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]>). This set of
874 characters is just a recommendation; the characters allowed in
875 the salt depend solely on your system's crypt library, and Perl can't
876 restrict what salts C<crypt()> accepts.
878 Here's an example that makes sure that whoever runs this program knows
881 $pwd = (getpwuid($<))[1];
885 chomp($word = <STDIN>);
889 if (crypt($word, $pwd) ne $pwd) {
895 Of course, typing in your own password to whoever asks you
898 The L<crypt|/crypt> function is unsuitable for encrypting large quantities
899 of data, not least of all because you can't get the information
900 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
901 on your favorite CPAN mirror for a slew of potentially useful
904 If using crypt() on a Unicode string (which I<potentially> has
905 characters with codepoints above 255), Perl tries to make sense
906 of the situation by trying to downgrade (a copy of the string)
907 the string back to an eight-bit byte string before calling crypt()
908 (on that copy). If that works, good. If not, crypt() dies with
909 C<Wide character in crypt>.
913 [This function has been largely superseded by the C<untie> function.]
915 Breaks the binding between a DBM file and a hash.
917 =item dbmopen HASH,DBNAME,MASK
919 [This function has been largely superseded by the C<tie> function.]
921 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
922 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
923 argument is I<not> a filehandle, even though it looks like one). DBNAME
924 is the name of the database (without the F<.dir> or F<.pag> extension if
925 any). If the database does not exist, it is created with protection
926 specified by MASK (as modified by the C<umask>). If your system supports
927 only the older DBM functions, you may perform only one C<dbmopen> in your
928 program. In older versions of Perl, if your system had neither DBM nor
929 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
932 If you don't have write access to the DBM file, you can only read hash
933 variables, not set them. If you want to test whether you can write,
934 either use file tests or try setting a dummy hash entry inside an C<eval>,
935 which will trap the error.
937 Note that functions such as C<keys> and C<values> may return huge lists
938 when used on large DBM files. You may prefer to use the C<each>
939 function to iterate over large DBM files. Example:
941 # print out history file offsets
942 dbmopen(%HIST,'/usr/lib/news/history',0666);
943 while (($key,$val) = each %HIST) {
944 print $key, ' = ', unpack('L',$val), "\n";
948 See also L<AnyDBM_File> for a more general description of the pros and
949 cons of the various dbm approaches, as well as L<DB_File> for a particularly
952 You can control which DBM library you use by loading that library
953 before you call dbmopen():
956 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
957 or die "Can't open netscape history file: $!";
963 Returns a Boolean value telling whether EXPR has a value other than
964 the undefined value C<undef>. If EXPR is not present, C<$_> will be
967 Many operations return C<undef> to indicate failure, end of file,
968 system error, uninitialized variable, and other exceptional
969 conditions. This function allows you to distinguish C<undef> from
970 other values. (A simple Boolean test will not distinguish among
971 C<undef>, zero, the empty string, and C<"0">, which are all equally
972 false.) Note that since C<undef> is a valid scalar, its presence
973 doesn't I<necessarily> indicate an exceptional condition: C<pop>
974 returns C<undef> when its argument is an empty array, I<or> when the
975 element to return happens to be C<undef>.
977 You may also use C<defined(&func)> to check whether subroutine C<&func>
978 has ever been defined. The return value is unaffected by any forward
979 declarations of C<&func>. Note that a subroutine which is not defined
980 may still be callable: its package may have an C<AUTOLOAD> method that
981 makes it spring into existence the first time that it is called -- see
984 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
985 used to report whether memory for that aggregate has ever been
986 allocated. This behavior may disappear in future versions of Perl.
987 You should instead use a simple test for size:
989 if (@an_array) { print "has array elements\n" }
990 if (%a_hash) { print "has hash members\n" }
992 When used on a hash element, it tells you whether the value is defined,
993 not whether the key exists in the hash. Use L</exists> for the latter
998 print if defined $switch{'D'};
999 print "$val\n" while defined($val = pop(@ary));
1000 die "Can't readlink $sym: $!"
1001 unless defined($value = readlink $sym);
1002 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
1003 $debugging = 0 unless defined $debugging;
1005 Note: Many folks tend to overuse C<defined>, and then are surprised to
1006 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
1007 defined values. For example, if you say
1011 The pattern match succeeds, and C<$1> is defined, despite the fact that it
1012 matched "nothing". But it didn't really match nothing--rather, it
1013 matched something that happened to be zero characters long. This is all
1014 very above-board and honest. When a function returns an undefined value,
1015 it's an admission that it couldn't give you an honest answer. So you
1016 should use C<defined> only when you're questioning the integrity of what
1017 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
1020 See also L</undef>, L</exists>, L</ref>.
1024 Given an expression that specifies a hash element, array element, hash slice,
1025 or array slice, deletes the specified element(s) from the hash or array.
1026 In the case of an array, if the array elements happen to be at the end,
1027 the size of the array will shrink to the highest element that tests
1028 true for exists() (or 0 if no such element exists).
1030 Returns each element so deleted or the undefined value if there was no such
1031 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
1032 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
1033 from a C<tie>d hash or array may not necessarily return anything.
1035 Deleting an array element effectively returns that position of the array
1036 to its initial, uninitialized state. Subsequently testing for the same
1037 element with exists() will return false. Note that deleting array
1038 elements in the middle of an array will not shift the index of the ones
1039 after them down--use splice() for that. See L</exists>.
1041 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
1043 foreach $key (keys %HASH) {
1047 foreach $index (0 .. $#ARRAY) {
1048 delete $ARRAY[$index];
1053 delete @HASH{keys %HASH};
1055 delete @ARRAY[0 .. $#ARRAY];
1057 But both of these are slower than just assigning the empty list
1058 or undefining %HASH or @ARRAY:
1060 %HASH = (); # completely empty %HASH
1061 undef %HASH; # forget %HASH ever existed
1063 @ARRAY = (); # completely empty @ARRAY
1064 undef @ARRAY; # forget @ARRAY ever existed
1066 Note that the EXPR can be arbitrarily complicated as long as the final
1067 operation is a hash element, array element, hash slice, or array slice
1070 delete $ref->[$x][$y]{$key};
1071 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1073 delete $ref->[$x][$y][$index];
1074 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1078 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1079 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1080 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1081 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1082 an C<eval(),> the error message is stuffed into C<$@> and the
1083 C<eval> is terminated with the undefined value. This makes
1084 C<die> the way to raise an exception.
1086 Equivalent examples:
1088 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1089 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1091 If the last element of LIST does not end in a newline, the current
1092 script line number and input line number (if any) are also printed,
1093 and a newline is supplied. Note that the "input line number" (also
1094 known as "chunk") is subject to whatever notion of "line" happens to
1095 be currently in effect, and is also available as the special variable
1096 C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
1098 Hint: sometimes appending C<", stopped"> to your message will cause it
1099 to make better sense when the string C<"at foo line 123"> is appended.
1100 Suppose you are running script "canasta".
1102 die "/etc/games is no good";
1103 die "/etc/games is no good, stopped";
1105 produce, respectively
1107 /etc/games is no good at canasta line 123.
1108 /etc/games is no good, stopped at canasta line 123.
1110 See also exit(), warn(), and the Carp module.
1112 If LIST is empty and C<$@> already contains a value (typically from a
1113 previous eval) that value is reused after appending C<"\t...propagated">.
1114 This is useful for propagating exceptions:
1117 die unless $@ =~ /Expected exception/;
1119 If LIST is empty and C<$@> contains an object reference that has a
1120 C<PROPAGATE> method, that method will be called with additional file
1121 and line number parameters. The return value replaces the value in
1122 C<$@>. ie. as if C<<$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };>>
1125 If C<$@> is empty then the string C<"Died"> is used.
1127 die() can also be called with a reference argument. If this happens to be
1128 trapped within an eval(), $@ contains the reference. This behavior permits
1129 a more elaborate exception handling implementation using objects that
1130 maintain arbitrary state about the nature of the exception. Such a scheme
1131 is sometimes preferable to matching particular string values of $@ using
1132 regular expressions. Here's an example:
1134 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1136 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1137 # handle Some::Module::Exception
1140 # handle all other possible exceptions
1144 Because perl will stringify uncaught exception messages before displaying
1145 them, you may want to overload stringification operations on such custom
1146 exception objects. See L<overload> for details about that.
1148 You can arrange for a callback to be run just before the C<die>
1149 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1150 handler will be called with the error text and can change the error
1151 message, if it sees fit, by calling C<die> again. See
1152 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1153 L<"eval BLOCK"> for some examples. Although this feature was meant
1154 to be run only right before your program was to exit, this is not
1155 currently the case--the C<$SIG{__DIE__}> hook is currently called
1156 even inside eval()ed blocks/strings! If one wants the hook to do
1157 nothing in such situations, put
1161 as the first line of the handler (see L<perlvar/$^S>). Because
1162 this promotes strange action at a distance, this counterintuitive
1163 behavior may be fixed in a future release.
1167 Not really a function. Returns the value of the last command in the
1168 sequence of commands indicated by BLOCK. When modified by a loop
1169 modifier, executes the BLOCK once before testing the loop condition.
1170 (On other statements the loop modifiers test the conditional first.)
1172 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1173 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1174 See L<perlsyn> for alternative strategies.
1176 =item do SUBROUTINE(LIST)
1178 A deprecated form of subroutine call. See L<perlsub>.
1182 Uses the value of EXPR as a filename and executes the contents of the
1183 file as a Perl script. Its primary use is to include subroutines
1184 from a Perl subroutine library.
1192 except that it's more efficient and concise, keeps track of the current
1193 filename for error messages, searches the @INC libraries, and updates
1194 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1195 variables. It also differs in that code evaluated with C<do FILENAME>
1196 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1197 same, however, in that it does reparse the file every time you call it,
1198 so you probably don't want to do this inside a loop.
1200 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1201 error. If C<do> can read the file but cannot compile it, it
1202 returns undef and sets an error message in C<$@>. If the file is
1203 successfully compiled, C<do> returns the value of the last expression
1206 Note that inclusion of library modules is better done with the
1207 C<use> and C<require> operators, which also do automatic error checking
1208 and raise an exception if there's a problem.
1210 You might like to use C<do> to read in a program configuration
1211 file. Manual error checking can be done this way:
1213 # read in config files: system first, then user
1214 for $file ("/share/prog/defaults.rc",
1215 "$ENV{HOME}/.someprogrc")
1217 unless ($return = do $file) {
1218 warn "couldn't parse $file: $@" if $@;
1219 warn "couldn't do $file: $!" unless defined $return;
1220 warn "couldn't run $file" unless $return;
1228 This function causes an immediate core dump. See also the B<-u>
1229 command-line switch in L<perlrun>, which does the same thing.
1230 Primarily this is so that you can use the B<undump> program (not
1231 supplied) to turn your core dump into an executable binary after
1232 having initialized all your variables at the beginning of the
1233 program. When the new binary is executed it will begin by executing
1234 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1235 Think of it as a goto with an intervening core dump and reincarnation.
1236 If C<LABEL> is omitted, restarts the program from the top.
1238 B<WARNING>: Any files opened at the time of the dump will I<not>
1239 be open any more when the program is reincarnated, with possible
1240 resulting confusion on the part of Perl.
1242 This function is now largely obsolete, partly because it's very
1243 hard to convert a core file into an executable, and because the
1244 real compiler backends for generating portable bytecode and compilable
1245 C code have superseded it. That's why you should now invoke it as
1246 C<CORE::dump()>, if you don't want to be warned against a possible
1249 If you're looking to use L<dump> to speed up your program, consider
1250 generating bytecode or native C code as described in L<perlcc>. If
1251 you're just trying to accelerate a CGI script, consider using the
1252 C<mod_perl> extension to B<Apache>, or the CPAN module, CGI::Fast.
1253 You might also consider autoloading or selfloading, which at least
1254 make your program I<appear> to run faster.
1258 When called in list context, returns a 2-element list consisting of the
1259 key and value for the next element of a hash, so that you can iterate over
1260 it. When called in scalar context, returns only the key for the next
1261 element in the hash.
1263 Entries are returned in an apparently random order. The actual random
1264 order is subject to change in future versions of perl, but it is guaranteed
1265 to be in the same order as either the C<keys> or C<values> function
1266 would produce on the same (unmodified) hash.
1268 When the hash is entirely read, a null array is returned in list context
1269 (which when assigned produces a false (C<0>) value), and C<undef> in
1270 scalar context. The next call to C<each> after that will start iterating
1271 again. There is a single iterator for each hash, shared by all C<each>,
1272 C<keys>, and C<values> function calls in the program; it can be reset by
1273 reading all the elements from the hash, or by evaluating C<keys HASH> or
1274 C<values HASH>. If you add or delete elements of a hash while you're
1275 iterating over it, you may get entries skipped or duplicated, so
1276 don't. Exception: It is always safe to delete the item most recently
1277 returned by C<each()>, which means that the following code will work:
1279 while (($key, $value) = each %hash) {
1281 delete $hash{$key}; # This is safe
1284 The following prints out your environment like the printenv(1) program,
1285 only in a different order:
1287 while (($key,$value) = each %ENV) {
1288 print "$key=$value\n";
1291 See also C<keys>, C<values> and C<sort>.
1293 =item eof FILEHANDLE
1299 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1300 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1301 gives the real filehandle. (Note that this function actually
1302 reads a character and then C<ungetc>s it, so isn't very useful in an
1303 interactive context.) Do not read from a terminal file (or call
1304 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1305 as terminals may lose the end-of-file condition if you do.
1307 An C<eof> without an argument uses the last file read. Using C<eof()>
1308 with empty parentheses is very different. It refers to the pseudo file
1309 formed from the files listed on the command line and accessed via the
1310 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1311 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1312 used will cause C<@ARGV> to be examined to determine if input is
1313 available. Similarly, an C<eof()> after C<< <> >> has returned
1314 end-of-file will assume you are processing another C<@ARGV> list,
1315 and if you haven't set C<@ARGV>, will read input from C<STDIN>;
1316 see L<perlop/"I/O Operators">.
1318 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1319 detect the end of each file, C<eof()> will only detect the end of the
1320 last file. Examples:
1322 # reset line numbering on each input file
1324 next if /^\s*#/; # skip comments
1327 close ARGV if eof; # Not eof()!
1330 # insert dashes just before last line of last file
1332 if (eof()) { # check for end of current file
1333 print "--------------\n";
1334 close(ARGV); # close or last; is needed if we
1335 # are reading from the terminal
1340 Practical hint: you almost never need to use C<eof> in Perl, because the
1341 input operators typically return C<undef> when they run out of data, or if
1348 In the first form, the return value of EXPR is parsed and executed as if it
1349 were a little Perl program. The value of the expression (which is itself
1350 determined within scalar context) is first parsed, and if there weren't any
1351 errors, executed in the lexical context of the current Perl program, so
1352 that any variable settings or subroutine and format definitions remain
1353 afterwards. Note that the value is parsed every time the eval executes.
1354 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1355 delay parsing and subsequent execution of the text of EXPR until run time.
1357 In the second form, the code within the BLOCK is parsed only once--at the
1358 same time the code surrounding the eval itself was parsed--and executed
1359 within the context of the current Perl program. This form is typically
1360 used to trap exceptions more efficiently than the first (see below), while
1361 also providing the benefit of checking the code within BLOCK at compile
1364 The final semicolon, if any, may be omitted from the value of EXPR or within
1367 In both forms, the value returned is the value of the last expression
1368 evaluated inside the mini-program; a return statement may be also used, just
1369 as with subroutines. The expression providing the return value is evaluated
1370 in void, scalar, or list context, depending on the context of the eval itself.
1371 See L</wantarray> for more on how the evaluation context can be determined.
1373 If there is a syntax error or runtime error, or a C<die> statement is
1374 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1375 error message. If there was no error, C<$@> is guaranteed to be a null
1376 string. Beware that using C<eval> neither silences perl from printing
1377 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1378 To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
1379 turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
1380 See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
1382 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1383 determining whether a particular feature (such as C<socket> or C<symlink>)
1384 is implemented. It is also Perl's exception trapping mechanism, where
1385 the die operator is used to raise exceptions.
1387 If the code to be executed doesn't vary, you may use the eval-BLOCK
1388 form to trap run-time errors without incurring the penalty of
1389 recompiling each time. The error, if any, is still returned in C<$@>.
1392 # make divide-by-zero nonfatal
1393 eval { $answer = $a / $b; }; warn $@ if $@;
1395 # same thing, but less efficient
1396 eval '$answer = $a / $b'; warn $@ if $@;
1398 # a compile-time error
1399 eval { $answer = }; # WRONG
1402 eval '$answer ='; # sets $@
1404 Due to the current arguably broken state of C<__DIE__> hooks, when using
1405 the C<eval{}> form as an exception trap in libraries, you may wish not
1406 to trigger any C<__DIE__> hooks that user code may have installed.
1407 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1408 as shown in this example:
1410 # a very private exception trap for divide-by-zero
1411 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1414 This is especially significant, given that C<__DIE__> hooks can call
1415 C<die> again, which has the effect of changing their error messages:
1417 # __DIE__ hooks may modify error messages
1419 local $SIG{'__DIE__'} =
1420 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1421 eval { die "foo lives here" };
1422 print $@ if $@; # prints "bar lives here"
1425 Because this promotes action at a distance, this counterintuitive behavior
1426 may be fixed in a future release.
1428 With an C<eval>, you should be especially careful to remember what's
1429 being looked at when:
1435 eval { $x }; # CASE 4
1437 eval "\$$x++"; # CASE 5
1440 Cases 1 and 2 above behave identically: they run the code contained in
1441 the variable $x. (Although case 2 has misleading double quotes making
1442 the reader wonder what else might be happening (nothing is).) Cases 3
1443 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1444 does nothing but return the value of $x. (Case 4 is preferred for
1445 purely visual reasons, but it also has the advantage of compiling at
1446 compile-time instead of at run-time.) Case 5 is a place where
1447 normally you I<would> like to use double quotes, except that in this
1448 particular situation, you can just use symbolic references instead, as
1451 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1452 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1454 Note that as a very special case, an C<eval ''> executed within the C<DB>
1455 package doesn't see the usual surrounding lexical scope, but rather the
1456 scope of the first non-DB piece of code that called it. You don't normally
1457 need to worry about this unless you are writing a Perl debugger.
1461 =item exec PROGRAM LIST
1463 The C<exec> function executes a system command I<and never returns>--
1464 use C<system> instead of C<exec> if you want it to return. It fails and
1465 returns false only if the command does not exist I<and> it is executed
1466 directly instead of via your system's command shell (see below).
1468 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1469 warns you if there is a following statement which isn't C<die>, C<warn>,
1470 or C<exit> (if C<-w> is set - but you always do that). If you
1471 I<really> want to follow an C<exec> with some other statement, you
1472 can use one of these styles to avoid the warning:
1474 exec ('foo') or print STDERR "couldn't exec foo: $!";
1475 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1477 If there is more than one argument in LIST, or if LIST is an array
1478 with more than one value, calls execvp(3) with the arguments in LIST.
1479 If there is only one scalar argument or an array with one element in it,
1480 the argument is checked for shell metacharacters, and if there are any,
1481 the entire argument is passed to the system's command shell for parsing
1482 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1483 If there are no shell metacharacters in the argument, it is split into
1484 words and passed directly to C<execvp>, which is more efficient.
1487 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1488 exec "sort $outfile | uniq";
1490 If you don't really want to execute the first argument, but want to lie
1491 to the program you are executing about its own name, you can specify
1492 the program you actually want to run as an "indirect object" (without a
1493 comma) in front of the LIST. (This always forces interpretation of the
1494 LIST as a multivalued list, even if there is only a single scalar in
1497 $shell = '/bin/csh';
1498 exec $shell '-sh'; # pretend it's a login shell
1502 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1504 When the arguments get executed via the system shell, results will
1505 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1508 Using an indirect object with C<exec> or C<system> is also more
1509 secure. This usage (which also works fine with system()) forces
1510 interpretation of the arguments as a multivalued list, even if the
1511 list had just one argument. That way you're safe from the shell
1512 expanding wildcards or splitting up words with whitespace in them.
1514 @args = ( "echo surprise" );
1516 exec @args; # subject to shell escapes
1518 exec { $args[0] } @args; # safe even with one-arg list
1520 The first version, the one without the indirect object, ran the I<echo>
1521 program, passing it C<"surprise"> an argument. The second version
1522 didn't--it tried to run a program literally called I<"echo surprise">,
1523 didn't find it, and set C<$?> to a non-zero value indicating failure.
1525 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1526 output before the exec, but this may not be supported on some platforms
1527 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1528 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1529 open handles in order to avoid lost output.
1531 Note that C<exec> will not call your C<END> blocks, nor will it call
1532 any C<DESTROY> methods in your objects.
1536 Given an expression that specifies a hash element or array element,
1537 returns true if the specified element in the hash or array has ever
1538 been initialized, even if the corresponding value is undefined. The
1539 element is not autovivified if it doesn't exist.
1541 print "Exists\n" if exists $hash{$key};
1542 print "Defined\n" if defined $hash{$key};
1543 print "True\n" if $hash{$key};
1545 print "Exists\n" if exists $array[$index];
1546 print "Defined\n" if defined $array[$index];
1547 print "True\n" if $array[$index];
1549 A hash or array element can be true only if it's defined, and defined if
1550 it exists, but the reverse doesn't necessarily hold true.
1552 Given an expression that specifies the name of a subroutine,
1553 returns true if the specified subroutine has ever been declared, even
1554 if it is undefined. Mentioning a subroutine name for exists or defined
1555 does not count as declaring it. Note that a subroutine which does not
1556 exist may still be callable: its package may have an C<AUTOLOAD>
1557 method that makes it spring into existence the first time that it is
1558 called -- see L<perlsub>.
1560 print "Exists\n" if exists &subroutine;
1561 print "Defined\n" if defined &subroutine;
1563 Note that the EXPR can be arbitrarily complicated as long as the final
1564 operation is a hash or array key lookup or subroutine name:
1566 if (exists $ref->{A}->{B}->{$key}) { }
1567 if (exists $hash{A}{B}{$key}) { }
1569 if (exists $ref->{A}->{B}->[$ix]) { }
1570 if (exists $hash{A}{B}[$ix]) { }
1572 if (exists &{$ref->{A}{B}{$key}}) { }
1574 Although the deepest nested array or hash will not spring into existence
1575 just because its existence was tested, any intervening ones will.
1576 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1577 into existence due to the existence test for the $key element above.
1578 This happens anywhere the arrow operator is used, including even:
1581 if (exists $ref->{"Some key"}) { }
1582 print $ref; # prints HASH(0x80d3d5c)
1584 This surprising autovivification in what does not at first--or even
1585 second--glance appear to be an lvalue context may be fixed in a future
1588 Use of a subroutine call, rather than a subroutine name, as an argument
1589 to exists() is an error.
1592 exists &sub(); # Error
1596 Evaluates EXPR and exits immediately with that value. Example:
1599 exit 0 if $ans =~ /^[Xx]/;
1601 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1602 universally recognized values for EXPR are C<0> for success and C<1>
1603 for error; other values are subject to interpretation depending on the
1604 environment in which the Perl program is running. For example, exiting
1605 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1606 the mailer to return the item undelivered, but that's not true everywhere.
1608 Don't use C<exit> to abort a subroutine if there's any chance that
1609 someone might want to trap whatever error happened. Use C<die> instead,
1610 which can be trapped by an C<eval>.
1612 The exit() function does not always exit immediately. It calls any
1613 defined C<END> routines first, but these C<END> routines may not
1614 themselves abort the exit. Likewise any object destructors that need to
1615 be called are called before the real exit. If this is a problem, you
1616 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1617 See L<perlmod> for details.
1623 Returns I<e> (the natural logarithm base) to the power of EXPR.
1624 If EXPR is omitted, gives C<exp($_)>.
1626 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1628 Implements the fcntl(2) function. You'll probably have to say
1632 first to get the correct constant definitions. Argument processing and
1633 value return works just like C<ioctl> below.
1637 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1638 or die "can't fcntl F_GETFL: $!";
1640 You don't have to check for C<defined> on the return from C<fnctl>.
1641 Like C<ioctl>, it maps a C<0> return from the system call into
1642 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1643 in numeric context. It is also exempt from the normal B<-w> warnings
1644 on improper numeric conversions.
1646 Note that C<fcntl> will produce a fatal error if used on a machine that
1647 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1648 manpage to learn what functions are available on your system.
1650 =item fileno FILEHANDLE
1652 Returns the file descriptor for a filehandle, or undefined if the
1653 filehandle is not open. This is mainly useful for constructing
1654 bitmaps for C<select> and low-level POSIX tty-handling operations.
1655 If FILEHANDLE is an expression, the value is taken as an indirect
1656 filehandle, generally its name.
1658 You can use this to find out whether two handles refer to the
1659 same underlying descriptor:
1661 if (fileno(THIS) == fileno(THAT)) {
1662 print "THIS and THAT are dups\n";
1665 (Filehandles connected to memory objects via new features of C<open> may
1666 return undefined even though they are open.)
1669 =item flock FILEHANDLE,OPERATION
1671 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1672 for success, false on failure. Produces a fatal error if used on a
1673 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1674 C<flock> is Perl's portable file locking interface, although it locks
1675 only entire files, not records.
1677 Two potentially non-obvious but traditional C<flock> semantics are
1678 that it waits indefinitely until the lock is granted, and that its locks
1679 B<merely advisory>. Such discretionary locks are more flexible, but offer
1680 fewer guarantees. This means that files locked with C<flock> may be
1681 modified by programs that do not also use C<flock>. See L<perlport>,
1682 your port's specific documentation, or your system-specific local manpages
1683 for details. It's best to assume traditional behavior if you're writing
1684 portable programs. (But if you're not, you should as always feel perfectly
1685 free to write for your own system's idiosyncrasies (sometimes called
1686 "features"). Slavish adherence to portability concerns shouldn't get
1687 in the way of your getting your job done.)
1689 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1690 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1691 you can use the symbolic names if you import them from the Fcntl module,
1692 either individually, or as a group using the ':flock' tag. LOCK_SH
1693 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1694 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1695 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1696 waiting for the lock (check the return status to see if you got it).
1698 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1699 before locking or unlocking it.
1701 Note that the emulation built with lockf(3) doesn't provide shared
1702 locks, and it requires that FILEHANDLE be open with write intent. These
1703 are the semantics that lockf(3) implements. Most if not all systems
1704 implement lockf(3) in terms of fcntl(2) locking, though, so the
1705 differing semantics shouldn't bite too many people.
1707 Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
1708 be open with read intent to use LOCK_SH and requires that it be open
1709 with write intent to use LOCK_EX.
1711 Note also that some versions of C<flock> cannot lock things over the
1712 network; you would need to use the more system-specific C<fcntl> for
1713 that. If you like you can force Perl to ignore your system's flock(2)
1714 function, and so provide its own fcntl(2)-based emulation, by passing
1715 the switch C<-Ud_flock> to the F<Configure> program when you configure
1718 Here's a mailbox appender for BSD systems.
1720 use Fcntl ':flock'; # import LOCK_* constants
1723 flock(MBOX,LOCK_EX);
1724 # and, in case someone appended
1725 # while we were waiting...
1730 flock(MBOX,LOCK_UN);
1733 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1734 or die "Can't open mailbox: $!";
1737 print MBOX $msg,"\n\n";
1740 On systems that support a real flock(), locks are inherited across fork()
1741 calls, whereas those that must resort to the more capricious fcntl()
1742 function lose the locks, making it harder to write servers.
1744 See also L<DB_File> for other flock() examples.
1748 Does a fork(2) system call to create a new process running the
1749 same program at the same point. It returns the child pid to the
1750 parent process, C<0> to the child process, or C<undef> if the fork is
1751 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1752 are shared, while everything else is copied. On most systems supporting
1753 fork(), great care has gone into making it extremely efficient (for
1754 example, using copy-on-write technology on data pages), making it the
1755 dominant paradigm for multitasking over the last few decades.
1757 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1758 output before forking the child process, but this may not be supported
1759 on some platforms (see L<perlport>). To be safe, you may need to set
1760 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1761 C<IO::Handle> on any open handles in order to avoid duplicate output.
1763 If you C<fork> without ever waiting on your children, you will
1764 accumulate zombies. On some systems, you can avoid this by setting
1765 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1766 forking and reaping moribund children.
1768 Note that if your forked child inherits system file descriptors like
1769 STDIN and STDOUT that are actually connected by a pipe or socket, even
1770 if you exit, then the remote server (such as, say, a CGI script or a
1771 backgrounded job launched from a remote shell) won't think you're done.
1772 You should reopen those to F</dev/null> if it's any issue.
1776 Declare a picture format for use by the C<write> function. For
1780 Test: @<<<<<<<< @||||| @>>>>>
1781 $str, $%, '$' . int($num)
1785 $num = $cost/$quantity;
1789 See L<perlform> for many details and examples.
1791 =item formline PICTURE,LIST
1793 This is an internal function used by C<format>s, though you may call it,
1794 too. It formats (see L<perlform>) a list of values according to the
1795 contents of PICTURE, placing the output into the format output
1796 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1797 Eventually, when a C<write> is done, the contents of
1798 C<$^A> are written to some filehandle, but you could also read C<$^A>
1799 yourself and then set C<$^A> back to C<"">. Note that a format typically
1800 does one C<formline> per line of form, but the C<formline> function itself
1801 doesn't care how many newlines are embedded in the PICTURE. This means
1802 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1803 You may therefore need to use multiple formlines to implement a single
1804 record format, just like the format compiler.
1806 Be careful if you put double quotes around the picture, because an C<@>
1807 character may be taken to mean the beginning of an array name.
1808 C<formline> always returns true. See L<perlform> for other examples.
1810 =item getc FILEHANDLE
1814 Returns the next character from the input file attached to FILEHANDLE,
1815 or the undefined value at end of file, or if there was an error (in
1816 the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
1817 STDIN. This is not particularly efficient. However, it cannot be
1818 used by itself to fetch single characters without waiting for the user
1819 to hit enter. For that, try something more like:
1822 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1825 system "stty", '-icanon', 'eol', "\001";
1831 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1834 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1838 Determination of whether $BSD_STYLE should be set
1839 is left as an exercise to the reader.
1841 The C<POSIX::getattr> function can do this more portably on
1842 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1843 module from your nearest CPAN site; details on CPAN can be found on
1848 Implements the C library function of the same name, which on most
1849 systems returns the current login from F</etc/utmp>, if any. If null,
1852 $login = getlogin || getpwuid($<) || "Kilroy";
1854 Do not consider C<getlogin> for authentication: it is not as
1855 secure as C<getpwuid>.
1857 =item getpeername SOCKET
1859 Returns the packed sockaddr address of other end of the SOCKET connection.
1862 $hersockaddr = getpeername(SOCK);
1863 ($port, $iaddr) = sockaddr_in($hersockaddr);
1864 $herhostname = gethostbyaddr($iaddr, AF_INET);
1865 $herstraddr = inet_ntoa($iaddr);
1869 Returns the current process group for the specified PID. Use
1870 a PID of C<0> to get the current process group for the
1871 current process. Will raise an exception if used on a machine that
1872 doesn't implement getpgrp(2). If PID is omitted, returns process
1873 group of current process. Note that the POSIX version of C<getpgrp>
1874 does not accept a PID argument, so only C<PID==0> is truly portable.
1878 Returns the process id of the parent process.
1880 Note for Linux users: on Linux, the C functions C<getpid()> and
1881 C<getppid()> return different values from different threads. In order to
1882 be portable, this behavior is not reflected by the perl-level function
1883 C<getppid()>, that returns a consistent value across threads. If you want
1884 to call the underlying C<getppid()>, you may use the CPAN module
1887 =item getpriority WHICH,WHO
1889 Returns the current priority for a process, a process group, or a user.
1890 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1891 machine that doesn't implement getpriority(2).
1897 =item gethostbyname NAME
1899 =item getnetbyname NAME
1901 =item getprotobyname NAME
1907 =item getservbyname NAME,PROTO
1909 =item gethostbyaddr ADDR,ADDRTYPE
1911 =item getnetbyaddr ADDR,ADDRTYPE
1913 =item getprotobynumber NUMBER
1915 =item getservbyport PORT,PROTO
1933 =item sethostent STAYOPEN
1935 =item setnetent STAYOPEN
1937 =item setprotoent STAYOPEN
1939 =item setservent STAYOPEN
1953 These routines perform the same functions as their counterparts in the
1954 system library. In list context, the return values from the
1955 various get routines are as follows:
1957 ($name,$passwd,$uid,$gid,
1958 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1959 ($name,$passwd,$gid,$members) = getgr*
1960 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1961 ($name,$aliases,$addrtype,$net) = getnet*
1962 ($name,$aliases,$proto) = getproto*
1963 ($name,$aliases,$port,$proto) = getserv*
1965 (If the entry doesn't exist you get a null list.)
1967 The exact meaning of the $gcos field varies but it usually contains
1968 the real name of the user (as opposed to the login name) and other
1969 information pertaining to the user. Beware, however, that in many
1970 system users are able to change this information and therefore it
1971 cannot be trusted and therefore the $gcos is tainted (see
1972 L<perlsec>). The $passwd and $shell, user's encrypted password and
1973 login shell, are also tainted, because of the same reason.
1975 In scalar context, you get the name, unless the function was a
1976 lookup by name, in which case you get the other thing, whatever it is.
1977 (If the entry doesn't exist you get the undefined value.) For example:
1979 $uid = getpwnam($name);
1980 $name = getpwuid($num);
1982 $gid = getgrnam($name);
1983 $name = getgrgid($num);
1987 In I<getpw*()> the fields $quota, $comment, and $expire are special
1988 cases in the sense that in many systems they are unsupported. If the
1989 $quota is unsupported, it is an empty scalar. If it is supported, it
1990 usually encodes the disk quota. If the $comment field is unsupported,
1991 it is an empty scalar. If it is supported it usually encodes some
1992 administrative comment about the user. In some systems the $quota
1993 field may be $change or $age, fields that have to do with password
1994 aging. In some systems the $comment field may be $class. The $expire
1995 field, if present, encodes the expiration period of the account or the
1996 password. For the availability and the exact meaning of these fields
1997 in your system, please consult your getpwnam(3) documentation and your
1998 F<pwd.h> file. You can also find out from within Perl what your
1999 $quota and $comment fields mean and whether you have the $expire field
2000 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
2001 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
2002 files are only supported if your vendor has implemented them in the
2003 intuitive fashion that calling the regular C library routines gets the
2004 shadow versions if you're running under privilege or if there exists
2005 the shadow(3) functions as found in System V ( this includes Solaris
2006 and Linux.) Those systems which implement a proprietary shadow password
2007 facility are unlikely to be supported.
2009 The $members value returned by I<getgr*()> is a space separated list of
2010 the login names of the members of the group.
2012 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
2013 C, it will be returned to you via C<$?> if the function call fails. The
2014 C<@addrs> value returned by a successful call is a list of the raw
2015 addresses returned by the corresponding system library call. In the
2016 Internet domain, each address is four bytes long and you can unpack it
2017 by saying something like:
2019 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
2021 The Socket library makes this slightly easier:
2024 $iaddr = inet_aton("127.1"); # or whatever address
2025 $name = gethostbyaddr($iaddr, AF_INET);
2027 # or going the other way
2028 $straddr = inet_ntoa($iaddr);
2030 If you get tired of remembering which element of the return list
2031 contains which return value, by-name interfaces are provided
2032 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
2033 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
2034 and C<User::grent>. These override the normal built-ins, supplying
2035 versions that return objects with the appropriate names
2036 for each field. For example:
2040 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
2042 Even though it looks like they're the same method calls (uid),
2043 they aren't, because a C<File::stat> object is different from
2044 a C<User::pwent> object.
2046 =item getsockname SOCKET
2048 Returns the packed sockaddr address of this end of the SOCKET connection,
2049 in case you don't know the address because you have several different
2050 IPs that the connection might have come in on.
2053 $mysockaddr = getsockname(SOCK);
2054 ($port, $myaddr) = sockaddr_in($mysockaddr);
2055 printf "Connect to %s [%s]\n",
2056 scalar gethostbyaddr($myaddr, AF_INET),
2059 =item getsockopt SOCKET,LEVEL,OPTNAME
2061 Returns the socket option requested, or undef if there is an error.
2067 In list context, returns a (possibly empty) list of filename expansions on
2068 the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
2069 scalar context, glob iterates through such filename expansions, returning
2070 undef when the list is exhausted. This is the internal function
2071 implementing the C<< <*.c> >> operator, but you can use it directly. If
2072 EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
2073 more detail in L<perlop/"I/O Operators">.
2075 Beginning with v5.6.0, this operator is implemented using the standard
2076 C<File::Glob> extension. See L<File::Glob> for details.
2080 Converts a time as returned by the time function to an 8-element list
2081 with the time localized for the standard Greenwich time zone.
2082 Typically used as follows:
2085 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2088 All list elements are numeric, and come straight out of the C `struct
2089 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2090 specified time. $mday is the day of the month, and $mon is the month
2091 itself, in the range C<0..11> with 0 indicating January and 11
2092 indicating December. $year is the number of years since 1900. That
2093 is, $year is C<123> in year 2023. $wday is the day of the week, with
2094 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2095 the year, in the range C<0..364> (or C<0..365> in leap years.)
2097 Note that the $year element is I<not> simply the last two digits of
2098 the year. If you assume it is, then you create non-Y2K-compliant
2099 programs--and you wouldn't want to do that, would you?
2101 The proper way to get a complete 4-digit year is simply:
2105 And to get the last two digits of the year (e.g., '01' in 2001) do:
2107 $year = sprintf("%02d", $year % 100);
2109 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2111 In scalar context, C<gmtime()> returns the ctime(3) value:
2113 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2115 Also see the C<timegm> function provided by the C<Time::Local> module,
2116 and the strftime(3) function available via the POSIX module.
2118 This scalar value is B<not> locale dependent (see L<perllocale>), but
2119 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2120 strftime(3) and mktime(3) functions available via the POSIX module. To
2121 get somewhat similar but locale dependent date strings, set up your
2122 locale environment variables appropriately (please see L<perllocale>)
2123 and try for example:
2125 use POSIX qw(strftime);
2126 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2128 Note that the C<%a> and C<%b> escapes, which represent the short forms
2129 of the day of the week and the month of the year, may not necessarily
2130 be three characters wide in all locales.
2138 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2139 execution there. It may not be used to go into any construct that
2140 requires initialization, such as a subroutine or a C<foreach> loop. It
2141 also can't be used to go into a construct that is optimized away,
2142 or to get out of a block or subroutine given to C<sort>.
2143 It can be used to go almost anywhere else within the dynamic scope,
2144 including out of subroutines, but it's usually better to use some other
2145 construct such as C<last> or C<die>. The author of Perl has never felt the
2146 need to use this form of C<goto> (in Perl, that is--C is another matter).
2147 (The difference being that C does not offer named loops combined with
2148 loop control. Perl does, and this replaces most structured uses of C<goto>
2149 in other languages.)
2151 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2152 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2153 necessarily recommended if you're optimizing for maintainability:
2155 goto ("FOO", "BAR", "GLARCH")[$i];
2157 The C<goto-&NAME> form is quite different from the other forms of
2158 C<goto>. In fact, it isn't a goto in the normal sense at all, and
2159 doesn't have the stigma associated with other gotos. Instead, it
2160 exits the current subroutine (losing any changes set by local()) and
2161 immediately calls in its place the named subroutine using the current
2162 value of @_. This is used by C<AUTOLOAD> subroutines that wish to
2163 load another subroutine and then pretend that the other subroutine had
2164 been called in the first place (except that any modifications to C<@_>
2165 in the current subroutine are propagated to the other subroutine.)
2166 After the C<goto>, not even C<caller> will be able to tell that this
2167 routine was called first.
2169 NAME needn't be the name of a subroutine; it can be a scalar variable
2170 containing a code reference, or a block which evaluates to a code
2173 =item grep BLOCK LIST
2175 =item grep EXPR,LIST
2177 This is similar in spirit to, but not the same as, grep(1) and its
2178 relatives. In particular, it is not limited to using regular expressions.
2180 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2181 C<$_> to each element) and returns the list value consisting of those
2182 elements for which the expression evaluated to true. In scalar
2183 context, returns the number of times the expression was true.
2185 @foo = grep(!/^#/, @bar); # weed out comments
2189 @foo = grep {!/^#/} @bar; # weed out comments
2191 Note that C<$_> is an alias to the list value, so it can be used to
2192 modify the elements of the LIST. While this is useful and supported,
2193 it can cause bizarre results if the elements of LIST are not variables.
2194 Similarly, grep returns aliases into the original list, much as a for
2195 loop's index variable aliases the list elements. That is, modifying an
2196 element of a list returned by grep (for example, in a C<foreach>, C<map>
2197 or another C<grep>) actually modifies the element in the original list.
2198 This is usually something to be avoided when writing clear code.
2200 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2206 Interprets EXPR as a hex string and returns the corresponding value.
2207 (To convert strings that might start with either 0, 0x, or 0b, see
2208 L</oct>.) If EXPR is omitted, uses C<$_>.
2210 print hex '0xAf'; # prints '175'
2211 print hex 'aF'; # same
2213 Hex strings may only represent integers. Strings that would cause
2214 integer overflow trigger a warning. Leading whitespace is not stripped,
2219 There is no builtin C<import> function. It is just an ordinary
2220 method (subroutine) defined (or inherited) by modules that wish to export
2221 names to another module. The C<use> function calls the C<import> method
2222 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2224 =item index STR,SUBSTR,POSITION
2226 =item index STR,SUBSTR
2228 The index function searches for one string within another, but without
2229 the wildcard-like behavior of a full regular-expression pattern match.
2230 It returns the position of the first occurrence of SUBSTR in STR at
2231 or after POSITION. If POSITION is omitted, starts searching from the
2232 beginning of the string. The return value is based at C<0> (or whatever
2233 you've set the C<$[> variable to--but don't do that). If the substring
2234 is not found, returns one less than the base, ordinarily C<-1>.
2240 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2241 You should not use this function for rounding: one because it truncates
2242 towards C<0>, and two because machine representations of floating point
2243 numbers can sometimes produce counterintuitive results. For example,
2244 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2245 because it's really more like -268.99999999999994315658 instead. Usually,
2246 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2247 functions will serve you better than will int().
2249 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2251 Implements the ioctl(2) function. You'll probably first have to say
2253 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2255 to get the correct function definitions. If F<ioctl.ph> doesn't
2256 exist or doesn't have the correct definitions you'll have to roll your
2257 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2258 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2259 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2260 written depending on the FUNCTION--a pointer to the string value of SCALAR
2261 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2262 has no string value but does have a numeric value, that value will be
2263 passed rather than a pointer to the string value. To guarantee this to be
2264 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2265 functions may be needed to manipulate the values of structures used by
2268 The return value of C<ioctl> (and C<fcntl>) is as follows:
2270 if OS returns: then Perl returns:
2272 0 string "0 but true"
2273 anything else that number
2275 Thus Perl returns true on success and false on failure, yet you can
2276 still easily determine the actual value returned by the operating
2279 $retval = ioctl(...) || -1;
2280 printf "System returned %d\n", $retval;
2282 The special string "C<0> but true" is exempt from B<-w> complaints
2283 about improper numeric conversions.
2285 Here's an example of setting a filehandle named C<REMOTE> to be
2286 non-blocking at the system level. You'll have to negotiate C<$|>
2287 on your own, though.
2289 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2291 $flags = fcntl(REMOTE, F_GETFL, 0)
2292 or die "Can't get flags for the socket: $!\n";
2294 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2295 or die "Can't set flags for the socket: $!\n";
2297 =item join EXPR,LIST
2299 Joins the separate strings of LIST into a single string with fields
2300 separated by the value of EXPR, and returns that new string. Example:
2302 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2304 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2305 first argument. Compare L</split>.
2309 Returns a list consisting of all the keys of the named hash. (In
2310 scalar context, returns the number of keys.) The keys are returned in
2311 an apparently random order. The actual random order is subject to
2312 change in future versions of perl, but it is guaranteed to be the same
2313 order as either the C<values> or C<each> function produces (given
2314 that the hash has not been modified). As a side effect, it resets
2317 Here is yet another way to print your environment:
2320 @values = values %ENV;
2322 print pop(@keys), '=', pop(@values), "\n";
2325 or how about sorted by key:
2327 foreach $key (sort(keys %ENV)) {
2328 print $key, '=', $ENV{$key}, "\n";
2331 The returned values are copies of the original keys in the hash, so
2332 modifying them will not affect the original hash. Compare L</values>.
2334 To sort a hash by value, you'll need to use a C<sort> function.
2335 Here's a descending numeric sort of a hash by its values:
2337 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2338 printf "%4d %s\n", $hash{$key}, $key;
2341 As an lvalue C<keys> allows you to increase the number of hash buckets
2342 allocated for the given hash. This can gain you a measure of efficiency if
2343 you know the hash is going to get big. (This is similar to pre-extending
2344 an array by assigning a larger number to $#array.) If you say
2348 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2349 in fact, since it rounds up to the next power of two. These
2350 buckets will be retained even if you do C<%hash = ()>, use C<undef
2351 %hash> if you want to free the storage while C<%hash> is still in scope.
2352 You can't shrink the number of buckets allocated for the hash using
2353 C<keys> in this way (but you needn't worry about doing this by accident,
2354 as trying has no effect).
2356 See also C<each>, C<values> and C<sort>.
2358 =item kill SIGNAL, LIST
2360 Sends a signal to a list of processes. Returns the number of
2361 processes successfully signaled (which is not necessarily the
2362 same as the number actually killed).
2364 $cnt = kill 1, $child1, $child2;
2367 If SIGNAL is zero, no signal is sent to the process. This is a
2368 useful way to check that the process is alive and hasn't changed
2369 its UID. See L<perlport> for notes on the portability of this
2372 Unlike in the shell, if SIGNAL is negative, it kills
2373 process groups instead of processes. (On System V, a negative I<PROCESS>
2374 number will also kill process groups, but that's not portable.) That
2375 means you usually want to use positive not negative signals. You may also
2376 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2382 The C<last> command is like the C<break> statement in C (as used in
2383 loops); it immediately exits the loop in question. If the LABEL is
2384 omitted, the command refers to the innermost enclosing loop. The
2385 C<continue> block, if any, is not executed:
2387 LINE: while (<STDIN>) {
2388 last LINE if /^$/; # exit when done with header
2392 C<last> cannot be used to exit a block which returns a value such as
2393 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2394 a grep() or map() operation.
2396 Note that a block by itself is semantically identical to a loop
2397 that executes once. Thus C<last> can be used to effect an early
2398 exit out of such a block.
2400 See also L</continue> for an illustration of how C<last>, C<next>, and
2407 Returns a lowercased version of EXPR. This is the internal function
2408 implementing the C<\L> escape in double-quoted strings. Respects
2409 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2410 and L<perlunicode> for more details about locale and Unicode support.
2412 If EXPR is omitted, uses C<$_>.
2418 Returns the value of EXPR with the first character lowercased. This
2419 is the internal function implementing the C<\l> escape in
2420 double-quoted strings. Respects current LC_CTYPE locale if C<use
2421 locale> in force. See L<perllocale> and L<perlunicode> for more
2422 details about locale and Unicode support.
2424 If EXPR is omitted, uses C<$_>.
2430 Returns the length in characters of the value of EXPR. If EXPR is
2431 omitted, returns length of C<$_>. Note that this cannot be used on
2432 an entire array or hash to find out how many elements these have.
2433 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2435 =item link OLDFILE,NEWFILE
2437 Creates a new filename linked to the old filename. Returns true for
2438 success, false otherwise.
2440 =item listen SOCKET,QUEUESIZE
2442 Does the same thing that the listen system call does. Returns true if
2443 it succeeded, false otherwise. See the example in
2444 L<perlipc/"Sockets: Client/Server Communication">.
2448 You really probably want to be using C<my> instead, because C<local> isn't
2449 what most people think of as "local". See
2450 L<perlsub/"Private Variables via my()"> for details.
2452 A local modifies the listed variables to be local to the enclosing
2453 block, file, or eval. If more than one value is listed, the list must
2454 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2455 for details, including issues with tied arrays and hashes.
2457 =item localtime EXPR
2459 Converts a time as returned by the time function to a 9-element list
2460 with the time analyzed for the local time zone. Typically used as
2464 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2467 All list elements are numeric, and come straight out of the C `struct
2468 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2469 specified time. $mday is the day of the month, and $mon is the month
2470 itself, in the range C<0..11> with 0 indicating January and 11
2471 indicating December. $year is the number of years since 1900. That
2472 is, $year is C<123> in year 2023. $wday is the day of the week, with
2473 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2474 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2475 is true if the specified time occurs during daylight savings time,
2478 Note that the $year element is I<not> simply the last two digits of
2479 the year. If you assume it is, then you create non-Y2K-compliant
2480 programs--and you wouldn't want to do that, would you?
2482 The proper way to get a complete 4-digit year is simply:
2486 And to get the last two digits of the year (e.g., '01' in 2001) do:
2488 $year = sprintf("%02d", $year % 100);
2490 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2492 In scalar context, C<localtime()> returns the ctime(3) value:
2494 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2496 This scalar value is B<not> locale dependent, see L<perllocale>, but
2497 instead a Perl builtin. Also see the C<Time::Local> module
2498 (to convert the second, minutes, hours, ... back to seconds since the
2499 stroke of midnight the 1st of January 1970, the value returned by
2500 time()), and the strftime(3) and mktime(3) functions available via the
2501 POSIX module. To get somewhat similar but locale dependent date
2502 strings, set up your locale environment variables appropriately
2503 (please see L<perllocale>) and try for example:
2505 use POSIX qw(strftime);
2506 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2508 Note that the C<%a> and C<%b>, the short forms of the day of the week
2509 and the month of the year, may not necessarily be three characters wide.
2513 This function places an advisory lock on a shared variable, or referenced
2514 object contained in I<THING> until the lock goes out of scope.
2516 lock() is a "weak keyword" : this means that if you've defined a function
2517 by this name (before any calls to it), that function will be called
2518 instead. (However, if you've said C<use threads>, lock() is always a
2519 keyword.) See L<threads>.
2525 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2526 returns log of C<$_>. To get the log of another base, use basic algebra:
2527 The base-N log of a number is equal to the natural log of that number
2528 divided by the natural log of N. For example:
2532 return log($n)/log(10);
2535 See also L</exp> for the inverse operation.
2541 Does the same thing as the C<stat> function (including setting the
2542 special C<_> filehandle) but stats a symbolic link instead of the file
2543 the symbolic link points to. If symbolic links are unimplemented on
2544 your system, a normal C<stat> is done. For much more detailed
2545 information, please see the documentation for C<stat>.
2547 If EXPR is omitted, stats C<$_>.
2551 The match operator. See L<perlop>.
2553 =item map BLOCK LIST
2557 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2558 C<$_> to each element) and returns the list value composed of the
2559 results of each such evaluation. In scalar context, returns the
2560 total number of elements so generated. Evaluates BLOCK or EXPR in
2561 list context, so each element of LIST may produce zero, one, or
2562 more elements in the returned value.
2564 @chars = map(chr, @nums);
2566 translates a list of numbers to the corresponding characters. And
2568 %hash = map { getkey($_) => $_ } @array;
2570 is just a funny way to write
2573 foreach $_ (@array) {
2574 $hash{getkey($_)} = $_;
2577 Note that C<$_> is an alias to the list value, so it can be used to
2578 modify the elements of the LIST. While this is useful and supported,
2579 it can cause bizarre results if the elements of LIST are not variables.
2580 Using a regular C<foreach> loop for this purpose would be clearer in
2581 most cases. See also L</grep> for an array composed of those items of
2582 the original list for which the BLOCK or EXPR evaluates to true.
2584 C<{> starts both hash references and blocks, so C<map { ...> could be either
2585 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2586 ahead for the closing C<}> it has to take a guess at which its dealing with
2587 based what it finds just after the C<{>. Usually it gets it right, but if it
2588 doesn't it won't realize something is wrong until it gets to the C<}> and
2589 encounters the missing (or unexpected) comma. The syntax error will be
2590 reported close to the C<}> but you'll need to change something near the C<{>
2591 such as using a unary C<+> to give perl some help:
2593 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2594 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2595 %hash = map { ("\L$_", 1) } @array # this also works
2596 %hash = map { lc($_), 1 } @array # as does this.
2597 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2599 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2601 or to force an anon hash constructor use C<+{>
2603 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2605 and you get list of anonymous hashes each with only 1 entry.
2607 =item mkdir FILENAME,MASK
2609 =item mkdir FILENAME
2611 Creates the directory specified by FILENAME, with permissions
2612 specified by MASK (as modified by C<umask>). If it succeeds it
2613 returns true, otherwise it returns false and sets C<$!> (errno).
2614 If omitted, MASK defaults to 0777.
2616 In general, it is better to create directories with permissive MASK,
2617 and let the user modify that with their C<umask>, than it is to supply
2618 a restrictive MASK and give the user no way to be more permissive.
2619 The exceptions to this rule are when the file or directory should be
2620 kept private (mail files, for instance). The perlfunc(1) entry on
2621 C<umask> discusses the choice of MASK in more detail.
2623 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2624 number of trailing slashes. Some operating and filesystems do not get
2625 this right, so Perl automatically removes all trailing slashes to keep
2628 =item msgctl ID,CMD,ARG
2630 Calls the System V IPC function msgctl(2). You'll probably have to say
2634 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2635 then ARG must be a variable which will hold the returned C<msqid_ds>
2636 structure. Returns like C<ioctl>: the undefined value for error,
2637 C<"0 but true"> for zero, or the actual return value otherwise. See also
2638 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2640 =item msgget KEY,FLAGS
2642 Calls the System V IPC function msgget(2). Returns the message queue
2643 id, or the undefined value if there is an error. See also
2644 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2646 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2648 Calls the System V IPC function msgrcv to receive a message from
2649 message queue ID into variable VAR with a maximum message size of
2650 SIZE. Note that when a message is received, the message type as a
2651 native long integer will be the first thing in VAR, followed by the
2652 actual message. This packing may be opened with C<unpack("l! a*")>.
2653 Taints the variable. Returns true if successful, or false if there is
2654 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2655 C<IPC::SysV::Msg> documentation.
2657 =item msgsnd ID,MSG,FLAGS
2659 Calls the System V IPC function msgsnd to send the message MSG to the
2660 message queue ID. MSG must begin with the native long integer message
2661 type, and be followed by the length of the actual message, and finally
2662 the message itself. This kind of packing can be achieved with
2663 C<pack("l! a*", $type, $message)>. Returns true if successful,
2664 or false if there is an error. See also C<IPC::SysV>
2665 and C<IPC::SysV::Msg> documentation.
2671 =item my EXPR : ATTRS
2673 =item my TYPE EXPR : ATTRS
2675 A C<my> declares the listed variables to be local (lexically) to the
2676 enclosing block, file, or C<eval>. If more than one value is listed,
2677 the list must be placed in parentheses.
2679 The exact semantics and interface of TYPE and ATTRS are still
2680 evolving. TYPE is currently bound to the use of C<fields> pragma,
2681 and attributes are handled using the C<attributes> pragma, or starting
2682 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
2683 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
2684 L<attributes>, and L<Attribute::Handlers>.
2690 The C<next> command is like the C<continue> statement in C; it starts
2691 the next iteration of the loop:
2693 LINE: while (<STDIN>) {
2694 next LINE if /^#/; # discard comments
2698 Note that if there were a C<continue> block on the above, it would get
2699 executed even on discarded lines. If the LABEL is omitted, the command
2700 refers to the innermost enclosing loop.
2702 C<next> cannot be used to exit a block which returns a value such as
2703 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2704 a grep() or map() operation.
2706 Note that a block by itself is semantically identical to a loop
2707 that executes once. Thus C<next> will exit such a block early.
2709 See also L</continue> for an illustration of how C<last>, C<next>, and
2712 =item no Module VERSION LIST
2714 =item no Module VERSION
2716 =item no Module LIST
2720 See the C<use> function, which C<no> is the opposite of.
2726 Interprets EXPR as an octal string and returns the corresponding
2727 value. (If EXPR happens to start off with C<0x>, interprets it as a
2728 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2729 binary string. Leading whitespace is ignored in all three cases.)
2730 The following will handle decimal, binary, octal, and hex in the standard
2733 $val = oct($val) if $val =~ /^0/;
2735 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2736 in octal), use sprintf() or printf():
2738 $perms = (stat("filename"))[2] & 07777;
2739 $oct_perms = sprintf "%lo", $perms;
2741 The oct() function is commonly used when a string such as C<644> needs
2742 to be converted into a file mode, for example. (Although perl will
2743 automatically convert strings into numbers as needed, this automatic
2744 conversion assumes base 10.)
2746 =item open FILEHANDLE,EXPR
2748 =item open FILEHANDLE,MODE,EXPR
2750 =item open FILEHANDLE,MODE,EXPR,LIST
2752 =item open FILEHANDLE,MODE,REFERENCE
2754 =item open FILEHANDLE
2756 Opens the file whose filename is given by EXPR, and associates it with
2759 (The following is a comprehensive reference to open(): for a gentler
2760 introduction you may consider L<perlopentut>.)
2762 If FILEHANDLE is an undefined scalar variable (or array or hash element)
2763 the variable is assigned a reference to a new anonymous filehandle,
2764 otherwise if FILEHANDLE is an expression, its value is used as the name of
2765 the real filehandle wanted. (This is considered a symbolic reference, so
2766 C<use strict 'refs'> should I<not> be in effect.)
2768 If EXPR is omitted, the scalar variable of the same name as the
2769 FILEHANDLE contains the filename. (Note that lexical variables--those
2770 declared with C<my>--will not work for this purpose; so if you're
2771 using C<my>, specify EXPR in your call to open.)
2773 If three or more arguments are specified then the mode of opening and
2774 the file name are separate. If MODE is C<< '<' >> or nothing, the file
2775 is opened for input. If MODE is C<< '>' >>, the file is truncated and
2776 opened for output, being created if necessary. If MODE is C<<< '>>' >>>,
2777 the file is opened for appending, again being created if necessary.
2779 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to
2780 indicate that you want both read and write access to the file; thus
2781 C<< '+<' >> is almost always preferred for read/write updates--the C<<
2782 '+>' >> mode would clobber the file first. You can't usually use
2783 either read-write mode for updating textfiles, since they have
2784 variable length records. See the B<-i> switch in L<perlrun> for a
2785 better approach. The file is created with permissions of C<0666>
2786 modified by the process' C<umask> value.
2788 These various prefixes correspond to the fopen(3) modes of C<'r'>,
2789 C<'r+'>, C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2791 In the 2-arguments (and 1-argument) form of the call the mode and
2792 filename should be concatenated (in this order), possibly separated by
2793 spaces. It is possible to omit the mode in these forms if the mode is
2796 If the filename begins with C<'|'>, the filename is interpreted as a
2797 command to which output is to be piped, and if the filename ends with a
2798 C<'|'>, the filename is interpreted as a command which pipes output to
2799 us. See L<perlipc/"Using open() for IPC">
2800 for more examples of this. (You are not allowed to C<open> to a command
2801 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2802 and L<perlipc/"Bidirectional Communication with Another Process">
2805 For three or more arguments if MODE is C<'|-'>, the filename is
2806 interpreted as a command to which output is to be piped, and if MODE
2807 is C<'-|'>, the filename is interpreted as a command which pipes
2808 output to us. In the 2-arguments (and 1-argument) form one should
2809 replace dash (C<'-'>) with the command.
2810 See L<perlipc/"Using open() for IPC"> for more examples of this.
2811 (You are not allowed to C<open> to a command that pipes both in I<and>
2812 out, but see L<IPC::Open2>, L<IPC::Open3>, and
2813 L<perlipc/"Bidirectional Communication"> for alternatives.)
2815 In the three-or-more argument form of pipe opens, if LIST is specified
2816 (extra arguments after the command name) then LIST becomes arguments
2817 to the command invoked if the platform supports it. The meaning of
2818 C<open> with more than three arguments for non-pipe modes is not yet
2819 specified. Experimental "layers" may give extra LIST arguments
2822 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2823 and opening C<< '>-' >> opens STDOUT.
2825 You may use the three-argument form of open to specify IO "layers"
2826 (sometimes also referred to as "disciplines") to be applied to the handle
2827 that affect how the input and output are processed (see L<open> and
2828 L<PerlIO> for more details). For example
2830 open(FH, "<:utf8", "file")
2832 will open the UTF-8 encoded file containing Unicode characters,
2833 see L<perluniintro>. (Note that if layers are specified in the
2834 three-arg form then default layers set by the C<open> pragma are
2837 Open returns nonzero upon success, the undefined value otherwise. If
2838 the C<open> involved a pipe, the return value happens to be the pid of
2841 If you're running Perl on a system that distinguishes between text
2842 files and binary files, then you should check out L</binmode> for tips
2843 for dealing with this. The key distinction between systems that need
2844 C<binmode> and those that don't is their text file formats. Systems
2845 like Unix, Mac OS, and Plan 9, which delimit lines with a single
2846 character, and which encode that character in C as C<"\n">, do not
2847 need C<binmode>. The rest need it.
2849 When opening a file, it's usually a bad idea to continue normal execution
2850 if the request failed, so C<open> is frequently used in connection with
2851 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2852 where you want to make a nicely formatted error message (but there are
2853 modules that can help with that problem)) you should always check
2854 the return value from opening a file. The infrequent exception is when
2855 working with an unopened filehandle is actually what you want to do.
2857 As a special case the 3 arg form with a read/write mode and the third
2858 argument being C<undef>:
2860 open(TMP, "+>", undef) or die ...
2862 opens a filehandle to an anonymous temporary file. Also using "+<"
2863 works for symmetry, but you really should consider writing something
2864 to the temporary file first. You will need to seek() to do the
2865 reading. Starting from Perl 5.8.1 the temporary files are created
2866 using the File::Temp module for greater portability, in Perl 5.8.0 the
2867 mkstemp() system call (which has known bugs in some platforms) was used.
2869 File handles can be opened to "in memory" files held in Perl scalars via:
2871 open($fh, '>', \$variable) || ..
2873 Though if you try to re-open C<STDOUT> or C<STDERR> as an "in memory"
2874 file, you have to close it first:
2877 open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
2882 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2883 while (<ARTICLE>) {...
2885 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2886 # if the open fails, output is discarded
2888 open(DBASE, '+<', 'dbase.mine') # open for update
2889 or die "Can't open 'dbase.mine' for update: $!";
2891 open(DBASE, '+<dbase.mine') # ditto
2892 or die "Can't open 'dbase.mine' for update: $!";
2894 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2895 or die "Can't start caesar: $!";
2897 open(ARTICLE, "caesar <$article |") # ditto
2898 or die "Can't start caesar: $!";
2900 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2901 or die "Can't start sort: $!";
2904 open(MEMORY,'>', \$var)
2905 or die "Can't open memory file: $!";
2906 print MEMORY "foo!\n"; # output will end up in $var
2908 # process argument list of files along with any includes
2910 foreach $file (@ARGV) {
2911 process($file, 'fh00');
2915 my($filename, $input) = @_;
2916 $input++; # this is a string increment
2917 unless (open($input, $filename)) {
2918 print STDERR "Can't open $filename: $!\n";
2923 while (<$input>) { # note use of indirection
2924 if (/^#include "(.*)"/) {
2925 process($1, $input);
2932 You may also, in the Bourne shell tradition, specify an EXPR beginning
2933 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2934 name of a filehandle (or file descriptor, if numeric) to be
2935 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2936 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2937 mode you specify should match the mode of the original filehandle.
2938 (Duping a filehandle does not take into account any existing contents of
2939 IO buffers.) If you use the 3 arg form then you can pass either a number,
2940 the name of a filehandle or the normal "reference to a glob".
2942 Here is a script that saves, redirects, and restores C<STDOUT> and
2943 C<STDERR> using various methods:
2946 open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
2947 open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
2949 open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
2950 open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
2952 select STDERR; $| = 1; # make unbuffered
2953 select STDOUT; $| = 1; # make unbuffered
2955 print STDOUT "stdout 1\n"; # this works for
2956 print STDERR "stderr 1\n"; # subprocesses too
2961 open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
2962 open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
2964 print STDOUT "stdout 2\n";
2965 print STDERR "stderr 2\n";
2967 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will
2968 do an equivalent of C's C<fdopen> of that file descriptor; this is
2969 more parsimonious of file descriptors. For example:
2971 open(FILEHANDLE, "<&=$fd")
2975 open(FILEHANDLE, "<&=", $fd)
2977 Note that if Perl is using the standard C libraries' fdopen() then on
2978 many UNIX systems, fdopen() is known to fail when file descriptors
2979 exceed a certain value, typically 255. If you need more file
2980 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2982 You can see whether Perl has been compiled with PerlIO or not by
2983 running C<perl -V> and looking for C<useperlio=> line. If C<useperlio>
2984 is C<define>, you have PerlIO, otherwise you don't.
2986 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2987 with 2-arguments (or 1-argument) form of open(), then
2988 there is an implicit fork done, and the return value of open is the pid
2989 of the child within the parent process, and C<0> within the child
2990 process. (Use C<defined($pid)> to determine whether the open was successful.)
2991 The filehandle behaves normally for the parent, but i/o to that
2992 filehandle is piped from/to the STDOUT/STDIN of the child process.
2993 In the child process the filehandle isn't opened--i/o happens from/to
2994 the new STDOUT or STDIN. Typically this is used like the normal
2995 piped open when you want to exercise more control over just how the
2996 pipe command gets executed, such as when you are running setuid, and
2997 don't want to have to scan shell commands for metacharacters.
2998 The following triples are more or less equivalent:
3000 open(FOO, "|tr '[a-z]' '[A-Z]'");
3001 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
3002 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
3003 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
3005 open(FOO, "cat -n '$file'|");
3006 open(FOO, '-|', "cat -n '$file'");
3007 open(FOO, '-|') || exec 'cat', '-n', $file;
3008 open(FOO, '-|', "cat", '-n', $file);
3010 The last example in each block shows the pipe as "list form", which is
3011 not yet supported on all platforms. A good rule of thumb is that if
3012 your platform has true C<fork()> (in other words, if your platform is
3013 UNIX) you can use the list form.
3015 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
3017 Beginning with v5.6.0, Perl will attempt to flush all files opened for
3018 output before any operation that may do a fork, but this may not be
3019 supported on some platforms (see L<perlport>). To be safe, you may need
3020 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
3021 of C<IO::Handle> on any open handles.
3023 On systems that support a close-on-exec flag on files, the flag will
3024 be set for the newly opened file descriptor as determined by the value
3025 of $^F. See L<perlvar/$^F>.
3027 Closing any piped filehandle causes the parent process to wait for the
3028 child to finish, and returns the status value in C<$?>.
3030 The filename passed to 2-argument (or 1-argument) form of open() will
3031 have leading and trailing whitespace deleted, and the normal
3032 redirection characters honored. This property, known as "magic open",
3033 can often be used to good effect. A user could specify a filename of
3034 F<"rsh cat file |">, or you could change certain filenames as needed:
3036 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
3037 open(FH, $filename) or die "Can't open $filename: $!";
3039 Use 3-argument form to open a file with arbitrary weird characters in it,
3041 open(FOO, '<', $file);
3043 otherwise it's necessary to protect any leading and trailing whitespace:
3045 $file =~ s#^(\s)#./$1#;
3046 open(FOO, "< $file\0");
3048 (this may not work on some bizarre filesystems). One should
3049 conscientiously choose between the I<magic> and 3-arguments form
3054 will allow the user to specify an argument of the form C<"rsh cat file |">,
3055 but will not work on a filename which happens to have a trailing space, while
3057 open IN, '<', $ARGV[0];
3059 will have exactly the opposite restrictions.
3061 If you want a "real" C C<open> (see L<open(2)> on your system), then you
3062 should use the C<sysopen> function, which involves no such magic (but
3063 may use subtly different filemodes than Perl open(), which is mapped
3064 to C fopen()). This is
3065 another way to protect your filenames from interpretation. For example:
3068 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
3069 or die "sysopen $path: $!";
3070 $oldfh = select(HANDLE); $| = 1; select($oldfh);
3071 print HANDLE "stuff $$\n";
3073 print "File contains: ", <HANDLE>;
3075 Using the constructor from the C<IO::Handle> package (or one of its
3076 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
3077 filehandles that have the scope of whatever variables hold references to
3078 them, and automatically close whenever and however you leave that scope:
3082 sub read_myfile_munged {
3084 my $handle = new IO::File;
3085 open($handle, "myfile") or die "myfile: $!";
3087 or return (); # Automatically closed here.
3088 mung $first or die "mung failed"; # Or here.
3089 return $first, <$handle> if $ALL; # Or here.
3093 See L</seek> for some details about mixing reading and writing.
3095 =item opendir DIRHANDLE,EXPR
3097 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
3098 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
3099 DIRHANDLE may be an expression whose value can be used as an indirect
3100 dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
3101 scalar variable (or array or hash element), the variable is assigned a
3102 reference to a new anonymous dirhandle.
3103 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
3109 Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
3110 or Unicode) value of the first character of EXPR. If EXPR is omitted,
3113 For the reverse, see L</chr>.
3114 See L<perlunicode> and L<encoding> for more about Unicode.
3120 =item our EXPR : ATTRS
3122 =item our TYPE EXPR : ATTRS
3124 An C<our> declares the listed variables to be valid globals within
3125 the enclosing block, file, or C<eval>. That is, it has the same
3126 scoping rules as a "my" declaration, but does not create a local
3127 variable. If more than one value is listed, the list must be placed
3128 in parentheses. The C<our> declaration has no semantic effect unless
3129 "use strict vars" is in effect, in which case it lets you use the
3130 declared global variable without qualifying it with a package name.
3131 (But only within the lexical scope of the C<our> declaration. In this
3132 it differs from "use vars", which is package scoped.)
3134 An C<our> declaration declares a global variable that will be visible
3135 across its entire lexical scope, even across package boundaries. The
3136 package in which the variable is entered is determined at the point
3137 of the declaration, not at the point of use. This means the following
3141 our $bar; # declares $Foo::bar for rest of lexical scope
3145 print $bar; # prints 20
3147 Multiple C<our> declarations in the same lexical scope are allowed
3148 if they are in different packages. If they happened to be in the same
3149 package, Perl will emit warnings if you have asked for them.
3153 our $bar; # declares $Foo::bar for rest of lexical scope
3157 our $bar = 30; # declares $Bar::bar for rest of lexical scope
3158 print $bar; # prints 30
3160 our $bar; # emits warning
3162 An C<our> declaration may also have a list of attributes associated
3165 The exact semantics and interface of TYPE and ATTRS are still
3166 evolving. TYPE is currently bound to the use of C<fields> pragma,
3167 and attributes are handled using the C<attributes> pragma, or starting
3168 from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
3169 L<perlsub/"Private Variables via my()"> for details, and L<fields>,
3170 L<attributes>, and L<Attribute::Handlers>.
3172 The only currently recognized C<our()> attribute is C<unique> which
3173 indicates that a single copy of the global is to be used by all
3174 interpreters should the program happen to be running in a
3175 multi-interpreter environment. (The default behaviour would be for
3176 each interpreter to have its own copy of the global.) Examples:
3178 our @EXPORT : unique = qw(foo);
3179 our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
3180 our $VERSION : unique = "1.00";
3182 Note that this attribute also has the effect of making the global
3183 readonly when the first new interpreter is cloned (for example,
3184 when the first new thread is created).
3186 Multi-interpreter environments can come to being either through the
3187 fork() emulation on Windows platforms, or by embedding perl in a
3188 multi-threaded application. The C<unique> attribute does nothing in
3189 all other environments.
3191 =item pack TEMPLATE,LIST
3193 Takes a LIST of values and converts it into a string using the rules
3194 given by the TEMPLATE. The resulting string is the concatenation of
3195 the converted values. Typically, each converted value looks
3196 like its machine-level representation. For example, on 32-bit machines
3197 a converted integer may be represented by a sequence of 4 bytes.
3199 The TEMPLATE is a sequence of characters that give the order and type
3200 of values, as follows:
3202 a A string with arbitrary binary data, will be null padded.
3203 A A text (ASCII) string, will be space padded.
3204 Z A null terminated (ASCIZ) string, will be null padded.
3206 b A bit string (ascending bit order inside each byte, like vec()).
3207 B A bit string (descending bit order inside each byte).
3208 h A hex string (low nybble first).
3209 H A hex string (high nybble first).
3211 c A signed char value.
3212 C An unsigned char value. Only does bytes. See U for Unicode.
3214 s A signed short value.
3215 S An unsigned short value.
3216 (This 'short' is _exactly_ 16 bits, which may differ from
3217 what a local C compiler calls 'short'. If you want
3218 native-length shorts, use the '!' suffix.)
3220 i A signed integer value.
3221 I An unsigned integer value.
3222 (This 'integer' is _at_least_ 32 bits wide. Its exact
3223 size depends on what a local C compiler calls 'int',
3224 and may even be larger than the 'long' described in
3227 l A signed long value.
3228 L An unsigned long value.
3229 (This 'long' is _exactly_ 32 bits, which may differ from
3230 what a local C compiler calls 'long'. If you want
3231 native-length longs, use the '!' suffix.)
3233 n An unsigned short in "network" (big-endian) order.
3234 N An unsigned long in "network" (big-endian) order.
3235 v An unsigned short in "VAX" (little-endian) order.
3236 V An unsigned long in "VAX" (little-endian) order.
3237 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3238 _exactly_ 32 bits, respectively.)
3240 q A signed quad (64-bit) value.
3241 Q An unsigned quad value.
3242 (Quads are available only if your system supports 64-bit
3243 integer values _and_ if Perl has been compiled to support those.
3244 Causes a fatal error otherwise.)
3246 j A signed integer value (a Perl internal integer, IV).
3247 J An unsigned integer value (a Perl internal unsigned integer, UV).
3249 f A single-precision float in the native format.
3250 d A double-precision float in the native format.
3252 F A floating point value in the native native format
3253 (a Perl internal floating point value, NV).
3254 D A long double-precision float in the native format.
3255 (Long doubles are available only if your system supports long
3256 double values _and_ if Perl has been compiled to support those.
3257 Causes a fatal error otherwise.)
3259 p A pointer to a null-terminated string.
3260 P A pointer to a structure (fixed-length string).
3262 u A uuencoded string.
3263 U A Unicode character number. Encodes to UTF-8 internally
3264 (or UTF-EBCDIC in EBCDIC platforms).
3266 w A BER compressed integer. Its bytes represent an unsigned
3267 integer in base 128, most significant digit first, with as
3268 few digits as possible. Bit eight (the high bit) is set
3269 on each byte except the last.
3273 @ Null fill to absolute position, counted from the start of
3274 the innermost ()-group.
3275 ( Start of a ()-group.
3277 The following rules apply:
3283 Each letter may optionally be followed by a number giving a repeat
3284 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3285 C<H>, C<@>, C<x>, C<X> and C<P> the pack function will gobble up that
3286 many values from the LIST. A C<*> for the repeat count means to use
3287 however many items are left, except for C<@>, C<x>, C<X>, where it is
3288 equivalent to C<0>, and C<u>, where it is equivalent to 1 (or 45, what
3289 is the same). A numeric repeat count may optionally be enclosed in
3290 brackets, as in C<pack 'C[80]', @arr>.
3292 One can replace the numeric repeat count by a template enclosed in brackets;
3293 then the packed length of this template in bytes is used as a count.
3294 For example, C<x[L]> skips a long (it skips the number of bytes in a long);
3295 the template C<$t X[$t] $t> unpack()s twice what $t unpacks.
3296 If the template in brackets contains alignment commands (such as C<x![d]>),
3297 its packed length is calculated as if the start of the template has the maximal
3300 When used with C<Z>, C<*> results in the addition of a trailing null
3301 byte (so the packed result will be one longer than the byte C<length>
3304 The repeat count for C<u> is interpreted as the maximal number of bytes
3305 to encode per line of output, with 0 and 1 replaced by 45.
3309 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3310 string of length count, padding with nulls or spaces as necessary. When
3311 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3312 after the first null, and C<a> returns data verbatim. When packing,
3313 C<a>, and C<Z> are equivalent.
3315 If the value-to-pack is too long, it is truncated. If too long and an
3316 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3317 by a null byte. Thus C<Z> always packs a trailing null byte under
3322 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3323 Each byte of the input field of pack() generates 1 bit of the result.
3324 Each result bit is based on the least-significant bit of the corresponding
3325 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3326 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3328 Starting from the beginning of the input string of pack(), each 8-tuple
3329 of bytes is converted to 1 byte of output. With format C<b>
3330 the first byte of the 8-tuple determines the least-significant bit of a
3331 byte, and with format C<B> it determines the most-significant bit of
3334 If the length of the input string is not exactly divisible by 8, the
3335 remainder is packed as if the input string were padded by null bytes
3336 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3338 If the input string of pack() is longer than needed, extra bytes are ignored.
3339 A C<*> for the repeat count of pack() means to use all the bytes of
3340 the input field. On unpack()ing the bits are converted to a string
3341 of C<"0">s and C<"1">s.
3345 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3346 representable as hexadecimal digits, 0-9a-f) long.
3348 Each byte of the input field of pack() generates 4 bits of the result.
3349 For non-alphabetical bytes the result is based on the 4 least-significant
3350 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3351 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3352 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3353 is compatible with the usual hexadecimal digits, so that C<"a"> and
3354 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3355 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3357 Starting from the beginning of the input string of pack(), each pair
3358 of bytes is converted to 1 byte of output. With format C<h> the
3359 first byte of the pair determines the least-significant nybble of the
3360 output byte, and with format C<H> it determines the most-significant
3363 If the length of the input string is not even, it behaves as if padded
3364 by a null byte at the end. Similarly, during unpack()ing the "extra"
3365 nybbles are ignored.
3367 If the input string of pack() is longer than needed, extra bytes are ignored.
3368 A C<*> for the repeat count of pack() means to use all the bytes of
3369 the input field. On unpack()ing the bits are converted to a string
3370 of hexadecimal digits.
3374 The C<p> type packs a pointer to a null-terminated string. You are
3375 responsible for ensuring the string is not a temporary value (which can
3376 potentially get deallocated before you get around to using the packed result).
3377 The C<P> type packs a pointer to a structure of the size indicated by the
3378 length. A NULL pointer is created if the corresponding value for C<p> or
3379 C<P> is C<undef>, similarly for unpack().
3383 The C</> template character allows packing and unpacking of strings where
3384 the packed structure contains a byte count followed by the string itself.
3385 You write I<length-item>C</>I<string-item>.
3387 The I<length-item> can be any C<pack> template letter, and describes
3388 how the length value is packed. The ones likely to be of most use are
3389 integer-packing ones like C<n> (for Java strings), C<w> (for ASN.1 or
3390 SNMP) and C<N> (for Sun XDR).
3392 For C<pack>, the I<string-item> must, at present, be C<"A*">, C<"a*"> or
3393 C<"Z*">. For C<unpack> the length of the string is obtained from the
3394 I<length-item>, but if you put in the '*' it will be ignored. For all other
3395 codes, C<unpack> applies the length value to the next item, which must not
3396 have a repeat count.
3398 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3399 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3400 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3402 The I<length-item> is not returned explicitly from C<unpack>.
3404 Adding a count to the I<length-item> letter is unlikely to do anything
3405 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3406 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3407 which Perl does not regard as legal in numeric strings.
3411 The integer types C<s>, C<S>, C<l>, and C<L> may be
3412 immediately followed by a C<!> suffix to signify native shorts or
3413 longs--as you can see from above for example a bare C<l> does mean
3414 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3415 may be larger. This is an issue mainly in 64-bit platforms. You can
3416 see whether using C<!> makes any difference by
3418 print length(pack("s")), " ", length(pack("s!")), "\n";
3419 print length(pack("l")), " ", length(pack("l!")), "\n";
3421 C<i!> and C<I!> also work but only because of completeness;
3422 they are identical to C<i> and C<I>.
3424 The actual sizes (in bytes) of native shorts, ints, longs, and long
3425 longs on the platform where Perl was built are also available via
3429 print $Config{shortsize}, "\n";
3430 print $Config{intsize}, "\n";
3431 print $Config{longsize}, "\n";
3432 print $Config{longlongsize}, "\n";
3434 (The C<$Config{longlongsize}> will be undefined if your system does
3435 not support long longs.)
3439 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J>
3440 are inherently non-portable between processors and operating systems
3441 because they obey the native byteorder and endianness. For example a
3442 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3443 (arranged in and handled by the CPU registers) into bytes as
3445 0x12 0x34 0x56 0x78 # big-endian
3446 0x78 0x56 0x34 0x12 # little-endian
3448 Basically, the Intel and VAX CPUs are little-endian, while everybody
3449 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3450 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3451 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3454 The names `big-endian' and `little-endian' are comic references to
3455 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3456 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3457 the egg-eating habits of the Lilliputians.
3459 Some systems may have even weirder byte orders such as
3464 You can see your system's preference with
3466 print join(" ", map { sprintf "%#02x", $_ }
3467 unpack("C*",pack("L",0x12345678))), "\n";
3469 The byteorder on the platform where Perl was built is also available
3473 print $Config{byteorder}, "\n";
3475 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3476 and C<'87654321'> are big-endian.
3478 If you want portable packed integers use the formats C<n>, C<N>,
3479 C<v>, and C<V>, their byte endianness and size are known.
3480 See also L<perlport>.
3484 Real numbers (floats and doubles) are in the native machine format only;
3485 due to the multiplicity of floating formats around, and the lack of a
3486 standard "network" representation, no facility for interchange has been
3487 made. This means that packed floating point data written on one machine
3488 may not be readable on another - even if both use IEEE floating point
3489 arithmetic (as the endian-ness of the memory representation is not part
3490 of the IEEE spec). See also L<perlport>.
3492 Note that Perl uses doubles internally for all numeric calculation, and
3493 converting from double into float and thence back to double again will
3494 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3499 If the pattern begins with a C<U>, the resulting string will be treated
3500 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3501 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3502 characters. If you don't want this to happen, you can begin your pattern
3503 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3504 string, and then follow this with a C<U*> somewhere in your pattern.
3508 You must yourself do any alignment or padding by inserting for example
3509 enough C<'x'>es while packing. There is no way to pack() and unpack()
3510 could know where the bytes are going to or coming from. Therefore
3511 C<pack> (and C<unpack>) handle their output and input as flat
3516 A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may
3517 take a repeat count, both as postfix, and for unpack() also via the C</>
3518 template character. Within each repetition of a group, positioning with
3519 C<@> starts again at 0. Therefore, the result of
3521 pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
3523 is the string "\0a\0\0bc".
3528 C<x> and C<X> accept C<!> modifier. In this case they act as
3529 alignment commands: they jump forward/back to the closest position
3530 aligned at a multiple of C<count> bytes. For example, to pack() or
3531 unpack() C's C<struct {char c; double d; char cc[2]}> one may need to
3532 use the template C<C x![d] d C[2]>; this assumes that doubles must be
3533 aligned on the double's size.
3535 For alignment commands C<count> of 0 is equivalent to C<count> of 1;
3536 both result in no-ops.
3540 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3541 White space may be used to separate pack codes from each other, but
3542 a C<!> modifier and a repeat count must follow immediately.
3546 If TEMPLATE requires more arguments to pack() than actually given, pack()
3547 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3548 to pack() than actually given, extra arguments are ignored.
3554 $foo = pack("CCCC",65,66,67,68);
3556 $foo = pack("C4",65,66,67,68);
3558 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3559 # same thing with Unicode circled letters
3561 $foo = pack("ccxxcc",65,66,67,68);
3564 # note: the above examples featuring "C" and "c" are true
3565 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3566 # and UTF-8. In EBCDIC the first example would be
3567 # $foo = pack("CCCC",193,194,195,196);
3569 $foo = pack("s2",1,2);
3570 # "\1\0\2\0" on little-endian
3571 # "\0\1\0\2" on big-endian
3573 $foo = pack("a4","abcd","x","y","z");
3576 $foo = pack("aaaa","abcd","x","y","z");
3579 $foo = pack("a14","abcdefg");
3580 # "abcdefg\0\0\0\0\0\0\0"
3582 $foo = pack("i9pl", gmtime);
3583 # a real struct tm (on my system anyway)
3585 $utmp_template = "Z8 Z8 Z16 L";
3586 $utmp = pack($utmp_template, @utmp1);
3587 # a struct utmp (BSDish)
3589 @utmp2 = unpack($utmp_template, $utmp);
3590 # "@utmp1" eq "@utmp2"
3593 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3596 $foo = pack('sx2l', 12, 34);
3597 # short 12, two zero bytes padding, long 34
3598 $bar = pack('s@4l', 12, 34);
3599 # short 12, zero fill to position 4, long 34
3602 The same template may generally also be used in unpack().
3604 =item package NAMESPACE
3608 Declares the compilation unit as being in the given namespace. The scope
3609 of the package declaration is from the declaration itself through the end
3610 of the enclosing block, file, or eval (the same as the C<my> operator).
3611 All further unqualified dynamic identifiers will be in this namespace.
3612 A package statement affects only dynamic variables--including those
3613 you've used C<local> on--but I<not> lexical variables, which are created
3614 with C<my>. Typically it would be the first declaration in a file to
3615 be included by the C<require> or C<use> operator. You can switch into a
3616 package in more than one place; it merely influences which symbol table
3617 is used by the compiler for the rest of that block. You can refer to
3618 variables and filehandles in other packages by prefixing the identifier
3619 with the package name and a double colon: C<$Package::Variable>.
3620 If the package name is null, the C<main> package as assumed. That is,
3621 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3622 still seen in older code).
3624 If NAMESPACE is omitted, then there is no current package, and all
3625 identifiers must be fully qualified or lexicals. However, you are
3626 strongly advised not to make use of this feature. Its use can cause
3627 unexpected behaviour, even crashing some versions of Perl. It is
3628 deprecated, and will be removed from a future release.
3630 See L<perlmod/"Packages"> for more information about packages, modules,
3631 and classes. See L<perlsub> for other scoping issues.
3633 =item pipe READHANDLE,WRITEHANDLE
3635 Opens a pair of connected pipes like the corresponding system call.
3636 Note that if you set up a loop of piped processes, deadlock can occur
3637 unless you are very careful. In addition, note that Perl's pipes use
3638 IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3639 after each command, depending on the application.
3641 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3642 for examples of such things.
3644 On systems that support a close-on-exec flag on files, the flag will be set
3645 for the newly opened file descriptors as determined by the value of $^F.
3652 Pops and returns the last value of the array, shortening the array by
3653 one element. Has an effect similar to
3657 If there are no elements in the array, returns the undefined value
3658 (although this may happen at other times as well). If ARRAY is
3659 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3660 array in subroutines, just like C<shift>.
3666 Returns the offset of where the last C<m//g> search left off for the variable
3667 in question (C<$_> is used when the variable is not specified). May be
3668 modified to change that offset. Such modification will also influence
3669 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3672 =item print FILEHANDLE LIST
3678 Prints a string or a list of strings. Returns true if successful.
3679 FILEHANDLE may be a scalar variable name, in which case the variable
3680 contains the name of or a reference to the filehandle, thus introducing
3681 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3682 the next token is a term, it may be misinterpreted as an operator
3683 unless you interpose a C<+> or put parentheses around the arguments.)
3684 If FILEHANDLE is omitted, prints by default to standard output (or
3685 to the last selected output channel--see L</select>). If LIST is
3686 also omitted, prints C<$_> to the currently selected output channel.
3687 To set the default output channel to something other than STDOUT
3688 use the select operation. The current value of C<$,> (if any) is
3689 printed between each LIST item. The current value of C<$\> (if
3690 any) is printed after the entire LIST has been printed. Because
3691 print takes a LIST, anything in the LIST is evaluated in list
3692 context, and any subroutine that you call will have one or more of
3693 its expressions evaluated in list context. Also be careful not to
3694 follow the print keyword with a left parenthesis unless you want
3695 the corresponding right parenthesis to terminate the arguments to
3696 the print--interpose a C<+> or put parentheses around all the
3699 Note that if you're storing FILEHANDLES in an array or other expression,
3700 you will have to use a block returning its value instead:
3702 print { $files[$i] } "stuff\n";
3703 print { $OK ? STDOUT : STDERR } "stuff\n";
3705 =item printf FILEHANDLE FORMAT, LIST
3707 =item printf FORMAT, LIST
3709 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3710 (the output record separator) is not appended. The first argument
3711 of the list will be interpreted as the C<printf> format. See C<sprintf>
3712 for an explanation of the format argument. If C<use locale> is in effect,
3713 the character used for the decimal point in formatted real numbers is
3714 affected by the LC_NUMERIC locale. See L<perllocale>.
3716 Don't fall into the trap of using a C<printf> when a simple
3717 C<print> would do. The C<print> is more efficient and less
3720 =item prototype FUNCTION
3722 Returns the prototype of a function as a string (or C<undef> if the
3723 function has no prototype). FUNCTION is a reference to, or the name of,
3724 the function whose prototype you want to retrieve.
3726 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3727 name for Perl builtin. If the builtin is not I<overridable> (such as
3728 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3729 C<system>) returns C<undef> because the builtin does not really behave
3730 like a Perl function. Otherwise, the string describing the equivalent
3731 prototype is returned.
3733 =item push ARRAY,LIST
3735 Treats ARRAY as a stack, and pushes the values of LIST
3736 onto the end of ARRAY. The length of ARRAY increases by the length of
3737 LIST. Has the same effect as
3740 $ARRAY[++$#ARRAY] = $value;
3743 but is more efficient. Returns the new number of elements in the array.
3755 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3757 =item quotemeta EXPR
3761 Returns the value of EXPR with all non-"word"
3762 characters backslashed. (That is, all characters not matching
3763 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3764 returned string, regardless of any locale settings.)
3765 This is the internal function implementing
3766 the C<\Q> escape in double-quoted strings.
3768 If EXPR is omitted, uses C<$_>.
3774 Returns a random fractional number greater than or equal to C<0> and less
3775 than the value of EXPR. (EXPR should be positive.) If EXPR is
3776 omitted, the value C<1> is used. Currently EXPR with the value C<0> is
3777 also special-cased as C<1> - this has not been documented before perl 5.8.0
3778 and is subject to change in future versions of perl. Automatically calls
3779 C<srand> unless C<srand> has already been called. See also C<srand>.
3781 Apply C<int()> to the value returned by C<rand()> if you want random
3782 integers instead of random fractional numbers. For example,
3786 returns a random integer between C<0> and C<9>, inclusive.
3788 (Note: If your rand function consistently returns numbers that are too
3789 large or too small, then your version of Perl was probably compiled
3790 with the wrong number of RANDBITS.)
3792 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3794 =item read FILEHANDLE,SCALAR,LENGTH
3796 Attempts to read LENGTH I<characters> of data into variable SCALAR
3797 from the specified FILEHANDLE. Returns the number of characters
3798 actually read, C<0> at end of file, or undef if there was an error (in
3799 the latter case C<$!> is also set). SCALAR will be grown or shrunk to
3800 the length actually read. If SCALAR needs growing, the new bytes will
3801 be zero bytes. An OFFSET may be specified to place the read data into
3802 some other place in SCALAR than the beginning. The call is actually
3803 implemented in terms of either Perl's or system's fread() call. To
3804 get a true read(2) system call, see C<sysread>.
3806 Note the I<characters>: depending on the status of the filehandle,
3807 either (8-bit) bytes or characters are read. By default all
3808 filehandles operate on bytes, but for example if the filehandle has
3809 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
3810 pragma, L<open>), the I/O will operate on characters, not bytes.
3812 =item readdir DIRHANDLE
3814 Returns the next directory entry for a directory opened by C<opendir>.
3815 If used in list context, returns all the rest of the entries in the
3816 directory. If there are no more entries, returns an undefined value in
3817 scalar context or a null list in list context.
3819 If you're planning to filetest the return values out of a C<readdir>, you'd
3820 better prepend the directory in question. Otherwise, because we didn't
3821 C<chdir> there, it would have been testing the wrong file.
3823 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3824 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3829 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3830 context, each call reads and returns the next line, until end-of-file is
3831 reached, whereupon the subsequent call returns undef. In list context,
3832 reads until end-of-file is reached and returns a list of lines. Note that
3833 the notion of "line" used here is however you may have defined it
3834 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3836 When C<$/> is set to C<undef>, when readline() is in scalar
3837 context (i.e. file slurp mode), and when an empty file is read, it
3838 returns C<''> the first time, followed by C<undef> subsequently.
3840 This is the internal function implementing the C<< <EXPR> >>
3841 operator, but you can use it directly. The C<< <EXPR> >>
3842 operator is discussed in more detail in L<perlop/"I/O Operators">.
3845 $line = readline(*STDIN); # same thing
3847 If readline encounters an operating system error, C<$!> will be set with the
3848 corresponding error message. It can be helpful to check C<$!> when you are
3849 reading from filehandles you don't trust, such as a tty or a socket. The
3850 following example uses the operator form of C<readline>, and takes the necessary
3851 steps to ensure that C<readline> was successful.
3855 unless (defined( $line = <> )) {
3866 Returns the value of a symbolic link, if symbolic links are
3867 implemented. If not, gives a fatal error. If there is some system
3868 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3869 omitted, uses C<$_>.
3873 EXPR is executed as a system command.
3874 The collected standard output of the command is returned.
3875 In scalar context, it comes back as a single (potentially
3876 multi-line) string. In list context, returns a list of lines
3877 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3878 This is the internal function implementing the C<qx/EXPR/>
3879 operator, but you can use it directly. The C<qx/EXPR/>
3880 operator is discussed in more detail in L<perlop/"I/O Operators">.
3882 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3884 Receives a message on a socket. Attempts to receive LENGTH characters
3885 of data into variable SCALAR from the specified SOCKET filehandle.
3886 SCALAR will be grown or shrunk to the length actually read. Takes the
3887 same flags as the system call of the same name. Returns the address
3888 of the sender if SOCKET's protocol supports this; returns an empty
3889 string otherwise. If there's an error, returns the undefined value.
3890 This call is actually implemented in terms of recvfrom(2) system call.
3891 See L<perlipc/"UDP: Message Passing"> for examples.
3893 Note the I<characters>: depending on the status of the socket, either
3894 (8-bit) bytes or characters are received. By default all sockets
3895 operate on bytes, but for example if the socket has been changed using
3896 binmode() to operate with the C<:utf8> I/O layer (see the C<open>
3897 pragma, L<open>), the I/O will operate on characters, not bytes.
3903 The C<redo> command restarts the loop block without evaluating the
3904 conditional again. The C<continue> block, if any, is not executed. If
3905 the LABEL is omitted, the command refers to the innermost enclosing
3906 loop. This command is normally used by programs that want to lie to
3907 themselves about what was just input:
3909 # a simpleminded Pascal comment stripper
3910 # (warning: assumes no { or } in strings)
3911 LINE: while (<STDIN>) {
3912 while (s|({.*}.*){.*}|$1 |) {}
3917 if (/}/) { # end of comment?
3926 C<redo> cannot be used to retry a block which returns a value such as
3927 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3928 a grep() or map() operation.
3930 Note that a block by itself is semantically identical to a loop
3931 that executes once. Thus C<redo> inside such a block will effectively
3932 turn it into a looping construct.
3934 See also L</continue> for an illustration of how C<last>, C<next>, and
3941 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3942 is not specified, C<$_> will be used. The value returned depends on the
3943 type of thing the reference is a reference to.
3944 Builtin types include:
3954 If the referenced object has been blessed into a package, then that package
3955 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3957 if (ref($r) eq "HASH") {
3958 print "r is a reference to a hash.\n";
3961 print "r is not a reference at all.\n";
3963 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3964 print "r is a reference to something that isa hash.\n";
3967 See also L<perlref>.
3969 =item rename OLDNAME,NEWNAME
3971 Changes the name of a file; an existing file NEWNAME will be
3972 clobbered. Returns true for success, false otherwise.
3974 Behavior of this function varies wildly depending on your system
3975 implementation. For example, it will usually not work across file system
3976 boundaries, even though the system I<mv> command sometimes compensates
3977 for this. Other restrictions include whether it works on directories,
3978 open files, or pre-existing files. Check L<perlport> and either the
3979 rename(2) manpage or equivalent system documentation for details.
3981 =item require VERSION
3987 Demands a version of Perl specified by VERSION, or demands some semantics
3988 specified by EXPR or by C<$_> if EXPR is not supplied.
3990 VERSION may be either a numeric argument such as 5.006, which will be
3991 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
3992 to C<$^V> (aka $PERL_VERSION). A fatal error is produced at run time if
3993 VERSION is greater than the version of the current Perl interpreter.
3994 Compare with L</use>, which can do a similar check at compile time.
3996 Specifying VERSION as a literal of the form v5.6.1 should generally be
3997 avoided, because it leads to misleading error messages under earlier
3998 versions of Perl which do not support this syntax. The equivalent numeric
3999 version should be used instead.
4001 require v5.6.1; # run time version check
4002 require 5.6.1; # ditto
4003 require 5.006_001; # ditto; preferred for backwards compatibility
4005 Otherwise, demands that a library file be included if it hasn't already
4006 been included. The file is included via the do-FILE mechanism, which is
4007 essentially just a variety of C<eval>. Has semantics similar to the following
4012 return 1 if $INC{$filename};
4013 my($realfilename,$result);
4015 foreach $prefix (@INC) {
4016 $realfilename = "$prefix/$filename";
4017 if (-f $realfilename) {
4018 $INC{$filename} = $realfilename;
4019 $result = do $realfilename;
4023 die "Can't find $filename in \@INC";
4025 delete $INC{$filename} if $@ || !$result;
4027 die "$filename did not return true value" unless $result;
4031 Note that the file will not be included twice under the same specified
4032 name. The file must return true as the last statement to indicate
4033 successful execution of any initialization code, so it's customary to
4034 end such a file with C<1;> unless you're sure it'll return true
4035 otherwise. But it's better just to put the C<1;>, in case you add more
4038 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
4039 replaces "F<::>" with "F</>" in the filename for you,
4040 to make it easy to load standard modules. This form of loading of
4041 modules does not risk altering your namespace.
4043 In other words, if you try this:
4045 require Foo::Bar; # a splendid bareword
4047 The require function will actually look for the "F<Foo/Bar.pm>" file in the
4048 directories specified in the C<@INC> array.
4050 But if you try this:
4052 $class = 'Foo::Bar';
4053 require $class; # $class is not a bareword
4055 require "Foo::Bar"; # not a bareword because of the ""
4057 The require function will look for the "F<Foo::Bar>" file in the @INC array and
4058 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
4060 eval "require $class";
4062 You can also insert hooks into the import facility, by putting directly
4063 Perl code into the @INC array. There are three forms of hooks: subroutine
4064 references, array references and blessed objects.
4066 Subroutine references are the simplest case. When the inclusion system
4067 walks through @INC and encounters a subroutine, this subroutine gets
4068 called with two parameters, the first being a reference to itself, and the
4069 second the name of the file to be included (e.g. "F<Foo/Bar.pm>"). The
4070 subroutine should return C<undef> or a filehandle, from which the file to
4071 include will be read. If C<undef> is returned, C<require> will look at
4072 the remaining elements of @INC.
4074 If the hook is an array reference, its first element must be a subroutine
4075 reference. This subroutine is called as above, but the first parameter is
4076 the array reference. This enables to pass indirectly some arguments to
4079 In other words, you can write:
4081 push @INC, \&my_sub;
4083 my ($coderef, $filename) = @_; # $coderef is \&my_sub
4089 push @INC, [ \&my_sub, $x, $y, ... ];
4091 my ($arrayref, $filename) = @_;
4092 # Retrieve $x, $y, ...
4093 my @parameters = @$arrayref[1..$#$arrayref];
4097 If the hook is an object, it must provide an INC method, that will be
4098 called as above, the first parameter being the object itself. (Note that
4099 you must fully qualify the sub's name, as it is always forced into package
4100 C<main>.) Here is a typical code layout:
4106 my ($self, $filename) = @_;
4110 # In the main program
4111 push @INC, new Foo(...);
4113 Note that these hooks are also permitted to set the %INC entry
4114 corresponding to the files they have loaded. See L<perlvar/%INC>.
4116 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
4122 Generally used in a C<continue> block at the end of a loop to clear
4123 variables and reset C<??> searches so that they work again. The
4124 expression is interpreted as a list of single characters (hyphens
4125 allowed for ranges). All variables and arrays beginning with one of
4126 those letters are reset to their pristine state. If the expression is
4127 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
4128 only variables or searches in the current package. Always returns
4131 reset 'X'; # reset all X variables
4132 reset 'a-z'; # reset lower case variables
4133 reset; # just reset ?one-time? searches
4135 Resetting C<"A-Z"> is not recommended because you'll wipe out your
4136 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
4137 variables--lexical variables are unaffected, but they clean themselves
4138 up on scope exit anyway, so you'll probably want to use them instead.
4145 Returns from a subroutine, C<eval>, or C<do FILE> with the value
4146 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
4147 context, depending on how the return value will be used, and the context
4148 may vary from one execution to the next (see C<wantarray>). If no EXPR
4149 is given, returns an empty list in list context, the undefined value in
4150 scalar context, and (of course) nothing at all in a void context.
4152 (Note that in the absence of an explicit C<return>, a subroutine, eval,
4153 or do FILE will automatically return the value of the last expression
4158 In list context, returns a list value consisting of the elements
4159 of LIST in the opposite order. In scalar context, concatenates the
4160 elements of LIST and returns a string value with all characters
4161 in the opposite order.
4163 print reverse <>; # line tac, last line first
4165 undef $/; # for efficiency of <>
4166 print scalar reverse <>; # character tac, last line tsrif
4168 This operator is also handy for inverting a hash, although there are some
4169 caveats. If a value is duplicated in the original hash, only one of those
4170 can be represented as a key in the inverted hash. Also, this has to
4171 unwind one hash and build a whole new one, which may take some time
4172 on a large hash, such as from a DBM file.
4174 %by_name = reverse %by_address; # Invert the hash
4176 =item rewinddir DIRHANDLE
4178 Sets the current position to the beginning of the directory for the
4179 C<readdir> routine on DIRHANDLE.
4181 =item rindex STR,SUBSTR,POSITION
4183 =item rindex STR,SUBSTR
4185 Works just like index() except that it returns the position of the LAST
4186 occurrence of SUBSTR in STR. If POSITION is specified, returns the
4187 last occurrence at or before that position.
4189 =item rmdir FILENAME
4193 Deletes the directory specified by FILENAME if that directory is empty. If it
4194 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
4195 FILENAME is omitted, uses C<$_>.
4199 The substitution operator. See L<perlop>.
4203 Forces EXPR to be interpreted in scalar context and returns the value
4206 @counts = ( scalar @a, scalar @b, scalar @c );
4208 There is no equivalent operator to force an expression to
4209 be interpolated in list context because in practice, this is never
4210 needed. If you really wanted to do so, however, you could use
4211 the construction C<@{[ (some expression) ]}>, but usually a simple
4212 C<(some expression)> suffices.
4214 Because C<scalar> is unary operator, if you accidentally use for EXPR a
4215 parenthesized list, this behaves as a scalar comma expression, evaluating
4216 all but the last element in void context and returning the final element
4217 evaluated in scalar context. This is seldom what you want.
4219 The following single statement:
4221 print uc(scalar(&foo,$bar)),$baz;
4223 is the moral equivalent of these two:
4226 print(uc($bar),$baz);
4228 See L<perlop> for more details on unary operators and the comma operator.
4230 =item seek FILEHANDLE,POSITION,WHENCE
4232 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
4233 FILEHANDLE may be an expression whose value gives the name of the
4234 filehandle. The values for WHENCE are C<0> to set the new position
4235 I<in bytes> to POSITION, C<1> to set it to the current position plus
4236 POSITION, and C<2> to set it to EOF plus POSITION (typically
4237 negative). For WHENCE you may use the constants C<SEEK_SET>,
4238 C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
4239 of the file) from the Fcntl module. Returns C<1> upon success, C<0>
4242 Note the I<in bytes>: even if the filehandle has been set to
4243 operate on characters (for example by using the C<:utf8> open
4244 layer), tell() will return byte offsets, not character offsets
4245 (because implementing that would render seek() and tell() rather slow).
4247 If you want to position file for C<sysread> or C<syswrite>, don't use
4248 C<seek>--buffering makes its effect on the file's system position
4249 unpredictable and non-portable. Use C<sysseek> instead.
4251 Due to the rules and rigors of ANSI C, on some systems you have to do a
4252 seek whenever you switch between reading and writing. Amongst other
4253 things, this may have the effect of calling stdio's clearerr(3).
4254 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
4258 This is also useful for applications emulating C<tail -f>. Once you hit
4259 EOF on your read, and then sleep for a while, you might have to stick in a
4260 seek() to reset things. The C<seek> doesn't change the current position,
4261 but it I<does> clear the end-of-file condition on the handle, so that the
4262 next C<< <FILE> >> makes Perl try again to read something. We hope.
4264 If that doesn't work (some IO implementations are particularly
4265 cantankerous), then you may need something more like this:
4268 for ($curpos = tell(FILE); $_ = <FILE>;
4269 $curpos = tell(FILE)) {
4270 # search for some stuff and put it into files
4272 sleep($for_a_while);
4273 seek(FILE, $curpos, 0);
4276 =item seekdir DIRHANDLE,POS
4278 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
4279 must be a value returned by C<telldir>. Has the same caveats about
4280 possible directory compaction as the corresponding system library
4283 =item select FILEHANDLE
4287 Returns the currently selected filehandle. Sets the current default
4288 filehandle for output, if FILEHANDLE is supplied. This has two
4289 effects: first, a C<write> or a C<print> without a filehandle will
4290 default to this FILEHANDLE. Second, references to variables related to
4291 output will refer to this output channel. For example, if you have to
4292 set the top of form format for more than one output channel, you might
4300 FILEHANDLE may be an expression whose value gives the name of the
4301 actual filehandle. Thus:
4303 $oldfh = select(STDERR); $| = 1; select($oldfh);
4305 Some programmers may prefer to think of filehandles as objects with
4306 methods, preferring to write the last example as:
4309 STDERR->autoflush(1);
4311 =item select RBITS,WBITS,EBITS,TIMEOUT
4313 This calls the select(2) system call with the bit masks specified, which
4314 can be constructed using C<fileno> and C<vec>, along these lines:
4316 $rin = $win = $ein = '';
4317 vec($rin,fileno(STDIN),1) = 1;
4318 vec($win,fileno(STDOUT),1) = 1;
4321 If you want to select on many filehandles you might wish to write a
4325 my(@fhlist) = split(' ',$_[0]);
4328 vec($bits,fileno($_),1) = 1;
4332 $rin = fhbits('STDIN TTY SOCK');
4336 ($nfound,$timeleft) =
4337 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4339 or to block until something becomes ready just do this
4341 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4343 Most systems do not bother to return anything useful in $timeleft, so
4344 calling select() in scalar context just returns $nfound.
4346 Any of the bit masks can also be undef. The timeout, if specified, is
4347 in seconds, which may be fractional. Note: not all implementations are
4348 capable of returning the $timeleft. If not, they always return
4349 $timeleft equal to the supplied $timeout.
4351 You can effect a sleep of 250 milliseconds this way:
4353 select(undef, undef, undef, 0.25);
4355 Note that whether C<select> gets restarted after signals (say, SIGALRM)
4356 is implementation-dependent.
4358 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4359 or <FH>) with C<select>, except as permitted by POSIX, and even
4360 then only on POSIX systems. You have to use C<sysread> instead.
4362 =item semctl ID,SEMNUM,CMD,ARG
4364 Calls the System V IPC function C<semctl>. You'll probably have to say
4368 first to get the correct constant definitions. If CMD is IPC_STAT or
4369 GETALL, then ARG must be a variable which will hold the returned
4370 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4371 the undefined value for error, "C<0 but true>" for zero, or the actual
4372 return value otherwise. The ARG must consist of a vector of native
4373 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4374 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4377 =item semget KEY,NSEMS,FLAGS
4379 Calls the System V IPC function semget. Returns the semaphore id, or
4380 the undefined value if there is an error. See also
4381 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4384 =item semop KEY,OPSTRING
4386 Calls the System V IPC function semop to perform semaphore operations
4387 such as signalling and waiting. OPSTRING must be a packed array of
4388 semop structures. Each semop structure can be generated with
4389 C<pack("s!3", $semnum, $semop, $semflag)>. The number of semaphore
4390 operations is implied by the length of OPSTRING. Returns true if
4391 successful, or false if there is an error. As an example, the
4392 following code waits on semaphore $semnum of semaphore id $semid:
4394 $semop = pack("s!3", $semnum, -1, 0);
4395 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4397 To signal the semaphore, replace C<-1> with C<1>. See also
4398 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4401 =item send SOCKET,MSG,FLAGS,TO
4403 =item send SOCKET,MSG,FLAGS
4405 Sends a message on a socket. Attempts to send the scalar MSG to the
4406 SOCKET filehandle. Takes the same flags as the system call of the
4407 same name. On unconnected sockets you must specify a destination to
4408 send TO, in which case it does a C C<sendto>. Returns the number of
4409 characters sent, or the undefined value if there is an error. The C
4410 system call sendmsg(2) is currently unimplemented. See
4411 L<perlipc/"UDP: Message Passing"> for examples.
4413 Note the I<characters>: depending on the status of the socket, either
4414 (8-bit) bytes or characters are sent. By default all sockets operate
4415 on bytes, but for example if the socket has been changed using
4416 binmode() to operate with the C<:utf8> I/O layer (see L</open>, or
4417 the C<open> pragma, L<open>), the I/O will operate on characters, not
4420 =item setpgrp PID,PGRP
4422 Sets the current process group for the specified PID, C<0> for the current
4423 process. Will produce a fatal error if used on a machine that doesn't
4424 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4425 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4426 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4429 =item setpriority WHICH,WHO,PRIORITY
4431 Sets the current priority for a process, a process group, or a user.
4432 (See setpriority(2).) Will produce a fatal error if used on a machine
4433 that doesn't implement setpriority(2).
4435 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4437 Sets the socket option requested. Returns undefined if there is an
4438 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4445 Shifts the first value of the array off and returns it, shortening the
4446 array by 1 and moving everything down. If there are no elements in the
4447 array, returns the undefined value. If ARRAY is omitted, shifts the
4448 C<@_> array within the lexical scope of subroutines and formats, and the
4449 C<@ARGV> array at file scopes or within the lexical scopes established by
4450 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4453 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4454 same thing to the left end of an array that C<pop> and C<push> do to the
4457 =item shmctl ID,CMD,ARG
4459 Calls the System V IPC function shmctl. You'll probably have to say
4463 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4464 then ARG must be a variable which will hold the returned C<shmid_ds>
4465 structure. Returns like ioctl: the undefined value for error, "C<0> but
4466 true" for zero, or the actual return value otherwise.
4467 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4469 =item shmget KEY,SIZE,FLAGS
4471 Calls the System V IPC function shmget. Returns the shared memory
4472 segment id, or the undefined value if there is an error.
4473 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4475 =item shmread ID,VAR,POS,SIZE
4477 =item shmwrite ID,STRING,POS,SIZE
4479 Reads or writes the System V shared memory segment ID starting at
4480 position POS for size SIZE by attaching to it, copying in/out, and
4481 detaching from it. When reading, VAR must be a variable that will
4482 hold the data read. When writing, if STRING is too long, only SIZE
4483 bytes are used; if STRING is too short, nulls are written to fill out
4484 SIZE bytes. Return true if successful, or false if there is an error.
4485 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4486 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4488 =item shutdown SOCKET,HOW
4490 Shuts down a socket connection in the manner indicated by HOW, which
4491 has the same interpretation as in the system call of the same name.
4493 shutdown(SOCKET, 0); # I/we have stopped reading data
4494 shutdown(SOCKET, 1); # I/we have stopped writing data
4495 shutdown(SOCKET, 2); # I/we have stopped using this socket
4497 This is useful with sockets when you want to tell the other
4498 side you're done writing but not done reading, or vice versa.
4499 It's also a more insistent form of close because it also
4500 disables the file descriptor in any forked copies in other
4507 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4508 returns sine of C<$_>.
4510 For the inverse sine operation, you may use the C<Math::Trig::asin>
4511 function, or use this relation:
4513 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4519 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4520 May be interrupted if the process receives a signal such as C<SIGALRM>.
4521 Returns the number of seconds actually slept. You probably cannot
4522 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4525 On some older systems, it may sleep up to a full second less than what
4526 you requested, depending on how it counts seconds. Most modern systems
4527 always sleep the full amount. They may appear to sleep longer than that,
4528 however, because your process might not be scheduled right away in a
4529 busy multitasking system.
4531 For delays of finer granularity than one second, you may use Perl's
4532 C<syscall> interface to access setitimer(2) if your system supports
4533 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4534 and starting from Perl 5.8 part of the standard distribution) may also
4537 See also the POSIX module's C<pause> function.
4539 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4541 Opens a socket of the specified kind and attaches it to filehandle
4542 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4543 the system call of the same name. You should C<use Socket> first
4544 to get the proper definitions imported. See the examples in
4545 L<perlipc/"Sockets: Client/Server Communication">.
4547 On systems that support a close-on-exec flag on files, the flag will
4548 be set for the newly opened file descriptor, as determined by the
4549 value of $^F. See L<perlvar/$^F>.
4551 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4553 Creates an unnamed pair of sockets in the specified domain, of the
4554 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4555 for the system call of the same name. If unimplemented, yields a fatal
4556 error. Returns true if successful.
4558 On systems that support a close-on-exec flag on files, the flag will
4559 be set for the newly opened file descriptors, as determined by the value
4560 of $^F. See L<perlvar/$^F>.
4562 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4563 to C<pipe(Rdr, Wtr)> is essentially:
4566 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4567 shutdown(Rdr, 1); # no more writing for reader
4568 shutdown(Wtr, 0); # no more reading for writer
4570 See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
4571 emulate socketpair using IP sockets to localhost if your system implements
4572 sockets but not socketpair.
4574 =item sort SUBNAME LIST
4576 =item sort BLOCK LIST
4580 In list context, this sorts the LIST and returns the sorted list value.
4581 In scalar context, the behaviour of C<sort()> is undefined.
4583 If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
4584 order. If SUBNAME is specified, it gives the name of a subroutine
4585 that returns an integer less than, equal to, or greater than C<0>,
4586 depending on how the elements of the list are to be ordered. (The C<<
4587 <=> >> and C<cmp> operators are extremely useful in such routines.)
4588 SUBNAME may be a scalar variable name (unsubscripted), in which case
4589 the value provides the name of (or a reference to) the actual
4590 subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
4591 an anonymous, in-line sort subroutine.
4593 If the subroutine's prototype is C<($$)>, the elements to be compared
4594 are passed by reference in C<@_>, as for a normal subroutine. This is
4595 slower than unprototyped subroutines, where the elements to be
4596 compared are passed into the subroutine
4597 as the package global variables $a and $b (see example below). Note that
4598 in the latter case, it is usually counter-productive to declare $a and
4601 In either case, the subroutine may not be recursive. The values to be
4602 compared are always passed by reference, so don't modify them.
4604 You also cannot exit out of the sort block or subroutine using any of the
4605 loop control operators described in L<perlsyn> or with C<goto>.
4607 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4608 current collation locale. See L<perllocale>.
4610 Perl 5.6 and earlier used a quicksort algorithm to implement sort.
4611 That algorithm was not stable, and I<could> go quadratic. (A I<stable> sort
4612 preserves the input order of elements that compare equal. Although
4613 quicksort's run time is O(NlogN) when averaged over all arrays of
4614 length N, the time can be O(N**2), I<quadratic> behavior, for some
4615 inputs.) In 5.7, the quicksort implementation was replaced with
4616 a stable mergesort algorithm whose worst case behavior is O(NlogN).
4617 But benchmarks indicated that for some inputs, on some platforms,
4618 the original quicksort was faster. 5.8 has a sort pragma for
4619 limited control of the sort. Its rather blunt control of the
4620 underlying algorithm may not persist into future perls, but the
4621 ability to characterize the input or output in implementation
4622 independent ways quite probably will. See L<sort>.
4627 @articles = sort @files;
4629 # same thing, but with explicit sort routine
4630 @articles = sort {$a cmp $b} @files;
4632 # now case-insensitively
4633 @articles = sort {uc($a) cmp uc($b)} @files;
4635 # same thing in reversed order
4636 @articles = sort {$b cmp $a} @files;
4638 # sort numerically ascending
4639 @articles = sort {$a <=> $b} @files;
4641 # sort numerically descending
4642 @articles = sort {$b <=> $a} @files;
4644 # this sorts the %age hash by value instead of key
4645 # using an in-line function
4646 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4648 # sort using explicit subroutine name
4650 $age{$a} <=> $age{$b}; # presuming numeric
4652 @sortedclass = sort byage @class;
4654 sub backwards { $b cmp $a }
4655 @harry = qw(dog cat x Cain Abel);
4656 @george = qw(gone chased yz Punished Axed);
4658 # prints AbelCaincatdogx
4659 print sort backwards @harry;
4660 # prints xdogcatCainAbel
4661 print sort @george, 'to', @harry;
4662 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4664 # inefficiently sort by descending numeric compare using
4665 # the first integer after the first = sign, or the
4666 # whole record case-insensitively otherwise
4669 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4674 # same thing, but much more efficiently;
4675 # we'll build auxiliary indices instead
4679 push @nums, /=(\d+)/;
4684 $nums[$b] <=> $nums[$a]
4686 $caps[$a] cmp $caps[$b]
4690 # same thing, but without any temps
4691 @new = map { $_->[0] }
4692 sort { $b->[1] <=> $a->[1]
4695 } map { [$_, /=(\d+)/, uc($_)] } @old;
4697 # using a prototype allows you to use any comparison subroutine
4698 # as a sort subroutine (including other package's subroutines)
4700 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4703 @new = sort other::backwards @old;
4705 # guarantee stability, regardless of algorithm
4707 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4709 # force use of mergesort (not portable outside Perl 5.8)
4710 use sort '_mergesort'; # note discouraging _
4711 @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
4713 If you're using strict, you I<must not> declare $a
4714 and $b as lexicals. They are package globals. That means
4715 if you're in the C<main> package and type
4717 @articles = sort {$b <=> $a} @files;
4719 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4720 but if you're in the C<FooPack> package, it's the same as typing
4722 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4724 The comparison function is required to behave. If it returns
4725 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4726 sometimes saying the opposite, for example) the results are not
4729 =item splice ARRAY,OFFSET,LENGTH,LIST
4731 =item splice ARRAY,OFFSET,LENGTH
4733 =item splice ARRAY,OFFSET
4737 Removes the elements designated by OFFSET and LENGTH from an array, and
4738 replaces them with the elements of LIST, if any. In list context,
4739 returns the elements removed from the array. In scalar context,
4740 returns the last element removed, or C<undef> if no elements are
4741 removed. The array grows or shrinks as necessary.
4742 If OFFSET is negative then it starts that far from the end of the array.
4743 If LENGTH is omitted, removes everything from OFFSET onward.
4744 If LENGTH is negative, removes the elements from OFFSET onward
4745 except for -LENGTH elements at the end of the array.
4746 If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
4747 past the end of the array, perl issues a warning, and splices at the
4750 The following equivalences hold (assuming C<< $[ == 0 and $#a >= $i >> )
4752 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4753 pop(@a) splice(@a,-1)
4754 shift(@a) splice(@a,0,1)
4755 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4756 $a[$i] = $y splice(@a,$i,1,$y)
4758 Example, assuming array lengths are passed before arrays:
4760 sub aeq { # compare two list values
4761 my(@a) = splice(@_,0,shift);
4762 my(@b) = splice(@_,0,shift);
4763 return 0 unless @a == @b; # same len?
4765 return 0 if pop(@a) ne pop(@b);
4769 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4771 =item split /PATTERN/,EXPR,LIMIT
4773 =item split /PATTERN/,EXPR
4775 =item split /PATTERN/
4779 Splits a string into a list of strings and returns that list. By default,
4780 empty leading fields are preserved, and empty trailing ones are deleted.
4782 In scalar context, returns the number of fields found and splits into
4783 the C<@_> array. Use of split in scalar context is deprecated, however,
4784 because it clobbers your subroutine arguments.
4786 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4787 splits on whitespace (after skipping any leading whitespace). Anything
4788 matching PATTERN is taken to be a delimiter separating the fields. (Note
4789 that the delimiter may be longer than one character.)
4791 If LIMIT is specified and positive, it represents the maximum number
4792 of fields the EXPR will be split into, though the actual number of
4793 fields returned depends on the number of times PATTERN matches within
4794 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4795 stripped (which potential users of C<pop> would do well to remember).
4796 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4797 had been specified. Note that splitting an EXPR that evaluates to the
4798 empty string always returns the empty list, regardless of the LIMIT
4801 A pattern matching the null string (not to be confused with
4802 a null pattern C<//>, which is just one member of the set of patterns
4803 matching a null string) will split the value of EXPR into separate
4804 characters at each point it matches that way. For example:
4806 print join(':', split(/ */, 'hi there'));
4808 produces the output 'h:i:t:h:e:r:e'.
4810 Using the empty pattern C<//> specifically matches the null string, and is
4811 not be confused with the use of C<//> to mean "the last successful pattern
4814 Empty leading (or trailing) fields are produced when there are positive width
4815 matches at the beginning (or end) of the string; a zero-width match at the
4816 beginning (or end) of the string does not produce an empty field. For
4819 print join(':', split(/(?=\w)/, 'hi there!'));
4821 produces the output 'h:i :t:h:e:r:e!'.
4823 The LIMIT parameter can be used to split a line partially
4825 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4827 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4828 one larger than the number of variables in the list, to avoid
4829 unnecessary work. For the list above LIMIT would have been 4 by
4830 default. In time critical applications it behooves you not to split
4831 into more fields than you really need.
4833 If the PATTERN contains parentheses, additional list elements are
4834 created from each matching substring in the delimiter.
4836 split(/([,-])/, "1-10,20", 3);
4838 produces the list value
4840 (1, '-', 10, ',', 20)
4842 If you had the entire header of a normal Unix email message in $header,
4843 you could split it up into fields and their values this way:
4845 $header =~ s/\n\s+/ /g; # fix continuation lines
4846 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4848 The pattern C</PATTERN/> may be replaced with an expression to specify
4849 patterns that vary at runtime. (To do runtime compilation only once,
4850 use C</$variable/o>.)
4852 As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
4853 white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
4854 be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
4855 will give you as many null initial fields as there are leading spaces.
4856 A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
4857 whitespace produces a null first field. A C<split> with no arguments
4858 really does a S<C<split(' ', $_)>> internally.
4860 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4865 open(PASSWD, '/etc/passwd');
4868 ($login, $passwd, $uid, $gid,
4869 $gcos, $home, $shell) = split(/:/);
4873 As with regular pattern matching, any capturing parentheses that are not
4874 matched in a C<split()> will be set to C<undef> when returned:
4876 @fields = split /(A)|B/, "1A2B3";
4877 # @fields is (1, 'A', 2, undef, 3)
4879 =item sprintf FORMAT, LIST
4881 Returns a string formatted by the usual C<printf> conventions of the C
4882 library function C<sprintf>. See below for more details
4883 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4884 the general principles.
4888 # Format number with up to 8 leading zeroes
4889 $result = sprintf("%08d", $number);
4891 # Round number to 3 digits after decimal point
4892 $rounded = sprintf("%.3f", $number);
4894 Perl does its own C<sprintf> formatting--it emulates the C
4895 function C<sprintf>, but it doesn't use it (except for floating-point
4896 numbers, and even then only the standard modifiers are allowed). As a
4897 result, any non-standard extensions in your local C<sprintf> are not
4898 available from Perl.
4900 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4901 pass it an array as your first argument. The array is given scalar context,
4902 and instead of using the 0th element of the array as the format, Perl will
4903 use the count of elements in the array as the format, which is almost never
4906 Perl's C<sprintf> permits the following universally-known conversions:
4909 %c a character with the given number
4911 %d a signed integer, in decimal
4912 %u an unsigned integer, in decimal
4913 %o an unsigned integer, in octal
4914 %x an unsigned integer, in hexadecimal
4915 %e a floating-point number, in scientific notation
4916 %f a floating-point number, in fixed decimal notation
4917 %g a floating-point number, in %e or %f notation
4919 In addition, Perl permits the following widely-supported conversions:
4921 %X like %x, but using upper-case letters
4922 %E like %e, but using an upper-case "E"
4923 %G like %g, but with an upper-case "E" (if applicable)
4924 %b an unsigned integer, in binary
4925 %p a pointer (outputs the Perl value's address in hexadecimal)
4926 %n special: *stores* the number of characters output so far
4927 into the next variable in the parameter list
4929 Finally, for backward (and we do mean "backward") compatibility, Perl
4930 permits these unnecessary but widely-supported conversions:
4933 %D a synonym for %ld
4934 %U a synonym for %lu
4935 %O a synonym for %lo
4938 Note that the number of exponent digits in the scientific notation produced
4939 by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4940 exponent less than 100 is system-dependent: it may be three or less
4941 (zero-padded as necessary). In other words, 1.23 times ten to the
4942 99th may be either "1.23e99" or "1.23e099".
4944 Between the C<%> and the format letter, you may specify a number of
4945 additional attributes controlling the interpretation of the format.
4946 In order, these are:
4950 =item format parameter index
4952 An explicit format parameter index, such as C<2$>. By default sprintf
4953 will format the next unused argument in the list, but this allows you
4954 to take the arguments out of order. Eg:
4956 printf '%2$d %1$d', 12, 34; # prints "34 12"
4957 printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
4962 space prefix positive number with a space
4963 + prefix positive number with a plus sign
4964 - left-justify within the field
4965 0 use zeros, not spaces, to right-justify
4966 # prefix non-zero octal with "0", non-zero hex with "0x",
4967 non-zero binary with "0b"
4971 printf '<% d>', 12; # prints "< 12>"
4972 printf '<%+d>', 12; # prints "<+12>"
4973 printf '<%6s>', 12; # prints "< 12>"
4974 printf '<%-6s>', 12; # prints "<12 >"
4975 printf '<%06s>', 12; # prints "<000012>"
4976 printf '<%#x>', 12; # prints "<0xc>"
4980 The vector flag C<v>, optionally specifying the join string to use.
4981 This flag tells perl to interpret the supplied string as a vector
4982 of integers, one for each character in the string, separated by
4983 a given string (a dot C<.> by default). This can be useful for
4984 displaying ordinal values of characters in arbitrary strings:
4986 printf "version is v%vd\n", $^V; # Perl's version
4988 Put an asterisk C<*> before the C<v> to override the string to
4989 use to separate the numbers:
4991 printf "address is %*vX\n", ":", $addr; # IPv6 address
4992 printf "bits are %0*v8b\n", " ", $bits; # random bitstring
4994 You can also explicitly specify the argument number to use for
4995 the join string using eg C<*2$v>:
4997 printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
4999 =item (minimum) width
5001 Arguments are usually formatted to be only as wide as required to
5002 display the given value. You can override the width by putting
5003 a number here, or get the width from the next argument (with C<*>)
5004 or from a specified argument (with eg C<*2$>):
5006 printf '<%s>', "a"; # prints "<a>"
5007 printf '<%6s>', "a"; # prints "< a>"
5008 printf '<%*s>', 6, "a"; # prints "< a>"
5009 printf '<%*2$s>', "a", 6; # prints "< a>"
5010 printf '<%2s>', "long"; # prints "<long>" (does not truncate)
5012 If a field width obtained through C<*> is negative, it has the same
5013 effect as the C<-> flag: left-justification.
5015 =item precision, or maximum width
5017 You can specify a precision (for numeric conversions) or a maximum
5018 width (for string conversions) by specifying a C<.> followed by a number.
5019 For floating point formats, with the exception of 'g' and 'G', this specifies
5020 the number of decimal places to show (the default being 6), eg:
5022 # these examples are subject to system-specific variation
5023 printf '<%f>', 1; # prints "<1.000000>"
5024 printf '<%.1f>', 1; # prints "<1.0>"
5025 printf '<%.0f>', 1; # prints "<1>"
5026 printf '<%e>', 10; # prints "<1.000000e+01>"
5027 printf '<%.1e>', 10; # prints "<1.0e+01>"
5029 For 'g' and 'G', this specifies the maximum number of digits to show,
5030 including prior to the decimal point as well as after it, eg:
5032 # these examples are subject to system-specific variation
5033 printf '<%g>', 1; # prints "<1>"
5034 printf '<%.10g>', 1; # prints "<1>"
5035 printf '<%g>', 100; # prints "<100>"
5036 printf '<%.1g>', 100; # prints "<1e+02>"
5037 printf '<%.2g>', 100.01; # prints "<1e+02>"
5038 printf '<%.5g>', 100.01; # prints "<100.01>"
5039 printf '<%.4g>', 100.01; # prints "<100>"
5041 For integer conversions, specifying a precision implies that the
5042 output of the number itself should be zero-padded to this width:
5044 printf '<%.6x>', 1; # prints "<000001>"
5045 printf '<%#.6x>', 1; # prints "<0x000001>"
5046 printf '<%-10.6x>', 1; # prints "<000001 >"
5048 For string conversions, specifying a precision truncates the string
5049 to fit in the specified width:
5051 printf '<%.5s>', "truncated"; # prints "<trunc>"
5052 printf '<%10.5s>', "truncated"; # prints "< trunc>"
5054 You can also get the precision from the next argument using C<.*>:
5056 printf '<%.6x>', 1; # prints "<000001>"
5057 printf '<%.*x>', 6, 1; # prints "<000001>"
5059 You cannot currently get the precision from a specified number,
5060 but it is intended that this will be possible in the future using
5063 printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
5067 For numeric conversions, you can specify the size to interpret the
5068 number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
5069 conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
5070 whatever the default integer size is on your platform (usually 32 or 64
5071 bits), but you can override this to use instead one of the standard C types,
5072 as supported by the compiler used to build Perl:
5074 l interpret integer as C type "long" or "unsigned long"
5075 h interpret integer as C type "short" or "unsigned short"
5076 q, L or ll interpret integer as C type "long long", "unsigned long long".
5077 or "quads" (typically 64-bit integers)
5079 The last will produce errors if Perl does not understand "quads" in your
5080 installation. (This requires that either the platform natively supports quads
5081 or Perl was specifically compiled to support quads.) You can find out
5082 whether your Perl supports quads via L<Config>:
5085 ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
5088 For floating point conversions (C<e f g E F G>), numbers are usually assumed
5089 to be the default floating point size on your platform (double or long double),
5090 but you can force 'long double' with C<q>, C<L>, or C<ll> if your
5091 platform supports them. You can find out whether your Perl supports long
5092 doubles via L<Config>:
5095 $Config{d_longdbl} eq 'define' && print "long doubles\n";
5097 You can find out whether Perl considers 'long double' to be the default
5098 floating point size to use on your platform via L<Config>:
5101 ($Config{uselongdouble} eq 'define') &&
5102 print "long doubles by default\n";
5104 It can also be the case that long doubles and doubles are the same thing:
5107 ($Config{doublesize} == $Config{longdblsize}) &&
5108 print "doubles are long doubles\n";
5110 The size specifier C<V> has no effect for Perl code, but it is supported
5111 for compatibility with XS code; it means 'use the standard size for
5112 a Perl integer (or floating-point number)', which is already the
5113 default for Perl code.
5115 =item order of arguments
5117 Normally, sprintf takes the next unused argument as the value to
5118 format for each format specification. If the format specification
5119 uses C<*> to require additional arguments, these are consumed from
5120 the argument list in the order in which they appear in the format
5121 specification I<before> the value to format. Where an argument is
5122 specified using an explicit index, this does not affect the normal
5123 order for the arguments (even when the explicitly specified index
5124 would have been the next argument in any case).
5128 printf '<%*.*s>', $a, $b, $c;
5130 would use C<$a> for the width, C<$b> for the precision and C<$c>
5131 as the value to format, while:
5133 print '<%*1$.*s>', $a, $b;
5135 would use C<$a> for the width and the precision, and C<$b> as the
5138 Here are some more examples - beware that when using an explicit
5139 index, the C<$> may need to be escaped:
5141 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
5142 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
5143 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
5144 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
5148 If C<use locale> is in effect, the character used for the decimal
5149 point in formatted real numbers is affected by the LC_NUMERIC locale.
5156 Return the square root of EXPR. If EXPR is omitted, returns square
5157 root of C<$_>. Only works on non-negative operands, unless you've
5158 loaded the standard Math::Complex module.
5161 print sqrt(-2); # prints 1.4142135623731i
5167 Sets the random number seed for the C<rand> operator.
5169 The point of the function is to "seed" the C<rand> function so that
5170 C<rand> can produce a different sequence each time you run your
5173 If srand() is not called explicitly, it is called implicitly at the
5174 first use of the C<rand> operator. However, this was not the case in
5175 versions of Perl before 5.004, so if your script will run under older
5176 Perl versions, it should call C<srand>.
5178 Most programs won't even call srand() at all, except those that
5179 need a cryptographically-strong starting point rather than the
5180 generally acceptable default, which is based on time of day,
5181 process ID, and memory allocation, or the F</dev/urandom> device,
5184 You can call srand($seed) with the same $seed to reproduce the
5185 I<same> sequence from rand(), but this is usually reserved for
5186 generating predictable results for testing or debugging.
5187 Otherwise, don't call srand() more than once in your program.
5189 Do B<not> call srand() (i.e. without an argument) more than once in
5190 a script. The internal state of the random number generator should
5191 contain more entropy than can be provided by any seed, so calling
5192 srand() again actually I<loses> randomness.
5194 Most implementations of C<srand> take an integer and will silently
5195 truncate decimal numbers. This means C<srand(42)> will usually
5196 produce the same results as C<srand(42.1)>. To be safe, always pass
5197 C<srand> an integer.
5199 In versions of Perl prior to 5.004 the default seed was just the
5200 current C<time>. This isn't a particularly good seed, so many old
5201 programs supply their own seed value (often C<time ^ $$> or C<time ^
5202 ($$ + ($$ << 15))>), but that isn't necessary any more.
5204 Note that you need something much more random than the default seed for
5205 cryptographic purposes. Checksumming the compressed output of one or more
5206 rapidly changing operating system status programs is the usual method. For
5209 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
5211 If you're particularly concerned with this, see the C<Math::TrulyRandom>
5214 Frequently called programs (like CGI scripts) that simply use
5218 for a seed can fall prey to the mathematical property that
5222 one-third of the time. So don't do that.
5224 =item stat FILEHANDLE
5230 Returns a 13-element list giving the status info for a file, either
5231 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
5232 it stats C<$_>. Returns a null list if the stat fails. Typically used
5235 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
5236 $atime,$mtime,$ctime,$blksize,$blocks)
5239 Not all fields are supported on all filesystem types. Here are the
5240 meaning of the fields:
5242 0 dev device number of filesystem
5244 2 mode file mode (type and permissions)
5245 3 nlink number of (hard) links to the file
5246 4 uid numeric user ID of file's owner
5247 5 gid numeric group ID of file's owner
5248 6 rdev the device identifier (special files only)
5249 7 size total size of file, in bytes
5250 8 atime last access time in seconds since the epoch
5251 9 mtime last modify time in seconds since the epoch
5252 10 ctime inode change time in seconds since the epoch (*)
5253 11 blksize preferred block size for file system I/O
5254 12 blocks actual number of blocks allocated
5256 (The epoch was at 00:00 January 1, 1970 GMT.)
5258 (*) The ctime field is non-portable, in particular you cannot expect
5259 it to be a "creation time", see L<perlport/"Files and Filesystems">
5262 If stat is passed the special filehandle consisting of an underline, no
5263 stat is done, but the current contents of the stat structure from the
5264 last stat or filetest are returned. Example:
5266 if (-x $file && (($d) = stat(_)) && $d < 0) {
5267 print "$file is executable NFS file\n";
5270 (This works on machines only for which the device number is negative
5273 Because the mode contains both the file type and its permissions, you
5274 should mask off the file type portion and (s)printf using a C<"%o">
5275 if you want to see the real permissions.
5277 $mode = (stat($filename))[2];
5278 printf "Permissions are %04o\n", $mode & 07777;
5280 In scalar context, C<stat> returns a boolean value indicating success
5281 or failure, and, if successful, sets the information associated with
5282 the special filehandle C<_>.
5284 The File::stat module provides a convenient, by-name access mechanism:
5287 $sb = stat($filename);
5288 printf "File is %s, size is %s, perm %04o, mtime %s\n",
5289 $filename, $sb->size, $sb->mode & 07777,
5290 scalar localtime $sb->mtime;
5292 You can import symbolic mode constants (C<S_IF*>) and functions
5293 (C<S_IS*>) from the Fcntl module:
5297 $mode = (stat($filename))[2];
5299 $user_rwx = ($mode & S_IRWXU) >> 6;
5300 $group_read = ($mode & S_IRGRP) >> 3;
5301 $other_execute = $mode & S_IXOTH;
5303 printf "Permissions are %04o\n", S_IMODE($mode), "\n";
5305 $is_setuid = $mode & S_ISUID;
5306 $is_setgid = S_ISDIR($mode);
5308 You could write the last two using the C<-u> and C<-d> operators.
5309 The commonly available S_IF* constants are
5311 # Permissions: read, write, execute, for user, group, others.
5313 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
5314 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
5315 S_IRWXO S_IROTH S_IWOTH S_IXOTH
5317 # Setuid/Setgid/Stickiness/SaveText.
5318 # Note that the exact meaning of these is system dependent.
5320 S_ISUID S_ISGID S_ISVTX S_ISTXT
5322 # File types. Not necessarily all are available on your system.
5324 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
5326 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
5328 S_IREAD S_IWRITE S_IEXEC
5330 and the S_IF* functions are
5332 S_IMODE($mode) the part of $mode containing the permission bits
5333 and the setuid/setgid/sticky bits
5335 S_IFMT($mode) the part of $mode containing the file type
5336 which can be bit-anded with e.g. S_IFREG
5337 or with the following functions
5339 # The operators -f, -d, -l, -b, -c, -p, and -s.
5341 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
5342 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
5344 # No direct -X operator counterpart, but for the first one
5345 # the -g operator is often equivalent. The ENFMT stands for
5346 # record flocking enforcement, a platform-dependent feature.
5348 S_ISENFMT($mode) S_ISWHT($mode)
5350 See your native chmod(2) and stat(2) documentation for more details
5351 about the S_* constants. To get status info for a symbolic link
5352 instead of the target file behind the link, use the C<lstat> function.
5358 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
5359 doing many pattern matches on the string before it is next modified.
5360 This may or may not save time, depending on the nature and number of
5361 patterns you are searching on, and on the distribution of character
5362 frequencies in the string to be searched--you probably want to compare
5363 run times with and without it to see which runs faster. Those loops
5364 which scan for many short constant strings (including the constant
5365 parts of more complex patterns) will benefit most. You may have only
5366 one C<study> active at a time--if you study a different scalar the first
5367 is "unstudied". (The way C<study> works is this: a linked list of every
5368 character in the string to be searched is made, so we know, for
5369 example, where all the C<'k'> characters are. From each search string,
5370 the rarest character is selected, based on some static frequency tables
5371 constructed from some C programs and English text. Only those places
5372 that contain this "rarest" character are examined.)
5374 For example, here is a loop that inserts index producing entries
5375 before any line containing a certain pattern:
5379 print ".IX foo\n" if /\bfoo\b/;
5380 print ".IX bar\n" if /\bbar\b/;
5381 print ".IX blurfl\n" if /\bblurfl\b/;
5386 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
5387 will be looked at, because C<f> is rarer than C<o>. In general, this is
5388 a big win except in pathological cases. The only question is whether
5389 it saves you more time than it took to build the linked list in the
5392 Note that if you have to look for strings that you don't know till
5393 runtime, you can build an entire loop as a string and C<eval> that to
5394 avoid recompiling all your patterns all the time. Together with
5395 undefining C<$/> to input entire files as one record, this can be very
5396 fast, often faster than specialized programs like fgrep(1). The following
5397 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
5398 out the names of those files that contain a match:
5400 $search = 'while (<>) { study;';
5401 foreach $word (@words) {
5402 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
5407 eval $search; # this screams
5408 $/ = "\n"; # put back to normal input delimiter
5409 foreach $file (sort keys(%seen)) {
5413 =item sub NAME BLOCK
5415 =item sub NAME (PROTO) BLOCK
5417 =item sub NAME : ATTRS BLOCK
5419 =item sub NAME (PROTO) : ATTRS BLOCK
5421 This is subroutine definition, not a real function I<per se>.
5422 Without a BLOCK it's just a forward declaration. Without a NAME,
5423 it's an anonymous function declaration, and does actually return
5424 a value: the CODE ref of the closure you just created.
5426 See L<perlsub> and L<perlref> for details about subroutines and
5427 references, and L<attributes> and L<Attribute::Handlers> for more
5428 information about attributes.
5430 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
5432 =item substr EXPR,OFFSET,LENGTH
5434 =item substr EXPR,OFFSET
5436 Extracts a substring out of EXPR and returns it. First character is at
5437 offset C<0>, or whatever you've set C<$[> to (but don't do that).
5438 If OFFSET is negative (or more precisely, less than C<$[>), starts
5439 that far from the end of the string. If LENGTH is omitted, returns
5440 everything to the end of the string. If LENGTH is negative, leaves that
5441 many characters off the end of the string.
5443 You can use the substr() function as an lvalue, in which case EXPR
5444 must itself be an lvalue. If you assign something shorter than LENGTH,
5445 the string will shrink, and if you assign something longer than LENGTH,
5446 the string will grow to accommodate it. To keep the string the same
5447 length you may need to pad or chop your value using C<sprintf>.
5449 If OFFSET and LENGTH specify a substring that is partly outside the
5450 string, only the part within the string is returned. If the substring
5451 is beyond either end of the string, substr() returns the undefined
5452 value and produces a warning. When used as an lvalue, specifying a
5453 substring that is entirely outside the string is a fatal error.
5454 Here's an example showing the behavior for boundary cases:
5457 substr($name, 4) = 'dy'; # $name is now 'freddy'
5458 my $null = substr $name, 6, 2; # returns '' (no warning)
5459 my $oops = substr $name, 7; # returns undef, with warning
5460 substr($name, 7) = 'gap'; # fatal error
5462 An alternative to using substr() as an lvalue is to specify the
5463 replacement string as the 4th argument. This allows you to replace
5464 parts of the EXPR and return what was there before in one operation,
5465 just as you can with splice().
5467 If the lvalue returned by substr is used after the EXPR is changed in
5468 any way, the behaviour may not be as expected and is subject to change.
5469 This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
5470 C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
5471 substring assignment, and then the substr is used again), or where a
5472 substr() is aliased via a C<foreach> loop or passed as a parameter or
5473 a reference to it is taken and then the alias, parameter, or deref'd
5474 reference either is used after the original EXPR has been changed or
5475 is assigned to and then used a second time.
5477 =item symlink OLDFILE,NEWFILE
5479 Creates a new filename symbolically linked to the old filename.
5480 Returns C<1> for success, C<0> otherwise. On systems that don't support
5481 symbolic links, produces a fatal error at run time. To check for that,
5484 $symlink_exists = eval { symlink("",""); 1 };
5488 Calls the system call specified as the first element of the list,
5489 passing the remaining elements as arguments to the system call. If
5490 unimplemented, produces a fatal error. The arguments are interpreted
5491 as follows: if a given argument is numeric, the argument is passed as
5492 an int. If not, the pointer to the string value is passed. You are
5493 responsible to make sure a string is pre-extended long enough to
5494 receive any result that might be written into a string. You can't use a
5495 string literal (or other read-only string) as an argument to C<syscall>
5496 because Perl has to assume that any string pointer might be written
5498 integer arguments are not literals and have never been interpreted in a
5499 numeric context, you may need to add C<0> to them to force them to look
5500 like numbers. This emulates the C<syswrite> function (or vice versa):
5502 require 'syscall.ph'; # may need to run h2ph
5504 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5506 Note that Perl supports passing of up to only 14 arguments to your system call,
5507 which in practice should usually suffice.
5509 Syscall returns whatever value returned by the system call it calls.
5510 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5511 Note that some system calls can legitimately return C<-1>. The proper
5512 way to handle such calls is to assign C<$!=0;> before the call and
5513 check the value of C<$!> if syscall returns C<-1>.
5515 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5516 number of the read end of the pipe it creates. There is no way
5517 to retrieve the file number of the other end. You can avoid this
5518 problem by using C<pipe> instead.
5520 =item sysopen FILEHANDLE,FILENAME,MODE
5522 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5524 Opens the file whose filename is given by FILENAME, and associates it
5525 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5526 the name of the real filehandle wanted. This function calls the
5527 underlying operating system's C<open> function with the parameters
5528 FILENAME, MODE, PERMS.
5530 The possible values and flag bits of the MODE parameter are
5531 system-dependent; they are available via the standard module C<Fcntl>.
5532 See the documentation of your operating system's C<open> to see which
5533 values and flag bits are available. You may combine several flags
5534 using the C<|>-operator.
5536 Some of the most common values are C<O_RDONLY> for opening the file in
5537 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5538 and C<O_RDWR> for opening the file in read-write mode, and.
5540 For historical reasons, some values work on almost every system
5541 supported by perl: zero means read-only, one means write-only, and two
5542 means read/write. We know that these values do I<not> work under
5543 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5544 use them in new code.
5546 If the file named by FILENAME does not exist and the C<open> call creates
5547 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5548 PERMS specifies the permissions of the newly created file. If you omit
5549 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5550 These permission values need to be in octal, and are modified by your
5551 process's current C<umask>.
5553 In many systems the C<O_EXCL> flag is available for opening files in
5554 exclusive mode. This is B<not> locking: exclusiveness means here that
5555 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5558 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5560 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5561 that takes away the user's option to have a more permissive umask.
5562 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5565 Note that C<sysopen> depends on the fdopen() C library function.
5566 On many UNIX systems, fdopen() is known to fail when file descriptors
5567 exceed a certain value, typically 255. If you need more file
5568 descriptors than that, consider rebuilding Perl to use the C<sfio>
5569 library, or perhaps using the POSIX::open() function.
5571 See L<perlopentut> for a kinder, gentler explanation of opening files.
5573 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5575 =item sysread FILEHANDLE,SCALAR,LENGTH
5577 Attempts to read LENGTH I<characters> of data into variable SCALAR
5578 from the specified FILEHANDLE, using the system call read(2). It
5579 bypasses buffered IO, so mixing this with other kinds of reads,
5580 C<print>, C<write>, C<seek>, C<tell>, or C<eof> can cause confusion
5581 because stdio usually buffers data. Returns the number of characters
5582 actually read, C<0> at end of file, or undef if there was an error (in
5583 the latter case C<$!> is also set). SCALAR will be grown or shrunk so
5584 that the last byte actually read is the last byte of the scalar after
5587 Note the I<characters>: depending on the status of the filehandle,
5588 either (8-bit) bytes or characters are read. By default all
5589 filehandles operate on bytes, but for example if the filehandle has
5590 been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
5591 pragma, L<open>), the I/O will operate on characters, not bytes.
5593 An OFFSET may be specified to place the read data at some place in the
5594 string other than the beginning. A negative OFFSET specifies
5595 placement at that many characters counting backwards from the end of
5596 the string. A positive OFFSET greater than the length of SCALAR
5597 results in the string being padded to the required size with C<"\0">
5598 bytes before the result of the read is appended.
5600 There is no syseof() function, which is ok, since eof() doesn't work
5601 very well on device files (like ttys) anyway. Use sysread() and check
5602 for a return value for 0 to decide whether you're done.
5604 =item sysseek FILEHANDLE,POSITION,WHENCE
5606 Sets FILEHANDLE's system position I<in bytes> using the system call
5607 lseek(2). FILEHANDLE may be an expression whose value gives the name
5608 of the filehandle. The values for WHENCE are C<0> to set the new
5609 position to POSITION, C<1> to set the it to the current position plus
5610 POSITION, and C<2> to set it to EOF plus POSITION (typically
5613 Note the I<in bytes>: even if the filehandle has been set to operate
5614 on characters (for example by using the C<:utf8> I/O layer), tell()
5615 will return byte offsets, not character offsets (because implementing
5616 that would render sysseek() very slow).
5618 sysseek() bypasses normal buffered io, so mixing this with reads (other
5619 than C<sysread>, for example >< or read()) C<print>, C<write>,
5620 C<seek>, C<tell>, or C<eof> may cause confusion.
5622 For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
5623 and C<SEEK_END> (start of the file, current position, end of the file)
5624 from the Fcntl module. Use of the constants is also more portable
5625 than relying on 0, 1, and 2. For example to define a "systell" function:
5627 use Fnctl 'SEEK_CUR';
5628 sub systell { sysseek($_[0], 0, SEEK_CUR) }
5630 Returns the new position, or the undefined value on failure. A position
5631 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5632 true on success and false on failure, yet you can still easily determine
5637 =item system PROGRAM LIST
5639 Does exactly the same thing as C<exec LIST>, except that a fork is
5640 done first, and the parent process waits for the child process to
5641 complete. Note that argument processing varies depending on the
5642 number of arguments. If there is more than one argument in LIST,
5643 or if LIST is an array with more than one value, starts the program
5644 given by the first element of the list with arguments given by the
5645 rest of the list. If there is only one scalar argument, the argument
5646 is checked for shell metacharacters, and if there are any, the
5647 entire argument is passed to the system's command shell for parsing
5648 (this is C</bin/sh -c> on Unix platforms, but varies on other
5649 platforms). If there are no shell metacharacters in the argument,
5650 it is split into words and passed directly to C<execvp>, which is
5653 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5654 output before any operation that may do a fork, but this may not be
5655 supported on some platforms (see L<perlport>). To be safe, you may need
5656 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5657 of C<IO::Handle> on any open handles.
5659 The return value is the exit status of the program as returned by the
5660 C<wait> call. To get the actual exit value shift right by eight (see below).
5661 See also L</exec>. This is I<not> what you want to use to capture
5662 the output from a command, for that you should use merely backticks or
5663 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5664 indicates a failure to start the program (inspect $! for the reason).
5666 Like C<exec>, C<system> allows you to lie to a program about its name if
5667 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5669 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
5670 killing the program they're running doesn't actually interrupt
5673 @args = ("command", "arg1", "arg2");
5675 or die "system @args failed: $?"
5677 You can check all the failure possibilities by inspecting
5680 $exit_value = $? >> 8;
5681 $signal_num = $? & 127;
5682 $dumped_core = $? & 128;
5684 or more portably by using the W*() calls of the POSIX extension;
5685 see L<perlport> for more information.
5687 When the arguments get executed via the system shell, results
5688 and return codes will be subject to its quirks and capabilities.
5689 See L<perlop/"`STRING`"> and L</exec> for details.
5691 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5693 =item syswrite FILEHANDLE,SCALAR,LENGTH
5695 =item syswrite FILEHANDLE,SCALAR
5697 Attempts to write LENGTH characters of data from variable SCALAR to
5698 the specified FILEHANDLE, using the system call write(2). If LENGTH
5699 is not specified, writes whole SCALAR. It bypasses buffered IO, so
5700 mixing this with reads (other than C<sysread())>, C<print>, C<write>,
5701 C<seek>, C<tell>, or C<eof> may cause confusion because stdio usually
5702 buffers data. Returns the number of characters actually written, or
5703 C<undef> if there was an error (in this case the errno variable C<$!>
5704 is also set). If the LENGTH is greater than the available data in the
5705 SCALAR after the OFFSET, only as much data as is available will be
5708 An OFFSET may be specified to write the data from some part of the
5709 string other than the beginning. A negative OFFSET specifies writing
5710 that many characters counting backwards from the end of the string.
5711 In the case the SCALAR is empty you can use OFFSET but only zero offset.
5713 Note the I<characters>: depending on the status of the filehandle,
5714 either (8-bit) bytes or characters are written. By default all
5715 filehandles operate on bytes, but for example if the filehandle has
5716 been opened with the C<:utf8> I/O layer (see L</open>, and the open
5717 pragma, L<open>), the I/O will operate on characters, not bytes.
5719 =item tell FILEHANDLE
5723 Returns the current position I<in bytes> for FILEHANDLE, or -1 on
5724 error. FILEHANDLE may be an expression whose value gives the name of
5725 the actual filehandle. If FILEHANDLE is omitted, assumes the file
5728 Note the I<in bytes>: even if the filehandle has been set to
5729 operate on characters (for example by using the C<:utf8> open
5730 layer), tell() will return byte offsets, not character offsets
5731 (because that would render seek() and tell() rather slow).
5733 The return value of tell() for the standard streams like the STDIN
5734 depends on the operating system: it may return -1 or something else.
5735 tell() on pipes, fifos, and sockets usually returns -1.
5737 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5739 Do not use tell() on a filehandle that has been opened using
5740 sysopen(), use sysseek() for that as described above. Why? Because
5741 sysopen() creates unbuffered, "raw", filehandles, while open() creates
5742 buffered filehandles. sysseek() make sense only on the first kind,
5743 tell() only makes sense on the second kind.
5745 =item telldir DIRHANDLE
5747 Returns the current position of the C<readdir> routines on DIRHANDLE.
5748 Value may be given to C<seekdir> to access a particular location in a
5749 directory. Has the same caveats about possible directory compaction as
5750 the corresponding system library routine.
5752 =item tie VARIABLE,CLASSNAME,LIST
5754 This function binds a variable to a package class that will provide the
5755 implementation for the variable. VARIABLE is the name of the variable
5756 to be enchanted. CLASSNAME is the name of a class implementing objects
5757 of correct type. Any additional arguments are passed to the C<new>
5758 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5759 or C<TIEHASH>). Typically these are arguments such as might be passed
5760 to the C<dbm_open()> function of C. The object returned by the C<new>
5761 method is also returned by the C<tie> function, which would be useful
5762 if you want to access other methods in CLASSNAME.
5764 Note that functions such as C<keys> and C<values> may return huge lists
5765 when used on large objects, like DBM files. You may prefer to use the
5766 C<each> function to iterate over such. Example:
5768 # print out history file offsets
5770 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5771 while (($key,$val) = each %HIST) {
5772 print $key, ' = ', unpack('L',$val), "\n";
5776 A class implementing a hash should have the following methods:
5778 TIEHASH classname, LIST
5780 STORE this, key, value
5785 NEXTKEY this, lastkey
5789 A class implementing an ordinary array should have the following methods:
5791 TIEARRAY classname, LIST
5793 STORE this, key, value
5795 STORESIZE this, count
5801 SPLICE this, offset, length, LIST
5806 A class implementing a file handle should have the following methods:
5808 TIEHANDLE classname, LIST
5809 READ this, scalar, length, offset
5812 WRITE this, scalar, length, offset
5814 PRINTF this, format, LIST
5818 SEEK this, position, whence
5820 OPEN this, mode, LIST
5825 A class implementing a scalar should have the following methods:
5827 TIESCALAR classname, LIST
5833 Not all methods indicated above need be implemented. See L<perltie>,
5834 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5836 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5837 for you--you need to do that explicitly yourself. See L<DB_File>
5838 or the F<Config> module for interesting C<tie> implementations.
5840 For further details see L<perltie>, L<"tied VARIABLE">.
5844 Returns a reference to the object underlying VARIABLE (the same value
5845 that was originally returned by the C<tie> call that bound the variable
5846 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5851 Returns the number of non-leap seconds since whatever time the system
5852 considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
5853 and 00:00:00 UTC, January 1, 1970 for most other systems).
5854 Suitable for feeding to C<gmtime> and C<localtime>.
5856 For measuring time in better granularity than one second,
5857 you may use either the Time::HiRes module (from CPAN, and starting from
5858 Perl 5.8 part of the standard distribution), or if you have
5859 gettimeofday(2), you may be able to use the C<syscall> interface of Perl.
5860 See L<perlfaq8> for details.
5864 Returns a four-element list giving the user and system times, in
5865 seconds, for this process and the children of this process.
5867 ($user,$system,$cuser,$csystem) = times;
5869 In scalar context, C<times> returns C<$user>.
5873 The transliteration operator. Same as C<y///>. See L<perlop>.
5875 =item truncate FILEHANDLE,LENGTH
5877 =item truncate EXPR,LENGTH
5879 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5880 specified length. Produces a fatal error if truncate isn't implemented
5881 on your system. Returns true if successful, the undefined value
5884 The behavior is undefined if LENGTH is greater than the length of the
5891 Returns an uppercased version of EXPR. This is the internal function
5892 implementing the C<\U> escape in double-quoted strings. Respects
5893 current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5894 and L<perlunicode> for more details about locale and Unicode support.
5895 It does not attempt to do titlecase mapping on initial letters. See
5896 C<ucfirst> for that.
5898 If EXPR is omitted, uses C<$_>.
5904 Returns the value of EXPR with the first character in uppercase
5905 (titlecase in Unicode). This is the internal function implementing
5906 the C<\u> escape in double-quoted strings. Respects current LC_CTYPE
5907 locale if C<use locale> in force. See L<perllocale> and L<perlunicode>
5908 for more details about locale and Unicode support.
5910 If EXPR is omitted, uses C<$_>.
5916 Sets the umask for the process to EXPR and returns the previous value.
5917 If EXPR is omitted, merely returns the current umask.
5919 The Unix permission C<rwxr-x---> is represented as three sets of three
5920 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5921 and isn't one of the digits). The C<umask> value is such a number
5922 representing disabled permissions bits. The permission (or "mode")
5923 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5924 even if you tell C<sysopen> to create a file with permissions C<0777>,
5925 if your umask is C<0022> then the file will actually be created with
5926 permissions C<0755>. If your C<umask> were C<0027> (group can't
5927 write; others can't read, write, or execute), then passing
5928 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5931 Here's some advice: supply a creation mode of C<0666> for regular
5932 files (in C<sysopen>) and one of C<0777> for directories (in
5933 C<mkdir>) and executable files. This gives users the freedom of
5934 choice: if they want protected files, they might choose process umasks
5935 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5936 Programs should rarely if ever make policy decisions better left to
5937 the user. The exception to this is when writing files that should be
5938 kept private: mail files, web browser cookies, I<.rhosts> files, and
5941 If umask(2) is not implemented on your system and you are trying to
5942 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5943 fatal error at run time. If umask(2) is not implemented and you are
5944 not trying to restrict access for yourself, returns C<undef>.
5946 Remember that a umask is a number, usually given in octal; it is I<not> a
5947 string of octal digits. See also L</oct>, if all you have is a string.
5953 Undefines the value of EXPR, which must be an lvalue. Use only on a
5954 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5955 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5956 will probably not do what you expect on most predefined variables or
5957 DBM list values, so don't do that; see L<delete>.) Always returns the
5958 undefined value. You can omit the EXPR, in which case nothing is
5959 undefined, but you still get an undefined value that you could, for
5960 instance, return from a subroutine, assign to a variable or pass as a
5961 parameter. Examples:
5964 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5968 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5969 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5970 select undef, undef, undef, 0.25;
5971 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5973 Note that this is a unary operator, not a list operator.
5979 Deletes a list of files. Returns the number of files successfully
5982 $cnt = unlink 'a', 'b', 'c';
5986 Note: C<unlink> will not delete directories unless you are superuser and
5987 the B<-U> flag is supplied to Perl. Even if these conditions are
5988 met, be warned that unlinking a directory can inflict damage on your
5989 filesystem. Use C<rmdir> instead.
5991 If LIST is omitted, uses C<$_>.
5993 =item unpack TEMPLATE,EXPR
5995 =item unpack TEMPLATE
5997 C<unpack> does the reverse of C<pack>: it takes a string
5998 and expands it out into a list of values.
5999 (In scalar context, it returns merely the first value produced.)
6001 If EXPR is omitted, unpacks the C<$_> string.
6003 The string is broken into chunks described by the TEMPLATE. Each chunk
6004 is converted separately to a value. Typically, either the string is a result
6005 of C<pack>, or the bytes of the string represent a C structure of some
6008 The TEMPLATE has the same format as in the C<pack> function.
6009 Here's a subroutine that does substring:
6012 my($what,$where,$howmuch) = @_;
6013 unpack("x$where a$howmuch", $what);
6018 sub ordinal { unpack("c",$_[0]); } # same as ord()
6020 In addition to fields allowed in pack(), you may prefix a field with
6021 a %<number> to indicate that
6022 you want a <number>-bit checksum of the items instead of the items
6023 themselves. Default is a 16-bit checksum. Checksum is calculated by
6024 summing numeric values of expanded values (for string fields the sum of
6025 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
6027 For example, the following
6028 computes the same number as the System V sum program:
6032 unpack("%32C*",<>) % 65535;
6035 The following efficiently counts the number of set bits in a bit vector:
6037 $setbits = unpack("%32b*", $selectmask);
6039 The C<p> and C<P> formats should be used with care. Since Perl
6040 has no way of checking whether the value passed to C<unpack()>
6041 corresponds to a valid memory location, passing a pointer value that's
6042 not known to be valid is likely to have disastrous consequences.
6044 If there are more pack codes or if the repeat count of a field or a group
6045 is larger than what the remainder of the input string allows, the result
6046 is not well defined: in some cases, the repeat count is decreased, or
6047 C<unpack()> will produce null strings or zeroes, or terminate with an
6048 error. If the input string is longer than one described by the TEMPLATE,
6049 the rest is ignored.
6051 See L</pack> for more examples and notes.
6053 =item untie VARIABLE
6055 Breaks the binding between a variable and a package. (See C<tie>.)
6056 Has no effect if the variable is not tied.
6058 =item unshift ARRAY,LIST
6060 Does the opposite of a C<shift>. Or the opposite of a C<push>,
6061 depending on how you look at it. Prepends list to the front of the
6062 array, and returns the new number of elements in the array.
6064 unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
6066 Note the LIST is prepended whole, not one element at a time, so the
6067 prepended elements stay in the same order. Use C<reverse> to do the
6070 =item use Module VERSION LIST
6072 =item use Module VERSION
6074 =item use Module LIST
6080 Imports some semantics into the current package from the named module,
6081 generally by aliasing certain subroutine or variable names into your
6082 package. It is exactly equivalent to
6084 BEGIN { require Module; import Module LIST; }
6086 except that Module I<must> be a bareword.
6088 VERSION may be either a numeric argument such as 5.006, which will be
6089 compared to C<$]>, or a literal of the form v5.6.1, which will be compared
6090 to C<$^V> (aka $PERL_VERSION. A fatal error is produced if VERSION is
6091 greater than the version of the current Perl interpreter; Perl will not
6092 attempt to parse the rest of the file. Compare with L</require>, which can
6093 do a similar check at run time.
6095 Specifying VERSION as a literal of the form v5.6.1 should generally be
6096 avoided, because it leads to misleading error messages under earlier
6097 versions of Perl which do not support this syntax. The equivalent numeric
6098 version should be used instead.
6100 use v5.6.1; # compile time version check
6102 use 5.006_001; # ditto; preferred for backwards compatibility
6104 This is often useful if you need to check the current Perl version before
6105 C<use>ing library modules that have changed in incompatible ways from
6106 older versions of Perl. (We try not to do this more than we have to.)
6108 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
6109 C<require> makes sure the module is loaded into memory if it hasn't been
6110 yet. The C<import> is not a builtin--it's just an ordinary static method
6111 call into the C<Module> package to tell the module to import the list of
6112 features back into the current package. The module can implement its
6113 C<import> method any way it likes, though most modules just choose to
6114 derive their C<import> method via inheritance from the C<Exporter> class that
6115 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
6116 method can be found then the call is skipped.
6118 If you do not want to call the package's C<import> method (for instance,
6119 to stop your namespace from being altered), explicitly supply the empty list:
6123 That is exactly equivalent to
6125 BEGIN { require Module }
6127 If the VERSION argument is present between Module and LIST, then the
6128 C<use> will call the VERSION method in class Module with the given
6129 version as an argument. The default VERSION method, inherited from
6130 the UNIVERSAL class, croaks if the given version is larger than the
6131 value of the variable C<$Module::VERSION>.
6133 Again, there is a distinction between omitting LIST (C<import> called
6134 with no arguments) and an explicit empty LIST C<()> (C<import> not
6135 called). Note that there is no comma after VERSION!
6137 Because this is a wide-open interface, pragmas (compiler directives)
6138 are also implemented this way. Currently implemented pragmas are:
6143 use sigtrap qw(SEGV BUS);
6144 use strict qw(subs vars refs);
6145 use subs qw(afunc blurfl);
6146 use warnings qw(all);
6147 use sort qw(stable _quicksort _mergesort);
6149 Some of these pseudo-modules import semantics into the current
6150 block scope (like C<strict> or C<integer>, unlike ordinary modules,
6151 which import symbols into the current package (which are effective
6152 through the end of the file).
6154 There's a corresponding C<no> command that unimports meanings imported
6155 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
6161 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
6162 for the C<-M> and C<-m> command-line options to perl that give C<use>
6163 functionality from the command-line.
6167 Changes the access and modification times on each file of a list of
6168 files. The first two elements of the list must be the NUMERICAL access
6169 and modification times, in that order. Returns the number of files
6170 successfully changed. The inode change time of each file is set
6171 to the current time. For example, this code has the same effect as the
6172 Unix touch(1) command when the files I<already exist>.
6176 utime $now, $now, @ARGV;
6178 B<Note:> Under NFS, touch(1) uses the time of the NFS server, not
6179 the time of the local machine. If there is a time synchronization
6180 problem, the NFS server and local machine will have different times.
6182 Since perl 5.7.2, if the first two elements of the list are C<undef>, then
6183 the utime(2) function in the C library will be called with a null second
6184 argument. On most systems, this will set the file's access and
6185 modification times to the current time (i.e. equivalent to the example
6188 utime undef, undef, @ARGV;
6192 Returns a list consisting of all the values of the named hash. (In a
6193 scalar context, returns the number of values.) The values are
6194 returned in an apparently random order. The actual random order is
6195 subject to change in future versions of perl, but it is guaranteed to
6196 be the same order as either the C<keys> or C<each> function would
6197 produce on the same (unmodified) hash.
6199 Note that the values are not copied, which means modifying them will
6200 modify the contents of the hash:
6202 for (values %hash) { s/foo/bar/g } # modifies %hash values
6203 for (@hash{keys %hash}) { s/foo/bar/g } # same
6205 As a side effect, calling values() resets the HASH's internal iterator.
6206 See also C<keys>, C<each>, and C<sort>.
6208 =item vec EXPR,OFFSET,BITS
6210 Treats the string in EXPR as a bit vector made up of elements of
6211 width BITS, and returns the value of the element specified by OFFSET
6212 as an unsigned integer. BITS therefore specifies the number of bits
6213 that are reserved for each element in the bit vector. This must
6214 be a power of two from 1 to 32 (or 64, if your platform supports
6217 If BITS is 8, "elements" coincide with bytes of the input string.
6219 If BITS is 16 or more, bytes of the input string are grouped into chunks
6220 of size BITS/8, and each group is converted to a number as with
6221 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
6222 for BITS==64). See L<"pack"> for details.
6224 If bits is 4 or less, the string is broken into bytes, then the bits
6225 of each byte are broken into 8/BITS groups. Bits of a byte are
6226 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
6227 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
6228 breaking the single input byte C<chr(0x36)> into two groups gives a list
6229 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
6231 C<vec> may also be assigned to, in which case parentheses are needed
6232 to give the expression the correct precedence as in
6234 vec($image, $max_x * $x + $y, 8) = 3;
6236 If the selected element is outside the string, the value 0 is returned.
6237 If an element off the end of the string is written to, Perl will first
6238 extend the string with sufficiently many zero bytes. It is an error
6239 to try to write off the beginning of the string (i.e. negative OFFSET).
6241 The string should not contain any character with the value > 255 (which
6242 can only happen if you're using UTF8 encoding). If it does, it will be
6243 treated as something which is not UTF8 encoded. When the C<vec> was
6244 assigned to, other parts of your program will also no longer consider the
6245 string to be UTF8 encoded. In other words, if you do have such characters
6246 in your string, vec() will operate on the actual byte string, and not the
6247 conceptual character string.
6249 Strings created with C<vec> can also be manipulated with the logical
6250 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
6251 vector operation is desired when both operands are strings.
6252 See L<perlop/"Bitwise String Operators">.
6254 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
6255 The comments show the string after each step. Note that this code works
6256 in the same way on big-endian or little-endian machines.
6259 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
6261 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
6262 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
6264 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
6265 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
6266 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
6267 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
6268 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
6269 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
6271 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
6272 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
6273 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
6276 To transform a bit vector into a string or list of 0's and 1's, use these:
6278 $bits = unpack("b*", $vector);
6279 @bits = split(//, unpack("b*", $vector));
6281 If you know the exact length in bits, it can be used in place of the C<*>.
6283 Here is an example to illustrate how the bits actually fall in place:
6289 unpack("V",$_) 01234567890123456789012345678901
6290 ------------------------------------------------------------------
6295 for ($shift=0; $shift < $width; ++$shift) {
6296 for ($off=0; $off < 32/$width; ++$off) {
6297 $str = pack("B*", "0"x32);
6298 $bits = (1<<$shift);
6299 vec($str, $off, $width) = $bits;
6300 $res = unpack("b*",$str);
6301 $val = unpack("V", $str);
6308 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6309 $off, $width, $bits, $val, $res
6313 Regardless of the machine architecture on which it is run, the above
6314 example should print the following table:
6317 unpack("V",$_) 01234567890123456789012345678901
6318 ------------------------------------------------------------------
6319 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
6320 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
6321 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
6322 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
6323 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
6324 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
6325 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
6326 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
6327 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
6328 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
6329 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
6330 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
6331 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
6332 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
6333 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
6334 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
6335 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
6336 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
6337 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
6338 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
6339 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
6340 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
6341 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
6342 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
6343 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
6344 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
6345 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
6346 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
6347 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
6348 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
6349 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
6350 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
6351 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
6352 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
6353 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
6354 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
6355 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
6356 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
6357 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
6358 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
6359 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
6360 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
6361 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
6362 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
6363 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
6364 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
6365 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
6366 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
6367 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
6368 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
6369 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
6370 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
6371 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
6372 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
6373 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
6374 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
6375 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
6376 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
6377 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
6378 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
6379 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
6380 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
6381 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
6382 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
6383 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
6384 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
6385 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
6386 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
6387 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
6388 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
6389 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
6390 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
6391 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
6392 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
6393 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
6394 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
6395 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
6396 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
6397 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
6398 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
6399 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
6400 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
6401 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
6402 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
6403 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
6404 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
6405 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
6406 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
6407 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
6408 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
6409 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
6410 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
6411 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
6412 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
6413 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
6414 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
6415 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
6416 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
6417 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
6418 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
6419 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
6420 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
6421 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
6422 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
6423 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
6424 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
6425 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
6426 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
6427 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
6428 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
6429 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
6430 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
6431 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
6432 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
6433 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
6434 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
6435 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
6436 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
6437 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
6438 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
6439 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
6440 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
6441 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
6442 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
6443 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
6444 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
6445 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
6446 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
6450 Behaves like the wait(2) system call on your system: it waits for a child
6451 process to terminate and returns the pid of the deceased process, or
6452 C<-1> if there are no child processes. The status is returned in C<$?>.
6453 Note that a return value of C<-1> could mean that child processes are
6454 being automatically reaped, as described in L<perlipc>.
6456 =item waitpid PID,FLAGS
6458 Waits for a particular child process to terminate and returns the pid of
6459 the deceased process, or C<-1> if there is no such child process. On some
6460 systems, a value of 0 indicates that there are processes still running.
6461 The status is returned in C<$?>. If you say
6463 use POSIX ":sys_wait_h";
6466 $kid = waitpid(-1, WNOHANG);
6469 then you can do a non-blocking wait for all pending zombie processes.
6470 Non-blocking wait is available on machines supporting either the
6471 waitpid(2) or wait4(2) system calls. However, waiting for a particular
6472 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
6473 system call by remembering the status values of processes that have
6474 exited but have not been harvested by the Perl script yet.)
6476 Note that on some systems, a return value of C<-1> could mean that child
6477 processes are being automatically reaped. See L<perlipc> for details,
6478 and for other examples.
6482 Returns true if the context of the currently executing subroutine is
6483 looking for a list value. Returns false if the context is looking
6484 for a scalar. Returns the undefined value if the context is looking
6485 for no value (void context).
6487 return unless defined wantarray; # don't bother doing more
6488 my @a = complex_calculation();
6489 return wantarray ? @a : "@a";
6491 This function should have been named wantlist() instead.
6495 Produces a message on STDERR just like C<die>, but doesn't exit or throw
6498 If LIST is empty and C<$@> already contains a value (typically from a
6499 previous eval) that value is used after appending C<"\t...caught">
6500 to C<$@>. This is useful for staying almost, but not entirely similar to
6503 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
6505 No message is printed if there is a C<$SIG{__WARN__}> handler
6506 installed. It is the handler's responsibility to deal with the message
6507 as it sees fit (like, for instance, converting it into a C<die>). Most
6508 handlers must therefore make arrangements to actually display the
6509 warnings that they are not prepared to deal with, by calling C<warn>
6510 again in the handler. Note that this is quite safe and will not
6511 produce an endless loop, since C<__WARN__> hooks are not called from
6514 You will find this behavior is slightly different from that of
6515 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
6516 instead call C<die> again to change it).
6518 Using a C<__WARN__> handler provides a powerful way to silence all
6519 warnings (even the so-called mandatory ones). An example:
6521 # wipe out *all* compile-time warnings
6522 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
6524 my $foo = 20; # no warning about duplicate my $foo,
6525 # but hey, you asked for it!
6526 # no compile-time or run-time warnings before here
6529 # run-time warnings enabled after here
6530 warn "\$foo is alive and $foo!"; # does show up
6532 See L<perlvar> for details on setting C<%SIG> entries, and for more
6533 examples. See the Carp module for other kinds of warnings using its
6534 carp() and cluck() functions.
6536 =item write FILEHANDLE
6542 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
6543 using the format associated with that file. By default the format for
6544 a file is the one having the same name as the filehandle, but the
6545 format for the current output channel (see the C<select> function) may be set
6546 explicitly by assigning the name of the format to the C<$~> variable.
6548 Top of form processing is handled automatically: if there is
6549 insufficient room on the current page for the formatted record, the
6550 page is advanced by writing a form feed, a special top-of-page format
6551 is used to format the new page header, and then the record is written.
6552 By default the top-of-page format is the name of the filehandle with
6553 "_TOP" appended, but it may be dynamically set to the format of your
6554 choice by assigning the name to the C<$^> variable while the filehandle is
6555 selected. The number of lines remaining on the current page is in
6556 variable C<$->, which can be set to C<0> to force a new page.
6558 If FILEHANDLE is unspecified, output goes to the current default output
6559 channel, which starts out as STDOUT but may be changed by the
6560 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
6561 is evaluated and the resulting string is used to look up the name of
6562 the FILEHANDLE at run time. For more on formats, see L<perlform>.
6564 Note that write is I<not> the opposite of C<read>. Unfortunately.
6568 The transliteration operator. Same as C<tr///>. See L<perlop>.