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.
361 If the file doesn't exist or can't be examined, it returns C<undef> and
362 sets C<$!> (errno). Despite the funny names, precedence is the same as any
363 other named unary operator. The 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 (false if symlinks aren't
382 supported by the file system).
383 -p File is a named pipe (FIFO), or Filehandle is a pipe.
385 -b File is a block special file.
386 -c File is a character special file.
387 -t Filehandle is opened to a tty.
389 -u File has setuid bit set.
390 -g File has setgid bit set.
391 -k File has sticky bit set.
393 -T File is an ASCII or UTF-8 text file (heuristic guess).
394 -B File is a "binary" file (opposite of -T).
396 -M Script start time minus file modification time, in days.
397 -A Same for access time.
398 -C Same for inode change time (Unix, may differ for other
405 next unless -f $_; # ignore specials
409 Note that C<-s/a/b/> does not do a negated substitution. Saying
410 C<-exp($foo)> still works as expected, however: only single letters
411 following a minus are interpreted as file tests.
413 These operators are exempt from the "looks like a function rule" described
414 above. That is, an opening parenthesis after the operator does not affect
415 how much of the following code constitutes the argument. Put the opening
416 parentheses before the operator to separate it from code that follows (this
417 applies only to operators with higher precedence than unary operators, of
420 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
421 (-s $file) + 1024 # correct
423 The interpretation of the file permission operators C<-r>, C<-R>,
424 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
425 of the file and the uids and gids of the user. There may be other
426 reasons you can't actually read, write, or execute the file: for
427 example network filesystem access controls, ACLs (access control lists),
428 read-only filesystems, and unrecognized executable formats. Note
429 that the use of these six specific operators to verify if some operation
430 is possible is usually a mistake, because it may be open to race
433 Also note that, for the superuser on the local filesystems, the C<-r>,
434 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
435 if any execute bit is set in the mode. Scripts run by the superuser
436 may thus need to do a stat() to determine the actual mode of the file,
437 or temporarily set their effective uid to something else.
439 If you are using ACLs, there is a pragma called C<filetest> that may
440 produce more accurate results than the bare stat() mode bits.
441 When under C<use filetest 'access'> the above-mentioned filetests
442 test whether the permission can(not) be granted using the
443 access(2) family of system calls. Also note that the C<-x> and C<-X> may
444 under this pragma return true even if there are no execute permission
445 bits set (nor any extra execute permission ACLs). This strangeness is
446 due to the underlying system calls' definitions. Note also that, due to
447 the implementation of C<use filetest 'access'>, the C<_> special
448 filehandle won't cache the results of the file tests when this pragma is
449 in effect. Read the documentation for the C<filetest> pragma for more
452 The C<-T> and C<-B> switches work as follows. The first block or so of
453 the file is examined to see if it is valid UTF-8 that includes non-ASCII
454 characters. If, so it's a C<-T> file. Otherwise, that same portion of
455 the file is examined for odd characters such as strange control codes or
456 characters with the high bit set. If more than a third of the
457 characters are strange, it's a C<-B> file; otherwise it's a C<-T> file.
458 Also, any file containing a zero byte in the examined portion is
459 considered a binary file. (If executed within the scope of a L<S<use
460 locale>|perllocale> which includes C<LC_CTYPE>, odd characters are
461 anything that isn't a printable nor space in the current locale.) If
462 C<-T> or C<-B> is used on a filehandle, the current IO buffer is
464 rather than the first block. Both C<-T> and C<-B> return true on an empty
465 file, or a file at EOF when testing a filehandle. Because you have to
466 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
467 against the file first, as in C<next unless -f $file && -T $file>.
469 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
470 the special filehandle consisting of a solitary underline, then the stat
471 structure of the previous file test (or stat operator) is used, saving
472 a system call. (This doesn't work with C<-t>, and you need to remember
473 that lstat() and C<-l> leave values in the stat structure for the
474 symbolic link, not the real file.) (Also, if the stat buffer was filled by
475 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
478 print "Can do.\n" if -r $a || -w _ || -x _;
481 print "Readable\n" if -r _;
482 print "Writable\n" if -w _;
483 print "Executable\n" if -x _;
484 print "Setuid\n" if -u _;
485 print "Setgid\n" if -g _;
486 print "Sticky\n" if -k _;
487 print "Text\n" if -T _;
488 print "Binary\n" if -B _;
490 As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file
491 test operators, in a way that C<-f -w -x $file> is equivalent to
492 C<-x $file && -w _ && -f _>. (This is only fancy syntax: if you use
493 the return value of C<-f $file> as an argument to another filetest
494 operator, no special magic will happen.)
496 Portability issues: L<perlport/-X>.
498 To avoid confusing would-be users of your code with mysterious
499 syntax errors, put something like this at the top of your script:
501 use 5.010; # so filetest ops can stack
508 =for Pod::Functions absolute value function
510 Returns the absolute value of its argument.
511 If VALUE is omitted, uses C<$_>.
513 =item accept NEWSOCKET,GENERICSOCKET
516 =for Pod::Functions accept an incoming socket connect
518 Accepts an incoming socket connect, just as accept(2)
519 does. Returns the packed address if it succeeded, false otherwise.
520 See the example in L<perlipc/"Sockets: Client/Server Communication">.
522 On systems that support a close-on-exec flag on files, the flag will
523 be set for the newly opened file descriptor, as determined by the
524 value of $^F. See L<perlvar/$^F>.
533 =for Pod::Functions schedule a SIGALRM
535 Arranges to have a SIGALRM delivered to this process after the
536 specified number of wallclock seconds has elapsed. If SECONDS is not
537 specified, the value stored in C<$_> is used. (On some machines,
538 unfortunately, the elapsed time may be up to one second less or more
539 than you specified because of how seconds are counted, and process
540 scheduling may delay the delivery of the signal even further.)
542 Only one timer may be counting at once. Each call disables the
543 previous timer, and an argument of C<0> may be supplied to cancel the
544 previous timer without starting a new one. The returned value is the
545 amount of time remaining on the previous timer.
547 For delays of finer granularity than one second, the Time::HiRes module
548 (from CPAN, and starting from Perl 5.8 part of the standard
549 distribution) provides ualarm(). You may also use Perl's four-argument
550 version of select() leaving the first three arguments undefined, or you
551 might be able to use the C<syscall> interface to access setitimer(2) if
552 your system supports it. See L<perlfaq8> for details.
554 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
555 C<sleep> may be internally implemented on your system with C<alarm>.
557 If you want to use C<alarm> to time out a system call you need to use an
558 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
559 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
560 restart system calls on some systems. Using C<eval>/C<die> always works,
561 modulo the caveats given in L<perlipc/"Signals">.
564 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
566 $nread = sysread SOCKET, $buffer, $size;
570 die unless $@ eq "alarm\n"; # propagate unexpected errors
577 For more information see L<perlipc>.
579 Portability issues: L<perlport/alarm>.
582 X<atan2> X<arctangent> X<tan> X<tangent>
584 =for Pod::Functions arctangent of Y/X in the range -PI to PI
586 Returns the arctangent of Y/X in the range -PI to PI.
588 For the tangent operation, you may use the C<Math::Trig::tan>
589 function, or use the familiar relation:
591 sub tan { sin($_[0]) / cos($_[0]) }
593 The return value for C<atan2(0,0)> is implementation-defined; consult
594 your atan2(3) manpage for more information.
596 Portability issues: L<perlport/atan2>.
598 =item bind SOCKET,NAME
601 =for Pod::Functions binds an address to a socket
603 Binds a network address to a socket, just as bind(2)
604 does. Returns true if it succeeded, false otherwise. NAME should be a
605 packed address of the appropriate type for the socket. See the examples in
606 L<perlipc/"Sockets: Client/Server Communication">.
608 =item binmode FILEHANDLE, LAYER
609 X<binmode> X<binary> X<text> X<DOS> X<Windows>
611 =item binmode FILEHANDLE
613 =for Pod::Functions prepare binary files for I/O
615 Arranges for FILEHANDLE to be read or written in "binary" or "text"
616 mode on systems where the run-time libraries distinguish between
617 binary and text files. If FILEHANDLE is an expression, the value is
618 taken as the name of the filehandle. Returns true on success,
619 otherwise it returns C<undef> and sets C<$!> (errno).
621 On some systems (in general, DOS- and Windows-based systems) binmode()
622 is necessary when you're not working with a text file. For the sake
623 of portability it is a good idea always to use it when appropriate,
624 and never to use it when it isn't appropriate. Also, people can
625 set their I/O to be by default UTF8-encoded Unicode, not bytes.
627 In other words: regardless of platform, use binmode() on binary data,
628 like images, for example.
630 If LAYER is present it is a single string, but may contain multiple
631 directives. The directives alter the behaviour of the filehandle.
632 When LAYER is present, using binmode on a text file makes sense.
634 If LAYER is omitted or specified as C<:raw> the filehandle is made
635 suitable for passing binary data. This includes turning off possible CRLF
636 translation and marking it as bytes (as opposed to Unicode characters).
637 Note that, despite what may be implied in I<"Programming Perl"> (the
638 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
639 Other layers that would affect the binary nature of the stream are
640 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
641 PERLIO environment variable.
643 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
644 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
645 establish default I/O layers. See L<open>.
647 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
648 in "Programming Perl, 3rd Edition". However, since the publishing of this
649 book, by many known as "Camel III", the consensus of the naming of this
650 functionality has moved from "discipline" to "layer". All documentation
651 of this version of Perl therefore refers to "layers" rather than to
652 "disciplines". Now back to the regularly scheduled documentation...>
654 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
655 C<:utf8> just marks the data as UTF-8 without further checking,
656 while C<:encoding(UTF-8)> checks the data for actually being valid
657 UTF-8. More details can be found in L<PerlIO::encoding>.
659 In general, binmode() should be called after open() but before any I/O
660 is done on the filehandle. Calling binmode() normally flushes any
661 pending buffered output data (and perhaps pending input data) on the
662 handle. An exception to this is the C<:encoding> layer that
663 changes the default character encoding of the handle; see L</open>.
664 The C<:encoding> layer sometimes needs to be called in
665 mid-stream, and it doesn't flush the stream. The C<:encoding>
666 also implicitly pushes on top of itself the C<:utf8> layer because
667 internally Perl operates on UTF8-encoded Unicode characters.
669 The operating system, device drivers, C libraries, and Perl run-time
670 system all conspire to let the programmer treat a single
671 character (C<\n>) as the line terminator, irrespective of external
672 representation. On many operating systems, the native text file
673 representation matches the internal representation, but on some
674 platforms the external representation of C<\n> is made up of more than
677 All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
678 a single character to end each line in the external representation of text
679 (even though that single character is CARRIAGE RETURN on old, pre-Darwin
680 flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
681 systems like OS/2, DOS, and the various flavors of MS-Windows, your program
682 sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
683 two characters C<\cM\cJ>. That means that if you don't use binmode() on
684 these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
685 input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
686 output. This is what you want for text files, but it can be disastrous for
689 Another consequence of using binmode() (on some systems) is that
690 special end-of-file markers will be seen as part of the data stream.
691 For systems from the Microsoft family this means that, if your binary
692 data contain C<\cZ>, the I/O subsystem will regard it as the end of
693 the file, unless you use binmode().
695 binmode() is important not only for readline() and print() operations,
696 but also when using read(), seek(), sysread(), syswrite() and tell()
697 (see L<perlport> for more details). See the C<$/> and C<$\> variables
698 in L<perlvar> for how to manually set your input and output
699 line-termination sequences.
701 Portability issues: L<perlport/binmode>.
703 =item bless REF,CLASSNAME
708 =for Pod::Functions create an object
710 This function tells the thingy referenced by REF that it is now an object
711 in the CLASSNAME package. If CLASSNAME is omitted, the current package
712 is used. Because a C<bless> is often the last thing in a constructor,
713 it returns the reference for convenience. Always use the two-argument
714 version if a derived class might inherit the function doing the blessing.
715 See L<perlobj> for more about the blessing (and blessings) of objects.
717 Consider always blessing objects in CLASSNAMEs that are mixed case.
718 Namespaces with all lowercase names are considered reserved for
719 Perl pragmata. Builtin types have all uppercase names. To prevent
720 confusion, you may wish to avoid such package names as well. Make sure
721 that CLASSNAME is a true value.
723 See L<perlmod/"Perl Modules">.
727 =for Pod::Functions +switch break out of a C<given> block
729 Break out of a C<given()> block.
731 This keyword is enabled by the C<"switch"> feature; see L<feature> for
732 more information on C<"switch">. You can also access it by prefixing it
733 with C<CORE::>. Alternatively, include a C<use v5.10> or later to the
737 X<caller> X<call stack> X<stack> X<stack trace>
741 =for Pod::Functions get context of the current subroutine call
743 Returns the context of the current pure perl subroutine call. In scalar
744 context, returns the caller's package name if there I<is> a caller (that is, if
745 we're in a subroutine or C<eval> or C<require>) and the undefined value
746 otherwise. caller never returns XS subs and they are skipped. The next pure
747 perl sub will appear instead of the XS
748 sub in caller's return values. In list
749 context, caller returns
752 ($package, $filename, $line) = caller;
754 With EXPR, it returns some extra information that the debugger uses to
755 print a stack trace. The value of EXPR indicates how many call frames
756 to go back before the current one.
759 ($package, $filename, $line, $subroutine, $hasargs,
762 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
765 Here, $subroutine is the function that the caller called (rather than the
766 function containing the caller). Note that $subroutine may be C<(eval)> if
767 the frame is not a subroutine call, but an C<eval>. In such a case
768 additional elements $evaltext and
769 C<$is_require> are set: C<$is_require> is true if the frame is created by a
770 C<require> or C<use> statement, $evaltext contains the text of the
771 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
772 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
773 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
774 frame.) $subroutine may also be C<(unknown)> if this particular
775 subroutine happens to have been deleted from the symbol table.
776 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
777 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
778 compiled with. C<$hints> corresponds to C<$^H>, and C<$bitmask>
779 corresponds to C<${^WARNING_BITS}>. The
780 C<$hints> and C<$bitmask> values are subject
781 to change between versions of Perl, and are not meant for external use.
783 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
784 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
785 of this hash, as they are the actual values stored in the optree.
787 Furthermore, when called from within the DB package in
788 list context, and with an argument, caller returns more
789 detailed information: it sets the list variable C<@DB::args> to be the
790 arguments with which the subroutine was invoked.
792 Be aware that the optimizer might have optimized call frames away before
793 C<caller> had a chance to get the information. That means that C<caller(N)>
794 might not return information about the call frame you expect it to, for
795 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
796 previous time C<caller> was called.
798 Be aware that setting C<@DB::args> is I<best effort>, intended for
799 debugging or generating backtraces, and should not be relied upon. In
800 particular, as C<@_> contains aliases to the caller's arguments, Perl does
801 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
802 subroutine makes to C<@_> or its contents, not the original values at call
803 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
804 elements, so under certain cases its elements may have become freed and
805 reallocated for other variables or temporary values. Finally, a side effect
806 of the current implementation is that the effects of C<shift @_> can
807 I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
808 reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
809 elements), so C<@DB::args> is actually a hybrid of the current state and
810 initial state of C<@_>. Buyer beware.
817 =item chdir FILEHANDLE
819 =item chdir DIRHANDLE
823 =for Pod::Functions change your current working directory
825 Changes the working directory to EXPR, if possible. If EXPR is omitted,
826 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
827 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
828 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
829 neither is set, C<chdir> does nothing. It returns true on success,
830 false otherwise. See the example under C<die>.
832 On systems that support fchdir(2), you may pass a filehandle or
833 directory handle as the argument. On systems that don't support fchdir(2),
834 passing handles raises an exception.
837 X<chmod> X<permission> X<mode>
839 =for Pod::Functions changes the permissions on a list of files
841 Changes the permissions of a list of files. The first element of the
842 list must be the numeric mode, which should probably be an octal
843 number, and which definitely should I<not> be a string of octal digits:
844 C<0644> is okay, but C<"0644"> is not. Returns the number of files
845 successfully changed. See also L</oct> if all you have is a string.
847 $cnt = chmod 0755, "foo", "bar";
848 chmod 0755, @executables;
849 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
851 $mode = "0644"; chmod oct($mode), "foo"; # this is better
852 $mode = 0644; chmod $mode, "foo"; # this is best
854 On systems that support fchmod(2), you may pass filehandles among the
855 files. On systems that don't support fchmod(2), passing filehandles raises
856 an exception. Filehandles must be passed as globs or glob references to be
857 recognized; barewords are considered filenames.
859 open(my $fh, "<", "foo");
860 my $perm = (stat $fh)[2] & 07777;
861 chmod($perm | 0600, $fh);
863 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
866 use Fcntl qw( :mode );
867 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
868 # Identical to the chmod 0755 of the example above.
870 Portability issues: L<perlport/chmod>.
873 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
879 =for Pod::Functions remove a trailing record separator from a string
881 This safer version of L</chop> removes any trailing string
882 that corresponds to the current value of C<$/> (also known as
883 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
884 number of characters removed from all its arguments. It's often used to
885 remove the newline from the end of an input record when you're worried
886 that the final record may be missing its newline. When in paragraph
887 mode (C<$/ = "">), it removes all trailing newlines from the string.
888 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
889 a reference to an integer or the like; see L<perlvar>) chomp() won't
891 If VARIABLE is omitted, it chomps C<$_>. Example:
894 chomp; # avoid \n on last field
899 If VARIABLE is a hash, it chomps the hash's values, but not its keys,
900 resetting the C<each> iterator in the process.
902 You can actually chomp anything that's an lvalue, including an assignment:
905 chomp($answer = <STDIN>);
907 If you chomp a list, each element is chomped, and the total number of
908 characters removed is returned.
910 Note that parentheses are necessary when you're chomping anything
911 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
912 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
913 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
914 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
924 =for Pod::Functions remove the last character from a string
926 Chops off the last character of a string and returns the character
927 chopped. It is much more efficient than C<s/.$//s> because it neither
928 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
929 If VARIABLE is a hash, it chops the hash's values, but not its keys,
930 resetting the C<each> iterator in the process.
932 You can actually chop anything that's an lvalue, including an assignment.
934 If you chop a list, each element is chopped. Only the value of the
935 last C<chop> is returned.
937 Note that C<chop> returns the last character. To return all but the last
938 character, use C<substr($string, 0, -1)>.
943 X<chown> X<owner> X<user> X<group>
945 =for Pod::Functions change the ownership on a list of files
947 Changes the owner (and group) of a list of files. The first two
948 elements of the list must be the I<numeric> uid and gid, in that
949 order. A value of -1 in either position is interpreted by most
950 systems to leave that value unchanged. Returns the number of files
951 successfully changed.
953 $cnt = chown $uid, $gid, 'foo', 'bar';
954 chown $uid, $gid, @filenames;
956 On systems that support fchown(2), you may pass filehandles among the
957 files. On systems that don't support fchown(2), passing filehandles raises
958 an exception. Filehandles must be passed as globs or glob references to be
959 recognized; barewords are considered filenames.
961 Here's an example that looks up nonnumeric uids in the passwd file:
964 chomp($user = <STDIN>);
966 chomp($pattern = <STDIN>);
968 ($login,$pass,$uid,$gid) = getpwnam($user)
969 or die "$user not in passwd file";
971 @ary = glob($pattern); # expand filenames
972 chown $uid, $gid, @ary;
974 On most systems, you are not allowed to change the ownership of the
975 file unless you're the superuser, although you should be able to change
976 the group to any of your secondary groups. On insecure systems, these
977 restrictions may be relaxed, but this is not a portable assumption.
978 On POSIX systems, you can detect this condition this way:
980 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
981 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
983 Portability issues: L<perlport/chown>.
986 X<chr> X<character> X<ASCII> X<Unicode>
990 =for Pod::Functions get character this number represents
992 Returns the character represented by that NUMBER in the character set.
993 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
994 chr(0x263a) is a Unicode smiley face.
996 Negative values give the Unicode replacement character (chr(0xfffd)),
997 except under the L<bytes> pragma, where the low eight bits of the value
998 (truncated to an integer) are used.
1000 If NUMBER is omitted, uses C<$_>.
1002 For the reverse, use L</ord>.
1004 Note that characters from 128 to 255 (inclusive) are by default
1005 internally not encoded as UTF-8 for backward compatibility reasons.
1007 See L<perlunicode> for more about Unicode.
1009 =item chroot FILENAME
1014 =for Pod::Functions make directory new root for path lookups
1016 This function works like the system call by the same name: it makes the
1017 named directory the new root directory for all further pathnames that
1018 begin with a C</> by your process and all its children. (It doesn't
1019 change your current working directory, which is unaffected.) For security
1020 reasons, this call is restricted to the superuser. If FILENAME is
1021 omitted, does a C<chroot> to C<$_>.
1023 B<NOTE:> It is good security practice to do C<chdir("/")> (to the root
1024 directory) immediately after a C<chroot()>.
1026 Portability issues: L<perlport/chroot>.
1028 =item close FILEHANDLE
1033 =for Pod::Functions close file (or pipe or socket) handle
1035 Closes the file or pipe associated with the filehandle, flushes the IO
1036 buffers, and closes the system file descriptor. Returns true if those
1037 operations succeed and if no error was reported by any PerlIO
1038 layer. Closes the currently selected filehandle if the argument is
1041 You don't have to close FILEHANDLE if you are immediately going to do
1042 another C<open> on it, because C<open> closes it for you. (See
1043 L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
1044 counter (C<$.>), while the implicit close done by C<open> does not.
1046 If the filehandle came from a piped open, C<close> returns false if one of
1047 the other syscalls involved fails or if its program exits with non-zero
1048 status. If the only problem was that the program exited non-zero, C<$!>
1049 will be set to C<0>. Closing a pipe also waits for the process executing
1050 on the pipe to exit--in case you wish to look at the output of the pipe
1051 afterwards--and implicitly puts the exit status value of that command into
1052 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
1054 If there are multiple threads running, C<close> on a filehandle from a
1055 piped open returns true without waiting for the child process to terminate,
1056 if the filehandle is still open in another thread.
1058 Closing the read end of a pipe before the process writing to it at the
1059 other end is done writing results in the writer receiving a SIGPIPE. If
1060 the other end can't handle that, be sure to read all the data before
1065 open(OUTPUT, '|sort >foo') # pipe to sort
1066 or die "Can't start sort: $!";
1067 #... # print stuff to output
1068 close OUTPUT # wait for sort to finish
1069 or warn $! ? "Error closing sort pipe: $!"
1070 : "Exit status $? from sort";
1071 open(INPUT, 'foo') # get sort's results
1072 or die "Can't open 'foo' for input: $!";
1074 FILEHANDLE may be an expression whose value can be used as an indirect
1075 filehandle, usually the real filehandle name or an autovivified handle.
1077 =item closedir DIRHANDLE
1080 =for Pod::Functions close directory handle
1082 Closes a directory opened by C<opendir> and returns the success of that
1085 =item connect SOCKET,NAME
1088 =for Pod::Functions connect to a remote socket
1090 Attempts to connect to a remote socket, just like connect(2).
1091 Returns true if it succeeded, false otherwise. NAME should be a
1092 packed address of the appropriate type for the socket. See the examples in
1093 L<perlipc/"Sockets: Client/Server Communication">.
1095 =item continue BLOCK
1100 =for Pod::Functions optional trailing block in a while or foreach
1102 When followed by a BLOCK, C<continue> is actually a
1103 flow control statement rather than a function. If
1104 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
1105 C<foreach>), it is always executed just before the conditional is about to
1106 be evaluated again, just like the third part of a C<for> loop in C. Thus
1107 it can be used to increment a loop variable, even when the loop has been
1108 continued via the C<next> statement (which is similar to the C C<continue>
1111 C<last>, C<next>, or C<redo> may appear within a C<continue>
1112 block; C<last> and C<redo> behave as if they had been executed within
1113 the main block. So will C<next>, but since it will execute a C<continue>
1114 block, it may be more entertaining.
1117 ### redo always comes here
1120 ### next always comes here
1122 # then back the top to re-check EXPR
1124 ### last always comes here
1126 Omitting the C<continue> section is equivalent to using an
1127 empty one, logically enough, so C<next> goes directly back
1128 to check the condition at the top of the loop.
1130 When there is no BLOCK, C<continue> is a function that
1131 falls through the current C<when> or C<default> block instead of iterating
1132 a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
1133 In Perl 5.14 and earlier, this form of C<continue> was
1134 only available when the C<"switch"> feature was enabled.
1135 See L<feature> and L<perlsyn/"Switch Statements"> for more
1139 X<cos> X<cosine> X<acos> X<arccosine>
1143 =for Pod::Functions cosine function
1145 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1146 takes the cosine of C<$_>.
1148 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1149 function, or use this relation:
1151 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1153 =item crypt PLAINTEXT,SALT
1154 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1155 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1157 =for Pod::Functions one-way passwd-style encryption
1159 Creates a digest string exactly like the crypt(3) function in the C
1160 library (assuming that you actually have a version there that has not
1161 been extirpated as a potential munition).
1163 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1164 into a short string, called a digest, which is returned. The same
1165 PLAINTEXT and SALT will always return the same string, but there is no
1166 (known) way to get the original PLAINTEXT from the hash. Small
1167 changes in the PLAINTEXT or SALT will result in large changes in the
1170 There is no decrypt function. This function isn't all that useful for
1171 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1172 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1173 primarily used to check if two pieces of text are the same without
1174 having to transmit or store the text itself. An example is checking
1175 if a correct password is given. The digest of the password is stored,
1176 not the password itself. The user types in a password that is
1177 crypt()'d with the same salt as the stored digest. If the two digests
1178 match, the password is correct.
1180 When verifying an existing digest string you should use the digest as
1181 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1182 to create the digest is visible as part of the digest. This ensures
1183 crypt() will hash the new string with the same salt as the digest.
1184 This allows your code to work with the standard L<crypt|/crypt> and
1185 with more exotic implementations. In other words, assume
1186 nothing about the returned string itself nor about how many bytes
1189 Traditionally the result is a string of 13 bytes: two first bytes of
1190 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1191 the first eight bytes of PLAINTEXT mattered. But alternative
1192 hashing schemes (like MD5), higher level security schemes (like C2),
1193 and implementations on non-Unix platforms may produce different
1196 When choosing a new salt create a random two character string whose
1197 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1198 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1199 characters is just a recommendation; the characters allowed in
1200 the salt depend solely on your system's crypt library, and Perl can't
1201 restrict what salts C<crypt()> accepts.
1203 Here's an example that makes sure that whoever runs this program knows
1206 $pwd = (getpwuid($<))[1];
1208 system "stty -echo";
1210 chomp($word = <STDIN>);
1214 if (crypt($word, $pwd) ne $pwd) {
1220 Of course, typing in your own password to whoever asks you
1223 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1224 of data, not least of all because you can't get the information
1225 back. Look at the L<Digest> module for more robust algorithms.
1227 If using crypt() on a Unicode string (which I<potentially> has
1228 characters with codepoints above 255), Perl tries to make sense
1229 of the situation by trying to downgrade (a copy of)
1230 the string back to an eight-bit byte string before calling crypt()
1231 (on that copy). If that works, good. If not, crypt() dies with
1232 C<Wide character in crypt>.
1234 Portability issues: L<perlport/crypt>.
1239 =for Pod::Functions breaks binding on a tied dbm file
1241 [This function has been largely superseded by the C<untie> function.]
1243 Breaks the binding between a DBM file and a hash.
1245 Portability issues: L<perlport/dbmclose>.
1247 =item dbmopen HASH,DBNAME,MASK
1248 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1250 =for Pod::Functions create binding on a tied dbm file
1252 [This function has been largely superseded by the
1253 L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
1255 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1256 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1257 argument is I<not> a filehandle, even though it looks like one). DBNAME
1258 is the name of the database (without the F<.dir> or F<.pag> extension if
1259 any). If the database does not exist, it is created with protection
1260 specified by MASK (as modified by the C<umask>). To prevent creation of
1261 the database if it doesn't exist, you may specify a MODE
1262 of 0, and the function will return a false value if it
1263 can't find an existing database. If your system supports
1264 only the older DBM functions, you may make only one C<dbmopen> call in your
1265 program. In older versions of Perl, if your system had neither DBM nor
1266 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1269 If you don't have write access to the DBM file, you can only read hash
1270 variables, not set them. If you want to test whether you can write,
1271 either use file tests or try setting a dummy hash entry inside an C<eval>
1274 Note that functions such as C<keys> and C<values> may return huge lists
1275 when used on large DBM files. You may prefer to use the C<each>
1276 function to iterate over large DBM files. Example:
1278 # print out history file offsets
1279 dbmopen(%HIST,'/usr/lib/news/history',0666);
1280 while (($key,$val) = each %HIST) {
1281 print $key, ' = ', unpack('L',$val), "\n";
1285 See also L<AnyDBM_File> for a more general description of the pros and
1286 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1287 rich implementation.
1289 You can control which DBM library you use by loading that library
1290 before you call dbmopen():
1293 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1294 or die "Can't open netscape history file: $!";
1296 Portability issues: L<perlport/dbmopen>.
1299 X<defined> X<undef> X<undefined>
1303 =for Pod::Functions test whether a value, variable, or function is defined
1305 Returns a Boolean value telling whether EXPR has a value other than
1306 the undefined value C<undef>. If EXPR is not present, C<$_> is
1309 Many operations return C<undef> to indicate failure, end of file,
1310 system error, uninitialized variable, and other exceptional
1311 conditions. This function allows you to distinguish C<undef> from
1312 other values. (A simple Boolean test will not distinguish among
1313 C<undef>, zero, the empty string, and C<"0">, which are all equally
1314 false.) Note that since C<undef> is a valid scalar, its presence
1315 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1316 returns C<undef> when its argument is an empty array, I<or> when the
1317 element to return happens to be C<undef>.
1319 You may also use C<defined(&func)> to check whether subroutine C<&func>
1320 has ever been defined. The return value is unaffected by any forward
1321 declarations of C<&func>. A subroutine that is not defined
1322 may still be callable: its package may have an C<AUTOLOAD> method that
1323 makes it spring into existence the first time that it is called; see
1326 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1327 used to report whether memory for that aggregate had ever been
1328 allocated. This behavior may disappear in future versions of Perl.
1329 You should instead use a simple test for size:
1331 if (@an_array) { print "has array elements\n" }
1332 if (%a_hash) { print "has hash members\n" }
1334 When used on a hash element, it tells you whether the value is defined,
1335 not whether the key exists in the hash. Use L</exists> for the latter
1340 print if defined $switch{D};
1341 print "$val\n" while defined($val = pop(@ary));
1342 die "Can't readlink $sym: $!"
1343 unless defined($value = readlink $sym);
1344 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1345 $debugging = 0 unless defined $debugging;
1347 Note: Many folks tend to overuse C<defined> and are then surprised to
1348 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1349 defined values. For example, if you say
1353 The pattern match succeeds and C<$1> is defined, although it
1354 matched "nothing". It didn't really fail to match anything. Rather, it
1355 matched something that happened to be zero characters long. This is all
1356 very above-board and honest. When a function returns an undefined value,
1357 it's an admission that it couldn't give you an honest answer. So you
1358 should use C<defined> only when questioning the integrity of what
1359 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1362 See also L</undef>, L</exists>, L</ref>.
1367 =for Pod::Functions deletes a value from a hash
1369 Given an expression that specifies an element or slice of a hash, C<delete>
1370 deletes the specified elements from that hash so that exists() on that element
1371 no longer returns true. Setting a hash element to the undefined value does
1372 not remove its key, but deleting it does; see L</exists>.
1374 In list context, returns the value or values deleted, or the last such
1375 element in scalar context. The return list's length always matches that of
1376 the argument list: deleting non-existent elements returns the undefined value
1377 in their corresponding positions.
1379 delete() may also be used on arrays and array slices, but its behavior is less
1380 straightforward. Although exists() will return false for deleted entries,
1381 deleting array elements never changes indices of existing values; use shift()
1382 or splice() for that. However, if any deleted elements fall at the end of an
1383 array, the array's size shrinks to the position of the highest element that
1384 still tests true for exists(), or to 0 if none do. In other words, an
1385 array won't have trailing nonexistent elements after a delete.
1387 B<WARNING:> Calling C<delete> on array values is strongly discouraged. The
1388 notion of deleting or checking the existence of Perl array elements is not
1389 conceptually coherent, and can lead to surprising behavior.
1391 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1392 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1393 or array may not necessarily return anything; it depends on the implementation
1394 of the C<tied> package's DELETE method, which may do whatever it pleases.
1396 The C<delete local EXPR> construct localizes the deletion to the current
1397 block at run time. Until the block exits, elements locally deleted
1398 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1399 of composite types">.
1401 %hash = (foo => 11, bar => 22, baz => 33);
1402 $scalar = delete $hash{foo}; # $scalar is 11
1403 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1404 @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
1406 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1408 foreach $key (keys %HASH) {
1412 foreach $index (0 .. $#ARRAY) {
1413 delete $ARRAY[$index];
1418 delete @HASH{keys %HASH};
1420 delete @ARRAY[0 .. $#ARRAY];
1422 But both are slower than assigning the empty list
1423 or undefining %HASH or @ARRAY, which is the customary
1424 way to empty out an aggregate:
1426 %HASH = (); # completely empty %HASH
1427 undef %HASH; # forget %HASH ever existed
1429 @ARRAY = (); # completely empty @ARRAY
1430 undef @ARRAY; # forget @ARRAY ever existed
1432 The EXPR can be arbitrarily complicated provided its
1433 final operation is an element or slice of an aggregate:
1435 delete $ref->[$x][$y]{$key};
1436 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1438 delete $ref->[$x][$y][$index];
1439 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1442 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1444 =for Pod::Functions raise an exception or bail out
1446 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1447 into C<$@> and the C<eval> is terminated with the undefined value.
1448 If the exception is outside of all enclosing C<eval>s, then the uncaught
1449 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1450 need to exit the process with a specific exit code, see L</exit>.
1452 Equivalent examples:
1454 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1455 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1457 If the last element of LIST does not end in a newline, the current
1458 script line number and input line number (if any) are also printed,
1459 and a newline is supplied. Note that the "input line number" (also
1460 known as "chunk") is subject to whatever notion of "line" happens to
1461 be currently in effect, and is also available as the special variable
1462 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1464 Hint: sometimes appending C<", stopped"> to your message will cause it
1465 to make better sense when the string C<"at foo line 123"> is appended.
1466 Suppose you are running script "canasta".
1468 die "/etc/games is no good";
1469 die "/etc/games is no good, stopped";
1471 produce, respectively
1473 /etc/games is no good at canasta line 123.
1474 /etc/games is no good, stopped at canasta line 123.
1476 If the output is empty and C<$@> already contains a value (typically from a
1477 previous eval) that value is reused after appending C<"\t...propagated">.
1478 This is useful for propagating exceptions:
1481 die unless $@ =~ /Expected exception/;
1483 If the output is empty and C<$@> contains an object reference that has a
1484 C<PROPAGATE> method, that method will be called with additional file
1485 and line number parameters. The return value replaces the value in
1486 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1489 If C<$@> is empty then the string C<"Died"> is used.
1491 If an uncaught exception results in interpreter exit, the exit code is
1492 determined from the values of C<$!> and C<$?> with this pseudocode:
1494 exit $! if $!; # errno
1495 exit $? >> 8 if $? >> 8; # child exit status
1496 exit 255; # last resort
1498 The intent is to squeeze as much possible information about the likely cause
1499 into the limited space of the system exit
1500 code. However, as C<$!> is the value
1501 of C's C<errno>, which can be set by any system call, this means that the value
1502 of the exit code used by C<die> can be non-predictable, so should not be relied
1503 upon, other than to be non-zero.
1505 You can also call C<die> with a reference argument, and if this is trapped
1506 within an C<eval>, C<$@> contains that reference. This permits more
1507 elaborate exception handling using objects that maintain arbitrary state
1508 about the exception. Such a scheme is sometimes preferable to matching
1509 particular string values of C<$@> with regular expressions. Because C<$@>
1510 is a global variable and C<eval> may be used within object implementations,
1511 be careful that analyzing the error object doesn't replace the reference in
1512 the global variable. It's easiest to make a local copy of the reference
1513 before any manipulations. Here's an example:
1515 use Scalar::Util "blessed";
1517 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1518 if (my $ev_err = $@) {
1519 if (blessed($ev_err)
1520 && $ev_err->isa("Some::Module::Exception")) {
1521 # handle Some::Module::Exception
1524 # handle all other possible exceptions
1528 Because Perl stringifies uncaught exception messages before display,
1529 you'll probably want to overload stringification operations on
1530 exception objects. See L<overload> for details about that.
1532 You can arrange for a callback to be run just before the C<die>
1533 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1534 handler is called with the error text and can change the error
1535 message, if it sees fit, by calling C<die> again. See
1536 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1537 L<"eval BLOCK"> for some examples. Although this feature was
1538 to be run only right before your program was to exit, this is not
1539 currently so: the C<$SIG{__DIE__}> hook is currently called
1540 even inside eval()ed blocks/strings! If one wants the hook to do
1541 nothing in such situations, put
1545 as the first line of the handler (see L<perlvar/$^S>). Because
1546 this promotes strange action at a distance, this counterintuitive
1547 behavior may be fixed in a future release.
1549 See also exit(), warn(), and the Carp module.
1554 =for Pod::Functions turn a BLOCK into a TERM
1556 Not really a function. Returns the value of the last command in the
1557 sequence of commands indicated by BLOCK. When modified by the C<while> or
1558 C<until> loop modifier, executes the BLOCK once before testing the loop
1559 condition. (On other statements the loop modifiers test the conditional
1562 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1563 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1564 See L<perlsyn> for alternative strategies.
1569 Uses the value of EXPR as a filename and executes the contents of the
1570 file as a Perl script.
1578 except that it's more concise, runs no external processes, keeps track of
1580 filename for error messages, searches the C<@INC> directories, and updates
1581 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1582 these variables. It also differs in that code evaluated with C<do FILENAME>
1583 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1584 same, however, in that it does reparse the file every time you call it,
1585 so you probably don't want to do this inside a loop.
1587 If C<do> can read the file but cannot compile it, it returns C<undef> and sets
1588 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1589 and sets C<$!> to the error. Always check C<$@> first, as compilation
1590 could fail in a way that also sets C<$!>. If the file is successfully
1591 compiled, C<do> returns the value of the last expression evaluated.
1593 Inclusion of library modules is better done with the
1594 C<use> and C<require> operators, which also do automatic error checking
1595 and raise an exception if there's a problem.
1597 You might like to use C<do> to read in a program configuration
1598 file. Manual error checking can be done this way:
1600 # read in config files: system first, then user
1601 for $file ("/share/prog/defaults.rc",
1602 "$ENV{HOME}/.someprogrc")
1604 unless ($return = do $file) {
1605 warn "couldn't parse $file: $@" if $@;
1606 warn "couldn't do $file: $!" unless defined $return;
1607 warn "couldn't run $file" unless $return;
1612 X<dump> X<core> X<undump>
1618 =for Pod::Functions create an immediate core dump
1620 This function causes an immediate core dump. See also the B<-u>
1621 command-line switch in L<perlrun>, which does the same thing.
1622 Primarily this is so that you can use the B<undump> program (not
1623 supplied) to turn your core dump into an executable binary after
1624 having initialized all your variables at the beginning of the
1625 program. When the new binary is executed it will begin by executing
1626 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1627 Think of it as a goto with an intervening core dump and reincarnation.
1628 If C<LABEL> is omitted, restarts the program from the top. The
1629 C<dump EXPR> form, available starting in Perl 5.18.0, allows a name to be
1630 computed at run time, being otherwise identical to C<dump LABEL>.
1632 B<WARNING>: Any files opened at the time of the dump will I<not>
1633 be open any more when the program is reincarnated, with possible
1634 resulting confusion by Perl.
1636 This function is now largely obsolete, mostly because it's very hard to
1637 convert a core file into an executable. That's why you should now invoke
1638 it as C<CORE::dump()>, if you don't want to be warned against a possible
1641 Unlike most named operators, this has the same precedence as assignment.
1642 It is also exempt from the looks-like-a-function rule, so
1643 C<dump ("foo")."bar"> will cause "bar" to be part of the argument to
1646 Portability issues: L<perlport/dump>.
1649 X<each> X<hash, iterator>
1656 =for Pod::Functions retrieve the next key/value pair from a hash
1658 When called on a hash in list context, returns a 2-element list
1659 consisting of the key and value for the next element of a hash. In Perl
1660 5.12 and later only, it will also return the index and value for the next
1661 element of an array so that you can iterate over it; older Perls consider
1662 this a syntax error. When called in scalar context, returns only the key
1663 (not the value) in a hash, or the index in an array.
1665 Hash entries are returned in an apparently random order. The actual random
1666 order is specific to a given hash; the exact same series of operations
1667 on two hashes may result in a different order for each hash. Any insertion
1668 into the hash may change the order, as will any deletion, with the exception
1669 that the most recent key returned by C<each> or C<keys> may be deleted
1670 without changing the order. So long as a given hash is unmodified you may
1671 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
1672 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
1673 details on why hash order is randomized. Aside from the guarantees
1674 provided here the exact details of Perl's hash algorithm and the hash
1675 traversal order are subject to change in any release of Perl.
1677 After C<each> has returned all entries from the hash or array, the next
1678 call to C<each> returns the empty list in list context and C<undef> in
1679 scalar context; the next call following I<that> one restarts iteration.
1680 Each hash or array has its own internal iterator, accessed by C<each>,
1681 C<keys>, and C<values>. The iterator is implicitly reset when C<each> has
1682 reached the end as just described; it can be explicitly reset by calling
1683 C<keys> or C<values> on the hash or array. If you add or delete a hash's
1684 elements while iterating over it, the effect on the iterator is
1685 unspecified; for example, entries may be skipped or duplicated--so don't
1686 do that. Exception: It is always safe to delete the item most recently
1687 returned by C<each()>, so the following code works properly:
1689 while (($key, $value) = each %hash) {
1691 delete $hash{$key}; # This is safe
1694 Tied hashes may have a different ordering behaviour to perl's hash
1697 This prints out your environment like the printenv(1) program,
1698 but in a different order:
1700 while (($key,$value) = each %ENV) {
1701 print "$key=$value\n";
1704 Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
1705 reference to an unblessed hash or array. The argument will be dereferenced
1706 automatically. This aspect of C<each> is considered highly experimental.
1707 The exact behaviour may change in a future version of Perl.
1709 while (($key,$value) = each $hashref) { ... }
1711 As of Perl 5.18 you can use a bare C<each> in a C<while> loop,
1712 which will set C<$_> on every iteration.
1715 print "$_=$ENV{$_}\n";
1718 To avoid confusing would-be users of your code who are running earlier
1719 versions of Perl with mysterious syntax errors, put this sort of thing at
1720 the top of your file to signal that your code will work I<only> on Perls of
1723 use 5.012; # so keys/values/each work on arrays
1724 use 5.014; # so keys/values/each work on scalars (experimental)
1725 use 5.018; # so each assigns to $_ in a lone while test
1727 See also C<keys>, C<values>, and C<sort>.
1729 =item eof FILEHANDLE
1738 =for Pod::Functions test a filehandle for its end
1740 Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
1741 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1742 gives the real filehandle. (Note that this function actually
1743 reads a character and then C<ungetc>s it, so isn't useful in an
1744 interactive context.) Do not read from a terminal file (or call
1745 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1746 as terminals may lose the end-of-file condition if you do.
1748 An C<eof> without an argument uses the last file read. Using C<eof()>
1749 with empty parentheses is different. It refers to the pseudo file
1750 formed from the files listed on the command line and accessed via the
1751 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1752 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1753 used will cause C<@ARGV> to be examined to determine if input is
1754 available. Similarly, an C<eof()> after C<< <> >> has returned
1755 end-of-file will assume you are processing another C<@ARGV> list,
1756 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1757 see L<perlop/"I/O Operators">.
1759 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1760 detect the end of each file, whereas C<eof()> will detect the end
1761 of the very last file only. Examples:
1763 # reset line numbering on each input file
1765 next if /^\s*#/; # skip comments
1768 close ARGV if eof; # Not eof()!
1771 # insert dashes just before last line of last file
1773 if (eof()) { # check for end of last file
1774 print "--------------\n";
1777 last if eof(); # needed if we're reading from a terminal
1780 Practical hint: you almost never need to use C<eof> in Perl, because the
1781 input operators typically return C<undef> when they run out of data or
1785 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1786 X<error, handling> X<exception, handling>
1792 =for Pod::Functions catch exceptions or compile and run code
1794 In the first form, often referred to as a "string eval", the return
1795 value of EXPR is parsed and executed as if it
1796 were a little Perl program. The value of the expression (which is itself
1797 determined within scalar context) is first parsed, and if there were no
1798 errors, executed as a block within the lexical context of the current Perl
1799 program. This means, that in particular, any outer lexical variables are
1800 visible to it, and any package variable settings or subroutine and format
1801 definitions remain afterwards.
1803 Note that the value is parsed every time the C<eval> executes.
1804 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1805 delay parsing and subsequent execution of the text of EXPR until run time.
1807 If the C<unicode_eval> feature is enabled (which is the default under a
1808 C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
1809 characters, so C<use utf8> declarations have no effect, and source filters
1810 are forbidden. In the absence of the C<unicode_eval> feature, the string
1811 will sometimes be treated as characters and sometimes as bytes, depending
1812 on the internal encoding, and source filters activated within the C<eval>
1813 exhibit the erratic, but historical, behaviour of affecting some outer file
1814 scope that is still compiling. See also the L</evalbytes> keyword, which
1815 always treats its input as a byte stream and works properly with source
1816 filters, and the L<feature> pragma.
1818 Problems can arise if the string expands a scalar containing a floating
1819 point number. That scalar can expand to letters, such as C<"NaN"> or
1820 C<"Infinity">; or, within the scope of a C<use locale>, the decimal
1821 point character may be something other than a dot (such as a comma).
1822 None of these are likely to parse as you are likely expecting.
1824 In the second form, the code within the BLOCK is parsed only once--at the
1825 same time the code surrounding the C<eval> itself was parsed--and executed
1826 within the context of the current Perl program. This form is typically
1827 used to trap exceptions more efficiently than the first (see below), while
1828 also providing the benefit of checking the code within BLOCK at compile
1831 The final semicolon, if any, may be omitted from the value of EXPR or within
1834 In both forms, the value returned is the value of the last expression
1835 evaluated inside the mini-program; a return statement may be also used, just
1836 as with subroutines. The expression providing the return value is evaluated
1837 in void, scalar, or list context, depending on the context of the C<eval>
1838 itself. See L</wantarray> for more on how the evaluation context can be
1841 If there is a syntax error or runtime error, or a C<die> statement is
1842 executed, C<eval> returns C<undef> in scalar context
1843 or an empty list in list context, and C<$@> is set to the error
1844 message. (Prior to 5.16, a bug caused C<undef> to be returned
1845 in list context for syntax errors, but not for runtime errors.)
1846 If there was no error, C<$@> is set to the empty string. A
1847 control flow operator like C<last> or C<goto> can bypass the setting of
1848 C<$@>. Beware that using C<eval> neither silences Perl from printing
1849 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1850 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1851 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1852 See L</warn>, L<perlvar>, and L<warnings>.
1854 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1855 determining whether a particular feature (such as C<socket> or C<symlink>)
1856 is implemented. It is also Perl's exception-trapping mechanism, where
1857 the die operator is used to raise exceptions.
1859 If you want to trap errors when loading an XS module, some problems with
1860 the binary interface (such as Perl version skew) may be fatal even with
1861 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1863 If the code to be executed doesn't vary, you may use the eval-BLOCK
1864 form to trap run-time errors without incurring the penalty of
1865 recompiling each time. The error, if any, is still returned in C<$@>.
1868 # make divide-by-zero nonfatal
1869 eval { $answer = $a / $b; }; warn $@ if $@;
1871 # same thing, but less efficient
1872 eval '$answer = $a / $b'; warn $@ if $@;
1874 # a compile-time error
1875 eval { $answer = }; # WRONG
1878 eval '$answer ='; # sets $@
1880 Using the C<eval{}> form as an exception trap in libraries does have some
1881 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1882 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1883 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1884 as this example shows:
1886 # a private exception trap for divide-by-zero
1887 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1890 This is especially significant, given that C<__DIE__> hooks can call
1891 C<die> again, which has the effect of changing their error messages:
1893 # __DIE__ hooks may modify error messages
1895 local $SIG{'__DIE__'} =
1896 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1897 eval { die "foo lives here" };
1898 print $@ if $@; # prints "bar lives here"
1901 Because this promotes action at a distance, this counterintuitive behavior
1902 may be fixed in a future release.
1904 With an C<eval>, you should be especially careful to remember what's
1905 being looked at when:
1911 eval { $x }; # CASE 4
1913 eval "\$$x++"; # CASE 5
1916 Cases 1 and 2 above behave identically: they run the code contained in
1917 the variable $x. (Although case 2 has misleading double quotes making
1918 the reader wonder what else might be happening (nothing is).) Cases 3
1919 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1920 does nothing but return the value of $x. (Case 4 is preferred for
1921 purely visual reasons, but it also has the advantage of compiling at
1922 compile-time instead of at run-time.) Case 5 is a place where
1923 normally you I<would> like to use double quotes, except that in this
1924 particular situation, you can just use symbolic references instead, as
1927 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1928 of localized variables, which means that for your code to run on older
1929 versions, a temporary is required if you want to mask some but not all
1932 # alter $@ on nefarious repugnancy only
1936 local $@; # protect existing $@
1937 eval { test_repugnancy() };
1938 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1939 $@ =~ /nefarious/ and $e = $@;
1941 die $e if defined $e
1944 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1945 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1947 An C<eval ''> executed within a subroutine defined
1948 in the C<DB> package doesn't see the usual
1949 surrounding lexical scope, but rather the scope of the first non-DB piece
1950 of code that called it. You don't normally need to worry about this unless
1951 you are writing a Perl debugger.
1953 =item evalbytes EXPR
1958 =for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream
1960 This function is like L</eval> with a string argument, except it always
1961 parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
1962 string containing characters whose ordinal value exceeds 255 results in an
1963 error. Source filters activated within the evaluated code apply to the
1966 This function is only available under the C<evalbytes> feature, a
1967 C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
1968 L<feature> for more information.
1973 =item exec PROGRAM LIST
1975 =for Pod::Functions abandon this program to run another
1977 The C<exec> function executes a system command I<and never returns>;
1978 use C<system> instead of C<exec> if you want it to return. It fails and
1979 returns false only if the command does not exist I<and> it is executed
1980 directly instead of via your system's command shell (see below).
1982 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1983 warns you if C<exec> is called in void context and if there is a following
1984 statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but
1985 you always do that, right?). If you I<really> want to follow an C<exec>
1986 with some other statement, you can use one of these styles to avoid the warning:
1988 exec ('foo') or print STDERR "couldn't exec foo: $!";
1989 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1991 If there is more than one argument in LIST, this calls execvp(3) with the
1992 arguments in LIST. If there is only one element in LIST, the argument is
1993 checked for shell metacharacters, and if there are any, the entire
1994 argument is passed to the system's command shell for parsing (this is
1995 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
1996 there are no shell metacharacters in the argument, it is split into words
1997 and passed directly to C<execvp>, which is more efficient. Examples:
1999 exec '/bin/echo', 'Your arguments are: ', @ARGV;
2000 exec "sort $outfile | uniq";
2002 If you don't really want to execute the first argument, but want to lie
2003 to the program you are executing about its own name, you can specify
2004 the program you actually want to run as an "indirect object" (without a
2005 comma) in front of the LIST, as in C<exec PROGRAM LIST>. (This always
2006 forces interpretation of the LIST as a multivalued list, even if there
2007 is only a single scalar in the list.) Example:
2009 $shell = '/bin/csh';
2010 exec $shell '-sh'; # pretend it's a login shell
2014 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
2016 When the arguments get executed via the system shell, results are
2017 subject to its quirks and capabilities. See L<perlop/"`STRING`">
2020 Using an indirect object with C<exec> or C<system> is also more
2021 secure. This usage (which also works fine with system()) forces
2022 interpretation of the arguments as a multivalued list, even if the
2023 list had just one argument. That way you're safe from the shell
2024 expanding wildcards or splitting up words with whitespace in them.
2026 @args = ( "echo surprise" );
2028 exec @args; # subject to shell escapes
2030 exec { $args[0] } @args; # safe even with one-arg list
2032 The first version, the one without the indirect object, ran the I<echo>
2033 program, passing it C<"surprise"> an argument. The second version didn't;
2034 it tried to run a program named I<"echo surprise">, didn't find it, and set
2035 C<$?> to a non-zero value indicating failure.
2037 On Windows, only the C<exec PROGRAM LIST> indirect object syntax will
2038 reliably avoid using the shell; C<exec LIST>, even with more than one
2039 element, will fall back to the shell if the first spawn fails.
2041 Perl attempts to flush all files opened for output before the exec,
2042 but this may not be supported on some platforms (see L<perlport>).
2043 To be safe, you may need to set C<$|> ($AUTOFLUSH in English) or
2044 call the C<autoflush()> method of C<IO::Handle> on any open handles
2045 to avoid lost output.
2047 Note that C<exec> will not call your C<END> blocks, nor will it invoke
2048 C<DESTROY> methods on your objects.
2050 Portability issues: L<perlport/exec>.
2053 X<exists> X<autovivification>
2055 =for Pod::Functions test whether a hash key is present
2057 Given an expression that specifies an element of a hash, returns true if the
2058 specified element in the hash has ever been initialized, even if the
2059 corresponding value is undefined.
2061 print "Exists\n" if exists $hash{$key};
2062 print "Defined\n" if defined $hash{$key};
2063 print "True\n" if $hash{$key};
2065 exists may also be called on array elements, but its behavior is much less
2066 obvious and is strongly tied to the use of L</delete> on arrays.
2068 B<WARNING:> Calling C<exists> on array values is strongly discouraged. The
2069 notion of deleting or checking the existence of Perl array elements is not
2070 conceptually coherent, and can lead to surprising behavior.
2072 print "Exists\n" if exists $array[$index];
2073 print "Defined\n" if defined $array[$index];
2074 print "True\n" if $array[$index];
2076 A hash or array element can be true only if it's defined and defined only if
2077 it exists, but the reverse doesn't necessarily hold true.
2079 Given an expression that specifies the name of a subroutine,
2080 returns true if the specified subroutine has ever been declared, even
2081 if it is undefined. Mentioning a subroutine name for exists or defined
2082 does not count as declaring it. Note that a subroutine that does not
2083 exist may still be callable: its package may have an C<AUTOLOAD>
2084 method that makes it spring into existence the first time that it is
2085 called; see L<perlsub>.
2087 print "Exists\n" if exists &subroutine;
2088 print "Defined\n" if defined &subroutine;
2090 Note that the EXPR can be arbitrarily complicated as long as the final
2091 operation is a hash or array key lookup or subroutine name:
2093 if (exists $ref->{A}->{B}->{$key}) { }
2094 if (exists $hash{A}{B}{$key}) { }
2096 if (exists $ref->{A}->{B}->[$ix]) { }
2097 if (exists $hash{A}{B}[$ix]) { }
2099 if (exists &{$ref->{A}{B}{$key}}) { }
2101 Although the most deeply nested array or hash element will not spring into
2102 existence just because its existence was tested, any intervening ones will.
2103 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
2104 into existence due to the existence test for the $key element above.
2105 This happens anywhere the arrow operator is used, including even here:
2108 if (exists $ref->{"Some key"}) { }
2109 print $ref; # prints HASH(0x80d3d5c)
2111 This surprising autovivification in what does not at first--or even
2112 second--glance appear to be an lvalue context may be fixed in a future
2115 Use of a subroutine call, rather than a subroutine name, as an argument
2116 to exists() is an error.
2119 exists &sub(); # Error
2122 X<exit> X<terminate> X<abort>
2126 =for Pod::Functions terminate this program
2128 Evaluates EXPR and exits immediately with that value. Example:
2131 exit 0 if $ans =~ /^[Xx]/;
2133 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
2134 universally recognized values for EXPR are C<0> for success and C<1>
2135 for error; other values are subject to interpretation depending on the
2136 environment in which the Perl program is running. For example, exiting
2137 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
2138 the mailer to return the item undelivered, but that's not true everywhere.
2140 Don't use C<exit> to abort a subroutine if there's any chance that
2141 someone might want to trap whatever error happened. Use C<die> instead,
2142 which can be trapped by an C<eval>.
2144 The exit() function does not always exit immediately. It calls any
2145 defined C<END> routines first, but these C<END> routines may not
2146 themselves abort the exit. Likewise any object destructors that need to
2147 be called are called before the real exit. C<END> routines and destructors
2148 can change the exit status by modifying C<$?>. If this is a problem, you
2149 can call C<POSIX::_exit($status)> to avoid END and destructor processing.
2150 See L<perlmod> for details.
2152 Portability issues: L<perlport/exit>.
2155 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
2159 =for Pod::Functions raise I<e> to a power
2161 Returns I<e> (the natural logarithm base) to the power of EXPR.
2162 If EXPR is omitted, gives C<exp($_)>.
2165 X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold>
2169 =for Pod::Functions +fc return casefolded version of a string
2171 Returns the casefolded version of EXPR. This is the internal function
2172 implementing the C<\F> escape in double-quoted strings.
2174 Casefolding is the process of mapping strings to a form where case
2175 differences are erased; comparing two strings in their casefolded
2176 form is effectively a way of asking if two strings are equal,
2179 Roughly, if you ever found yourself writing this
2181 lc($this) eq lc($that) # Wrong!
2183 uc($this) eq uc($that) # Also wrong!
2185 $this =~ /^\Q$that\E\z/i # Right!
2189 fc($this) eq fc($that)
2191 And get the correct results.
2193 Perl only implements the full form of casefolding,
2194 but you can access the simple folds using L<Unicode::UCD/casefold()> and
2195 L<Unicode::UCD/prop_invmap()>.
2196 For further information on casefolding, refer to
2197 the Unicode Standard, specifically sections 3.13 C<Default Case Operations>,
2198 4.2 C<Case-Normative>, and 5.18 C<Case Mappings>,
2199 available at L<http://www.unicode.org/versions/latest/>, as well as the
2200 Case Charts available at L<http://www.unicode.org/charts/case/>.
2202 If EXPR is omitted, uses C<$_>.
2204 This function behaves the same way under various pragma, such as within
2205 S<C<"use feature 'unicode_strings">>, as L</lc> does, with the single
2206 exception of C<fc> of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the
2207 scope of S<C<use locale>>. The foldcase of this character would
2208 normally be C<"ss">, but as explained in the L</lc> section, case
2209 changes that cross the 255/256 boundary are problematic under locales,
2210 and are hence prohibited. Therefore, this function under locale returns
2211 instead the string C<"\x{17F}\x{17F}">, which is the LATIN SMALL LETTER
2212 LONG S. Since that character itself folds to C<"s">, the string of two
2213 of them together should be equivalent to a single U+1E9E when foldcased.
2215 While the Unicode Standard defines two additional forms of casefolding,
2216 one for Turkic languages and one that never maps one character into multiple
2217 characters, these are not provided by the Perl core; However, the CPAN module
2218 C<Unicode::Casing> may be used to provide an implementation.
2220 This keyword is available only when the C<"fc"> feature is enabled,
2221 or when prefixed with C<CORE::>; See L<feature>. Alternately,
2222 include a C<use v5.16> or later to the current scope.
2224 =item fcntl FILEHANDLE,FUNCTION,SCALAR
2227 =for Pod::Functions file control system call
2229 Implements the fcntl(2) function. You'll probably have to say
2233 first to get the correct constant definitions. Argument processing and
2234 value returned work just like C<ioctl> below.
2238 fcntl($filehandle, F_GETFL, $packed_return_buffer)
2239 or die "can't fcntl F_GETFL: $!";
2241 You don't have to check for C<defined> on the return from C<fcntl>.
2242 Like C<ioctl>, it maps a C<0> return from the system call into
2243 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
2244 in numeric context. It is also exempt from the normal B<-w> warnings
2245 on improper numeric conversions.
2247 Note that C<fcntl> raises an exception if used on a machine that
2248 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
2249 manpage to learn what functions are available on your system.
2251 Here's an example of setting a filehandle named C<REMOTE> to be
2252 non-blocking at the system level. You'll have to negotiate C<$|>
2253 on your own, though.
2255 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2257 $flags = fcntl(REMOTE, F_GETFL, 0)
2258 or die "Can't get flags for the socket: $!\n";
2260 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2261 or die "Can't set flags for the socket: $!\n";
2263 Portability issues: L<perlport/fcntl>.
2268 =for Pod::Functions the name of the current source file
2270 A special token that returns the name of the file in which it occurs.
2272 =item fileno FILEHANDLE
2275 =for Pod::Functions return file descriptor from filehandle
2277 Returns the file descriptor for a filehandle, or undefined if the
2278 filehandle is not open. If there is no real file descriptor at the OS
2279 level, as can happen with filehandles connected to memory objects via
2280 C<open> with a reference for the third argument, -1 is returned.
2282 This is mainly useful for constructing
2283 bitmaps for C<select> and low-level POSIX tty-handling operations.
2284 If FILEHANDLE is an expression, the value is taken as an indirect
2285 filehandle, generally its name.
2287 You can use this to find out whether two handles refer to the
2288 same underlying descriptor:
2290 if (fileno(THIS) != -1 && fileno(THIS) == fileno(THAT)) {
2291 print "THIS and THAT are dups\n";
2292 } elsif (fileno(THIS) != -1 && fileno(THAT) != -1) {
2293 print "THIS and THAT have different " .
2294 "underlying file descriptors\n";
2296 print "At least one of THIS and THAT does " .
2297 "not have a real file descriptor\n";
2300 =item flock FILEHANDLE,OPERATION
2301 X<flock> X<lock> X<locking>
2303 =for Pod::Functions lock an entire file with an advisory lock
2305 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
2306 for success, false on failure. Produces a fatal error if used on a
2307 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
2308 C<flock> is Perl's portable file-locking interface, although it locks
2309 entire files only, not records.
2311 Two potentially non-obvious but traditional C<flock> semantics are
2312 that it waits indefinitely until the lock is granted, and that its locks
2313 are B<merely advisory>. Such discretionary locks are more flexible, but
2314 offer fewer guarantees. This means that programs that do not also use
2315 C<flock> may modify files locked with C<flock>. See L<perlport>,
2316 your port's specific documentation, and your system-specific local manpages
2317 for details. It's best to assume traditional behavior if you're writing
2318 portable programs. (But if you're not, you should as always feel perfectly
2319 free to write for your own system's idiosyncrasies (sometimes called
2320 "features"). Slavish adherence to portability concerns shouldn't get
2321 in the way of your getting your job done.)
2323 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
2324 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
2325 you can use the symbolic names if you import them from the L<Fcntl> module,
2326 either individually, or as a group using the C<:flock> tag. LOCK_SH
2327 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
2328 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
2329 LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
2330 waiting for the lock; check the return status to see if you got it.
2332 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
2333 before locking or unlocking it.
2335 Note that the emulation built with lockf(3) doesn't provide shared
2336 locks, and it requires that FILEHANDLE be open with write intent. These
2337 are the semantics that lockf(3) implements. Most if not all systems
2338 implement lockf(3) in terms of fcntl(2) locking, though, so the
2339 differing semantics shouldn't bite too many people.
2341 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
2342 be open with read intent to use LOCK_SH and requires that it be open
2343 with write intent to use LOCK_EX.
2345 Note also that some versions of C<flock> cannot lock things over the
2346 network; you would need to use the more system-specific C<fcntl> for
2347 that. If you like you can force Perl to ignore your system's flock(2)
2348 function, and so provide its own fcntl(2)-based emulation, by passing
2349 the switch C<-Ud_flock> to the F<Configure> program when you configure
2350 and build a new Perl.
2352 Here's a mailbox appender for BSD systems.
2354 # import LOCK_* and SEEK_END constants
2355 use Fcntl qw(:flock SEEK_END);
2359 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2361 # and, in case someone appended while we were waiting...
2362 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2367 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2370 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2371 or die "Can't open mailbox: $!";
2374 print $mbox $msg,"\n\n";
2377 On systems that support a real flock(2), locks are inherited across fork()
2378 calls, whereas those that must resort to the more capricious fcntl(2)
2379 function lose their locks, making it seriously harder to write servers.
2381 See also L<DB_File> for other flock() examples.
2383 Portability issues: L<perlport/flock>.
2386 X<fork> X<child> X<parent>
2388 =for Pod::Functions create a new process just like this one
2390 Does a fork(2) system call to create a new process running the
2391 same program at the same point. It returns the child pid to the
2392 parent process, C<0> to the child process, or C<undef> if the fork is
2393 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2394 are shared, while everything else is copied. On most systems supporting
2395 fork(), great care has gone into making it extremely efficient (for
2396 example, using copy-on-write technology on data pages), making it the
2397 dominant paradigm for multitasking over the last few decades.
2399 Perl attempts to flush all files opened for
2400 output before forking the child process, but this may not be supported
2401 on some platforms (see L<perlport>). To be safe, you may need to set
2402 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2403 C<IO::Handle> on any open handles to avoid duplicate output.
2405 If you C<fork> without ever waiting on your children, you will
2406 accumulate zombies. On some systems, you can avoid this by setting
2407 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2408 forking and reaping moribund children.
2410 Note that if your forked child inherits system file descriptors like
2411 STDIN and STDOUT that are actually connected by a pipe or socket, even
2412 if you exit, then the remote server (such as, say, a CGI script or a
2413 backgrounded job launched from a remote shell) won't think you're done.
2414 You should reopen those to F</dev/null> if it's any issue.
2416 On some platforms such as Windows, where the fork() system call is not available,
2417 Perl can be built to emulate fork() in the Perl interpreter.
2418 The emulation is designed, at the level of the Perl program,
2419 to be as compatible as possible with the "Unix" fork().
2420 However it has limitations that have to be considered in code intended to be portable.
2421 See L<perlfork> for more details.
2423 Portability issues: L<perlport/fork>.
2428 =for Pod::Functions declare a picture format with use by the write() function
2430 Declare a picture format for use by the C<write> function. For
2434 Test: @<<<<<<<< @||||| @>>>>>
2435 $str, $%, '$' . int($num)
2439 $num = $cost/$quantity;
2443 See L<perlform> for many details and examples.
2445 =item formline PICTURE,LIST
2448 =for Pod::Functions internal function used for formats
2450 This is an internal function used by C<format>s, though you may call it,
2451 too. It formats (see L<perlform>) a list of values according to the
2452 contents of PICTURE, placing the output into the format output
2453 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2454 Eventually, when a C<write> is done, the contents of
2455 C<$^A> are written to some filehandle. You could also read C<$^A>
2456 and then set C<$^A> back to C<"">. Note that a format typically
2457 does one C<formline> per line of form, but the C<formline> function itself
2458 doesn't care how many newlines are embedded in the PICTURE. This means
2459 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2460 You may therefore need to use multiple formlines to implement a single
2461 record format, just like the C<format> compiler.
2463 Be careful if you put double quotes around the picture, because an C<@>
2464 character may be taken to mean the beginning of an array name.
2465 C<formline> always returns true. See L<perlform> for other examples.
2467 If you are trying to use this instead of C<write> to capture the output,
2468 you may find it easier to open a filehandle to a scalar
2469 (C<< open $fh, ">", \$output >>) and write to that instead.
2471 =item getc FILEHANDLE
2472 X<getc> X<getchar> X<character> X<file, read>
2476 =for Pod::Functions get the next character from the filehandle
2478 Returns the next character from the input file attached to FILEHANDLE,
2479 or the undefined value at end of file or if there was an error (in
2480 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2481 STDIN. This is not particularly efficient. However, it cannot be
2482 used by itself to fetch single characters without waiting for the user
2483 to hit enter. For that, try something more like:
2486 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2489 system "stty", '-icanon', 'eol', "\001";
2495 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2498 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2502 Determination of whether $BSD_STYLE should be set
2503 is left as an exercise to the reader.
2505 The C<POSIX::getattr> function can do this more portably on
2506 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2507 module from your nearest L<CPAN|http://www.cpan.org> site.
2510 X<getlogin> X<login>
2512 =for Pod::Functions return who logged in at this tty
2514 This implements the C library function of the same name, which on most
2515 systems returns the current login from F</etc/utmp>, if any. If it
2516 returns the empty string, use C<getpwuid>.
2518 $login = getlogin || getpwuid($<) || "Kilroy";
2520 Do not consider C<getlogin> for authentication: it is not as
2521 secure as C<getpwuid>.
2523 Portability issues: L<perlport/getlogin>.
2525 =item getpeername SOCKET
2526 X<getpeername> X<peer>
2528 =for Pod::Functions find the other end of a socket connection
2530 Returns the packed sockaddr address of the other end of the SOCKET
2534 $hersockaddr = getpeername(SOCK);
2535 ($port, $iaddr) = sockaddr_in($hersockaddr);
2536 $herhostname = gethostbyaddr($iaddr, AF_INET);
2537 $herstraddr = inet_ntoa($iaddr);
2542 =for Pod::Functions get process group
2544 Returns the current process group for the specified PID. Use
2545 a PID of C<0> to get the current process group for the
2546 current process. Will raise an exception if used on a machine that
2547 doesn't implement getpgrp(2). If PID is omitted, returns the process
2548 group of the current process. Note that the POSIX version of C<getpgrp>
2549 does not accept a PID argument, so only C<PID==0> is truly portable.
2551 Portability issues: L<perlport/getpgrp>.
2554 X<getppid> X<parent> X<pid>
2556 =for Pod::Functions get parent process ID
2558 Returns the process id of the parent process.
2560 Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work
2561 around non-POSIX thread semantics the minority of Linux systems (and
2562 Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation
2563 has since been removed. See the documentation for L<$$|perlvar/$$> for
2566 Portability issues: L<perlport/getppid>.
2568 =item getpriority WHICH,WHO
2569 X<getpriority> X<priority> X<nice>
2571 =for Pod::Functions get current nice value
2573 Returns the current priority for a process, a process group, or a user.
2574 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
2575 machine that doesn't implement getpriority(2).
2577 Portability issues: L<perlport/getpriority>.
2580 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2581 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2582 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2583 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2584 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2585 X<endnetent> X<endprotoent> X<endservent>
2587 =for Pod::Functions get passwd record given user login name
2591 =for Pod::Functions get group record given group name
2593 =item gethostbyname NAME
2595 =for Pod::Functions get host record given name
2597 =item getnetbyname NAME
2599 =for Pod::Functions get networks record given name
2601 =item getprotobyname NAME
2603 =for Pod::Functions get protocol record given name
2607 =for Pod::Functions get passwd record given user ID
2611 =for Pod::Functions get group record given group user ID
2613 =item getservbyname NAME,PROTO
2615 =for Pod::Functions get services record given its name
2617 =item gethostbyaddr ADDR,ADDRTYPE
2619 =for Pod::Functions get host record given its address
2621 =item getnetbyaddr ADDR,ADDRTYPE
2623 =for Pod::Functions get network record given its address
2625 =item getprotobynumber NUMBER
2627 =for Pod::Functions get protocol record numeric protocol
2629 =item getservbyport PORT,PROTO
2631 =for Pod::Functions get services record given numeric port
2635 =for Pod::Functions get next passwd record
2639 =for Pod::Functions get next group record
2643 =for Pod::Functions get next hosts record
2647 =for Pod::Functions get next networks record
2651 =for Pod::Functions get next protocols record
2655 =for Pod::Functions get next services record
2659 =for Pod::Functions prepare passwd file for use
2663 =for Pod::Functions prepare group file for use
2665 =item sethostent STAYOPEN
2667 =for Pod::Functions prepare hosts file for use
2669 =item setnetent STAYOPEN
2671 =for Pod::Functions prepare networks file for use
2673 =item setprotoent STAYOPEN
2675 =for Pod::Functions prepare protocols file for use
2677 =item setservent STAYOPEN
2679 =for Pod::Functions prepare services file for use
2683 =for Pod::Functions be done using passwd file
2687 =for Pod::Functions be done using group file
2691 =for Pod::Functions be done using hosts file
2695 =for Pod::Functions be done using networks file
2699 =for Pod::Functions be done using protocols file
2703 =for Pod::Functions be done using services file
2705 These routines are the same as their counterparts in the
2706 system C library. In list context, the return values from the
2707 various get routines are as follows:
2710 ( $name, $passwd, $gid, $members ) = getgr*
2711 ( $name, $aliases, $addrtype, $net ) = getnet*
2712 ( $name, $aliases, $port, $proto ) = getserv*
2713 ( $name, $aliases, $proto ) = getproto*
2714 ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
2715 ( $name, $passwd, $uid, $gid, $quota,
2716 $comment, $gcos, $dir, $shell, $expire ) = getpw*
2719 (If the entry doesn't exist you get an empty list.)
2721 The exact meaning of the $gcos field varies but it usually contains
2722 the real name of the user (as opposed to the login name) and other
2723 information pertaining to the user. Beware, however, that in many
2724 system users are able to change this information and therefore it
2725 cannot be trusted and therefore the $gcos is tainted (see
2726 L<perlsec>). The $passwd and $shell, user's encrypted password and
2727 login shell, are also tainted, for the same reason.
2729 In scalar context, you get the name, unless the function was a
2730 lookup by name, in which case you get the other thing, whatever it is.
2731 (If the entry doesn't exist you get the undefined value.) For example:
2733 $uid = getpwnam($name);
2734 $name = getpwuid($num);
2736 $gid = getgrnam($name);
2737 $name = getgrgid($num);
2741 In I<getpw*()> the fields $quota, $comment, and $expire are special
2742 in that they are unsupported on many systems. If the
2743 $quota is unsupported, it is an empty scalar. If it is supported, it
2744 usually encodes the disk quota. If the $comment field is unsupported,
2745 it is an empty scalar. If it is supported it usually encodes some
2746 administrative comment about the user. In some systems the $quota
2747 field may be $change or $age, fields that have to do with password
2748 aging. In some systems the $comment field may be $class. The $expire
2749 field, if present, encodes the expiration period of the account or the
2750 password. For the availability and the exact meaning of these fields
2751 in your system, please consult getpwnam(3) and your system's
2752 F<pwd.h> file. You can also find out from within Perl what your
2753 $quota and $comment fields mean and whether you have the $expire field
2754 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2755 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2756 files are supported only if your vendor has implemented them in the
2757 intuitive fashion that calling the regular C library routines gets the
2758 shadow versions if you're running under privilege or if there exists
2759 the shadow(3) functions as found in System V (this includes Solaris
2760 and Linux). Those systems that implement a proprietary shadow password
2761 facility are unlikely to be supported.
2763 The $members value returned by I<getgr*()> is a space-separated list of
2764 the login names of the members of the group.
2766 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2767 C, it will be returned to you via C<$?> if the function call fails. The
2768 C<@addrs> value returned by a successful call is a list of raw
2769 addresses returned by the corresponding library call. In the
2770 Internet domain, each address is four bytes long; you can unpack it
2771 by saying something like:
2773 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2775 The Socket library makes this slightly easier:
2778 $iaddr = inet_aton("127.1"); # or whatever address
2779 $name = gethostbyaddr($iaddr, AF_INET);
2781 # or going the other way
2782 $straddr = inet_ntoa($iaddr);
2784 In the opposite way, to resolve a hostname to the IP address
2788 $packed_ip = gethostbyname("www.perl.org");
2789 if (defined $packed_ip) {
2790 $ip_address = inet_ntoa($packed_ip);
2793 Make sure C<gethostbyname()> is called in SCALAR context and that
2794 its return value is checked for definedness.
2796 The C<getprotobynumber> function, even though it only takes one argument,
2797 has the precedence of a list operator, so beware:
2799 getprotobynumber $number eq 'icmp' # WRONG
2800 getprotobynumber($number eq 'icmp') # actually means this
2801 getprotobynumber($number) eq 'icmp' # better this way
2803 If you get tired of remembering which element of the return list
2804 contains which return value, by-name interfaces are provided
2805 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2806 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2807 and C<User::grent>. These override the normal built-ins, supplying
2808 versions that return objects with the appropriate names
2809 for each field. For example:
2813 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2815 Even though it looks as though they're the same method calls (uid),
2816 they aren't, because a C<File::stat> object is different from
2817 a C<User::pwent> object.
2819 Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
2821 =item getsockname SOCKET
2824 =for Pod::Functions retrieve the sockaddr for a given socket
2826 Returns the packed sockaddr address of this end of the SOCKET connection,
2827 in case you don't know the address because you have several different
2828 IPs that the connection might have come in on.
2831 $mysockaddr = getsockname(SOCK);
2832 ($port, $myaddr) = sockaddr_in($mysockaddr);
2833 printf "Connect to %s [%s]\n",
2834 scalar gethostbyaddr($myaddr, AF_INET),
2837 =item getsockopt SOCKET,LEVEL,OPTNAME
2840 =for Pod::Functions get socket options on a given socket
2842 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2843 Options may exist at multiple protocol levels depending on the socket
2844 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2845 C<Socket> module) will exist. To query options at another level the
2846 protocol number of the appropriate protocol controlling the option
2847 should be supplied. For example, to indicate that an option is to be
2848 interpreted by the TCP protocol, LEVEL should be set to the protocol
2849 number of TCP, which you can get using C<getprotobyname>.
2851 The function returns a packed string representing the requested socket
2852 option, or C<undef> on error, with the reason for the error placed in
2853 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2854 consult getsockopt(2) for details. A common case is that the option is an
2855 integer, in which case the result is a packed integer, which you can decode
2856 using C<unpack> with the C<i> (or C<I>) format.
2858 Here's an example to test whether Nagle's algorithm is enabled on a socket:
2860 use Socket qw(:all);
2862 defined(my $tcp = getprotobyname("tcp"))
2863 or die "Could not determine the protocol number for tcp";
2864 # my $tcp = IPPROTO_TCP; # Alternative
2865 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2866 or die "getsockopt TCP_NODELAY: $!";
2867 my $nodelay = unpack("I", $packed);
2868 print "Nagle's algorithm is turned ",
2869 $nodelay ? "off\n" : "on\n";
2871 Portability issues: L<perlport/getsockopt>.
2874 X<glob> X<wildcard> X<filename, expansion> X<expand>
2878 =for Pod::Functions expand filenames using wildcards
2880 In list context, returns a (possibly empty) list of filename expansions on
2881 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2882 scalar context, glob iterates through such filename expansions, returning
2883 undef when the list is exhausted. This is the internal function
2884 implementing the C<< <*.c> >> operator, but you can use it directly. If
2885 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2886 more detail in L<perlop/"I/O Operators">.
2888 Note that C<glob> splits its arguments on whitespace and treats
2889 each segment as separate pattern. As such, C<glob("*.c *.h")>
2890 matches all files with a F<.c> or F<.h> extension. The expression
2891 C<glob(".* *")> matches all files in the current working directory.
2892 If you want to glob filenames that might contain whitespace, you'll
2893 have to use extra quotes around the spacey filename to protect it.
2894 For example, to glob filenames that have an C<e> followed by a space
2895 followed by an C<f>, use either of:
2897 @spacies = <"*e f*">;
2898 @spacies = glob '"*e f*"';
2899 @spacies = glob q("*e f*");
2901 If you had to get a variable through, you could do this:
2903 @spacies = glob "'*${var}e f*'";
2904 @spacies = glob qq("*${var}e f*");
2906 If non-empty braces are the only wildcard characters used in the
2907 C<glob>, no filenames are matched, but potentially many strings
2908 are returned. For example, this produces nine strings, one for
2909 each pairing of fruits and colors:
2911 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2913 This operator is implemented using the standard
2914 C<File::Glob> extension. See L<File::Glob> for details, including
2915 C<bsd_glob> which does not treat whitespace as a pattern separator.
2917 Portability issues: L<perlport/glob>.
2920 X<gmtime> X<UTC> X<Greenwich>
2924 =for Pod::Functions convert UNIX time into record or string using Greenwich time
2926 Works just like L</localtime> but the returned values are
2927 localized for the standard Greenwich time zone.
2929 Note: When called in list context, $isdst, the last value
2930 returned by gmtime, is always C<0>. There is no
2931 Daylight Saving Time in GMT.
2933 Portability issues: L<perlport/gmtime>.
2936 X<goto> X<jump> X<jmp>
2942 =for Pod::Functions create spaghetti code
2944 The C<goto LABEL> form finds the statement labeled with LABEL and
2945 resumes execution there. It can't be used to get out of a block or
2946 subroutine given to C<sort>. It can be used to go almost anywhere
2947 else within the dynamic scope, including out of subroutines, but it's
2948 usually better to use some other construct such as C<last> or C<die>.
2949 The author of Perl has never felt the need to use this form of C<goto>
2950 (in Perl, that is; C is another matter). (The difference is that C
2951 does not offer named loops combined with loop control. Perl does, and
2952 this replaces most structured uses of C<goto> in other languages.)
2954 The C<goto EXPR> form expects to evaluate C<EXPR> to a code reference or
2955 a label name. If it evaluates to a code reference, it will be handled
2956 like C<goto &NAME>, below. This is especially useful for implementing
2957 tail recursion via C<goto __SUB__>.
2959 If the expression evaluates to a label name, its scope will be resolved
2960 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2961 necessarily recommended if you're optimizing for maintainability:
2963 goto ("FOO", "BAR", "GLARCH")[$i];
2965 As shown in this example, C<goto EXPR> is exempt from the "looks like a
2966 function" rule. A pair of parentheses following it does not (necessarily)
2967 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2968 Also, unlike most named operators, this has the same precedence as
2971 Use of C<goto LABEL> or C<goto EXPR> to jump into a construct is
2972 deprecated and will issue a warning. Even then, it may not be used to
2973 go into any construct that requires initialization, such as a
2974 subroutine or a C<foreach> loop. It also can't be used to go into a
2975 construct that is optimized away.
2977 The C<goto &NAME> form is quite different from the other forms of
2978 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2979 doesn't have the stigma associated with other gotos. Instead, it
2980 exits the current subroutine (losing any changes set by local()) and
2981 immediately calls in its place the named subroutine using the current
2982 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2983 load another subroutine and then pretend that the other subroutine had
2984 been called in the first place (except that any modifications to C<@_>
2985 in the current subroutine are propagated to the other subroutine.)
2986 After the C<goto>, not even C<caller> will be able to tell that this
2987 routine was called first.
2989 NAME needn't be the name of a subroutine; it can be a scalar variable
2990 containing a code reference or a block that evaluates to a code
2993 =item grep BLOCK LIST
2996 =item grep EXPR,LIST
2998 =for Pod::Functions locate elements in a list test true against a given criterion
3000 This is similar in spirit to, but not the same as, grep(1) and its
3001 relatives. In particular, it is not limited to using regular expressions.
3003 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3004 C<$_> to each element) and returns the list value consisting of those
3005 elements for which the expression evaluated to true. In scalar
3006 context, returns the number of times the expression was true.
3008 @foo = grep(!/^#/, @bar); # weed out comments
3012 @foo = grep {!/^#/} @bar; # weed out comments
3014 Note that C<$_> is an alias to the list value, so it can be used to
3015 modify the elements of the LIST. While this is useful and supported,
3016 it can cause bizarre results if the elements of LIST are not variables.
3017 Similarly, grep returns aliases into the original list, much as a for
3018 loop's index variable aliases the list elements. That is, modifying an
3019 element of a list returned by grep (for example, in a C<foreach>, C<map>
3020 or another C<grep>) actually modifies the element in the original list.
3021 This is usually something to be avoided when writing clear code.
3023 If C<$_> is lexical in the scope where the C<grep> appears (because it has
3024 been declared with the deprecated C<my $_> construct)
3025 then, in addition to being locally aliased to
3026 the list elements, C<$_> keeps being lexical inside the block; i.e., it
3027 can't be seen from the outside, avoiding any potential side-effects.
3029 See also L</map> for a list composed of the results of the BLOCK or EXPR.
3032 X<hex> X<hexadecimal>
3036 =for Pod::Functions convert a string to a hexadecimal number
3038 Interprets EXPR as a hex string and returns the corresponding value.
3039 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
3040 L</oct>.) If EXPR is omitted, uses C<$_>.
3042 print hex '0xAf'; # prints '175'
3043 print hex 'aF'; # same
3045 Hex strings may only represent integers. Strings that would cause
3046 integer overflow trigger a warning. Leading whitespace is not stripped,
3047 unlike oct(). To present something as hex, look into L</printf>,
3048 L</sprintf>, and L</unpack>.
3053 =for Pod::Functions patch a module's namespace into your own
3055 There is no builtin C<import> function. It is just an ordinary
3056 method (subroutine) defined (or inherited) by modules that wish to export
3057 names to another module. The C<use> function calls the C<import> method
3058 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
3060 =item index STR,SUBSTR,POSITION
3061 X<index> X<indexOf> X<InStr>
3063 =item index STR,SUBSTR
3065 =for Pod::Functions find a substring within a string
3067 The index function searches for one string within another, but without
3068 the wildcard-like behavior of a full regular-expression pattern match.
3069 It returns the position of the first occurrence of SUBSTR in STR at
3070 or after POSITION. If POSITION is omitted, starts searching from the
3071 beginning of the string. POSITION before the beginning of the string
3072 or after its end is treated as if it were the beginning or the end,
3073 respectively. POSITION and the return value are based at zero.
3074 If the substring is not found, C<index> returns -1.
3077 X<int> X<integer> X<truncate> X<trunc> X<floor>
3081 =for Pod::Functions get the integer portion of a number
3083 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
3084 You should not use this function for rounding: one because it truncates
3085 towards C<0>, and two because machine representations of floating-point
3086 numbers can sometimes produce counterintuitive results. For example,
3087 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
3088 because it's really more like -268.99999999999994315658 instead. Usually,
3089 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
3090 functions will serve you better than will int().
3092 =item ioctl FILEHANDLE,FUNCTION,SCALAR
3095 =for Pod::Functions system-dependent device control system call
3097 Implements the ioctl(2) function. You'll probably first have to say
3099 require "sys/ioctl.ph"; # probably in
3100 # $Config{archlib}/sys/ioctl.ph
3102 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
3103 exist or doesn't have the correct definitions you'll have to roll your
3104 own, based on your C header files such as F<< <sys/ioctl.h> >>.
3105 (There is a Perl script called B<h2ph> that comes with the Perl kit that
3106 may help you in this, but it's nontrivial.) SCALAR will be read and/or
3107 written depending on the FUNCTION; a C pointer to the string value of SCALAR
3108 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
3109 has no string value but does have a numeric value, that value will be
3110 passed rather than a pointer to the string value. To guarantee this to be
3111 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
3112 functions may be needed to manipulate the values of structures used by
3115 The return value of C<ioctl> (and C<fcntl>) is as follows:
3117 if OS returns: then Perl returns:
3119 0 string "0 but true"
3120 anything else that number
3122 Thus Perl returns true on success and false on failure, yet you can
3123 still easily determine the actual value returned by the operating
3126 $retval = ioctl(...) || -1;
3127 printf "System returned %d\n", $retval;
3129 The special string C<"0 but true"> is exempt from B<-w> complaints
3130 about improper numeric conversions.
3132 Portability issues: L<perlport/ioctl>.
3134 =item join EXPR,LIST
3137 =for Pod::Functions join a list into a string using a separator
3139 Joins the separate strings of LIST into a single string with fields
3140 separated by the value of EXPR, and returns that new string. Example:
3142 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
3144 Beware that unlike C<split>, C<join> doesn't take a pattern as its
3145 first argument. Compare L</split>.
3154 =for Pod::Functions retrieve list of indices from a hash
3156 Called in list context, returns a list consisting of all the keys of the
3157 named hash, or in Perl 5.12 or later only, the indices of an array. Perl
3158 releases prior to 5.12 will produce a syntax error if you try to use an
3159 array argument. In scalar context, returns the number of keys or indices.
3161 Hash entries are returned in an apparently random order. The actual random
3162 order is specific to a given hash; the exact same series of operations
3163 on two hashes may result in a different order for each hash. Any insertion
3164 into the hash may change the order, as will any deletion, with the exception
3165 that the most recent key returned by C<each> or C<keys> may be deleted
3166 without changing the order. So long as a given hash is unmodified you may
3167 rely on C<keys>, C<values> and C<each> to repeatedly return the same order
3168 as each other. See L<perlsec/"Algorithmic Complexity Attacks"> for
3169 details on why hash order is randomized. Aside from the guarantees
3170 provided here the exact details of Perl's hash algorithm and the hash
3171 traversal order are subject to change in any release of Perl. Tied hashes
3172 may behave differently to Perl's hashes with respect to changes in order on
3173 insertion and deletion of items.
3175 As a side effect, calling keys() resets the internal iterator of the HASH or
3176 ARRAY (see L</each>). In particular, calling keys() in void context resets
3177 the iterator with no other overhead.
3179 Here is yet another way to print your environment:
3182 @values = values %ENV;
3184 print pop(@keys), '=', pop(@values), "\n";
3187 or how about sorted by key:
3189 foreach $key (sort(keys %ENV)) {
3190 print $key, '=', $ENV{$key}, "\n";
3193 The returned values are copies of the original keys in the hash, so
3194 modifying them will not affect the original hash. Compare L</values>.
3196 To sort a hash by value, you'll need to use a C<sort> function.
3197 Here's a descending numeric sort of a hash by its values:
3199 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
3200 printf "%4d %s\n", $hash{$key}, $key;
3203 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
3204 allocated for the given hash. This can gain you a measure of efficiency if
3205 you know the hash is going to get big. (This is similar to pre-extending
3206 an array by assigning a larger number to $#array.) If you say
3210 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
3211 in fact, since it rounds up to the next power of two. These
3212 buckets will be retained even if you do C<%hash = ()>, use C<undef
3213 %hash> if you want to free the storage while C<%hash> is still in scope.
3214 You can't shrink the number of buckets allocated for the hash using
3215 C<keys> in this way (but you needn't worry about doing this by accident,
3216 as trying has no effect). C<keys @array> in an lvalue context is a syntax
3219 Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
3220 a reference to an unblessed hash or array. The argument will be
3221 dereferenced automatically. This aspect of C<keys> is considered highly
3222 experimental. The exact behaviour may change in a future version of Perl.
3224 for (keys $hashref) { ... }
3225 for (keys $obj->get_arrayref) { ... }
3227 To avoid confusing would-be users of your code who are running earlier
3228 versions of Perl with mysterious syntax errors, put this sort of thing at
3229 the top of your file to signal that your code will work I<only> on Perls of
3232 use 5.012; # so keys/values/each work on arrays
3233 use 5.014; # so keys/values/each work on scalars (experimental)
3235 See also C<each>, C<values>, and C<sort>.
3237 =item kill SIGNAL, LIST
3242 =for Pod::Functions send a signal to a process or process group
3244 Sends a signal to a list of processes. Returns the number of arguments
3245 that were successfully used to signal (which is not necessarily the same
3246 as the number of processes actually killed, e.g. where a process group is
3249 $cnt = kill 'HUP', $child1, $child2;
3250 kill 'KILL', @goners;
3252 SIGNAL may be either a signal name (a string) or a signal number. A signal
3253 name may start with a C<SIG> prefix, thus C<FOO> and C<SIGFOO> refer to the
3254 same signal. The string form of SIGNAL is recommended for portability because
3255 the same signal may have different numbers in different operating systems.
3257 A list of signal names supported by the current platform can be found in
3258 C<$Config{sig_name}>, which is provided by the C<Config> module. See L<Config>
3261 A negative signal name is the same as a negative signal number, killing process
3262 groups instead of processes. For example, C<kill '-KILL', $pgrp> and
3263 C<kill -9, $pgrp> will send C<SIGKILL> to
3264 the entire process group specified. That
3265 means you usually want to use positive not negative signals.
3267 If SIGNAL is either the number 0 or the string C<ZERO> (or C<SIGZERO>),
3268 no signal is sent to
3269 the process, but C<kill> checks whether it's I<possible> to send a signal to it
3270 (that means, to be brief, that the process is owned by the same user, or we are
3271 the super-user). This is useful to check that a child process is still
3272 alive (even if only as a zombie) and hasn't changed its UID. See
3273 L<perlport> for notes on the portability of this construct.
3275 The behavior of kill when a I<PROCESS> number is zero or negative depends on
3276 the operating system. For example, on POSIX-conforming systems, zero will
3277 signal the current process group, -1 will signal all processes, and any
3278 other negative PROCESS number will act as a negative signal number and
3279 kill the entire process group specified.
3281 If both the SIGNAL and the PROCESS are negative, the results are undefined.
3282 A warning may be produced in a future version.
3284 See L<perlipc/"Signals"> for more details.
3286 On some platforms such as Windows where the fork() system call is not
3287 available, Perl can be built to emulate fork() at the interpreter level.
3288 This emulation has limitations related to kill that have to be considered,
3289 for code running on Windows and in code intended to be portable.
3291 See L<perlfork> for more details.
3293 If there is no I<LIST> of processes, no signal is sent, and the return
3294 value is 0. This form is sometimes used, however, because it causes
3295 tainting checks to be run. But see
3296 L<perlsec/Laundering and Detecting Tainted Data>.
3298 Portability issues: L<perlport/kill>.
3307 =for Pod::Functions exit a block prematurely
3309 The C<last> command is like the C<break> statement in C (as used in
3310 loops); it immediately exits the loop in question. If the LABEL is
3311 omitted, the command refers to the innermost enclosing
3312 loop. The C<last EXPR> form, available starting in Perl
3313 5.18.0, allows a label name to be computed at run time,
3314 and is otherwise identical to C<last LABEL>. The
3315 C<continue> block, if any, is not executed:
3317 LINE: while (<STDIN>) {
3318 last LINE if /^$/; # exit when done with header
3322 C<last> cannot be used to exit a block that returns a value such as
3323 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3324 a grep() or map() operation.
3326 Note that a block by itself is semantically identical to a loop
3327 that executes once. Thus C<last> can be used to effect an early
3328 exit out of such a block.
3330 See also L</continue> for an illustration of how C<last>, C<next>, and
3333 Unlike most named operators, this has the same precedence as assignment.
3334 It is also exempt from the looks-like-a-function rule, so
3335 C<last ("foo")."bar"> will cause "bar" to be part of the argument to
3343 =for Pod::Functions return lower-case version of a string
3345 Returns a lowercased version of EXPR. This is the internal function
3346 implementing the C<\L> escape in double-quoted strings.
3348 If EXPR is omitted, uses C<$_>.
3350 What gets returned depends on several factors:
3354 =item If C<use bytes> is in effect:
3356 The results follow ASCII rules. Only the characters C<A-Z> change,
3357 to C<a-z> respectively.
3359 =item Otherwise, if C<use locale> for C<LC_CTYPE> is in effect:
3361 Respects current C<LC_CTYPE> locale for code points < 256; and uses Unicode
3362 rules for the remaining code points (this last can only happen if
3363 the UTF8 flag is also set). See L<perllocale>.
3365 Starting in v5.20, Perl wil use full Unicode rules if the locale is
3366 UTF-8. Otherwise, there is a deficiency in this scheme, which is that
3367 case changes that cross the 255/256
3368 boundary are not well-defined. For example, the lower case of LATIN CAPITAL
3369 LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
3370 platforms). But under C<use locale> (prior to v5.20 or not a UTF-8
3371 locale), the lower case of U+1E9E is
3372 itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the
3373 current locale, and Perl has no way of knowing if that character even
3374 exists in the locale, much less what code point it is. Perl returns
3375 a result that is above 255 (almost always the input character unchanged,
3376 for all instances (and there aren't many) where the 255/256 boundary
3377 would otherwise be crossed; and starting in v5.22, it raises a
3378 L<locale|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".> warning.
3380 =item Otherwise, If EXPR has the UTF8 flag set:
3382 Unicode rules are used for the case change.
3384 =item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'> is in effect:
3386 Unicode rules are used for the case change.
3390 ASCII rules are used for the case change. The lowercase of any character
3391 outside the ASCII range is the character itself.
3396 X<lcfirst> X<lowercase>
3400 =for Pod::Functions return a string with just the next letter in lower case
3402 Returns the value of EXPR with the first character lowercased. This
3403 is the internal function implementing the C<\l> escape in
3404 double-quoted strings.
3406 If EXPR is omitted, uses C<$_>.
3408 This function behaves the same way under various pragmata, such as in a locale,
3416 =for Pod::Functions return the number of characters in a string
3418 Returns the length in I<characters> of the value of EXPR. If EXPR is
3419 omitted, returns the length of C<$_>. If EXPR is undefined, returns
3422 This function cannot be used on an entire array or hash to find out how
3423 many elements these have. For that, use C<scalar @array> and C<scalar keys
3424 %hash>, respectively.
3426 Like all Perl character operations, length() normally deals in logical
3427 characters, not physical bytes. For how many bytes a string encoded as
3428 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
3429 to C<use Encode> first). See L<Encode> and L<perlunicode>.
3434 =for Pod::Functions the current source line number
3436 A special token that compiles to the current line number.
3438 =item link OLDFILE,NEWFILE
3441 =for Pod::Functions create a hard link in the filesystem
3443 Creates a new filename linked to the old filename. Returns true for
3444 success, false otherwise.
3446 Portability issues: L<perlport/link>.
3448 =item listen SOCKET,QUEUESIZE
3451 =for Pod::Functions register your socket as a server
3453 Does the same thing that the listen(2) system call does. Returns true if
3454 it succeeded, false otherwise. See the example in
3455 L<perlipc/"Sockets: Client/Server Communication">.
3460 =for Pod::Functions create a temporary value for a global variable (dynamic scoping)
3462 You really probably want to be using C<my> instead, because C<local> isn't
3463 what most people think of as "local". See
3464 L<perlsub/"Private Variables via my()"> for details.
3466 A local modifies the listed variables to be local to the enclosing
3467 block, file, or eval. If more than one value is listed, the list must
3468 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
3469 for details, including issues with tied arrays and hashes.
3471 The C<delete local EXPR> construct can also be used to localize the deletion
3472 of array/hash elements to the current block.
3473 See L<perlsub/"Localized deletion of elements of composite types">.
3475 =item localtime EXPR
3476 X<localtime> X<ctime>
3480 =for Pod::Functions convert UNIX time into record or string using local time
3482 Converts a time as returned by the time function to a 9-element list
3483 with the time analyzed for the local time zone. Typically used as
3487 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
3490 All list elements are numeric and come straight out of the C `struct
3491 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
3492 of the specified time.
3494 C<$mday> is the day of the month and C<$mon> the month in
3495 the range C<0..11>, with 0 indicating January and 11 indicating December.
3496 This makes it easy to get a month name from a list:
3498 my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
3499 print "$abbr[$mon] $mday";
3500 # $mon=9, $mday=18 gives "Oct 18"
3502 C<$year> contains the number of years since 1900. To get a 4-digit
3507 To get the last two digits of the year (e.g., "01" in 2001) do:
3509 $year = sprintf("%02d", $year % 100);
3511 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
3512 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
3513 (or C<0..365> in leap years.)
3515 C<$isdst> is true if the specified time occurs during Daylight Saving
3516 Time, false otherwise.
3518 If EXPR is omitted, C<localtime()> uses the current time (as returned
3521 In scalar context, C<localtime()> returns the ctime(3) value:
3523 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
3525 The format of this scalar value is B<not> locale-dependent
3526 but built into Perl. For GMT instead of local
3527 time use the L</gmtime> builtin. See also the
3528 C<Time::Local> module (for converting seconds, minutes, hours, and such back to
3529 the integer value returned by time()), and the L<POSIX> module's strftime(3)
3530 and mktime(3) functions.
3532 To get somewhat similar but locale-dependent date strings, set up your
3533 locale environment variables appropriately (please see L<perllocale>) and
3536 use POSIX qw(strftime);
3537 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
3538 # or for GMT formatted appropriately for your locale:
3539 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
3541 Note that the C<%a> and C<%b>, the short forms of the day of the week
3542 and the month of the year, may not necessarily be three characters wide.
3544 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
3545 by-name access mechanism to the gmtime() and localtime() functions,
3548 For a comprehensive date and time representation look at the
3549 L<DateTime> module on CPAN.
3551 Portability issues: L<perlport/localtime>.
3556 =for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method
3558 This function places an advisory lock on a shared variable or referenced
3559 object contained in I<THING> until the lock goes out of scope.
3561 The value returned is the scalar itself, if the argument is a scalar, or a
3562 reference, if the argument is a hash, array or subroutine.
3564 lock() is a "weak keyword" : this means that if you've defined a function
3565 by this name (before any calls to it), that function will be called
3566 instead. If you are not under C<use threads::shared> this does nothing.
3567 See L<threads::shared>.
3570 X<log> X<logarithm> X<e> X<ln> X<base>
3574 =for Pod::Functions retrieve the natural logarithm for a number
3576 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
3577 returns the log of C<$_>. To get the
3578 log of another base, use basic algebra:
3579 The base-N log of a number is equal to the natural log of that number
3580 divided by the natural log of N. For example:
3584 return log($n)/log(10);
3587 See also L</exp> for the inverse operation.
3589 =item lstat FILEHANDLE
3594 =item lstat DIRHANDLE
3598 =for Pod::Functions stat a symbolic link
3600 Does the same thing as the C<stat> function (including setting the
3601 special C<_> filehandle) but stats a symbolic link instead of the file
3602 the symbolic link points to. If symbolic links are unimplemented on
3603 your system, a normal C<stat> is done. For much more detailed
3604 information, please see the documentation for C<stat>.
3606 If EXPR is omitted, stats C<$_>.
3608 Portability issues: L<perlport/lstat>.
3612 =for Pod::Functions match a string with a regular expression pattern
3614 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3616 =item map BLOCK LIST
3621 =for Pod::Functions apply a change to a list to get back a new list with the changes
3623 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3624 C<$_> to each element) and returns the list value composed of the
3625 results of each such evaluation. In scalar context, returns the
3626 total number of elements so generated. Evaluates BLOCK or EXPR in
3627 list context, so each element of LIST may produce zero, one, or
3628 more elements in the returned value.
3630 @chars = map(chr, @numbers);
3632 translates a list of numbers to the corresponding characters.
3634 my @squares = map { $_ * $_ } @numbers;
3636 translates a list of numbers to their squared values.
3638 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3640 shows that number of returned elements can differ from the number of
3641 input elements. To omit an element, return an empty list ().
3642 This could also be achieved by writing
3644 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3646 which makes the intention more clear.
3648 Map always returns a list, which can be
3649 assigned to a hash such that the elements
3650 become key/value pairs. See L<perldata> for more details.
3652 %hash = map { get_a_key_for($_) => $_ } @array;
3654 is just a funny way to write
3658 $hash{get_a_key_for($_)} = $_;
3661 Note that C<$_> is an alias to the list value, so it can be used to
3662 modify the elements of the LIST. While this is useful and supported,
3663 it can cause bizarre results if the elements of LIST are not variables.
3664 Using a regular C<foreach> loop for this purpose would be clearer in
3665 most cases. See also L</grep> for an array composed of those items of
3666 the original list for which the BLOCK or EXPR evaluates to true.
3668 If C<$_> is lexical in the scope where the C<map> appears (because it has
3669 been declared with the deprecated C<my $_> construct),
3670 then, in addition to being locally aliased to
3671 the list elements, C<$_> keeps being lexical inside the block; that is, it
3672 can't be seen from the outside, avoiding any potential side-effects.
3674 C<{> starts both hash references and blocks, so C<map { ...> could be either
3675 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3676 ahead for the closing C<}> it has to take a guess at which it's dealing with
3677 based on what it finds just after the
3678 C<{>. Usually it gets it right, but if it
3679 doesn't it won't realize something is wrong until it gets to the C<}> and
3680 encounters the missing (or unexpected) comma. The syntax error will be
3681 reported close to the C<}>, but you'll need to change something near the C<{>
3682 such as using a unary C<+> or semicolon to give Perl some help:
3684 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3685 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3686 %hash = map {; "\L$_" => 1 } @array # this also works
3687 %hash = map { ("\L$_" => 1) } @array # as does this
3688 %hash = map { lc($_) => 1 } @array # and this.
3689 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3691 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3693 or to force an anon hash constructor use C<+{>:
3695 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
3698 to get a list of anonymous hashes each with only one entry apiece.
3700 =item mkdir FILENAME,MASK
3701 X<mkdir> X<md> X<directory, create>
3703 =item mkdir FILENAME
3707 =for Pod::Functions create a directory
3709 Creates the directory specified by FILENAME, with permissions
3710 specified by MASK (as modified by C<umask>). If it succeeds it
3711 returns true; otherwise it returns false and sets C<$!> (errno).
3712 MASK defaults to 0777 if omitted, and FILENAME defaults
3713 to C<$_> if omitted.
3715 In general, it is better to create directories with a permissive MASK
3716 and let the user modify that with their C<umask> than it is to supply
3717 a restrictive MASK and give the user no way to be more permissive.
3718 The exceptions to this rule are when the file or directory should be
3719 kept private (mail files, for instance). The perlfunc(1) entry on
3720 C<umask> discusses the choice of MASK in more detail.
3722 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3723 number of trailing slashes. Some operating and filesystems do not get
3724 this right, so Perl automatically removes all trailing slashes to keep
3727 To recursively create a directory structure, look at
3728 the C<make_path> function of the L<File::Path> module.
3730 =item msgctl ID,CMD,ARG
3733 =for Pod::Functions SysV IPC message control operations
3735 Calls the System V IPC function msgctl(2). You'll probably have to say
3739 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3740 then ARG must be a variable that will hold the returned C<msqid_ds>
3741 structure. Returns like C<ioctl>: the undefined value for error,
3742 C<"0 but true"> for zero, or the actual return value otherwise. See also
3743 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3746 Portability issues: L<perlport/msgctl>.
3748 =item msgget KEY,FLAGS
3751 =for Pod::Functions get SysV IPC message queue
3753 Calls the System V IPC function msgget(2). Returns the message queue
3754 id, or C<undef> on error. See also
3755 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3758 Portability issues: L<perlport/msgget>.
3760 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3763 =for Pod::Functions receive a SysV IPC message from a message queue
3765 Calls the System V IPC function msgrcv to receive a message from
3766 message queue ID into variable VAR with a maximum message size of
3767 SIZE. Note that when a message is received, the message type as a
3768 native long integer will be the first thing in VAR, followed by the
3769 actual message. This packing may be opened with C<unpack("l! a*")>.
3770 Taints the variable. Returns true if successful, false
3771 on error. See also L<perlipc/"SysV IPC"> and the documentation for
3772 C<IPC::SysV> and C<IPC::SysV::Msg>.
3774 Portability issues: L<perlport/msgrcv>.
3776 =item msgsnd ID,MSG,FLAGS
3779 =for Pod::Functions send a SysV IPC message to a message queue
3781 Calls the System V IPC function msgsnd to send the message MSG to the
3782 message queue ID. MSG must begin with the native long integer message
3783 type, be followed by the length of the actual message, and then finally
3784 the message itself. This kind of packing can be achieved with
3785 C<pack("l! a*", $type, $message)>. Returns true if successful,
3786 false on error. See also the C<IPC::SysV>
3787 and C<IPC::SysV::Msg> documentation.
3789 Portability issues: L<perlport/msgsnd>.
3794 =item my TYPE VARLIST
3796 =item my VARLIST : ATTRS
3798 =item my TYPE VARLIST : ATTRS
3800 =for Pod::Functions declare and assign a local variable (lexical scoping)
3802 A C<my> declares the listed variables to be local (lexically) to the
3803 enclosing block, file, or C<eval>. If more than one variable is listed,
3804 the list must be placed in parentheses.
3806 The exact semantics and interface of TYPE and ATTRS are still
3807 evolving. TYPE may be a bareword, a constant declared
3808 with C<use constant>, or C<__PACKAGE__>. It is
3809 currently bound to the use of the C<fields> pragma,
3810 and attributes are handled using the C<attributes> pragma, or starting
3811 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3812 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3813 L<attributes>, and L<Attribute::Handlers>.
3815 Note that with a parenthesised list, C<undef> can be used as a dummy
3816 placeholder, for example to skip assignment of initial values:
3818 my ( undef, $min, $hour ) = localtime;
3827 =for Pod::Functions iterate a block prematurely
3829 The C<next> command is like the C<continue> statement in C; it starts
3830 the next iteration of the loop:
3832 LINE: while (<STDIN>) {
3833 next LINE if /^#/; # discard comments
3837 Note that if there were a C<continue> block on the above, it would get
3838 executed even on discarded lines. If LABEL is omitted, the command
3839 refers to the innermost enclosing loop. The C<next EXPR> form, available
3840 as of Perl 5.18.0, allows a label name to be computed at run time, being
3841 otherwise identical to C<next LABEL>.
3843 C<next> cannot be used to exit a block which returns a value such as
3844 C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
3845 a grep() or map() operation.
3847 Note that a block by itself is semantically identical to a loop
3848 that executes once. Thus C<next> will exit such a block early.
3850 See also L</continue> for an illustration of how C<last>, C<next>, and
3853 Unlike most named operators, this has the same precedence as assignment.
3854 It is also exempt from the looks-like-a-function rule, so
3855 C<next ("foo")."bar"> will cause "bar" to be part of the argument to
3858 =item no MODULE VERSION LIST
3862 =item no MODULE VERSION
3864 =item no MODULE LIST
3870 =for Pod::Functions unimport some module symbols or semantics at compile time
3872 See the C<use> function, of which C<no> is the opposite.
3875 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3879 =for Pod::Functions convert a string to an octal number
3881 Interprets EXPR as an octal string and returns the corresponding
3882 value. (If EXPR happens to start off with C<0x>, interprets it as a
3883 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3884 binary string. Leading whitespace is ignored in all three cases.)
3885 The following will handle decimal, binary, octal, and hex in standard
3888 $val = oct($val) if $val =~ /^0/;
3890 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3891 in octal), use sprintf() or printf():
3893 $dec_perms = (stat("filename"))[2] & 07777;
3894 $oct_perm_str = sprintf "%o", $perms;
3896 The oct() function is commonly used when a string such as C<644> needs
3897 to be converted into a file mode, for example. Although Perl
3898 automatically converts strings into numbers as needed, this automatic
3899 conversion assumes base 10.
3901 Leading white space is ignored without warning, as too are any trailing
3902 non-digits, such as a decimal point (C<oct> only handles non-negative
3903 integers, not negative integers or floating point).
3905 =item open FILEHANDLE,EXPR
3906 X<open> X<pipe> X<file, open> X<fopen>
3908 =item open FILEHANDLE,MODE,EXPR
3910 =item open FILEHANDLE,MODE,EXPR,LIST
3912 =item open FILEHANDLE,MODE,REFERENCE
3914 =item open FILEHANDLE
3916 =for Pod::Functions open a file, pipe, or descriptor
3918 Opens the file whose filename is given by EXPR, and associates it with
3921 Simple examples to open a file for reading:
3923 open(my $fh, "<", "input.txt")
3924 or die "cannot open < input.txt: $!";
3928 open(my $fh, ">", "output.txt")
3929 or die "cannot open > output.txt: $!";
3931 (The following is a comprehensive reference to open(): for a gentler
3932 introduction you may consider L<perlopentut>.)
3934 If FILEHANDLE is an undefined scalar variable (or array or hash element), a
3935 new filehandle is autovivified, meaning that the variable is assigned a
3936 reference to a newly allocated anonymous filehandle. Otherwise if
3937 FILEHANDLE is an expression, its value is the real filehandle. (This is
3938 considered a symbolic reference, so C<use strict "refs"> should I<not> be
3941 If three (or more) arguments are specified, the open mode (including
3942 optional encoding) in the second argument are distinct from the filename in
3943 the third. If MODE is C<< < >> or nothing, the file is opened for input.
3944 If MODE is C<< > >>, the file is opened for output, with existing files
3945 first being truncated ("clobbered") and nonexisting files newly created.
3946 If MODE is C<<< >> >>>, the file is opened for appending, again being
3947 created if necessary.
3949 You can put a C<+> in front of the C<< > >> or C<< < >> to
3950 indicate that you want both read and write access to the file; thus
3951 C<< +< >> is almost always preferred for read/write updates--the
3952 C<< +> >> mode would clobber the file first. You can't usually use
3953 either read-write mode for updating textfiles, since they have
3954 variable-length records. See the B<-i> switch in L<perlrun> for a
3955 better approach. The file is created with permissions of C<0666>
3956 modified by the process's C<umask> value.
3958 These various prefixes correspond to the fopen(3) modes of C<r>,
3959 C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
3961 In the one- and two-argument forms of the call, the mode and filename
3962 should be concatenated (in that order), preferably separated by white
3963 space. You can--but shouldn't--omit the mode in these forms when that mode
3964 is C<< < >>. It is always safe to use the two-argument form of C<open> if
3965 the filename argument is a known literal.
3967 For three or more arguments if MODE is C<|->, the filename is
3968 interpreted as a command to which output is to be piped, and if MODE
3969 is C<-|>, the filename is interpreted as a command that pipes
3970 output to us. In the two-argument (and one-argument) form, one should
3971 replace dash (C<->) with the command.
3972 See L<perlipc/"Using open() for IPC"> for more examples of this.
3973 (You are not allowed to C<open> to a command that pipes both in I<and>
3974 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3975 L<perlipc/"Bidirectional Communication with Another Process"> for
3978 In the form of pipe opens taking three or more arguments, if LIST is specified
3979 (extra arguments after the command name) then LIST becomes arguments
3980 to the command invoked if the platform supports it. The meaning of
3981 C<open> with more than three arguments for non-pipe modes is not yet
3982 defined, but experimental "layers" may give extra LIST arguments
3985 In the two-argument (and one-argument) form, opening C<< <- >>
3986 or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
3988 You may (and usually should) use the three-argument form of open to specify
3989 I/O layers (sometimes referred to as "disciplines") to apply to the handle
3990 that affect how the input and output are processed (see L<open> and
3991 L<PerlIO> for more details). For example:
3993 open(my $fh, "<:encoding(UTF-8)", "filename")
3994 || die "can't open UTF-8 encoded filename: $!";
3996 opens the UTF8-encoded file containing Unicode characters;
3997 see L<perluniintro>. Note that if layers are specified in the
3998 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3999 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
4000 Those layers will also be ignored if you specifying a colon with no name
4001 following it. In that case the default layer for the operating system
4002 (:raw on Unix, :crlf on Windows) is used.
4004 Open returns nonzero on success, the undefined value otherwise. If
4005 the C<open> involved a pipe, the return value happens to be the pid of
4008 If you're running Perl on a system that distinguishes between text
4009 files and binary files, then you should check out L</binmode> for tips
4010 for dealing with this. The key distinction between systems that need
4011 C<binmode> and those that don't is their text file formats. Systems
4012 like Unix, Mac OS, and Plan 9, that end lines with a single
4013 character and encode that character in C as C<"\n"> do not
4014 need C<binmode>. The rest need it.
4016 When opening a file, it's seldom a good idea to continue
4017 if the request failed, so C<open> is frequently used with
4018 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
4019 where you want to format a suitable error message (but there are
4020 modules that can help with that problem)) always check
4021 the return value from opening a file.
4023 The filehandle will be closed when its reference count reaches zero.
4024 If it is a lexically scoped variable declared with C<my>, that usually
4025 means the end of the enclosing scope. However, this automatic close
4026 does not check for errors, so it is better to explicitly close
4027 filehandles, especially those used for writing:
4030 || warn "close failed: $!";
4032 An older style is to use a bareword as the filehandle, as
4034 open(FH, "<", "input.txt")
4035 or die "cannot open < input.txt: $!";
4037 Then you can use C<FH> as the filehandle, in C<< close FH >> and C<<
4038 <FH> >> and so on. Note that it's a global variable, so this form is
4039 not recommended in new code.
4041 As a shortcut a one-argument call takes the filename from the global
4042 scalar variable of the same name as the filehandle:
4045 open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
4047 Here C<$ARTICLE> must be a global (package) scalar variable - not one
4048 declared with C<my> or C<state>.
4050 As a special case the three-argument form with a read/write mode and the third
4051 argument being C<undef>:
4053 open(my $tmp, "+>", undef) or die ...
4055 opens a filehandle to an anonymous temporary file. Also using C<< +< >>
4056 works for symmetry, but you really should consider writing something
4057 to the temporary file first. You will need to seek() to do the
4060 Perl is built using PerlIO by default; Unless you've
4061 changed this (such as building Perl with C<Configure -Uuseperlio>), you can
4062 open filehandles directly to Perl scalars via:
4064 open($fh, ">", \$variable) || ..
4066 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
4069 open(STDOUT, ">", \$variable)
4070 or die "Can't open STDOUT: $!";
4074 open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
4075 # if the open fails, output is discarded
4077 open(my $dbase, "+<", "dbase.mine") # open for update
4078 or die "Can't open 'dbase.mine' for update: $!";
4080 open(my $dbase, "+<dbase.mine") # ditto
4081 or die "Can't open 'dbase.mine' for update: $!";
4083 open(ARTICLE, "-|", "caesar <$article") # decrypt article
4084 or die "Can't start caesar: $!";
4086 open(ARTICLE, "caesar <$article |") # ditto
4087 or die "Can't start caesar: $!";
4089 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
4090 or die "Can't start sort: $!";
4093 open(MEMORY, ">", \$var)
4094 or die "Can't open memory file: $!";
4095 print MEMORY "foo!\n"; # output will appear in $var
4097 # process argument list of files along with any includes
4099 foreach $file (@ARGV) {
4100 process($file, "fh00");
4104 my($filename, $input) = @_;
4105 $input++; # this is a string increment
4106 unless (open($input, "<", $filename)) {
4107 print STDERR "Can't open $filename: $!\n";
4112 while (<$input>) { # note use of indirection
4113 if (/^#include "(.*)"/) {
4114 process($1, $input);
4121 See L<perliol> for detailed info on PerlIO.
4123 You may also, in the Bourne shell tradition, specify an EXPR beginning
4124 with C<< >& >>, in which case the rest of the string is interpreted
4125 as the name of a filehandle (or file descriptor, if numeric) to be
4126 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
4127 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
4128 The mode you specify should match the mode of the original filehandle.
4129 (Duping a filehandle does not take into account any existing contents
4130 of IO buffers.) If you use the three-argument
4131 form, then you can pass either a
4132 number, the name of a filehandle, or the normal "reference to a glob".
4134 Here is a script that saves, redirects, and restores C<STDOUT> and
4135 C<STDERR> using various methods:
4138 open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
4139 open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
4141 open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
4142 open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
4144 select STDERR; $| = 1; # make unbuffered
4145 select STDOUT; $| = 1; # make unbuffered
4147 print STDOUT "stdout 1\n"; # this works for
4148 print STDERR "stderr 1\n"; # subprocesses too
4150 open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
4151 open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
4153 print STDOUT "stdout 2\n";
4154 print STDERR "stderr 2\n";
4156 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
4157 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
4158 that file descriptor (and not call C<dup(2)>); this is more
4159 parsimonious of file descriptors. For example:
4161 # open for input, reusing the fileno of $fd
4162 open(FILEHANDLE, "<&=$fd")
4166 open(FILEHANDLE, "<&=", $fd)
4170 # open for append, using the fileno of OLDFH
4171 open(FH, ">>&=", OLDFH)
4175 open(FH, ">>&=OLDFH")
4177 Being parsimonious on filehandles is also useful (besides being
4178 parsimonious) for example when something is dependent on file
4179 descriptors, like for example locking using flock(). If you do just
4180 C<< open(A, ">>&B") >>, the filehandle A will not have the same file
4181 descriptor as B, and therefore flock(A) will not flock(B) nor vice
4182 versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
4183 the same underlying system file descriptor.
4185 Note that under Perls older than 5.8.0, Perl uses the standard C library's'
4186 fdopen() to implement the C<=> functionality. On many Unix systems,
4187 fdopen() fails when file descriptors exceed a certain value, typically 255.
4188 For Perls 5.8.0 and later, PerlIO is (most often) the default.
4190 You can see whether your Perl was built with PerlIO by running C<perl -V>
4191 and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
4192 have PerlIO; otherwise you don't.
4194 If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
4195 with the one- or two-argument forms of C<open>),
4196 an implicit C<fork> is done, so C<open> returns twice: in the parent
4197 process it returns the pid
4198 of the child process, and in the child process it returns (a defined) C<0>.
4199 Use C<defined($pid)> or C<//> to determine whether the open was successful.
4201 For example, use either
4203 $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
4207 $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
4213 # either write TO_KID or else read FROM_KID
4215 waitpid $child_pid, 0;
4217 # am the child; use STDIN/STDOUT normally
4222 The filehandle behaves normally for the parent, but I/O to that
4223 filehandle is piped from/to the STDOUT/STDIN of the child process.
4224 In the child process, the filehandle isn't opened--I/O happens from/to
4225 the new STDOUT/STDIN. Typically this is used like the normal
4226 piped open when you want to exercise more control over just how the
4227 pipe command gets executed, such as when running setuid and
4228 you don't want to have to scan shell commands for metacharacters.
4230 The following blocks are more or less equivalent:
4232 open(FOO, "|tr '[a-z]' '[A-Z]'");
4233 open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
4234 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
4235 open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
4237 open(FOO, "cat -n '$file'|");
4238 open(FOO, "-|", "cat -n '$file'");
4239 open(FOO, "-|") || exec "cat", "-n", $file;
4240 open(FOO, "-|", "cat", "-n", $file);
4242 The last two examples in each block show the pipe as "list form", which is
4243 not yet supported on all platforms. A good rule of thumb is that if
4244 your platform has a real C<fork()> (in other words, if your platform is
4245 Unix, including Linux and MacOS X), you can use the list form. You would
4246 want to use the list form of the pipe so you can pass literal arguments
4247 to the command without risk of the shell interpreting any shell metacharacters
4248 in them. However, this also bars you from opening pipes to commands
4249 that intentionally contain shell metacharacters, such as:
4251 open(FOO, "|cat -n | expand -4 | lpr")
4252 // die "Can't open pipeline to lpr: $!";
4254 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
4256 Perl will attempt to flush all files opened for
4257 output before any operation that may do a fork, but this may not be
4258 supported on some platforms (see L<perlport>). To be safe, you may need
4259 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4260 of C<IO::Handle> on any open handles.
4262 On systems that support a close-on-exec flag on files, the flag will
4263 be set for the newly opened file descriptor as determined by the value
4264 of C<$^F>. See L<perlvar/$^F>.
4266 Closing any piped filehandle causes the parent process to wait for the
4267 child to finish, then returns the status value in C<$?> and
4268 C<${^CHILD_ERROR_NATIVE}>.
4270 The filename passed to the one- and two-argument forms of open() will
4271 have leading and trailing whitespace deleted and normal
4272 redirection characters honored. This property, known as "magic open",
4273 can often be used to good effect. A user could specify a filename of
4274 F<"rsh cat file |">, or you could change certain filenames as needed:
4276 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
4277 open(FH, $filename) or die "Can't open $filename: $!";
4279 Use the three-argument form to open a file with arbitrary weird characters in it,
4281 open(FOO, "<", $file)
4282 || die "can't open < $file: $!";
4284 otherwise it's necessary to protect any leading and trailing whitespace:
4286 $file =~ s#^(\s)#./$1#;
4287 open(FOO, "< $file\0")
4288 || die "open failed: $!";
4290 (this may not work on some bizarre filesystems). One should
4291 conscientiously choose between the I<magic> and I<three-argument> form
4294 open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
4296 will allow the user to specify an argument of the form C<"rsh cat file |">,
4297 but will not work on a filename that happens to have a trailing space, while
4299 open(IN, "<", $ARGV[0])
4300 || die "can't open < $ARGV[0]: $!";
4302 will have exactly the opposite restrictions.
4304 If you want a "real" C C<open> (see L<open(2)> on your system), then you
4305 should use the C<sysopen> function, which involves no such magic (but may
4306 use subtly different filemodes than Perl open(), which is mapped to C
4307 fopen()). This is another way to protect your filenames from
4308 interpretation. For example:
4311 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
4312 or die "sysopen $path: $!";
4313 $oldfh = select(HANDLE); $| = 1; select($oldfh);
4314 print HANDLE "stuff $$\n";
4316 print "File contains: ", <HANDLE>;
4318 See L</seek> for some details about mixing reading and writing.
4320 Portability issues: L<perlport/open>.
4322 =item opendir DIRHANDLE,EXPR
4325 =for Pod::Functions open a directory
4327 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
4328 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
4329 DIRHANDLE may be an expression whose value can be used as an indirect
4330 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
4331 scalar variable (or array or hash element), the variable is assigned a
4332 reference to a new anonymous dirhandle; that is, it's autovivified.
4333 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
4335 See the example at C<readdir>.
4342 =for Pod::Functions find a character's numeric representation
4344 Returns the numeric value of the first character of EXPR.
4345 If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
4346 (Note I<character>, not byte.)
4348 For the reverse, see L</chr>.
4349 See L<perlunicode> for more about Unicode.
4354 =item our TYPE VARLIST
4356 =item our VARLIST : ATTRS
4358 =item our TYPE VARLIST : ATTRS
4360 =for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping)
4362 C<our> makes a lexical alias to a package (i.e. global) variable of the
4363 same name in the current package for use within the current lexical scope.
4365 C<our> has the same scoping rules as C<my> or C<state>, meaning that it is
4366 only valid within a lexical scope. Unlike C<my> and C<state>, which both
4367 declare new (lexical) variables, C<our> only creates an alias to an
4368 existing variable: a package variable of the same name.
4370 This means that when C<use strict 'vars'> is in effect, C<our> lets you use
4371 a package variable without qualifying it with the package name, but only within
4372 the lexical scope of the C<our> declaration.
4380 our $foo; # alias to $Foo::foo
4381 print $foo; # prints 23
4384 print $Foo::foo; # prints 23
4386 print $foo; # ERROR: requires explicit package name
4388 This works even if the package variable has not been used before, as
4389 package variables spring into existence when first used.
4394 our $foo = 23; # just like $Foo::foo = 23
4396 print $Foo::foo; # prints 23
4398 If more than one variable is listed, the list must be placed
4403 An C<our> declaration declares an alias for a package variable that will be visible
4404 across its entire lexical scope, even across package boundaries. The
4405 package in which the variable is entered is determined at the point
4406 of the declaration, not at the point of use. This means the following
4410 our $bar; # declares $Foo::bar for rest of lexical scope
4414 print $bar; # prints 20, as it refers to $Foo::bar
4416 Multiple C<our> declarations with the same name in the same lexical
4417 scope are allowed if they are in different packages. If they happen
4418 to be in the same package, Perl will emit warnings if you have asked
4419 for them, just like multiple C<my> declarations. Unlike a second
4420 C<my> declaration, which will bind the name to a fresh variable, a
4421 second C<our> declaration in the same package, in the same scope, is
4426 our $bar; # declares $Foo::bar for rest of lexical scope
4430 our $bar = 30; # declares $Bar::bar for rest of lexical scope
4431 print $bar; # prints 30
4433 our $bar; # emits warning but has no other effect
4434 print $bar; # still prints 30
4436 An C<our> declaration may also have a list of attributes associated
4439 The exact semantics and interface of TYPE and ATTRS are still
4440 evolving. TYPE is currently bound to the use of the C<fields> pragma,
4441 and attributes are handled using the C<attributes> pragma, or, starting
4442 from Perl 5.8.0, also via the C<Attribute::Handlers> module. See
4443 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
4444 L<attributes>, and L<Attribute::Handlers>.
4446 Note that with a parenthesised list, C<undef> can be used as a dummy
4447 placeholder, for example to skip assignment of initial values:
4449 our ( undef, $min, $hour ) = localtime;
4451 C<our> differs from C<use vars>, which allows use of an unqualified name
4452 I<only> within the affected package, but across scopes.
4454 =item pack TEMPLATE,LIST
4457 =for Pod::Functions convert a list into a binary representation
4459 Takes a LIST of values and converts it into a string using the rules
4460 given by the TEMPLATE. The resulting string is the concatenation of
4461 the converted values. Typically, each converted value looks
4462 like its machine-level representation. For example, on 32-bit machines
4463 an integer may be represented by a sequence of 4 bytes, which will in
4464 Perl be presented as a string that's 4 characters long.
4466 See L<perlpacktut> for an introduction to this function.
4468 The TEMPLATE is a sequence of characters that give the order and type
4469 of values, as follows:
4471 a A string with arbitrary binary data, will be null padded.
4472 A A text (ASCII) string, will be space padded.
4473 Z A null-terminated (ASCIZ) string, will be null padded.
4475 b A bit string (ascending bit order inside each byte,
4477 B A bit string (descending bit order inside each byte).
4478 h A hex string (low nybble first).
4479 H A hex string (high nybble first).
4481 c A signed char (8-bit) value.
4482 C An unsigned char (octet) value.
4483 W An unsigned char value (can be greater than 255).
4485 s A signed short (16-bit) value.
4486 S An unsigned short value.
4488 l A signed long (32-bit) value.
4489 L An unsigned long value.
4491 q A signed quad (64-bit) value.
4492 Q An unsigned quad value.
4493 (Quads are available only if your system supports 64-bit
4494 integer values _and_ if Perl has been compiled to support
4495 those. Raises an exception otherwise.)
4497 i A signed integer value.
4498 I A unsigned integer value.
4499 (This 'integer' is _at_least_ 32 bits wide. Its exact
4500 size depends on what a local C compiler calls 'int'.)
4502 n An unsigned short (16-bit) in "network" (big-endian) order.
4503 N An unsigned long (32-bit) in "network" (big-endian) order.
4504 v An unsigned short (16-bit) in "VAX" (little-endian) order.
4505 V An unsigned long (32-bit) in "VAX" (little-endian) order.
4507 j A Perl internal signed integer value (IV).
4508 J A Perl internal unsigned integer value (UV).
4510 f A single-precision float in native format.
4511 d A double-precision float in native format.
4513 F A Perl internal floating-point value (NV) in native format
4514 D A float of long-double precision in native format.
4515 (Long doubles are available only if your system supports
4516 long double values _and_ if Perl has been compiled to
4517 support those. Raises an exception otherwise.
4518 Note that there are different long double formats.)
4520 p A pointer to a null-terminated string.
4521 P A pointer to a structure (fixed-length string).
4523 u A uuencoded string.
4524 U A Unicode character number. Encodes to a character in char-
4525 acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
4528 w A BER compressed integer (not an ASN.1 BER, see perlpacktut
4529 for details). Its bytes represent an unsigned integer in
4530 base 128, most significant digit first, with as few digits
4531 as possible. Bit eight (the high bit) is set on each byte
4534 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
4536 @ Null-fill or truncate to absolute position, counted from the
4537 start of the innermost ()-group.
4538 . Null-fill or truncate to absolute position specified by
4540 ( Start of a ()-group.
4542 One or more modifiers below may optionally follow certain letters in the
4543 TEMPLATE (the second column lists letters for which the modifier is valid):
4545 ! sSlLiI Forces native (short, long, int) sizes instead
4546 of fixed (16-/32-bit) sizes.
4548 ! xX Make x and X act as alignment commands.
4550 ! nNvV Treat integers as signed instead of unsigned.
4552 ! @. Specify position as byte offset in the internal
4553 representation of the packed string. Efficient
4556 > sSiIlLqQ Force big-endian byte-order on the type.
4557 jJfFdDpP (The "big end" touches the construct.)
4559 < sSiIlLqQ Force little-endian byte-order on the type.
4560 jJfFdDpP (The "little end" touches the construct.)
4562 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
4563 to force a particular byte-order on all components in that group,
4564 including all its subgroups.
4568 Larry recalls that the hex and bit string formats (H, h, B, b) were added to
4569 pack for processing data from NASA's Magellan probe. Magellan was in an
4570 elliptical orbit, using the antenna for the radar mapping when close to
4571 Venus and for communicating data back to Earth for the rest of the orbit.
4572 There were two transmission units, but one of these failed, and then the
4573 other developed a fault whereby it would randomly flip the sense of all the
4574 bits. It was easy to automatically detect complete records with the correct
4575 sense, and complete records with all the bits flipped. However, this didn't
4576 recover the records where the sense flipped midway. A colleague of Larry's
4577 was able to pretty much eyeball where the records flipped, so they wrote an
4578 editor named kybble (a pun on the dog food Kibbles 'n Bits) to enable him to
4579 manually correct the records and recover the data. For this purpose pack
4580 gained the hex and bit string format specifiers.
4582 git shows that they were added to perl 3.0 in patch #44 (Jan 1991, commit
4583 27e2fb84680b9cc1), but the patch description makes no mention of their
4584 addition, let alone the story behind them.
4588 The following rules apply:
4594 Each letter may optionally be followed by a number indicating the repeat
4595 count. A numeric repeat count may optionally be enclosed in brackets, as
4596 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
4597 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
4598 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
4599 something else, described below. Supplying a C<*> for the repeat count
4600 instead of a number means to use however many items are left, except for:
4606 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
4610 <.>, where it means relative to the start of the string.
4614 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
4618 One can replace a numeric repeat count with a template letter enclosed in
4619 brackets to use the packed byte length of the bracketed template for the
4622 For example, the template C<x[L]> skips as many bytes as in a packed long,
4623 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
4624 variable-expanded) unpacks. If the template in brackets contains alignment
4625 commands (such as C<x![d]>), its packed length is calculated as if the
4626 start of the template had the maximal possible alignment.
4628 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
4629 trailing null byte, so the resulting string is always one byte longer than
4630 the byte length of the item itself.
4632 When used with C<@>, the repeat count represents an offset from the start
4633 of the innermost C<()> group.
4635 When used with C<.>, the repeat count determines the starting position to
4636 calculate the value offset as follows:
4642 If the repeat count is C<0>, it's relative to the current position.
4646 If the repeat count is C<*>, the offset is relative to the start of the
4651 And if it's an integer I<n>, the offset is relative to the start of the
4652 I<n>th innermost C<( )> group, or to the start of the string if I<n> is
4653 bigger then the group level.
4657 The repeat count for C<u> is interpreted as the maximal number of bytes
4658 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
4659 count should not be more than 65.
4663 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
4664 string of length count, padding with nulls or spaces as needed. When
4665 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
4666 after the first null, and C<a> returns data with no stripping at all.
4668 If the value to pack is too long, the result is truncated. If it's too
4669 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
4670 followed by a null byte. Thus C<Z> always packs a trailing null, except
4671 when the count is 0.
4675 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
4676 Each such format generates 1 bit of the result. These are typically followed
4677 by a repeat count like C<B8> or C<B64>.
4679 Each result bit is based on the least-significant bit of the corresponding
4680 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
4681 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
4683 Starting from the beginning of the input string, each 8-tuple
4684 of characters is converted to 1 character of output. With format C<b>,
4685 the first character of the 8-tuple determines the least-significant bit of a
4686 character; with format C<B>, it determines the most-significant bit of
4689 If the length of the input string is not evenly divisible by 8, the
4690 remainder is packed as if the input string were padded by null characters
4691 at the end. Similarly during unpacking, "extra" bits are ignored.