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 5.9.1, 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
725 L<feature> for more information. You can also access it by
726 prefixing it with C<CORE::>. Alternately, include a C<use
727 v5.10> or later to the current scope.
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,
1551 a scalar variable or a subroutine beginning with C<&>.
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>
1602 =for Pod::Functions create an immediate core dump
1604 This function causes an immediate core dump. See also the B<-u>
1605 command-line switch in L<perlrun>, which does the same thing.
1606 Primarily this is so that you can use the B<undump> program (not
1607 supplied) to turn your core dump into an executable binary after
1608 having initialized all your variables at the beginning of the
1609 program. When the new binary is executed it will begin by executing
1610 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1611 Think of it as a goto with an intervening core dump and reincarnation.
1612 If C<LABEL> is omitted, restarts the program from the top.
1614 B<WARNING>: Any files opened at the time of the dump will I<not>
1615 be open any more when the program is reincarnated, with possible
1616 resulting confusion by Perl.
1618 This function is now largely obsolete, mostly because it's very hard to
1619 convert a core file into an executable. That's why you should now invoke
1620 it as C<CORE::dump()>, if you don't want to be warned against a possible
1623 Portability issues: L<perlport/dump>.
1626 X<each> X<hash, iterator>
1633 =for Pod::Functions retrieve the next key/value pair from a hash
1635 When called on a hash in list context, returns a 2-element list
1636 consisting of the key and value for the next element of a hash. In Perl
1637 5.12 and later only, it will also return the index and value for the next
1638 element of an array so that you can iterate over it; older Perls consider
1639 this a syntax error. When called in scalar context, returns only the key
1640 (not the value) in a hash, or the index in an array.
1642 Hash entries are returned in an apparently random order. The actual random
1643 order is subject to change in future versions of Perl, but it is
1644 guaranteed to be in the same order as either the C<keys> or C<values>
1645 function would produce on the same (unmodified) hash. Since Perl
1646 5.8.2 the ordering can be different even between different runs of Perl
1647 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1649 After C<each> has returned all entries from the hash or array, the next
1650 call to C<each> returns the empty list in list context and C<undef> in
1651 scalar context; the next call following I<that> one restarts iteration.
1652 Each hash or array has its own internal iterator, accessed by C<each>,
1653 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1654 reached the end as just described; it can be explicitly reset by calling
1655 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1656 elements while iterating over it, entries may be skipped or duplicated--so
1657 don't do that. Exception: In the current implementation, it is always safe
1658 to delete the item most recently returned by C<each()>, so the following
1659 code works properly:
1661 while (($key, $value) = each %hash) {
1663 delete $hash{$key}; # This is safe
1666 This prints out your environment like the printenv(1) program,
1667 but in a different order:
1669 while (($key,$value) = each %ENV) {
1670 print "$key=$value\n";
1673 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1674 reference to an unblessed hash or array. The argument will be dereferenced
1675 automatically. This aspect of C<each> is considered highly experimental.
1676 The exact behaviour may change in a future version of Perl.
1678 while (($key,$value) = each $hashref) { ... }
1680 To avoid confusing would-be users of your code who are running earlier
1681 versions of Perl with mysterious syntax errors, put this sort of thing at
1682 the top of your file to signal that your code will work I<only> on Perls of
1685 use 5.012; # so keys/values/each work on arrays
1686 use 5.014; # so keys/values/each work on scalars (experimental)
1688 See also C<keys>, C<values>, and C<sort>.
1690 =item eof FILEHANDLE
1699 =for Pod::Functions test a filehandle for its end
1701 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1702 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1703 gives the real filehandle. (Note that this function actually
1704 reads a character and then C<ungetc>s it, so isn't useful in an
1705 interactive context.) Do not read from a terminal file (or call
1706 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1707 as terminals may lose the end-of-file condition if you do.
1709 An C<eof> without an argument uses the last file read. Using C<eof()>
1710 with empty parentheses is different. It refers to the pseudo file
1711 formed from the files listed on the command line and accessed via the
1712 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1713 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1714 used will cause C<@ARGV> to be examined to determine if input is
1715 available. Similarly, an C<eof()> after C<< <> >> has returned
1716 end-of-file will assume you are processing another C<@ARGV> list,
1717 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1718 see L<perlop/"I/O Operators">.
1720 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1721 detect the end of each file, whereas C<eof()> will detect the end
1722 of the very last file only. Examples:
1724 # reset line numbering on each input file
1726 next if /^\s*#/; # skip comments
1729 close ARGV if eof; # Not eof()!
1732 # insert dashes just before last line of last file
1734 if (eof()) { # check for end of last file
1735 print "--------------\n";
1738 last if eof(); # needed if we're reading from a terminal
1741 Practical hint: you almost never need to use C<eof> in Perl, because the
1742 input operators typically return C<undef> when they run out of data or
1746 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1747 X<error, handling> X<exception, handling>
1753 =for Pod::Functions catch exceptions or compile and run code
1755 In the first form, the return value of EXPR is parsed and executed as if it
1756 were a little Perl program. The value of the expression (which is itself
1757 determined within scalar context) is first parsed, and if there were no
1758 errors, executed as a block within the lexical context of the current Perl
1759 program. This means, that in particular, any outer lexical variables are
1760 visible to it, and any package variable settings or subroutine and format
1761 definitions remain afterwards.
1763 Note that the value is parsed every time the C<eval> executes.
1764 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1765 delay parsing and subsequent execution of the text of EXPR until run time.
1767 If the C<unicode_eval> feature is enabled (which is the default under a
1768 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1769 characters, so C<use utf8> declarations have no effect, and source filters
1770 are forbidden. In the absence of the C<unicode_eval> feature, the string
1771 will sometimes be treated as characters and sometimes as bytes, depending
1772 on the internal encoding, and source filters activated within the C<eval>
1773 exhibit the erratic, but historical, behaviour of affecting some outer file
1774 scope that is still compiling. See also the L</evalbytes> keyword, which
1775 always treats its input as a byte stream and works properly with source
1776 filters, and the L<feature> pragma.
1778 In the second form, the code within the BLOCK is parsed only once--at the
1779 same time the code surrounding the C<eval> itself was parsed--and executed
1780 within the context of the current Perl program. This form is typically
1781 used to trap exceptions more efficiently than the first (see below), while
1782 also providing the benefit of checking the code within BLOCK at compile
1785 The final semicolon, if any, may be omitted from the value of EXPR or within
1788 In both forms, the value returned is the value of the last expression
1789 evaluated inside the mini-program; a return statement may be also used, just
1790 as with subroutines. The expression providing the return value is evaluated
1791 in void, scalar, or list context, depending on the context of the C<eval>
1792 itself. See L</wantarray> for more on how the evaluation context can be
1795 If there is a syntax error or runtime error, or a C<die> statement is
1796 executed, C<eval> returns C<undef> in scalar context
1797 or an empty list in list context, and C<$@> is set to the error
1798 message. (Prior to 5.16, a bug caused C<undef> to be returned
1799 in list context for syntax errors, but not for runtime errors.)
1800 If there was no error, C<$@> is set to the empty string. A
1801 control flow operator like C<last> or C<goto> can bypass the setting of
1802 C<$@>. Beware that using C<eval> neither silences Perl from printing
1803 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1804 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1805 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1806 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1808 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1809 determining whether a particular feature (such as C<socket> or C<symlink>)
1810 is implemented. It is also Perl's exception-trapping mechanism, where
1811 the die operator is used to raise exceptions.
1813 If you want to trap errors when loading an XS module, some problems with
1814 the binary interface (such as Perl version skew) may be fatal even with
1815 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1817 If the code to be executed doesn't vary, you may use the eval-BLOCK
1818 form to trap run-time errors without incurring the penalty of
1819 recompiling each time. The error, if any, is still returned in C<$@>.
1822 # make divide-by-zero nonfatal
1823 eval { $answer = $a / $b; }; warn $@ if $@;
1825 # same thing, but less efficient
1826 eval '$answer = $a / $b'; warn $@ if $@;
1828 # a compile-time error
1829 eval { $answer = }; # WRONG
1832 eval '$answer ='; # sets $@
1834 Using the C<eval{}> form as an exception trap in libraries does have some
1835 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1836 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1837 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1838 as this example shows:
1840 # a private exception trap for divide-by-zero
1841 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1844 This is especially significant, given that C<__DIE__> hooks can call
1845 C<die> again, which has the effect of changing their error messages:
1847 # __DIE__ hooks may modify error messages
1849 local $SIG{'__DIE__'} =
1850 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1851 eval { die "foo lives here" };
1852 print $@ if $@; # prints "bar lives here"
1855 Because this promotes action at a distance, this counterintuitive behavior
1856 may be fixed in a future release.
1858 With an C<eval>, you should be especially careful to remember what's
1859 being looked at when:
1865 eval { $x }; # CASE 4
1867 eval "\$$x++"; # CASE 5
1870 Cases 1 and 2 above behave identically: they run the code contained in
1871 the variable $x. (Although case 2 has misleading double quotes making
1872 the reader wonder what else might be happening (nothing is).) Cases 3
1873 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1874 does nothing but return the value of $x. (Case 4 is preferred for
1875 purely visual reasons, but it also has the advantage of compiling at
1876 compile-time instead of at run-time.) Case 5 is a place where
1877 normally you I<would> like to use double quotes, except that in this
1878 particular situation, you can just use symbolic references instead, as
1881 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1882 of localized variables, which means that for your code to run on older
1883 versions, a temporary is required if you want to mask some but not all
1886 # alter $@ on nefarious repugnancy only
1890 local $@; # protect existing $@
1891 eval { test_repugnancy() };
1892 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1893 $@ =~ /nefarious/ and $e = $@;
1895 die $e if defined $e
1898 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1899 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1901 An C<eval ''> executed within the C<DB> package doesn't see the usual
1902 surrounding lexical scope, but rather the scope of the first non-DB piece
1903 of code that called it. You don't normally need to worry about this unless
1904 you are writing a Perl debugger.
1906 =item evalbytes EXPR
1911 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1913 This function is like L</eval> with a string argument, except it always
1914 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1915 string containing characters whose ordinal value exceeds 255 results in an
1916 error. Source filters activated within the evaluated code apply to the
1919 This function is only available under the C<evalbytes> feature, a
1920 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1921 L<feature> for more information.
1926 =item exec PROGRAM LIST
1928 =for Pod::Functions abandon this program to run another
1930 The C<exec> function executes a system command I<and never returns>;
1931 use C<system> instead of C<exec> if you want it to return. It fails and
1932 returns false only if the command does not exist I<and> it is executed
1933 directly instead of via your system's command shell (see below).
1935 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1936 warns you if C<exec> is called in void context and if there is a following
1937 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1938 you always do that, right?). If you I<really> want to follow an C<exec>
1939 with some other statement, you can use one of these styles to avoid the warning:
1941 exec ('foo') or print STDERR "couldn't exec foo: $!";
1942 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1944 If there is more than one argument in LIST, or if LIST is an array
1945 with more than one value, calls execvp(3) with the arguments in LIST.
1946 If there is only one scalar argument or an array with one element in it,
1947 the argument is checked for shell metacharacters, and if there are any,
1948 the entire argument is passed to the system's command shell for parsing
1949 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1950 If there are no shell metacharacters in the argument, it is split into
1951 words and passed directly to C<execvp>, which is more efficient.
1954 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1955 exec "sort $outfile | uniq";
1957 If you don't really want to execute the first argument, but want to lie
1958 to the program you are executing about its own name, you can specify
1959 the program you actually want to run as an "indirect object" (without a
1960 comma) in front of the LIST. (This always forces interpretation of the
1961 LIST as a multivalued list, even if there is only a single scalar in
1964 $shell = '/bin/csh';
1965 exec $shell '-sh'; # pretend it's a login shell
1969 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1971 When the arguments get executed via the system shell, results are
1972 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1975 Using an indirect object with C<exec> or C<system> is also more
1976 secure. This usage (which also works fine with system()) forces
1977 interpretation of the arguments as a multivalued list, even if the
1978 list had just one argument. That way you're safe from the shell
1979 expanding wildcards or splitting up words with whitespace in them.
1981 @args = ( "echo surprise" );
1983 exec @args; # subject to shell escapes
1985 exec { $args[0] } @args; # safe even with one-arg list
1987 The first version, the one without the indirect object, ran the I<echo>
1988 program, passing it C<"surprise"> an argument. The second version didn't;
1989 it tried to run a program named I<"echo surprise">, didn't find it, and set
1990 C<$?> to a non-zero value indicating failure.
1992 Beginning with v5.6.0, Perl attempts to flush all files opened for
1993 output before the exec, but this may not be supported on some platforms
1994 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1995 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1996 open handles to avoid lost output.
1998 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1999 C<DESTROY> methods on your objects.
2001 Portability issues: L<perlport/exec>.
2004 X<exists> X<autovivification>
2006 =for Pod::Functions test whether a hash key is present
2008 Given an expression that specifies an element of a hash, returns true if the
2009 specified element in the hash has ever been initialized, even if the
2010 corresponding value is undefined.
2012 print "Exists\n" if exists $hash{$key};
2013 print "Defined\n" if defined $hash{$key};
2014 print "True\n" if $hash{$key};
2016 exists may also be called on array elements, but its behavior is much less
2017 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2018 that calling exists on array values is deprecated and likely to be removed in
2019 a future version of Perl.
2021 print "Exists\n" if exists $array[$index];
2022 print "Defined\n" if defined $array[$index];
2023 print "True\n" if $array[$index];
2025 A hash or array element can be true only if it's defined and defined only if
2026 it exists, but the reverse doesn't necessarily hold true.
2028 Given an expression that specifies the name of a subroutine,
2029 returns true if the specified subroutine has ever been declared, even
2030 if it is undefined. Mentioning a subroutine name for exists or defined
2031 does not count as declaring it. Note that a subroutine that does not
2032 exist may still be callable: its package may have an C<AUTOLOAD>
2033 method that makes it spring into existence the first time that it is
2034 called; see L<perlsub>.
2036 print "Exists\n" if exists &subroutine;
2037 print "Defined\n" if defined &subroutine;
2039 Note that the EXPR can be arbitrarily complicated as long as the final
2040 operation is a hash or array key lookup or subroutine name:
2042 if (exists $ref->{A}->{B}->{$key}) { }
2043 if (exists $hash{A}{B}{$key}) { }
2045 if (exists $ref->{A}->{B}->[$ix]) { }
2046 if (exists $hash{A}{B}[$ix]) { }
2048 if (exists &{$ref->{A}{B}{$key}}) { }
2050 Although the most deeply nested array or hash element will not spring into
2051 existence just because its existence was tested, any intervening ones will.
2052 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2053 into existence due to the existence test for the $key element above.
2054 This happens anywhere the arrow operator is used, including even here:
2057 if (exists $ref->{"Some key"}) { }
2058 print $ref; # prints HASH(0x80d3d5c)
2060 This surprising autovivification in what does not at first--or even
2061 second--glance appear to be an lvalue context may be fixed in a future
2064 Use of a subroutine call, rather than a subroutine name, as an argument
2065 to exists() is an error.
2068 exists &sub(); # Error
2071 X<exit> X<terminate> X<abort>
2075 =for Pod::Functions terminate this program
2077 Evaluates EXPR and exits immediately with that value. Example:
2080 exit 0 if $ans =~ /^[Xx]/;
2082 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2083 universally recognized values for EXPR are C<0> for success and C<1>
2084 for error; other values are subject to interpretation depending on the
2085 environment in which the Perl program is running. For example, exiting
2086 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2087 the mailer to return the item undelivered, but that's not true everywhere.
2089 Don't use C<exit> to abort a subroutine if there's any chance that
2090 someone might want to trap whatever error happened. Use C<die> instead,
2091 which can be trapped by an C<eval>.
2093 The exit() function does not always exit immediately. It calls any
2094 defined C<END> routines first, but these C<END> routines may not
2095 themselves abort the exit. Likewise any object destructors that need to
2096 be called are called before the real exit. C<END> routines and destructors
2097 can change the exit status by modifying C<$?>. If this is a problem, you
2098 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2099 See L<perlmod> for details.
2101 Portability issues: L<perlport/exit>.
2104 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2108 =for Pod::Functions raise I<e> to a power
2110 Returns I<e> (the natural logarithm base) to the power of EXPR.
2111 If EXPR is omitted, gives C<exp($_)>.
2114 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2118 =for Pod::Functions +fc return casefolded version of a string
2120 Returns the casefolded version of EXPR. This is the internal function
2121 implementing the C<\F> escape in double-quoted strings.
2123 Casefolding is the process of mapping strings to a form where case
2124 differences are erased; comparing two strings in their casefolded
2125 form is effectively a way of asking if two strings are equal,
2128 Roughly, if you ever found yourself writing this
2130 lc($this) eq lc($that) # Wrong!
2132 uc($this) eq uc($that) # Also wrong!
2134 $this =~ /^\Q$that\E\z/i # Right!
2138 fc($this) eq fc($that)
2140 And get the correct results.
2142 Perl only implements the full form of casefolding,
2143 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2144 L<Unicode::UCD/prop_invmap()>.
2145 For further information on casefolding, refer to
2146 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2147 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2148 available at L<http://www.unicode.org/versions/latest/>, as well as the
2149 Case Charts available at L<http://www.unicode.org/charts/case/>.
2151 If EXPR is omitted, uses C<$_>.
2153 This function behaves the same way under various pragma, such as in a locale,
2156 While the Unicode Standard defines two additional forms of casefolding,
2157 one for Turkic languages and one that never maps one character into multiple
2158 characters, these are not provided by the Perl core; However, the CPAN module
2159 C<Unicode::Casing> may be used to provide an implementation.
2161 This keyword is available only when the C<"fc"> feature is enabled,
2162 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2163 include a C<use v5.16> or later to the current scope.
2165 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2168 =for Pod::Functions file control system call
2170 Implements the fcntl(2) function. You'll probably have to say
2174 first to get the correct constant definitions. Argument processing and
2175 value returned work just like C<ioctl> below.
2179 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2180 or die "can't fcntl F_GETFL: $!";
2182 You don't have to check for C<defined> on the return from C<fcntl>.
2183 Like C<ioctl>, it maps a C<0> return from the system call into
2184 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2185 in numeric context. It is also exempt from the normal B<-w> warnings
2186 on improper numeric conversions.
2188 Note that C<fcntl> raises an exception if used on a machine that
2189 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2190 manpage to learn what functions are available on your system.
2192 Here's an example of setting a filehandle named C<REMOTE> to be
2193 non-blocking at the system level. You'll have to negotiate C<$|>
2194 on your own, though.
2196 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2198 $flags = fcntl(REMOTE, F_GETFL, 0)
2199 or die "Can't get flags for the socket: $!\n";
2201 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2202 or die "Can't set flags for the socket: $!\n";
2204 Portability issues: L<perlport/fcntl>.
2209 =for Pod::Functions the name of the current source file
2211 A special token that returns the name of the file in which it occurs.
2213 =item fileno FILEHANDLE
2216 =for Pod::Functions return file descriptor from filehandle
2218 Returns the file descriptor for a filehandle, or undefined if the
2219 filehandle is not open. If there is no real file descriptor at the OS
2220 level, as can happen with filehandles connected to memory objects via
2221 C<open> with a reference for the third argument, -1 is returned.
2223 This is mainly useful for constructing
2224 bitmaps for C<select> and low-level POSIX tty-handling operations.
2225 If FILEHANDLE is an expression, the value is taken as an indirect
2226 filehandle, generally its name.
2228 You can use this to find out whether two handles refer to the
2229 same underlying descriptor:
2231 if (fileno(THIS) == fileno(THAT)) {
2232 print "THIS and THAT are dups\n";
2235 =item flock FILEHANDLE,OPERATION
2236 X<flock> X<lock> X<locking>
2238 =for Pod::Functions lock an entire file with an advisory lock
2240 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2241 for success, false on failure. Produces a fatal error if used on a
2242 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2243 C<flock> is Perl's portable file-locking interface, although it locks
2244 entire files only, not records.
2246 Two potentially non-obvious but traditional C<flock> semantics are
2247 that it waits indefinitely until the lock is granted, and that its locks
2248 are B<merely advisory>. Such discretionary locks are more flexible, but
2249 offer fewer guarantees. This means that programs that do not also use
2250 C<flock> may modify files locked with C<flock>. See L<perlport>,
2251 your port's specific documentation, and your system-specific local manpages
2252 for details. It's best to assume traditional behavior if you're writing
2253 portable programs. (But if you're not, you should as always feel perfectly
2254 free to write for your own system's idiosyncrasies (sometimes called
2255 "features"). Slavish adherence to portability concerns shouldn't get
2256 in the way of your getting your job done.)
2258 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2259 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2260 you can use the symbolic names if you import them from the L<Fcntl> module,
2261 either individually, or as a group using the C<:flock> tag. LOCK_SH
2262 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2263 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2264 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2265 waiting for the lock; check the return status to see if you got it.
2267 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2268 before locking or unlocking it.
2270 Note that the emulation built with lockf(3) doesn't provide shared
2271 locks, and it requires that FILEHANDLE be open with write intent. These
2272 are the semantics that lockf(3) implements. Most if not all systems
2273 implement lockf(3) in terms of fcntl(2) locking, though, so the
2274 differing semantics shouldn't bite too many people.
2276 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2277 be open with read intent to use LOCK_SH and requires that it be open
2278 with write intent to use LOCK_EX.
2280 Note also that some versions of C<flock> cannot lock things over the
2281 network; you would need to use the more system-specific C<fcntl> for
2282 that. If you like you can force Perl to ignore your system's flock(2)
2283 function, and so provide its own fcntl(2)-based emulation, by passing
2284 the switch C<-Ud_flock> to the F<Configure> program when you configure
2285 and build a new Perl.
2287 Here's a mailbox appender for BSD systems.
2289 # import LOCK_* and SEEK_END constants
2290 use Fcntl qw(:flock SEEK_END);
2294 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2296 # and, in case someone appended while we were waiting...
2297 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2302 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2305 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2306 or die "Can't open mailbox: $!";
2309 print $mbox $msg,"\n\n";
2312 On systems that support a real flock(2), locks are inherited across fork()
2313 calls, whereas those that must resort to the more capricious fcntl(2)
2314 function lose their locks, making it seriously harder to write servers.
2316 See also L<DB_File> for other flock() examples.
2318 Portability issues: L<perlport/flock>.
2321 X<fork> X<child> X<parent>
2323 =for Pod::Functions create a new process just like this one
2325 Does a fork(2) system call to create a new process running the
2326 same program at the same point. It returns the child pid to the
2327 parent process, C<0> to the child process, or C<undef> if the fork is
2328 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2329 are shared, while everything else is copied. On most systems supporting
2330 fork(), great care has gone into making it extremely efficient (for
2331 example, using copy-on-write technology on data pages), making it the
2332 dominant paradigm for multitasking over the last few decades.
2334 Beginning with v5.6.0, Perl attempts to flush all files opened for
2335 output before forking the child process, but this may not be supported
2336 on some platforms (see L<perlport>). To be safe, you may need to set
2337 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2338 C<IO::Handle> on any open handles to avoid duplicate output.
2340 If you C<fork> without ever waiting on your children, you will
2341 accumulate zombies. On some systems, you can avoid this by setting
2342 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2343 forking and reaping moribund children.
2345 Note that if your forked child inherits system file descriptors like
2346 STDIN and STDOUT that are actually connected by a pipe or socket, even
2347 if you exit, then the remote server (such as, say, a CGI script or a
2348 backgrounded job launched from a remote shell) won't think you're done.
2349 You should reopen those to F</dev/null> if it's any issue.
2351 On some platforms such as Windows, where the fork() system call is not available,
2352 Perl can be built to emulate fork() in the Perl interpreter.
2353 The emulation is designed, at the level of the Perl program,
2354 to be as compatible as possible with the "Unix" fork().
2355 However it has limitations that have to be considered in code intended to be portable.
2356 See L<perlfork> for more details.
2358 Portability issues: L<perlport/fork>.
2363 =for Pod::Functions declare a picture format with use by the write() function
2365 Declare a picture format for use by the C<write> function. For
2369 Test: @<<<<<<<< @||||| @>>>>>
2370 $str, $%, '$' . int($num)
2374 $num = $cost/$quantity;
2378 See L<perlform> for many details and examples.
2380 =item formline PICTURE,LIST
2383 =for Pod::Functions internal function used for formats
2385 This is an internal function used by C<format>s, though you may call it,
2386 too. It formats (see L<perlform>) a list of values according to the
2387 contents of PICTURE, placing the output into the format output
2388 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2389 Eventually, when a C<write> is done, the contents of
2390 C<$^A> are written to some filehandle. You could also read C<$^A>
2391 and then set C<$^A> back to C<"">. Note that a format typically
2392 does one C<formline> per line of form, but the C<formline> function itself
2393 doesn't care how many newlines are embedded in the PICTURE. This means
2394 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2395 You may therefore need to use multiple formlines to implement a single
2396 record format, just like the C<format> compiler.
2398 Be careful if you put double quotes around the picture, because an C<@>
2399 character may be taken to mean the beginning of an array name.
2400 C<formline> always returns true. See L<perlform> for other examples.
2402 If you are trying to use this instead of C<write> to capture the output,
2403 you may find it easier to open a filehandle to a scalar
2404 (C<< open $fh, ">", \$output >>) and write to that instead.
2406 =item getc FILEHANDLE
2407 X<getc> X<getchar> X<character> X<file, read>
2411 =for Pod::Functions get the next character from the filehandle
2413 Returns the next character from the input file attached to FILEHANDLE,
2414 or the undefined value at end of file or if there was an error (in
2415 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2416 STDIN. This is not particularly efficient. However, it cannot be
2417 used by itself to fetch single characters without waiting for the user
2418 to hit enter. For that, try something more like:
2421 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2424 system "stty", '-icanon', 'eol', "\001";
2430 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2433 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2437 Determination of whether $BSD_STYLE should be set
2438 is left as an exercise to the reader.
2440 The C<POSIX::getattr> function can do this more portably on
2441 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2442 module from your nearest CPAN site; details on CPAN can be found under
2446 X<getlogin> X<login>
2448 =for Pod::Functions return who logged in at this tty
2450 This implements the C library function of the same name, which on most
2451 systems returns the current login from F</etc/utmp>, if any. If it
2452 returns the empty string, use C<getpwuid>.
2454 $login = getlogin || getpwuid($<) || "Kilroy";
2456 Do not consider C<getlogin> for authentication: it is not as
2457 secure as C<getpwuid>.
2459 Portability issues: L<perlport/getlogin>.
2461 =item getpeername SOCKET
2462 X<getpeername> X<peer>
2464 =for Pod::Functions find the other end of a socket connection
2466 Returns the packed sockaddr address of the other end of the SOCKET
2470 $hersockaddr = getpeername(SOCK);
2471 ($port, $iaddr) = sockaddr_in($hersockaddr);
2472 $herhostname = gethostbyaddr($iaddr, AF_INET);
2473 $herstraddr = inet_ntoa($iaddr);
2478 =for Pod::Functions get process group
2480 Returns the current process group for the specified PID. Use
2481 a PID of C<0> to get the current process group for the
2482 current process. Will raise an exception if used on a machine that
2483 doesn't implement getpgrp(2). If PID is omitted, returns the process
2484 group of the current process. Note that the POSIX version of C<getpgrp>
2485 does not accept a PID argument, so only C<PID==0> is truly portable.
2487 Portability issues: L<perlport/getpgrp>.
2490 X<getppid> X<parent> X<pid>
2492 =for Pod::Functions get parent process ID
2494 Returns the process id of the parent process.
2496 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2497 around non-POSIX thread semantics the minority of Linux systems (and
2498 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2499 has since been removed. See the documentation for L<$$|perlvar/$$> for
2502 Portability issues: L<perlport/getppid>.
2504 =item getpriority WHICH,WHO
2505 X<getpriority> X<priority> X<nice>
2507 =for Pod::Functions get current nice value
2509 Returns the current priority for a process, a process group, or a user.
2510 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2511 machine that doesn't implement getpriority(2).
2513 Portability issues: L<perlport/getpriority>.
2516 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2517 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2518 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2519 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2520 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2521 X<endnetent> X<endprotoent> X<endservent>
2523 =for Pod::Functions get passwd record given user login name
2527 =for Pod::Functions get group record given group name
2529 =item gethostbyname NAME
2531 =for Pod::Functions get host record given name
2533 =item getnetbyname NAME
2535 =for Pod::Functions get networks record given name
2537 =item getprotobyname NAME
2539 =for Pod::Functions get protocol record given name
2543 =for Pod::Functions get passwd record given user ID
2547 =for Pod::Functions get group record given group user ID
2549 =item getservbyname NAME,PROTO
2551 =for Pod::Functions get services record given its name
2553 =item gethostbyaddr ADDR,ADDRTYPE
2555 =for Pod::Functions get host record given its address
2557 =item getnetbyaddr ADDR,ADDRTYPE
2559 =for Pod::Functions get network record given its address
2561 =item getprotobynumber NUMBER
2563 =for Pod::Functions get protocol record numeric protocol
2565 =item getservbyport PORT,PROTO
2567 =for Pod::Functions get services record given numeric port
2571 =for Pod::Functions get next passwd record
2575 =for Pod::Functions get next group record
2579 =for Pod::Functions get next hosts record
2583 =for Pod::Functions get next networks record
2587 =for Pod::Functions get next protocols record
2591 =for Pod::Functions get next services record
2595 =for Pod::Functions prepare passwd file for use
2599 =for Pod::Functions prepare group file for use
2601 =item sethostent STAYOPEN
2603 =for Pod::Functions prepare hosts file for use
2605 =item setnetent STAYOPEN
2607 =for Pod::Functions prepare networks file for use
2609 =item setprotoent STAYOPEN
2611 =for Pod::Functions prepare protocols file for use
2613 =item setservent STAYOPEN
2615 =for Pod::Functions prepare services file for use
2619 =for Pod::Functions be done using passwd file
2623 =for Pod::Functions be done using group file
2627 =for Pod::Functions be done using hosts file
2631 =for Pod::Functions be done using networks file
2635 =for Pod::Functions be done using protocols file
2639 =for Pod::Functions be done using services file
2641 These routines are the same as their counterparts in the
2642 system C library. In list context, the return values from the
2643 various get routines are as follows:
2645 ($name,$passwd,$uid,$gid,
2646 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2647 ($name,$passwd,$gid,$members) = getgr*
2648 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2649 ($name,$aliases,$addrtype,$net) = getnet*
2650 ($name,$aliases,$proto) = getproto*
2651 ($name,$aliases,$port,$proto) = getserv*
2653 (If the entry doesn't exist you get an empty list.)
2655 The exact meaning of the $gcos field varies but it usually contains
2656 the real name of the user (as opposed to the login name) and other
2657 information pertaining to the user. Beware, however, that in many
2658 system users are able to change this information and therefore it
2659 cannot be trusted and therefore the $gcos is tainted (see
2660 L<perlsec>). The $passwd and $shell, user's encrypted password and
2661 login shell, are also tainted, for the same reason.
2663 In scalar context, you get the name, unless the function was a
2664 lookup by name, in which case you get the other thing, whatever it is.
2665 (If the entry doesn't exist you get the undefined value.) For example:
2667 $uid = getpwnam($name);
2668 $name = getpwuid($num);
2670 $gid = getgrnam($name);
2671 $name = getgrgid($num);
2675 In I<getpw*()> the fields $quota, $comment, and $expire are special
2676 in that they are unsupported on many systems. If the
2677 $quota is unsupported, it is an empty scalar. If it is supported, it
2678 usually encodes the disk quota. If the $comment field is unsupported,
2679 it is an empty scalar. If it is supported it usually encodes some
2680 administrative comment about the user. In some systems the $quota
2681 field may be $change or $age, fields that have to do with password
2682 aging. In some systems the $comment field may be $class. The $expire
2683 field, if present, encodes the expiration period of the account or the
2684 password. For the availability and the exact meaning of these fields
2685 in your system, please consult getpwnam(3) and your system's
2686 F<pwd.h> file. You can also find out from within Perl what your
2687 $quota and $comment fields mean and whether you have the $expire field
2688 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2689 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2690 files are supported only if your vendor has implemented them in the
2691 intuitive fashion that calling the regular C library routines gets the
2692 shadow versions if you're running under privilege or if there exists
2693 the shadow(3) functions as found in System V (this includes Solaris
2694 and Linux). Those systems that implement a proprietary shadow password
2695 facility are unlikely to be supported.
2697 The $members value returned by I<getgr*()> is a space-separated list of
2698 the login names of the members of the group.
2700 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2701 C, it will be returned to you via C<$?> if the function call fails. The
2702 C<@addrs> value returned by a successful call is a list of raw
2703 addresses returned by the corresponding library call. In the
2704 Internet domain, each address is four bytes long; you can unpack it
2705 by saying something like:
2707 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2709 The Socket library makes this slightly easier:
2712 $iaddr = inet_aton("127.1"); # or whatever address
2713 $name = gethostbyaddr($iaddr, AF_INET);
2715 # or going the other way
2716 $straddr = inet_ntoa($iaddr);
2718 In the opposite way, to resolve a hostname to the IP address
2722 $packed_ip = gethostbyname("www.perl.org");
2723 if (defined $packed_ip) {
2724 $ip_address = inet_ntoa($packed_ip);
2727 Make sure C<gethostbyname()> is called in SCALAR context and that
2728 its return value is checked for definedness.
2730 The C<getprotobynumber> function, even though it only takes one argument,
2731 has the precedence of a list operator, so beware:
2733 getprotobynumber $number eq 'icmp' # WRONG
2734 getprotobynumber($number eq 'icmp') # actually means this
2735 getprotobynumber($number) eq 'icmp' # better this way
2737 If you get tired of remembering which element of the return list
2738 contains which return value, by-name interfaces are provided
2739 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2740 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2741 and C<User::grent>. These override the normal built-ins, supplying
2742 versions that return objects with the appropriate names
2743 for each field. For example:
2747 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2749 Even though it looks as though they're the same method calls (uid),
2750 they aren't, because a C<File::stat> object is different from
2751 a C<User::pwent> object.
2753 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2755 =item getsockname SOCKET
2758 =for Pod::Functions retrieve the sockaddr for a given socket
2760 Returns the packed sockaddr address of this end of the SOCKET connection,
2761 in case you don't know the address because you have several different
2762 IPs that the connection might have come in on.
2765 $mysockaddr = getsockname(SOCK);
2766 ($port, $myaddr) = sockaddr_in($mysockaddr);
2767 printf "Connect to %s [%s]\n",
2768 scalar gethostbyaddr($myaddr, AF_INET),
2771 =item getsockopt SOCKET,LEVEL,OPTNAME
2774 =for Pod::Functions get socket options on a given socket
2776 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2777 Options may exist at multiple protocol levels depending on the socket
2778 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2779 C<Socket> module) will exist. To query options at another level the
2780 protocol number of the appropriate protocol controlling the option
2781 should be supplied. For example, to indicate that an option is to be
2782 interpreted by the TCP protocol, LEVEL should be set to the protocol
2783 number of TCP, which you can get using C<getprotobyname>.
2785 The function returns a packed string representing the requested socket
2786 option, or C<undef> on error, with the reason for the error placed in
2787 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2788 consult getsockopt(2) for details. A common case is that the option is an
2789 integer, in which case the result is a packed integer, which you can decode
2790 using C<unpack> with the C<i> (or C<I>) format.
2792 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2794 use Socket qw(:all);
2796 defined(my $tcp = getprotobyname("tcp"))
2797 or die "Could not determine the protocol number for tcp";
2798 # my $tcp = IPPROTO_TCP; # Alternative
2799 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2800 or die "getsockopt TCP_NODELAY: $!";
2801 my $nodelay = unpack("I", $packed);
2802 print "Nagle's algorithm is turned ",
2803 $nodelay ? "off\n" : "on\n";
2805 Portability issues: L<perlport/getsockopt>.
2808 X<glob> X<wildcard> X<filename, expansion> X<expand>
2812 =for Pod::Functions expand filenames using wildcards
2814 In list context, returns a (possibly empty) list of filename expansions on
2815 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2816 scalar context, glob iterates through such filename expansions, returning
2817 undef when the list is exhausted. This is the internal function
2818 implementing the C<< <*.c> >> operator, but you can use it directly. If
2819 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2820 more detail in L<perlop/"I/O Operators">.
2822 Note that C<glob> splits its arguments on whitespace and treats
2823 each segment as separate pattern. As such, C<glob("*.c *.h")>
2824 matches all files with a F<.c> or F<.h> extension. The expression
2825 C<glob(".* *")> matches all files in the current working directory.
2826 If you want to glob filenames that might contain whitespace, you'll
2827 have to use extra quotes around the spacey filename to protect it.
2828 For example, to glob filenames that have an C<e> followed by a space
2829 followed by an C<f>, use either of:
2831 @spacies = <"*e f*">;
2832 @spacies = glob '"*e f*"';
2833 @spacies = glob q("*e f*");
2835 If you had to get a variable through, you could do this:
2837 @spacies = glob "'*${var}e f*'";
2838 @spacies = glob qq("*${var}e f*");
2840 If non-empty braces are the only wildcard characters used in the
2841 C<glob>, no filenames are matched, but potentially many strings
2842 are returned. For example, this produces nine strings, one for
2843 each pairing of fruits and colors:
2845 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2847 Beginning with v5.6.0, this operator is implemented using the standard
2848 C<File::Glob> extension. See L<File::Glob> for details, including
2849 C<bsd_glob> which does not treat whitespace as a pattern separator.
2851 Portability issues: L<perlport/glob>.
2854 X<gmtime> X<UTC> X<Greenwich>
2858 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2860 Works just like L</localtime> but the returned values are
2861 localized for the standard Greenwich time zone.
2863 Note: When called in list context, $isdst, the last value
2864 returned by gmtime, is always C<0>. There is no
2865 Daylight Saving Time in GMT.
2867 Portability issues: L<perlport/gmtime>.
2870 X<goto> X<jump> X<jmp>
2876 =for Pod::Functions create spaghetti code
2878 The C<goto-LABEL> form finds the statement labeled with LABEL and
2879 resumes execution there. It can't be used to get out of a block or
2880 subroutine given to C<sort>. It can be used to go almost anywhere
2881 else within the dynamic scope, including out of subroutines, but it's
2882 usually better to use some other construct such as C<last> or C<die>.
2883 The author of Perl has never felt the need to use this form of C<goto>
2884 (in Perl, that is; C is another matter). (The difference is that C
2885 does not offer named loops combined with loop control. Perl does, and
2886 this replaces most structured uses of C<goto> in other languages.)
2888 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2889 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2890 necessarily recommended if you're optimizing for maintainability:
2892 goto ("FOO", "BAR", "GLARCH")[$i];
2894 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2895 function" rule. A pair of parentheses following it does not (necessarily)
2896 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2898 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2899 deprecated and will issue a warning. Even then, it may not be used to
2900 go into any construct that requires initialization, such as a
2901 subroutine or a C<foreach> loop. It also can't be used to go into a
2902 construct that is optimized away.
2904 The C<goto-&NAME> form is quite different from the other forms of
2905 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2906 doesn't have the stigma associated with other gotos. Instead, it
2907 exits the current subroutine (losing any changes set by local()) and
2908 immediately calls in its place the named subroutine using the current
2909 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2910 load another subroutine and then pretend that the other subroutine had
2911 been called in the first place (except that any modifications to C<@_>
2912 in the current subroutine are propagated to the other subroutine.)
2913 After the C<goto>, not even C<caller> will be able to tell that this
2914 routine was called first.
2916 NAME needn't be the name of a subroutine; it can be a scalar variable
2917 containing a code reference or a block that evaluates to a code
2920 =item grep BLOCK LIST
2923 =item grep EXPR,LIST
2925 =for Pod::Functions locate elements in a list test true against a given criterion
2927 This is similar in spirit to, but not the same as, grep(1) and its
2928 relatives. In particular, it is not limited to using regular expressions.
2930 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2931 C<$_> to each element) and returns the list value consisting of those
2932 elements for which the expression evaluated to true. In scalar
2933 context, returns the number of times the expression was true.
2935 @foo = grep(!/^#/, @bar); # weed out comments
2939 @foo = grep {!/^#/} @bar; # weed out comments
2941 Note that C<$_> is an alias to the list value, so it can be used to
2942 modify the elements of the LIST. While this is useful and supported,
2943 it can cause bizarre results if the elements of LIST are not variables.
2944 Similarly, grep returns aliases into the original list, much as a for
2945 loop's index variable aliases the list elements. That is, modifying an
2946 element of a list returned by grep (for example, in a C<foreach>, C<map>
2947 or another C<grep>) actually modifies the element in the original list.
2948 This is usually something to be avoided when writing clear code.
2950 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2951 been declared with C<my $_>) then, in addition to being locally aliased to
2952 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2953 can't be seen from the outside, avoiding any potential side-effects.
2955 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2958 X<hex> X<hexadecimal>
2962 =for Pod::Functions convert a string to a hexadecimal number
2964 Interprets EXPR as a hex string and returns the corresponding value.
2965 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2966 L</oct>.) If EXPR is omitted, uses C<$_>.
2968 print hex '0xAf'; # prints '175'
2969 print hex 'aF'; # same
2971 Hex strings may only represent integers. Strings that would cause
2972 integer overflow trigger a warning. Leading whitespace is not stripped,
2973 unlike oct(). To present something as hex, look into L</printf>,
2974 L</sprintf>, and L</unpack>.
2979 =for Pod::Functions patch a module's namespace into your own
2981 There is no builtin C<import> function. It is just an ordinary
2982 method (subroutine) defined (or inherited) by modules that wish to export
2983 names to another module. The C<use> function calls the C<import> method
2984 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2986 =item index STR,SUBSTR,POSITION
2987 X<index> X<indexOf> X<InStr>
2989 =item index STR,SUBSTR
2991 =for Pod::Functions find a substring within a string
2993 The index function searches for one string within another, but without
2994 the wildcard-like behavior of a full regular-expression pattern match.
2995 It returns the position of the first occurrence of SUBSTR in STR at
2996 or after POSITION. If POSITION is omitted, starts searching from the
2997 beginning of the string. POSITION before the beginning of the string
2998 or after its end is treated as if it were the beginning or the end,
2999 respectively. POSITION and the return value are based at zero.
3000 If the substring is not found, C<index> returns -1.
3003 X<int> X<integer> X<truncate> X<trunc> X<floor>
3007 =for Pod::Functions get the integer portion of a number
3009 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3010 You should not use this function for rounding: one because it truncates
3011 towards C<0>, and two because machine representations of floating-point
3012 numbers can sometimes produce counterintuitive results. For example,
3013 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3014 because it's really more like -268.99999999999994315658 instead. Usually,
3015 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3016 functions will serve you better than will int().
3018 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3021 =for Pod::Functions system-dependent device control system call
3023 Implements the ioctl(2) function. You'll probably first have to say
3025 require "sys/ioctl.ph"; # probably in
3026 # $Config{archlib}/sys/ioctl.ph
3028 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3029 exist or doesn't have the correct definitions you'll have to roll your
3030 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3031 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3032 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3033 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3034 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3035 has no string value but does have a numeric value, that value will be
3036 passed rather than a pointer to the string value. To guarantee this to be
3037 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3038 functions may be needed to manipulate the values of structures used by
3041 The return value of C<ioctl> (and C<fcntl>) is as follows:
3043 if OS returns: then Perl returns:
3045 0 string "0 but true"
3046 anything else that number
3048 Thus Perl returns true on success and false on failure, yet you can
3049 still easily determine the actual value returned by the operating
3052 $retval = ioctl(...) || -1;
3053 printf "System returned %d\n", $retval;
3055 The special string C<"0 but true"> is exempt from B<-w> complaints
3056 about improper numeric conversions.
3058 Portability issues: L<perlport/ioctl>.
3060 =item join EXPR,LIST
3063 =for Pod::Functions join a list into a string using a separator
3065 Joins the separate strings of LIST into a single string with fields
3066 separated by the value of EXPR, and returns that new string. Example:
3068 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3070 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3071 first argument. Compare L</split>.
3080 =for Pod::Functions retrieve list of indices from a hash
3082 Called in list context, returns a list consisting of all the keys of the
3083 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3084 releases prior to 5.12 will produce a syntax error if you try to use an
3085 array argument. In scalar context, returns the number of keys or indices.
3087 The keys of a hash are returned in an apparently random order. The actual
3088 random order is subject to change in future versions of Perl, but it
3089 is guaranteed to be the same order as either the C<values> or C<each>
3090 function produces (given that the hash has not been modified). Since
3091 Perl 5.8.1 the ordering can be different even between different runs of
3092 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
3095 As a side effect, calling keys() resets the internal iterator of the HASH or
3096 ARRAY (see L</each>). In particular, calling keys() in void context resets
3097 the iterator with no other overhead.
3099 Here is yet another way to print your environment:
3102 @values = values %ENV;
3104 print pop(@keys), '=', pop(@values), "\n";
3107 or how about sorted by key:
3109 foreach $key (sort(keys %ENV)) {
3110 print $key, '=', $ENV{$key}, "\n";
3113 The returned values are copies of the original keys in the hash, so
3114 modifying them will not affect the original hash. Compare L</values>.
3116 To sort a hash by value, you'll need to use a C<sort> function.
3117 Here's a descending numeric sort of a hash by its values:
3119 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3120 printf "%4d %s\n", $hash{$key}, $key;
3123 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3124 allocated for the given hash. This can gain you a measure of efficiency if
3125 you know the hash is going to get big. (This is similar to pre-extending
3126 an array by assigning a larger number to $#array.) If you say
3130 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3131 in fact, since it rounds up to the next power of two. These
3132 buckets will be retained even if you do C<%hash = ()>, use C<undef
3133 %hash> if you want to free the storage while C<%hash> is still in scope.
3134 You can't shrink the number of buckets allocated for the hash using
3135 C<keys> in this way (but you needn't worry about doing this by accident,
3136 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3139 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3140 a reference to an unblessed hash or array. The argument will be
3141 dereferenced automatically. This aspect of C<keys> is considered highly
3142 experimental. The exact behaviour may change in a future version of Perl.
3144 for (keys $hashref) { ... }
3145 for (keys $obj->get_arrayref) { ... }
3147 To avoid confusing would-be users of your code who are running earlier
3148 versions of Perl with mysterious syntax errors, put this sort of thing at
3149 the top of your file to signal that your code will work I<only> on Perls of
3152 use 5.012; # so keys/values/each work on arrays
3153 use 5.014; # so keys/values/each work on scalars (experimental)
3155 See also C<each>, C<values>, and C<sort>.
3157 =item kill SIGNAL, LIST
3162 =for Pod::Functions send a signal to a process or process group
3164 Sends a signal to a list of processes. Returns the number of
3165 processes successfully signaled (which is not necessarily the
3166 same as the number actually killed).
3168 $cnt = kill 1, $child1, $child2;
3171 If SIGNAL is zero, no signal is sent to the process, but C<kill>
3172 checks whether it's I<possible> to send a signal to it (that
3173 means, to be brief, that the process is owned by the same user, or we are
3174 the super-user). This is useful to check that a child process is still
3175 alive (even if only as a zombie) and hasn't changed its UID. See
3176 L<perlport> for notes on the portability of this construct.
3178 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
3179 of processes. That means you usually
3180 want to use positive not negative signals.
3182 You may also use a signal name in quotes. A negative signal name is the
3183 same as a negative signal number, killing process groups instead of processes.
3184 For example, C<kill -KILL, $pgrp> will send C<SIGKILL> to the entire process
3187 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3188 the operating system. For example, on POSIX-conforming systems, zero will
3189 signal the current process group, -1 will signal all processes, and any
3190 other negative PROCESS number will act as a negative signal number and
3191 kill the entire process group specified.
3193 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3194 A warning may be produced in a future version.
3196 See L<perlipc/"Signals"> for more details.
3198 On some platforms such as Windows where the fork() system call is not available.
3199 Perl can be built to emulate fork() at the interpreter level.
3200 This emulation has limitations related to kill that have to be considered,
3201 for code running on Windows and in code intended to be portable.
3203 See L<perlfork> for more details.
3205 If there is no I<LIST> of processes, no signal is sent, and the return
3206 value is 0. This form is sometimes used, however, because it causes
3207 tainting checks to be run. But see
3208 L<perlsec/Laundering and Detecting Tainted Data>.
3210 Portability issues: L<perlport/kill>.
3217 =for Pod::Functions exit a block prematurely
3219 The C<last> command is like the C<break> statement in C (as used in
3220 loops); it immediately exits the loop in question. If the LABEL is
3221 omitted, the command refers to the innermost enclosing loop. The
3222 C<continue> block, if any, is not executed:
3224 LINE: while (<STDIN>) {
3225 last LINE if /^$/; # exit when done with header
3229 C<last> cannot be used to exit a block that returns a value such as
3230 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3231 a grep() or map() operation.
3233 Note that a block by itself is semantically identical to a loop
3234 that executes once. Thus C<last> can be used to effect an early
3235 exit out of such a block.
3237 See also L</continue> for an illustration of how C<last>, C<next>, and
3245 =for Pod::Functions return lower-case version of a string
3247 Returns a lowercased version of EXPR. This is the internal function
3248 implementing the C<\L> escape in double-quoted strings.
3250 If EXPR is omitted, uses C<$_>.
3252 What gets returned depends on several factors:
3256 =item If C<use bytes> is in effect:
3260 =item On EBCDIC platforms
3262 The results are what the C language system call C<tolower()> returns.
3264 =item On ASCII platforms
3266 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3271 =item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3273 Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3274 semantics for the remaining code points (this last can only happen if
3275 the UTF8 flag is also set). See L<perllocale>.
3277 A deficiency in this is that case changes that cross the 255/256
3278 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3279 LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3280 platforms). But under C<use locale>, the lower case of U+1E9E is
3281 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3282 current locale, and Perl has no way of knowing if that character even
3283 exists in the locale, much less what code point it is. Perl returns
3284 the input character unchanged, for all instances (and there aren't
3285 many) where the 255/256 boundary would otherwise be crossed.
3287 =item Otherwise, If EXPR has the UTF8 flag set:
3289 Unicode semantics are used for the case change.
3291 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect:
3293 Unicode semantics are used for the case change.
3299 =item On EBCDIC platforms
3301 The results are what the C language system call C<tolower()> returns.
3303 =item On ASCII platforms
3305 ASCII semantics are used for the case change. The lowercase of any character
3306 outside the ASCII range is the character itself.
3313 X<lcfirst> X<lowercase>
3317 =for Pod::Functions return a string with just the next letter in lower case
3319 Returns the value of EXPR with the first character lowercased. This
3320 is the internal function implementing the C<\l> escape in
3321 double-quoted strings.
3323 If EXPR is omitted, uses C<$_>.
3325 This function behaves the same way under various pragmata, such as in a locale,
3333 =for Pod::Functions return the number of bytes in a string
3335 Returns the length in I<characters> of the value of EXPR. If EXPR is
3336 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3339 This function cannot be used on an entire array or hash to find out how
3340 many elements these have. For that, use C<scalar @array> and C<scalar keys
3341 %hash>, respectively.
3343 Like all Perl character operations, length() normally deals in logical
3344 characters, not physical bytes. For how many bytes a string encoded as
3345 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3346 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3351 =for Pod::Functions the current source line number
3353 A special token that compiles to the current line number.
3355 =item link OLDFILE,NEWFILE
3358 =for Pod::Functions create a hard link in the filesystem
3360 Creates a new filename linked to the old filename. Returns true for
3361 success, false otherwise.
3363 Portability issues: L<perlport/link>.
3365 =item listen SOCKET,QUEUESIZE
3368 =for Pod::Functions register your socket as a server
3370 Does the same thing that the listen(2) system call does. Returns true if
3371 it succeeded, false otherwise. See the example in
3372 L<perlipc/"Sockets: Client/Server Communication">.
3377 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3379 You really probably want to be using C<my> instead, because C<local> isn't
3380 what most people think of as "local". See
3381 L<perlsub/"Private Variables via my()"> for details.
3383 A local modifies the listed variables to be local to the enclosing
3384 block, file, or eval. If more than one value is listed, the list must
3385 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3386 for details, including issues with tied arrays and hashes.
3388 The C<delete local EXPR> construct can also be used to localize the deletion
3389 of array/hash elements to the current block.
3390 See L<perlsub/"Localized deletion of elements of composite types">.
3392 =item localtime EXPR
3393 X<localtime> X<ctime>
3397 =for Pod::Functions convert UNIX time into record or string using local time
3399 Converts a time as returned by the time function to a 9-element list
3400 with the time analyzed for the local time zone. Typically used as
3404 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3407 All list elements are numeric and come straight out of the C `struct
3408 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3409 of the specified time.
3411 C<$mday> is the day of the month and C<$mon> the month in
3412 the range C<0..11>, with 0 indicating January and 11 indicating December.
3413 This makes it easy to get a month name from a list:
3415 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3416 print "$abbr[$mon] $mday";
3417 # $mon=9, $mday=18 gives "Oct 18"
3419 C<$year> contains the number of years since 1900. To get a 4-digit
3424 To get the last two digits of the year (e.g., "01" in 2001) do:
3426 $year = sprintf("%02d", $year % 100);
3428 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3429 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3430 (or C<0..365> in leap years.)
3432 C<$isdst> is true if the specified time occurs during Daylight Saving
3433 Time, false otherwise.
3435 If EXPR is omitted, C<localtime()> uses the current time (as returned
3438 In scalar context, C<localtime()> returns the ctime(3) value:
3440 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3442 The format of this scalar value is B<not> locale-dependent
3443 but built into Perl. For GMT instead of local
3444 time use the L</gmtime> builtin. See also the
3445 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3446 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3447 and mktime(3) functions.
3449 To get somewhat similar but locale-dependent date strings, set up your
3450 locale environment variables appropriately (please see L<perllocale>) and
3453 use POSIX qw(strftime);
3454 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3455 # or for GMT formatted appropriately for your locale:
3456 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3458 Note that the C<%a> and C<%b>, the short forms of the day of the week
3459 and the month of the year, may not necessarily be three characters wide.
3461 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3462 by-name access mechanism to the gmtime() and localtime() functions,
3465 For a comprehensive date and time representation look at the
3466 L<DateTime> module on CPAN.
3468 Portability issues: L<perlport/localtime>.
3473 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3475 This function places an advisory lock on a shared variable or referenced
3476 object contained in I<THING> until the lock goes out of scope.
3478 The value returned is the scalar itself, if the argument is a scalar, or a
3479 reference, if the argument is a hash, array or subroutine.
3481 lock() is a "weak keyword" : this means that if you've defined a function
3482 by this name (before any calls to it), that function will be called
3483 instead. If you are not under C<use threads::shared> this does nothing.
3484 See L<threads::shared>.
3487 X<log> X<logarithm> X<e> X<ln> X<base>
3491 =for Pod::Functions retrieve the natural logarithm for a number
3493 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3494 returns the log of C<$_>. To get the
3495 log of another base, use basic algebra:
3496 The base-N log of a number is equal to the natural log of that number
3497 divided by the natural log of N. For example:
3501 return log($n)/log(10);
3504 See also L</exp> for the inverse operation.
3506 =item lstat FILEHANDLE
3511 =item lstat DIRHANDLE
3515 =for Pod::Functions stat a symbolic link
3517 Does the same thing as the C<stat> function (including setting the
3518 special C<_> filehandle) but stats a symbolic link instead of the file
3519 the symbolic link points to. If symbolic links are unimplemented on
3520 your system, a normal C<stat> is done. For much more detailed
3521 information, please see the documentation for C<stat>.
3523 If EXPR is omitted, stats C<$_>.
3525 Portability issues: L<perlport/lstat>.
3529 =for Pod::Functions match a string with a regular expression pattern
3531 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3533 =item map BLOCK LIST
3538 =for Pod::Functions apply a change to a list to get back a new list with the changes
3540 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3541 C<$_> to each element) and returns the list value composed of the
3542 results of each such evaluation. In scalar context, returns the
3543 total number of elements so generated. Evaluates BLOCK or EXPR in
3544 list context, so each element of LIST may produce zero, one, or
3545 more elements in the returned value.
3547 @chars = map(chr, @numbers);
3549 translates a list of numbers to the corresponding characters.
3551 my @squares = map { $_ * $_ } @numbers;
3553 translates a list of numbers to their squared values.
3555 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3557 shows that number of returned elements can differ from the number of
3558 input elements. To omit an element, return an empty list ().
3559 This could also be achieved by writing
3561 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3563 which makes the intention more clear.
3565 Map always returns a list, which can be
3566 assigned to a hash such that the elements
3567 become key/value pairs. See L<perldata> for more details.
3569 %hash = map { get_a_key_for($_) => $_ } @array;
3571 is just a funny way to write
3575 $hash{get_a_key_for($_)} = $_;
3578 Note that C<$_> is an alias to the list value, so it can be used to
3579 modify the elements of the LIST. While this is useful and supported,
3580 it can cause bizarre results if the elements of LIST are not variables.
3581 Using a regular C<foreach> loop for this purpose would be clearer in
3582 most cases. See also L</grep> for an array composed of those items of
3583 the original list for which the BLOCK or EXPR evaluates to true.
3585 If C<$_> is lexical in the scope where the C<map> appears (because it has
3586 been declared with C<my $_>), then, in addition to being locally aliased to
3587 the list elements, C<$_> keeps being lexical inside the block; that is, it
3588 can't be seen from the outside, avoiding any potential side-effects.
3590 C<{> starts both hash references and blocks, so C<map { ...> could be either
3591 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3592 ahead for the closing C<}> it has to take a guess at which it's dealing with
3593 based on what it finds just after the
3594 C<{>. Usually it gets it right, but if it
3595 doesn't it won't realize something is wrong until it gets to the C<}> and
3596 encounters the missing (or unexpected) comma. The syntax error will be
3597 reported close to the C<}>, but you'll need to change something near the C<{>
3598 such as using a unary C<+> to give Perl some help:
3600 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3601 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3602 %hash = map { ("\L$_" => 1) } @array # this also works
3603 %hash = map { lc($_) => 1 } @array # as does this.
3604 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3606 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3608 or to force an anon hash constructor use C<+{>:
3610 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3613 to get a list of anonymous hashes each with only one entry apiece.
3615 =item mkdir FILENAME,MASK
3616 X<mkdir> X<md> X<directory, create>
3618 =item mkdir FILENAME
3622 =for Pod::Functions create a directory
3624 Creates the directory specified by FILENAME, with permissions
3625 specified by MASK (as modified by C<umask>). If it succeeds it
3626 returns true; otherwise it returns false and sets C<$!> (errno).
3627 MASK defaults to 0777 if omitted, and FILENAME defaults
3628 to C<$_> if omitted.
3630 In general, it is better to create directories with a permissive MASK
3631 and let the user modify that with their C<umask> than it is to supply
3632 a restrictive MASK and give the user no way to be more permissive.
3633 The exceptions to this rule are when the file or directory should be
3634 kept private (mail files, for instance). The perlfunc(1) entry on
3635 C<umask> discusses the choice of MASK in more detail.
3637 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3638 number of trailing slashes. Some operating and filesystems do not get
3639 this right, so Perl automatically removes all trailing slashes to keep
3642 To recursively create a directory structure, look at
3643 the C<mkpath> function of the L<File::Path> module.
3645 =item msgctl ID,CMD,ARG
3648 =for Pod::Functions SysV IPC message control operations
3650 Calls the System V IPC function msgctl(2). You'll probably have to say
3654 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3655 then ARG must be a variable that will hold the returned C<msqid_ds>
3656 structure. Returns like C<ioctl>: the undefined value for error,
3657 C<"0 but true"> for zero, or the actual return value otherwise. See also
3658 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3661 Portability issues: L<perlport/msgctl>.
3663 =item msgget KEY,FLAGS
3666 =for Pod::Functions get SysV IPC message queue
3668 Calls the System V IPC function msgget(2). Returns the message queue
3669 id, or C<undef> on error. See also
3670 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3673 Portability issues: L<perlport/msgget>.
3675 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3678 =for Pod::Functions receive a SysV IPC message from a message queue
3680 Calls the System V IPC function msgrcv to receive a message from
3681 message queue ID into variable VAR with a maximum message size of
3682 SIZE. Note that when a message is received, the message type as a
3683 native long integer will be the first thing in VAR, followed by the
3684 actual message. This packing may be opened with C<unpack("l! a*")>.
3685 Taints the variable. Returns true if successful, false
3686 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3687 C<IPC::SysV> and C<IPC::SysV::Msg>.
3689 Portability issues: L<perlport/msgrcv>.
3691 =item msgsnd ID,MSG,FLAGS
3694 =for Pod::Functions send a SysV IPC message to a message queue
3696 Calls the System V IPC function msgsnd to send the message MSG to the
3697 message queue ID. MSG must begin with the native long integer message
3698 type, be followed by the length of the actual message, and then finally
3699 the message itself. This kind of packing can be achieved with
3700 C<pack("l! a*", $type, $message)>. Returns true if successful,
3701 false on error. See also the C<IPC::SysV>
3702 and C<IPC::SysV::Msg> documentation.
3704 Portability issues: L<perlport/msgsnd>.
3711 =item my EXPR : ATTRS
3713 =item my TYPE EXPR : ATTRS
3715 =for Pod::Functions declare and assign a local variable (lexical scoping)
3717 A C<my> declares the listed variables to be local (lexically) to the
3718 enclosing block, file, or C<eval>. If more than one value is listed,
3719 the list must be placed in parentheses.
3721 The exact semantics and interface of TYPE and ATTRS are still
3722 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3723 and attributes are handled using the C<attributes> pragma, or starting
3724 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3725 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3726 L<attributes>, and L<Attribute::Handlers>.
3733 =for Pod::Functions iterate a block prematurely
3735 The C<next> command is like the C<continue> statement in C; it starts
3736 the next iteration of the loop:
3738 LINE: while (<STDIN>) {
3739 next LINE if /^#/; # discard comments
3743 Note that if there were a C<continue> block on the above, it would get
3744 executed even on discarded lines. If LABEL is omitted, the command
3745 refers to the innermost enclosing loop.
3747 C<next> cannot be used to exit a block which returns a value such as
3748 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3749 a grep() or map() operation.
3751 Note that a block by itself is semantically identical to a loop
3752 that executes once. Thus C<next> will exit such a block early.
3754 See also L</continue> for an illustration of how C<last>, C<next>, and
3757 =item no MODULE VERSION LIST
3761 =item no MODULE VERSION
3763 =item no MODULE LIST
3769 =for Pod::Functions unimport some module symbols or semantics at compile time
3771 See the C<use> function, of which C<no> is the opposite.
3774 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3778 =for Pod::Functions convert a string to an octal number
3780 Interprets EXPR as an octal string and returns the corresponding
3781 value. (If EXPR happens to start off with C<0x>, interprets it as a
3782 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3783 binary string. Leading whitespace is ignored in all three cases.)
3784 The following will handle decimal, binary, octal, and hex in standard
3787 $val = oct($val) if $val =~ /^0/;
3789 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3790 in octal), use sprintf() or printf():
3792 $dec_perms = (stat("filename"))[2] & 07777;
3793 $oct_perm_str = sprintf "%o", $perms;
3795 The oct() function is commonly used when a string such as C<644> needs
3796 to be converted into a file mode, for example. Although Perl
3797 automatically converts strings into numbers as needed, this automatic
3798 conversion assumes base 10.
3800 Leading white space is ignored without warning, as too are any trailing
3801 non-digits, such as a decimal point (C<oct> only handles non-negative
3802 integers, not negative integers or floating point).
3804 =item open FILEHANDLE,EXPR
3805 X<open> X<pipe> X<file, open> X<fopen>
3807 =item open FILEHANDLE,MODE,EXPR
3809 =item open FILEHANDLE,MODE,EXPR,LIST
3811 =item open FILEHANDLE,MODE,REFERENCE
3813 =item open FILEHANDLE
3815 =for Pod::Functions open a file, pipe, or descriptor
3817 Opens the file whose filename is given by EXPR, and associates it with
3820 Simple examples to open a file for reading:
3822 open(my $fh, "<", "input.txt")
3823 or die "cannot open < input.txt: $!";
3827 open(my $fh, ">", "output.txt")
3828 or die "cannot open > output.txt: $!";
3830 (The following is a comprehensive reference to open(): for a gentler
3831 introduction you may consider L<perlopentut>.)
3833 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3834 new filehandle is autovivified, meaning that the variable is assigned a
3835 reference to a newly allocated anonymous filehandle. Otherwise if
3836 FILEHANDLE is an expression, its value is the real filehandle. (This is
3837 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3840 If EXPR is omitted, the global (package) scalar variable of the same
3841 name as the FILEHANDLE contains the filename. (Note that lexical
3842 variables--those declared with C<my> or C<state>--will not work for this
3843 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3846 If three (or more) arguments are specified, the open mode (including
3847 optional encoding) in the second argument are distinct from the filename in
3848 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3849 If MODE is C<< > >>, the file is opened for output, with existing files
3850 first being truncated ("clobbered") and nonexisting files newly created.
3851 If MODE is C<<< >> >>>, the file is opened for appending, again being
3852 created if necessary.
3854 You can put a C<+> in front of the C<< > >> or C<< < >> to
3855 indicate that you want both read and write access to the file; thus
3856 C<< +< >> is almost always preferred for read/write updates--the
3857 C<< +> >> mode would clobber the file first. You can't usually use
3858 either read-write mode for updating textfiles, since they have
3859 variable-length records. See the B<-i> switch in L<perlrun> for a
3860 better approach. The file is created with permissions of C<0666>
3861 modified by the process's C<umask> value.
3863 These various prefixes correspond to the fopen(3) modes of C<r>,
3864 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3866 In the one- and two-argument forms of the call, the mode and filename
3867 should be concatenated (in that order), preferably separated by white
3868 space. You can--but shouldn't--omit the mode in these forms when that mode
3869 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3870 the filename argument is a known literal.
3872 For three or more arguments if MODE is C<|->, the filename is
3873 interpreted as a command to which output is to be piped, and if MODE
3874 is C<-|>, the filename is interpreted as a command that pipes
3875 output to us. In the two-argument (and one-argument) form, one should
3876 replace dash (C<->) with the command.
3877 See L<perlipc/"Using open() for IPC"> for more examples of this.
3878 (You are not allowed to C<open> to a command that pipes both in I<and>
3879 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3880 L<perlipc/"Bidirectional Communication with Another Process"> for
3883 In the form of pipe opens taking three or more arguments, if LIST is specified
3884 (extra arguments after the command name) then LIST becomes arguments
3885 to the command invoked if the platform supports it. The meaning of
3886 C<open> with more than three arguments for non-pipe modes is not yet
3887 defined, but experimental "layers" may give extra LIST arguments
3890 In the two-argument (and one-argument) form, opening C<< <- >>
3891 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3893 You may (and usually should) use the three-argument form of open to specify
3894 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3895 that affect how the input and output are processed (see L<open> and
3896 L<PerlIO> for more details). For example:
3898 open(my $fh, "<:encoding(UTF-8)", "filename")
3899 || die "can't open UTF-8 encoded filename: $!";
3901 opens the UTF8-encoded file containing Unicode characters;
3902 see L<perluniintro>. Note that if layers are specified in the
3903 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3904 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3905 Those layers will also be ignored if you specifying a colon with no name
3906 following it. In that case the default layer for the operating system
3907 (:raw on Unix, :crlf on Windows) is used.
3909 Open returns nonzero on success, the undefined value otherwise. If
3910 the C<open> involved a pipe, the return value happens to be the pid of
3913 If you're running Perl on a system that distinguishes between text
3914 files and binary files, then you should check out L</binmode> for tips
3915 for dealing with this. The key distinction between systems that need
3916 C<binmode> and those that don't is their text file formats. Systems
3917 like Unix, Mac OS, and Plan 9, that end lines with a single
3918 character and encode that character in C as C<"\n"> do not
3919 need C<binmode>. The rest need it.
3921 When opening a file, it's seldom a good idea to continue
3922 if the request failed, so C<open> is frequently used with
3923 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3924 where you want to format a suitable error message (but there are
3925 modules that can help with that problem)) always check
3926 the return value from opening a file.
3928 As a special case the three-argument form with a read/write mode and the third
3929 argument being C<undef>:
3931 open(my $tmp, "+>", undef) or die ...
3933 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3934 works for symmetry, but you really should consider writing something
3935 to the temporary file first. You will need to seek() to do the
3938 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3939 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3940 open filehandles directly to Perl scalars via:
3942 open($fh, ">", \$variable) || ..
3944 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3947 open(STDOUT, ">", \$variable)
3948 or die "Can't open STDOUT: $!";
3953 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3954 while (<ARTICLE>) {...
3956 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3957 # if the open fails, output is discarded
3959 open(my $dbase, "+<", "dbase.mine") # open for update
3960 or die "Can't open 'dbase.mine' for update: $!";
3962 open(my $dbase, "+<dbase.mine") # ditto
3963 or die "Can't open 'dbase.mine' for update: $!";
3965 open(ARTICLE, "-|", "caesar <$article") # decrypt article
3966 or die "Can't start caesar: $!";
3968 open(ARTICLE, "caesar <$article |") # ditto
3969 or die "Can't start caesar: $!";
3971 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3972 or die "Can't start sort: $!";
3975 open(MEMORY, ">", \$var)
3976 or die "Can't open memory file: $!";
3977 print MEMORY "foo!\n"; # output will appear in $var
3979 # process argument list of files along with any includes
3981 foreach $file (@ARGV) {
3982 process($file, "fh00");
3986 my($filename, $input) = @_;
3987 $input++; # this is a string increment
3988 unless (open($input, "<", $filename)) {
3989 print STDERR "Can't open $filename: $!\n";
3994 while (<$input>) { # note use of indirection
3995 if (/^#include "(.*)"/) {
3996 process($1, $input);
4003 See L<perliol> for detailed info on PerlIO.
4005 You may also, in the Bourne shell tradition, specify an EXPR beginning
4006 with C<< >& >>, in which case the rest of the string is interpreted
4007 as the name of a filehandle (or file descriptor, if numeric) to be
4008 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4009 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4010 The mode you specify should match the mode of the original filehandle.
4011 (Duping a filehandle does not take into account any existing contents
4012 of IO buffers.) If you use the three-argument
4013 form, then you can pass either a
4014 number, the name of a filehandle, or the normal "reference to a glob".
4016 Here is a script that saves, redirects, and restores C<STDOUT> and
4017 C<STDERR> using various methods:
4020 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4021 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4023 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4024 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4026 select STDERR; $| = 1; # make unbuffered
4027 select STDOUT; $| = 1; # make unbuffered
4029 print STDOUT "stdout 1\n"; # this works for
4030 print STDERR "stderr 1\n"; # subprocesses too
4032 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4033 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4035 print STDOUT "stdout 2\n";
4036 print STDERR "stderr 2\n";
4038 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4039 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4040 that file descriptor (and not call C<dup(2)>); this is more
4041 parsimonious of file descriptors. For example:
4043 # open for input, reusing the fileno of $fd
4044 open(FILEHANDLE, "<&=$fd")
4048 open(FILEHANDLE, "<&=", $fd)
4052 # open for append, using the fileno of OLDFH
4053 open(FH, ">>&=", OLDFH)
4057 open(FH, ">>&=OLDFH")
4059 Being parsimonious on filehandles is also useful (besides being
4060 parsimonious) for example when something is dependent on file
4061 descriptors, like for example locking using flock(). If you do just
4062 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4063 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4064 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4065 the same underlying system file descriptor.
4067 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4068 fdopen() to implement the C<=> functionality. On many Unix systems,
4069 fdopen() fails when file descriptors exceed a certain value, typically 255.
4070 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4072 You can see whether your Perl was built with PerlIO by running C<perl -V>
4073 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4074 have PerlIO; otherwise you don't.
4076 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4077 with the one- or two-argument forms of C<open>),
4078 an implicit C<fork> is done, so C<open> returns twice: in the parent
4079 process it returns the pid
4080 of the child process, and in the child process it returns (a defined) C<0>.
4081 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4083 For example, use either
4085 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4089 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4095 # either write TO_KID or else read FROM_KID
4097 waitpid $child_pid, 0;
4099 # am the child; use STDIN/STDOUT normally
4104 The filehandle behaves normally for the parent, but I/O to that
4105 filehandle is piped from/to the STDOUT/STDIN of the child process.
4106 In the child process, the filehandle isn't opened--I/O happens from/to
4107 the new STDOUT/STDIN. Typically this is used like the normal
4108 piped open when you want to exercise more control over just how the
4109 pipe command gets executed, such as when running setuid and
4110 you don't want to have to scan shell commands for metacharacters.
4112 The following blocks are more or less equivalent:
4114 open(FOO, "|tr '[a-z]' '[A-Z]'");
4115 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4116 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4117 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4119 open(FOO, "cat -n '$file'|");
4120 open(FOO, "-|", "cat -n '$file'");
4121 open(FOO, "-|") || exec "cat", "-n", $file;
4122 open(FOO, "-|", "cat", "-n", $file);
4124 The last two examples in each block show the pipe as "list form", which is
4125 not yet supported on all platforms. A good rule of thumb is that if
4126 your platform has a real C<fork()> (in other words, if your platform is
4127 Unix, including Linux and MacOS X), you can use the list form. You would
4128 want to use the list form of the pipe so you can pass literal arguments
4129 to the command without risk of the shell interpreting any shell metacharacters
4130 in them. However, this also bars you from opening pipes to commands
4131 that intentionally contain shell metacharacters, such as:
4133 open(FOO, "|cat -n | expand -4 | lpr")
4134 // die "Can't open pipeline to lpr: $!";
4136 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4138 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4139 output before any operation that may do a fork, but this may not be
4140 supported on some platforms (see L<perlport>). To be safe, you may need
4141 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4142 of C<IO::Handle> on any open handles.
4144 On systems that support a close-on-exec flag on files, the flag will
4145 be set for the newly opened file descriptor as determined by the value
4146 of C<$^F>. See L<perlvar/$^F>.
4148 Closing any piped filehandle causes the parent process to wait for the
4149 child to finish, then returns the status value in C<$?> and
4150 C<${^CHILD_ERROR_NATIVE}>.
4152 The filename passed to the one- and two-argument forms of open() will
4153 have leading and trailing whitespace deleted and normal
4154 redirection characters honored. This property, known as "magic open",
4155 can often be used to good effect. A user could specify a filename of
4156 F<"rsh cat file |">, or you could change certain filenames as needed:
4158 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4159 open(FH, $filename) or die "Can't open $filename: $!";
4161 Use the three-argument form to open a file with arbitrary weird characters in it,
4163 open(FOO, "<", $file)
4164 || die "can't open < $file: $!";
4166 otherwise it's necessary to protect any leading and trailing whitespace:
4168 $file =~ s#^(\s)#./$1#;
4169 open(FOO, "< $file\0")
4170 || die "open failed: $!";
4172 (this may not work on some bizarre filesystems). One should
4173 conscientiously choose between the I<magic> and I<three-argument> form
4176 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4178 will allow the user to specify an argument of the form C<"rsh cat file |">,
4179 but will not work on a filename that happens to have a trailing space, while
4181 open(IN, "<", $ARGV[0])
4182 || die "can't open < $ARGV[0]: $!";
4184 will have exactly the opposite restrictions.
4186 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4187 should use the C<sysopen> function, which involves no such magic (but may
4188 use subtly different filemodes than Perl open(), which is mapped to C
4189 fopen()). This is another way to protect your filenames from
4190 interpretation. For example:
4193 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4194 or die "sysopen $path: $!";
4195 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4196 print HANDLE "stuff $$\n";
4198 print "File contains: ", <HANDLE>;
4200 Using the constructor from the C<IO::Handle> package (or one of its
4201 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
4202 filehandles that have the scope of the variables used to hold them, then
4203 automatically (but silently) close once their reference counts become
4204 zero, typically at scope exit:
4208 sub read_myfile_munged {
4210 # or just leave it undef to autoviv
4211 my $handle = IO::File->new;
4212 open($handle, "<", "myfile") or die "myfile: $!";
4214 or return (); # Automatically closed here.
4215 mung($first) or die "mung failed"; # Or here.
4216 return (first, <$handle>) if $ALL; # Or here.
4217 return $first; # Or here.
4220 B<WARNING:> The previous example has a bug because the automatic
4221 close that happens when the refcount on C<handle> reaches zero does not
4222 properly detect and report failures. I<Always> close the handle
4223 yourself and inspect the return value.
4226 || warn "close failed: $!";
4228 See L</seek> for some details about mixing reading and writing.
4230 Portability issues: L<perlport/open>.
4232 =item opendir DIRHANDLE,EXPR
4235 =for Pod::Functions open a directory
4237 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4238 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4239 DIRHANDLE may be an expression whose value can be used as an indirect
4240 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4241 scalar variable (or array or hash element), the variable is assigned a
4242 reference to a new anonymous dirhandle; that is, it's autovivified.
4243 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4245 See the example at C<readdir>.
4252 =for Pod::Functions find a character's numeric representation
4254 Returns the numeric value of the first character of EXPR.
4255 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4256 (Note I<character>, not byte.)
4258 For the reverse, see L</chr>.
4259 See L<perlunicode> for more about Unicode.
4266 =item our EXPR : ATTRS
4268 =item our TYPE EXPR : ATTRS
4270 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4272 C<our> associates a simple name with a package variable in the current
4273 package for use within the current scope. When C<use strict 'vars'> is in
4274 effect, C<our> lets you use declared global variables without qualifying
4275 them with package names, within the lexical scope of the C<our> declaration.
4276 In this way C<our> differs from C<use vars>, which is package-scoped.
4278 Unlike C<my> or C<state>, which allocates storage for a variable and
4279 associates a simple name with that storage for use within the current
4280 scope, C<our> associates a simple name with a package (read: global)
4281 variable in the current package, for use within the current lexical scope.
4282 In other words, C<our> has the same scoping rules as C<my> or C<state>, but
4283 does not necessarily create a variable.
4285 If more than one value is listed, the list must be placed
4291 An C<our> declaration declares a global variable that will be visible
4292 across its entire lexical scope, even across package boundaries. The
4293 package in which the variable is entered is determined at the point
4294 of the declaration, not at the point of use. This means the following
4298 our $bar; # declares $Foo::bar for rest of lexical scope
4302 print $bar; # prints 20, as it refers to $Foo::bar
4304 Multiple C<our> declarations with the same name in the same lexical
4305 scope are allowed if they are in different packages. If they happen
4306 to be in the same package, Perl will emit warnings if you have asked
4307 for them, just like multiple C<my> declarations. Unlike a second
4308 C<my> declaration, which will bind the name to a fresh variable, a
4309 second C<our> declaration in the same package, in the same scope, is
4314 our $bar; # declares $Foo::bar for rest of lexical scope
4318 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4319 print $bar; # prints 30
4321 our $bar; # emits warning but has no other effect
4322 print $bar; # still prints 30
4324 An C<our> declaration may also have a list of attributes associated
4327 The exact semantics and interface of TYPE and ATTRS are still
4328 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4329 and attributes are handled using the C<attributes> pragma, or, starting
4330 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4331 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4332 L<attributes>, and L<Attribute::Handlers>.
4334 =item pack TEMPLATE,LIST
4337 =for Pod::Functions convert a list into a binary representation
4339 Takes a LIST of values and converts it into a string using the rules
4340 given by the TEMPLATE. The resulting string is the concatenation of
4341 the converted values. Typically, each converted value looks
4342 like its machine-level representation. For example, on 32-bit machines
4343 an integer may be represented by a sequence of 4 bytes, which will in
4344 Perl be presented as a string that's 4 characters long.
4346 See L<perlpacktut> for an introduction to this function.
4348 The TEMPLATE is a sequence of characters that give the order and type
4349 of values, as follows:
4351 a A string with arbitrary binary data, will be null padded.
4352 A A text (ASCII) string, will be space padded.
4353 Z A null-terminated (ASCIZ) string, will be null padded.
4355 b A bit string (ascending bit order inside each byte,
4357 B A bit string (descending bit order inside each byte).
4358 h A hex string (low nybble first).
4359 H A hex string (high nybble first).
4361 c A signed char (8-bit) value.
4362 C An unsigned char (octet) value.
4363 W An unsigned char value (can be greater than 255).
4365 s A signed short (16-bit) value.
4366 S An unsigned short value.
4368 l A signed long (32-bit) value.
4369 L An unsigned long value.
4371 q A signed quad (64-bit) value.
4372 Q An unsigned quad value.
4373 (Quads are available only if your system supports 64-bit
4374 integer values _and_ if Perl has been compiled to support
4375 those. Raises an exception otherwise.)
4377 i A signed integer value.
4378 I A unsigned integer value.
4379 (This 'integer' is _at_least_ 32 bits wide. Its exact
4380 size depends on what a local C compiler calls 'int'.)
4382 n An unsigned short (16-bit) in "network" (big-endian) order.
4383 N An unsigned long (32-bit) in "network" (big-endian) order.
4384 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4385 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4387 j A Perl internal signed integer value (IV).
4388 J A Perl internal unsigned integer value (UV).
4390 f A single-precision float in native format.
4391 d A double-precision float in native format.
4393 F A Perl internal floating-point value (NV) in native format
4394 D A float of long-double precision in native format.
4395 (Long doubles are available only if your system supports
4396 long double values _and_ if Perl has been compiled to
4397 support those. Raises an exception otherwise.)
4399 p A pointer to a null-terminated string.
4400 P A pointer to a structure (fixed-length string).
4402 u A uuencoded string.
4403 U A Unicode character number. Encodes to a character in char-
4404 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4407 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4408 for details). Its bytes represent an unsigned integer in
4409 base 128, most significant digit first, with as few digits
4410 as possible. Bit eight (the high bit) is set on each byte
4413 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4415 @ Null-fill or truncate to absolute position, counted from the
4416 start of the innermost ()-group.
4417 . Null-fill or truncate to absolute position specified by
4419 ( Start of a ()-group.
4421 One or more modifiers below may optionally follow certain letters in the
4422 TEMPLATE (the second column lists letters for which the modifier is valid):
4424 ! sSlLiI Forces native (short, long, int) sizes instead
4425 of fixed (16-/32-bit) sizes.
4427 xX Make x and X act as alignment commands.
4429 nNvV Treat integers as signed instead of unsigned.
4431 @. Specify position as byte offset in the internal
4432 representation of the packed string. Efficient
4435 > sSiIlLqQ Force big-endian byte-order on the type.
4436 jJfFdDpP (The "big end" touches the construct.)
4438 < sSiIlLqQ Force little-endian byte-order on the type.
4439 jJfFdDpP (The "little end" touches the construct.)
4441 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4442 to force a particular byte-order on all components in that group,
4443 including all its subgroups.
4445 The following rules apply:
4451 Each letter may optionally be followed by a number indicating the repeat
4452 count. A numeric repeat count may optionally be enclosed in brackets, as
4453 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4454 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4455 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4456 something else, described below. Supplying a C<*> for the repeat count
4457 instead of a number means to use however many items are left, except for:
4463 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4467 <.>, where it means relative to the start of the string.
4471 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4475 One can replace a numeric repeat count with a template letter enclosed in
4476 brackets to use the packed byte length of the bracketed template for the
4479 For example, the template C<x[L]> skips as many bytes as in a packed long,
4480 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4481 variable-expanded) unpacks. If the template in brackets contains alignment
4482 commands (such as C<x![d]>), its packed length is calculated as if the
4483 start of the template had the maximal possible alignment.
4485 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4486 trailing null byte, so the resulting string is always one byte longer than
4487 the byte length of the item itself.
4489 When used with C<@>, the repeat count represents an offset from the start
4490 of the innermost C<()> group.
4492 When used with C<.>, the repeat count determines the starting position to
4493 calculate the value offset as follows:
4499 If the repeat count is C<0>, it's relative to the current position.
4503 If the repeat count is C<*>, the offset is relative to the start of the
4508 And if it's an integer I<n>, the offset is relative to the start of the
4509 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4510 bigger then the group level.
4514 The repeat count for C<u> is interpreted as the maximal number of bytes
4515 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4516 count should not be more than 65.
4520 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4521 string of length count, padding with nulls or spaces as needed. When
4522 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4523 after the first null, and C<a> returns data with no stripping at all.
4525 If the value to pack is too long, the result is truncated. If it's too
4526 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4527 followed by a null byte. Thus C<Z> always packs a trailing null, except
4528 when the count is 0.
4532 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4533 Each such format generates 1 bit of the result. These are typically followed
4534 by a repeat count like C<B8> or C<B64>.
4536 Each result bit is based on the least-significant bit of the corresponding
4537 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4538 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4540 Starting from the beginning of the input string, each 8-tuple
4541 of characters is converted to 1 character of output. With format C<b>,
4542 the first character of the 8-tuple determines the least-significant bit of a
4543 character; with format C<B>, it determines the most-significant bit of
4546 If the length of the input string is not evenly divisible by 8, the
4547 remainder is packed as if the input string were padded by null characters
4548 at the end. Similarly during unpacking, "extra" bits are ignored.
4550 If the input string is longer than needed, remaining characters are ignored.
4552 A C<*> for the repeat count uses all characters of the input field.
4553 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4557 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4558 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4560 For each such format, pack() generates 4 bits of result.
4561 With non-alphabetical characters, the result is based on the 4 least-significant
4562 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4563 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4564 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4565 is compatible with the usual hexadecimal digits, so that C<"a"> and
4566 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4567 characters with this format.
4569 Starting from the beginning of the template to pack(), each pair
4570 of characters is converted to 1 character of output. With format C<h>, the
4571 first character of the pair determines the least-significant nybble of the
4572 output character; with format C<H>, it determines the most-significant
4575 If the length of the input string is not even, it behaves as if padded by
4576 a null character at the end. Similarly, "extra" nybbles are ignored during
4579 If the input string is longer than needed, extra characters are ignored.
4581 A C<*> for the repeat count uses all characters of the input field. For
4582 unpack(), nybbles are converted to a string of hexadecimal digits.
4586 The C<p> format packs a pointer to a null-terminated string. You are
4587 responsible for ensuring that the string is not a temporary value, as that
4588 could potentially get deallocated before you got around to using the packed
4589 result. The C<P> format packs a pointer to a structure of the size indicated
4590 by the length. A null pointer is created if the corresponding value for
4591 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4592 unpacks into C<undef>.
4594 If your system has a strange pointer size--meaning a pointer is neither as
4595 big as an int nor as big as a long--it may not be possible to pack or
4596 unpack pointers in big- or little-endian byte order. Attempting to do
4597 so raises an exception.
4601 The C</> template character allows packing and unpacking of a sequence of
4602 items where the packed structure contains a packed item count followed by
4603 the packed items themselves. This is useful when the structure you're
4604 unpacking has encoded the sizes or repeat counts for some of its fields
4605 within the structure itself as separate fields.
4607 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4608 I<length-item> describes how the length value is packed. Formats likely
4609 to be of most use are integer-packing ones like C<n> for Java strings,
4610 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4612 For C<pack>, I<sequence-item> may have a repeat count, in which case
4613 the minimum of that and the number of available items is used as the argument
4614 for I<length-item>. If it has no repeat count or uses a '*', the number
4615 of available items is used.
4617 For C<unpack>, an internal stack of integer arguments unpacked so far is
4618 used. You write C</>I<sequence-item> and the repeat count is obtained by
4619 popping off the last element from the stack. The I<sequence-item> must not
4620 have a repeat count.
4622 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4623 the I<length-item> is the string length, not the number of strings. With
4624 an explicit repeat count for pack, the packed string is adjusted to that
4625 length. For example:
4627 This code: gives this result:
4629 unpack("W/a", "\004Gurusamy") ("Guru")
4630 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4631 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4633 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4634 pack("a/W2", ord("a") .. ord("z")) "2ab"
4636 The I<length-item> is not returned explicitly from C<unpack>.
4638 Supplying a count to the I<length-item> format letter is only useful with
4639 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4640 introduce C<"\000"> characters, which Perl does not regard as legal in
4645 The integer types C<s>, C<S>, C<l>, and C<L> may be
4646 followed by a C<!> modifier to specify native shorts or
4647 longs. As shown in the example above, a bare C<l> means
4648 exactly 32 bits, although the native C<long> as seen by the local C compiler
4649 may be larger. This is mainly an issue on 64-bit platforms. You can
4650 see whether using C<!> makes any difference this way:
4652 printf "format s is %d, s! is %d\n",
4653 length pack("s"), length pack("s!");
4655 printf "format l is %d, l! is %d\n",
4656 length pack("l"), length pack("l!");
4659 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4660 they are identical to C<i> and C<I>.
4662 The actual sizes (in bytes) of native shorts, ints, longs, and long
4663 longs on the platform where Perl was built are also available from
4666 $ perl -V:{short,int,long{,long}}size
4672 or programmatically via the C<Config> module:
4675 print $Config{shortsize}, "\n";
4676 print $Config{intsize}, "\n";
4677 print $Config{longsize}, "\n";
4678 print $Config{longlongsize}, "\n";
4680 C<$Config{longlongsize}> is undefined on systems without
4685 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4686 inherently non-portable between processors and operating systems because
4687 they obey native byteorder and endianness. For example, a 4-byte integer
4688 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4689 handled by the CPU registers) into bytes as
4691 0x12 0x34 0x56 0x78 # big-endian
4692 0x78 0x56 0x34 0x12 # little-endian
4694 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4695 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4696 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4697 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4699 The names I<big-endian> and I<little-endian> are comic references to the
4700 egg-eating habits of the little-endian Lilliputians and the big-endian
4701 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4702 This entered computer lingo via the paper "On Holy Wars and a Plea for
4703 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4705 Some systems may have even weirder byte orders such as
4710 You can determine your system endianness with this incantation:
4712 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4714 The byteorder on the platform where Perl was built is also available
4718 print "$Config{byteorder}\n";
4720 or from the command line:
4724 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4725 and C<"87654321"> are big-endian.
4727 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4728 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4729 immediately below. See also L<perlport>.
4733 Starting with Perl 5.9.2, integer and floating-point formats, along with
4734 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4735 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4736 or little-endian byte-order. These modifiers are especially useful
4737 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4738 64-bit integers, or floating-point values.
4740 Here are some concerns to keep in mind when using an endianness modifier:
4746 Exchanging signed integers between different platforms works only
4747 when all platforms store them in the same format. Most platforms store
4748 signed integers in two's-complement notation, so usually this is not an issue.
4752 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4753 formats on big- or little-endian machines. Otherwise, attempting to
4754 use them raises an exception.
4758 Forcing big- or little-endian byte-order on floating-point values for
4759 data exchange can work only if all platforms use the same
4760 binary representation such as IEEE floating-point. Even if all
4761 platforms are using IEEE, there may still be subtle differences. Being able
4762 to use C<< > >> or C<< < >> on floating-point values can be useful,
4763 but also dangerous if you don't know exactly what you're doing.
4764 It is not a general way to portably store floating-point values.
4768 When using C<< > >> or C<< < >> on a C<()> group, this affects
4769 all types inside the group that accept byte-order modifiers,
4770 including all subgroups. It is silently ignored for all other
4771 types. You are not allowed to override the byte-order within a group
4772 that already has a byte-order modifier suffix.
4778 Real numbers (floats and doubles) are in native machine format only.
4779 Due to the multiplicity of floating-point formats and the lack of a
4780 standard "network" representation for them, no facility for interchange has been
4781 made. This means that packed floating-point data written on one machine
4782 may not be readable on another, even if both use IEEE floating-point
4783 arithmetic (because the endianness of the memory representation is not part
4784 of the IEEE spec). See also L<perlport>.
4786 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4787 modifiers to force big- or little-endian byte-order on floating-point values.
4789 Because Perl uses doubles (or long doubles, if configured) internally for
4790 all numeric calculation, converting from double into float and thence
4791 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4792 will not in general equal $foo.
4796 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4797 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4798 where the packed string is processed in its UTF-8-encoded Unicode form on
4799 a byte-by-byte basis. Character mode is the default
4800 unless the format string starts with C<U>. You
4801 can always switch mode mid-format with an explicit
4802 C<C0> or C<U0> in the format. This mode remains in effect until the next
4803 mode change, or until the end of the C<()> group it (directly) applies to.
4805 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4806 bytes is not necessarily obvious. Probably only the first of these
4809 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4810 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4812 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4813 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4815 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4816 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4818 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4819 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4820 C3.8E.C2.B1.C3.8F.C2.89
4822 Those examples also illustrate that you should not try to use
4823 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4827 You must yourself do any alignment or padding by inserting, for example,
4828 enough C<"x">es while packing. There is no way for pack() and unpack()
4829 to know where characters are going to or coming from, so they
4830 handle their output and input as flat sequences of characters.
4834 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4835 take a repeat count either as postfix, or for unpack(), also via the C</>
4836 template character. Within each repetition of a group, positioning with
4837 C<@> starts over at 0. Therefore, the result of
4839 pack("@1A((@2A)@3A)", qw[X Y Z])
4841 is the string C<"\0X\0\0YZ">.
4845 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4846 jump forward or back to the closest position aligned at a multiple of C<count>
4847 characters. For example, to pack() or unpack() a C structure like
4850 char c; /* one signed, 8-bit character */
4855 one may need to use the template C<c x![d] d c[2]>. This assumes that
4856 doubles must be aligned to the size of double.
4858 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4863 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4864 represent signed 16-/32-bit integers in big-/little-endian order.
4865 This is portable only when all platforms sharing packed data use the
4866 same binary representation for signed integers; for example, when all
4867 platforms use two's-complement representation.
4871 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4872 White space can separate pack codes from each other, but modifiers and
4873 repeat counts must follow immediately. Breaking complex templates into
4874 individual line-by-line components, suitably annotated, can do as much to
4875 improve legibility and maintainability of pack/unpack formats as C</x> can
4876 for complicated pattern matches.
4880 If TEMPLATE requires more arguments than pack() is given, pack()
4881 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4882 than given, extra arguments are ignored.
4888 $foo = pack("WWWW",65,66,67,68);
4890 $foo = pack("W4",65,66,67,68);
4892 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4893 # same thing with Unicode circled letters.
4894 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4895 # same thing with Unicode circled letters. You don't get the
4896 # UTF-8 bytes because the U at the start of the format caused
4897 # a switch to U0-mode, so the UTF-8 bytes get joined into
4899 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4900 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4901 # This is the UTF-8 encoding of the string in the
4904 $foo = pack("ccxxcc",65,66,67,68);
4907 # NOTE: The examples above featuring "W" and "c" are true
4908 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4909 # and UTF-8. On EBCDIC systems, the first example would be
4910 # $foo = pack("WWWW",193,194,195,196);
4912 $foo = pack("s2",1,2);
4913 # "\001\000\002\000" on little-endian
4914 # "\000\001\000\002" on big-endian
4916 $foo = pack("a4","abcd","x","y","z");
4919 $foo = pack("aaaa","abcd","x","y","z");
4922 $foo = pack("a14","abcdefg");
4923 # "abcdefg\0\0\0\0\0\0\0"
4925 $foo = pack("i9pl", gmtime);
4926 # a real struct tm (on my system anyway)
4928 $utmp_template = "Z8 Z8 Z16 L";
4929 $utmp = pack($utmp_template, @utmp1);
4930 # a struct utmp (BSDish)
4932 @utmp2 = unpack($utmp_template, $utmp);
4933 # "@utmp1" eq "@utmp2"
4936 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4939 $foo = pack('sx2l', 12, 34);
4940 # short 12, two zero bytes padding, long 34
4941 $bar = pack('s@4l', 12, 34);
4942 # short 12, zero fill to position 4, long 34
4944 $baz = pack('s.l', 12, 4, 34);
4945 # short 12, zero fill to position 4, long 34
4947 $foo = pack('nN', 42, 4711);
4948 # pack big-endian 16- and 32-bit unsigned integers
4949 $foo = pack('S>L>', 42, 4711);
4951 $foo = pack('s<l<', -42, 4711);
4952 # pack little-endian 16- and 32-bit signed integers
4953 $foo = pack('(sl)<', -42, 4711);
4956 The same template may generally also be used in unpack().
4958 =item package NAMESPACE
4960 =item package NAMESPACE VERSION
4961 X<package> X<module> X<namespace> X<version>
4963 =item package NAMESPACE BLOCK
4965 =item package NAMESPACE VERSION BLOCK
4966 X<package> X<module> X<namespace> X<version>
4968 =for Pod::Functions declare a separate global namespace
4970 Declares the BLOCK or the rest of the compilation unit as being in the
4971 given namespace. The scope of the package declaration is either the
4972 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4973 itself through the end of current scope (the enclosing block, file, or
4974 C<eval>). That is, the forms without a BLOCK are operative through the end
4975 of the current scope, just like the C<my>, C<state>, and C<our> operators.
4976 All unqualified dynamic identifiers in this scope will be in the given
4977 namespace, except where overridden by another C<package> declaration or
4978 when they're one of the special identifiers that qualify into C<main::>,
4979 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
4981 A package statement affects dynamic variables only, including those
4982 you've used C<local> on, but I<not> lexical variables, which are created
4983 with C<my>, C<state>, or C<our>. Typically it would be the first
4984 declaration in a file included by C<require> or C<use>. You can switch into a
4985 package in more than one place, since this only determines which default
4986 symbol table the compiler uses for the rest of that block. You can refer to
4987 identifiers in other packages than the current one by prefixing the identifier
4988 with the package name and a double colon, as in C<$SomePack::var>
4989 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4990 package as assumed. That is, C<$::sail> is equivalent to
4991 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4992 code, mostly from Perl 4).
4994 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4995 namespace to a L<version> object with the VERSION provided. VERSION must be a
4996 "strict" style version number as defined by the L<version> module: a positive
4997 decimal number (integer or decimal-fraction) without exponentiation or else a
4998 dotted-decimal v-string with a leading 'v' character and at least three
4999 components. You should set C<$VERSION> only once per package.
5001 See L<perlmod/"Packages"> for more information about packages, modules,
5002 and classes. See L<perlsub> for other scoping issues.
5007 =for Pod::Functions +5.004 the current package
5009 A special token that returns the name of the package in which it occurs.
5011 =item pipe READHANDLE,WRITEHANDLE
5014 =for Pod::Functions open a pair of connected filehandles
5016 Opens a pair of connected pipes like the corresponding system call.
5017 Note that if you set up a loop of piped processes, deadlock can occur
5018 unless you are very careful. In addition, note that Perl's pipes use
5019 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5020 after each command, depending on the application.
5022 Returns true on success.
5024 See L<IPC::Open2>, L<IPC::Open3>, and
5025 L<perlipc/"Bidirectional Communication with Another Process">
5026 for examples of such things.
5028 On systems that support a close-on-exec flag on files, that flag is set
5029 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5030 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5039 =for Pod::Functions remove the last element from an array and return it
5041 Pops and returns the last value of the array, shortening the array by
5044 Returns the undefined value if the array is empty, although this may also
5045 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5046 main program, but the C<@_> array in subroutines, just like C<shift>.
5048 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
5049 reference to an unblessed array. The argument will be dereferenced
5050 automatically. This aspect of C<pop> is considered highly experimental.
5051 The exact behaviour may change in a future version of Perl.
5053 To avoid confusing would-be users of your code who are running earlier
5054 versions of Perl with mysterious syntax errors, put this sort of thing at
5055 the top of your file to signal that your code will work I<only> on Perls of
5058 use 5.014; # so push/pop/etc work on scalars (experimental)
5061 X<pos> X<match, position>
5065 =for Pod::Functions find or set the offset for the last/next m//g search
5067 Returns the offset of where the last C<m//g> search left off for the
5068 variable in question (C<$_> is used when the variable is not
5069 specified). Note that 0 is a valid match offset. C<undef> indicates
5070 that the search position is reset (usually due to match failure, but
5071 can also be because no match has yet been run on the scalar).
5073 C<pos> directly accesses the location used by the regexp engine to
5074 store the offset, so assigning to C<pos> will change that offset, and
5075 so will also influence the C<\G> zero-width assertion in regular
5076 expressions. Both of these effects take place for the next match, so
5077 you can't affect the position with C<pos> during the current match,
5078 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5080 Setting C<pos> also resets the I<matched with zero-length> flag, described
5081 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5083 Because a failed C<m//gc> match doesn't reset the offset, the return
5084 from C<pos> won't change either in this case. See L<perlre> and
5087 =item print FILEHANDLE LIST
5090 =item print FILEHANDLE
5096 =for Pod::Functions output a list to a filehandle
5098 Prints a string or a list of strings. Returns true if successful.
5099 FILEHANDLE may be a scalar variable containing the name of or a reference
5100 to the filehandle, thus introducing one level of indirection. (NOTE: If
5101 FILEHANDLE is a variable and the next token is a term, it may be
5102 misinterpreted as an operator unless you interpose a C<+> or put
5103 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5104 last selected (see L</select>) output handle. If LIST is omitted, prints
5105 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5106 print the content of C<$_> to it, you must use a real filehandle like
5107 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5108 to something other than STDOUT, use the select operation.
5110 The current value of C<$,> (if any) is printed between each LIST item. The
5111 current value of C<$\> (if any) is printed after the entire LIST has been
5112 printed. Because print takes a LIST, anything in the LIST is evaluated in
5113 list context, including any subroutines whose return lists you pass to
5114 C<print>. Be careful not to follow the print keyword with a left
5115 parenthesis unless you want the corresponding right parenthesis to
5116 terminate the arguments to the print; put parentheses around all arguments
5117 (or interpose a C<+>, but that doesn't look as good).
5119 If you're storing handles in an array or hash, or in general whenever
5120 you're using any expression more complex than a bareword handle or a plain,
5121 unsubscripted scalar variable to retrieve it, you will have to use a block
5122 returning the filehandle value instead, in which case the LIST may not be
5125 print { $files[$i] } "stuff\n";
5126 print { $OK ? STDOUT : STDERR } "stuff\n";
5128 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5129 L<perlipc> for more on signal handling.
5131 =item printf FILEHANDLE FORMAT, LIST
5134 =item printf FILEHANDLE
5136 =item printf FORMAT, LIST
5140 =for Pod::Functions output a formatted list to a filehandle
5142 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5143 (the output record separator) is not appended. The first argument of the
5144 list will be interpreted as the C<printf> format. See
5145 L<sprintf|/sprintf FORMAT, LIST> for an
5146 explanation of the format argument. If you omit the LIST, C<$_> is used;
5147 to use FILEHANDLE without a LIST, you must use a real filehandle like
5148 C<FH>, not an indirect one like C<$fh>. If C<use locale> (including
5149 C<use locale ':not_characters'>) is in effect and
5150 POSIX::setlocale() has been called, the character used for the decimal
5151 separator in formatted floating-point numbers is affected by the LC_NUMERIC
5152 locale setting. See L<perllocale> and L<POSIX>.
5154 Don't fall into the trap of using a C<printf> when a simple
5155 C<print> would do. The C<print> is more efficient and less
5158 =item prototype FUNCTION
5161 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5163 Returns the prototype of a function as a string (or C<undef> if the
5164 function has no prototype). FUNCTION is a reference to, or the name of,
5165 the function whose prototype you want to retrieve.
5167 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5168 name for a Perl builtin. If the builtin's arguments
5169 cannot be adequately expressed by a prototype
5170 (such as C<system>), prototype() returns C<undef>, because the builtin
5171 does not really behave like a Perl function. Otherwise, the string
5172 describing the equivalent prototype is returned.
5174 =item push ARRAY,LIST
5177 =item push EXPR,LIST
5179 =for Pod::Functions append one or more elements to an array
5181 Treats ARRAY as a stack by appending the values of LIST to the end of
5182 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5186 $ARRAY[++$#ARRAY] = $value;
5189 but is more efficient. Returns the number of elements in the array following
5190 the completed C<push>.
5192 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
5193 reference to an unblessed array. The argument will be dereferenced
5194 automatically. This aspect of C<push> is considered highly experimental.
5195 The exact behaviour may change in a future version of Perl.
5197 To avoid confusing would-be users of your code who are running earlier
5198 versions of Perl with mysterious syntax errors, put this sort of thing at
5199 the top of your file to signal that your code will work I<only> on Perls of
5202 use 5.014; # so push/pop/etc work on scalars (experimental)
5206 =for Pod::Functions singly quote a string
5210 =for Pod::Functions doubly quote a string
5214 =for Pod::Functions quote a list of words
5218 =for Pod::Functions backquote quote a string
5220 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5224 =for Pod::Functions +5.005 compile pattern
5226 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5228 =item quotemeta EXPR
5229 X<quotemeta> X<metacharacter>
5233 =for Pod::Functions quote regular expression magic characters
5235 Returns the value of EXPR with all the ASCII non-"word"
5236 characters backslashed. (That is, all ASCII characters not matching
5237 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5238 returned string, regardless of any locale settings.)
5239 This is the internal function implementing
5240 the C<\Q> escape in double-quoted strings.
5241 (See below for the behavior on non-ASCII code points.)
5243 If EXPR is omitted, uses C<$_>.
5245 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5246 regular expressions, because by default an interpolated variable will be
5247 considered a mini-regular expression. For example:
5249 my $sentence = 'The quick brown fox jumped over the lazy dog';
5250 my $substring = 'quick.*?fox';
5251 $sentence =~ s{$substring}{big bad wolf};
5253 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5257 my $sentence = 'The quick brown fox jumped over the lazy dog';
5258 my $substring = 'quick.*?fox';
5259 $sentence =~ s{\Q$substring\E}{big bad wolf};
5263 my $sentence = 'The quick brown fox jumped over the lazy dog';
5264 my $substring = 'quick.*?fox';
5265 my $quoted_substring = quotemeta($substring);
5266 $sentence =~ s{$quoted_substring}{big bad wolf};
5268 Will both leave the sentence as is.
5269 Normally, when accepting literal string
5270 input from the user, quotemeta() or C<\Q> must be used.
5272 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5273 strings, but not quoted in UTF-8 strings.
5275 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5276 quoting non-ASCII characters; the quoting of ASCII characters is
5279 Also unchanged is the quoting of non-UTF-8 strings when outside the
5280 scope of a C<use feature 'unicode_strings'>, which is to quote all
5281 characters in the upper Latin1 range. This provides complete backwards
5282 compatibility for old programs which do not use Unicode. (Note that
5283 C<unicode_strings> is automatically enabled within the scope of a
5284 S<C<use v5.12>> or greater.)
5286 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5287 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5288 above, locale does not affect the quoting of ASCII-range characters.
5289 This protects against those locales where characters such as C<"|"> are
5290 considered to be word characters.
5292 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5293 Unicode (see L<http://www.unicode.org/reports/tr31/>.)
5294 The only code points that are quoted are those that have any of the
5295 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5296 Default_Ignorable_Code_Point, or General_Category=Control.
5298 Of these properties, the two important ones are Pattern_Syntax and
5299 Pattern_White_Space. They have been set up by Unicode for exactly this
5300 purpose of deciding which characters in a regular expression pattern
5301 should be quoted. No character that can be in an identifier has these
5304 Perl promises, that if we ever add regular expression pattern
5305 metacharacters to the dozen already defined
5306 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5307 Pattern_Syntax property. Perl also promises, that if we ever add
5308 characters that are considered to be white space in regular expressions
5309 (currently mostly affected by C</x>), they will all have the
5310 Pattern_White_Space property.
5312 Unicode promises that the set of code points that have these two
5313 properties will never change, so something that is not quoted in v5.16
5314 will never need to be quoted in any future Perl release. (Not all the
5315 code points that match Pattern_Syntax have actually had characters
5316 assigned to them; so there is room to grow, but they are quoted
5317 whether assigned or not. Perl, of course, would never use an
5318 unassigned code point as an actual metacharacter.)
5320 Quoting characters that have the other 3 properties is done to enhance
5321 the readability of the regular expression and not because they actually
5322 need to be quoted for regular expression purposes (characters with the
5323 White_Space property are likely to be indistinguishable on the page or
5324 screen from those with the Pattern_White_Space property; and the other
5325 two properties contain non-printing characters).
5332 =for Pod::Functions retrieve the next pseudorandom number
5334 Returns a random fractional number greater than or equal to C<0> and less
5335 than the value of EXPR. (EXPR should be positive.) If EXPR is
5336 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5337 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5338 and is subject to change in future versions of Perl). Automatically calls
5339 C<srand> unless C<srand> has already been called. See also C<srand>.
5341 Apply C<int()> to the value returned by C<rand()> if you want random
5342 integers instead of random fractional numbers. For example,
5346 returns a random integer between C<0> and C<9>, inclusive.
5348 (Note: If your rand function consistently returns numbers that are too
5349 large or too small, then your version of Perl was probably compiled
5350 with the wrong number of RANDBITS.)
5352 B<C<rand()> is not cryptographically secure. You should not rely
5353 on it in security-sensitive situations.> As of this writing, a
5354 number of third-party CPAN modules offer random number generators
5355 intended by their authors to be cryptographically secure,
5356 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5357 and L<Math::TrulyRandom>.
5359 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5360 X<read> X<file, read>
5362 =item read FILEHANDLE,SCALAR,LENGTH
5364 =for Pod::Functions fixed-length buffered input from a filehandle
5366 Attempts to read LENGTH I<characters> of data into variable SCALAR
5367 from the specified FILEHANDLE. Returns the number of characters
5368 actually read, C<0> at end of file, or undef if there was an error (in
5369 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5370 so that the last character actually read is the last character of the
5371 scalar after the read.
5373 An OFFSET may be specified to place the read data at some place in the
5374 string other than the beginning. A negative OFFSET specifies
5375 placement at that many characters counting backwards from the end of
5376 the string. A positive OFFSET greater than the length of SCALAR
5377 results in the string being padded to the required size with C<"\0">
5378 bytes before the result of the read is appended.
5380 The call is implemented in terms of either Perl's or your system's native
5381 fread(3) library function. To get a true read(2) system call, see
5382 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5384 Note the I<characters>: depending on the status of the filehandle,
5385 either (8-bit) bytes or characters are read. By default, all
5386 filehandles operate on bytes, but for example if the filehandle has
5387 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5388 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5389 characters, not bytes. Similarly for the C<:encoding> pragma:
5390 in that case pretty much any characters can be read.
5392 =item readdir DIRHANDLE
5395 =for Pod::Functions get a directory from a directory handle
5397 Returns the next directory entry for a directory opened by C<opendir>.
5398 If used in list context, returns all the rest of the entries in the
5399 directory. If there are no more entries, returns the undefined value in
5400 scalar context and the empty list in list context.
5402 If you're planning to filetest the return values out of a C<readdir>, you'd
5403 better prepend the directory in question. Otherwise, because we didn't
5404 C<chdir> there, it would have been testing the wrong file.
5406 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5407 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5410 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
5411 which will set C<$_> on every iteration.
5413 opendir(my $dh, $some_dir) || die;
5414 while(readdir $dh) {
5415 print "$some_dir/$_\n";
5419 To avoid confusing would-be users of your code who are running earlier
5420 versions of Perl with mysterious failures, put this sort of thing at the
5421 top of your file to signal that your code will work I<only> on Perls of a
5424 use 5.012; # so readdir assigns to $_ in a lone while test
5429 X<readline> X<gets> X<fgets>
5431 =for Pod::Functions fetch a record from a file
5433 Reads from the filehandle whose typeglob is contained in EXPR (or from
5434 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5435 returns the next line until end-of-file is reached, whereupon the
5436 subsequent call returns C<undef>. In list context, reads until end-of-file
5437 is reached and returns a list of lines. Note that the notion of "line"
5438 used here is whatever you may have defined with C<$/> or
5439 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5441 When C<$/> is set to C<undef>, when C<readline> is in scalar
5442 context (i.e., file slurp mode), and when an empty file is read, it
5443 returns C<''> the first time, followed by C<undef> subsequently.
5445 This is the internal function implementing the C<< <EXPR> >>
5446 operator, but you can use it directly. The C<< <EXPR> >>
5447 operator is discussed in more detail in L<perlop/"I/O Operators">.
5450 $line = readline(*STDIN); # same thing
5452 If C<readline> encounters an operating system error, C<$!> will be set
5453 with the corresponding error message. It can be helpful to check
5454 C<$!> when you are reading from filehandles you don't trust, such as a
5455 tty or a socket. The following example uses the operator form of
5456 C<readline> and dies if the result is not defined.
5458 while ( ! eof($fh) ) {
5459 defined( $_ = <$fh> ) or die "readline failed: $!";
5463 Note that you have can't handle C<readline> errors that way with the
5464 C<ARGV> filehandle. In that case, you have to open each element of
5465 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5467 foreach my $arg (@ARGV) {
5468 open(my $fh, $arg) or warn "Can't open $arg: $!";
5470 while ( ! eof($fh) ) {
5471 defined( $_ = <$fh> )
5472 or die "readline failed for $arg: $!";
5482 =for Pod::Functions determine where a symbolic link is pointing
5484 Returns the value of a symbolic link, if symbolic links are
5485 implemented. If not, raises an exception. If there is a system
5486 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5487 omitted, uses C<$_>.
5489 Portability issues: L<perlport/readlink>.
5496 =for Pod::Functions execute a system command and collect standard output
5498 EXPR is executed as a system command.
5499 The collected standard output of the command is returned.
5500 In scalar context, it comes back as a single (potentially
5501 multi-line) string. In list context, returns a list of lines
5502 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5503 This is the internal function implementing the C<qx/EXPR/>
5504 operator, but you can use it directly. The C<qx/EXPR/>
5505 operator is discussed in more detail in L<perlop/"I/O Operators">.
5506 If EXPR is omitted, uses C<$_>.
5508 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5511 =for Pod::Functions receive a message over a Socket
5513 Receives a message on a socket. Attempts to receive LENGTH characters
5514 of data into variable SCALAR from the specified SOCKET filehandle.
5515 SCALAR will be grown or shrunk to the length actually read. Takes the
5516 same flags as the system call of the same name. Returns the address
5517 of the sender if SOCKET's protocol supports this; returns an empty
5518 string otherwise. If there's an error, returns the undefined value.
5519 This call is actually implemented in terms of recvfrom(2) system call.
5520 See L<perlipc/"UDP: Message Passing"> for examples.
5522 Note the I<characters>: depending on the status of the socket, either
5523 (8-bit) bytes or characters are received. By default all sockets
5524 operate on bytes, but for example if the socket has been changed using
5525 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5526 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5527 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5528 case pretty much any characters can be read.
5535 =for Pod::Functions start this loop iteration over again
5537 The C<redo> command restarts the loop block without evaluating the
5538 conditional again. The C<continue> block, if any, is not executed. If
5539 the LABEL is omitted, the command refers to the innermost enclosing
5540 loop. Programs that want to lie to themselves about what was just input
5541 normally use this command:
5543 # a simpleminded Pascal comment stripper
5544 # (warning: assumes no { or } in strings)
5545 LINE: while (<STDIN>) {
5546 while (s|({.*}.*){.*}|$1 |) {}
5551 if (/}/) { # end of comment?
5560 C<redo> cannot be used to retry a block that returns a value such as
5561 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5562 a grep() or map() operation.
5564 Note that a block by itself is semantically identical to a loop
5565 that executes once. Thus C<redo> inside such a block will effectively
5566 turn it into a looping construct.
5568 See also L</continue> for an illustration of how C<last>, C<next>, and
5576 =for Pod::Functions find out the type of thing being referenced
5578 Returns a non-empty string if EXPR is a reference, the empty
5579 string otherwise. If EXPR
5580 is not specified, C<$_> will be used. The value returned depends on the
5581 type of thing the reference is a reference to.
5582 Builtin types include:
5596 If the referenced object has been blessed into a package, then that package
5597 name is returned instead. You can think of C<ref> as a C<typeof> operator.
5599 if (ref($r) eq "HASH") {
5600 print "r is a reference to a hash.\n";
5603 print "r is not a reference at all.\n";
5606 The return value C<LVALUE> indicates a reference to an lvalue that is not
5607 a variable. You get this from taking the reference of function calls like
5608 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5609 to a L<version string|perldata/"Version Strings">.
5611 The result C<Regexp> indicates that the argument is a regular expression
5612 resulting from C<qr//>.
5614 See also L<perlref>.
5616 =item rename OLDNAME,NEWNAME
5617 X<rename> X<move> X<mv> X<ren>
5619 =for Pod::Functions change a filename
5621 Changes the name of a file; an existing file NEWNAME will be
5622 clobbered. Returns true for success, false otherwise.
5624 Behavior of this function varies wildly depending on your system
5625 implementation. For example, it will usually not work across file system
5626 boundaries, even though the system I<mv> command sometimes compensates
5627 for this. Other restrictions include whether it works on directories,
5628 open files, or pre-existing files. Check L<perlport> and either the
5629 rename(2) manpage or equivalent system documentation for details.
5631 For a platform independent C<move> function look at the L<File::Copy>
5634 Portability issues: L<perlport/rename>.
5636 =item require VERSION
5643 =for Pod::Functions load in external functions from a library at runtime
5645 Demands a version of Perl specified by VERSION, or demands some semantics
5646 specified by EXPR or by C<$_> if EXPR is not supplied.
5648 VERSION may be either a numeric argument such as 5.006, which will be
5649 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5650 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5651 VERSION is greater than the version of the current Perl interpreter.
5652 Compare with L</use>, which can do a similar check at compile time.
5654 Specifying VERSION as a literal of the form v5.6.1 should generally be
5655 avoided, because it leads to misleading error messages under earlier
5656 versions of Perl that do not support this syntax. The equivalent numeric
5657 version should be used instead.
5659 require v5.6.1; # run time version check
5660 require 5.6.1; # ditto
5661 require 5.006_001; # ditto; preferred for backwards
5664 Otherwise, C<require> demands that a library file be included if it
5665 hasn't already been included. The file is included via the do-FILE
5666 mechanism, which is essentially just a variety of C<eval> with the
5667 caveat that lexical variables in the invoking script will be invisible
5668 to the included code. Has semantics similar to the following subroutine:
5671 my ($filename) = @_;
5672 if (exists $INC{$filename}) {
5673 return 1 if $INC{$filename};
5674 die "Compilation failed in require";
5676 my ($realfilename,$result);
5678 foreach $prefix (@INC) {
5679 $realfilename = "$prefix/$filename";
5680 if (-f $realfilename) {
5681 $INC{$filename} = $realfilename;
5682 $result = do $realfilename;
5686 die "Can't find $filename in \@INC";
5689 $INC{$filename} = undef;
5691 } elsif (!$result) {
5692 delete $INC{$filename};
5693 die "$filename did not return true value";
5699 Note that the file will not be included twice under the same specified
5702 The file must return true as the last statement to indicate
5703 successful execution of any initialization code, so it's customary to
5704 end such a file with C<1;> unless you're sure it'll return true
5705 otherwise. But it's better just to put the C<1;>, in case you add more
5708 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5709 replaces "F<::>" with "F</>" in the filename for you,
5710 to make it easy to load standard modules. This form of loading of
5711 modules does not risk altering your namespace.
5713 In other words, if you try this:
5715 require Foo::Bar; # a splendid bareword
5717 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5718 directories specified in the C<@INC> array.
5720 But if you try this:
5722 $class = 'Foo::Bar';
5723 require $class; # $class is not a bareword
5725 require "Foo::Bar"; # not a bareword because of the ""
5727 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5728 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5730 eval "require $class";
5732 Now that you understand how C<require> looks for files with a
5733 bareword argument, there is a little extra functionality going on behind
5734 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5735 first look for a similar filename with a "F<.pmc>" extension. If this file
5736 is found, it will be loaded in place of any file ending in a "F<.pm>"
5739 You can also insert hooks into the import facility by putting Perl code
5740 directly into the @INC array. There are three forms of hooks: subroutine
5741 references, array references, and blessed objects.
5743 Subroutine references are the simplest case. When the inclusion system
5744 walks through @INC and encounters a subroutine, this subroutine gets
5745 called with two parameters, the first a reference to itself, and the
5746 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5747 subroutine should return either nothing or else a list of up to three
5748 values in the following order:
5754 A filehandle, from which the file will be read.
5758 A reference to a subroutine. If there is no filehandle (previous item),
5759 then this subroutine is expected to generate one line of source code per
5760 call, writing the line into C<$_> and returning 1, then finally at end of
5761 file returning 0. If there is a filehandle, then the subroutine will be
5762 called to act as a simple source filter, with the line as read in C<$_>.
5763 Again, return 1 for each valid line, and 0 after all lines have been
5768 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5769 reference to the subroutine itself is passed in as C<$_[0]>.
5773 If an empty list, C<undef>, or nothing that matches the first 3 values above
5774 is returned, then C<require> looks at the remaining elements of @INC.
5775 Note that this filehandle must be a real filehandle (strictly a typeglob
5776 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5777 will be ignored and processing will stop there.
5779 If the hook is an array reference, its first element must be a subroutine
5780 reference. This subroutine is called as above, but the first parameter is
5781 the array reference. This lets you indirectly pass arguments to
5784 In other words, you can write:
5786 push @INC, \&my_sub;
5788 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5794 push @INC, [ \&my_sub, $x, $y, ... ];
5796 my ($arrayref, $filename) = @_;
5797 # Retrieve $x, $y, ...
5798 my @parameters = @$arrayref[1..$#$arrayref];
5802 If the hook is an object, it must provide an INC method that will be
5803 called as above, the first parameter being the object itself. (Note that
5804 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5805 into package C<main>.) Here is a typical code layout:
5811 my ($self, $filename) = @_;
5815 # In the main program
5816 push @INC, Foo->new(...);
5818 These hooks are also permitted to set the %INC entry
5819 corresponding to the files they have loaded. See L<perlvar/%INC>.
5821 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5828 =for Pod::Functions clear all variables of a given name
5830 Generally used in a C<continue> block at the end of a loop to clear
5831 variables and reset C<??> searches so that they work again. The
5832 expression is interpreted as a list of single characters (hyphens
5833 allowed for ranges). All variables and arrays beginning with one of
5834 those letters are reset to their pristine state. If the expression is
5835 omitted, one-match searches (C<?pattern?>) are reset to match again.
5836 Only resets variables or searches in the current package. Always returns
5839 reset 'X'; # reset all X variables
5840 reset 'a-z'; # reset lower case variables
5841 reset; # just reset ?one-time? searches
5843 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5844 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5845 variables; lexical variables are unaffected, but they clean themselves
5846 up on scope exit anyway, so you'll probably want to use them instead.
5854 =for Pod::Functions get out of a function early
5856 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5857 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5858 context, depending on how the return value will be used, and the context
5859 may vary from one execution to the next (see L</wantarray>). If no EXPR
5860 is given, returns an empty list in list context, the undefined value in
5861 scalar context, and (of course) nothing at all in void context.
5863 (In the absence of an explicit C<return>, a subroutine, eval,
5864 or do FILE automatically returns the value of the last expression
5868 X<reverse> X<rev> X<invert>
5870 =for Pod::Functions flip a string or a list
5872 In list context, returns a list value consisting of the elements
5873 of LIST in the opposite order. In scalar context, concatenates the
5874 elements of LIST and returns a string value with all characters
5875 in the opposite order.
5877 print join(", ", reverse "world", "Hello"); # Hello, world
5879 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5881 Used without arguments in scalar context, reverse() reverses C<$_>.
5883 $_ = "dlrow ,olleH";
5884 print reverse; # No output, list context
5885 print scalar reverse; # Hello, world
5887 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5888 preserve non-existent elements whenever possible, i.e., for non magical
5889 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
5891 This operator is also handy for inverting a hash, although there are some
5892 caveats. If a value is duplicated in the original hash, only one of those
5893 can be represented as a key in the inverted hash. Also, this has to
5894 unwind one hash and build a whole new one, which may take some time
5895 on a large hash, such as from a DBM file.
5897 %by_name = reverse %by_address; # Invert the hash
5899 =item rewinddir DIRHANDLE
5902 =for Pod::Functions reset directory handle
5904 Sets the current position to the beginning of the directory for the
5905 C<readdir> routine on DIRHANDLE.
5907 Portability issues: L<perlport/rewinddir>.
5909 =item rindex STR,SUBSTR,POSITION
5912 =item rindex STR,SUBSTR
5914 =for Pod::Functions right-to-left substring search
5916 Works just like index() except that it returns the position of the I<last>
5917 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5918 last occurrence beginning at or before that position.
5920 =item rmdir FILENAME
5921 X<rmdir> X<rd> X<directory, remove>
5925 =for Pod::Functions remove a directory
5927 Deletes the directory specified by FILENAME if that directory is
5928 empty. If it succeeds it returns true; otherwise it returns false and
5929 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5931 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5932 the C<rmtree> function of the L<File::Path> module.
5936 =for Pod::Functions replace a pattern with a string
5938 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5940 =item say FILEHANDLE LIST
5943 =item say FILEHANDLE
5949 =for Pod::Functions +say output a list to a filehandle, appending a newline
5951 Just like C<print>, but implicitly appends a newline. C<say LIST> is
5952 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
5953 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
5954 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
5956 This keyword is available only when the C<"say"> feature
5957 is enabled, or when prefixed with C<CORE::>; see
5958 L<feature>. Alternately, include a C<use v5.10> or later to the current
5962 X<scalar> X<context>
5964 =for Pod::Functions force a scalar context
5966 Forces EXPR to be interpreted in scalar context and returns the value
5969 @counts = ( scalar @a, scalar @b, scalar @c );
5971 There is no equivalent operator to force an expression to
5972 be interpolated in list context because in practice, this is never
5973 needed. If you really wanted to do so, however, you could use
5974 the construction C<@{[ (some expression) ]}>, but usually a simple
5975 C<(some expression)> suffices.
5977 Because C<scalar> is a unary operator, if you accidentally use a
5978 parenthesized list for the EXPR, this behaves as a scalar comma expression,
5979 evaluating all but the last element in void context and returning the final
5980 element evaluated in scalar context. This is seldom what you want.
5982 The following single statement:
5984 print uc(scalar(&foo,$bar)),$baz;
5986 is the moral equivalent of these two:
5989 print(uc($bar),$baz);
5991 See L<perlop> for more details on unary operators and the comma operator.
5993 =item seek FILEHANDLE,POSITION,WHENCE
5994 X<seek> X<fseek> X<filehandle, position>
5996 =for Pod::Functions reposition file pointer for random-access I/O
5998 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5999 FILEHANDLE may be an expression whose value gives the name of the
6000 filehandle. The values for WHENCE are C<0> to set the new position
6001 I<in bytes> to POSITION; C<1> to set it to the current position plus
6002 POSITION; and C<2> to set it to EOF plus POSITION, typically
6003 negative. For WHENCE you may use the constants C<SEEK_SET>,
6004 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6005 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6008 Note the I<in bytes>: even if the filehandle has been set to
6009 operate on characters (for example by using the C<:encoding(utf8)> open
6010 layer), tell() will return byte offsets, not character offsets
6011 (because implementing that would render seek() and tell() rather slow).
6013 If you want to position the file for C<sysread> or C<syswrite>, don't use
6014 C<seek>, because buffering makes its effect on the file's read-write position
6015 unpredictable and non-portable. Use C<sysseek> instead.
6017 Due to the rules and rigors of ANSI C, on some systems you have to do a
6018 seek whenever you switch between reading and writing. Amongst other
6019 things, this may have the effect of calling stdio's clearerr(3).
6020 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6024 This is also useful for applications emulating C<tail -f>. Once you hit
6025 EOF on your read and then sleep for a while, you (probably) have to stick in a
6026 dummy seek() to reset things. The C<seek> doesn't change the position,
6027 but it I<does> clear the end-of-file condition on the handle, so that the
6028 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6030 If that doesn't work (some I/O implementations are particularly
6031 cantankerous), you might need something like this:
6034 for ($curpos = tell(FILE); $_ = <FILE>;
6035 $curpos = tell(FILE)) {
6036 # search for some stuff and put it into files
6038 sleep($for_a_while);
6039 seek(FILE, $curpos, 0);
6042 =item seekdir DIRHANDLE,POS
6045 =for Pod::Functions reposition directory pointer
6047 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6048 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6049 about possible directory compaction as the corresponding system library
6052 =item select FILEHANDLE
6053 X<select> X<filehandle, default>
6057 =for Pod::Functions reset default output or do I/O multiplexing
6059 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6060 sets the new current default filehandle for output. This has two
6061 effects: first, a C<write> or a C<print> without a filehandle
6062 default to this FILEHANDLE. Second, references to variables related to
6063 output will refer to this output channel.
6065 For example, to set the top-of-form format for more than one
6066 output channel, you might do the following:
6073 FILEHANDLE may be an expression whose value gives the name of the
6074 actual filehandle. Thus:
6076 $oldfh = select(STDERR); $| = 1; select($oldfh);
6078 Some programmers may prefer to think of filehandles as objects with
6079 methods, preferring to write the last example as:
6082 STDERR->autoflush(1);
6084 Portability issues: L<perlport/select>.
6086 =item select RBITS,WBITS,EBITS,TIMEOUT
6089 This calls the select(2) syscall with the bit masks specified, which
6090 can be constructed using C<fileno> and C<vec>, along these lines:
6092 $rin = $win = $ein = '';
6093 vec($rin, fileno(STDIN), 1) = 1;
6094 vec($win, fileno(STDOUT), 1) = 1;
6097 If you want to select on many filehandles, you may wish to write a
6098 subroutine like this:
6103 for my $fh (@fhlist) {
6104 vec($bits, fileno($fh), 1) = 1;
6108 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6112 ($nfound,$timeleft) =
6113 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6115 or to block until something becomes ready just do this
6117 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6119 Most systems do not bother to return anything useful in $timeleft, so
6120 calling select() in scalar context just returns $nfound.
6122 Any of the bit masks can also be undef. The timeout, if specified, is
6123 in seconds, which may be fractional. Note: not all implementations are
6124 capable of returning the $timeleft. If not, they always return
6125 $timeleft equal to the supplied $timeout.
6127 You can effect a sleep of 250 milliseconds this way:
6129 select(undef, undef, undef, 0.25);
6131 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6132 is implementation-dependent. See also L<perlport> for notes on the
6133 portability of C<select>.
6135 On error, C<select> behaves just like select(2): it returns
6138 On some Unixes, select(2) may report a socket file descriptor as "ready for
6139 reading" even when no data is available, and thus any subsequent C<read>
6140 would block. This can be avoided if you always use O_NONBLOCK on the
6141 socket. See select(2) and fcntl(2) for further details.
6143 The standard C<IO::Select> module provides a user-friendlier interface
6144 to C<select>, mostly because it does all the bit-mask work for you.
6146 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6147 or <FH>) with C<select>, except as permitted by POSIX, and even
6148 then only on POSIX systems. You have to use C<sysread> instead.
6150 Portability issues: L<perlport/select>.
6152 =item semctl ID,SEMNUM,CMD,ARG
6155 =for Pod::Functions SysV semaphore control operations
6157 Calls the System V IPC function semctl(2). You'll probably have to say
6161 first to get the correct constant definitions. If CMD is IPC_STAT or
6162 GETALL, then ARG must be a variable that will hold the returned
6163 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6164 the undefined value for error, "C<0 but true>" for zero, or the actual
6165 return value otherwise. The ARG must consist of a vector of native
6166 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6167 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6170 Portability issues: L<perlport/semctl>.
6172 =item semget KEY,NSEMS,FLAGS
6175 =for Pod::Functions get set of SysV semaphores
6177 Calls the System V IPC function semget(2). Returns the semaphore id, or
6178 the undefined value on error. See also
6179 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6182 Portability issues: L<perlport/semget>.
6184 =item semop KEY,OPSTRING
6187 =for Pod::Functions SysV semaphore operations
6189 Calls the System V IPC function semop(2) for semaphore operations
6190 such as signalling and waiting. OPSTRING must be a packed array of
6191 semop structures. Each semop structure can be generated with
6192 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6193 implies the number of semaphore operations. Returns true if
6194 successful, false on error. As an example, the
6195 following code waits on semaphore $semnum of semaphore id $semid:
6197 $semop = pack("s!3", $semnum, -1, 0);
6198 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6200 To signal the semaphore, replace C<-1> with C<1>. See also
6201 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6204 Portability issues: L<perlport/semop>.
6206 =item send SOCKET,MSG,FLAGS,TO
6209 =item send SOCKET,MSG,FLAGS
6211 =for Pod::Functions send a message over a socket
6213 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6214 filehandle. Takes the same flags as the system call of the same name. On
6215 unconnected sockets, you must specify a destination to I<send to>, in which
6216 case it does a sendto(2) syscall. Returns the number of characters sent,
6217 or the undefined value on error. The sendmsg(2) syscall is currently
6218 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6220 Note the I<characters>: depending on the status of the socket, either
6221 (8-bit) bytes or characters are sent. By default all sockets operate
6222 on bytes, but for example if the socket has been changed using
6223 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6224 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6225 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6226 pragma: in that case pretty much any characters can be sent.
6228 =item setpgrp PID,PGRP
6231 =for Pod::Functions set the process group of a process
6233 Sets the current process group for the specified PID, C<0> for the current
6234 process. Raises an exception when used on a machine that doesn't
6235 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6236 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6237 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6240 Portability issues: L<perlport/setpgrp>.
6242 =item setpriority WHICH,WHO,PRIORITY
6243 X<setpriority> X<priority> X<nice> X<renice>
6245 =for Pod::Functions set a process's nice value
6247 Sets the current priority for a process, a process group, or a user.
6248 (See setpriority(2).) Raises an exception when used on a machine
6249 that doesn't implement setpriority(2).
6251 Portability issues: L<perlport/setpriority>.
6253 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6256 =for Pod::Functions set some socket options
6258 Sets the socket option requested. Returns C<undef> on error.
6259 Use integer constants provided by the C<Socket> module for
6260 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6261 getprotobyname. OPTVAL might either be a packed string or an integer.
6262 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6264 An example disabling Nagle's algorithm on a socket:
6266 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6267 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6269 Portability issues: L<perlport/setsockopt>.
6278 =for Pod::Functions remove the first element of an array, and return it
6280 Shifts the first value of the array off and returns it, shortening the
6281 array by 1 and moving everything down. If there are no elements in the
6282 array, returns the undefined value. If ARRAY is omitted, shifts the
6283 C<@_> array within the lexical scope of subroutines and formats, and the
6284 C<@ARGV> array outside a subroutine and also within the lexical scopes
6285 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6286 C<UNITCHECK {}>, and C<END {}> constructs.
6288 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
6289 reference to an unblessed array. The argument will be dereferenced
6290 automatically. This aspect of C<shift> is considered highly experimental.
6291 The exact behaviour may change in a future version of Perl.
6293 To avoid confusing would-be users of your code who are running earlier
6294 versions of Perl with mysterious syntax errors, put this sort of thing at
6295 the top of your file to signal that your code will work I<only> on Perls of
6298 use 5.014; # so push/pop/etc work on scalars (experimental)
6300 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6301 same thing to the left end of an array that C<pop> and C<push> do to the
6304 =item shmctl ID,CMD,ARG
6307 =for Pod::Functions SysV shared memory operations
6309 Calls the System V IPC function shmctl. You'll probably have to say
6313 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6314 then ARG must be a variable that will hold the returned C<shmid_ds>
6315 structure. Returns like ioctl: C<undef> for error; "C<0> but
6316 true" for zero; and the actual return value otherwise.
6317 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6319 Portability issues: L<perlport/shmctl>.
6321 =item shmget KEY,SIZE,FLAGS
6324 =for Pod::Functions get SysV shared memory segment identifier
6326 Calls the System V IPC function shmget. Returns the shared memory
6327 segment id, or C<undef> on error.
6328 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6330 Portability issues: L<perlport/shmget>.
6332 =item shmread ID,VAR,POS,SIZE
6336 =for Pod::Functions read SysV shared memory
6338 =item shmwrite ID,STRING,POS,SIZE
6340 =for Pod::Functions write SysV shared memory
6342 Reads or writes the System V shared memory segment ID starting at
6343 position POS for size SIZE by attaching to it, copying in/out, and
6344 detaching from it. When reading, VAR must be a variable that will
6345 hold the data read. When writing, if STRING is too long, only SIZE
6346 bytes are used; if STRING is too short, nulls are written to fill out
6347 SIZE bytes. Return true if successful, false on error.
6348 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6349 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6351 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6353 =item shutdown SOCKET,HOW
6356 =for Pod::Functions close down just half of a socket connection
6358 Shuts down a socket connection in the manner indicated by HOW, which
6359 has the same interpretation as in the syscall of the same name.
6361 shutdown(SOCKET, 0); # I/we have stopped reading data
6362 shutdown(SOCKET, 1); # I/we have stopped writing data
6363 shutdown(SOCKET, 2); # I/we have stopped using this socket
6365 This is useful with sockets when you want to tell the other
6366 side you're done writing but not done reading, or vice versa.
6367 It's also a more insistent form of close because it also
6368 disables the file descriptor in any forked copies in other
6371 Returns C<1> for success; on error, returns C<undef> if
6372 the first argument is not a valid filehandle, or returns C<0> and sets
6373 C<$!> for any other failure.
6376 X<sin> X<sine> X<asin> X<arcsine>
6380 =for Pod::Functions return the sine of a number
6382 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6383 returns sine of C<$_>.
6385 For the inverse sine operation, you may use the C<Math::Trig::asin>
6386 function, or use this relation:
6388 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6395 =for Pod::Functions block for some number of seconds
6397 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6398 argument is given. Returns the integer number of seconds actually slept.
6400 May be interrupted if the process receives a signal such as C<SIGALRM>.
6403 local $SIG{ALARM} = sub { die "Alarm!\n" };
6406 die $@ unless $@ eq "Alarm!\n";
6408 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6409 is often implemented using C<alarm>.
6411 On some older systems, it may sleep up to a full second less than what
6412 you requested, depending on how it counts seconds. Most modern systems
6413 always sleep the full amount. They may appear to sleep longer than that,
6414 however, because your process might not be scheduled right away in a
6415 busy multitasking system.
6417 For delays of finer granularity than one second, the Time::HiRes module
6418 (from CPAN, and starting from Perl 5.8 part of the standard
6419 distribution) provides usleep(). You may also use Perl's four-argument
6420 version of select() leaving the first three arguments undefined, or you
6421 might be able to use the C<syscall> interface to access setitimer(2) if
6422 your system supports it. See L<perlfaq8> for details.
6424 See also the POSIX module's C<pause> function.
6426 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6429 =for Pod::Functions create a socket
6431 Opens a socket of the specified kind and attaches it to filehandle
6432 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6433 the syscall of the same name. You should C<use Socket> first
6434 to get the proper definitions imported. See the examples in
6435 L<perlipc/"Sockets: Client/Server Communication">.
6437 On systems that support a close-on-exec flag on files, the flag will
6438 be set for the newly opened file descriptor, as determined by the
6439 value of $^F. See L<perlvar/$^F>.
6441 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6444 =for Pod::Functions create a pair of sockets
6446 Creates an unnamed pair of sockets in the specified domain, of the
6447 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6448 for the syscall of the same name. If unimplemented, raises an exception.
6449 Returns true if successful.
6451 On systems that support a close-on-exec flag on files, the flag will
6452 be set for the newly opened file descriptors, as determined by the value
6453 of $^F. See L<perlvar/$^F>.
6455 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6456 to C<pipe(Rdr, Wtr)> is essentially:
6459 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6460 shutdown(Rdr, 1); # no more writing for reader
6461 shutdown(Wtr, 0); # no more reading for writer
6463 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6464 emulate socketpair using IP sockets to localhost if your system implements
6465 sockets but not socketpair.
6467 Portability issues: L<perlport/socketpair>.
6469 =item sort SUBNAME LIST
6470 X<sort> X<qsort> X<quicksort> X<mergesort>
6472 =item sort BLOCK LIST
6476 =for Pod::Functions sort a list of values
6478 In list context, this sorts the LIST and returns the sorted list value.
6479 In scalar context, the behaviour of C<sort()> is undefined.
6481 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6482 order. If SUBNAME is specified, it gives the name of a subroutine
6483 that returns an integer less than, equal to, or greater than C<0>,
6484 depending on how the elements of the list are to be ordered. (The
6485 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6486 SUBNAME may be a scalar variable name (unsubscripted), in which case
6487 the value provides the name of (or a reference to) the actual
6488 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6489 an anonymous, in-line sort subroutine.
6491 If the subroutine's prototype is C<($$)>, the elements to be compared are
6492 passed by reference in C<@_>, as for a normal subroutine. This is slower
6493 than unprototyped subroutines, where the elements to be compared are passed
6494 into the subroutine as the package global variables $a and $b (see example
6495 below). Note that in the latter case, it is usually highly counter-productive
6496 to declare $a and $b as lexicals.
6498 If the subroutine is an XSUB, the elements to be compared are pushed on to
6499 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6502 The values to be compared are always passed by reference and should not
6505 You also cannot exit out of the sort block or subroutine using any of the
6506 loop control operators described in L<perlsyn> or with C<goto>.
6508 When C<use locale> (but not C<use locale 'not_characters'>) is in
6509 effect, C<sort LIST> sorts LIST according to the
6510 current collation locale. See L<perllocale>.
6512 sort() returns aliases into the original list, much as a for loop's index
6513 variable aliases the list elements. That is, modifying an element of a
6514 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6515 actually modifies the element in the original list. This is usually
6516 something to be avoided when writing clear code.
6518 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6519 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6520 preserves the input order of elements that compare equal. Although
6521 quicksort's run time is O(NlogN) when averaged over all arrays of
6522 length N, the time can be O(N**2), I<quadratic> behavior, for some
6523 inputs.) In 5.7, the quicksort implementation was replaced with
6524 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6525 But benchmarks indicated that for some inputs, on some platforms,
6526 the original quicksort was faster. 5.8 has a sort pragma for
6527 limited control of the sort. Its rather blunt control of the
6528 underlying algorithm may not persist into future Perls, but the
6529 ability to characterize the input or output in implementation
6530 independent ways quite probably will. See L<the sort pragma|sort>.
6535 @articles = sort @files;
6537 # same thing, but with explicit sort routine
6538 @articles = sort {$a cmp $b} @files;
6540 # now case-insensitively
6541 @articles = sort {fc($a) cmp fc($b)} @files;
6543 # same thing in reversed order
6544 @articles = sort {$b cmp $a} @files;
6546 # sort numerically ascending
6547 @articles = sort {$a <=> $b} @files;
6549 # sort numerically descending
6550 @articles = sort {$b <=> $a} @files;
6552 # this sorts the %age hash by value instead of key
6553 # using an in-line function
6554 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6556 # sort using explicit subroutine name
6558 $age{$a} <=> $age{$b}; # presuming numeric
6560 @sortedclass = sort byage @class;
6562 sub backwards { $b cmp $a }
6563 @harry = qw(dog cat x Cain Abel);
6564 @george = qw(gone chased yz Punished Axed);
6566 # prints AbelCaincatdogx
6567 print sort backwards @harry;
6568 # prints xdogcatCainAbel
6569 print sort @george, 'to', @harry;
6570 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6572 # inefficiently sort by descending numeric compare using
6573 # the first integer after the first = sign, or the
6574 # whole record case-insensitively otherwise
6577 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6582 # same thing, but much more efficiently;
6583 # we'll build auxiliary indices instead
6585 my @nums = @caps = ();
6587 push @nums, ( /=(\d+)/ ? $1 : undef );
6591 my @new = @old[ sort {
6592 $nums[$b] <=> $nums[$a]
6594 $caps[$a] cmp $caps[$b]
6598 # same thing, but without any temps
6599 @new = map { $_->[0] }
6600 sort { $b->[1] <=> $a->[1]
6603 } map { [$_, /=(\d+)/, fc($_)] } @old;
6605 # using a prototype allows you to use any comparison subroutine
6606 # as a sort subroutine (including other package's subroutines)
6608 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6611 @new = sort other::backwards @old;
6613 # guarantee stability, regardless of algorithm
6615 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6617 # force use of mergesort (not portable outside Perl 5.8)
6618 use sort '_mergesort'; # note discouraging _
6619 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6621 Warning: syntactical care is required when sorting the list returned from
6622 a function. If you want to sort the list returned by the function call
6623 C<find_records(@key)>, you can use:
6625 @contact = sort { $a cmp $b } find_records @key;
6626 @contact = sort +find_records(@key);
6627 @contact = sort &find_records(@key);
6628 @contact = sort(find_records(@key));
6630 If instead you want to sort the array @key with the comparison routine
6631 C<find_records()> then you can use:
6633 @contact = sort { find_records() } @key;
6634 @contact = sort find_records(@key);
6635 @contact = sort(find_records @key);
6636 @contact = sort(find_records (@key));
6638 If you're using strict, you I<must not> declare $a
6639 and $b as lexicals. They are package globals. That means
6640 that if you're in the C<main> package and type
6642 @articles = sort {$b <=> $a} @files;
6644 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6645 but if you're in the C<FooPack> package, it's the same as typing
6647 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6649 The comparison function is required to behave. If it returns
6650 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6651 sometimes saying the opposite, for example) the results are not
6654 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6655 (not-a-number), be careful when sorting with a
6656 comparison function like C<< $a <=> $b >> any lists that might contain a
6657 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6658 eliminate any C<NaN>s from the input list.
6660 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6662 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6665 =item splice ARRAY or EXPR,OFFSET,LENGTH
6667 =item splice ARRAY or EXPR,OFFSET
6669 =item splice ARRAY or EXPR
6671 =for Pod::Functions add or remove elements anywhere in an array
6673 Removes the elements designated by OFFSET and LENGTH from an array, and
6674 replaces them with the elements of LIST, if any. In list context,
6675 returns the elements removed from the array. In scalar context,
6676 returns the last element removed, or C<undef> if no elements are
6677 removed. The array grows or shrinks as necessary.
6678 If OFFSET is negative then it starts that far from the end of the array.
6679 If LENGTH is omitted, removes everything from OFFSET onward.
6680 If LENGTH is negative, removes the elements from OFFSET onward
6681 except for -LENGTH elements at the end of the array.
6682 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6683 past the end of the array, Perl issues a warning, and splices at the
6686 The following equivalences hold (assuming C<< $#a >= $i >> )
6688 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6689 pop(@a) splice(@a,-1)
6690 shift(@a) splice(@a,0,1)
6691 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6692 $a[$i] = $y splice(@a,$i,1,$y)
6694 Example, assuming array lengths are passed before arrays:
6696 sub aeq { # compare two list values
6697 my(@a) = splice(@_,0,shift);
6698 my(@b) = splice(@_,0,shift);
6699 return 0 unless @a == @b; # same len?
6701 return 0 if pop(@a) ne pop(@b);
6705 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6707 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6708 reference to an unblessed array. The argument will be dereferenced
6709 automatically. This aspect of C<splice> is considered highly experimental.
6710 The exact behaviour may change in a future version of Perl.
6712 To avoid confusing would-be users of your code who are running earlier
6713 versions of Perl with mysterious syntax errors, put this sort of thing at
6714 the top of your file to signal that your code will work I<only> on Perls of
6717 use 5.014; # so push/pop/etc work on scalars (experimental)
6719 =item split /PATTERN/,EXPR,LIMIT
6722 =item split /PATTERN/,EXPR
6724 =item split /PATTERN/
6728 =for Pod::Functions split up a string using a regexp delimiter
6730 Splits the string EXPR into a list of strings and returns the
6731 list in list context, or the size of the list in scalar context.
6733 If only PATTERN is given, EXPR defaults to C<$_>.
6735 Anything in EXPR that matches PATTERN is taken to be a separator
6736 that separates the EXPR into substrings (called "I<fields>") that
6737 do B<not> include the separator. Note that a separator may be
6738 longer than one character or even have no characters at all (the
6739 empty string, which is a zero-width match).
6741 The PATTERN need not be constant; an expression may be used
6742 to specify a pattern that varies at runtime.
6744 If PATTERN matches the empty string, the EXPR is split at the match
6745 position (between characters). As an example, the following:
6747 print join(':', split('b', 'abc')), "\n";
6749 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6752 print join(':', split('', 'abc')), "\n";
6754 uses empty string matches as separators to produce the output
6755 'a:b:c'; thus, the empty string may be used to split EXPR into a
6756 list of its component characters.
6758 As a special case for C<split>, the empty pattern given in
6759 L<match operator|perlop/"m/PATTERN/msixpodualgc"> syntax (C<//>) specifically matches the empty string, which is contrary to its usual
6760 interpretation as the last successful match.
6762 If PATTERN is C</^/>, then it is treated as if it used the
6763 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
6764 isn't much use otherwise.
6766 As another special case, C<split> emulates the default behavior of the
6767 command line tool B<awk> when the PATTERN is either omitted or a I<literal
6768 string> composed of a single space character (such as S<C<' '>> or
6769 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
6770 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
6771 instead treated as if it were C</\s+/>; in particular, this means that
6772 I<any> contiguous whitespace (not just a single space character) is used as
6773 a separator. However, this special treatment can be avoided by specifying
6774 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
6775 only a single space character to be a separator.
6777 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
6778 the previously described I<awk> emulation.
6780 If LIMIT is specified and positive, it represents the maximum number
6781 of fields into which the EXPR may be split; in other words, LIMIT is
6782 one greater than the maximum number of times EXPR may be split. Thus,
6783 the LIMIT value C<1> means that EXPR may be split a maximum of zero
6784 times, producing a maximum of one field (namely, the entire value of
6785 EXPR). For instance:
6787 print join(':', split(//, 'abc', 1)), "\n";
6789 produces the output 'abc', and this:
6791 print join(':', split(//, 'abc', 2)), "\n";
6793 produces the output 'a:bc', and each of these:
6795 print join(':', split(//, 'abc', 3)), "\n";
6796 print join(':', split(//, 'abc', 4)), "\n";
6798 produces the output 'a:b:c'.
6800 If LIMIT is negative, it is treated as if it were instead arbitrarily
6801 large; as many fields as possible are produced.
6803 If LIMIT is omitted (or, equivalently, zero), then it is usually
6804 treated as if it were instead negative but with the exception that
6805 trailing empty fields are stripped (empty leading fields are always
6806 preserved); if all fields are empty, then all fields are considered to
6807 be trailing (and are thus stripped in this case). Thus, the following:
6809 print join(':', split(',', 'a,b,c,,,')), "\n";
6811 produces the output 'a:b:c', but the following:
6813 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
6815 produces the output 'a:b:c:::'.
6817 In time-critical applications, it is worthwhile to avoid splitting
6818 into more fields than necessary. Thus, when assigning to a list,
6819 if LIMIT is omitted (or zero), then LIMIT is treated as though it
6820 were one larger than the number of variables in the list; for the
6821 following, LIMIT is implicitly 3:
6823 ($login, $passwd) = split(/:/);
6825 Note that splitting an EXPR that evaluates to the empty string always
6826 produces zero fields, regardless of the LIMIT specified.
6828 An empty leading field is produced when there is a positive-width
6829 match at the beginning of EXPR. For instance:
6831 print join(':', split(/ /, ' abc')), "\n";
6833 produces the output ':abc'. However, a zero-width match at the
6834 beginning of EXPR never produces an empty field, so that:
6836 print join(':', split(//, ' abc'));
6838 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
6840 An empty trailing field, on the other hand, is produced when there is a
6841 match at the end of EXPR, regardless of the length of the match
6842 (of course, unless a non-zero LIMIT is given explicitly, such fields are
6843 removed, as in the last example). Thus:
6845 print join(':', split(//, ' abc', -1)), "\n";
6847 produces the output S<' :a:b:c:'>.
6849 If the PATTERN contains
6850 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
6851 then for each separator, an additional field is produced for each substring
6852 captured by a group (in the order in which the groups are specified,
6853 as per L<backreferences|perlretut/Backreferences>); if any group does not
6854 match, then it captures the C<undef> value instead of a substring. Also,
6855 note that any such additional field is produced whenever there is a
6856 separator (that is, whenever a split occurs), and such an additional field
6857 does B<not> count towards the LIMIT. Consider the following expressions
6858 evaluated in list context (each returned list is provided in the associated
6861 split(/-|,/, "1-10,20", 3)
6864 split(/(-|,)/, "1-10,20", 3)
6865 # ('1', '-', '10', ',', '20')
6867 split(/-|(,)/, "1-10,20", 3)
6868 # ('1', undef, '10', ',', '20')
6870 split(/(-)|,/, "1-10,20", 3)
6871 # ('1', '-', '10', undef, '20')
6873 split(/(-)|(,)/, "1-10,20", 3)
6874 # ('1', '-', undef, '10', undef, ',', '20')
6876 =item sprintf FORMAT, LIST
6879 =for Pod::Functions formatted print into a string
6881 Returns a string formatted by the usual C<printf> conventions of the C
6882 library function C<sprintf>. See below for more details
6883 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
6884 the general principles.
6888 # Format number with up to 8 leading zeroes
6889 $result = sprintf("%08d", $number);
6891 # Round number to 3 digits after decimal point
6892 $rounded = sprintf("%.3f", $number);
6894 Perl does its own C<sprintf> formatting: it emulates the C
6895 function sprintf(3), but doesn't use it except for floating-point
6896 numbers, and even then only standard modifiers are allowed.
6897 Non-standard extensions in your local sprintf(3) are
6898 therefore unavailable from Perl.
6900 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
6901 pass it an array as your first argument.
6902 The array is given scalar context,
6903 and instead of using the 0th element of the array as the format, Perl will
6904 use the count of elements in the array as the format, which is almost never
6907 Perl's C<sprintf> permits the following universally-known conversions:
6910 %c a character with the given number
6912 %d a signed integer, in decimal
6913 %u an unsigned integer, in decimal
6914 %o an unsigned integer, in octal
6915 %x an unsigned integer, in hexadecimal
6916 %e a floating-point number, in scientific notation
6917 %f a floating-point number, in fixed decimal notation
6918 %g a floating-point number, in %e or %f notation
6920 In addition, Perl permits the following widely-supported conversions:
6922 %X like %x, but using upper-case letters
6923 %E like %e, but using an upper-case "E"
6924 %G like %g, but with an upper-case "E" (if applicable)
6925 %b an unsigned integer, in binary
6926 %B like %b, but using an upper-case "B" with the # flag
6927 %p a pointer (outputs the Perl value's address in hexadecimal)
6928 %n special: *stores* the number of characters output so far
6929 into the next argument in the parameter list
6931 Finally, for backward (and we do mean "backward") compatibility, Perl
6932 permits these unnecessary but widely-supported conversions:
6935 %D a synonym for %ld
6936 %U a synonym for %lu
6937 %O a synonym for %lo
6940 Note that the number of exponent digits in the scientific notation produced
6941 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
6942 exponent less than 100 is system-dependent: it may be three or less
6943 (zero-padded as necessary). In other words, 1.23 times ten to the
6944 99th may be either "1.23e99" or "1.23e099".
6946 Between the C<%> and the format letter, you may specify several
6947 additional attributes controlling the interpretation of the format.
6948 In order, these are:
6952 =item format parameter index
6954 An explicit format parameter index, such as C<2$>. By default sprintf
6955 will format the next unused argument in the list, but this allows you
6956 to take the arguments out of order:
6958 printf '%2$d %1$d', 12, 34; # prints "34 12"
6959 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
6965 space prefix non-negative number with a space
6966 + prefix non-negative number with a plus sign
6967 - left-justify within the field
6968 0 use zeros, not spaces, to right-justify
6969 # ensure the leading "0" for any octal,
6970 prefix non-zero hexadecimal with "0x" or "0X",
6971 prefix non-zero binary with "0b" or "0B"
6975 printf '<% d>', 12; # prints "< 12>"
6976 printf '<%+d>', 12; # prints "<+12>"
6977 printf '<%6s>', 12; # prints "< 12>"
6978 printf '<%-6s>', 12; # prints "<12 >"
6979 printf '<%06s>', 12; # prints "<000012>"
6980 printf '<%#o>', 12; # prints "<014>"
6981 printf '<%#x>', 12; # prints "<0xc>"
6982 printf '<%#X>', 12; # prints "<0XC>"
6983 printf '<%#b>', 12; # prints "<0b1100>"
6984 printf '<%#B>', 12; # prints "<0B1100>"
6986 When a space and a plus sign are given as the flags at once,
6987 a plus sign is used to prefix a positive number.
6989 printf '<%+ d>', 12; # prints "<+12>"
6990 printf '<% +d>', 12; # prints "<+12>"
6992 When the # flag and a precision are given in the %o conversion,
6993 the precision is incremented if it's necessary for the leading "0".
6995 printf '<%#.5o>', 012; # prints "<00012>"
6996 printf '<%#.5o>', 012345; # prints "<012345>"
6997 printf '<%#.0o>', 0; # prints "<0>"
7001 This flag tells Perl to interpret the supplied string as a vector of
7002 integers, one for each character in the string. Perl applies the format to
7003 each integer in turn, then joins the resulting strings with a separator (a
7004 dot C<.> by default). This can be useful for displaying ordinal values of
7005 characters in arbitrary strings:
7007 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7008 printf "version is v%vd\n", $^V; # Perl's version
7010 Put an asterisk C<*> before the C<v> to override the string to
7011 use to separate the numbers:
7013 printf "address is %*vX\n", ":", $addr; # IPv6 address
7014 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7016 You can also explicitly specify the argument number to use for
7017 the join string using something like C<*2$v>; for example:
7019 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7022 =item (minimum) width
7024 Arguments are usually formatted to be only as wide as required to
7025 display the given value. You can override the width by putting
7026 a number here, or get the width from the next argument (with C<*>)
7027 or from a specified argument (e.g., with C<*2$>):
7029 printf "<%s>", "a"; # prints "<a>"
7030 printf "<%6s>", "a"; # prints "< a>"
7031 printf "<%*s>", 6, "a"; # prints "< a>"
7032 printf "<%*2$s>", "a", 6; # prints "< a>"
7033 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7035 If a field width obtained through C<*> is negative, it has the same
7036 effect as the C<-> flag: left-justification.
7038 =item precision, or maximum width
7041 You can specify a precision (for numeric conversions) or a maximum
7042 width (for string conversions) by specifying a C<.> followed by a number.
7043 For floating-point formats except C<g> and C<G>, this specifies
7044 how many places right of the decimal point to show (the default being 6).
7047 # these examples are subject to system-specific variation
7048 printf '<%f>', 1; # prints "<1.000000>"
7049 printf '<%.1f>', 1; # prints "<1.0>"
7050 printf '<%.0f>', 1; # prints "<1>"
7051 printf '<%e>', 10; # prints "<1.000000e+01>"
7052 printf '<%.1e>', 10; # prints "<1.0e+01>"
7054 For "g" and "G", this specifies the maximum number of digits to show,
7055 including those prior to the decimal point and those after it; for
7058 # These examples are subject to system-specific variation.
7059 printf '<%g>', 1; # prints "<1>"
7060 printf '<%.10g>', 1; # prints "<1>"
7061 printf '<%g>', 100; # prints "<100>"
7062 printf '<%.1g>', 100; # prints "<1e+02>"
7063 printf '<%.2g>', 100.01; # prints "<1e+02>"
7064 printf '<%.5g>', 100.01; # prints "<100.01>"
7065 printf '<%.4g>', 100.01; # prints "<100>"
7067 For integer conversions, specifying a precision implies that the
7068 output of the number itself should be zero-padded to this width,
7069 where the 0 flag is ignored:
7071 printf '<%.6d>', 1; # prints "<000001>"
7072 printf '<%+.6d>', 1; # prints "<+000001>"
7073 printf '<%-10.6d>', 1; # prints "<000001 >"
7074 printf '<%10.6d>', 1; # prints "< 000001>"
7075 printf '<%010.6d>', 1; # prints "< 000001>"
7076 printf '<%+10.6d>', 1; # prints "< +000001>"
7078 printf '<%.6x>', 1; # prints "<000001>"
7079 printf '<%#.6x>', 1; # prints "<0x000001>"
7080 printf '<%-10.6x>', 1; # prints "<000001 >"
7081 printf '<%10.6x>', 1; # prints "< 000001>"
7082 printf '<%010.6x>', 1; # prints "< 000001>"
7083 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7085 For string conversions, specifying a precision truncates the string
7086 to fit the specified width:
7088 printf '<%.5s>', "truncated"; # prints "<trunc>"
7089 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7091 You can also get the precision from the next argument using C<.*>:
7093 printf '<%.6x>', 1; # prints "<000001>"
7094 printf '<%.*x>', 6, 1; # prints "<000001>"
7096 If a precision obtained through C<*> is negative, it counts
7097 as having no precision at all.
7099 printf '<%.*s>', 7, "string"; # prints "<string>"
7100 printf '<%.*s>', 3, "string"; # prints "<str>"
7101 printf '<%.*s>', 0, "string"; # prints "<>"
7102 printf '<%.*s>', -1, "string"; # prints "<string>"
7104 printf '<%.*d>', 1, 0; # prints "<0>"
7105 printf '<%.*d>', 0, 0; # prints "<>"
7106 printf '<%.*d>', -1, 0; # prints "<0>"
7108 You cannot currently get the precision from a specified number,
7109 but it is intended that this will be possible in the future, for
7110 example using C<.*2$>:
7112 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print
7117 For numeric conversions, you can specify the size to interpret the
7118 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7119 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7120 whatever the default integer size is on your platform (usually 32 or 64
7121 bits), but you can override this to use instead one of the standard C types,
7122 as supported by the compiler used to build Perl:
7124 hh interpret integer as C type "char" or "unsigned
7125 char" on Perl 5.14 or later
7126 h interpret integer as C type "short" or
7128 j interpret integer as C type "intmax_t" on Perl
7129 5.14 or later, and only with a C99 compiler
7131 l interpret integer as C type "long" or
7133 q, L, or ll interpret integer as C type "long long",
7134 "unsigned long long", or "quad" (typically
7136 t interpret integer as C type "ptrdiff_t" on Perl
7138 z interpret integer as C type "size_t" on Perl 5.14
7141 As of 5.14, none of these raises an exception if they are not supported on
7142 your platform. However, if warnings are enabled, a warning of the
7143 C<printf> warning class is issued on an unsupported conversion flag.
7144 Should you instead prefer an exception, do this:
7146 use warnings FATAL => "printf";
7148 If you would like to know about a version dependency before you
7149 start running the program, put something like this at its top:
7151 use 5.014; # for hh/j/t/z/ printf modifiers
7153 You can find out whether your Perl supports quads via L<Config>:
7156 if ($Config{use64bitint} eq "define"
7157 || $Config{longsize} >= 8) {
7158 print "Nice quads!\n";
7161 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7162 to be the default floating-point size on your platform (double or long double),
7163 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7164 platform supports them. You can find out whether your Perl supports long
7165 doubles via L<Config>:
7168 print "long doubles\n" if $Config{d_longdbl} eq "define";
7170 You can find out whether Perl considers "long double" to be the default
7171 floating-point size to use on your platform via L<Config>:
7174 if ($Config{uselongdouble} eq "define") {
7175 print "long doubles by default\n";
7178 It can also be that long doubles and doubles are the same thing:
7181 ($Config{doublesize} == $Config{longdblsize}) &&
7182 print "doubles are long doubles\n";
7184 The size specifier C<V> has no effect for Perl code, but is supported for
7185 compatibility with XS code. It means "use the standard size for a Perl
7186 integer or floating-point number", which is the default.
7188 =item order of arguments
7190 Normally, sprintf() takes the next unused argument as the value to
7191 format for each format specification. If the format specification
7192 uses C<*> to require additional arguments, these are consumed from
7193 the argument list in the order they appear in the format
7194 specification I<before> the value to format. Where an argument is
7195 specified by an explicit index, this does not affect the normal
7196 order for the arguments, even when the explicitly specified index
7197 would have been the next argument.
7201 printf "<%*.*s>", $a, $b, $c;
7203 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7204 as the value to format; while:
7206 printf "<%*1$.*s>", $a, $b;
7208 would use C<$a> for the width and precision, and C<$b> as the
7211 Here are some more examples; be aware that when using an explicit
7212 index, the C<$> may need escaping:
7214 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7215 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7216 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7217 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7221 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7222 and POSIX::setlocale() has been called,
7223 the character used for the decimal separator in formatted floating-point
7224 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
7228 X<sqrt> X<root> X<square root>
7232 =for Pod::Functions square root function
7234 Return the positive square root of EXPR. If EXPR is omitted, uses
7235 C<$_>. Works only for non-negative operands unless you've
7236 loaded the C<Math::Complex> module.
7239 print sqrt(-4); # prints 2i
7242 X<srand> X<seed> X<randseed>
7246 =for Pod::Functions seed the random number generator
7248 Sets and returns the random number seed for the C<rand> operator.
7250 The point of the function is to "seed" the C<rand> function so that C<rand>
7251 can produce a different sequence each time you run your program. When
7252 called with a parameter, C<srand> uses that for the seed; otherwise it
7253 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7254 it returns the seed. To signal that your code will work I<only> on Perls
7255 of a recent vintage:
7257 use 5.014; # so srand returns the seed
7259 If C<srand()> is not called explicitly, it is called implicitly without a
7260 parameter at the first use of the C<rand> operator. However, this was not true
7261 of versions of Perl before 5.004, so if your script will run under older
7262 Perl versions, it should call C<srand>; otherwise most programs won't call
7265 But there are a few situations in recent Perls where programs are likely to
7266 want to call C<srand>. One is for generating predictable results, generally for
7267 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7268 each time. Another case is that you may want to call C<srand()>
7269 after a C<fork()> to avoid child processes sharing the same seed value as the
7270 parent (and consequently each other).
7272 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7273 process. The internal state of the random number generator should
7274 contain more entropy than can be provided by any seed, so calling
7275 C<srand()> again actually I<loses> randomness.
7277 Most implementations of C<srand> take an integer and will silently
7278 truncate decimal numbers. This means C<srand(42)> will usually
7279 produce the same results as C<srand(42.1)>. To be safe, always pass
7280 C<srand> an integer.
7282 In versions of Perl prior to 5.004 the default seed was just the
7283 current C<time>. This isn't a particularly good seed, so many old
7284 programs supply their own seed value (often C<time ^ $$> or C<time ^
7285 ($$ + ($$ << 15))>), but that isn't necessary any more.
7287 Frequently called programs (like CGI scripts) that simply use
7291 for a seed can fall prey to the mathematical property that
7295 one-third of the time. So don't do that.
7297 A typical use of the returned seed is for a test program which has too many
7298 combinations to test comprehensively in the time available to it each run. It
7299 can test a random subset each time, and should there be a failure, log the seed
7300 used for that run so that it can later be used to reproduce the same results.
7302 B<C<rand()> is not cryptographically secure. You should not rely
7303 on it in security-sensitive situations.> As of this writing, a
7304 number of third-party CPAN modules offer random number generators
7305 intended by their authors to be cryptographically secure,
7306 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7307 and L<Math::TrulyRandom>.
7309 =item stat FILEHANDLE
7310 X<stat> X<file, status> X<ctime>
7314 =item stat DIRHANDLE
7318 =for Pod::Functions get a file's status information
7320 Returns a 13-element list giving the status info for a file, either
7321 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7322 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7325 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7326 $atime,$mtime,$ctime,$blksize,$blocks)
7329 Not all fields are supported on all filesystem types. Here are the
7330 meanings of the fields:
7332 0 dev device number of filesystem
7334 2 mode file mode (type and permissions)
7335 3 nlink number of (hard) links to the file
7336 4 uid numeric user ID of file's owner
7337 5 gid numeric group ID of file's owner
7338 6 rdev the device identifier (special files only)
7339 7 size total size of file, in bytes
7340 8 atime last access time in seconds since the epoch
7341 9 mtime last modify time in seconds since the epoch
7342 10 ctime inode change time in seconds since the epoch (*)
7343 11 blksize preferred block size for file system I/O
7344 12 blocks actual number of blocks allocated
7346 (The epoch was at 00:00 January 1, 1970 GMT.)
7348 (*) Not all fields are supported on all filesystem types. Notably, the
7349 ctime field is non-portable. In particular, you cannot expect it to be a
7350 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7352 If C<stat> is passed the special filehandle consisting of an underline, no
7353 stat is done, but the current contents of the stat structure from the
7354 last C<stat>, C<lstat>, or filetest are returned. Example:
7356 if (-x $file && (($d) = stat(_)) && $d < 0) {
7357 print "$file is executable NFS file\n";
7360 (This works on machines only for which the device number is negative
7363 Because the mode contains both the file type and its permissions, you
7364 should mask off the file type portion and (s)printf using a C<"%o">
7365 if you want to see the real permissions.
7367 $mode = (stat($filename))[2];
7368 printf "Permissions are %04o\n", $mode & 07777;
7370 In scalar context, C<stat> returns a boolean value indicating success
7371 or failure, and, if successful, sets the information associated with
7372 the special filehandle C<_>.
7374 The L<File::stat> module provides a convenient, by-name access mechanism:
7377 $sb = stat($filename);
7378 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7379 $filename, $sb->size, $sb->mode & 07777,
7380 scalar localtime $sb->mtime;
7382 You can import symbolic mode constants (C<S_IF*>) and functions
7383 (C<S_IS*>) from the Fcntl module:
7387 $mode = (stat($filename))[2];
7389 $user_rwx = ($mode & S_IRWXU) >> 6;
7390 $group_read = ($mode & S_IRGRP) >> 3;
7391 $other_execute = $mode & S_IXOTH;
7393 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7395 $is_setuid = $mode & S_ISUID;
7396 $is_directory = S_ISDIR($mode);
7398 You could write the last two using the C<-u> and C<-d> operators.
7399 Commonly available C<S_IF*> constants are:
7401 # Permissions: read, write, execute, for user, group, others.
7403 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7404 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7405 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7407 # Setuid/Setgid/Stickiness/SaveText.
7408 # Note that the exact meaning of these is system-dependent.
7410 S_ISUID S_ISGID S_ISVTX S_ISTXT
7412 # File types. Not all are necessarily available on
7415 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7416 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7418 # The following are compatibility aliases for S_IRUSR,
7419 # S_IWUSR, and S_IXUSR.
7421 S_IREAD S_IWRITE S_IEXEC
7423 and the C<S_IF*> functions are
7425 S_IMODE($mode) the part of $mode containing the permission
7426 bits and the setuid/setgid/sticky bits
7428 S_IFMT($mode) the part of $mode containing the file type
7429 which can be bit-anded with (for example)
7430 S_IFREG or with the following functions
7432 # The operators -f, -d, -l, -b, -c, -p, and -S.
7434 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7435 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7437 # No direct -X operator counterpart, but for the first one
7438 # the -g operator is often equivalent. The ENFMT stands for
7439 # record flocking enforcement, a platform-dependent feature.
7441 S_ISENFMT($mode) S_ISWHT($mode)
7443 See your native chmod(2) and stat(2) documentation for more details
7444 about the C<S_*> constants. To get status info for a symbolic link
7445 instead of the target file behind the link, use the C<lstat> function.
7447 Portability issues: L<perlport/stat>.
7452 =item state TYPE EXPR
7454 =item state EXPR : ATTRS
7456 =item state TYPE EXPR : ATTRS
7458 =for Pod::Functions +state declare and assign a persistent lexical variable
7460 C<state> declares a lexically scoped variable, just like C<my>.
7461 However, those variables will never be reinitialized, contrary to
7462 lexical variables that are reinitialized each time their enclosing block
7464 See L<perlsub/"Persistent Private Variables"> for details.
7466 C<state> variables are enabled only when the C<use feature "state"> pragma
7467 is in effect, unless the keyword is written as C<CORE::state>.
7468 See also L<feature>.
7475 =for Pod::Functions optimize input data for repeated searches
7477 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
7478 doing many pattern matches on the string before it is next modified.
7479 This may or may not save time, depending on the nature and number of
7480 patterns you are searching and the distribution of character
7481 frequencies in the string to be searched; you probably want to compare
7482 run times with and without it to see which is faster. Those loops
7483 that scan for many short constant strings (including the constant
7484 parts of more complex patterns) will benefit most.
7485 (The way C<study> works is this: a linked list of every
7486 character in the string to be searched is made, so we know, for
7487 example, where all the C<'k'> characters are. From each search string,
7488 the rarest character is selected, based on some static frequency tables
7489 constructed from some C programs and English text. Only those places
7490 that contain this "rarest" character are examined.)
7492 For example, here is a loop that inserts index producing entries
7493 before any line containing a certain pattern:
7497 print ".IX foo\n" if /\bfoo\b/;
7498 print ".IX bar\n" if /\bbar\b/;
7499 print ".IX blurfl\n" if /\bblurfl\b/;
7504 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7505 will be looked at, because C<f> is rarer than C<o>. In general, this is
7506 a big win except in pathological cases. The only question is whether
7507 it saves you more time than it took to build the linked list in the
7510 Note that if you have to look for strings that you don't know till
7511 runtime, you can build an entire loop as a string and C<eval> that to
7512 avoid recompiling all your patterns all the time. Together with
7513 undefining C<$/> to input entire files as one record, this can be quite
7514 fast, often faster than specialized programs like fgrep(1). The following
7515 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7516 out the names of those files that contain a match:
7518 $search = 'while (<>) { study;';
7519 foreach $word (@words) {
7520 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7525 eval $search; # this screams
7526 $/ = "\n"; # put back to normal input delimiter
7527 foreach $file (sort keys(%seen)) {
7531 =item sub NAME BLOCK
7534 =item sub NAME (PROTO) BLOCK
7536 =item sub NAME : ATTRS BLOCK
7538 =item sub NAME (PROTO) : ATTRS BLOCK
7540 =for Pod::Functions declare a subroutine, possibly anonymously
7542 This is subroutine definition, not a real function I<per se>. Without a
7543 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7544 function declaration, so does return a value: the CODE ref of the closure
7547 See L<perlsub> and L<perlref> for details about subroutines and
7548 references; see L<attributes> and L<Attribute::Handlers> for more
7549 information about attributes.
7554 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7556 A special token that returns the a reference to the current subroutine, or
7557 C<undef> outside of a subroutine.
7559 This token is only available under C<use v5.16> or the "current_sub"
7560 feature. See L<feature>.
7562 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7563 X<substr> X<substring> X<mid> X<left> X<right>
7565 =item substr EXPR,OFFSET,LENGTH
7567 =item substr EXPR,OFFSET
7569 =for Pod::Functions get or alter a portion of a string
7571 Extracts a substring out of EXPR and returns it. First character is at
7572 offset zero. If OFFSET is negative, starts
7573 that far back from the end of the string. If LENGTH is omitted, returns
7574 everything through the end of the string. If LENGTH is negative, leaves that
7575 many characters off the end of the string.
7577 my $s = "The black cat climbed the green tree";
7578 my $color = substr $s, 4, 5; # black
7579 my $middle = substr $s, 4, -11; # black cat climbed the
7580 my $end = substr $s, 14; # climbed the green tree
7581 my $tail = substr $s, -4; # tree
7582 my $z = substr $s, -4, 2; # tr
7584 You can use the substr() function as an lvalue, in which case EXPR
7585 must itself be an lvalue. If you assign something shorter than LENGTH,
7586 the string will shrink, and if you assign something longer than LENGTH,
7587 the string will grow to accommodate it. To keep the string the same
7588 length, you may need to pad or chop your value using C<sprintf>.
7590 If OFFSET and LENGTH specify a substring that is partly outside the
7591 string, only the part within the string is returned. If the substring
7592 is beyond either end of the string, substr() returns the undefined
7593 value and produces a warning. When used as an lvalue, specifying a
7594 substring that is entirely outside the string raises an exception.
7595 Here's an example showing the behavior for boundary cases:
7598 substr($name, 4) = 'dy'; # $name is now 'freddy'
7599 my $null = substr $name, 6, 2; # returns "" (no warning)
7600 my $oops = substr $name, 7; # returns undef, with warning
7601 substr($name, 7) = 'gap'; # raises an exception
7603 An alternative to using substr() as an lvalue is to specify the
7604 replacement string as the 4th argument. This allows you to replace
7605 parts of the EXPR and return what was there before in one operation,
7606 just as you can with splice().
7608 my $s = "The black cat climbed the green tree";
7609 my $z = substr $s, 14, 7, "jumped from"; # climbed
7610 # $s is now "The black cat jumped from the green tree"
7612 Note that the lvalue returned by the three-argument version of substr() acts as
7613 a 'magic bullet'; each time it is assigned to, it remembers which part
7614 of the original string is being modified; for example:
7617 for (substr($x,1,2)) {
7618 $_ = 'a'; print $x,"\n"; # prints 1a4
7619 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7621 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7624 With negative offsets, it remembers its position from the end of the string
7625 when the target string is modified:
7628 for (substr($x, -3, 2)) {
7629 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7631 print $_,"\n"; # prints f
7634 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7635 unspecified. Prior to 5.16, the result with negative offsets was
7638 =item symlink OLDFILE,NEWFILE
7639 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7641 =for Pod::Functions create a symbolic link to a file
7643 Creates a new filename symbolically linked to the old filename.
7644 Returns C<1> for success, C<0> otherwise. On systems that don't support
7645 symbolic links, raises an exception. To check for that,
7648 $symlink_exists = eval { symlink("",""); 1 };
7650 Portability issues: L<perlport/symlink>.
7652 =item syscall NUMBER, LIST
7653 X<syscall> X<system call>
7655 =for Pod::Functions execute an arbitrary system call
7657 Calls the system call specified as the first element of the list,
7658 passing the remaining elements as arguments to the system call. If
7659 unimplemented, raises an exception. The arguments are interpreted
7660 as follows: if a given argument is numeric, the argument is passed as
7661 an int. If not, the pointer to the string value is passed. You are
7662 responsible to make sure a string is pre-extended long enough to
7663 receive any result that might be written into a string. You can't use a
7664 string literal (or other read-only string) as an argument to C<syscall>
7665 because Perl has to assume that any string pointer might be written
7667 integer arguments are not literals and have never been interpreted in a
7668 numeric context, you may need to add C<0> to them to force them to look
7669 like numbers. This emulates the C<syswrite> function (or vice versa):
7671 require 'syscall.ph'; # may need to run h2ph
7673 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7675 Note that Perl supports passing of up to only 14 arguments to your syscall,
7676 which in practice should (usually) suffice.
7678 Syscall returns whatever value returned by the system call it calls.
7679 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7680 Note that some system calls I<can> legitimately return C<-1>. The proper
7681 way to handle such calls is to assign C<$!=0> before the call, then
7682 check the value of C<$!> if C<syscall> returns C<-1>.
7684 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7685 number of the read end of the pipe it creates, but there is no way
7686 to retrieve the file number of the other end. You can avoid this
7687 problem by using C<pipe> instead.
7689 Portability issues: L<perlport/syscall>.
7691 =item sysopen FILEHANDLE,FILENAME,MODE
7694 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7696 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7698 Opens the file whose filename is given by FILENAME, and associates it with
7699 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7700 filehandle wanted; an undefined scalar will be suitably autovivified. This
7701 function calls the underlying operating system's I<open>(2) function with the
7702 parameters FILENAME, MODE, and PERMS.
7704 The possible values and flag bits of the MODE parameter are
7705 system-dependent; they are available via the standard module C<Fcntl>. See
7706 the documentation of your operating system's I<open>(2) syscall to see
7707 which values and flag bits are available. You may combine several flags
7708 using the C<|>-operator.
7710 Some of the most common values are C<O_RDONLY> for opening the file in
7711 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7712 and C<O_RDWR> for opening the file in read-write mode.
7713 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7715 For historical reasons, some values work on almost every system
7716 supported by Perl: 0 means read-only, 1 means write-only, and 2
7717 means read/write. We know that these values do I<not> work under
7718 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
7719 use them in new code.
7721 If the file named by FILENAME does not exist and the C<open> call creates
7722 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7723 PERMS specifies the permissions of the newly created file. If you omit
7724 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7725 These permission values need to be in octal, and are modified by your
7726 process's current C<umask>.
7729 In many systems the C<O_EXCL> flag is available for opening files in
7730 exclusive mode. This is B<not> locking: exclusiveness means here that
7731 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7732 on network filesystems, and has no effect unless the C<O_CREAT> flag
7733 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7734 being opened if it is a symbolic link. It does not protect against
7735 symbolic links in the file's path.
7738 Sometimes you may want to truncate an already-existing file. This
7739 can be done using the C<O_TRUNC> flag. The behavior of
7740 C<O_TRUNC> with C<O_RDONLY> is undefined.
7743 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7744 that takes away the user's option to have a more permissive umask.
7745 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7748 Note that C<sysopen> depends on the fdopen() C library function.
7749 On many Unix systems, fdopen() is known to fail when file descriptors
7750 exceed a certain value, typically 255. If you need more file
7751 descriptors than that, consider rebuilding Perl to use the C<sfio>
7752 library, or perhaps using the POSIX::open() function.
7754 See L<perlopentut> for a kinder, gentler explanation of opening files.
7756 Portability issues: L<perlport/sysopen>.
7758 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7761 =item sysread FILEHANDLE,SCALAR,LENGTH
7763 =for Pod::Functions fixed-length unbuffered input from a filehandle
7765 Attempts to read LENGTH bytes of data into variable SCALAR from the
7766 specified FILEHANDLE, using the read(2). It bypasses
7767 buffered IO, so mixing this with other kinds of reads, C<print>,
7768 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7769 perlio or stdio layers usually buffers data. Returns the number of
7770 bytes actually read, C<0> at end of file, or undef if there was an
7771 error (in the latter case C<$!> is also set). SCALAR will be grown or
7772 shrunk so that the last byte actually read is the last byte of the
7773 scalar after the read.
7775 An OFFSET may be specified to place the read data at some place in the
7776 string other than the beginning. A negative OFFSET specifies
7777 placement at that many characters counting backwards from the end of
7778 the string. A positive OFFSET greater than the length of SCALAR
7779 results in the string being padded to the required size with C<"\0">
7780 bytes before the result of the read is appended.
7782 There is no syseof() function, which is ok, since eof() doesn't work
7783 well on device files (like ttys) anyway. Use sysread() and check
7784 for a return value for 0 to decide whether you're done.
7786 Note that if the filehandle has been marked as C<:utf8> Unicode
7787 characters are read instead of bytes (the LENGTH, OFFSET, and the
7788 return value of sysread() are in Unicode characters).
7789 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7790 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7792 =item sysseek FILEHANDLE,POSITION,WHENCE
7795 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
7797 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7798 be an expression whose value gives the name of the filehandle. The values
7799 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7800 to the current position plus POSITION; and C<2> to set it to EOF plus
7801 POSITION, typically negative.
7803 Note the I<in bytes>: even if the filehandle has been set to operate
7804 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7805 tell() will return byte offsets, not character offsets (because
7806 implementing that would render sysseek() unacceptably slow).
7808 sysseek() bypasses normal buffered IO, so mixing it with reads other
7809 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7810 C<seek>, C<tell>, or C<eof> may cause confusion.
7812 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7813 and C<SEEK_END> (start of the file, current position, end of the file)
7814 from the Fcntl module. Use of the constants is also more portable
7815 than relying on 0, 1, and 2. For example to define a "systell" function:
7817 use Fcntl 'SEEK_CUR';
7818 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7820 Returns the new position, or the undefined value on failure. A position
7821 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7822 true on success and false on failure, yet you can still easily determine
7828 =item system PROGRAM LIST
7830 =for Pod::Functions run a separate program
7832 Does exactly the same thing as C<exec LIST>, except that a fork is
7833 done first and the parent process waits for the child process to
7834 exit. Note that argument processing varies depending on the
7835 number of arguments. If there is more than one argument in LIST,
7836 or if LIST is an array with more than one value, starts the program
7837 given by the first element of the list with arguments given by the
7838 rest of the list. If there is only one scalar argument, the argument
7839 is checked for shell metacharacters, and if there are any, the
7840 entire argument is passed to the system's command shell for parsing
7841 (this is C</bin/sh -c> on Unix platforms, but varies on other
7842 platforms). If there are no shell metacharacters in the argument,
7843 it is split into words and passed directly to C<execvp>, which is
7846 Beginning with v5.6.0, Perl will attempt to flush all files opened for
7847 output before any operation that may do a fork, but this may not be
7848 supported on some platforms (see L<perlport>). To be safe, you may need
7849 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7850 of C<IO::Handle> on any open handles.
7852 The return value is the exit status of the program as returned by the
7853 C<wait> call. To get the actual exit value, shift right by eight (see
7854 below). See also L</exec>. This is I<not> what you want to use to capture
7855 the output from a command; for that you should use merely backticks or
7856 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7857 indicates a failure to start the program or an error of the wait(2) system
7858 call (inspect $! for the reason).
7860 If you'd like to make C<system> (and many other bits of Perl) die on error,
7861 have a look at the L<autodie> pragma.
7863 Like C<exec>, C<system> allows you to lie to a program about its name if
7864 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
7866 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
7867 C<system>, if you expect your program to terminate on receipt of these
7868 signals you will need to arrange to do so yourself based on the return
7871 @args = ("command", "arg1", "arg2");
7873 or die "system @args failed: $?"
7875 If you'd like to manually inspect C<system>'s failure, you can check all
7876 possible failure modes by inspecting C<$?> like this:
7879 print "failed to execute: $!\n";
7882 printf "child died with signal %d, %s coredump\n",
7883 ($? & 127), ($? & 128) ? 'with' : 'without';
7886 printf "child exited with value %d\n", $? >> 8;
7889 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
7890 with the C<W*()> calls from the POSIX module.
7892 When C<system>'s arguments are executed indirectly by the shell,
7893 results and return codes are subject to its quirks.
7894 See L<perlop/"`STRING`"> and L</exec> for details.
7896 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
7897 handler. See L<perlipc> for details.
7899 Portability issues: L<perlport/system>.
7901 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
7904 =item syswrite FILEHANDLE,SCALAR,LENGTH
7906 =item syswrite FILEHANDLE,SCALAR
7908 =for Pod::Functions fixed-length unbuffered output to a filehandle
7910 Attempts to write LENGTH bytes of data from variable SCALAR to the
7911 specified FILEHANDLE, using write(2). If LENGTH is
7912 not specified, writes whole SCALAR. It bypasses buffered IO, so
7913 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
7914 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
7915 stdio layers usually buffer data. Returns the number of bytes
7916 actually written, or C<undef> if there was an error (in this case the
7917 errno variable C<$!> is also set). If the LENGTH is greater than the
7918 data available in the SCALAR after the OFFSET, only as much data as is
7919 available will be written.
7921 An OFFSET may be specified to write the data from some part of the
7922 string other than the beginning. A negative OFFSET specifies writing
7923 that many characters counting backwards from the end of the string.
7924 If SCALAR is of length zero, you can only use an OFFSET of 0.
7926 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
7927 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
7928 return value of syswrite() are in (UTF8-encoded Unicode) characters.
7929 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7930 Alternately, if the handle is not marked with an encoding but you
7931 attempt to write characters with code points over 255, raises an exception.
7932 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7934 =item tell FILEHANDLE
7939 =for Pod::Functions get current seekpointer on a filehandle
7941 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
7942 error. FILEHANDLE may be an expression whose value gives the name of
7943 the actual filehandle. If FILEHANDLE is omitted, assumes the file
7946 Note the I<in bytes>: even if the filehandle has been set to
7947 operate on characters (for example by using the C<:encoding(utf8)> open
7948 layer), tell() will return byte offsets, not character offsets (because
7949 that would render seek() and tell() rather slow).
7951 The return value of tell() for the standard streams like the STDIN
7952 depends on the operating system: it may return -1 or something else.
7953 tell() on pipes, fifos, and sockets usually returns -1.
7955 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
7957 Do not use tell() (or other buffered I/O operations) on a filehandle
7958 that has been manipulated by sysread(), syswrite(), or sysseek().
7959 Those functions ignore the buffering, while tell() does not.
7961 =item telldir DIRHANDLE
7964 =for Pod::Functions get current seekpointer on a directory handle
7966 Returns the current position of the C<readdir> routines on DIRHANDLE.
7967 Value may be given to C<seekdir> to access a particular location in a
7968 directory. C<telldir> has the same caveats about possible directory
7969 compaction as the corresponding system library routine.
7971 =item tie VARIABLE,CLASSNAME,LIST
7974 =for Pod::Functions +5.002 bind a variable to an object class
7976 This function binds a variable to a package class that will provide the
7977 implementation for the variable. VARIABLE is the name of the variable
7978 to be enchanted. CLASSNAME is the name of a class implementing objects
7979 of correct type. Any additional arguments are passed to the C<new>
7980 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
7981 or C<TIEHASH>). Typically these are arguments such as might be passed
7982 to the C<dbm_open()> function of C. The object returned by the C<new>
7983 method is also returned by the C<tie> function, which would be useful
7984 if you want to access other methods in CLASSNAME.
7986 Note that functions such as C<keys> and C<values> may return huge lists
7987 when used on large objects, like DBM files. You may prefer to use the
7988 C<each> function to iterate over such. Example:
7990 # print out history file offsets
7992 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
7993 while (($key,$val) = each %HIST) {
7994 print $key, ' = ', unpack('L',$val), "\n";
7998 A class implementing a hash should have the following methods:
8000 TIEHASH classname, LIST
8002 STORE this, key, value
8007 NEXTKEY this, lastkey
8012 A class implementing an ordinary array should have the following methods:
8014 TIEARRAY classname, LIST
8016 STORE this, key, value
8018 STORESIZE this, count
8024 SPLICE this, offset, length, LIST
8029 A class implementing a filehandle should have the following methods:
8031 TIEHANDLE classname, LIST
8032 READ this, scalar, length, offset
8035 WRITE this, scalar, length, offset
8037 PRINTF this, format, LIST
8041 SEEK this, position, whence
8043 OPEN this, mode, LIST
8048 A class implementing a scalar should have the following methods:
8050 TIESCALAR classname, LIST
8056 Not all methods indicated above need be implemented. See L<perltie>,
8057 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8059 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8060 for you; you need to do that explicitly yourself. See L<DB_File>
8061 or the F<Config> module for interesting C<tie> implementations.
8063 For further details see L<perltie>, L<"tied VARIABLE">.
8068 =for Pod::Functions get a reference to the object underlying a tied variable
8070 Returns a reference to the object underlying VARIABLE (the same value
8071 that was originally returned by the C<tie> call that bound the variable
8072 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8078 =for Pod::Functions return number of seconds since 1970
8080 Returns the number of non-leap seconds since whatever time the system
8081 considers to be the epoch, suitable for feeding to C<gmtime> and
8082 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8083 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8084 1904 in the current local time zone for its epoch.
8086 For measuring time in better granularity than one second, use the
8087 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8088 if you have gettimeofday(2), you may be able to use the C<syscall>
8089 interface of Perl. See L<perlfaq8> for details.
8091 For date and time processing look at the many related modules on CPAN.
8092 For a comprehensive date and time representation look at the
8098 =for Pod::Functions return elapsed time for self and child processes
8100 Returns a four-element list giving the user and system times in
8101 seconds for this process and any exited children of this process.
8103 ($user,$system,$cuser,$csystem) = times;
8105 In scalar context, C<times> returns C<$user>.
8107 Children's times are only included for terminated children.
8109 Portability issues: L<perlport/times>.
8113 =for Pod::Functions transliterate a string
8115 The transliteration operator. Same as C<y///>. See
8116 L<perlop/"Quote and Quote-like Operators">.
8118 =item truncate FILEHANDLE,LENGTH
8121 =item truncate EXPR,LENGTH
8123 =for Pod::Functions shorten a file
8125 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8126 specified length. Raises an exception if truncate isn't implemented
8127 on your system. Returns true if successful, C<undef> on error.
8129 The behavior is undefined if LENGTH is greater than the length of the
8132 The position in the file of FILEHANDLE is left unchanged. You may want to
8133 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8135 Portability issues: L<perlport/truncate>.
8138 X<uc> X<uppercase> X<toupper>
8142 =for Pod::Functions return upper-case version of a string
8144 Returns an uppercased version of EXPR. This is the internal function
8145 implementing the C<\U> escape in double-quoted strings.
8146 It does not attempt to do titlecase mapping on initial letters. See
8147 L</ucfirst> for that.
8149 If EXPR is omitted, uses C<$_>.
8151 This function behaves the same way under various pragma, such as in a locale,
8155 X<ucfirst> X<uppercase>
8159 =for Pod::Functions return a string with just the next letter in upper case
8161 Returns the value of EXPR with the first character in uppercase
8162 (titlecase in Unicode). This is the internal function implementing
8163 the C<\u> escape in double-quoted strings.
8165 If EXPR is omitted, uses C<$_>.
8167 This function behaves the same way under various pragma, such as in a locale,
8175 =for Pod::Functions set file creation mode mask
8177 Sets the umask for the process to EXPR and returns the previous value.
8178 If EXPR is omitted, merely returns the current umask.
8180 The Unix permission C<rwxr-x---> is represented as three sets of three
8181 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8182 and isn't one of the digits). The C<umask> value is such a number
8183 representing disabled permissions bits. The permission (or "mode")
8184 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8185 even if you tell C<sysopen> to create a file with permissions C<0777>,
8186 if your umask is C<0022>, then the file will actually be created with
8187 permissions C<0755>. If your C<umask> were C<0027> (group can't
8188 write; others can't read, write, or execute), then passing
8189 C<sysopen> C<0666> would create a file with mode C<0640> (because
8190 C<0666 &~ 027> is C<0640>).
8192 Here's some advice: supply a creation mode of C<0666> for regular
8193 files (in C<sysopen>) and one of C<0777> for directories (in
8194 C<mkdir>) and executable files. This gives users the freedom of
8195 choice: if they want protected files, they might choose process umasks
8196 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8197 Programs should rarely if ever make policy decisions better left to
8198 the user. The exception to this is when writing files that should be
8199 kept private: mail files, web browser cookies, I<.rhosts> files, and
8202 If umask(2) is not implemented on your system and you are trying to
8203 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8204 raises an exception. If umask(2) is not implemented and you are
8205 not trying to restrict access for yourself, returns C<undef>.
8207 Remember that a umask is a number, usually given in octal; it is I<not> a
8208 string of octal digits. See also L</oct>, if all you have is a string.
8210 Portability issues: L<perlport/umask>.
8213 X<undef> X<undefine>
8217 =for Pod::Functions remove a variable or function definition
8219 Undefines the value of EXPR, which must be an lvalue. Use only on a
8220 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8221 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8222 will probably not do what you expect on most predefined variables or
8223 DBM list values, so don't do that; see L</delete>. Always returns the
8224 undefined value. You can omit the EXPR, in which case nothing is
8225 undefined, but you still get an undefined value that you could, for
8226 instance, return from a subroutine, assign to a variable, or pass as a
8227 parameter. Examples:
8230 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8234 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8235 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8236 select undef, undef, undef, 0.25;
8237 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8239 Note that this is a unary operator, not a list operator.
8242 X<unlink> X<delete> X<remove> X<rm> X<del>
8246 =for Pod::Functions remove one link to a file
8248 Deletes a list of files. On success, it returns the number of files
8249 it successfully deleted. On failure, it returns false and sets C<$!>
8252 my $unlinked = unlink 'a', 'b', 'c';
8254 unlink glob "*.bak";
8256 On error, C<unlink> will not tell you which files it could not remove.
8257 If you want to know which files you could not remove, try them one
8260 foreach my $file ( @goners ) {
8261 unlink $file or warn "Could not unlink $file: $!";
8264 Note: C<unlink> will not attempt to delete directories unless you are
8265 superuser and the B<-U> flag is supplied to Perl. Even if these
8266 conditions are met, be warned that unlinking a directory can inflict
8267 damage on your filesystem. Finally, using C<unlink> on directories is
8268 not supported on many operating systems. Use C<rmdir> instead.
8270 If LIST is omitted, C<unlink> uses C<$_>.
8272 =item unpack TEMPLATE,EXPR
8275 =item unpack TEMPLATE
8277 =for Pod::Functions convert binary structure into normal perl variables
8279 C<unpack> does the reverse of C<pack>: it takes a string
8280 and expands it out into a list of values.
8281 (In scalar context, it returns merely the first value produced.)
8283 If EXPR is omitted, unpacks the C<$_> string.
8284 See L<perlpacktut> for an introduction to this function.
8286 The string is broken into chunks described by the TEMPLATE. Each chunk
8287 is converted separately to a value. Typically, either the string is a result
8288 of C<pack>, or the characters of the string represent a C structure of some
8291 The TEMPLATE has the same format as in the C<pack> function.
8292 Here's a subroutine that does substring:
8295 my($what,$where,$howmuch) = @_;
8296 unpack("x$where a$howmuch", $what);
8301 sub ordinal { unpack("W",$_[0]); } # same as ord()
8303 In addition to fields allowed in pack(), you may prefix a field with
8304 a %<number> to indicate that
8305 you want a <number>-bit checksum of the items instead of the items
8306 themselves. Default is a 16-bit checksum. Checksum is calculated by
8307 summing numeric values of expanded values (for string fields the sum of
8308 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8310 For example, the following
8311 computes the same number as the System V sum program:
8315 unpack("%32W*",<>) % 65535;
8318 The following efficiently counts the number of set bits in a bit vector:
8320 $setbits = unpack("%32b*", $selectmask);
8322 The C<p> and C<P> formats should be used with care. Since Perl
8323 has no way of checking whether the value passed to C<unpack()>
8324 corresponds to a valid memory location, passing a pointer value that's
8325 not known to be valid is likely to have disastrous consequences.
8327 If there are more pack codes or if the repeat count of a field or a group
8328 is larger than what the remainder of the input string allows, the result
8329 is not well defined: the repeat count may be decreased, or
8330 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8331 If the input string is longer than one described by the TEMPLATE,
8332 the remainder of that input string is ignored.
8334 See L</pack> for more examples and notes.
8336 =item unshift ARRAY,LIST
8339 =item unshift EXPR,LIST
8341 =for Pod::Functions prepend more elements to the beginning of a list
8343 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8344 depending on how you look at it. Prepends list to the front of the
8345 array and returns the new number of elements in the array.
8347 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8349 Note the LIST is prepended whole, not one element at a time, so the
8350 prepended elements stay in the same order. Use C<reverse> to do the
8353 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
8354 a reference to an unblessed array. The argument will be dereferenced
8355 automatically. This aspect of C<unshift> is considered highly
8356 experimental. The exact behaviour may change in a future version of Perl.
8358 To avoid confusing would-be users of your code who are running earlier
8359 versions of Perl with mysterious syntax errors, put this sort of thing at
8360 the top of your file to signal that your code will work I<only> on Perls of
8363 use 5.014; # so push/pop/etc work on scalars (experimental)
8365 =item untie VARIABLE
8368 =for Pod::Functions break a tie binding to a variable
8370 Breaks the binding between a variable and a package.
8371 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8372 Has no effect if the variable is not tied.
8374 =item use Module VERSION LIST
8375 X<use> X<module> X<import>
8377 =item use Module VERSION
8379 =item use Module LIST
8385 =for Pod::Functions load in a module at compile time and import its namespace
8387 Imports some semantics into the current package from the named module,
8388 generally by aliasing certain subroutine or variable names into your
8389 package. It is exactly equivalent to
8391 BEGIN { require Module; Module->import( LIST ); }
8393 except that Module I<must> be a bareword.
8394 The importation can be made conditional; see L<if>.
8396 In the peculiar C<use VERSION> form, VERSION may be either a positive
8397 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8398 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8399 exception is raised if VERSION is greater than the version of the
8400 current Perl interpreter; Perl will not attempt to parse the rest of the
8401 file. Compare with L</require>, which can do a similar check at run time.
8402 Symmetrically, C<no VERSION> allows you to specify that you want a version
8403 of Perl older than the specified one.
8405 Specifying VERSION as a literal of the form v5.6.1 should generally be
8406 avoided, because it leads to misleading error messages under earlier
8407 versions of Perl (that is, prior to 5.6.0) that do not support this
8408 syntax. The equivalent numeric version should be used instead.
8410 use v5.6.1; # compile time version check
8412 use 5.006_001; # ditto; preferred for backwards compatibility
8414 This is often useful if you need to check the current Perl version before
8415 C<use>ing library modules that won't work with older versions of Perl.
8416 (We try not to do this more than we have to.)
8418 C<use VERSION> also enables all features available in the requested
8419 version as defined by the C<feature> pragma, disabling any features
8420 not in the requested version's feature bundle. See L<feature>.
8421 Similarly, if the specified Perl version is greater than or equal to
8422 5.11.0, strictures are enabled lexically as
8423 with C<use strict>. Any explicit use of
8424 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8425 before it. In both cases, the F<feature.pm> and F<strict.pm> files are
8426 not actually loaded.
8428 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8429 C<require> makes sure the module is loaded into memory if it hasn't been
8430 yet. The C<import> is not a builtin; it's just an ordinary static method
8431 call into the C<Module> package to tell the module to import the list of
8432 features back into the current package. The module can implement its
8433 C<import> method any way it likes, though most modules just choose to
8434 derive their C<import> method via inheritance from the C<Exporter> class that
8435 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8436 method can be found then the call is skipped, even if there is an AUTOLOAD
8439 If you do not want to call the package's C<import> method (for instance,
8440 to stop your namespace from being altered), explicitly supply the empty list:
8444 That is exactly equivalent to
8446 BEGIN { require Module }
8448 If the VERSION argument is present between Module and LIST, then the
8449 C<use> will call the VERSION method in class Module with the given
8450 version as an argument. The default VERSION method, inherited from
8451 the UNIVERSAL class, croaks if the given version is larger than the
8452 value of the variable C<$Module::VERSION>.
8454 Again, there is a distinction between omitting LIST (C<import> called
8455 with no arguments) and an explicit empty LIST C<()> (C<import> not
8456 called). Note that there is no comma after VERSION!
8458 Because this is a wide-open interface, pragmas (compiler directives)
8459 are also implemented this way. Currently implemented pragmas are:
8464 use sigtrap qw(SEGV BUS);
8465 use strict qw(subs vars refs);
8466 use subs qw(afunc blurfl);
8467 use warnings qw(all);
8468 use sort qw(stable _quicksort _mergesort);
8470 Some of these pseudo-modules import semantics into the current
8471 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8472 which import symbols into the current package (which are effective
8473 through the end of the file).
8475 Because C<use> takes effect at compile time, it doesn't respect the
8476 ordinary flow control of the code being compiled. In particular, putting
8477 a C<use> inside the false branch of a conditional doesn't prevent it
8478 from being processed. If a module or pragma only needs to be loaded
8479 conditionally, this can be done using the L<if> pragma:
8481 use if $] < 5.008, "utf8";
8482 use if WANT_WARNINGS, warnings => qw(all);
8484 There's a corresponding C<no> declaration that unimports meanings imported
8485 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8486 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8487 or no unimport method being found.
8493 Care should be taken when using the C<no VERSION> form of C<no>. It is
8494 I<only> meant to be used to assert that the running Perl is of a earlier
8495 version than its argument and I<not> to undo the feature-enabling side effects
8498 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8499 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8500 functionality from the command-line.
8505 =for Pod::Functions set a file's last access and modify times
8507 Changes the access and modification times on each file of a list of
8508 files. The first two elements of the list must be the NUMERIC access
8509 and modification times, in that order. Returns the number of files
8510 successfully changed. The inode change time of each file is set
8511 to the current time. For example, this code has the same effect as the
8512 Unix touch(1) command when the files I<already exist> and belong to
8513 the user running the program:
8516 $atime = $mtime = time;
8517 utime $atime, $mtime, @ARGV;
8519 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
8520 the utime(2) syscall from your C library is called with a null second
8521 argument. On most systems, this will set the file's access and
8522 modification times to the current time (i.e., equivalent to the example
8523 above) and will work even on files you don't own provided you have write
8527 utime(undef, undef, $file)
8528 || warn "couldn't touch $file: $!";
8531 Under NFS this will use the time of the NFS server, not the time of
8532 the local machine. If there is a time synchronization problem, the
8533 NFS server and local machine will have different times. The Unix
8534 touch(1) command will in fact normally use this form instead of the
8535 one shown in the first example.
8537 Passing only one of the first two elements as C<undef> is
8538 equivalent to passing a 0 and will not have the effect
8539 described when both are C<undef>. This also triggers an
8540 uninitialized warning.
8542 On systems that support futimes(2), you may pass filehandles among the
8543 files. On systems that don't support futimes(2), passing filehandles raises
8544 an exception. Filehandles must be passed as globs or glob references to be
8545 recognized; barewords are considered filenames.
8547 Portability issues: L<perlport/utime>.
8556 =for Pod::Functions return a list of the values in a hash
8558 In list context, returns a list consisting of all the values of the named
8559 hash. In Perl 5.12 or later only, will also return a list of the values of
8560 an array; prior to that release, attempting to use an array argument will
8561 produce a syntax error. In scalar context, returns the number of values.
8563 When called on a hash, the values are returned in an apparently random
8564 order. The actual random order is subject to change in future versions of
8565 Perl, but it is guaranteed to be the same order as either the C<keys> or
8566 C<each> function would produce on the same (unmodified) hash. Since Perl
8567 5.8.1 the ordering is different even between different runs of Perl for
8568 security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
8570 As a side effect, calling values() resets the HASH or ARRAY's internal
8571 iterator, see L</each>. (In particular, calling values() in void context
8572 resets the iterator with no other overhead. Apart from resetting the
8573 iterator, C<values @array> in list context is the same as plain C<@array>.
8574 (We recommend that you use void context C<keys @array> for this, but
8575 reasoned that taking C<values @array> out would require more
8576 documentation than leaving it in.)
8578 Note that the values are not copied, which means modifying them will
8579 modify the contents of the hash:
8581 for (values %hash) { s/foo/bar/g } # modifies %hash values
8582 for (@hash{keys %hash}) { s/foo/bar/g } # same
8584 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
8585 a reference to an unblessed hash or array. The argument will be
8586 dereferenced automatically. This aspect of C<values> is considered highly
8587 experimental. The exact behaviour may change in a future version of Perl.
8589 for (values $hashref) { ... }
8590 for (values $obj->get_arrayref) { ... }
8592 To avoid confusing would-be users of your code who are running earlier
8593 versions of Perl with mysterious syntax errors, put this sort of thing at
8594 the top of your file to signal that your code will work I<only> on Perls of
8597 use 5.012; # so keys/values/each work on arrays
8598 use 5.014; # so keys/values/each work on scalars (experimental)
8600 See also C<keys>, C<each>, and C<sort>.
8602 =item vec EXPR,OFFSET,BITS
8603 X<vec> X<bit> X<bit vector>
8605 =for Pod::Functions test or set particular bits in a string
8607 Treats the string in EXPR as a bit vector made up of elements of
8608 width BITS and returns the value of the element specified by OFFSET
8609 as an unsigned integer. BITS therefore specifies the number of bits
8610 that are reserved for each element in the bit vector. This must
8611 be a power of two from 1 to 32 (or 64, if your platform supports
8614 If BITS is 8, "elements" coincide with bytes of the input string.
8616 If BITS is 16 or more, bytes of the input string are grouped into chunks
8617 of size BITS/8, and each group is converted to a number as with
8618 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8619 for BITS==64). See L<"pack"> for details.
8621 If bits is 4 or less, the string is broken into bytes, then the bits
8622 of each byte are broken into 8/BITS groups. Bits of a byte are
8623 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8624 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8625 breaking the single input byte C<chr(0x36)> into two groups gives a list
8626 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8628 C<vec> may also be assigned to, in which case parentheses are needed
8629 to give the expression the correct precedence as in
8631 vec($image, $max_x * $x + $y, 8) = 3;
8633 If the selected element is outside the string, the value 0 is returned.
8634 If an element off the end of the string is written to, Perl will first
8635 extend the string with sufficiently many zero bytes. It is an error
8636 to try to write off the beginning of the string (i.e., negative OFFSET).
8638 If the string happens to be encoded as UTF-8 internally (and thus has
8639 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8640 internal byte string, not the conceptual character string, even if you
8641 only have characters with values less than 256.
8643 Strings created with C<vec> can also be manipulated with the logical
8644 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8645 vector operation is desired when both operands are strings.
8646 See L<perlop/"Bitwise String Operators">.
8648 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8649 The comments show the string after each step. Note that this code works
8650 in the same way on big-endian or little-endian machines.
8653 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8655 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8656 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8658 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8659 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8660 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8661 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8662 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8663 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8665 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8666 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8667 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8670 To transform a bit vector into a string or list of 0's and 1's, use these:
8672 $bits = unpack("b*", $vector);
8673 @bits = split(//, unpack("b*", $vector));
8675 If you know the exact length in bits, it can be used in place of the C<*>.
8677 Here is an example to illustrate how the bits actually fall in place:
8683 unpack("V",$_) 01234567890123456789012345678901
8684 ------------------------------------------------------------------
8689 for ($shift=0; $shift < $width; ++$shift) {
8690 for ($off=0; $off < 32/$width; ++$off) {
8691 $str = pack("B*", "0"x32);
8692 $bits = (1<<$shift);
8693 vec($str, $off, $width) = $bits;
8694 $res = unpack("b*",$str);
8695 $val = unpack("V", $str);
8702 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8703 $off, $width, $bits, $val, $res
8707 Regardless of the machine architecture on which it runs, the
8708 example above should print the following table:
8711 unpack("V",$_) 01234567890123456789012345678901
8712 ------------------------------------------------------------------
8713 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8714 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8715 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8716 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8717 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8718 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8719 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8720 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8721 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8722 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8723 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8724 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8725 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8726 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8727 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8728 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8729 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8730 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8731 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8732 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8733 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8734 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8735 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8736 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8737 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8738 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8739 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8740 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8741 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8742 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8743 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8744 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8745 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8746 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8747 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8748 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8749 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8750 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8751 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8752 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8753 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8754 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8755 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8756 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8757 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8758 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8759 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8760 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8761 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8762 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8763 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8764 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8765 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8766 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8767 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8768 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8769 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8770 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8771 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8772 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8773 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8774 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8775 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8776 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8777 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8778 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8779 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8780 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8781 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8782 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8783 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8784 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8785 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8786 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8787 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8788 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8789 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8790 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8791 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8792 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8793 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8794 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8795 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8796 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8797 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8798 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8799 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8800 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8801 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8802 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8803 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8804 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8805 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8806 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8807 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8808 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8809 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8810 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8811 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8812 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8813 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8814 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8815 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8816 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8817 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8818 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8819 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8820 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8821 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8822 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8823 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8824 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8825 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8826 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8827 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8828 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8829 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8830 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8831 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8832 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8833 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8834 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8835 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8836 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8837 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8838 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8839 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8840 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8845 =for Pod::Functions wait for any child process to die
8847 Behaves like wait(2) on your system: it waits for a child
8848 process to terminate and returns the pid of the deceased process, or
8849 C<-1> if there are no child processes. The status is returned in C<$?>
8850 and C<${^CHILD_ERROR_NATIVE}>.
8851 Note that a return value of C<-1> could mean that child processes are
8852 being automatically reaped, as described in L<perlipc>.
8854 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8855 child created by qx() or system(). See L<perlipc> for details.
8857 Portability issues: L<perlport/wait>.
8859 =item waitpid PID,FLAGS
8862 =for Pod::Functions wait for a particular child process to die
8864 Waits for a particular child process to terminate and returns the pid of
8865 the deceased process, or C<-1> if there is no such child process. On some
8866 systems, a value of 0 indicates that there are processes still running.
8867 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
8869 use POSIX ":sys_wait_h";
8872 $kid = waitpid(-1, WNOHANG);
8875 then you can do a non-blocking wait for all pending zombie processes.
8876 Non-blocking wait is available on machines supporting either the
8877 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
8878 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
8879 system call by remembering the status values of processes that have
8880 exited but have not been harvested by the Perl script yet.)
8882 Note that on some systems, a return value of C<-1> could mean that child
8883 processes are being automatically reaped. See L<perlipc> for details,
8884 and for other examples.
8886 Portability issues: L<perlport/waitpid>.
8889 X<wantarray> X<context>
8891 =for Pod::Functions get void vs scalar vs list context of current subroutine call
8893 Returns true if the context of the currently executing subroutine or
8894 C<eval> is looking for a list value. Returns false if the context is
8895 looking for a scalar. Returns the undefined value if the context is
8896 looking for no value (void context).
8898 return unless defined wantarray; # don't bother doing more
8899 my @a = complex_calculation();
8900 return wantarray ? @a : "@a";
8902 C<wantarray()>'s result is unspecified in the top level of a file,
8903 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
8904 in a C<DESTROY> method.
8906 This function should have been named wantlist() instead.
8909 X<warn> X<warning> X<STDERR>
8911 =for Pod::Functions print debugging info
8913 Prints the value of LIST to STDERR. If the last element of LIST does
8914 not end in a newline, it appends the same file/line number text as C<die>
8917 If the output is empty and C<$@> already contains a value (typically from a
8918 previous eval) that value is used after appending C<"\t...caught">
8919 to C<$@>. This is useful for staying almost, but not entirely similar to
8922 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
8924 No message is printed if there is a C<$SIG{__WARN__}> handler
8925 installed. It is the handler's responsibility to deal with the message
8926 as it sees fit (like, for instance, converting it into a C<die>). Most
8927 handlers must therefore arrange to actually display the
8928 warnings that they are not prepared to deal with, by calling C<warn>
8929 again in the handler. Note that this is quite safe and will not
8930 produce an endless loop, since C<__WARN__> hooks are not called from
8933 You will find this behavior is slightly different from that of
8934 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
8935 instead call C<die> again to change it).
8937 Using a C<__WARN__> handler provides a powerful way to silence all
8938 warnings (even the so-called mandatory ones). An example:
8940 # wipe out *all* compile-time warnings
8941 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
8943 my $foo = 20; # no warning about duplicate my $foo,
8944 # but hey, you asked for it!
8945 # no compile-time or run-time warnings before here
8948 # run-time warnings enabled after here
8949 warn "\$foo is alive and $foo!"; # does show up
8951 See L<perlvar> for details on setting C<%SIG> entries and for more
8952 examples. See the Carp module for other kinds of warnings using its
8953 carp() and cluck() functions.
8955 =item write FILEHANDLE
8962 =for Pod::Functions print a picture record
8964 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
8965 using the format associated with that file. By default the format for
8966 a file is the one having the same name as the filehandle, but the
8967 format for the current output channel (see the C<select> function) may be set
8968 explicitly by assigning the name of the format to the C<$~> variable.
8970 Top of form processing is handled automatically: if there is insufficient
8971 room on the current page for the formatted record, the page is advanced by
8972 writing a form feed, a special top-of-page format is used to format the new
8973 page header before the record is written. By default, the top-of-page
8974 format is the name of the filehandle with "_TOP" appended. This would be a
8975 problem with autovivified filehandles, but it may be dynamically set to the
8976 format of your choice by assigning the name to the C<$^> variable while
8977 that filehandle is selected. The number of lines remaining on the current
8978 page is in variable C<$->, which can be set to C<0> to force a new page.
8980 If FILEHANDLE is unspecified, output goes to the current default output
8981 channel, which starts out as STDOUT but may be changed by the
8982 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
8983 is evaluated and the resulting string is used to look up the name of
8984 the FILEHANDLE at run time. For more on formats, see L<perlform>.
8986 Note that write is I<not> the opposite of C<read>. Unfortunately.
8990 =for Pod::Functions transliterate a string
8992 The transliteration operator. Same as C<tr///>. See
8993 L<perlop/"Quote and Quote-like Operators">.
8997 =head2 Non-function Keywords by Cross-reference
9007 These keywords are documented in L<perldata/"Special Literals">.
9025 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9035 This method keyword is documented in L<perlobj/"Destructors">.
9069 These operators are documented in L<perlop>.
9079 This keyword is documented in L<perlsub/"Autoloading">.
9103 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9115 These flow-control keywords related to the experimental switch feature are
9116 documented in L<perlsyn/"Switch Statements"> .