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 a 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 a scalar context by
59 returning the undefined value, and in a 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 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
111 C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
112 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
114 =item Regular expressions and pattern matching
115 X<regular expression> X<regex> X<regexp>
117 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
119 =item Numeric functions
120 X<numeric> X<number> X<trigonometric> X<trigonometry>
122 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
123 C<sin>, C<sqrt>, C<srand>
125 =item Functions for real @ARRAYs
128 C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
130 =item Functions for list data
133 C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
135 =item Functions for real %HASHes
138 C<delete>, C<each>, C<exists>, C<keys>, C<values>
140 =item Input and output functions
141 X<I/O> X<input> X<output> X<dbm>
143 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
144 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
145 C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
146 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
149 =item Functions for fixed-length data or records
151 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
153 =item Functions for filehandles, files, or directories
154 X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
156 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
157 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
158 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
159 C<umask>, C<unlink>, C<utime>
161 =item Keywords related to the control flow of your Perl program
164 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
165 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
167 =item Keywords related to switch
169 C<break>, C<continue>, C<given>, C<when>, C<default>
171 (These are available only if you enable the C<"switch"> feature.
172 See L<feature> and L<perlsyn/"Switch statements">.)
174 =item Keywords related to scoping
176 C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
179 (C<state> is available only if the C<"state"> feature is enabled. See
182 =item Miscellaneous functions
184 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>,
185 C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
187 =item Functions for processes and process groups
188 X<process> X<pid> X<process id>
190 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
191 C<pipe>, C<qx//>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
192 C<times>, C<wait>, C<waitpid>
194 =item Keywords related to Perl modules
197 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
199 =item Keywords related to classes and object-orientation
200 X<object> X<class> X<package>
202 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
205 =item Low-level socket functions
208 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
209 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
210 C<socket>, C<socketpair>
212 =item System V interprocess communication functions
213 X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
215 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
216 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
218 =item Fetching user and group info
219 X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
221 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
222 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
223 C<getpwuid>, C<setgrent>, C<setpwent>
225 =item Fetching network info
226 X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
228 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
229 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
231 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
232 C<setnetent>, C<setprotoent>, C<setservent>
234 =item Time-related functions
237 C<gmtime>, C<localtime>, C<time>, C<times>
239 =item Functions new in perl5
242 C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
243 C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
244 C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
245 C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
246 C<ucfirst>, C<untie>, C<use>, C<when>
248 * C<sub> was a keyword in Perl 4, but in Perl 5 it is an
249 operator, which can be used in expressions.
251 =item Functions obsoleted in perl5
253 C<dbmclose>, C<dbmopen>
258 X<portability> X<Unix> X<portable>
260 Perl was born in Unix and can therefore access all common Unix
261 system calls. In non-Unix environments, the functionality of some
262 Unix system calls may not be available, or details of the available
263 functionality may differ slightly. The Perl functions affected
266 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
267 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
268 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
269 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
270 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
271 C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
272 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
273 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
274 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
275 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
276 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
277 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
278 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
279 C<shmwrite>, C<socket>, C<socketpair>,
280 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
281 C<times>, C<truncate>, C<umask>, C<unlink>,
282 C<utime>, C<wait>, C<waitpid>
284 For more information about the portability of these functions, see
285 L<perlport> and other available platform-specific documentation.
287 =head2 Alphabetical Listing of Perl Functions
292 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>
293 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
301 A file test, where X is one of the letters listed below. This unary
302 operator takes one argument, either a filename, a filehandle, or a dirhandle,
303 and tests the associated file to see if something is true about it. If the
304 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
305 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
306 the undefined value if the file doesn't exist. Despite the funny
307 names, precedence is the same as any other named unary operator. The
308 operator may be any of:
310 -r File is readable by effective uid/gid.
311 -w File is writable by effective uid/gid.
312 -x File is executable by effective uid/gid.
313 -o File is owned by effective uid.
315 -R File is readable by real uid/gid.
316 -W File is writable by real uid/gid.
317 -X File is executable by real uid/gid.
318 -O File is owned by real uid.
321 -z File has zero size (is empty).
322 -s File has nonzero size (returns size in bytes).
324 -f File is a plain file.
325 -d File is a directory.
326 -l File is a symbolic link.
327 -p File is a named pipe (FIFO), or Filehandle is a pipe.
329 -b File is a block special file.
330 -c File is a character special file.
331 -t Filehandle is opened to a tty.
333 -u File has setuid bit set.
334 -g File has setgid bit set.
335 -k File has sticky bit set.
337 -T File is an ASCII text file (heuristic guess).
338 -B File is a "binary" file (opposite of -T).
340 -M Script start time minus file modification time, in days.
341 -A Same for access time.
342 -C Same for inode change time (Unix, may differ for other platforms)
348 next unless -f $_; # ignore specials
352 Note that C<-s/a/b/> does not do a negated substitution. Saying
353 C<-exp($foo)> still works as expected, however: only single letters
354 following a minus are interpreted as file tests.
356 These operators are exempt from the "looks like a function rule" described
357 above. That is, an opening parenthesis after the operator does not affect
358 how much of the following code constitutes the argument. Put the opening
359 parentheses before the operator to separate it from code that follows (this
360 applies only to operators with higher precedence than unary operators, of
363 -s($file) + 1024 # probably wrong; same as -s($file + 1024)
364 (-s $file) + 1024 # correct
366 The interpretation of the file permission operators C<-r>, C<-R>,
367 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
368 of the file and the uids and gids of the user. There may be other
369 reasons you can't actually read, write, or execute the file: for
370 example network filesystem access controls, ACLs (access control lists),
371 read-only filesystems, and unrecognized executable formats. Note
372 that the use of these six specific operators to verify if some operation
373 is possible is usually a mistake, because it may be open to race
376 Also note that, for the superuser on the local filesystems, the C<-r>,
377 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
378 if any execute bit is set in the mode. Scripts run by the superuser
379 may thus need to do a stat() to determine the actual mode of the file,
380 or temporarily set their effective uid to something else.
382 If you are using ACLs, there is a pragma called C<filetest> that may
383 produce more accurate results than the bare stat() mode bits.
384 When under C<use filetest 'access'> the above-mentioned filetests
385 test whether the permission can(not) be granted using the
386 access(2) family of system calls. Also note that the C<-x> and C<-X> may
387 under this pragma return true even if there are no execute permission
388 bits set (nor any extra execute permission ACLs). This strangeness is
389 due to the underlying system calls' definitions. Note also that, due to
390 the implementation of C<use filetest 'access'>, the C<_> special
391 filehandle won't cache the results of the file tests when this pragma is
392 in effect. Read the documentation for the C<filetest> pragma for more
395 The C<-T> and C<-B> switches work as follows. The first block or so of the
396 file is examined for odd characters such as strange control codes or
397 characters with the high bit set. If too many strange characters (>30%)
398 are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
399 containing a zero byte in the first block is considered a binary file. If C<-T>
400 or C<-B> is used on a filehandle, the current IO buffer is examined
401 rather than the first block. Both C<-T> and C<-B> return true on an empty
402 file, or a file at EOF when testing a filehandle. Because you have to
403 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
404 against the file first, as in C<next unless -f $file && -T $file>.
406 If any of the file tests (or either the C<stat> or C<lstat> operator) is given
407 the special filehandle consisting of a solitary underline, then the stat
408 structure of the previous file test (or stat operator) is used, saving
409 a system call. (This doesn't work with C<-t>, and you need to remember
410 that lstat() and C<-l> leave values in the stat structure for the
411 symbolic link, not the real file.) (Also, if the stat buffer was filled by
412 an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
415 print "Can do.\n" if -r $a || -w _ || -x _;
418 print "Readable\n" if -r _;
419 print "Writable\n" if -w _;
420 print "Executable\n" if -x _;
421 print "Setuid\n" if -u _;
422 print "Setgid\n" if -g _;
423 print "Sticky\n" if -k _;
424 print "Text\n" if -T _;
425 print "Binary\n" if -B _;
427 As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
428 test operators, in a way that C<-f -w -x $file> is equivalent to
429 C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
430 the return value of C<-f $file> as an argument to another filetest
431 operator, no special magic will happen.)
438 Returns the absolute value of its argument.
439 If VALUE is omitted, uses C<$_>.
441 =item accept NEWSOCKET,GENERICSOCKET
444 Accepts an incoming socket connect, just as accept(2)
445 does. Returns the packed address if it succeeded, false otherwise.
446 See the example in L<perlipc/"Sockets: Client/Server Communication">.
448 On systems that support a close-on-exec flag on files, the flag will
449 be set for the newly opened file descriptor, as determined by the
450 value of $^F. See L<perlvar/$^F>.
459 Arranges to have a SIGALRM delivered to this process after the
460 specified number of wallclock seconds has elapsed. If SECONDS is not
461 specified, the value stored in C<$_> is used. (On some machines,
462 unfortunately, the elapsed time may be up to one second less or more
463 than you specified because of how seconds are counted, and process
464 scheduling may delay the delivery of the signal even further.)
466 Only one timer may be counting at once. Each call disables the
467 previous timer, and an argument of C<0> may be supplied to cancel the
468 previous timer without starting a new one. The returned value is the
469 amount of time remaining on the previous timer.
471 For delays of finer granularity than one second, the Time::HiRes module
472 (from CPAN, and starting from Perl 5.8 part of the standard
473 distribution) provides ualarm(). You may also use Perl's four-argument
474 version of select() leaving the first three arguments undefined, or you
475 might be able to use the C<syscall> interface to access setitimer(2) if
476 your system supports it. See L<perlfaq8> for details.
478 It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
479 C<sleep> may be internally implemented on your system with C<alarm>.
481 If you want to use C<alarm> to time out a system call you need to use an
482 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
483 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
484 restart system calls on some systems. Using C<eval>/C<die> always works,
485 modulo the caveats given in L<perlipc/"Signals">.
488 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
490 $nread = sysread SOCKET, $buffer, $size;
494 die unless $@ eq "alarm\n"; # propagate unexpected errors
501 For more information see L<perlipc>.
504 X<atan2> X<arctangent> X<tan> X<tangent>
506 Returns the arctangent of Y/X in the range -PI to PI.
508 For the tangent operation, you may use the C<Math::Trig::tan>
509 function, or use the familiar relation:
511 sub tan { sin($_[0]) / cos($_[0]) }
513 The return value for C<atan2(0,0)> is implementation-defined; consult
514 your atan2(3) manpage for more information.
516 =item bind SOCKET,NAME
519 Binds a network address to a socket, just as bind(2)
520 does. Returns true if it succeeded, false otherwise. NAME should be a
521 packed address of the appropriate type for the socket. See the examples in
522 L<perlipc/"Sockets: Client/Server Communication">.
524 =item binmode FILEHANDLE, LAYER
525 X<binmode> X<binary> X<text> X<DOS> X<Windows>
527 =item binmode FILEHANDLE
529 Arranges for FILEHANDLE to be read or written in "binary" or "text"
530 mode on systems where the run-time libraries distinguish between
531 binary and text files. If FILEHANDLE is an expression, the value is
532 taken as the name of the filehandle. Returns true on success,
533 otherwise it returns C<undef> and sets C<$!> (errno).
535 On some systems (in general, DOS and Windows-based systems) binmode()
536 is necessary when you're not working with a text file. For the sake
537 of portability it is a good idea always to use it when appropriate,
538 and never to use it when it isn't appropriate. Also, people can
539 set their I/O to be by default UTF-8 encoded Unicode, not bytes.
541 In other words: regardless of platform, use binmode() on binary data,
542 like images, for example.
544 If LAYER is present it is a single string, but may contain multiple
545 directives. The directives alter the behaviour of the filehandle.
546 When LAYER is present, using binmode on a text file makes sense.
548 If LAYER is omitted or specified as C<:raw> the filehandle is made
549 suitable for passing binary data. This includes turning off possible CRLF
550 translation and marking it as bytes (as opposed to Unicode characters).
551 Note that, despite what may be implied in I<"Programming Perl"> (the
552 Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
553 Other layers that would affect the binary nature of the stream are
554 I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
555 PERLIO environment variable.
557 The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
558 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
559 establish default I/O layers. See L<open>.
561 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
562 in "Programming Perl, 3rd Edition". However, since the publishing of this
563 book, by many known as "Camel III", the consensus of the naming of this
564 functionality has moved from "discipline" to "layer". All documentation
565 of this version of Perl therefore refers to "layers" rather than to
566 "disciplines". Now back to the regularly scheduled documentation...>
568 To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(utf8)>.
569 C<:utf8> just marks the data as UTF-8 without further checking,
570 while C<:encoding(utf8)> checks the data for actually being valid
571 UTF-8. More details can be found in L<PerlIO::encoding>.
573 In general, binmode() should be called after open() but before any I/O
574 is done on the filehandle. Calling binmode() normally flushes any
575 pending buffered output data (and perhaps pending input data) on the
576 handle. An exception to this is the C<:encoding> layer that
577 changes the default character encoding of the handle; see L</open>.
578 The C<:encoding> layer sometimes needs to be called in
579 mid-stream, and it doesn't flush the stream. The C<:encoding>
580 also implicitly pushes on top of itself the C<:utf8> layer because
581 internally Perl operates on UTF8-encoded Unicode characters.
583 The operating system, device drivers, C libraries, and Perl run-time
584 system all work together to let the programmer treat a single
585 character (C<\n>) as the line terminator, irrespective of the external
586 representation. On many operating systems, the native text file
587 representation matches the internal representation, but on some
588 platforms the external representation of C<\n> is made up of more than
591 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
592 character to end each line in the external representation of text (even
593 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
594 on Unix and most VMS files). In other systems like OS/2, DOS and the
595 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
596 but what's stored in text files are the two characters C<\cM\cJ>. That
597 means that, if you don't use binmode() on these systems, C<\cM\cJ>
598 sequences on disk will be converted to C<\n> on input, and any C<\n> in
599 your program will be converted back to C<\cM\cJ> on output. This is what
600 you want for text files, but it can be disastrous for binary files.
602 Another consequence of using binmode() (on some systems) is that
603 special end-of-file markers will be seen as part of the data stream.
604 For systems from the Microsoft family this means that, if your binary
605 data contain C<\cZ>, the I/O subsystem will regard it as the end of
606 the file, unless you use binmode().
608 binmode() is important not only for readline() and print() operations,
609 but also when using read(), seek(), sysread(), syswrite() and tell()
610 (see L<perlport> for more details). See the C<$/> and C<$\> variables
611 in L<perlvar> for how to manually set your input and output
612 line-termination sequences.
614 =item bless REF,CLASSNAME
619 This function tells the thingy referenced by REF that it is now an object
620 in the CLASSNAME package. If CLASSNAME is omitted, the current package
621 is used. Because a C<bless> is often the last thing in a constructor,
622 it returns the reference for convenience. Always use the two-argument
623 version if a derived class might inherit the function doing the blessing.
624 See L<perltoot> and L<perlobj> for more about the blessing (and blessings)
627 Consider always blessing objects in CLASSNAMEs that are mixed case.
628 Namespaces with all lowercase names are considered reserved for
629 Perl pragmata. Builtin types have all uppercase names. To prevent
630 confusion, you may wish to avoid such package names as well. Make sure
631 that CLASSNAME is a true value.
633 See L<perlmod/"Perl Modules">.
637 Break out of a C<given()> block.
639 This keyword is enabled by the C<"switch"> feature: see L<feature>
640 for more information.
643 X<caller> X<call stack> X<stack> X<stack trace>
647 Returns the context of the current subroutine call. In scalar context,
648 returns the caller's package name if there I<is> a caller (that is, if
649 we're in a subroutine or C<eval> or C<require>) and the undefined value
650 otherwise. In list context, returns
653 ($package, $filename, $line) = caller;
655 With EXPR, it returns some extra information that the debugger uses to
656 print a stack trace. The value of EXPR indicates how many call frames
657 to go back before the current one.
660 ($package, $filename, $line, $subroutine, $hasargs,
663 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
666 Here $subroutine may be C<(eval)> if the frame is not a subroutine
667 call, but an C<eval>. In such a case additional elements $evaltext and
668 C<$is_require> are set: C<$is_require> is true if the frame is created by a
669 C<require> or C<use> statement, $evaltext contains the text of the
670 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
671 $subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
672 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
673 frame.) $subroutine may also be C<(unknown)> if this particular
674 subroutine happens to have been deleted from the symbol table.
675 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
676 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
677 compiled with. The C<$hints> and C<$bitmask> values are subject to change
678 between versions of Perl, and are not meant for external use.
680 C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
681 caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
682 of this hash, as they are the actual values stored in the optree.
684 Furthermore, when called from within the DB package, caller returns more
685 detailed information: it sets the list variable C<@DB::args> to be the
686 arguments with which the subroutine was invoked.
688 Be aware that the optimizer might have optimized call frames away before
689 C<caller> had a chance to get the information. That means that C<caller(N)>
690 might not return information about the call frame you expect it to, for
691 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
692 previous time C<caller> was called.
694 Also be aware that setting C<@DB::args> is I<best effort>, intended for
695 debugging or generating backtraces, and should not be relied upon. In
696 particular, as C<@_> contains aliases to the caller's arguments, Perl does
697 not take a copy of C<@_>, so C<@DB::args> will contain modifications the
698 subroutine makes to C<@_> or its contents, not the original values at call
699 time. C<@DB::args>, like C<@_>, does not hold explicit references to its
700 elements, so under certain cases its elements may have become freed and
701 reallocated for other variables or temporary values. Finally, a side effect
702 of the current implementation is that the effects of C<shift @_> can
703 I<normally> be undone (but not C<pop @_> or other splicing, and not if a
704 reference to C<@_> has been taken, and subject to the caveat about reallocated
705 elements), so C<@DB::args> is actually a hybrid of the current state and
706 initial state of C<@_>. Buyer beware.
713 =item chdir FILEHANDLE
715 =item chdir DIRHANDLE
719 Changes the working directory to EXPR, if possible. If EXPR is omitted,
720 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
721 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
722 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
723 neither is set, C<chdir> does nothing. It returns true on success,
724 false otherwise. See the example under C<die>.
726 On systems that support fchdir(2), you may pass a filehandle or
727 directory handle as the argument. On systems that don't support fchdir(2),
728 passing handles raises an exception.
731 X<chmod> X<permission> X<mode>
733 Changes the permissions of a list of files. The first element of the
734 list must be the numerical mode, which should probably be an octal
735 number, and which definitely should I<not> be a string of octal digits:
736 C<0644> is okay, but C<"0644"> is not. Returns the number of files
737 successfully changed. See also L</oct>, if all you have is a string.
739 $cnt = chmod 0755, "foo", "bar";
740 chmod 0755, @executables;
741 $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
743 $mode = "0644"; chmod oct($mode), "foo"; # this is better
744 $mode = 0644; chmod $mode, "foo"; # this is best
746 On systems that support fchmod(2), you may pass filehandles among the
747 files. On systems that don't support fchmod(2), passing filehandles raises
748 an exception. Filehandles must be passed as globs or glob references to be
749 recognized; barewords are considered filenames.
751 open(my $fh, "<", "foo");
752 my $perm = (stat $fh)[2] & 07777;
753 chmod($perm | 0600, $fh);
755 You can also import the symbolic C<S_I*> constants from the C<Fcntl>
758 use Fcntl qw( :mode );
759 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
760 # Identical to the chmod 0755 of the example above.
763 X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
769 This safer version of L</chop> removes any trailing string
770 that corresponds to the current value of C<$/> (also known as
771 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
772 number of characters removed from all its arguments. It's often used to
773 remove the newline from the end of an input record when you're worried
774 that the final record may be missing its newline. When in paragraph
775 mode (C<$/ = "">), it removes all trailing newlines from the string.
776 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
777 a reference to an integer or the like; see L<perlvar>) chomp() won't
779 If VARIABLE is omitted, it chomps C<$_>. Example:
782 chomp; # avoid \n on last field
787 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
789 You can actually chomp anything that's an lvalue, including an assignment:
792 chomp($answer = <STDIN>);
794 If you chomp a list, each element is chomped, and the total number of
795 characters removed is returned.
797 Note that parentheses are necessary when you're chomping anything
798 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
799 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
800 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
801 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
811 Chops off the last character of a string and returns the character
812 chopped. It is much more efficient than C<s/.$//s> because it neither
813 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
814 If VARIABLE is a hash, it chops the hash's values, but not its keys.
816 You can actually chop anything that's an lvalue, including an assignment.
818 If you chop a list, each element is chopped. Only the value of the
819 last C<chop> is returned.
821 Note that C<chop> returns the last character. To return all but the last
822 character, use C<substr($string, 0, -1)>.
827 X<chown> X<owner> X<user> X<group>
829 Changes the owner (and group) of a list of files. The first two
830 elements of the list must be the I<numeric> uid and gid, in that
831 order. A value of -1 in either position is interpreted by most
832 systems to leave that value unchanged. Returns the number of files
833 successfully changed.
835 $cnt = chown $uid, $gid, 'foo', 'bar';
836 chown $uid, $gid, @filenames;
838 On systems that support fchown(2), you may pass filehandles among the
839 files. On systems that don't support fchown(2), passing filehandles raises
840 an exception. Filehandles must be passed as globs or glob references to be
841 recognized; barewords are considered filenames.
843 Here's an example that looks up nonnumeric uids in the passwd file:
846 chomp($user = <STDIN>);
848 chomp($pattern = <STDIN>);
850 ($login,$pass,$uid,$gid) = getpwnam($user)
851 or die "$user not in passwd file";
853 @ary = glob($pattern); # expand filenames
854 chown $uid, $gid, @ary;
856 On most systems, you are not allowed to change the ownership of the
857 file unless you're the superuser, although you should be able to change
858 the group to any of your secondary groups. On insecure systems, these
859 restrictions may be relaxed, but this is not a portable assumption.
860 On POSIX systems, you can detect this condition this way:
862 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
863 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
866 X<chr> X<character> X<ASCII> X<Unicode>
870 Returns the character represented by that NUMBER in the character set.
871 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
872 chr(0x263a) is a Unicode smiley face.
874 Negative values give the Unicode replacement character (chr(0xfffd)),
875 except under the L<bytes> pragma, where the low eight bits of the value
876 (truncated to an integer) are used.
878 If NUMBER is omitted, uses C<$_>.
880 For the reverse, use L</ord>.
882 Note that characters from 128 to 255 (inclusive) are by default
883 internally not encoded as UTF-8 for backward compatibility reasons.
885 See L<perlunicode> for more about Unicode.
887 =item chroot FILENAME
892 This function works like the system call by the same name: it makes the
893 named directory the new root directory for all further pathnames that
894 begin with a C</> by your process and all its children. (It doesn't
895 change your current working directory, which is unaffected.) For security
896 reasons, this call is restricted to the superuser. If FILENAME is
897 omitted, does a C<chroot> to C<$_>.
899 =item close FILEHANDLE
904 Closes the file or pipe associated with the filehandle, flushes the IO
905 buffers, and closes the system file descriptor. Returns true if those
906 operations have succeeded and if no error was reported by any PerlIO
907 layer. Closes the currently selected filehandle if the argument is
910 You don't have to close FILEHANDLE if you are immediately going to do
911 another C<open> on it, because C<open> closes it for you. (See
912 C<open>.) However, an explicit C<close> on an input file resets the line
913 counter (C<$.>), while the implicit close done by C<open> does not.
915 If the filehandle came from a piped open, C<close> returns false if one of
916 the other syscalls involved fails or if its program exits with non-zero
917 status. If the only problem was that the program exited non-zero, C<$!>
918 will be set to C<0>. Closing a pipe also waits for the process executing
919 on the pipe to exit--in case you wish to look at the output of the pipe
920 afterwards--and implicitly puts the exit status value of that command into
921 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
923 If there are multiple threads running, C<close> on a filehandle from a
924 piped open returns true without waiting for the child process to terminate,
925 if the filehandle is still open in another thread.
927 Closing the read end of a pipe before the process writing to it at the
928 other end is done writing results in the writer receiving a SIGPIPE. If
929 the other end can't handle that, be sure to read all the data before
934 open(OUTPUT, '|sort >foo') # pipe to sort
935 or die "Can't start sort: $!";
936 #... # print stuff to output
937 close OUTPUT # wait for sort to finish
938 or warn $! ? "Error closing sort pipe: $!"
939 : "Exit status $? from sort";
940 open(INPUT, 'foo') # get sort's results
941 or die "Can't open 'foo' for input: $!";
943 FILEHANDLE may be an expression whose value can be used as an indirect
944 filehandle, usually the real filehandle name.
946 =item closedir DIRHANDLE
949 Closes a directory opened by C<opendir> and returns the success of that
952 =item connect SOCKET,NAME
955 Attempts to connect to a remote socket, just like connect(2).
956 Returns true if it succeeded, false otherwise. NAME should be a
957 packed address of the appropriate type for the socket. See the examples in
958 L<perlipc/"Sockets: Client/Server Communication">.
965 C<continue> is actually a flow control statement rather than a function. If
966 there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
967 C<foreach>), it is always executed just before the conditional is about to
968 be evaluated again, just like the third part of a C<for> loop in C. Thus
969 it can be used to increment a loop variable, even when the loop has been
970 continued via the C<next> statement (which is similar to the C C<continue>
973 C<last>, C<next>, or C<redo> may appear within a C<continue>
974 block; C<last> and C<redo> behave as if they had been executed within
975 the main block. So will C<next>, but since it will execute a C<continue>
976 block, it may be more entertaining.
979 ### redo always comes here
982 ### next always comes here
984 # then back the top to re-check EXPR
986 ### last always comes here
988 Omitting the C<continue> section is equivalent to using an
989 empty one, logically enough, so C<next> goes directly back
990 to check the condition at the top of the loop.
992 If the C<"switch"> feature is enabled, C<continue> is also a
993 function that exits the current C<when> (or C<default>) block and
994 falls through to the next one. See L<feature> and
995 L<perlsyn/"Switch statements"> for more information.
999 X<cos> X<cosine> X<acos> X<arccosine>
1003 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
1004 takes the cosine of C<$_>.
1006 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
1007 function, or use this relation:
1009 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
1011 =item crypt PLAINTEXT,SALT
1012 X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
1013 X<decrypt> X<cryptography> X<passwd> X<encrypt>
1015 Creates a digest string exactly like the crypt(3) function in the C
1016 library (assuming that you actually have a version there that has not
1017 been extirpated as a potential munition).
1019 crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
1020 into a short string, called a digest, which is returned. The same
1021 PLAINTEXT and SALT will always return the same string, but there is no
1022 (known) way to get the original PLAINTEXT from the hash. Small
1023 changes in the PLAINTEXT or SALT will result in large changes in the
1026 There is no decrypt function. This function isn't all that useful for
1027 cryptography (for that, look for F<Crypt> modules on your nearby CPAN
1028 mirror) and the name "crypt" is a bit of a misnomer. Instead it is
1029 primarily used to check if two pieces of text are the same without
1030 having to transmit or store the text itself. An example is checking
1031 if a correct password is given. The digest of the password is stored,
1032 not the password itself. The user types in a password that is
1033 crypt()'d with the same salt as the stored digest. If the two digests
1034 match, the password is correct.
1036 When verifying an existing digest string you should use the digest as
1037 the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
1038 to create the digest is visible as part of the digest. This ensures
1039 crypt() will hash the new string with the same salt as the digest.
1040 This allows your code to work with the standard L<crypt|/crypt> and
1041 with more exotic implementations. In other words, do not assume
1042 anything about the returned string itself, or how many bytes in the
1045 Traditionally the result is a string of 13 bytes: two first bytes of
1046 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
1047 the first eight bytes of PLAINTEXT mattered. But alternative
1048 hashing schemes (like MD5), higher level security schemes (like C2),
1049 and implementations on non-Unix platforms may produce different
1052 When choosing a new salt create a random two character string whose
1053 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
1054 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
1055 characters is just a recommendation; the characters allowed in
1056 the salt depend solely on your system's crypt library, and Perl can't
1057 restrict what salts C<crypt()> accepts.
1059 Here's an example that makes sure that whoever runs this program knows
1062 $pwd = (getpwuid($<))[1];
1064 system "stty -echo";
1066 chomp($word = <STDIN>);
1070 if (crypt($word, $pwd) ne $pwd) {
1076 Of course, typing in your own password to whoever asks you
1079 The L<crypt|/crypt> function is unsuitable for hashing large quantities
1080 of data, not least of all because you can't get the information
1081 back. Look at the L<Digest> module for more robust algorithms.
1083 If using crypt() on a Unicode string (which I<potentially> has
1084 characters with codepoints above 255), Perl tries to make sense
1085 of the situation by trying to downgrade (a copy of)
1086 the string back to an eight-bit byte string before calling crypt()
1087 (on that copy). If that works, good. If not, crypt() dies with
1088 C<Wide character in crypt>.
1093 [This function has been largely superseded by the C<untie> function.]
1095 Breaks the binding between a DBM file and a hash.
1097 =item dbmopen HASH,DBNAME,MASK
1098 X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
1100 [This function has been largely superseded by the C<tie> function.]
1102 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
1103 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
1104 argument is I<not> a filehandle, even though it looks like one). DBNAME
1105 is the name of the database (without the F<.dir> or F<.pag> extension if
1106 any). If the database does not exist, it is created with protection
1107 specified by MASK (as modified by the C<umask>). If your system supports
1108 only the older DBM functions, you may make only one C<dbmopen> call in your
1109 program. In older versions of Perl, if your system had neither DBM nor
1110 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
1113 If you don't have write access to the DBM file, you can only read hash
1114 variables, not set them. If you want to test whether you can write,
1115 either use file tests or try setting a dummy hash entry inside an C<eval>
1118 Note that functions such as C<keys> and C<values> may return huge lists
1119 when used on large DBM files. You may prefer to use the C<each>
1120 function to iterate over large DBM files. Example:
1122 # print out history file offsets
1123 dbmopen(%HIST,'/usr/lib/news/history',0666);
1124 while (($key,$val) = each %HIST) {
1125 print $key, ' = ', unpack('L',$val), "\n";
1129 See also L<AnyDBM_File> for a more general description of the pros and
1130 cons of the various dbm approaches, as well as L<DB_File> for a particularly
1131 rich implementation.
1133 You can control which DBM library you use by loading that library
1134 before you call dbmopen():
1137 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
1138 or die "Can't open netscape history file: $!";
1141 X<defined> X<undef> X<undefined>
1145 Returns a Boolean value telling whether EXPR has a value other than
1146 the undefined value C<undef>. If EXPR is not present, C<$_> is
1149 Many operations return C<undef> to indicate failure, end of file,
1150 system error, uninitialized variable, and other exceptional
1151 conditions. This function allows you to distinguish C<undef> from
1152 other values. (A simple Boolean test will not distinguish among
1153 C<undef>, zero, the empty string, and C<"0">, which are all equally
1154 false.) Note that since C<undef> is a valid scalar, its presence
1155 doesn't I<necessarily> indicate an exceptional condition: C<pop>
1156 returns C<undef> when its argument is an empty array, I<or> when the
1157 element to return happens to be C<undef>.
1159 You may also use C<defined(&func)> to check whether subroutine C<&func>
1160 has ever been defined. The return value is unaffected by any forward
1161 declarations of C<&func>. A subroutine that is not defined
1162 may still be callable: its package may have an C<AUTOLOAD> method that
1163 makes it spring into existence the first time that it is called; see
1166 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
1167 used to report whether memory for that aggregate had ever been
1168 allocated. This behavior may disappear in future versions of Perl.
1169 You should instead use a simple test for size:
1171 if (@an_array) { print "has array elements\n" }
1172 if (%a_hash) { print "has hash members\n" }
1174 When used on a hash element, it tells you whether the value is defined,
1175 not whether the key exists in the hash. Use L</exists> for the latter
1180 print if defined $switch{'D'};
1181 print "$val\n" while defined($val = pop(@ary));
1182 die "Can't readlink $sym: $!"
1183 unless defined($value = readlink $sym);
1184 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1185 $debugging = 0 unless defined $debugging;
1187 Note: Many folks tend to overuse C<defined>, and then are surprised to
1188 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1189 defined values. For example, if you say
1193 The pattern match succeeds and C<$1> is defined, although it
1194 matched "nothing". It didn't really fail to match anything. Rather, it
1195 matched something that happened to be zero characters long. This is all
1196 very above-board and honest. When a function returns an undefined value,
1197 it's an admission that it couldn't give you an honest answer. So you
1198 should use C<defined> only when questioning the integrity of what
1199 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1202 See also L</undef>, L</exists>, L</ref>.
1207 Given an expression that specifies an element or slice of a hash, C<delete>
1208 deletes the specified elements from that hash so that exists() on that element
1209 no longer returns true. Setting a hash element to the undefined value does
1210 not remove its key, but deleting it does; see L</exists>.
1212 It returns the value or values deleted in list context, or the last such
1213 element in scalar context. The return list's length always matches that of
1214 the argument list: deleting non-existent elements returns the undefined value
1215 in their corresponding positions.
1217 delete() may also be used on arrays and array slices, but its behavior is less
1218 straightforward. Although exists() will return false for deleted entries,
1219 deleting array elements never changes indices of existing values; use shift()
1220 or splice() for that. However, if all deleted elements fall at the end of an
1221 array, the array's size shrinks to the position of the highest element that
1222 still tests true for exists(), or to 0 if none do.
1224 B<Be aware> that calling delete on array values is deprecated and likely to
1225 be removed in a future version of Perl.
1227 Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
1228 a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
1229 or array may not necessarily return anything; it depends on the implementation
1230 of the C<tied> package's DELETE method, which may do whatever it pleases.
1232 The C<delete local EXPR> construct localizes the deletion to the current
1233 block at run time. Until the block exits, elements locally deleted
1234 temporarily no longer exist. See L<perlsub/"Localized deletion of elements
1235 of composite types">.
1237 %hash = (foo => 11, bar => 22, baz => 33);
1238 $scalar = delete $hash{foo}; # $scalar is 11
1239 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
1240 @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
1242 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1244 foreach $key (keys %HASH) {
1248 foreach $index (0 .. $#ARRAY) {
1249 delete $ARRAY[$index];
1254 delete @HASH{keys %HASH};
1256 delete @ARRAY[0 .. $#ARRAY];
1258 But both are slower than assigning the empty list
1259 or undefining %HASH or @ARRAY, which is the customary
1260 way to empty out an aggregate:
1262 %HASH = (); # completely empty %HASH
1263 undef %HASH; # forget %HASH ever existed
1265 @ARRAY = (); # completely empty @ARRAY
1266 undef @ARRAY; # forget @ARRAY ever existed
1268 The EXPR can be arbitrarily complicated provided its
1269 final operation is an element or slice of an aggregate:
1271 delete $ref->[$x][$y]{$key};
1272 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1274 delete $ref->[$x][$y][$index];
1275 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1278 X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
1280 C<die> raises an exception. Inside an C<eval> the error message is stuffed
1281 into C<$@> and the C<eval> is terminated with the undefined value.
1282 If the exception is outside of all enclosing C<eval>s, then the uncaught
1283 exception prints LIST to C<STDERR> and exits with a non-zero value. If you
1284 need to exit the process with a specific exit code, see L</exit>.
1286 Equivalent examples:
1288 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1289 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1291 If the last element of LIST does not end in a newline, the current
1292 script line number and input line number (if any) are also printed,
1293 and a newline is supplied. Note that the "input line number" (also
1294 known as "chunk") is subject to whatever notion of "line" happens to
1295 be currently in effect, and is also available as the special variable
1296 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1298 Hint: sometimes appending C<", stopped"> to your message will cause it
1299 to make better sense when the string C<"at foo line 123"> is appended.
1300 Suppose you are running script "canasta".
1302 die "/etc/games is no good";
1303 die "/etc/games is no good, stopped";
1305 produce, respectively
1307 /etc/games is no good at canasta line 123.
1308 /etc/games is no good, stopped at canasta line 123.
1310 If the output is empty and C<$@> already contains a value (typically from a
1311 previous eval) that value is reused after appending C<"\t...propagated">.
1312 This is useful for propagating exceptions:
1315 die unless $@ =~ /Expected exception/;
1317 If the output is empty and C<$@> contains an object reference that has a
1318 C<PROPAGATE> method, that method will be called with additional file
1319 and line number parameters. The return value replaces the value in
1320 C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
1323 If C<$@> is empty then the string C<"Died"> is used.
1325 If an uncaught exception results in interpreter exit, the exit code is
1326 determined from the values of C<$!> and C<$?> with this pseudocode:
1328 exit $! if $!; # errno
1329 exit $? >> 8 if $? >> 8; # child exit status
1330 exit 255; # last resort
1332 The intent is to squeeze as much possible information about the likely cause
1333 into the limited space of the system exit code. However, as C<$!> is the value
1334 of C's C<errno>, which can be set by any system call, this means that the value
1335 of the exit code used by C<die> can be non-predictable, so should not be relied
1336 upon, other than to be non-zero.
1338 You can also call C<die> with a reference argument, and if this is trapped
1339 within an C<eval>, C<$@> contains that reference. This permits more
1340 elaborate exception handling using objects that maintain arbitrary state
1341 about the exception. Such a scheme is sometimes preferable to matching
1342 particular string values of C<$@> with regular expressions. Because C<$@>
1343 is a global variable and C<eval> may be used within object implementations,
1344 be careful that analyzing the error object doesn't replace the reference in
1345 the global variable. It's easiest to make a local copy of the reference
1346 before any manipulations. Here's an example:
1348 use Scalar::Util "blessed";
1350 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1351 if (my $ev_err = $@) {
1352 if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
1353 # handle Some::Module::Exception
1356 # handle all other possible exceptions
1360 Because Perl stringifies uncaught exception messages before display,
1361 you'll probably want to overload stringification operations on
1362 exception objects. See L<overload> for details about that.
1364 You can arrange for a callback to be run just before the C<die>
1365 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1366 handler is called with the error text and can change the error
1367 message, if it sees fit, by calling C<die> again. See
1368 L<perlvar/%SIG> for details on setting C<%SIG> entries, and
1369 L<"eval BLOCK"> for some examples. Although this feature was
1370 to be run only right before your program was to exit, this is not
1371 currently so: the C<$SIG{__DIE__}> hook is currently called
1372 even inside eval()ed blocks/strings! If one wants the hook to do
1373 nothing in such situations, put
1377 as the first line of the handler (see L<perlvar/$^S>). Because
1378 this promotes strange action at a distance, this counterintuitive
1379 behavior may be fixed in a future release.
1381 See also exit(), warn(), and the Carp module.
1386 Not really a function. Returns the value of the last command in the
1387 sequence of commands indicated by BLOCK. When modified by the C<while> or
1388 C<until> loop modifier, executes the BLOCK once before testing the loop
1389 condition. (On other statements the loop modifiers test the conditional
1392 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1393 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1394 See L<perlsyn> for alternative strategies.
1396 =item do SUBROUTINE(LIST)
1399 This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
1400 a scalar variable or a subroutine beginning with C<&>.
1405 Uses the value of EXPR as a filename and executes the contents of the
1406 file as a Perl script.
1414 except that it's more efficient and concise, keeps track of the current
1415 filename for error messages, searches the C<@INC> directories, and updates
1416 C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
1417 these variables. It also differs in that code evaluated with C<do FILENAME>
1418 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1419 same, however, in that it does reparse the file every time you call it,
1420 so you probably don't want to do this inside a loop.
1422 If C<do> can read the file but cannot compile it, it returns undef and sets
1423 an error message in C<$@>. If C<do> cannot read the file, it returns undef
1424 and sets C<$!> to the error. Always check C<$@> first, as compilation
1425 could fail in a way that also sets C<$!>. If the file is successfully
1426 compiled, C<do> returns the value of the last expression evaluated.
1428 Inclusion of library modules is better done with the
1429 C<use> and C<require> operators, which also do automatic error checking
1430 and raise an exception if there's a problem.
1432 You might like to use C<do> to read in a program configuration
1433 file. Manual error checking can be done this way:
1435 # read in config files: system first, then user
1436 for $file ("/share/prog/defaults.rc",
1437 "$ENV{HOME}/.someprogrc")
1439 unless ($return = do $file) {
1440 warn "couldn't parse $file: $@" if $@;
1441 warn "couldn't do $file: $!" unless defined $return;
1442 warn "couldn't run $file" unless $return;
1447 X<dump> X<core> X<undump>
1451 This function causes an immediate core dump. See also the B<-u>
1452 command-line switch in L<perlrun>, which does the same thing.
1453 Primarily this is so that you can use the B<undump> program (not
1454 supplied) to turn your core dump into an executable binary after
1455 having initialized all your variables at the beginning of the
1456 program. When the new binary is executed it will begin by executing
1457 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1458 Think of it as a goto with an intervening core dump and reincarnation.
1459 If C<LABEL> is omitted, restarts the program from the top.
1461 B<WARNING>: Any files opened at the time of the dump will I<not>
1462 be open any more when the program is reincarnated, with possible
1463 resulting confusion by Perl.
1465 This function is now largely obsolete, mostly because it's very hard to
1466 convert a core file into an executable. That's why you should now invoke
1467 it as C<CORE::dump()>, if you don't want to be warned against a possible
1471 X<each> X<hash, iterator>
1476 When called in list context, returns a 2-element list consisting of the key
1477 and value for the next element of a hash, or the index and value for the
1478 next element of an array, so that you can iterate over it. When called in
1479 scalar context, returns only the key (not the value) in a hash, or the index
1482 Hash entries are returned in an apparently random order. The actual random
1483 order is subject to change in future versions of Perl, but it is
1484 guaranteed to be in the same order as either the C<keys> or C<values>
1485 function would produce on the same (unmodified) hash. Since Perl
1486 5.8.2 the ordering can be different even between different runs of Perl
1487 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
1489 After C<each> has returned all entries from the hash or array, the next
1490 call to C<each> returns the empty list in list context and C<undef> in
1491 scalar context. The next call following that one restarts iteration. Each
1492 hash or array has its own internal iterator, accessed by C<each>, C<keys>,
1493 and C<values>. The iterator is implicitly reset when C<each> has reached
1494 the end as just described; it can be explicitly reset by calling C<keys> or
1495 C<values> on the hash or array. If you add or delete a hash's elements
1496 while iterating over it, entries may be skipped or duplicated--so don't do
1497 that. Exception: It is always safe to delete the item most recently
1498 returned by C<each()>, so the following code works properly:
1500 while (($key, $value) = each %hash) {
1502 delete $hash{$key}; # This is safe
1505 This prints out your environment like the printenv(1) program,
1506 but in a different order:
1508 while (($key,$value) = each %ENV) {
1509 print "$key=$value\n";
1512 Starting with Perl 5.14, C<each> can take a reference to an unblessed hash
1513 or array. The argument will be dereferenced automatically. This aspect of
1514 C<each> is considered highly experimental. The exact behaviour may change
1515 in a future version of Perl.
1517 while (($key,$value) = each $hashref) { ... }
1519 See also C<keys>, C<values> and C<sort>.
1521 =item eof FILEHANDLE
1530 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1531 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1532 gives the real filehandle. (Note that this function actually
1533 reads a character and then C<ungetc>s it, so isn't useful in an
1534 interactive context.) Do not read from a terminal file (or call
1535 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1536 as terminals may lose the end-of-file condition if you do.
1538 An C<eof> without an argument uses the last file read. Using C<eof()>
1539 with empty parentheses is different. It refers to the pseudo file
1540 formed from the files listed on the command line and accessed via the
1541 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1542 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1543 used will cause C<@ARGV> to be examined to determine if input is
1544 available. Similarly, an C<eof()> after C<< <> >> has returned
1545 end-of-file will assume you are processing another C<@ARGV> list,
1546 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1547 see L<perlop/"I/O Operators">.
1549 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1550 detect the end of each file, C<eof()> will detect the end of only the
1551 last file. Examples:
1553 # reset line numbering on each input file
1555 next if /^\s*#/; # skip comments
1558 close ARGV if eof; # Not eof()!
1561 # insert dashes just before last line of last file
1563 if (eof()) { # check for end of last file
1564 print "--------------\n";
1567 last if eof(); # needed if we're reading from a terminal
1570 Practical hint: you almost never need to use C<eof> in Perl, because the
1571 input operators typically return C<undef> when they run out of data, or if
1575 X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
1576 X<error, handling> X<exception, handling>
1582 In the first form, the return value of EXPR is parsed and executed as if it
1583 were a little Perl program. The value of the expression (which is itself
1584 determined within scalar context) is first parsed, and if there weren't any
1585 errors, executed in the lexical context of the current Perl program, so
1586 that any variable settings or subroutine and format definitions remain
1587 afterwards. Note that the value is parsed every time the C<eval> executes.
1588 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1589 delay parsing and subsequent execution of the text of EXPR until run time.
1591 In the second form, the code within the BLOCK is parsed only once--at the
1592 same time the code surrounding the C<eval> itself was parsed--and executed
1593 within the context of the current Perl program. This form is typically
1594 used to trap exceptions more efficiently than the first (see below), while
1595 also providing the benefit of checking the code within BLOCK at compile
1598 The final semicolon, if any, may be omitted from the value of EXPR or within
1601 In both forms, the value returned is the value of the last expression
1602 evaluated inside the mini-program; a return statement may be also used, just
1603 as with subroutines. The expression providing the return value is evaluated
1604 in void, scalar, or list context, depending on the context of the C<eval>
1605 itself. See L</wantarray> for more on how the evaluation context can be
1608 If there is a syntax error or runtime error, or a C<die> statement is
1609 executed, C<eval> returns an undefined value in scalar context
1610 or an empty list--or, for syntax errors, a list containing a single
1611 undefined value--in list context, and C<$@> is set to the error
1612 message. The discrepancy in the return values in list context is
1613 considered a bug by some, and will probably be fixed in a future
1614 release. If there was no error, C<$@> is guaranteed to be the empty
1615 string. Beware that using C<eval> neither silences Perl from printing
1616 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1617 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1618 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1619 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1621 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1622 determining whether a particular feature (such as C<socket> or C<symlink>)
1623 is implemented. It is also Perl's exception-trapping mechanism, where
1624 the die operator is used to raise exceptions.
1626 If you want to trap errors when loading an XS module, some problems with
1627 the binary interface (such as Perl version skew) may be fatal even with
1628 C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
1630 If the code to be executed doesn't vary, you may use the eval-BLOCK
1631 form to trap run-time errors without incurring the penalty of
1632 recompiling each time. The error, if any, is still returned in C<$@>.
1635 # make divide-by-zero nonfatal
1636 eval { $answer = $a / $b; }; warn $@ if $@;
1638 # same thing, but less efficient
1639 eval '$answer = $a / $b'; warn $@ if $@;
1641 # a compile-time error
1642 eval { $answer = }; # WRONG
1645 eval '$answer ='; # sets $@
1647 Using the C<eval{}> form as an exception trap in libraries does have some
1648 issues. Due to the current arguably broken state of C<__DIE__> hooks, you
1649 may wish not to trigger any C<__DIE__> hooks that user code may have installed.
1650 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1651 as this example shows:
1653 # a private exception trap for divide-by-zero
1654 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1657 This is especially significant, given that C<__DIE__> hooks can call
1658 C<die> again, which has the effect of changing their error messages:
1660 # __DIE__ hooks may modify error messages
1662 local $SIG{'__DIE__'} =
1663 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1664 eval { die "foo lives here" };
1665 print $@ if $@; # prints "bar lives here"
1668 Because this promotes action at a distance, this counterintuitive behavior
1669 may be fixed in a future release.
1671 With an C<eval>, you should be especially careful to remember what's
1672 being looked at when:
1678 eval { $x }; # CASE 4
1680 eval "\$$x++"; # CASE 5
1683 Cases 1 and 2 above behave identically: they run the code contained in
1684 the variable $x. (Although case 2 has misleading double quotes making
1685 the reader wonder what else might be happening (nothing is).) Cases 3
1686 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1687 does nothing but return the value of $x. (Case 4 is preferred for
1688 purely visual reasons, but it also has the advantage of compiling at
1689 compile-time instead of at run-time.) Case 5 is a place where
1690 normally you I<would> like to use double quotes, except that in this
1691 particular situation, you can just use symbolic references instead, as
1694 Before Perl 5.14, the assignment to C<$@> occurred before restoration
1695 of localised variables, which means that for your code to run on older
1696 versions, a temporary is required if you want to mask some but not all
1699 # alter $@ on nefarious repugnancy only
1703 local $@; # protect existing $@
1704 eval { test_repugnancy() };
1705 # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
1706 $@ =~ /nefarious/ and $e = $@;
1708 die $e if defined $e
1711 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1712 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1714 An C<eval ''> executed within the C<DB> package doesn't see the usual
1715 surrounding lexical scope, but rather the scope of the first non-DB piece
1716 of code that called it. You don't normally need to worry about this unless
1717 you are writing a Perl debugger.
1722 =item exec PROGRAM LIST
1724 The C<exec> function executes a system command I<and never returns>;
1725 use C<system> instead of C<exec> if you want it to return. It fails and
1726 returns false only if the command does not exist I<and> it is executed
1727 directly instead of via your system's command shell (see below).
1729 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1730 warns you if there is a following statement that isn't C<die>, C<warn>,
1731 or C<exit> (if C<-w> is set--but you always do that, right?). If you
1732 I<really> want to follow an C<exec> with some other statement, you
1733 can use one of these styles to avoid the warning:
1735 exec ('foo') or print STDERR "couldn't exec foo: $!";
1736 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1738 If there is more than one argument in LIST, or if LIST is an array
1739 with more than one value, calls execvp(3) with the arguments in LIST.
1740 If there is only one scalar argument or an array with one element in it,
1741 the argument is checked for shell metacharacters, and if there are any,
1742 the entire argument is passed to the system's command shell for parsing
1743 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1744 If there are no shell metacharacters in the argument, it is split into
1745 words and passed directly to C<execvp>, which is more efficient.
1748 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1749 exec "sort $outfile | uniq";
1751 If you don't really want to execute the first argument, but want to lie
1752 to the program you are executing about its own name, you can specify
1753 the program you actually want to run as an "indirect object" (without a
1754 comma) in front of the LIST. (This always forces interpretation of the
1755 LIST as a multivalued list, even if there is only a single scalar in
1758 $shell = '/bin/csh';
1759 exec $shell '-sh'; # pretend it's a login shell
1763 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1765 When the arguments get executed via the system shell, results are
1766 subject to its quirks and capabilities. See L<perlop/"`STRING`">
1769 Using an indirect object with C<exec> or C<system> is also more
1770 secure. This usage (which also works fine with system()) forces
1771 interpretation of the arguments as a multivalued list, even if the
1772 list had just one argument. That way you're safe from the shell
1773 expanding wildcards or splitting up words with whitespace in them.
1775 @args = ( "echo surprise" );
1777 exec @args; # subject to shell escapes
1779 exec { $args[0] } @args; # safe even with one-arg list
1781 The first version, the one without the indirect object, ran the I<echo>
1782 program, passing it C<"surprise"> an argument. The second version didn't;
1783 it tried to run a program named I<"echo surprise">, didn't find it, and set
1784 C<$?> to a non-zero value indicating failure.
1786 Beginning with v5.6.0, Perl attempts to flush all files opened for
1787 output before the exec, but this may not be supported on some platforms
1788 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1789 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1790 open handles to avoid lost output.
1792 Note that C<exec> will not call your C<END> blocks, nor will it invoke
1793 C<DESTROY> methods on your objects.
1796 X<exists> X<autovivification>
1798 Given an expression that specifies an element of a hash, returns true if the
1799 specified element in the hash has ever been initialized, even if the
1800 corresponding value is undefined.
1802 print "Exists\n" if exists $hash{$key};
1803 print "Defined\n" if defined $hash{$key};
1804 print "True\n" if $hash{$key};
1806 exists may also be called on array elements, but its behavior is much less
1807 obvious, and is strongly tied to the use of L</delete> on arrays. B<Be aware>
1808 that calling exists on array values is deprecated and likely to be removed in
1809 a future version of Perl.
1811 print "Exists\n" if exists $array[$index];
1812 print "Defined\n" if defined $array[$index];
1813 print "True\n" if $array[$index];
1815 A hash or array element can be true only if it's defined, and defined if
1816 it exists, but the reverse doesn't necessarily hold true.
1818 Given an expression that specifies the name of a subroutine,
1819 returns true if the specified subroutine has ever been declared, even
1820 if it is undefined. Mentioning a subroutine name for exists or defined
1821 does not count as declaring it. Note that a subroutine that does not
1822 exist may still be callable: its package may have an C<AUTOLOAD>
1823 method that makes it spring into existence the first time that it is
1824 called; see L<perlsub>.
1826 print "Exists\n" if exists &subroutine;
1827 print "Defined\n" if defined &subroutine;
1829 Note that the EXPR can be arbitrarily complicated as long as the final
1830 operation is a hash or array key lookup or subroutine name:
1832 if (exists $ref->{A}->{B}->{$key}) { }
1833 if (exists $hash{A}{B}{$key}) { }
1835 if (exists $ref->{A}->{B}->[$ix]) { }
1836 if (exists $hash{A}{B}[$ix]) { }
1838 if (exists &{$ref->{A}{B}{$key}}) { }
1840 Although the mostly deeply nested array or hash will not spring into
1841 existence just because its existence was tested, any intervening ones will.
1842 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1843 into existence due to the existence test for the $key element above.
1844 This happens anywhere the arrow operator is used, including even here:
1847 if (exists $ref->{"Some key"}) { }
1848 print $ref; # prints HASH(0x80d3d5c)
1850 This surprising autovivification in what does not at first--or even
1851 second--glance appear to be an lvalue context may be fixed in a future
1854 Use of a subroutine call, rather than a subroutine name, as an argument
1855 to exists() is an error.
1858 exists &sub(); # Error
1861 X<exit> X<terminate> X<abort>
1865 Evaluates EXPR and exits immediately with that value. Example:
1868 exit 0 if $ans =~ /^[Xx]/;
1870 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1871 universally recognized values for EXPR are C<0> for success and C<1>
1872 for error; other values are subject to interpretation depending on the
1873 environment in which the Perl program is running. For example, exiting
1874 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1875 the mailer to return the item undelivered, but that's not true everywhere.
1877 Don't use C<exit> to abort a subroutine if there's any chance that
1878 someone might want to trap whatever error happened. Use C<die> instead,
1879 which can be trapped by an C<eval>.
1881 The exit() function does not always exit immediately. It calls any
1882 defined C<END> routines first, but these C<END> routines may not
1883 themselves abort the exit. Likewise any object destructors that need to
1884 be called are called before the real exit. C<END> routines and destructors
1885 can change the exit status by modifying C<$?>. If this is a problem, you
1886 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1887 See L<perlmod> for details.
1890 X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
1894 Returns I<e> (the natural logarithm base) to the power of EXPR.
1895 If EXPR is omitted, gives C<exp($_)>.
1897 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1900 Implements the fcntl(2) function. You'll probably have to say
1904 first to get the correct constant definitions. Argument processing and
1905 value returned work just like C<ioctl> below.
1909 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1910 or die "can't fcntl F_GETFL: $!";
1912 You don't have to check for C<defined> on the return from C<fcntl>.
1913 Like C<ioctl>, it maps a C<0> return from the system call into
1914 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1915 in numeric context. It is also exempt from the normal B<-w> warnings
1916 on improper numeric conversions.
1918 Note that C<fcntl> raises an exception if used on a machine that
1919 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1920 manpage to learn what functions are available on your system.
1922 Here's an example of setting a filehandle named C<REMOTE> to be
1923 non-blocking at the system level. You'll have to negotiate C<$|>
1924 on your own, though.
1926 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
1928 $flags = fcntl(REMOTE, F_GETFL, 0)
1929 or die "Can't get flags for the socket: $!\n";
1931 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
1932 or die "Can't set flags for the socket: $!\n";
1934 =item fileno FILEHANDLE
1937 Returns the file descriptor for a filehandle, or undefined if the
1938 filehandle is not open. If there is no real file descriptor at the OS
1939 level, as can happen with filehandles connected to memory objects via
1940 C<open> with a reference for the third argument, -1 is returned.
1942 This is mainly useful for constructing
1943 bitmaps for C<select> and low-level POSIX tty-handling operations.
1944 If FILEHANDLE is an expression, the value is taken as an indirect
1945 filehandle, generally its name.
1947 You can use this to find out whether two handles refer to the
1948 same underlying descriptor:
1950 if (fileno(THIS) == fileno(THAT)) {
1951 print "THIS and THAT are dups\n";
1954 =item flock FILEHANDLE,OPERATION
1955 X<flock> X<lock> X<locking>
1957 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1958 for success, false on failure. Produces a fatal error if used on a
1959 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1960 C<flock> is Perl's portable file-locking interface, although it locks
1961 entire files only, not records.
1963 Two potentially non-obvious but traditional C<flock> semantics are
1964 that it waits indefinitely until the lock is granted, and that its locks
1965 are B<merely advisory>. Such discretionary locks are more flexible, but
1966 offer fewer guarantees. This means that programs that do not also use
1967 C<flock> may modify files locked with C<flock>. See L<perlport>,
1968 your port's specific documentation, or your system-specific local manpages
1969 for details. It's best to assume traditional behavior if you're writing
1970 portable programs. (But if you're not, you should as always feel perfectly
1971 free to write for your own system's idiosyncrasies (sometimes called
1972 "features"). Slavish adherence to portability concerns shouldn't get
1973 in the way of your getting your job done.)
1975 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1976 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1977 you can use the symbolic names if you import them from the Fcntl module,
1978 either individually, or as a group using the ':flock' tag. LOCK_SH
1979 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1980 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1981 LOCK_SH or LOCK_EX then C<flock> returns immediately rather than blocking
1982 waiting for the lock; check the return status to see if you got it.
1984 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1985 before locking or unlocking it.
1987 Note that the emulation built with lockf(3) doesn't provide shared
1988 locks, and it requires that FILEHANDLE be open with write intent. These
1989 are the semantics that lockf(3) implements. Most if not all systems
1990 implement lockf(3) in terms of fcntl(2) locking, though, so the
1991 differing semantics shouldn't bite too many people.
1993 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1994 be open with read intent to use LOCK_SH and requires that it be open
1995 with write intent to use LOCK_EX.
1997 Note also that some versions of C<flock> cannot lock things over the
1998 network; you would need to use the more system-specific C<fcntl> for
1999 that. If you like you can force Perl to ignore your system's flock(2)
2000 function, and so provide its own fcntl(2)-based emulation, by passing
2001 the switch C<-Ud_flock> to the F<Configure> program when you configure
2004 Here's a mailbox appender for BSD systems.
2006 use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
2010 flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
2012 # and, in case someone appended while we were waiting...
2013 seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
2018 flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
2021 open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
2022 or die "Can't open mailbox: $!";
2025 print $mbox $msg,"\n\n";
2028 On systems that support a real flock(2), locks are inherited across fork()
2029 calls, whereas those that must resort to the more capricious fcntl(2)
2030 function lose their locks, making it seriously harder to write servers.
2032 See also L<DB_File> for other flock() examples.
2035 X<fork> X<child> X<parent>
2037 Does a fork(2) system call to create a new process running the
2038 same program at the same point. It returns the child pid to the
2039 parent process, C<0> to the child process, or C<undef> if the fork is
2040 unsuccessful. File descriptors (and sometimes locks on those descriptors)
2041 are shared, while everything else is copied. On most systems supporting
2042 fork(), great care has gone into making it extremely efficient (for
2043 example, using copy-on-write technology on data pages), making it the
2044 dominant paradigm for multitasking over the last few decades.
2046 Beginning with v5.6.0, Perl attempts to flush all files opened for
2047 output before forking the child process, but this may not be supported
2048 on some platforms (see L<perlport>). To be safe, you may need to set
2049 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
2050 C<IO::Handle> on any open handles to avoid duplicate output.
2052 If you C<fork> without ever waiting on your children, you will
2053 accumulate zombies. On some systems, you can avoid this by setting
2054 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
2055 forking and reaping moribund children.
2057 Note that if your forked child inherits system file descriptors like
2058 STDIN and STDOUT that are actually connected by a pipe or socket, even
2059 if you exit, then the remote server (such as, say, a CGI script or a
2060 backgrounded job launched from a remote shell) won't think you're done.
2061 You should reopen those to F</dev/null> if it's any issue.
2066 Declare a picture format for use by the C<write> function. For
2070 Test: @<<<<<<<< @||||| @>>>>>
2071 $str, $%, '$' . int($num)
2075 $num = $cost/$quantity;
2079 See L<perlform> for many details and examples.
2081 =item formline PICTURE,LIST
2084 This is an internal function used by C<format>s, though you may call it,
2085 too. It formats (see L<perlform>) a list of values according to the
2086 contents of PICTURE, placing the output into the format output
2087 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
2088 Eventually, when a C<write> is done, the contents of
2089 C<$^A> are written to some filehandle. You could also read C<$^A>
2090 and then set C<$^A> back to C<"">. Note that a format typically
2091 does one C<formline> per line of form, but the C<formline> function itself
2092 doesn't care how many newlines are embedded in the PICTURE. This means
2093 that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
2094 You may therefore need to use multiple formlines to implement a single
2095 record format, just like the C<format> compiler.
2097 Be careful if you put double quotes around the picture, because an C<@>
2098 character may be taken to mean the beginning of an array name.
2099 C<formline> always returns true. See L<perlform> for other examples.
2101 If you are trying to use this instead of C<write> to capture the output,
2102 you may find it easier to open a filehandle to a scalar
2103 (C<< open $fh, ">", \$output >>) and write to that instead.
2105 =item getc FILEHANDLE
2106 X<getc> X<getchar> X<character> X<file, read>
2110 Returns the next character from the input file attached to FILEHANDLE,
2111 or the undefined value at end of file or if there was an error (in
2112 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
2113 STDIN. This is not particularly efficient. However, it cannot be
2114 used by itself to fetch single characters without waiting for the user
2115 to hit enter. For that, try something more like:
2118 system "stty cbreak </dev/tty >/dev/tty 2>&1";
2121 system "stty", '-icanon', 'eol', "\001";
2127 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
2130 system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
2134 Determination of whether $BSD_STYLE should be set
2135 is left as an exercise to the reader.
2137 The C<POSIX::getattr> function can do this more portably on
2138 systems purporting POSIX compliance. See also the C<Term::ReadKey>
2139 module from your nearest CPAN site; details on CPAN can be found under
2143 X<getlogin> X<login>
2145 This implements the C library function of the same name, which on most
2146 systems returns the current login from F</etc/utmp>, if any. If it
2147 returns the empty string, use C<getpwuid>.
2149 $login = getlogin || getpwuid($<) || "Kilroy";
2151 Do not consider C<getlogin> for authentication: it is not as
2152 secure as C<getpwuid>.
2154 =item getpeername SOCKET
2155 X<getpeername> X<peer>
2157 Returns the packed sockaddr address of the other end of the SOCKET
2161 $hersockaddr = getpeername(SOCK);
2162 ($port, $iaddr) = sockaddr_in($hersockaddr);
2163 $herhostname = gethostbyaddr($iaddr, AF_INET);
2164 $herstraddr = inet_ntoa($iaddr);
2169 Returns the current process group for the specified PID. Use
2170 a PID of C<0> to get the current process group for the
2171 current process. Will raise an exception if used on a machine that
2172 doesn't implement getpgrp(2). If PID is omitted, returns the process
2173 group of the current process. Note that the POSIX version of C<getpgrp>
2174 does not accept a PID argument, so only C<PID==0> is truly portable.
2177 X<getppid> X<parent> X<pid>
2179 Returns the process id of the parent process.
2181 Note for Linux users: on Linux, the C functions C<getpid()> and
2182 C<getppid()> return different values from different threads. In order to
2183 be portable, this behavior is not reflected by the Perl-level function
2184 C<getppid()>, that returns a consistent value across threads. If you want
2185 to call the underlying C<getppid()>, you may use the CPAN module
2188 =item getpriority WHICH,WHO
2189 X<getpriority> X<priority> X<nice>
2191 Returns the current priority for a process, a process group, or a user.
2192 (See C<getpriority(2)>.) Will raise a fatal exception if used on a
2193 machine that doesn't implement getpriority(2).
2196 X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
2197 X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
2198 X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
2199 X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
2200 X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
2201 X<endnetent> X<endprotoent> X<endservent>
2205 =item gethostbyname NAME
2207 =item getnetbyname NAME
2209 =item getprotobyname NAME
2215 =item getservbyname NAME,PROTO
2217 =item gethostbyaddr ADDR,ADDRTYPE
2219 =item getnetbyaddr ADDR,ADDRTYPE
2221 =item getprotobynumber NUMBER
2223 =item getservbyport PORT,PROTO
2241 =item sethostent STAYOPEN
2243 =item setnetent STAYOPEN
2245 =item setprotoent STAYOPEN
2247 =item setservent STAYOPEN
2261 These routines are the same as their counterparts in the
2262 system C library. In list context, the return values from the
2263 various get routines are as follows:
2265 ($name,$passwd,$uid,$gid,
2266 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
2267 ($name,$passwd,$gid,$members) = getgr*
2268 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
2269 ($name,$aliases,$addrtype,$net) = getnet*
2270 ($name,$aliases,$proto) = getproto*
2271 ($name,$aliases,$port,$proto) = getserv*
2273 (If the entry doesn't exist you get an empty list.)
2275 The exact meaning of the $gcos field varies but it usually contains
2276 the real name of the user (as opposed to the login name) and other
2277 information pertaining to the user. Beware, however, that in many
2278 system users are able to change this information and therefore it
2279 cannot be trusted and therefore the $gcos is tainted (see
2280 L<perlsec>). The $passwd and $shell, user's encrypted password and
2281 login shell, are also tainted, for the same reason.
2283 In scalar context, you get the name, unless the function was a
2284 lookup by name, in which case you get the other thing, whatever it is.
2285 (If the entry doesn't exist you get the undefined value.) For example:
2287 $uid = getpwnam($name);
2288 $name = getpwuid($num);
2290 $gid = getgrnam($name);
2291 $name = getgrgid($num);
2295 In I<getpw*()> the fields $quota, $comment, and $expire are special
2296 in that they are unsupported on many systems. If the
2297 $quota is unsupported, it is an empty scalar. If it is supported, it
2298 usually encodes the disk quota. If the $comment field is unsupported,
2299 it is an empty scalar. If it is supported it usually encodes some
2300 administrative comment about the user. In some systems the $quota
2301 field may be $change or $age, fields that have to do with password
2302 aging. In some systems the $comment field may be $class. The $expire
2303 field, if present, encodes the expiration period of the account or the
2304 password. For the availability and the exact meaning of these fields
2305 in your system, please consult your getpwnam(3) documentation and your
2306 F<pwd.h> file. You can also find out from within Perl what your
2307 $quota and $comment fields mean and whether you have the $expire field
2308 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2309 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2310 files are supported only if your vendor has implemented them in the
2311 intuitive fashion that calling the regular C library routines gets the
2312 shadow versions if you're running under privilege or if there exists
2313 the shadow(3) functions as found in System V (this includes Solaris
2314 and Linux). Those systems that implement a proprietary shadow password
2315 facility are unlikely to be supported.
2317 The $members value returned by I<getgr*()> is a space-separated list of
2318 the login names of the members of the group.
2320 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2321 C, it will be returned to you via C<$?> if the function call fails. The
2322 C<@addrs> value returned by a successful call is a list of raw
2323 addresses returned by the corresponding library call. In the
2324 Internet domain, each address is four bytes long; you can unpack it
2325 by saying something like:
2327 ($a,$b,$c,$d) = unpack('W4',$addr[0]);
2329 The Socket library makes this slightly easier:
2332 $iaddr = inet_aton("127.1"); # or whatever address
2333 $name = gethostbyaddr($iaddr, AF_INET);
2335 # or going the other way
2336 $straddr = inet_ntoa($iaddr);
2338 In the opposite way, to resolve a hostname to the IP address
2342 $packed_ip = gethostbyname("www.perl.org");
2343 if (defined $packed_ip) {
2344 $ip_address = inet_ntoa($packed_ip);
2347 Make sure <gethostbyname()> is called in SCALAR context and that
2348 its return value is checked for definedness.
2350 If you get tired of remembering which element of the return list
2351 contains which return value, by-name interfaces are provided
2352 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2353 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2354 and C<User::grent>. These override the normal built-ins, supplying
2355 versions that return objects with the appropriate names
2356 for each field. For example:
2360 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2362 Even though it looks as though they're the same method calls (uid),
2363 they aren't, because a C<File::stat> object is different from
2364 a C<User::pwent> object.
2366 =item getsockname SOCKET
2369 Returns the packed sockaddr address of this end of the SOCKET connection,
2370 in case you don't know the address because you have several different
2371 IPs that the connection might have come in on.
2374 $mysockaddr = getsockname(SOCK);
2375 ($port, $myaddr) = sockaddr_in($mysockaddr);
2376 printf "Connect to %s [%s]\n",
2377 scalar gethostbyaddr($myaddr, AF_INET),
2380 =item getsockopt SOCKET,LEVEL,OPTNAME
2383 Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
2384 Options may exist at multiple protocol levels depending on the socket
2385 type, but at least the uppermost socket level SOL_SOCKET (defined in the
2386 C<Socket> module) will exist. To query options at another level the
2387 protocol number of the appropriate protocol controlling the option
2388 should be supplied. For example, to indicate that an option is to be
2389 interpreted by the TCP protocol, LEVEL should be set to the protocol
2390 number of TCP, which you can get using C<getprotobyname>.
2392 The function returns a packed string representing the requested socket
2393 option, or C<undef> on error, with the reason for the error placed in
2394 C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
2395 consult getsockopt(2) for details. A common case is that the option is an
2396 integer, in which case the result is a packed integer, which you can decode
2397 using C<unpack> with the C<i> (or C<I>) format.
2399 An example to test whether Nagle's algorithm is turned on on a socket:
2401 use Socket qw(:all);
2403 defined(my $tcp = getprotobyname("tcp"))
2404 or die "Could not determine the protocol number for tcp";
2405 # my $tcp = IPPROTO_TCP; # Alternative
2406 my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
2407 or die "getsockopt TCP_NODELAY: $!";
2408 my $nodelay = unpack("I", $packed);
2409 print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
2413 X<glob> X<wildcard> X<filename, expansion> X<expand>
2417 In list context, returns a (possibly empty) list of filename expansions on
2418 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2419 scalar context, glob iterates through such filename expansions, returning
2420 undef when the list is exhausted. This is the internal function
2421 implementing the C<< <*.c> >> operator, but you can use it directly. If
2422 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2423 more detail in L<perlop/"I/O Operators">.
2425 Note that C<glob> splits its arguments on whitespace and treats
2426 each segment as separate pattern. As such, C<glob("*.c *.h")>
2427 matches all files with a F<.c> or F<.h> extension. The expression
2428 C<glob(".* *")> matches all files in the current working directory.
2430 If non-empty braces are the only wildcard characters used in the
2431 C<glob>, no filenames are matched, but potentially many strings
2432 are returned. For example, this produces nine strings, one for
2433 each pairing of fruits and colors:
2435 @many = glob "{apple,tomato,cherry}={green,yellow,red}";
2437 Beginning with v5.6.0, this operator is implemented using the standard
2438 C<File::Glob> extension. See L<File::Glob> for details, including
2439 C<bsd_glob> which does not treat whitespace as a pattern separator.
2442 X<gmtime> X<UTC> X<Greenwich>
2446 Works just like L<localtime> but the returned values are
2447 localized for the standard Greenwich time zone.
2449 Note: When called in list context, $isdst, the last value
2450 returned by gmtime, is always C<0>. There is no
2451 Daylight Saving Time in GMT.
2453 See L<perlport/gmtime> for portability concerns.
2456 X<goto> X<jump> X<jmp>
2462 The C<goto-LABEL> form finds the statement labeled with LABEL and
2463 resumes execution there. It can't be used to get out of a block or
2464 subroutine given to C<sort>. It can be used to go almost anywhere
2465 else within the dynamic scope, including out of subroutines, but it's
2466 usually better to use some other construct such as C<last> or C<die>.
2467 The author of Perl has never felt the need to use this form of C<goto>
2468 (in Perl, that is; C is another matter). (The difference is that C
2469 does not offer named loops combined with loop control. Perl does, and
2470 this replaces most structured uses of C<goto> in other languages.)
2472 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2473 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2474 necessarily recommended if you're optimizing for maintainability:
2476 goto ("FOO", "BAR", "GLARCH")[$i];
2478 As shown in this example, C<goto-EXPR> is exempt from the "looks like a
2479 function" rule. A pair of parentheses following it does not (necessarily)
2480 delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
2482 Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
2483 deprecated and will issue a warning. Even then, it may not be used to
2484 go into any construct that requires initialization, such as a
2485 subroutine or a C<foreach> loop. It also can't be used to go into a
2486 construct that is optimized away.
2488 The C<goto-&NAME> form is quite different from the other forms of
2489 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2490 doesn't have the stigma associated with other gotos. Instead, it
2491 exits the current subroutine (losing any changes set by local()) and
2492 immediately calls in its place the named subroutine using the current
2493 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2494 load another subroutine and then pretend that the other subroutine had
2495 been called in the first place (except that any modifications to C<@_>
2496 in the current subroutine are propagated to the other subroutine.)
2497 After the C<goto>, not even C<caller> will be able to tell that this
2498 routine was called first.
2500 NAME needn't be the name of a subroutine; it can be a scalar variable
2501 containing a code reference, or a block that evaluates to a code
2504 =item grep BLOCK LIST
2507 =item grep EXPR,LIST
2509 This is similar in spirit to, but not the same as, grep(1) and its
2510 relatives. In particular, it is not limited to using regular expressions.
2512 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2513 C<$_> to each element) and returns the list value consisting of those
2514 elements for which the expression evaluated to true. In scalar
2515 context, returns the number of times the expression was true.
2517 @foo = grep(!/^#/, @bar); # weed out comments
2521 @foo = grep {!/^#/} @bar; # weed out comments
2523 Note that C<$_> is an alias to the list value, so it can be used to
2524 modify the elements of the LIST. While this is useful and supported,
2525 it can cause bizarre results if the elements of LIST are not variables.
2526 Similarly, grep returns aliases into the original list, much as a for
2527 loop's index variable aliases the list elements. That is, modifying an
2528 element of a list returned by grep (for example, in a C<foreach>, C<map>
2529 or another C<grep>) actually modifies the element in the original list.
2530 This is usually something to be avoided when writing clear code.
2532 If C<$_> is lexical in the scope where the C<grep> appears (because it has
2533 been declared with C<my $_>) then, in addition to being locally aliased to
2534 the list elements, C<$_> keeps being lexical inside the block; i.e., it
2535 can't be seen from the outside, avoiding any potential side-effects.
2537 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2540 X<hex> X<hexadecimal>
2544 Interprets EXPR as a hex string and returns the corresponding value.
2545 (To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
2546 L</oct>.) If EXPR is omitted, uses C<$_>.
2548 print hex '0xAf'; # prints '175'
2549 print hex 'aF'; # same
2551 Hex strings may only represent integers. Strings that would cause
2552 integer overflow trigger a warning. Leading whitespace is not stripped,
2553 unlike oct(). To present something as hex, look into L</printf>,
2554 L</sprintf>, or L</unpack>.
2559 There is no builtin C<import> function. It is just an ordinary
2560 method (subroutine) defined (or inherited) by modules that wish to export
2561 names to another module. The C<use> function calls the C<import> method
2562 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2564 =item index STR,SUBSTR,POSITION
2565 X<index> X<indexOf> X<InStr>
2567 =item index STR,SUBSTR
2569 The index function searches for one string within another, but without
2570 the wildcard-like behavior of a full regular-expression pattern match.
2571 It returns the position of the first occurrence of SUBSTR in STR at
2572 or after POSITION. If POSITION is omitted, starts searching from the
2573 beginning of the string. POSITION before the beginning of the string
2574 or after its end is treated as if it were the beginning or the end,
2575 respectively. POSITION and the return value are based at C<0> (or whatever
2576 you've set the C<$[> variable to--but don't do that). If the substring
2577 is not found, C<index> returns one less than the base, ordinarily C<-1>.
2580 X<int> X<integer> X<truncate> X<trunc> X<floor>
2584 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2585 You should not use this function for rounding: one because it truncates
2586 towards C<0>, and two because machine representations of floating-point
2587 numbers can sometimes produce counterintuitive results. For example,
2588 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2589 because it's really more like -268.99999999999994315658 instead. Usually,
2590 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2591 functions will serve you better than will int().
2593 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2596 Implements the ioctl(2) function. You'll probably first have to say
2598 require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
2600 to get the correct function definitions. If F<sys/ioctl.ph> doesn't
2601 exist or doesn't have the correct definitions you'll have to roll your
2602 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2603 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2604 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2605 written depending on the FUNCTION; a C pointer to the string value of SCALAR
2606 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2607 has no string value but does have a numeric value, that value will be
2608 passed rather than a pointer to the string value. To guarantee this to be
2609 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2610 functions may be needed to manipulate the values of structures used by
2613 The return value of C<ioctl> (and C<fcntl>) is as follows:
2615 if OS returns: then Perl returns:
2617 0 string "0 but true"
2618 anything else that number
2620 Thus Perl returns true on success and false on failure, yet you can
2621 still easily determine the actual value returned by the operating
2624 $retval = ioctl(...) || -1;
2625 printf "System returned %d\n", $retval;
2627 The special string C<"0 but true"> is exempt from B<-w> complaints
2628 about improper numeric conversions.
2630 =item join EXPR,LIST
2633 Joins the separate strings of LIST into a single string with fields
2634 separated by the value of EXPR, and returns that new string. Example:
2636 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2638 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2639 first argument. Compare L</split>.
2646 Returns a list consisting of all the keys of the named hash, or the indices
2647 of an array. (In scalar context, returns the number of keys or indices.)
2649 The keys of a hash are returned in an apparently random order. The actual
2650 random order is subject to change in future versions of Perl, but it
2651 is guaranteed to be the same order as either the C<values> or C<each>
2652 function produces (given that the hash has not been modified). Since
2653 Perl 5.8.1 the ordering can be different even between different runs of
2654 Perl for security reasons (see L<perlsec/"Algorithmic Complexity
2657 As a side effect, calling keys() resets the HASH or ARRAY's internal iterator
2658 (see L</each>). In particular, calling keys() in void context resets
2659 the iterator with no other overhead.
2661 Here is yet another way to print your environment:
2664 @values = values %ENV;
2666 print pop(@keys), '=', pop(@values), "\n";
2669 or how about sorted by key:
2671 foreach $key (sort(keys %ENV)) {
2672 print $key, '=', $ENV{$key}, "\n";
2675 The returned values are copies of the original keys in the hash, so
2676 modifying them will not affect the original hash. Compare L</values>.
2678 To sort a hash by value, you'll need to use a C<sort> function.
2679 Here's a descending numeric sort of a hash by its values:
2681 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2682 printf "%4d %s\n", $hash{$key}, $key;
2685 Used as an lvalue, C<keys> allows you to increase the number of hash buckets
2686 allocated for the given hash. This can gain you a measure of efficiency if
2687 you know the hash is going to get big. (This is similar to pre-extending
2688 an array by assigning a larger number to $#array.) If you say
2692 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2693 in fact, since it rounds up to the next power of two. These
2694 buckets will be retained even if you do C<%hash = ()>, use C<undef
2695 %hash> if you want to free the storage while C<%hash> is still in scope.
2696 You can't shrink the number of buckets allocated for the hash using
2697 C<keys> in this way (but you needn't worry about doing this by accident,
2698 as trying has no effect). C<keys @array> in an lvalue context is a syntax
2701 Starting with Perl 5.14, C<keys> can take a reference to an unblessed hash
2702 or array. The argument will be dereferenced automatically. This aspect of
2703 C<keys> is considered highly experimental. The exact behaviour may change
2704 in a future version of Perl.
2706 for (keys $hashref) { ... }
2707 for (keys $obj->get_arrayref) { ... }
2709 See also C<each>, C<values> and C<sort>.
2711 =item kill SIGNAL, LIST
2714 Sends a signal to a list of processes. Returns the number of
2715 processes successfully signaled (which is not necessarily the
2716 same as the number actually killed).
2718 $cnt = kill 1, $child1, $child2;
2721 If SIGNAL is zero, no signal is sent to the process, but C<kill>
2722 checks whether it's I<possible> to send a signal to it (that
2723 means, to be brief, that the process is owned by the same user, or we are
2724 the super-user). This is useful to check that a child process is still
2725 alive (even if only as a zombie) and hasn't changed its UID. See
2726 L<perlport> for notes on the portability of this construct.
2728 Unlike in the shell, if SIGNAL is negative, it kills process groups instead
2729 of processes. That means you usually want to use positive not negative signals.
2730 You may also use a signal name in quotes.
2732 The behavior of kill when a I<PROCESS> number is zero or negative depends on
2733 the operating system. For example, on POSIX-conforming systems, zero will
2734 signal the current process group and -1 will signal all processes.
2736 See L<perlipc/"Signals"> for more details.
2743 The C<last> command is like the C<break> statement in C (as used in
2744 loops); it immediately exits the loop in question. If the LABEL is
2745 omitted, the command refers to the innermost enclosing loop. The
2746 C<continue> block, if any, is not executed:
2748 LINE: while (<STDIN>) {
2749 last LINE if /^$/; # exit when done with header
2753 C<last> cannot be used to exit a block that returns a value such as
2754 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2755 a grep() or map() operation.
2757 Note that a block by itself is semantically identical to a loop
2758 that executes once. Thus C<last> can be used to effect an early
2759 exit out of such a block.
2761 See also L</continue> for an illustration of how C<last>, C<next>, and
2769 Returns a lowercased version of EXPR. This is the internal function
2770 implementing the C<\L> escape in double-quoted strings.
2772 If EXPR is omitted, uses C<$_>.
2774 What gets returned depends on several factors:
2778 =item If C<use bytes> is in effect:
2782 =item On EBCDIC platforms
2784 The results are what the C language system call C<tolower()> returns.
2786 =item On ASCII platforms
2788 The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
2793 =item Otherwise, If EXPR has the UTF8 flag set
2795 If the current package has a subroutine named C<ToLower>, it will be used to
2797 (See L<perlunicode/"User-Defined Case Mappings (for serious hackers only)">.)
2798 Otherwise Unicode semantics are used for the case change.
2800 =item Otherwise, if C<use locale> is in effect
2802 Respects current LC_CTYPE locale. See L<perllocale>.
2804 =item Otherwise, if C<use feature 'unicode_strings'> is in effect:
2806 Unicode semantics are used for the case change. Any subroutine named
2807 C<ToLower> will be ignored.
2813 =item On EBCDIC platforms
2815 The results are what the C language system call C<tolower()> returns.
2817 =item On ASCII platforms
2819 ASCII semantics are used for the case change. The lowercase of any character
2820 outside the ASCII range is the character itself.
2827 X<lcfirst> X<lowercase>
2831 Returns the value of EXPR with the first character lowercased. This
2832 is the internal function implementing the C<\l> escape in
2833 double-quoted strings.
2835 If EXPR is omitted, uses C<$_>.
2837 This function behaves the same way under various pragmata, such as in a locale,
2845 Returns the length in I<characters> of the value of EXPR. If EXPR is
2846 omitted, returns the length of C<$_>. If EXPR is undefined, returns
2849 This function cannot be used on an entire array or hash to find out how
2850 many elements these have. For that, use C<scalar @array> and C<scalar keys
2851 %hash>, respectively.
2853 Like all Perl character operations, length() normally deals in logical
2854 characters, not physical bytes. For how many bytes a string encoded as
2855 UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
2856 to C<use Encode> first). See L<Encode> and L<perlunicode>.
2858 =item link OLDFILE,NEWFILE
2861 Creates a new filename linked to the old filename. Returns true for
2862 success, false otherwise.
2864 =item listen SOCKET,QUEUESIZE
2867 Does the same thing that the listen(2) system call does. Returns true if
2868 it succeeded, false otherwise. See the example in
2869 L<perlipc/"Sockets: Client/Server Communication">.
2874 You really probably want to be using C<my> instead, because C<local> isn't
2875 what most people think of as "local". See
2876 L<perlsub/"Private Variables via my()"> for details.
2878 A local modifies the listed variables to be local to the enclosing
2879 block, file, or eval. If more than one value is listed, the list must
2880 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2881 for details, including issues with tied arrays and hashes.
2883 The C<delete local EXPR> construct can also be used to localize the deletion
2884 of array/hash elements to the current block.
2885 See L<perlsub/"Localized deletion of elements of composite types">.
2887 =item localtime EXPR
2888 X<localtime> X<ctime>
2892 Converts a time as returned by the time function to a 9-element list
2893 with the time analyzed for the local time zone. Typically used as
2897 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2900 All list elements are numeric, and come straight out of the C `struct
2901 tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
2902 of the specified time.
2904 C<$mday> is the day of the month, and C<$mon> is the month itself, in
2905 the range C<0..11> with 0 indicating January and 11 indicating December.
2906 This makes it easy to get a month name from a list:
2908 my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
2909 print "$abbr[$mon] $mday";
2910 # $mon=9, $mday=18 gives "Oct 18"
2912 C<$year> is the number of years since 1900, not just the last two digits
2913 of the year. That is, C<$year> is C<123> in year 2023. The proper way
2914 to get a 4-digit year is simply:
2918 Otherwise you create non-Y2K-compliant programs--and you wouldn't want
2919 to do that, would you?
2921 To get the last two digits of the year (e.g., '01' in 2001) do:
2923 $year = sprintf("%02d", $year % 100);
2925 C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
2926 Wednesday. C<$yday> is the day of the year, in the range C<0..364>
2927 (or C<0..365> in leap years.)
2929 C<$isdst> is true if the specified time occurs during Daylight Saving
2930 Time, false otherwise.
2932 If EXPR is omitted, C<localtime()> uses the current time (as returned
2935 In scalar context, C<localtime()> returns the ctime(3) value:
2937 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2939 This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
2940 instead of local time use the L</gmtime> builtin. See also the
2941 C<Time::Local> module (to convert the seconds, minutes, hours, ... back to
2942 the integer value returned by time()), and the L<POSIX> module's strftime(3)
2943 and mktime(3) functions.
2945 To get somewhat similar but locale-dependent date strings, set up your
2946 locale environment variables appropriately (please see L<perllocale>) and
2949 use POSIX qw(strftime);
2950 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2951 # or for GMT formatted appropriately for your locale:
2952 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2954 Note that the C<%a> and C<%b>, the short forms of the day of the week
2955 and the month of the year, may not necessarily be three characters wide.
2957 See L<perlport/localtime> for portability concerns.
2959 The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
2960 by-name access mechanism to the gmtime() and localtime() functions,
2963 For a comprehensive date and time representation look at the
2964 L<DateTime> module on CPAN.
2969 This function places an advisory lock on a shared variable or referenced
2970 object contained in I<THING> until the lock goes out of scope.
2972 lock() is a "weak keyword" : this means that if you've defined a function
2973 by this name (before any calls to it), that function will be called
2974 instead. If you are not under C<use threads::shared> this does nothing.
2975 See L<threads::shared>.
2978 X<log> X<logarithm> X<e> X<ln> X<base>
2982 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2983 returns the log of C<$_>. To get the
2984 log of another base, use basic algebra:
2985 The base-N log of a number is equal to the natural log of that number
2986 divided by the natural log of N. For example:
2990 return log($n)/log(10);
2993 See also L</exp> for the inverse operation.
3000 Does the same thing as the C<stat> function (including setting the
3001 special C<_> filehandle) but stats a symbolic link instead of the file
3002 the symbolic link points to. If symbolic links are unimplemented on
3003 your system, a normal C<stat> is done. For much more detailed
3004 information, please see the documentation for C<stat>.
3006 If EXPR is omitted, stats C<$_>.
3010 The match operator. See L<perlop/"Regexp Quote-Like Operators">.
3012 =item map BLOCK LIST
3017 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
3018 C<$_> to each element) and returns the list value composed of the
3019 results of each such evaluation. In scalar context, returns the
3020 total number of elements so generated. Evaluates BLOCK or EXPR in
3021 list context, so each element of LIST may produce zero, one, or
3022 more elements in the returned value.
3024 @chars = map(chr, @numbers);
3026 translates a list of numbers to the corresponding characters.
3028 my @squares = map { $_ * $_ } @numbers;
3030 translates a list of numbers to their squared values.
3032 my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
3034 shows that number of returned elements can differ from the number of
3035 input elements. To omit an element, return an empty list ().
3036 This could also be achieved by writing
3038 my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
3040 which makes the intention more clear.
3042 Map always returns a list, which can be
3043 assigned to a hash such that the elements
3044 become key/value pairs. See L<perldata> for more details.
3046 %hash = map { get_a_key_for($_) => $_ } @array;
3048 is just a funny way to write
3052 $hash{get_a_key_for($_)} = $_;
3055 Note that C<$_> is an alias to the list value, so it can be used to
3056 modify the elements of the LIST. While this is useful and supported,
3057 it can cause bizarre results if the elements of LIST are not variables.
3058 Using a regular C<foreach> loop for this purpose would be clearer in
3059 most cases. See also L</grep> for an array composed of those items of
3060 the original list for which the BLOCK or EXPR evaluates to true.
3062 If C<$_> is lexical in the scope where the C<map> appears (because it has
3063 been declared with C<my $_>), then, in addition to being locally aliased to
3064 the list elements, C<$_> keeps being lexical inside the block; that is, it
3065 can't be seen from the outside, avoiding any potential side-effects.
3067 C<{> starts both hash references and blocks, so C<map { ...> could be either
3068 the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
3069 ahead for the closing C<}> it has to take a guess at which it's dealing with
3070 based on what it finds just after the C<{>. Usually it gets it right, but if it
3071 doesn't it won't realize something is wrong until it gets to the C<}> and
3072 encounters the missing (or unexpected) comma. The syntax error will be
3073 reported close to the C<}>, but you'll need to change something near the C<{>
3074 such as using a unary C<+> to give Perl some help:
3076 %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
3077 %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
3078 %hash = map { ("\L$_" => 1) } @array # this also works
3079 %hash = map { lc($_) => 1 } @array # as does this.
3080 %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
3082 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
3084 or to force an anon hash constructor use C<+{>:
3086 @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
3088 to get a list of anonymous hashes each with only one entry apiece.
3090 =item mkdir FILENAME,MASK
3091 X<mkdir> X<md> X<directory, create>
3093 =item mkdir FILENAME
3097 Creates the directory specified by FILENAME, with permissions
3098 specified by MASK (as modified by C<umask>). If it succeeds it
3099 returns true, otherwise it returns false and sets C<$!> (errno).
3100 If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
3103 In general, it is better to create directories with a permissive MASK,
3104 and let the user modify that with their C<umask>, than it is to supply
3105 a restrictive MASK and give the user no way to be more permissive.
3106 The exceptions to this rule are when the file or directory should be
3107 kept private (mail files, for instance). The perlfunc(1) entry on
3108 C<umask> discusses the choice of MASK in more detail.
3110 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
3111 number of trailing slashes. Some operating and filesystems do not get
3112 this right, so Perl automatically removes all trailing slashes to keep
3115 To recursively create a directory structure, look at
3116 the C<mkpath> function of the L<File::Path> module.
3118 =item msgctl ID,CMD,ARG
3121 Calls the System V IPC function msgctl(2). You'll probably have to say
3125 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3126 then ARG must be a variable that will hold the returned C<msqid_ds>
3127 structure. Returns like C<ioctl>: the undefined value for error,
3128 C<"0 but true"> for zero, or the actual return value otherwise. See also
3129 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3132 =item msgget KEY,FLAGS
3135 Calls the System V IPC function msgget(2). Returns the message queue
3136 id, or the undefined value if there is an error. See also
3137 L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
3140 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
3143 Calls the System V IPC function msgrcv to receive a message from
3144 message queue ID into variable VAR with a maximum message size of
3145 SIZE. Note that when a message is received, the message type as a
3146 native long integer will be the first thing in VAR, followed by the
3147 actual message. This packing may be opened with C<unpack("l! a*")>.
3148 Taints the variable. Returns true if successful, or false if there is
3149 an error. See also L<perlipc/"SysV IPC"> and the documentation for
3150 C<IPC::SysV> and C<IPC::SysV::Msg>.
3152 =item msgsnd ID,MSG,FLAGS
3155 Calls the System V IPC function msgsnd to send the message MSG to the
3156 message queue ID. MSG must begin with the native long integer message
3157 type, and be followed by the length of the actual message, and finally
3158 the message itself. This kind of packing can be achieved with
3159 C<pack("l! a*", $type, $message)>. Returns true if successful,
3160 or false if there is an error. See also the C<IPC::SysV>
3161 and C<IPC::SysV::Msg> documentation.
3168 =item my EXPR : ATTRS
3170 =item my TYPE EXPR : ATTRS
3172 A C<my> declares the listed variables to be local (lexically) to the
3173 enclosing block, file, or C<eval>. If more than one value is listed,
3174 the list must be placed in parentheses.
3176 The exact semantics and interface of TYPE and ATTRS are still
3177 evolving. TYPE is currently bound to the use of the C<fields> pragma,
3178 and attributes are handled using the C<attributes> pragma, or starting
3179 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3180 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3181 L<attributes>, and L<Attribute::Handlers>.
3188 The C<next> command is like the C<continue> statement in C; it starts
3189 the next iteration of the loop:
3191 LINE: while (<STDIN>) {
3192 next LINE if /^#/; # discard comments
3196 Note that if there were a C<continue> block on the above, it would get
3197 executed even on discarded lines. If LABEL is omitted, the command
3198 refers to the innermost enclosing loop.
3200 C<next> cannot be used to exit a block which returns a value such as
3201 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3202 a grep() or map() operation.
3204 Note that a block by itself is semantically identical to a loop
3205 that executes once. Thus C<next> will exit such a block early.
3207 See also L</continue> for an illustration of how C<last>, C<next>, and
3210 =item no MODULE VERSION LIST
3214 =item no MODULE VERSION
3216 =item no MODULE LIST
3222 See the C<use> function, of which C<no> is the opposite.
3225 X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
3229 Interprets EXPR as an octal string and returns the corresponding
3230 value. (If EXPR happens to start off with C<0x>, interprets it as a
3231 hex string. If EXPR starts off with C<0b>, it is interpreted as a
3232 binary string. Leading whitespace is ignored in all three cases.)
3233 The following will handle decimal, binary, octal, and hex in standard
3236 $val = oct($val) if $val =~ /^0/;
3238 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
3239 in octal), use sprintf() or printf():
3241 $dec_perms = (stat("filename"))[2] & 07777;
3242 $oct_perm_str = sprintf "%o", $perms;
3244 The oct() function is commonly used when a string such as C<644> needs
3245 to be converted into a file mode, for example. Although Perl
3246 automatically converts strings into numbers as needed, this automatic
3247 conversion assumes base 10.
3249 Leading white space is ignored without warning, as too are any trailing
3250 non-digits, such as a decimal point (C<oct> only handles non-negative
3251 integers, not negative integers or floating point).
3253 =item open FILEHANDLE,EXPR
3254 X<open> X<pipe> X<file, open> X<fopen>
3256 =item open FILEHANDLE,MODE,EXPR
3258 =item open FILEHANDLE,MODE,EXPR,LIST
3260 =item open FILEHANDLE,MODE,REFERENCE
3262 =item open FILEHANDLE
3264 Opens the file whose filename is given by EXPR, and associates it with
3267 Simple examples to open a file for reading:
3269 open(my $fh, '<', "input.txt") or die $!;
3273 open(my $fh, '>', "output.txt") or die $!;
3275 (The following is a comprehensive reference to open(): for a gentler
3276 introduction you may consider L<perlopentut>.)
3278 If FILEHANDLE is an undefined scalar variable (or array or hash element)
3279 the variable is assigned a reference to a new anonymous filehandle,
3280 otherwise if FILEHANDLE is an expression, its value is used as the name of
3281 the real filehandle wanted. (This is considered a symbolic reference, so
3282 C<use strict 'refs'> should I<not> be in effect.)
3284 If EXPR is omitted, the scalar variable of the same name as the
3285 FILEHANDLE contains the filename. (Note that lexical variables--those
3286 declared with C<my>--will not work for this purpose; so if you're
3287 using C<my>, specify EXPR in your call to open.)
3289 If three or more arguments are specified then the mode of opening and
3290 the filename are separate. If MODE is C<< '<' >> or nothing, the file
3291 is opened for input. If MODE is C<< '>' >>, the file is truncated and
3292 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
3293 the file is opened for appending, again being created if necessary.
3295 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
3296 indicate that you want both read and write access to the file; thus
3297 C<< '+<' >> is almost always preferred for read/write updates--the
3298 C<< '+>' >> mode would clobber the file first. You can't usually use
3299 either read-write mode for updating textfiles, since they have
3300 variable-length records. See the B<-i> switch in L<perlrun> for a
3301 better approach. The file is created with permissions of C<0666>
3302 modified by the process's C<umask> value.
3304 These various prefixes correspond to the fopen(3) modes of C<'r'>,
3305 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
3307 In the two-argument (and one-argument) form of the call, the mode and
3308 filename should be concatenated (in that order), possibly separated by
3309 spaces. You may omit the mode in these forms when that mode is
3312 For three or more arguments if MODE is C<'|-'>, the filename is
3313 interpreted as a command to which output is to be piped, and if MODE
3314 is C<'-|'>, the filename is interpreted as a command that pipes
3315 output to us. In the two-argument (and one-argument) form, one should
3316 replace dash (C<'-'>) with the command.
3317 See L<perlipc/"Using open() for IPC"> for more examples of this.
3318 (You are not allowed to C<open> to a command that pipes both in I<and>
3319 out, but see L<IPC::Open2>, L<IPC::Open3>, and
3320 L<perlipc/"Bidirectional Communication with Another Process"> for
3323 In the form of pipe opens taking three or more arguments, if LIST is specified
3324 (extra arguments after the command name) then LIST becomes arguments
3325 to the command invoked if the platform supports it. The meaning of
3326 C<open> with more than three arguments for non-pipe modes is not yet
3327 defined, but experimental "layers" may give extra LIST arguments
3330 In the two-argument (and one-argument) form, opening C<< '<-' >>
3331 or C<'-'> opens STDIN and opening C<< '>-' >> opens STDOUT.
3333 You may use the three-argument form of open to specify I/O layers
3334 (sometimes referred to as "disciplines") to apply to the handle
3335 that affect how the input and output are processed (see L<open> and
3336 L<PerlIO> for more details). For example:
3338 open(my $fh, "<:encoding(UTF-8)", "filename")
3339 || die "can't open UTF-8 encoded filename: $!";
3341 opens the UTF-8 encoded file containing Unicode characters;
3342 see L<perluniintro>. Note that if layers are specified in the
3343 three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
3344 usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
3346 Open returns nonzero on success, the undefined value otherwise. If
3347 the C<open> involved a pipe, the return value happens to be the pid of
3350 If you're running Perl on a system that distinguishes between text
3351 files and binary files, then you should check out L</binmode> for tips
3352 for dealing with this. The key distinction between systems that need
3353 C<binmode> and those that don't is their text file formats. Systems
3354 like Unix, Mac OS, and Plan 9, that end lines with a single
3355 character and encode that character in C as C<"\n"> do not
3356 need C<binmode>. The rest need it.
3358 When opening a file, it's seldom a good idea to continue
3359 if the request failed, so C<open> is frequently used with
3360 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
3361 where you want to format a suitable error message (but there are
3362 modules that can help with that problem)) always check
3363 the return value from opening a file.
3365 As a special case the 3-arg form with a read/write mode and the third
3366 argument being C<undef>:
3368 open(my $tmp, "+>", undef) or die ...
3370 opens a filehandle to an anonymous temporary file. Also using "+<"
3371 works for symmetry, but you really should consider writing something
3372 to the temporary file first. You will need to seek() to do the
3375 Since v5.8.0, Perl has built using PerlIO by default. Unless you've
3376 changed this (i.e., Configure -Uuseperlio), you can open filehandles
3377 directly to Perl scalars via:
3379 open($fh, '>', \$variable) || ..
3381 To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
3384 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
3389 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
3390 while (<ARTICLE>) {...
3392 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
3393 # if the open fails, output is discarded
3395 open(my $dbase, '+<', 'dbase.mine') # open for update
3396 or die "Can't open 'dbase.mine' for update: $!";
3398 open(my $dbase, '+<dbase.mine') # ditto
3399 or die "Can't open 'dbase.mine' for update: $!";
3401 open(ARTICLE, '-|', "caesar <$article") # decrypt article
3402 or die "Can't start caesar: $!";
3404 open(ARTICLE, "caesar <$article |") # ditto
3405 or die "Can't start caesar: $!";
3407 open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
3408 or die "Can't start sort: $!";
3411 open(MEMORY,'>', \$var)
3412 or die "Can't open memory file: $!";
3413 print MEMORY "foo!\n"; # output will appear in $var
3415 # process argument list of files along with any includes
3417 foreach $file (@ARGV) {
3418 process($file, 'fh00');
3422 my($filename, $input) = @_;
3423 $input++; # this is a string increment
3424 unless (open($input, $filename)) {
3425 print STDERR "Can't open $filename: $!\n";
3430 while (<$input>) { # note use of indirection
3431 if (/^#include "(.*)"/) {
3432 process($1, $input);
3439 See L<perliol> for detailed info on PerlIO.
3441 You may also, in the Bourne shell tradition, specify an EXPR beginning
3442 with C<< '>&' >>, in which case the rest of the string is interpreted
3443 as the name of a filehandle (or file descriptor, if numeric) to be
3444 duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
3445 C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
3446 The mode you specify should match the mode of the original filehandle.
3447 (Duping a filehandle does not take into account any existing contents
3448 of IO buffers.) If you use the 3-arg form then you can pass either a
3449 number, the name of a filehandle or the normal "reference to a glob".
3451 Here is a script that saves, redirects, and restores C<STDOUT> and
3452 C<STDERR> using various methods:
3455 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
3456 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
3458 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
3459 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
3461 select STDERR; $| = 1; # make unbuffered
3462 select STDOUT; $| = 1; # make unbuffered
3464 print STDOUT "stdout 1\n"; # this works for
3465 print STDERR "stderr 1\n"; # subprocesses too
3467 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
3468 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
3470 print STDOUT "stdout 2\n";
3471 print STDERR "stderr 2\n";
3473 If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
3474 or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
3475 that file descriptor (and not call C<dup(2)>); this is more
3476 parsimonious of file descriptors. For example:
3478 # open for input, reusing the fileno of $fd
3479 open(FILEHANDLE, "<&=$fd")
3483 open(FILEHANDLE, "<&=", $fd)
3487 # open for append, using the fileno of OLDFH
3488 open(FH, ">>&=", OLDFH)
3492 open(FH, ">>&=OLDFH")
3494 Being parsimonious on filehandles is also useful (besides being
3495 parsimonious) for example when something is dependent on file
3496 descriptors, like for example locking using flock(). If you do just
3497 C<< open(A, '>>&B') >>, the filehandle A will not have the same file
3498 descriptor as B, and therefore flock(A) will not flock(B), and vice
3499 versa. But with C<< open(A, '>>&=B') >> the filehandles will share
3500 the same file descriptor.
3502 Note that if you are using Perls older than 5.8.0, Perl will be using
3503 the standard C libraries' fdopen() to implement the "=" functionality.
3504 On many Unix systems fdopen() fails when file descriptors exceed a
3505 certain value, typically 255. For Perls 5.8.0 and later, PerlIO is
3506 most often the default.
3508 You can see whether Perl has been compiled with PerlIO or not by
3509 running C<perl -V> and looking for the C<useperlio=> line. If C<useperlio>
3510 is C<define>, you have PerlIO; otherwise you don't.
3512 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
3513 with the 2-argument (or 1-argument) form of open(), then
3514 there is an implicit fork done, and the return value of open is the pid
3515 of the child within the parent process, and C<0> within the child
3516 process. (Use C<defined($pid)> to determine whether the open was successful.)
3517 The filehandle behaves normally for the parent, but I/O to that
3518 filehandle is piped from/to the STDOUT/STDIN of the child process.
3519 In the child process, the filehandle isn't opened--I/O happens from/to
3520 the new STDOUT/STDIN. Typically this is used like the normal
3521 piped open when you want to exercise more control over just how the
3522 pipe command gets executed, such as when running setuid and
3523 you don't want to have to scan shell commands for metacharacters.
3525 The following blocks are more or less equivalent:
3527 open(FOO, "|tr '[a-z]' '[A-Z]'");
3528 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3529 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3530 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3532 open(FOO, "cat -n '$file'|");
3533 open(FOO, '-|', "cat -n '$file'");
3534 open(FOO, '-|') || exec 'cat', '-n', $file;
3535 open(FOO, '-|', "cat", '-n', $file);
3537 The last two examples in each block shows the pipe as "list form", which is
3538 not yet supported on all platforms. A good rule of thumb is that if
3539 your platform has true C<fork()> (in other words, if your platform is
3540 Unix) you can use the list form.
3542 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3544 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3545 output before any operation that may do a fork, but this may not be
3546 supported on some platforms (see L<perlport>). To be safe, you may need
3547 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3548 of C<IO::Handle> on any open handles.
3550 On systems that support a close-on-exec flag on files, the flag will
3551 be set for the newly opened file descriptor as determined by the value
3552 of $^F. See L<perlvar/$^F>.
3554 Closing any piped filehandle causes the parent process to wait for the
3555 child to finish, and returns the status value in C<$?> and
3556 C<${^CHILD_ERROR_NATIVE}>.
3558 The filename passed to the 2-argument (or 1-argument) form of open() will
3559 have leading and trailing whitespace deleted, and the normal
3560 redirection characters honored. This property, known as "magic open",
3561 can often be used to good effect. A user could specify a filename of
3562 F<"rsh cat file |">, or you could change certain filenames as needed:
3564 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3565 open(FH, $filename) or die "Can't open $filename: $!";
3567 Use 3-argument form to open a file with arbitrary weird characters in it,
3569 open(FOO, '<', $file);
3571 otherwise it's necessary to protect any leading and trailing whitespace:
3573 $file =~ s#^(\s)#./$1#;
3574 open(FOO, "< $file\0");
3576 (this may not work on some bizarre filesystems). One should
3577 conscientiously choose between the I<magic> and 3-arguments form
3582 will allow the user to specify an argument of the form C<"rsh cat file |">,
3583 but will not work on a filename that happens to have a trailing space, while
3585 open IN, '<', $ARGV[0];
3587 will have exactly the opposite restrictions.
3589 If you want a "real" C C<open> (see C<open(2)> on your system), then you
3590 should use the C<sysopen> function, which involves no such magic (but
3591 may use subtly different filemodes than Perl open(), which is mapped
3592 to C fopen()). This is
3593 another way to protect your filenames from interpretation. For example:
3596 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3597 or die "sysopen $path: $!";
3598 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3599 print HANDLE "stuff $$\n";
3601 print "File contains: ", <HANDLE>;
3603 Using the constructor from the C<IO::Handle> package (or one of its
3604 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3605 filehandles that have the scope of whatever variables hold references to
3606 them, and automatically close whenever and however you leave that scope:
3610 sub read_myfile_munged {
3612 my $handle = IO::File->new;
3613 open($handle, "myfile") or die "myfile: $!";
3615 or return (); # Automatically closed here.
3616 mung $first or die "mung failed"; # Or here.
3617 return $first, <$handle> if $ALL; # Or here.
3621 See L</seek> for some details about mixing reading and writing.
3623 =item opendir DIRHANDLE,EXPR
3626 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3627 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3628 DIRHANDLE may be an expression whose value can be used as an indirect
3629 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3630 scalar variable (or array or hash element), the variable is assigned a
3631 reference to a new anonymous dirhandle.
3632 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3634 See the example at C<readdir>.
3641 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3642 or Unicode) value of the first character of EXPR. If EXPR is an empty
3643 string, returns 0. If EXPR is omitted, uses C<$_>.
3645 For the reverse, see L</chr>.
3646 See L<perlunicode> for more about Unicode.
3653 =item our EXPR : ATTRS
3655 =item our TYPE EXPR : ATTRS
3657 C<our> associates a simple name with a package variable in the current
3658 package for use within the current scope. When C<use strict 'vars'> is in
3659 effect, C<our> lets you use declared global variables without qualifying
3660 them with package names, within the lexical scope of the C<our> declaration.
3661 In this way C<our> differs from C<use vars>, which is package-scoped.
3663 Unlike C<my>, which both allocates storage for a variable and associates
3664 a simple name with that storage for use within the current scope, C<our>
3665 associates a simple name with a package variable in the current package,
3666 for use within the current scope. In other words, C<our> has the same
3667 scoping rules as C<my>, but does not necessarily create a
3670 If more than one value is listed, the list must be placed
3676 An C<our> declaration declares a global variable that will be visible
3677 across its entire lexical scope, even across package boundaries. The
3678 package in which the variable is entered is determined at the point
3679 of the declaration, not at the point of use. This means the following
3683 our $bar; # declares $Foo::bar for rest of lexical scope
3687 print $bar; # prints 20, as it refers to $Foo::bar
3689 Multiple C<our> declarations with the same name in the same lexical
3690 scope are allowed if they are in different packages. If they happen
3691 to be in the same package, Perl will emit warnings if you have asked
3692 for them, just like multiple C<my> declarations. Unlike a second
3693 C<my> declaration, which will bind the name to a fresh variable, a
3694 second C<our> declaration in the same package, in the same scope, is
3699 our $bar; # declares $Foo::bar for rest of lexical scope
3703 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3704 print $bar; # prints 30
3706 our $bar; # emits warning but has no other effect
3707 print $bar; # still prints 30
3709 An C<our> declaration may also have a list of attributes associated
3712 The exact semantics and interface of TYPE and ATTRS are still
3713 evolving. TYPE is currently bound to the use of C<fields> pragma,
3714 and attributes are handled using the C<attributes> pragma, or starting
3715 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3716 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3717 L<attributes>, and L<Attribute::Handlers>.
3719 =item pack TEMPLATE,LIST
3722 Takes a LIST of values and converts it into a string using the rules
3723 given by the TEMPLATE. The resulting string is the concatenation of
3724 the converted values. Typically, each converted value looks
3725 like its machine-level representation. For example, on 32-bit machines
3726 an integer may be represented by a sequence of 4 bytes, which will in
3727 Perl be presented as a string that's 4 characters long.
3729 See L<perlpacktut> for an introduction to this function.
3731 The TEMPLATE is a sequence of characters that give the order and type
3732 of values, as follows:
3734 a A string with arbitrary binary data, will be null padded.
3735 A A text (ASCII) string, will be space padded.
3736 Z A null-terminated (ASCIZ) string, will be null padded.
3738 b A bit string (ascending bit order inside each byte, like vec()).
3739 B A bit string (descending bit order inside each byte).
3740 h A hex string (low nybble first).
3741 H A hex string (high nybble first).
3743 c A signed char (8-bit) value.
3744 C An unsigned char (octet) value.
3745 W An unsigned char value (can be greater than 255).
3747 s A signed short (16-bit) value.
3748 S An unsigned short value.
3750 l A signed long (32-bit) value.
3751 L An unsigned long value.
3753 q A signed quad (64-bit) value.
3754 Q An unsigned quad value.
3755 (Quads are available only if your system supports 64-bit
3756 integer values _and_ if Perl has been compiled to support those.
3757 Raises an exception otherwise.)
3759 i A signed integer value.
3760 I A unsigned integer value.
3761 (This 'integer' is _at_least_ 32 bits wide. Its exact
3762 size depends on what a local C compiler calls 'int'.)
3764 n An unsigned short (16-bit) in "network" (big-endian) order.
3765 N An unsigned long (32-bit) in "network" (big-endian) order.
3766 v An unsigned short (16-bit) in "VAX" (little-endian) order.
3767 V An unsigned long (32-bit) in "VAX" (little-endian) order.
3769 j A Perl internal signed integer value (IV).
3770 J A Perl internal unsigned integer value (UV).
3772 f A single-precision float in native format.
3773 d A double-precision float in native format.
3775 F A Perl internal floating-point value (NV) in native format
3776 D A float of long-double precision in native format.
3777 (Long doubles are available only if your system supports long
3778 double values _and_ if Perl has been compiled to support those.
3779 Raises an exception otherwise.)
3781 p A pointer to a null-terminated string.
3782 P A pointer to a structure (fixed-length string).
3784 u A uuencoded string.
3785 U A Unicode character number. Encodes to a character in character mode
3786 and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
3788 w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
3789 details). Its bytes represent an unsigned integer in base 128,
3790 most significant digit first, with as few digits as possible. Bit
3791 eight (the high bit) is set on each byte except the last.
3793 x A null byte (a.k.a ASCII NUL, "\000", chr(0))
3795 @ Null-fill or truncate to absolute position, counted from the
3796 start of the innermost ()-group.
3797 . Null-fill or truncate to absolute position specified by the value.
3798 ( Start of a ()-group.
3800 One or more modifiers below may optionally follow certain letters in the
3801 TEMPLATE (the second column lists letters for which the modifier is valid):
3803 ! sSlLiI Forces native (short, long, int) sizes instead
3804 of fixed (16-/32-bit) sizes.
3806 xX Make x and X act as alignment commands.
3808 nNvV Treat integers as signed instead of unsigned.
3810 @. Specify position as byte offset in the internal
3811 representation of the packed string. Efficient but
3814 > sSiIlLqQ Force big-endian byte-order on the type.
3815 jJfFdDpP (The "big end" touches the construct.)
3817 < sSiIlLqQ Force little-endian byte-order on the type.
3818 jJfFdDpP (The "little end" touches the construct.)
3820 The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
3821 to force a particular byte-order on all components in that group,
3822 including all its subgroups.
3824 The following rules apply:
3830 Each letter may optionally be followed by a number indicating the repeat
3831 count. A numeric repeat count may optionally be enclosed in brackets, as
3832 in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
3833 the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
3834 C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
3835 something else, dscribed below. Supplying a C<*> for the repeat count
3836 instead of a number means to use however many items are left, except for:
3842 C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
3846 <.>, where it means relative to the start of the string.
3850 C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
3854 One can replace a numeric repeat count with a template letter enclosed in
3855 brackets to use the packed byte length of the bracketed template for the
3858 For example, the template C<x[L]> skips as many bytes as in a packed long,
3859 and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
3860 variable-expanded) unpacks. If the template in brackets contains alignment
3861 commands (such as C<x![d]>), its packed length is calculated as if the
3862 start of the template had the maximal possible alignment.
3864 When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
3865 trailing null byte, so the resulting string is always one byte longer than
3866 the byte length of the item itself.
3868 When used with C<@>, the repeat count represents an offset from the start
3869 of the innermost C<()> group.
3871 When used with C<.>, the repeat count determines the starting position to
3872 calculate the value offset as follows:
3878 If the repeat count is C<0>, it's relative to the current position.
3882 If the repeat count is C<*>, the offset is relative to the start of the
3887 And if it's an integer I<n>, the offset is relative to the start of the
3888 I<n>th innermost C<()> group, or to the start of the string if I<n> is
3889 bigger then the group level.
3893 The repeat count for C<u> is interpreted as the maximal number of bytes
3894 to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
3895 count should not be more than 65.
3899 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3900 string of length count, padding with nulls or spaces as needed. When
3901 unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
3902 after the first null, and C<a> returns data without any sort of trimming.
3904 If the value to pack is too long, the result is truncated. If it's too
3905 long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
3906 followed by a null byte. Thus C<Z> always packs a trailing null, except
3907 for when the count is 0.
3911 Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
3912 Each such format generates 1 bit of the result.
3914 Each result bit is based on the least-significant bit of the corresponding
3915 input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
3916 and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
3918 Starting from the beginning of the input string, each 8-tuple
3919 of characters is converted to 1 character of output. With format C<b>,
3920 the first character of the 8-tuple determines the least-significant bit of a
3921 character; with format C<B>, it determines the most-significant bit of
3924 If the length of the input string is not evenly divisible by 8, the
3925 remainder is packed as if the input string were padded by null characters
3926 at the end. Similarly during unpacking, "extra" bits are ignored.
3928 If the input string is longer than needed, remaining characters are ignored.
3930 A C<*> for the repeat count uses all characters of the input field.
3931 On unpacking, bits are converted to a string of C<"0">s and C<"1">s.
3935 The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
3936 representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
3938 For each such format, pack() generates 4 bits of the result.
3939 With non-alphabetical characters, the result is based on the 4 least-significant
3940 bits of the input character, i.e., on C<ord($char)%16>. In particular,
3941 characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3942 C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
3943 is compatible with the usual hexadecimal digits, so that C<"a"> and
3944 C<"A"> both generate the nybble C<0xa==10>. Do not use any characters
3945 but these with this format.
3947 Starting from the beginning of the template to pack(), each pair
3948 of characters is converted to 1 character of output. With format C<h>, the
3949 first character of the pair determines the least-significant nybble of the
3950 output character; with format C<H>, it determines the most-significant
3953 If the length of the input string is not even, it behaves as if padded by
3954 a null character at the end. Similarly, "extra" nybbles are ignored during
3957 If the input string is longer than needed, extra characters are ignored.
3959 A C<*> for the repeat count uses all characters of the input field. For
3960 unpack(), nybbles are converted to a string of hexadecimal digits.
3964 The C<p> format packs a pointer to a null-terminated string. You are
3965 responsible for ensuring that the string is not a temporary value, as that
3966 could potentially get deallocated before you got around to using the packed
3967 result. The C<P> format packs a pointer to a structure of the size indicated
3968 by the length. A null pointer is created if the corresponding value for
3969 C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
3970 unpacks into C<undef>.
3972 If your system has a strange pointer size--meaning a pointer is neither as
3973 big as an int nor as big as a long--it may not be possible to pack or
3974 unpack pointers in big- or little-endian byte order. Attempting to do
3975 so raises an exception.
3979 The C</> template character allows packing and unpacking of a sequence of
3980 items where the packed structure contains a packed item count followed by
3981 the packed items themselves. This is useful when the structure you're
3982 unpacking has encoded the sizes or repeat counts for some of its fields
3983 within the structure itself as separate fields.
3985 For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
3986 I<length-item> describes how the length value is packed. Formats likely
3987 to be of most use are integer-packing ones like C<n> for Java strings,
3988 C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
3990 For C<pack>, I<sequence-item> may have a repeat count, in which case
3991 the minimum of that and the number of available items is used as the argument
3992 for I<length-item>. If it has no repeat count or uses a '*', the number
3993 of available items is used.
3995 For C<unpack>, an internal stack of integer arguments unpacked so far is
3996 used. You write C</>I<sequence-item> and the repeat count is obtained by
3997 popping off the last element from the stack. The I<sequence-item> must not
3998 have a repeat count.
4000 If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
4001 the I<length-item> is the string length, not the number of strings. With
4002 an explicit repeat count for pack, the packed string is adjusted to that
4003 length. For example:
4005 unpack("W/a", "\004Gurusamy") gives ("Guru")
4006 unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
4007 unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
4009 pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
4010 pack("a/W2", ord("a") .. ord("z")) gives "2ab"
4012 The I<length-item> is not returned explicitly from C<unpack>.
4014 Supplying a count to the I<length-item> format letter is only useful with
4015 C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
4016 introduce C<"\000"> characters, which Perl does not regard as legal in
4021 The integer types C<s>, C<S>, C<l>, and C<L> may be
4022 followed by a C<!> modifier to specify native shorts or
4023 longs. As shown in the example above, a bare C<l> means
4024 exactly 32 bits, although the native C<long> as seen by the local C compiler
4025 may be larger. This is mainly an issue on 64-bit platforms. You can
4026 see whether using C<!> makes any difference this way:
4028 printf "format s is %d, s! is %d\n",
4029 length pack("s"), length pack("s!");
4031 printf "format l is %d, l! is %d\n",
4032 length pack("l"), length pack("l!");
4035 C<i!> and C<I!> are also allowed, but only for completeness' sake:
4036 they are identical to C<i> and C<I>.
4038 The actual sizes (in bytes) of native shorts, ints, longs, and long
4039 longs on the platform where Perl was built are also available from
4042 $ perl -V:{short,int,long{,long}}size
4048 or programmatically via the C<Config> module:
4051 print $Config{shortsize}, "\n";
4052 print $Config{intsize}, "\n";
4053 print $Config{longsize}, "\n";
4054 print $Config{longlongsize}, "\n";
4056 C<$Config{longlongsize}> is undefined on systems without
4061 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
4062 inherently non-portable between processors and operating systems because
4063 they obey native byteorder and endianness. For example, a 4-byte integer
4064 0x12345678 (305419896 decimal) would be ordered natively (arranged in and
4065 handled by the CPU registers) into bytes as
4067 0x12 0x34 0x56 0x78 # big-endian
4068 0x78 0x56 0x34 0x12 # little-endian
4070 Basically, Intel and VAX CPUs are little-endian, while everybody else,
4071 including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
4072 big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in
4073 little-endian mode, but SGI/Cray uses them in big-endian mode.
4075 The names I<big-endian> and I<little-endian> are comic references to the
4076 egg-eating habits of the little-endian Lilliputians and the big-endian
4077 Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
4078 This entered computer lingo via the paper "On Holy Wars and a Plea for
4079 Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
4081 Some systems may have even weirder byte orders such as
4086 You can determine your system endianness with this incantation:
4088 printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
4090 The byteorder on the platform where Perl was built is also available
4094 print "$Config{byteorder}\n";
4096 or from the command line:
4100 Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
4101 and C<"87654321"> are big-endian.
4103 For portably packed integers, either use the formats C<n>, C<N>, C<v>,
4104 and C<V> or else use the C<< > >> and C<< < >> modifiers described
4105 immediately below. See also L<perlport>.
4109 Starting with Perl 5.9.2, integer and floating-point formats, along with
4110 the C<p> and C<P> formats and C<()> groups, may all be followed by the
4111 C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
4112 or little-endian byte-order. These modifiers are especially useful
4113 given how C<n>, C<N>, C<v> and C<V> don't cover signed integers,
4114 64-bit integers, or floating-point values.
4116 Here are some concerns to keep in mind when using an endianness modifier:
4122 Exchanging signed integers between different platforms works only
4123 when all platforms store them in the same format. Most platforms store
4124 signed integers in two's-complement notation, so usually this is not an issue.
4128 The C<< > >> or C<< < >> modifiers can only be used on floating-point
4129 formats on big- or little-endian machines. Otherwise, attempting to
4130 use them raises an exception.
4134 Forcing big- or little-endian byte-order on floating-point values for
4135 data exchange can work only if all platforms use the same
4136 binary representation such as IEEE floating-point. Even if all
4137 platforms are using IEEE, there may still be subtle differences. Being able
4138 to use C<< > >> or C<< < >> on floating-point values can be useful,
4139 but also dangerous if you don't know exactly what you're doing.
4140 It is not a general way to portably store floating-point values.
4144 When using C<< > >> or C<< < >> on a C<()> group, this affects
4145 all types inside the group that accept byte-order modifiers,
4146 including all subgroups. It is silently ignored for all other
4147 types. You are not allowed to override the byte-order within a group
4148 that already has a byte-order modifier suffix.
4154 Real numbers (floats and doubles) are in native machine format only.
4155 Due to the multiplicity of floating-point formats and the lack of a
4156 standard "network" representation for them, no facility for interchange has been
4157 made. This means that packed floating-point data written on one machine
4158 may not be readable on another, even if both use IEEE floating-point
4159 arithmetic (because the endianness of the memory representation is not part
4160 of the IEEE spec). See also L<perlport>.
4162 If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
4163 modifiers to force big- or little-endian byte-order on floating-point values.
4165 Because Perl uses doubles (or long doubles, if configured) internally for
4166 all numeric calculation, converting from double into float and thence
4167 to double again loses precision, so C<unpack("f", pack("f", $foo)>)
4168 will not in general equal $foo.
4172 Pack and unpack can operate in two modes: character mode (C<C0> mode) where
4173 the packed string is processed per character, and UTF-8 mode (C<U0> mode)
4174 where the packed string is processed in its UTF-8-encoded Unicode form on
4175 a byte-by-byte basis. Character mode is the default unless the format string
4176 starts with C<U>. You can always switch mode mid-format with an explicit
4177 C<C0> or C<U0> in the format. This mode remains in effect until the next
4178 mode change, or until the end of the C<()> group it (directly) applies to.
4182 You must yourself do any alignment or padding by inserting, for example,
4183 enough C<"x">es while packing. There is no way for pack() and unpack()
4184 to know where characters are going to or coming from, so they
4185 handle their output and input as flat sequences of characters.
4189 A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
4190 take a repeat count either as postfix, or for unpack(), also via the C</>
4191 template character. Within each repetition of a group, positioning with
4192 C<@> starts over at 0. Therefore, the result of
4194 pack("@1A((@2A)@3A)", qw[X Y Z])
4196 is the string C<"\0X\0\0YZ">.
4200 C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
4201 jump forward or back to the closest position aligned at a multiple of C<count>
4202 characters. For example, to pack() or unpack() a C structure like
4205 char c; /* one signed, 8-bit character */
4210 one may need to use the template C<c x![d] d c[2]>. This assumes that
4211 doubles must be aligned to the size of double.
4213 For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
4218 C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
4219 represent signed 16-/32-bit integers in big-/little-endian order.
4220 This is portable only when all platforms sharing packed data use the
4221 same binary representation for signed integers; for example, when all
4222 platforms use two's-complement representation.
4226 Comments can be embedded in a TEMPLATE using C<#> through the end of line.
4227 White space can separate pack codes from each other, but modifiers and
4228 repeat counts must follow immediately. Breaking complex templates into
4229 individual line-by-line components, suitably annotated, can do as much to
4230 improve legibility and maintainability of pack/unpack formats as C</x> can
4231 for complicated pattern matches.
4235 If TEMPLATE requires more arguments than pack() is given, pack()
4236 assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
4237 than given, extra arguments are ignored.
4243 $foo = pack("WWWW",65,66,67,68);
4245 $foo = pack("W4",65,66,67,68);
4247 $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
4248 # same thing with Unicode circled letters.
4249 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
4250 # same thing with Unicode circled letters. You don't get the UTF-8
4251 # bytes because the U at the start of the format caused a switch to
4252 # U0-mode, so the UTF-8 bytes get joined into characters
4253 $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
4254 # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
4255 # This is the UTF-8 encoding of the string in the previous example
4257 $foo = pack("ccxxcc",65,66,67,68);
4260 # NOTE: The examples above featuring "W" and "c" are true
4261 # only on ASCII and ASCII-derived systems such as ISO Latin 1
4262 # and UTF-8. On EBCDIC systems, the first example would be
4263 # $foo = pack("WWWW",193,194,195,196);
4265 $foo = pack("s2",1,2);
4266 # "\001\000\002\000" on little-endian
4267 # "\000\001\000\002" on big-endian
4269 $foo = pack("a4","abcd","x","y","z");
4272 $foo = pack("aaaa","abcd","x","y","z");
4275 $foo = pack("a14","abcdefg");
4276 # "abcdefg\0\0\0\0\0\0\0"
4278 $foo = pack("i9pl", gmtime);
4279 # a real struct tm (on my system anyway)
4281 $utmp_template = "Z8 Z8 Z16 L";
4282 $utmp = pack($utmp_template, @utmp1);
4283 # a struct utmp (BSDish)
4285 @utmp2 = unpack($utmp_template, $utmp);
4286 # "@utmp1" eq "@utmp2"
4289 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
4292 $foo = pack('sx2l', 12, 34);
4293 # short 12, two zero bytes padding, long 34
4294 $bar = pack('s@4l', 12, 34);
4295 # short 12, zero fill to position 4, long 34
4297 $baz = pack('s.l', 12, 4, 34);
4298 # short 12, zero fill to position 4, long 34
4300 $foo = pack('nN', 42, 4711);
4301 # pack big-endian 16- and 32-bit unsigned integers
4302 $foo = pack('S>L>', 42, 4711);
4304 $foo = pack('s<l<', -42, 4711);
4305 # pack little-endian 16- and 32-bit signed integers
4306 $foo = pack('(sl)<', -42, 4711);
4309 The same template may generally also be used in unpack().
4311 =item package NAMESPACE VERSION
4312 X<package> X<module> X<namespace> X<version>
4314 =item package NAMESPACE
4316 =item package NAMESPACE VERSION BLOCK
4317 X<package> X<module> X<namespace> X<version>
4319 =item package NAMESPACE BLOCK
4321 Declares the BLOCK, or the rest of the compilation unit, as being in
4322 the given namespace. The scope of the package declaration is either the
4323 supplied code BLOCK or, in the absence of a BLOCK, from the declaration
4324 itself through the end of the enclosing block, file, or eval (the same
4325 as the C<my> operator). All unqualified dynamic identifiers in this
4326 scope will be in the given namespace, except where overridden by another
4327 C<package> declaration.
4329 A package statement affects dynamic variables only, including those
4330 you've used C<local> on, but I<not> lexical variables, which are created
4331 with C<my> (or C<our> (or C<state>)). Typically it would be the first
4332 declaration in a file included by C<require> or C<use>. You can switch into a
4333 package in more than one place, since this only determines which default
4334 symbol table the compiler uses for the rest of that block. You can refer to
4335 identifiers in other packages than the current one by prefixing the identifier
4336 with the package name and a double colon, as in C<$SomePack::var>
4337 or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
4338 package as assumed. That is, C<$::sail> is equivalent to
4339 C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
4340 code, mostly from Perl 4).
4342 If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
4343 namespace to a L<version> object with the VERSION provided. VERSION must be a
4344 "strict" style version number as defined by the L<version> module: a positive
4345 decimal number (integer or decimal-fraction) without exponentiation or else a
4346 dotted-decimal v-string with a leading 'v' character and at least three
4347 components. You should set C<$VERSION> only once per package.
4349 See L<perlmod/"Packages"> for more information about packages, modules,
4350 and classes. See L<perlsub> for other scoping issues.
4352 =item pipe READHANDLE,WRITEHANDLE
4355 Opens a pair of connected pipes like the corresponding system call.
4356 Note that if you set up a loop of piped processes, deadlock can occur
4357 unless you are very careful. In addition, note that Perl's pipes use
4358 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
4359 after each command, depending on the application.
4361 See L<IPC::Open2>, L<IPC::Open3>, and
4362 L<perlipc/"Bidirectional Communication with Another Process">
4363 for examples of such things.
4365 On systems that support a close-on-exec flag on files, that flag is set
4366 on all newly opened file descriptors whose C<fileno>s are I<higher> than
4367 the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
4374 Pops and returns the last value of the array, shortening the array by
4377 Returns the undefined value if the array is empty, although this may also
4378 happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
4379 main program, but the C<@_> array in subroutines, just like C<shift>.
4381 Starting with Perl 5.14, C<pop> can take a reference to an unblessed array.
4382 The argument will be dereferenced automatically. This aspect of C<pop> is
4383 considered highly experimental. The exact behaviour may change in a future
4387 X<pos> X<match, position>
4391 Returns the offset of where the last C<m//g> search left off for the
4392 variable in question (C<$_> is used when the variable is not
4393 specified). Note that 0 is a valid match offset. C<undef> indicates
4394 that the search position is reset (usually due to match failure, but
4395 can also be because no match has yet been run on the scalar).
4397 C<pos> directly accesses the location used by the regexp engine to
4398 store the offset, so assigning to C<pos> will change that offset, and
4399 so will also influence the C<\G> zero-width assertion in regular
4400 expressions. Both of these effects take place for the next match, so
4401 you can't affect the position with C<pos> during the current match,
4402 such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
4404 Setting C<pos> also resets the I<matched with zero-length> flag, described
4405 under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
4407 Because a failed C<m//gc> match doesn't reset the offset, the return
4408 from C<pos> won't change either in this case. See L<perlre> and
4411 =item print FILEHANDLE LIST
4418 Prints a string or a list of strings. Returns true if successful.
4419 FILEHANDLE may be a scalar variable containing
4420 the name of or a reference to the filehandle, thus introducing
4421 one level of indirection. (NOTE: If FILEHANDLE is a variable and
4422 the next token is a term, it may be misinterpreted as an operator
4423 unless you interpose a C<+> or put parentheses around the arguments.)
4424 If FILEHANDLE is omitted, prints to standard output by default, or
4425 to the last selected output channel; see L</select>. If LIST is
4426 also omitted, prints C<$_> to the currently selected output handle.
4427 To set the default output handle to something other than STDOUT
4428 use the select operation. The current value of C<$,> (if any) is
4429 printed between each LIST item. The current value of C<$\> (if
4430 any) is printed after the entire LIST has been printed. Because
4431 print takes a LIST, anything in the LIST is evaluated in list
4432 context, and any subroutine that you call will have one or more of
4433 its expressions evaluated in list context. Also be careful not to
4434 follow the print keyword with a left parenthesis unless you want
4435 the corresponding right parenthesis to terminate the arguments to
4436 the print; put parentheses around all the arguments
4437 (or interpose a C<+>, but that doesn't look as good).
4439 Note that if you're storing FILEHANDLEs in an array, or if you're using
4440 any other expression more complex than a scalar variable to retrieve it,
4441 you will have to use a block returning the filehandle value instead:
4443 print { $files[$i] } "stuff\n";
4444 print { $OK ? STDOUT : STDERR } "stuff\n";
4446 Printing to a closed pipe or socket will generate a SIGPIPE signal. See
4447 L<perlipc> for more on signal handling.
4449 =item printf FILEHANDLE FORMAT, LIST
4452 =item printf FORMAT, LIST
4454 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
4455 (the output record separator) is not appended. The first argument
4456 of the list will be interpreted as the C<printf> format. See C<sprintf>
4457 for an explanation of the format argument. If C<use locale> is in effect,
4458 and POSIX::setlocale() has been called, the character used for the decimal
4459 separator in formatted floating-point numbers is affected by the LC_NUMERIC
4460 locale. See L<perllocale> and L<POSIX>.
4462 Don't fall into the trap of using a C<printf> when a simple
4463 C<print> would do. The C<print> is more efficient and less
4466 =item prototype FUNCTION
4469 Returns the prototype of a function as a string (or C<undef> if the
4470 function has no prototype). FUNCTION is a reference to, or the name of,
4471 the function whose prototype you want to retrieve.
4473 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
4474 name for a Perl builtin. If the builtin is not I<overridable> (such as
4475 C<qw//>) or if its arguments cannot be adequately expressed by a prototype
4476 (such as C<system>), prototype() returns C<undef>, because the builtin
4477 does not really behave like a Perl function. Otherwise, the string
4478 describing the equivalent prototype is returned.
4480 =item push ARRAY,LIST
4483 Treats ARRAY as a stack, and pushes the values of LIST
4484 onto the end of ARRAY. The length of ARRAY increases by the length of
4485 LIST. Has the same effect as
4488 $ARRAY[++$#ARRAY] = $value;
4491 but is more efficient. Returns the number of elements in the array following
4492 the completed C<push>.
4494 Starting with Perl 5.14, C<push> can take a reference to an unblessed
4495 array. The argument will be dereferenced automatically. This aspect of
4496 C<push> is considered highly experimental. The exact behaviour may change
4497 in a future version of Perl.
4507 Generalized quotes. See L<perlop/"Quote-Like Operators">.
4511 Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
4513 =item quotemeta EXPR
4514 X<quotemeta> X<metacharacter>
4518 Returns the value of EXPR with all non-"word"
4519 characters backslashed. (That is, all characters not matching
4520 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
4521 returned string, regardless of any locale settings.)
4522 This is the internal function implementing
4523 the C<\Q> escape in double-quoted strings.
4525 If EXPR is omitted, uses C<$_>.
4527 quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
4528 regular expressions, because by default an interpolated variable will be
4529 considered a mini-regular expression. For example:
4531 my $sentence = 'The quick brown fox jumped over the lazy dog';
4532 my $substring = 'quick.*?fox';
4533 $sentence =~ s{$substring}{big bad wolf};
4535 Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
4539 my $sentence = 'The quick brown fox jumped over the lazy dog';
4540 my $substring = 'quick.*?fox';
4541 $sentence =~ s{\Q$substring\E}{big bad wolf};
4545 my $sentence = 'The quick brown fox jumped over the lazy dog';
4546 my $substring = 'quick.*?fox';
4547 my $quoted_substring = quotemeta($substring);
4548 $sentence =~ s{$quoted_substring}{big bad wolf};
4550 Will both leave the sentence as is. Normally, when accepting string input from
4551 the user, quotemeta() or C<\Q> must be used.
4553 In Perl 5.14, all characters whose code points are above 127 are not
4554 quoted in UTF-8 encoded strings, but all are quoted in UTF-8 strings.
4555 It is planned to change this behavior in 5.16, but the exact rules
4556 haven't been determined yet.
4563 Returns a random fractional number greater than or equal to C<0> and less
4564 than the value of EXPR. (EXPR should be positive.) If EXPR is
4565 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
4566 also special-cased as C<1> (this was undocumented before Perl 5.8.0
4567 and is subject to change in future versions of Perl). Automatically calls
4568 C<srand> unless C<srand> has already been called. See also C<srand>.
4570 Apply C<int()> to the value returned by C<rand()> if you want random
4571 integers instead of random fractional numbers. For example,
4575 returns a random integer between C<0> and C<9>, inclusive.
4577 (Note: If your rand function consistently returns numbers that are too
4578 large or too small, then your version of Perl was probably compiled
4579 with the wrong number of RANDBITS.)
4581 B<C<rand()> is not cryptographically secure. You should not rely
4582 on it in security-sensitive situations.> As of this writing, a
4583 number of third-party CPAN modules offer random number generators
4584 intended by their authors to be cryptographically secure,
4585 including: L<Math::Random::Secure>, L<Math::Random::MT::Perl>, and
4586 L<Math::TrulyRandom>.
4588 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
4589 X<read> X<file, read>
4591 =item read FILEHANDLE,SCALAR,LENGTH
4593 Attempts to read LENGTH I<characters> of data into variable SCALAR
4594 from the specified FILEHANDLE. Returns the number of characters
4595 actually read, C<0> at end of file, or undef if there was an error (in
4596 the latter case C<$!> is also set). SCALAR will be grown or shrunk
4597 so that the last character actually read is the last character of the
4598 scalar after the read.
4600 An OFFSET may be specified to place the read data at some place in the
4601 string other than the beginning. A negative OFFSET specifies
4602 placement at that many characters counting backwards from the end of
4603 the string. A positive OFFSET greater than the length of SCALAR
4604 results in the string being padded to the required size with C<"\0">
4605 bytes before the result of the read is appended.
4607 The call is implemented in terms of either Perl's or your system's native
4608 fread(3) library function. To get a true read(2) system call, see C<sysread>.
4610 Note the I<characters>: depending on the status of the filehandle,
4611 either (8-bit) bytes or characters are read. By default all
4612 filehandles operate on bytes, but for example if the filehandle has
4613 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
4614 pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4615 characters, not bytes. Similarly for the C<:encoding> pragma:
4616 in that case pretty much any characters can be read.
4618 =item readdir DIRHANDLE
4621 Returns the next directory entry for a directory opened by C<opendir>.
4622 If used in list context, returns all the rest of the entries in the
4623 directory. If there are no more entries, returns the undefined value in
4624 scalar context and the empty list in list context.
4626 If you're planning to filetest the return values out of a C<readdir>, you'd
4627 better prepend the directory in question. Otherwise, because we didn't
4628 C<chdir> there, it would have been testing the wrong file.
4630 opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
4631 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
4634 As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
4635 which will set C<$_> on every iteration.
4637 opendir(my $dh, $some_dir) || die;
4638 while(readdir $dh) {
4639 print "$some_dir/$_\n";
4646 X<readline> X<gets> X<fgets>
4648 Reads from the filehandle whose typeglob is contained in EXPR (or from
4649 *ARGV if EXPR is not provided). In scalar context, each call reads and
4650 returns the next line until end-of-file is reached, whereupon the
4651 subsequent call returns C<undef>. In list context, reads until end-of-file
4652 is reached and returns a list of lines. Note that the notion of "line"
4653 used here is whatever you may have defined with C<$/> or
4654 C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
4656 When C<$/> is set to C<undef>, when C<readline> is in scalar
4657 context (i.e., file slurp mode), and when an empty file is read, it
4658 returns C<''> the first time, followed by C<undef> subsequently.
4660 This is the internal function implementing the C<< <EXPR> >>
4661 operator, but you can use it directly. The C<< <EXPR> >>
4662 operator is discussed in more detail in L<perlop/"I/O Operators">.
4665 $line = readline(*STDIN); # same thing
4667 If C<readline> encounters an operating system error, C<$!> will be set
4668 with the corresponding error message. It can be helpful to check
4669 C<$!> when you are reading from filehandles you don't trust, such as a
4670 tty or a socket. The following example uses the operator form of
4671 C<readline> and dies if the result is not defined.
4673 while ( ! eof($fh) ) {
4674 defined( $_ = <$fh> ) or die "readline failed: $!";
4678 Note that you have can't handle C<readline> errors that way with the
4679 C<ARGV> filehandle. In that case, you have to open each element of
4680 C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
4682 foreach my $arg (@ARGV) {
4683 open(my $fh, $arg) or warn "Can't open $arg: $!";
4685 while ( ! eof($fh) ) {
4686 defined( $_ = <$fh> )
4687 or die "readline failed for $arg: $!";
4697 Returns the value of a symbolic link, if symbolic links are
4698 implemented. If not, raises an exception. If there is a system
4699 error, returns the undefined value and sets C<$!> (errno). If EXPR is
4700 omitted, uses C<$_>.
4707 EXPR is executed as a system command.
4708 The collected standard output of the command is returned.
4709 In scalar context, it comes back as a single (potentially
4710 multi-line) string. In list context, returns a list of lines
4711 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
4712 This is the internal function implementing the C<qx/EXPR/>
4713 operator, but you can use it directly. The C<qx/EXPR/>
4714 operator is discussed in more detail in L<perlop/"I/O Operators">.
4715 If EXPR is omitted, uses C<$_>.
4717 =item recv SOCKET,SCALAR,LENGTH,FLAGS
4720 Receives a message on a socket. Attempts to receive LENGTH characters
4721 of data into variable SCALAR from the specified SOCKET filehandle.
4722 SCALAR will be grown or shrunk to the length actually read. Takes the
4723 same flags as the system call of the same name. Returns the address
4724 of the sender if SOCKET's protocol supports this; returns an empty
4725 string otherwise. If there's an error, returns the undefined value.
4726 This call is actually implemented in terms of recvfrom(2) system call.
4727 See L<perlipc/"UDP: Message Passing"> for examples.
4729 Note the I<characters>: depending on the status of the socket, either
4730 (8-bit) bytes or characters are received. By default all sockets
4731 operate on bytes, but for example if the socket has been changed using
4732 binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
4733 C<open> pragma, L<open>), the I/O will operate on UTF-8 encoded Unicode
4734 characters, not bytes. Similarly for the C<:encoding> pragma: in that
4735 case pretty much any characters can be read.
4742 The C<redo> command restarts the loop block without evaluating the
4743 conditional again. The C<continue> block, if any, is not executed. If
4744 the LABEL is omitted, the command refers to the innermost enclosing
4745 loop. Programs that want to lie to themselves about what was just input
4746 normally use this command:
4748 # a simpleminded Pascal comment stripper
4749 # (warning: assumes no { or } in strings)
4750 LINE: while (<STDIN>) {
4751 while (s|({.*}.*){.*}|$1 |) {}
4756 if (/}/) { # end of comment?
4765 C<redo> cannot be used to retry a block that returns a value such as
4766 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
4767 a grep() or map() operation.
4769 Note that a block by itself is semantically identical to a loop
4770 that executes once. Thus C<redo> inside such a block will effectively
4771 turn it into a looping construct.
4773 See also L</continue> for an illustration of how C<last>, C<next>, and
4781 Returns a non-empty string if EXPR is a reference, the empty
4782 string otherwise. If EXPR
4783 is not specified, C<$_> will be used. The value returned depends on the
4784 type of thing the reference is a reference to.
4785 Builtin types include:
4799 If the referenced object has been blessed into a package, then that package
4800 name is returned instead. You can think of C<ref> as a C<typeof> operator.
4802 if (ref($r) eq "HASH") {
4803 print "r is a reference to a hash.\n";
4806 print "r is not a reference at all.\n";
4809 The return value C<LVALUE> indicates a reference to an lvalue that is not
4810 a variable. You get this from taking the reference of function calls like
4811 C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
4812 to a L<version string|perldata/"Version Strings">.
4814 The result C<Regexp> indicates that the argument is a regular expression
4815 resulting from C<qr//>.
4817 See also L<perlref>.
4819 =item rename OLDNAME,NEWNAME
4820 X<rename> X<move> X<mv> X<ren>
4822 Changes the name of a file; an existing file NEWNAME will be
4823 clobbered. Returns true for success, false otherwise.
4825 Behavior of this function varies wildly depending on your system
4826 implementation. For example, it will usually not work across file system
4827 boundaries, even though the system I<mv> command sometimes compensates
4828 for this. Other restrictions include whether it works on directories,
4829 open files, or pre-existing files. Check L<perlport> and either the
4830 rename(2) manpage or equivalent system documentation for details.
4832 For a platform independent C<move> function look at the L<File::Copy>
4835 =item require VERSION
4842 Demands a version of Perl specified by VERSION, or demands some semantics
4843 specified by EXPR or by C<$_> if EXPR is not supplied.
4845 VERSION may be either a numeric argument such as 5.006, which will be
4846 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
4847 to C<$^V> (aka $PERL_VERSION). An exception is raised if
4848 VERSION is greater than the version of the current Perl interpreter.
4849 Compare with L</use>, which can do a similar check at compile time.
4851 Specifying VERSION as a literal of the form v5.6.1 should generally be
4852 avoided, because it leads to misleading error messages under earlier
4853 versions of Perl that do not support this syntax. The equivalent numeric
4854 version should be used instead.
4856 require v5.6.1; # run time version check
4857 require 5.6.1; # ditto
4858 require 5.006_001; # ditto; preferred for backwards compatibility
4860 Otherwise, C<require> demands that a library file be included if it
4861 hasn't already been included. The file is included via the do-FILE
4862 mechanism, which is essentially just a variety of C<eval> with the
4863 caveat that lexical variables in the invoking script will be invisible
4864 to the included code. Has semantics similar to the following subroutine:
4867 my ($filename) = @_;
4868 if (exists $INC{$filename}) {
4869 return 1 if $INC{$filename};
4870 die "Compilation failed in require";
4872 my ($realfilename,$result);
4874 foreach $prefix (@INC) {
4875 $realfilename = "$prefix/$filename";
4876 if (-f $realfilename) {
4877 $INC{$filename} = $realfilename;
4878 $result = do $realfilename;
4882 die "Can't find $filename in \@INC";
4885 $INC{$filename} = undef;
4887 } elsif (!$result) {
4888 delete $INC{$filename};
4889 die "$filename did not return true value";
4895 Note that the file will not be included twice under the same specified
4898 The file must return true as the last statement to indicate
4899 successful execution of any initialization code, so it's customary to
4900 end such a file with C<1;> unless you're sure it'll return true
4901 otherwise. But it's better just to put the C<1;>, in case you add more
4904 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4905 replaces "F<::>" with "F</>" in the filename for you,
4906 to make it easy to load standard modules. This form of loading of
4907 modules does not risk altering your namespace.
4909 In other words, if you try this:
4911 require Foo::Bar; # a splendid bareword
4913 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4914 directories specified in the C<@INC> array.
4916 But if you try this:
4918 $class = 'Foo::Bar';
4919 require $class; # $class is not a bareword
4921 require "Foo::Bar"; # not a bareword because of the ""
4923 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4924 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4926 eval "require $class";
4928 Now that you understand how C<require> looks for files with a
4929 bareword argument, there is a little extra functionality going on behind
4930 the scenes. Before C<require> looks for a "F<.pm>" extension, it will
4931 first look for a similar filename with a "F<.pmc>" extension. If this file
4932 is found, it will be loaded in place of any file ending in a "F<.pm>"
4935 You can also insert hooks into the import facility, by putting Perl code
4936 directly into the @INC array. There are three forms of hooks: subroutine
4937 references, array references and blessed objects.
4939 Subroutine references are the simplest case. When the inclusion system
4940 walks through @INC and encounters a subroutine, this subroutine gets
4941 called with two parameters, the first a reference to itself, and the
4942 second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
4943 subroutine should return either nothing or else a list of up to three
4944 values in the following order:
4950 A filehandle, from which the file will be read.
4954 A reference to a subroutine. If there is no filehandle (previous item),
4955 then this subroutine is expected to generate one line of source code per
4956 call, writing the line into C<$_> and returning 1, then returning 0 at
4957 end of file. If there is a filehandle, then the subroutine will be
4958 called to act as a simple source filter, with the line as read in C<$_>.
4959 Again, return 1 for each valid line, and 0 after all lines have been
4964 Optional state for the subroutine. The state is passed in as C<$_[1]>. A
4965 reference to the subroutine itself is passed in as C<$_[0]>.
4969 If an empty list, C<undef>, or nothing that matches the first 3 values above
4970 is returned, then C<require> looks at the remaining elements of @INC.
4971 Note that this filehandle must be a real filehandle (strictly a typeglob
4972 or reference to a typeglob, blessed or unblessed); tied filehandles will be
4973 ignored and return value processing will stop there.
4975 If the hook is an array reference, its first element must be a subroutine
4976 reference. This subroutine is called as above, but the first parameter is
4977 the array reference. This lets you indirectly pass arguments to
4980 In other words, you can write:
4982 push @INC, \&my_sub;
4984 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4990 push @INC, [ \&my_sub, $x, $y, ... ];
4992 my ($arrayref, $filename) = @_;
4993 # Retrieve $x, $y, ...
4994 my @parameters = @$arrayref[1..$#$arrayref];
4998 If the hook is an object, it must provide an INC method that will be
4999 called as above, the first parameter being the object itself. (Note that
5000 you must fully qualify the sub's name, as unqualified C<INC> is always forced
5001 into package C<main>.) Here is a typical code layout:
5007 my ($self, $filename) = @_;
5011 # In the main program
5012 push @INC, Foo->new(...);
5014 These hooks are also permitted to set the %INC entry
5015 corresponding to the files they have loaded. See L<perlvar/%INC>.
5017 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
5024 Generally used in a C<continue> block at the end of a loop to clear
5025 variables and reset C<??> searches so that they work again. The
5026 expression is interpreted as a list of single characters (hyphens
5027 allowed for ranges). All variables and arrays beginning with one of
5028 those letters are reset to their pristine state. If the expression is
5029 omitted, one-match searches (C<?pattern?>) are reset to match again.
5030 Only resets variables or searches in the current package. Always returns
5033 reset 'X'; # reset all X variables
5034 reset 'a-z'; # reset lower case variables
5035 reset; # just reset ?one-time? searches
5037 Resetting C<"A-Z"> is not recommended because you'll wipe out your
5038 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
5039 variables; lexical variables are unaffected, but they clean themselves
5040 up on scope exit anyway, so you'll probably want to use them instead.
5048 Returns from a subroutine, C<eval>, or C<do FILE> with the value
5049 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
5050 context, depending on how the return value will be used, and the context
5051 may vary from one execution to the next (see C<wantarray>). If no EXPR
5052 is given, returns an empty list in list context, the undefined value in
5053 scalar context, and (of course) nothing at all in void context.
5055 (In the absence of an explicit C<return>, a subroutine, eval,
5056 or do FILE automatically returns the value of the last expression
5060 X<reverse> X<rev> X<invert>
5062 In list context, returns a list value consisting of the elements
5063 of LIST in the opposite order. In scalar context, concatenates the
5064 elements of LIST and returns a string value with all characters
5065 in the opposite order.
5067 print join(", ", reverse "world", "Hello"); # Hello, world
5069 print scalar reverse "dlrow ,", "olleH"; # Hello, world
5071 Used without arguments in scalar context, reverse() reverses C<$_>.
5073 $_ = "dlrow ,olleH";
5074 print reverse; # No output, list context
5075 print scalar reverse; # Hello, world
5077 Note that reversing an array to itself (as in C<@a = reverse @a>) will
5078 preserve non-existent elements whenever possible, i.e., for non magical
5079 arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
5081 This operator is also handy for inverting a hash, although there are some
5082 caveats. If a value is duplicated in the original hash, only one of those
5083 can be represented as a key in the inverted hash. Also, this has to
5084 unwind one hash and build a whole new one, which may take some time
5085 on a large hash, such as from a DBM file.
5087 %by_name = reverse %by_address; # Invert the hash
5089 =item rewinddir DIRHANDLE
5092 Sets the current position to the beginning of the directory for the
5093 C<readdir> routine on DIRHANDLE.
5095 =item rindex STR,SUBSTR,POSITION
5098 =item rindex STR,SUBSTR
5100 Works just like index() except that it returns the position of the I<last>
5101 occurrence of SUBSTR in STR. If POSITION is specified, returns the
5102 last occurrence beginning at or before that position.
5104 =item rmdir FILENAME
5105 X<rmdir> X<rd> X<directory, remove>
5109 Deletes the directory specified by FILENAME if that directory is
5110 empty. If it succeeds it returns true, otherwise it returns false and
5111 sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
5113 To remove a directory tree recursively (C<rm -rf> on Unix) look at
5114 the C<rmtree> function of the L<File::Path> module.
5118 The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
5120 =item say FILEHANDLE LIST
5127 Just like C<print>, but implicitly appends a newline.
5128 C<say LIST> is simply an abbreviation for C<{ local $\ = "\n"; print
5131 This keyword is available only when the "say" feature is
5132 enabled: see L<feature>.
5135 X<scalar> X<context>
5137 Forces EXPR to be interpreted in scalar context and returns the value
5140 @counts = ( scalar @a, scalar @b, scalar @c );
5142 There is no equivalent operator to force an expression to
5143 be interpolated in list context because in practice, this is never
5144 needed. If you really wanted to do so, however, you could use
5145 the construction C<@{[ (some expression) ]}>, but usually a simple
5146 C<(some expression)> suffices.
5148 Because C<scalar> is a unary operator, if you accidentally use for EXPR a
5149 parenthesized list, this behaves as a scalar comma expression, evaluating
5150 all but the last element in void context and returning the final element
5151 evaluated in scalar context. This is seldom what you want.
5153 The following single statement:
5155 print uc(scalar(&foo,$bar)),$baz;
5157 is the moral equivalent of these two:
5160 print(uc($bar),$baz);
5162 See L<perlop> for more details on unary operators and the comma operator.
5164 =item seek FILEHANDLE,POSITION,WHENCE
5165 X<seek> X<fseek> X<filehandle, position>
5167 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
5168 FILEHANDLE may be an expression whose value gives the name of the
5169 filehandle. The values for WHENCE are C<0> to set the new position
5170 I<in bytes> to POSITION, C<1> to set it to the current position plus
5171 POSITION, and C<2> to set it to EOF plus POSITION (typically
5172 negative). For WHENCE you may use the constants C<SEEK_SET>,
5173 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
5174 of the file) from the Fcntl module. Returns C<1> on success, C<0>
5177 Note the I<in bytes>: even if the filehandle has been set to
5178 operate on characters (for example by using the C<:encoding(utf8)> open
5179 layer), tell() will return byte offsets, not character offsets
5180 (because implementing that would render seek() and tell() rather slow).
5182 If you want to position the file for C<sysread> or C<syswrite>, don't use
5183 C<seek>, because buffering makes its effect on the file's read-write position
5184 unpredictable and non-portable. Use C<sysseek> instead.
5186 Due to the rules and rigors of ANSI C, on some systems you have to do a
5187 seek whenever you switch between reading and writing. Amongst other
5188 things, this may have the effect of calling stdio's clearerr(3).
5189 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
5193 This is also useful for applications emulating C<tail -f>. Once you hit
5194 EOF on your read and then sleep for a while, you (probably) have to stick in a
5195 dummy seek() to reset things. The C<seek> doesn't change the position,
5196 but it I<does> clear the end-of-file condition on the handle, so that the
5197 next C<< <FILE> >> makes Perl try again to read something. (We hope.)
5199 If that doesn't work (some I/O implementations are particularly
5200 cantankerous), you might need something like this:
5203 for ($curpos = tell(FILE); $_ = <FILE>;
5204 $curpos = tell(FILE)) {
5205 # search for some stuff and put it into files
5207 sleep($for_a_while);
5208 seek(FILE, $curpos, 0);
5211 =item seekdir DIRHANDLE,POS
5214 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
5215 must be a value returned by C<telldir>. C<seekdir> also has the same caveats
5216 about possible directory compaction as the corresponding system library
5219 =item select FILEHANDLE
5220 X<select> X<filehandle, default>
5224 Returns the currently selected filehandle. If FILEHANDLE is supplied,
5225 sets the new current default filehandle for output. This has two
5226 effects: first, a C<write> or a C<print> without a filehandle will
5227 default to this FILEHANDLE. Second, references to variables related to
5228 output will refer to this output channel. For example, if you have to
5229 set the top of form format for more than one output channel, you might
5237 FILEHANDLE may be an expression whose value gives the name of the
5238 actual filehandle. Thus:
5240 $oldfh = select(STDERR); $| = 1; select($oldfh);
5242 Some programmers may prefer to think of filehandles as objects with
5243 methods, preferring to write the last example as:
5246 STDERR->autoflush(1);
5248 =item select RBITS,WBITS,EBITS,TIMEOUT
5251 This calls the select(2) syscall with the bit masks specified, which
5252 can be constructed using C<fileno> and C<vec>, along these lines:
5254 $rin = $win = $ein = '';
5255 vec($rin,fileno(STDIN),1) = 1;
5256 vec($win,fileno(STDOUT),1) = 1;
5259 If you want to select on many filehandles, you may wish to write a
5260 subroutine like this:
5263 my(@fhlist) = split(' ',$_[0]);
5266 vec($bits,fileno($_),1) = 1;
5270 $rin = fhbits('STDIN TTY SOCK');
5274 ($nfound,$timeleft) =
5275 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
5277 or to block until something becomes ready just do this
5279 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
5281 Most systems do not bother to return anything useful in $timeleft, so
5282 calling select() in scalar context just returns $nfound.
5284 Any of the bit masks can also be undef. The timeout, if specified, is
5285 in seconds, which may be fractional. Note: not all implementations are
5286 capable of returning the $timeleft. If not, they always return
5287 $timeleft equal to the supplied $timeout.
5289 You can effect a sleep of 250 milliseconds this way:
5291 select(undef, undef, undef, 0.25);
5293 Note that whether C<select> gets restarted after signals (say, SIGALRM)
5294 is implementation-dependent. See also L<perlport> for notes on the
5295 portability of C<select>.
5297 On error, C<select> behaves like select(2): it returns
5300 On some Unixes, select(2) may report a socket file
5301 descriptor as "ready for reading" when no data is available, and
5302 thus a subsequent read blocks. This can be avoided if you always use
5303 O_NONBLOCK on the socket. See select(2) and fcntl(2) for further
5306 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
5307 or <FH>) with C<select>, except as permitted by POSIX, and even
5308 then only on POSIX systems. You have to use C<sysread> instead.
5310 =item semctl ID,SEMNUM,CMD,ARG
5313 Calls the System V IPC function semctl(2). You'll probably have to say
5317 first to get the correct constant definitions. If CMD is IPC_STAT or
5318 GETALL, then ARG must be a variable that will hold the returned
5319 semid_ds structure or semaphore value array. Returns like C<ioctl>:
5320 the undefined value for error, "C<0 but true>" for zero, or the actual
5321 return value otherwise. The ARG must consist of a vector of native
5322 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
5323 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
5326 =item semget KEY,NSEMS,FLAGS
5329 Calls the System V IPC function semget(2). Returns the semaphore id, or
5330 the undefined value if there is an error. See also
5331 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
5334 =item semop KEY,OPSTRING
5337 Calls the System V IPC function semop(2) for semaphore operations
5338 such as signalling and waiting. OPSTRING must be a packed array of
5339 semop structures. Each semop structure can be generated with
5340 C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
5341 implies the number of semaphore operations. Returns true if
5342 successful, or false if there is an error. As an example, the
5343 following code waits on semaphore $semnum of semaphore id $semid:
5345 $semop = pack("s!3", $semnum, -1, 0);
5346 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
5348 To signal the semaphore, replace C<-1> with C<1>. See also
5349 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
5352 =item send SOCKET,MSG,FLAGS,TO
5355 =item send SOCKET,MSG,FLAGS
5357 Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
5358 filehandle. Takes the same flags as the system call of the same name. On
5359 unconnected sockets, you must specify a destination to I<send to>, in which
5360 case it does a sendto(2) syscall. Returns the number of characters sent,
5361 or the undefined value on error. The sendmsg(2) syscall is currently
5362 unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
5364 Note the I<characters>: depending on the status of the socket, either
5365 (8-bit) bytes or characters are sent. By default all sockets operate
5366 on bytes, but for example if the socket has been changed using
5367 binmode() to operate with the C<:encoding(utf8)> I/O layer (see
5368 L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
5369 encoded Unicode characters, not bytes. Similarly for the C<:encoding>
5370 pragma: in that case pretty much any characters can be sent.
5372 =item setpgrp PID,PGRP
5375 Sets the current process group for the specified PID, C<0> for the current
5376 process. Raises an exception when used on a machine that doesn't
5377 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
5378 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
5379 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
5382 =item setpriority WHICH,WHO,PRIORITY
5383 X<setpriority> X<priority> X<nice> X<renice>
5385 Sets the current priority for a process, a process group, or a user.
5386 (See setpriority(2).) Raises an exception when used on a machine
5387 that doesn't implement setpriority(2).
5389 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
5392 Sets the socket option requested. Returns undefined if there is an
5393 error. Use integer constants provided by the C<Socket> module for
5394 LEVEL and OPNAME. Values for LEVEL can also be obtained from
5395 getprotobyname. OPTVAL might either be a packed string or an integer.
5396 An integer OPTVAL is shorthand for pack("i", OPTVAL).
5398 An example disabling Nagle's algorithm on a socket:
5400 use Socket qw(IPPROTO_TCP TCP_NODELAY);
5401 setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
5408 Shifts the first value of the array off and returns it, shortening the
5409 array by 1 and moving everything down. If there are no elements in the
5410 array, returns the undefined value. If ARRAY is omitted, shifts the
5411 C<@_> array within the lexical scope of subroutines and formats, and the
5412 C<@ARGV> array outside a subroutine and also within the lexical scopes
5413 established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
5414 C<UNITCHECK {}> and C<END {}> constructs.
5416 Starting with Perl 5.14, C<shift> can take a reference to an unblessed
5417 array. The argument will be dereferenced automatically. This aspect of
5418 C<shift> is considered highly experimental. The exact behaviour may change
5419 in a future version of Perl.
5421 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
5422 same thing to the left end of an array that C<pop> and C<push> do to the
5425 =item shmctl ID,CMD,ARG
5428 Calls the System V IPC function shmctl. You'll probably have to say
5432 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
5433 then ARG must be a variable that will hold the returned C<shmid_ds>
5434 structure. Returns like ioctl: the undefined value for error, "C<0> but
5435 true" for zero, or the actual return value otherwise.
5436 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5438 =item shmget KEY,SIZE,FLAGS
5441 Calls the System V IPC function shmget. Returns the shared memory
5442 segment id, or the undefined value if there is an error.
5443 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
5445 =item shmread ID,VAR,POS,SIZE
5449 =item shmwrite ID,STRING,POS,SIZE
5451 Reads or writes the System V shared memory segment ID starting at
5452 position POS for size SIZE by attaching to it, copying in/out, and
5453 detaching from it. When reading, VAR must be a variable that will
5454 hold the data read. When writing, if STRING is too long, only SIZE
5455 bytes are used; if STRING is too short, nulls are written to fill out
5456 SIZE bytes. Return true if successful, or false if there is an error.
5457 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
5458 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
5460 =item shutdown SOCKET,HOW
5463 Shuts down a socket connection in the manner indicated by HOW, which
5464 has the same interpretation as in the syscall of the same name.
5466 shutdown(SOCKET, 0); # I/we have stopped reading data
5467 shutdown(SOCKET, 1); # I/we have stopped writing data
5468 shutdown(SOCKET, 2); # I/we have stopped using this socket
5470 This is useful with sockets when you want to tell the other
5471 side you're done writing but not done reading, or vice versa.
5472 It's also a more insistent form of close because it also
5473 disables the file descriptor in any forked copies in other
5476 Returns C<1> for success; on error, returns C<undef> if
5477 the first argument is not a valid filehandle, or returns C<0> and sets
5478 C<$!> for any other failure.
5481 X<sin> X<sine> X<asin> X<arcsine>
5485 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
5486 returns sine of C<$_>.
5488 For the inverse sine operation, you may use the C<Math::Trig::asin>
5489 function, or use this relation:
5491 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
5498 Causes the script to sleep for (integer) EXPR seconds, or forever if no
5499 argument is given. Returns the integer number of seconds actually slept.
5501 May be interrupted if the process receives a signal such as C<SIGALRM>.
5504 local $SIG{ALARM} = sub { die "Alarm!\n" };
5507 die $@ unless $@ eq "Alarm!\n";
5509 You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
5510 is often implemented using C<alarm>.
5512 On some older systems, it may sleep up to a full second less than what
5513 you requested, depending on how it counts seconds. Most modern systems
5514 always sleep the full amount. They may appear to sleep longer than that,
5515 however, because your process might not be scheduled right away in a
5516 busy multitasking system.
5518 For delays of finer granularity than one second, the Time::HiRes module
5519 (from CPAN, and starting from Perl 5.8 part of the standard
5520 distribution) provides usleep(). You may also use Perl's four-argument
5521 version of select() leaving the first three arguments undefined, or you
5522 might be able to use the C<syscall> interface to access setitimer(2) if
5523 your system supports it. See L<perlfaq8> for details.
5525 See also the POSIX module's C<pause> function.
5527 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
5530 Opens a socket of the specified kind and attaches it to filehandle
5531 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
5532 the syscall of the same name. You should C<use Socket> first
5533 to get the proper definitions imported. See the examples in
5534 L<perlipc/"Sockets: Client/Server Communication">.
5536 On systems that support a close-on-exec flag on files, the flag will
5537 be set for the newly opened file descriptor, as determined by the
5538 value of $^F. See L<perlvar/$^F>.
5540 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
5543 Creates an unnamed pair of sockets in the specified domain, of the
5544 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
5545 for the syscall of the same name. If unimplemented, raises an exception.
5546 Returns true if successful.
5548 On systems that support a close-on-exec flag on files, the flag will
5549 be set for the newly opened file descriptors, as determined by the value
5550 of $^F. See L<perlvar/$^F>.
5552 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
5553 to C<pipe(Rdr, Wtr)> is essentially:
5556 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
5557 shutdown(Rdr, 1); # no more writing for reader
5558 shutdown(Wtr, 0); # no more reading for writer
5560 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
5561 emulate socketpair using IP sockets to localhost if your system implements
5562 sockets but not socketpair.
5564 =item sort SUBNAME LIST
5565 X<sort> X<qsort> X<quicksort> X<mergesort>
5567 =item sort BLOCK LIST
5571 In list context, this sorts the LIST and returns the sorted list value.
5572 In scalar context, the behaviour of C<sort()> is undefined.
5574 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
5575 order. If SUBNAME is specified, it gives the name of a subroutine
5576 that returns an integer less than, equal to, or greater than C<0>,
5577 depending on how the elements of the list are to be ordered. (The
5578 C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
5579 SUBNAME may be a scalar variable name (unsubscripted), in which case
5580 the value provides the name of (or a reference to) the actual
5581 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
5582 an anonymous, in-line sort subroutine.
5584 If the subroutine's prototype is C<($$)>, the elements to be compared
5585 are passed by reference in C<@_>, as for a normal subroutine. This is
5586 slower than unprototyped subroutines, where the elements to be
5587 compared are passed into the subroutine
5588 as the package global variables $a and $b (see example below). Note that
5589 in the latter case, it is usually counter-productive to declare $a and
5592 The values to be compared are always passed by reference and should not
5595 You also cannot exit out of the sort block or subroutine using any of the
5596 loop control operators described in L<perlsyn> or with C<goto>.
5598 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
5599 current collation locale. See L<perllocale>.
5601 sort() returns aliases into the original list, much as a for loop's index
5602 variable aliases the list elements. That is, modifying an element of a
5603 list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
5604 actually modifies the element in the original list. This is usually
5605 something to be avoided when writing clear code.
5607 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
5608 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
5609 preserves the input order of elements that compare equal. Although
5610 quicksort's run time is O(NlogN) when averaged over all arrays of
5611 length N, the time can be O(N**2), I<quadratic> behavior, for some
5612 inputs.) In 5.7, the quicksort implementation was replaced with
5613 a stable mergesort algorithm whose worst-case behavior is O(NlogN).
5614 But benchmarks indicated that for some inputs, on some platforms,
5615 the original quicksort was faster. 5.8 has a sort pragma for
5616 limited control of the sort. Its rather blunt control of the
5617 underlying algorithm may not persist into future Perls, but the
5618 ability to characterize the input or output in implementation
5619 independent ways quite probably will. See L<the sort pragma|sort>.
5624 @articles = sort @files;
5626 # same thing, but with explicit sort routine
5627 @articles = sort {$a cmp $b} @files;
5629 # now case-insensitively
5630 @articles = sort {uc($a) cmp uc($b)} @files;
5632 # same thing in reversed order
5633 @articles = sort {$b cmp $a} @files;
5635 # sort numerically ascending
5636 @articles = sort {$a <=> $b} @files;
5638 # sort numerically descending
5639 @articles = sort {$b <=> $a} @files;
5641 # this sorts the %age hash by value instead of key
5642 # using an in-line function
5643 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
5645 # sort using explicit subroutine name
5647 $age{$a} <=> $age{$b}; # presuming numeric
5649 @sortedclass = sort byage @class;
5651 sub backwards { $b cmp $a }
5652 @harry = qw(dog cat x Cain Abel);
5653 @george = qw(gone chased yz Punished Axed);
5655 # prints AbelCaincatdogx
5656 print sort backwards @harry;
5657 # prints xdogcatCainAbel
5658 print sort @george, 'to', @harry;
5659 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
5661 # inefficiently sort by descending numeric compare using
5662 # the first integer after the first = sign, or the
5663 # whole record case-insensitively otherwise
5666 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
5671 # same thing, but much more efficiently;
5672 # we'll build auxiliary indices instead
5674 my @nums = @caps = ();
5676 push @nums, ( /=(\d+)/ ? $1 : undef );
5680 my @new = @old[ sort {
5681 $nums[$b] <=> $nums[$a]
5683 $caps[$a] cmp $caps[$b]
5687 # same thing, but without any temps
5688 @new = map { $_->[0] }
5689 sort { $b->[1] <=> $a->[1]
5692 } map { [$_, /=(\d+)/, uc($_)] } @old;
5694 # using a prototype allows you to use any comparison subroutine
5695 # as a sort subroutine (including other package's subroutines)
5697 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
5700 @new = sort other::backwards @old;
5702 # guarantee stability, regardless of algorithm
5704 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5706 # force use of mergesort (not portable outside Perl 5.8)
5707 use sort '_mergesort'; # note discouraging _
5708 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
5710 Warning: syntactical care is required when sorting the list returned from
5711 a function. If you want to sort the list returned by the function call
5712 C<find_records(@key)>, you can use:
5714 @contact = sort { $a cmp $b } find_records @key;
5715 @contact = sort +find_records(@key);
5716 @contact = sort &find_records(@key);
5717 @contact = sort(find_records(@key));
5719 If instead you want to sort the array @key with the comparison routine
5720 C<find_records()> then you can use:
5722 @contact = sort { find_records() } @key;
5723 @contact = sort find_records(@key);
5724 @contact = sort(find_records @key);
5725 @contact = sort(find_records (@key));
5727 If you're using strict, you I<must not> declare $a
5728 and $b as lexicals. They are package globals. That means
5729 that if you're in the C<main> package and type
5731 @articles = sort {$b <=> $a} @files;
5733 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
5734 but if you're in the C<FooPack> package, it's the same as typing
5736 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
5738 The comparison function is required to behave. If it returns
5739 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
5740 sometimes saying the opposite, for example) the results are not
5743 Because C<< <=> >> returns C<undef> when either operand is C<NaN>
5744 (not-a-number), and because C<sort> raises an exception unless the
5745 result of a comparison is defined, when sorting with a comparison function
5746 like C<< $a <=> $b >>, be careful about lists that might contain a C<NaN>.
5747 The following example takes advantage that C<NaN != NaN> to
5748 eliminate any C<NaN>s from the input list.
5750 @result = sort { $a <=> $b } grep { $_ == $_ } @input;
5752 =item splice ARRAY,OFFSET,LENGTH,LIST
5755 =item splice ARRAY,OFFSET,LENGTH
5757 =item splice ARRAY,OFFSET
5761 Removes the elements designated by OFFSET and LENGTH from an array, and
5762 replaces them with the elements of LIST, if any. In list context,
5763 returns the elements removed from the array. In scalar context,
5764 returns the last element removed, or C<undef> if no elements are
5765 removed. The array grows or shrinks as necessary.
5766 If OFFSET is negative then it starts that far from the end of the array.
5767 If LENGTH is omitted, removes everything from OFFSET onward.
5768 If LENGTH is negative, removes the elements from OFFSET onward
5769 except for -LENGTH elements at the end of the array.
5770 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
5771 past the end of the array, Perl issues a warning, and splices at the
5774 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
5776 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
5777 pop(@a) splice(@a,-1)
5778 shift(@a) splice(@a,0,1)
5779 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
5780 $a[$i] = $y splice(@a,$i,1,$y)
5782 Example, assuming array lengths are passed before arrays:
5784 sub aeq { # compare two list values
5785 my(@a) = splice(@_,0,shift);
5786 my(@b) = splice(@_,0,shift);
5787 return 0 unless @a == @b; # same len?
5789 return 0 if pop(@a) ne pop(@b);
5793 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
5795 Starting with Perl 5.14, C<splice> can take a reference to an unblessed
5796 array. The argument will be dereferenced automatically. This aspect of
5797 C<splice> is considered highly experimental. The exact behaviour may
5798 change in a future version of Perl.
5800 =item split /PATTERN/,EXPR,LIMIT
5803 =item split /PATTERN/,EXPR
5805 =item split /PATTERN/
5809 Splits the string EXPR into a list of strings and returns that list. By
5810 default, empty leading fields are preserved, and empty trailing ones are
5811 deleted. (If all fields are empty, they are considered to be trailing.)
5813 In scalar context, returns the number of fields found.
5815 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
5816 splits on whitespace (after skipping any leading whitespace). Anything
5817 matching PATTERN is taken to be a delimiter separating the fields. (Note
5818 that the delimiter may be longer than one character.)
5820 If LIMIT is specified and positive, it represents the maximum number
5821 of fields the EXPR will be split into, though the actual number of
5822 fields returned depends on the number of times PATTERN matches within
5823 EXPR. If LIMIT is unspecified or zero, trailing null fields are
5824 stripped (which potential users of C<pop> would do well to remember).
5825 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
5826 had been specified. Note that splitting an EXPR that evaluates to the
5827 empty string always returns the empty list, regardless of the LIMIT
5830 A pattern matching the empty string (not to be confused with
5831 an empty pattern C<//>, which is just one member of the set of patterns
5832 matching the epmty string), splits EXPR into individual
5833 characters. For example:
5835 print join(':', split(/ */, 'hi there')), "\n";
5837 produces the output 'h:i:t:h:e:r:e'.
5839 As a special case for C<split>, the empty pattern C<//> specifically
5840 matches the empty string; this is not be confused with the normal use
5841 of an empty pattern to mean the last successful match. So to split
5842 a string into individual characters, the following:
5844 print join(':', split(//, 'hi there')), "\n";
5846 produces the output 'h:i: :t:h:e:r:e'.
5848 Empty leading fields are produced when there are positive-width matches at
5849 the beginning of the string; a zero-width match at the beginning of
5850 the string does not produce an empty field. For example:
5852 print join(':', split(/(?=\w)/, 'hi there!'));
5854 produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
5855 hand, are produced when there is a match at the end of the string (and
5856 when LIMIT is given and is not 0), regardless of the length of the match.
5859 print join(':', split(//, 'hi there!', -1)), "\n";
5860 print join(':', split(/\W/, 'hi there!', -1)), "\n";
5862 produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
5863 both with an empty trailing field.
5865 The LIMIT parameter can be used to split a line partially
5867 ($login, $passwd, $remainder) = split(/:/, $_, 3);
5869 When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
5870 a LIMIT one larger than the number of variables in the list, to avoid
5871 unnecessary work. For the list above LIMIT would have been 4 by
5872 default. In time critical applications it behooves you not to split
5873 into more fields than you really need.
5875 If the PATTERN contains parentheses, additional list elements are
5876 created from each matching substring in the delimiter.
5878 split(/([,-])/, "1-10,20", 3);
5880 produces the list value
5882 (1, '-', 10, ',', 20)
5884 If you had the entire header of a normal Unix email message in $header,
5885 you could split it up into fields and their values this way:
5887 $header =~ s/\n(?=\s)//g; # fix continuation lines
5888 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
5890 The pattern C</PATTERN/> may be replaced with an expression to specify
5891 patterns that vary at runtime. (To do runtime compilation only once,
5892 use C</$variable/o>.)
5894 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
5895 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
5896 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
5897 will give you as many initial null fields (empty string) as there are leading spaces.
5898 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
5899 whitespace produces a null first field. A C<split> with no arguments
5900 really does a S<C<split(' ', $_)>> internally.
5902 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
5907 open(PASSWD, '/etc/passwd');
5910 ($login, $passwd, $uid, $gid,
5911 $gcos, $home, $shell) = split(/:/);
5915 As with regular pattern matching, any capturing parentheses that are not
5916 matched in a C<split()> will be set to C<undef> when returned:
5918 @fields = split /(A)|B/, "1A2B3";
5919 # @fields is (1, 'A', 2, undef, 3)
5921 =item sprintf FORMAT, LIST
5924 Returns a string formatted by the usual C<printf> conventions of the C
5925 library function C<sprintf>. See below for more details
5926 and see C<sprintf(3)> or C<printf(3)> on your system for an explanation of
5927 the general principles.
5931 # Format number with up to 8 leading zeroes
5932 $result = sprintf("%08d", $number);
5934 # Round number to 3 digits after decimal point
5935 $rounded = sprintf("%.3f", $number);
5937 Perl does its own C<sprintf> formatting: it emulates the C
5938 function sprintf(3), but doesn't use it except for floating-point
5939 numbers, and even then only standard modifiers are allowed.
5940 Non-standard extensions in your local sprintf(3) are
5941 therefore unavailable from Perl.
5943 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
5944 pass it an array as your first argument. The array is given scalar context,
5945 and instead of using the 0th element of the array as the format, Perl will
5946 use the count of elements in the array as the format, which is almost never
5949 Perl's C<sprintf> permits the following universally-known conversions:
5952 %c a character with the given number
5954 %d a signed integer, in decimal
5955 %u an unsigned integer, in decimal
5956 %o an unsigned integer, in octal
5957 %x an unsigned integer, in hexadecimal
5958 %e a floating-point number, in scientific notation
5959 %f a floating-point number, in fixed decimal notation
5960 %g a floating-point number, in %e or %f notation
5962 In addition, Perl permits the following widely-supported conversions:
5964 %X like %x, but using upper-case letters
5965 %E like %e, but using an upper-case "E"
5966 %G like %g, but with an upper-case "E" (if applicable)
5967 %b an unsigned integer, in binary
5968 %B like %b, but using an upper-case "B" with the # flag
5969 %p a pointer (outputs the Perl value's address in hexadecimal)
5970 %n special: *stores* the number of characters output so far
5971 into the next variable in the parameter list
5973 Finally, for backward (and we do mean "backward") compatibility, Perl
5974 permits these unnecessary but widely-supported conversions:
5977 %D a synonym for %ld
5978 %U a synonym for %lu
5979 %O a synonym for %lo
5982 Note that the number of exponent digits in the scientific notation produced
5983 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
5984 exponent less than 100 is system-dependent: it may be three or less
5985 (zero-padded as necessary). In other words, 1.23 times ten to the
5986 99th may be either "1.23e99" or "1.23e099".
5988 Between the C<%> and the format letter, you may specify several
5989 additional attributes controlling the interpretation of the format.
5990 In order, these are:
5994 =item format parameter index
5996 An explicit format parameter index, such as C<2$>. By default sprintf
5997 will format the next unused argument in the list, but this allows you
5998 to take the arguments out of order:
6000 printf '%2$d %1$d', 12, 34; # prints "34 12"
6001 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
6007 space prefix non-negative number with a space
6008 + prefix non-negative number with a plus sign
6009 - left-justify within the field
6010 0 use zeros, not spaces, to right-justify
6011 # ensure the leading "0" for any octal,
6012 prefix non-zero hexadecimal with "0x" or "0X",
6013 prefix non-zero binary with "0b" or "0B"
6017 printf '<% d>', 12; # prints "< 12>"
6018 printf '<%+d>', 12; # prints "<+12>"
6019 printf '<%6s>', 12; # prints "< 12>"
6020 printf '<%-6s>', 12; # prints "<12 >"
6021 printf '<%06s>', 12; # prints "<000012>"
6022 printf '<%#o>', 12; # prints "<014>"
6023 printf '<%#x>', 12; # prints "<0xc>"
6024 printf '<%#X>', 12; # prints "<0XC>"
6025 printf '<%#b>', 12; # prints "<0b1100>"
6026 printf '<%#B>', 12; # prints "<0B1100>"
6028 When a space and a plus sign are given as the flags at once,
6029 a plus sign is used to prefix a positive number.
6031 printf '<%+ d>', 12; # prints "<+12>"
6032 printf '<% +d>', 12; # prints "<+12>"
6034 When the # flag and a precision are given in the %o conversion,
6035 the precision is incremented if it's necessary for the leading "0".
6037 printf '<%#.5o>', 012; # prints "<00012>"
6038 printf '<%#.5o>', 012345; # prints "<012345>"
6039 printf '<%#.0o>', 0; # prints "<0>"
6043 This flag tells Perl to interpret the supplied string as a vector of
6044 integers, one for each character in the string. Perl applies the format to
6045 each integer in turn, then joins the resulting strings with a separator (a
6046 dot C<.> by default). This can be useful for displaying ordinal values of
6047 characters in arbitrary strings:
6049 printf "%vd", "AB\x{100}"; # prints "65.66.256"
6050 printf "version is v%vd\n", $^V; # Perl's version
6052 Put an asterisk C<*> before the C<v> to override the string to
6053 use to separate the numbers:
6055 printf "address is %*vX\n", ":", $addr; # IPv6 address
6056 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
6058 You can also explicitly specify the argument number to use for
6059 the join string using something like C<*2$v>; for example:
6061 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
6063 =item (minimum) width
6065 Arguments are usually formatted to be only as wide as required to
6066 display the given value. You can override the width by putting
6067 a number here, or get the width from the next argument (with C<*>)
6068 or from a specified argument (e.g., with C<*2$>):
6070 printf '<%s>', "a"; # prints "<a>"
6071 printf '<%6s>', "a"; # prints "< a>"
6072 printf '<%*s>', 6, "a"; # prints "< a>"
6073 printf '<%*2$s>', "a", 6; # prints "< a>"
6074 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
6076 If a field width obtained through C<*> is negative, it has the same
6077 effect as the C<-> flag: left-justification.
6079 =item precision, or maximum width
6082 You can specify a precision (for numeric conversions) or a maximum
6083 width (for string conversions) by specifying a C<.> followed by a number.
6084 For floating-point formats except 'g' and 'G', this specifies
6085 how many places right of the decimal point to show (the default being 6).
6088 # these examples are subject to system-specific variation
6089 printf '<%f>', 1; # prints "<1.000000>"
6090 printf '<%.1f>', 1; # prints "<1.0>"
6091 printf '<%.0f>', 1; # prints "<1>"
6092 printf '<%e>', 10; # prints "<1.000000e+01>"
6093 printf '<%.1e>', 10; # prints "<1.0e+01>"
6095 For "g" and "G", this specifies the maximum number of digits to show,
6096 including thoe prior to the decimal point and those after it; for
6099 # These examples are subject to system-specific variation.
6100 printf '<%g>', 1; # prints "<1>"
6101 printf '<%.10g>', 1; # prints "<1>"
6102 printf '<%g>', 100; # prints "<100>"
6103 printf '<%.1g>', 100; # prints "<1e+02>"
6104 printf '<%.2g>', 100.01; # prints "<1e+02>"
6105 printf '<%.5g>', 100.01; # prints "<100.01>"
6106 printf '<%.4g>', 100.01; # prints "<100>"
6108 For integer conversions, specifying a precision implies that the
6109 output of the number itself should be zero-padded to this width,
6110 where the 0 flag is ignored:
6112 printf '<%.6d>', 1; # prints "<000001>"
6113 printf '<%+.6d>', 1; # prints "<+000001>"
6114 printf '<%-10.6d>', 1; # prints "<000001 >"
6115 printf '<%10.6d>', 1; # prints "< 000001>"
6116 printf '<%010.6d>', 1; # prints "< 000001>"
6117 printf '<%+10.6d>', 1; # prints "< +000001>"
6119 printf '<%.6x>', 1; # prints "<000001>"
6120 printf '<%#.6x>', 1; # prints "<0x000001>"
6121 printf '<%-10.6x>', 1; # prints "<000001 >"
6122 printf '<%10.6x>', 1; # prints "< 000001>"
6123 printf '<%010.6x>', 1; # prints "< 000001>"
6124 printf '<%#10.6x>', 1; # prints "< 0x000001>"
6126 For string conversions, specifying a precision truncates the string
6127 to fit the specified width:
6129 printf '<%.5s>', "truncated"; # prints "<trunc>"
6130 printf '<%10.5s>', "truncated"; # prints "< trunc>"
6132 You can also get the precision from the next argument using C<.*>:
6134 printf '<%.6x>', 1; # prints "<000001>"
6135 printf '<%.*x>', 6, 1; # prints "<000001>"
6137 If a precision obtained through C<*> is negative, it counts
6138 as having no precision at all.
6140 printf '<%.*s>', 7, "string"; # prints "<string>"
6141 printf '<%.*s>', 3, "string"; # prints "<str>"
6142 printf '<%.*s>', 0, "string"; # prints "<>"
6143 printf '<%.*s>', -1, "string"; # prints "<string>"
6145 printf '<%.*d>', 1, 0; # prints "<0>"
6146 printf '<%.*d>', 0, 0; # prints "<>"
6147 printf '<%.*d>', -1, 0; # prints "<0>"
6149 You cannot currently get the precision from a specified number,
6150 but it is intended that this will be possible in the future, for
6151 example using C<.*2$>:
6153 printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
6157 For numeric conversions, you can specify the size to interpret the
6158 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
6159 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
6160 whatever the default integer size is on your platform (usually 32 or 64
6161 bits), but you can override this to use instead one of the standard C types,
6162 as supported by the compiler used to build Perl:
6164 l interpret integer as C type "long" or "unsigned long"
6165 h interpret integer as C type "short" or "unsigned short"
6166 q, L or ll interpret integer as C type "long long", "unsigned long long".
6167 or "quads" (typically 64-bit integers)
6169 The last will raise an exception if Perl does not understand "quads" in your
6170 installation. (This requires either that the platform natively support quads,
6171 or that Perl were specifically compiled to support quads.) You can find out
6172 whether your Perl supports quads via L<Config>:
6175 if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
6176 print "Nice quads!\n";
6179 For floating-point conversions (C<e f g E F G>), numbers are usually assumed
6180 to be the default floating-point size on your platform (double or long double),
6181 but you can force "long double" with C<q>, C<L>, or C<ll> if your
6182 platform supports them. You can find out whether your Perl supports long
6183 doubles via L<Config>:
6186 print "long doubles\n" if $Config{d_longdbl} eq "define";
6188 You can find out whether Perl considers "long double" to be the default
6189 floating-point size to use on your platform via L<Config>:
6192 if ($Config{uselongdouble} eq "define") {
6193 print "long doubles by default\n";
6196 It can also be that long doubles and doubles are the same thing:
6199 ($Config{doublesize} == $Config{longdblsize}) &&
6200 print "doubles are long doubles\n";
6202 The size specifier C<V> has no effect for Perl code, but is supported for
6203 compatibility with XS code. It means "use the standard size for a Perl
6204 integer or floating-point number", which is the default.
6206 =item order of arguments
6208 Normally, sprintf() takes the next unused argument as the value to
6209 format for each format specification. If the format specification
6210 uses C<*> to require additional arguments, these are consumed from
6211 the argument list in the order they appear in the format
6212 specification I<before> the value to format. Where an argument is
6213 specified by an explicit index, this does not affect the normal
6214 order for the arguments, even when the explicitly specified index
6215 would have been the next argument.
6219 printf "<%*.*s>", $a, $b, $c;
6221 uses C<$a> for the width, C<$b> for the precision, and C<$c>
6222 as the value to format; while:
6224 printf "<%*1$.*s>", $a, $b;
6226 would use C<$a> for the width and precision, and C<$b> as the
6229 Here are some more examples; be aware that when using an explicit
6230 index, the C<$> may need escaping:
6232 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
6233 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
6234 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
6235 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
6239 If C<use locale> is in effect and POSIX::setlocale() has been called,
6240 the character used for the decimal separator in formatted floating-point
6241 numbers is affected by the LC_NUMERIC locale. See L<perllocale>
6245 X<sqrt> X<root> X<square root>
6249 Return the positive square root of EXPR. If EXPR is omitted, uses
6250 C<$_>. Works only for non-negative operands unless you've
6251 loaded the C<Math::Complex> module.
6254 print sqrt(-4); # prints 2i
6257 X<srand> X<seed> X<randseed>
6261 Sets and returns the random number seed for the C<rand> operator.
6263 The point of the function is to "seed" the C<rand> function so that
6264 C<rand> can produce a different sequence each time you run your
6265 program. When called with a parameter, C<srand> uses that for the seed;
6266 otherwise it (semi-)randomly chooses a seed. In either case, starting with
6267 Perl 5.14, it returns the seed.
6269 If C<srand()> is not called explicitly, it is called implicitly without a
6270 parameter at the first use of the C<rand> operator. However, this was not true
6271 of versions of Perl before 5.004, so if your script will run under older
6272 Perl versions, it should call C<srand>; otherwise most programs won't call
6275 But there are a few situations in recent Perls where programs are likely to
6276 want to call C<srand>. One is for generating predictable results generally for
6277 testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
6278 each time. Another other case is where you need a cryptographically-strong
6279 starting point rather than the generally acceptable default, which is based on
6280 time of day, process ID, and memory allocation, or the F</dev/urandom> device
6281 if available. And still another case is that you may want to call C<srand()>
6282 after a C<fork()> to avoid child processes sharing the same seed value as the
6283 parent (and consequently each other).
6285 Do B<not> call C<srand()> (i.e., without an argument) more than once per
6286 process. The internal state of the random number generator should
6287 contain more entropy than can be provided by any seed, so calling
6288 C<srand()> again actually I<loses> randomness.
6290 Most implementations of C<srand> take an integer and will silently
6291 truncate decimal numbers. This means C<srand(42)> will usually
6292 produce the same results as C<srand(42.1)>. To be safe, always pass
6293 C<srand> an integer.
6295 In versions of Perl prior to 5.004 the default seed was just the
6296 current C<time>. This isn't a particularly good seed, so many old
6297 programs supply their own seed value (often C<time ^ $$> or C<time ^
6298 ($$ + ($$ << 15))>), but that isn't necessary any more.
6300 For cryptographic purposes, however, you need something much more random
6301 than the default seed. Checksumming the compressed output of one or more
6302 rapidly changing operating system status programs is the usual method. For
6305 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
6307 If you're particularly concerned with this, search the CPAN for
6308 random number generator modules instead of rolling out your own.
6310 Frequently called programs (like CGI scripts) that simply use
6314 for a seed can fall prey to the mathematical property that
6318 one-third of the time. So don't do that.
6320 A typical use of the returned seed is for a test program which has too many
6321 combinations to test comprehensively in the time available to it each run. It
6322 can test a random subset each time, and should there be a failure, log the seed
6323 used for that run so that it can later be used to reproduce the exact results.
6325 =item stat FILEHANDLE
6326 X<stat> X<file, status> X<ctime>
6330 =item stat DIRHANDLE
6334 Returns a 13-element list giving the status info for a file, either
6335 the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
6336 omitted, it stats C<$_>. Returns the empty list if C<stat> fails. Typically
6339 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
6340 $atime,$mtime,$ctime,$blksize,$blocks)
6343 Not all fields are supported on all filesystem types. Here are the
6344 meanings of the fields:
6346 0 dev device number of filesystem
6348 2 mode file mode (type and permissions)
6349 3 nlink number of (hard) links to the file
6350 4 uid numeric user ID of file's owner
6351 5 gid numeric group ID of file's owner
6352 6 rdev the device identifier (special files only)
6353 7 size total size of file, in bytes
6354 8 atime last access time in seconds since the epoch
6355 9 mtime last modify time in seconds since the epoch
6356 10 ctime inode change time in seconds since the epoch (*)
6357 11 blksize preferred block size for file system I/O
6358 12 blocks actual number of blocks allocated
6360 (The epoch was at 00:00 January 1, 1970 GMT.)
6362 (*) Not all fields are supported on all filesystem types. Notably, the
6363 ctime field is non-portable. In particular, you cannot expect it to be a
6364 "creation time", see L<perlport/"Files and Filesystems"> for details.
6366 If C<stat> is passed the special filehandle consisting of an underline, no
6367 stat is done, but the current contents of the stat structure from the
6368 last C<stat>, C<lstat>, or filetest are returned. Example:
6370 if (-x $file && (($d) = stat(_)) && $d < 0) {
6371 print "$file is executable NFS file\n";
6374 (This works on machines only for which the device number is negative
6377 Because the mode contains both the file type and its permissions, you
6378 should mask off the file type portion and (s)printf using a C<"%o">
6379 if you want to see the real permissions.
6381 $mode = (stat($filename))[2];
6382 printf "Permissions are %04o\n", $mode & 07777;
6384 In scalar context, C<stat> returns a boolean value indicating success
6385 or failure, and, if successful, sets the information associated with
6386 the special filehandle C<_>.
6388 The L<File::stat> module provides a convenient, by-name access mechanism:
6391 $sb = stat($filename);
6392 printf "File is %s, size is %s, perm %04o, mtime %s\n",
6393 $filename, $sb->size, $sb->mode & 07777,
6394 scalar localtime $sb->mtime;
6396 You can import symbolic mode constants (C<S_IF*>) and functions
6397 (C<S_IS*>) from the Fcntl module:
6401 $mode = (stat($filename))[2];
6403 $user_rwx = ($mode & S_IRWXU) >> 6;
6404 $group_read = ($mode & S_IRGRP) >> 3;
6405 $other_execute = $mode & S_IXOTH;
6407 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
6409 $is_setuid = $mode & S_ISUID;
6410 $is_directory = S_ISDIR($mode);
6412 You could write the last two using the C<-u> and C<-d> operators.
6413 Commonly available C<S_IF*> constants are:
6415 # Permissions: read, write, execute, for user, group, others.
6417 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
6418 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
6419 S_IRWXO S_IROTH S_IWOTH S_IXOTH
6421 # Setuid/Setgid/Stickiness/SaveText.
6422 # Note that the exact meaning of these is system dependent.
6424 S_ISUID S_ISGID S_ISVTX S_ISTXT
6426 # File types. Not necessarily all are available on your system.
6428 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
6430 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
6432 S_IREAD S_IWRITE S_IEXEC
6434 and the C<S_IF*> functions are
6436 S_IMODE($mode) the part of $mode containing the permission bits
6437 and the setuid/setgid/sticky bits
6439 S_IFMT($mode) the part of $mode containing the file type
6440 which can be bit-anded with (for example) S_IFREG
6441 or with the following functions
6443 # The operators -f, -d, -l, -b, -c, -p, and -S.
6445 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
6446 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
6448 # No direct -X operator counterpart, but for the first one
6449 # the -g operator is often equivalent. The ENFMT stands for
6450 # record flocking enforcement, a platform-dependent feature.
6452 S_ISENFMT($mode) S_ISWHT($mode)
6454 See your native chmod(2) and stat(2) documentation for more details
6455 about the C<S_*> constants. To get status info for a symbolic link
6456 instead of the target file behind the link, use the C<lstat> function.
6461 =item state TYPE EXPR
6463 =item state EXPR : ATTRS
6465 =item state TYPE EXPR : ATTRS
6467 C<state> declares a lexically scoped variable, just like C<my> does.
6468 However, those variables will never be reinitialized, contrary to
6469 lexical variables that are reinitialized each time their enclosing block
6472 C<state> variables are enabled only when the C<use feature "state"> pragma
6473 is in effect. See L<feature>.
6480 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
6481 doing many pattern matches on the string before it is next modified.
6482 This may or may not save time, depending on the nature and number of
6483 patterns you are searching on, and on the distribution of character
6484 frequencies in the string to be searched; you probably want to compare
6485 run times with and without it to see which runs faster. Those loops
6486 that scan for many short constant strings (including the constant
6487 parts of more complex patterns) will benefit most. You may have only
6488 one C<study> active at a time: if you study a different scalar the first
6489 is "unstudied". (The way C<study> works is this: a linked list of every
6490 character in the string to be searched is made, so we know, for
6491 example, where all the C<'k'> characters are. From each search string,
6492 the rarest character is selected, based on some static frequency tables
6493 constructed from some C programs and English text. Only those places
6494 that contain this "rarest" character are examined.)
6496 For example, here is a loop that inserts index producing entries
6497 before any line containing a certain pattern:
6501 print ".IX foo\n" if /\bfoo\b/;
6502 print ".IX bar\n" if /\bbar\b/;
6503 print ".IX blurfl\n" if /\bblurfl\b/;
6508 In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
6509 will be looked at, because C<f> is rarer than C<o>. In general, this is
6510 a big win except in pathological cases. The only question is whether
6511 it saves you more time than it took to build the linked list in the
6514 Note that if you have to look for strings that you don't know till
6515 runtime, you can build an entire loop as a string and C<eval> that to
6516 avoid recompiling all your patterns all the time. Together with
6517 undefining C<$/> to input entire files as one record, this can be quite
6518 fast, often faster than specialized programs like fgrep(1). The following
6519 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
6520 out the names of those files that contain a match:
6522 $search = 'while (<>) { study;';
6523 foreach $word (@words) {
6524 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
6529 eval $search; # this screams
6530 $/ = "\n"; # put back to normal input delimiter
6531 foreach $file (sort keys(%seen)) {
6535 =item sub NAME BLOCK
6538 =item sub NAME (PROTO) BLOCK
6540 =item sub NAME : ATTRS BLOCK
6542 =item sub NAME (PROTO) : ATTRS BLOCK
6544 This is subroutine definition, not a real function I<per se>.
6545 Without a BLOCK it's just a forward declaration. Without a NAME,
6546 it's an anonymous function declaration, and does actually return
6547 a value: the CODE ref of the closure you just created.
6549 See L<perlsub> and L<perlref> for details about subroutines and
6550 references, and L<attributes> and L<Attribute::Handlers> for more
6551 information about attributes.
6553 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
6554 X<substr> X<substring> X<mid> X<left> X<right>
6556 =item substr EXPR,OFFSET,LENGTH
6558 =item substr EXPR,OFFSET
6560 Extracts a substring out of EXPR and returns it. First character is at
6561 offset C<0>, or whatever you've set C<$[> to (but don't do that).
6562 If OFFSET is negative (or more precisely, less than C<$[>), starts
6563 that far from the end of the string. If LENGTH is omitted, returns
6564 everything to the end of the string. If LENGTH is negative, leaves that
6565 many characters off the end of the string.
6567 my $s = "The black cat climbed the green tree";
6568 my $color = substr $s, 4, 5; # black
6569 my $middle = substr $s, 4, -11; # black cat climbed the
6570 my $end = substr $s, 14; # climbed the green tree
6571 my $tail = substr $s, -4; # tree
6572 my $z = substr $s, -4, 2; # tr
6574 You can use the substr() function as an lvalue, in which case EXPR
6575 must itself be an lvalue. If you assign something shorter than LENGTH,
6576 the string will shrink, and if you assign something longer than LENGTH,
6577 the string will grow to accommodate it. To keep the string the same
6578 length, you may need to pad or chop your value using C<sprintf>.
6580 If OFFSET and LENGTH specify a substring that is partly outside the
6581 string, only the part within the string is returned. If the substring
6582 is beyond either end of the string, substr() returns the undefined
6583 value and produces a warning. When used as an lvalue, specifying a
6584 substring that is entirely outside the string raises an exception.
6585 Here's an example showing the behavior for boundary cases:
6588 substr($name, 4) = 'dy'; # $name is now 'freddy'
6589 my $null = substr $name, 6, 2; # returns "" (no warning)
6590 my $oops = substr $name, 7; # returns undef, with warning
6591 substr($name, 7) = 'gap'; # raises an exception
6593 An alternative to using substr() as an lvalue is to specify the
6594 replacement string as the 4th argument. This allows you to replace
6595 parts of the EXPR and return what was there before in one operation,
6596 just as you can with splice().
6598 my $s = "The black cat climbed the green tree";
6599 my $z = substr $s, 14, 7, "jumped from"; # climbed
6600 # $s is now "The black cat jumped from the green tree"
6602 Note that the lvalue returned by the 3-arg version of substr() acts as
6603 a 'magic bullet'; each time it is assigned to, it remembers which part
6604 of the original string is being modified; for example:
6607 for (substr($x,1,2)) {
6608 $_ = 'a'; print $x,"\n"; # prints 1a4
6609 $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
6611 $_ = 'pq'; print $x,"\n"; # prints 5pq9
6614 Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
6617 =item symlink OLDFILE,NEWFILE
6618 X<symlink> X<link> X<symbolic link> X<link, symbolic>
6620 Creates a new filename symbolically linked to the old filename.
6621 Returns C<1> for success, C<0> otherwise. On systems that don't support
6622 symbolic links, raises an exception. To check for that,
6625 $symlink_exists = eval { symlink("",""); 1 };
6627 =item syscall NUMBER, LIST
6628 X<syscall> X<system call>
6630 Calls the system call specified as the first element of the list,
6631 passing the remaining elements as arguments to the system call. If
6632 unimplemented, raises an exception. The arguments are interpreted
6633 as follows: if a given argument is numeric, the argument is passed as
6634 an int. If not, the pointer to the string value is passed. You are
6635 responsible to make sure a string is pre-extended long enough to
6636 receive any result that might be written into a string. You can't use a
6637 string literal (or other read-only string) as an argument to C<syscall>
6638 because Perl has to assume that any string pointer might be written
6640 integer arguments are not literals and have never been interpreted in a
6641 numeric context, you may need to add C<0> to them to force them to look
6642 like numbers. This emulates the C<syswrite> function (or vice versa):
6644 require 'syscall.ph'; # may need to run h2ph
6646 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
6648 Note that Perl supports passing of up to only 14 arguments to your syscall,
6649 which in practice should (usually) suffice.
6651 Syscall returns whatever value returned by the system call it calls.
6652 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
6653 Note that some system calls can legitimately return C<-1>. The proper
6654 way to handle such calls is to assign C<$!=0;> before the call and
6655 check the value of C<$!> if syscall returns C<-1>.
6657 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
6658 number of the read end of the pipe it creates. There is no way
6659 to retrieve the file number of the other end. You can avoid this
6660 problem by using C<pipe> instead.
6662 =item sysopen FILEHANDLE,FILENAME,MODE
6665 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
6667 Opens the file whose filename is given by FILENAME, and associates it
6668 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
6669 the name of the real filehandle wanted. This function calls the
6670 underlying operating system's C<open> function with the parameters
6671 FILENAME, MODE, PERMS.
6673 The possible values and flag bits of the MODE parameter are
6674 system-dependent; they are available via the standard module C<Fcntl>.
6675 See the documentation of your operating system's C<open> to see which
6676 values and flag bits are available. You may combine several flags
6677 using the C<|>-operator.
6679 Some of the most common values are C<O_RDONLY> for opening the file in
6680 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
6681 and C<O_RDWR> for opening the file in read-write mode.
6682 X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
6684 For historical reasons, some values work on almost every system
6685 supported by Perl: 0 means read-only, 1 means write-only, and 2
6686 means read/write. We know that these values do I<not> work under
6687 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
6688 use them in new code.
6690 If the file named by FILENAME does not exist and the C<open> call creates
6691 it (typically because MODE includes the C<O_CREAT> flag), then the value of
6692 PERMS specifies the permissions of the newly created file. If you omit
6693 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
6694 These permission values need to be in octal, and are modified by your
6695 process's current C<umask>.
6698 In many systems the C<O_EXCL> flag is available for opening files in
6699 exclusive mode. This is B<not> locking: exclusiveness means here that
6700 if the file already exists, sysopen() fails. C<O_EXCL> may not work
6701 on network filesystems, and has no effect unless the C<O_CREAT> flag
6702 is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
6703 being opened if it is a symbolic link. It does not protect against
6704 symbolic links in the file's path.
6707 Sometimes you may want to truncate an already-existing file. This
6708 can be done using the C<O_TRUNC> flag. The behavior of
6709 C<O_TRUNC> with C<O_RDONLY> is undefined.
6712 You should seldom if ever use C<0644> as argument to C<sysopen>, because
6713 that takes away the user's option to have a more permissive umask.
6714 Better to omit it. See the perlfunc(1) entry on C<umask> for more
6717 Note that C<sysopen> depends on the fdopen() C library function.
6718 On many Unix systems, fdopen() is known to fail when file descriptors
6719 exceed a certain value, typically 255. If you need more file
6720 descriptors than that, consider rebuilding Perl to use the C<sfio>
6721 library, or perhaps using the POSIX::open() function.
6723 See L<perlopentut> for a kinder, gentler explanation of opening files.
6725 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
6728 =item sysread FILEHANDLE,SCALAR,LENGTH
6730 Attempts to read LENGTH bytes of data into variable SCALAR from the
6731 specified FILEHANDLE, using the read(2). It bypasses
6732 buffered IO, so mixing this with other kinds of reads, C<print>,
6733 C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
6734 perlio or stdio layers usually buffers data. Returns the number of
6735 bytes actually read, C<0> at end of file, or undef if there was an
6736 error (in the latter case C<$!> is also set). SCALAR will be grown or
6737 shrunk so that the last byte actually read is the last byte of the
6738 scalar after the read.
6740 An OFFSET may be specified to place the read data at some place in the
6741 string other than the beginning. A negative OFFSET specifies
6742 placement at that many characters counting backwards from the end of
6743 the string. A positive OFFSET greater than the length of SCALAR
6744 results in the string being padded to the required size with C<"\0">
6745 bytes before the result of the read is appended.
6747 There is no syseof() function, which is ok, since eof() doesn't work
6748 well on device files (like ttys) anyway. Use sysread() and check
6749 for a return value for 0 to decide whether you're done.
6751 Note that if the filehandle has been marked as C<:utf8> Unicode
6752 characters are read instead of bytes (the LENGTH, OFFSET, and the
6753 return value of sysread() are in Unicode characters).
6754 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6755 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6757 =item sysseek FILEHANDLE,POSITION,WHENCE
6760 Sets FILEHANDLE's system position in bytes using
6761 lseek(2). FILEHANDLE may be an expression whose value gives the name
6762 of the filehandle. The values for WHENCE are C<0> to set the new
6763 position to POSITION, C<1> to set the it to the current position plus
6764 POSITION, and C<2> to set it to EOF plus POSITION (typically
6767 Note the I<in bytes>: even if the filehandle has been set to operate
6768 on characters (for example by using the C<:encoding(utf8)> I/O layer),
6769 tell() will return byte offsets, not character offsets (because
6770 implementing that would render sysseek() unacceptably slow).
6772 sysseek() bypasses normal buffered IO, so mixing this with reads (other
6773 than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
6774 C<seek>, C<tell>, or C<eof> may cause confusion.
6776 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
6777 and C<SEEK_END> (start of the file, current position, end of the file)
6778 from the Fcntl module. Use of the constants is also more portable
6779 than relying on 0, 1, and 2. For example to define a "systell" function:
6781 use Fcntl 'SEEK_CUR';
6782 sub systell { sysseek($_[0], 0, SEEK_CUR) }
6784 Returns the new position, or the undefined value on failure. A position
6785 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
6786 true on success and false on failure, yet you can still easily determine
6792 =item system PROGRAM LIST
6794 Does exactly the same thing as C<exec LIST>, except that a fork is
6795 done first, and the parent process waits for the child process to
6796 exit. Note that argument processing varies depending on the
6797 number of arguments. If there is more than one argument in LIST,
6798 or if LIST is an array with more than one value, starts the program
6799 given by the first element of the list with arguments given by the
6800 rest of the list. If there is only one scalar argument, the argument
6801 is checked for shell metacharacters, and if there are any, the
6802 entire argument is passed to the system's command shell for parsing
6803 (this is C</bin/sh -c> on Unix platforms, but varies on other
6804 platforms). If there are no shell metacharacters in the argument,
6805 it is split into words and passed directly to C<execvp>, which is
6808 Beginning with v5.6.0, Perl will attempt to flush all files opened for
6809 output before any operation that may do a fork, but this may not be
6810 supported on some platforms (see L<perlport>). To be safe, you may need
6811 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
6812 of C<IO::Handle> on any open handles.
6814 The return value is the exit status of the program as returned by the
6815 C<wait> call. To get the actual exit value, shift right by eight (see
6816 below). See also L</exec>. This is I<not> what you want to use to capture
6817 the output from a command, for that you should use merely backticks or
6818 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
6819 indicates a failure to start the program or an error of the wait(2) system
6820 call (inspect $! for the reason).
6822 If you'd like to make C<system> (and many other bits of Perl) die on error,
6823 have a look at the L<autodie> pragma.
6825 Like C<exec>, C<system> allows you to lie to a program about its name if
6826 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
6828 Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
6829 C<system>, if you expect your program to terminate on receipt of these
6830 signals you will need to arrange to do so yourself based on the return
6833 @args = ("command", "arg1", "arg2");
6835 or die "system @args failed: $?"
6837 If you'd like to manually inspect C<system>'s failure, you can check all
6838 possible failure modes by inspecting C<$?> like this:
6841 print "failed to execute: $!\n";
6844 printf "child died with signal %d, %s coredump\n",
6845 ($? & 127), ($? & 128) ? 'with' : 'without';
6848 printf "child exited with value %d\n", $? >> 8;
6851 Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
6852 with the C<W*()> calls from the POSIX module.
6854 When C<system>'s arguments are executed indirectly by the shell,
6855 results and return codes are subject to its quirks.
6856 See L<perlop/"`STRING`"> and L</exec> for details.
6858 Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
6859 handler. See L<perlipc> for details.
6861 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
6864 =item syswrite FILEHANDLE,SCALAR,LENGTH
6866 =item syswrite FILEHANDLE,SCALAR
6868 Attempts to write LENGTH bytes of data from variable SCALAR to the
6869 specified FILEHANDLE, using write(2). If LENGTH is
6870 not specified, writes whole SCALAR. It bypasses buffered IO, so
6871 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
6872 C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
6873 stdio layers usually buffers data. Returns the number of bytes
6874 actually written, or C<undef> if there was an error (in this case the
6875 errno variable C<$!> is also set). If the LENGTH is greater than the
6876 data available in the SCALAR after the OFFSET, only as much data as is
6877 available will be written.
6879 An OFFSET may be specified to write the data from some part of the
6880 string other than the beginning. A negative OFFSET specifies writing
6881 that many characters counting backwards from the end of the string.
6882 If SCALAR is of length zero, you can only use an OFFSET of 0.
6884 B<Warning>: If the filehandle is marked C<:utf8>, Unicode characters
6885 encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
6886 return value of syswrite() are in (UTF-8 encoded Unicode) characters.
6887 The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
6888 See L</binmode>, L</open>, and the C<open> pragma, L<open>.
6890 =item tell FILEHANDLE
6895 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
6896 error. FILEHANDLE may be an expression whose value gives the name of
6897 the actual filehandle. If FILEHANDLE is omitted, assumes the file
6900 Note the I<in bytes>: even if the filehandle has been set to
6901 operate on characters (for example by using the C<:encoding(utf8)> open
6902 layer), tell() will return byte offsets, not character offsets (because
6903 that would render seek() and tell() rather slow).
6905 The return value of tell() for the standard streams like the STDIN
6906 depends on the operating system: it may return -1 or something else.
6907 tell() on pipes, fifos, and sockets usually returns -1.
6909 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
6911 Do not use tell() (or other buffered I/O operations) on a filehandle
6912 that has been manipulated by sysread(), syswrite() or sysseek().
6913 Those functions ignore the buffering, while tell() does not.
6915 =item telldir DIRHANDLE
6918 Returns the current position of the C<readdir> routines on DIRHANDLE.
6919 Value may be given to C<seekdir> to access a particular location in a
6920 directory. C<telldir> has the same caveats about possible directory
6921 compaction as the corresponding system library routine.
6923 =item tie VARIABLE,CLASSNAME,LIST
6926 This function binds a variable to a package class that will provide the
6927 implementation for the variable. VARIABLE is the name of the variable
6928 to be enchanted. CLASSNAME is the name of a class implementing objects
6929 of correct type. Any additional arguments are passed to the C<new>
6930 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
6931 or C<TIEHASH>). Typically these are arguments such as might be passed
6932 to the C<dbm_open()> function of C. The object returned by the C<new>
6933 method is also returned by the C<tie> function, which would be useful
6934 if you want to access other methods in CLASSNAME.
6936 Note that functions such as C<keys> and C<values> may return huge lists
6937 when used on large objects, like DBM files. You may prefer to use the
6938 C<each> function to iterate over such. Example:
6940 # print out history file offsets
6942 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
6943 while (($key,$val) = each %HIST) {
6944 print $key, ' = ', unpack('L',$val), "\n";
6948 A class implementing a hash should have the following methods:
6950 TIEHASH classname, LIST
6952 STORE this, key, value
6957 NEXTKEY this, lastkey
6962 A class implementing an ordinary array should have the following methods:
6964 TIEARRAY classname, LIST
6966 STORE this, key, value
6968 STORESIZE this, count
6974 SPLICE this, offset, length, LIST
6979 A class implementing a filehandle should have the following methods:
6981 TIEHANDLE classname, LIST
6982 READ this, scalar, length, offset
6985 WRITE this, scalar, length, offset
6987 PRINTF this, format, LIST
6991 SEEK this, position, whence
6993 OPEN this, mode, LIST
6998 A class implementing a scalar should have the following methods:
7000 TIESCALAR classname, LIST
7006 Not all methods indicated above need be implemented. See L<perltie>,
7007 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
7009 Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
7010 for you; you need to do that explicitly yourself. See L<DB_File>
7011 or the F<Config> module for interesting C<tie> implementations.
7013 For further details see L<perltie>, L<"tied VARIABLE">.
7018 Returns a reference to the object underlying VARIABLE (the same value
7019 that was originally returned by the C<tie> call that bound the variable
7020 to a package.) Returns the undefined value if VARIABLE isn't tied to a
7026 Returns the number of non-leap seconds since whatever time the system
7027 considers to be the epoch, suitable for feeding to C<gmtime> and
7028 C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
7029 a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
7030 1904 in the current local time zone for its epoch.
7032 For measuring time in better granularity than one second,
7033 you may use either the L<Time::HiRes> module (from CPAN, and starting from
7034 Perl 5.8 part of the standard distribution), or if you have
7035 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
7036 See L<perlfaq8> for details.
7038 For date and time processing look at the many related modules on CPAN.
7039 For a comprehensive date and time representation look at the
7045 Returns a four-element list giving the user and system times, in
7046 seconds, for this process and the children of this process.
7048 ($user,$system,$cuser,$csystem) = times;
7050 In scalar context, C<times> returns C<$user>.
7052 Children's times are only included for terminated children.
7056 The transliteration operator. Same as C<y///>. See
7057 L<perlop/"Quote and Quote-like Operators">.
7059 =item truncate FILEHANDLE,LENGTH
7062 =item truncate EXPR,LENGTH
7064 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
7065 specified length. Raises an exception if truncate isn't implemented
7066 on your system. Returns true if successful, the undefined value
7069 The behavior is undefined if LENGTH is greater than the length of the
7072 The position in the file of FILEHANDLE is left unchanged. You may want to
7073 call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
7076 X<uc> X<uppercase> X<toupper>
7080 Returns an uppercased version of EXPR. This is the internal function
7081 implementing the C<\U> escape in double-quoted strings.
7082 It does not attempt to do titlecase mapping on initial letters. See
7083 L</ucfirst> for that.
7085 If EXPR is omitted, uses C<$_>.
7087 This function behaves the same way under various pragma, such as in a locale,
7091 X<ucfirst> X<uppercase>
7095 Returns the value of EXPR with the first character in uppercase
7096 (titlecase in Unicode). This is the internal function implementing
7097 the C<\u> escape in double-quoted strings.
7099 If EXPR is omitted, uses C<$_>.
7101 This function behaves the same way under various pragma, such as in a locale,
7109 Sets the umask for the process to EXPR and returns the previous value.
7110 If EXPR is omitted, merely returns the current umask.
7112 The Unix permission C<rwxr-x---> is represented as three sets of three
7113 bits, or three octal digits: C<0750> (the leading 0 indicates octal
7114 and isn't one of the digits). The C<umask> value is such a number
7115 representing disabled permissions bits. The permission (or "mode")
7116 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
7117 even if you tell C<sysopen> to create a file with permissions C<0777>,
7118 if your umask is C<0022> then the file will actually be created with
7119 permissions C<0755>. If your C<umask> were C<0027> (group can't
7120 write; others can't read, write, or execute), then passing
7121 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
7124 Here's some advice: supply a creation mode of C<0666> for regular
7125 files (in C<sysopen>) and one of C<0777> for directories (in
7126 C<mkdir>) and executable files. This gives users the freedom of
7127 choice: if they want protected files, they might choose process umasks
7128 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
7129 Programs should rarely if ever make policy decisions better left to
7130 the user. The exception to this is when writing files that should be
7131 kept private: mail files, web browser cookies, I<.rhosts> files, and
7134 If umask(2) is not implemented on your system and you are trying to
7135 restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
7136 raises an exception. If umask(2) is not implemented and you are
7137 not trying to restrict access for yourself, returns C<undef>.
7139 Remember that a umask is a number, usually given in octal; it is I<not> a
7140 string of octal digits. See also L</oct>, if all you have is a string.
7143 X<undef> X<undefine>
7147 Undefines the value of EXPR, which must be an lvalue. Use only on a
7148 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
7149 (using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
7150 will probably not do what you expect on most predefined variables or
7151 DBM list values, so don't do that; see L<delete>. Always returns the
7152 undefined value. You can omit the EXPR, in which case nothing is
7153 undefined, but you still get an undefined value that you could, for
7154 instance, return from a subroutine, assign to a variable, or pass as a
7155 parameter. Examples:
7158 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
7162 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
7163 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
7164 select undef, undef, undef, 0.25;
7165 ($a, $b, undef, $c) = &foo; # Ignore third value returned
7167 Note that this is a unary operator, not a list operator.
7170 X<unlink> X<delete> X<remove> X<rm> X<del>
7174 Deletes a list of files. On success, it returns the number of files
7175 it successfully deleted. On failure, it returns false and sets C<$!>
7178 my $unlinked = unlink 'a', 'b', 'c';
7180 unlink glob "*.bak";
7182 On error, C<unlink> will not tell you which files it could not remove.
7183 If you want to know which files you could not remove, try them one
7186 foreach my $file ( @goners ) {
7187 unlink $file or warn "Could not unlink $file: $!";
7190 Note: C<unlink> will not attempt to delete directories unless you are
7191 superuser and the B<-U> flag is supplied to Perl. Even if these
7192 conditions are met, be warned that unlinking a directory can inflict
7193 damage on your filesystem. Finally, using C<unlink> on directories is
7194 not supported on many operating systems. Use C<rmdir> instead.
7196 If LIST is omitted, C<unlink> uses C<$_>.
7198 =item unpack TEMPLATE,EXPR
7201 =item unpack TEMPLATE
7203 C<unpack> does the reverse of C<pack>: it takes a string
7204 and expands it out into a list of values.
7205 (In scalar context, it returns merely the first value produced.)
7207 If EXPR is omitted, unpacks the C<$_> string.
7208 See L<perlpacktut> for an introduction to this function.
7210 The string is broken into chunks described by the TEMPLATE. Each chunk
7211 is converted separately to a value. Typically, either the string is a result
7212 of C<pack>, or the characters of the string represent a C structure of some
7215 The TEMPLATE has the same format as in the C<pack> function.
7216 Here's a subroutine that does substring:
7219 my($what,$where,$howmuch) = @_;
7220 unpack("x$where a$howmuch", $what);
7225 sub ordinal { unpack("W",$_[0]); } # same as ord()
7227 In addition to fields allowed in pack(), you may prefix a field with
7228 a %<number> to indicate that
7229 you want a <number>-bit checksum of the items instead of the items
7230 themselves. Default is a 16-bit checksum. Checksum is calculated by
7231 summing numeric values of expanded values (for string fields the sum of
7232 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
7234 For example, the following
7235 computes the same number as the System V sum program:
7239 unpack("%32W*",<>) % 65535;
7242 The following efficiently counts the number of set bits in a bit vector:
7244 $setbits = unpack("%32b*", $selectmask);
7246 The C<p> and C<P> formats should be used with care. Since Perl
7247 has no way of checking whether the value passed to C<unpack()>
7248 corresponds to a valid memory location, passing a pointer value that's
7249 not known to be valid is likely to have disastrous consequences.
7251 If there are more pack codes or if the repeat count of a field or a group
7252 is larger than what the remainder of the input string allows, the result
7253 is not well defined: the repeat count may be decreased, or
7254 C<unpack()> may produce empty strings or zeros, or it may raise an exception.
7255 If the input string is longer than one described by the TEMPLATE,
7256 the remainder of that input string is ignored.
7258 See L</pack> for more examples and notes.
7260 =item untie VARIABLE
7263 Breaks the binding between a variable and a package. (See C<tie>.)
7264 Has no effect if the variable is not tied.
7266 =item unshift ARRAY,LIST
7269 Does the opposite of a C<shift>. Or the opposite of a C<push>,
7270 depending on how you look at it. Prepends list to the front of the
7271 array, and returns the new number of elements in the array.
7273 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
7275 Note the LIST is prepended whole, not one element at a time, so the
7276 prepended elements stay in the same order. Use C<reverse> to do the
7279 Starting with Perl 5.14, C<unshift> can take a reference to an unblessed
7280 array. The argument will be dereferenced automatically. This aspect of
7281 C<unshift> is considered highly experimental. The exact behaviour may
7282 change in a future version of Perl.
7284 =item use Module VERSION LIST
7285 X<use> X<module> X<import>
7287 =item use Module VERSION
7289 =item use Module LIST
7295 Imports some semantics into the current package from the named module,
7296 generally by aliasing certain subroutine or variable names into your
7297 package. It is exactly equivalent to
7299 BEGIN { require Module; Module->import( LIST ); }
7301 except that Module I<must> be a bareword.
7303 In the peculiar C<use VERSION> form, VERSION may be either a positive
7304 decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
7305 of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
7306 exception is raised if VERSION is greater than the version of the
7307 current Perl interpreter; Perl will not attempt to parse the rest of the
7308 file. Compare with L</require>, which can do a similar check at run time.
7309 Symmetrically, C<no VERSION> allows you to specify that you want a version
7310 of Perl older than the specified one.
7312 Specifying VERSION as a literal of the form v5.6.1 should generally be
7313 avoided, because it leads to misleading error messages under earlier
7314 versions of Perl (that is, prior to 5.6.0) that do not support this
7315 syntax. The equivalent numeric version should be used instead.
7317 use v5.6.1; # compile time version check
7319 use 5.006_001; # ditto; preferred for backwards compatibility
7321 This is often useful if you need to check the current Perl version before
7322 C<use>ing library modules that won't work with older versions of Perl.
7323 (We try not to do this more than we have to.)
7325 Also, if the specified Perl version is greater than or equal to 5.9.5,
7326 C<use VERSION> will also load the C<feature> pragma and enable all
7327 features available in the requested version. See L<feature>.
7328 Similarly, if the specified Perl version is greater than or equal to
7329 5.11.0, strictures are enabled lexically as with C<use strict> (except
7330 that the F<strict.pm> file is not actually loaded).
7332 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
7333 C<require> makes sure the module is loaded into memory if it hasn't been
7334 yet. The C<import> is not a builtin; it's just an ordinary static method
7335 call into the C<Module> package to tell the module to import the list of
7336 features back into the current package. The module can implement its
7337 C<import> method any way it likes, though most modules just choose to
7338 derive their C<import> method via inheritance from the C<Exporter> class that
7339 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
7340 method can be found then the call is skipped, even if there is an AUTOLOAD
7343 If you do not want to call the package's C<import> method (for instance,
7344 to stop your namespace from being altered), explicitly supply the empty list:
7348 That is exactly equivalent to
7350 BEGIN { require Module }
7352 If the VERSION argument is present between Module and LIST, then the
7353 C<use> will call the VERSION method in class Module with the given
7354 version as an argument. The default VERSION method, inherited from
7355 the UNIVERSAL class, croaks if the given version is larger than the
7356 value of the variable C<$Module::VERSION>.
7358 Again, there is a distinction between omitting LIST (C<import> called
7359 with no arguments) and an explicit empty LIST C<()> (C<import> not
7360 called). Note that there is no comma after VERSION!
7362 Because this is a wide-open interface, pragmas (compiler directives)
7363 are also implemented this way. Currently implemented pragmas are:
7368 use sigtrap qw(SEGV BUS);
7369 use strict qw(subs vars refs);
7370 use subs qw(afunc blurfl);
7371 use warnings qw(all);
7372 use sort qw(stable _quicksort _mergesort);
7374 Some of these pseudo-modules import semantics into the current
7375 block scope (like C<strict> or C<integer>, unlike ordinary modules,
7376 which import symbols into the current package (which are effective
7377 through the end of the file).
7379 Because C<use> takes effect at compile time, it doesn't respect the
7380 ordinary flow control of the code being compiled. In particular, putting
7381 a C<use> inside the false branch of a conditional doesn't prevent it
7382 from being processed. If a module or pragma only needs to be loaded
7383 conditionally, this can be done using the L<if> pragma:
7385 use if $] < 5.008, "utf8";
7386 use if WANT_WARNINGS, warnings => qw(all);
7388 There's a corresponding C<no> command that unimports meanings imported
7389 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
7390 It behaves just as C<import> does with VERSION, an omitted or empty LIST,
7391 or no unimport method being found.
7397 Care should be taken when using the C<no VERSION> form of C<no>. It is
7398 I<only> meant to be used to assert that the running perl is of a earlier
7399 version than its argument and I<not> to undo the feature-enabling side effects
7402 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
7403 for the C<-M> and C<-m> command-line options to Perl that give C<use>
7404 functionality from the command-line.
7409 Changes the access and modification times on each file of a list of
7410 files. The first two elements of the list must be the NUMERICAL access
7411 and modification times, in that order. Returns the number of files
7412 successfully changed. The inode change time of each file is set
7413 to the current time. For example, this code has the same effect as the
7414 Unix touch(1) command when the files I<already exist> and belong to
7415 the user running the program:
7418 $atime = $mtime = time;
7419 utime $atime, $mtime, @ARGV;
7421 Since Perl 5.7.2, if the first two elements of the list are C<undef>,
7422 the utime(2) syscall from your C library is called with a null second
7423 argument. On most systems, this will set the file's access and
7424 modification times to the current time (i.e., equivalent to the example
7425 above) and will work even on files you don't own provided you have write
7429 utime(undef, undef, $file)
7430 || warn "couldn't touch $file: $!";
7433 Under NFS this will use the time of the NFS server, not the time of
7434 the local machine. If there is a time synchronization problem, the
7435 NFS server and local machine will have different times. The Unix
7436 touch(1) command will in fact normally use this form instead of the
7437 one shown in the first example.
7439 Passing only one of the first two elements as C<undef> is
7440 equivalent to passing a 0 and will not have the effect
7441 described when both are C<undef>. This also triggers an
7442 uninitialized warning.
7444 On systems that support futimes(2), you may pass filehandles among the
7445 files. On systems that don't support futimes(2), passing filehandles raises
7446 an exception. Filehandles must be passed as globs or glob references to be
7447 recognized; barewords are considered filenames.
7454 Returns a list consisting of all the values of the named hash, or the values
7455 of an array. (In a scalar context, returns the number of values.)
7457 The values are returned in an apparently random order. The actual
7458 random order is subject to change in future versions of Perl, but it
7459 is guaranteed to be the same order as either the C<keys> or C<each>
7460 function would produce on the same (unmodified) hash. Since Perl
7461 5.8.1 the ordering is different even between different runs of Perl
7462 for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
7464 As a side effect, calling values() resets the HASH or ARRAY's internal
7466 see L</each>. (In particular, calling values() in void context resets
7467 the iterator with no other overhead. Apart from resetting the iterator,
7468 C<values @array> in list context is the same as plain C<@array>.
7469 We recommend that you use void context C<keys @array> for this, but reasoned
7470 that it taking C<values @array> out would require more documentation than
7474 Note that the values are not copied, which means modifying them will
7475 modify the contents of the hash:
7477 for (values %hash) { s/foo/bar/g } # modifies %hash values
7478 for (@hash{keys %hash}) { s/foo/bar/g } # same
7480 Starting with Perl 5.14, C<values> can take a reference to an unblessed
7481 hash or array. The argument will be dereferenced automatically. This
7482 aspect of C<values> is considered highly experimental. The exact behaviour
7483 may change in a future version of Perl.
7485 for (values $hashref) { ... }
7486 for (values $obj->get_arrayref) { ... }
7488 See also C<keys>, C<each>, and C<sort>.
7490 =item vec EXPR,OFFSET,BITS
7491 X<vec> X<bit> X<bit vector>
7493 Treats the string in EXPR as a bit vector made up of elements of
7494 width BITS, and returns the value of the element specified by OFFSET
7495 as an unsigned integer. BITS therefore specifies the number of bits
7496 that are reserved for each element in the bit vector. This must
7497 be a power of two from 1 to 32 (or 64, if your platform supports
7500 If BITS is 8, "elements" coincide with bytes of the input string.
7502 If BITS is 16 or more, bytes of the input string are grouped into chunks
7503 of size BITS/8, and each group is converted to a number as with
7504 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
7505 for BITS==64). See L<"pack"> for details.
7507 If bits is 4 or less, the string is broken into bytes, then the bits
7508 of each byte are broken into 8/BITS groups. Bits of a byte are
7509 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
7510 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
7511 breaking the single input byte C<chr(0x36)> into two groups gives a list
7512 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
7514 C<vec> may also be assigned to, in which case parentheses are needed
7515 to give the expression the correct precedence as in
7517 vec($image, $max_x * $x + $y, 8) = 3;
7519 If the selected element is outside the string, the value 0 is returned.
7520 If an element off the end of the string is written to, Perl will first
7521 extend the string with sufficiently many zero bytes. It is an error
7522 to try to write off the beginning of the string (i.e., negative OFFSET).
7524 If the string happens to be encoded as UTF-8 internally (and thus has
7525 the UTF8 flag set), this is ignored by C<vec>, and it operates on the
7526 internal byte string, not the conceptual character string, even if you
7527 only have characters with values less than 256.
7529 Strings created with C<vec> can also be manipulated with the logical
7530 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
7531 vector operation is desired when both operands are strings.
7532 See L<perlop/"Bitwise String Operators">.
7534 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
7535 The comments show the string after each step. Note that this code works
7536 in the same way on big-endian or little-endian machines.
7539 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
7541 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
7542 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
7544 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
7545 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
7546 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
7547 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
7548 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
7549 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
7551 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
7552 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
7553 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
7556 To transform a bit vector into a string or list of 0's and 1's, use these:
7558 $bits = unpack("b*", $vector);
7559 @bits = split(//, unpack("b*", $vector));
7561 If you know the exact length in bits, it can be used in place of the C<*>.
7563 Here is an example to illustrate how the bits actually fall in place:
7569 unpack("V",$_) 01234567890123456789012345678901
7570 ------------------------------------------------------------------
7575 for ($shift=0; $shift < $width; ++$shift) {
7576 for ($off=0; $off < 32/$width; ++$off) {
7577 $str = pack("B*", "0"x32);
7578 $bits = (1<<$shift);
7579 vec($str, $off, $width) = $bits;
7580 $res = unpack("b*",$str);
7581 $val = unpack("V", $str);
7588 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7589 $off, $width, $bits, $val, $res
7593 Regardless of the machine architecture on which it runs, the
7594 example above should print the following table:
7597 unpack("V",$_) 01234567890123456789012345678901
7598 ------------------------------------------------------------------
7599 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
7600 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
7601 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
7602 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
7603 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
7604 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
7605 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
7606 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
7607 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
7608 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
7609 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
7610 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
7611 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
7612 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
7613 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
7614 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
7615 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
7616 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
7617 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
7618 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
7619 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
7620 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
7621 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
7622 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
7623 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
7624 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
7625 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
7626 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
7627 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
7628 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
7629 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
7630 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
7631 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
7632 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
7633 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
7634 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
7635 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
7636 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
7637 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
7638 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
7639 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
7640 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
7641 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
7642 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
7643 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
7644 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
7645 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
7646 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
7647 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
7648 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
7649 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
7650 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
7651 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
7652 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
7653 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
7654 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
7655 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
7656 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
7657 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
7658 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
7659 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
7660 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
7661 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
7662 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
7663 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
7664 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
7665 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
7666 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
7667 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
7668 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
7669 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
7670 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
7671 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
7672 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
7673 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
7674 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
7675 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
7676 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
7677 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
7678 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
7679 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
7680 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
7681 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
7682 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
7683 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
7684 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
7685 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
7686 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
7687 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
7688 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
7689 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
7690 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
7691 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
7692 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
7693 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
7694 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
7695 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
7696 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
7697 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
7698 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
7699 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
7700 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
7701 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
7702 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
7703 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
7704 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
7705 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
7706 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
7707 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
7708 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
7709 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
7710 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
7711 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
7712 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
7713 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
7714 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
7715 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
7716 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
7717 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
7718 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
7719 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
7720 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
7721 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
7722 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
7723 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
7724 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
7725 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
7726 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
7731 Behaves like wait(2) on your system: it waits for a child
7732 process to terminate and returns the pid of the deceased process, or
7733 C<-1> if there are no child processes. The status is returned in C<$?>
7734 and C<${^CHILD_ERROR_NATIVE}>.
7735 Note that a return value of C<-1> could mean that child processes are
7736 being automatically reaped, as described in L<perlipc>.
7738 If you use wait in your handler for $SIG{CHLD} it may accidentally for the
7739 child created by qx() or system(). See L<perlipc> for details.
7741 =item waitpid PID,FLAGS
7744 Waits for a particular child process to terminate and returns the pid of
7745 the deceased process, or C<-1> if there is no such child process. On some
7746 systems, a value of 0 indicates that there are processes still running.
7747 The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
7749 use POSIX ":sys_wait_h";
7752 $kid = waitpid(-1, WNOHANG);
7755 then you can do a non-blocking wait for all pending zombie processes.
7756 Non-blocking wait is available on machines supporting either the
7757 waitpid(2) or wait4(2) syscalls. However, waiting for a particular
7758 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
7759 system call by remembering the status values of processes that have
7760 exited but have not been harvested by the Perl script yet.)
7762 Note that on some systems, a return value of C<-1> could mean that child
7763 processes are being automatically reaped. See L<perlipc> for details,
7764 and for other examples.
7767 X<wantarray> X<context>
7769 Returns true if the context of the currently executing subroutine or
7770 C<eval> is looking for a list value. Returns false if the context is
7771 looking for a scalar. Returns the undefined value if the context is
7772 looking for no value (void context).
7774 return unless defined wantarray; # don't bother doing more
7775 my @a = complex_calculation();
7776 return wantarray ? @a : "@a";
7778 C<wantarray()>'s result is unspecified in the top level of a file,
7779 in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
7780 in a C<DESTROY> method.
7782 This function should have been named wantlist() instead.
7785 X<warn> X<warning> X<STDERR>
7787 Prints the value of LIST to STDERR. If the last element of LIST does
7788 not end in a newline, it appends the same file/line number text as C<die>
7791 If the output is empty and C<$@> already contains a value (typically from a
7792 previous eval) that value is used after appending C<"\t...caught">
7793 to C<$@>. This is useful for staying almost, but not entirely similar to
7796 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
7798 No message is printed if there is a C<$SIG{__WARN__}> handler
7799 installed. It is the handler's responsibility to deal with the message
7800 as it sees fit (like, for instance, converting it into a C<die>). Most
7801 handlers must therefore arrange to actually display the
7802 warnings that they are not prepared to deal with, by calling C<warn>
7803 again in the handler. Note that this is quite safe and will not
7804 produce an endless loop, since C<__WARN__> hooks are not called from
7807 You will find this behavior is slightly different from that of
7808 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
7809 instead call C<die> again to change it).
7811 Using a C<__WARN__> handler provides a powerful way to silence all
7812 warnings (even the so-called mandatory ones). An example:
7814 # wipe out *all* compile-time warnings
7815 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
7817 my $foo = 20; # no warning about duplicate my $foo,
7818 # but hey, you asked for it!
7819 # no compile-time or run-time warnings before here
7822 # run-time warnings enabled after here
7823 warn "\$foo is alive and $foo!"; # does show up
7825 See L<perlvar> for details on setting C<%SIG> entries, and for more
7826 examples. See the Carp module for other kinds of warnings using its
7827 carp() and cluck() functions.
7829 =item write FILEHANDLE
7836 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
7837 using the format associated with that file. By default the format for
7838 a file is the one having the same name as the filehandle, but the
7839 format for the current output channel (see the C<select> function) may be set
7840 explicitly by assigning the name of the format to the C<$~> variable.
7842 Top of form processing is handled automatically: if there is
7843 insufficient room on the current page for the formatted record, the
7844 page is advanced by writing a form feed, a special top-of-page format
7845 is used to format the new page header, and then the record is written.
7846 By default the top-of-page format is the name of the filehandle with
7847 "_TOP" appended, but it may be dynamically set to the format of your
7848 choice by assigning the name to the C<$^> variable while the filehandle is
7849 selected. The number of lines remaining on the current page is in
7850 variable C<$->, which can be set to C<0> to force a new page.
7852 If FILEHANDLE is unspecified, output goes to the current default output
7853 channel, which starts out as STDOUT but may be changed by the
7854 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
7855 is evaluated and the resulting string is used to look up the name of
7856 the FILEHANDLE at run time. For more on formats, see L<perlform>.
7858 Note that write is I<not> the opposite of C<read>. Unfortunately.
7862 The transliteration operator. Same as C<tr///>. See
7863 L<perlop/"Quote and Quote-like Operators">.