3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 A named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
140 C<umask>, C<unlink>, C<utime>
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
228 C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 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>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file (heuristic guess).
295 -B File is a "binary" file (opposite of -T).
297 -M Script start time minus file modification time, in days.
298 -A Same for access time.
299 -C Same for inode change time (Unix, may differ for other platforms)
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current IO buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) (Also, if the stat buffer was filled by
354 a C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
357 print "Can do.\n" if -r $a || -w _ || -x _;
360 print "Readable\n" if -r _;
361 print "Writable\n" if -w _;
362 print "Executable\n" if -x _;
363 print "Setuid\n" if -u _;
364 print "Setgid\n" if -g _;
365 print "Sticky\n" if -k _;
366 print "Text\n" if -T _;
367 print "Binary\n" if -B _;
373 Returns the absolute value of its argument.
374 If VALUE is omitted, uses C<$_>.
376 =item accept NEWSOCKET,GENERICSOCKET
378 Accepts an incoming socket connect, just as the accept(2) system call
379 does. Returns the packed address if it succeeded, false otherwise.
380 See the example in L<perlipc/"Sockets: Client/Server Communication">.
382 On systems that support a close-on-exec flag on files, the flag will
383 be set for the newly opened file descriptor, as determined by the
384 value of $^F. See L<perlvar/$^F>.
390 Arranges to have a SIGALRM delivered to this process after the
391 specified number of wallclock seconds have elapsed. If SECONDS is not
392 specified, the value stored in C<$_> is used. (On some machines,
393 unfortunately, the elapsed time may be up to one second less or more
394 than you specified because of how seconds are counted, and process
395 scheduling may delay the delivery of the signal even further.)
397 Only one timer may be counting at once. Each call disables the
398 previous timer, and an argument of C<0> may be supplied to cancel the
399 previous timer without starting a new one. The returned value is the
400 amount of time remaining on the previous timer.
402 For delays of finer granularity than one second, you may use Perl's
403 four-argument version of select() leaving the first three arguments
404 undefined, or you might be able to use the C<syscall> interface to
405 access setitimer(2) if your system supports it. The Time::HiRes
406 module (from CPAN, and starting from Perl 5.8 part of the standard
407 distribution) may also prove useful.
409 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
410 (C<sleep> may be internally implemented in your system with C<alarm>)
412 If you want to use C<alarm> to time out a system call you need to use an
413 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
414 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
415 restart system calls on some systems. Using C<eval>/C<die> always works,
416 modulo the caveats given in L<perlipc/"Signals">.
419 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
421 $nread = sysread SOCKET, $buffer, $size;
425 die unless $@ eq "alarm\n"; # propagate unexpected errors
432 For more information see L<perlipc>.
436 Returns the arctangent of Y/X in the range -PI to PI.
438 For the tangent operation, you may use the C<Math::Trig::tan>
439 function, or use the familiar relation:
441 sub tan { sin($_[0]) / cos($_[0]) }
443 =item bind SOCKET,NAME
445 Binds a network address to a socket, just as the bind system call
446 does. Returns true if it succeeded, false otherwise. NAME should be a
447 packed address of the appropriate type for the socket. See the examples in
448 L<perlipc/"Sockets: Client/Server Communication">.
450 =item binmode FILEHANDLE, LAYER
452 =item binmode FILEHANDLE
454 Arranges for FILEHANDLE to be read or written in "binary" or "text"
455 mode on systems where the run-time libraries distinguish between
456 binary and text files. If FILEHANDLE is an expression, the value is
457 taken as the name of the filehandle. Returns true on success,
458 otherwise it returns C<undef> and sets C<$!> (errno).
460 If LAYER is omitted or specified as C<:raw> the filehandle is made
461 suitable for passing binary data. This includes turning off possible CRLF
462 translation and marking it as bytes (as opposed to Unicode characters).
463 Note that as desipite what may be implied in I<"Programming Perl">
464 (the Camel) or elsewhere C<:raw> is I<not> the simply inverse of C<:crlf>
465 -- other layers which would affect binary nature of the stream are
466 I<also> disabled. See L<PerlIO>, L<perlrun> and the discussion about the
467 PERLIO environment variable.
469 I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
470 in "Programming Perl, 3rd Edition". However, since the publishing of this
471 book, by many known as "Camel III", the consensus of the naming of this
472 functionality has moved from "discipline" to "layer". All documentation
473 of this version of Perl therefore refers to "layers" rather than to
474 "disciplines". Now back to the regularly scheduled documentation...>
476 On some systems (in general, DOS and Windows-based systems) binmode()
477 is necessary when you're not working with a text file. For the sake
478 of portability it is a good idea to always use it when appropriate,
479 and to never use it when it isn't appropriate.
481 In other words: regardless of platform, use binmode() on binary files
482 (like for example images).
484 If LAYER is present it is a single string, but may contain
485 multiple directives. The directives alter the behaviour of the
486 file handle. When LAYER is present using binmode on text
489 To mark FILEHANDLE as UTF-8, use C<:utf8>.
491 The C<:bytes>, C<:crlf>, and C<:utf8>, and any other directives of the
492 form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
493 establish default I/O layers. See L<open>.
495 In general, binmode() should be called after open() but before any I/O
496 is done on the filehandle. Calling binmode() will normally flush any
497 pending buffered output data (and perhaps pending input data) on the
498 handle. An exception to this is the C<:encoding> layer that
499 changes the default character encoding of the handle, see L<open>.
500 The C<:encoding> layer sometimes needs to be called in
501 mid-stream, and it doesn't flush the stream.
503 The operating system, device drivers, C libraries, and Perl run-time
504 system all work together to let the programmer treat a single
505 character (C<\n>) as the line terminator, irrespective of the external
506 representation. On many operating systems, the native text file
507 representation matches the internal representation, but on some
508 platforms the external representation of C<\n> is made up of more than
511 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
512 character to end each line in the external representation of text (even
513 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
514 on Unix and most VMS files). In other systems like OS/2, DOS and the
515 various flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>,
516 but what's stored in text files are the two characters C<\cM\cJ>. That
517 means that, if you don't use binmode() on these systems, C<\cM\cJ>
518 sequences on disk will be converted to C<\n> on input, and any C<\n> in
519 your program will be converted back to C<\cM\cJ> on output. This is what
520 you want for text files, but it can be disastrous for binary files.
522 Another consequence of using binmode() (on some systems) is that
523 special end-of-file markers will be seen as part of the data stream.
524 For systems from the Microsoft family this means that if your binary
525 data contains C<\cZ>, the I/O subsystem will regard it as the end of
526 the file, unless you use binmode().
528 binmode() is not only important for readline() and print() operations,
529 but also when using read(), seek(), sysread(), syswrite() and tell()
530 (see L<perlport> for more details). See the C<$/> and C<$\> variables
531 in L<perlvar> for how to manually set your input and output
532 line-termination sequences.
534 =item bless REF,CLASSNAME
538 This function tells the thingy referenced by REF that it is now an object
539 in the CLASSNAME package. If CLASSNAME is omitted, the current package
540 is used. Because a C<bless> is often the last thing in a constructor,
541 it returns the reference for convenience. Always use the two-argument
542 version if the function doing the blessing might be inherited by a
543 derived class. See L<perltoot> and L<perlobj> for more about the blessing
544 (and blessings) of objects.
546 Consider always blessing objects in CLASSNAMEs that are mixed case.
547 Namespaces with all lowercase names are considered reserved for
548 Perl pragmata. Builtin types have all uppercase names, so to prevent
549 confusion, you may wish to avoid such package names as well. Make sure
550 that CLASSNAME is a true value.
552 See L<perlmod/"Perl Modules">.
558 Returns the context of the current subroutine call. In scalar context,
559 returns the caller's package name if there is a caller, that is, if
560 we're in a subroutine or C<eval> or C<require>, and the undefined value
561 otherwise. In list context, returns
563 ($package, $filename, $line) = caller;
565 With EXPR, it returns some extra information that the debugger uses to
566 print a stack trace. The value of EXPR indicates how many call frames
567 to go back before the current one.
569 ($package, $filename, $line, $subroutine, $hasargs,
570 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
572 Here $subroutine may be C<(eval)> if the frame is not a subroutine
573 call, but an C<eval>. In such a case additional elements $evaltext and
574 C<$is_require> are set: C<$is_require> is true if the frame is created by a
575 C<require> or C<use> statement, $evaltext contains the text of the
576 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
577 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
578 each C<use> statement creates a C<require> frame inside an C<eval EXPR>
579 frame.) $subroutine may also be C<(unknown)> if this particular
580 subroutine happens to have been deleted from the symbol table.
581 C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
582 C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
583 compiled with. The C<$hints> and C<$bitmask> values are subject to change
584 between versions of Perl, and are not meant for external use.
586 Furthermore, when called from within the DB package, caller returns more
587 detailed information: it sets the list variable C<@DB::args> to be the
588 arguments with which the subroutine was invoked.
590 Be aware that the optimizer might have optimized call frames away before
591 C<caller> had a chance to get the information. That means that C<caller(N)>
592 might not return information about the call frame you expect it do, for
593 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
594 previous time C<caller> was called.
598 Changes the working directory to EXPR, if possible. If EXPR is omitted,
599 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
600 changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
601 variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
602 neither is set, C<chdir> does nothing. It returns true upon success,
603 false otherwise. See the example under C<die>.
607 Changes the permissions of a list of files. The first element of the
608 list must be the numerical mode, which should probably be an octal
609 number, and which definitely should I<not> a string of octal digits:
610 C<0644> is okay, C<'0644'> is not. Returns the number of files
611 successfully changed. See also L</oct>, if all you have is a string.
613 $cnt = chmod 0755, 'foo', 'bar';
614 chmod 0755, @executables;
615 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
617 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
618 $mode = 0644; chmod $mode, 'foo'; # this is best
620 You can also import the symbolic C<S_I*> constants from the Fcntl
625 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
626 # This is identical to the chmod 0755 of the above example.
634 This safer version of L</chop> removes any trailing string
635 that corresponds to the current value of C<$/> (also known as
636 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
637 number of characters removed from all its arguments. It's often used to
638 remove the newline from the end of an input record when you're worried
639 that the final record may be missing its newline. When in paragraph
640 mode (C<$/ = "">), it removes all trailing newlines from the string.
641 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
642 a reference to an integer or the like, see L<perlvar>) chomp() won't
644 If VARIABLE is omitted, it chomps C<$_>. Example:
647 chomp; # avoid \n on last field
652 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
654 You can actually chomp anything that's an lvalue, including an assignment:
657 chomp($answer = <STDIN>);
659 If you chomp a list, each element is chomped, and the total number of
660 characters removed is returned.
662 Note that parentheses are necessary when you're chomping anything
663 that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
664 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
665 C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
666 C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
675 Chops off the last character of a string and returns the character
676 chopped. It is much more efficient than C<s/.$//s> because it neither
677 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
678 If VARIABLE is a hash, it chops the hash's values, but not its keys.
680 You can actually chop anything that's an lvalue, including an assignment.
682 If you chop a list, each element is chopped. Only the value of the
683 last C<chop> is returned.
685 Note that C<chop> returns the last character. To return all but the last
686 character, use C<substr($string, 0, -1)>.
692 Changes the owner (and group) of a list of files. The first two
693 elements of the list must be the I<numeric> uid and gid, in that
694 order. A value of -1 in either position is interpreted by most
695 systems to leave that value unchanged. Returns the number of files
696 successfully changed.
698 $cnt = chown $uid, $gid, 'foo', 'bar';
699 chown $uid, $gid, @filenames;
701 Here's an example that looks up nonnumeric uids in the passwd file:
704 chomp($user = <STDIN>);
706 chomp($pattern = <STDIN>);
708 ($login,$pass,$uid,$gid) = getpwnam($user)
709 or die "$user not in passwd file";
711 @ary = glob($pattern); # expand filenames
712 chown $uid, $gid, @ary;
714 On most systems, you are not allowed to change the ownership of the
715 file unless you're the superuser, although you should be able to change
716 the group to any of your secondary groups. On insecure systems, these
717 restrictions may be relaxed, but this is not a portable assumption.
718 On POSIX systems, you can detect this condition this way:
720 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
721 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
727 Returns the character represented by that NUMBER in the character set.
728 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
729 chr(0x263a) is a Unicode smiley face. Note that characters from 127
730 to 255 (inclusive) are by default not encoded in Unicode for backward
731 compatibility reasons (but see L<encoding>).
733 For the reverse, use L</ord>.
734 See L<perlunicode> and L<encoding> for more about Unicode.
736 If NUMBER is omitted, uses C<$_>.
738 =item chroot FILENAME
742 This function works like the system call by the same name: it makes the
743 named directory the new root directory for all further pathnames that
744 begin with a C</> by your process and all its children. (It doesn't
745 change your current working directory, which is unaffected.) For security
746 reasons, this call is restricted to the superuser. If FILENAME is
747 omitted, does a C<chroot> to C<$_>.
749 =item close FILEHANDLE
753 Closes the file or pipe associated with the file handle, returning
754 true only if IO buffers are successfully flushed and closes the system
755 file descriptor. Closes the currently selected filehandle if the
758 You don't have to close FILEHANDLE if you are immediately going to do
759 another C<open> on it, because C<open> will close it for you. (See
760 C<open>.) However, an explicit C<close> on an input file resets the line
761 counter (C<$.>), while the implicit close done by C<open> does not.
763 If the file handle came from a piped open C<close> will additionally
764 return false if one of the other system calls involved fails or if the
765 program exits with non-zero status. (If the only problem was that the
766 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
767 also waits for the process executing on the pipe to complete, in case you
768 want to look at the output of the pipe afterwards, and
769 implicitly puts the exit status value of that command into C<$?>.
771 Prematurely closing the read end of a pipe (i.e. before the process
772 writing to it at the other end has closed it) will result in a
773 SIGPIPE being delivered to the writer. If the other end can't
774 handle that, be sure to read all the data before closing the pipe.
778 open(OUTPUT, '|sort >foo') # pipe to sort
779 or die "Can't start sort: $!";
780 #... # print stuff to output
781 close OUTPUT # wait for sort to finish
782 or warn $! ? "Error closing sort pipe: $!"
783 : "Exit status $? from sort";
784 open(INPUT, 'foo') # get sort's results
785 or die "Can't open 'foo' for input: $!";
787 FILEHANDLE may be an expression whose value can be used as an indirect
788 filehandle, usually the real filehandle name.
790 =item closedir DIRHANDLE
792 Closes a directory opened by C<opendir> and returns the success of that
795 =item connect SOCKET,NAME
797 Attempts to connect to a remote socket, just as the connect system call
798 does. Returns true if it succeeded, false otherwise. NAME should be a
799 packed address of the appropriate type for the socket. See the examples in
800 L<perlipc/"Sockets: Client/Server Communication">.
804 Actually a flow control statement rather than a function. If there is a
805 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
806 C<foreach>), it is always executed just before the conditional is about to
807 be evaluated again, just like the third part of a C<for> loop in C. Thus
808 it can be used to increment a loop variable, even when the loop has been
809 continued via the C<next> statement (which is similar to the C C<continue>
812 C<last>, C<next>, or C<redo> may appear within a C<continue>
813 block. C<last> and C<redo> will behave as if they had been executed within
814 the main block. So will C<next>, but since it will execute a C<continue>
815 block, it may be more entertaining.
818 ### redo always comes here
821 ### next always comes here
823 # then back the top to re-check EXPR
825 ### last always comes here
827 Omitting the C<continue> section is semantically equivalent to using an
828 empty one, logically enough. In that case, C<next> goes directly back
829 to check the condition at the top of the loop.
835 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
836 takes cosine of C<$_>.
838 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
839 function, or use this relation:
841 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
843 =item crypt PLAINTEXT,SALT
845 Encrypts a string exactly like the crypt(3) function in the C library
846 (assuming that you actually have a version there that has not been
847 extirpated as a potential munition). This can prove useful for checking
848 the password file for lousy passwords, amongst other things. Only the
849 guys wearing white hats should do this.
851 Note that L<crypt|/crypt> is intended to be a one-way function, much like
852 breaking eggs to make an omelette. There is no (known) corresponding
853 decrypt function (in other words, the crypt() is a one-way hash
854 function). As a result, this function isn't all that useful for
855 cryptography. (For that, see your nearby CPAN mirror.)
857 When verifying an existing encrypted string you should use the
858 encrypted text as the salt (like C<crypt($plain, $crypted) eq
859 $crypted>). This allows your code to work with the standard L<crypt|/crypt>
860 and with more exotic implementations. In other words, do not assume
861 anything about the returned string itself, or how many bytes in
862 the encrypted string matter.
864 Traditionally the result is a string of 13 bytes: two first bytes of
865 the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
866 the first eight bytes of the encrypted string mattered, but
867 alternative hashing schemes (like MD5), higher level security schemes
868 (like C2), and implementations on non-UNIX platforms may produce
871 When choosing a new salt create a random two character string whose
872 characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
873 '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
875 Here's an example that makes sure that whoever runs this program knows
878 $pwd = (getpwuid($<))[1];
882 chomp($word = <STDIN>);
886 if (crypt($word, $pwd) ne $pwd) {
892 Of course, typing in your own password to whoever asks you
895 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
896 of data, not least of all because you can't get the information
897 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
898 on your favorite CPAN mirror for a slew of potentially useful
901 If using crypt() on a Unicode string (which I<potentially> has
902 characters with codepoints above 255), Perl tries to make sense
903 of the situation by trying to downgrade (a copy of the string)
904 the string back to an eight-bit byte string before calling crypt()
905 (on that copy). If that works, good. If not, crypt() dies with
906 C<Wide character in crypt>.
910 [This function has been largely superseded by the C<untie> function.]
912 Breaks the binding between a DBM file and a hash.
914 =item dbmopen HASH,DBNAME,MASK
916 [This function has been largely superseded by the C<tie> function.]
918 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
919 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
920 argument is I<not> a filehandle, even though it looks like one). DBNAME
921 is the name of the database (without the F<.dir> or F<.pag> extension if
922 any). If the database does not exist, it is created with protection
923 specified by MASK (as modified by the C<umask>). If your system supports
924 only the older DBM functions, you may perform only one C<dbmopen> in your
925 program. In older versions of Perl, if your system had neither DBM nor
926 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
929 If you don't have write access to the DBM file, you can only read hash
930 variables, not set them. If you want to test whether you can write,
931 either use file tests or try setting a dummy hash entry inside an C<eval>,
932 which will trap the error.
934 Note that functions such as C<keys> and C<values> may return huge lists
935 when used on large DBM files. You may prefer to use the C<each>
936 function to iterate over large DBM files. Example:
938 # print out history file offsets
939 dbmopen(%HIST,'/usr/lib/news/history',0666);
940 while (($key,$val) = each %HIST) {
941 print $key, ' = ', unpack('L',$val), "\n";
945 See also L<AnyDBM_File> for a more general description of the pros and
946 cons of the various dbm approaches, as well as L<DB_File> for a particularly
949 You can control which DBM library you use by loading that library
950 before you call dbmopen():
953 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
954 or die "Can't open netscape history file: $!";
960 Returns a Boolean value telling whether EXPR has a value other than
961 the undefined value C<undef>. If EXPR is not present, C<$_> will be
964 Many operations return C<undef> to indicate failure, end of file,
965 system error, uninitialized variable, and other exceptional
966 conditions. This function allows you to distinguish C<undef> from
967 other values. (A simple Boolean test will not distinguish among
968 C<undef>, zero, the empty string, and C<"0">, which are all equally
969 false.) Note that since C<undef> is a valid scalar, its presence
970 doesn't I<necessarily> indicate an exceptional condition: C<pop>
971 returns C<undef> when its argument is an empty array, I<or> when the
972 element to return happens to be C<undef>.
974 You may also use C<defined(&func)> to check whether subroutine C<&func>
975 has ever been defined. The return value is unaffected by any forward
976 declarations of C<&func>. Note that a subroutine which is not defined
977 may still be callable: its package may have an C<AUTOLOAD> method that
978 makes it spring into existence the first time that it is called -- see
981 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
982 used to report whether memory for that aggregate has ever been
983 allocated. This behavior may disappear in future versions of Perl.
984 You should instead use a simple test for size:
986 if (@an_array) { print "has array elements\n" }
987 if (%a_hash) { print "has hash members\n" }
989 When used on a hash element, it tells you whether the value is defined,
990 not whether the key exists in the hash. Use L</exists> for the latter
995 print if defined $switch{'D'};
996 print "$val\n" while defined($val = pop(@ary));
997 die "Can't readlink $sym: $!"
998 unless defined($value = readlink $sym);
999 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1000 $debugging = 0 unless defined $debugging;
1002 Note: Many folks tend to overuse C<defined>, and then are surprised to
1003 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1004 defined values. For example, if you say
1008 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1009 matched "nothing". But it didn't really match nothing--rather, it
1010 matched something that happened to be zero characters long. This is all
1011 very above-board and honest. When a function returns an undefined value,
1012 it's an admission that it couldn't give you an honest answer. So you
1013 should use C<defined> only when you're questioning the integrity of what
1014 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1017 See also L</undef>, L</exists>, L</ref>.
1021 Given an expression that specifies a hash element, array element, hash slice,
1022 or array slice, deletes the specified element(s) from the hash or array.
1023 In the case of an array, if the array elements happen to be at the end,
1024 the size of the array will shrink to the highest element that tests
1025 true for exists() (or 0 if no such element exists).
1027 Returns each element so deleted or the undefined value if there was no such
1028 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1029 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1030 from a C<tie>d hash or array may not necessarily return anything.
1032 Deleting an array element effectively returns that position of the array
1033 to its initial, uninitialized state. Subsequently testing for the same
1034 element with exists() will return false. Note that deleting array
1035 elements in the middle of an array will not shift the index of the ones
1036 after them down--use splice() for that. See L</exists>.
1038 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1040 foreach $key (keys %HASH) {
1044 foreach $index (0 .. $#ARRAY) {
1045 delete $ARRAY[$index];
1050 delete @HASH{keys %HASH};
1052 delete @ARRAY[0 .. $#ARRAY];
1054 But both of these are slower than just assigning the empty list
1055 or undefining %HASH or @ARRAY:
1057 %HASH = (); # completely empty %HASH
1058 undef %HASH; # forget %HASH ever existed
1060 @ARRAY = (); # completely empty @ARRAY
1061 undef @ARRAY; # forget @ARRAY ever existed
1063 Note that the EXPR can be arbitrarily complicated as long as the final
1064 operation is a hash element, array element, hash slice, or array slice
1067 delete $ref->[$x][$y]{$key};
1068 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1070 delete $ref->[$x][$y][$index];
1071 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1075 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1076 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1077 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1078 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1079 an C<eval(),> the error message is stuffed into C<$@> and the
1080 C<eval> is terminated with the undefined value. This makes
1081 C<die> the way to raise an exception.
1083 Equivalent examples:
1085 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1086 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1088 If the last element of LIST does not end in a newline, the current
1089 script line number and input line number (if any) are also printed,
1090 and a newline is supplied. Note that the "input line number" (also
1091 known as "chunk") is subject to whatever notion of "line" happens to
1092 be currently in effect, and is also available as the special variable
1093 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1095 Hint: sometimes appending C<", stopped"> to your message will cause it
1096 to make better sense when the string C<"at foo line 123"> is appended.
1097 Suppose you are running script "canasta".
1099 die "/etc/games is no good";
1100 die "/etc/games is no good, stopped";
1102 produce, respectively
1104 /etc/games is no good at canasta line 123.
1105 /etc/games is no good, stopped at canasta line 123.
1107 See also exit(), warn(), and the Carp module.
1109 If LIST is empty and C<$@> already contains a value (typically from a
1110 previous eval) that value is reused after appending C<"\t...propagated">.
1111 This is useful for propagating exceptions:
1114 die unless $@ =~ /Expected exception/;
1116 If LIST is empty and C<$@> contains an object reference that has a
1117 C<PROPAGATE> method, that method will be called with additional file
1118 and line number parameters. The return value replaces the value in
1119 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1122 If C<$@> is empty then the string C<"Died"> is used.
1124 die() can also be called with a reference argument. If this happens to be
1125 trapped within an eval(), $@ contains the reference. This behavior permits
1126 a more elaborate exception handling implementation using objects that
1127 maintain arbitrary state about the nature of the exception. Such a scheme
1128 is sometimes preferable to matching particular string values of $@ using
1129 regular expressions. Here's an example:
1131 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1133 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1134 # handle Some::Module::Exception
1137 # handle all other possible exceptions
1141 Because perl will stringify uncaught exception messages before displaying
1142 them, you may want to overload stringification operations on such custom
1143 exception objects. See L<overload> for details about that.
1145 You can arrange for a callback to be run just before the C<die>
1146 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1147 handler will be called with the error text and can change the error
1148 message, if it sees fit, by calling C<die> again. See
1149 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1150 L<"eval BLOCK"> for some examples. Although this feature was meant
1151 to be run only right before your program was to exit, this is not
1152 currently the case--the C<$SIG{__DIE__}> hook is currently called
1153 even inside eval()ed blocks/strings! If one wants the hook to do
1154 nothing in such situations, put
1158 as the first line of the handler (see L<perlvar/$^S>). Because
1159 this promotes strange action at a distance, this counterintuitive
1160 behavior may be fixed in a future release.
1164 Not really a function. Returns the value of the last command in the
1165 sequence of commands indicated by BLOCK. When modified by a loop
1166 modifier, executes the BLOCK once before testing the loop condition.
1167 (On other statements the loop modifiers test the conditional first.)
1169 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1170 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1171 See L<perlsyn> for alternative strategies.
1173 =item do SUBROUTINE(LIST)
1175 A deprecated form of subroutine call. See L<perlsub>.
1179 Uses the value of EXPR as a filename and executes the contents of the
1180 file as a Perl script. Its primary use is to include subroutines
1181 from a Perl subroutine library.
1189 except that it's more efficient and concise, keeps track of the current
1190 filename for error messages, searches the @INC libraries, and updates
1191 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1192 variables. It also differs in that code evaluated with C<do FILENAME>
1193 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1194 same, however, in that it does reparse the file every time you call it,
1195 so you probably don't want to do this inside a loop.
1197 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1198 error. If C<do> can read the file but cannot compile it, it
1199 returns undef and sets an error message in C<$@>. If the file is
1200 successfully compiled, C<do> returns the value of the last expression
1203 Note that inclusion of library modules is better done with the
1204 C<use> and C<require> operators, which also do automatic error checking
1205 and raise an exception if there's a problem.
1207 You might like to use C<do> to read in a program configuration
1208 file. Manual error checking can be done this way:
1210 # read in config files: system first, then user
1211 for $file ("/share/prog/defaults.rc",
1212 "$ENV{HOME}/.someprogrc")
1214 unless ($return = do $file) {
1215 warn "couldn't parse $file: $@" if $@;
1216 warn "couldn't do $file: $!" unless defined $return;
1217 warn "couldn't run $file" unless $return;
1225 This function causes an immediate core dump. See also the B<-u>
1226 command-line switch in L<perlrun>, which does the same thing.
1227 Primarily this is so that you can use the B<undump> program (not
1228 supplied) to turn your core dump into an executable binary after
1229 having initialized all your variables at the beginning of the
1230 program. When the new binary is executed it will begin by executing
1231 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1232 Think of it as a goto with an intervening core dump and reincarnation.
1233 If C<LABEL> is omitted, restarts the program from the top.
1235 B<WARNING>: Any files opened at the time of the dump will I<not>
1236 be open any more when the program is reincarnated, with possible
1237 resulting confusion on the part of Perl.
1239 This function is now largely obsolete, partly because it's very
1240 hard to convert a core file into an executable, and because the
1241 real compiler backends for generating portable bytecode and compilable
1242 C code have superseded it. That's why you should now invoke it as
1243 C<CORE::dump()>, if you don't want to be warned against a possible
1246 If you're looking to use L<dump> to speed up your program, consider
1247 generating bytecode or native C code as described in L<perlcc>. If
1248 you're just trying to accelerate a CGI script, consider using the
1249 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1250 You might also consider autoloading or selfloading, which at least
1251 make your program I<appear> to run faster.
1255 When called in list context, returns a 2-element list consisting of the
1256 key and value for the next element of a hash, so that you can iterate over
1257 it. When called in scalar context, returns only the key for the next
1258 element in the hash.
1260 Entries are returned in an apparently random order. The actual random
1261 order is subject to change in future versions of perl, but it is guaranteed
1262 to be in the same order as either the C<keys> or C<values> function
1263 would produce on the same (unmodified) hash.
1265 When the hash is entirely read, a null array is returned in list context
1266 (which when assigned produces a false (C<0>) value), and C<undef> in
1267 scalar context. The next call to C<each> after that will start iterating
1268 again. There is a single iterator for each hash, shared by all C<each>,
1269 C<keys>, and C<values> function calls in the program; it can be reset by
1270 reading all the elements from the hash, or by evaluating C<keys HASH> or
1271 C<values HASH>. If you add or delete elements of a hash while you're
1272 iterating over it, you may get entries skipped or duplicated, so
1273 don't. Exception: It is always safe to delete the item most recently
1274 returned by C<each()>, which means that the following code will work:
1276 while (($key, $value) = each %hash) {
1278 delete $hash{$key}; # This is safe
1281 The following prints out your environment like the printenv(1) program,
1282 only in a different order:
1284 while (($key,$value) = each %ENV) {
1285 print "$key=$value\n";
1288 See also C<keys>, C<values> and C<sort>.
1290 =item eof FILEHANDLE
1296 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1297 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1298 gives the real filehandle. (Note that this function actually
1299 reads a character and then C<ungetc>s it, so isn't very useful in an
1300 interactive context.) Do not read from a terminal file (or call
1301 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1302 as terminals may lose the end-of-file condition if you do.
1304 An C<eof> without an argument uses the last file read. Using C<eof()>
1305 with empty parentheses is very different. It refers to the pseudo file
1306 formed from the files listed on the command line and accessed via the
1307 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1308 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1309 used will cause C<@ARGV> to be examined to determine if input is
1310 available. Similarly, an C<eof()> after C<< <> >> has returned
1311 end-of-file will assume you are processing another C<@ARGV> list,
1312 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1313 see L<perlop/"I/O Operators">.
1315 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1316 detect the end of each file, C<eof()> will only detect the end of the
1317 last file. Examples:
1319 # reset line numbering on each input file
1321 next if /^\s*#/; # skip comments
1324 close ARGV if eof; # Not eof()!
1327 # insert dashes just before last line of last file
1329 if (eof()) { # check for end of current file
1330 print "--------------\n";
1331 close(ARGV); # close or last; is needed if we
1332 # are reading from the terminal
1337 Practical hint: you almost never need to use C<eof> in Perl, because the
1338 input operators typically return C<undef> when they run out of data, or if
1345 In the first form, the return value of EXPR is parsed and executed as if it
1346 were a little Perl program. The value of the expression (which is itself
1347 determined within scalar context) is first parsed, and if there weren't any
1348 errors, executed in the lexical context of the current Perl program, so
1349 that any variable settings or subroutine and format definitions remain
1350 afterwards. Note that the value is parsed every time the eval executes.
1351 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1352 delay parsing and subsequent execution of the text of EXPR until run time.
1354 In the second form, the code within the BLOCK is parsed only once--at the
1355 same time the code surrounding the eval itself was parsed--and executed
1356 within the context of the current Perl program. This form is typically
1357 used to trap exceptions more efficiently than the first (see below), while
1358 also providing the benefit of checking the code within BLOCK at compile
1361 The final semicolon, if any, may be omitted from the value of EXPR or within
1364 In both forms, the value returned is the value of the last expression
1365 evaluated inside the mini-program; a return statement may be also used, just
1366 as with subroutines. The expression providing the return value is evaluated
1367 in void, scalar, or list context, depending on the context of the eval itself.
1368 See L</wantarray> for more on how the evaluation context can be determined.
1370 If there is a syntax error or runtime error, or a C<die> statement is
1371 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1372 error message. If there was no error, C<$@> is guaranteed to be a null
1373 string. Beware that using C<eval> neither silences perl from printing
1374 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1375 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1376 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1377 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1379 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1380 determining whether a particular feature (such as C<socket> or C<symlink>)
1381 is implemented. It is also Perl's exception trapping mechanism, where
1382 the die operator is used to raise exceptions.
1384 If the code to be executed doesn't vary, you may use the eval-BLOCK
1385 form to trap run-time errors without incurring the penalty of
1386 recompiling each time. The error, if any, is still returned in C<$@>.
1389 # make divide-by-zero nonfatal
1390 eval { $answer = $a / $b; }; warn $@ if $@;
1392 # same thing, but less efficient
1393 eval '$answer = $a / $b'; warn $@ if $@;
1395 # a compile-time error
1396 eval { $answer = }; # WRONG
1399 eval '$answer ='; # sets $@
1401 Due to the current arguably broken state of C<__DIE__> hooks, when using
1402 the C<eval{}> form as an exception trap in libraries, you may wish not
1403 to trigger any C<__DIE__> hooks that user code may have installed.
1404 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1405 as shown in this example:
1407 # a very private exception trap for divide-by-zero
1408 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1411 This is especially significant, given that C<__DIE__> hooks can call
1412 C<die> again, which has the effect of changing their error messages:
1414 # __DIE__ hooks may modify error messages
1416 local $SIG{'__DIE__'} =
1417 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1418 eval { die "foo lives here" };
1419 print $@ if $@; # prints "bar lives here"
1422 Because this promotes action at a distance, this counterintuitive behavior
1423 may be fixed in a future release.
1425 With an C<eval>, you should be especially careful to remember what's
1426 being looked at when:
1432 eval { $x }; # CASE 4
1434 eval "\$$x++"; # CASE 5
1437 Cases 1 and 2 above behave identically: they run the code contained in
1438 the variable $x. (Although case 2 has misleading double quotes making
1439 the reader wonder what else might be happening (nothing is).) Cases 3
1440 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1441 does nothing but return the value of $x. (Case 4 is preferred for
1442 purely visual reasons, but it also has the advantage of compiling at
1443 compile-time instead of at run-time.) Case 5 is a place where
1444 normally you I<would> like to use double quotes, except that in this
1445 particular situation, you can just use symbolic references instead, as
1448 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1449 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1451 Note that as a very special case, an C<eval ''> executed within the C<DB>
1452 package doesn't see the usual surrounding lexical scope, but rather the
1453 scope of the first non-DB piece of code that called it. You don't normally
1454 need to worry about this unless you are writing a Perl debugger.
1458 =item exec PROGRAM LIST
1460 The C<exec> function executes a system command I<and never returns>--
1461 use C<system> instead of C<exec> if you want it to return. It fails and
1462 returns false only if the command does not exist I<and> it is executed
1463 directly instead of via your system's command shell (see below).
1465 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1466 warns you if there is a following statement which isn't C<die>, C<warn>,
1467 or C<exit> (if C<-w> is set - but you always do that). If you
1468 I<really> want to follow an C<exec> with some other statement, you
1469 can use one of these styles to avoid the warning:
1471 exec ('foo') or print STDERR "couldn't exec foo: $!";
1472 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1474 If there is more than one argument in LIST, or if LIST is an array
1475 with more than one value, calls execvp(3) with the arguments in LIST.
1476 If there is only one scalar argument or an array with one element in it,
1477 the argument is checked for shell metacharacters, and if there are any,
1478 the entire argument is passed to the system's command shell for parsing
1479 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1480 If there are no shell metacharacters in the argument, it is split into
1481 words and passed directly to C<execvp>, which is more efficient.
1484 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1485 exec "sort $outfile | uniq";
1487 If you don't really want to execute the first argument, but want to lie
1488 to the program you are executing about its own name, you can specify
1489 the program you actually want to run as an "indirect object" (without a
1490 comma) in front of the LIST. (This always forces interpretation of the
1491 LIST as a multivalued list, even if there is only a single scalar in
1494 $shell = '/bin/csh';
1495 exec $shell '-sh'; # pretend it's a login shell
1499 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1501 When the arguments get executed via the system shell, results will
1502 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1505 Using an indirect object with C<exec> or C<system> is also more
1506 secure. This usage (which also works fine with system()) forces
1507 interpretation of the arguments as a multivalued list, even if the
1508 list had just one argument. That way you're safe from the shell
1509 expanding wildcards or splitting up words with whitespace in them.
1511 @args = ( "echo surprise" );
1513 exec @args; # subject to shell escapes
1515 exec { $args[0] } @args; # safe even with one-arg list
1517 The first version, the one without the indirect object, ran the I<echo>
1518 program, passing it C<"surprise"> an argument. The second version
1519 didn't--it tried to run a program literally called I<"echo surprise">,
1520 didn't find it, and set C<$?> to a non-zero value indicating failure.
1522 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1523 output before the exec, but this may not be supported on some platforms
1524 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1525 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1526 open handles in order to avoid lost output.
1528 Note that C<exec> will not call your C<END> blocks, nor will it call
1529 any C<DESTROY> methods in your objects.
1533 Given an expression that specifies a hash element or array element,
1534 returns true if the specified element in the hash or array has ever
1535 been initialized, even if the corresponding value is undefined. The
1536 element is not autovivified if it doesn't exist.
1538 print "Exists\n" if exists $hash{$key};
1539 print "Defined\n" if defined $hash{$key};
1540 print "True\n" if $hash{$key};
1542 print "Exists\n" if exists $array[$index];
1543 print "Defined\n" if defined $array[$index];
1544 print "True\n" if $array[$index];
1546 A hash or array element can be true only if it's defined, and defined if
1547 it exists, but the reverse doesn't necessarily hold true.
1549 Given an expression that specifies the name of a subroutine,
1550 returns true if the specified subroutine has ever been declared, even
1551 if it is undefined. Mentioning a subroutine name for exists or defined
1552 does not count as declaring it. Note that a subroutine which does not
1553 exist may still be callable: its package may have an C<AUTOLOAD>
1554 method that makes it spring into existence the first time that it is
1555 called -- see L<perlsub>.
1557 print "Exists\n" if exists &subroutine;
1558 print "Defined\n" if defined &subroutine;
1560 Note that the EXPR can be arbitrarily complicated as long as the final
1561 operation is a hash or array key lookup or subroutine name:
1563 if (exists $ref->{A}->{B}->{$key}) { }
1564 if (exists $hash{A}{B}{$key}) { }
1566 if (exists $ref->{A}->{B}->[$ix]) { }
1567 if (exists $hash{A}{B}[$ix]) { }
1569 if (exists &{$ref->{A}{B}{$key}}) { }
1571 Although the deepest nested array or hash will not spring into existence
1572 just because its existence was tested, any intervening ones will.
1573 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1574 into existence due to the existence test for the $key element above.
1575 This happens anywhere the arrow operator is used, including even:
1578 if (exists $ref->{"Some key"}) { }
1579 print $ref; # prints HASH(0x80d3d5c)
1581 This surprising autovivification in what does not at first--or even
1582 second--glance appear to be an lvalue context may be fixed in a future
1585 Use of a subroutine call, rather than a subroutine name, as an argument
1586 to exists() is an error.
1589 exists &sub(); # Error
1593 Evaluates EXPR and exits immediately with that value. Example:
1596 exit 0 if $ans =~ /^[Xx]/;
1598 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1599 universally recognized values for EXPR are C<0> for success and C<1>
1600 for error; other values are subject to interpretation depending on the
1601 environment in which the Perl program is running. For example, exiting
1602 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1603 the mailer to return the item undelivered, but that's not true everywhere.
1605 Don't use C<exit> to abort a subroutine if there's any chance that
1606 someone might want to trap whatever error happened. Use C<die> instead,
1607 which can be trapped by an C<eval>.
1609 The exit() function does not always exit immediately. It calls any
1610 defined C<END> routines first, but these C<END> routines may not
1611 themselves abort the exit. Likewise any object destructors that need to
1612 be called are called before the real exit. If this is a problem, you
1613 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1614 See L<perlmod> for details.
1620 Returns I<e> (the natural logarithm base) to the power of EXPR.
1621 If EXPR is omitted, gives C<exp($_)>.
1623 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1625 Implements the fcntl(2) function. You'll probably have to say
1629 first to get the correct constant definitions. Argument processing and
1630 value return works just like C<ioctl> below.
1634 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1635 or die "can't fcntl F_GETFL: $!";
1637 You don't have to check for C<defined> on the return from C<fnctl>.
1638 Like C<ioctl>, it maps a C<0> return from the system call into
1639 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1640 in numeric context. It is also exempt from the normal B<-w> warnings
1641 on improper numeric conversions.
1643 Note that C<fcntl> will produce a fatal error if used on a machine that
1644 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1645 manpage to learn what functions are available on your system.
1647 =item fileno FILEHANDLE
1649 Returns the file descriptor for a filehandle, or undefined if the
1650 filehandle is not open. This is mainly useful for constructing
1651 bitmaps for C<select> and low-level POSIX tty-handling operations.
1652 If FILEHANDLE is an expression, the value is taken as an indirect
1653 filehandle, generally its name.
1655 You can use this to find out whether two handles refer to the
1656 same underlying descriptor:
1658 if (fileno(THIS) == fileno(THAT)) {
1659 print "THIS and THAT are dups\n";
1662 (Filehandles connected to memory objects via new features of C<open> may
1663 return undefined even though they are open.)
1666 =item flock FILEHANDLE,OPERATION
1668 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1669 for success, false on failure. Produces a fatal error if used on a
1670 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1671 C<flock> is Perl's portable file locking interface, although it locks
1672 only entire files, not records.
1674 Two potentially non-obvious but traditional C<flock> semantics are
1675 that it waits indefinitely until the lock is granted, and that its locks
1676 B<merely advisory>. Such discretionary locks are more flexible, but offer
1677 fewer guarantees. This means that files locked with C<flock> may be
1678 modified by programs that do not also use C<flock>. See L<perlport>,
1679 your port's specific documentation, or your system-specific local manpages
1680 for details. It's best to assume traditional behavior if you're writing
1681 portable programs. (But if you're not, you should as always feel perfectly
1682 free to write for your own system's idiosyncrasies (sometimes called
1683 "features"). Slavish adherence to portability concerns shouldn't get
1684 in the way of your getting your job done.)
1686 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1687 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1688 you can use the symbolic names if you import them from the Fcntl module,
1689 either individually, or as a group using the ':flock' tag. LOCK_SH
1690 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1691 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1692 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1693 waiting for the lock (check the return status to see if you got it).
1695 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1696 before locking or unlocking it.
1698 Note that the emulation built with lockf(3) doesn't provide shared
1699 locks, and it requires that FILEHANDLE be open with write intent. These
1700 are the semantics that lockf(3) implements. Most if not all systems
1701 implement lockf(3) in terms of fcntl(2) locking, though, so the
1702 differing semantics shouldn't bite too many people.
1704 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1705 be open with read intent to use LOCK_SH and requires that it be open
1706 with write intent to use LOCK_EX.
1708 Note also that some versions of C<flock> cannot lock things over the
1709 network; you would need to use the more system-specific C<fcntl> for
1710 that. If you like you can force Perl to ignore your system's flock(2)
1711 function, and so provide its own fcntl(2)-based emulation, by passing
1712 the switch C<-Ud_flock> to the F<Configure> program when you configure
1715 Here's a mailbox appender for BSD systems.
1717 use Fcntl ':flock'; # import LOCK_* constants
1720 flock(MBOX,LOCK_EX);
1721 # and, in case someone appended
1722 # while we were waiting...
1727 flock(MBOX,LOCK_UN);
1730 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1731 or die "Can't open mailbox: $!";
1734 print MBOX $msg,"\n\n";
1737 On systems that support a real flock(), locks are inherited across fork()
1738 calls, whereas those that must resort to the more capricious fcntl()
1739 function lose the locks, making it harder to write servers.
1741 See also L<DB_File> for other flock() examples.
1745 Does a fork(2) system call to create a new process running the
1746 same program at the same point. It returns the child pid to the
1747 parent process, C<0> to the child process, or C<undef> if the fork is
1748 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1749 are shared, while everything else is copied. On most systems supporting
1750 fork(), great care has gone into making it extremely efficient (for
1751 example, using copy-on-write technology on data pages), making it the
1752 dominant paradigm for multitasking over the last few decades.
1754 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1755 output before forking the child process, but this may not be supported
1756 on some platforms (see L<perlport>). To be safe, you may need to set
1757 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1758 C<IO::Handle> on any open handles in order to avoid duplicate output.
1760 If you C<fork> without ever waiting on your children, you will
1761 accumulate zombies. On some systems, you can avoid this by setting
1762 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1763 forking and reaping moribund children.
1765 Note that if your forked child inherits system file descriptors like
1766 STDIN and STDOUT that are actually connected by a pipe or socket, even
1767 if you exit, then the remote server (such as, say, a CGI script or a
1768 backgrounded job launched from a remote shell) won't think you're done.
1769 You should reopen those to F</dev/null> if it's any issue.
1773 Declare a picture format for use by the C<write> function. For
1777 Test: @<<<<<<<< @||||| @>>>>>
1778 $str, $%, '$' . int($num)
1782 $num = $cost/$quantity;
1786 See L<perlform> for many details and examples.
1788 =item formline PICTURE,LIST
1790 This is an internal function used by C<format>s, though you may call it,
1791 too. It formats (see L<perlform>) a list of values according to the
1792 contents of PICTURE, placing the output into the format output
1793 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1794 Eventually, when a C<write> is done, the contents of
1795 C<$^A> are written to some filehandle, but you could also read C<$^A>
1796 yourself and then set C<$^A> back to C<"">. Note that a format typically
1797 does one C<formline> per line of form, but the C<formline> function itself
1798 doesn't care how many newlines are embedded in the PICTURE. This means
1799 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1800 You may therefore need to use multiple formlines to implement a single
1801 record format, just like the format compiler.
1803 Be careful if you put double quotes around the picture, because an C<@>
1804 character may be taken to mean the beginning of an array name.
1805 C<formline> always returns true. See L<perlform> for other examples.
1807 =item getc FILEHANDLE
1811 Returns the next character from the input file attached to FILEHANDLE,
1812 or the undefined value at end of file, or if there was an error (in
1813 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1814 STDIN. This is not particularly efficient. However, it cannot be
1815 used by itself to fetch single characters without waiting for the user
1816 to hit enter. For that, try something more like:
1819 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1822 system "stty", '-icanon', 'eol', "\001";
1828 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1831 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1835 Determination of whether $BSD_STYLE should be set
1836 is left as an exercise to the reader.
1838 The C<POSIX::getattr> function can do this more portably on
1839 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1840 module from your nearest CPAN site; details on CPAN can be found on
1845 Implements the C library function of the same name, which on most
1846 systems returns the current login from F</etc/utmp>, if any. If null,
1849 $login = getlogin || getpwuid($<) || "Kilroy";
1851 Do not consider C<getlogin> for authentication: it is not as
1852 secure as C<getpwuid>.
1854 =item getpeername SOCKET
1856 Returns the packed sockaddr address of other end of the SOCKET connection.
1859 $hersockaddr = getpeername(SOCK);
1860 ($port, $iaddr) = sockaddr_in($hersockaddr);
1861 $herhostname = gethostbyaddr($iaddr, AF_INET);
1862 $herstraddr = inet_ntoa($iaddr);
1866 Returns the current process group for the specified PID. Use
1867 a PID of C<0> to get the current process group for the
1868 current process. Will raise an exception if used on a machine that
1869 doesn't implement getpgrp(2). If PID is omitted, returns process
1870 group of current process. Note that the POSIX version of C<getpgrp>
1871 does not accept a PID argument, so only C<PID==0> is truly portable.
1875 Returns the process id of the parent process.
1877 Note for Linux users: on Linux, the C functions C<getpid()> and
1878 C<getppid()> return different values from different threads. In order to
1879 be portable, this behavior is not reflected by the perl-level function
1880 C<getppid()>, that returns a consistent value across threads. If you want
1881 to call the underlying C<getppid()>, you may use the CPAN module
1884 =item getpriority WHICH,WHO
1886 Returns the current priority for a process, a process group, or a user.
1887 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1888 machine that doesn't implement getpriority(2).
1894 =item gethostbyname NAME
1896 =item getnetbyname NAME
1898 =item getprotobyname NAME
1904 =item getservbyname NAME,PROTO
1906 =item gethostbyaddr ADDR,ADDRTYPE
1908 =item getnetbyaddr ADDR,ADDRTYPE
1910 =item getprotobynumber NUMBER
1912 =item getservbyport PORT,PROTO
1930 =item sethostent STAYOPEN
1932 =item setnetent STAYOPEN
1934 =item setprotoent STAYOPEN
1936 =item setservent STAYOPEN
1950 These routines perform the same functions as their counterparts in the
1951 system library. In list context, the return values from the
1952 various get routines are as follows:
1954 ($name,$passwd,$uid,$gid,
1955 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1956 ($name,$passwd,$gid,$members) = getgr*
1957 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1958 ($name,$aliases,$addrtype,$net) = getnet*
1959 ($name,$aliases,$proto) = getproto*
1960 ($name,$aliases,$port,$proto) = getserv*
1962 (If the entry doesn't exist you get a null list.)
1964 The exact meaning of the $gcos field varies but it usually contains
1965 the real name of the user (as opposed to the login name) and other
1966 information pertaining to the user. Beware, however, that in many
1967 system users are able to change this information and therefore it
1968 cannot be trusted and therefore the $gcos is tainted (see
1969 L<perlsec>). The $passwd and $shell, user's encrypted password and
1970 login shell, are also tainted, because of the same reason.
1972 In scalar context, you get the name, unless the function was a
1973 lookup by name, in which case you get the other thing, whatever it is.
1974 (If the entry doesn't exist you get the undefined value.) For example:
1976 $uid = getpwnam($name);
1977 $name = getpwuid($num);
1979 $gid = getgrnam($name);
1980 $name = getgrgid($num);
1984 In I<getpw*()> the fields $quota, $comment, and $expire are special
1985 cases in the sense that in many systems they are unsupported. If the
1986 $quota is unsupported, it is an empty scalar. If it is supported, it
1987 usually encodes the disk quota. If the $comment field is unsupported,
1988 it is an empty scalar. If it is supported it usually encodes some
1989 administrative comment about the user. In some systems the $quota
1990 field may be $change or $age, fields that have to do with password
1991 aging. In some systems the $comment field may be $class. The $expire
1992 field, if present, encodes the expiration period of the account or the
1993 password. For the availability and the exact meaning of these fields
1994 in your system, please consult your getpwnam(3) documentation and your
1995 F<pwd.h> file. You can also find out from within Perl what your
1996 $quota and $comment fields mean and whether you have the $expire field
1997 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1998 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1999 files are only supported if your vendor has implemented them in the
2000 intuitive fashion that calling the regular C library routines gets the
2001 shadow versions if you're running under privilege or if there exists
2002 the shadow(3) functions as found in System V ( this includes Solaris
2003 and Linux.) Those systems which implement a proprietary shadow password
2004 facility are unlikely to be supported.
2006 The $members value returned by I<getgr*()> is a space separated list of
2007 the login names of the members of the group.
2009 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2010 C, it will be returned to you via C<$?> if the function call fails. The
2011 C<@addrs> value returned by a successful call is a list of the raw
2012 addresses returned by the corresponding system library call. In the
2013 Internet domain, each address is four bytes long and you can unpack it
2014 by saying something like:
2016 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2018 The Socket library makes this slightly easier:
2021 $iaddr = inet_aton("127.1"); # or whatever address
2022 $name = gethostbyaddr($iaddr, AF_INET);
2024 # or going the other way
2025 $straddr = inet_ntoa($iaddr);
2027 If you get tired of remembering which element of the return list
2028 contains which return value, by-name interfaces are provided
2029 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2030 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2031 and C<User::grent>. These override the normal built-ins, supplying
2032 versions that return objects with the appropriate names
2033 for each field. For example:
2037 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2039 Even though it looks like they're the same method calls (uid),
2040 they aren't, because a C<File::stat> object is different from
2041 a C<User::pwent> object.
2043 =item getsockname SOCKET
2045 Returns the packed sockaddr address of this end of the SOCKET connection,
2046 in case you don't know the address because you have several different
2047 IPs that the connection might have come in on.
2050 $mysockaddr = getsockname(SOCK);
2051 ($port, $myaddr) = sockaddr_in($mysockaddr);
2052 printf "Connect to %s [%s]\n",
2053 scalar gethostbyaddr($myaddr, AF_INET),
2056 =item getsockopt SOCKET,LEVEL,OPTNAME
2058 Returns the socket option requested, or undef if there is an error.
2064 In list context, returns a (possibly empty) list of filename expansions on
2065 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2066 scalar context, glob iterates through such filename expansions, returning
2067 undef when the list is exhausted. This is the internal function
2068 implementing the C<< <*.c> >> operator, but you can use it directly. If
2069 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2070 more detail in L<perlop/"I/O Operators">.
2072 Beginning with v5.6.0, this operator is implemented using the standard
2073 C<File::Glob> extension. See L<File::Glob> for details.
2077 Converts a time as returned by the time function to an 8-element list
2078 with the time localized for the standard Greenwich time zone.
2079 Typically used as follows:
2082 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2085 All list elements are numeric, and come straight out of the C `struct
2086 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2087 specified time. $mday is the day of the month, and $mon is the month
2088 itself, in the range C<0..11> with 0 indicating January and 11
2089 indicating December. $year is the number of years since 1900. That
2090 is, $year is C<123> in year 2023. $wday is the day of the week, with
2091 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2092 the year, in the range C<0..364> (or C<0..365> in leap years.)
2094 Note that the $year element is I<not> simply the last two digits of
2095 the year. If you assume it is, then you create non-Y2K-compliant
2096 programs--and you wouldn't want to do that, would you?
2098 The proper way to get a complete 4-digit year is simply:
2102 And to get the last two digits of the year (e.g., '01' in 2001) do:
2104 $year = sprintf("%02d", $year % 100);
2106 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2108 In scalar context, C<gmtime()> returns the ctime(3) value:
2110 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2112 Also see the C<timegm> function provided by the C<Time::Local> module,
2113 and the strftime(3) function available via the POSIX module.
2115 This scalar value is B<not> locale dependent (see L<perllocale>), but
2116 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2117 strftime(3) and mktime(3) functions available via the POSIX module. To
2118 get somewhat similar but locale dependent date strings, set up your
2119 locale environment variables appropriately (please see L<perllocale>)
2120 and try for example:
2122 use POSIX qw(strftime);
2123 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2125 Note that the C<%a> and C<%b> escapes, which represent the short forms
2126 of the day of the week and the month of the year, may not necessarily
2127 be three characters wide in all locales.
2135 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2136 execution there. It may not be used to go into any construct that
2137 requires initialization, such as a subroutine or a C<foreach> loop. It
2138 also can't be used to go into a construct that is optimized away,
2139 or to get out of a block or subroutine given to C<sort>.
2140 It can be used to go almost anywhere else within the dynamic scope,
2141 including out of subroutines, but it's usually better to use some other
2142 construct such as C<last> or C<die>. The author of Perl has never felt the
2143 need to use this form of C<goto> (in Perl, that is--C is another matter).
2144 (The difference being that C does not offer named loops combined with
2145 loop control. Perl does, and this replaces most structured uses of C<goto>
2146 in other languages.)
2148 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2149 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2150 necessarily recommended if you're optimizing for maintainability:
2152 goto ("FOO", "BAR", "GLARCH")[$i];
2154 The C<goto-&NAME> form is quite different from the other forms of
2155 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2156 doesn't have the stigma associated with other gotos. Instead, it
2157 exits the current subroutine (losing any changes set by local()) and
2158 immediately calls in its place the named subroutine using the current
2159 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2160 load another subroutine and then pretend that the other subroutine had
2161 been called in the first place (except that any modifications to C<@_>
2162 in the current subroutine are propagated to the other subroutine.)
2163 After the C<goto>, not even C<caller> will be able to tell that this
2164 routine was called first.
2166 NAME needn't be the name of a subroutine; it can be a scalar variable
2167 containing a code reference, or a block which evaluates to a code
2170 =item grep BLOCK LIST
2172 =item grep EXPR,LIST
2174 This is similar in spirit to, but not the same as, grep(1) and its
2175 relatives. In particular, it is not limited to using regular expressions.
2177 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2178 C<$_> to each element) and returns the list value consisting of those
2179 elements for which the expression evaluated to true. In scalar
2180 context, returns the number of times the expression was true.
2182 @foo = grep(!/^#/, @bar); # weed out comments
2186 @foo = grep {!/^#/} @bar; # weed out comments
2188 Note that C<$_> is an alias to the list value, so it can be used to
2189 modify the elements of the LIST. While this is useful and supported,
2190 it can cause bizarre results if the elements of LIST are not variables.
2191 Similarly, grep returns aliases into the original list, much as a for
2192 loop's index variable aliases the list elements. That is, modifying an
2193 element of a list returned by grep (for example, in a C<foreach>, C<map>
2194 or another C<grep>) actually modifies the element in the original list.
2195 This is usually something to be avoided when writing clear code.
2197 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2203 Interprets EXPR as a hex string and returns the corresponding value.
2204 (To convert strings that might start with either 0, 0x, or 0b, see
2205 L</oct>.) If EXPR is omitted, uses C<$_>.
2207 print hex '0xAf'; # prints '175'
2208 print hex 'aF'; # same
2210 Hex strings may only represent integers. Strings that would cause
2211 integer overflow trigger a warning. Leading whitespace is not stripped,
2216 There is no builtin C<import> function. It is just an ordinary
2217 method (subroutine) defined (or inherited) by modules that wish to export
2218 names to another module. The C<use> function calls the C<import> method
2219 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2221 =item index STR,SUBSTR,POSITION
2223 =item index STR,SUBSTR
2225 The index function searches for one string within another, but without
2226 the wildcard-like behavior of a full regular-expression pattern match.
2227 It returns the position of the first occurrence of SUBSTR in STR at
2228 or after POSITION. If POSITION is omitted, starts searching from the
2229 beginning of the string. The return value is based at C<0> (or whatever
2230 you've set the C<$[> variable to--but don't do that). If the substring
2231 is not found, returns one less than the base, ordinarily C<-1>.
2237 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2238 You should not use this function for rounding: one because it truncates
2239 towards C<0>, and two because machine representations of floating point
2240 numbers can sometimes produce counterintuitive results. For example,
2241 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2242 because it's really more like -268.99999999999994315658 instead. Usually,
2243 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2244 functions will serve you better than will int().
2246 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2248 Implements the ioctl(2) function. You'll probably first have to say
2250 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2252 to get the correct function definitions. If F<ioctl.ph> doesn't
2253 exist or doesn't have the correct definitions you'll have to roll your
2254 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2255 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2256 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2257 written depending on the FUNCTION--a pointer to the string value of SCALAR
2258 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2259 has no string value but does have a numeric value, that value will be
2260 passed rather than a pointer to the string value. To guarantee this to be
2261 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2262 functions may be needed to manipulate the values of structures used by
2265 The return value of C<ioctl> (and C<fcntl>) is as follows:
2267 if OS returns: then Perl returns:
2269 0 string "0 but true"
2270 anything else that number
2272 Thus Perl returns true on success and false on failure, yet you can
2273 still easily determine the actual value returned by the operating
2276 $retval = ioctl(...) || -1;
2277 printf "System returned %d\n", $retval;
2279 The special string "C<0> but true" is exempt from B<-w> complaints
2280 about improper numeric conversions.
2282 Here's an example of setting a filehandle named C<REMOTE> to be
2283 non-blocking at the system level. You'll have to negotiate C<$|>
2284 on your own, though.
2286 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2288 $flags = fcntl(REMOTE, F_GETFL, 0)
2289 or die "Can't get flags for the socket: $!\n";
2291 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2292 or die "Can't set flags for the socket: $!\n";
2294 =item join EXPR,LIST
2296 Joins the separate strings of LIST into a single string with fields
2297 separated by the value of EXPR, and returns that new string. Example:
2299 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2301 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2302 first argument. Compare L</split>.
2306 Returns a list consisting of all the keys of the named hash. (In
2307 scalar context, returns the number of keys.) The keys are returned in
2308 an apparently random order. The actual random order is subject to
2309 change in future versions of perl, but it is guaranteed to be the same
2310 order as either the C<values> or C<each> function produces (given
2311 that the hash has not been modified). As a side effect, it resets
2314 Here is yet another way to print your environment:
2317 @values = values %ENV;
2319 print pop(@keys), '=', pop(@values), "\n";
2322 or how about sorted by key:
2324 foreach $key (sort(keys %ENV)) {
2325 print $key, '=', $ENV{$key}, "\n";
2328 The returned values are copies of the original keys in the hash, so
2329 modifying them will not affect the original hash. Compare L</values>.
2331 To sort a hash by value, you'll need to use a C<sort> function.
2332 Here's a descending numeric sort of a hash by its values:
2334 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2335 printf "%4d %s\n", $hash{$key}, $key;
2338 As an lvalue C<keys> allows you to increase the number of hash buckets
2339 allocated for the given hash. This can gain you a measure of efficiency if
2340 you know the hash is going to get big. (This is similar to pre-extending
2341 an array by assigning a larger number to $#array.) If you say
2345 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2346 in fact, since it rounds up to the next power of two. These
2347 buckets will be retained even if you do C<%hash = ()>, use C<undef
2348 %hash> if you want to free the storage while C<%hash> is still in scope.
2349 You can't shrink the number of buckets allocated for the hash using
2350 C<keys> in this way (but you needn't worry about doing this by accident,
2351 as trying has no effect).
2353 See also C<each>, C<values> and C<sort>.
2355 =item kill SIGNAL, LIST
2357 Sends a signal to a list of processes. Returns the number of
2358 processes successfully signaled (which is not necessarily the
2359 same as the number actually killed).
2361 $cnt = kill 1, $child1, $child2;
2364 If SIGNAL is zero, no signal is sent to the process. This is a
2365 useful way to check that the process is alive and hasn't changed
2366 its UID. See L<perlport> for notes on the portability of this
2369 Unlike in the shell, if SIGNAL is negative, it kills
2370 process groups instead of processes. (On System V, a negative I<PROCESS>
2371 number will also kill process groups, but that's not portable.) That
2372 means you usually want to use positive not negative signals. You may also
2373 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2379 The C<last> command is like the C<break> statement in C (as used in
2380 loops); it immediately exits the loop in question. If the LABEL is
2381 omitted, the command refers to the innermost enclosing loop. The
2382 C<continue> block, if any, is not executed:
2384 LINE: while (<STDIN>) {
2385 last LINE if /^$/; # exit when done with header
2389 C<last> cannot be used to exit a block which returns a value such as
2390 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2391 a grep() or map() operation.
2393 Note that a block by itself is semantically identical to a loop
2394 that executes once. Thus C<last> can be used to effect an early
2395 exit out of such a block.
2397 See also L</continue> for an illustration of how C<last>, C<next>, and
2404 Returns a lowercased version of EXPR. This is the internal function
2405 implementing the C<\L> escape in double-quoted strings. Respects
2406 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2407 and L<perlunicode> for more details about locale and Unicode support.
2409 If EXPR is omitted, uses C<$_>.
2415 Returns the value of EXPR with the first character lowercased. This
2416 is the internal function implementing the C<\l> escape in
2417 double-quoted strings. Respects current LC_CTYPE locale if C<use
2418 locale> in force. See L<perllocale> and L<perlunicode> for more
2419 details about locale and Unicode support.
2421 If EXPR is omitted, uses C<$_>.
2427 Returns the length in characters of the value of EXPR. If EXPR is
2428 omitted, returns length of C<$_>. Note that this cannot be used on
2429 an entire array or hash to find out how many elements these have.
2430 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2432 =item link OLDFILE,NEWFILE
2434 Creates a new filename linked to the old filename. Returns true for
2435 success, false otherwise.
2437 =item listen SOCKET,QUEUESIZE
2439 Does the same thing that the listen system call does. Returns true if
2440 it succeeded, false otherwise. See the example in
2441 L<perlipc/"Sockets: Client/Server Communication">.
2445 You really probably want to be using C<my> instead, because C<local> isn't
2446 what most people think of as "local". See
2447 L<perlsub/"Private Variables via my()"> for details.
2449 A local modifies the listed variables to be local to the enclosing
2450 block, file, or eval. If more than one value is listed, the list must
2451 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2452 for details, including issues with tied arrays and hashes.
2454 =item localtime EXPR
2456 Converts a time as returned by the time function to a 9-element list
2457 with the time analyzed for the local time zone. Typically used as
2461 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2464 All list elements are numeric, and come straight out of the C `struct
2465 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2466 specified time. $mday is the day of the month, and $mon is the month
2467 itself, in the range C<0..11> with 0 indicating January and 11
2468 indicating December. $year is the number of years since 1900. That
2469 is, $year is C<123> in year 2023. $wday is the day of the week, with
2470 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2471 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2472 is true if the specified time occurs during daylight savings time,
2475 Note that the $year element is I<not> simply the last two digits of
2476 the year. If you assume it is, then you create non-Y2K-compliant
2477 programs--and you wouldn't want to do that, would you?
2479 The proper way to get a complete 4-digit year is simply:
2483 And to get the last two digits of the year (e.g., '01' in 2001) do:
2485 $year = sprintf("%02d", $year % 100);
2487 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2489 In scalar context, C<localtime()> returns the ctime(3) value:
2491 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2493 This scalar value is B<not> locale dependent, see L<perllocale>, but
2494 instead a Perl builtin. Also see the C<Time::Local> module
2495 (to convert the second, minutes, hours, ... back to seconds since the
2496 stroke of midnight the 1st of January 1970, the value returned by
2497 time()), and the strftime(3) and mktime(3) functions available via the
2498 POSIX module. To get somewhat similar but locale dependent date
2499 strings, set up your locale environment variables appropriately
2500 (please see L<perllocale>) and try for example:
2502 use POSIX qw(strftime);
2503 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2505 Note that the C<%a> and C<%b>, the short forms of the day of the week
2506 and the month of the year, may not necessarily be three characters wide.
2510 This function places an advisory lock on a shared variable, or referenced
2511 object contained in I<THING> until the lock goes out of scope.
2513 lock() is a "weak keyword" : this means that if you've defined a function
2514 by this name (before any calls to it), that function will be called
2515 instead. (However, if you've said C<use threads>, lock() is always a
2516 keyword.) See L<threads>.
2522 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2523 returns log of C<$_>. To get the log of another base, use basic algebra:
2524 The base-N log of a number is equal to the natural log of that number
2525 divided by the natural log of N. For example:
2529 return log($n)/log(10);
2532 See also L</exp> for the inverse operation.
2538 Does the same thing as the C<stat> function (including setting the
2539 special C<_> filehandle) but stats a symbolic link instead of the file
2540 the symbolic link points to. If symbolic links are unimplemented on
2541 your system, a normal C<stat> is done. For much more detailed
2542 information, please see the documentation for C<stat>.
2544 If EXPR is omitted, stats C<$_>.
2548 The match operator. See L<perlop>.
2550 =item map BLOCK LIST
2554 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2555 C<$_> to each element) and returns the list value composed of the
2556 results of each such evaluation. In scalar context, returns the
2557 total number of elements so generated. Evaluates BLOCK or EXPR in
2558 list context, so each element of LIST may produce zero, one, or
2559 more elements in the returned value.
2561 @chars = map(chr, @nums);
2563 translates a list of numbers to the corresponding characters. And
2565 %hash = map { getkey($_) => $_ } @array;
2567 is just a funny way to write
2570 foreach $_ (@array) {
2571 $hash{getkey($_)} = $_;
2574 Note that C<$_> is an alias to the list value, so it can be used to
2575 modify the elements of the LIST. While this is useful and supported,
2576 it can cause bizarre results if the elements of LIST are not variables.
2577 Using a regular C<foreach> loop for this purpose would be clearer in
2578 most cases. See also L</grep> for an array composed of those items of
2579 the original list for which the BLOCK or EXPR evaluates to true.
2581 C<{> starts both hash references and blocks, so C<map { ...> could be either
2582 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2583 ahead for the closing C<}> it has to take a guess at which its dealing with
2584 based what it finds just after the C<{>. Usually it gets it right, but if it
2585 doesn't it won't realize something is wrong until it gets to the C<}> and
2586 encounters the missing (or unexpected) comma. The syntax error will be
2587 reported close to the C<}> but you'll need to change something near the C<{>
2588 such as using a unary C<+> to give perl some help:
2590 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2591 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2592 %hash = map { ("\L$_", 1) } @array # this also works
2593 %hash = map { lc($_), 1 } @array # as does this.
2594 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2596 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2598 or to force an anon hash constructor use C<+{>
2600 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2602 and you get list of anonymous hashes each with only 1 entry.
2604 =item mkdir FILENAME,MASK
2606 =item mkdir FILENAME
2608 Creates the directory specified by FILENAME, with permissions
2609 specified by MASK (as modified by C<umask>). If it succeeds it
2610 returns true, otherwise it returns false and sets C<$!> (errno).
2611 If omitted, MASK defaults to 0777.
2613 In general, it is better to create directories with permissive MASK,
2614 and let the user modify that with their C<umask>, than it is to supply
2615 a restrictive MASK and give the user no way to be more permissive.
2616 The exceptions to this rule are when the file or directory should be
2617 kept private (mail files, for instance). The perlfunc(1) entry on
2618 C<umask> discusses the choice of MASK in more detail.
2620 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2621 number of trailing slashes. Some operating and filesystems do not get
2622 this right, so Perl automatically removes all trailing slashes to keep
2625 =item msgctl ID,CMD,ARG
2627 Calls the System V IPC function msgctl(2). You'll probably have to say
2631 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2632 then ARG must be a variable which will hold the returned C<msqid_ds>
2633 structure. Returns like C<ioctl>: the undefined value for error,
2634 C<"0 but true"> for zero, or the actual return value otherwise. See also
2635 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2637 =item msgget KEY,FLAGS
2639 Calls the System V IPC function msgget(2). Returns the message queue
2640 id, or the undefined value if there is an error. See also
2641 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2643 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2645 Calls the System V IPC function msgrcv to receive a message from
2646 message queue ID into variable VAR with a maximum message size of
2647 SIZE. Note that when a message is received, the message type as a
2648 native long integer will be the first thing in VAR, followed by the
2649 actual message. This packing may be opened with C<unpack("l! a*")>.
2650 Taints the variable. Returns true if successful, or false if there is
2651 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2652 C<IPC::SysV::Msg> documentation.
2654 =item msgsnd ID,MSG,FLAGS
2656 Calls the System V IPC function msgsnd to send the message MSG to the
2657 message queue ID. MSG must begin with the native long integer message
2658 type, and be followed by the length of the actual message, and finally
2659 the message itself. This kind of packing can be achieved with
2660 C<pack("l! a*", $type, $message)>. Returns true if successful,
2661 or false if there is an error. See also C<IPC::SysV>
2662 and C<IPC::SysV::Msg> documentation.
2668 =item my EXPR : ATTRS
2670 =item my TYPE EXPR : ATTRS
2672 A C<my> declares the listed variables to be local (lexically) to the
2673 enclosing block, file, or C<eval>. If more than one value is listed,
2674 the list must be placed in parentheses.
2676 The exact semantics and interface of TYPE and ATTRS are still
2677 evolving. TYPE is currently bound to the use of C<fields> pragma,
2678 and attributes are handled using the C<attributes> pragma, or starting
2679 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2680 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2681 L<attributes>, and L<Attribute::Handlers>.
2687 The C<next> command is like the C<continue> statement in C; it starts
2688 the next iteration of the loop:
2690 LINE: while (<STDIN>) {
2691 next LINE if /^#/; # discard comments
2695 Note that if there were a C<continue> block on the above, it would get
2696 executed even on discarded lines. If the LABEL is omitted, the command
2697 refers to the innermost enclosing loop.
2699 C<next> cannot be used to exit a block which returns a value such as
2700 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2701 a grep() or map() operation.
2703 Note that a block by itself is semantically identical to a loop
2704 that executes once. Thus C<next> will exit such a block early.
2706 See also L</continue> for an illustration of how C<last>, C<next>, and
2709 =item no Module VERSION LIST
2711 =item no Module VERSION
2713 =item no Module LIST
2717 See the C<use> function, which C<no> is the opposite of.
2723 Interprets EXPR as an octal string and returns the corresponding
2724 value. (If EXPR happens to start off with C<0x>, interprets it as a
2725 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2726 binary string. Leading whitespace is ignored in all three cases.)
2727 The following will handle decimal, binary, octal, and hex in the standard
2730 $val = oct($val) if $val =~ /^0/;
2732 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2733 in octal), use sprintf() or printf():
2735 $perms = (stat("filename"))[2] & 07777;
2736 $oct_perms = sprintf "%lo", $perms;
2738 The oct() function is commonly used when a string such as C<644> needs
2739 to be converted into a file mode, for example. (Although perl will
2740 automatically convert strings into numbers as needed, this automatic
2741 conversion assumes base 10.)
2743 =item open FILEHANDLE,EXPR
2745 =item open FILEHANDLE,MODE,EXPR
2747 =item open FILEHANDLE,MODE,EXPR,LIST
2749 =item open FILEHANDLE,MODE,REFERENCE
2751 =item open FILEHANDLE
2753 Opens the file whose filename is given by EXPR, and associates it with
2756 (The following is a comprehensive reference to open(): for a gentler
2757 introduction you may consider L<perlopentut>.)
2759 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2760 the variable is assigned a reference to a new anonymous filehandle,
2761 otherwise if FILEHANDLE is an expression, its value is used as the name of
2762 the real filehandle wanted. (This is considered a symbolic reference, so
2763 C<use strict 'refs'> should I<not> be in effect.)
2765 If EXPR is omitted, the scalar variable of the same name as the
2766 FILEHANDLE contains the filename. (Note that lexical variables--those
2767 declared with C<my>--will not work for this purpose; so if you're
2768 using C<my>, specify EXPR in your call to open.)
2770 If three or more arguments are specified then the mode of opening and
2771 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2772 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2773 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2774 the file is opened for appending, again being created if necessary.
2776 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2777 indicate that you want both read and write access to the file; thus
2778 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2779 '+>' >> mode would clobber the file first. You can't usually use
2780 either read-write mode for updating textfiles, since they have
2781 variable length records. See the B<-i> switch in L<perlrun> for a
2782 better approach. The file is created with permissions of C<0666>
2783 modified by the process' C<umask> value.
2785 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2786 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2788 In the 2-arguments (and 1-argument) form of the call the mode and
2789 filename should be concatenated (in this order), possibly separated by
2790 spaces. It is possible to omit the mode in these forms if the mode is
2793 If the filename begins with C<'|'>, the filename is interpreted as a
2794 command to which output is to be piped, and if the filename ends with a
2795 C<'|'>, the filename is interpreted as a command which pipes output to
2796 us. See L<perlipc/"Using open() for IPC">
2797 for more examples of this. (You are not allowed to C<open> to a command
2798 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2799 and L<perlipc/"Bidirectional Communication with Another Process">
2802 For three or more arguments if MODE is C<'|-'>, the filename is
2803 interpreted as a command to which output is to be piped, and if MODE
2804 is C<'-|'>, the filename is interpreted as a command which pipes
2805 output to us. In the 2-arguments (and 1-argument) form one should
2806 replace dash (C<'-'>) with the command.
2807 See L<perlipc/"Using open() for IPC"> for more examples of this.
2808 (You are not allowed to C<open> to a command that pipes both in I<and>
2809 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2810 L<perlipc/"Bidirectional Communication"> for alternatives.)
2812 In the three-or-more argument form of pipe opens, if LIST is specified
2813 (extra arguments after the command name) then LIST becomes arguments
2814 to the command invoked if the platform supports it. The meaning of
2815 C<open> with more than three arguments for non-pipe modes is not yet
2816 specified. Experimental "layers" may give extra LIST arguments
2819 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2820 and opening C<< '>-' >> opens STDOUT.
2822 You may use the three-argument form of open to specify IO "layers"
2823 (sometimes also referred to as "disciplines") to be applied to the handle
2824 that affect how the input and output are processed (see L<open> and
2825 L<PerlIO> for more details). For example
2827 open(FH, "<:utf8", "file")
2829 will open the UTF-8 encoded file containing Unicode characters,
2830 see L<perluniintro>. (Note that if layers are specified in the
2831 three-arg form then default layers set by the C<open> pragma are
2834 Open returns nonzero upon success, the undefined value otherwise. If
2835 the C<open> involved a pipe, the return value happens to be the pid of
2838 If you're running Perl on a system that distinguishes between text
2839 files and binary files, then you should check out L</binmode> for tips
2840 for dealing with this. The key distinction between systems that need
2841 C<binmode> and those that don't is their text file formats. Systems
2842 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2843 character, and which encode that character in C as C<"\n">, do not
2844 need C<binmode>. The rest need it.
2846 When opening a file, it's usually a bad idea to continue normal execution
2847 if the request failed, so C<open> is frequently used in connection with
2848 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2849 where you want to make a nicely formatted error message (but there are
2850 modules that can help with that problem)) you should always check
2851 the return value from opening a file. The infrequent exception is when
2852 working with an unopened filehandle is actually what you want to do.
2854 As a special case the 3 arg form with a read/write mode and the third
2855 argument being C<undef>:
2857 open(TMP, "+>", undef) or die ...
2859 opens a filehandle to an anonymous temporary file. Also using "+<"
2860 works for symmetry, but you really should consider writing something
2861 to the temporary file first. You will need to seek() to do the
2862 reading. Starting from Perl 5.8.1 the temporary files are created
2863 using the File::Temp module for greater portability, in Perl 5.8.0 the
2864 mkstemp() system call (which has known bugs in some platforms) was used.
2866 File handles can be opened to "in memory" files held in Perl scalars via:
2868 open($fh, '>', \$variable) || ..
2870 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2871 file, you have to close it first:
2874 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2879 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2880 while (<ARTICLE>) {...
2882 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2883 # if the open fails, output is discarded
2885 open(DBASE, '+<', 'dbase.mine') # open for update
2886 or die "Can't open 'dbase.mine' for update: $!";
2888 open(DBASE, '+<dbase.mine') # ditto
2889 or die "Can't open 'dbase.mine' for update: $!";
2891 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2892 or die "Can't start caesar: $!";
2894 open(ARTICLE, "caesar <$article |") # ditto
2895 or die "Can't start caesar: $!";
2897 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2898 or die "Can't start sort: $!";
2901 open(MEMORY,'>', \$var)
2902 or die "Can't open memory file: $!";
2903 print MEMORY "foo!\n"; # output will end up in $var
2905 # process argument list of files along with any includes
2907 foreach $file (@ARGV) {
2908 process($file, 'fh00');
2912 my($filename, $input) = @_;
2913 $input++; # this is a string increment
2914 unless (open($input, $filename)) {
2915 print STDERR "Can't open $filename: $!\n";
2920 while (<$input>) { # note use of indirection
2921 if (/^#include "(.*)"/) {
2922 process($1, $input);
2929 You may also, in the Bourne shell tradition, specify an EXPR beginning
2930 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2931 name of a filehandle (or file descriptor, if numeric) to be
2932 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2933 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2934 mode you specify should match the mode of the original filehandle.
2935 (Duping a filehandle does not take into account any existing contents of
2936 IO buffers.) If you use the 3 arg form then you can pass either a number,
2937 the name of a filehandle or the normal "reference to a glob".
2939 Here is a script that saves, redirects, and restores C<STDOUT> and
2940 C<STDERR> using various methods:
2943 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2944 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2946 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2947 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2949 select STDERR; $| = 1; # make unbuffered
2950 select STDOUT; $| = 1; # make unbuffered
2952 print STDOUT "stdout 1\n"; # this works for
2953 print STDERR "stderr 1\n"; # subprocesses too
2958 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2959 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2961 print STDOUT "stdout 2\n";
2962 print STDERR "stderr 2\n";
2964 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2965 do an equivalent of C's C<fdopen> of that file descriptor; this is
2966 more parsimonious of file descriptors. For example:
2968 open(FILEHANDLE, "<&=$fd")
2972 open(FILEHANDLE, "<&=", $fd)
2974 Note that if Perl is using the standard C libraries' fdopen() then on
2975 many UNIX systems, fdopen() is known to fail when file descriptors
2976 exceed a certain value, typically 255. If you need more file
2977 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2979 You can see whether Perl has been compiled with PerlIO or not by
2980 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2981 is C<define>, you have PerlIO, otherwise you don't.
2983 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2984 with 2-arguments (or 1-argument) form of open(), then
2985 there is an implicit fork done, and the return value of open is the pid
2986 of the child within the parent process, and C<0> within the child
2987 process. (Use C<defined($pid)> to determine whether the open was successful.)
2988 The filehandle behaves normally for the parent, but i/o to that
2989 filehandle is piped from/to the STDOUT/STDIN of the child process.
2990 In the child process the filehandle isn't opened--i/o happens from/to
2991 the new STDOUT or STDIN. Typically this is used like the normal
2992 piped open when you want to exercise more control over just how the
2993 pipe command gets executed, such as when you are running setuid, and
2994 don't want to have to scan shell commands for metacharacters.
2995 The following triples are more or less equivalent:
2997 open(FOO, "|tr '[a-z]' '[A-Z]'");
2998 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2999 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3000 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3002 open(FOO, "cat -n '$file'|");
3003 open(FOO, '-|', "cat -n '$file'");
3004 open(FOO, '-|') || exec 'cat', '-n', $file;
3005 open(FOO, '-|', "cat", '-n', $file);
3007 The last example in each block shows the pipe as "list form", which is
3008 not yet supported on all platforms. A good rule of thumb is that if
3009 your platform has true C<fork()> (in other words, if your platform is
3010 UNIX) you can use the list form.
3012 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3014 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3015 output before any operation that may do a fork, but this may not be
3016 supported on some platforms (see L<perlport>). To be safe, you may need
3017 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3018 of C<IO::Handle> on any open handles.
3020 On systems that support a close-on-exec flag on files, the flag will
3021 be set for the newly opened file descriptor as determined by the value
3022 of $^F. See L<perlvar/$^F>.
3024 Closing any piped filehandle causes the parent process to wait for the
3025 child to finish, and returns the status value in C<$?>.
3027 The filename passed to 2-argument (or 1-argument) form of open() will
3028 have leading and trailing whitespace deleted, and the normal
3029 redirection characters honored. This property, known as "magic open",
3030 can often be used to good effect. A user could specify a filename of
3031 F<"rsh cat file |">, or you could change certain filenames as needed:
3033 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3034 open(FH, $filename) or die "Can't open $filename: $!";
3036 Use 3-argument form to open a file with arbitrary weird characters in it,
3038 open(FOO, '<', $file);
3040 otherwise it's necessary to protect any leading and trailing whitespace:
3042 $file =~ s#^(\s)#./$1#;
3043 open(FOO, "< $file\0");
3045 (this may not work on some bizarre filesystems). One should
3046 conscientiously choose between the I<magic> and 3-arguments form
3051 will allow the user to specify an argument of the form C<"rsh cat file |">,
3052 but will not work on a filename which happens to have a trailing space, while
3054 open IN, '<', $ARGV[0];
3056 will have exactly the opposite restrictions.
3058 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3059 should use the C<sysopen> function, which involves no such magic (but
3060 may use subtly different filemodes than Perl open(), which is mapped
3061 to C fopen()). This is
3062 another way to protect your filenames from interpretation. For example:
3065 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3066 or die "sysopen $path: $!";
3067 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3068 print HANDLE "stuff $$\n";
3070 print "File contains: ", <HANDLE>;
3072 Using the constructor from the C<IO::Handle> package (or one of its
3073 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3074 filehandles that have the scope of whatever variables hold references to
3075 them, and automatically close whenever and however you leave that scope:
3079 sub read_myfile_munged {
3081 my $handle = new IO::File;
3082 open($handle, "myfile") or die "myfile: $!";
3084 or return (); # Automatically closed here.
3085 mung $first or die "mung failed"; # Or here.
3086 return $first, <$handle> if $ALL; # Or here.
3090 See L</seek> for some details about mixing reading and writing.
3092 =item opendir DIRHANDLE,EXPR
3094 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3095 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3096 DIRHANDLE may be an expression whose value can be used as an indirect
3097 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3098 scalar variable (or array or hash element), the variable is assigned a
3099 reference to a new anonymous dirhandle.
3100 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3106 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3107 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3110 For the reverse, see L</chr>.
3111 See L<perlunicode> and L<encoding> for more about Unicode.
3117 =item our EXPR : ATTRS
3119 =item our TYPE EXPR : ATTRS
3121 An C<our> declares the listed variables to be valid globals within
3122 the enclosing block, file, or C<eval>. That is, it has the same
3123 scoping rules as a "my" declaration, but does not create a local
3124 variable. If more than one value is listed, the list must be placed
3125 in parentheses. The C<our> declaration has no semantic effect unless
3126 "use strict vars" is in effect, in which case it lets you use the
3127 declared global variable without qualifying it with a package name.
3128 (But only within the lexical scope of the C<our> declaration. In this
3129 it differs from "use vars", which is package scoped.)
3131 An C<our> declaration declares a global variable that will be visible
3132 across its entire lexical scope, even across package boundaries. The
3133 package in which the variable is entered is determined at the point
3134 of the declaration, not at the point of use. This means the following
3138 our $bar; # declares $Foo::bar for rest of lexical scope
3142 print $bar; # prints 20
3144 Multiple C<our> declarations in the same lexical scope are allowed
3145 if they are in different packages. If they happened to be in the same
3146 package, Perl will emit warnings if you have asked for them.
3150 our $bar; # declares $Foo::bar for rest of lexical scope
3154 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3155 print $bar; # prints 30
3157 our $bar; # emits warning
3159 An C<our> declaration may also have a list of attributes associated
3162 The exact semantics and interface of TYPE and ATTRS are still
3163 evolving. TYPE is currently bound to the use of C<fields> pragma,
3164 and attributes are handled using the C<attributes> pragma, or starting
3165 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3166 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3167 L<attributes>, and L<Attribute::Handlers>.
3169 The only currently recognized C<our()> attribute is C<unique> which
3170 indicates that a single copy of the global is to be used by all
3171 interpreters should the program happen to be running in a
3172 multi-interpreter environment. (The default behaviour would be for
3173 each interpreter to have its own copy of the global.) Examples:
3175 our @EXPORT : unique = qw(foo);
3176 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3177 our $VERSION : unique = "1.00";
3179 Note that this attribute also has the effect of making the global
3180 readonly when the first new interpreter is cloned (for example,
3181 when the first new thread is created).
3183 Multi-interpreter environments can come to being either through the
3184 fork() emulation on Windows platforms, or by embedding perl in a
3185 multi-threaded application. The C<unique> attribute does nothing in
3186 all other environments.
3188 =item pack TEMPLATE,LIST
3190 Takes a LIST of values and converts it into a string using the rules
3191 given by the TEMPLATE. The resulting string is the concatenation of
3192 the converted values. Typically, each converted value looks
3193 like its machine-level representation. For example, on 32-bit machines
3194 a converted integer may be represented by a sequence of 4 bytes.
3196 The TEMPLATE is a sequence of characters that give the order and type
3197 of values, as follows:
3199 a A string with arbitrary binary data, will be null padded.
3200 A A text (ASCII) string, will be space padded.
3201 Z A null terminated (ASCIZ) string, will be null padded.
3203 b A bit string (ascending bit order inside each byte, like vec()).
3204 B A bit string (descending bit order inside each byte).
3205 h A hex string (low nybble first).
3206 H A hex string (high nybble first).
3208 c A signed char value.
3209 C An unsigned char value. Only does bytes. See U for Unicode.
3211 s A signed short value.
3212 S An unsigned short value.
3213 (This 'short' is _exactly_ 16 bits, which may differ from
3214 what a local C compiler calls 'short'. If you want
3215 native-length shorts, use the '!' suffix.)
3217 i A signed integer value.
3218 I An unsigned integer value.
3219 (This 'integer' is _at_least_ 32 bits wide. Its exact
3220 size depends on what a local C compiler calls 'int',
3221 and may even be larger than the 'long' described in
3224 l A signed long value.
3225 L An unsigned long value.
3226 (This 'long' is _exactly_ 32 bits, which may differ from
3227 what a local C compiler calls 'long'. If you want
3228 native-length longs, use the '!' suffix.)
3230 n An unsigned short in "network" (big-endian) order.
3231 N An unsigned long in "network" (big-endian) order.
3232 v An unsigned short in "VAX" (little-endian) order.
3233 V An unsigned long in "VAX" (little-endian) order.
3234 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3235 _exactly_ 32 bits, respectively.)
3237 q A signed quad (64-bit) value.
3238 Q An unsigned quad value.
3239 (Quads are available only if your system supports 64-bit
3240 integer values _and_ if Perl has been compiled to support those.
3241 Causes a fatal error otherwise.)
3243 j A signed integer value (a Perl internal integer, IV).
3244 J An unsigned integer value (a Perl internal unsigned integer, UV).
3246 f A single-precision float in the native format.
3247 d A double-precision float in the native format.
3249 F A floating point value in the native native format
3250 (a Perl internal floating point value, NV).
3251 D A long double-precision float in the native format.
3252 (Long doubles are available only if your system supports long
3253 double values _and_ if Perl has been compiled to support those.
3254 Causes a fatal error otherwise.)
3256 p A pointer to a null-terminated string.
3257 P A pointer to a structure (fixed-length string).
3259 u A uuencoded string.
3260 U A Unicode character number. Encodes to UTF-8 internally
3261 (or UTF-EBCDIC in EBCDIC platforms).
3263 w A BER compressed integer. Its bytes represent an unsigned
3264 integer in base 128, most significant digit first, with as
3265 few digits as possible. Bit eight (the high bit) is set
3266 on each byte except the last.
3270 @ Null fill to absolute position, counted from the start of
3271 the innermost ()-group.
3272 ( Start of a ()-group.
3274 The following rules apply:
3280 Each letter may optionally be followed by a number giving a repeat
3281 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3282 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3283 many values from the LIST. A C<*> for the repeat count means to use
3284 however many items are left, except for C<@>, C<x>, C<X>, where it is
3285 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3286 is the same). A numeric repeat count may optionally be enclosed in
3287 brackets, as in C<pack 'C[80]', @arr>.
3289 One can replace the numeric repeat count by a template enclosed in brackets;
3290 then the packed length of this template in bytes is used as a count.
3291 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3292 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3293 If the template in brackets contains alignment commands (such as C<x![d]>),
3294 its packed length is calculated as if the start of the template has the maximal
3297 When used with C<Z>, C<*> results in the addition of a trailing null
3298 byte (so the packed result will be one longer than the byte C<length>
3301 The repeat count for C<u> is interpreted as the maximal number of bytes
3302 to encode per line of output, with 0 and 1 replaced by 45.
3306 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3307 string of length count, padding with nulls or spaces as necessary. When
3308 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3309 after the first null, and C<a> returns data verbatim. When packing,
3310 C<a>, and C<Z> are equivalent.
3312 If the value-to-pack is too long, it is truncated. If too long and an
3313 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3314 by a null byte. Thus C<Z> always packs a trailing null byte under
3319 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3320 Each byte of the input field of pack() generates 1 bit of the result.
3321 Each result bit is based on the least-significant bit of the corresponding
3322 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3323 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3325 Starting from the beginning of the input string of pack(), each 8-tuple
3326 of bytes is converted to 1 byte of output. With format C<b>
3327 the first byte of the 8-tuple determines the least-significant bit of a
3328 byte, and with format C<B> it determines the most-significant bit of
3331 If the length of the input string is not exactly divisible by 8, the
3332 remainder is packed as if the input string were padded by null bytes
3333 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3335 If the input string of pack() is longer than needed, extra bytes are ignored.
3336 A C<*> for the repeat count of pack() means to use all the bytes of
3337 the input field. On unpack()ing the bits are converted to a string
3338 of C<"0">s and C<"1">s.
3342 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3343 representable as hexadecimal digits, 0-9a-f) long.
3345 Each byte of the input field of pack() generates 4 bits of the result.
3346 For non-alphabetical bytes the result is based on the 4 least-significant
3347 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3348 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3349 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3350 is compatible with the usual hexadecimal digits, so that C<"a"> and
3351 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3352 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3354 Starting from the beginning of the input string of pack(), each pair
3355 of bytes is converted to 1 byte of output. With format C<h> the
3356 first byte of the pair determines the least-significant nybble of the
3357 output byte, and with format C<H> it determines the most-significant
3360 If the length of the input string is not even, it behaves as if padded
3361 by a null byte at the end. Similarly, during unpack()ing the "extra"
3362 nybbles are ignored.
3364 If the input string of pack() is longer than needed, extra bytes are ignored.
3365 A C<*> for the repeat count of pack() means to use all the bytes of
3366 the input field. On unpack()ing the bits are converted to a string
3367 of hexadecimal digits.
3371 The C<p> type packs a pointer to a null-terminated string. You are
3372 responsible for ensuring the string is not a temporary value (which can
3373 potentially get deallocated before you get around to using the packed result).
3374 The C<P> type packs a pointer to a structure of the size indicated by the
3375 length. A NULL pointer is created if the corresponding value for C<p> or
3376 C<P> is C<undef>, similarly for unpack().
3380 The C</> template character allows packing and unpacking of strings where
3381 the packed structure contains a byte count followed by the string itself.
3382 You write I<length-item>C</>I<string-item>.
3384 The I<length-item> can be any C<pack> template letter, and describes
3385 how the length value is packed. The ones likely to be of most use are
3386 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3387 SNMP) and C<N> (for Sun XDR).
3389 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3390 C<"Z*">. For C<unpack> the length of the string is obtained from the
3391 I<length-item>, but if you put in the '*' it will be ignored. For all other
3392 codes, C<unpack> applies the length value to the next item, which must not
3393 have a repeat count.
3395 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3396 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3397 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3399 The I<length-item> is not returned explicitly from C<unpack>.
3401 Adding a count to the I<length-item> letter is unlikely to do anything
3402 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3403 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3404 which Perl does not regard as legal in numeric strings.
3408 The integer types C<s>, C<S>, C<l>, and C<L> may be
3409 immediately followed by a C<!> suffix to signify native shorts or
3410 longs--as you can see from above for example a bare C<l> does mean
3411 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3412 may be larger. This is an issue mainly in 64-bit platforms. You can
3413 see whether using C<!> makes any difference by
3415 print length(pack("s")), " ", length(pack("s!")), "\n";
3416 print length(pack("l")), " ", length(pack("l!")), "\n";
3418 C<i!> and C<I!> also work but only because of completeness;
3419 they are identical to C<i> and C<I>.
3421 The actual sizes (in bytes) of native shorts, ints, longs, and long
3422 longs on the platform where Perl was built are also available via
3426 print $Config{shortsize}, "\n";
3427 print $Config{intsize}, "\n";
3428 print $Config{longsize}, "\n";
3429 print $Config{longlongsize}, "\n";
3431 (The C<$Config{longlongsize}> will be undefined if your system does
3432 not support long longs.)
3436 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3437 are inherently non-portable between processors and operating systems
3438 because they obey the native byteorder and endianness. For example a
3439 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3440 (arranged in and handled by the CPU registers) into bytes as
3442 0x12 0x34 0x56 0x78 # big-endian
3443 0x78 0x56 0x34 0x12 # little-endian
3445 Basically, the Intel and VAX CPUs are little-endian, while everybody
3446 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3447 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3448 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3451 The names `big-endian' and `little-endian' are comic references to
3452 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3453 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3454 the egg-eating habits of the Lilliputians.
3456 Some systems may have even weirder byte orders such as
3461 You can see your system's preference with
3463 print join(" ", map { sprintf "%#02x", $_ }
3464 unpack("C*",pack("L",0x12345678))), "\n";
3466 The byteorder on the platform where Perl was built is also available
3470 print $Config{byteorder}, "\n";
3472 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3473 and C<'87654321'> are big-endian.
3475 If you want portable packed integers use the formats C<n>, C<N>,
3476 C<v>, and C<V>, their byte endianness and size are known.
3477 See also L<perlport>.
3481 Real numbers (floats and doubles) are in the native machine format only;
3482 due to the multiplicity of floating formats around, and the lack of a
3483 standard "network" representation, no facility for interchange has been
3484 made. This means that packed floating point data written on one machine
3485 may not be readable on another - even if both use IEEE floating point
3486 arithmetic (as the endian-ness of the memory representation is not part
3487 of the IEEE spec). See also L<perlport>.
3489 Note that Perl uses doubles internally for all numeric calculation, and
3490 converting from double into float and thence back to double again will
3491 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3496 If the pattern begins with a C<U>, the resulting string will be treated
3497 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3498 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3499 characters. If you don't want this to happen, you can begin your pattern
3500 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3501 string, and then follow this with a C<U*> somewhere in your pattern.
3505 You must yourself do any alignment or padding by inserting for example
3506 enough C<'x'>es while packing. There is no way to pack() and unpack()
3507 could know where the bytes are going to or coming from. Therefore
3508 C<pack> (and C<unpack>) handle their output and input as flat
3513 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3514 take a repeat count, both as postfix, and for unpack() also via the C</>
3515 template character. Within each repetition of a group, positioning with
3516 C<@> starts again at 0. Therefore, the result of
3518 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3520 is the string "\0a\0\0bc".
3525 C<x> and C<X> accept C<!> modifier. In this case they act as
3526 alignment commands: they jump forward/back to the closest position
3527 aligned at a multiple of C<count> bytes. For example, to pack() or
3528 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3529 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3530 aligned on the double's size.
3532 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3533 both result in no-ops.
3537 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3538 White space may be used to separate pack codes from each other, but
3539 a C<!> modifier and a repeat count must follow immediately.
3543 If TEMPLATE requires more arguments to pack() than actually given, pack()
3544 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3545 to pack() than actually given, extra arguments are ignored.
3551 $foo = pack("CCCC",65,66,67,68);
3553 $foo = pack("C4",65,66,67,68);
3555 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3556 # same thing with Unicode circled letters
3558 $foo = pack("ccxxcc",65,66,67,68);
3561 # note: the above examples featuring "C" and "c" are true
3562 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3563 # and UTF-8. In EBCDIC the first example would be
3564 # $foo = pack("CCCC",193,194,195,196);
3566 $foo = pack("s2",1,2);
3567 # "\1\0\2\0" on little-endian
3568 # "\0\1\0\2" on big-endian
3570 $foo = pack("a4","abcd","x","y","z");
3573 $foo = pack("aaaa","abcd","x","y","z");
3576 $foo = pack("a14","abcdefg");
3577 # "abcdefg\0\0\0\0\0\0\0"
3579 $foo = pack("i9pl", gmtime);
3580 # a real struct tm (on my system anyway)
3582 $utmp_template = "Z8 Z8 Z16 L";
3583 $utmp = pack($utmp_template, @utmp1);
3584 # a struct utmp (BSDish)
3586 @utmp2 = unpack($utmp_template, $utmp);
3587 # "@utmp1" eq "@utmp2"
3590 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3593 $foo = pack('sx2l', 12, 34);
3594 # short 12, two zero bytes padding, long 34
3595 $bar = pack('s@4l', 12, 34);
3596 # short 12, zero fill to position 4, long 34
3599 The same template may generally also be used in unpack().
3601 =item package NAMESPACE
3605 Declares the compilation unit as being in the given namespace. The scope
3606 of the package declaration is from the declaration itself through the end
3607 of the enclosing block, file, or eval (the same as the C<my> operator).
3608 All further unqualified dynamic identifiers will be in this namespace.
3609 A package statement affects only dynamic variables--including those
3610 you've used C<local> on--but I<not> lexical variables, which are created
3611 with C<my>. Typically it would be the first declaration in a file to
3612 be included by the C<require> or C<use> operator. You can switch into a
3613 package in more than one place; it merely influences which symbol table
3614 is used by the compiler for the rest of that block. You can refer to
3615 variables and filehandles in other packages by prefixing the identifier
3616 with the package name and a double colon: C<$Package::Variable>.
3617 If the package name is null, the C<main> package as assumed. That is,
3618 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3619 still seen in older code).
3621 If NAMESPACE is omitted, then there is no current package, and all
3622 identifiers must be fully qualified or lexicals. However, you are
3623 strongly advised not to make use of this feature. Its use can cause
3624 unexpected behaviour, even crashing some versions of Perl. It is
3625 deprecated, and will be removed from a future release.
3627 See L<perlmod/"Packages"> for more information about packages, modules,
3628 and classes. See L<perlsub> for other scoping issues.
3630 =item pipe READHANDLE,WRITEHANDLE
3632 Opens a pair of connected pipes like the corresponding system call.
3633 Note that if you set up a loop of piped processes, deadlock can occur
3634 unless you are very careful. In addition, note that Perl's pipes use
3635 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3636 after each command, depending on the application.
3638 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3639 for examples of such things.
3641 On systems that support a close-on-exec flag on files, the flag will be set
3642 for the newly opened file descriptors as determined by the value of $^F.
3649 Pops and returns the last value of the array, shortening the array by
3650 one element. Has an effect similar to
3654 If there are no elements in the array, returns the undefined value
3655 (although this may happen at other times as well). If ARRAY is
3656 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3657 array in subroutines, just like C<shift>.
3663 Returns the offset of where the last C<m//g> search left off for the variable
3664 in question (C<$_> is used when the variable is not specified). May be
3665 modified to change that offset. Such modification will also influence
3666 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3669 =item print FILEHANDLE LIST
3675 Prints a string or a list of strings. Returns true if successful.
3676 FILEHANDLE may be a scalar variable name, in which case the variable
3677 contains the name of or a reference to the filehandle, thus introducing
3678 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3679 the next token is a term, it may be misinterpreted as an operator
3680 unless you interpose a C<+> or put parentheses around the arguments.)
3681 If FILEHANDLE is omitted, prints by default to standard output (or
3682 to the last selected output channel--see L</select>). If LIST is
3683 also omitted, prints C<$_> to the currently selected output channel.
3684 To set the default output channel to something other than STDOUT
3685 use the select operation. The current value of C<$,> (if any) is
3686 printed between each LIST item. The current value of C<$\> (if
3687 any) is printed after the entire LIST has been printed. Because
3688 print takes a LIST, anything in the LIST is evaluated in list
3689 context, and any subroutine that you call will have one or more of
3690 its expressions evaluated in list context. Also be careful not to
3691 follow the print keyword with a left parenthesis unless you want
3692 the corresponding right parenthesis to terminate the arguments to
3693 the print--interpose a C<+> or put parentheses around all the
3696 Note that if you're storing FILEHANDLES in an array or other expression,
3697 you will have to use a block returning its value instead:
3699 print { $files[$i] } "stuff\n";
3700 print { $OK ? STDOUT : STDERR } "stuff\n";
3702 =item printf FILEHANDLE FORMAT, LIST
3704 =item printf FORMAT, LIST
3706 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3707 (the output record separator) is not appended. The first argument
3708 of the list will be interpreted as the C<printf> format. See C<sprintf>
3709 for an explanation of the format argument. If C<use locale> is in effect,
3710 the character used for the decimal point in formatted real numbers is
3711 affected by the LC_NUMERIC locale. See L<perllocale>.
3713 Don't fall into the trap of using a C<printf> when a simple
3714 C<print> would do. The C<print> is more efficient and less
3717 =item prototype FUNCTION
3719 Returns the prototype of a function as a string (or C<undef> if the
3720 function has no prototype). FUNCTION is a reference to, or the name of,
3721 the function whose prototype you want to retrieve.
3723 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3724 name for Perl builtin. If the builtin is not I<overridable> (such as
3725 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3726 C<system>) returns C<undef> because the builtin does not really behave
3727 like a Perl function. Otherwise, the string describing the equivalent
3728 prototype is returned.
3730 =item push ARRAY,LIST
3732 Treats ARRAY as a stack, and pushes the values of LIST
3733 onto the end of ARRAY. The length of ARRAY increases by the length of
3734 LIST. Has the same effect as
3737 $ARRAY[++$#ARRAY] = $value;
3740 but is more efficient. Returns the new number of elements in the array.
3752 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3754 =item quotemeta EXPR
3758 Returns the value of EXPR with all non-"word"
3759 characters backslashed. (That is, all characters not matching
3760 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3761 returned string, regardless of any locale settings.)
3762 This is the internal function implementing
3763 the C<\Q> escape in double-quoted strings.
3765 If EXPR is omitted, uses C<$_>.
3771 Returns a random fractional number greater than or equal to C<0> and less
3772 than the value of EXPR. (EXPR should be positive.) If EXPR is
3773 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3774 also special-cased as C<1> - this has not been documented before perl 5.8.0
3775 and is subject to change in future versions of perl. Automatically calls
3776 C<srand> unless C<srand> has already been called. See also C<srand>.
3778 Apply C<int()> to the value returned by C<rand()> if you want random
3779 integers instead of random fractional numbers. For example,
3783 returns a random integer between C<0> and C<9>, inclusive.
3785 (Note: If your rand function consistently returns numbers that are too
3786 large or too small, then your version of Perl was probably compiled
3787 with the wrong number of RANDBITS.)
3789 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3791 =item read FILEHANDLE,SCALAR,LENGTH
3793 Attempts to read LENGTH I<characters> of data into variable SCALAR
3794 from the specified FILEHANDLE. Returns the number of characters
3795 actually read, C<0> at end of file, or undef if there was an error (in
3796 the latter case C<$!> is also set). SCALAR will be grown or shrunk to
3797 the length actually read. If SCALAR needs growing, the new bytes will
3798 be zero bytes. An OFFSET may be specified to place the read data into
3799 some other place in SCALAR than the beginning. The call is actually
3800 implemented in terms of either Perl's or system's fread() call. To
3801 get a true read(2) system call, see C<sysread>.
3803 Note the I<characters>: depending on the status of the filehandle,
3804 either (8-bit) bytes or characters are read. By default all
3805 filehandles operate on bytes, but for example if the filehandle has
3806 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3807 pragma, L<open>), the I/O will operate on characters, not bytes.
3809 =item readdir DIRHANDLE
3811 Returns the next directory entry for a directory opened by C<opendir>.
3812 If used in list context, returns all the rest of the entries in the
3813 directory. If there are no more entries, returns an undefined value in
3814 scalar context or a null list in list context.
3816 If you're planning to filetest the return values out of a C<readdir>, you'd
3817 better prepend the directory in question. Otherwise, because we didn't
3818 C<chdir> there, it would have been testing the wrong file.
3820 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3821 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3826 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3827 context, each call reads and returns the next line, until end-of-file is
3828 reached, whereupon the subsequent call returns undef. In list context,
3829 reads until end-of-file is reached and returns a list of lines. Note that
3830 the notion of "line" used here is however you may have defined it
3831 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3833 When C<$/> is set to C<undef>, when readline() is in scalar
3834 context (i.e. file slurp mode), and when an empty file is read, it
3835 returns C<''> the first time, followed by C<undef> subsequently.
3837 This is the internal function implementing the C<< <EXPR> >>
3838 operator, but you can use it directly. The C<< <EXPR> >>
3839 operator is discussed in more detail in L<perlop/"I/O Operators">.
3842 $line = readline(*STDIN); # same thing
3844 If readline encounters an operating system error, C<$!> will be set with the
3845 corresponding error message. It can be helpful to check C<$!> when you are
3846 reading from filehandles you don't trust, such as a tty or a socket. The
3847 following example uses the operator form of C<readline>, and takes the necessary
3848 steps to ensure that C<readline> was successful.
3852 unless (defined( $line = <> )) {
3863 Returns the value of a symbolic link, if symbolic links are
3864 implemented. If not, gives a fatal error. If there is some system
3865 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3866 omitted, uses C<$_>.
3870 EXPR is executed as a system command.
3871 The collected standard output of the command is returned.
3872 In scalar context, it comes back as a single (potentially
3873 multi-line) string. In list context, returns a list of lines
3874 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3875 This is the internal function implementing the C<qx/EXPR/>
3876 operator, but you can use it directly. The C<qx/EXPR/>
3877 operator is discussed in more detail in L<perlop/"I/O Operators">.
3879 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3881 Receives a message on a socket. Attempts to receive LENGTH characters
3882 of data into variable SCALAR from the specified SOCKET filehandle.
3883 SCALAR will be grown or shrunk to the length actually read. Takes the
3884 same flags as the system call of the same name. Returns the address
3885 of the sender if SOCKET's protocol supports this; returns an empty
3886 string otherwise. If there's an error, returns the undefined value.
3887 This call is actually implemented in terms of recvfrom(2) system call.
3888 See L<perlipc/"UDP: Message Passing"> for examples.
3890 Note the I<characters>: depending on the status of the socket, either
3891 (8-bit) bytes or characters are received. By default all sockets
3892 operate on bytes, but for example if the socket has been changed using
3893 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3894 pragma, L<open>), the I/O will operate on characters, not bytes.
3900 The C<redo> command restarts the loop block without evaluating the
3901 conditional again. The C<continue> block, if any, is not executed. If
3902 the LABEL is omitted, the command refers to the innermost enclosing
3903 loop. This command is normally used by programs that want to lie to
3904 themselves about what was just input:
3906 # a simpleminded Pascal comment stripper
3907 # (warning: assumes no { or } in strings)
3908 LINE: while (<STDIN>) {
3909 while (s|({.*}.*){.*}|$1 |) {}
3914 if (/}/) { # end of comment?
3923 C<redo> cannot be used to retry a block which returns a value such as
3924 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3925 a grep() or map() operation.
3927 Note that a block by itself is semantically identical to a loop
3928 that executes once. Thus C<redo> inside such a block will effectively
3929 turn it into a looping construct.
3931 See also L</continue> for an illustration of how C<last>, C<next>, and
3938 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3939 is not specified, C<$_> will be used. The value returned depends on the
3940 type of thing the reference is a reference to.
3941 Builtin types include:
3951 If the referenced object has been blessed into a package, then that package
3952 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3954 if (ref($r) eq "HASH") {
3955 print "r is a reference to a hash.\n";
3958 print "r is not a reference at all.\n";
3960 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3961 print "r is a reference to something that isa hash.\n";
3964 See also L<perlref>.
3966 =item rename OLDNAME,NEWNAME
3968 Changes the name of a file; an existing file NEWNAME will be
3969 clobbered. Returns true for success, false otherwise.
3971 Behavior of this function varies wildly depending on your system
3972 implementation. For example, it will usually not work across file system
3973 boundaries, even though the system I<mv> command sometimes compensates
3974 for this. Other restrictions include whether it works on directories,
3975 open files, or pre-existing files. Check L<perlport> and either the
3976 rename(2) manpage or equivalent system documentation for details.
3978 =item require VERSION
3984 Demands a version of Perl specified by VERSION, or demands some semantics
3985 specified by EXPR or by C<$_> if EXPR is not supplied.
3987 VERSION may be either a numeric argument such as 5.006, which will be
3988 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3989 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3990 VERSION is greater than the version of the current Perl interpreter.
3991 Compare with L</use>, which can do a similar check at compile time.
3993 Specifying VERSION as a literal of the form v5.6.1 should generally be
3994 avoided, because it leads to misleading error messages under earlier
3995 versions of Perl which do not support this syntax. The equivalent numeric
3996 version should be used instead.
3998 require v5.6.1; # run time version check
3999 require 5.6.1; # ditto
4000 require 5.006_001; # ditto; preferred for backwards compatibility
4002 Otherwise, demands that a library file be included if it hasn't already
4003 been included. The file is included via the do-FILE mechanism, which is
4004 essentially just a variety of C<eval>. Has semantics similar to the following
4009 return 1 if $INC{$filename};
4010 my($realfilename,$result);
4012 foreach $prefix (@INC) {
4013 $realfilename = "$prefix/$filename";
4014 if (-f $realfilename) {
4015 $INC{$filename} = $realfilename;
4016 $result = do $realfilename;
4020 die "Can't find $filename in \@INC";
4022 delete $INC{$filename} if $@ || !$result;
4024 die "$filename did not return true value" unless $result;
4028 Note that the file will not be included twice under the same specified
4029 name. The file must return true as the last statement to indicate
4030 successful execution of any initialization code, so it's customary to
4031 end such a file with C<1;> unless you're sure it'll return true
4032 otherwise. But it's better just to put the C<1;>, in case you add more
4035 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4036 replaces "F<::>" with "F</>" in the filename for you,
4037 to make it easy to load standard modules. This form of loading of
4038 modules does not risk altering your namespace.
4040 In other words, if you try this:
4042 require Foo::Bar; # a splendid bareword
4044 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4045 directories specified in the C<@INC> array.
4047 But if you try this:
4049 $class = 'Foo::Bar';
4050 require $class; # $class is not a bareword
4052 require "Foo::Bar"; # not a bareword because of the ""
4054 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4055 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4057 eval "require $class";
4059 You can also insert hooks into the import facility, by putting directly
4060 Perl code into the @INC array. There are three forms of hooks: subroutine
4061 references, array references and blessed objects.
4063 Subroutine references are the simplest case. When the inclusion system
4064 walks through @INC and encounters a subroutine, this subroutine gets
4065 called with two parameters, the first being a reference to itself, and the
4066 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4067 subroutine should return C<undef> or a filehandle, from which the file to
4068 include will be read. If C<undef> is returned, C<require> will look at
4069 the remaining elements of @INC.
4071 If the hook is an array reference, its first element must be a subroutine
4072 reference. This subroutine is called as above, but the first parameter is
4073 the array reference. This enables to pass indirectly some arguments to
4076 In other words, you can write:
4078 push @INC, \&my_sub;
4080 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4086 push @INC, [ \&my_sub, $x, $y, ... ];
4088 my ($arrayref, $filename) = @_;
4089 # Retrieve $x, $y, ...
4090 my @parameters = @$arrayref[1..$#$arrayref];
4094 If the hook is an object, it must provide an INC method, that will be
4095 called as above, the first parameter being the object itself. (Note that
4096 you must fully qualify the sub's name, as it is always forced into package
4097 C<main>.) Here is a typical code layout:
4103 my ($self, $filename) = @_;
4107 # In the main program
4108 push @INC, new Foo(...);
4110 Note that these hooks are also permitted to set the %INC entry
4111 corresponding to the files they have loaded. See L<perlvar/%INC>.
4113 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4119 Generally used in a C<continue> block at the end of a loop to clear
4120 variables and reset C<??> searches so that they work again. The
4121 expression is interpreted as a list of single characters (hyphens
4122 allowed for ranges). All variables and arrays beginning with one of
4123 those letters are reset to their pristine state. If the expression is
4124 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4125 only variables or searches in the current package. Always returns
4128 reset 'X'; # reset all X variables
4129 reset 'a-z'; # reset lower case variables
4130 reset; # just reset ?one-time? searches
4132 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4133 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4134 variables--lexical variables are unaffected, but they clean themselves
4135 up on scope exit anyway, so you'll probably want to use them instead.
4142 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4143 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4144 context, depending on how the return value will be used, and the context
4145 may vary from one execution to the next (see C<wantarray>). If no EXPR
4146 is given, returns an empty list in list context, the undefined value in
4147 scalar context, and (of course) nothing at all in a void context.
4149 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4150 or do FILE will automatically return the value of the last expression
4155 In list context, returns a list value consisting of the elements
4156 of LIST in the opposite order. In scalar context, concatenates the
4157 elements of LIST and returns a string value with all characters
4158 in the opposite order.
4160 print reverse <>; # line tac, last line first
4162 undef $/; # for efficiency of <>
4163 print scalar reverse <>; # character tac, last line tsrif
4165 This operator is also handy for inverting a hash, although there are some
4166 caveats. If a value is duplicated in the original hash, only one of those
4167 can be represented as a key in the inverted hash. Also, this has to
4168 unwind one hash and build a whole new one, which may take some time
4169 on a large hash, such as from a DBM file.
4171 %by_name = reverse %by_address; # Invert the hash
4173 =item rewinddir DIRHANDLE
4175 Sets the current position to the beginning of the directory for the
4176 C<readdir> routine on DIRHANDLE.
4178 =item rindex STR,SUBSTR,POSITION
4180 =item rindex STR,SUBSTR
4182 Works just like index() except that it returns the position of the LAST
4183 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4184 last occurrence at or before that position.
4186 =item rmdir FILENAME
4190 Deletes the directory specified by FILENAME if that directory is empty. If it
4191 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4192 FILENAME is omitted, uses C<$_>.
4196 The substitution operator. See L<perlop>.
4200 Forces EXPR to be interpreted in scalar context and returns the value
4203 @counts = ( scalar @a, scalar @b, scalar @c );
4205 There is no equivalent operator to force an expression to
4206 be interpolated in list context because in practice, this is never
4207 needed. If you really wanted to do so, however, you could use
4208 the construction C<@{[ (some expression) ]}>, but usually a simple
4209 C<(some expression)> suffices.
4211 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4212 parenthesized list, this behaves as a scalar comma expression, evaluating
4213 all but the last element in void context and returning the final element
4214 evaluated in scalar context. This is seldom what you want.
4216 The following single statement:
4218 print uc(scalar(&foo,$bar)),$baz;
4220 is the moral equivalent of these two:
4223 print(uc($bar),$baz);
4225 See L<perlop> for more details on unary operators and the comma operator.
4227 =item seek FILEHANDLE,POSITION,WHENCE
4229 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4230 FILEHANDLE may be an expression whose value gives the name of the
4231 filehandle. The values for WHENCE are C<0> to set the new position
4232 I<in bytes> to POSITION, C<1> to set it to the current position plus
4233 POSITION, and C<2> to set it to EOF plus POSITION (typically
4234 negative). For WHENCE you may use the constants C<SEEK_SET>,
4235 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4236 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4239 Note the I<in bytes>: even if the filehandle has been set to
4240 operate on characters (for example by using the C<:utf8> open
4241 layer), tell() will return byte offsets, not character offsets
4242 (because implementing that would render seek() and tell() rather slow).
4244 If you want to position file for C<sysread> or C<syswrite>, don't use
4245 C<seek>--buffering makes its effect on the file's system position
4246 unpredictable and non-portable. Use C<sysseek> instead.
4248 Due to the rules and rigors of ANSI C, on some systems you have to do a
4249 seek whenever you switch between reading and writing. Amongst other
4250 things, this may have the effect of calling stdio's clearerr(3).
4251 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4255 This is also useful for applications emulating C<tail -f>. Once you hit
4256 EOF on your read, and then sleep for a while, you might have to stick in a
4257 seek() to reset things. The C<seek> doesn't change the current position,
4258 but it I<does> clear the end-of-file condition on the handle, so that the
4259 next C<< <FILE> >> makes Perl try again to read something. We hope.
4261 If that doesn't work (some IO implementations are particularly
4262 cantankerous), then you may need something more like this:
4265 for ($curpos = tell(FILE); $_ = <FILE>;
4266 $curpos = tell(FILE)) {
4267 # search for some stuff and put it into files
4269 sleep($for_a_while);
4270 seek(FILE, $curpos, 0);
4273 =item seekdir DIRHANDLE,POS
4275 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4276 must be a value returned by C<telldir>. Has the same caveats about
4277 possible directory compaction as the corresponding system library
4280 =item select FILEHANDLE
4284 Returns the currently selected filehandle. Sets the current default
4285 filehandle for output, if FILEHANDLE is supplied. This has two
4286 effects: first, a C<write> or a C<print> without a filehandle will
4287 default to this FILEHANDLE. Second, references to variables related to
4288 output will refer to this output channel. For example, if you have to
4289 set the top of form format for more than one output channel, you might
4297 FILEHANDLE may be an expression whose value gives the name of the
4298 actual filehandle. Thus:
4300 $oldfh = select(STDERR); $| = 1; select($oldfh);
4302 Some programmers may prefer to think of filehandles as objects with
4303 methods, preferring to write the last example as:
4306 STDERR->autoflush(1);
4308 =item select RBITS,WBITS,EBITS,TIMEOUT
4310 This calls the select(2) system call with the bit masks specified, which
4311 can be constructed using C<fileno> and C<vec>, along these lines:
4313 $rin = $win = $ein = '';
4314 vec($rin,fileno(STDIN),1) = 1;
4315 vec($win,fileno(STDOUT),1) = 1;
4318 If you want to select on many filehandles you might wish to write a
4322 my(@fhlist) = split(' ',$_[0]);
4325 vec($bits,fileno($_),1) = 1;
4329 $rin = fhbits('STDIN TTY SOCK');
4333 ($nfound,$timeleft) =
4334 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4336 or to block until something becomes ready just do this
4338 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4340 Most systems do not bother to return anything useful in $timeleft, so
4341 calling select() in scalar context just returns $nfound.
4343 Any of the bit masks can also be undef. The timeout, if specified, is
4344 in seconds, which may be fractional. Note: not all implementations are
4345 capable of returning the $timeleft. If not, they always return
4346 $timeleft equal to the supplied $timeout.
4348 You can effect a sleep of 250 milliseconds this way:
4350 select(undef, undef, undef, 0.25);
4352 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4353 is implementation-dependent.
4355 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4356 or <FH>) with C<select>, except as permitted by POSIX, and even
4357 then only on POSIX systems. You have to use C<sysread> instead.
4359 =item semctl ID,SEMNUM,CMD,ARG
4361 Calls the System V IPC function C<semctl>. You'll probably have to say
4365 first to get the correct constant definitions. If CMD is IPC_STAT or
4366 GETALL, then ARG must be a variable which will hold the returned
4367 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4368 the undefined value for error, "C<0 but true>" for zero, or the actual
4369 return value otherwise. The ARG must consist of a vector of native
4370 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4371 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4374 =item semget KEY,NSEMS,FLAGS
4376 Calls the System V IPC function semget. Returns the semaphore id, or
4377 the undefined value if there is an error. See also
4378 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4381 =item semop KEY,OPSTRING
4383 Calls the System V IPC function semop to perform semaphore operations
4384 such as signalling and waiting. OPSTRING must be a packed array of
4385 semop structures. Each semop structure can be generated with
4386 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4387 operations is implied by the length of OPSTRING. Returns true if
4388 successful, or false if there is an error. As an example, the
4389 following code waits on semaphore $semnum of semaphore id $semid:
4391 $semop = pack("s!3", $semnum, -1, 0);
4392 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4394 To signal the semaphore, replace C<-1> with C<1>. See also
4395 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4398 =item send SOCKET,MSG,FLAGS,TO
4400 =item send SOCKET,MSG,FLAGS
4402 Sends a message on a socket. Attempts to send the scalar MSG to the
4403 SOCKET filehandle. Takes the same flags as the system call of the
4404 same name. On unconnected sockets you must specify a destination to
4405 send TO, in which case it does a C C<sendto>. Returns the number of
4406 characters sent, or the undefined value if there is an error. The C
4407 system call sendmsg(2) is currently unimplemented. See
4408 L<perlipc/"UDP: Message Passing"> for examples.
4410 Note the I<characters>: depending on the status of the socket, either
4411 (8-bit) bytes or characters are sent. By default all sockets operate
4412 on bytes, but for example if the socket has been changed using
4413 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4414 the C<open> pragma, L<open>), the I/O will operate on characters, not
4417 =item setpgrp PID,PGRP
4419 Sets the current process group for the specified PID, C<0> for the current
4420 process. Will produce a fatal error if used on a machine that doesn't
4421 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4422 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4423 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4426 =item setpriority WHICH,WHO,PRIORITY
4428 Sets the current priority for a process, a process group, or a user.
4429 (See setpriority(2).) Will produce a fatal error if used on a machine
4430 that doesn't implement setpriority(2).
4432 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4434 Sets the socket option requested. Returns undefined if there is an
4435 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4442 Shifts the first value of the array off and returns it, shortening the
4443 array by 1 and moving everything down. If there are no elements in the
4444 array, returns the undefined value. If ARRAY is omitted, shifts the
4445 C<@_> array within the lexical scope of subroutines and formats, and the
4446 C<@ARGV> array at file scopes or within the lexical scopes established by
4447 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4450 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4451 same thing to the left end of an array that C<pop> and C<push> do to the
4454 =item shmctl ID,CMD,ARG
4456 Calls the System V IPC function shmctl. You'll probably have to say
4460 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4461 then ARG must be a variable which will hold the returned C<shmid_ds>
4462 structure. Returns like ioctl: the undefined value for error, "C<0> but
4463 true" for zero, or the actual return value otherwise.
4464 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4466 =item shmget KEY,SIZE,FLAGS
4468 Calls the System V IPC function shmget. Returns the shared memory
4469 segment id, or the undefined value if there is an error.
4470 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4472 =item shmread ID,VAR,POS,SIZE
4474 =item shmwrite ID,STRING,POS,SIZE
4476 Reads or writes the System V shared memory segment ID starting at
4477 position POS for size SIZE by attaching to it, copying in/out, and
4478 detaching from it. When reading, VAR must be a variable that will
4479 hold the data read. When writing, if STRING is too long, only SIZE
4480 bytes are used; if STRING is too short, nulls are written to fill out
4481 SIZE bytes. Return true if successful, or false if there is an error.
4482 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4483 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4485 =item shutdown SOCKET,HOW
4487 Shuts down a socket connection in the manner indicated by HOW, which
4488 has the same interpretation as in the system call of the same name.
4490 shutdown(SOCKET, 0); # I/we have stopped reading data
4491 shutdown(SOCKET, 1); # I/we have stopped writing data
4492 shutdown(SOCKET, 2); # I/we have stopped using this socket
4494 This is useful with sockets when you want to tell the other
4495 side you're done writing but not done reading, or vice versa.
4496 It's also a more insistent form of close because it also
4497 disables the file descriptor in any forked copies in other
4504 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4505 returns sine of C<$_>.
4507 For the inverse sine operation, you may use the C<Math::Trig::asin>
4508 function, or use this relation:
4510 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4516 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4517 May be interrupted if the process receives a signal such as C<SIGALRM>.
4518 Returns the number of seconds actually slept. You probably cannot
4519 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4522 On some older systems, it may sleep up to a full second less than what
4523 you requested, depending on how it counts seconds. Most modern systems
4524 always sleep the full amount. They may appear to sleep longer than that,
4525 however, because your process might not be scheduled right away in a
4526 busy multitasking system.
4528 For delays of finer granularity than one second, you may use Perl's
4529 C<syscall> interface to access setitimer(2) if your system supports
4530 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4531 and starting from Perl 5.8 part of the standard distribution) may also
4534 See also the POSIX module's C<pause> function.
4536 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4538 Opens a socket of the specified kind and attaches it to filehandle
4539 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4540 the system call of the same name. You should C<use Socket> first
4541 to get the proper definitions imported. See the examples in
4542 L<perlipc/"Sockets: Client/Server Communication">.
4544 On systems that support a close-on-exec flag on files, the flag will
4545 be set for the newly opened file descriptor, as determined by the
4546 value of $^F. See L<perlvar/$^F>.
4548 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4550 Creates an unnamed pair of sockets in the specified domain, of the
4551 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4552 for the system call of the same name. If unimplemented, yields a fatal
4553 error. Returns true if successful.
4555 On systems that support a close-on-exec flag on files, the flag will
4556 be set for the newly opened file descriptors, as determined by the value
4557 of $^F. See L<perlvar/$^F>.
4559 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4560 to C<pipe(Rdr, Wtr)> is essentially:
4563 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4564 shutdown(Rdr, 1); # no more writing for reader
4565 shutdown(Wtr, 0); # no more reading for writer
4567 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4568 emulate socketpair using IP sockets to localhost if your system implements
4569 sockets but not socketpair.
4571 =item sort SUBNAME LIST
4573 =item sort BLOCK LIST
4577 In list context, this sorts the LIST and returns the sorted list value.
4578 In scalar context, the behaviour of C<sort()> is undefined.
4580 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4581 order. If SUBNAME is specified, it gives the name of a subroutine
4582 that returns an integer less than, equal to, or greater than C<0>,
4583 depending on how the elements of the list are to be ordered. (The C<<
4584 <=> >> and C<cmp> operators are extremely useful in such routines.)
4585 SUBNAME may be a scalar variable name (unsubscripted), in which case
4586 the value provides the name of (or a reference to) the actual
4587 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4588 an anonymous, in-line sort subroutine.
4590 If the subroutine's prototype is C<($$)>, the elements to be compared
4591 are passed by reference in C<@_>, as for a normal subroutine. This is
4592 slower than unprototyped subroutines, where the elements to be
4593 compared are passed into the subroutine
4594 as the package global variables $a and $b (see example below). Note that
4595 in the latter case, it is usually counter-productive to declare $a and
4598 In either case, the subroutine may not be recursive. The values to be
4599 compared are always passed by reference, so don't modify them.
4601 You also cannot exit out of the sort block or subroutine using any of the
4602 loop control operators described in L<perlsyn> or with C<goto>.
4604 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4605 current collation locale. See L<perllocale>.
4607 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4608 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4609 preserves the input order of elements that compare equal. Although
4610 quicksort's run time is O(NlogN) when averaged over all arrays of
4611 length N, the time can be O(N**2), I<quadratic> behavior, for some
4612 inputs.) In 5.7, the quicksort implementation was replaced with
4613 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4614 But benchmarks indicated that for some inputs, on some platforms,
4615 the original quicksort was faster. 5.8 has a sort pragma for
4616 limited control of the sort. Its rather blunt control of the
4617 underlying algorithm may not persist into future perls, but the
4618 ability to characterize the input or output in implementation
4619 independent ways quite probably will. See L<sort>.
4624 @articles = sort @files;
4626 # same thing, but with explicit sort routine
4627 @articles = sort {$a cmp $b} @files;
4629 # now case-insensitively
4630 @articles = sort {uc($a) cmp uc($b)} @files;
4632 # same thing in reversed order
4633 @articles = sort {$b cmp $a} @files;
4635 # sort numerically ascending
4636 @articles = sort {$a <=> $b} @files;
4638 # sort numerically descending
4639 @articles = sort {$b <=> $a} @files;
4641 # this sorts the %age hash by value instead of key
4642 # using an in-line function
4643 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4645 # sort using explicit subroutine name
4647 $age{$a} <=> $age{$b}; # presuming numeric
4649 @sortedclass = sort byage @class;
4651 sub backwards { $b cmp $a }
4652 @harry = qw(dog cat x Cain Abel);
4653 @george = qw(gone chased yz Punished Axed);
4655 # prints AbelCaincatdogx
4656 print sort backwards @harry;
4657 # prints xdogcatCainAbel
4658 print sort @george, 'to', @harry;
4659 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4661 # inefficiently sort by descending numeric compare using
4662 # the first integer after the first = sign, or the
4663 # whole record case-insensitively otherwise
4666 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4671 # same thing, but much more efficiently;
4672 # we'll build auxiliary indices instead
4676 push @nums, /=(\d+)/;
4681 $nums[$b] <=> $nums[$a]
4683 $caps[$a] cmp $caps[$b]
4687 # same thing, but without any temps
4688 @new = map { $_->[0] }
4689 sort { $b->[1] <=> $a->[1]
4692 } map { [$_, /=(\d+)/, uc($_)] } @old;
4694 # using a prototype allows you to use any comparison subroutine
4695 # as a sort subroutine (including other package's subroutines)
4697 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4700 @new = sort other::backwards @old;
4702 # guarantee stability, regardless of algorithm
4704 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4706 # force use of mergesort (not portable outside Perl 5.8)
4707 use sort '_mergesort'; # note discouraging _
4708 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4710 If you're using strict, you I<must not> declare $a
4711 and $b as lexicals. They are package globals. That means
4712 if you're in the C<main> package and type
4714 @articles = sort {$b <=> $a} @files;
4716 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4717 but if you're in the C<FooPack> package, it's the same as typing
4719 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4721 The comparison function is required to behave. If it returns
4722 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4723 sometimes saying the opposite, for example) the results are not
4726 =item splice ARRAY,OFFSET,LENGTH,LIST
4728 =item splice ARRAY,OFFSET,LENGTH
4730 =item splice ARRAY,OFFSET
4734 Removes the elements designated by OFFSET and LENGTH from an array, and
4735 replaces them with the elements of LIST, if any. In list context,
4736 returns the elements removed from the array. In scalar context,
4737 returns the last element removed, or C<undef> if no elements are
4738 removed. The array grows or shrinks as necessary.
4739 If OFFSET is negative then it starts that far from the end of the array.
4740 If LENGTH is omitted, removes everything from OFFSET onward.
4741 If LENGTH is negative, removes the elements from OFFSET onward
4742 except for -LENGTH elements at the end of the array.
4743 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4744 past the end of the array, perl issues a warning, and splices at the
4747 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4749 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4750 pop(@a) splice(@a,-1)
4751 shift(@a) splice(@a,0,1)
4752 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4753 $a[$i] = $y splice(@a,$i,1,$y)
4755 Example, assuming array lengths are passed before arrays:
4757 sub aeq { # compare two list values
4758 my(@a) = splice(@_,0,shift);
4759 my(@b) = splice(@_,0,shift);
4760 return 0 unless @a == @b; # same len?
4762 return 0 if pop(@a) ne pop(@b);
4766 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4768 =item split /PATTERN/,EXPR,LIMIT
4770 =item split /PATTERN/,EXPR
4772 =item split /PATTERN/
4776 Splits a string into a list of strings and returns that list. By default,
4777 empty leading fields are preserved, and empty trailing ones are deleted.
4779 In scalar context, returns the number of fields found and splits into
4780 the C<@_> array. Use of split in scalar context is deprecated, however,
4781 because it clobbers your subroutine arguments.
4783 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4784 splits on whitespace (after skipping any leading whitespace). Anything
4785 matching PATTERN is taken to be a delimiter separating the fields. (Note
4786 that the delimiter may be longer than one character.)
4788 If LIMIT is specified and positive, it represents the maximum number
4789 of fields the EXPR will be split into, though the actual number of
4790 fields returned depends on the number of times PATTERN matches within
4791 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4792 stripped (which potential users of C<pop> would do well to remember).
4793 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4794 had been specified. Note that splitting an EXPR that evaluates to the
4795 empty string always returns the empty list, regardless of the LIMIT
4798 A pattern matching the null string (not to be confused with
4799 a null pattern C<//>, which is just one member of the set of patterns
4800 matching a null string) will split the value of EXPR into separate
4801 characters at each point it matches that way. For example:
4803 print join(':', split(/ */, 'hi there'));
4805 produces the output 'h:i:t:h:e:r:e'.
4807 Using the empty pattern C<//> specifically matches the null string, and is
4808 not be confused with the use of C<//> to mean "the last successful pattern
4811 Empty leading (or trailing) fields are produced when there are positive width
4812 matches at the beginning (or end) of the string; a zero-width match at the
4813 beginning (or end) of the string does not produce an empty field. For
4816 print join(':', split(/(?=\w)/, 'hi there!'));
4818 produces the output 'h:i :t:h:e:r:e!'.
4820 The LIMIT parameter can be used to split a line partially
4822 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4824 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4825 one larger than the number of variables in the list, to avoid
4826 unnecessary work. For the list above LIMIT would have been 4 by
4827 default. In time critical applications it behooves you not to split
4828 into more fields than you really need.
4830 If the PATTERN contains parentheses, additional list elements are
4831 created from each matching substring in the delimiter.
4833 split(/([,-])/, "1-10,20", 3);
4835 produces the list value
4837 (1, '-', 10, ',', 20)
4839 If you had the entire header of a normal Unix email message in $header,
4840 you could split it up into fields and their values this way:
4842 $header =~ s/\n\s+/ /g; # fix continuation lines
4843 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4845 The pattern C</PATTERN/> may be replaced with an expression to specify
4846 patterns that vary at runtime. (To do runtime compilation only once,
4847 use C</$variable/o>.)
4849 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4850 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4851 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4852 will give you as many null initial fields as there are leading spaces.
4853 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4854 whitespace produces a null first field. A C<split> with no arguments
4855 really does a S<C<split(' ', $_)>> internally.
4857 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4862 open(PASSWD, '/etc/passwd');
4865 ($login, $passwd, $uid, $gid,
4866 $gcos, $home, $shell) = split(/:/);
4870 As with regular pattern matching, any capturing parentheses that are not
4871 matched in a C<split()> will be set to C<undef> when returned:
4873 @fields = split /(A)|B/, "1A2B3";
4874 # @fields is (1, 'A', 2, undef, 3)
4876 =item sprintf FORMAT, LIST
4878 Returns a string formatted by the usual C<printf> conventions of the C
4879 library function C<sprintf>. See below for more details
4880 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4881 the general principles.
4885 # Format number with up to 8 leading zeroes
4886 $result = sprintf("%08d", $number);
4888 # Round number to 3 digits after decimal point
4889 $rounded = sprintf("%.3f", $number);
4891 Perl does its own C<sprintf> formatting--it emulates the C
4892 function C<sprintf>, but it doesn't use it (except for floating-point
4893 numbers, and even then only the standard modifiers are allowed). As a
4894 result, any non-standard extensions in your local C<sprintf> are not
4895 available from Perl.
4897 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4898 pass it an array as your first argument. The array is given scalar context,
4899 and instead of using the 0th element of the array as the format, Perl will
4900 use the count of elements in the array as the format, which is almost never
4903 Perl's C<sprintf> permits the following universally-known conversions:
4906 %c a character with the given number
4908 %d a signed integer, in decimal
4909 %u an unsigned integer, in decimal
4910 %o an unsigned integer, in octal
4911 %x an unsigned integer, in hexadecimal
4912 %e a floating-point number, in scientific notation
4913 %f a floating-point number, in fixed decimal notation
4914 %g a floating-point number, in %e or %f notation
4916 In addition, Perl permits the following widely-supported conversions:
4918 %X like %x, but using upper-case letters
4919 %E like %e, but using an upper-case "E"
4920 %G like %g, but with an upper-case "E" (if applicable)
4921 %b an unsigned integer, in binary
4922 %p a pointer (outputs the Perl value's address in hexadecimal)
4923 %n special: *stores* the number of characters output so far
4924 into the next variable in the parameter list
4926 Finally, for backward (and we do mean "backward") compatibility, Perl
4927 permits these unnecessary but widely-supported conversions:
4930 %D a synonym for %ld
4931 %U a synonym for %lu
4932 %O a synonym for %lo
4935 Note that the number of exponent digits in the scientific notation produced
4936 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4937 exponent less than 100 is system-dependent: it may be three or less
4938 (zero-padded as necessary). In other words, 1.23 times ten to the
4939 99th may be either "1.23e99" or "1.23e099".
4941 Between the C<%> and the format letter, you may specify a number of
4942 additional attributes controlling the interpretation of the format.
4943 In order, these are:
4947 =item format parameter index
4949 An explicit format parameter index, such as C<2$>. By default sprintf
4950 will format the next unused argument in the list, but this allows you
4951 to take the arguments out of order. Eg:
4953 printf '%2$d %1$d', 12, 34; # prints "34 12"
4954 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4959 space prefix positive number with a space
4960 + prefix positive number with a plus sign
4961 - left-justify within the field
4962 0 use zeros, not spaces, to right-justify
4963 # prefix non-zero octal with "0", non-zero hex with "0x",
4964 non-zero binary with "0b"
4968 printf '<% d>', 12; # prints "< 12>"
4969 printf '<%+d>', 12; # prints "<+12>"
4970 printf '<%6s>', 12; # prints "< 12>"
4971 printf '<%-6s>', 12; # prints "<12 >"
4972 printf '<%06s>', 12; # prints "<000012>"
4973 printf '<%#x>', 12; # prints "<0xc>"
4977 The vector flag C<v>, optionally specifying the join string to use.
4978 This flag tells perl to interpret the supplied string as a vector
4979 of integers, one for each character in the string, separated by
4980 a given string (a dot C<.> by default). This can be useful for
4981 displaying ordinal values of characters in arbitrary strings:
4983 printf "version is v%vd\n", $^V; # Perl's version
4985 Put an asterisk C<*> before the C<v> to override the string to
4986 use to separate the numbers:
4988 printf "address is %*vX\n", ":", $addr; # IPv6 address
4989 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
4991 You can also explicitly specify the argument number to use for
4992 the join string using eg C<*2$v>:
4994 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
4996 =item (minimum) width
4998 Arguments are usually formatted to be only as wide as required to
4999 display the given value. You can override the width by putting
5000 a number here, or get the width from the next argument (with C<*>)
5001 or from a specified argument (with eg C<*2$>):
5003 printf '<%s>', "a"; # prints "<a>"
5004 printf '<%6s>', "a"; # prints "< a>"
5005 printf '<%*s>', 6, "a"; # prints "< a>"
5006 printf '<%*2$s>', "a", 6; # prints "< a>"
5007 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5009 If a field width obtained through C<*> is negative, it has the same
5010 effect as the C<-> flag: left-justification.
5012 =item precision, or maximum width
5014 You can specify a precision (for numeric conversions) or a maximum
5015 width (for string conversions) by specifying a C<.> followed by a number.
5016 For floating point formats, with the exception of 'g' and 'G', this specifies
5017 the number of decimal places to show (the default being 6), eg:
5019 # these examples are subject to system-specific variation
5020 printf '<%f>', 1; # prints "<1.000000>"
5021 printf '<%.1f>', 1; # prints "<1.0>"
5022 printf '<%.0f>', 1; # prints "<1>"
5023 printf '<%e>', 10; # prints "<1.000000e+01>"
5024 printf '<%.1e>', 10; # prints "<1.0e+01>"
5026 For 'g' and 'G', this specifies the maximum number of digits to show,
5027 including prior to the decimal point as well as after it, eg:
5029 # these examples are subject to system-specific variation
5030 printf '<%g>', 1; # prints "<1>"
5031 printf '<%.10g>', 1; # prints "<1>"
5032 printf '<%g>', 100; # prints "<100>"
5033 printf '<%.1g>', 100; # prints "<1e+02>"
5034 printf '<%.2g>', 100.01; # prints "<1e+02>"
5035 printf '<%.5g>', 100.01; # prints "<100.01>"
5036 printf '<%.4g>', 100.01; # prints "<100>"
5038 For integer conversions, specifying a precision implies that the
5039 output of the number itself should be zero-padded to this width:
5041 printf '<%.6x>', 1; # prints "<000001>"
5042 printf '<%#.6x>', 1; # prints "<0x000001>"
5043 printf '<%-10.6x>', 1; # prints "<000001 >"
5045 For string conversions, specifying a precision truncates the string
5046 to fit in the specified width:
5048 printf '<%.5s>', "truncated"; # prints "<trunc>"
5049 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5051 You can also get the precision from the next argument using C<.*>:
5053 printf '<%.6x>', 1; # prints "<000001>"
5054 printf '<%.*x>', 6, 1; # prints "<000001>"
5056 You cannot currently get the precision from a specified number,
5057 but it is intended that this will be possible in the future using
5060 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5064 For numeric conversions, you can specify the size to interpret the
5065 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5066 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5067 whatever the default integer size is on your platform (usually 32 or 64
5068 bits), but you can override this to use instead one of the standard C types,
5069 as supported by the compiler used to build Perl:
5071 l interpret integer as C type "long" or "unsigned long"
5072 h interpret integer as C type "short" or "unsigned short"
5073 q, L or ll interpret integer as C type "long long", "unsigned long long".
5074 or "quads" (typically 64-bit integers)
5076 The last will produce errors if Perl does not understand "quads" in your
5077 installation. (This requires that either the platform natively supports quads
5078 or Perl was specifically compiled to support quads.) You can find out
5079 whether your Perl supports quads via L<Config>:
5082 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5085 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5086 to be the default floating point size on your platform (double or long double),
5087 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5088 platform supports them. You can find out whether your Perl supports long
5089 doubles via L<Config>:
5092 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5094 You can find out whether Perl considers 'long double' to be the default
5095 floating point size to use on your platform via L<Config>:
5098 ($Config{uselongdouble} eq 'define') &&
5099 print "long doubles by default\n";
5101 It can also be the case that long doubles and doubles are the same thing:
5104 ($Config{doublesize} == $Config{longdblsize}) &&
5105 print "doubles are long doubles\n";
5107 The size specifier C<V> has no effect for Perl code, but it is supported
5108 for compatibility with XS code; it means 'use the standard size for
5109 a Perl integer (or floating-point number)', which is already the
5110 default for Perl code.
5112 =item order of arguments
5114 Normally, sprintf takes the next unused argument as the value to
5115 format for each format specification. If the format specification
5116 uses C<*> to require additional arguments, these are consumed from
5117 the argument list in the order in which they appear in the format
5118 specification I<before> the value to format. Where an argument is
5119 specified using an explicit index, this does not affect the normal
5120 order for the arguments (even when the explicitly specified index
5121 would have been the next argument in any case).
5125 printf '<%*.*s>', $a, $b, $c;
5127 would use C<$a> for the width, C<$b> for the precision and C<$c>
5128 as the value to format, while:
5130 print '<%*1$.*s>', $a, $b;
5132 would use C<$a> for the width and the precision, and C<$b> as the
5135 Here are some more examples - beware that when using an explicit
5136 index, the C<$> may need to be escaped:
5138 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5139 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5140 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5141 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5145 If C<use locale> is in effect, the character used for the decimal
5146 point in formatted real numbers is affected by the LC_NUMERIC locale.
5153 Return the square root of EXPR. If EXPR is omitted, returns square
5154 root of C<$_>. Only works on non-negative operands, unless you've
5155 loaded the standard Math::Complex module.
5158 print sqrt(-2); # prints 1.4142135623731i
5164 Sets the random number seed for the C<rand> operator.
5166 The point of the function is to "seed" the C<rand> function so that
5167 C<rand> can produce a different sequence each time you run your
5170 If srand() is not called explicitly, it is called implicitly at the
5171 first use of the C<rand> operator. However, this was not the case in
5172 versions of Perl before 5.004, so if your script will run under older
5173 Perl versions, it should call C<srand>.
5175 Most programs won't even call srand() at all, except those that
5176 need a cryptographically-strong starting point rather than the
5177 generally acceptable default, which is based on time of day,
5178 process ID, and memory allocation, or the F</dev/urandom> device,
5181 You can call srand($seed) with the same $seed to reproduce the
5182 I<same> sequence from rand(), but this is usually reserved for
5183 generating predictable results for testing or debugging.
5184 Otherwise, don't call srand() more than once in your program.
5186 Do B<not> call srand() (i.e. without an argument) more than once in
5187 a script. The internal state of the random number generator should
5188 contain more entropy than can be provided by any seed, so calling
5189 srand() again actually I<loses> randomness.
5191 Most implementations of C<srand> take an integer and will silently
5192 truncate decimal numbers. This means C<srand(42)> will usually
5193 produce the same results as C<srand(42.1)>. To be safe, always pass
5194 C<srand> an integer.
5196 In versions of Perl prior to 5.004 the default seed was just the
5197 current C<time>. This isn't a particularly good seed, so many old
5198 programs supply their own seed value (often C<time ^ $$> or C<time ^
5199 ($$ + ($$ << 15))>), but that isn't necessary any more.
5201 Note that you need something much more random than the default seed for
5202 cryptographic purposes. Checksumming the compressed output of one or more
5203 rapidly changing operating system status programs is the usual method. For
5206 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5208 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5211 Frequently called programs (like CGI scripts) that simply use
5215 for a seed can fall prey to the mathematical property that
5219 one-third of the time. So don't do that.
5221 =item stat FILEHANDLE
5227 Returns a 13-element list giving the status info for a file, either
5228 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5229 it stats C<$_>. Returns a null list if the stat fails. Typically used
5232 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5233 $atime,$mtime,$ctime,$blksize,$blocks)
5236 Not all fields are supported on all filesystem types. Here are the
5237 meaning of the fields:
5239 0 dev device number of filesystem
5241 2 mode file mode (type and permissions)
5242 3 nlink number of (hard) links to the file
5243 4 uid numeric user ID of file's owner
5244 5 gid numeric group ID of file's owner
5245 6 rdev the device identifier (special files only)
5246 7 size total size of file, in bytes
5247 8 atime last access time in seconds since the epoch
5248 9 mtime last modify time in seconds since the epoch
5249 10 ctime inode change time in seconds since the epoch (*)
5250 11 blksize preferred block size for file system I/O
5251 12 blocks actual number of blocks allocated
5253 (The epoch was at 00:00 January 1, 1970 GMT.)
5255 (*) The ctime field is non-portable, in particular you cannot expect
5256 it to be a "creation time", see L<perlport/"Files and Filesystems">
5259 If stat is passed the special filehandle consisting of an underline, no
5260 stat is done, but the current contents of the stat structure from the
5261 last stat or filetest are returned. Example:
5263 if (-x $file && (($d) = stat(_)) && $d < 0) {
5264 print "$file is executable NFS file\n";
5267 (This works on machines only for which the device number is negative
5270 Because the mode contains both the file type and its permissions, you
5271 should mask off the file type portion and (s)printf using a C<"%o">
5272 if you want to see the real permissions.
5274 $mode = (stat($filename))[2];
5275 printf "Permissions are %04o\n", $mode & 07777;
5277 In scalar context, C<stat> returns a boolean value indicating success
5278 or failure, and, if successful, sets the information associated with
5279 the special filehandle C<_>.
5281 The File::stat module provides a convenient, by-name access mechanism:
5284 $sb = stat($filename);
5285 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5286 $filename, $sb->size, $sb->mode & 07777,
5287 scalar localtime $sb->mtime;
5289 You can import symbolic mode constants (C<S_IF*>) and functions
5290 (C<S_IS*>) from the Fcntl module:
5294 $mode = (stat($filename))[2];
5296 $user_rwx = ($mode & S_IRWXU) >> 6;
5297 $group_read = ($mode & S_IRGRP) >> 3;
5298 $other_execute = $mode & S_IXOTH;
5300 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5302 $is_setuid = $mode & S_ISUID;
5303 $is_setgid = S_ISDIR($mode);
5305 You could write the last two using the C<-u> and C<-d> operators.
5306 The commonly available S_IF* constants are
5308 # Permissions: read, write, execute, for user, group, others.
5310 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5311 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5312 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5314 # Setuid/Setgid/Stickiness/SaveText.
5315 # Note that the exact meaning of these is system dependent.
5317 S_ISUID S_ISGID S_ISVTX S_ISTXT
5319 # File types. Not necessarily all are available on your system.
5321 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5323 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5325 S_IREAD S_IWRITE S_IEXEC
5327 and the S_IF* functions are
5329 S_IMODE($mode) the part of $mode containing the permission bits
5330 and the setuid/setgid/sticky bits
5332 S_IFMT($mode) the part of $mode containing the file type
5333 which can be bit-anded with e.g. S_IFREG
5334 or with the following functions
5336 # The operators -f, -d, -l, -b, -c, -p, and -s.
5338 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5339 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5341 # No direct -X operator counterpart, but for the first one
5342 # the -g operator is often equivalent. The ENFMT stands for
5343 # record flocking enforcement, a platform-dependent feature.
5345 S_ISENFMT($mode) S_ISWHT($mode)
5347 See your native chmod(2) and stat(2) documentation for more details
5348 about the S_* constants. To get status info for a symbolic link
5349 instead of the target file behind the link, use the C<lstat> function.
5355 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5356 doing many pattern matches on the string before it is next modified.
5357 This may or may not save time, depending on the nature and number of
5358 patterns you are searching on, and on the distribution of character
5359 frequencies in the string to be searched--you probably want to compare
5360 run times with and without it to see which runs faster. Those loops
5361 which scan for many short constant strings (including the constant
5362 parts of more complex patterns) will benefit most. You may have only
5363 one C<study> active at a time--if you study a different scalar the first
5364 is "unstudied". (The way C<study> works is this: a linked list of every
5365 character in the string to be searched is made, so we know, for
5366 example, where all the C<'k'> characters are. From each search string,
5367 the rarest character is selected, based on some static frequency tables
5368 constructed from some C programs and English text. Only those places
5369 that contain this "rarest" character are examined.)
5371 For example, here is a loop that inserts index producing entries
5372 before any line containing a certain pattern:
5376 print ".IX foo\n" if /\bfoo\b/;
5377 print ".IX bar\n" if /\bbar\b/;
5378 print ".IX blurfl\n" if /\bblurfl\b/;
5383 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5384 will be looked at, because C<f> is rarer than C<o>. In general, this is
5385 a big win except in pathological cases. The only question is whether
5386 it saves you more time than it took to build the linked list in the
5389 Note that if you have to look for strings that you don't know till
5390 runtime, you can build an entire loop as a string and C<eval> that to
5391 avoid recompiling all your patterns all the time. Together with
5392 undefining C<$/> to input entire files as one record, this can be very
5393 fast, often faster than specialized programs like fgrep(1). The following
5394 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5395 out the names of those files that contain a match:
5397 $search = 'while (<>) { study;';
5398 foreach $word (@words) {
5399 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5404 eval $search; # this screams
5405 $/ = "\n"; # put back to normal input delimiter
5406 foreach $file (sort keys(%seen)) {
5410 =item sub NAME BLOCK
5412 =item sub NAME (PROTO) BLOCK
5414 =item sub NAME : ATTRS BLOCK
5416 =item sub NAME (PROTO) : ATTRS BLOCK
5418 This is subroutine definition, not a real function I<per se>.
5419 Without a BLOCK it's just a forward declaration. Without a NAME,
5420 it's an anonymous function declaration, and does actually return
5421 a value: the CODE ref of the closure you just created.
5423 See L<perlsub> and L<perlref> for details about subroutines and
5424 references, and L<attributes> and L<Attribute::Handlers> for more
5425 information about attributes.
5427 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5429 =item substr EXPR,OFFSET,LENGTH
5431 =item substr EXPR,OFFSET
5433 Extracts a substring out of EXPR and returns it. First character is at
5434 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5435 If OFFSET is negative (or more precisely, less than C<$[>), starts
5436 that far from the end of the string. If LENGTH is omitted, returns
5437 everything to the end of the string. If LENGTH is negative, leaves that
5438 many characters off the end of the string.
5440 You can use the substr() function as an lvalue, in which case EXPR
5441 must itself be an lvalue. If you assign something shorter than LENGTH,
5442 the string will shrink, and if you assign something longer than LENGTH,
5443 the string will grow to accommodate it. To keep the string the same
5444 length you may need to pad or chop your value using C<sprintf>.
5446 If OFFSET and LENGTH specify a substring that is partly outside the
5447 string, only the part within the string is returned. If the substring
5448 is beyond either end of the string, substr() returns the undefined
5449 value and produces a warning. When used as an lvalue, specifying a
5450 substring that is entirely outside the string is a fatal error.
5451 Here's an example showing the behavior for boundary cases:
5454 substr($name, 4) = 'dy'; # $name is now 'freddy'
5455 my $null = substr $name, 6, 2; # returns '' (no warning)
5456 my $oops = substr $name, 7; # returns undef, with warning
5457 substr($name, 7) = 'gap'; # fatal error
5459 An alternative to using substr() as an lvalue is to specify the
5460 replacement string as the 4th argument. This allows you to replace
5461 parts of the EXPR and return what was there before in one operation,
5462 just as you can with splice().
5464 =item symlink OLDFILE,NEWFILE
5466 Creates a new filename symbolically linked to the old filename.
5467 Returns C<1> for success, C<0> otherwise. On systems that don't support
5468 symbolic links, produces a fatal error at run time. To check for that,
5471 $symlink_exists = eval { symlink("",""); 1 };
5475 Calls the system call specified as the first element of the list,
5476 passing the remaining elements as arguments to the system call. If
5477 unimplemented, produces a fatal error. The arguments are interpreted
5478 as follows: if a given argument is numeric, the argument is passed as
5479 an int. If not, the pointer to the string value is passed. You are
5480 responsible to make sure a string is pre-extended long enough to
5481 receive any result that might be written into a string. You can't use a
5482 string literal (or other read-only string) as an argument to C<syscall>
5483 because Perl has to assume that any string pointer might be written
5485 integer arguments are not literals and have never been interpreted in a
5486 numeric context, you may need to add C<0> to them to force them to look
5487 like numbers. This emulates the C<syswrite> function (or vice versa):
5489 require 'syscall.ph'; # may need to run h2ph
5491 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5493 Note that Perl supports passing of up to only 14 arguments to your system call,
5494 which in practice should usually suffice.
5496 Syscall returns whatever value returned by the system call it calls.
5497 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5498 Note that some system calls can legitimately return C<-1>. The proper
5499 way to handle such calls is to assign C<$!=0;> before the call and
5500 check the value of C<$!> if syscall returns C<-1>.
5502 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5503 number of the read end of the pipe it creates. There is no way
5504 to retrieve the file number of the other end. You can avoid this
5505 problem by using C<pipe> instead.
5507 =item sysopen FILEHANDLE,FILENAME,MODE
5509 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5511 Opens the file whose filename is given by FILENAME, and associates it
5512 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5513 the name of the real filehandle wanted. This function calls the
5514 underlying operating system's C<open> function with the parameters
5515 FILENAME, MODE, PERMS.
5517 The possible values and flag bits of the MODE parameter are
5518 system-dependent; they are available via the standard module C<Fcntl>.
5519 See the documentation of your operating system's C<open> to see which
5520 values and flag bits are available. You may combine several flags
5521 using the C<|>-operator.
5523 Some of the most common values are C<O_RDONLY> for opening the file in
5524 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5525 and C<O_RDWR> for opening the file in read-write mode, and.
5527 For historical reasons, some values work on almost every system
5528 supported by perl: zero means read-only, one means write-only, and two
5529 means read/write. We know that these values do I<not> work under
5530 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5531 use them in new code.
5533 If the file named by FILENAME does not exist and the C<open> call creates
5534 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5535 PERMS specifies the permissions of the newly created file. If you omit
5536 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5537 These permission values need to be in octal, and are modified by your
5538 process's current C<umask>.
5540 In many systems the C<O_EXCL> flag is available for opening files in
5541 exclusive mode. This is B<not> locking: exclusiveness means here that
5542 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5545 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5547 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5548 that takes away the user's option to have a more permissive umask.
5549 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5552 Note that C<sysopen> depends on the fdopen() C library function.
5553 On many UNIX systems, fdopen() is known to fail when file descriptors
5554 exceed a certain value, typically 255. If you need more file
5555 descriptors than that, consider rebuilding Perl to use the C<sfio>
5556 library, or perhaps using the POSIX::open() function.
5558 See L<perlopentut> for a kinder, gentler explanation of opening files.
5560 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5562 =item sysread FILEHANDLE,SCALAR,LENGTH
5564 Attempts to read LENGTH I<characters> of data into variable SCALAR
5565 from the specified FILEHANDLE, using the system call read(2). It
5566 bypasses buffered IO, so mixing this with other kinds of reads,
5567 C<print>, C<write>, C<seek>, C<tell>, or C<eof> can cause confusion
5568 because stdio usually buffers data. Returns the number of characters
5569 actually read, C<0> at end of file, or undef if there was an error (in
5570 the latter case C<$!> is also set). SCALAR will be grown or shrunk so
5571 that the last byte actually read is the last byte of the scalar after
5574 Note the I<characters>: depending on the status of the filehandle,
5575 either (8-bit) bytes or characters are read. By default all
5576 filehandles operate on bytes, but for example if the filehandle has
5577 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5578 pragma, L<open>), the I/O will operate on characters, not bytes.
5580 An OFFSET may be specified to place the read data at some place in the
5581 string other than the beginning. A negative OFFSET specifies
5582 placement at that many characters counting backwards from the end of
5583 the string. A positive OFFSET greater than the length of SCALAR
5584 results in the string being padded to the required size with C<"\0">
5585 bytes before the result of the read is appended.
5587 There is no syseof() function, which is ok, since eof() doesn't work
5588 very well on device files (like ttys) anyway. Use sysread() and check
5589 for a return value for 0 to decide whether you're done.
5591 =item sysseek FILEHANDLE,POSITION,WHENCE
5593 Sets FILEHANDLE's system position I<in bytes> using the system call
5594 lseek(2). FILEHANDLE may be an expression whose value gives the name
5595 of the filehandle. The values for WHENCE are C<0> to set the new
5596 position to POSITION, C<1> to set the it to the current position plus
5597 POSITION, and C<2> to set it to EOF plus POSITION (typically
5600 Note the I<in bytes>: even if the filehandle has been set to operate
5601 on characters (for example by using the C<:utf8> I/O layer), tell()
5602 will return byte offsets, not character offsets (because implementing
5603 that would render sysseek() very slow).
5605 sysseek() bypasses normal buffered io, so mixing this with reads (other
5606 than C<sysread>, for example >< or read()) C<print>, C<write>,
5607 C<seek>, C<tell>, or C<eof> may cause confusion.
5609 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5610 and C<SEEK_END> (start of the file, current position, end of the file)
5611 from the Fcntl module. Use of the constants is also more portable
5612 than relying on 0, 1, and 2. For example to define a "systell" function:
5614 use Fnctl 'SEEK_CUR';
5615 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5617 Returns the new position, or the undefined value on failure. A position
5618 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5619 true on success and false on failure, yet you can still easily determine
5624 =item system PROGRAM LIST
5626 Does exactly the same thing as C<exec LIST>, except that a fork is
5627 done first, and the parent process waits for the child process to
5628 complete. Note that argument processing varies depending on the
5629 number of arguments. If there is more than one argument in LIST,
5630 or if LIST is an array with more than one value, starts the program
5631 given by the first element of the list with arguments given by the
5632 rest of the list. If there is only one scalar argument, the argument
5633 is checked for shell metacharacters, and if there are any, the
5634 entire argument is passed to the system's command shell for parsing
5635 (this is C</bin/sh -c> on Unix platforms, but varies on other
5636 platforms). If there are no shell metacharacters in the argument,
5637 it is split into words and passed directly to C<execvp>, which is
5640 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5641 output before any operation that may do a fork, but this may not be
5642 supported on some platforms (see L<perlport>). To be safe, you may need
5643 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5644 of C<IO::Handle> on any open handles.
5646 The return value is the exit status of the program as returned by the
5647 C<wait> call. To get the actual exit value shift right by eight (see below).
5648 See also L</exec>. This is I<not> what you want to use to capture
5649 the output from a command, for that you should use merely backticks or
5650 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5651 indicates a failure to start the program (inspect $! for the reason).
5653 Like C<exec>, C<system> allows you to lie to a program about its name if
5654 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5656 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5657 killing the program they're running doesn't actually interrupt
5660 @args = ("command", "arg1", "arg2");
5662 or die "system @args failed: $?"
5664 You can check all the failure possibilities by inspecting
5667 $exit_value = $? >> 8;
5668 $signal_num = $? & 127;
5669 $dumped_core = $? & 128;
5671 or more portably by using the W*() calls of the POSIX extension;
5672 see L<perlport> for more information.
5674 When the arguments get executed via the system shell, results
5675 and return codes will be subject to its quirks and capabilities.
5676 See L<perlop/"`STRING`"> and L</exec> for details.
5678 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5680 =item syswrite FILEHANDLE,SCALAR,LENGTH
5682 =item syswrite FILEHANDLE,SCALAR
5684 Attempts to write LENGTH characters of data from variable SCALAR to
5685 the specified FILEHANDLE, using the system call write(2). If LENGTH
5686 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5687 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5688 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5689 buffers data. Returns the number of characters actually written, or
5690 C<undef> if there was an error (in this case the errno variable C<$!>
5691 is also set). If the LENGTH is greater than the available data in the
5692 SCALAR after the OFFSET, only as much data as is available will be
5695 An OFFSET may be specified to write the data from some part of the
5696 string other than the beginning. A negative OFFSET specifies writing
5697 that many characters counting backwards from the end of the string.
5698 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5700 Note the I<characters>: depending on the status of the filehandle,
5701 either (8-bit) bytes or characters are written. By default all
5702 filehandles operate on bytes, but for example if the filehandle has
5703 been opened with the C<:utf8> I/O layer (see L</open>, and the open
5704 pragma, L<open>), the I/O will operate on characters, not bytes.
5706 =item tell FILEHANDLE
5710 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5711 error. FILEHANDLE may be an expression whose value gives the name of
5712 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5715 Note the I<in bytes>: even if the filehandle has been set to
5716 operate on characters (for example by using the C<:utf8> open
5717 layer), tell() will return byte offsets, not character offsets
5718 (because that would render seek() and tell() rather slow).
5720 The return value of tell() for the standard streams like the STDIN
5721 depends on the operating system: it may return -1 or something else.
5722 tell() on pipes, fifos, and sockets usually returns -1.
5724 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5726 Do not use tell() on a filehandle that has been opened using
5727 sysopen(), use sysseek() for that as described above. Why? Because
5728 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5729 buffered filehandles. sysseek() make sense only on the first kind,
5730 tell() only makes sense on the second kind.
5732 =item telldir DIRHANDLE
5734 Returns the current position of the C<readdir> routines on DIRHANDLE.
5735 Value may be given to C<seekdir> to access a particular location in a
5736 directory. Has the same caveats about possible directory compaction as
5737 the corresponding system library routine.
5739 =item tie VARIABLE,CLASSNAME,LIST
5741 This function binds a variable to a package class that will provide the
5742 implementation for the variable. VARIABLE is the name of the variable
5743 to be enchanted. CLASSNAME is the name of a class implementing objects
5744 of correct type. Any additional arguments are passed to the C<new>
5745 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5746 or C<TIEHASH>). Typically these are arguments such as might be passed
5747 to the C<dbm_open()> function of C. The object returned by the C<new>
5748 method is also returned by the C<tie> function, which would be useful
5749 if you want to access other methods in CLASSNAME.
5751 Note that functions such as C<keys> and C<values> may return huge lists
5752 when used on large objects, like DBM files. You may prefer to use the
5753 C<each> function to iterate over such. Example:
5755 # print out history file offsets
5757 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5758 while (($key,$val) = each %HIST) {
5759 print $key, ' = ', unpack('L',$val), "\n";
5763 A class implementing a hash should have the following methods:
5765 TIEHASH classname, LIST
5767 STORE this, key, value
5772 NEXTKEY this, lastkey
5776 A class implementing an ordinary array should have the following methods:
5778 TIEARRAY classname, LIST
5780 STORE this, key, value
5782 STORESIZE this, count
5788 SPLICE this, offset, length, LIST
5793 A class implementing a file handle should have the following methods:
5795 TIEHANDLE classname, LIST
5796 READ this, scalar, length, offset
5799 WRITE this, scalar, length, offset
5801 PRINTF this, format, LIST
5805 SEEK this, position, whence
5807 OPEN this, mode, LIST
5812 A class implementing a scalar should have the following methods:
5814 TIESCALAR classname, LIST
5820 Not all methods indicated above need be implemented. See L<perltie>,
5821 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5823 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5824 for you--you need to do that explicitly yourself. See L<DB_File>
5825 or the F<Config> module for interesting C<tie> implementations.
5827 For further details see L<perltie>, L<"tied VARIABLE">.
5831 Returns a reference to the object underlying VARIABLE (the same value
5832 that was originally returned by the C<tie> call that bound the variable
5833 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5838 Returns the number of non-leap seconds since whatever time the system
5839 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5840 and 00:00:00 UTC, January 1, 1970 for most other systems).
5841 Suitable for feeding to C<gmtime> and C<localtime>.
5843 For measuring time in better granularity than one second,
5844 you may use either the Time::HiRes module (from CPAN, and starting from
5845 Perl 5.8 part of the standard distribution), or if you have
5846 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5847 See L<perlfaq8> for details.
5851 Returns a four-element list giving the user and system times, in
5852 seconds, for this process and the children of this process.
5854 ($user,$system,$cuser,$csystem) = times;
5856 In scalar context, C<times> returns C<$user>.
5860 The transliteration operator. Same as C<y///>. See L<perlop>.
5862 =item truncate FILEHANDLE,LENGTH
5864 =item truncate EXPR,LENGTH
5866 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5867 specified length. Produces a fatal error if truncate isn't implemented
5868 on your system. Returns true if successful, the undefined value
5871 The behavior is undefined if LENGTH is greater than the length of the
5878 Returns an uppercased version of EXPR. This is the internal function
5879 implementing the C<\U> escape in double-quoted strings. Respects
5880 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5881 and L<perlunicode> for more details about locale and Unicode support.
5882 It does not attempt to do titlecase mapping on initial letters. See
5883 C<ucfirst> for that.
5885 If EXPR is omitted, uses C<$_>.
5891 Returns the value of EXPR with the first character in uppercase
5892 (titlecase in Unicode). This is the internal function implementing
5893 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5894 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5895 for more details about locale and Unicode support.
5897 If EXPR is omitted, uses C<$_>.
5903 Sets the umask for the process to EXPR and returns the previous value.
5904 If EXPR is omitted, merely returns the current umask.
5906 The Unix permission C<rwxr-x---> is represented as three sets of three
5907 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5908 and isn't one of the digits). The C<umask> value is such a number
5909 representing disabled permissions bits. The permission (or "mode")
5910 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5911 even if you tell C<sysopen> to create a file with permissions C<0777>,
5912 if your umask is C<0022> then the file will actually be created with
5913 permissions C<0755>. If your C<umask> were C<0027> (group can't
5914 write; others can't read, write, or execute), then passing
5915 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5918 Here's some advice: supply a creation mode of C<0666> for regular
5919 files (in C<sysopen>) and one of C<0777> for directories (in
5920 C<mkdir>) and executable files. This gives users the freedom of
5921 choice: if they want protected files, they might choose process umasks
5922 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5923 Programs should rarely if ever make policy decisions better left to
5924 the user. The exception to this is when writing files that should be
5925 kept private: mail files, web browser cookies, I<.rhosts> files, and
5928 If umask(2) is not implemented on your system and you are trying to
5929 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5930 fatal error at run time. If umask(2) is not implemented and you are
5931 not trying to restrict access for yourself, returns C<undef>.
5933 Remember that a umask is a number, usually given in octal; it is I<not> a
5934 string of octal digits. See also L</oct>, if all you have is a string.
5940 Undefines the value of EXPR, which must be an lvalue. Use only on a
5941 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5942 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5943 will probably not do what you expect on most predefined variables or
5944 DBM list values, so don't do that; see L<delete>.) Always returns the
5945 undefined value. You can omit the EXPR, in which case nothing is
5946 undefined, but you still get an undefined value that you could, for
5947 instance, return from a subroutine, assign to a variable or pass as a
5948 parameter. Examples:
5951 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5955 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5956 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5957 select undef, undef, undef, 0.25;
5958 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5960 Note that this is a unary operator, not a list operator.
5966 Deletes a list of files. Returns the number of files successfully
5969 $cnt = unlink 'a', 'b', 'c';
5973 Note: C<unlink> will not delete directories unless you are superuser and
5974 the B<-U> flag is supplied to Perl. Even if these conditions are
5975 met, be warned that unlinking a directory can inflict damage on your
5976 filesystem. Use C<rmdir> instead.
5978 If LIST is omitted, uses C<$_>.
5980 =item unpack TEMPLATE,EXPR
5982 =item unpack TEMPLATE
5984 C<unpack> does the reverse of C<pack>: it takes a string
5985 and expands it out into a list of values.
5986 (In scalar context, it returns merely the first value produced.)
5988 If EXPR is omitted, unpacks the C<$_> string.
5990 The string is broken into chunks described by the TEMPLATE. Each chunk
5991 is converted separately to a value. Typically, either the string is a result
5992 of C<pack>, or the bytes of the string represent a C structure of some
5995 The TEMPLATE has the same format as in the C<pack> function.
5996 Here's a subroutine that does substring:
5999 my($what,$where,$howmuch) = @_;
6000 unpack("x$where a$howmuch", $what);
6005 sub ordinal { unpack("c",$_[0]); } # same as ord()
6007 In addition to fields allowed in pack(), you may prefix a field with
6008 a %<number> to indicate that
6009 you want a <number>-bit checksum of the items instead of the items
6010 themselves. Default is a 16-bit checksum. Checksum is calculated by
6011 summing numeric values of expanded values (for string fields the sum of
6012 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6014 For example, the following
6015 computes the same number as the System V sum program:
6019 unpack("%32C*",<>) % 65535;
6022 The following efficiently counts the number of set bits in a bit vector:
6024 $setbits = unpack("%32b*", $selectmask);
6026 The C<p> and C<P> formats should be used with care. Since Perl
6027 has no way of checking whether the value passed to C<unpack()>
6028 corresponds to a valid memory location, passing a pointer value that's
6029 not known to be valid is likely to have disastrous consequences.
6031 If there are more pack codes or if the repeat count of a field or a group
6032 is larger than what the remainder of the input string allows, the result
6033 is not well defined: in some cases, the repeat count is decreased, or
6034 C<unpack()> will produce null strings or zeroes, or terminate with an
6035 error. If the input string is longer than one described by the TEMPLATE,
6036 the rest is ignored.
6038 See L</pack> for more examples and notes.
6040 =item untie VARIABLE
6042 Breaks the binding between a variable and a package. (See C<tie>.)
6043 Has no effect if the variable is not tied.
6045 =item unshift ARRAY,LIST
6047 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6048 depending on how you look at it. Prepends list to the front of the
6049 array, and returns the new number of elements in the array.
6051 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6053 Note the LIST is prepended whole, not one element at a time, so the
6054 prepended elements stay in the same order. Use C<reverse> to do the
6057 =item use Module VERSION LIST
6059 =item use Module VERSION
6061 =item use Module LIST
6067 Imports some semantics into the current package from the named module,
6068 generally by aliasing certain subroutine or variable names into your
6069 package. It is exactly equivalent to
6071 BEGIN { require Module; import Module LIST; }
6073 except that Module I<must> be a bareword.
6075 VERSION may be either a numeric argument such as 5.006, which will be
6076 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6077 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6078 greater than the version of the current Perl interpreter; Perl will not
6079 attempt to parse the rest of the file. Compare with L</require>, which can
6080 do a similar check at run time.
6082 Specifying VERSION as a literal of the form v5.6.1 should generally be
6083 avoided, because it leads to misleading error messages under earlier
6084 versions of Perl which do not support this syntax. The equivalent numeric
6085 version should be used instead.
6087 use v5.6.1; # compile time version check
6089 use 5.006_001; # ditto; preferred for backwards compatibility
6091 This is often useful if you need to check the current Perl version before
6092 C<use>ing library modules that have changed in incompatible ways from
6093 older versions of Perl. (We try not to do this more than we have to.)
6095 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6096 C<require> makes sure the module is loaded into memory if it hasn't been
6097 yet. The C<import> is not a builtin--it's just an ordinary static method
6098 call into the C<Module> package to tell the module to import the list of
6099 features back into the current package. The module can implement its
6100 C<import> method any way it likes, though most modules just choose to
6101 derive their C<import> method via inheritance from the C<Exporter> class that
6102 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6103 method can be found then the call is skipped.
6105 If you do not want to call the package's C<import> method (for instance,
6106 to stop your namespace from being altered), explicitly supply the empty list:
6110 That is exactly equivalent to
6112 BEGIN { require Module }
6114 If the VERSION argument is present between Module and LIST, then the
6115 C<use> will call the VERSION method in class Module with the given
6116 version as an argument. The default VERSION method, inherited from
6117 the UNIVERSAL class, croaks if the given version is larger than the
6118 value of the variable C<$Module::VERSION>.
6120 Again, there is a distinction between omitting LIST (C<import> called
6121 with no arguments) and an explicit empty LIST C<()> (C<import> not
6122 called). Note that there is no comma after VERSION!
6124 Because this is a wide-open interface, pragmas (compiler directives)
6125 are also implemented this way. Currently implemented pragmas are:
6130 use sigtrap qw(SEGV BUS);
6131 use strict qw(subs vars refs);
6132 use subs qw(afunc blurfl);
6133 use warnings qw(all);
6134 use sort qw(stable _quicksort _mergesort);
6136 Some of these pseudo-modules import semantics into the current
6137 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6138 which import symbols into the current package (which are effective
6139 through the end of the file).
6141 There's a corresponding C<no> command that unimports meanings imported
6142 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6148 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6149 for the C<-M> and C<-m> command-line options to perl that give C<use>
6150 functionality from the command-line.
6154 Changes the access and modification times on each file of a list of
6155 files. The first two elements of the list must be the NUMERICAL access
6156 and modification times, in that order. Returns the number of files
6157 successfully changed. The inode change time of each file is set
6158 to the current time. For example, this code has the same effect as the
6159 Unix touch(1) command when the files I<already exist>.
6163 utime $now, $now, @ARGV;
6165 B<Note:> Under NFS, touch(1) uses the time of the NFS server, not
6166 the time of the local machine. If there is a time synchronization
6167 problem, the NFS server and local machine will have different times.
6169 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6170 the utime(2) function in the C library will be called with a null second
6171 argument. On most systems, this will set the file's access and
6172 modification times to the current time (i.e. equivalent to the example
6175 utime undef, undef, @ARGV;
6179 Returns a list consisting of all the values of the named hash. (In a
6180 scalar context, returns the number of values.) The values are
6181 returned in an apparently random order. The actual random order is
6182 subject to change in future versions of perl, but it is guaranteed to
6183 be the same order as either the C<keys> or C<each> function would
6184 produce on the same (unmodified) hash.
6186 Note that the values are not copied, which means modifying them will
6187 modify the contents of the hash:
6189 for (values %hash) { s/foo/bar/g } # modifies %hash values
6190 for (@hash{keys %hash}) { s/foo/bar/g } # same
6192 As a side effect, calling values() resets the HASH's internal iterator.
6193 See also C<keys>, C<each>, and C<sort>.
6195 =item vec EXPR,OFFSET,BITS
6197 Treats the string in EXPR as a bit vector made up of elements of
6198 width BITS, and returns the value of the element specified by OFFSET
6199 as an unsigned integer. BITS therefore specifies the number of bits
6200 that are reserved for each element in the bit vector. This must
6201 be a power of two from 1 to 32 (or 64, if your platform supports
6204 If BITS is 8, "elements" coincide with bytes of the input string.
6206 If BITS is 16 or more, bytes of the input string are grouped into chunks
6207 of size BITS/8, and each group is converted to a number as with
6208 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6209 for BITS==64). See L<"pack"> for details.
6211 If bits is 4 or less, the string is broken into bytes, then the bits
6212 of each byte are broken into 8/BITS groups. Bits of a byte are
6213 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6214 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6215 breaking the single input byte C<chr(0x36)> into two groups gives a list
6216 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6218 C<vec> may also be assigned to, in which case parentheses are needed
6219 to give the expression the correct precedence as in
6221 vec($image, $max_x * $x + $y, 8) = 3;
6223 If the selected element is outside the string, the value 0 is returned.
6224 If an element off the end of the string is written to, Perl will first
6225 extend the string with sufficiently many zero bytes. It is an error
6226 to try to write off the beginning of the string (i.e. negative OFFSET).
6228 The string should not contain any character with the value > 255 (which
6229 can only happen if you're using UTF8 encoding). If it does, it will be
6230 treated as something which is not UTF8 encoded. When the C<vec> was
6231 assigned to, other parts of your program will also no longer consider the
6232 string to be UTF8 encoded. In other words, if you do have such characters
6233 in your string, vec() will operate on the actual byte string, and not the
6234 conceptual character string.
6236 Strings created with C<vec> can also be manipulated with the logical
6237 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6238 vector operation is desired when both operands are strings.
6239 See L<perlop/"Bitwise String Operators">.
6241 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6242 The comments show the string after each step. Note that this code works
6243 in the same way on big-endian or little-endian machines.
6246 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6248 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6249 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6251 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6252 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6253 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6254 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6255 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6256 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6258 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6259 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6260 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6263 To transform a bit vector into a string or list of 0's and 1's, use these:
6265 $bits = unpack("b*", $vector);
6266 @bits = split(//, unpack("b*", $vector));
6268 If you know the exact length in bits, it can be used in place of the C<*>.
6270 Here is an example to illustrate how the bits actually fall in place:
6276 unpack("V",$_) 01234567890123456789012345678901
6277 ------------------------------------------------------------------
6282 for ($shift=0; $shift < $width; ++$shift) {
6283 for ($off=0; $off < 32/$width; ++$off) {
6284 $str = pack("B*", "0"x32);
6285 $bits = (1<<$shift);
6286 vec($str, $off, $width) = $bits;
6287 $res = unpack("b*",$str);
6288 $val = unpack("V", $str);
6295 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6296 $off, $width, $bits, $val, $res
6300 Regardless of the machine architecture on which it is run, the above
6301 example should print the following table:
6304 unpack("V",$_) 01234567890123456789012345678901
6305 ------------------------------------------------------------------
6306 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6307 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6308 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6309 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6310 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6311 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6312 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6313 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6314 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6315 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6316 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6317 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6318 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6319 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6320 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6321 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6322 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6323 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6324 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6325 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6326 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6327 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6328 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6329 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6330 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6331 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6332 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6333 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6334 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6335 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6336 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6337 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6338 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6339 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6340 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6341 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6342 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6343 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6344 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6345 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6346 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6347 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6348 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6349 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6350 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6351 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6352 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6353 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6354 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6355 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6356 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6357 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6358 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6359 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6360 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6361 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6362 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6363 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6364 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6365 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6366 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6367 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6368 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6369 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6370 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6371 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6372 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6373 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6374 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6375 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6376 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6377 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6378 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6379 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6380 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6381 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6382 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6383 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6384 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6385 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6386 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6387 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6388 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6389 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6390 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6391 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6392 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6393 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6394 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6395 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6396 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6397 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6398 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6399 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6400 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6401 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6402 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6403 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6404 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6405 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6406 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6407 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6408 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6409 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6410 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6411 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6412 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6413 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6414 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6415 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6416 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6417 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6418 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6419 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6420 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6421 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6422 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6423 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6424 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6425 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6426 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6427 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6428 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6429 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6430 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6431 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6432 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6433 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6437 Behaves like the wait(2) system call on your system: it waits for a child
6438 process to terminate and returns the pid of the deceased process, or
6439 C<-1> if there are no child processes. The status is returned in C<$?>.
6440 Note that a return value of C<-1> could mean that child processes are
6441 being automatically reaped, as described in L<perlipc>.
6443 =item waitpid PID,FLAGS
6445 Waits for a particular child process to terminate and returns the pid of
6446 the deceased process, or C<-1> if there is no such child process. On some
6447 systems, a value of 0 indicates that there are processes still running.
6448 The status is returned in C<$?>. If you say
6450 use POSIX ":sys_wait_h";
6453 $kid = waitpid(-1, WNOHANG);
6456 then you can do a non-blocking wait for all pending zombie processes.
6457 Non-blocking wait is available on machines supporting either the
6458 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6459 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6460 system call by remembering the status values of processes that have
6461 exited but have not been harvested by the Perl script yet.)
6463 Note that on some systems, a return value of C<-1> could mean that child
6464 processes are being automatically reaped. See L<perlipc> for details,
6465 and for other examples.
6469 Returns true if the context of the currently executing subroutine is
6470 looking for a list value. Returns false if the context is looking
6471 for a scalar. Returns the undefined value if the context is looking
6472 for no value (void context).
6474 return unless defined wantarray; # don't bother doing more
6475 my @a = complex_calculation();
6476 return wantarray ? @a : "@a";
6478 This function should have been named wantlist() instead.
6482 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6485 If LIST is empty and C<$@> already contains a value (typically from a
6486 previous eval) that value is used after appending C<"\t...caught">
6487 to C<$@>. This is useful for staying almost, but not entirely similar to
6490 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6492 No message is printed if there is a C<$SIG{__WARN__}> handler
6493 installed. It is the handler's responsibility to deal with the message
6494 as it sees fit (like, for instance, converting it into a C<die>). Most
6495 handlers must therefore make arrangements to actually display the
6496 warnings that they are not prepared to deal with, by calling C<warn>
6497 again in the handler. Note that this is quite safe and will not
6498 produce an endless loop, since C<__WARN__> hooks are not called from
6501 You will find this behavior is slightly different from that of
6502 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6503 instead call C<die> again to change it).
6505 Using a C<__WARN__> handler provides a powerful way to silence all
6506 warnings (even the so-called mandatory ones). An example:
6508 # wipe out *all* compile-time warnings
6509 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6511 my $foo = 20; # no warning about duplicate my $foo,
6512 # but hey, you asked for it!
6513 # no compile-time or run-time warnings before here
6516 # run-time warnings enabled after here
6517 warn "\$foo is alive and $foo!"; # does show up
6519 See L<perlvar> for details on setting C<%SIG> entries, and for more
6520 examples. See the Carp module for other kinds of warnings using its
6521 carp() and cluck() functions.
6523 =item write FILEHANDLE
6529 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6530 using the format associated with that file. By default the format for
6531 a file is the one having the same name as the filehandle, but the
6532 format for the current output channel (see the C<select> function) may be set
6533 explicitly by assigning the name of the format to the C<$~> variable.
6535 Top of form processing is handled automatically: if there is
6536 insufficient room on the current page for the formatted record, the
6537 page is advanced by writing a form feed, a special top-of-page format
6538 is used to format the new page header, and then the record is written.
6539 By default the top-of-page format is the name of the filehandle with
6540 "_TOP" appended, but it may be dynamically set to the format of your
6541 choice by assigning the name to the C<$^> variable while the filehandle is
6542 selected. The number of lines remaining on the current page is in
6543 variable C<$->, which can be set to C<0> to force a new page.
6545 If FILEHANDLE is unspecified, output goes to the current default output
6546 channel, which starts out as STDOUT but may be changed by the
6547 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6548 is evaluated and the resulting string is used to look up the name of
6549 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6551 Note that write is I<not> the opposite of C<read>. Unfortunately.
6555 The transliteration operator. Same as C<tr///>. See L<perlop>.