<$/> is set to C<undef>, when C<readline> is in scalar
5512 context (i.e., file slurp mode), and when an empty file is read, it
5513 returns C<''> the first time, followed by C<undef> subsequently.
5515 This is the internal function implementing the C<< <EXPR> >>
5516 operator, but you can use it directly. The C<< <EXPR> >>
5517 operator is discussed in more detail in L<perlop/"I/O Operators">.
5520 $line = readline(*STDIN); # same thing
5522 If C<readline> encounters an operating system error, C<$!> will be set
5523 with the corresponding error message. It can be helpful to check
5524 C<$!> when you are reading from filehandles you don't trust, such as a
5525 tty or a socket. The following example uses the operator form of
5526 C<readline> and dies if the result is not defined.
5528 while ( ! eof($fh) ) {
5529 defined( $_ = <$fh> ) or die "readline failed: $!";
5533 Note that you have can't handle C<readline> errors that way with the
5534 C<ARGV> filehandle. In that case, you have to open each element of
5535 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5537 foreach my $arg (@ARGV) {
5538 open(my $fh, $arg) or warn "Can't open $arg: $!";
5540 while ( ! eof($fh) ) {
5541 defined( $_ = <$fh> )
5542 or die "readline failed for $arg: $!";
5552 =for Pod::Functions determine where a symbolic link is pointing
5554 Returns the value of a symbolic link, if symbolic links are
5555 implemented. If not, raises an exception. If there is a system
5556 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5557 omitted, uses C<$_>.
5559 Portability issues: L<perlport/readlink>.
5566 =for Pod::Functions execute a system command and collect standard output
5568 EXPR is executed as a system command.
5569 The collected standard output of the command is returned.
5570 In scalar context, it comes back as a single (potentially
5571 multi-line) string. In list context, returns a list of lines
5572 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5573 This is the internal function implementing the C<qx/EXPR/>
5574 operator, but you can use it directly. The C<qx/EXPR/>
5575 operator is discussed in more detail in L<perlop/"I/O Operators">.
5576 If EXPR is omitted, uses C<$_>.
5578 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5581 =for Pod::Functions receive a message over a Socket
5583 Receives a message on a socket. Attempts to receive LENGTH characters
5584 of data into variable SCALAR from the specified SOCKET filehandle.
5585 SCALAR will be grown or shrunk to the length actually read. Takes the
5586 same flags as the system call of the same name. Returns the address
5587 of the sender if SOCKET's protocol supports this; returns an empty
5588 string otherwise. If there's an error, returns the undefined value.
5589 This call is actually implemented in terms of recvfrom(2) system call.
5590 See L<perlipc/"UDP: Message Passing"> for examples.
5592 Note the I<characters>: depending on the status of the socket, either
5593 (8-bit) bytes or characters are received. By default all sockets
5594 operate on bytes, but for example if the socket has been changed using
5595 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5596 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5597 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5598 case pretty much any characters can be read.
5607 =for Pod::Functions start this loop iteration over again
5609 The C<redo> command restarts the loop block without evaluating the
5610 conditional again. The C<continue> block, if any, is not executed. If
5611 the LABEL is omitted, the command refers to the innermost enclosing
5612 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
5613 label name to be computed at run time, and is otherwise identical to C<redo
5614 LABEL>. Programs that want to lie to themselves about what was just input
5615 normally use this command:
5617 # a simpleminded Pascal comment stripper
5618 # (warning: assumes no { or } in strings)
5619 LINE: while (<STDIN>) {
5620 while (s|({.*}.*){.*}|$1 |) {}
5625 if (/}/) { # end of comment?
5634 C<redo> cannot be used to retry a block that returns a value such as
5635 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5636 a grep() or map() operation.
5638 Note that a block by itself is semantically identical to a loop
5639 that executes once. Thus C<redo> inside such a block will effectively
5640 turn it into a looping construct.
5642 See also L</continue> for an illustration of how C<last>, C<next>, and
5645 Unlike most named operators, this has the same precedence as assignment.
5646 It is also exempt from the looks-like-a-function rule, so
5647 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
5655 =for Pod::Functions find out the type of thing being referenced
5657 Returns a non-empty string if EXPR is a reference, the empty
5658 string otherwise. If EXPR
5659 is not specified, C<$_> will be used. The value returned depends on the
5660 type of thing the reference is a reference to.
5661 Builtin types include:
5675 If the referenced object has been blessed into a package, then that package
5676 name is returned instead. You can think of C<ref> as a C<typeof> operator.
5678 if (ref($r) eq "HASH") {
5679 print "r is a reference to a hash.\n";
5682 print "r is not a reference at all.\n";
5685 The return value C<LVALUE> indicates a reference to an lvalue that is not
5686 a variable. You get this from taking the reference of function calls like
5687 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5688 to a L<version string|perldata/"Version Strings">.
5690 The result C<Regexp> indicates that the argument is a regular expression
5691 resulting from C<qr//>.
5693 See also L<perlref>.
5695 =item rename OLDNAME,NEWNAME
5696 X<rename> X<move> X<mv> X<ren>
5698 =for Pod::Functions change a filename
5700 Changes the name of a file; an existing file NEWNAME will be
5701 clobbered. Returns true for success, false otherwise.
5703 Behavior of this function varies wildly depending on your system
5704 implementation. For example, it will usually not work across file system
5705 boundaries, even though the system I<mv> command sometimes compensates
5706 for this. Other restrictions include whether it works on directories,
5707 open files, or pre-existing files. Check L<perlport> and either the
5708 rename(2) manpage or equivalent system documentation for details.
5710 For a platform independent C<move> function look at the L<File::Copy>
5713 Portability issues: L<perlport/rename>.
5715 =item require VERSION
5722 =for Pod::Functions load in external functions from a library at runtime
5724 Demands a version of Perl specified by VERSION, or demands some semantics
5725 specified by EXPR or by C<$_> if EXPR is not supplied.
5727 VERSION may be either a numeric argument such as 5.006, which will be
5728 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5729 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5730 VERSION is greater than the version of the current Perl interpreter.
5731 Compare with L</use>, which can do a similar check at compile time.
5733 Specifying VERSION as a literal of the form v5.6.1 should generally be
5734 avoided, because it leads to misleading error messages under earlier
5735 versions of Perl that do not support this syntax. The equivalent numeric
5736 version should be used instead.
5738 require v5.6.1; # run time version check
5739 require 5.6.1; # ditto
5740 require 5.006_001; # ditto; preferred for backwards
5743 Otherwise, C<require> demands that a library file be included if it
5744 hasn't already been included. The file is included via the do-FILE
5745 mechanism, which is essentially just a variety of C<eval> with the
5746 caveat that lexical variables in the invoking script will be invisible
5747 to the included code. Has semantics similar to the following subroutine:
5750 my ($filename) = @_;
5751 if (exists $INC{$filename}) {
5752 return 1 if $INC{$filename};
5753 die "Compilation failed in require";
5755 my ($realfilename,$result);
5757 foreach $prefix (@INC) {
5758 $realfilename = "$prefix/$filename";
5759 if (-f $realfilename) {
5760 $INC{$filename} = $realfilename;
5761 $result = do $realfilename;
5765 die "Can't find $filename in \@INC";
5768 $INC{$filename} = undef;
5770 } elsif (!$result) {
5771 delete $INC{$filename};
5772 die "$filename did not return true value";
5778 Note that the file will not be included twice under the same specified
5781 The file must return true as the last statement to indicate
5782 successful execution of any initialization code, so it's customary to
5783 end such a file with C<1;> unless you're sure it'll return true
5784 otherwise. But it's better just to put the C<1;>, in case you add more
5787 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5788 replaces "F<::>" with "F</>" in the filename for you,
5789 to make it easy to load standard modules. This form of loading of
5790 modules does not risk altering your namespace.
5792 In other words, if you try this:
5794 require Foo::Bar; # a splendid bareword
5796 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5797 directories specified in the C<@INC> array.
5799 But if you try this:
5801 $class = 'Foo::Bar';
5802 require $class; # $class is not a bareword
5804 require "Foo::Bar"; # not a bareword because of the ""
5806 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5807 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5809 eval "require $class";
5811 Now that you understand how C<require> looks for files with a
5812 bareword argument, there is a little extra functionality going on behind
5813 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5814 first look for a similar filename with a "F<.pmc>" extension. If this file
5815 is found, it will be loaded in place of any file ending in a "F<.pm>"
5818 You can also insert hooks into the import facility by putting Perl code
5819 directly into the @INC array. There are three forms of hooks: subroutine
5820 references, array references, and blessed objects.
5822 Subroutine references are the simplest case. When the inclusion system
5823 walks through @INC and encounters a subroutine, this subroutine gets
5824 called with two parameters, the first a reference to itself, and the
5825 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5826 subroutine should return either nothing or else a list of up to three
5827 values in the following order:
5833 A filehandle, from which the file will be read.
5837 A reference to a subroutine. If there is no filehandle (previous item),
5838 then this subroutine is expected to generate one line of source code per
5839 call, writing the line into C<$_> and returning 1, then finally at end of
5840 file returning 0. If there is a filehandle, then the subroutine will be
5841 called to act as a simple source filter, with the line as read in C<$_>.
5842 Again, return 1 for each valid line, and 0 after all lines have been
5847 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5848 reference to the subroutine itself is passed in as C<$_[0]>.
5852 If an empty list, C<undef>, or nothing that matches the first 3 values above
5853 is returned, then C<require> looks at the remaining elements of @INC.
5854 Note that this filehandle must be a real filehandle (strictly a typeglob
5855 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5856 will be ignored and processing will stop there.
5858 If the hook is an array reference, its first element must be a subroutine
5859 reference. This subroutine is called as above, but the first parameter is
5860 the array reference. This lets you indirectly pass arguments to
5863 In other words, you can write:
5865 push @INC, \&my_sub;
5867 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5873 push @INC, [ \&my_sub, $x, $y, ... ];
5875 my ($arrayref, $filename) = @_;
5876 # Retrieve $x, $y, ...
5877 my @parameters = @$arrayref[1..$#$arrayref];
5881 If the hook is an object, it must provide an INC method that will be
5882 called as above, the first parameter being the object itself. (Note that
5883 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5884 into package C<main>.) Here is a typical code layout:
5890 my ($self, $filename) = @_;
5894 # In the main program
5895 push @INC, Foo->new(...);
5897 These hooks are also permitted to set the %INC entry
5898 corresponding to the files they have loaded. See L<perlvar/%INC>.
5900 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5907 =for Pod::Functions clear all variables of a given name
5909 Generally used in a C<continue> block at the end of a loop to clear
5910 variables and reset C<??> searches so that they work again. The
5911 expression is interpreted as a list of single characters (hyphens
5912 allowed for ranges). All variables and arrays beginning with one of
5913 those letters are reset to their pristine state. If the expression is
5914 omitted, one-match searches (C<?pattern?>) are reset to match again.
5915 Only resets variables or searches in the current package. Always returns
5918 reset 'X'; # reset all X variables
5919 reset 'a-z'; # reset lower case variables
5920 reset; # just reset ?one-time? searches
5922 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5923 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5924 variables; lexical variables are unaffected, but they clean themselves
5925 up on scope exit anyway, so you'll probably want to use them instead.
5933 =for Pod::Functions get out of a function early
5935 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5936 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5937 context, depending on how the return value will be used, and the context
5938 may vary from one execution to the next (see L</wantarray>). If no EXPR
5939 is given, returns an empty list in list context, the undefined value in
5940 scalar context, and (of course) nothing at all in void context.
5942 (In the absence of an explicit C<return>, a subroutine, eval,
5943 or do FILE automatically returns the value of the last expression
5946 Unlike most named operators, this is also exempt from the
5947 looks-like-a-function rule, so C<return ("foo")."bar"> will
5948 cause "bar" to be part of the argument to C<return>.
5951 X<reverse> X<rev> X<invert>
5953 =for Pod::Functions flip a string or a list
5955 In list context, returns a list value consisting of the elements
5956 of LIST in the opposite order. In scalar context, concatenates the
5957 elements of LIST and returns a string value with all characters
5958 in the opposite order.
5960 print join(", ", reverse "world", "Hello"); # Hello, world
5962 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5964 Used without arguments in scalar context, reverse() reverses C<$_>.
5966 $_ = "dlrow ,olleH";
5967 print reverse; # No output, list context
5968 print scalar reverse; # Hello, world
5970 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5971 preserve non-existent elements whenever possible; i.e., for non-magical
5972 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
5974 This operator is also handy for inverting a hash, although there are some
5975 caveats. If a value is duplicated in the original hash, only one of those
5976 can be represented as a key in the inverted hash. Also, this has to
5977 unwind one hash and build a whole new one, which may take some time
5978 on a large hash, such as from a DBM file.
5980 %by_name = reverse %by_address; # Invert the hash
5982 =item rewinddir DIRHANDLE
5985 =for Pod::Functions reset directory handle
5987 Sets the current position to the beginning of the directory for the
5988 C<readdir> routine on DIRHANDLE.
5990 Portability issues: L<perlport/rewinddir>.
5992 =item rindex STR,SUBSTR,POSITION
5995 =item rindex STR,SUBSTR
5997 =for Pod::Functions right-to-left substring search
5999 Works just like index() except that it returns the position of the I<last>
6000 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6001 last occurrence beginning at or before that position.
6003 =item rmdir FILENAME
6004 X<rmdir> X<rd> X<directory, remove>
6008 =for Pod::Functions remove a directory
6010 Deletes the directory specified by FILENAME if that directory is
6011 empty. If it succeeds it returns true; otherwise it returns false and
6012 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
6014 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6015 the C<rmtree> function of the L<File::Path> module.
6019 =for Pod::Functions replace a pattern with a string
6021 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6023 =item say FILEHANDLE LIST
6026 =item say FILEHANDLE
6032 =for Pod::Functions +say output a list to a filehandle, appending a newline
6034 Just like C<print>, but implicitly appends a newline. C<say LIST> is
6035 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
6036 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
6037 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
6039 This keyword is available only when the C<"say"> feature
6040 is enabled, or when prefixed with C<CORE::>; see
6041 L<feature>. Alternately, include a C<use v5.10> or later to the current
6045 X<scalar> X<context>
6047 =for Pod::Functions force a scalar context
6049 Forces EXPR to be interpreted in scalar context and returns the value
6052 @counts = ( scalar @a, scalar @b, scalar @c );
6054 There is no equivalent operator to force an expression to
6055 be interpolated in list context because in practice, this is never
6056 needed. If you really wanted to do so, however, you could use
6057 the construction C<@{[ (some expression) ]}>, but usually a simple
6058 C<(some expression)> suffices.
6060 Because C<scalar> is a unary operator, if you accidentally use a
6061 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6062 evaluating all but the last element in void context and returning the final
6063 element evaluated in scalar context. This is seldom what you want.
6065 The following single statement:
6067 print uc(scalar(&foo,$bar)),$baz;
6069 is the moral equivalent of these two:
6072 print(uc($bar),$baz);
6074 See L<perlop> for more details on unary operators and the comma operator.
6076 =item seek FILEHANDLE,POSITION,WHENCE
6077 X<seek> X<fseek> X<filehandle, position>
6079 =for Pod::Functions reposition file pointer for random-access I/O
6081 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
6082 FILEHANDLE may be an expression whose value gives the name of the
6083 filehandle. The values for WHENCE are C<0> to set the new position
6084 I<in bytes> to POSITION; C<1> to set it to the current position plus
6085 POSITION; and C<2> to set it to EOF plus POSITION, typically
6086 negative. For WHENCE you may use the constants C<SEEK_SET>,
6087 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6088 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6091 Note the I<in bytes>: even if the filehandle has been set to
6092 operate on characters (for example by using the C<:encoding(utf8)> open
6093 layer), tell() will return byte offsets, not character offsets
6094 (because implementing that would render seek() and tell() rather slow).
6096 If you want to position the file for C<sysread> or C<syswrite>, don't use
6097 C<seek>, because buffering makes its effect on the file's read-write position
6098 unpredictable and non-portable. Use C<sysseek> instead.
6100 Due to the rules and rigors of ANSI C, on some systems you have to do a
6101 seek whenever you switch between reading and writing. Amongst other
6102 things, this may have the effect of calling stdio's clearerr(3).
6103 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6107 This is also useful for applications emulating C<tail -f>. Once you hit
6108 EOF on your read and then sleep for a while, you (probably) have to stick in a
6109 dummy seek() to reset things. The C<seek> doesn't change the position,
6110 but it I<does> clear the end-of-file condition on the handle, so that the
6111 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6113 If that doesn't work (some I/O implementations are particularly
6114 cantankerous), you might need something like this:
6117 for ($curpos = tell(FILE); $_ = <FILE>;
6118 $curpos = tell(FILE)) {
6119 # search for some stuff and put it into files
6121 sleep($for_a_while);
6122 seek(FILE, $curpos, 0);
6125 =item seekdir DIRHANDLE,POS
6128 =for Pod::Functions reposition directory pointer
6130 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6131 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6132 about possible directory compaction as the corresponding system library
6135 =item select FILEHANDLE
6136 X<select> X<filehandle, default>
6140 =for Pod::Functions reset default output or do I/O multiplexing
6142 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6143 sets the new current default filehandle for output. This has two
6144 effects: first, a C<write> or a C<print> without a filehandle
6145 default to this FILEHANDLE. Second, references to variables related to
6146 output will refer to this output channel.
6148 For example, to set the top-of-form format for more than one
6149 output channel, you might do the following:
6156 FILEHANDLE may be an expression whose value gives the name of the
6157 actual filehandle. Thus:
6159 $oldfh = select(STDERR); $| = 1; select($oldfh);
6161 Some programmers may prefer to think of filehandles as objects with
6162 methods, preferring to write the last example as:
6165 STDERR->autoflush(1);
6167 Portability issues: L<perlport/select>.
6169 =item select RBITS,WBITS,EBITS,TIMEOUT
6172 This calls the select(2) syscall with the bit masks specified, which
6173 can be constructed using C<fileno> and C<vec>, along these lines:
6175 $rin = $win = $ein = '';
6176 vec($rin, fileno(STDIN), 1) = 1;
6177 vec($win, fileno(STDOUT), 1) = 1;
6180 If you want to select on many filehandles, you may wish to write a
6181 subroutine like this:
6186 for my $fh (@fhlist) {
6187 vec($bits, fileno($fh), 1) = 1;
6191 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6195 ($nfound,$timeleft) =
6196 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6198 or to block until something becomes ready just do this
6200 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6202 Most systems do not bother to return anything useful in $timeleft, so
6203 calling select() in scalar context just returns $nfound.
6205 Any of the bit masks can also be undef. The timeout, if specified, is
6206 in seconds, which may be fractional. Note: not all implementations are
6207 capable of returning the $timeleft. If not, they always return
6208 $timeleft equal to the supplied $timeout.
6210 You can effect a sleep of 250 milliseconds this way:
6212 select(undef, undef, undef, 0.25);
6214 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6215 is implementation-dependent. See also L<perlport> for notes on the
6216 portability of C<select>.
6218 On error, C<select> behaves just like select(2): it returns
6221 On some Unixes, select(2) may report a socket file descriptor as "ready for
6222 reading" even when no data is available, and thus any subsequent C<read>
6223 would block. This can be avoided if you always use O_NONBLOCK on the
6224 socket. See select(2) and fcntl(2) for further details.
6226 The standard C<IO::Select> module provides a user-friendlier interface
6227 to C<select>, mostly because it does all the bit-mask work for you.
6229 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6230 or <FH>) with C<select>, except as permitted by POSIX, and even
6231 then only on POSIX systems. You have to use C<sysread> instead.
6233 Portability issues: L<perlport/select>.
6235 =item semctl ID,SEMNUM,CMD,ARG
6238 =for Pod::Functions SysV semaphore control operations
6240 Calls the System V IPC function semctl(2). You'll probably have to say
6244 first to get the correct constant definitions. If CMD is IPC_STAT or
6245 GETALL, then ARG must be a variable that will hold the returned
6246 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6247 the undefined value for error, "C<0 but true>" for zero, or the actual
6248 return value otherwise. The ARG must consist of a vector of native
6249 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6250 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6253 Portability issues: L<perlport/semctl>.
6255 =item semget KEY,NSEMS,FLAGS
6258 =for Pod::Functions get set of SysV semaphores
6260 Calls the System V IPC function semget(2). Returns the semaphore id, or
6261 the undefined value on error. See also
6262 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6265 Portability issues: L<perlport/semget>.
6267 =item semop KEY,OPSTRING
6270 =for Pod::Functions SysV semaphore operations
6272 Calls the System V IPC function semop(2) for semaphore operations
6273 such as signalling and waiting. OPSTRING must be a packed array of
6274 semop structures. Each semop structure can be generated with
6275 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6276 implies the number of semaphore operations. Returns true if
6277 successful, false on error. As an example, the
6278 following code waits on semaphore $semnum of semaphore id $semid:
6280 $semop = pack("s!3", $semnum, -1, 0);
6281 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6283 To signal the semaphore, replace C<-1> with C<1>. See also
6284 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6287 Portability issues: L<perlport/semop>.
6289 =item send SOCKET,MSG,FLAGS,TO
6292 =item send SOCKET,MSG,FLAGS
6294 =for Pod::Functions send a message over a socket
6296 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6297 filehandle. Takes the same flags as the system call of the same name. On
6298 unconnected sockets, you must specify a destination to I<send to>, in which
6299 case it does a sendto(2) syscall. Returns the number of characters sent,
6300 or the undefined value on error. The sendmsg(2) syscall is currently
6301 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6303 Note the I<characters>: depending on the status of the socket, either
6304 (8-bit) bytes or characters are sent. By default all sockets operate
6305 on bytes, but for example if the socket has been changed using
6306 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6307 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6308 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6309 pragma: in that case pretty much any characters can be sent.
6311 =item setpgrp PID,PGRP
6314 =for Pod::Functions set the process group of a process
6316 Sets the current process group for the specified PID, C<0> for the current
6317 process. Raises an exception when used on a machine that doesn't
6318 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6319 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6320 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6323 Portability issues: L<perlport/setpgrp>.
6325 =item setpriority WHICH,WHO,PRIORITY
6326 X<setpriority> X<priority> X<nice> X<renice>
6328 =for Pod::Functions set a process's nice value
6330 Sets the current priority for a process, a process group, or a user.
6331 (See setpriority(2).) Raises an exception when used on a machine
6332 that doesn't implement setpriority(2).
6334 Portability issues: L<perlport/setpriority>.
6336 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6339 =for Pod::Functions set some socket options
6341 Sets the socket option requested. Returns C<undef> on error.
6342 Use integer constants provided by the C<Socket> module for
6343 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6344 getprotobyname. OPTVAL might either be a packed string or an integer.
6345 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6347 An example disabling Nagle's algorithm on a socket:
6349 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6350 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6352 Portability issues: L<perlport/setsockopt>.
6361 =for Pod::Functions remove the first element of an array, and return it
6363 Shifts the first value of the array off and returns it, shortening the
6364 array by 1 and moving everything down. If there are no elements in the
6365 array, returns the undefined value. If ARRAY is omitted, shifts the
6366 C<@_> array within the lexical scope of subroutines and formats, and the
6367 C<@ARGV> array outside a subroutine and also within the lexical scopes
6368 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6369 C<UNITCHECK {}>, and C<END {}> constructs.
6371 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
6372 reference to an unblessed array. The argument will be dereferenced
6373 automatically. This aspect of C<shift> is considered highly experimental.
6374 The exact behaviour may change in a future version of Perl.
6376 To avoid confusing would-be users of your code who are running earlier
6377 versions of Perl with mysterious syntax errors, put this sort of thing at
6378 the top of your file to signal that your code will work I<only> on Perls of
6381 use 5.014; # so push/pop/etc work on scalars (experimental)
6383 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6384 same thing to the left end of an array that C<pop> and C<push> do to the
6387 =item shmctl ID,CMD,ARG
6390 =for Pod::Functions SysV shared memory operations
6392 Calls the System V IPC function shmctl. You'll probably have to say
6396 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6397 then ARG must be a variable that will hold the returned C<shmid_ds>
6398 structure. Returns like ioctl: C<undef> for error; "C<0> but
6399 true" for zero; and the actual return value otherwise.
6400 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6402 Portability issues: L<perlport/shmctl>.
6404 =item shmget KEY,SIZE,FLAGS
6407 =for Pod::Functions get SysV shared memory segment identifier
6409 Calls the System V IPC function shmget. Returns the shared memory
6410 segment id, or C<undef> on error.
6411 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6413 Portability issues: L<perlport/shmget>.
6415 =item shmread ID,VAR,POS,SIZE
6419 =for Pod::Functions read SysV shared memory
6421 =item shmwrite ID,STRING,POS,SIZE
6423 =for Pod::Functions write SysV shared memory
6425 Reads or writes the System V shared memory segment ID starting at
6426 position POS for size SIZE by attaching to it, copying in/out, and
6427 detaching from it. When reading, VAR must be a variable that will
6428 hold the data read. When writing, if STRING is too long, only SIZE
6429 bytes are used; if STRING is too short, nulls are written to fill out
6430 SIZE bytes. Return true if successful, false on error.
6431 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6432 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6434 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6436 =item shutdown SOCKET,HOW
6439 =for Pod::Functions close down just half of a socket connection
6441 Shuts down a socket connection in the manner indicated by HOW, which
6442 has the same interpretation as in the syscall of the same name.
6444 shutdown(SOCKET, 0); # I/we have stopped reading data
6445 shutdown(SOCKET, 1); # I/we have stopped writing data
6446 shutdown(SOCKET, 2); # I/we have stopped using this socket
6448 This is useful with sockets when you want to tell the other
6449 side you're done writing but not done reading, or vice versa.
6450 It's also a more insistent form of close because it also
6451 disables the file descriptor in any forked copies in other
6454 Returns C<1> for success; on error, returns C<undef> if
6455 the first argument is not a valid filehandle, or returns C<0> and sets
6456 C<$!> for any other failure.
6459 X<sin> X<sine> X<asin> X<arcsine>
6463 =for Pod::Functions return the sine of a number
6465 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6466 returns sine of C<$_>.
6468 For the inverse sine operation, you may use the C<Math::Trig::asin>
6469 function, or use this relation:
6471 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6478 =for Pod::Functions block for some number of seconds
6480 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6481 argument is given. Returns the integer number of seconds actually slept.
6483 May be interrupted if the process receives a signal such as C<SIGALRM>.
6486 local $SIG{ALARM} = sub { die "Alarm!\n" };
6489 die $@ unless $@ eq "Alarm!\n";
6491 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6492 is often implemented using C<alarm>.
6494 On some older systems, it may sleep up to a full second less than what
6495 you requested, depending on how it counts seconds. Most modern systems
6496 always sleep the full amount. They may appear to sleep longer than that,
6497 however, because your process might not be scheduled right away in a
6498 busy multitasking system.
6500 For delays of finer granularity than one second, the Time::HiRes module
6501 (from CPAN, and starting from Perl 5.8 part of the standard
6502 distribution) provides usleep(). You may also use Perl's four-argument
6503 version of select() leaving the first three arguments undefined, or you
6504 might be able to use the C<syscall> interface to access setitimer(2) if
6505 your system supports it. See L<perlfaq8> for details.
6507 See also the POSIX module's C<pause> function.
6509 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6512 =for Pod::Functions create a socket
6514 Opens a socket of the specified kind and attaches it to filehandle
6515 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6516 the syscall of the same name. You should C<use Socket> first
6517 to get the proper definitions imported. See the examples in
6518 L<perlipc/"Sockets: Client/Server Communication">.
6520 On systems that support a close-on-exec flag on files, the flag will
6521 be set for the newly opened file descriptor, as determined by the
6522 value of $^F. See L<perlvar/$^F>.
6524 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6527 =for Pod::Functions create a pair of sockets
6529 Creates an unnamed pair of sockets in the specified domain, of the
6530 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6531 for the syscall of the same name. If unimplemented, raises an exception.
6532 Returns true if successful.
6534 On systems that support a close-on-exec flag on files, the flag will
6535 be set for the newly opened file descriptors, as determined by the value
6536 of $^F. See L<perlvar/$^F>.
6538 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6539 to C<pipe(Rdr, Wtr)> is essentially:
6542 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6543 shutdown(Rdr, 1); # no more writing for reader
6544 shutdown(Wtr, 0); # no more reading for writer
6546 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6547 emulate socketpair using IP sockets to localhost if your system implements
6548 sockets but not socketpair.
6550 Portability issues: L<perlport/socketpair>.
6552 =item sort SUBNAME LIST
6553 X<sort> X<qsort> X<quicksort> X<mergesort>
6555 =item sort BLOCK LIST
6559 =for Pod::Functions sort a list of values
6561 In list context, this sorts the LIST and returns the sorted list value.
6562 In scalar context, the behaviour of C<sort()> is undefined.
6564 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6565 order. If SUBNAME is specified, it gives the name of a subroutine
6566 that returns an integer less than, equal to, or greater than C<0>,
6567 depending on how the elements of the list are to be ordered. (The
6568 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6569 SUBNAME may be a scalar variable name (unsubscripted), in which case
6570 the value provides the name of (or a reference to) the actual
6571 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6572 an anonymous, in-line sort subroutine.
6574 If the subroutine's prototype is C<($$)>, the elements to be compared are
6575 passed by reference in C<@_>, as for a normal subroutine. This is slower
6576 than unprototyped subroutines, where the elements to be compared are passed
6577 into the subroutine as the package global variables $a and $b (see example
6578 below). Note that in the latter case, it is usually highly counter-productive
6579 to declare $a and $b as lexicals.
6581 If the subroutine is an XSUB, the elements to be compared are pushed on to
6582 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6585 The values to be compared are always passed by reference and should not
6588 You also cannot exit out of the sort block or subroutine using any of the
6589 loop control operators described in L<perlsyn> or with C<goto>.
6591 When C<use locale> (but not C<use locale 'not_characters'>) is in
6592 effect, C<sort LIST> sorts LIST according to the
6593 current collation locale. See L<perllocale>.
6595 sort() returns aliases into the original list, much as a for loop's index
6596 variable aliases the list elements. That is, modifying an element of a
6597 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6598 actually modifies the element in the original list. This is usually
6599 something to be avoided when writing clear code.
6601 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6602 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6603 preserves the input order of elements that compare equal. Although
6604 quicksort's run time is O(NlogN) when averaged over all arrays of
6605 length N, the time can be O(N**2), I<quadratic> behavior, for some
6606 inputs.) In 5.7, the quicksort implementation was replaced with
6607 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6608 But benchmarks indicated that for some inputs, on some platforms,
6609 the original quicksort was faster. 5.8 has a sort pragma for
6610 limited control of the sort. Its rather blunt control of the
6611 underlying algorithm may not persist into future Perls, but the
6612 ability to characterize the input or output in implementation
6613 independent ways quite probably will. See L<the sort pragma|sort>.
6618 @articles = sort @files;
6620 # same thing, but with explicit sort routine
6621 @articles = sort {$a cmp $b} @files;
6623 # now case-insensitively
6624 @articles = sort {fc($a) cmp fc($b)} @files;
6626 # same thing in reversed order
6627 @articles = sort {$b cmp $a} @files;
6629 # sort numerically ascending
6630 @articles = sort {$a <=> $b} @files;
6632 # sort numerically descending
6633 @articles = sort {$b <=> $a} @files;
6635 # this sorts the %age hash by value instead of key
6636 # using an in-line function
6637 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6639 # sort using explicit subroutine name
6641 $age{$a} <=> $age{$b}; # presuming numeric
6643 @sortedclass = sort byage @class;
6645 sub backwards { $b cmp $a }
6646 @harry = qw(dog cat x Cain Abel);
6647 @george = qw(gone chased yz Punished Axed);
6649 # prints AbelCaincatdogx
6650 print sort backwards @harry;
6651 # prints xdogcatCainAbel
6652 print sort @george, 'to', @harry;
6653 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6655 # inefficiently sort by descending numeric compare using
6656 # the first integer after the first = sign, or the
6657 # whole record case-insensitively otherwise
6660 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6665 # same thing, but much more efficiently;
6666 # we'll build auxiliary indices instead
6668 my @nums = @caps = ();
6670 push @nums, ( /=(\d+)/ ? $1 : undef );
6674 my @new = @old[ sort {
6675 $nums[$b] <=> $nums[$a]
6677 $caps[$a] cmp $caps[$b]
6681 # same thing, but without any temps
6682 @new = map { $_->[0] }
6683 sort { $b->[1] <=> $a->[1]
6686 } map { [$_, /=(\d+)/, fc($_)] } @old;
6688 # using a prototype allows you to use any comparison subroutine
6689 # as a sort subroutine (including other package's subroutines)
6691 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6694 @new = sort other::backwards @old;
6696 # guarantee stability, regardless of algorithm
6698 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6700 # force use of mergesort (not portable outside Perl 5.8)
6701 use sort '_mergesort'; # note discouraging _
6702 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6704 Warning: syntactical care is required when sorting the list returned from
6705 a function. If you want to sort the list returned by the function call
6706 C<find_records(@key)>, you can use:
6708 @contact = sort { $a cmp $b } find_records @key;
6709 @contact = sort +find_records(@key);
6710 @contact = sort &find_records(@key);
6711 @contact = sort(find_records(@key));
6713 If instead you want to sort the array @key with the comparison routine
6714 C<find_records()> then you can use:
6716 @contact = sort { find_records() } @key;
6717 @contact = sort find_records(@key);
6718 @contact = sort(find_records @key);
6719 @contact = sort(find_records (@key));
6721 If you're using strict, you I<must not> declare $a
6722 and $b as lexicals. They are package globals. That means
6723 that if you're in the C<main> package and type
6725 @articles = sort {$b <=> $a} @files;
6727 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6728 but if you're in the C<FooPack> package, it's the same as typing
6730 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6732 The comparison function is required to behave. If it returns
6733 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6734 sometimes saying the opposite, for example) the results are not
6737 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6738 (not-a-number), be careful when sorting with a
6739 comparison function like C<< $a <=> $b >> any lists that might contain a
6740 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6741 eliminate any C<NaN>s from the input list.
6743 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6745 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6748 =item splice ARRAY or EXPR,OFFSET,LENGTH
6750 =item splice ARRAY or EXPR,OFFSET
6752 =item splice ARRAY or EXPR
6754 =for Pod::Functions add or remove elements anywhere in an array
6756 Removes the elements designated by OFFSET and LENGTH from an array, and
6757 replaces them with the elements of LIST, if any. In list context,
6758 returns the elements removed from the array. In scalar context,
6759 returns the last element removed, or C<undef> if no elements are
6760 removed. The array grows or shrinks as necessary.
6761 If OFFSET is negative then it starts that far from the end of the array.
6762 If LENGTH is omitted, removes everything from OFFSET onward.
6763 If LENGTH is negative, removes the elements from OFFSET onward
6764 except for -LENGTH elements at the end of the array.
6765 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6766 past the end of the array, Perl issues a warning, and splices at the
6769 The following equivalences hold (assuming C<< $#a >= $i >> )
6771 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6772 pop(@a) splice(@a,-1)
6773 shift(@a) splice(@a,0,1)
6774 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6775 $a[$i] = $y splice(@a,$i,1,$y)
6777 Example, assuming array lengths are passed before arrays:
6779 sub aeq { # compare two list values
6780 my(@a) = splice(@_,0,shift);
6781 my(@b) = splice(@_,0,shift);
6782 return 0 unless @a == @b; # same len?
6784 return 0 if pop(@a) ne pop(@b);
6788 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6790 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6791 reference to an unblessed array. The argument will be dereferenced
6792 automatically. This aspect of C<splice> is considered highly experimental.
6793 The exact behaviour may change in a future version of Perl.
6795 To avoid confusing would-be users of your code who are running earlier
6796 versions of Perl with mysterious syntax errors, put this sort of thing at
6797 the top of your file to signal that your code will work I<only> on Perls of
6800 use 5.014; # so push/pop/etc work on scalars (experimental)
6802 =item split /PATTERN/,EXPR,LIMIT
6805 =item split /PATTERN/,EXPR
6807 =item split /PATTERN/
6811 =for Pod::Functions split up a string using a regexp delimiter
6813 Splits the string EXPR into a list of strings and returns the
6814 list in list context, or the size of the list in scalar context.
6816 If only PATTERN is given, EXPR defaults to C<$_>.
6818 Anything in EXPR that matches PATTERN is taken to be a separator
6819 that separates the EXPR into substrings (called "I<fields>") that
6820 do B<not> include the separator. Note that a separator may be
6821 longer than one character or even have no characters at all (the
6822 empty string, which is a zero-width match).
6824 The PATTERN need not be constant; an expression may be used
6825 to specify a pattern that varies at runtime.
6827 If PATTERN matches the empty string, the EXPR is split at the match
6828 position (between characters). As an example, the following:
6830 print join(':', split('b', 'abc')), "\n";
6832 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6835 print join(':', split('', 'abc')), "\n";
6837 uses empty string matches as separators to produce the output
6838 'a:b:c'; thus, the empty string may be used to split EXPR into a
6839 list of its component characters.
6841 As a special case for C<split>, the empty pattern given in
6842 L<match operator|perlop/"m/PATTERN/msixpodualgc"> syntax (C<//>) specifically matches the empty string, which is contrary to its usual
6843 interpretation as the last successful match.
6845 If PATTERN is C</^/>, then it is treated as if it used the
6846 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
6847 isn't much use otherwise.
6849 As another special case, C<split> emulates the default behavior of the
6850 command line tool B<awk> when the PATTERN is either omitted or a I<literal
6851 string> composed of a single space character (such as S<C<' '>> or
6852 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
6853 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
6854 instead treated as if it were C</\s+/>; in particular, this means that
6855 I<any> contiguous whitespace (not just a single space character) is used as
6856 a separator. However, this special treatment can be avoided by specifying
6857 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
6858 only a single space character to be a separator.
6860 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
6861 the previously described I<awk> emulation.
6863 If LIMIT is specified and positive, it represents the maximum number
6864 of fields into which the EXPR may be split; in other words, LIMIT is
6865 one greater than the maximum number of times EXPR may be split. Thus,
6866 the LIMIT value C<1> means that EXPR may be split a maximum of zero
6867 times, producing a maximum of one field (namely, the entire value of
6868 EXPR). For instance:
6870 print join(':', split(//, 'abc', 1)), "\n";
6872 produces the output 'abc', and this:
6874 print join(':', split(//, 'abc', 2)), "\n";
6876 produces the output 'a:bc', and each of these:
6878 print join(':', split(//, 'abc', 3)), "\n";
6879 print join(':', split(//, 'abc', 4)), "\n";
6881 produces the output 'a:b:c'.
6883 If LIMIT is negative, it is treated as if it were instead arbitrarily
6884 large; as many fields as possible are produced.
6886 If LIMIT is omitted (or, equivalently, zero), then it is usually
6887 treated as if it were instead negative but with the exception that
6888 trailing empty fields are stripped (empty leading fields are always
6889 preserved); if all fields are empty, then all fields are considered to
6890 be trailing (and are thus stripped in this case). Thus, the following:
6892 print join(':', split(',', 'a,b,c,,,')), "\n";
6894 produces the output 'a:b:c', but the following:
6896 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
6898 produces the output 'a:b:c:::'.
6900 In time-critical applications, it is worthwhile to avoid splitting
6901 into more fields than necessary. Thus, when assigning to a list,
6902 if LIMIT is omitted (or zero), then LIMIT is treated as though it
6903 were one larger than the number of variables in the list; for the
6904 following, LIMIT is implicitly 3:
6906 ($login, $passwd) = split(/:/);
6908 Note that splitting an EXPR that evaluates to the empty string always
6909 produces zero fields, regardless of the LIMIT specified.
6911 An empty leading field is produced when there is a positive-width
6912 match at the beginning of EXPR. For instance:
6914 print join(':', split(/ /, ' abc')), "\n";
6916 produces the output ':abc'. However, a zero-width match at the
6917 beginning of EXPR never produces an empty field, so that:
6919 print join(':', split(//, ' abc'));
6921 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
6923 An empty trailing field, on the other hand, is produced when there is a
6924 match at the end of EXPR, regardless of the length of the match
6925 (of course, unless a non-zero LIMIT is given explicitly, such fields are
6926 removed, as in the last example). Thus:
6928 print join(':', split(//, ' abc', -1)), "\n";
6930 produces the output S<' :a:b:c:'>.
6932 If the PATTERN contains
6933 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
6934 then for each separator, an additional field is produced for each substring
6935 captured by a group (in the order in which the groups are specified,
6936 as per L<backreferences|perlretut/Backreferences>); if any group does not
6937 match, then it captures the C<undef> value instead of a substring. Also,
6938 note that any such additional field is produced whenever there is a
6939 separator (that is, whenever a split occurs), and such an additional field
6940 does B<not> count towards the LIMIT. Consider the following expressions
6941 evaluated in list context (each returned list is provided in the associated
6944 split(/-|,/, "1-10,20", 3)
6947 split(/(-|,)/, "1-10,20", 3)
6948 # ('1', '-', '10', ',', '20')
6950 split(/-|(,)/, "1-10,20", 3)
6951 # ('1', undef, '10', ',', '20')
6953 split(/(-)|,/, "1-10,20", 3)
6954 # ('1', '-', '10', undef, '20')
6956 split(/(-)|(,)/, "1-10,20", 3)
6957 # ('1', '-', undef, '10', undef, ',', '20')
6959 =item sprintf FORMAT, LIST
6962 =for Pod::Functions formatted print into a string
6964 Returns a string formatted by the usual C<printf> conventions of the C
6965 library function C<sprintf>. See below for more details
6966 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
6967 the general principles.
6971 # Format number with up to 8 leading zeroes
6972 $result = sprintf("%08d", $number);
6974 # Round number to 3 digits after decimal point
6975 $rounded = sprintf("%.3f", $number);
6977 Perl does its own C<sprintf> formatting: it emulates the C
6978 function sprintf(3), but doesn't use it except for floating-point
6979 numbers, and even then only standard modifiers are allowed.
6980 Non-standard extensions in your local sprintf(3) are
6981 therefore unavailable from Perl.
6983 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
6984 pass it an array as your first argument.
6985 The array is given scalar context,
6986 and instead of using the 0th element of the array as the format, Perl will
6987 use the count of elements in the array as the format, which is almost never
6990 Perl's C<sprintf> permits the following universally-known conversions:
6993 %c a character with the given number
6995 %d a signed integer, in decimal
6996 %u an unsigned integer, in decimal
6997 %o an unsigned integer, in octal
6998 %x an unsigned integer, in hexadecimal
6999 %e a floating-point number, in scientific notation
7000 %f a floating-point number, in fixed decimal notation
7001 %g a floating-point number, in %e or %f notation
7003 In addition, Perl permits the following widely-supported conversions:
7005 %X like %x, but using upper-case letters
7006 %E like %e, but using an upper-case "E"
7007 %G like %g, but with an upper-case "E" (if applicable)
7008 %b an unsigned integer, in binary
7009 %B like %b, but using an upper-case "B" with the # flag
7010 %p a pointer (outputs the Perl value's address in hexadecimal)
7011 %n special: *stores* the number of characters output so far
7012 into the next argument in the parameter list
7014 Finally, for backward (and we do mean "backward") compatibility, Perl
7015 permits these unnecessary but widely-supported conversions:
7018 %D a synonym for %ld
7019 %U a synonym for %lu
7020 %O a synonym for %lo
7023 Note that the number of exponent digits in the scientific notation produced
7024 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7025 exponent less than 100 is system-dependent: it may be three or less
7026 (zero-padded as necessary). In other words, 1.23 times ten to the
7027 99th may be either "1.23e99" or "1.23e099".
7029 Between the C<%> and the format letter, you may specify several
7030 additional attributes controlling the interpretation of the format.
7031 In order, these are:
7035 =item format parameter index
7037 An explicit format parameter index, such as C<2$>. By default sprintf
7038 will format the next unused argument in the list, but this allows you
7039 to take the arguments out of order:
7041 printf '%2$d %1$d', 12, 34; # prints "34 12"
7042 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7048 space prefix non-negative number with a space
7049 + prefix non-negative number with a plus sign
7050 - left-justify within the field
7051 0 use zeros, not spaces, to right-justify
7052 # ensure the leading "0" for any octal,
7053 prefix non-zero hexadecimal with "0x" or "0X",
7054 prefix non-zero binary with "0b" or "0B"
7058 printf '<% d>', 12; # prints "< 12>"
7059 printf '<%+d>', 12; # prints "<+12>"
7060 printf '<%6s>', 12; # prints "< 12>"
7061 printf '<%-6s>', 12; # prints "<12 >"
7062 printf '<%06s>', 12; # prints "<000012>"
7063 printf '<%#o>', 12; # prints "<014>"
7064 printf '<%#x>', 12; # prints "<0xc>"
7065 printf '<%#X>', 12; # prints "<0XC>"
7066 printf '<%#b>', 12; # prints "<0b1100>"
7067 printf '<%#B>', 12; # prints "<0B1100>"
7069 When a space and a plus sign are given as the flags at once,
7070 a plus sign is used to prefix a positive number.
7072 printf '<%+ d>', 12; # prints "<+12>"
7073 printf '<% +d>', 12; # prints "<+12>"
7075 When the # flag and a precision are given in the %o conversion,
7076 the precision is incremented if it's necessary for the leading "0".
7078 printf '<%#.5o>', 012; # prints "<00012>"
7079 printf '<%#.5o>', 012345; # prints "<012345>"
7080 printf '<%#.0o>', 0; # prints "<0>"
7084 This flag tells Perl to interpret the supplied string as a vector of
7085 integers, one for each character in the string. Perl applies the format to
7086 each integer in turn, then joins the resulting strings with a separator (a
7087 dot C<.> by default). This can be useful for displaying ordinal values of
7088 characters in arbitrary strings:
7090 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7091 printf "version is v%vd\n", $^V; # Perl's version
7093 Put an asterisk C<*> before the C<v> to override the string to
7094 use to separate the numbers:
7096 printf "address is %*vX\n", ":", $addr; # IPv6 address
7097 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7099 You can also explicitly specify the argument number to use for
7100 the join string using something like C<*2$v>; for example:
7102 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7105 =item (minimum) width
7107 Arguments are usually formatted to be only as wide as required to
7108 display the given value. You can override the width by putting
7109 a number here, or get the width from the next argument (with C<*>)
7110 or from a specified argument (e.g., with C<*2$>):
7112 printf "<%s>", "a"; # prints "<a>"
7113 printf "<%6s>", "a"; # prints "< a>"
7114 printf "<%*s>", 6, "a"; # prints "< a>"
7115 printf '<%*2$s>', "a", 6; # prints "< a>"
7116 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7118 If a field width obtained through C<*> is negative, it has the same
7119 effect as the C<-> flag: left-justification.
7121 =item precision, or maximum width
7124 You can specify a precision (for numeric conversions) or a maximum
7125 width (for string conversions) by specifying a C<.> followed by a number.
7126 For floating-point formats except C<g> and C<G>, this specifies
7127 how many places right of the decimal point to show (the default being 6).
7130 # these examples are subject to system-specific variation
7131 printf '<%f>', 1; # prints "<1.000000>"
7132 printf '<%.1f>', 1; # prints "<1.0>"
7133 printf '<%.0f>', 1; # prints "<1>"
7134 printf '<%e>', 10; # prints "<1.000000e+01>"
7135 printf '<%.1e>', 10; # prints "<1.0e+01>"
7137 For "g" and "G", this specifies the maximum number of digits to show,
7138 including those prior to the decimal point and those after it; for
7141 # These examples are subject to system-specific variation.
7142 printf '<%g>', 1; # prints "<1>"
7143 printf '<%.10g>', 1; # prints "<1>"
7144 printf '<%g>', 100; # prints "<100>"
7145 printf '<%.1g>', 100; # prints "<1e+02>"
7146 printf '<%.2g>', 100.01; # prints "<1e+02>"
7147 printf '<%.5g>', 100.01; # prints "<100.01>"
7148 printf '<%.4g>', 100.01; # prints "<100>"
7150 For integer conversions, specifying a precision implies that the
7151 output of the number itself should be zero-padded to this width,
7152 where the 0 flag is ignored:
7154 printf '<%.6d>', 1; # prints "<000001>"
7155 printf '<%+.6d>', 1; # prints "<+000001>"
7156 printf '<%-10.6d>', 1; # prints "<000001 >"
7157 printf '<%10.6d>', 1; # prints "< 000001>"
7158 printf '<%010.6d>', 1; # prints "< 000001>"
7159 printf '<%+10.6d>', 1; # prints "< +000001>"
7161 printf '<%.6x>', 1; # prints "<000001>"
7162 printf '<%#.6x>', 1; # prints "<0x000001>"
7163 printf '<%-10.6x>', 1; # prints "<000001 >"
7164 printf '<%10.6x>', 1; # prints "< 000001>"
7165 printf '<%010.6x>', 1; # prints "< 000001>"
7166 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7168 For string conversions, specifying a precision truncates the string
7169 to fit the specified width:
7171 printf '<%.5s>', "truncated"; # prints "<trunc>"
7172 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7174 You can also get the precision from the next argument using C<.*>:
7176 printf '<%.6x>', 1; # prints "<000001>"
7177 printf '<%.*x>', 6, 1; # prints "<000001>"
7179 If a precision obtained through C<*> is negative, it counts
7180 as having no precision at all.
7182 printf '<%.*s>', 7, "string"; # prints "<string>"
7183 printf '<%.*s>', 3, "string"; # prints "<str>"
7184 printf '<%.*s>', 0, "string"; # prints "<>"
7185 printf '<%.*s>', -1, "string"; # prints "<string>"
7187 printf '<%.*d>', 1, 0; # prints "<0>"
7188 printf '<%.*d>', 0, 0; # prints "<>"
7189 printf '<%.*d>', -1, 0; # prints "<0>"
7191 You cannot currently get the precision from a specified number,
7192 but it is intended that this will be possible in the future, for
7193 example using C<.*2$>:
7195 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print
7200 For numeric conversions, you can specify the size to interpret the
7201 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7202 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7203 whatever the default integer size is on your platform (usually 32 or 64
7204 bits), but you can override this to use instead one of the standard C types,
7205 as supported by the compiler used to build Perl:
7207 hh interpret integer as C type "char" or "unsigned
7208 char" on Perl 5.14 or later
7209 h interpret integer as C type "short" or
7211 j interpret integer as C type "intmax_t" on Perl
7212 5.14 or later, and only with a C99 compiler
7214 l interpret integer as C type "long" or
7216 q, L, or ll interpret integer as C type "long long",
7217 "unsigned long long", or "quad" (typically
7219 t interpret integer as C type "ptrdiff_t" on Perl
7221 z interpret integer as C type "size_t" on Perl 5.14
7224 As of 5.14, none of these raises an exception if they are not supported on
7225 your platform. However, if warnings are enabled, a warning of the
7226 C<printf> warning class is issued on an unsupported conversion flag.
7227 Should you instead prefer an exception, do this:
7229 use warnings FATAL => "printf";
7231 If you would like to know about a version dependency before you
7232 start running the program, put something like this at its top:
7234 use 5.014; # for hh/j/t/z/ printf modifiers
7236 You can find out whether your Perl supports quads via L<Config>:
7239 if ($Config{use64bitint} eq "define"
7240 || $Config{longsize} >= 8) {
7241 print "Nice quads!\n";
7244 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7245 to be the default floating-point size on your platform (double or long double),
7246 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7247 platform supports them. You can find out whether your Perl supports long
7248 doubles via L<Config>:
7251 print "long doubles\n" if $Config{d_longdbl} eq "define";
7253 You can find out whether Perl considers "long double" to be the default
7254 floating-point size to use on your platform via L<Config>:
7257 if ($Config{uselongdouble} eq "define") {
7258 print "long doubles by default\n";
7261 It can also be that long doubles and doubles are the same thing:
7264 ($Config{doublesize} == $Config{longdblsize}) &&
7265 print "doubles are long doubles\n";
7267 The size specifier C<V> has no effect for Perl code, but is supported for
7268 compatibility with XS code. It means "use the standard size for a Perl
7269 integer or floating-point number", which is the default.
7271 =item order of arguments
7273 Normally, sprintf() takes the next unused argument as the value to
7274 format for each format specification. If the format specification
7275 uses C<*> to require additional arguments, these are consumed from
7276 the argument list in the order they appear in the format
7277 specification I<before> the value to format. Where an argument is
7278 specified by an explicit index, this does not affect the normal
7279 order for the arguments, even when the explicitly specified index
7280 would have been the next argument.
7284 printf "<%*.*s>", $a, $b, $c;
7286 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7287 as the value to format; while:
7289 printf '<%*1$.*s>', $a, $b;
7291 would use C<$a> for the width and precision, and C<$b> as the
7294 Here are some more examples; be aware that when using an explicit
7295 index, the C<$> may need escaping:
7297 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7298 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7299 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7300 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7304 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7305 and POSIX::setlocale() has been called,
7306 the character used for the decimal separator in formatted floating-point
7307 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
7311 X<sqrt> X<root> X<square root>
7315 =for Pod::Functions square root function
7317 Return the positive square root of EXPR. If EXPR is omitted, uses
7318 C<$_>. Works only for non-negative operands unless you've
7319 loaded the C<Math::Complex> module.
7322 print sqrt(-4); # prints 2i
7325 X<srand> X<seed> X<randseed>
7329 =for Pod::Functions seed the random number generator
7331 Sets and returns the random number seed for the C<rand> operator.
7333 The point of the function is to "seed" the C<rand> function so that C<rand>
7334 can produce a different sequence each time you run your program. When
7335 called with a parameter, C<srand> uses that for the seed; otherwise it
7336 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7337 it returns the seed. To signal that your code will work I<only> on Perls
7338 of a recent vintage:
7340 use 5.014; # so srand returns the seed
7342 If C<srand()> is not called explicitly, it is called implicitly without a
7343 parameter at the first use of the C<rand> operator.
7344 However, there are a few situations where programs are likely to
7345 want to call C<srand>. One is for generating predictable results, generally for
7346 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7347 each time. Another case is that you may want to call C<srand()>
7348 after a C<fork()> to avoid child processes sharing the same seed value as the
7349 parent (and consequently each other).
7351 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7352 process. The internal state of the random number generator should
7353 contain more entropy than can be provided by any seed, so calling
7354 C<srand()> again actually I<loses> randomness.
7356 Most implementations of C<srand> take an integer and will silently
7357 truncate decimal numbers. This means C<srand(42)> will usually
7358 produce the same results as C<srand(42.1)>. To be safe, always pass
7359 C<srand> an integer.
7361 A typical use of the returned seed is for a test program which has too many
7362 combinations to test comprehensively in the time available to it each run. It
7363 can test a random subset each time, and should there be a failure, log the seed
7364 used for that run so that it can later be used to reproduce the same results.
7366 B<C<rand()> is not cryptographically secure. You should not rely
7367 on it in security-sensitive situations.> As of this writing, a
7368 number of third-party CPAN modules offer random number generators
7369 intended by their authors to be cryptographically secure,
7370 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7371 and L<Math::TrulyRandom>.
7373 =item stat FILEHANDLE
7374 X<stat> X<file, status> X<ctime>
7378 =item stat DIRHANDLE
7382 =for Pod::Functions get a file's status information
7384 Returns a 13-element list giving the status info for a file, either
7385 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7386 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7389 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7390 $atime,$mtime,$ctime,$blksize,$blocks)
7393 Not all fields are supported on all filesystem types. Here are the
7394 meanings of the fields:
7396 0 dev device number of filesystem
7398 2 mode file mode (type and permissions)
7399 3 nlink number of (hard) links to the file
7400 4 uid numeric user ID of file's owner
7401 5 gid numeric group ID of file's owner
7402 6 rdev the device identifier (special files only)
7403 7 size total size of file, in bytes
7404 8 atime last access time in seconds since the epoch
7405 9 mtime last modify time in seconds since the epoch
7406 10 ctime inode change time in seconds since the epoch (*)
7407 11 blksize preferred I/O size in bytes for interacting with the
7408 file (may vary from file to file)
7409 12 blocks actual number of system-specific blocks allocated
7410 on disk (often, but not always, 512 bytes each)
7412 (The epoch was at 00:00 January 1, 1970 GMT.)
7414 (*) Not all fields are supported on all filesystem types. Notably, the
7415 ctime field is non-portable. In particular, you cannot expect it to be a
7416 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7418 If C<stat> is passed the special filehandle consisting of an underline, no
7419 stat is done, but the current contents of the stat structure from the
7420 last C<stat>, C<lstat>, or filetest are returned. Example:
7422 if (-x $file && (($d) = stat(_)) && $d < 0) {
7423 print "$file is executable NFS file\n";
7426 (This works on machines only for which the device number is negative
7429 Because the mode contains both the file type and its permissions, you
7430 should mask off the file type portion and (s)printf using a C<"%o">
7431 if you want to see the real permissions.
7433 $mode = (stat($filename))[2];
7434 printf "Permissions are %04o\n", $mode & 07777;
7436 In scalar context, C<stat> returns a boolean value indicating success
7437 or failure, and, if successful, sets the information associated with
7438 the special filehandle C<_>.
7440 The L<File::stat> module provides a convenient, by-name access mechanism:
7443 $sb = stat($filename);
7444 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7445 $filename, $sb->size, $sb->mode & 07777,
7446 scalar localtime $sb->mtime;
7448 You can import symbolic mode constants (C<S_IF*>) and functions
7449 (C<S_IS*>) from the Fcntl module:
7453 $mode = (stat($filename))[2];
7455 $user_rwx = ($mode & S_IRWXU) >> 6;
7456 $group_read = ($mode & S_IRGRP) >> 3;
7457 $other_execute = $mode & S_IXOTH;
7459 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7461 $is_setuid = $mode & S_ISUID;
7462 $is_directory = S_ISDIR($mode);
7464 You could write the last two using the C<-u> and C<-d> operators.
7465 Commonly available C<S_IF*> constants are:
7467 # Permissions: read, write, execute, for user, group, others.
7469 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7470 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7471 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7473 # Setuid/Setgid/Stickiness/SaveText.
7474 # Note that the exact meaning of these is system-dependent.
7476 S_ISUID S_ISGID S_ISVTX S_ISTXT
7478 # File types. Not all are necessarily available on
7481 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7482 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7484 # The following are compatibility aliases for S_IRUSR,
7485 # S_IWUSR, and S_IXUSR.
7487 S_IREAD S_IWRITE S_IEXEC
7489 and the C<S_IF*> functions are
7491 S_IMODE($mode) the part of $mode containing the permission
7492 bits and the setuid/setgid/sticky bits
7494 S_IFMT($mode) the part of $mode containing the file type
7495 which can be bit-anded with (for example)
7496 S_IFREG or with the following functions
7498 # The operators -f, -d, -l, -b, -c, -p, and -S.
7500 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7501 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7503 # No direct -X operator counterpart, but for the first one
7504 # the -g operator is often equivalent. The ENFMT stands for
7505 # record flocking enforcement, a platform-dependent feature.
7507 S_ISENFMT($mode) S_ISWHT($mode)
7509 See your native chmod(2) and stat(2) documentation for more details
7510 about the C<S_*> constants. To get status info for a symbolic link
7511 instead of the target file behind the link, use the C<lstat> function.
7513 Portability issues: L<perlport/stat>.
7518 =item state TYPE EXPR
7520 =item state EXPR : ATTRS
7522 =item state TYPE EXPR : ATTRS
7524 =for Pod::Functions +state declare and assign a persistent lexical variable
7526 C<state> declares a lexically scoped variable, just like C<my>.
7527 However, those variables will never be reinitialized, contrary to
7528 lexical variables that are reinitialized each time their enclosing block
7530 See L<perlsub/"Persistent Private Variables"> for details.
7532 C<state> variables are enabled only when the C<use feature "state"> pragma
7533 is in effect, unless the keyword is written as C<CORE::state>.
7534 See also L<feature>.
7541 =for Pod::Functions optimize input data for repeated searches
7543 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
7544 doing many pattern matches on the string before it is next modified.
7545 This may or may not save time, depending on the nature and number of
7546 patterns you are searching and the distribution of character
7547 frequencies in the string to be searched; you probably want to compare
7548 run times with and without it to see which is faster. Those loops
7549 that scan for many short constant strings (including the constant
7550 parts of more complex patterns) will benefit most.
7551 (The way C<study> works is this: a linked list of every
7552 character in the string to be searched is made, so we know, for
7553 example, where all the C<'k'> characters are. From each search string,
7554 the rarest character is selected, based on some static frequency tables
7555 constructed from some C programs and English text. Only those places
7556 that contain this "rarest" character are examined.)
7558 For example, here is a loop that inserts index producing entries
7559 before any line containing a certain pattern:
7563 print ".IX foo\n" if /\bfoo\b/;
7564 print ".IX bar\n" if /\bbar\b/;
7565 print ".IX blurfl\n" if /\bblurfl\b/;
7570 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7571 will be looked at, because C<f> is rarer than C<o>. In general, this is
7572 a big win except in pathological cases. The only question is whether
7573 it saves you more time than it took to build the linked list in the
7576 Note that if you have to look for strings that you don't know till
7577 runtime, you can build an entire loop as a string and C<eval> that to
7578 avoid recompiling all your patterns all the time. Together with
7579 undefining C<$/> to input entire files as one record, this can be quite
7580 fast, often faster than specialized programs like fgrep(1). The following
7581 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7582 out the names of those files that contain a match:
7584 $search = 'while (<>) { study;';
7585 foreach $word (@words) {
7586 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7591 eval $search; # this screams
7592 $/ = "\n"; # put back to normal input delimiter
7593 foreach $file (sort keys(%seen)) {
7597 =item sub NAME BLOCK
7600 =item sub NAME (PROTO) BLOCK
7602 =item sub NAME : ATTRS BLOCK
7604 =item sub NAME (PROTO) : ATTRS BLOCK
7606 =for Pod::Functions declare a subroutine, possibly anonymously
7608 This is subroutine definition, not a real function I<per se>. Without a
7609 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7610 function declaration, so does return a value: the CODE ref of the closure
7613 See L<perlsub> and L<perlref> for details about subroutines and
7614 references; see L<attributes> and L<Attribute::Handlers> for more
7615 information about attributes.
7620 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7622 A special token that returns the a reference to the current subroutine, or
7623 C<undef> outside of a subroutine.
7625 This token is only available under C<use v5.16> or the "current_sub"
7626 feature. See L<feature>.
7628 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7629 X<substr> X<substring> X<mid> X<left> X<right>
7631 =item substr EXPR,OFFSET,LENGTH
7633 =item substr EXPR,OFFSET
7635 =for Pod::Functions get or alter a portion of a string
7637 Extracts a substring out of EXPR and returns it. First character is at
7638 offset zero. If OFFSET is negative, starts
7639 that far back from the end of the string. If LENGTH is omitted, returns
7640 everything through the end of the string. If LENGTH is negative, leaves that
7641 many characters off the end of the string.
7643 my $s = "The black cat climbed the green tree";
7644 my $color = substr $s, 4, 5; # black
7645 my $middle = substr $s, 4, -11; # black cat climbed the
7646 my $end = substr $s, 14; # climbed the green tree
7647 my $tail = substr $s, -4; # tree
7648 my $z = substr $s, -4, 2; # tr
7650 You can use the substr() function as an lvalue, in which case EXPR
7651 must itself be an lvalue. If you assign something shorter than LENGTH,
7652 the string will shrink, and if you assign something longer than LENGTH,
7653 the string will grow to accommodate it. To keep the string the same
7654 length, you may need to pad or chop your value using C<sprintf>.
7656 If OFFSET and LENGTH specify a substring that is partly outside the
7657 string, only the part within the string is returned. If the substring
7658 is beyond either end of the string, substr() returns the undefined
7659 value and produces a warning. When used as an lvalue, specifying a
7660 substring that is entirely outside the string raises an exception.
7661 Here's an example showing the behavior for boundary cases:
7664 substr($name, 4) = 'dy'; # $name is now 'freddy'
7665 my $null = substr $name, 6, 2; # returns "" (no warning)
7666 my $oops = substr $name, 7; # returns undef, with warning
7667 substr($name, 7) = 'gap'; # raises an exception
7669 An alternative to using substr() as an lvalue is to specify the
7670 replacement string as the 4th argument. This allows you to replace
7671 parts of the EXPR and return what was there before in one operation,
7672 just as you can with splice().
7674 my $s = "The black cat climbed the green tree";
7675 my $z = substr $s, 14, 7, "jumped from"; # climbed
7676 # $s is now "The black cat jumped from the green tree"
7678 Note that the lvalue returned by the three-argument version of substr() acts as
7679 a 'magic bullet'; each time it is assigned to, it remembers which part
7680 of the original string is being modified; for example:
7683 for (substr($x,1,2)) {
7684 $_ = 'a'; print $x,"\n"; # prints 1a4
7685 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7687 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7690 With negative offsets, it remembers its position from the end of the string
7691 when the target string is modified:
7694 for (substr($x, -3, 2)) {
7695 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7697 print $_,"\n"; # prints f
7700 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7701 unspecified. Prior to 5.16, the result with negative offsets was
7704 =item symlink OLDFILE,NEWFILE
7705 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7707 =for Pod::Functions create a symbolic link to a file
7709 Creates a new filename symbolically linked to the old filename.
7710 Returns C<1> for success, C<0> otherwise. On systems that don't support
7711 symbolic links, raises an exception. To check for that,
7714 $symlink_exists = eval { symlink("",""); 1 };
7716 Portability issues: L<perlport/symlink>.
7718 =item syscall NUMBER, LIST
7719 X<syscall> X<system call>
7721 =for Pod::Functions execute an arbitrary system call
7723 Calls the system call specified as the first element of the list,
7724 passing the remaining elements as arguments to the system call. If
7725 unimplemented, raises an exception. The arguments are interpreted
7726 as follows: if a given argument is numeric, the argument is passed as
7727 an int. If not, the pointer to the string value is passed. You are
7728 responsible to make sure a string is pre-extended long enough to
7729 receive any result that might be written into a string. You can't use a
7730 string literal (or other read-only string) as an argument to C<syscall>
7731 because Perl has to assume that any string pointer might be written
7733 integer arguments are not literals and have never been interpreted in a
7734 numeric context, you may need to add C<0> to them to force them to look
7735 like numbers. This emulates the C<syswrite> function (or vice versa):
7737 require 'syscall.ph'; # may need to run h2ph
7739 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7741 Note that Perl supports passing of up to only 14 arguments to your syscall,
7742 which in practice should (usually) suffice.
7744 Syscall returns whatever value returned by the system call it calls.
7745 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7746 Note that some system calls I<can> legitimately return C<-1>. The proper
7747 way to handle such calls is to assign C<$!=0> before the call, then
7748 check the value of C<$!> if C<syscall> returns C<-1>.
7750 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7751 number of the read end of the pipe it creates, but there is no way
7752 to retrieve the file number of the other end. You can avoid this
7753 problem by using C<pipe> instead.
7755 Portability issues: L<perlport/syscall>.
7757 =item sysopen FILEHANDLE,FILENAME,MODE
7760 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7762 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7764 Opens the file whose filename is given by FILENAME, and associates it with
7765 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7766 filehandle wanted; an undefined scalar will be suitably autovivified. This
7767 function calls the underlying operating system's I<open>(2) function with the
7768 parameters FILENAME, MODE, and PERMS.
7770 The possible values and flag bits of the MODE parameter are
7771 system-dependent; they are available via the standard module C<Fcntl>. See
7772 the documentation of your operating system's I<open>(2) syscall to see
7773 which values and flag bits are available. You may combine several flags
7774 using the C<|>-operator.
7776 Some of the most common values are C<O_RDONLY> for opening the file in
7777 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7778 and C<O_RDWR> for opening the file in read-write mode.
7779 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7781 For historical reasons, some values work on almost every system
7782 supported by Perl: 0 means read-only, 1 means write-only, and 2
7783 means read/write. We know that these values do I<not> work under
7784 OS/390 and on the Macintosh; you probably don't want to
7785 use them in new code.
7787 If the file named by FILENAME does not exist and the C<open> call creates
7788 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7789 PERMS specifies the permissions of the newly created file. If you omit
7790 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7791 These permission values need to be in octal, and are modified by your
7792 process's current C<umask>.
7795 In many systems the C<O_EXCL> flag is available for opening files in
7796 exclusive mode. This is B<not> locking: exclusiveness means here that
7797 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7798 on network filesystems, and has no effect unless the C<O_CREAT> flag
7799 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7800 being opened if it is a symbolic link. It does not protect against
7801 symbolic links in the file's path.
7804 Sometimes you may want to truncate an already-existing file. This
7805 can be done using the C<O_TRUNC> flag. The behavior of
7806 C<O_TRUNC> with C<O_RDONLY> is undefined.
7809 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7810 that takes away the user's option to have a more permissive umask.
7811 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7814 Note that C<sysopen> depends on the fdopen() C library function.
7815 On many Unix systems, fdopen() is known to fail when file descriptors
7816 exceed a certain value, typically 255. If you need more file
7817 descriptors than that, consider rebuilding Perl to use the C<sfio>
7818 library, or perhaps using the POSIX::open() function.
7820 See L<perlopentut> for a kinder, gentler explanation of opening files.
7822 Portability issues: L<perlport/sysopen>.
7824 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7827 =item sysread FILEHANDLE,SCALAR,LENGTH
7829 =for Pod::Functions fixed-length unbuffered input from a filehandle
7831 Attempts to read LENGTH bytes of data into variable SCALAR from the
7832 specified FILEHANDLE, using the read(2). It bypasses
7833 buffered IO, so mixing this with other kinds of reads, C<print>,
7834 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7835 perlio or stdio layers usually buffers data. Returns the number of
7836 bytes actually read, C<0> at end of file, or undef if there was an
7837 error (in the latter case C<$!> is also set). SCALAR will be grown or
7838 shrunk so that the last byte actually read is the last byte of the
7839 scalar after the read.
7841 An OFFSET may be specified to place the read data at some place in the
7842 string other than the beginning. A negative OFFSET specifies
7843 placement at that many characters counting backwards from the end of
7844 the string. A positive OFFSET greater than the length of SCALAR
7845 results in the string being padded to the required size with C<"\0">
7846 bytes before the result of the read is appended.
7848 There is no syseof() function, which is ok, since eof() doesn't work
7849 well on device files (like ttys) anyway. Use sysread() and check
7850 for a return value for 0 to decide whether you're done.
7852 Note that if the filehandle has been marked as C<:utf8> Unicode
7853 characters are read instead of bytes (the LENGTH, OFFSET, and the
7854 return value of sysread() are in Unicode characters).
7855 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7856 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7858 =item sysseek FILEHANDLE,POSITION,WHENCE
7861 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
7863 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7864 be an expression whose value gives the name of the filehandle. The values
7865 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7866 to the current position plus POSITION; and C<2> to set it to EOF plus
7867 POSITION, typically negative.
7869 Note the I<in bytes>: even if the filehandle has been set to operate
7870 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7871 tell() will return byte offsets, not character offsets (because
7872 implementing that would render sysseek() unacceptably slow).
7874 sysseek() bypasses normal buffered IO, so mixing it with reads other
7875 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7876 C<seek>, C<tell>, or C<eof> may cause confusion.
7878 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7879 and C<SEEK_END> (start of the file, current position, end of the file)
7880 from the Fcntl module. Use of the constants is also more portable
7881 than relying on 0, 1, and 2. For example to define a "systell" function:
7883 use Fcntl 'SEEK_CUR';
7884 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7886 Returns the new position, or the undefined value on failure. A position
7887 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7888 true on success and false on failure, yet you can still easily determine
7894 =item system PROGRAM LIST
7896 =for Pod::Functions run a separate program
7898 Does exactly the same thing as C<exec LIST>, except that a fork is
7899 done first and the parent process waits for the child process to
7900 exit. Note that argument processing varies depending on the
7901 number of arguments. If there is more than one argument in LIST,
7902 or if LIST is an array with more than one value, starts the program
7903 given by the first element of the list with arguments given by the
7904 rest of the list. If there is only one scalar argument, the argument
7905 is checked for shell metacharacters, and if there are any, the
7906 entire argument is passed to the system's command shell for parsing
7907 (this is C</bin/sh -c> on Unix platforms, but varies on other
7908 platforms). If there are no shell metacharacters in the argument,
7909 it is split into words and passed directly to C<execvp>, which is
7912 Perl will attempt to flush all files opened for
7913 output before any operation that may do a fork, but this may not be
7914 supported on some platforms (see L<perlport>). To be safe, you may need
7915 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7916 of C<IO::Handle> on any open handles.
7918 The return value is the exit status of the program as returned by the
7919 C<wait> call. To get the actual exit value, shift right by eight (see
7920 below). See also L</exec>. This is I<not> what you want to use to capture
7921 the output from a command; for that you should use merely backticks or
7922 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7923 indicates a failure to start the program or an error of the wait(2) system
7924 call (inspect $! for the reason).
7926 If you'd like to make C<system> (and many other bits of Perl) die on error,
7927 have a look at the L<autodie> pragma.
7929 Like C<exec>, C<system> allows you to lie to a program about its name if
7930 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
7932 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
7933 C<system>, if you expect your program to terminate on receipt of these
7934 signals you will need to arrange to do so yourself based on the return
7937 @args = ("command", "arg1", "arg2");
7939 or die "system @args failed: $?"
7941 If you'd like to manually inspect C<system>'s failure, you can check all
7942 possible failure modes by inspecting C<$?> like this:
7945 print "failed to execute: $!\n";
7948 printf "child died with signal %d, %s coredump\n",
7949 ($? & 127), ($? & 128) ? 'with' : 'without';
7952 printf "child exited with value %d\n", $? >> 8;
7955 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
7956 with the C<W*()> calls from the POSIX module.
7958 When C<system>'s arguments are executed indirectly by the shell,
7959 results and return codes are subject to its quirks.
7960 See L<perlop/"`STRING`"> and L</exec> for details.
7962 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
7963 handler. See L<perlipc> for details.
7965 Portability issues: L<perlport/system>.
7967 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7970 =item syswrite FILEHANDLE,SCALAR,LENGTH
7972 =item syswrite FILEHANDLE,SCALAR
7974 =for Pod::Functions fixed-length unbuffered output to a filehandle
7976 Attempts to write LENGTH bytes of data from variable SCALAR to the
7977 specified FILEHANDLE, using write(2). If LENGTH is
7978 not specified, writes whole SCALAR. It bypasses buffered IO, so
7979 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
7980 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
7981 stdio layers usually buffer data. Returns the number of bytes
7982 actually written, or C<undef> if there was an error (in this case the
7983 errno variable C<$!> is also set). If the LENGTH is greater than the
7984 data available in the SCALAR after the OFFSET, only as much data as is
7985 available will be written.
7987 An OFFSET may be specified to write the data from some part of the
7988 string other than the beginning. A negative OFFSET specifies writing
7989 that many characters counting backwards from the end of the string.
7990 If SCALAR is of length zero, you can only use an OFFSET of 0.
7992 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
7993 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
7994 return value of syswrite() are in (UTF8-encoded Unicode) characters.
7995 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7996 Alternately, if the handle is not marked with an encoding but you
7997 attempt to write characters with code points over 255, raises an exception.
7998 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8000 =item tell FILEHANDLE
8005 =for Pod::Functions get current seekpointer on a filehandle
8007 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8008 error. FILEHANDLE may be an expression whose value gives the name of
8009 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8012 Note the I<in bytes>: even if the filehandle has been set to
8013 operate on characters (for example by using the C<:encoding(utf8)> open
8014 layer), tell() will return byte offsets, not character offsets (because
8015 that would render seek() and tell() rather slow).
8017 The return value of tell() for the standard streams like the STDIN
8018 depends on the operating system: it may return -1 or something else.
8019 tell() on pipes, fifos, and sockets usually returns -1.
8021 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
8023 Do not use tell() (or other buffered I/O operations) on a filehandle
8024 that has been manipulated by sysread(), syswrite(), or sysseek().
8025 Those functions ignore the buffering, while tell() does not.
8027 =item telldir DIRHANDLE
8030 =for Pod::Functions get current seekpointer on a directory handle
8032 Returns the current position of the C<readdir> routines on DIRHANDLE.
8033 Value may be given to C<seekdir> to access a particular location in a
8034 directory. C<telldir> has the same caveats about possible directory
8035 compaction as the corresponding system library routine.
8037 =item tie VARIABLE,CLASSNAME,LIST
8040 =for Pod::Functions +5.002 bind a variable to an object class
8042 This function binds a variable to a package class that will provide the
8043 implementation for the variable. VARIABLE is the name of the variable
8044 to be enchanted. CLASSNAME is the name of a class implementing objects
8045 of correct type. Any additional arguments are passed to the C<new>
8046 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8047 or C<TIEHASH>). Typically these are arguments such as might be passed
8048 to the C<dbm_open()> function of C. The object returned by the C<new>
8049 method is also returned by the C<tie> function, which would be useful
8050 if you want to access other methods in CLASSNAME.
8052 Note that functions such as C<keys> and C<values> may return huge lists
8053 when used on large objects, like DBM files. You may prefer to use the
8054 C<each> function to iterate over such. Example:
8056 # print out history file offsets
8058 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8059 while (($key,$val) = each %HIST) {
8060 print $key, ' = ', unpack('L',$val), "\n";
8064 A class implementing a hash should have the following methods:
8066 TIEHASH classname, LIST
8068 STORE this, key, value
8073 NEXTKEY this, lastkey
8078 A class implementing an ordinary array should have the following methods:
8080 TIEARRAY classname, LIST
8082 STORE this, key, value
8084 STORESIZE this, count
8090 SPLICE this, offset, length, LIST
8095 A class implementing a filehandle should have the following methods:
8097 TIEHANDLE classname, LIST
8098 READ this, scalar, length, offset
8101 WRITE this, scalar, length, offset
8103 PRINTF this, format, LIST
8107 SEEK this, position, whence
8109 OPEN this, mode, LIST
8114 A class implementing a scalar should have the following methods:
8116 TIESCALAR classname, LIST
8122 Not all methods indicated above need be implemented. See L<perltie>,
8123 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8125 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8126 for you; you need to do that explicitly yourself. See L<DB_File>
8127 or the F<Config> module for interesting C<tie> implementations.
8129 For further details see L<perltie>, L<"tied VARIABLE">.
8134 =for Pod::Functions get a reference to the object underlying a tied variable
8136 Returns a reference to the object underlying VARIABLE (the same value
8137 that was originally returned by the C<tie> call that bound the variable
8138 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8144 =for Pod::Functions return number of seconds since 1970
8146 Returns the number of non-leap seconds since whatever time the system
8147 considers to be the epoch, suitable for feeding to C<gmtime> and
8148 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8149 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8150 1904 in the current local time zone for its epoch.
8152 For measuring time in better granularity than one second, use the
8153 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8154 if you have gettimeofday(2), you may be able to use the C<syscall>
8155 interface of Perl. See L<perlfaq8> for details.
8157 For date and time processing look at the many related modules on CPAN.
8158 For a comprehensive date and time representation look at the
8164 =for Pod::Functions return elapsed time for self and child processes
8166 Returns a four-element list giving the user and system times in
8167 seconds for this process and any exited children of this process.
8169 ($user,$system,$cuser,$csystem) = times;
8171 In scalar context, C<times> returns C<$user>.
8173 Children's times are only included for terminated children.
8175 Portability issues: L<perlport/times>.
8179 =for Pod::Functions transliterate a string
8181 The transliteration operator. Same as C<y///>. See
8182 L<perlop/"Quote and Quote-like Operators">.
8184 =item truncate FILEHANDLE,LENGTH
8187 =item truncate EXPR,LENGTH
8189 =for Pod::Functions shorten a file
8191 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8192 specified length. Raises an exception if truncate isn't implemented
8193 on your system. Returns true if successful, C<undef> on error.
8195 The behavior is undefined if LENGTH is greater than the length of the
8198 The position in the file of FILEHANDLE is left unchanged. You may want to
8199 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8201 Portability issues: L<perlport/truncate>.
8204 X<uc> X<uppercase> X<toupper>
8208 =for Pod::Functions return upper-case version of a string
8210 Returns an uppercased version of EXPR. This is the internal function
8211 implementing the C<\U> escape in double-quoted strings.
8212 It does not attempt to do titlecase mapping on initial letters. See
8213 L</ucfirst> for that.
8215 If EXPR is omitted, uses C<$_>.
8217 This function behaves the same way under various pragma, such as in a locale,
8221 X<ucfirst> X<uppercase>
8225 =for Pod::Functions return a string with just the next letter in upper case
8227 Returns the value of EXPR with the first character in uppercase
8228 (titlecase in Unicode). This is the internal function implementing
8229 the C<\u> escape in double-quoted strings.
8231 If EXPR is omitted, uses C<$_>.
8233 This function behaves the same way under various pragma, such as in a locale,
8241 =for Pod::Functions set file creation mode mask
8243 Sets the umask for the process to EXPR and returns the previous value.
8244 If EXPR is omitted, merely returns the current umask.
8246 The Unix permission C<rwxr-x---> is represented as three sets of three
8247 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8248 and isn't one of the digits). The C<umask> value is such a number
8249 representing disabled permissions bits. The permission (or "mode")
8250 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8251 even if you tell C<sysopen> to create a file with permissions C<0777>,
8252 if your umask is C<0022>, then the file will actually be created with
8253 permissions C<0755>. If your C<umask> were C<0027> (group can't
8254 write; others can't read, write, or execute), then passing
8255 C<sysopen> C<0666> would create a file with mode C<0640> (because
8256 C<0666 &~ 027> is C<0640>).
8258 Here's some advice: supply a creation mode of C<0666> for regular
8259 files (in C<sysopen>) and one of C<0777> for directories (in
8260 C<mkdir>) and executable files. This gives users the freedom of
8261 choice: if they want protected files, they might choose process umasks
8262 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8263 Programs should rarely if ever make policy decisions better left to
8264 the user. The exception to this is when writing files that should be
8265 kept private: mail files, web browser cookies, I<.rhosts> files, and
8268 If umask(2) is not implemented on your system and you are trying to
8269 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8270 raises an exception. If umask(2) is not implemented and you are
8271 not trying to restrict access for yourself, returns C<undef>.
8273 Remember that a umask is a number, usually given in octal; it is I<not> a
8274 string of octal digits. See also L</oct>, if all you have is a string.
8276 Portability issues: L<perlport/umask>.
8279 X<undef> X<undefine>
8283 =for Pod::Functions remove a variable or function definition
8285 Undefines the value of EXPR, which must be an lvalue. Use only on a
8286 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8287 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8288 will probably not do what you expect on most predefined variables or
8289 DBM list values, so don't do that; see L</delete>. Always returns the
8290 undefined value. You can omit the EXPR, in which case nothing is
8291 undefined, but you still get an undefined value that you could, for
8292 instance, return from a subroutine, assign to a variable, or pass as a
8293 parameter. Examples:
8296 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8300 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8301 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8302 select undef, undef, undef, 0.25;
8303 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8305 Note that this is a unary operator, not a list operator.
8308 X<unlink> X<delete> X<remove> X<rm> X<del>
8312 =for Pod::Functions remove one link to a file
8314 Deletes a list of files. On success, it returns the number of files
8315 it successfully deleted. On failure, it returns false and sets C<$!>
8318 my $unlinked = unlink 'a', 'b', 'c';
8320 unlink glob "*.bak";
8322 On error, C<unlink> will not tell you which files it could not remove.
8323 If you want to know which files you could not remove, try them one
8326 foreach my $file ( @goners ) {
8327 unlink $file or warn "Could not unlink $file: $!";
8330 Note: C<unlink> will not attempt to delete directories unless you are
8331 superuser and the B<-U> flag is supplied to Perl. Even if these
8332 conditions are met, be warned that unlinking a directory can inflict
8333 damage on your filesystem. Finally, using C<unlink> on directories is
8334 not supported on many operating systems. Use C<rmdir> instead.
8336 If LIST is omitted, C<unlink> uses C<$_>.
8338 =item unpack TEMPLATE,EXPR
8341 =item unpack TEMPLATE
8343 =for Pod::Functions convert binary structure into normal perl variables
8345 C<unpack> does the reverse of C<pack>: it takes a string
8346 and expands it out into a list of values.
8347 (In scalar context, it returns merely the first value produced.)
8349 If EXPR is omitted, unpacks the C<$_> string.
8350 See L<perlpacktut> for an introduction to this function.
8352 The string is broken into chunks described by the TEMPLATE. Each chunk
8353 is converted separately to a value. Typically, either the string is a result
8354 of C<pack>, or the characters of the string represent a C structure of some
8357 The TEMPLATE has the same format as in the C<pack> function.
8358 Here's a subroutine that does substring:
8361 my($what,$where,$howmuch) = @_;
8362 unpack("x$where a$howmuch", $what);
8367 sub ordinal { unpack("W",$_[0]); } # same as ord()
8369 In addition to fields allowed in pack(), you may prefix a field with
8370 a %<number> to indicate that
8371 you want a <number>-bit checksum of the items instead of the items
8372 themselves. Default is a 16-bit checksum. Checksum is calculated by
8373 summing numeric values of expanded values (for string fields the sum of
8374 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8376 For example, the following
8377 computes the same number as the System V sum program:
8381 unpack("%32W*",<>) % 65535;
8384 The following efficiently counts the number of set bits in a bit vector:
8386 $setbits = unpack("%32b*", $selectmask);
8388 The C<p> and C<P> formats should be used with care. Since Perl
8389 has no way of checking whether the value passed to C<unpack()>
8390 corresponds to a valid memory location, passing a pointer value that's
8391 not known to be valid is likely to have disastrous consequences.
8393 If there are more pack codes or if the repeat count of a field or a group
8394 is larger than what the remainder of the input string allows, the result
8395 is not well defined: the repeat count may be decreased, or
8396 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8397 If the input string is longer than one described by the TEMPLATE,
8398 the remainder of that input string is ignored.
8400 See L</pack> for more examples and notes.
8402 =item unshift ARRAY,LIST
8405 =item unshift EXPR,LIST
8407 =for Pod::Functions prepend more elements to the beginning of a list
8409 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8410 depending on how you look at it. Prepends list to the front of the
8411 array and returns the new number of elements in the array.
8413 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8415 Note the LIST is prepended whole, not one element at a time, so the
8416 prepended elements stay in the same order. Use C<reverse> to do the
8419 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
8420 a reference to an unblessed array. The argument will be dereferenced
8421 automatically. This aspect of C<unshift> is considered highly
8422 experimental. The exact behaviour may change in a future version of Perl.
8424 To avoid confusing would-be users of your code who are running earlier
8425 versions of Perl with mysterious syntax errors, put this sort of thing at
8426 the top of your file to signal that your code will work I<only> on Perls of
8429 use 5.014; # so push/pop/etc work on scalars (experimental)
8431 =item untie VARIABLE
8434 =for Pod::Functions break a tie binding to a variable
8436 Breaks the binding between a variable and a package.
8437 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8438 Has no effect if the variable is not tied.
8440 =item use Module VERSION LIST
8441 X<use> X<module> X<import>
8443 =item use Module VERSION
8445 =item use Module LIST
8451 =for Pod::Functions load in a module at compile time and import its namespace
8453 Imports some semantics into the current package from the named module,
8454 generally by aliasing certain subroutine or variable names into your
8455 package. It is exactly equivalent to
8457 BEGIN { require Module; Module->import( LIST ); }
8459 except that Module I<must> be a bareword.
8460 The importation can be made conditional by using the L<if> module.
8462 In the peculiar C<use VERSION> form, VERSION may be either a positive
8463 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8464 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8465 exception is raised if VERSION is greater than the version of the
8466 current Perl interpreter; Perl will not attempt to parse the rest of the
8467 file. Compare with L</require>, which can do a similar check at run time.
8468 Symmetrically, C<no VERSION> allows you to specify that you want a version
8469 of Perl older than the specified one.
8471 Specifying VERSION as a literal of the form v5.6.1 should generally be
8472 avoided, because it leads to misleading error messages under earlier
8473 versions of Perl (that is, prior to 5.6.0) that do not support this
8474 syntax. The equivalent numeric version should be used instead.
8476 use v5.6.1; # compile time version check
8478 use 5.006_001; # ditto; preferred for backwards compatibility
8480 This is often useful if you need to check the current Perl version before
8481 C<use>ing library modules that won't work with older versions of Perl.
8482 (We try not to do this more than we have to.)
8484 C<use VERSION> also enables all features available in the requested
8485 version as defined by the C<feature> pragma, disabling any features
8486 not in the requested version's feature bundle. See L<feature>.
8487 Similarly, if the specified Perl version is greater than or equal to
8488 5.12.0, strictures are enabled lexically as
8489 with C<use strict>. Any explicit use of
8490 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8491 before it. In both cases, the F<feature.pm> and F<strict.pm> files are
8492 not actually loaded.
8494 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8495 C<require> makes sure the module is loaded into memory if it hasn't been
8496 yet. The C<import> is not a builtin; it's just an ordinary static method
8497 call into the C<Module> package to tell the module to import the list of
8498 features back into the current package. The module can implement its
8499 C<import> method any way it likes, though most modules just choose to
8500 derive their C<import> method via inheritance from the C<Exporter> class that
8501 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8502 method can be found then the call is skipped, even if there is an AUTOLOAD
8505 If you do not want to call the package's C<import> method (for instance,
8506 to stop your namespace from being altered), explicitly supply the empty list:
8510 That is exactly equivalent to
8512 BEGIN { require Module }
8514 If the VERSION argument is present between Module and LIST, then the
8515 C<use> will call the VERSION method in class Module with the given
8516 version as an argument. The default VERSION method, inherited from
8517 the UNIVERSAL class, croaks if the given version is larger than the
8518 value of the variable C<$Module::VERSION>.
8520 Again, there is a distinction between omitting LIST (C<import> called
8521 with no arguments) and an explicit empty LIST C<()> (C<import> not
8522 called). Note that there is no comma after VERSION!
8524 Because this is a wide-open interface, pragmas (compiler directives)
8525 are also implemented this way. Currently implemented pragmas are:
8530 use sigtrap qw(SEGV BUS);
8531 use strict qw(subs vars refs);
8532 use subs qw(afunc blurfl);
8533 use warnings qw(all);
8534 use sort qw(stable _quicksort _mergesort);
8536 Some of these pseudo-modules import semantics into the current
8537 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8538 which import symbols into the current package (which are effective
8539 through the end of the file).
8541 Because C<use> takes effect at compile time, it doesn't respect the
8542 ordinary flow control of the code being compiled. In particular, putting
8543 a C<use> inside the false branch of a conditional doesn't prevent it
8544 from being processed. If a module or pragma only needs to be loaded
8545 conditionally, this can be done using the L<if> pragma:
8547 use if $] < 5.008, "utf8";
8548 use if WANT_WARNINGS, warnings => qw(all);
8550 There's a corresponding C<no> declaration that unimports meanings imported
8551 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8552 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8553 or no unimport method being found.
8559 Care should be taken when using the C<no VERSION> form of C<no>. It is
8560 I<only> meant to be used to assert that the running Perl is of a earlier
8561 version than its argument and I<not> to undo the feature-enabling side effects
8564 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8565 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8566 functionality from the command-line.
8571 =for Pod::Functions set a file's last access and modify times
8573 Changes the access and modification times on each file of a list of
8574 files. The first two elements of the list must be the NUMERIC access
8575 and modification times, in that order. Returns the number of files
8576 successfully changed. The inode change time of each file is set
8577 to the current time. For example, this code has the same effect as the
8578 Unix touch(1) command when the files I<already exist> and belong to
8579 the user running the program:
8582 $atime = $mtime = time;
8583 utime $atime, $mtime, @ARGV;
8585 Since Perl 5.8.0, if the first two elements of the list are C<undef>,
8586 the utime(2) syscall from your C library is called with a null second
8587 argument. On most systems, this will set the file's access and
8588 modification times to the current time (i.e., equivalent to the example
8589 above) and will work even on files you don't own provided you have write
8593 utime(undef, undef, $file)
8594 || warn "couldn't touch $file: $!";
8597 Under NFS this will use the time of the NFS server, not the time of
8598 the local machine. If there is a time synchronization problem, the
8599 NFS server and local machine will have different times. The Unix
8600 touch(1) command will in fact normally use this form instead of the
8601 one shown in the first example.
8603 Passing only one of the first two elements as C<undef> is
8604 equivalent to passing a 0 and will not have the effect
8605 described when both are C<undef>. This also triggers an
8606 uninitialized warning.
8608 On systems that support futimes(2), you may pass filehandles among the
8609 files. On systems that don't support futimes(2), passing filehandles raises
8610 an exception. Filehandles must be passed as globs or glob references to be
8611 recognized; barewords are considered filenames.
8613 Portability issues: L<perlport/utime>.
8622 =for Pod::Functions return a list of the values in a hash
8624 In list context, returns a list consisting of all the values of the named
8625 hash. In Perl 5.12 or later only, will also return a list of the values of
8626 an array; prior to that release, attempting to use an array argument will
8627 produce a syntax error. In scalar context, returns the number of values.
8629 When called on a hash, the values are returned in an apparently random
8630 order. The actual random order is subject to change in future versions of
8631 Perl, but it is guaranteed to be the same order as either the C<keys> or
8632 C<each> function would produce on the same (unmodified) hash. Since Perl
8633 5.8.1 the ordering is different even between different runs of Perl for
8634 security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
8636 As a side effect, calling values() resets the HASH or ARRAY's internal
8637 iterator, see L</each>. (In particular, calling values() in void context
8638 resets the iterator with no other overhead. Apart from resetting the
8639 iterator, C<values @array> in list context is the same as plain C<@array>.
8640 (We recommend that you use void context C<keys @array> for this, but
8641 reasoned that taking C<values @array> out would require more
8642 documentation than leaving it in.)
8644 Note that the values are not copied, which means modifying them will
8645 modify the contents of the hash:
8647 for (values %hash) { s/foo/bar/g } # modifies %hash values
8648 for (@hash{keys %hash}) { s/foo/bar/g } # same
8650 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
8651 a reference to an unblessed hash or array. The argument will be
8652 dereferenced automatically. This aspect of C<values> is considered highly
8653 experimental. The exact behaviour may change in a future version of Perl.
8655 for (values $hashref) { ... }
8656 for (values $obj->get_arrayref) { ... }
8658 To avoid confusing would-be users of your code who are running earlier
8659 versions of Perl with mysterious syntax errors, put this sort of thing at
8660 the top of your file to signal that your code will work I<only> on Perls of
8663 use 5.012; # so keys/values/each work on arrays
8664 use 5.014; # so keys/values/each work on scalars (experimental)
8666 See also C<keys>, C<each>, and C<sort>.
8668 =item vec EXPR,OFFSET,BITS
8669 X<vec> X<bit> X<bit vector>
8671 =for Pod::Functions test or set particular bits in a string
8673 Treats the string in EXPR as a bit vector made up of elements of
8674 width BITS and returns the value of the element specified by OFFSET
8675 as an unsigned integer. BITS therefore specifies the number of bits
8676 that are reserved for each element in the bit vector. This must
8677 be a power of two from 1 to 32 (or 64, if your platform supports
8680 If BITS is 8, "elements" coincide with bytes of the input string.
8682 If BITS is 16 or more, bytes of the input string are grouped into chunks
8683 of size BITS/8, and each group is converted to a number as with
8684 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8685 for BITS==64). See L<"pack"> for details.
8687 If bits is 4 or less, the string is broken into bytes, then the bits
8688 of each byte are broken into 8/BITS groups. Bits of a byte are
8689 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8690 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8691 breaking the single input byte C<chr(0x36)> into two groups gives a list
8692 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8694 C<vec> may also be assigned to, in which case parentheses are needed
8695 to give the expression the correct precedence as in
8697 vec($image, $max_x * $x + $y, 8) = 3;
8699 If the selected element is outside the string, the value 0 is returned.
8700 If an element off the end of the string is written to, Perl will first
8701 extend the string with sufficiently many zero bytes. It is an error
8702 to try to write off the beginning of the string (i.e., negative OFFSET).
8704 If the string happens to be encoded as UTF-8 internally (and thus has
8705 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8706 internal byte string, not the conceptual character string, even if you
8707 only have characters with values less than 256.
8709 Strings created with C<vec> can also be manipulated with the logical
8710 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8711 vector operation is desired when both operands are strings.
8712 See L<perlop/"Bitwise String Operators">.
8714 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8715 The comments show the string after each step. Note that this code works
8716 in the same way on big-endian or little-endian machines.
8719 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8721 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8722 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8724 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8725 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8726 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8727 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8728 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8729 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8731 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8732 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8733 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8736 To transform a bit vector into a string or list of 0's and 1's, use these:
8738 $bits = unpack("b*", $vector);
8739 @bits = split(//, unpack("b*", $vector));
8741 If you know the exact length in bits, it can be used in place of the C<*>.
8743 Here is an example to illustrate how the bits actually fall in place:
8749 unpack("V",$_) 01234567890123456789012345678901
8750 ------------------------------------------------------------------
8755 for ($shift=0; $shift < $width; ++$shift) {
8756 for ($off=0; $off < 32/$width; ++$off) {
8757 $str = pack("B*", "0"x32);
8758 $bits = (1<<$shift);
8759 vec($str, $off, $width) = $bits;
8760 $res = unpack("b*",$str);
8761 $val = unpack("V", $str);
8768 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8769 $off, $width, $bits, $val, $res
8773 Regardless of the machine architecture on which it runs, the
8774 example above should print the following table:
8777 unpack("V",$_) 01234567890123456789012345678901
8778 ------------------------------------------------------------------
8779 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8780 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8781 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8782 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8783 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8784 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8785 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8786 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8787 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8788 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8789 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8790 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8791 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8792 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8793 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8794 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8795 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8796 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8797 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8798 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8799 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8800 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8801 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8802 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8803 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8804 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8805 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8806 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8807 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8808 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8809 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8810 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8811 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8812 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8813 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8814 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8815 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8816 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8817 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8818 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8819 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8820 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8821 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8822 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8823 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8824 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8825 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8826 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8827 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8828 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8829 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8830 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8831 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8832 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8833 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8834 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8835 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8836 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8837 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8838 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8839 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8840 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8841 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8842 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8843 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8844 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8845 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8846 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8847 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8848 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8849 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8850 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8851 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8852 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8853 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8854 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8855 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8856 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8857 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8858 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8859 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8860 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8861 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8862 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8863 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8864 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8865 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8866 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8867 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8868 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8869 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8870 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8871 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8872 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8873 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8874 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8875 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8876 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8877 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8878 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8879 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8880 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8881 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8882 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8883 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8884 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8885 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8886 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8887 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8888 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8889 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8890 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8891 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8892 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8893 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8894 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8895 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8896 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8897 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8898 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8899 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8900 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8901 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8902 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8903 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8904 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8905 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8906 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8911 =for Pod::Functions wait for any child process to die
8913 Behaves like wait(2) on your system: it waits for a child
8914 process to terminate and returns the pid of the deceased process, or
8915 C<-1> if there are no child processes. The status is returned in C<$?>
8916 and C<${^CHILD_ERROR_NATIVE}>.
8917 Note that a return value of C<-1> could mean that child processes are
8918 being automatically reaped, as described in L<perlipc>.
8920 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8921 child created by qx() or system(). See L<perlipc> for details.
8923 Portability issues: L<perlport/wait>.
8925 =item waitpid PID,FLAGS
8928 =for Pod::Functions wait for a particular child process to die
8930 Waits for a particular child process to terminate and returns the pid of
8931 the deceased process, or C<-1> if there is no such child process. On some
8932 systems, a value of 0 indicates that there are processes still running.
8933 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
8935 use POSIX ":sys_wait_h";
8938 $kid = waitpid(-1, WNOHANG);
8941 then you can do a non-blocking wait for all pending zombie processes.
8942 Non-blocking wait is available on machines supporting either the
8943 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
8944 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
8945 system call by remembering the status values of processes that have
8946 exited but have not been harvested by the Perl script yet.)
8948 Note that on some systems, a return value of C<-1> could mean that child
8949 processes are being automatically reaped. See L<perlipc> for details,
8950 and for other examples.
8952 Portability issues: L<perlport/waitpid>.
8955 X<wantarray> X<context>
8957 =for Pod::Functions get void vs scalar vs list context of current subroutine call
8959 Returns true if the context of the currently executing subroutine or
8960 C<eval> is looking for a list value. Returns false if the context is
8961 looking for a scalar. Returns the undefined value if the context is
8962 looking for no value (void context).
8964 return unless defined wantarray; # don't bother doing more
8965 my @a = complex_calculation();
8966 return wantarray ? @a : "@a";
8968 C<wantarray()>'s result is unspecified in the top level of a file,
8969 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
8970 in a C<DESTROY> method.
8972 This function should have been named wantlist() instead.
8975 X<warn> X<warning> X<STDERR>
8977 =for Pod::Functions print debugging info
8979 Prints the value of LIST to STDERR. If the last element of LIST does
8980 not end in a newline, it appends the same file/line number text as C<die>
8983 If the output is empty and C<$@> already contains a value (typically from a
8984 previous eval) that value is used after appending C<"\t...caught">
8985 to C<$@>. This is useful for staying almost, but not entirely similar to
8988 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
8990 No message is printed if there is a C<$SIG{__WARN__}> handler
8991 installed. It is the handler's responsibility to deal with the message
8992 as it sees fit (like, for instance, converting it into a C<die>). Most
8993 handlers must therefore arrange to actually display the
8994 warnings that they are not prepared to deal with, by calling C<warn>
8995 again in the handler. Note that this is quite safe and will not
8996 produce an endless loop, since C<__WARN__> hooks are not called from
8999 You will find this behavior is slightly different from that of
9000 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
9001 instead call C<die> again to change it).
9003 Using a C<__WARN__> handler provides a powerful way to silence all
9004 warnings (even the so-called mandatory ones). An example:
9006 # wipe out *all* compile-time warnings
9007 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9009 my $foo = 20; # no warning about duplicate my $foo,
9010 # but hey, you asked for it!
9011 # no compile-time or run-time warnings before here
9014 # run-time warnings enabled after here
9015 warn "\$foo is alive and $foo!"; # does show up
9017 See L<perlvar> for details on setting C<%SIG> entries and for more
9018 examples. See the Carp module for other kinds of warnings using its
9019 carp() and cluck() functions.
9021 =item write FILEHANDLE
9028 =for Pod::Functions print a picture record
9030 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9031 using the format associated with that file. By default the format for
9032 a file is the one having the same name as the filehandle, but the
9033 format for the current output channel (see the C<select> function) may be set
9034 explicitly by assigning the name of the format to the C<$~> variable.
9036 Top of form processing is handled automatically: if there is insufficient
9037 room on the current page for the formatted record, the page is advanced by
9038 writing a form feed, a special top-of-page format is used to format the new
9039 page header before the record is written. By default, the top-of-page
9040 format is the name of the filehandle with "_TOP" appended. This would be a
9041 problem with autovivified filehandles, but it may be dynamically set to the
9042 format of your choice by assigning the name to the C<$^> variable while
9043 that filehandle is selected. The number of lines remaining on the current
9044 page is in variable C<$->, which can be set to C<0> to force a new page.
9046 If FILEHANDLE is unspecified, output goes to the current default output
9047 channel, which starts out as STDOUT but may be changed by the
9048 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
9049 is evaluated and the resulting string is used to look up the name of
9050 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9052 Note that write is I<not> the opposite of C<read>. Unfortunately.
9056 =for Pod::Functions transliterate a string
9058 The transliteration operator. Same as C<tr///>. See
9059 L<perlop/"Quote and Quote-like Operators">.
9063 =head2 Non-function Keywords by Cross-reference
9073 These keywords are documented in L<perldata/"Special Literals">.
9091 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9101 This method keyword is documented in L<perlobj/"Destructors">.
9135 These operators are documented in L<perlop>.
9145 This keyword is documented in L<perlsub/"Autoloading">.
9169 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9181 These flow-control keywords related to the experimental switch feature are
9182 documented in L<perlsyn/"Switch Statements"> .