4 perlfunc - Perl builtin functions
8 The functions in this section can serve as terms in an expression.
9 They fall into two major categories: list operators and named unary
10 operators. These differ in their precedence relationship with a
11 following comma. (See the precedence table in L<perlop>.) List
12 operators take more than one argument, while unary operators can never
13 take more than one argument. Thus, a comma terminates the argument of
14 a unary operator, but merely separates the arguments of a list
15 operator. A unary operator generally provides scalar context to its
16 argument, while a list operator may provide either scalar or list
17 contexts for its arguments. If it does both, scalar arguments
18 come first and list argument follow, and there can only ever
19 be one such list argument. For instance, splice() has three scalar
20 arguments followed by a list, whereas gethostbyname() has four scalar
23 In the syntax descriptions that follow, list operators that expect a
24 list (and provide list context for elements of the list) are shown
25 with LIST as an argument. Such a list may consist of any combination
26 of scalar arguments or list values; the list values will be included
27 in the list as if each individual element were interpolated at that
28 point in the list, forming a longer single-dimensional list value.
29 Commas should separate literal elements of the LIST.
31 Any function in the list below may be used either with or without
32 parentheses around its arguments. (The syntax descriptions omit the
33 parentheses.) If you use parentheses, the simple but occasionally
34 surprising rule is this: It I<looks> like a function, therefore it I<is> a
35 function, and precedence doesn't matter. Otherwise it's a list
36 operator or unary operator, and precedence does matter. Whitespace
37 between the function and left parenthesis doesn't count, so sometimes
38 you need to be careful:
40 print 1+2+4; # Prints 7.
41 print(1+2) + 4; # Prints 3.
42 print (1+2)+4; # Also prints 3!
43 print +(1+2)+4; # Prints 7.
44 print ((1+2)+4); # Prints 7.
46 If you run Perl with the B<-w> switch it can warn you about this. For
47 example, the third line above produces:
49 print (...) interpreted as function at - line 1.
50 Useless use of integer addition in void context at - line 1.
52 A few functions take no arguments at all, and therefore work as neither
53 unary nor list operators. These include such functions as C<time>
54 and C<endpwent>. For example, C<time+86_400> always means
57 For functions that can be used in either a scalar or list context,
58 nonabortive failure is generally indicated in scalar context by
59 returning the undefined value, and in list context by returning the
62 Remember the following important rule: There is B<no rule> that relates
63 the behavior of an expression in list context to its behavior in scalar
64 context, or vice versa. It might do two totally different things.
65 Each operator and function decides which sort of value would be most
66 appropriate to return in scalar context. Some operators return the
67 length of the list that would have been returned in list context. Some
68 operators return the first value in the list. Some operators return the
69 last value in the list. Some operators return a count of successful
70 operations. In general, they do what you want, unless you want
74 A named array in scalar context is quite different from what would at
75 first glance appear to be a list in scalar context. You can't get a list
76 like C<(1,2,3)> into being in scalar context, because the compiler knows
77 the context at compile time. It would generate the scalar comma operator
78 there, not the list construction version of the comma. That means it
79 was never a list to start with.
81 In general, functions in Perl that serve as wrappers for system calls ("syscalls")
82 of the same name (like chown(2), fork(2), closedir(2), etc.) return
83 true when they succeed and C<undef> otherwise, as is usually mentioned
84 in the descriptions below. This is different from the C interfaces,
85 which return C<-1> on failure. Exceptions to this rule include C<wait>,
86 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
87 variable on failure. Other functions do not, except accidentally.
89 Extension modules can also hook into the Perl parser to define new
90 kinds of keyword-headed expression. These may look like functions, but
91 may also look completely different. The syntax following the keyword
92 is defined entirely by the extension. If you are an implementor, see
93 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
94 a module, see the module's documentation for details of the syntax that
97 =head2 Perl Functions by Category
100 Here are Perl's functions (including things that look like
101 functions, like some keywords and named operators)
102 arranged by category. Some functions appear in more
107 =item Functions for SCALARs or strings
108 X<scalar> X<string> X<character>
110 =for Pod::Functions =String
112 C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>,
113 C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
114 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
116 C<fc> is available only if the C<"fc"> feature is enabled or if it is
117 prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically
118 with a C<use v5.16> (or higher) declaration in the current scope.
121 =item Regular expressions and pattern matching
122 X<regular expression> X<regex> X<regexp>
124 =for Pod::Functions =Regexp
126 C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study>
128 =item Numeric functions
129 X<numeric> X<number> X<trigonometric> X<trigonometry>
131 =for Pod::Functions =Math
133 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
134 C<sin>, C<sqrt>, C<srand>
136 =item Functions for real @ARRAYs
139 =for Pod::Functions =ARRAY
141 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
143 =item Functions for list data
146 =for Pod::Functions =LIST
148 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
150 =item Functions for real %HASHes
153 =for Pod::Functions =HASH
155 C<delete>, C<each>, C<exists>, C<keys>, C<values>
157 =item Input and output functions
158 X<I/O> X<input> X<output> X<dbm>
160 =for Pod::Functions =I/O
162 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
163 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
164 C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>,
165 C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>,
166 C<truncate>, C<warn>, C<write>
168 C<say> is available only if the C<"say"> feature is enabled or if it is
169 prefixed with C<CORE::>. The C<"say"> feature is enabled automatically
170 with a C<use v5.10> (or higher) declaration in the current scope.
172 =item Functions for fixed-length data or records
174 =for Pod::Functions =Binary
176 C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>,
179 =item Functions for filehandles, files, or directories
180 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
182 =for Pod::Functions =File
184 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
185 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
186 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
187 C<umask>, C<unlink>, C<utime>
189 =item Keywords related to the control flow of your Perl program
192 =for Pod::Functions =Flow
194 C<break>, C<caller>, C<continue>, C<die>, C<do>,
195 C<dump>, C<eval>, C<evalbytes> C<exit>,
196 C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
197 C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
199 C<break> is available only if you enable the experimental C<"switch">
200 feature or use the C<CORE::> prefix. The C<"switch"> feature also enables
201 the C<default>, C<given> and C<when> statements, which are documented in
202 L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled
203 automatically with a C<use v5.10> (or higher) declaration in the current
204 scope. In Perl v5.14 and earlier, C<continue> required the C<"switch">
205 feature, like the other keywords.
207 C<evalbytes> is only available with the C<"evalbytes"> feature (see
208 L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available
209 with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both
210 the C<"evalbytes"> and C<"current_sub"> features are enabled automatically
211 with a C<use v5.16> (or higher) declaration in the current scope.
213 =item Keywords related to scoping
215 =for Pod::Functions =Namespace
217 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
219 C<state> is available only if the C<"state"> feature is enabled or if it is
220 prefixed with C<CORE::>. The C<"state"> feature is enabled automatically
221 with a C<use v5.10> (or higher) declaration in the current scope.
223 =item Miscellaneous functions
225 =for Pod::Functions =Misc
227 C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef>
229 =item Functions for processes and process groups
230 X<process> X<pid> X<process id>
232 =for Pod::Functions =Process
234 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
235 C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
236 C<setpriority>, C<sleep>, C<system>,
237 C<times>, C<wait>, C<waitpid>
239 =item Keywords related to Perl modules
242 =for Pod::Functions =Modules
244 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
246 =item Keywords related to classes and object-orientation
247 X<object> X<class> X<package>
249 =for Pod::Functions =Objects
251 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
254 =item Low-level socket functions
257 =for Pod::Functions =Socket
259 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
260 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
261 C<socket>, C<socketpair>
263 =item System V interprocess communication functions
264 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
266 =for Pod::Functions =SysV
268 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
269 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
271 =item Fetching user and group info
272 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
274 =for Pod::Functions =User
276 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
277 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
278 C<getpwuid>, C<setgrent>, C<setpwent>
280 =item Fetching network info
281 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
283 =for Pod::Functions =Network
285 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
286 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
287 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
288 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
289 C<setnetent>, C<setprotoent>, C<setservent>
291 =item Time-related functions
294 =for Pod::Functions =Time
296 C<gmtime>, C<localtime>, C<time>, C<times>
298 =item Non-function keywords
300 =for Pod::Functions =!Non-functions
302 C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>,
303 C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>,
304 C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>,
305 C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>,
306 C<while>, C<x>, C<xor>
311 X<portability> X<Unix> X<portable>
313 Perl was born in Unix and can therefore access all common Unix
314 system calls. In non-Unix environments, the functionality of some
315 Unix system calls may not be available or details of the available
316 functionality may differ slightly. The Perl functions affected
319 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
320 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
321 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
322 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
323 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
324 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
325 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
326 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
327 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
328 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
329 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
330 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
331 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
332 C<shmwrite>, C<socket>, C<socketpair>,
333 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
334 C<times>, C<truncate>, C<umask>, C<unlink>,
335 C<utime>, C<wait>, C<waitpid>
337 For more information about the portability of these functions, see
338 L<perlport> and other available platform-specific documentation.
340 =head2 Alphabetical Listing of Perl Functions
345 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
346 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
354 =for Pod::Functions a file test (-r, -x, etc)
356 A file test, where X is one of the letters listed below. This unary
357 operator takes one argument, either a filename, a filehandle, or a dirhandle,
358 and tests the associated file to see if something is true about it. If the
359 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
360 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
361 the undefined value if the file doesn't exist. Despite the funny
362 names, precedence is the same as any other named unary operator. The
363 operator may be any of:
365 -r File is readable by effective uid/gid.
366 -w File is writable by effective uid/gid.
367 -x File is executable by effective uid/gid.
368 -o File is owned by effective uid.
370 -R File is readable by real uid/gid.
371 -W File is writable by real uid/gid.
372 -X File is executable by real uid/gid.
373 -O File is owned by real uid.
376 -z File has zero size (is empty).
377 -s File has nonzero size (returns size in bytes).
379 -f File is a plain file.
380 -d File is a directory.
381 -l File is a symbolic link.
382 -p File is a named pipe (FIFO), or Filehandle is a pipe.
384 -b File is a block special file.
385 -c File is a character special file.
386 -t Filehandle is opened to a tty.
388 -u File has setuid bit set.
389 -g File has setgid bit set.
390 -k File has sticky bit set.
392 -T File is an ASCII text file (heuristic guess).
393 -B File is a "binary" file (opposite of -T).
395 -M Script start time minus file modification time, in days.
396 -A Same for access time.
397 -C Same for inode change time (Unix, may differ for other
404 next unless -f $_; # ignore specials
408 Note that C<-s/a/b/> does not do a negated substitution. Saying
409 C<-exp($foo)> still works as expected, however: only single letters
410 following a minus are interpreted as file tests.
412 These operators are exempt from the "looks like a function rule" described
413 above. That is, an opening parenthesis after the operator does not affect
414 how much of the following code constitutes the argument. Put the opening
415 parentheses before the operator to separate it from code that follows (this
416 applies only to operators with higher precedence than unary operators, of
419 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
420 (-s $file) + 1024 # correct
422 The interpretation of the file permission operators C<-r>, C<-R>,
423 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
424 of the file and the uids and gids of the user. There may be other
425 reasons you can't actually read, write, or execute the file: for
426 example network filesystem access controls, ACLs (access control lists),
427 read-only filesystems, and unrecognized executable formats. Note
428 that the use of these six specific operators to verify if some operation
429 is possible is usually a mistake, because it may be open to race
432 Also note that, for the superuser on the local filesystems, the C<-r>,
433 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
434 if any execute bit is set in the mode. Scripts run by the superuser
435 may thus need to do a stat() to determine the actual mode of the file,
436 or temporarily set their effective uid to something else.
438 If you are using ACLs, there is a pragma called C<filetest> that may
439 produce more accurate results than the bare stat() mode bits.
440 When under C<use filetest 'access'> the above-mentioned filetests
441 test whether the permission can(not) be granted using the
442 access(2) family of system calls. Also note that the C<-x> and C<-X> may
443 under this pragma return true even if there are no execute permission
444 bits set (nor any extra execute permission ACLs). This strangeness is
445 due to the underlying system calls' definitions. Note also that, due to
446 the implementation of C<use filetest 'access'>, the C<_> special
447 filehandle won't cache the results of the file tests when this pragma is
448 in effect. Read the documentation for the C<filetest> pragma for more
451 The C<-T> and C<-B> switches work as follows. The first block or so of the
452 file is examined for odd characters such as strange control codes or
453 characters with the high bit set. If too many strange characters (>30%)
454 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
455 containing a zero byte in the first block is considered a binary file. If C<-T>
456 or C<-B> is used on a filehandle, the current IO buffer is examined
457 rather than the first block. Both C<-T> and C<-B> return true on an empty
458 file, or a file at EOF when testing a filehandle. Because you have to
459 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
460 against the file first, as in C<next unless -f $file && -T $file>.
462 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
463 the special filehandle consisting of a solitary underline, then the stat
464 structure of the previous file test (or stat operator) is used, saving
465 a system call. (This doesn't work with C<-t>, and you need to remember
466 that lstat() and C<-l> leave values in the stat structure for the
467 symbolic link, not the real file.) (Also, if the stat buffer was filled by
468 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
471 print "Can do.\n" if -r $a || -w _ || -x _;
474 print "Readable\n" if -r _;
475 print "Writable\n" if -w _;
476 print "Executable\n" if -x _;
477 print "Setuid\n" if -u _;
478 print "Setgid\n" if -g _;
479 print "Sticky\n" if -k _;
480 print "Text\n" if -T _;
481 print "Binary\n" if -B _;
483 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
484 test operators, in a way that C<-f -w -x $file> is equivalent to
485 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
486 the return value of C<-f $file> as an argument to another filetest
487 operator, no special magic will happen.)
489 Portability issues: L<perlport/-X>.
491 To avoid confusing would-be users of your code with mysterious
492 syntax errors, put something like this at the top of your script:
494 use 5.010; # so filetest ops can stack
501 =for Pod::Functions absolute value function
503 Returns the absolute value of its argument.
504 If VALUE is omitted, uses C<$_>.
506 =item accept NEWSOCKET,GENERICSOCKET
509 =for Pod::Functions accept an incoming socket connect
511 Accepts an incoming socket connect, just as accept(2)
512 does. Returns the packed address if it succeeded, false otherwise.
513 See the example in L<perlipc/"Sockets: Client/Server Communication">.
515 On systems that support a close-on-exec flag on files, the flag will
516 be set for the newly opened file descriptor, as determined by the
517 value of $^F. See L<perlvar/$^F>.
526 =for Pod::Functions schedule a SIGALRM
528 Arranges to have a SIGALRM delivered to this process after the
529 specified number of wallclock seconds has elapsed. If SECONDS is not
530 specified, the value stored in C<$_> is used. (On some machines,
531 unfortunately, the elapsed time may be up to one second less or more
532 than you specified because of how seconds are counted, and process
533 scheduling may delay the delivery of the signal even further.)
535 Only one timer may be counting at once. Each call disables the
536 previous timer, and an argument of C<0> may be supplied to cancel the
537 previous timer without starting a new one. The returned value is the
538 amount of time remaining on the previous timer.
540 For delays of finer granularity than one second, the Time::HiRes module
541 (from CPAN, and starting from Perl 5.8 part of the standard
542 distribution) provides ualarm(). You may also use Perl's four-argument
543 version of select() leaving the first three arguments undefined, or you
544 might be able to use the C<syscall> interface to access setitimer(2) if
545 your system supports it. See L<perlfaq8> for details.
547 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
548 C<sleep> may be internally implemented on your system with C<alarm>.
550 If you want to use C<alarm> to time out a system call you need to use an
551 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
552 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
553 restart system calls on some systems. Using C<eval>/C<die> always works,
554 modulo the caveats given in L<perlipc/"Signals">.
557 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
559 $nread = sysread SOCKET, $buffer, $size;
563 die unless $@ eq "alarm\n"; # propagate unexpected errors
570 For more information see L<perlipc>.
572 Portability issues: L<perlport/alarm>.
575 X<atan2> X<arctangent> X<tan> X<tangent>
577 =for Pod::Functions arctangent of Y/X in the range -PI to PI
579 Returns the arctangent of Y/X in the range -PI to PI.
581 For the tangent operation, you may use the C<Math::Trig::tan>
582 function, or use the familiar relation:
584 sub tan { sin($_[0]) / cos($_[0]) }
586 The return value for C<atan2(0,0)> is implementation-defined; consult
587 your atan2(3) manpage for more information.
589 Portability issues: L<perlport/atan2>.
591 =item bind SOCKET,NAME
594 =for Pod::Functions binds an address to a socket
596 Binds a network address to a socket, just as bind(2)
597 does. Returns true if it succeeded, false otherwise. NAME should be a
598 packed address of the appropriate type for the socket. See the examples in
599 L<perlipc/"Sockets: Client/Server Communication">.
601 =item binmode FILEHANDLE, LAYER
602 X<binmode> X<binary> X<text> X<DOS> X<Windows>
604 =item binmode FILEHANDLE
606 =for Pod::Functions prepare binary files for I/O
608 Arranges for FILEHANDLE to be read or written in "binary" or "text"
609 mode on systems where the run-time libraries distinguish between
610 binary and text files. If FILEHANDLE is an expression, the value is
611 taken as the name of the filehandle. Returns true on success,
612 otherwise it returns C<undef> and sets C<$!> (errno).
614 On some systems (in general, DOS- and Windows-based systems) binmode()
615 is necessary when you're not working with a text file. For the sake
616 of portability it is a good idea always to use it when appropriate,
617 and never to use it when it isn't appropriate. Also, people can
618 set their I/O to be by default UTF8-encoded Unicode, not bytes.
620 In other words: regardless of platform, use binmode() on binary data,
621 like images, for example.
623 If LAYER is present it is a single string, but may contain multiple
624 directives. The directives alter the behaviour of the filehandle.
625 When LAYER is present, using binmode on a text file makes sense.
627 If LAYER is omitted or specified as C<:raw> the filehandle is made
628 suitable for passing binary data. This includes turning off possible CRLF
629 translation and marking it as bytes (as opposed to Unicode characters).
630 Note that, despite what may be implied in I<"Programming Perl"> (the
631 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
632 Other layers that would affect the binary nature of the stream are
633 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
634 PERLIO environment variable.
636 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
637 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
638 establish default I/O layers. See L<open>.
640 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
641 in "Programming Perl, 3rd Edition". However, since the publishing of this
642 book, by many known as "Camel III", the consensus of the naming of this
643 functionality has moved from "discipline" to "layer". All documentation
644 of this version of Perl therefore refers to "layers" rather than to
645 "disciplines". Now back to the regularly scheduled documentation...>
647 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
648 C<:utf8> just marks the data as UTF-8 without further checking,
649 while C<:encoding(UTF-8)> checks the data for actually being valid
650 UTF-8. More details can be found in L<PerlIO::encoding>.
652 In general, binmode() should be called after open() but before any I/O
653 is done on the filehandle. Calling binmode() normally flushes any
654 pending buffered output data (and perhaps pending input data) on the
655 handle. An exception to this is the C<:encoding> layer that
656 changes the default character encoding of the handle; see L</open>.
657 The C<:encoding> layer sometimes needs to be called in
658 mid-stream, and it doesn't flush the stream. The C<:encoding>
659 also implicitly pushes on top of itself the C<:utf8> layer because
660 internally Perl operates on UTF8-encoded Unicode characters.
662 The operating system, device drivers, C libraries, and Perl run-time
663 system all conspire to let the programmer treat a single
664 character (C<\n>) as the line terminator, irrespective of external
665 representation. On many operating systems, the native text file
666 representation matches the internal representation, but on some
667 platforms the external representation of C<\n> is made up of more than
670 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
671 a single character to end each line in the external representation of text
672 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
673 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
674 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
675 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
676 two characters C<\cM\cJ>. That means that if you don't use binmode() on
677 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
678 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
679 output. This is what you want for text files, but it can be disastrous for
682 Another consequence of using binmode() (on some systems) is that
683 special end-of-file markers will be seen as part of the data stream.
684 For systems from the Microsoft family this means that, if your binary
685 data contain C<\cZ>, the I/O subsystem will regard it as the end of
686 the file, unless you use binmode().
688 binmode() is important not only for readline() and print() operations,
689 but also when using read(), seek(), sysread(), syswrite() and tell()
690 (see L<perlport> for more details). See the C<$/> and C<$\> variables
691 in L<perlvar> for how to manually set your input and output
692 line-termination sequences.
694 Portability issues: L<perlport/binmode>.
696 =item bless REF,CLASSNAME
701 =for Pod::Functions create an object
703 This function tells the thingy referenced by REF that it is now an object
704 in the CLASSNAME package. If CLASSNAME is omitted, the current package
705 is used. Because a C<bless> is often the last thing in a constructor,
706 it returns the reference for convenience. Always use the two-argument
707 version if a derived class might inherit the function doing the blessing.
708 See L<perlobj> for more about the blessing (and blessings) of objects.
710 Consider always blessing objects in CLASSNAMEs that are mixed case.
711 Namespaces with all lowercase names are considered reserved for
712 Perl pragmata. Builtin types have all uppercase names. To prevent
713 confusion, you may wish to avoid such package names as well. Make sure
714 that CLASSNAME is a true value.
716 See L<perlmod/"Perl Modules">.
720 =for Pod::Functions +switch break out of a C<given> block
722 Break out of a C<given()> block.
724 This keyword is enabled by the C<"switch"> feature; see L<feature> for
725 more information on C<"switch">. You can also access it by prefixing it
726 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
730 X<caller> X<call stack> X<stack> X<stack trace>
734 =for Pod::Functions get context of the current subroutine call
736 Returns the context of the current subroutine call. In scalar context,
737 returns the caller's package name if there I<is> a caller (that is, if
738 we're in a subroutine or C<eval> or C<require>) and the undefined value
739 otherwise. In list context, returns
742 ($package, $filename, $line) = caller;
744 With EXPR, it returns some extra information that the debugger uses to
745 print a stack trace. The value of EXPR indicates how many call frames
746 to go back before the current one.
749 ($package, $filename, $line, $subroutine, $hasargs,
752 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
755 Here $subroutine may be C<(eval)> if the frame is not a subroutine
756 call, but an C<eval>. In such a case additional elements $evaltext and
757 C<$is_require> are set: C<$is_require> is true if the frame is created by a
758 C<require> or C<use> statement, $evaltext contains the text of the
759 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
760 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
761 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
762 frame.) $subroutine may also be C<(unknown)> if this particular
763 subroutine happens to have been deleted from the symbol table.
764 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
765 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
766 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
767 corresponds to C<${^WARNING_BITS}>. The
768 C<$hints> and C<$bitmask> values are subject
769 to change between versions of Perl, and are not meant for external use.
771 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
772 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
773 of this hash, as they are the actual values stored in the optree.
775 Furthermore, when called from within the DB package in
776 list context, and with an argument, caller returns more
777 detailed information: it sets the list variable C<@DB::args> to be the
778 arguments with which the subroutine was invoked.
780 Be aware that the optimizer might have optimized call frames away before
781 C<caller> had a chance to get the information. That means that C<caller(N)>
782 might not return information about the call frame you expect it to, for
783 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
784 previous time C<caller> was called.
786 Be aware that setting C<@DB::args> is I<best effort>, intended for
787 debugging or generating backtraces, and should not be relied upon. In
788 particular, as C<@_> contains aliases to the caller's arguments, Perl does
789 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790 subroutine makes to C<@_> or its contents, not the original values at call
791 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
792 elements, so under certain cases its elements may have become freed and
793 reallocated for other variables or temporary values. Finally, a side effect
794 of the current implementation is that the effects of C<shift @_> can
795 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
797 elements), so C<@DB::args> is actually a hybrid of the current state and
798 initial state of C<@_>. Buyer beware.
805 =item chdir FILEHANDLE
807 =item chdir DIRHANDLE
811 =for Pod::Functions change your current working directory
813 Changes the working directory to EXPR, if possible. If EXPR is omitted,
814 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
815 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817 neither is set, C<chdir> does nothing. It returns true on success,
818 false otherwise. See the example under C<die>.
820 On systems that support fchdir(2), you may pass a filehandle or
821 directory handle as the argument. On systems that don't support fchdir(2),
822 passing handles raises an exception.
825 X<chmod> X<permission> X<mode>
827 =for Pod::Functions changes the permissions on a list of files
829 Changes the permissions of a list of files. The first element of the
830 list must be the numeric mode, which should probably be an octal
831 number, and which definitely should I<not> be a string of octal digits:
832 C<0644> is okay, but C<"0644"> is not. Returns the number of files
833 successfully changed. See also L</oct> if all you have is a string.
835 $cnt = chmod 0755, "foo", "bar";
836 chmod 0755, @executables;
837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
842 On systems that support fchmod(2), you may pass filehandles among the
843 files. On systems that don't support fchmod(2), passing filehandles raises
844 an exception. Filehandles must be passed as globs or glob references to be
845 recognized; barewords are considered filenames.
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
851 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
854 use Fcntl qw( :mode );
855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
856 # Identical to the chmod 0755 of the example above.
858 Portability issues: L<perlport/chmod>.
861 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
867 =for Pod::Functions remove a trailing record separator from a string
869 This safer version of L</chop> removes any trailing string
870 that corresponds to the current value of C<$/> (also known as
871 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872 number of characters removed from all its arguments. It's often used to
873 remove the newline from the end of an input record when you're worried
874 that the final record may be missing its newline. When in paragraph
875 mode (C<$/ = "">), it removes all trailing newlines from the string.
876 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
877 a reference to an integer or the like; see L<perlvar>) chomp() won't
879 If VARIABLE is omitted, it chomps C<$_>. Example:
882 chomp; # avoid \n on last field
887 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
889 You can actually chomp anything that's an lvalue, including an assignment:
892 chomp($answer = <STDIN>);
894 If you chomp a list, each element is chomped, and the total number of
895 characters removed is returned.
897 Note that parentheses are necessary when you're chomping anything
898 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
899 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
900 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
901 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
911 =for Pod::Functions remove the last character from a string
913 Chops off the last character of a string and returns the character
914 chopped. It is much more efficient than C<s/.$//s> because it neither
915 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
916 If VARIABLE is a hash, it chops the hash's values, but not its keys.
918 You can actually chop anything that's an lvalue, including an assignment.
920 If you chop a list, each element is chopped. Only the value of the
921 last C<chop> is returned.
923 Note that C<chop> returns the last character. To return all but the last
924 character, use C<substr($string, 0, -1)>.
929 X<chown> X<owner> X<user> X<group>
931 =for Pod::Functions change the ownership on a list of files
933 Changes the owner (and group) of a list of files. The first two
934 elements of the list must be the I<numeric> uid and gid, in that
935 order. A value of -1 in either position is interpreted by most
936 systems to leave that value unchanged. Returns the number of files
937 successfully changed.
939 $cnt = chown $uid, $gid, 'foo', 'bar';
940 chown $uid, $gid, @filenames;
942 On systems that support fchown(2), you may pass filehandles among the
943 files. On systems that don't support fchown(2), passing filehandles raises
944 an exception. Filehandles must be passed as globs or glob references to be
945 recognized; barewords are considered filenames.
947 Here's an example that looks up nonnumeric uids in the passwd file:
950 chomp($user = <STDIN>);
952 chomp($pattern = <STDIN>);
954 ($login,$pass,$uid,$gid) = getpwnam($user)
955 or die "$user not in passwd file";
957 @ary = glob($pattern); # expand filenames
958 chown $uid, $gid, @ary;
960 On most systems, you are not allowed to change the ownership of the
961 file unless you're the superuser, although you should be able to change
962 the group to any of your secondary groups. On insecure systems, these
963 restrictions may be relaxed, but this is not a portable assumption.
964 On POSIX systems, you can detect this condition this way:
966 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
967 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
969 Portability issues: L<perlport/chmod>.
972 X<chr> X<character> X<ASCII> X<Unicode>
976 =for Pod::Functions get character this number represents
978 Returns the character represented by that NUMBER in the character set.
979 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
980 chr(0x263a) is a Unicode smiley face.
982 Negative values give the Unicode replacement character (chr(0xfffd)),
983 except under the L<bytes> pragma, where the low eight bits of the value
984 (truncated to an integer) are used.
986 If NUMBER is omitted, uses C<$_>.
988 For the reverse, use L</ord>.
990 Note that characters from 128 to 255 (inclusive) are by default
991 internally not encoded as UTF-8 for backward compatibility reasons.
993 See L<perlunicode> for more about Unicode.
995 =item chroot FILENAME
1000 =for Pod::Functions make directory new root for path lookups
1002 This function works like the system call by the same name: it makes the
1003 named directory the new root directory for all further pathnames that
1004 begin with a C</> by your process and all its children. (It doesn't
1005 change your current working directory, which is unaffected.) For security
1006 reasons, this call is restricted to the superuser. If FILENAME is
1007 omitted, does a C<chroot> to C<$_>.
1009 Portability issues: L<perlport/chroot>.
1011 =item close FILEHANDLE
1016 =for Pod::Functions close file (or pipe or socket) handle
1018 Closes the file or pipe associated with the filehandle, flushes the IO
1019 buffers, and closes the system file descriptor. Returns true if those
1020 operations succeed and if no error was reported by any PerlIO
1021 layer. Closes the currently selected filehandle if the argument is
1024 You don't have to close FILEHANDLE if you are immediately going to do
1025 another C<open> on it, because C<open> closes it for you. (See
1026 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1027 counter (C<$.>), while the implicit close done by C<open> does not.
1029 If the filehandle came from a piped open, C<close> returns false if one of
1030 the other syscalls involved fails or if its program exits with non-zero
1031 status. If the only problem was that the program exited non-zero, C<$!>
1032 will be set to C<0>. Closing a pipe also waits for the process executing
1033 on the pipe to exit--in case you wish to look at the output of the pipe
1034 afterwards--and implicitly puts the exit status value of that command into
1035 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1037 If there are multiple threads running, C<close> on a filehandle from a
1038 piped open returns true without waiting for the child process to terminate,
1039 if the filehandle is still open in another thread.
1041 Closing the read end of a pipe before the process writing to it at the
1042 other end is done writing results in the writer receiving a SIGPIPE. If
1043 the other end can't handle that, be sure to read all the data before
1048 open(OUTPUT, '|sort >foo') # pipe to sort
1049 or die "Can't start sort: $!";
1050 #... # print stuff to output
1051 close OUTPUT # wait for sort to finish
1052 or warn $! ? "Error closing sort pipe: $!"
1053 : "Exit status $? from sort";
1054 open(INPUT, 'foo') # get sort's results
1055 or die "Can't open 'foo' for input: $!";
1057 FILEHANDLE may be an expression whose value can be used as an indirect
1058 filehandle, usually the real filehandle name or an autovivified handle.
1060 =item closedir DIRHANDLE
1063 =for Pod::Functions close directory handle
1065 Closes a directory opened by C<opendir> and returns the success of that
1068 =item connect SOCKET,NAME
1071 =for Pod::Functions connect to a remote socket
1073 Attempts to connect to a remote socket, just like connect(2).
1074 Returns true if it succeeded, false otherwise. NAME should be a
1075 packed address of the appropriate type for the socket. See the examples in
1076 L<perlipc/"Sockets: Client/Server Communication">.
1078 =item continue BLOCK
1083 =for Pod::Functions optional trailing block in a while or foreach
1085 When followed by a BLOCK, C<continue> is actually a
1086 flow control statement rather than a function. If
1087 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1088 C<foreach>), it is always executed just before the conditional is about to
1089 be evaluated again, just like the third part of a C<for> loop in C. Thus
1090 it can be used to increment a loop variable, even when the loop has been
1091 continued via the C<next> statement (which is similar to the C C<continue>
1094 C<last>, C<next>, or C<redo> may appear within a C<continue>
1095 block; C<last> and C<redo> behave as if they had been executed within
1096 the main block. So will C<next>, but since it will execute a C<continue>
1097 block, it may be more entertaining.
1100 ### redo always comes here
1103 ### next always comes here
1105 # then back the top to re-check EXPR
1107 ### last always comes here
1109 Omitting the C<continue> section is equivalent to using an
1110 empty one, logically enough, so C<next> goes directly back
1111 to check the condition at the top of the loop.
1113 When there is no BLOCK, C<continue> is a function that
1114 falls through the current C<when> or C<default> block instead of iterating
1115 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1116 In Perl 5.14 and earlier, this form of C<continue> was
1117 only available when the C<"switch"> feature was enabled.
1118 See L<feature> and L<perlsyn/"Switch Statements"> for more
1122 X<cos> X<cosine> X<acos> X<arccosine>
1126 =for Pod::Functions cosine function
1128 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1129 takes the cosine of C<$_>.
1131 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1132 function, or use this relation:
1134 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1136 =item crypt PLAINTEXT,SALT
1137 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1138 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1140 =for Pod::Functions one-way passwd-style encryption
1142 Creates a digest string exactly like the crypt(3) function in the C
1143 library (assuming that you actually have a version there that has not
1144 been extirpated as a potential munition).
1146 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1147 into a short string, called a digest, which is returned. The same
1148 PLAINTEXT and SALT will always return the same string, but there is no
1149 (known) way to get the original PLAINTEXT from the hash. Small
1150 changes in the PLAINTEXT or SALT will result in large changes in the
1153 There is no decrypt function. This function isn't all that useful for
1154 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1155 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1156 primarily used to check if two pieces of text are the same without
1157 having to transmit or store the text itself. An example is checking
1158 if a correct password is given. The digest of the password is stored,
1159 not the password itself. The user types in a password that is
1160 crypt()'d with the same salt as the stored digest. If the two digests
1161 match, the password is correct.
1163 When verifying an existing digest string you should use the digest as
1164 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1165 to create the digest is visible as part of the digest. This ensures
1166 crypt() will hash the new string with the same salt as the digest.
1167 This allows your code to work with the standard L<crypt|/crypt> and
1168 with more exotic implementations. In other words, assume
1169 nothing about the returned string itself nor about how many bytes
1172 Traditionally the result is a string of 13 bytes: two first bytes of
1173 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1174 the first eight bytes of PLAINTEXT mattered. But alternative
1175 hashing schemes (like MD5), higher level security schemes (like C2),
1176 and implementations on non-Unix platforms may produce different
1179 When choosing a new salt create a random two character string whose
1180 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1181 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1182 characters is just a recommendation; the characters allowed in
1183 the salt depend solely on your system's crypt library, and Perl can't
1184 restrict what salts C<crypt()> accepts.
1186 Here's an example that makes sure that whoever runs this program knows
1189 $pwd = (getpwuid($<))[1];
1191 system "stty -echo";
1193 chomp($word = <STDIN>);
1197 if (crypt($word, $pwd) ne $pwd) {
1203 Of course, typing in your own password to whoever asks you
1206 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1207 of data, not least of all because you can't get the information
1208 back. Look at the L<Digest> module for more robust algorithms.
1210 If using crypt() on a Unicode string (which I<potentially> has
1211 characters with codepoints above 255), Perl tries to make sense
1212 of the situation by trying to downgrade (a copy of)
1213 the string back to an eight-bit byte string before calling crypt()
1214 (on that copy). If that works, good. If not, crypt() dies with
1215 C<Wide character in crypt>.
1217 Portability issues: L<perlport/crypt>.
1222 =for Pod::Functions breaks binding on a tied dbm file
1224 [This function has been largely superseded by the C<untie> function.]
1226 Breaks the binding between a DBM file and a hash.
1228 Portability issues: L<perlport/dbmclose>.
1230 =item dbmopen HASH,DBNAME,MASK
1231 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1233 =for Pod::Functions create binding on a tied dbm file
1235 [This function has been largely superseded by the
1236 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1238 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1239 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1240 argument is I<not> a filehandle, even though it looks like one). DBNAME
1241 is the name of the database (without the F<.dir> or F<.pag> extension if
1242 any). If the database does not exist, it is created with protection
1243 specified by MASK (as modified by the C<umask>). To prevent creation of
1244 the database if it doesn't exist, you may specify a MODE
1245 of 0, and the function will return a false value if it
1246 can't find an existing database. If your system supports
1247 only the older DBM functions, you may make only one C<dbmopen> call in your
1248 program. In older versions of Perl, if your system had neither DBM nor
1249 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1252 If you don't have write access to the DBM file, you can only read hash
1253 variables, not set them. If you want to test whether you can write,
1254 either use file tests or try setting a dummy hash entry inside an C<eval>
1257 Note that functions such as C<keys> and C<values> may return huge lists
1258 when used on large DBM files. You may prefer to use the C<each>
1259 function to iterate over large DBM files. Example:
1261 # print out history file offsets
1262 dbmopen(%HIST,'/usr/lib/news/history',0666);
1263 while (($key,$val) = each %HIST) {
1264 print $key, ' = ', unpack('L',$val), "\n";
1268 See also L<AnyDBM_File> for a more general description of the pros and
1269 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1270 rich implementation.
1272 You can control which DBM library you use by loading that library
1273 before you call dbmopen():
1276 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1277 or die "Can't open netscape history file: $!";
1279 Portability issues: L<perlport/dbmopen>.
1282 X<defined> X<undef> X<undefined>
1286 =for Pod::Functions test whether a value, variable, or function is defined
1288 Returns a Boolean value telling whether EXPR has a value other than
1289 the undefined value C<undef>. If EXPR is not present, C<$_> is
1292 Many operations return C<undef> to indicate failure, end of file,
1293 system error, uninitialized variable, and other exceptional
1294 conditions. This function allows you to distinguish C<undef> from
1295 other values. (A simple Boolean test will not distinguish among
1296 C<undef>, zero, the empty string, and C<"0">, which are all equally
1297 false.) Note that since C<undef> is a valid scalar, its presence
1298 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1299 returns C<undef> when its argument is an empty array, I<or> when the
1300 element to return happens to be C<undef>.
1302 You may also use C<defined(&func)> to check whether subroutine C<&func>
1303 has ever been defined. The return value is unaffected by any forward
1304 declarations of C<&func>. A subroutine that is not defined
1305 may still be callable: its package may have an C<AUTOLOAD> method that
1306 makes it spring into existence the first time that it is called; see
1309 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1310 used to report whether memory for that aggregate had ever been
1311 allocated. This behavior may disappear in future versions of Perl.
1312 You should instead use a simple test for size:
1314 if (@an_array) { print "has array elements\n" }
1315 if (%a_hash) { print "has hash members\n" }
1317 When used on a hash element, it tells you whether the value is defined,
1318 not whether the key exists in the hash. Use L</exists> for the latter
1323 print if defined $switch{D};
1324 print "$val\n" while defined($val = pop(@ary));
1325 die "Can't readlink $sym: $!"
1326 unless defined($value = readlink $sym);
1327 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1328 $debugging = 0 unless defined $debugging;
1330 Note: Many folks tend to overuse C<defined> and are then surprised to
1331 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1332 defined values. For example, if you say
1336 The pattern match succeeds and C<$1> is defined, although it
1337 matched "nothing". It didn't really fail to match anything. Rather, it
1338 matched something that happened to be zero characters long. This is all
1339 very above-board and honest. When a function returns an undefined value,
1340 it's an admission that it couldn't give you an honest answer. So you
1341 should use C<defined> only when questioning the integrity of what
1342 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1345 See also L</undef>, L</exists>, L</ref>.
1350 =for Pod::Functions deletes a value from a hash
1352 Given an expression that specifies an element or slice of a hash, C<delete>
1353 deletes the specified elements from that hash so that exists() on that element
1354 no longer returns true. Setting a hash element to the undefined value does
1355 not remove its key, but deleting it does; see L</exists>.
1357 In list context, returns the value or values deleted, or the last such
1358 element in scalar context. The return list's length always matches that of
1359 the argument list: deleting non-existent elements returns the undefined value
1360 in their corresponding positions.
1362 delete() may also be used on arrays and array slices, but its behavior is less
1363 straightforward. Although exists() will return false for deleted entries,
1364 deleting array elements never changes indices of existing values; use shift()
1365 or splice() for that. However, if all deleted elements fall at the end of an
1366 array, the array's size shrinks to the position of the highest element that
1367 still tests true for exists(), or to 0 if none do.
1369 B<WARNING:> Calling delete on array values is deprecated and likely to
1370 be removed in a future version of Perl.
1372 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1373 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1374 or array may not necessarily return anything; it depends on the implementation
1375 of the C<tied> package's DELETE method, which may do whatever it pleases.
1377 The C<delete local EXPR> construct localizes the deletion to the current
1378 block at run time. Until the block exits, elements locally deleted
1379 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1380 of composite types">.
1382 %hash = (foo => 11, bar => 22, baz => 33);
1383 $scalar = delete $hash{foo}; # $scalar is 11
1384 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1385 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1387 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1389 foreach $key (keys %HASH) {
1393 foreach $index (0 .. $#ARRAY) {
1394 delete $ARRAY[$index];
1399 delete @HASH{keys %HASH};
1401 delete @ARRAY[0 .. $#ARRAY];
1403 But both are slower than assigning the empty list
1404 or undefining %HASH or @ARRAY, which is the customary
1405 way to empty out an aggregate:
1407 %HASH = (); # completely empty %HASH
1408 undef %HASH; # forget %HASH ever existed
1410 @ARRAY = (); # completely empty @ARRAY
1411 undef @ARRAY; # forget @ARRAY ever existed
1413 The EXPR can be arbitrarily complicated provided its
1414 final operation is an element or slice of an aggregate:
1416 delete $ref->[$x][$y]{$key};
1417 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1419 delete $ref->[$x][$y][$index];
1420 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1423 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1425 =for Pod::Functions raise an exception or bail out
1427 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1428 into C<$@> and the C<eval> is terminated with the undefined value.
1429 If the exception is outside of all enclosing C<eval>s, then the uncaught
1430 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1431 need to exit the process with a specific exit code, see L</exit>.
1433 Equivalent examples:
1435 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1436 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1438 If the last element of LIST does not end in a newline, the current
1439 script line number and input line number (if any) are also printed,
1440 and a newline is supplied. Note that the "input line number" (also
1441 known as "chunk") is subject to whatever notion of "line" happens to
1442 be currently in effect, and is also available as the special variable
1443 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1445 Hint: sometimes appending C<", stopped"> to your message will cause it
1446 to make better sense when the string C<"at foo line 123"> is appended.
1447 Suppose you are running script "canasta".
1449 die "/etc/games is no good";
1450 die "/etc/games is no good, stopped";
1452 produce, respectively
1454 /etc/games is no good at canasta line 123.
1455 /etc/games is no good, stopped at canasta line 123.
1457 If the output is empty and C<$@> already contains a value (typically from a
1458 previous eval) that value is reused after appending C<"\t...propagated">.
1459 This is useful for propagating exceptions:
1462 die unless $@ =~ /Expected exception/;
1464 If the output is empty and C<$@> contains an object reference that has a
1465 C<PROPAGATE> method, that method will be called with additional file
1466 and line number parameters. The return value replaces the value in
1467 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1470 If C<$@> is empty then the string C<"Died"> is used.
1472 If an uncaught exception results in interpreter exit, the exit code is
1473 determined from the values of C<$!> and C<$?> with this pseudocode:
1475 exit $! if $!; # errno
1476 exit $? >> 8 if $? >> 8; # child exit status
1477 exit 255; # last resort
1479 The intent is to squeeze as much possible information about the likely cause
1480 into the limited space of the system exit
1481 code. However, as C<$!> is the value
1482 of C's C<errno>, which can be set by any system call, this means that the value
1483 of the exit code used by C<die> can be non-predictable, so should not be relied
1484 upon, other than to be non-zero.
1486 You can also call C<die> with a reference argument, and if this is trapped
1487 within an C<eval>, C<$@> contains that reference. This permits more
1488 elaborate exception handling using objects that maintain arbitrary state
1489 about the exception. Such a scheme is sometimes preferable to matching
1490 particular string values of C<$@> with regular expressions. Because C<$@>
1491 is a global variable and C<eval> may be used within object implementations,
1492 be careful that analyzing the error object doesn't replace the reference in
1493 the global variable. It's easiest to make a local copy of the reference
1494 before any manipulations. Here's an example:
1496 use Scalar::Util "blessed";
1498 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1499 if (my $ev_err = $@) {
1500 if (blessed($ev_err)
1501 && $ev_err->isa("Some::Module::Exception")) {
1502 # handle Some::Module::Exception
1505 # handle all other possible exceptions
1509 Because Perl stringifies uncaught exception messages before display,
1510 you'll probably want to overload stringification operations on
1511 exception objects. See L<overload> for details about that.
1513 You can arrange for a callback to be run just before the C<die>
1514 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1515 handler is called with the error text and can change the error
1516 message, if it sees fit, by calling C<die> again. See
1517 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1518 L<"eval BLOCK"> for some examples. Although this feature was
1519 to be run only right before your program was to exit, this is not
1520 currently so: the C<$SIG{__DIE__}> hook is currently called
1521 even inside eval()ed blocks/strings! If one wants the hook to do
1522 nothing in such situations, put
1526 as the first line of the handler (see L<perlvar/$^S>). Because
1527 this promotes strange action at a distance, this counterintuitive
1528 behavior may be fixed in a future release.
1530 See also exit(), warn(), and the Carp module.
1535 =for Pod::Functions turn a BLOCK into a TERM
1537 Not really a function. Returns the value of the last command in the
1538 sequence of commands indicated by BLOCK. When modified by the C<while> or
1539 C<until> loop modifier, executes the BLOCK once before testing the loop
1540 condition. (On other statements the loop modifiers test the conditional
1543 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1544 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1545 See L<perlsyn> for alternative strategies.
1547 =item do SUBROUTINE(LIST)
1550 This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1556 Uses the value of EXPR as a filename and executes the contents of the
1557 file as a Perl script.
1565 except that it's more concise, runs no external processes, keeps track of
1567 filename for error messages, searches the C<@INC> directories, and updates
1568 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1569 these variables. It also differs in that code evaluated with C<do FILENAME>
1570 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1571 same, however, in that it does reparse the file every time you call it,
1572 so you probably don't want to do this inside a loop.
1574 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1575 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1576 and sets C<$!> to the error. Always check C<$@> first, as compilation
1577 could fail in a way that also sets C<$!>. If the file is successfully
1578 compiled, C<do> returns the value of the last expression evaluated.
1580 Inclusion of library modules is better done with the
1581 C<use> and C<require> operators, which also do automatic error checking
1582 and raise an exception if there's a problem.
1584 You might like to use C<do> to read in a program configuration
1585 file. Manual error checking can be done this way:
1587 # read in config files: system first, then user
1588 for $file ("/share/prog/defaults.rc",
1589 "$ENV{HOME}/.someprogrc")
1591 unless ($return = do $file) {
1592 warn "couldn't parse $file: $@" if $@;
1593 warn "couldn't do $file: $!" unless defined $return;
1594 warn "couldn't run $file" unless $return;
1599 X<dump> X<core> X<undump>
1605 =for Pod::Functions create an immediate core dump
1607 This function causes an immediate core dump. See also the B<-u>
1608 command-line switch in L<perlrun>, which does the same thing.
1609 Primarily this is so that you can use the B<undump> program (not
1610 supplied) to turn your core dump into an executable binary after
1611 having initialized all your variables at the beginning of the
1612 program. When the new binary is executed it will begin by executing
1613 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1614 Think of it as a goto with an intervening core dump and reincarnation.
1615 If C<LABEL> is omitted, restarts the program from the top. The
1616 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1617 computed at run time, being otherwise identical to C<dump LABEL>.
1619 B<WARNING>: Any files opened at the time of the dump will I<not>
1620 be open any more when the program is reincarnated, with possible
1621 resulting confusion by Perl.
1623 This function is now largely obsolete, mostly because it's very hard to
1624 convert a core file into an executable. That's why you should now invoke
1625 it as C<CORE::dump()>, if you don't want to be warned against a possible
1628 Unlike most named operators, this has the same precedence as assignment.
1629 It is also exempt from the looks-like-a-function rule, so
1630 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1633 Portability issues: L<perlport/dump>.
1636 X<each> X<hash, iterator>
1643 =for Pod::Functions retrieve the next key/value pair from a hash
1645 When called on a hash in list context, returns a 2-element list
1646 consisting of the key and value for the next element of a hash. In Perl
1647 5.12 and later only, it will also return the index and value for the next
1648 element of an array so that you can iterate over it; older Perls consider
1649 this a syntax error. When called in scalar context, returns only the key
1650 (not the value) in a hash, or the index in an array.
1652 Hash entries are returned in an apparently random order. The actual random
1653 order is subject to change in future versions of Perl, but it is
1654 guaranteed to be in the same order as either the C<keys> or C<values>
1655 function would produce on the same (unmodified) hash. Since Perl
1656 5.8.2 the ordering can be different even between different runs of Perl
1657 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1659 After C<each> has returned all entries from the hash or array, the next
1660 call to C<each> returns the empty list in list context and C<undef> in
1661 scalar context; the next call following I<that> one restarts iteration.
1662 Each hash or array has its own internal iterator, accessed by C<each>,
1663 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1664 reached the end as just described; it can be explicitly reset by calling
1665 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1666 elements while iterating over it, entries may be skipped or duplicated--so
1667 don't do that. Exception: In the current implementation, it is always safe
1668 to delete the item most recently returned by C<each()>, so the following
1669 code works properly:
1671 while (($key, $value) = each %hash) {
1673 delete $hash{$key}; # This is safe
1676 This prints out your environment like the printenv(1) program,
1677 but in a different order:
1679 while (($key,$value) = each %ENV) {
1680 print "$key=$value\n";
1683 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1684 reference to an unblessed hash or array. The argument will be dereferenced
1685 automatically. This aspect of C<each> is considered highly experimental.
1686 The exact behaviour may change in a future version of Perl.
1688 while (($key,$value) = each $hashref) { ... }
1690 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1691 which will set C<$_> on every iteration.
1694 print "$_=$ENV{$_}\n";
1697 To avoid confusing would-be users of your code who are running earlier
1698 versions of Perl with mysterious syntax errors, put this sort of thing at
1699 the top of your file to signal that your code will work I<only> on Perls of
1702 use 5.012; # so keys/values/each work on arrays
1703 use 5.014; # so keys/values/each work on scalars (experimental)
1704 use 5.018; # so each assigns to $_ in a lone while test
1706 See also C<keys>, C<values>, and C<sort>.
1708 =item eof FILEHANDLE
1717 =for Pod::Functions test a filehandle for its end
1719 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1720 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1721 gives the real filehandle. (Note that this function actually
1722 reads a character and then C<ungetc>s it, so isn't useful in an
1723 interactive context.) Do not read from a terminal file (or call
1724 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1725 as terminals may lose the end-of-file condition if you do.
1727 An C<eof> without an argument uses the last file read. Using C<eof()>
1728 with empty parentheses is different. It refers to the pseudo file
1729 formed from the files listed on the command line and accessed via the
1730 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1731 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1732 used will cause C<@ARGV> to be examined to determine if input is
1733 available. Similarly, an C<eof()> after C<< <> >> has returned
1734 end-of-file will assume you are processing another C<@ARGV> list,
1735 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1736 see L<perlop/"I/O Operators">.
1738 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1739 detect the end of each file, whereas C<eof()> will detect the end
1740 of the very last file only. Examples:
1742 # reset line numbering on each input file
1744 next if /^\s*#/; # skip comments
1747 close ARGV if eof; # Not eof()!
1750 # insert dashes just before last line of last file
1752 if (eof()) { # check for end of last file
1753 print "--------------\n";
1756 last if eof(); # needed if we're reading from a terminal
1759 Practical hint: you almost never need to use C<eof> in Perl, because the
1760 input operators typically return C<undef> when they run out of data or
1764 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1765 X<error, handling> X<exception, handling>
1771 =for Pod::Functions catch exceptions or compile and run code
1773 In the first form, the return value of EXPR is parsed and executed as if it
1774 were a little Perl program. The value of the expression (which is itself
1775 determined within scalar context) is first parsed, and if there were no
1776 errors, executed as a block within the lexical context of the current Perl
1777 program. This means, that in particular, any outer lexical variables are
1778 visible to it, and any package variable settings or subroutine and format
1779 definitions remain afterwards.
1781 Note that the value is parsed every time the C<eval> executes.
1782 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1783 delay parsing and subsequent execution of the text of EXPR until run time.
1785 If the C<unicode_eval> feature is enabled (which is the default under a
1786 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1787 characters, so C<use utf8> declarations have no effect, and source filters
1788 are forbidden. In the absence of the C<unicode_eval> feature, the string
1789 will sometimes be treated as characters and sometimes as bytes, depending
1790 on the internal encoding, and source filters activated within the C<eval>
1791 exhibit the erratic, but historical, behaviour of affecting some outer file
1792 scope that is still compiling. See also the L</evalbytes> keyword, which
1793 always treats its input as a byte stream and works properly with source
1794 filters, and the L<feature> pragma.
1796 In the second form, the code within the BLOCK is parsed only once--at the
1797 same time the code surrounding the C<eval> itself was parsed--and executed
1798 within the context of the current Perl program. This form is typically
1799 used to trap exceptions more efficiently than the first (see below), while
1800 also providing the benefit of checking the code within BLOCK at compile
1803 The final semicolon, if any, may be omitted from the value of EXPR or within
1806 In both forms, the value returned is the value of the last expression
1807 evaluated inside the mini-program; a return statement may be also used, just
1808 as with subroutines. The expression providing the return value is evaluated
1809 in void, scalar, or list context, depending on the context of the C<eval>
1810 itself. See L</wantarray> for more on how the evaluation context can be
1813 If there is a syntax error or runtime error, or a C<die> statement is
1814 executed, C<eval> returns C<undef> in scalar context
1815 or an empty list in list context, and C<$@> is set to the error
1816 message. (Prior to 5.16, a bug caused C<undef> to be returned
1817 in list context for syntax errors, but not for runtime errors.)
1818 If there was no error, C<$@> is set to the empty string. A
1819 control flow operator like C<last> or C<goto> can bypass the setting of
1820 C<$@>. Beware that using C<eval> neither silences Perl from printing
1821 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1822 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1823 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1824 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1826 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1827 determining whether a particular feature (such as C<socket> or C<symlink>)
1828 is implemented. It is also Perl's exception-trapping mechanism, where
1829 the die operator is used to raise exceptions.
1831 If you want to trap errors when loading an XS module, some problems with
1832 the binary interface (such as Perl version skew) may be fatal even with
1833 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1835 If the code to be executed doesn't vary, you may use the eval-BLOCK
1836 form to trap run-time errors without incurring the penalty of
1837 recompiling each time. The error, if any, is still returned in C<$@>.
1840 # make divide-by-zero nonfatal
1841 eval { $answer = $a / $b; }; warn $@ if $@;
1843 # same thing, but less efficient
1844 eval '$answer = $a / $b'; warn $@ if $@;
1846 # a compile-time error
1847 eval { $answer = }; # WRONG
1850 eval '$answer ='; # sets $@
1852 Using the C<eval{}> form as an exception trap in libraries does have some
1853 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1854 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1855 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1856 as this example shows:
1858 # a private exception trap for divide-by-zero
1859 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1862 This is especially significant, given that C<__DIE__> hooks can call
1863 C<die> again, which has the effect of changing their error messages:
1865 # __DIE__ hooks may modify error messages
1867 local $SIG{'__DIE__'} =
1868 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1869 eval { die "foo lives here" };
1870 print $@ if $@; # prints "bar lives here"
1873 Because this promotes action at a distance, this counterintuitive behavior
1874 may be fixed in a future release.
1876 With an C<eval>, you should be especially careful to remember what's
1877 being looked at when:
1883 eval { $x }; # CASE 4
1885 eval "\$$x++"; # CASE 5
1888 Cases 1 and 2 above behave identically: they run the code contained in
1889 the variable $x. (Although case 2 has misleading double quotes making
1890 the reader wonder what else might be happening (nothing is).) Cases 3
1891 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1892 does nothing but return the value of $x. (Case 4 is preferred for
1893 purely visual reasons, but it also has the advantage of compiling at
1894 compile-time instead of at run-time.) Case 5 is a place where
1895 normally you I<would> like to use double quotes, except that in this
1896 particular situation, you can just use symbolic references instead, as
1899 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1900 of localized variables, which means that for your code to run on older
1901 versions, a temporary is required if you want to mask some but not all
1904 # alter $@ on nefarious repugnancy only
1908 local $@; # protect existing $@
1909 eval { test_repugnancy() };
1910 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1911 $@ =~ /nefarious/ and $e = $@;
1913 die $e if defined $e
1916 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1917 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1919 An C<eval ''> executed within a subroutine defined
1920 in the C<DB> package doesn't see the usual
1921 surrounding lexical scope, but rather the scope of the first non-DB piece
1922 of code that called it. You don't normally need to worry about this unless
1923 you are writing a Perl debugger.
1925 =item evalbytes EXPR
1930 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1932 This function is like L</eval> with a string argument, except it always
1933 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1934 string containing characters whose ordinal value exceeds 255 results in an
1935 error. Source filters activated within the evaluated code apply to the
1938 This function is only available under the C<evalbytes> feature, a
1939 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1940 L<feature> for more information.
1945 =item exec PROGRAM LIST
1947 =for Pod::Functions abandon this program to run another
1949 The C<exec> function executes a system command I<and never returns>;
1950 use C<system> instead of C<exec> if you want it to return. It fails and
1951 returns false only if the command does not exist I<and> it is executed
1952 directly instead of via your system's command shell (see below).
1954 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1955 warns you if C<exec> is called in void context and if there is a following
1956 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1957 you always do that, right?). If you I<really> want to follow an C<exec>
1958 with some other statement, you can use one of these styles to avoid the warning:
1960 exec ('foo') or print STDERR "couldn't exec foo: $!";
1961 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1963 If there is more than one argument in LIST, or if LIST is an array
1964 with more than one value, calls execvp(3) with the arguments in LIST.
1965 If there is only one scalar argument or an array with one element in it,
1966 the argument is checked for shell metacharacters, and if there are any,
1967 the entire argument is passed to the system's command shell for parsing
1968 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1969 If there are no shell metacharacters in the argument, it is split into
1970 words and passed directly to C<execvp>, which is more efficient.
1973 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1974 exec "sort $outfile | uniq";
1976 If you don't really want to execute the first argument, but want to lie
1977 to the program you are executing about its own name, you can specify
1978 the program you actually want to run as an "indirect object" (without a
1979 comma) in front of the LIST. (This always forces interpretation of the
1980 LIST as a multivalued list, even if there is only a single scalar in
1983 $shell = '/bin/csh';
1984 exec $shell '-sh'; # pretend it's a login shell
1988 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1990 When the arguments get executed via the system shell, results are
1991 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1994 Using an indirect object with C<exec> or C<system> is also more
1995 secure. This usage (which also works fine with system()) forces
1996 interpretation of the arguments as a multivalued list, even if the
1997 list had just one argument. That way you're safe from the shell
1998 expanding wildcards or splitting up words with whitespace in them.
2000 @args = ( "echo surprise" );
2002 exec @args; # subject to shell escapes
2004 exec { $args[0] } @args; # safe even with one-arg list
2006 The first version, the one without the indirect object, ran the I<echo>
2007 program, passing it C<"surprise"> an argument. The second version didn't;
2008 it tried to run a program named I<"echo surprise">, didn't find it, and set
2009 C<$?> to a non-zero value indicating failure.
2011 Perl attempts to flush all files opened for output before the exec,
2012 but this may not be supported on some platforms (see L<perlport>).
2013 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2014 call the C<autoflush()> method of C<IO::Handle> on any open handles
2015 to avoid lost output.
2017 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2018 C<DESTROY> methods on your objects.
2020 Portability issues: L<perlport/exec>.
2023 X<exists> X<autovivification>
2025 =for Pod::Functions test whether a hash key is present
2027 Given an expression that specifies an element of a hash, returns true if the
2028 specified element in the hash has ever been initialized, even if the
2029 corresponding value is undefined.
2031 print "Exists\n" if exists $hash{$key};
2032 print "Defined\n" if defined $hash{$key};
2033 print "True\n" if $hash{$key};
2035 exists may also be called on array elements, but its behavior is much less
2036 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2037 that calling exists on array values is deprecated and likely to be removed in
2038 a future version of Perl.
2040 print "Exists\n" if exists $array[$index];
2041 print "Defined\n" if defined $array[$index];
2042 print "True\n" if $array[$index];
2044 A hash or array element can be true only if it's defined and defined only if
2045 it exists, but the reverse doesn't necessarily hold true.
2047 Given an expression that specifies the name of a subroutine,
2048 returns true if the specified subroutine has ever been declared, even
2049 if it is undefined. Mentioning a subroutine name for exists or defined
2050 does not count as declaring it. Note that a subroutine that does not
2051 exist may still be callable: its package may have an C<AUTOLOAD>
2052 method that makes it spring into existence the first time that it is
2053 called; see L<perlsub>.
2055 print "Exists\n" if exists &subroutine;
2056 print "Defined\n" if defined &subroutine;
2058 Note that the EXPR can be arbitrarily complicated as long as the final
2059 operation is a hash or array key lookup or subroutine name:
2061 if (exists $ref->{A}->{B}->{$key}) { }
2062 if (exists $hash{A}{B}{$key}) { }
2064 if (exists $ref->{A}->{B}->[$ix]) { }
2065 if (exists $hash{A}{B}[$ix]) { }
2067 if (exists &{$ref->{A}{B}{$key}}) { }
2069 Although the most deeply nested array or hash element will not spring into
2070 existence just because its existence was tested, any intervening ones will.
2071 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2072 into existence due to the existence test for the $key element above.
2073 This happens anywhere the arrow operator is used, including even here:
2076 if (exists $ref->{"Some key"}) { }
2077 print $ref; # prints HASH(0x80d3d5c)
2079 This surprising autovivification in what does not at first--or even
2080 second--glance appear to be an lvalue context may be fixed in a future
2083 Use of a subroutine call, rather than a subroutine name, as an argument
2084 to exists() is an error.
2087 exists &sub(); # Error
2090 X<exit> X<terminate> X<abort>
2094 =for Pod::Functions terminate this program
2096 Evaluates EXPR and exits immediately with that value. Example:
2099 exit 0 if $ans =~ /^[Xx]/;
2101 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2102 universally recognized values for EXPR are C<0> for success and C<1>
2103 for error; other values are subject to interpretation depending on the
2104 environment in which the Perl program is running. For example, exiting
2105 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2106 the mailer to return the item undelivered, but that's not true everywhere.
2108 Don't use C<exit> to abort a subroutine if there's any chance that
2109 someone might want to trap whatever error happened. Use C<die> instead,
2110 which can be trapped by an C<eval>.
2112 The exit() function does not always exit immediately. It calls any
2113 defined C<END> routines first, but these C<END> routines may not
2114 themselves abort the exit. Likewise any object destructors that need to
2115 be called are called before the real exit. C<END> routines and destructors
2116 can change the exit status by modifying C<$?>. If this is a problem, you
2117 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2118 See L<perlmod> for details.
2120 Portability issues: L<perlport/exit>.
2123 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2127 =for Pod::Functions raise I<e> to a power
2129 Returns I<e> (the natural logarithm base) to the power of EXPR.
2130 If EXPR is omitted, gives C<exp($_)>.
2133 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2137 =for Pod::Functions +fc return casefolded version of a string
2139 Returns the casefolded version of EXPR. This is the internal function
2140 implementing the C<\F> escape in double-quoted strings.
2142 Casefolding is the process of mapping strings to a form where case
2143 differences are erased; comparing two strings in their casefolded
2144 form is effectively a way of asking if two strings are equal,
2147 Roughly, if you ever found yourself writing this
2149 lc($this) eq lc($that) # Wrong!
2151 uc($this) eq uc($that) # Also wrong!
2153 $this =~ /^\Q$that\E\z/i # Right!
2157 fc($this) eq fc($that)
2159 And get the correct results.
2161 Perl only implements the full form of casefolding,
2162 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2163 L<Unicode::UCD/prop_invmap()>.
2164 For further information on casefolding, refer to
2165 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2166 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2167 available at L<http://www.unicode.org/versions/latest/>, as well as the
2168 Case Charts available at L<http://www.unicode.org/charts/case/>.
2170 If EXPR is omitted, uses C<$_>.
2172 This function behaves the same way under various pragma, such as in a locale,
2175 While the Unicode Standard defines two additional forms of casefolding,
2176 one for Turkic languages and one that never maps one character into multiple
2177 characters, these are not provided by the Perl core; However, the CPAN module
2178 C<Unicode::Casing> may be used to provide an implementation.
2180 This keyword is available only when the C<"fc"> feature is enabled,
2181 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2182 include a C<use v5.16> or later to the current scope.
2184 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2187 =for Pod::Functions file control system call
2189 Implements the fcntl(2) function. You'll probably have to say
2193 first to get the correct constant definitions. Argument processing and
2194 value returned work just like C<ioctl> below.
2198 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2199 or die "can't fcntl F_GETFL: $!";
2201 You don't have to check for C<defined> on the return from C<fcntl>.
2202 Like C<ioctl>, it maps a C<0> return from the system call into
2203 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2204 in numeric context. It is also exempt from the normal B<-w> warnings
2205 on improper numeric conversions.
2207 Note that C<fcntl> raises an exception if used on a machine that
2208 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2209 manpage to learn what functions are available on your system.
2211 Here's an example of setting a filehandle named C<REMOTE> to be
2212 non-blocking at the system level. You'll have to negotiate C<$|>
2213 on your own, though.
2215 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2217 $flags = fcntl(REMOTE, F_GETFL, 0)
2218 or die "Can't get flags for the socket: $!\n";
2220 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2221 or die "Can't set flags for the socket: $!\n";
2223 Portability issues: L<perlport/fcntl>.
2228 =for Pod::Functions the name of the current source file
2230 A special token that returns the name of the file in which it occurs.
2232 =item fileno FILEHANDLE
2235 =for Pod::Functions return file descriptor from filehandle
2237 Returns the file descriptor for a filehandle, or undefined if the
2238 filehandle is not open. If there is no real file descriptor at the OS
2239 level, as can happen with filehandles connected to memory objects via
2240 C<open> with a reference for the third argument, -1 is returned.
2242 This is mainly useful for constructing
2243 bitmaps for C<select> and low-level POSIX tty-handling operations.
2244 If FILEHANDLE is an expression, the value is taken as an indirect
2245 filehandle, generally its name.
2247 You can use this to find out whether two handles refer to the
2248 same underlying descriptor:
2250 if (fileno(THIS) == fileno(THAT)) {
2251 print "THIS and THAT are dups\n";
2254 =item flock FILEHANDLE,OPERATION
2255 X<flock> X<lock> X<locking>
2257 =for Pod::Functions lock an entire file with an advisory lock
2259 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2260 for success, false on failure. Produces a fatal error if used on a
2261 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2262 C<flock> is Perl's portable file-locking interface, although it locks
2263 entire files only, not records.
2265 Two potentially non-obvious but traditional C<flock> semantics are
2266 that it waits indefinitely until the lock is granted, and that its locks
2267 are B<merely advisory>. Such discretionary locks are more flexible, but
2268 offer fewer guarantees. This means that programs that do not also use
2269 C<flock> may modify files locked with C<flock>. See L<perlport>,
2270 your port's specific documentation, and your system-specific local manpages
2271 for details. It's best to assume traditional behavior if you're writing
2272 portable programs. (But if you're not, you should as always feel perfectly
2273 free to write for your own system's idiosyncrasies (sometimes called
2274 "features"). Slavish adherence to portability concerns shouldn't get
2275 in the way of your getting your job done.)
2277 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2278 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2279 you can use the symbolic names if you import them from the L<Fcntl> module,
2280 either individually, or as a group using the C<:flock> tag. LOCK_SH
2281 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2282 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2283 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2284 waiting for the lock; check the return status to see if you got it.
2286 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2287 before locking or unlocking it.
2289 Note that the emulation built with lockf(3) doesn't provide shared
2290 locks, and it requires that FILEHANDLE be open with write intent. These
2291 are the semantics that lockf(3) implements. Most if not all systems
2292 implement lockf(3) in terms of fcntl(2) locking, though, so the
2293 differing semantics shouldn't bite too many people.
2295 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2296 be open with read intent to use LOCK_SH and requires that it be open
2297 with write intent to use LOCK_EX.
2299 Note also that some versions of C<flock> cannot lock things over the
2300 network; you would need to use the more system-specific C<fcntl> for
2301 that. If you like you can force Perl to ignore your system's flock(2)
2302 function, and so provide its own fcntl(2)-based emulation, by passing
2303 the switch C<-Ud_flock> to the F<Configure> program when you configure
2304 and build a new Perl.
2306 Here's a mailbox appender for BSD systems.
2308 # import LOCK_* and SEEK_END constants
2309 use Fcntl qw(:flock SEEK_END);
2313 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2315 # and, in case someone appended while we were waiting...
2316 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2321 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2324 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2325 or die "Can't open mailbox: $!";
2328 print $mbox $msg,"\n\n";
2331 On systems that support a real flock(2), locks are inherited across fork()
2332 calls, whereas those that must resort to the more capricious fcntl(2)
2333 function lose their locks, making it seriously harder to write servers.
2335 See also L<DB_File> for other flock() examples.
2337 Portability issues: L<perlport/flock>.
2340 X<fork> X<child> X<parent>
2342 =for Pod::Functions create a new process just like this one
2344 Does a fork(2) system call to create a new process running the
2345 same program at the same point. It returns the child pid to the
2346 parent process, C<0> to the child process, or C<undef> if the fork is
2347 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2348 are shared, while everything else is copied. On most systems supporting
2349 fork(), great care has gone into making it extremely efficient (for
2350 example, using copy-on-write technology on data pages), making it the
2351 dominant paradigm for multitasking over the last few decades.
2353 Perl attempts to flush all files opened for
2354 output before forking the child process, but this may not be supported
2355 on some platforms (see L<perlport>). To be safe, you may need to set
2356 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2357 C<IO::Handle> on any open handles to avoid duplicate output.
2359 If you C<fork> without ever waiting on your children, you will
2360 accumulate zombies. On some systems, you can avoid this by setting
2361 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2362 forking and reaping moribund children.
2364 Note that if your forked child inherits system file descriptors like
2365 STDIN and STDOUT that are actually connected by a pipe or socket, even
2366 if you exit, then the remote server (such as, say, a CGI script or a
2367 backgrounded job launched from a remote shell) won't think you're done.
2368 You should reopen those to F</dev/null> if it's any issue.
2370 On some platforms such as Windows, where the fork() system call is not available,
2371 Perl can be built to emulate fork() in the Perl interpreter.
2372 The emulation is designed, at the level of the Perl program,
2373 to be as compatible as possible with the "Unix" fork().
2374 However it has limitations that have to be considered in code intended to be portable.
2375 See L<perlfork> for more details.
2377 Portability issues: L<perlport/fork>.
2382 =for Pod::Functions declare a picture format with use by the write() function
2384 Declare a picture format for use by the C<write> function. For
2388 Test: @<<<<<<<< @||||| @>>>>>
2389 $str, $%, '$' . int($num)
2393 $num = $cost/$quantity;
2397 See L<perlform> for many details and examples.
2399 =item formline PICTURE,LIST
2402 =for Pod::Functions internal function used for formats
2404 This is an internal function used by C<format>s, though you may call it,
2405 too. It formats (see L<perlform>) a list of values according to the
2406 contents of PICTURE, placing the output into the format output
2407 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2408 Eventually, when a C<write> is done, the contents of
2409 C<$^A> are written to some filehandle. You could also read C<$^A>
2410 and then set C<$^A> back to C<"">. Note that a format typically
2411 does one C<formline> per line of form, but the C<formline> function itself
2412 doesn't care how many newlines are embedded in the PICTURE. This means
2413 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2414 You may therefore need to use multiple formlines to implement a single
2415 record format, just like the C<format> compiler.
2417 Be careful if you put double quotes around the picture, because an C<@>
2418 character may be taken to mean the beginning of an array name.
2419 C<formline> always returns true. See L<perlform> for other examples.
2421 If you are trying to use this instead of C<write> to capture the output,
2422 you may find it easier to open a filehandle to a scalar
2423 (C<< open $fh, ">", \$output >>) and write to that instead.
2425 =item getc FILEHANDLE
2426 X<getc> X<getchar> X<character> X<file, read>
2430 =for Pod::Functions get the next character from the filehandle
2432 Returns the next character from the input file attached to FILEHANDLE,
2433 or the undefined value at end of file or if there was an error (in
2434 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2435 STDIN. This is not particularly efficient. However, it cannot be
2436 used by itself to fetch single characters without waiting for the user
2437 to hit enter. For that, try something more like:
2440 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2443 system "stty", '-icanon', 'eol', "\001";
2449 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2452 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2456 Determination of whether $BSD_STYLE should be set
2457 is left as an exercise to the reader.
2459 The C<POSIX::getattr> function can do this more portably on
2460 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2461 module from your nearest CPAN site; details on CPAN can be found under
2465 X<getlogin> X<login>
2467 =for Pod::Functions return who logged in at this tty
2469 This implements the C library function of the same name, which on most
2470 systems returns the current login from F</etc/utmp>, if any. If it
2471 returns the empty string, use C<getpwuid>.
2473 $login = getlogin || getpwuid($<) || "Kilroy";
2475 Do not consider C<getlogin> for authentication: it is not as
2476 secure as C<getpwuid>.
2478 Portability issues: L<perlport/getlogin>.
2480 =item getpeername SOCKET
2481 X<getpeername> X<peer>
2483 =for Pod::Functions find the other end of a socket connection
2485 Returns the packed sockaddr address of the other end of the SOCKET
2489 $hersockaddr = getpeername(SOCK);
2490 ($port, $iaddr) = sockaddr_in($hersockaddr);
2491 $herhostname = gethostbyaddr($iaddr, AF_INET);
2492 $herstraddr = inet_ntoa($iaddr);
2497 =for Pod::Functions get process group
2499 Returns the current process group for the specified PID. Use
2500 a PID of C<0> to get the current process group for the
2501 current process. Will raise an exception if used on a machine that
2502 doesn't implement getpgrp(2). If PID is omitted, returns the process
2503 group of the current process. Note that the POSIX version of C<getpgrp>
2504 does not accept a PID argument, so only C<PID==0> is truly portable.
2506 Portability issues: L<perlport/getpgrp>.
2509 X<getppid> X<parent> X<pid>
2511 =for Pod::Functions get parent process ID
2513 Returns the process id of the parent process.
2515 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2516 around non-POSIX thread semantics the minority of Linux systems (and
2517 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2518 has since been removed. See the documentation for L<$$|perlvar/$$> for
2521 Portability issues: L<perlport/getppid>.
2523 =item getpriority WHICH,WHO
2524 X<getpriority> X<priority> X<nice>
2526 =for Pod::Functions get current nice value
2528 Returns the current priority for a process, a process group, or a user.
2529 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2530 machine that doesn't implement getpriority(2).
2532 Portability issues: L<perlport/getpriority>.
2535 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2536 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2537 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2538 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2539 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2540 X<endnetent> X<endprotoent> X<endservent>
2542 =for Pod::Functions get passwd record given user login name
2546 =for Pod::Functions get group record given group name
2548 =item gethostbyname NAME
2550 =for Pod::Functions get host record given name
2552 =item getnetbyname NAME
2554 =for Pod::Functions get networks record given name
2556 =item getprotobyname NAME
2558 =for Pod::Functions get protocol record given name
2562 =for Pod::Functions get passwd record given user ID
2566 =for Pod::Functions get group record given group user ID
2568 =item getservbyname NAME,PROTO
2570 =for Pod::Functions get services record given its name
2572 =item gethostbyaddr ADDR,ADDRTYPE
2574 =for Pod::Functions get host record given its address
2576 =item getnetbyaddr ADDR,ADDRTYPE
2578 =for Pod::Functions get network record given its address
2580 =item getprotobynumber NUMBER
2582 =for Pod::Functions get protocol record numeric protocol
2584 =item getservbyport PORT,PROTO
2586 =for Pod::Functions get services record given numeric port
2590 =for Pod::Functions get next passwd record
2594 =for Pod::Functions get next group record
2598 =for Pod::Functions get next hosts record
2602 =for Pod::Functions get next networks record
2606 =for Pod::Functions get next protocols record
2610 =for Pod::Functions get next services record
2614 =for Pod::Functions prepare passwd file for use
2618 =for Pod::Functions prepare group file for use
2620 =item sethostent STAYOPEN
2622 =for Pod::Functions prepare hosts file for use
2624 =item setnetent STAYOPEN
2626 =for Pod::Functions prepare networks file for use
2628 =item setprotoent STAYOPEN
2630 =for Pod::Functions prepare protocols file for use
2632 =item setservent STAYOPEN
2634 =for Pod::Functions prepare services file for use
2638 =for Pod::Functions be done using passwd file
2642 =for Pod::Functions be done using group file
2646 =for Pod::Functions be done using hosts file
2650 =for Pod::Functions be done using networks file
2654 =for Pod::Functions be done using protocols file
2658 =for Pod::Functions be done using services file
2660 These routines are the same as their counterparts in the
2661 system C library. In list context, the return values from the
2662 various get routines are as follows:
2664 ($name,$passwd,$uid,$gid,
2665 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2666 ($name,$passwd,$gid,$members) = getgr*
2667 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2668 ($name,$aliases,$addrtype,$net) = getnet*
2669 ($name,$aliases,$proto) = getproto*
2670 ($name,$aliases,$port,$proto) = getserv*
2672 (If the entry doesn't exist you get an empty list.)
2674 The exact meaning of the $gcos field varies but it usually contains
2675 the real name of the user (as opposed to the login name) and other
2676 information pertaining to the user. Beware, however, that in many
2677 system users are able to change this information and therefore it
2678 cannot be trusted and therefore the $gcos is tainted (see
2679 L<perlsec>). The $passwd and $shell, user's encrypted password and
2680 login shell, are also tainted, for the same reason.
2682 In scalar context, you get the name, unless the function was a
2683 lookup by name, in which case you get the other thing, whatever it is.
2684 (If the entry doesn't exist you get the undefined value.) For example:
2686 $uid = getpwnam($name);
2687 $name = getpwuid($num);
2689 $gid = getgrnam($name);
2690 $name = getgrgid($num);
2694 In I<getpw*()> the fields $quota, $comment, and $expire are special
2695 in that they are unsupported on many systems. If the
2696 $quota is unsupported, it is an empty scalar. If it is supported, it
2697 usually encodes the disk quota. If the $comment field is unsupported,
2698 it is an empty scalar. If it is supported it usually encodes some
2699 administrative comment about the user. In some systems the $quota
2700 field may be $change or $age, fields that have to do with password
2701 aging. In some systems the $comment field may be $class. The $expire
2702 field, if present, encodes the expiration period of the account or the
2703 password. For the availability and the exact meaning of these fields
2704 in your system, please consult getpwnam(3) and your system's
2705 F<pwd.h> file. You can also find out from within Perl what your
2706 $quota and $comment fields mean and whether you have the $expire field
2707 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2708 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2709 files are supported only if your vendor has implemented them in the
2710 intuitive fashion that calling the regular C library routines gets the
2711 shadow versions if you're running under privilege or if there exists
2712 the shadow(3) functions as found in System V (this includes Solaris
2713 and Linux). Those systems that implement a proprietary shadow password
2714 facility are unlikely to be supported.
2716 The $members value returned by I<getgr*()> is a space-separated list of
2717 the login names of the members of the group.
2719 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2720 C, it will be returned to you via C<$?> if the function call fails. The
2721 C<@addrs> value returned by a successful call is a list of raw
2722 addresses returned by the corresponding library call. In the
2723 Internet domain, each address is four bytes long; you can unpack it
2724 by saying something like:
2726 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2728 The Socket library makes this slightly easier:
2731 $iaddr = inet_aton("127.1"); # or whatever address
2732 $name = gethostbyaddr($iaddr, AF_INET);
2734 # or going the other way
2735 $straddr = inet_ntoa($iaddr);
2737 In the opposite way, to resolve a hostname to the IP address
2741 $packed_ip = gethostbyname("www.perl.org");
2742 if (defined $packed_ip) {
2743 $ip_address = inet_ntoa($packed_ip);
2746 Make sure C<gethostbyname()> is called in SCALAR context and that
2747 its return value is checked for definedness.
2749 The C<getprotobynumber> function, even though it only takes one argument,
2750 has the precedence of a list operator, so beware:
2752 getprotobynumber $number eq 'icmp' # WRONG
2753 getprotobynumber($number eq 'icmp') # actually means this
2754 getprotobynumber($number) eq 'icmp' # better this way
2756 If you get tired of remembering which element of the return list
2757 contains which return value, by-name interfaces are provided
2758 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2759 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2760 and C<User::grent>. These override the normal built-ins, supplying
2761 versions that return objects with the appropriate names
2762 for each field. For example:
2766 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2768 Even though it looks as though they're the same method calls (uid),
2769 they aren't, because a C<File::stat> object is different from
2770 a C<User::pwent> object.
2772 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2774 =item getsockname SOCKET
2777 =for Pod::Functions retrieve the sockaddr for a given socket
2779 Returns the packed sockaddr address of this end of the SOCKET connection,
2780 in case you don't know the address because you have several different
2781 IPs that the connection might have come in on.
2784 $mysockaddr = getsockname(SOCK);
2785 ($port, $myaddr) = sockaddr_in($mysockaddr);
2786 printf "Connect to %s [%s]\n",
2787 scalar gethostbyaddr($myaddr, AF_INET),
2790 =item getsockopt SOCKET,LEVEL,OPTNAME
2793 =for Pod::Functions get socket options on a given socket
2795 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2796 Options may exist at multiple protocol levels depending on the socket
2797 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2798 C<Socket> module) will exist. To query options at another level the
2799 protocol number of the appropriate protocol controlling the option
2800 should be supplied. For example, to indicate that an option is to be
2801 interpreted by the TCP protocol, LEVEL should be set to the protocol
2802 number of TCP, which you can get using C<getprotobyname>.
2804 The function returns a packed string representing the requested socket
2805 option, or C<undef> on error, with the reason for the error placed in
2806 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2807 consult getsockopt(2) for details. A common case is that the option is an
2808 integer, in which case the result is a packed integer, which you can decode
2809 using C<unpack> with the C<i> (or C<I>) format.
2811 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2813 use Socket qw(:all);
2815 defined(my $tcp = getprotobyname("tcp"))
2816 or die "Could not determine the protocol number for tcp";
2817 # my $tcp = IPPROTO_TCP; # Alternative
2818 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2819 or die "getsockopt TCP_NODELAY: $!";
2820 my $nodelay = unpack("I", $packed);
2821 print "Nagle's algorithm is turned ",
2822 $nodelay ? "off\n" : "on\n";
2824 Portability issues: L<perlport/getsockopt>.
2827 X<glob> X<wildcard> X<filename, expansion> X<expand>
2831 =for Pod::Functions expand filenames using wildcards
2833 In list context, returns a (possibly empty) list of filename expansions on
2834 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2835 scalar context, glob iterates through such filename expansions, returning
2836 undef when the list is exhausted. This is the internal function
2837 implementing the C<< <*.c> >> operator, but you can use it directly. If
2838 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2839 more detail in L<perlop/"I/O Operators">.
2841 Note that C<glob> splits its arguments on whitespace and treats
2842 each segment as separate pattern. As such, C<glob("*.c *.h")>
2843 matches all files with a F<.c> or F<.h> extension. The expression
2844 C<glob(".* *")> matches all files in the current working directory.
2845 If you want to glob filenames that might contain whitespace, you'll
2846 have to use extra quotes around the spacey filename to protect it.
2847 For example, to glob filenames that have an C<e> followed by a space
2848 followed by an C<f>, use either of:
2850 @spacies = <"*e f*">;
2851 @spacies = glob '"*e f*"';
2852 @spacies = glob q("*e f*");
2854 If you had to get a variable through, you could do this:
2856 @spacies = glob "'*${var}e f*'";
2857 @spacies = glob qq("*${var}e f*");
2859 If non-empty braces are the only wildcard characters used in the
2860 C<glob>, no filenames are matched, but potentially many strings
2861 are returned. For example, this produces nine strings, one for
2862 each pairing of fruits and colors:
2864 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2866 This operator is implemented using the standard
2867 C<File::Glob> extension. See L<File::Glob> for details, including
2868 C<bsd_glob> which does not treat whitespace as a pattern separator.
2870 Portability issues: L<perlport/glob>.
2873 X<gmtime> X<UTC> X<Greenwich>
2877 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2879 Works just like L</localtime> but the returned values are
2880 localized for the standard Greenwich time zone.
2882 Note: When called in list context, $isdst, the last value
2883 returned by gmtime, is always C<0>. There is no
2884 Daylight Saving Time in GMT.
2886 Portability issues: L<perlport/gmtime>.
2889 X<goto> X<jump> X<jmp>
2895 =for Pod::Functions create spaghetti code
2897 The C<goto-LABEL> form finds the statement labeled with LABEL and
2898 resumes execution there. It can't be used to get out of a block or
2899 subroutine given to C<sort>. It can be used to go almost anywhere
2900 else within the dynamic scope, including out of subroutines, but it's
2901 usually better to use some other construct such as C<last> or C<die>.
2902 The author of Perl has never felt the need to use this form of C<goto>
2903 (in Perl, that is; C is another matter). (The difference is that C
2904 does not offer named loops combined with loop control. Perl does, and
2905 this replaces most structured uses of C<goto> in other languages.)
2907 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2908 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2909 necessarily recommended if you're optimizing for maintainability:
2911 goto ("FOO", "BAR", "GLARCH")[$i];
2913 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2914 function" rule. A pair of parentheses following it does not (necessarily)
2915 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2916 Also, unlike most named operators, this has the same precedence as
2919 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2920 deprecated and will issue a warning. Even then, it may not be used to
2921 go into any construct that requires initialization, such as a
2922 subroutine or a C<foreach> loop. It also can't be used to go into a
2923 construct that is optimized away.
2925 The C<goto-&NAME> form is quite different from the other forms of
2926 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2927 doesn't have the stigma associated with other gotos. Instead, it
2928 exits the current subroutine (losing any changes set by local()) and
2929 immediately calls in its place the named subroutine using the current
2930 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2931 load another subroutine and then pretend that the other subroutine had
2932 been called in the first place (except that any modifications to C<@_>
2933 in the current subroutine are propagated to the other subroutine.)
2934 After the C<goto>, not even C<caller> will be able to tell that this
2935 routine was called first.
2937 NAME needn't be the name of a subroutine; it can be a scalar variable
2938 containing a code reference or a block that evaluates to a code
2941 =item grep BLOCK LIST
2944 =item grep EXPR,LIST
2946 =for Pod::Functions locate elements in a list test true against a given criterion
2948 This is similar in spirit to, but not the same as, grep(1) and its
2949 relatives. In particular, it is not limited to using regular expressions.
2951 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2952 C<$_> to each element) and returns the list value consisting of those
2953 elements for which the expression evaluated to true. In scalar
2954 context, returns the number of times the expression was true.
2956 @foo = grep(!/^#/, @bar); # weed out comments
2960 @foo = grep {!/^#/} @bar; # weed out comments
2962 Note that C<$_> is an alias to the list value, so it can be used to
2963 modify the elements of the LIST. While this is useful and supported,
2964 it can cause bizarre results if the elements of LIST are not variables.
2965 Similarly, grep returns aliases into the original list, much as a for
2966 loop's index variable aliases the list elements. That is, modifying an
2967 element of a list returned by grep (for example, in a C<foreach>, C<map>
2968 or another C<grep>) actually modifies the element in the original list.
2969 This is usually something to be avoided when writing clear code.
2971 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2972 been declared with the deprecated C<my $_> construct)
2973 then, in addition to being locally aliased to
2974 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2975 can't be seen from the outside, avoiding any potential side-effects.
2977 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2980 X<hex> X<hexadecimal>
2984 =for Pod::Functions convert a string to a hexadecimal number
2986 Interprets EXPR as a hex string and returns the corresponding value.
2987 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2988 L</oct>.) If EXPR is omitted, uses C<$_>.
2990 print hex '0xAf'; # prints '175'
2991 print hex 'aF'; # same
2993 Hex strings may only represent integers. Strings that would cause
2994 integer overflow trigger a warning. Leading whitespace is not stripped,
2995 unlike oct(). To present something as hex, look into L</printf>,
2996 L</sprintf>, and L</unpack>.
3001 =for Pod::Functions patch a module's namespace into your own
3003 There is no builtin C<import> function. It is just an ordinary
3004 method (subroutine) defined (or inherited) by modules that wish to export
3005 names to another module. The C<use> function calls the C<import> method
3006 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3008 =item index STR,SUBSTR,POSITION
3009 X<index> X<indexOf> X<InStr>
3011 =item index STR,SUBSTR
3013 =for Pod::Functions find a substring within a string
3015 The index function searches for one string within another, but without
3016 the wildcard-like behavior of a full regular-expression pattern match.
3017 It returns the position of the first occurrence of SUBSTR in STR at
3018 or after POSITION. If POSITION is omitted, starts searching from the
3019 beginning of the string. POSITION before the beginning of the string
3020 or after its end is treated as if it were the beginning or the end,
3021 respectively. POSITION and the return value are based at zero.
3022 If the substring is not found, C<index> returns -1.
3025 X<int> X<integer> X<truncate> X<trunc> X<floor>
3029 =for Pod::Functions get the integer portion of a number
3031 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3032 You should not use this function for rounding: one because it truncates
3033 towards C<0>, and two because machine representations of floating-point
3034 numbers can sometimes produce counterintuitive results. For example,
3035 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3036 because it's really more like -268.99999999999994315658 instead. Usually,
3037 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3038 functions will serve you better than will int().
3040 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3043 =for Pod::Functions system-dependent device control system call
3045 Implements the ioctl(2) function. You'll probably first have to say
3047 require "sys/ioctl.ph"; # probably in
3048 # $Config{archlib}/sys/ioctl.ph
3050 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3051 exist or doesn't have the correct definitions you'll have to roll your
3052 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3053 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3054 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3055 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3056 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3057 has no string value but does have a numeric value, that value will be
3058 passed rather than a pointer to the string value. To guarantee this to be
3059 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3060 functions may be needed to manipulate the values of structures used by
3063 The return value of C<ioctl> (and C<fcntl>) is as follows:
3065 if OS returns: then Perl returns:
3067 0 string "0 but true"
3068 anything else that number
3070 Thus Perl returns true on success and false on failure, yet you can
3071 still easily determine the actual value returned by the operating
3074 $retval = ioctl(...) || -1;
3075 printf "System returned %d\n", $retval;
3077 The special string C<"0 but true"> is exempt from B<-w> complaints
3078 about improper numeric conversions.
3080 Portability issues: L<perlport/ioctl>.
3082 =item join EXPR,LIST
3085 =for Pod::Functions join a list into a string using a separator
3087 Joins the separate strings of LIST into a single string with fields
3088 separated by the value of EXPR, and returns that new string. Example:
3090 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3092 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3093 first argument. Compare L</split>.
3102 =for Pod::Functions retrieve list of indices from a hash
3104 Called in list context, returns a list consisting of all the keys of the
3105 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3106 releases prior to 5.12 will produce a syntax error if you try to use an
3107 array argument. In scalar context, returns the number of keys or indices.
3109 The keys of a hash are returned in an apparently random order. The actual
3110 random order is subject to change in future versions of Perl, but it
3111 is guaranteed to be the same order as either the C<values> or C<each>
3112 function produces (given that the hash has not been modified). Since
3113 Perl 5.8.1 the ordering can be different even between different runs of
3114 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
3117 As a side effect, calling keys() resets the internal iterator of the HASH or
3118 ARRAY (see L</each>). In particular, calling keys() in void context resets
3119 the iterator with no other overhead.
3121 Here is yet another way to print your environment:
3124 @values = values %ENV;
3126 print pop(@keys), '=', pop(@values), "\n";
3129 or how about sorted by key:
3131 foreach $key (sort(keys %ENV)) {
3132 print $key, '=', $ENV{$key}, "\n";
3135 The returned values are copies of the original keys in the hash, so
3136 modifying them will not affect the original hash. Compare L</values>.
3138 To sort a hash by value, you'll need to use a C<sort> function.
3139 Here's a descending numeric sort of a hash by its values:
3141 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3142 printf "%4d %s\n", $hash{$key}, $key;
3145 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3146 allocated for the given hash. This can gain you a measure of efficiency if
3147 you know the hash is going to get big. (This is similar to pre-extending
3148 an array by assigning a larger number to $#array.) If you say
3152 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3153 in fact, since it rounds up to the next power of two. These
3154 buckets will be retained even if you do C<%hash = ()>, use C<undef
3155 %hash> if you want to free the storage while C<%hash> is still in scope.
3156 You can't shrink the number of buckets allocated for the hash using
3157 C<keys> in this way (but you needn't worry about doing this by accident,
3158 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3161 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3162 a reference to an unblessed hash or array. The argument will be
3163 dereferenced automatically. This aspect of C<keys> is considered highly
3164 experimental. The exact behaviour may change in a future version of Perl.
3166 for (keys $hashref) { ... }
3167 for (keys $obj->get_arrayref) { ... }
3169 To avoid confusing would-be users of your code who are running earlier
3170 versions of Perl with mysterious syntax errors, put this sort of thing at
3171 the top of your file to signal that your code will work I<only> on Perls of
3174 use 5.012; # so keys/values/each work on arrays
3175 use 5.014; # so keys/values/each work on scalars (experimental)
3177 See also C<each>, C<values>, and C<sort>.
3179 =item kill SIGNAL, LIST
3184 =for Pod::Functions send a signal to a process or process group
3186 Sends a signal to a list of processes. Returns the number of
3187 processes successfully signaled (which is not necessarily the
3188 same as the number actually killed).
3190 $cnt = kill 1, $child1, $child2;
3193 If SIGNAL is zero, no signal is sent to the process, but C<kill>
3194 checks whether it's I<possible> to send a signal to it (that
3195 means, to be brief, that the process is owned by the same user, or we are
3196 the super-user). This is useful to check that a child process is still
3197 alive (even if only as a zombie) and hasn't changed its UID. See
3198 L<perlport> for notes on the portability of this construct.
3200 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
3201 of processes. That means you usually
3202 want to use positive not negative signals.
3204 You may also use a signal name in quotes. A negative signal name is the
3205 same as a negative signal number, killing process groups instead of processes.
3206 For example, C<kill -KILL, $pgrp> will send C<SIGKILL> to the entire process
3209 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3210 the operating system. For example, on POSIX-conforming systems, zero will
3211 signal the current process group, -1 will signal all processes, and any
3212 other negative PROCESS number will act as a negative signal number and
3213 kill the entire process group specified.
3215 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3216 A warning may be produced in a future version.
3218 See L<perlipc/"Signals"> for more details.
3220 On some platforms such as Windows where the fork() system call is not available.
3221 Perl can be built to emulate fork() at the interpreter level.
3222 This emulation has limitations related to kill that have to be considered,
3223 for code running on Windows and in code intended to be portable.
3225 See L<perlfork> for more details.
3227 If there is no I<LIST> of processes, no signal is sent, and the return
3228 value is 0. This form is sometimes used, however, because it causes
3229 tainting checks to be run. But see
3230 L<perlsec/Laundering and Detecting Tainted Data>.
3232 Portability issues: L<perlport/kill>.
3241 =for Pod::Functions exit a block prematurely
3243 The C<last> command is like the C<break> statement in C (as used in
3244 loops); it immediately exits the loop in question. If the LABEL is
3245 omitted, the command refers to the innermost enclosing
3246 loop. The C<last EXPR> form, available starting in Perl
3247 5.18.0, allows a label name to be computed at run time,
3248 and is otherwise identical to C<last LABEL>. The
3249 C<continue> block, if any, is not executed:
3251 LINE: while (<STDIN>) {
3252 last LINE if /^$/; # exit when done with header
3256 C<last> cannot be used to exit a block that returns a value such as
3257 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3258 a grep() or map() operation.
3260 Note that a block by itself is semantically identical to a loop
3261 that executes once. Thus C<last> can be used to effect an early
3262 exit out of such a block.
3264 See also L</continue> for an illustration of how C<last>, C<next>, and
3267 Unlike most named operators, this has the same precedence as assignment.
3268 It is also exempt from the looks-like-a-function rule, so
3269 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3277 =for Pod::Functions return lower-case version of a string
3279 Returns a lowercased version of EXPR. This is the internal function
3280 implementing the C<\L> escape in double-quoted strings.
3282 If EXPR is omitted, uses C<$_>.
3284 What gets returned depends on several factors:
3288 =item If C<use bytes> is in effect:
3292 =item On EBCDIC platforms
3294 The results are what the C language system call C<tolower()> returns.
3296 =item On ASCII platforms
3298 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3303 =item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3305 Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3306 semantics for the remaining code points (this last can only happen if
3307 the UTF8 flag is also set). See L<perllocale>.
3309 A deficiency in this is that case changes that cross the 255/256
3310 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3311 LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3312 platforms). But under C<use locale>, the lower case of U+1E9E is
3313 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3314 current locale, and Perl has no way of knowing if that character even
3315 exists in the locale, much less what code point it is. Perl returns
3316 the input character unchanged, for all instances (and there aren't
3317 many) where the 255/256 boundary would otherwise be crossed.
3319 =item Otherwise, If EXPR has the UTF8 flag set:
3321 Unicode semantics are used for the case change.
3323 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect:
3325 Unicode semantics are used for the case change.
3331 =item On EBCDIC platforms
3333 The results are what the C language system call C<tolower()> returns.
3335 =item On ASCII platforms
3337 ASCII semantics are used for the case change. The lowercase of any character
3338 outside the ASCII range is the character itself.
3345 X<lcfirst> X<lowercase>
3349 =for Pod::Functions return a string with just the next letter in lower case
3351 Returns the value of EXPR with the first character lowercased. This
3352 is the internal function implementing the C<\l> escape in
3353 double-quoted strings.
3355 If EXPR is omitted, uses C<$_>.
3357 This function behaves the same way under various pragmata, such as in a locale,
3365 =for Pod::Functions return the number of characters in a string
3367 Returns the length in I<characters> of the value of EXPR. If EXPR is
3368 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3371 This function cannot be used on an entire array or hash to find out how
3372 many elements these have. For that, use C<scalar @array> and C<scalar keys
3373 %hash>, respectively.
3375 Like all Perl character operations, length() normally deals in logical
3376 characters, not physical bytes. For how many bytes a string encoded as
3377 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3378 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3383 =for Pod::Functions the current source line number
3385 A special token that compiles to the current line number.
3387 =item link OLDFILE,NEWFILE
3390 =for Pod::Functions create a hard link in the filesystem
3392 Creates a new filename linked to the old filename. Returns true for
3393 success, false otherwise.
3395 Portability issues: L<perlport/link>.
3397 =item listen SOCKET,QUEUESIZE
3400 =for Pod::Functions register your socket as a server
3402 Does the same thing that the listen(2) system call does. Returns true if
3403 it succeeded, false otherwise. See the example in
3404 L<perlipc/"Sockets: Client/Server Communication">.
3409 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3411 You really probably want to be using C<my> instead, because C<local> isn't
3412 what most people think of as "local". See
3413 L<perlsub/"Private Variables via my()"> for details.
3415 A local modifies the listed variables to be local to the enclosing
3416 block, file, or eval. If more than one value is listed, the list must
3417 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3418 for details, including issues with tied arrays and hashes.
3420 The C<delete local EXPR> construct can also be used to localize the deletion
3421 of array/hash elements to the current block.
3422 See L<perlsub/"Localized deletion of elements of composite types">.
3424 =item localtime EXPR
3425 X<localtime> X<ctime>
3429 =for Pod::Functions convert UNIX time into record or string using local time
3431 Converts a time as returned by the time function to a 9-element list
3432 with the time analyzed for the local time zone. Typically used as
3436 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3439 All list elements are numeric and come straight out of the C `struct
3440 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3441 of the specified time.
3443 C<$mday> is the day of the month and C<$mon> the month in
3444 the range C<0..11>, with 0 indicating January and 11 indicating December.
3445 This makes it easy to get a month name from a list:
3447 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3448 print "$abbr[$mon] $mday";
3449 # $mon=9, $mday=18 gives "Oct 18"
3451 C<$year> contains the number of years since 1900. To get a 4-digit
3456 To get the last two digits of the year (e.g., "01" in 2001) do:
3458 $year = sprintf("%02d", $year % 100);
3460 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3461 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3462 (or C<0..365> in leap years.)
3464 C<$isdst> is true if the specified time occurs during Daylight Saving
3465 Time, false otherwise.
3467 If EXPR is omitted, C<localtime()> uses the current time (as returned
3470 In scalar context, C<localtime()> returns the ctime(3) value:
3472 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3474 The format of this scalar value is B<not> locale-dependent
3475 but built into Perl. For GMT instead of local
3476 time use the L</gmtime> builtin. See also the
3477 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3478 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3479 and mktime(3) functions.
3481 To get somewhat similar but locale-dependent date strings, set up your
3482 locale environment variables appropriately (please see L<perllocale>) and
3485 use POSIX qw(strftime);
3486 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3487 # or for GMT formatted appropriately for your locale:
3488 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3490 Note that the C<%a> and C<%b>, the short forms of the day of the week
3491 and the month of the year, may not necessarily be three characters wide.
3493 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3494 by-name access mechanism to the gmtime() and localtime() functions,
3497 For a comprehensive date and time representation look at the
3498 L<DateTime> module on CPAN.
3500 Portability issues: L<perlport/localtime>.
3505 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3507 This function places an advisory lock on a shared variable or referenced
3508 object contained in I<THING> until the lock goes out of scope.
3510 The value returned is the scalar itself, if the argument is a scalar, or a
3511 reference, if the argument is a hash, array or subroutine.
3513 lock() is a "weak keyword" : this means that if you've defined a function
3514 by this name (before any calls to it), that function will be called
3515 instead. If you are not under C<use threads::shared> this does nothing.
3516 See L<threads::shared>.
3519 X<log> X<logarithm> X<e> X<ln> X<base>
3523 =for Pod::Functions retrieve the natural logarithm for a number
3525 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3526 returns the log of C<$_>. To get the
3527 log of another base, use basic algebra:
3528 The base-N log of a number is equal to the natural log of that number
3529 divided by the natural log of N. For example:
3533 return log($n)/log(10);
3536 See also L</exp> for the inverse operation.
3538 =item lstat FILEHANDLE
3543 =item lstat DIRHANDLE
3547 =for Pod::Functions stat a symbolic link
3549 Does the same thing as the C<stat> function (including setting the
3550 special C<_> filehandle) but stats a symbolic link instead of the file
3551 the symbolic link points to. If symbolic links are unimplemented on
3552 your system, a normal C<stat> is done. For much more detailed
3553 information, please see the documentation for C<stat>.
3555 If EXPR is omitted, stats C<$_>.
3557 Portability issues: L<perlport/lstat>.
3561 =for Pod::Functions match a string with a regular expression pattern
3563 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3565 =item map BLOCK LIST
3570 =for Pod::Functions apply a change to a list to get back a new list with the changes
3572 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3573 C<$_> to each element) and returns the list value composed of the
3574 results of each such evaluation. In scalar context, returns the
3575 total number of elements so generated. Evaluates BLOCK or EXPR in
3576 list context, so each element of LIST may produce zero, one, or
3577 more elements in the returned value.
3579 @chars = map(chr, @numbers);
3581 translates a list of numbers to the corresponding characters.
3583 my @squares = map { $_ * $_ } @numbers;
3585 translates a list of numbers to their squared values.
3587 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3589 shows that number of returned elements can differ from the number of
3590 input elements. To omit an element, return an empty list ().
3591 This could also be achieved by writing
3593 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3595 which makes the intention more clear.
3597 Map always returns a list, which can be
3598 assigned to a hash such that the elements
3599 become key/value pairs. See L<perldata> for more details.
3601 %hash = map { get_a_key_for($_) => $_ } @array;
3603 is just a funny way to write
3607 $hash{get_a_key_for($_)} = $_;
3610 Note that C<$_> is an alias to the list value, so it can be used to
3611 modify the elements of the LIST. While this is useful and supported,
3612 it can cause bizarre results if the elements of LIST are not variables.
3613 Using a regular C<foreach> loop for this purpose would be clearer in
3614 most cases. See also L</grep> for an array composed of those items of
3615 the original list for which the BLOCK or EXPR evaluates to true.
3617 If C<$_> is lexical in the scope where the C<map> appears (because it has
3618 been declared with the deprecated C<my $_> construct),
3619 then, in addition to being locally aliased to
3620 the list elements, C<$_> keeps being lexical inside the block; that is, it
3621 can't be seen from the outside, avoiding any potential side-effects.
3623 C<{> starts both hash references and blocks, so C<map { ...> could be either
3624 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3625 ahead for the closing C<}> it has to take a guess at which it's dealing with
3626 based on what it finds just after the
3627 C<{>. Usually it gets it right, but if it
3628 doesn't it won't realize something is wrong until it gets to the C<}> and
3629 encounters the missing (or unexpected) comma. The syntax error will be
3630 reported close to the C<}>, but you'll need to change something near the C<{>
3631 such as using a unary C<+> to give Perl some help:
3633 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3634 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3635 %hash = map { ("\L$_" => 1) } @array # this also works
3636 %hash = map { lc($_) => 1 } @array # as does this.
3637 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3639 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3641 or to force an anon hash constructor use C<+{>:
3643 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3646 to get a list of anonymous hashes each with only one entry apiece.
3648 =item mkdir FILENAME,MASK
3649 X<mkdir> X<md> X<directory, create>
3651 =item mkdir FILENAME
3655 =for Pod::Functions create a directory
3657 Creates the directory specified by FILENAME, with permissions
3658 specified by MASK (as modified by C<umask>). If it succeeds it
3659 returns true; otherwise it returns false and sets C<$!> (errno).
3660 MASK defaults to 0777 if omitted, and FILENAME defaults
3661 to C<$_> if omitted.
3663 In general, it is better to create directories with a permissive MASK
3664 and let the user modify that with their C<umask> than it is to supply
3665 a restrictive MASK and give the user no way to be more permissive.
3666 The exceptions to this rule are when the file or directory should be
3667 kept private (mail files, for instance). The perlfunc(1) entry on
3668 C<umask> discusses the choice of MASK in more detail.
3670 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3671 number of trailing slashes. Some operating and filesystems do not get
3672 this right, so Perl automatically removes all trailing slashes to keep
3675 To recursively create a directory structure, look at
3676 the C<mkpath> function of the L<File::Path> module.
3678 =item msgctl ID,CMD,ARG
3681 =for Pod::Functions SysV IPC message control operations
3683 Calls the System V IPC function msgctl(2). You'll probably have to say
3687 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3688 then ARG must be a variable that will hold the returned C<msqid_ds>
3689 structure. Returns like C<ioctl>: the undefined value for error,
3690 C<"0 but true"> for zero, or the actual return value otherwise. See also
3691 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3694 Portability issues: L<perlport/msgctl>.
3696 =item msgget KEY,FLAGS
3699 =for Pod::Functions get SysV IPC message queue
3701 Calls the System V IPC function msgget(2). Returns the message queue
3702 id, or C<undef> on error. See also
3703 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3706 Portability issues: L<perlport/msgget>.
3708 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3711 =for Pod::Functions receive a SysV IPC message from a message queue
3713 Calls the System V IPC function msgrcv to receive a message from
3714 message queue ID into variable VAR with a maximum message size of
3715 SIZE. Note that when a message is received, the message type as a
3716 native long integer will be the first thing in VAR, followed by the
3717 actual message. This packing may be opened with C<unpack("l! a*")>.
3718 Taints the variable. Returns true if successful, false
3719 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3720 C<IPC::SysV> and C<IPC::SysV::Msg>.
3722 Portability issues: L<perlport/msgrcv>.
3724 =item msgsnd ID,MSG,FLAGS
3727 =for Pod::Functions send a SysV IPC message to a message queue
3729 Calls the System V IPC function msgsnd to send the message MSG to the
3730 message queue ID. MSG must begin with the native long integer message
3731 type, be followed by the length of the actual message, and then finally
3732 the message itself. This kind of packing can be achieved with
3733 C<pack("l! a*", $type, $message)>. Returns true if successful,
3734 false on error. See also the C<IPC::SysV>
3735 and C<IPC::SysV::Msg> documentation.
3737 Portability issues: L<perlport/msgsnd>.
3744 =item my EXPR : ATTRS
3746 =item my TYPE EXPR : ATTRS
3748 =for Pod::Functions declare and assign a local variable (lexical scoping)
3750 A C<my> declares the listed variables to be local (lexically) to the
3751 enclosing block, file, or C<eval>. If more than one value is listed,
3752 the list must be placed in parentheses.
3754 The exact semantics and interface of TYPE and ATTRS are still
3755 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3756 and attributes are handled using the C<attributes> pragma, or starting
3757 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3758 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3759 L<attributes>, and L<Attribute::Handlers>.
3768 =for Pod::Functions iterate a block prematurely
3770 The C<next> command is like the C<continue> statement in C; it starts
3771 the next iteration of the loop:
3773 LINE: while (<STDIN>) {
3774 next LINE if /^#/; # discard comments
3778 Note that if there were a C<continue> block on the above, it would get
3779 executed even on discarded lines. If LABEL is omitted, the command
3780 refers to the innermost enclosing loop. The C<next EXPR> form, available
3781 as of Perl 5.18.0, allows a label name to be computed at run time, being
3782 otherwise identical to C<next LABEL>.
3784 C<next> cannot be used to exit a block which returns a value such as
3785 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3786 a grep() or map() operation.
3788 Note that a block by itself is semantically identical to a loop
3789 that executes once. Thus C<next> will exit such a block early.
3791 See also L</continue> for an illustration of how C<last>, C<next>, and
3794 Unlike most named operators, this has the same precedence as assignment.
3795 It is also exempt from the looks-like-a-function rule, so
3796 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3799 =item no MODULE VERSION LIST
3803 =item no MODULE VERSION
3805 =item no MODULE LIST
3811 =for Pod::Functions unimport some module symbols or semantics at compile time
3813 See the C<use> function, of which C<no> is the opposite.
3816 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3820 =for Pod::Functions convert a string to an octal number
3822 Interprets EXPR as an octal string and returns the corresponding
3823 value. (If EXPR happens to start off with C<0x>, interprets it as a
3824 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3825 binary string. Leading whitespace is ignored in all three cases.)
3826 The following will handle decimal, binary, octal, and hex in standard
3829 $val = oct($val) if $val =~ /^0/;
3831 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3832 in octal), use sprintf() or printf():
3834 $dec_perms = (stat("filename"))[2] & 07777;
3835 $oct_perm_str = sprintf "%o", $perms;
3837 The oct() function is commonly used when a string such as C<644> needs
3838 to be converted into a file mode, for example. Although Perl
3839 automatically converts strings into numbers as needed, this automatic
3840 conversion assumes base 10.
3842 Leading white space is ignored without warning, as too are any trailing
3843 non-digits, such as a decimal point (C<oct> only handles non-negative
3844 integers, not negative integers or floating point).
3846 =item open FILEHANDLE,EXPR
3847 X<open> X<pipe> X<file, open> X<fopen>
3849 =item open FILEHANDLE,MODE,EXPR
3851 =item open FILEHANDLE,MODE,EXPR,LIST
3853 =item open FILEHANDLE,MODE,REFERENCE
3855 =item open FILEHANDLE
3857 =for Pod::Functions open a file, pipe, or descriptor
3859 Opens the file whose filename is given by EXPR, and associates it with
3862 Simple examples to open a file for reading:
3864 open(my $fh, "<", "input.txt")
3865 or die "cannot open < input.txt: $!";
3869 open(my $fh, ">", "output.txt")
3870 or die "cannot open > output.txt: $!";
3872 (The following is a comprehensive reference to open(): for a gentler
3873 introduction you may consider L<perlopentut>.)
3875 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3876 new filehandle is autovivified, meaning that the variable is assigned a
3877 reference to a newly allocated anonymous filehandle. Otherwise if
3878 FILEHANDLE is an expression, its value is the real filehandle. (This is
3879 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3882 If EXPR is omitted, the global (package) scalar variable of the same
3883 name as the FILEHANDLE contains the filename. (Note that lexical
3884 variables--those declared with C<my> or C<state>--will not work for this
3885 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3888 If three (or more) arguments are specified, the open mode (including
3889 optional encoding) in the second argument are distinct from the filename in
3890 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3891 If MODE is C<< > >>, the file is opened for output, with existing files
3892 first being truncated ("clobbered") and nonexisting files newly created.
3893 If MODE is C<<< >> >>>, the file is opened for appending, again being
3894 created if necessary.
3896 You can put a C<+> in front of the C<< > >> or C<< < >> to
3897 indicate that you want both read and write access to the file; thus
3898 C<< +< >> is almost always preferred for read/write updates--the
3899 C<< +> >> mode would clobber the file first. You can't usually use
3900 either read-write mode for updating textfiles, since they have
3901 variable-length records. See the B<-i> switch in L<perlrun> for a
3902 better approach. The file is created with permissions of C<0666>
3903 modified by the process's C<umask> value.
3905 These various prefixes correspond to the fopen(3) modes of C<r>,
3906 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3908 In the one- and two-argument forms of the call, the mode and filename
3909 should be concatenated (in that order), preferably separated by white
3910 space. You can--but shouldn't--omit the mode in these forms when that mode
3911 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3912 the filename argument is a known literal.
3914 For three or more arguments if MODE is C<|->, the filename is
3915 interpreted as a command to which output is to be piped, and if MODE
3916 is C<-|>, the filename is interpreted as a command that pipes
3917 output to us. In the two-argument (and one-argument) form, one should
3918 replace dash (C<->) with the command.
3919 See L<perlipc/"Using open() for IPC"> for more examples of this.
3920 (You are not allowed to C<open> to a command that pipes both in I<and>
3921 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3922 L<perlipc/"Bidirectional Communication with Another Process"> for
3925 In the form of pipe opens taking three or more arguments, if LIST is specified
3926 (extra arguments after the command name) then LIST becomes arguments
3927 to the command invoked if the platform supports it. The meaning of
3928 C<open> with more than three arguments for non-pipe modes is not yet
3929 defined, but experimental "layers" may give extra LIST arguments
3932 In the two-argument (and one-argument) form, opening C<< <- >>
3933 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3935 You may (and usually should) use the three-argument form of open to specify
3936 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3937 that affect how the input and output are processed (see L<open> and
3938 L<PerlIO> for more details). For example:
3940 open(my $fh, "<:encoding(UTF-8)", "filename")
3941 || die "can't open UTF-8 encoded filename: $!";
3943 opens the UTF8-encoded file containing Unicode characters;
3944 see L<perluniintro>. Note that if layers are specified in the
3945 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3946 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3947 Those layers will also be ignored if you specifying a colon with no name
3948 following it. In that case the default layer for the operating system
3949 (:raw on Unix, :crlf on Windows) is used.
3951 Open returns nonzero on success, the undefined value otherwise. If
3952 the C<open> involved a pipe, the return value happens to be the pid of
3955 If you're running Perl on a system that distinguishes between text
3956 files and binary files, then you should check out L</binmode> for tips
3957 for dealing with this. The key distinction between systems that need
3958 C<binmode> and those that don't is their text file formats. Systems
3959 like Unix, Mac OS, and Plan 9, that end lines with a single
3960 character and encode that character in C as C<"\n"> do not
3961 need C<binmode>. The rest need it.
3963 When opening a file, it's seldom a good idea to continue
3964 if the request failed, so C<open> is frequently used with
3965 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3966 where you want to format a suitable error message (but there are
3967 modules that can help with that problem)) always check
3968 the return value from opening a file.
3970 As a special case the three-argument form with a read/write mode and the third
3971 argument being C<undef>:
3973 open(my $tmp, "+>", undef) or die ...
3975 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3976 works for symmetry, but you really should consider writing something
3977 to the temporary file first. You will need to seek() to do the
3980 Perl is built using PerlIO by default; Unless you've
3981 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3982 open filehandles directly to Perl scalars via:
3984 open($fh, ">", \$variable) || ..
3986 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3989 open(STDOUT, ">", \$variable)
3990 or die "Can't open STDOUT: $!";
3995 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3996 while (<ARTICLE>) {...
3998 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3999 # if the open fails, output is discarded
4001 open(my $dbase, "+<", "dbase.mine") # open for update
4002 or die "Can't open 'dbase.mine' for update: $!";
4004 open(my $dbase, "+<dbase.mine") # ditto
4005 or die "Can't open 'dbase.mine' for update: $!";
4007 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4008 or die "Can't start caesar: $!";
4010 open(ARTICLE, "caesar <$article |") # ditto
4011 or die "Can't start caesar: $!";
4013 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4014 or die "Can't start sort: $!";
4017 open(MEMORY, ">", \$var)
4018 or die "Can't open memory file: $!";
4019 print MEMORY "foo!\n"; # output will appear in $var
4021 # process argument list of files along with any includes
4023 foreach $file (@ARGV) {
4024 process($file, "fh00");
4028 my($filename, $input) = @_;
4029 $input++; # this is a string increment
4030 unless (open($input, "<", $filename)) {
4031 print STDERR "Can't open $filename: $!\n";
4036 while (<$input>) { # note use of indirection
4037 if (/^#include "(.*)"/) {
4038 process($1, $input);
4045 See L<perliol> for detailed info on PerlIO.
4047 You may also, in the Bourne shell tradition, specify an EXPR beginning
4048 with C<< >& >>, in which case the rest of the string is interpreted
4049 as the name of a filehandle (or file descriptor, if numeric) to be
4050 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4051 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4052 The mode you specify should match the mode of the original filehandle.
4053 (Duping a filehandle does not take into account any existing contents
4054 of IO buffers.) If you use the three-argument
4055 form, then you can pass either a
4056 number, the name of a filehandle, or the normal "reference to a glob".
4058 Here is a script that saves, redirects, and restores C<STDOUT> and
4059 C<STDERR> using various methods:
4062 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4063 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4065 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4066 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4068 select STDERR; $| = 1; # make unbuffered
4069 select STDOUT; $| = 1; # make unbuffered
4071 print STDOUT "stdout 1\n"; # this works for
4072 print STDERR "stderr 1\n"; # subprocesses too
4074 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4075 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4077 print STDOUT "stdout 2\n";
4078 print STDERR "stderr 2\n";
4080 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4081 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4082 that file descriptor (and not call C<dup(2)>); this is more
4083 parsimonious of file descriptors. For example:
4085 # open for input, reusing the fileno of $fd
4086 open(FILEHANDLE, "<&=$fd")
4090 open(FILEHANDLE, "<&=", $fd)
4094 # open for append, using the fileno of OLDFH
4095 open(FH, ">>&=", OLDFH)
4099 open(FH, ">>&=OLDFH")
4101 Being parsimonious on filehandles is also useful (besides being
4102 parsimonious) for example when something is dependent on file
4103 descriptors, like for example locking using flock(). If you do just
4104 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4105 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4106 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4107 the same underlying system file descriptor.
4109 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4110 fdopen() to implement the C<=> functionality. On many Unix systems,
4111 fdopen() fails when file descriptors exceed a certain value, typically 255.
4112 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4114 You can see whether your Perl was built with PerlIO by running C<perl -V>
4115 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4116 have PerlIO; otherwise you don't.
4118 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4119 with the one- or two-argument forms of C<open>),
4120 an implicit C<fork> is done, so C<open> returns twice: in the parent
4121 process it returns the pid
4122 of the child process, and in the child process it returns (a defined) C<0>.
4123 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4125 For example, use either
4127 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4131 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4137 # either write TO_KID or else read FROM_KID
4139 waitpid $child_pid, 0;
4141 # am the child; use STDIN/STDOUT normally
4146 The filehandle behaves normally for the parent, but I/O to that
4147 filehandle is piped from/to the STDOUT/STDIN of the child process.
4148 In the child process, the filehandle isn't opened--I/O happens from/to
4149 the new STDOUT/STDIN. Typically this is used like the normal
4150 piped open when you want to exercise more control over just how the
4151 pipe command gets executed, such as when running setuid and
4152 you don't want to have to scan shell commands for metacharacters.
4154 The following blocks are more or less equivalent:
4156 open(FOO, "|tr '[a-z]' '[A-Z]'");
4157 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4158 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4159 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4161 open(FOO, "cat -n '$file'|");
4162 open(FOO, "-|", "cat -n '$file'");
4163 open(FOO, "-|") || exec "cat", "-n", $file;
4164 open(FOO, "-|", "cat", "-n", $file);
4166 The last two examples in each block show the pipe as "list form", which is
4167 not yet supported on all platforms. A good rule of thumb is that if
4168 your platform has a real C<fork()> (in other words, if your platform is
4169 Unix, including Linux and MacOS X), you can use the list form. You would
4170 want to use the list form of the pipe so you can pass literal arguments
4171 to the command without risk of the shell interpreting any shell metacharacters
4172 in them. However, this also bars you from opening pipes to commands
4173 that intentionally contain shell metacharacters, such as:
4175 open(FOO, "|cat -n | expand -4 | lpr")
4176 // die "Can't open pipeline to lpr: $!";
4178 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4180 Perl will attempt to flush all files opened for
4181 output before any operation that may do a fork, but this may not be
4182 supported on some platforms (see L<perlport>). To be safe, you may need
4183 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4184 of C<IO::Handle> on any open handles.
4186 On systems that support a close-on-exec flag on files, the flag will
4187 be set for the newly opened file descriptor as determined by the value
4188 of C<$^F>. See L<perlvar/$^F>.
4190 Closing any piped filehandle causes the parent process to wait for the
4191 child to finish, then returns the status value in C<$?> and
4192 C<${^CHILD_ERROR_NATIVE}>.
4194 The filename passed to the one- and two-argument forms of open() will
4195 have leading and trailing whitespace deleted and normal
4196 redirection characters honored. This property, known as "magic open",
4197 can often be used to good effect. A user could specify a filename of
4198 F<"rsh cat file |">, or you could change certain filenames as needed:
4200 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4201 open(FH, $filename) or die "Can't open $filename: $!";
4203 Use the three-argument form to open a file with arbitrary weird characters in it,
4205 open(FOO, "<", $file)
4206 || die "can't open < $file: $!";
4208 otherwise it's necessary to protect any leading and trailing whitespace:
4210 $file =~ s#^(\s)#./$1#;
4211 open(FOO, "< $file\0")
4212 || die "open failed: $!";
4214 (this may not work on some bizarre filesystems). One should
4215 conscientiously choose between the I<magic> and I<three-argument> form
4218 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4220 will allow the user to specify an argument of the form C<"rsh cat file |">,
4221 but will not work on a filename that happens to have a trailing space, while
4223 open(IN, "<", $ARGV[0])
4224 || die "can't open < $ARGV[0]: $!";
4226 will have exactly the opposite restrictions.
4228 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4229 should use the C<sysopen> function, which involves no such magic (but may
4230 use subtly different filemodes than Perl open(), which is mapped to C
4231 fopen()). This is another way to protect your filenames from
4232 interpretation. For example:
4235 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4236 or die "sysopen $path: $!";
4237 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4238 print HANDLE "stuff $$\n";
4240 print "File contains: ", <HANDLE>;
4242 Using the constructor from the C<IO::Handle> package (or one of its
4243 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
4244 filehandles that have the scope of the variables used to hold them, then
4245 automatically (but silently) close once their reference counts become
4246 zero, typically at scope exit:
4250 sub read_myfile_munged {
4252 # or just leave it undef to autoviv
4253 my $handle = IO::File->new;
4254 open($handle, "<", "myfile") or die "myfile: $!";
4256 or return (); # Automatically closed here.
4257 mung($first) or die "mung failed"; # Or here.
4258 return (first, <$handle>) if $ALL; # Or here.
4259 return $first; # Or here.
4262 B<WARNING:> The previous example has a bug because the automatic
4263 close that happens when the refcount on C<handle> reaches zero does not
4264 properly detect and report failures. I<Always> close the handle
4265 yourself and inspect the return value.
4268 || warn "close failed: $!";
4270 See L</seek> for some details about mixing reading and writing.
4272 Portability issues: L<perlport/open>.
4274 =item opendir DIRHANDLE,EXPR
4277 =for Pod::Functions open a directory
4279 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4280 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4281 DIRHANDLE may be an expression whose value can be used as an indirect
4282 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4283 scalar variable (or array or hash element), the variable is assigned a
4284 reference to a new anonymous dirhandle; that is, it's autovivified.
4285 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4287 See the example at C<readdir>.
4294 =for Pod::Functions find a character's numeric representation
4296 Returns the numeric value of the first character of EXPR.
4297 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4298 (Note I<character>, not byte.)
4300 For the reverse, see L</chr>.
4301 See L<perlunicode> for more about Unicode.
4308 =item our EXPR : ATTRS
4310 =item our TYPE EXPR : ATTRS
4312 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4314 C<our> makes a lexical alias to a package variable of the same name in the current
4315 package for use within the current lexical scope.
4317 C<our> has the same scoping rules as C<my> or C<state>, but C<our> only
4318 declares an alias, whereas C<my> or C<state> both declare a variable name and
4319 allocate storage for that name within the current scope.
4321 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4322 a package variable without qualifying it with the package name, but only within
4323 the lexical scope of the C<our> declaration. In this way, C<our> differs from
4324 C<use vars>, which allows use of an unqualified name I<only> within the
4325 affected package, but across scopes.
4327 If more than one value is listed, the list must be placed
4333 An C<our> declaration declares an alias for a package variable that will be visible
4334 across its entire lexical scope, even across package boundaries. The
4335 package in which the variable is entered is determined at the point
4336 of the declaration, not at the point of use. This means the following
4340 our $bar; # declares $Foo::bar for rest of lexical scope
4344 print $bar; # prints 20, as it refers to $Foo::bar
4346 Multiple C<our> declarations with the same name in the same lexical
4347 scope are allowed if they are in different packages. If they happen
4348 to be in the same package, Perl will emit warnings if you have asked
4349 for them, just like multiple C<my> declarations. Unlike a second
4350 C<my> declaration, which will bind the name to a fresh variable, a
4351 second C<our> declaration in the same package, in the same scope, is
4356 our $bar; # declares $Foo::bar for rest of lexical scope
4360 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4361 print $bar; # prints 30
4363 our $bar; # emits warning but has no other effect
4364 print $bar; # still prints 30
4366 An C<our> declaration may also have a list of attributes associated
4369 The exact semantics and interface of TYPE and ATTRS are still
4370 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4371 and attributes are handled using the C<attributes> pragma, or, starting
4372 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4373 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4374 L<attributes>, and L<Attribute::Handlers>.
4376 =item pack TEMPLATE,LIST
4379 =for Pod::Functions convert a list into a binary representation
4381 Takes a LIST of values and converts it into a string using the rules
4382 given by the TEMPLATE. The resulting string is the concatenation of
4383 the converted values. Typically, each converted value looks
4384 like its machine-level representation. For example, on 32-bit machines
4385 an integer may be represented by a sequence of 4 bytes, which will in
4386 Perl be presented as a string that's 4 characters long.
4388 See L<perlpacktut> for an introduction to this function.
4390 The TEMPLATE is a sequence of characters that give the order and type
4391 of values, as follows:
4393 a A string with arbitrary binary data, will be null padded.
4394 A A text (ASCII) string, will be space padded.
4395 Z A null-terminated (ASCIZ) string, will be null padded.
4397 b A bit string (ascending bit order inside each byte,
4399 B A bit string (descending bit order inside each byte).
4400 h A hex string (low nybble first).
4401 H A hex string (high nybble first).
4403 c A signed char (8-bit) value.
4404 C An unsigned char (octet) value.
4405 W An unsigned char value (can be greater than 255).
4407 s A signed short (16-bit) value.
4408 S An unsigned short value.
4410 l A signed long (32-bit) value.
4411 L An unsigned long value.
4413 q A signed quad (64-bit) value.
4414 Q An unsigned quad value.
4415 (Quads are available only if your system supports 64-bit
4416 integer values _and_ if Perl has been compiled to support
4417 those. Raises an exception otherwise.)
4419 i A signed integer value.
4420 I A unsigned integer value.
4421 (This 'integer' is _at_least_ 32 bits wide. Its exact
4422 size depends on what a local C compiler calls 'int'.)
4424 n An unsigned short (16-bit) in "network" (big-endian) order.
4425 N An unsigned long (32-bit) in "network" (big-endian) order.
4426 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4427 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4429 j A Perl internal signed integer value (IV).
4430 J A Perl internal unsigned integer value (UV).
4432 f A single-precision float in native format.
4433 d A double-precision float in native format.
4435 F A Perl internal floating-point value (NV) in native format
4436 D A float of long-double precision in native format.
4437 (Long doubles are available only if your system supports
4438 long double values _and_ if Perl has been compiled to
4439 support those. Raises an exception otherwise.)
4441 p A pointer to a null-terminated string.
4442 P A pointer to a structure (fixed-length string).
4444 u A uuencoded string.
4445 U A Unicode character number. Encodes to a character in char-
4446 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4449 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4450 for details). Its bytes represent an unsigned integer in
4451 base 128, most significant digit first, with as few digits
4452 as possible. Bit eight (the high bit) is set on each byte
4455 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4457 @ Null-fill or truncate to absolute position, counted from the
4458 start of the innermost ()-group.
4459 . Null-fill or truncate to absolute position specified by
4461 ( Start of a ()-group.
4463 One or more modifiers below may optionally follow certain letters in the
4464 TEMPLATE (the second column lists letters for which the modifier is valid):
4466 ! sSlLiI Forces native (short, long, int) sizes instead
4467 of fixed (16-/32-bit) sizes.
4469 xX Make x and X act as alignment commands.
4471 nNvV Treat integers as signed instead of unsigned.
4473 @. Specify position as byte offset in the internal
4474 representation of the packed string. Efficient
4477 > sSiIlLqQ Force big-endian byte-order on the type.
4478 jJfFdDpP (The "big end" touches the construct.)
4480 < sSiIlLqQ Force little-endian byte-order on the type.
4481 jJfFdDpP (The "little end" touches the construct.)
4483 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4484 to force a particular byte-order on all components in that group,
4485 including all its subgroups.
4489 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4490 pack for processing data from NASA's Magellan probe. Magellan was in an
4491 elliptical orbit, using the antenna for the radar mapping when close to
4492 Venus and for communicating data back to Earth for the rest of the orbit.
4493 There were two transmission units, but one of these failed, and then the
4494 other developed a fault whereby it would randomly flip the sense of all the
4495 bits. It was easy to automatically detect complete records with the correct
4496 sense, and complete records with all the bits flipped. However, this didn't
4497 recover the records where the sense flipped midway. A colleague of Larry's
4498 was able to pretty much eyeball where the records flipped, so they wrote an
4499 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4500 manually correct the records and recover the data. For this purpose pack
4501 gained the hex and bit string format specifiers.
4503 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4504 27e2fb84680b9cc1), but the patch description makes no mention of their
4505 addition, let alone the story behind them.
4509 The following rules apply:
4515 Each letter may optionally be followed by a number indicating the repeat
4516 count. A numeric repeat count may optionally be enclosed in brackets, as
4517 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4518 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4519 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4520 something else, described below. Supplying a C<*> for the repeat count
4521 instead of a number means to use however many items are left, except for:
4527 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4531 <.>, where it means relative to the start of the string.
4535 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4539 One can replace a numeric repeat count with a template letter enclosed in
4540 brackets to use the packed byte length of the bracketed template for the
4543 For example, the template C<x[L]> skips as many bytes as in a packed long,
4544 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4545 variable-expanded) unpacks. If the template in brackets contains alignment
4546 commands (such as C<x![d]>), its packed length is calculated as if the
4547 start of the template had the maximal possible alignment.
4549 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4550 trailing null byte, so the resulting string is always one byte longer than
4551 the byte length of the item itself.
4553 When used with C<@>, the repeat count represents an offset from the start
4554 of the innermost C<()> group.
4556 When used with C<.>, the repeat count determines the starting position to
4557 calculate the value offset as follows:
4563 If the repeat count is C<0>, it's relative to the current position.
4567 If the repeat count is C<*>, the offset is relative to the start of the
4572 And if it's an integer I<n>, the offset is relative to the start of the
4573 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4574 bigger then the group level.
4578 The repeat count for C<u> is interpreted as the maximal number of bytes
4579 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4580 count should not be more than 65.
4584 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4585 string of length count, padding with nulls or spaces as needed. When
4586 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4587 after the first null, and C<a> returns data with no stripping at all.
4589 If the value to pack is too long, the result is truncated. If it's too
4590 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4591 followed by a null byte. Thus C<Z> always packs a trailing null, except
4592 when the count is 0.
4596 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4597 Each such format generates 1 bit of the result. These are typically followed
4598 by a repeat count like C<B8> or C<B64>.
4600 Each result bit is based on the least-significant bit of the corresponding
4601 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4602 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4604 Starting from the beginning of the input string, each 8-tuple
4605 of characters is converted to 1 character of output. With format C<b>,
4606 the first character of the 8-tuple determines the least-significant bit of a
4607 character; with format C<B>, it determines the most-significant bit of
4610 If the length of the input string is not evenly divisible by 8, the
4611 remainder is packed as if the input string were padded by null characters
4612 at the end. Similarly during unpacking, "extra" bits are ignored.
4614 If the input string is longer than needed, remaining characters are ignored.
4616 A C<*> for the repeat count uses all characters of the input field.
4617 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4621 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4622 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4624 For each such format, pack() generates 4 bits of result.
4625 With non-alphabetical characters, the result is based on the 4 least-significant
4626 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4627 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4628 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4629 is compatible with the usual hexadecimal digits, so that C<"a"> and
4630 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4631 characters with this format.
4633 Starting from the beginning of the template to pack(), each pair
4634 of characters is converted to 1 character of output. With format C<h>, the
4635 first character of the pair determines the least-significant nybble of the
4636 output character; with format C<H>, it determines the most-significant
4639 If the length of the input string is not even, it behaves as if padded by
4640 a null character at the end. Similarly, "extra" nybbles are ignored during
4643 If the input string is longer than needed, extra characters are ignored.
4645 A C<*> for the repeat count uses all characters of the input field. For
4646 unpack(), nybbles are converted to a string of hexadecimal digits.
4650 The C<p> format packs a pointer to a null-terminated string. You are
4651 responsible for ensuring that the string is not a temporary value, as that
4652 could potentially get deallocated before you got around to using the packed
4653 result. The C<P> format packs a pointer to a structure of the size indicated
4654 by the length. A null pointer is created if the corresponding value for
4655 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4656 unpacks into C<undef>.
4658 If your system has a strange pointer size--meaning a pointer is neither as
4659 big as an int nor as big as a long--it may not be possible to pack or
4660 unpack pointers in big- or little-endian byte order. Attempting to do
4661 so raises an exception.
4665 The C</> template character allows packing and unpacking of a sequence of
4666 items where the packed structure contains a packed item count followed by
4667 the packed items themselves. This is useful when the structure you're
4668 unpacking has encoded the sizes or repeat counts for some of its fields
4669 within the structure itself as separate fields.
4671 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4672 I<length-item> describes how the length value is packed. Formats likely
4673 to be of most use are integer-packing ones like C<n> for Java strings,
4674 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4676 For C<pack>, I<sequence-item> may have a repeat count, in which case
4677 the minimum of that and the number of available items is used as the argument
4678 for I<length-item>. If it has no repeat count or uses a '*', the number
4679 of available items is used.
4681 For C<unpack>, an internal stack of integer arguments unpacked so far is
4682 used. You write C</>I<sequence-item> and the repeat count is obtained by
4683 popping off the last element from the stack. The I<sequence-item> must not
4684 have a repeat count.
4686 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4687 the I<length-item> is the string length, not the number of strings. With
4688 an explicit repeat count for pack, the packed string is adjusted to that
4689 length. For example:
4691 This code: gives this result:
4693 unpack("W/a", "\004Gurusamy") ("Guru")
4694 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4695 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4697 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4698 pack("a/W2", ord("a") .. ord("z")) "2ab"
4700 The I<length-item> is not returned explicitly from C<unpack>.
4702 Supplying a count to the I<length-item> format letter is only useful with
4703 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4704 introduce C<"\000"> characters, which Perl does not regard as legal in