4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate literal elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use parentheses, the simple but occasionally
34 surprising rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. Whitespace
37 between the function and left parenthesis doesn't count, so sometimes
38 you need to be careful:
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in scalar context by
59 returning the undefined value, and in list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls ("syscalls")
82 of the same name (like chown(2), fork(2), closedir(2), etc.) return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule include C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 Extension modules can also hook into the Perl parser to define new
90 kinds of keyword-headed expression. These may look like functions, but
91 may also look completely different. The syntax following the keyword
92 is defined entirely by the extension. If you are an implementor, see
93 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94 a module, see the module's documentation for details of the syntax that
97 =head2 Perl Functions by Category
100 Here are Perl's functions (including things that look like
101 functions, like some keywords and named operators)
102 arranged by category. Some functions appear in more
107 =item Functions for SCALARs or strings
108 X<scalar> X<string> X<character>
110 =for Pod::Functions =String
112 C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113 C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
114 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
116 C<fc> is available only if the C<"fc"> feature is enabled or if it is
117 prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
118 with a C<use v5.16> (or higher) declaration in the current scope.
121 =item Regular expressions and pattern matching
122 X<regular expression> X<regex> X<regexp>
124 =for Pod::Functions =Regexp
126 C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
128 =item Numeric functions
129 X<numeric> X<number> X<trigonometric> X<trigonometry>
131 =for Pod::Functions =Math
133 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134 C<sin>, C<sqrt>, C<srand>
136 =item Functions for real @ARRAYs
139 =for Pod::Functions =ARRAY
141 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
143 =item Functions for list data
146 =for Pod::Functions =LIST
148 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
150 =item Functions for real %HASHes
153 =for Pod::Functions =HASH
155 C<delete>, C<each>, C<exists>, C<keys>, C<values>
157 =item Input and output functions
158 X<I/O> X<input> X<output> X<dbm>
160 =for Pod::Functions =I/O
162 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
164 C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165 C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166 C<truncate>, C<warn>, C<write>
168 C<say> is available only if the C<"say"> feature is enabled or if it is
169 prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170 with a C<use v5.10> (or higher) declaration in the current scope.
172 =item Functions for fixed-length data or records
174 =for Pod::Functions =Binary
176 C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
179 =item Functions for filehandles, files, or directories
180 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
182 =for Pod::Functions =File
184 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
185 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
186 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187 C<umask>, C<unlink>, C<utime>
189 =item Keywords related to the control flow of your Perl program
192 =for Pod::Functions =Flow
194 C<break>, C<caller>, C<continue>, C<die>, C<do>,
195 C<dump>, C<eval>, C<evalbytes> C<exit>,
196 C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
197 C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
199 C<break> is available only if you enable the experimental C<"switch">
200 feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201 the C<default>, C<given> and C<when> statements, which are documented in
202 L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203 automatically with a C<use v5.10> (or higher) declaration in the current
204 scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205 feature, like the other keywords.
207 C<evalbytes> is only available with the C<"evalbytes"> feature (see
208 L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
209 with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
210 the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211 with a C<use v5.16> (or higher) declaration in the current scope.
213 =item Keywords related to scoping
215 =for Pod::Functions =Namespace
217 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
219 C<state> is available only if the C<"state"> feature is enabled or if it is
220 prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221 with a C<use v5.10> (or higher) declaration in the current scope.
223 =item Miscellaneous functions
225 =for Pod::Functions =Misc
227 C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
229 =item Functions for processes and process groups
230 X<process> X<pid> X<process id>
232 =for Pod::Functions =Process
234 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
235 C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236 C<setpriority>, C<sleep>, C<system>,
237 C<times>, C<wait>, C<waitpid>
239 =item Keywords related to Perl modules
242 =for Pod::Functions =Modules
244 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
246 =item Keywords related to classes and object-orientation
247 X<object> X<class> X<package>
249 =for Pod::Functions =Objects
251 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
254 =item Low-level socket functions
257 =for Pod::Functions =Socket
259 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
261 C<socket>, C<socketpair>
263 =item System V interprocess communication functions
264 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
266 =for Pod::Functions =SysV
268 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
271 =item Fetching user and group info
272 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
274 =for Pod::Functions =User
276 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278 C<getpwuid>, C<setgrent>, C<setpwent>
280 =item Fetching network info
281 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
283 =for Pod::Functions =Network
285 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289 C<setnetent>, C<setprotoent>, C<setservent>
291 =item Time-related functions
294 =for Pod::Functions =Time
296 C<gmtime>, C<localtime>, C<time>, C<times>
298 =item Non-function keywords
300 =for Pod::Functions =!Non-functions
302 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
303 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306 C<while>, C<x>, C<xor>
311 X<portability> X<Unix> X<portable>
313 Perl was born in Unix and can therefore access all common Unix
314 system calls. In non-Unix environments, the functionality of some
315 Unix system calls may not be available or details of the available
316 functionality may differ slightly. The Perl functions affected
319 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
322 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
324 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
325 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
328 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
329 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
332 C<shmwrite>, C<socket>, C<socketpair>,
333 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334 C<times>, C<truncate>, C<umask>, C<unlink>,
335 C<utime>, C<wait>, C<waitpid>
337 For more information about the portability of these functions, see
338 L<perlport> and other available platform-specific documentation.
340 =head2 Alphabetical Listing of Perl Functions
345 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
354 =for Pod::Functions a file test (-r, -x, etc)
356 A file test, where X is one of the letters listed below. This unary
357 operator takes one argument, either a filename, a filehandle, or a dirhandle,
358 and tests the associated file to see if something is true about it. If the
359 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
360 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
361 the undefined value if the file doesn't exist. Despite the funny
362 names, precedence is the same as any other named unary operator. The
363 operator may be any of:
365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
397 -C Same for inode change time (Unix, may differ for other
404 next unless -f $_; # ignore specials
408 Note that C<-s/a/b/> does not do a negated substitution. Saying
409 C<-exp($foo)> still works as expected, however: only single letters
410 following a minus are interpreted as file tests.
412 These operators are exempt from the "looks like a function rule" described
413 above. That is, an opening parenthesis after the operator does not affect
414 how much of the following code constitutes the argument. Put the opening
415 parentheses before the operator to separate it from code that follows (this
416 applies only to operators with higher precedence than unary operators, of
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
422 The interpretation of the file permission operators C<-r>, C<-R>,
423 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424 of the file and the uids and gids of the user. There may be other
425 reasons you can't actually read, write, or execute the file: for
426 example network filesystem access controls, ACLs (access control lists),
427 read-only filesystems, and unrecognized executable formats. Note
428 that the use of these six specific operators to verify if some operation
429 is possible is usually a mistake, because it may be open to race
432 Also note that, for the superuser on the local filesystems, the C<-r>,
433 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
434 if any execute bit is set in the mode. Scripts run by the superuser
435 may thus need to do a stat() to determine the actual mode of the file,
436 or temporarily set their effective uid to something else.
438 If you are using ACLs, there is a pragma called C<filetest> that may
439 produce more accurate results than the bare stat() mode bits.
440 When under C<use filetest 'access'> the above-mentioned filetests
441 test whether the permission can(not) be granted using the
442 access(2) family of system calls. Also note that the C<-x> and C<-X> may
443 under this pragma return true even if there are no execute permission
444 bits set (nor any extra execute permission ACLs). This strangeness is
445 due to the underlying system calls' definitions. Note also that, due to
446 the implementation of C<use filetest 'access'>, the C<_> special
447 filehandle won't cache the results of the file tests when this pragma is
448 in effect. Read the documentation for the C<filetest> pragma for more
451 The C<-T> and C<-B> switches work as follows. The first block or so of the
452 file is examined for odd characters such as strange control codes or
453 characters with the high bit set. If too many strange characters (>30%)
454 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
455 containing a zero byte in the first block is considered a binary file. If C<-T>
456 or C<-B> is used on a filehandle, the current IO buffer is examined
457 rather than the first block. Both C<-T> and C<-B> return true on an empty
458 file, or a file at EOF when testing a filehandle. Because you have to
459 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460 against the file first, as in C<next unless -f $file && -T $file>.
462 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
463 the special filehandle consisting of a solitary underline, then the stat
464 structure of the previous file test (or stat operator) is used, saving
465 a system call. (This doesn't work with C<-t>, and you need to remember
466 that lstat() and C<-l> leave values in the stat structure for the
467 symbolic link, not the real file.) (Also, if the stat buffer was filled by
468 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
471 print "Can do.\n" if -r $a || -w _ || -x _;
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
483 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
484 test operators, in a way that C<-f -w -x $file> is equivalent to
485 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
486 the return value of C<-f $file> as an argument to another filetest
487 operator, no special magic will happen.)
489 Portability issues: L<perlport/-X>.
491 To avoid confusing would-be users of your code with mysterious
492 syntax errors, put something like this at the top of your script:
494 use 5.010; # so filetest ops can stack
501 =for Pod::Functions absolute value function
503 Returns the absolute value of its argument.
504 If VALUE is omitted, uses C<$_>.
506 =item accept NEWSOCKET,GENERICSOCKET
509 =for Pod::Functions accept an incoming socket connect
511 Accepts an incoming socket connect, just as accept(2)
512 does. Returns the packed address if it succeeded, false otherwise.
513 See the example in L<perlipc/"Sockets: Client/Server Communication">.
515 On systems that support a close-on-exec flag on files, the flag will
516 be set for the newly opened file descriptor, as determined by the
517 value of $^F. See L<perlvar/$^F>.
526 =for Pod::Functions schedule a SIGALRM
528 Arranges to have a SIGALRM delivered to this process after the
529 specified number of wallclock seconds has elapsed. If SECONDS is not
530 specified, the value stored in C<$_> is used. (On some machines,
531 unfortunately, the elapsed time may be up to one second less or more
532 than you specified because of how seconds are counted, and process
533 scheduling may delay the delivery of the signal even further.)
535 Only one timer may be counting at once. Each call disables the
536 previous timer, and an argument of C<0> may be supplied to cancel the
537 previous timer without starting a new one. The returned value is the
538 amount of time remaining on the previous timer.
540 For delays of finer granularity than one second, the Time::HiRes module
541 (from CPAN, and starting from Perl 5.8 part of the standard
542 distribution) provides ualarm(). You may also use Perl's four-argument
543 version of select() leaving the first three arguments undefined, or you
544 might be able to use the C<syscall> interface to access setitimer(2) if
545 your system supports it. See L<perlfaq8> for details.
547 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548 C<sleep> may be internally implemented on your system with C<alarm>.
550 If you want to use C<alarm> to time out a system call you need to use an
551 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
552 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
553 restart system calls on some systems. Using C<eval>/C<die> always works,
554 modulo the caveats given in L<perlipc/"Signals">.
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
559 $nread = sysread SOCKET, $buffer, $size;
563 die unless $@ eq "alarm\n"; # propagate unexpected errors
570 For more information see L<perlipc>.
572 Portability issues: L<perlport/alarm>.
575 X<atan2> X<arctangent> X<tan> X<tangent>
577 =for Pod::Functions arctangent of Y/X in the range -PI to PI
579 Returns the arctangent of Y/X in the range -PI to PI.
581 For the tangent operation, you may use the C<Math::Trig::tan>
582 function, or use the familiar relation:
584 sub tan { sin($_[0]) / cos($_[0]) }
586 The return value for C<atan2(0,0)> is implementation-defined; consult
587 your atan2(3) manpage for more information.
589 Portability issues: L<perlport/atan2>.
591 =item bind SOCKET,NAME
594 =for Pod::Functions binds an address to a socket
596 Binds a network address to a socket, just as bind(2)
597 does. Returns true if it succeeded, false otherwise. NAME should be a
598 packed address of the appropriate type for the socket. See the examples in
599 L<perlipc/"Sockets: Client/Server Communication">.
601 =item binmode FILEHANDLE, LAYER
602 X<binmode> X<binary> X<text> X<DOS> X<Windows>
604 =item binmode FILEHANDLE
606 =for Pod::Functions prepare binary files for I/O
608 Arranges for FILEHANDLE to be read or written in "binary" or "text"
609 mode on systems where the run-time libraries distinguish between
610 binary and text files. If FILEHANDLE is an expression, the value is
611 taken as the name of the filehandle. Returns true on success,
612 otherwise it returns C<undef> and sets C<$!> (errno).
614 On some systems (in general, DOS- and Windows-based systems) binmode()
615 is necessary when you're not working with a text file. For the sake
616 of portability it is a good idea always to use it when appropriate,
617 and never to use it when it isn't appropriate. Also, people can
618 set their I/O to be by default UTF8-encoded Unicode, not bytes.
620 In other words: regardless of platform, use binmode() on binary data,
621 like images, for example.
623 If LAYER is present it is a single string, but may contain multiple
624 directives. The directives alter the behaviour of the filehandle.
625 When LAYER is present, using binmode on a text file makes sense.
627 If LAYER is omitted or specified as C<:raw> the filehandle is made
628 suitable for passing binary data. This includes turning off possible CRLF
629 translation and marking it as bytes (as opposed to Unicode characters).
630 Note that, despite what may be implied in I<"Programming Perl"> (the
631 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632 Other layers that would affect the binary nature of the stream are
633 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
634 PERLIO environment variable.
636 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
637 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638 establish default I/O layers. See L<open>.
640 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641 in "Programming Perl, 3rd Edition". However, since the publishing of this
642 book, by many known as "Camel III", the consensus of the naming of this
643 functionality has moved from "discipline" to "layer". All documentation
644 of this version of Perl therefore refers to "layers" rather than to
645 "disciplines". Now back to the regularly scheduled documentation...>
647 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
648 C<:utf8> just marks the data as UTF-8 without further checking,
649 while C<:encoding(UTF-8)> checks the data for actually being valid
650 UTF-8. More details can be found in L<PerlIO::encoding>.
652 In general, binmode() should be called after open() but before any I/O
653 is done on the filehandle. Calling binmode() normally flushes any
654 pending buffered output data (and perhaps pending input data) on the
655 handle. An exception to this is the C<:encoding> layer that
656 changes the default character encoding of the handle; see L</open>.
657 The C<:encoding> layer sometimes needs to be called in
658 mid-stream, and it doesn't flush the stream. The C<:encoding>
659 also implicitly pushes on top of itself the C<:utf8> layer because
660 internally Perl operates on UTF8-encoded Unicode characters.
662 The operating system, device drivers, C libraries, and Perl run-time
663 system all conspire to let the programmer treat a single
664 character (C<\n>) as the line terminator, irrespective of external
665 representation. On many operating systems, the native text file
666 representation matches the internal representation, but on some
667 platforms the external representation of C<\n> is made up of more than
670 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671 a single character to end each line in the external representation of text
672 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
673 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
674 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676 two characters C<\cM\cJ>. That means that if you don't use binmode() on
677 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679 output. This is what you want for text files, but it can be disastrous for
682 Another consequence of using binmode() (on some systems) is that
683 special end-of-file markers will be seen as part of the data stream.
684 For systems from the Microsoft family this means that, if your binary
685 data contain C<\cZ>, the I/O subsystem will regard it as the end of
686 the file, unless you use binmode().
688 binmode() is important not only for readline() and print() operations,
689 but also when using read(), seek(), sysread(), syswrite() and tell()
690 (see L<perlport> for more details). See the C<$/> and C<$\> variables
691 in L<perlvar> for how to manually set your input and output
692 line-termination sequences.
694 Portability issues: L<perlport/binmode>.
696 =item bless REF,CLASSNAME
701 =for Pod::Functions create an object
703 This function tells the thingy referenced by REF that it is now an object
704 in the CLASSNAME package. If CLASSNAME is omitted, the current package
705 is used. Because a C<bless> is often the last thing in a constructor,
706 it returns the reference for convenience. Always use the two-argument
707 version if a derived class might inherit the function doing the blessing.
708 See L<perlobj> for more about the blessing (and blessings) of objects.
710 Consider always blessing objects in CLASSNAMEs that are mixed case.
711 Namespaces with all lowercase names are considered reserved for
712 Perl pragmata. Builtin types have all uppercase names. To prevent
713 confusion, you may wish to avoid such package names as well. Make sure
714 that CLASSNAME is a true value.
716 See L<perlmod/"Perl Modules">.
720 =for Pod::Functions +switch break out of a C<given> block
722 Break out of a C<given()> block.
724 This keyword is enabled by the C<"switch"> feature; see L<feature> for
725 more information on C<"switch">. You can also access it by prefixing it
726 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
730 X<caller> X<call stack> X<stack> X<stack trace>
734 =for Pod::Functions get context of the current subroutine call
736 Returns the context of the current pure perl subroutine call. In scalar
737 context, 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. caller never returns XS subs and they are skipped. The next pure
740 perl sub will appear instead of the XS sub in caller's return values. In list
741 context, caller returns
744 ($package, $filename, $line) = caller;
746 With EXPR, it returns some extra information that the debugger uses to
747 print a stack trace. The value of EXPR indicates how many call frames
748 to go back before the current one.
751 ($package, $filename, $line, $subroutine, $hasargs,
754 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
757 Here $subroutine may be C<(eval)> if the frame is not a subroutine
758 call, but an C<eval>. In such a case additional elements $evaltext and
759 C<$is_require> are set: C<$is_require> is true if the frame is created by a
760 C<require> or C<use> statement, $evaltext contains the text of the
761 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
762 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
763 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
764 frame.) $subroutine may also be C<(unknown)> if this particular
765 subroutine happens to have been deleted from the symbol table.
766 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
767 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
768 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
769 corresponds to C<${^WARNING_BITS}>. The
770 C<$hints> and C<$bitmask> values are subject
771 to change between versions of Perl, and are not meant for external use.
773 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
774 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
775 of this hash, as they are the actual values stored in the optree.
777 Furthermore, when called from within the DB package in
778 list context, and with an argument, caller returns more
779 detailed information: it sets the list variable C<@DB::args> to be the
780 arguments with which the subroutine was invoked.
782 Be aware that the optimizer might have optimized call frames away before
783 C<caller> had a chance to get the information. That means that C<caller(N)>
784 might not return information about the call frame you expect it to, for
785 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
786 previous time C<caller> was called.
788 Be aware that setting C<@DB::args> is I<best effort>, intended for
789 debugging or generating backtraces, and should not be relied upon. In
790 particular, as C<@_> contains aliases to the caller's arguments, Perl does
791 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
792 subroutine makes to C<@_> or its contents, not the original values at call
793 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
794 elements, so under certain cases its elements may have become freed and
795 reallocated for other variables or temporary values. Finally, a side effect
796 of the current implementation is that the effects of C<shift @_> can
797 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
798 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
799 elements), so C<@DB::args> is actually a hybrid of the current state and
800 initial state of C<@_>. Buyer beware.
807 =item chdir FILEHANDLE
809 =item chdir DIRHANDLE
813 =for Pod::Functions change your current working directory
815 Changes the working directory to EXPR, if possible. If EXPR is omitted,
816 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
817 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
818 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
819 neither is set, C<chdir> does nothing. It returns true on success,
820 false otherwise. See the example under C<die>.
822 On systems that support fchdir(2), you may pass a filehandle or
823 directory handle as the argument. On systems that don't support fchdir(2),
824 passing handles raises an exception.
827 X<chmod> X<permission> X<mode>
829 =for Pod::Functions changes the permissions on a list of files
831 Changes the permissions of a list of files. The first element of the
832 list must be the numeric mode, which should probably be an octal
833 number, and which definitely should I<not> be a string of octal digits:
834 C<0644> is okay, but C<"0644"> is not. Returns the number of files
835 successfully changed. See also L</oct> if all you have is a string.
837 $cnt = chmod 0755, "foo", "bar";
838 chmod 0755, @executables;
839 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
841 $mode = "0644"; chmod oct($mode), "foo"; # this is better
842 $mode = 0644; chmod $mode, "foo"; # this is best
844 On systems that support fchmod(2), you may pass filehandles among the
845 files. On systems that don't support fchmod(2), passing filehandles raises
846 an exception. Filehandles must be passed as globs or glob references to be
847 recognized; barewords are considered filenames.
849 open(my $fh, "<", "foo");
850 my $perm = (stat $fh)[2] & 07777;
851 chmod($perm | 0600, $fh);
853 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
856 use Fcntl qw( :mode );
857 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
858 # Identical to the chmod 0755 of the example above.
860 Portability issues: L<perlport/chmod>.
863 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
869 =for Pod::Functions remove a trailing record separator from a string
871 This safer version of L</chop> removes any trailing string
872 that corresponds to the current value of C<$/> (also known as
873 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
874 number of characters removed from all its arguments. It's often used to
875 remove the newline from the end of an input record when you're worried
876 that the final record may be missing its newline. When in paragraph
877 mode (C<$/ = "">), it removes all trailing newlines from the string.
878 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
879 a reference to an integer or the like; see L<perlvar>) chomp() won't
881 If VARIABLE is omitted, it chomps C<$_>. Example:
884 chomp; # avoid \n on last field
889 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
890 resetting the C<each> iterator in the process.
892 You can actually chomp anything that's an lvalue, including an assignment:
895 chomp($answer = <STDIN>);
897 If you chomp a list, each element is chomped, and the total number of
898 characters removed is returned.
900 Note that parentheses are necessary when you're chomping anything
901 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
902 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
903 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
904 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
914 =for Pod::Functions remove the last character from a string
916 Chops off the last character of a string and returns the character
917 chopped. It is much more efficient than C<s/.$//s> because it neither
918 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
919 If VARIABLE is a hash, it chops the hash's values, but not its keys,
920 resetting the C<each> iterator in the process.
922 You can actually chop anything that's an lvalue, including an assignment.
924 If you chop a list, each element is chopped. Only the value of the
925 last C<chop> is returned.
927 Note that C<chop> returns the last character. To return all but the last
928 character, use C<substr($string, 0, -1)>.
933 X<chown> X<owner> X<user> X<group>
935 =for Pod::Functions change the ownership on a list of files
937 Changes the owner (and group) of a list of files. The first two
938 elements of the list must be the I<numeric> uid and gid, in that
939 order. A value of -1 in either position is interpreted by most
940 systems to leave that value unchanged. Returns the number of files
941 successfully changed.
943 $cnt = chown $uid, $gid, 'foo', 'bar';
944 chown $uid, $gid, @filenames;
946 On systems that support fchown(2), you may pass filehandles among the
947 files. On systems that don't support fchown(2), passing filehandles raises
948 an exception. Filehandles must be passed as globs or glob references to be
949 recognized; barewords are considered filenames.
951 Here's an example that looks up nonnumeric uids in the passwd file:
954 chomp($user = <STDIN>);
956 chomp($pattern = <STDIN>);
958 ($login,$pass,$uid,$gid) = getpwnam($user)
959 or die "$user not in passwd file";
961 @ary = glob($pattern); # expand filenames
962 chown $uid, $gid, @ary;
964 On most systems, you are not allowed to change the ownership of the
965 file unless you're the superuser, although you should be able to change
966 the group to any of your secondary groups. On insecure systems, these
967 restrictions may be relaxed, but this is not a portable assumption.
968 On POSIX systems, you can detect this condition this way:
970 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
971 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
973 Portability issues: L<perlport/chmod>.
976 X<chr> X<character> X<ASCII> X<Unicode>
980 =for Pod::Functions get character this number represents
982 Returns the character represented by that NUMBER in the character set.
983 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
984 chr(0x263a) is a Unicode smiley face.
986 Negative values give the Unicode replacement character (chr(0xfffd)),
987 except under the L<bytes> pragma, where the low eight bits of the value
988 (truncated to an integer) are used.
990 If NUMBER is omitted, uses C<$_>.
992 For the reverse, use L</ord>.
994 Note that characters from 128 to 255 (inclusive) are by default
995 internally not encoded as UTF-8 for backward compatibility reasons.
997 See L<perlunicode> for more about Unicode.
999 =item chroot FILENAME
1004 =for Pod::Functions make directory new root for path lookups
1006 This function works like the system call by the same name: it makes the
1007 named directory the new root directory for all further pathnames that
1008 begin with a C</> by your process and all its children. (It doesn't
1009 change your current working directory, which is unaffected.) For security
1010 reasons, this call is restricted to the superuser. If FILENAME is
1011 omitted, does a C<chroot> to C<$_>.
1013 Portability issues: L<perlport/chroot>.
1015 =item close FILEHANDLE
1020 =for Pod::Functions close file (or pipe or socket) handle
1022 Closes the file or pipe associated with the filehandle, flushes the IO
1023 buffers, and closes the system file descriptor. Returns true if those
1024 operations succeed and if no error was reported by any PerlIO
1025 layer. Closes the currently selected filehandle if the argument is
1028 You don't have to close FILEHANDLE if you are immediately going to do
1029 another C<open> on it, because C<open> closes it for you. (See
1030 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1031 counter (C<$.>), while the implicit close done by C<open> does not.
1033 If the filehandle came from a piped open, C<close> returns false if one of
1034 the other syscalls involved fails or if its program exits with non-zero
1035 status. If the only problem was that the program exited non-zero, C<$!>
1036 will be set to C<0>. Closing a pipe also waits for the process executing
1037 on the pipe to exit--in case you wish to look at the output of the pipe
1038 afterwards--and implicitly puts the exit status value of that command into
1039 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1041 If there are multiple threads running, C<close> on a filehandle from a
1042 piped open returns true without waiting for the child process to terminate,
1043 if the filehandle is still open in another thread.
1045 Closing the read end of a pipe before the process writing to it at the
1046 other end is done writing results in the writer receiving a SIGPIPE. If
1047 the other end can't handle that, be sure to read all the data before
1052 open(OUTPUT, '|sort >foo') # pipe to sort
1053 or die "Can't start sort: $!";
1054 #... # print stuff to output
1055 close OUTPUT # wait for sort to finish
1056 or warn $! ? "Error closing sort pipe: $!"
1057 : "Exit status $? from sort";
1058 open(INPUT, 'foo') # get sort's results
1059 or die "Can't open 'foo' for input: $!";
1061 FILEHANDLE may be an expression whose value can be used as an indirect
1062 filehandle, usually the real filehandle name or an autovivified handle.
1064 =item closedir DIRHANDLE
1067 =for Pod::Functions close directory handle
1069 Closes a directory opened by C<opendir> and returns the success of that
1072 =item connect SOCKET,NAME
1075 =for Pod::Functions connect to a remote socket
1077 Attempts to connect to a remote socket, just like connect(2).
1078 Returns true if it succeeded, false otherwise. NAME should be a
1079 packed address of the appropriate type for the socket. See the examples in
1080 L<perlipc/"Sockets: Client/Server Communication">.
1082 =item continue BLOCK
1087 =for Pod::Functions optional trailing block in a while or foreach
1089 When followed by a BLOCK, C<continue> is actually a
1090 flow control statement rather than a function. If
1091 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1092 C<foreach>), it is always executed just before the conditional is about to
1093 be evaluated again, just like the third part of a C<for> loop in C. Thus
1094 it can be used to increment a loop variable, even when the loop has been
1095 continued via the C<next> statement (which is similar to the C C<continue>
1098 C<last>, C<next>, or C<redo> may appear within a C<continue>
1099 block; C<last> and C<redo> behave as if they had been executed within
1100 the main block. So will C<next>, but since it will execute a C<continue>
1101 block, it may be more entertaining.
1104 ### redo always comes here
1107 ### next always comes here
1109 # then back the top to re-check EXPR
1111 ### last always comes here
1113 Omitting the C<continue> section is equivalent to using an
1114 empty one, logically enough, so C<next> goes directly back
1115 to check the condition at the top of the loop.
1117 When there is no BLOCK, C<continue> is a function that
1118 falls through the current C<when> or C<default> block instead of iterating
1119 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1120 In Perl 5.14 and earlier, this form of C<continue> was
1121 only available when the C<"switch"> feature was enabled.
1122 See L<feature> and L<perlsyn/"Switch Statements"> for more
1126 X<cos> X<cosine> X<acos> X<arccosine>
1130 =for Pod::Functions cosine function
1132 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1133 takes the cosine of C<$_>.
1135 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1136 function, or use this relation:
1138 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1140 =item crypt PLAINTEXT,SALT
1141 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1142 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1144 =for Pod::Functions one-way passwd-style encryption
1146 Creates a digest string exactly like the crypt(3) function in the C
1147 library (assuming that you actually have a version there that has not
1148 been extirpated as a potential munition).
1150 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1151 into a short string, called a digest, which is returned. The same
1152 PLAINTEXT and SALT will always return the same string, but there is no
1153 (known) way to get the original PLAINTEXT from the hash. Small
1154 changes in the PLAINTEXT or SALT will result in large changes in the
1157 There is no decrypt function. This function isn't all that useful for
1158 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1159 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1160 primarily used to check if two pieces of text are the same without
1161 having to transmit or store the text itself. An example is checking
1162 if a correct password is given. The digest of the password is stored,
1163 not the password itself. The user types in a password that is
1164 crypt()'d with the same salt as the stored digest. If the two digests
1165 match, the password is correct.
1167 When verifying an existing digest string you should use the digest as
1168 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1169 to create the digest is visible as part of the digest. This ensures
1170 crypt() will hash the new string with the same salt as the digest.
1171 This allows your code to work with the standard L<crypt|/crypt> and
1172 with more exotic implementations. In other words, assume
1173 nothing about the returned string itself nor about how many bytes
1176 Traditionally the result is a string of 13 bytes: two first bytes of
1177 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1178 the first eight bytes of PLAINTEXT mattered. But alternative
1179 hashing schemes (like MD5), higher level security schemes (like C2),
1180 and implementations on non-Unix platforms may produce different
1183 When choosing a new salt create a random two character string whose
1184 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1185 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1186 characters is just a recommendation; the characters allowed in
1187 the salt depend solely on your system's crypt library, and Perl can't
1188 restrict what salts C<crypt()> accepts.
1190 Here's an example that makes sure that whoever runs this program knows
1193 $pwd = (getpwuid($<))[1];
1195 system "stty -echo";
1197 chomp($word = <STDIN>);
1201 if (crypt($word, $pwd) ne $pwd) {
1207 Of course, typing in your own password to whoever asks you
1210 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1211 of data, not least of all because you can't get the information
1212 back. Look at the L<Digest> module for more robust algorithms.
1214 If using crypt() on a Unicode string (which I<potentially> has
1215 characters with codepoints above 255), Perl tries to make sense
1216 of the situation by trying to downgrade (a copy of)
1217 the string back to an eight-bit byte string before calling crypt()
1218 (on that copy). If that works, good. If not, crypt() dies with
1219 C<Wide character in crypt>.
1221 Portability issues: L<perlport/crypt>.
1226 =for Pod::Functions breaks binding on a tied dbm file
1228 [This function has been largely superseded by the C<untie> function.]
1230 Breaks the binding between a DBM file and a hash.
1232 Portability issues: L<perlport/dbmclose>.
1234 =item dbmopen HASH,DBNAME,MASK
1235 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1237 =for Pod::Functions create binding on a tied dbm file
1239 [This function has been largely superseded by the
1240 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1242 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1243 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1244 argument is I<not> a filehandle, even though it looks like one). DBNAME
1245 is the name of the database (without the F<.dir> or F<.pag> extension if
1246 any). If the database does not exist, it is created with protection
1247 specified by MASK (as modified by the C<umask>). To prevent creation of
1248 the database if it doesn't exist, you may specify a MODE
1249 of 0, and the function will return a false value if it
1250 can't find an existing database. If your system supports
1251 only the older DBM functions, you may make only one C<dbmopen> call in your
1252 program. In older versions of Perl, if your system had neither DBM nor
1253 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1256 If you don't have write access to the DBM file, you can only read hash
1257 variables, not set them. If you want to test whether you can write,
1258 either use file tests or try setting a dummy hash entry inside an C<eval>
1261 Note that functions such as C<keys> and C<values> may return huge lists
1262 when used on large DBM files. You may prefer to use the C<each>
1263 function to iterate over large DBM files. Example:
1265 # print out history file offsets
1266 dbmopen(%HIST,'/usr/lib/news/history',0666);
1267 while (($key,$val) = each %HIST) {
1268 print $key, ' = ', unpack('L',$val), "\n";
1272 See also L<AnyDBM_File> for a more general description of the pros and
1273 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1274 rich implementation.
1276 You can control which DBM library you use by loading that library
1277 before you call dbmopen():
1280 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1281 or die "Can't open netscape history file: $!";
1283 Portability issues: L<perlport/dbmopen>.
1286 X<defined> X<undef> X<undefined>
1290 =for Pod::Functions test whether a value, variable, or function is defined
1292 Returns a Boolean value telling whether EXPR has a value other than
1293 the undefined value C<undef>. If EXPR is not present, C<$_> is
1296 Many operations return C<undef> to indicate failure, end of file,
1297 system error, uninitialized variable, and other exceptional
1298 conditions. This function allows you to distinguish C<undef> from
1299 other values. (A simple Boolean test will not distinguish among
1300 C<undef>, zero, the empty string, and C<"0">, which are all equally
1301 false.) Note that since C<undef> is a valid scalar, its presence
1302 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1303 returns C<undef> when its argument is an empty array, I<or> when the
1304 element to return happens to be C<undef>.
1306 You may also use C<defined(&func)> to check whether subroutine C<&func>
1307 has ever been defined. The return value is unaffected by any forward
1308 declarations of C<&func>. A subroutine that is not defined
1309 may still be callable: its package may have an C<AUTOLOAD> method that
1310 makes it spring into existence the first time that it is called; see
1313 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1314 used to report whether memory for that aggregate had ever been
1315 allocated. This behavior may disappear in future versions of Perl.
1316 You should instead use a simple test for size:
1318 if (@an_array) { print "has array elements\n" }
1319 if (%a_hash) { print "has hash members\n" }
1321 When used on a hash element, it tells you whether the value is defined,
1322 not whether the key exists in the hash. Use L</exists> for the latter
1327 print if defined $switch{D};
1328 print "$val\n" while defined($val = pop(@ary));
1329 die "Can't readlink $sym: $!"
1330 unless defined($value = readlink $sym);
1331 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1332 $debugging = 0 unless defined $debugging;
1334 Note: Many folks tend to overuse C<defined> and are then surprised to
1335 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1336 defined values. For example, if you say
1340 The pattern match succeeds and C<$1> is defined, although it
1341 matched "nothing". It didn't really fail to match anything. Rather, it
1342 matched something that happened to be zero characters long. This is all
1343 very above-board and honest. When a function returns an undefined value,
1344 it's an admission that it couldn't give you an honest answer. So you
1345 should use C<defined> only when questioning the integrity of what
1346 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1349 See also L</undef>, L</exists>, L</ref>.
1354 =for Pod::Functions deletes a value from a hash
1356 Given an expression that specifies an element or slice of a hash, C<delete>
1357 deletes the specified elements from that hash so that exists() on that element
1358 no longer returns true. Setting a hash element to the undefined value does
1359 not remove its key, but deleting it does; see L</exists>.
1361 In list context, returns the value or values deleted, or the last such
1362 element in scalar context. The return list's length always matches that of
1363 the argument list: deleting non-existent elements returns the undefined value
1364 in their corresponding positions.
1366 delete() may also be used on arrays and array slices, but its behavior is less
1367 straightforward. Although exists() will return false for deleted entries,
1368 deleting array elements never changes indices of existing values; use shift()
1369 or splice() for that. However, if all deleted elements fall at the end of an
1370 array, the array's size shrinks to the position of the highest element that
1371 still tests true for exists(), or to 0 if none do.
1373 B<WARNING:> Calling delete on array values is deprecated and likely to
1374 be removed in a future version of Perl.
1376 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1377 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1378 or array may not necessarily return anything; it depends on the implementation
1379 of the C<tied> package's DELETE method, which may do whatever it pleases.
1381 The C<delete local EXPR> construct localizes the deletion to the current
1382 block at run time. Until the block exits, elements locally deleted
1383 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1384 of composite types">.
1386 %hash = (foo => 11, bar => 22, baz => 33);
1387 $scalar = delete $hash{foo}; # $scalar is 11
1388 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1389 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1391 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1393 foreach $key (keys %HASH) {
1397 foreach $index (0 .. $#ARRAY) {
1398 delete $ARRAY[$index];
1403 delete @HASH{keys %HASH};
1405 delete @ARRAY[0 .. $#ARRAY];
1407 But both are slower than assigning the empty list
1408 or undefining %HASH or @ARRAY, which is the customary
1409 way to empty out an aggregate:
1411 %HASH = (); # completely empty %HASH
1412 undef %HASH; # forget %HASH ever existed
1414 @ARRAY = (); # completely empty @ARRAY
1415 undef @ARRAY; # forget @ARRAY ever existed
1417 The EXPR can be arbitrarily complicated provided its
1418 final operation is an element or slice of an aggregate:
1420 delete $ref->[$x][$y]{$key};
1421 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1423 delete $ref->[$x][$y][$index];
1424 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1427 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1429 =for Pod::Functions raise an exception or bail out
1431 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1432 into C<$@> and the C<eval> is terminated with the undefined value.
1433 If the exception is outside of all enclosing C<eval>s, then the uncaught
1434 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1435 need to exit the process with a specific exit code, see L</exit>.
1437 Equivalent examples:
1439 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1440 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1442 If the last element of LIST does not end in a newline, the current
1443 script line number and input line number (if any) are also printed,
1444 and a newline is supplied. Note that the "input line number" (also
1445 known as "chunk") is subject to whatever notion of "line" happens to
1446 be currently in effect, and is also available as the special variable
1447 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1449 Hint: sometimes appending C<", stopped"> to your message will cause it
1450 to make better sense when the string C<"at foo line 123"> is appended.
1451 Suppose you are running script "canasta".
1453 die "/etc/games is no good";
1454 die "/etc/games is no good, stopped";
1456 produce, respectively
1458 /etc/games is no good at canasta line 123.
1459 /etc/games is no good, stopped at canasta line 123.
1461 If the output is empty and C<$@> already contains a value (typically from a
1462 previous eval) that value is reused after appending C<"\t...propagated">.
1463 This is useful for propagating exceptions:
1466 die unless $@ =~ /Expected exception/;
1468 If the output is empty and C<$@> contains an object reference that has a
1469 C<PROPAGATE> method, that method will be called with additional file
1470 and line number parameters. The return value replaces the value in
1471 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1474 If C<$@> is empty then the string C<"Died"> is used.
1476 If an uncaught exception results in interpreter exit, the exit code is
1477 determined from the values of C<$!> and C<$?> with this pseudocode:
1479 exit $! if $!; # errno
1480 exit $? >> 8 if $? >> 8; # child exit status
1481 exit 255; # last resort
1483 The intent is to squeeze as much possible information about the likely cause
1484 into the limited space of the system exit
1485 code. However, as C<$!> is the value
1486 of C's C<errno>, which can be set by any system call, this means that the value
1487 of the exit code used by C<die> can be non-predictable, so should not be relied
1488 upon, other than to be non-zero.
1490 You can also call C<die> with a reference argument, and if this is trapped
1491 within an C<eval>, C<$@> contains that reference. This permits more
1492 elaborate exception handling using objects that maintain arbitrary state
1493 about the exception. Such a scheme is sometimes preferable to matching
1494 particular string values of C<$@> with regular expressions. Because C<$@>
1495 is a global variable and C<eval> may be used within object implementations,
1496 be careful that analyzing the error object doesn't replace the reference in
1497 the global variable. It's easiest to make a local copy of the reference
1498 before any manipulations. Here's an example:
1500 use Scalar::Util "blessed";
1502 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1503 if (my $ev_err = $@) {
1504 if (blessed($ev_err)
1505 && $ev_err->isa("Some::Module::Exception")) {
1506 # handle Some::Module::Exception
1509 # handle all other possible exceptions
1513 Because Perl stringifies uncaught exception messages before display,
1514 you'll probably want to overload stringification operations on
1515 exception objects. See L<overload> for details about that.
1517 You can arrange for a callback to be run just before the C<die>
1518 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1519 handler is called with the error text and can change the error
1520 message, if it sees fit, by calling C<die> again. See
1521 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1522 L<"eval BLOCK"> for some examples. Although this feature was
1523 to be run only right before your program was to exit, this is not
1524 currently so: the C<$SIG{__DIE__}> hook is currently called
1525 even inside eval()ed blocks/strings! If one wants the hook to do
1526 nothing in such situations, put
1530 as the first line of the handler (see L<perlvar/$^S>). Because
1531 this promotes strange action at a distance, this counterintuitive
1532 behavior may be fixed in a future release.
1534 See also exit(), warn(), and the Carp module.
1539 =for Pod::Functions turn a BLOCK into a TERM
1541 Not really a function. Returns the value of the last command in the
1542 sequence of commands indicated by BLOCK. When modified by the C<while> or
1543 C<until> loop modifier, executes the BLOCK once before testing the loop
1544 condition. (On other statements the loop modifiers test the conditional
1547 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1548 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1549 See L<perlsyn> for alternative strategies.
1551 =item do SUBROUTINE(LIST)
1554 This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1560 Uses the value of EXPR as a filename and executes the contents of the
1561 file as a Perl script.
1569 except that it's more concise, runs no external processes, keeps track of
1571 filename for error messages, searches the C<@INC> directories, and updates
1572 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1573 these variables. It also differs in that code evaluated with C<do FILENAME>
1574 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1575 same, however, in that it does reparse the file every time you call it,
1576 so you probably don't want to do this inside a loop.
1578 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1579 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1580 and sets C<$!> to the error. Always check C<$@> first, as compilation
1581 could fail in a way that also sets C<$!>. If the file is successfully
1582 compiled, C<do> returns the value of the last expression evaluated.
1584 Inclusion of library modules is better done with the
1585 C<use> and C<require> operators, which also do automatic error checking
1586 and raise an exception if there's a problem.
1588 You might like to use C<do> to read in a program configuration
1589 file. Manual error checking can be done this way:
1591 # read in config files: system first, then user
1592 for $file ("/share/prog/defaults.rc",
1593 "$ENV{HOME}/.someprogrc")
1595 unless ($return = do $file) {
1596 warn "couldn't parse $file: $@" if $@;
1597 warn "couldn't do $file: $!" unless defined $return;
1598 warn "couldn't run $file" unless $return;
1603 X<dump> X<core> X<undump>
1609 =for Pod::Functions create an immediate core dump
1611 This function causes an immediate core dump. See also the B<-u>
1612 command-line switch in L<perlrun>, which does the same thing.
1613 Primarily this is so that you can use the B<undump> program (not
1614 supplied) to turn your core dump into an executable binary after
1615 having initialized all your variables at the beginning of the
1616 program. When the new binary is executed it will begin by executing
1617 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1618 Think of it as a goto with an intervening core dump and reincarnation.
1619 If C<LABEL> is omitted, restarts the program from the top. The
1620 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1621 computed at run time, being otherwise identical to C<dump LABEL>.
1623 B<WARNING>: Any files opened at the time of the dump will I<not>
1624 be open any more when the program is reincarnated, with possible
1625 resulting confusion by Perl.
1627 This function is now largely obsolete, mostly because it's very hard to
1628 convert a core file into an executable. That's why you should now invoke
1629 it as C<CORE::dump()>, if you don't want to be warned against a possible
1632 Unlike most named operators, this has the same precedence as assignment.
1633 It is also exempt from the looks-like-a-function rule, so
1634 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1637 Portability issues: L<perlport/dump>.
1640 X<each> X<hash, iterator>
1647 =for Pod::Functions retrieve the next key/value pair from a hash
1649 When called on a hash in list context, returns a 2-element list
1650 consisting of the key and value for the next element of a hash. In Perl
1651 5.12 and later only, it will also return the index and value for the next
1652 element of an array so that you can iterate over it; older Perls consider
1653 this a syntax error. When called in scalar context, returns only the key
1654 (not the value) in a hash, or the index in an array.
1656 Hash entries are returned in an apparently random order. The actual random
1657 order is specific to a given hash; the exact same series of operations
1658 on two hashes may result in a different order for each hash. Any insertion
1659 into the hash may change the order, as will any deletion, with the exception
1660 that the most recent key returned by C<each> or C<keys> may be deleted
1661 without changing the order. So long as a given hash is unmodified you may
1662 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1663 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1664 details on why hash order is randomized. Aside from the guarantees
1665 provided here the exact details of Perl's hash algorithm and the hash
1666 traversal order are subject to change in any release of Perl.
1668 After C<each> has returned all entries from the hash or array, the next
1669 call to C<each> returns the empty list in list context and C<undef> in
1670 scalar context; the next call following I<that> one restarts iteration.
1671 Each hash or array has its own internal iterator, accessed by C<each>,
1672 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1673 reached the end as just described; it can be explicitly reset by calling
1674 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1675 elements while iterating over it, entries may be skipped or duplicated--so
1676 don't do that. Exception: In the current implementation, it is always safe
1677 to delete the item most recently returned by C<each()>, so the following
1678 code works properly:
1680 while (($key, $value) = each %hash) {
1682 delete $hash{$key}; # This is safe
1685 This prints out your environment like the printenv(1) program,
1686 but in a different order:
1688 while (($key,$value) = each %ENV) {
1689 print "$key=$value\n";
1692 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1693 reference to an unblessed hash or array. The argument will be dereferenced
1694 automatically. This aspect of C<each> is considered highly experimental.
1695 The exact behaviour may change in a future version of Perl.
1697 while (($key,$value) = each $hashref) { ... }
1699 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1700 which will set C<$_> on every iteration.
1703 print "$_=$ENV{$_}\n";
1706 To avoid confusing would-be users of your code who are running earlier
1707 versions of Perl with mysterious syntax errors, put this sort of thing at
1708 the top of your file to signal that your code will work I<only> on Perls of
1711 use 5.012; # so keys/values/each work on arrays
1712 use 5.014; # so keys/values/each work on scalars (experimental)
1713 use 5.018; # so each assigns to $_ in a lone while test
1715 See also C<keys>, C<values>, and C<sort>.
1717 =item eof FILEHANDLE
1726 =for Pod::Functions test a filehandle for its end
1728 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1729 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1730 gives the real filehandle. (Note that this function actually
1731 reads a character and then C<ungetc>s it, so isn't useful in an
1732 interactive context.) Do not read from a terminal file (or call
1733 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1734 as terminals may lose the end-of-file condition if you do.
1736 An C<eof> without an argument uses the last file read. Using C<eof()>
1737 with empty parentheses is different. It refers to the pseudo file
1738 formed from the files listed on the command line and accessed via the
1739 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1740 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1741 used will cause C<@ARGV> to be examined to determine if input is
1742 available. Similarly, an C<eof()> after C<< <> >> has returned
1743 end-of-file will assume you are processing another C<@ARGV> list,
1744 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1745 see L<perlop/"I/O Operators">.
1747 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1748 detect the end of each file, whereas C<eof()> will detect the end
1749 of the very last file only. Examples:
1751 # reset line numbering on each input file
1753 next if /^\s*#/; # skip comments
1756 close ARGV if eof; # Not eof()!
1759 # insert dashes just before last line of last file
1761 if (eof()) { # check for end of last file
1762 print "--------------\n";
1765 last if eof(); # needed if we're reading from a terminal
1768 Practical hint: you almost never need to use C<eof> in Perl, because the
1769 input operators typically return C<undef> when they run out of data or
1773 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1774 X<error, handling> X<exception, handling>
1780 =for Pod::Functions catch exceptions or compile and run code
1782 In the first form, often referred to as a "string eval", the return
1783 value of EXPR is parsed and executed as if it
1784 were a little Perl program. The value of the expression (which is itself
1785 determined within scalar context) is first parsed, and if there were no
1786 errors, executed as a block within the lexical context of the current Perl
1787 program. This means, that in particular, any outer lexical variables are
1788 visible to it, and any package variable settings or subroutine and format
1789 definitions remain afterwards.
1791 Note that the value is parsed every time the C<eval> executes.
1792 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1793 delay parsing and subsequent execution of the text of EXPR until run time.
1795 If the C<unicode_eval> feature is enabled (which is the default under a
1796 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1797 characters, so C<use utf8> declarations have no effect, and source filters
1798 are forbidden. In the absence of the C<unicode_eval> feature, the string
1799 will sometimes be treated as characters and sometimes as bytes, depending
1800 on the internal encoding, and source filters activated within the C<eval>
1801 exhibit the erratic, but historical, behaviour of affecting some outer file
1802 scope that is still compiling. See also the L</evalbytes> keyword, which
1803 always treats its input as a byte stream and works properly with source
1804 filters, and the L<feature> pragma.
1806 Problems can arise if the string expands a scalar containing a floating
1807 point number. That scalar can expand to letters, such as C<"NaN"> or
1808 C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1809 point character may be something other than a dot (such as a comma).
1810 None of these are likely to parse as you are likely expecting.
1812 In the second form, the code within the BLOCK is parsed only once--at the
1813 same time the code surrounding the C<eval> itself was parsed--and executed
1814 within the context of the current Perl program. This form is typically
1815 used to trap exceptions more efficiently than the first (see below), while
1816 also providing the benefit of checking the code within BLOCK at compile
1819 The final semicolon, if any, may be omitted from the value of EXPR or within
1822 In both forms, the value returned is the value of the last expression
1823 evaluated inside the mini-program; a return statement may be also used, just
1824 as with subroutines. The expression providing the return value is evaluated
1825 in void, scalar, or list context, depending on the context of the C<eval>
1826 itself. See L</wantarray> for more on how the evaluation context can be
1829 If there is a syntax error or runtime error, or a C<die> statement is
1830 executed, C<eval> returns C<undef> in scalar context
1831 or an empty list in list context, and C<$@> is set to the error
1832 message. (Prior to 5.16, a bug caused C<undef> to be returned
1833 in list context for syntax errors, but not for runtime errors.)
1834 If there was no error, C<$@> is set to the empty string. A
1835 control flow operator like C<last> or C<goto> can bypass the setting of
1836 C<$@>. Beware that using C<eval> neither silences Perl from printing
1837 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1838 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1839 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1840 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1842 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1843 determining whether a particular feature (such as C<socket> or C<symlink>)
1844 is implemented. It is also Perl's exception-trapping mechanism, where
1845 the die operator is used to raise exceptions.
1847 If you want to trap errors when loading an XS module, some problems with
1848 the binary interface (such as Perl version skew) may be fatal even with
1849 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1851 If the code to be executed doesn't vary, you may use the eval-BLOCK
1852 form to trap run-time errors without incurring the penalty of
1853 recompiling each time. The error, if any, is still returned in C<$@>.
1856 # make divide-by-zero nonfatal
1857 eval { $answer = $a / $b; }; warn $@ if $@;
1859 # same thing, but less efficient
1860 eval '$answer = $a / $b'; warn $@ if $@;
1862 # a compile-time error
1863 eval { $answer = }; # WRONG
1866 eval '$answer ='; # sets $@
1868 Using the C<eval{}> form as an exception trap in libraries does have some
1869 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1870 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1871 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1872 as this example shows:
1874 # a private exception trap for divide-by-zero
1875 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1878 This is especially significant, given that C<__DIE__> hooks can call
1879 C<die> again, which has the effect of changing their error messages:
1881 # __DIE__ hooks may modify error messages
1883 local $SIG{'__DIE__'} =
1884 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1885 eval { die "foo lives here" };
1886 print $@ if $@; # prints "bar lives here"
1889 Because this promotes action at a distance, this counterintuitive behavior
1890 may be fixed in a future release.
1892 With an C<eval>, you should be especially careful to remember what's
1893 being looked at when:
1899 eval { $x }; # CASE 4
1901 eval "\$$x++"; # CASE 5
1904 Cases 1 and 2 above behave identically: they run the code contained in
1905 the variable $x. (Although case 2 has misleading double quotes making
1906 the reader wonder what else might be happening (nothing is).) Cases 3
1907 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1908 does nothing but return the value of $x. (Case 4 is preferred for
1909 purely visual reasons, but it also has the advantage of compiling at
1910 compile-time instead of at run-time.) Case 5 is a place where
1911 normally you I<would> like to use double quotes, except that in this
1912 particular situation, you can just use symbolic references instead, as
1915 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1916 of localized variables, which means that for your code to run on older
1917 versions, a temporary is required if you want to mask some but not all
1920 # alter $@ on nefarious repugnancy only
1924 local $@; # protect existing $@
1925 eval { test_repugnancy() };
1926 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1927 $@ =~ /nefarious/ and $e = $@;
1929 die $e if defined $e
1932 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1933 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1935 An C<eval ''> executed within a subroutine defined
1936 in the C<DB> package doesn't see the usual
1937 surrounding lexical scope, but rather the scope of the first non-DB piece
1938 of code that called it. You don't normally need to worry about this unless
1939 you are writing a Perl debugger.
1941 =item evalbytes EXPR
1946 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1948 This function is like L</eval> with a string argument, except it always
1949 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1950 string containing characters whose ordinal value exceeds 255 results in an
1951 error. Source filters activated within the evaluated code apply to the
1954 This function is only available under the C<evalbytes> feature, a
1955 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1956 L<feature> for more information.
1961 =item exec PROGRAM LIST
1963 =for Pod::Functions abandon this program to run another
1965 The C<exec> function executes a system command I<and never returns>;
1966 use C<system> instead of C<exec> if you want it to return. It fails and
1967 returns false only if the command does not exist I<and> it is executed
1968 directly instead of via your system's command shell (see below).
1970 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1971 warns you if C<exec> is called in void context and if there is a following
1972 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1973 you always do that, right?). If you I<really> want to follow an C<exec>
1974 with some other statement, you can use one of these styles to avoid the warning:
1976 exec ('foo') or print STDERR "couldn't exec foo: $!";
1977 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1979 If there is more than one argument in LIST, or if LIST is an array
1980 with more than one value, calls execvp(3) with the arguments in LIST.
1981 If there is only one scalar argument or an array with one element in it,
1982 the argument is checked for shell metacharacters, and if there are any,
1983 the entire argument is passed to the system's command shell for parsing
1984 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1985 If there are no shell metacharacters in the argument, it is split into
1986 words and passed directly to C<execvp>, which is more efficient.
1989 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1990 exec "sort $outfile | uniq";
1992 If you don't really want to execute the first argument, but want to lie
1993 to the program you are executing about its own name, you can specify
1994 the program you actually want to run as an "indirect object" (without a
1995 comma) in front of the LIST. (This always forces interpretation of the
1996 LIST as a multivalued list, even if there is only a single scalar in
1999 $shell = '/bin/csh';
2000 exec $shell '-sh'; # pretend it's a login shell
2004 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2006 When the arguments get executed via the system shell, results are
2007 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2010 Using an indirect object with C<exec> or C<system> is also more
2011 secure. This usage (which also works fine with system()) forces
2012 interpretation of the arguments as a multivalued list, even if the
2013 list had just one argument. That way you're safe from the shell
2014 expanding wildcards or splitting up words with whitespace in them.
2016 @args = ( "echo surprise" );
2018 exec @args; # subject to shell escapes
2020 exec { $args[0] } @args; # safe even with one-arg list
2022 The first version, the one without the indirect object, ran the I<echo>
2023 program, passing it C<"surprise"> an argument. The second version didn't;
2024 it tried to run a program named I<"echo surprise">, didn't find it, and set
2025 C<$?> to a non-zero value indicating failure.
2027 Perl attempts to flush all files opened for output before the exec,
2028 but this may not be supported on some platforms (see L<perlport>).
2029 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2030 call the C<autoflush()> method of C<IO::Handle> on any open handles
2031 to avoid lost output.
2033 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2034 C<DESTROY> methods on your objects.
2036 Portability issues: L<perlport/exec>.
2039 X<exists> X<autovivification>
2041 =for Pod::Functions test whether a hash key is present
2043 Given an expression that specifies an element of a hash, returns true if the
2044 specified element in the hash has ever been initialized, even if the
2045 corresponding value is undefined.
2047 print "Exists\n" if exists $hash{$key};
2048 print "Defined\n" if defined $hash{$key};
2049 print "True\n" if $hash{$key};
2051 exists may also be called on array elements, but its behavior is much less
2052 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2053 that calling exists on array values is deprecated and likely to be removed in
2054 a future version of Perl.
2056 print "Exists\n" if exists $array[$index];
2057 print "Defined\n" if defined $array[$index];
2058 print "True\n" if $array[$index];
2060 A hash or array element can be true only if it's defined and defined only if
2061 it exists, but the reverse doesn't necessarily hold true.
2063 Given an expression that specifies the name of a subroutine,
2064 returns true if the specified subroutine has ever been declared, even
2065 if it is undefined. Mentioning a subroutine name for exists or defined
2066 does not count as declaring it. Note that a subroutine that does not
2067 exist may still be callable: its package may have an C<AUTOLOAD>
2068 method that makes it spring into existence the first time that it is
2069 called; see L<perlsub>.
2071 print "Exists\n" if exists &subroutine;
2072 print "Defined\n" if defined &subroutine;
2074 Note that the EXPR can be arbitrarily complicated as long as the final
2075 operation is a hash or array key lookup or subroutine name:
2077 if (exists $ref->{A}->{B}->{$key}) { }
2078 if (exists $hash{A}{B}{$key}) { }
2080 if (exists $ref->{A}->{B}->[$ix]) { }
2081 if (exists $hash{A}{B}[$ix]) { }
2083 if (exists &{$ref->{A}{B}{$key}}) { }
2085 Although the most deeply nested array or hash element will not spring into
2086 existence just because its existence was tested, any intervening ones will.
2087 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2088 into existence due to the existence test for the $key element above.
2089 This happens anywhere the arrow operator is used, including even here:
2092 if (exists $ref->{"Some key"}) { }
2093 print $ref; # prints HASH(0x80d3d5c)
2095 This surprising autovivification in what does not at first--or even
2096 second--glance appear to be an lvalue context may be fixed in a future
2099 Use of a subroutine call, rather than a subroutine name, as an argument
2100 to exists() is an error.
2103 exists &sub(); # Error
2106 X<exit> X<terminate> X<abort>
2110 =for Pod::Functions terminate this program
2112 Evaluates EXPR and exits immediately with that value. Example:
2115 exit 0 if $ans =~ /^[Xx]/;
2117 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2118 universally recognized values for EXPR are C<0> for success and C<1>
2119 for error; other values are subject to interpretation depending on the
2120 environment in which the Perl program is running. For example, exiting
2121 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2122 the mailer to return the item undelivered, but that's not true everywhere.
2124 Don't use C<exit> to abort a subroutine if there's any chance that
2125 someone might want to trap whatever error happened. Use C<die> instead,
2126 which can be trapped by an C<eval>.
2128 The exit() function does not always exit immediately. It calls any
2129 defined C<END> routines first, but these C<END> routines may not
2130 themselves abort the exit. Likewise any object destructors that need to
2131 be called are called before the real exit. C<END> routines and destructors
2132 can change the exit status by modifying C<$?>. If this is a problem, you
2133 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2134 See L<perlmod> for details.
2136 Portability issues: L<perlport/exit>.
2139 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2143 =for Pod::Functions raise I<e> to a power
2145 Returns I<e> (the natural logarithm base) to the power of EXPR.
2146 If EXPR is omitted, gives C<exp($_)>.
2149 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2153 =for Pod::Functions +fc return casefolded version of a string
2155 Returns the casefolded version of EXPR. This is the internal function
2156 implementing the C<\F> escape in double-quoted strings.
2158 Casefolding is the process of mapping strings to a form where case
2159 differences are erased; comparing two strings in their casefolded
2160 form is effectively a way of asking if two strings are equal,
2163 Roughly, if you ever found yourself writing this
2165 lc($this) eq lc($that) # Wrong!
2167 uc($this) eq uc($that) # Also wrong!
2169 $this =~ /^\Q$that\E\z/i # Right!
2173 fc($this) eq fc($that)
2175 And get the correct results.
2177 Perl only implements the full form of casefolding,
2178 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2179 L<Unicode::UCD/prop_invmap()>.
2180 For further information on casefolding, refer to
2181 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2182 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2183 available at L<http://www.unicode.org/versions/latest/>, as well as the
2184 Case Charts available at L<http://www.unicode.org/charts/case/>.
2186 If EXPR is omitted, uses C<$_>.
2188 This function behaves the same way under various pragma, such as within
2189 S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2190 exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2191 scope of S<C<use locale>>. The foldcase of this character would
2192 normally be C<"ss">, but as explained in the L</lc> section, case
2193 changes that cross the 255/256 boundary are problematic under locales,
2194 and are hence prohibited. Therefore, this function under locale returns
2195 instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2196 LONG S. Since that character itself folds to C<"s">, the string of two
2197 of them together should be equivalent to a single U+1E9E when foldcased.
2199 While the Unicode Standard defines two additional forms of casefolding,
2200 one for Turkic languages and one that never maps one character into multiple
2201 characters, these are not provided by the Perl core; However, the CPAN module
2202 C<Unicode::Casing> may be used to provide an implementation.
2204 This keyword is available only when the C<"fc"> feature is enabled,
2205 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2206 include a C<use v5.16> or later to the current scope.
2208 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2211 =for Pod::Functions file control system call
2213 Implements the fcntl(2) function. You'll probably have to say
2217 first to get the correct constant definitions. Argument processing and
2218 value returned work just like C<ioctl> below.
2222 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2223 or die "can't fcntl F_GETFL: $!";
2225 You don't have to check for C<defined> on the return from C<fcntl>.
2226 Like C<ioctl>, it maps a C<0> return from the system call into
2227 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2228 in numeric context. It is also exempt from the normal B<-w> warnings
2229 on improper numeric conversions.
2231 Note that C<fcntl> raises an exception if used on a machine that
2232 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2233 manpage to learn what functions are available on your system.
2235 Here's an example of setting a filehandle named C<REMOTE> to be
2236 non-blocking at the system level. You'll have to negotiate C<$|>
2237 on your own, though.
2239 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2241 $flags = fcntl(REMOTE, F_GETFL, 0)
2242 or die "Can't get flags for the socket: $!\n";
2244 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2245 or die "Can't set flags for the socket: $!\n";
2247 Portability issues: L<perlport/fcntl>.
2252 =for Pod::Functions the name of the current source file
2254 A special token that returns the name of the file in which it occurs.
2256 =item fileno FILEHANDLE
2259 =for Pod::Functions return file descriptor from filehandle
2261 Returns the file descriptor for a filehandle, or undefined if the
2262 filehandle is not open. If there is no real file descriptor at the OS
2263 level, as can happen with filehandles connected to memory objects via
2264 C<open> with a reference for the third argument, -1 is returned.
2266 This is mainly useful for constructing
2267 bitmaps for C<select> and low-level POSIX tty-handling operations.
2268 If FILEHANDLE is an expression, the value is taken as an indirect
2269 filehandle, generally its name.
2271 You can use this to find out whether two handles refer to the
2272 same underlying descriptor:
2274 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
2275 print "THIS and THAT are dups\n";
2276 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2277 print "THIS and THAT have different " .
2278 "underlying file descriptors\n";
2280 print "At least one of THIS and THAT does " .
2281 "not have a real file descriptor\n";
2284 =item flock FILEHANDLE,OPERATION
2285 X<flock> X<lock> X<locking>
2287 =for Pod::Functions lock an entire file with an advisory lock
2289 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2290 for success, false on failure. Produces a fatal error if used on a
2291 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2292 C<flock> is Perl's portable file-locking interface, although it locks
2293 entire files only, not records.
2295 Two potentially non-obvious but traditional C<flock> semantics are
2296 that it waits indefinitely until the lock is granted, and that its locks
2297 are B<merely advisory>. Such discretionary locks are more flexible, but
2298 offer fewer guarantees. This means that programs that do not also use
2299 C<flock> may modify files locked with C<flock>. See L<perlport>,
2300 your port's specific documentation, and your system-specific local manpages
2301 for details. It's best to assume traditional behavior if you're writing
2302 portable programs. (But if you're not, you should as always feel perfectly
2303 free to write for your own system's idiosyncrasies (sometimes called
2304 "features"). Slavish adherence to portability concerns shouldn't get
2305 in the way of your getting your job done.)
2307 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2308 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2309 you can use the symbolic names if you import them from the L<Fcntl> module,
2310 either individually, or as a group using the C<:flock> tag. LOCK_SH
2311 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2312 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2313 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2314 waiting for the lock; check the return status to see if you got it.
2316 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2317 before locking or unlocking it.
2319 Note that the emulation built with lockf(3) doesn't provide shared
2320 locks, and it requires that FILEHANDLE be open with write intent. These
2321 are the semantics that lockf(3) implements. Most if not all systems
2322 implement lockf(3) in terms of fcntl(2) locking, though, so the
2323 differing semantics shouldn't bite too many people.
2325 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2326 be open with read intent to use LOCK_SH and requires that it be open
2327 with write intent to use LOCK_EX.
2329 Note also that some versions of C<flock> cannot lock things over the
2330 network; you would need to use the more system-specific C<fcntl> for
2331 that. If you like you can force Perl to ignore your system's flock(2)
2332 function, and so provide its own fcntl(2)-based emulation, by passing
2333 the switch C<-Ud_flock> to the F<Configure> program when you configure
2334 and build a new Perl.
2336 Here's a mailbox appender for BSD systems.
2338 # import LOCK_* and SEEK_END constants
2339 use Fcntl qw(:flock SEEK_END);
2343 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2345 # and, in case someone appended while we were waiting...
2346 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2351 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2354 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2355 or die "Can't open mailbox: $!";
2358 print $mbox $msg,"\n\n";
2361 On systems that support a real flock(2), locks are inherited across fork()
2362 calls, whereas those that must resort to the more capricious fcntl(2)
2363 function lose their locks, making it seriously harder to write servers.
2365 See also L<DB_File> for other flock() examples.
2367 Portability issues: L<perlport/flock>.
2370 X<fork> X<child> X<parent>
2372 =for Pod::Functions create a new process just like this one
2374 Does a fork(2) system call to create a new process running the
2375 same program at the same point. It returns the child pid to the
2376 parent process, C<0> to the child process, or C<undef> if the fork is
2377 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2378 are shared, while everything else is copied. On most systems supporting
2379 fork(), great care has gone into making it extremely efficient (for
2380 example, using copy-on-write technology on data pages), making it the
2381 dominant paradigm for multitasking over the last few decades.
2383 Perl attempts to flush all files opened for
2384 output before forking the child process, but this may not be supported
2385 on some platforms (see L<perlport>). To be safe, you may need to set
2386 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2387 C<IO::Handle> on any open handles to avoid duplicate output.
2389 If you C<fork> without ever waiting on your children, you will
2390 accumulate zombies. On some systems, you can avoid this by setting
2391 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2392 forking and reaping moribund children.
2394 Note that if your forked child inherits system file descriptors like
2395 STDIN and STDOUT that are actually connected by a pipe or socket, even
2396 if you exit, then the remote server (such as, say, a CGI script or a
2397 backgrounded job launched from a remote shell) won't think you're done.
2398 You should reopen those to F</dev/null> if it's any issue.
2400 On some platforms such as Windows, where the fork() system call is not available,
2401 Perl can be built to emulate fork() in the Perl interpreter.
2402 The emulation is designed, at the level of the Perl program,
2403 to be as compatible as possible with the "Unix" fork().
2404 However it has limitations that have to be considered in code intended to be portable.
2405 See L<perlfork> for more details.
2407 Portability issues: L<perlport/fork>.
2412 =for Pod::Functions declare a picture format with use by the write() function
2414 Declare a picture format for use by the C<write> function. For
2418 Test: @<<<<<<<< @||||| @>>>>>
2419 $str, $%, '$' . int($num)
2423 $num = $cost/$quantity;
2427 See L<perlform> for many details and examples.
2429 =item formline PICTURE,LIST
2432 =for Pod::Functions internal function used for formats
2434 This is an internal function used by C<format>s, though you may call it,
2435 too. It formats (see L<perlform>) a list of values according to the
2436 contents of PICTURE, placing the output into the format output
2437 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2438 Eventually, when a C<write> is done, the contents of
2439 C<$^A> are written to some filehandle. You could also read C<$^A>
2440 and then set C<$^A> back to C<"">. Note that a format typically
2441 does one C<formline> per line of form, but the C<formline> function itself
2442 doesn't care how many newlines are embedded in the PICTURE. This means
2443 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2444 You may therefore need to use multiple formlines to implement a single
2445 record format, just like the C<format> compiler.
2447 Be careful if you put double quotes around the picture, because an C<@>
2448 character may be taken to mean the beginning of an array name.
2449 C<formline> always returns true. See L<perlform> for other examples.
2451 If you are trying to use this instead of C<write> to capture the output,
2452 you may find it easier to open a filehandle to a scalar
2453 (C<< open $fh, ">", \$output >>) and write to that instead.
2455 =item getc FILEHANDLE
2456 X<getc> X<getchar> X<character> X<file, read>
2460 =for Pod::Functions get the next character from the filehandle
2462 Returns the next character from the input file attached to FILEHANDLE,
2463 or the undefined value at end of file or if there was an error (in
2464 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2465 STDIN. This is not particularly efficient. However, it cannot be
2466 used by itself to fetch single characters without waiting for the user
2467 to hit enter. For that, try something more like:
2470 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2473 system "stty", '-icanon', 'eol', "\001";
2479 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2482 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2486 Determination of whether $BSD_STYLE should be set
2487 is left as an exercise to the reader.
2489 The C<POSIX::getattr> function can do this more portably on
2490 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2491 module from your nearest L<CPAN|http://www.cpan.org> site.
2494 X<getlogin> X<login>
2496 =for Pod::Functions return who logged in at this tty
2498 This implements the C library function of the same name, which on most
2499 systems returns the current login from F</etc/utmp>, if any. If it
2500 returns the empty string, use C<getpwuid>.
2502 $login = getlogin || getpwuid($<) || "Kilroy";
2504 Do not consider C<getlogin> for authentication: it is not as
2505 secure as C<getpwuid>.
2507 Portability issues: L<perlport/getlogin>.
2509 =item getpeername SOCKET
2510 X<getpeername> X<peer>
2512 =for Pod::Functions find the other end of a socket connection
2514 Returns the packed sockaddr address of the other end of the SOCKET
2518 $hersockaddr = getpeername(SOCK);
2519 ($port, $iaddr) = sockaddr_in($hersockaddr);
2520 $herhostname = gethostbyaddr($iaddr, AF_INET);
2521 $herstraddr = inet_ntoa($iaddr);
2526 =for Pod::Functions get process group
2528 Returns the current process group for the specified PID. Use
2529 a PID of C<0> to get the current process group for the
2530 current process. Will raise an exception if used on a machine that
2531 doesn't implement getpgrp(2). If PID is omitted, returns the process
2532 group of the current process. Note that the POSIX version of C<getpgrp>
2533 does not accept a PID argument, so only C<PID==0> is truly portable.
2535 Portability issues: L<perlport/getpgrp>.
2538 X<getppid> X<parent> X<pid>
2540 =for Pod::Functions get parent process ID
2542 Returns the process id of the parent process.
2544 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2545 around non-POSIX thread semantics the minority of Linux systems (and
2546 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2547 has since been removed. See the documentation for L<$$|perlvar/$$> for
2550 Portability issues: L<perlport/getppid>.
2552 =item getpriority WHICH,WHO
2553 X<getpriority> X<priority> X<nice>
2555 =for Pod::Functions get current nice value
2557 Returns the current priority for a process, a process group, or a user.
2558 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2559 machine that doesn't implement getpriority(2).
2561 Portability issues: L<perlport/getpriority>.
2564 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2565 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2566 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2567 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2568 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2569 X<endnetent> X<endprotoent> X<endservent>
2571 =for Pod::Functions get passwd record given user login name
2575 =for Pod::Functions get group record given group name
2577 =item gethostbyname NAME
2579 =for Pod::Functions get host record given name
2581 =item getnetbyname NAME
2583 =for Pod::Functions get networks record given name
2585 =item getprotobyname NAME
2587 =for Pod::Functions get protocol record given name
2591 =for Pod::Functions get passwd record given user ID
2595 =for Pod::Functions get group record given group user ID
2597 =item getservbyname NAME,PROTO
2599 =for Pod::Functions get services record given its name
2601 =item gethostbyaddr ADDR,ADDRTYPE
2603 =for Pod::Functions get host record given its address
2605 =item getnetbyaddr ADDR,ADDRTYPE
2607 =for Pod::Functions get network record given its address
2609 =item getprotobynumber NUMBER
2611 =for Pod::Functions get protocol record numeric protocol
2613 =item getservbyport PORT,PROTO
2615 =for Pod::Functions get services record given numeric port
2619 =for Pod::Functions get next passwd record
2623 =for Pod::Functions get next group record
2627 =for Pod::Functions get next hosts record
2631 =for Pod::Functions get next networks record
2635 =for Pod::Functions get next protocols record
2639 =for Pod::Functions get next services record
2643 =for Pod::Functions prepare passwd file for use
2647 =for Pod::Functions prepare group file for use
2649 =item sethostent STAYOPEN
2651 =for Pod::Functions prepare hosts file for use
2653 =item setnetent STAYOPEN
2655 =for Pod::Functions prepare networks file for use
2657 =item setprotoent STAYOPEN
2659 =for Pod::Functions prepare protocols file for use
2661 =item setservent STAYOPEN
2663 =for Pod::Functions prepare services file for use
2667 =for Pod::Functions be done using passwd file
2671 =for Pod::Functions be done using group file
2675 =for Pod::Functions be done using hosts file
2679 =for Pod::Functions be done using networks file
2683 =for Pod::Functions be done using protocols file
2687 =for Pod::Functions be done using services file
2689 These routines are the same as their counterparts in the
2690 system C library. In list context, the return values from the
2691 various get routines are as follows:
2693 ($name,$passwd,$uid,$gid,
2694 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2695 ($name,$passwd,$gid,$members) = getgr*
2696 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2697 ($name,$aliases,$addrtype,$net) = getnet*
2698 ($name,$aliases,$proto) = getproto*
2699 ($name,$aliases,$port,$proto) = getserv*
2701 (If the entry doesn't exist you get an empty list.)
2703 The exact meaning of the $gcos field varies but it usually contains
2704 the real name of the user (as opposed to the login name) and other
2705 information pertaining to the user. Beware, however, that in many
2706 system users are able to change this information and therefore it
2707 cannot be trusted and therefore the $gcos is tainted (see
2708 L<perlsec>). The $passwd and $shell, user's encrypted password and
2709 login shell, are also tainted, for the same reason.
2711 In scalar context, you get the name, unless the function was a
2712 lookup by name, in which case you get the other thing, whatever it is.
2713 (If the entry doesn't exist you get the undefined value.) For example:
2715 $uid = getpwnam($name);
2716 $name = getpwuid($num);
2718 $gid = getgrnam($name);
2719 $name = getgrgid($num);
2723 In I<getpw*()> the fields $quota, $comment, and $expire are special
2724 in that they are unsupported on many systems. If the
2725 $quota is unsupported, it is an empty scalar. If it is supported, it
2726 usually encodes the disk quota. If the $comment field is unsupported,
2727 it is an empty scalar. If it is supported it usually encodes some
2728 administrative comment about the user. In some systems the $quota
2729 field may be $change or $age, fields that have to do with password
2730 aging. In some systems the $comment field may be $class. The $expire
2731 field, if present, encodes the expiration period of the account or the
2732 password. For the availability and the exact meaning of these fields
2733 in your system, please consult getpwnam(3) and your system's
2734 F<pwd.h> file. You can also find out from within Perl what your
2735 $quota and $comment fields mean and whether you have the $expire field
2736 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2737 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2738 files are supported only if your vendor has implemented them in the
2739 intuitive fashion that calling the regular C library routines gets the
2740 shadow versions if you're running under privilege or if there exists
2741 the shadow(3) functions as found in System V (this includes Solaris
2742 and Linux). Those systems that implement a proprietary shadow password
2743 facility are unlikely to be supported.
2745 The $members value returned by I<getgr*()> is a space-separated list of
2746 the login names of the members of the group.
2748 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2749 C, it will be returned to you via C<$?> if the function call fails. The
2750 C<@addrs> value returned by a successful call is a list of raw
2751 addresses returned by the corresponding library call. In the
2752 Internet domain, each address is four bytes long; you can unpack it
2753 by saying something like:
2755 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2757 The Socket library makes this slightly easier:
2760 $iaddr = inet_aton("127.1"); # or whatever address
2761 $name = gethostbyaddr($iaddr, AF_INET);
2763 # or going the other way
2764 $straddr = inet_ntoa($iaddr);
2766 In the opposite way, to resolve a hostname to the IP address
2770 $packed_ip = gethostbyname("www.perl.org");
2771 if (defined $packed_ip) {
2772 $ip_address = inet_ntoa($packed_ip);
2775 Make sure C<gethostbyname()> is called in SCALAR context and that
2776 its return value is checked for definedness.
2778 The C<getprotobynumber> function, even though it only takes one argument,
2779 has the precedence of a list operator, so beware:
2781 getprotobynumber $number eq 'icmp' # WRONG
2782 getprotobynumber($number eq 'icmp') # actually means this
2783 getprotobynumber($number) eq 'icmp' # better this way
2785 If you get tired of remembering which element of the return list
2786 contains which return value, by-name interfaces are provided
2787 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2788 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2789 and C<User::grent>. These override the normal built-ins, supplying
2790 versions that return objects with the appropriate names
2791 for each field. For example:
2795 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2797 Even though it looks as though they're the same method calls (uid),
2798 they aren't, because a C<File::stat> object is different from
2799 a C<User::pwent> object.
2801 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2803 =item getsockname SOCKET
2806 =for Pod::Functions retrieve the sockaddr for a given socket
2808 Returns the packed sockaddr address of this end of the SOCKET connection,
2809 in case you don't know the address because you have several different
2810 IPs that the connection might have come in on.
2813 $mysockaddr = getsockname(SOCK);
2814 ($port, $myaddr) = sockaddr_in($mysockaddr);
2815 printf "Connect to %s [%s]\n",
2816 scalar gethostbyaddr($myaddr, AF_INET),
2819 =item getsockopt SOCKET,LEVEL,OPTNAME
2822 =for Pod::Functions get socket options on a given socket
2824 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2825 Options may exist at multiple protocol levels depending on the socket
2826 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2827 C<Socket> module) will exist. To query options at another level the
2828 protocol number of the appropriate protocol controlling the option
2829 should be supplied. For example, to indicate that an option is to be
2830 interpreted by the TCP protocol, LEVEL should be set to the protocol
2831 number of TCP, which you can get using C<getprotobyname>.
2833 The function returns a packed string representing the requested socket
2834 option, or C<undef> on error, with the reason for the error placed in
2835 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2836 consult getsockopt(2) for details. A common case is that the option is an
2837 integer, in which case the result is a packed integer, which you can decode
2838 using C<unpack> with the C<i> (or C<I>) format.
2840 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2842 use Socket qw(:all);
2844 defined(my $tcp = getprotobyname("tcp"))
2845 or die "Could not determine the protocol number for tcp";
2846 # my $tcp = IPPROTO_TCP; # Alternative
2847 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2848 or die "getsockopt TCP_NODELAY: $!";
2849 my $nodelay = unpack("I", $packed);
2850 print "Nagle's algorithm is turned ",
2851 $nodelay ? "off\n" : "on\n";
2853 Portability issues: L<perlport/getsockopt>.
2856 X<glob> X<wildcard> X<filename, expansion> X<expand>
2860 =for Pod::Functions expand filenames using wildcards
2862 In list context, returns a (possibly empty) list of filename expansions on
2863 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2864 scalar context, glob iterates through such filename expansions, returning
2865 undef when the list is exhausted. This is the internal function
2866 implementing the C<< <*.c> >> operator, but you can use it directly. If
2867 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2868 more detail in L<perlop/"I/O Operators">.
2870 Note that C<glob> splits its arguments on whitespace and treats
2871 each segment as separate pattern. As such, C<glob("*.c *.h")>
2872 matches all files with a F<.c> or F<.h> extension. The expression
2873 C<glob(".* *")> matches all files in the current working directory.
2874 If you want to glob filenames that might contain whitespace, you'll
2875 have to use extra quotes around the spacey filename to protect it.
2876 For example, to glob filenames that have an C<e> followed by a space
2877 followed by an C<f>, use either of:
2879 @spacies = <"*e f*">;
2880 @spacies = glob '"*e f*"';
2881 @spacies = glob q("*e f*");
2883 If you had to get a variable through, you could do this:
2885 @spacies = glob "'*${var}e f*'";
2886 @spacies = glob qq("*${var}e f*");
2888 If non-empty braces are the only wildcard characters used in the
2889 C<glob>, no filenames are matched, but potentially many strings
2890 are returned. For example, this produces nine strings, one for
2891 each pairing of fruits and colors:
2893 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2895 This operator is implemented using the standard
2896 C<File::Glob> extension. See L<File::Glob> for details, including
2897 C<bsd_glob> which does not treat whitespace as a pattern separator.
2899 Portability issues: L<perlport/glob>.
2902 X<gmtime> X<UTC> X<Greenwich>
2906 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2908 Works just like L</localtime> but the returned values are
2909 localized for the standard Greenwich time zone.
2911 Note: When called in list context, $isdst, the last value
2912 returned by gmtime, is always C<0>. There is no
2913 Daylight Saving Time in GMT.
2915 Portability issues: L<perlport/gmtime>.
2918 X<goto> X<jump> X<jmp>
2924 =for Pod::Functions create spaghetti code
2926 The C<goto LABEL> form finds the statement labeled with LABEL and
2927 resumes execution there. It can't be used to get out of a block or
2928 subroutine given to C<sort>. It can be used to go almost anywhere
2929 else within the dynamic scope, including out of subroutines, but it's
2930 usually better to use some other construct such as C<last> or C<die>.
2931 The author of Perl has never felt the need to use this form of C<goto>
2932 (in Perl, that is; C is another matter). (The difference is that C
2933 does not offer named loops combined with loop control. Perl does, and
2934 this replaces most structured uses of C<goto> in other languages.)
2936 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
2937 a label name. If it evaluates to a code reference, it will be handled
2938 like C<goto &NAME>, below. This is especially useful for implementing
2939 tail recursion via C<goto __SUB__>.
2941 If the expression evaluates to a label name, its scope will be resolved
2942 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2943 necessarily recommended if you're optimizing for maintainability:
2945 goto ("FOO", "BAR", "GLARCH")[$i];
2947 As shown in this example, C<goto EXPR> is exempt from the "looks like a
2948 function" rule. A pair of parentheses following it does not (necessarily)
2949 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2950 Also, unlike most named operators, this has the same precedence as
2953 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
2954 deprecated and will issue a warning. Even then, it may not be used to
2955 go into any construct that requires initialization, such as a
2956 subroutine or a C<foreach> loop. It also can't be used to go into a
2957 construct that is optimized away.
2959 The C<goto &NAME> form is quite different from the other forms of
2960 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2961 doesn't have the stigma associated with other gotos. Instead, it
2962 exits the current subroutine (losing any changes set by local()) and
2963 immediately calls in its place the named subroutine using the current
2964 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2965 load another subroutine and then pretend that the other subroutine had
2966 been called in the first place (except that any modifications to C<@_>
2967 in the current subroutine are propagated to the other subroutine.)
2968 After the C<goto>, not even C<caller> will be able to tell that this
2969 routine was called first.
2971 NAME needn't be the name of a subroutine; it can be a scalar variable
2972 containing a code reference or a block that evaluates to a code
2975 =item grep BLOCK LIST
2978 =item grep EXPR,LIST
2980 =for Pod::Functions locate elements in a list test true against a given criterion
2982 This is similar in spirit to, but not the same as, grep(1) and its
2983 relatives. In particular, it is not limited to using regular expressions.
2985 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2986 C<$_> to each element) and returns the list value consisting of those
2987 elements for which the expression evaluated to true. In scalar
2988 context, returns the number of times the expression was true.
2990 @foo = grep(!/^#/, @bar); # weed out comments
2994 @foo = grep {!/^#/} @bar; # weed out comments
2996 Note that C<$_> is an alias to the list value, so it can be used to
2997 modify the elements of the LIST. While this is useful and supported,
2998 it can cause bizarre results if the elements of LIST are not variables.
2999 Similarly, grep returns aliases into the original list, much as a for
3000 loop's index variable aliases the list elements. That is, modifying an
3001 element of a list returned by grep (for example, in a C<foreach>, C<map>
3002 or another C<grep>) actually modifies the element in the original list.
3003 This is usually something to be avoided when writing clear code.
3005 If C<$_> is lexical in the scope where the C<grep> appears (because it has
3006 been declared with the deprecated C<my $_> construct)
3007 then, in addition to being locally aliased to
3008 the list elements, C<$_> keeps being lexical inside the block; i.e., it
3009 can't be seen from the outside, avoiding any potential side-effects.
3011 See also L</map> for a list composed of the results of the BLOCK or EXPR.
3014 X<hex> X<hexadecimal>
3018 =for Pod::Functions convert a string to a hexadecimal number
3020 Interprets EXPR as a hex string and returns the corresponding value.
3021 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
3022 L</oct>.) If EXPR is omitted, uses C<$_>.
3024 print hex '0xAf'; # prints '175'
3025 print hex 'aF'; # same
3027 Hex strings may only represent integers. Strings that would cause
3028 integer overflow trigger a warning. Leading whitespace is not stripped,
3029 unlike oct(). To present something as hex, look into L</printf>,
3030 L</sprintf>, and L</unpack>.
3035 =for Pod::Functions patch a module's namespace into your own
3037 There is no builtin C<import> function. It is just an ordinary
3038 method (subroutine) defined (or inherited) by modules that wish to export
3039 names to another module. The C<use> function calls the C<import> method
3040 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3042 =item index STR,SUBSTR,POSITION
3043 X<index> X<indexOf> X<InStr>
3045 =item index STR,SUBSTR
3047 =for Pod::Functions find a substring within a string
3049 The index function searches for one string within another, but without
3050 the wildcard-like behavior of a full regular-expression pattern match.
3051 It returns the position of the first occurrence of SUBSTR in STR at
3052 or after POSITION. If POSITION is omitted, starts searching from the
3053 beginning of the string. POSITION before the beginning of the string
3054 or after its end is treated as if it were the beginning or the end,
3055 respectively. POSITION and the return value are based at zero.
3056 If the substring is not found, C<index> returns -1.
3059 X<int> X<integer> X<truncate> X<trunc> X<floor>
3063 =for Pod::Functions get the integer portion of a number
3065 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3066 You should not use this function for rounding: one because it truncates
3067 towards C<0>, and two because machine representations of floating-point
3068 numbers can sometimes produce counterintuitive results. For example,
3069 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3070 because it's really more like -268.99999999999994315658 instead. Usually,
3071 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3072 functions will serve you better than will int().
3074 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3077 =for Pod::Functions system-dependent device control system call
3079 Implements the ioctl(2) function. You'll probably first have to say
3081 require "sys/ioctl.ph"; # probably in
3082 # $Config{archlib}/sys/ioctl.ph
3084 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3085 exist or doesn't have the correct definitions you'll have to roll your
3086 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3087 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3088 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3089 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3090 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3091 has no string value but does have a numeric value, that value will be
3092 passed rather than a pointer to the string value. To guarantee this to be
3093 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3094 functions may be needed to manipulate the values of structures used by
3097 The return value of C<ioctl> (and C<fcntl>) is as follows:
3099 if OS returns: then Perl returns:
3101 0 string "0 but true"
3102 anything else that number
3104 Thus Perl returns true on success and false on failure, yet you can
3105 still easily determine the actual value returned by the operating
3108 $retval = ioctl(...) || -1;
3109 printf "System returned %d\n", $retval;
3111 The special string C<"0 but true"> is exempt from B<-w> complaints
3112 about improper numeric conversions.
3114 Portability issues: L<perlport/ioctl>.
3116 =item join EXPR,LIST
3119 =for Pod::Functions join a list into a string using a separator
3121 Joins the separate strings of LIST into a single string with fields
3122 separated by the value of EXPR, and returns that new string. Example:
3124 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3126 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3127 first argument. Compare L</split>.
3136 =for Pod::Functions retrieve list of indices from a hash
3138 Called in list context, returns a list consisting of all the keys of the
3139 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3140 releases prior to 5.12 will produce a syntax error if you try to use an
3141 array argument. In scalar context, returns the number of keys or indices.
3143 Hash entries are returned in an apparently random order. The actual random
3144 order is specific to a given hash; the exact same series of operations
3145 on two hashes may result in a different order for each hash. Any insertion
3146 into the hash may change the order, as will any deletion, with the exception
3147 that the most recent key returned by C<each> or C<keys> may be deleted
3148 without changing the order. So long as a given hash is unmodified you may
3149 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
3150 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3151 details on why hash order is randomized. Aside from the guarantees
3152 provided here the exact details of Perl's hash algorithm and the hash
3153 traversal order are subject to change in any release of Perl.
3155 As a side effect, calling keys() resets the internal iterator of the HASH or
3156 ARRAY (see L</each>). In particular, calling keys() in void context resets
3157 the iterator with no other overhead.
3159 Here is yet another way to print your environment:
3162 @values = values %ENV;
3164 print pop(@keys), '=', pop(@values), "\n";
3167 or how about sorted by key:
3169 foreach $key (sort(keys %ENV)) {
3170 print $key, '=', $ENV{$key}, "\n";
3173 The returned values are copies of the original keys in the hash, so
3174 modifying them will not affect the original hash. Compare L</values>.
3176 To sort a hash by value, you'll need to use a C<sort> function.
3177 Here's a descending numeric sort of a hash by its values:
3179 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3180 printf "%4d %s\n", $hash{$key}, $key;
3183 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3184 allocated for the given hash. This can gain you a measure of efficiency if
3185 you know the hash is going to get big. (This is similar to pre-extending
3186 an array by assigning a larger number to $#array.) If you say
3190 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3191 in fact, since it rounds up to the next power of two. These
3192 buckets will be retained even if you do C<%hash = ()>, use C<undef
3193 %hash> if you want to free the storage while C<%hash> is still in scope.
3194 You can't shrink the number of buckets allocated for the hash using
3195 C<keys> in this way (but you needn't worry about doing this by accident,
3196 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3199 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3200 a reference to an unblessed hash or array. The argument will be
3201 dereferenced automatically. This aspect of C<keys> is considered highly
3202 experimental. The exact behaviour may change in a future version of Perl.
3204 for (keys $hashref) { ... }
3205 for (keys $obj->get_arrayref) { ... }
3207 To avoid confusing would-be users of your code who are running earlier
3208 versions of Perl with mysterious syntax errors, put this sort of thing at
3209 the top of your file to signal that your code will work I<only> on Perls of
3212 use 5.012; # so keys/values/each work on arrays
3213 use 5.014; # so keys/values/each work on scalars (experimental)
3215 See also C<each>, C<values>, and C<sort>.
3217 =item kill SIGNAL, LIST
3222 =for Pod::Functions send a signal to a process or process group
3224 Sends a signal to a list of processes. Returns the number of
3225 processes successfully signaled (which is not necessarily the
3226 same as the number actually killed).
3228 $cnt = kill 'HUP', $child1, $child2;
3229 kill 'KILL', @goners;
3231 SIGNAL may be either a signal name (a string) or a signal number. A signal
3232 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3233 same signal. The string form of SIGNAL is recommended for portability because
3234 the same signal may have different numbers in different operating systems.
3236 A list of signal names supported by the current platform can be found in
3237 C<$Config{sig_name}>, which is provided by the C<Config> module. See L<Config>
3240 A negative signal name is the same as a negative signal number, killing process
3241 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3242 C<kill -9, $pgrp> will send C<SIGKILL> to
3243 the entire process group specified. That
3244 means you usually want to use positive not negative signals.
3246 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3247 no signal is sent to
3248 the process, but C<kill> checks whether it's I<possible> to send a signal to it
3249 (that means, to be brief, that the process is owned by the same user, or we are
3250 the super-user). This is useful to check that a child process is still
3251 alive (even if only as a zombie) and hasn't changed its UID. See
3252 L<perlport> for notes on the portability of this construct.
3254 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3255 the operating system. For example, on POSIX-conforming systems, zero will
3256 signal the current process group, -1 will signal all processes, and any
3257 other negative PROCESS number will act as a negative signal number and
3258 kill the entire process group specified.
3260 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3261 A warning may be produced in a future version.
3263 See L<perlipc/"Signals"> for more details.
3265 On some platforms such as Windows where the fork() system call is not available.
3266 Perl can be built to emulate fork() at the interpreter level.
3267 This emulation has limitations related to kill that have to be considered,
3268 for code running on Windows and in code intended to be portable.
3270 See L<perlfork> for more details.
3272 If there is no I<LIST> of processes, no signal is sent, and the return
3273 value is 0. This form is sometimes used, however, because it causes
3274 tainting checks to be run. But see
3275 L<perlsec/Laundering and Detecting Tainted Data>.
3277 Portability issues: L<perlport/kill>.
3286 =for Pod::Functions exit a block prematurely
3288 The C<last> command is like the C<break> statement in C (as used in
3289 loops); it immediately exits the loop in question. If the LABEL is
3290 omitted, the command refers to the innermost enclosing
3291 loop. The C<last EXPR> form, available starting in Perl
3292 5.18.0, allows a label name to be computed at run time,
3293 and is otherwise identical to C<last LABEL>. The
3294 C<continue> block, if any, is not executed:
3296 LINE: while (<STDIN>) {
3297 last LINE if /^$/; # exit when done with header
3301 C<last> cannot be used to exit a block that returns a value such as
3302 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3303 a grep() or map() operation.
3305 Note that a block by itself is semantically identical to a loop
3306 that executes once. Thus C<last> can be used to effect an early
3307 exit out of such a block.
3309 See also L</continue> for an illustration of how C<last>, C<next>, and
3312 Unlike most named operators, this has the same precedence as assignment.
3313 It is also exempt from the looks-like-a-function rule, so
3314 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3322 =for Pod::Functions return lower-case version of a string
3324 Returns a lowercased version of EXPR. This is the internal function
3325 implementing the C<\L> escape in double-quoted strings.
3327 If EXPR is omitted, uses C<$_>.
3329 What gets returned depends on several factors:
3333 =item If C<use bytes> is in effect:
3335 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3338 =item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3340 Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3341 semantics for the remaining code points (this last can only happen if
3342 the UTF8 flag is also set). See L<perllocale>.
3344 A deficiency in this is that case changes that cross the 255/256
3345 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3346 LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3347 platforms). But under C<use locale>, the lower case of U+1E9E is
3348 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3349 current locale, and Perl has no way of knowing if that character even
3350 exists in the locale, much less what code point it is. Perl returns
3351 the input character unchanged, for all instances (and there aren't
3352 many) where the 255/256 boundary would otherwise be crossed.
3354 =item Otherwise, If EXPR has the UTF8 flag set:
3356 Unicode semantics are used for the case change.
3358 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3360 Unicode semantics are used for the case change.
3364 ASCII semantics are used for the case change. The lowercase of any character
3365 outside the ASCII range is the character itself.
3370 X<lcfirst> X<lowercase>
3374 =for Pod::Functions return a string with just the next letter in lower case
3376 Returns the value of EXPR with the first character lowercased. This
3377 is the internal function implementing the C<\l> escape in
3378 double-quoted strings.
3380 If EXPR is omitted, uses C<$_>.
3382 This function behaves the same way under various pragmata, such as in a locale,
3390 =for Pod::Functions return the number of characters in a string
3392 Returns the length in I<characters> of the value of EXPR. If EXPR is
3393 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3396 This function cannot be used on an entire array or hash to find out how
3397 many elements these have. For that, use C<scalar @array> and C<scalar keys
3398 %hash>, respectively.
3400 Like all Perl character operations, length() normally deals in logical
3401 characters, not physical bytes. For how many bytes a string encoded as
3402 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3403 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3408 =for Pod::Functions the current source line number
3410 A special token that compiles to the current line number.
3412 =item link OLDFILE,NEWFILE
3415 =for Pod::Functions create a hard link in the filesystem
3417 Creates a new filename linked to the old filename. Returns true for
3418 success, false otherwise.
3420 Portability issues: L<perlport/link>.
3422 =item listen SOCKET,QUEUESIZE
3425 =for Pod::Functions register your socket as a server
3427 Does the same thing that the listen(2) system call does. Returns true if
3428 it succeeded, false otherwise. See the example in
3429 L<perlipc/"Sockets: Client/Server Communication">.
3434 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3436 You really probably want to be using C<my> instead, because C<local> isn't
3437 what most people think of as "local". See
3438 L<perlsub/"Private Variables via my()"> for details.
3440 A local modifies the listed variables to be local to the enclosing
3441 block, file, or eval. If more than one value is listed, the list must
3442 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3443 for details, including issues with tied arrays and hashes.
3445 The C<delete local EXPR> construct can also be used to localize the deletion
3446 of array/hash elements to the current block.
3447 See L<perlsub/"Localized deletion of elements of composite types">.
3449 =item localtime EXPR
3450 X<localtime> X<ctime>
3454 =for Pod::Functions convert UNIX time into record or string using local time
3456 Converts a time as returned by the time function to a 9-element list
3457 with the time analyzed for the local time zone. Typically used as
3461 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3464 All list elements are numeric and come straight out of the C `struct
3465 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3466 of the specified time.
3468 C<$mday> is the day of the month and C<$mon> the month in
3469 the range C<0..11>, with 0 indicating January and 11 indicating December.
3470 This makes it easy to get a month name from a list:
3472 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3473 print "$abbr[$mon] $mday";
3474 # $mon=9, $mday=18 gives "Oct 18"
3476 C<$year> contains the number of years since 1900. To get a 4-digit
3481 To get the last two digits of the year (e.g., "01" in 2001) do:
3483 $year = sprintf("%02d", $year % 100);
3485 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3486 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3487 (or C<0..365> in leap years.)
3489 C<$isdst> is true if the specified time occurs during Daylight Saving
3490 Time, false otherwise.
3492 If EXPR is omitted, C<localtime()> uses the current time (as returned
3495 In scalar context, C<localtime()> returns the ctime(3) value:
3497 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3499 The format of this scalar value is B<not> locale-dependent
3500 but built into Perl. For GMT instead of local
3501 time use the L</gmtime> builtin. See also the
3502 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3503 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3504 and mktime(3) functions.
3506 To get somewhat similar but locale-dependent date strings, set up your
3507 locale environment variables appropriately (please see L<perllocale>) and
3510 use POSIX qw(strftime);
3511 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3512 # or for GMT formatted appropriately for your locale:
3513 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3515 Note that the C<%a> and C<%b>, the short forms of the day of the week
3516 and the month of the year, may not necessarily be three characters wide.
3518 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3519 by-name access mechanism to the gmtime() and localtime() functions,
3522 For a comprehensive date and time representation look at the
3523 L<DateTime> module on CPAN.
3525 Portability issues: L<perlport/localtime>.
3530 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3532 This function places an advisory lock on a shared variable or referenced
3533 object contained in I<THING> until the lock goes out of scope.
3535 The value returned is the scalar itself, if the argument is a scalar, or a
3536 reference, if the argument is a hash, array or subroutine.
3538 lock() is a "weak keyword" : this means that if you've defined a function
3539 by this name (before any calls to it), that function will be called
3540 instead. If you are not under C<use threads::shared> this does nothing.
3541 See L<threads::shared>.
3544 X<log> X<logarithm> X<e> X<ln> X<base>
3548 =for Pod::Functions retrieve the natural logarithm for a number
3550 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3551 returns the log of C<$_>. To get the
3552 log of another base, use basic algebra:
3553 The base-N log of a number is equal to the natural log of that number
3554 divided by the natural log of N. For example:
3558 return log($n)/log(10);
3561 See also L</exp> for the inverse operation.
3563 =item lstat FILEHANDLE
3568 =item lstat DIRHANDLE
3572 =for Pod::Functions stat a symbolic link
3574 Does the same thing as the C<stat> function (including setting the
3575 special C<_> filehandle) but stats a symbolic link instead of the file
3576 the symbolic link points to. If symbolic links are unimplemented on
3577 your system, a normal C<stat> is done. For much more detailed
3578 information, please see the documentation for C<stat>.
3580 If EXPR is omitted, stats C<$_>.
3582 Portability issues: L<perlport/lstat>.
3586 =for Pod::Functions match a string with a regular expression pattern
3588 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3590 =item map BLOCK LIST
3595 =for Pod::Functions apply a change to a list to get back a new list with the changes
3597 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3598 C<$_> to each element) and returns the list value composed of the
3599 results of each such evaluation. In scalar context, returns the
3600 total number of elements so generated. Evaluates BLOCK or EXPR in
3601 list context, so each element of LIST may produce zero, one, or
3602 more elements in the returned value.
3604 @chars = map(chr, @numbers);
3606 translates a list of numbers to the corresponding characters.
3608 my @squares = map { $_ * $_ } @numbers;
3610 translates a list of numbers to their squared values.
3612 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3614 shows that number of returned elements can differ from the number of
3615 input elements. To omit an element, return an empty list ().
3616 This could also be achieved by writing
3618 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3620 which makes the intention more clear.
3622 Map always returns a list, which can be
3623 assigned to a hash such that the elements
3624 become key/value pairs. See L<perldata> for more details.
3626 %hash = map { get_a_key_for($_) => $_ } @array;
3628 is just a funny way to write
3632 $hash{get_a_key_for($_)} = $_;
3635 Note that C<$_> is an alias to the list value, so it can be used to
3636 modify the elements of the LIST. While this is useful and supported,
3637 it can cause bizarre results if the elements of LIST are not variables.
3638 Using a regular C<foreach> loop for this purpose would be clearer in
3639 most cases. See also L</grep> for an array composed of those items of
3640 the original list for which the BLOCK or EXPR evaluates to true.
3642 If C<$_> is lexical in the scope where the C<map> appears (because it has
3643 been declared with the deprecated C<my $_> construct),
3644 then, in addition to being locally aliased to
3645 the list elements, C<$_> keeps being lexical inside the block; that is, it
3646 can't be seen from the outside, avoiding any potential side-effects.
3648 C<{> starts both hash references and blocks, so C<map { ...> could be either
3649 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3650 ahead for the closing C<}> it has to take a guess at which it's dealing with
3651 based on what it finds just after the
3652 C<{>. Usually it gets it right, but if it
3653 doesn't it won't realize something is wrong until it gets to the C<}> and
3654 encounters the missing (or unexpected) comma. The syntax error will be
3655 reported close to the C<}>, but you'll need to change something near the C<{>
3656 such as using a unary C<+> to give Perl some help:
3658 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3659 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3660 %hash = map { ("\L$_" => 1) } @array # this also works
3661 %hash = map { lc($_) => 1 } @array # as does this.
3662 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3664 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3666 or to force an anon hash constructor use C<+{>:
3668 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3671 to get a list of anonymous hashes each with only one entry apiece.
3673 =item mkdir FILENAME,MASK
3674 X<mkdir> X<md> X<directory, create>
3676 =item mkdir FILENAME
3680 =for Pod::Functions create a directory
3682 Creates the directory specified by FILENAME, with permissions
3683 specified by MASK (as modified by C<umask>). If it succeeds it
3684 returns true; otherwise it returns false and sets C<$!> (errno).
3685 MASK defaults to 0777 if omitted, and FILENAME defaults
3686 to C<$_> if omitted.
3688 In general, it is better to create directories with a permissive MASK
3689 and let the user modify that with their C<umask> than it is to supply
3690 a restrictive MASK and give the user no way to be more permissive.
3691 The exceptions to this rule are when the file or directory should be
3692 kept private (mail files, for instance). The perlfunc(1) entry on
3693 C<umask> discusses the choice of MASK in more detail.
3695 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3696 number of trailing slashes. Some operating and filesystems do not get
3697 this right, so Perl automatically removes all trailing slashes to keep
3700 To recursively create a directory structure, look at
3701 the C<mkpath> function of the L<File::Path> module.
3703 =item msgctl ID,CMD,ARG
3706 =for Pod::Functions SysV IPC message control operations
3708 Calls the System V IPC function msgctl(2). You'll probably have to say
3712 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3713 then ARG must be a variable that will hold the returned C<msqid_ds>
3714 structure. Returns like C<ioctl>: the undefined value for error,
3715 C<"0 but true"> for zero, or the actual return value otherwise. See also
3716 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3719 Portability issues: L<perlport/msgctl>.
3721 =item msgget KEY,FLAGS
3724 =for Pod::Functions get SysV IPC message queue
3726 Calls the System V IPC function msgget(2). Returns the message queue
3727 id, or C<undef> on error. See also
3728 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3731 Portability issues: L<perlport/msgget>.
3733 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3736 =for Pod::Functions receive a SysV IPC message from a message queue
3738 Calls the System V IPC function msgrcv to receive a message from
3739 message queue ID into variable VAR with a maximum message size of
3740 SIZE. Note that when a message is received, the message type as a
3741 native long integer will be the first thing in VAR, followed by the
3742 actual message. This packing may be opened with C<unpack("l! a*")>.
3743 Taints the variable. Returns true if successful, false
3744 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3745 C<IPC::SysV> and C<IPC::SysV::Msg>.
3747 Portability issues: L<perlport/msgrcv>.
3749 =item msgsnd ID,MSG,FLAGS
3752 =for Pod::Functions send a SysV IPC message to a message queue
3754 Calls the System V IPC function msgsnd to send the message MSG to the
3755 message queue ID. MSG must begin with the native long integer message
3756 type, be followed by the length of the actual message, and then finally
3757 the message itself. This kind of packing can be achieved with
3758 C<pack("l! a*", $type, $message)>. Returns true if successful,
3759 false on error. See also the C<IPC::SysV>
3760 and C<IPC::SysV::Msg> documentation.
3762 Portability issues: L<perlport/msgsnd>.
3767 =item my TYPE VARLIST
3769 =item my VARLIST : ATTRS
3771 =item my TYPE VARLIST : ATTRS
3773 =for Pod::Functions declare and assign a local variable (lexical scoping)
3775 A C<my> declares the listed variables to be local (lexically) to the
3776 enclosing block, file, or C<eval>. If more than one variable is listed,
3777 the list must be placed in parentheses.
3779 The exact semantics and interface of TYPE and ATTRS are still
3780 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3781 and attributes are handled using the C<attributes> pragma, or starting
3782 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3783 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3784 L<attributes>, and L<Attribute::Handlers>.
3786 Note that with a parenthesised list, C<undef> can be used as a dummy
3787 placeholder, for example to skip assignment of initial values:
3789 my ( undef, $min, $hour ) = localtime;
3798 =for Pod::Functions iterate a block prematurely
3800 The C<next> command is like the C<continue> statement in C; it starts
3801 the next iteration of the loop:
3803 LINE: while (<STDIN>) {
3804 next LINE if /^#/; # discard comments
3808 Note that if there were a C<continue> block on the above, it would get
3809 executed even on discarded lines. If LABEL is omitted, the command
3810 refers to the innermost enclosing loop. The C<next EXPR> form, available
3811 as of Perl 5.18.0, allows a label name to be computed at run time, being
3812 otherwise identical to C<next LABEL>.
3814 C<next> cannot be used to exit a block which returns a value such as
3815 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3816 a grep() or map() operation.
3818 Note that a block by itself is semantically identical to a loop
3819 that executes once. Thus C<next> will exit such a block early.
3821 See also L</continue> for an illustration of how C<last>, C<next>, and
3824 Unlike most named operators, this has the same precedence as assignment.
3825 It is also exempt from the looks-like-a-function rule, so
3826 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3829 =item no MODULE VERSION LIST
3833 =item no MODULE VERSION
3835 =item no MODULE LIST
3841 =for Pod::Functions unimport some module symbols or semantics at compile time
3843 See the C<use> function, of which C<no> is the opposite.
3846 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3850 =for Pod::Functions convert a string to an octal number
3852 Interprets EXPR as an octal string and returns the corresponding
3853 value. (If EXPR happens to start off with C<0x>, interprets it as a
3854 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3855 binary string. Leading whitespace is ignored in all three cases.)
3856 The following will handle decimal, binary, octal, and hex in standard
3859 $val = oct($val) if $val =~ /^0/;
3861 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3862 in octal), use sprintf() or printf():
3864 $dec_perms = (stat("filename"))[2] & 07777;
3865 $oct_perm_str = sprintf "%o", $perms;
3867 The oct() function is commonly used when a string such as C<644> needs
3868 to be converted into a file mode, for example. Although Perl
3869 automatically converts strings into numbers as needed, this automatic
3870 conversion assumes base 10.
3872 Leading white space is ignored without warning, as too are any trailing
3873 non-digits, such as a decimal point (C<oct> only handles non-negative
3874 integers, not negative integers or floating point).
3876 =item open FILEHANDLE,EXPR
3877 X<open> X<pipe> X<file, open> X<fopen>
3879 =item open FILEHANDLE,MODE,EXPR
3881 =item open FILEHANDLE,MODE,EXPR,LIST
3883 =item open FILEHANDLE,MODE,REFERENCE
3885 =item open FILEHANDLE
3887 =for Pod::Functions open a file, pipe, or descriptor
3889 Opens the file whose filename is given by EXPR, and associates it with
3892 Simple examples to open a file for reading:
3894 open(my $fh, "<", "input.txt")
3895 or die "cannot open < input.txt: $!";
3899 open(my $fh, ">", "output.txt")
3900 or die "cannot open > output.txt: $!";
3902 (The following is a comprehensive reference to open(): for a gentler
3903 introduction you may consider L<perlopentut>.)
3905 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3906 new filehandle is autovivified, meaning that the variable is assigned a
3907 reference to a newly allocated anonymous filehandle. Otherwise if
3908 FILEHANDLE is an expression, its value is the real filehandle. (This is
3909 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3912 If three (or more) arguments are specified, the open mode (including
3913 optional encoding) in the second argument are distinct from the filename in
3914 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3915 If MODE is C<< > >>, the file is opened for output, with existing files
3916 first being truncated ("clobbered") and nonexisting files newly created.
3917 If MODE is C<<< >> >>>, the file is opened for appending, again being
3918 created if necessary.
3920 You can put a C<+> in front of the C<< > >> or C<< < >> to
3921 indicate that you want both read and write access to the file; thus
3922 C<< +< >> is almost always preferred for read/write updates--the
3923 C<< +> >> mode would clobber the file first. You can't usually use
3924 either read-write mode for updating textfiles, since they have
3925 variable-length records. See the B<-i> switch in L<perlrun> for a
3926 better approach. The file is created with permissions of C<0666>
3927 modified by the process's C<umask> value.
3929 These various prefixes correspond to the fopen(3) modes of C<r>,
3930 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3932 In the one- and two-argument forms of the call, the mode and filename
3933 should be concatenated (in that order), preferably separated by white
3934 space. You can--but shouldn't--omit the mode in these forms when that mode
3935 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3936 the filename argument is a known literal.
3938 For three or more arguments if MODE is C<|->, the filename is
3939 interpreted as a command to which output is to be piped, and if MODE
3940 is C<-|>, the filename is interpreted as a command that pipes
3941 output to us. In the two-argument (and one-argument) form, one should
3942 replace dash (C<->) with the command.
3943 See L<perlipc/"Using open() for IPC"> for more examples of this.
3944 (You are not allowed to C<open> to a command that pipes both in I<and>
3945 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3946 L<perlipc/"Bidirectional Communication with Another Process"> for
3949 In the form of pipe opens taking three or more arguments, if LIST is specified
3950 (extra arguments after the command name) then LIST becomes arguments
3951 to the command invoked if the platform supports it. The meaning of
3952 C<open> with more than three arguments for non-pipe modes is not yet
3953 defined, but experimental "layers" may give extra LIST arguments
3956 In the two-argument (and one-argument) form, opening C<< <- >>
3957 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3959 You may (and usually should) use the three-argument form of open to specify
3960 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3961 that affect how the input and output are processed (see L<open> and
3962 L<PerlIO> for more details). For example:
3964 open(my $fh, "<:encoding(UTF-8)", "filename")
3965 || die "can't open UTF-8 encoded filename: $!";
3967 opens the UTF8-encoded file containing Unicode characters;
3968 see L<perluniintro>. Note that if layers are specified in the
3969 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3970 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3971 Those layers will also be ignored if you specifying a colon with no name
3972 following it. In that case the default layer for the operating system
3973 (:raw on Unix, :crlf on Windows) is used.
3975 Open returns nonzero on success, the undefined value otherwise. If
3976 the C<open> involved a pipe, the return value happens to be the pid of
3979 If you're running Perl on a system that distinguishes between text
3980 files and binary files, then you should check out L</binmode> for tips
3981 for dealing with this. The key distinction between systems that need
3982 C<binmode> and those that don't is their text file formats. Systems
3983 like Unix, Mac OS, and Plan 9, that end lines with a single
3984 character and encode that character in C as C<"\n"> do not
3985 need C<binmode>. The rest need it.
3987 When opening a file, it's seldom a good idea to continue
3988 if the request failed, so C<open> is frequently used with
3989 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3990 where you want to format a suitable error message (but there are
3991 modules that can help with that problem)) always check
3992 the return value from opening a file.
3994 The filehandle will be closed when its reference count reaches zero.
3995 If it is a lexically scoped variable declared with C<my>, that usually
3996 means the end of the enclosing scope. However, this automatic close
3997 does not check for errors, so it is better to explicitly close
3998 filehandles, especially those used for writing:
4001 || warn "close failed: $!";
4003 An older style is to use a bareword as the filehandle, as
4005 open(FH, "<", "input.txt")
4006 or die "cannot open < input.txt: $!";
4008 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4009 <FH> >> and so on. Note that it's a global variable, so this form is
4010 not recommended in new code.
4012 As a shortcut a one-argument call takes the filename from the global
4013 scalar variable of the same name as the filehandle:
4016 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4018 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4019 declared with C<my> or C<state>.
4021 As a special case the three-argument form with a read/write mode and the third
4022 argument being C<undef>:
4024 open(my $tmp, "+>", undef) or die ...
4026 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4027 works for symmetry, but you really should consider writing something
4028 to the temporary file first. You will need to seek() to do the
4031 Perl is built using PerlIO by default; Unless you've
4032 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4033 open filehandles directly to Perl scalars via:
4035 open($fh, ">", \$variable) || ..
4037 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4040 open(STDOUT, ">", \$variable)
4041 or die "Can't open STDOUT: $!";
4045 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
4046 # if the open fails, output is discarded
4048 open(my $dbase, "+<", "dbase.mine") # open for update
4049 or die "Can't open 'dbase.mine' for update: $!";
4051 open(my $dbase, "+<dbase.mine") # ditto
4052 or die "Can't open 'dbase.mine' for update: $!";
4054 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4055 or die "Can't start caesar: $!";
4057 open(ARTICLE, "caesar <$article |") # ditto
4058 or die "Can't start caesar: $!";
4060 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4061 or die "Can't start sort: $!";
4064 open(MEMORY, ">", \$var)
4065 or die "Can't open memory file: $!";
4066 print MEMORY "foo!\n"; # output will appear in $var
4068 # process argument list of files along with any includes
4070 foreach $file (@ARGV) {
4071 process($file, "fh00");
4075 my($filename, $input) = @_;
4076 $input++; # this is a string increment
4077 unless (open($input, "<", $filename)) {
4078 print STDERR "Can't open $filename: $!\n";
4083 while (<$input>) { # note use of indirection
4084 if (/^#include "(.*)"/) {
4085 process($1, $input);
4092 See L<perliol> for detailed info on PerlIO.
4094 You may also, in the Bourne shell tradition, specify an EXPR beginning
4095 with C<< >& >>, in which case the rest of the string is interpreted
4096 as the name of a filehandle (or file descriptor, if numeric) to be
4097 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4098 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4099 The mode you specify should match the mode of the original filehandle.
4100 (Duping a filehandle does not take into account any existing contents
4101 of IO buffers.) If you use the three-argument
4102 form, then you can pass either a
4103 number, the name of a filehandle, or the normal "reference to a glob".
4105 Here is a script that saves, redirects, and restores C<STDOUT> and
4106 C<STDERR> using various methods:
4109 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4110 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4112 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4113 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4115 select STDERR; $| = 1; # make unbuffered
4116 select STDOUT; $| = 1; # make unbuffered
4118 print STDOUT "stdout 1\n"; # this works for
4119 print STDERR "stderr 1\n"; # subprocesses too
4121 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4122 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4124 print STDOUT "stdout 2\n";
4125 print STDERR "stderr 2\n";
4127 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4128 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4129 that file descriptor (and not call C<dup(2)>); this is more
4130 parsimonious of file descriptors. For example:
4132 # open for input, reusing the fileno of $fd
4133 open(FILEHANDLE, "<&=$fd")
4137 open(FILEHANDLE, "<&=", $fd)
4141 # open for append, using the fileno of OLDFH
4142 open(FH, ">>&=", OLDFH)
4146 open(FH, ">>&=OLDFH")
4148 Being parsimonious on filehandles is also useful (besides being
4149 parsimonious) for example when something is dependent on file
4150 descriptors, like for example locking using flock(). If you do just
4151 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4152 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4153 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4154 the same underlying system file descriptor.
4156 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4157 fdopen() to implement the C<=> functionality. On many Unix systems,
4158 fdopen() fails when file descriptors exceed a certain value, typically 255.
4159 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4161 You can see whether your Perl was built with PerlIO by running C<perl -V>
4162 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4163 have PerlIO; otherwise you don't.
4165 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4166 with the one- or two-argument forms of C<open>),
4167 an implicit C<fork> is done, so C<open> returns twice: in the parent
4168 process it returns the pid
4169 of the child process, and in the child process it returns (a defined) C<0>.
4170 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4172 For example, use either
4174 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4178 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4184 # either write TO_KID or else read FROM_KID
4186 waitpid $child_pid, 0;
4188 # am the child; use STDIN/STDOUT normally
4193 The filehandle behaves normally for the parent, but I/O to that
4194 filehandle is piped from/to the STDOUT/STDIN of the child process.
4195 In the child process, the filehandle isn't opened--I/O happens from/to
4196 the new STDOUT/STDIN. Typically this is used like the normal
4197 piped open when you want to exercise more control over just how the
4198 pipe command gets executed, such as when running setuid and
4199 you don't want to have to scan shell commands for metacharacters.
4201 The following blocks are more or less equivalent:
4203 open(FOO, "|tr '[a-z]' '[A-Z]'");
4204 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4205 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4206 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4208 open(FOO, "cat -n '$file'|");
4209 open(FOO, "-|", "cat -n '$file'");
4210 open(FOO, "-|") || exec "cat", "-n", $file;
4211 open(FOO, "-|", "cat", "-n", $file);
4213 The last two examples in each block show the pipe as "list form", which is
4214 not yet supported on all platforms. A good rule of thumb is that if
4215 your platform has a real C<fork()> (in other words, if your platform is
4216 Unix, including Linux and MacOS X), you can use the list form. You would
4217 want to use the list form of the pipe so you can pass literal arguments
4218 to the command without risk of the shell interpreting any shell metacharacters
4219 in them. However, this also bars you from opening pipes to commands
4220 that intentionally contain shell metacharacters, such as:
4222 open(FOO, "|cat -n | expand -4 | lpr")
4223 // die "Can't open pipeline to lpr: $!";
4225 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4227 Perl will attempt to flush all files opened for
4228 output before any operation that may do a fork, but this may not be
4229 supported on some platforms (see L<perlport>). To be safe, you may need
4230 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4231 of C<IO::Handle> on any open handles.
4233 On systems that support a close-on-exec flag on files, the flag will
4234 be set for the newly opened file descriptor as determined by the value
4235 of C<$^F>. See L<perlvar/$^F>.
4237 Closing any piped filehandle causes the parent process to wait for the
4238 child to finish, then returns the status value in C<$?> and
4239 C<${^CHILD_ERROR_NATIVE}>.
4241 The filename passed to the one- and two-argument forms of open() will
4242 have leading and trailing whitespace deleted and normal
4243 redirection characters honored. This property, known as "magic open",
4244 can often be used to good effect. A user could specify a filename of
4245 F<"rsh cat file |">, or you could change certain filenames as needed:
4247 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4248 open(FH, $filename) or die "Can't open $filename: $!";
4250 Use the three-argument form to open a file with arbitrary weird characters in it,
4252 open(FOO, "<", $file)
4253 || die "can't open < $file: $!";
4255 otherwise it's necessary to protect any leading and trailing whitespace:
4257 $file =~ s#^(\s)#./$1#;
4258 open(FOO, "< $file\0")
4259 || die "open failed: $!";
4261 (this may not work on some bizarre filesystems). One should
4262 conscientiously choose between the I<magic> and I<three-argument> form
4265 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4267 will allow the user to specify an argument of the form C<"rsh cat file |">,
4268 but will not work on a filename that happens to have a trailing space, while
4270 open(IN, "<", $ARGV[0])
4271 || die "can't open < $ARGV[0]: $!";
4273 will have exactly the opposite restrictions.
4275 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4276 should use the C<sysopen> function, which involves no such magic (but may
4277 use subtly different filemodes than Perl open(), which is mapped to C
4278 fopen()). This is another way to protect your filenames from
4279 interpretation. For example:
4282 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4283 or die "sysopen $path: $!";
4284 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4285 print HANDLE "stuff $$\n";
4287 print "File contains: ", <HANDLE>;
4289 See L</seek> for some details about mixing reading and writing.
4291 Portability issues: L<perlport/open>.
4293 =item opendir DIRHANDLE,EXPR
4296 =for Pod::Functions open a directory
4298 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4299 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4300 DIRHANDLE may be an expression whose value can be used as an indirect
4301 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4302 scalar variable (or array or hash element), the variable is assigned a
4303 reference to a new anonymous dirhandle; that is, it's autovivified.
4304 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4306 See the example at C<readdir>.
4313 =for Pod::Functions find a character's numeric representation
4315 Returns the numeric value of the first character of EXPR.
4316 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4317 (Note I<character>, not byte.)
4319 For the reverse, see L</chr>.
4320 See L<perlunicode> for more about Unicode.
4325 =item our TYPE VARLIST
4327 =item our VARLIST : ATTRS
4329 =item our TYPE VARLIST : ATTRS
4331 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4333 C<our> makes a lexical alias to a package variable of the same name in the current
4334 package for use within the current lexical scope.
4336 C<our> has the same scoping rules as C<my> or C<state>, but C<our> only
4337 declares an alias, whereas C<my> or C<state> both declare a variable name and
4338 allocate storage for that name within the current scope.
4340 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4341 a package variable without qualifying it with the package name, but only within
4342 the lexical scope of the C<our> declaration. In this way, C<our> differs from
4343 C<use vars>, which allows use of an unqualified name I<only> within the
4344 affected package, but across scopes.
4346 If more than one variable is listed, the list must be placed
4352 An C<our> declaration declares an alias for a package variable that will be visible
4353 across its entire lexical scope, even across package boundaries. The
4354 package in which the variable is entered is determined at the point
4355 of the declaration, not at the point of use. This means the following
4359 our $bar; # declares $Foo::bar for rest of lexical scope
4363 print $bar; # prints 20, as it refers to $Foo::bar
4365 Multiple C<our> declarations with the same name in the same lexical
4366 scope are allowed if they are in different packages. If they happen
4367 to be in the same package, Perl will emit warnings if you have asked
4368 for them, just like multiple C<my> declarations. Unlike a second
4369 C<my> declaration, which will bind the name to a fresh variable, a
4370 second C<our> declaration in the same package, in the same scope, is
4375 our $bar; # declares $Foo::bar for rest of lexical scope
4379 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4380 print $bar; # prints 30
4382 our $bar; # emits warning but has no other effect
4383 print $bar; # still prints 30
4385 An C<our> declaration may also have a list of attributes associated
4388 The exact semantics and interface of TYPE and ATTRS are still
4389 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4390 and attributes are handled using the C<attributes> pragma, or, starting
4391 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4392 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4393 L<attributes>, and L<Attribute::Handlers>.
4395 Note that with a parenthesised list, C<undef> can be used as a dummy
4396 placeholder, for example to skip assignment of initial values:
4398 our ( undef, $min, $hour ) = localtime;
4400 =item pack TEMPLATE,LIST
4403 =for Pod::Functions convert a list into a binary representation
4405 Takes a LIST of values and converts it into a string using the rules
4406 given by the TEMPLATE. The resulting string is the concatenation of
4407 the converted values. Typically, each converted value looks
4408 like its machine-level representation. For example, on 32-bit machines
4409 an integer may be represented by a sequence of 4 bytes, which will in
4410 Perl be presented as a string that's 4 characters long.
4412 See L<perlpacktut> for an introduction to this function.
4414 The TEMPLATE is a sequence of characters that give the order and type
4415 of values, as follows:
4417 a A string with arbitrary binary data, will be null padded.
4418 A A text (ASCII) string, will be space padded.
4419 Z A null-terminated (ASCIZ) string, will be null padded.
4421 b A bit string (ascending bit order inside each byte,
4423 B A bit string (descending bit order inside each byte).
4424 h A hex string (low nybble first).
4425 H A hex string (high nybble first).
4427 c A signed char (8-bit) value.
4428 C An unsigned char (octet) value.
4429 W An unsigned char value (can be greater than 255).
4431 s A signed short (16-bit) value.
4432 S An unsigned short value.
4434 l A signed long (32-bit) value.
4435 L An unsigned long value.
4437 q A signed quad (64-bit) value.
4438 Q An unsigned quad value.
4439 (Quads are available only if your system supports 64-bit
4440 integer values _and_ if Perl has been compiled to support
4441 those. Raises an exception otherwise.)
4443 i A signed integer value.
4444 I A unsigned integer value.
4445 (This 'integer' is _at_least_ 32 bits wide. Its exact
4446 size depends on what a local C compiler calls 'int'.)
4448 n An unsigned short (16-bit) in "network" (big-endian) order.
4449 N An unsigned long (32-bit) in "network" (big-endian) order.
4450 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4451 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4453 j A Perl internal signed integer value (IV).
4454 J A Perl internal unsigned integer value (UV).
4456 f A single-precision float in native format.
4457 d A double-precision float in native format.
4459 F A Perl internal floating-point value (NV) in native format
4460 D A float of long-double precision in native format.
4461 (Long doubles are available only if your system supports
4462 long double values _and_ if Perl has been compiled to
4463 support those. Raises an exception otherwise.)
4465 p A pointer to a null-terminated string.
4466 P A pointer to a structure (fixed-length string).
4468 u A uuencoded string.
4469 U A Unicode character number. Encodes to a character in char-
4470 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4473 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4474 for details). Its bytes represent an unsigned integer in
4475 base 128, most significant digit first, with as few digits
4476 as possible. Bit eight (the high bit) is set on each byte
4479 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4481 @ Null-fill or truncate to absolute position, counted from the
4482 start of the innermost ()-group.
4483 . Null-fill or truncate to absolute position specified by
4485 ( Start of a ()-group.
4487 One or more modifiers below may optionally follow certain letters in the
4488 TEMPLATE (the second column lists letters for which the modifier is valid):
4490 ! sSlLiI Forces native (short, long, int) sizes instead
4491 of fixed (16-/32-bit) sizes.
4493 xX Make x and X act as alignment commands.
4495 nNvV Treat integers as signed instead of unsigned.
4497 @. Specify position as byte offset in the internal
4498 representation of the packed string. Efficient
4501 > sSiIlLqQ Force big-endian byte-order on the type.
4502 jJfFdDpP (The "big end" touches the construct.)
4504 < sSiIlLqQ Force little-endian byte-order on the type.
4505 jJfFdDpP (The "little end" touches the construct.)
4507 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4508 to force a particular byte-order on all components in that group,
4509 including all its subgroups.
4513 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4514 pack for processing data from NASA's Magellan probe. Magellan was in an
4515 elliptical orbit, using the antenna for the radar mapping when close to
4516 Venus and for communicating data back to Earth for the rest of the orbit.
4517 There were two transmission units, but one of these failed, and then the
4518 other developed a fault whereby it would randomly flip the sense of all the
4519 bits. It was easy to automatically detect complete records with the correct
4520 sense, and complete records with all the bits flipped. However, this didn't
4521 recover the records where the sense flipped midway. A colleague of Larry's
4522 was able to pretty much eyeball where the records flipped, so they wrote an
4523 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4524 manually correct the records and recover the data. For this purpose pack
4525 gained the hex and bit string format specifiers.
4527 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4528 27e2fb84680b9cc1), but the patch description makes no mention of their
4529 addition, let alone the story behind them.
4533 The following rules apply:
4539 Each letter may optionally be followed by a number indicating the repeat
4540 count. A numeric repeat count may optionally be enclosed in brackets, as
4541 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4542 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4543 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4544 something else, described below. Supplying a C<*> for the repeat count
4545 instead of a number means to use however many items are left, except for:
4551 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4555 <.>, where it means relative to the start of the string.
4559 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4563 One can replace a numeric repeat count with a template letter enclosed in
4564 brackets to use the packed byte length of the bracketed template for the
4567 For example, the template C<x[L]> skips as many bytes as in a packed long,
4568 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4569 variable-expanded) unpacks. If the template in brackets contains alignment
4570 commands (such as C<x![d]>), its packed length is calculated as if the
4571 start of the template had the maximal possible alignment.
4573 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4574 trailing null byte, so the resulting string is always one byte longer than
4575 the byte length of the item itself.
4577 When used with C<@>, the repeat count represents an offset from the start
4578 of the innermost C<()> group.
4580 When used with C<.>, the repeat count determines the starting position to
4581 calculate the value offset as follows:
4587 If the repeat count is C<0>, it's relative to the current position.
4591 If the repeat count is C<*>, the offset is relative to the start of the
4596 And if it's an integer I<n>, the offset is relative to the start of the
4597 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4598 bigger then the group level.
4602 The repeat count for C<u> is interpreted as the maximal number of bytes
4603 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4604 count should not be more than 65.
4608 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4609 string of length count, padding with nulls or spaces as needed. When
4610 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4611 after the first null, and C<a> returns data with no stripping at all.
4613 If the value to pack is too long, the result is truncated. If it's too
4614 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4615 followed by a null byte. Thus C<Z> always packs a trailing null, except
4616 when the count is 0.
4620 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4621 Each such format generates 1 bit of the result. These are typically followed
4622 by a repeat count like C<B8> or C<B64>.
4624 Each result bit is based on the least-significant bit of the corresponding
4625 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4626 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4628 Starting from the beginning of the input string, each 8-tuple
4629 of characters is converted to 1 character of output. With format C<b>,
4630 the first character of the 8-tuple determines the least-significant bit of a
4631 character; with format C<B>, it determines the most-significant bit of
4634 If the length of the input string is not evenly divisible by 8, the
4635 remainder is packed as if the input string were padded by null characters
4636 at the end. Similarly during unpacking, "extra" bits are ignored.
4638 If the input string is longer than needed, remaining characters are ignored.
4640 A C<*> for the repeat count uses all characters of the input field.
4641 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4645 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4646 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4648 For each such format, pack() generates 4 bits of result.
4649 With non-alphabetical characters, the result is based on the 4 least-significant
4650 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4651 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4652 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4653 is compatible with the usual hexadecimal digits, so that C<"a"> and
4654 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4655 characters with this format.
4657 Starting from the beginning of the template to pack(), each pair
4658 of characters is converted to 1 character of output. With format C<h>, the
4659 first character of the pair determines the least-significant nybble of the
4660 output character; with format C<H>, it determines the most-significant
4663 If the length of the input string is not even, it behaves as if padded by
4664 a null character at the end. Similarly, "extra" nybbles are ignored during
4667 If the input string is longer than needed, extra characters are ignored.
4669 A C<*> for the repeat count uses all characters of the input field. For
4670 unpack(), nybbles are converted to a string of hexadecimal digits.
4674 The C<p> format packs a pointer to a null-terminated string. You are
4675 responsible for ensuring that the string is not a temporary value, as that
4676 could potentially get deallocated before you got around to using the packed
4677 result. The C<P> format packs a pointer to a structure of the size indicated
4678 by the length. A null pointer is created if the corresponding value for
4679 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4680 unpacks into C<undef>.
4682 If your system has a strange pointer size--meaning a pointer is neither as
4683 big as an int nor as big as a long--it may not be possible to pack or
4684 unpack pointers in big- or little-endian byte order. Attempting to do
4685 so raises an exception.
4689 The C</> template character allows packing and unpacking of a sequence of
4690 items where the packed structure contains a packed item count followed by
4691 the packed items themselves. This is useful when the structure you're
4692 unpacking has encoded the sizes or repeat counts for some of its fields
4693 within the structure itself as separate fields.
4695 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4696 I<length-item> describes how the length value is packed. Formats likely
4697 to be of most use are integer-packing ones like C<n> for Java strings,
4698 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4700 For C<pack>, I<sequence-item> may have a repeat count, in which case
4701 the minimum of that and the number of available items is used as the argument
4702 for I<length-item>. If it has no repeat count or uses a '*', the number
4703 of available items is used.
4705 For C<unpack>, an internal stack of integer arguments unpacked so far is
4706 used. You write C</>I<sequence-item> and the repeat count is obtained by
4707 popping off the last element from the stack. The I<sequence-item> must not
4708 have a repeat count.
4710 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4711 the I<length-item> is the string length, not the number of strings. With
4712 an explicit repeat count for pack, the packed string is adjusted to that
4713 length. For example:
4715 This code: gives this result:
4717 unpack("W/a", "\004Gurusamy") ("Guru")
4718 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4719 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4721 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4722 pack("a/W2", ord("a") .. ord("z")) "2ab"
4724 The I<length-item> is not returned explicitly from C<unpack>.
4726 Supplying a count to the I<length-item> format letter is only useful with
4727 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4728 introduce C<"\000"> characters, which Perl does not regard as legal in
4733 The integer types C<s>, C<S>, C<l>, and C<L> may be
4734 followed by a C<!> modifier to specify native shorts or
4735 longs. As shown in the example above, a bare C<l> means
4736 exactly 32 bits, although the native C<long> as seen by the local C compiler
4737 may be larger. This is mainly an issue on 64-bit platforms. You can
4738 see whether using C<!> makes any difference this way:
4740 printf "format s is %d, s! is %d\n",
4741 length pack("s"), length pack("s!");
4743 printf "format l is %d, l! is %d\n",
4744 length pack("l"), length pack("l!");
4747 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4748 they are identical to C<i> and C<I>.
4750 The actual sizes (in bytes) of native shorts, ints, longs, and long
4751 longs on the platform where Perl was built are also available from
4754 $ perl -V:{short,int,long{,long}}size
4760 or programmatically via the C<Config> module:
4763 print $Config{shortsize}, "\n";
4764 print $Config{intsize}, "\n";
4765 print $Config{longsize}, "\n";
4766 print $Config{longlongsize}, "\n";
4768 C<$Config{longlongsize}> is undefined on systems without
4773 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4774 inherently non-portable between processors and operating systems because
4775 they obey native byteorder and endianness. For example, a 4-byte integer
4776 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4777 handled by the CPU registers) into bytes as
4779 0x12 0x34 0x56 0x78 # big-endian
4780 0x78 0x56 0x34 0x12 # little-endian
4782 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4783 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4784 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4785 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4787 The names I<big-endian> and I<little-endian> are comic references to the
4788 egg-eating habits of the little-endian Lilliputians and the big-endian
4789 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4790 This entered computer lingo via the paper "On Holy Wars and a Plea for
4791 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4793 Some systems may have even weirder byte orders such as
4798 You can determine your system endianness with this incantation:
4800 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4802 The byteorder on the platform where Perl was built is also available
4806 print "$Config{byteorder}\n";
4808 or from the command line:
4812 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4813 and C<"87654321"> are big-endian.
4815 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4816 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4817 immediately below. See also L<perlport>.
4821 Starting with Perl 5.10.0, integer and floating-point formats, along with
4822 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4823 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4824 or little-endian byte-order. These modifiers are especially useful
4825 given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
4826 64-bit integers, or floating-point values.
4828 Here are some concerns to keep in mind when using an endianness modifier:
4834 Exchanging signed integers between different platforms works only
4835 when all platforms store them in the same format. Most platforms store
4836 signed integers in two's-complement notation, so usually this is not an issue.
4840 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4841 formats on big- or little-endian machines. Otherwise, attempting to
4842 use them raises an exception.
4846 Forcing big- or little-endian byte-order on floating-point values for
4847 data exchange can work only if all platforms use the same
4848 binary representation such as IEEE floating-point. Even if all
4849 platforms are using IEEE, there may still be subtle differences. Being able
4850 to use C<< > >> or C<< < >> on floating-point values can be useful,
4851 but also dangerous if you don't know exactly what you're doing.
4852 It is not a general way to portably store floating-point values.
4856 When using C<< > >> or C<< < >> on a C<()> group, this affects
4857 all types inside the group that accept byte-order modifiers,
4858 including all subgroups. It is silently ignored for all other
4859 types. You are not allowed to override the byte-order within a group
4860 that already has a byte-order modifier suffix.
4866 Real numbers (floats and doubles) are in native machine format only.
4867 Due to the multiplicity of floating-point formats and the lack of a
4868 standard "network" representation for them, no facility for interchange has been
4869 made. This means that packed floating-point data written on one machine
4870 may not be readable on another, even if both use IEEE floating-point
4871 arithmetic (because the endianness of the memory representation is not part
4872 of the IEEE spec). See also L<perlport>.
4874 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4875 modifiers to force big- or little-endian byte-order on floating-point values.
4877 Because Perl uses doubles (or long doubles, if configured) internally for
4878 all numeric calculation, converting from double into float and thence
4879 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4880 will not in general equal $foo.
4884 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4885 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4886 where the packed string is processed in its UTF-8-encoded Unicode form on
4887 a byte-by-byte basis. Character mode is the default
4888 unless the format string starts with C<U>. You
4889 can always switch mode mid-format with an explicit
4890 C<C0> or C<U0> in the format. This mode remains in effect until the next
4891 mode change, or until the end of the C<()> group it (directly) applies to.
4893 Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
4894 bytes is not necessarily obvious. Probably only the first of these
4897 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4898 perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
4900 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4901 perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4903 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4904 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
4906 $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
4907 perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
4908 C3.8E.C2.B1.C3.8F.C2.89
4910 Those examples also illustrate that you should not try to use
4911 C<pack>/C<unpack> as a substitute for the L<Encode> module.
4915 You must yourself do any alignment or padding by inserting, for example,
4916 enough C<"x">es while packing. There is no way for pack() and unpack()
4917 to know where characters are going to or coming from, so they
4918 handle their output and input as flat sequences of characters.
4922 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4923 take a repeat count either as postfix, or for unpack(), also via the C</>
4924 template character. Within each repetition of a group, positioning with
4925 C<@> starts over at 0. Therefore, the result of
4927 pack("@1A((@2A)@3A)", qw[X Y Z])
4929 is the string C<"\0X\0\0YZ">.
4933 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4934 jump forward or back to the closest position aligned at a multiple of C<count>
4935 characters. For example, to pack() or unpack() a C structure like
4938 char c; /* one signed, 8-bit character */
4943 one may need to use the template C<c x![d] d c[2]>. This assumes that
4944 doubles must be aligned to the size of double.
4946 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4951 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4952 represent signed 16-/32-bit integers in big-/little-endian order.
4953 This is portable only when all platforms sharing packed data use the
4954 same binary representation for signed integers; for example, when all
4955 platforms use two's-complement representation.
4959 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4960 White space can separate pack codes from each other, but modifiers and
4961 repeat counts must follow immediately. Breaking complex templates into
4962 individual line-by-line components, suitably annotated, can do as much to
4963 improve legibility and maintainability of pack/unpack formats as C</x> can
4964 for complicated pattern matches.
4968 If TEMPLATE requires more arguments than pack() is given, pack()
4969 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4970 than given, extra arguments are ignored.
4976 $foo = pack("WWWW",65,66,67,68);
4978 $foo = pack("W4",65,66,67,68);
4980 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4981 # same thing with Unicode circled letters.
4982 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4983 # same thing with Unicode circled letters. You don't get the
4984 # UTF-8 bytes because the U at the start of the format caused
4985 # a switch to U0-mode, so the UTF-8 bytes get joined into
4987 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4988 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4989 # This is the UTF-8 encoding of the string in the
4992 $foo = pack("ccxxcc",65,66,67,68);
4995 # NOTE: The examples above featuring "W" and "c" are true
4996 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4997 # and UTF-8. On EBCDIC systems, the first example would be
4998 # $foo = pack("WWWW",193,194,195,196);
5000 $foo = pack("s2",1,2);
5001 # "\001\000\002\000" on little-endian
5002 # "\000\001\000\002" on big-endian
5004 $foo = pack("a4","abcd","x","y","z");
5007 $foo = pack("aaaa","abcd","x","y","z");
5010 $foo = pack("a14","abcdefg");
5011 # "abcdefg\0\0\0\0\0\0\0"
5013 $foo = pack("i9pl", gmtime);
5014 # a real struct tm (on my system anyway)
5016 $utmp_template = "Z8 Z8 Z16 L";
5017 $utmp = pack($utmp_template, @utmp1);
5018 # a struct utmp (BSDish)
5020 @utmp2 = unpack($utmp_template, $utmp);
5021 # "@utmp1" eq "@utmp2"
5024 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
5027 $foo = pack('sx2l', 12, 34);
5028 # short 12, two zero bytes padding, long 34
5029 $bar = pack('s@4l', 12, 34);
5030 # short 12, zero fill to position 4, long 34
5032 $baz = pack('s.l', 12, 4, 34);
5033 # short 12, zero fill to position 4, long 34
5035 $foo = pack('nN', 42, 4711);
5036 # pack big-endian 16- and 32-bit unsigned integers
5037 $foo = pack('S>L>', 42, 4711);
5039 $foo = pack('s<l<', -42, 4711);
5040 # pack little-endian 16- and 32-bit signed integers
5041 $foo = pack('(sl)<', -42, 4711);
5044 The same template may generally also be used in unpack().
5046 =item package NAMESPACE
5048 =item package NAMESPACE VERSION
5049 X<package> X<module> X<namespace> X<version>
5051 =item package NAMESPACE BLOCK
5053 =item package NAMESPACE VERSION BLOCK
5054 X<package> X<module> X<namespace> X<version>
5056 =for Pod::Functions declare a separate global namespace
5058 Declares the BLOCK or the rest of the compilation unit as being in the
5059 given namespace. The scope of the package declaration is either the
5060 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
5061 itself through the end of current scope (the enclosing block, file, or
5062 C<eval>). That is, the forms without a BLOCK are operative through the end
5063 of the current scope, just like the C<my>, C<state>, and C<our> operators.
5064 All unqualified dynamic identifiers in this scope will be in the given
5065 namespace, except where overridden by another C<package> declaration or
5066 when they're one of the special identifiers that qualify into C<main::>,
5067 like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
5069 A package statement affects dynamic variables only, including those
5070 you've used C<local> on, but I<not> lexically-scoped variables, which are created
5071 with C<my>, C<state>, or C<our>. Typically it would be the first
5072 declaration in a file included by C<require> or C<use>. You can switch into a
5073 package in more than one place, since this only determines which default
5074 symbol table the compiler uses for the rest of that block. You can refer to
5075 identifiers in other packages than the current one by prefixing the identifier
5076 with the package name and a double colon, as in C<$SomePack::var>
5077 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
5078 package as assumed. That is, C<$::sail> is equivalent to
5079 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
5080 code, mostly from Perl 4).
5082 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
5083 namespace to a L<version> object with the VERSION provided. VERSION must be a
5084 "strict" style version number as defined by the L<version> module: a positive
5085 decimal number (integer or decimal-fraction) without exponentiation or else a
5086 dotted-decimal v-string with a leading 'v' character and at least three
5087 components. You should set C<$VERSION> only once per package.
5089 See L<perlmod/"Packages"> for more information about packages, modules,
5090 and classes. See L<perlsub> for other scoping issues.
5095 =for Pod::Functions +5.004 the current package
5097 A special token that returns the name of the package in which it occurs.
5099 =item pipe READHANDLE,WRITEHANDLE
5102 =for Pod::Functions open a pair of connected filehandles
5104 Opens a pair of connected pipes like the corresponding system call.
5105 Note that if you set up a loop of piped processes, deadlock can occur
5106 unless you are very careful. In addition, note that Perl's pipes use
5107 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
5108 after each command, depending on the application.
5110 Returns true on success.
5112 See L<IPC::Open2>, L<IPC::Open3>, and
5113 L<perlipc/"Bidirectional Communication with Another Process">
5114 for examples of such things.
5116 On systems that support a close-on-exec flag on files, that flag is set
5117 on all newly opened file descriptors whose C<fileno>s are I<higher> than
5118 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
5127 =for Pod::Functions remove the last element from an array and return it
5129 Pops and returns the last value of the array, shortening the array by
5132 Returns the undefined value if the array is empty, although this may also
5133 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
5134 main program, but the C<@_> array in subroutines, just like C<shift>.
5136 Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
5137 reference to an unblessed array. The argument will be dereferenced
5138 automatically. This aspect of C<pop> is considered highly experimental.
5139 The exact behaviour may change in a future version of Perl.
5141 To avoid confusing would-be users of your code who are running earlier
5142 versions of Perl with mysterious syntax errors, put this sort of thing at
5143 the top of your file to signal that your code will work I<only> on Perls of
5146 use 5.014; # so push/pop/etc work on scalars (experimental)
5149 X<pos> X<match, position>
5153 =for Pod::Functions find or set the offset for the last/next m//g search
5155 Returns the offset of where the last C<m//g> search left off for the
5156 variable in question (C<$_> is used when the variable is not
5157 specified). Note that 0 is a valid match offset. C<undef> indicates
5158 that the search position is reset (usually due to match failure, but
5159 can also be because no match has yet been run on the scalar).
5161 C<pos> directly accesses the location used by the regexp engine to
5162 store the offset, so assigning to C<pos> will change that offset, and
5163 so will also influence the C<\G> zero-width assertion in regular
5164 expressions. Both of these effects take place for the next match, so
5165 you can't affect the position with C<pos> during the current match,
5166 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
5168 Setting C<pos> also resets the I<matched with zero-length> flag, described
5169 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
5171 Because a failed C<m//gc> match doesn't reset the offset, the return
5172 from C<pos> won't change either in this case. See L<perlre> and
5175 =item print FILEHANDLE LIST
5178 =item print FILEHANDLE
5184 =for Pod::Functions output a list to a filehandle
5186 Prints a string or a list of strings. Returns true if successful.
5187 FILEHANDLE may be a scalar variable containing the name of or a reference
5188 to the filehandle, thus introducing one level of indirection. (NOTE: If
5189 FILEHANDLE is a variable and the next token is a term, it may be
5190 misinterpreted as an operator unless you interpose a C<+> or put
5191 parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
5192 last selected (see L</select>) output handle. If LIST is omitted, prints
5193 C<$_> to the currently selected output handle. To use FILEHANDLE alone to
5194 print the content of C<$_> to it, you must use a real filehandle like
5195 C<FH>, not an indirect one like C<$fh>. To set the default output handle
5196 to something other than STDOUT, use the select operation.
5198 The current value of C<$,> (if any) is printed between each LIST item. The
5199 current value of C<$\> (if any) is printed after the entire LIST has been
5200 printed. Because print takes a LIST, anything in the LIST is evaluated in
5201 list context, including any subroutines whose return lists you pass to
5202 C<print>. Be careful not to follow the print keyword with a left
5203 parenthesis unless you want the corresponding right parenthesis to
5204 terminate the arguments to the print; put parentheses around all arguments
5205 (or interpose a C<+>, but that doesn't look as good).
5207 If you're storing handles in an array or hash, or in general whenever
5208 you're using any expression more complex than a bareword handle or a plain,
5209 unsubscripted scalar variable to retrieve it, you will have to use a block
5210 returning the filehandle value instead, in which case the LIST may not be
5213 print { $files[$i] } "stuff\n";
5214 print { $OK ? STDOUT : STDERR } "stuff\n";
5216 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
5217 L<perlipc> for more on signal handling.
5219 =item printf FILEHANDLE FORMAT, LIST
5222 =item printf FILEHANDLE
5224 =item printf FORMAT, LIST
5228 =for Pod::Functions output a formatted list to a filehandle
5230 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
5231 (the output record separator) is not appended. The FORMAT and the
5232 LIST are actually parsed as a single list. The first argument
5233 of the list will be interpreted as the C<printf> format. This
5234 means that C<printf(@_)> will use C<$_[0]> as the format. See
5235 L<sprintf|/sprintf FORMAT, LIST> for an
5236 explanation of the format argument. If C<use locale> (including
5237 C<use locale ':not_characters'>) is in effect and
5238 POSIX::setlocale() has been called, the character used for the decimal
5239 separator in formatted floating-point numbers is affected by the LC_NUMERIC
5240 locale setting. See L<perllocale> and L<POSIX>.
5242 For historical reasons, if you omit the list, C<$_> is used as the format;
5243 to use FILEHANDLE without a list, you must use a real filehandle like
5244 C<FH>, not an indirect one like C<$fh>. However, this will rarely do what
5245 you want; if $_ contains formatting codes, they will be replaced with the
5246 empty string and a warning will be emitted if warnings are enabled. Just
5247 use C<print> if you want to print the contents of $_.
5249 Don't fall into the trap of using a C<printf> when a simple
5250 C<print> would do. The C<print> is more efficient and less
5253 =item prototype FUNCTION
5256 =for Pod::Functions +5.002 get the prototype (if any) of a subroutine
5258 Returns the prototype of a function as a string (or C<undef> if the
5259 function has no prototype). FUNCTION is a reference to, or the name of,
5260 the function whose prototype you want to retrieve.
5262 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
5263 name for a Perl builtin. If the builtin's arguments
5264 cannot be adequately expressed by a prototype
5265 (such as C<system>), prototype() returns C<undef>, because the builtin
5266 does not really behave like a Perl function. Otherwise, the string
5267 describing the equivalent prototype is returned.
5269 =item push ARRAY,LIST
5272 =item push EXPR,LIST
5274 =for Pod::Functions append one or more elements to an array
5276 Treats ARRAY as a stack by appending the values of LIST to the end of
5277 ARRAY. The length of ARRAY increases by the length of LIST. Has the same
5281 $ARRAY[++$#ARRAY] = $value;
5284 but is more efficient. Returns the number of elements in the array following
5285 the completed C<push>.
5287 Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
5288 reference to an unblessed array. The argument will be dereferenced
5289 automatically. This aspect of C<push> is considered highly experimental.
5290 The exact behaviour may change in a future version of Perl.
5292 To avoid confusing would-be users of your code who are running earlier
5293 versions of Perl with mysterious syntax errors, put this sort of thing at
5294 the top of your file to signal that your code will work I<only> on Perls of
5297 use 5.014; # so push/pop/etc work on scalars (experimental)
5301 =for Pod::Functions singly quote a string
5305 =for Pod::Functions doubly quote a string
5309 =for Pod::Functions quote a list of words
5313 =for Pod::Functions backquote quote a string
5315 Generalized quotes. See L<perlop/"Quote-Like Operators">.
5319 =for Pod::Functions +5.005 compile pattern
5321 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
5323 =item quotemeta EXPR
5324 X<quotemeta> X<metacharacter>
5328 =for Pod::Functions quote regular expression magic characters
5330 Returns the value of EXPR with all the ASCII non-"word"
5331 characters backslashed. (That is, all ASCII characters not matching
5332 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
5333 returned string, regardless of any locale settings.)
5334 This is the internal function implementing
5335 the C<\Q> escape in double-quoted strings.
5336 (See below for the behavior on non-ASCII code points.)
5338 If EXPR is omitted, uses C<$_>.
5340 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
5341 regular expressions, because by default an interpolated variable will be
5342 considered a mini-regular expression. For example:
5344 my $sentence = 'The quick brown fox jumped over the lazy dog';
5345 my $substring = 'quick.*?fox';
5346 $sentence =~ s{$substring}{big bad wolf};
5348 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
5352 my $sentence = 'The quick brown fox jumped over the lazy dog';
5353 my $substring = 'quick.*?fox';
5354 $sentence =~ s{\Q$substring\E}{big bad wolf};
5358 my $sentence = 'The quick brown fox jumped over the lazy dog';
5359 my $substring = 'quick.*?fox';
5360 my $quoted_substring = quotemeta($substring);
5361 $sentence =~ s{$quoted_substring}{big bad wolf};
5363 Will both leave the sentence as is.
5364 Normally, when accepting literal string
5365 input from the user, quotemeta() or C<\Q> must be used.
5367 In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded
5368 strings, but not quoted in UTF-8 strings.
5370 Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for
5371 quoting non-ASCII characters; the quoting of ASCII characters is
5374 Also unchanged is the quoting of non-UTF-8 strings when outside the
5375 scope of a C<use feature 'unicode_strings'>, which is to quote all
5376 characters in the upper Latin1 range. This provides complete backwards
5377 compatibility for old programs which do not use Unicode. (Note that
5378 C<unicode_strings> is automatically enabled within the scope of a
5379 S<C<use v5.12>> or greater.)
5381 Within the scope of C<use locale>, all non-ASCII Latin1 code points
5382 are quoted whether the string is encoded as UTF-8 or not. As mentioned
5383 above, locale does not affect the quoting of ASCII-range characters.
5384 This protects against those locales where characters such as C<"|"> are
5385 considered to be word characters.
5387 Otherwise, Perl quotes non-ASCII characters using an adaptation from
5388 Unicode (see L<http://www.unicode.org/reports/tr31/>).
5389 The only code points that are quoted are those that have any of the
5390 Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space,
5391 Default_Ignorable_Code_Point, or General_Category=Control.
5393 Of these properties, the two important ones are Pattern_Syntax and
5394 Pattern_White_Space. They have been set up by Unicode for exactly this
5395 purpose of deciding which characters in a regular expression pattern
5396 should be quoted. No character that can be in an identifier has these
5399 Perl promises, that if we ever add regular expression pattern
5400 metacharacters to the dozen already defined
5401 (C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the
5402 Pattern_Syntax property. Perl also promises, that if we ever add
5403 characters that are considered to be white space in regular expressions
5404 (currently mostly affected by C</x>), they will all have the
5405 Pattern_White_Space property.
5407 Unicode promises that the set of code points that have these two
5408 properties will never change, so something that is not quoted in v5.16
5409 will never need to be quoted in any future Perl release. (Not all the
5410 code points that match Pattern_Syntax have actually had characters
5411 assigned to them; so there is room to grow, but they are quoted
5412 whether assigned or not. Perl, of course, would never use an
5413 unassigned code point as an actual metacharacter.)
5415 Quoting characters that have the other 3 properties is done to enhance
5416 the readability of the regular expression and not because they actually
5417 need to be quoted for regular expression purposes (characters with the
5418 White_Space property are likely to be indistinguishable on the page or
5419 screen from those with the Pattern_White_Space property; and the other
5420 two properties contain non-printing characters).
5427 =for Pod::Functions retrieve the next pseudorandom number
5429 Returns a random fractional number greater than or equal to C<0> and less
5430 than the value of EXPR. (EXPR should be positive.) If EXPR is
5431 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
5432 also special-cased as C<1> (this was undocumented before Perl 5.8.0
5433 and is subject to change in future versions of Perl). Automatically calls
5434 C<srand> unless C<srand> has already been called. See also C<srand>.
5436 Apply C<int()> to the value returned by C<rand()> if you want random
5437 integers instead of random fractional numbers. For example,
5441 returns a random integer between C<0> and C<9>, inclusive.
5443 (Note: If your rand function consistently returns numbers that are too
5444 large or too small, then your version of Perl was probably compiled
5445 with the wrong number of RANDBITS.)
5447 B<C<rand()> is not cryptographically secure. You should not rely
5448 on it in security-sensitive situations.> As of this writing, a
5449 number of third-party CPAN modules offer random number generators
5450 intended by their authors to be cryptographically secure,
5451 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
5452 and L<Math::TrulyRandom>.
5454 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
5455 X<read> X<file, read>
5457 =item read FILEHANDLE,SCALAR,LENGTH
5459 =for Pod::Functions fixed-length buffered input from a filehandle
5461 Attempts to read LENGTH I<characters> of data into variable SCALAR
5462 from the specified FILEHANDLE. Returns the number of characters
5463 actually read, C<0> at end of file, or undef if there was an error (in
5464 the latter case C<$!> is also set). SCALAR will be grown or shrunk
5465 so that the last character actually read is the last character of the
5466 scalar after the read.
5468 An OFFSET may be specified to place the read data at some place in the
5469 string other than the beginning. A negative OFFSET specifies
5470 placement at that many characters counting backwards from the end of
5471 the string. A positive OFFSET greater than the length of SCALAR
5472 results in the string being padded to the required size with C<"\0">
5473 bytes before the result of the read is appended.
5475 The call is implemented in terms of either Perl's or your system's native
5476 fread(3) library function. To get a true read(2) system call, see
5477 L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
5479 Note the I<characters>: depending on the status of the filehandle,
5480 either (8-bit) bytes or characters are read. By default, all
5481 filehandles operate on bytes, but for example if the filehandle has
5482 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5483 pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5484 characters, not bytes. Similarly for the C<:encoding> pragma:
5485 in that case pretty much any characters can be read.
5487 =item readdir DIRHANDLE
5490 =for Pod::Functions get a directory from a directory handle
5492 Returns the next directory entry for a directory opened by C<opendir>.
5493 If used in list context, returns all the rest of the entries in the
5494 directory. If there are no more entries, returns the undefined value in
5495 scalar context and the empty list in list context.
5497 If you're planning to filetest the return values out of a C<readdir>, you'd
5498 better prepend the directory in question. Otherwise, because we didn't
5499 C<chdir> there, it would have been testing the wrong file.
5501 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
5502 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
5505 As of Perl 5.12 you can use a bare C<readdir> in a C<while> loop,
5506 which will set C<$_> on every iteration.
5508 opendir(my $dh, $some_dir) || die;
5509 while(readdir $dh) {
5510 print "$some_dir/$_\n";
5514 To avoid confusing would-be users of your code who are running earlier
5515 versions of Perl with mysterious failures, put this sort of thing at the
5516 top of your file to signal that your code will work I<only> on Perls of a
5519 use 5.012; # so readdir assigns to $_ in a lone while test
5524 X<readline> X<gets> X<fgets>
5526 =for Pod::Functions fetch a record from a file
5528 Reads from the filehandle whose typeglob is contained in EXPR (or from
5529 C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
5530 returns the next line until end-of-file is reached, whereupon the
5531 subsequent call returns C<undef>. In list context, reads until end-of-file
5532 is reached and returns a list of lines. Note that the notion of "line"
5533 used here is whatever you may have defined with C<$/> or
5534 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
5536 When C<$/> is set to C<undef>, when C<readline> is in scalar
5537 context (i.e., file slurp mode), and when an empty file is read, it
5538 returns C<''> the first time, followed by C<undef> subsequently.
5540 This is the internal function implementing the C<< <EXPR> >>
5541 operator, but you can use it directly. The C<< <EXPR> >>
5542 operator is discussed in more detail in L<perlop/"I/O Operators">.
5545 $line = readline(*STDIN); # same thing
5547 If C<readline> encounters an operating system error, C<$!> will be set
5548 with the corresponding error message. It can be helpful to check
5549 C<$!> when you are reading from filehandles you don't trust, such as a
5550 tty or a socket. The following example uses the operator form of
5551 C<readline> and dies if the result is not defined.
5553 while ( ! eof($fh) ) {
5554 defined( $_ = <$fh> ) or die "readline failed: $!";
5558 Note that you have can't handle C<readline> errors that way with the
5559 C<ARGV> filehandle. In that case, you have to open each element of
5560 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
5562 foreach my $arg (@ARGV) {
5563 open(my $fh, $arg) or warn "Can't open $arg: $!";
5565 while ( ! eof($fh) ) {
5566 defined( $_ = <$fh> )
5567 or die "readline failed for $arg: $!";
5577 =for Pod::Functions determine where a symbolic link is pointing
5579 Returns the value of a symbolic link, if symbolic links are
5580 implemented. If not, raises an exception. If there is a system
5581 error, returns the undefined value and sets C<$!> (errno). If EXPR is
5582 omitted, uses C<$_>.
5584 Portability issues: L<perlport/readlink>.
5591 =for Pod::Functions execute a system command and collect standard output
5593 EXPR is executed as a system command.
5594 The collected standard output of the command is returned.
5595 In scalar context, it comes back as a single (potentially
5596 multi-line) string. In list context, returns a list of lines
5597 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
5598 This is the internal function implementing the C<qx/EXPR/>
5599 operator, but you can use it directly. The C<qx/EXPR/>
5600 operator is discussed in more detail in L<perlop/"I/O Operators">.
5601 If EXPR is omitted, uses C<$_>.
5603 =item recv SOCKET,SCALAR,LENGTH,FLAGS
5606 =for Pod::Functions receive a message over a Socket
5608 Receives a message on a socket. Attempts to receive LENGTH characters
5609 of data into variable SCALAR from the specified SOCKET filehandle.
5610 SCALAR will be grown or shrunk to the length actually read. Takes the
5611 same flags as the system call of the same name. Returns the address
5612 of the sender if SOCKET's protocol supports this; returns an empty
5613 string otherwise. If there's an error, returns the undefined value.
5614 This call is actually implemented in terms of recvfrom(2) system call.
5615 See L<perlipc/"UDP: Message Passing"> for examples.
5617 Note the I<characters>: depending on the status of the socket, either
5618 (8-bit) bytes or characters are received. By default all sockets
5619 operate on bytes, but for example if the socket has been changed using
5620 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
5621 C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
5622 characters, not bytes. Similarly for the C<:encoding> pragma: in that
5623 case pretty much any characters can be read.
5632 =for Pod::Functions start this loop iteration over again
5634 The C<redo> command restarts the loop block without evaluating the
5635 conditional again. The C<continue> block, if any, is not executed. If
5636 the LABEL is omitted, the command refers to the innermost enclosing
5637 loop. The C<redo EXPR> form, available starting in Perl 5.18.0, allows a
5638 label name to be computed at run time, and is otherwise identical to C<redo
5639 LABEL>. Programs that want to lie to themselves about what was just input
5640 normally use this command:
5642 # a simpleminded Pascal comment stripper
5643 # (warning: assumes no { or } in strings)
5644 LINE: while (<STDIN>) {
5645 while (s|({.*}.*){.*}|$1 |) {}
5650 if (/}/) { # end of comment?
5659 C<redo> cannot be used to retry a block that returns a value such as
5660 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
5661 a grep() or map() operation.
5663 Note that a block by itself is semantically identical to a loop
5664 that executes once. Thus C<redo> inside such a block will effectively
5665 turn it into a looping construct.
5667 See also L</continue> for an illustration of how C<last>, C<next>, and
5670 Unlike most named operators, this has the same precedence as assignment.
5671 It is also exempt from the looks-like-a-function rule, so
5672 C<redo ("foo")."bar"> will cause "bar" to be part of the argument to
5680 =for Pod::Functions find out the type of thing being referenced
5682 Returns a non-empty string if EXPR is a reference, the empty
5683 string otherwise. If EXPR is not specified, C<$_> will be used. The
5684 value returned depends on the type of thing the reference is a reference to.
5686 Builtin types include:
5700 You can think of C<ref> as a C<typeof> operator.
5702 if (ref($r) eq "HASH") {
5703 print "r is a reference to a hash.\n";
5706 print "r is not a reference at all.\n";
5709 The return value C<LVALUE> indicates a reference to an lvalue that is not
5710 a variable. You get this from taking the reference of function calls like
5711 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
5712 to a L<version string|perldata/"Version Strings">.
5714 The result C<Regexp> indicates that the argument is a regular expression
5715 resulting from C<qr//>.
5717 If the referenced object has been blessed into a package, then that package
5718 name is returned instead. But don't use that, as it's now considered
5719 "bad practice". For one reason, an object could be using a class called
5720 C<Regexp> or C<IO>, or even C<HASH>. Also, C<ref> doesn't take into account
5721 subclasses, like C<isa> does.
5723 Instead, use C<blessed> (in the L<Scalar::Util> module) for boolean
5724 checks, C<isa> for specific class checks and C<reftype> (also from
5725 L<Scalar::Util>) for type checks. (See L<perlobj> for details and a
5726 C<blessed/isa> example.)
5728 See also L<perlref>.
5730 =item rename OLDNAME,NEWNAME
5731 X<rename> X<move> X<mv> X<ren>
5733 =for Pod::Functions change a filename
5735 Changes the name of a file; an existing file NEWNAME will be
5736 clobbered. Returns true for success, false otherwise.
5738 Behavior of this function varies wildly depending on your system
5739 implementation. For example, it will usually not work across file system
5740 boundaries, even though the system I<mv> command sometimes compensates
5741 for this. Other restrictions include whether it works on directories,
5742 open files, or pre-existing files. Check L<perlport> and either the
5743 rename(2) manpage or equivalent system documentation for details.
5745 For a platform independent C<move> function look at the L<File::Copy>
5748 Portability issues: L<perlport/rename>.
5750 =item require VERSION
5757 =for Pod::Functions load in external functions from a library at runtime
5759 Demands a version of Perl specified by VERSION, or demands some semantics
5760 specified by EXPR or by C<$_> if EXPR is not supplied.
5762 VERSION may be either a numeric argument such as 5.006, which will be
5763 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
5764 to C<$^V> (aka $PERL_VERSION). An exception is raised if
5765 VERSION is greater than the version of the current Perl interpreter.
5766 Compare with L</use>, which can do a similar check at compile time.
5768 Specifying VERSION as a literal of the form v5.6.1 should generally be
5769 avoided, because it leads to misleading error messages under earlier
5770 versions of Perl that do not support this syntax. The equivalent numeric
5771 version should be used instead.
5773 require v5.6.1; # run time version check
5774 require 5.6.1; # ditto
5775 require 5.006_001; # ditto; preferred for backwards
5778 Otherwise, C<require> demands that a library file be included if it
5779 hasn't already been included. The file is included via the do-FILE
5780 mechanism, which is essentially just a variety of C<eval> with the
5781 caveat that lexical variables in the invoking script will be invisible
5782 to the included code. If it were implemented in pure Perl, it
5783 would have semantics similar to the following:
5789 my ($filename) = @_;
5790 if ( my $version = eval { version->parse($filename) } ) {
5791 if ( $version > $^V ) {
5792 my $vn = $version->normal;
5793 croak "Perl $vn required--this is only $^V, stopped";
5797 if (exists $INC{$filename}) {
5798 return 1 if $INC{$filename};
5799 croak "Compilation failed in require";
5801 my ($realfilename,$result);
5803 foreach $prefix (@INC) {
5804 $realfilename = "$prefix/$filename";
5805 if (-f $realfilename) {
5806 $INC{$filename} = $realfilename;
5807 my $caller = caller;
5808 my $do_as_caller = eval qq{
5812 $result = $do_as_caller->($realfilename);
5816 croak "Can't locate $filename in \@INC";
5819 $INC{$filename} = undef;
5821 } elsif (!$result) {
5822 delete $INC{$filename};
5823 croak "$filename did not return true value";
5830 Note that the file will not be included twice under the same specified
5833 The file must return true as the last statement to indicate
5834 successful execution of any initialization code, so it's customary to
5835 end such a file with C<1;> unless you're sure it'll return true
5836 otherwise. But it's better just to put the C<1;>, in case you add more
5839 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
5840 replaces "F<::>" with "F</>" in the filename for you,
5841 to make it easy to load standard modules. This form of loading of
5842 modules does not risk altering your namespace.
5844 In other words, if you try this:
5846 require Foo::Bar; # a splendid bareword
5848 The require function will actually look for the "F<Foo/Bar.pm>" file in the
5849 directories specified in the C<@INC> array.
5851 But if you try this:
5853 $class = 'Foo::Bar';
5854 require $class; # $class is not a bareword
5856 require "Foo::Bar"; # not a bareword because of the ""
5858 The require function will look for the "F<Foo::Bar>" file in the @INC array and
5859 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
5861 eval "require $class";
5863 Now that you understand how C<require> looks for files with a
5864 bareword argument, there is a little extra functionality going on behind
5865 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
5866 first look for a similar filename with a "F<.pmc>" extension. If this file
5867 is found, it will be loaded in place of any file ending in a "F<.pm>"
5870 You can also insert hooks into the import facility by putting Perl code
5871 directly into the @INC array. There are three forms of hooks: subroutine
5872 references, array references, and blessed objects.
5874 Subroutine references are the simplest case. When the inclusion system
5875 walks through @INC and encounters a subroutine, this subroutine gets
5876 called with two parameters, the first a reference to itself, and the
5877 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
5878 subroutine should return either nothing or else a list of up to four
5879 values in the following order:
5885 A reference to a scalar, containing any initial source code to prepend to
5886 the file or generator output.
5890 A filehandle, from which the file will be read.
5894 A reference to a subroutine. If there is no filehandle (previous item),
5895 then this subroutine is expected to generate one line of source code per
5896 call, writing the line into C<$_> and returning 1, then finally at end of
5897 file returning 0. If there is a filehandle, then the subroutine will be
5898 called to act as a simple source filter, with the line as read in C<$_>.
5899 Again, return 1 for each valid line, and 0 after all lines have been
5904 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
5905 reference to the subroutine itself is passed in as C<$_[0]>.
5909 If an empty list, C<undef>, or nothing that matches the first 3 values above
5910 is returned, then C<require> looks at the remaining elements of @INC.
5911 Note that this filehandle must be a real filehandle (strictly a typeglob
5912 or reference to a typeglob, whether blessed or unblessed); tied filehandles
5913 will be ignored and processing will stop there.
5915 If the hook is an array reference, its first element must be a subroutine
5916 reference. This subroutine is called as above, but the first parameter is
5917 the array reference. This lets you indirectly pass arguments to
5920 In other words, you can write:
5922 push @INC, \&my_sub;
5924 my ($coderef, $filename) = @_; # $coderef is \&my_sub
5930 push @INC, [ \&my_sub, $x, $y, ... ];
5932 my ($arrayref, $filename) = @_;
5933 # Retrieve $x, $y, ...
5934 my @parameters = @$arrayref[1..$#$arrayref];
5938 If the hook is an object, it must provide an INC method that will be
5939 called as above, the first parameter being the object itself. (Note that
5940 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5941 into package C<main>.) Here is a typical code layout:
5947 my ($self, $filename) = @_;
5951 # In the main program
5952 push @INC, Foo->new(...);
5954 These hooks are also permitted to set the %INC entry
5955 corresponding to the files they have loaded. See L<perlvar/%INC>.
5957 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5964 =for Pod::Functions clear all variables of a given name
5966 Generally used in a C<continue> block at the end of a loop to clear
5967 variables and reset C<??> searches so that they work again. The
5968 expression is interpreted as a list of single characters (hyphens
5969 allowed for ranges). All variables and arrays beginning with one of
5970 those letters are reset to their pristine state. If the expression is
5971 omitted, one-match searches (C<?pattern?>) are reset to match again.
5972 Only resets variables or searches in the current package. Always returns
5975 reset 'X'; # reset all X variables
5976 reset 'a-z'; # reset lower case variables
5977 reset; # just reset ?one-time? searches
5979 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5980 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5981 variables; lexical variables are unaffected, but they clean themselves
5982 up on scope exit anyway, so you'll probably want to use them instead.
5990 =for Pod::Functions get out of a function early
5992 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5993 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5994 context, depending on how the return value will be used, and the context
5995 may vary from one execution to the next (see L</wantarray>). If no EXPR
5996 is given, returns an empty list in list context, the undefined value in
5997 scalar context, and (of course) nothing at all in void context.
5999 (In the absence of an explicit C<return>, a subroutine, eval,
6000 or do FILE automatically returns the value of the last expression
6003 Unlike most named operators, this is also exempt from the
6004 looks-like-a-function rule, so C<return ("foo")."bar"> will
6005 cause "bar" to be part of the argument to C<return>.
6008 X<reverse> X<rev> X<invert>
6010 =for Pod::Functions flip a string or a list
6012 In list context, returns a list value consisting of the elements
6013 of LIST in the opposite order. In scalar context, concatenates the
6014 elements of LIST and returns a string value with all characters
6015 in the opposite order.
6017 print join(", ", reverse "world", "Hello"); # Hello, world
6019 print scalar reverse "dlrow ,", "olleH"; # Hello, world
6021 Used without arguments in scalar context, reverse() reverses C<$_>.
6023 $_ = "dlrow ,olleH";
6024 print reverse; # No output, list context
6025 print scalar reverse; # Hello, world
6027 Note that reversing an array to itself (as in C<@a = reverse @a>) will
6028 preserve non-existent elements whenever possible; i.e., for non-magical
6029 arrays or for tied arrays with C<EXISTS> and C<DELETE> methods.
6031 This operator is also handy for inverting a hash, although there are some
6032 caveats. If a value is duplicated in the original hash, only one of those
6033 can be represented as a key in the inverted hash. Also, this has to
6034 unwind one hash and build a whole new one, which may take some time
6035 on a large hash, such as from a DBM file.
6037 %by_name = reverse %by_address; # Invert the hash
6039 =item rewinddir DIRHANDLE
6042 =for Pod::Functions reset directory handle
6044 Sets the current position to the beginning of the directory for the
6045 C<readdir> routine on DIRHANDLE.
6047 Portability issues: L<perlport/rewinddir>.
6049 =item rindex STR,SUBSTR,POSITION
6052 =item rindex STR,SUBSTR
6054 =for Pod::Functions right-to-left substring search
6056 Works just like index() except that it returns the position of the I<last>
6057 occurrence of SUBSTR in STR. If POSITION is specified, returns the
6058 last occurrence beginning at or before that position.
6060 =item rmdir FILENAME
6061 X<rmdir> X<rd> X<directory, remove>
6065 =for Pod::Functions remove a directory
6067 Deletes the directory specified by FILENAME if that directory is
6068 empty. If it succeeds it returns true; otherwise it returns false and
6069 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
6071 To remove a directory tree recursively (C<rm -rf> on Unix) look at
6072 the C<rmtree> function of the L<File::Path> module.
6076 =for Pod::Functions replace a pattern with a string
6078 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
6080 =item say FILEHANDLE LIST
6083 =item say FILEHANDLE
6089 =for Pod::Functions +say output a list to a filehandle, appending a newline
6091 Just like C<print>, but implicitly appends a newline. C<say LIST> is
6092 simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
6093 FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
6094 use a real filehandle like C<FH>, not an indirect one like C<$fh>.
6096 This keyword is available only when the C<"say"> feature
6097 is enabled, or when prefixed with C<CORE::>; see
6098 L<feature>. Alternately, include a C<use v5.10> or later to the current
6102 X<scalar> X<context>
6104 =for Pod::Functions force a scalar context
6106 Forces EXPR to be interpreted in scalar context and returns the value
6109 @counts = ( scalar @a, scalar @b, scalar @c );
6111 There is no equivalent operator to force an expression to
6112 be interpolated in list context because in practice, this is never
6113 needed. If you really wanted to do so, however, you could use
6114 the construction C<@{[ (some expression) ]}>, but usually a simple
6115 C<(some expression)> suffices.
6117 Because C<scalar> is a unary operator, if you accidentally use a
6118 parenthesized list for the EXPR, this behaves as a scalar comma expression,
6119 evaluating all but the last element in void context and returning the final
6120 element evaluated in scalar context. This is seldom what you want.
6122 The following single statement:
6124 print uc(scalar(&foo,$bar)),$baz;
6126 is the moral equivalent of these two:
6129 print(uc($bar),$baz);
6131 See L<perlop> for more details on unary operators and the comma operator.
6133 =item seek FILEHANDLE,POSITION,WHENCE
6134 X<seek> X<fseek> X<filehandle, position>
6136 =for Pod::Functions reposition file pointer for random-access I/O
6138 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
6139 FILEHANDLE may be an expression whose value gives the name of the
6140 filehandle. The values for WHENCE are C<0> to set the new position
6141 I<in bytes> to POSITION; C<1> to set it to the current position plus
6142 POSITION; and C<2> to set it to EOF plus POSITION, typically
6143 negative. For WHENCE you may use the constants C<SEEK_SET>,
6144 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
6145 of the file) from the L<Fcntl> module. Returns C<1> on success, false
6148 Note the I<in bytes>: even if the filehandle has been set to
6149 operate on characters (for example by using the C<:encoding(utf8)> open
6150 layer), tell() will return byte offsets, not character offsets
6151 (because implementing that would render seek() and tell() rather slow).
6153 If you want to position the file for C<sysread> or C<syswrite>, don't use
6154 C<seek>, because buffering makes its effect on the file's read-write position
6155 unpredictable and non-portable. Use C<sysseek> instead.
6157 Due to the rules and rigors of ANSI C, on some systems you have to do a
6158 seek whenever you switch between reading and writing. Amongst other
6159 things, this may have the effect of calling stdio's clearerr(3).
6160 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
6164 This is also useful for applications emulating C<tail -f>. Once you hit
6165 EOF on your read and then sleep for a while, you (probably) have to stick in a
6166 dummy seek() to reset things. The C<seek> doesn't change the position,
6167 but it I<does> clear the end-of-file condition on the handle, so that the
6168 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
6170 If that doesn't work (some I/O implementations are particularly
6171 cantankerous), you might need something like this:
6174 for ($curpos = tell(FILE); $_ = <FILE>;
6175 $curpos = tell(FILE)) {
6176 # search for some stuff and put it into files
6178 sleep($for_a_while);
6179 seek(FILE, $curpos, 0);
6182 =item seekdir DIRHANDLE,POS
6185 =for Pod::Functions reposition directory pointer
6187 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
6188 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
6189 about possible directory compaction as the corresponding system library
6192 =item select FILEHANDLE
6193 X<select> X<filehandle, default>
6197 =for Pod::Functions reset default output or do I/O multiplexing
6199 Returns the currently selected filehandle. If FILEHANDLE is supplied,
6200 sets the new current default filehandle for output. This has two
6201 effects: first, a C<write> or a C<print> without a filehandle
6202 default to this FILEHANDLE. Second, references to variables related to
6203 output will refer to this output channel.
6205 For example, to set the top-of-form format for more than one
6206 output channel, you might do the following:
6213 FILEHANDLE may be an expression whose value gives the name of the
6214 actual filehandle. Thus:
6216 $oldfh = select(STDERR); $| = 1; select($oldfh);
6218 Some programmers may prefer to think of filehandles as objects with
6219 methods, preferring to write the last example as:
6222 STDERR->autoflush(1);
6224 Portability issues: L<perlport/select>.
6226 =item select RBITS,WBITS,EBITS,TIMEOUT
6229 This calls the select(2) syscall with the bit masks specified, which
6230 can be constructed using C<fileno> and C<vec>, along these lines:
6232 $rin = $win = $ein = '';
6233 vec($rin, fileno(STDIN), 1) = 1;
6234 vec($win, fileno(STDOUT), 1) = 1;
6237 If you want to select on many filehandles, you may wish to write a
6238 subroutine like this:
6243 for my $fh (@fhlist) {
6244 vec($bits, fileno($fh), 1) = 1;
6248 $rin = fhbits(*STDIN, *TTY, *MYSOCK);
6252 ($nfound,$timeleft) =
6253 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
6255 or to block until something becomes ready just do this
6257 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
6259 Most systems do not bother to return anything useful in $timeleft, so
6260 calling select() in scalar context just returns $nfound.
6262 Any of the bit masks can also be undef. The timeout, if specified, is
6263 in seconds, which may be fractional. Note: not all implementations are
6264 capable of returning the $timeleft. If not, they always return
6265 $timeleft equal to the supplied $timeout.
6267 You can effect a sleep of 250 milliseconds this way:
6269 select(undef, undef, undef, 0.25);
6271 Note that whether C<select> gets restarted after signals (say, SIGALRM)
6272 is implementation-dependent. See also L<perlport> for notes on the
6273 portability of C<select>.
6275 On error, C<select> behaves just like select(2): it returns
6278 On some Unixes, select(2) may report a socket file descriptor as "ready for
6279 reading" even when no data is available, and thus any subsequent C<read>
6280 would block. This can be avoided if you always use O_NONBLOCK on the
6281 socket. See select(2) and fcntl(2) for further details.
6283 The standard C<IO::Select> module provides a user-friendlier interface
6284 to C<select>, mostly because it does all the bit-mask work for you.
6286 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
6287 or <FH>) with C<select>, except as permitted by POSIX, and even
6288 then only on POSIX systems. You have to use C<sysread> instead.
6290 Portability issues: L<perlport/select>.
6292 =item semctl ID,SEMNUM,CMD,ARG
6295 =for Pod::Functions SysV semaphore control operations
6297 Calls the System V IPC function semctl(2). You'll probably have to say
6301 first to get the correct constant definitions. If CMD is IPC_STAT or
6302 GETALL, then ARG must be a variable that will hold the returned
6303 semid_ds structure or semaphore value array. Returns like C<ioctl>:
6304 the undefined value for error, "C<0 but true>" for zero, or the actual
6305 return value otherwise. The ARG must consist of a vector of native
6306 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
6307 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
6310 Portability issues: L<perlport/semctl>.
6312 =item semget KEY,NSEMS,FLAGS
6315 =for Pod::Functions get set of SysV semaphores
6317 Calls the System V IPC function semget(2). Returns the semaphore id, or
6318 the undefined value on error. See also
6319 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
6322 Portability issues: L<perlport/semget>.
6324 =item semop KEY,OPSTRING
6327 =for Pod::Functions SysV semaphore operations
6329 Calls the System V IPC function semop(2) for semaphore operations
6330 such as signalling and waiting. OPSTRING must be a packed array of
6331 semop structures. Each semop structure can be generated with
6332 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
6333 implies the number of semaphore operations. Returns true if
6334 successful, false on error. As an example, the
6335 following code waits on semaphore $semnum of semaphore id $semid:
6337 $semop = pack("s!3", $semnum, -1, 0);
6338 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
6340 To signal the semaphore, replace C<-1> with C<1>. See also
6341 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
6344 Portability issues: L<perlport/semop>.
6346 =item send SOCKET,MSG,FLAGS,TO
6349 =item send SOCKET,MSG,FLAGS
6351 =for Pod::Functions send a message over a socket
6353 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
6354 filehandle. Takes the same flags as the system call of the same name. On
6355 unconnected sockets, you must specify a destination to I<send to>, in which
6356 case it does a sendto(2) syscall. Returns the number of characters sent,
6357 or the undefined value on error. The sendmsg(2) syscall is currently
6358 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
6360 Note the I<characters>: depending on the status of the socket, either
6361 (8-bit) bytes or characters are sent. By default all sockets operate
6362 on bytes, but for example if the socket has been changed using
6363 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
6364 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
6365 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
6366 pragma: in that case pretty much any characters can be sent.
6368 =item setpgrp PID,PGRP
6371 =for Pod::Functions set the process group of a process
6373 Sets the current process group for the specified PID, C<0> for the current
6374 process. Raises an exception when used on a machine that doesn't
6375 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
6376 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
6377 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
6380 Portability issues: L<perlport/setpgrp>.
6382 =item setpriority WHICH,WHO,PRIORITY
6383 X<setpriority> X<priority> X<nice> X<renice>
6385 =for Pod::Functions set a process's nice value
6387 Sets the current priority for a process, a process group, or a user.
6388 (See setpriority(2).) Raises an exception when used on a machine
6389 that doesn't implement setpriority(2).
6391 Portability issues: L<perlport/setpriority>.
6393 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
6396 =for Pod::Functions set some socket options
6398 Sets the socket option requested. Returns C<undef> on error.
6399 Use integer constants provided by the C<Socket> module for
6400 LEVEL and OPNAME. Values for LEVEL can also be obtained from
6401 getprotobyname. OPTVAL might either be a packed string or an integer.
6402 An integer OPTVAL is shorthand for pack("i", OPTVAL).
6404 An example disabling Nagle's algorithm on a socket:
6406 use Socket qw(IPPROTO_TCP TCP_NODELAY);
6407 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
6409 Portability issues: L<perlport/setsockopt>.
6418 =for Pod::Functions remove the first element of an array, and return it
6420 Shifts the first value of the array off and returns it, shortening the
6421 array by 1 and moving everything down. If there are no elements in the
6422 array, returns the undefined value. If ARRAY is omitted, shifts the
6423 C<@_> array within the lexical scope of subroutines and formats, and the
6424 C<@ARGV> array outside a subroutine and also within the lexical scopes
6425 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
6426 C<UNITCHECK {}>, and C<END {}> constructs.
6428 Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
6429 reference to an unblessed array. The argument will be dereferenced
6430 automatically. This aspect of C<shift> is considered highly experimental.
6431 The exact behaviour may change in a future version of Perl.
6433 To avoid confusing would-be users of your code who are running earlier
6434 versions of Perl with mysterious syntax errors, put this sort of thing at
6435 the top of your file to signal that your code will work I<only> on Perls of
6438 use 5.014; # so push/pop/etc work on scalars (experimental)
6440 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
6441 same thing to the left end of an array that C<pop> and C<push> do to the
6444 =item shmctl ID,CMD,ARG
6447 =for Pod::Functions SysV shared memory operations
6449 Calls the System V IPC function shmctl. You'll probably have to say
6453 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
6454 then ARG must be a variable that will hold the returned C<shmid_ds>
6455 structure. Returns like ioctl: C<undef> for error; "C<0> but
6456 true" for zero; and the actual return value otherwise.
6457 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6459 Portability issues: L<perlport/shmctl>.
6461 =item shmget KEY,SIZE,FLAGS
6464 =for Pod::Functions get SysV shared memory segment identifier
6466 Calls the System V IPC function shmget. Returns the shared memory
6467 segment id, or C<undef> on error.
6468 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
6470 Portability issues: L<perlport/shmget>.
6472 =item shmread ID,VAR,POS,SIZE
6476 =for Pod::Functions read SysV shared memory
6478 =item shmwrite ID,STRING,POS,SIZE
6480 =for Pod::Functions write SysV shared memory
6482 Reads or writes the System V shared memory segment ID starting at
6483 position POS for size SIZE by attaching to it, copying in/out, and
6484 detaching from it. When reading, VAR must be a variable that will
6485 hold the data read. When writing, if STRING is too long, only SIZE
6486 bytes are used; if STRING is too short, nulls are written to fill out
6487 SIZE bytes. Return true if successful, false on error.
6488 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
6489 C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
6491 Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
6493 =item shutdown SOCKET,HOW
6496 =for Pod::Functions close down just half of a socket connection
6498 Shuts down a socket connection in the manner indicated by HOW, which
6499 has the same interpretation as in the syscall of the same name.
6501 shutdown(SOCKET, 0); # I/we have stopped reading data
6502 shutdown(SOCKET, 1); # I/we have stopped writing data
6503 shutdown(SOCKET, 2); # I/we have stopped using this socket
6505 This is useful with sockets when you want to tell the other
6506 side you're done writing but not done reading, or vice versa.
6507 It's also a more insistent form of close because it also
6508 disables the file descriptor in any forked copies in other
6511 Returns C<1> for success; on error, returns C<undef> if
6512 the first argument is not a valid filehandle, or returns C<0> and sets
6513 C<$!> for any other failure.
6516 X<sin> X<sine> X<asin> X<arcsine>
6520 =for Pod::Functions return the sine of a number
6522 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
6523 returns sine of C<$_>.
6525 For the inverse sine operation, you may use the C<Math::Trig::asin>
6526 function, or use this relation:
6528 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
6535 =for Pod::Functions block for some number of seconds
6537 Causes the script to sleep for (integer) EXPR seconds, or forever if no
6538 argument is given. Returns the integer number of seconds actually slept.
6540 May be interrupted if the process receives a signal such as C<SIGALRM>.
6543 local $SIG{ALARM} = sub { die "Alarm!\n" };
6546 die $@ unless $@ eq "Alarm!\n";
6548 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
6549 is often implemented using C<alarm>.
6551 On some older systems, it may sleep up to a full second less than what
6552 you requested, depending on how it counts seconds. Most modern systems
6553 always sleep the full amount. They may appear to sleep longer than that,
6554 however, because your process might not be scheduled right away in a
6555 busy multitasking system.
6557 For delays of finer granularity than one second, the Time::HiRes module
6558 (from CPAN, and starting from Perl 5.8 part of the standard
6559 distribution) provides usleep(). You may also use Perl's four-argument
6560 version of select() leaving the first three arguments undefined, or you
6561 might be able to use the C<syscall> interface to access setitimer(2) if
6562 your system supports it. See L<perlfaq8> for details.
6564 See also the POSIX module's C<pause> function.
6566 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
6569 =for Pod::Functions create a socket
6571 Opens a socket of the specified kind and attaches it to filehandle
6572 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
6573 the syscall of the same name. You should C<use Socket> first
6574 to get the proper definitions imported. See the examples in
6575 L<perlipc/"Sockets: Client/Server Communication">.
6577 On systems that support a close-on-exec flag on files, the flag will
6578 be set for the newly opened file descriptor, as determined by the
6579 value of $^F. See L<perlvar/$^F>.
6581 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
6584 =for Pod::Functions create a pair of sockets
6586 Creates an unnamed pair of sockets in the specified domain, of the
6587 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
6588 for the syscall of the same name. If unimplemented, raises an exception.
6589 Returns true if successful.
6591 On systems that support a close-on-exec flag on files, the flag will
6592 be set for the newly opened file descriptors, as determined by the value
6593 of $^F. See L<perlvar/$^F>.
6595 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
6596 to C<pipe(Rdr, Wtr)> is essentially:
6599 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
6600 shutdown(Rdr, 1); # no more writing for reader
6601 shutdown(Wtr, 0); # no more reading for writer
6603 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
6604 emulate socketpair using IP sockets to localhost if your system implements
6605 sockets but not socketpair.
6607 Portability issues: L<perlport/socketpair>.
6609 =item sort SUBNAME LIST
6610 X<sort> X<qsort> X<quicksort> X<mergesort>
6612 =item sort BLOCK LIST
6616 =for Pod::Functions sort a list of values
6618 In list context, this sorts the LIST and returns the sorted list value.
6619 In scalar context, the behaviour of C<sort()> is undefined.
6621 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
6622 order. If SUBNAME is specified, it gives the name of a subroutine
6623 that returns an integer less than, equal to, or greater than C<0>,
6624 depending on how the elements of the list are to be ordered. (The
6625 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
6626 SUBNAME may be a scalar variable name (unsubscripted), in which case
6627 the value provides the name of (or a reference to) the actual
6628 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
6629 an anonymous, in-line sort subroutine.
6631 If the subroutine's prototype is C<($$)>, the elements to be compared are
6632 passed by reference in C<@_>, as for a normal subroutine. This is slower
6633 than unprototyped subroutines, where the elements to be compared are passed
6634 into the subroutine as the package global variables $a and $b (see example
6635 below). Note that in the latter case, it is usually highly counter-productive
6636 to declare $a and $b as lexicals.
6638 If the subroutine is an XSUB, the elements to be compared are pushed on to
6639 the stack, the way arguments are usually passed to XSUBs. $a and $b are
6642 The values to be compared are always passed by reference and should not
6645 You also cannot exit out of the sort block or subroutine using any of the
6646 loop control operators described in L<perlsyn> or with C<goto>.
6648 When C<use locale> (but not C<use locale 'not_characters'>) is in
6649 effect, C<sort LIST> sorts LIST according to the
6650 current collation locale. See L<perllocale>.
6652 sort() returns aliases into the original list, much as a for loop's index
6653 variable aliases the list elements. That is, modifying an element of a
6654 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
6655 actually modifies the element in the original list. This is usually
6656 something to be avoided when writing clear code.
6658 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
6659 That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
6660 preserves the input order of elements that compare equal. Although
6661 quicksort's run time is O(NlogN) when averaged over all arrays of
6662 length N, the time can be O(N**2), I<quadratic> behavior, for some
6663 inputs.) In 5.7, the quicksort implementation was replaced with
6664 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
6665 But benchmarks indicated that for some inputs, on some platforms,
6666 the original quicksort was faster. 5.8 has a sort pragma for
6667 limited control of the sort. Its rather blunt control of the
6668 underlying algorithm may not persist into future Perls, but the
6669 ability to characterize the input or output in implementation
6670 independent ways quite probably will. See L<the sort pragma|sort>.
6675 @articles = sort @files;
6677 # same thing, but with explicit sort routine
6678 @articles = sort {$a cmp $b} @files;
6680 # now case-insensitively
6681 @articles = sort {fc($a) cmp fc($b)} @files;
6683 # same thing in reversed order
6684 @articles = sort {$b cmp $a} @files;
6686 # sort numerically ascending
6687 @articles = sort {$a <=> $b} @files;
6689 # sort numerically descending
6690 @articles = sort {$b <=> $a} @files;
6692 # this sorts the %age hash by value instead of key
6693 # using an in-line function
6694 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
6696 # sort using explicit subroutine name
6698 $age{$a} <=> $age{$b}; # presuming numeric
6700 @sortedclass = sort byage @class;
6702 sub backwards { $b cmp $a }
6703 @harry = qw(dog cat x Cain Abel);
6704 @george = qw(gone chased yz Punished Axed);
6706 # prints AbelCaincatdogx
6707 print sort backwards @harry;
6708 # prints xdogcatCainAbel
6709 print sort @george, 'to', @harry;
6710 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
6712 # inefficiently sort by descending numeric compare using
6713 # the first integer after the first = sign, or the
6714 # whole record case-insensitively otherwise
6717 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
6722 # same thing, but much more efficiently;
6723 # we'll build auxiliary indices instead
6725 my @nums = @caps = ();
6727 push @nums, ( /=(\d+)/ ? $1 : undef );
6731 my @new = @old[ sort {
6732 $nums[$b] <=> $nums[$a]
6734 $caps[$a] cmp $caps[$b]
6738 # same thing, but without any temps
6739 @new = map { $_->[0] }
6740 sort { $b->[1] <=> $a->[1]
6743 } map { [$_, /=(\d+)/, fc($_)] } @old;
6745 # using a prototype allows you to use any comparison subroutine
6746 # as a sort subroutine (including other package's subroutines)
6748 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
6751 @new = sort other::backwards @old;
6753 # guarantee stability, regardless of algorithm
6755 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6757 # force use of mergesort (not portable outside Perl 5.8)
6758 use sort '_mergesort'; # note discouraging _
6759 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
6761 Warning: syntactical care is required when sorting the list returned from
6762 a function. If you want to sort the list returned by the function call
6763 C<find_records(@key)>, you can use:
6765 @contact = sort { $a cmp $b } find_records @key;
6766 @contact = sort +find_records(@key);
6767 @contact = sort &find_records(@key);
6768 @contact = sort(find_records(@key));
6770 If instead you want to sort the array @key with the comparison routine
6771 C<find_records()> then you can use:
6773 @contact = sort { find_records() } @key;
6774 @contact = sort find_records(@key);
6775 @contact = sort(find_records @key);
6776 @contact = sort(find_records (@key));
6778 If you're using strict, you I<must not> declare $a
6779 and $b as lexicals. They are package globals. That means
6780 that if you're in the C<main> package and type
6782 @articles = sort {$b <=> $a} @files;
6784 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
6785 but if you're in the C<FooPack> package, it's the same as typing
6787 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
6789 The comparison function is required to behave. If it returns
6790 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
6791 sometimes saying the opposite, for example) the results are not
6794 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
6795 (not-a-number), be careful when sorting with a
6796 comparison function like C<< $a <=> $b >> any lists that might contain a
6797 C<NaN>. The following example takes advantage that C<NaN != NaN> to
6798 eliminate any C<NaN>s from the input list.
6800 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
6802 =item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
6805 =item splice ARRAY or EXPR,OFFSET,LENGTH
6807 =item splice ARRAY or EXPR,OFFSET
6809 =item splice ARRAY or EXPR
6811 =for Pod::Functions add or remove elements anywhere in an array
6813 Removes the elements designated by OFFSET and LENGTH from an array, and
6814 replaces them with the elements of LIST, if any. In list context,
6815 returns the elements removed from the array. In scalar context,
6816 returns the last element removed, or C<undef> if no elements are
6817 removed. The array grows or shrinks as necessary.
6818 If OFFSET is negative then it starts that far from the end of the array.
6819 If LENGTH is omitted, removes everything from OFFSET onward.
6820 If LENGTH is negative, removes the elements from OFFSET onward
6821 except for -LENGTH elements at the end of the array.
6822 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
6823 past the end of the array and a LENGTH was provided, Perl issues a warning,
6824 and splices at the end of the array.
6826 The following equivalences hold (assuming C<< $#a >= $i >> )
6828 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
6829 pop(@a) splice(@a,-1)
6830 shift(@a) splice(@a,0,1)
6831 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
6832 $a[$i] = $y splice(@a,$i,1,$y)
6834 Example, assuming array lengths are passed before arrays:
6836 sub aeq { # compare two list values
6837 my(@a) = splice(@_,0,shift);
6838 my(@b) = splice(@_,0,shift);
6839 return 0 unless @a == @b; # same len?
6841 return 0 if pop(@a) ne pop(@b);
6845 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
6847 Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
6848 reference to an unblessed array. The argument will be dereferenced
6849 automatically. This aspect of C<splice> is considered highly experimental.
6850 The exact behaviour may change in a future version of Perl.
6852 To avoid confusing would-be users of your code who are running earlier
6853 versions of Perl with mysterious syntax errors, put this sort of thing at
6854 the top of your file to signal that your code will work I<only> on Perls of
6857 use 5.014; # so push/pop/etc work on scalars (experimental)
6859 =item split /PATTERN/,EXPR,LIMIT
6862 =item split /PATTERN/,EXPR
6864 =item split /PATTERN/
6868 =for Pod::Functions split up a string using a regexp delimiter
6870 Splits the string EXPR into a list of strings and returns the
6871 list in list context, or the size of the list in scalar context.
6873 If only PATTERN is given, EXPR defaults to C<$_>.
6875 Anything in EXPR that matches PATTERN is taken to be a separator
6876 that separates the EXPR into substrings (called "I<fields>") that
6877 do B<not> include the separator. Note that a separator may be
6878 longer than one character or even have no characters at all (the
6879 empty string, which is a zero-width match).
6881 The PATTERN need not be constant; an expression may be used
6882 to specify a pattern that varies at runtime.
6884 If PATTERN matches the empty string, the EXPR is split at the match
6885 position (between characters). As an example, the following:
6887 print join(':', split('b', 'abc')), "\n";
6889 uses the 'b' in 'abc' as a separator to produce the output 'a:c'.
6892 print join(':', split('', 'abc')), "\n";
6894 uses empty string matches as separators to produce the output
6895 'a:b:c'; thus, the empty string may be used to split EXPR into a
6896 list of its component characters.
6898 As a special case for C<split>, the empty pattern given in
6899 L<match operator|perlop/"m/PATTERN/msixpodualgc"> syntax (C<//>) specifically matches the empty string, which is contrary to its usual
6900 interpretation as the last successful match.
6902 If PATTERN is C</^/>, then it is treated as if it used the
6903 L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it
6904 isn't much use otherwise.
6906 As another special case, C<split> emulates the default behavior of the
6907 command line tool B<awk> when the PATTERN is either omitted or a I<literal
6908 string> composed of a single space character (such as S<C<' '>> or
6909 S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading
6910 whitespace in EXPR is removed before splitting occurs, and the PATTERN is
6911 instead treated as if it were C</\s+/>; in particular, this means that
6912 I<any> contiguous whitespace (not just a single space character) is used as
6913 a separator. However, this special treatment can be avoided by specifying
6914 the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing
6915 only a single space character to be a separator. In earlier Perls this
6916 special case was restricted to the use of a plain S<C<" ">> as the
6917 pattern argument to split, in Perl 5.18.0 and later this special case is
6918 triggered by any expression which evaluates as the simple string S<C<" ">>.
6920 If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering
6921 the previously described I<awk> emulation.
6923 If LIMIT is specified and positive, it represents the maximum number
6924 of fields into which the EXPR may be split; in other words, LIMIT is
6925 one greater than the maximum number of times EXPR may be split. Thus,
6926 the LIMIT value C<1> means that EXPR may be split a maximum of zero
6927 times, producing a maximum of one field (namely, the entire value of
6928 EXPR). For instance:
6930 print join(':', split(//, 'abc', 1)), "\n";
6932 produces the output 'abc', and this:
6934 print join(':', split(//, 'abc', 2)), "\n";
6936 produces the output 'a:bc', and each of these:
6938 print join(':', split(//, 'abc', 3)), "\n";
6939 print join(':', split(//, 'abc', 4)), "\n";
6941 produces the output 'a:b:c'.
6943 If LIMIT is negative, it is treated as if it were instead arbitrarily
6944 large; as many fields as possible are produced.
6946 If LIMIT is omitted (or, equivalently, zero), then it is usually
6947 treated as if it were instead negative but with the exception that
6948 trailing empty fields are stripped (empty leading fields are always
6949 preserved); if all fields are empty, then all fields are considered to
6950 be trailing (and are thus stripped in this case). Thus, the following:
6952 print join(':', split(',', 'a,b,c,,,')), "\n";
6954 produces the output 'a:b:c', but the following:
6956 print join(':', split(',', 'a,b,c,,,', -1)), "\n";
6958 produces the output 'a:b:c:::'.
6960 In time-critical applications, it is worthwhile to avoid splitting
6961 into more fields than necessary. Thus, when assigning to a list,
6962 if LIMIT is omitted (or zero), then LIMIT is treated as though it
6963 were one larger than the number of variables in the list; for the
6964 following, LIMIT is implicitly 3:
6966 ($login, $passwd) = split(/:/);
6968 Note that splitting an EXPR that evaluates to the empty string always
6969 produces zero fields, regardless of the LIMIT specified.
6971 An empty leading field is produced when there is a positive-width
6972 match at the beginning of EXPR. For instance:
6974 print join(':', split(/ /, ' abc')), "\n";
6976 produces the output ':abc'. However, a zero-width match at the
6977 beginning of EXPR never produces an empty field, so that:
6979 print join(':', split(//, ' abc'));
6981 produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>).
6983 An empty trailing field, on the other hand, is produced when there is a
6984 match at the end of EXPR, regardless of the length of the match
6985 (of course, unless a non-zero LIMIT is given explicitly, such fields are
6986 removed, as in the last example). Thus:
6988 print join(':', split(//, ' abc', -1)), "\n";
6990 produces the output S<' :a:b:c:'>.
6992 If the PATTERN contains
6993 L<capturing groups|perlretut/Grouping things and hierarchical matching>,
6994 then for each separator, an additional field is produced for each substring
6995 captured by a group (in the order in which the groups are specified,
6996 as per L<backreferences|perlretut/Backreferences>); if any group does not
6997 match, then it captures the C<undef> value instead of a substring. Also,
6998 note that any such additional field is produced whenever there is a
6999 separator (that is, whenever a split occurs), and such an additional field
7000 does B<not> count towards the LIMIT. Consider the following expressions
7001 evaluated in list context (each returned list is provided in the associated
7004 split(/-|,/, "1-10,20", 3)
7007 split(/(-|,)/, "1-10,20", 3)
7008 # ('1', '-', '10', ',', '20')
7010 split(/-|(,)/, "1-10,20", 3)
7011 # ('1', undef, '10', ',', '20')
7013 split(/(-)|,/, "1-10,20", 3)
7014 # ('1', '-', '10', undef, '20')
7016 split(/(-)|(,)/, "1-10,20", 3)
7017 # ('1', '-', undef, '10', undef, ',', '20')
7019 =item sprintf FORMAT, LIST
7022 =for Pod::Functions formatted print into a string
7024 Returns a string formatted by the usual C<printf> conventions of the C
7025 library function C<sprintf>. See below for more details
7026 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
7027 the general principles.
7031 # Format number with up to 8 leading zeroes
7032 $result = sprintf("%08d", $number);
7034 # Round number to 3 digits after decimal point
7035 $rounded = sprintf("%.3f", $number);
7037 Perl does its own C<sprintf> formatting: it emulates the C
7038 function sprintf(3), but doesn't use it except for floating-point
7039 numbers, and even then only standard modifiers are allowed.
7040 Non-standard extensions in your local sprintf(3) are
7041 therefore unavailable from Perl.
7043 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
7044 pass it an array as your first argument.
7045 The array is given scalar context,
7046 and instead of using the 0th element of the array as the format, Perl will
7047 use the count of elements in the array as the format, which is almost never
7050 Perl's C<sprintf> permits the following universally-known conversions:
7053 %c a character with the given number
7055 %d a signed integer, in decimal
7056 %u an unsigned integer, in decimal
7057 %o an unsigned integer, in octal
7058 %x an unsigned integer, in hexadecimal
7059 %e a floating-point number, in scientific notation
7060 %f a floating-point number, in fixed decimal notation
7061 %g a floating-point number, in %e or %f notation
7063 In addition, Perl permits the following widely-supported conversions:
7065 %X like %x, but using upper-case letters
7066 %E like %e, but using an upper-case "E"
7067 %G like %g, but with an upper-case "E" (if applicable)
7068 %b an unsigned integer, in binary
7069 %B like %b, but using an upper-case "B" with the # flag
7070 %p a pointer (outputs the Perl value's address in hexadecimal)
7071 %n special: *stores* the number of characters output so far
7072 into the next argument in the parameter list
7074 Finally, for backward (and we do mean "backward") compatibility, Perl
7075 permits these unnecessary but widely-supported conversions:
7078 %D a synonym for %ld
7079 %U a synonym for %lu
7080 %O a synonym for %lo
7083 Note that the number of exponent digits in the scientific notation produced
7084 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
7085 exponent less than 100 is system-dependent: it may be three or less
7086 (zero-padded as necessary). In other words, 1.23 times ten to the
7087 99th may be either "1.23e99" or "1.23e099".
7089 Between the C<%> and the format letter, you may specify several
7090 additional attributes controlling the interpretation of the format.
7091 In order, these are:
7095 =item format parameter index
7097 An explicit format parameter index, such as C<2$>. By default sprintf
7098 will format the next unused argument in the list, but this allows you
7099 to take the arguments out of order:
7101 printf '%2$d %1$d', 12, 34; # prints "34 12"
7102 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
7108 space prefix non-negative number with a space
7109 + prefix non-negative number with a plus sign
7110 - left-justify within the field
7111 0 use zeros, not spaces, to right-justify
7112 # ensure the leading "0" for any octal,
7113 prefix non-zero hexadecimal with "0x" or "0X",
7114 prefix non-zero binary with "0b" or "0B"
7118 printf '<% d>', 12; # prints "< 12>"
7119 printf '<%+d>', 12; # prints "<+12>"
7120 printf '<%6s>', 12; # prints "< 12>"
7121 printf '<%-6s>', 12; # prints "<12 >"
7122 printf '<%06s>', 12; # prints "<000012>"
7123 printf '<%#o>', 12; # prints "<014>"
7124 printf '<%#x>', 12; # prints "<0xc>"
7125 printf '<%#X>', 12; # prints "<0XC>"
7126 printf '<%#b>', 12; # prints "<0b1100>"
7127 printf '<%#B>', 12; # prints "<0B1100>"
7129 When a space and a plus sign are given as the flags at once,
7130 a plus sign is used to prefix a positive number.
7132 printf '<%+ d>', 12; # prints "<+12>"
7133 printf '<% +d>', 12; # prints "<+12>"
7135 When the # flag and a precision are given in the %o conversion,
7136 the precision is incremented if it's necessary for the leading "0".
7138 printf '<%#.5o>', 012; # prints "<00012>"
7139 printf '<%#.5o>', 012345; # prints "<012345>"
7140 printf '<%#.0o>', 0; # prints "<0>"
7144 This flag tells Perl to interpret the supplied string as a vector of
7145 integers, one for each character in the string. Perl applies the format to
7146 each integer in turn, then joins the resulting strings with a separator (a
7147 dot C<.> by default). This can be useful for displaying ordinal values of
7148 characters in arbitrary strings:
7150 printf "%vd", "AB\x{100}"; # prints "65.66.256"
7151 printf "version is v%vd\n", $^V; # Perl's version
7153 Put an asterisk C<*> before the C<v> to override the string to
7154 use to separate the numbers:
7156 printf "address is %*vX\n", ":", $addr; # IPv6 address
7157 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
7159 You can also explicitly specify the argument number to use for
7160 the join string using something like C<*2$v>; for example:
7162 printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
7165 =item (minimum) width
7167 Arguments are usually formatted to be only as wide as required to
7168 display the given value. You can override the width by putting
7169 a number here, or get the width from the next argument (with C<*>)
7170 or from a specified argument (e.g., with C<*2$>):
7172 printf "<%s>", "a"; # prints "<a>"
7173 printf "<%6s>", "a"; # prints "< a>"
7174 printf "<%*s>", 6, "a"; # prints "< a>"
7175 printf '<%*2$s>', "a", 6; # prints "< a>"
7176 printf "<%2s>", "long"; # prints "<long>" (does not truncate)
7178 If a field width obtained through C<*> is negative, it has the same
7179 effect as the C<-> flag: left-justification.
7181 =item precision, or maximum width
7184 You can specify a precision (for numeric conversions) or a maximum
7185 width (for string conversions) by specifying a C<.> followed by a number.
7186 For floating-point formats except C<g> and C<G>, this specifies
7187 how many places right of the decimal point to show (the default being 6).
7190 # these examples are subject to system-specific variation
7191 printf '<%f>', 1; # prints "<1.000000>"
7192 printf '<%.1f>', 1; # prints "<1.0>"
7193 printf '<%.0f>', 1; # prints "<1>"
7194 printf '<%e>', 10; # prints "<1.000000e+01>"
7195 printf '<%.1e>', 10; # prints "<1.0e+01>"
7197 For "g" and "G", this specifies the maximum number of digits to show,
7198 including those prior to the decimal point and those after it; for
7201 # These examples are subject to system-specific variation.
7202 printf '<%g>', 1; # prints "<1>"
7203 printf '<%.10g>', 1; # prints "<1>"
7204 printf '<%g>', 100; # prints "<100>"
7205 printf '<%.1g>', 100; # prints "<1e+02>"
7206 printf '<%.2g>', 100.01; # prints "<1e+02>"
7207 printf '<%.5g>', 100.01; # prints "<100.01>"
7208 printf '<%.4g>', 100.01; # prints "<100>"
7210 For integer conversions, specifying a precision implies that the
7211 output of the number itself should be zero-padded to this width,
7212 where the 0 flag is ignored:
7214 printf '<%.6d>', 1; # prints "<000001>"
7215 printf '<%+.6d>', 1; # prints "<+000001>"
7216 printf '<%-10.6d>', 1; # prints "<000001 >"
7217 printf '<%10.6d>', 1; # prints "< 000001>"
7218 printf '<%010.6d>', 1; # prints "< 000001>"
7219 printf '<%+10.6d>', 1; # prints "< +000001>"
7221 printf '<%.6x>', 1; # prints "<000001>"
7222 printf '<%#.6x>', 1; # prints "<0x000001>"
7223 printf '<%-10.6x>', 1; # prints "<000001 >"
7224 printf '<%10.6x>', 1; # prints "< 000001>"
7225 printf '<%010.6x>', 1; # prints "< 000001>"
7226 printf '<%#10.6x>', 1; # prints "< 0x000001>"
7228 For string conversions, specifying a precision truncates the string
7229 to fit the specified width:
7231 printf '<%.5s>', "truncated"; # prints "<trunc>"
7232 printf '<%10.5s>', "truncated"; # prints "< trunc>"
7234 You can also get the precision from the next argument using C<.*>:
7236 printf '<%.6x>', 1; # prints "<000001>"
7237 printf '<%.*x>', 6, 1; # prints "<000001>"
7239 If a precision obtained through C<*> is negative, it counts
7240 as having no precision at all.
7242 printf '<%.*s>', 7, "string"; # prints "<string>"
7243 printf '<%.*s>', 3, "string"; # prints "<str>"
7244 printf '<%.*s>', 0, "string"; # prints "<>"
7245 printf '<%.*s>', -1, "string"; # prints "<string>"
7247 printf '<%.*d>', 1, 0; # prints "<0>"
7248 printf '<%.*d>', 0, 0; # prints "<>"
7249 printf '<%.*d>', -1, 0; # prints "<0>"
7251 You cannot currently get the precision from a specified number,
7252 but it is intended that this will be possible in the future, for
7253 example using C<.*2$>:
7255 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print
7260 For numeric conversions, you can specify the size to interpret the
7261 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
7262 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
7263 whatever the default integer size is on your platform (usually 32 or 64
7264 bits), but you can override this to use instead one of the standard C types,
7265 as supported by the compiler used to build Perl:
7267 hh interpret integer as C type "char" or "unsigned
7268 char" on Perl 5.14 or later
7269 h interpret integer as C type "short" or
7271 j interpret integer as C type "intmax_t" on Perl
7272 5.14 or later, and only with a C99 compiler
7274 l interpret integer as C type "long" or
7276 q, L, or ll interpret integer as C type "long long",
7277 "unsigned long long", or "quad" (typically
7279 t interpret integer as C type "ptrdiff_t" on Perl
7281 z interpret integer as C type "size_t" on Perl 5.14
7284 As of 5.14, none of these raises an exception if they are not supported on
7285 your platform. However, if warnings are enabled, a warning of the
7286 C<printf> warning class is issued on an unsupported conversion flag.
7287 Should you instead prefer an exception, do this:
7289 use warnings FATAL => "printf";
7291 If you would like to know about a version dependency before you
7292 start running the program, put something like this at its top:
7294 use 5.014; # for hh/j/t/z/ printf modifiers
7296 You can find out whether your Perl supports quads via L<Config>:
7299 if ($Config{use64bitint} eq "define"
7300 || $Config{longsize} >= 8) {
7301 print "Nice quads!\n";
7304 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
7305 to be the default floating-point size on your platform (double or long double),
7306 but you can force "long double" with C<q>, C<L>, or C<ll> if your
7307 platform supports them. You can find out whether your Perl supports long
7308 doubles via L<Config>:
7311 print "long doubles\n" if $Config{d_longdbl} eq "define";
7313 You can find out whether Perl considers "long double" to be the default
7314 floating-point size to use on your platform via L<Config>:
7317 if ($Config{uselongdouble} eq "define") {
7318 print "long doubles by default\n";
7321 It can also be that long doubles and doubles are the same thing:
7324 ($Config{doublesize} == $Config{longdblsize}) &&
7325 print "doubles are long doubles\n";
7327 The size specifier C<V> has no effect for Perl code, but is supported for
7328 compatibility with XS code. It means "use the standard size for a Perl
7329 integer or floating-point number", which is the default.
7331 =item order of arguments
7333 Normally, sprintf() takes the next unused argument as the value to
7334 format for each format specification. If the format specification
7335 uses C<*> to require additional arguments, these are consumed from
7336 the argument list in the order they appear in the format
7337 specification I<before> the value to format. Where an argument is
7338 specified by an explicit index, this does not affect the normal
7339 order for the arguments, even when the explicitly specified index
7340 would have been the next argument.
7344 printf "<%*.*s>", $a, $b, $c;
7346 uses C<$a> for the width, C<$b> for the precision, and C<$c>
7347 as the value to format; while:
7349 printf '<%*1$.*s>', $a, $b;
7351 would use C<$a> for the width and precision, and C<$b> as the
7354 Here are some more examples; be aware that when using an explicit
7355 index, the C<$> may need escaping:
7357 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
7358 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
7359 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
7360 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
7364 If C<use locale> (including C<use locale 'not_characters'>) is in effect
7365 and POSIX::setlocale() has been called,
7366 the character used for the decimal separator in formatted floating-point
7367 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
7371 X<sqrt> X<root> X<square root>
7375 =for Pod::Functions square root function
7377 Return the positive square root of EXPR. If EXPR is omitted, uses
7378 C<$_>. Works only for non-negative operands unless you've
7379 loaded the C<Math::Complex> module.
7382 print sqrt(-4); # prints 2i
7385 X<srand> X<seed> X<randseed>
7389 =for Pod::Functions seed the random number generator
7391 Sets and returns the random number seed for the C<rand> operator.
7393 The point of the function is to "seed" the C<rand> function so that C<rand>
7394 can produce a different sequence each time you run your program. When
7395 called with a parameter, C<srand> uses that for the seed; otherwise it
7396 (semi-)randomly chooses a seed. In either case, starting with Perl 5.14,
7397 it returns the seed. To signal that your code will work I<only> on Perls
7398 of a recent vintage:
7400 use 5.014; # so srand returns the seed
7402 If C<srand()> is not called explicitly, it is called implicitly without a
7403 parameter at the first use of the C<rand> operator.
7404 However, there are a few situations where programs are likely to
7405 want to call C<srand>. One is for generating predictable results, generally for
7406 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
7407 each time. Another case is that you may want to call C<srand()>
7408 after a C<fork()> to avoid child processes sharing the same seed value as the
7409 parent (and consequently each other).
7411 Do B<not> call C<srand()> (i.e., without an argument) more than once per
7412 process. The internal state of the random number generator should
7413 contain more entropy than can be provided by any seed, so calling
7414 C<srand()> again actually I<loses> randomness.
7416 Most implementations of C<srand> take an integer and will silently
7417 truncate decimal numbers. This means C<srand(42)> will usually
7418 produce the same results as C<srand(42.1)>. To be safe, always pass
7419 C<srand> an integer.
7421 A typical use of the returned seed is for a test program which has too many
7422 combinations to test comprehensively in the time available to it each run. It
7423 can test a random subset each time, and should there be a failure, log the seed
7424 used for that run so that it can later be used to reproduce the same results.
7426 B<C<rand()> is not cryptographically secure. You should not rely
7427 on it in security-sensitive situations.> As of this writing, a
7428 number of third-party CPAN modules offer random number generators
7429 intended by their authors to be cryptographically secure,
7430 including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
7431 and L<Math::TrulyRandom>.
7433 =item stat FILEHANDLE
7434 X<stat> X<file, status> X<ctime>
7438 =item stat DIRHANDLE
7442 =for Pod::Functions get a file's status information
7444 Returns a 13-element list giving the status info for a file, either
7445 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
7446 omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
7449 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
7450 $atime,$mtime,$ctime,$blksize,$blocks)
7453 Not all fields are supported on all filesystem types. Here are the
7454 meanings of the fields:
7456 0 dev device number of filesystem
7458 2 mode file mode (type and permissions)
7459 3 nlink number of (hard) links to the file
7460 4 uid numeric user ID of file's owner
7461 5 gid numeric group ID of file's owner
7462 6 rdev the device identifier (special files only)
7463 7 size total size of file, in bytes
7464 8 atime last access time in seconds since the epoch
7465 9 mtime last modify time in seconds since the epoch
7466 10 ctime inode change time in seconds since the epoch (*)
7467 11 blksize preferred I/O size in bytes for interacting with the
7468 file (may vary from file to file)
7469 12 blocks actual number of system-specific blocks allocated
7470 on disk (often, but not always, 512 bytes each)
7472 (The epoch was at 00:00 January 1, 1970 GMT.)
7474 (*) Not all fields are supported on all filesystem types. Notably, the
7475 ctime field is non-portable. In particular, you cannot expect it to be a
7476 "creation time"; see L<perlport/"Files and Filesystems"> for details.
7478 If C<stat> is passed the special filehandle consisting of an underline, no
7479 stat is done, but the current contents of the stat structure from the
7480 last C<stat>, C<lstat>, or filetest are returned. Example:
7482 if (-x $file && (($d) = stat(_)) && $d < 0) {
7483 print "$file is executable NFS file\n";
7486 (This works on machines only for which the device number is negative
7489 Because the mode contains both the file type and its permissions, you
7490 should mask off the file type portion and (s)printf using a C<"%o">
7491 if you want to see the real permissions.
7493 $mode = (stat($filename))[2];
7494 printf "Permissions are %04o\n", $mode & 07777;
7496 In scalar context, C<stat> returns a boolean value indicating success
7497 or failure, and, if successful, sets the information associated with
7498 the special filehandle C<_>.
7500 The L<File::stat> module provides a convenient, by-name access mechanism:
7503 $sb = stat($filename);
7504 printf "File is %s, size is %s, perm %04o, mtime %s\n",
7505 $filename, $sb->size, $sb->mode & 07777,
7506 scalar localtime $sb->mtime;
7508 You can import symbolic mode constants (C<S_IF*>) and functions
7509 (C<S_IS*>) from the Fcntl module:
7513 $mode = (stat($filename))[2];
7515 $user_rwx = ($mode & S_IRWXU) >> 6;
7516 $group_read = ($mode & S_IRGRP) >> 3;
7517 $other_execute = $mode & S_IXOTH;
7519 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
7521 $is_setuid = $mode & S_ISUID;
7522 $is_directory = S_ISDIR($mode);
7524 You could write the last two using the C<-u> and C<-d> operators.
7525 Commonly available C<S_IF*> constants are:
7527 # Permissions: read, write, execute, for user, group, others.
7529 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
7530 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
7531 S_IRWXO S_IROTH S_IWOTH S_IXOTH
7533 # Setuid/Setgid/Stickiness/SaveText.
7534 # Note that the exact meaning of these is system-dependent.
7536 S_ISUID S_ISGID S_ISVTX S_ISTXT
7538 # File types. Not all are necessarily available on
7541 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
7542 S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
7544 # The following are compatibility aliases for S_IRUSR,
7545 # S_IWUSR, and S_IXUSR.
7547 S_IREAD S_IWRITE S_IEXEC
7549 and the C<S_IF*> functions are
7551 S_IMODE($mode) the part of $mode containing the permission
7552 bits and the setuid/setgid/sticky bits
7554 S_IFMT($mode) the part of $mode containing the file type
7555 which can be bit-anded with (for example)
7556 S_IFREG or with the following functions
7558 # The operators -f, -d, -l, -b, -c, -p, and -S.
7560 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
7561 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
7563 # No direct -X operator counterpart, but for the first one
7564 # the -g operator is often equivalent. The ENFMT stands for
7565 # record flocking enforcement, a platform-dependent feature.
7567 S_ISENFMT($mode) S_ISWHT($mode)
7569 See your native chmod(2) and stat(2) documentation for more details
7570 about the C<S_*> constants. To get status info for a symbolic link
7571 instead of the target file behind the link, use the C<lstat> function.
7573 Portability issues: L<perlport/stat>.
7578 =item state TYPE VARLIST
7580 =item state VARLIST : ATTRS
7582 =item state TYPE VARLIST : ATTRS
7584 =for Pod::Functions +state declare and assign a persistent lexical variable
7586 C<state> declares a lexically scoped variable, just like C<my>.
7587 However, those variables will never be reinitialized, contrary to
7588 lexical variables that are reinitialized each time their enclosing block
7590 See L<perlsub/"Persistent Private Variables"> for details.
7592 If more than one variable is listed, the list must be placed in
7593 parentheses. With a parenthesised list, C<undef> can be used as a
7594 dummy placeholder. However, since initialization of state variables in
7595 list context is currently not possible this would serve no purpose.
7597 C<state> variables are enabled only when the C<use feature "state"> pragma
7598 is in effect, unless the keyword is written as C<CORE::state>.
7599 See also L<feature>.
7606 =for Pod::Functions optimize input data for repeated searches
7608 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
7609 doing many pattern matches on the string before it is next modified.
7610 This may or may not save time, depending on the nature and number of
7611 patterns you are searching and the distribution of character
7612 frequencies in the string to be searched; you probably want to compare
7613 run times with and without it to see which is faster. Those loops
7614 that scan for many short constant strings (including the constant
7615 parts of more complex patterns) will benefit most.
7616 (The way C<study> works is this: a linked list of every
7617 character in the string to be searched is made, so we know, for
7618 example, where all the C<'k'> characters are. From each search string,
7619 the rarest character is selected, based on some static frequency tables
7620 constructed from some C programs and English text. Only those places
7621 that contain this "rarest" character are examined.)
7623 For example, here is a loop that inserts index producing entries
7624 before any line containing a certain pattern:
7628 print ".IX foo\n" if /\bfoo\b/;
7629 print ".IX bar\n" if /\bbar\b/;
7630 print ".IX blurfl\n" if /\bblurfl\b/;
7635 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
7636 will be looked at, because C<f> is rarer than C<o>. In general, this is
7637 a big win except in pathological cases. The only question is whether
7638 it saves you more time than it took to build the linked list in the
7641 Note that if you have to look for strings that you don't know till
7642 runtime, you can build an entire loop as a string and C<eval> that to
7643 avoid recompiling all your patterns all the time. Together with
7644 undefining C<$/> to input entire files as one record, this can be quite
7645 fast, often faster than specialized programs like fgrep(1). The following
7646 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
7647 out the names of those files that contain a match:
7649 $search = 'while (<>) { study;';
7650 foreach $word (@words) {
7651 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
7656 eval $search; # this screams
7657 $/ = "\n"; # put back to normal input delimiter
7658 foreach $file (sort keys(%seen)) {
7662 =item sub NAME BLOCK
7665 =item sub NAME (PROTO) BLOCK
7667 =item sub NAME : ATTRS BLOCK
7669 =item sub NAME (PROTO) : ATTRS BLOCK
7671 =for Pod::Functions declare a subroutine, possibly anonymously
7673 This is subroutine definition, not a real function I<per se>. Without a
7674 BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
7675 function declaration, so does return a value: the CODE ref of the closure
7678 See L<perlsub> and L<perlref> for details about subroutines and
7679 references; see L<attributes> and L<Attribute::Handlers> for more
7680 information about attributes.
7685 =for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine
7687 A special token that returns a reference to the current subroutine, or
7688 C<undef> outside of a subroutine.
7690 The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>)
7691 is subject to change.
7693 This token is only available under C<use v5.16> or the "current_sub"
7694 feature. See L<feature>.
7696 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
7697 X<substr> X<substring> X<mid> X<left> X<right>
7699 =item substr EXPR,OFFSET,LENGTH
7701 =item substr EXPR,OFFSET
7703 =for Pod::Functions get or alter a portion of a string
7705 Extracts a substring out of EXPR and returns it. First character is at
7706 offset zero. If OFFSET is negative, starts
7707 that far back from the end of the string. If LENGTH is omitted, returns
7708 everything through the end of the string. If LENGTH is negative, leaves that
7709 many characters off the end of the string.
7711 my $s = "The black cat climbed the green tree";
7712 my $color = substr $s, 4, 5; # black
7713 my $middle = substr $s, 4, -11; # black cat climbed the
7714 my $end = substr $s, 14; # climbed the green tree
7715 my $tail = substr $s, -4; # tree
7716 my $z = substr $s, -4, 2; # tr
7718 You can use the substr() function as an lvalue, in which case EXPR
7719 must itself be an lvalue. If you assign something shorter than LENGTH,
7720 the string will shrink, and if you assign something longer than LENGTH,
7721 the string will grow to accommodate it. To keep the string the same
7722 length, you may need to pad or chop your value using C<sprintf>.
7724 If OFFSET and LENGTH specify a substring that is partly outside the
7725 string, only the part within the string is returned. If the substring
7726 is beyond either end of the string, substr() returns the undefined
7727 value and produces a warning. When used as an lvalue, specifying a
7728 substring that is entirely outside the string raises an exception.
7729 Here's an example showing the behavior for boundary cases:
7732 substr($name, 4) = 'dy'; # $name is now 'freddy'
7733 my $null = substr $name, 6, 2; # returns "" (no warning)
7734 my $oops = substr $name, 7; # returns undef, with warning
7735 substr($name, 7) = 'gap'; # raises an exception
7737 An alternative to using substr() as an lvalue is to specify the
7738 replacement string as the 4th argument. This allows you to replace
7739 parts of the EXPR and return what was there before in one operation,
7740 just as you can with splice().
7742 my $s = "The black cat climbed the green tree";
7743 my $z = substr $s, 14, 7, "jumped from"; # climbed
7744 # $s is now "The black cat jumped from the green tree"
7746 Note that the lvalue returned by the three-argument version of substr() acts as
7747 a 'magic bullet'; each time it is assigned to, it remembers which part
7748 of the original string is being modified; for example:
7751 for (substr($x,1,2)) {
7752 $_ = 'a'; print $x,"\n"; # prints 1a4
7753 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
7755 $_ = 'pq'; print $x,"\n"; # prints 5pq9
7758 With negative offsets, it remembers its position from the end of the string
7759 when the target string is modified:
7762 for (substr($x, -3, 2)) {
7763 $_ = 'a'; print $x,"\n"; # prints 1a4, as above
7765 print $_,"\n"; # prints f
7768 Prior to Perl version 5.10, the result of using an lvalue multiple times was
7769 unspecified. Prior to 5.16, the result with negative offsets was
7772 =item symlink OLDFILE,NEWFILE
7773 X<symlink> X<link> X<symbolic link> X<link, symbolic>
7775 =for Pod::Functions create a symbolic link to a file
7777 Creates a new filename symbolically linked to the old filename.
7778 Returns C<1> for success, C<0> otherwise. On systems that don't support
7779 symbolic links, raises an exception. To check for that,
7782 $symlink_exists = eval { symlink("",""); 1 };
7784 Portability issues: L<perlport/symlink>.
7786 =item syscall NUMBER, LIST
7787 X<syscall> X<system call>
7789 =for Pod::Functions execute an arbitrary system call
7791 Calls the system call specified as the first element of the list,
7792 passing the remaining elements as arguments to the system call. If
7793 unimplemented, raises an exception. The arguments are interpreted
7794 as follows: if a given argument is numeric, the argument is passed as
7795 an int. If not, the pointer to the string value is passed. You are
7796 responsible to make sure a string is pre-extended long enough to
7797 receive any result that might be written into a string. You can't use a
7798 string literal (or other read-only string) as an argument to C<syscall>
7799 because Perl has to assume that any string pointer might be written
7801 integer arguments are not literals and have never been interpreted in a
7802 numeric context, you may need to add C<0> to them to force them to look
7803 like numbers. This emulates the C<syswrite> function (or vice versa):
7805 require 'syscall.ph'; # may need to run h2ph
7807 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
7809 Note that Perl supports passing of up to only 14 arguments to your syscall,
7810 which in practice should (usually) suffice.
7812 Syscall returns whatever value returned by the system call it calls.
7813 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
7814 Note that some system calls I<can> legitimately return C<-1>. The proper
7815 way to handle such calls is to assign C<$!=0> before the call, then
7816 check the value of C<$!> if C<syscall> returns C<-1>.
7818 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
7819 number of the read end of the pipe it creates, but there is no way
7820 to retrieve the file number of the other end. You can avoid this
7821 problem by using C<pipe> instead.
7823 Portability issues: L<perlport/syscall>.
7825 =item sysopen FILEHANDLE,FILENAME,MODE
7828 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
7830 =for Pod::Functions +5.002 open a file, pipe, or descriptor
7832 Opens the file whose filename is given by FILENAME, and associates it with
7833 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
7834 filehandle wanted; an undefined scalar will be suitably autovivified. This
7835 function calls the underlying operating system's I<open>(2) function with the
7836 parameters FILENAME, MODE, and PERMS.
7838 The possible values and flag bits of the MODE parameter are
7839 system-dependent; they are available via the standard module C<Fcntl>. See
7840 the documentation of your operating system's I<open>(2) syscall to see
7841 which values and flag bits are available. You may combine several flags
7842 using the C<|>-operator.
7844 Some of the most common values are C<O_RDONLY> for opening the file in
7845 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
7846 and C<O_RDWR> for opening the file in read-write mode.
7847 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
7849 For historical reasons, some values work on almost every system
7850 supported by Perl: 0 means read-only, 1 means write-only, and 2
7851 means read/write. We know that these values do I<not> work under
7852 OS/390 and on the Macintosh; you probably don't want to
7853 use them in new code.
7855 If the file named by FILENAME does not exist and the C<open> call creates
7856 it (typically because MODE includes the C<O_CREAT> flag), then the value of
7857 PERMS specifies the permissions of the newly created file. If you omit
7858 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
7859 These permission values need to be in octal, and are modified by your
7860 process's current C<umask>.
7863 In many systems the C<O_EXCL> flag is available for opening files in
7864 exclusive mode. This is B<not> locking: exclusiveness means here that
7865 if the file already exists, sysopen() fails. C<O_EXCL> may not work
7866 on network filesystems, and has no effect unless the C<O_CREAT> flag
7867 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
7868 being opened if it is a symbolic link. It does not protect against
7869 symbolic links in the file's path.
7872 Sometimes you may want to truncate an already-existing file. This
7873 can be done using the C<O_TRUNC> flag. The behavior of
7874 C<O_TRUNC> with C<O_RDONLY> is undefined.
7877 You should seldom if ever use C<0644> as argument to C<sysopen>, because
7878 that takes away the user's option to have a more permissive umask.
7879 Better to omit it. See the perlfunc(1) entry on C<umask> for more
7882 Note that C<sysopen> depends on the fdopen() C library function.
7883 On many Unix systems, fdopen() is known to fail when file descriptors
7884 exceed a certain value, typically 255. If you need more file
7885 descriptors than that, consider rebuilding Perl to use the C<sfio>
7886 library, or perhaps using the POSIX::open() function.
7888 See L<perlopentut> for a kinder, gentler explanation of opening files.
7890 Portability issues: L<perlport/sysopen>.
7892 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
7895 =item sysread FILEHANDLE,SCALAR,LENGTH
7897 =for Pod::Functions fixed-length unbuffered input from a filehandle
7899 Attempts to read LENGTH bytes of data into variable SCALAR from the
7900 specified FILEHANDLE, using the read(2). It bypasses
7901 buffered IO, so mixing this with other kinds of reads, C<print>,
7902 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
7903 perlio or stdio layers usually buffers data. Returns the number of
7904 bytes actually read, C<0> at end of file, or undef if there was an
7905 error (in the latter case C<$!> is also set). SCALAR will be grown or
7906 shrunk so that the last byte actually read is the last byte of the
7907 scalar after the read.
7909 An OFFSET may be specified to place the read data at some place in the
7910 string other than the beginning. A negative OFFSET specifies
7911 placement at that many characters counting backwards from the end of
7912 the string. A positive OFFSET greater than the length of SCALAR
7913 results in the string being padded to the required size with C<"\0">
7914 bytes before the result of the read is appended.
7916 There is no syseof() function, which is ok, since eof() doesn't work
7917 well on device files (like ttys) anyway. Use sysread() and check
7918 for a return value for 0 to decide whether you're done.
7920 Note that if the filehandle has been marked as C<:utf8> Unicode
7921 characters are read instead of bytes (the LENGTH, OFFSET, and the
7922 return value of sysread() are in Unicode characters).
7923 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
7924 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
7926 =item sysseek FILEHANDLE,POSITION,WHENCE
7929 =for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite
7931 Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
7932 be an expression whose value gives the name of the filehandle. The values
7933 for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
7934 to the current position plus POSITION; and C<2> to set it to EOF plus
7935 POSITION, typically negative.
7937 Note the I<in bytes>: even if the filehandle has been set to operate
7938 on characters (for example by using the C<:encoding(utf8)> I/O layer),
7939 tell() will return byte offsets, not character offsets (because
7940 implementing that would render sysseek() unacceptably slow).
7942 sysseek() bypasses normal buffered IO, so mixing it with reads other
7943 than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
7944 C<seek>, C<tell>, or C<eof> may cause confusion.
7946 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
7947 and C<SEEK_END> (start of the file, current position, end of the file)
7948 from the Fcntl module. Use of the constants is also more portable
7949 than relying on 0, 1, and 2. For example to define a "systell" function:
7951 use Fcntl 'SEEK_CUR';
7952 sub systell { sysseek($_[0], 0, SEEK_CUR) }
7954 Returns the new position, or the undefined value on failure. A position
7955 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
7956 true on success and false on failure, yet you can still easily determine
7962 =item system PROGRAM LIST
7964 =for Pod::Functions run a separate program
7966 Does exactly the same thing as C<exec LIST>, except that a fork is
7967 done first and the parent process waits for the child process to
7968 exit. Note that argument processing varies depending on the
7969 number of arguments. If there is more than one argument in LIST,
7970 or if LIST is an array with more than one value, starts the program
7971 given by the first element of the list with arguments given by the
7972 rest of the list. If there is only one scalar argument, the argument
7973 is checked for shell metacharacters, and if there are any, the
7974 entire argument is passed to the system's command shell for parsing
7975 (this is C</bin/sh -c> on Unix platforms, but varies on other
7976 platforms). If there are no shell metacharacters in the argument,
7977 it is split into words and passed directly to C<execvp>, which is
7980 Perl will attempt to flush all files opened for
7981 output before any operation that may do a fork, but this may not be
7982 supported on some platforms (see L<perlport>). To be safe, you may need
7983 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
7984 of C<IO::Handle> on any open handles.
7986 The return value is the exit status of the program as returned by the
7987 C<wait> call. To get the actual exit value, shift right by eight (see
7988 below). See also L</exec>. This is I<not> what you want to use to capture
7989 the output from a command; for that you should use merely backticks or
7990 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
7991 indicates a failure to start the program or an error of the wait(2) system
7992 call (inspect $! for the reason).
7994 If you'd like to make C<system> (and many other bits of Perl) die on error,
7995 have a look at the L<autodie> pragma.
7997 Like C<exec>, C<system> allows you to lie to a program about its name if
7998 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
8000 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
8001 C<system>, if you expect your program to terminate on receipt of these
8002 signals you will need to arrange to do so yourself based on the return
8005 @args = ("command", "arg1", "arg2");
8007 or die "system @args failed: $?"
8009 If you'd like to manually inspect C<system>'s failure, you can check all
8010 possible failure modes by inspecting C<$?> like this:
8013 print "failed to execute: $!\n";
8016 printf "child died with signal %d, %s coredump\n",
8017 ($? & 127), ($? & 128) ? 'with' : 'without';
8020 printf "child exited with value %d\n", $? >> 8;
8023 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
8024 with the C<W*()> calls from the POSIX module.
8026 When C<system>'s arguments are executed indirectly by the shell,
8027 results and return codes are subject to its quirks.
8028 See L<perlop/"`STRING`"> and L</exec> for details.
8030 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
8031 handler. See L<perlipc> for details.
8033 Portability issues: L<perlport/system>.
8035 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
8038 =item syswrite FILEHANDLE,SCALAR,LENGTH
8040 =item syswrite FILEHANDLE,SCALAR
8042 =for Pod::Functions fixed-length unbuffered output to a filehandle
8044 Attempts to write LENGTH bytes of data from variable SCALAR to the
8045 specified FILEHANDLE, using write(2). If LENGTH is
8046 not specified, writes whole SCALAR. It bypasses buffered IO, so
8047 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
8048 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
8049 stdio layers usually buffer data. Returns the number of bytes
8050 actually written, or C<undef> if there was an error (in this case the
8051 errno variable C<$!> is also set). If the LENGTH is greater than the
8052 data available in the SCALAR after the OFFSET, only as much data as is
8053 available will be written.
8055 An OFFSET may be specified to write the data from some part of the
8056 string other than the beginning. A negative OFFSET specifies writing
8057 that many characters counting backwards from the end of the string.
8058 If SCALAR is of length zero, you can only use an OFFSET of 0.
8060 B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
8061 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
8062 return value of syswrite() are in (UTF8-encoded Unicode) characters.
8063 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
8064 Alternately, if the handle is not marked with an encoding but you
8065 attempt to write characters with code points over 255, raises an exception.
8066 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
8068 =item tell FILEHANDLE
8073 =for Pod::Functions get current seekpointer on a filehandle
8075 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
8076 error. FILEHANDLE may be an expression whose value gives the name of
8077 the actual filehandle. If FILEHANDLE is omitted, assumes the file
8080 Note the I<in bytes>: even if the filehandle has been set to
8081 operate on characters (for example by using the C<:encoding(utf8)> open
8082 layer), tell() will return byte offsets, not character offsets (because
8083 that would render seek() and tell() rather slow).
8085 The return value of tell() for the standard streams like the STDIN
8086 depends on the operating system: it may return -1 or something else.
8087 tell() on pipes, fifos, and sockets usually returns -1.
8089 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
8091 Do not use tell() (or other buffered I/O operations) on a filehandle
8092 that has been manipulated by sysread(), syswrite(), or sysseek().
8093 Those functions ignore the buffering, while tell() does not.
8095 =item telldir DIRHANDLE
8098 =for Pod::Functions get current seekpointer on a directory handle
8100 Returns the current position of the C<readdir> routines on DIRHANDLE.
8101 Value may be given to C<seekdir> to access a particular location in a
8102 directory. C<telldir> has the same caveats about possible directory
8103 compaction as the corresponding system library routine.
8105 =item tie VARIABLE,CLASSNAME,LIST
8108 =for Pod::Functions +5.002 bind a variable to an object class
8110 This function binds a variable to a package class that will provide the
8111 implementation for the variable. VARIABLE is the name of the variable
8112 to be enchanted. CLASSNAME is the name of a class implementing objects
8113 of correct type. Any additional arguments are passed to the
8114 appropriate constructor
8115 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
8116 or C<TIEHASH>). Typically these are arguments such as might be passed
8117 to the C<dbm_open()> function of C. The object returned by the
8118 constructor is also returned by the C<tie> function, which would be useful
8119 if you want to access other methods in CLASSNAME.
8121 Note that functions such as C<keys> and C<values> may return huge lists
8122 when used on large objects, like DBM files. You may prefer to use the
8123 C<each> function to iterate over such. Example:
8125 # print out history file offsets
8127 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
8128 while (($key,$val) = each %HIST) {
8129 print $key, ' = ', unpack('L',$val), "\n";
8133 A class implementing a hash should have the following methods:
8135 TIEHASH classname, LIST
8137 STORE this, key, value
8142 NEXTKEY this, lastkey
8147 A class implementing an ordinary array should have the following methods:
8149 TIEARRAY classname, LIST
8151 STORE this, key, value
8153 STORESIZE this, count
8159 SPLICE this, offset, length, LIST
8166 A class implementing a filehandle should have the following methods:
8168 TIEHANDLE classname, LIST
8169 READ this, scalar, length, offset
8172 WRITE this, scalar, length, offset
8174 PRINTF this, format, LIST
8178 SEEK this, position, whence
8180 OPEN this, mode, LIST
8185 A class implementing a scalar should have the following methods:
8187 TIESCALAR classname, LIST
8193 Not all methods indicated above need be implemented. See L<perltie>,
8194 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
8196 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
8197 for you; you need to do that explicitly yourself. See L<DB_File>
8198 or the F<Config> module for interesting C<tie> implementations.
8200 For further details see L<perltie>, L<"tied VARIABLE">.
8205 =for Pod::Functions get a reference to the object underlying a tied variable
8207 Returns a reference to the object underlying VARIABLE (the same value
8208 that was originally returned by the C<tie> call that bound the variable
8209 to a package.) Returns the undefined value if VARIABLE isn't tied to a
8215 =for Pod::Functions return number of seconds since 1970
8217 Returns the number of non-leap seconds since whatever time the system
8218 considers to be the epoch, suitable for feeding to C<gmtime> and
8219 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
8220 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
8221 1904 in the current local time zone for its epoch.
8223 For measuring time in better granularity than one second, use the
8224 L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
8225 if you have gettimeofday(2), you may be able to use the C<syscall>
8226 interface of Perl. See L<perlfaq8> for details.
8228 For date and time processing look at the many related modules on CPAN.
8229 For a comprehensive date and time representation look at the
8235 =for Pod::Functions return elapsed time for self and child processes
8237 Returns a four-element list giving the user and system times in
8238 seconds for this process and any exited children of this process.
8240 ($user,$system,$cuser,$csystem) = times;
8242 In scalar context, C<times> returns C<$user>.
8244 Children's times are only included for terminated children.
8246 Portability issues: L<perlport/times>.
8250 =for Pod::Functions transliterate a string
8252 The transliteration operator. Same as C<y///>. See
8253 L<perlop/"Quote and Quote-like Operators">.
8255 =item truncate FILEHANDLE,LENGTH
8258 =item truncate EXPR,LENGTH
8260 =for Pod::Functions shorten a file
8262 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
8263 specified length. Raises an exception if truncate isn't implemented
8264 on your system. Returns true if successful, C<undef> on error.
8266 The behavior is undefined if LENGTH is greater than the length of the
8269 The position in the file of FILEHANDLE is left unchanged. You may want to
8270 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
8272 Portability issues: L<perlport/truncate>.
8275 X<uc> X<uppercase> X<toupper>
8279 =for Pod::Functions return upper-case version of a string
8281 Returns an uppercased version of EXPR. This is the internal function
8282 implementing the C<\U> escape in double-quoted strings.
8283 It does not attempt to do titlecase mapping on initial letters. See
8284 L</ucfirst> for that.
8286 If EXPR is omitted, uses C<$_>.
8288 This function behaves the same way under various pragma, such as in a locale,
8292 X<ucfirst> X<uppercase>
8296 =for Pod::Functions return a string with just the next letter in upper case
8298 Returns the value of EXPR with the first character in uppercase
8299 (titlecase in Unicode). This is the internal function implementing
8300 the C<\u> escape in double-quoted strings.
8302 If EXPR is omitted, uses C<$_>.
8304 This function behaves the same way under various pragma, such as in a locale,
8312 =for Pod::Functions set file creation mode mask
8314 Sets the umask for the process to EXPR and returns the previous value.
8315 If EXPR is omitted, merely returns the current umask.
8317 The Unix permission C<rwxr-x---> is represented as three sets of three
8318 bits, or three octal digits: C<0750> (the leading 0 indicates octal
8319 and isn't one of the digits). The C<umask> value is such a number
8320 representing disabled permissions bits. The permission (or "mode")
8321 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
8322 even if you tell C<sysopen> to create a file with permissions C<0777>,
8323 if your umask is C<0022>, then the file will actually be created with
8324 permissions C<0755>. If your C<umask> were C<0027> (group can't
8325 write; others can't read, write, or execute), then passing
8326 C<sysopen> C<0666> would create a file with mode C<0640> (because
8327 C<0666 &~ 027> is C<0640>).
8329 Here's some advice: supply a creation mode of C<0666> for regular
8330 files (in C<sysopen>) and one of C<0777> for directories (in
8331 C<mkdir>) and executable files. This gives users the freedom of
8332 choice: if they want protected files, they might choose process umasks
8333 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
8334 Programs should rarely if ever make policy decisions better left to
8335 the user. The exception to this is when writing files that should be
8336 kept private: mail files, web browser cookies, I<.rhosts> files, and
8339 If umask(2) is not implemented on your system and you are trying to
8340 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
8341 raises an exception. If umask(2) is not implemented and you are
8342 not trying to restrict access for yourself, returns C<undef>.
8344 Remember that a umask is a number, usually given in octal; it is I<not> a
8345 string of octal digits. See also L</oct>, if all you have is a string.
8347 Portability issues: L<perlport/umask>.
8350 X<undef> X<undefine>
8354 =for Pod::Functions remove a variable or function definition
8356 Undefines the value of EXPR, which must be an lvalue. Use only on a
8357 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
8358 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
8359 will probably not do what you expect on most predefined variables or
8360 DBM list values, so don't do that; see L</delete>. Always returns the
8361 undefined value. You can omit the EXPR, in which case nothing is
8362 undefined, but you still get an undefined value that you could, for
8363 instance, return from a subroutine, assign to a variable, or pass as a
8364 parameter. Examples:
8367 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
8371 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
8372 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
8373 select undef, undef, undef, 0.25;
8374 ($a, $b, undef, $c) = &foo; # Ignore third value returned
8376 Note that this is a unary operator, not a list operator.
8379 X<unlink> X<delete> X<remove> X<rm> X<del>
8383 =for Pod::Functions remove one link to a file
8385 Deletes a list of files. On success, it returns the number of files
8386 it successfully deleted. On failure, it returns false and sets C<$!>
8389 my $unlinked = unlink 'a', 'b', 'c';
8391 unlink glob "*.bak";
8393 On error, C<unlink> will not tell you which files it could not remove.
8394 If you want to know which files you could not remove, try them one
8397 foreach my $file ( @goners ) {
8398 unlink $file or warn "Could not unlink $file: $!";
8401 Note: C<unlink> will not attempt to delete directories unless you are
8402 superuser and the B<-U> flag is supplied to Perl. Even if these
8403 conditions are met, be warned that unlinking a directory can inflict
8404 damage on your filesystem. Finally, using C<unlink> on directories is
8405 not supported on many operating systems. Use C<rmdir> instead.
8407 If LIST is omitted, C<unlink> uses C<$_>.
8409 =item unpack TEMPLATE,EXPR
8412 =item unpack TEMPLATE
8414 =for Pod::Functions convert binary structure into normal perl variables
8416 C<unpack> does the reverse of C<pack>: it takes a string
8417 and expands it out into a list of values.
8418 (In scalar context, it returns merely the first value produced.)
8420 If EXPR is omitted, unpacks the C<$_> string.
8421 See L<perlpacktut> for an introduction to this function.
8423 The string is broken into chunks described by the TEMPLATE. Each chunk
8424 is converted separately to a value. Typically, either the string is a result
8425 of C<pack>, or the characters of the string represent a C structure of some
8428 The TEMPLATE has the same format as in the C<pack> function.
8429 Here's a subroutine that does substring:
8432 my($what,$where,$howmuch) = @_;
8433 unpack("x$where a$howmuch", $what);
8438 sub ordinal { unpack("W",$_[0]); } # same as ord()
8440 In addition to fields allowed in pack(), you may prefix a field with
8441 a %<number> to indicate that
8442 you want a <number>-bit checksum of the items instead of the items
8443 themselves. Default is a 16-bit checksum. Checksum is calculated by
8444 summing numeric values of expanded values (for string fields the sum of
8445 C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
8447 For example, the following
8448 computes the same number as the System V sum program:
8452 unpack("%32W*",<>) % 65535;
8455 The following efficiently counts the number of set bits in a bit vector:
8457 $setbits = unpack("%32b*", $selectmask);
8459 The C<p> and C<P> formats should be used with care. Since Perl
8460 has no way of checking whether the value passed to C<unpack()>
8461 corresponds to a valid memory location, passing a pointer value that's
8462 not known to be valid is likely to have disastrous consequences.
8464 If there are more pack codes or if the repeat count of a field or a group
8465 is larger than what the remainder of the input string allows, the result
8466 is not well defined: the repeat count may be decreased, or
8467 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
8468 If the input string is longer than one described by the TEMPLATE,
8469 the remainder of that input string is ignored.
8471 See L</pack> for more examples and notes.
8473 =item unshift ARRAY,LIST
8476 =item unshift EXPR,LIST
8478 =for Pod::Functions prepend more elements to the beginning of a list
8480 Does the opposite of a C<shift>. Or the opposite of a C<push>,
8481 depending on how you look at it. Prepends list to the front of the
8482 array and returns the new number of elements in the array.
8484 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
8486 Note the LIST is prepended whole, not one element at a time, so the
8487 prepended elements stay in the same order. Use C<reverse> to do the
8490 Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
8491 a reference to an unblessed array. The argument will be dereferenced
8492 automatically. This aspect of C<unshift> is considered highly
8493 experimental. The exact behaviour may change in a future version of Perl.
8495 To avoid confusing would-be users of your code who are running earlier
8496 versions of Perl with mysterious syntax errors, put this sort of thing at
8497 the top of your file to signal that your code will work I<only> on Perls of
8500 use 5.014; # so push/pop/etc work on scalars (experimental)
8502 =item untie VARIABLE
8505 =for Pod::Functions break a tie binding to a variable
8507 Breaks the binding between a variable and a package.
8508 (See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
8509 Has no effect if the variable is not tied.
8511 =item use Module VERSION LIST
8512 X<use> X<module> X<import>
8514 =item use Module VERSION
8516 =item use Module LIST
8522 =for Pod::Functions load in a module at compile time and import its namespace
8524 Imports some semantics into the current package from the named module,
8525 generally by aliasing certain subroutine or variable names into your
8526 package. It is exactly equivalent to
8528 BEGIN { require Module; Module->import( LIST ); }
8530 except that Module I<must> be a bareword.
8531 The importation can be made conditional by using the L<if> module.
8533 In the peculiar C<use VERSION> form, VERSION may be either a positive
8534 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
8535 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
8536 exception is raised if VERSION is greater than the version of the
8537 current Perl interpreter; Perl will not attempt to parse the rest of the
8538 file. Compare with L</require>, which can do a similar check at run time.
8539 Symmetrically, C<no VERSION> allows you to specify that you want a version
8540 of Perl older than the specified one.
8542 Specifying VERSION as a literal of the form v5.6.1 should generally be
8543 avoided, because it leads to misleading error messages under earlier
8544 versions of Perl (that is, prior to 5.6.0) that do not support this
8545 syntax. The equivalent numeric version should be used instead.
8547 use v5.6.1; # compile time version check
8549 use 5.006_001; # ditto; preferred for backwards compatibility
8551 This is often useful if you need to check the current Perl version before
8552 C<use>ing library modules that won't work with older versions of Perl.
8553 (We try not to do this more than we have to.)
8555 C<use VERSION> also enables all features available in the requested
8556 version as defined by the C<feature> pragma, disabling any features
8557 not in the requested version's feature bundle. See L<feature>.
8558 Similarly, if the specified Perl version is greater than or equal to
8559 5.12.0, strictures are enabled lexically as
8560 with C<use strict>. Any explicit use of
8561 C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
8562 before it. In both cases, the F<feature.pm> and F<strict.pm> files are
8563 not actually loaded.
8565 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
8566 C<require> makes sure the module is loaded into memory if it hasn't been
8567 yet. The C<import> is not a builtin; it's just an ordinary static method
8568 call into the C<Module> package to tell the module to import the list of
8569 features back into the current package. The module can implement its
8570 C<import> method any way it likes, though most modules just choose to
8571 derive their C<import> method via inheritance from the C<Exporter> class that
8572 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
8573 method can be found then the call is skipped, even if there is an AUTOLOAD
8576 If you do not want to call the package's C<import> method (for instance,
8577 to stop your namespace from being altered), explicitly supply the empty list:
8581 That is exactly equivalent to
8583 BEGIN { require Module }
8585 If the VERSION argument is present between Module and LIST, then the
8586 C<use> will call the VERSION method in class Module with the given
8587 version as an argument. The default VERSION method, inherited from
8588 the UNIVERSAL class, croaks if the given version is larger than the
8589 value of the variable C<$Module::VERSION>.
8591 Again, there is a distinction between omitting LIST (C<import> called
8592 with no arguments) and an explicit empty LIST C<()> (C<import> not
8593 called). Note that there is no comma after VERSION!
8595 Because this is a wide-open interface, pragmas (compiler directives)
8596 are also implemented this way. Currently implemented pragmas are:
8601 use sigtrap qw(SEGV BUS);
8602 use strict qw(subs vars refs);
8603 use subs qw(afunc blurfl);
8604 use warnings qw(all);
8605 use sort qw(stable _quicksort _mergesort);
8607 Some of these pseudo-modules import semantics into the current
8608 block scope (like C<strict> or C<integer>, unlike ordinary modules,
8609 which import symbols into the current package (which are effective
8610 through the end of the file).
8612 Because C<use> takes effect at compile time, it doesn't respect the
8613 ordinary flow control of the code being compiled. In particular, putting
8614 a C<use> inside the false branch of a conditional doesn't prevent it
8615 from being processed. If a module or pragma only needs to be loaded
8616 conditionally, this can be done using the L<if> pragma:
8618 use if $] < 5.008, "utf8";
8619 use if WANT_WARNINGS, warnings => qw(all);
8621 There's a corresponding C<no> declaration that unimports meanings imported
8622 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
8623 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
8624 or no unimport method being found.
8630 Care should be taken when using the C<no VERSION> form of C<no>. It is
8631 I<only> meant to be used to assert that the running Perl is of a earlier
8632 version than its argument and I<not> to undo the feature-enabling side effects
8635 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
8636 for the C<-M> and C<-m> command-line options to Perl that give C<use>
8637 functionality from the command-line.
8642 =for Pod::Functions set a file's last access and modify times
8644 Changes the access and modification times on each file of a list of
8645 files. The first two elements of the list must be the NUMERIC access
8646 and modification times, in that order. Returns the number of files
8647 successfully changed. The inode change time of each file is set
8648 to the current time. For example, this code has the same effect as the
8649 Unix touch(1) command when the files I<already exist> and belong to
8650 the user running the program:
8653 $atime = $mtime = time;
8654 utime $atime, $mtime, @ARGV;
8656 Since Perl 5.8.0, if the first two elements of the list are C<undef>,
8657 the utime(2) syscall from your C library is called with a null second
8658 argument. On most systems, this will set the file's access and
8659 modification times to the current time (i.e., equivalent to the example
8660 above) and will work even on files you don't own provided you have write
8664 utime(undef, undef, $file)
8665 || warn "couldn't touch $file: $!";
8668 Under NFS this will use the time of the NFS server, not the time of
8669 the local machine. If there is a time synchronization problem, the
8670 NFS server and local machine will have different times. The Unix
8671 touch(1) command will in fact normally use this form instead of the
8672 one shown in the first example.
8674 Passing only one of the first two elements as C<undef> is
8675 equivalent to passing a 0 and will not have the effect
8676 described when both are C<undef>. This also triggers an
8677 uninitialized warning.
8679 On systems that support futimes(2), you may pass filehandles among the
8680 files. On systems that don't support futimes(2), passing filehandles raises
8681 an exception. Filehandles must be passed as globs or glob references to be
8682 recognized; barewords are considered filenames.
8684 Portability issues: L<perlport/utime>.
8693 =for Pod::Functions return a list of the values in a hash
8695 In list context, returns a list consisting of all the values of the named
8696 hash. In Perl 5.12 or later only, will also return a list of the values of
8697 an array; prior to that release, attempting to use an array argument will
8698 produce a syntax error. In scalar context, returns the number of values.
8700 Hash entries are returned in an apparently random order. The actual random
8701 order is specific to a given hash; the exact same series of operations
8702 on two hashes may result in a different order for each hash. Any insertion
8703 into the hash may change the order, as will any deletion, with the exception
8704 that the most recent key returned by C<each> or C<keys> may be deleted
8705 without changing the order. So long as a given hash is unmodified you may
8706 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
8707 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
8708 details on why hash order is randomized. Aside from the guarantees
8709 provided here the exact details of Perl's hash algorithm and the hash
8710 traversal order are subject to change in any release of Perl.
8712 As a side effect, calling values() resets the HASH or ARRAY's internal
8713 iterator, see L</each>. (In particular, calling values() in void context
8714 resets the iterator with no other overhead. Apart from resetting the
8715 iterator, C<values @array> in list context is the same as plain C<@array>.
8716 (We recommend that you use void context C<keys @array> for this, but
8717 reasoned that taking C<values @array> out would require more
8718 documentation than leaving it in.)
8720 Note that the values are not copied, which means modifying them will
8721 modify the contents of the hash:
8723 for (values %hash) { s/foo/bar/g } # modifies %hash values
8724 for (@hash{keys %hash}) { s/foo/bar/g } # same
8726 Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
8727 a reference to an unblessed hash or array. The argument will be
8728 dereferenced automatically. This aspect of C<values> is considered highly
8729 experimental. The exact behaviour may change in a future version of Perl.
8731 for (values $hashref) { ... }
8732 for (values $obj->get_arrayref) { ... }
8734 To avoid confusing would-be users of your code who are running earlier
8735 versions of Perl with mysterious syntax errors, put this sort of thing at
8736 the top of your file to signal that your code will work I<only> on Perls of
8739 use 5.012; # so keys/values/each work on arrays
8740 use 5.014; # so keys/values/each work on scalars (experimental)
8742 See also C<keys>, C<each>, and C<sort>.
8744 =item vec EXPR,OFFSET,BITS
8745 X<vec> X<bit> X<bit vector>
8747 =for Pod::Functions test or set particular bits in a string
8749 Treats the string in EXPR as a bit vector made up of elements of
8750 width BITS and returns the value of the element specified by OFFSET
8751 as an unsigned integer. BITS therefore specifies the number of bits
8752 that are reserved for each element in the bit vector. This must
8753 be a power of two from 1 to 32 (or 64, if your platform supports
8756 If BITS is 8, "elements" coincide with bytes of the input string.
8758 If BITS is 16 or more, bytes of the input string are grouped into chunks
8759 of size BITS/8, and each group is converted to a number as with
8760 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
8761 for BITS==64). See L<"pack"> for details.
8763 If bits is 4 or less, the string is broken into bytes, then the bits
8764 of each byte are broken into 8/BITS groups. Bits of a byte are
8765 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
8766 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
8767 breaking the single input byte C<chr(0x36)> into two groups gives a list
8768 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
8770 C<vec> may also be assigned to, in which case parentheses are needed
8771 to give the expression the correct precedence as in
8773 vec($image, $max_x * $x + $y, 8) = 3;
8775 If the selected element is outside the string, the value 0 is returned.
8776 If an element off the end of the string is written to, Perl will first
8777 extend the string with sufficiently many zero bytes. It is an error
8778 to try to write off the beginning of the string (i.e., negative OFFSET).
8780 If the string happens to be encoded as UTF-8 internally (and thus has
8781 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
8782 internal byte string, not the conceptual character string, even if you
8783 only have characters with values less than 256.
8785 Strings created with C<vec> can also be manipulated with the logical
8786 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
8787 vector operation is desired when both operands are strings.
8788 See L<perlop/"Bitwise String Operators">.
8790 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
8791 The comments show the string after each step. Note that this code works
8792 in the same way on big-endian or little-endian machines.
8795 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
8797 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
8798 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
8800 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
8801 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
8802 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
8803 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
8804 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
8805 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
8807 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
8808 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
8809 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
8812 To transform a bit vector into a string or list of 0's and 1's, use these:
8814 $bits = unpack("b*", $vector);
8815 @bits = split(//, unpack("b*", $vector));
8817 If you know the exact length in bits, it can be used in place of the C<*>.
8819 Here is an example to illustrate how the bits actually fall in place:
8825 unpack("V",$_) 01234567890123456789012345678901
8826 ------------------------------------------------------------------
8831 for ($shift=0; $shift < $width; ++$shift) {
8832 for ($off=0; $off < 32/$width; ++$off) {
8833 $str = pack("B*", "0"x32);
8834 $bits = (1<<$shift);
8835 vec($str, $off, $width) = $bits;
8836 $res = unpack("b*",$str);
8837 $val = unpack("V", $str);
8844 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
8845 $off, $width, $bits, $val, $res
8849 Regardless of the machine architecture on which it runs, the
8850 example above should print the following table:
8853 unpack("V",$_) 01234567890123456789012345678901
8854 ------------------------------------------------------------------
8855 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
8856 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
8857 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
8858 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
8859 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
8860 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
8861 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
8862 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
8863 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
8864 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
8865 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
8866 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
8867 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
8868 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
8869 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
8870 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
8871 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
8872 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
8873 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
8874 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
8875 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
8876 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
8877 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
8878 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
8879 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
8880 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
8881 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
8882 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
8883 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
8884 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
8885 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
8886 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
8887 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
8888 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
8889 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
8890 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
8891 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
8892 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
8893 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
8894 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
8895 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
8896 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
8897 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
8898 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
8899 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
8900 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
8901 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
8902 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
8903 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
8904 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
8905 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
8906 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
8907 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
8908 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
8909 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
8910 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
8911 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
8912 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
8913 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
8914 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
8915 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
8916 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
8917 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
8918 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
8919 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
8920 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
8921 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
8922 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
8923 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
8924 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
8925 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
8926 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
8927 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
8928 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
8929 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
8930 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
8931 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
8932 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
8933 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
8934 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
8935 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
8936 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
8937 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
8938 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
8939 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
8940 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
8941 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
8942 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
8943 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
8944 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
8945 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
8946 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
8947 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
8948 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
8949 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
8950 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
8951 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
8952 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
8953 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
8954 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
8955 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
8956 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
8957 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
8958 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
8959 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
8960 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
8961 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
8962 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
8963 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
8964 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
8965 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
8966 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
8967 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
8968 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
8969 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
8970 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
8971 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
8972 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
8973 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
8974 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
8975 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
8976 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
8977 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
8978 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
8979 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
8980 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
8981 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
8982 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
8987 =for Pod::Functions wait for any child process to die
8989 Behaves like wait(2) on your system: it waits for a child
8990 process to terminate and returns the pid of the deceased process, or
8991 C<-1> if there are no child processes. The status is returned in C<$?>
8992 and C<${^CHILD_ERROR_NATIVE}>.
8993 Note that a return value of C<-1> could mean that child processes are
8994 being automatically reaped, as described in L<perlipc>.
8996 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
8997 child created by qx() or system(). See L<perlipc> for details.
8999 Portability issues: L<perlport/wait>.
9001 =item waitpid PID,FLAGS
9004 =for Pod::Functions wait for a particular child process to die
9006 Waits for a particular child process to terminate and returns the pid of
9007 the deceased process, or C<-1> if there is no such child process. On some
9008 systems, a value of 0 indicates that there are processes still running.
9009 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
9011 use POSIX ":sys_wait_h";
9014 $kid = waitpid(-1, WNOHANG);
9017 then you can do a non-blocking wait for all pending zombie processes.
9018 Non-blocking wait is available on machines supporting either the
9019 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
9020 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
9021 system call by remembering the status values of processes that have
9022 exited but have not been harvested by the Perl script yet.)
9024 Note that on some systems, a return value of C<-1> could mean that child
9025 processes are being automatically reaped. See L<perlipc> for details,
9026 and for other examples.
9028 Portability issues: L<perlport/waitpid>.
9031 X<wantarray> X<context>
9033 =for Pod::Functions get void vs scalar vs list context of current subroutine call
9035 Returns true if the context of the currently executing subroutine or
9036 C<eval> is looking for a list value. Returns false if the context is
9037 looking for a scalar. Returns the undefined value if the context is
9038 looking for no value (void context).
9040 return unless defined wantarray; # don't bother doing more
9041 my @a = complex_calculation();
9042 return wantarray ? @a : "@a";
9044 C<wantarray()>'s result is unspecified in the top level of a file,
9045 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
9046 in a C<DESTROY> method.
9048 This function should have been named wantlist() instead.
9051 X<warn> X<warning> X<STDERR>
9053 =for Pod::Functions print debugging info
9055 Prints the value of LIST to STDERR. If the last element of LIST does
9056 not end in a newline, it appends the same file/line number text as C<die>
9059 If the output is empty and C<$@> already contains a value (typically from a
9060 previous eval) that value is used after appending C<"\t...caught">
9061 to C<$@>. This is useful for staying almost, but not entirely similar to
9064 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
9066 No message is printed if there is a C<$SIG{__WARN__}> handler
9067 installed. It is the handler's responsibility to deal with the message
9068 as it sees fit (like, for instance, converting it into a C<die>). Most
9069 handlers must therefore arrange to actually display the
9070 warnings that they are not prepared to deal with, by calling C<warn>
9071 again in the handler. Note that this is quite safe and will not
9072 produce an endless loop, since C<__WARN__> hooks are not called from
9075 You will find this behavior is slightly different from that of
9076 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
9077 instead call C<die> again to change it).
9079 Using a C<__WARN__> handler provides a powerful way to silence all
9080 warnings (even the so-called mandatory ones). An example:
9082 # wipe out *all* compile-time warnings
9083 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
9085 my $foo = 20; # no warning about duplicate my $foo,
9086 # but hey, you asked for it!
9087 # no compile-time or run-time warnings before here
9090 # run-time warnings enabled after here
9091 warn "\$foo is alive and $foo!"; # does show up
9093 See L<perlvar> for details on setting C<%SIG> entries and for more
9094 examples. See the Carp module for other kinds of warnings using its
9095 carp() and cluck() functions.
9097 =item write FILEHANDLE
9104 =for Pod::Functions print a picture record
9106 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
9107 using the format associated with that file. By default the format for
9108 a file is the one having the same name as the filehandle, but the
9109 format for the current output channel (see the C<select> function) may be set
9110 explicitly by assigning the name of the format to the C<$~> variable.
9112 Top of form processing is handled automatically: if there is insufficient
9113 room on the current page for the formatted record, the page is advanced by
9114 writing a form feed and a special top-of-page
9115 format is used to format the new
9116 page header before the record is written. By default, the top-of-page
9117 format is the name of the filehandle with "_TOP" appended, or "top"
9118 in the current package if the former does not exist. This would be a
9119 problem with autovivified filehandles, but it may be dynamically set to the
9120 format of your choice by assigning the name to the C<$^> variable while
9121 that filehandle is selected. The number of lines remaining on the current
9122 page is in variable C<$->, which can be set to C<0> to force a new page.
9124 If FILEHANDLE is unspecified, output goes to the current default output
9125 channel, which starts out as STDOUT but may be changed by the
9126 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
9127 is evaluated and the resulting string is used to look up the name of
9128 the FILEHANDLE at run time. For more on formats, see L<perlform>.
9130 Note that write is I<not> the opposite of C<read>. Unfortunately.
9134 =for Pod::Functions transliterate a string
9136 The transliteration operator. Same as C<tr///>. See
9137 L<perlop/"Quote and Quote-like Operators">.
9141 =head2 Non-function Keywords by Cross-reference
9151 These keywords are documented in L<perldata/"Special Literals">.
9169 These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.
9179 This method keyword is documented in L<perlobj/"Destructors">.
9213 These operators are documented in L<perlop>.
9223 This keyword is documented in L<perlsub/"Autoloading">.
9247 These flow-control keywords are documented in L<perlsyn/"Compound Statements">.
9259 These flow-control keywords related to the experimental switch feature are
9260 documented in L<perlsyn/"Switch Statements">.