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
725 L<feature> for more information. You can also access it by
726 prefixing it with C<CORE::>. Alternately, include a C<use
727 v5.10> or later to the current scope.
730 X<caller> X<call stack> X<stack> X<stack trace>
734 =for Pod::Functions get context of the current subroutine call
736 Returns the context of the current subroutine call. In scalar context,
737 returns the caller's package name if there I<is> a caller (that is, if
738 we're in a subroutine or C<eval> or C<require>) and the undefined value
739 otherwise. In list context, returns
742 ($package, $filename, $line) = caller;
744 With EXPR, it returns some extra information that the debugger uses to
745 print a stack trace. The value of EXPR indicates how many call frames
746 to go back before the current one.
749 ($package, $filename, $line, $subroutine, $hasargs,
752 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
755 Here $subroutine may be C<(eval)> if the frame is not a subroutine
756 call, but an C<eval>. In such a case additional elements $evaltext and
757 C<$is_require> are set: C<$is_require> is true if the frame is created by a
758 C<require> or C<use> statement, $evaltext contains the text of the
759 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
760 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
761 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
762 frame.) $subroutine may also be C<(unknown)> if this particular
763 subroutine happens to have been deleted from the symbol table.
764 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
765 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
766 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
767 corresponds to C<${^WARNING_BITS}>. The
768 C<$hints> and C<$bitmask> values are subject
769 to change between versions of Perl, and are not meant for external use.
771 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
772 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
773 of this hash, as they are the actual values stored in the optree.
775 Furthermore, when called from within the DB package in
776 list context, and with an argument, caller returns more
777 detailed information: it sets the list variable C<@DB::args> to be the
778 arguments with which the subroutine was invoked.
780 Be aware that the optimizer might have optimized call frames away before
781 C<caller> had a chance to get the information. That means that C<caller(N)>
782 might not return information about the call frame you expect it to, for
783 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
784 previous time C<caller> was called.
786 Be aware that setting C<@DB::args> is I<best effort>, intended for
787 debugging or generating backtraces, and should not be relied upon. In
788 particular, as C<@_> contains aliases to the caller's arguments, Perl does
789 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
790 subroutine makes to C<@_> or its contents, not the original values at call
791 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
792 elements, so under certain cases its elements may have become freed and
793 reallocated for other variables or temporary values. Finally, a side effect
794 of the current implementation is that the effects of C<shift @_> can
795 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
796 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
797 elements), so C<@DB::args> is actually a hybrid of the current state and
798 initial state of C<@_>. Buyer beware.
805 =item chdir FILEHANDLE
807 =item chdir DIRHANDLE
811 =for Pod::Functions change your current working directory
813 Changes the working directory to EXPR, if possible. If EXPR is omitted,
814 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
815 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
816 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
817 neither is set, C<chdir> does nothing. It returns true on success,
818 false otherwise. See the example under C<die>.
820 On systems that support fchdir(2), you may pass a filehandle or
821 directory handle as the argument. On systems that don't support fchdir(2),
822 passing handles raises an exception.
825 X<chmod> X<permission> X<mode>
827 =for Pod::Functions changes the permissions on a list of files
829 Changes the permissions of a list of files. The first element of the
830 list must be the numeric mode, which should probably be an octal
831 number, and which definitely should I<not> be a string of octal digits:
832 C<0644> is okay, but C<"0644"> is not. Returns the number of files
833 successfully changed. See also L</oct> if all you have is a string.
835 $cnt = chmod 0755, "foo", "bar";
836 chmod 0755, @executables;
837 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
839 $mode = "0644"; chmod oct($mode), "foo"; # this is better
840 $mode = 0644; chmod $mode, "foo"; # this is best
842 On systems that support fchmod(2), you may pass filehandles among the
843 files. On systems that don't support fchmod(2), passing filehandles raises
844 an exception. Filehandles must be passed as globs or glob references to be
845 recognized; barewords are considered filenames.
847 open(my $fh, "<", "foo");
848 my $perm = (stat $fh)[2] & 07777;
849 chmod($perm | 0600, $fh);
851 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
854 use Fcntl qw( :mode );
855 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
856 # Identical to the chmod 0755 of the example above.
858 Portability issues: L<perlport/chmod>.
861 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
867 =for Pod::Functions remove a trailing record separator from a string
869 This safer version of L</chop> removes any trailing string
870 that corresponds to the current value of C<$/> (also known as
871 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
872 number of characters removed from all its arguments. It's often used to
873 remove the newline from the end of an input record when you're worried
874 that the final record may be missing its newline. When in paragraph
875 mode (C<$/ = "">), it removes all trailing newlines from the string.
876 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
877 a reference to an integer or the like; see L<perlvar>) chomp() won't
879 If VARIABLE is omitted, it chomps C<$_>. Example:
882 chomp; # avoid \n on last field
887 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
889 You can actually chomp anything that's an lvalue, including an assignment:
892 chomp($answer = <STDIN>);
894 If you chomp a list, each element is chomped, and the total number of
895 characters removed is returned.
897 Note that parentheses are necessary when you're chomping anything
898 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
899 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
900 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
901 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
911 =for Pod::Functions remove the last character from a string
913 Chops off the last character of a string and returns the character
914 chopped. It is much more efficient than C<s/.$//s> because it neither
915 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
916 If VARIABLE is a hash, it chops the hash's values, but not its keys.
918 You can actually chop anything that's an lvalue, including an assignment.
920 If you chop a list, each element is chopped. Only the value of the
921 last C<chop> is returned.
923 Note that C<chop> returns the last character. To return all but the last
924 character, use C<substr($string, 0, -1)>.
929 X<chown> X<owner> X<user> X<group>
931 =for Pod::Functions change the ownership on a list of files
933 Changes the owner (and group) of a list of files. The first two
934 elements of the list must be the I<numeric> uid and gid, in that
935 order. A value of -1 in either position is interpreted by most
936 systems to leave that value unchanged. Returns the number of files
937 successfully changed.
939 $cnt = chown $uid, $gid, 'foo', 'bar';
940 chown $uid, $gid, @filenames;
942 On systems that support fchown(2), you may pass filehandles among the
943 files. On systems that don't support fchown(2), passing filehandles raises
944 an exception. Filehandles must be passed as globs or glob references to be
945 recognized; barewords are considered filenames.
947 Here's an example that looks up nonnumeric uids in the passwd file:
950 chomp($user = <STDIN>);
952 chomp($pattern = <STDIN>);
954 ($login,$pass,$uid,$gid) = getpwnam($user)
955 or die "$user not in passwd file";
957 @ary = glob($pattern); # expand filenames
958 chown $uid, $gid, @ary;
960 On most systems, you are not allowed to change the ownership of the
961 file unless you're the superuser, although you should be able to change
962 the group to any of your secondary groups. On insecure systems, these
963 restrictions may be relaxed, but this is not a portable assumption.
964 On POSIX systems, you can detect this condition this way:
966 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
967 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
969 Portability issues: L<perlport/chmod>.
972 X<chr> X<character> X<ASCII> X<Unicode>
976 =for Pod::Functions get character this number represents
978 Returns the character represented by that NUMBER in the character set.
979 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
980 chr(0x263a) is a Unicode smiley face.
982 Negative values give the Unicode replacement character (chr(0xfffd)),
983 except under the L<bytes> pragma, where the low eight bits of the value
984 (truncated to an integer) are used.
986 If NUMBER is omitted, uses C<$_>.
988 For the reverse, use L</ord>.
990 Note that characters from 128 to 255 (inclusive) are by default
991 internally not encoded as UTF-8 for backward compatibility reasons.
993 See L<perlunicode> for more about Unicode.
995 =item chroot FILENAME
1000 =for Pod::Functions make directory new root for path lookups
1002 This function works like the system call by the same name: it makes the
1003 named directory the new root directory for all further pathnames that
1004 begin with a C</> by your process and all its children. (It doesn't
1005 change your current working directory, which is unaffected.) For security
1006 reasons, this call is restricted to the superuser. If FILENAME is
1007 omitted, does a C<chroot> to C<$_>.
1009 Portability issues: L<perlport/chroot>.
1011 =item close FILEHANDLE
1016 =for Pod::Functions close file (or pipe or socket) handle
1018 Closes the file or pipe associated with the filehandle, flushes the IO
1019 buffers, and closes the system file descriptor. Returns true if those
1020 operations succeed and if no error was reported by any PerlIO
1021 layer. Closes the currently selected filehandle if the argument is
1024 You don't have to close FILEHANDLE if you are immediately going to do
1025 another C<open> on it, because C<open> closes it for you. (See
1026 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1027 counter (C<$.>), while the implicit close done by C<open> does not.
1029 If the filehandle came from a piped open, C<close> returns false if one of
1030 the other syscalls involved fails or if its program exits with non-zero
1031 status. If the only problem was that the program exited non-zero, C<$!>
1032 will be set to C<0>. Closing a pipe also waits for the process executing
1033 on the pipe to exit--in case you wish to look at the output of the pipe
1034 afterwards--and implicitly puts the exit status value of that command into
1035 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1037 If there are multiple threads running, C<close> on a filehandle from a
1038 piped open returns true without waiting for the child process to terminate,
1039 if the filehandle is still open in another thread.
1041 Closing the read end of a pipe before the process writing to it at the
1042 other end is done writing results in the writer receiving a SIGPIPE. If
1043 the other end can't handle that, be sure to read all the data before
1048 open(OUTPUT, '|sort >foo') # pipe to sort
1049 or die "Can't start sort: $!";
1050 #... # print stuff to output
1051 close OUTPUT # wait for sort to finish
1052 or warn $! ? "Error closing sort pipe: $!"
1053 : "Exit status $? from sort";
1054 open(INPUT, 'foo') # get sort's results
1055 or die "Can't open 'foo' for input: $!";
1057 FILEHANDLE may be an expression whose value can be used as an indirect
1058 filehandle, usually the real filehandle name or an autovivified handle.
1060 =item closedir DIRHANDLE
1063 =for Pod::Functions close directory handle
1065 Closes a directory opened by C<opendir> and returns the success of that
1068 =item connect SOCKET,NAME
1071 =for Pod::Functions connect to a remote socket
1073 Attempts to connect to a remote socket, just like connect(2).
1074 Returns true if it succeeded, false otherwise. NAME should be a
1075 packed address of the appropriate type for the socket. See the examples in
1076 L<perlipc/"Sockets: Client/Server Communication">.
1078 =item continue BLOCK
1083 =for Pod::Functions optional trailing block in a while or foreach
1085 When followed by a BLOCK, C<continue> is actually a
1086 flow control statement rather than a function. If
1087 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1088 C<foreach>), it is always executed just before the conditional is about to
1089 be evaluated again, just like the third part of a C<for> loop in C. Thus
1090 it can be used to increment a loop variable, even when the loop has been
1091 continued via the C<next> statement (which is similar to the C C<continue>
1094 C<last>, C<next>, or C<redo> may appear within a C<continue>
1095 block; C<last> and C<redo> behave as if they had been executed within
1096 the main block. So will C<next>, but since it will execute a C<continue>
1097 block, it may be more entertaining.
1100 ### redo always comes here
1103 ### next always comes here
1105 # then back the top to re-check EXPR
1107 ### last always comes here
1109 Omitting the C<continue> section is equivalent to using an
1110 empty one, logically enough, so C<next> goes directly back
1111 to check the condition at the top of the loop.
1113 When there is no BLOCK, C<continue> is a function that
1114 falls through the current C<when> or C<default> block instead of iterating
1115 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1116 In Perl 5.14 and earlier, this form of C<continue> was
1117 only available when the C<"switch"> feature was enabled.
1118 See L<feature> and L<perlsyn/"Switch Statements"> for more
1122 X<cos> X<cosine> X<acos> X<arccosine>
1126 =for Pod::Functions cosine function
1128 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1129 takes the cosine of C<$_>.
1131 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1132 function, or use this relation:
1134 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1136 =item crypt PLAINTEXT,SALT
1137 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1138 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1140 =for Pod::Functions one-way passwd-style encryption
1142 Creates a digest string exactly like the crypt(3) function in the C
1143 library (assuming that you actually have a version there that has not
1144 been extirpated as a potential munition).
1146 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1147 into a short string, called a digest, which is returned. The same
1148 PLAINTEXT and SALT will always return the same string, but there is no
1149 (known) way to get the original PLAINTEXT from the hash. Small
1150 changes in the PLAINTEXT or SALT will result in large changes in the
1153 There is no decrypt function. This function isn't all that useful for
1154 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1155 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1156 primarily used to check if two pieces of text are the same without
1157 having to transmit or store the text itself. An example is checking
1158 if a correct password is given. The digest of the password is stored,
1159 not the password itself. The user types in a password that is
1160 crypt()'d with the same salt as the stored digest. If the two digests
1161 match, the password is correct.
1163 When verifying an existing digest string you should use the digest as
1164 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1165 to create the digest is visible as part of the digest. This ensures
1166 crypt() will hash the new string with the same salt as the digest.
1167 This allows your code to work with the standard L<crypt|/crypt> and
1168 with more exotic implementations. In other words, assume
1169 nothing about the returned string itself nor about how many bytes
1172 Traditionally the result is a string of 13 bytes: two first bytes of
1173 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1174 the first eight bytes of PLAINTEXT mattered. But alternative
1175 hashing schemes (like MD5), higher level security schemes (like C2),
1176 and implementations on non-Unix platforms may produce different
1179 When choosing a new salt create a random two character string whose
1180 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1181 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1182 characters is just a recommendation; the characters allowed in
1183 the salt depend solely on your system's crypt library, and Perl can't
1184 restrict what salts C<crypt()> accepts.
1186 Here's an example that makes sure that whoever runs this program knows
1189 $pwd = (getpwuid($<))[1];
1191 system "stty -echo";
1193 chomp($word = <STDIN>);
1197 if (crypt($word, $pwd) ne $pwd) {
1203 Of course, typing in your own password to whoever asks you
1206 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1207 of data, not least of all because you can't get the information
1208 back. Look at the L<Digest> module for more robust algorithms.
1210 If using crypt() on a Unicode string (which I<potentially> has
1211 characters with codepoints above 255), Perl tries to make sense
1212 of the situation by trying to downgrade (a copy of)
1213 the string back to an eight-bit byte string before calling crypt()
1214 (on that copy). If that works, good. If not, crypt() dies with
1215 C<Wide character in crypt>.
1217 Portability issues: L<perlport/crypt>.
1222 =for Pod::Functions breaks binding on a tied dbm file
1224 [This function has been largely superseded by the C<untie> function.]
1226 Breaks the binding between a DBM file and a hash.
1228 Portability issues: L<perlport/dbmclose>.
1230 =item dbmopen HASH,DBNAME,MASK
1231 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1233 =for Pod::Functions create binding on a tied dbm file
1235 [This function has been largely superseded by the
1236 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1238 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1239 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1240 argument is I<not> a filehandle, even though it looks like one). DBNAME
1241 is the name of the database (without the F<.dir> or F<.pag> extension if
1242 any). If the database does not exist, it is created with protection
1243 specified by MASK (as modified by the C<umask>). To prevent creation of
1244 the database if it doesn't exist, you may specify a MODE
1245 of 0, and the function will return a false value if it
1246 can't find an existing database. If your system supports
1247 only the older DBM functions, you may make only one C<dbmopen> call in your
1248 program. In older versions of Perl, if your system had neither DBM nor
1249 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1252 If you don't have write access to the DBM file, you can only read hash
1253 variables, not set them. If you want to test whether you can write,
1254 either use file tests or try setting a dummy hash entry inside an C<eval>
1257 Note that functions such as C<keys> and C<values> may return huge lists
1258 when used on large DBM files. You may prefer to use the C<each>
1259 function to iterate over large DBM files. Example:
1261 # print out history file offsets
1262 dbmopen(%HIST,'/usr/lib/news/history',0666);
1263 while (($key,$val) = each %HIST) {
1264 print $key, ' = ', unpack('L',$val), "\n";
1268 See also L<AnyDBM_File> for a more general description of the pros and
1269 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1270 rich implementation.
1272 You can control which DBM library you use by loading that library
1273 before you call dbmopen():
1276 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1277 or die "Can't open netscape history file: $!";
1279 Portability issues: L<perlport/dbmopen>.
1282 X<defined> X<undef> X<undefined>
1286 =for Pod::Functions test whether a value, variable, or function is defined
1288 Returns a Boolean value telling whether EXPR has a value other than
1289 the undefined value C<undef>. If EXPR is not present, C<$_> is
1292 Many operations return C<undef> to indicate failure, end of file,
1293 system error, uninitialized variable, and other exceptional
1294 conditions. This function allows you to distinguish C<undef> from
1295 other values. (A simple Boolean test will not distinguish among
1296 C<undef>, zero, the empty string, and C<"0">, which are all equally
1297 false.) Note that since C<undef> is a valid scalar, its presence
1298 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1299 returns C<undef> when its argument is an empty array, I<or> when the
1300 element to return happens to be C<undef>.
1302 You may also use C<defined(&func)> to check whether subroutine C<&func>
1303 has ever been defined. The return value is unaffected by any forward
1304 declarations of C<&func>. A subroutine that is not defined
1305 may still be callable: its package may have an C<AUTOLOAD> method that
1306 makes it spring into existence the first time that it is called; see
1309 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1310 used to report whether memory for that aggregate had ever been
1311 allocated. This behavior may disappear in future versions of Perl.
1312 You should instead use a simple test for size:
1314 if (@an_array) { print "has array elements\n" }
1315 if (%a_hash) { print "has hash members\n" }
1317 When used on a hash element, it tells you whether the value is defined,
1318 not whether the key exists in the hash. Use L</exists> for the latter
1323 print if defined $switch{D};
1324 print "$val\n" while defined($val = pop(@ary));
1325 die "Can't readlink $sym: $!"
1326 unless defined($value = readlink $sym);
1327 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1328 $debugging = 0 unless defined $debugging;
1330 Note: Many folks tend to overuse C<defined> and are then surprised to
1331 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1332 defined values. For example, if you say
1336 The pattern match succeeds and C<$1> is defined, although it
1337 matched "nothing". It didn't really fail to match anything. Rather, it
1338 matched something that happened to be zero characters long. This is all
1339 very above-board and honest. When a function returns an undefined value,
1340 it's an admission that it couldn't give you an honest answer. So you
1341 should use C<defined> only when questioning the integrity of what
1342 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1345 See also L</undef>, L</exists>, L</ref>.
1350 =for Pod::Functions deletes a value from a hash
1352 Given an expression that specifies an element or slice of a hash, C<delete>
1353 deletes the specified elements from that hash so that exists() on that element
1354 no longer returns true. Setting a hash element to the undefined value does
1355 not remove its key, but deleting it does; see L</exists>.
1357 In list context, returns the value or values deleted, or the last such
1358 element in scalar context. The return list's length always matches that of
1359 the argument list: deleting non-existent elements returns the undefined value
1360 in their corresponding positions.
1362 delete() may also be used on arrays and array slices, but its behavior is less
1363 straightforward. Although exists() will return false for deleted entries,
1364 deleting array elements never changes indices of existing values; use shift()
1365 or splice() for that. However, if all deleted elements fall at the end of an
1366 array, the array's size shrinks to the position of the highest element that
1367 still tests true for exists(), or to 0 if none do.
1369 B<WARNING:> Calling delete on array values is deprecated and likely to
1370 be removed in a future version of Perl.
1372 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1373 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1374 or array may not necessarily return anything; it depends on the implementation
1375 of the C<tied> package's DELETE method, which may do whatever it pleases.
1377 The C<delete local EXPR> construct localizes the deletion to the current
1378 block at run time. Until the block exits, elements locally deleted
1379 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1380 of composite types">.
1382 %hash = (foo => 11, bar => 22, baz => 33);
1383 $scalar = delete $hash{foo}; # $scalar is 11
1384 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1385 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1387 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1389 foreach $key (keys %HASH) {
1393 foreach $index (0 .. $#ARRAY) {
1394 delete $ARRAY[$index];
1399 delete @HASH{keys %HASH};
1401 delete @ARRAY[0 .. $#ARRAY];
1403 But both are slower than assigning the empty list
1404 or undefining %HASH or @ARRAY, which is the customary
1405 way to empty out an aggregate:
1407 %HASH = (); # completely empty %HASH
1408 undef %HASH; # forget %HASH ever existed
1410 @ARRAY = (); # completely empty @ARRAY
1411 undef @ARRAY; # forget @ARRAY ever existed
1413 The EXPR can be arbitrarily complicated provided its
1414 final operation is an element or slice of an aggregate:
1416 delete $ref->[$x][$y]{$key};
1417 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1419 delete $ref->[$x][$y][$index];
1420 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1423 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1425 =for Pod::Functions raise an exception or bail out
1427 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1428 into C<$@> and the C<eval> is terminated with the undefined value.
1429 If the exception is outside of all enclosing C<eval>s, then the uncaught
1430 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1431 need to exit the process with a specific exit code, see L</exit>.
1433 Equivalent examples:
1435 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1436 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1438 If the last element of LIST does not end in a newline, the current
1439 script line number and input line number (if any) are also printed,
1440 and a newline is supplied. Note that the "input line number" (also
1441 known as "chunk") is subject to whatever notion of "line" happens to
1442 be currently in effect, and is also available as the special variable
1443 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1445 Hint: sometimes appending C<", stopped"> to your message will cause it
1446 to make better sense when the string C<"at foo line 123"> is appended.
1447 Suppose you are running script "canasta".
1449 die "/etc/games is no good";
1450 die "/etc/games is no good, stopped";
1452 produce, respectively
1454 /etc/games is no good at canasta line 123.
1455 /etc/games is no good, stopped at canasta line 123.
1457 If the output is empty and C<$@> already contains a value (typically from a
1458 previous eval) that value is reused after appending C<"\t...propagated">.
1459 This is useful for propagating exceptions:
1462 die unless $@ =~ /Expected exception/;
1464 If the output is empty and C<$@> contains an object reference that has a
1465 C<PROPAGATE> method, that method will be called with additional file
1466 and line number parameters. The return value replaces the value in
1467 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1470 If C<$@> is empty then the string C<"Died"> is used.
1472 If an uncaught exception results in interpreter exit, the exit code is
1473 determined from the values of C<$!> and C<$?> with this pseudocode:
1475 exit $! if $!; # errno
1476 exit $? >> 8 if $? >> 8; # child exit status
1477 exit 255; # last resort
1479 The intent is to squeeze as much possible information about the likely cause
1480 into the limited space of the system exit
1481 code. However, as C<$!> is the value
1482 of C's C<errno>, which can be set by any system call, this means that the value
1483 of the exit code used by C<die> can be non-predictable, so should not be relied
1484 upon, other than to be non-zero.
1486 You can also call C<die> with a reference argument, and if this is trapped
1487 within an C<eval>, C<$@> contains that reference. This permits more
1488 elaborate exception handling using objects that maintain arbitrary state
1489 about the exception. Such a scheme is sometimes preferable to matching
1490 particular string values of C<$@> with regular expressions. Because C<$@>
1491 is a global variable and C<eval> may be used within object implementations,
1492 be careful that analyzing the error object doesn't replace the reference in
1493 the global variable. It's easiest to make a local copy of the reference
1494 before any manipulations. Here's an example:
1496 use Scalar::Util "blessed";
1498 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1499 if (my $ev_err = $@) {
1500 if (blessed($ev_err)
1501 && $ev_err->isa("Some::Module::Exception")) {
1502 # handle Some::Module::Exception
1505 # handle all other possible exceptions
1509 Because Perl stringifies uncaught exception messages before display,
1510 you'll probably want to overload stringification operations on
1511 exception objects. See L<overload> for details about that.
1513 You can arrange for a callback to be run just before the C<die>
1514 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1515 handler is called with the error text and can change the error
1516 message, if it sees fit, by calling C<die> again. See
1517 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1518 L<"eval BLOCK"> for some examples. Although this feature was
1519 to be run only right before your program was to exit, this is not
1520 currently so: the C<$SIG{__DIE__}> hook is currently called
1521 even inside eval()ed blocks/strings! If one wants the hook to do
1522 nothing in such situations, put
1526 as the first line of the handler (see L<perlvar/$^S>). Because
1527 this promotes strange action at a distance, this counterintuitive
1528 behavior may be fixed in a future release.
1530 See also exit(), warn(), and the Carp module.
1535 =for Pod::Functions turn a BLOCK into a TERM
1537 Not really a function. Returns the value of the last command in the
1538 sequence of commands indicated by BLOCK. When modified by the C<while> or
1539 C<until> loop modifier, executes the BLOCK once before testing the loop
1540 condition. (On other statements the loop modifiers test the conditional
1543 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1544 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1545 See L<perlsyn> for alternative strategies.
1547 =item do SUBROUTINE(LIST)
1550 This form of subroutine call is deprecated. SUBROUTINE can be a bareword
1556 Uses the value of EXPR as a filename and executes the contents of the
1557 file as a Perl script.
1565 except that it's more efficient and concise, keeps track of the current
1566 filename for error messages, searches the C<@INC> directories, and updates
1567 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1568 these variables. It also differs in that code evaluated with C<do FILENAME>
1569 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1570 same, however, in that it does reparse the file every time you call it,
1571 so you probably don't want to do this inside a loop.
1573 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1574 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1575 and sets C<$!> to the error. Always check C<$@> first, as compilation
1576 could fail in a way that also sets C<$!>. If the file is successfully
1577 compiled, C<do> returns the value of the last expression evaluated.
1579 Inclusion of library modules is better done with the
1580 C<use> and C<require> operators, which also do automatic error checking
1581 and raise an exception if there's a problem.
1583 You might like to use C<do> to read in a program configuration
1584 file. Manual error checking can be done this way:
1586 # read in config files: system first, then user
1587 for $file ("/share/prog/defaults.rc",
1588 "$ENV{HOME}/.someprogrc")
1590 unless ($return = do $file) {
1591 warn "couldn't parse $file: $@" if $@;
1592 warn "couldn't do $file: $!" unless defined $return;
1593 warn "couldn't run $file" unless $return;
1598 X<dump> X<core> X<undump>
1602 =for Pod::Functions create an immediate core dump
1604 This function causes an immediate core dump. See also the B<-u>
1605 command-line switch in L<perlrun>, which does the same thing.
1606 Primarily this is so that you can use the B<undump> program (not
1607 supplied) to turn your core dump into an executable binary after
1608 having initialized all your variables at the beginning of the
1609 program. When the new binary is executed it will begin by executing
1610 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1611 Think of it as a goto with an intervening core dump and reincarnation.
1612 If C<LABEL> is omitted, restarts the program from the top.
1614 B<WARNING>: Any files opened at the time of the dump will I<not>
1615 be open any more when the program is reincarnated, with possible
1616 resulting confusion by Perl.
1618 This function is now largely obsolete, mostly because it's very hard to
1619 convert a core file into an executable. That's why you should now invoke
1620 it as C<CORE::dump()>, if you don't want to be warned against a possible
1623 Portability issues: L<perlport/dump>.
1626 X<each> X<hash, iterator>
1633 =for Pod::Functions retrieve the next key/value pair from a hash
1635 When called on a hash in list context, returns a 2-element list
1636 consisting of the key and value for the next element of a hash. In Perl
1637 5.12 and later only, it will also return the index and value for the next
1638 element of an array so that you can iterate over it; older Perls consider
1639 this a syntax error. When called in scalar context, returns only the key
1640 (not the value) in a hash, or the index in an array.
1642 Hash entries are returned in an apparently random order. The actual random
1643 order is subject to change in future versions of Perl, but it is
1644 guaranteed to be in the same order as either the C<keys> or C<values>
1645 function would produce on the same (unmodified) hash. Since Perl
1646 5.8.2 the ordering can be different even between different runs of Perl
1647 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1649 After C<each> has returned all entries from the hash or array, the next
1650 call to C<each> returns the empty list in list context and C<undef> in
1651 scalar context; the next call following I<that> one restarts iteration.
1652 Each hash or array has its own internal iterator, accessed by C<each>,
1653 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1654 reached the end as just described; it can be explicitly reset by calling
1655 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1656 elements while iterating over it, entries may be skipped or duplicated--so
1657 don't do that. Exception: In the current implementation, it is always safe
1658 to delete the item most recently returned by C<each()>, so the following
1659 code works properly:
1661 while (($key, $value) = each %hash) {
1663 delete $hash{$key}; # This is safe
1666 This prints out your environment like the printenv(1) program,
1667 but in a different order:
1669 while (($key,$value) = each %ENV) {
1670 print "$key=$value\n";
1673 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1674 reference to an unblessed hash or array. The argument will be dereferenced
1675 automatically. This aspect of C<each> is considered highly experimental.
1676 The exact behaviour may change in a future version of Perl.
1678 while (($key,$value) = each $hashref) { ... }
1680 To avoid confusing would-be users of your code who are running earlier
1681 versions of Perl with mysterious syntax errors, put this sort of thing at
1682 the top of your file to signal that your code will work I<only> on Perls of
1685 use 5.012; # so keys/values/each work on arrays
1686 use 5.014; # so keys/values/each work on scalars (experimental)
1688 See also C<keys>, C<values>, and C<sort>.
1690 =item eof FILEHANDLE
1699 =for Pod::Functions test a filehandle for its end
1701 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1702 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1703 gives the real filehandle. (Note that this function actually
1704 reads a character and then C<ungetc>s it, so isn't useful in an
1705 interactive context.) Do not read from a terminal file (or call
1706 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1707 as terminals may lose the end-of-file condition if you do.
1709 An C<eof> without an argument uses the last file read. Using C<eof()>
1710 with empty parentheses is different. It refers to the pseudo file
1711 formed from the files listed on the command line and accessed via the
1712 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1713 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1714 used will cause C<@ARGV> to be examined to determine if input is
1715 available. Similarly, an C<eof()> after C<< <> >> has returned
1716 end-of-file will assume you are processing another C<@ARGV> list,
1717 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1718 see L<perlop/"I/O Operators">.
1720 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1721 detect the end of each file, whereas C<eof()> will detect the end
1722 of the very last file only. Examples:
1724 # reset line numbering on each input file
1726 next if /^\s*#/; # skip comments
1729 close ARGV if eof; # Not eof()!
1732 # insert dashes just before last line of last file
1734 if (eof()) { # check for end of last file
1735 print "--------------\n";
1738 last if eof(); # needed if we're reading from a terminal
1741 Practical hint: you almost never need to use C<eof> in Perl, because the
1742 input operators typically return C<undef> when they run out of data or
1746 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1747 X<error, handling> X<exception, handling>
1753 =for Pod::Functions catch exceptions or compile and run code
1755 In the first form, the return value of EXPR is parsed and executed as if it
1756 were a little Perl program. The value of the expression (which is itself
1757 determined within scalar context) is first parsed, and if there were no
1758 errors, executed as a block within the lexical context of the current Perl
1759 program. This means, that in particular, any outer lexical variables are
1760 visible to it, and any package variable settings or subroutine and format
1761 definitions remain afterwards.
1763 Note that the value is parsed every time the C<eval> executes.
1764 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1765 delay parsing and subsequent execution of the text of EXPR until run time.
1767 If the C<unicode_eval> feature is enabled (which is the default under a
1768 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1769 characters, so C<use utf8> declarations have no effect, and source filters
1770 are forbidden. In the absence of the C<unicode_eval> feature, the string
1771 will sometimes be treated as characters and sometimes as bytes, depending
1772 on the internal encoding, and source filters activated within the C<eval>
1773 exhibit the erratic, but historical, behaviour of affecting some outer file
1774 scope that is still compiling. See also the L</evalbytes> keyword, which
1775 always treats its input as a byte stream and works properly with source
1776 filters, and the L<feature> pragma.
1778 In the second form, the code within the BLOCK is parsed only once--at the
1779 same time the code surrounding the C<eval> itself was parsed--and executed
1780 within the context of the current Perl program. This form is typically
1781 used to trap exceptions more efficiently than the first (see below), while
1782 also providing the benefit of checking the code within BLOCK at compile
1785 The final semicolon, if any, may be omitted from the value of EXPR or within
1788 In both forms, the value returned is the value of the last expression
1789 evaluated inside the mini-program; a return statement may be also used, just
1790 as with subroutines. The expression providing the return value is evaluated
1791 in void, scalar, or list context, depending on the context of the C<eval>
1792 itself. See L</wantarray> for more on how the evaluation context can be
1795 If there is a syntax error or runtime error, or a C<die> statement is
1796 executed, C<eval> returns C<undef> in scalar context
1797 or an empty list in list context, and C<$@> is set to the error
1798 message. (Prior to 5.16, a bug caused C<undef> to be returned
1799 in list context for syntax errors, but not for runtime errors.)
1800 If there was no error, C<$@> is set to the empty string. A
1801 control flow operator like C<last> or C<goto> can bypass the setting of
1802 C<$@>. Beware that using C<eval> neither silences Perl from printing
1803 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1804 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1805 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1806 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1808 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1809 determining whether a particular feature (such as C<socket> or C<symlink>)
1810 is implemented. It is also Perl's exception-trapping mechanism, where
1811 the die operator is used to raise exceptions.
1813 If you want to trap errors when loading an XS module, some problems with
1814 the binary interface (such as Perl version skew) may be fatal even with
1815 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1817 If the code to be executed doesn't vary, you may use the eval-BLOCK
1818 form to trap run-time errors without incurring the penalty of
1819 recompiling each time. The error, if any, is still returned in C<$@>.
1822 # make divide-by-zero nonfatal
1823 eval { $answer = $a / $b; }; warn $@ if $@;
1825 # same thing, but less efficient
1826 eval '$answer = $a / $b'; warn $@ if $@;
1828 # a compile-time error
1829 eval { $answer = }; # WRONG
1832 eval '$answer ='; # sets $@
1834 Using the C<eval{}> form as an exception trap in libraries does have some
1835 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1836 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1837 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1838 as this example shows:
1840 # a private exception trap for divide-by-zero
1841 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1844 This is especially significant, given that C<__DIE__> hooks can call
1845 C<die> again, which has the effect of changing their error messages:
1847 # __DIE__ hooks may modify error messages
1849 local $SIG{'__DIE__'} =
1850 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1851 eval { die "foo lives here" };
1852 print $@ if $@; # prints "bar lives here"
1855 Because this promotes action at a distance, this counterintuitive behavior
1856 may be fixed in a future release.
1858 With an C<eval>, you should be especially careful to remember what's
1859 being looked at when:
1865 eval { $x }; # CASE 4
1867 eval "\$$x++"; # CASE 5
1870 Cases 1 and 2 above behave identically: they run the code contained in
1871 the variable $x. (Although case 2 has misleading double quotes making
1872 the reader wonder what else might be happening (nothing is).) Cases 3
1873 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1874 does nothing but return the value of $x. (Case 4 is preferred for
1875 purely visual reasons, but it also has the advantage of compiling at
1876 compile-time instead of at run-time.) Case 5 is a place where
1877 normally you I<would> like to use double quotes, except that in this
1878 particular situation, you can just use symbolic references instead, as
1881 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1882 of localized variables, which means that for your code to run on older
1883 versions, a temporary is required if you want to mask some but not all
1886 # alter $@ on nefarious repugnancy only
1890 local $@; # protect existing $@
1891 eval { test_repugnancy() };
1892 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1893 $@ =~ /nefarious/ and $e = $@;
1895 die $e if defined $e
1898 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1899 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1901 An C<eval ''> executed within the C<DB> package doesn't see the usual
1902 surrounding lexical scope, but rather the scope of the first non-DB piece
1903 of code that called it. You don't normally need to worry about this unless
1904 you are writing a Perl debugger.
1906 =item evalbytes EXPR
1911 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1913 This function is like L</eval> with a string argument, except it always
1914 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1915 string containing characters whose ordinal value exceeds 255 results in an
1916 error. Source filters activated within the evaluated code apply to the
1919 This function is only available under the C<evalbytes> feature, a
1920 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1921 L<feature> for more information.
1926 =item exec PROGRAM LIST
1928 =for Pod::Functions abandon this program to run another
1930 The C<exec> function executes a system command I<and never returns>;
1931 use C<system> instead of C<exec> if you want it to return. It fails and
1932 returns false only if the command does not exist I<and> it is executed
1933 directly instead of via your system's command shell (see below).
1935 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1936 warns you if C<exec> is called in void context and if there is a following
1937 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1938 you always do that, right?). If you I<really> want to follow an C<exec>
1939 with some other statement, you can use one of these styles to avoid the warning:
1941 exec ('foo') or print STDERR "couldn't exec foo: $!";
1942 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1944 If there is more than one argument in LIST, or if LIST is an array
1945 with more than one value, calls execvp(3) with the arguments in LIST.
1946 If there is only one scalar argument or an array with one element in it,
1947 the argument is checked for shell metacharacters, and if there are any,
1948 the entire argument is passed to the system's command shell for parsing
1949 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1950 If there are no shell metacharacters in the argument, it is split into
1951 words and passed directly to C<execvp>, which is more efficient.
1954 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1955 exec "sort $outfile | uniq";
1957 If you don't really want to execute the first argument, but want to lie
1958 to the program you are executing about its own name, you can specify
1959 the program you actually want to run as an "indirect object" (without a
1960 comma) in front of the LIST. (This always forces interpretation of the
1961 LIST as a multivalued list, even if there is only a single scalar in
1964 $shell = '/bin/csh';
1965 exec $shell '-sh'; # pretend it's a login shell
1969 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1971 When the arguments get executed via the system shell, results are
1972 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1975 Using an indirect object with C<exec> or C<system> is also more
1976 secure. This usage (which also works fine with system()) forces
1977 interpretation of the arguments as a multivalued list, even if the
1978 list had just one argument. That way you're safe from the shell
1979 expanding wildcards or splitting up words with whitespace in them.
1981 @args = ( "echo surprise" );
1983 exec @args; # subject to shell escapes
1985 exec { $args[0] } @args; # safe even with one-arg list
1987 The first version, the one without the indirect object, ran the I<echo>
1988 program, passing it C<"surprise"> an argument. The second version didn't;
1989 it tried to run a program named I<"echo surprise">, didn't find it, and set
1990 C<$?> to a non-zero value indicating failure.
1992 Perl attempts to flush all files opened for output before the exec,
1993 but this may not be supported on some platforms (see L<perlport>).
1994 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
1995 call the C<autoflush()> method of C<IO::Handle> on any open handles
1996 to avoid lost output.
1998 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1999 C<DESTROY> methods on your objects.
2001 Portability issues: L<perlport/exec>.
2004 X<exists> X<autovivification>
2006 =for Pod::Functions test whether a hash key is present
2008 Given an expression that specifies an element of a hash, returns true if the
2009 specified element in the hash has ever been initialized, even if the
2010 corresponding value is undefined.
2012 print "Exists\n" if exists $hash{$key};
2013 print "Defined\n" if defined $hash{$key};
2014 print "True\n" if $hash{$key};
2016 exists may also be called on array elements, but its behavior is much less
2017 obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
2018 that calling exists on array values is deprecated and likely to be removed in
2019 a future version of Perl.
2021 print "Exists\n" if exists $array[$index];
2022 print "Defined\n" if defined $array[$index];
2023 print "True\n" if $array[$index];
2025 A hash or array element can be true only if it's defined and defined only if
2026 it exists, but the reverse doesn't necessarily hold true.
2028 Given an expression that specifies the name of a subroutine,
2029 returns true if the specified subroutine has ever been declared, even
2030 if it is undefined. Mentioning a subroutine name for exists or defined
2031 does not count as declaring it. Note that a subroutine that does not
2032 exist may still be callable: its package may have an C<AUTOLOAD>
2033 method that makes it spring into existence the first time that it is
2034 called; see L<perlsub>.
2036 print "Exists\n" if exists &subroutine;
2037 print "Defined\n" if defined &subroutine;
2039 Note that the EXPR can be arbitrarily complicated as long as the final
2040 operation is a hash or array key lookup or subroutine name:
2042 if (exists $ref->{A}->{B}->{$key}) { }
2043 if (exists $hash{A}{B}{$key}) { }
2045 if (exists $ref->{A}->{B}->[$ix]) { }
2046 if (exists $hash{A}{B}[$ix]) { }
2048 if (exists &{$ref->{A}{B}{$key}}) { }
2050 Although the most deeply nested array or hash element will not spring into
2051 existence just because its existence was tested, any intervening ones will.
2052 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2053 into existence due to the existence test for the $key element above.
2054 This happens anywhere the arrow operator is used, including even here:
2057 if (exists $ref->{"Some key"}) { }
2058 print $ref; # prints HASH(0x80d3d5c)
2060 This surprising autovivification in what does not at first--or even
2061 second--glance appear to be an lvalue context may be fixed in a future
2064 Use of a subroutine call, rather than a subroutine name, as an argument
2065 to exists() is an error.
2068 exists &sub(); # Error
2071 X<exit> X<terminate> X<abort>
2075 =for Pod::Functions terminate this program
2077 Evaluates EXPR and exits immediately with that value. Example:
2080 exit 0 if $ans =~ /^[Xx]/;
2082 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2083 universally recognized values for EXPR are C<0> for success and C<1>
2084 for error; other values are subject to interpretation depending on the
2085 environment in which the Perl program is running. For example, exiting
2086 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2087 the mailer to return the item undelivered, but that's not true everywhere.
2089 Don't use C<exit> to abort a subroutine if there's any chance that
2090 someone might want to trap whatever error happened. Use C<die> instead,
2091 which can be trapped by an C<eval>.
2093 The exit() function does not always exit immediately. It calls any
2094 defined C<END> routines first, but these C<END> routines may not
2095 themselves abort the exit. Likewise any object destructors that need to
2096 be called are called before the real exit. C<END> routines and destructors
2097 can change the exit status by modifying C<$?>. If this is a problem, you
2098 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2099 See L<perlmod> for details.
2101 Portability issues: L<perlport/exit>.
2104 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2108 =for Pod::Functions raise I<e> to a power
2110 Returns I<e> (the natural logarithm base) to the power of EXPR.
2111 If EXPR is omitted, gives C<exp($_)>.
2114 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2118 =for Pod::Functions +fc return casefolded version of a string
2120 Returns the casefolded version of EXPR. This is the internal function
2121 implementing the C<\F> escape in double-quoted strings.
2123 Casefolding is the process of mapping strings to a form where case
2124 differences are erased; comparing two strings in their casefolded
2125 form is effectively a way of asking if two strings are equal,
2128 Roughly, if you ever found yourself writing this
2130 lc($this) eq lc($that) # Wrong!
2132 uc($this) eq uc($that) # Also wrong!
2134 $this =~ /^\Q$that\E\z/i # Right!
2138 fc($this) eq fc($that)
2140 And get the correct results.
2142 Perl only implements the full form of casefolding,
2143 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2144 L<Unicode::UCD/prop_invmap()>.
2145 For further information on casefolding, refer to
2146 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2147 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2148 available at L<http://www.unicode.org/versions/latest/>, as well as the
2149 Case Charts available at L<http://www.unicode.org/charts/case/>.
2151 If EXPR is omitted, uses C<$_>.
2153 This function behaves the same way under various pragma, such as in a locale,
2156 While the Unicode Standard defines two additional forms of casefolding,
2157 one for Turkic languages and one that never maps one character into multiple
2158 characters, these are not provided by the Perl core; However, the CPAN module
2159 C<Unicode::Casing> may be used to provide an implementation.
2161 This keyword is available only when the C<"fc"> feature is enabled,
2162 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2163 include a C<use v5.16> or later to the current scope.
2165 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2168 =for Pod::Functions file control system call
2170 Implements the fcntl(2) function. You'll probably have to say
2174 first to get the correct constant definitions. Argument processing and
2175 value returned work just like C<ioctl> below.
2179 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2180 or die "can't fcntl F_GETFL: $!";
2182 You don't have to check for C<defined> on the return from C<fcntl>.
2183 Like C<ioctl>, it maps a C<0> return from the system call into
2184 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2185 in numeric context. It is also exempt from the normal B<-w> warnings
2186 on improper numeric conversions.
2188 Note that C<fcntl> raises an exception if used on a machine that
2189 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2190 manpage to learn what functions are available on your system.
2192 Here's an example of setting a filehandle named C<REMOTE> to be
2193 non-blocking at the system level. You'll have to negotiate C<$|>
2194 on your own, though.
2196 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2198 $flags = fcntl(REMOTE, F_GETFL, 0)
2199 or die "Can't get flags for the socket: $!\n";
2201 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2202 or die "Can't set flags for the socket: $!\n";
2204 Portability issues: L<perlport/fcntl>.
2209 =for Pod::Functions the name of the current source file
2211 A special token that returns the name of the file in which it occurs.
2213 =item fileno FILEHANDLE
2216 =for Pod::Functions return file descriptor from filehandle
2218 Returns the file descriptor for a filehandle, or undefined if the
2219 filehandle is not open. If there is no real file descriptor at the OS
2220 level, as can happen with filehandles connected to memory objects via
2221 C<open> with a reference for the third argument, -1 is returned.
2223 This is mainly useful for constructing
2224 bitmaps for C<select> and low-level POSIX tty-handling operations.
2225 If FILEHANDLE is an expression, the value is taken as an indirect
2226 filehandle, generally its name.
2228 You can use this to find out whether two handles refer to the
2229 same underlying descriptor:
2231 if (fileno(THIS) == fileno(THAT)) {
2232 print "THIS and THAT are dups\n";
2235 =item flock FILEHANDLE,OPERATION
2236 X<flock> X<lock> X<locking>
2238 =for Pod::Functions lock an entire file with an advisory lock
2240 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2241 for success, false on failure. Produces a fatal error if used on a
2242 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2243 C<flock> is Perl's portable file-locking interface, although it locks
2244 entire files only, not records.
2246 Two potentially non-obvious but traditional C<flock> semantics are
2247 that it waits indefinitely until the lock is granted, and that its locks
2248 are B<merely advisory>. Such discretionary locks are more flexible, but
2249 offer fewer guarantees. This means that programs that do not also use
2250 C<flock> may modify files locked with C<flock>. See L<perlport>,
2251 your port's specific documentation, and your system-specific local manpages
2252 for details. It's best to assume traditional behavior if you're writing
2253 portable programs. (But if you're not, you should as always feel perfectly
2254 free to write for your own system's idiosyncrasies (sometimes called
2255 "features"). Slavish adherence to portability concerns shouldn't get
2256 in the way of your getting your job done.)
2258 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2259 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2260 you can use the symbolic names if you import them from the L<Fcntl> module,
2261 either individually, or as a group using the C<:flock> tag. LOCK_SH
2262 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2263 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2264 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2265 waiting for the lock; check the return status to see if you got it.
2267 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2268 before locking or unlocking it.
2270 Note that the emulation built with lockf(3) doesn't provide shared
2271 locks, and it requires that FILEHANDLE be open with write intent. These
2272 are the semantics that lockf(3) implements. Most if not all systems
2273 implement lockf(3) in terms of fcntl(2) locking, though, so the
2274 differing semantics shouldn't bite too many people.
2276 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2277 be open with read intent to use LOCK_SH and requires that it be open
2278 with write intent to use LOCK_EX.
2280 Note also that some versions of C<flock> cannot lock things over the
2281 network; you would need to use the more system-specific C<fcntl> for
2282 that. If you like you can force Perl to ignore your system's flock(2)
2283 function, and so provide its own fcntl(2)-based emulation, by passing
2284 the switch C<-Ud_flock> to the F<Configure> program when you configure
2285 and build a new Perl.
2287 Here's a mailbox appender for BSD systems.
2289 # import LOCK_* and SEEK_END constants
2290 use Fcntl qw(:flock SEEK_END);
2294 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2296 # and, in case someone appended while we were waiting...
2297 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2302 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2305 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2306 or die "Can't open mailbox: $!";
2309 print $mbox $msg,"\n\n";
2312 On systems that support a real flock(2), locks are inherited across fork()
2313 calls, whereas those that must resort to the more capricious fcntl(2)
2314 function lose their locks, making it seriously harder to write servers.
2316 See also L<DB_File> for other flock() examples.
2318 Portability issues: L<perlport/flock>.
2321 X<fork> X<child> X<parent>
2323 =for Pod::Functions create a new process just like this one
2325 Does a fork(2) system call to create a new process running the
2326 same program at the same point. It returns the child pid to the
2327 parent process, C<0> to the child process, or C<undef> if the fork is
2328 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2329 are shared, while everything else is copied. On most systems supporting
2330 fork(), great care has gone into making it extremely efficient (for
2331 example, using copy-on-write technology on data pages), making it the
2332 dominant paradigm for multitasking over the last few decades.
2334 Perl attempts to flush all files opened for
2335 output before forking the child process, but this may not be supported
2336 on some platforms (see L<perlport>). To be safe, you may need to set
2337 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2338 C<IO::Handle> on any open handles to avoid duplicate output.
2340 If you C<fork> without ever waiting on your children, you will
2341 accumulate zombies. On some systems, you can avoid this by setting
2342 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2343 forking and reaping moribund children.
2345 Note that if your forked child inherits system file descriptors like
2346 STDIN and STDOUT that are actually connected by a pipe or socket, even
2347 if you exit, then the remote server (such as, say, a CGI script or a
2348 backgrounded job launched from a remote shell) won't think you're done.
2349 You should reopen those to F</dev/null> if it's any issue.
2351 On some platforms such as Windows, where the fork() system call is not available,
2352 Perl can be built to emulate fork() in the Perl interpreter.
2353 The emulation is designed, at the level of the Perl program,
2354 to be as compatible as possible with the "Unix" fork().
2355 However it has limitations that have to be considered in code intended to be portable.
2356 See L<perlfork> for more details.
2358 Portability issues: L<perlport/fork>.
2363 =for Pod::Functions declare a picture format with use by the write() function
2365 Declare a picture format for use by the C<write> function. For
2369 Test: @<<<<<<<< @||||| @>>>>>
2370 $str, $%, '$' . int($num)
2374 $num = $cost/$quantity;
2378 See L<perlform> for many details and examples.
2380 =item formline PICTURE,LIST
2383 =for Pod::Functions internal function used for formats
2385 This is an internal function used by C<format>s, though you may call it,
2386 too. It formats (see L<perlform>) a list of values according to the
2387 contents of PICTURE, placing the output into the format output
2388 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2389 Eventually, when a C<write> is done, the contents of
2390 C<$^A> are written to some filehandle. You could also read C<$^A>
2391 and then set C<$^A> back to C<"">. Note that a format typically
2392 does one C<formline> per line of form, but the C<formline> function itself
2393 doesn't care how many newlines are embedded in the PICTURE. This means
2394 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2395 You may therefore need to use multiple formlines to implement a single
2396 record format, just like the C<format> compiler.
2398 Be careful if you put double quotes around the picture, because an C<@>
2399 character may be taken to mean the beginning of an array name.
2400 C<formline> always returns true. See L<perlform> for other examples.
2402 If you are trying to use this instead of C<write> to capture the output,
2403 you may find it easier to open a filehandle to a scalar
2404 (C<< open $fh, ">", \$output >>) and write to that instead.
2406 =item getc FILEHANDLE
2407 X<getc> X<getchar> X<character> X<file, read>
2411 =for Pod::Functions get the next character from the filehandle
2413 Returns the next character from the input file attached to FILEHANDLE,
2414 or the undefined value at end of file or if there was an error (in
2415 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2416 STDIN. This is not particularly efficient. However, it cannot be
2417 used by itself to fetch single characters without waiting for the user
2418 to hit enter. For that, try something more like:
2421 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2424 system "stty", '-icanon', 'eol', "\001";
2430 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2433 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2437 Determination of whether $BSD_STYLE should be set
2438 is left as an exercise to the reader.
2440 The C<POSIX::getattr> function can do this more portably on
2441 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2442 module from your nearest CPAN site; details on CPAN can be found under
2446 X<getlogin> X<login>
2448 =for Pod::Functions return who logged in at this tty
2450 This implements the C library function of the same name, which on most
2451 systems returns the current login from F</etc/utmp>, if any. If it
2452 returns the empty string, use C<getpwuid>.
2454 $login = getlogin || getpwuid($<) || "Kilroy";
2456 Do not consider C<getlogin> for authentication: it is not as
2457 secure as C<getpwuid>.
2459 Portability issues: L<perlport/getlogin>.
2461 =item getpeername SOCKET
2462 X<getpeername> X<peer>
2464 =for Pod::Functions find the other end of a socket connection
2466 Returns the packed sockaddr address of the other end of the SOCKET
2470 $hersockaddr = getpeername(SOCK);
2471 ($port, $iaddr) = sockaddr_in($hersockaddr);
2472 $herhostname = gethostbyaddr($iaddr, AF_INET);
2473 $herstraddr = inet_ntoa($iaddr);
2478 =for Pod::Functions get process group
2480 Returns the current process group for the specified PID. Use
2481 a PID of C<0> to get the current process group for the
2482 current process. Will raise an exception if used on a machine that
2483 doesn't implement getpgrp(2). If PID is omitted, returns the process
2484 group of the current process. Note that the POSIX version of C<getpgrp>
2485 does not accept a PID argument, so only C<PID==0> is truly portable.
2487 Portability issues: L<perlport/getpgrp>.
2490 X<getppid> X<parent> X<pid>
2492 =for Pod::Functions get parent process ID
2494 Returns the process id of the parent process.
2496 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2497 around non-POSIX thread semantics the minority of Linux systems (and
2498 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2499 has since been removed. See the documentation for L<$$|perlvar/$$> for
2502 Portability issues: L<perlport/getppid>.
2504 =item getpriority WHICH,WHO
2505 X<getpriority> X<priority> X<nice>
2507 =for Pod::Functions get current nice value
2509 Returns the current priority for a process, a process group, or a user.
2510 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2511 machine that doesn't implement getpriority(2).
2513 Portability issues: L<perlport/getpriority>.
2516 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2517 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2518 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2519 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2520 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2521 X<endnetent> X<endprotoent> X<endservent>
2523 =for Pod::Functions get passwd record given user login name
2527 =for Pod::Functions get group record given group name
2529 =item gethostbyname NAME
2531 =for Pod::Functions get host record given name
2533 =item getnetbyname NAME
2535 =for Pod::Functions get networks record given name
2537 =item getprotobyname NAME
2539 =for Pod::Functions get protocol record given name
2543 =for Pod::Functions get passwd record given user ID
2547 =for Pod::Functions get group record given group user ID
2549 =item getservbyname NAME,PROTO
2551 =for Pod::Functions get services record given its name
2553 =item gethostbyaddr ADDR,ADDRTYPE
2555 =for Pod::Functions get host record given its address
2557 =item getnetbyaddr ADDR,ADDRTYPE
2559 =for Pod::Functions get network record given its address
2561 =item getprotobynumber NUMBER
2563 =for Pod::Functions get protocol record numeric protocol
2565 =item getservbyport PORT,PROTO
2567 =for Pod::Functions get services record given numeric port
2571 =for Pod::Functions get next passwd record
2575 =for Pod::Functions get next group record
2579 =for Pod::Functions get next hosts record
2583 =for Pod::Functions get next networks record
2587 =for Pod::Functions get next protocols record
2591 =for Pod::Functions get next services record
2595 =for Pod::Functions prepare passwd file for use
2599 =for Pod::Functions prepare group file for use
2601 =item sethostent STAYOPEN
2603 =for Pod::Functions prepare hosts file for use
2605 =item setnetent STAYOPEN
2607 =for Pod::Functions prepare networks file for use
2609 =item setprotoent STAYOPEN
2611 =for Pod::Functions prepare protocols file for use
2613 =item setservent STAYOPEN
2615 =for Pod::Functions prepare services file for use
2619 =for Pod::Functions be done using passwd file
2623 =for Pod::Functions be done using group file
2627 =for Pod::Functions be done using hosts file
2631 =for Pod::Functions be done using networks file
2635 =for Pod::Functions be done using protocols file
2639 =for Pod::Functions be done using services file
2641 These routines are the same as their counterparts in the
2642 system C library. In list context, the return values from the
2643 various get routines are as follows:
2645 ($name,$passwd,$uid,$gid,
2646 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2647 ($name,$passwd,$gid,$members) = getgr*
2648 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2649 ($name,$aliases,$addrtype,$net) = getnet*
2650 ($name,$aliases,$proto) = getproto*
2651 ($name,$aliases,$port,$proto) = getserv*
2653 (If the entry doesn't exist you get an empty list.)
2655 The exact meaning of the $gcos field varies but it usually contains
2656 the real name of the user (as opposed to the login name) and other
2657 information pertaining to the user. Beware, however, that in many
2658 system users are able to change this information and therefore it
2659 cannot be trusted and therefore the $gcos is tainted (see
2660 L<perlsec>). The $passwd and $shell, user's encrypted password and
2661 login shell, are also tainted, for the same reason.
2663 In scalar context, you get the name, unless the function was a
2664 lookup by name, in which case you get the other thing, whatever it is.
2665 (If the entry doesn't exist you get the undefined value.) For example:
2667 $uid = getpwnam($name);
2668 $name = getpwuid($num);
2670 $gid = getgrnam($name);
2671 $name = getgrgid($num);
2675 In I<getpw*()> the fields $quota, $comment, and $expire are special
2676 in that they are unsupported on many systems. If the
2677 $quota is unsupported, it is an empty scalar. If it is supported, it
2678 usually encodes the disk quota. If the $comment field is unsupported,
2679 it is an empty scalar. If it is supported it usually encodes some
2680 administrative comment about the user. In some systems the $quota
2681 field may be $change or $age, fields that have to do with password
2682 aging. In some systems the $comment field may be $class. The $expire
2683 field, if present, encodes the expiration period of the account or the
2684 password. For the availability and the exact meaning of these fields
2685 in your system, please consult getpwnam(3) and your system's
2686 F<pwd.h> file. You can also find out from within Perl what your
2687 $quota and $comment fields mean and whether you have the $expire field
2688 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2689 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2690 files are supported only if your vendor has implemented them in the
2691 intuitive fashion that calling the regular C library routines gets the
2692 shadow versions if you're running under privilege or if there exists
2693 the shadow(3) functions as found in System V (this includes Solaris
2694 and Linux). Those systems that implement a proprietary shadow password
2695 facility are unlikely to be supported.
2697 The $members value returned by I<getgr*()> is a space-separated list of
2698 the login names of the members of the group.
2700 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2701 C, it will be returned to you via C<$?> if the function call fails. The
2702 C<@addrs> value returned by a successful call is a list of raw
2703 addresses returned by the corresponding library call. In the
2704 Internet domain, each address is four bytes long; you can unpack it
2705 by saying something like:
2707 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2709 The Socket library makes this slightly easier:
2712 $iaddr = inet_aton("127.1"); # or whatever address
2713 $name = gethostbyaddr($iaddr, AF_INET);
2715 # or going the other way
2716 $straddr = inet_ntoa($iaddr);
2718 In the opposite way, to resolve a hostname to the IP address
2722 $packed_ip = gethostbyname("www.perl.org");
2723 if (defined $packed_ip) {
2724 $ip_address = inet_ntoa($packed_ip);
2727 Make sure C<gethostbyname()> is called in SCALAR context and that
2728 its return value is checked for definedness.
2730 The C<getprotobynumber> function, even though it only takes one argument,
2731 has the precedence of a list operator, so beware:
2733 getprotobynumber $number eq 'icmp' # WRONG
2734 getprotobynumber($number eq 'icmp') # actually means this
2735 getprotobynumber($number) eq 'icmp' # better this way
2737 If you get tired of remembering which element of the return list
2738 contains which return value, by-name interfaces are provided
2739 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2740 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2741 and C<User::grent>. These override the normal built-ins, supplying
2742 versions that return objects with the appropriate names
2743 for each field. For example:
2747 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2749 Even though it looks as though they're the same method calls (uid),
2750 they aren't, because a C<File::stat> object is different from
2751 a C<User::pwent> object.
2753 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2755 =item getsockname SOCKET
2758 =for Pod::Functions retrieve the sockaddr for a given socket
2760 Returns the packed sockaddr address of this end of the SOCKET connection,
2761 in case you don't know the address because you have several different
2762 IPs that the connection might have come in on.
2765 $mysockaddr = getsockname(SOCK);
2766 ($port, $myaddr) = sockaddr_in($mysockaddr);
2767 printf "Connect to %s [%s]\n",
2768 scalar gethostbyaddr($myaddr, AF_INET),
2771 =item getsockopt SOCKET,LEVEL,OPTNAME
2774 =for Pod::Functions get socket options on a given socket
2776 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2777 Options may exist at multiple protocol levels depending on the socket
2778 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2779 C<Socket> module) will exist. To query options at another level the
2780 protocol number of the appropriate protocol controlling the option
2781 should be supplied. For example, to indicate that an option is to be
2782 interpreted by the TCP protocol, LEVEL should be set to the protocol
2783 number of TCP, which you can get using C<getprotobyname>.
2785 The function returns a packed string representing the requested socket
2786 option, or C<undef> on error, with the reason for the error placed in
2787 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2788 consult getsockopt(2) for details. A common case is that the option is an
2789 integer, in which case the result is a packed integer, which you can decode
2790 using C<unpack> with the C<i> (or C<I>) format.
2792 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2794 use Socket qw(:all);
2796 defined(my $tcp = getprotobyname("tcp"))
2797 or die "Could not determine the protocol number for tcp";
2798 # my $tcp = IPPROTO_TCP; # Alternative
2799 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2800 or die "getsockopt TCP_NODELAY: $!";
2801 my $nodelay = unpack("I", $packed);
2802 print "Nagle's algorithm is turned ",
2803 $nodelay ? "off\n" : "on\n";
2805 Portability issues: L<perlport/getsockopt>.
2808 X<glob> X<wildcard> X<filename, expansion> X<expand>
2812 =for Pod::Functions expand filenames using wildcards
2814 In list context, returns a (possibly empty) list of filename expansions on
2815 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2816 scalar context, glob iterates through such filename expansions, returning
2817 undef when the list is exhausted. This is the internal function
2818 implementing the C<< <*.c> >> operator, but you can use it directly. If
2819 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2820 more detail in L<perlop/"I/O Operators">.
2822 Note that C<glob> splits its arguments on whitespace and treats
2823 each segment as separate pattern. As such, C<glob("*.c *.h")>
2824 matches all files with a F<.c> or F<.h> extension. The expression
2825 C<glob(".* *")> matches all files in the current working directory.
2826 If you want to glob filenames that might contain whitespace, you'll
2827 have to use extra quotes around the spacey filename to protect it.
2828 For example, to glob filenames that have an C<e> followed by a space
2829 followed by an C<f>, use either of:
2831 @spacies = <"*e f*">;
2832 @spacies = glob '"*e f*"';
2833 @spacies = glob q("*e f*");
2835 If you had to get a variable through, you could do this:
2837 @spacies = glob "'*${var}e f*'";
2838 @spacies = glob qq("*${var}e f*");
2840 If non-empty braces are the only wildcard characters used in the
2841 C<glob>, no filenames are matched, but potentially many strings
2842 are returned. For example, this produces nine strings, one for
2843 each pairing of fruits and colors:
2845 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2847 This operator is implemented using the standard
2848 C<File::Glob> extension. See L<File::Glob> for details, including
2849 C<bsd_glob> which does not treat whitespace as a pattern separator.
2851 Portability issues: L<perlport/glob>.
2854 X<gmtime> X<UTC> X<Greenwich>
2858 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2860 Works just like L</localtime> but the returned values are
2861 localized for the standard Greenwich time zone.
2863 Note: When called in list context, $isdst, the last value
2864 returned by gmtime, is always C<0>. There is no
2865 Daylight Saving Time in GMT.
2867 Portability issues: L<perlport/gmtime>.
2870 X<goto> X<jump> X<jmp>
2876 =for Pod::Functions create spaghetti code
2878 The C<goto-LABEL> form finds the statement labeled with LABEL and
2879 resumes execution there. It can't be used to get out of a block or
2880 subroutine given to C<sort>. It can be used to go almost anywhere
2881 else within the dynamic scope, including out of subroutines, but it's
2882 usually better to use some other construct such as C<last> or C<die>.
2883 The author of Perl has never felt the need to use this form of C<goto>
2884 (in Perl, that is; C is another matter). (The difference is that C
2885 does not offer named loops combined with loop control. Perl does, and
2886 this replaces most structured uses of C<goto> in other languages.)
2888 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2889 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2890 necessarily recommended if you're optimizing for maintainability:
2892 goto ("FOO", "BAR", "GLARCH")[$i];
2894 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2895 function" rule. A pair of parentheses following it does not (necessarily)
2896 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2898 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2899 deprecated and will issue a warning. Even then, it may not be used to
2900 go into any construct that requires initialization, such as a
2901 subroutine or a C<foreach> loop. It also can't be used to go into a
2902 construct that is optimized away.
2904 The C<goto-&NAME> form is quite different from the other forms of
2905 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2906 doesn't have the stigma associated with other gotos. Instead, it
2907 exits the current subroutine (losing any changes set by local()) and
2908 immediately calls in its place the named subroutine using the current
2909 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2910 load another subroutine and then pretend that the other subroutine had
2911 been called in the first place (except that any modifications to C<@_>
2912 in the current subroutine are propagated to the other subroutine.)
2913 After the C<goto>, not even C<caller> will be able to tell that this
2914 routine was called first.
2916 NAME needn't be the name of a subroutine; it can be a scalar variable
2917 containing a code reference or a block that evaluates to a code
2920 =item grep BLOCK LIST
2923 =item grep EXPR,LIST
2925 =for Pod::Functions locate elements in a list test true against a given criterion
2927 This is similar in spirit to, but not the same as, grep(1) and its
2928 relatives. In particular, it is not limited to using regular expressions.
2930 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2931 C<$_> to each element) and returns the list value consisting of those
2932 elements for which the expression evaluated to true. In scalar
2933 context, returns the number of times the expression was true.
2935 @foo = grep(!/^#/, @bar); # weed out comments
2939 @foo = grep {!/^#/} @bar; # weed out comments
2941 Note that C<$_> is an alias to the list value, so it can be used to
2942 modify the elements of the LIST. While this is useful and supported,
2943 it can cause bizarre results if the elements of LIST are not variables.
2944 Similarly, grep returns aliases into the original list, much as a for
2945 loop's index variable aliases the list elements. That is, modifying an
2946 element of a list returned by grep (for example, in a C<foreach>, C<map>
2947 or another C<grep>) actually modifies the element in the original list.
2948 This is usually something to be avoided when writing clear code.
2950 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2951 been declared with C<my $_>) then, in addition to being locally aliased to
2952 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2953 can't be seen from the outside, avoiding any potential side-effects.
2955 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2958 X<hex> X<hexadecimal>
2962 =for Pod::Functions convert a string to a hexadecimal number
2964 Interprets EXPR as a hex string and returns the corresponding value.
2965 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2966 L</oct>.) If EXPR is omitted, uses C<$_>.
2968 print hex '0xAf'; # prints '175'
2969 print hex 'aF'; # same
2971 Hex strings may only represent integers. Strings that would cause
2972 integer overflow trigger a warning. Leading whitespace is not stripped,
2973 unlike oct(). To present something as hex, look into L</printf>,
2974 L</sprintf>, and L</unpack>.
2979 =for Pod::Functions patch a module's namespace into your own
2981 There is no builtin C<import> function. It is just an ordinary
2982 method (subroutine) defined (or inherited) by modules that wish to export
2983 names to another module. The C<use> function calls the C<import> method
2984 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2986 =item index STR,SUBSTR,POSITION
2987 X<index> X<indexOf> X<InStr>
2989 =item index STR,SUBSTR
2991 =for Pod::Functions find a substring within a string
2993 The index function searches for one string within another, but without
2994 the wildcard-like behavior of a full regular-expression pattern match.
2995 It returns the position of the first occurrence of SUBSTR in STR at
2996 or after POSITION. If POSITION is omitted, starts searching from the
2997 beginning of the string. POSITION before the beginning of the string
2998 or after its end is treated as if it were the beginning or the end,
2999 respectively. POSITION and the return value are based at zero.
3000 If the substring is not found, C<index> returns -1.
3003 X<int> X<integer> X<truncate> X<trunc> X<floor>
3007 =for Pod::Functions get the integer portion of a number
3009 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3010 You should not use this function for rounding: one because it truncates
3011 towards C<0>, and two because machine representations of floating-point
3012 numbers can sometimes produce counterintuitive results. For example,
3013 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3014 because it's really more like -268.99999999999994315658 instead. Usually,
3015 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3016 functions will serve you better than will int().
3018 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3021 =for Pod::Functions system-dependent device control system call
3023 Implements the ioctl(2) function. You'll probably first have to say
3025 require "sys/ioctl.ph"; # probably in
3026 # $Config{archlib}/sys/ioctl.ph
3028 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3029 exist or doesn't have the correct definitions you'll have to roll your
3030 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3031 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3032 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3033 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3034 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3035 has no string value but does have a numeric value, that value will be
3036 passed rather than a pointer to the string value. To guarantee this to be
3037 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3038 functions may be needed to manipulate the values of structures used by
3041 The return value of C<ioctl> (and C<fcntl>) is as follows:
3043 if OS returns: then Perl returns:
3045 0 string "0 but true"
3046 anything else that number
3048 Thus Perl returns true on success and false on failure, yet you can
3049 still easily determine the actual value returned by the operating
3052 $retval = ioctl(...) || -1;
3053 printf "System returned %d\n", $retval;
3055 The special string C<"0 but true"> is exempt from B<-w> complaints
3056 about improper numeric conversions.
3058 Portability issues: L<perlport/ioctl>.
3060 =item join EXPR,LIST
3063 =for Pod::Functions join a list into a string using a separator
3065 Joins the separate strings of LIST into a single string with fields
3066 separated by the value of EXPR, and returns that new string. Example:
3068 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3070 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3071 first argument. Compare L</split>.
3080 =for Pod::Functions retrieve list of indices from a hash
3082 Called in list context, returns a list consisting of all the keys of the
3083 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3084 releases prior to 5.12 will produce a syntax error if you try to use an
3085 array argument. In scalar context, returns the number of keys or indices.
3087 The keys of a hash are returned in an apparently random order. The actual
3088 random order is subject to change in future versions of Perl, but it
3089 is guaranteed to be the same order as either the C<values> or C<each>
3090 function produces (given that the hash has not been modified). Since
3091 Perl 5.8.1 the ordering can be different even between different runs of
3092 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
3095 As a side effect, calling keys() resets the internal iterator of the HASH or
3096 ARRAY (see L</each>). In particular, calling keys() in void context resets
3097 the iterator with no other overhead.
3099 Here is yet another way to print your environment:
3102 @values = values %ENV;
3104 print pop(@keys), '=', pop(@values), "\n";
3107 or how about sorted by key:
3109 foreach $key (sort(keys %ENV)) {
3110 print $key, '=', $ENV{$key}, "\n";
3113 The returned values are copies of the original keys in the hash, so
3114 modifying them will not affect the original hash. Compare L</values>.
3116 To sort a hash by value, you'll need to use a C<sort> function.
3117 Here's a descending numeric sort of a hash by its values:
3119 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3120 printf "%4d %s\n", $hash{$key}, $key;
3123 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3124 allocated for the given hash. This can gain you a measure of efficiency if
3125 you know the hash is going to get big. (This is similar to pre-extending
3126 an array by assigning a larger number to $#array.) If you say
3130 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3131 in fact, since it rounds up to the next power of two. These
3132 buckets will be retained even if you do C<%hash = ()>, use C<undef
3133 %hash> if you want to free the storage while C<%hash> is still in scope.
3134 You can't shrink the number of buckets allocated for the hash using
3135 C<keys> in this way (but you needn't worry about doing this by accident,
3136 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3139 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3140 a reference to an unblessed hash or array. The argument will be
3141 dereferenced automatically. This aspect of C<keys> is considered highly
3142 experimental. The exact behaviour may change in a future version of Perl.
3144 for (keys $hashref) { ... }
3145 for (keys $obj->get_arrayref) { ... }
3147 To avoid confusing would-be users of your code who are running earlier
3148 versions of Perl with mysterious syntax errors, put this sort of thing at
3149 the top of your file to signal that your code will work I<only> on Perls of
3152 use 5.012; # so keys/values/each work on arrays
3153 use 5.014; # so keys/values/each work on scalars (experimental)
3155 See also C<each>, C<values>, and C<sort>.
3157 =item kill SIGNAL, LIST
3162 =for Pod::Functions send a signal to a process or process group
3164 Sends a signal to a list of processes. Returns the number of
3165 processes successfully signaled (which is not necessarily the
3166 same as the number actually killed).
3168 $cnt = kill 1, $child1, $child2;
3171 If SIGNAL is zero, no signal is sent to the process, but C<kill>
3172 checks whether it's I<possible> to send a signal to it (that
3173 means, to be brief, that the process is owned by the same user, or we are
3174 the super-user). This is useful to check that a child process is still
3175 alive (even if only as a zombie) and hasn't changed its UID. See
3176 L<perlport> for notes on the portability of this construct.
3178 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
3179 of processes. That means you usually
3180 want to use positive not negative signals.
3182 You may also use a signal name in quotes. A negative signal name is the
3183 same as a negative signal number, killing process groups instead of processes.
3184 For example, C<kill -KILL, $pgrp> will send C<SIGKILL> to the entire process
3187 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3188 the operating system. For example, on POSIX-conforming systems, zero will
3189 signal the current process group, -1 will signal all processes, and any
3190 other negative PROCESS number will act as a negative signal number and
3191 kill the entire process group specified.
3193 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3194 A warning may be produced in a future version.
3196 See L<perlipc/"Signals"> for more details.
3198 On some platforms such as Windows where the fork() system call is not available.
3199 Perl can be built to emulate fork() at the interpreter level.
3200 This emulation has limitations related to kill that have to be considered,
3201 for code running on Windows and in code intended to be portable.
3203 See L<perlfork> for more details.
3205 If there is no I<LIST> of processes, no signal is sent, and the return
3206 value is 0. This form is sometimes used, however, because it causes
3207 tainting checks to be run. But see
3208 L<perlsec/Laundering and Detecting Tainted Data>.
3210 Portability issues: L<perlport/kill>.
3217 =for Pod::Functions exit a block prematurely
3219 The C<last> command is like the C<break> statement in C (as used in
3220 loops); it immediately exits the loop in question. If the LABEL is
3221 omitted, the command refers to the innermost enclosing loop. The
3222 C<continue> block, if any, is not executed:
3224 LINE: while (<STDIN>) {
3225 last LINE if /^$/; # exit when done with header
3229 C<last> cannot be used to exit a block that returns a value such as
3230 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3231 a grep() or map() operation.
3233 Note that a block by itself is semantically identical to a loop
3234 that executes once. Thus C<last> can be used to effect an early
3235 exit out of such a block.
3237 See also L</continue> for an illustration of how C<last>, C<next>, and
3245 =for Pod::Functions return lower-case version of a string
3247 Returns a lowercased version of EXPR. This is the internal function
3248 implementing the C<\L> escape in double-quoted strings.
3250 If EXPR is omitted, uses C<$_>.
3252 What gets returned depends on several factors:
3256 =item If C<use bytes> is in effect:
3260 =item On EBCDIC platforms
3262 The results are what the C language system call C<tolower()> returns.
3264 =item On ASCII platforms
3266 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
3271 =item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect:
3273 Respects current LC_CTYPE locale for code points < 256; and uses Unicode
3274 semantics for the remaining code points (this last can only happen if
3275 the UTF8 flag is also set). See L<perllocale>.
3277 A deficiency in this is that case changes that cross the 255/256
3278 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3279 LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII
3280 platforms). But under C<use locale>, the lower case of U+1E9E is
3281 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3282 current locale, and Perl has no way of knowing if that character even
3283 exists in the locale, much less what code point it is. Perl returns
3284 the input character unchanged, for all instances (and there aren't
3285 many) where the 255/256 boundary would otherwise be crossed.
3287 =item Otherwise, If EXPR has the UTF8 flag set:
3289 Unicode semantics are used for the case change.
3291 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect:
3293 Unicode semantics are used for the case change.
3299 =item On EBCDIC platforms
3301 The results are what the C language system call C<tolower()> returns.
3303 =item On ASCII platforms
3305 ASCII semantics are used for the case change. The lowercase of any character
3306 outside the ASCII range is the character itself.
3313 X<lcfirst> X<lowercase>
3317 =for Pod::Functions return a string with just the next letter in lower case
3319 Returns the value of EXPR with the first character lowercased. This
3320 is the internal function implementing the C<\l> escape in
3321 double-quoted strings.
3323 If EXPR is omitted, uses C<$_>.
3325 This function behaves the same way under various pragmata, such as in a locale,
3333 =for Pod::Functions return the number of bytes in a string
3335 Returns the length in I<characters> of the value of EXPR. If EXPR is
3336 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3339 This function cannot be used on an entire array or hash to find out how
3340 many elements these have. For that, use C<scalar @array> and C<scalar keys
3341 %hash>, respectively.
3343 Like all Perl character operations, length() normally deals in logical
3344 characters, not physical bytes. For how many bytes a string encoded as
3345 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3346 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3351 =for Pod::Functions the current source line number
3353 A special token that compiles to the current line number.
3355 =item link OLDFILE,NEWFILE
3358 =for Pod::Functions create a hard link in the filesystem
3360 Creates a new filename linked to the old filename. Returns true for
3361 success, false otherwise.
3363 Portability issues: L<perlport/link>.
3365 =item listen SOCKET,QUEUESIZE
3368 =for Pod::Functions register your socket as a server
3370 Does the same thing that the listen(2) system call does. Returns true if
3371 it succeeded, false otherwise. See the example in
3372 L<perlipc/"Sockets: Client/Server Communication">.
3377 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3379 You really probably want to be using C<my> instead, because C<local> isn't
3380 what most people think of as "local". See
3381 L<perlsub/"Private Variables via my()"> for details.
3383 A local modifies the listed variables to be local to the enclosing
3384 block, file, or eval. If more than one value is listed, the list must
3385 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3386 for details, including issues with tied arrays and hashes.
3388 The C<delete local EXPR> construct can also be used to localize the deletion
3389 of array/hash elements to the current block.
3390 See L<perlsub/"Localized deletion of elements of composite types">.
3392 =item localtime EXPR
3393 X<localtime> X<ctime>
3397 =for Pod::Functions convert UNIX time into record or string using local time
3399 Converts a time as returned by the time function to a 9-element list
3400 with the time analyzed for the local time zone. Typically used as
3404 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3407 All list elements are numeric and come straight out of the C `struct
3408 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3409 of the specified time.
3411 C<$mday> is the day of the month and C<$mon> the month in
3412 the range C<0..11>, with 0 indicating January and 11 indicating December.
3413 This makes it easy to get a month name from a list:
3415 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3416 print "$abbr[$mon] $mday";
3417 # $mon=9, $mday=18 gives "Oct 18"
3419 C<$year> contains the number of years since 1900. To get a 4-digit
3424 To get the last two digits of the year (e.g., "01" in 2001) do:
3426 $year = sprintf("%02d", $year % 100);
3428 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3429 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3430 (or C<0..365> in leap years.)
3432 C<$isdst> is true if the specified time occurs during Daylight Saving
3433 Time, false otherwise.
3435 If EXPR is omitted, C<localtime()> uses the current time (as returned
3438 In scalar context, C<localtime()> returns the ctime(3) value:
3440 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3442 The format of this scalar value is B<not> locale-dependent
3443 but built into Perl. For GMT instead of local
3444 time use the L</gmtime> builtin. See also the
3445 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3446 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3447 and mktime(3) functions.
3449 To get somewhat similar but locale-dependent date strings, set up your
3450 locale environment variables appropriately (please see L<perllocale>) and
3453 use POSIX qw(strftime);
3454 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3455 # or for GMT formatted appropriately for your locale:
3456 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3458 Note that the C<%a> and C<%b>, the short forms of the day of the week
3459 and the month of the year, may not necessarily be three characters wide.
3461 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3462 by-name access mechanism to the gmtime() and localtime() functions,
3465 For a comprehensive date and time representation look at the
3466 L<DateTime> module on CPAN.
3468 Portability issues: L<perlport/localtime>.
3473 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3475 This function places an advisory lock on a shared variable or referenced
3476 object contained in I<THING> until the lock goes out of scope.
3478 The value returned is the scalar itself, if the argument is a scalar, or a
3479 reference, if the argument is a hash, array or subroutine.
3481 lock() is a "weak keyword" : this means that if you've defined a function
3482 by this name (before any calls to it), that function will be called
3483 instead. If you are not under C<use threads::shared> this does nothing.
3484 See L<threads::shared>.
3487 X<log> X<logarithm> X<e> X<ln> X<base>
3491 =for Pod::Functions retrieve the natural logarithm for a number
3493 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3494 returns the log of C<$_>. To get the
3495 log of another base, use basic algebra:
3496 The base-N log of a number is equal to the natural log of that number
3497 divided by the natural log of N. For example:
3501 return log($n)/log(10);
3504 See also L</exp> for the inverse operation.
3506 =item lstat FILEHANDLE
3511 =item lstat DIRHANDLE
3515 =for Pod::Functions stat a symbolic link
3517 Does the same thing as the C<stat> function (including setting the
3518 special C<_> filehandle) but stats a symbolic link instead of the file
3519 the symbolic link points to. If symbolic links are unimplemented on
3520 your system, a normal C<stat> is done. For much more detailed
3521 information, please see the documentation for C<stat>.
3523 If EXPR is omitted, stats C<$_>.
3525 Portability issues: L<perlport/lstat>.
3529 =for Pod::Functions match a string with a regular expression pattern
3531 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3533 =item map BLOCK LIST
3538 =for Pod::Functions apply a change to a list to get back a new list with the changes
3540 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3541 C<$_> to each element) and returns the list value composed of the
3542 results of each such evaluation. In scalar context, returns the
3543 total number of elements so generated. Evaluates BLOCK or EXPR in
3544 list context, so each element of LIST may produce zero, one, or
3545 more elements in the returned value.
3547 @chars = map(chr, @numbers);
3549 translates a list of numbers to the corresponding characters.
3551 my @squares = map { $_ * $_ } @numbers;
3553 translates a list of numbers to their squared values.
3555 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3557 shows that number of returned elements can differ from the number of
3558 input elements. To omit an element, return an empty list ().
3559 This could also be achieved by writing
3561 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3563 which makes the intention more clear.
3565 Map always returns a list, which can be
3566 assigned to a hash such that the elements
3567 become key/value pairs. See L<perldata> for more details.
3569 %hash = map { get_a_key_for($_) => $_ } @array;
3571 is just a funny way to write
3575 $hash{get_a_key_for($_)} = $_;
3578 Note that C<$_> is an alias to the list value, so it can be used to
3579 modify the elements of the LIST. While this is useful and supported,
3580 it can cause bizarre results if the elements of LIST are not variables.
3581 Using a regular C<foreach> loop for this purpose would be clearer in
3582 most cases. See also L</grep> for an array composed of those items of
3583 the original list for which the BLOCK or EXPR evaluates to true.
3585 If C<$_> is lexical in the scope where the C<map> appears (because it has
3586 been declared with C<my $_>), then, in addition to being locally aliased to
3587 the list elements, C<$_> keeps being lexical inside the block; that is, it
3588 can't be seen from the outside, avoiding any potential side-effects.
3590 C<{> starts both hash references and blocks, so C<map { ...> could be either
3591 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3592 ahead for the closing C<}> it has to take a guess at which it's dealing with
3593 based on what it finds just after the
3594 C<{>. Usually it gets it right, but if it
3595 doesn't it won't realize something is wrong until it gets to the C<}> and
3596 encounters the missing (or unexpected) comma. The syntax error will be
3597 reported close to the C<}>, but you'll need to change something near the C<{>
3598 such as using a unary C<+> to give Perl some help:
3600 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3601 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3602 %hash = map { ("\L$_" => 1) } @array # this also works
3603 %hash = map { lc($_) => 1 } @array # as does this.
3604 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3606 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3608 or to force an anon hash constructor use C<+{>:
3610 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3613 to get a list of anonymous hashes each with only one entry apiece.
3615 =item mkdir FILENAME,MASK
3616 X<mkdir> X<md> X<directory, create>
3618 =item mkdir FILENAME
3622 =for Pod::Functions create a directory
3624 Creates the directory specified by FILENAME, with permissions
3625 specified by MASK (as modified by C<umask>). If it succeeds it
3626 returns true; otherwise it returns false and sets C<$!> (errno).
3627 MASK defaults to 0777 if omitted, and FILENAME defaults
3628 to C<$_> if omitted.
3630 In general, it is better to create directories with a permissive MASK
3631 and let the user modify that with their C<umask> than it is to supply
3632 a restrictive MASK and give the user no way to be more permissive.
3633 The exceptions to this rule are when the file or directory should be
3634 kept private (mail files, for instance). The perlfunc(1) entry on
3635 C<umask> discusses the choice of MASK in more detail.
3637 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3638 number of trailing slashes. Some operating and filesystems do not get
3639 this right, so Perl automatically removes all trailing slashes to keep
3642 To recursively create a directory structure, look at
3643 the C<mkpath> function of the L<File::Path> module.
3645 =item msgctl ID,CMD,ARG
3648 =for Pod::Functions SysV IPC message control operations
3650 Calls the System V IPC function msgctl(2). You'll probably have to say
3654 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3655 then ARG must be a variable that will hold the returned C<msqid_ds>
3656 structure. Returns like C<ioctl>: the undefined value for error,
3657 C<"0 but true"> for zero, or the actual return value otherwise. See also
3658 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3661 Portability issues: L<perlport/msgctl>.
3663 =item msgget KEY,FLAGS
3666 =for Pod::Functions get SysV IPC message queue
3668 Calls the System V IPC function msgget(2). Returns the message queue
3669 id, or C<undef> on error. See also
3670 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3673 Portability issues: L<perlport/msgget>.
3675 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3678 =for Pod::Functions receive a SysV IPC message from a message queue
3680 Calls the System V IPC function msgrcv to receive a message from
3681 message queue ID into variable VAR with a maximum message size of
3682 SIZE. Note that when a message is received, the message type as a
3683 native long integer will be the first thing in VAR, followed by the
3684 actual message. This packing may be opened with C<unpack("l! a*")>.
3685 Taints the variable. Returns true if successful, false
3686 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3687 C<IPC::SysV> and C<IPC::SysV::Msg>.
3689 Portability issues: L<perlport/msgrcv>.
3691 =item msgsnd ID,MSG,FLAGS
3694 =for Pod::Functions send a SysV IPC message to a message queue
3696 Calls the System V IPC function msgsnd to send the message MSG to the
3697 message queue ID. MSG must begin with the native long integer message
3698 type, be followed by the length of the actual message, and then finally
3699 the message itself. This kind of packing can be achieved with
3700 C<pack("l! a*", $type, $message)>. Returns true if successful,
3701 false on error. See also the C<IPC::SysV>
3702 and C<IPC::SysV::Msg> documentation.
3704 Portability issues: L<perlport/msgsnd>.
3711 =item my EXPR : ATTRS
3713 =item my TYPE EXPR : ATTRS
3715 =for Pod::Functions declare and assign a local variable (lexical scoping)
3717 A C<my> declares the listed variables to be local (lexically) to the
3718 enclosing block, file, or C<eval>. If more than one value is listed,
3719 the list must be placed in parentheses.
3721 The exact semantics and interface of TYPE and ATTRS are still
3722 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3723 and attributes are handled using the C<attributes> pragma, or starting
3724 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3725 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3726 L<attributes>, and L<Attribute::Handlers>.
3733 =for Pod::Functions iterate a block prematurely
3735 The C<next> command is like the C<continue> statement in C; it starts
3736 the next iteration of the loop:
3738 LINE: while (<STDIN>) {
3739 next LINE if /^#/; # discard comments
3743 Note that if there were a C<continue> block on the above, it would get
3744 executed even on discarded lines. If LABEL is omitted, the command
3745 refers to the innermost enclosing loop.
3747 C<next> cannot be used to exit a block which returns a value such as
3748 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3749 a grep() or map() operation.
3751 Note that a block by itself is semantically identical to a loop
3752 that executes once. Thus C<next> will exit such a block early.
3754 See also L</continue> for an illustration of how C<last>, C<next>, and
3757 =item no MODULE VERSION LIST
3761 =item no MODULE VERSION
3763 =item no MODULE LIST
3769 =for Pod::Functions unimport some module symbols or semantics at compile time
3771 See the C<use> function, of which C<no> is the opposite.
3774 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3778 =for Pod::Functions convert a string to an octal number
3780 Interprets EXPR as an octal string and returns the corresponding
3781 value. (If EXPR happens to start off with C<0x>, interprets it as a
3782 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3783 binary string. Leading whitespace is ignored in all three cases.)
3784 The following will handle decimal, binary, octal, and hex in standard
3787 $val = oct($val) if $val =~ /^0/;
3789 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3790 in octal), use sprintf() or printf():
3792 $dec_perms = (stat("filename"))[2] & 07777;
3793 $oct_perm_str = sprintf "%o", $perms;
3795 The oct() function is commonly used when a string such as C<644> needs
3796 to be converted into a file mode, for example. Although Perl
3797 automatically converts strings into numbers as needed, this automatic
3798 conversion assumes base 10.
3800 Leading white space is ignored without warning, as too are any trailing
3801 non-digits, such as a decimal point (C<oct> only handles non-negative
3802 integers, not negative integers or floating point).
3804 =item open FILEHANDLE,EXPR
3805 X<open> X<pipe> X<file, open> X<fopen>
3807 =item open FILEHANDLE,MODE,EXPR
3809 =item open FILEHANDLE,MODE,EXPR,LIST
3811 =item open FILEHANDLE,MODE,REFERENCE
3813 =item open FILEHANDLE
3815 =for Pod::Functions open a file, pipe, or descriptor
3817 Opens the file whose filename is given by EXPR, and associates it with
3820 Simple examples to open a file for reading:
3822 open(my $fh, "<", "input.txt")
3823 or die "cannot open < input.txt: $!";
3827 open(my $fh, ">", "output.txt")
3828 or die "cannot open > output.txt: $!";
3830 (The following is a comprehensive reference to open(): for a gentler
3831 introduction you may consider L<perlopentut>.)
3833 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3834 new filehandle is autovivified, meaning that the variable is assigned a
3835 reference to a newly allocated anonymous filehandle. Otherwise if
3836 FILEHANDLE is an expression, its value is the real filehandle. (This is
3837 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3840 If EXPR is omitted, the global (package) scalar variable of the same
3841 name as the FILEHANDLE contains the filename. (Note that lexical
3842 variables--those declared with C<my> or C<state>--will not work for this
3843 purpose; so if you're using C<my> or C<state>, specify EXPR in your
3846 If three (or more) arguments are specified, the open mode (including
3847 optional encoding) in the second argument are distinct from the filename in
3848 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3849 If MODE is C<< > >>, the file is opened for output, with existing files
3850 first being truncated ("clobbered") and nonexisting files newly created.
3851 If MODE is C<<< >> >>>, the file is opened for appending, again being
3852 created if necessary.
3854 You can put a C<+> in front of the C<< > >> or C<< < >> to
3855 indicate that you want both read and write access to the file; thus
3856 C<< +< >> is almost always preferred for read/write updates--the
3857 C<< +> >> mode would clobber the file first. You can't usually use
3858 either read-write mode for updating textfiles, since they have
3859 variable-length records. See the B<-i> switch in L<perlrun> for a
3860 better approach. The file is created with permissions of C<0666>
3861 modified by the process's C<umask> value.
3863 These various prefixes correspond to the fopen(3) modes of C<r>,
3864 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3866 In the one- and two-argument forms of the call, the mode and filename
3867 should be concatenated (in that order), preferably separated by white
3868 space. You can--but shouldn't--omit the mode in these forms when that mode
3869 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3870 the filename argument is a known literal.
3872 For three or more arguments if MODE is C<|->, the filename is
3873 interpreted as a command to which output is to be piped, and if MODE
3874 is C<-|>, the filename is interpreted as a command that pipes
3875 output to us. In the two-argument (and one-argument) form, one should
3876 replace dash (C<->) with the command.
3877 See L<perlipc/"Using open() for IPC"> for more examples of this.
3878 (You are not allowed to C<open> to a command that pipes both in I<and>
3879 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3880 L<perlipc/"Bidirectional Communication with Another Process"> for
3883 In the form of pipe opens taking three or more arguments, if LIST is specified
3884 (extra arguments after the command name) then LIST becomes arguments
3885 to the command invoked if the platform supports it. The meaning of
3886 C<open> with more than three arguments for non-pipe modes is not yet
3887 defined, but experimental "layers" may give extra LIST arguments
3890 In the two-argument (and one-argument) form, opening C<< <- >>
3891 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3893 You may (and usually should) use the three-argument form of open to specify
3894 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3895 that affect how the input and output are processed (see L<open> and
3896 L<PerlIO> for more details). For example:
3898 open(my $fh, "<:encoding(UTF-8)", "filename")
3899 || die "can't open UTF-8 encoded filename: $!";
3901 opens the UTF8-encoded file containing Unicode characters;
3902 see L<perluniintro>. Note that if layers are specified in the
3903 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3904 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3905 Those layers will also be ignored if you specifying a colon with no name
3906 following it. In that case the default layer for the operating system
3907 (:raw on Unix, :crlf on Windows) is used.
3909 Open returns nonzero on success, the undefined value otherwise. If
3910 the C<open> involved a pipe, the return value happens to be the pid of
3913 If you're running Perl on a system that distinguishes between text
3914 files and binary files, then you should check out L</binmode> for tips
3915 for dealing with this. The key distinction between systems that need
3916 C<binmode> and those that don't is their text file formats. Systems
3917 like Unix, Mac OS, and Plan 9, that end lines with a single
3918 character and encode that character in C as C<"\n"> do not
3919 need C<binmode>. The rest need it.
3921 When opening a file, it's seldom a good idea to continue
3922 if the request failed, so C<open> is frequently used with
3923 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3924 where you want to format a suitable error message (but there are
3925 modules that can help with that problem)) always check
3926 the return value from opening a file.
3928 As a special case the three-argument form with a read/write mode and the third
3929 argument being C<undef>:
3931 open(my $tmp, "+>", undef) or die ...
3933 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
3934 works for symmetry, but you really should consider writing something
3935 to the temporary file first. You will need to seek() to do the
3938 Perl is built using PerlIO by default; Unless you've
3939 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
3940 open filehandles directly to Perl scalars via:
3942 open($fh, ">", \$variable) || ..
3944 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3947 open(STDOUT, ">", \$variable)
3948 or die "Can't open STDOUT: $!";
3953 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
3954 while (<ARTICLE>) {...
3956 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
3957 # if the open fails, output is discarded
3959 open(my $dbase, "+<", "dbase.mine") # open for update
3960 or die "Can't open 'dbase.mine' for update: $!";
3962 open(my $dbase, "+<dbase.mine") # ditto
3963 or die "Can't open 'dbase.mine' for update: $!";
3965 open(ARTICLE, "-|", "caesar <$article") # decrypt article
3966 or die "Can't start caesar: $!";
3968 open(ARTICLE, "caesar <$article |") # ditto
3969 or die "Can't start caesar: $!";
3971 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3972 or die "Can't start sort: $!";
3975 open(MEMORY, ">", \$var)
3976 or die "Can't open memory file: $!";
3977 print MEMORY "foo!\n"; # output will appear in $var
3979 # process argument list of files along with any includes
3981 foreach $file (@ARGV) {
3982 process($file, "fh00");
3986 my($filename, $input) = @_;
3987 $input++; # this is a string increment
3988 unless (open($input, "<", $filename)) {
3989 print STDERR "Can't open $filename: $!\n";
3994 while (<$input>) { # note use of indirection
3995 if (/^#include "(.*)"/) {
3996 process($1, $input);
4003 See L<perliol> for detailed info on PerlIO.
4005 You may also, in the Bourne shell tradition, specify an EXPR beginning
4006 with C<< >& >>, in which case the rest of the string is interpreted
4007 as the name of a filehandle (or file descriptor, if numeric) to be
4008 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4009 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4010 The mode you specify should match the mode of the original filehandle.
4011 (Duping a filehandle does not take into account any existing contents
4012 of IO buffers.) If you use the three-argument
4013 form, then you can pass either a
4014 number, the name of a filehandle, or the normal "reference to a glob".
4016 Here is a script that saves, redirects, and restores C<STDOUT> and
4017 C<STDERR> using various methods:
4020 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4021 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4023 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4024 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4026 select STDERR; $| = 1; # make unbuffered
4027 select STDOUT; $| = 1; # make unbuffered
4029 print STDOUT "stdout 1\n"; # this works for
4030 print STDERR "stderr 1\n"; # subprocesses too
4032 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4033 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4035 print STDOUT "stdout 2\n";
4036 print STDERR "stderr 2\n";
4038 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4039 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4040 that file descriptor (and not call C<dup(2)>); this is more
4041 parsimonious of file descriptors. For example:
4043 # open for input, reusing the fileno of $fd
4044 open(FILEHANDLE, "<&=$fd")
4048 open(FILEHANDLE, "<&=", $fd)
4052 # open for append, using the fileno of OLDFH
4053 open(FH, ">>&=", OLDFH)
4057 open(FH, ">>&=OLDFH")
4059 Being parsimonious on filehandles is also useful (besides being
4060 parsimonious) for example when something is dependent on file
4061 descriptors, like for example locking using flock(). If you do just
4062 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4063 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4064 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4065 the same underlying system file descriptor.
4067 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4068 fdopen() to implement the C<=> functionality. On many Unix systems,
4069 fdopen() fails when file descriptors exceed a certain value, typically 255.
4070 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4072 You can see whether your Perl was built with PerlIO by running C<perl -V>
4073 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4074 have PerlIO; otherwise you don't.
4076 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4077 with the one- or two-argument forms of C<open>),
4078 an implicit C<fork> is done, so C<open> returns twice: in the parent
4079 process it returns the pid
4080 of the child process, and in the child process it returns (a defined) C<0>.
4081 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4083 For example, use either
4085 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4089 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4095 # either write TO_KID or else read FROM_KID
4097 waitpid $child_pid, 0;
4099 # am the child; use STDIN/STDOUT normally
4104 The filehandle behaves normally for the parent, but I/O to that
4105 filehandle is piped from/to the STDOUT/STDIN of the child process.
4106 In the child process, the filehandle isn't opened--I/O happens from/to
4107 the new STDOUT/STDIN. Typically this is used like the normal
4108 piped open when you want to exercise more control over just how the
4109 pipe command gets executed, such as when running setuid and
4110 you don't want to have to scan shell commands for metacharacters.
4112 The following blocks are more or less equivalent:
4114 open(FOO, "|tr '[a-z]' '[A-Z]'");
4115 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4116 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4117 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4119 open(FOO, "cat -n '$file'|");
4120 open(FOO, "-|", "cat -n '$file'");
4121 open(FOO, "-|") || exec "cat", "-n", $file;
4122 open(FOO, "-|", "cat", "-n", $file);
4124 The last two examples in each block show the pipe as "list form", which is
4125 not yet supported on all platforms. A good rule of thumb is that if
4126 your platform has a real C<fork()> (in other words, if your platform is
4127 Unix, including Linux and MacOS X), you can use the list form. You would
4128 want to use the list form of the pipe so you can pass literal arguments
4129 to the command without risk of the shell interpreting any shell metacharacters
4130 in them. However, this also bars you from opening pipes to commands
4131 that intentionally contain shell metacharacters, such as:
4133 open(FOO, "|cat -n | expand -4 | lpr")
4134 // die "Can't open pipeline to lpr: $!";
4136 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4138 Perl will attempt to flush all files opened for
4139 output before any operation that may do a fork, but this may not be
4140 supported on some platforms (see L<perlport>). To be safe, you may need
4141 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4142 of C<IO::Handle> on any open handles.
4144 On systems that support a close-on-exec flag on files, the flag will
4145 be set for the newly opened file descriptor as determined by the value
4146 of C<$^F>. See L<perlvar/$^F>.
4148 Closing any piped filehandle causes the parent process to wait for the
4149 child to finish, then returns the status value in C<$?> and
4150 C<${^CHILD_ERROR_NATIVE}>.
4152 The filename passed to the one- and two-argument forms of open() will
4153 have leading and trailing whitespace deleted and normal
4154 redirection characters honored. This property, known as "magic open",
4155 can often be used to good effect. A user could specify a filename of
4156 F<"rsh cat file |">, or you could change certain filenames as needed:
4158 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4159 open(FH, $filename) or die "Can't open $filename: $!";
4161 Use the three-argument form to open a file with arbitrary weird characters in it,
4163 open(FOO, "<", $file)
4164 || die "can't open < $file: $!";
4166 otherwise it's necessary to protect any leading and trailing whitespace:
4168 $file =~ s#^(\s)#./$1#;
4169 open(FOO, "< $file\0")
4170 || die "open failed: $!";
4172 (this may not work on some bizarre filesystems). One should
4173 conscientiously choose between the I<magic> and I<three-argument> form
4176 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4178 will allow the user to specify an argument of the form C<"rsh cat file |">,
4179 but will not work on a filename that happens to have a trailing space, while
4181 open(IN, "<", $ARGV[0])
4182 || die "can't open < $ARGV[0]: $!";
4184 will have exactly the opposite restrictions.
4186 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4187 should use the C<sysopen> function, which involves no such magic (but may
4188 use subtly different filemodes than Perl open(), which is mapped to C
4189 fopen()). This is another way to protect your filenames from
4190 interpretation. For example:
4193 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4194 or die "sysopen $path: $!";
4195 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4196 print HANDLE "stuff $$\n";
4198 print "File contains: ", <HANDLE>;
4200 Using the constructor from the C<IO::Handle> package (or one of its
4201 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
4202 filehandles that have the scope of the variables used to hold them, then
4203 automatically (but silently) close once their reference counts become
4204 zero, typically at scope exit:
4208 sub read_myfile_munged {
4210 # or just leave it undef to autoviv
4211 my $handle = IO::File->new;
4212 open($handle, "<", "myfile") or die "myfile: $!";
4214 or return (); # Automatically closed here.
4215 mung($first) or die "mung failed"; # Or here.
4216 return (first, <$handle>) if $ALL; # Or here.
4217 return $first; # Or here.
4220 B<WARNING:> The previous example has a bug because the automatic
4221 close that happens when the refcount on C<handle> reaches zero does not
4222 properly detect and report failures. I<Always> close the handle
4223 yourself and inspect the return value.
4226 || warn "close failed: $!";
4228 See L</seek> for some details about mixing reading and writing.
4230 Portability issues: L<perlport/open>.
4232 =item opendir DIRHANDLE,EXPR
4235 =for Pod::Functions open a directory
4237 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4238 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4239 DIRHANDLE may be an expression whose value can be used as an indirect
4240 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4241 scalar variable (or array or hash element), the variable is assigned a
4242 reference to a new anonymous dirhandle; that is, it's autovivified.
4243 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4245 See the example at C<readdir>.
4252 =for Pod::Functions find a character's numeric representation
4254 Returns the numeric value of the first character of EXPR.
4255 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4256 (Note I<character>, not byte.)
4258 For the reverse, see L</chr>.
4259 See L<perlunicode> for more about Unicode.
4266 =item our EXPR : ATTRS
4268 =item our TYPE EXPR : ATTRS
4270 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4272 C<our> associates a simple name with a package variable in the current
4273 package for use within the current scope. When C<use strict 'vars'> is in
4274 effect, C<our> lets you use declared global variables without qualifying
4275 them with package names, within the lexical scope of the C<our> declaration.
4276 In this way C<our> differs from C<use vars>, which is package-scoped.
4278 Unlike C<my> or C<state>, which allocates storage for a variable and
4279 associates a simple name with that storage for use within the current
4280 scope, C<our> associates a simple name with a package (read: global)
4281 variable in the current package, for use within the current lexical scope.
4282 In other words, C<our> has the same scoping rules as C<my> or C<state>, but
4283 does not necessarily create a variable.
4285 If more than one value is listed, the list must be placed
4291 An C<our> declaration declares a global variable that will be visible
4292 across its entire lexical scope, even across package boundaries. The
4293 package in which the variable is entered is determined at the point
4294 of the declaration, not at the point of use. This means the following
4298 our $bar; # declares $Foo::bar for rest of lexical scope
4302 print $bar; # prints 20, as it refers to $Foo::bar
4304 Multiple C<our> declarations with the same name in the same lexical
4305 scope are allowed if they are in different packages. If they happen
4306 to be in the same package, Perl will emit warnings if you have asked
4307 for them, just like multiple C<my> declarations. Unlike a second
4308 C<my> declaration, which will bind the name to a fresh variable, a
4309 second C<our> declaration in the same package, in the same scope, is
4314 our $bar; # declares $Foo::bar for rest of lexical scope
4318 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4319 print $bar; # prints 30
4321 our $bar; # emits warning but has no other effect
4322 print $bar; # still prints 30
4324 An C<our> declaration may also have a list of attributes associated
4327 The exact semantics and interface of TYPE and ATTRS are still
4328 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4329 and attributes are handled using the C<attributes> pragma, or, starting
4330 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4331 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4332 L<attributes>, and L<Attribute::Handlers>.
4334 =item pack TEMPLATE,LIST
4337 =for Pod::Functions convert a list into a binary representation
4339 Takes a LIST of values and converts it into a string using the rules
4340 given by the TEMPLATE. The resulting string is the concatenation of
4341 the converted values. Typically, each converted value looks
4342 like its machine-level representation. For example, on 32-bit machines
4343 an integer may be represented by a sequence of 4 bytes, which will in
4344 Perl be presented as a string that's 4 characters long.
4346 See L<perlpacktut> for an introduction to this function.
4348 The TEMPLATE is a sequence of characters that give the order and type
4349 of values, as follows:
4351 a A string with arbitrary binary data, will be null padded.
4352 A A text (ASCII) string, will be space padded.
4353 Z A null-terminated (ASCIZ) string, will be null padded.
4355 b A bit string (ascending bit order inside each byte,
4357 B A bit string (descending bit order inside each byte).
4358 h A hex string (low nybble first).
4359 H A hex string (high nybble first).
4361 c A signed char (8-bit) value.
4362 C An unsigned char (octet) value.
4363 W An unsigned char value (can be greater than 255).
4365 s A signed short (16-bit) value.
4366 S An unsigned short value.
4368 l A signed long (32-bit) value.
4369 L An unsigned long value.
4371 q A signed quad (64-bit) value.
4372 Q An unsigned quad value.
4373 (Quads are available only if your system supports 64-bit
4374 integer values _and_ if Perl has been compiled to support
4375 those. Raises an exception otherwise.)
4377 i A signed integer value.
4378 I A unsigned integer value.
4379 (This 'integer' is _at_least_ 32 bits wide. Its exact
4380 size depends on what a local C compiler calls 'int'.)
4382 n An unsigned short (16-bit) in "network" (big-endian) order.
4383 N An unsigned long (32-bit) in "network" (big-endian) order.
4384 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4385 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4387 j A Perl internal signed integer value (IV).
4388 J A Perl internal unsigned integer value (UV).
4390 f A single-precision float in native format.
4391 d A double-precision float in native format.
4393 F A Perl internal floating-point value (NV) in native format
4394 D A float of long-double precision in native format.
4395 (Long doubles are available only if your system supports
4396 long double values _and_ if Perl has been compiled to
4397 support those. Raises an exception otherwise.)
4399 p A pointer to a null-terminated string.
4400 P A pointer to a structure (fixed-length string).
4402 u A uuencoded string.
4403 U A Unicode character number. Encodes to a character in char-
4404 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4407 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4408 for details). Its bytes represent an unsigned integer in
4409 base 128, most significant digit first, with as few digits
4410 as possible. Bit eight (the high bit) is set on each byte
4413 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4415 @ Null-fill or truncate to absolute position, counted from the
4416 start of the innermost ()-group.
4417 . Null-fill or truncate to absolute position specified by
4419 ( Start of a ()-group.
4421 One or more modifiers below may optionally follow certain letters in the
4422 TEMPLATE (the second column lists letters for which the modifier is valid):
4424 ! sSlLiI Forces native (short, long, int) sizes instead
4425 of fixed (16-/32-bit) sizes.
4427 xX Make x and X act as alignment commands.
4429 nNvV Treat integers as signed instead of unsigned.
4431 @. Specify position as byte offset in the internal
4432 representation of the packed string. Efficient
4435 > sSiIlLqQ Force big-endian byte-order on the type.
4436 jJfFdDpP (The "big end" touches the construct.)
4438 < sSiIlLqQ Force little-endian byte-order on the type.
4439 jJfFdDpP (The "little end" touches the construct.)
4441 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4442 to force a particular byte-order on all components in that group,
4443 including all its subgroups.
4445 The following rules apply:
4451 Each letter may optionally be followed by a number indicating the repeat
4452 count. A numeric repeat count may optionally be enclosed in brackets, as
4453 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4454 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4455 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4456 something else, described below. Supplying a C<*> for the repeat count
4457 instead of a number means to use however many items are left, except for:
4463 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4467 <.>, where it means relative to the start of the string.
4471 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4475 One can replace a numeric repeat count with a template letter enclosed in
4476 brackets to use the packed byte length of the bracketed template for the
4479 For example, the template C<x[L]> skips as many bytes as in a packed long,
4480 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4481 variable-expanded) unpacks. If the template in brackets contains alignment
4482 commands (such as C<x![d]>), its packed length is calculated as if the
4483 start of the template had the maximal possible alignment.
4485 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4486 trailing null byte, so the resulting string is always one byte longer than
4487 the byte length of the item itself.
4489 When used with C<@>, the repeat count represents an offset from the start
4490 of the innermost C<()> group.
4492 When used with C<.>, the repeat count determines the starting position to
4493 calculate the value offset as follows:
4499 If the repeat count is C<0>, it's relative to the current position.
4503 If the repeat count is C<*>, the offset is relative to the start of the
4508 And if it's an integer I<n>, the offset is relative to the start of the
4509 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4510 bigger then the group level.
4514 The repeat count for C<u> is interpreted as the maximal number of bytes
4515 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4516 count should not be more than 65.
4520 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4521 string of length count, padding with nulls or spaces as needed. When
4522 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4523 after the first null, and C<a> returns data with no stripping at all.
4525 If the value to pack is too long, the result is truncated. If it's too
4526 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4527 followed by a null byte. Thus C<Z> always packs a trailing null, except
4528 when the count is 0.
4532 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4533 Each such format generates 1 bit of the result. These are typically followed
4534 by a repeat count like C<B8> or C<B64>.
4536 Each result bit is based on the least-significant bit of the corresponding
4537 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4538 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4540 Starting from the beginning of the input string, each 8-tuple
4541 of characters is converted to 1 character of output. With format C<b>,
4542 the first character of the 8-tuple determines the least-significant bit of a
4543 character; with format C<B>, it determines the most-significant bit of
4546 If the length of the input string is not evenly divisible by 8, the
4547 remainder is packed as if the input string were padded by null characters
4548 at the end. Similarly during unpacking, "extra" bits are ignored.
4550 If the input string is longer than needed, remaining characters are ignored.
4552 A C<*> for the repeat count uses all characters of the input field.
4553 On unpacking, bits are converted to a string of C<0>s and C<1>s.
4557 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
4558 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
4560 For each such format, pack() generates 4 bits of result.
4561 With non-alphabetical characters, the result is based on the 4 least-significant
4562 bits of the input character, i.e., on C<ord($char)%16>. In particular,
4563 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
4564 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
4565 is compatible with the usual hexadecimal digits, so that C<"a"> and
4566 C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
4567 characters with this format.
4569 Starting from the beginning of the template to pack(), each pair
4570 of characters is converted to 1 character of output. With format C<h>, the
4571 first character of the pair determines the least-significant nybble of the
4572 output character; with format C<H>, it determines the most-significant
4575 If the length of the input string is not even, it behaves as if padded by
4576 a null character at the end. Similarly, "extra" nybbles are ignored during
4579 If the input string is longer than needed, extra characters are ignored.
4581 A C<*> for the repeat count uses all characters of the input field. For
4582 unpack(), nybbles are converted to a string of hexadecimal digits.
4586 The C<p> format packs a pointer to a null-terminated string. You are
4587 responsible for ensuring that the string is not a temporary value, as that
4588 could potentially get deallocated before you got around to using the packed
4589 result. The C<P> format packs a pointer to a structure of the size indicated
4590 by the length. A null pointer is created if the corresponding value for
4591 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
4592 unpacks into C<undef>.
4594 If your system has a strange pointer size--meaning a pointer is neither as
4595 big as an int nor as big as a long--it may not be possible to pack or
4596 unpack pointers in big- or little-endian byte order. Attempting to do
4597 so raises an exception.
4601 The C</> template character allows packing and unpacking of a sequence of
4602 items where the packed structure contains a packed item count followed by
4603 the packed items themselves. This is useful when the structure you're
4604 unpacking has encoded the sizes or repeat counts for some of its fields
4605 within the structure itself as separate fields.
4607 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
4608 I<length-item> describes how the length value is packed. Formats likely
4609 to be of most use are integer-packing ones like C<n> for Java strings,
4610 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
4612 For C<pack>, I<sequence-item> may have a repeat count, in which case
4613 the minimum of that and the number of available items is used as the argument
4614 for I<length-item>. If it has no repeat count or uses a '*', the number
4615 of available items is used.
4617 For C<unpack>, an internal stack of integer arguments unpacked so far is
4618 used. You write C</>I<sequence-item> and the repeat count is obtained by
4619 popping off the last element from the stack. The I<sequence-item> must not
4620 have a repeat count.
4622 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4623 the I<length-item> is the string length, not the number of strings. With
4624 an explicit repeat count for pack, the packed string is adjusted to that
4625 length. For example:
4627 This code: gives this result:
4629 unpack("W/a", "\004Gurusamy") ("Guru")
4630 unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
4631 unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
4633 pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
4634 pack("a/W2", ord("a") .. ord("z")) "2ab"
4636 The I<length-item> is not returned explicitly from C<unpack>.
4638 Supplying a count to the I<length-item> format letter is only useful with
4639 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4640 introduce C<"\000"> characters, which Perl does not regard as legal in
4645 The integer types C<s>, C<S>, C<l>, and C<L> may be
4646 followed by a C<!> modifier to specify native shorts or
4647 longs. As shown in the example above, a bare C<l> means
4648 exactly 32 bits, although the native C<long> as seen by the local C compiler
4649 may be larger. This is mainly an issue on 64-bit platforms. You can
4650 see whether using C<!> makes any difference this way:
4652 printf "format s is %d, s! is %d\n",
4653 length pack("s"), length pack("s!");
4655 printf "format l is %d, l! is %d\n",
4656 length pack("l"), length pack("l!");
4659 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4660 they are identical to C<i> and C<I>.
4662 The actual sizes (in bytes) of native shorts, ints, longs, and long
4663 longs on the platform where Perl was built are also available from
4666 $ perl -V:{short,int,long{,long}}size
4672 or programmatically via the C<Config> module:
4675 print $Config{shortsize}, "\n";
4676 print $Config{intsize}, "\n";
4677 print $Config{longsize}, "\n";
4678 print $Config{longlongsize}, "\n";
4680 C<$Config{longlongsize}> is undefined on systems without
4685 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4686 inherently non-portable between processors and operating systems because
4687 they obey native byteorder and endianness. For example, a 4-byte integer
4688 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4689 handled by the CPU registers) into bytes as
4691 0x12 0x34 0x56 0x78 # big-endian
4692 0x78 0x56 0x34 0x12 # little-endian
4694 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4695 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4696 big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
4697 them in little-endian mode, but SGI/Cray uses them in big-endian mode.
4699 The names I<big-endian> and I<little-endian> are comic references to the
4700 egg-eating habits of the little-endian Lilliputians and the big-endian
4701 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4702 This entered computer lingo via the paper "On Holy Wars and a Plea for
4703 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4705 Some systems may have even weirder byte orders such as